Rapid recovery of cardiac beta-adrenergic receptors after isoproterenol-induced "down"-regulation

Senescence-like Phenotype After Chronic Exposure to Isoproterenol in Primary Quiescent Immune Cells

Chronic stress is associated with a higher risk for carcinogenesis as well as age-related diseases and immune dysfunction. There is evidence showing that psychological stress can contribute to premature immunosenescence. Therefore, the question arose whether chronic exposure to catecholamine could drive immune cells into senescence. Peripheral blood mononuclear cells were isolated from whole blood. After repeated ex vivo treatment with isoproterenol, an epinephrine analog, well-established senescence biomarkers were assessed. We found (i) DNA double-strand break induction, (ii) telomere shortening, (iii) failure to proliferate, (iv) higher senescence-associated β-galactosidase activity, (v) decreases in caspases 3 and 7 activity, and (vi) strong upregulation of the proteoglycan versican accompanied by increased cellular adhesion suggesting the induction of a senescence-like phenotype. These results emphasize the complexity of the effect of isoproterenol on multiple cellular processes and provide insights into the molecular mechanisms of stress leading to immunosenescence.

Cardiac microtubules in health and heart disease

Cardiomyocytes are large (∼40,000 µm3), rod-shaped muscle cells that provide the working force behind each heartbeat. These highly structured cells are packed with dense cytoskeletal networks that can be divided into two groups—the contractile (i.e. sarcomeric) cytoskeleton that consists of filamentous actin-myosin arrays organized into myofibrils, and the non-sarcomeric cytoskeleton, which is composed of β- and γ-actin, microtubules, and intermediate filaments. Together, microtubules and intermediate filaments form a cross-linked scaffold, and these networks are responsible for the delivery of intracellular cargo, the transmission of mechanical signals, the shaping of membrane systems, and the organization of myofibrils and organelles. Microtubules are extensively altered as part of both adaptive and pathological cardiac remodeling, which has diverse ramifications for the structure and function of the cardiomyocyte. In heart failure, the proliferation and post-translational modification of the microtubule network is linked to a number of maladaptive processes, including the mechanical impediment of cardiomyocyte contraction and relaxation. This raises the possibility that reversing microtubule alterations could improve cardiac performance, yet therapeutic efforts will strongly benefit from a deeper understanding of basic microtubule biology in the heart. The aim of this review is to summarize the known physiological roles of the cardiomyocyte microtubule network, the consequences of its pathological remodeling, and to highlight the open and intriguing questions regarding cardiac microtubules.

Impact statement

Advancements in cell biological and biophysical approaches and super-resolution imaging have greatly broadened our view of tubulin biology over the last decade. In the heart, microtubules and microtubule-based transport help to organize and maintain key structures within the cardiomyocyte, including the sarcomere, intercalated disc, protein clearance machinery and transverse-tubule and sarcoplasmic reticulum membranes. It has become increasingly clear that post translational regulation of microtubules is a key determinant of their sub-cellular functionality. Alterations in microtubule network density, stability, and post-translational modifications are hallmarks of pathological cardiac remodeling, and modified microtubules can directly impede cardiomyocyte contractile function in various forms of heart disease. This review summarizes the functional roles and multi-leveled regulation of the cardiac microtubule cytoskeleton and highlights how refined experimental techniques are shedding mechanistic clarity on the regionally specified roles of microtubules in cardiac physiology and pathophysiology.

The significant role of the Golgi apparatus in cardiovascular diseases

The Golgi apparatus (GA) is a ribbon-like system of stacks which consist of multiple closely apposed flattened cisternae and vesicles usually localized in the juxta-nuclear area. As for the biological functions, the GA plays a major role in protein biosynthesis, post-translational modification, and sorting protein from ER to plasma membrane and other destinations. Structural changes and functional disorder of the GA is associated with various diseases. Moreover, increasing evidence revealed that swelling, poor development, and other morphological alterations of the GA are linked to cardiovascular diseases such as heart failure (HF), arrhythmia, and dilated cardiomyopathy. Furthermore, dysfunction of the GA is also related to cardiovascular diseases since the GA is extremely responsible for transport, glycosylation, biosynthesis, and subcellular distribution of cardiovascular proteins. This review gives a brief overview of the intricate relationship between the GA and cardiovascular diseases. In addition, we provide a further prospective that the GA may provide diagnosis reference for cardiovascular diseases, and changes in the ultrastructure and morphology of the GA such as swelling, poor development, and fragmentation may serve as a reliable index for cardiovascular diseases.

Temporal response of ectopic activity in guinea pig ventricular myocardium in response to isoproterenol and acetylcholine

Both β adrenergic and muscarinic receptor stimulation independently potentiate arrhythmogenesis. However, the effect of simultaneous stimulation on arrhythmogenesis is not well known. The purpose of this study was to determine the temporal response of arrhythmia risk to individual and combined autonomic agonists. Guinea pig hearts were excised and Langendorff-perfused. The β adrenergic receptor and muscarinic receptor agonists were isoproterenol (ISO, 0.6 μM) and acetylcholine (ACh, 10 μM), respectively. All measurements with agonists occurred over 21 min. ISO induced ectopic activity for the first 8 min. ISO also transiently shortened and then prolonged R-R interval over a similar time course. ACh added after ISO transiently induced ectopic activity for 12 min, while R-R interval invariantly prolonged. ACh alone produced few ectopic beats, while invariantly prolonging R-R interval. In contrast to ISO alone, ISO following ACh significantly increased ectopic activity and shortened R-R interval for the duration of the experiment. Animals aged 17–19 months exhibited sustained arrhythmogenesis while those aged 11–14 did not. When ACh was removed in older hearts while ISO perfused, a transient increase in ectopic activity and decreased R-R interval was observed, similar to ISO alone. These data suggest that pre-treating with and maintaining ACh perfusion can sustain ISO sensitivity, in contrast to ISO perfusion alone.

