PHARMACOLOGY

Adrenergic Pharmacology

Synthesis of Norepinephrine

Norepinephrine is synthesized from tyrosine, which is actively transported into the varicosity of the postganglionic

Figure 16-9 Biosynthesis of norepinephrine and epinephrine in sympathetic nerve terminal (and adrenal medulla). A, Perspective view of molecules. B, Enzymatic processes. (From Tollenaeré JP: Atlas of the Three-Dimensional Structure of Drugs. Amsterdam, Elsevier North-Holland, 1979 as modified by Vanhoutte PM: Adrenergic neuroeffector interaction in the blood vessel wall. Fed Proc 37:181, 1978.)

A series of steps results in the conversion of tyrosine to norepinephrine and epinephrine (in the adrenal medulla). The first of these steps involves tyrosine hydroxylase (TH). This cytoplasmic enzyme is the rate-controlling step for norepinephrine biosynthesis. High levels of norepinephrine inhibit TH, and low levels stimulate the enzyme. During sympathetic nervous system stimulation, an increased supply of tyrosine also increases synthesis of norepinephrine. TH activity is modified by phosphorylation. TH depends on a pteridine

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Dopamine can and does act as a neurotransmitter in some cells, but in most adrenergic neurons, dopamine is catabolized quickly by the enzyme monoamine oxidase (MAO), found particularly in mitochondria. Subsequently, dopamine is β-hydroxylated within the vesicles to norepinephrine by the enzyme dopamine β-hydroxylase (DBH). In the adrenal medulla and to a limited extent in discrete regions of the brain, another enzyme, phenylethanolamine N-methyl transferase (PNMT), methylates about 85% of the norepinephrine to epinephrine. Glucocorticoids from the adrenal cortex pass through the adrenal medulla and can activate the system, and stress-induced steroid release can increase epinephrine production. This local circulation amplifies the effects of glucocorticoid release.^[51]

Storage of Norepinephrine

Norepinephrine is stored within large, dense-core vesicles. Electron microscopy demonstrates that the dense cores in these vesicles are not filled with norepinephrine but perhaps with other binding proteins. The vesicles also contain calcium and a variety of peptides and ATP. Depending on the nature and frequency of physiologic stimuli, the ATP can be selectively released for an immediate postsynaptic effect through purinoreceptors.

Synaptic vesicles are heterogeneous and exist within functionally defined compartments. There appears to be an actively recycling population of synaptic vesicles and a reserve population of vesicles that is mobilized only on extensive stimulation. Newly synthesized or taken up transmitters are preferentially incorporated into the actively recycling vesicles and are preferentially released on stimulation. Drugs that mimic the neurotransmitter and are taken up presynaptically may be disproportionately represented in release. Functionally, norepinephrine is stored in compartments, of which 10% is readily releasable. In general, 1% of stored norepinephrine is released with each depolarization, implying a significant functional reserve. On stimulation, the contents of the vesicle are released into the synaptic cleft. Approximately 10% of stored norepinephrine is resistant to depletion, such as occurs with reserpine.

Synaptic vesicles have two fundamentally different functions: They take up and store neurotransmitters, and they fuse with and bud from the presynaptic plasma terminal membrane. The proteins of synaptic vesicles can be divided into two functionally discrete classes. The first class, transport proteins, provides the channels and pumps for the uptake and storage of neurotransmitters. The second class of proteins directs movement and docking reactions of the synaptic vesicle membrane.

Release of Norepinephrine

There are several different processes by which the contents of the vesicle enter the synaptic cleft. In exocytosis, the dominant physiologic mechanism of release, the vesicle responds to the entry of calcium by initiating vesicle docking, fusion, and endocytosis (the process by which vesicular membrane and proteins are recaptured) ( Fig. 16-10 ). ^[49] The entire contents of the vesicle are liberated on nerve stimulation. The biology of vesicular release is not a random event, but a highly differentiated process. The fact that exocytosis is so highly conserved from species to species indicates its biologic importance. Angiotensin II, prostacyclin, and histamine may potentiate release, whereas acetylcholine and prostaglandin E inhibit release. Because of its generalized importance in neurotransmitter release, the process of exocytosis has been extensively investigated.

