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82
J AM COLL CARDIOL
1983; 1:82-9
Recent Advances in Knowledge About Beta-Adrenergic Receptors:
Application to Clinical Cardiology
AUGUST M. WATANABE, MD, FACC
Indianapolis. Indiana
The properties of beta-adrenergic receptors in the cardiovascular system have been studied in the past by two
experimental approaches, which can be termed pharmacologic and biochemical. In the pharmacologic approach, the nature of a drug interaction with receptors
is deduced from alterations in the physiologic properties
of the tissue caused by administration of various concentrations of the drug. Many important concepts about
beta-adrenergic receptors have come from such indirect
pharmacologic studies. The biochemical approach directly assesses the interaction of drugs with beta-adrenergic receptors by studying the binding of radiolabeled
antagonists and agonists with the receptor. This rela-
The properties of receptors for hormones. neurotransmitters
and drugs have come to be analyzed by two distinct experimental approaches, which generally can be termed pharmacologic and biochemical (I). The pharmacologic approach utilizes intact functioning organs or tissues which,
under specified experimental conditions. are exposed to a
series of drugs for the generation of concentration-response
data. The response measured is a change in a physiologic
characteristic of the organ or tissue under study. The data
are then analyzed and interpreted utilizing mathematical
models of drug-receptor interactions (l). Since the original
definition of beta-adrenergic receptors developed by Ahlquist (2) in 1948 with the pharmacologic approach. much
important information regarding beta-adrenergic receptors
has been gleaned from studies with functioning tissues. For
From the Krannert Institute of Cardiology, the Departments of Medicine
and Pharmacology and Toxicology. Indiana University School of Medicine, and the Richard L. Roudebush Veterans Administration Medical
Center. Indianapolis, Indiana.
The author's research cited in this paper was supported by the Herman
C. Krannert Fund. Indianapolis, Indiana, by grants HLI8795, HL06308,
HL07182 from the National Heart, Lung, and Blood Institute; National
Institutes of Health, Bethesda. Maryland, and by the American Heart
Association, Indiana Affiliate, Indianapolis, Indiana.
Address for reprints; August M. Watanabe, MD. Indiana University
School of Medicine, 1100 West Michigan Street. Indianapolis. Indiana
46223.
© 1983 by the American College of Cardiology
tively new approach has provided a large amount of new
information regarding the intrinsic properties of betaadrenergic receptors and modification of these properties by physiologic stresses, administration of drugs and
disease states. The biochemical approach has also been
applied recently to the study of beta-adrenergic receptors in human beings. In the future, substantial clinically
relevant new information regarding the nature of betaadrenergic receptors in physiologic and pathologic conditions should result from application of a combination
of the biochemical and physiologicapproaches to studies
in human beings.
example, the different chemical domains on catecholamine
molecules which determine the affinity of beta-adrenergic
receptors for these compounds as well as the intrinsic pharmacologic activity of the amines were distinguished by the
pharmacologic approach (3). The classification of beta-adrenergic receptors into two subtypes, designated beta) and
beta-, was originally accomplished by detailed concentration-response studies with intact tissues (4).
Although many such important concepts about beta-adrenergic receptors have been deduced from pharmacologic
studies, the development and application of the biochemical
approach to the study of beta-adrenergic receptors has initiated a new era of discovery concerning the intrinsic properties of these receptors. The technology for the biochemical
approach was first developed and validated only in 1974.
but the discoveries based on this approach already have been
voluminous. These recent discoveries dealt not only with
the fundamental biochemistry and molecular pharmacology
of receptors but also with clinically relevant findings in
animal models of disease or drug therapy and in human
beings. Thus, in a relatively short time the biochemical
approach to the study of adrenergic receptors has been translated into application to clinical studies. This brief review
focuses on new clinically relevant findings of particular
interest to cardiologists which have resulted from the application of the biochemical approach, sometimes coupled
with the pharmacologic approach, to the study of beta-ad0735-1097183/010082-08$03.00
CARDIOVASCULAR BETA-ADRENERGIC RECEPTORS
J AM COLL CARDIOL
1983;1:82-9
renergic receptors. More generalized discussions of recent
discoveries regarding beta-adrenergic receptors can be found
in numerous review articles (5-14).
