Medical Physiology [3 ed.] 9781455743773, 9780323427968, 2016005260

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Medical Physiology [3 ed.]
 9781455743773, 9780323427968, 2016005260

Table of contents :
Medical Physiology
Copyright Page
Contributors
Video Table of Contents
Preface to the Third Edition
The eBook
Acknowledgments
Preface to the First Edition
Target Audience
Content of the Textbook
Emphasis of the Textbook
Creating the Textbook
Special Features
Acknowledgments
Section I: Introduction
Chapter 1: Foundations of Physiology
What is physiology?
Physiological genomics is the link between the organ and the gene
Cells live in a highly protected milieu intérieur
Homeostatic mechanisms—operating through sophisticated feedback control mechanisms— are responsible for maintaining the constancy of the milieu intérieur
Medicine is the study of “physiology gone awry”
References
Section II: Physiology of Cells and Molecules
Chapter 2: Functional Organization of the Cell
Structure of Biological Membranes
The surface of the cell is defined by a membrane
The cell membrane is composed primarily of phospholipids
Phospholipids form complex structures in aqueous solution
The diffusion of individual lipids within a leaflet of a bilayer is determined by the chemical makeup of its constituents
Phospholipid bilayer membranes are impermeable to charged molecules
The plasma membrane is a bilayer
Membrane proteins can be integrally or peripherally associated with the plasma membrane
The membrane-spanning portions of transmembrane proteins are usually hydrophobic α helices
Some membrane proteins are mobile in the plane of the bilayer
Function of Membrane Proteins
Integral membrane proteins can serve as receptors
Integral membrane proteins can serve as adhesion molecules
Integral membrane proteins can carry out the transmembrane movement of water-soluble substances
Integral membrane proteins can also be enzymes
Integral membrane proteins can participate in intracellular signaling
Peripheral membrane proteins participate in intracellular signaling and can form a submembranous cytoskeleton
Cellular Organelles and the Cytoskeleton
The cell is composed of discrete organelles that subserve distinct functions
The nucleus stores, replicates, and reads the cell’s genetic information
Lysosomes digest material derived from the interior and exterior of the cell
The mitochondrion is the site of oxidative energy production
The cytoplasm is not amorphous but is organized by the cytoskeleton
Intermediate filaments provide cells with structural support
Microtubules provide structural support and provide the basis for several types of subcellular motility
Thin filaments (actin) and thick filaments (myosin) are present in almost every cell type
Synthesis and Recycling of Membrane Proteins
Secretory and membrane proteins are synthesized in association with the rough ER
Simultaneous protein synthesis and translocation through the rough ER membrane requires machinery for signal recognition and protein translocation
Proper insertion of membrane proteins requires start- and stop-transfer sequences
Newly synthesized secretory and membrane proteins undergo post-translational modification and folding in the lumen of the rough ER
Secretory and membrane proteins follow the secretory pathway through the cell
Carrier vesicles control the traffic between the organelles of the secretory pathway
Specialized protein complexes, such as clathrin and coatamers, mediate the formation and fusion of vesicles in the secretory pathway
Vesicle Formation in the Secretory Pathway
Vesicle Fusion in the Secretory Pathway
Newly synthesized secretory and membrane proteins are processed during their passage through the secretory pathway
Newly synthesized proteins are sorted in the trans-Golgi network
A mannose-6-phosphate recognition marker is required to target newly synthesized hydrolytic enzymes to lysosomes
Cells internalize extracellular material and plasma membrane through the process of endocytosis
Receptor-mediated endocytosis is responsible for internalizing specific proteins
Endocytosed proteins can be targeted to lysosomes or recycled to the cell surface
Certain molecules are internalized through an alternative process that involves caveolae
Specialized Cell Types
Epithelial cells form a barrier between the internal and external milieu
Tight Junctions
Adhering Junctions
Gap Junctions
Desmosomes
Epithelial cells are polarized
References
Books and Reviews
Journal Articles
Chapter 3: Signal Transduction
Mechanisms of Cellular Communication
Cells can communicate with one another via chemical signals
Soluble chemical signals interact with target cells via binding to surface or intracellular receptors
Cells can also communicate by direct interactions—juxtacrine signaling
Gap Junctions
Adhering and Tight Junctions
Membrane-Associated Ligands
Ligands in the Extracellular Matrix
Second-messenger systems amplify signals and integrate responses among cell types
Receptors That are Ion Channels
Ligand-gated ion channels transduce a chemical signal into an electrical signal
Receptors Coupled to G Proteins
General Properties of G Proteins
G proteins are heterotrimers that exist in many combinations of different α, β, and γ subunits
G-protein activation follows a cycle
Activated α subunits couple to a variety of downstream effectors, including enzymes and ion channels
βγ subunits can activate downstream effectors
Small GTP-binding proteins are involved in a vast number of cellular processes
G-Protein Second Messengers: Cyclic Nucleotides
cAMP usually exerts its effect by increasing the activity of protein kinase A
Protein phosphatases reverse the action of kinases
cGMP exerts its effect by stimulating a nonselective cation channel in the retina
G-Protein Second Messengers: Products of Phosphoinositide Breakdown
Many messengers bind to receptors that activate phosphoinositide breakdown
IP3 liberates Ca2+ from intracellular stores
Calcium activates calmodulin-dependent protein kinases
DAGs and Ca2+ activate protein kinase C
G-Protein Second Messengers: Arachidonic Acid Metabolites
Phospholipase A2 is the primary enzyme responsible for releasing AA
Cyclooxygenases, lipoxygenases, and epoxygenases mediate the formation of biologically active eicosanoids
Prostaglandins, prostacyclins, and thromboxanes (cyclooxygenase products) are vasoactive, regulate platelet action, and modulate ion transport N3-16
The leukotrienes (5-lipoxygenase products) play a major role in inflammatory responses
The HETEs and EETs (epoxygenase products) tend to enhance Ca2+ release from intracellular stores and to enhance cell proliferation
Degradation of the eicosanoids terminates their activity
Receptors That are Catalytic
The receptor guanylyl cyclase transduces the activity of atrial natriuretic peptide, whereas a soluble guanylyl cyclase transduces the activity of nitric oxide
Receptor (Membrane-Bound) Guanylyl Cyclase
Soluble Guanylyl Cyclase
Some catalytic receptors are serine/threonine kinases
RTKs produce phosphotyrosine motifs recognized by SH2 and phosphotyrosine-binding domains of downstream effectors
Creation of Phosphotyrosine Motifs
Recognition of pY Motifs by SH2 and Phosphotyrosine-Binding Domains
The MAPK Pathway
The Phosphatidylinositol-3-Kinase Pathway
Tyrosine kinase–associated receptors activate cytosolic tyrosine kinases such as Src and JAK
Receptor tyrosine phosphatases are required for lymphocyte activation
Nuclear Receptors
Steroid and thyroid hormones enter the cell and bind to members of the nuclear receptor superfamily in the cytoplasm or nucleus
Activated nuclear receptors bind to sequence elements in the regulatory region of responsive genes and either activate or repress DNA transcription
References
Books and Reviews
Journal Articles
Chapter 4: Regulation of Gene Expression
From Genes to Proteins
Gene expression differs among tissues and—in any tissue—may vary in response to external stimuli
Genetic information flows from DNA to proteins
The gene consists of a transcription unit
DNA is packaged into chromatin
Gene expression may be regulated at multiple steps
Transcription factors are proteins that regulate gene transcription
The Promoter and Regulatory Elements
The basal transcriptional machinery mediates gene transcription
The promoter determines the initiation site and direction of transcription
Positive and negative regulatory elements modulate gene transcription
Locus control regions and insulator elements influence transcription within multigene chromosomal domains
Transcription Factors
DNA-binding transcription factors recognize specific DNA sequences
Transcription factors that bind to DNA can be grouped into families based on tertiary structure
Zinc Finger
Basic Zipper
Basic Helix-Loop-Helix
Helix-Turn-Helix
Coactivators and corepressors are transcription factors that do not bind to DNA
Transcriptional activators stimulate transcription by three mechanisms
Recruitment of the Basal Transcriptional Machinery
Chromatin Remodeling
Stimulation of Pol II
Transcriptional activators act in combination
Transcriptional repressors act by competition, quenching, or active repression
The activity of transcription factors may be regulated by post-translational modifications
Phosphorylation
Site-Specific Proteolysis
Other Post-Translational Modifications
The expression of some transcription factors is tissue specific
Regulation of Inducible Gene Expression by Signal-Transduction Pathways
cAMP regulates transcription via the transcription factors CREB and CBP
Receptor tyrosine kinases regulate transcription via a Ras-dependent cascade of protein kinases
Tyrosine kinase–associated receptors can regulate transcription via JAK-STAT
Nuclear receptors are transcription factors
Modular Construction
Dimerization
Activation of Transcription
Repression of Transcription
Physiological stimuli can modulate transcription factors, which can coordinate complex cellular responses
Epigenetic Regulation of Gene Expression
Epigenetic regulation can result in long-term gene silencing
Alterations in chromatin structure may mediate epigenetic regulation, stimulating or inhibiting gene transcription
Histone methylation may stimulate or inhibit gene expression
DNA methylation is associated with gene inactivation
Post-Translational Regulation of Gene Expression
Alternative splicing generates diversity from single genes
Retained Intron
Alternative 3′ Splice Sites
Alternative 5′ Splice Sites
Cassette Exons
Mutually Exclusive Exons
Alternative 5′ Ends
Alternative 3′ Ends
Regulatory elements in the 3′ untranslated region control mRNA stability
MicroRNAs regulate mRNA abundance and translation
References
Books and Reviews
Journal Articles
Glossary
Chapter 5: Transport of Solutes and Water
The Intracellular and Extracellular Fluids
Total-body water is the sum of the ICF and ECF volumes
Plasma Volume
Interstitial Fluid
Transcellular Fluid
ICF is rich in K+, whereas ECF is rich in Na+ and Cl−
Volume Occupied by Plasma Proteins
Effect of Protein Charge
All body fluids have approximately the same osmolality, and each fluid has equal numbers of positive and negative charges
Osmolality
Electroneutrality
Solute Transport Across Cell Membranes
In passive, noncoupled transport across a permeable membrane, a solute moves down its electrochemical gradient
At equilibrium, the chemical and electrical potential energy differences across the membrane are equal but opposite
(Vm − EX) is the net electrochemical driving force acting on an ion
In simple diffusion, the flux of an uncharged substance through membrane lipid is directly proportional to its concentration difference
Some substances cross the membrane passively through intrinsic membrane proteins that can form pores, channels, or carriers
Water-filled pores can allow molecules, some as large as 45 kDa, to cross membranes passively
Gated channels, which alternately open and close, allow ions to cross the membrane passively
Na+ Channels
K+ Channels
Ca2+ Channels
Proton Channels
Anion Channels
Some carriers facilitate the passive diffusion of small solutes such as glucose
The physical structures of pores, channels, and carriers are quite similar
The Na-K pump, the most important primary active transporter in animal cells, uses the energy of ATP to extrude Na+ and take up K+
Besides the Na-K pump, other P-type ATPases include the H-K and Ca pumps
H-K Pump
Ca Pumps
Other Pumps
The F-type and the V-type ATPases transport H+
F-type or FoF1 ATPases
V-type H Pump
ATP-binding cassette transporters can act as pumps, channels, or regulators
ABCA Subfamily
MDR Subfamily
MRP/CFTR Subfamily
Cotransporters, one class of secondary active transporters, are generally driven by the energy of the inwardly directed Na+ gradient
Na/Glucose Cotransporter
Na+-Driven Cotransporters for Organic Solutes
Na/HCO3 Cotransporters
Na+-Driven Cotransporters for Other Inorganic Anions
Na/K/Cl Cotransporter
Na/Cl Cotransporter
K/Cl Cotransporter
H+-Driven Cotransporters
Exchangers, another class of secondary active transporters, exchange ions for one another
Na-Ca Exchanger
Na-H Exchanger
Na+-Driven Cl-HCO3 Exchanger
Cl-HCO3 Exchanger
Other Anion Exchangers
Regulation of Intracellular Ion Concentrations
The Na-K pump keeps [Na+] inside the cell low and [K+] high
The Ca pump and the Na-Ca exchanger keep intracellular [Ca2+] four orders of magnitude lower than extracellular [Ca2+]
Ca Pump (SERCA) in Organelle Membranes
Ca Pump (PMCA) on the Plasma Membrane
Na-Ca Exchanger (NCX) on the Plasma Membrane
In most cells, [Cl−] is modestly above equilibrium because Cl− uptake by the Cl-HCO3 exchanger and Na/K/Cl cotransporter balances passive Cl− efflux through channels
The Na-H exchanger and Na+-driven transporters keep the intracellular pH and [] above their equilibrium values
Water Transport and the Regulation of Cell Volume
Water transport is driven by osmotic and hydrostatic pressure differences across membranes
Because of the presence of impermeant, negatively charged proteins within the cell, Donnan forces will lead to cell swelling
The Na-K pump maintains cell volume by doing osmotic work that counteracts the passive Donnan forces
Cell volume changes trigger rapid changes in ion channels or transporters, returning volume toward normal
Response to Cell Shrinkage
Response to Cell Swelling
Cells respond to long-term hyperosmolality by accumulating new intracellular organic solutes
The gradient in tonicity—or effective osmolality—determines the osmotic flow of water across a cell membrane
Water Exchange Across Cell Membranes
Water Exchange Across the Capillary Wall
Adding isotonic saline, pure water, or pure NaCl to the ECF will increase ECF volume but will have divergent effects on ICF volume and ECF osmolality
Infusion of Isotonic Saline
Infusion of “Solute-Free” Water
Ingestion of Pure NaCl Salt
Whole-body Na+ content determines ECF volume, whereas whole-body water content determines osmolality
Transport of Solutes and Water Across Epithelia
The epithelial cell generally has different electrochemical gradients across its apical and basolateral membranes
Tight and leaky epithelia differ in the permeabilities of their tight junctions
Epithelial cells can absorb or secrete different solutes by inserting specific channels or transporters at either the apical or basolateral membrane
Na+ Absorption
K+ Secretion
Glucose Absorption
Cl− Secretion
Water transport across epithelia passively follows solute transport
Absorption of a Hyperosmotic Fluid
Absorption of an Isosmotic Fluid
Absorption of a Hypo-osmotic Fluid
Epithelia can regulate transport by controlling transport proteins, tight junctions, and the supply of the transported substances
Increased Synthesis (or Degradation) of Transport Proteins
Recruitment of Transport Proteins to the Cell Membrane
Post-translational Modification of Pre-existing Transport Proteins
Changes in the Paracellular Pathway
Luminal Supply of Transported Species and Flow Rate
References
Books and Reviews
Journal Articles
Chapter 6: Electrophysiology of the Cell Membrane
Ionic Basis of Membrane Potentials
Principles of electrostatics explain why aqueous pores formed by channel proteins are needed for ion diffusion across cell membranes
Membrane potentials can be measured with microelectrodes as well as dyes or fluorescent proteins that are voltage sensitive
Membrane potential is generated by ion gradients
For mammalian cells, Nernst potentials for ions typically range from −100 mV for K+ to +100 mV for Ca2+
Currents carried by ions across membranes depend on the concentration of ions on both sides of the membrane, the membrane potential, and the permeability of the membrane to each ion
Membrane potential depends on ionic concentration gradients and permeabilities
Electrical Model of a Cell Membrane
The cell membrane model includes various ionic conductances and electromotive forces in parallel with a capacitor
The separation of relatively few charges across the bilayer capacitance maintains the membrane potential
Ionic current is directly proportional to the electrochemical driving force (Ohm’s law)
Capacitative current is proportional to the rate of voltage change
A voltage clamp measures currents across cell membranes
The patch-clamp technique resolves unitary currents through single channel molecules
Single channel currents sum to produce macroscopic membrane currents
Single channels can fluctuate between open and closed states
Molecular Physiology of Ion Channels
Classes of ion channels can be distinguished on the basis of electrophysiology, pharmacological and physiological ligands, intracellular messengers, and sequence homology
Electrophysiology
Pharmacological Ligands
Physiological Ligands
Intracellular Messengers
Sequence Homology
Many channels are formed by a radially symmetric arrangement of subunits or domains around a central pore
Gap junction channels are made up of two connexons, each of which has six identical subunits called connexins
An evolutionary tree called a dendrogram illustrates the relatedness of ion channels
Hydrophobic domains of channel proteins can predict how these proteins weave through the membrane
Protein superfamilies, subfamilies, and subtypes are the structural bases of channel diversity
Connexins
K+ Channels
HCN, CNG, and TRP Channels
NAADP Receptor
Voltage-Gated Na+ Channels
Voltage-Gated Ca2+ Channels
CatSper Channels
Hv Channels
Ligand-Gated Channels
Other Ion Channels
References
Books and Reviews
Journal Articles
Chapter 7: Electrical Excitability and Action Potentials
Mechanisms of Nerve and Muscle Action Potentials
An action potential is a transient depolarization triggered by a depolarization beyond a threshold
In contrast to an action potential, a graded response is proportional to stimulus intensity and decays with distance along the axon
Excitation of a nerve or muscle depends on the product (strength × duration) of the stimulus and on the refractory period
The action potential arises from changes in membrane conductance to Na+ and K+
The Na+ and K+ currents that flow during the action potential are time and voltage dependent
Time Dependence of Na+ and K+ Currents
Voltage Dependence of Na+ and K+ Currents
Macroscopic Na+ and K+ currents result from the opening and closing of many channels
The Hodgkin-Huxley model predicts macroscopic currents and the shape of the action potential
Physiology of Voltage-Gated Channels and Their Relatives
A large superfamily of structurally related membrane proteins includes voltage-gated and related channels
Na+ channels generate the rapid initial depolarization of the action potential
Na+ channels are blocked by neurotoxins and local anesthetics
Ca2+ channels contribute to action potentials in some cells and also function in electrical and chemical coupling mechanisms
Ca2+ channels are characterized as L-, T-, P/Q-, N-, and R-type channels on the basis of kinetic properties and inhibitor sensitivity
K+ channels determine resting potential and regulate the frequency and termination of action potentials
The Kv (or Shaker-related) family of K+ channels mediates both the delayed outward-rectifier current and the transient A-type current
Two families of KCa K+ channels mediate Ca2+-activated K+ currents
The Kir K+ channels mediate inward-rectifier K+ currents, and K2P channels may sense stress
Propagation of Action Potentials
The propagation of electrical signals in the nervous system involves local current loops
Myelin improves the efficiency with which axons conduct action potentials
The cable properties of the membrane and cytoplasm determine the velocity of signal propagation
References
Books and Reviews
Journal Articles
Chapter 8: Synaptic Transmission and the Neuromuscular Junction
Mechanisms of Synaptic Transmission
Electrical continuity between cells is established by electrical or chemical synapses
Electrical synapses directly link the cytoplasm of adjacent cells
Chemical synapses use neurotransmitters to provide electrical continuity between adjacent cells
Neurotransmitters can activate ionotropic or metabotropic receptors
Synaptic Transmission at the Neuromuscular Junction
Neuromuscular junctions are specialized synapses between motor neurons and skeletal muscle
ACh activates nicotinic AChRs to produce an excitatory end-plate current
The nicotinic AChR is a member of the pentameric Cys-loop receptor family of ligand-gated ion channels
Activation of AChR channels requires binding of two ACh molecules
Miniature EPPs reveal the quantal nature of transmitter release from the presynaptic terminals
Direct sensing of extracellular transmitter also shows quantal release of transmitter
Synaptic vesicles package, store, and deliver neurotransmitters
Neurotransmitter release occurs by exocytosis of synaptic vesicles
Re-uptake or cleavage of the neurotransmitter terminates its action
Toxins and Drugs Affecting Synaptic Transmission
Guanidinium neurotoxins such as tetrodotoxin prevent depolarization of the nerve terminal, whereas dendrotoxins inhibit repolarization
ω-Conotoxin blocks Ca2+ channels that mediate Ca2+ influx into nerve terminals, inhibiting synaptic transmission
Bacterial toxins such as tetanus and botulinum toxins cleave proteins involved in exocytosis, preventing fusion of synaptic vesicles
Both agonists and antagonists of the nicotinic AChR can prevent synaptic transmission
Inhibitors of AChE prolong and magnify the EPP
References
Books and Reviews
Journal Articles
Chapter 9: Cellular Physiology of Skeletal, Cardiac, and Smooth Muscle
Skeletal Muscle
Contraction of skeletal muscle is initiated by motor neurons that innervate motor units
Action potentials propagate from the sarcolemma to the interior of muscle fibers along the transverse tubule network
Depolarization of the T-tubule membrane results in Ca2+ release from the SR at the triad
Striations of skeletal muscle fibers correspond to ordered arrays of thick and thin filaments within myofibrils
Thin and thick filaments are supramolecular assemblies of protein subunits
Thin Filaments
Thick Filaments
During the cross-bridge cycle, contractile proteins convert the energy of ATP hydrolysis into mechanical energy
An increase in [Ca2+]i triggers contraction by removing the inhibition of cross-bridge cycling
Termination of contraction requires re-uptake of Ca2+ into the SR
Muscle contractions produce force under isometric conditions and force with shortening under isotonic conditions
Muscle length influences tension development by determining the degree of overlap between actin and myosin filaments
At higher loads, the velocity of shortening is lower because more cross-bridges are simultaneously active
In a single skeletal muscle fiber, the force developed may be increased by summing multiple twitches in time
In a whole skeletal muscle, the force developed may be increased by summing the contractions of multiple fibers
Cardiac Muscle
Action potentials propagate between adjacent cardiac myocytes through gap junctions
Cardiac contraction requires Ca2+ entry through L-type Ca2+ channels
Cross-bridge cycling and termination of cardiac muscle contraction are similar to the events in skeletal muscle
In cardiac muscle, increasing the entry of Ca2+ enhances the contractile force
Smooth Muscle
Smooth muscles may contract in response to synaptic transmission or electrical coupling
Action potentials of smooth muscles may be brief or prolonged
Some smooth-muscle cells spontaneously generate either pacemaker currents or slow waves
Some smooth muscles contract without action potentials
In smooth muscle, both entry of extracellular Ca2+ and intracellular Ca2+ spark activate contraction
Ca2+ Entry via Voltage-Gated Channels
Ca2+ Release from the SR
Ca2+ Entry through Store-Operated Ca2+ Channels (SOCs)
Ca2+-dependent phosphorylation of the myosin regulatory light chain activates cross-bridge cycling in smooth muscle
Termination of smooth-muscle contraction requires dephosphorylation of myosin light chain
Smooth-muscle contraction may also occur independently of increases in [Ca2+]i
In smooth muscle, increases in both [Ca2+]i and the Ca2+ sensitivity of the contractile apparatus enhance contractile force
Smooth muscle maintains high force at low energy consumption
Diversity among Muscles
Skeletal muscle is composed of slow-twitch and fast-twitch fibers
The properties of cardiac cells vary with location in the heart
The properties of smooth-muscle cells differ markedly among tissues and may adapt with time
Smooth-muscle cells express a wide variety of neurotransmitter and hormone receptors
References
Books and Reviews
Journal Articles
Section III: The Nervous System
Chapter 10: Organization of the Nervous System
The nervous system can be divided into central, peripheral, and autonomic nervous systems
Each area of the nervous system has unique nerve cells and a different function
Cells of the Nervous System
The neuron doctrine first asserted that the nervous system is composed of many individual signaling units—the neurons
Nerve cells have four specialized regions: cell body, dendrites, axon, and presynaptic terminals
Cell Body
Dendrites
Axon
Presynaptic Terminals
The cytoskeleton helps compartmentalize the neuron and also provides the tracks along which material travels between different parts of the neuron
Fast Axoplasmic Transport
Fast Retrograde Transport
Slow Axoplasmic Transport
Neurons can be classified on the basis of their axonal projection, their dendritic geometry, and the number of processes emanating from the cell body
Axonal Projection
Dendritic Geometry
Number of Processes
Glial cells provide a physiological environment for neurons
Development of Neurons and Glial Cells
Neurons differentiate from the neuroectoderm
Neurons and glial cells originate from cells in the proliferating germinal matrix near the ventricles
Neurons migrate to their correct anatomical position in the brain with the help of adhesion molecules
Neurons do not regenerate
Neurons
Axons
Glia
Subdivisions of the Nervous System
The CNS consists of the telencephalon, cerebellum, diencephalon, midbrain, pons, medulla, and spinal cord
Telencephalon
Cerebellum
Diencephalon
Brainstem (Midbrain, Pons, and Medulla)
Spinal Cord
The PNS comprises the cranial and spinal nerves, their associated sensory ganglia, and various sensory receptors
The ANS innervates effectors that are not under voluntary control
References
Books and Reviews
Journal Articles
Chapter 11: The Neuronal Microenvironment
Extracellular fluid in the brain provides a highly regulated environment for central nervous system neurons
The brain is physically and metabolically fragile
Cerebrospinal Fluid
CSF fills the ventricles and subarachnoid space
The brain floats in CSF, which acts as a shock absorber
The choroid plexuses secrete CSF into the ventricles, and the arachnoid granulations absorb it
The epithelial cells of the choroid plexus secrete the CSF
Brain Extracellular Space
Neurons, glia, and capillaries are packed tightly together in the CNS
The CSF communicates freely with the BECF, which stabilizes the composition of the neuronal microenvironment
The ion fluxes that accompany neural activity cause large changes in extracellular ion concentration
The Blood-Brain Barrier
The blood-brain barrier prevents some blood constituents from entering the brain extracellular space
Continuous tight junctions link brain capillary endothelial cells
Uncharged and lipid-soluble molecules more readily pass through the blood-brain barrier
Transport by capillary endothelial cells contributes to the blood-brain barrier
Glial Cells
Glial cells constitute half the volume of the brain and outnumber neurons
Astrocytes supply fuel to neurons in the form of lactic acid
Astrocytes are predominantly permeable to K+ and also help regulate [K+]o
Gap junctions couple astrocytes to one another, allowing diffusion of small solutes
Astrocytes synthesize neurotransmitters, take them up from the extracellular space, and have neurotransmitter receptors
Astrocytes secrete trophic factors that promote neuronal survival and synaptogenesis
Astrocytic endfeet modulate cerebral blood flow
Oligodendrocytes and Schwann cells make and sustain myelin
Oligodendrocytes are involved in pH regulation and iron metabolism in the brain
Microglial cells are the macrophages of the CNS
References
Books and Reviews
Journal Articles
Chapter 12: Physiology of Neurons
Neurons receive, combine, transform, store, and send information
Neural information flows from dendrite to soma to axon to synapse
Signal Conduction in Dendrites
Dendrites attenuate synaptic potentials
Dendritic membranes have voltage-gated ion channels
Control of Spiking Patterns in the Soma
Neurons can transform a simple input into a variety of output patterns
Intrinsic firing patterns are determined by a variety of ion currents with relatively slow kinetics
Axonal Conduction
Axons are specialized for rapid, reliable, and efficient transmission of electrical signals
Action potentials are usually initiated at the initial segment
Conduction velocity of a myelinated axon increases linearly with diameter
Demyelinated axons conduct action potentials slowly, unreliably, or not at all
References
Books and Reviews
Journal Articles
Chapter 13: Synaptic Transmission in the Nervous System
Neuronal Synapses
The molecular mechanisms of neuronal synapses are similar but not identical to those of the neuromuscular junction
Presynaptic terminals may contact neurons at the dendrite, soma, or axon and may contain both clear vesicles and dense-core granules
The postsynaptic membrane contains transmitter receptors and numerous proteins clustered in the postsynaptic density
Some transmitters are used by diffusely distributed systems of neurons to modulate the general excitability of the brain
Electrical synapses serve specialized functions in the mammalian nervous system
Neurotransmitter Systems of the Brain
Most of the brain’s transmitters are common biochemicals
Synaptic transmitters can stimulate, inhibit, or modulate the postsynaptic neuron
Excitatory Synapses
Inhibitory Synapses
Modulatory Synapses
G proteins may affect ion channels directly, or indirectly through second messengers
Signaling cascades allow amplification, regulation, and a long duration of transmitter responses
Neurotransmitters may have both convergent and divergent effects
Fast Amino Acid–Mediated Synapses in the CNS
Most EPSPs in the brain are mediated by two types of glutamate-gated channels
Most IPSPs in the brain are mediated by the GABAA receptor, which is activated by several classes of drugs
The ionotropic receptors for ACh, serotonin, GABA, and glycine belong to the superfamily of ligand-gated/pentameric channels
Most neuronal synapses release a very small number of transmitter quanta with each action potential
When multiple transmitters colocalize to the same synapse, the exocytosis of large vesicles requires high-frequency stimulation
Plasticity of Central Synapses
Use-dependent changes in synaptic strength underlie many forms of learning
Short-term synaptic plasticity usually reflects presynaptic changes
Long-term potentiation in the hippocampus may last for days or weeks
Long-term depression exists in multiple forms
Long-term depression in the cerebellum may be important for motor learning
References
Books and Reviews
Journal Articles
Chapter 14: The Autonomic Nervous System
Organization of the Visceral Control System
The ANS has sympathetic, parasympathetic, and enteric divisions
Sympathetic preganglionic neurons originate from spinal segments T1 to L3 and synapse with postganglionic neurons in paravertebral or prevertebral ganglia
Preganglionic Neurons
Paravertebral Ganglia
Prevertebral Ganglia
Postganglionic Neurons
Cranial Nerves III, VII, and IX
Cranial Nerve X
Sacral Nerves
The visceral control system also has an important afferent limb
The enteric division is a self-contained nervous system of the GI tract and receives sympathetic and parasympathetic input
Synaptic Physiology of the Autonomic Nervous System
The sympathetic and parasympathetic divisions have opposite effects on most visceral targets
All preganglionic neurons—both sympathetic and parasympathetic—release acetylcholine and stimulate N2 nicotinic receptors on postganglionic neurons
All postganglionic parasympathetic neurons release ACh and stimulate muscarinic receptors on visceral targets
Most postganglionic sympathetic neurons release norepinephrine onto visceral targets
Postganglionic sympathetic and parasympathetic neurons often have muscarinic as well as nicotinic receptors
Nonclassic transmitters can be released at each level of the ANS
Two of the most unusual nonclassic neurotransmitters, ATP and nitric oxide, were first identified in the ANS
ATP
Nitric Oxide
Central Nervous System Control of the Viscera
Sympathetic output can be massive and nonspecific, as in the fight-or-flight response, or selective for specific target organs
Parasympathetic neurons participate in many simple involuntary reflexes
A variety of brainstem nuclei provide basic control of the ANS
The forebrain can modulate autonomic output, and reciprocally, visceral sensory input integrated in the brainstem can influence or even overwhelm the forebrain
CNS control centers oversee visceral feedback loops and orchestrate a feed-forward response to meet anticipated needs
The ANS has multiple levels of reflex loops
References
Books and Reviews
Journal Articles
Chapter 15: Sensory Transduction
Sensory receptors convert environmental energy into neural signals
Sensory transduction uses adaptations of common molecular signaling mechanisms
Sensory transduction requires detection and amplification, usually followed by a local receptor potential
Chemoreception
Chemoreceptors are ubiquitous, diverse, and evolutionarily ancient
Taste receptors are modified epithelial cells, whereas olfactory receptors are neurons
Taste Receptor Cells
Olfactory Receptor Cells
Complex flavors are derived from a few basic types of taste receptors, with contributions from sensory receptors of smell, temperature, texture, and pain
Taste transduction involves many types of molecular signaling systems
Salty
Sour
Sweet
Bitter
Amino Acids
Olfactory transduction involves specific receptors, G protein–coupled signaling, and a cyclic nucleotide–gated ion channel
Visual Transduction
The optical components of the eye collect light and focus it onto the retina
The retina is a small, displaced part of the CNS
There are three primary types of photoreceptors: rods, cones, and intrinsically photosensitive ganglion cells
Rods and cones hyperpolarize in response to light
Rhodopsin is a G protein–coupled “receptor” for light
The eye uses a variety of mechanisms to adapt to a wide range of light levels
Color vision depends on the different spectral sensitivities of the three types of cones
The ipRGCs have unique properties and functions
Vestibular and Auditory Transduction: Hair Cells
Bending the stereovilli of hair cells along one axis causes cation channels to open or to close
The otolithic organs (saccule and utricle) detect the orientation and linear acceleration of the head
The semicircular canals detect the angular acceleration of the head
The outer and middle ears collect and condition air pressure waves for transduction within the inner ear
Outer Ear
Middle Ear
The cochlea is a spiral of three parallel, fluid-filled tubes
Inner hair cells transduce sound, whereas the active movements of outer hair cells amplify the signal
The frequency sensitivity of auditory hair cells depends on their position along the basilar membrane of the cochlea
Somatic Sensory Receptors, Proprioception, and Pain
A variety of sensory endings in the skin transduce mechanical, thermal, and chemical stimuli
Mechanoreceptors in the skin provide sensitivity to specific stimuli such as vibration and steady pressure
Separate thermoreceptors detect warmth and cold
Nociceptors are specialized sensory endings that transduce painful stimuli
Muscle spindles sense changes in the length of skeletal muscle fibers, whereas Golgi tendon organs gauge the muscle’s force
References
Books and Reviews
Journal Articles
Chapter 16: Circuits of the Central Nervous System
Elements of Neural Circuits
Neural circuits process sensory information, generate motor output, and create spontaneous activity
Nervous systems have several levels of organization
Most local circuits have three elements: input axons, interneurons, and projection (output) neurons
Simple, Stereotyped Responses: Spinal Reflex Circuits
Passive stretching of a skeletal muscle causes a reflexive contraction of that same muscle and relaxation of the antagonist muscles
Force applied to the Golgi tendon organ regulates muscle contractile strength
Noxious stimuli can evoke complex reflexive movements
Spinal reflexes are strongly influenced by control centers within the brain
Rhythmic Activity: Central Pattern Generators
Central pattern generators in the spinal cord can create a complex motor program even without sensory feedback
Pacemaker cells and synaptic interconnections both contribute to central pattern generation
Central pattern generators in the spinal cord take advantage of sensory feedback, interconnections among spinal segments, and interactions with brainstem control centers
Spatial Representations: Sensory and Motor Maps in the Brain
The nervous system contains maps of sensory and motor information
The cerebral cortex has multiple visuotopic maps
Maps of somatic sensory information magnify some parts of the body more than others
The cerebral cortex has a motor map that is adjacent to and well aligned with the somatosensory map
Sensory and motor maps are fuzzy and plastic
Temporal Representations: Time-Measuring Circuits
To localize sound, the brain compares the timing and intensity of input to the ears
The brain measures interaural timing by a combination of neural delay lines and coincidence detectors
References
Books and Reviews
Journal Articles
Section IV: The Cardiovascular System
Chapter 17: Organization of the Cardiovascular System
Elements of the Cardiovascular System
The circulation is an evolutionary consequence of body size
The heart is a dual pump that drives the blood in two serial circuits: the systemic and the pulmonary circulations
Hemodynamics
Blood flow is driven by a constant pressure head across variable resistances
Blood pressure is always measured as a pressure difference between two points
Total blood flow, or cardiac output, is the product (heart rate) × (stroke volume)
Flow in an idealized vessel increases with the fourth power of radius (Poiseuille equation)
Viscous resistance to flow is proportional to the viscosity of blood but does not depend on properties of the blood vessel walls
The viscosity of blood is a measure of the internal slipperiness between layers of fluid
How Blood Flows
Blood flow is laminar
Pressure and flow oscillate with each heartbeat between maximum systolic and minimum diastolic values
Origins of Pressure in the Circulation
Gravity causes a hydrostatic pressure difference when there is a difference in height
Low compliance of a vessel causes the transmural pressure to increase when the vessel blood volume is increased
The viscous resistance of blood causes an axial pressure difference when there is flow
The inertia of the blood and vessels causes pressure to decrease when the velocity of blood flow increases
How to Measure Blood Pressure, Blood Flow, and Cardiac Volumes
Blood pressure can be measured directly by puncturing the vessel
Blood pressure can be measured indirectly by use of a sphygmomanometer
Blood flow can be measured directly by electromagnetic and ultrasound flowmeters
Invasive Methods
Noninvasive Methods
Cardiac output can be measured indirectly by the Fick method, which is based on the conservation of mass
Cardiac output can be measured indirectly by dilution methods
Regional blood flow can be measured indirectly by “clearance” methods
Ventricular dimensions, ventricular volumes, and volume changes can be measured by angiography and echocardiography
References
Books and Reviews
Journal Articles
Chapter 18: Blood
Blood Composition
Whole blood is a suspension of cellular elements in plasma
Bone marrow is the source of most blood cells
RBCs are mainly composed of hemoglobin
Leukocytes defend against infections
Neutrophils
Eosinophils
Basophils
Lymphocytes
Monocytes
Platelets are nucleus-free fragments
Blood Viscosity
Whole blood has an anomalous viscosity
Blood viscosity increases with the hematocrit and the fibrinogen plasma concentration
Fibrinogen
Hematocrit
Vessel Radius
Velocity of Flow
Temperature
Hemostasis and Fibrinolysis
Platelets can plug holes in small vessels
Adhesion
Activation
Aggregation
A controlled cascade of proteolysis creates a blood clot
Intrinsic Pathway (Surface Contact Activation)
Extrinsic Pathway (Tissue Factor Activation)
Common Pathway
Coagulation as a Connected Diagram
Anticoagulants keep the clotting network in check
Paracrine Factors
Anticoagulant Factors
Fibrinolysis breaks up clots
References
Books and Reviews
Journal Articles
Chapter 19: Arteries and Veins
Arterial Distribution and Venous Collection Systems
Physical properties of vessels closely follow the level of branching in the circuit
Most of the blood volume resides in the systemic veins
The intravascular pressures along the systemic circuit are higher than those along the pulmonary circuit
Under normal conditions, the steepest pressure drop in the systemic circulation occurs in arterioles, the site of greatest vascular resistance
Local intravascular pressure depends on the distribution of vascular resistance
Elastic Properties of Blood Vessels
Blood vessels are elastic tubes
Because of the elastic properties of vessels, the pressure-flow relationship of passive vascular beds is nonlinear
Contraction of smooth muscle halts blood flow when driving pressure falls below the critical closing pressure
Elastic and collagen fibers determine the distensibility and compliance of vessels
Differences in compliance cause arteries to act as resistors and veins to act as capacitors
Laplace’s law describes how tension in the vessel wall increases with transmural pressure
The vascular wall is adapted to withstand wall tension, not transmural pressure
Elastin and collagen separately contribute to the wall tension of vessels
Aging reduces the distensibility of arteries
Active tension from smooth-muscle activity adds to the elastic tension of vessels
Elastic tension helps stabilize vessels under vasomotor control
References
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Journal Articles
Chapter 20: The Microcirculation
The microcirculation serves both nutritional and non-nutritional roles
The microcirculation extends from the arterioles to the venules
Capillary Exchange of Solutes
The exchange of O2 and CO2 across capillaries depends on the diffusional properties of the surrounding tissue
The O2 extraction ratio of a whole organ depends primarily on blood flow and metabolic demand
According to Fick’s law, the diffusion of small water-soluble solutes across a capillary wall depends on both the permeability and the concentration gradient
The whole-organ extraction ratio for small hydrophilic solutes provides an estimate of the solute permeability of capillaries
Small polar molecules have a relatively low permeability because they can traverse the capillary wall only by diffusing through water-filled pores (small-pore effect)
The exchange of macromolecules across capillaries can occur by transcytosis (large-pore effect)
Capillary Exchange of Water
Fluid transfer across capillaries is convective and depends on net hydrostatic and osmotic forces (i.e., Starling forces)
Capillary blood pressure (Pc) falls from ~35 mm Hg at the arteriolar end to ~15 mm Hg at the venular end
Arteriolar (Pa) and Venular (Pv) Pressure
Location
Time
Gravity
Interstitial fluid pressure (Pif) is slightly negative, except in encapsulated organs
Capillary colloid osmotic pressure (πc), which reflects the presence of plasma proteins, is ~25 mm Hg
Interstitial fluid colloid osmotic pressure (πif) varies between 0 and 10 mm Hg among different organs
The Starling principle predicts ultrafiltration at the arteriolar end and absorption at the venular end of most capillary beds
For continuous capillaries, the endothelial barrier for fluid exchange is more complex than considered by Starling
Lymphatics
Lymphatics return excess interstitial fluid to the blood
Flow in Initial Lymphatics
Flow in Collecting Lymphatics
Transport of Proteins and Cells
The circulation of extracellular fluids involves three convective loops: blood, interstitial fluid, and lymph
Regulation of the Microcirculation
The active contraction of vascular smooth muscle regulates precapillary resistance, which controls capillary blood flow
Contraction of Vascular Smooth Muscle
Relaxation of Vascular Smooth Muscle
Tissue metabolites regulate local blood flow in specific vascular beds, independently of the systemic regulation
The endothelium of capillary beds is the source of several vasoactive compounds, including nitric oxide, endothelium-derived hyperpolarizing factor, and endothelin
Nitric Oxide
Endothelium-Derived Hyperpolarizing Factor
Prostacyclin (Prostaglandin I2)
Endothelins
Thromboxane A2
Other Endothelial Factors
Autoregulation stabilizes blood flow despite large fluctuations in systemic arterial pressure
Blood vessels proliferate in response to growth factors by a process known as angiogenesis
Promoters of Vessel Growth
Inhibitors of Vessel Growth
References
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Journal Articles
Chapter 21: Cardiac Electrophysiology and the Electrocardiogram
Electrophysiology of Cardiac Cells
The cardiac action potential starts in specialized muscle cells of the sinoatrial node and then propagates in an orderly fashion throughout the heart
The cardiac action potential conducts from cell to cell via gap junctions
Cardiac action potentials have as many as five distinctive phases
The Na+ current is the largest current in the heart
The Ca2+ current in the heart passes primarily through L-type Ca2+ channels
The repolarizing K+ current turns on slowly
Early Outward K+ Current (A-type Current)
G Protein–Activated K+ Current
KATP Current
The If current is mediated by a nonselective cation channel
Different cardiac tissues uniquely combine ionic currents to produce distinctive action potentials
The SA node is the primary pacemaker of the heart
The Concept of Pacemaker Activity
SA Node
AV Node
Purkinje Fibers
Atrial and ventricular myocytes fire action potentials but do not have pacemaker activity
Atrial Muscle
Ventricular Muscle
Acetylcholine and catecholamines modulate pacemaker activity, conduction velocity, and contractility
Acetylcholine
Catecholamines
The Electrocardiogram
An ECG generally includes five waves
A pair of ECG electrodes defines a lead
The Limb Leads
The Precordial Leads
A simple two-cell model can explain how a simple ECG can arise
Cardiac Arrhythmias
Conduction abnormalities are a major cause of arrhythmias
Partial (or Incomplete) Conduction Block
Complete Conduction Block
Re-Entry
Accessory Conduction Pathways
Fibrillation
Altered automaticity can originate from the sinus node or from an ectopic locus
Depolarization-Dependent Triggered Activity
Long QT Syndrome
Ca2+ overload and metabolic changes can also cause arrhythmias
Ca2+ Overload
Metabolism-Dependent Conduction Changes
Electromechanical Dissociation
References
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Chapter 22: The Heart as a Pump
The Cardiac Cycle
The closing and opening of the cardiac valves define four phases of the cardiac cycle
Changes in ventricular volume, pressure, and flow accompany the four phases of the cardiac cycle N22-1
Diastasis Period (Middle of Phase 1)
Atrial Contraction (End of Phase 1)
Isovolumetric Contraction (Phase 2)
Ejection or Outflow (Phase 3)
Isovolumetric Relaxation (Phase 4)
Rapid Ventricular Filling Period (Beginning of Phase 1)
The ECG, phonocardiogram, and echocardiogram all follow the cyclic pattern of the cardiac cycle
Aortic Blood Flow
Jugular Venous Pulse
Electrocardiogram
Phonocardiogram and Heart Sounds
Echocardiogram
The cardiac cycle causes flow waves in the aorta and peripheral vessels
Aortic Arch
Thoracic-Abdominal Aorta and Large Arteries
The cardiac cycle also causes pressure waves in the aorta and peripheral vessels
Terminal Arteries and Arterioles
Capillaries
Distortion of pressure waves is the result of their propagation along the arterial tree
Effect of Frequency on Wave Velocity and Damping
Effect of Wall Stiffness on Wave Velocity
Pressure waves in veins do not originate from arterial waves
Effect of the Cardiac Cycle
Effect of the Respiratory Cycle
Effect of Skeletal Muscle Contraction (“Muscle Pump”)
Cardiac Dynamics
The right ventricle contracts like a bellows, whereas the left ventricle contracts like a hand squeezing a tube of toothpaste
The right atrium contracts before the left, but the left ventricle contracts before the right
Atrial Contraction
Initiation of Ventricular Contraction
Ventricular Ejection
Ventricular Relaxation
Measurements of ventricular volumes, pressures, and flows allow clinicians to judge cardiac performance
Definitions of Cardiac Volumes
Measurements of Cardiac Volumes
Measurement of Ventricular Pressures
Measurement of Flows
The pressure-volume loop of a ventricle illustrates the ejection work of the ventricle
Segment AB
Segment BC
Segment CD
Segment DE
Segment EF
Segment FA
The “pumping work” done by the heart accounts for a small fraction of the total energy the heart consumes
From Contractile Filaments to a Regulated Pump
The entry of Ca2+ from the outside triggers Ca2+-induced Ca2+ release from the sarcoplasmic reticulum
A global rise in [Ca2+]i initiates contraction of cardiac myocytes
Phosphorylation of phospholamban and of troponin I speeds cardiac muscle relaxation
Extrusion of Ca2+ into the ECF
Reuptake of Ca2+ by the SR
Uptake of Ca2+ by Mitochondria
Dissociation of Ca2+ from Troponin C
The overlap of thick and thin filaments cannot explain the unusual shape of the cardiac length-tension diagram
Starling’s law states that a greater fiber length (i.e., greater ventricular volume) causes the heart to deliver more mechanical energy
The velocity of cardiac muscle shortening falls when the contraction occurs against a greater opposing force (or pressure) or at a shorter muscle length (or lower volume)
Increases in heart rate enhance myocardial tension
Contractility is an intrinsic measure of cardiac performance
Effect of Changes in Contractility
Effect of Changes in Preload (i.e., Initial Sarcomere Length)
Effect of Changes in Afterload
Positive inotropic agents increase myocardial contractility by raising [Ca2+]i
Positive Inotropic Agents
Negative Inotropic Agents
References
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Journal Articles
Chapter 23: Regulation of Arterial Pressure and Cardiac Output
Short-Term Regulation of Arterial Pressure
Systemic mean arterial blood pressure is the principal variable that the cardiovascular system controls
Neural reflexes mediate the short-term regulation of mean arterial blood pressure
High-pressure baroreceptors at the carotid sinus and aortic arch are stretch receptors that sense changes in arterial pressure
Increased arterial pressure raises the firing rate of afferent baroreceptor nerves
The medulla coordinates afferent baroreceptor signals
The efferent pathways of the baroreceptor response include both sympathetic and parasympathetic divisions of the autonomic nervous system
Sympathetic Efferents
Parasympathetic Efferents
The principal effectors in the neural control of arterial pressure are the heart, the arteries, the veins, and the adrenal medulla
Sympathetic Input to the Heart (Cardiac Nerves)
Parasympathetic Input to the Heart (Vagus Nerve)
Sympathetic Input to Blood Vessels (Vasoconstrictor Response)
Parasympathetic Input to Blood Vessels (Vasodilator Response)
Sympathetic Input to Blood Vessels in Skeletal Muscle (Vasodilator Response)
Adrenal Medulla
The unique combination of agonists and receptors determines the end response in cardiac and vascular effector cells
Adrenergic Receptors in the Heart
Cholinergic Receptors in the Heart
Adrenergic Receptors in Blood Vessels
Cholinergic Receptors in or near Blood Vessels
Nonadrenergic, Noncholinergic Receptors in Blood Vessels
The medullary cardiovascular center tonically maintains blood pressure and is under the control of higher brain centers
Secondary neural regulation of arterial blood pressure depends on chemoreceptors
Carotid Bodies
Aortic Bodies
Afferent Fiber Input to the Medulla
Physiological Role of the Peripheral Chemoreceptors in Cardiovascular Control
Central Chemoreceptors
Regulation of Cardiac Output
Mechanisms intrinsic to the heart modulate both heart rate and stroke volume
Intrinsic Control of Heart Rate
Intrinsic Control of Stroke Volume
Mechanisms extrinsic to the heart also modulate heart rate and stroke volume
Baroreceptor Regulation
Chemoreceptor Regulation
Low-pressure baroreceptors in the atria respond to increased “fullness” of the vascular system, triggering tachycardia, renal vasodilation, and diuresis
Atrial Receptors
Ventricular Receptors
Cardiac output is roughly proportional to effective circulating blood volume
Matching of Venous Return and Cardiac Output
Increases in cardiac output cause right atrial pressure to fall
Changes in blood volume shift the vascular function curve to different RAPs, whereas changes in arteriolar tone alter the slope of the curve
Because vascular function and cardiac function depend on each other, cardiac output and venous return match at exactly one value of RAP
Intermediate- and Long-Term Control of the Circulation
Endocrine and paracrine vasoactive compounds control the circulatory system on an intermediate- to long-term basis
Biogenic Amines
Peptides
Prostaglandins
Nitric Oxide
Pathways for the renal control of ECF volume are the primary long-term regulators of mean arterial pressure
References
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Journal Articles
Chapter 24: Special Circulations
The blood flow to individual organs must vary to meet the needs of the particular organ, as well as of the whole body
Neural, myogenic, metabolic, and endothelial mechanisms control regional blood flow
Neural Mechanisms
Myogenic Mechanisms
Metabolic Mechanisms
Endothelial Mechanisms
The Brain
Anastomoses at the circle of Willis and among the branches of distributing arteries protect the blood supply to the brain, which is ~15% of resting cardiac output
Arteries
Veins
Capillaries
Lymphatics
Vascular Volume
Neural, metabolic, and myogenic mechanisms control blood flow to the brain
Neural Control
Metabolic Control
Myogenic Control
The neurovascular unit matches blood flow to local brain activity
Autoregulation maintains a fairly constant cerebral blood flow across a broad range of perfusion pressures
The Heart
The coronary circulation receives 5% of the resting cardiac output from the left heart and mostly returns it to the right heart
Extravascular compression impairs coronary blood flow during systole
Myocardial blood flow parallels myocardial metabolism
Although sympathetic stimulation directly constricts coronary vessels, accompanying metabolic effects predominate, producing an overall vasodilation
Collateral vessel growth can provide blood flow to ischemic regions
Vasodilator drugs may compromise myocardial flow through “coronary steal”
The Skeletal Muscle
A microvascular unit is the capillary bed supplied by a single terminal arteriole
Metabolites released by active muscle trigger vasodilation and an increase in blood flow
Sympathetic innervation increases the intrinsic tone of resistance vessels
Rhythmic contraction promotes blood flow through the “muscle pump”
The Splanchnic Organs
The vascular supply to the gut is highly interconnected
Blood flow to the gastrointestinal tract increases up to eight-fold after a meal (postprandial hyperemia)
Sympathetic activity directly constricts splanchnic blood vessels, whereas parasympathetic activity indirectly dilates them
Changes in the splanchnic circulation regulate total peripheral resistance and the distribution of blood volume
Exercise and hemorrhage can substantially reduce splanchnic blood flow
The liver receives its blood flow from both the systemic and the portal circulation
The Skin
The skin is the largest organ of the body
Specialized arteriovenous anastomoses in apical skin help control heat loss
Apical Skin
Nonapical Skin
Mechanical stimuli elicit local vascular responses in the skin
White Reaction
“Triple Response”
References
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Chapter 25: Integrated Control of the Cardiovascular System
Interaction among the Different Cardiovascular Control Systems
The control of the cardiovascular system involves “linear,” “branched,” and “connected” interactions
Regulation of the entire cardiovascular system depends on the integrated action of multiple subsystem controls as well as noncardiovascular controls
Response to Erect Posture
Because of gravity, standing up (orthostasis) tends to shift blood from the head and heart to veins in the legs
The ANS mediates an “orthostatic response” that raises heart rate and peripheral vascular resistance and thus tends to restore mean arterial pressure
Nonuniform Initial Distribution of Blood
Nonuniform Distensibility of the Vessels
Muscle Pumps
Autonomic Reflexes
Postural Hypotension
Temperature Effects
Responses to Acute Emotional Stress
The fight-or-flight reaction is a sympathetic response that is centrally controlled in the cortex and hypothalamus
The common faint reflects mainly a parasympathetic response caused by sudden emotional stress
Response to Exercise
Early physiologists suggested that muscle contraction leads to mechanical and chemical changes that trigger an increase in cardiac output
Mechanical Response: Increased Venous Return
Chemical Response: Local Vasodilation in Active Muscle
Central command organizes an integrated cardiovascular response to exercise
Muscle and baroreceptor reflexes, metabolites, venous return, histamine, epinephrine, and increased temperature reinforce the response to exercise
Response to Hemorrhage
After hemorrhage, cardiovascular reflexes restore mean arterial pressure
Tachycardia and Increased Contractility
Arteriolar Constriction
Venous Constriction
Circulating Vasoactive Agonists
After hemorrhage, transcapillary refill, fluid conservation, and thirst restore the blood volume
Transcapillary Refill
Renal Conservation of Salt and Water
Thirst
Positive-feedback mechanisms cause irreversible hemorrhagic shock
Failure of the Vasoconstrictor Response
Failure of the Capillary Refill
Failure of the Heart
CNS Depression
References
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Journal Articles
Section V: The Respiratory System
Chapter 26: Organization of the Respiratory System
Comparative Physiology of Respiration
External respiration is the exchange of O2 and CO2 between the atmosphere and the mitochondria
Diffusion is the major mechanism of external respiration for small aquatic organisms
Convection enhances diffusion by producing steeper gradients across the diffusion barrier
Surface area amplification enhances diffusion
Respiratory pigments such as hemoglobin increase the carrying capacity of the blood for both O2 and CO2
Pathophysiology recapitulates phylogeny … in reverse
Organization of the Respiratory System in Humans
Humans optimize each aspect of external respiration—ventilation, circulation, area amplification, gas carriage, local control, and central control
Conducting airways deliver fresh air to the alveolar spaces
Alveolar air spaces are the site of gas exchange
The lungs play important nonrespiratory roles, including filtering the blood, serving as a reservoir for the left ventricle, and performing several biochemical conversions
Olfaction
Processing of Inhaled Air Before It Reaches the Alveoli
Left Ventricular Reservoir
Filtering Small Emboli from the Blood
Biochemical Reactions
Lung Volumes and Capacities
The spirometer measures changes in lung volume
The volume of distribution of helium or nitrogen in the lung is an estimate of the RV
Helium-Dilution Technique
N2-Washout Method
The plethysmograph, together with Boyle’s law, is a tool for estimation of RV
References
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Chapter 27: Mechanics of Ventilation
Static Properties of the Lung
The balance between the outward elastic recoil of the chest wall and the inward elastic recoil of the lungs generates a subatmospheric intrapleural pressure
Contraction of the diaphragm and selected intercostal muscles increases the volume of the thorax, producing an inspiration
Relaxation of the muscles of inspiration produces a quiet expiration
An increase of the static compliance makes it easier to inflate the lungs
Surface tension at the air-water interface of the airways accounts for most of the elastic recoil of the lungs
Pulmonary surfactant is a mixture of lipids—mainly dipalmitoylphosphatidylcholine—and apoproteins
Pulmonary surfactant reduces surface tension and increases compliance
Dynamic Properties of the Lung
Airflow is proportional to the difference between alveolar and atmospheric pressure, but inversely proportional to airway resistance
In the lung, airflow is transitional in most of the tracheobronchial tree
The smallest airways contribute only slightly to total airway resistance in healthy lungs
Vagal tone, histamine, and reduced lung volume all increase airway resistance
Intrapleural pressure has a static component (−PTP) that determines lung volume and a dynamic component (Pa) that determines airflow
Transpulmonary Pressure
Alveolar Pressure
During inspiration, a sustained negative shift in PIP causes Pa to become transiently more negative
Dynamic compliance falls as respiratory frequency rises
Transmural pressure differences cause airways to dilate during inspiration and to compress during expiration
Static Conditions
Inspiration
Expiration
Because of airway collapse, expiratory flow rates become independent of effort at low lung volumes
References
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Chapter 28: Acid-Base Physiology
pH and Buffers
pH values vary enormously among different intracellular and extracellular compartments
Buffers minimize the size of the pH changes produced by adding acid or alkali to a solution
According to the Henderson-Hasselbalch equation, pH depends on the ratio [CO2]/[]
has a far higher buffering power in an open than in a closed system
Acid-Base Chemistry When Is the Only Buffer
In the absence of other buffers, doubling causes pH to fall by 0.3 but causes almost no change in []
In the absence of other buffers, doubling [] causes pH to rise by 0.3
Acid-Base Chemistry in the Presence of and Buffers—The Davenport Diagram
The Davenport diagram is a graphical tool for interpreting acid-base disturbances in blood
The Buffer
Buffers
Solving the Problem
The amount of formed or consumed during “respiratory” acid-base disturbances increases with
Adding or removing an acid or base—at a constant —produces a “metabolic” acid-base disturbance
During metabolic disturbances, makes a greater contribution to total buffering when pH and are high and when is low
A metabolic change can compensate for a respiratory disturbance
A respiratory change can compensate for a metabolic disturbance
Position on a Davenport diagram defines the nature of an acid-base disturbance
pH Regulation of Intracellular Fluid
Ion transporters at the plasma membrane closely regulate the pH inside of cells
Indirect interactions between K+ and H+ make it appear as if cells have a K-H exchanger
Changes in intracellular pH are often a sign of changes in extracellular pH, and vice versa
References
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Journal Articles
Chapter 29: Transport of Oxygen and Carbon Dioxide in the Blood
Carriage of O2
The amount of O2 dissolved in blood is far too small to meet the metabolic demands of the body
Hemoglobin consists of two α and two β subunits, each of which has an iron-containing “heme” and a polypeptide “globin”
The Hb-O2 dissociation curve has a sigmoidal shape because of cooperativity among the four subunits of the Hb molecule
Increases in temperature, [CO2], and [H+], all of which are characteristic of metabolically active tissues, cause Hb to dump O2
Temperature
Acid
Carbon Dioxide
2,3-Diphosphoglycerate reduces the affinity of adult, but not of fetal, Hb
Carriage of CO2
Blood carries “total CO2” mainly as
CO2 transport depends critically on carbonic anhydrase, the Cl-HCO3 exchanger, and Hb
The high in the lungs causes the blood to dump CO2
The O2-CO2 diagram describes the interaction of and in the blood
References
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Chapter 30: Gas Exchange in the Lungs
Diffusion of Gases
Gas flow across a barrier is proportional to diffusing capacity and concentration gradient (Fick’s law)
The total flux of a gas between alveolar air and blood is the summation of multiple diffusion events along each pulmonary capillary during the respiratory cycle
The flow of O2, CO, and CO2 between alveolar air and blood depends on the interaction of these gases with red blood cells
Diffusion and Perfusion Limitations on Gas Transport
The diffusing capacity normally limits the uptake of CO from alveolar air to blood
Perfusion normally limits the uptake of N2O from alveolar air to blood
In principle, CO transport could become perfusion limited and N2O transport could become diffusion limited under special conditions
The uptake of CO provides an estimate of DL
For both O2 and CO2, transport is normally perfusion limited
Uptake of O2
Escape of CO2
Pathological changes that reduce DL do not necessarily produce hypoxia
References
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Chapter 31: Ventilation and Perfusion of the Lungs
Ventilation
About 30% of total ventilation in a respiratory cycle is wasted ventilating anatomical dead space (i.e., conducting airways)
The Fowler single-breath N2-washout technique estimates anatomical dead space
The Bohr expired-[CO2] approach estimates physiological dead space
Alveolar ventilation is the ratio of CO2 production rate to CO2 mole fraction in alveolar air
Alveolar and arterial are inversely proportional to alveolar ventilation
Alveolar and arterial rise with increased alveolar ventilation
Because of the action of gravity on the lung, regional ventilation in an upright subject is normally greater at the base than the apex
Restrictive and obstructive pulmonary diseases can exacerbate the nonuniformity of ventilation
Restrictive Pulmonary Disease
Obstructive Pulmonary Disease
Perfusion of the Lung
The pulmonary circulation has low pressure and resistance but high compliance
Overall pulmonary vascular resistance is minimal at FRC
Alveolar Vessels
Extra-Alveolar Vessels
Increases in pulmonary arterial pressure reduce pulmonary vascular resistance by recruiting and distending pulmonary capillaries
Recruitment
Distention
Hypoxia is a strong vasoconstrictor, opposite to its effect in the systemic circulation
Oxygen
Carbon Dioxide and Low pH
Autonomic Nervous System
Hormones and Other Humoral Agents
Because of gravity, regional perfusion in an upright subject is far greater near the base than the apex of the lung
Zone 1: Pa > PPA > PPV
Zone 2: PPA > Pa > PPV
Zone 3: PPA > PPV > Pa
Zone 4: PPA > PPV > Pa
Matching Ventilation and Perfusion
The greater the ventilation-perfusion ratio, the higher the and the lower the in the alveolar air
Because of the action of gravity, the regional ratio in an upright subject is greater at the apex of the lung than at the base
The ventilation of unperfused alveoli (local = ∞) triggers compensatory bronchoconstriction and a fall in surfactant production
Alveolar Dead-Space Ventilation
Redirection of Blood Flow
Regulation of Local Ventilation
The perfusion of unventilated alveoli (local = 0) triggers a compensatory hypoxic vasoconstriction
Shunt
Redirection of Airflow
Asthma
Normal Anatomical Shunts
Pathological Shunts
Regulation of Local Perfusion
Even if whole-lung and are normal, exaggerated local mismatches produce hypoxia and respiratory acidosis
Normal Lungs
Alveolar Dead-Space Ventilation Affecting One Lung
Shunt Affecting One Lung
Mixed Mismatches
References
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Chapter 32: Control of Ventilation
Overview of the Respiratory Control System
Automatic centers in the brainstem activate the respiratory muscles rhythmically and subconsciously
Peripheral and central chemoreceptors—which sense , , and pH—drive the CPG
Other receptors as well as higher brain centers also modulate ventilation
Neurons That Control Ventilation
The neurons that generate the respiratory rhythm are located in the medulla
The pons modulates—but is not essential for—respiratory output
The dorsal and ventral respiratory groups contain many neurons that fire in phase with respiratory motor output
The dorsal respiratory group processes sensory input and contains primarily inspiratory neurons
The ventral respiratory group is primarily motor and contains both inspiratory and expiratory neurons
Generation of the Respiratory Rhythm
Different RRNs fire at different times during inspiration and expiration
The firing patterns of RRNs depend on the ion channels in their membranes and the synaptic inputs they receive
Intrinsic Membrane Properties
Synaptic Input
Pacemaker properties and synaptic interactions may both contribute to the generation of the respiratory rhythm
Pacemaker Activity
Synaptic Interactions
The respiratory CPG for eupnea could reside in a single site or in multiple sites, or could emerge from a complex network
Restricted-Site Model
Distributed Oscillator Models
Emergent Property Model
Chemical Control of Ventilation
Peripheral Chemoreceptors
Peripheral chemoreceptors (carotid and aortic bodies) respond to hypoxia, hypercapnia, and acidosis
Sensitivity to Decreased Arterial
Sensitivity to Increased Arterial
Sensitivity to Decreased Arterial pH
The glomus cell is the chemosensor in the carotid and aortic bodies
Hypoxia, hypercapnia, and acidosis inhibit K+ channels, raise glomus cell [Ca2+]i, and release neurotransmitters
Hypoxia N32-17
Hypercapnia
Extracellular Acidosis
Central Chemoreceptors
The blood-brain barrier separates the central chemoreceptors in the medulla from arterial blood
Central chemoreceptors are located in the ventrolateral medulla and other brainstem regions
Some neurons of the medullary raphé and VLM are unusually pH sensitive
Integrated Responses to Hypoxia, Hypercapnia, and Acidosis
Hypoxia accentuates the acute response to respiratory acidosis
Respiratory Acidosis
Metabolic Acidosis
Respiratory acidosis accentuates the acute response to hypoxia
Modulation of Ventilatory Control
Stretch and chemical/irritant receptors in the airways and lung parenchyma provide feedback about lung volume and the presence of irritants
Slowly Adapting Pulmonary Stretch Receptors
Rapidly Adapting Pulmonary Stretch (Irritant) Receptors
C-Fiber Receptors
Higher brain centers coordinate ventilation with other behaviors and can override the brainstem’s control of breathing
Coordination with Voluntary Behaviors That Use Respiratory Muscles
Coordination with Complex Nonventilatory Behaviors
Modification by Affective States
Balancing Conflicting Demands of Gas Exchange and Other Behaviors
References
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Section VI: The Urinary System
Chapter 33: Organization of the Urinary System
Functional Anatomy of the Kidney
The kidneys are paired, retroperitoneal organs with vascular and epithelial elements
The kidneys have a very high blood flow and glomerular capillaries flanked by afferent and efferent arterioles
The functional unit of the kidney is the nephron
The renal corpuscle has three components: vascular elements, the mesangium, and Bowman’s capsule and space
The tubule components of the nephron include the proximal tubule, loop of Henle, distal tubule, and collecting duct
The tightness of tubule epithelia increases from the proximal to the medullary collecting tubule
Main Elements of Renal Function
The nephron forms an ultrafiltrate of the blood plasma and then selectively reabsorbs the tubule fluid or secretes solutes into it
The JGA is a region where each thick ascending limb contacts its glomerulus
Sympathetic nerve fibers to the kidney regulate renal blood flow, glomerular filtration, and tubule reabsorption
The kidneys, as endocrine organs, produce renin, 1,25-dihydroxyvitamin D, erythropoietin, prostaglandins, and bradykinin
Measuring Renal Clearance and Transport
The clearance of a solute is the virtual volume of plasma that would be totally cleared of a solute in a given time
A solute’s urinary excretion is the algebraic sum of its filtered load, reabsorption by tubules, and secretion by tubules
Microscopic techniques make it possible to measure single-nephron rates of filtration, absorption, and secretion
Single-Nephron GFR
Handling of Water by Tubule Segments in a Single Nephron
Handling of Solutes by Tubule Segments in a Single Nephron
The Ureters and Bladder
The ureters propel urine from the renal pelvis to the bladder by peristaltic waves conducted along a syncytium of smooth-muscle cells
Sympathetic, parasympathetic, and somatic fibers innervate the urinary bladder and its sphincters
Bladder filling activates stretch receptors, initiating the micturition reflex, a spinal reflex under control of higher central nervous system centers
References
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Chapter 34: Glomerular Filtration and Renal Blood Flow
Glomerular Filtration
A high glomerular filtration rate is essential for maintaining stable and optimal extracellular levels of solutes and water
The clearance of inulin is a measure of GFR
The clearance of creatinine is a useful clinical index of GFR
Molecular size and electrical charge determine the filterability of solutes across the glomerular filtration barrier
Hydrostatic pressure in glomerular capillaries favors glomerular ultrafiltration, whereas oncotic pressure in capillaries and hydrostatic pressure in Bowman’s space oppose it
Renal Blood Flow
Increased glomerular plasma flow leads to an increase in GFR
Afferent and efferent arteriolar resistances control both glomerular plasma flow and GFR
Peritubular capillaries provide tubules with nutrients and retrieve reabsorbed fluid
Blood flow in the renal cortex exceeds that in the renal medulla
The clearance of para-aminohippurate is a measure of RPF
Control of Renal Blood Flow and Glomerular Filtration
Autoregulation keeps RBF and GFR relatively constant
Myogenic Response
Tubuloglomerular Feedback
Volume expansion and a high-protein diet increase GFR by reducing TGF
Four factors that modulate RBF and GFR play key roles in regulating effective circulating volume
Renin-Angiotensin-Aldosterone Axis
Sympathetic Nerves
Arginine Vasopressin
Atrial Natriuretic Peptide
Other vasoactive agents modulate RBF and GFR
Epinephrine
Dopamine
Endothelins
Prostaglandins
Leukotrienes
Nitric Oxide
References
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Chapter 35: Transport of Sodium and Chloride
Na+ and Cl− Transport by Different Segments of The Nephron
Na+ and Cl− reabsorption decreases from proximal tubules to Henle’s loops to classic distal tubules to collecting tubules and ducts
The tubule reabsorbs Na+ via both the transcellular and the paracellular pathways
Transcellular Na+ Reabsorption
Paracellular Na+ Reabsorption
Na+ and Cl−, and Water Transport at the Cellular and Molecular Level
Na+ reabsorption involves apical transporters or ENaCs and a basolateral Na-K pump
Proximal Tubule
Thin Limbs of Henle’s Loop
Thick Ascending Limb
Distal Convoluted Tubule
Initial and Cortical Collecting Tubules
Medullary Collecting Duct
Cl− reabsorption involves both paracellular and transcellular pathways
Proximal Tubule
Thick Ascending Limb
Distal Convoluted Tubule
Collecting Ducts
Water reabsorption is passive and secondary to solute transport
Proximal Tubule
Loop of Henle and Distal Nephron
The kidney’s high O2 consumption reflects a high level of active Na+ transport
Regulation of Na+ and Cl− Transport
Glomerulotubular balance stabilizes fractional Na+ reabsorption by the proximal tubule in the face of changes in the filtered Na+ load
The proximal tubule achieves GT balance by both peritubular and luminal mechanisms
Peritubular Factors in the Proximal Tubule
Luminal Factors in the Proximal Tubule
ECF volume contraction or expansion upsets GT balance
The distal nephron also increases Na+ reabsorption in response to an increased Na+ load
Four parallel pathways that regulate effective circulating volume all modulate Na+ reabsorption
Renin-Angiotensin-Aldosterone Axis
Sympathetic Division of the Autonomic Nervous System
Arginine Vasopressin (Antidiuretic Hormone)
Atrial Natriuretic Peptide
Dopamine, elevated plasma [Ca2+], an endogenous steroid, prostaglandins, and bradykinin all decrease Na+ reabsorption
Dopamine
Elevated Plasma [Ca2+]
Endogenous Na-K Pump Inhibitor
Prostaglandins
Bradykinin
References
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Chapter 36: Transport of Urea, Glucose, Phosphate, Calcium, Magnesium, and Organic Solutes
Urea
The kidney filters, reabsorbs, and secretes urea
Urea excretion rises with increasing urinary flow
Glucose
The proximal tubule reabsorbs glucose via apical, electrogenic Na/glucose cotransport and basolateral facilitated diffusion
Glucose excretion in the urine occurs only when the plasma concentration exceeds a threshold
Other Organic Solutes
The proximal tubule reabsorbs amino acids using a wide variety of apical and basolateral transporters
An H+-driven cotransporter takes up oligopeptides across the apical membrane, whereas endocytosis takes up proteins and other large organic molecules
Oligopeptides
Proteins
Two separate apical Na+-driven cotransporters reabsorb monocarboxylates and dicarboxylates/tricarboxylates
The proximal tubule secretes PAH and a variety of other organic anions
PAH secretion is an example of a Tm-limited mechanism
The proximal tubule both reabsorbs and secretes urate
Reabsorption
Secretion
The late proximal tubule secretes several organic cations
Nonionic diffusion of neutral weak acids and bases across tubules explains why their excretion is pH dependent
Phosphate
The proximal tubule reabsorbs phosphate via apical Na/phosphate cotransporters
Phosphate excretion in the urine already occurs at physiological plasma concentrations
PTH inhibits apical Na/phosphate uptake, promoting phosphate excretion
Fibroblast growth factor 23 and other phosphatonins also inhibit apical Na/phosphate uptake, promoting phosphate excretion
Calcium
Binding to plasma proteins and formation of Ca2+-anion complexes influence the filtration and reabsorption of Ca2+
The proximal tubule reabsorbs two thirds of filtered Ca2+, with more distal segments reabsorbing nearly all of the remainder
Proximal Tubule
Thick Ascending Limb
Distal Convoluted Tubule
Transcellular Ca2+ movement is a two-step process, involving passive Ca2+ entry through apical channels and basolateral extrusion by electrogenic Na/Ca exchange and a Ca pump
PTH and vitamin D stimulate—whereas high plasma Ca2+ inhibits—Ca2+ reabsorption
Parathyroid Hormone
Vitamin D
Plasma Ca2+ Levels
Diuretics
Magnesium
Most Mg2+ reabsorption takes place along the TAL
Mg2+ reabsorption increases with depletion of Mg2+ or Ca2+, or with elevated PTH levels
Mg2+ Depletion
Hypermagnesemia and Hypercalcemia
Hormones
Diuretics
References
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Chapter 37: Transport of Potassium
Potassium Balance and the Overall Renal Handling of Potassium
Changes in K+ concentrations can have major effects on cell and organ function
K+ homeostasis involves external K+ balance between environment and body, and internal K+ balance between intracellular and extracellular compartments
External K+ Balance
Internal K+ Balance
Ingested K+ moves transiently into cells for storage before excretion by the kidney
The kidney excretes K+ by a combination of filtration, reabsorption, and secretion
Potassium Transport by Different Segments of the Nephron
The proximal tubule reabsorbs most of the filtered K+, whereas the distal nephron reabsorbs or secretes K+, depending on K+ intake
Low Dietary K+
Normal or High Dietary K+
Medullary trapping of K+ helps to maximize K+ excretion when K+ intake is high
Potassium Transport at the Cellular and Molecular Levels
Passive K+ reabsorption along the proximal tubule follows Na+ and fluid movements
K+ reabsorption along the TAL occurs predominantly via a transcellular route that exploits secondary active Na/K/Cl cotransport
K+ secretion by principal and intercalated cells of the ICT and CCT involves active K+ uptake across the basolateral membrane
K+ reabsorption by intercalated cells involves apical uptake via an H-K pump
K+ reabsorption along the MCD is both passive and active
Regulation of Renal Potassium Excretion
Increased luminal flow increases K+ secretion
An increased lumen-negative transepithelial potential increases K+ secretion
Low luminal [Cl−] enhances K+ secretion
Aldosterone increases K+ secretion
Mineralocorticoids
Glucocorticoids
High K+ intake promotes renal K+ secretion
Dietary K+ Loading
Dietary K+ Deprivation
Acidosis decreases K+ secretion
Epinephrine reduces and AVP enhances K+ excretion
Opposing factors stabilize K+ secretion
Attenuating Effects
Additive Effects
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Chapter 38: Urine Concentration and Dilution
Water Balance and the Overall Renal Handling of Water
The kidney can generate a urine as dilute as 40 mOsm (one seventh of plasma osmolality) or as concentrated as 1200 mOsm (four times plasma osmolality)
Free-water clearance () is positive if the kidney produces urine that is less concentrated than plasma and negative if the kidney produces urine that is more concentrated than plasma
Isosmotic Urine
Dilute Urine
Concentrated Urine
Water Transport by Different Segments of the Nephron
The kidney concentrates urine by driving water via osmosis from the tubule lumen into a hyperosmotic interstitium
Tubule fluid is isosmotic in the proximal tubule, becomes dilute in the loop of Henle, and then either remains dilute or becomes concentrated by the end of the collecting duct
Generation of a Hyperosmotic Medulla and Urine
The renal medulla is hyperosmotic to blood plasma during both antidiuresis (low urine flow) and water diuresis
NaCl transport generates only a ~200-mOsm gradient across any portion of the ascending limb, but countercurrent exchange can multiply this single effect to produce a 900-mOsm gradient between cortex and papilla
The single effect is the result of passive NaCl reabsorption in the thin ascending limb and active NaCl reabsorption in the TAL
The IMCD reabsorbs urea, producing high levels of urea in the interstitium of the inner medulla
Urea Handling
Urea Recycling
The vasa recta’s countercurrent exchange and relatively low blood flow minimize washout of medullary hyperosmolality
The MCD produces a concentrated urine by osmosis, driven by the osmotic gradient between the medullary interstitium and the lumen
Regulation by Arginine Vasopressin
AVP increases water permeability in all nephron segments beyond the DCT
AVP, via cAMP, causes vesicles containing AQP2 to fuse with apical membranes of principal cells of collecting tubules and ducts
AVP increases NaCl reabsorption in the outer medulla and urea reabsorption in the IMCD, enhancing urinary concentrating ability
References
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Chapter 39: Transport of Acids and Bases
Acid-Base Balance and the Overall Renal Handling of Acid
Whereas the lungs excrete the large amount of CO2 formed by metabolism, the kidneys are crucial for excreting nonvolatile acids
To maintain acid-base balance, the kidney must not only reabsorb virtually all filtered but also secrete generated nonvolatile acids
Secreted H+ titrates to CO2 ( reabsorption) and also titrates filtered buffers and endogenously produced NH3
Titration of Filtered (“ Reabsorption”)
Titration of Filtered Buffers (Titratable-Acid Formation)
Titration of Filtered and Secreted NH3 (Ammonium Excretion)
Acid-Base Transport by Different Segments of the Nephron
The nephron reclaims virtually all the filtered in the proximal tubule (~80%), thick ascending limb (~10%), and distal nephron (~10%)
The nephron generates new , mostly in the proximal tubule
Formation of Titratable Acid
Excretion
Acid-Base Transport at the Cellular and Molecular Levels
H+ moves across the apical membrane from tubule cell to lumen by Na-H exchange, electrogenic H pumping, and K-H pumping
Na-H Exchanger
Electrogenic H Pump
H-K Exchange Pump
CAs in the lumen and cytosol stimulate H+ secretion by accelerating the interconversion of CO2 and
Apical CA (CA IV)
Cytoplasmic CA (CA II)
Basolateral CA (CA IV and CA XII)
Inhibition of CA
efflux across the basolateral membrane takes place by electrogenic Na/HCO3 cotransport and Cl-HCO3 exchange
Electrogenic Na/HCO3 Cotransport
Cl-HCO3 Exchange
is synthesized by proximal tubules, partly reabsorbed in the loop of Henle, and secreted passively into papillary collecting ducts
Regulation of Renal Acid Secretion
Respiratory acidosis stimulates renal H+ secretion
Metabolic acidosis stimulates both proximal H+ secretion and NH3 production
Metabolic alkalosis reduces proximal H+ secretion and, in the CCT, may even provoke secretion
A rise in GFR increases delivery to the tubules, enhancing reabsorption (glomerulotubular balance for )
Extracellular volume contraction—via ANG II, aldosterone, and sympathetic activity—stimulates renal H+ secretion
Hypokalemia increases renal H+ secretion
Both glucocorticoids and mineralocorticoids stimulate acid secretion
Diuretics can change H+ secretion, depending on how they affect transepithelial voltage, ECF volume, and plasma [K+]
References
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Chapter 40: Integration of Salt and Water Balance
Sodium Balance
Water Balance
Control of Extracellular Fluid Volume
In the steady state, Na+ intake via the gastrointestinal tract equals Na+ output from renal and extrarenal pathways
The kidneys increase Na+ excretion in response to an increase in ECF volume, not to an increase in extracellular Na+ concentration
It is not the ECF volume as a whole, but the effective circulating volume, that regulates Na+ excretion
Decreases in effective circulating volume trigger four parallel effector pathways to decrease renal Na+ excretion
Increased activity of the renin-angiotensin-aldosterone axis is the first of four parallel pathways that correct a low effective circulating volume
Increased sympathetic nerve activity, increased AVP, and decreased ANP are the other three parallel pathways that correct a low effective circulating volume
Renal Sympathetic Nerve Activity
Arginine Vasopressin (Antidiuretic Hormone)
Atrial Natriuretic Peptide
High arterial pressure raises Na+ excretion by hemodynamic mechanisms, independent of changes in effective circulating volume
Large and Acute Decrease in Arterial Blood Pressure
Large Increase in Arterial Pressure
Control of Water Content (Extracellular Osmolality)
Increased plasma osmolality stimulates hypothalamic osmoreceptors that trigger the release of AVP, inhibiting water excretion
Hypothalamic neurons synthesize AVP and transport it along their axons to the posterior pituitary, where they store it in nerve terminals prior to release
Increased osmolality stimulates a second group of osmoreceptors that trigger thirst, which promotes water intake
Several nonosmotic stimuli also enhance AVP secretion
Reduced Effective Circulating Volume
Volume Expansion
Pregnancy
Other Factors
Decreased effective circulating volume and low arterial pressure also trigger thirst
Defense of the effective circulating volume usually has priority over defense of osmolality
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Section VII: The Gastrointestinal System
Chapter 41: Organization of the Gastrointestinal System
Overview of Digestive Processes
The gastrointestinal tract is a tube that is specialized along its length for the sequential processing of food
Assimilation of dietary food substances requires digestion as well as absorption
Digestion requires enzymes secreted in the mouth, stomach, pancreas, and small intestine
Ingestion of food initiates multiple endocrine, neural, and paracrine responses
In addition to its function in nutrition, the GI tract plays important roles in excretion, fluid and electrolyte balance, and immunity
Regulation of Gastrointestinal Function
The ENS is a “minibrain” with sensory neurons, interneurons, and motor neurons
ACh, peptides, and bioactive amines are the ENS neurotransmitters that regulate epithelial and motor function
The brain-gut axis is a bidirectional system that controls GI function via the ANS, GI hormones, and the immune system
Gastrointestinal Motility
Tonic and rhythmic contractions of smooth muscle are responsible for churning, peristalsis, and reservoir action
Segments of the GI tract have both longitudinal and circular arrays of muscles and are separated by sphincters that consist of specialized circular muscles
Location of a sphincter determines its function
Upper Esophageal Sphincter
Lower Esophageal Sphincter
Pyloric Sphincter
Ileocecal Sphincter
Internal and External Anal Sphincters
Motility of the small intestine achieves both churning and propulsive movement, and its temporal pattern differs in the fed and fasted states
Motility of the large intestine achieves both propulsive movement and a reservoir function
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Chapter 42: Gastric Function
Functional Anatomy of the Stomach
The mucosa is composed of surface epithelial cells and glands
With increasing rates of secretion of gastric juice, the H+ concentration rises and the Na+ concentration falls
The proximal portion of the stomach secretes acid, pepsinogens, intrinsic factor, bicarbonate, and mucus, whereas the distal part releases gastrin and somatostatin
Corpus
Antrum
The stomach accommodates food, mixes it with gastric secretions, grinds it, and empties the chyme into the duodenum
Acid Secretion
The parietal cell has a specialized tubulovesicular structure that increases apical membrane area when the cell is stimulated to secrete acid
An H-K pump is responsible for gastric acid secretion by parietal cells
Three secretagogues (acetylcholine, gastrin, and histamine) directly and indirectly induce acid secretion by parietal cells
The three acid secretagogues act through either Ca2+/diacylglycerol or cAMP
Antral and duodenal G cells release gastrin, whereas ECL cells in the corpus release histamine
Gastric D cells release somatostatin, the central inhibitor of acid secretion
Several enteric hormones (“enterogastrone”) and prostaglandins inhibit gastric acid secretion
A meal triggers three phases of acid secretion
Basal State
Cephalic Phase
Gastric Phase
Intestinal Phase
Pepsinogen Secretion
Chief cells, triggered by both cAMP and Ca2+ pathways, secrete multiple pepsinogens that initiate protein digestion
Agonists Acting via cAMP
Agonists Acting via Ca2+
Low pH is required for both pepsinogen activation and pepsin activity
Protection of the Gastric Surface Epithelium and Neutralization of Acid in the Duodenum
Vagal stimulation and irritation stimulate gastric mucous cells to secrete mucins
Gastric surface cells secrete , stimulated by acetylcholine, acids, and prostaglandins
Mucus protects the gastric surface epithelium by trapping an -rich fluid near the apical border of these cells
Acid entry into the duodenum induces S cells to release secretin, triggering the pancreas and duodenum to secrete
Filling and Emptying of the Stomach
Gastric motor activity plays a role in filling, churning, and emptying
Filling of the stomach is facilitated by both receptive relaxation and gastric accommodation
The stomach churns its contents until the particles are small enough to be gradually emptied into the duodenum
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Chapter 43: Pancreatic and Salivary Glands
Overview of Exocrine Gland Physiology
The pancreas and major salivary glands are compound exocrine glands
Acinar cells are specialized protein-synthesizing cells
Duct cells are epithelial cells specialized for fluid and electrolyte transport
Goblet cells contribute to mucin production in exocrine glands
Pancreatic Acinar Cell
The acinar cell secretes digestive proteins in response to stimulation
Acetylcholine and cholecystokinin mediate the regulated secretion of proteins by pancreatic acinar cells
Ca2+ is the major second messenger for the secretion of proteins by pancreatic acinar cells
Ca2+
cAMP
Effectors
In addition to proteins, the pancreatic acinar cell secretes a plasma-like fluid
Pancreatic Duct Cell
The pancreatic duct cell secretes isotonic NaHCO3
Secretin (via cAMP) and ACh (via Ca2+) stimulate secretion by pancreatic ducts
Apical membrane chloride channels are important sites of neurohumoral regulation
Pancreatic duct cells may also secrete glycoproteins
Composition, Function, and Control of Pancreatic Secretion
Pancreatic juice is a protein-rich, alkaline secretion
In the fasting state, levels of secreted pancreatic enzymes oscillate at low levels
CCK from duodenal I cells stimulates acinar enzyme secretion, and secretin from S cells stimulates and fluid secretion by ducts
A meal triggers cephalic, gastric, and intestinal phases of pancreatic secretion
Cephalic Phase
Gastric Phase
Intestinal Phase
The pancreas has large reserves of digestive enzymes for carbohydrates and proteins, but not for lipids
Fat in the distal part of the small intestine inhibits pancreatic secretion
Several mechanisms protect the pancreas from autodigestion
Salivary Acinar Cell
Different salivary acinar cells secrete different proteins
Cholinergic and adrenergic neural pathways are the most important physiological activators of regulated secretion by salivary acinar cells
Both cAMP and Ca2+ mediate salivary acinar secretion
Salivary Duct Cell
Salivary duct cells produce a hypotonic fluid that is poor in NaCl and rich in KHCO3
Parasympathetic stimulation decreases Na+ absorption, whereas aldosterone increases Na+ absorption by duct cells
Salivary duct cells also secrete and take up proteins
Composition, Function, and Control of Salivary Secretion
Depending on protein composition, salivary secretions can be serous, seromucous, or mucous
At low flow rates, the saliva is hypotonic and rich in K+, whereas at higher flow rates, its composition approaches that of plasma
Parasympathetic stimulation increases salivary secretion
Parasympathetic Control
Sympathetic Control
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Chapter 44: Intestinal Fluid and Electrolyte Movement
Functional Anatomy
Both the small and large intestine absorb and secrete fluid and electrolytes, whereas only the small intestine absorbs nutrients
The small intestine has a villus-crypt organization, whereas the colon has surface epithelial cells with interspersed crypts
The surface area of the small intestine is amplified by folds, villi, and microvilli; amplification is less marked in the colon
Overview of Fluid and Electrolyte Movement in the Intestines
The small intestine absorbs ~6.5 L/day of an ~8.5-L fluid load that is presented to it, and the colon absorbs ~1.9 L/day
The small intestine absorbs net amounts of water, Na+, Cl−, and K+ and secretes , whereas the colon absorbs net amounts of water, Na+, and Cl− and secretes both K+ and
The intestines absorb and secrete solutes by both active and passive mechanisms
Intestinal fluid movement is always coupled to solute movement, and sometimes solute movement is coupled to fluid movement by solvent drag
The resistance of the tight junctions primarily determines the transepithelial resistance of intestinal epithelia
Cellular Mechanisms of Na+ Absorption
Na/glucose and Na/amino-acid cotransport in the small intestine is a major mechanism for postprandial Na+ absorption
Electroneutral Na-H exchange in the duodenum and jejunum is responsible for Na+ absorption that is stimulated by luminal alkalinity
Parallel Na-H and Cl-HCO3 exchange in the ileum and proximal part of the colon is the primary mechanism of Na+ absorption during the interdigestive period
Epithelial Na+ channels are the primary mechanism of “electrogenic” Na+ absorption in the distal part of the colon
Cellular Mechanisms of Cl− Absorption and Secretion
Voltage-dependent Cl− absorption represents coupling of Cl− absorption to electrogenic Na+ absorption in both the small intestine and the large intestine
Electroneutral Cl-HCO3 exchange results in Cl− absorption and secretion in the ileum and colon
Parallel Na-H and Cl-HCO3 exchange in the ileum and the proximal part of the colon mediates Cl− absorption during the interdigestive period
Electrogenic Cl− secretion occurs in crypts of both the small and the large intestine
Cellular Mechanisms of K+ Absorption and Secretion
Overall net transepithelial K+ movement is absorptive in the small intestine and secretory in the colon
K+ absorption in the small intestine probably occurs via solvent drag
Passive K+ secretion is the primary mechanism for net colonic secretion
Active K+ secretion is also present throughout the large intestine and is induced both by aldosterone and by cAMP
Aldosterone
cAMP and Ca2+
Active K+ absorption takes place only in the distal portion of the colon and is energized by an apical H-K pump
Regulation of Intestinal Ion Transport
Chemical mediators from the enteric nervous system, endocrine cells, and immune cells in the lamina propria may be either secretagogues or absorptagogues
Secretagogues can be classified by their type and by the intracellular second-messenger system that they stimulate
Mineralocorticoids, glucocorticoids, and somatostatin are absorptagogues
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Chapter 45: Nutrient Digestion and Absorption
Carbohydrate Digestion
Carbohydrates, providing ~45% of total energy needs of Western diets, require hydrolysis to monosaccharides before absorption
Luminal digestion begins with the action of salivary amylase and finishes with pancreatic amylase
“Membrane digestion” involves hydrolysis of oligosaccharides to monosaccharides by brush-border disaccharidases
Carbohydrate Absorption
SGLT1 is responsible for the Na+-coupled uptake of glucose and galactose across the apical membrane
The GLUT transporters mediate the facilitated diffusion of fructose at the apical membrane and of all three monosaccharides at the basolateral membrane
Protein Digestion
Proteins require hydrolysis to oligopeptides or amino acids before absorption in the small intestine
Luminal digestion of protein involves both gastric and pancreatic proteases, and yields amino acids and oligopeptides
Brush-border peptidases fully digest some oligopeptides to amino acids, whereas cytosolic peptidases digest oligopeptides that directly enter the enterocyte
Protein, Peptide, and Amino-Acid Absorption
Absorption of whole protein by apical endocytosis occurs primarily during the neonatal period
The apical absorption of dipeptides, tripeptides, and tetrapeptides occurs via an H+-driven cotransporter
Amino acids enter enterocytes via one or more group-specific apical transporters
At the basolateral membrane, amino acids exit enterocytes via Na+-independent transporters and enter via Na+-dependent transporters
Lipid Digestion
Natural lipids of biological origin are sparingly soluble in water
Dietary lipids are predominantly TAGs
Endogenous lipids are phospholipids and cholesterol from bile and membrane lipids from desquamated intestinal epithelial cells
The mechanical disruption of dietary lipids in the mouth and stomach produces an emulsion of lipid particles
Lingual and gastric (acid) lipase initiate lipid digestion
Pancreatic (alkaline) lipase, colipase, milk lipase, and other esterases—aided by bile salts—complete lipid hydrolysis in the duodenum and jejunum
Lipid Absorption
Products of lipolysis enter the bulk water phase of the intestinal lumen as vesicles, mixed micelles, and monomers
Lipids diffuse as mixed micelles and monomers through unstirred layers before crossing the jejunal enterocyte brush border
The enterocyte re-esterifies lipid components and assembles them into chylomicrons
The enterocyte secretes chylomicrons into the lymphatics during feeding and secretes VLDLs during fasting
Digestion and Absorption of Vitamins and Minerals
Intestinal absorption of fat-soluble vitamins follows the pathways of lipid absorption and transport
Dietary folate (PteGlu7) must be deconjugated by a brush-border enzyme before absorption by an anion exchanger at the apical membrane
Vitamin B12 (cobalamin) binds to haptocorrin in the stomach and then to intrinsic factor in the small intestine before endocytosis by enterocytes in the ileum
Ca2+ absorption, regulated primarily by vitamin D, occurs by active transport in the duodenum and by diffusion throughout the small intestine
Mg2+ absorption occurs by an active process in the ileum
Heme and nonheme iron are absorbed in the duodenum by distinct cellular mechanisms
Nonheme Iron
Heme Iron
Nutritional Requirements
No absolute daily requirement for carbohydrate or fat intake exists
The daily protein requirement for adult humans is typically 0.8 g/kg body weight but is higher in pregnant women, postsurgical patients, and athletes
Minerals and vitamins are not energy sources but are necessary for certain enzymatic reactions, for protein complexes, or as precursors for biomolecules
Minerals
Vitamins
Excessive intake of vitamins and minerals has mixed effects on bodily function
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Chapter 46: Hepatobiliary Function
Overview of Liver Physiology
The liver biotransforms and degrades substances taken up from blood and either returns them to the circulation or excretes them into bile
The liver stores carbohydrates, lipids, vitamins, and minerals; it synthesizes carbohydrates, protein, and intermediary metabolites
Functional Anatomy of the Liver and Biliary Tree
Hepatocytes are secretory epithelial cells separating the lumen of bile canaliculi from the fenestrated endothelium of sinusoids
The liver contains endothelial cells, macrophages (Kupffer cells), and stellate cells (Ito cells) within the sinusoidal spaces
The liver has a dual blood supply, but a single venous drainage system
Hepatocytes can be thought of as being arranged as classic hepatic lobules, portal lobules, or acinar units
Periportal hepatocytes specialize in oxidative metabolism, whereas pericentral hepatocytes detoxify drugs
Bile drains from canaliculi into small terminal ductules, then into larger ducts, and eventually, via a single common duct, into the duodenum
Uptake, Processing, and Secretion of Compounds by Hepatocytes
An Na-K pump at the basolateral membranes of hepatocytes provides the energy for transporting a wide variety of solutes via channels and transporters
Hepatocytes take up bile acids, other organic anions, and organic cations across their basolateral (sinusoidal) membranes
Bile Acids and Salts
Organic Anions
Bilirubin
Organic Cations
Neutral Organic Compounds
Inside the hepatocyte, the basolateral-to-apical movement of many compounds occurs by protein-bound or vesicular routes
Bile Salts
Bilirubin
In phase I of the biotransformation of organic anions and other compounds, hepatocytes use mainly cytochrome P-450 enzymes
In phase II of biotransformation, conjugation of phase I products makes them more water soluble for secretion into blood or bile
In phase III of biotransformation, hepatocytes excrete products of phase I and II into bile or sinusoidal blood
The interactions of xenobiotics with nuclear receptors control phase I, II, and III
Hepatocytes secrete bile acids, organic anions, organic cations, and lipids across their apical (canalicular) membranes
Bile Salts
Organic Anions
Organic Cations
Biliary Lipids
Hepatocytes take up proteins across their basolateral membranes by receptor-mediated endocytosis and fluid-phase endocytosis
Bile Formation
The secretion of canalicular bile is active and isotonic
Major organic molecules in bile include bile acids, cholesterol, and phospholipids
Canalicular bile flow has a constant component driven by the secretion of small organic molecules and a variable component driven by the secretion of bile acids
Bile Acid–Independent Flow in the Canaliculi
Bile Acid–Dependent Flow in the Canaliculi
Secretin stimulates the cholangiocytes of ductules and ducts to secrete a watery, -rich fluid
The gallbladder stores bile and delivers it to the duodenum during a meal
The relative tones of the gallbladder and sphincter of Oddi determine whether bile flows from the common hepatic duct into the gallbladder or into the duodenum
Enterohepatic Circulation of Bile Acids
The enterohepatic circulation of bile acids is a loop consisting of secretion by the liver, reabsorption by the intestine, and return to the liver in portal blood for repeat secretion into bile
Efficient intestinal conservation of bile acids depends on active apical absorption in the terminal ileum and passive absorption throughout the intestinal tract
The Liver as a Metabolic Organ
The liver can serve as either a source or a sink for glucose
The liver synthesizes a variety of important plasma proteins (e.g., albumin, coagulation factors, and carriage proteins) and metabolizes dietary amino acids
Protein Synthesis
Amino-Acid Uptake
Amino-Acid Metabolism
The liver obtains dietary triacylglycerols and cholesterol by taking up remnant chylomicrons via receptor-mediated endocytosis
Cholesterol, synthesized primarily in the liver, is an important component of cell membranes and serves as a precursor for bile acids and steroid hormones
Synthesis of Cholesterol
The liver is the central organ for cholesterol homeostasis and for the synthesis and degradation of LDL
The liver is the prime site for metabolism and storage of the fat-soluble vitamins A, D, E, and K
Vitamin A
Vitamin D
Vitamin E
Vitamin K
The liver stores copper and iron
Copper
Iron
References
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Journal Articles
Section VIII: The Endocrine System
Chapter 47: Organization of Endocrine Control
Principles of Endocrine Function
Chemical signaling can occur through endocrine, paracrine, or autocrine pathways
Endocrine Glands
Paracrine Factors
Hormones may be peptides, metabolites of single amino acids, or metabolites of cholesterol
Hormones can circulate either free or bound to carrier proteins
Immunoassays allow measurement of circulating hormones
Hormones can have complementary and antagonistic actions
Endocrine regulation occurs through feedback control
Endocrine regulation can involve hierarchic levels of control
The anterior pituitary regulates reproduction, growth, energy metabolism, and stress responses
The posterior pituitary regulates water balance and uterine contraction
Peptide Hormones
Specialized endocrine cells synthesize, store, and secrete peptide hormones
Peptide hormones bind to cell-surface receptors and activate a variety of signal-transduction systems
G Proteins Coupled to Adenylyl Cyclase
G Proteins Coupled to Phospholipase C
G Proteins Coupled to Phospholipase A2
Guanylyl Cyclase
Receptor Tyrosine Kinases
Tyrosine Kinase–Associated Receptors
Amine Hormones
Amine hormones are made from tyrosine and tryptophan
Amine hormones act via surface receptors
Steroid and Thyroid Hormones
Cholesterol is the precursor for the steroid hormones: cortisol, aldosterone, estradiol, progesterone, and testosterone
Steroid hormones bind to intracellular receptors that regulate gene transcription
Thyroid hormones bind to intracellular receptors that regulate metabolic rate
Steroid and thyroid hormones can also have nongenomic actions
References
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Journal Articles
Chapter 48: Endocrine Regulation of Growth and Body Mass
Growth Hormone
GH, secreted by somatotrophs in the anterior pituitary, is the principal endocrine regulator of growth
GH is in a family of hormones with overlapping activity
Somatotrophs secrete GH in pulses
GH secretion is under hierarchical control by GH–releasing hormone and somatostatin
GH-Releasing Hormone
GHRH Receptor
Ghrelin
Ghrelin Receptor
Somatostatin
SS Receptor
Both GH and IGF-1 negatively feed back on GH secretion by somatotrophs
GH has short-term anti-insulin metabolic effects as well as long-term growth-promoting effects mediated by IGF-1
GH Receptor
Short-Term Effects of GH
Long-Term Effects of GH via IGF-1
Growth-Promoting Hormones
IGF-1 is the principal mediator of the growth-promoting action of GH
IGF-2 acts similarly to IGF-1 but is less dependent on GH
Growth rate parallels plasma levels of IGF-1 except early and late in life
Thyroid hormones, steroids, and insulin also promote growth
Thyroid Hormones
Sex Steroids
Glucocorticoids
Insulin
The musculoskeletal system responds to growth stimuli of the GHRH–GH–IGF-1 axis
Regulation of Body Mass
The balance between energy intake and expenditure determines body mass
Energy expenditure comprises resting metabolic rate, activity-related energy expenditure, and diet-induced thermogenesis
Hypothalamic centers control the sensations of satiety and hunger
Leptin tells the brain how much fat is stored
Leptin and insulin are anorexigenic (i.e., satiety) signals for the hypothalamus
POMC Neurons
NPY/AgRP Neurons
Secondary Neurons
Ghrelin is an orexigenic signal for the hypothalamus
Plasma nutrient levels and enteric hormones are short-term factors that regulate feeding
References
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Chapter 49: The Thyroid Gland
Synthesis of Thyroid Hormones
T4 and T3, made by iodination of tyrosine residues on thyroglobulin, are stored as part of thyroglobulin molecules in thyroid follicles
Follicular cells take up iodinated thyroglobulin, hydrolyze it, and release T4 and T3 into the blood for binding to plasma proteins
Peripheral tissues deiodinate T4 to produce T3
Action of Thyroid Hormones
Thyroid hormones act through nuclear receptors in target tissues
Thyroid hormones can also act by nongenomic pathways
Thyroid hormones increase basal metabolic rate by stimulating futile cycles of catabolism and anabolism
Carbohydrate Metabolism
Protein Metabolism
Lipid Metabolism
Na-K Pump Activity
Thermogenesis
Thyroid hormones are essential for normal growth and development
Hypothalamic-Pituitary-Thyroid Axis
TRH from the hypothalamus stimulates thyrotrophs of the anterior pituitary to secrete TSH, which stimulates T4/T3 synthesis
Thyrotropin-Releasing Hormone
TRH Receptor
Thyrotropin
TSH Receptor
T3 exerts negative feedback on TSH secretion
References
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Chapter 50: The Adrenal Gland
The Adrenal Cortex: Cortisol
Cortisol is the primary glucocorticoid hormone in humans
Target Tissues
Actions
The adrenal zona fasciculata converts cholesterol to cortisol
Cortisol binds to a cytoplasmic receptor that translocates to the nucleus and modulates transcription in multiple tissues
Corticotropin-releasing hormone from the hypothalamus stimulates anterior pituitary corticotrophs to secrete ACTH, which stimulates the adrenal cortex to synthesize and secrete cortisol
Corticotropin-Releasing Hormone
CRH Receptor
Arginine Vasopressin
Adrenocorticotropic Hormone
ACTH Receptor
Cortisol exerts negative feedback on CRH and ACTH secretion, whereas stress acts through higher CNS centers to stimulate the axis
Feedback to the Anterior Pituitary
Feedback to the Hypothalamus
Control by a Higher CNS Center
The Adrenal Cortex: Aldosterone
The mineralocorticoid aldosterone is the primary regulator of salt balance and extracellular volume
The glomerulosa cells of the adrenal cortex synthesize aldosterone from cholesterol via progesterone
Aldosterone stimulates Na+ reabsorption and K+ excretion by the renal tubule
Angiotensin II, K+, and ACTH all stimulate aldosterone secretion
Angiotensin II
Potassium
Adrenocorticotropic Hormone
Aldosterone exerts indirect negative feedback on the renin-angiotensin axis by increasing effective circulating volume and by lowering plasma [K+]
Renin-Angiotensin Axis
Potassium
Role of Aldosterone in Normal Physiology
Role of Aldosterone in Disease
The Adrenal Medulla
The adrenal medulla bridges the endocrine and sympathetic nervous systems
Only chromaffin cells of the adrenal medulla have the enzyme for epinephrine synthesis
Catecholamines bind to α and β adrenoceptors on the cell surface and act through heterotrimeric G proteins
The CNS-epinephrine axis provides integrated control of multiple functions
References
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Chapter 51: The Endocrine Pancreas
The islets of Langerhans are endocrine and paracrine tissue
Insulin
Insulin replenishes fuel reserves in muscle, liver, and adipose tissue
β cells synthesize and secrete insulin
The Insulin Gene
Insulin Synthesis
Secretion of Insulin, Proinsulin, and C Peptide
Glucose is the major regulator of insulin secretion
Metabolism of glucose by the β cell triggers insulin secretion
Neural and humoral factors modulate insulin secretion
Exercise
Feeding
The insulin receptor is a receptor tyrosine kinase
High levels of insulin lead to downregulation of insulin receptors
In liver, insulin promotes conversion of glucose to glycogen stores or to triacylglycerols
Glycogen Synthesis and Glycogenolysis
Glycolysis and Gluconeogenesis
Lipogenesis
Protein Metabolism
In muscle, insulin promotes the uptake of glucose and its storage as glycogen
In adipocytes, insulin promotes glucose uptake and conversion to TAGs for storage
Glucagon
Pancreatic α cells secrete glucagon in response to ingested protein
Pancreatic α Cells
Intestinal L Cells
Glucagon, acting through cAMP, promotes the synthesis of glucose by the liver
Glucagon promotes oxidation of fat in the liver, which can lead to ketogenesis
Somatostatin
Somatostatin inhibits the secretion of growth hormone, insulin, and other hormones
References
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Journal Articles
Chapter 52: The Parathyroid Glands and Vitamin D
Calcium and Phosphate Balance
The gut, kidneys, and bone regulate calcium balance
The gut, kidneys, and bone also regulate phosphate balance
Physiology of Bone
Dense cortical bone and the more reticulated trabecular bone are the two major bone types
The extracellular matrix forms the nidus for the nucleation of hydroxyapatite crystals
Bone remodeling depends on the closely coupled activities of osteoblasts and osteoclasts
Parathyroid Hormone
Plasma Ca2+ regulates the synthesis and secretion of PTH
PTH Synthesis and Vitamin D
Processing of PTH
Metabolism of PTH
High plasma [Ca2+] inhibits the synthesis and release of PTH
The PTH receptor couples via G proteins to either adenylyl cyclase or phospholipase C
In the kidney, PTH promotes Ca2+ reabsorption, phosphate loss, and 1-hydroxylation of 25-hydroxyvitamin D
Stimulation of Ca2+ Reabsorption
Inhibition of Phosphate Reabsorption
Stimulation of the Last Step of Synthesis of 1,25- Dihydroxyvitamin D
In bone, PTH can promote net resorption or net deposition
Bone Resorption by Indirect Stimulation of Osteoclasts
Bone Resorption by Reduction in Bone Matrix
Bone Deposition
Vitamin D
The active form of vitamin D is its 1,25-dihydroxy metabolite
Vitamin D, by acting on the small intestine and kidney, raises plasma [Ca2+] and thus promotes bone mineralization
Small Intestine
Kidney
Bone
Calcium ingestion lowers—whereas phosphate ingestion raises—levels of both PTH and 1,25-dihydroxyvitamin D
Calcium Ingestion
Phosphate Ingestion
Calcitonin and Other Hormones
Calcitonin inhibits osteoclasts, but its effects are transitory
Sex steroid hormones promote bone deposition, whereas glucocorticoids promote resorption
PTHrP, encoded by a gene that is entirely distinct from that for PTH, can cause hypercalcemia in certain malignancies
References
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Section IX: The Reproductive System
Chapter 53: Sexual Differentiation
Genetic Aspects of Sexual Differentiation
Meiosis occurs only in germ cells and gives rise to male and female gametes
Fertilization of an oocyte by an X- or Y-bearing sperm establishes the zygote’s genotypic sex
Genotypic sex determines differentiation of the indifferent gonad into either an ovary or a testis
The testis-determining gene is located on the Y chromosome
Endocrine and paracrine messengers modulate phenotypic differentiation
Differentiation of the Gonads
Primordial germ cells migrate from the yolk sac to the primordial gonad
The primitive testis develops from the medulla of the primordial gonad
The primitive ovary develops from the cortex of the primordial gonad
Development of the Accessory Sex Organs
The embryonic gonad determines the development of the internal genitalia and the external sexual phenotype
Embryos of both sexes have a double set of embryonic genital ducts
In males, the wolffian ducts become the epididymis, vas deferens, seminal vesicles, and ejaculatory duct
In females, the müllerian ducts become the fallopian tubes, the uterus, and the upper third of the vagina
In males, development of the wolffian ducts requires testosterone
In males, antimüllerian hormone causes regression of the müllerian ducts
Differentiation of the External Genitalia
The urogenital sinus develops into the urinary bladder, the urethra, and, in females, the vestibule of the vagina
The external genitalia of both sexes develop from common anlagen
Endocrine and Paracrine Control of Sexual Differentiation
The SRY gene triggers development of the testis, which makes the androgens and AMH necessary for male sexual differentiation
Testosterone Production
Androgen Receptor
DHT Formation
Antimüllerian Hormone
Androgens direct the male pattern of sexual differentiation of the internal ducts, the urogenital sinus, and the external genitalia
Differentiation of the Duct System
Differentiation of the Urogenital Sinus and External Genitalia
Androgens and estrogens influence sexual differentiation of the brain
Puberty
Puberty involves steroid hormones produced by the gonads and the adrenals
Hypothalamic gonadotropin-releasing hormone secretion controls puberty
Multiple factors control the timing of puberty
Androgens and estrogens influence secondary sex characteristics at puberty
Males
Females
The appearance of secondary sex characteristics at puberty completes sexual differentiation and development
References
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Chapter 54: The Male Reproductive System
Hypothalamic-Pituitary-Gonadal Axis
The hypothalamus secretes GnRH, which acts on gonadotrophs in the anterior pituitary
Under the control of GnRH, gonadotrophs in the anterior pituitary secrete LH and FSH
LH stimulates the Leydig cells of the testis to produce testosterone
FSH stimulates Sertoli cells to synthesize hormones that influence Leydig cells and spermatogenesis
The hypothalamic-pituitary-testicular axis is under feedback inhibition by testicular steroids and inhibins
Testosterone
Leydig cells convert cholesterol to testosterone
Adipose tissue, skin, and the adrenal cortex also produce testosterone and other androgens
Testosterone acts on target organs by binding to a nuclear receptor
Metabolism of testosterone occurs primarily in the liver and prostate
Biology of Spermatogenesis and Semen
Spermatogenesis includes mitotic divisions of spermatogonia, meiotic divisions of spermatocytes to spermatids, and maturation to spermatozoa N54-7
The Sertoli cells support spermatogenesis
Sperm maturation occurs in the epididymis
Spermatozoa are the only independently motile cells in the human body
The accessory male sex glands—the seminal vesicles, prostate, and bulbourethral glands—produce the seminal plasma
Male Sex Act
The sympathetic and parasympathetic divisions of the autonomic nervous system control the male genital system
Sympathetic Division of the ANS
Parasympathetic Division of the ANS
Visceral Afferents
Erection is primarily under parasympathetic control
Parasympathetic Innervation
Sympathetic Innervation
Somatic Innervation
Afferent Innervation
Emission is primarily under sympathetic control
Motor Activity of the Duct System
Secretory Activity of the Accessory Glands
Ejaculation is under the control of a spinal reflex
References
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Chapter 55: The Female Reproductive System
Female reproductive organs include the ovaries and accessory sex organs
Reproductive function in the human female is cyclic
Hypothalamic-Pituitary-Gonadal Axis and Control of the Menstrual Cycle
The human menstrual cycle coordinates changes in both the ovary and endometrium
Follicular/Proliferative Phase
Ovulation
Luteal/Secretory Phase
Menses
The hypothalamic-pituitary-ovarian axis drives the menstrual cycle
Neurons in the hypothalamus release GnRH in a pulsatile fashion
GnRH stimulates gonadotrophs in the anterior pituitary to secrete FSH and LH
The ovarian steroids (estrogens and progestins) feed back on the hypothalamic-pituitary axis
Negative Feedback by Ovarian Steroids
Positive Feedback by Ovarian Steroids
Ovaries produce peptide hormones—inhibins, activins, and follistatins—that modulate FSH secretion
Negative Feedback by the Inhibins
Positive Feedback by the Activins
Modulation of gonadotropin secretion by positive and negative ovarian feedback produces the normal menstrual rhythm
Ovarian Steroids
Starting from cholesterol, the ovary synthesizes estradiol, the major estrogen, and progesterone, the major progestin
Estrogen biosynthesis requires two ovarian cells and two gonadotropins, whereas progestin synthesis requires only a single cell
Estrogens stimulate cellular proliferation and growth of sex organs and other tissues related to reproduction
The Ovarian Cycle: Folliculogenesis, Ovulation, and Formation of the Corpus Luteum
Female reproductive life span is determined by the number of primordial follicles established during fetal life
Primary Oocytes
Primordial Follicles
Primary Follicles
Secondary Follicles
Tertiary Follicles
Graafian Follicles
The oocyte grows and matures during folliculogenesis
FSH and LH stimulate the growth of a cohort of follicles
Each month, one follicle achieves dominance
Estradiol secretion by the dominant follicle triggers the LH surge and thus ovulation
After ovulation, theca and granulosa cells of the follicle differentiate into theca-lutein and granulosa-lutein cells of the corpus luteum
Growth and involution of the corpus luteum produce the rise and fall in estradiol and progesterone during the luteal phase
The Endometrial Cycle
The ovarian hormones drive the morphological and functional changes of the endometrium during the monthly cycle
The Menstrual Phase
The Proliferative Phase
The Secretory Phase
The effective implantation window is 3 to 4 days
Female Sex Act
The female sex response occurs in four distinct phases
Excitement
Plateau
Orgasm
Resolution
Both the sympathetic and the parasympathetic divisions control the female sex response
The female sex response facilitates sperm transport through the female reproductive tract
Menopause
Only a few functioning follicles remain in the ovaries of a menopausal woman
During menopause, levels of the ovarian steroids fall, whereas gonadotropin levels rise
References
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Chapter 56: Fertilization, Pregnancy, and Lactation
Transport of Gametes and Fertilization
Cilia and smooth muscle transport the egg and sperm within the female genital tract
The “capacitation” of the spermatozoa that occurs in the female genital tract enhances the ability of the sperm cell to fertilize the ovum
Fertilization begins as the sperm cell attaches to the zona pellucida and undergoes the acrosomal reaction, and it ends with the fusion of the male and female pronuclei
Implantation of the Developing Embryo
The presence of an embryo leads to decidualization of the endometrium
Uterine secretions nourish the preimplantation embryo, promote growth, and prepare it for implantation
The blastocyst secretes substances that facilitate implantation
During implantation, the blastocyst apposes itself to the endometrium, adheres to epithelial cells, and finally invades the stroma
Apposition
Adhesion
Invasion
Physiology of the Placenta
At the placenta, the space between the fetus’s chorionic villi and the mother’s endometrial wall contains a continuously renewed pool of extravasated maternal blood
Maternal Blood Flow
Fetal Blood Flow
Gases and other solutes move across the placenta
O2 and CO2 Transport
Other Solutes
The placenta makes a variety of peptide hormones, including hCG and human chorionic somatomammotropin
The Maternal-Placental-Fetal Unit
During pregnancy, progesterone and estrogens rise to levels that are substantially higher than their peaks in a normal cycle
After 8 weeks of gestation, the maternal-placental-fetal unit maintains high levels of progesterone and estrogens
Response of the Mother to Pregnancy
Both maternal cardiac output and blood volume increase during pregnancy
Increased levels of progesterone during pregnancy increase alveolar ventilation
Pregnancy increases the demand for dietary protein, iron, and folic acid
Less than one third of the total maternal weight gain during pregnancy represents the fetus
Parturition
Human birth usually occurs at around the 40th week of gestation
Parturition occurs in distinct stages, numbered 0 to 3
Stage 0—Quiescence
Stage 1—Transformation/Activation
Stage 2—Active Labor
Stage 3—Involution
Reciprocal decreases in progesterone receptors and increases in estrogen receptors are critical for the onset of labor
Signals from the fetus may initiate labor
PGs initiate uterine contractions, and both PGs and OT sustain labor
Prostaglandins
Oxytocin
Relaxin
Mechanical Factors
Positive Feedback
Lactation
The epithelial alveolar cells of the mammary gland secrete the complex mixture of sugars, proteins, lipids, and other substances that constitute milk
PRL is essential for milk production, and suckling is a powerful stimulus for PRL secretion
OT and psychic stimuli initiate milk ejection (“let-down”)
Suckling inhibits the ovarian cycle
References
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Chapter 57: Fetal and Neonatal Physiology
Biology of Fetal Growth
Two distinct circulations—fetoplacental and uteroplacental—underlie the transfer of gases and nutrients
Growth occurs by hyperplasia and hypertrophy
Growth depends primarily on genetic factors during the first half of gestation and on epigenetic factors thereafter
Increases in placental mass parallel periods of rapid fetal growth
Insulin, the insulin-like growth factors, and thyroxine stimulate fetal growth
Glucocorticoids and Insulin
Insulin-Like Growth Factors
Epidermal Growth Factor
Thyroid Hormones
Peptide Hormones
Many fetal tissues produce red blood cells early in gestation
The fetal gastrointestinal and urinary systems excrete products into the amniotic fluid by midpregnancy
A surge in protein synthesis, with an increase in muscle mass, is a major factor in the rapid fetal weight gain during the third trimester
Fetal lipid stores increase rapidly during the third trimester
Development and Maturation of the Cardiopulmonary System
Fetal lungs develop by repetitive branching of both bronchial and pulmonary arterial trees
An increase in cortisol, with other hormones, triggers surfactant production in the third trimester
Fetal respiratory movements begin near the end of the first trimester but wane just before birth
The fetal circulation has four unique pathways—placenta, ductus venosus, foramen ovale, and ductus arteriosus—to facilitate gas and nutrient exchange
Placenta
Ductus Venosus
Foramen Ovale
Ductus Arteriosus
Cardiopulmonary Adjustments at Birth
Loss of the placental circulation requires the newborn to breathe on its own
Mild hypoxia and hypercapnia, as well as tactile stimuli and cold skin, trigger the first breath
At birth, removal of the placenta increases systemic vascular resistance, whereas lung expansion decreases pulmonary vascular resistance
Removal of the Placental Circulation
Increase in Pulmonary Blood Flow
Closure of the ductus venosus within the first days of life forces portal blood to perfuse the liver
Closure of the foramen ovale occurs as left atrial pressure begins to exceed right atrial pressure
Closure of the ductus arteriosus completes the separation between the pulmonary and systemic circulations
Neonatal Physiology
Although the newborn is prone to hypothermia, nonshivering thermogenesis in brown fat helps to keep the neonate warm
The neonate mobilizes glucose and FAs soon after delivery
Carbohydrate Metabolism
Fat Metabolism
Metabolic Rate
Breast milk from a mother with a balanced diet satisfies all of the infant’s nutritional requirements during the first several months of life
The neonate is at special risk of developing fluid and acid-base imbalances
Humoral and cellular immune responses begin at early stages of development in the fetus
Fetus
Neonate
In premature newborns, immaturity of organ systems and fragility of homeostatic mechanisms exacerbate postnatal risks
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Section X: Physiology of Everyday Life
Chapter 58: Metabolism
Forms of Energy
Energy Balance
Energy input to the body is the sum of energy output and storage
The inefficiency of chemical reactions leads to loss of the energy available for metabolic processes
Free energy, conserved as high-energy bonds in ATP, provides the energy for cellular functions
Energy Interconversion From Cycling between 6-Carbon and 3-Carbon Molecules
Glycolysis converts the 6-carbon glucose molecule to two 3-carbon pyruvate molecules
Gluconeogenesis converts nonhexose precursors to the 6-carbon glucose molecule
Reciprocal regulation of glycolysis and gluconeogenesis minimizes futile cycling
Allosteric Regulation
Transcriptional Regulation
Cells can convert glucose or amino acids into FAs
The body permits only certain energy interconversions
Energy Capture (Anabolism)
After a carbohydrate meal, the body burns some ingested glucose and incorporates the rest into glycogen or TAGs
Liver
Muscle
Adipose Tissue
After a protein meal, the body burns some ingested amino acids and incorporates the rest into proteins
After a fatty meal, the body burns some ingested FAs and incorporates the rest into TAGs
Energy Liberation (Catabolism)
The first step in energy catabolism is to break down glycogen or TAGs to simpler compounds
Skeletal Muscle
Liver
Adipocytes
The second step in TAG catabolism is β-oxidation of FAs
The final common steps in oxidizing carbohydrates, TAGs, and proteins to CO2 are the citric acid cycle and oxidative phosphorylation
Citric Acid Cycle
Oxidative Phosphorylation
Ketogenesis
Oxidizing different fuels yields similar amounts of energy per unit O2 consumed
Integrative Metabolism During Fasting
During an overnight fast, glycogenolysis and gluconeogenesis maintain plasma glucose levels
Requirement for Glucose
Gluconeogenesis versus Glycogenolysis
Gluconeogenesis: The Cori Cycle
Gluconeogenesis: The Glucose-Alanine Cycle
Lipolysis
Starvation beyond an overnight fast enhances gluconeogenesis and lipolysis
Enhanced Gluconeogenesis
Enhanced Lipolysis
Prolonged starvation moderates proteolysis but accelerates lipolysis, thereby releasing ketone bodies
Decreased Proteolysis
Decreased Hepatic Gluconeogenesis
Increased Renal Gluconeogenesis
Increased Lipolysis and Ketogenesis
References
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Chapter 59: Regulation of Body Temperature
Heat and Temperature: Advantages of Homeothermy
Homeotherms maintain their activities over a wide range of environmental temperatures
Body core temperature depends on time of day, physical activity, time in the menstrual cycle, and age
The body’s rate of heat production can vary from ~70 kcal/hr at rest to 600 kcal/hr during exercise
Modes of Heat Transfer
Maintaining a relatively constant body temperature requires a fine balance between heat production and heat losses
Heat moves from the body core to the skin, primarily by convection
Heat moves from the skin to the environment by radiation, conduction, convection, and evaporation
Radiation
Conduction
Convection
Evaporation
When heat gain exceeds heat loss, body core temperature rises
Clothing insulates the body from the environment and limits heat transfer from the body to the environment
Active Regulation of Body Temperature by the Central Nervous System
Thermoreceptors in the skin and temperature-sensitive neurons in the hypothalamus respond to changes in their local temperature
Skin Thermoreceptors
Hypothalamic Temperature-Sensitive Neurons
The CNS thermoregulatory network integrates thermal information and directs changes in efferent activity to modify rates of heat transfer and production
Thermal effectors include behavior, cutaneous circulation, sweat glands, and skeletal muscles responsible for shivering
Hypothermia, Hyperthermia, and Fever
Hypothermia or hyperthermia occurs when heat transfer to or from the environment overwhelms the body’s thermoregulatory capacity
Exercise raises heat production, which is followed by a matching rise in heat loss, but at the cost of a steady-state hyperthermia of exercise
Fever is a regulated hyperthermia
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Chapter 60: Exercise Physiology and Sports Science
Motor Units and Muscle Function
The motor unit is the functional element of muscle contraction
Muscle force rises with the recruitment of motor units and an increase in their firing frequency
Compared with type I motor units, type II units are faster and stronger but more fatigable
As external forces stretch muscle, series elastic elements contribute a larger fraction of total tension
The action of a muscle depends on the axis of its fibers and its origin and insertion on the skeleton
Fluid and energetically efficient movements require learning
Strength versus endurance training differentially alters the properties of motor units N60-3
Conversion of Chemical Energy to Mechanical Work
ATP and PCr provide immediate but limited energy
Anaerobic glycolysis provides a rapid but self-limited source of ATP
Oxidation of glucose, lactate, and fatty acids provides a slower but long-term source of ATP
Oxidation of Nonmuscle Glucose
Oxidation of Lactate
Gluconeogenesis
Oxidation of Nonmuscle Lipid
Choice of Fuel Sources
Muscle Fatigue
Fatigued muscle produces less force and has a reduced velocity of shortening
Changes in the CNS produce central fatigue
Impaired excitability and impaired Ca2+ release can produce peripheral fatigue
High-Frequency Fatigue
Low-Frequency Fatigue
Fatigue can result from ATP depletion, lactic acid accumulation, and glycogen depletion
ATP Depletion
Lactic Acid Accumulation
Glycogen Depletion
Determinants of Maximal O2 Uptake and Consumption
Maximal O2 uptake by the lungs can exceed resting O2 uptake by more than 20-fold
O2 uptake by muscle is the product of muscle blood flow and O2 extraction
O2 delivery by the cardiovascular system is the limiting step for maximal O2 utilization
Limited O2 Uptake by the Lungs
Limited O2 Delivery by the Cardiovascular System
Limited O2 Extraction by Muscle
Effective circulating volume takes priority over cutaneous blood flow for thermoregulation
Sweating
Eccrine, but not apocrine, sweat glands contribute to temperature regulation
Eccrine sweat glands are tubules comprising a secretory coiled gland and a reabsorptive duct
Secretion by Coil Cells
Reabsorption by Duct Cells
The NaCl content of sweat increases with the rate of secretion but decreases with acclimatization to heat
Flow Dependence
Cystic Fibrosis
Replenishment
Acclimatization
The hyperthermia of exercise stimulates eccrine sweat glands
Endurance (Aerobic) Training
Aerobic training requires regular periods of stress and recovery
Aerobic training increases maximal O2 delivery by increasing plasma volume and maximal cardiac output
Maximizing Arterial O2 Content
Maximizing Cardiac Output
Aerobic training enhances O2 diffusion into muscle
Aerobic training increases mitochondrial content
References
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Chapter 61: Environmental Physiology
The Environment
Voluntary feedback control mechanisms can modulate the many layers of our external environment
Environmental temperature provides conscious clues for triggering voluntary feedback mechanisms
Room ventilation should maintain , , and levels of toxic substances within acceptable limits
Acceptable Limits for and
Measuring Room Ventilation
Carbon Monoxide
Threshold Limit Values and Biological Exposure Indices
Tissues must resist the G force produced by gravity and other mechanisms of acceleration
The partial pressures of gases—other than water—inside the body depend on Pb
Diving Physiology
Immersion raises Pb, thereby compressing gases in the lungs
SCUBA divers breathe compressed air to maintain normal lung expansion
Increased alveolar can cause narcosis
Increased alveolar can lead to O2 toxicity
Using helium to replace inspired N2 and O2 avoids nitrogen narcosis and O2 toxicity
After an extended dive, one must decompress slowly to avoid decompression illness
High-Altitude Physiology
Pb and ambient on top of Mount Everest are approximately one third of their values at sea level
Everest Base Camp
Peak of Mount Everest
Air Travel
Up to modest altitudes, arterial O2 content falls relatively less than Pb due to the shape of the Hb-O2 dissociation curve
During the first few days at altitude, compensatory adjustments to hypoxemia include tachycardia and hyperventilation
Long-term adaptations to altitude include increases in hematocrit, pulmonary diffusing capacity, capillarity, and oxidative enzymes
Hematocrit
Pulmonary Diffusing Capacity
Capillary Density
Oxidative Enzymes
High altitude causes mild symptoms in most people and acute or chronic mountain sickness in susceptible individuals
Symptoms of Hypoxia
Acute Mountain Sickness
Chronic Mountain Sickness
Flight and Space Physiology
Acceleration in one direction shifts the blood volume in the opposite direction
“Weightlessness” causes a cephalad shift of the blood volume and an increase in urine output
Space flight leads to motion sickness and to decreases in muscle and bone mass
Exercise partially overcomes the deconditioning of muscles during space flight
Return to earth requires special measures to maintain arterial blood pressure
References
Books and Reviews
Journal Articles
Chapter 62: The Physiology of Aging
Concepts in Aging
During the 20th century, the age structure of populations in developed nations shifted toward older individuals
The definition, occurrence, and measurement of aging are fundamental but controversial issues
Aging is an evolved trait
Human aging studies can be cross-sectional or longitudinal
Cross-Sectional Design
Longitudinal Design
Whether age-associated diseases are an integral part of aging remains controversial
Cellular and Molecular Mechanisms of Aging
Oxidative stress and related processes that damage macromolecules may have a causal role in aging
Reactive Oxygen Species
Glycation and Glycoxidation
Mitochondrial Damage
Somatic Mutations
Inadequacy of repair processes may contribute to the aging phenotype
DNA Repair
Protein Homeostasis
Autophagy
Dysfunction of the homeostasis of cell number may be a major factor in aging
Limitations in Cell Division
Cell Removal
Aging of the Human Physiological Systems
Aging people lose height and lean body mass but gain and redistribute fat
Aging thins the skin and causes the musculoskeletal system to become weak, brittle, and stiff
Skin
Skeletal Muscle
Bone
Synovial Joints
The healthy elderly experience deficits in sensory transduction and speed of central processing
Sensory Functions
Motor Functions
Cognitive Functions
Aging causes decreased arterial compliance and increased ventilation-perfusion mismatching
Cardiovascular Function
Pulmonary Function
Exercise
Glomerular filtration rate falls with age in many but not all people
Aging has only minor effects on gastrointestinal function
Aging causes modest declines in most endocrine functions
Insulin
Growth Hormone and IGF-1
Adrenal Steroids
Thyroid Hormones
Parathyroid Hormone
Gonadal Hormones
Aging Slowly
Caloric restriction slows aging and extends life in several species, including some mammals
Genetic alterations can extend life in several species
Proposed interventions to slow aging and extend human life are controversial
References
Books and Reviews
Journal Articles

