Molecular mechanisms of receptor desensitization using the beta-adrenergic receptor-coupled adenylate cyclase system as a model

The avian allantois: a depot for stress-released catecholamines

Plasma and amniotic and allantoic fluid of 10- and 14-day-old chicken embryos contain free dopamine (DA), norepinephrine (NE), and epinephrine (E). Compared with postnatal chickens, concentrations of DA and E in the plasma are very high, and they are even higher in the allantoic fluid. In contrast, the allantoic concentration of NE is below the plasma level. In the amniotic fluid, the concentrations of all three catecholamines (CAs) are below the plasma levels. High concentrations of DA and E in the allantoic fluid after opening of the egg shell decline during the following 24 hr, which indicates that they are due to stress. Asphyxia, handling, disturbance of allantoic fluid, and cooling are also perceived as stress and are followed by immediate accumulation of CAs in the allantoic fluid. DA and E respond to stress in like manner, while NE often responds with an opposite trend. It appears that the avian allantois, in addition to its role in respiration and urea disposal, also serves the instant CA removal from the circulation. Both the amniotic and the allantoic membranes of the chicken should be ideal models for the study of CA transport mechanisms.

Pharmacokinetic-pharmacodynamic modeling: time-dependent protein binding--an alternative interpretation of clockwise and counterclockwise hysteresis

Development of effect compartment model theory has greatly enhanced our understanding of the relationship between pharmacokinetics and pharmacodynamics. When effect versus concentration in serum (usually total concentration) is plotted and counterclockwise hysteresis is observed, an initial disequilibrium between receptor(s) and serum is generally presumed and an effect compartment model is used; alternatively, clockwise hysteresis may infer tolerance, which may be characterized by an adaptation model. In this simulation study, the influence of time-dependent binding to serum protein on the relationship between effect and concentration in serum was investigated. In these simulations, time-dependent protein binding occurred as a result of an increase in protein concentration in serum or displacement by a metabolite. When concentration of free drug in serum was responsible for the pharmacological response, and response versus total drug concentration in serum was plotted, counterclockwise hysteresis, consistent with an effect compartment, occurred with a time-dependent decrease in binding to serum protein. Clockwise hysteresis, consistent with tolerance, occurred with a time-dependent increase in binding to serum protein. For both sets of simulations, no hysteresis was observed when response was plotted against concentration of free drug in serum. These results indicate that, when response is related to concentration of free drug, measurement of concentration of free drug may allow a clearer interpretation of the pharmacokinetic-pharmacodynamic relationship.

Gastric chief cells: receptors and signal-transduction mechanisms

Elucidation of receptors and mediators regulating gastric pepsinogen secretion has lagged behind understanding of the factors that control acid secretion. During the past decade, as a consequence of the development of in vitro models for studying the control of pepsinogen secretion at the cellular level, much information about chief cell receptors and signal-transduction mechanisms has been obtained, including the identification and characterization of receptors for secretin, vasoactive intestinal polypeptide, cholinergic agonists, gastrin, cholecystokinin, peptide YY, and cholera toxin. Moreover, these cell preparations have permitted secretagogue-induced changes in chief-cell calcium concentration, protein kinase C distribution, and phosphoinositide and cyclic nucleotide content to be measured and related to changes in pepsinogen secretion. This article reviews these advances, discusses areas of uncertainty and controversy, and indicates areas for future investigation.

Differential short-term desensitization to vasopressin, isoproterenol, glucagon, parathyroid hormone and calcitonin in the thick ascending limb of rat kidney

Short-term desensitization to hormone-induced cAMP accumulation was investigated in the medullary (MTAL) and the cortical (CTAL) thick ascending limbs of Henle's loop isolated by microdissection from the rat kidney. The following agonists were studied: vasopressin, glucagon and human calcitonin in the MTAL, and vasopressin, glucagon, human calcitonin, parathyroid hormone (PTH) and the beta-adrenergic agonist isoproterenol in the CTAL. Isolated tubules were preincubated in vitro for 60 min in the presence or absence of a maximal concentration of one of the five agonists (vasopressin 10 nM, glucagon 10 nM, calcitonin 100 nM, PTH 10 nM, isoproterenol 1 microM). Desensitization induced by each agent to its own action was then quantified by measuring the amount of cAMP accumulating in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine and the same agonist concentration as that used during preincubation. In the MTAL, as previously reported, preincubation with vasopressin led to a marked (80%-85%) desensitization to this hormone. A significant hormone self-induced desensitization of about 45% was also obtained with glucagon, but not with calcitonin. In the CTAL, the following order of potency to elicit desensitization was observed: vasopressin (80%) greater than isoproterenol (50%) greater than glucagon (30%) greater than PTH (20%, NS) greater than calcitonin (10%, NS). Thus, the magnitude of desensitization varied greatly from one hormone to another, but for a given hormone, was of roughly similar extent in both MTAL and CTAL.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of carbachol- and histamine-induced inositol phospholipid hydrolysis in a human oligodendroglioma

A stable cell line derived from a human oligodendroglioma (HOG) was used to study the regulation of muscarinic- and histamine receptor-mediated phosphoinositide hydrolysis. Both carbachol and histamine increased inositol monophosphate (InsP) accumulation in a dose- and time-dependent manner in the presence of lithium and the effect of simultaneous addition of carbachol and histamine was additive, implying independent signal transduction pathways. Homologous desensitization of muscarinic, but not histamine receptors, could be demonstrated although neither receptor type appeared to be heterologously desensitized. [3H]InsP accumulation in HOG cells was also stimulated by fluoride, suggesting guanosine triphosphate (GTP)-binding protein involvement, but phosphoinositide (PtdIns) hydrolysis was not sensitive to pertussis toxin. Phorbol ester-activation of protein kinase C (PKC) inhibited both muscarinic and histamine receptor-stimulated InsP release but did not attenuate either the fluoride-induced release of InsP nor beta-adrenergic receptor-mediated stimulation of adenylate cyclase activity. Taken together, we conclude that muscarinic and histamine receptors are differentially regulated through both PKC-dependent and -independent mechanisms, and that feedback inhibition of PtdIns turnover occurs proximal to the GTP binding proteins.

