Comparison of rat and human left ventricle beta-adrenergic receptors: subtype heterogeneity delineated by direct radioligand binding

Beta adrenergic receptors were identified in rat myocardial left ventricle and human papillary muscle by using the antagonist radioligand 3H-dihydroalprenolol. The number (37.3 and 44.5 fmol/mg of protein, respectively in rat and man), and the KD (1.6 and 2.8 nM, respectively in rat and man) of beta receptors were not significantly different. Adrenergic receptors of both beta 1 and beta 2 subtypes were found to coexist in the left ventricle. The relative proportions of the two beta receptor subtypes were determined by the use of competition radioligand selective binding and computer modelling techniques employing the subtype selective antagonists ICI 118,551 (beta 2 selective) and atenolol (beta 1 selective) in rat or metoprolol (beta 1 selective) in man. The rat left ventricle contained about 74% beta 1 and 26% beta 2 adrenergic receptors, human left ventricle papillary muscles contained about 69% beta 1 and 31% beta 2. Human and rat left ventricles contain both beta 1 and beta 2 adrenergic receptors with similar affinities. Rat might be a model for the study of human myocardial beta adrenergic receptors.     

An unexpected negative inotropic effect of prostaglandin F2alpha in the rat heart

Prostaglandin F(2alpha) (PGF(2alpha)) is produced during myocardial inflammation and many of the insults that trigger contractile dysfunction also activate prostaglandin synthesis and production. However, although PGF(2alpha) plays a significant role in the cardiac response to inflammation, the effect of this particular compound on the heart was largely studied at the cellular level and probably no due attention was paid to the effect of PGF(2alpha) on the whole heart contractility. Therefore, in the present study we have investigated the effect of PGF(2alpha) on isolated right ventricle of the rat heart. PGF(2alpha) (1nM-1microM) induced concentration-dependent decrease of the amplitude of contractions of the ventricular muscle. Real time RT-PCR has revealed that prostaglandin FP receptors are expressed in the rat myocardium and the level of expression was similar to those of creatine kinase and adenylate kinase, which are proteins abundantly present in the heart. An antagonist of FP receptors, PGF(2alpha) dimetilamide (10nM), abolished negative inotropic effect induced by PGF(2alpha). To examine the possibility that PGF(2alpha) could activate non-FP prostaglandin receptor, we have measured the level of expression of all known prostaglandin receptors in the rat heart. These experiments have shown that the order of expression of prostaglandin receptors in the rat heart is FP>EP1=TP>EP4>EP3>DP=IP. Based on the obtained results we conclude that PGF(2alpha) induces negative inotropic effect on rat heart by activating FP prostaglandin receptors. This effect of PGF(2alpha) could contribute to cardiac dysfunction in conditions of systemic and myocardial inflammation.     

Lack of specific (3H) prazosin binding sites in dog and rabbit cerebral arteries

In order to explore the characteristics of alpha adrenergic receptors on cerebrovascular smooth muscle, specific binding sites for the alpha 1 adrenergic ligand, (3H) prazosin, were studied in blood vessel homogenates. No specific (3H) prazosin binding was found in either rabbit or dog cerebral arteries, but specific binding was demonstrated in the rabbit saphenous and ear arteries. In the ear artery 3H-prazosin binding was saturable with a Kd of 0.51 +/- 0.20 nM and a Bmax of 89 +/- 29 fmoles/mg protein. To confirm the adequacy of our membrane preparation, homogenates of both dog and rabbit cerebral arteries showed saturable specific binding with two different ligands: one for muscarinic receptors, [3H](-) quinuclidinyl benzilate (QNB) and one for alpha 2 adrenergic receptors, (3H) yohimbine. The results of these studies demonstrate a lack of alpha 1 adrenergic receptors on cerebral blood vessels, confirming functional studies showing only a weak contractile response to norepinephrine.     