The biochemical approach. This approach to the study
of beta-adrenergic receptors is similar in principle to that
of a variety of other hormone and neurotransmitter receptor
assays and to many of the principles of radioimmunoassay
(6,7,14,15). In all of these cases the interaction of a protein
(receptor or antibody) with a ligand (drug or antigen) is
examined. To allow detection and quantification of the interaction, the ligand is radiolabeled-thus the use of the
terms radiolabeled ligand binding assays and radioimmunoassays in describing these approaches. In the case of
receptor assays, a suitable organ or tissue source of receptors
is obtained, generally the organ is homogenized, portions
of the homogenate are then incubated with radiolabeled
ligand, unbound radiolabeled ligand is separated from that
which has interacted with receptors and the amount bound
to receptors is quantified by counting radioactivity (14, 15).
To verify that the radiolabeled ligand is bound to a receptor, certain criteria must be fulfilled (5-15). First, drugs
should compete with the radio labeled ligand for interaction
with the receptor having the same rank order of potency
they possess in intact tissue pharmacologic studies. Second,
the biologically active stereoisomer should compete with
the radiolabeled ligand for binding more potently than the
inactive stereoisomer. Third, it should be possible to saturate
all specific binding sites. Finally, only those organs or tissues that pharmacologic studies have shown to possess the
receptors of interest should bind the radiolabeled ligand
specifically. If these criteria of pharmacologic specificity,
stereospecificity, saturability and appropriate localization of
binding and certain other criteria are met, it can be assumed
with reasonable confidence that the radiolabeled ligand is
bound specifically to the receptor.
Application to cardiac tissues. Because the cardiovascular system is regulated by the sympathic nervous system.
many of the radiolabeled ligand binding studies of betaadrenergic receptors have been performed in cardiac tissues.
In fact, one of the first radiolabeled ligand binding studies
of beta-adrenergic receptors was done in homogenates of
canine heart (6). Subsequently, many studies of beta-adrenergic receptors derived from mammalian sources have
been done on the heart (17-20). Although the results of
many of these experiments in animals have been relevant
to clinical medicine, it is desirable to test some of the conclusions from animal studies directly in human beings. To
perform studies on beta-adrenergic receptors in human subjects, a readily available tissue source of receptors is required. The membranes of circulating leukocytes (polymorphonuclear leukocytes and lymphocytes) of human beings
and certain animals contain beta-adrenergic receptors that
can be studied directly with radio labeled ligand binding
assays (21). Accumulating evidence suggests that certain
83
properties of the receptors in circulating leukocytes, such
as density per unit surface area of membrane, reflect the
properties of receptors in cardiovascular tissues. For example, administration of propranolol to rats caused an increase in density of beta-adrenergic receptors not only in
lymphocytes but also in the ventricles and lungs of the same
animals from which the lymphocytes were taken (22). In
clinical studies. manipulations that altered the density of
beta-adrenergic receptors in circulating leukocytes changed
the heart rate response either to physiologic stress or to
administration of catecholamines in a manner consistent
with the conclusion that beta-adrenergic receptors in the
heart change in a qualitatively similar fashion as those on
leukocyte membranes. For example, in subjects who had
infusion of isoproterenol for 30 to 60 minutes. beta-adrenergic receptor density in leukocytes was increased and the
elevation in heart rate in response to a "pulse" infusion of
isoproterenol was augmented (23). Subjects treated with
propranolol had more leukocyte beta-adrenergic receptors
and significantly greater increases in heart rate and heart
rate-blood pressure product on standing (24). Persons whose
diets were changed from a low to a high sodium content
showed an elevation in leukocyte beta-adrenergic receptor
density as well as an increase in sensitivity to the positive
chronotropic effects of isoproterenol (25). Thus. it appears
to be possible to examine the effects of diseases and drugs
on the beta-adrenergic receptors of the cardiovascular system in human beings by studying the receptors on the membranes of circulating leukocytes. However, the validity of
this approach will have to be verified carefully in each
clinical setting in which it is utilized.
Clinically Relevant Information From
Radiolabeled Ligand Binding Studies of
Beta-Adrenergic Receptors
Verification of Physiologic Principles
Beta, and beta, receptors. The pharmacologic subclassification of beta-adrenergic receptors into the subtypes
beta, and beta, has facilitated the development of subtypespecific agonists and antagonists. Thus, clinicians now have
the option of using beta.vselective antagonists to treat angina
pectoris, certain cardiac arrhythmias, hypertension and other
cardiovascular diseases with reduced concern about blocking beta--receptors in bronchioles and peripheral arterioles.