Quantification of Gi alpha-proteins in the failing and nonfailing human myocardium

Heterotrimeric Gi-proteins play an important role in the regulation of cardiac adenylate cyclase. Besides a downregulation of beta-adrenoceptors with an accompanying reduction of the positive inotropic effects of cAMP-dependent positive inotropic agents, an increase of pertussis toxin substrates (Gi alpha-proteins) has been observed. The increase of Gi alpha has been reported to be associated with a reduced adenylate cyclase activity in dilated cardiomyopathy from hearts with heart failure class NYHA IV. Since the quantification of Gi alpha-proteins with the pertussis toxin labeling method is hampered by a number of biological and technical factors, Gi alpha-proteins were quantified radioimmunologically using the iodinated C-terminus 125I-KENLKDCGLF as tracer, purified retinal transducin alpha as standard, and an antiserum (DS 4) raised against the same peptide. With this technique Gi alpha-proteins were increased by 118% in dilated cardiomyopathy and 48% in ischemic cardiomyopathy, although pertussis toxin substrates were only increased by 40% in dilated cardiomyopathy and no change was observed in ischemic cardiomyopathy. In cardiomyopathic tissue, an inverse relationship was observed between the increase of Gi alpha and the positive inotropic effects of isoprenaline or milrinone. These data provide evidence for a functional role of Gi alpha in the reduced positive inotropic effects of cAMP-dependent positive inotropic agents. In addition, results obtained with pertussis toxin labeling for quantification of Gi alpha-proteins do not necessarily reflect the expression of Gi alpha-proteins in the human myocardium.

Cyclic AMP modulates sensory-neural communication at the vestibular end organ

Adenosine 3':5'-cyclic phosphate (cAMP) is a second messenger that plays an important role in mediating neuronal interactions in many systems. A possible role for cAMP in sensorineural communication at the vestibular end organ was studied. The putative roles for cAMP action investigated here were: the ability of cAMP to act as the second messenger for the efferent transmitter, acetylcholine, and the possible involvement of cAMP in modulating spontaneous or mechanically-evoked afferent nerve firing. Levels of cAMP were increased pharmacologically with forskolin, 3-isobutyl-1-methyl xanthine (IBMX) and dibutyryl cAMP. Changes in multiunit afferent nerve firing measured from the ampullar nerve of the semicircular canal, and the transepithelial potential measured across the neuroepithelium of the semicircular canal were recorded. At selected doses, all drugs produced a similar increase in spontaneous multiunit afferent nerve firing with a concomitant decrease in the transepithelial potential. Mechanically-evoked hair cell activity and the response to exogenously applied acetylcholine were unaffected by these drugs. We are suggesting that the excitatory aspects of the acetylcholine response are not mediated via a cAMP-dependent mechanism. However, cAMP does play an important role in modulating spontaneous afferent nerve firing in the semicircular canal. The finding that spontaneous afferent nerve firing can be biochemically modulated without altering mechanically-induced afferent firing is novel and deserves further investigation.

The change in the threshold for short-term desensitization in isolated smooth muscle cells showing an all-or-none response to acetylcholine

1. Isolated smooth muscle cells from guinea-pig taenia caecum were prepared by collagenase digestion. The cells showed an all-or-none response to acetylcholine (ACh) under our experimental conditions. 2. Desensitized cells showed an all-or-none response but required a higher concentration of ACh for induction of contraction, indicating that the desensitization was due to a change in the threshold concentration. 3. In [3H]-quinuclidinyl benzilate ([3H]-QNB) binding to the desensitized cells, KD and Bmax were not significantly different from those estimated in the control cells. The competitive inhibition curve for specific binding of [3H]-QNB by ACh in the desensitized cells was in agreement with that of control cells. 4. The ACh-stimulated increase of the 45Ca2+ influx was very rapid and correlated well with the contraction of the cells. The concentrations of ACh inducing the maximal 45Ca2+ influx were increased by desensitization. 5. These results indicated that although the binding of ACh to the receptor was not changed by desensitization, the threshold concentration of ACh for their contraction was raised by desensitization, and the 45Ca2+ influx accompanying the contraction was shifted to the side of high concentration of ACh. 6. These results suggest that the development of short-term desensitization is due to an uncoupling of the receptor from the mechanism for initiation of the contraction.

cAMP-dependent down-regulation of endothelin-1 receptors on rat astrocytoma C6 cells

The density of endothelin-1 (ET-1) receptors on rat astrocytoma C6 cells is down-regulated by activation of protein kinase C (PKC). We have investigated whether intracellular accumulation of cyclic adenosine monophosphate (cAMP) may also modulate surface ET-1 receptor number. The density of ET-1 receptors was measured by binding of [125I]ET-1 on rat astrocytoma C6 intact cells exposed to catecholamines, dibutyryl-cAMP or forskolin. Prolonged exposure of the cells to the beta-adrenergic agonists, isoproterenol or noradrenaline, results in a time- and dose-dependent decrease in cell surface ET-1 receptor number. This decrease proceeds slowly: maximal down-regulation is obtained by 6-7 h and sustained for up to 24 h in the presence of 10 microM isoproterenol. Since this down-regulation is mimicked by dibutyryl-cAMP (4 microM) and by forskolin (10 microM), we conclude that ET-1 receptors are susceptible to down-regulation through a cAMP-dependent pathway.