Reciprocal expressions of alpha 1- and beta-adrenergic receptors, but constant expression of glucagon receptor by rat hepatocytes during development and primary culture

The stimulations of cyclic AMP formation and adenylate cyclase activity by glucagon and isoproterenol were both found to be highest in neonatal rat hepatocytes and to decrease during development. Adult hepatocytes still showed a considerable response to glucagon, but a negligible response to isoproterenol. The decrease in cyclic AMP formation during development can be explained in the case of the response to beta-adrenergic agonist as due to decrease of its receptor number, judging from binding of [125I]iodocyanopindolol to purified plasma membranes. But in the case of the glucagon response, the decrease in the response may be due to change of post-receptor components of the adenylate cyclase system, because the receptor number tended to increase during development, as shown by binding of [125I]iodoglucagon. Similarly, alpha 1-adrenergic receptors increased in number during development, but their IC50 value did not change, as measured by binding of [3H]prazosin to plasma membranes. Previous studies on primary cultures of adult rat hepatocytes showed that the beta-adrenergic response and its receptor number increased markedly during short-term culture (Nakamura, T., Tomomura, A., Noda, C., Shimoji, M., & Ichihara, A. (1983) J. Biol. Chem. 258, 9283-9289). However, in this work the amount of alpha 1-adrenergic receptor of adult rat hepatocytes was found to decrease by one third during 1-2 days culture. Therefore, changes of alpha 1- and beta-adrenergic receptors during development of rat liver and during primary culture of adult rat hepatocytes were reciprocal, although the directions of change in the two conditions were opposite. The additions of various hormones to primary cultures of adult rat hepatocytes did not affect the reciprocal changes of adrenergic receptors, suggesting that these hormones did not regulate the changes of the receptors.     

室旁核在剌激猫肾神经传入纤维引起的血压效应中的应用

The purpose of this study is to investigate the role of paraventricular nucleus of the hypothalamus (PVN) and alpha 1 adrenergic receptor of PVN in the pressor responses to stimulation of renal afferent nerve in alpha 1-chloralose-anesthetized cats with carotid sinoaortic denervation and vagotomy. The pressor response to stimulation of renal afferent nerve consisted of a primary and a second components. The primary component response was completely blocked while the second component was not blocked by autonomic blocking agents (hexomethonium and atropine). Bilateral lesions of PVN greatly attenuated the pressor response before and after autonomic blockade. Intracerebroventricular and PVN injection alpha 1, adrenergic antagonist (prazosin) significantly decreased in the pressor response to stimulation of renal afferent nerve. These results indicate that paraventricular nucleus of the hypothalamus and alpha 1 adrenergic receptors in central nervous system, especially in PVN, play an important role in the pressor responses to stimulation of renal afferent nerve.     

Involvement of norepinephrine activity in the regulation of alpha 1 adrenergic receptors in the medial preoptic nucleus of estradiol-treated rats

To establish whether the diurnal decrease in the density of alpha 1 receptors observed in the medial preoptic nucleus (MPN) of estrogen (E2)-treated rats is related to the concomitant diurnal increase in norepinephrine (NE) turnover rates, we quantitated the density of [3H]-Prazosin binding to alpha 1 receptors after blockade of NE turnover with alpha-methyl-paratyrosine (alpha MPT). A series of preliminary studies was performed to rule out an interference of this drug with [3H]-Prazosin binding to alpha 1 adrenergic receptors in vitro and in vivo. Incubation of brain slices with alpha MPT produced a dose-dependent inhibition of [3H]-Prazosin binding to alpha 1 adrenergic receptors with an IC50 of approximately 6 mM. Scatchard analysis demonstrated that alpha MPT exhibited a simple competitive interaction with [3H]-Prazosin binding sites as shown by an increase in the apparent dissociation constant (Kd) of the ligand and no change in the number of alpha 1 receptors (Bmax). In contrast, preincubation of brain slices with alpha MPT and prior in vivo administration of alpha MPT did not affect [3H]-Prazosin binding to alpha 1 adrenergic receptors. Once we established that alpha MPT could be used to suppress NE turnover without interfering with the measurement of alpha 1 receptor densities, we repeatedly injected this drug to ovariectomized (OVX) and E2-implanted rats. The density of alpha 1 adrenergic receptors in MPN was quantitated autoradiographically. Blockade of NE turnover with alpha MPT only partially prevented the reduction in alpha 1 receptor density observed in the E2-treated rats, suggesting that the decrease in the level of [3H]-Prazosin binding sites cannot be completely ascribed to increased NE turnover rates.     