Similarly, bronchoconstriction can be treated with beta2selective agonists with less concern about producing tachycardia from stimulation of beta.-receptors in the heart. Thus.
the subclassification of beta-receptors is not only of pharmacologic interest, but also has important applications in
the clinical management of patients. Although this subclassification was originally proposed on the basis of indirect
pharmacologic studies (4). the true nature of subtypes of
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beta-receptors was not revealed until the advent of radiolabeled ligand binding studies. This biochemical approach
has confirmed directly the existence of two subtypes of betaadrenergic receptors (20,26-28) and more recently suggested that the molecular properties of beta,- and betar
receptors probably differ (29). Thus, the two subtypes of
beta-adrenergic receptors appear to be distinct molecular
entities (29). Biochemical studies, therefore, have verified
at the molecular level the concept of beta-adrenergic receptor subtypes that was originally deduced only from indirect
pharmacologic studies.
Elucidation of changes in responsiveness to sympathetic stimulation. Changes in responsiveness of animals
or human beings to sympathetic stimulation after treatment
with certain drugs, after certain surgical procedures such as
denervation or in association with certain diseases have been
observed by pharmacologists and clinicians for many years.
With the development of biochemical assays of beta-adrenergic receptors, some of the mechanisms for these changes
are being elucidated (11). It is now established that betaadrenergic receptors in plasma membranes of cells are dynamic entities. Physiologic stresses and pharmacologic or
pathologic perturbations can result in alterations in the number of beta-adrenergic receptor binding sites, changes in the
affinity of receptors for catecholamines or modulation of
the "coupling" of the receptor to its proximate biochemical
effector enzyme, adenylate cyclase (11). As a broad generalization, the density of beta-adrenergic receptors on target
organs appears to be inversely related to the magnitude of
stimulation of those receptors by agonists (10-13). Thus,
when sympathetic nervous system activity (either neural
activity or circulating catecholamines, or both) is high, the
density of beta-adrenergic receptors in target organs will
tend to be low. This decrease in receptor number has been
termed down regulation. Conversely, in situations where
sympathetic nervous system activity is low, beta-adrenergic
receptor density will tend to be high. This is called up
regulation. Recent clinical studies have shown that such
changes in beta-adrenergic receptor density can occur even
in response to normal physiologic alterations in sympathetic
activity (25). Normal volunteers were fed low and high
sodium diets either to stimulate or to suppress sympathetic
nervous system activity. which was monitored by measurement of plasma and urinary catecholamine levels (25). When
sodium intake was increased from 10 to 400 mEq daily, a
change that reduced sympathetic nervous system activity,
the density of beta-adrenergic receptors on leukocyte membranes increased by approximately 50% (25). That the changes
in leukocyte receptor density reflected receptor changes in
the cardiovascular system was suggested by the observation
that the subjects' sensitivity to the positive chronotropic
effects of isoproterenol was also increased (25).
An additional observation was that the density of betaadrenergic receptors on leukocytes of these normal subjects
was inversely correlated with circulating and 24 hour urinary
catecholamine levels (25). The levels of catecholamines in
the blood and urine presumably reflected sympathetic nerve
activity. Thus. the higher the level of sympathetic nervous
system activity (the greater the concentration of catecholamines in the receptor milieu), the more down regulation
had occurred (that is, the lower the density of receptors).
These results provide a plausible explanation for the substantial interindividual variation in density of receptors on
leukocytes of normal subjects (25,30). They also provide
one possible explanation for the differences in response of
individual patients to certain drugs (see later).
Changes with aging. Resting heart rate and cardiac output tend to diminish with age (31,32). It is reasonable to
hypothesize that one possible mechanism for this trend is a
change in beta-adrenergic receptors in the heart. It has been
observed that elderly persons have decreased responsiveness
to infusions of isoproterenol (33) and a lesser number of
beta-adrenergic receptors on lymphocytes (34). A possible
mechanism for reduced numbers of beta-adrenergic receptors with senescence is diminished rate of receptor synthesis.
In senescent rats, the rate of synthesis of beta-adrenergic
receptors in the heart was lower than that in young rats (35).
It is thus possible that some of the cardiac functional and
electrophysiologic changes that occur with aging are related
to changes in beta-adrenergic receptor density in the heart,
although this subject needs to be investigated further.