In this model, the vesicle merges with the cell membrane, a process that depends on microfilaments and influenced by calcium. A widely accepted view of the role of synaptic vesicles in transmitter release is as follows. When an action potential reaches a nerve terminal, the presynaptic plasma membrane depolarizes, and the voltage-gated calcium channels open at the active zone. Although only small amounts of calcium are required to initiate the process, the concentration of calcium in the specialized zones of active release, nanodomains, is very high. This concept of calcium nanodomains is postulated to represent the biologic explanation underlying augmentation and post-tetanic potentiation^[52] in that the gradual diffusion of these high calcium levels from the nanodomains recruits new vesicles for release on subsequent stimulations (see Chapter 22 ). That calcium levels cause this phenomenon appears to be well accepted. ^[53] The ensuing rise in intracellular calcium triggers exocytosis of synaptic vesicles, resulting in the release of neurotransmitters.^[54] The synaptic vesicle membranes are reclaimed from the plasma membrane by endocytosis, and the vesicles eventually refill with neurotransmitters. A diagram of this process can be seen in Figure 16-11 . ^[55]

Various specific soluble and membrane-bound proteins have been identified that participate in docking, fusion, and endocytosis. Synaptotagmin serves as the intermediary between calcium entry and docking, because it binds calcium. ^[56] Microinjection of this protein participates in

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Figure 16-10 Release and reuptake of norepinephrine at sympathetic nerve terminals. aad, aromatic L-amino decarboxylase: DβH, dopamine β-hydroxylase; dopa, L-dihydroxyphenyalanine; NE, norepinephrine; tyr hyd, tyrosine hydroxylase; solid circle, active carrier. (From Vanhoutte PM: Adrenergic neuroeffector interaction in the blood vessel wall. Fed Proc 37:181, 1978 as modified by Shepherd J, Vanhoutte P: Neurohumoral regulation. In Shepherd S, Vanhoutte P [eds]: The Human Cardiovascular System: Facts and Concepts. New York, Raven Press, 1979, p 107.)

Figure 16-11 Pathway of synaptic vesicle movement in the nerve terminal. Synaptic vesicles accumulate neurotransmitters (NT) by active transport (stage I) and then move to the plasma membrane (stage II), where they become docked at the active zone (stage III). Calcium (Ca²⁺ ) influx after membrane depolarization triggers synaptic vesicle exocytosis and release of neurotransmitters (stage IV), after which the empty synaptic vesicles are endocytosed by clathrin-coated pits (stage V) and recycled (stage VI) by means of an endosomal intermediate (stage VII). Stages V and VII have not been definitely proved but are probable on the basis of morphologic observations. (From Südhof TC, Jahn R: Proteins of synaptic vesicles involved in exocytosis and membrane recycling. Neuron 6:665, 1991.)

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Exocytosis is distinct from the generalized secretory process in that it is far faster and independent of organelles such as the Golgi apparatus. The entire process of exocytosis, endocytosis, and vesicular reconstitution occurs in seconds.^{[66] [67] [68] [69]} Neurotransmitters from a single nerve terminal can be released 50 times per second, requiring a coordinated and tightly linked regulation of underlying biochemical processes. Proof for exocytosis as the dominant mechanism of release is derived from elegant experiments in which nerve stimulation caused release of the biosynthetic enzyme dopamine β-hydroxylase and norepinephrine in the same proportions that are contained within the vesicles.

Exocytosis accounts for most norepinephrine release, although leakage is continuous from the cytoplasm into the neuroeffector junction. Release is indirect with drugs such as ephedrine or bretylium, which displace norepinephrine from the vesicles. Drugs that inhibit vesicular uptake, such as reserpine, also facilitate indirect release. Norepinephrine leakage from storage vesicles into the axoplasm exceeds leakage into the presynaptic cleft by 100-fold and presynaptic reuptake by 10-fold, which explains the initial hypertensive effect of drugs such as reserpine that cause release of neurotransmitter from this compartment.^[70] Another hypothesis of vesicular release called kiss and run was proposed by Stevens and Williams.^[71] Although this mechanism is thought to account for less than 15% of the total release of norepinephrine, it may be functionally important in facilitating immediacy of transmission. Studies of cultured hippocampal cells found a resting pool of 180 vesicles and a recycling pool of approximately 25 vesicles, of which only 5 to 8 were in the readily releasable subset.^[72] This results in a very small active pool and a large inactive reserve pool.^{[60] [71]} After release, the fusion pores seal rapidly, and the vesicles immediately refill with transmitters, the so-called kiss and run subset.^[73]