Ontogeny of alpha 1- and beta-adrenergic receptors in rat lung

The binding characteristics of the alpha 1-selective adrenergic ligand [3H]-prazosin were determined in particulate membranes of rat lung from day 18 of gestation to adulthood. Specific binding was present at all ages studied, was reversible and inhibition of specific binding by agonists followed the order of potency: (-)-epinephrine = (-)-norepinephrine much greater than (-)-isoproterenol greater than (+)-norepinephrine. Inhibition by antagonists followed the order of potency: prazosin greater than WB4101, much greater than yohimbine. Binding capacity increased during the neonatal period from 52 +/- 9 fmoles x mg-1 protein in lung preparations on day 18 of a 21 day gestation increasing to 105 +/- 4 fmoles x mg-1 protein (mean +/- SE) by postnatal day 15. Binding activity decreased thereafter, reaching adult levels by 28 days of postnatal age, 62 +/- 3 fmoles x mg-1 protein. This pattern of alpha 1-adrenergic receptor density was distinct from that of beta-adrenergic receptors identified in rat lung membrane with the beta- adrenergic antagonist, (-)-[3H]dihydroalprenolol ((-)-[3H]DHA). (-)-[3H]DHA binding increased dramatically during this same time period, from 46 +/- 4 fmoles x mg-1 protein on day 18 of gestation to 496 +/- 44 fmoles x mg-1 protein in the adult lung. Affinity for [3H]-prazosin and (-)-[3H]DHA did not change with age. Pulmonary alpha 1-adrenergic receptors are present as early as 18 days of gestation in the rat and alpha 1-adrenergic receptor density is maximal by 15 days of postnatal age. The timing of the changes in alpha 1-adrenergic receptors correlates with the timing of increased sympathetic innervation of the developing rat lung and is distinct from that of beta-adrenergic receptor sites.     

Effects of androgen on alpha- and beta-adrenergic receptors in membranes from the rat seminal vesicle.

The effects of castration and androgen-replacement on adrenergic receptors in membranes from the rat seminal vesicle were studied. Membranes from seminal vesicles showed saturable and high-affinity binding sites for the beta-adrenergic receptor antagonist, [3H]dihydroalprenolol ([3H]DHA), and the alpha 1-adrenergic receptor antagonist, [3H]prazosin. Castration markedly reduced beta-adrenergic receptors with decreasing the effect of GTP modulating the receptor-ligand affinity, suggesting defects in both the receptor per se and the guanine-nucleotides-regulating mechanism after castration. In contrast, castration increased alpha 1-adrenergic receptors and androgen-replacement reversed this change. The effects of GTP decreasing the alpha 1-receptor binding affinity to the radioligand were observed to a similar extent in the castrated and control membranes. These results demonstrate an inverse regulation by androgen on beta- and alpha 1-adrenergic receptors in membranes of the rat seminal vesicle.     

Glutaraldehyde pretreatment blocks phospholipase A2 modulation of adrenergic receptors

Treatment of rat cerebral cortical membranes with phospholipase A2 affects, in a parallel fashion, beta-, alpha 1- and alpha 2-adrenergic receptor binding, but not the affinity of these receptors for their respective ligands. Pretreatment of membranes with 0.1 percent glutaraldehyde blocks the effects of phospholipase A2 on adrenergic receptor binding. The results support the hypothesis that desensitization or "masking" of adrenergic receptors may involve changes in membrane lipid composition. Furthermore, glutaraldehyde may prove a useful tool in the investigation of the dynamic roles of lipids in receptor function and more specifically, their regulation and coupling to physiological events.     

Identification of alpha 1 adrenergic receptors in rabbit aorta with [125I] BE2254

Alpha-adrenergic receptors may play an important role in regulating vascular tone and reactivity. To study alpha-adrenergic receptors in blood vessels, we have developed a method to characterize and quantitate alpha-adrenergic receptors in a particulate fraction of individual rabbit aortas using the high specific activity alpha antagonist [125I] BE2254. [125I] BE2254 specifically labels a single class of binding sites with a dissociation constant of 286 pM and a maximal binding capacity of 16.7 fmoles/mg protein. Catecholamines compete for [125I] BE2254 binding stereospecifically and with the characteristic alpha-adrenergic potency series of (-)epinephrine greater than or equal to (-)norepinephrine much greater than (-)isoproterenol. The alpha 1-selective antagonist prazosin (KD = 0.7 nM) is much more potent in competing for [125I] BE2254 binding than is the alpha 2-selective antagonist yohimbine (KD = 1000 nM), which suggests that the alpha adrenergic receptor identified is predominantly of the alpha 1 subtype. Also, the dissociation constants from these binding studies were in good agreement with those reported in rabbit aorta from classical pharmacological experiments where contraction was found to be mediated via alpha 1 receptors. This extension of radioligand binding techniques to individual rabbit aortas should simplify the study of vascular alpha adrenergic receptor regulation, and provide a basis for broadening the understanding of vascular alpha adrenergic receptors.     