Citation preview

Medical Physiology

Medical Physiology THIRD

3

EDITION

WALTER F. BORON, MD, PhD

EMILE L. BOULPAEP, MD

Professor David N. and Inez Myers/Antonio Scarpa Chairman Department of Physiology and Biophysics Case Western Reserve University Cleveland, Ohio

Professor Department of Cellular and Molecular Physiology Yale University School of Medicine New Haven, Connecticut

1600 John F. Kennedy Blvd. Ste 1800 Philadelphia, PA 19103-2899

MEDICAL PHYSIOLOGY, THIRD EDITION INTERNATIONAL EDITION

ISBN: 978-1-4557-4377-3 ISBN: 978-0-323-42796-8

Copyright © 2017 by Elsevier, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. With respect to any drug or pharmaceutical products identified, readers are advised to check the most current information provided (i) on procedures featured or (ii) by the manufacturer of each product to be administered, to verify the recommended dose or formula, the method and duration of administration, and contraindications. It is the responsibility of practitioners, relying on their own experience and knowledge of their patients, to make diagnoses, to determine dosages and the best treatment for each individual patient, and to take all appropriate safety precautions. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Previous editions copyrighted 2012, 2009, 2005, and 2003. Library of Congress Cataloging-in-Publication Data Names: Boron, Walter F., editor. | Boulpaep, Emile L., editor. Title: Medical physiology / [edited by] Walter F. Boron, Emile L. Boulpaep. Other titles: Medical physiology (Boron) Description: Edition 3. | Philadelphia, PA : Elsevier, [2017] | Includes bibliographical references and index. Identifiers: LCCN 2016005260| ISBN 9781455743773 (hardcover : alk. paper) | ISBN 9780323427968 (International ed.) Subjects: | MESH: Physiological Phenomena | Cell Physiological Phenomena Classification: LCC QP34.5 | NLM QT 104 | DDC 612—dc23 LC record available at http://lccn.loc.gov/2016005260

Executive Content Strategist: Elyse O’Grady Senior Content Development Specialist: Marybeth Thiel Publishing Services Manager: Julie Eddy Senior Project Manager: David Stein Design Direction: Julia Dummitt

Printed in China Last digit is the print number:  9  8  7  6  5  4  3  2  1

CONTRIBUTORS Peter S. Aronson, MD

C.N.H. Long Professor of Internal Medicine Professor of Cellular and Molecular Physiology Section of Nephrology Department of Internal Medicine Yale University School of Medicine New Haven, Connecticut

Eugene J. Barrett, MD, PhD

Professor Departments of Medicine and Pharmacology University of Virginia School of Medicine Charlottesville, Virginia

Paula Q. Barrett, PhD

Professor Department of Pharmacology University of Virginia School of Medicine Charlottesville, Virginia

Henry J. Binder, MD

Professor Emeritus of Medicine Department of Internal Medicine—Digestive Diseases Yale University School of Medicine New Haven, Connecticut

Walter F. Boron, MD, PhD

Professor David N. and Inez Myers/Antonio Scarpa Chairman Department of Physiology and Biophysics Case Western Reserve University Cleveland, Ohio

Emile L. Boulpaep, MD

Professor Department of Cellular and Molecular Physiology Yale University School of Medicine New Haven, Connecticut

Lloyd Cantley, MD, FASN

Professor Department of Internal Medicine Department of Cellular and Molecular Physiology Yale University School of Medicine New Haven, Connecticut

Michael J. Caplan, MD, PhD

C.N.H. Long Professor and Chair Department of Cellular and Molecular Physiology Yale University School of Medicine New Haven, Connecticut

Barry W. Connors, PhD

Professor and Chair Department of Neuroscience Alpert Medical School Brown University Providence, Rhode Island

Arthur DuBois, MD

Professor Emeritus of Epidemiology and Public Health and Cellular and Molecular Physiology John B. Pierce Laboratory New Haven, Connecticut

Gerhard Giebisch, MD

Professor Emeritus of Cellular and Molecular Physiology Department of Cellular and Molecular Physiology Yale University School of Medicine New Haven, Connecticut

Fred S. Gorelick, MD

Professor Departments of Internal Medicine and Cell Biology Yale University School of Medicine New Haven, Connecticut

Peter Igarashi, MD

Nesbitt Chair and Head Department of Medicine University of Minnesota Minneapolis, Minnesota

Ervin E. Jones, MD, PhD

Retired Department of Obstetrics and Gynecology Yale University School of Medicine New Haven, Connecticut

W. Jonathan Lederer, MD, PhD

Director and Professor, Center for Biomedical Engineering and Technology and Department of Physiology University of Maryland School of Medicine Baltimore, Maryland

George Lister, MD

Jean McLean Wallace Professor of Pediatrics Professor of Cellular and Molecular Physiology Yale School of Medicine New Haven, Connecticut

v

vi

Contributors

Charles M. Mansbach II, MD†

Professor of Medicine and Physiology University of Tennessee Health Science Center Memphis, Tennessee

Christopher R. Marino, MD

Professor of Medicine University of Tennessee Health Science Center Chief of Staff VA Medical Center Memphis, Tennessee

Edward J. Masoro, PhD

Professor Emeritus of Physiology University of Texas Health Science Center at San Antonio San Antonio, Texas

Steven S. Segal, PhD

Professor Department of Medical Pharmacology and Physiology University of Missouri School of Medicine Columbia, Missouri

Gerald I. Shulman, MD, PhD, FACP, MACE

Investigator, Howard Hughes Medical Institute George R. Cowgill Professor of Physiological Chemistry Professor of Medicine (Endocrinology/Metabolism) and Cellular & Molecular Physiology Co-Director, Yale Diabetes Research Center Yale University School of Medicine New Haven, Connecticut

Frederick J. Suchy, MD

Professor Department of Reproductive Biology Case Western Reserve University Cleveland, Ohio

Chief Research Officer Director, Children’s Hospital Colorado Research Institute Professor of Pediatrics Associate Dean for Child Health Research University of Colorado School of Medicine Aurora, Colorado

Edward G. Moczydlowski, PhD

Erich E. Windhager, MD

Sam Mesiano, PhD

Senior Associate Dean of Academic Affairs & Professor of Physiology College of Health Sciences California Northstate University Elk Grove, California

Shaun F. Morrison, PhD

Professor Department of Neurological Surgery Oregon Health & Science University Portland, Oregon

Kitt Falk Petersen, MD

Professor Section of Endocrinology Department of Internal Medicine Yale University School of Medicine New Haven, Connecticut

Bruce R. Ransom, MD, PhD

Magnuson Professor and Chair Department of Neurology Department of Physiology and Biophysics University of Washington Health Sciences Center Seattle, Washington

George B. Richerson, MD, PhD

Professor & Chairman Department of Neurology University of Iowa Carver College of Medicine Iowa City, Iowa



Deceased.

Professor Department of Physiology and Biophysics Weill Medical College Cornell University New York, New York

VIDEO TABLE OF CONTENTS 7-1 8-1 9-1 10-1 13-1 22-1 27-1 38-1 41-1 55-1

Action Potential Chemical Synaptic Transmission The Cross Bridge Cycle Chemotaxis Chemical Synaptic Transmission The Cardiac Cycle Pressures during Respiration The Countercurrent Multiplier Peristalsis The Menstrual Cycle

PREFACE TO THE THIRD EDITION We are delighted that the physiological community so eagerly welcomed the Second Edition of our book. The 3-fold philosophy that has guided us in the previous editions has endured as we prepared the Third Edition. First, we combine the expertise of several authors with the consistency of a single pen. In the First Edition, we achieved this singleness of pen by sitting—shoulder to shoulder—at a computer as we rewrote the primary copy of our authors, line by line. By the time we began editing the Third Edition, one of us had moved from New Haven to Cleveland. Even so, we continued to edit jointly and in real time—monitor to monitor—using desktop-sharing software. After more than two decades, we have become so accustomed to each other’s writing styles that we can literally finish each other’s sentences. Second, we still integrate physiological concepts from the level of DNA and epigenetics to the human body, and everything in between. Third, we complete the presentation of important physiological principles by pairing them with illustrations from pathophysiology, thereby putting physiology in a clinical context. In this Third Edition, we have updated the entire book to reflect new molecular insights. In the process, we have shortened the printed version of the book by 40 pages. The Third Edition contains 20 new or redrawn figures as well as enhancements to 125 others. Similarly, we included over 190 tables. In the First Edition, we launched the concept of online-only Notes—electronic footnotes that were available on the Student Consult website. These Notes (indicated by icons in the print version of the book) amplify concepts in the text, provide details and derivations of equations, add clinical illustrations, and include interesting facts (e.g., biographies of famous physiologists). With the increased use of online materials and eBooks, our readers may welcome our updating of the previous Notes as well as a 13% increase in the total number of Notes for the Third Edition, for a total of about 750. In the Second Edition, we provided the reader with numerous crosslinks to explanatory materials within the book by providing chapter numbers. In the Third Edition, we greatly expand the number of such crosslinks—but now refer the reader to specific pages in the print, and link the reader to specific paragraphs in the eBook. The eBook provides references to scientific literature. In Section II (Physiology of Cells and Molecules), fresh insights led to substantial revisions in Chapter 4 (Regulation of Gene Expression), including the subchapter on epigenetics, and another on posttranslational modifications. Moreover, advances in physiological genomics and the understanding of genetic diseases led to major expansions of two tables— one on the SLC family of transporters (Table 5-4 in the chapter on Transport of Solutes and Water) and the other on ion channels (Table 6-2 in the chapter on Electrophysiology

of the Cell Membrane). In both tables, our updates help the reader navigate through what sometimes are multiple systems of terminology. In Section III (The Nervous System), new molecular developments led to major changes in Chapter 15 (Sensory Transduction), including the transduction of taste. In Section IV (The Cardiovascular System), we have improved the molecular underpinning of the ionic currents in Chapter 21 (Cardiac Electrophysiology and the Electrocardiogram). In Section VI (The Urinary System), we welcome Peter Aronson as a new co-author. Improved molecular insights led to major improvements in Chapter 36, including the subchapters on urea, urate, phosphate, and calcium. In Section VII (The Gastrointestinal System), Chapter 43 (Pancreatic and Salivary Glands) underwent significant modernization, including an expansion of the treatment of salivary glands. In Chapter 45 (Nutrient Digestion and Absorption), we welcome Charles Mansbach as a new co-author. Section VIII (The Endocrine System) underwent significant updating, including the treatment of phosphate handling in Chapter 52 (The Parathyroid Glands and Vitamin D). In Section IX (The Reproductive System), we welcome two new authors. Sam Mesiano extensively reworked Chapters 53 (Sexual Differentiation) through Chapter 56 (Fertilization, Pregnancy, and Lactation), and George Lister has similarly rewritten Chapter 57 (Fetal and Neonatal Physiology). Finally, in Section X (Physiology of Everyday Life), we welcome Shaun Morrison, who extensively rewrote Chapter 59 (Regulation of Body Temperature). Chapter 62 (The Physiology of Aging) underwent extensive changes, including new treatments of necroptosis and frailty.

THE eBOOK Although you can still enjoy our book while reading the print version, you can also access the extended content at your computer via the website www.StudentConsult.com. The eBook is also available through the Inkling app on tablets and smart phones. Regardless of the platform for accessing the eBook, the student may access Notes, crosslinks, and references as noted above, and also can “follow” professors and see their highlights and annotations within the text.

ACKNOWLEDGMENTS A textbook is the culmination of successful collaborations among many individuals. First, we thank our chapter authors, who are listed under Contributors on pages v and vi. We also thank other colleagues who wrote WebNotes, or provided other valuable materials or input. Roberto vii

viii

Preface to the Third Edition

Dominguez provided Figure 9-5A, and Slavek Filipek and Kris Palczewski provided Figure 15-12. Philine Wangemann made invaluable suggestions for the Vestibular and Auditory Transduction subchapter in Chapter 15. George Dubyak responded to numerous queries. We thank all our readers who sent us their suggestions or corrections; we list them in the accompanying  NP-1. At the art studio DNA Illustrations, Inc, we thank David and Alex Baker for developing new figures and updating others, while maintaining the textbook’s aesthetic appeal, originally established by JB Woolsey and Associates. At Elsevier, we are most grateful to Elyse O’Grady— Executive Content Strategist—for her trust and endurance. Marybeth Thiel—Senior Content Development Specialist— was the project’s communications hub, responsible for coordinating all parties working on the textbook, and for assembling the many elements that comprised the final product. Her meticulous care was indispensible. We thank David Stein—Senior Project Manager—for overseeing

production of the textbook. Striving for consistency, Elsevier did us the favor of assigning a single copyeditor—Janet E. Lincoln—to the entire project. We were especially impressed with her meticulous copyediting. Moreover, because she read the manuscript as a dedicated student, she identified several logical or scientific errors, including inconsistencies between chapters. Finally, we thank four editorial assistants. Charleen Bertolini used every ounce of her friendly, good-humored, and tenacious personality to keep our authors—and us—on track during the first few years as we prepared the Third Edition. Later, three students in the MS in Medical Physiology Program at Case Western Reserve University took the reins from Charleen—Evan Rotar, Alisha Bouzaher, and Anne Jessica Roe. As we did for the first two editions, we again invite the reader to enjoy learning physiology. If you are pleased with our effort, tell others. If not, tell us.

Preface to the Third Edition

viii.e1

NP-1  List of Readers Who Made Suggestions Faculty Raif Musa Aziz, PhD, Assistant Professor, Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of Sao Paulo, Sao Paulo, Brazil Mark Borden, Colorado

PhD,

Associate

Professor,

University

of

Gerald DiBona, MD, Professor Emeritus of Medicine and Molecular Physiology and Biophysics, Carver College of Medicine, University of Iowa Roberto Dominguez, PhD, Professor of Physiology, Perelman School of Medicine, University of Pennsylvania George Dubyak, PhD, Professor, Department of Physiology and Biophysics, Case Western Reserve University

Andrea Romani, MD, PhD, Associate Professor, Department of Physiology and Biophysics, Case Western Reserve University Corey Smith, PhD, Professor, Department of Physiology and Biophysics, Case Western Reserve University Julian Stelzer, PhD, Assistant Professor, Department of Physiology and Biophysics, Case Western Reserve University Funabashi Toshiya, MD, PhD, Professor, Department of Physiology, St. Marianna University School of Medicine, Kawasaki, Japan Philine Wangemann, PhD, University Distinguished Professor, Department of Anatomy & Physiology, Kansas State University Ernest Wright, PhD, Professor, David Geffen School of Medicine, University of California—Los Angeles

Mikael Esmann, PhD, Professor of Physiology and Biophysics, Aarhus University

Students

Slavek Filipek, PhD, Department of Pharmacology, School of Medicine, Case Western Reserve University

Taylor Burch

Gabriel Haddad, MD, Chairman of Pediatrics, University of California—San Diego Ulrich Hopfer, MD, PhD, Professor Emeritus, Department of Physiology and Biophysics, Case Western Reserve University Norman Javitt, MD, PhD, Professor of Medicine and Pediatrics, New York University Medical Center Bhanu Jena, PhD, DSc, Professor of Physiology, School of Medicine, Wayne State University Stephen Jones, PhD, Professor, Department of Physiology and Biophysics, Case Western Reserve University Alan Kay, PhD, Professor of Biology, University of Iowa Rossana Occhipinti, PhD, Department of Physiology and Biophysics, Case Western Reserve University

Natthew Arunthamakun

Tung Chu Xiaoke Feng Clare Fewtrell Trevor Hall Jeffery Jeong Hani Khadra Bob Lee Shannon Li Sarabjot Makkar Claire Miller

Krzysztof Palczewski, PhD, Professor and Chair, Department of Pharmacology, School of Medicine, Case Western Reserve University

Pamela Moorehead

Mark Parker, PhD, Assistant Professor, Department of Physiology and Biophysics, SUNY at Buffalo

Sarah Sheldon

D. Narayan Rao, PhD, Department of Physiology, Faculty of Medicine, Benghazi University

Amalia Namath

Sadia Tahir Eunji Yim

PREFACE TO THE FIRST EDITION

We were intrigued by an idea suggested to us by W.B. Saunders: write a modern textbook of physiology that combines the expertise of a multi-author book with the consistency of a single pen. Our approach has been, first, to recruit as writers mainly professors who teach medical physiology at the Yale University School of Medicine, and then to recast the professors’ manuscripts in a uniform style. After much effort, we now present our book, which we hope will bring physiology to life and at the same time be a reliable resource for students.

transmission in the nervous system, sensory transduction, and neural circuits. In addition, Part III also treats two subjects—the autonomic nervous system and the neuronal microenvironment—that are important for understanding other physiological systems. Finally, Part X (The Physiology of Everyday Life) is an integrated, multisystem approach to metabolism, temperature regulation, exercise, and adaptations to special environments.

TARGET AUDIENCE

Some important aspects of physiology remain as fundamentally important today as when the pioneers of physiology discovered them a century or more ago. These early observations were generally phenomenological descriptions that physiologists have since been trying to understand at a mechanistic level. Where possible, a goal of this textbook is to extend this understanding all the way to the cell and molecule. Moreover, although some areas are evolving rapidly, we have tried to be as up to date as practical. To make room for the cellular and molecular bricks, we have omitted some classic experimental observations, especially when they were of a “black-box” nature. Just as each major Part of the textbook begins with an introductory chapter, each chapter generally first describes— at the level of the whole body or organ system (e.g., the kidney)—how the body performs a certain task and/or controls a certain parameter (e.g., plasma K+ concentration). As appropriate, our discussion then progresses in a reductionistic fashion from organ to tissue to cell and organelles, and ultimately to the molecules that underlie the physiology. Finally, most chapters include a discussion of how the body regulates the parameter of interest at all levels of integration, from molecules to the whole body.

We wrote Medical Physiology primarily as an introductory text for medical students, although it should also be valuable for students in the allied health professions and for graduate students in the physiological sciences. The book should continue to be useful for the advanced medical student who is learning pathophysiology and clinical medicine. Finally, we hope that physicians in training, clinical fellows, and clinical faculty will find the book worthwhile for reviewing principles and becoming updated on new information pertinent for understanding the physiological basis of human disease.

CONTENT OF THE TEXTBOOK Aside from Part I, which is a brief introduction to the discipline of physiology, the book consists of nine major Parts. Part II (Physiology of Cells and Molecules) reflects that, increasingly, the underpinnings of modern physiology have become cellular and molecular. Chapters 2, 4, and 5 would not be present in a traditional physiology text. Chapter 2 (Functional Organization of the Cell), Chapter 4 (Signal Transduction), and Chapter 5 (Regulation of Gene Expression) provide the essentials of cell biology and molecular biology necessary for understanding cell and organ function. The other chapters in Part II cover the cellular physiology of transport, excitability, and muscle—all of which are classic topics for traditional physiology texts. In this book we have extended each of these subjects to the molecular level. The remainder of the book will frequently send the reader back to the principles introduced in Part II. Parts III to IX address individual organ systems. In each case, the first chapter provides a general introduction to the system. Part III (Cellular Physiology of the Nervous System) is untraditional in that it deliberately omits those aspects of the physiology of the central nervous system that neuroscience courses generally treat and that require extensive knowledge of neuroanatomical pathways. Rather, Part III focuses on cellular neurophysiology, including synaptic

EMPHASIS OF THE TEXTBOOK

CREATING THE TEXTBOOK The first draft of each chapter was written by authors with extensive research and/or teaching experience in that field. The editors, sitting shoulder to shoulder at a computer, then largely rewrote all chapters line by line. The goal of this exercise was for the reader to recognize, throughout the entire book, a single voice—a unity provided by consistency in style, in organization, in the sequence for presenting concepts, and in terminology and notation, as well as in consistency in the expression of standard values (e.g., a cardiac output of 5 liters/min). The editors also attempted to minimize overlap among chapters by making extensive use of cross references (by page, figure, or table number) to principles introduced elsewhere in the book. ix

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Preface to the First Edition

After the first round of editing, Dr. Malcolm Thaler—a practicing physician and accomplished author in his own right—improved the readability of the text and sometimes added clinical examples. Afterwards, the editors again went through the entire text line by line to decide on the material to be included in specific illustrations, and to match the main text of the book with the content of each figure. The editors then traveled to Philadelphia to visit the art studio of JB Woolsey and Associates. Over many visits, John Woolsey and the editors together developed the content and format for each of the approximately 760 full-color illustrations used in the textbook. These meetings were unique intellectual and pedagogical dialogues concerning the design of the figures. To a large extent, the figures owe their pedagogical style to the creativity of John Woolsey. The illustrations evolved through several iterations of figure editing, based on suggestions from both the editors and authors. This evolution, as well as text changes requested by authors, led to yet a third round of editing of the entire book, often line by line. Throughout this seemingly endless process, our goal has been to achieve the proper balance among reader friendliness, depth, and accuracy.

SPECIAL FEATURES Compared with other major textbooks of physiology, a much larger fraction of the space in this book is devoted to illustrations. Thus, although our textbook may appear thick, it actually has fewer text words than most other leading medical physiology books. Virtually all illustrations in our book are in full color, conceived de novo, with consistent style and pedagogy. Many of the figures feature “dialogue balloons” that tell a story. The illustrations are also available in digital format on the Evolve Web site (http:// evolve.elsevier.com/productPages/s_417.html) for use in the classroom. The textbook makes considerable use of clinical boxes— highlighted on a color background—that present examples of diseases illustrating important physiological principles. The text includes over 2000 cross references that send the reader from the current page to specific pages, figures, or tables elsewhere in the book for relevant concepts or data. The text also includes hundreds of web icons, which direct the reader to our website at http://www.wbsaunders.com/ MERLIN/BandB/. These web links provide derivations of mathematical equations, amplification of concepts, material that was deleted for the sake of brevity from earlier drafts of the textbook, and clinical illustrations not included in the clinical boxes.

The website will also contain several other features, including summaries for each subchapter, an expanded list of references (sometimes with direct links to the primary literature), other links that may be of interest to the physiology student (e.g., biographies of famous physiologists), latebreaking scientific developments that occur after publication of the book, and—alas—the correction of errors. Finally, we invite the reader to visit our website to comment on our book, to point out errors, and to make other helpful suggestions.

ACKNOWLEDGMENTS A textbook is the culmination of successful collaborations among many individuals. First, we would like to thank our authors. Second, we acknowledge the expert input of Dr. Malcolm Thaler, both in terms of style and clinical insight. We also thank Dr. Thaler for emphasizing the importance of telling a “good story.” The textbook’s aesthetic appeal is largely attributable to JB Woolsey and Associates, particularly John Woolsey and Joel Dubin. At W.B. Saunders, we are especially thankful to William R. Schmitt—Acquisitions Editor—for his trust and patience over the years that this book has been in gestation. At the times when the seas were rough, he steered a safe course. Melissa Dudlick—Developmental Editor at W.B. Saunders— was the project’s nerve center, responsible for day-to-day communication among all parties working on the textbook, and for assembling all of the many components that went into making the final product. Her good humor and careful attention to detail greatly facilitated the creation of the textbook. We thank Frank Polizzano—Publishing Services Manager at W.B. Saunders—for overseeing production of the textbook. Before this textbook was completed, the author of Part X (The Physiology of Everyday Life), Ethan Nadel, passed away. We are indebted to those who generously stepped up to carefully check the nearly finished manuscripts for the final four chapters: Dr. Gerald Shulman for Chapter 57, Dr. John Stitt for Chapter 58, the late Dr. Carl Gisolfi for Chapter 59, and Dr. Arthur DuBois for Chapter 60. In addition, Dr. George Lister provided expert advice for Chapter 56. We are also grateful to Dr. Bruce Davis for researching the sequences of the polypeptide hormones, to Mr. Duncan Wong for expert information-technology services, and to Mrs. Leisa Strohmaier for administrative assistance. We now invite the reader to enjoy the experience of learning physiology. If you are pleased with our effort, tell others. If not, tell us.

CHAPTER

1 

FOUNDATIONS OF PHYSIOLOGY Emile L. Boulpaep and Walter F. Boron

What is physiology? Physiology is the dynamic study of life. Physiology describes the “vital” functions of living organisms and their organs, cells, and molecules. For centuries, the discipline of physiology has been closely intertwined with medicine. Although physiology is not primarily concerned with structure—as is the case for anatomy, histology, and structural biology— structure and function are inextricably linked because the living structures perform the functions. For some, physiology is the function of the whole person (e.g., exercise physiology). For many practicing clinicians, physiology may be the function of an individual organ system, such as the cardiovascular, respiratory, or gastrointestinal system. For still others, physiology may focus on the cellular principles that are common to the function of all organs and tissues. This last field has traditionally been called general physiology, a term that is now supplanted by cellular and molecular physiology. Although one can divide physiology according to varying degrees of reductionism, it is also possible to define a branch of physiology—for example, comparative physiology—that focuses on differences and similarities among different species. Indeed, comparative physiology may deal with all degrees of reductionism, from molecule to whole organism. In a similar way, medical physiology deals with how the human body functions, which depends on how the individual organ systems function, which depends on how the component cells function, which in turn depends on the interactions among subcellular organelles and countless molecules. Thus, medical physiology takes a global view of the human body; but in doing so, it requires an integrated understanding of events at the level of molecules, cells, and organs. Physiology is the mother of several biological sciences, having given birth to the disciplines of biochemistry, biophysics, and neuroscience, as well as their corresponding scientific societies and journals. Thus, it should come as no surprise that the boundaries of physiology are not sharply delineated. Conversely, physiology has its unique attributes. For example, physiology has evolved over the centuries from a more qualitative to a more quantitative science. Indeed, many of the leading physiologists were— and still are—trained as chemists, physicists, mathematicians, or engineers. 2

Physiological genomics is the link between the organ and the gene The life of the human body requires not only that individual organ systems do their jobs but also that these organ systems work “hand in hand” with each other. They must share information. Their actions must be interdependent. The cells within an organ or a tissue often share information, and certainly the individual cells must act in concert to perform the proper function of the organ or tissue. In fact, cells in one organ must often share information with cells in another organ and make decisions that are appropriate for the health of the individual cell as well as for the health of the whole person. In most cases, the sharing of information between organs and between cells takes place at the level of atoms or molecules. Cell-to-cell messengers or intracellular messengers may be as simple as H+ or K+ or Ca2+. The messengers may also be more complex chemicals. A cell may release a molecule that acts on a neighboring cell or that enters the bloodstream and acts on other cells a great distance away. In other cases, a neuron may send an axon a centimeter or even a meter away and rapidly modulate, through a neurotransmitter molecule, the activity of another cell or another organ. Cells and organs must interact with one another, and the method of communication is almost always molecular. The grand organizer—the master that controls the molecules, the cells, and the organs and the way they interact—is the genome with its epigenetic modifications. Traditionally, the discipline of physiology has, in its reductionistic journey, always stopped at about the level of cells and certain subcellular organelles as well as their component and controlling molecules. The discipline of physiology left to molecular biology and molecular genetics the business of how the cell controls itself through its DNA. The modern discipline of physiology has become closely intertwined with molecular biology, however, because DNA encodes the proteins in which physiologists are most interested. Very often, physiologists painstakingly develop elegant strategies for cloning the genes relevant to physiology. Sometimes brute-force approaches, such as the Human Genome Project in the United States, hand the physiologist a candidate gene, homologous to one of known function, on a silver platter. In still other cases, molecular biologists may clone a gene with

CHAPTER 1  •  Foundations of Physiology

no known function. In this case, it may be up to the physiologist to determine the function of the gene product; that is, to determine its physiology. Physiological genomics (or functional genomics) is a new branch of physiology devoted to the understanding of the roles that genes play in physiology. Traditionally, physiologists have moved in a reductionistic direction from organ to cell to molecule to gene. One of the most fascinating aspects of physiological genomics is that it has closed the circle and linked organ physiology directly with molecular biology. Perhaps one of the most striking examples is the knockout mouse. Knocking out the gene encoding a protein that, according to conventional wisdom, is very important will sometimes have no obvious effect or sometimes unexpected effects. It is up to the physiologist, at least in part, to figure out why. It is perhaps rather sobering to consider that to truly understand the impact of a transgene or a knockout on the physiology of a mouse, one would have to carefully re-evaluate the totality of mouse physiology. To grasp the function of a gene product, the physiologist must retrace the steps up the reductionistic road and achieve an integrated understanding of that gene’s function at the level of the cells, organs, and whole body. Physiology is unique among the basic medical sciences in that it is both broad in its scope (i.e., it deals with multiple systems) and integrative in its outlook. In some cases, important physiological parameters, such as blood pressure, may be under the control of many genes. Certain polymorphisms in several of these many genes could have a cumulative effect that produces high blood pressure. How would one identify which polymorphisms of which genes may underlie high blood pressure? This sort of complex problem does not easily lend itself to a physiologist’s controlled studies. One approach would be to study a population of people, or strains of experimental animals, and use statistical tools to determine which polymorphisms correlate with high blood pressure in a population. Indeed, epidemiologists use statistical tools to study group effects in populations. However, even after the identification of variants in various genes, each of which may make a small contribution to high blood pressure, the physiologist has an important role. First, the physiologist, performing controlled experiments, must determine whether a particular genetic variant does indeed have at least the potential to modulate blood pressure. Second, the physiologist must determine the mechanism of the effect.

Cells live in a highly protected milieu intérieur In his lectures on the phenomena of life, Claude Bernard noted in 1878 on the conditions of the constancy of life, which he considered a property of higher forms of life. According to Bernard, animals have two environments: the “milieu extérieur” that physically surrounds the whole organism; and the “milieu intérieur,” in which the tissues and cells of the organism live. This internal environment is neither the air nor the water in which an organism lives but rather—in the case of the human body—the well-controlled liquid environment that Bernard called “the organic liquid that circulates and bathes all the anatomic elements of the tissues, the lymph or the plasma.” In short, this internal environment is what we today call the extracellular fluid. He

3

argued that physiological functions continue in a manner indifferent to the changing environment because the milieu intérieur isolates the organs and tissues of the body from the vagaries of the physical conditions of the environment. Indeed, Bernard described the milieu intérieur as if an organism had placed itself in a greenhouse. According to Bernard’s concept of milieu intérieur, some fluids contained within the body are not really inside the body at all. For example, the contents of the gastrointestinal tract, sweat ducts, and renal tubules are all outside the body. They are all continuous with the milieu extérieur. Bernard compares a complex organism to an ensemble of anatomical elements that live together inside the milieu intérieur. Therefore, in Section II of this textbook, we examine the physiology of these cells and molecules. In Chapter 2 (“Functional Organization of the Cell”), we begin our journey through physiology with a discussion of the biology of the cells that are the individual elements of the body. Chapter 3 (“Signal Transduction”) discusses how cells communicate directly through gap junctions or indirectly by molecules released into the extracellular fluid. These released molecules can bind to receptors on the cell membrane and initiate signal-transduction cascades that can modify gene transcription (a genomic response) and a wide range of other cell functions (nongenomic responses). Alternatively, these released molecules can bind to receptors in the cytoplasm or nucleus and alter the transcription of genes. In Chapter 4 (“Regulation of Gene Expression”), we examine the response of the nucleus. Chapter 5 (“Transport of Solutes and Water”) addresses how the plasma membrane separates the cell interior from Bernard’s milieu intérieur and establishes the composition of the cell interior. In the process of establishing the composition of the intracellular fluid, the plasma membrane also sets up ion and voltage gradients across itself. Excitable cells—mainly nerve and muscle cells—can exploit these gradients for the long-distance “electrical” transmission of information. The property of “excitability,” which requires both the perception of a change (a signal) and the reaction to it, is the topic of Chapters 6 to 9. In Section III, we examine how the nervous system exploits excitability to process information. Another theme developed by Bernard was that the “fixité du milieu intérieur” (the constancy of the extracellular fluid) is the condition of “free, independent life.” He explains that organ differentiation is the exclusive property of higher organisms and that each organ contributes to “compensate and equilibrate” against changes in the external environment. In that sense, each of the systems discussed in Sections IV to VIII permits the body to live within an adverse external environment because the cardiovascular system, the respiratory system, the urinary system, the gastrointestinal system, and the endocrine system create and maintain a constant internal environment. Individual cell types in various organ systems act in concert to support the constancy of the internal milieu, and the internal milieu in turn provides these cells with a culture medium in which they can thrive. The discipline of physiology also deals with those characteristics that are the property of a living organism as opposed to a nonliving organism. Four fundamental properties distinguish the living body. First, only living organisms exchange matter and energy with the environment to continue their

4

SECTION I  •  Introduction

existence. Several organ systems of the body participate in these exchanges. Second, only living organisms can receive signals from their environment and react accordingly. The principles of sensory perception, processing by the nervous system, and reaction are discussed in the chapters on excitability and the nervous system. Third, what distinguishes a living organism is the life cycle of growth and reproduction, as discussed in the chapters on reproduction (Section IX). Finally, the living organism is able to adapt to changing circumstances. This is a theme that is developed throughout this textbook but especially in the chapters on everyday life (Section X).

Homeostatic mechanisms—operating through sophisticated feedback control mechanisms— are responsible for maintaining the constancy of the milieu intérieur Homeostasis is the control of a vital parameter. The body carefully controls a seemingly endless list of vital parameters. Examples of tightly controlled parameters that affect nearly the whole body are arterial pressure and blood volume. At the level of the milieu intérieur, tightly regulated parameters include body core temperature and plasma levels of oxygen, glucose, potassium ions (K+), calcium ions (Ca2+), and hydrogen ions (H+). Homeostasis also occurs at the level of the single cell. Thus, cells regulate many of the same parameters that the body as a whole regulates: volume, the concentrations of many small inorganic ions (e.g., Na+, Ca2+, H+), and energy levels (e.g., ATP). One of the most common themes in physiology is the negative-feedback mechanism responsible for homeostasis. Negative feedback requires at least four elements. First, the system must be able to sense the vital parameter (e.g., glucose level) or something related to it. Second, the system must be able to compare the input signal with some internal reference value called a set-point, thereby forming a difference signal. Third, the system must multiply the error signal by some proportionality factor (i.e., the gain) to produce some sort of output signal (e.g., release of insulin). Fourth, the output signal must be able to activate an effector mechanism (e.g., glucose uptake and metabolism) that opposes the source of the input signal and thereby brings the vital parameter closer to the set-point (e.g., decrease of blood glucose levels back to normal).  N1-1  Sometimes the body controls a parameter, in part, by cleverly employing positive-feedback loops. A single feedback loop often does not operate in isolation but rather as part of a larger network of controls. Thus, a complex interplay may exist among feedback loops within single cells, within a tissue, within an organ or organ system, or at the level of the whole body. After studying these individual feedback loops in isolation, the physiologist may find that two feedback loops act either synergistically or antagonistically. For example, insulin lowers blood glucose levels, whereas epinephrine and cortisol have the opposite effect. Thus, the physiologist must determine the relative weights of feedback loops in competition with one another. Finally, the physiologist must also establish hierarchy among various feedback loops. For example, the hypothalamus controls the anterior pituitary, which controls the adrenal

cortex, which releases cortisol, which helps control blood glucose levels. Another theme of homeostasis is redundancy. The more vital a parameter is, the more systems the body mobilizes to regulate it. If one system should fail, others are there to help maintain homeostasis. It is probably for this reason that genetic knockouts sometimes fail to have their expected deleterious effects. The result of many homeostatic systems controlling many vital parameters is a milieu intérieur with a stable composition. Whether at the level of the milieu intérieur or the cytoplasm of a single cell, homeostasis occurs at a price: energy. When a vital parameter (e.g., the blood glucose level) is well regulated, that parameter is not in equilibrium. Equilibrium is a state that does not involve energy consumption. Instead, a well-regulated parameter is generally in a steady state. That is, its value is constant because the body or the cell carefully matches actions that lower the parameter value with other actions that raise it. The net effect is that the vital parameter is held at a constant value. An important principle in physiology, to which we have already alluded, is that each cell plays a specialized role in the overall function of the body. In return, the body—which is the sum of all these cells—provides the milieu intérieur appropriate for the life of each cell. As part of the bargain, each cell or organ must respect the needs of the body as a whole and not run amok for its own greedy interests. For example, during exercise, the system that controls body core temperature sheds heat by elaborating sweat for evaporation. However, the production of sweat ultimately reduces blood volume. Because the body as a whole places a higher priority on the control of blood volume than on the control of body core temperature, at some point the system that controls blood volume will instruct the system that controls body core temperature to reduce the production of sweat. Unfortunately, this juggling of priorities works only if the individual stops exercising; if not, the result may be heat stroke. The adaptability of an organism depends on its ability to alter its response. Indeed, flexible feedback loops are at the root of many forms of physiological adaptation. For instance, at sea level, experimentally lowering the level of oxygen (the sensory stimulus) in the inspired air causes an increase in breathing (the response). However, after acclimatization at high altitude to low oxygen levels, the same low level of oxygen (the same sensory stimulus) causes one to breathe much faster (a greater response). Thus, the response may depend on the previous history and therefore the “state” of the system. In addition to acclimatization, genetic factors can also contribute to the ability to respond to an environmental stress. For example, certain populations of humans who have lived for generations at high altitude withstand hypoxia better than lowlanders do, even after the lowlanders have fully acclimatized.

Medicine is the study of “physiology gone awry” Medicine borrows its physicochemical principles from physiology. Medicine also uses physiology as a reference state: it is essential to know how organs and systems function in the healthy person to grasp which components may be

CHAPTER 1  •  Foundations of Physiology

4.e1

N1-1  Feedback Control Contributed by Arthur DuBois In proportional control, the set-point is not reached because the difference signal would disappear, and control would come to an end. Engineers devised a way around this. They took the time integral of the difference signal and used that to activate the effector mechanism to achieve integral control that would allow return to the set-point. There was another problem. Since there is a time delay in processing the input signal, there is a delay in returning to the set-point. Engineers also had a way around that. They took the time-derivative of the difference signal and added that to the corrective signal, speeding up the return toward the set-point. Another problem turned up. If you have a heater and a cooler, each with its own thermostat, and you want the room to be 23°C to 25°C, you must set one thermostat to turn on the heater at

temperatures 25°C but shut it off at ≤25°C to avoid running the heater and cooler both at once. If the room is cold, the heater will warm it up to 23°C, then shut off. If the room is warm, the cooler will cool it down to 25°C, then shut off. By analogy, the body has separate systems for shivering and sweating, so both do not occur at once. One can picture that anabolic and catabolic pathways should cycle separately and not simultaneously. Many body systems such as respiratory and circulatory controls oscillate between slightly above and slightly below the desired average, hunting for it rather than sitting on a single ideal value. In a case in which the control system is less precise, the swings become wider, as they do when a drunk driver wanders back and forth across the road proceeding home.

CHAPTER 1  •  Foundations of Physiology

malfunctioning in a patient. A large part of clinical medicine is simply dealing with the abnormal physiology brought about by a disease process. One malfunction (e.g., heart failure) can lead to a primary pathological effect (e.g., a decrease in cardiac output) that—in chain-reaction style— leads to a series of secondary effects (e.g., fluid overload) that are the appropriate responses of physiological feedback loops. Indeed, as clinician-physiologists have explored the basis of disease, they have discovered a great deal about physiology. For this reason, we have tried to illustrate physiological principles with clinical examples, some of which are displayed in clinical boxes in this text. Physiologists have developed many tools and tests to examine normal function. A large number of functional tests—used in diagnosis of a disease, monitoring of the evolution of an illness, and evaluation of the progress of therapy—are direct transfers of technology developed in

5

the physiology laboratory. Typical examples are cardiac monitoring, pulmonary function tests, and renal clearance tests as well as the assays used to measure plasma levels of various ions, gases, and hormones. Refinements of such technology in the hospital environment, in turn, benefit the study of physiology. Thus, the exchange of information between medicine and physiology is a two-way street. The understanding of physiology summarized in this book comes from some experiments on humans but mostly from research on other mammals and even on squids and slime molds. However, our ultimate focus is on the human body.

REFERENCES The reference list is available at www.StudentConsult.com.

CHAPTER 1  •  Foundations of Physiology

REFERENCES Bernard C: Leçons sur les phénomènes de la vie communs aux animaux et aux végétaux. Cours de physiologie générale du Museum d’Histoire Naturelle. Paris, Baillière et Fils, 1878. Cannon WB: The Wisdom of the Body. New York, WW Norton, 1932. Smith HW: From Fish to Philosopher. New York, Doubleday, 1961.

5.e1

CHAPTER

2 

FUNCTIONAL ORGANIZATION OF THE CELL Michael J. Caplan

In the minds of many students, the discipline of physiology is linked inextricably to images from its past. This prejudice is not surprising because many experiments from physiology’s proud history, such as those of Pavlov on his dogs, have transcended mere scientific renown and entered the realm of popular culture. Some might believe that the science of physiology devotes itself exclusively to the study of whole animals and is therefore an antique relic in this era of molecular reductionism. Nothing could be further from the truth. Physiology is and always has been the study of the homeostatic mechanisms that allow an organism to persist despite the ever-changing pressures imposed by a hostile environment. These mechanisms can be appreciated at many different levels of resolution. Certainly it would be difficult to understand how the body operates unless one appreciates the functions of its organs and the communication between these organs that allows them to influence one another’s behaviors. It would also be difficult to understand how an organ performs its particular tasks unless one is familiar with the properties of its constituent cells and molecules. The modern treatment of physiology that is presented in this textbook is as much about the interactions of molecules in cells as it is about the interactions of organs in organisms. It is necessary, therefore, at the outset to discuss the structure and characteristics of the cell. Our discussion focuses first on the architectural and dynamic features of a generic cell. We then examine how this generic cell can be adapted to serve in diverse physiological capacities. Through adaptations at the cellular level, organs acquire the machinery necessary to perform their individual metabolic tasks.

STRUCTURE OF BIOLOGICAL MEMBRANES The surface of the cell is defined by a membrane The chemical composition of the cell interior is very different from that of its surroundings. This observation applies equally to unicellular paramecia that swim freely in a freshwater pond and to neurons that are densely packed in the cerebral cortex of the human brain. The biochemical processes involved in cell function require the maintenance of a precisely regulated intracellular environment. The cytoplasm is an extraordinarily complex solution, the constituents of which include myriad proteins, nucleic acids, nucleotides, 8

and sugars that the cell synthesizes or accumulates at great metabolic cost. The cell also expends tremendous energy to regulate the intracellular concentrations of numerous ions. If there were no barrier surrounding the cell to prevent exchange between the intracellular and extracellular spaces, all of the cytoplasm’s hard-won compositional uniqueness would be lost by diffusion in a few seconds. The requisite barrier is provided by the plasma membrane, which forms the cell’s outer skin. The plasma membrane is impermeable to large molecules such as proteins and nucleic acids, thus ensuring their retention within the cytosol. It is selectively permeable to small molecules such as ions and metabolites. However, the metabolic requirements of the cell demand a plasma membrane that is much more sophisticated than a simple passive barrier that allows various substances to leak through at different rates. Frequently, the concentration of a nutrient in the extracellular fluid (ECF) is several orders of magnitude lower than that required inside the cell. If the cell wishes to use such a substance, therefore, it must be able to accumulate it against a concentration gradient. A simple pore in the membrane cannot concentrate anything; it can only modulate the rate at which a gradient dissipates. To accomplish the more sophisticated feat of creating a concentration gradient, the membrane must be endowed with special machinery that uses metabolic energy to drive the uphill movements of substances— active transport—into or out of the cell. In addition, it would be useful to rapidly modulate the permeability properties of the plasma membrane in response to various metabolic stimuli. Active transport and the ability to control passive permeabilities underlie a wide range of physiological processes, from the electrical excitability of neurons to the resorptive and secretory functions of the kidney. In Chapter 5, we will explore how cells actively transport solutes across the plasma membrane. The mechanisms through which the plasma membrane’s dynamic selectivity is achieved, modified, and regulated are discussed briefly below in this chapter and in greater detail in Chapter 7.

The cell membrane is composed primarily of phospholipids Our understanding of biological membrane structure is based on studies of red blood cells, or erythrocytes, that were conducted in the early part of the 20th century. The

CHAPTER 2  •  Functional Organization of the Cell

A—PHOSPHATIDYLETHANOLAMINE

B—PHOSPHOLIPID ICON This icon is used in this text to represent this and other phospholipid molecules.

+

NH3

Ethanolamine

CH2 CH2

C—MONOLAYER

O

Phosphate O

P

Hydrophobic lipid tails Hydrophilic head groups



O

O

Glycerol

CH2

CH

O

O

C CH2

9

O

C

CH2

Water

O

CH2

D—PHOSPHOLIPID BILAYER

Fatty acid

In an aqueous environment, polar hydrophilic head groups orient toward the polar water… …and nonpolar (hydrophobic) tails orient away from the water. Thus, a phospholipid bilayer is formed.

R1

R2

Figure 2-1  Phospholipids.

erythrocyte lacks the nucleus and other complicated intracellular structures that are characteristic of most animal cells. It consists of a plasma membrane surrounding a cytoplasm that is rich in hemoglobin. It is possible to break open erythrocytes and release their cytoplasmic contents. The plasma membranes can then be recovered by centrifugation to provide a remarkably pure preparation of cell surface membrane. Biochemical analysis reveals that this membrane is composed of two principal constituents: lipid and protein. Most of the lipid associated with erythrocyte plasma membranes belongs to the molecular family of phospholipids. In general, phospholipids share a glycerol backbone, two hydroxyl groups of which are esterified to various fatty-acid or acyl groups (Fig. 2-1A). These acyl groups may have different numbers of carbon atoms and also may have double bonds between carbons. For glycerol-based phospholipids, the third glycerolic hydroxyl group is esterified to a phosphate group, which is in turn esterified to a small molecule referred to as a head group. The identity of the head group determines the name as well as many of the properties of the individual phospholipids. For instance, glycerol-based phospholipids that bear an ethanolamine molecule in the head group position are categorized as phosphatidylethanolamines (see Fig. 2-1A).

Phospholipids form complex structures in aqueous solution The unique structure and physical chemistry of each phospholipid (see Fig. 2-1B) underlie the formation of biological membranes and explain many of their most important properties. Fatty acids are nonpolar molecules. Their long carbon

chains lack the charged groups that would facilitate interactions with water, which is polar. Consequently, fatty acids dissolve poorly in water but readily in organic solvents; thus, fatty acids are hydrophobic. On the other hand, the head groups of most phospholipids are charged or polar. These head groups interact well with water and consequently are very water soluble. Thus, the head groups are hydrophilic. Because phospholipids combine hydrophilic heads with hydrophobic tails, their interaction with water is referred to as amphipathic. When mixed with water, phospholipids organize themselves into structures that prevent their hydrophobic tails from making contact with water while simultaneously permitting their hydrophilic head groups to be fully dissolved. When added to water at fairly low concentrations, phospholipids form a monolayer (see Fig. 2-1C) on the water’s surface at the air-water interface. It is energetically less costly to the system for the hydrophobic tails to stick up in the air than to interact with the solvent. At higher concentrations, phospholipids assemble into micelles. The hydrophilic head groups form the surfaces of these small spheres, whereas the hydrophobic tails point toward their centers. In this geometry, the tails are protected from any contact with water and instead are able to participate in energetically favorable interactions among themselves. At still higher concentrations, phospholipids spontaneously form bilayers (see Fig. 2-1D). In these structures, the phospholipid molecules arrange themselves into two parallel sheets or leaflets that face each other tail to tail. The hydrophilic head groups form the surfaces of the bilayer; the hydrophobic tails form the center of the sandwich. The hydrophilic surfaces insulate the hydrophobic tails from

10

SECTION II  •  Physiology of Cells and Molecules

contact with the solvent, leaving the tails free to associate exclusively with one another. The physical characteristics of a lipid bilayer largely depend on the chemical composition of its constituent phospholipid molecules. For example, the width of the bilayer is determined by the length of the fatty-acid side chains. Dihexadecanoic phospholipids (whose two fatty-acid chains are each 16 carbons long) produce bilayers that are 2.47 nm wide; ditetradecanoic phospholipids (bearing 14-carbon fatty acids) generate 2.3-nm bilayers. Similarly, the nature of the head groups determines how densely packed adjacent phospholipid molecules are in each leaflet of the membrane. Detergents can dissolve phospholipid membranes because, like the phospholipids themselves, they are amphipathic. They possess very hydrophilic head groups and hydrophobic tails and are water soluble at much higher concentrations than are the phospholipids. When mixed together in aqueous solutions, detergent and phospholipid molecules interact through their hydrophobic tails, and the resulting complexes are water soluble, either as individual dimers or in mixed micelles. Therefore, adding sufficient concentrations of detergent to phospholipid bilayer membranes disrupts the membranes and dissolves the lipids. Detergents are extremely useful tools in research into the structure and composition of lipid membranes.

The diffusion of individual lipids within a leaflet of a bilayer is determined by the chemical makeup of its constituents Despite its highly organized appearance, a phospholipid bilayer is a fluid structure. An individual phospholipid molecule is free to diffuse within the entire leaflet in which it resides. The rate at which this two-dimensional diffusion occurs is extremely temperature dependent. At high temperatures, the thermal energy of any given lipid molecule is greater than the interaction energy that would tend to hold adjacent lipid molecules together. Under these con­ ditions, lateral diffusion can proceed rapidly, and the lipid is said to be in the sol state. At lower temperatures, inter­ action energies exceed the thermal energies of most individual molecules. Thus, phospholipids diffuse slowly because they lack the energy to free themselves from the embraces of their neighbors. This behavior is characteristic of the gel state. The temperature at which the bilayer membrane converts from the gel to the sol phase (and vice versa) is referred to as the transition temperature. The transition temperature is another characteristic that depends on the chemical makeup of the phospholipids in the bilayer. Phospholipids with long, saturated fatty-acid chains can extensively interact with one another. Consequently, a fair amount of thermal energy is required to overcome these interactions and permit diffusion. Not surprisingly, such bilayers have relatively high transition temperatures. For example, the transition temperature for dioctadecanoic phosphatidylcholine (which has two 18-carbon fatty-acid chains, fully saturated) is 55.5°C. In contrast, phospholipids that have shorter fatty-acid chains or double bonds (which introduce kinks) cannot line up next to each other as well and hence do not interact as well.

Considerably less energy is required to induce them to participate in diffusion. For example, if we reduce the length of the carbon chain from 18 to 14, the transition temperature falls to 23°C. If we retain 18 carbons but introduce one double bond (making the fatty-acid chains monounsaturated), the transition temperature also falls dramatically. By mixing other types of lipid molecules into phos­ pholipid bilayers, we can markedly alter the membrane’s fluidity properties. The glycerol-based phospholipids, the most common membrane lipids, include the phosphatidylethanolamines described above (see Fig. 2-1A), as well as the phosphatidylinositols (Fig. 2-2A), phosphatidylserines (see Fig. 2-2B), and phosphatidylcholines (see Fig. 2-2C). The second major class of membrane lipids, the sphingo­ lipids (derivatives of sphingosine), is made up of three subgroups: sphingomyelins (see Fig. 2-2D),  N2-1  gly­ cosphingolipids such as the galactocerebrosides (see Fig. 2-2E), and gangliosides (not shown in figure). Cholesterol (see Fig. 2-2F) is another important membrane lipid. Because these other molecules are not shaped exactly like the glycerolbased phospholipids, they participate to different degrees in intermolecular interactions with phospholipid side chains.  N2-2  The presence of these alternative lipids changes the strength of the interactions that prevents lipid molecules from diffusing. Consequently, the membrane has a different fluidity and a different transition temperature. This behavior is especially characteristic of the cholesterol molecule, whose rigid steroid ring binds to and partially immobilizes fattyacid side chains. Therefore, at modest concentrations, cholesterol decreases fluidity. However, when it is present in high concentrations, cholesterol can substantially disrupt the ability of the phospholipids to interact among themselves, which increases fluidity and lowers the gel-sol transition temperature. This issue is significant because animal cell plasma membranes can contain substantial quantities of cholesterol. Bilayers composed of several different lipids do not undergo the transition from gel to sol at a single, well-defined temperature. Instead, they interconvert more gradually over a temperature range that is defined by the composition of the mixture. Within this transition range in such multicomponent bilayers, the membrane can become divided into compositionally distinct zones. The phospholipids with long-chain, saturated fatty acids will adhere to one another relatively tightly, which results in the formation of regions with gel-like properties. Phospholipids bearing short-chain, unsaturated fatty acids will be excluded from these regions and migrate to sol-like regions. Hence, “lakes” of lipids with markedly different physical properties can exist side by side in the plane of a phospholipid membrane. Thus, the same thermodynamic forces that form the elegant bilayer structure can partition distinct lipid domains within the bilayer. As discussed below, the segregation of lipid lakes in the plane of the membrane may be important for sorting membrane proteins to different parts of the cell. Although phospholipids can diffuse in the plane of a lipid bilayer membrane, they do not diffuse between adjacent leaflets (Fig. 2-3). The rate at which phospholipids spontaneously “flip-flop” from one leaflet of a bilayer to the other is extremely low. As mentioned above, the center of a bilayer membrane consists of the fatty-acid tails of the phospholipid

CHAPTER 2  •  Functional Organization of the Cell

N2-1  Sphingomyelins Contributed by Emile Boulpaep and Walter Boron The polar head group of sphingomyelins can be either phosphocholine, as shown in Figure 2-2D, or phosphoethanolamine (analogous to the phosphoethanolamine moiety in Fig. 2-1A). Note that sphingomyelins are both (1) sphingolipids because they contain sphingosine, and (2) phospholipids because they contain a phosphate group as do the glycerol-based phospholipids shown in Figures 2-1A and 2-2A–C.

N2-2  Diversity of Lipids in a Bilayer Contributed by Michael Caplan

Hydrophilic heads

Hydrophobic tails

Cholesterol aids in stiffening the membrane.

eFigure 2-1  The upper leaflet of this lipid bilayer contains, from left to right, phosphatidylinositol, phosphatidylserine, cholesterol, phosphatidylinositol, phosphatidylcholine, and cholesterol.

10.e1

11

CHAPTER 2  •  Functional Organization of the Cell

A

B

PHOSPHATIDYLINOSITOL

PHOSPHATIDYLSERINE

C

PHOSPHATIDYLCHOLINE CH3

+

NH3 OH

Inositol

OH

H

Serine

N +

O

OH O

Choline

Phosphate



O

O

CH

O

O O

C

O O

P

O

CH2

CH2

O

P



O

O

CH2

CH

O

O O

C

C



O

P

O

CH2

CH3

CH2

CH2

HO

C



COO

C

OH

Glycerol

H3C

O

CH2

O

CH2

CH

O

O

C

O

C

CH2

CH2

CH2

CH2

CH2

CH2

R1

R2

R1

R2

R1

R2

CH2

O

Fatty acid

D

E

SPHINGOMYELIN CH3

Choline

+

N

CH2OH

Galactose

OH

Sphingosine

Sphingosine

OH

H

H

CH

CH

N

CH



O CH2

OH

O CH2

O

P

OH

OH

CH2 O

CH

CH

N

CH

C

O

CHOLESTEROL

O

HO

CH3

CH2

O

F

GALACTOCEREBROSIDE

CH3

C

O

CH3

CH3 CH3

CH

CH

CH2

CH2 CH2 CH2

CH H3C

CH2

CH CH3

Figure 2-2  Structures of some common membrane lipids.

molecules and is an extremely hydrophobic environment. For a phospholipid molecule to jump from one leaflet to the other, its highly hydrophilic head group would have to transit this central hydrophobic core, which would have an extremely high energy cost. This caveat does not apply to cholesterol (see Fig. 2-3), whose polar head is a single hydroxyl group. The energy cost of dragging this small polar hydroxyl group through the bilayer is relatively low, which permits relatively rapid cholesterol flip-flop.

Phospholipid bilayer membranes are impermeable to charged molecules The lipid bilayer is ideally suited to separate two aqueous compartments. Its hydrophilic head groups interact well with water at both membrane surfaces, whereas the hydrophobic center ensures that the energetic cost of crossing the membrane is prohibitive for charged atoms or molecules. Pure phospholipid bilayer membranes are extremely

12

SECTION II  •  Physiology of Cells and Molecules

PM

Phospholipids can move laterally, rotate, or flex. Rarely do they flip to the other leaflet.

ER M

PM

Cholesterol aids in stiffening the membrane and can flip easily. Figure 2-3  Mobility of lipids within a bilayer.

impermeable to almost any charged water-soluble substance. Ions such as Na+, K+, Cl–, and Ca2+ are insoluble in the hydrophobic membrane core and consequently cannot travel from the aqueous environment on one side of the membrane to the aqueous environment on the opposite side. The same is true of large water-soluble molecules, such as proteins, nucleic acids, sugars, and nucleotides. Whereas phospholipid membranes are impermeable to water-soluble molecules, small uncharged polar molecules can cross fairly freely. This is often true for O2, CO2, NH3, and, remarkably, water itself. Water molecules may, at least in part, traverse the membrane through transient cracks between the hydrophobic tails of the phospholipids without having to surmount an enormous energetic barrier. The degree of permeability of water (and perhaps that of CO2 and NH3 as well) varies extensively with lipid composition; some phospholipids (especially those with short or kinked fattyacid chains) permit a much greater rate of transbilayer water diffusion than others do.

E

Figure 2-4  Transmission electron micrograph of a cell membrane. The

The plasma membrane is a bilayer

photograph shows two adjacent cells of the pancreas of a frog (original magnification ×43,000). The inset is a high-magnification view (original magnification ×216,000) of the plasma membranes (PM) of the cells. Note that each membrane includes two dense layers with an intermediate layer of lower density. The dense layers represent the interaction of the polar head groups of the phospholipids with the OsO4 used to stain the preparation. E, nuclear envelope; M, mitochondrion. (From Porter KR, Bonneville MR: Fine Structure of Cells and Tissues, 4th ed. Philadelphia, Lea & Febiger, 1973.)

As may be inferred from the preceding discussion, the membrane at the cell surface is, in fact, a phospholipid bilayer. The truth of this statement was established by a remarkably straightforward experiment. In 1925, Gorter and Grendel measured the surface area of the lipids they extracted from erythrocyte plasma membranes. They used a device called a Langmuir trough in which the lipids are allowed to line up at an air-water interface (see Fig. 2-1C) and are then packed together into a continuous monolayer by a sliding bar that decreases the surface available to them. The area of the monolayer that was created by the erythrocyte lipids was exactly twice the surface area of the erythrocytes from which they were derived. Therefore, the plasma membrane must be a bilayer. Confirmation of the bilayer structure of biological membranes has come from x-ray diffraction studies performed on the repetitive whorls of membrane that form the myelin sheaths surrounding neuronal axons (see pp. 292–293). The membrane’s bilayer structure can be visualized directly in the high-magnification electron micrograph depicted in Figure 2-4. The osmium tetraoxide molecule (OsO4) with which the membrane is stained binds to the head groups of phospholipids. Thus, both surfaces of a phospholipid bilayer appear black in electron micrographs,

whereas the membrane’s unstained central core appears white. The phospholipid compositions of the two leaflets of the plasma membrane are not identical. Labeling studies performed on erythrocyte plasma membranes reveal that the surface that faces the cytoplasm contains phospha­ tidylethanolamine and phosphatidylserine, whereas the outward-facing leaflet is composed almost exclusively of phosphatidylcholine. As is discussed below in this chapter, this asymmetry is created during the biosynthesis of the phospholipid molecules. It is not entirely clear what advantage this distribution provides to the cell. The interactions between certain proteins and the plasma membrane may require this segregation. The lipid asymmetry may be especially important for those phospholipids that are involved in second-messenger cascades. Phosphatidylinositols, for example, give rise to phosphoinositides, which play critical roles in signaling pathways (see pp. 58–61). In addition, the phosphatidylinositol composition of the cytoplasmic face of an organelle helps to define the identity of the organelle and to govern its trafficking and targeting properties. Finally, the phospholipids that are characteristic of animal cell plasma membranes generally have one saturated and one

CHAPTER 2  •  Functional Organization of the Cell

Peripheral protein

13

Integral proteins

Extracellular space

Some proteins are linked to membrane phospholipids via an oligosaccharide...

Most integral membrane proteins have membrane-spanning α-helical domains of about 20 amino acids.

Peripheral proteins are noncovalently bonded with integral proteins.

Some have multiple membranespanning domains.

A

B

C

E

D H N

Integral protein

P

C O

P

F

…or are linked directly to fatty acids or prenyl groups.

Cytosol Figure 2-5  Classes of membrane proteins. In E, protein is coupled via a GPI linkage.

unsaturated fatty-acid residue. Consequently, they are less likely to partition into sol-like or gel-like lipid domains than are phospholipids that bear identical fatty-acid chains.  N2-3

Membrane proteins can be integrally or peripherally associated with the plasma membrane The demonstration that the plasma membrane’s lipid components form a bilayer leaves open the question of how the membrane’s protein constituents are organized. Membrane proteins can belong to either of two broad classes, peripheral or integral. Peripherally associated membrane proteins are neither embedded within the membrane nor attached to it by covalent bonds; instead, they adhere tightly to the cytoplasmic or extracellular surfaces of the plasma membrane (Fig. 2-5A). They can be removed from the membrane, however, by mild treatments that disrupt ionic bonds (very high salt concentrations) or hydrogen bonds (very low salt concentrations). In contrast, integral membrane proteins are intimately associated with the lipid bilayer. They cannot be eluted from the membrane by these high- or low-salt washes. For integral membrane proteins to be dislodged, the membrane itself must be dissolved by adding detergents. Integral membrane proteins can be associated with the lipid bilayer in any of three ways. First, some proteins actually span the lipid bilayer once or several times (see Fig. 2-5B, C) and hence are referred to as transmembrane proteins. Experiments performed on erythrocyte membranes reveal that these proteins can be labeled with protein-tagging reagents applied to either side of the bilayer.

The second group of integral membrane proteins is embedded in the bilayer without actually crossing it (see Fig. 2-5D). A third group of membrane-associated proteins is not actually embedded in the bilayer at all. Instead, these lipid-anchored proteins are attached to the membrane by a covalent bond that links them either to a lipid component of the membrane or to a fatty-acid derivative that intercalates into the membrane. For example, proteins can be linked to a special type of glycosylated phospholipid molecule (see Fig. 2-5E), which is most often glycosylphosphatidylinositol (GPI), on the outer leaflet of the membrane. This family is referred to collectively as the glycophospholipidlinked proteins. Another example is a direct linkage to a fatty acid (e.g., a myristyl group) or a prenyl (e.g., farnesyl) group that intercalates into the inner leaflet of the membrane (see Fig. 2-5F).

The membrane-spanning portions of transmembrane proteins are usually hydrophobic α helices How can membrane-spanning proteins remain stably associated with the bilayer in a conformation that requires at least some portion of their amino-acid sequence to be in continuous contact with the membrane’s hydrophobic central core? The answer to this question can be found in the special structures of those protein domains that actually span the membrane. The side chains of the eight amino acids listed in the upper portion of Table 2-1 are hydrophobic. These aromatic or uncharged aliphatic groups are almost as difficult to solvate in water as are the fatty-acid side chains of the membrane phospholipids themselves. Not surprisingly, therefore,

CHAPTER 2  •  Functional Organization of the Cell

13.e1

N2-3  Membrane Microdomains Contributed by Michael Caplan According to current models (see Anderson and Jacobson, 2002; Edidin, 2003), lipids and proteins are not uniformly distributed in the plane of the membranes that surround cells and organelles. Instead, certain lipids and associated proteins cluster to form microdomains that differ in composition, structure, and function from the rest of the membrane that surrounds them. These microdomains can be thought of as small islands bordered by the “lake” of lipids and proteins that constitute the bulk of the membrane. These two-dimensional structures are composed of lipids that tend to form close interactions with one another, resulting in the self-assembly of organized domains that include specific types of lipids and exclude others. The lipids that tend to be found in microdomains include sphingomyelin, cholesterol, and glycolipids. Proteins that are able to interact closely with microdomainforming lipids can also become selectively incorporated into these microdomains. A number of different names are used to refer to these microdomains, the most common of which are caveolae and rafts. Caveolae (see pp. 42–43) were originally identified in the electron microscope as flask-shaped invaginations of the plasma membrane. They carry a coat composed of proteins called caveolins, and they tend to be at least 50 to 80 nm in diameter. Caveolae have been shown to participate in endocytosis of specific subsets of proteins and are also richly endowed with signaling molecules, such as receptor tyrosine kinases. Rafts are less well understood structures, which are defined by the biochemical behaviors of their constituents when the surrounding membrane is dissolved in nonionic detergents. Lipid microdomains rich in sphingomyelin, cholesterol, and glycolipids tend to resist solubilization in these detergents under certain

conditions and can be recovered intact by density centrifugation. Once again, a number of interesting proteins involved in cell signaling and communication, including kinases, ion channels, and G proteins, tend to be concentrated in rafts, or to become associated with rafts upon the activation of specific signaltransduction pathways. Rafts are thought to collect signaling proteins into small, highly concentrated zones, thereby facilitating their interactions and hence their ability to activate particular pathways. Rafts are also involved in membrane trafficking processes. In polarized epithelial cells, the sorting of a number of proteins to the apical plasma membrane is dependent upon their ability to partition into lipid rafts that form in the plane of the membrane of the trans-Golgi network. Little is known about what lipid rafts actually look like in cell membranes in situ. It is currently thought that they are fairly small (1000 members either known or predicted from genome sequences. GPCRs mediate cellular responses to a diverse array of signaling molecules, such as hormones, neurotransmitters, vasoactive peptides, odorants, tastants, and other local mediators. Despite the chemical diversity of their ligands, most receptors of this class have a similar structure (Fig. 3-3). They consist of a single polypeptide chain with seven membranespanning α-helical segments, an extracellular N terminus that is glycosylated, a large cytoplasmic loop that is composed mainly of hydrophilic amino acids between helices 5 and 6, and a hydrophilic domain at the cytoplasmic C terminus. Most small ligands (e.g., epinephrine) bind in the plane of the membrane at a site that involves several membrane-spanning segments. In the case of larger protein ligands, a portion of the extracellular N terminus also participates in ligand binding. The 5,6-cytoplasmic loop appears to be the major site of interaction with the intracellular G protein, although the 3,4-cytoplasmic loop and the cytoplasmic C terminus also contribute to binding in some cases. Binding of the GPCR to its extracellular ligand regulates this interaction between the receptor and the G proteins, thus transmitting a signal to downstream effectors. In the next four sections of this subchapter, we discuss the general

CHAPTER 3  •  Signal Transduction

N3-4  Compartmentalization of Second-Messenger Effects Contributed by Laurie Roman In the textbook, we referred only to whole-cell levels of intracellular second messengers (e.g., cAMP), as if these messengers were uniformly distributed throughout the cell. However, some cell physiologists and cell biologists believe that local effects of intracellular second messengers may be extremely important in governing how signal-transduction processes work. One piece of evidence for such local effects is that the receptors for hormones and other extracellular agonists often are a part of macromolecular clusters of proteins that share a common physiological role. For example, a hormone receptor, its downstream heterotrimeric G protein, an amplifying enzyme (e.g., adenylyl cyclase) that generates the intracellular second messenger (e.g. cAMP), other proteins (e.g., the A kinase anchoring protein [or AKAP]), and the effector molecule (e.g., protein kinase A) may all reside in a microdomain at the cell membrane. Thus, it is possible that a particular hormone could act by locally raising [cAMP]i to levels much higher than in neighboring areas, so that—of all the cellular proteins potentially sensitive to cAMP—only a local subset of these targets may be activated by the newly formed cAMP. A second piece of evidence for the local effects of cAMP is the wide distribution of phosphodiesterases, which would be expected to break down cAMP and limit its ability to spread throughout the cell.

51.e1

52

SECTION II  •  Physiology of Cells and Molecules

Extracellular space

BOX 3-1  Action of Toxins on Heterotrimeric G Proteins

I

N

C G protein binding Cytosol Figure 3-3  G protein–coupled receptor.

principles of how G proteins function and then consider three major second-messenger systems that G proteins trigger.

GENERAL PROPERTIES OF G PROTEINS

nfectious diarrheal disease has a multitude of causes. Cholera toxin, a secretory product of the bacterium Vibrio cholerae, is responsible in part for the devastating characteristics of cholera. The toxin is an oligomeric protein composed of one A subunit and five B subunits (AB5). After cholera toxin enters intestinal epithelial cells, the A subunit separates from the B subunits and becomes activated by proteolytic cleavage. The resulting active A1 fragment catalyzes the ADP ribosylation of Gαs. This ribosylation, which involves transfer of the ADP-ribose moiety from the oxidized form of nicotinamide adenine dinucleotide (NAD+) to the α subunit, inhibits the GTPase activity of Gαs. As a result of this modification, Gαs remains in its activated, GTP-bound form and can activate adenylyl cyclase. In intestinal epithelial cells, the constitutively activated Gαs elevates levels of cAMP, which causes an increase in Cl− conductance and water flow and thereby contributes to the large fluid loss characteristic of this disease. A related bacterial product is pertussis toxin, which is also an AB5 protein. It is produced by Bordetella pertussis, the causative agent of whooping cough. Pertussis toxin ADPribosylates Gαi. This ADP-ribosylated Gαi cannot exchange its GDP (inactive state) for GTP. Thus, αi remains in its GDPbound inactive state. As a result, receptor occupancy can no longer release the active αi-GTP, so adenylyl cyclase cannot be inhibited. Thus, both cholera toxin and pertussis toxin increase the generation of cAMP.

G proteins are heterotrimers that exist in many combinations of different α, β, and γ subunits G proteins are members of a superfamily of GTP-binding proteins. This superfamily includes the classic heterotrimeric G proteins that bind to GPCRs as well as the so-called small GTP-binding proteins, such as Ras. Both the heterotrimeric and small G proteins can hydrolyze GTP and switch between an active GTP-bound state and an inactive GDPbound state. Heterotrimeric G proteins are composed of three subunits, α, β, and γ. At least 16 different α subunits (~42 to 50 kDa), 5 β subunits (~33 to 35 kDa), and 11 γ subunits (~8 to 10 kDa) are present in mammalian tissue. The α subunit binds and hydrolyzes GTP and also interacts with “downstream” effector proteins such as adenylyl cyclase. Historically, the α subunits were thought to provide the principal specificity to each type of G protein, with the βγ complex functioning to anchor the trimeric complex to the membrane. However, it is now clear that the βγ complex also functions in signal transduction by interacting with effector molecules distinct from those regulated by the α subunits. Moreover, both the α and γ subunits are involved in anchoring the complex to the membrane. The α subunit is held to the membrane by either a myristyl or a palmitoyl group, whereas the γ subunit is held via a prenyl group. The multiple α, β, and γ subunits demonstrate distinct tissue distributions and interact with different receptors and effectors (Table 3-2). Because of the potential for several hundred combinations of the known α, β, and γ subunits, G proteins are ideally suited to link a diversity of receptors to

a diversity of effectors. The many classes of G proteins, in conjunction with the presence of several receptor types for a single ligand, provide a mechanism whereby a common signal can elicit the appropriate physiological response in different tissues. For example, when epinephrine binds β1 adrenergic receptors in the heart, it stimulates adenylyl cyclase, which increases heart rate and the force of contraction. However, in the periphery, epinephrine acts on α2 adrenergic receptors coupled to a G protein that inhibits adenylyl cyclase, thereby increasing peripheral vascular resistance and consequently increasing venous return and blood pressure. Among the first effectors found to be sensitive to G proteins was the enzyme adenylyl cyclase. The heterotrimeric G protein known as Gs was so named because it stimulates adenylyl cyclase. A separate class of G proteins was given the name Gi because it is responsible for the ligand-dependent inhibition of adenylyl cyclase. Identification of these classes of G proteins was greatly facilitated by the observation that the α subunits of individual G proteins are substrates for ADP ribosylation catalyzed by bacterial toxins. The toxin from Vibrio cholerae activates Gs, whereas the toxin from Bordetella pertussis inactivates the cyclase-inhibiting Gi (Box 3-1). For their work in identifying G proteins and elucidating the physiological role of these proteins, Alfred Gilman and Martin Rodbell received the 1994 Nobel Prize in Physiology or Medicine.  N3-5

CHAPTER 3  •  Signal Transduction

N3-5  Alfred Gilman and Martin Rodbell For more information about Alfred Gilman and Martin Rodbell and the work that led to their Nobel Prize, visit http:// www.nobel.se/medicine/laureates/1994/index.html (accessed October 2014).

52.e1

CHAPTER 3  •  Signal Transduction

53

TABLE 3-2  Families of G Proteins FAMILY/SUBUNIT

TOXIN

DISTRIBUTION

RECEPTOR

EFFECTOR/ROLE

100

CTX

Ubiquitous

β adrenergic, TSH, glucagon, others

↑ Adenylyl cyclase ↑ Ca2+ channel

88

CTX

Olfactory epithelium

Odorant

↑ Adenylyl cyclase Open K+ channel

100 88

PTX PTX PTX

~Ubiquitous Ubiquitous ~Ubiquitous

M2, α2 adrenergic, others

↑ IP3, DAG, Ca2+, and AA, ↓ adenylyl cyclase

αO1A αO1B

73 73

PTX PTX

Brain, others Brain, others

Met-enkephalin, α2 adrenergic, others

αt1 αt2

68 68

PTX, CTX PTX, CTX

Retinal rods Retinal cones

Rhodopsin Cone opsin

αg αz

67 60

PTX, CTX

Taste buds Brain, adrenal, platelet

Taste ?

Gq αq α11 α14 α15 α16

100 88 79 57 58

~Ubiquitous ~Ubiquitous Lung, kidney, liver B cell, myeloid T cell, myeloid

M1, α1 adrenergic, others

↑ PLCβ1, PLC β2, PLC β3

Several receptors

↑ PLCβ1, PLC β2, PLC β3

G12 α12 α13

100 67

Ubiquitous Ubiquitous

Gs (αs) αs(s) αs(l) αolf Gi (αi) αi1 αi2 αi3

% IDENTITY

↑ cGMP-phosphodiesterase

↓ Adenylyl cyclase

CTX, cholera toxin; PTX, pertussis toxin.

G-protein activation follows a cycle In their inactive state, heterotrimeric G proteins are a complex of α, β, and γ subunits in which GDP occupies the guanine nucleotide–binding site of the α subunit. After ligand binding to the GPCR (Fig. 3-4, step 1), a conformational change in the receptor–G protein complex facilitates the release of bound GDP and simultaneous binding of GTP to the α subunit (see Fig. 3-4, step 2). This GDP-GTP exchange stimulates dissociation of the complex from the receptor (see Fig. 3-4, step 3) and causes disassembly of the trimer into a free GTP-bound α subunit and separate βγ complex (see Fig. 3-4, step 4). The GTP-bound α subunit interacts in the plane of the membrane with downstream effectors such as adenylyl cyclase and phospholipases (see Fig. 3-4, step 5), or cleavage of its myristoyl or palmitoyl group can release the α subunit from the membrane. Similarly, the βγ subunit can activate ion channels or other effectors. The α subunit is itself an enzyme that catalyzes the hydrolysis of GTP to GDP and inorganic phosphate (Pi). The result is an inactive α-GDP complex that dissociates from its downstream effector and reassociates with a βγ subunit (see Fig. 3-4, step 6); this reassociation terminates signaling and brings the system back to resting state (see Fig. 3-4, step 1). The βγ subunit stabilizes α-GDP and thereby substantially slows the rate of GDP-GTP exchange (see Fig. 3-4, step 2) and dampens signal transmission in the resting state.

The RGS (for “regulation of G-protein signaling”) family of proteins appears to enhance the intrinsic GTPase activity of some but not all α subunits. Investigators have identified at least 19 mammalian RGS proteins and shown that they interact with specific α subunits. RGS proteins promote GTP hydrolysis and thus the termination of signaling.

Activated α subunits couple to a variety of downstream effectors, including enzymes and ion channels Activated α subunits can couple to a variety of enzymes. A major enzyme that acts as an effector downstream of activated α subunits is adenylyl cyclase (Fig. 3-5A), which catalyzes the conversion of ATP to cAMP. This enzyme can be either activated or inhibited by G-protein signaling, depending on whether it associates with the GTP-bound form of Gαs (stimulatory) or Gαi (inhibitory). Thus, different ligands—acting through different combinations of GPCRs and G proteins—can have opposing effects on the same intracellular signaling pathway.  N3-4 G proteins can also activate enzymes that break down cyclic nucleotides. For example, the G protein called transducin contains an αt subunit that activates the cGMP phosphodiesterase, which in turn catalyzes the breakdown of cGMP to GMP (see Fig. 3-5B). This pathway plays a key role in phototransduction in the retina (see p. 368). G proteins can also couple to phospholipases. These enzymes catabolize phospholipids, as discussed in detail below in the section on G-protein second messengers.

54

SECTION II  •  Physiology of Cells and Molecules

N

Receptor (R) consists of seven membranespanning segments.

C 1 In the resting state, the receptor associates with the inactive G protein heterotrimer.

2 Upon ligand binding, the receptor-G protein complex undergoes a conformational change that promotes the exchange of GDP for GTP.

Extracellular space E1

E1 E1

γ

β

R

α

E2

G protein

γ

β

R

α

E2

Cytosol

3 G protein dissociates from the receptor.

4

α-GTP and βγ

subunits dissociate.

E1

R R

γ

β

α

E2

γ

R R

E1

α

β

E2

6

5 Both α-GTP and βγ can now interact with their appropriate effectors (E1, E2).

α-catalyzed hydrolysis of GTP to GDP inactivates α and promotes reassembly of the trimer.

E1

E1

R R

α

γ

β

E2

R R

γ

α

β

Pi

Figure 3-4  Enzymatic cycle of heterotrimeric G proteins.

RGS

E2

Members of the RGS family of G-protein regulators stimulate GTP hydrolysis with some but not all α subunits.

CHAPTER 3  •  Signal Transduction

A G PROTEINS ACTING VIA ADENYLYL CYCLASE Extracellular space

Adenylyl cyclase

γ

αs

β

αs

αi

AC

G protein complex (stimulatory)

cAMP

β

γ

G protein complex (inhibitory) NH2

Cyclic AMP activates protein kinase A.

Cytosol B

PKA

Adenine N

N

N

N CH2 O

G PROTEIN ACTING VIA A PHOSPHODIESTERASE Light

H

O O

Extracellular space

P

H

H

H

OH

O –

Phosphodiesterase

O

Cyclic AMP γ

Cytosol

αt

αt

β

G protein complex (transducin)

PDE

cGMP

GMP O

The breakdown of cGMP leads to the closure of cGMP-dependent channels. O O



H2N

P O

N

CH2 O H

H

H

H

GMP

Phospholipase C

β

N

OH OH

G PROTEIN ACTING VIA A PHOSPHOLIPASE

γ

O



Extracellular space C

N

N

cGMP

Guanine

C

αq

αq

PIP2

PLC

DAG

PKC

DAG activates the enzyme protein kinase C.

PKC

Ca2+

G protein complex

IP3

IP3 signals the release of Ca2+ from the ER. ER Figure 3-5  Downstream effects of activated G-protein α subunits. A, When a ligand binds to a receptor

coupled to αs, adenylyl cyclase (AC) is activated, whereas when a ligand binds to a receptor coupled to αi, the enzyme is inhibited. The activated enzyme converts ATP to cAMP, which then can activate PKA. B, In phototransduction, a photon interacts with the receptor and activates the G protein transducin. The αt activates phosphodiesterase (PDE), which in turn hydrolyzes cGMP; this lowers the intracellular concentrations of cGMP and therefore closes the cGMP-activated channels. C, In this example, the ligand binds to a receptor that is coupled to αq, which activates PLC. This enzyme converts PIP2 to IP3 and DAG. The IP3 leads to the release of Ca2+ from intracellular stores, whereas the DAG activates PKC.

55

56

SECTION II  •  Physiology of Cells and Molecules

This superfamily of phospholipases can be grouped into phospholipases A2, C, or D on the basis of the site at which the enzyme cleaves the phospholipid. G proteins that include the αq subunit activate phospholipase C, which breaks phosphatidylinositol 4,5-bisphosphate into two intracellular messengers, membrane-associated diacylglycerol and cytosolic IP3 (see Fig. 3-5C). Diacylglycerol stimulates protein kinase C, whereas IP3 binds to a receptor on the endoplasmic reticulum (ER) membrane and triggers the release of Ca2+ from intracellular stores. Some G proteins interact with ion channels. Agonists that bind to the β adrenergic receptor activate the L-type Ca2+ channel (see pp. 190–193) in the heart and skeletal muscle. The α subunit of the G protein Gs binds to and directly stimulates L-type Ca2+ channels and also indirectly stimulates this channel via a signal-transduction cascade that involves cAMP-dependent phosphorylation of the channel.

βγ subunits can activate downstream effectors Following activation and disassociation of the heterotrimeric G protein, βγ subunits can also interact with downstream effectors. The neurotransmitter ACh released from the vagus nerve reduces the rate and strength of heart contraction. This action in the atria of the heart is mediated by muscarinic M2 AChRs, members of the GPCR family (see p. 341). These receptors can be activated by muscarine, an alkaloid found in certain poisonous mushrooms. Muscarinic AChRs are very different from the nicotinic AChRs discussed above, which are ligand-gated ion channels. Binding of ACh to the muscarinic M2 receptor in the atria activates a heterotrimeric G protein, which results in the generation of both activated Gαi as well as a free βγ subunit complex. The βγ complex then interacts with a particular class of K+ channels, increasing their permeability. This increase in K+ permeability keeps the membrane potential relatively negative and thus renders the cell more resistant to excitation. The βγ subunit complex also modulates the activity of adenylyl cyclase and phospholipase C and stimulates phospholipase A2. Such effects of βγ can be independent of, synergize with, or antagonize the action of the α subunit. For example, studies using various isoforms of adenylyl cyclase have demonstrated that purified βγ stimulates some isoforms, inhibits others, and has no effect on still others. Different combinations of βγ isoforms may have different activities. For example, β1γ1 is one tenth as efficient at stimulating type II adenylyl cyclase as is β1γ2. Some βγ complexes can bind to a special protein kinase called the β adrenergic receptor kinase (βARK). As a result of this interaction, βARK translocates to the plasma membrane, where it phosphorylates the ligand-receptor complex (but not the unbound receptor). This phosphorylation results in the recruitment of β-arrestin to the GPCR, which in turn mediates disassociation of the receptor-ligand complex and thus attenuates the activity of the same β adrenergic receptors that gave rise to the βγ complex in the first place. This action is an example of receptor desensitization. These phosphorylated receptors eventually undergo endocytosis, which transiently reduces the number of receptors that are available on the cell surface. This endocytosis is an important step in resensitization of the receptor system.