Although chromaffin cells in the adrenal medulla synthesize epinephrine and norepinephrine, the two compounds are stored in and secreted from distinct chromaffin cell subtypes. Pharmacologic differences between cells containing norepinephrine and epinephrine have been described, and data suggest that there may be a preferential release from one or another form of chromaffin cell contingent on the nature of the stimulus.^[74] Nicotinic agonists or depolarizing agents may cause the preferential release of norepinephrine, whereas histamine elicits predominantly epinephrine release.^{[75] [76] [77]} Protein kinase C plays an important role in regulating catecholamine secretion from norepinephrine-containing chromaffin cells. ^[78]

Inactivation

Most of the norepinephrine released is rapidly removed from the synaptic cleft by an amine mechanism (i.e., uptake-1 mechanism) or by nonneuronal tissue (i.e., uptake-2 mechanism). If a transmitter is to exert fine control over an effector system, as when norepinephrine controls blood pressure through the baroreceptor reflex, its half-life in the biophase (i.e., the extracellular space close to the receptor) must be very short. The uptake-1 mechanism represents the first and most important step in the inactivation of released norepinephrine. Most released norepinephrine is transported into the storage vesicle for reuse. This neurotransmitter uptake into synaptic vesicles is driven by an electrochemical proton gradient across the synaptic vesicle membrane. The vacuolar proton pump is a large, hetero-oligomeric complex, containing eight to nine different subunits. After reuptake, the small amounts of norepinephrine not taken up into the vesicle are deaminated by MAO. There are several organ-specific forms of this enzyme.

Since its isolation and cloning in 1991, considerable information on the human norepinephrine transporter has been developed.^{[79] [80]} The pharmacologic characteristics of this binding protein identify it as the cocaine binding site, although tricyclic antidepressants (i.e., desipramine and nortriptyline) were also potent antagonists.

Uptake of norepinephrine into the nerve varicosity and its return to the storage vesicle, albeit efficient, is not specific for the neurotransmitter. Some compounds structurally similar to norepinephrine may enter the nerve by the same mechanism and may result in depletion of the neurotransmitter. These false transmitters can be of great clinical importance. Moreover, some drugs that block reuptake into the vesicle or into the synaptic ending itself may enhance response to catecholamines; that is, more norepinephrine is available to receptors. These drugs include cocaine and tricyclic antidepressants ( Table 16-5 ).

Activity of the uptake-1 system varies greatly among different tissues. Because of anatomic barriers, peripheral blood vessels have almost no reuptake of norepinephrine, but they have the highest rate of synthesis in the body. The highest rate of reuptake is found in the heart. Drugs or disease states that alter biosynthesis or storage (e.g., methyldopa decreases storage) would be expected to have a more profound effect on blood pressure; those that affect reuptake (e.g., cocaine) would be expected to affect cardiac rate and rhythm.

Typically, the lungs remove 25% of the norepinephrine that passes through their circulation, whereas epinephrine and dopamine pass through unchanged. Pulmonary uptake of norepinephrine appears to be a sodium-dependent, facilitated-transport process in the endothelial cells of precapillary and postcapillary vessels and pulmonary veins. There is no significant uptake by nerve endings. Pulmonary hypertension diminishes norepinephrine uptake, presumably because of concomitant thickening of the pulmonary vasculature.^[81] Uptake is diminished in patients with primary or secondary pulmonary hypertension and elevated pulmonary vascular resistance. Although the functional significance of the endothelial uptake mechanism of the pulmonary vasculature is unknown, uptake of other powerful vasoactive compounds suggests that the pulmonary endothelium functions to protect the left heart.

Defects in the ANS are common in patients with congestive heart failure (CHF). The heart is depleted of catecholamines, and the reuptake of norepinephrine is decreased.^[82] Sustained sympathoexcitation results in increased neuronal release of norepinephrine.^[83] Cardiac norepinephrine spillover rates differ widely, even among patients with end-stage heart failure awaiting cardiac transplantation, but some studies suggest that plasma

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Metabolism

During storage and reuptake, a small amount of norepinephrine escapes uptake into the nerve ending and enters the circulation, where it is metabolized by MAO or catechol-O-methyl transferase (COMT), or both, in the blood, liver, and kidney ( Fig. 16-12 ).^[88]

