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Beta blockers and exercise: Physiologic and biochemical definitions and new concepts
Beta Blockers and Exercise: Physiologic and Biochemical Definitions and New Concepts DONALD C. HARRISON, MD
Exercise training effects occur in man afler chronic exposure to aerobic exercise, which should be defined quantitatively to permit a precise mechanistic understanding. Exercise adaptations result from circulatory and metabolic changes that involve altered responsiveness to neurohumoral transmitters at the receptor level. The adrenergic mechanisms are most important and are linked to new understandings of the adrenergic receptor and its coupling with biochemical processes in the cell. The adren-
ergic receptor is a specialized protein in the cell membrane linked to the catalytic moiety of the enzyme adenylate cyclase by a coupling protein controlled by guanine nucleotides. The sensitivity of this receptor mechanism may be altered by exposure to agonists and antagonists and by circulatory and metabolic diseases. The effects of /% adrenergic blockers on exercise adaptation and the clinical sequeiae are emphasized. (Am J Cardiol 1985;55:29D33D)
The importance of neurohumoral mechanisms in the physiological adaptation to muscular exercise has long been appreciated. Extensive investigations into the human response to exercise in both health and disease have been published; in this article the interaction between exercise and p blockers will be considered. After the concept of selective adrenergic stimulation and blockade through a series of specific receptors was introduced by Ahlquist in 194&r little progress in our understanding occurred until the early 196Os, when the first competitive &blocking agents were developed and introduced for pharmacologic and physiologic investigation.2*3 The clinical utility of these new agents in the treatment of patients with cardiovascular disease was soon recognized, and extensive studies in man were performed.M These investigations led to greater understanding of the role of the sympathetic nervous system in the biologic adaptation to stress. Initially the definition of p receptors had a pharmacologic basis but more recently biochemical techniques have permitted a tentative molecular biologic explanation for the action of hormones and drugs that stimulate or block the adrenergic nervous system at subcellular and cellular levels.7-e These concepts relate
exercise adaptation to new hypotheses for the role of the /3-adrenergic receptor system. The objectives of this article are to emphasize both the importance of standardizing definitions to understand the action of P-blocking drugs in exercise, and the importance of quantitating the metabolic, respiratory and circulatory responses to exercise in man. I will also discuss some of the new biochemical observations on the cellular and subcellular responses to adrenergic stimulation by pharmacologic agents and exercise. Exercise in Man With the development of sophisticated noninvasive techniques to evaluate man’s circulatory system during stress, many new findings relating to adaptation to exercise have been reported. The definitions of exercise and exercise capacity must now include a quantitative measure of the level of exercise and the period of time over which it occurs. It is also important to define upright versus supine exercise and delineate the role of exercise in regulating left ventricular function by changing heart rate, afterload and ejection fraction in normal subjects and patients with cardiovascular disorders. Considerable attention has been paid to the exercise training effect, which has been defined as having 2 impartant circulatory components. The first is a decrease in the heart rate at rest and an increase in the heart rate in response to a given quantitative exercise stress. The second is an increase in workload capacity in persons who show a training effect. In addition, training induces
From the Division of Cardiology, Stanfoid University School of kdicine, Stanford, California. This study was supported in part by Grants HL 29762-02 and HL 07265-07 from the Natiorial Institutes of Health, Betl-mda, Marylard, end a grant from the Stuart Pharmaceutical Company, Wilmington, Delaware. Address for reprints: Donald C. Harrison, MD, Division of Cardiology, Stanford University School of Medicine, Stanford, California 94305. 290
SOD
A SYMPOSIUM:
TABLE I
BETA
BLOCKERS
AND
EXERCISE
Factors Regulating Adrenerglc Receptors
Physiological Exercise Adrenergic agonists Denervation Glucocorticoids Thyroid hormone Estrogen and progesterone Maturation and aging Diseases or drugs Adrenergic antagonists lschemia Heart failure Hypertension and cardiac Alcohol withdrawal Psychotropic drugs
hypertrophy
changes in the metabolic and endocrine responses to exercise. Beta blockade would be expected to have considerable effects and produce changes antithetical for: responding acutely to exercise or obtaining an aerobic training effect, since the two major determinants of the heart’s oxygen consumption (which must increase for acute response to exercise or aerobic training effect) are the rise in pulse rate and blood pressure, which are attenuated by /3 blockade. The following questions should be kept in mind as new studies of exercise training in patients receiving P-blocking drugs are reported. What are the biochemical and physiological mechanisms accounting for the circulatory and metabolic training effects in man? Does the training effect depend on the type of exercise performed, its duration and its variability among persons? Is the training effect or the adaptation to exercise in normal persons modified by long-term administration of @-adrenergic blockers? Is the specificity of /lr to /?z blockade important? Do nonselective and selective &blocking drugs produce different cardiovascular and metabolic consequences ? Do patients with cardiac disease, ventricular dysfunction and cardiac failure differ from normal persons or from patients with mild hypertension in their response to exercise training after P-adrenergic blocker therapy? And, lastly, what rec-
ommendations can be made for exercise training in patients receiving P-adrenergic blocking drugs on a long-term basis? (Many are being treated for hypertension and for recurrent infarction after acute myocardial infarction, as well as angina and arrhythmias.) The Beta Receptor: A Pharmacologic Concept The concept of p blockade was initially developed by Ahlquist, who observed differential responses to a series of synthetic compounds with catecholamine activity on the circulatory system.r Because von Euler had already demonstrated that norepinephrine was the primary neurotransmitter for the adrenergic nervous system, a new explanation differing from Cannon’s concept of 2 neurotransmitters (sympathin I and sympathin E) to explain the inhibitory and stimulatory action of the adrenergic nervous system was necessary.lO During the 1950s a series of new agonists simulating the activity of the adrenergic nervous system was developed for both /l and (Y receptors. Specific blocking agents for (Y receptors were also developed. However, little progress was made in understanding the /3-adrenergic receptor concept until Powell and Slate+ introduced dichloroisoproterenol in 1958. Unfortunately this compound had both agonist and antagonist properties and did not receive much attention. The development by Black et al of pronethalo12 and propranoloP in the 1960s opened the door to a complete understanding of the pharmacologic concepts of p blockade. Beta blockade has been pharmacologically defined as the inhibition of heart rate changes in a whole organism produced by selected doses of isoproterenol. Several studies using isoproterenol to quantitate the degree of blockade permitted comparison of the activity of several new competitive P-blocking drugs during the 1960s. Exercise was also frequently used as a stimulus to determine the degree of blockade before and after the administration of one of these fl blockers. These definitions of 6 blockade and the potency ratio for various P-blocking drugs became commonplace in cardiovascular and metabolic investigations in man. Although it was appreciated that these pharmacologic responses had a biochemical basis, little progress was made toward their understanding until the mid-1970s. Beta Blockade: A Biochemical Concept In the 197Os, biochemical techniques using radioactive-labeled P-blocking agents, which could be bound
FIGURE 1. The hormone-responsive adenylate cyclase system. The coupling protein is regulated by guanine nucleotides such as guanosine triphosphate (GTP) being reduced to guanosine diphosphate (GDP). The catalytic moiety is for the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (c-AMP). lt is postulated that 2 structurally related receptors and coupling proteins exist, so that lnhibitory or stimulatory responses can .be induced through the same : system. Modified from Lefkowitz et al7 and reprinted with permission from N Engl J Med.