These are examples of concepts originally derived from
indirect pharmacologic studies that have been verified or
illuminated by application of direct biochemical studies of
beta-adrenergic receptors. It is likely that other physiologic
concepts will be verified directly as more studies are done
in the future.
Alterations of Cardiovascular Beta-Adrenergic
Receptors in Disease
Cardiovascular disease. Myocardial ischemia. A few
studies have examined changes in beta-adrenergic receptors
in the heart or blood vessels in cardiovascular disease. In a
canine model of experimental myocardial ischemia (36), the
number of beta-adrenergic receptors was found to be increased in the ischemic myocardium in association with a
marked reduction in tissue norepinephrine content. The affinity of the receptors for the radiolabeled antagonist was
unchanged. The density of muscarinic receptors in the same
region was also unchanged. In a subsequent study (37), it
was found that the responsiveness of the ischemic region to
isoproterenol infusion was enhanced. In response to the
catecholamine, tissue cyclic adenosine monophosphate
(cAMP) levels were elevated more and phosphorylase was
activated to a greater extent in the ischemic region, which
had increased beta-adrenergic receptor density, than in the
normally perfused control region (37). The mechanism for
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1983;1:82-9
the increase in beta-adrenergic receptor density, whether
related to a direct effect of ischemia or secondary to the
functional sympathetic denervation, is as yet unknown. Also,
the pathophysiologic significance of these findings remains
to be established. However, these are interesting results that
show a relatively rapid, selective change in beta-adrenergic
receptor density in ischemic myocardium. One could speculate that these changes may participate in the generation
of cardiac arrhythmias during myocardial ischemia.
Ventricular arrhythmias. In a cat model of myocardial
ischemia and reperfusion, a variety of experiments suggested that ventricular arrhythmias that occurred during coronary reperfusion were related to enhanced alpha-adrenergic
responsiveness (38). In this model, radiolabeled ligand binding studies demonstrated a twofold increase in density of
alpha.vadrenergic receptors early after occlusion of the coronary artery and during the first few minutes of reperfusion
(39). In this model, beta-adrenergic receptor density did not
change (39). It was hypothesized that the change in alphaladrenergic receptor density might be related to the enhanced
alpha-adrenergic receptor responsiveness that appeared to
cause the lethal arrhythmias during reperfusion. The mechanism for the change in alpha-adrenergic receptor density
and the reason for the difference between the results in this
study and those in the previously mentioned canine study
are unknown. These experiments in dogs and cats were the
first attempts to study changes in beta-adrenergic receptors
in myocardial ischemia. The ultimate significance of these
results and their relevance to coronary artery disease in
human beings remains to be established. However, in view
of the large incidence of coronary artery disease and the
wide use of beta-adrenergic receptor blocking agents, which
may modify receptor density in these patients, this should
be an important and fruitful area of future research.
Heart failure. The properties of beta-adrenergic receptors in the failing heart have been assessed in ventricles
taken from human heart transplant recipients (40). Receptors
in ventricular tissue of transplant donors were studied as
controls. It was found that the density of beta-adrenergic
receptors in failing left ventricles was approximately 50%
less than that in donor control nonfailing hearts (40). This
reduction in receptor density was accompanied by a diminution in isoproterenol-mediated stimulation of adenylate
cyclase activity and muscle contraction (40). It seems possible that this reduction in receptor density is related to an
increase in circulating and neurally released catecholamines
that is known to occur in response to heart failure (down
regulation). The reduced beta-adrenergic receptor density
may account, at least partially, for the diminished contractile
function reserve seen in some forms of chronic heart failure.
Hypertension. The properties of beta-adrenergic receptors in cardiac and vascular tissues have been examined in
the spontaneously hypertensive rat. This animal model of
hypertension is known to have abnormally high efferent
85
sympathetic nerve activity that presumably contributes to
the hypertension (41-43). the density of beta-adrenergic
receptors in the heart and vascular tissues of these animals
was reduced modestly compared with the density in normotensive control animals (44,45). It seems likely that these
changes in density are related to the increased stimulation
of receptors by catecholamines as a result of the augmented
nerve activity (down regulation). Whether these changes (in
arterioles, for example) contribute to the hypertension remains unknown.