Agonist-induced modulation of agonist binding to alpha 1-adrenoceptors in bovine aorta

Prolonged exposure of tissues to hormone agonists results in a subsequent reduction in the sensitivity of the tissue through a process known as desensitization. The desensitization response, either homologous or heterologous, has been shown to be correlated with receptor phosphorylation. Recently we have provided evidence that protein kinase C, when activated by a phorbol ester, regulates alpha 1-adrenoceptor coupling to a G-protein. In the present study, the effects of epinephrine (10 microM) pretreatment on the binding behavior of the alpha 1-adrenoceptor were determined from radioligand binding assays at 25 degrees and 2 degrees C. Pretreatment of tissues with epinephrine for 25 min moderately decreased [3H]prazosin binding by 12% (Bmax 121.5 fmol/mg) in comparison to control (139.3 fmol/mg) with no change in its affinity. The second consequence of desensitization by epinephrine is a decrease in the affinity of agonist binding to alpha 1-adrenoceptors associated with uncoupling of the receptors from the G-protein. In control membranes, at 25 degrees C, epinephrine defined two different affinity states of the receptor, viz. high affinity (KDH 16.5 nM, % RH 21) and low affinity (KDL 710 nM, % RL 79). The high affinity state formed at 25 degrees C is stabilized by forming a ternary complex with a G-protein. Addition of guanylylimidodiphosphate (Gpp(NH)p) reduced the stability of this complex resulting in a loss of high affinity sites in control membranes. On the other hand, epinephrine treated membranes exhibited only a single class of low affinity agonist binding (KDH 659 nM) and further, Gpp(NH)p had no significant effect on binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Different mechanisms of beta-adrenoceptor down-regulation by chronic imipramine and electroconvulsive treatment: possible role for protein kinase C

The aim of this study was to find out how protein kinase C (PKC) is involved in down-regulation of the beta-adrenoceptor in cortical slices of rats subjected to antidepressant treatments. The responses of the cyclic AMP generating system to forskolin, isoproterenol, and noradrenaline were tested in the absence and presence of a PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA). The antidepressive treatments applied were chronic administration of imipramine and electroconvulsive shock. The potentiating effect of the phorbol ester on cyclic AMP response to isoproterenol was retained in imipramine-treated animals and even accentuated in rats subjected to electroconvulsive treatment; the TPA effect on noradrenaline-induced cyclic AMP response was blunted in rats receiving imipramine, but augmented in those receiving electroconvulsive treatment. In imipramine-treated rats the beta-down-regulation was still evident in the presence of TPA; after electroconvulsive treatment the phorbol ester-induced potentiation was so high that no significant beta-down-regulation could be observed. No procedure affected the response to forskolin. The beta-down-regulation that develops during chronic imipramine treatment differs from that caused by chronic electroconvulsive treatment; in both cases it is not related to the direct effect on adenylate cyclase.

Attenuation of circadian variation by combined antianginal therapy with suppression of morning and evening increases in transient myocardial ischemia

The circadian variation of total ischemic activity was examined during 3289 hours of ambulatory ECG monitoring in 101 patients with stable angina pectoris and proved coronary artery disease, who were not receiving any prophylactic antianginal therapy. The 101 patients displayed 411 episodes of ischemia, 312 (76%) of which were silent; a circadian rhythm was noted for the occurrence of total and silent ischemia. Thirty-eight percent of the ischemic episodes occurred between 6 AM and 12 noon, and total and silent ischemia were significantly more frequent during this period compared with the other three 6-hour periods (p less than 0.01); a lesser peak was noted in the evening. The effects of metoprolol and combined therapy with metoprolol and nifedipine on the circadian variation of ischemic activity were studied in two subgroups of patients in a random, double-blind study design (31 patients receiving metoprolol and 42 receiving combined therapy). During therapy with metoprolol the morning increase in ischemic activity was attenuated, and the highest frequency of ischemia was then noted in the evening (6 AM to 12 noon compared with 6 PM to 12 midnight; p less than 0.05). Combined therapy abolished the morning peak as did metoprolol monotherapy, but even the evening increase in ischemic activity was attenuated (p less than 0.05). The diurnal distribution of the mean heart rate at the onset of ischemia, when patients were off therapy, showed a morning increase similar to the increase in ischemic activity but no second peak in the evening.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a 7-day treatment with idazoxan and its 2-methoxy derivative RX 821002 [correction of RX 821001] on alpha 2-adrenoceptors and non-adrenoceptor idazoxan binding sites in rabbits

1. The present study investigates the influence of a 7-day treatment with 2 mg kg-1, s.c., twice daily of RX 821002 (an alpha 2-adrenoceptor antagonist which binds only to alpha 2-adrenoceptors) or idazoxan (alpha 2-antagonist which binds to alpha 2-adrenoceptors and also to non-adrenoceptor idazoxan binding sites: NAIBS) on alpha 2-adrenoceptor (labelled with [3H]-RX 821002) and NAIBS (labelled with [3H]-idazoxan) number in three tissues (adipocytes, colocytes and platelets) in the rabbit. 2. Acute administration of RX 821002 or idazoxan increased plasma non-esterified fatty acids (NEFA) and catecholamine levels with no change in plasma glucose levels. 3. The 7-day treatment with RX 821002 or idazoxan failed to influence food intake, total body weight or perirenal adipose tissue weight. 4. RX 821002 and idazoxan increased the number of [3H]-RX 821002 binding sites in adipose tissue with no change in colocytes or platelets. 5. RX 821002 and idazoxan failed to modify [3H]-idazoxan binding sites on adipocytes and colocytes. No significant [3H]-idazoxan binding was detected on rabbit platelets. 6. The results show that a 7-day treatment with alpha 2-antagonists induces an up-regulation in adipocyte alpha 2-adrenoceptors. In contrast, this phenomenon does not involve all the tissues since colocytes and platelets escape the effects of alpha 2-antagonists. The data suggest a differential regulation of alpha 2-adrenoceptors according to their location. 7. The fact that NAIBS did not vary suggests that alpha 2-adrenoceptors and NAIBS are two different entities. Finally, since RX 821002 and idazoxan exert similar effects after either acute or chronic treatment, it is suggested that NAIBS are not involved in the control of catecholamine release or in NEFA or glucose metabolism.