FIGURE 2. The biochemical interrelationships explaining the coupling of a hormone(H)respons adenylate cyclase system are shown in a ternary model. R = the receptor; C = the catalytic moiety of the enzyme adenylate cyclase; N = the coupling protein. Other abbreviations as in Figure 1. Reprinted with permission from N Engl J Med.
April 26. 1985
specifically to cellular and subcellular fractions of cells from the circulation or in various organs, permitted determination of the affinity of molecular structures in the membrane for the radiolabeled ligand of the antagonist.7J2J3 Thus the concept of “receptor density,” which is equated with numbers, permits a biochemical definition of receptor binding.13 In contrast, “affinity” defines the avidity of a cell or cell membrane for binding radiolabeled ligands of antagonists (e.g., alprenolol or pindolol for most of the adrenergic nervous system studies). The properties of both receptor density and affinity are derived from the binding of radiolabeled ligands of antagonists. These study techniques permitted the definition of the number of receptors on a given cell or group of cells as femtomole per gram of protein or per cell. It became clear that receptors and receptor density were not static but could change rapidly. While it was recognized that receptors could change their affinity or density substantially within a short period of time, the mechanisms by which receptors produce a biologic action are still not fully understood, but new studies that will be discussed outline the most likely and rational explanations. Some of the physiologic conditions, diseases and drugs that alter the receptor for adrenergic agonists are listed in Table I.‘Jpnr During the past 4 years, biochemical techniques have allowed selective proteins from cell membranes to be isolated and identified as receptors for the adrenergic nervous system.7Js21 Receptor sensitivity has been defined as the rate at which an agonist for the adrenergic nervous system produces activation of a cellular biological process. This hypothesis requires that the receptor (R) be coupled to an intermediatory coupling protein (N), regulated by guanine nucleotides and closely related to a cellular activating unit (effector system) likely to be adenylate cyclase (C) (Fig. 1). A mechanism for receptor-effector coupling has been hypothesized.lgp21 The components of the hormoneresponsive adenylate cyclase system have been identified:22 the hormone receptor, e.g., the cellular padrenergic receptor (R); the catalytic moiety of the enzyme adenylate cyclase (C), which converts adenosine triphosphate to cyclic adenosine monophosphate (CAMP); and a coupling protein (N) (which appears to be regulated by guanine nucleotides such as guanosine triphosphate)zs (Fig. 1). There are 2 forms of the coupling protein (Ns and Nr) that have either stimulatory or inhibitory effects when coupled to the catalytic moiety.7 These coupling proteins are in turn coupled to the stimulatory receptor protein (Rs) or to the inhibitory receptor protein (RI). The entire system is found in the plasma membrane7 and has been presented as a ternary-complex model for activation of adenylate cyclase by ,8-adrenergic agonists and guanine nucleotides (Fig. 2).24 Williams25 discusses this concept for a variety of polypeptide hormones and the adrenergic neurotransmitter elsewhere in this issue. At present, many receptors for neurohumoral substances can only be defined in terms of binding based on radioligand studies, or on their ability to activate a biological event based on sophisticated biochemical study. Only a few have been as well characterized as the
THE AMERICAN
Table II
JOURNAL
OF CARDIOLOGY
Volume
55
310
Mechanisms for Altering Receptor Responses
Change in the number of receptors (primary loss or secondary to desensitization) Altered sensitivity to agonist (desensitization) Uncoupling from effector components Internalization Intracellular processing and destruction of the receptor by lysosomal enzymes Covalent modification and uncoupling Phosphorylation?