Epinephrine, which is released by the adrenal medulla, is inactivated by the same enzymes. The final metabolic product of inactivation is vanillylmandelic acid (VMA). The two catabolic enzymes and the vigorous uptake system account for an efficient clearance of catecholamines. Because of this rapid clearance, the half-life of norepinephrine (and most biogenic amines) in plasma is very short, less than 1 minute. This short half-life necessitates administration of these agents by infusion. Another consequence of their short half-life is that a more ideal measure of catecholamine production may be metabolic products, rather than catecholamines themselves. For example, screening for a norepinephrine-producing pheochromocytoma is frequently done by measuring urine metanephrine and VMA. Only a small percentage of norepinephrine appears in the urine for assay.

Inhibition of MAO would be expected to have a great impact on the sympathetic function of a patient. MAO inhibitors (MAOIs) are generally well tolerated, but the stability of the patient belies the fact that amine handling is fundamentally changed. Clinically important, life-threatening drug interactions are discussed in "Drugs and the Autonomic Nervous System."

Other compounds can be metabolized by catabolic enzymes to produce false transmitters. Although it is not used therapeutically, tyramine is the prototypic drug studied. Tyramine is present in many foods, particularly aged cheese and wines, and it can be synthesized from tyrosine. Tyrosine is decarboxylated in the liver and gut. Tyramine enters the sympathetic nerve terminal through the uptake-1 mechanism, displacing norepinephrine from the vesicles into the cytoplasm. Released norepinephrine leaks out from the cytoplasm and is responsible for the sympathomimetic effect of tyramine. However, a secondary effect can occur. In the vesicle, tyramine is converted by DBH into octopamine, which is eventually released as a false transmitter in place of norepinephrine, but without the expected effects, because it has only 10% of the potency of norepinephrine.^[89] Sodium plays a key role in the transport of norepinephrine into the cell.^[90]

Adrenergic Receptors

Ahlquist originally identified α- and β-adrenergic receptors by their different responses to pharmacologic agents. Initially, α-adrenergic receptors were distinguished from β-adrenergic receptors by their greater response to epinephrine and norepinephrine than to isoproterenol.^[24] The development of α- and β-antagonists further supported the existence of separate α-receptors. The advent of radioligand binding techniques signaled an era of pharmacology in which subtypes of receptors could be more readily assessed.

Traditionally, adrenergic receptors have been classified as α or β and more recently as α₁ , α₂ , β₁ , or β₂ based on available drugs. With the advent of molecular biology, classification evolved to three major subtypes and nine sub-subtypes^[91] ( Fig. 16-13 ). Justification for such a scheme is derived from pharmacologic analyses of drug-affinity patterns, functional differences in signal transduction

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Figure 16-12 Metabolism of catecholamines. (From Lake CR, Chernow B, Feuerstein G, et al: The sympathetic nervous system in man, its evaluation and the measurement of plasma norepinephrine. In Ziegler M, Lake C [eds]: Frontiers of Clinical Neuroscience, vol 2. Baltimore, Williams & Wilkins, 1984, p 1.)

Figure 16-13 Classification of adrenergic receptors. HR, heart rate.

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α-Adrenergic Receptors

Radioligand binding affinities have shown that at the α₁ -receptor prazosin is more potent than yohimbine; the reverse is true at α₂ -receptors. Functional and binding assays and molecular biologic methods have unequivocally confirmed the classification of α-adrenoceptors into subtypes.^[92] Within α₁ -adrenergic receptors, α_1A/D -, α_1B -, and α_1C -receptors have been characterized. Several α₂ -isoreceptors (α_2A , α_2B , and α_2C ) have been described. The α₂ -receptors can be expressed presynaptically or even in non-neuronal tissue. The α₂ -receptors are found in the peripheral nervous system, in the CNS, and in a variety of organs, including platelets, liver, pancreas, kidney, and eyes, where specific physiologic functions have been identified.^[93] Later, the predominant α₂ -receptor of the human spinal cord was identified as the α_2A -subtype.^{[91] [94]} It appears that mammalian genomes contain two sets of at least three unique genes encoding the α-adrenoceptors. The genes encoding α₂ -receptors have been localized in chromosomes 2, 4, and 10.