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Decreased beta-adrenergic receptor density in rat myocardium during hemorrhagic shock

We investigated alterations in the number and affinity of cardiac beta-adrenergic receptors during hemorrhagic shock. Forty male Wistar rats were divided into two groups: (1) a shock group (n = 20), in which mean arterial blood pressure was decreased to 40-50 mmHg by bleeding and kept constant for 6 h; and (2) a control group (n = 20), which underwent a sham operation. We used (-)[(3)H]dihydroalprenolol for the determination of the number and affinity of beta-adrenergic receptors in myocardial membranes. An additional 25 rats were used for determination of plasma epinephrine and norepinephrine concentrations. Scatchard analysis showed a 20% reduction ( P < 0.05) in beta-adrenergic receptor density in the shock group (70.3 +/- 3.5 fmol.mg(-1) protein) compared to the control group (90.0 +/- 4.8 fmol.mg(-1) protein) but no significant change in the affinity (2.52 +/- 0.06 vs. 2.31 +/- 0.09 nmol. l(-1), control vs. shock). Plasma catecholamine concentrations were increased significantly at 1, 2, 4 and 6 h after the start of hypotension. These data suggest that increased levels of plasma catecholamines in hemorrhagic shock may be correlated a significant loss of beta-adrenergic receptors in rat myocardium.

Inhibition of beta-adrenergic receptor trafficking in adult cardiocytes by MAP4 decoration of microtubules

Decreased beta-adrenergic receptor (beta-AR) number occurs both in animal models of cardiac hypertrophy and failure and in patients. beta-AR recycling is an important mechanism for the beta-AR resensitization that maintains a normal complement of cell surface beta-ARs. We have shown that 1) in severe pressure overload cardiac hypertrophy, there is extensive microtubule-associated protein 4 (MAP4) decoration of a dense microtubule network; and 2) MAP4 microtubule decoration inhibits muscarinic acetylcholine receptor recycling in neuroblastoma cells. We asked here whether MAP4 microtubule decoration inhibits beta-AR recycling in adult cardiocytes. [(3)H]CGP-12177 was used as a beta-AR ligand, and feline cardiocytes were isolated and infected with adenovirus containing MAP4 (AdMAP4) or beta-galactosidase (Adbeta-gal) cDNA. MAP4 decorated the microtubules extensively only in AdMAP4 cardiocytes. beta-AR agonist exposure reduced cell surface beta-AR number comparably in AdMAP4 and Adbeta-gal cardiocytes; however, after agonist withdrawal, the cell surface beta-AR number recovered to 78.4 +/- 2.9% of the pretreatment value in Adbeta-gal cardiocytes but only to 56.8 +/- 1.4% in AdMAP4 cardiocytes (P < 0.01). This result was confirmed in cardiocytes isolated from transgenic mice having cardiac-restricted MAP4 overexpression. In functional terms of cAMP generation, beta-AR agonist responsiveness of AdMAP4 cells was 47% less than that of Adbeta-gal cells. We conclude that MAP4 microtubule decoration interferes with beta-AR recycling and that this may be one mechanism for beta-AR downregulation in heart failure.

Review of mechanisms involved in the apparent differential desensitization of beta1- and beta2-adrenoceptor-mediated functional responses

1. There has been considerable debate whether responses mediated via beta1- and beta2-adrenoceptors (beta1ARs and beta2ARs) display the same degree of desensitization after prolonged or repeated exposure to agonists. 2. Examples are provided for selective desensitization of functional responses and loss of binding sites for beta1ARs. Equally, examples are given of selective desensitization and down-regulation involving beta2ARs. 3. This review examines whether receptor subtype-selective desensitization of betaAR-mediated responses can occur and whether even within the same subtype, there may be tissue-selective desensitization. Possible reasons why apparent selectivity of desensitization of functional responses may occur are considered and are divided into methodological and non-methodological factors. 4. Methodological factors discussed are: the concentration of agonist used for inducing desensitization and the washout times before construction of the post-incubation concentration-response curve (CRC), the need for correction of CRCs from time-matched controls, and the methods adopted for plotting CRCs. 5. Four non-methodological factors are considered. Firstly, the roles of different receptor reserves for the responses of each tissue can have an important effect on whether desensitization is apparent; a large reserve will make desensitization less likely to be apparent. Secondly, there is more than one site at which desensitization occurs; receptors are uncoupled from adenylyl cyclase activation, there is an additional site at the level of stimulation of cyclic AMP-dependent protein kinase and betaARs may ultimately be down-regulated. These processes may differ depending on the tissue and conditions and this may influence whether differential desensitization occurs between tissues. Thirdly, the apparent degree of desensitization after washout of an agonist can depend upon the rate of resensitization. Experiments to overcome this problem are described which demonstrate betaAR desensitization in the continued presence of agonist. Finally, the role of up-regulation of PDE in desensitization is discussed.

Dissociation between regional dysfunction and beta-adrenergic receptor signaling in heart failure

We have previously shown that left ventricular (LV) pacing-induced heart failure is associated with preserved wall thickening in the interventricular septum (IVS) compared with the posterolateral wall (PLW). The current study focuses on the relationship between regional myocardial function and altered beta-adrenergic receptor (beta-AR) signaling. We studied 15 pigs: 6 controls and 9 paced from the left ventricle (225 beats/min, 26 +/- 3 days). Heart failure was documented by decreased LV fractional shortening (P < 0.0001) and increased left atrial pressure (P < 0.0001). In heart failure, despite marked differences in basal regional function (percent wall thickening: IVS, 33 +/- 10% vs. PLW, 13 +/- 7%; P = 0.0003), there were no differences between the two regions in beta-AR responsiveness, measured by regional wall thickening in response to dobutamine infusion and any measurement of adrenergic signaling. Adenylyl cyclase activity, beta-AR number, and beta-AR/Gs coupling were markedly reduced in failing LV without regional differences. In animals with heart failure, LV G protein receptor kinase (GRK) isoform 2 content was unchanged and GRK5, the other major GRK isoform, was increased more than threefold (IVS, 0.51 +/- 0.20 vs. 0. 12 +/- 0.12 arbitrary densitometric units, P = 0.01; PLW, 0.47 +/- 0. 15 vs. 0.13 +/- 0.09 arbitrary densitometric units, P = 0.03), but again, there were no regional differences. These data indicate that systemic rather than regional factors govern LV adrenergic signaling and that regional adrenergic signaling abnormalities poorly predict wall thickening in the same regions.