coupling of the @-adrenergic receptor to the adenylate cyclase activity.*Receptor binding for agonist and antagonist by the same protein may be quite different. For the agonist the biological event is thought to result from the complex coupling of receptor with the agonist; in the adrenergic nervous system this is norepinephrine being bound to a & receptor in the myocardial cell. Acting through coupling proteins, CAMP is activated, resulting in an increase in contractile strength of the myocardial cell when stimulated. Physiological adaptation to various stresses or diseases may alter receptor number or sensitivity through a variety of means (Table II). The mechanisms by which regulation of the adrenergic receptor occurs have recently been elucidated.7~s There may be an altered number of receptors in the cell membrane, either because of a primary change in absolute numbers or desensitization of receptors. The altered cellular sensitivity to pharmacologic or hormonal stimulation is manifested clinically as desensitization, tolerance, tachyphylaxis or refractoriness.7 Many clinical examples of desensitization have been described, including the waning effects of fladrenergic stimulation in patients with congestive heart failure. Our group at Stanford has been particularly interested in adrenergic receptor alterations in patients with congestive heart failure.1s~26 We showed significant adrenergic receptor density reduction in myocardial cell membranes using ligand-binding studies that corresponded to the severity of left ventricular dysfunction.‘s In contrast, direct stimulation by fluoride (not through receptor activation) of adenylate cyclase activity did not show an effect. In these same patients, there was a reduced response to isoproterenol when compared with normal control subjects but no significant reduction in response with increasing calcium concentration in muscle bath solutions.26 This implies that only the receptor-mediated effects are altered, and those that bypass the receptor remain intact. Several mechanisms may explain desensitization. The first is that uncoupling of the receptor from its effector components is necessary to produce activation of adenylate cyclase (Fig. 3). This may result from the sequestration of the receptor by the cell membrane, uncoupling it from its effector elements in the cell surface7*gJs (Fig. 3). This sequestration into a cellular compartment is not well defined, but the receptor in the sequestrated compartment is removed from contact with the effector components of the system. Previous studiesisJ7 have shown the sequestered receptors to be fully intact but to be functionally inadequate because of their physical deprivation from the normal bio-
32D
A SYMPCYWM:
BETA
BLOCKERS
AND
EXERCISE
chemical effector. This occurs when receptors are exposed acutely to the agonist for that receptor. Once the agonist is removed, the receptor may recycle to the cell surface and become recoupled to its effector mechanisms.? If agonist-induced desensitization is permitted to occur for a long term, then the sequestered receptors are destroyed within the cell by lysosomal enzymes. Once this happens, recycling does not occur and new receptor synthesis is required for a restoration of action of the surface receptors.7 In avian erythrocyte models, a second mechanism for receptor desensitization without down regulation of receptors has been shown.2sp2g Long-term exposure to isoproterenol may result in uncoupling of the receptor from its effector mechanism without down regulation. This is mediated by phosphorylation of the /3 receptor by CAMP protein kinase or other kinases activated by the CAMP-dependent protein kinase.7p2g Thus the /I-adrenergic receptors are phosphorylated and are no longer coupled to the effector cell mechanisms for activating CAMP (Fig. 4). In other biological systems, phosphorylation of enzymes is a widespread mechanism for control of physiologic activities. However, the importance of the 2 mechanisms in changing the sensitivity of receptors without changing their numbers has not been fully defined in intact mammalian systems. There appear to be 2 operative mechanisms that control the function of P-adrenergic receptors during desensitization. They are: the removal of the receptor from the cell surface, which would be measured as a down regulation with the sequestration of the receptor away from the adenylate cyclase within the cell; or the alteration
PAR A. Correct
-
coupling hi
-
N -C
PAR B. Uncoupled
of the function of the receptor by a process of direct phosphorylation.7 Several important clinical implications can be drawn from this new biochemical understanding of agonistinduced receptor desensitization. First, agonist therapy for /3-adrenergic stimulation should be intermittent. For example, the use of salbutamol in treating bronchospasm should be intermittent to avoid tolerance. Second, the smallest dose of agonist that will produce the physiologic effect should be used to minimize development of tachyphylaxis. Understanding these mechanisms may also permit the development of a method for activating biochemical effector processes such as CAMP within the cell by circumventing the adrenergic receptor mechanisms entirely. Such an approach would permit new forms of treatment for patients with congestive heart failure. Finally, understanding of adrenergic receptor regulation permits hypotheses to explain a number of important clinical syndromes such as /3adrenergic withdrawal, adrenergic manifestation of hyperthyroidism, arrhythmic manifestations of ischemic heart disease and acceleration of congestive heart failure without further structural changes in the heart. With these concepts and definitions for the adrenergic nervous system in mind, the articles herein answer a series of questions relating to /I blockers and exercise. The questions are: (1) What are the mechanisms by which long-term exercise produces a training effect? (2) Are these training effects the result of alterations of adrenergic and her-monal receptors regulating metabolic and circulatory pathways for energy production? (3) Are alterations in the receptor number or in the sensitivity of receptors altered by long-term ,6 blockade? (4) Are these changes due to alterations in the number or affinity of receptors at the cell surface, or to a desensitization is outlined in Table II? In summary, our understanding of adrenergic receptor mechanisms has progressed from a pharmacologic to a biochemical definition. This in turn has allowed a better understanding of the myocardial cellular response in cardiovascular adaption to exercise and /I-ADRENERGIC
C.
Internalized 0
RECEPTOR COVALENT MODIFICATION
N -aA. Coupled to c-AMP
D.
ATP
Intracellular processing
c-A’MP
PAR B. Phosphofylation uncoupled
FIGURE 3. Desensitization of the P-adrenergic receptor. The & adrenergic receptor (PAR), the coupling protein (N), and the catalytic moiety of the enzyme adenylate cyclase (C) are shown in the cell membrane. Illustrated are normal coupling, uncoupled @AR from N and C, internalized PAR, and sequestered BAR, which are altered or destroyed by lysosomal enzymes if this state continues. Modified from Lefkowltz et al7 and reprinted with permission from N Engl J Med.
and =y-
N-Coo
Pea cl FIGURE 4. A second
mechanism for p-adrenergic receptor desensitization, involving phosphorylation of the receptor. which uncouples it from N and C. Abbreviations as in Figure 3. Modified from Lefkowitz et al7 and reprinted with permission from N Engl J Med.
April 28,198s
disease. Further, these receptor mechanisms play an important role in the endocrine and metabolic adaption to exercise, ‘and are also modified by /3 blockers. Thus, the mechanisms whereby exercise alters the biochemical pathways, permitting physiologic adaptation of the circulatory and metabolic systems, and how these are altered by p-blocker therapy, have important clinical implications. Perhaps, in better understanding these mechanisms, the basis for a new form of therapy for cells damaged by disease can be elucidated. Receptor mechanisms may help explain the postulated effects of exercise in primary and secondary prevention of atherosclerosis. Although our understanding is imprecise, it is advancing rapidly, and the interfaces between physiology and pharmacology with molecular biology and biochemistry are becoming less distinct.
5.
9. 7. 9. 9.
Study of adrenotropic
receptors.
14.
15. 19. 17. 19.