Angiotensin-induced desensitization of the phosphoinositide pathway in cardiac cells occurs at the level of the receptor

Previous studies show that angiotensin II (Ang II) increases phosphoinositide turnover in cultured neonatal heart cells. Ang II has also been shown to transiently increase spontaneous beating behavior in these cells. In this study we seek to identify the molecular mechanism underlying this rapid (3-5-minute) desensitization. Time-course studies on the accumulation of [3H]inositol phosphates indicate that the loss in functional responsiveness correlates with reduced efficacy of Ang II to activate the phosphoinositide path. Binding studies with 125I-Ang II revealed that there was no change in surface receptor binding capacity during the time in which desensitization developed. Normal phosphoinositide and functional responses are observed when desensitized cells are treated with probes that activate the cardiac phosphoinositide pathway at discrete steps. These studies reveal that the functional status of the major components of the phosphoinositide signaling pathway, including G proteins, phospholipase C, and protein kinase C (PKC), are normal during maintained Ang II desensitization. To study the potential role of PKC in Ang II desensitization, the cells are treated with TPA for 24 hours, which downregulates PKC activity. PKC-depleted cells rapidly desensitize after Ang II application. We conclude that the selective Ang II-evoked biochemical/functional desensitization involves inhibition at the level of the receptor, rather than at a component downstream in the path, and that this process is independent of PKC and loss of surface binding capacity.

Reduced responsiveness of adenylate cyclase in alveolar macrophages from patients with asthma

Alveolar macrophages from patients with asthma accumulated less cyclic adenosine monophosphate when these macrophages were exposed to isobutyl methylxanthine, salbutamol, or prostaglandin E2, compared to cells from control subjects without asthma, and the degree of the hyporesponsiveness was related to the severity of asthma. In addition, a significantly lower adenylate cyclase activity was observed in crude membrane fractions of macrophages from the group with asthma in the presence of salbutamol and prostaglandin E2. The refractoriness observed in patients with asthma is thus not accounted for by a specific beta-adrenergic desensitization at the adenylate cyclase receptor level but should rather be explained by a cyclic adenosine monophosphate-dependent postreceptor mechanism.

Lithium sensitive G protein hyperfunction: a dynamic model for the pathogenesis of bipolar affective disorder

Guanine nucleotide binding proteins (G proteins) play a pivotal role in information transduction from various membrane receptors to a variety of intracellular effector systems. By influencing the metabolism of adenylate cyclase and phosphatidylinositol, G proteins affect the activities of both cAMP-dependent protein kinase (kinase A) and protein kinase C. The hypothesis of this present study addresses the oscillatory behavior of symptoms observed in manic-depressive patients by suggesting that the cellular phosphorylation state in the central nervous system, which results from the relative activity of protein kinase A and protein kinase C, determines the affective state. From this hypothesis, we developed a kinetic model based on self- and inter-regulatory steps between these two protein kinase systems. The solutions of the differential equations governing this kinetic model can describe oscillatory pathological affective states. More specifically, we show that hyperfunction of G proteins leads to an unstable 'catastrophic' dynamic system characteristic of a manic or depressive state, and that lithium treatment attenuates G protein function and damps the oscillatory system to yield a stable state.

Relationship between membrane fluidity and adrenoceptor binding in depression

Membrane fluidity and adrenergic receptor binding were studied in platelets of depressed patients before and during treatment with desmethylimipramine to investigate the relationship between the alpha 2-adrenergic receptor and its membrane environment in depression. Most samples came from a previous study in which we observed higher 3H-para-aminoclonidine (3H-PAC) binding in platelets from depressed patients compared to healthy subjects. Fluidity was measured by steady state diphenylhexatriene (DPH) anisotropy in both purified plasma membranes and in intracellular membrane preparations from platelets. No differences were observed in DPH membrane fluidity, per se, indicating that fluidity changes probably do not underlie either the increased alpha 2-adrenergic receptor binding in depression or the normalization of binding during treatment. However, lower intracellular membrane fluidity was correlated with higher binding to 3H-PAC site-1 in healthy subjects, but not in depressed patients. Thus, during depression there may be a disruption in the normal relationship between the adrenergic receptor and its membrane environment.

Enhancement of beta-adrenoceptor function after reperfusion following cardioplegic arrest in rat hearts

We investigated alterations in a beta-adrenoceptor (BAR) system after reperfusion following hypothermic ischemia induced by a high-potassium (18 meq/liter) cardioplegic solution in isolated rat hearts. Materials were divided into two groups: the reperfusion group (Gr-R, n = 5) with 40 min reperfusion following 40 min cardioplegic arrest (10 degrees C) and the control group (Gr-C, n = 5) with no ischemia as time-matched perfused control. BAR and adenylate cyclase activities in crude membrane fractions were compared. Results showed that basal, NaF-, and forskolin-stimulated adenylate cyclase activity did not differ between the two groups. The maximal enzyme activity in the presence of 10(-4) M (-)-isoproterenol was higher in Gr-R than in Gr-C, while the net activity stimulated by (-)-isoproterenol was 74% higher in Gr-R than in Gr-C. The [125I]Iodocyanopindolol [( 125I]CYP) binding assay showed that BAR density was 14% higher in Gr-R than in Gr-C, while the affinity was not significantly different. The IC50 values of (-)-isoproterenol for [125I]CYP binding were lower in Gr-R than in Gr-C and the proportion of high-affinity binding sites was higher in Gr-R than in Gr-C. These data showed that 40 min reperfusion following hypothermic cardioplegic arrest (40 min) resulted in significant increases in myocardial BAR density and maximal (-)-isoproterenol-stimulated adenylate cyclase activity, and enhancement of BAR affinity for beta-adrenergic agonists due to the increase in the proportion of high-affinity binding sites.