There is more than theoretical relevance in subclassification of receptors.^[95] For example, the α-adrenergic receptors in the prostate gland are predominantly α_1A . Therapy with selective α_1A -antagonists for benign prostatic hypertrophy may avoid some of the postural hypotension and other deleterious effects that occur with less specific α-antagonists. Localization of β₃ -receptors to fat cells suggests a new therapy for obesity. Polymorphism of this β-receptor subtype is associated with obesity and the potential for the development of diabetes. ^{[96] [97] [98]} Point mutations in genes encoding β₂ -receptors are correlated with decreased downregulation of β-receptors and nocturnal asthma.^{[99] [100]} Of particular clinical interest to anesthesiologists is the demonstration that mutations that decrease α_2C presynaptic function and enhance β₁ receptor linkage cause adrenergic hyperactivity and predispose to CHF.^{[101] [102]}

Amino acid sequence comparisons indicate that α-receptors are members of the seven transmembrane segment gene superfamily using G protein for signal

TABLE 16-6 -- Distribution of α- and β-receptors

Receptor	Distribution	Response	Agonist	Antagonist
α1	Smooth muscle	Constriction	Methoxamine	Prazosin
			Phenylephrine
α2	Presynaptic	Inhibit norepinephrine release	Clonidine	Yohimbine
			Dexmedetomidine
β1	Heart	Inotropy	Dobutamine	Metoprolol
		Chronotropy
β2	Smooth muscle	Dilation	Terbutaline
		Relaxation

transduction. A core of 175 amino acids constitutes the seven transmembrane regions that are highly conserved among different family members.^[103] The plethora of receptor subtypes remains incompletely explained, although the observation that different signal transduction mechanisms are used suggests finer control and physiologic significance. It may be important that there is considerable variability in α-adrenergic receptor subtypes among species.^[104]

Receptors can be presynaptic as well as postsynaptic. Presynaptic receptors may act as heteroreceptors or autoreceptors. An autoreceptor is a presynaptic receptor that reacts with the neurotransmitter released from its own nerve terminal, providing feedback regulation. A heteroreceptor is a presynaptic receptor that responds to substances other than the neurotransmitter released from that specific nerve terminal. This regulatory scheme is present throughout the nervous system but is particularly important in the sympathetic nervous system. ^[105]

Although several presynaptic receptors have been identified, the α₂ -receptor may be of the greatest clinical import. Presynaptic α₂ -receptors regulate the release of norepinephrine and ATP through a negative-feedback mechanism.^[106] Activation of presynaptic α₂ -receptors by norepinephrine inhibits subsequent norepinephrine release in response to nerve stimulation. Clonidine is a prototype α₂ -agonist. In the human brain, ligand studies reveal a high density of α₂ -receptors, particularly in cerebral cortex and medulla.^[107] This latter distribution may account for the bradycardiac and hypotensive responses to α₂ -agonist drugs. In summary, presynaptic α₂ -receptors and cholinergic receptors inhibit release, and presynaptic β-receptors stimulate release of norepinephrine.

β-Adrenergic Receptors

The structure of the β-adrenergic receptors was among the first to be ascertained and is well characterized. Like the α-receptor, the β-receptor is one of the superfamily of proteins that have seven helices woven through the cellular membrane. These transmembrane domains are labeled M₁ through M₇ ; antagonists have specific binding sites, whereas agonists are more diffusely attached to hydrophobic membrane-spanning domains ( Fig. 16-14 ). The extracellular portion of the receptor ends in an amino group. A carboxyl group occupies the intracellular terminus, where phosphorylation occurs. At these cytoplasmic domains, there is interaction with G proteins and kinases, including β-adrenergic receptor kinase. The β-receptor has

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Figure 16-14 Molecular structure of the β-adrenergic receptor. Notice the three domains. The transmembrane domains act as a ligand-binding pocket. Cytoplasmic domains can interact with G proteins and kinases, such as β-adrenergic receptor kinase (β-ARK). The latter can phosphorylate and desensitize the receptor. (From Opie L: Receptors and signal transduction. In Opie LH [ed]: The Heart: Physiology and Metabolism. New York, Raven Press, 1991.)