19. 20. 21.
References $l&J;u9RP.
13.
Am J Physiol
1948; 153:
Black Ji, Btephens~ JS. Pharmacology of new adrenergic beta-receptor-blocking compound (nethalide). Lancet 1992;231 I-314. Black JW, Crowther AF, Shanks RG, Smtth LH, Dornhcd AC. New adrenergic beta receptor antagonist. Lancet 199* 1: 1090-1084. Ha&on DC, Grlffl~ JR, Flene TJ. Effects of beta-adrenergic blockade wlth propranolol in patients wlth atrial arrhythmias. N Engl J Med 1995; 273410-415. H-n DC, Braunwakl E, Glkk 0, Mason DT, Chklsay CA, Ross J Jr. Effects of beta adrenergic blockade on the circulation, with particular reference to observations in oatients wlth hvoertroohlc subaortic stenosis. I. . Circulation 1994;29:84-98: Harriron DC, Grfffln JR. Metabolic and circulatory res nses to selective adrenergic stlrnulatlon and blockade. Circulation 196 r ;34:219-225. Lefkowfts RJ, Caron MO. Stiles GL. Mechanisms of membrane-receptor regutation: biochemical, physiological, and clinical insights derlved from studies of the adre ic receptors. N Engl J Med 1984;310:1570-1579. Leffrowftz RJ. Cllnica “B physiology of adrenerglc receptor regulation. Am J Physiol 1992;243:E43-E47. Stltes GL, Caron MO, Le(kowF RJ. The beta-adrenergic receptor: bio;;~rt mechanisms of physrologlcal regulation. Physlol Rev 1984;94:
10. Moran NC, Ptiins ME. Adrenerglc blockade of mammalian heart by dichforo anatogus of isoproterenol. J Pharmacol Exp Ther 1958; 124:2230. 11. PoweS CE, Sbter HI. BWing of inhibitory ackenergtc receptm by dichkxo anatog of isoproterenol. J Pharamacol Exp Thsr 1959;122:490-488. 12. Hoffman BB, Lefkowltz RJ. Radlollga~nd binding studies of adrenergic re-
THE AMERICAN
JOURNAL
OF CARDIOLOGY
Volume
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33D
ceptors: new insights into molecular and physiological regulation. Annu Rev Pharmacol Toxlcol 1980:20:581-608. Kant RS, DeLean A, Lefkowftz RJ. Guaitiiie analysis of beta-arkenergic receptor lnteractlons: resolution of high and low affinity states of the receptor by computer modeling of ligand binding data. Mol Pharmacol 1980;17: 14-23. S&Mel JM, DeLean A, Lefkowk RJ. A high affinky agonist Mgic receptor complex is an intermediate for catecholamine stimulation of adenytate cyciase in turkey and frog erythrocyte membranes. J Biol Chem 1980:255:1436-1441. Harden TK. Agonist-induced desenskixaticn of the beta-ackenergic receptor linked to adenylate cyclase. Pharmacol Rev 1983;35:5-32. Grlbnau TW, Vlssef J, Nlvard RJF, eds. Affinity Chromatography and Ra lated Techniques: Theoretical Aspects/Industrial and Biomedical Applications. New York: Elsevler, 1982. Gullfory RJ, Jeng SJ. Photoaffinity labeling: theory and practice. Fed Proc 1983;42:2826-2830. B&tow MR, Glnsburg R, Mlnobe W, Cublcclottl RS, Sageman WS, Lurle K, Bllllngham ME, HarrMn DC, Stlnson EB. Decreased catecholamine sensitivtty and beta-adrenergic-receptor density in failing human hearts. N Enal J Med 1982:307:205-2 11. Lefk~wltz RJ, Stabel JY, Caron MO. Adenylate cyclasacoupled betaadrenerglc receptors: structure and mechanisms of activation and desensitization. Annu Rev Blochem 1983;52:159-186. Hardan TK, Cotton CU, Waldo GL, Lutton JK, Perkins JP. Catecholamine-induced alteration in ths sedimentation behavior of membrane bound beta-adrener ic receptors. Science 1980;210:441-443. Ceriona RA, &r ulovkl B, Benovlc JL, Lefkowltz RJ, Canm MG The pure betaadrenerglc receptor: a single polypeptide confers catecholamine reg&o? to an adenylate cyclase system. Nature 1983;306:562-
22. Bu Y-F, Johnson GL, Cubeddo I, Leichlln BH, Otlmann R, Perklns JP. Regulation of adenosine 3’:5’-monophosphate content of human astrocytonia cells: mechanism of agonist-specific desensitization. J Cyclic N& cleotlde Res 1976:2:271-285. 23. Llmblrd LE, 0111 DM, Lefkowltz RJ. Agonist-promoted coupling of the beta-adrenergic receptor with the guanlne nucleotide regulatory protein ;;;he adenylate cyclase system. Proc Natl Acad SCI USA 1980;77:77524. De Lean A, Stadel JM, Lefkowltz RJ. A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-couplsd betaedreneraic receotor. J Biol Chem 1980:255:7108-7117. 25. Wllllams RS.-Role of receptor mechanisms in the exercise adaptive resoonse. Am J Cardiol1985:55:88D-73D. 29. &burg R, Brlstow YR, ~llllngham ME, Stlnson EB, Schroeder JS, Harrhran DC. Study of the normal failing isolated human heart: decreased r&r$onse of failing heart to isoproterenol. Am Hearl J 1983;106:53527. St&vkl B, StarJel JM, Lefkowftz RJ. Functional integrity of desensitized beta-adrenergic receptors: internalized receptors reconstitute catecholamine-stimulated adenylate cyctase activity. J Biol Chem 1983258: 6410-6414. 29. Glass DB, Krebs EG. Protein phosphorylation catalyzed by cyclic AMPdependent and cyclic GMPdependent protein kinases. Annu Rev Pharmacol Toxicol 1980;20:363-388. 29. Stadel JM, Nambl P, Shon RGL, Sawyer DF, Caron MO, Lefkowltz RJ. Catecholamine-induced desensitization of turkey ervthrocvte adenylate cyclase Is associated with phosphorytaticn of the beta&en&gii receptor. Proc Natl Acsd Sci 1983;80:3173-3177.