Biology of pancreatic cancer

Pancreatic cancer is the fifth leading cause of death from malignant disease in Western society. Apart from the fortunate few patients who present with a resectable small pancreatic adenocarcinoma, conventional treatment offers no hope of cure and has little palliative value. Over the past two decades major steps have been made in our understanding of the biology of pancreatic growth and neoplasia. This review sets out to explore these advances, firstly in the regulation of normal pancreatic growth, and secondly the mechanism which may be involved in malignant change of the exocrine pancreas. From an understanding of this new biology, new treatment strategies may be possible for patients with pancreatic cancer.

Low doses of l-sulpiride down-regulate striatal and cortical dopamine receptors and beta-adrenoceptors

There is now clinical evidence that l-sulpiride has antidepressant effects when administered at low, non-neuroleptic doses. Down-regulation of beta-receptor-linked adenylate cyclase is a well-documented adaptive response to chronic administration of antidepressant drugs. In this study, we investigated dopamine receptor and beta-adrenoceptor changes induced by chronic administration of low doses of l-sulpiride. The data indicate that striatal D1 and D2 receptor function was desensitized by the treatment, which suggests that at low doses l-sulpiride preferentially blocks D2 autoreceptors, leading to increased dopamine release. l-Sulpiride also induced a selective down-regulation of beta-receptor-associated adenylate cyclase activity in the frontal cortex, but not in the striatum, which does not receive norepinephrine projections. Taken together these data suggest that cortical noradrenergic terminals may be endowed with dopamine D2 receptors controlling norepinephrine release and that blockade of this dopaminergic inhibitory modulation may be involved in the antidepressant effects of l-sulpiride.

Isoprenaline-induced increase in mRNA levels of inhibitory G-protein alpha-subunits in rat heart

Long-term beta-adrenergic stimulation has been shown to desensitize the beta-adrenoceptor/adenylyl cyclase signalling pathway at both the receptor and the G-protein level. To further elucidate the cellular mechanism of G-protein regulation we investigated the influence of prolonged infusion of isoprenaline (2.4 mg/kg.d) on myocardial mRNA levels of different G-protein alpha-subunits in rats. For comparison rats were treated with triiodothyronine (T3; 0.5 mg/kg.d) which induces cardiac hypertrophy like isoprenaline but has different effects on the adenylyl cyclase system. Isoprenaline- and T3-treated animals developed an increase in heart/body weight ratio of 41 +/- 3% and 27 +/- 4%, respectively (P less than 0.05). Isoprenaline increased myocardial total RNA concentration by 39 +/- 6% (P less than 0.05). Hybridization with 32P-labeled rat cDNAs demonstrated an expression rank order of Gs alpha-mRNA greater than Gi alpha-2-mRNA greater than Gi alpha-3-mRNA and no detectable expression of Gi alpha-1-mRNA in rat myocardium. mRNA levels of Gs alpha, Gi alpha-2 and Gi alpha-3 were 36.9 +/- 1.28, 10.7 +/- 1.07 and 3.7 +/- 0.19 pg/micrograms total RNA, respectively. Isoprenaline increased Gi alpha-2- and Gi alpha-3-mRNA concentrations per microgram total RNA by 49 +/- 18% and 27 +/- 7%, respectively (P less than 0.05). This effect was abolished by simultaneously administered propranolol (9.9 mg/kg.d), indicating a beta-adrenoceptor-mediated mechanism. In contrast, T3-induced cardiac hypertrophy was not accompanied by changes in Gi alpha-mRNA expression. Gs alpha-mRNA levels were unaffected by either treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of adenosine and adenosine transport in ethanol-induced cellular tolerance and dependence. Possible biologic and genetic markers of alcoholism

Acute exposure to ethanol in culture inhibits adenosine uptake into cells, thereby increasing the concentration of extracellular adenosine. Extracellular adenosine then reacts with adenosine A2 receptors to stimulate intracellular cAMP production. During prolonged exposure to ethanol, the increase in cAMP is followed by the development of heterologous desensitization of receptors coupled to adenylyl cyclase via Gs, the stimulatory GTP-binding protein. Ethanol-induced heterologous desensitization appears to be due to a reduction in mRNA and protein for G alpha s, a subunit of Gs. This is an example of cellular dependence on ethanol. The important implication of these findings is that a selective inhibitory effect of ethanol on adenosine uptake can lead to desensitization of diverse receptors coupled to cAMP production. Such changes could contribute to the pleiotropic effects of ethanol in the brain and other organs. Prolonged exposure to ethanol also alters the nucleoside transport system. While ethanol inhibits adenosine uptake into naive cells, ethanol no longer inhibits adenosine uptake into cells that have adapted to ethanol. This resistance to ethanol inhibition appears to be a form of cellular tolerance to ethanol. Thus, there appears to be a synergism between ethanol-induced heterologous desensitization of receptor-stimulated cAMP production (cellular dependence) and resistance to ethanol inhibition of adenosine uptake (cellular tolerance), because both lead to reduced intracellular levels of cAMP. Our studies on cAMP signal transduction in cell culture are directly relevant to the pathophysiology of human alcoholism. Heterologous desensitization of cAMP production is demonstrable in lymphocytes taken from actively drinking alcoholics; this measurement appears to be a biologic marker of active alcohol consumption. In addition, regulation of adenosine receptor-dependent cAMP production may be altered in patients at risk to develop alcoholism because of genetic factors. Thus, lymphocytes from alcoholics cultured many generations in the absence of ethanol show increased adenosine receptor-dependent cAMP production and increased sensitivity to ethanol-induced heterologous desensitization. These persistent phenotypic abnormalities in cell culture could be used as genetic markers for alcoholism. Studies are under way to test this possibility.