Angiotensin-converting enzyme inhibitor up-regulates cardiac beta-receptors in cultured neonatal rat myocytes

In patients with congestive heart failure (CHF), beta-receptor up-regulation is regarded as one of the mechanisms leading to improved function and prognosis. To clarify whether beta-receptor up-regulation is involved in the mechanisms underlying the beneficial effects of angiotensin-converting enzyme (ACE) inhibitors, we investigated the actions of ACE inhibitors and an angiotensin II type 1 receptor (AT1) antagonist on beta-receptors of neonatal rat cultured cardiac myocytes. Angiotensin II (A-II) increased the spontaneous beating frequency of the cells, and the effect was completely antagonized by the AT1 antagonist CV-11974. Under control conditions, beta-receptor density (Bmax) and affinity (Kd) were measured by radiobinding assay with the hydrophilic ligand [3H]CGP-12177, and were 103 +/- 11 fmol/mg protein and 3.4 +/- 0.4 nmol/L, respectively. Captopril increased the beta-receptor density of myocytes and augmented the response to isoproterenol. Bmax was increased by 34% after 24 h treatment with 10(-6) mol/L captopril. CV-3480, and ACE inhibitor that contains no sulfhydryl group, but neither A-II nor the AT1 antagonist, also up-regulated beta-receptors. The results suggest that beta-receptor up-regulation contributes at least partly to the beneficial cardiac effects of ACE inhibitors in patients with CHF. ACE inhibitors and AT1 antagonists seem to play different roles in clinical practice.

In vivo effect of methyl-quinuclidinyl-benzylate on myocardial beta-adrenoceptor density

The muscarinic receptor antagonist methyl-quinuclidinyl-benzylate decreased myocardial beta-adrenoceptor density Bmax: 20.4 +/- 2.4 pmol/ml tissue versus 33.3 +/- 4 pmol/ml tissue in control dogs (P < 0.001), as assessed by using [11C]CGP-12177 (((2S)-4-(3-t-butyl-amino-2 hydroxypropoxy)-benzimidazol-2-one)) and positron emission tomography. In contrast, atropine did not induce any change in Bmax: 33.7 +/- 3.6 pmol/ml tissue. We hypothetized that methyl-quinuclidinyl-benzylate induced the release of norepinephrine from sympathetic nerve terminals, an effect which could be blocked by guanethidine. Guanethidine alone (10 mg/kg) did not change Bmax: 35.5 +/- 6 pmol/ml tissue. Guanethidine + methyl-quinuclidinyl-benzylate did not induce any significant change in Bmax: 31.5 +/- 5.1 pmol/ml tissue. Therefore, it seems likely that methyl-quinuclidinyl-benzylate acts at the presynaptic level, probably inducing the release of norepinephrine which then causes a down-regulation of beta-adrenoceptors.

Diabetes-induced changes in the Gi-modulated muscarinic receptor-adenylyl cyclase system in rat myocardium

The inhibitory guanine nucleotide binding regulatory protein (Gi)-mediated muscarinic receptor-adenylyl cyclase system was studied in myocardium from adult male Wistar rats with 10 weeks of diabetes induced by a single intravenous injection of streptozotocin (60 mg/kg). Neither the messenger ribonucleic acid level nor the amount of Gi was changed in the streptozotocin diabetic group as compared to the control group. The activity of the adenylyl cyclase stimulated by guanyliminodiphosphate was decreased by 48% in the streptozotocin diabetic group whereas stimulated activities of adenylyl cyclase by sodium fluoride and forskolin remained unchanged. The inhibition of forskolin-stimulated adenylyl cyclase activity by carbachol was more potent in membranes from the streptozotocin diabetic group than that in membranes from the control group. The competition binding curve between (3H)- quinuclidinyl benzilate and carbachol obtained from the streptozotocin diabetic group was shifted to the left as compared to the control group. These results suggest that the myocardium of streptozotocin-induced diabetic rats exhibited an increase in Gi function as demonstrated by the increased inhibition of guanyliminodiphosphate-mediated adenylyl cyclase and the superhigh affinity for carbachol of the muscarinic receptors. As there were signs, similar to those seen in clinical heart failure, in the streptozotocin diabetic group, these results demonstrate that functional alteration of Gi might underlie, at least in part, the cardiac dysfunction that is associated with diabetes.

Abnormalities in sympathoneuronal regulation are localized to failing myocardium in rabbit heart

OBJECTIVES

This study investigated the differences in sympathoneuronal regulation between acute left ventricular failure and chronic biventricular failure to determine whether an increase in plasma norepinephrine concentration plays a primary role in the genesis of the desensitization phenomenon in heart failure.

BACKGROUND

It remains to be determined whether plasma norepinephrine plays a primary role in the pathogenesis of sympathetic desensitization in heart failure in vivo.

METHODS

Acute left ventricular failure was induced by aortic regurgitation in seven rabbits. Chronic heart failure was induced by adriamycin treatment in another seven rabbits.

RESULTS

Cardiac output was lower in rabbits with aortic regurgitation than in seven sham-operated rabbits. Left ventricular end-diastolic pressure was higher in rabbits with aortic regurgitation, but no significant difference in right ventricular end-diastolic pressure was observed. Beta-adrenoceptor density and norepinephrine concentration in the left ventricular myocardium were lower in rabbits with aortic regurgitation; no such differences were observed for the right ventricular myocardium. Cardiac output was lower in adriamycin-treated rabbits than in seven control rabbits. Both left and right ventricular end-diastolic pressures were higher in experimental rabbits than in control rabbits. Myocardial beta-adrenoceptor density and norepinephrine content were reduced in both ventricles.

CONCLUSIONS

In chronic heart failure induced by adriamycin, sympathoneuronal activity was altered in both ventricles, whereas in acute left ventricular failure induced by aortic regurgitation, sympathoneuronal activity was affected only in the left ventricle despite a similar increase in plasma norepinephrine concentration in both animal models. Local abnormalities in sympathoneuronal regulation in failing myocardium therefore appear to be responsible for these phenomena.