Exercise training effects occur in man afler chronic exposure to aerobic exercise, which should be defined quantitatively to permit a precise mechanistic understanding. Exercise adaptations result from circulatory and metabolic changes that involve altered responsiveness to neurohumoral transmitters at the receptor level. The adrenergic mechanisms are most important and are linked to new understandings of the adrenergic receptor and its coupling with biochemical processes in the cell. The adren-
ergic receptor is a specialized protein in the cell membrane linked to the catalytic moiety of the enzyme adenylate cyclase by a coupling protein controlled by guanine nucleotides. The sensitivity of this receptor mechanism may be altered by exposure to agonists and antagonists and by circulatory and metabolic diseases. The effects of /% adrenergic blockers on exercise adaptation and the clinical sequeiae are emphasized. (Am J Cardiol 1985;55:29D33D)
The importance of neurohumoral mechanisms in the physiological adaptation to muscular exercise has long been appreciated. Extensive investigations into the human response to exercise in both health and disease have been published; in this article the interaction between exercise and p blockers will be considered. After the concept of selective adrenergic stimulation and blockade through a series of specific receptors was introduced by Ahlquist in 194&r little progress in our understanding occurred until the early 196Os, when the first competitive &blocking agents were developed and introduced for pharmacologic and physiologic investigation.2*3 The clinical utility of these new agents in the treatment of patients with cardiovascular disease was soon recognized, and extensive studies in man were performed.M These investigations led to greater understanding of the role of the sympathetic nervous system in the biologic adaptation to stress. Initially the definition of p receptors had a pharmacologic basis but more recently biochemical techniques have permitted a tentative molecular biologic explanation for the action of hormones and drugs that stimulate or block the adrenergic nervous system at subcellular and cellular levels.7-e These concepts relate
exercise adaptation to new hypotheses for the role of the /3-adrenergic receptor system. The objectives of this article are to emphasize both the importance of standardizing definitions to understand the action of P-blocking drugs in exercise, and the importance of quantitating the metabolic, respiratory and circulatory responses to exercise in man. I will also discuss some of the new biochemical observations on the cellular and subcellular responses to adrenergic stimulation by pharmacologic agents and exercise. Exercise in Man With the development of sophisticated noninvasive techniques to evaluate man’s circulatory system during stress, many new findings relating to adaptation to exercise have been reported. The definitions of exercise and exercise capacity must now include a quantitative measure of the level of exercise and the period of time over which it occurs. It is also important to define upright versus supine exercise and delineate the role of exercise in regulating left ventricular function by changing heart rate, afterload and ejection fraction in normal subjects and patients with cardiovascular disorders. Considerable attention has been paid to the exercise training effect, which has been defined as having 2 impartant circulatory components. The first is a decrease in the heart rate at rest and an increase in the heart rate in response to a given quantitative exercise stress. The second is an increase in workload capacity in persons who show a training effect. In addition, training induces
From the Division of Cardiology, Stanfoid University School of kdicine, Stanford, California. This study was supported in part by Grants HL 29762-02 and HL 07265-07 from the Natiorial Institutes of Health, Betl-mda, Marylard, end a grant from the Stuart Pharmaceutical Company, Wilmington, Delaware. Address for reprints: Donald C. Harrison, MD, Division of Cardiology, Stanford University School of Medicine, Stanford, California 94305. 290
SOD
A SYMPOSIUM:
TABLE I
BETA
BLOCKERS
AND
EXERCISE
Factors Regulating Adrenerglc Receptors
Physiological Exercise Adrenergic agonists Denervation Glucocorticoids Thyroid hormone Estrogen and progesterone Maturation and aging Diseases or drugs Adrenergic antagonists lschemia Heart failure Hypertension and cardiac Alcohol withdrawal Psychotropic drugs
hypertrophy
changes in the metabolic and endocrine responses to exercise. Beta blockade would be expected to have considerable effects and produce changes antithetical for: responding acutely to exercise or obtaining an aerobic training effect, since the two major determinants of the heart’s oxygen consumption (which must increase for acute response to exercise or aerobic training effect) are the rise in pulse rate and blood pressure, which are attenuated by /3 blockade. The following questions should be kept in mind as new studies of exercise training in patients receiving P-blocking drugs are reported. What are the biochemical and physiological mechanisms accounting for the circulatory and metabolic training effects in man? Does the training effect depend on the type of exercise performed, its duration and its variability among persons? Is the training effect or the adaptation to exercise in normal persons modified by long-term administration of @-adrenergic blockers? Is the specificity of /lr to /?z blockade important? Do nonselective and selective &blocking drugs produce different cardiovascular and metabolic consequences ? Do patients with cardiac disease, ventricular dysfunction and cardiac failure differ from normal persons or from patients with mild hypertension in their response to exercise training after P-adrenergic blocker therapy? And, lastly, what rec-
ommendations can be made for exercise training in patients receiving P-adrenergic blocking drugs on a long-term basis? (Many are being treated for hypertension and for recurrent infarction after acute myocardial infarction, as well as angina and arrhythmias.) The Beta Receptor: A Pharmacologic Concept The concept of p blockade was initially developed by Ahlquist, who observed differential responses to a series of synthetic compounds with catecholamine activity on the circulatory system.r Because von Euler had already demonstrated that norepinephrine was the primary neurotransmitter for the adrenergic nervous system, a new explanation differing from Cannon’s concept of 2 neurotransmitters (sympathin I and sympathin E) to explain the inhibitory and stimulatory action of the adrenergic nervous system was necessary.lO During the 1950s a series of new agonists simulating the activity of the adrenergic nervous system was developed for both /l and (Y receptors. Specific blocking agents for (Y receptors were also developed. However, little progress was made in understanding the /3-adrenergic receptor concept until Powell and Slate+ introduced dichloroisoproterenol in 1958. Unfortunately this compound had both agonist and antagonist properties and did not receive much attention. The development by Black et al of pronethalo12 and propranoloP in the 1960s opened the door to a complete understanding of the pharmacologic concepts of p blockade. Beta blockade has been pharmacologically defined as the inhibition of heart rate changes in a whole organism produced by selected doses of isoproterenol. Several studies using isoproterenol to quantitate the degree of blockade permitted comparison of the activity of several new competitive P-blocking drugs during the 1960s. Exercise was also frequently used as a stimulus to determine the degree of blockade before and after the administration of one of these fl blockers. These definitions of 6 blockade and the potency ratio for various P-blocking drugs became commonplace in cardiovascular and metabolic investigations in man. Although it was appreciated that these pharmacologic responses had a biochemical basis, little progress was made toward their understanding until the mid-1970s. Beta Blockade: A Biochemical Concept In the 197Os, biochemical techniques using radioactive-labeled P-blocking agents, which could be bound
FIGURE 1. The hormone-responsive adenylate cyclase system. The coupling protein is regulated by guanine nucleotides such as guanosine triphosphate (GTP) being reduced to guanosine diphosphate (GDP). The catalytic moiety is for the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (c-AMP). lt is postulated that 2 structurally related receptors and coupling proteins exist, so that lnhibitory or stimulatory responses can .be induced through the same : system. Modified from Lefkowitz et al7 and reprinted with permission from N Engl J Med.