Significance of the changes in beta-adrenergic receptors. Beta-adrenergic receptors have now been studied in
several cardiovascular diseases, and the receptors do seem
to change in number in these pathologic states. Many more
studies will have to be done, in both animal models and
human beings, before the significance of these changes will
be established. Questions that will have to be addressed
include: I) What is the general relevance of the findings
from animal models? Are these results applicable to the
clinical situation, or are they species-specific or even specific to the model used? 2) Are changes in receptors causally
related to the cardiovascular diseases with which they are
associated? Are they parallel events that contribute to the
clinical manifestations of the disease? Or, are the changes
merely parallel alterations without any pathophysiologic significance? 3) In conditions such as heart failure and hypertension which have multiple pathologic mechanisms and
etiologies, are receptor changes similar or qualitatively different depending on the etiology or mechanism of the disease? 4) What will be the utility and validity of studying
beta-adrenergic receptors in readily available blood elements
(leukocytes) and, from these data, drawing conclusions about
receptors in cardiac or vascular tissues?
Noncardiovascular disease. Hyperthyroidism. Some
diseases that do not primarily occur in cardiac or vascular
tissues are known to alter cardiac responsiveness to sympathetic nervous system stimulation. Perhaps one of the best
known of such associations is that seen with altered thyroid
state, particularly hyperthyroidism. Many of the clinical
features of hyperthyroidism are suggestive of a hyperactive
sympathetic nervous system. Certain of these features, such
as tachycardia, can be readily treated with antisympathetic
drugs such as reserpine or beta-adrenergic receptor blocking
agents. However, many studies done over the years failed
to demonstrate any clear evidence of excessive sympathetic
nervous system activity; thus the role of the sympathetic
nervous system in the clinical features of hyperthyroidism
has remained controversial (46). Data from radiolabeled
ligand binding assays have shed more light on this issue.
Rats treated with exogenous thyroid hormone demonstrate
the features of hyperthyroidism. Beta-adrenergic receptor
density in the hearts taken from these animals is increased
from 50 to 100% (47-49). Beta-adrenergic receptor density
in membranes of cultured heart cells is also increased by
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treatment with thyroid hormone (50 ). The biologi c significance of these finding s is supported by the observation that
there is an augmented catecholamin e-induced stimulation
of cyclic AMP levels in the triiod othyronine-treated cells
(50) .
Clinical studies suggest that these observations are relevant to hyperthyroidism in human beings. Triiodothyronine
was administered to normal volunteers for I week and leukocyte beta-adrenerg ic receptor den sity was measured (30) .
The numb er of receptors in leukocytes of treated subjects
was approximately twofold greater than the number in placebo-treated control subjects. From these results, taken together with the animal thyrotoxic osis studies and the evidence that changes in leukocyte beta- adrenergic receptors
reflect changes in the heart, it is reasonable to conclude that
some of the cardiovascular man ifesta tions of hyperthyroidism are due to an increase in the numb er of beta-adrenergic
recept ors in the heart.
Hypothyroidism , In contrast, patient s with hypoth yro idism appear to have r educed sympathetic nervou s sys tem
activity. For example, a prominent clinical feature of hypoth yroidism is brad ycardia. The number of beta-adrenergic
receptors in heart s from rats made hypoth yroid with propylthi ouracil was decreased modestly (49, 5 1). The chan ges
induced by hypothyroidism , however , appeared more com plex and diffu se than the changes caused by hyperth yroidism . There was also a reduction in the number of alph aadrenergic receptors and the activity of enz yme s in sarcolemm a and sarcoplasmic reticulum was altered (49) . Hypothyroidism also appeared to alter the interaction betwe en
the beta-adrenergic receptor and adenylate cyclase, suggesting decreased or impaired "coupling" of the receptor
with the enzyme (51). This might lead to diminished cellular
generation of cyclic AMP for a given magnitude of betareceptor stimulation . Thus, in hypothyroidism there appears
to be a reduction in the number of beta-adrenergic receptors
and also diminished effectiveness in the translation of the
effe cts of receptor stimulation to subsequent biochemical
changes. The se findings suggest a plausible mechanism for
the bradycardia and diminished sympathetic respon siveness
of the heart in patients with hypothyroid ism .
In summary . patients with thyroid dysfunction manifest
clinic al signs suggestive of altered sympathetic nervous system activity or respon sivene ss to sympathetic stimulation.
The changes in beta-adrenergic receptors in response to the
altered thyroid state summarized here provide possible
mechan isms for these clinical signs.