Demonstration of the suitability of CGP 12177 for in vivo studies of beta-adrenoceptors

1. Positron emission tomography (PET) with appropriate radioligands offers the possibility of studying receptors non-invasively in man. The suitability of CGP 12177, a hydrophilic non-selective beta-adrenoceptor antagonist which can be labelled with the positron emitter 11C, as a ligand for in vivo studies of beta-adrenoceptors was assessed in rats. 2. [3H]-CGP 12177 was injected into the tail veins of restrained conscious rats. Serial blood samples were taken from tail arteries to determine clearance from plasma. Rats were killed and tissues removed to determine tissue uptake. Radioactivity was assessed by liquid scintillation counting. 3. The uptake of (-)-[3H]-CGP 12177 in various tissues was compared to that of (+/-)-[3H]-CGP 12177. Maximum tissue:plasma ratios obtained for the (-)-enantiomer in lung, heart and liver were 170, 42 and 13 compared with 60, 15 and 12 for the racemate. Prior injection of excess unlabelled (+/-)-CGP 12177 blocked the uptake of both (-)- and (+/-)-[3H]-CGP 12177 in lung and heart but not liver, tissue:plasma ratios for both tracers being reduced to 7, 3 and 7 respectively. 4. Clearance of (-)-[3H]-CGP 12177 from plasma was rapid during the first 5 min but showed only small changes during 5 to 90 min. Uptake in lung and heart reached a maximum at 1 to 5 min and showed a slow decrease during 5-90 min. Prior injection of unlabelled (+/-)-CGP 12177 reduced uptake in lung and heart to 10% and 20% respectively. Injection of unlabelled ( +/-)-CGP 12177 at 15 mind is placed ~75% of the radioactivity by 90 min.5. ( +/- )-Propranolol had a similar effect to that of unlabelled ( +/-)-CGP 12177. Prior injection reduced uptake of radioactivity in lung and heart to 15% and 20% respectively. Injection of ( +/- )-propranolol at 15 min displaced ~ 60% of the radioactivity by 90 min indicating that the tracer binds to beta-adrenoceptor sites in vivo.6. In vivo saturation curves, obtained by injection of (-)-[3H]-CGP 12177 with increasing amounts of unlabelled (- )-CGP 12177, gave values of Bmax for lung of ~45 pmol per g wet weight of tissue and for heart of ~6 pmol per g wet weight of tissue. KD could only be expressed as nmol injected per kg bodyweight, that for lung (2.5 nmol kg-1) being greater than that for heart (1.3 nmol kg-1).7. Competition studies carried out by co-injecting (-)-[3H]-CGP 12177 with unlabelled (+/- )-CGP12177 or (-)-propranolol gave similar values for Bmax (lung 44 pmol g-1, heart 6 pmol g-1,). Values of KD for (+/-)-CGP 12177 (lung 4.7 pmol kg-1, heart 2.6 pmol kg-1) were approximately twice those for(-)-CGP 12177. Values of KD for (-)-propranolol (lung 38 nmol kg-1, heart 104 nmol kg-1) were greater.8. The results show that (-)-[3H]-CGP 12177 is a suitable ligand for assessing beta-adrenoceptors in vivo.

Recovery of beta-receptors and adenylate cyclase from desensitization induced by short term heat exposure in rat parotid glands

1. The recovery of rat parotid beta-adrenergic receptors (beta-AR) and adenylate cyclase (AC) from heat (33 degrees C)-induced desensitization was studied. 2. Down-regulated cell surface beta-AR and AC activity in response to isoprenaline (IPR) returned to the control level 120 hr after the termination of heat exposure. 3. However, beta-AR in parotid crude membranes increased over the control level for 48-120 hr. 4. Coupling between beta-AR and G protein(s) was attenuated at 120 hr. 5. These data suggest that beta-AR on the cell surface, but not those internalized, can transduce biological responses.

Characterization of cell-surface ?-adrenergic ([3H]CGP-12177) binding in adult rat ventricular myocytes: lack of regulation by ?-agonists at physiological concentrations

The major focus of this paper is the characterization and quantification of rat cardiomyocyte, cell-surface beta-adrenergic receptors labelled with the hydrophilic radioligand [3H]CGP-12177. The ventricular cardiomyocytes used in these experiments have previously been extensively studied in our laboratory and confirmed to be functionally compatible with similar cells in vivo. Specific binding of [3H]CGP was stereospecific, saturable and of high affinity. Binding of [3H]CGP was also readily reversible, demonstrated appropriate drug specificity and positively correlated with increasing cell concentrations. The potency of the beta 1-antagonist atenolol was almost 100 times higher than that of the beta 2-antagonist ICI-118.551 in binding to the [3H]CGP binding site. This preparation appears ideal for the investigation of beta-adrenergic receptor regulation in heart cells. Indeed, our initial experiments show clearly that pharmacological concentrations of isoproterenol, and norepinephrine, can reduce (down-regulate) the number of specific [3H]CGP binding sites. This result is in agreement with many other reports on similar experiments in a variety of cell types. However, physiologically relevant concentrations of these two agonists (1-100 nM) do not induce down-regulation of the beta-adrenergic receptors in short-term (2 h) incubations. Nevertheless, the high-affinity receptors that we have described mediate a contractile response to isoproterenol in the nanomolar concentration range (EC50 = 3.6 +/- 0.3 nM). This is approximately 300 times lower than the concentration needed to produce down-regulation. Thus, our data indicate that short-term down-regulation of cardiomyocyte beta-adrenergic receptors can only be observed with high, pharmacological concentrations of isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic ethanol treatment of rats and the myocardial β-adrenoceptors

We examined the effect of chronic treatment with ethanol on the dynamics of beta-adrenoceptor binding in left ventricular myocardium of rats. After treatment with BAAM (20 mg/kg i.p.), an irreversible inhibitor of beta-adrenoceptors, the inhibition of beta-adrenoceptor binding was less, and the recovery of receptor binding was faster in chronically ethanol-treated rats compared to the control animals given equicaloric dextrin maltose treatment. When intracellular beta-adrenoceptor recycling was inhibited with colchicine, cytoplasmic left ventricular beta-adrenoceptor binding was greater in ethanol-treated compared to dextrin maltose-treated animals. We conclude that the previously reported decreased functional activity of the beta-adrenoceptor-mediated system probably reflects the contribution of ethanol-mediated effects not entirely restricted to the receptor-binding mechanisms.