Circadian rhythms in neurotransmitter receptors in discrete rat brain regions

Circadian rhythms were measured in alpha 1-, alpha 2- and beta-adrenergic, acetylcholine muscarinic (ACh), and benzodiazepine (BDZ) receptor binding in small regions of rat brain. Rhythms in alpha 1-receptor binding were measured in olfactory bulb, frontal, cingulate, piriform, parietal, temporal and occipital cortex, hypothalamus, hippocampus, pons-medulla, caudate-putamen and thalamus-septum. No rhythm was found in cerebellum. Rhythms in alpha 2-receptor binding were measured in frontal, parietal and temporal cortex, and pons-medulla. No rhythm was found in cingulate, piriform or occipital cortex, or hypothalamus. Rhythms in binding to beta-receptors were measured in olfactory bulb, piriform, insular, parietal and temporal cortex, hypothalamus and cerebellum. No rhythms were found in frontal, entorhinal, cingulate, or occipital cortex, hippocampus, caudate-putamen, or pons-medulla. Rhythms in ACh receptor binding were measured in olfactory bulb, parietal cortex and caudate-putamen. No rhythms were found in frontal or occipital cortex, nucleus accumbens, hippocampus, thalamus-septum, pons-medulla or cerebellum. Rhythms in BDZ receptor binding were measured in olfactory bulb, olfactory and occipital cortex, olfactory tubercle, nucleus accumbens, amygdala, caudate-putamen, hippocampus and cerebellum. No rhythms were found in parietal cortex, pons-medulla or thalamus-septum. The 24-hr mean binding to receptors varied between 3- and 10-fold, the highest in cortex and the lowest, usually, in cerebellum. The piriform cortex was particularly high in alpha 1- and alpha 2-adrenergic receptors; the nucleus accumbens and caudate, in ACh receptors; and the amygdala, in BDZ receptors. Most adrenergic and ACh receptor rhythms peaked in subjective night (the period when lights were off under L:D conditions), whereas most BDZ receptor rhythms peaked in subjective day (the time lights were on in L:D). Perhaps in the rat, a nocturnal animal, the adrenergic and ACh receptors mediate activity and the functions that accompany it, and the BDZ receptors mediate rest, and with it, sleep.     

Characterization of adrenergic receptors on proximal tubular basolateral membranes

Alpha adrenergic receptors are identified on basolateral plasma membranes derived from proximal tubular epithelial cells. The density of alpha 2 receptors was over two-fold greater than alpha 1 receptors. The basolateral membranes were devoid of beta receptors. These results support previous demonstrations of alpha adrenergic receptors in rat renal cortex and concur with studies which suggest a limited presence of beta receptors on rat proximal tubules.     

Cross-regulation between G-protein-coupled receptors. Activation of beta 2-adrenergic receptors increases alpha 1-adrenergic receptor mRNA levels.

Stimulation of DDT1 MF-2 vas deferens cells with epinephrine resulted in a time- and dose-dependent loss of alpha 1-adrenergic receptor-specific ligand binding. Regulation of alpha 1-adrenergic receptor mRNA was characterized. In monolayer culture, cells displayed 0.7 +/- 0.05 amol of alpha 1-adrenergic receptor mRNA/microgram of total cellular RNA. Epinephrine, which acts at both alpha 1- and beta 2-adrenergic receptors of DDT1 MF-2 cells, induced a short term (2-8 h) increase (50-70%) in the abundance of alpha 1-adrenergic receptor mRNA. Propranolol, a beta 2-adrenergic receptor antagonist, attenuated the epinephrine-mediated increase in alpha 1-adrenergic receptor mRNA but did not affect the decrease in alpha 1-adrenergic receptor-specific ligand binding. Phentolamine, an alpha 1-adrenergic receptor antagonist, did not attenuate the epinephrine-mediated increase in alpha 1-adrenergic receptor mRNA at 4 h but did block the decrease in alpha 1-adrenergic receptor-specific ligand binding. The half-life of the alpha 1-adrenergic receptor mRNA was approximately 7 h in untreated cells as well as in cells challenged with epinephrine. The epinephrine-promoted increase in alpha 1-adrenergic receptor mRNA was found to result from cross-regulation via beta 2-adrenergic receptors. Cholera toxin, forskolin, as well as the cyclic AMP analog CPT cAMP (8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate) increased the alpha 1-adrenergic receptor mRNA at 4 h, as did epinephrine in the presence of alpha 1-antagonists but not in the presence of a beta-adrenergic antagonist. This is the first report of heterologous up-regulation of mRNA levels of adrenergic receptors. Cross-regulation between alpha 1- and beta 2-adrenergic receptor-mediated pathways at 4 h occurs at the level of mRNA whereas later down-regulation of alpha 1-receptor mRNA and binding proceed via agonist activation of alpha 1-adrenergic receptors.     