Small GTP-binding proteins are involved in a vast number of cellular processes A distinct group of proteins that are structurally related to the α subunit of the heterotrimeric G proteins are the small GTP-binding proteins. More than 100 of these have been identified to date, and they have been divided into five groups: Ras, Rho, Rab, Arf, and Ran families. These 21-kDa proteins can be membrane associated (e.g., Ras) or may translocate between the membrane and the cytosol (e.g., Rho). The three isoforms of Ras (NRas, HRas, and KRas) relay signals from the plasma membrane to the nucleus via an elaborate kinase cascade (see pp. 89–90), thereby regulating gene transcription. In some tumors, mutation of the genes encoding Ras proteins results in constitutively active Ras. These mutated genes are called oncogenes because the altered Ras gene product promotes the malignant transformation of a cell and can contribute to the development of cancer (oncogenesis). In contrast, Rho family members are primarily involved in rearrangement of the actin cytoskeleton. Rab and Arf regulate vesicle trafficking, whereas Ran regulates nucleocytoplasmic transport. Similarly to the α subunit of heterotrimeric G proteins, the small GTP-binding proteins switch between an inactive GDP-bound form and an active GTP-bound form. Two classes of regulatory proteins modulate the activity of these small GTP-binding proteins. The first of these includes the GTPase-activating proteins (GAPs) and neurofibromin (a product of the neurofibromatosis type 1 gene). GAPs increase the rate at which small GTP-binding proteins hydrolyze bound GTP and thus result in more rapid inactivation. Counteracting the activity of GAPs are guanine nucleotide exchange factors (GEFs) such as “son of sevenless” or SOS (see p. 69), which promote the conversion of inactive Ras-GDP to active Ras-GTP. Interestingly, cAMP directly activates several GEFs, such as Epac (exchange protein activated by cAMP); this demonstrates crosstalk between a classical heterotrimeric G-protein signaling pathway and the small Ras-like G proteins.

G-PROTEIN SECOND MESSENGERS: CYCLIC NUCLEOTIDES cAMP usually exerts its effect by increasing the activity of protein kinase A Activation of Gs-coupled receptors results in the stimulation of adenylyl cyclase, which can cause [cAMP]i to rise 5-fold in ~5 seconds (see Fig. 3-5A). This sudden rise is counteracted by cAMP breakdown to AMP by cAMP phosphodiesterase. The downstream effects of this increase in [cAMP]i depend on the cellular microdomains in which [cAMP]i rises as well as the specialized functions that the responding cell carries out in the organism. For example, in the adrenal cortex, ACTH stimulation of cAMP production results in the secretion of aldosterone and cortisol (see p. 1023); in the kidney, a vasopressin-induced rise in cAMP levels facilitates water reabsorption (see p. 818). Excess cAMP is also responsible for certain pathological conditions, such as cholera (see Box 3-1). Another pathological process associated with

CHAPTER 3  •  Signal Transduction

excess cAMP is McCune-Albright syndrome, characterized by a triad of (1) variable hyperfunction of multiple endocrine glands, including precocious puberty in girls; (2) bone lesions; and (3) pigmented skin lesions (café au lait spots). This disorder is caused by a somatic mutation during development that constitutively activates the G-protein αs subunit in a mosaic pattern. cAMP exerts many of its effects through cAMP-dependent protein kinase A (PKA). This enzyme catalyzes transfer of the terminal phosphate of ATP to specific serine or threonine residues on substrate proteins. PKA phosphorylation sites are present in a multitude of intracellular proteins, including ion channels, receptors, metabolic enzymes, and signaling pathway proteins. Phosphorylation of these sites can influence either the localization or the activity of the substrate. For example, phosphorylation of the β2 adrenergic receptor by PKA causes receptor desensitization in neurons, whereas phosphorylation of the cystic fibrosis transmembrane conductance regulator (CFTR) increases its Cl− channel activity. To enhance regulation of phosphorylation events, the cell tightly controls the activity of PKA so that the enzyme can respond to subtle—and local—variations in cAMP levels. One important control mechanism is the use of regulatory subunits that constitutively inhibit PKA. In the absence of cAMP, two catalytic subunits of PKA associate with two of these regulatory subunits; the result is a heterotetrameric protein complex that has a low level of catalytic activity (Fig. 3-6). Binding of cAMP to the regulatory subunits induces a conformational change that diminishes their affinity for the catalytic subunits, and the subsequent dissociation of the complex results in activation of kinase activity. Not only can PKA activation have the short-term effects noted above, but the free catalytic subunit of PKA can also enter the nucleus, where substrate phosphorylation can activate the transcription of specific PKA-dependent genes (see p. 89). Although most cells use the same catalytic subunit, different regulatory subunits are found in different cell types.

cAMP-dependent kinase (PKA) is composed of two regulatory (R) and 2 catalytic (C) subunits. Binding of cAMP to the regulatory subunits induces a conformational change that reduces their affinity for the catalytic subunits. cAMP cAMP

R

C

C cAMP

cAMP

C

R

R

R cAMP

cAMP cAMP

PKA

C

cAMP

The complex dissociates and the catalytic subunits are free to catalyze the phosphorylation of protein substrates. Figure 3-6  Activation of PKA by cAMP.

57

Another mechanism that contributes to regulation of PKA is the targeting of the enzyme to specific subcellular locations. Such targeting promotes the preferential phosphorylation of substrates that are confined to precise locations within the cell. PKA targeting is achieved by the association of a PKA regulatory subunit with an A kinase anchoring protein (AKAP), which in turn binds to cytoskeletal elements or to components of cellular subcompartments. More than 35 AKAPs are known. The specificity of PKA targeting is highlighted by the observation that, in neurons, PKA is localized to postsynaptic densities through its association with AKAP79. This anchoring protein also targets calcineurin—a protein phosphatase—to the same site. This targeting of both PKA and calcineurin to the same postsynaptic site makes it possible for the cell to tightly regulate the phosphorylation state of important neuronal substrates. The cAMP generated by adenylyl cyclase can interact with effectors other than PKA. For example, olfactory receptors (see pp. 358–359) activate a member of the Gs family called Golf. The subsequent rise in [cAMP]i activates a cyclic nucleotide–gated (CNG) ion channel (see Table 6-2, family No. 4). Na+ influx through this channel leads to membrane depolarization and the initiation of a nerve impulse. For his work in elucidating the role played by cAMP as a second messenger in regulating glycogen metabolism (see Fig. 58-9), Earl Sutherland received the 1971 Nobel Prize in Physiology or Medicine.  N3-6  In 1992, Edmond Fischer and Edwin Krebs shared the prize for their part in demonstrating the role of protein phosphorylation in the signaltransduction process.  N3-7 This coordinated set of phosphorylation and dephosphorylation reactions has several physiological advantages. First, it allows a single molecule (e.g., cAMP) to regulate a range of enzymatic reactions. Second, it affords a large amplification to a small signal. The concentration of epinephrine needed to stimulate glycogenolysis in muscle is ~10−10 M. This subnanomolar level of hormone can raise [cAMP]i to ~10−6 M. Thus, the catalytic cascades amplify the hormone signal 10,000-fold, which results in the liberation of enough glucose to raise blood glucose levels from ~5 to ~8 mM. Although the effects of cAMP on the synthesis and degradation of glycogen are confined to muscle and liver, a wide variety of cells use cAMP-mediated acti­vation cascades in the response to a wide variety of hormones.

Protein phosphatases reverse the action of kinases As discussed above, one way that the cell can terminate a cAMP signal is to use a phosphodiesterase to degrade cAMP. In this way, the subsequent steps along the signaling pathway can also be terminated. However, because the downstream effects of cAMP often involve phosphorylation of effector proteins at serine and threonine residues by kinases such as PKA, another powerful way to terminate the action of cAMP is to dephosphorylate these effector proteins. Such dephosphorylation events are mediated by enzymes called serine/ threonine phosphoprotein phosphatases. Four groups of serine/threonine phosphoprotein phosphatases (PPs) are known: 1, 2a, 2b, and 2c. These enzymes themselves are regulated by phosphorylation at their serine, threonine, and tyrosine residues. The balance between kinase

CHAPTER 3  •  Signal Transduction

N3-6  Earl W. Sutherland, Jr. For more information about Earl W. Sutherland, Jr., and the work that led to his Nobel Prize, visit http://www.nobel.se/ medicine/laureates/1971/index.html (accessed October 2014).

57.e1

N3-7  Edmond H. Fischer and Edwin S. Krebs For more information about Edmond H. Fischer and Edwin S. Krebs and the work that led to their Nobel Prize, visit http:// www.nobel.se/medicine/laureates/1992/index.html (accessed October 2014).

58

SECTION II  •  Physiology of Cells and Molecules

phosphorylated tyrosine groups and thus acts to recruit the phosphatase to its target substrate. Many of the PTPs are themselves regulated by phosphorylation.

PKA (active) C cAMP

R

cAMP

cGMP exerts its effect by stimulating a nonselective cation channel in the retina

cAMP

cGMP is another cyclic nucleotide that is involved in G-protein signaling events. In the outer segments of rods and cones in the visual system, the G protein does not couple to an enzyme that generates cGMP but, as noted above, couples to an enzyme that breaks it down. As discussed beginning on page 367, light activates a GPCR called rhodopsin, which activates the G protein transducin (see p. 368), which in turn activates the cGMP phosphodiesterase (see p. 368) that lowers [cGMP]i. The fall in [cGMP]i closes cGMP-gated nonselective cation channels that are members of the same family of CNG ion channels that cAMP activates in olfactory signaling (see pp. 358–359).

R

cAMP

C

I-1

I-1

P

PP1

I-1

P

Inactive PP1

Phosphoprotein phosphatase (active)

Figure 3-7  Inactivation of PP1 by PKA.

and phosphatase activity plays a major role in the control of signaling events. PP1 dephosphorylates many proteins phosphorylated by PKA, including those phosphorylated in response to epinephrine (see Fig. 58-9). Another protein, phosphoprotein phosphatase inhibitor 1 (I-1), can bind to and inhibit PP1. Interestingly, PKA phosphorylates and induces I-1 binding to PP1 (Fig. 3-7), thereby inhibiting PP1 and preserving the phosphate groups added by PKA in the first place. PP2a, which is less specific than PP1, appears to be the main phosphatase responsible for reversing the action of other protein serine/threonine kinases. The Ca2+-dependent PP2b, also known as calcineurin, is prevalent in the brain, muscle, and immune cells and is also the pharmacological target of the immunosuppressive reagents FK-506 (tacrolimus) and cyclosporine. The substrates for PP2c include the DNA checkpoint regulators Chk1 and Chk2, which normally sense DNA damage in the setting of organ injury and temporarily stop cell proliferation. Dephosphorylation of these kinases by PP2c inactivates them and allows the cell to re-enter the cell cycle during the repair process. In addition to serine/threonine kinases such as PKA, a second group of kinases involved in regulating signaling pathways (discussed beginning on pp. 68–70) are tyrosine kinases that phosphorylate their substrate proteins on tyrosine residues. The enzymes that remove phosphates from these tyrosine residues—phosphotyrosine phosphatases (PTPs)—are much more variable than the serine and threonine phosphatases. The first PTP to be characterized was the cytosolic enzyme PTP1B from human placenta. PTP1B has a high degree of homology with CD45, a membrane protein that is both a receptor and a tyrosine phosphatase. The large family of PTPs can be divided into two classes: membrane-spanning receptor-like proteins such as CD45 and cytosolic tyrosine phosphatases such as PTP1B. A number of intracellular PTPs contain Src homology 2 (SH2) domains, a peptide sequence or motif that interacts with

G-PROTEIN SECOND MESSENGERS: PRODUCTS OF PHOSPHOINOSITIDE BREAKDOWN Many messengers bind to receptors that activate phosphoinositide breakdown Although the phosphatidylinositols (PIs) are minor constituents of cell membranes, they are largely distributed in the internal leaflet of the membrane and play an important role in signal transduction. The inositol sugar moiety of PI molecules (see Fig. 2-2A) can be phosphorylated to yield two major phosphoinositides involved in signal transduction: phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2 or PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3 or PIP3; see p. 69).  N3-8 Certain membrane-associated receptors act through G proteins (e.g., Gq) that stimulate phospholipase C (PLC) to cleave PIP2 into inositol 1,4,5-trisphosphate (or P3) and diacylglycerol (DAG), as shown in Figure 3-8A. PLCs are classified into three families (β, γ, δ) that differ in their catalytic properties, cell-type–specific expression, and modes of activation. PLCβ is typically activated downstream of certain G proteins (e.g., Gq), whereas PLCγ contains an SH2 domain and is activated downstream of certain tyrosine kinases. Stimulation of PLCβ results in a rapid increase in cytosolic IP3 levels as well as an early peak in DAG levels (see Fig. 3-8B). Both products are second messengers. The watersoluble IP3 travels through the cytosol to stimulate Ca2+ release from intracellular stores (see next section). DAG remains in the plane of the membrane to activate protein kinase C, which migrates from the cytosol and binds to DAG in the membrane (see pp. 60–61). Phosphatidylcholines (PCs), which—unlike PI—are an abundant phospholipid in the cell membrane, are also a source of DAG. The cell can produce DAG from PC by either of two mechanisms (see Fig. 3-8C). First, PLC can directly convert PC to phosphocholine and DAG. Second, phospholipase D (PLD), by cleaving the phosphoester bond on the other side of the phosphate, can convert PC to choline and phosphatidic acid (PA; also phospho-DAG). This PA

CHAPTER 3  •  Signal Transduction

N3-8  Acyl Groups Contributed by Emile Boulpaep and Walter Boron As noted in the text, phosphatidylinositols (PIs) (see p. 10) and phosphatidylcholines (PCs) (see p. 10) can each contain a variety of acyl groups. Therefore, the phosphoinositides derived from them can also contain a variety of acyl groups. A phosphoinositide is a PI derivative containing one, two, or three additional phosphate groups. Because there are three possible attachment sites (at sites 3, 4, or 5), there are a total of seven combinations possible.

Seven Combinations • Three monophosphates: • PI3P • PI4P • PI5P • Three bisphosphates called PIP2 • PI(3,4)P2 • PI(4,5)P2 • PI(3,5)P2 • One trisphosphate called PIP3 • PI(3,4,5)P3

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CHAPTER 3  •  Signal Transduction

PIP2

DAG 13p6

A

PRODUCTION OF IP3 AND DAG

Binding of a hormone to a cell surface G protein–coupled receptor activates phospholipase Cβ.

O

PLC cleaves the polar head group here.

C

P

O

O

O

Cytosol

O CH2

O O





O

OH HO

3 O

O

P

5

O OH

1

4

2



5 4 O

3

IP3

O



OH HO

O P

O

P O

6

O

OH

2

Phospholipase Cβ hydrolyzes PIP2 into IP3 and DAG.

Plasma membrane

O

OH O

P O

6

O

O C

CH2 CH

O

1

P

O

O



O

O

DAG

Plasma membrane Cytosol

O

CH2

O –

C O

CH2 CH

O

Extracellular space

O

C

γ

α

β

α

PKC

PLCβ PKC

PIP2

Receptor–G protein complex

Active

IP3

C IP3 interacts with its receptor (ITPR) in the membrane of the ER, which allows the release + of Ca2 into the cytosol.

ER H+

Ca

BREAKDOWN OF PHOSPHATIDYLCHOLINE BY PLC AND PLD R1

R2

The SERCA Ca pump transports the + Ca2 back into the ER.

2+

CH2 C

B

TIME COURSE OF IP3 AND DAG LEVELS

Response

DAG

IP3 The early DAG peak is caused by DAG released from PIP2 by PLCβ.

H2C

PLC

C

O

O

CH

CH2

P

O

O

PLD

CH2

Choline

CH2 H3C

N

+

CH3

CH3

Seconds

Minutes

Hours

Figure 3-8  Second messengers in the DAG/IP3 pathway. ER, endoplasmic reticulum; SERCA, sarcoplasmic and endoplasmic reticulum Ca-ATPase.

O

O –

O

The slow DAG wave is caused by DAG released by PLCβ and PLD from phosphatidylcholine (PC).

CH2 O

59

60

SECTION II  •  Physiology of Cells and Molecules

can then be converted to DAG via PA-phosphohydrolase. Production of DAG from PC, either directly (via PLC) or indirectly (via PLD), produces the slow wave of increasing cytosolic DAG shown in Figure 3-8B. Thus, in some systems, the formation of DAG is biphasic and consists of an early peak that is transient and parallels the formation of IP3, followed by a late phase that is slow in onset but sustained for several minutes.

IP3 liberates Ca2+ from intracellular stores As discussed on page 126, three major transport mechanisms keep free intracellular Ca2+ ([Ca2+]i) below ~100 nM. Increases in [Ca2+]i from this extremely low baseline allow Ca2+ to function as an important second messenger. IP3 generated by the metabolism of membrane phospholipids travels through the cytosol and binds to the IP3 receptor, a ligandgated Ca2+ channel located in the membrane of the endoplasmic reticulum (see Fig. 3-8A). The result is a release of Ca2+ from intracellular stores and a rise in [Ca2+]i. Indeed, it was within this system that Ca2+ was first identified as a messenger mediating the stimulus-response coupling of endocrine cells. The IP3 receptor (ITPR) is a tetramer composed of subunits of ~260 kDa. At least three genes encode the subunits of the receptor. These genes are subject to alternative splicing, which further increases the potential for receptor diversity. The receptor is a substrate for phosphorylation by protein kinases A and C as well as calcium-calmodulin (Ca2+-CaM)–dependent protein kinases.  N3-9 Interaction of IP3 with its receptor results in passive efflux of Ca2+ from the ER and thus a rapid rise in the free cytosolic Ca2+ concentration. The IP3-induced changes in [Ca2+]i exhibit complex temporal and spatial patterns. The rise in [Ca2+]i, which can be brief or persistent, can oscillate repetitively or spread across groups of cells coupled by gap junctions. In at least some systems, the frequency of [Ca2+]i oscillations seems to be physiologically important. For example, in isolated pancreatic acinar cells, graded increases in the concentration of ACh produce graded increases in the frequency—but not the magnitude—of repetitive [Ca2+]i spikes. The mechanisms responsible for [Ca2+]i oscillations and waves are complex. It appears that both propagation and oscillation depend on positive-feedback mechanisms, in which high [Ca2+]i facilitates Ca2+ release, as well as on negative-feedback mechanisms, in which high [Ca2+]i inhibits further Ca2+ release. Structurally related to ITPRs are the Ca2+-release channels known as ryanodine receptors (RYRs; see p. 230). Because cytosolic Ca2+ activates RYRs, these channels play an important role in elevating [Ca2+]i in certain cells by a process known as calcium-induced Ca2+ release (CICR; see pp. 242–243)—an example of the positive feedback noted above. For example, RYRs are responsible for releasing Ca2+ from the sarcoplasmic reticulum of muscle and thereby switching on muscle contraction (see pp. 229–230). Moreover, cyclic ADP ribose (cADPR), the product of ADP-ribosylcyclases, increases the sensitivity of RYR to cytosolic Ca2+, thereby augmenting CICR. [Ca2+]i can increase as the result not only of Ca2+ release from intracellular stores, but also of enhanced influx through Ca2+ channels in the plasma membrane. By whatever mecha-

nism, increased [Ca2+]i exerts its effects by binding to cellular proteins and changing their activity, as discussed in the next two sections. Some Ca2+-dependent signaling events are so sensitive to Ca2+ that a [Ca2+]i increase of as little as 100 nM can trigger a vast array of cellular responses. These responses include secretion of digestive enzymes by pancreatic acinar cells, release of insulin by β cells, contraction of vascular smooth muscle, conversion of glycogen to glucose in the liver, release of histamine by mast cells, aggregation of platelets, and DNA synthesis and cell division in fibroblasts. The same mechanisms that normally keep [Ca2+]i at extremely low levels (see p. 126) are also responsible for reversing the increases in [Ca2+]i that occur during signaling events. Increases in [Ca2+]i activate an ATP-fueled Ca pump (SERCA; see p. 118) that begins pumping Ca2+ back into the ER. In addition, a Ca pump (see p. 118) and Na-Ca exchanger (see pp. 123–124) at the plasma membrane extrude excess Ca2+ from the cell. These processes are much slower than Ca2+ release, so [Ca2+]i remains high until IP3 is dephosphorylated, terminating Ca2+ release via ITPR and thereby allowing the transporters to restore [Ca2+]i to basal levels.

Calcium activates calmodulin-dependent protein kinases How does an increase in [Ca2+]i lead to downstream responses in the signal-transduction cascade? The effects of changes in [Ca2+]i are mediated by Ca2+-binding proteins, the most important of which is calmodulin (CaM). CaM is a highaffinity cytoplasmic Ca2+-binding protein of 148 amino acids. Each molecule of CaM cooperatively binds four calcium ions. Ca2+ binding induces a major conformational change in CaM that allows it to bind to other proteins (Fig. 3-9). Although CaM does not have intrinsic enzymatic activity, it forms a complex with a number of enzymes and thereby confers a Ca2+ dependence on their activity. For example, binding of the Ca2+-CaM complex activates the enzyme that degrades cAMP, cAMP phosphodiesterase. Many of the effects of CaM occur as the Ca2+-CaM complex binds to and activates a family of Ca2+-CaM–dependent kinases known as CaM kinases (CaMKs). These kinases phosphorylate specific serine and threonine residues of a variety of proteins. An important CaMK in smooth-muscle cells is myosin light-chain kinase (MLCK) (see p. 247). Another CaMK is glycogen phosphorylase kinase (PK), which plays a role in glycogen degradation (see p. 1182). MLCK, PK, and some other CaMKs have a rather narrow substrate specificity. The ubiquitous CaM kinase II (CaMKII), on the other hand, has a broad substrate specificity. Especially high levels of this multifunctional enzyme are present at the synaptic terminals of neurons. One of the actions of CaMKII is to phosphorylate and thereby activate the rate-limiting enzyme (tyrosine hydroxylase; see Fig. 13-8) in the synthesis of catecholamine neurotransmitters. CaMKII can also phosphorylate itself, which allows it to remain active in the absence of Ca2+.

DAGs and Ca2+ activate protein kinase C As noted above, hydrolysis of PIP2 by PLC yields not only the IP3 that leads to Ca2+ release from internal stores but also

CHAPTER 3  •  Signal Transduction

N3-9  IP3 Receptor Diversity Contributed by Laurie Roman As noted in the text, the IP3 receptor (IPTR) is a tetramer composed of subunits of ~260 kDa, and at least four different genes encode the receptor subunits. These genes are subject to alternative splicing, further increasing the potential for receptor diversity. IP3 receptors bind their ligand with high affinity (the dissociation constant KD = 2–10 nM) or low affinity (KD = 40 nM). However, the extent to which these different affinities correlate with particular forms of the receptor has not been established.

60.e1

CHAPTER 3  •  Signal Transduction

61

Inactive protein

2+

Ca

Active protein

Calmodulin

Ca2+-Calmodulin

Ca2+-Calmodulin−dependent protein kinase

Figure 3-9  CaM. After four intracellular Ca2+ ions bind to CaM, the Ca2+-CaM complex can bind to and activate another protein. In this example, the activated protein is a Ca2+-CaM–dependent kinase.

DAG (see Fig. 3-8A). The most important function of DAG is to activate protein kinase C (PKC), an intracellular serine/ threonine kinase. In mammals, the PKC family comprises at least 10 members that differ in their tissue and cellular localization. This family is further subdivided into three groups that all require membrane-associated phosphatidylserine but have different requirements for Ca2+ and DAG. The classical PKC family members PKCα, PKCβ, and PKCγ require both DAG and Ca2+ for activation, whereas the novel PKCs (such as PKCδ, PKCε, and PKCη) require DAG but are independent of Ca2+, and the atypical PKCs (PKCζ and PKCλ) appear to be independent of both DAG and Ca2+. As a consequence, the signals generated by the PKC pathway depend on the isoforms of the enzyme that a cell expresses as well as on the levels of Ca2+ and DAG at specific locations at the cell membrane. Moreover, proteins such as receptor for activated C-kinase (RACK) and receptor for inactivated C-kinase (RICK) can target specific PKC isoforms to specific cellular compartments. In its basal state, PKCα is an inactive, soluble cytosolic protein. When a GPCR activates PLC, both DAG (generated in the inner leaflet of the plasma membrane) and Ca2+ (released in response to IP3) bind to the PKC regulatory domain; this results in translocation of PKCα to the membrane and activation of the PKC kinase domain. Even though the initial Ca2+ signal is transient, PKCα activation can be sustained, resulting in activation of physiological responses, such as proliferation and differentiation. Elevated levels of active PKCα are maintained by a slow wave of elevated DAG (see Fig. 3-8B), which is due to the hydrolysis of PC by PLC and PLD. Physiological stimulation of the classical and novel PKCs by DAG can be mimicked by the exogenous application of a class of tumor promoters called phorbol esters. These plant products bind to the regulatory domain of PKCs and thus specifically activate them even in the absence of DAG. Among the major substrates of PKC are the myristoylated, alanine-rich C-kinase substrate proteins, known as MARCKS proteins. These acidic proteins contain consensus sites for PKC phosphorylation as well as CaM- and actin-binding

sites. MARCKS proteins cross-link actin filaments and thus appear to play a role in translating extracellular signals into actin plasticity and changes in cell shape. Unphosphorylated MARCKS proteins are associated with the plasma membrane, and they cross-link actin. Phosphorylation of the MARCKS proteins causes them to translocate into the cytosol, where they are no longer able to cross-link actin. Thus, mitogenic growth factors that activate PKC may produce morphological changes and anchorage-independent cell proliferation in part by modifying the activity of MARCKS proteins. PKC can also directly or indirectly modulate transcription factors and thereby enhance the transcription of specific genes (see p. 86). Such genomic actions of PKC explain why phorbol esters are tumor promoters.

G-PROTEIN SECOND MESSENGERS: ARACHIDONIC ACID METABOLITES In addition to DAG, other hydrolysis products of membrane phospholipids can act as signaling molecules.  N3-10  The best characterized of these hydrolysis products is arachidonic acid (AA), which is attached by an ester bond to the second carbon of the glycerol backbone of membrane phospholipids (Fig. 3-10). Phospholipase A2 initiates the cellular actions of AA by releasing this fatty acid from glycerol-based phospholipids.  N3-11  A series of enzymes subsequently convert AA into a family of biologically active metabolites that are collectively called eicosanoids (from the Greek eikosi [20]) because, like AA, they all have 20 carbon atoms. Three major pathways can convert AA into these eicosanoids (Fig. 3-11). In the first pathway, cyclooxygenase (COX) enzymes produce thromboxanes (TXs), prostaglandins (PGs), and prostacyclins. In the second pathway, 5-lipoxygenase enzymes produce leukotrienes (LTs) and some hydroxyeicosatetraenoic acid (HETE) compounds. In the third pathway, the epoxygenase enzymes, which are members of the cytochrome P-450 class, produce other HETE compounds as well as cis-epoxyeicosatrienoic acid (EET) compounds. These

CHAPTER 3  •  Signal Transduction

N3-10  Platelet-Activating Factor

61.e1

N3-11  Phospholipase A2

Contributed by Ed Moczydlowski

Contributed by Laurie Roman

Although it is not a member of the arachidonic acid (AA) family, platelet-activating factor (PAF) is an important lipid signaling molecule. PAF is an ether lipid that the cell synthesizes either de novo or by remodeling of a membrane-bound precursor. PAF occurs in a wide variety of organisms and mediates many biological activities. In mammals, PAF is a potent inducer of platelet aggregation and stimulates the chemotaxis and degranulation of neutrophils, thereby facilitating the release of LTB4 and 5-HETE. PAF is involved in several aspects of allergic reactions; for example, it stimulates histamine release and enhances the secretion of immunoglobulin E, immunoglobulin A, and tumor necrosis factor. Endothelial cells are also an important target of PAF; PAF causes a negative shift of Vm in these cells by activating Ca2+-dependent K+ channels. PAF also enhances vascular permeability and the adhesion of neutrophils and platelets to endothelial cells. PAF exerts its effects by binding to a specific receptor on the plasma membrane. A major consequence of PAF binding to its GPCR is formation of IP3 and stimulation of a group of MAPKs. PAF acetylhydrolase terminates the action of this signaling lipid.

Phospholipase A2 (PLA2) catalyzes the hydrolytic cleavage of glycerol-based phospholipids (see Fig. 2-2A–C) at the second carbon of the glycerol backbone, yielding AA and a lysophospholipid (see Fig. 3-10). Some of the cytosolic PLA2 enzymes require Ca2+ for activity. In addition, raising [Ca2+]i from the physiological level of ~100 nM to ~300 nM facilitates the association of cytoplasmic PLA2 with cell membranes, where the PLA2 can be activated by specific G proteins.

62

SECTION II  •  Physiology of Cells and Molecules

BOX 3-2  Eicosanoid Nomenclature

Polar head group

T

O O–

O

P O

CH2

H C

O

OH

C

CH2

O

Polar head group O O–

P

O

he nomenclature of the eicosanoids is not as arcane as it might first appear. The numerical subscript 2 (as in PGH2) or 4 (as in LTA4) refers to the number of double bonds in the eicosanoid backbone. For example, AA has four double bonds, as do the leukotrienes. For the cyclooxygenase metabolites, the letter (A to I) immediately preceding the 2 refers to the structure of the 5-carbon ring that is formed about halfway along the 20-carbon chain of the eicosanoid. For the leukotrienes, the letters A and B that immediately precede the 4 refer to differences in the eicosanoid backbone. For the cysteinyl leukotrienes, the letter C refers to the full glutathione conjugate (see Fig. 46-8). Removal of glutamate from LTC4 yields LTD4, and removal of glycine from LTD4 yields LTE4, leaving behind only cysteine. For 5-HPETE and 5-HETE, the fifth carbon atom (counting the carboxyl group as number 1) is derivatized with a hydroperoxy or hydroxy group, respectively.

O

Phospholipase A2 cleaves here.

O

CH2

CH2

O

O

C

C

Phospholipase A2 is the primary enzyme responsible for releasing AA

CH2

O

Lysophospholipid

COOH

The arachidonic acid is always found esterified to the second carbon atom of the glycerol backbone. Phospholipid

COOH

Arachidonic acid Figure 3-10  Release of AA from membrane phospholipids by PLA2. AA

is esterified to membrane phospholipids at the second carbon of the glycerol backbone. PLA2 cleaves the phospholipid at the indicated position and releases AA as well as a lysophospholipid.

three enzymes catalyze the stereospecific insertion of molecular O2 into various positions in AA. The cyclooxygenases, lipoxygenases, and epoxygenases are selectively distributed in different cell types, which further increases the complexity of eicosanoid biology. Eicosanoids have powerful biological activities, including effects on allergic and inflammatory processes, platelet aggregation, vascular smooth muscle, and gastric acid secretion.

The first step in the phospholipase A2 (PLA2) signaltransduction cascade is binding of an extracellular agonist to a membrane receptor (see Fig. 3-11). These receptors include those for serotonin (5-HT2 receptors), glutamate (mGLUR1 receptors), fibroblast growth factor-β, interferon-α (IFN-α), IFN-β, and IFN-γ. Once the receptor is occupied by its agonist, it can activate a G protein that belongs to the Gi/Go family. The mechanism by which this activated G protein stimulates PLA2 is not well understood. It does not appear that a G-protein α subunit is involved. The G-protein βγ dimer may stimulate PLA2 either directly or via mitogenactivated protein kinases (MAPKs) (see p. 69), which phosphorylates PLA2 at a serine residue. The result is rapid hydrolysis of phospholipids that contain AA. In contrast to the direct pathway just mentioned, agonists acting on other receptors may promote AA release indirectly. First, a ligand may bind to a receptor coupled to PLC, which would lead to the release of DAG (see Fig. 3-11). As noted above, DAG lipase can cleave DAG to yield AA and a monoacylglycerol (MAG). Agonists that act via this pathway include dopamine (D2 receptors), adenosine (A1 receptors), norepinephrine (α2 adrenergic receptors), and serotonin (5-HT1 receptors). Second, any agonist that raises [Ca2+]i can promote AA formation because Ca2+ can stimulate some cytosolic forms of PLA2. Third, any signal-transduction pathway that activates MAPK can also enhance AA release because MAPK phosphorylates PLA2.

Cyclooxygenases, lipoxygenases, and epoxygenases mediate the formation of biologically active eicosanoids Once it is released from the membrane, AA can diffuse out of the cell, be reincorporated into membrane phospholipids, or be metabolized (see Fig. 3-11). In the first pathway of AA metabolism (see Fig. 3-11), cyclooxygenases  N3-12  catalyze the stepwise conversion of AA into the intermediates prostaglandin G2 (PGG2) and

CHAPTER 3  •  Signal Transduction

N3-12  Cyclooxygenase Contributed by Laurie Roman Cyclooxygenase catalyzes the stepwise conversion of AA into the intermediates PGG2 and PGH2. Thus, this enzyme is also referred to as prostaglandin H synthetase (PGHS). As noted in Box 3-3, it is the same enzyme that catalyzes both reactions. Cyclooxygenase exists in three isoforms, COX-1 (a transcript of 2.8 kilobases [kb]), COX-2 (a 4.1-kb transcript), and COX-3 (a splice variant of COX-1 that is also known as COX-1b).

62.e1

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CHAPTER 3  •  Signal Transduction

INDIRECT PATHWAYS

DIRECT PATHWAY Extracellular space Phospholipase A2

γ

α

β

Phospholipid

Lysophospholipid

MAG

DAG

Phospholipase Cβ

DAG lipase

PLA2

PLCβ

α

α

Receptor-G protein complex 1

γ

Receptor-G protein complex 2

Reincorporation of AACoA

Cytosol

IP3

Ca2+

ARACHIDONIC ACID Cyclooxygenase (COX)

ASA

ER

Epoxygenase (Cytochrome P450)

COOH 5-Lipoxygenase

Other HETEs EETs

5-HPETE PGG2

COOH OOH

COX

Dehydrase

LTA4

PGH2

Peroxidase

5-HETE LTA4 Hydrolase

LTB4

OH

O

COOH

O Thromboxane synthase

Prostacyclin synthase

Glutathione-S-transferase

TXA2 (active, unstable)

PGI2 (active, unstable)

TXB2 (inactive)

6-keto-PGF1α (weak)

PGD2

PGE2

HO

LTD4

LTE4

LEUKOTRIENES

NH2

PGE2 HO

O

LTC4

LTE4

OH

OH

COOH

PGF2α

PROSTAGLANDINS

PGI2

TXA2 O O

β

COOH

OH

S COOH

COOH O

COOH OH

Figure 3-11  AA signaling pathways. In the direct pathway, an agonist binds to a receptor that activates PLA2, which releases AA from a membrane phospholipid (see Fig. 3-10). In one of three indirect pathways, an agonist binds to a different receptor that activates PLC and thereby leads to the formation of DAG and IP3, as in Figure 3-8; DAG lipase then releases the AA from DAG. In a second indirect pathway, the IP3 releases Ca2+ from internal stores, which leads to the activation of PLA2 (see the direct pathway). In a third indirect pathway (not shown), MAPK stimulates PLA2. Regardless of its source, the AA may follow any of three pathways to form a wide array of eicosanoids. The cyclooxygenase pathway produces thromboxanes (TXA2 and TXB2), prostacyclin (i.e., PGI2), and prostaglandins. The 5-lipoxygenase pathway produces 5-HETE and the leukotrienes. The epoxygenase pathway leads to the production of other HETEs and EETs. AACoA, arachidonic-Acid–coenzyme A; ASA, acetylsalicylic acid.

64

SECTION II  •  Physiology of Cells and Molecules

BOX 3-3  Therapeutic Inhibition of Cyclooxygenase Isoforms

C

yclooxygenase is a bifunctional enzyme that first oxidizes AA to PGG2 through its cyclooxygenase activity and then peroxidizes this compound to PGH2. X-ray crystallographic studies of COX-1 reveal that the sites for the two enzymatic activities (i.e., cyclooxygenase and peroxidase) are adjacent but spatially distinct. The cyclooxygenase site is a long hydrophobic channel. Aspirin (acetylsalicylic acid) irreversibly inhibits COX-1 by acetylating a serine residue at the top of this channel. Several of the other NSAIDs interact, via their carboxyl groups, with other amino acids in the same region. COX-1 activation plays an important role in intravascular thrombosis because it leads to TXA2 synthesis by platelets. Inhibition of this process by low-dose aspirin is a mainstay for prevention of coronary thrombosis in patients with atherosclerotic coronary artery disease. However, COX-1 activation is also important for producing the cytoprotective prostanoids PGE2 (a prostaglandin) and PGI2 (a prostacyclin) in the gastric mucosa. It is the loss of these compounds that can lead to the unwanted side effect of gastrointestinal bleeding after long-term aspirin use.  N3-15 Inflammatory stimuli induce COX-2 in a number of cell types, and it is inhibition of COX-2 that provides the antiinflammatory actions of high-dose aspirin (a weak COX-2 inhibitor) and other nonselective cyclooxygenase inhibitors such as ibuprofen. Because the two enzymes are only 60% homologous, pharmaceutical companies have now generated compounds that specifically inhibit COX-2, such as celecoxib. COX-2 inhibitors work well as anti-inflammatory agents and have a reduced likelihood of causing gastrointestinal bleeding because they do not inhibit COX-1–dependent prostacyclin production. COX-2 inhibitors have been reported to increase the risk of thrombotic cardiovascular events when they are taken for long periods.

prostaglandin H2 (PGH2). PGH2 is the precursor of the other prostaglandins, the prostacyclins and the thromboxanes. As noted in Box 3-3, cyclooxygenase exists in two predominant isoforms, cyclooxygenase 1 (COX-1) and COX-2, as well as the COX-1b spice variant of COX-1. In many cells, COX-1 is expressed in a constitutive fashion, whereas COX-2 levels can be induced by specific stimuli. For example, in monocytes stimulated by inflammatory agents such as interleukin-1β (IL-1β), only levels of COX-2 increase. These observations have led to the concept that expression of COX-1 is important for homeostatic prostaglandin functions such as platelet aggregation and regulation of vascular tone, whereas upregulation of COX-2 is primarily important for mediating prostaglandin-dependent inflammatory responses. However, as selective inhibitors of COX-2 have become available, it has become clear that this is an oversimplification. In the second pathway of AA metabolism, 5-lipoxygenase initiates the conversion of AA into biologically active leu­ kotrienes. For example, in myeloid cells, 5-lipoxygenase converts AA to 5-hydroperoxyeicosatetraenoic acid (5-HPETE),  N3-13  which is short-lived and rapidly degraded by a peroxidase to the corresponding alcohol 5-HETE. Alter­natively, a dehydrase can convert 5-HPETE to

an unstable epoxide, leukotriene A4 (LTA4), which can be either further metabolized by LTA4 hydrolase to LTB4 or coupled (“conjugated”) by LTC4 synthase to the tripeptide glutathione (see p. 955). This conjugation—via the cysteine residue of glutathione—yields LTC4. Enzymes sequentially remove portions of the glutathione moiety to produce LTD4 and LTE4. LTC4, LTD4, and LTE4 are the “cysteinyl” leukotrienes; they participate in allergic and inflammatory responses and make up the mixture previously described as the slow-reacting substance of anaphylaxis. The third pathway of AA metabolism begins with the transformation of AA by epoxygenase (a cytochrome P-450 oxidase).  N3-14  Molecular O2 is a substrate in this reaction. The epoxygenase pathway converts AA into two major products, HETEs and EETs. Members of both groups display a diverse array of biological activities. Moreover, the cells of different tissues (e.g., liver, kidney, eye, and pituitary) use different biosynthetic pathways to generate different epoxygenase products.

Prostaglandins, prostacyclins, and thromboxanes (cyclooxygenase products) are vasoactive, regulate platelet action, and modulate ion transport  N3-16 The metabolism of PGH2 to generate selected prostanoid derivatives is cell specific. For example, platelets convert PGH2 to thromboxane A2 (TXA2), a short-lived compound that can aggregate platelets, bring about the platelet release reaction, and constrict small blood vessels. In contrast, endothelial cells convert PGH2 to prostacyclin I2 (PGI2), which inhibits platelet aggregation and dilates blood vessels. Many cell types convert PGH2 to prostaglandins. Acting locally in a paracrine or autocrine fashion, prostaglandins are involved in such processes as platelet aggregation, airway constriction, renin release, and inflammation.  N3-16  Prostaglandin synthesis has also been implicated in the pathophysiological mechanism of cardiovascular disease, cancer, and inflammatory diseases. Nonsteroidal antiinflammatory drugs (NSAIDs) such as aspirin, acetaminophen, ibuprofen, indomethacin, and naproxen directly target cyclooxygenase. NSAID inhibition of cyclooxygenase is a useful tool in the treatment of inflammation and fever and, at least in the case of aspirin, in the prevention of heart disease. The diverse cellular responses to prostanoids are mediated by a family of G protein–coupled prostanoid receptors. This family currently has nine proposed members, including receptors for thromboxane/PGH2 (TP), PGI2 (IP), PGE2 (EP1 to EP4), PGD2 (DP and CRTH2), and PGF2α (FP). These prostanoid receptors signal via Gq, Gi, or Gs, depending on cell type. These in turn regulate intracellular adenylyl cyclase (see p. 53) and phospholipases (see p. 58).

The leukotrienes (5-lipoxygenase products) play a major role in inflammatory responses Many lipoxygenase metabolites of AA have a role in allergic and inflammatory diseases (Table 3-3).  N3-17  LTB4 is produced by inflammatory cells such as neutrophils and macrophages. The cysteinyl leukotrienes including LTC4 and LTE4 are synthesized by mast cells, basophils, and

CHAPTER 3  •  Signal Transduction

N3-13  Names of Arachidonic Acid Metabolites

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N3-14  Epoxygenase Contributed by Emile Boulpaep and Walter Boron

Contributed by Emile Boulpaep and Walter Boron 5-HPETE = 5-S-hydroperoxy-6-8-trans-11,14-ciseicosatetraenoic acid 5-HETE = 5-hydroxyeicosatetraenoic acid EET = cis-epoxyeicosatrienoic acid

As shown in Figure 3-11, one pathway of arachidonic-acid (AA) metabolism begins with the transformation of AA by epoxygenase (a cytochrome P-450 oxidase) to two major products: HETEs and EETs. Epoxygenase requires molecular oxygen (i.e., it is an oxidase) and has several required cofactors, including cytochrome P-450 reductase, NADPH/NADP+ (reduced/oxidized forms of nicotinamide adenine dinucleotide phosphate), or NADH/NAD+ (reduced/oxidized forms of nicotinamide adenine dinucleotide).

N3-15  Side Effects of Cyclooxygenase Inhibitors Contributed by Emile Boulpaep and Walter Boron Both COX-1 and COX-2 appear to be required for production of PGE2 in the renal glomerulus, a process that is important in maintaining normal glomerular perfusion in the event of decreased renal blood flow. Thus, another risk of cyclooxygenase inhibitors is diminished renal function in patients with heart failure or volume depletion. Similar to the nonselective cyclooxygenase inhibitors, COX-2 inhibitors have been shown to decrease renal perfusion and increase the risk of hemodynamic acute renal failure in susceptible individuals.

REFERENCE Schnermann J, Chou C-L, Ma T, et al: Defective proximal tubular fluid reabsorption in transgenic aquaporin-1 null mice. Proc Natl Acad Sci U S A 95:9660–9664, 1998.

N3-16  Actions of Prostanoids Contributed by Laurie Roman The prostanoids may participate in regulation of the Na-K pump, which plays a central role in salt and water transport in the kidney and the maintenance of ion gradients in all cell types. For example, the inhibition of the Na-K pump produced by IL-1 appears to be mediated by the formation of PGE2. Indeed, IL-1 stimulates the formation of PGE2, and application of exogenous PGE2 inhibits Na-K pump activity directly. Moreover, cyclooxygenase blockers prevent the Na-K pump inhibition induced by IL-1. This action on the Na-K pump is not limited to the kidney; AA metabolites also inhibit the pump in the brain. Prostaglandins also are vasoactive and are important in the regulation of renal blood flow.

N3-17  Actions of Leukotrienes Contributed by Laurie Roman LTC4, LTD4, LTE4, and LTF4 are often referred to as the “cysteinyl leukotrienes” or sometimes as the “peptidyl leukotrienes.” As summarized in Figure 3-11, the enzyme glutathione-Stransferase (GST) conjugates LTA4, which is unstable, to the sulfhydryl group of the cysteine in glutathione (glutathione, also abbreviated GSH, is the branched tripeptide Glu-Cys-Gly) to produce LTC4. (See page 955 to learn how the liver uses GSH for conjugation reactions.) The enzyme γ-glutamyl transferase clips off the glutamate residue of LTC4 to produce LTD4 (which is conjugated to -Cys-Gly). A dipeptidase clips the dipeptide bond between Cys and Gly to release the terminal Gly as well as LTE4 (which is conjugated to only the -Cys). Leukotrienes have multiple effects on the vascular endothelium during inflammation. Various regulatory processes may interact at the level of the small blood vessels to increase the margination (i.e., the attachment to the vessel wall) of subgroups of leukocytes, increase the permeability at the postcapillary venule, and evoke diapedesis (i.e., the migration of the cell through the endothelium) of the adherent leukocytes to create a focus of interstitial inflammation. Each of these steps can be affected by leukotrienes as well as other agents. The infiltration of leukocytes begins when the cells adhere to the endothelium of the postcapillary venule. Mediators that can increase the adhesiveness of leukocytes include LTB4 and several of the cysteinyl leukotrienes. Increased vascular permeability,

influenced by the pulling apart of adjacent endothelial cells, can occur in response to LTC4, LTD4, and LTE4. After adherent leukocytes accumulate—and the size of the interendothelial cell pores increases—a stimulus for diapedesis produces an influx of leukocytes into the interstitial space. Once in the interstitial space, the leukocytes come under the influence of LTB4, a potent chemotactic factor (i.e., chemical attractant) for neutrophils (a type of white blood cell that phagocytoses invading organisms) and less so for eosinophils (another type of white blood cell). LTB4 is also chemokinetic (i.e., speeds up chemotaxis) for eosinophils. In the lungs, the cysteinyl leukotrienes appear to stimulate the secretion of mucus by the bronchial mucosa. Nanomolar concentrations of LTC4 and LTD4 stimulate the contraction of the smooth muscles of bronchi as well as smaller airways. Both LTB4 (generated by a hydrolase from the unstable LTA4) and the cysteinyl leukotrienes (i.e., LTC4, LTD4, and LTE4) act as growth or differentiation factors for a number of cell types in vitro. LTB4 stimulates myelopoiesis (formation of white blood cells) in human bone marrow, whereas LTC4 and LTD4 stimulate the proliferation of glomerular epithelial cells in the kidney. Picomolar concentrations of LTB4 stimulate the differentiation of a particular type of T lymphocytes referred to as competent suppressor or CD8+ lymphocytes. Additional immunological regulatory functions that may be subserved by LTB4 include the stimulation of IFN-γ and IL-2 production by T cells.

CHAPTER 3  •  Signal Transduction

65

TABLE 3-3  Involvement of Leukotrienes in Human Disease DISEASE

EVIDENCE

Asthma

Bronchoconstriction from inhaled LTE4; identification of LTC4, LTD4, and LTE4 in serum or urine or both

Psoriasis

Detection of LTB4 and LTE2 in fluids from psoriatic lesions

Adult respiratory distress syndrome (ARDS)

Elevated levels of LTB4 in plasma

Allergic rhinitis

Elevated levels of LTB4 in nasal fluids

Gout

Detection of LTB4 in joint fluid

Rheumatoid arthritis

Elevated levels of LTB4 in joint fluids and serum

Inflammatory bowel disease (ulcerative colitis and Crohn disease)

Identification of LTB4 in gastrointestinal fluids and LTE4 in urine

BOX 3-4  Role of Leukotrienes in Disease

S

ince the original description of the slow-reacting substance of anaphylaxis, which is generated during antigenic challenge of a sensitized lung, leukotrienes have been presumed to play a part in allergic disease of the airways (see Table 3-3). The involvement of cells (mast cells, basophils, and eosinophils) that produce cysteinyl leukotrienes (LTC4 through LTF4) in these pathobiological processes supports this concept. In addition, the levels of LTC4, LTD4, and LTE4 are increased in lavage fluid from the nares of patients with allergic rhinitis after the application of specific antigens to the nasal airways. Introducing LTC4 or LTD4 into the airways as an aerosol (nebulizer concentration of only 10 µM) causes maximal expiratory airflow (a rough measure of airway resistance; see p. 602) to decline by ~30%. This bronchoconstrictor effect is 1000-fold more potent than that of histamine, the “reference” agonist. Leukotrienes affect both large and small airways; histamine affects relatively smaller airways. Activation of the cysLT1 receptor in mast cells and eosinophils results in the chemotaxis of these cells to sites of inflammation. Because antagonists of the cysLT1 receptor (e.g., montelukast sodium) can partially block these bronchoconstrictive and proinflammatory effects, these agents are useful in the treatment of allergen-induced asthma and rhinitis. In addition to being involved in allergic disease, several of the leukotrienes are associated with other inflammatory disorders. Synovial fluid from patients with rheumatoid arthritis contains 5-lipoxygenase products. Another example is the skin disease psoriasis. In patients with active psoriasis, LTB4, LTC4, and LTD4 have been recovered from skin chambers overlying abraded lesions. Leukotrienes also appear to be involved in inflammatory bowel disease. LTB4 and other leukotrienes are generated and released in vitro from intestinal mucosa obtained from patients with ulcerative colitis or Crohn disease.

the receptor in mast cells and eosinophils causes release of the proinflammatory cytokines histamine and tumor necrosis factor-alpha (TNF-α). In addition to playing a role in the inflammatory response, the lipoxygenase metabolites can also influence the activity of many ion channels, either directly or by regulating protein kinases. For example, in synaptic nerve endings, lipoxygenase metabolites decrease the excitability of cells by activating K+ channels. Lipoxygenase products may also regulate secretion. In pancreatic islet cells, free AA generated in response to glucose appears to be part of a negative-feedback loop that prevents excess insulin secretion by inhibiting CaM kinase II.

The HETEs and EETs (epoxygenase products) tend to enhance Ca2+ release from intracellular stores and to enhance cell proliferation The epoxygenase pathway leads to the production of HETEs other than 5-HETE as well as EETs. HETEs and EETs have been implicated in a wide variety of processes, some of which are summarized in Table 3-4. For example, in stimulated mononuclear leukocytes, HETEs enhance Ca2+ release from intracellular stores and promote cell proliferation. In smoothmuscle cells, HETEs increase proliferation and migration; these AA metabolites may be one of the primary factors involved in the formation of atherosclerotic plaque. In blood vessels, HETEs can be potent vasoconstrictors. EETs enhance the release of Ca2+ from intracellular stores, increase Na-H exchange, and stimulate cell proliferation. In blood vessels, EETs primarily induce vasodilation and angiogenesis, although they have vasoconstrictive properties in the smaller pulmonary blood vessels.

Degradation of the eicosanoids terminates their activity eosinophils, cells that are commonly associated with allergic inflammatory responses such as asthma and urticaria. The cysteinyl leukotriene receptors cysLT1 and cysLT2 are GPCRs found on airway smooth-muscle cells as well as on eosinophils, mast cells, and lymphocytes. CysLT1, which couples to both pertussis toxin–sensitive and pertussis toxin–insensitive G proteins, mediates phospholipasedependent increases in [Ca2+]i. In the airways, these events produce a potent bronchoconstriction, whereas activation of

Inactivation of the products of eicosanoids is an important mechanism for terminating their biological action. In the case of COX products, the enzyme 15-hydroxyprostaglandin dehydrogenase catalyzes the initial reactions that convert biologically active prostaglandins into their inactive 15-keto metabolites. This enzyme also appears to be active in the catabolism of thromboxanes. As far as the 5-lipoxygenase products are concerned, the specificity and cellular distribution of the enzymes that metabolize leukotrienes parallel the diversity of the enzymes

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SECTION II  •  Physiology of Cells and Molecules

TABLE 3-4  Actions of Epoxygenase Products HETEs

CELL/TISSUE

ACTION

Stimulated mononuclear leukocytes

↑ Cell proliferation ↑ Ca2+ release from intracellular stores ↓TNFα production Implicated in the destruction of these cells in type 1 (juvenileonset) diabetes mellitus ↓ Release of fibrinolytic factors ↓ Binding of antithrombin ↑ Cell proliferation ↑ Migration Formation of atherosclerotic plaque? Potent vasoconstrictors “Myogenic” vasoconstrictive response of renal and cerebral arteries

β cells of pancreatic islets Endothelial cells Vascular smooth-muscle cells Blood vessels

EETs

Cells, general

Endocrine cells Toad bladder

Blood vessels Endothelium Platelets

↑ Ca release from intracellular stores ↑ Na-H exchange ↑ Cell proliferation ↓ Cyclooxygenase activity ↓ Release of somatostatin, insulin, glucagon ↓ Vasopressin-stimulated H2O permeability ↓ Renin release Vasodilation Angiogenesis ↑ Tumor cell adhesion ↓ Aggregation 2+

involved in their synthesis. For example, 20-hydrolase-LTB4, a member of the P-450 family, catalyzes the ω-oxidation of LTB4, thereby terminating its biological activity. LTC4 is metabolized through two pathways. One oxidizes the LTC4. The other pathway first removes the glutamic acid residue of the conjugated glutathione, which yields LTD4, and then removes the glycine residue, which yields LTE4; the latter is readily excreted into the urine. The metabolic breakdown of the HETE and EET products of epoxygenase (cytochrome P-450) is rapid and complex. The predominant pathway of inactivation appears to be hydrolysis via soluble epoxide hydrolase to form dihydroxyeicosatrienoic acids (DHETs), which themselves can induce biological responses, such as vasodilation. Once formed, DHETs can be excreted intact in the urine or can form conjugates with reduced glutathione (GSH). In addition, both EETs and DHETs can undergo β-oxidation to form epoxy fatty acids or can be metabolized by cyclooxygenase to generate various prostaglandin analogs.

RECEPTORS THAT ARE CATALYTIC A number of hormones and growth factors bind to cellsurface proteins that have—or are associated with— enzymatic activity on the cytoplasmic side of the membrane.

Here we discuss five classes of such catalytic receptors (Fig. 3-12): 1. Receptor guanylyl cyclases catalyze the generation of cGMP from GTP. 2. Receptor serine/threonine kinases phosphorylate serine or threonine residues on cellular proteins 3. Receptor tyrosine kinases (RTKs) phosphorylate tyrosine residues on themselves and other proteins. 4. Tyrosine kinase–associated receptors interact with cytosolic (i.e., not membrane-bound) tyrosine kinases. 5. Receptor tyrosine phosphatases cleave phosphate groups from tyrosine groups of cellular proteins.

The receptor guanylyl cyclase transduces the activity of atrial natriuretic peptide, whereas a soluble guanylyl cyclase transduces the activity of nitric oxide Receptor (Membrane-Bound) Guanylyl Cyclase  Some of the best-characterized examples of a transmembrane protein with guanylyl cyclase activity (see Fig. 3-12A) are the receptors for the natriuretic peptides.  N3-18  These ligands are a family of related small proteins (~28 amino acids) including atrial natriuretic peptide (ANP), B-type or brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). For example, in response to atrial and ventricular stretch that occur with intravascular volume expansion, cardiac myocytes release ANP and BNP, which act through receptor guanylyl cyclases. Their action is to relax vascular smooth muscle and dilate blood vessels (see ANP in Table 20-7, and p. 553) as well as to enhance Na+ excretion into urine (natriuresis; see p. 843). Both activities contribute to lowering of effective circulating blood volume and thus blood pressure (see pp. 554–555). Natriuretic peptide receptors NPRA and NPRB are membrane proteins with an extracellular ligand-binding domain and a single membrane-spanning segment (see Fig. 3-12A). The intracellular domain has two consensus catalytic domains for guanylyl cyclase activity. Binding of a natriuretic peptide induces a conformational change in the receptor that causes receptor dimerization and activation. Thus, binding of ANP to its receptor causes the conversion of GTP to cGMP and raises intracellular levels of cGMP. In turn, cGMP activates a cGMP-dependent kinase (PKG or cGK) that phosphorylates proteins at certain serine and threonine residues. In the renal medullary collecting duct, the cGMP generated in response to ANP may act not only via PKG but also by directly modulating ion channels (see p. 768). Soluble Guanylyl Cyclase  In contrast to the receptor guanylyl cyclase, which is activated by ANP, the cytosolic soluble guanylyl cyclase (sGC) is activated by nitric oxide (NO). This sGC is unrelated to the receptor guanylyl cyclase and contains a heme moiety that binds NO. NO is a highly reactive, short-lived free radical. This dissolved gas arises from a family of NO synthase (NOS) enzymes that catalyze the reaction L-arginine + 1.5 NADPH + H + + 2O2 (3-1) → NO + citrulline + 1.5 NADP +

Here, NADPH and NADP+ are the reduced and oxidized forms of nicotinamide adenine dinucleotide phosphate,

CHAPTER 3  •  Signal Transduction

66.e1

N3-18  Atrial Natriuretic Peptide Contributed by Emile Boulpaep Granular inclusions in atrial myocytes, called Palade bodies, contain pro-ANP, the precursor of atrial natriuretic peptide (ANP; also called atrial natriuretic factor, ANF). Pro-ANP, comprising 126 amino acids, is derived from the precursor known as prepro-ANP (151 residues in the human). The converting enzyme corin—a cardiac transmembrane serine protease—cleaves the pro-ANP during or after release from the atria, which yields the inactive N-terminal fragment of 98 residues and the active C-terminal 28–amino-acid peptide called ANP. Release is primarily caused by stretch of the atrial myocytes. Hormones such as angiotensin, endothelins, arginine vasopressin, and glucocorticoid modulate ANP expression and release. It is noteworthy that expression of corin is reduced in heart failure, which blunts the release of ANP in the failing heart. This blunting might contribute to the inappropriate increase of extracellular fluid volume in heart failure. ANP is a member of the NP (natriuretic peptide) family of peptides. The biological effects of ANP are potent vasodilation, diuresis, natriuresis, and kaliuresis, as well as inhibition of the renin-angiotensin-aldosterone system. At least three types of natriuretic peptide receptors (NPRs) exist: NPRA (also called GC-A—GC for guanylyl cyclase), NPRB (also called GC-B), and NPR-C. NPRA and NPRB are receptors

with a single transmembrane domain coupled to a cytosolic guanylyl cyclase (see p. 66). Activation of NPRA or NPRB leads to the intracellular generation of cGMP. In smooth muscle, intracellular cGMP activates the cGMP-dependent protein kinase that phosphorylates MLCK. Phosphorylation of MLCK inactivates MLCK; this leads to the dephosphorylation of myosin light chains, which allows muscle relaxation. The ANP C-type receptor NPRC is not coupled to a messenger system but serves mainly to clear the natriuretic peptides from the circulation. The heart, brain, pituitary, and lung synthesize an ANP-like compound termed BNP, originally known as brain natriuretic peptide (32 residues in the human). The biological actions of BNP are similar to those of ANP. The hypothalamus, pituitary, and kidney synthesize C-type natriuretic peptide or CNP, which is highly homologous to ANP and BNP. CNP binds only to NPRB and is only a weak natriuretic but a strong vasodilator. The kidney also synthesizes an ANP-like natriuretic compound known as urodilatin or URO. URO has four additional amino acids compared to ANP and also binds to the ANP A-type receptor. Its biological effect in the target tissue is also transduced by cGMP.

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CHAPTER 3  •  Signal Transduction

A

C

RECEPTOR GUANYLYL CYCLASES

B

Extracellular space N

RECEPTOR SERINE/ THREONINE KINASES

RECEPTOR TYROSINE KINASES (RTKs)

D

TYROSINE-KINASE– ASSOCIATED RECEPTORS N

Ligand

N

RECEPTOR TYROSINE PHOSPHATASES

Carbohydrate groups

N

Ligand N

E

N N

N

Ligand

Serinethreonine kinase domain C C

C

Type I

Guanylyl cyclase domains Cytosol ANP RECEPTOR

JAK2

JAK2

C

Type II TGF- RECEPTOR

C

C

Tyrosine kinase domains

This is the kinase that phosphorylates NGF downstream RECEPTOR effectors.

Tyrosine kinases

C

C

GROWTH HORMONE RECEPTOR

Tyrosine phosphatase domain

C

CD45

Figure 3-12  Catalytic receptors. A, Receptor guanylyl cyclases have an extracellular ligand-binding domain. B, Receptor serine/threonine kinases have two subunits. The ligand binds only to the type II subunit. C, RTKs similar to the NGF receptor dimerize on binding a ligand. D, Tyrosine kinase–associated receptors have no intrinsic enzyme activity but associate noncovalently with soluble nonreceptor tyrosine kinases. E, Receptor tyrosine phosphatases have intrinsic tyrosine phosphatase activity.

respectively. Tetrahydrobiopterin is a cofactor. The NOS family includes neuronal or nNOS (NOS1), inducible or iNOS (NOS2), and endothelial or eNOS (NOS3). nNOS and iNOS are soluble enzymes, whereas eNOS is linked to the plasma membrane. The activation of NOS begins as an extracellular agonist (e.g., ACh) binds to a plasmamembrane receptor, triggering the entry of Ca2+, which binds to cytosolic CaM and then stimulates NOS. In smooth muscle, NO stimulates the sGC, which then converts GTP to cGMP, activating PKG, which leads to smooth-muscle relaxation. Why NO is so ubiquitous and when its release is important are not known. However, abnormalities of the NO system are involved in the pathophysiological processes of adult respiratory distress syndrome, high-altitude pulmonary edema, stroke, and other diseases. For example, the importance of NO in the control of blood flow had long been exploited unwittingly to treat angina pectoris. Angina is the classic chest pain that accompanies inadequate blood flow to the heart muscle, usually as a result of coronary artery atherosclerosis. Nitroglycerin relieves this pain by spontaneously breaking down and releasing NO, which relaxes the smooth muscles of peripheral arterioles, thereby reducing the work of the heart and relieving the associated pain. Understanding the physiological and pathophysiological

roles of NO has led to the introduction of clinical treatments that modulate the NO system. In addition to the use of NO generators for treatment of angina, examples include the use of gaseous NO for treatment of pulmonary edema and inhibitors of cGMP phosphodiesterase (see p. 53) such as sildenafil (Viagra) for treatment of erectile dysfunction. In addition to acting as a chemical signal in blood vessels, NO generated by iNOS appears to play an important role in the destruction of invading organisms by macrophages and neutrophils. NO generated by nNOS also serves as a neurotransmitter (see pp. 315–317) and may play a role in learning and memory. The importance of the NO signaling pathway was recognized by the awarding of the 1998 Nobel Prize for Physiology or Medicine to R.F. Furchgott, L.J. Ignarro, and F. Murad for their discoveries concerning NO as a signaling molecule in the cardiovascular system.  N3-18A

Some catalytic receptors are serine/threonine kinases We have already discussed how activation of various G protein–linked receptors can initiate a cascade that eventually activates kinases (e.g., PKA, PKC) that phosphorylate proteins at serine and threonine residues. In addition, some receptors are themselves serine/threonine kinases—such as

CHAPTER 3  •  Signal Transduction

N3-18A  Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad For more information about Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad and the work that led to their Nobel Prize, visit http://www.nobelprize.org/nobel_prizes/medicine/ laureates/1998/ (Accessed March 2015).

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SECTION II  •  Physiology of Cells and Molecules

the one for transforming growth factor-β (TGF-β)—and are thus catalytic receptors. The TGF-β superfamily includes a large group of cytokines, including five TGF-βs, antimüllerian hormone (see p. 1080), as well as the inhibins and activins (see p. 1113), bone morphogenic proteins, and other glycoproteins, all of which control cell growth and differentiation. Members of this family participate in embryogenesis, suppress epithelialcell growth, promote wound repair, and influence immune and endocrine functions. Unchecked TGF-β signaling is important in progressive fibrotic disorders (e.g., liver cirrhosis, idiopathic pulmonary fibrosis) that result in replacement of normal organ tissue by deposits of collagen and other matrix components. The receptors for TGF-β and related factors are glycoproteins with a single membrane-spanning segment and intrinsic serine/threonine-kinase activity. Receptor types I and II (see Fig. 3-12B) are required for ligand binding and catalytic activity. The type II receptor first binds the ligand, and this binding is followed by the formation of a stable ternary complex of ligand, type II receptor, and type I receptor. After recruitment of the type I receptor into the complex, the type II receptor phosphorylates the type I receptor, thereby activating the serine/threonine kinase activity of the type I receptor. The principal targets of this kinase activity are SMAD proteins, which fall into three groups.  N3-19  The largest group is the receptor-activated SMADs (SMADs 1, 2, 3, 5, and 8), which—after phosphorylation by activated type I receptors—association with SMAD4, the only member of the second group. This heterodimeric complex translocates to the nucleus, where it regulates transcription of target genes. The third group (SMAD6, SMAD7) is the inhibitory SMADs, which can bind to type I receptors and prevent the phosphorylation of the receptor-activated SMADs.

RTKs produce phosphotyrosine motifs recognized by SH2 and phosphotyrosine-binding domains of downstream effectors In addition to the class of receptors with intrinsic serine/ threonine kinase activity, other plasma-membrane receptors have intrinsic tyrosine kinase activity. All RTKs discovered to date phosphorylate themselves in addition to other cellular proteins. Epidermal growth factor (EGF), plateletderived growth factor (PDGF), vascular endothelial growth factor (VEGF), insulin and insulin-like growth factor type 1 (IGF-1), fibroblast growth factor (FGF), and nerve growth factor (NGF) can all bind to receptors that possess intrinsic tyrosine kinase activity. Creation of Phosphotyrosine Motifs  Most RTKs are singlepass transmembrane proteins that have an extracellular ligand-binding domain and a single intracellular kinase domain (see Fig. 3-12C). Binding of a ligand, such as NGF, facilitates the formation of receptor dimers that in turn promote the direct association and trans-phosphorylation of the adjacent kinase domains; the result is activation of the receptor complex. The activated receptors then catalyze the addition of phosphate to tyrosine (Y) residues on the receptor itself as well as specific membrane-associated and cytoplasmic proteins. The resulting phosphotyrosine (pY)

TABLE 3-5  Tyrosine Phosphopeptides of the PDGF Receptor That Are Recognized by SH2 Domains on Various Proteins TYROSINE (Y) THAT IS PHOSPHORYLATED IN PDGF RECEPTOR

pY MOTIF RECOGNIZED BY SH2-CONTAINING PROTEIN

SH2CONTAINING PROTEIN

Y579

pYIYVD

Src family kinases

Y708

pYMDMS

p85

Y719

pYVPML

p85

Y739

pYNAPY

GTPase-activating protein

Y1021

pYIIPY

PLCγ

motifs serve as high-affinity binding sites for the recruitment of a number of intracellular signaling molecules, discussed in the next paragraph. These interactions lead to the for­ mation of a signaling complex and the activation of downstream effectors. Some RTKs, such as the insulin and IGF-1 receptors,  N3-20  exist as dimers even before ligand binding but undergo a conformational change that promotes autophosphorylation and activation of the kinase domains (see pp. 1041–1042). Recognition of pY Motifs by SH2 and PhosphotyrosineBinding Domains  The pY motifs created by tyrosine kinases

serve as high-affinity binding sites for the recruitment of cytoplasmic or membrane-associated proteins that contain either an SH2 domain or PTB (phosphotyrosine-binding) domain. SH2 domains are ~100 amino acids in length. They are composed of relatively well conserved residues that form the binding pocket for pY motifs as well as more variable residues that are implicated in binding specificity. These residues that confer binding specificity primarily recognize the three amino acids located on the C-terminal side of the phosphotyrosine. For example, the activated PDGF receptor has five such pY motifs (Table 3-5), each of which interacts with a specific SH2-containing protein. In contrast to SH2 and PTB domains, which interact with highly regulated pY motifs, Src homology 3 (SH3) domains interact constitutively with proline-rich regions in other proteins in a manner that does not require phosphorylation of the motif. However, phosphorylation at distant sites can change the conformation near the proline-rich region and thereby regulate the interaction. Like SH2 interactions, SH3 interactions appear to be responsible for targeting of signaling molecules to specific subcellular locations. SH2- or SH3-containing proteins include growth factor receptor– bound protein 2 (GRB2), PLCγ, and the p85 subunit of the phosphatidylinositol-3-kinase. The MAPK Pathway  A common pathway by which activated RTKs transduce their signal to cytosol and even to the nucleus is a cascade of events that increase the activity of the small GTP-binding protein Ras. This Ras-dependent signaling pathway involves the following steps (Fig. 3-13):

Step 1: A ligand binds to the extracellular domain of a specific RTK, thus causing receptor dimerization.

CHAPTER 3  •  Signal Transduction

N3-19  SMADs

68.e1

N3-20  Insulin and IGF-1 Receptors

Contributed by Ed Moczydlowski

Contributed by Emile Boulpaep and Walter Boron

The largest group of SMAD proteins is the receptor-activated SMADs (SMADs 1, 2, 3, 5, and 8), which have a type I receptor–interacting domain that is phosphorylated by the activated type I receptor; this phosphorylation results in their disassociation from the receptor and subsequent association with the regulatory SMAD, SMAD4. This heterodimeric complex translocates to the nucleus where it can regulate transcription of target genes by both direct and indirect mechanisms. The signaling specificity of this system comes from two mechanisms. First, distinct members of the receptor-activated SMAD group interact with specific type I receptors. For example SMAD2 and SMAD3 associate with the TGF-β type I receptor ALK-5, whereas SMAD1 associates with bone morphogenic protein (BMP) type I receptors such as ALK-2 and ALK-3. Second, the receptor-activated SMAD/SMAD4 heterodimer regulates not only downstream effector gene expression but also the expression of a third group of SMADs, the inhibitory SMADs. These proteins (SMAD6, SMAD7), once expressed, can bind to type I receptors and prevent the association and activation of receptor-activated SMADs.

The insulin receptor (see Fig. 51-5) and the IGF-1 receptor are activated by somewhat different mechanisms, as we discuss on pages 1041–1042 for the insulin receptor and on page 996 for the IGF-1 receptor. In brief, these receptors are tetrameric; they are composed of two α and two β subunits. The α subunit contains a cysteine-rich region and functions in ligand binding. The β subunit is a single-pass transmembrane protein with a cytoplasmic tyrosine kinase domain. The α and β subunits are held together by disulfide bonds (as are the two α subunits), forming a heterotetramer. Ligand binding produces conformational changes that appear to cause allosteric interactions between the two α and β pairs, as opposed to the dimerization characteristic of the first class of RTKs (see Fig. 3-12C). Thus, insulin binding results in the autophosphorylation of tyrosine residues in the catalytic domains of the β subunits. The activated insulin receptor also phosphorylates cytoplasmic substrates such as IRS-1 (insulin-receptor substrate 1; see Fig. 51-6), which, once phosphorylated, serves as a docking site for additional signaling proteins.

CHAPTER 3  •  Signal Transduction

6 The activated GTP-Ras recruits Raf-1 and activates it.

1 Ligand binding causes receptor dimerization. Ligand

Ligand

Extracellular space

5 SOS activates Ras by causing GTP to replace GDP on Ras.

8 MEK phosphorylates and activates MAPK.

9 MAPK works as an important effector molecule by phosphorylating many cellular proteins.

7 Raf-1 phosphorylates and activates MEK.

Receptor

Plasma membrane SH2 domain

Cytosol Tyrosine kinase domain

Ras

Ras Raf-1

P

P

69

MEK

P

P

MAPK P

Cytosolic proteins

GRB2 SOS

2 The activated RTK phosphorylates itself. 3 GRB2, an SH2-containing protein, recognizes the phosphotyrosine residues.

Ra

f-1

S

SO

4 The binding of GRB2 recruits SOS.

Nucleus Modulation of transcription

Inactive transcription factor MAPK

P

P

Active transcription factor

10 MAPK translocates to the nucleus where it phosphorylates a transcription factor.

Figure 3-13  Regulation of transcription by the Ras pathway. A ligand, such as a growth factor, binds to a specific RTK, and this leads to an increase in gene transcription in a 10-step process.

Step 2: The now-activated RTK phosphorylates itself on tyrosine residues of the cytoplasmic domain (autophosphorylation). Step 3: GRB2, an SH2-containing protein, recognizes pY residues on the activated receptor. Step 4: Because GRB2 constitutively associates with the guanine nucleotide exchange factor SOS (son of sevenless), via an SH3-proline interaction, the recruitment of GRB2 automatically results in the recruitment of SOS as well. Step 5: SOS activates the small G protein Ras by catalyzing the replacement of GDP with GTP. Step 6: The activated GTP-Ras complex activates other proteins by physically recruiting them to the plasma membrane. In particular, active GTP-Ras interacts with the N-terminal portion of the cytosolic serine/threonine kinase Raf-1 (also known as MAP kinase kinase kinase or MAPKKK or MAP3K), which is the first in a series of sequentially activated protein kinases that ultimately transmits the activation signal. Step 7: Raf-1 phosphorylates and activates a protein kinase called MEK (also known as MAP kinase kinase or MAPKK). MEK is a multifunctional protein kinase that phosphorylates substrates on both tyrosine and serine/ threonine residues. Step 8: MEK phosphorylates MAPKs, cytosolic serine/ threonine kinases also called extracellular signal– regulated kinases (ERK1, ERK2). Activation of MAPK requires dual phosphorylation on neighboring serine and tyrosine residues. Raf, MEK, and MAPK typically assemble on a scaffolding protein at the inner side of the

cell membrane to facilitate interaction/phosphorylation during the activation process. Step 9: MAPK is an important effector molecule in Rasdependent signaling to the cytoskeleton. MAPK phosphorylates multiple proteins involved in actin cytoskeletal assembly and cell-matrix interactions; this phosphorylation leads to Ras-dependent changes in cell morphology and cell migration. Step 10: Once activated, MAPK disassociates from the scaffold and translocates primarily to the nucleus, where it phosphorylates a number of nuclear proteins that are transcription factors. The result is either enhancement or repression of the DNA binding and transcriptional activity of these nuclear proteins.  N3-21 Two other signal-transduction pathways (cAMP and Ca2+) can modulate the activity of some of the protein intermediates in this MAPK cascade, which suggests multiple points of integration for the various signaling systems. The Phosphatidylinositol-3-Kinase Pathway  The phosphatidylinositol-3-kinase (PI3K) is an SH2 domain–containing protein that commonly signals downstream of RTKs. PI3K is a heterodimer consisting of a p85 regulatory subunit and p110 catalytic subunit. p85 has an SH2 domain for targeting the complex to activated receptors and an SH3 domain that mediates constitutive association with p110. p110 is a lipid kinase that phosphorylates PIP2 (see p. 58) on the 3 position of the inositol ring to form PIP3. PIP2 is a relatively common lipid in the inner leaflet of the cell membrane, whereas PIP3 constitutes NH +4 ≫ Cs+ > Li+, Na+, Ca2+. Under normal physiological conditions, the permeability ratio PK/PNa is >100, and Na+ can block many K+ channels. Some K+ channels can pass Na+ current in the complete absence of K+. This characteristic is analogous to the behavior of Ca2+ channels, which can pass Na+ and K+ currents in the absence of Ca2+. Given such strong K+ selectivity and an equilibrium potential near −80 mV, the primary role of K+ channels in excitable cells is to oppose the action of excitatory Na+ and Ca2+ channels and stabilize the resting state. Whereas some K+ channels are major determinants of the resting potential, other K+ channels mediate the repolarizing phase and shape of action potentials, control firing frequency, and define the bursting behavior of rhythmic firing. Such functions are broadly important in regulating the strength and frequency of all types of muscle contraction, in terminating transmitter release at nerve terminals, in attenuating the strength of synaptic connections, and in coding the intensity of sensory stimuli. Finally, in epithelia, K+ channels also function in K+ absorption and secretion. Before understanding the molecular nature of K+ channels, electrophysiologists classified K+ currents according to their functional properties and gating behavior, grouping macroscopic K+ currents into four major types: 1. Delayed outward rectifiers 2. Transient outward rectifiers (A-type currents) 3. Ca2+-activated K+ currents 4. Inward rectifiers These four fundamental K+ currents are the macroscopic manifestation of five distinct families of genes (see Table 6-2, family No. 2): 1. Kv channels (voltage-gated K+ channels related to the Shaker family) 2. Small- and intermediate-conductance KCa channels (Ca2+-calmodulin–activated K+ channels), including SKCa and IKCa channels 3. Large-conductance KCa channels (Ca2+-activated BKCa channels and related Na+- and H+- activated K+ channels)

193

4. Kir channels (inward-rectifier K+ channels) 5. K2P channels (two-pore K+ channels) In the next three sections, we discuss the various families of K+ channels and their associated macroscopic currents.

The Kv (or Shaker-related) family of K+ channels mediates both the delayed outward-rectifier current and the transient A-type current The K+ current in the HH voltage-clamp analysis of the squid giant axon (see pp. 177–178) is an example of a delayed outward rectifier. Figure 7-18A shows that this current activates with a sigmoidal lag phase (i.e., it is delayed in time, as in Fig. 7-6C). Figure 7-18B is an I-V plot of peak currents obtained in experiments such as that presented in Figure 7-18A and shows that the outward current rises steeply at positive voltages (i.e., it is an outward rectifier). A second variety of K+ current that is also outwardly rectifying is the transient A-type K+ current. This current was first characterized in mollusk neurons, but similar currents are common in the vertebrate nervous system. A-type currents are activated and inactivated over a relatively rapid time scale. Because their voltage activation range is typically more negative than that of other K+ currents, they are activated in the negative Vm range that prevails during the afterhyperpolarizing phase of action potentials. In neurons that spike repetitively, this A-type current can be very important in determining the interval between successive spikes and thus the timing of repetitive action potentials. For example, if the A-type current is small, Vm rises relatively quickly toward the threshold, and consequently the interspike interval is short and the firing frequency is high (see Fig. 7-18C). However, if the A-type current is large, Vm rises slowly toward the threshold, and therefore the interspike interval is long and the firing frequency is low (see Fig. 7-18D). Because the nervous system often encodes sensory information as a frequency-modulated signal, these A-type currents play a critical role. The channels responsible for both the delayed outwardrectifier and the transient A-type currents belong to the Kv channel family (where v stands for voltage-gated). The prototypic protein subunit of these channels is the Shaker channel of Drosophila (see Fig. 7-12C). All channels belonging to this family contain the conserved S1 to S6 core that is characteristic of the Shaker channel (see Fig. 7-10), but may differ extensively in the length and sequence of their intracellular N-terminal and C-terminal domains. The voltagesensing element in the S4 segment underlies activation by depolarization; the S4 segment actually moves outward across the membrane with depolarizing voltage, thus increasing the probability of the channel’s being open.  N7-13 The Kv channel family has multiple subclasses (see Table 6-2, family No. 2). Individual members of this Kv channel family, whether in Drosophila or humans, exhibit profound differences in gating kinetics that are analogous to delayedrectifier (slow activation) or A-type (rapid inactivation) currents. For example, Figure 7-18E shows the macroscopic currents of four subtypes of rat brain Kv1 (or Shaker) channels heterologously expressed in frog oocytes. All of these Kv1 channel subtypes (Kv1.1 to Kv1.4) exhibit sigmoidal activation kinetics when examined on a brief time scale—

194

SECTION II  •  Physiology of Cells and Molecules

A

DELAYED ACTIVATION OF Kv CHANNELS +30 mV –60 mV The activation of the current is delayed.

1 2 Peak current

+

K current

B

OUTWARD RECTIFICATION OF Kv CHANNELS Peak K current (IK)

3

4 5 +

6

P

N

C

C

A-TYPE OUTWARD RECTIFIER: SMALL CURRENT Small A-type current

Outward The current flows only in the outward direction. That is, the channel is an “outward rectifier.”

Vm

E

A-TYPE OUTWARD RECTIFIER: LARGE CURRENT Long interspike interval Large A-type current

DIFFERENCES IN GATING KINETICS AMONG Kv-TYPE DELAYED OUTWARD RECTIFIERS Kv1.1

150 pA

Kv1.1

Kv1.2

200 pA

Kv1.2

Kv1.3

6 pA

Kv1.4

400 pA

0 F

D

25 Time (ms)

50

0

500 pA

300 pA

Kv1.3

10 pA

Kv1.4

200 pA 1

2 Time (s)

INACTIVATION OF Kv-TYPE CHANNELS 4 α subunits

4 α subunits

4 α subunits

β

The N-terminal domain ball moves in and blocks the channel.

4 α subunits

β

The β subunit moves in and blocks the channel.

3

Chapter 7    •  Electrical Excitability and Action Potentials

195

BOX 7-3  Congenital and Drug-Induced Cardiac Arrhythmias Linked to K+ Channels Congenital Long QT Syndromes As discussed in Box 7-1, congenital cardiac abnormality in some people results in lengthening of the QT interval of the electrocardiographic signal—long QT syndrome—which corresponds to a prolonged cardiac action potential. Affected children and young adults can exhibit an arrhythmic disturbance of the ventricular heartbeat that results in sudden death. As we have already seen in Box 7-1, one form of a long QT syndrome—LQT3—involves gain-of-function mutations of the cardiac Na+ channel Nav1.5 (SCN5A) that prolong Na+ channel opening. However, at least six forms of long QT syndrome—LQT1, LQT2, LQT5, LQT6, LQT7, and LQT13—are caused by loss-of-function mutations in cardiac K+ channels (see Table 6-2, family No. 2) or their accessory proteins. LQT1 is due to mutations in the KCNQ1 gene encoding KvLQT1, a 581-residue protein belonging to the Kv family of voltage-gated K+ channels. Another form of this disease, LQT2, involves mutations in the KCNH2 gene encoding HERG (for human ether-à-go-go) which is related to the gene defective in the ether-à-go-go Drosophila mutation, in which flies convulsively shake under ether anesthesia. Both KvLQT1 and HERG K+ channels participate in repolarization of the human cardiac action potential (see p. 488). KvLQT1 mediates the slowly activating delayed-rectifier component (IKs) of cardiac action potential repolarization; HERG mediates the rapidly activating repolarization component (IKr). Both LQT1 and LQT2 result from loss-offunction effects associated with decreased K+ channel expression in cardiac myocytes.

in the millisecond range (left side of Fig. 7-18E). That is, these channels display some degree of “delayed” activation. Different Kv channels exhibit different rates of activation. Thus, these currents can modulate action potential duration by either keeping it short (e.g., in nerve and skeletal muscle) when the delayed rectifier turns on quickly or keeping it long (e.g., in heart) when the delayed rectifier turns on slowly. Kv1 channels also differ markedly in their inactivation kinetics when observed over a long time scale—in the range of seconds (right side of Fig. 7-18E). Kv1.1 exhibits little time-dependent inactivation (i.e., the current is sustained throughout the stimulus). On the other hand, the Kv1.4

KvLQT1 associates with minK, a small, single-span membrane protein encoded by the KCNE1 gene. minK modulates the gating kinetics of KvLQT1, and mutations in minK cause LQT5. Three other human proteins closely related to minK are known as MiRP1, MiRP2, and MiRP3 (minK-related proteins)—the products of the genes KCNE2, KCNE3, and KCNE4, respectively. MiRP1 associates with HERG, and mutations in MiRP1 are linked to LQT6. Two other K+ channel genes also cause long QT syndromes. Mutations in Kir2.1, encoded by the gene KCNJ2, cause LQT7, whereas mutations in GIRK4, encoded by KCNJ5, cause LQT13.

Acquired Long QT Syndrome The HERG channel is notorious for its sensitivity to blockade by many classes of therapeutic drugs, including antihistamines (e.g., terfenadine), antipsychotics (e.g., sertindole), and gastrointestinal drugs (e.g., cisapride). Blockade of HERG can readily mimic the genetic condition of LQT2. The promiscuous drug sensitivity of the HERG K+ channel appears to result from particular structural features of the internal aspect of the channel pore that favor binding of many hydrophobic small molecules. People who have natural variations in ion channel genes that cause a subclinical propensity for long QT intervals or who have deficiencies in drugmetabolizing enzymes appear to be especially at risk. Many drugs have been banned or limited for therapeutic use because of the risk of HERG channel block. Today all new drugs proposed for clinical use must first undergo screening for HERG blockade in order to prevent deaths by acquired long QT syndrome.

channel completely inactivates in 10 degrees away from the center of the fovea and thus the center of gaze).

SECTION III  •  The Nervous System

To optic nerve

Nerve fiber layer

Light Ganglion cell

ipRGC

Ganglion cell layer

Amacrine cell

Inner plexiform layer

Proximal

Retina Inner nuclear layer

Outer plexiform layer Light

Vertical information flow

364

Lateral information flow Horizontal cell

Bipolar cells

Rod Outer nuclear layer

Cone

Cone

Rod

Rod

Cone

Distal

Photoreceptor outer segment Pigment epithelium

Ciliary stalk

Synaptic terminals

Folding of outer cell membrane

Inner segment

Connecting cilium

Nucleus Mitochondria Freefloating disks

Cilium

Outer segment

Disks Cone

Rod

Cytoplasmic space

Figure 15-9  Neural circuits in the primate retina. Notice that the incoming light reaches intrinsically photosensitive retinal ganglion cells (ipRGCs) immediately, but hits rod and cone photoreceptor cells only after passing through several thin, transparent layers of other neurons. The pigment epithelium absorbs the light not absorbed by the photoreceptor cells and thus minimizes reflections of stray light. The ganglion cells communicate to the thalamus by sending action potentials down their axons. However, the photoreceptor cells and other neurons communicate by graded synaptic potentials that are conducted electrotonically.

Chapter 15  •  Sensory Transduction

A CENTER OF THE RETINA (FOVEA)

B

PERIPHERY OF THE RETINA

Ganglion cells

365

membrane-bound intracellular organelles that have pinched off from the outer membrane. Cone outer segments have similarly stacked membranes, except that they are infolded and remain continuous with the outer membrane. The disk membranes contain the photopigments—rhodopsin in rods and molecules related to rhodopsin in cones. Rhodopsin moves from its synthesis site in the inner segment through the stalk and into the outer segment through small vesicles whose membranes are packed with rhodopsin to be incorporated into the disks.

Rods and cones hyperpolarize in response to light

Bipolar cells

Cone

Cone Rod

Photoreceptors

Receptive fields of ganglion cells

The receptive field of ganglion cells at the retinal periphery is much larger than that at the fovea.

Figure 15-10  Comparison of the synaptic connections and receptive fields in the fovea and periphery of the retina.

Rods and cones are elongated cells with synaptic terminals, an inner segment, and an outer segment (see Fig. 15-9). The synaptic terminals connect to the inner segment by a short axon. The inner segment contains the nucleus and metabolic machinery; it synthesizes the photopigments and has a high density of mitochondria. The inner segment also serves an optical function—its high density funnels photons into the outer segment. A thin ciliary stalk connects the inner segment to the outer segment. The outer segment is the transduction site, although it is the last part of the cell to see the light. Structurally, the outer segment is a highly modified cilium. Each rod outer segment has ~1000 tightly packed stacks of disk membranes, which are flattened,

The remarkable psychophysical experiments of Hecht and colleagues in 1942 demonstrated that five to seven photons, each acting on only a single rod, are sufficient to evoke a sensation of light in humans. Thus, the rod is performing at the edge of its physical limits because there is no light level smaller than 1 photon. To detect a single photon requires a prodigious feat of signal amplification. As Denis Baylor has pointed out, “the sensitivity of rod vision is so great that the energy needed to lift a sugar cube one centimeter, if converted to a blue-green light, would suffice to give an intense sensation of a flash to every human who ever existed.” Phototransduction involves a cascade of chemical and electrical events to detect, to amplify, and to signal a response to light. As do many other sensory receptors, photoreceptors use electrical events (receptor potentials) to carry the visual signal from the outer segment to their synapses. Chemical messengers diffusing over such a distance would simply be too slow. A surprising fact about the receptor potential of rods and cones is that it is hyperpolarizing. Light causes the cell’s Vm to become more negative than the resting potential that it maintains in the dark (Fig. 15-11A). At low light intensities, the size of the receptor potential rises linearly with light intensity; but at higher intensities, the response saturates. Hyperpolarization is an essential step in relaying the visual signal because it directly modulates the rate of transmitter release from the photoreceptor onto its postsynaptic neurons. This synapse is conventional in that it releases more transmitter—in this case glutamate—when its presynaptic terminal is depolarized and less when it is hyperpolarized. Thus, a flash of light causes a decrease in transmitter secretion. The upshot is that the vertebrate photoreceptor is most active in the dark. How is the light-induced hyperpolarization generated? Figure 15-11B shows a method to measure the current flowing across the membrane of the outer segment of a single rod. In the dark, each photoreceptor produces an ionic current that flows steadily into the outer segment and out of the inner segment. This dark current is carried mainly by inwardly directed Na+ ions in the outer segment and by outwardly directed K+ ions from the inner segment (see Fig. 15-11C). Na+ flows through a nonselective cation channel of the outer segment, which light indirectly regulates, and K+ flows through a K+ channel in the inner segment, which light does not regulate. Na+ carries ~90% of the dark current in the outer segment, and Ca2+, ~10%. In the dark, Vm is about −40 mV. Na-K pumps, primarily located within the inner segments, remove the Na+ and import K+. An Na-Ca exchanger removes Ca2+ from the outer segment.

366

SECTION III  •  The Nervous System

A

B

LIGHT-EVOKED HYPERPOLARIZATIONS

LIGHT STIMULATING A SINGLE ROD

Light flash Response to the least intense light flash –40 –45 Membrane –50 potential (mV) –55 Response to the most intense light flash

–60 –65 0 C

100

200 300 400 Time (ms)

DARK

500

600

D

LIGHT

Depolarized: high transmitter release

Hyperpolarized: low transmitter release

Rod

Rod

Inner segment

Inner segment

Nonselective cation channel K+

Na+

cGMP +

Na

cGMP

Outer segment

Na+

Outer segment

Figure 15-11  Phototransduction. A, The experiment for which results are summarized here was performed on a red-sensitive cone from a turtle. A brief flash of light causes a hyperpolarization of the photoreceptor cell. The size of the peak and the duration of the receptor potential increase with the increasing intensity of the flash. At low light intensities, the magnitude of the peak increases linearly with light intensity. At high intensities, the peak response saturates, but the plateau becomes longer. B, A single rod has been sucked into a pipette, which allows the investigators to monitor the current. The horizontal white band is the light used to stimulate the rod. C, In the absence of light, Na+ enters the outer segment of the rod through cGMPgated channels and depolarizes the cell. The electrical circuit for this dark current is completed by K+ leaving the inner segment. The dark current, which depolarizes the cell, leads to constant transmitter release. D, In the presence of light, Na+ can no longer enter the cell because cGMP levels are low, and the cGMP-gated channel closes. The photoreceptor cell thus hyperpolarizes, and transmitter release decreases. (A, Data from Baylor DA, Hodgkin AL, Lamb TD: The electrical response of turtle cones to flashes and steps of light. J Physiol 242:685–727, 1974; B, from Baylor DA, Lamb TD, Yau K-W: Responses of retinal rods to single photons. J Physiol 288:613–634, 1979.)

Chapter 15  •  Sensory Transduction

Figure 15-12  Rhodopsin, transducin, and signal transduction at the molecular

level. A, The opsin molecule is a classic seven-transmembrane receptor that couples to transducin, a G protein. When the opsin is attached to retinal (magenta structure) via amino-acid residue 296 in the seventh (i.e., most C-terminal) membrane-spanning segment of opsin, the assembly is called rhodopsin. B, The absorption of a photon by 11-cis retinal causes the molecule to isomerize to all-trans retinal. C, After rhodopsin absorbs a photon of light, it activates many transducins. The activated α subunit of transducin (Gαt) in turn activates phosphodiesterase, which hydrolyzes cGMP. The resultant decrease in [cGMP]i closes cGMP-gated channels and produces a hyperpolarization (receptor potential). GMP, 5′-guanylate monophosphate; NCKX1, the Na+/(Ca2+-K+) exchanger (SLC24A1). (A, Data from Palczewsk K, Kumasaka T, Miyano, M et al: Crystal structure of rhodopsin: A G protein-coupled receptor. Science 289(5480):739–745, 2000. Reconstructed figure is courtesy of S. Filipek and K. Palczewski.)

Absorption of photons leads to closure of the nonselective cation channels in the outer segment. The total conductance of the cell membrane decreases. Because the K+ channels of the inner segment remain open, K+ continues to flow out of the cell, and this outward current causes the cell to hyperpolarize (see Fig. 15-11D). The number of cation channels that close depends on the number of photons that are absorbed. The range of one rod’s sensitivity is 1 to ~1000 photons. Baylor and colleagues measured the minimum amount of light required to produce a change in receptor current (see Fig. 15-11B). They found that absorption of 1 photon suppresses a surprisingly large current, equivalent to the entry of >106 Na+ ions, and thus represents an enormous amplification of energy. The single-photon response is also much larger than the background electrical noise in the rod, as it must be to produce the rod’s high sensitivity to dim light. Cones respond similarly to single photons, but they are inherently noisier and their response is only ~ 1 50 the size of that in the rod. Cone responses do not saturate, even at the brightest levels of natural light. Cones also respond faster than rods.

Rhodopsin is a G protein–coupled “receptor” for light

RHODOPSIN

Disk interior N

2 3

1

6

7

Cytosol Retinal B

5

4

Attachment site for retinal

C

RETINAL 11-cis retinal H3C CH3 H

CH3 H H

H CH3

H

H H3C

Light

C

O

All-trans retinal H3C CH3 H CH3 H

H

O

CH3 C

H CH3 C

H

H

H

H

VISUAL TRANSDUCTION

Visual pigment (rhodopsin)

Light

Disk interior

Cytosol

γ

Disk

Disk membrane

Phosphodiesterase

α

β

α

Transducin

GMP cG

Ca2+

Extracellular space

P M

Guanylyl cyclase cG

MP K+

P

Rod outer segment membrane

M cG

How can a single photon stop the flow of 1 million Na+ ions across the membrane of a rod cell? The process begins when the photon is absorbed by rhodopsin, the light receptor molecule. Rhodopsin is one of the most tightly packed proteins in the body, with a density of ~30,000 molecules per square micrometer in the disk membranes. Thus, the packing ratio is 1 protein molecule for every 60 lipid molecules! One rod contains ~109 rhodopsin molecules. This staggering density ensures an optimized capture rate for photons passing through a photoreceptor. Even so, only ~10% of the light entering the eye is used by the receptors. The rest is either absorbed by the optical components of the eye or passes between or through the receptors. Rhodopsin has two key components: retinal and the protein opsin. Retinal is the aldehyde of vitamin A, or retinol (~500 Da). Opsin is a single polypeptide (~41 kDa) with seven membranespanning segments (Fig. 15-12A). It is a member of the superfamily of GPCRs (see pp. 51–52) that includes many neurotransmitter receptors as well as the olfactory receptor molecules. To be transduced, photons are actually absorbed by retinal, which is responsible for rhodopsin’s color. The tail

A

367

NCKX1 4 Na+

Na+ Ca2+ Nonselective cation channel

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SECTION III  •  The Nervous System

of retinal can twist into a variety of geometric configurations, one of which is a kinked and unstable version called 11-cis retinal (see Fig. 15-12B). The cis form sits within a pocket of the opsin (comparable to the ligand-binding site of other GPCRs) and is covalently bound to it. However, because of its instability, the cis form can exist only in the dark. If 11-cis retinal absorbs a photon, it isomerizes within 1 ps to a straighter and more stable version called all-trans retinal. This isomerization in turn triggers a series of conformational changes in the opsin that lead to a form called metarhodopsin II, which can activate an attached molecule called transducin. Transducin carries the signal forward in the cascade and causes a reduction in Na+ conductance. Soon after isomerization, all-trans retinal and opsin separate in a process called bleaching; this separation causes the color to change from the rosy red of rhodopsin (rhodon is Greek for the color “rose”) to the pale yellow of opsin. The photoreceptor cell converts all-trans retinal to retinol (vitamin A), which then translocates to the pigment epithelium and becomes 11-cis retinal. This compound makes its way back to the outer segment, where it recombines with opsin. This cycle of rhodopsin regeneration takes a few minutes. Transducin is so named because it transduces the lightactivated signal from rhodopsin into the photoreceptor membrane’s response (see Fig. 15-12C). Transducin was the first of the large family of GTP-binding proteins (G proteins; see p. 52) to be identified, and its amino-acid sequence is very similar to that of other G proteins (see Table 3-2). When it is activated by metarhodopsin, the α subunit of transducin exchanges a bound GDP for a GTP and then diffuses within the plane of the membrane to stimulate a phosphodiesterase that hydrolyzes cGMP to 5′-guanylate monophosphate. cGMP is the diffusible second messenger that links the light-activated events of the disk membranes to the electrical events of the outer membrane. A key discovery by Fesenko and colleagues in 1985 showed that the “light-sensitive” cation channel of rods is actually a cGMP-gated cation channel (see pp. 169–172). This CNG channel was the first of its kind to be discovered (we have already discussed a similar channel in olfactory receptors). In the dark, a constitutively active guanylyl cyclase that synthesizes cGMP from GTP keeps cGMP levels high within the photoreceptor cytoplasm. This high [cGMP]i causes the cGMP-gated cation channels to spend much of their time open and accounts for the dark current (see Fig. 15-11C). Because light stimulates the phosphodiesterase and thus decreases [cGMP]i, light reduces the number of open cGMP-gated cation channels and thus reduces the dark current. The photoreceptor then hyperpolarizes, transmitter release falls, and a visual signal is passed to retinal neurons. Strong amplification occurs along the phototransduction pathway. The absorption of 1 photon activates 1 metarhodopsin molecule, which can activate ~700 transducin molecules within ~100 ms. These transducin molecules activate phosphodiesterase, which increases the rate of cGMP hydrolysis by ~100-fold. One photon leads to the hydrolysis of ~1400 cGMP molecules by the peak of the response, thus reducing [cGMP] by ~8% in the cytoplasm around the activated disk. This decrease in [cGMP]i closes ~230 of the 11,000 cGMP-gated channels that are open in the dark. As a result, the dark current falls by ~2%.

The cGMP-gated channel has additional interesting properties. It responds within milliseconds when [cGMP]i rises, and it does not desensitize in response to cGMP. The concentration-response curve is very steep at low [cGMP]i because opening requires the simultaneous binding of three cGMP molecules. Thus, the channel has switch-like behavior at physiological levels of cGMP. Ion conductance through the channel also has steep voltage dependence because Ca2+ and Mg2+ strongly block the channel (as well as permeate it) within its physiological voltage range. This open-channel block (see Fig. 7-20D) makes the normal single-channel conductance very small, among the smallest of any ion channel; the open channel normally carries a current of only 3 × 10−15 A (3 fA)! The currents of ion channels are inherently “noisy” as they flicker open and closed. However, the 11,000 channels—each with currents of 3 fA—summate to a rather noise-free dark current of 11,000 channels × 3 fA per channel = 33 pA. In contrast, if 11 channels—each with currents of 3 pA—carried the dark current of 33 pA, the 2% change in this signal (0.66 pA) would be smaller than the noise produced by the opening and closing of a single channel (3 pA). Thus, the small channels give the photoreceptor a high signal-to-noise ratio. The [cGMP]i in the photoreceptor cell represents a dynamic balance between the synthesis of cGMP by guanylyl cyclase and the breakdown of cGMP by phosphodiesterase. Ca2+, which enters through the relatively nonselective cGMPgated channel, synergistically inhibits the guanylyl cyclase and stimulates the phosphodiesterase. These Ca2+ sensitivities set up a negative-feedback system. In the dark, the incoming Ca2+ prevents runaway increases in [cGMP]i. In the light, the ensuing decrease in [Ca2+]i relieves the inhibition on guanylyl cyclase, inhibits the phosphodiesterase, increases [cGMP]i, and thus poises the system for channel reopening. When a light stimulus terminates, the activated forms of each component of the transduction cascade must be inactivated. One mechanism of this termination process appears to involve the channels themselves. As described in the preceding paragraph, closure of the cGMP-gated channels in the light leads to a fall in [Ca2+]i, which helps replenish cGMP and facilitates channel reopening. Two additional mechanisms involve the proteins rhodopsin kinase and arrestin. Rhodopsin kinase phosphorylates light-activated rhodopsin and allows it to be recognized by arrestin. Arrestin, an abundant cytosolic protein, binds to the phosphorylated light-activated rhodopsin and completely terminates its ability to activate transducin.

The eye uses a variety of mechanisms to adapt to a wide range of light levels The human eye can operate effectively over a 1010-fold range of light intensities, which is the equivalent of going from almost total darkness to bright sunlight on snow. However, moving from a bright to a dark environment, or vice versa, requires time for adaptation before the eye can respond optimally. Adaptation is mediated by several mechanisms. One mechanism mentioned above is regulation of the size of the pupil by the iris, which can change light sensitivity by ~16fold. That still leaves the vast majority of the range to account

Chapter 15  •  Sensory Transduction

Cones adapt to low light quickly… 8

…but still have a relatively high threshold. Thus, by themselves, cones do not provide effective dark-adapted vision.

7 6 Log relative 5 light threshold 4 3 0

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However, rods adapt to much dimmer light, although they do so more slowly. Figure 15-13  Effect of dark adaptation on the visual threshold. The subject was exposed to light at a level of 1600 millilumens and then switched to the dark. The graph is a plot of the time course of the subject’s relative threshold (on a log scale) for detecting a light stimulus. (Data from Hecht S, Shlaer S, Smith EL, et al: The visual functions of the complete color blind. J Gen Physiol 31:459–472, 1948.)

for. During dark adaptation, two additional mechanisms with very different time courses are evident, as we can see from a test of the detection threshold for the human eye (Fig. 15-13). The first phase of adaptation is finished within ~10 minutes and is a property of the cones; the second takes at least 30 minutes and is attributed to the rods. A fully darkadapted retina, relying on rods, can have a light threshold that is as much as 15,000 times lower than a retina relying on cones. In essence, then, the human eye has two retinas in one, a rod retina for low light levels and a cone retina for high light levels. These two systems can operate at the same time; when dark adapted, the rods can respond to the lowest light levels, but cones are available to respond when brighter stimuli appear. The rapid and slow phases of adaptation that are discussed in the preceding paragraph have both neural and photoreceptor mechanisms. The neural mechanisms are relatively fast, operate at relatively low ambient light levels, and involve multiple mechanisms within the neuronal network of the retina. The photoreceptor mechanisms involve some of the processes that are described in the previous section. Thus, in bright sunlight, rods become ineffective because most of their rhodopsin remains inactivated, or bleached. cGMP-gated channels are closed and thus Ca2+ entry is blocked, so [Ca2+]i falls to a few nanomolar as Ca2+ is removed by the Na+/(Ca2+-K+) exchanger NCKX1 (SLC24A1; see Table 5-4). After returning to darkness, the rods slowly regenerate rhodopsin and become sensitive once again. However, a component of the cGMP system also regulates photoreceptor sensitivity. In the dark, when baseline [cGMP]i is relatively high, substantial amounts of Ca2+ enter through cGMP-gated channels. The resultant high [Ca2+]i (several

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hundred nanomolars) inhibits guanylyl cyclase and stimulates phosphodiesterase, thereby preventing [cGMP]i from rising too high. Conversely, when background light levels are high, this same feedback system causes baseline [cGMP]i to remain high so that [cGMP]i can fall in response to further increases in light levels. Otherwise, the signal-transduction system would become saturated. In other words, the photoreceptor adapts to the increased back­ground light intensity and remains responsive to small changes. Additional adaptation mechanisms regulate the sensitivity of rhodopsin, guanylyl cyclase, and the cGMP-gated channel. Clearly, adaptation involves an intricate network of molecular interactions.

Color vision depends on the different spectral sensitivities of the three types of cones The human eye responds only to a small region of the electromagnetic spectrum (see Fig. 15-5), but within it, we are exquisitely sensitive to the light’s wavelength. We see assorted colors in a daytime panorama because objects absorb some wavelengths while reflecting, refracting, or transmitting others. Different sources of light may also affect the colors of a scene; the light from tungsten bulbs is reddish, whereas that of fluorescent bulbs is bluish. Research on color vision has a long history. In 1801, Thomas Young first outlined the trichromatic theory of color vision, which was championed later in the 19th century by Hermann von Helmholtz. These investigators found that they could reproduce a particular sample hue by mixing the correct intensities of three lights with the primary hues blue, green, and red. They proposed that color vision, with its wide range of distinct, perceived hues, is based on only three different pigments in the eye, each absorbing a different range of wavelengths. Microspectrophotometry of single cones in 1964 amply confirmed this scheme. Thus, although analysis of color by the human brain is sophisticated and complex, it all derives from the responses of only three types of photo­ pigments in cones. Our sensitivity to the wavelength of light depends on the retina’s state of adaptation. When it is dark adapted (also called scotopic conditions), the spectral sensitivity curve for human vision is shifted toward shorter wavelengths compared with the curve obtained after light adaptation (photopic conditions; Fig. 15-14A). The absolute sensitivity to light can also be several orders of magnitude higher under scotopic conditions (see Fig. 15-13). The primary reason for the difference in these curves is that rods are doing the transduction of dim light under dark-adapted conditions, whereas cones transduce in the light-adapted eye. As we would predict, the spectral sensitivity curve for scotopic vision is quite similar to the absorption spectrum of the rods’ rhodopsin, with a peak at 500 nm. The spectral sensitivity of the light-adapted eye depends on the photopigments in the cones. Humans have three different kinds of cones, and each expresses a photopigment with a different absorbance spectrum. The peaks of their absorbance curves fall at ~420, 530, and 560 nm, which correspond to the violet, yellow-green, and yellow-red regions of the spectrum (see Fig. 15-14B). The three cones and their pigments were historically called blue, green, and red,

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SPECTRAL SENSITIVITY UNDER DARK- AND LIGHT-ADAPTED CONDITIONS Under conditions of dark adaptation, sensitivity to light depends mainly on rods… 500

...whereas under conditions of light adaptation, sensitivity depends mainly on cones.