Alterations in the beta-adrenergic receptor system after hypothermic ischemia in hearts with preischemic beta-receptor desensitization

Although hypothermic cardioplegic arrest is a basic method of myocardial protection in cardiac surgery, the beta-adrenergic receptor (BAR) system has been little investigated in the heart subjected to hypothermic ischemia. Additionally, although the hypothermic arrest is often induced in hearts with preischemic desensitization of the BAR system by preceding congestive heart failure, the functional state of the BAR system after ischemia has not been studied in these hearts. We investigated alterations in the BAR system after hypothermic ischemia in normal rat hearts and in those with preischemic desensitization of the BAR system produced with isoproterenol (ISP: 400 micrograms/kg/hr for 24 hr). Both normal and BAR-desensitized hearts were isolated and subjected either to 40 min of hypothermic (10 degrees C) global ischemia followed by 40 min of reperfusion or subjected to time-matched aerobic perfusion with modified Krebs-Henseleit solution. At the end of perfusion (1) BAR binding properties with [3H]CGP-12177 and adenylate cyclase activity were measured in crude membrane fraction and (2) the inotropic response to ISP (delta LV + dP/dtmax) was evaluated in an isovolumetric contracting heart preparation. Following reperfusion, normal hearts without desensitized BAR showed a higher Bmax value than those of nonischemic time-matched hearts (41.8 +/- 3.1 vs 35.4 +/- 2.4 fmole/mg protein, P less than 0.05), whereas the Kd value was in a similar range in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of immune modulation of beta-adrenergic pathways in human dilated cardiomyopathy: influence of methodologic factors

Sera from patients with dilated cardiomyopathy contain autoantibodies modifying cardiac beta-adrenergic pathways. The influence of the methodology used to determine the prevalence of these antibodies was examined by comparing in 51 patients with idiopathic dilated cardiomyopathy the results of three assays: (1) isoproterenol-sensitive adenylate cyclase; (2) ligand binding inhibition; (3) enzyme-linked immunoassay (ELISA) of beta-receptor peptides. In 20% of the patients, all three tests gave positive results, while the concordance of the adenylate cyclase assay with either of the other two tests was 65%. ELISA was positive for a beta 1-receptor peptide corresponding to the second extracellular loop, but negative for a beta 2-receptor peptide. The results suggest that autoantibodies in dilated cardiomyopathy sera interact with several components of the beta-receptor-adenylate cyclase, and therefore a combination of methodologic approaches is needed to evaluate the prevalence and consequences of beta-receptor autoimmunity.

Catecholamine-induced cardiac hypertrophy uncouples beta-adrenoceptors from slow calcium channels

Changes in the parameters of Ca(2+)-dependent slow action potentials (APs) and in their sensitivity to noradrenaline, forskolin, dibutyryl-cAMP and extracellular Ca2+ concentration were studied and compared in left ventricular trabeculae from normal control rats and rats with cardiac hypertrophy. Cytochemical studies were also carried out to determine changes in the activity of membrane-bound adenylate cyclase. Hypertrophy was induced by administration of 5 mg/kg isoproterenol once daily for 7 days. In hypertrophied cardiac muscle, the overshoot of the slow APs was increased by 75%, the maximum rate of rise (Vmax) increased by 76% and the AP duration at 50% repolarization (APD50) prolonged by 56%. The Vmax, an indicator of the slow inward Ca2+ current, increased, in a dose-dependent manner, in response to the beta-adrenoceptor agonist noradrenaline, the adenylate cyclase activator forskolin, the protein kinase activator cAMP and elevated Ca2+ concentration in normal control preparations, whereas in hypertrophied myocardium, the beta-agonist noradrenaline and the adenylate cyclase activator forskolin had no effect. In cytochemical studies with ATP as substrate, adenylate cyclase activity was localized in the sarcolemma, and significantly fewer reaction products appeared on the outer side of the cell membrane in hypertrophied myocytes than in control myocytes. The results suggest that catecholamine-induced cardiac hypertrophy damages the catalytic subunit of membrane-bound adenylate cyclase, thus uncoupling beta-adrenoceptors from slow Ca2+ channels in the transmembrane signalling process.

Effect of antireceptor antibodies in dilated cardiomyopathy on the cycling of cardiac beta receptors

A substantial proportion of patients with dilated cardiomyopathy have circulating autoantibodies directed against the cardiac beta-adrenoceptor. These antireceptor antibodies inhibit both ligand binding to membrane beta-receptors and isoproterenol-sensitive adenylate cyclase. The functional consequences of antibody-receptor interactions were further studied by examining their influence on beta-adrenoceptor cycling. Sera from eight patients with cardiomyopathy induced a loss of beta-receptors from the surface of cardiac myocytes consistent with internalization. This loss was inhibited by concanavalin A, suggesting that receptor sequestration preceded internalization but was unaffected by the cytoskeleton inhibitors colchicine and cytochalasin. In cell-free preparations, serum-induced desensitization of beta-receptors was prevented by heparin but not the inhibitor of protein kinase A; this is consistent with a requirement for receptor phosphorylation by the beta-receptor kinase. In contrast to isoproterenol-mediated endocytosis, beta-receptors internalized under the influence of cardiomyopathic sera do not recycle to the plasma membrane. These results indicate that antireceptor antibodies in human dilated cardiomyopathy induced downregulation by interfering at several steps in the cycling of beta-receptors. These effects would contribute to the reported decline in beta-receptor responsiveness in cardiomyopathic myocardium.

Downregulation of tumor necrosis factor receptors on macrophages and endothelial cells by microtubule depolymerizing agents

Exposure of murine and human macrophages and human umbilical vein endothelial cells to micromolar concentrations of five microtubule (MT)-depolymerizing agents (colchicine, nocodazole, podophyllotoxin, vincristine, and vinblastine) resulted in a loss of binding sites for iodinated TNF-alpha. The reduction amounted to 40-60% by 1 h and approximately 75% by 2-4 h. In 1 h, specific binding was reduced 50% by 0.1-5 microM of these drugs at 37 degrees C, but not at 4 degrees C. Inactive isomers of colchicine were ineffective, as were microfilament-destabilizing cytochalasins. The active agents did not compete with TNF-alpha R for binding. Antiserum against TNF-alpha did not neutralize the effect of colchicine and nocodazole. PGE1 and dibutyryl-cAMP could not mimic, and cyclooxygenase inhibitors could not prevent the drug effects. All the binding sites were regenerated within 3 h after removal of nocodazole, which binds tubulin reversibly, whereas little recovery was found even 18 h after the removal of colchicine, which binds tubulin irreversibly. These findings suggested that MT disassembly was responsible for the observed downregulation of TNF-alpha R. The protein synthesis inhibitor cycloheximide inhibited binding of TNF-alpha to a similar extent and with a similar time course as colchicine in the absence of added ligand. Neither drug affected binding of IFN-gamma to macrophages, nor binding of TNF-alpha to human polymorphonuclear leukocytes. Thus, an intact MT network appears to be important in maintenance of the steady state of TNF-alpha R on those cells in which TNF-alpha R turns over rapidly in the absence of ligand. The antiinflammatory actions of MT-depolymerizing agents may result in part from their interference with the ability of such cells to respond to TNF-alpha.