Blockade by the local anaesthetic, tetracaine, of desensitization of Ca-induced Ca release after muscarinic stimulation in smooth muscle

1. Desensitization of contractile responses dependent on release of intracellularly stored Ca elicited by carbachol, histamine or caffeine was measured after desensitizing treatment with carbachol or histamine in the presence or absence of local anaesthetics in Ca-free solution containing 2 mM EGTA in the smooth muscle of guinea-pig taenia caecum. 2. Histamine-induced homologous desensitization was inhibited by tetracaine and procainamide. Dibucaine did not exert an inhibitory effect on the desensitization. This is consistent with our previous findings concerning the effects of local anaesthetics on the desensitization of histamine H1-receptors measured under normal physiological conditions. 3. Carbachol induced a functional change of intracellular Ca stores which resulted in heterologous desensitization. Tetracaine completely blocked carbachol-induced desensitization of the caffeine-elicited contraction, but in the case of carbachol-induced desensitization of carbachol- and histamine-elicited contractions, this blocking effect of tetracaine was very weak and absent, respectively. The other local anaesthetics used did not affect the desensitization. These results suggest that the Ca-induced and inositol trisphosphate-induced Ca release mechanisms were both desensitized by carbachol and that the desensitization of the Ca-induced Ca release mechanism was selectively blocked by tetracaine.

Adrenergic receptor responsiveness and congestive heart failure

Our knowledge and understanding of the structure, mechanism of action and regulation of receptor-adenylate cyclase systems have increased dramatically in the last few years. A family of receptors (including the beta-adrenergic receptors) and guanine nucleotide regulatory proteins (G proteins) have been purified and cloned. Structure-function studies are beginning to provide insight into how the various components of the transmembrane signaling apparatus interact to promote alterations in the activity of the effector systems. Much effort has been devoted to understanding how various pathophysiologic conditions, such as ischemia or congestive heart failure, and the therapeutic methods used to treat such conditions perturb or regulate receptor systems. It has become abundantly clear that such regulation does occur but is not restricted to simple alterations in receptor number, and may well involve covalent modification (phosphorylation) of receptors or alteration in the ability of receptors to interact with G proteins. In addition, regulation of the quantity or functionality of the various G proteins and the catalytic unit of adenylate cyclase itself appear to occur. For example, recent evidence suggests that congestive heart failure in humans is associated with a decreased number of beta-adrenergic receptors as well as an increased quantity of the inhibitory G protein (Gi). These alterations may provide important insight into how to develop new therapeutic methods. Mechanisms generally responsible for transmembrane signaling, how the components are regulated by pathophysiologic conditions, and drugs used to treat disease states are discussed in detail.

cAMP-dependent protein kinase activation affects vasopressin V2-receptor number and internalization in LLC-PK1 renal epithelial cells

The relationship between activation of the cAMP-dependent protein kinase (cAMP-PK) and ligand binding and internalization by the vasopressin renal (V2-type) receptor of LLC-PK1 renal epithelial cells was examined. Upon cAMP-PK activation through 1 h treatment with the cAMP analogue 8-bromo-cAMP (BrcA), a marked reduction in V2-receptor steady state number and internalization in LLC-PK1 cells was effected. In cells treated for 17 h with BrcA and hence down-regulated for cAMP-PK, the V2-receptor number was normal but internalization was markedly reduced. Cells of the LLC-PK1 mutant FIB4, which possesses about 10% parental cAMP-PK catalytic subunit activity, exhibited lower V2-receptor steady state number and internalization in comparison to untreated LLC-PK1 cells. A negative correlation was thus evident between cAMP-PK activation and V2-receptor number, and internalization. Phosphorylation by cAMP-PK may effect ligand-independent removal of receptor from the plasma membrane.

Ionic mechanism of the outward current induced by extracellular ejection of interleukin-1 onto identified neurons of Aplysia

The ionic mechanism of the effect of extracellularly ejected recombinant human interleukin-1-beta (rhIL-1) on the membrane of identified neurons R9 and R10 of Aplysia was investigated with voltage-clamp, micropressure-ejection, and ion substitution techniques. Micropressure-ejected rhIL-1 caused a marked hyperpolarization in the unclamped neuron. Clamping the same neuron at its resting potential level (-60 mV) and reejecting rhIL-1 with the same dose produced a slow outward current (I0(IL-1), 20-30 s in duration, 3-5 nA in amplitude) associated with a decrease in input membrane conductance. I0(IL-1) was decreased by depolarization and increased by hyperpolarization. The extrapolated reversal potential of I0(IL-1) was approximately +15 mV. I0(IL-1) was sensitive to changes in the external Na+ concentration but not to changes in K+, Ca2+ and Cl- concentrations, and was resistant to tetraethylammonium (5 mM) and 4-aminopyridine (5 mM). Neither perfusion of the neuron with 50 microM tetrodotoxin nor perfusion with 10 mM Co2+ seawater caused any changes in I0(IL-1). I0(IL-1) was partially reduced by 50 microM ouabain. These results suggest that extracellular IL-1 can induce a slow outward current associated with a decrease in Na+ conductance and the immunomodulator IL-1 can act directly on the nervous system as well as on the immune system.