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Figure 15-14  Sensitivity of vision and photoreceptors at different wavelengths of light. A, The graph shows the results of a psychophysical experiment. Under dark-adapted (scotopic) conditions, the human eye is maximally sensitive at ~500 nm. Under light-adapted (photopic) conditions, the eye is maximally sensitive at ~560 nm. B, The spectral sensitivity of rods (obtained with a spectrophotometer) peaks at ~500 nm; that of the three types of cones peaks at ~420 nm for the S (blue) cone, ~530 nm for the M (green) cone, and ~560 nm for the L (red) cone; and that of melanopsin peaks at ~475 nm. Each absorbance spectrum has been normalized to its peak sensitivity. (A, Data from Knowles A: The biochemical aspects of vision. In Barlow HB, Mollon JD [eds]: The Senses. Cambridge, UK, Cambridge University Press, 1982, pp 82–101; B, rhodopsin data from Dartnell HJ, Bowmaker JK, Mollon JD: Microspectrophotometry of human photoreceptors. In Mollon JD, Sharpe LT [eds]: Colour Vision. London, Academic Press, 1983, pp 69–80; melanopsin data from Matsuyama T, Yamashita T, Imamoto Y, Shichida Y: Photochemical properties of mammalian melanopsin. Biochemistry 51:5454–5462, 2012.)

respectively. They are now more commonly called S, M, and L (for short, medium, and long wavelengths); we use this terminology here. Because the absolute sensitivity of the short-wavelength cone is only one tenth that of the other two, the spectral sensitivity of photopic human vision is dominated by the two longer-wavelength cones (compare the spectral sensitivity functions in Fig. 15-14A with the absorbance spectra of the cones in Fig. 15-14B). Single cones do not encode the wavelength of a light stimulus. If a cone responds to a photon, it generates the same response regardless of the wavelength of that photon. A glance at Figure 15-14B shows that each type of cone pigment can absorb a wide range of wavelengths. The pigment in a cone is more likely to absorb photons when their wavelength is at its peak absorbance, but light hitting the cone on the fringe of its absorbance range can still generate a large response if the light’s intensity is sufficiently high. This property of response univariance is the reason that vision in an eye with only one functioning pigment (e.g., scotopic vision using only rods) can only be monochromatic. With a single pigment system, the distinction between different colors and between differences in intensity is confounded. Two different cones (as in most New World monkeys), each with a different but overlapping range of wavelength sensitivities, remove much of the ambiguity in encoding the wavelength of light stimuli. With three overlapping pigments (as in Old World monkeys and humans), light of a single wavelength stimulates each of the three cones to different degrees, and light of any other wavelength stimulates these cones with a distinctly different pattern. Because the nervous system can compare the relative stimulation of the three cone types to decode the wavelength, it can also distinguish changes in the intensity (luminance) of the light from changes in its wavelength. Color capabilities are not constant across the retina. The use of multiple cones is not compatible with fine spatial discrimination because of wavelength-dependent differences in the eye’s ability to focus light, known as chromatic aberration, and because very small objects may stimulate only single cones. The fovea has only M and L cones, which limits its color discrimination in comparison to the peripheral portions of the retina but leaves it best adapted to discriminate fine spatial detail (Box 15-1). The four different human visual pigments have a similar structure. The presence of retinal and the mechanisms of its photoisomerization are essentially identical in each. The main difference is the primary structure of the attached protein, the opsin. M and L opsins share 96% of their amino acids. Pairwise comparisons among the other opsins show only 44% or lower sequence similarity, however. Apparently, the different amino-acid structures of the opsins affect their charge distributions in the region of the 11-cis retinal and shift its absorption spectrum to give the different pigments their specific spectral sensitivities.

The ipRGCs have unique properties and functions The ipRGC, the third retinal photoreceptor, differs from rods and cones in fundamental ways. First, instead of expressing rhodopsin or cone opsins, ipRGCs use a related but unique light-sensitive protein called melanopsin that is most

Chapter 15  •  Sensory Transduction

BOX 15-1  Inherited Defects in Color Vision

I

nherited defects in color vision are relatively common, and many are caused by mutations in visual pigment genes. For example, 8% of white males and 1% of white females have some defect in their L or M pigments caused by X-linked recessive mutations. A single abnormal pigment can lead to either dichromacy (the absence of one functional pigment) or anomalous trichromacy (a shift in the absorption spectrum of one pigment relative to normal), often with a consequent inability to distinguish certain colors. Jeremy Nathans and colleagues found that men have only one copy of the L pigment gene; but located right next to it on the X chromosome, they may have one to three copies of the M pigment gene. He proposed that homologous recombination could account for the gene duplication, loss of a gene, or production of the hybrid L-M genes that occur in red-green color blindness. Hybrid L-M pigments have spectral properties intermediate between those of the two normal pigments, probably because their opsins possess a combination of the traits of the two normal pigments. Lack of two of the three functional cone pigments leads to monochromacy. The number of people who have such true color blindness is very small, 100,000 Hz. A continuous pure tone (see p. 376) produces a wave that travels along the basilar membrane and has different amplitudes at different points along the base-apex axis (Fig. 15-24A). Increases in sound amplitude cause an increase in the rate of action potentials in auditory nerve axons—rate coding.  N15-14  The frequency of the sound determines where along the cochlea the cochlear membranes vibrate most—high frequencies at one end and low at the other— and thus which hair cells are stimulated. This selectivity is the basis for place coding in the auditory system; that is, the frequency selectivity of a hair cell depends mainly on its longitudinal position along the cochlear membranes. The cochlea is essentially a spectral analyzer that evaluates a complex sound according to its pure tonal components, with each pure tone stimulating a specific region of the cochlea.

Chapter 15  •  Sensory Transduction

N15-12  Otoacoustic Emissions

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N15-13  Auditory Range

Contributed by Philine Wangemann

Contributed by Philine Wangemann

Amplification by the outer hair cells evokes vibrations of the basilar membrane that travel through the middle ear, set the tympanic membrane in motion, and produce a sound that comes out of the ear canal. Clinically most relevant are transient otoacoustic emissions and distortion-product otoacoustic emissions. Transient otoacoustic emissions are sounds that are detected in the ear canal milliseconds after a very brief stimulus. Amplification by the outer hair cells is nonlinear, which means that the cochlea produces and emits distortion products. Distortion products in response to two pure tones at nearby frequencies (f1 and f2) relate to these stimuli by simple math, for example, 2f1 − f2 or 2f2 − f1. Transient otoacoustic emissions and distortion-product otoacoustic emissions provide useful clues for the evaluation of outer hair cell function.

The auditory frequency range of the human ear is well adapted to the perception of speech, which encompasses frequencies between 60 and 12,000 Hz. We can comfortably hear sounds with amplitudes from 0 to 120 dB SPL. Higher sound pres­ sure levels cause pain and destruction of the ear  N15-9.  Typical sound pressure levels are 20 dB SPL for whispering, 60 dB SPL for normal conversation, 80 dB SPL for loud traffic, and 120 dB SPL for a nearby train horn.

N15-14  Rate Coding Contributed by Philine Wangemann Amplitude information is transmitted by rate coding. Rate coding refers to the principle that increases in sound amplitude result in an increase the rate of action potentials. Cooperation between neurons is required to code the full range of sound pressure levels from 0 to 120 dB SPL.

Chapter 15  •  Sensory Transduction

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EFFECT OF FREQUENCY ON ENVELOPE POSITION

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Apex Helicotrema

…than its apical end.

Figure 15-24  Waves along the basilar membrane of the cochlea. A, As a wave generated by a sound of a single frequency travels along the basilar membrane, its amplitude changes. The green and yellow curves represent a sample wave at two different times. The upper and lower broken lines (i.e., the envelope) encompass all maximum amplitudes of all waves, at all points in time. Thus, a wave can never escape the envelope. The figure exaggerates the amplitudes of the traveling waves ~1 million-fold. B, For a pure tone of 10,000 Hz, the envelope is confined to a short region of the basilar membrane near the stapes. For pure tones of 4000 Hz and 200 Hz, the widest part of the envelope moves closer to the helicotrema. C, The cochlea narrows in diameter from base to apex, whereas the basilar membrane tapers in the opposite direction.

Using optical methods to study cadaver ears, Georg von Békésy found that sounds of a particular frequency generate relatively localized waves in the basilar membrane and that the envelope of these waves changes position according to the frequency of the sound (see Fig. 15-24B). Low frequencies generate their maximal amplitudes near the apex. As sound frequency increases, the envelope shifts progressively toward the basal end (i.e., near the oval and round windows). For his work, von Békésy  N15-15  received the 1961 Nobel Prize in Physiology or Medicine. Two properties of the basilar membrane underlie the low-apical to high-basal gradient of resonance: taper and stiffness (see Fig. 15-24C). If we could unwind the cochlea

and stretch it straight, we would see that it tapers from base to apex. The basilar membrane tapers in the opposite direction—wider at the apex, narrower at the base. More important, the narrow basal end is ~100-fold stiffer than its wide and floppy apical end. Thus, the basilar membrane resembles a harp. At one end—the base, near the oval and round windows—it has short, taut strings that vibrate at high frequencies. At the other end—the apex—it has longer, looser strings that vibrate at low frequencies. Although von Békésy’s experiments were illuminating, they were also paradoxical. A variety of experimental data suggested that the tuning of living hair cells is considerably sharper than the broad envelopes of von Békésy’s traveling

Chapter 15  •  Sensory Transduction

N15-15  Georg von Békésy For more information about Georg von Békésy and the work that led to his Nobel Prize, visit http://www.nobel.se/medicine/ laureates/1961/index.html (accessed December 2014).

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Figure 15-25  Peak movement of the basilar membrane. The graph illustrates the displacement of the basilar membrane in response to a pure tone as a function of distance along the base-to-apex axis. The dashed line indicates the displacement threshold for triggering of an electrical response. (Data from Ashmore JF: Mammalian hearing and the cellular mechanisms of the cochlear amplifier. In Corey DP, Roper SD [eds]: Sensory Transduction. New York, Rockefeller University Press, 1992, pp 396–412.)

waves on the basilar membrane could possibly produce.  N15-16  Recordings from primary auditory nerve cells are also very sharp, implying that this tuning must occur within the cochlea, not in the CNS. Some enhancement of tuning comes from the structure of the inner hair cells themselves. Those near the base have shorter, stiffer stereovilli, which makes them resonate to higher frequencies than possible with the longer, floppier stereovilli on cells near the apex. The blue curve in Figure 15-25 approximates von Békésy’s envelope of traveling waves for a passive basilar membrane from cadavers. It is important to note that von Békésy used unnaturally loud sounds. With reasonable sound levels, the maximum passive displacement of the basilar membrane would be slightly more than 0.1 nm. This distance is less than the pore diameter of an ion channel and also less than the threshold (0.3 to 0.4 nm) for an electrical response from a hair cell. However, measurements from the basilar membrane in living animals (the orange curve in Fig. 15-25) by very sensitive methods show that movements of the basilar membrane are much more localized and much larger than predicted by von Békésy. The maximal physiological displacement is ~20-fold greater than threshold and ~40-fold greater than that predicted by the passive von Békésy model. Moreover, the physiological displacement decays sharply on either side of the peak, >100-fold within ~0.5 mm (recall that the human basilar membrane has a total length of >30 mm). Both the extremely large physiological excursions of the basilar membrane and the exquisitely sharp tuning of the cochlea depend on the cochlear amplifier (see p. 380). Indeed, selectively damaging outer hair cells—with large doses of certain antibiotics, for example—considerably dulls the sharpness of cochlear tuning and dramatically reduces the amplification. The brain can control the tuning of hair cells. Axons that arise in the superior olivary complex in the brainstem

BOX 15-2  Cochlear Implants

T

he most common cause of human deafness is damage to the hair cells of the cochlea.  N15-18  This damage can be caused by genetic factors, a variety of drugs (e.g., some antibiotics, including quinine), chronic exposure to excessively loud sounds, and other types of disease. Even when all hair cells have been destroyed, if the auditory nerve is intact, it is often possible to restore substantial hearing with a cochlear implant. A cochlear implant  N15-19  is essentially an electronic cochlea. Most of the system resides outside the body. The user wears a headpiece with a microphone, which is connected to a small, battery-powered digital speech processor. This processor sends signals to a miniature radio transmitter next to the scalp, which transmits digitally encoded signals— no wires penetrate the skin—to a receiver/decoder that is surgically implanted in the mastoid bone behind the ear. A very thin and flexible set of wires carries the signals through a tiny hole into the basal end of the cochlea, where an array of 8 to 22 electrodes lies adjacent to the auditory nerve endings (where healthy hair cells would normally be) along the cochlea. Each electrode activates a small portion of the auditory nerve axons. The cochlear implant exploits the tonotopic arrangement of auditory nerve fibers. By stimulating near the base of the cochlea, it is possible to trigger a perception of high-frequency sounds; stimulation toward the apex evokes low-frequency sounds. The efficacy of the implant can be extraordinary. Users require training of a few months or longer, and in many cases, they achieve very good comprehension of spoken speech, even as it comes across on a telephone. As the technology and safety of cochlear implants have improved, so has their popularity. By 2010, >200,000 people were using cochlear implants worldwide, ~80,000 of them infants and children. The best candidates for cochlear implants are young children (optimally as young as 1 year) and older children or adults whose deafness was acquired after they learned some speech. Children older than ~7 years and adults whose deafness preceded any experience with speech generally do not fare as well with cochlear implants. The systems of sensory neurons in the brain, including the auditory system, need to experience normal inputs at a young age to develop properly. When the auditory system is deprived of sounds early in life, it can never develop completely normal function even if sensory inputs are restored during adulthood.

synapse mainly on the outer hair cells and, sparsely, on the afferent axons that innervate the inner hair cells.  N15-17  Stimulation of these olivocochlear efferent fibers suppresses the responsiveness of the cochlea to sound and is thought to provide auditory focus by suppressing responsiveness to unwanted sounds—allowing us to hear better in noisy environments (Box 15-2). The main efferent neurotransmitter is acetylcholine (ACh), which activates ionotropic ACh receptors (see pp. 206–207)—nonselective cation channels—and triggers an entry of Ca2+. The influx of Ca2+ activates Ca2+-activated K+ channels, causing a hyperpolarization—effectively an inhibitory postsynaptic potential—that suppresses the electromotility of outer hair cells and action potentials in afferent dendrites. Thus, the efferent axons allow the brain to control the gain of the inner ear.

Chapter 15  •  Sensory Transduction

N15-16  Sharpening of Cochlear Tuning

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N15-17  Central Processing of Auditory Patterns

Contributed by Philine Wangemann Outer hair cells express the motor protein prestin along the lateral cell wall, which is responsible for electromotility. Transduction-mediated depolarization of outer hair cells during upward movements of the basilar membrane causes prestin to contract; this shortens the hair cell body and increases the upward movement of the basilar membrane (see Fig. 15-23). Conversely, hyperpolarization during downward movements of the basilar membrane expands prestin, elongates the outer hair cells, and enlarges the downward movement of the basilar membrane. This electromotility, which amplifies and sharpens the peak of the sound-induced traveling wave, is a prerequisite for the sensitivity of hearing and the ability to sharply discriminate frequencies (see Fig. 15-25).

N15-18  Conductive Hearing Loss

Contributed by Philine Wangemann Auditory patterns are analyzed in the medial geniculate and the auditory cortex. Neurons in these areas are often highly specialized and respond only to a specific frequency and intensity pattern. Interpretation of sound elements requires cortical input beyond the auditory cortex. Central processing is clinically evaluated by auditory brainstem recordings. The coordinated firing of groups of neurons in responses to brief stimuli (clicks or tone pips) produces transient voltage fluctuations that can be detected with surface electrodes. Distinctive voltage fluctuations occur 2 to 12 ms after the stimulus and can be associated with neuronal activity in the auditory pathway including the cochlear nerve, cochlear nucleus, and superior olivary complex.

N15-19  Cochlear Implants

Contributed by Philine Wangemann

Contributed by Emile Boulpaep and Walter Boron

Conductive hearing losses are disorders that compromise the conduction of sound through the external ear, tympanic membrane, or middle ear. Pressure differences across the tympanic membrane (eardrum) can rupture it. Accumulations of fluid in the middle ear can lead to conductive hearing losses that are seen particularly often in children with middle ear infections (otitis media). With proper treatment, the hearing loss due to otitis media is usually self-limited. Otosclerosis, which stiffens the ossicular chain, is another common cause of conductive hearing loss. Treatments for conductive hearing loss encompass a palette of devices including hearing aids and middle ear implants. Hearing aids amplify the sound in the external ear canal. Prosthetic devices can replace the tympanic membrane and the ossicular chain. Middle ear implants are clamped onto the incus and enhance the vibrations of the ossicular chain.

See the following websites for more information on cochlear implants: http://www.nidcd.nih.gov/health/hearing/pages/coch.aspx http://ecs.utdallas.edu/loizou/cimplants/tutorial/

Chapter 15  •  Sensory Transduction

SOMATIC SENSORY RECEPTORS, PROPRIOCEPTION, AND PAIN

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Somatic sensation is the most widespread and diverse of the body’s sensory systems (soma means “body” in Greek). Its receptors are distributed throughout the body instead of being condensed into small and specialized sensory surfaces, as most other sensory systems are arranged. Somatosensory receptors cover the skin, subcutaneous tissue, skeletal muscles, bones and joints, major internal organs, epithelia, and cardiovascular system. These receptors also vary widely in their specificity. The body has mechanoreceptors to transduce pressure, stretch, vibration, and tissue damage; thermoreceptors to gauge temperature; and chemoreceptors to sense a variety of substances. Somatic sensation (or somesthesia) is usually considered to be a combination of at least four sensory modalities: the senses of touch, temperature, body position (proprioception), and pain (nociception).

Mechanoreceptors, which are sensitive to physical distortion such as bending or stretching, account for many of the somatic sensory receptors. They exist throughout our bodies and monitor the following: physical contact with the skin, blood pressure in the heart and vessels, stretching of the gut and bladder, and pressure on the teeth. The transduction site of these mechanoreceptors is one or more unmyelinated axon branches. Our progress in understanding the molecular nature of mechanosensory transduction has been relatively slow. Similar to the transduction process in hair cells, that in cutaneous mechanoreceptive nerve endings probably involves the gating of ion channels. Some of these channels belong to the TRP superfamily (see Table 6-2, family No. 5). Thermoreceptors respond best to changes in temperature, whereas chemoreceptors are sensitive to various kinds of chemical alterations. In the next three sections, we discuss mechanoreceptors, thermoreceptors, and chemoreceptors that are located in the skin.

A variety of sensory endings in the skin transduce mechanical, thermal, and chemical stimuli

Mechanoreceptors in the skin provide sensitivity to specific stimuli such as vibration and steady pressure

To meet a wide array of sensory demands, many kinds of specialized receptors are required. Somatic sensory receptors range from simple bare nerve endings to complex combinations of nerve, muscle, connective tissue, and supporting cells. As we have seen, the other major sensory systems have only one type of sensory receptor or a set of very similar subtypes.

Skin protects us from our environment by preventing evaporation of body fluids, invasion by microbes, abrasion, and damage from sunlight. However, skin also provides our most direct contact with the world. The two major types of mammalian skin are hairy and glabrous. Glabrous skin (or hairless skin) is found on the palms of our hands and fingertips and on the soles of our feet and pads of our toes (Fig. 15-26A).

A

B

GLABROUS (HAIRLESS) SKIN

PACINI’S CORPUSCLE Stimulus

Intact corpuscle

Papillary ridges

Epidermis Most intense stimulus

300

Dermis

Extracellular recording of 200 receptor potential 100 (µV)

Least intense stimulus

Pacini’s corpuscle is a rapidly adapting receptor.

0 Ruffini’s endings

Subcutaneous tissue

5 ms

Brief stimulus Subpapillary plexus

Free nerve endings

Sweat gland

Meissner’s Pacini’s corpuscle corpuscle

Merkel’s disk

Figure 15-26  Sensors in the skin. (Data from Mendelson M, Loewenstein WR: Mechanisms of receptor adaptation. Science 144:554–555, 1964.)

100 ms

Long stimulus

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Hairy skin makes up most of the rest and differs widely in its hairiness. Both types of skin have an outer layer, the epidermis, and an inner layer, the dermis, and sensory receptors innervate both. The receptors in the skin are sensitive to many types of stimuli and respond when the skin is vibrated, pressed, pricked, or stroked, or when its hairs are bent or pulled. These are quite different kinds of mechanical energy, yet we can feel them all and easily tell them apart. Skin also has exquisite sensitivity; for example, we can reliably feel a dot only 0.006 mm high and 0.04 mm across when it is stroked across a fingertip. The standard Braille dot is 167 times higher! The sensory endings in the skin take many shapes, and most of them are named after the 19th-century European histologists who observed them and made them popular. The largest and best-studied mechanoreceptor is Pacini’s corpuscle, which is up to 2 mm long and almost 1 mm in diameter (see Fig. 15-26B). Pacini’s corpuscle is located in the subcutaneous tissue of both glabrous and hairy skin. It has an ovoid capsule with 20 to 70 onion-like, concentric layers of connective tissue and a nerve terminal in the middle. The capsule is responsible for the rapidly adapting response of the Pacini’s corpuscle. When the capsule is compressed, energy is transferred to the nerve terminal, its membrane is deformed, and mechanosensitive channels open. Current flowing through the channels generates a depolarizing receptor potential that, if large enough, causes the axon to fire an action potential (see Fig. 15-26B, left panel). However, the capsule layers are slick, with viscous fluid between them. If the stimulus pressure is maintained, the layers slip past one another and transfer the stimulus energy away so that the underlying axon terminal is no longer deformed and the receptor potential dissipates (see Fig. 15-26B, right panel). When pressure is released, the events reverse themselves and the terminal is depolarized again. In this way, the non-neural covering of Pacini’s corpuscle specializes the corpuscle for sensing of vibrations and makes it almost unresponsive to steady pressure. Pacini’s corpuscle is most sensitive to vibrations of 200 to 300 Hz, and its threshold increases dramatically below 50 Hz and above ~500 Hz. The sensation evoked by stimulation of Pacini’s corpuscle is a poorly localized humming feeling. Werner Loewenstein and colleagues in the 1960s showed the importance of the Pacini corpuscle’s capsule to its frequency sensitivity. With fine microdissection, they were able to strip away the capsule from single corpuscles. They found that the resultant naked nerve terminal is much less sensitive to vibrating stimuli and much more sensitive to steady pressure. Clearly, the capsule modifies the sensitivity of the bare mechanoreceptive axon. The encapsulated Pacini corpuscle is an example of a rapidly adapting sensor, whereas the decapsulated nerve ending behaves like a slowly adapting sensor. Several other types of encapsulated mechanoreceptors are located in the dermis, but none has been studied as well as Pacini’s corpuscle. Meissner’s corpuscles (see Fig. 15-26A) are located in the ridges of glabrous skin and are about one tenth the size of Pacini’s corpuscles. They are rapidly adapting, although less so than Pacini’s corpuscles. Ruffini’s corpuscles resemble diminutive Pacini’s corpuscles and, like Pacini’s corpuscles, occur in the subcutaneous tissue of both

hairy and glabrous skin. Their preferred stimuli might be called “fluttering” vibrations. As relatively slowly adapting receptors, they respond best to low frequencies. Merkel’s disks are also slowly adapting receptors made from a flattened, non-neural epithelial cell that synapses on a nerve terminal. They lie at the border of the dermis and epidermis of glabrous skin. It is not clear whether it is the nerve terminal or epithelial cell that is mechanosensitive. The nerve terminals of Krause’s end bulbs appear knotted. They innervate the border areas of dry skin and mucous membranes (e.g., around the lips and external genitalia) and are probably rapidly adapting mechanoreceptors. The receptive fields of different types of skin receptors vary greatly in size. Pacini’s corpuscles have extremely broad receptive fields (Fig. 15-27A), whereas those of Meissner’s corpuscles (see Fig. 15-27B) and Merkel’s disks are very small. The last two seem to be responsible for the ability of the fingertips to make very fine tactile discriminations. Small receptive fields are an important factor in achieving high spatial resolution. Resolution varies widely, a fact easily demonstrated by measuring the skin’s two-point discrimination. Bend a paper clip into a U shape. Vary the distance between the tips and test how easily you can distinguish the touch of one tip versus two on your palm, your fingertips, your lips, your back, and your foot. To avoid bias, a colleague—rather than you—should apply the stimulus. Compare the results with standardized data (see Fig. 15-27C). The identities of somatosensory transduction molecules remain elusive. A variety of TRP channel subtypes transduce mechanical stimuli in invertebrate species (e.g., Drosophila, Caenorhabditis elegans). In mammals, rapidly adapting ion channels are associated with receptors for light touch, and several of the TRPC channels appear to be involved in sensitivity to light touch in mice. A non-TRP protein named Piezo2 is associated with rapidly adapting mechanosensory currents in mouse sensory neurons, and knocking down the expression of Piezo2 expression causes deficits in touch. Other mechanosensory channels are expressed in some sensory neurons, including TRPA1 and TRPV4, two-pore potassium channels (KCNKs), and degenerin/epithelial sodium channels (especially ASIC1 to ASIC3 and their accessory proteins), but their roles in mammalian mechanosensation are still controversial. One reason it is difficult to identify mechanosensory channels is that they often need to be associated with other cellular components in order to be sensitive to mechanical stimuli. The mechanisms by which mechanical force is transferred from cells and their membranes to mechano­ sensitive channels are unclear. Ion channels may be physically coupled to either extracellular structures (e.g., collagen fibers) or cytoskeletal components (e.g., actin, microtubules) that transfer energy from deformation of the cell to the gating mechanism of the channel. Mechani­ cally gated ion channels of sensory neurons, including those requiring Piezo2, depend on the actin cytoskeleton. Some channels may be sensitive to stress, sheer, or curvature of the lipid bilayer itself and require no other types of anchoring proteins. Other channels may respond to mechanically triggered second messengers such as DAG (acting directly on the channel) or IP3 (acting indirectly via an IP3 receptor).

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Chapter 15  •  Sensory Transduction

A

RECEPTIVE FIELD OF PACINI’S CORPUSCLES

B RECEPTIVE FIELD OF MEISSNER’S CORPUSCLES

C

TWO-POINT DISCRIMINATION ACROSS THE SKIN 4 3 Fingers 2 1 Thumb Palm Forearm Forehead Cheek Nose Upper lip

Upper arm Shoulder Breast Back Belly

Thigh

Pacini’s corpuscles

Meissner’s corpuscles

Calf

Sole Toe 0 5 10 15 20 25 30 35 40 45 50 Two-point discrimination threshold (mm) Figure 15-27  Receptive fields and spatial discrimination of skin mechanoreceptors. A, Each of the two black dots indicates an area of maximal sensitivity of a single Pacini corpuscle. Each blue-green area is the receptive field of a corpuscle (i.e., the corpuscle responds when stimulus strength increases sufficiently anywhere within the area). B, Each dot represents the entire receptive field of a single Meissner corpuscle. Note that the fields are much smaller than in A. C, The horizontal bars represent the minimum distance at which two points can be perceived as distinct at various locations over the body. Spatial discrimination depends on both receptor density and receptive-field size. (A and B, Data from Vallbo AB, Johansson RS: Properties of cutaneous mechanoreceptors in the human hand related to touch sensation. Hum Neurobiol 3:3–14, 1984; C, data from Weinstein S: Intensive and extensive aspects of tactile sensitivity as a function of body part, sex and laterality. In Kenshalo DR [ed]: The Skin Senses. Springfield, IL, Charles C Thomas, 1968.)

Two things determine the sensitivity of spatial dis­ crimination in an area of skin. The first is the size of the receptors’ receptive fields—if they are small, the two tips of your paper clip are more likely to stimulate different sets of receptors. The second parameter that determines spatial discrimination is the density of the receptors in the skin. Indeed, two-point discrimination of the fingertips is better than that of the palm, even though their receptive fields are the same size. The key to finer discrimination in the fingertips is their higher density of receptors. Crowding more receptors into each square millimeter of fingertip has a second advantage: because the CNS receives more information per stimulus, it has a better chance of detecting very small stimuli. Although we rarely think about it, hair is a sensitive part of our somatic sensory system. For some animals, hairs are a major sensory system. Rodents whisk long facial vibrissae (hairs) and feel the texture, distance, and shape of their local environment. Hairs grow from follicles embedded in the skin, and each follicle is richly innervated by free

mechanoreceptive nerve endings that either wrap around it or run parallel to it. Bending of the hair causes deformation of the follicle and surrounding tissue, which stretches, bends, or flattens the nerve endings and increases or decreases their firing frequency. Various mechanoreceptors innervate hair follicles, and they may be either slowly or rapidly adapting.

Separate thermoreceptors detect warmth and cold Neurons are sensitive to changes in temperature, as are all of life’s chemical reactions. Neuronal temperature sensitivity has two consequences: first, neurons can measure temperature; but second, to work properly, most neural circuits need to be kept at a relatively stable temperature. Neurons of the mammalian CNS are especially vulnerable to temperature changes. Whereas skin tissue temperatures can range from 20°C to 40°C without harm or discomfort, brain temperature must be near 37°C to avoid serious dysfunction. The body has complex systems to control brain (i.e., body core)

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temperature tightly (see pp. 1198–1201). Even though all neurons are sensitive to temperature, not all neurons are ther­moreceptors. Because of specific membrane mechanisms, some neurons are extremely sensitive to temperature and seem to be adapted to the job of sensing it. Although many temperature-sensitive neurons are present in the skin, they are also clustered in the hypothalamus and the spinal cord (see pp. 1198–1199). The hypothalamic temperature sensors, like their cutaneous counterparts, are important for regulation of the physiological responses that maintain stable body temperature. Perceptions of temperature apparently reflect warmth and cold receptors located in the skin. Thermoreceptors, like mechanoreceptors, are not spread uniformly across the skin. When you map the skin’s sensitivity to temperature with a small cold or warm probe, you find spots ~1 mm across that are especially sensitive to either warmth or cold, but not to both. In addition, some areas of skin in between are relatively insensitive. The spatial dissociation of the hot and cold maps shows that they are separate submodalities, with separate receptors to encode each. Recordings from single sensory fibers have confirmed this conclusion. The responses of both warmth and cold thermoreceptors adapt during long stimuli, as many sensory receptors commonly do. Most cutaneous thermoreceptors are probably free nerve endings, without obvious specialization. Their axons are small, either unmyelinated C fibers or the smallest-diameter myelinated Aδ fibers (see Table 12-1). We can perceive changes in our average skin temperature of as little as 0.01°C. Within the skin are separate types of thermoreceptors that are sensitive to a range of relatively hot or cold temperatures. Figure 15-28A shows how the steady discharge rate of both types of receptors varies with temperature. Warmth receptors begin firing above ~30°C and increase their firing rate until 44°C to 46°C, beyond which the rate falls off steeply and a sensation of pain begins, presumably mediated by nociceptive endings (see the next section). Cold receptors have a much broader temperature response. They are relatively quiet at skin temperatures of ~40°C, but their steady discharge rate increases as the temperature falls to 24°C to 28°C. Further decreases in temperature cause the steady discharge rate of the cold receptors to decrease until the temperature falls to ~10°C. Below that temperature, firing ceases and cold becomes an effective local anesthetic. In addition to the tonic response just described (i.e., the steady discharge rate), cold receptors also have a phasic response that enables them to report changes in temperature. As shown in Figure 15-28B, when the temperature suddenly shifts from 20.5°C to 15.2°C (both points are to the left of the peak in Fig. 15-28A), the firing rate transiently increases (i.e., the phasic response). However, the new steady-state level is lower, as suggested by the left pair of points in Figure 15-28A. When the temperature suddenly shifts from 35°C to 31.5°C (both points are to the right of the peak in Fig. 15-28A), the firing rate transiently increases, and the new steady-state level is higher, as suggested by the right pair of points in Figure 15-28A. The transduction of relatively warm temperatures is carried out by several types of TRPV channels (specifically TRPV1 to TRPV4—see Table 6-2, family No. 5) expressed in

A

STEADY (TONIC) RESPONSES Warmth receptor fibers

30 Mean steady firing rate 20 (impulses/s) 10

0 15

B

Cold receptor fibers

20 30 40 Skin temperature (°C)

50

TRANSIENT (PHASIC) RESPONSES OF “COLD” FIBERS 35° 31.5°

Firing rate (impulses/s) 20.5°

Temperature “step” 15.2°

Figure 15-28  Temperature sensitivity of cutaneous thermoreceptors.

A, The curves represent the mean steady firing rates of neurons from warmth receptors and cold receptors. B, These data from two experiments on cold receptors show the effects of cooling steps of similar magnitude but starting from different temperatures (20.5°C and 35°C). In both instances, the transient (phasic) responses are the same: an increase in the firing rate. When the starting temperature is 20.5°C, the final firing rate is less than the initial one. However, when the initial temperature is 35°C, the final rate is greater than the initial one. (Data from Somjen GG: Sensory Coding in the Mammalian Nervous System. New York, Appleton-Century-Crofts, 1972.)

thermoreceptors. TRPV1 is a vanilloid receptor—it is activated by the vanilloid class of compounds that includes capsaicin, the pungent ingredient that gives spicy foods their burning quality. Aptly enough, chili peppers taste “hot” because they activate some of the same ion channels that heat itself activates! TRPV1 and TRPV2 channels have painfully high temperature thresholds (~43°C and ~50°C, respectively) and thus help mediate the noxious aspects of thermoreception (see p. 387). Other TRPV channels (TRPV3 and TRPV4) are activated at more moderate temperatures and presumably provide our sensations of warmth. Yet another TRP channel, TRPM8, mediates sensations of moderate cold. TRPM8 channels begin to open at temperatures below ~27°C and are maximally activated at 8°C. In a remarkable analogy to the hot-sensitive TRPV1 channel (the capsaicin receptor), the cool-sensitive TRPM8 channel is a menthol receptor. Menthol evokes sensations of cold because it activates the same ion channel that is opened by cold temperatures.

Chapter 15  •  Sensory Transduction

Nociceptors are specialized sensory endings that transduce painful stimuli Physical energy that is informative at low and moderate levels can be destructive at higher intensity. Sensations of pain motivate us to avoid such situations. Nociceptors are the receptors mediating acutely painful feelings to warn us that body tissue is being damaged or is at risk of being damaged (as the Latin roots imply: nocere [to hurt] + recipere [to receive]). The pain-sensing system is entirely separate from the other modalities we have discussed; it has its own peripheral receptors and a complex, dispersed, chemically unique set of central circuits. Nociceptors are free nerve endings, widely distributed throughout the body. They innervate the skin, bone, muscle, most internal organs, blood vessels, and heart. Ironically, nociceptors are generally absent from the brain substance itself, although they are in the meninges. Nociceptors vary in their selectivity. Mechanical nociceptors, some of which are quite selective, respond to strong pressure—in particular, pressure from sharp objects. A subset of nociceptors expresses Mas-related G protein– coupled receptor D (MrgprD); genetic ablation of just these neurons makes mice insensitive to noxious mechanical stimuli without affecting their responses to painful heat or cold. TRPA1 channels are involved in some forms of painrelated mechanosensation, and they may transduce stimuli that trigger pain originating from viscera such as the colon and bladder. Thermal nociceptors signal either burning heat (above ~45°C, when tissues begin to be destroyed) or unhealthy cold; the heat-sensitive nociceptive neurons express the TRPV1 and TRPV2 channels, whereas the cold-sensitive nociceptors express TRPA1 and TRPM8 channels. A uniquely cold-resistant Na+ channel, Nav1.8, allows cold-sensitive nociceptors to continue firing action potentials even at temperatures low enough to silence other neurons. Chemical nociceptors, which are mechanically insensitive, respond to a variety of agents, including K+, extremes of pH, neuroactive substances such as histamine and bradykinin from the body itself, and various irritants from the environment. Some chemosensitive nociceptors may express TRP channels that respond to, among other things, plant-derived irritants such as capsaicin (TRPV1), menthol (TRPM8), and the pungent derivatives of mustard and garlic (TRPA1). Finally, polymodal nociceptors are single nerve endings that are sensitive to combinations of mechanical, thermal, and chemical stimuli. Nociceptive axons include both fast Aδ fibers and slow, unmyelinated C fibers. Aδ axons mediate sensations of sharp, intense pain; C fibers elicit more persistent feelings of dull, burning pain. The Na+ channel Nav1.7 has a particularly interesting relationship to pain. Patients with loss-of-function mutations of Nav1.7 are insensitive to noxious stimuli and experience repeated injuries because they lack protective reflexes. Several gain-of-function Nav1.7 mutations cause channel hyperexcitability and syndromes of intense chronic pain. Sensations of pain can be modulated in a variety of ways. Skin, joints, or muscles that have been damaged or inflamed are unusually sensitive to further stimuli. This phenomenon

387

Tissue damage creates an "inflammatory soup." Substance P, released from nerve endings, increases capillary permeability and contributes to inflammation.

Skin

Blood vessel

H+

Substance P

Bradykinin Serotonin Lesion K+ Nociceptive axon Axon reflex

Prostaglandins Histamine

NGF

Substance P

Mast cell

Substance P causes mast cells to release histamine, which in turn activates nociceptor endings. Figure 15-29  Hyperalgesia of inflammation.

is called hyperalgesia, and it can be manifested as a reduced threshold for pain, an increase in perceived intensity of painful stimuli, or spontaneous pain. Primary hyperalgesia occurs within the area of damaged tissue, but within ~20 minutes after an injury, tissues surrounding a damaged area may become supersensitive by a process called secondary hyperalgesia. Hyperalgesia seems to involve processes near peripheral receptors (Fig. 15-29) as well as mechanisms in the CNS. Damaged skin releases a variety of chemical substances from its many cell types, blood cells, and nerve endings. These substances—sometimes called the inflammatory soup—include neurotransmitters (e.g., glutamate, serotonin, adenosine, ATP), peptides (e.g., substance P, bradykinin), various lipids (e.g., prostaglandins, endocannabinoids), proteases, neurotrophins, cytokines, and chemokines, K+, H+, and others; they trigger the set of local responses that we know as inflammation. As a result, blood vessels become more leaky and cause tissue swelling (or edema) and redness (see Box 20-1). Nearby mast cells release the chemical histamine, which directly excites nociceptors. By a mechanism called the axon reflex, action potentials can propagate along nociceptive axons from the site of an injury into side branches of the same axon that innervate neighboring regions of skin. The spreading axon branches of the nociceptors themselves may release substances that sensitize nociceptive terminals and make them responsive to

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previously nonpainful stimuli. Such “silent” nociceptors among our small Aδ and C fibers are normally unresponsive to stimuli—even destructive ones. Only after sensitization do they become responsive to mechanical or chemical stimuli and contribute greatly to hyperalgesia. For example, the neurotrophin nerve growth factor (NGF)—part of the inflammatory soup—triggers strong hypersensitivity to heat and mechanical stimuli by modulating TRPV1 channels. Activation of TRPA1 and ASICs are also important in hyperalgesia. The cytokine tumor necrosis factor-alpha (TNF-α) potentiates the inflammatory response directly and enhances release of substances that sensitize nociceptors. Drugs that interfere with neurotrophin and cytokine actions can be effective treatments for the pain of inflammatory diseases. The cognitive sensations of pain are under remarkably potent control by the brain, more so than other sensory system. In some cases, nociceptors may fire wildly, although perceptions of pain are absent; on the other hand, pain may be crippling although nociceptors are silent. Chronic activation of nociceptors can lead to central sensitization, a chronic enhancement of central pain-processing circuits. Prolonged activity in nociceptive axons and their spinal cord synapses causes increased glutamate release, strong activation of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)– and NMDA (N-methyl-D-aspartate)–type glutamate receptors, and eventually a form of long-term potentiation (see pp. 329–337). Nonpainful sensory input and neural activity from various nuclei within the brain can modify pain. For example, pain evoked by activity in nociceptors (Aδ and C fibers) can be reduced by simultaneous activity in low-threshold mechanoreceptors (Aα and Aβ fibers). This phenomenon is a familiar experience—some of the discomfort of a burn, cut, or bruise can be relieved by gentle massage or rubbing (stimulating mechanoreceptors) around the injured area. In 1965, Melzack and Wall proposed that this phenomenon involves a circuit in the spinal cord that can “gate” the transmission of nociceptive information to the brain; control of the gate could be provided by other sensory information (e.g., tactile stimulation) or by descending control from the brain itself. Gate-like regulation of pain may arise from the modulation of gammaaminobutyric acid (GABA)–mediated and glycine-mediated inhibitory circuits in the spinal cord. A second mechanism for modifying the sensation of pain involves the relatively small peptides called endorphins. In the 1970s, it was discovered that a class of drugs called opioids (including morphine, heroin, and codeine) act by binding tightly and specifically to opioid receptors in the brain and, furthermore, that the brain itself manufactures “endogenous morphine-like substances,” collectively called endorphins (see p. 315).

Muscle spindles sense changes in the length of skeletal muscle fibers, whereas Golgi tendon organs gauge the muscle’s force The somatic sensory receptors described thus far provide information about the external environment. However, the body also needs detailed information about itself to know where each of its parts is in space, whether it is moving, and if so, in which direction and how fast. Proprioception

provides this sense of self and serves two main purposes. First, knowledge of the positions of our limbs as they move helps us judge the identity of external objects. It is much easier to recognize an object if you can actively palpate it than if it is placed passively into your hand so that your skin is stimulated but you are not allowed to personally guide your fingers around it. Second, proprioceptive information is essential for accurately guiding many movements, especially while they are being learned. Skeletal muscles, which mediate voluntary movement, have two mechanosensitive proprioceptors: the muscle spindles (or stretch receptors) and Golgi tendon organs (Fig. 15-30). Muscle spindles measure the length and rate of stretch of the muscles, whereas the Golgi tendon organs gauge the force generated by a muscle by measuring the tension in its tendon. Together, they provide a full description of the dynamic state of each muscle. The different sensitivities of the spindle and the tendon organ are due partly to their structures but also to their placement: spindles are located in modified muscle fibers called intrafusal muscle fibers, which are aligned in parallel with the “ordinary” forcegenerating or extrafusal skeletal muscle fibers. On the other hand, Golgi tendon organs are aligned in series with the extrafusal fibers. The Golgi tendon organ consists of bare nerve endings of group Ib axons (see Table 12-1). These endings intimately invest an encapsulated collagen matrix and usually sit at the junction between skeletal muscle fibers and the tendon. When tension develops in the muscle as a result of either passive stretch or active contraction, the collagen fibers tend to squeeze and distort the mechanosensitive nerve endings, triggering them to fire action potentials. The mammalian muscle spindle is a complex of modified skeletal muscle fibers (intrafusal fibers) combined with both afferent and efferent innervation. The spindle does not contribute significant force generation to the muscle but serves a purely sensory function. A simplified summary of the muscle spindle is that it contains two kinds of intrafusal muscle fibers (bag and chain), with two kinds of sensory endings entwined about them (the primary and secondary endings). The different viscoelastic properties of the muscle fibers make them differentially sensitive to the consequences of muscle stretch. Because the primary sensory endings of group Ia axons coil around and strongly innervate individual bag muscle fibers (in addition to chain fibers), they are very sensitive to the dynamics of muscle length (i.e., changes in its length). The secondary sensory endings of group II axons mainly innervate the chain fibers and most accurately transduce the static length of the muscle; in other words, they are slowly adapting receptors. The discharge rate of afferent neurons increases when the whole muscle—and therefore the spindle—is stretched. ENaC and ASIC2 channels may contribute to the stretch sensitivity of the sensory nerve terminals in muscle spindles. What is the function of the motor innervation of the muscle spindle? Consider what happens when the α motor neurons stimulate the force-generating extrafusal fibers and the muscle contracts. The spindle, connected in parallel to the extrafusal fibers, quickly tends to go slack, which makes it insensitive to further changes in length. To avoid this situation and to continue to maintain control over the sensitivity

Chapter 15  •  Sensory Transduction

Muscle spindle (intrafusal fibers) Extrafusal muscle fiber Muscle spindle capsule (cut open)

Golgi tendon organ (GTO) GTO capsule

Spindle afferent neuron Spindle efferent neuron

Neuromuscular synapse from α motor neuron to extrafusal fibers

Neuromuscular synapse from α motor neuron to extrafusal fibers

Collagen fibrils

GTO afferent (Ib) neuron

Tendon

389

Chain fiber Bag fiber Primary afferent (Ia) neuron γ Motor neurons to intrafusal fibers Secondary afferent (II) neuron

Figure 15-30  Golgi tendon organ and muscle spindle fibers. A muscle contains two kinds of muscle fibers, extrafusal fibers (ordinary muscle fibers that cause contraction) and intrafusal fibers (aligned in parallel with the extrafusal fibers). Some of the extrafusal fibers have Golgi tendon organs located in series between the end of the muscle fiber and the macroscopic tendon. The intrafusal fibers contain muscle spindles, which receive both afferent (sensory) and efferent (motor) innervation. The spindle (inset) contains both bag fibers, with nuclei bunched together, and chain fibers, with nuclei in a row.

of the spindle, γ motor neurons cause the intrafusal muscle fibers to contract in parallel with the extrafusal fibers. This ability of the spindle’s intrafusal fibers to change their length as necessary greatly increases the range of lengths over which the spindle can work. It also means that the sensory responses of the spindle depend not only on the length of the whole muscle in which the spindle sits but also on the contractile state of its own intrafusal muscle fibers. Presumably, the ambiguity in this code is sorted out centrally by circuits that simultaneously keep track of the spindle’s sensory output and the activity of its motor nerve supply. In addition to the muscle receptors, various mechanoreceptors are found in the connective tissues of joints, especially within the capsules and ligaments. Many resemble Ruffini, Golgi, and Pacini end organs; others are free nerve endings. They respond to changes in the angle, direction, and velocity of movement in a joint. Most are rapidly adapting,

which means that sensory information about a moving joint is rich. Nerves encoding the resting position of a joint are few. We are nevertheless quite good at judging the position of a joint, even with our eyes closed. It seems that information from joint receptors is combined with that from muscle spindles and Golgi tendon organs, and probably from cutaneous receptors as well, to estimate joint angle. Removal of one source of information can be compensated by use of the other sources. When an arthritic hip is replaced with a steel and plastic one, patients are still able to tell the angle between their thigh and their pelvis, even though all hip joint mechanoreceptors are long gone.

REFERENCES The reference list is available at www.StudentConsult.com.

Chapter 15  •  Sensory Transduction

REFERENCES Books and Reviews Alper SL, Sharma AK: The SLC26 gene family of anion transporters and channels. Mol Aspects Med 34:494–515, 2013. Bowmaker JK: Visual pigments and molecular genetics of color blindness. News Physiol Sci 13:63–69, 1998. Copeland BJ, Pillsbury HC 3rd: Cochlear implantation for the treatment of deafness. Annu Rev Med 55:157–167, 2004. Corey DP: New TRP channels in hearing and mechanosensation. Neuron 39:585–588, 2003. Eatock RA, Songer JE: Vestibular hair cells and afferents: Two channels for head motion signals. Annu Rev Neurosci 34:501–534, 2011. Eijkelkamp N, Quick K, Wood JN: Transient receptor potential channels and mechanosensation. Annu Rev Neurosci 36:519– 546, 2013. Fain GL: Sensory Transduction. Sunderland, MA, Sinauer, 2003. Guinan JJ, Salt A, Cheatham MA: Progress in cochlear physiology after Békésy. Hearing Res 293:12–20, 2012. Hudspeth AJ: How hearing happens. Neuron 19:947–950, 1997. Hunt CC: Mammalian muscle spindle: Peripheral mechanisms. Physiol Rev 70:643–663, 1990. Jordt SE, McKemy DD, Julius D: Lessons from peppers and peppermint: The molecular logic of thermosensation. Curr Opin Neurobiol 13:487–492, 2003. Kazmierczak P, Müller U: Sensing sound: Molecules that orchestrate mechanotransduction by hair cells. Trends Neurosci 35: 220–229, 2011. Kinnamon SC, Margolskee RF: Mechanisms of taste transduction. Curr Opin Neurobiol 6:506–513, 1996. Kral A, Sharma A: Developmental neuroplasticity after cochlear implantation. Trends Neurosci 35:111–122, 2012. Kung C: A possible unifying principle for mechanosensation. Nature 436:647–654, 2005. Lin SY, Corey DP: TRP channels in mechanosensation. Curr Opin Neurobiol 15:350–357, 2005. Lucas RJ: Mammalian inner retinal photoreception. Curr Biol 23:R125–R133, 2013. Mattes RD: Accumulating evidence supports a taste component for free fatty acids in humans. Physiol Behav 104:624–631, 2011. Mombaerts P: Genes and ligands for odorant, vomeronasal and taste receptors. Nat Rev Neurosci 5:263–278, 2004. Nathans J: In the eye of the beholder: Visual pigments and inherited variation in human vision. Cell 78:357–360, 1994. Nobili R, Mammano F, Ashmore J: How well do we understand the cochlea? Trends Neurosci 21:159–167, 1998. Peng AW, Salles FT, Pan B, Ricci AJ: Integrating the biophysical and molecular mechanisms of auditory hair cell mechanotransduction. Nat Commun 2:523, 2011. Santos-Sacchi J: New tunes from Corti’s organ: The outer hair cell boogie rules. Curr Opin Neurobiol 13:459–468, 2003. Schnetkamp PPM: The SLC24 gene family of Na+/Ca2+–K+ exchangers: From sight and smell to memory consolidation and skin pigmentation. Mol Aspects Med 34:455–464, 2013.

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Shepherd GM: Neurogastronomy: How the Brain Creates Flavor and Why It Matters. New York, Columbia University Press, 2011, p 288. Tsunozaki M, Bautista DM: Mammalian somatosensory mechanotransduction. Curr Opin Neurobiol 19:1–8, 2009. Journal Articles Buck L, Axel R: A novel multigene family may encode odorant receptors: A molecular basis for odor recognition. Cell 65:175– 187, 1991. Chandrashekar J, Kuhn C, Oka Y, et al: The cells and peripheral representation of sodium taste in mice. Nature 464:297–301, 2010. Coste B, Mathur J, Schmidt M, et al: Piezo1 and Piezo2 are essential components of distinct mechanically activated cation channels. Science 330:55–60, 2010. Crawford AC, Fettiplace R: Auditory nerve responses to imposed displacements of the turtle basilar membrane. Hear Res 12:199– 208, 1983. Eijkelkamp N, Linley JE, Torres JM, et al: A role for Piezo2 in EPAC1-dependent mechanical allodynia. Nat Commun 4:1682, 2013. Hecht S, Shlaer S, Pirenne MH: Energy, quanta, and vision. J Gen Physiol 25:819–840, 1942. Hudspeth AJ: How hearing happens. Neuron 19:947–950, 1997. Ishimaru Y, Inada H, Kubota M, et al: Transient receptor potential family members PKD1L3 and PKD2L1 form a candidate sour taste receptor. Proc Natl Acad Sci U S A 103:12569–12574, 2006. Jianga P, Josuea J, Lia X, et al: Major taste loss in carnivorous mammals. Proc Natl Acad Sci U S A 109:4956–4961, 2012. Kawaguchi H, Yamanaka A, Uchida K, et al: Activation of polycystic kidney disease-2-like 1 (PKD2L1)-PKD1L3 complex by acid in mouse taste cells. J Biol Chem 285:17277–17281, 2010. Liberman MC, Gao J, He DZ, et al: Prestin is required for electromotility of the outer hair cell and for the cochlear amplifier. Nature 419:300–304, 2002. Nakamura T, Gold GH: A cyclic nucleotide–gated conductance in olfactory receptor cilia. Nature 325:442–444, 1987. Nelson G, Hoon MA, Chandrashekar J, et al: Mammalian sweet taste receptors. Cell 106:381–390, 2001. Oka Y, Butnaru M, von Buchholtz L, et al: High salt recruits aversive taste pathways. Nature 494:472–475, 2013. Taruno A, Vingtdeux V, Ohmoto M, et al: CALHM1 ion channel mediates purinergic neurotransmission of sweet, bitter and umami tastes. Nature 495:223–226, 2013. Yu Y, Ulbrich MH, Li M-H, et al: Molecular mechanism of the assembly of an acid-sensing receptor ion channel complex. Nat Commun 3:1252, 2012. Zimmerman K, Leffler A, Babes A, et al: Sensory neuron sodium channel Nav1.8 is essential for pain at low temperatures. Nature 447:855–858, 2007. Zhao H, Ivic L, Otaki JM, et al: Functional expression of a mammalian odorant receptor. Science 279:237–242, 1998. Zhao GQ, Zhang Y, Hoon MA, et al: The receptors for mammalian sweet and umami taste. Cell 115:255–266, 2003.

C H A P T E R 16  CIRCUITS OF THE CENTRAL NERVOUS SYSTEM Barry W. Connors

ELEMENTS OF NEURAL CIRCUITS Neural circuits process sensory information, generate motor output, and create spontaneous activity A neuron never works alone. Even in the most primitive nervous systems, all neurons participate in synaptically interconnected networks called circuits. In some hydrozoans (small jellyfish), the major neurons lack specialization and are multifunctional. They serve simultaneously as photodetectors, pattern generators for swimming rhythms, and motor neurons. Groups of these cells are repetitively interconnected by two-way electrical synapses into simple ringlike arrangements, and these networks coordinate the rhythmic contraction of the animal’s muscles during swimming. This simple neural network also has the flexibility to command defensive changes in swimming patterns when a shadow passes over the animal. Thus, neuronal circuits have profound advantages over unconnected neurons. In more complex animals, each neuron within a circuit may have very specialized properties. By the interconnection of various specialized neurons, even a simple neuronal circuit may accomplish astonishingly intricate functions. Some neural circuits may be primarily sensory (e.g., the retina) or motor (e.g., the ventral horns of the spinal cord). Many circuits combine features of both, with some neurons dedicated to providing and processing sensory input, others to commanding motor output, and many neurons (perhaps most) doing both. Neural circuits may also generate their own intrinsic signals, with no need for any sensory or central input to activate them. The brain does more than just respond reflexively to sensory input, as a moment’s introspection will amply demonstrate. Some neural functions—such as walking, running, breathing, chewing, talking, and piano playing—require precise timing, with coordination of rhythmic temporal patterns across hundreds of outputs. These basic rhythms may be generated by neurons and neural circuits called pacemakers because of their clock-like capabilities. The patterns and rhythms generated by a pacemaking circuit can always be modulated—stopped, started, or altered—by input from sensory or central pathways. Neuronal circuits that produce rhythmic motor output are sometimes called central pattern generators; we discuss these in a section below. This chapter introduces the basic principles of neural circuits in the mammalian central nervous system (CNS). We 390

describe a few examples of specific systems in detail to illuminate general principles as well as the diversity of neural solutions to life’s complex problems. However, this topic is enormous, and we have necessarily been selective and somewhat arbitrary in our presentation.

Nervous systems have several levels of organization The function of a nervous system is to generate adaptive behaviors. Because different species face unique problems, we expect brains to differ in their organization and mechanisms. Nevertheless, certain principles apply to most nervous systems. It is useful to define various levels of organization.  N16-1  We can analyze a complex behavior—reading the words on this page—in a simple way, with progressively finer detail, down to the level of ion channels, receptors, messengers, and the genes that control them. At the highest level, we recognize neural subsystems and pathways (see Chapter 10), which in this case include the sensory input from the retina (see Chapter 15) leading to the visual cortex, the central processing regions that make sense of the visual information and the motor systems that coordinate movement of the eyes and head. Many of these systems can be recognized in the gross anatomy of the brain. Each specific brain region is extensively interconnected with other regions that serve different primary functions. These regions tend to have profuse connections that send information in both directions along most sensory/central motor pathways. The advantages of this complexity are obvious; while you are interpreting visual information, for example, it can be very useful simultaneously to analyze sound and to know where your eyes are pointing and how your body is oriented. The systems of the brain can be more deeply understood by studying their organization at the cellular level. Within a local brain region, the arrangement of neurons and their synaptic connections is called a local circuit. A local circuit typically includes the set of inputs, outputs, and all the interconnected neurons that are essential to functions of the local brain region. Many regions of the brain are composed of a large number of stereotyped local circuits, almost modular in their interchangeability, that are themselves interconnected. Within the local circuits are finer arrangements of neurons and synapses sometimes called microcircuits. Microcircuits may be repeated numerous times within a local circuit, and they determine the transformations of

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N16-1  Levels of Organization of the Nervous System Contributed by Barry Connors A

BEHAVIOR

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SYSTEMS AND PATHWAYS

LOCAL CIRCUITS

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eFigure 16-1  (Data from Shepherd GM: Neurobiology, 3rd ed. New York, Oxford University Press, 1994.)

CHAPTER 16  •  Circuits of the Central Nervous System

information that occur within small areas of dendrites and the collection of synapses impinging on them. At even finer resolution, neural systems can be understood by the properties of their individual neurons (see Chapter 12), synapses, membranes, molecules (e.g., neurotransmitters and neuromodulators), and ions as well as the genes that encode and control the system’s molecular biology.

Most local circuits have three elements: input axons, interneurons, and projection (output) neurons One of the most fascinating things about the nervous system is the wide array of different local circuits that have evolved for different behavioral functions. Despite this diversity, we can define a few general components of local circuits, which we illustrate with two examples from very different parts of the CNS: the ventral horn of the spinal cord and the cerebral neocortex. Some of the functions of these circuits are described in subsequent sections; here, we examine their cellular anatomy. All local circuits have some form of input, which is usually a set of axons that originate elsewhere and terminate in synapses within the local circuit. A major input to the spinal cord (Fig. 16-1) is the afferent sensory axons in the dorsal roots. These axons carry information from somatic sensory receptors in the skin, connective tissue, and muscles (see pp. 383–389). However, local circuits in the spinal cord also have many other sources of input, including descending input from the brain and input from the spinal cord itself, both from the contralateral side and from spinal segments

Dorsal horn

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above and below. Input to the local circuits of the neocortex (Fig. 16-2) is also easily identified; relay neurons of the thalamus send axons into particular layers of the cortex to bring a range of information about sensation, motor systems, and the body’s internal state. By far, the most numerous type of input to the local circuits of the neocortex comes from the neocortex itself—from adjacent local circuits, distant areas of cortex, and the contralateral hemisphere. These two systems illustrate a basic principle: local circuits receive multiple types of input. Output is usually achieved with a subset of cells known as projection neurons, or principal neurons, which send axons to one or more targets. The most obvious spinal output comes from the α motor neurons, which send their axons out through the ventral roots to innervate skeletal muscle fibers. Output axons from the neocortex come mainly from large pyramidal neurons in layer V, which innervate many targets in the brainstem, spinal cord, and other structures, as well as from neurons in layer VI, which make their synapses back onto the cells of the thalamus. However, as was true with inputs, most local circuits have multiple types of outputs. Thus, spinal neurons innervate other regions of the spinal cord and the brain, whereas neocortical circuits make most of their connections to other neocortical circuits. Rare, indeed, is the neural circuit that has only input and output cells. Local processing is achieved by additional neurons whose axonal connections remain within the local circuit. These neurons are usually called interneurons or intrinsic neurons. Interneurons vary widely in structure and function, and a single local circuit may have many different

Dorsal root

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Figure 16-1  Local circuits in the spinal cord. A basic local circuit in the spinal cord consists of inputs (e.g., sensory axons of the dorsal roots), interneurons (both excitatory and inhibitory), and output neurons (e.g., α motor neurons that send their axons through the ventral roots).

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I II and III

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From To other Outside thalamus neocortex neocortex (input) and thalamus Figure 16-2  Local circuits in the neocortex. A basic local circuit in the neocortex consists of inputs (e.g., afferent axons from the thalamus), excitatory and inhibitory interneurons, and output neurons (e.g., pyramidal cells).

types. Both the spinal cord and neocortex have excitatory and inhibitory interneurons, interneurons that make very specific or widely divergent connections, and interneurons that either receive direct contact from input axons or process only information from other interneurons. In many parts of the brain, interneurons vastly outnumber output neurons. To take an extreme example, the cerebellum has ~1011 granule cells—a type of excitatory interneuron—which is more than the total number of all other types of neurons in the entire brain! The “principles” of local circuits outlined here have many variations. For example, a projection cell may have some of the characteristics of an interneuron, as when a branch of its output axon stays within the local circuit and makes synaptic connections. This branching is the case for the projection cells of both the neocortex (pyramidal cells) and the spinal cord (α motor neurons). On the other hand, some interneurons may entirely lack an axon and instead make their local synaptic connections through very short neurites or even dendrites. In some rare cases, the source of the input to a local circuit may not be purely synaptic but chemical (as with CO2-sensitive neurons in the medulla; see p. 714) or physical (as with temperature-sensitive neurons in the hypothalamus; see p. 1199). Although the main neurons within a generic local circuit are wired in series (see Figs. 16-1 and 16-2), local circuits, often in massive numbers, operate in parallel with one another. Furthermore, these circuits usually

demonstrate a tremendous amount of crosstalk; information from each circuit is shared mutually, and each circuit continually influences neighboring circuits. Indeed, one of the things that makes analysis of local neural circuits so exceptionally difficult is that they operate in highly interactive, simultaneously interdependent, and expansive networks.

SIMPLE, STEREOTYPED RESPONSES: SPINAL REFLEX CIRCUITS Passive stretching of a skeletal muscle causes a reflexive contraction of that same muscle and relaxation of the antagonist muscles Reflexes are among the most basic of neural functions and involve some of the simplest neuronal circuits. A motor reflex is a rapid, stereotyped motor response to a particular sensory stimulus. Although the existence of reflexes had been long appreciated, it was Sir Charles Sherrington  N10-2  who, beginning in the 1890s, first defined the anatomical and physiological bases for some simple spinal reflexes. So meticulous were Sherrington’s observations of reflexes and their timing that they offered him compelling evidence for the existence of synapses, a term he originated. Reflexes are essential, if rudimentary, elements of behavior. Because of their relative simplicity, more than a century of research has taught us a lot about their biological basis. However, reflexes are also important for understanding more complex behaviors. Intricate behaviors may sometimes be built up from sequences of simple reflexive responses. In addition, neural circuits that generate reflexes almost always mediate or participate in much more complex behaviors. Here we examine a relatively well understood example of reflex-mediating circuitry. The CNS commands the body to move about by activating motor neurons, which excite skeletal muscles (Sherrington called motor neurons the final common path). Motor neurons receive synaptic input from many sources within the brain and spinal cord, and the output of large numbers of motor neurons must be closely coordinated to achieve even uncomplicated actions such as walking. However, in some circumstances, motor neurons can be commanded directly by a simple sensory stimulus—muscle stretch—with only the minimum of neural machinery intervening between the sensory cell and motor neuron: one synapse. Understanding of this simplest of reflexes, the stretch reflex or myotatic reflex, first requires knowledge of some anatomy. Each motor neuron, with its soma in the spinal cord or brainstem, commands a group of skeletal muscle cells; a single motor neuron and the muscle cells that it synapses on are collectively called a motor unit (see pp. 241–242). Each muscle cell belongs to only one motor unit. The size of motor units varies dramatically and depends on muscle function. In small muscles that generate finely controlled movements, such as the extraocular muscles of the eye, motor units tend to be small and may contain just a few muscle fibers. Large muscles that generate strong forces, such as the gastrocnemius muscle of the leg, tend to have large motor units with as many as several thousand muscle fibers. There are two types of motor neurons (see Table 12-1): α motor neurons

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Inhibitory interneuron

Primary sensory neuron Excitatory synapse

The reflex stimulates— in the spinal cord—the motor neurons to the extensor muscle...

Ia axon from the stretch receptor Inhibitory Spinal α Motor synapse cord neuron Ventral cell bodies root

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...and inhibits the motor neurons to the flexor muscle. Patellar tendon

Flexor (semitendinosus) Extensor (quadriceps)

Knee jerk

Figure 16-3  Knee-jerk (myotatic) reflex. Tapping the patellar tendon with a percussion hammer elicits a reflexive knee jerk caused by contraction of the quadriceps muscle: the stretch reflex. Stretching the tendon pulls on the muscle spindle, exciting the primary sensory afferents, which convey their information via group Ia axons. These axons make monosynaptic connections to the α motor neurons that innervate the quadriceps, resulting in the contraction of this muscle. The Ia axons also excite inhibitory interneurons that recip­ rocally innervate the motor neurons of the antagonist muscle of the quadriceps (the flexor), resulting in relaxation of the semitendinosus muscle. Thus, the reflex relaxation of the antagonistic muscle is polysynaptic.

innervate the main force-generating muscle fibers (the extrafusal fibers), whereas γ motor neurons innervate only the fibers of the muscle spindles. The group of all motor neurons innervating a single muscle is called a motor neuron pool (see pp. 241–242). When a skeletal muscle is abruptly stretched, a rapid, reflexive contraction of the same muscle often occurs. The contraction increases muscle tension and opposes the stretch. This stretch reflex is particularly strong in physiological extensor muscles—those that resist gravity—and it is sometimes called the myotatic reflex because it is specific for the same muscle that is stretched. The most familiar version is the knee jerk, which is elicited by a light tap on the patellar tendon. The tap deflects the tendon, which then pulls on and briefly stretches the quadriceps femoris muscle. A reflexive contraction of the quadriceps quickly follows (Fig. 16-3). Stretch reflexes are also easily demonstrated in the biceps of the arm and the muscles that close the jaw. Sherrington

showed that the stretch reflex depends on the nervous system and requires sensory feedback from the muscle. For example, cutting the dorsal (sensory) roots to the lumbar spinal cord abolishes the stretch reflex in the quadriceps muscle. The basic circuit for the stretch reflex begins with the primary sensory axons from the muscle spindles (see p. 388) in the muscle itself. Increasing the length of the muscle stimulates the spindle afferents, particularly the large group Ia axons from the primary sensory endings. In the spinal cord, these group Ia sensory axons terminate monosynaptically onto the α motor neurons that innervate the same (i.e., the homonymous) muscle from which the group Ia axons originated. Thus, stretching a muscle causes rapid feedback excitation of the same muscle through the minimum possible circuit: one sensory neuron, one central synapse, and one motor neuron (Box 16-1). Monosynaptic connections account for much of the rapid component of the stretch reflex, but they are only the

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BOX 16-1  Motor System Injury

T

he motor control systems, because of their extended anatomy, are especially susceptible to damage from trauma or disease. The nature of a patient’s motor deficits often allows the neurologist to diagnose the site of neural damage with great accuracy. When injury occurs to lower parts of the motor system, such as motor neurons or their axons, deficits may be very localized. If the motor nerve to a muscle is damaged, that muscle may develop paresis (weakness) or complete paralysis (loss of motor function). When motor axons cannot trigger contractions, there can be no reflexes (areflexia). Normal muscles are slightly contracted even at rest—they have some tone. If their motor nerves are transected, muscles become flaccid (atonia) and eventually develop profound atrophy (loss of muscle mass) because of the absence of trophic influences from the nerves. Motor neurons normally receive strong excitatory influences from the upper parts of the motor system, including regions of the spinal cord, the brainstem, and the cerebral cortex. When upper regions of the motor system are injured by stroke, trauma, or demyelinating disease, for example, the signs and symptoms are distinctly different from those caused by lower damage. Complete transection of the spinal cord leads to profound paralysis below the level of the lesion. This is called paraplegia when only both legs are selectively affected, hemiplegia when one side of the body is affected, and quadriplegia when the legs, trunk, and arms are involved. For a few days after an acute injury, there is also areflexia and reduced muscle tone (hypotonia), a condition called spinal shock. The muscles are limp and cannot be controlled by the brain or by the remaining circuits of the spinal cord. Spinal shock is temporary; after days to months, it is replaced by both an exaggerated muscle tone (hypertonia) and heightened stretch reflexes (hyperreflexia) with related signs—this combination is called spasticity. The biological mechanisms of spasticity are poorly understood, although the hypertonia is the consequence of tonically overactive stretch reflex circuitry, driven by spinal neurons that have become chronically hyperexcitable.

beginning of the story. At the same time the stretched muscle is being stimulated to contract, parallel circuits are inhibiting the α motor neurons of its antagonist muscles (i.e., those muscles that move a joint in the opposite direction). Thus, as the knee-jerk reflex causes contraction of the quadriceps muscle, it simultaneously causes relaxation of its antagonists, including the semitendinosus muscle (see Fig. 16-3). To achieve inhibition, branches of the group Ia sensory axons excite specific interneurons that inhibit the α motor neurons of the antagonists. This reciprocal innervation increases the effectiveness of the stretch reflex by minimizing the antagonistic forces of the antagonist muscles.

Force applied to the Golgi tendon organ regulates muscle contractile strength Skeletal muscle contains another mechanosensory transducer in addition to the stretch receptor: the Golgi tendon organ (see p. 388). Tendon organs are aligned in series with the muscle; they are exquisitely sensitive to the tension

within a tendon and thus respond to the force generated by the muscle rather than to muscle length. Tendon organs may respond during passive muscle stretch, but they are stimulated particularly well during active contractions of a muscle. The group Ib sensory axons of the tendon organs excite both excitatory and inhibitory interneurons within the spinal cord (Fig. 16-4). In some cases, this interneuron circuitry inhibits the muscle in which tension has increased and excites the antagonistic muscle; therefore, activity in the tendon organs can yield effects that are almost the opposite of the stretch reflex. Under other circumstances, particularly during rapid movements such as locomotion, sensory input from Golgi tendon organs actually excites the motor neurons activating the same muscle. The reflex effects of Golgi tendon organ activity vary because the interneurons receiving input from Ib axons also receive input from other sensory endings in the muscle and skin, and from axons descending from the brain. In general, reflexes mediated by the Golgi tendon organs serve to control the force within muscles and the stability of particular joints.

Noxious stimuli can evoke complex reflexive movements Sensations from the skin and connective tissue can also evoke strong spinal reflexes. Imagine walking on a beach and stepping on a sharp piece of shell. Your response is swift and coordinated and does not require thoughtful reflection: you rapidly withdraw the wounded foot by activating the leg flexors and inhibiting the extensors. To keep from falling, you also extend your opposite leg by activating its extensors and inhibiting its flexors (Fig. 16-5). This response is an example of a flexion-withdrawal reflex. The original stimulus for the reflex came from fast pain afferent neurons in the skin, primarily the group Aδ axons. This bilateral flexor reflex response is coordinated by sets of inhibitory and excitatory interneurons within the spinal gray matter. Note that this coordination requires circuitry not only on the side of the cord ipsilateral to the wounded side but also on the contralateral side. That is, while you withdraw the foot that hurts, you must also extend the opposite leg to support your body weight. Flexor reflexes can be activated by most of the various sensory afferents that detect noxious stimuli. Motor output spreads widely up and down the spinal cord, as it must to orchestrate so much of the body’s musculature into an effective response. A remarkable feature of flexor reflexes is their specificity. Touching a hot surface, for example, elicits reflexive withdrawal of the hand in the direction opposite the side of the stimulus, and the strength of the reflex is related to the intensity of the stimulus. Unlike simple stretch reflexes, flexor reflexes coordinate the movement of entire limbs and even pairs of limbs. Such coordination requires precise and widespread wiring of the spinal interneurons.

Spinal reflexes are strongly influenced by control centers within the brain Axons descend from numerous centers within the brainstem and the cerebral cortex and synapse primarily on the spinal interneurons, with some direct input to the motor neurons.

CHAPTER 16  •  Circuits of the Central Nervous System

Dorsal root ganglion

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Inhibitory interneuron

Primary sensory neuron

Ib axon from Golgi tendon organ The reflex inhibits—in the spinal cord—the motor neurons to the extensor muscle...

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α Motor neuron axons ...and stimulates the motor neurons to the flexor muscle. Flexor (semitendinosus) Extensor (quadriceps)

Patellar tendon Golgi tendon organ Figure 16-4  Golgi tendon organ reflex. Contraction of the quadriceps muscle can elicit a reflexive relaxation of this muscle and contraction of the antagonistic semitendinosus muscle. Contraction of the muscle pulls on the tendon; this squeezes and excites the sensory endings of the Golgi tendon organ, which convey their information via group Ib axons. These axons synapse on both inhibitory and excitatory interneurons in the spinal cord. The inhibitory interneurons innervate α motor neurons to the quadriceps, relaxing this muscle. The excitatory interneurons innervate α motor neurons to the antagonistic semitendinosus muscle, contracting it. Thus, both limbs of the reflex are polysynaptic.

This descending control is essential for all conscious (and much unconscious) command of movement, a topic beyond the scope of this chapter. Less obvious is that the descending pathways can alter the strength of reflexes. For example, to heighten an anxious patient’s stretch reflexes, a neurologist will sometimes ask the patient to perform the Jendrassik maneuver. The patient clasps his or her hands together and pulls; while the patient is distracted with that task, the examiner tests the stretch reflexes of the leg. Another example of the brain’s modulation of a stretch reflex occurs when you catch a falling ball. If a ball were to fall unexpectedly from the sky and hit your outstretched hand, the force applied to your arm would cause a rapid stretch reflex—contraction in the stretched muscles and reciprocal inhibition in the antagonist muscles. The result would be that your hand would slap the ball back up into the air. However, if you anticipate catching the falling ball, for a short period around the time of impact (about ±60 ms), both your stretched muscles and the antagonist muscles contract! This maneuver stiffens your arm just when you need to squeeze that ball to avoid dropping it. Stretch reflexes of the leg also vary dramatically during each step as we walk, thereby facilitating movement of the legs. Like stretch reflexes, flexor reflexes can also be strongly affected by descending pathways. With mental effort,

painful stimuli can be tolerated and withdrawal reflexes sup­pressed. On the other hand, anticipation of a painful stimulus may heighten the vigor of a withdrawal reflex when the stimulus actually arrives. Most of the brain’s influence on spinal circuitry is achieved by control of the many spinal interneurons. Spinal reflexes are frequently studied in isolation from one another, and textbooks often describe them this way. However, under realistic conditions, many reflex systems operate simultaneously, and motor output from the spinal cord depends on interactions among them as well as on the state of controlling influences descending from the brain. It is now well accepted that reflexes do not simply correct for external perturbations of the body; in addition, they play a key role in the control of all movements. The neurons involved in reflexes are the same neurons that generate other behaviors. Think again of the flexor response to the sharp shell—the pricked foot is withdrawn while the opposite leg extends. Now imagine that a crab pinches that opposite foot—you respond with the opposite pattern of withdrawal and extension. Repeat this a few times, crabs pinching you left and right, and you have achieved the basic pattern necessary for walking! Indeed, rhythmic locomotor patterns use components of these same spinal reflex circuits, as discussed next.

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Cutaneous afferent fiber from the nociceptor (Aδ) Extensor (quadriceps) In the leg that feels the pain, the reflex inhibits— in the spinal cord—the motor neurons to the extensor muscle… …and stimulates the motor neurons to the flexor muscle.

Flexor (semitendinosus) In the opposite leg, the reflex stimulates—in the spinal cord—the motor neurons to the extensor muscle… Extensor (quadriceps)

Stimulated leg withdraws Cutaneous receptor for sharp pain

…and inhibits the motor neurons to the flexor muscle.

Opposite leg supports

Figure 16-5  Flexion-withdrawal reflex. A painful stimulus to the right foot elicits a reflexive flexion of the right knee and an extension of the left knee. The noxious stimulus activates nociceptor afferents, which convey their information via group Aδ axons. These axons synapse on both inhibitory and excitatory interneurons. The inhibitory interneurons that project to the right side of the spinal cord innervate α motor neurons to the quadriceps and relax this muscle. The excitatory interneurons that project to the right side of the spinal cord innervate α motor neurons to the antagonistic semitendinosus muscle and contract it. The net effect is a coordinated flexion of the right knee. Similarly, the inhibitory interneurons that project to the left side of the spinal cord innervate α motor neurons to the left semitendinosus muscle and relax this muscle. The excitatory interneurons that project to the left side of the spinal cord innervate α motor neurons to the left quadriceps and contract it. The net effect is a coordinated extension of the left knee.

RHYTHMIC ACTIVITY: CENTRAL PATTERN GENERATORS Central pattern generators in the spinal cord can create a complex motor program even without sensory feedback A common feature of motor control is the motor program, a set of structured muscle commands that are determined by the nervous system before a movement begins and that can be sent to the muscles with the appropriate timing so that a sequence of movements occurs without any need for sensory feedback. The best evidence for the existence of motor programs is that the brain or spinal cord can command a variety of voluntary and automatic movements, such as walking and breathing (see pp. 706–709), even in the complete absence

of sensory feedback from the periphery. The existence of motor programs certainly does not mean that sensory information is unimportant; on the contrary, motor behavior without sensory feedback is always different from that with normal feedback. The neural circuits responsible for various motor programs have been defined in a wide range of species. Although the details vary endlessly, certain broad principles emerge, even when vertebrates and invertebrates are compared. Here we focus on central pattern generators, wellstudied circuits that underlie many of the rhythmic motor activities that are central to animal behavior. Rhythmic behavior includes walking, running, swimming, breathing, chewing, certain eye movements, shivering, and even scratching. The central pattern generators driving each of these activities share certain basic properties. At their core is a set of cyclic, coordinated timing signals that are

CHAPTER 16  •  Circuits of the Central Nervous System

A

ALTERNATING CONTRACTIONS IN A SINGLE LIMB

B

LH LF RH RF

When the extensors contract…

the flexors relax…

…and the foot is planted.

LH

RH

LF

RF

Extensors (LH)

Flexors (LH)

Foot planted Foot lifted

LH LF RH RF LH LF RH RF LH LF RH RF

397

STEPPING PATTERN OF A CAT DURING VARIOUS GAITS WALK

Foot planted

Foot lifted

TROT

PACE

GALLOP

Time

Figure 16-6  Rhythmic patterns during locomotion. A, The experimental tracings are electromyograms (EMGs)—extracellular recordings of the electrical activity of muscles—from the extensor and flexor muscles of the left hind limb of a walking cat. The pink bars indicate that the foot is lifted; the purple bars indicate that the foot is planted. B, The walk, trot, pace, and gallop not only represent different patterns and frequencies of planting and lifting for a single leg but also different patterns of coordination among the legs. LF, left front; LH, left hind; RF, right front; RH, right hind. (Data from Pearson K: The control of walking. Sci Am 2:72–86, 1976.)

generated by a cluster of interconnected neurons. These basic signals are used to command as many as several hundred muscles, each precisely contracting or relaxing during a particular phase of the cycle; for example, with each walking step, the knee must first be flexed and then extended. Figure 16-6A shows how the extensor and flexor muscles of the left hind limb of a cat contract rhythmically—and out of phase with one another—while the animal walks. Rhythms must also be coordinated with other rhythms; for humans to walk, one leg must move forward while the other thrusts backward, then vice versa, and the arms must swing in time with the legs, but with the opposite phase. For four-footed animals, the rhythms are even more complicated and must be able to accommodate changes in gait (see Fig. 16-6B). For coordination to be achieved among the various limbs, sets of central pattern generators must be interconnected. The motor patterns must also have great flexibility so that they can be altered on a moment’s notice—consider the adjustments necessary when one foot strikes an obstacle while walking or the changing motor patterns necessary to go from walking, to trotting, to running, to jumping. Finally, reliable methods must be available for regulating the speed of the patterns and for turning them on and off. The central pattern generators for some rhythmic functions, such as breathing, are in the brainstem (see p. 706). Surprisingly, those responsible for locomotion reside in the spinal cord itself. Even with the spinal cord transected so that the lumbar segments are isolated from all higher centers, cats on a treadmill can generate well-coordinated stepping

movements. Furthermore, stimulation of sensory afferents or descending tracts can induce the spinal pattern generators in four-footed animals to switch rapidly from walking, to trotting, to galloping patterns by altering not only the frequency of motor commands but also their pattern and coordination. During walking and trotting and pacing, the hind legs alternate their movements, but during galloping, they both flex and extend simultaneously (compare the different leg patterns in Fig. 16-6B). Grillner and colleagues showed that each limb has at least one central pattern generator. If one leg is prevented from stepping, the other continues stepping normally. Under most circumstances, the various spinal pattern generators are coupled to one another, although the nature of the coupling must change to explain, for example, the switch from trotting to galloping patterns.

Pacemaker cells and synaptic interconnections both contribute to central pattern generation How do neural circuits generate rhythmic patterns of activity? There is no single answer, and different circuits use different mechanisms. The simplest pattern generators are single neurons whose membrane characteristics endow them with pacemaker properties that are analogous to those of cardiac muscle cells (see p. 489) and smooth muscle cells (see p. 244). Even when experimentally isolated from other neurons, pacemaker neurons may be able to generate rhythmic activity by relying only on their intrinsic membrane conductances (see Fig. 12-4). It is easy to imagine how

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SECTION III  •  The Nervous System

Excitatory interneuron

Flexor motor neuron

Inhibitory interneurons

Tonic descending or afferent input

Excitatory interneuron

Extensor motor neuron

Figure 16-7  Half-center model for alternating rhythm generation in flexor and extensor motor neurons. Stimulating the upper excitatory interneuron has two effects. First, the stimulated excitatory interneuron excites the motor neuron to the flexor muscle. Second, the stimulated excitatory interneuron excites an inhibitory interneuron, which inhibits the lower pathway. Stimulating the lower excitatory interneuron has the opposite effects. Thus, when one motor neuron is active, the opposite one is inhibited.

intrinsic pacemaker neurons might act as the primary rhythmic driving force for sets of motor neurons that in turn command cyclic behavior. Among vertebrates, however, pacemaker neurons may contribute to some central pattern generators, but they do not appear to be solely responsible for generating rhythms. Instead, pacemakers are embedded within interconnected circuits, and it is the combination of intrinsic pacemaker properties and synaptic interconnections that generates rhythms. Neural circuits without pacemaker neurons can also generate rhythmic output. In 1911, T. Graham Brown proposed a pattern-generating circuit for locomotion. The essence of Brown’s half-center model is a set of excitatory and inhibitory interneurons arranged to inhibit one another reciprocally (Fig. 16-7). The half-centers are the two halves of the circuit, each commanding one of a pair of antagonist muscles. For the circuit to work, a tonic (i.e., nonrhythmic) drive must be applied to the excitatory interneurons; this drive could come from axons originating outside the circuit (e.g., from neurons in the brain) or from the intrinsic excitability of the neurons themselves. Furthermore, some built-in mechanism must limit the duration of the inhibitory activity so that excitability can cyclically switch from one half-center to the other. Note that feedback from the muscles is not needed for the rhythms to proceed indefinitely. In fact, studies of >50 vertebrate and invertebrate motor circuits have confirmed that rhythm generation can continue in the absence of sensory information.

Central pattern generators in the spinal cord take advantage of sensory feedback, interconnections among spinal segments, and interactions with brainstem control centers The half-center model can produce rhythmic, alternating neural activity, but it is clearly too simplistic to account for most features of locomotor pattern generation. Analysis of vertebrate pattern generators is a daunting task, made difficult by the complexity of the circuits and the behaviors they control. In one of the most detailed investigations, Grillner and colleagues studied a simple model of vertebrate locomotion circuits: the spinal cord of the sea lamprey. Lampreys

are among the simplest fish, and they swim with undulating motions of their body by using precisely coordinated waves of contractions of body muscles. At each spinal segment, muscle activity alternates—one side contracts as the other relaxes. As in mammals, the rhythmic pattern is generated within the spinal cord, and neurons in the brainstem control the initiation and speed of the patterns. The basic patterngenerating circuit for the lamprey spinal cord is repeated in each of the animal’s 100 or so spinal segments. The lamprey pattern-generating circuit improves on the half-center model in three ways. The first is sensory feedback. The lamprey has two kinds of stretch receptor neurons in the lateral margin of the spinal cord itself. These neurons sense stretching of the cord and body, which occurs as the animal bends during swimming. One type of stretch receptor excites the pattern generator interneurons on that same side and facilitates contraction, whereas the other type inhibits the pattern generator on the contralateral side and suppresses contraction. Because stretching occurs on the side of the cord that is currently relaxed, the effect of both stretch receptors is to terminate activity on the contracted side of the body and to initiate contraction on the relaxed side. The second improvement of the lamprey circuit over the half-center model is the interconnection of spinal segments, which ensures the smooth progression of contractions down the length of the body, so that swimming can be efficient. Specifically, each segment must command its muscles to contract slightly later than the one anterior to it, with a lag of ~1% of a full activity cycle for normal forward swimming. Under some circumstances, the animal can also reverse the sequence of intersegment coordination to allow it to swim backward! A third improvement over the half-center model is the reciprocal communication between the lamprey spinal pattern generators and control centers in the brainstem. Not only does the brainstem use numerous pathways and transmitters to modulate the generators, but the spinal generators also inform the brainstem of their activity. The features outlined for swimming lampreys are relevant to walking cats and humans. All use spinal pattern generators to produce rhythms. All use sensory feedback to modulate locomotor rhythms (in mammals, feedback from muscle,

CHAPTER 16  •  Circuits of the Central Nervous System

joint, and cutaneous receptors is all-important). All coordinate the spinal pattern generators across segments, and all maintain reciprocal communication between spinal generators and brainstem control centers.

SPATIAL REPRESENTATIONS: SENSORY AND MOTOR MAPS IN THE BRAIN We have already seen that the spinal cord can receive sensory input, integrate it, and produce motor output that is totally independent of the brain. The brain also receives this sensory information and uses it to control the motor activity of the spinal reflexes and central pattern generators. How does the brain organize this sensory input and motor output? In many cases, it organizes these functions spatially with neural maps. In everyday life, we use maps to represent spatial locations. You may use endless ways to construct a map, depending on which features of an area you want to highlight and what sort of transformation you make as you take measurements from the source (the thing being mapped) and place them on the target (the map). Maps of the earth may emphasize topography, the road system, political boundaries, distributions of air temperature and wind direction, population density, or vegetation. A map is a model of a part of the world—and a very limited model at that. The brain also builds maps, most of which represent very selected aspects of our sensory information about the environment or the motor systems controlling our body. These maps can represent spatial qualities of various sensory modalities (e.g., a place in the visual field) or nonspatial qualities (e.g., smell).

The nervous system contains maps of sensory and motor information Almost all sensory receptors are laid out in planar sheets. In some cases, these receptor sheets are straightforward spatial maps of the sensory environment that they encode. For example, the somatic sensory receptors of the skin literally form a map of the body surface. Similarly, a tiny version of the visual scene is projected onto the mosaic of retinal photoreceptors. The topographies of other sensory receptor sheets represent qualities other than spatial features of the sensory stimuli. For example, the position of a hair cell along the basilar membrane in the cochlea determines the range of sound frequencies to which it will respond. Thus, the sheet of hair cells is a frequency map of sound rather than a map of the location of sounds in space. Olfactory and taste receptors also do not encode stimulus position; instead, because the receptor specificity varies topographically, the receptor sheets may be chemical maps of the types of stimuli. The most interesting thing about sensory receptor maps is that they often project onto many different regions of the CNS. In fact, each sensory surface may be mapped and remapped many times within the brain, the characteristics of each map being unique. In some cases, the brain constructs maps of stimulus features even when these features are not mapped at the level of the receptors themselves. Sound localization is a good example of this property (see the next section). Some neural maps may also combine the features of other neural

399

maps, for example, overlaying visual information with auditory information.

The cerebral cortex has multiple visuotopic maps Some of the best examples of brain maps are those of the visual fields. Figure 16-8A shows the basic anatomical pathway extending from the retina to the lateral geniculate nucleus of the thalamus and on to the primary visual cortex (area V1). Note that area V1 actually maps the visual thalamus, which in turn maps the retina, the first visuotopic map in the brain. Thus, the V1 map is sometimes referred to as a retinotopic map. Figure 16-8B shows how the visual fields are mapped onto cortical area V1. The first thing to notice is that the left half of the visual field is represented on the right cortex and the upper half of the visual field is represented on the lower portions of the cortex. This orientation is strictly determined by the system’s anatomy. For example, all the retinal axons from the left-most halves of both eyes (which are stimulated by light from the right visual hemifield) project to the left half of the brain. Compare the red and blue pathways in Figure 16-8A. During development, each axon must therefore make an unerring decision about which side of the brain to innervate when it reaches the optic chiasm! The second thing to notice is that scaling of the visual fields onto the visual cortex—often called the magnification factor—is not constant. In particular, the central region of the visual fields—the fovea—is greatly magnified on the cortical surface. Behavioral importance ultimately determines mapping in the brain. Primates require vision of particularly high resolution in the center of their gaze; photoreceptors and ganglion cells are thus packed as densely as possible into the central retinal region (see p. 363). About half of the primary visual cortex is devoted to input from the relatively small fovea and the retinal area just surrounding it. Understanding a visual scene requires us to analyze many of its features simultaneously. An object may have shape, color, motion, location, and context, and the brain can usually organize these features to present a seamless interpretation, or image. The details of this process are only now being worked out, but it appears that the task is accomplished with the help of numerous visual areas within the cerebral cortex. Studies of monkey cortex by a variety of electrophysiological and anatomical methods have identified >25 areas that are mainly visual in function, most of which are in the vicinity of area V1. According to recent estimates, humans devote almost half of their neocortex primarily to the processing of visual information. Several features of a visual scene, such as motion, form, and color, are processed in parallel and, to some extent, in separate stages of processing. The neural mechanisms by which these separate features are somehow melded into one image or concept of an object remain unknown, but they depend on strong and reciprocal interconnections between the visual maps in various areas of the brain. The apparently simple topography of a sensory map looks much more complex and discontinuous when it is examined in detail. Many cortical areas can be described as maps on maps. Such an arrangement is especially striking in the visual system. For example, within area V1 of Old World monkeys and humans, the visuotopic maps of the two eyes remain

400

SECTION III  •  The Nervous System

A

CORONAL SECTION SHOWING PROJECTION OF THE RETINA TO THE PRIMARY VISUAL CORTEX Visual fields Left

Right

B

Fixation point

SAGITTAL SECTIONS SHOWING VISUOTOPY OF THE PRIMARY VISUAL CORTEX Foveal visual field

Left visual field

Right visual field

Right eye Retina Optic chiasm Right optic tract Right LGN Optic radiation Fovea

Fovea

Primary visual cortex (area V1) Figure 16-8  Visual fields and visual maps. A, The right sides of both retinas (which sense the left visual hemifield) project to the right lateral geniculate nucleus (LGN), which in turn projects to the right primary visual cortex (area V1). B, The upper parts of the visual fields project to lower parts of the contralateral visual cortex, and vice versa. Although the fovea represents only a small part of the visual field, its representation is greatly magnified in the primary visual cortex, which reflects the large number of retinal ganglion cells that are devoted to the fovea.

segregated. In layer IV of the primary visual cortex, this segregation is accomplished by having visual input derived from the left eye alternate every 0.25 to 0.5 mm with visual input from the right. Thus, two sets of information, one from the left eye and one from the right eye, remain separated but adjacent. Viewed edge on, these left-right alternations look like columns (Fig. 16-9A); hence their name: ocular dominance columns, which were identified by David Hubel and Torsten Wiesel, who shared half of the 1981 Nobel Prize in Physiology or Medicine.   N16-2  Viewed from the surface of the brain, this alternating left-right array of inputs looks like bands or zebra stripes (see Fig. 16-9B). Superimposed on the zebra-stripe ocular dominance pattern in layer IV of the primary visual cortex, but quite distinct from these zebra stripes, layers II and III have structures called blobs. These blobs are visible when the cortex is stained for the mitochondrial enzyme cytochrome oxidase. Viewed edge on, these blobs look like round pegs (see Fig. 16-9). Viewed from the surface of the brain (see Fig. 16-9), the blobs appear as a polka-dot pattern of small dots that are ~0.2 mm in diameter. Adjacent to the primary visual cortex (V1) is the secondary visual cortex (V2), which has, instead of blobs, a

series of thick and thin stripes that are separated by pale interstripes. Some other higher-order visual areas also have striped patterns. Whereas ocular dominance columns demarcate the left and right eyes, blobs and stripes seem to demarcate clusters of neurons that process and channel different types of visual information between areas V1 and V2 and pass them on to other visual regions of the cortex. For example, neurons within the blobs of area V1 seem to be especially attuned to information about color and project to neurons in the thin stripes of V2. Other neurons throughout area V1 are very sensitive to motion but are insensitive to color. They channel their information mainly to neurons of the thick stripes in V2.

Maps of somatic sensory information magnify some parts of the body more than others One of the most famous depictions of a neural map came from studies of the human somatosensory cortex by Penfield and colleagues. Penfield stimulated small sites on the cortical surface in locally anesthetized but conscious patients during neurosurgical procedures; from their verbal descriptions of the position of their sensations, he drew a

CHAPTER 16  •  Circuits of the Central Nervous System

N16-2  David Hubel and Torsten Wiesel David H. Hubel and Torsten N. Wiesel shared the 1981 Nobel Prize in Physiology or Medicine with Roger W. Sperry. Hubel and Wiesel were cited “for their discoveries concerning information processing in the visual system.” For more informa­tion visit, http://nobelprize.org/nobel_prizes/medicine/laureates/ 1981/.

400.e1

CHAPTER 16  •  Circuits of the Central Nervous System

A

B

LAYERS OF AREA V1

Cytochrome oxidase blobs

401

SPLIT-OPEN VIEW

Cytochrome oxidase blobs

I

II, III

II, III

Righ t eye

Left eye

Righ t eye

IV

IV

V, VI

V, VI

Left eye

Ocular dominance columns

Right eye

Left eye

Ocular dominance columns

Figure 16-9  Ocular dominance columns and blobs in the primary visual cortex (area V1). A, Ocular dominance columns are shown as alternating black (right eye) and gray (left eye) structures in layer IV. The alternating light and dark bands are visible in an autoradiograph taken 2 weeks after injecting one eye with 3H-labeled proline and fucose. The 3H label moved from the optic nerve to neurons in the lateral geniculate nucleus and then to the axon terminals in the V1 cortex that are represented in this figure. The blobs are shown as tealcolored pegs in layers II and III. They represent the regular distribution of cytochrome oxidase–rich neurons and are organized in pillar-shaped clusters. B, Cutting the brain parallel to its surface, but between layers III and IV, reveals a polka-dot pattern of blobs in layer II/III and zebra-like stripes in layer IV. (Data from Hubel D: Eye, Brain and Vision. New York, WH Freeman, 1988.)

homunculus, a little person representing the somatotopy— mapping of the body surface—of the primary somatic sensory cortex (Fig. 16-10A). The basic features of Penfield’s map have been confirmed with other methods, including recording from neurons while the body surface is stimulated and modern brain-imaging methods, such as positron emission tomography and functional magnetic resonance imaging. The human somatotopic map resembles a trapeze artist hanging upside down—the legs are hooked over the top of the postcentral gyrus and dangle into the medial cortex between the hemispheres, and the trunk, upper limbs, and head are draped over the lateral aspect of the postcentral gyrus. Two interesting features should be noticed about the somatotopic map in Figure 16-10A. First, mapping of the body surface is not always continuous. For example, the representation of the hand separates those of the head and face. Second, the map is not scaled like the human body. Instead, it looks like a cartoon character: the mouth, tongue, and fingers are very large, whereas the trunk, arms, and legs are tiny. As was the case for mapping of the visual fields onto the visual cortex, it is clear in Penfield’s map that the magnification factor for the body surface is not a constant but varies for different parts of the body. Fingertips are magnified on the cortex much more than the tips of the toes. The relative size of cortex that is devoted to each body part is correlated with the density of sensory input received from that part, and 1 mm2 of fingertip skin has many more sensory endings than a similar patch on the buttocks. Size on the map is also related to the importance of the sensory input from that part

of the body; information from the tip of the tongue is more useful than that from the elbow. The mouth representation is probably large because tactile sensations are important in the production of speech, and the lips and tongue are one of the last lines of defense in deciding whether a morsel is a potential piece of food or poison. The importance of each body part differs among species, and indeed, some species have body parts that others do not. For example, the sensory nerves from the facial whisker follicles of rodents have a huge representation on the cortex, whereas the digits of the paws receive relatively little. Rodent behavior explains this paradox. Most are nocturnal, and to navigate they actively sweep their whiskers about as they move. By touching their local environment, they can sense shapes, textures, and movement with remarkable acuity. For a rat or mouse, seeing things with its eyes is often less important than “seeing” things with its whiskers. As we have already seen for the visual system, other sensory systems usually map their information numerous times. Maps may be carried through many anatomical levels. The somatotopic maps in the cortex begin with the primary somatic sensory axons (see Table 12-1) that enter the spinal cord or the brainstem, each at the spinal segment appropriate to the site of the information that it carries. The sensory axons synapse on second-order neurons, and these cells project their axons into various nuclei of the thalamus and form synapses. Thalamic relay neurons in turn send their axons into the neocortex. The topographical order of the body surface (i.e., somatotopy) is maintained at each anatomical stage, and somatotopic maps are located within the

402 A

SECTION III  •  The Nervous System

SOMATOSENSORY

Arm Head Elbow Neck Trunk Forearm Hip Hand Genitals Fingers Leg Thumb

Eye Nose Face

runs through the postcentral gyrus of the cerebral cortex, shown as a blue band on the image of the brain. B, The plane of section runs through the precentral gyrus of the cerebral cortex, shown as a violet band on the image of the brain. (Data from Penfield W, Rasmussen T: The Cerebral Cortex of Man. New York, Macmillan, 1952.)

Foot Toes

Upper lips Lips Lower lips Teeth

spinal cord, the brainstem, and the thalamus as well as in the somatosensory cortex. Within the cortex, the somatic sensory system has several maps of the body, each unique and each concerned with different types of somatotopic information. Multiple maps are the rule in the brain.

The cerebral cortex has a motor map that is adjacent to and well aligned with the somatosensory map

Gums Jaw Tongue Midline

Pharynx Intra-abdominal

B

Figure 16-10  Somatosensory and motor maps. A, The plane of section

MOTOR

Lateral

Hand Wrist Little Ring Elbow Middle Shoulder Trunk Index

Thumb Neck

Hip

Eyebrow

Ankle

Eyelid and eyeball Face

Toes

Lips Jaw Tongue Swallowing Midline Lateral

Neural maps are not limited to sensory systems; they also appear regularly in brain structures that are considered to have primarily motor functions. Studies done in the 1860s by Fritsch and Hitzig showed that stimulation of particular parts of the cerebral cortex evokes specific muscle contractions in dogs. Penfield and colleagues generated maps of the primary motor cortex in humans (see Fig. 16-10B) by microstimulating and observing the evoked movements. They noted an orderly relationship between the site of cortical stimulation and the body part that moved. Penfield’s motor maps look remarkably like his somatosensory maps, which lie in the adjacent cortical gyrus (see Fig. 16-10A). Note that the sensory and motor maps are adjacent and similar in basic layout (legs represented medially and head laterally), and both have a striking magnification of the head and hand regions. Not surprisingly, there are myriad axonal interconnections between the primary motor and primary somatosensory areas. However, functional magnetic resonance imaging of the human motor cortex shows that the motor map for hand movements is not nearly as simple and somatotopic as Penfield’s drawings might imply. Movements of individual fingers or the wrist that are initiated by the individual activate specific and widely distributed regions of motor cortex, but these regions also overlap one another. Rather than following an obvious somatotopic progression, it instead appears that neurons in the arm area of the motor cortex form distributed and cooperative networks that control collections of arm muscles. Other regions of the motor cortex also have a distributed organization when they are examined on a fine scale, although Penfield’s somatotopic maps still suffice to describe the gross organization of the motor cortex. In other parts of the brain, motor and sensory functions may even occupy the same tissue, and precise alignment of the motor and sensory maps is usually the case. For example, a paired midbrain structure called the superior colliculus receives direct retinotopic connections from the retina as well as input from the visual cortex. Accordingly, a spot of light in the visual field activates a particular patch of neurons in the colliculus. The same patch of collicular neurons can also command, through other brainstem connections, eye and head movements that bring the image of the light spot into the center of the visual field so that it is imaged onto the

CHAPTER 16  •  Circuits of the Central Nervous System

A

VISUAL MAP

l Nasa al r o p Tem

erio Infe r rior

Superior colliculus of a cat

S up

fovea. The motor map for orientation of the eyes is in precise register with the visual response map. In addition, the superior colliculus has maps of both auditory and somatosensory information superimposed on its visual and motor maps; the four aligned maps work in concert to represent points in polysensory space and help control an animal’s orienting responses to prominent stimuli (Fig. 16-11).

403

Sensory and motor maps are fuzzy and plastic B

l Nasa ral o p Tem

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Superior Inferior

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SOMATOSENSORY MAP

l

Vent ra

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We have described a sample of the sensory and motor maps in the brain, but we are left to wonder just why neural maps are so ubiquitous, elaborate, and varied. What is the advantage of mapping neural functions in an orderly way? You could imagine other arrangements: spatial information might be widely scattered about on a neural structure, much as the bytes of one large digital file may be scattered across the array of memory elements in a computer. Various explanations may be proposed for the phenomenon of orderly mapping in the nervous system, although most remain speculations. Maps may be the most efficient way of generating nearest-neighbor relationships between neurons that must be interconnected for proper function. For example, the collicular neurons that participate in sensing stimuli 10 degrees up and 20 degrees to the left and other collicular neurons that command eye movements toward that point undoubtedly need to be strongly interconnected. Orderly collicular mapping enforces togetherness for those cells and minimizes the length of axons necessary to interconnect them. In addition, if brain structures are arranged topographically, neighboring neurons will be most likely to become activated synchronously. Neighboring neurons are very likely to be interconnected in structures such as the cortex, and their synchronous activity serves to reinforce the strength of their interconnections because of the inherent rules governing synaptic plasticity (see pp. 328–333). An additional advantage of mapping is that it may simplify establishment of the proper connections between neurons during development. For example, it is easier for an axon from neuron A to find neuron B if distances are short. Maps may thus make it easier to establish interconnections precisely among the neurons that represent the three sensory maps and one motor map in the superior colliculus. Another advantage of maps may be to facilitate the effectiveness of inhibitory connections. Perception of the edge of a stimulus (edge detection) is heightened by lateral connections that suppress the activity of neurons representing the space slightly away from the edge. If sensory areas are mapped, it is a simple matter to arrange the inhibitory connections onto nearby neurons and thereby construct an edge-detector circuit. It is worth clarifying several general points about neural maps. “The map is not the territory,” as the philosopher Alfred Korzybski pointed out. In other words, all maps, including neural maps, are abstract representations. They are also distorted by the shortcomings of particular experimental measurements. A problem with neural maps is that different experimenters, using different methods, may sometimes generate quite different maps of the same part of the brain. As more and better-refined methods become available, our understanding of these maps is evolving. Moreover,

Here we show all three maps superimposed. Rostral D

SUPERIMPOSITION

Lateral

Medial

Caudal Figure 16-11  Polysensory space in the superior colliculus. A, The representation of visual space projected onto the right superior colliculus of a cat. Note that visual space is divided into nasal versus temporal space and superior versus inferior space. B and C, Comparable auditory and somatosensory maps, respectively. D, Superimposition of the preceding three maps. Note the approximate correspondence among the visual (red), auditory (green), and somatosensory (blue) maps. The motor map for orienting the eyes (not shown) is in almost perfect register with the visual map in A. (Data from Stein BE, Wallace MT, Meredith MA: Neural mechanisms mediating attention and orientation to multisensory cues. In Gazzaniga M [ed]: The Cognitive Neurosciences. Cambridge, MA, MIT Press, 1995.)

404 A

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SOMATOSENSORY REORGANIZATION

Normal organization

Owl monkey

Median (input) nerve cut

Dorsum

3b (S1 proper)

D4

Body

Dorsum H

Face

I

P3 I P2 P1 T

P2 D2

I

P3

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Deprived

P3 H

D1

Portion—corresponding to the palm—deprived by the median nerve section…

...now corresponds to the dorsal (hairy) skin of the first three digits.