Independent sensitization of beta-adrenoceptors and adenylate cyclase in acute myocardial ischaemia

1. Acute myocardial ischaemia provokes sensitization of the adenylate cyclase system. This sensitization could be differentiated in a receptor-linked and an enzyme-linked sensitization. The increase in the number of beta-adrenoceptors in the plasma membranes was observed already after 15 min of global ischaemia (50 +/- 2 to 67 +/- 6 fmol mg-1 protein) and persisted after 50 min of ischaemia. The maximally isoprenaline-stimulated adenylate cyclase activity rose from 66 +/- 7 to 100 +/- 10 pmol cAMP min-1 mg-1 protein after 15 min of global ischaemia indicating the receptor-mediated sensitization of the beta-adrenergic system. However, after 50 min of ischaemia the isoprenaline-stimulated adenylate cyclase was reduced by about 50% despite the continuous increase of beta-adrenoceptors in the plasma membranes. 2. Additionally direct stimulation of the adenylate cyclase by forskolin revealed an increased enzyme activity after 15 min of global ischaemia (300 +/- 20 vs 378 +/- 25 pmol cAMP min-1 mg-1). Prolonged periods of ischaemia, however, caused a decline of the total adenylate cyclase activity (232 +/- 24 pmol cAMP min-1 mg-1 protein). This demonstrates an enzyme-specific sensitization of the adenylate cyclase, which in contrast to the rise in beta-adrenoceptors is only transient. This enzyme-specific sensitization or the late inactivation of the enzyme occur independently of receptor activation and cannot be prevented by beta-adrenoceptor blockade (10(-6) M alprenolol) prior to the ischaemic insult.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of beta-adrenergic receptors in A431 cells in situ. Effect of chronic exposure to agonist

The status of beta-adrenergic receptors was investigated in A431 cells exposed to chronic stimulation by the beta-adrenergic agonist, (-)-isoproterenol. Specific binding of beta-adrenergic antagonist (-)-[125I]iodocyanopindolol declined to 60-80% below control values within 12 h of agonist treatment. This decline in ligand binding was also observed in high-speed membrane fractions prepared from agonist-treated cells. Immunoblots probed with anti-receptor antibodies revealed both that beta-adrenergic receptors from untreated and treated cells migrated as 65,000-Mr peptides and that the cellular complement of receptor was unchanged. Indirect immunofluorescence localization of beta-adrenergic receptors was comparable in control (untreated) cells and cells challenged with (-)-isoproterenol for 1, 12, or 24 h. Thus receptor complement, migration on SDS/polyacrylamide-gel electrophoresis, and localization in situ are largely unaffected by agonist stimulation. Receptor binding of antagonist radioligands, in contrast, is markedly down-regulated in cells stimulated chronically with beta-adrenergic agonists. These data argue in favour of agonist-induced alteration(s) in the conformation of the receptor that preclude radioligand binding rather than agonist-induced receptor sequestration and/or degradation.

Coregulation of calcium channels and beta-adrenergic receptors in cultured chick embryo ventricular cells

To examine mechanisms whereby the abundance of functional Ca channels may be regulated in excitable tissue, Ca channel number was estimated by binding of the dihydropyridine (DHP) antagonist 3H (+)PN200-110 to monolayers of intact myocytes from chick embryo ventricle. Beta adrenergic receptor properties were studied in cultured myocytes using [3H]CGP12177, an antagonist ligand. Physiological correlates for alterations in DHP binding site number included 45Ca uptake and contractile response to (+)BAYk 8644, a specific L-type Ca channel activator. All binding and physiological determinations were performed in similar intact cell preparations under identical conditions. 4-h exposure to 1 microM isoproterenol reduced cell surface beta-adrenergic receptor number from 44 +/- 3 to 17 +/- 2 fmol/mg (P less than 0.05); DHP binding sites declined in number from 113 +/- 25 to 73 +/- 30 fmol/mg (P less than 0.03). When protein kinase A was activated by a non-receptor-dependent mechanism, DHP binding declined similarly to 68% of control. Exposure to diltiazem, a Ca channel antagonist, for 18-24 h had no effect on number of DHP binding sites. After 4-h isoproterenol exposure, 45Ca uptake stimulated by BAYk 8644 declined from 3.3 +/- 0.2 nmol/mg to 2.9 +/- 0.3 nmol/mg (P less than 0.01) and BAYk 8644-stimulated increase in amplitude of contraction declined from 168 +/- 7 to 134 +/- 11% (P = 0.02). Thus, elevation of [cAMP] in myocytes is associated with a time-dependent decline in Ca channel abundance as estimated by DHP binding and a decline in physiological responses that are in part dependent on abundance of Ca channels. Binding of a directly acting Ca channel antagonist for 18-24 h does not modulate the number of DHP binding sites.

Plasma membrane-specific deficiency in cardiac beta-adrenergic receptor in streptozocin-diabetic rats

Cell surface [3H]CGP 12177 binding sites in 10-wk streptozocin-diabetic rats decreased by 41% (P less than 0.01) compared with that in the control rats. In contrast, there was no difference in the total cell receptor concentration between the control and the diabetic rats, which was measured by hydrophobic antagonist [125I]-iodocyanopindolol binding. Forty-eight-hour in vivo insulin treatment significantly (P less than 0.05) increased cell surface beta-adrenergic receptor concentration by 37% above that in diabetic rats without any change in total receptor concentration in the cells. However in vitro treatment of 8 nM insulin, 33 mM glucose, or 10 mM 3-hydroxybutyrate for 2 h showed no effect on [3H]CGP 12177 binding. In contrast, 10 microM isoproterenol-dependent decrease and the recovery of cell surface receptors after the removal of the agonist were significantly (P less than 0.01) impaired in diabetic rats compared with those of control rats. These results indicate that only cell surface beta-adrenergic receptors decrease in diabetic rats, which may be associated with abnormalities in the receptor distribution. The decrease in cell surface receptor number closely associates with the diabetic state and is reversed by the short-term insulin treatment.