Cloning and expression of the mouse homolog of the human alpha 2-C2 adrenergic receptor.

Three subtypes of alpha 2 adrenergic receptors have been identified in the human and rat. The subtype located on human chromosome 2 (alpha 2-C2) is unique in that it is expressed mainly in the peripheral tissues and lacks sites for N-linked glycosylation. We isolated the gene encoding the mouse homolog of the human alpha 2-C2 adrenergic receptor (M alpha 2-2H). The deduced amino acid sequence of the M alpha 2-2H shows 82% and 96% identity to the human alpha 2-C2 and the rat RNG alpha 2 adrenergic receptors, respectively. Southern blot analysis demonstrated that the M alpha 2-2H was encoded by a single copy gene and was distinct from the mouse homologs of the alpha 2-C4 and alpha 2-C10 adrenergic receptors. When expressed in COS-7 cells, the M alpha 2-2H exhibited a pharmacological profile similar to the human alpha 2-C2 and rat RNG alpha 2 receptors.     

Regulation of alpha- and beta-adrenergic receptor subclasses by gonadal steroids in human myometrium

Both alpha- and beta-adrenergic receptors have been identified in the human myometrium by radioligand binding. Both adrenergic receptor subclasses have been shown to mediate the contractile response of the uterus upon catecholamine stimulation: alpha-adrenergic receptors cause uterine contraction while beta-adrenergic receptors induce relaxation. We have identified alpha 1- and alpha 2-adrenergic receptors in myometrial membranes using the newly developed radiolabelled specific antagonists [3H]-prazosin and [3H]-rauwolscine. This enabled us to characterize both receptor subclasses individually. Beta adrenergic receptors were identified using the radiolabelled antagonist (-)-[3H]-dihydroalprenolol. Binding of radioligands to the myometrial membrane receptors was rapid, readily reversible, of high affinity and stereoselective. The total number of alpha 1-, alpha 2- and beta-receptors was determined by Scatchard analysis of radioligand saturation binding and the beta/beta 2-receptor ratio was determined by computer analysis of the beta 2-selective antagonist ICI 118 551) (-)-[3H]-dihydroalprenolol competition binding curves. This enabled us to study the regulation of both alpha- and beta-receptor subclasses under various physiological and pharmacological conditions in the human, i.e., during different phases of the menstrual cycle, in postmenopausal women and during depo-progestin (Medroxyprogesterone acetate) therapy. Only the alpha 2- and beta 1-adrenergic receptor concentrations were found to be subjected to gonadal steroid regulation. The number of alpha 2- and beta 1-adrenergic receptors increased concomitantly with circulating plasma oestradiol levels. This effect was counteracted by progesterone. The number of alpha 1- and beta 2-adrenergic receptors was unaffected by the gonadal steroid environment. These results are an example of the heteroregulation of membrane receptors by oestrogens and progesterone and cast new light on the regulatory mechanisms involved in uterine contractility in the human.     

Differentiation of Alpha-Adrenergic Receptors Using Pharmacological Evaluation and Molecular Modeling of Selective Adrenergic Agents

Abstract

Subtypes of alpha adrenergic receptors were studied using selective adrenergic agonists. A-53693, A-54741, and related compounds were evaluated for their affinity for alpha receptor subtypes using radioligand binding techniques. Efficacy and potency were also evaluated using in vitro bioassays of alpha-1 receptors in rabbit aorta smooth muscle and alpha-2 receptors in the phenoxybenzamine-pretreated canine saphenous vein. Active and inactive compounds were then submitted for computer-assisted molecular modeling evaluation to ascertain the structural requirements for optimal potency and selectivity. Rigid catecholamines such as A-53693 display a high degree of selectivity for alpha-2 compared to alpha-1 receptors, probably because of the unique regions of space at the ligand binding site occupied by active compounds. Imidazolines such as A-54741 also interact with extremely high affinity and potency for alpha-2 receptors, and to a lesser extent at alpha-1 receptors. The spatial domains occupied by phenethylamines and imidazolines differ, each having unique regions of permissable space at alpha receptors. Compounds such as A-53693 and A-54741 are extremely useful probes of the molecular interactions of alpha agonistic compounds which will help in the design of even more selective drugs for alpha adrenergic receptors.     