MOTOR-SYSTEM REORGANIZATION un

k

Hind limb

Hind limb

FL N

Jaw

Jaw

Normal organization

Facial nerve cut

Weeks

elim

b

Ey

For

Ey

e

elim

b

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For

iss

e

br

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Vi

b elim For

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M1

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Ey

iss

e

a

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Tr

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k

Hind limb

Tr

k

M1

Tr

Rat cortex

un

B

I

D4

P3

H P4

D5

I

D3

P1 T

D1

D1

D4

Dorsum

H P4

D5

I

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D3

D5 D4 D3 D2

P4

Dorsum

H P4

D5

Reorganization

Jaw

Reorganization

Figure 16-12  Plasticity of maps. A, The first panel on the right, labeled “Normal organization,” shows the somatotopic organization of the right hand in the left somatosensory cortex of the monkey brain. The colors correspond to different regions of the hand (viewed from the palm side, except for portions labeled “Dorsum”). The second panel shows (in gray) the territory that is deprived of input by sectioning the median nerve. The third panel shows that the cortical map is greatly changed several months after nerve section. The nerve was not allowed to regrow, but the previously deprived cortical region now responds to the dorsal skin of D3, D2, and D1. Notice that responses to regions P1, P2, and T have disappeared; region I has encroached; and regions H and P3 have suddenly appeared at a second location. B, The first panel on the right, labeled “Normal organization,” shows the somatotopic organization of the left motor cortex (M1) of the rat brain. The colors correspond to the muscles that control different regions of the body. The second panel shows (in gray) the territory that normally provides motor output to the facial nerve, which has been severed. The third panel shows that, after several weeks, the deprived cortical territory is now remapped. Notice that the deprived territory that once evoked whisker movements now evokes eye, eyelid, and forelimb movements. FL, additional representation of forelimb; N, neck area. (A, Data from Kaas JH: The reorganization of sensory and motor maps in adult mammals. In Gazzaniga M [ed]: The Cognitive Neurosciences. Cambridge, MA, MIT Press, 1995; B, data from Sanes J, Suner S, Donoghue JP: Dynamic organization of primary motor cortex output to target muscles in adult rats: Long-term patterns of reorganization following motor or mixed peripheral nerve lesions. Exp Brain Res 79:479–491, 1990.)

the brain itself muddies its maps. Maps of sensory space onto a brain area are not point-to-point representations. On the contrary, a point in sensory space (e.g., a spot of light) activates a relatively large group of neurons in a sensory region of the brain. However, such activation of many neurons is not due to errors of connectivity; the spatial dissemination of activity is part of the mechanism used to encode and to process information. The strength of activation is most intense within the center of the activated neuronal group, but the population of more weakly activated neurons may encompass a large portion of an entire brain. This diversity in strength of activation means that a point in sensory space is unlikely to be encoded by the activity of a single neuron;

instead it is represented by the distributed activity in a large population of neurons. Such a distributed code has computational advantages, and some redundancy also guards against errors, damage, and loss of information. Finally, maps may change with time. All sensory and motor maps are clearly dynamic and can be reorganized rapidly and substantially as a function of development, behavioral state, training, or damage to the brain or periphery. Such changes are referred to as plasticity. Figure 16-12 illustrates two examples of dramatic changes in neocortical mapping, one sensory and one motor, after damage to peripheral nerves. In both cases, severing a peripheral nerve causes the part of the map that normally relates to the body

CHAPTER 16  •  Circuits of the Central Nervous System

part served by this severed nerve to become remapped to another body part. Although the mechanisms of these reorganizations are only partially known, they probably reflect the same types of processes that underlie our ability to learn sensorimotor skills with practice and to adjust and improve after neural damage from trauma or stroke.

Neural circuits are very good at resolving time intervals, in some cases down to microseconds or less. One of the most demanding tasks of timing is performed by the auditory system as it localizes the source of certain sounds. Sound localization is an important skill, whether you are prey, predator, or pedestrian. Vertebrates use several different strategies for localization of sound, depending on the species, the frequency of the sound, and whether the task is to localize the source in the horizontal (left-right) or vertical (up-down) plane. In this subchapter, we briefly review general strategies of sound localization and then explain the mechanism by which a brainstem circuit measures the relative timing of low-frequency sounds so that the source of the sounds can be localized with precision. Sound localization along the vertical plane (the degree of elevation) depends, in humans at least, on the distinctive shape of the external ear, the pinna. Much of the sound that we hear enters the auditory canal directly, and its energy is transferred to the cochlea. However, some sound reflects off the curves and folds of the pinna and tragus before it enters the canal and thus takes slightly longer to reach the cochlea. Notice what happens when the vertical direction of the sound changes. Because of the arcing shape of the pinna, the reflected path of sounds coming from above is shorter than that of sounds from below (Fig. 16-13). The two sets of sounds (the direct and, slightly delayed, the reflected) combine to create sounds that are slightly different on entering the auditory canal. Because of the interference patterns created by the direct and reflected sounds, the combined sound has spectral properties that are characteristic of the elevation of the sound source. This mechanism of vertical sound localization works well even with one ear at a time, although its precise neural mechanisms are not clear. For humans, accurate determination of the direction of a sound along the horizontal plane (the azimuth) requires two working ears. Sounds must first be processed by the cochlea in each ear and then compared by neurons within the CNS to estimate horizontal direction. But what exactly is compared? For sounds that are relatively high in frequency (~2 to 20 kHz), the important measure is the interaural (i.e., ear-to-ear) intensity difference. Stated simply, the ear facing the sound hears it as louder than the ear facing away because the head casts a “sound shadow” (Fig. 16-14A). If the sound is directly to the right or left of the listener, this difference is maximal; if the sound is straight ahead, no difference is heard; and if the sound comes from an oblique direction, intensity differences are intermediate. Note that this system can be fooled. A sound source straight ahead gives the

Pinna

Path 1 direct sound Path 1 reflected sound

TEMPORAL REPRESENTATIONS: TIME-MEASURING CIRCUITS To localize sound, the brain compares the timing and intensity of input to the ears

405

Tragus Auditory canal

Path 2 direct sound Path 2 reflected sound

Path 3 reflected sound Path 3 direct sound

Figure 16-13  Detection of sound in the vertical plane. The detection of sound in the vertical plane requires only one ear. Regardless of the source of a sound, the sound reaches the auditory canal by both direct and reflected pathways. The brain localizes the source of the sound in the vertical plane by detecting differences in the combined sounds from the direct and reflected pathways.

same intensity difference (i.e., none) as a sound source directly behind. The interaural intensity difference is not helpful at lower frequencies. Sounds below ~2 kHz have a wavelength that is longer than the width of the head itself. Longer sound waves are diffracted around the head, and differences in interaural intensity no longer occur. At low frequencies, the nervous system uses another strategy—it measures interaural delay (see Fig. 16-14B). Consider a 200-Hz sound coming directly from the right. Its peak-to-peak distance (i.e., the wavelength) is ~172 cm, which is considerably more than the 20-cm width of the head. Each sound wave peak will reach the right ear ~0.6 ms before it reaches the left ear. If the sound comes from a 45-degree angle ahead, the interaural delay is ~0.3 ms; if it comes from straight ahead (or directly behind), the delay is 0 ms. Delays of small fractions of a millisecond are well within the capabilities of certain brainstem auditory neurons to detect. Sounds need not be continuous for the interaural delay to be detected. Sound onset or offset, clicks, or any abrupt changes in the sound give opportunities for interaural time comparisons. Obviously, measurement of interaural delay is subject to the same front-back ambiguity as interaural intensity, and indeed, it is sometimes difficult to distinguish whether a sound is in front of or behind your head.

The brain measures interaural timing by a combination of neural delay lines and coincidence detectors How does the auditory system measure interaural timing? Surprisingly, to detect very small time differences, the nervous system uses a precise arrangement of neurons in space. Figure 16-15A summarizes the neuroanatomy of the

406 A

SECTION III  •  The Nervous System

HIGH-FREQUENCY SOUND

Sound shadow

B

The CNS detects a difference in intensity between the two ears.

Sound waves

LOW-FREQUENCY SOUND

The CNS detects the sound delay between the two ears.

Sound waves 0.6 ms 172 cm (200 Hz) Figure 16-14  Sound detection in a horizontal plane. A, Two ears are necessary for the detection of sound in a horizontal plane. For frequencies between 2 kHz and 20 kHz, the CNS detects the ear-to-ear intensity difference. In this example, the sound comes from the right. The left ear hears a weaker sound because it is in the shadow of the head. B, For frequencies 0 ∆V>0 ∆P>0

Compliance = ∆V ∆P Figure 17-9  Compliance: changes in pressure with vessels of different compliances.

we assume that the entire body is at the level of the heart, we do not need to add a hydrostatic pressure component to the various intravascular pressures. Thus, the mean pressure in the aorta is 95 mm Hg, and—because it takes a driving pressure of ~5 mm Hg to pump blood into the end of the large arteries—the mean pressure at the end of the large arteries in the foot and head is 90 mm Hg. Similarly, the mean pressure in the large veins draining the foot and head is 5 mm Hg, and—because it takes a driving pressure of ~3 mm Hg to pump blood to the right atrium—the mean pressure in the right atrium is 2 mm Hg. When a 180-cm tall person is standing (see Fig. 17-8B), we must add a 130-cm column of blood (the Δh between the heart and large vessels in the foot) to the pressure pre­vailing in the large arteries and veins of the foot. Because a water column of 130 cm is equivalent to 95 mm Hg, the mean pressure for a large artery in the foot will be 90 + 95 = 185 mm Hg, and the mean pressure for a large vein in the foot will be 5 + 95 = 100 mm Hg. On the other hand, we must subtract a 50-cm column of blood from the pressure prevailing in the head. Because a water column of 50 cm is equivalent to 37 mm Hg, the mean pressure for a large artery in the head will be 90 − 37 = 53 mm Hg, and the mean pressure for a large vein in the head will be 5 − 37 = −32 mm Hg. Of course, this “negative” value really means that the pressure in a large vein in the head is 32 mm Hg lower than the reference pressure at the level of the heart. In this example, we have simplified things somewhat by ignoring the valves that interrupt the blood column. In reality, the veins of the limbs have a series of one-way valves that allow blood to flow only toward the heart. These valves act like a series of relay stations, so that the contraction of skeletal muscle around the veins pushes blood from one valve to another (see p. 516). Thus, veins in the foot do not “see” the full hydrostatic column of 95 mm Hg when the leg muscles pump blood away from the foot veins. Although the absolute arterial and venous pressures are much higher in the foot than in the head, the ΔP that drives blood flow is the same in the vascular beds of the foot and head. Thus, in the horizontal position, the ΔP across the vascular beds in the foot or head is 90 − 5 = 85 mm Hg. In the upright position, the ΔP for the foot is 185 − 100 = 85 mm Hg, and for the head, 53 − (−32) = 85 mm Hg. Thus,

gravity does not affect the driving pressure that governs flow. On the other hand, in “dependent” areas of the body (i.e., vessels “below” the heart in a gravitational sense), the hydrostatic pressure does tend to increase the transmural pressure (intravascular versus extravascular “tissue” pressure) and thus the diameter of distensible vessels. Because various anatomical barriers separate different tissue compartments, it is assumed that gravity does not appreciably affect this tissue pressure.

Low compliance of a vessel causes the transmural pressure to increase when the vessel blood volume is increased Until now, we have considered blood vessels to be rigid tubes, which, by definition, have fixed volumes. If we were to try to inject a volume of fluid into a truly rigid tube with closed ends, we could in principle increase the pressure to infinity without increasing the volume of the tube (Fig. 17-9A). At the other extreme, if the wall of the tube were to offer no resistance to deformation (i.e., infinite compliance), we could inject an infinite volume of fluid without increasing the pressure at all (see Fig. 17-9B). Blood vessels lie between these two extremes; they are distensible but have a finite compliance (see p. 454). Thus, if we were to inject a volume of blood into the vessel, the volume of the vessel would increase by the same amount (ΔV), and the intravascular pressure would also increase (see Fig. 17-9C). The ΔP accompanying a given ΔV is greater if the compliance of the vessel is lower. The relationship between ΔP and ΔV is a static property of the vessel wall and holds whether or not there is flow in the vessel. Thus, if we were to infuse blood into a patient’s blood vessels, the intravascular pressure would rise throughout the circulation, even if the heart were stopped.

The viscous resistance of blood causes an axial pressure difference when there is flow As we saw in Ohm’s law of hydrodynamics (see Equation 17-1), during steady flow down the axis of a tube (see Fig. 17-2), the driving pressure (ΔP) is proportional to both flow and resistance. Viewed differently, if we want to achieve a constant flow, then the greater the resistance, the greater the

420

SECTION IV  •  The Cardiovascular System

ΔP that we must apply along the axis of flow. Of the four sources of pressure in the circulatory system, this ΔP due to viscous resistance is the only one that appears in Poiseuille’s law (see Equation 17-9).

P = 100

The inertia of the blood and vessels causes pressure to decrease when the velocity of blood flow increases For the most part, we have been assuming that the flow of blood as well as its mean linear velocity is steady. However, as we have already noted, blood flow in the circulation is not steady; the heart imparts its energy in a pulsatile manner, with each heartbeat. Therefore, v in the aorta increases and reaches a maximum during systole and falls off during diastole. As we shall shortly see, these changes in velocity lead to compensatory changes in intravascular pressure. The tradeoff between velocity and pressure reflects the conversion between two forms of energy. Although we generally state that fluids flow from a higher to a lower pressure, it is more accurate to say that fluids flow from a higher to a lower total energy. This energy is made up of both the pressure or potential energy and the kinetic energy (KE = 1 2 mv 2). The impact of the interconversion between these two forms of energy is manifested by the familiar Bernoulli effect. As fluid flows along a horizontal tube with a narrow central region, which has half the diameter of the two ends, the pressure in the central region is actually lower than the pressure at the distal end of the tube (Fig. 17-10). How can the fluid paradoxically flow against the pressure gradient from the lower-pressure central to the higherpressure distal region of the tube? We saw above that flow is the product of cross-sectional area and velocity (see Equation 17-8). Because the flow is the same in both portions of the tube, but the cross-sectional area in the center is lower by a factor of 4, the velocity in the central region must be 4-fold higher (see table at bottom of Fig. 17-10). Although the blood in the central region has a lower potential energy (pressure = 60) than the blood at the distal end of the tube (pressure = 80), it has a 16-fold higher kinetic energy. Thus, the total energy of the fluid in the center exceeds that in the distal region, so that the fluid does indeed flow down the energy gradient. This example illustrates an interconversion between potential energy (pressure) and kinetic energy (velocity) in space because velocity changes along the length of a tube even though flow is constant. We will see on pages 511–513 that during ejection of blood from the left ventricle into the aorta, the flow and velocity of blood change with time at any point within the aorta. These changes in velocity contribute to the changes in pressure inside the aorta. The Bernoulli effect has important practical implications for measurement of blood pressure with an open-tipped catheter. The pressure recorded with the open tip facing the flow is higher than the actual pressure by an amount corresponding to the kinetic energy of the oncoming fluid (Fig. 17-11). Conversely, the pressure recorded with the open tip facing away from the flow is lower than the actual pressure by an equal amount. The measured pressure is correct only when the opening is on the side of the catheter, perpendicular to the flow of blood.

P = 90

P = 80

v

P = 100

P = 80 P = 60

v

A

A/ 4

Cross-sectional area (A)

1

1 4

1

P

100

60

80

Velocity (v )

2

8

2

Kinetic energy (ρv 2 /2)

2

32

2

Total energy (E)

102

92

82

Figure 17-10  Bernoulli effect. For the top tube, which has a uniform radius, velocity (v) is uniform and transmural pressure (P) falls linearly with the length, which we artificially compress to fit in the available space. The bottom tube has the same upstream and downstream pressures but a constriction in the middle that is short enough so as not to increase overall resistance or overall fall in P. The constriction has crosssectional area that is only one fourth that of the two ends. Thus, velocity in the narrow portion must be 4-fold higher than it is at the ends. Although the total energy of fluid falls linearly along the tube, pressure is lower in the middle than at the distal end.

HOW TO MEASURE BLOOD PRESSURE, BLOOD FLOW, AND CARDIAC VOLUMES Blood pressure can be measured directly by puncturing the vessel One can record blood pressure anywhere along the circulation—inside a heart chamber, inside an artery, within a capillary, or within a vein. Clinicians are generally concerned with the intravascular pressure at a particular site (e.g., in a systemic artery) in reference to the barometric pressure outside the body and not with pressure differences between two sites. The most direct approach for measurement of pressure is to introduce a needle or a catheter into a vessel and position

CHAPTER 17  •  Organization of the Cardiovascular System

97 mm Hg Facing upstream

Blood

93 mm Hg Facing downstream

95 mm Hg Side pressure

Figure 17-11  Effects of kinetic energy on the measurement of blood pressure with catheters.

Strain gauge on flexible diaphragm

Brachial artery

Catheter

Membrane Amplifier

Recorder

Figure 17-12  Direct method for determining blood pressure.

the open tip at a particular site. In the first measurements of blood pressure ever performed, Stephen Hales in 1733 found that a column of blood from a presumably agitated horse rose to fill a brass pipe to a height of 3 m. It was Poiseuille who measured blood pressure for the first time by connecting a mercury-filled U-tube to arteries through a tube containing a solution of saturated NaHCO3. In modern times, a saline-filled transmission or conduit system connects the blood vessel to a pressure transducer. In the most primitive form of this system, a catheter was connected to a closed chamber, one wall of which was a deformable diaphragm. Nowadays, the pressure transducer is a stiff diaphragm bonded to a strain gauge that converts mechanical strain into a change in electrical resistance, capacitance, or inductance (Fig. 17-12). The opposite face of the diaphragm is open to the atmosphere, so that the blood pressure is referenced to barometric pressure. The overall performance of the system depends largely on the properties of the catheter and the strain gauge. The presence of air bubbles and a long or

421

narrow catheter can decrease the displacement, velocity, and acceleration of the fluid in the catheter. Together, these properties determine overall performance characteristics such as sensitivity, linearity, damping of the pressure wave, and frequency response. To avoid problems with fluid transmission in the catheter, some high-fidelity devices employ a solidstate pressure transducer at the catheter tip. In catheterizations of the right heart, the clinician begins by sliding a fluid-filled catheter into an antecubital vein and, while continuously recording pressure, advances the catheter tip into the superior vena cava, through the right atrium and the right ventricle, and past the pulmonary valve into the pulmonary artery. Eventually, the tip reaches and snugly fits into a smaller branch of the pulmonary artery, recording the pulmonary wedge pressure (see p. 519). The wedge pressure effectively measures the pressure downstream from the catheter tip, that is, the left atrial pressure. In catheterizations of the left heart, the clinician slides a catheter into the brachial artery or femoral artery, obtaining the systemic arterial blood pressure. From there, the catheter is advanced into the aorta, the left ventricle, and finally the left atrium. Clinical measurements of venous pressure are typically made by inserting a catheter into the jugular vein. Because of the low pressures, these venous measurements require very sensitive pressure transducers or water manometers. In the research laboratory, one can measure capillary pressure in exposed capillary beds by inserting a micropipette that is pressurized just enough (with a known pressure) to keep fluid from entering or leaving the pipette.

Blood pressure can be measured indirectly by use of a sphygmomanometer In clinical practice, one may measure arterial pressure indirectly by use of a manual sphygmomanometer (Fig. 17-13). An inextensible cuff containing an inflatable bag is wrapped around the arm (or occasionally, the thigh). Inflation of the bag by means of a rubber squeeze bulb to a pressure level above the expected systolic pressure occludes the underlying brachial artery and halts blood flow downstream. The pressure in the cuff, measured by means of a mercury or aneroid manometer, is then allowed to slowly decline (see Fig. 17-13, diagonal red line). The physician can use either of two methods to monitor the blood flow downstream of the slowly deflating cuff. In the palpatory method, the physician detects the pulse as an indicator of flow by feeling the radial artery at the wrist. In the auscultatory method, the physician detects flow by using a stethoscope to detect the changing character of Korotkoff sounds over the brachial artery in the antecubital space. The palpatory method permits determination of the systolic pressure; that is, the pressure in the cuff below which it is just possible to detect a radial pulse. Because of limited sensitivity of the finger, palpation probably slightly underestimates systolic pressure. The auscultatory method permits the detection of both systolic and diastolic pressure. The sounds heard during the slow deflation of the cuff can be divided into five phases (see Fig. 17-13). During phase I, there is a sharp tapping sound, indicating that a spurt of blood is escaping under the cuff when cuff pressure is just

422

SECTION IV  •  The Cardiovascular System

The systolic pressure corresponds to the first tapping sound. 110

Time

130

90 70

110

50

Cuff pressure (mm Hg) 90

30 10

Mercury reservoir Inflation bulb

Sphygmomanometer cuff

14 mm Hg

20 mm Hg 6 mm Hg 5 mm Hg

Arterial 70 pressure

Cuff pressure

10

The diastolic pressure corresponds to the muffling of the sounds.

Relative intensity 5 of sounds

0

Silence

Phase I

II

V

e nc le Si g flin uf ng pi

IV

um

s

ur

m

ur

M

g in pp s Ta ne to

III

M

Palpation of radial artery

Th

Auscultation of brachial artery

40 mm Hg

Figure 17-13  Sphygmomanometry. The clinician inflates the cuff to a pressure that is higher than the anticipated systolic pressure and then slowly releases the pressure in the cuff.

below systolic pressure. The pressure at which these taps are first heard closely represents systolic pressure. In phase II, the sound becomes a blowing or swishing murmur. During phase III, the sound becomes a louder thumping. In phase IV, as the cuff pressure falls toward the diastolic level, the sound becomes muffled and softer. Finally, in phase V, the sound disappears. Although some debate persists about whether the point of muffling or the point of silence is the correct diastolic pressure, most favor the point of muffling as being more consistent. Actual diastolic pressure may be somewhat overestimated by the point of muffling but underestimated by the point of silence. Practical problems arise when a sphygmomanometer is used with children or obese adults or when it is used to obtain a measurement on a thigh. Ideally, one would like to use a pressure cuff wide enough to ensure that the pressure inside the cuff is the same as that in the tissue surrounding the artery. In 1967, the American Heart Association recommended that the pneumatic bag within the cuff be 20% wider than the diameter of the limb, extend at least halfway around the limb, and be centered over the artery. More recent studies indicate that accuracy and reliability improve when the pneumatic bag completely encircles the limb, as long as the width of the pneumatic bag is at least the limb diameter.

Blood flow can be measured directly by electromagnetic and ultrasound flowmeters The spectrum of blood flow measurements in the circulation ranges from determinations of total blood flow (cardiac

output) to assessment of flow within an organ or a particular tissue within an organ. Moreover, one can average blood flow measurements over time or record continuously. Examples of continuous recording include recordings of the phasic blood flow that occurs during the cardiac cycle or any other periodic event (e.g., breathing). We discuss both invasive and noninvasive approaches. Invasive Methods  Invasive approaches require direct access to the vessel under study and are thus useful only in research laboratories. The earliest measurements of blood flow involved collecting venous outflow into a graduated cylinder and timing the collection with a stopwatch. This direct approach was limited to short time intervals to minimize blood loss and the resulting changes in hemodynamics. Blood loss could be avoided by ingenious but now antiquated devices that returned the blood to the circulation, in either a manual or a semiautomated fashion. The most frequently used modern instruments for measurement of blood flow in the research laboratory are electromagnetic flowmeters based on the electromagnetic induction principle (Fig. 17-14). The vessel is placed in a magnetic field. According to Faraday’s induction law, moving any conductor (including an electrolyte solution, such as blood) at right angles to lines of the magnetic field generates a voltage difference between two points along an axis perpendicular to both the axis of the movement and the axis of the magnetic field. The induced voltage is



E = BvD

(17-14)

CHAPTER 17  •  Organization of the Cardiovascular System

sections include discussions of two indirect methods that clinicians use to measure mean blood flow.

The movement of an electrical conductor (i.e., blood in a vessel) through a magnetic field induces a voltage between two points (e1 and e2) along an axis that is mutually perpendicular to both the axis of the magnetic field and the axis of blood flow. Axis of magnetic field

Cardiac output can be measured indirectly by the Fick method, which is based on the conservation of mass The Fick method requires that a substance be removed from or added to the blood during its flow through an organ. The rate at which X passes a checkpoint in the circulation ( Q ) is simply the product of the rate at which blood volume passes the checkpoint (F) and the concentration of X in that blood:

e1 +

N

S

Q = F ⋅[ X ] moles liters moles Units: = ⋅ s s liter

Voltmeter

D –

e2 Magnet

v

423

Blood vessel Axis of blood flow Figure 17-14  Electromagnetic flowmeter.

where B is the density of magnetic flux, v is the average linear velocity, and D is the diameter of the moving column of blood. Ultrasound flowmeters employ a pair of probes placed at two sites along a vessel. One probe emits an ultrasound signal, and the other records it. The linear velocity of blood in the vessel either induces a change in the frequency of the ultrasound signal (Doppler effect) or alters the transit time of the ultrasound signal. Both the electromagnetic and ultrasound methods measure linear velocity, not flow per se. Noninvasive Methods  The electromagnetic or ultrasonic

flowmeters require the surgical isolation of a vessel. How­ ever, ultrasonic methods are also widely used transcutaneously on surface vessels in humans. This method is based on recording of the backscattering of the ultrasound signal from moving red blood cells. To the extent that the red blood cells move, the reflected sound has a frequency different from that of the emitted sound (Doppler effect). This frequency difference may thus be calibrated to measure flow. Plethysmographic methods are noninvasive approaches for measurement of changes in the volume of a limb or even of a whole person (see p. 617). Inflation of a pressure cuff enough to occlude veins but not arteries allows blood to continue to flow into (but not out of) a limb or an organ, so that the volume increases with time. The record of this rise in volume, as recorded by the plethysmograph, is a measure of blood flow. With the exception of transcutaneous ultrasonography, the direct methods discussed for measurement of blood flow are largely confined to research laboratories. The next two

(17-15)

The Fick principle is a restatement of the law of conservation of mass. The amount of X per unit time that passes a downstream checkpoint (Q B) minus the amount of X that passes an upstream checkpoint (Q A) must equal the amount of X added or subtracted per unit time (Q added subtracted) between these two checkpoints (Fig. 17-15A): Q added subtracted = Q B − Q A



(17-16)

Q added subtracted is positive for the addition of X. If the volume flow is identical at both checkpoints, combining Equation 17-15 and Equation 17-16 yields the Fick equation: Q added subtracted = F ([ X]B − [ X]A )



(17-17)

We can calculate flow from the amount of X added or subtracted and the concentrations of X at the two checkpoints: F=



Q added subtracted [ X]B − [ X]A

=

moles min moles liter

=

liters min

(17-18)

It is easiest to apply the Fick principle to the blood flow through the lungs, which is the cardiac output (see Fig. 17-15B). The quantity added to the bloodstream is the O2 uptake (Q O2) by the lungs, which we obtain by measuring the subject’s O2 consumption. This value is typically 250 mL of O2 gas per minute. The upstream checkpoint is the pulmonary artery (point A), where the O2 content ([O2]A) is typically 15 mL of O2 per deciliter of blood. The sample for this checkpoint must reflect the O2 content of mixed venous blood, obtained by means of a catheter within the right atrium or the right ventricle or pulmonary artery. The downstream checkpoint is a pulmonary vein (point B), where the O2 content ([O2]B) is typically 20 mL O2 per deciliter of blood. We can obtain the sample for this checkpoint from any systemic artery. Using these particular values, we calculate a cardiac output of 5 L/min:



250 mL O2 min Q O2 = [O2 ]B − [O2 ]A (20 − 15) mL O2 dL blood (17-19) = 5 L blood min

F=

424

SECTION IV  •  The Cardiovascular System

A

THE FICK PRINCIPLE ˙ added/subtracted Q

˙B Q

˙A Q

Checkpoint A B

Checkpoint B

MEASUREMENT OF CARDIAC OUTPUT 250 mL O2 / min

[O2]A =15 mL O2 / dL blood Pulmonary artery

˙A Q

Lung [O2]B = 20 mL O2 / dL blood Pulmonary vein

˙B Q

˙O Q 2

Left heart

Right heart

˙A Q Vena cava

˙O Q 2

˙B Q

250 mL O2 / min

Aorta

Tissue Figure 17-15  Fick method for determining cardiac output.

Cardiac output can be measured indirectly by dilution methods The dye dilution method, inaugurated by G.N. Stewart in 1897 and extended by W.F. Hamilton in 1932, is a variation of the Fick procedure. One injects a known quantity of a substance (X) into a systemic vein (e.g., antecubital vein) at site A while simultaneously monitoring the concentration downstream at site B (Fig. 17-16A). It is important that the substance not leave the vascular circuit and that it be easy to follow the concentration, by either successive sampling or continuous monitoring. If we inject a single known amount (QX) of the indicator, an observer downstream at checkpoint B will see a rising concentration of X, which, after reaching its peak, falls off exponentially. Concentration measurements provide the interval (Δt) between the time the dye makes its first appearance at site B and the time the dye finally disappears there. If site B is in the pulmonary artery, then the entire amount QX that we injected into the peripheral vein must pass site B during the interval Δt, carried by the entire cardiac output. We can deduce the average concentration [X] during the interval Δt from the concentration-versus-time curve in Figure 17-16B. From the conservation of mass, we know that

Q X = V ⋅[ X ]

Units: moles = liters ⋅

moles liter

(17-20)

Because the volume of blood (V) that flowed through the pulmonary artery during the interval Δt is, by definition, the product of cardiac output and the time interval (CO · Δt),

QX = CO ⋅ ∆t ⋅[ X] liters moles Units: moles = ⋅s⋅ s liter

(17-21)

Note that the product Δt · [ X ] is the area under the concentration-versus-time curve in Figure 17-16B. Solving for CO, we have

CO =

QX Q = X [ X] ⋅ ∆t Area

(17-22)

In practice, cardiologists monitor [X] in the brachial artery. Obviously, only a fraction of the cardiac output passes through a brachial artery; however, this fraction is the same as the fraction of QX that passes through the brachial artery. If we were to re-derive Equation 17-22 for the brachial artery,

425

CHAPTER 17  •  Organization of the Cardiovascular System

A

PRINCIPLE OF DYE DILUTION

˙X Q Checkpoint A Right heart

Left heart

Brachial artery

Systemic vein

˙A Q

˙ B = >0 Q Checkpoint B Photocell

Pulmonary artery

Dye stream

Lamp Recorder B

C

CONCENTRATION PROFILE AT PULMONARY ARTERY 25

[X]

20

D

CONCENTRATION PROFILE AT BRACHIAL ARTERY (WITHOUT RECIRCULATION)

CONCENTRATION PROFILE AT BRACHIAL ARTERY (WITH RECIRCULATION)

10

10 Mixing in total cardiac output

[X] mg/liter

Amount injected 2

[X]

2

0 3 ∆t

Time (s)

Recirculation

Dispersion

30

0 3

Time (s)

30

2

0 3

Time (s)

30

∆t Figure 17-16  Dye dilution method for determining blood flow. In B, C, and D, the areas underneath the three red curves—as well as the three green areas—are all the same.

we would end up multiplying both the CO and QX terms by this same fraction. Therefore, even though only a small portion of both cardiac output and injected dye passes through any single systemic artery, we can still use Equa­ tion 17-22 to compute cardiac output with data from that artery. Compared with the [X] profile in the pulmonary artery, the [X] profile in the brachial artery is not as tall and is more spread out, so that [X] is smaller and Δt is longer. However, the product [X] · Δt in the brachial artery—or any other systemic artery—is the same as that in the pulmonary artery. Indocyanine green dye (Cardiogreen) is the most common dye employed. Because the liver removes this dye from the

circulation, it is possible to repeat the injections, after a sufficient wait, without progressive accumulation of dye in the plasma. Imagine that after we inject 5 mg of the dye, [X] under the curve is 2 mg/L and Δt is 0.5 min. Thus,

CO =

5 mg 2 mg L × 0.5 min

= 5 L min

(17-23)

A practical problem is that after we inject a marker into a systemic vein, blood moves more quickly through some pulmonary beds than others, so that the marker arrives at checkpoint B at different times. This process, known as dispersion, is the main cause of the flattening of the [X] profile

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SECTION IV  •  The Cardiovascular System

in the brachial artery (see Fig. 17-16C) versus the pulmonary artery (see Fig. 17-16B). If we injected the dye into the left atrium and monitored it in the systemic veins, the dispersion would be far worse because of longer and more varied path lengths in the systemic circulation compared with the pulmonary circulation. In fact, the concentration curve would be so flattened that it would be difficult to resolve the area underneath the [X] profile. A second practical problem with a closed circulatory system is that before the initial [X] wave has waned, recirculation causes the injected indicator to appear for a second time in front of the sensor at checkpoint B (see Fig. 17-16D). Extrapolation of the exponential decay of the first wave can correct for this problem. The thermodilution technique is a convenient alternative approach to the dye technique. In this method, one injects a bolus of cold saline and an indwelling thermistor is used to follow the dilution of these “negative calories” as a change of temperature at the downstream site. In the thermodilution technique, a temperature-versus-time profile replaces the concentration-versus-time profile. During cardiac catheterization, the cardiologist injects a bolus of cold saline into the right atrium and records the temperature change in the pulmonary artery. The distance between upstream injection and downstream recording site is kept short to avoid heat exchange in the pulmonary capillary bed. The advantages of this method are that (1) the injection of cold saline can be repeated without harm, (2) a single venous (versus venous and arterial) puncture allows access to both the upstream and the downstream sites, (3) less dispersion occurs because no capillary beds are involved, and (4) less recirculation occurs because of adequate temperature equilibration in the pulmonary and systemic capillary beds. A potential drawback is incomplete mixing, which may result from the proximity between injection and detection sites.

Regional blood flow can be measured indirectly by “clearance” methods The methods used to measure regional blood flow are often called clearance methods, although the term here has a meaning somewhat different from its meaning in kidney physiology. Clearance methods are another application of the Fick principle, using the rate of uptake or elimination of a substance by an organ together with a determination of the difference in concentration of the indicator between the arterial inflow and venous outflow (i.e., the a-v difference). By analogy with Equation 17-18, we can compute the blood flow through an organ (F) from the rate at which the organ removes the test substance X from the blood (Q X ) and the concentrations of the substance in arterial blood ([X]a) and venous blood ([X]v):

F=

moles min Q X = = liters min (17-24) [ X]a − [ X]v moles liter

One can determine hepatic blood flow with the use of BSP (bromsulphthalein), a dye that the liver almost completely clears and excretes into the bile (see p. 951). Here, Q X is the rate of removal of BSP from the blood, estimated as the rate at which BSP appears in the bile. [X]a is the

BOX 17-2  Thallium Scanning for Assessment of Coronary Blood Flow

T

hallium is an ion that acts as a potassium analog and enters cells through the same channels or transporters as K+ does. Active cardiac muscle takes up injected 201Tl, provided there is adequate blood flow. Therefore, the rate of uptake of the 201Tl isotope by the heart is a useful qualitative measure of coronary blood flow. Complete 201Tl myocardial imaging is possible by two-dimensional scanning of the emitted gamma rays or by computed tomography for a threedimensional image. Thus, in those portions of myocardial tissue supplied by stenotic coronary vessels, the uptake is slower, and these areas appear as defects on a thallium scan. Thallium scans are used to detect coronary artery disease during exercise stress tests.

concentration of BSP in a systemic artery, and [X]v is the concentration of BSP in the hepatic vein. In a similar manner, one can determine renal blood flow with the use of PAH (para-aminohippurate). The kidneys almost completely remove this compound from the blood and secrete it into the urine (see pp. 749–750). It is possible to determine coronary blood flow or regional blood flow through skeletal muscle from the tissue clearance of rapidly diffusing inert gases, such as the radioisotopes 133Xe and 85Kr. Finally, one can use the rate of disappearance of nitrous oxide (N2O), a gas that is historically important as the first anesthetic, to compute cerebral blood flow. A similar although qualitative approach is thallium scanning to assess coronary blood flow. Here one measures the uptake of an isotope by the heart muscle, rather than its clearance (Box 17-2).

Ventricular dimensions, ventricular volumes, and volume changes can be measured by angiography and echocardiography Clinicians can use a variety of approaches to examine the cardiac chambers. Gated radionuclide imaging employs N17-6  compounds of the gamma-emitting isotope 99mTc,  which has a half-life of 6 hours. After 99mTc is injected, a gamma camera provides imaging of the cardiac chambers. Electrocardiogram (ECG) gating (i.e., synchronization to a particular spot on the ECG) allows the apparatus to snap a picture at a specific part of the cardiac cycle and to sum these pictures over many cycles. Because this method does not provide a high-resolution image, it yields only a relative ventricular volume. From the difference between the count at the maximally filled state (end-diastolic volume) and at its minimally filled state (end-systolic volume), the cardiologist can estimate the fraction of ventricular blood that is ejected during systole—the ejection fraction—which is an important measure of cardiac function. Angiography can accurately provide the linear dimensions of the ventricle, allowing the cardiologist to calculate absolute ventricular volumes. A catheter is threaded into either the left or the right ventricle, and saline containing a

CHAPTER 17  •  Organization of the Cardiovascular System

N17-6 

99m

Tc Scanning

Contributed by Emile Boulpaep Several compounds labeled with 99mTc—for instance, technetium Tc 99m sestamibi and technetium Tc 99m tetrofosmin— have been introduced for imaging myocardial perfusion. The 99m Tc label emits gamma radiation at 140 keV by an isomeric transition (indicated by the m in 99m); it has a half-life of 6 hours. Following injection, the initial distribution of these agents in the myocardium is proportional to the relative distribution of myocardial blood flow. The radiochemical enters cardiac myocytes passively in such a way that about 30% to 40% of the chemical is extracted by the myocardium. Extraction may be enhanced by administering nitrates prior to injection. Because the radiochemical leaves the myocyte rather slowly (over several hours), one can perform the imaging with the gamma camera over a time period of hours. Note that absolute measurements of myocardial blood flow would require positron-emission tomography (PET), which can quantitate counts per unit volume of tissue. It is possible to use 99mTc-labeled compounds not only for assessing myocardial perfusion but also for assessing myocardial function. In single-photon emission computed tomography (SPECT), the computer acquires imaging data synchronized with the R wave of the ECG (see Fig. 21-7). This gated imaging allows one to display end-diastolic and endsystolic images along various axes of the heart. These enddiastolic and end-systolic dimensions can then be compared to assess ejection fraction, stroke volume, regional wall motion, and regional wall thickening.

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CHAPTER 17  •  Organization of the Cardiovascular System

contrast substance (i.e., a chemical opaque to x-rays) is injected into the ventricle. This approach provides a twodimensional projection of the ventricular volume as a function of time. In magnetic resonance imaging, the physician obtains a nuclear magnetic resonance (NMR) image of the protons in the water of the heart muscle and blood. However, because standard NMR requires long data-acquisition times, it does not provide good time resolution. Echocardiography, which exploits ultrasonic waves to visualize the heart and great vessels, can be used in two modes. In M-mode echocardiography (M is for motion), the technician places a single transducer in a fixed position on the chest wall and obtains a one-dimensional view of heart components. As shown in the upper portion of Figure 17-17A, the ultrasonic beam transects the anterior wall of the right ventricle, the right ventricle, the septum, the left ventricle, the leaflets of the mitral valve, and the posterior wall of the left ventricle. The lower portion of Figure 17-17A shows the positions of the borders between these structures (x-axis) during a single cardiac cycle (y-axis) and thus how the size of the left ventricle—along the axis of the beam— changes with time. Of course, the technician can obtain other views by changing the orientation of the beam. In two-dimensional echocardiography, the probe auto­ matically and rapidly pivots, scanning the heart in a single anatomical slice or plane (see Fig. 17-17A, area between the two broken lines) and providing a true cross section. This approach is therefore superior to angiography, which provides only a two-dimensional projection. Because cardiac output is the product of heart rate and stroke volume, one can calculate cardiac output from echocardiographic measurements of ventricular end-diastolic and end-systolic volume. A problem common to angiography and M-mode echocardiography is that it is impossible to compute ventricular volume from a single dimension because the ventricle is not a simple sphere. As is shown in Figure 17-17B, the left ventricle is often assumed to be a prolate ellipse, with a long axis L and two short axes D1 and D2. To simplify the calculation and to allow ventricular volume to be computed from a single measurement, it is sometimes assumed that D1 and D2 are identical and that D1 is half of L. Unfortunately, use of this algorithm and just a single dimension, as provided by M-mode echocardiography,  N17-7  often yields grossly erroneous volumes. Use of two-dimensional echocardiography to sum information from several parallel slices through the ventricle, or from planes that are at a known angle to one another, can yield more accurate volumes. In addition to ultrasound methods and angiography, the technique of magnetic resonance angiography, an application of magnetic resonance tomography, is used to obtain twodimensional images of slices of ventricular volumes or of blood vessels. In contrast to standard echocardiography, Doppler echocardiography provides information on the velocity, direction, and character of blood flow, just as police radar monitors traffic. In Doppler echocardiography (as with police radar), most information is obtained with the beam parallel to the flow of blood. In the simplest application of Doppler flow measurements, one can continuously monitor the velocity of flowing blood in a blood vessel or part of the heart. On such a record, the x-axis represents time, and the y-axis

A

427

PRINCIPLE OF ECHOCARDIOGRAPHY Right ventricle

Aorta

Transducer can rotate to produce views of other axes.

Mitral valves

Left ventricle

Sound waves Reflected waves 0

Time (s)

0.5

1.0

B

Motion of boundaries

ASSUMED VENTRICULAR GEOMETRY

L D1 D2

Figure 17-17  M-mode and two-dimensional echocardiography. In A, the tracing on the bottom shows the result of an M-mode echocardiogram (i.e., transducer in a single position) during one cardiac cycle. The waves represent motion (M) of heart boundaries transected by a stationary ultrasonic beam. In two-dimensional echocardiography (upper panel), the probe rapidly rotates between the two extremes (broken lines), producing an image of a slice through the heart at one instant in time.

represents the spectrum of velocities of the moving red blood cells (i.e., different cells can be moving at different velocities). Flow toward the transducer appears above baseline, whereas flow away from the transducer appears below baseline. The intensity of the record at a single point on the y-axis (encoded by a gray scale or false color) represents the strength of the returning signal, which depends on the number of red blood cells moving at that velocity. Thus, Doppler echocardiography is able to distinguish the

428

SECTION IV  •  The Cardiovascular System

B

A

Figure 17-18  The colors, which encode the velocity of blood flow, are superimposed on a two-dimensional echocardiogram, which is shown in a gray scale. A, Blood moves through the mitral valve and into the left ventricle during diastole. Because blood is flowing toward the transducer, its velocity is encoded as red. B, Blood moves out of the ventricle and toward the aortic valve during systole. Because blood is flowing away from the transducer, its velocity is encoded as blue. (From Feigenbaum H: Echocardiography. In Braunwald E [ed]: Heart Disease: A Textbook of Cardiovascular Medicine, 5th ed. Philadelphia, WB Saunders, 1997.)

character of flow: laminar versus turbulent. Alternatively, at one instant in time, the Doppler technician can scan a region of a vessel or the heart, obtaining a two-dimensional, color-encoded map of blood velocities. If we overlay such two-dimensional Doppler data on a two-dimensional echocardiogram, which shows the position of the vessel or cardiac structures, the result is a color flow Doppler echocardiogram (Fig. 17-18).

Finally, a magnetic resonance scanner can also be used in two-dimensional phase-contrast mapping to yield quantitative measurements of blood flow velocity.

REFERENCES The reference list is available at www.StudentConsult.com.

CHAPTER 17  •  Organization of the Cardiovascular System

N17-7  Ventricular Volume from M-Mode Echocardiography Contributed by Emile Boulpaep As shown in Figure 17-17B, the left ventricle is often assumed to be a prolate ellipse, with a long axis L and two short axes D1 and D2. To simplify the calculation, and to allow ventricular volume to be computed from a single measurement, it is sometimes assumed that D1 and D2 are identical, and that D1 is half of L. Unfortunately, use of this algorithm and just a single dimension, as provided by M-mode echocardiography, often yields grossly erroneous volumes. One can obtain a more accurate estimate of ventricular volume by including an independent measurement of a second dimension, as is done in two-dimensional echocardiography. For example, one could obtain the long axis (L) in addition to the short axes (D1 and D2, which are assumed to be the same in the simple calculation). However, the ventricle often does not resemble a prolate ellipse, certainly not in pathological states. Thus, cardiologists have used more complex geometric models (e.g., bullet shape).

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428.e2

SECTION IV  •  The Cardiovascular System

REFERENCES Books and Reviews Badeer HS: Hemodynamics for medical students. Adv Physiol Educ 25:44–52, 2001. Caro CG, Pedley TJ, Schroter RC, Seed WA: The Mechanics of the Circulation. Oxford, UK, Oxford University Press, 1978. Lassen NA, Henriksen O, Sejrsen P: Indicator methods for measurement of organ and tissue blood flow. In Handbook of Physiology, Section 2: The Cardiovascular System, vol 3. Bethesda, MD, American Physiological Society, 1979, pp 21–63. Levine RA, Gillam LD, Weyman AE: Echocardiography in cardiac research. In Fozzard HA, Haber E, Jennings RB, et al (eds): The Heart and Cardiovascular System. New York, Raven Press, 1986, pp 369–452. Maeda N, Shiga T: Velocity of oxygen transfer and erythrocyte rheology. News Physiol Sci 9:22–27, 1994. Rowland T, Obert P: Doppler echocardiography for the estimation of cardiac output with exercise. Sports Med 32:973–986, 2002. Journal Articles Coulter NA Jr, Pappenheimer JR: Development of turbulence in flowing blood. Am J Physiol 159:401–408, 1949.

Cournand A, Ranges HA: Catheterization of the right auricle. Proc Soc Exp Biol Med 46:462–466, 1941. Fähraeus R, Lindqvist T: The viscosity of the blood in narrow capillary tubes. Am J Physiol 96:562–568, 1931. Hamilton WF, Moore JW, Kinsman JM, Spurling RG: Studies on the circulation. IV. Further analysis of the injection method and of changes in hemodynamics under physiological and pathological conditions. Am J Physiol 99:534–551, 1932. Haynes RH: Physical basis of the dependence of blood viscosity on tube radius. Am J Physiol 198:1193–1200, 1960. Poiseuille JLM: Recherches expérimentales sur le mouvement des liquides dans les tubes de très petits diamètres. Mem Savant Etrangers Paris 9:433–544, 1846. Reynolds O: An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinusoid, and of the law of resistance in parallel channels. Philos Trans R Soc Lond B Biol Sci 174:935–982, 1883. Thury A, van Langenhove G, Carlier SG, et al: High shear stress after successful balloon angioplasty is associated with restenosis and target lesion revascularization. Am Heart J 144:136–143, 2002.

C H A P T E R 18  BLOOD Emile L. Boulpaep

Blood is a complex fluid consisting of plasma—extracellular fluid rich in proteins—and of formed elements—red blood cells (RBCs), white blood cells (WBCs), and platelets. Total blood volume is ~70 mL/kg body weight in the adult woman and ~80 mL/kg body weight in the adult man (see Table 5-1).

BLOOD COMPOSITION Whole blood is a suspension of cellular elements in plasma If you spin down a sample of blood containing an anticoagulant for ~5 minutes at 10,000 g, the bottom fraction contains formed elements—RBCs (or erythrocytes), WBCs (leukocytes, which include granulocytes, lymphocytes, and monocytes), and platelets (thrombocytes); the top fraction is blood plasma (Fig. 18-1). The RBCs having the highest density are at the bottom of the tube, whereas most of the WBCs and platelets form a whitish gray layer—the buffy coat—between the RBCs and plasma. Only a small amount of WBCs, platelets, and plasma is trapped in the bottom column of RBCs. The hematocrit (see p. 102) is the fraction of the total column occupied by RBCs. The normal hematocrit is ~40% for adult women and ~45% for adult men. The hematocrit in the newborn is ~55% and falls to ~35% at 2 months of age, from which time it rises during development to reach adult values at puberty. The hematocrit is a measure of concentration of RBCs, not of total body red cell mass. Expansion of plasma volume in a pregnant woman reduces the hematocrit, whereas her total red cell volume also increases but less than plasma volume (see p. 1142). Immediately after hemorrhage, the hematocrit may be normal despite the loss of blood volume (see pp. 585–586). Total RBC volume is ~28 mL/kg body weight in the adult woman and ~36 mL/kg body weight in the adult man. Plasma is a pale-white watery solution of electrolytes, plasma proteins, carbohydrates, and lipids. Pink-colored plasma suggests the presence of hemoglobin caused by hemolysis (lysis of RBCs) and release of hemoglobin into the plasma. A brown-green color may reflect elevated bilirubin levels (see Box 46-1). Plasma can also be cloudy in cryoglobulinemias (see pp. 438–439). The electrolyte composition of plasma differs only slightly from that of interstitial

fluid on account of the volume occupied by proteins and their electrical charge (see Table 5-2). Plasma proteins at a normal concentration of ~7.0 g/dL account for a colloid osmotic pressure or oncotic pressure of ~25 mm Hg (see p. 470). Principal plasma proteins are albumin, fibrinogen, globulins, and other coagulation factors. The molecular weights of plasma proteins range up to 970 kDa (Table 18-1). The plasma concentration of albumin ranges from 3.5 to 5.5 g/dL, which provides the body with a total plasma albumin pool of ~135 g. Albumin is synthesized by the liver at a rate of ~120 mg/kg body weight per day and, due to catabolism, has a half-life in the circulation of ~20 days. Urinary losses of albumin are normally negligible (40 mm thick in certain inflammatory disorders). This rate of fall is called the erythrocyte sedimentation rate (ESR). Although it is nonspecific because so many different conditions can cause it to increase, the ESR is still widely used by clinicians to assess the presence and severity of inflammation. It is a simple technique, easily performed in a physician’s office. As an example of its utility, a patient with an inflammatory process that naturally waxes and wanes, such as lupus erythematosus, may present with nonspecific complaints such as fatigue, weakness, and achiness. An elevated ESR would suggest that these complaints are due to the reactivation of the disease and not just to a poor night’s sleep or depression.

FUNCTION

Transthyretin

62

Binds T3 and T4 Binds vitamin A

Albumin

69

Oncotic pressure Binds steroids, T3, bilirubin, bile salts, fatty acids

α1-antitrypsin (α1AT)

54

Protease inhibitor Deficiency causes emphysema

α2-macroglobulin

725

Broad-spectrum protease inhibitor Synthesized by liver

Haptoglobin

100

Binds hemoglobin

β-lipoprotein = low-density lipoprotein (LDL)

380

Binds lipid

80

Binds iron

Transferrin

Hematocrit = Height of RBCs Total height

MOLECULAR WEIGHT (kDa)

Complement C3

185

Third component of complement system

Fibrinogen

340

Clotting protein Precursor of fibrin

Immunoglobulin A (IgA)

160

Mucosal immunity Synthesized by plasma cells in exocrine glands

Immunoglobulin D (IgD)

170

Synthesized by B lymphocytes

Immunoglobulin E (IgE)

190

Synthesized by B lymphocytes Binds to mast cells or basophils

Immunoglobulin G (IgG)

150

Humoral immunity Synthesized by plasma cells

Immunoglobulin M (IgM)

970

Humoral immunity Synthesized by B lymphocytes

*The proteins are listed in the approximate order of decreasing electrophoretic mobility.

mobility (Fig. 18-2A): albumin, α1-globulins, α2-globulins, β-globulins, fibrinogen, and γ-globulins. The three most abundant peaks are albumin, fibrinogen, and γ-globulins. The γ-globulins include the immunoglobulins or antibodies, which can be separated into IgA, IgD, IgE, IgG, and IgM. Immunoglobulins are synthesized by B lymphocytes and plasma cells. Clinical laboratories most often perform electrophoresis of blood proteins on serum instead of plasma (see Fig. 18-2B). Table 18-1 shows the major protein components

Chapter 18  •  Blood

A

PLASMA PROTEINS Alb

2

1

Gc

Bone marrow is the source of most blood cells

12

AT3

Hpt

-Lp



Pl CRP C1q

1Ac Tf C3 Fibr C4 C5 -LP IgM C1Inh Hpx C1s IgA IgD(E) Cer 2M

1At

IaTl 1Ag

Pre A

Alb

IgG FB

Fibrinogen

12



C1r B

SERUM PROTEINS Alb

1

2

Gc

AT3

Hpt

-Lp

Pl C1q

CRP

1Ac 2M

1At IaTl 1Ag

Pre A

-LP C1Inh C1s Cer

Tf

C4 C5

IgA

Alb

Notice absence of fibrinogen.

C3

Hpx

IgM IgD(E) IgG

C1r

431

FB

Figure 18-2  Electrophoretic pattern of human plasma and serum proteins. Normal concentration ranges are as follows: total protein, 6 to 8 g/dL; albumin, 3.1 to 5.4 g/dL; α1-globulins, 0.1 to 0.4 g/dL; α2-globulins, 0.4 to 1.1 g/dL; β-globulins, 0.5 to 1.2 g/dL; γ-globulins, 0.7 to 1.7 g/dL.

that are readily resolved by electrophoresis. Proteins present in plasma at low concentrations are identified by immunological techniques, such as radioimmunoassay (see p. 976) or enzyme-linked immunosorbent assay. Not listed in Table 18-1 are several important carrier proteins present in plasma: ceruloplasmin (see p. 970), transcobalamin (see p. 937), corticosteroid-binding globulin (CBG; see p. 1021), insulin-like growth factor (IGF)–binding proteins (see p. 996), sex hormone–binding globulin (SHBG or TeBG; see pp. 1119–1120), thyroid-binding globulin (see pp. 1008– 1009), and vitamin D–binding protein (see p. 1064). The liver synthesizes most of the globulins and coagulation factors.  N18-1

If you spread a drop of anticoagulated blood thinly on a glass slide, you can detect under the microscope the cellular elements of blood. In such a peripheral blood smear, the following mature cell types are easily recognized: erythrocytes; granulocytes divided in neutrophils, eosinophils, and basophils; lymphocytes; monocytes; and platelets (Fig. 18-3). Hematopoiesis is the process of generation of all the cell types present in blood. Because of the diversity of cell types generated, hematopoiesis serves multiple roles ranging from the carriage of gases to immune responses and hemostasis. Pluripotent long-term hematopoietic stem cells (LT-HSCs) constitute a population of adult stem cells found in bone marrow that are multipotent and able to self-renew. The short-term hematopoietic stem cells (ST-HSCs) give rise to committed stem cells or progenitors, which after proliferation are able to differentiate into lineages that in turn give rise to burst-forming units (BFUs) or colony-forming units (CFUs), each of which ultimately will produce one or a limited number of mature cell types: erythrocytes, the megakaryocytes that give rise to platelets, eosinophils, basophils, neutrophils, monocytes-macrophages/dendritic cells, and B or T lymphocytes and natural killer cells (Fig. 18-4). Soluble factors known as cytokines guide the development of each lineage. The research of Donald Metcalf demonstrated the importance of a family of hematopoietic cytokines that stimulate colony formation by progenitor cells, the colonystimulating factors. The main colony-stimulating factors are granulocyte-macrophage colony-stimulating factor (GM-CSF; see p. 70), granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF), interleukin-3 (IL-3) and IL-5 (see p. 70), thrombopoietin (TPO), and erythropoietin (EPO; see pp. 431–433). GM-CSF is a glycoprotein that stimulates proliferation of a common myeloid progenitor and promotes the production of neutrophils, eosinophils, and monocytes-macrophages. Recombinant GM-CSF (sargramostim [Leukine]) is used clinically after bone marrow transplantation and in certain acute leukemias. G-CSF and M-CSF are glycoproteins that guide the ultimate development of granulocytes and monocytesmacrophages/dendritic cells, respectively. Recombinant GCSF (filgrastim [Neupogen]) is used therapeutically in neutropenia (e.g., after chemotherapy). M-CSF is also required for osteoclast development (see p. 1057 and Fig. 52-4). IL-3 (also known as multi-CSF) has a broad effect on multiple lineages. The liver and the kidney constitutively produce this glycoprotein. IL-5 (colony-stimulating factor, eosinophil), a homodimeric glycoprotein, sustains the terminal differentiation of eosinophilic precursors. TPO binds to a TPO receptor called c-Mpl, which is the cellular homolog of the viral oncogene v-mpl (murine myeloproliferative leukemia virus). On stimulation by TPO, the Mpl receptor induces an increase in the number and size of megakaryocytes—the cells that produce platelets— which thereby greatly augments the number of circulating platelets. N18-2  which is homologous to TPO, is proEPO,  duced by the kidney and to a lesser extent by the liver. This cytokine supports erythropoiesis or red cell development

Chapter 18  •  Blood

431.e1

N18-1  Plasma Proteins Contributed by Emile Boulpaep Protein

Conventional Units

International Units

Protein, total   Electrophoresis

6.4–8.3 g/dL Albumin: 3.5–5.0 g/dL α1-globulin: 0.1–0.3 g/dL α2-globulin: 0.6–1.0 g/dL β-globulin: 0.7–1.1 g/dL γ-globulin: 0.8–1.6 g/dL

64.0–83.0 g/L 35–50 g/L 1–3 g/L 6–10 g/L 7–11 g/L 8–16 g/L M: 2.5–11.7 U/L F: 0.3–9.2 U/L 0.17–0.68 µkat/L 0.12–0.60 µkat/L 34–48 g/L Adult (>20 yr) 0.43–1.70 µkat/L 0.46–2.23 µkat/L 1.2 mM

1

CO2 40 mm Hg

2A

3A

CO2 40 mm Hg

CO2 1.2 mM

Equilibrate

H2O

H2O

HCO–3 H+ + 0.000,068 mM 14 mM

HCO–3 PPV > PA 0 cm H2O

50

0

–3 cm H2O

+7 cm H 2O

5

10 15 Distance (cm)

0 cm H2O

+10 cm H2O

20 Apex

0 cm H2O

+5 cm H2O

C—LUNG ZONES Zone 1 conditions occur only when PA is high (e.g., positive-pressure ventilation) or when PPA is low (e.g., hemorrhage).

+20 cm H2O

+10 cm H2O

Zone 2 +15 cm H2O at midpoint

Zone 3 Zone 4

PIP leads to partial collapse of extraalveolar vessels.

Zone 4 PPA > PPV > PA

Lungs normally have Zones 2 through 4. Smaller regional lung volume leads to less mechanical tethering. Figure 31-9  Physiological nonuniformity of pulmonary perfusion.

0 cm H2O

Chapter 31  •  Ventilation and Perfusion of the Lungs

pressures. We define the first three zones based on how alveolar blood vessels are affected by the relative values of three different pressures: alveolar pressure (PA), the pressure inside pulmonary arterioles (PPA), and the pressure inside pulmonary venules (PPV). In the fourth zone, we instead focus on how extra-alveolar vessels are affected by intrapleural pressure (PIP). Zone 1: PA > PPA > PPV  These conditions prevail at the apex of the lung under certain circumstances. The defining characteristic of a zone 1 alveolar vessel is that PPA and PPV are so low that they have fallen below PA. At the level of the left atrium (the reference point for the pressure measurements), the mean PPA is ~15 mm Hg (see Table 31-1), which—because mercury is 13.6-fold more dense than water—corresponds to ~20 cm H2O (see Fig. 31-9C, lower illustration for zone 3). Similarly, mean PPV is ~8 mm Hg, or ~10 cm H2O. As we move upward closer to the apex of an upright lung, the actual pressures in the lumens of pulmonary arterioles and venules fall by 1 cm H2O for each 1 cm of vertical ascent. In the hypothetical case in which alveoli at the lung apex are 20 cm above the level of the left atrium, the mean PPA of these alveoli would be 0 cm H2O (see Fig. 31-9C, zone 1). The corresponding PPV would be about −10 cm H2O. The pressure inside the pulmonary capillary (Pc) would be intermediate, perhaps −5 cm H2O. In principle, blood would still flow through this capillary—the driving pressure would be ~10 cm H2O— were it not for the pressure inside the surrounding alveoli, which is 0 cm H2O between breaths. Therefore, because PA is much higher than Pc, the negative PTM (see p. 414) would tend to crush the capillary and reduce blood flow. Fortunately, zone 1 conditions do not exist for normal people at rest. However, they can arise if there is either a sufficient decrease in PPA (e.g., in hemorrhage) or a sufficient increase in PA (e.g., in positive-pressure ventilation). Zone 2: PPA > PA > PPV  These conditions normally prevail from the apex to the mid-lung. The defining characteristic of zone 2 is that mean PPA and PPV are high enough so that they sandwich PA (see Fig. 31-9C, zone 2). Thus, at the arteriolar end, the positive PTM causes the alveolar vessel to dilate. Further down the capillary, though, luminal pressure gradually falls below PA, so that the negative PTM squeezes the vessel, raising resistance and thus reducing flow. As we move downward in zone 2, the crushing force decreases because the hydrostatic pressures in the arteriole, capillary, and venule all rise in parallel by 1 cm H2O for each 1 cm of descent (see Fig. 31-9C, upper → lower illustrations for zone 2). Simultaneously, resistance decreases. The conversion of a closed vessel (or one that is open but not conducting) to a conducting one by increased PPA and PPV is an example of recruitment. Zone 3: PPA > PPV > PA  These conditions prevail in the middle to lower lung. The defining characteristic of zone 3 is that mean PPA and PPV are so high that they both exceed PA (see Fig. 31-9C, zone 3). Thus, PTM is positive along the entire length of the alveolar vessel, tending to dilate it. As we move downward in zone 3, the hydrostatic pressures in the arteriole, capillary, and venule all continue to rise by 1 cm

689

H2O for each 1 cm of descent. Because PA between breaths does not vary with height in the lung, the gradually increasing pressure of the alveolar vessel produces a greater and greater PTM, causing the vessel to dilate more and more—an example of distention (see Fig. 31-9C, upper → lower illustrations for zone 3). This distention causes a gradual decrease in resistance of the capillaries as we move downward in zone 3. Hence, although the driving force (PPA − PPV) remains constant, perfusion increases toward the base of the lung. The arrangement in which a variable PTM controls flow is known as a Starling resistor. Keep in mind that the driving force (PPA − PPV) is constant in all of the zones. Zone 4: PPA > PPV > PA  These conditions prevail at the extreme base of the lungs. In zone 4, the alveolar vessels behave as in zone 3; they dilate more as we descend toward the base of the lung. However, the extra-alveolar vessels behave differently. At the base of the lung, PIP is least negative (see Fig. 31-5C). Thus, as we approach the extreme base of the lung, the distending forces acting on the extra-alveolar blood vessels fade, and the resistance of these extra-alveolar vessels increases (see Fig. 31-9C, zone 4). Recall that we saw a similar effect—at the level of the whole lung (see Fig. 31-7B, blue curve)—where resistance of the extra-alveolar vessels increased as lung volume fell (i.e., as PIP became less negative). Because these extra-alveolar vessels feed or drain  begins to fall from its peak as we the alveolar vessels, Q approach the extreme base of the lungs (see Fig. 31-9B). These lung zones are physiological, not anatomical. The boundaries between the zones are neither fixed nor sharp. For example, the boundaries can move downward with positive-pressure ventilation (which increases PA) and can move upward with exercise (which increases PPA). In our discussion of lung zones, we have tacitly assumed that PA is always zero and that the values of PPA and PPV are stable and depend only on height in the lung. In real life, of course, things are more complicated. During the respiratory cycle, PA becomes negative during inspiration (promoting dilation of alveolar vessels) but positive during expiration. During the cardiac cycle, the pressure inside the arterioles and pulmonary capillaries is greatest during systole (promoting dilation of the vessel) and lowest during diastole. Thus, we would expect blood flow through an alveolar vessel to be greatest when inspiration coincides with systole.

MATCHING VENTILATION AND PERFUSION The greater the ventilation-perfusion ratio, the higher the PO2 and the lower the PCO2 in the alveolar air In Figure 31-4 we saw that, all other factors being equal, alveolar ventilation determines alveolar PO2 and PCO2. The greater the ventilation, the more closely PA O2 and PA CO2 approach their respective values in inspired air. However, in Figure 31-4 we were really focusing on total alveolar ventilation and how this influences the average, or idealized, alveolar PO2 and PCO2. In fact, we have already learned that both ventilation and perfusion vary among alveoli. In any group of alveoli, the greater the local ventilation, the more closely the composition of local alveolar air approaches that of the inspired air. Similarly, because blood flow removes O2

690

A

SECTION V  •  The Respiratory System

· · DEPENDENCE OF VA/Q ON HEIGHT IN LUNG Base Apex 4 3

· VA · 2 Q

· · VA/Q

· Q

200

· VA

THE O2-CO2 DIAGRAM

Mixedvenous blood: · · VA/Q = 0

5

4 3 Rib number

·

0

·

Low VA/Q Arterial blood

20

0

2

60

40 PCO2 (mm Hg)

100

1 0

· · VA or Q Unit volume (arbitrary units)

B

40

60

High · V

A /Q

100 120 PO2 (mm Hg)

80

·

Inspired air: · · VA/Q = ∞

140

160

 ratio and alveolar gas composition. (Data from West JB: Ventilation/ Figure 31-10  Regional differences in V A /Q Blood Flow and Gas Exchange. Oxford, UK, Blackwell, 1985.)

TABLE 31-3  Effect of Regional Differences in V A /Q on the Composition of Alveolar Air and Pulmonary-Capillary Blood LOCATION Apex Base Overall

FRACTION OF TOTAL LUNG VOLUME

V A /Q

PO2 (mm Hg)

PCO2 (mm Hg)

pH

Q (L/min)

7%

3.3

132

28

7.55

0.07

13% 100%

0.6 0.84*

89

42

7.38

1.3

100

40

7.40

5.0

*Because the transport of both O2 and CO2 is perfusion limited, we assume that end-capillary values of PO2 and PCO2 are the same as their respective alveolar  ratio for the values. If the overall alveolar ventilation for the two lungs is 4.2 L/min, and if the cardiac output (i.e., perfusion) is 5 L/min, then the overall V A /Q two lungs is (4.2 L/min)/(5 L/min) = 0.84. Modified from West JB: Ventilation/Blood Flow and Gas Exchange. Oxford, UK, Blackwell, 1989.

from the alveolar air and adds CO2, the greater the perfusion, the more closely the composition of local alveolar air approaches that of mixed-venous blood. Thus, the local  ) determines the local  A /Q ventilation-perfusion ratio (V PA O2 and PA CO2. You might view the alveoli as a sports venue where ventilation and perfusion are engaged in a continuous struggle over control of the composition of alveolar air. To the extent that ventilation gains the upper hand, PA O2 rises and PA CO2 falls. To the extent that perfusion holds sway, these parameters change in the opposite direction. As a physical analog of this struggle over control of alveo A ) from a lar PO2 , consider water flowing (analogous to V  through a faucet into a sink (alveoli); the water exits (Q) drain with an adjustable opening. If the drain opening is in midposition and we begin flowing water moderately fast, then the water level (PA O2) will gradually increase and reach  A ) will cause a steady state. Increasing the inflow of water ( V the water level (PA O2 ) to rise until the product of pressure head and drain conductance is high enough to drive water down the drain as fast as the water flows in. If we increase  then the drain opening and thus the outflow of water (Q), the water level (PA O2 ) will fall until the decrease in the pressure head matches the increase in drain conductance, so that once again water inflow and outflow are balanced. Just as a high faucet-drain ratio will raise the water level, a high  ratio will increase alveolar PO .  A /Q V 2

Because of the action of gravity, the regional V A /Q ratio in an upright subject is greater at the apex of the lung than at the base We have already seen that when a subject is upright in a gravitational field, ventilation falls from the base to the apex of the lung (see Fig. 31-5B) and that perfusion also falls, but more steeply (see Fig. 31-9B). Thus, it is not surprising  itself varies with height in the lung  A /Q that the ratio V  is lowest near the base, where Q  exceeds  A /Q (Fig. 31-10A). V  A. The ratio gradually increases to 1 at about the level of V the third rib and further increases toward the apex, where  falls more precipitously than V  A. Q  at the apex and  A /Q Table 31-3 shows how differences in V base of the lungs influence the regional composition of alveolar air. At the apex (the most rostral 7% of lung volume in  is highest, alveolar PO and PCO  A /Q this example), where V 2 2 most closely approach their values in inspired air. Because both O2 transport and CO2 transport across the blood-gas barrier are perfusion limited (see pp. 671–673), O2 and CO2 have completely equilibrated between the alveolar air and the blood by the end of the pulmonary capillaries. Thus, blood leaving the apex has the same high PO2 and low PCO2 as the alveolar air. Of course, the relatively low PCO2 produces a respiratory alkalosis (see p. 634) in the blood leaving the apex. The situation is just the opposite near the base of the lung (the most caudal 13% of lung volume in this example).

Chapter 31  •  Ventilation and Perfusion of the Lungs

A

ALVEOLAR DEAD-SPACE VENTILATION WITHOUT COMPENSATION B

3 Perfusion of other lung increases, causing · · V/Q.

1 Because perfusion to this lung stops, while ventilation continues, · · V/Q ∞. PO2 PCO2

691

COMPENSATION: BRONCHIOLAR CONSTRICTION 1 PO2, PCO2, and pH around smooth muscle causes bronchiolar constriction, diverting airflow to “normal” airways. 2 In response to blood flow, alveolar type II pneumocytes produce less surfactant, causing compliance and ventilation (alveoli shrink!).

PO2 = 149 PCO2 = 0

2 The alveolar gas assumes the composition of inspired air.  mismatch and compensatory response—alveolar dead-space ventilation. Figure 31-11  Extreme V A /Q

 here is lowest, alveolar PO and PCO tend more  A /Q Because V 2 2 toward their values in mixed-venous blood. What impact do these different regions of the lung, each with its own  ratio, have on the composition of systemic arterial  A /Q V blood? Each region makes a contribution that is proportional to its blood flow (see the rightmost column in Table 31-3). Because the apex is poorly perfused, it makes only a small contribution to the overall composition of arterial blood. On the other hand, pulmonary tissue at the base of the lungs, which receives ~26% of total cardiac output, makes a major contribution. As a result, the average composition of blood exiting the lung more closely reflects the composition of the blood that had equilibrated with the air in the base of the lung. The O2-CO2 diagram introduced as Figure 29-11 is a  ratios through A /Q helpful tool for depicting how different V out the lung produce different blood-gas compositions. The curve in Figure 31-10B represents all possible combinations of PO2 and PCO2 in the alveolar air or end-pulmonarycapillary blood. The H2O-saturated inspired air (PO2 = 149, PCO2 = ~0 mm Hg) represents the rightmost extreme of the  ratio of inspired air is ∞,  A /Q diagram. By definition, the V because it does not come into contact with pulmonarycapillary blood. The mixed-venous blood (PO2 = 40, PCO2 = 46 mm Hg) represents the other extreme. By definition, the  ratio of mixed-venous blood is zero, because it has not  A /Q V yet come into contact with alveolar air. With the end points of the diagram established, we can now predict—with the help of the alveolar gas equation (see Equation 31-17), the Bohr effect (see p. 652), and the Haldane effect (see p. 657)— all possible combinations of PO2 and PCO2 throughout the lung. As shown in Figure 31-10B, the base, midportion, and apex of the lungs correspond to points along the O2-CO2 diagram between mixed-venous blood at one extreme and inspired air at the other.

The ventilation of unperfused alveoli (local V A /Q = ∞) triggers compensatory bronchoconstriction and a fall in surfactant production The effects of gravity on ventilation and perfusion cause  to vary widely, even in idealized lungs (see  A /Q regional V Fig. 31-10A). However, microscopic or local physiological and pathological variations in ventilation and perfusion can  , the extremes of  A /Q cause even greater mismatches of V which are alveolar dead-space ventilation (this section) and shunt (next section). Alveolar Dead-Space Ventilation  At one end of the spec mismatches is the elimination of blood  A /Q trum of V flow to a group of alveoli. For example, if we ligated the pulmonary artery feeding one lung, the affected alveoli would receive no perfusion even though ventilation would initially continue normally (Fig. 31-11A). Above, we saw that such alveolar dead space together with the anatomical dead space constitute the physiological dead space (see Equation 31-8). The ventilation of the unperfused alveoli is called alveolar dead-space ventilation because it does not contribute to gas exchange. Thus, these alveoli behave like conducting airways.  N31-12 A natural cause of alveolar dead-space ventilation is a pulmonary embolism, which obstructs blood flow to a group of alveoli. Because one task of the lung is to filter small emboli from the blood (see p. 600), the lung must deal with small regions of alveolar dead-space ventilation on a recurring basis. At the instant the blood flow ceases, the alveoli supplied by the affected vessel(s) contain normal alveolar air. However, each cycle of inspiration and expiration replaces some stale alveolar air with fresh, inspired air. Because no exchange of O2 and CO2 occurs between these unperfused alveoli and pulmonary-capillary blood, the alveolar gas

Chapter 31  •  Ventilation and Perfusion of the Lungs

N31-12  Notes on the Differences Between Anatomical and Physiological Dead Space Contributed by Emile Boulpaep and Walter Boron There is a fundamental difference between the anatomical and alveolar dead space. The conducting airways are in series with and upstream from (proximal to) the alveoli. The conducting airways have the composition of inspired air only after an inspiration; after an expiration, they have the composition of alveolar air. On the other hand, unperfused alveoli are in parallel with normal alveoli and have the composition of inspired air, regardless of the position in the respiratory cycle.

691.e1

692

SECTION V  •  The Respiratory System

A

SHUNT WITHOUT COMPENSATION

B COMPENSATION: VASOCONSTRICTION

1 Because ventilation to this lung stops, while perfusion continues, · · V/Q 0.

3 Ventilation of other lung increases, · · causing V/Q.

In response to local alveolar hypoxia, the arterioles feeding the alveoli constrict: hypoxic vasoconstriction.

PO2 PCO2

2 The alveolar gas assumes the composition of mixed-venous blood.  mismatch and compensatory response—shunt. Figure 31-12  Extreme V A /Q

gradually achieves the composition of moist inspired air, with alveolar PO2 rising to ~149 mm Hg and PCO2 falling to ~0 mm Hg (see Fig. 31-11A, step 2). By definition, alveolar  ratio of ∞, as described by the  A /Q dead space has a V “Inspired air” point on the x-axis of an O2-CO2 diagram (see Fig. 31-10B). Redirection of Blood Flow  Blocking blood flow to one group of alveoli diverts blood to other “normal” alveoli, which then become somewhat hyperperfused. Thus, the  in alveoli downstream  A /Q blockage not only increases V  in other regions.  A /Q from the blockage, but also decreases V Redirection of blood flow thus accentuates the nonuniformity of ventilation. Regulation of Local Ventilation  Because alveolar deadspace ventilation causes alveolar PCO2 to fall to ~0 mm Hg in downstream alveoli, it leads to a respiratory alkalosis (see p. 634) in the surrounding interstitial fluid. These local changes trigger a compensatory bronchiolar constriction in the adjacent tissues (see Fig. 31-11B), so that over a period of seconds to minutes, airflow partially diverts away from the unperfused alveoli and toward normal alveoli, to which blood flow is also being diverted. This compensation makes  shift  A /Q teleological sense, because it tends to correct the V in both the unperfused and normal alveoli. The precise mechanism of bronchiolar constriction is unknown, although bronchiolar smooth muscle may contract—at least in part— in response to a high extracellular pH.  N31-13 In addition to a local respiratory alkalosis, the elimination of perfusion has a second consequence. Downstream from the blockage, alveolar type II pneumocytes become starved for various nutrients, including the lipids they need to make surfactant. (These cells never become starved for O2!) As a result of the decreased blood flow, surfactant production falls over a period of hours to days. The result is a local decrease in compliance, further reducing local ventilation.

These compensatory responses—bronchiolar constriction (i.e., increased resistance, a property of conducting airways) and reduced surfactant production (i.e., decreased compliance, a property of alveoli)—work well only if the alveolar dead space is relatively small, so that an ample volume of healthy tissue remains into which the airflow can divert.

The perfusion of unventilated alveoli (local V A /Q = 0) triggers a compensatory hypoxic vasoconstriction Shunt  Alveolar dead-space ventilation is at one end of the  mismatches. At the opposite end is  A /Q spectrum of V shunt—the flow of blood past unventilated alveoli. For example, if we ligate a mainstem bronchus, then inspired air cannot refresh alveoli distal to the obstruction (see Fig. 31-12A). As a result, mixed-venous blood perfusing the unventilated alveoli “shunts” from the right heart to the left heart, without benefit of ventilation. When the low-O2 shunted blood mixes with high-O2 unshunted blood (which is ventilated), the result is that the mixture has a lower-thannormal PO2 , causing hypoxia in the systemic arteries. It is possible to calculate the extent of the shunt from the degree of hypoxia.  N31-14 Natural causes of airway obstruction include aspiration of a foreign body or the presence of a tumor in the lumen of a conducting airway. The collapse of alveoli (atelectasis) also produces a right-to-left shunt, a pathological example of which is pneumothorax (see p. 608). Atelectasis also occurs naturally in dependent regions of the lungs, where PIP is not so negative (see Fig. 31-5C) and surfactant levels gradually decline. Sighing or yawning stimulates surfactant release (see p. 615) and can reverse physiological atelectasis. Imagine that an infant aspirates a peanut. Initially, the air trapped distal to the obstruction has the composition of normal alveolar air. However, pulmonary-capillary blood gradually extracts O2 from the trapped air and adds CO2.

Chapter 31  •  Ventilation and Perfusion of the Lungs

692.e1

N31-13  Bronchiolar Constriction during Alveolar Dead-Space Ventilation Contributed by Emile Boulpaep and Walter Boron The precise mechanism of bronchiolar constriction in response to alveolar dead-space ventilation is unknown. However, it is intriguing to speculate that, at least in part, the mechanism may parallel that for the autoregulation of blood flow in the brain. The vascular smooth-muscle cells (VSMCs) of the penetrating cerebral arterioles constrict in response to respiratory alkalosis— which is why one feels dizzy after hyperventilating. This

constriction of the VSMCs occurs when one imposes an alkalosis in the complete absence of CO2 /HCO3−. Furthermore, the alkalosis-induced vasoconstriction is due entirely to a pH decrease on the outside of the VSMC. In other words, these cells have some sort of an extracellular pH sensor. A pH increase on the inside of the cell actually has the opposite effect: vasodilation. During extracellular acidosis, the vessels dilate.

N31-14  The Shunt Equation Contributed by Emile Boulpaep and Walter Boron   mismatch and arises when A shunt is one extreme of a V/Q blood perfuses unventilated alveoli. Alveoli may be unventilated because they are downstream from an obstructed conducting airway. Regardless of the mechanism that prevents airflow to these alveoli, the resulting right-to-left shunt causes mixedvenous blood to remain relatively unoxygenated and to go directly to the left heart, where it mixes with oxygenated “arterial” blood. This process is known as venous admixture. Imagine that 80% of the blood flow to the lungs goes to alveoli that are appropriately ventilated but that 20% goes to alveoli that are downstream from completely obstructed conduct . The shunt ing airways. The total perfusion of the lungs is Q T  , 20% in this example, perfusion of the unventilated alveoli is Q S and the shunted blood has an O2 content (units: mL O2/dL) identi −Q  ) cal to that of mixed-venous blood (Cv ). The difference (Q T S is the perfusion to the normally ventilated alveoli, 80% in our example, and this unshunted blood has an O2 content appropriate for the end of a pulmonary capillary (Cc′). The blood emerging from the lungs is a mixture of shunted and unshunted blood so that the O2 emerging from the lung is partially O2 carried by the shunted blood and partially O2 carried by the unshunted blood: Total O2 leaving lungs = Shunted O2 + Unshunted O2 (NE 31-35)  How much O2 per minute emerges from the lungs in the systemic arterial blood? This amount is the product of the O2 content of this arterial blood (Ca) and the total blood flow out the  ): lungs (Q T

. Total O2 leaving lungs = Ca × QT

(NE 31-36) 

Similarly, the O2 contributed by the shunted blood is the product of the O2 content and the flow of shunted blood:

O2 contributed by shunted blood = mL O2 /min

Cv

. × QS

mL O2 100 mL blood

mL blood min

(NE 31-37) 

Finally, the amount of O2 contributed per minute by the un­ shunted blood is . . O2 contributed by unshunted blood = Cc' × (Q T – QS)

(NE 31-38) 

Inserting the expressions for each of the terms in Equations NE 31-36 through NE 31-38 into Equation NE 31-35, we have

. . . . Ca × QT = Cv × QS + Ca × QT – QS

(

TotalO2 leaving lungs

)

O2 carried by shunted blood

(

O2 carried by unshunted blood

)

(NE 31-39) 

Rearranging this equation and solving for the fraction of total  /Q  ), we have blood flow that is represented by the shunt (Q S T

. QS Cc' − Ca . = QT Cc' − Cv

(NE 31-40) 

This expression is known as the shunt equation.  /Q  in our What does Equation NE 31-40 predict for Q S T example? We will assume that the O2 content of mixed-venous blood is 15 mL O2/dL blood, whereas that for blood at the end of the pulmonary capillaries is 20 mL O2/dL blood. These values are similar to those given in Table 29-3. If our hypothetical subject—who is affected by a 20% shunt—has systemic arterial blood with an O2 content of 19 mL O2/dL blood, then the shunt equation predicts

. QS Cc' − Ca 20−19 = =20% . = QT Cc' − Cv 20−15

(NE 31-41) 

Thus, the shunt equation predicts that the shunt is 20% of the total blood flow, which is reasonable, inasmuch as we started the example by assuming that 80% of the blood flowed through properly ventilated alveoli.

Chapter 31  •  Ventilation and Perfusion of the Lungs

Eventually, the PO2 and PCO2 of the trapped air drift to their values in mixed-venous blood. If the shunt is small, so that it does not materially affect the PO2 or PCO2 of the systemic arterial blood, then the alveoli will have a PO2 of 40 mm Hg and a PCO2 of 46 mm Hg. By definition, shunted alveoli have  of zero and are represented by the “Mixed-venous  A /Q aV blood” point on an O2-CO2 diagram (see Fig. 31-10B). Redirection of Airflow  Blocking airflow to one group of alveoli simultaneously diverts air to normal parts of the lung, which then become somewhat hyperventilated. Thus, shunt  in unventilated alveoli, but also  A /Q not only decreases V  in other regions. The net effect is a widening  A /Q increases V  ratios.  A /Q of the nonuniformity of V Asthma  Although less dramatic than complete airway

.  A /Q obstruction, an incomplete occlusion also decreases V An example is asthma, in which hyperreactivity of airway smooth muscle increases local airway resistance and decreases ventilation of alveoli distal to the pathology. Normal Anatomical Shunts  The thebesian veins drain some of the venous blood from the heart muscle, particularly the left ventricle, directly into the corresponding cardiac chamber. Thus, delivery of deoxygenated blood from thebesian veins into the left ventricle (10 times a day) to the scrutiny of the renaltubule epithelium. If it were not for such a high turnover of the ECF, only small volumes of blood would be “cleared” per unit time (see p. 731) of certain solutes and water. Such a low clearance would have two harmful consequences for the renal excretion of solutes that renal tubules cannot adequately secrete. First, in the face of a sudden increase in the plasma level of a toxic material—originating either from metabolism or from food or fluid intake—the excretion of the material would be delayed. A high blood flow and a high GFR allow the kidneys to eliminate harmful materials rapidly by filtration. A second consequence of low clearance would be that steady-state plasma levels would be very high for waste materials that depend on filtration for excretion. The following example by Robert Pitts, a major contributor to renal physiology, illustrates the importance of this concept. Consider two individuals consuming a diet that contains 70 g/day of protein, one with normal renal function (e.g., GFR of

180 L/day) and the other a renal patient with sharply reduced glomerular filtration (e.g., GFR of 18 L/day). Each individual produces 12 g/day of nitrogen in the form of urea (urea nitrogen) derived from dietary protein and must excrete this into the urine. However, these two individuals achieve urea balance at very different blood urea levels. We make the simplifying assumption that the tubules neither absorb nor secrete urea, so that only filtered urea can be excreted, and all filtered urea is excreted. The normal individual can excrete 12 g/day of urea nitrogen from 180 L of blood plasma having a [blood urea nitrogen] of 12 g/180 L, or 6.7 mg/dL. In the patient with end-stage renal disease (ESRD), whose GFR may be only 10% of normal, excreting 12 g/day of urea nitrogen requires that each of the 18 L of filtered blood plasma have a blood urea nitrogen level that is 10 times higher, or 67 mg/dL. Thus, excreting the same amount of urea nitrogen—to maintain a steady state—requires a much higher plasma blood urea nitrogen concentration in the ESRD patient than in the normal individual.

The clearance of inulin is a measure of GFR The ideal glomerular marker for measuring GFR would be a substance X that has the same concentration in the glomerular filtrate as in plasma and that also is not reabsorbed, secreted, synthesized, broken down, or accumulated by the tubules (Table 34-1). In Equation 33-4, we saw that Input into Bowman’s space



Output into urine

 PX ⋅GFR = U X ⋅ V mg mL

mL min

(34-1) 

mg mL mL min

PX is the concentration of the solute in plasma, GFR is the sum of volume flow of filtrate from the plasma into all Bowman’s spaces, UX is the urine concentration of the solute,  is the urine flow. Rearranging this equation, we have and V



U X × V PX mL (mg/mL) × (mL/min) = min (mg/mL)

GFR =

(34-2) 

Note that Equation 34-2 has the same form as the clearance equation (see Equation 33-3) and is identical to Equation 739

740

SECTION VI  •  The Urinary System

33-5. Thus, the plasma clearance of a glomerular marker is the GFR.  N34-1 Inulin is an exogenous starch-like fructose polymer that is extracted from the Jerusalem artichoke and has a molecular weight of 5000 Da. Inulin is freely filtered at the glomerulus, but neither reabsorbed nor secreted by the renal tubules (Fig. 34-1A). Inulin also fulfills the additional requirements listed in Table 34-1 for an ideal glomerular marker. Assuming that GFR does not change, three tests demonstrate that inulin clearance is an accurate marker of GFR. First, as shown in Figure 34-1B, the rate of inulin excretion TABLE 34-1  Criteria for Use of a Substance to Measure GFR 1. Substance must be freely filterable in the glomeruli. 2. Substance must be neither reabsorbed nor secreted by the renal tubules. 3. Substance must not be synthesized, broken down, or accumulated by the kidney. 4. Substance must be physiologically inert (not toxic and without effect on renal function).

A HANDLING OF INULIN

Efferent arteriole

Afferent arteriole

Glomerular capillary

 ) is directly proportional to the plasma inulin con(U In ⋅ V centration (PIn), as implied by Equation 34-2. The slope in Figure 34-1B is the inulin clearance. Second, inulin clearance is independent of the plasma inulin concentration (see Fig. 34-1C). This conclusion was already implicit in Figure 34-1B, in which the slope (i.e., inulin clearance) does not vary with PIn. Third, inulin clearance is independent of urine flow (see Fig. 34-1D). Given a particular PIn, after the renal corpuscles filter the inulin, the total amount of inulin in the urine does not change. Thus, diluting this glomerular marker in a large amount of urine, or concentrating it in a small volume, does  ). If the not affect the total amount of inulin excreted (U In ⋅ V urine flow is high, the urine inulin concentration will be  ) is fixed, proportionally low, and vice versa. Because (U In ⋅ V  (U In ⋅ V)/PIn is also fixed. Two lines of evidence provide direct proof that inulin clearance represents GFR. First, by collecting filtrate from single glomeruli, Richards and coworkers showed in 1941 that the concentration of inulin in Bowman’s space of the mammalian kidney is the same as that in plasma. Thus, inulin is freely filtered. Second, by perfusing single tubules with known amounts of labeled inulin, Marsh and B

DEPENDENCE OF INULIN EXCRETION ON PLASMA [INULIN] 2500 The slope is the 2000 clearance of inulin. . 1500 UIn · V (mg/min) 1000 500 0

Bowman’s space Amount of inulin filtered is PIn · GFR.

C Peritubular capillary

4

8 12 PIn (mg/mL)

16

20

16

20

DEPENDENCE OF INULIN CLEARANCE ON PLASMA [INULIN] 250

.

200

UIn · V/PIn 150 (mL/min) 100

Because inulin is not reabsorbed…

50 0

…and not secreted… D …the amount excreted . in the urine (UIn · V) is the same as the amount filtered.

0

Renal vein

0

4

8 12 PIn (mg/mL)

DEPENDENCE OF INULIN CLEARANCE ON URINE FLOW 250 200

.

UIn · V/PIn 150 (mL/min) 100 50 0

Figure 34-1  Clearance of inulin.

0

1

2

3

.4 5 6 V (mL/min)

7

8

9

10

Chapter 34  •  Glomerular Filtration and Renal Blood Flow

N34-1  Units of Clearance Contributed by Erich Windhager and Gerhard Giebisch Clearance values are conventionally given in milliliters of total plasma per minute, even though plasma consists of 93% “water” and 7% protein, with only the “plasma water”—that is, the protein-free plasma solution, including all solutes small enough to undergo filtration—undergoing glomerular filtration. As pointed out in Chapter 5 (see Table 5-2) the concentrations of plasma solutes can be expressed in millimoles per liter of total plasma, or millimoles per liter of protein-free plasma (i.e., plasma water). Customarily, clinical laboratories report values in millimoles (or milligrams) per deciliter of plasma, not plasma water. When we say that the GFR is 125 mL/min, we mean that each minute the kidney filters all ions and small solutes contained in 125 mL of plasma. However, because the glomerular capillary blood retains the proteins, only 0.93 × 125 mL = 116 mL of plasma water appear in Bowman’s capsule. Nevertheless, GFR is defined in terms of volume of blood plasma filtered per minute rather than in terms of the volume of protein-free plasma solution that actually arrives in Bowman’s space (i.e., the filtrate).

740.e1

Chapter 34  •  Glomerular Filtration and Renal Blood Flow

Frasier showed that the renal tubules neither secrete nor reabsorb inulin. Although the inulin clearance is the most reliable method for measuring GFR, it is not practical for clinical use. One must administer inulin intravenously to achieve reasonably constant plasma inulin levels. Another deterrent is that the chemical analysis for determining inulin levels in plasma and urine is sufficiently demanding to render inulin unsuitable for routine use in a clinical laboratory. The normal value for GFR in a 70-kg man is ~125 mL/ min. Population studies show that GFR is proportional to body surface area. Because the surface area of an average 70-kg man is 1.73 m2, the normal GFR in men is often reported as 125 mL/min per 1.73 m2 of body surface area. In women, this figure is 110 mL/min per 1.73 m2. Age is a second variable. GFR is very low in the newborn, owing to incomplete development of functioning glomerular units. Beginning at ~2 years of age, GFR normalizes for body surface area and gradually falls off with age as a consequence of progressive loss of functioning nephrons.

The clearance of creatinine is a useful clinical index of GFR Because inulin is not a convenient marker for routine clinical testing, nephrologists use other compounds that have clearances similar to those of inulin. The most commonly used compound in human studies is 125I-iothalamate. However, even 125I-iothalamate must be infused intravenously and is generally used only in clinical research studies rather than in routine patient care. The problems of intravenous infusion of a GFR marker can be completely avoided by using an endogenous substance with inulin-like properties. Creatinine is such a substance, and creatinine clearance (CCr) is commonly used to estimate GFR in humans. Tubules, to a variable degree, secrete creatinine, which, by itself, would lead to a ~20% overestimation of GFR in humans. Moreover, when GFR falls to low levels with chronic kidney disease, the overestimation of GFR by CCr becomes more appreciable. In clinical practice, determining CCr is an easy and reliable means of assessing the GFR, and such determination avoids the need to inject anything into the patient. One merely obtains samples of venous blood and urine, analyzes them for creatinine concentration, and makes a simple calculation (see Equation 34-3 below). Although CCr may overestimate the absolute level of GFR, assessing changes in CCr is extremely useful for monitoring relative changes in GFR in patients. The source of plasma creatinine is the normal metabolism of creatine phosphate in muscle. In men, this metabolism generates creatinine at the rate of 20 to 25 mg/kg body weight per day (i.e., ~1.5 g/day in a 70-kg man). In women, the value is 15 to 20 mg/kg body weight per day (i.e., ~1.2  g/day in a 70-kg woman), owing to a lower muscle mass. In the steady state, the rate of urinary creatinine excretion equals this rate of metabolic production. Because metabolic production of creatinine largely depends on muscle mass, the daily excretion of creatinine depends strongly not only on gender but also on age, because elderly patients tend to have lower muscle mass. For a CCr measurement, the patient generally collects urine over an entire 24-hour period, and

741

· PCr · CCr = UCr · V = Constant 20

200

15 Plasma creatinine concentration 10 (mg/dL)

150 Blood urea 100 nitrogen (mg/dL) 50

5 0

0

0 25 50 75 100 125 GFR (mL/min)

Figure 34-2  Dependence of plasma creatinine and blood urea nitrogen on the GFR. In the steady state, the amount of creatinine appearing in the urine per day (UCr ⋅ V ) equals the production rate. Because all filtered creatinine (PCr · CCr) appears in the urine, (PCr · CCr) equals (UCr ⋅ V ), which is constant. Thus, PCr must increase as CCr (i.e., GFR) decreases, and vice versa. If we assume that the kidney handles urea in the same way that it handles inulin, then a plot of blood urea nitrogen versus GFR will have the same shape as that of creatinine concentration versus GFR.

the plasma sample is obtained by venipuncture at one time during the day based on the assumption that creatinine production and excretion are in a steady state. Frequently, clinicians make a further simplification, using the endogenous plasma concentration of creatinine (PCr), normally 1 mg/dL, as an instant index of GFR. This use rests on the inverse relationship between PCr and CCr:

CCr =

 U Cr ⋅ V ≈ GFR PCr

(34-3) 

In the steady state, when metabolic production in muscle  ) of creatinine, and equals the urinary excretion rate (U Cr ⋅ V both remain fairly constant, this equation predicts that a plot of PCr versus CCr (i.e., PCr versus GFR) is a rectangular hyperbola (Fig. 34-2). For example, in a healthy person whose GFR is 100 mL/min, plasma creatinine concentration is ~1 mg/dL. The product of GFR (100 mL/min) and PCr (1 mg/dL) is thus 1 mg/min, which is the rate both of creatinine production and of creatinine excretion. If GFR suddenly drops to 50 mL/min (Fig. 34-3, top), the kidneys will initially filter and excrete less creatinine (see Fig. 34-3, middle), although the production rate is unchanged. As a result, the plasma creatinine level will rise to a new steady state, which is reached at a PCr of 2 mg/dL (see Fig. 34-3, bottom). At this point, the product of the reduced GFR (50 mL/min) and the elevated PCr (2 mg/dL) will again equal 1 mg/min, the rate of endogenous production of creatinine. Similarly, if GFR were to fall to one fourth of normal, PCr would rise to 4 mg/dL. This concept is reflected in the rightrectangular hyperbola of Figure 34-2.  N34-2

Molecular size and electrical charge determine the filterability of solutes across the glomerular filtration barrier The glomerular filtration barrier consists of four elements (see p. 726): (1) the glycocalyx overlying the endothelial cells, (2) endothelial cells, (3) the glomerular basement membrane, and (4) epithelial podocytes. Layers 1, 3, and 4 are

Chapter 34  •  Glomerular Filtration and Renal Blood Flow

741.e1

N34-2  Calculating Estimated Glomerular Filtration Rate Contributed by Gerhard Giebisch, Peter Aronson, Walter Boron, and Emile Boulpaep Clinicians can use the plasma creatinine concentration (PCr) to calculate CCr—that is, the estimated GFR (eGFR)—without the necessity of collecting urine. Researches have derived empirical equations for calculating eGFR based on patient data, including not only PCr, but also parameters that include patient age, weight, eGFR



(

mL/ min⋅1.73 m2

)

= 175 ( PCr )

−1.154

⋅ ( Age)

mg/dL

−0.203

gender, and race. In using these equations, we recognize that daily creatinine excretion depends on muscle mass, which in turn depends on age, weight, sex, and race. An example is the Modification of Diet in Renal Disease (MDRD) Study equation:

⋅ (0.742[if female]) ⋅ (1.212[if African American])

Improving upon the MDRD equation was the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) calculator for eGFR (http://www.qxmd.com/calculate-online/nephrology/ ckd-epi-egfr):

Thus, the MDRD calculation takes into account PCr, age, sex, and—in the United States—whether or not the person is African American. Because MDRD is normalized to body surface area, it does not include body weight. eGFR



(

mL/ min ⋅1.73m 2

)

P  = 141⋅ smaller of  Cr  or 1  k 

a

(NE 34-1) 

Years

P  ⋅ larger of  Cr  or 1  k 

−1.209

Here, k is 0.7 for females and 0.9 for males, and a is −0.329 for females and −0.411 for males. In the first bracketed term, we take the larger of (PCr/k) or 1, whereas in the second bracketed term, we take the smaller of (PCr/k) or 1. Like the MDRD

⋅ (0.993)

Age

⋅ (1.018[if female]) ⋅ (1.159[if African American])

(NE 34-2)  calculation, the CKD-EPI eGFR is normalized to body surface area (i.e., it does not include body weight). The Cockcroft-Gault calculator for eGFR,

kg

eGFR =



mL/min

140 (Age)⋅(Weight)⋅(0.85[if female]) 72 ( PCr )

(NE 34-3) 

mg/dL

takes into account PCr, weight (ideally, lean body mass), sex, and age. For example, for a male aged 22 and weighing 60 kg, the Cockcroft-Gault calculator kg



eGFR = mL/ min

140 (22)⋅(60 kg)⋅(1[for a male]) 72 (1.0)

= 122mL/min = 175L/day

(NE 34-4) 

mg/dL

yields an eGFR of 122 mL/min. The National Kidney Foundation (NKF) recommends that one calculate eGFR with each determination of PCr.

REFERENCES Cockcroft D, Gault MD: Prediction of creatinine clearance from serum creatinine. Nephron 16:31–41, 1976. Levey AS, Stevens LA, Schmid CH, et al; for the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI): A new equation to estimate glomerular filtration rate. Ann Intern Med 150:604–612, 2009. National Kidney Disease Education Program: GFR calculators. Last updated April 25, 2012. http://nkdep.nih.gov/lab -evaluation/gfr-calculators.shtml. National Kidney Disease Education Program: GFR MDRD calculator for adults (conventional units). Last updated

March 1, 2012. http://www.niddk.nih.gov/health-information/ health-communication-programs/nkdep/lab-evaluation/gfr -calculators/Pages/gfr-calculators.aspx. Accessed October 2015. National Kidney Foundation: Calculators for health care professionals. http://www.kidney.org/professionals/KDOQI/ gfr_calculator. Accessed October 2015. QxMD: CKD-EPI eGFR. http://www.qxmd.com/calculate-online/ nephrology/ckd-epi-egfr. Accessed October 2015.

742

SECTION VI  •  The Urinary System

covered with negative charges from anionic proteoglycans. The gene mutations that cause excessive urinary excretion of albumin (nephrotic syndrome; see p. 727) generally affect slit diaphragm proteins, which suggests that the junctions between adjacent podocytes are the predominant barrier to filtration of macromolecules. Table 34-2 summarizes the permselectivity of the glomerular barrier for different solutes, as estimated by the ratio of solute concentration in the ultrafiltrate versus the plasma (UFX/PX). The ratio UFX/PX, also known as the sieving coefficient for the solute X, depends on molecular weight

100 GFR (mL/min) 50 0

Production

1.0 Production and excretion of creatinine 0.5 (mg/min) 0 Plasma creatinine concentration (mg/dL)

Excretion

2 1 0

0

1

2 Days

3

4

Figure 34-3  Effect of suddenly decreasing the GFR on plasma creatinine concentration.

and effective molecular radius. Investigators have used two approaches to estimate UFX/PX. The first, which is valid for all solutes, is the micropuncture technique (see Fig. 33-9A). Sampling fluid from Bowman’s space yields a direct measurement of UFX, from which we can compute UFX/PX. The second approach, which is valid only for solutes that the kidney neither absorbs nor secretes, is to compute the clear­ ance ratio (see p. 733),  N34-3 the ratio of the clearances of X (CX) and inulin (CIn). Inspection of Table 34-2 shows that substances of low molecular weight ( [urea] H2O [NaCl] < [NaCl] H2O Urea H2O

INNER MEDULLA H 2O

600 mOsm NaCl

600 mOsm NaCl 300 mOsm Urea

900

Urea H2O

600 mOsm NaCl 600 mOsm Urea

1200

Figure 38-7  Opposing effects of NaCl and urea gradients on urine concentrating ability during antidiuresis. The numbers in the green boxes indicate the osmolalities (in mOsm) of the interstitial fluid.

causes luminal [urea] to increase in these segments. Because the interstitial [urea] is low in the cortex, a rising luminal [urea] in the ICT and CCT opposes water reabsorption in these segments. Even when the tubule crosses the corticomedullary junction, courses toward the papilla, and is surrounded by interstitial fluid with an ever-increasing [urea], the transepithelial urea gradient still favors water movement into the lumen, which is a handicap for the osmotic concentration of the tubule fluid. The IMCD partially compensates for this problem by acquiring, in response to AVP, a high permeability to urea. The result is a relatively low reflection coefficient for urea (σurea; see p. 468), which converts any transepithelial difference in [urea] into a smaller difference in effective osmotic pressure (see pp. 132–133). Thus, water reabsorption continues from the IMCD even though [urea] in tubule fluid exceeds that in the interstitium. The combination of a high interstitial [NaCl] and high σNaCl (σNaCl = 1.0), along with a low σurea (σurea = 0.74), promotes NaCl-driven water reabsorption. The high AVP-induced urea permeability has the additional effect of raising interstitial [urea], which further reduces the adverse effect of the high luminal [urea] on water reabsorption. If luminal urea opposes the formation of a concentrated urine, why did the mammalian kidney evolve to have high levels of urea in the lumen of the collecting tubules and ducts? At least two reasons are apparent. First, because urea is the body’s major excretable nitrogenous waste, the kidney’s ability to achieve high urinary [urea] reduces the necessity to excrete large volumes of water to excrete nitrogenous waste. Second, as we have already seen, the kidney actually takes advantage of urea—indirectly—to generate maximally concentrated urine. In the presence of AVP, the permeability of the IMCD to urea is high, so that large amounts of urea can enter the medullary interstitium. The high interstitial [urea] energizes the increase in luminal [NaCl] in the tDLH, which, in turn, fuels the single effect in the tALH, thus creating the high inner-medullary [NaCl] that is directly responsible for concentrating the urine. As discussed above in this section, the composition of the inner medullary interstitium determines the composition of the final urine. However, to some extent, the composition of the final urine, as well as the rate of urine flow, also influences the composition of the interstitium. Figure 38-2 shows that the medullary interstitial osmolality is much lower, and the stratification of osmolality from cortex to papillary tip is much less, during water diuresis than during antidiuresis. Two factors contribute to the lesser degree of osmotic stratification under conditions of water diuresis, when levels of AVP are low. First, less urea moves from the IMCD lumen to the interstitium, both because of the low urea permeability of the IMCD and because of the low water permeability of the upstream segments that would otherwise concentrate urea. Second, the MCDs reabsorb some water despite the low AVP levels, and this water dilutes the medullary interstitium. The reasons for this apparent paradox are as follows: (1) even when AVP is low, the water permeability is not zero; (2) the ICT and CCT present a much larger fluid volume to the MCD, because they reabsorb less water when AVP levels are low; and (3) the tubule fluid is more hypo-osmotic, which results in a larger osmotic gradient for transepithelial water

Chapter 38  •  Urine Concentration and Dilution

TABLE 38-2  Factors that Modulate Urinary Concentration and Dilution

movement. With low AVP levels, this larger osmotic gradient overrides the effect of the lower water permeability. Table 38-2 summarizes factors that modulate urinary concentration ability.

1. Osmotic gradient of medullary interstitium from corticomedullary junction to papilla: a. Length of loops of Henle: Species with long loops (e.g., desert rodents) concentrate more than those with short loops (e.g., beaver). b. Rate of active NaCl reabsorption in the TAL: Increased luminal Na+ delivery to the TAL (high GFR or filtration fraction, and low proximal-tubule Na+ reabsorption) enhances NaCl reabsorption, whereas low Na+ delivery (low GFR, increased proximal Na+ and fluid reabsorption) reduces concentrating ability. High Na-K pump turnover enhances NaCl reabsorption, whereas inhibiting transport (e.g., loop diuretics) reduces concentrating ability. c. Protein content of diet: High-protein diet, up to a point, promotes urea production and thus accumulation in the inner medullary interstitium, and increased concentrating ability. 2. Medullary blood flow: Low blood flow promotes high interstitial osmolality. High blood flow washes out medullary solutes. 3. Osmotic permeability of the collecting tubules and ducts to water: AVP enhances water permeability and thus water reabsorption. 4. Luminal flow in the loop of Henle and the collecting duct: High flow (osmotic diuresis) diminishes the efficiency of the countercurrent multiplier, and thus reduces the osmolality of the medullary interstitium. In the MCD, high flow reduces the time available for equilibration of water and urea. 5. Pathophysiology: Central diabetes insipidus (DI) reduces plasma AVP levels, whereas nephrogenic DI reduces renal responsiveness to AVP (see Box 38-1).

REGULATION BY ARGININE VASOPRESSIN Large-bodied neurons in the paraventricular and supraoptic nuclei of the hypothalamus synthesize AVP, a nonapeptide also known as ADH. These neurons package the AVP and transport it along their axons to the posterior pituitary, where they release AVP through a breech in the blood-brain barrier into the systemic circulation (see pp. 844–845). In Chapter 40, we discuss how increased plasma osmolality and decreased effective circulating volume increase AVP release. AVP has synergistic effects on two target organs. First, at rather high circulating levels, such as those seen in hypo­ volemic shock, AVP acts on vascular smooth muscle to cause vasoconstriction (see p. 553) and thus to increase blood pressure. Second, and more importantly, AVP acts on the kidney, where it is the major regulator of water excretion. AVP increases water reabsorption by increasing (1) the water permeabilities of the collecting tubules and ducts, (2) NaCl reabsorption in the TAL, and (3) urea reabsorption by the IMCD.