FIGURE 2. The biochemical interrelationships explaining the coupling of a hormone(H)respons adenylate cyclase system are shown in a ternary model. R = the receptor; C = the catalytic moiety of the enzyme adenylate cyclase; N = the coupling protein. Other abbreviations as in Figure 1. Reprinted with permission from N Engl J Med.
April 26. 1985
specifically to cellular and subcellular fractions of cells from the circulation or in various organs, permitted determination of the affinity of molecular structures in the membrane for the radiolabeled ligand of the antagonist.7J2J3 Thus the concept of “receptor density,” which is equated with numbers, permits a biochemical definition of receptor binding.13 In contrast, “affinity” defines the avidity of a cell or cell membrane for binding radiolabeled ligands of antagonists (e.g., alprenolol or pindolol for most of the adrenergic nervous system studies). The properties of both receptor density and affinity are derived from the binding of radiolabeled ligands of antagonists. These study techniques permitted the definition of the number of receptors on a given cell or group of cells as femtomole per gram of protein or per cell. It became clear that receptors and receptor density were not static but could change rapidly. While it was recognized that receptors could change their affinity or density substantially within a short period of time, the mechanisms by which receptors produce a biologic action are still not fully understood, but new studies that will be discussed outline the most likely and rational explanations. Some of the physiologic conditions, diseases and drugs that alter the receptor for adrenergic agonists are listed in Table I.‘Jpnr During the past 4 years, biochemical techniques have allowed selective proteins from cell membranes to be isolated and identified as receptors for the adrenergic nervous system.7Js21 Receptor sensitivity has been defined as the rate at which an agonist for the adrenergic nervous system produces activation of a cellular biological process. This hypothesis requires that the receptor (R) be coupled to an intermediatory coupling protein (N), regulated by guanine nucleotides and closely related to a cellular activating unit (effector system) likely to be adenylate cyclase (C) (Fig. 1). A mechanism for receptor-effector coupling has been hypothesized.lgp21 The components of the hormoneresponsive adenylate cyclase system have been identified:22 the hormone receptor, e.g., the cellular padrenergic receptor (R); the catalytic moiety of the enzyme adenylate cyclase (C), which converts adenosine triphosphate to cyclic adenosine monophosphate (CAMP); and a coupling protein (N) (which appears to be regulated by guanine nucleotides such as guanosine triphosphate)zs (Fig. 1). There are 2 forms of the coupling protein (Ns and Nr) that have either stimulatory or inhibitory effects when coupled to the catalytic moiety.7 These coupling proteins are in turn coupled to the stimulatory receptor protein (Rs) or to the inhibitory receptor protein (RI). The entire system is found in the plasma membrane7 and has been presented as a ternary-complex model for activation of adenylate cyclase by ,8-adrenergic agonists and guanine nucleotides (Fig. 2).24 Williams25 discusses this concept for a variety of polypeptide hormones and the adrenergic neurotransmitter elsewhere in this issue. At present, many receptors for neurohumoral substances can only be defined in terms of binding based on radioligand studies, or on their ability to activate a biological event based on sophisticated biochemical study. Only a few have been as well characterized as the
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Table II
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Mechanisms for Altering Receptor Responses
Change in the number of receptors (primary loss or secondary to desensitization) Altered sensitivity to agonist (desensitization) Uncoupling from effector components Internalization Intracellular processing and destruction of the receptor by lysosomal enzymes Covalent modification and uncoupling Phosphorylation?