Alterations of Cardio vascular Beta-Adrenergic
Receptors With Drug Therapy
Effects of antisympathetic agents. It is well established
that sympathetic denervation leads to enhanced tissue responsiveness to catecholamines, a phenomenon called de-
nervation hypersensitivity. In de nervation produ ced surgically or chemically (for example, with 6-hydroxydopaminel.
the hypersensitivity could be attributed to changes on at
least two levels, prejun ctionally at the sympathetic nerve
termin al that normall y remo ves catecholamines from the
neuroeffector junctions and postjunctionally on the effe ctor
cell. Radiolabeled ligand bindin g studies have shown that
with denervation the den sity of beta-adrenergic receptors on
effector cells increases, presumably because in this condition the concentration of catec holamines in the receptor
milieu is low (52). Treatment with drugs that ant agonize
sympathetic nervous system activity also results in increa sed
receptor density. Administration of guanethidine, which both
blocks release of norepinephrine from nerve terminals and
depletes norepinephrine stores, resulted in increased betaadrenergic receptor density in hearts of rats (53).
Beta-receptor blockade. Blockade of beta-adrenergic
receptors also leads to increases in receptor den sity (up
regulation ) (22 ,54). The se observations are particularly important clinically because of the prominent role of betaadrenergic receptor blockade in the treatment of card iovascular disease . Som e patients who discontinue treatment with
beta-receptor blocking drugs exhibit clinical findin gs
suggestive of a hyperactive sympathetic nervous system (5558). Patients with coronary artery disease may occa sionall y
develop unstable angina or eve n myocardi al infarction after
sudden withdrawal of beta-adrenergic receptor blocking agents
(55-58). Results from clinical studies sugges t that this syndrome may be related to hyper-responsiveness of beta- adrenergic receptors to stimulation by catecholamines . It has
been shown that human beings will exhibit hyper-responsiveness to infused catechol amine s for up to 13 days after
term inating beta-receptor blockade (59 ,60). A possible explanat ion for this is the previously described observation
that beta-adrenergic receptors are increased in the hearts of
animals treated with beta-adrenergic receptor blocking drug s
(22, 54). That similar increa ses may also occur in patients
treated with beta-receptor block ing drugs is suggested by
the observation that human subjects treated with propranol ol
exhibited increas es in the density of beta-adrenergic receptors on leukocytes (24 ,25) . After withdrawal of propranolol
the subjects also showed augmented orthostatic chan ges in
heart rate and heart rate-bl ood pressure product. suggesting
that the chan ges in leukocyte recept ors reflected chan ges in
cardiac receptors (24) .
Mechanism of propranolol withdrawal syndrome.
Incre ases in beta-adrenergic receptors in the heart may thus
be one mechani sm to explain the relatively unusual entity
referred to as the propranolol withdrawal syndrome (58) . It
is unkno wn why this phenomenon occ urs only in a minority
of patients treated with beta- receptor blocking dru gs . On e
explanation may be differences in the level of sympathetic
nervous system activity before initiation of beta-receptor
blockade. In a given subje ct with high levels of sympathetic
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activity , and thus high concentrations of circulating catecholamines, beta-adrenergic receptor density would be expected to be greatly reduced (25). If this subject is then
treated with a beta-receptor blocking agent , which would
release the down regulation , beta-receptor density might be
expected to increase substantially (25). If beta-receptor
blockade is then suddenly withdr awn this subject, who still
has a high level of sympathetic nervou s system activity and
now a relativel y high densit y of receptors, might be a prime
candidate for developing the propranolol withdrawal syndrome . Although some of these thought s are speculati ve,
this would appear to be an important clinical issue, particularly with the increasing use of beta-receptor blocking agents
in patients with angina pectoris and after myocardial infarction.
Beta-adrenergic receptor stimulation. If beta-adrenergic receptors can be regulated in response to ambient
concentrations of catecholamines under physiologic conditions, then administration of exogenous beta-agonists would
be expected to diminish the number of receptor s on effector
organs and tissues . It has been clearly established in model
systems that exposure of cells to catecholamines leads to a
reduction in the number of beta-adrenergic receptor s in the
membrane s of the cells (down regulat ion) (61). Additionally, it has been shown that after catecholamine expo sure ,
the abilit y of beta-agonists to " couple" the receptorto adenylate cyclase is diminished (6 1- 63). ln intact cells , this
agonist-induced down regulation and desensitization is manifested as a diminished physiologic response to catechol amines (63). Clinical studies suggest that receptor number
can change in response to admini stration of exogenous betaagonists to human being s. Whether or not " coupling" is
altered by the administration of catecholamines remain s to
be established. Infusion of isoprot erenol in normal subjects
produced a biphasic effect on leukocyte beta-adrenergic receptor number, an initial increase in density that lasted only
about I hour followed by a later reduction in density (23) .