β-receptors have been further divided into β₁ -, β₂ -, and β₃ -subtypes, all of which increase cyclic adenosine monophosphate (cAMP) through adenylate cyclase and the mediation of G proteins. ^{[108] [109]} Traditionally, the β₁ -receptors were thought to be isolated to cardiac tissue, and β₂ -receptors were believed to be restricted to vascular and bronchial smooth muscle. Although this model of distribution is still useful because it reflects the primary clinical effects of pharmacologic manipulation of the β₁ - and β₂ -receptor subtypes, β₂ -receptors are more important in cardiac function than indicated by this model. The β₂ -receptor population in human cardiac tissue is substantial, accounting for 15% of the β-receptors in the ventricles and 30% to 40% in the atria.^[110] β₂ -receptors may help to compensate for disease by maintaining response to catecholamine stimulation when β₁ -receptors are downregulated during chronic catecholamine stimulation and in CHF.^[111] The β₂ -population is almost unaffected in end-stage congestive cardiomyopathy. ^[112] In addition to positive inotropic effects, β₂ -receptors in the human atria participate in the regulation of heart rate. The generation of cAMP in the human heart appears to be mediated primarily by β₂ -receptors, although this may be an artifact related to lability of β₁ -receptors.^[112] The β₂ -agonism may have significant effects on cardiac contractility and rate.^[111]

Dopamine Receptors

Dopamine exists as an intermediate in norepinephrine biosynthesis, and it exerts α- or β-adrenergic effects (depending on the dose administered). Goldberg and Rajfer^[113] demonstrated physiologically distinct dopamine type 1 (DA₁ ) and dopamine type 2 (DA₂ ) receptors, the most important of the five dopamine receptors cloned ( Fig. 16-15 ). DA₁ receptors are postsynaptic and act on renal, mesenteric, splenic, and coronary vascular smooth muscle to mediate vasodilation through stimulated adenylate cyclase and increased cAMP production. The vasodilatory effect tends to be strongest in the renal arteries. It is for this action, particularly the redistribution of renal blood flow, that dopamine is most frequently used. Additional renal DA₁ receptors located in the tubules modulate natriuresis through the sodium-potassium ATPase pump and the sodium-hydrogen exchanger.^{[113] [114] [115] [116]} The DA₂ receptors are presynaptic; they may inhibit norepinephrine and perhaps acetylcholine release. There are also central DA₂ receptors that may mediate nausea and vomiting. The antiemetic activity of droperidol is thought to be related to its DA₂ activity.

G Proteins

After adrenergic receptor stimulation, the extracellular signal is transformed into an intracellular signal by a process known as signal transduction in which α₁ - and β-receptors are coupled to G proteins. When activated, the G proteins can modulate the synthesis or the availability of intracellular second messengers ( Fig. 16-16 ). The activated second messenger diffuses through the cytoplasm and stimulates an enzymatic cascade. The sequence of first messenger → receptor → G protein → effector → second messenger → enzymatic cascade is found in a wide variety of cells; the specific entities that fulfill the separate roles vary from cell to cell.^[117] G proteins located on the inner surface of the cell membrane can also directly modify the activity of transmembrane ion channels.

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Figure 16-15 Location of dopamine-1 (DA₁ ) receptors, α₁ - and α₂ -adrenoceptors on postganglionic vascular effector cells, and DA₂ receptors and α₂ -adrenoceptors on the prejunctional sympathetic nerve terminal. When dopamine is administered, activation of DA₁ receptors causes vasodilation, whereas activation of DA₂ receptors causes inhibition (-) of norepinephrine (NE) release from storage granules. A larger dose of dopamine activates α₁ - and α₂ -adrenoceptors on the postjunctional effector cells to cause vasoconstriction and on α₂ -adrenoceptors on the prejunctional sympathetic terminal to inhibit release of NE. NE released from the prejunctional sympathetic terminal also acts on α₁ - and α₂ -adrenoceptors. (From Goldberg LI, Rajfer SI: Dopamine receptors: Applications in clinical cardiology. Circulation 72:245, 1985.)