coupling of the @-adrenergic receptor to the adenylate cyclase activity.*Receptor binding for agonist and antagonist by the same protein may be quite different. For the agonist the biological event is thought to result from the complex coupling of receptor with the agonist; in the adrenergic nervous system this is norepinephrine being bound to a & receptor in the myocardial cell. Acting through coupling proteins, CAMP is activated, resulting in an increase in contractile strength of the myocardial cell when stimulated. Physiological adaptation to various stresses or diseases may alter receptor number or sensitivity through a variety of means (Table II). The mechanisms by which regulation of the adrenergic receptor occurs have recently been elucidated.7~s There may be an altered number of receptors in the cell membrane, either because of a primary change in absolute numbers or desensitization of receptors. The altered cellular sensitivity to pharmacologic or hormonal stimulation is manifested clinically as desensitization, tolerance, tachyphylaxis or refractoriness.7 Many clinical examples of desensitization have been described, including the waning effects of fladrenergic stimulation in patients with congestive heart failure. Our group at Stanford has been particularly interested in adrenergic receptor alterations in patients with congestive heart failure.1s~26 We showed significant adrenergic receptor density reduction in myocardial cell membranes using ligand-binding studies that corresponded to the severity of left ventricular dysfunction.‘s In contrast, direct stimulation by fluoride (not through receptor activation) of adenylate cyclase activity did not show an effect. In these same patients, there was a reduced response to isoproterenol when compared with normal control subjects but no significant reduction in response with increasing calcium concentration in muscle bath solutions.26 This implies that only the receptor-mediated effects are altered, and those that bypass the receptor remain intact. Several mechanisms may explain desensitization. The first is that uncoupling of the receptor from its effector components is necessary to produce activation of adenylate cyclase (Fig. 3). This may result from the sequestration of the receptor by the cell membrane, uncoupling it from its effector elements in the cell surface7*gJs (Fig. 3). This sequestration into a cellular compartment is not well defined, but the receptor in the sequestrated compartment is removed from contact with the effector components of the system. Previous studiesisJ7 have shown the sequestered receptors to be fully intact but to be functionally inadequate because of their physical deprivation from the normal bio-
32D
A SYMPCYWM:
BETA
BLOCKERS
AND
EXERCISE
chemical effector. This occurs when receptors are exposed acutely to the agonist for that receptor. Once the agonist is removed, the receptor may recycle to the cell surface and become recoupled to its effector mechanisms.? If agonist-induced desensitization is permitted to occur for a long term, then the sequestered receptors are destroyed within the cell by lysosomal enzymes. Once this happens, recycling does not occur and new receptor synthesis is required for a restoration of action of the surface receptors.7 In avian erythrocyte models, a second mechanism for receptor desensitization without down regulation of receptors has been shown.2sp2g Long-term exposure to isoproterenol may result in uncoupling of the receptor from its effector mechanism without down regulation. This is mediated by phosphorylation of the /3 receptor by CAMP protein kinase or other kinases activated by the CAMP-dependent protein kinase.7p2g Thus the /I-adrenergic receptors are phosphorylated and are no longer coupled to the effector cell mechanisms for activating CAMP (Fig. 4). In other biological systems, phosphorylation of enzymes is a widespread mechanism for control of physiologic activities. However, the importance of the 2 mechanisms in changing the sensitivity of receptors without changing their numbers has not been fully defined in intact mammalian systems. There appear to be 2 operative mechanisms that control the function of P-adrenergic receptors during desensitization. They are: the removal of the receptor from the cell surface, which would be measured as a down regulation with the sequestration of the receptor away from the adenylate cyclase within the cell; or the alteration
PAR A. Correct
-
coupling hi
-
N -C
PAR B. Uncoupled
of the function of the receptor by a process of direct phosphorylation.7 Several important clinical implications can be drawn from this new biochemical understanding of agonistinduced receptor desensitization. First, agonist therapy for /3-adrenergic stimulation should be intermittent. For example, the use of salbutamol in treating bronchospasm should be intermittent to avoid tolerance. Second, the smallest dose of agonist that will produce the physiologic effect should be used to minimize development of tachyphylaxis. Understanding these mechanisms may also permit the development of a method for activating biochemical effector processes such as CAMP within the cell by circumventing the adrenergic receptor mechanisms entirely. Such an approach would permit new forms of treatment for patients with congestive heart failure. Finally, understanding of adrenergic receptor regulation permits hypotheses to explain a number of important clinical syndromes such as /3adrenergic withdrawal, adrenergic manifestation of hyperthyroidism, arrhythmic manifestations of ischemic heart disease and acceleration of congestive heart failure without further structural changes in the heart. With these concepts and definitions for the adrenergic nervous system in mind, the articles herein answer a series of questions relating to /I blockers and exercise. The questions are: (1) What are the mechanisms by which long-term exercise produces a training effect? (2) Are these training effects the result of alterations of adrenergic and her-monal receptors regulating metabolic and circulatory pathways for energy production? (3) Are alterations in the receptor number or in the sensitivity of receptors altered by long-term ,6 blockade? (4) Are these changes due to alterations in the number or affinity of receptors at the cell surface, or to a desensitization is outlined in Table II? In summary, our understanding of adrenergic receptor mechanisms has progressed from a pharmacologic to a biochemical definition. This in turn has allowed a better understanding of the myocardial cellular response in cardiovascular adaption to exercise and /I-ADRENERGIC
C.
Internalized 0
RECEPTOR COVALENT MODIFICATION
N -aA. Coupled to c-AMP
D.
ATP
Intracellular processing
c-A’MP
PAR B. Phosphofylation uncoupled
FIGURE 3. Desensitization of the P-adrenergic receptor. The & adrenergic receptor (PAR), the coupling protein (N), and the catalytic moiety of the enzyme adenylate cyclase (C) are shown in the cell membrane. Illustrated are normal coupling, uncoupled @AR from N and C, internalized PAR, and sequestered BAR, which are altered or destroyed by lysosomal enzymes if this state continues. Modified from Lefkowltz et al7 and reprinted with permission from N Engl J Med.
and =y-
N-Coo
Pea cl FIGURE 4. A second
mechanism for p-adrenergic receptor desensitization, involving phosphorylation of the receptor. which uncouples it from N and C. Abbreviations as in Figure 3. Modified from Lefkowitz et al7 and reprinted with permission from N Engl J Med.
April 28,198s
disease. Further, these receptor mechanisms play an important role in the endocrine and metabolic adaption to exercise, ‘and are also modified by /3 blockers. Thus, the mechanisms whereby exercise alters the biochemical pathways, permitting physiologic adaptation of the circulatory and metabolic systems, and how these are altered by p-blocker therapy, have important clinical implications. Perhaps, in better understanding these mechanisms, the basis for a new form of therapy for cells damaged by disease can be elucidated. Receptor mechanisms may help explain the postulated effects of exercise in primary and secondary prevention of atherosclerosis. Although our understanding is imprecise, it is advancing rapidly, and the interfaces between physiology and pharmacology with molecular biology and biochemistry are becoming less distinct.
5.
9. 7. 9. 9.
Study of adrenotropic
receptors.
14.
15. 19. 17. 19.
19. 20. 21.
References $l&J;u9RP.
13.