Intracellular distribution of adrenoceptors in the failing human myocardium

Although it is increasingly recognized that the density of cardiac membrane-bound beta adrenoceptors declines in heart failure, the mechanisms involved are unclear. Furthermore, it is not known whether cardiac alpha-1 adrenoceptors are similarly affected. Inasmuch as agonist-induced desensitization results in translocation of adrenoceptors from the plasma membrane to an intracellular vesicular fraction, we determined the intracellular distribution of cardiac adrenoceptors in two groups: group 1 (n = 9) consisted of papillary muscles from patients with mild-to-moderate heart failure undergoing valve replacement, and group 2 (n = 8) consisted of severely failing hearts removed during orthotopic cardiac transplantation. The density of cardiac beta adrenoceptors was lower in membranes from group 2 (17.8 +/- 3.3 fmol/mg protein vs 27.8 +/- 3.7 fmol/mg in group 1; (p less than 0.01), and the percentage of beta receptors recovered in the vesicular fraction was higher in group 2 (47.1 +/- 3.3% vs 36.8 +/- 5.0% in group 1; p less than 0.01). In group 1 but not group 2 there was a significant inverse correlation (r = -0.87; p less than 0.001) between the density of membrane-bound beta receptors and the percentage of beta receptors recovered in the vesicular fraction. Alpha-1 adrenoceptors were lower in both membrane and vesicular fraction of group 2 compared to group 1; in group 2 but not group 1 there was a significant negative correlation between the density of membrane-bound alpha-1 adrenoceptors and the percentage of alpha-1 receptors in the vesicular fraction (r = -0.8; p less than 0.01). These results suggest that the regulation of alpha-1 and beta adrenoceptors differs in the failing myocardium. Furthermore, agonist-induced desensitization may play a predominant role only in mild-to-moderate heart failure.

Effects of chronic dobutamine administration on the response to acute exercise in dogs

The effects of chronic dobutamine administration on haemodynamic and metabolic responses to submaximal and maximal exercise were studied in dogs. Dobutamine was infused at a rate of 40 micrograms/kg min-1, 2 h day-1, 5 days week-1 for a period of 6 weeks. Acute infusion of dobutamine for 1 h increased heart rate by 73 +/- 30 beats min-1 and cardiac output by 143 +/- 141 ml/min kg-1, reduced mean arterial blood pressure by 12 +/- 10 mmHg and arterial-venous O2 difference by 1.5 +/- 1 vol%. Maximal oxygen consumption, heart rate, stroke volume, cardiac output and arterial-venous O2 difference were unchanged after 6 weeks of treatment. Reductions in heart rate at rest and during submaximal exercise following chronic dobutamine treatment were small and significant only at the lowest exercise level studied. Mixed venous lactate concentrations measured at rest, during submaximal and maximal exercise and at 2 min of recovery were not different after dobutamine treatment. Chronic dobutamine infusion did not change the citrate synthase activity in the lateral gastrocnemius muscle. These results suggest that chronic dobutamine therapy in healthy dogs does not produce aerobic training responses.

In vivo regulation of beta-adrenergic receptors on mononuclear leukocytes and heart. Assessment of receptor compartmentation after agonist infusion and acute aortic constriction in guinea pigs

In animals injected with a bolus of isoproterenol, beta-adrenergic receptors in both mononuclear leukocytes (MNL) and heart were sequestered away from the cell surface, and the time course (0-120 min) and dose-response patterns were similar in the two tissues. In guinea pigs given a constant infusion of isoproterenol, 0.15 mg/(kg.h), down-regulation of total receptor number occurred more quickly and to a greater extent in the MNL than in the heart. We also compared receptor sequestration after aortic constriction-induced acute heart failure. Negligible sequestration (9%) of beta-adrenergic receptors occurred in the MNL of animals treated in this manner, whereas the number of receptors in the sarcolemmal fraction decreased 61%. This selective sequestration of cardiac receptors may result from the action of high concentrations of norepinephrine (which is selective for beta 1 over beta 2 receptors) present at sympathetic nerve-cardiac cell synapses. We conclude that although receptor redistribution occurs similarly in MNL and heart in response to a circulating nonselective agonist, beta-adrenergic receptor redistribution may occur selectively in the heart in response to such stimuli as aortic constriction-induced acute heart failure that activate the sympathetic nervous system.

[3H]-8-cyclopentyl-1,3-dipropylxanthine binding to A1 adenosine receptors of intact rat ventricular myocytes

The purpose of the present study was the identification of A1 adenosine receptors in intact rat ventricular myocytes, which are thought to mediate the negative inotropic effects of adenosine. The adenosine receptor antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine was used as radioligand. Binding of the radioligand to intact myocytes was rapid, reversible, and saturable with a binding capacity of 40,000 binding sites per cell. The dissociation constant of the radioligand was 0.48 nM. The adenosine receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine, "xanthine amine congener," and theophylline were competitive inhibitors with affinities in agreement with results obtained for A1 receptors in other tissues. Competition experiments using the adenosine receptor agonists R-N(6)-phenylisopropyladenosine, 5'-N-ethylcarboxamidoadenosine, and S-N(6)-phenylisopropyladenosine gave monophasic displacement curves with Ki values of 50 nM, 440 nM, and 4,300 nM, which agreed well with the GTP-inducible low affinity state in cardiac membranes. The low affinity for agonists was not due to agonist-induced desensitization, and correlated well with the corresponding IC50 values for the inhibition of cyclic AMP accumulation by isoprenaline. It is suggested that only a low affinity state of A1 receptors can be detected in intact rat myocytes due to the presence of high concentrations of guanine nucleotides in intact cells.

Homologous regulation of prostaglandin E2 receptors in rat renal medulla

Prostaglandin (PG) receptors are present on enzymatically dissociated cells from the rat renal medulla and are subject to homologous regulation both in vivo and in vitro. One hour after injection of 100 micrograms of 16,16'-dimethyl-PGE2, the number of PGE2 binding sites on renal cells declines to 40% of controls. In vitro exposure of renal cells to PGE2 or dimethyl-PGE2 also results in a time- and concentration-dependent "down" regulation of prostaglandin receptors. In the absence of indomethacin in the incubation medium, endogenously synthesized prostaglandins mediate a similar time-dependent loss of cell-associated receptors. This loss is reversible since, after agonist removal and reincubation of the cells at 37 degrees C, there is a rapid (within 15 min) reappearance of PGE2 receptors (to 60-93% of controls). Reappearance occurs whether down regulation is induced in vitro by endogenously synthesized prostaglandins, added PGE2 or dimethyl-PGE2, or in vivo after injection of dimethyl-PGE2. Cycloheximide does not affect down regulation but significantly prevents subsequent recovery of the receptors. In contrast, neither colchicine nor chloroquine influences homologous regulation of renal prostaglandin receptors. These results document an agonist-induced reversible cycling of renal prostaglandin receptors which may determine the effectiveness of prostaglandin action in normal and pathologic states.