Myocardial beta-adrenoceptor density and the distribution of beta 1- and beta 2-adrenoceptor subpopulations in children with congenital heart disease

Twenty-six infants and children with congenital heart disease (CHD) undergoing cardiac surgery were investigated for alterations in myocardial beta-adrenoceptor density. The patients were divided into three groups according to type and severity of CHD: group I consisted of 6 patients with acyanotic shunt lesions of moderate severity; group II comprised 13 children with severe acyanotic shunt and valve lesions and group III included 7 children with cyanotic CHD. The myocardial beta-adrenoceptor density was determined using (-)3-[125I]Iodocyanopindolol [( 125I]ICYP) and was reduced by approximately 50% in severe acyanotic CHD (33.6 fmol/mg protein) and cyanotic CHD (35.3 fmol/mg protein) in comparison with the group with less severe acyanotic shunt defects (64.4 fmol/mg protein). The affinity dissociation constant (Kd.ICYP) did not differ statistically between the groups. The proportion of beta 1- and beta 2-subpopulations was evaluated by ICI 118,551-[125I]ICYP competition studies. In group II (61.5%) and group III (69.1%) significant lower portions of beta 1-adrenoceptors were found compared with group I (78.2%). This shift of subpopulations was due to a decreased beta 1-receptor density while beta 2-receptor density was unchanged in all groups. While the plasma noradrenaline levels of group I were similar to those of a control group of 13 healthy children, respective values of group II and III were significantly elevated. A significant negative correlation was found between plasma noradrenaline levels and myocardial beta-adrenoceptor density. It is concluded that exposure of these receptors to increased circulating catecholamines, due to an enhanced sympathetic tone, leads to a reduction of their density.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic treatment with vasopressin analogues alters affinity of vasopressin receptors in the septum and amygdala of the rat brain

Accurel devices were used to administer AVP or AVP-antagonist into the lateral ventricle of the rat brain for 7 days. Neither AVP nor antagonist altered total binding site concentration in either septum or amygdala. However, treatment with antagonist caused a marked decrease in the affinity of the receptor for agonist in both the septum and amygdala.

Degradation of a-factor by a Saccharomyces cerevisiae alpha-mating-type-specific endopeptidase: evidence for a role in recovery of cells from G1 arrest

Mating response between opposite mating types of Saccharomyces cerevisiae is dependent upon alpha factor, a tridecapeptide, and a-factor, an isoprenylated, methyl esterified dodecapeptide whose interaction with the alpha target cell has not been characterized. We report on the first biochemical and physiological evidence of an alpha-mating-type-specific a-factor-degrading activity. Radioiodinated a-factor was used to identify the a-factor-degrading activity, which is cell associated, endoproteolytic, and not required for response to pheromone. a-factor degradation was not energy dependent, nor did it require pheromone internalization or interaction with its receptor. Phenylmethylsulfonyl fluoride and tosyl-L-arginyl-methyl ester inhibited degradation of a-factor and increased the time required by alpha cells to recover from a-factor-induced growth arrest and morphological alteration, providing evidence that a-factor degradation plays a role in the recovery of alpha cells from the pheromone response.

Beta-adrenergic function in aging. Basic mechanisms and clinical implications

Catecholamines have an important endocrine and neuroendocrine role in mediating a variety of autonomic functions. One consequence of normal aging, in particular in the cardiovascular system, is a decline in beta-adrenergic function associated with an alteration in responsiveness to beta-adrenergic therapy. The intrinsic ability for muscle contractility or relaxation is maintained with age and there appears to be an alteration in the process linking the receptor with the contractile or relaxation mechanisms. In rats, beta-adrenergic receptor density decreases with age in adipose tissues and most brain areas, is unchanged in lymphocytes, heart and lung, and increases in the liver. In humans, there are no receptor changes with age in either lymphocytes or brain. In contrast, the number of high-affinity receptors (or coupled receptors) decreases with age in most tissues. In addition, there is a decrease in membrane adenylate cyclase activity or cellular production of cyclic adenosine monophosphate (adenosine 3',5'-cyclic phosphate; cAMP). Plasma noradrenaline (norepinephrine) concentration increases with age. The reduced receptor number in some tissues (down-regulation), the reduced high-affinity receptors and the reduced hormone-stimulated adenylate cyclase activity with age suggests receptor desensitisation to increased plasma noradrenaline concentration. The inability of older animals to desensitise to beta-adrenergic agonists further supports this hypothesis. However, there is an additional post-receptor reduction in catalytic unit activity with age independent of desensitisation. Medications directed at the beta-adrenergic system are commonly used in the elderly. Many of the data on the impact of age on clinical responses are conflicting or unavailable. Concomitant disease, functional status, nutritional state and polypharmacy may play an even greater role than age. However, the available data can be used to guide the selection of therapy, anticipate side effects, and predict potential interactions with other medications and diseases.