The structure of G proteins has been the subject of intense scrutiny. Three types of subunits (α, β, and γ) have been described. The α-subunit is most variable and determines the activity of the protein, whether it is stimulatory (G_s ), inhibitory (G_i ), G_o , or G_q/11 .^{[118] [119]} The α-subunit may split off and behave independently, whereas the β- and γ-subunits remain together. Although 20 α-subunits, 5 β-subunits, and 6 γ-subunits have been cloned, the constellation of G proteins used by any individual receptor is more limited. Each class of adrenergic receptor couples to a different major subfamily of G proteins, which are linked to different effectors. The major subtypes of α₁ -, α₂ -, and β-receptors are linked to G_q , G_i ,

Figure 16-16 Epinephrine-stimulated glycogenolysis in a liver cell demonstrates the role of G proteins in cellular function. The first messenger (epinephrine) binds to its specific receptor, stimulating the G protein (in this case, Gs) to activate the effector, adenylyl cyclase. This enzyme converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP), the second messenger, which then triggers a cascade of enzymatic reactions that stimulates the enzyme phosphorylase (phos-a) to convert glycogen into glucose, which the cell extrudes. (From Linder ME, Gilman AG: G Proteins. Sci Am 267:56, 1992.)

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In its resting state, the G protein is bound to guanosine diphosphate (GDP) and is not in contact with the receptor. When the receptor is activated by the first messenger, it stimulates the G protein to release GDP and bind GTP to its α-subunit, activating itself. The bound GTP signals the G protein to split into two parts consisting of the α-GTP structure and the βγ-subunit. The released α-subunit binds to the effector and activates it, and then converts its attached GTP to GDP, returning itself to the resting state. The α-subunit joins with the βγ-unit, and the reconstructed G protein again waits at the inner membrane.

β-Receptor stimulation activates G proteins, enhancing adenylate cyclase activity and cAMP formation. The briefest encounter of plasma membrane β-adrenergic receptors with epinephrine or norepinephrine results in profound increases (up to 400-fold higher than the basal level within minutes) in the intracellular levels of cAMP. Increased cAMP synthesis activates protein kinases, which phosphorylate target proteins. Phosphorylation elicits various cellular responses that complete the path between receptor and effect. Stimulation of α₂ -receptors results in G_i inhibition of the adenylate cyclase. There is a relative abundance of G proteins, resulting in amplification of receptor agonism at the signal transduction step. The number of G protein molecules greatly exceeds the number of β-adrenergic receptors and adenylate cyclase molecules. It is the receptor concentration and ultimately adenylate cyclase activity that limit the response to catecholamines, perhaps explaining the efficacy of phosphodiesterase inhibitors. ^{[121] [122]}

Other transduction pathways are known. The α_1B -receptor acts through G proteins, but it activates phospholipase C in the inner cell membrane, which then increases hydrolysis of the diphosphate form of phosphoinositol to the triphosphate and diacylglycerol. These two compounds then mobilize intracellular calcium stores from the sarcoplasmic reticulum and probably the subsarcolemma, which markedly increases intracellular calcium ion concentration. Ultimately, the calcium ions bind to calmodulin. This calcium-sensitive intracellular protein then activates a myosin light chain kinase that phosphorylates the myosin light chain and facilitates the interaction between actin and myosin, resulting in the contraction of the smooth muscle. In other cells, calmodulin stimulates different kinases, eliciting different effector activity.

Myocardial cells respond to receptor stimulation differently depending on the identity of the first messenger. Two opposing effects, inhibition or stimulation of contractility, are produced by the sequence of receptor → G protein → effector → enzymatic cascade, but the identity of the chemicals in the sequence differs.^[123] Norepinephrine causes myocardial cells to contract with more vigor when the α-subunit of the stimulatory protein (G_s ) activates adenylate cyclase. The α-subunits of this protein cause potassium channels to open and permit efflux of potassium ion. The force of contraction is diminished when acetylcholine acts as a first messenger, stimulating its receptor to activate the inhibitory protein G_i or G_o . Clinically important, second-to-second changes in heart rate can be explained by the simultaneous activation of G_s and G_o . The current caused by G_o is larger than that of G_s , which explains the clinical impression that vagal inhibition of heart rate is augmented in the presence of sympathetic stimulation, such as may occur in unpremedicated patients.^[123]

Interaction of anesthetics with G proteins has been suggested as a mechanism for the negative inotropic effects of halothane and other volatile anesthetics. Halothane attenuates neurotransmitter release from peripheral sympathetic neurons,^{[124] [125] [126] [127]} but other important postsynaptic effects may be involved in its negative inotropic action. ^{[128] [129] [130] [131] [132]}

Although the changes in cAMP formation caused by halothane would be a plausible explanation for halothane's negative inotropic effects, studies suggest that this alteration is unrelated to adenylate cyclase.^[133] Halothane blocks slow calcium channels in the heart,^{[134] [135]} alters calcium fluxes in sarcoplasmic reticulum, ^{[136] [137]} and inhibits cAMP-dependent protein kinase.^[129] It appears that the negative inotropic effect of inhaled anesthetics occurs at several sites.