Down-regulation of surface beta-adrenoceptors on intact human mononuclear leukocytes. Time-course and isoproterenol concentration dependence

Incubation of human mononuclear leukocytes (MNL) in vitro with isoproterenol resulted in a rapid loss of surface beta-adrenoceptors, determined by radioligand binding at 4 degrees. Isoproterenol concentrations in the range of 10 nM to 100 microM resulted in significant down-regulation of beta-adrenoceptors. At a concentration of 1 microM isoproterenol, the time-dependent loss of surface beta-adrenoceptors closely paralleled the loss in isoproterenol-stimulated adenylate cyclase activity. If receptor number in intact cells was determined at 32 degrees, hardly any loss in receptor number was observed, due to reversal of down-regulation during the incubation period. When beta-adrenoceptor number in broken cell preparations was determined by [125I]cyanopindolol binding at 37 degrees no significant loss was observed, even after 2 hr of isoproterenol treatment, while [3H]CGP-12177 binding resulted in a similar reduction in binding sites as in intact cells. Reversal of loss in surface beta-adrenoceptors was rapid after 1 hr pretreatment with isoproterenol, but followed a biphasic time course after 4 hr pretreatment, with an initial rapid return of about 40% of the down-regulated receptors, followed by a slow, gradual reappearance of receptors. The results indicate that catecholamine exposure leads to a rapid sequestration of MNL surface beta-adrenoceptors away from the cell surface, to a compartment where they are inaccessible to the hydrophilic ligand [3H]CGP-12177 as well as to the lipophilic ligand [125I]cyanopindolol at 4 degrees. Up to 2 hr of isoproterenol treatment does not lead to any breakdown of sequestered beta-adrenoceptors, as they are still recognized by [125I]cyanopindolol binding in broken cell preparations.

Effect of a high carbohydrate diet on cardiac alpha-1 and beta adrenoceptors

Short-term (2 weeks) administration of a high-fructose diet to euthyroid Sprague-Dawley rats results in a significant (18%) increase of cardiac beta-adrenoceptors without any change in their affinities or their distribution between the plasma membrane and a vesicular fraction. A much smaller increase occurs in the number of alpha 1-adrenoceptors (8% higher than in rats fed a regular diet). The high-carbohydrate diet induced a 39% increase in beta-adrenoceptor numbers/heart in hypothyroid animals and a 19% increase in the total number of alpha 1-adrenoceptors. These results strongly suggest that changes in cardiac performance after dietary manipulations may be mediated, in part, through enhancement of adrenergic pathways.

Alterations of beta-adrenoceptors subsequent to myocardial infarction

Elevations and reductions of the number of beta-adrenoceptor binding sites are dependent on the strength and the duration of receptor interaction with respective agonists. In the paper presented here, results obtained by the authors concerning biosynthesis, storage and release of catecholamines following experimentally induced infarction of the myocardium in rats are compared with those of other authors for other species. Principally, storage and release of noradrenaline from ischemic hearts do not differ with the mode of inducing tissue hypoxia (stopped-flow ischemia, coronary artery ligation, occlusion of the great cardiac vein), nor for various species (rat, dog, guinea-pig). Differences are, however, present in the results of several beta-adrenoceptor binding studies performed after experimental myocardial infarction. Following acute infarction, an increase in the number of beta-adrenoceptor binding sites is generally observed, which is explained on the basis of an externalization of receptors from the cytoplasm ot the sarcolemmal membrane. Results pertaining to 2-3 days after infarction are not uniform: in guinea-pig hearts a marked drop in the number of beta-adrenoceptors has been reported, a mild rise in the number has been detected in the left and right ventricle of rat hearts. These divergent observations could arise from the experimental protocol employed, for instance in the binding assay and in the pretreatment given to the hearts prior to assay.

Kinetic analysis of cardiac beta-receptors in perfused working rabbit hearts

Myocardial adrenergic beta-receptors were isolated and partially purified from the nonischemic perfused working rabbit hearts. Using highly radioactive beta-receptor antagonist, dihydroalprenolol (DHA), properties of beta-receptors were investigated by kinetic equilibrium analysis when the physiological function of the heart appeared to be normal. At the concentration of 10 nM DHA dissociation constant (Kd) was 14.9 nM and there were at least two distinctly different DHA binding sites, based on the analysis of the dissociation rate of DHA-receptor complex. Identification of the two distinctly different DHA binding sites was not obvious from the analysis of Scatchard plot.

Carbachol induces desensitization of cardiac beta-adrenergic receptors through muscarinic M1 receptors

Incubation of enzymatically dissociated cardiac myocytes with carbachol leads to a time- and concentration-dependent loss of beta-receptors assayable with [3H]CGP-12177. This loss is due to a redistribution of beta-receptors from the plasma membrane to a cytosol-derived vesicular fraction, consistent with an internalization process. The carbachol effects are not influenced by gallamine or oxotremorine which interact with the high-affinity (M2) muscarinic receptors. These results suggest that carbachol-induced desensitization is secondary to activation of protein kinase C by diacylglycerols generated through M1 receptor-linked phosphoinositide hydrolysis.

Isoproterenol-induced desensitization to the positive inotropic effect of isoproterenol in ventricular strips isolated from carp heart (Cyprinus carpio)

Whether the positive inotropic effect of isoproterenol in ventricular strips of carp heart is altered by previous exposure to the agonist was studied. Isoproterenol produced a concentration-dependent positive inotropic effect in these preparations which was competitively antagonized by propranolol. Isoproterenol dose-response curves were shifted significantly downward and to the right after previous treatment with and removal of isoproterenol. The desensitization could be demonstrated after 5 min incubation and near maximal desensitization was observed after 30 min exposure to 1000 nM isoproterenol. The responsiveness was only partially recovered after 180 min incubation in drug-free medium. Isoproterenol produces a rapidly developing desensitization to inotropic effects of the beta-adrenergic agonist. This phenomenon may be mediated by alterations in beta-adrenergic receptors, adenylate cyclase and/or the beta receptor-adenylate cyclase coupling mechanism.