Am J Physiol
1948; 153:
Black Ji, Btephens~ JS. Pharmacology of new adrenergic beta-receptor-blocking compound (nethalide). Lancet 1992;231 I-314. Black JW, Crowther AF, Shanks RG, Smtth LH, Dornhcd AC. New adrenergic beta receptor antagonist. Lancet 199* 1: 1090-1084. Ha&on DC, Grlffl~ JR, Flene TJ. Effects of beta-adrenergic blockade wlth propranolol in patients wlth atrial arrhythmias. N Engl J Med 1995; 273410-415. H-n DC, Braunwakl E, Glkk 0, Mason DT, Chklsay CA, Ross J Jr. Effects of beta adrenergic blockade on the circulation, with particular reference to observations in oatients wlth hvoertroohlc subaortic stenosis. I. . Circulation 1994;29:84-98: Harriron DC, Grfffln JR. Metabolic and circulatory res nses to selective adrenergic stlrnulatlon and blockade. Circulation 196 r ;34:219-225. Lefkowfts RJ, Caron MO. Stiles GL. Mechanisms of membrane-receptor regutation: biochemical, physiological, and clinical insights derlved from studies of the adre ic receptors. N Engl J Med 1984;310:1570-1579. Leffrowftz RJ. Cllnica “B physiology of adrenerglc receptor regulation. Am J Physiol 1992;243:E43-E47. Stltes GL, Caron MO, Le(kowF RJ. The beta-adrenergic receptor: bio;;~rt mechanisms of physrologlcal regulation. Physlol Rev 1984;94:
10. Moran NC, Ptiins ME. Adrenerglc blockade of mammalian heart by dichforo anatogus of isoproterenol. J Pharmacol Exp Ther 1958; 124:2230. 11. PoweS CE, Sbter HI. BWing of inhibitory ackenergtc receptm by dichkxo anatog of isoproterenol. J Pharamacol Exp Thsr 1959;122:490-488. 12. Hoffman BB, Lefkowltz RJ. Radlollga~nd binding studies of adrenergic re-
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ceptors: new insights into molecular and physiological regulation. Annu Rev Pharmacol Toxlcol 1980:20:581-608. Kant RS, DeLean A, Lefkowftz RJ. Guaitiiie analysis of beta-arkenergic receptor lnteractlons: resolution of high and low affinity states of the receptor by computer modeling of ligand binding data. Mol Pharmacol 1980;17: 14-23. S&Mel JM, DeLean A, Lefkowk RJ. A high affinky agonist Mgic receptor complex is an intermediate for catecholamine stimulation of adenytate cyciase in turkey and frog erythrocyte membranes. J Biol Chem 1980:255:1436-1441. Harden TK. Agonist-induced desenskixaticn of the beta-ackenergic receptor linked to adenylate cyclase. Pharmacol Rev 1983;35:5-32. Grlbnau TW, Vlssef J, Nlvard RJF, eds. Affinity Chromatography and Ra lated Techniques: Theoretical Aspects/Industrial and Biomedical Applications. New York: Elsevler, 1982. Gullfory RJ, Jeng SJ. Photoaffinity labeling: theory and practice. Fed Proc 1983;42:2826-2830. B&tow MR, Glnsburg R, Mlnobe W, Cublcclottl RS, Sageman WS, Lurle K, Bllllngham ME, HarrMn DC, Stlnson EB. Decreased catecholamine sensitivtty and beta-adrenergic-receptor density in failing human hearts. N Enal J Med 1982:307:205-2 11. Lefk~wltz RJ, Stabel JY, Caron MO. Adenylate cyclasacoupled betaadrenerglc receptors: structure and mechanisms of activation and desensitization. Annu Rev Blochem 1983;52:159-186. Hardan TK, Cotton CU, Waldo GL, Lutton JK, Perkins JP. Catecholamine-induced alteration in ths sedimentation behavior of membrane bound beta-adrener ic receptors. Science 1980;210:441-443. Ceriona RA, &r ulovkl B, Benovlc JL, Lefkowltz RJ, Canm MG The pure betaadrenerglc receptor: a single polypeptide confers catecholamine reg&o? to an adenylate cyclase system. Nature 1983;306:562-
22. Bu Y-F, Johnson GL, Cubeddo I, Leichlln BH, Otlmann R, Perklns JP. Regulation of adenosine 3’:5’-monophosphate content of human astrocytonia cells: mechanism of agonist-specific desensitization. J Cyclic N& cleotlde Res 1976:2:271-285. 23. Llmblrd LE, 0111 DM, Lefkowltz RJ. Agonist-promoted coupling of the beta-adrenergic receptor with the guanlne nucleotide regulatory protein ;;;he adenylate cyclase system. Proc Natl Acad SCI USA 1980;77:77524. De Lean A, Stadel JM, Lefkowltz RJ. A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-couplsd betaedreneraic receotor. J Biol Chem 1980:255:7108-7117. 25. Wllllams RS.-Role of receptor mechanisms in the exercise adaptive resoonse. Am J Cardiol1985:55:88D-73D. 29. &burg R, Brlstow YR, ~llllngham ME, Stlnson EB, Schroeder JS, Harrhran DC. Study of the normal failing isolated human heart: decreased r&r$onse of failing heart to isoproterenol. Am Hearl J 1983;106:53527. St&vkl B, StarJel JM, Lefkowftz RJ. Functional integrity of desensitized beta-adrenergic receptors: internalized receptors reconstitute catecholamine-stimulated adenylate cyctase activity. J Biol Chem 1983258: 6410-6414. 29. Glass DB, Krebs EG. Protein phosphorylation catalyzed by cyclic AMPdependent and cyclic GMPdependent protein kinases. Annu Rev Pharmacol Toxicol 1980;20:363-388. 29. Stadel JM, Nambl P, Shon RGL, Sawyer DF, Caron MO, Lefkowltz RJ. Catecholamine-induced desensitization of turkey ervthrocvte adenylate cyclase Is associated with phosphorytaticn of the beta&en&gii receptor. Proc Natl Acsd Sci 1983;80:3173-3177.