Prolonged administration in vivo of alpha and beta adrenergic agonists decreases insulin binding to rat myocardial membranes in vitro by different mechanisms.

Male Sprague Dawley rats were continuously treated in vivo for 6, 12 and 20 hours with a combination of an alpha- (beta-) adrenoreceptor agonist and a beta- (alpha-) adrenoreceptor antagonist in subcutaneously implanted depot tablets. Crude membranes prepared from myocardial cells exhibited a decreased maximum binding of [125I]-insulin after 20 hours irrespective of the treatment applied. Scatchard and non-linear regression analysis of the displacement curves assuming two non-cooperative binding sites revealed a downregulation of the high affinity receptors for about 85% and a concomitant 2.5-fold increased receptor affinity under beta-adrenergic influence. In contrast, alpha-adrenergic treatment did not affect the receptor number but decreased the high affinity by 70%. The low affinity binding sites were virtually unaffected by the different treatments. The phospholipid and cholesterol contents of the membranes were not significantly altered. The phospholipid/cholesterol ratios after 12 and 20 hours of alpha-adrenergic treatment, however, were decreased. We suggest that the decreased binding activity of insulin receptors on rat myocardial membranes after continuous in vivo treatment with alpha- and beta- adrenergic agonists is mediated by different mechanisms.     

alpha 1- and beta 2-adrenergic receptor expression in the Madin-Darby canine kidney epithelial cell line

The Madin-Darby canine kidney (MDCK) cell line, derived from distal tubule/collecting duct, expresses differentiated properties of renal tubule epithelium in culture. We studied the expression of adrenergic receptors in MDCK to examine the role of catecholamines in the regulation of renal function. Radioligand-binding studies demonstrated, on the basis of receptor affinities of subtype-selective adrenergic agonists and antagonists, that MDCK cells have both alpha 1- and beta 2- adrenergic receptors. To determine whether these receptor types were expressed by the same cell, we developed a number of clonal MDCK cell lines. The clonal lines had stable but unique morphologies reflecting heterogeneity in the parent cell line. Some clones expressed only beta 2-adrenergic receptors and were nonmotile, whereas others expressed both alpha 1- and beta 2-receptors and demonstrated motility on the culture substrate at low cell densities. In one clone, alpha- and beta- receptor expression was stable for more than 50 passages. Catecholamine agonists increased phosphatidylinositol turnover by activating alpha- adrenergic receptors and cellular cyclic adenosine monophosphate accumulation by activating beta-adrenergic receptors. Guanine nucleotide decreased the affinity of isoproterenol for the beta 2- receptor but did not alter the affinity of epinephrine for the alpha 1- receptor. These results show that alpha 1- and beta 2-receptors can be expressed by a single renal tubular cell and that the two receptors behave as distinct entities in terms of cellular response and receptor regulation. Heterogeneity of adrenergic receptor expression in MDCK clones may reflect properties of different types of renal tubule cells.     

Adrenergic receptor characteristics of cardiac myocytes cultured in serum-free medium: comparison with serum-supplemented medium

Neonatal rat ventricular myocytes cultured in serum-free medium coexpress both alpha 1 and beta 1 receptors as determined by radioligand binding studies. In cells exposed to serum for 48 hr surface area increased 3.69 fold, but the maximum number of binding sites ([125I]-iodocynanopindolol) only increased 1.5 fold from 12956 +/- 7579 to 19676 +/- 5181 sites/cell (n = 5, p less than .05) yielding a value of 2.48 sites/um2 for cells grown in serum-supplemented medium compared with 6.96 sites/um2 for cells grown in serum-free medium. Thus serum-induced hypertrophy is associated with a decrease in beta 1 receptor density relative to cell size; however, adenylate cyclase response is unaffected. This cell culture system constitutes an excellent model for studying interventions that may influence the regulation of cardiac myocyte hypertrophy by nonhemodynamic factors, particularly through the adrenergic receptor system.