AVP increases water permeability in all nephron segments beyond the DCT Of the water remaining in the DCT, the kidney reabsorbs a variable fraction in the segments from the ICT to the end of the nephron. Absorption of this final fraction of water is under the control of circulating AVP. Figure 38-8 summarizes the water permeability of various nephron segments. The water permeability is highest in the

GFR, glomerular filtration rate.

Proximal convoluted tubule (PCT) Proximal straight tubule (PST) Thin descending limb (tDLH) Thin ascending limb (tALH) Nephron segments

Medullary thick ascending limb (mTAL) No AVP

Cortical thick ascending limb (cTAL)

AVP

Distal convoluted tubule (DCT) Connecting tubule (CNT) Initial and cortical collecting tubules (ICT & CCT) Outer medullary collecting duct (OMCD) Inner medullary collecting duct (IMCD) 10

100

817

1000

10,000

Osmotic water permeability (µm/s) Figure 38-8  Water permeability in different nephron segments. Note that the x-axis scale is logarithmic. (Modified from Knepper MA, Rector FC: Urine concentration and dilution. In Brenner BM [ed]: The Kidney. Philadelphia, WB Saunders, 1996, pp 532–570.)

818

SECTION VI  •  The Urinary System

proximal tubule and tDLH. The constitutively high water permeability in these segments reflects the abundant presence of AQP1 water channels (see p. 110) in the apical and basolateral cell membranes. In marked contrast to the proximal tubule and tDLH, the following few segments—from the tALH to the connecting tubule—constitutively have very low water permeabilities. In the absence of AVP, the next tubule segments, the ICT and CCT, have rather low water permeabilities, whereas the MCDs are virtually impermeable to water. However, AVP dramatically increases the water permeabilities of the collecting tubules (ICT and CCT) and ducts (OMCD and IMCD) by causing AQP2 water channels to insert into the apical membrane (see below). A third type of water channel, AQP3, is present in the basolateral cell membranes of MCDs. Like AQP1, AQP3 is insensitive to AVP. Given the favorable osmotic gradients discussed in the preceding subchapter, high levels of AVP cause substantial water reabsorption to occur in AVP-sensitive nephron segments. In contrast, when circulating levels of AVP are low, for instance after ingestion of large amounts of water, the water permeability of these nephron segments remains low. Therefore, the fluid leaving the DCT remains hypo-osmotic as it flows down more distal nephron segments. In fact, in the absence of AVP, continued NaCl absorption makes the tubule fluid even more hypo-osmotic, which results in a large volume of dilute urine (see Fig. 38-1).

AVP, via cAMP, causes vesicles containing AQP2 to fuse with apical membranes of principal cells of collecting tubules and ducts AVP binds to V2 receptors in the basolateral membrane of the principal cells from the ICT to the end of the nephron (Fig. 38-9). Receptor binding activates the Gs heterotrimeric G protein, stimulating adenylyl cyclase to generate cAMP (see pp. 56–57). The latter activates protein kinase A, which phosphorylates AQP2 and additional proteins that play a role in the trafficking of intracellular vesicles containing AQP2 and the fusion of these vesicles with the apical membrane. These water channels are AVP sensitive, not in the sense that AVP increases their single channel water conductance, but rather that it increases their density in the apical membrane.  N38-6  In conditions of low AVP, AQP2 water channels are mainly in the membrane of intracellular vesicles just beneath the apical membrane. In the membrane of these vesicles, the AQP2 water channels are present as aggregophores—aggregates of AQP2 proteins. Under the influence of AVP, the vesicles containing AQP2 move to the apical membrane of principal cells of the collecting tubules and ducts. By exocytosis (see pp. 34–35), these vesicles fuse with the apical membrane, thus increasing the density of AQP2. When AVP levels in the blood decline, endocytosis retrieves the water channel–containing aggregates from the apical membrane and shuttles them back to the cytoplasmic vesicle pool. The apical water permeability of principal cells depends not only on AVP levels but also on certain other factors. For example, high [Ca2+]i and high [Li+] both inhibit adenylyl cyclase, thus decreasing [cAMP]i, reducing water permeability, and producing a diuresis. A similar inhibition of AQP2

Clusters of AQP2

V2 receptor Prostaglandins Calcium Protein kinase C Other agents

P Exocytosis

β

γ

AVP

α α

P

AC

Vesicle

Endocytosis

Other proteins

cAMP

Protein kinase A

Protein phosphorylation

Phosphodiesterase

5´ AMP

AQP3 and AQP4

AQP2 synthesis Nucleus P

CREB (CRE-binding protein) DNA

Tubule lumen

CRE site AP1 site

Interstitial space

Figure 38-9  Cellular mechanism of AVP action in the collecting tubules and ducts. AC, adenylyl cyclase; AP1, activator protein 1; CRE, cAMP response element.

insertion, and hence a decrease in water permeability, occurs when agents such as colchicine disrupt the integrity of the cytoskeleton. Conversely, inhibitors of phosphodiesterase (e.g., theophylline), which increase [cAMP]i, tend to increase the osmotic water permeability. In addition to regulating AQP2 trafficking in and out of the apical membrane in the short term, AVP regulates AQP2 protein abundance over the longer term.

AVP increases NaCl reabsorption in the outer medulla and urea reabsorption in the IMCD, enhancing urinary concentrating ability AVP promotes water reabsorption not only by increasing the water permeability of the collecting tubules and ducts, but also by enhancing the osmotic gradients across the walls of the IMCD and perhaps the OMCD. In the outer medulla, AVP acts through the cAMP pathway to increase NaCl reabsorption by the TAL. AVP acts by stimulating apical Na/K/Cl cotransport and K+ recycling across the apical membrane (see p. 768). The net effect is to increase the osmolality of the outer medullary interstitium and thus enhance the osmotic gradient favoring water reabsorption by the OMCD. In addition, AVP stimulates the growth of TAL cells in animals that are genetically devoid of AVP. This hormone also stimulates Na+ reabsorption in the CCT, largely by activating apical Na+ channels (ENaCs). These

Chapter 38  •  Urine Concentration and Dilution

N38-6  Multiple Effects of Arginine Vasopressin on AQP2 Activity Contributed by Erich Windhager and Gerhard Giebisch On page 818, we mentioned that AVP acts through cAMP and protein kinase A (PKA) to phosphorylate AQP2 and other proteins. One result of these phosphorylation events is to increase the trafficking of AQP2 from vesicular pools to the apical membrane of the collecting-duct cells. Thus, AVP increases AQP2 density in the apical membrane; that is, the number of water channels per unit area of apical membrane. In addition, PKA also phosphorylates AQP2 itself as well as cAMP response element–binding protein (CREB; see p. 89). The phosphorylation of CREB, in the longer term, stimulates AQP2 synthesis, as indicated in Figure 38-9.

818.e1

Chapter 38  •  Urine Concentration and Dilution

819

BOX 38-1  Diabetes Insipidus

D

iabetes insipidus (DI) is a fairly rare disorder that occurs in two varieties. The first, neurogenic or central DI, is caused by failure of AVP secretion. The lesion can be at the level of either the hypothalamus (where neurons synthesize AVP) or the pituitary gland (where neurons release AVP). Central DI can be idiopathic, familial, or caused by any disorder of the hypothalamus or pituitary, such as injury, a tumor, infection, or autoimmune processes. In the second variety, nephrogenic DI (NDI), the kidneys respond inadequately to normal or even elevated levels of circulating AVP due to familial or acquired defects. Ninety percent of the familial cases are due to mutations in the X-linked AVPR2 gene that encodes the V2 receptor, and 10%, to mutations in the AQP2 gene. Acquired NDI may be associated with electrolyte abnormalities (e.g., states of K+ depletion or high plasma [Ca2+]  N36-14), the renal disease associated with sickle cell anemia, and various drugs (notably Li+ salts and colchicine). In both central and nephrogenic DI, patients present with polyuria and polydipsia. If patients cannot gain access to water on their own (e.g., infants, bedridden elderly), the disorder can result in marked hypernatremia, hypotension, and shock. Often the physician first suspects the diagnosis when the patient is deprived of access to water or other fluids. The patient may then quickly become dehydrated, and a random determination of plasma [Na+] may yield a very high value. The physician can confirm the diagnosis of DI most easily by a fluid-deprivation test. The patient will continue to produce a large output of dilute urine, despite the need to conserve fluids.

If the patient has central DI, administering a subcutaneous dose of AVP will rapidly increase urine osmolality by >50%. In patients with nephrogenic DI, on the other hand, the increase in urine osmolality will be less. The treatment for central DI is desmopressin acetate (DDAVP) (see Fig. 56-10), a synthetic AVP analog that patients can take intranasally. Nephrogenic DI, in which the kidneys are resistant to the effects of the hormone, does not respond to DDAVP therapy. In these patients, it is best to treat the underlying disease. It can also be helpful to administer a diuretic (to produce natriuresis) and restrict dietary Na+ to induce a state of volume depletion, which in turn enhances proximal NaCl and water reabsorption and thereby moderates the polyuria. The high urine flow in DI is associated with low rates of solute excretion. Therefore, the physician must distinguish DI from states of polyuria accompanied by high rates of solute excretion in the urine (osmotic diuresis). The most frequent cause of the latter is untreated diabetes mellitus. In that case, the polyuria occurs because the high plasma [glucose] leads to the filtration of an amount of glucose that exceeds the capacity of the proximal tubule to retrieve it from the lumen (see pp. 772–773). Another cause of osmotic diuresis is the administration of poorly reabsorbable solutes, such as mannitol. In an entirely distinct class of polyurias is primary polydipsia, a psychoneurotic disorder in which patients drink large amounts of fluid. Whereas simple water deprivation benefits a patient with primary polydipsia, it aggravates the condition of a patient with DI.

BOX 38-2  Role of Aquaporins in Renal Water Transport

W

hereas AQP1 is the water channel responsible for a large amount of transcellular fluid movement in the proximal tubule and the tDLH, three related isoforms of the water channel protein—AQP2, AQP3, and AQP4—are present in the principal cells of the collecting ducts. These channels regulate water transport in collecting tubules and ducts. Apical AQP2 is the basis for AVP-regulated water permeability. AQP3 and AQP4 are present in the basolateral membrane of principal cells, where they provide an exit pathway for water movement into the peritubular fluid. Short-term and long-term regulation of water permeability depends on an intact AQP2 system. In short-term regulation, AVP—via cAMP—causes water channel–containing vesicles from a subapical pool to fuse with the apical membrane (see Fig. 38-9). As a result, the number of channels and the water permeability sharply increase. In long-term regulation, AVP—by enhancing transcription of the AQP2 gene—increases the abundance of AQP2 protein in principal cells.

observations on the TAL and CCT were all made on rodents. In humans these TAL and CCT mechanisms may have only minor significance. In the inner medulla, AVP enhances the urea permea­ bility of the terminal two thirds of the IMCD (see pp. 811– 813). The AVP-dependent increase in [cAMP]i that triggers the apical insertion of AQP2-containing vesicles also leads

Mutations of several AQP genes lead to loss of function and marked abnormalities of water balance. Examples include sharply decreased fluid absorption along the proximal tubule in AQP1 knockout animals and nephrogenic diabetes insipidus (see Box 38-1) in patients with mutations of the gene for AQP2. An interesting situation may develop during the third trimester of pregnancy, when elevated plasma levels of vasopressinase— a placental aminopeptidase that degrades AVP—may lead to a clinical picture of central diabetes insipidus. An acquired increase of AQP2 expression often accompanies states of abnormal fluid retention, such as congestive heart failure, hepatic cirrhosis, nephrotic syndrome, and pregnancy. In addition, some conditions—including acute and chronic renal failure, primary polydipsia, consumption of a low-protein diet, and syndrome of inappropriate antidiuretic hormone secretion (see Box 38-3)—are associated with increased AQP2 levels in the apical membrane.

to a phosphorylation of apical UT-A1 and basolateral UT-A3 urea transporters (see p. 770), increasing their activity. The result is a substantial increase in urea reabsorption and thus the high interstitial [urea] that is indirectly responsible (see p. 816) for generating the osmotic gradient that drives water reabsorption in the inner medulla (Boxes 38-1, 38-2, and 38-3).

820

SECTION VI  •  The Urinary System

BOX 38-3  Syndrome of Inappropriate Antidiuretic Hormone Secretion

T

he syndrome of inappropriate ADH secretion (SIADH) is the opposite of diabetes insipidus. Patients with SIADH secrete ADH (i.e., AVP) or AVP-like substances at levels that are inappropriately high, given the low plasma osmolality and lack of hypovolemia. Thus, the urine osmolality is inappro­ priately high and patients are unable to excrete ingested water loads normally. As a result, total-body water increases, the blood becomes hypo-osmolar, plasma [Na+] drops (hyponatremia), and cells swell. If plasma [Na+] falls substantially, cell swelling can cause headaches, nausea, vomiting, and behavioral changes. Eventually, stupor, coma, and seizures may ensue. Before making the diagnosis of SIADH, the physician must rule out other causes of hyponatremia in which AVP levels may be appropriate. In Chapter 40, we discuss how plasma osmolality (see p. 844) and effective circulating volume (see p. 843) appropriately regulate AVP secretion. SIADH has four major causes: 1. Certain malignant tumors (e.g., bronchogenic carcinoma, sarcomas, lymphomas, and leukemias) release AVP or AVP-like substances.

REFERENCES The reference list is available at www.StudentConsult.com.

2. Cranial disorders (e.g., head trauma, meningitis, and brain abscesses) can increase AVP release. 3. Nonmalignant pulmonary disorders (e.g., tuberculosis, pneumonia, and abscesses) and positive-pressure ventilation also can cause SIADH.  N38-7 4. Several drugs can either stimulate AVP release (e.g., clofibrate, phenothiazines), increase the sensitivity of renal tubules to AVP (e.g., chlorpropamide), or both (e.g., carbamazepine). Treatment is best directed at the underlying disorder, combined, if necessary and clinically appropriate, with fluid restriction. Patients with severe hyponatremia and marked symptoms must receive urgent attention. Infusion of hyperosmotic Na+ is usually effective, but the correction must be gradual or severe neurological damage can result owing to rapid changes in the volume of neurons, especially in the pontine area of the brainstem.

Chapter 38  •  Urine Concentration and Dilution

N38-7  Pulmonary Disorders Causing Syndrome of Inappropriate Antidiuretic Hormone Secretion Contributed by Emile Boulpaep and Walter Boron Several chronic, nonmalignant pulmonary conditions, including positive-pressure ventilation, impede venous return. The result is reduced stretch of the atrial receptors (see Fig. 23-7). As discussed on page 547, the afferent fibers from these stretch receptors project not only to the medulla (where they produce cardiovascular effects) but also to the hypothalamic neurons that synthesize and release AVP. Decreased atrial stretch increases AVP release. Thus, the aforementioned pulmonary conditions result in a syndrome of inappropriate AVP (ADH) release—SIADH.

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Chapter 38  •  Urine Concentration and Dilution

REFERENCES Books and Reviews Agre P, Preston GM, Smyth BL, et al: Aquaporin CHIP: The archetypal molecular water channel. Am J Physiol 265:F463–F476, 1993. Greger R: Transport mechanisms in thick ascending limb of Henle’s loop of mammalian nephron. Physiol Rev 65:760–797, 1985. Knepper MA, Saidel GM, Hascall VC, Dwyer T: Concentration of solutes in the renal inner medulla: Interstitial hyaluronan as a mechano-osmotic transducer. Am J Physiol Renal Physiol 284:F433–F446, 2003. Moeller HB, Fenton RA: Cell biology of vasopressin-regulated aquaporin-2 trafficking. Pflugers Arch 464:133–144, 2012. Sands JM, Layton HE: Urine concentrating mechanism and its regulation. In Seldin DW, Giebisch G (eds): The Kidney: Physiology and Pathophysiology, 3rd ed. Philadelphia, Lippincott Williams & Wilkins, 2000. Sands JM, Layton HE: The physiology of urinary concentration: An update. Semin Nephrol 29:178–195, 2009. Sasaki W, Ishibashi K, Marumo F: Aquaporin-2 and -3: Representatives of two subgroups of the aquaporin family colocalized in the kidney collecting duct. Annu Rev Physiol 60:199– 220, 1998. Seldin DW, Giebisch G (eds): The Kidney: Physiology and Pathophysiology, 3rd ed. Philadelphia, Lippincott Williams & Wilkins, 2000. Shayakul C, Hediger MA: The SLC14 gene family of urea transporters. Pflugers Arch 447:603–609, 2004.

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Tsukaguchi H, Shayakul C, Berger UV, Hediger MA: Urea transporters in kidney: Molecular analysis and contribution to the urinary concentrating process. Am J Physiol 275:F319–F324, 1998. Journal Articles Deen PMT, Verdijk MAJ, Knoers NVAM, et al: Requirement of human renal water channel AQP-2 for vasopressin-dependent concentration of urine. Science 264:92–95, 1994. Gottschalk CW, Mylle M: Micropuncture study of composition of loop of Henle fluid in desert rodents. Am J Physiol 204:532–535, 1959. Grantham JJ, Burg MB: Effect of vasopressin and cyclic AMP on permeability of isolated collecting tubules. J Clin Invest 49:1815– 1826, 1970. Lassiter WE, Gottschalk CW, Mylle M: Micropuncture study of net transtubular movement of water and urea in nondiuretic kidney. Am J Physiol 200:1139–1146, 1964. Potter EA, Stewart G, Smith CP: Urea flux across MDCK-mUT-A2 monolayers is acutely sensitive to AVP, cAMP, and [Ca2+]i. Am J Physiol Renal Physiol 291:F122–128, 2006. Sanjana VM, Robertson CR, Jamison RL: Water extraction from the inner medullary collecting tubule system: A role for urea. Kidney Int 10:139–146, 1976. Stewart GS, King SL, Potter EA, Smith CP: Acute regulation of mUT-A3 urea transporter expressed in a MDCK cell line. Am J Physiol Renal Physiol 292:F1157–1163, 2007. Pallone TL, Edwards A, Ma, T, et al: The intrarenal distribution of blood flow. Adv Organ Biol 9:75–92, 2000.

C H A P T E R 39  TRANSPORT OF ACIDS AND BASES Gerhard Giebisch, Erich E. Windhager, and Peter S. Aronson

The lungs and the kidneys are largely responsible for regulating the acid-base balance of the blood (see Chapter 28). They do so by independently controlling the two major components of the body’s major buffering system: CO2 and HCO3− (Fig. 39-1). Chapter 31 focuses on how the lungs control plasma [CO2]. In this chapter we see how the kidneys control plasma [HCO3− ].

ACID-BASE BALANCE AND THE OVERALL RENAL HANDLING OF ACID Whereas the lungs excrete the large amount of CO2 formed by metabolism, the kidneys are crucial for excreting nonvolatile acids The kidneys play a critical role in helping the body rid itself of excess acid that accompanies the intake of food or that forms in certain metabolic reactions. By far, the largest potential source of acid is CO2 production (Table 39-1, section A), which occurs during oxidation of carbohydrates, fats, and most amino acids (see pp. 1185–1187). An adult ingesting a typical Western diet produces ~15,000 mmol/day of CO2. This CO2 would act as an acid if it went on to form H+ and HCO3− (see p. 630). Fortunately, the lungs excrete this prodigious amount of CO2 by diffusion across the alveolar-capillary barrier (see p. 673), preventing the CO2 from forming H+. However, metabolism also generates nonvolatile acids— such as sulfuric acid, phosphoric acid, and various organic acids—that the lungs cannot handle (see Table 39-1, section B). In addition, metabolism generates nonvolatile bases, which end up as HCO3− (see Table 39-1, section C). Sub­ tracting the metabolically generated base from the meta­ bolically generated acid leaves a net endogenous H+ production of ~40 mmol/day for a person weighing 70 kg. The strong acids contained in a typical Western acid-ash diet (20 mmol/day of H+ gained) and the obligatory loss of bases in stool (10 mmol/day of OH− lost) represent an additional acid load to the body of 30 mmol/day. Thus, the body is faced with a total load of nonvolatile acids (i.e., not CO2) of ~70 mmol/day—or ~1 mmol/kg body weight—derived from metabolism, diet, and intestinal losses. The kidneys handle this acid load by “dividing” 70 mmol/day of carbonic acid (H2CO3): excreting ~70 mmol/day of H+ into the urine and simultaneously transporting 70 mmol/day of new

HCO3− into the blood. Once in the blood, this new HCO3− neutralizes the daily load of 70 mmol of nonvolatile acid. Were it not for the tightly controlled excretion of H+ by the kidney, the daily load of ~70 mmol of nonvolatile acids would progressively lower plasma pH and, in the process, exhaust the body’s stores of bases, especially HCO3− . The result would be death by relentless acidification. Indeed, one of the characteristic symptoms of renal failure is severe acidosis caused by acid retention.  N39-1  The kidneys continuously monitor the acid-base parameters of the extracellular fluid (ECF) and adjust their rate of acid secretion to maintain the pH of ECF within narrow limits. In summary, although the lungs excrete an extremely large amount of a potential acid in the form of CO2, the kidneys play an equally essential role in the defense of the normal acid-base equilibrium, because they are the sole effective route for neutralizing nonvolatile acids.

To maintain acid-base balance, the kidney must not only reabsorb virtually all filtered HCO3- but also secrete generated nonvolatile acids In terms of acid-base balance, the major task of the kidney is to secrete acid into the urine and thus to neutralize the nonvolatile acids that metabolism produces. However, before the kidney can begin to achieve this goal, it must deal with a related and even more serious problem: retrieving from the tubule fluid virtually all HCO3− filtered by the glomeruli. Each day, the glomeruli filter 180 L of blood plasma, each liter containing 24 mmol of HCO3− , so that the daily filtered load of HCO3− is 180 L × 24 mM = 4320 mmol. If this filtered HCO3− were all left behind in the urine, the result would be equivalent to an acid load in the blood of 4320 mmol, or a catastrophic metabolic acidosis (see p. 635). The kidneys avoid this problem by reclaiming virtually all the filtered HCO3− through secretion of H+ into the tubule lumen and titration of the 4320 mmol/day of filtered HCO3− to CO2 and H2O. After the kidney reclaims virtually all the filtered HCO3− (i.e., 4320 mmol/day), how does it deal with the acid load of 70 mmol/day produced by metabolism, diet, and intestinal losses? If we simply poured 70 mmol of nonvolatile acid into the ~1.5 L of “unbuffered” urine produced each day, urinary [H+] would be 0.070 mol/1.5 L = 0.047 M, which would correspond to a pH of ~1.3. The lowest urine pH that the kidney 821

Chapter 39  •  Transport of Acids and Bases

N39-1  Acidoses of Renal Origin Contributed by Erich Windhager and Gerhard Giebisch Any overall decrease in the ability of the kidneys to excrete the daily load of ~70 mmol of nonvolatile acids will lead to metabolic acidosis. In the strict sense of the term, renal tubular acidosis (RTA) is an acidosis that develops secondary to the dysfunction of renal tubules. In addition, an overall decrease in useful renal mass and GFR—as occurs in endstage renal disease—also leads to an acidosis of renal origin. One system of organizing these maladies recognizes five types of RTAs: • Uremic acidosis or RTA of glomerular insufficiency. The fundamental problem is a decrease in the total amount of NH3 that the proximal tubule can synthesize from glutamine (see pp. 829–831). • Proximal (type 2) RTA. A specific dysfunction of the proximal tubule reduces the total amount of HCO3− that these nephron segments reabsorb. • Classical distal (type 1) RTA. A specific dysfunction of the distal tubule reduces the total amount of HCO3− that these nephron segments reabsorb. The mechanisms can include mutations of key proteins involved in distal H+ secretion, such as H pumps and Cl-HCO3 exchangers. • Generalized (type 4) RTA. A global dysfunction of the distal tubule—secondary to aldosterone deficiency or aldosterone resistance (see p. 835)—leads to a reduced net excretion of acid. • Type 3 RTA. Rare defects in CAII lead to defects in both proximal and distal H+ secretion.

821.e1

822

SECTION VI  •  The Urinary System

Net uptake of acid = 30 mmol/day

Diet + 20 mmol H /day

Absorbed + 20 mmol H /day

15,000 mmol CO2/day ECF pH 7.4

Lung

Metabolism

Gut + H as nonvolatile acids 40 mmol/day

Secreted_ 10 mmol OH /day Feces 10 mmol OH–/day

Reabsorbed 4320 mmol HCO3–/day “New” HCO–3 70 mmol/day

Filtered 4320 mmol HCO3–/day

Expired air 15,000 mmol CO2/day

Kidneys

+

40 mmol NH4/day

30 mmol titratable acid/day Urine + 70 mmol H /day

Figure 39-1  Acid-base balance. All values are for a 70-kg human consuming a typical Western acid-ash diet. The values in the boxes are approximations.

TABLE 39-1  Metabolic Sources of Nonvolatile Acids and Bases A.  Reactions Producing CO2 (merely a potential acid)

1. Complete oxidation of neutral carbohydrate and fat → CO2 + H2O 2. Oxidation of most neutral amino acids → Urea + CO2 + H2O

B.  Reactions Producing Nonvolatile Acids

1. Oxidation of sulfur-containing amino acids → Urea + CO2 + H2O + H2SO4 → 2 H+ + SO2− 4 (Examples: methionine, cysteine) 2. Metabolism of phosphorus-containing compounds → H3PO4 → H+ + H2PO−4 3. Oxidation of cationic amino acids → Urea + CO2 + H2O + H+ (e.g., lysine+, arginine+) 4. Production of nonmetabolizable organic acids → HA → H+ + A− (e.g., uric acid, oxalic acid) 5. Incomplete oxidation of carbohydrate and fat → HA → H+ + A− (e.g., lactic acid, keto acids)

phosphate, creatinine, and urate. Because of its favorable pK of 6.8 and its relatively high rate of excretion, phosphate is the most important nonvolatile filtered buffer. The other major urinary buffer is NH3 /NH +4 , which the kidney synthesizes. After diffusing into the tubule lumen, the NH3 reacts with secreted H+ to form NH +4 . Through adaptive increases in the synthesis of NH3 and excretion of NH +4 , the kidneys can respond to the body’s need to excrete increased loads of H+. The kidney does not simply eliminate the 70 mmol/day of nonvolatile acids by filtering and then excreting them in the urine. Rather, the body deals with the 70-mmol/day acid challenge in three steps: Step 1: Extracellular HCO3− neutralizes most of the H+ load:

can achieve is ~4.4, which corresponds to an [H+] that is three orders of magnitude lower than required to excrete the 70 mmol/day of nonvolatile acids. The kidneys solve this problem by binding the H+ to buffers that the kidney can excrete within the physiological range of urinary pH values. Some of these buffers the kidney filters—for example,

(39-1) 

Acid load

Thus, HCO3− decreases by an amount that is equal to the H+ it consumes, and an equal amount of CO2 is produced in the process. Non-HCO3− buffers (see p. 635) in the blood neutralize most of the remaining H+ load:

C.  Reactions Producing Nonvolatile Bases

1. Oxidation of anionic amino acids → Urea + CO2 + H2O + HCO3− (e.g., glutamate−, aspartate−) 2. Oxidation of organic anions → CO2 + H2O + HCO3− (e.g., lactate−, acetate−)

+ HCO3− + H  → CO2 + H2 O



+ B− + H  → BH

(39-2) 

Acid load

Thus, B−, too, decreases by an amount that is equal to the H+ it consumes. A very tiny fraction of the H+ load (99.9%). As discussed beginning on page 825, the kidney reabsorbs HCO3− at specialized sites along the nephron. However, regardless of the site, the basic mechanism of HCO3− reabsorption is the same (Fig. 39-2A): H+ transported into the lumen by the tubule cell titrates filtered HCO3− to CO2 plus H2O. One way that this titration can occur is by H+ interacting with HCO3− to form H2CO3, which in turn dissociates to yield H2O and CO2. However, the reaction H2CO3 → H2O + CO2 is far too slow to convert the entire filtered load of HCO3− to CO2 plus H2O. The enzyme carbonic anhydrase (CA)  N18-3—which is present in many tubule segments—bypasses this slow reaction by splitting HCO3− into CO2 and OH− (see Table 39-1). The secreted H+ neutralizes this OH− so that the net effect is to accelerate the production of H2O and CO2. The apical membranes of these H+-secreting tubules are highly permeable to CO2, so that the CO2 produced in the lumen, as well as the H2O, diffuses into the tubule cell. Inside the tubule cell, the CO2 and H2O regenerate intra­ cellular H+ and HCO3− with the aid of CA. Finally, the cell exports these two products, thereby moving the H+ out across the apical membrane into the tubule lumen and the HCO3− out across the basolateral membrane into the blood. Thus, for each H+ secreted into the lumen, one HCO3− disappears from the lumen, and one HCO3− appears in the blood. However, the HCO3− that disappears from the lumen and the HCO3− that appears in the blood are not the same molecule! To secrete H+ and yet keep intracellular pH within narrow physiological limits (see pp. 644–645), the cell closely coordinates the apical secretion of H+ and the basolateral exit of HCO3− . Two points are worth re-emphasizing. First, HCO3− reabsorption does not represent net H+ excretion into the urine. It merely prevents the loss of the filtered alkali. Second, even though HCO3− reabsorption is simply a reclamation effort, this process consumes by far the largest fraction of the H+ secreted into the tubule lumen. For example, reclaiming the 4320 mmol of HCO3− filtered each day requires 4320 mmol

Chapter 39  •  Transport of Acids and Bases

N39-2  Urinary Excretion of Carboxylates Contributed by Peter Aronson and Gerhard Giebisch In addition to the loss of filtered HCO3− in the urine, the excretion of organic anions that can undergo conversion to HCO3− (e.g., lactate, citrate) would represent a loss of alkali into the urine, which in principle would need to be taken into account in computing net renal acid excretion. Because the proximal tubule normally reabsorbs nearly all of these carboxylates (see p. 779), this component of alkali loss is minor under most circumstances.

823.e1

824

A

SECTION VI  •  The Urinary System

Figure 39-2  Titration of luminal buffers by secreted H+. A and B, Generic

HCO3– REABSORPTION Tubule lumen

Interstitial space

HCO3–

models of H+ secretion at various sites along the nephron. The red arrows represent diverse transport mechanisms. C, Ammonium handling by the proximal tubule.

Carbonic anhydrases

CO2 –

OH

OH

+

+

H

H

of H+ secretion, far more than the additional 70 mmol/day of H+ secretion necessary for neutralizing nonvolatile acids.

CA IV and XIV CO2 CA II HCO–3



Retrieved HCO3–

Titration of Filtered Non-HCO3- Buffers (Titratable-Acid For­ mation)  The H+ secreted into the tubules can interact with

buffers other than HCO3− and NH3. The titration of the nonNH3, non-HCO3− buffers (B−)—mainly HPO2− 4 , creatinine, and urate—to their conjugate weak acids (HB) constitutes the titratable acid discussed on page 823.

H2O



H+ + B− →

BH



(39-4) 

Titratable acid

B

FORMATION OF TITRATABLE ACID Tubule lumen

Interstitial space

CO2 CA II HCO3–



+

H

OH

+

H

New HCO3–

HPO42–

H2PO4–

C

AMMONIUM EXCRETION Tubule lumen

Interstitial space

CO2 CA II HCO3– +

H

NH3

+

H

NH3

+

NH4

OH–

Metabolism + NH4

New HCO3–

The major proton acceptor in this category of buffers excreted in the urine is HPO2− 4 , although creatinine also makes an important contribution; urate and other buffers contribute to a lesser extent. Figure 39-2B shows the fate of H+ as it protonates phosphate from its divalent form (HPO2− 4 ) to its monovalent form (H2 PO−4 ). Because low luminal pH inhibits the apical Na/phosphate cotransporter (NaPi) in the proximal tubule, and NaPi carries H2 PO−4 less effectively than + HPO2− 4 (see pp. 785–786), the kidneys tend to excrete H + bound phosphate in the urine. For each H it transfers to the lumen to titrate HPO2− 4 , the tubule cell generates one new HCO3− and transfers it to the blood (see Fig. 39-2B). How much does the “titratable acid” contribute to net acid excretion? The following three factors determine the rate at which these buffers act as vehicles for excreting acid: 1. The amount of the buffer in the glomerular filtrate and final urine. The filtered load (see p. 732) of HPO2− 4 , for example, is the product of plasma [HPO2− 4 ] and glomerular filtration rate (GFR). Plasma phosphate levels may range from 0.8 to 1.5 mM (see p. 1054). Therefore, increasing plasma [HPO2− 4 ] allows the kidneys to excrete more H+ in the urine as H2 PO−4 . Conversely, decreasing the GFR (as in chronic renal failure) reduces the amount of HPO2− 4 available for buffering, lowers the excretion of titratable acid, and thus contributes to metabolic acidosis. Ultimately, the key parameter is the amount of buffer excreted in the urine. In the case of phosphate, the fraction of the filtered load that the kidney excretes increases markedly as plasma [phosphate] exceeds the maximum saturation (Tm; see p. 786). For a plasma [phosphate] of 1.3 mM, the kidneys reabsorb ~90%, and ~30 mmol/day appear in the urine. 2. The pK of the buffer. To be most effective at accepting H+, the buffer (e.g., phosphate, creatinine, urate) should have a pK value that is between the pH of the glomerular filtrate and the pH of the final urine. For example, if blood plasma has a pH of 7.4, then only ~20% of its phosphate (pK = 6.8) will be in the form of H2 PO−4 (Table 39-3). Even if the final urine were only mildly acidic, with a pH of 6.2, ~80% of the phosphate in the urine would be in the form of H2 PO−4 . In other words, the kidney would

Chapter 39  •  Transport of Acids and Bases

TABLE 39-3  Titration of Buffers % PROTONATED BUFFER pH

PHOSPHATE (pK = 6.8)

URATE (pK = 5.8)

CREATININE (pK = 5.0)

7.4

20.1

2.5

0.4

6.2

79.9

28.5

5.9

4.4

99.6

96.2

79.9

have titrated ~60% of the filtered phosphate from HPO2− 4 to H2 PO−4 . Because creatinine has a pK of 5.0, lowering the pH of the tubule fluid from 7.4 to 6.2 increases the fractional protonation of creatinine from ~0.4% to only ~6%. However, urate has a pK of 5.8, so lowering pH from 7.4 to 6.2 would increase its fractional protonation from 2.5% to 28.5%. 3. The pH of the urine. Regardless of the pK of the buffer, the lower is the urinary pH, the more protonated is the buffer and the greater is the amount of acid excreted with this buffer. As discussed, lowering the pH of the tubule fluid from 7.4 to 6.2 increases the protonation of creatinine from 0.4% to only ~6%. However, if the pH of the final urine is 4.4, the fractional protonation of creatinine increases to ~80% (see Table 39-3). Thus, creatinine becomes a much more effective buffer during acidosis, when the kidney maximally acidifies the urine. Titration of Filtered and Secreted NH3 (Ammonium Excre­ tion)  The third class of acceptors of luminal H+ is NH3.

However, unlike either HCO3− or the bases that give rise to “titratable acid” (e.g., HPO2− 4 ), glomerular filtration contributes only a negligible quantity of NH3 because plasma [NH3] concentration is exceedingly low. Instead, urinary NH3 derives mainly from diffusion into the lumen from the proximal-tubule cell (see Fig. 39-2C), with some NH +4 entering the lumen directly via the apical Na-H exchanger NHE3. In the case of the proximal tubule, the conversion of glutamine to α-ketoglutarate (α-KG) generates two NH +4 ions, which form two NH3 and two H+ ions. In addition, the metabolism of α-KG generates two OH− ions, which CA converts to HCO3− ions. This new HCO3− then enters the blood.  N39-3 In summary, when renal-tubule cells secrete H+ into the lumen, this H+ simultaneously titrates three kinds of buffers: (1) HCO3− , (2) HPO2− 4 and other buffers that become the “titratable acid,” and (3) NH3. Each of these three buffers competes with the other two for available H+. In our example, the kidneys secrete 4390 mmol/day of H+ into the tubule lumen. The kidneys use most of this secreted acid— 4320 mmol/day or ~98% of the total—to reclaim filtered HCO3−. The balance of the total secreted H+, 70 mmol/day, the kidneys use to generate new HCO3− .

ACID-BASE TRANSPORT BY DIFFERENT SEGMENTS OF THE NEPHRON Most nephron segments secrete H+ to varying degrees.

825

The nephron reclaims virtually all the filtered HCO3in the proximal tubule (~80%), thick ascending limb (~10%), and distal nephron (~10%) The kidney reabsorbs the largest fraction of filtered HCO3− (~80%) along the proximal tubule (Fig. 39-3A). By the end of the proximal tubule, luminal pH falls to ~6.8, which represents only a modest transepithelial H+ gradient compared with the plasma pH of 7.4. Thus, the proximal tubule is a high-capacity, low-gradient system for H+ secretion. The thick ascending limb of the loop of Henle (TAL) reabsorbs an additional 10% of filtered HCO3− , so that by the time the tubule fluid reaches the distal convoluted tubule (DCT), the kidney has reclaimed ~90% of the filtered HCO3− . The rest of the distal nephron—from the DCT to the inner medullary collecting duct (IMCD)—reabsorbs almost all the remaining ~10% of the filtered HCO3− . Although the latter portion of the nephron reabsorbs only a small fraction of the filtered HCO3− , it can lower luminal pH to ~4.4. Thus, the collecting tubules and ducts are a low-capacity, high-gradient system for H+ transport. The amount of HCO3− lost in the urine depends on urine pH. If the [CO2] in the urine were the same as that in the blood, and if urine pH were 5.4, the [HCO3− ] in the urine would be 0.24 mM, which is 1% of the 24 mM in blood (see p. 630). For a urine production of 1.5 L/day, the kidneys would excrete 0.36 mmol/day of HCO3− . For a filtered HCO3− load of 4320 mmol/day, this loss represents a fractional excretion of ~0.01%. In other words, the kidneys reclaim ~99.99% of the filtered HCO3−. Similarly, at a nearly maximally acidic urine pH of 4.4, urine [HCO3− ] would be only 0.024 mM. Therefore, the kidneys would excrete only 36 µmol/day of filtered HCO3− and would reabsorb ~99.999%.

The nephron generates new HCO3- , mostly in the proximal tubule The kidney generates new HCO3− in two ways (see Fig. 39-3B). It titrates filtered buffers such as HPO2− 4 to produce “titratable acid,” and it titrates secreted NH3 to NH +4 . In healthy people, NH +4 excretion is the more important of the two and contributes ~60% of net acid excretion or new HCO3−. Formation of Titratable Acid  The extent to which a particular buffer contributes to titratable acid (see Fig. 39-2B) depends on the amount of buffer in the lumen and luminal pH. The titratable acid due to phosphate is already substantial at the end of the proximal tubule (Table 39-4), even though the proximal tubule reabsorbs ~80% of the filtered phosphate. The reason is that the luminal pH equals the pK of the buffer at the end of the proximal tubule. The titratable acid due to phosphate rises only slightly along the classical distal tubule (i.e., DCT, connecting tubule [CNT], and initial collecting tubule [ICT]), because acid secretion slightly exceeds phosphate reabsorption. The titratable acid due to phosphate rises further as luminal pH falls to 4.4 along the collecting ducts in the absence of significant phosphate reabsorption. Although the late proximal tubule secretes creatinine, the titratable acid due to creatinine (see Table 39-4) is minuscule at the end of the proximal tubule, because luminal pH is so

Chapter 39  •  Transport of Acids and Bases

825.e1

N39-3  Ammonium Secretion by the Medullary Collecting Duct Contributed by Erich Windhager, Gerhard Giebisch, Emile Boulpaep, and Walter Boron Ammonium secretion by the medullary collecting duct is critical for renal NH+4 excretion. As described in Figure 39-5C, the TAL of juxtamedullary nephrons reabsorbs some NH+4 and deposits this NH+4 in the medullary interstitium, where it is partitioned between ammonium and ammonia according to the equilibrium NH+4 ⇌ NH3 + H+. As pointed out in Figure 39-5D, this interstitial NH+4 (and NH3) can have three fates: (1) some recycles back to the late proximal tubule and descending thin limb of Henle, (2) some bypasses the cortex by being secreted into the medullary collecting duct, and (3) some is washed out by the blood for export to the liver. The mechanism of pathway (2) is depicted in Figure 39-5E. NH3 diffuses from the medullary interstitium, through the tubule cell and into the lumen. The NH3 moves via members of the Rh family at both the basolateral and apical membranes. The parallel extrusion of H+ across the apical membrane of the collecting-duct cell provides the luminal H+ that then titrates the luminal NH3 to NH+4 , which is excreted. This luminal H pumping also generates OH− inside the cells. Although not shown in Figure 39-5E, intracellular CA converts this newly created OH− (along with H2O) to HCO3− , and basolateral Cl-HCO3 exchangers then export this Tubule lumen

newly created HCO3− to the interstitium. The HCO3− , of course, ultimately is washed out by the blood. Thus, for each NH+4 formed in the lumen of the collecting duct by this route, the tubule cell transfers one “new” HCO3− to the blood. Figure 39-5E also shows that the Na-K pump can also transport NH+4 directly into the collecting-duct cell. This intracellular NH+4 can then dissociate into NH3 (which can diffuse into the lumen) and H+ (which moves into the lumen via the apical H pump), with the ultimate formation of NH+4 in the lumen. The NH+4 that enters the collecting-duct lumen by this route does not generate a new HCO3− ion. eFigure 39-1 shows the most recent model for how the TAL handles NH3 and CO2.

REFERENCES Geyer RR, Musa-Aziz R, Qin X, Boron WF: Relative CO2/NH3 selectivities of mammalian aquaporins 0-9. Am J Physiol Cell Physiol 304:C985–C994, 2013. Weiner ID, Verlander JW: Ammonia transport in the kidney by Rhesus glycoproteins. Am J Physiol Renal Physiol 306(10): F1107–F1120, 2014.

Thick Ascending Limb Cell

Collecting Duct

Cl–

Tubule lumen pH ~ 5.5

 intercalated cell

Interstitial space pH ~ 7.4

AE1

pH ~ 7.1

K+ ATPase

H+ + HCO3–

2Cl– NKCC2 Na+ NH4+

NH4+

+

NH4

H+ CA II

H+ NH3

HCO3–

H2O

CO2

K+ channel

RhBG

CO2

diffusion

CO2 NH3

RhCG

CO2 NH3

NH3

HCO3–

H+ ATPase

H2O CO2

NH3 RhCG

NH3

diffusion

NH4+ NH4+

eFigure 39-1  Proposed model for CO2 and NH3 transport across the apical and basolateral membranes of TAL and α-intercalated cells in the collecting duct. Dashed arrows represent the possible diffusion of CO2 or NH3 across plasma membranes. NKCC2, Na/K/Cl cotransporter 2. (Republished with permission from Geyer RR, Parker MD, Toye AM, et al: Relative CO2/NH3 permeabilities of human RhAG, RhBG and RhCG. J Membrane Biology 246(12):915-926, F8, 2013.)

826

A

SECTION VI  •  The Urinary System



B

RECLAMATION OF FILTERED HCO3 ALONG THE NEPHRON GFR = 180 L/day – Plasma [HCO3 ] = 24 mM pH = 7.4 Filtered load = 4320 mmol/day 100% remaining

1 80% reabsorbed

2–

Plasma [HPO4 ] ≅ 1.04 mM – Plasma [H2PO4 ] ≅ 0.26 mM + Plasma [NH4 ] ≅ 0 mM

4% remaining 3

6% reabsorbed Distal convoluted tubule

Cortical collecting tubule

H+ to form TA 5 mmol/day H+ to form TA 15 mmol/day



New HCO3 5 mmol/day

H+ + NH3 40 mmol/day

10% remaining



Proximal convoluted tubule Proximal straight tubule



GENERATION OF NEW HCO3

New HCO3 55 mmol/day

2

10% reabsorbed 20% remaining

Thick ascending limb of Henle’s loop (TAL)

4 ~4% reabsorbed

+

NH 4

Outer medullary collecting duct

Inner medullary collecting duct

+

NH 4

H+ to form TA 10 mmol/day –

New HCO 3 10 mmol/day

· V = 1.5 L/day UHCO3– = 0.24 mM pH = 5.4 Urinary excretion of HCO3– ≅ 0.36 mmol/day ~0.01% of filtered load remaining

Excretion of 70 mmol/day of H+ corresponds to the generation of 70 mmol/day of new HCO3–.

Urinary excretion of H+ = 70 mmol/day

Figure 39-3  Acid-base handling along the nephron. In A, the numbered yellow boxes indicate the fraction of the filtered load reabsorbed by various nephron segments. The green boxes indicate the fraction of the filtered load that remains in the lumen at various sites. In B, the red boxes indicate the moieties of acid secretion associated with either the formation of titratable acid (TA) or the secretion of NH+4. The yellow boxes indicate the formation of new HCO3− or NH+4 reabsorption by the TAL. The values in the boxes are approximations.

much higher than creatinine’s pK. However, the titratable acidity due to creatinine increases substantially along the collecting ducts as luminal pH plummets. The urine contains the protonated form of other small organic acids (e.g., uric, lactic, pyruvic, and citric acids) that also contribute to titratable acid. NH+4 Excretion  Of the new HCO3− that the nephron gen­

erates, ~60% (~40 mmol/day) is the product of net NH +4

excretion (see Fig. 39-3B), which is the result of five processes: (1) the proximal tubule actually secretes slightly more than ~40 mmol/day of NH +4 , (2) the TAL reabsorbs some NH +4 and deposits it in the interstitium, (3) some of this interstitial NH +4 recycles back to the proximal tubule and thin descending limb (tDLH), (4) some of the interstitial NH +4 enters the lumen of the collecting duct, and finally, (5) some of the interstitial NH +4 enters the vasa recta and leaves the kidney. As we shall see on p. 831, the liver uses some of

Chapter 39  •  Transport of Acids and Bases

827

TABLE 39-4  Titratable Acidity of Creatinine and Phosphate Along the Nephron* PHOSPHATE

Bowman’s space

pH

FILTERED LOAD REMAINING (%)

7.4

100

CREATININE

TITRATABLE ACID DUE TO Pi (mmol/day) 0

FILTERED LOAD REMAINING (%) 100

End of PT

6.8

20

14.0

120

End of ICT

6.0

10

15.5

Final urine

5.4

10

17.8

TITRATABLE ACID DUE TO CREATININE (mmol/day) 0



SUM OF TITRATABLE ACID DUE TO PHOSPHATE AND CREATININE (mmol/day) 0

0.2

14.2

120

1.7

17.2

120

5.5

23.3

*Note that other buffers in the urine contribute to the total titratable acid, which increases with the excreted amount of each buffer and with decreases in urine pH. In this example, we assume a plasma [phosphate] of 1.3 mM, a plasma [creatinine] of 0.09 mM, and a GFR of 180 L/day. † We assume that the proximal tubule secretes an amount of creatinine that is equivalent to 20% of the filtered load. Pi, inorganic phosphate; PT, proximal tubule.

this NH +4 to generate urea, a process that consumes HCO3− . Thus, the net amount of new HCO3− attributable to NH +4 excretion is (1) − (2) + (3) + (4) − (5).

ACID-BASE TRANSPORT AT THE CELLULAR AND MOLECULAR LEVELS The secretion of acid from the blood to the lumen—whether for reabsorption of filtered HCO3− , formation of titratable acid, or NH +4 excretion—shares three steps: (1) transport of H+ (derived from H2O) from tubule cell to lumen, which leaves behind intracellular OH−; (2) conversion of intracellular OH− to HCO3− , catalyzed by CA; and (3) transport of newly formed HCO3− from tubule cell to blood. In addition, because the buffering power of filtered non-HCO3− buffers is not high enough for these buffers to accept sufficient luminal H+, the adequate formation of new HCO3− requires that the kidney generate buffer de novo. This buffer is NH3.

H+ moves across the apical membrane from tubule cell to lumen by Na-H exchange, electrogenic H pumping, and K-H pumping Although the kidney could, in principle, acidify the tubule fluid either by secreting H+ or by reabsorbing OH− or HCO3− , the secretion of H+ appears to be solely responsible for acidifying tubule fluid. At least three mechanisms can extrude H+ across the apical membrane; not all of these are present in any one cell. Na-H Exchanger  Of the known NHE isoforms (see p. 124), NHE3 is particularly relevant for the kidney because it moves more H+ from tubule cell to lumen than any other transporter.  N39-4  NHE3 is present not only throughout the proximal tubule (Fig. 39-4A, B) but also in the TAL (see Fig. 39-4C) and DCT. The apical NHE3 secretes H+ in exchange for luminal + Na . Because a steep lumen-to-cell Na+ gradient drives this exchange process (see p. 115), apical H+ secretion ultimately depends on the activity of the basolateral Na-K pump. The carboxyl termini of the NHEs have phosphorylation sites for various protein kinases. For example, protein kinase

A (PKA) phosphorylates apical NHE in the proximal tubule, inhibiting it. Both parathyroid hormone and dopamine inhibit NHE3 via PKA. Electrogenic H Pump  A second mechanism for apical H+ secretion by tubule cells is the electrogenic H pump, a vacuolar-type ATPase (see pp. 118–119). The ATP-driven H pump can establish steep transepithelial H+ concentration gradients, thus lowering the urine pH to ~4.0 to 5.0. In contrast, NHE3, which depends on the 10-fold Na+ gradient across the apical membrane, cannot generate an H+ gradient in excess of ~1 pH unit. The apical electrogenic H pumps are located mainly in a subpopulation of intercalated cells (α cells) of the CNT, ICT, and cortical collecting tubule (CCT) and in cells of the IMCD and outer medullary collecting duct (OMCD; Fig. 39-4D). However, H pumps are also present in the apical membrane of the proximal tubule (see Fig. 39-4A, B), the TAL (see Fig. 39-4C), and the DCT. In addition, an electrogenic H pump is also present in the basolateral membrane of β-intercalated cells.  N39-5  Mutations in genes encoding subunits of this H pump cause a metabolic acidosis (see p. 635) in the blood—a distal renal tubular acidosis (dRTA). The regulation of the apical H pump involves several mechanisms. First, the transepithelial electrical potential may modulate the H pump rate. For instance, aldosterone induces increased apical Na+ uptake by the principal cells in the CCT (see pp. 765–766), thus causing an increase in the lumen-negative potential, which in turn stimulates the H pump. Second, aldosterone stimulates the H pump independently of changes in voltage. Third, acidosis increases the recruitment and targeting of pump molecules to the apical membranes of α-intercalated cells in the CNT, ICT and CCT, whereas alkalosis has the opposite effect. H-K Exchange Pump  A third type of H+-secretory mecha-

nism is present in the ICT, the CCT, and the OMCD (see Fig. 39-4D): an electroneutral H-K pump (see pp. 117–118) that is related to the Na-K pump. Several isoforms of the H-K pump are present in the kidney and exhibit differential sensitivities to inhibition by drugs such as omeprazole, SCH-28080, and ouabain. The H-K pump probably does not contribute significantly to acid secretion under normal conditions.

Chapter 39  •  Transport of Acids and Bases

N39-4  Renal NHEs

827.e1

N39-5  The β-Intercalated Cell

Contributed by Peter Aronson, Emile Boulpaep, and Walter Boron

Contributed by Walter Boron, Peter Aronson, and Emile Boulpaep

As described on page 124 of the text, several related genes encode NHEs.  N5-20 In the renal proximal tubule, Na-H exchange is blocked by the removal of Na+ from the lumen. Although all NHEs are far less sensitive to amiloride than the ENaC epithelial Na+ channels (see pp. 758–759 and Fig. 35-4D), the apical NHE3 isoform in the proximal tubule is even less amiloride sensitive than the ubiquitous or “housekeeping” NHE1. The NHE1 isoform is present in the basolateral membranes of several nephron segments. The role of basolateral NHEs in acidsecreting nephron segments, such as the proximal tubule, is unclear; they may help regulate pHi independently of transepithelial H+ secretion. Given a 10 : 1 concentration gradient for Na+ from the proximal tubule lumen to the cell interior, a maximal pH gradient of 1 pH unit can be achieved by this gradient. Indeed, the late proximal tubule may have a luminal pH as low as ~6.4. The NHE2 isoform is present at the apical membrane of the DCT, where it may participate in the apical step of H+ secretion.

Electrogenic H pumps are present in β-intercalated cells (see Fig. 39-9B), which, to a first approximation, are backward α-intercalated cells (see Fig. 39-4D). We discuss β-intercalated cells (β-ICs) in the text on page 834. In β-ICs, the electrogenic H pump is present in the basolateral membrane, and the Cl-HCO3 exchanger is in the apical membrane. Thus, unlike the α-ICs, which engage in net HCO3− reabsorption, the β-ICs engage in net HCO3− secretion. An interesting difference between the α-ICs and the β-ICs is that in the α cells, the Cl-HCO3 exchanger is a variant of AE1 (the Cl-HCO3 exchanger in red blood cells, and a member of the SLC4 family), whereas in the β cells the Cl-HCO3 exchanger is molecularly quite different, being a member of the SLC26 family. In addition to the switch from α-IC to β-IC, HCO3− secretion can also be stimulated by increased luminal delivery of Cl−, which promotes the exchange of luminal Cl− for intracellular HCO3− via the apical Cl-HCO3 exchanger. A molecule by the name of hensin controls the conversion from β- to α-intercalated cells. Genetic deletion of hensin in the tubule causes a distal renal tubular acidosis (dRTA) because the mice secrete HCO3− inappropriately and therefore become HCO3− deficient in the blood.

REFERENCE Al-Awqati Q: 2007 Homer W. Smith Award: Control of terminal differentiation in epithelia. J Am Soc Nephrol 19:443– 449, 2008.

828

SECTION VI  •  The Urinary System

A EARLY PROXIMAL CONVOLUTED TUBULE (S1) Tubule lumen Interstitial – space HCO3 OH– CO 2 + Na H

CA IV and XIV CO CA II 3 +

H+

H

Interstitial space



HCO3



HCO3



H+

OH

OH– H+

CO2

H2O

D Interstitial space

Tubule lumen –

HCO3

+

H+

CA II

Cl–

Na+ 3



HCO3

CA IV

THICK ASCENDING LIMB (TAL)

Na



HCO3

CA II

+

Na

H2O

C

LATE PROXIMAL STRAIGHT TUBULE (S3)

Tubule lumen

Na+

2

+

B

AE2

α-INTERCALATED AND MEDULLARY COLLECTING-DUCT CELLS

Tubule lumen –

HCO3

H

Cl–

+

CA II

Interstitial space AE1 Cl–

+

K H+

H+

Figure 39-4  Cell models of H+ secretion.

However, K+ depletion (see p. 803) induces expression of the H-K pump, which retrieves luminal K+ and, as a side effect, enhances H+ secretion. This H+ secretion contributes to the metabolic alkalosis often observed in patients with hypokalemia—hypokalemic metabolic alkalosis.

CAs in the lumen and cytosol stimulate H+ secretion by accelerating the interconversion of CO2 and HCO3The CAs  N18-3  play an important role in renal acidification by catalyzing the interconversion of CO2 to HCO3−. Inhibition of CAs by sulfonamides, such as acetazolamide, profoundly slows acid secretion. CAs may act at three distinct sites of acid-secreting tubule cells (see Fig. 39-4): the extracellular face of the apical membrane, the cytoplasm, and the extracellular face of the basolateral membrane. Two CAs are especially important for tubule cells. The soluble CA II is present in the cytoplasm, whereas CA IV is coupled via a GPI linkage (see p. 13) to the outside of the apical and basolateral membranes, predominantly in proximal-tubule cells. Apical CA (CA IV)  In the absence of apical CA, the H+

secreted accumulates in the lumen, and Na-H exchange and H+ secretion are inhibited. By promoting the conversion of luminal HCO3− to CO2 plus OH−, apical CA prevents the lumen from becoming overly acidic and thus substantially

relieves this inhibition. Thus, CA promotes high rates of HCO3− reabsorption along the early proximal tubule (see Fig. 39-4A). In the distal nephron (see Fig. 39-4D), H+ secretion is less dependent on luminal CA than it is in the early proximal tubule for two reasons. First, the H+ secretion rate is lower than that in the proximal tubule. Thus, the uncatalyzed conversion of luminal H+ and HCO3− to CO2 and H2O can more easily keep up with the lower H+ secretion rate. Second, in the collecting tubules and ducts the electrogenic H pump can extrude H+ against a very high gradient. Therefore, even in the absence of CA, the collecting ducts can raise luminal [H+] substantially, thereby accelerating the uncatalyzed reaction by mass action. Cytoplasmic CA (CA II)  Cytoplasmic CA accelerates the conversion of intracellular CO2 and OH− to HCO3− (see Fig. 39-4). As a result, CA II increases the supply of H+ for apical H+ extrusion and the supply of HCO3− for the basolateral HCO3− exit step. In the CNT, ICT, and CCT, the intercalated cells (which engage in acid-base transport) contain CA II, whereas the principal cells do not. Basolateral CA (CA IV and CA XII)  The role played by basolateral CA IV and CA XII (an integral membrane protein with an extracellular catalytic domain) is not yet understood.  N39-6

Chapter 39  •  Transport of Acids and Bases

828.e1

N39-6  Carbonic Anhydrase at the Basolateral Membrane Contributed by Walter Boron Although it has been known for years that carbonic anhydrases (CAs) are present at the basolateral membrane of the proximal tubule (CA IV, CA XII), only recently has research begun to shed light on the significance of this observation. Two distinct classes of CAs are present near or at the basolateral membrane: (1) the cytosolic or soluble CA II, and (2) one or more membrane-bound CAs (e.g., CA IV, CA XII) with the catalytic domain facing the interstitial fluid. Renal CA XIV is abundant in rodents but is virtually undetectable in human and rabbit kidneys. The role of CA XIV in rodents may be an adaptation to the relatively low activity of rodent CA IV, owing to a G63Q substitution.

CA II According to several reports, the soluble CA II binds reversibly to a site on the cytosolic carboxyl termini of certain HCO3− transporters in the SLC4 family. Among these is the electrogenic Na/ HCO3 cotransporter NBCe1, which is responsible for the vast majority of HCO3− efflux across the basolateral membrane of the proximal tubule (see Fig. 39-4A). According to one viewpoint, the function of the bound CA II is to provide HCO3− as a substrate for the NBCe1 to export to the basolateral side of the tubule cell according to the reaction

CA II CO2 + OH −   → HCO3−

(NE 39-1) 

Published data are consistent with the hypothesis that bound—but not free—CA II increases HCO3− transport. According to an alternate view that is emerging from the laboratory of Walter Boron, the role of CA II is very different. Preliminary data suggest that NBCe1 transports CO2− 3 . Thus, when operating with an apparent Na+:HCO3− stoichiometry of 1 : 3, as it appears to do in the proximal tubule, NBCe1 might actually − transport 1 Na+, 1 CO2− 3 , and 1 HCO3 out of the cell across the basolateral membrane. You might imagine that 1 Na+ and 3 HCO3− ions approach the basolateral membrane from the bulk cytosol. NBCe1 directly extrudes the Na+ and 1 HCO3− . The second HCO3− dissociates to provide the CO2− 3 that NBCe1 will export:

HCO3− → CO23 − + H+

(NE 39-2) 

The third HCO3− , in a reaction catalyzed by CA II, produces an OH−,

CA II HCO3−   → CO2 + OH −

(NE 39-3) 

and this OH− neutralizes the newly formed H+:

H+ + OH− → H2O



(NE 39-4) 

As a result, NBCe1 would export 1 Na+, 1 HCO3− , and 1 CO2− 3 . Of the original 3 HCO3− ions that approached the basolateral membrane, 1 carbon atom, 2 hydrogen atoms, and 3 oxygen atoms would be left behind in the cytosol in the form of CO2 + H2O. According to the alternate view proposed by the Boron laboratory, the CO2 and H2O would exit across the basolateral membrane via another route. Also according to the alternate view, the role of the bound CA II would be to buffer the H+ ions that accumulate on the intracellular side of the membrane as − CO2− 3 forms from HCO3 . Preliminary data from the Boron laboratory indicate that the presence of CA II does not stimulate the electrical current carried by NBCe1, at least as expressed in Xenopus oocytes.

Extracellular CAs According to the classical view, the role of CAs that face the basolateral ECF would be to consume the HCO3− exported by NBCe1 according to the following reaction:

HCO3−

CA

CO2 + OH −

(NE 39-5) 

According to this view, in consuming the newly exported HCO3− , the CA would stimulate the NBCe1. According to the alternate hypothesis put forward by the Boron laboratory, the role of these extracellular CAs is just the opposite of that of the cytoplasmic CA II. Recall that this hypothesis proposes that NBCe1 directly exports 1 Na+, 1 HCO3− , and 1 CO2− 3 , and that 1 CO2 and 1 H2O exit by a parallel route. The extracellular CA would assist in the reassembly of 1 CO2, 1 H2O, − and 1 CO2− 3 to form 2 HCO3 ions, which would then diffuse away from the membrane into the bulk ECF. Indeed, preliminary data show that expressing CA IV on the surface of a Xenopus oocyte greatly reduces the magnitude of the alkalinization produced as NBCe1 exports “HCO3− ” from the cell. Moreover, blocking the CA IV with acetazolamide increases the magnitude of the alkalinization by more than twofold. Finally, preliminary data show that blockade of the CA IV has virtually no effect on the current carried by NBCe1. Thus, it may be that the role of the extracellular CA is not to stimulate NBCe1, but to minimize the size of pH changes on the extracellular surface of the cell.

Chapter 39  •  Transport of Acids and Bases

Inhibition of CA  The administration of drugs that block CAs, such as acetazolamide, strongly inhibits HCO3− reabsorption along the nephron, leading to the excretion of an alkaline urine. Because acetazolamide reduces the reabsorption of Na+, HCO3− , and water, this drug is also a diuretic (i.e., it promotes urine output).  N39-7  How­ ever, a small amount of H+ secretion and HCO3− reabsorption remains despite the complete inhibition of CA. This remaining transport is related in part to the slow uncatalyzed hydration-dehydration reactions and in part to a buildup of luminal H2CO3, which may diffuse into the cell across the apical membrane (mimicking the uptake of CO2 and H2O).

HCO3- efflux across the basolateral membrane takes place by electrogenic Na/HCO3 cotransport and Cl-HCO3 exchange The regulation of the intracellular pH of acid-secreting tubule cells requires that H+ secretion across the apical membrane be tightly linked to, and matched by, the extrusion of HCO3− across the basolateral membrane. Two mechanisms are responsible for HCO3− transport from the cell into the peritubular fluid: electrogenic Na/HCO3 cotransport and Cl-HCO3 exchange. Electrogenic Na/HCO3 Cotransport  In proximal-tubule cells, the electrogenic Na/HCO3 cotransporter NBCe1 (see p. 122) is responsible for much of the HCO3− transport across the basolateral membrane. NBCe1 (SLC4A4) is expressed at highest levels in the S1 portion of the proximal tubule (see Fig. 39-4A) and gradually becomes less abundant in the more distal proximal-tubule segments (see Fig. 39-4B). NBCe1 is a 1035–amino-acid protein with a molecular weight of ~130 kDa. 4,4′-Diisothiocyanostilbene-2,2′disulfonate (DIDS), an inhibitor of most HCO3− transporters, also inhibits NBCe1. Because, in proximal-tubule cells, this transporter usually transports three HCO3− ions for each Na+ ion, the electrochemical driving forces cause it to carry these ions from cell to blood. Renal NBCe1 carries two net negative charges and is thus electrogenic. Human mutations that reduce either NBCe1 activity or NBCe1 targeting to the basolateral membrane cause a severe metabolic acidosis—proximal renal tubular acidosis N39-8 (pRTA).  Chronic metabolic and respiratory acidosis, hypokalemia, and hyperfiltration all increase NBCe1 activity. As would be expected, several factors cause parallel changes in the activities of the apical NHE3 and basolateral Na/HCO3 cotransporter, minimizing changes in cell pH and [Na+]. Thus, angiotensin II (ANG II) and protein kinase C (PKC) stimulate both transporters, whereas parathyroid hormone and PKA markedly inhibit both. Cl-HCO3 Exchange  In the S3 segment of the proximal tubule, as well as in the TAL and collecting tubules and ducts, Cl-HCO3 exchangers participate in transepithelial acid-base transport. The AE1 anion exchanger (see pp. 124–125) is found in the basolateral membranes of α-intercalated cells of the CNT, the ICT, and the CCT (see Fig. 39-4D). Basolateral AE2 is present in the TAL (see Fig. 39-4C) and the DCT.

829

NH+4 is synthesized by proximal tubules, partly reabsorbed in the loop of Henle, and secreted passively into papillary collecting ducts As we saw in our discussion of the segmental handling of NH +4 (see pp. 826–827 and Fig. 39-3B), the proximal tubule is the main site of renal NH +4 synthesis, although almost all other tubule segments have the capacity to form NH +4 . The proximal tubule forms NH +4 largely from glutamine (Fig. 39-5A), which enters tubule cells both from luminal and peritubular fluid via Na+-coupled cotransporters. Inside the mitochondria, glutaminase splits glutamine into NH +4 and glutamate, and then glutamate dehydrogenase splits the glutamate into α-KG and a second NH +4 . Ammonium is a weak acid that can dissociate to form H+ and NH3. Because the pK of the NH3 /NH +4 equilibrium is ~9.2, the NH3 /NH +4 ratio is 1 : 100 at a pH of 7.2. Whereas the cationic NH +4 does not rapidly cross most cell membranes, the neutral NH3 readily diffuses through most, but not all, cell membranes via gas channels.  N39-3  When NH3 diffuses from a relatively alkaline proximal-tubule or collecting-duct cell into the more acidic lumen, the NH3 becomes “trapped” in the lumen after buffering the newly secreted H+ to form the relatively impermeant NH +4 (see Fig. 39-5A). Not only does NH3 diffuse across the apical membrane, but the apical NHE3 directly secretes some NH +4 into the proximal tubule lumen (with NH +4 taking the place of H+). A second consequence of NH +4 synthesis is that the byproduct α-KG participates in gluconeogenesis, which indirectly generates HCO3− ions. As shown in Figure 39-5A, the metabolism of two glutamines generates four NH3 and two α-KG. Gluconeogenesis of these two α-KG, along with four H+, forms one glucose and four HCO3− ions. Accordingly, for each NH +4 secreted into the tubule lumen, the cell secretes one new HCO3− into the peritubular fluid. In juxtamedullary nephrons, which have long loops of Henle, the tDLH may both reabsorb and secrete NH3, with the secretion dominating. Tubule fluid may become alkaline along the tDLH, titrating NH +4 to NH3 and promoting NH3 reabsorption. On the other hand, reabsorption of NH +4 by the TAL (see following paragraph) creates a gradient favoring NH3 diffusion from the interstitium into the lumen of the tDLH. Modeling of these processes predicts net secretion of NH3 into the tDLH in the outer medulla (see Fig. 39-5D). In the thin ascending limb, NH +4 reabsorption may occur by diffusion of NH +4 into the interstitium. In contrast to the earlier segments, the TAL reabsorbs NH +4 (see Fig. 39-5C). Thus, much of the NH +4 secreted by the proximal tubule and tDLH does not reach the DCT. Because the apical membrane of the TAL is unusual in having a very low NH3 permeability, the TAL takes up NH +4 across the apical membrane by using two transport mechanisms, the Na/K/Cl cotransporter and the K+ channels. Indeed, inhibiting the Na/K/Cl cotransporter blocks a significant fraction of NH +4 reabsorption, which suggests that NH +4 can replace K+ on the cotransporter. Ammonium leaves the cell across the basolateral membrane—probably as NH3, via a gas channel, and as NH +4 carried by the NHE—which leads to accumulation of NH +4 in the renal medulla. The NH +4 that has accumulated in the interstitium of the medulla has three possible fates (see Fig. 39-5D). First,

Chapter 39  •  Transport of Acids and Bases

829.e1

N39-7  Diuretic Action of the CA Inhibitor Acetazolamide Contributed by Gerhard Giebisch and Erich Windhager As described in Box 40-3 and in Table 40-3, the drug acetazolamide (a potent inhibitor of CAs) produces diuresis by inhibiting the component of proximal-tubule Na+ reabsorption that is coupled to HCO3− reabsorption. For further discussion of CAs, consult  N18-3.

N39-8  The Electrogenic Na/HCO3 Cotransporter NBCe1 Contributed by Walter Boron NBCe1 is a member of the SLC4 family of solute transporters. It is believed that all of the family members have the same topology: (1) a large cytoplasmic N terminus (Nt) that comprises about 40% of the protein, (2) a large transmembrane domain (TMD) that includes 10 to 14 transmembrane segments (TMs) and comprises ~50% of the protein, and (3) a short cytoplasmic C terminus (Ct) that comprises ~10% of the protein. The gene SLC4A4 encodes three known variants of NBCe1, which differ from one another at their extreme Nt and Ct. The proximal tubule expresses the variant NBCe1-A, which has a very high functional activity. The other variants—the more ubiquitous NBCe1-B and the “brain” form NBCe1-C—have a different Nt. This difference endows these transporters with a low functional activity—due to either reduced trafficking to the membrane or reduced intrinsic activity. However, a soluble protein called IRBIT appear to reverse this inhibition. The NBCe1-A variant in the proximal tubule, however, is the fast variant. In the proximal tubule, NBCe1-A appears to operate with a stoichiometry of 1 Na+ for 3 HCO3− ions. Thus, each transport event moves two negative charges out of the cell and thereby makes the basolateral membrane potential (Vbl) more positive. The reversal potential for NBCe1-A is very close to Vbl. Accord­ ingly, cell depolarization inhibits Na/HCO3 efflux or can even reverse the direction of transport and cause basolateral Na/HCO3 uptake. At least 10 naturally occurring human mutations of NBCe1 are known. From a molecular perspective, these mutations cause

poor function or poor targeting to the appropriate plasma membrane (i.e., the basolateral membrane in the case of NBCe1-A in the proximal-tubule cell). From a clinical perspective, these naturally occurring mutations have a devastating effect on the patient, causing a severe pRTA and other problems that may— depending on the mutation—lead to short stature, mental retardation, and ocular deficits.

REFERENCES Boron WF, Boulpaep EL: Intracellular pH regulation in the renal proximal tubule of the salamander. Basolateral HCO3− transport. J Gen Physiol 81:53–94, 1983. Parker MD, Boron WF: The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters. Physiol Rev 93:803–959, 2013. Parker MD, Boron WF: Sodium-coupled bicarbonate transporters. In Alpern RJ, Hebert SC (ed): The Kidney. Burlington, MA, Academic Press, pp 1481–1497, 2007. Romero MF, Fulton CM, Boron WF: The SLC4 family of HCO3− transporters. Pflugers Arch 477:495–509, 2004. Romero MF, Hediger MA, Boulpaep EL, Boron WF: Expression cloning of the renal electrogenic Na/HCO3 cotransporter. Nature 387:409–413, 1997. Toye AM, Parker MD, Daly CM, et al: The Human NBCe1-A mutant R881C, associated with proximal renal tubular acidosis, retains function but is mistargeted in polarized renal epithelia. Am J Physiol Cell Physiol 291:788–801, 2006.

830 A

SECTION VI  •  The Urinary System

B

EARLY PROXIMAL TUBULE

Tubule lumen

Glutamine

Mitochondrion

Na+

NH3

2 Glutamine

NH3

CA II

4 NH3

4 NH3

4

NH4+

2

4 OH–

NH4+ 2 Glutamate

4

HCO3–

HCO3–

NBC

H+

H+

Glutamate dehydrogenase

2 NH4+

AQP1

H

+

4 CO2



3

+

Na

NH+4

H2O GLUT

+ 4 H

Glucose

2 α-ketoglutarate2 – +

+

H

H

+

Na

4

Interstitial space

Tubule lumen

SNAT3 Na+ (SLC38A3)

SNAT

Glutamine

DESCENDING LIMB OF HENLE (tDLH)

3 Na+

Oxaloacetate2 –

NH4+

2 K+

PEPCK 2 PEP3



Interstitial space

C

D

THICK ASCENDING LIMB (TAL)

NH4+

NH4+

3

FATES OF MEDULLARY NH3/NH4+

NH3

+

Na

2 K+

+

H

2

NH3

Cl– +

H2O CO2

Na

NH4+

NH3 +

H

HCO3–

Na+ HCO3–

E

NH4+

MEDULLARY COLLECTING DUCT Interstitial space

Tubule lumen RhCG

NH3

NH4+

H+

2

1

3 To liver

NH3

NH3

1

2

1

2

RhBG NH3

NH3 H+

1

3

Na+

NH4+

RhCG 2 NH4+

40 mmol/day NH3

NH4+

Chapter 39  •  Transport of Acids and Bases

831

Figure 39-5  Ammonium handling. B, In juxtamedullary nephrons, the secretion of NH+4 into the tubule lumen

of the tDLH occurs mainly in the outer portion of the medulla. In D, the three numbered boxes indicate the three fates of the NH+4 reabsorbed by the TAL. GLUT, glucose transporter; NBC, Na/HCO3 cotransporter; PEP, phosphoenolpyruvate.

Metabolism of amino acids ~940 mmol/day of amino groups Amino acids

Glutamate– 20 mmol/ day

NH4+ α keto acid Aspartate 20 mmol/ day

NH2

450 mmol/ day

O

450 mmol/day NH3 + HCO3–

+

H

900 mmol/day of nitrogen Glutamate– + NH4+

Fumarate

C H2N NH2 Urea

Glutamine + H2O

40 mmol/day of nitrogen LIVER KIDNEY

Glutamine 20 mmol/ day

Urea 450 mmol/ day

Glutamine Renal artery 2 NH4+ Renal vein 2

HCO–3 Ureter

α-Ketoglutarate

Urinary excretion of: + NH4 = 40 mmol/day Urea = 450 mmol/day Figure 39-6  Cooperation between the liver and kidney in excreting nitrogen derived from amino-acid breakdown. In this example, we assume a release of 940 mmol/day of amino groups, resulting in the urinary excretion of 450 mmol/day of urea (900 mmol/day of amino nitrogen) and 40 mmol/day of NH+4 . The values in the boxes are approximations.

some dissociates into H+ and NH3, which then enters the lumen of the late proximal tubule and the early tDLH (see Fig. 39-5D). This NH3 probably diffuses across the aquaporin 1 (AQP1) water channel (see Chapter 5) that is present in both the basolateral and apical membranes of these tubules. Luminal H+ then traps the NH3 as NH +4 (see Fig. 39-5B).

Thus, NH +4 recycles between the proximal tubule/tDLH and the TAL. Second, some of the interstitial NH +4 dissociates into H+ and NH3, which then enters the lumen of the medullary collecting ducts (see Fig. 39-5D). NH3 diffuses into the cell across the basolateral membrane via the gas channels RhBG and RhCG, and then enters the lumen via RhCG, where the NH3 combines with secreted H+ to form NH +4 (see Fig. 39-5E). In addition, the Na-K pump may carry NH +4 (in place of K+) into cells of the medullary collecting ducts. To the extent that NH +4 moves directly from the TAL to the medullary collecting duct, it engages in a bypass of the cortical portions of the distal nephron. This bypass prevents cortical portions of the distal nephron from losing NH3 by diffusion from the lumen into the cortical interstitium, and thus minimizes the entry of the toxic NH3 into the circulation. Third, a small fraction of medullary NH +4 enters the vasa recta. This NH +4 washout returns the nitrogen to the systemic circulation for eventual detoxification by the liver. In the steady state, the buildup of NH +4 in the medulla leads to a sharp increase in [NH +4 ] along the corticomedullary axis. Because the liver synthesizes glutamine (see p. 965), the main starting material for NH +4 production in the kidney, hepatorenal interactions are important in the overall process of NH +4 excretion (Fig. 39-6). The liver disposes of ~1000 mmol/day of amino groups during the catabolism of amino acids. Some of these amino groups become NH +4 via deamination reactions, and some end up as amino groups on either glutamate or aspartate via transamination reactions. Of the ~1000 mmol/day of catabolized amino groups, the liver detoxifies ~95% by producing urea (see p. 965), which the kidneys excrete (see p. 770). One −NH2 in urea comes from an NH +4 that had dissociated to form NH3 and H+, the other −NH2 comes from aspartate, and the C=O comes from HCO3− (see Fig. 39-6). The net result is the generation of urea and—considering that the generated H+ consumes another HCO3− —the consumption of two HCO3− . The liver detoxifies the remaining ~5% of catabolized amino groups by converting NH +4 and glutamate to glutamine (see Fig. 39-6). This reaction does not generate acidbase equivalents. The proximal-tubule cells take up this hepatic glutamine and use it as the source of the NH +4 that they secrete into the tubule lumen as they generate one new HCO3− (see Fig. 39-5A). Thus, the two hepatorenal mechanisms for disposing of catabolized amino groups have opposite effects on HCO3− . For each catabolized amino group excreted as urea, the liver consumes the equivalent of one HCO3−. For each catabolized amino group excreted as NH +4 via the glutamine pathway, the proximal tubule produces one new HCO3− (see Fig. 39-6). To the extent that the kidney excretes NH +4 , the liver consumes less HCO3− as it synthesizes urea (Box 39-1).  N39-9

Chapter 39  •  Transport of Acids and Bases

N39-9  Net Renal Ammonium Excretion Contributed by Peter Aronson and Gerhard Giebisch As noted in the text, one component of the “new HCO3− ” created by the proximal tubule parallels the generation of NH+4 in the proximal-tubule lumen. However, this generation of new HCO3− is reversed to the extent that the NH+4 reabsorbed by the TAL into the medullary interstitium is then picked up by the vasa recta and carried back to the liver for urea production (see Fig. 39-6). Thus, the resecretion of NH+4 from the medullary interstitium into the collecting-duct lumen (for excretion into the urine) is crucial to optimize the efficiency of new-HCO3− generation by the kidney and thus to balance net acid production.

831.e1

832

SECTION VI  •  The Urinary System

BOX 39-1  Renal Tubular Acidosis Contributed by Mark D. Parker

R

enal tubular acidosis (RTA) is a broad label applied to a group of disorders that compromise renal acid-base handling. RTA is characterized by a reduced ability to eliminate H+ in the urine or by HCO3− wasting, both of which can result in a lowered plasma pH (i.e., metabolic acidosis) and, in children, severe impairment of physical and intellectual development. RTA can follow a more generalized disruption of renal function (e.g., as a side effect of medication, autoimmune disease, multiple myeloma) or can result from mutations in genes that encode renal acid-base–handling proteins. RTA is classified into four types, each of which has a different set of causes and clinical manifestations. In addition, we can define a fifth type of RTA that is associated with end-stage renal disease.

Type 1 or Distal RTA

Distal RTA (dRTA) results from defective H+ excretion by distal segments of the nephron. Consequently, dRTA patients cannot appropriately acidify their urine and may exhibit a metabolic acidosis. Genetic causes of dRTA include mutations in the Cl-HCO3 exchanger AE1 and in subunits of the H pump, both of which are key components of the H+-secretory machinery in α-intercalated cells (see Fig. 39-4D). In patients with incomplete dRTA, blood pH is unaffected because compensatory mechanisms (e.g., proximal-tubule function) remain intact; in these individuals, metabolic acidosis occurs only following an acid load. Manifestations of dRTA can include hypokalemia, kidney stones, hemolytic anemia (due to loss of AE1 function in red cells), and hearing loss (due to loss of H pump function in the cochlea).

Type 2 or Proximal RTA Proximal RTA (pRTA) results from the inability of proximal-tubule cells to reabsorb filtered HCO3− or to generate new HCO3− . Conse­ quently, pRTA patients exhibit a severe metabolic acidosis and a wasting of HCO3− into the urine. Genetic defects in the Na/HCO3 cotransporter NBCe1 cause pRTA because of the key role of

REGULATION OF RENAL ACID SECRETION A variety of physiological and pathophysiological stimuli can modulate renal H+ secretion as well as NH3 synthesis. Most of these factors produce coordinated changes in apical and basolateral acid-base transport, as well as in NH3 production.

Respiratory acidosis stimulates renal H+ secretion The four fundamental pH disturbances are respiratory acidosis and alkalosis, and metabolic acidosis and alkalosis (see Fig. 28-11A). In each case, the initial and almost instantaneous line of defense is the action of buffers—both in the extracellular and intracellular compartments—to minimize the magnitude of the pH changes (see pp. 628–629). However, restoring the pH to a value as close to “normal” as possible requires slower compensatory responses from the lungs or kidneys. In respiratory acidosis, in which the primary disturbance is an increase in arterial PCO2 , the compensatory response

that protein in mediating HCO3− movement into the bloodstream (see Fig. 39-4A). Other causes include Fanconi syndrome (e.g., due to multiple myeloma, lead poisoning) and acetazolamide toxicity. Manifestations of pRTA can include hypokalemia and—in children—developmental defects, including ocular problems and poor dentition (considered in part to be due to loss of NBCe1 function in the eye and enamel organ).

Type 3 RTA Type 3 RTA is a rare combination of type 1 and type 2 RTAs and is associated with defects in CA II, a shared component of the acid-base–handling mechanisms in the distal and proximal tubules. Clinical manifestations include osteopetrosis due to loss of CA II function in osteoclasts (see p. 1056).

Type 4 or Hyperkalemic RTA (Hypoaldosteronism) Hyperkalemic RTA is a mild form of acidosis caused by aldosterone insufficiency or renal insensitivity to aldosterone. Insufficient stimulation of mineralocorticoid receptors in α-intercalated cells reduces H+ directly (see p. 835); insufficient stimulation of these receptors in principal cells reduces K+ secretion, leading to hyperkalemia, which causes metabolic acidosis by several mechanisms (see p. 835).

Uremic Acidosis In end-stage renal disease, a loss of functional renal mass compromises total ammoniagenesis.  N39-1

Treatment Treatments for RTA vary depending on the clinical signs in each case but generally focus on correcting the metabolic acidosis by administration of HCO3− or citrate salts (oral base therapy). Additional therapies include administration of diuretics (e.g., hydrochlorothiazide) to stimulate renal H+ secretion.

is an increase in renal H+ secretion, which translates to increased production of new HCO3− via NH +4 excretion. The opposite occurs in respiratory alkalosis. These changes in H+ secretion tend to correct the distorted [HCO3− ]/ [CO2] ratios that occur in primary respiratory acid-base derangements. Respiratory acidosis stimulates H+ secretion in at least three ways. First, an acute elevated PCO2 directly stimulates proximal-tubule cells to secrete H+, as shown by applying solutions in which it is possible to change PCO2 without altering basolateral pH or [HCO3−].  N39-10  Thus, proximaltubule cells directly sense basolateral CO2. In part, the mechanism is the exocytotic insertion of H pumps into the apical membranes of proximal-tubule cells. Second, acute respiratory acidosis also causes exocytotic insertion of H pumps into the apical membranes of intercalated cells in distal nephron segments. Third, chronic respiratory acidosis leads to adaptive responses that upregulate acid-base transporters. For example, respiratory acidosis increases the activities of apical NHE3 and basolateral NBCe1 in proximal tubule. These adaptive changes allow the kidney to

Chapter 39  •  Transport of Acids and Bases

832.e1

N39-10  Use of Out-of-Equilibrium Solutions to Probe the Chemosensitivity of the Proximal Tubule Contributed by Walter Boron As described in  N28-4, the laboratory of Walter Boron developed a rapid-mixing technique that makes it possible to generate out-of-equilibrium (OOE) CO2/HCO3− solutions with virtually any combination of [CO2], [HCO3− ], and pH—as long as the desired pH is not more than a few pH units from neutrality. Recently, the laboratory has applied the OOE CO2/HCO3− solutions to learn more about how the proximal tubule senses acute acid-base disturbances and translates that information to alter the rate at which the tubule reabsorbs HCO3− (i.e., moves HCO3− from the lumen to the basolateral side of the tubule). The approach was to isolate a single proximal tubule and perfuse its lumen with a solution of 5% CO2/22 mM HCO3− /pH 7.4 as well as 3 H-methoxyinulin as a volume marker. By collecting the fluid after it had flowed down the lumen and then analyzing this fluid for [HCO3− ] and [3H-methoxyinulin], the investigators were able to compute the rate of volume reabsorption (JV—that is, the rate at which the tubule moves water from the lumen to the basolateral surface of the tubule, measured in nanoliters per minute per millimeter of tubule length) and the rate of HCO3− reabsorption (JHCO3 —measured in picomoles per minute per millimeter of tubule length). The investigators superfused the basolateral (bl) surface of the tubule with a rapidly flowing solution that was either the “standard” equilibrated 5% CO2/22 mM HCO3− /pH 7.4 solution or an OOE solution in which they varied—one at a time—[CO2]bl, [HCO3− ]bl, or pHbl. Thus, they were able to observe the effects of altering basolateral acid-base composition on JV and JHCO3 . What they found was rather striking. When the investigators raised [CO2]bl from 0 to 4.8 mM—at a fixed [HCO3− ]bl of 22 mM and a fixed pHbl of 7.40—they found that JHCO3 increased in a graded fashion. This result is what one might expect from what we learned about a metabolic compensation to a respiratory acidosis (see p. 641). That is, the kidney ought to respond to a rise in [CO2]bl—the “respiratory” part of a respiratory acidosis— by reabsorbing more HCO3− and thereby tending to restore blood pH to a more alkaline value. However, the investigators were quite surprised to find that increases in JHCO3 were not accompanied by the expected increases in JV (i.e., the extra NaHCO3 reabsorbed by the proximal tubule should have been accompanied by osmotically obligated water, which should have raised JV appreciably). When the investigators raised [HCO3− ]bl from 0 to 44 mM— at a fixed [CO2]bl of 1.2 mM and a fixed pHbl of 7.40—they found

that JHCO3 decreased in a graded fashion. This result is what one might expect for the kidney’s response to a metabolic alkalosis caused by an abnormality outside of the kidney. That is, the kidney ought to respond to a rise in [HCO3− ]bl —the “metabolic” part of a metabolic alkalosis—by reabsorbing less HCO3− and thereby tending to restore blood pH to a more acidic value. However, the investigators were quite surprised to find that decreases in JHCO3 were not accompanied by the expected decreases in JV (i.e., because the tubule reabsorbed less NaHCO3, it should also have reabsorbed less osmotically obligated water, so that JV should have fallen appreciably). Finally, when the investigators raised pHbl from 6.8 to 8.0 mM—at a fixed [CO2]bl of 1.2 mM and a fixed [HCO3− ]bl of 22 mM—they found that JHCO3 did not change! One might have expected that a basolateral alkalosis (the “alkalosis” part of a respiratory or metabolic alkalosis) would have caused the tubule to reabsorb less HCO3− and thereby tend to restore blood pH to a more acidic value. In these experiments, the intracellular pH of the tubule cells changed appreciably, but neither change in pH, intracellular or basolateral, triggered a change in JHCO3 or JV. These experiments led the investigators to conclude that the proximal tubule cannot sense pH per se. Instead, they propose that the proximal-tubule cell has sensors for both basolateral CO2 and basolateral HCO3− . In other words, the proximal tubule seems to regulate blood pH by sensing the body’s two most important buffers. When activated, the CO2 sensors would trigger an increase in NaHCO3 reabsorption but a compensating decrease in the reabsorption of other solutes. When activated, the HCO3− sensors would trigger a decrease in NaHCO3 reabsorption but a compensating increase in the reabsorption of other solutes. The compensating effects on the other solutes would serve to stabilize blood pressure.

REFERENCES Zhao J, Zhou Y, Boron WF: Effect of isolated removal of either basolateral HCO3− or basolateral CO2 on HCO3− reabsorption by rabbit S2 proximal tubule. Am J Physiol Renal Physiol 285: F359–F369, 2003. Zhou Y, Zhao J, Bouyer P, Boron WF: Evidence from renal proximal tubules that HCO3− and solute reabsorption are acutely regulated not by pH but by basolateral HCO3− and CO2. Proc Natl Acad Sci U S A 102(10):3875–3880, 2005. Epub February 22, 2005.

Chapter 39  •  Transport of Acids and Bases

↓ pHo

833

0.10

↓ pHi

Protein kinase C

↑ NHE3

Tyrosine kinase pathways

↑ NBCe1

↑ Na/Citrate cotransporter

Immediate early genes

↑ Ammoniagenic enzymes

Chronic metabolic acidosis

Urinary excretion + of NH4 plus NH3 0.05 (mmol/min) Normal

Figure 39-7  Effects of chronic acidosis on proximal-tubule function. Enhanced Na citrate reabsorption is a defense against acidosis by conversion of citrate to HCO3− . The price paid is enhanced stone formation because luminal citrate reduces stone formation by complexing with Ca2+. Indeed, acidotic patients tend to get calcium-containing kidney stones.

0 5.0

6.0

7.0

8.0

Urinary pH Figure 39-8  Effect of chronic metabolic acidosis on total NH+4 excretion

pro­duce a metabolic compensation to the respiratory acidosis (see p. 641).

Metabolic acidosis stimulates both proximal H+ secretion and NH3 production The first compensatory response to metabolic acidosis is increased alveolar ventilation, which blows off CO2 (see p. 710) and thus corrects the distorted [HCO3− ]/[CO2] ratio in a primary metabolic acidosis. The kidneys can also participate in the compensatory response—assuming, of course, that the acidosis is not the consequence of renal disease. Proximal-tubule cells can directly sense an acute fall in basolateral [HCO3− ], which results in a stimulation of proximal H+ secretion.  N39-10  In intercalated cells in the distal nephron, metabolic acidosis stimulates apical membrane H pump insertion and activity. The mechanism may be protonsensitive G protein–coupled receptors on the basolateral membrane of intercalated cells, and an HCO3−-sensitive soluble adenylyl cyclase (sAC) in the cytosol. In chronic metabolic acidosis, the adaptive responses of the proximal tubule are probably similar to those outlined above for chronic respiratory acidosis. These include upregulation of apical NHE3 and electrogenic H pumps, as well as basolateral NBCe1 (Fig. 39-7), perhaps reflecting increases in the number of transporters on the surface membranes. The parallel activation of apical and basolateral transporters may minimize changes in pHi, while increasing transepithelial HCO3− reabsorption. This upregulation appears to involve intracellular protein kinases, including the Src family of receptor-associated tyrosine kinases (see p. 70). Endothelin appears to be essential for the upregulation of NHE3 in chronic metabolic acidosis. In addition to increased H+ secretion, the other ingredi­ ent needed to produce new HCO3− is enhanced NH3 production. Together, the two increase NH +4 excretion. Indeed, the excretion of NH +4 into the urine increases markedly as a result of the adaptive response to chronic metabolic acidosis

into final urine. (Data from Pitts RF: Renal excretion of acid. Fed Proc 7:418–426, 1948.)

(Fig. 39-8). Thus, the ability to increase NH3 synthesis is an important element in the kidney’s defense against acidotic challenges. Indeed, as chronic metabolic acidosis develops, the kidneys progressively excrete a larger fraction of urinary H+ as NH +4 . As a consequence, the excretion of titratable acid becomes a progressively smaller fraction of total acid excretion. The adaptive stimulation of NH3 synthesis, which occurs in response to a fall in pHi, involves a stimulation of both glutaminase and phosphoenolpyruvate carboxykinase (PEPCK). The stimulation of mitochondrial glutaminase increases the conversion of glutamine to NH +4 and glutamate (see Fig. 39-5A). The stimulation of PEPCK enhances gluconeogenesis and thus the conversion of α-KG (the product of glutamate deamination) to glucose.

Metabolic alkalosis reduces proximal H+ secretion and, in the CCT, may even provoke HCO3- secretion Figure 39-9A illustrates the response of the proximal tubule to metabolic alkalosis. As shown in the upper curve, when the peritubular capillaries have a physiological [HCO3−], increasing the luminal [HCO3− ] causes H+ secretion to increase steeply up to a luminal [HCO3− ] of ~45 mM. The reason is that the incremental luminal HCO3− is an additional buffer that minimizes the luminal acidification in the vicinity of the apical H+ transporters. As shown in the lower curve in Figure 39-9A, when [HCO3− ] in the peritubular blood is higher than normal— that is, during metabolic alkalosis—H+ secretion is lower for any luminal [HCO3− ]. The likely explanation is that the proximal-tubule cell directly senses the increase in plasma [HCO3− ], depressing the rates at which NHE3 moves H+ from cell to lumen and NBCe1 moves HCO3− from cell to blood.

834

A

SECTION VI  •  The Urinary System

EFFECT OF BASOLATERAL ALKALOSIS ON H+ SECRETION BY PROXIMAL TUBULES

200

A rise in GFR increases HCO3- delivery to the tubules, enhancing HCO3- reabsorption (glomerulotubular balance for HCO3- )

Normal

+

H secretion pmole mm tubule × min

100 Basolateral metabolic alkalosis 0

B

0

10

20 30 40 50 60 Mean luminal [HCO–3] (millimoles)

70

80

CORTICAL COLLECTING TUBULE (CCT): β INTERCALATED CELL

CCT

Lumen

– NDCBE (SLC4A8)

CO2

H2O

CA

Cl–

H+

OH–

+

Na

2 HCO3– Cl–

HCO3–

Cl– Pendrin (SLC26A4)

Figure 39-9  Effect of chronic metabolic alkalosis on renal acid-base transport. (Data from Alpern RJ, Cogan MG, Rector FC: Effects of extracellular fluid volume and plasma bicarbonate concentration on proximal acidification in the rat. J Clin Invest 71:736–746, 1983.)

So far, we have discussed the effect of metabolic alkalosis on H+ secretion by the proximal tubule. In the ICT and CCT, metabolic alkalosis can cause the tubule to switch from secreting H+ to secreting HCO3− into the lumen. The α-intercalated cells in the ICT and CCT secrete H+ by using an apical H pump and a basolateral Cl-HCO3 exchanger, which is AE1 (SLC4A1; see Fig. 39-4D). Metabolic alkalosis, over a period of days, shifts the intercalated-cell population, increasing the proportion of β-intercalated cells (see Fig. 39-9B)  N39-5  at the expense of α cells. Because β cells have the opposite apical-versus-basolateral distribution of H pumps and Cl-HCO3 exchangers, they secrete HCO3− into the lumen and tend to correct the metabolic alkalosis. The apical Cl-HCO3 exchanger in β cells is pendrin (SLC26A4; see Table 5-4). In contrast to chronic alkalosis, chronic acidosis alters the distribution of intercalated cell types in the distal nephron in favor of acid-secreting α cells (see Fig. 39-4D) over the base-secreting β-intercalated cells.

Increasing either luminal flow or luminal [HCO3− ] significantly enhances HCO3− reabsorption,  N39-11  probably by raising effective [HCO3−] (and thus pH) in the micro­ environment of H+ transporters in the brush-border microvilli. Because a high luminal pH stimulates NHE3 and the H pumps located in the microvilli of the proximal tubule, increased flow translates to enhanced H+ secretion. This flow dependence, an example of glomerulotubular (GT) balance (see p. 763), is important because it minimizes HCO3− loss, and thus the development of a metabolic acidosis, when GFR increases. Conversely, this GT balance of HCO3− reabsorption also prevents metabolic alkalosis when GFR decreases. The flow dependence of HCO3− reabsorption also accounts for the stimulation of H+ transport that occurs after uninephrectomy (i.e., surgical removal of one kidney), when GFR in the remnant kidney rises in response to the loss of renal tissue.

Extracellular volume contraction—via ANG II, aldosterone, and sympathetic activity—stimulates renal H+ secretion A decrease in effective circulating volume stimulates Na+ reabsorption by four parallel pathways (see pp. 838–840), including activation of the renin-angiotensin-aldosterone axis (and thus an increase in ANG II levels) and stimulation of renal sympathetic nerves (and thus the release of norepinephrine). Both ANG II and norepinephrine stimulate Na-H exchange in the proximal tubule. Because the proximal tubule couples Na+ and H+ transport, volume contraction increases not only Na+ reabsorption but also H+ secretion. Similarly, ANG II stimulates acid secretion by α-intercalated cells in the distal nephron. Volume expansion has the opposite effect. On a longer time scale, volume depletion also increases aldosterone levels, thereby enhancing H+ secretion in cortical and medullary collecting ducts (see below). Thus, the regulation of effective circulating volume takes precedence over the regulation of plasma pH.  N39-12

Hypokalemia increases renal H+ secretion As discussed on page 803, acid-base disturbances can cause changes in K+ homeostasis. The opposite is also true. Because a side effect of K+ depletion is increased renal H+ secretion, K+ depletion is frequently associated with metabolic alkalosis. Several lines of evidence indicate that, in the proximal tubule, hypokalemia leads to a marked increase in apical Na-H exchange and basolateral Na/HCO3 cotransport. As in other cells, in tubule cells the pH falls during K+ depletion (see p. 645). The resulting chronic cell acidification may lead to adaptive responses that activate Na-H exchange and electrogenic Na/HCO3 cotransport, presumably by the same mechanisms that stimulate H+ secretion in chronic acidosis (see Fig. 39-7). In the proximal tubule, K+ depletion also markedly increases NH3 synthesis and NH +4 excretion, thus increasing urinary H+ excretion as NH +4 . Finally, K+ depletion stimulates apical K-H exchange in α-intercalated cells

Chapter 39  •  Transport of Acids and Bases

N39-11  Flow Dependence of HCO3- Reabsorption

834.e1

N39-12  Effect of Dietary Na+ Intake on Proximal-Tubule NHE3 Activity

Contributed by Gerhard Giebisch and Erich Windhager

Contributed by Gerhard Giebisch and Erich Windhager

In the text, we point out that raising either luminal [HCO3− ] or luminal flow increases HCO3− reabsorption. One likely mechanism is mentioned in the text: The higher the flow or the higher the bulk luminal [HCO3− ], the higher the pH and [HCO3− ] in the unstirred layer that surrounds the microvilli on the apical membrane. In addition, increasing the flow also increases the shear force that acts on the central cilium present on every proximal-tubule cell. It is believed that the more the cilium bends with flow, the greater the signal to increase the reabsorption of solutes (including NaHCO3) and water. This hypothesis would account for at least a portion of the glomerulotubular balance for both HCO3− reabsorption (see p. 834) and fractional Na+ reabsorption (see p. 763).

Decreased dietary Na+ intake causes a decrease in effective circulating volume (i.e., volume contraction), resulting in increased activity of the apical NHE3. This effect is evident even if one assesses the activity in brush-border membrane vesicles removed from the animal. Consumption of a high-Na+ diet has the opposite effect.

Chapter 39  •  Transport of Acids and Bases

of the ICT and CCT (see p. 799) and enhances H+ secretion as a side effect of K+ retention. Just as hypokalemia can maintain metabolic alkalosis, hyperkalemia is often associated with metabolic acidosis. A contributory factor may be reduced NH +4 excretion, perhaps because of lower synthesis in proximal-tubule cells, possibly due to a higher intracellular pH. In addition, with high luminal [K+] in the TAL, K+ competes with NH +4 for uptake by apical Na/K/Cl cotransporters and K+ channels, thereby reducing NH +4 reabsorption. As a result, the reduced NH +4 levels in the medullary interstitium provide less NH3 for diffusion into the medullary collecting duct. Finally, with high [K+] in the medullary interstitium, K+ competes with NH +4 for uptake by basolateral Na-K pumps in the medullary collecting duct. The net effects are reduced NH +4 excretion and acidosis.

Both glucocorticoids and mineralocorticoids stimulate acid secretion Chronic adrenal insufficiency (see p. 1019) leads to acid retention and, potentially, to life-threatening metabolic acidosis. Both glucocorticoids and mineralocorticoids stimulate H+ secretion, but at different sites along the nephron. Glucocorticoids (e.g., cortisol) enhance the activity of Na-H exchange in the proximal tubule and thus stimulate H+ secretion. In addition, they inhibit phosphate reabsorption, raising the luminal availability of buffer anions for titration by secreted H+. Mineralocorticoids (e.g., aldosterone) stimulate H+ secretion by three coordinated mechanisms—one direct and two indirect. First, mineralocorticoids directly stimulate H+ secretion in the collecting tubules and ducts by increas­ ing the activity of the apical electrogenic H pump and basolateral Cl-HCO3 exchanger (see Fig. 39-4D). Second, mineralocorticoids indirectly stimulate H+ secretion by enhancing Na+ reabsorption in the collecting ducts (see p. 766), which increases the lumen-negative voltage. This increased negativity may stimulate the apical electrogenic H pump in α-intercalated cells to secrete acid. Third, mineralocorticoids—particularly when administered for longer periods of time and accompanied by high Na+ intake—cause K+ depletion and indirectly increase H+ secretion (see pp. 834–835).

835

Diuretics can change H+ secretion, depending on how they affect transepithelial voltage, ECF volume, and plasma [K+] The effects of diuretics on renal H+ secretion  N39-13  vary substantially from one diuretic to another, depending on both the site and the mechanism of action. From the point of view of acid-base balance, diuretics fall broadly into two groups: those that promote the excretion of a relatively alkaline urine and those that have the opposite effect. To the first group belong CA inhibitors and K+-sparing diuretics. The CA inhibitors lead to excretion of an alkaline urine by inhibiting H+ secretion. Their greatest effect is in the proximal tubule, but they also inhibit H+ secretion by the TAL and intercalated cells in the distal nephron. K+-sparing diuretics—including amiloride, triamterene, and the spironolactones—also reduce acid excretion. Both amiloride and triamterene inhibit the apical epithelial Na+ channels (ENaCs; see pp. 758–759) in the collecting tubules and ducts, rendering the lumen more positive so that it is more difficult for the electrogenic H pump to secrete H+ ions into the lumen. Spironolactones decrease H+ secretion by interfering with the action of aldosterone. The second group of diuretics—those that tend to increase urinary acid excretion and often induce alkalosis—includes loop diuretics such as furosemide (which inhibits the apical Na/K/Cl cotransporter in the TAL) and thiazide diuretics such as chlorothiazide (which inhibits the apical Na/Cl cotransporter in the DCT). These diuretics act by three mechanisms. First, all cause some degree of volume contraction, and thus lead to increased levels of ANG II and aldosterone (see pp. 841–842), both of which enhance H+ secretion. Second, these diuretics enhance Na+ delivery to the collecting tubules and ducts, thereby increasing the electrogenic uptake of Na+, raising lumen-negative voltage, and enhancing H+ secretion. Third, this group of diuretics causes K+ wasting; as discussed on pages 834–835, K+ depletion enhances H+ secretion.

REFERENCES The reference list is available at www.StudentConsult.com.

Chapter 39  •  Transport of Acids and Bases

N39-13  Effect of Diuretics on Renal H+ Excretion Contributed by Erich Windhager and Gerhard Giebisch Box 40-3, as well as Table 40-3, summarizes some of the effects of various classes of diuretics and lists the protein targets of these diuretics in the kidney.

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REFERENCES Books and Reviews Alper SL: Genetic diseases of acid-base transporters. Annu Rev Physiol 64:899–923, 2002. Bobulescu IA, Moe OW: Na+/H+ exchangers in renal regulation of acid-base balance. Semin Nephrol 26:334–344, 2006. Brown D, Wagner CA: Molecular mechanisms of acid-base sensing by the kidney. J Am Soc Nephrol 23:774–780, 2012. Fry AC, Karet FE: Inherited renal acidoses. Physiology (Bethesda) 22:202–211, 2007. Good DW: Ammonium transport by the thick ascending limb of Henle’s loop. Annu Rev Physiol 56:623–647, 1994. Igarashi T, Inatomi J, Sekine T, et al: Mutations in SLC4A4 cause permanent isolated proximal renal tubular acidosis with ocular abnormalities. Nat Genet 23:264–266, 1999. Karet FE: Mechanisms in hyperkalemic renal tubular acidosis. J Am Soc Nephrol 20:251–254, 2009. Laing CM, Toye AM, Capasso G, Unwin RJ: Renal tubular acidosis: Developments in our understanding of the molecular basis. Int J Biochem Cell Biol 37:1151–1161, 2005. Moe OW: Acute regulation of proximal tubule apical membrane Na/H exchanger NHE-3: Role of phosphorylation, protein, trafficking, and regulatory factors. J Am Soc Nephrol 10:2412–2425, 1999. Purkerson JM, Schwartz GJ: The role of carbonic anhydrases in renal physiology. Kidney Int 71:103–115, 2007. Rose BD, Post TW: Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed. New York, McGraw-Hill, 2001. Seldin DW, Giebisch G (eds): The Kidney: Physiology and Pathophysiology, 3rd ed. Philadelphia, Lippincott Williams & Wilkins, 2000. Stone DK, Xie XS: Proton translocating ATPases: Issues in structure and function. Kidney Int 33:767–774, 1988. Wakabayashi S, Shigekawa M, Pouysségur J: Molecular physiology of vertebrate Na+/H+ exchangers. Physiol Rev 77:51–74, 1997. Wall SM: Recent advances in our understanding of intercalated cells. Curr Opin Nephrol Hypertens 14:480–484, 2005. Journal Articles Aronson PS, Nee J, Suhm MA: Modifier role of internal H in activating the Na-H exchanger in renal microvillus membrane vesicles. Nature 299:161–163, 1982. Boron WF, Boulpaep EL: Intracellular pH regulation in the renal proximal tubule of the salamander: Basolateral HCO3− transport. J Gen Physiol 81:53–94, 1983. Bruce LJ, Cope DL, Jones GK, et al: Familial distal renal tubular acidosis is associated with mutations in the red cell anion exchanger (Band 3, AE1) gene. J Clin Invest 100:1693–1707, 1997. Fry AC, Karet FE: Inherited renal acidoses. Physiology (Bethesda) 22:202–211, 2007. Geibel J, Giebisch G, Boron WF: Angiotensin II stimulates both Na-H exchange and Na/HCO3 cotransport in the rabbit proximal tubule. Proc Natl Acad Sci U S A 87:7917–7920, 1990. Igarashi T, Inatomi J, Sekine T, et al: Mutations in SLC4A4 cause permanent isolated proximal renal tubular acidosis with ocular abnormalities. Nat Genet 23:264–266, 1999.