Upregulation and Downregulation

The β-adrenergic receptors are not fixed; they change significantly in dynamic response to the amount of norepinephrine present in the synaptic cleft or in plasma. For β-adrenergic receptors, this response is fast; within 30 minutes of denervation or adrenergic blockade, the number of receptors increases. This upregulation may explain why sudden discontinuation of β-adrenergic receptor blocking drugs causes rebound tachycardia and increases the incidence of MI and ischemia. Many chronic phenomena, such as varicose veins^[138] or aging, can decrease adrenergic receptor number or responsiveness systemically.

Clinically and at the cellular level, responses to many hormones and neurotransmitters wane rapidly despite continuous exposure to adrenergic agonists. ^[139] This phenomenon, called desensitization, has been particularly well studied for the stimulation of cAMP levels by plasma membrane β-adrenergic receptors.^[103] Mechanisms postulated for desensitization include uncoupling (e.g., phosphorylation), sequestration, and downregulation. The molecular mechanisms underlying rapid β-adrenergic receptor desensitization do not appear to require internalization of the receptors, but rather an alteration in the functioning of β-receptors themselves that uncouples the receptors from the stimulatory G_s protein. Agonist-induced desensitization involves phosphorylation of G protein-coupled receptors by two classes of serine-threonine kinases. One of these initiates receptor-specific or homologous desensitization. The other works through second messenger-dependent kinases, mediating a general cellular hyporesponsiveness, called heterologous desensitization. Ultimately, an inhibitory arrestin protein binds to the phosphorylated receptor, causing desensitization by blocking signal transduction. Because enzymatic phosphorylation occurs only in the activated state, transient β-blockade has been used in states of receptor desensitization such as CHF or cardiopulmonary bypass to achieve a "a receptor holiday."^{[12] [104] [140]} Regeneration of a functional β-adrenergic receptor is contingent on sequestration of the receptor, with dephosphorylation and presumed recycling. There has been some evidence

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Chronic CHF is one of the most important and best-studied pathophysiologic situations in which tolerance or downregulation occurs. It was initially observed that the density of cardiac β-receptors decreased markedly in patients with terminal heart failure in response to the elevated plasma catecholamine levels. This finding explained why administration of exogenous β-agonists was relatively ineffectual in this syndrome. With the demonstration that β₁ - and β₂ -receptors coexisted in human ventricles, Bristow and coworkers, ^[143] using radioligand techniques, documented that β₁ -receptor density decreased without change in the density of β₂ -receptors in human ventricles affected by CHF. Consequently, β₂ -agonism accounted for 60% of the total inotropic response stimulated by isoproterenol in the failing heart, compared with 40% in the nonfailing heart.^[144] Receptor polymorphisms associated with decreased α_2C presynaptic function and enhanced β₁ -G protein linkage lead to adrenergic hyperactivity and predispose to the development of CHF.^[102]

Figure 16-17 Synthesis of acetylcholine (ACh). Choline and acetyl coenzyme A (CoA) bind to the surface of choline acetyltransferase (ChAT). An imidazole on ChAT promotes proton removal and generates a more nucleophilic choline, facilitating condensation with the acetyl group of acetyl-CoA. The products of the reaction are coenzyme A and ACh, which are rapidly packaged in vesicles for immediate release on proper stimulation. ChAT also catalyzes the reverse reaction between ACh and CoASH, although at a much slower rate than the forward reaction. (From Doukas PH: Drugs affecting the parasympathetic nervous system. In Wingard L, Brody T, Larner J, et al [eds]: Human Pharmacology: Molecular to Clinical. St. Louis, Mosby-Year Book, 1991.)

Another disease in which adrenergic receptor function is altered is hyperthyroidism. The activity of the thyroid gland influences the receptor density, with hyperthyroidism increasing density and hypothyroidism decreasing density. There is some evidence that corticosteroids decrease receptor density.^[25] Consequently, the reaction of the body to well-characterized sympathetic agonists may be considerably different, depending on the pathologic and environmental circumstances. ^[145] However, the structural similarity of thyroid hormone and tyrosine suggests that false transmitters may play a role.^[146]