When patients with heart failure were treated with the betaagonist pirbuterol, a decrease in beta-adrenergic receptor
density in lymphocytes was found (64) . Treatment of subjects with the noncatecholamine beta--adrenergic receptor
agonist terbutaline decrea sed betaj-adrenergic receptor density in polymorphonuclear leuko cytes (65). Thes e observations suggest one possible mechanism for the phenomenon of tachyphylaxis. With sustained administration of
catecholamines in a clinical setting, the decrea sed responsiveness over time that is sometimes observed may be partially related to a reduction in the number of beta-adrenergic
receptors.
Summary and Conclusions
Although the technology for biochemical assessment of betaadrenergic receptors (developed largely in nonmammalian
model systems) was introduced only in 1974, it is now
87
applied widely to clinicall y relevant studies in animals and
human beings. The studies discussed are only a beginnin g
and much remains to be done utiliz ing radiolabeled ligand
bindin g assays in clinical studies of health and disease. For
example, what are the effect s of age , exercise , physical
conditioning, diet and posture on cardio vascular beta-ad renergic receptors? The diseases that have begun to be studied need more detailed examination. In the studies cited ,
the main emphasis has been on measuring receptor number
and affinity of the receptor for antagonists. It is possible
that in addition to or instead of changes in number, there
are changes in receptor "coupling" to adenylate cyclase so
that for a given amount of receptor stimulation, adenylate
cyclase will be activated to a greater or lesser extent in
certain disease states. Such changes can be detected only
by careful analysis of agonist interaction with the receptor
and the ability of the receptor to form guanine nucleotidedependent high affinity intermedi ate states ( I I ).
It must be remembered that the beta-adrenergic receptor
is only one of a series of components in the receptor-cyclaseprotein kinase system that mediate the intracellular effects
of catecholamines (66- 71). In addition to the receptor, there
are adenylate cyclase , cyclic AMP-d ependent protein kinase
and phosphatases which remove phosphate from protein s
phosphorylated by protein kinase (66-68,70 .72, 73). Calcium ions. the intracellular handling of which is altered by
beta-agonists, also participate in the mediation of intracel lular effects of catecholamines (68,69). One or more of these
other components in the system may be altered in cardiovascular disease or in response to drug therap y: until each
of the components is examined , mechanistic conclu sions
about disease or drug effects must be reached with caution.
Finally, diseases must be examined more specifically in
terms of pathophysiologic mechanisms or etiologic cause s
of the diseases. For example, heart failure can be caused
by multiple factors and the role and nature of changes in
beta-adrenergic receptors may vary depending on the cau se.
The study of other cardi ac diseases such as cardiomyopathies and cardiac arrhythmia s should reveal interesting results. Thus, continued application of radiolabeled ligand
binding assays to clinical cardiology should yield substantial, interesting and important new information.
The use of these assays will also be important in cardiovascular pharm acology. both for understandin g better
the actions of existing drugs and fo r new drug developm ent .
The effects of noncatechol amines and nonbeta-adrenergic
receptor blocking agents on beta-adrenergic receptors can
be assessed expeditiou sly with radiolabeled ligand binding
studies (74 ,75). New chemi cals, thought to act on betaadrenergic receptors, can be efficiently screened and characterized with in vitro binding assays. The se assays can
reveal whether a compound is an agonist or antagoni st , the
intrinsic sympathomimetic activity of agonists, the affinity
of beta-adrenergic receptor s for the compound and the se-
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lectivity of the compound for beta,- and beta-receptors
(14). Radiolabeled ligand binding studies of alpha-adrenergic and muscarinic receptors make it possible to determine
if a compound has any activity on these other receptors of
the autonomic nervous system as well as on beta-adrenergic
receptors (14,74).
Radiolabeled ligand binding assays of beta-adrenergic
receptors thus represent a major advance in the science of
sympathetic nervous system physiology and pharmacology.
The application of these assays should continue to make
major contributions toward progress in cardiovascular physiology, pathology and pharmacology.
I thank Terri Butcher for excellent secretarial assistance.
References
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