Karet FE: Mechanisms in hyperkalemic renal tubular acidosis. J Am Soc Nephrol 20:251–254, 2009. Karet FE, Finberg KE, Nelson RD, et al: Mutations in the gene encoding B1 subunit of H+-ATPase cause renal tubular acidosis with sensorineural deafness. Nat Genet 21:84–90, 1999. Laing CM, Toye AM, Capasso G, Unwin RJ: Renal tubular acidosis: Developments in our understanding of the molecular basis. Int J Biochem Cell Biol 37:1151–1161, 2005. McKinney TD, Burg MB: Bicarbonate transport by rabbit cortical collecting tubules: Effect of acid and alkali loads in vivo on transport in vitro. J Clin Invest 60:766–768, 1977. Petrovic S, Wang Z, Ma L, Soleimani M: Regulation of the apical Cl − /HCO3− exchanger pendrin in rat cortical collecting duct in metabolic acidosis. Am J Physiol Renal Physiol 284:F103–F112, 2003. Piermarini PM, Verlander JW, Royaux IE, Evans DH: Pendrin immunoreactivity in the gill epithelium of a euryhaline elasmobranch. Am J Physiol Regul Integr Comp Physiol 283:R983– R992, 2002. Quentin F, Chambrey R, Trinh-Trang-Tan MM, et al: The Cl − /HCO3− exchanger pendrin in the rat kidney is regulated in response to chronic alterations in chloride balance. Am J Physiol Renal Physiol 287:F1179–1188, 2004. Romero MF, Hediger MA, Boulpaep EL, Boron WF: Expression cloning of the renal electrogenic Na/HCO3 cotransporter. Nature 387:409–413, 1997. Royaux IE, Wall SM, Karniski LP, et al: Pendrin, encoded by the Pendred syndrome gene, resides in the apical region of renal intercalated cells and mediates bicarbonate secretion. Proc Natl Acad Sci U S A 98:4221–4226, 2001. Schwartz GJ, Al-Awqati Q: Carbon dioxide causes exocytosis of vesicles containing H+ pumps in isolated perfused proximal and collecting tubules. J Clin Invest 75:1638–1644, 1985. Sly WS, Hewett-Emmett D, Whyte MP, et al: Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. Proc Natl Acad Sci U S A 80:2752– 2756, 1983. Smith AN, Skaug J, Choate KA, et al: Mutations in ATP6N1B, encoding a new kidney vacuolar proton pump 116-kD subunit, cause recessive distal renal tubular acidosis with preserved hearing. Nat Genet 26:71–75, 2000. Sun X, Yang LV, Tiegs BC, et al: Deletion of the pH sensor GPR4 decreases renal acid excretion. J Am Soc Nephrol 21:1745–1755, 2010. Verlander JW, Hassell KA, Royaux IE, et al: Deoxycorticosterone upregulates PDS (Slc26a4) in mouse kidney. Role of pendrin in mineralocorticoid-induced hypertension. Hypertension 42: 356–362, 2003. Wall SM, Hassell KA, Royaux IE, et al: Localization of pendrin in mouse kidney. Am J Physiol Renal Physiol 284:F229–F241, 2003. Wang T, Malnic G, Giebisch G, Chan YL: Renal bicarbonate reabsorption in the rat. IV. Bicarbonate transport mechanisms in the early and late distal tubule. J Clin Invest 91:2776–2784, 1993. Zhou Y, Zhao J, Bouyer P, Boron WF: Evidence from renal proximal tubules that HCO3− and solute reabsorption are acutely regulated not by pH but by basolateral HCO3− and CO2. Proc Natl Acad Sci U S A 102:3875–3880, 2005.

C H A P T E R 40  INTEGRATION OF SALT AND WATER BALANCE Gerhard Giebisch, Erich E. Windhager, and Peter S. Aronson

Two separate but closely interrelated control systems regulate the volume and osmolality of the extracellular fluid (ECF). It is important to regulate the ECF volume to maintain blood pressure, which is essential for adequate tissue perfusion and function. The body regulates ECF volume by adjusting the total-body content of NaCl. It is important to regulate the extracellular osmolality because hypotonic (see pp. 131–132) or hypertonic (see p. 131) osmolalities cause changes in cell volume that seriously compromise cell function, especially in the central nervous system (CNS). The body regulates extracellular osmolality by adjusting totalbody water content. These two homeostatic mechanisms— for ECF volume and osmolality—use different sensors, different hormonal transducers, and different effectors (Table 40-1). However, they have one thing in common: some of their effectors, although different, are located in the kidney. In the case of the ECF volume, the control system modulates the urinary excretion of Na+. In the case of osmolality, the control system modulates the urinary excretion of solutefree water or simply free water (see pp. 806–807). Sodium Balance  The maintenance of the ECF volume, or Na+ balance, depends on signals that reflect the adequacy of the circulation—the so-called effective circulating volume, discussed below. Low- and high-pressure baroreceptors send afferent signals to the brain (see pp. 536–537), which translates this “volume signal” into several responses that can affect ECF volume or blood pressure over either the short or the long term. The short-term effects (over a period of seconds to minutes) occur as the autonomic nervous system and humoral mechanisms modulate the activity of the heart and blood vessels to control blood pressure. The long-term effects (over a period of hours to days) consist of nervous, humoral, and hemodynamic mechanisms that modulate renal Na+ excretion (see pp. 763–769). In the first part of this chapter, we discuss the entire feedback loop, of which Na+ excretion is the effector. Why is the Na+ content of the body the main determinant of the ECF volume? Na+, with its associated anions, Cl− and HCO3− , is the main osmotic constituent of the ECF volume; when Na salts move, water must follow. Because the body generally maintains ECF osmolality within narrow limits (e.g., ~290 milliosmoles/kg, or 290 mOsm), it follows that whole-body Na+ content—which the kidneys control—must be the major determinant of the ECF volume. A simple example illustrates the point. If the kidney were to enhance

836

the excretion of Na+ and its accompanying anions by 145 milliequivalents (meq) each—the amount of solute normally present in 1 L of ECF—the kidneys would have to excrete an additional liter of urine to prevent a serious fall in osmolality. Alternatively, the addition of 145 mmol of “dry” NaCl to the ECF obligates the addition of 1 L of water to the ECF; this addition can be accomplished by ingestion of water or reduction of renal excretion of free water. Relatively small changes in Na+ excretion lead to marked alterations in the ECF volume. Thus, precise and sensitive control mechanisms are needed to safeguard and regulate the body’s content of Na+. Water Balance  The maintenance of osmolality, or water balance, depends on receptors in the hypothalamus that detect changes in the plasma osmolality. These receptors send signals to areas of the brain that (1) control thirst and thus regulate free-water intake and (2) control the production of arginine vasopressin (AVP)—also known as antidiuretic hormone (ADH)—and thus regulate free-water excretion by the kidneys. We discuss renal water excretion beginning on page 806. In the second part of this chapter, we discuss the entire feedback loop, of which water excretion is merely the end point. Why is the water content of the body the main determinant of osmolality? Total-body osmolality is defined as the ratio of total-body osmoles to total-body water (see p. 102). Although the ECF volume control system can regulate the amount of extracellular osmoles, it has little effect on totalbody osmoles. Total-body osmoles are largely a function of the intracellular milieu because the intracellular compartment is larger than the ECF and its solute composition is highly regulated. Total-body osmoles do not change substantially except during growth or during certain disease states, such as diabetes mellitus (in which excess glucose increases total-body osmolality). Only by controlling water independent of Na+ control can the body control osmolality.

CONTROL OF EXTRACELLULAR FLUID VOLUME In the steady state, Na+ intake via the gastrointestinal tract equals Na+ output from renal and extrarenal pathways The two principal solutes in the ECF are Na+ and Cl−. Sodium is one of the most abundant ions in the body, totaling

Chapter 40  •  Integration of Salt and Water Balance

837

TABLE 40-1  Comparison of the Systems Controlling ECF Volume and Osmolality REGULATION OF ECF VOLUME AND BLOOD PRESSURE

REGULATION OF OSMOLALITY

What is sensed?

Effective circulating volume

Plasma osmolality

Sensors

Carotid sinus, aortic arch, renal afferent arteriole, atria

Hypothalamic osmoreceptors

Efferent pathways

Renin-angiotensin-aldosterone axis, sympathetic nervous system, AVP, ANP

AVP

Thirst

Effector

Short term: Heart, blood vessels Long term: Kidney

Kidney

Brain: drinking behavior

What is affected?

Short term: Blood pressure Long term: Na+ excretion

Renal water excretion

Water intake

~58 meq/kg body weight. Approximately 65% of the total Na+ is located in the ECF, and an additional 5% to 10%, in the intracellular fluid (ICF). Extracellular and intracellular Na+, comprising 70% to 75% of the total-body pool, is readily exchangeable, as determined by its ability to equilibrate rapidly with injected radioactive Na+. The remaining 25% to 30% of the body’s Na+ pool is bound as Na+-apatites in bone. The concentration of Na+ in the plasma and interstitial fluid typically ranges between 135 and 145 mM. Chloride totals ~33 meq/kg body weight. Approximately 85% is extracellular, and the remaining 15% is intracellular. Thus, all Cl− is readily exchangeable. The [Cl−] of plasma and interstitial fluid normally varies between 100 and 108 mM. Changes in total-body Cl− are usually influenced by the same factors, and in the same direction, as changes in total-body Na+. Exceptions arise during acid-base disturbances, when Cl− metabolism may change independently of Na+. By definition, in the steady state, the total-body content of water and electrolytes is constant. For Na+, this concept can be expressed as

Oral Na + intake = Renal Na + output + Extrarenal Na + output

(40-1) 

Under normal circumstances, extrarenal Na+ output is negligible. However, large fluid losses from the gastrointestinal tract (e.g., vomiting, diarrhea) or skin (e.g., excessive sweating, extensive burns) can represent substantial extrarenal Na+ losses. The kidney responds to such deficits by reducing renal Na+ excretion. Conversely, in conditions of excessive Na+ intake, the kidneys excrete the surfeit of Na+.

The kidneys increase Na+ excretion in response to an increase in ECF volume, not to an increase in extracellular Na+ concentration In contrast to many other renal mechanisms of electrolyte excretion, the renal excretion of Na+ depends on the amount of Na+ in the body and not on the Na+ concentration in ECF. Because the amount of Na+ is the product of ECF volume and the extracellular Na+ concentration, and because the osmoregulatory system keeps plasma osmolality constant within very narrow limits, it is actually the volume of ECF that acts as the signal for Na+ homeostasis. Figure 40-1A demonstrates the renal response to an abrupt step increase and step decrease in Na+ intake. A subject weighing 70 kg starts with an unusually low Na+

intake of 10 mmol/day, matched by an equally low urinary output. When the individual abruptly increases dietary Na+ intake from 10 to 150 mmol/day—and maintains it at this level for several days—urinary Na+ output also increases, but at first it lags behind intake. This initial period during which Na+ intake exceeds Na+ output is a state of positive Na+ balance. After ~5 days, urinary Na+ output rises to match dietary intake, after which total-body Na+ does not increase further. In this example, we assume that the cumulative retention of Na+ amounts to 140 mmol. The abrupt increase in dietary Na+ initially elevates plasma osmolality, thus stimulating thirst and release of AVP. Because the subject has free access to water, and because the kidneys salvage water in response to AVP (see pp. 817–819), the volume of free water rises. This increase in free water not only prevents a rise in [Na+] and osmolality, but also produces a weight gain that, in this example, is 1 kg (see Fig. 40-1A). This weight gain corresponds, in our example, to the accumulation of 140 mmol of Na+ and the accompanying free water, which makes 1 L of isotonic saline. In the new steady state, only the extracellular compartment has increased in volume. Intracellular volume does not change because, in the end, no driving force exists for water to cross cell membranes (i.e., extracellular osmolality is normal). Instead, the slight expansion of ECF volume signals the kidney to increase its rate of Na+ excretion. The extracellular Na+ concentration is unchanged during this period and thus cannot be the signal to increase Na+ excretion. When the subject in our example later reduces Na+ intake to the initial level of 10 mmol/day (see Fig. 40-1A), Na+ excretion diminishes until the initial balanced state (input = output) is established once again. Immediately after the reduction in Na+ intake, Na+ is temporarily out of balance. This time, we have a period of negative Na+ balance, in which output exceeds input. During this period, the ECF volume falls by 1 L, and body weight returns to normal. Again, the extracellular Na+ concentration is unchanged during this transient period. Ingestion of increasingly larger amounts of Na+ results in retention of progressively larger amounts in the steady state and thus accumulation of progressively more ECF volume. Urinary Na+ excretion increases linearly with this rise in retained Na+, as shown in Figure 40-1B. The control system that so tightly links urinary Na+ excretion to ECF volume is extremely sensitive. In our hypothetical example (see Fig. 40-1A)—a 70-kg individual with an initial ECF volume of 17 L—expanding ECF volume by 1 L, or ~6%, triggers a

838

A

SECTION VI  •  The Urinary System

+ EFFECT OF ABRUPT CHANGES IN Na INTAKE

Weight (kg)

B

EFFECT OF POSITIVE Na+ BALANCE ON Na+ EXCRETION 1400

71 1200

70

1000

150 +

Na (mmol/ day)

100 50 10

Output Intake

Negative balance

Positive balance

–3 –2 –1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Days

Incremental urinary Na+ excretion (mmol/day)

800 600 400 200 0

0

4 Gain in extracellular water (L)

8

0

800 1000 1200 200 400 600 + Amount of Na retained by body (mmol cumulative Na+ balance)

Figure 40-1  Na+ balance. In A, the red curve shows the time course of dietary Na+ intake, and the green

curve shows Na+ excretion. The gold area between the two curves at the beginning of the experiment corresponds to the accumulated total-body Na+ of 140 mmol. This additional Na+, dissolved in ~1 L of ECF, accounts for the 1-kg gain in body weight (blue curve). (B, Data from Walser M: Phenomenological analysis of renal regulation of sodium and potassium balance. Kidney Int 27:837–841, 1985.)

15-fold increase in steady-state urinary Na+ excretion (i.e., from 10 mmol/day to 150 mmol/day in Fig. 40-1A). Physiologically normal individuals can be in Na+ balance on a nearly Na+-free diet (1 to 2 mmol/day) without overt signs of ECF volume depletion. Conversely, even with consumption of a high-Na+ diet (200 mmol/day versus the “normal” ~100 mmol/day for a Western diet), clinical signs of ECF volume excess, such as edema, are absent.

in Na+ retention. For example, in congestive heart failure, particularly when edema is extensive, the total ECF volume is greatly increased. However, the low cardiac output fails to expand the key blood-filled compartments. As a result, Na+ reabsorption by the renal tubules remains high (i.e., urinary Na+ excretion is inappropriately low compared with Na+ intake), which exacerbates the systemic congestion (Box 40-1).  N40-1 

It is not the ECF volume as a whole, but the effective circulating volume, that regulates Na+ excretion

Decreases in effective circulating volume trigger four parallel effector pathways to decrease renal Na+ excretion

Although we have referred to the overall expansion of the ECF volume as the signal for increased urinary Na+ excretion, this is an oversimplification. Only certain regions of the ECF compartment are important for this signaling. For an expansion in ECF volume to stimulate Na+ excretion—either acutely or chronically—the expansion must make itself evident in parts of the ECF compartment where the ECF volume sensors are located, namely, in blood-filled compartments. ECF volume per se is not the critical factor in regulating renal Na+ excretion. The critical parameter that the body recognizes is the effective circulating volume (see pp. 554–555)—not something that we can identify anatomically. Rather, effective circulating volume is a functional blood volume that reflects the extent of tissue perfusion in specific regions, as evidenced by the fullness or pressure within their blood vessels. Normally, changes in effective circulating volume parallel those in total ECF volume. However, this relationship may be distorted in certain diseases, such as congestive heart failure, nephrotic syndrome, or liver cirrhosis. In all three cases, total ECF volume is grossly expanded (e.g., edema or ascites). In contrast, the effective circulating volume is low, resulting

Figure 40-2 shows the elements of the feedback loop that controls the effective circulating volume. As summarized in Table 40-2, sensors that monitor changes in effective circulating volume are baroreceptors located in both highpressure (see pp. 534–536) and low-pressure (see pp. 546– 547) areas of the circulation. Although most are located within the vascular tree of the thorax, additional baroreceptors are present in the kidney—particularly in the afferent arterioles (see p. 730)—as well as in the CNS and liver. Of the pressures at these sites, it is renal perfusion pressure that is most important for long-term regulation of Na+ excretion, and thus blood pressure, because increased resistance to renal blood flow (e.g., renal artery stenosis) causes sustained hypertension (Box 40-2). The sensors shown in Figure 40-2 generate four distinct hormonal or neural signals (pathways 1 to 4 in the figure). In the first pathway, the kidney itself senses a reduced effective circulating volume and directly stimulates a hormonal effector pathway, the renin-angiotensinaldosterone system, discussed in the section beginning on page 841. In addition, increased renal perfusion pressure

Chapter 40  •  Integration of Salt and Water Balance

N40-1  Effect of Posture and Water Immersion on Na+ Excretion Contributed by Gerhard Giebisch and Erich Windhager On page 838, we introduce congestive heart failure as an example of the nonparallel behavior of ECF volume on the one hand and effective circulating volume on the other. Two additional examples that depend upon gravity are posture and water immersion. Urinary Na+ excretion is lowest when one is standing (i.e., when thoracic perfusion is lowest), higher when one is lying down (recumbency), and highest when one is immersed up to the chin for several hours in warm water. During immersion, the hydrostatic pressure of the water compresses the tissues—and thus the vessels, particularly the veins—in the extremities and abdomen and consequently enhances venous return to the thorax. Recumbency—and, to a greater extent, water immersion—shifts blood into the thoracic vessels, increasing the so-called central blood volume (see p. 449). In contrast, the upright position depletes the intrathoracic blood volume. The thoracic vessels are immune to this compression because their extravascular pressure (i.e., intrapleural pressure; see p. 606) is unaffected by the water. Thus, it is the enhanced venous return alone that stimulates vascular sensors to increase Na+ excretion. This example clearly demonstrates that only special portions within the ECF compartment play critical roles in the sensing of ECF volume.

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Chapter 40  •  Integration of Salt and Water Balance

BOX 40-1  Volume Expansion and Contraction

W

hen Na+ intake persists in the face of impaired renal Na+ excretion (e.g., during renal failure), the body retains isosmotic fluid. The result is an expansion of plasma volume and of the interstitial fluid compartment. In the extreme, the interstitial volume increase can become so severe that the subepidermal tissues swell (e.g., around the ankles). When the physician presses with a finger against the skin and then removes the finger, the finger imprint remains in the tissue—pitting edema. Not all cases of lower-extremity edema reflect total-body Na+ and fluid retention. For example, venous obstruction to return of blood from the lower extremities can cause local edema in the lower legs. Patients with this condition should elevate their feet whenever possible and should wear compression stockings. Fluid can also accumulate in certain transcellular spaces (see p. 102), such as the pleural cavity (pleural effusion) or the peritoneal cavity (ascites); such conditions reflect derangements of local Starling forces or an increase in protein permeability due to inflammation, which alters the fluid distribution between the plasma and the ECF (see Box 20-1). In cases of abnormal Na+ retention, putting the subject on a low-Na+ diet can partially correct the edema. Administration of diuretics  N40-2  can also reduce volume overload, as long as the kidney retains sufficient function to respond to them. An excessive loss of Na+ into the urine can be caused by disturbances of Na+ reabsorption along the nephron and leads to a dramatic shrinkage of the ECF volume. Because the plasma volume is part of the ECF volume, significant reductions can severely affect the circulation, culminating in hypovolemic shock (see p. 583). Renal causes of reduced ECF volume include the prolonged use of powerful loop diuretics (see p. 757), osmotic diuresis (see Box 35-1) during poorly controlled diabetes mellitus, adrenal insufficiency with low aldosterone levels, and the recovery phase following acute renal failure or relief of urinary obstruction.

TABLE 40-2  ECF Volume Receptors “Central” vascular sensors High pressure JGA (renal afferent arteriole) Carotid sinus Aortic arch Low pressure Cardiac atria Pulmonary vasculature Sensors in the CNS (less important) Sensors in the liver (less important)

itself can increase Na+ excretion independent of the reninangiotensin-aldosterone system, as we shall see beginning on page 843. The second and third effector pathways are neural. Baroreceptors detect decreases in effective circulating volume and communicate these via afferent neurons to the medulla of the brainstem. Emerging from the medulla are two types of efferent signals that ultimately act on the kidney. In one, increased activity of the sympathetic division of the autonomic nervous system reduces renal blood flow and directly

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BOX 40-2  Renal Hypertension

I

n the 1930s, Goldblatt produced hypertension experimentally in unilaterally nephrectomized animals by placing a surgical clip around the renal artery of the remaining kidney (one-kidney Goldblatt hypertension). The constriction can be adjusted so that it results not in renal ischemia, but only in a reduction of the perfusion pressure distal to the clip. This maneuver stimulates the renal baroreceptors, leading to a rapid increase in synthesis and secretion of renin from the clipped kidney. The renin release reaches a peak after 1 hour. As renin cleaves ANG I from angiotensinogen, systemic ANG I levels rise quickly. ACE, present mainly in the lungs but also in the kidneys, then rapidly converts ANG I into ANG II. Thus, within minutes of clamping the renal artery, one observes a sustained rise in systemic arterial pressure. The newly established stable elevation in systemic pressure then normalizes the pressure in the renal artery downstream from the constriction. From this time onward, circulating renin and ANG II levels decline toward normal over 5 to 7 days, while the systemic arterial pressure remains abnormally high. The early rise in blood pressure is the result of the renin-angiotensin vasoconstrictor mechanism, which is activated by the experimentally induced reduction in pressure and flow in the renal artery distal to the constriction. The later phase of systemic hypertension is the result of aldosterone release and of the retention of salt and water. Unilateral partial clamping of a renal artery in an otherwise healthy animal also produces hypertension (two-kidney Goldblatt hypertension). As in the one-kidney model, the clipped kidney increases its synthesis and secretion of renin. Renin then causes ANG II levels to increase systemically and will, in addition to the effect on the clamped kidney, cause the nonclamped contralateral kidney to retain salt and water. As in the one-kidney model, the resulting hypertension has an early vasoconstrictive phase and a delayed volume-dependent phase. These models of hypertension show that the kidney can be critical as a long-term baroreceptor. Thus, when increased resistance in a renal artery leads to reduced intrarenal perfusion pressure, the rest of the body, including central baroreceptors, experiences—and cannot counteract—the sustained hypertension. In both types of Goldblatt hypertension, administration of ACE inhibitors can lower arterial blood pressure. In fact, inhibiting ACE is therapeutically effective even after circulating renin and ANG II levels have normalized. The reason is that maintained hypertension involves an increased intrarenal conversion of ANG I to ANG II (via renal ACE), with the ANG II enhancing proximal Na+ reabsorption. Indeed, direct measurements show that, even after circulating renin and ANG II levels have returned to normal, the intrarenal levels of ACE and ANG II are elevated. ACE inhibitors lower systemic and intrarenal ANG II levels. These experimental models correspond to some forms of human hypertension, including hypertension produced by renin-secreting tumors of the JGA and by all types of pathological impairment of renal arterial blood supply. Thus, coarctation of the aorta, in which the aorta is constricted above the renal arteries but below the arteries to the head and upper extremities, invariably leads to hypertension. Renal hypertension also results from stenosis of a renal artery, caused, for example, by arteriosclerotic thickening of the vessel wall.

Chapter 40  •  Integration of Salt and Water Balance

N40-2  Sensitivity of the Natriuretic Response to Increased Extracellular Fluid Volume Contributed by Erich Windhager and Gerhard Giebisch Figure 40-1B shows a hypothetical example of how urinary Na+ excretion (y-axis) changes in response to increases in isotonic extracellular water volume (upper x-axis) or amount of Na+ retained by the body (lower x-axis). In the example in the figure, the urinary Na+ excretion increases by 120 mmol/ day for every 100 mmol of cumulative Na+ retention. This proportionality is indicated by the slope of the line. However, this slope need not be the same for every person. In a patient with abnormal Na+ retention, the natriuretic response must be less sensitive than normal (i.e., the slope of the line in Fig. 40-1B must be less steep). In other words, in response to an increase in Na+ intake, the patient would have to accumulate more Na+ and water (i.e., he or she would have to become more volume expanded than would a normal person) in order to sufficiently stimulate the kidneys to elicit the natriuretic response necessary for coming into Na+ balance (i.e., achieving a steady state in which urinary excretion balances dietary intake).

839.e1

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SECTION VI  •  The Urinary System

Increased renal Na+ retention counteracts decreased effective circulating volume.

Effective circulating volume

Renal baroreceptor

Liver

Aortic arch

Carotid sinus

Cardiac atria

Central nervous system

Atrial myocytes

GFR

Juxtaglomerular apparatus (JGA)

Atrial lowpressure receptor

Pulmonary lowpressure receptor

Brain 1 Renin

Angiotensin II (ANG II)

2 Sympathetic division of ANS

3 Posterior pituitary

Aldosterone

4

Atrial natriuretic peptide (ANP)

Arginine vasopressin (AVP or ADH)

Changes in hemodynamics and tubule transport

Na+ excretion Figure 40-2  Feedback control of effective circulating volume. A low effective circulating volume triggers four parallel effector pathways (numbered 1 to 4) that act on the kidney, either by changing the hemodynamics or by changing Na+ transport by the renal-tubule cells. ANS, autonomic nervous system.

stimulates Na+ reabsorption, thereby reducing Na+ excretion (discussed on pp. 842–843). In the other effector pathway, the posterior pituitary increases its secretion of AVP, which leads to conservation of water (discussed on p. 843). This AVP mechanism becomes active only after large declines in effective circulating volume. The final pathway is hormonal. Reduced effective circulating volume decreases the release of atrial natriuretic peptide (ANP), thus reducing Na+ excretion (discussed on p. 843). All four parallel effector pathways correct the primary change in effective circulating blood volume. An increase in effective circulating volume promotes Na+ excretion (thus

reducing ECF volume), whereas a decrease in effective circulating volume inhibits Na+ excretion (thus raising ECF volume). An important feature of renal Na+ excretion is the two-way redundancy of control mechanisms. First, efferent pathways may act in concert on a single effector within the kidney. For instance, both sympathetic input and hemodynamic/ physical factors often act on proximal tubules. Second, one efferent pathway may act at different effector sites. For example, angiotensin II (ANG II) enhances Na+ retention directly by stimulating apical Na-H exchange in tubule cells (see Fig. 35-4) and indirectly by lowering renal plasma flow (see p. 746).

Chapter 40  •  Integration of Salt and Water Balance

841

Effective circulating volume Lungs

ACE

Angiotensin II

Adrenal Increased renal Na+ retention counteracts decreased effective circulating volume.

Hypothalamus

Aldosterone Angiotensin I

Kidneys

Thirst and AVP Renin

JGA

Angiotensinogen

Liver

+

Na excretion H2O excretion

Figure 40-3  Renin-angiotensin-aldosterone axis.

Increased activity of the renin-angiotensin-aldosterone axis is the first of four parallel pathways that correct a low effective circulating volume The renin-angiotensin-aldosterone axis (Fig. 40-3) promotes Na+ retention via the actions of both ANG II and aldosterone. For a consideration of this axis in the context of the physiology of the adrenal cortex, see page 1029. Angiotensinogen,  N23-12  also known as renin substrate, is an α2-globulin that is synthesized by the liver and released into the systemic circulation. The liver contains only small stores of angiotensinogen. Another protein, renin,  N40-4  is produced and stored in distinctive granules by the granular cells of the renal juxtaglomerular apparatus (JGA; see p. 727). As discussed below (see p. 841), decreases in effective circulating volume stimulate these cells to release renin, which is a protease that cleaves a peptide bond near the C terminus of angiotensinogen, releasing the decapeptide angiotensin I (ANG I). Angiotensin-converting enzyme (ACE) rapidly removes the two C-terminal amino acids from the physiologically inactive ANG I to form the physiologically active octapeptide ANG II. ACE is present on the luminal surface of vascular endothelia throughout the body and is abundantly present in the endothelium-rich lungs. ACE in the kidney—particularly in the endothelial cells of the afferent and efferent arterioles, and also in the proximal tubule—can produce enough ANG II to exert local vascular effects. Thus, the kidney receives ANG II from three sources: (1) Systemic ANG II comes from the general circulation, originating largely from the pulmonary circulation. (2) Renal vessels generate ANG II from ANG I. (3) Proximaltubule cells, which contain renin and ACE, secrete ANG II into its lumen. Both in the circulation and in the tubule

lumen, aminopeptidases further cleave ANG II to the heptapeptides ANG III [ANG-(2-8)] and ANG-(1-7), which are biologically active. The principal factor controlling plasma ANG II levels is renin release from JGA granular cells. A decrease in effective circulating volume manifests itself to the JGA—and thus stimulates renin release—in three ways (see Fig. 40-2): 1. Decreased systemic blood pressure (sympathetic effect on JGA). A low effective circulating volume, sensed by baroreceptors located in the central arterial circulation (see p. 534), signals medullary control centers to increase sympathetic outflow to the JGA, which in turn increases renin release. Renal denervation or β-adrenergic blocking drugs (e.g., propranolol) inhibit renin release. 2. Decreased NaCl concentration at the macula densa (NaCl sensor). Decreased effective circulating volume tends to increase filtration fraction (the inverse of the sequence shown in Fig. 34-10), thereby increasing Na+ and fluid reabsorption by the proximal tubule (see p. 842) and reducing the flow of tubule fluid through the loop of Henle. Na+ reabsorption in the thick ascending limb (TAL) then decreases luminal [Na+] more than if tubular flow were higher. The resulting decrease in luminal [NaCl] at the macula densa stimulates renin release. 3. Decreased renal perfusion pressure (renal baroreceptor). Stretch receptors in the granular cells (see p. 727) of the afferent arterioles sense the decreased distention associated with low effective circulating volume. This decreased stretch lowers [Ca2+]i, which increases renin release and initiates a cascade that tends to promote Na+ reabsorption and thus increase blood pressure. Conversely, increased distention (high extracellular volume) inhibits renin release.

Chapter 40  •  Integration of Salt and Water Balance

N40-4  Renin Release from Granular Cells Contributed by Erich Windhager and Gerhard Giebisch As pointed out in the text, the granular cells are one of two cell types in which the exocytosis of a hormone decreases in response to a rise in [Ca2+]i. For example, if one raises [K+]o, the granular cell depolarizes. This depolarization probably opens voltage-gated Ca2+ channels (see p. 190) or decreases Ca2+ extrusion via an Na-Ca exchanger (see pp. 123–124). In either case, [Ca2+]i rises and blocks renin release. Similarly, applying Ca2+ ionophores—compounds that increase the permeability of the cell membrane to Ca2+—also raises [Ca2+]i and reduces renin release. Increases in intracellular levels of cAMP have the opposite effect of raising [Ca2+]i—increases in [cAMP]i stimulate renin release from granular cells. Conversely, agents that inhibit adenylyl cyclase activity (e.g., β-adrenergic antagonists, α-adrenergic agonists, and A1 adenosine receptor agonists) decrease [cAMP]i and thereby inhibit renin release.

REFERENCE Kurtz A: Cellular control of renin secretion. Rev Physiol Biochem Pharmacol 113:1–38, 1989.

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SECTION VI  •  The Urinary System

Renal blood flow Afferent Efferent arteriolar resistance

Filtration fraction

Peritubular capillary colloid osmotic pressure

Proximal Na+ reabsorption

Peritubular capillary hydrostatic pressure

Na+ excretion H2O excretion

ANG II Vasa recta blood flow

Washout of urea from medullary interstitium

[Urea] [Na+] in medullary interstitium

Gradient for passive NaCl reabsorption by the thin ascending limb of Henle

+

Loop Na reabsorption

Figure 40-4  Hemodynamic actions of ANG II on Na+ reabsorption.

The above stimulation of renin release by a decrease in [Ca2+]i  N40-4  stands in contrast to most Ca2+-activated secretory processes, in which an increase in [Ca2+]i stimulates secretion (see p. 221). Another exception is the chief cell of the parathyroid gland, in which an increase in [Ca2+]i inhibits secretion of parathyroid hormone (see pp. 1060–1061). Intracellular cAMP also appears to be a second messen­ ger for renin release. Agents that activate adenylyl cyclase  N40-5  enhance renin secretion, presumably via protein kinase A. The question whether the effects of [cAMP]i and [Ca2+]i are independent or sequential remains open.  N40-3

Additional factors also modulate renin release. Prosta­ glandins E2 and I2 and endothelin all activate renin release. Agents that blunt renin release include ANG II (which represents a short feedback loop), AVP, thromboxane A2, high plasma levels of K+, and nitric oxide. ANG II has several important actions as follows: 1. Stimulation of aldosterone release from glomerulosa cells in the adrenal cortex (see p. 1028). In turn, aldosterone promotes Na+ reabsorption in the distal tubule and collecting tubules and ducts (see p. 766). 2. Vasoconstriction of renal and other systemic vessels. ANG II increases Na+ reabsorption by altering renal hemodynamics, probably in two ways (Fig. 40-4). First, at high concentrations, ANG II constricts the efferent more than the afferent arterioles, thus increasing filtration fraction and reducing the hydrostatic pressure in the downstream peritubular capillaries. The increased filtration fraction also increases the protein concentration in the downstream blood and hence raises the colloid osmotic pressure of the peritubular capillaries. The changes in each of these two Starling forces favor the uptake of reabsorbate from peritubular interstitium into peri­tubular capillaries (see pp. 763–765) and hence enhance the reabsorption of Na+ and fluid by the proximal tubule. Second, ANG II decreases medullary blood flow through the vasa recta. Low blood flow decreases the medullary washout of NaCl and urea (see pp. 813–815), thus raising [urea] in the medullary interstitium and enhancing Na+ reabsorption along the thin ascending limb of Henle’s loop (see p. 811). 3. Enhanced tubuloglomerular feedback. ANG II raises the sensitivity and lowers the set-point of the tubuloglo-

merular feedback mechanism (see pp. 750–751), so that an increase in Na+ and fluid delivery to the macula densa elicits a more pronounced fall in the glomerular filtration rate (GFR). 4. Enhanced Na-H exchange. ANG II promotes Na+ reabsorption in the proximal tubule, TAL, and initial collecting tubule (see pp. 765–766). 5. Renal hypertrophy. Over a prolonged time, ANG II induces hypertrophy of renal-tubule cells. 6. Stimulated thirst and AVP release. ANG II acts on the hypothalamus, where it increases the sensation of thirst and stimulates secretion of AVP from the posterior pituitary, both of which increase total-body free water. This ANG II effect represents an intersection between the systems for regulating effective circulating volume and osmolality.

Increased sympathetic nerve activity, increased AVP, and decreased ANP are the other three parallel pathways that correct a low effective circulating volume Renal Sympathetic Nerve Activity  The second of the four parallel effector pathways for the control of effective circulating volume is the sympathetic nervous system. Enhanced activity of the renal sympathetic nerves has two direct effects on Na+ reabsorption (see pp. 766–768): (1) increased renal vascular resistance, and (2) increased Na+ reabsorption by tubule cells. In addition, increased sympathetic tone has an indirect effect—enhancing renin release from granular cells (see previous section). These multiple actions of sympathetic traffic to the kidney reduce GFR and enhance Na+ reabsorption, thereby increasing Na+ retention and increasing effective circulating volume. In everyday life (i.e., the unstressed state), the role of sympathetic nerve activity in kidney function appears to be modest at best. However, sympathetic innervation may play a role during challenges to volume homeostasis. For example, low Na+ intake triggers reduced renal Na+ excretion; renal denervation blunts this response. Another example is hemorrhage, in which renal sympathetic nerves emerge as important participants in preserving ECF volume.

Chapter 40  •  Integration of Salt and Water Balance

842.e1

N40-3  Systemic versus Local Roles of the Juxtaglomerular Apparatus Contributed by Emile Boulpaep and Walter Boron The JGA performs two apparently opposite functions: main­ taining a constant GFR (tubuloglomerular feedback, or TGF) and maintaining a constant whole-body blood pressure by modulating renin release. TGF (see pp. 750–751) is a local phenomenon, whereas the release of renin has systemic consequences (see pp. 841–842). In the case of tubuloglomerular feedback (i.e., the local response), decreased renal perfusion pressure, reduced filtered load, or enhanced proximal fluid reabsorption all lead to a decrease in the flow of tubule fluid past the macula densa, as well as to a decrease in Na+ delivery and Na+ concentration. Within seconds after such a transient disturbance, and by an unknown mechanism, TGF dilates the afferent arteriole of the same nephron in an attempt to increase single-nephron glomerular filtration rate (SNGFR) and restore fluid and Na+ to that particular macula densa. In the case of renin release (i.e., the systemic response), by contrast, a sustained fall in arterial pressure or a contraction of the extracellular volume reduces fluid delivery to many maculae

N40-5  Other Factors that Activate Adenylyl Cyclase in Granular Cells Contributed by Gerhard Giebisch and Erich Windhager Agents that activate adenylyl cyclase in the granular cells of the JGA—and thus stimulate renin release—include forskolin, β-adrenergic agonists, A2 adenosine receptor agonists, dopamine, and glucagon. In addition, exogenous cAMP and phosphodiesterase inhibitors enhance renin secretion. All of these agents presumably act through protein kinase A.

densae, leading to the release of renin. Renin, in turn, causes an increase in local and systemic concentrations of ANG II. Besides causing general vasoconstriction, ANG II constricts the afferent and efferent glomerular arterioles, thereby decreasing GFR. This effect is opposite to that of TGF: TGF dilates a single afferent arteriole, whereas renin release constricts many afferent and efferent arterioles. TGF may be viewed as a mechanism designed to maintain a constant SNGFR, whereas renin release is aimed at maintaining blood pressure by both systemic and renal vasoconstriction (i.e., hemodynamic effects), as well as by reducing SNGFR and enhancing tubule Na+ reabsorption (Na+-retaining effects). TGF is a minute-to-minute, fine control of SNGFR that can be superseded by the intermediate- to long-term effects of the powerful renin response, which comes into play whenever plasma volume and blood pressure are jeopardized. It must be emphasized that renin release is governed not only by the JGA but also by other mechanisms, in particular by changes in the activity of sympathetic nerves (see pp. 842–843).

Chapter 40  •  Integration of Salt and Water Balance

Conversely, expansion of the intravascular volume increases renal Na+ excretion; renal denervation sharply reduces this response as well. Arginine Vasopressin (Antidiuretic Hormone)  As discussed below (see p. 844), the posterior pituitary releases AVP primarily in response to increases in extracellular osmolality. Indeed, AVP mainly increases distal-nephron water per­ meability, promoting water retention (see pp. 817–818). However, the posterior pituitary also releases AVP in response to large reductions in effective circulating volume (e.g., hemorrhage), and secondary actions of AVP— vasoconstriction (see p. 553) and promotion of renal Na+ retention (see p. 768)—are appropriate for this stimulus. Atrial Natriuretic Peptide  Of the four parallel effectors

that correct a low effective circulating volume (see Fig. 40-2), ANP is the only one that does so by decreasing its activity. As its name implies, ANP promotes natriuresis (i.e., Na+ excretion). Atrial myocytes synthesize and store ANP and release ANP in response to stretch (a low-pressure volume sensor; see p. 547). Thus, reduced effective circulating volume inhibits ANP release and reduces Na+ excretion. ANP plays a role in the diuretic response to the redistribution of ECF and plasma volume into the thorax that occurs during water immersion and space flight (see p. 1233). Acting through a receptor guanylyl cyclase (see pp. 66– 67), ANP has many synergistic effects (see p. 768) on renal hemodynamics and on transport by renal tubules that promote renal Na+ and water excretion.  N40-6  Although ANP directly inhibits Na+ transport in the inner medullary collecting duct, its major actions are hemodynamic— increased GFR and increased cortical and medullary blood flow. ANP also decreases the release of renin, independently inhibits aldosterone secretion by the adrenal gland, and decreases release of AVP. In summary, a decrease in effective circulating volume leads to a fall in ANP release and a net decrease in Na+ and water excretion.

High arterial pressure raises Na+ excretion by hemodynamic mechanisms, independent of changes in effective circulating volume We have seen that expanding the effective circulating volume stimulates sensors that increase Na+ excretion via four parallel effector pathways (see Fig. 40-2). However, the kidney can also modulate Na+ excretion in response to purely hemodynamic changes, as in the following two examples. Large and Acute Decrease in Arterial Blood Pressure  If glomerulotubular (GT) balance (see p. 763) were perfect, decreasing the GFR would cause Na+ excretion to fall linearly (Fig. 40-5, blue line). However, acutely lowering GFR by partial clamping of the aorta causes a steep, nonlinear decrease in urinary Na+ excretion (see Fig. 40-5, red curve). When GFR falls sufficiently, the kidneys excrete only traces of Na+ in a small volume of urine. This response primarily reflects the transport of the classical distal tubule (see p. 765), which continues to reabsorb Na+ at a high rate despite the decreased Na+ delivery.

843

+ As GFR rises, Na excretion rises very rapidly (“pressure diuresis”).

175

150

Normal GFR and + Na excretion

125

Percent 100 of Na+ excretion

Ideal GT balance (i.e., fractional excretion of Na+ is constant)

75

50

As GFR falls, Na+ excretion falls even faster.

25

0

0

25 50 75 100 Percent of control glomerular filtration rate

125

Figure 40-5  Effect of changes in GFR on urinary Na+ excretion. The blue line represents ideal glomerulotubular (GT) balance. The red curve summarizes data from dogs. The investigators reduced GFR by inflating a balloon in the aorta, above the level of the renal arteries. They increased GFR by compressing the carotid arteries and thus increased blood pressure. (Data from Thompson DD, Pitts RF: Effects of alterations of renal arterial pressure on sodium and water excretion. Am J Physiol 168:490– 499, 1952.)

Large Increase in Arterial Pressure  In some cases, an increased effective circulating volume is accompanied by an increase in arterial pressure. Examples include primary hyperaldosteronism and Liddle disease  N23-14,  states of abnormally high distal Na+ reabsorption. The excess Na+ reabsorption leads to high blood pressure and compensatory pressure-induced natriuresis. One reason for this pressure diuresis is that hypertension increases GFR, increasing the filtered load of Na+, which by itself would increase urinary Na+ excretion (see Fig. 40-5, blue line). However, at least four other mechanisms contribute to the natriuresis (see Fig. 40-5, red curve). First, the increased effective circulating volume inhibits the renin-angiotensin-aldosterone axis and thus reduces Na+ reabsorption (see pp. 765–766). Second, the high blood pressure augments blood flow in the vasa recta, thereby washing out medullary solutes and reducing interstitial hypertonicity in the medulla (see pp. 813–815) and ultimately reducing passive Na+ reabsorption in the thin ascending limb (see p. 811). Third, an increase in arterial pressure leads, by an unknown mechanism, to prompt reduction in the number of apical Na-H exchangers in the proximal tubule. Normalizing the blood pressure rapidly reverses this effect. Finally, hypertension leads to increased pressure in the peritubular capillaries, thereby reducing proximal-tubule reabsorption (physical factors; see p. 763).

Chapter 40  •  Integration of Salt and Water Balance

N40-6  Renal Sites of Action of Atrial Natriuretic Peptide Contributed by Erich Windhager and Gerhard Giebisch 1 GFR is increased. 2 + Na reabsorption is directly or indirectly inhibited. 3 ANG II−stimulated Na+ reabsorption is inhibited.

10 Na+ reabsorption is inhibited owing to decrease in plasma aldosterone levels.

Na+ Na Na+

+

Macula densa

5 Passive water efflux is decreased.

Hypertonicity of the medullary interstitium is decreased. 6

Cl– Na+

12 Na+ load to inner medullary collecting duct is increased.

8 Load to macula densa is increased. Outer medulla Inner medulla

H2O

Thin descending limb of loop of Henle

Na+

9 Renin secretion is inhibited.

Glomerulus

Proximal convoluted tubule Thick ascending limb of loop of Henle

4 Na+ load to loop of Henle is increased.

11 Thiazide-sensitive Na/Cl cotransport is inhibited.

Na+

+

Na

13 Amiloridesensitive Na+ reabsorption is inhibited.

7 Passive Na+ efflux is decreased. Urinary Na+ excretion is increased. 15

Na+ –

Cl K+

⋅ UNaV

14 Furosemidesensitive Na/K/Cl cotransport in basolateral membranes is stimulated.

 urinary sodium excretion rate. (Data from Atlas SA, Maack T: eFigure 40-1  Sites of action of ANP. UNaV, Atrial natriuretic factor. In Windhager E (ed): Handbook of Physiology, Section 8: Renal Physiology. New York, Oxford University Press [for American Physiological Society], 1992, pp 1577–1674.)

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SECTION VI  •  The Urinary System

CONTROL OF WATER CONTENT (EXTRACELLULAR OSMOLALITY) Water accounts for half or more of body weight (~60% in men and 50% in women; see p. 102) and is distributed between the ICF and ECF compartments. Changes in totalbody water content in the absence of changes in total-body solute content lead to changes in osmolality, to which the CNS is extremely sensitive. Osmolality deviations of ±15% lead to severe disturbances of CNS function. Thus, osmoregulation is critical. Two elements control water content and thus whole-body osmolality: (1) the kidneys, which control water excretion (see pp. 806–807); and (2) thirst mechanisms, which control the oral intake of water. These two effector mechanisms are part of negative-feedback loops that begin within the hypothalamus. An increase in osmolality stimulates separate osmoreceptors to secrete AVP (which reduces renal excretion of free water) and to trigger thirst (which, if fulfilled, increases intake of free water). As a result, the two complementary feedback loops stabilize osmolality and thus [Na+].

Increased plasma osmolality stimulates hypothalamic osmoreceptors that trigger the release of AVP, inhibiting water excretion An increase in the osmolality of the ECF is the primary signal for the secretion of AVP from the posterior pituitary gland. An elegant series of animal studies by Verney in the 1940s established that infusing a hyperosmotic NaCl solution into the carotid artery abruptly terminates an established water diuresis (Fig. 40-6A). Infusing the same quantity of hyperosmotic NaCl into the peripheral circulation has little effect because the hyperosmolar solution becomes diluted by the time it reaches the cerebral vessels. Therefore, the osmosensitive site is intracranial. Surgically removing the posterior pituitary abolishes the effect of infusing hyperosmotic NaCl into the carotid artery (see Fig. 40-6B). However, injecting posterior-pituitary extracts into the animal inhibits the diuresis, regardless of whether the posterior pituitary is intact. Later work showed that Verney’s posterior-pituitary extract contained an “antidiuretic hormone”—now known to be AVP—that the posterior pituitary secretes in response to increased plasma osmolality. Ingesting large volumes of water causes plasma osmolality to fall, thus leading to reduced AVP secretion. In healthy individuals, plasma osmolality is ~290 mOsm. The threshold for AVP release is somewhat lower, ~280 mOsm (Fig. 40-7, red curve). Increasing the osmolality by only 1% higher than this level is sufficient to produce a detectable increase in plasma [AVP], which rises steeply with further increases in osmolality. Thus, hyperosmolality leads to increased levels of AVP, which completes the feedback loop by causing the kidneys to retain free water (see pp. 817–818). Although changes in plasma [NaCl] are usually responsible for changes in plasma osmolality, other solutes can do the same. For example, hypertonic mannitol resembles NaCl in stimulating AVP release. However, an equivalent increase in extracellular osmolality by urea has little effect on plasma AVP levels. The reason is that urea readily permeates cell membranes and hence exerts a low effective

A

BEFORE REMOVAL OF POSTERIOR PITUITARY H2O (p.o.)

6

Hyperosmotic Posterior pituitary NaCl (i.a.) extract (i.v.)

4 Urine flow (mL/min) 2

0

B

0

60

120 180 Time (min)

240

300

AFTER REMOVAL OF POSTERIOR PITUITARY H2O (p.o.)

6

Hyperosmotic Posterior pituitary NaCl (i.a.) extract (i.v.)

4 Urine flow (mL/min) 2

0

0

60

120 180 Time (min)

240

300

Figure 40-6  Sensing of blood osmolality in the dog brain. i.a., intraarterial (carotid) injection; i.v., intravenous injection; p.o., per os (by mouth). (Data from Verney EG: The antidiuretic hormone and the factors which determine its release. Proc Royal Soc Lond B 135:25–106, 1947.)

12 Volume contraction 8 Plasma AVP (pg/mL)

Volume expansion

4

0 260

Euvolemia (normal)

270

280 290 300 Plasma osmolality (mOsm)

310

Figure 40-7  Dependence of AVP release on plasma osmolality. (Data from Robertson GL, Aycinena P, Zerbe RL: Neurogenic disorders of osmoregulation. Am J Med 72:339–353, 1982.)

osmolality or tonicity (see pp. 132–133) and is thus poorly effective in shrinking cells.

Hypothalamic neurons synthesize AVP and transport it along their axons to the posterior pituitary, where they store it in nerve terminals prior to release Osmoreceptors of the CNS appear to be located in two areas that breech the blood-brain barrier: the organum

Chapter 40  •  Integration of Salt and Water Balance

Osmoreceptors in OVLT and SFO Paraventricular nucleus Magnocellular neurons

Hypothalamus Baroreceptor input from NTS

Supraoptic nucleus

Anterior lobe of pituitary Posterior lobe of pituitary

AVP

Preproneurophysin II Proneurophysin II 1

2

3

4 Arg

Gly Lys Arg

10 11 12 1. Signal peptide 19 aa 2. AVP 9 aa 3. Neurophysin II 95 aa AVP

4. Glycopeptide 39 aa

S

Gly in the 10 position (which is removed) is necessary for amidation of the Gly residue in the 9 position of AVP.

S

N– Cys Tyr Phe Gln Asn Cys Pro Arg Gly

–Amide

1 2 3 4 5 6 7 8 9 Arginine vasopressin Figure 40-8  Control of AVP synthesis and release by osmoreceptors. Osmoreceptors are located in the OVLT and SFO, two areas that breech the blood-brain barrier. Signals from atrial low-pressure baroreceptors travel with the vagus nerve to the nucleus tractus solitarii (NTS); a second neuron carries the signal to the hypothalamus. aa, amino acids.

vasculosum of the lamina terminalis (OVLT) and the subfornical organ (SFO), two of the circumventricular organs (see pp. 284–285). Specific neurons in these regions (Fig. 40-8) are able to sense changes in plasma osmolality. Elevated osmolality increases the activity of mechanosensitive cation

845

channels located in the neuronal membrane, which results in depolarization and thus an increased frequency of action potentials. Hypo-osmolality causes a striking decrease of frequency. The osmosensitive neurons project to large-diameter neurons in the supraoptic and paraventricular nuclei of the anterior hypothalamus (see Fig. 40-8). These neurons synthesize AVP, package it into granules, and transport the granules along their axons to nerve terminals in the posterior lobe of the pituitary, which is part of the brain (see pp. 979– 981). When stimulated by the osmosensitive neurons, these magnocellular neurons release the stored AVP into the posterior pituitary—an area that also lacks a blood-brain barrier—and AVP enters the general circulation. In humans and most mammals, the antidiuretic hormone is AVP, which is encoded by the messenger RNA for pre­ proneurophysin II. After cleavage of the signal peptide, the resulting prohormone proneurophysin II contains AVP, neurophysin II (NpII), and a glycopeptide (see Fig. 40-8). Cleavage of the prohormone within the secretory granule yields these three components. AVP has nine amino acids, with a disulfide bridge connecting two cysteine residues. Mutations of NpII impair AVP secretion, which suggests that NpII assists in the processing or secretion of AVP. Levels of circulating AVP depend on both the rate of AVP release from the posterior pituitary and the rate of AVP degradation. The major factor controlling AVP release is plasma osmolality. However, as discussed below, other factors also can modulate AVP secretion. Two organs, the liver and the kidney, contribute to the breakdown of AVP and the rapid decline of AVP levels when secretion has ceased. The half-life of AVP in the circulation is 18 minutes. Diseases of the liver and kidney may impair AVP degradation and may thereby contribute to water retention. For example, the congestion of the liver and impairment of renal function that accompany heart failure can compromise AVP breakdown, leading to inappropriately high circulating levels of AVP. Conversely, in pregnancy, placental vasopressinase activity can accelerate degradation of AVP.

Increased osmolality stimulates a second group of osmoreceptors that trigger thirst, which promotes water intake The second efferent pathway of the osmoregulatory system is thirst, which regulates the oral intake of water. Like the osmoreceptors that trigger AVP release, the osmoreceptors that trigger thirst are located in two circumventricular organs, the OVLT and the SFO. Also like the osmoreceptors that trigger AVP release, those that trigger thirst respond to the cell shrinkage that is caused by hyperosmolar solutions. However, these thirst osmoreceptor neurons are distinct from the adjacent AVP osmoreceptor neurons in the OVLT and SFO. Hyperosmolality triggers two parallel feedback-control mechanisms that have a common end point (Fig. 40-9): an increase in whole-body free water. In response to hyperosmolality, the AVP osmoreceptors in the hypothalamus trigger other neurons to release AVP. The result is the insertion of aquaporin 2 (AQP2) water channels in the collecting duct of the kidney, an increase in the reabsorption of water, and, therefore, a reduced excretion of free water. In response

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SECTION VI  •  The Urinary System

Increased oral intake and renal recovery of free water counteract hyperosmolality.

Arterial pressure

Osmolality Effective circulating volume

Brain Carotid sinus

Juxtaglomerular apparatus (JGA)

OVLT & SFO Thirst osmoreceptor

AVP osmoreceptor

Thirst

AVP neurons (PVN & SON)

Atrial low-pressure receptors Na+ appetite

Renin

Angiotensin II

Increased Na+ intake counteracts decreased effective circulating volume.

AVP

Kidneys

Figure 40-9  Feedback systems involved in the control of osmolality. PVN, paraventricular nucleus; SON, supraoptic nucleus of the hypothalamus.

H2O intake

H2O excretion

Free water

to hyperosmolality, the thirst osmoreceptors stimulate an appetite for water that leads to the increased intake of free water. The net effect is an increase in whole-body free water and, therefore, a reduction in osmolality.

Several nonosmotic stimuli also enhance AVP secretion Although an increase in plasma osmolality is the primary trigger for AVP release, several other stimuli increase AVP release, including a decrease in effective circulating volume or arterial pressure and pregnancy. Conversely, volume expansion diminishes AVP release. Reduced Effective Circulating Volume  As noted above on page 846, a mere 1% rise in plasma osmolality stimulates AVP release by a detectable amount. However, fairly large reductions in effective circulating volume (5% to 10%) are required to stimulate AVP release of similar amounts. Nevertheless, once the rather high threshold for nonosmotic release of AVP is exceeded, AVP release rises steeply with further volume depletion. The interaction between osmotic and volume stimuli on AVP release is illustrated in Figure 40-7, which shows that the effective circulating volume modifies the slope of the relationship between plasma AVP levels

and osmolality, as well as the osmotic threshold for AVP release. At a fixed osmolality, volume contraction (see Fig. 40-7, green curve) increases the rate of AVP release. Therefore, during volume depletion, a low plasma osmolality (e.g., 280 mOsm) that would normally suppress AVP release allows AVP secretion to continue (see Fig. 40-7, green dot). This leftward shift of the osmolality threshold for AVP release is accompanied by an increased slope, reflecting an increased sensitivity of the osmoreceptors to changes in osmolality. Figure 40-9 summarizes the three pathways by which decreased effective circulating volume and low arterial pressure enhance AVP release: (1) A reduction in left atrial pressure—produced by volume depletion—via low-pressure receptors in the left atrium decreases the firing rate of vagal afferents (see p. 547). These afferents signal brainstem neurons in the nucleus tractus solitarii, causing magnocellular neu­rons in the hypothalamus to release AVP (see Fig. 40-8). Indeed, at constant osmolality, AVP secretion varies inversely with left atrial pressure. (2) Low effective circulating volume triggers granular cells in the JGA to release renin. This leads to the formation of ANG II, which acts on receptors in the OVLT and the SFO to stimulate AVP release. (3) More importantly, a fall in the arterial pressure similarly causes high-pressure carotid sinus baroreceptors to stimulate AVP release (see pp. 534–536).

Chapter 40  •  Integration of Salt and Water Balance

Two clinical examples in which reduced effective circulating volume leads to increases in AVP are severe hemorrhagic shock and hypovolemic shock (e.g., shock resulting from excessive loss of ECF, as in cholera). In both cases, the water retention caused by AVP release accounts for the accompanying hyponatremia. In the first part of this chapter, we said that the appropriate renal response to decreased effective circulating volume is to retain Na+ (i.e., isotonic saline). Why is it that, in response to shock, the body also retains free water? Compared with isotonic saline, free water is less effective as an expander of the ECF volume (see p. 135). Nevertheless, in times of profound need, the body uses free-water retention to help expand extracellular (and plasma) volume. Clearly, the body is willing to tolerate some hypo-osmolality of the body fluids as the price for maintaining an adequate blood volume. A clinical example in which reduced effective circulating volume can lead to an inappropriate increase in AVP levels is congestive heart failure (see p. 838). In this situation, the water retention may be so severe that the patient develops hyponatremia (i.e., hypo-osmolality).

847

Volume Expansion  In contrast to volume contraction, chronic volume expansion reduces AVP secretion, as a consequence of the rightward shift of the threshold to higher osmolalities and of a decline in the slope (see Fig. 40-7, blue curve). In other words, volume expansion decreases the sensitivity of the central osmoreceptors to changes in plasma osmolality. A clinical example is hyperaldosteronism. With normal thirst and water excretion, the chronic Na+ retention resulting from the hyperaldosteronism would expand the ECF volume isotonically, thus leaving plasma [Na+] unchanged. However, because chronic volume expansion downregulates AVP release, the kidneys do not retain adequate water, which results in slight hypernatremia (i.e., elevated plasma [Na+]) and very modest hyperosmolality (Box 40-3). Pregnancy  Leftward shifts in the threshold for AVP release and thirst often occur during pregnancy. These changes probably reflect the action of chorionic gonadotropin on the sensitivity of the osmoreceptors. Pregnancy is therefore often associated with a decrease of 8 to 10 mOsm

BOX 40-3  Diuretics

D

iuretics reversibly inhibit Na+ reabsorption at specific sites along the nephron, increasing the excretion of Na+ and water, creating a state of negative Na+ balance, and thereby contracting ECF volume. Properly speaking, these agents should be called natriuretic to emphasize this use to promote Na+ excretion. This is in contrast to aquaretic agents (e.g., vasopressin receptor antagonists, or VRAs) that promote water excretion with little or no effect on Na+ excretion. Nevertheless, it has been customary to refer to natriuretics as diuretics. Clinicians use diuretics to treat hypertension as well as edema (see Box 20-1) caused by heart failure, cirrhosis of the liver, or nephrotic syndrome. Common to these latter edematous diseases is an abnormal shift of ECF away from the effective circulating volume, which thereby activates the feedback pathways. The results are Na+ retention and expansion of total extracellular volume. However, this expansion, which results in edema formation, falls short of correcting the underlying decrease in the effective circulating volume. The reason that most of this added extracellular volume remains ineffective—and does not restore the effective circulating volume—is not intuitive but reflects the underlying pathologic condition that initiated the edema in the first place. Thus, treating these edematous diseases requires generating a negative Na+ balance, which can often be achieved by rigid dietary Na+ restriction or the use of diuretics. Diuretics are also useful in treating hypertension. Even though the primary cause of the hypertension may not always be an increase in the effective circulating volume, enhanced Na+ excretion is frequently effective in lowering blood pressure.

Classification The site and mechanism of a diuretic’s action determine the magnitude and nature of the response (Table 40-3). Both chemically and functionally, diuretics are very heterogeneous. For example, acetazolamide produces diuresis by inhibiting carbonic anhydrase and thus the component of proximal-tubule Na+

reabsorption that is coupled to HCO3− reabsorption. The diuretic effect of hydrochlorothiazide is largely the result of its ability to inhibit Na/Cl cotransport in the distal convoluted tubule. Spironolactone (which resembles aldosterone) competitively inhibits mineralocorticoid receptors in principal cells of the initial and cortical collecting tubule. Mannitol (reduced fructose) is a powerful osmotic diuretic (see Box 35-1) that reduces net Na+ transport in the proximal tubule and TAL by causing retention of water in the lumen and reduction in luminal [Na+]. An ideal diuretic should promote the excretion of urine whose composition resembles that of the ECF. Such diuretics do not exist. In reality, diuretics not only inhibit the reabsorption of Na+ and its osmotically obligated water, but also interfere with the renal handling of Cl−, H+, K+, and Ca2+, as well as with urinary concentrating ability.  N40-7  Thus, many diuretics disturb the normal plasma electrolyte pattern. Table 40-4 summarizes the most frequent side effects of diuretic use on the electrolyte composition of the ECF. These electrolyte derangements are the predictable consequences of the mechanism of action of individual diuretics at specific tubule sites.

Delivery of Diuretics to Their Sites of Action Diuretics generally inhibit transporters or channels at the apical membranes of tubule cells. How do the diuretics get there? Plasma proteins bind many diuretics so that the free concentration of the diuretic in plasma water may be fairly low. Thus, glomerular filtration may deliver only a modest amount to the tubule fluid. However, organic anion or organic cation trans­ porters in the S3 segment of the proximal tubule can secrete diuretics and can thereby produce high luminal concentrations. For example, the basolateral organic anion transporter system that carries para-aminohippurate (see pp. 779–781) also secretes thiazide diuretics, furosemide, and ethacrynic acid. Organic cation transporters (see pp. 783–784) secrete amiloride. The subsequent reabsorption of fluid along the nephron further concentrates diuretics in the tubule lumen. Not surprisingly, renal Continued

Chapter 40  •  Integration of Salt and Water Balance

847.e1

N40-7  Secondary Effects of Diuretic Drugs Contributed by Erich Windhager and Gerhard Giebisch As noted in the text, the perfect diuretic—which does not exist— would produce an increase in the urinary excretion of protein-free fluid with a composition otherwise identical to that of the ECL. However, diuretics not only inhibit the reabsorption of Na+ and the osmotically obligated water, but also interfere with the renal handling of Cl−, H+, K+, and Ca2+, as well as with urinary concentrating ability. 1. Urine [Cl−]. With the exception of carbonic anhydrase inhibitors, all diuretics promote the excretion of urine having a high [Cl−]. The ratio [Cl−]/[Na+] is greater in the urine than in the plasma. 2. Urine pH. Because of its inhibition of proximal-tubule HCO3− reabsorption, acetazolamide leads to excretion of a relatively alkaline urine. Thus, acetazolamide produces a mild metabolic acidosis. In contrast, the loop diuretics and thiazides cause the excretion of a Cl−-rich, HCO3− -poor urine, which tends to induce a metabolic alkalosis. 3. Urine [K+]. Some diuretics are called K+-sparing because they tend to conserve body K+. These diuretics—which include amiloride, triamterene, and spironolactone—block

only a small fraction of Na+ reabsorption, but reduce K+ secretion through apical K+ channels by hyperpolarizing the apical cell membrane. By inhibiting passive cation movement, they may induce hyperkalemia. This hyperkalemia may lead to metabolic acidosis (see p. 835). 4. Urine [Ca2+]. With the exception of the chlorothiazides, most diuretics enhance Ca2+ excretion. They interfere with the passive reabsorption of Ca2+ through the paracellular pathway in both the proximal tubule and TAL (see p. 787). In the proximal tubule, the high luminal flow rate produced by the diuresis reduces the reabsorption of Ca2+ via solvent drag. In the TAL, loop diuretics diminish the lumen-positive potential that normally drives the passive reabsorption of Ca2+. 5. Urine osmolality. Loop diuretics diminish the urinary concentrating ability by inhibiting Na+ transport in the TAL (see p. 811). Clinical side effects of diuretic therapy are summarized in Table 40-4.

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SECTION VI  •  The Urinary System

BOX 40-3  Diuretics—cont’d disease may compromise the delivery of diuretics and cause resistance to the actions of diuretics.  N40-8

Response of Nephron Segments Downstream from a Diuretic’s Site of Action

The proximal tubule reabsorbs the largest fraction of filtered Na+; the loop of Henle, the distal convoluted tubule, and the collecting ducts retrieve smaller fractions. Thus, intuition could suggest that the proximal tubule would be the best target for diuretics. However, secondary effects in downstream nephron segments can substantially mitigate the primary effect of a diuretic. Inhibiting Na+ transport by the proximal tubule raises Na+ delivery to downstream segments and almost always stimulates Na+ reabsorption there (see p. 765). As a result of this downstream Na+ reclamation, the overall diuretic action of proximally acting diuretics (e.g., acetazolamide) is relatively weak. A diuretic is most potent if it acts downstream of the proximal tubule, a condition met by loop diuretics, which inhibit Na+ transport along the TAL. Although the TAL normally reabsorbs only 15% to 25% of the filtered load of Na+, the reabsorptive capacity of the more distal nephron segments is limited. Thus, the loop diuretics are currently the most powerful diuretic agents. Because nephron segments distal to the TAL have only modest rates of

Na+ reabsorption, diuretics that target these segments are not as potent as loop diuretics. Nevertheless, distally acting diuretics are important because their effects are long lasting. Moreover, agents acting on the connecting and collecting tubules are K+ sparing (i.e., they tend to conserve body K+). It is sometimes advantageous to use two diuretics that act at different sites along the nephron, generating a synergistic effect. Thus, if a loop diuretic alone is providing inadequate diuresis, one could complement its action by adding a thiazide, which will block the compensating effect of the distal convoluted tubule to reabsorb Na+.

Blunting of Diuretic Action with Long-Term Use The prolonged administration of a diuretic may lead to a sustained loss of body weight but only transient natriuresis.  N40-9  Most of the decline in Na+ excretion occurs because the druginduced fall in effective circulating volume triggers Na+ retention mediated by increased sympathetic outflow to the kidneys (which lowers GFR), increased secretion of ANG II and aldosterone, and decreased secretion of ANP. Hypertrophy or increased activity of tubule segments downstream of the main site of action of the diuretic can also contribute to the diminished efficacy of the drug during long-term administration.

TABLE 40-3  Action of Diuretics PHYSIOLOGICAL REGULATION OF “TARGET”

PAGE REFERENCE FOR TARGET

SITE

DRUG

FINAL MOLECULAR “TARGET”

PCT

Acetazolamide

Carbonic anhydrase

PCT

Dopamine

Na-H exchanger (NHE3)

ANG II, sympathetic nerve activity, α-adrenergic agonists

Dopamine

p. 827

TAL

Loop diuretics:   Furosemide   Bumetanide   Ethacrynic acid

Na/K/Cl cotransporter (NKCC2)

Aldosterone

PGE2

p. 757

DCT

Thiazides Metolazone

Na/Cl cotransporter (NCC)

ANG II Aldosterone

CCT

Amiloride Triamterene

Na+ channel (ENaC)

ANG II

CCT

Spironolactone

Mineralocorticoid receptor

Aldosterone

IMCD

Amiloride

cGMP-gated cation channel

Aldosterone

Water-permeable segments

Osmotic diuretics (mannitol)

STIMULATOR

INHIBITOR

pp. 828–829

p. 758 PGE2

pp. 758–759 p. 766

ANP

p. 768

CCT, cortical collecting tubule; DCT, distal convoluted tubule; IMCD, inner medullary collecting duct; PCT, proximal convoluted tubule; PGE2, prostaglandin E2.

in plasma osmolality. A similar but smaller change may also occur in the late phase of the menstrual cycle. Other Factors  Pain, nausea, and several drugs (e.g., morphine, nicotine, and high doses of barbiturates) stimulate AVP secretion. In contrast, alcohol and drugs that block the effect of morphine (opiate antagonists) inhibit AVP secretion

and thus promote diuresis. Of great clinical importance is the hypersecretion of AVP that may occur postoperatively. In addition, some malignant tumors secrete large amounts of AVP. Such secretion of inappropriate amounts of “antidiuretic hormone” leads to pathological retention of water with dilution of the plasma electrolytes, particularly Na+. If progressive and uncorrected, this condition may lead

Chapter 40  •  Integration of Salt and Water Balance

N40-8  Reduced Delivery of Diuretics in Renal Disease

848.e1

N40-9  Blunting of Diuretic Action Contributed by Erich Windhager and Gerhard Giebisch

Contributed by Erich Windhager and Gerhard Giebisch As noted in the text, diuretics cannot have their intended effects unless they have appropriate access to their protein targets in the tubule cells. The two access routes are filtration and secretion, of which secretion is usually the most important. Not surprisingly, renal disease may compromise the net secretion of diuretics in three ways. First, the capability of the diseased cells to secrete diuretics may be impaired (i.e., decreased transport). Second, renal failure leads to a buildup in the blood of organic anions that would otherwise be secreted. These organic anions may competitively inhibit the transport of diuretics by the proximal tubule (i.e., competition). Third, in renal diseases in which breakdown of the glomerular filtration barrier leads to proteinuria, albumin and other proteins not normally present in the tubule lumen bind the diuretics and greatly reduce the concentration of unbound drug (i.e., binding).

Let us assume that a patient has a fixed daily intake of Na+. As noted in the text, the administration of a diuretic will cause an initial period of increased Na+ excretion (negative Na+ balance), peaking within a few days, that leads to a loss in weight. During prolonged administration of the diuretic, urinary Na+ excretion will fall back toward normal over a period of many days, and the patient will reach a steady state (neutral Na+ balance) in which Na+ intake and excretion are equal, and in which the initial weight loss is maintained. When the drug is discontinued, the patient will experience a transient period of diminished urinary Na+ excretion, reaching a nadir after a few days. During this time he or she is in positive Na+ balance. As a result, the patient will regain the weight that was lost during the initial phase of the diuretic treatment. However, over a period of many days, the Na+ excretion eventually rises back to a normal level as the patient achieves a new steady state (neutral Na+ balance) in which Na+ intake and excretion are again equal, and the patient maintains a prediuretic weight.

Chapter 40  •  Integration of Salt and Water Balance

849

TABLE 40-4  Complications of Diuretic Therapy COMPLICATION

CAUSATIVE DIURETICS

SYMPTOMS

CAUSATIVE FACTORS

ECF volume depletion

Loop diuretics and thiazides

Lassitude, thirst, muscle cramps, hypotension

Rapid reduction of plasma volume

K+ depletion

Acetazolamide, loop diuretics, thiazides

Muscle weakness, paralysis, cardiac arrhythmias

Flow and Na+-related stimulation of distal K+ secretion

K+ retention

Amiloride, triamterene, spironolactone

Cardiac arrhythmias, muscle cramps, paralysis

Block of ENaC in the collecting duct

Hyponatremia

Thiazides, furosemide

CNS symptoms, coma

Block of Na+ transport in waterimpermeable nephron segment

Metabolic alkalosis

Loop diuretics, thiazides

Cardiac arrhythmias, CNS symptoms

Excessive Cl− excretion, secondary volume contraction

Metabolic acidosis

Acetazolamide, amiloride, triamterene

Hyperventilation, muscular and neurological disturbances

Interference with H+ secretion

Hypercalcemia

Thiazides

Abnormal tissue calcification, disturbances of nerve and muscle function

Increased Ca2+ reabsorption in distal convoluted tubule

Hyperuricemia

Thiazides, loop diuretics

Gout

Decreased ECF volume, which activates proximal fluid and uric acid reabsorption

ENaC, epithelial Na+ channel.

to life-threatening deterioration of cerebral function (see Box 38-3).

Defense of the effective circulating volume usually has priority over defense of osmolality

Decreased effective circulating volume and low arterial pressure also trigger thirst

Under physiological conditions, the body regulates plasma volume and plasma osmolality independently. However, as discussed on page 847, this clear separation of defense mechanisms against volume and osmotic challenges breaks down when more dramatic derangements of fluid or salt metabolism occur. In general, the body defends volume at the expense of osmolality. Examples include severe reductions in absolute blood volume (e.g., hemorrhage) and decreases in effective circulating volume even when absolute ECF volume may be expanded (e.g., congestive heart failure, nephrotic syndrome, and liver cirrhosis). All are conditions that strongly stimulate both Na+- and water-retaining mechanisms. However, hyponatremia can be the consequence.  N40-10

Large decreases in effective circulating volume and blood pressure not only stimulate the release of AVP, they also profoundly stimulate the sensation of thirst. In fact, hemorrhage is one of the most powerful stimuli of hypovolemic thirst: “Thirst among the wounded on the battlefield is legendary” (Fitzsimons). Therefore, three distinct stimuli— hyperosmolality, profound volume contraction, and large decreases in blood pressure—lead to the sensation of thirst. Low effective circulating volume and low blood pressure stimulate thirst centers in the hypothalamus via the same pathways by which they stimulate AVP release (see Fig. 40-9). In addition to stimulating thirst, some of these hypothalamic areas are also involved in stimulating the desire to ingest salt (i.e., Na+ appetite). We discuss the role of the hypothalamus in the control of appetite on page 1001.

REFERENCES The reference list is available at www.StudentConsult.com.

Chapter 40  •  Integration of Salt and Water Balance

849.e1

N40-10  Defense of Osmolality at the Expense of Effective Circulating Volume During Dehydration Contributed by Gerhard Giebisch, Erich Windhager, Emile Boulpaep, and Walter Boron An exception to the rule of defending volume over osmolality occurs during severe water loss (i.e., dehydration; see p. 1215). In this case, the hyperosmolality that accompanies the dehydration maximally stimulates AVP secretion and thirst (see Fig. 40-9). Of course, severe dehydration also reduces total-body volume. However, this loss of free water occurs at the expense of both intracellular water (~60%) and extracellular water (~40%). Thus, dehydration does not put the effective circulating volume at as great a risk as the acute loss of an equivalent volume of blood. Because dehydration reduces effective circulating volume, one might think that the renin-angiotensin-aldosterone axis would lead to Na+ retention during dehydration. However, the opposite effect may occur, possibly because hyperosmolality makes the glomerulosa cells of the adrenal medulla less sensitive to ANG II and thereby reduces the release of aldosterone. Thus, the kidneys fail to retain Na+ appropriately. Accordingly, in severe

dehydration, the net effect is an attempt to correct hyperosmolality by both water intake and retention, as well as by the loss of Na+ (i.e., natriuresis) that occurs because aldosterone levels are inappropriately low for the effective circulating volume. Therefore, in severe dehydration, the body violates the principle of defending volume over osmolality. If the dehydration occurs during exercise, the drive to preserve effective circulating volume will trump temperature regulation (see p. 1215), offsetting the earlier vasodilation of the skin and active muscle. We can infer that the exercise-induced dehydration, by triggering thirst and AVP secretion (see previous paragraph), leads to a correction of the hyperosmolality and an increase in effective circulating volume that, once again, allows the individual to sweat and effectively regulate whole-body temperature.

849.e2

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REFERENCES

Schrier RW: Use of diuretics in heart failure and cirrhosis. Semin Nephrol 31(6):503–512, 2011.

Books and Reviews Bernstein PL, Ellison DH: Diuretics and salt transport along the nephron. Semin Nephrol 31(6):475–482, 2011. Bonny O, Rossier BC: Disturbances of Na/K balance: Pseudohypoaldosteronism revised. J Am Soc Nephrol 13:2399– 2414, 2002. Bourque CW: Central mechanisms of osmosensation and systemic osmoregulation. Nat Rev Neurosci 9(7):519–531, 2008. Epub May 29, 2008. Bourque CW, Oliet SHR: Osmoreceptors in the central nervous system. Annu Rev Physiol 59:601–619, 1997. Crowley SD, Coffman TM: In hypertension, the kidney rules. Curr Hypertens Rep 9(2):148–153, 2007. DiBona GF: Physiology in perspective: The wisdom of the body. Neural control of the kidney. Am J Physiol Regul Integr Comp Physiol 289(3):R633–R641, 2005. Fitzsimons JT: Angiotensin, thirst and sodium appetite. Physiol Rev 78:583–686, 1998. Gutkowska J, Antunes-Rodrigues J, McCann SM: Atrial natriuretic peptide in brain and pituitary gland. Physiol Rev 77:465–515, 1997. Nader PC, Thompson JR, Alpern RJ: Complications of diuretic use. Semin Nephrol 8:365–387, 1988. Navar LG, Zou L, Von Thun A, et al: Unraveling the mystery of Goldblatt hypertension. News Physiol Sci 13:170–176, 1998. Rennke HG, Denker BD: Renal Pathophysiology: The Essentials, 3rd ed. Baltimore, MD, Lippincott Williams & Wilkins, 2009. Rolls BJ, Rolls ET: Thirst. Cambridge, UK, Cambridge University Press, 1982.

Journal Articles Chou CL, Marsh DJ: Role of proximal convoluted tubule in pressure diuresis in the rat. Am J Physiol 251:F283–F289, 1986. Clark BA, Brown RS, Epstein FH: Effect of atrial natriuretic peptide on potassium-stimulated aldosterone secretion: Potential relevance to hypoaldosteronism in man. J Clin Endocrinol Metab 75:399–403, 1992. Gurley SB, Riquier-Brison AD, Schnermann J, et al: AT1A angiotensin receptors in the renal proximal tubule regulate blood pressure. Cell Metab 13(4):469–475, 2011. Iino Y, Imai M: Effects of prostaglandins on Na transport in isolated collecting tubules. Pflugers Arch 373(2):125–132, 1978. Mason WT: Supraoptic neurones of rat hypothalamus are osmosensitive. Nature 287:154–157, 1980. Oliet SHR, Bourque CW: Mechanosensitive channels transduce osmosensitivity in supraoptic neurons. Nature 364:341–343, 1993. Rabkin R, Share L, Payne PA, et al: The handling of immunoreactive vasopressin by the isolated perfused rat kidney. J Clin Invest 63:6–13, 1979. Verney EG: The antidiuretic hormone and the factors which determine its release. Proc R Soc London B Biol Sci 135:25–106, 1947. Yang LE, Maunsbach AB, Leong PKK, McDonough AA: Differential traffic of proximal tubule Na+ transporters during hypertension or PTH: NHE3 to base of microvilli vs. NaPi2 to endosomes. Am J Physiol Renal Physiol 287:F896–F906, 2004.

C H A P T E R 41  ORGANIZATION OF THE GASTROINTESTINAL SYSTEM Henry J. Binder

OVERVIEW OF DIGESTIVE PROCESSES The gastrointestinal tract is a tube that is specialized along its length for the sequential processing of food The gastrointestinal (GI) tract consists of both the series of hollow organs stretching from the mouth to the anus and the several accessory glands and organs that add secretions to these hollow organs (Fig. 41-1). Each of these hollow organs, which are separated from each other at key locations by sphincters, has evolved to serve a specialized function. The mouth and oropharynx are responsible for chopping food into small pieces, lubricating it, initiating carbohydrate and fat digestion, and propelling the food into the esophagus. The esophagus acts as a conduit to the stomach. The stomach (see Chapter 42) temporarily stores food and also initiates digestion by churning and by secreting proteases and acid. The small intestine (see Chapters 44 and 45) continues the work of digestion and is the primary site for the absorption of nutrients. The large intestine (see Chapters 44 and 45) reabsorbs fluids and electrolytes and also stores the fecal matter before expulsion from the body. The accessory glands and organs include the salivary glands, pancreas, and liver. The pancreas (see Chapter 43) secretes digestive enzymes into the duodenum, in addition to secreting HCO3− to neutralize gastric acid. The liver secretes bile (see Chapter 46), which the gallbladder stores for future delivery to the duodenum during a meal. Bile contains bile acids, which play a key role in the digestion of fats. Although the anatomy of the wall of the GI tract varies along its length, certain organizational themes are common to all segments. Figure 41-2, a cross section through a generic piece of stomach or intestine, shows the characteristic layered structure of mucosa, submucosa, muscle, and serosa. The mucosa consists of the epithelial layer, as well as an underlying layer of loose connective tissue known as the lamina propria, which contains capillaries, enteric neurons, and immune cells (e.g., mast cells), as well as a thin layer of smooth muscle known as the lamina muscularis mucosae (literally, the muscle layer of the mucosa). The surface area of the epithelial layer is amplified by several mechanisms. Most cells have microvilli on their apical surfaces. In addition, the layer of epithelial cells can be evaginated to form villi or invaginated to form crypts (or glands). Finally, on a larger scale, the mucosa is organized into large folds. The submucosa consists of loose connective tissue and larger blood vessels. The submucosa may also contain glands that secrete material into the GI lumen. 852

The muscle layer, the muscularis externa, includes two layers of smooth muscle. The inner layer is circular, whereas the outer layer is longitudinal. Enteric neurons are present between these two muscle layers. The serosa is an enveloping layer of connective tissue that is covered with squamous epithelial cells.

Assimilation of dietary food substances requires digestion as well as absorption The sedentary human body requires ~30 kcal/kg body weight each day (see p. 1170). This nutrient requirement is normally acquired by the oral intake of multiple food substances that the GI tract then assimilates. Although antigenic amounts of protein enter the body via the skin and across the pulmonary epithelium, caloric uptake by routes other than the GI tract is not thought to occur. Both the small and large intestines absorb water and electrolytes, but only the small intestine absorbs lipids, carbohydrates, and amino acids. However, even without effective GI function, parenteral (i.e., intravenous) alimentation can provide sufficient calories to sustain adults and to support growth in premature infants. Total parenteral nutrition has been used successfully on a long-term basis in many clinical settings in which oral intake is impossible or undesirable. Food substances are not necessarily—and often are— consumed in a chemical form that the small intestine can directly absorb. To facilitate absorption, the GI tract digests the food by both mechanical and chemical processes. Mechanical disruption of ingested food begins in the mouth with chewing (mastication). Individuals without teeth usually require their solid food to be cut into smaller pieces before eating. The mechanical processes that alter food composition to facilitate absorption continue in the stomach (see p. 865), both to initiate protein and lipid enzymatic digestion and to allow passage of gastric contents through the pylorus into the duodenum. This change in the size and consistency of gastric contents is necessary because solids that are >2 mm in diameter do not pass through the pylorus. The chemical form in which different nutrients are ingested and absorbed varies according to the specific nutrient in question. For example, although most lipids are consumed in the form of triacylglycerols, it is fatty acids and monoacylglycerols, not triacylglycerols, that are absorbed by the small intestine. Thus, a complex series of chemical reactions (i.e., lipid digestion) are required to convert dietary

CHAPTER 41  •  Organization of the Gastrointestinal System

Parotid gland

Mouth Salivary glands

Upper esophageal sphincter (UES)

Esophagus

Lower esophageal sphincter (LES)

Liver Gallbladder Pyloric sphincter

Stomach Pancreas

Duodenum

Transverse colon

Ascending colon

Jejunum Descending colon Ileum

Haustra

Anus

Ileocecal sphincter Appendix

Internal and external anal sphincters Figure 41-1  Major components of the human digestive system.

triacylglycerols to these smaller lipid forms (see pp. 927– 928). Similarly, amino acids are present in food as proteins and large peptides, but only amino acids and small peptides— primarily dipeptides and tripeptides—are absorbed by the small intestine. Carbohydrates are present in the diet as starch, disaccharides, and monosaccharides (e.g., glucose). However, because the small intestine absorbs all carbohydrates as monosaccharides, most dietary carbohydrates require chemical digestion before their absorption.

Digestion requires enzymes secreted in the mouth, stomach, pancreas, and small intestine Digestion involves the conversion of dietary food nutrients to a form that the small intestine can absorb. For carbohydrates and lipids, these digestive processes are initiated in the mouth by salivary and lingual enzymes: amylase for carbohydrates and lipase for lipids. Protein digestion is initiated in the stomach by gastric proteases (i.e., pepsins), whereas additional lipid digestion in the stomach occurs primarily as a result of the lingual lipase that is swallowed, although some gastric lipase is also secreted. Carbohydrate digestion does not involve any secreted gastric enzymes. Digestion is completed in the small intestine by the action of both pancreatic enzymes and enzymes at the brush border of the small intestine. Pancreatic enzymes, which include

853

lipase, chymotrypsin, and amylase, are critical for the digestion of lipids, protein, and carbohydrates, respectively. The enzymes on the luminal surface of the small intestine (e.g., brush-border disaccharidases and dipeptidases) complete the digestion of carbohydrates and proteins. Digestion by these brush-border enzymes is referred to as membrane digestion. The material presented to the small intestine includes both dietary intake and secretory products. The food material entering the small intestine differs considerably from the ingested material because of the mechanical and chemical changes just discussed. The load to the small intestine is also significantly greater than that of the ingested material. Dietary fluid intake is 1.5 to 2.5 L/day, whereas the fluid load presented to the small intestine is 8 to 9 L/day. The increased volume results from substantial quantities of salivary, gastric, biliary, pancreatic, and small-intestinal secretions. These secretions contain large amounts of protein, primarily in the form of the digestive enzymes discussed above.

Ingestion of food initiates multiple endocrine, neural, and paracrine responses Digestion of food involves multiple secretory, enzymatic, and motor processes that are closely coordinated with one another. The necessary control is achieved by neural and hormonal processes that are initiated by dietary food substances; the result is a coordinated series of motor and secretory responses. For example, chemoreceptors, osmoreceptors, and mechanoreceptors in the mucosa in large part generate the afferent stimuli that induce gastric and pancreatic secretions. These receptors sense the luminal contents and initiate a neurohumoral response. Endocrine, neural, and paracrine mechanisms all contribute to digestion. All three include sensor and transmitter processes. An endocrine mechanism (see p. 47) involves the release of a transmitter (e.g., peptide) into the blood. For example, protein in the stomach stimulates the release of gastrin from antral G cells. Gastrin then enters the blood and stimulates H+ release from parietal cells in the body of the stomach. A neural mechanism involves the activation of nerves and neurotransmitters that influence either secretory or motor activity. Neural transmission of these responses may involve the enteric nervous system (ENS; see pp. 339– 340) or the central nervous system (CNS). An example of neural control is activation of the vagus nerve in response to the smell of food. The resultant release of the neurotransmitter acetylcholine (ACh) also releases H+ from parietal cells in the stomach. The third mechanism of neurohumoral control is paracrine (see p. 47). In this mechanism, a transmitter is released from a sensor cell, and it affects adjacent cells without either entering the blood or activating neurons. For example, paracrine mechanisms help regulate gastric acid secretion by parietal cells: the histamine released from so-called enterochromaffin-like (ECL) cells in the body of the stomach stimulates H+ release from neighboring parietal cells. In addition to the primary response that leads to the release of one or more digestive enzymes, other signals terminate these secretory responses. Enteric neurons are

854 A

SECTION VII  •  The Gastrointestinal System

MACROSCOPIC VIEW OF THE WALL OF THE DUODENUM Stomach Common bile duct Mesentery

Submucosa Submucosal blood vessels Lamina propria

Gland in submucosa

Serosa Intestinal villi with epithelial lining Muscularis externa

B

Outer longitudinal muscle layer Inner circular muscle layer

Muscularis mucosae

Gland in lamina propria Mucosa

MICROSCOPIC VIEW OF THE WALL OF THE COLON

Large intestine Surface absorptive cell

Crypt of ¨ Lieberkuhn

Goblet cell

Lamina propria Enteric endocrine cell

Muscularis mucosae Submucosa Circular muscle of muscularis externa Longitudinal muscle of muscularis externa

Crypt

Stem/progenitor cell

Undifferentiated crypt cell

Figure 41-2  Wall of the GI tract. A, The wall of a segment of the duodenum consists of the following structures, from inside to outside: an epithelial layer with crypts, lamina propria, muscularis mucosae, submucosa, circular and then longitudinal layer of the muscularis externa, and serosa. B, The colon has the same basic structure as the small intestine. Some of the epithelial cells are on the surface and others are in the crypts that penetrate into the wall of the colon.

CHAPTER 41  •  Organization of the Gastrointestinal System

important throughout the initiation and termination of the responses. Although the endocrine, neural, and paracrine responses are most often studied separately, with considerable effort made to isolate individual events, these responses do not occur as isolated events. Rather, each type is part of an integrated response to a meal that results in the digestion and absorption of food. This entire series of events that results from the ingestion of food can best be described as an integrated response that includes both afferent and efferent limbs.

In addition to its function in nutrition, the GI tract plays important roles in excretion, fluid and electrolyte balance, and immunity Although its primary roles are digesting and absorbing nutrients, the GI tract also excretes waste material. Fecal material includes nondigested/nonabsorbed dietary food products, colonic bacteria and their metabolic products, and several excretory products. These excretory products include (1) heavy metals such as iron and copper, whose major route of excretion is in bile; and (2) several organic anions and cations, including drugs, that are excreted in bile but are reabsorbed either poorly or not at all by either the small or large intestine. As noted above, the small intestine is presented with 8 to 9 L/day of fluid, an amount that includes ~1 L/day that the intestine itself secretes. Almost all this water is reabsorbed in the small and large intestine; therefore, stool has relatively small amounts of water (~0.1 L/day). Diarrhea (an increase in stool liquidity and weight, >200 g/day) results from either increased fluid secretion by the small or large intestine, or decreased fluid reabsorption by the intestines. An important clinical example of diarrhea is cholera, especially in developing countries. Cholera can be fatal because of the water and electrolyte imbalance that it creates. Thus, the GI tract plays a crucial role in maintaining overall fluid and electrolyte balance (see Chapter 44). The GI tract also contributes to immune function. The mucosal immune system, or gut-associated lymphoid tissue (GALT), consists of both organized aggregates of lymphoid tissue (e.g., Peyer’s patches; see Fig. 41-2B) and diffuse populations of immune cells. These immune cells include lymphocytes that reside between the epithelial cells lining the gut, as well as lymphocytes and mast cells in the lamina propria. GALT has two primary functions: (1) to protect against potential microbial pathogens, including bacteria, protozoans, and viruses; and (2) to permit immunological tolerance to both the potentially immunogenic dietary substances and the bacteria that normally reside primarily in the lumen of the large intestine. The mucosal immune system is important because the GI tract has the largest area of the body in potential direct contact with infectious, toxic, and immunogenic material. Approximately 80% of the immunoglobulin-producing cells are found in the small intestine. Although GALT has some interaction with the systemic immune system, GALT is operationally distinct. Finally, evidence indicates communication between the GALT and mucosal immune systems at other mucosal surfaces, such as the pulmonary epithelia.

855

Certain nonimmunological defense processes are also important in protecting against potential luminal pathogens and in limiting the uptake of macromolecules from the GI tract. The nonimmunological mechanisms that are critical in maintaining the ecology of intestinal flora include gastric acid secretion, intestinal mucin, peristalsis, and the epithelial-cell permeability barrier. Thus, whereas relatively low levels of aerobic bacteria are present in the lumen of the small intestine of physiologically normal subjects, individuals with impaired small-intestinal peristalsis often have substantially higher levels of both aerobic and anaerobic bacteria in their small intestine. A consequence may be diarrhea or steatorrhea (i.e., increased fecal fat excretion). The clinical manifestation of impaired intestinal peristalsis is referred to as either blind loop syndrome or stagnant bowel syndrome.

REGULATION OF GASTROINTESTINAL FUNCTION The ENS is a “minibrain” with sensory neurons, interneurons, and motor neurons The ENS (see pp. 339–340) is the primary neural mechanism that controls GI function and is one of the three divisions of the autonomic nervous system (ANS), along with the sympathetic and parasympathetic divisions. One indication of the importance of the ENS is the number of neurons consigned to it. The ENS consists of ~100 million neurons, roughly the number in the spinal cord or in the rest of the entire ANS. The ENS is located solely within GI tissue, but it can be modified by input from the brain. Neurons of the ENS are primarily, but not exclusively, clustered in one of two collections of neurons (Fig. 41-3A): the submucosal plexus and the myenteric plexus. The submucosal (or Meissner’s) plexus is found in the submucosa only in the small and large intestine. The myenteric (or Auerbach’s) plexus is located between the circular and longitudinal muscle layers throughout the GI tract from the proximal end of the esophagus to the rectum. The ENS is a complete reflex circuit and can operate totally within the GI tract, without the participation of either the spinal cord or the cephalic brain. As with other neurons, the activity of the ENS is the result of the generation of action potentials by single neurons and the release of chemical neurotransmitters that affect either other neurons or effector cells (i.e., epithelial or muscle cells). The ENS consists of sensory circuits, interneuronal connections, and secretomotor neurons (see Fig. 41-3B). Sensory (or afferent) neurons monitor changes in luminal activity, including distention (i.e., smooth-muscle tension), chemistry (e.g., pH, osmolality, levels of specific nutrients), and mechanical stimulation. These sensory neurons activate interneurons, which relay signals that activate efferent secretomotor neurons that in turn stimulate or inhibit a wide range of effector cells: smooth-muscle cells, epithelial cells that secrete or absorb fluid and electrolytes, submucosal blood vessels, and enteric endocrine cells. The largely independent function of the ENS has given rise to the concept of a GI “minibrain.” Because the efferent responses to several different stimuli are often quite similar,

856 A

SECTION VII  •  The Gastrointestinal System

B

LOCATION OF THE ENS

Longitudinal muscle of muscularis externa

Paravascular nerve

CONNECTIONS OF ENS NEURONS Longitudinal muscle

Perivascular nerve

Circular muscle

SENSORY

Myenteric (Auerbach’s) plexus

Blood vessels

Sensory

Tertiary plexus

Muscularis mucosae

Endocrine cells

PARASYMPATHETIC Motor

Circular muscle of muscularis externa

Vagus nerve

Mechanoreceptors Motor

Deep muscular plexus

Motor

Submucosal (Meissner’s) plexus Submucosal artery

Chemoreceptors

Pelvic nerve Muscularis mucosae

Mucosal plexus

Secretory cells

SYMPATHETIC

Mucosa

Motor

Motor

Brainstem Sympathetic or spinal cord ganglia

Myenteric plexus

Submucosal plexus

Mucosa

Figure 41-3  Schematic representation of the ENS. A, The submucosal (or Meissner’s) plexus is located between the muscularis mucosae and the circular muscle of the muscularis externa. The myenteric (or Auerbach’s) plexus is located between the circular and longitudinal layers of the muscularis externa. In addition to these two plexuses that have ganglia, three others—the mucosal, deep muscular, and tertiary plexuses—are present. B, The ENS consists of sensory neurons, interneurons, and motor neurons. Some sensory signals travel centrally from the ENS. Both the parasympathetic and the sympathetic divisions of the ANS modulate the ENS. This figure illustrates some of the typical circuitry of ENS neurons.

a generalized concept has developed that the ENS possesses multiple preprogrammed responses. For example, both mechanical distention of the jejunum and the presence of a bacterial enterotoxin in the jejunum can elicit identical responses: stimulation of profuse fluid and electrolyte secretion, together with propagated, propulsive, coordinated smooth-muscle contractions. Such preprogrammed efferent responses are probably initiated by sensory input to the enteric interneuronal connections. However, efferent responses controlled by the ENS may also be modified by input from autonomic ganglia, which are in turn under the influence of the spinal cord and brain (see p. 336).  N41-1  In addition, the ENS receives input directly from the brain via parasympathetic nerves (i.e., the vagus nerve).

ACh, peptides, and bioactive amines are the ENS neurotransmitters that regulate epithelial and motor function ACh is the primary preganglionic and postganglionic neurotransmitter regulating both secretory function and smooth-muscle activity in the GI tract. In addition, many other neurotransmitters are present in enteric neurons. Among the peptides, vasoactive intestinal peptide (VIP)

has an important role in both inhibition of intestinal smooth muscle and stimulation of intestinal fluid and electrolyte secretion. Although VIP was first identified in the GI tract, it is now appreciated that VIP is also an important neurotransmitter in the brain (see Table 13-1). Also playing an important role in GI regulation are other peptides (e.g., enkephalins, somatostatin, and substance P), amines (e.g., serotonin), and nitric oxide (NO). Our understanding of ENS neurotransmitters is evolving, and the list of identified agonists grows ever longer. In addition, substantial species differences exist. Frequently, chemical neurotransmitters are identified in neurons without a clear-cut demonstration of their physiological role in the regulation of organ function. More than one neurotransmitter has been identified within single neurons, a finding suggesting that regulation of some cell functions may require more than one neurotransmitter.

The brain-gut axis is a bidirectional system that controls GI function via the ANS, GI hormones, and the immune system Well recognized, but poorly understood, is the modification of several different aspects of GI function by the brain. In

CHAPTER 41  •  Organization of the Gastrointestinal System

N41-1  Hierarchical Reflex Loops in the ANS Contributed by George Richerson 5 Higher CNS centers (hypothalamus)

Descending

Ascending 4 Brainstem Descending Ascending Afferent ascending

3 Spinal cord

Preganglionic 2 Autonomic ganglion

Postganglionic

End organ (colon)

1 ENS

eFigure 41-1  At the lowest level, the ENS is an independent system consisting of afferent neurons, interneurons, and motor neurons. One level up, the autonomic ganglia control the autonomic end organs, including the ENS. One further level up, the spinal cord controls certain autonomic ganglia and integrates response among different levels of the spinal cord. The brainstem receives inputs from visceral afferents and coordinates the control of all viscera. Finally, forebrain CNS centers receive input from the brainstem and coordinate the activity of the ANS via input to the brainstem.

856.e1

CHAPTER 41  •  Organization of the Gastrointestinal System

other words, neural control of the GI tract is a function of not only intrinsic nerves (i.e., the ENS) but also nerves that are extrinsic to the GI tract. These extrinsic pathways are composed of elements of both the parasympathetic and, to a lesser extent, the sympathetic nervous system and are under the control of autonomic centers in the brainstem (see p. 338). Parasympathetic innervation of the GI tract from the pharynx to the distal colon is through the vagus nerve; the distal third of the colon receives its parasympathetic innervation from the pelvic nerves (see Fig. 14-4). The preganglionic fibers of the parasympathetic nerves use ACh as their neurotransmitter and synapse on some neurons of the ENS (see Fig. 41-3B). These ENS neurons are thus postganglionic parasympathetic fibers, and their cell bodies are, in a sense, the parasympathetic ganglion. These postganglionic parasympathetic fibers use mainly ACh as their neurotransmitter; however, as noted in the previous section, many other neurotransmitters are also present. Parasympathetic stimulation—after one or more synapses in a very complex ENS network—increases secretion and motility. The parasympathetic nerves also contain afferent fibers (see p. 339) that carry information to autonomic centers in the medulla from chemoreceptors, osmoreceptors, and mechanoreceptors in the mucosa. The loop that is initiated by these afferents, integrated by central autonomic centers, and completed

TABLE 41-1  GI Peptide Hormones 

857

by the aforementioned parasympathetic efferents, is known as a vagovagal reflex. The preganglionic sympathetic fibers to the GI tract synapse on postganglionic neurons in the prevertebral ganglia (see Fig. 14-3); the neurotransmitter at this synapse is ACh (see p. 341). The postganglionic sympathetic fibers either synapse in the ENS or directly innervate effector cells (see Fig. 41-3B). In addition to the control that is entirely within the ENS, as well as control via autonomic centers in the medulla, the GI tract is also under the control of higher CNS centers. Examples of cerebral function that affects GI behavior include the fight-or-flight response, which reduces blood flow to the GI tract, and the sight and smell of food, which increase gastric acid secretion. Communication between the GI tract and higher CNS centers is bidirectional. For example, cholecystokinin from the GI tract mediates, in part, the development of food satiety in the brain. In addition, gastrin-releasing peptide, a neurotransmitter made in ENS cells (see p. 868), inhibits gastric acid secretion when experimentally injected into the ventricles of the brain. Table 41-1 summarizes peptide hormones made by the GI tract as well as their major actions. In addition to the “hard-wired” communications involved in sensory input and motor output, communication via the

N41-2

HORMONE

SOURCE

TARGET

ACTION

Cholecystokinin

I cells in duodenum and jejunum and neurons in ileum and colon

Pancreas Gall bladder

↑ Enzyme secretion ↑ Contraction

Gastric inhibitory peptide

K cells in duodenum and jejunum

Pancreas

Exocrine: ↓ fluid absorption Endocrine: ↑ insulin release

Gastrin

G cells, antrum of stomach

Parietal cells in body of stomach

↑ H+ secretion

Gastrin-releasing peptide

Vagal nerve endings

G cells in antrum of stomach

↑ Gastrin release

Guanylin

Ileum and colon

Small and large intestine

↑ Fluid absorption

Motilin

Endocrine cells in upper GI tract

Esophageal sphincter Stomach Duodenum

↑ Smooth-muscle contraction

Neurotensin

Endocrine cells, widespread in GI tract

Intestinal smooth muscle

Vasoactive stimulation of histamine release

Peptide YY

Endocrine cells in ileum and colon

Stomach Pancreas

↓ Vagally mediated acid secretion ↓ Enzyme and fluid secretion

Secretin

S cells in small intestine

Pancreas

↑ HCO3− and fluid secretion by pancreatic ducts ↓ Gastric acid secretion

Somatostatin

D cells of stomach and duodenum, δ cells of pancreatic islets

Stomach Intestine

Stomach

Pancreas Liver

↓ ↑ ↑ ↓ ↓

Gastrin release Fluid absorption/↓ secretion Smooth-muscle contraction Endocrine/exocrine secretions Bile flow

Substance P

Enteric neurons

Enteric neurons

Neurotransmitter

VIP

ENS neurons

Small intestine

↑ Smooth-muscle relaxation ↑ Secretion by small intestine ↑ Secretion by pancreas

Pancreas

CHAPTER 41  •  Organization of the Gastrointestinal System

N41-2  GI Peptide Hormones Contributed by Emile Boulpaep and Walter Boron The amino-acid sequences of several of the peptide hormones listed in Table 41-1 are presented elsewhere in the text or below: • Cholecystokinin (CCK): The amino-acid sequence is presented in Figure 42-7C. • Cholecystokinin-like peptide (CCK-8): The amino-acid sequence is presented in Figure 13-9. This is one of several cleavage products of CCK. • Gastric inhibitory peptide: See Table 41-1. A peptide consisting of 42 amino acids. The single-letter code for these amino acids is YAEGTFISD YSIAMDKIHQ QDFVNWLLAQ KGKKNDWKHN ITQ. • Gastrin (“little” and “big”): The amino-acid sequences are presented in Figure 42-7. • Gastrin-releasing peptide (GRP): The amino-acid sequence is presented in Figure 13-9. • Guanylin (guanylyl cyclase activator 2A): A peptide consisting of 15 amino acids. The single-letter code for these amino acids is PGTCEICAYA ACTGC. • Neurotensin: The amino-acid sequence is presented in Figure 13-9. • Peptide YY: Peptide YY (also known as PYY-I) consists of 36 amino acids. The single-letter code for these amino acids is YP IKPEAPGEDA SPEELNRYYA SLRHYLNLVT RQRY. Notice that the sequence starts and ends with a Y (i.e., tyrosine). PYY-II lacks the first two residues of PYY-I (i.e., YP) and thus is only 34 residues in length (see p. 1005). • Secretin: This peptide (see p. 876) consists of 27 amino acids: HSD GTFTSELSRL REGARLQRLL QGLV. • Somatostatin: The amino-acid sequence is presented in Figure 13-9. • Substance P: The amino-acid sequence is presented in Figure 13-9. • Vasoactive intestinal peptide (VIP): The amino-acid sequence is presented in Figure 13-9.

857.e1

858

SECTION VII  •  The Gastrointestinal System

gut-brain axis also requires significant participation of the immune system. Neuroimmune regulation of both epithelial and motor function in the small and large intestine primarily involves mast cells in the lamina propria of the intestine. Because the mast cells are sensitive to neurotransmitters, they can process information from the brain to the ENS and can also respond to signals from interneurons of the ENS. Mast cells also monitor sensory input from the intestinal lumen by participating in the immune response to foreign antigens. In turn, chemical mediators released by mast cells (e.g., histamine) directly affect both intestinal smooth-muscle cells and epithelial cells. Our understanding of how the immune system modulates the neural control of GI function is rapidly evolving. In conclusion, three parallel components of the gut-brain axis—the ENS, GI hormones, and the immune system— control GI function, an arrangement that provides substantial redundancy. Such redundancy permits fine-tuning of the regulation of digestive processes and provides “backup” or “fail-safe” mechanisms that ensure the integrity of GI function, especially at times of impaired function (i.e., during disease).

GASTROINTESTINAL MOTILITY Tonic and rhythmic contractions of smooth muscle are responsible for churning, peristalsis, and reservoir action The motor activity of the GI tract performs three primary functions. First, it produces segmental contractions that are associated with nonpropulsive movement of the luminal contents. The result is the increased mixing—or churning—that enhances the digestion and absorption of dietary nutrients. Second, GI motor activity produces peristalsis, a progressive wave of relaxation followed by contraction. The result is propulsion, or the propagated movement of food and its digestive products in a caudal direction, ultimately eliminating nondigested, nonabsorbed material. Third, motor activity allows some hollow organs— particularly the stomach and large intestine—to hold the luminal content, exerting a reservoir function. This reservoir function is made possible by sphincters that separate the organs of the GI tract. All these functions are primarily accomplished by the coordinated activity of smooth muscle (see pp. 243–249). The electrical and mechanical properties of intestinal smooth muscle needed for these functions include both tonic (i.e., sustained) contractions and rhythmic contractions (i.e., alternating contraction and relaxation) of individual muscle cells. The intrinsic rhythmic contractility is a function of the membrane voltage (Vm) of the smoothmuscle cell. Vm can either oscillate in a subthreshold range at a low frequency (several cycles per minute), referred to as slow-wave activity, or reach a threshold for initiating a true action potential (see Fig. 9-14). The integrated effect of the slow waves and action potentials determines the smoothmuscle activity of the GI tract. Slow-wave activity apparently occurs as voltage-gated Ca2+ channels depolarize the cell and increase [Ca2+]i, followed by the opening of Ca2+-activated K+ channels, which repolarize the cell (see p. 244).

These activities are regulated, in large part, by both neural and hormonal stimuli. Modulation of intestinal smoothmuscle contraction is largely a function of [Ca2+]i (see pp. 246–247). Several agonists regulate [Ca2+]i by one of two mechanisms: (1) activating G protein–linked receptors, which results in the formation of inositol 1,4,5-trisphosphate (IP3) and the release of Ca2+ from intracellular stores; or (2) opening and closing of plasma-membrane Ca2+ channels. Both excitatory and inhibitory neurotransmitters can modulate smooth-muscle [Ca2+]i and thus contractility. In general, ACh is the predominant neurotransmitter of excitatory motor neurons, whereas VIP and NO are the neurotransmitters of inhibitory motor neurons. Different neural or hormonal inputs probably increase (or decrease) the frequency with which Vm exceeds threshold and produces an action potential and thus increases (or decreases) muscle contractility. An additional, unique factor in the aforementioned regulatory control is that luminal food and digestive products activate mucosal chemoreceptors and mechanoreceptors, as discussed above, thus inducing hormone release or stimulating the ENS and controlling smooth-muscle function. For example, gastric contents with elevated osmolality or a high lipid content entering the duodenum activate mucosal osmoreceptors and chemoreceptors that increase the release of cholecystokinin and thus delay gastric emptying (see p. 878).

Segments of the GI tract have both longitudinal and circular arrays of muscles and are separated by sphincters that consist of specialized circular muscles The muscle layers of the GI tract consist almost entirely of smooth muscle. Exceptions are the striated muscle of (1) the upper esophageal sphincter (UES), which separates the hypopharynx from the esophagus; (2) the upper third of the esophagus; and (3) the external anal sphincter. As shown above in Figure 41-2, the two smooth-muscle layers are arranged as an inner circular layer and an outer longitudinal layer. The myenteric ganglia of the ENS are located between the two muscle layers. The segments of the GI tract through which food products pass are hollow, low-pressure organs that are separated by specialized circular muscles or sphincters. These sphincters function as barriers to flow by maintaining a positive resting pressure that serves to separate the two adjacent organs, in which lower pressures prevail. Sphincters thus regulate both antegrade (forward) and retrograde (reverse) movement. For example, the resting pressure of the pyloric sphincter controls, in part, the emptying of gastric contents into the duodenum. On the opposite end of the stomach, the resting pressure of the lower esophageal sphincter (LES) prevents gastric contents from refluxing back into the esophagus and causing gastroesophageal reflux disease (GERD). As a general rule, stimuli proximal to a sphincter cause sphincteric relaxation, whereas stimuli distal to a sphincter induce sphincteric contraction. Changes in sphincter pressure are coordinated with the smooth-muscle contractions in the organs on either side. This coordination depends on both the intrinsic properties of sphincteric smooth muscle and neurohumoral stimuli. Sphincters effectively serve as one-way valves. Thus, the act of deglutition (or swallowing) induces relaxation of the

CHAPTER 41  •  Organization of the Gastrointestinal System

UES, whereas the LES remains contracted. Only when the UES returns to its initial pressure does the LES begin to relax, ~3 seconds after the start of deglutition. Disturbances in sphincter activity are often associated with alterations in one or more of these regulatory processes.

Dry swallow At rest After swallowing

UES

100 mm Hg 0

1

100 mm Hg 0

Location of a sphincter determines its function Six sphincters are present in the GI tract (see Fig. 41-1), each with a different resting pressure and different response to various stimuli. An additional sphincter, the sphincter of Oddi, regulates movement of the contents of the common bile duct into the duodenum. Upper Esophageal Sphincter  Separating the pharynx and the upper part of the esophagus is the UES, which consists of striated muscle and has the highest resting pressure of all the GI sphincters. The swallowing mechanism, which involves the oropharynx and the UES, is largely under the control of the swallowing center in the medulla via cranial nerves V (trigeminal), IX (glossopharyngeal), X (vagus), and XII (hypoglossal). Respiration and deglutition are closely integrated (see p. 720). The UES is closed during inspiration, thereby diverting atmospheric air to the glottis and away from the esophagus. During swallowing, the situation reverses, with closure of the glottis and inhibition of respiration, but with relaxation of the UES (Fig. 41-4). These changes permit the entry of food contents into the esophagus and not into the airways of the respiratory tract. Lower Esophageal Sphincter  The esophagus is separated from the stomach by the LES, which is composed of specialized smooth muscle that is both anatomically and physiologically distinct from adjacent smooth muscle in the distal end of the esophagus and proximal portion of the stomach. The primary functions of the LES are (1) to permit coordinated movement of ingested food into the stomach from the esophagus after swallowing or deglutition, and (2) to prevent reflux of gastric contents into the esophagus. Either deglutition or distention of the esophagus results in a reduction in LES pressure (see Fig. 41-4), thereby permitting entry of food into the stomach. Relaxation of the LES occurs after the UES has already returned to its resting pressure. The LES maintains a resting tone that is the result of both intrinsic myogenic properties of the sphincteric muscle and cholinergic regulation. Relaxation of the LES is mediated both by the vagus nerve and by intrinsic properties of the smooth muscle, including important inhibitory effects by VIP and by NO. Abnormalities of both resting LES pressure and its relaxation in response to deglutition are often associated with significant symptoms. Thus, a reduced resting LES pressure often results in gastroesophageal reflux, which may cause esophagitis (i.e., inflammation of the esophageal mucosa). A defect in LES relaxation is a major component of a condition called achalasia (Box 41-1), which often results in dilation of the esophagus (megaesophagus) and is associated with difficulty in swallowing (dysphagia). Swallowing and the function of the UES and LES are closely integrated into the function of the esophagus. Under normal circumstances, esophageal muscle contractions are

859

2

100 mm Hg 0 3

100 mm Hg 0

4 Diaphragm LES

6

100 mm Hg 0

5

100 mm Hg 0 0 5s

Figure 41-4  Esophageal pressures during swallowing. The swallowing center in the medulla that initiates deglutition includes the nucleus ambiguus (cranial nerves [CN] IX and X), the dorsal motor nucleus of the vagus (CN X), and others. Shown are recordings of intraluminal pressures at different sites along the esophagus, from the UES (record 1) to the LES (record 6). The left side of the graph shows the pressures at rest. As shown on the right side, after a dry swallow, the pressure wave of a “primary peristalsis” moves sequentially down the esophagus. (Data from Conklin JL, Christensen J: Motor functions of the pharynx and esophagus. In Johnson LR [ed]: Physiology of the Gastrointestinal Tract, 3rd ed. New York, Lippincott-Raven, 1994, pp 903–928.)

BOX 41-1  Achalasia

A

chalasia is a relatively uncommon condition associated with difficulty swallowing (dysphagia) and a dilated esophagus proximal to a narrowed, tapered area at the gastroesophageal junction. The term achalasia is derived from Greek words meaning “absence of relaxation.” The distal narrowed area of the esophagus suggests the presence of a stricture. However, it is easy to introduce an esophagoscope into the stomach through the narrowed area. Subsequent studies of esophageal motility in which investigators measured intraesophageal pressure demonstrated the presence of two defects in patients with achalasia: (1) failure of the LES to relax, and (2) impaired peristalsis in the distal two thirds of the body of the esophagus (i.e., the portion that consists of smooth muscle). Peristalsis is intact in the proximal third of the esophagus, which consists of striated muscle. In essence, the smooth-muscle portions of the esophagus behave as a denervated structure. The fundamental defect in achalasia is likely related to selective loss of intramural inhibitory neurons that regulate the LES, the neurotransmitters for which are VIP and NO. Treatment is either physical distention (or stretching) of the LES with a pneumatic-bag dilator or surgical cutting of the LES (i.e., an esophageal Heller myotomy via a laparoscopic approach).

860

SECTION VII  •  The Gastrointestinal System

almost exclusively peristaltic and are initiated by swallowing. Deglutition initiates relaxation of the UES and propagated contractions, first of the UES and then of the muscles along the esophagus (see Fig. 41-4). In the meantime, the LES has already relaxed. The result of the advancing peristaltic wave is the caudad propulsion of a bolus toward the stomach. Distention of the esophagus (in the absence of swallowing) also initiates propulsive esophageal contractions distal to the site of distention, as well as relaxation of the LES. Reflux of gastric contents into the lower part of the esophagus also produces such a local distention, without a swallow, and elicits the same response: peristaltic contractions that clear the esophagus of refluxed gastric material. Peristalsis that is initiated by swallowing is called primary peristalsis, whereas that elicited by distention of the esophagus is referred to as secondary peristalsis. Esophageal contractions after a swallow are regulated by the medullary swallowing center, intramural esophageal plexuses, the vagus nerve, and intrinsic myogenic processes. Pyloric Sphincter  The pylorus is the sphincter that sepa-

rates the stomach from the duodenum. The pressure of the pyloric sphincter regulates, in part, gastric emptying and prevents duodenal-gastric reflux. However, although a specific pyloric sphincter is present, it is quite short and is a relatively poor barrier (i.e., it can resist only a small pressure gradient). The stomach, duodenum, biliary tract, and pancreas—which are closely related embryologically— function as an integrated unit. Indeed, coordinated contraction and relaxation of the antrum, pylorus, and duodenum (which is sometimes referred to as the antroduodenal cluster unit) are probably more important than simply the pressure produced by the pyloric smooth muscle per se. Regulation of gastric emptying is discussed further on pp. 877–878. Ileocecal Sphincter  The valve-like structure that separates the ileum and cecum is called the ileocecal sphincter. Similar to other GI sphincters, the ileocecal sphincter maintains a positive resting pressure and is under the control of the vagus nerve, sympathetic nerves, and the ENS. Distention of the ileum results in relaxation of the sphincter, whereas distention of the proximal (ascending) colon causes contraction of the ileocecal sphincter. As a consequence, ileal flow into the colon is regulated by luminal contents and pressure, both proximal and distal to the ileocecal sphincter. Internal and External Anal Sphincters  The “anal sphincter” actually consists of both an internal and an external sphincter. The internal sphincter has both circular and longitudinal smooth muscle and is under involuntary control. The external sphincter, which encircles the rectum, contains only striated muscle but is controlled by both voluntary and involuntary mechanisms. The high resting pressure of the overall anal sphincter predominantly reflects the resting tone of the internal anal sphincter. Distention of the rectum (Fig. 41-5A), either by colonic contents (i.e., stool) or experimentally by balloon inflation, initiates the rectosphincteric reflex by relaxing the internal sphincter (see Fig. 41-5B). If defecation is not desired, continence is maintained by an involuntary reflex—orchestrated by the sacral spinal cord— that contracts the external anal sphincter (see Fig. 41-5C). If

A

If passive distention of the rectum is sufficiently large…

RECTUM

Rectal distention Active

40 30

Passive

Change in 20 pressure (mm Hg) 10

…it triggers an active contraction of the rectal smooth muscles.

0 Time B

INTERNAL ANAL SPHINCTER Passive rectal distention also triggers relaxation of the smooth muscle of the internal anal sphincter (rectosphincteric reflex).

Change in 0 pressure (mm Hg) –10 –20 Time C

EXTERNAL ANAL SPHINCTER

30

If defecation is not desired, the skeletal muscle of the external anal sphincter contracts by an involuntary reflex.

15 Change in pressure 10 (mm Hg) 5 0 Time Figure 41-5  Pressure changes initiated by rectal distention. (Data from Schuster MM: Simultaneous manometric recording of internal and external anal sphincteric reflexes. Johns Hopkins Med J 116:70–88, 1965.)

defecation is desired, a series of both voluntary and involuntary events occurs that includes relaxation of the external anal sphincter, contraction of abdominal wall muscles, and relaxation of pelvic wall muscles. Flexure of the hips and descent of the pelvic floor then facilitate defecation by minimizing the angle between the rectum and anus. In contrast, if a delay in defecation is needed or desired, voluntary contraction of the external anal sphincter is usually sufficient to override the series of reflexes initiated by rectal distention.

Motility of the small intestine achieves both churning and propulsive movement, and its temporal pattern differs in the fed and fasted states Digestion and absorption of dietary nutrients are the primary functions of the small intestine, and the motor activity of the small intestine is closely integrated with its digestive

CHAPTER 41  •  Organization of the Gastrointestinal System

861

Beginning of jejunum

Distance (cm) from duodenum

Duodenum

CONTRACTILE ACTIVITY Feeding Migrating motor complex

0 20 70 120 170 220 270 300

Fasting 0

1

2

3 4 Time (hr)

5

6

7

8

Figure 41-6  Mechanical activity in the fasting and fed states. Shown are records of intraluminal pressure along the small intestine of a conscious dog. Before feeding (left side), the pattern is one of MMCs. Feeding triggers a switch to a different pattern, characterized by both segmental contractions that churn the contents and peristaltic contractions that propel the contents along the small intestine. (Data from Itoh Z, Sekiguchi T: Interdigestive motor activity in health and disease. Scand J Gastroenterol Suppl 82:121–134, 1983.)

and absorptive roles. The two classes of small-intestinal motor activity are churning (or mixing) and propulsion of the bolus of luminal contents. Churning—which is accomplished by segmental, nonpropulsive contractions—mixes the luminal contents with pancreatic, biliary, and smallintestinal secretions, thus enhancing the digestion of dietary nutrients in the lumen. These segmental contractions also decrease the unstirred water layer that is adjacent to the apical membranes of the small-intestine cells, thus promoting absorption. Churning or mixing movements occur following eating and are the result of contractions of circular muscle in segments flanked at either end by receiving segments that relax. Churning, however, does not advance the luminal contents along the small intestine. In contrast, propulsion—which is accomplished by propagated, peri­ staltic contractions—results in caudad movement of the intestinal luminal contents, either for absorption at more distal sites of the small or large intestine or for elimination in stool. Peristaltic propulsion occurs as a result of contraction of the circular muscle and relaxation of the longitudinal muscle in the propulsive or upstream segment, together with relaxation of the circular muscle and contraction of the longitudinal muscle in the downstream receiving segment. Thus, circular smooth muscle in the small intestine participates in both churning and propulsion. The Vm changes of intestinal smooth-muscle cells consist of both action potentials (see p. 244) and slow-wave activity (see p. 244). The patterns of electrical and mechanical activ-

ity differ in the fasting and fed states. In the fasting state, the small intestine is relatively quiescent but exhibits synchronized, rhythmic changes in both electrical and motor activity (Fig. 41-6). The interdigestive myoelectric or migrating motor complex (MMC) is the term used to describe these rhythmic contractions of the small intestine that are observed in the fasting state. MMCs in humans occur at intervals of 90 to 120 minutes and consist of four distinct phases: (1) a prolonged quiescent period, (2) a period of increasing action potential frequency and contractility, (3) a period of peak electrical and mechanical activity that lasts a few minutes, and (4) a period of declining activity that merges into the next quiescent period. During the interdigestive period, particles >2 mm in diameter can pass from the stomach into the duodenum, which permits emptying of ingested material from the stomach (e.g., bones, coins) that could not be reduced in size to 0.2 L/24 hr). Diarrhea has many causes and can be classified in various ways. One classification divides diarrheas by the causative factor. The causative factor can be failure to absorb a dietary nutrient, in which case the result is an osmotic diarrhea. An example of osmotic diarrhea is that caused by primary lactase deficiency. Alternatively, the causative factor may not be lack of absorption of a dietary nutrient, but rather endogenous secretion of fluid and electrolytes from the intestine, in which case the result is secretory diarrhea. The leading causes of secretory diarrhea include infections with E. coli (the major cause of traveler’s diarrhea) and cholera (a cause of substantial morbidity and mortality in developing countries). In these and other infectious diarrheas, an enterotoxin produced by one of many bacterial organisms raises intracellular concentrations of cAMP, cGMP, or Ca2+ (see Table 44-2). A second group of secretory diarrheas includes those produced by different, relatively uncommon hormone-producing tumors. Examples of such tumors include those that produce VIP (Verner-Morrison syndrome), glucagon (glucagonomas), and serotonin (carcinoid syndrome). These secretagogues act by raising either [cAMP]i or [Ca2+]i (see Table 44-2). When tumors produce these secretagogues in abundance, the resulting diarrhea can be copious and explosive. As we have seen, the secretory diarrheas have in common their ability to increase [cAMP]i, [cGMP]i, or [Ca2+]i. Table 44-4 summarizes the mechanisms by which these second messengers produce the secretory diarrhea. Because the second messengers do not alter the function of nutrient-coupled Na+ absorption, administration of an oral rehydration solution containing glucose and Na+ is effective in the treatment of enterotoxin-mediated diarrhea (see Box 44-1).

response to the same protein kinases that increase Cl− conductance. The net result of all these changes is the initiation of active Cl− secretion across the epithelial cell. The induction of apical membrane Cl− channels is extremely important in the pathophysiology of many diarrheal disorders. Box 44-3 discusses the changes in ion transport that occur in secretory diarrheas such as that associated with cholera. A central role in cystic fibrosis has been posited for the CFTR Cl− channel in the apical membrane (see p. 122). However, other Cl− channels, including the Ca2+-activated CaCC (see Table 6-2, family No. 17) are likely present in the intestine and may contribute to active Cl− secretion.

CELLULAR MECHANISMS OF K+ ABSORPTION AND SECRETION Overall net transepithelial K+ movement is absorptive in the small intestine and secretory in the colon The gastrointestinal tract participates in overall K+ balance, although compared with the kidneys, the small intestine and

large intestine play relatively modest roles, especially in healthy individuals. The pattern of intestinal K+ movement parallels that of the kidney: (1) the intestines have the capacity for both K+ absorption and secretion, and (2) the intestines absorb K+ in the proximal segments but secrete it in the distal segments. Dietary K+ furnishes 80 to 120 mmol/day, whereas stool K+ output is only ~10 mmol/day. The kidney is responsible for disposal of the remainder of the daily K+ intake (see p. 795). Substantial quantities of K+ are secreted in gastric, pancreatic, and biliary fluid. Therefore, the total K+ load presented to the small intestine is considerably greater than that represented by the diet. The concentration of K+ in stool is frequently >100 mM. This high stool [K+] is the result of several factors, including both colonic K+ secretion and water absorption, especially in the distal part of the colon.

K+ absorption in the small intestine probably occurs via solvent drag Experiments in which a plasma-like solution perfused segments of the intestine have established that K+ is absorbed in the jejunum and ileum of the small intestine and is secreted in the large intestine. Although the small intestine absorbs substantial amounts of K+, no evidence has been presented to suggest that K+ absorption in the jejunum and ileum is an active transport process or even carrier mediated. Thus, K+ absorption in the small intestine is probably passive, most likely a result of solvent drag (i.e., pulled along by bulk water movement; see p. 908), as illustrated in Figure 44-6A. Although changes in dietary Na+ and K+ and alterations in hydration influence K+ movement in the colon, similar physiological events do not appear to affect K+ absorption in the small intestine.

Passive K+ secretion is the primary mechanism for net colonic secretion In contrast to the small intestine, the human colon is a net secretor of K+. This secretion occurs by two mechanisms: a passive transport process that is discussed in this section and an active process that is discussed in the next. Together, these two K+ secretory pathways are greater than the modest component of active K+ absorption in the distal part of the colon and thus account for the overall secretion of K+ by the colon. Passive K+ secretion, which is the pathway that is primarily responsible for overall net colonic K+ secretion, is driven by the lumen-negative Vte of 15 to 25 mV. The route of passive K+ secretion is predominantly paracellular, not transcellular (see Fig. 44-6B). Because Vte is the primary determinant of passive K+ secretion, it is not surprising that passive K+ secretion is greatest in the distal end of the colon, where Vte difference is most negative. Similarly, increases in the lumen-negative Vte that occur as an adaptive response to dehydration—secondary to an elevation in aldosterone secretion (see the next section)—result in an enhanced rate of passive K+ secretion. Information is not available regarding the distribution of passive K+ secretion between surface epithelial and crypt cells.

Chapter 44  •  Intestinal Fluid and Electrolyte Movement

+ PASSIVE K ABSORPTION

A

Active K+ secretion is also present throughout the large intestine and is induced both by aldosterone and by cAMP

3 Na+ Jejunum

+

2K

H2O

Ileum

K+

+ PASSIVE K SECRETION

B

+

3 Na

2 K+



K

+

Proximal colon

The lumen potential is –25 mV.

Distal colon

ACTIVE K+ SECRETION

C

K

+

3 Na

+

+

2K

BK +

Na

+

K

Proximal colon Distal colon

2 Cl– NKCC1 + ACTIVE K ABSORPTION

D

3 Na+ H+ +

K

909

+

2K ?

Distal colon High transport

Low transport

Moderate transport

Very low transport

Figure 44-6  Cellular mechanisms of K+ secretion and absorption. A, In

the small intestine, K+ absorption occurs via solvent drag. B, Throughout the colon, passive K+ secretion occurs via tight junctions, driven by a lumen-negative transepithelial voltage. C, Throughout the colon, active K+ secretion is transcellular. D, In the distal colon, active K+ absorption is transcellular. The thickness of the arrows in the insets indicates the relative magnitude of K+ flux in different segments.

In addition to passive K+ secretion, active K+ transport processes—both secretory and absorptive—are also present in the colon. However, active transport of K+ is subject to considerable segmental variation in the colon. Whereas active K+ secretion occurs throughout the colon, active K+ absorption is present only in the distal segments of the large intestine. Thus, in the rectosigmoid colon, active K+ absorption and active K+ secretion are both operative and appear to contribute to total-body homeostasis. The model of active K+ secretion in the colon is quite similar to that of active Cl− secretion (see Fig. 44-5) and is also parallel to that of active K+ secretion in the renal distal nephron (see p. 799). The general paradigm of active K+ transport in the colon is a “pump-leak” model (see Fig. 44-6C). Uptake of K+ across the basolateral membrane is a result of both the Na-K pump and the Na/K/Cl cotransporter (NKCC1), which is energized by the low [Na+]i that is created by the Na-K pump. Once K+ enters the cell across the basolateral membrane, it may exit either across the apical membrane (K+ secretion) or across the basolateral membrane (K+ recycling). The cell controls the extent to which secretion occurs, in part by K+ channels present in both the apical and the basolateral membranes. When apical K+ channel activity is less than basolateral channel activity, K+ recycling dominates. Indeed, in the basal state, the rate of active K+ secretion is low because the apical K+ channel activity is minimal in comparison with the K+ channel activity in the basolateral membrane. It is likely that aldosterone stimulates active K+ secretion in surface epithelial cells of the large intestine, whereas cAMP enhances active K+ secretion in crypt cells. In both cases, the rate-limiting step is the apical BK K+ channel, and both secretagogues act by increasing K+ channel activity. Aldosterone  The mineralocorticoid aldosterone enhances overall net K+ secretion by two mechanisms. First, it increases passive K+ secretion by increasing Na-K pump activity and thus increasing electrogenic Na+ absorption (see Fig. 44-3D). The net effects are to increase the lumen-negative Vte and to enhance passive K+ secretion (see Fig. 44-6B). Second, aldosterone stimulates active K+ secretion by increasing the activity of both apical K+ channels and basolateral Na-K pumps (see Fig. 44-6C). cAMP and Ca2+  VIP and cholera enterotoxin both

increase [cAMP]i and thus stimulate K+ secretion. Increases in [Ca2+]i—induced, for example, by serotonin (or 5hydroxytryptamine [5-HT])—also stimulate active K+ secretion. In contrast to aldosterone, neither of these second messengers has an effect on the Na-K pump; rather, they increase the activity of both the apical and the basolateral K+ channels. Because the stimulation of K+ channels is greater at the apical than at the basolateral membrane, the result is an increase in K+ exit from the epithelial cell across the apical membrane (i.e., secretion). Stimulation of K+ secretion by cAMP and Ca2+, both of which also induce

910

SECTION VII  •  The Gastrointestinal System

active Cl− secretion (see Fig. 44-5), contributes to the significant fecal K+ losses that occur in many diarrheal diseases.

Active K+ absorption takes place only in the distal portion of the colon and is energized by an apical H-K pump As noted above, not only does the distal end of the colon actively secrete K+, it also actively absorbs K+. The balance between the two processes plays a role in overall K+ homeostasis. Increases in dietary K+ enhance both passive and active K+ secretion (see Fig. 44-6B, C). However, dietary K+ depletion enhances active K+ absorption (see Fig. 44-6D). The mechanism of active K+ absorption appears to be an exchange of luminal K+ for intracellular H+ across the apical membrane, mediated by an H-K pump (see pp. 117–118). The colonic H-K pump is ~60% identical at the amino-acid level to both the Na-K pump and the gastric parietal-cell H-K pump. Thus, active colonic K+ absorption occurs via a trans­ cellular route, in contrast to the paracellular route that characterizes K+ absorption in the small intestine (see Fig. 44-6A). The mechanism of K+ exit across the basolateral membrane may involve K/Cl cotransport. Not known is whether active K+ secretion (see Fig. 44-6C) and active K+ absorption (see Fig. 44-6D) occur in the same cell or in different cells.

REGULATION OF INTESTINAL ION TRANSPORT Chemical mediators from the enteric nervous system, endocrine cells, and immune cells in the lamina propria may be either secretagogues or absorptagogues Numerous chemical mediators from several different sources regulate intestinal electrolyte transport. Some of these agonists are important both in health and in diarrheal disorders, and at times only quantitative differences separate normal regulatory control from the pathophysiology of diarrhea. These mediators may function in one or more modes: neural, endocrine, paracrine, and perhaps autocrine (see p. 47). Most of these agonists (i.e., secretagogues) promote secretion, whereas some others (i.e., absorptagogues) enhance absorption. The enteric nervous system (ENS), discussed on pages 339–340 and 855–856, is important in the normal regulation of intestinal epithelial electrolyte transport. Activation of enteric secretomotor neurons results in the release of acetylcholine from mucosal neurons and in the induction of active Cl− secretion (see Fig. 44-5). Additional neurotransmitters, including VIP, 5-HT, and histamine, mediate ENS regulation of epithelial ion transport. An example of regulation mediated by the endocrine system is the release of aldosterone from the adrenal cortex and the subsequent formation of angiotensin II; both dehydration and volume contraction stimulate this reninangiotensin-aldosterone axis (see pp. 841–842). Both angiotensin and aldosterone regulate total-body Na+ homeostasis by stimulating Na+ absorption, angiotensin in the small intestine, and aldosterone in the colon. Their effects on cellular Na+ absorption differ. In the small intestine, angiotensin

TABLE 44-3  Products of Lamina Propria Cells that Affect Intestinal Ion Transport CELL

PRODUCT

Macrophages

Prostaglandins O2 radicals

Mast cells

Histamine

Neutrophils

Eicosanoids Platelet-activating factor

Fibroblasts

Eicosanoids Bradykinin

enhances electroneutral NaCl absorption (see Fig. 44-3C), probably by upregulating apical membrane Na-H exchange. In the colon, aldosterone stimulates electrogenic Na+ absorption (see Fig. 44-3D). The response of the intestine to angiotensin and aldo­ sterone represents a classic endocrine feedback loop: dehydration results in increased levels of angiotensin and aldosterone, the primary effects of which are to stimulate fluid and Na+ absorption by both the renal tubules (see pp. 765–766) and the intestines. The result is restoration of total-body fluid and Na+ content. Regulation of intestinal transport also occurs by paracrine effects. Endocrine cells constitute a small fraction of the total population of mucosal cells in the intestines. These endocrine cells contain several peptides and bioactive amines that are released in response to various stimuli. Relatively little is known about the biology of these cells, but gut distention can induce the release of one or more of these agonists (e.g., 5-HT). The effect of these agonists on adjacent surface epithelial cells represents a paracrine action. Another example of paracrine regulation of intestinal fluid and electrolyte transport is the influence of immune cells in the lamina propria (see Fig. 44-1). Table 44-3 lists these immune cells and some of the agonists that they release. The same agonist may be released from more than one cell, and individual cells produce multiple agonists. These agonists may activate epithelial cells directly or may activate other immune cells or enteric neurons. For example, reactive oxygen radicals released by mast cells affect epithelial-cell function by acting on enteric neurons and fibroblasts, and they also have direct action on surface and crypt epithelial cells. A single agonist usually has multiple sites of action. For example, the histamine released from mast cells can induce fluid secretion as a result of its interaction with receptors on surface epithelial cells (Fig. 44-7). However, histamine can also activate ENS motor neurons, which can in turn alter epithelial-cell ion transport as well as intestinal smoothmuscle tone and blood flow. As a consequence, the effects of histamine on intestinal ion transport are multiple and amplified.

Secretagogues can be classified by their type and by the intracellular second-messenger system that they stimulate Several agonists induce the accumulation of fluid and electrolytes in the intestinal lumen (i.e., net secretion).

Chapter 44  •  Intestinal Fluid and Electrolyte Movement

Exterior milieu

911

Interstitial space Antigen

Epithelial cell

Histamine



Receptor

Cl

PGE2

Myofibroblasts

Antibody

IL-1

cAMP

EP2 receptor ACh

Histamine ACh



Cl

Enteric neuron

Ca2+

M3 receptor

Intestinal smooth muscle

Mast cell in lamina propria

Figure 44-7  Mast cell activation. Activation of mast cells in the lamina propria triggers the release of histamine, which either directly affects epithelial cells or stimulates an enteric neuron and thus has an indirect effect. The neuron modulates the epithelium (secretion), intestinal smooth muscle (motility), or vascular smooth muscle (blood flow). ACh, acetylcholine; EP2 receptor, prostaglandin E2 receptor; IL-1, interleukin-1; PGE2, prostaglandin E2.

These secretagogues are a diverse, heterogeneous group of compounds, but they can be effectively classified in two different ways: by the type of secretagogue and by the intracellular second messenger that these agonists activate. Grouped according to type, the secretagogues fall into four categories: (1) bacterial exotoxins (i.e., enterotoxins), (2) hormones and neurotransmitters, (3) products of cells of the immune system, and (4) laxatives. Table 44-2 provides a partial list of these secretagogues. A bacterial exotoxin is a peptide that is produced and excreted by bacteria that can produce effects independently of the bacteria. An enterotoxin is an exotoxin that induces changes in intestinal fluid and electrolyte movement. For example, E. coli produces two distinct enterotoxins (the so-called heat-labile and heatstable toxins) that induce fluid and electrolyte secretion via two distinct receptors and second-messenger systems. We can also classify secretagogues according to the signaltransduction system that they activate after binding to a specific membrane receptor. As summarized in Table 44-2, the second messengers of these signal-transduction systems include cAMP, cGMP, and Ca2+. For example, the heat-labile toxin of E. coli binds to apical membrane receptors, becomes internalized, and then activates basolateral adenylyl cyclase. The resulting increase in [cAMP]i activates protein kinase A. VIP also acts by this route (Fig. 44-8). The heat-stable toxin of E. coli binds to and activates an apical receptor guanylyl cyclase, similar to the atrial natriuretic peptide (ANP) receptor (see p. 66). The newly produced cGMP activates protein kinase G and may also activate protein kinase A. The natural agonist for this pathway is guanylin, a 15–amino-acid peptide secreted by mucosal cells of the small and large intestine. Still other secretory agonists (e.g., 5-HT) produce their effects by increasing [Ca2+]i and thus activating protein kinase C or Ca2+-calmodulin–dependent protein kinases. One way that secretagogues can increase [Ca2+]i is by stimulating phospholipase C, which leads to the production of inositol

TABLE 44-4  End Effects of Second Messengers on Intestinal Transport SECOND MESSENGER

INCREASED ANION SECRETION

INHIBITED NaCl ABSORPTION

cAMP

+++

+++

cGMP

+

+++

2+

+++

+++

Ca

1,4,5-trisphosphate (IP3) and the release of Ca2+ from intracellular stores (see p. 60). Secretagogues can also increase [Ca2+]i by activating protein kinases, which may stimulate basolateral Ca2+ channels. Although the secretagogues listed in Table 44-2 stimulate fluid and electrolyte secretion via one of three distinct second messengers (i.e., cAMP, cGMP, and Ca2+), the end effects are quite similar. As summarized in Table 44-4, all three second-messenger systems stimulate active Cl− secretion (see Fig. 44-5) and inhibit electroneutral NaCl absorption (see Fig. 44-3C). The abilities of cAMP and Ca2+ to stimulate Cl− secretion and inhibit electroneutral NaCl absorption are almost identical. In contrast, cGMP’s ability to stimulate Cl− secretion is somewhat less, although its effects on electroneutral NaCl absorption are quantitatively similar to those of cAMP and Ca2+. Both stimulation of Cl− secretion and inhibition of electroneutral NaCl absorption have the same overall effect: net secretion of fluid and electrolytes. It is uncertain whether the observed decrease in electroneutral NaCl absorption is the result of inhibiting Na-H exchange, Cl-HCO3 exchange, or both inasmuch as electroneutral NaCl absorption represents the coupling of separate Na-H and Cl-HCO3 exchange processes via pHi (see Fig. 44-3C).

912

SECTION VII  •  The Gastrointestinal System

PKA catalytic subunits phosphorylate apical membrane proteins.

Epithelial cell

External milieu

Interstitial space PKA regulatory subunit P

Active PKG phosphorylates apical membrane proteins. Receptor guanylyl cyclase Heat-stable toxin (STa)

cAMP

cAMP

cAMP

cAMP

PKA (active)

P

cAMP

PKA catalytic subunit

PKA

cGMP

P

PKG II

CaM kinase

AC

Active PKG type II

PKG II

cGMP

2 Some secretagogues bind to a receptor that generates cGMP.

Gs

Gq

Secretagogue (e.g., VIP, heatlabile toxin) 1 Some secretagogues bind to GPCRs, coupled to Gs, activating adenylyl cyclase. Secretagogue (e.g., serotonin) 3 Other secretagogues bind to GPCRs, coupled to Gq, activating phospholipase C.

Calmodulin

Calcium calmodulin

PLC

Active CaM kinase phosphorylates apical membrane proteins.

PIP2 Active CaM kinase

Ca2+

Ca2+

ER IP3 PKC

PKC

DAG Active PKC

Figure 44-8  Action of secretagogues. Secretagogues (agents that stimulate the net secretion of fluid and electrolytes into the intestinal lumen) act by any of the mechanisms numbered 1, 2, or 3. AC, adenylyl cyclase; CaM, calmodulin; DAG, diacylglycerol; ER, endoplasmic reticulum; Gq and Gs, α-subunit types of G proteins; GPCRs, G protein–coupled receptors; PIP2, phosphatidylinositol 4,5-bisphosphate; PKA, protein kinase A; PKC, protein kinase C; PKG, cGMP-dependent protein kinase; PLC, phospholipase C.

Mineralocorticoids, glucocorticoids, and somatostatin are absorptagogues Although multiple secretagogues exist, relatively few agonists can be found that enhance fluid and electrolyte absorption. The cellular effects of these absorptagogues are less well understood than those of the secretagogues. Those few absorptagogues that have been identified increase intestinal fluid and electrolyte absorption by either a paracrine or an endocrine mechanism. Corticosteroids are the primary hormones that enhance intestinal fluid and electrolyte absorption. Mineralocorticoids (e.g., aldosterone) stimulate Na+ absorption and K+ secretion in the distal end of the colon; they do not affect ion transport in the small intestine. Their cellular actions are outlined on page 1027. Aldosterone induces both apical membrane Na+ channels (a process that is inhibited by the diuretic amiloride) and basolateral Na-K pumps; this

action results in substantial enhancement of colonic “electrogenic” Na+ absorption. Although the effects of glucocorticoids on ion transport have most often been considered a result of crossover binding to the mineralocorticoid receptor (see p. 766), it is now evident that glucocorticoids also have potent actions on ion transport via their own receptor and that these changes in ion transport are distinct from those of the mineralocorticoids. Glucocorticoids stimulate electroneutral NaCl absorption (see Fig. 44-3C) throughout the large and small intestine without any effect on either K+ secretion or electrogenic Na+ absorption. Both corticosteroids act, at least in part, by genomic mechanisms (see pp. 71–72). Other agonists appear to stimulate fluid and electrolyte absorption by stimulating electroneutral NaCl absorption and inhibiting electrogenic HCO3− secretion; both these changes enhance fluid absorption. Among these absorptagogues are somatostatin, which is released from endocrine

Chapter 44  •  Intestinal Fluid and Electrolyte Movement

cells in the intestinal mucosa (see pp. 993–994), and the enkephalins and norepinephrine, which are neurotransmitters of enteric neurons. The limited information available suggests that these agonists affect ion transport by decreasing [Ca2+]i, probably by blocking Ca2+ channels. Thus, it appears that fluctuations in [Ca2+]i regulate Na+ and Cl− transport in both the absorptive (low [Ca2+]i) and secretory (high [Ca2+]i)

913

directions. Therefore, Ca2+ is clearly a critical modulator of intestinal ion transport.

REFERENCES The reference list is available at www.StudentConsult.com.

Chapter 44  •  Intestinal Fluid and Electrolyte Movement

REFERENCES Books and Reviews Alper SL, Sharma AK: The SLC26 gene family of anion transporters and channels. Mol Aspects Med 34:494–515, 2013. Arroyo JP, Kahle KT, Gamba G: The SLC12 family of electroneutral cation-coupled chloride cotransporters. Mol Aspects Med 34:288–298, 2013. Binder HJ, Sandle GI: Electrolyte transport in the mammalian colon. In Johnson LR (ed): Physiology of the Gastrointestinal Tract, 3rd ed. New York, Raven Press, 1994, pp 2133–2172. Donowitz M, Ming Tse C, Fuster D: SLC9/NHE gene family, a plasma membrane and organellar family of Na+/H+ exchangers. Mol Aspects Med 34:236–251, 2013. Farthing MJG: Oral rehydration therapy. Pharmacol Ther 64:477– 492, 1994. Field M, Semrad CE: Toxigenic diarrheas, congenital diarrheas, and cystic fibrosis: Disorders of intestinal ion transport. Annu Rev Physiol 55:631–655, 1993. Greger R, Bleich M, Leipziger J, et al: Regulation of ion transport in colonic crypts. News Physiol Sci 12:62–66, 1997. Kaunitz JD, Barrett KE, McRoberts JA: Electrolyte secretion and absorption: Small intestine and colon. In Yamada T (ed): Textbook of Gastroenterology, vol 1, 2nd ed. Philadelphia, JB Lippincott, 1995, pp 326–361. Montrose MH, Keely SJ, Barrett KE: Electrolyte secretion and absorption. Small intestine and colon. In Yamada T (ed):

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Textbook of Gastroenterology, vol. 1, 4th ed. Philadelphia, Lippincott Williams & Wilkins, 2003, pp 308–340. Palacin M, Estevez R, Bertran J, Zorzano A: Molecular biology of mammalian plasma membrane amino acid transporters. Physiol Rev 78:969–1054, 1998. Rao MC: Oral rehydration therapy: New explanations for an old remedy. Annu Rev Physiol 66:385–417, 2004. Zachos NC, Tse M, Donowitz M: Molecular physiology of intestinal Na/H exchange. Annu Rev Physiol 67:411–443, 2005. Journal Articles Canessa CM, Horisberger J-D, Rossier BC: Epithelial sodium channel related to proteins involved in neurodegeneration. Nature 361:467–470, 1993. Knickelbein RG, Aronson PS, Schron CM, et al: Sodium and chloride transport across rabbit ileal brush border. II. Evidence for Cl-HCO3 exchange and mechanism of coupling. Am J Physiol 249:G236–G245, 1985. Moseley RH, Hoglund P, Wu GD, et al: Downregulated in adenoma gene encodes a chloride transporter defective in congenital chloride diarrhea. Am J Physiol 276:G185–G192, 1999. Schulz S, Green CK, Yuen PST, Garbers DL: Guanylyl cyclase is a heat-stable enterotoxin receptor. Cell 63:941–948, 1990. Singh SK, Binder HJ, Boron WF, Geibel JP: Fluid absorption in isolated perfused colonic crypts. J Clin Invest 96:2373–2379, 1995.

C H A P T E R 45  NUTRIENT DIGESTION AND ABSORPTION Henry J. Binder and Charles M. Mansbach II

In general, the digestive-absorptive processes for most of the constituents of our diet are highly efficient. For example, normal adult intestine absorbs ~95% of dietary lipid. How­ ever, we ingest most of the constituents of dietary food in a form that the intestine cannot readily absorb. Multiple digestive processes convert dietary food to a form that can be absorbed—primarily in the small intestine, but also, to a much smaller extent, in the colon. The digestive process—the enzymatic conversion of complex dietary substances to a form that can be absorbed— is initiated by the sight, smell, and taste of food. Although some digestion (that of carbohydrates) begins in the mouth and additional digestion may occur within the lumen of the stomach, most digestive processes occur in the small intestine. Digestion within the small intestine occurs either in the lumen, mediated by pancreatic enzymes, or at the smallintestinal brush-border membrane (membrane digestion), mediated by brush-border enzymes. Several different patterns of luminal, brush-border, and cytosolic digestion exist (Fig. 45-1). Some of the dietary carbohydrate and protein that escape digestion and absorption in the small intestine are altered in the large intestine by bacterial enzymes to short-chain fatty acids (SCFAs)  N45-1  that are absorbed by the colon. The digestive processes for carbohydrates, proteins, and lipids result in the conversion of dietary nutrients to chemical forms for which intestinal absorptive processes exist. As a consequence, the digestive-absorptive processes for the several dietary constituents are closely integrated and regulated biological events that ensure survival. Multiple diseases can alter these digestive-absorptive processes and can thereby impair nutrient assimilation (i.e., the overall process of digestion and absorption). Because of the substantial segmental distribution of nutrient absorption along the gastrointestinal tract (Fig. 45-2), the clinical manifestations of disease (Table 45-1) often reflect these segmental differences.

CARBOHYDRATE DIGESTION Carbohydrates, providing ~45% of total energy needs of Western diets, require hydrolysis to monosaccharides before absorption We can classify dietary carbohydrates into two major groups: (1) the monosaccharides (monomers), and (2) the oligo­ saccharides (short polymers) and polysaccharides (long 914

polymers). The small intestine can directly absorb the monomers but not the polymers. Some polymers are digestible, that is, the body can digest them to form the monomers that the small intestine can absorb. Other polymers are non­ digestible, or “fiber.” The composition of dietary carbohydrate is quite varied and is a function of culture. The diet of individuals in so-called developed countries contains considerable amounts of “refined” sugar and, compared with individuals in most developing countries, less fiber. Such differences in the fiber content of the Western diet may account for several diseases that are more prevalent in these societies (e.g., colon carcinoma and atherosclerosis). As a consequence, the consumption of fiber by the health-conscious public in the United States has increased during the past 3 decades. In general, increased amounts of fiber in the diet are associated with increased stool weight and frequency. Approximately 45% to 60% of dietary carbohydrate is in the form of starch, which is a polysaccharide. Starch is a storage form for carbohydrates that is primarily found in plants, and it consists of both amylose and amylopectin. In contrast, the storage form of carbohydrates in animal tissues is glycogen, which is consumed in much smaller amounts. Amylose is a straight-chain glucose polymer that typically contains multiple glucose residues, connected by α-1,4 linkages. In contrast, amylopectin is a massive branched glucose polymer that may contain 1 million glucose residues. In addition to the α-1,4 linkages, amylopectin has frequent α-1,6 linkages at the branch points. Amylopectins are usually present in much greater quantities (perhaps 4-fold higher) than amylose. Glycogen—the “animal starch”—has α-1,4 and α-1,6 linkages like amylopectin. However, glycogen is more highly branched (i.e., more α-1,6 linkages). Most dietary oligosaccharides are the disaccharides sucrose and lactose, which represent 30% to 40% of dietary carbohydrates. Sucrose is table sugar, derived from sugar cane and sugar beets, whereas lactose is the sugar found in milk. The remaining carbohydrates are the monosaccharides fructose and glucose, which make up 5% to 10% of total carbohydrate intake. There is no evidence of any intestinal absorption of either starches or disaccharides. Because the small intestine can absorb only monosaccharides, all dietary carbohydrate must be digested to monosaccharides before absorption. The colon cannot absorb monosaccharides. Dietary fiber consists of both soluble and insoluble forms and includes lignins, pectins, and cellulose. These fibers are primarily present in fruits, vegetables, and cereals. Cellulose

Chapter 45  •  Nutrient Digestion and Absorption

N45-1  Fatty Acids: Chain Length Contributed by Emile Boulpaep and Walter Boron Name

Abbreviation

Short-chain fatty acid Medium-chain fatty acid Long-chain fatty acid Very-long-chain fatty acid

SCFA MCFA LCFA VLCFA

Number of Carbon Atoms 21

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Chapter 45  •  Nutrient Digestion and Absorption

Lumen DIGESTION

Epithelium

915

Interstitial space

EXAMPLE

None

Glucose

Luminal hydrolysis of polymer to monomers

Glucose

Protein

Amino acids (AA)

AA

Glucose

Glucose

Sucrose

Brushborder hydrolysis of oligomer to monomers

Fructose

Fructose Intracellular hydrolysis Peptide

Luminal hydrolysis followed by intracellular resynthesis

AA

Glycerol Triacylglycerol

Triacylglycerol Fatty acids

Figure 45-1  General mechanisms of digestion and absorption. Digestion-absorption can follow any of five patterns. First, the substance (e.g., glucose) may not require digestion; the intestinal cells may absorb the nutrient as ingested. Second, a polymer (e.g., protein) may be digested in the lumen to its constituent monomers (e.g., amino acids) by pancreatic enzymes prior to absorption. Third, an oligomer (e.g., sucrose) is digested into its constituent monomers (e.g., monosaccharides) by brush-border enzymes prior to absorption. When in free solution, fructose is present primarily as the pyranose (6-membered ring) form and less so as the furanose (5-membered ring) form. Fourth, an oligomer (e.g., oligopeptide) may be directly absorbed by the cell and then broken down into monomers (e.g., amino acids) inside the cell. Finally, a substance (e.g., TAG) may be broken down into its constituent components prior to absorption; the cell may then resynthesize the original molecule.

TABLE 45-1  Major Gastrointestinal Diseases and Nutritional Deficiencies DISEASE

ORGAN SITE OF PREDOMINANT PATHOLOGY

DEFECTIVE PROCESS

Celiac disease (see Box 45-5)

Duodenum and jejunum

Fat absorption, lactose hydrolysis

Chronic pancreatitis

Exocrine pancreas

Fat digestion

Surgical resection of ileum; Crohn disease of ileum

Ileum

Cobalamin and bile-acid absorption

Primary lactase deficiency

Small intestine

Lactose hydrolysis

is a glucose polymer connected by β-1,4 linkages, which cannot be digested by mammalian enzymes. However, enzymes from colonic bacteria may degrade fiber. This process is carried out with varying efficiency; pectins, gum, and mucilages are metabolized to a much greater degree than either cellulose or hemicellulose. In contrast, lignins, which are aromatic polymers and not carbohydrates, are not altered by microbial enzymes in the colonic lumen and are excreted unaltered in stool.

As we discuss below, the digestive process for dietary carbohydrates has two steps: (1) intraluminal hydrolysis of starch to oligosaccharides by salivary and pancreatic amylases (Fig. 45-3), and (2) so-called membrane digestion of oligosaccharides to monosaccharides by brush-border disaccharidases. The resulting carbohydrates are absorbed by transport processes that are specific for certain monosaccharides. These transport pathways are located in the apical membrane of the small-intestinal villous epithelial cells.

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SECTION VII  •  The Gastrointestinal System

A

CARBOHYDRATES, PROTEINS AND LIPIDS

B

CALCIUM, IRON AND FOLATE Calcium Iron

Carbohydrates, proteins, lipids

Folate

Duodenum Jejunum Ileum

Duodenum Calcium Calcium

High absorption Moderate absorption C

D

BILE ACIDS

Low absorption

COBALAMIN

Very low absorption

Bile acids

Duodenum Jejunum Ileum

Cobalamin

Ileum

Figure 45-2  Sites of nutrient absorption. A, The entire small intestine absorbs carbohydrates, proteins, and lipids. However, the absorption is greatest in the duodenum, somewhat less in the jejunum, and much less in the ileum. The thickness of the arrows indicates the relative magnitude of total absorption at the indicated site in vivo (see inset). The maximal absorptive capacity of a specific segment under optimized experimental conditions (e.g., substrate concentrations) may be greater. B, Some substances are actively absorbed only in the duodenum. C, Bile acids are absorbed along the entire small intestine, but active absorption occurs only in the ileum. D, The vitamin cobalamin is absorbed only in the ileum.

Luminal digestion begins with the action of salivary amylase and finishes with pancreatic amylase Acinar cells from both the salivary glands (see pp. 893–894) and pancreas (see p. 882) synthesize and secrete α-amylases. Salivary and pancreatic amylases, unlike most of the pancreatic proteases that we discuss below, are secreted not in an inactive proenzyme form, but rather in an active form. Salivary and pancreatic α-amylases have similar enzymatic function, and their amino-acid sequences are 94% identical. Salivary α-amylase in the mouth initiates starch digestion; in healthy adults, this step is of relatively limited importance. Salivary amylase is inactivated by gastric acid but can be partially protected by complexing with oligosaccharides. Pancreatic α-amylase completes starch digestion in the lumen of the small intestine. Although amylase binds to the apical membrane of enterocytes, this localization does not provide any kinetic advantage for starch hydrolysis. Cholecystokinin (CCK; see pp. 882–883) stimulates the secretion of pancreatic α-amylase by pancreatic acinar cells. α-amylase is an endoenzyme that hydrolyzes internal α-1,4 linkages (see Fig. 45-3A). α-amylase does not cleave terminal α-1,4 linkages, α-1,6 linkages (i.e., branch points), or α-1,4 linkages that are immediately adjacent to α-1,6 linkages. As a result, starch hydrolysis products are maltose,

maltotriose, and α-limit dextrins. Because α-amylase has no activity against terminal α-1,4 linkages, glucose is not a product of starch digestion. The intestine cannot absorb these products of amylase digestion of starch, and thus further digestion is required to produce substrates (i.e., monosaccharides) that the small intestine can absorb by specific transport mechanisms.

“Membrane digestion” involves hydrolysis of oligosaccharides to monosaccharides by brush-border disaccharidases The human small intestine has three brush-border proteins with oligo­saccharidase activity: lactase, glucoamylase (most often called maltase), and sucrase-isomaltase. These are all integral membrane proteins whose catalytic domains face the intestinal lumen (see Fig. 45-3B). Sucrase-isomaltase is actually two enzymes—sucrase and isomaltase (also known as α-dextrinase or debranching enzyme)—bound together. Thus, four oligosaccharidase entities are present at the brush border. Lactase has only one substrate; it breaks lactose into glucose and galactose. The other three enzymes have more complicated substrate spectra. All will cleave the terminal α-1,4 linkages of maltose, maltotriose, and α-limit dextrins. In addition, each of these three enzymes has at least one

Chapter 45  •  Nutrient Digestion and Absorption

Epithelium

Lumen

A

DIGESTION OF OLIGOSACCHARIDES AT BRUSH BORDER Lumen

α-Amylase

Amylose

Interstitial space

B

DIGESTION OF STARCH IN LUMEN

Lactase Amylopectin

Cytoplasm Lactase splits lactose. Both monomers are transported via SGLT1.

Lactose Terminal α-1,4 link Cannot be cut by amylase

Adjacent α-1,6 linkage Adjacent Terminal α-1,4 link (branching) α-1,4 link α-1,4 link Cannot be cut by amylase

917

SGLT1

+

Glucoamylase (also known as maltase) removes glucose monomers for transport.

2 Na+ +

Glucoamylase

Maltotriose

Maltotriose or maltose

α-Limit dextrins

+

Sucrase-isomaltase is actually two enzymes. The sucrase moiety splits sucrose, as well as maltose and maltotriose.

Maltose

C

SGLT1

+ 2 Na

+

GLUT5

ABSORPTION OF MONOSACCHARIDES Lumen

Epithelium

SGLT1 Galactose Glucose +

2 Na

Glucose 3 Na+

GLUT2

+

Sucrase-isomaltase Sucrose

Sucrase Isomaltase

Maltose

Sucrase Isomaltase

Maltotriose

The isomaltase moiety splits α-limit dextrins, as well as maltose and maltotriose.

2 K+

Fructose GLUT5

Interstitial space

Fructose

GLUT2

α-limit dextrins

+

Maltose Maltotriose

2 Na+

Figure 45-3  Digestion of carbohydrates to monosaccharides. A, Salivary and pancreatic α-amylase are

endoenzymes. They can digest the linear “internal” α-1,4 linkages between glucose residues but cannot break “terminal” α-1,4 linkages between the last two sugars in the chain. They also cannot split the α-1,6 linkages at the branch points of amylopectin or the adjacent α-1,4 linkages. As a result, the products of α-amylase action are linear glucose oligomers, maltotriose (a linear glucose trimer), maltose (a linear glucose dimer), and α-limit dextrins (which contain an α-1,6 branching linkage). B, The brush-border oligosaccharidases are intrinsic membrane proteins with their catalytic domains facing the lumen. Sucrase-isomaltase is actually two enzymes, and therefore, there are a total of four oligosaccharidases that split the oligosaccharides produced by α-amylase into monosaccharides. C, SGLT1 is the Na+-coupled transporter that mediates the uptake of glucose or galactose from the lumen of the small intestine into the enterocyte. GLUT5 mediates the facilitated diffusion of fructose into the enterocyte. Once the monosaccharides are inside the enterocyte, GLUT2 mediates their efflux across the basolateral membrane into the interstitial space.

SGLT1

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SECTION VII  •  The Gastrointestinal System

A PRESENCE OF LACTASE ACTIVITY Plasma glucose rises after glucose or lactose ingestion…

140

Plasma glucose

…and subsequent H2 excreted by lungs is low.

120

Breath H2

Glucose ingested 100

Lactose ingested

Lactose

80 0

Glucose 0

1

2

3

0

1

Hours

2

3

Hours

B LACTASE DEFICIENCY 140

Plasma glucose

…and colonic bacteria metabolize the lactose that enters the colon, resulting in higher H2 excretion.

Lactase-deficient individuals hydrolyze less lactose to glucose… Glucose ingested

120

Lactose

Breath H2

100 Glucose Lactose ingested

80 0 0

1

2

3

Hours

0

1

2

3

Hours

Figure 45-4  Effects of lactase deficiency on levels of glucose in the plasma and H2 in the breath. A, In an individual with normal lactase activity, blood glucose levels rise after the ingestion of either glucose or lactose. Thus, the small intestine can split the lactose into glucose and galactose, and absorb the two monosaccharides. At the same time, H2 in the breath is low. B, In an adult with low lactase activity, the rise in blood levels is less pronounced after ingesting lactose. Because the rise is normal after ingesting glucose, we can conclude that the difference is due to lactase activity. Conversely, the individual with lactase deficiency excretes large amounts of H2 into the breath. This H2 is the product of lactose catabolism by colonic bacteria.

other activity. Maltase can also degrade the α-1,4 linkages in straight-chain oligosaccharides up to nine monomers in length. However, maltase cannot split either sucrose or lactose. The sucrase moiety of sucrase-isomaltase is required to split sucrose into glucose and fructose. The isomaltase moiety of sucrase-isomaltase is critical; it is the only enzyme that can split the branching α-1,6 linkages of α-limit dextrins.  N45-2 The action of the four oligosaccharidases generates several monosaccharides. Whereas the hydrolysis products of maltose are two glucose residues, those of sucrose are glucose and fructose. The hydrolysis of lactose by lactase yields glucose and galactose. The activities of the hydrolysis reactions of sucrase-isomaltase and maltase are considerably greater than the rates at which the various transporters can absorb the resulting monosaccharides. Thus, uptake, not hydrolysis, is the rate-limiting step. In contrast, lactase activity is considerably less than that of the other oligo­ saccharidases and is rate limiting for overall lactose digestion-absorption. The oligosaccharidases have a varying spatial distribution throughout the small intestine. In general, the abundance

and activity of oligosaccharidases peak in the proximal jejunum (i.e., at the ligament of Treitz) and are considerably less in the duodenum and distal ileum. Oligosaccharidases are absent in the large intestine. The distribution of oligosaccharidase activity parallels that of active glucose transport. These oligosaccharidases are affected by developmental and dietary factors in different ways. In many nonwhite ethnic groups, as well as in almost all other mammals, lactase activity markedly decreases after weaning in the postnatal period. The regulation of this decreased lactase activity is genetically determined.  N45-3  The other oligosaccharidases do not decrease in the postnatal period. In addition, long-term feeding of sucrose upregulates sucrase activity. In contrast, fasting reduces sucrase activity much more than it reduces lactase activity. In general, lactase activity is both more susceptible to enterocyte injury (e.g., following viral enteritis) and slower to recover from damage than is other oligosaccharidase activity. Thus, reduced lactase activity (as a consequence of both genetic regulation and environmental effects) has substantial clinical significance in that lactose ingestion may result in a range of symptoms in affected individuals (Fig. 45-4A, Box 45-1).

Chapter 45  •  Nutrient Digestion and Absorption

918.e1

N45-2  Oligosaccharidases Contributed by Emile Boulpaep and Walter Boron The oligosaccharidases are large integral membrane proteins that are anchored to the apical membrane by a transmembrane stalk; >90% of the protein is extracellular. Villous epithelial cells synthe­ size the disaccharidases via the secretory pathway (see pp. 34–35). The proteins undergo extensive N-linked and O-linked glycosylation in the Golgi and then traffic to the apical membrane. Sucrase-isomaltase is a special case. After the insertion of the single sucrase-isomaltase peptide (including its transmembrane stalk) into the brush-border membrane, pancreatic proteases cleave the peptide between the sucrase and isomaltase moieties. After this cleavage, the isomaltase moiety remains

continuous with the transmembrane stalk, and the sucrase moiety remains attached to the isomaltase moiety by van der Waals forces. Thus, sucrase-isomaltase differs from the other two oligosaccharidases in that the mature protein consists of two peptide chains (encoded by the same mRNA nonetheless), each with a distinct catalytic site and distinct substrate specificities. See eFigure 45-1 for a summary of the composition of sugars and oligosaccharides. As we saw in the text, sucrase is unique in splitting sucrose, and the isomaltase is unique in splitting the α-1,6 linkage of α-limit dextrins. The table lists the enzymatic specificities for each of the brush-border oligosaccharidases.

Specificities of Oligosaccharidases SUBSTRATES

TERMINAL α-1,4 LINKAGES

INTERNAL α-1,4 LINKAGES IN OLIGOSACCHARIDES UP TO 9 MONOMERS IN LENGTH

Maltase





Sucrase*



Isomaltase*



ENZYME

LACTOSE (SPLITTING THE β-1,4 LINKAGE BETWEEN D-GALACTOSE AND D-GLUCOSE)

Lactase



SUCRASE (SPLITTING α-1,2 LINKAGES BETWEEN D-GLUCOSE AND D-GALACTOSE)

α-1,6 (BRANCHING) LINKAGES OF α-LIMIT DEXTRINS

✓ ✓

*Sucrase and isomaltase are separate peptides, held together by van der Waals forces and anchored to the membrane via the transmembrane stalk of the isomaltase.

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SECTION VII  •  The Gastrointestinal System

N45-2  Oligosaccharidases—cont’d SUGAR

LINKAGE

COMPONENTS D-Galactose—D-Glucose

β-1,4*

Sucrose

D-Glucose—D-Fructose

α-1,2

Maltose

D-Glucose—D-Glucose

α-1,4

Isomaltose

D-Glucose

α-1,6

Maltotriose

D-Glucose—D-Glucose—D-Glucose

Both α-1,4

-limit dextrins

By definition, these are small D-glucose polymers that have been exhaustively digested by enzymes (e.g., amylase) that cannot attack -1,6 branch points, -1,4 linkages that are adjacent to -1,6 branch points, or terminal -1,4 linkages. According to the rules summarized in the text, these are all the possibilities:

Coupled mainly by 1,4 linkages, but containing one internal 1,6 linkage (highlighted in yellow) from a former branch point. Note: “internal” means that at least one 1,4 linkage separates the 1,6 linkage from an end.



Lactose

D-Glucose



D-Glucose—D-Glucose

D-Glucose—D-Glucose



D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose—D-Glucose

D-Glucose—D-Glucose



D-Glucose—D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose—D-Glucose



D-Glucose—D-Glucose—D-Glucose

D-Glucose—D-Glucose—D-Glucose—D-Glucose—D-Glucose

*For a 1,4 linkage, the linkage is between the number 1 carbon atom of the leftmost sugar in the “Components” column and the number 4 carbon atom of the rightmost sugar.

eFigure 45-1  Composition of common oligosaccharide.

Chapter 45  •  Nutrient Digestion and Absorption

N45-3  Lactose Intolerance Contributed by Henry Binder Some authors object to the statement that lactose intolerance in adults is a lactase “deficiency” and instead propose that the normal course of events is for lactase activity to decline after weaning. According to one view, lactase “persistence” evolved in certain human populations after the domestication of herd animals allowed the consumption of nonhuman milk. This hypothesis could account for the geographical distribution of lactose intolerance in humans.

918.e3

Chapter 45  •  Nutrient Digestion and Absorption

BOX 45-1  Lactase Deficiency

P

rimary lactase deficiency is most prevalent in nonwhites, and it also occurs in some whites. Primary lactase deficiency represents an isolated deficiency of lactase, with all other brush-border enzymes being at normal levels and without any histological abnormalities. Lactase activity decreases after weaning; the time course of its reduction is determined by hereditary factors. Ingestion of lactose in the form of milk and milk products by individuals with decreased amounts of small-intestinal lactase activity may be associated with a range of gastrointestinal symptoms, including diarrhea, cramps, and flatus, or with no discernible symptoms. Several factors determine whether individuals with lactase deficiency experience symptoms after ingestion of lactose, including rate of gastric emptying, transit time through the small intestine, and, most importantly, the ability of colonic bacteria to metabolize lactose to SCFAs,  N45-1  CO2, and H2. Figure 45-4A shows the rise of plasma [glucose] following the ingestion of either lactose or glucose in adults with normal lactase levels. This figure also shows that the [H2] in the breath rises only slightly following the ingestion of either lactose or glucose in individuals with normal lactase levels. Figure 45-4B shows that in individuals with primary lactase deficiency, the ingestion of lactose leads to a much smaller rise in plasma [glucose], although the ingestion of glucose itself leads to a normal rise in plasma [glucose]. Thus, no defect in glucose absorption per se is present, but simply a markedly reduced capacity to hydrolyze lactose to glucose and galactose. In lactase-deficient individuals, breath H2 is markedly increased after lactose ingestion because nonabsorbed lactose is metabolized by colonic bacteria to H2, which is absorbed into the blood and is subsequently excreted by the lungs. In contrast, the rise in breath H2 after the ingestion of glucose is normal in these individuals. Treatment for symptomatic individuals with primary lactase deficiency is reduction or elimination of consumption of milk and milk products or the use of milk products treated with a commercial lactase preparation.

CARBOHYDRATE ABSORPTION The three monosaccharide products of carbohydrate digestion—glucose, galactose, and fructose—are absorbed by the small intestine in a two-step process involving their uptake across the apical membrane into the epithelial cell and their coordinated exit across the basolateral membrane (see Fig. 45-3C). Na/glucose transporter 1 (SGLT1) is the membrane protein responsible for glucose and galactose uptake at the apical membrane. The exit of all three monosaccharides across the basolateral membrane uses a facilitated sugar transporter (GLUT2). Because SGLT1 cannot carry fructose, the apical step of fructose absorption occurs by the facilitated diffusion of fructose via GLUT5. Thus, although two different apical membrane transport mechanisms exist for glucose and fructose uptake, a single transporter (GLUT2) is responsible for the movement of both monosaccharides across the basolateral membrane.

919

BOX 45-2  Glucose-Galactose Malabsorption

M

olecular studies have been performed on jejunal mucosa from patients with so-called glucose-galactose malabsorption (or monosaccharide malabsorption). These individuals have diarrhea when they ingest dietary sugars that are normally absorbed by SGLT1. This diarrhea results from both reduced small-intestinal Na+ and fluid absorption (as a consequence of the defect in Na+-coupled monosaccharide absorption) and fluid secretion secondary to the osmotic effects of nonabsorbed monosaccharide. Eliminating the monosaccharides glucose and galactose, as well as the disaccharide lactose (i.e., glucose + galactose), from the diet eliminates the diarrhea. The monosaccharide fructose, which crosses the apical membrane via GLUT5, does not induce diarrhea. Early studies identified the abnormality in this hereditary disorder as a defect at the apical membrane that is presumably related to defective or absent SGLT1. Molecular studies of SGLT1 have revealed multiple mutations that result in single amino-acid substitutions in SGLT1, each of which prevents the transport of glucose by SGLT1 in affected individuals. Patients with glucose-galactose malabsorption do not have glycosuria (i.e., glucose in the urine), because glucose reabsorption by the proximal tubule normally occurs via both SGLT1 and SGLT2 (see p. 772).

SGLT1 is responsible for the Na+-coupled uptake of glucose and galactose across the apical membrane The uptake of glucose across the apical membrane via SGLT1 (Fig. 45-5A) represents active transport, because the glucose influx occurs against the glucose concentration gradient (see pp. 121–122). Glucose uptake across the apical membrane is energized by the electrochemical Na+ gradient, which in turn is maintained by the extrusion of Na+ across the basolateral membrane by the Na-K pump. This type of Na+-driven glucose transport is an example of secondary active transport (see p. 115). Inhibition of the Na-K pump reduces active glucose absorption by decreasing the apical membrane Na+ gradient and thus decreasing the driving force for glucose entry. The affinity of SGLT1 for glucose is markedly reduced in the absence of Na+. The varied affinity of SGLT1 for different monosaccharides reflects its preference for specific molecular configurations. SGLT1 has two structural requirements for monosaccharides: (1) a hexose in a D config­ uration, and (2) a hexose that can form a six-membered pyranose ring (see Fig. 45-5B). SGLT1 does not absorb L-glucose, which has the wrong stereochemistry, and it does not absorb D-fructose, which forms a five-membered ring (Box 45-2).  N45-4

The GLUT transporters mediate the facilitated diffusion of fructose at the apical membrane and of all three monosaccharides at the basolateral membrane Early work showed that fructose absorption is independent of Na+ but has characteristics of both a carrier-mediated and a passive process. These observations show that the small intestine has separate transport systems for glucose

Chapter 45  •  Nutrient Digestion and Absorption

N45-4  Na/Glucose Cotransporters Contributed by Emile Boulpaep and Walter Boron Because the membrane potential across the luminal membrane is 40 to 50 mV (cell interior negative), and intracellular [Na+] is far less than luminal [Na+], a “downhill” electrochemical Na+ gradient exists across the apical membrane that is the primary driving force for the uptake of glucose (and other actively transported monosaccharides) by SGLT1 (see pp. 121–122). Glucose uptake at the apical membrane has other characteristics of a carrier-mediated active transport process, including saturation kinetics, competitive inhibition, and energy dependence. SGLT1 belongs to the SLC5 family of transporters that couple Na+ to monosaccharides and other small molecules. These membrane proteins have 14 predicted membranespanning segments. The gene for SGLT1 has been localized to human chromosome 22. Kinetic studies of the SGLT1 expressed in host cells have confirmed many of the characteristics of the Na/glucose cotransport system that had been identified in native tissue. Expression studies have established that the Na+:sugar stoichiometry of SGLT1 is a 2 : 1 ratio. Its cousins SGLT2 and SGLT3 both have an Na+:sugar stoichiometry of 1 : 1. For a discussion of the stereospecificity of sugars, see the biochemistry text by Voet and Voet, page 254 (Fig. 10–4).

REFERENCES Voet D, Voet J: Biochemistry, ed 2. New York, Wiley, 1995. Wright EM, Turk E: The sodium/glucose cotransport family SLC5. Pflugers Arch 447:510–518, 2004.

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A

SECTION VII  •  The Gastrointestinal System

Epithelium

STRUCTURE OF SGLT1 Lumen

Extracellular space N

Interstitial space One of many brush-border peptidases

C

7

2

6 1

3

4

5

7

2

11

9

8

10

13 12

14

(AA)4

Gastric and pancreatic peptidases

Tripeptidase

(AA)3 Oligopeptides (AA)n

(AA)3

Proteins

Cytosol B

AA

H+

AA PepT1

+

Amino acids

AA

AA

(AA)2

(AA)2 H+

STRUCTURAL REQUIREMENTS OF SUGAR

+ AA

Dipeptidase

6

CH2OH

H

5 4

HO

OH 3

H

O H

2

H

AA

+ AA

AA

1

OH

OH

Pyranose ring in D configuration. Figure 45-5  Na+-coupled hexose transporter. A, The SGLT family of proteins has 14 membrane-spanning segments. This diagram represents the structure of the vSGLT Na/galactose cotransporter from the bacterium Vibrio parahaemolyticus. B, SGLT1 transports only hexoses in a D configuration and with a pyranose ring. This figure shows D-glucose; D-galactose is identical, except that the H and OH on carbon 4 are inverted. (A, Data from Faham S, Watanabe A, Besserer GM, et al: The crystal structure of a sodium galactose transporter reveals mechanistic insights into Na+/sugar symport. Science 321:810–814, 2008.)

and fructose. Subsequent studies established that facilitated diffusion is responsible for fructose absorption. Fructose uptake across the apical membrane is mediated by GLUT5, a member of the GLUT family of transport proteins (see p. 114). GLUT5 is present mainly in the jejunum.  N45-5 The efflux of glucose, fructose, and galactose across the basolateral membrane also occurs by facilitated diffusion. The characteristics of the basolateral sugar transporter, identified as GLUT2, are similar to those of other sugar transport systems in erythrocytes, fibroblasts, and adipocytes. GLUT2 has no homology to SGLT1 but is 41% identical to GLUT5, which is responsible for the uptake of fructose from the lumen.

PROTEIN DIGESTION Proteins require hydrolysis to oligopeptides or amino acids before absorption in the small intestine With the exception of antigenic amounts of dietary protein that are absorbed intact, proteins must first be digested

One of many AA transporters

Na+

Figure 45-6  Action of luminal, brush-border, and cytosolic peptidases. Pepsin from the stomach and the five pancreatic proteases hydrolyze proteins—both dietary and endogenous—to single amino acids, AA, or to oligopeptides, (AA)n. These reactions occur in the lumen of the stomach or small intestine. A variety of peptidases at the brush borders of enterocytes then progressively hydrolyze oligopeptides to amino acids. The enterocyte directly absorbs some of the small oligopeptides via the action of the H/oligopeptide cotransporter PepT1. These small peptides are digested to amino acids by peptidases in the cytoplasm of the enterocyte.

into their constituent oligopeptides and amino acids before being taken up by the enterocytes. Digestion-absorption occurs through four major pathways. First, several luminal enzymes (i.e., proteases) from the stomach and pancreas may hydrolyze proteins to peptides and then to amino acids, which are then absorbed (Fig. 45-6). Second, lumi­ nal enzymes may digest proteins to peptides, but enzymes present at the brush border digest the peptides to amino acids, which are then absorbed. Third, luminal enzymes may digest proteins to peptides, which are themselves taken up as oligopeptides by the enterocytes. Further digestion of the oligopeptides by cytosolic enzymes yields intracellular amino acids, which are moved by transporters across the basolateral membrane into the blood. Fourth, luminal enzymes may digest dietary proteins to oligopeptides, which are taken up by enterocytes via an endocytotic process (Fig. 45-7) and moved directly into the blood. Overall, protein digestion-absorption is very efficient; 7) do not have increased fecal nitrogen excretion. Five pancreatic enzymes (Table 45-2) participate in protein digestion and are secreted as inactive proenzymes. Trypsinogen is initially activated by a jejunal brush-border enzyme, enterokinase (enteropeptidase), by the cleavage of a hexapeptide, thereby yielding trypsin. Trypsin not only autoactivates trypsinogen but also activates the other pancreatic proteolytic proenzymes. The secretion of proteolytic enzymes as proenzymes, with subsequent luminal activation,

TABLE 45-2  Pancreatic Peptidases PROENZYME

ACTIVATING AGENT

ACTIVE ENZYME

ACTION

PRODUCTS

Trypsinogen

Enteropeptidase (i.e., enterokinase from jejunum) and trypsin

Trypsin

Endopeptidase

Oligopeptides (2–6 amino acids)

Chymotrypsinogen

Trypsin

Chymotrypsin

Endopeptidase

Oligopeptides (2–6 amino acids)

Proelastase

Trypsin

Elastase

Endopeptidase

Oligopeptides (2–6 amino acids)

Procarboxypeptidase A

Trypsin

Carboxypeptidase A

Exopeptidase

Single amino acids

Procarboxypeptidase B

Trypsin

Carboxypeptidase B

Exopeptidase

Single amino acids

Chapter 45  •  Nutrient Digestion and Absorption

N45-6  Pernicious Anemia Contributed by Henry Binder The close relationship between acid and gastrin release is clearly manifested in individuals with impaired acid secretion. In pernicious anemia, atrophy of the gastric mucosa in the corpus and an absence of parietal cells result in a lack in the secretion of both gastric acid and intrinsic factor (IF). Many patients with pernicious anemia exhibit antibody-mediated immunity against their parietal cells, and many of these patients also produce anti-IF autoantibodies. Because IF is required for cobalamin absorption in the ileum, the result is impaired cobalamin absorption. In contrast, the antrum is normal. Moreover, plasma gastrin levels are markedly elevated as a result of the absence of intraluminal acid, which normally triggers gastric D cells to release somatostatin (see pp. 868–870); this, in turn, inhibits antral gastrin release (see Box 42-1). Because parietal cells are absent, the elevated plasma gastrin levels are not associated with enhanced gastric acid secretion. The clinical complications of cobalamin deficiency evolve over a period of years. Patients develop megaloblastic anemia (in which the circulating red blood cells are enlarged), a distinctive form of glossitis, and a neuropathy. The earliest neurological findings are those of peripheral neuropathy, as manifested by paresthesias and slow reflexes, as well as impaired senses of touch, vibration, and temperature. If untreated, the disease will ultimately involve the spinal cord, particularly the dorsal columns, thus producing weakness and ataxia. Memory impairment, depression, and dementia can also result. Parenteral administration of cobalamin reverses and prevents the manifestations of pernicious anemia, but it does not influence parietal cells or restore gastric secretion of either IF or intraluminal acid.

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SECTION VII  •  The Gastrointestinal System

prevents pancreatic autodigestion before enzyme secretion into the intestine. Pancreatic proteolytic enzymes are either exopeptidases or endopeptidases and function in an integrated manner. Trypsin, chymotrypsin, and elastase are endopeptidases with affinity for peptide bonds adjacent to specific amino acids, so that their action results in the production of oligopeptides with two to six amino acids. In contrast, the exopeptidases—carboxypeptidase A and carboxypeptidase B—hydrolyze peptide bonds adjacent to the carboxyl (C) terminus, which results in the release of individual amino acids. The coordinated action of these pancreatic proteases converts ~70% of luminal amino nitrogen to oligopeptides and ~30% to free amino acids.

Brush-border peptidases fully digest some oligopeptides to amino acids, whereas cytosolic peptidases digest oligopeptides that directly enter the enterocyte Small peptides present in the small-intestinal lumen after digestion by gastric and pancreatic proteases undergo further hydrolysis by peptidases at the brush border (see Fig. 45-6). Multiple peptidases are present both on the brush border and in the cytoplasm of villous epithelial cells. This distribution of cell-associated peptidases stands in contrast to that of the oligosaccharidases, which are found only at the brush border. Because each peptidase recognizes only a limited repertoire of peptide bonds, and because the oligopeptides to be digested contain 24 different amino acids, large numbers of peptidases are required to ensure the hydrolysis of peptides. As we discuss below, a transporter on the apical membrane of enterocytes can take up small oligopeptides, primarily dipeptides and tripeptides. Once inside the cell, these oligopeptides may be further digested by cytoplasmic peptidases. The brush-border and cytoplasmic peptidases have substantially different characteristics. For example, the brush-border peptidases have affinity for relatively larger oligopeptides (three to eight amino acids), whereas the cytoplasmic peptidases primarily hydrolyze dipeptides and tripeptides. Because the brush-border and cytoplasmic enzymes often have different biochemical properties (e.g., heat lability and electrophoretic mobility), it is evident that the peptidases in the brush border and cytoplasm are distinct, independently regulated molecules. Like the pancreatic proteases, each of the several brushborder peptidases is an endopeptidase, an exopeptidase, or a dipeptidase with affinity for specific peptide bonds. The exopeptidases are either carboxypeptidases, which release C-terminal amino acids, or aminopeptidases, which hydrolyze the amino acids at the amino (N)–terminal end. Cyto­ plasmic peptidases are relatively less numerous.

PROTEIN, PEPTIDE, AND AMINO-ACID ABSORPTION Absorption of whole protein by apical endocytosis occurs primarily during the neonatal period During the postnatal period, intestinal epithelial cells absorb protein by endocytosis, a process that provides a mechanism

for transfer of passive immunity from mother to child. The uptake of intact protein by the epithelial cell ceases by the sixth month; the cessation of this protein uptake, called closure, is hormonally mediated. For example, administration of corticosteroids during the postnatal period induces closure and reduces the time that the intestine can absorb significant amounts of whole protein. The adult intestine can absorb finite amounts of intact protein and polypeptides. Uncertainty exists regarding the cellular route of absorption, as well as the relationship of the mechanism of protein uptake in adults to that in neonates. Enterocytes can take up by endocytosis a small amount of intact protein, most of which is degraded in lysosomes (see Fig. 45-7). A small amount of intact protein appears in the interstitial space. The uptake of intact protein also occurs through a second, more specialized route. In the small intestine, immediately overlying Peyer’s patches (follicles of lymphoid tissue in the lamina propria), M cells replace the usual enterocytes on the surface of the gut. M cells have few microvilli and are specialized for protein uptake. They have limited ability for lysosomal protein degradation; rather, they package ingested proteins (i.e., antigens) in clathrin-coated vesicles, which they secrete at their basolateral membranes into the lamina propria. There, immunocompetent cells process the target antigens and transfer them to lymphocytes to initiate an immune response. Although protein uptake in adults may not have nutritional value, such uptake is clearly important in mucosal immunity and probably is involved in one or more disease processes.

The apical absorption of dipeptides, tripeptides, and tetrapeptides occurs via an H+-driven cotransporter Virtually all absorbed protein products exit the villous epithelial cell and enter the blood as individual amino acids. Substantial portions of these amino acids are released in the lumen of the small intestine by luminal proteases and brush-border peptidases and, as we discuss below, move across the apical membranes of enterocytes via several amino-acid transport systems (see Fig. 45-6). However, substantial amounts of protein are absorbed from the intestinal lumen as dipeptides, tripeptides, or tetrapeptides and then hydrolyzed to amino acids by intracellular peptidases. The transporter responsible for the uptake of luminal oligopeptides (Fig. 45-8A) is distinct from the various aminoacid transporters. Furthermore, administering an amino acid as a peptide (e.g., the dipeptide glycylglycine) results in a higher blood level of the amino acid than administering an equivalent amount of the same amino acid as a monomer (e.g., glycine; see Fig. 45-8B). One possible explanation for this effect is that the oligopeptide cotransporter, which carries multiple amino acids rather than a single amino acid into the cell, may simply be more effective than amino-acid transporters in transferring amino-acid monomers into the cell. This accelerated peptide absorption has been referred to as a kinetic advantage and raises the question of the usefulness of the enteral administration of crystalline amino acids to patients with impaired intestinal function or catabolic deficiencies. The evidence for a specific transport process for dipeptides, tripeptides, and tetrapeptides comes from direct

Chapter 45  •  Nutrient Digestion and Absorption

A

B OLIGOPEPTIDE ABSORPTION Epithelium Lumen

923

“KINETIC ADVANTAGE” OF PEPTIDE ABSORPTION

Interstitial space PepT1 3 Na

Peptide

+

+

l yc Gl yc

Gl yc

ine

H+

ine

Na+

Peptidases

yl g

H

2K

+

Glycine appearance in blood

Figure 45-8  Absorption of oligopeptides. A, The H/oligopeptide cotransporter PepT1 moves dipeptides, tripeptides, and tetrapeptides into the enterocyte, across the apical membrane. Peptidases in the cytoplasm hydrolyze the oligopeptides into their constituent amino acids, which then exit across the basolateral membrane via one of three Na+-independent amino-acid transporters. B, If glycine is present in the lumen only as a free amino acid, then the enterocyte absorbs it only via apical amino-acid transporters. However, if the same amount of glycine is present in the lumen in the form of the dipeptide glycylglycine, the rate of appearance of glycine in the blood is about twice as high. Thus, PepT1, which moves several amino-acid monomers for each turnover of the transporter, is an effective mechanism for absorbing “amino acids.”

measurements of oligopeptide transport, molecular identification of the transporter, and studies of the hereditary disorders of amino-acid transport, cystinuria, and Hartnup disease. Oligopeptide uptake is an active process driven not by an Na+ gradient, but by a proton gradient. Oligopeptide uptake occurs via an H/oligopeptide cotransporter known as PepT1 (SLC15A1; see p. 123), which is also present in the renal proximal tubule. PepT1 also appears to be responsible for the intestinal uptake of certain dipeptide-like antibiotics (e.g., oral amino-substituted cephalosporins). As noted above, after their uptake, dipeptides, tripeptides, and tetrapeptides are usually hydrolyzed by cytoplasmic peptidases to their constituent amino acids, the forms in which they are transported out of the cell across the basolateral membrane. Because peptides are almost completely hydrolyzed to amino acids intracellularly, few peptides appear in the portal vein. Proline-containing dipeptides, which are relatively resistant to hydrolysis, are the primary peptides present in the circulation.

Amino acids enter enterocytes via one or more group-specific apical transporters Multiple amino-acid transport systems have been identified and characterized in various nonepithelial cells. The absorption of amino acids across the small intestine requires sequential movement across both the apical and basolateral membranes of the villous epithelial cell. Although the amino-acid transport systems have overlapping affinities for various amino acids, the consensus is that at least seven distinct transport systems are present at the apical membrane (see Table 36-1); we discuss the basolateral amino-acid

transporters in the next section. Whereas many apical amino-acid transporters are probably unique to epithelial cells, some of those at the basolateral membrane are probably the same as in nonepithelial cells. The predominant apical amino-acid transport system is system B0 (SLC6A19, SLC6A15; see Table 36-1) and results in Na+-dependent uptake of neutral amino acids. As is the case for glucose uptake (see p. 919), uphill movement of neutral amino acids is driven by an inwardly directed Na+ gradient that is maintained by the basolateral Na-K pump. The uptake of amino acids by system B0 is an electrogenic process and represents another example of secondary active transport. It transports amino acids with an L-stereo configuration and an amino group in the α position. System B0+ (SLC6A14) is similar to system B0 but has broader substrate specificity. System b0+ (SLC7A9/SLC3A1 dimer) differs from B0+ mainly in being independent of Na+. Other apical carrier-mediated transport mechanisms exist for anionic (i.e., acidic) α amino acids, cationic (i.e., basic) α amino acids, β amino acids, and imino acids (see Table 36-1). Because these transporters have overlapping affinities for amino acids, and because of species differences as well as segmental and developmental differences among the transporters, it has been difficult to establish a comprehensive model of apical membrane amino-acid transport in the mammalian small intestine (Box 45-3).

At the basolateral membrane, amino acids exit enterocytes via Na+-independent transporters and enter via Na+-dependent transporters Amino acids appear in the cytosol of intestinal villous cells as the result either of their uptake across the apical

924

SECTION VII  •  The Gastrointestinal System

BOX 45-3  Defects in Apical Amino-Acid Transport: Hartnup Disease and Cystinuria

H

artnup disease and cystinuria are hereditary disorders of amino-acid transport across the apical membrane. These autosomal recessive disorders are associated with both small-intestine and renal-tubule abnormalities (see Box 36-1) in the absorption of neutral amino acids in the case of Hartnup disease and of cationic (i.e., basic) amino acids and cystine in the case of cystinuria. The clinical signs of Hartnup disease are most evident in children and include the skin changes of pellagra, cerebellar ataxia, and psychiatric abnormalities. In Hartnup disease, the absorption of neutral amino acids by system B0 (SLC6A19) in the small intestine is markedly reduced, whereas that of cationic amino acids is intact (Fig. 45-9). The principal manifestation of cystinuria is the formation of kidney stones. In cystinuria, the absorption of cationic amino acids by system b0+ (SLC7A9/SLC3A1 dimer) is abnormal as a result of mutations in SLC7A9 or SLC3A1, but absorption of neutral amino acids is normal. Because neither of these diseases involves the oligopeptide cotransporter, the absorption of oligopeptides containing either

neutral or cationic amino acids is normal in both diseases. Only 10% of patients with Hartnup disease have clinical evidence of protein deficiency (e.g., pellagra) commonly associated with defects in protein or amino-acid absorption. The lack of evidence of protein deficiency is a consequence of the presence of more than one transport system for different amino acids, as well as a separate transporter for oligopeptides. Thus, oligopeptides containing neutral amino acids are absorbed normally in Hartnup disease, and oligopeptides with cationic amino acids are absorbed normally in cystinuria. These two genetic diseases also emphasize the existence of amino-acid transport mechanisms on the basolateral membrane that are distinct and separate from the apical amino-acid transporters. Thus, in both Hartnup disease and cystinuria, oligopeptides are transported normally across the apical membrane and are hydrolyzed to amino acids in the cytosol, and the resulting neutral and cationic amino acids are readily transported out of the cell across the basolateral membrane.

A HARTNUP DISEASE

B CYSTINURIA

L-Phenylalanine

L-Alanine

L-Arginine or cystine

L-Arginine or cystine

Amino acid transporter Oligopeptide cotransporter

L-PhenylalanylL-leucine L-ArginylL-leucine

L-PhenylalanylL-leucine

Oligopeptide cotransporter

L-ArginylL-leucine

Normal subjects Hartnup disease Amino acid absorption

Substrate

Amino acid transporter

Normal subjects Cystinuric patients Amino acid absorption

L-Phe

L-Arg

L-PhenylalanylL-leucine

L-Ala

L-Arg

L-ArginylL-leucine

Figure 45-9  Genetic disorders of apical amino-acid transport. A, In Hartnup disease, an autosomal recessive

disorder, the apical system B0 (SLC6A19) is defective. As a result, the absorption of neutral amino acids, such as L-phenylalanine, is reduced. However, the absorption of L-cystine (i.e., Cys-S-S-Cys) and cationic (i.e., basic) amino acids (e.g., L-arginine) remains intact. The enterocyte can absorb L-phenylalanine normally if the amino acid is present in the form of the dipeptide L-phenylalanyl-L-leucine, inasmuch as the oligopeptide cotransporter PepT1 is normal. B, In cystinuria, an autosomal recessive disorder, the apical system b0+ (SLC7A9/SLC3A1 dimer) is defective. As a result, the absorption of L-cystine and cationic amino acids (e.g., L-arginine) is reduced. However, the absorption of amino acids that use System B0 (e.g., L-alanine) is normal. The enterocyte can absorb L-arginine normally if the amino acid is present in the form of the dipeptide L-arginyl-L-leucine.

Chapter 45  •  Nutrient Digestion and Absorption

BOX 45-4  Defect in Basolateral Amino-Acid Transport:

Lumen

Epithelium

925

Interstitial space

Lysinuric Protein Intolerance

L

ysinuric protein intolerance is a rare autosomal recessive disorder of amino-acid transport across the basolateral membrane (Fig. 45-10). Evidence indicates impaired cationic amino-acid transport, and symptoms of malnutrition are seen. It appears that the defect is in system y+L, which is located solely on the basolateral membrane. System y+L has two subtypes, y+LAT1 (SLC7A7/SLC3A2 dimer) and y+LAT2 (SLC7A6/SLC3A2 dimer). Mutations in the SLC7A7 gene (subtype y+LAT1) cause the disease lysinuric protein intolerance. These patients exhibit normal absorption of cationic amino acids across the apical membrane. Unlike in Hartnup disease or cystinuria, in which the enterocytes can absorb the amino acid normally if it is presented as an oligopeptide, in lysinuric protein intolerance the enterocytes cannot absorb the amino acid regardless of whether the amino acid is “free” or is part of an oligopeptide. These observations are best explained by hypothesizing that the patients hydrolyze intracellular oligopeptides properly but have a defect in the transport of cationic amino acids across the basolateral membrane. This defect is present not only in the small intestine but also in hepatocytes and kidney cells, and perhaps in nonepithelial cells as well.

membrane or of the hydrolysis of oligopeptides that had entered the apical membrane (see Fig. 45-6). The enterocyte subsequently uses ~10% of the absorbed amino acids for intracellular protein synthesis. Movement of amino acids across the basolateral membrane is bidirectional; the movement of any one amino acid can occur via one or more amino-acid transporters. At least six amino-acid transport systems are present in the basolateral membrane (see Table 36-1). Three amino-acid transport processes on the basolateral membrane mediate amino-acid exit from the cell into the blood and thus complete the process of protein assimilation. Two other amino-acid transporters mediate uptake from the blood for the purposes of cell nutrition. The three Na+-independent amino-acid transport systems appear to mediate amino-acid movement out of the epithelial cell into blood. The two Na+dependent processes facilitate their movement into the epithelial cell. Indeed, these two Na+-dependent transporters resemble those that are also present in nonpolar cells. In general, the amino acids incorporated into protein within villous cells are derived more from those that enter across the apical membrane than from those that enter across the basolateral membrane. In contrast, epithelial cells in the intestinal crypt derive almost all their amino acids for protein synthesis from the circulation; crypt cells do not take up amino acids across their apical membrane (Box 45-4).

H

+

Lysine-XX Na+

Lysine-XX PepT1 Lysine

SLC7A7 defective

SLC3A2

Lysine

Figure 45-10  Genetic disorder of basolateral amino-acid transport. Lysinuric protein intolerance is an autosomal recessive defect in which the Na+-independent y+L amino-acid transporter on the apical and basolateral membranes is defective. However, the absence of apical y+L (SLC7A6/ SLC3A2 or SLC7A7/SLC3A2 dimers) does not present a problem because Na+-dependent amino-acid transporters can take up lysine, and PepT1 can take up lysine-containing oligopeptides (Lys-XX). However, there is no other mechanism for moving lysine out of the enterocyte across the basolateral membrane.

of oxygen. Some lipids also contain small but functionally important amounts of nitrogen and phosphorus (Fig. 45-11). Lipids are typified by their preferential solubility in organic solvents compared with water. A widely used indicator of the lipidic nature of a compound is its octanol-water partition coefficient, which for most lipids is between 104 and 107. The biological fate of lipids depends critically on their chemical structure as well as on their interactions with water and other lipids in aqueous body fluids (e.g., intestinal contents and bile). Thus, lipids have been classified according to their physicochemical interactions with water. Lipids may be either nonpolar and thus very insoluble in water (e.g., cholesteryl esters and carotene) or polar and thus interacting with water to some degree. Even polar lipids are only amphiphilic; that is, they have both polar (hydrophilic) and nonpolar (hydrophobic) groups. Polar lipids range from the insoluble, nonswelling amphiphiles (e.g., triacylglycerols) to the soluble amphiphiles (e.g., bile acids). Added in small amounts, insoluble polar lipids form stable monolayers on the surface of water (see Fig. 2-1C), whereas the soluble amphiphiles do not. The physicochemical behavior in bulk solution varies from insolubility—as is the case with triacylglycerols (TAGs) and cholesterol—to the formation of various macroaggregates, such as liquid crystals and micelles. Less-soluble lipids may be incorporated into the macroaggregates of the more polar lipids and are thus stably maintained in aqueous solutions.

Dietary lipids are predominantly TAGs

LIPID DIGESTION Natural lipids of biological origin are sparingly soluble in water Lipids in the diet are derived from animals or plants and are composed of carbon, hydrogen, and a smaller amount

The term fat is generally used to refer to TAGs—formerly called triglycerides—but it is also used loosely to refer to lipids in general. Of the fat in an adult diet, >90% is TAGs, which are commonly long-chain fatty acyl esters of glycerol, a trihydroxyl alcohol. The three esterification (i.e., acylation) positions on the glycerol backbone that are occupied by

926

B

OH C

C

GLYCEROL

H O

H

H

H

C

C

C

H

H

OH OH OH

CH2

CH3

F

PHOSPHATIDYLCHOLINE

G

H3C

N

+

CH3 H3C

O

O

C CH2

O

CH

CH2

C

O

CH2

C

O

O

O

C

O C

O C

CH2

CH2

CH3

CH3

H

H

O

CH2

CH3

H

H

H

H

C

C

C

O

O

C

O C

sn2-MONOACYLGLYCEROL H

H

H

H

H

C

C

C

OH O

OH O

C

CH2

CH2

CH2

CH3

CH3

CH3

I

CHOLESTEROL

CHOLESTERYL ESTER

J

H

OH O

CHOLIC ACID

HO

CH3

OH

+

N

CH3

CH3

CH3

OH HO

P

O C

CH2

CH

CH2 CH2 H3C

CH2

CH3

CH2

CH2

CH2

OH O

CH3

CH3 CH3



O

O

CH2

O

C

O



O

O CH

C

E

DIACYLGLYCEROL

CH2

O

CH2

H

CH2

CH2

P

H

CH3

CH2

O

H

LYSOPHOSPHATIDYLCHOLINE

CH3

D

TRIACYLGLYCEROL

CH2

CH2

C

O

O

FATTY ACID

O

A

SECTION VII  •  The Gastrointestinal System

C OH

CH CH3 CH3

CH3 CH 3 CH CH2 CH3

CH2

CH3 CH3

CH2 H3C

CH CH3

Figure 45-11  Chemical formulas of some common lipids. The example in A is stearic acid, a fully saturated

fatty acid with 18 carbon atoms. B shows glycerol, a trihydroxy alcohol, with hydroxyl groups in positions sn1, sn2, and sn3. In C, the left sn1– and center sn2–fatty acids are palmitic acid, a fully saturated fatty acid with 16 carbon atoms. The rightmost sn3–fatty acid is palmitoleic acid, which is also a 16-carbon structure, but with a double bond between carbons 9 and 10. In F, the left sn1–fatty acid is palmitic acid, and the right sn2–fatty acid is palmitoleic acid. In I, the example is the result of esterifying cholesterol and palmitic acid.

hydroxyl groups are designated sn1, sn2, and sn3, according to a stereochemical numbering system adopted by an international committee on biochemical nomenclature (see Fig. 45-11C–E). At body temperature, fats are usually liquid droplets. Dietary fat is the body’s only source of essential fatty acids, and its hydrolytic products promote the absorption of fat-soluble vitamins (the handling of which is

discussed on p. 933). Fat is also the major nutrient responsible for postprandial satiety. Typical adult Western diets contain ~140 g of fat per day (providing ~60% of the energy), which is more than the recommended intake of less than ~70 g of fat per day (2 km of bile ductules and ducts, with a volume of ~20 cm3 and a macroscopic surface area of

~400 cm2. Microvilli at the apical surface magnify this area by ~5.5-fold. As noted above, the canaliculi into which bile is secreted form a three-dimensional polygonal meshwork of tubes between hepatocytes, with many anastomotic interconnections (see Fig. 46-1). From the canaliculi, the bile enters the small terminal bile ductules (i.e., canals of Hering), which have a basement membrane and in cross section are surrounded by three to six ductal epithelial cells or hepatocytes (Fig. 46-4A). The canals of Hering then empty into a system of perilobular ducts, which, in turn, drain into interlobular bile ducts. The interlobular bile ducts form a richly anastomosing network that closely surrounds the branches of the portal vein. These bile ducts are lined by a layer of cuboidal or columnar epithelium that has microvillous architecture on its luminal surface. The cells have a prominent Golgi apparatus and numerous vesicles, which probably participate in the exchange of substances among the cytoplasm, bile, and blood plasma through exocytosis and endocytosis. The interlobular bile ducts unite to form larger and larger ducts, first the septal ducts and then the lobar ducts, two hepatic ducts, and finally a common hepatic duct (see Fig. 46-4B). Along the biliary tree, the biliary epithelial cells, or cholangiocytes, are similar in their fine structure except for size and height. However, as discussed below (see pp. 960–961), in terms of their functional properties, cholangiocytes and bile ducts of different sizes are heterogeneous in their expression of enzymes, receptors, and transporters. Increasing emphasis has been placed on the absorptive and secretory properties of the biliary epithelial cells, properties that contribute significantly to the process of bile formation. As with other epithelial cells, cholangiocytes are highly cohesive, with the lateral plasma membranes of contiguous cells

949

Chapter 46  •  Hepatobiliary Function

A

DUCTULES AND SMALL DUCTS Canaliculi

Terminal bile ductules

B

LARGE DUCTS AND GALLBLADDER

Perilobular bile duct

Right hepatic duct

Interlobular bile duct Liver

Left hepatic duct

Cystic duct

Common hepatic duct Common bile duct

Section of liver lobule

Pancreatic duct

Gallbladder Duodenum Ampulla (of Vater)

Hepatocytes

Sphincter of Oddi

Bile flow Figure 46-4  Structure of the biliary tree. A, The bile canaliculi, which are formed by the apical membranes of adjacent hepatocytes, eventually merge with terminal bile ductules (canals of Hering). The ductules eventually merge into perilobular ducts, and then interlobular ducts. B, The interlobular ducts merge into septal ducts and lobar ducts (not shown), and eventually the right and left hepatic ducts, which combine as the common hepatic duct. The confluence of the common hepatic duct and the cystic duct gives rise to the common bile duct. The common bile duct may merge with the pancreatic duct and form the ampulla of Vater before entering the duodenum, as shown in the figure, or have a completely independent lumen. In either case, there is a common sphincter—the sphincter of Oddi—that simultaneously regulates flow out of the common bile duct and the pancreatic duct.

forming tortuous interdigitations. Tight junctions seal contacts between cells that are close to the luminal region and thus limit the exchange of water and solutes between plasma and bile. The common hepatic duct emerges from the porta hepatis after the union of the right and left hepatic ducts. It merges with the cystic duct emanating from the gallbladder to form the common bile duct. In adults, the common bile duct is quite large, ~7 cm in length and ~0.5 to 1.5 cm in diameter. In most individuals, the common bile duct and the pancreatic duct merge before forming a common antrum known as the ampulla of Vater. At the point of transit through the duodenal wall, this common channel is surrounded by a thickening of both the longitudinal and the circular layers of smooth muscle, the so-called sphincter of Oddi. This sphincter constricts the lumen of the bile duct and thus regulates the flow of bile into the duodenum. The hormone cholecystokinin (CCK) relaxes the sphincter of Oddi via a nonadrenergic, noncholinergic neural pathway (see pp. 344–345) involving vasoactive intestinal peptide (VIP). The gallbladder lies in a fossa beneath the right lobe of the liver. This distensible pear-shaped structure has a capacity of 30 to 50 mL in adults. The absorptive surface of the gallbladder is enhanced by numerous prominent folds that

are important for concentrative transport activity, as discussed below. The gallbladder is connected at its neck to the cystic duct, which empties into the common bile duct (see Fig. 46-4B). The cystic duct maintains continuity with the surface columnar epithelium, lamina propria, muscularis, and serosa of the gallbladder. Instead of a sphincter, the gallbladder has, at its neck, a spiral valve—the valve of Heister—formed by the mucous membrane. This valve regulates flow into and out of the gallbladder.

UPTAKE, PROCESSING, AND SECRETION OF COMPOUNDS BY HEPATOCYTES The liver metabolizes an enormous variety of compounds that are brought to it by the portal and systemic circulations. These compounds include endogenous molecules (e.g., bile salts and bilirubin, which are key ingredients of bile) and exogenous molecules (e.g., drugs and toxins). The hepatocyte handles these molecules in four major steps (Fig. 46-5A): (1) the hepatocyte imports the compound from the blood across its basolateral (i.e., sinusoidal) membrane, (2) the hepatocyte transports the material within the cell, (3) the hepatocyte may chemically modify or degrade the

950 A

SECTION VII  •  The Gastrointestinal System

C

HEPATOCYTE Space of Disse

SECRETION OF BILE ACIDS AND SALTS Conjugated and unconjugated bile salts BA– H+ + BA– Charged Unconjugated (salt) BA-Z– + Na BA– Neutral OATP1B1, H.BA (acid) 1B3 NTCP MRP4 (ABCC4) Pool of BA– and BA-Z– HCO3– . H BA

Sinusoid endothelium 4

Bile canaliculus 2

1

3 Chemical modification

Binding protein

BA-Z– BP

Blood in sinusoid

+

BA-Y– BA-Z–

H+

Y–

BP.BA-Z– BA-Y– Hepatocyte

Bile canaliculus

BA– H2O

AA

MRP2 (ABCC2)

Z

B HOUSEKEEPING TRANSPORTERS Na+

BA–

+

Na Glucose

2 HCO3–

Na CO2

BSEP (ABCB11)

BA– or BA-Z–

+

BP GLUT2 H+

H2O CO2 OH–

HCO3–



SO4–

D

SECRETION OF OTHER ORGANIC ANIONS

Carbonic anhydrase

Other organic acid OA–

OATP1B1, 1B3

HCO3–

Bile salts Cholesterol Phospholipids Bilirubin Cl – HCO3–

OA–

OATP

MRP4 (ABCC4)



GSH–

HCO3

Y

Cl –

OA–

GSH–

Glutathione

OA-Y

GSH– 3 Na+

Ca++

+

Cl –

K

MRP2 (ABCC2)

GSH-Y

Bile canaliculus

H+

2 K+

BCRP (ABCG2)

Y +

Na

GSH–

S-Y

Y

S

Figure 46-5  Transporters in hepatocyte. A, The hepatocyte can process compounds in four steps: (1) uptake from blood across the basolateral (i.e., sinusoidal) membrane; (2) transport within the cell; (3) control chemical modification or degradation; and (4) export into the bile across the apical (i.e., canalicular) membrane. B, The hepatocyte has a full complement of housekeeping transporters. C, Bile acids can enter the hepatocyte in any of several forms: the unconjugated salt (BA−); the neutral, protonated bile acid (H ⋅ BA); or the bile salt conjugated to taurine or glycine (BA-Z−, where Z represents taurine or glycine). The three pathways for bile acid entry across the basolateral membrane are the Na+-driven transporter NTCP, which prefers BA-Z− but also carries BA−; nonionic diffusion of H ⋅ BA; and an OATP. Binding proteins (BPs) may ferry conjugated bile acids across the cytoplasm. Some bile acids are conjugated to sulfate or glucuronate (Y); these exit the cell across the canalicular membrane via the MRP2 (multidrug resistance–associated protein 2) transporter. Most bile acids are conjugated to glycine or taurine (Z) prior to their extrusion into the bile via BSEP. D, Organic anions (OA), including bile acids and bilirubin, may enter across the basolateral membrane via an OATP. After conjugation with sulfate or glucuronate (Y), these compounds may be extruded into the bile by MRP2. GSH synthesized in the hepatocyte, after conjugation to Y, can enter the canaliculus via MRP2. Unconjugated GSH can enter the canaliculus via an unidentified transporter. GSH can exit the hepatocyte across the basolateral membrane via an OATP. AA, amino acid.

Chapter 46  •  Hepatobiliary Function

compound intracellularly, and (4) the hepatocyte excretes the molecule or its product or products into the bile across the apical (i.e., canalicular) membrane. Thus, compounds are secreted in a vectorial manner through the hepatocyte.

An Na-K pump at the basolateral membranes of hepatocytes provides the energy for transporting a wide variety of solutes via channels and transporters Like other epithelial cells, the hepatocyte is endowed with a host of transporters that are necessary for basic housekeeping functions.  N46-2  To the extent that these transporters are restricted to either the apical or basolateral membrane, they have the potential of participating in net transepithelial transport. For example, the Na-K pump (see pp. 115–117) at the basolateral membrane of hepatocytes maintains a low [Na+]i and high [K+]i (see Fig. 46-5B). A basolateral Ca pump (see p. 118) maintains [Ca2+]i at an extremely low level, ~100 nM, as in other cells. The hepatocyte uses the inwardly directed Na+ gradient to fuel numerous active transporters, such as the Na-H exchanger, Na/HCO3 cotransporter, and Na+-driven amino-acid transporters. As discussed below, the Na+ gradient also drives one of the bile acid transporters. The hepatocyte takes up glucose via the GLUT2 facilitateddiffusion mechanism (see p. 114), which is insensitive to regulation by insulin. The basolateral membrane has both K+ and Cl− channels. The resting membrane potential (Vm) of −30 to −40 mV is considerably more positive than the equilibrium potential for K+ (EK) because of the presence of numerous “leak” pathways, such as the aforementioned electrogenic Na+-driven transporters as well as Cl− channels (ECl = Vm).

Hepatocytes take up bile acids, other organic anions, and organic cations across their basolateral (sinusoidal) membranes Bile Acids and Salts  The primary bile acids are cholic acid and chenodeoxycholic acid, both of which ar