A novel LLC-PK1 renal epithelial cell mutant impaired in in vivo down-regulation of cAMP-mediated hormonal response

A novel "cAMP-resistant" variant of LLC-PK1 renal epithelial cells which is impaired in in vivo down-regulation of response following hormonal stimulation of adenylate cyclase (AC) is described. Compared to parental cells, the BIB27 mutant exhibited markedly higher in vivo activation of cAMP-dependent protein kinase (cAMP-PK) in response to the hormones salmon calcitonin (SCT) or [Arg8]-vasopressin (AVP) or the AC activator forskolin. The activation of cAMP-PK subsequent to agonist stimulation also persisted much longer in the mutant than in LLC-PK1 cells, although the cAMP-PK of BIB27 cells was normal in terms of both absolute levels and regulation by cAMP in vitro. Intracellular cAMP accumulation was also much higher in BIB27 than in LLC-PK1 cells following agonist stimulation. Production of cAMP could be detected in BIB27 cells even 12 h after treatment with AVP or SCT, whereas cAMP production in LLC-PK1 had returned to basal within 1 and 8 h, respectively. High levels of free cAMP-PK catalytic (C) subunit in BIB27 persisted even 12 h after hormone addition, meaning that the higher cAMP production in BIB27 did not result in the normal down-regulation of cAMP-PK C subunit levels. In vitro AC activity in BIB27 cell homogenates could be stimulated by hormones or receptor-independent agonists, but to a lesser extent than in LLC-PK1 cell homogenates. The SCT and AVP concentrations promoting half-maximal AC activation in BIB27 cells were about 10- and 3-fold higher than parental, respectively. BIB27 accordingly appeared to possess a mutation in AC responsible for the impairment of both in vitro response to agonists and the normal in vivo down-regulation processes following hormonal stimulation.

Pheromone-induced phosphorylation of a G protein beta subunit in S. cerevisiae is associated with an adaptive response to mating pheromone

The mating pheromone response in S. cerevisiae is activated by a G protein-mediated signaling pathway in which G beta gamma is the active transducer of the signal. When exogenous pheromone is added to vegetatively growing cells, G beta is rapidly phosphorylated at several sites; phosphorylation does not require de novo protein synthesis. A mutation in G beta was constructed that eliminates signal-induced phosphorylation. This mutation leads to enhanced sensitivity to and impaired ability to recover from pheromone, but does not affect the ability of G beta gamma to transmit the mating signal. These phenotypes suggest that G protein phosphorylation mediates an adaptive response to pheromone-induced signaling. G beta phosphorylation does not require either the pheromone receptor C-terminus or the product of the SST2 gene, both of which mediate separate adaptive responses to pheromone. However, G beta phosphorylation is greatly facilitated by the presence of the G alpha subunit, which has also been shown to participate in an adaptation to pheromone.

Stimulation of cAMP and phosphomonoester production by melanotropin in melanoma cells: 31P NMR studies

A major part of the present understanding of the molecular basis of signal transduction has been gained from in vitro studies using classical biochemical methods. In this study, we used 31P NMR spectroscopy to investigate the response of live M2R mouse melanoma cells to stimulation by melanocyte-stimulating hormone (MSH; melanotropin). In the presence of 3-isobutyl-1-methylxanthine and a synergistic dose of forskolin (1.67 microM), MSH induced a transient (approximately 60-min) rise in the cellular concentration of 3',5'-cyclic adenosine monophosphate (cAMP), which coincided in time with an equivalent decrease (approximately 40%) in ATP. However, no detectable change in phosphocreatine concentration was observed. Concomitantly, MSH induced a striking and unexpected increase in the concentration of three phosphomonoester (PME) metabolites (approximately 2-fold increase in total PME signal area); one signal has been assigned to phosphoethanolamine. The levels of the PMEs remained high for 2-4 hr and declined slowly (approximately 10 hr) to basal level, following perfusion with fresh culture medium. The increase in PME was also observed after stimulation with MSH alone. In contrast, stimulation with a high dose of forskolin (50 microM) and isobutylmethylxanthine (0.2 mM), although effective in stimulating the production of cAMP, did not induce the PME response. Evaluation of the cells' energetics indicated that the enhanced production of phosphoethanolamine is probably not due to ethanolamine phosphorylation. Therefore, it is likely to result from hydrolysis of phosphatidylethanolamine by a specific phospholipase C. The response of the PMEs appears to be regulated by a cAMP-independent process, suggesting the existence of an alternative transduction pathway controlled by MSH.

Degradation of a-factor by a Saccharomyces cerevisiae alpha-mating-type-specific endopeptidase: evidence for a role in recovery of cells from G1 arrest

Mating response between opposite mating types of Saccharomyces cerevisiae is dependent upon alpha factor, a tridecapeptide, and a-factor, an isoprenylated, methyl esterified dodecapeptide whose interaction with the alpha target cell has not been characterized. We report on the first biochemical and physiological evidence of an alpha-mating-type-specific a-factor-degrading activity. Radioiodinated a-factor was used to identify the a-factor-degrading activity, which is cell associated, endoproteolytic, and not required for response to pheromone. a-factor degradation was not energy dependent, nor did it require pheromone internalization or interaction with its receptor. Phenylmethylsulfonyl fluoride and tosyl-L-arginyl-methyl ester inhibited degradation of a-factor and increased the time required by alpha cells to recover from a-factor-induced growth arrest and morphological alteration, providing evidence that a-factor degradation plays a role in the recovery of alpha cells from the pheromone response.

Adenylate cyclase agonist properties of CGP-12177A in brown fat: evidence for atypical beta-adrenergic receptors

Thermogenesis in brown adipose tissue (BAT) is stimulated by catecholamine activation of adenylate cyclase through the beta-adrenergic receptor. Recently it was reported that the beta-adrenergic antagonist CGP-12177A stimulates oxygen consumption in BAT. To investigate the mechanism of action of CGP-12177A in BAT, we assessed the inhibitory and stimulatory affects of CGP-12177A on the adenylate cyclase system in myocardial and BAT membranes from rats. CGP-1277A inhibited isoproterenol-stimulated adenylate cyclase activity in a dose-dependent manner, with an inhibitory constant (Ki) of 1.94 +/- 0.18 microM in BAT and 0.49 +/- 0.11 microM in the heart. However, in the absence of isoproterenol, CGP-12177A stimulated adenylate cyclase in BAT with two components of activation, and half-maximal stimulation occurred at 1 microM and 1.5 mM. In contrast, CGP-12177A did not stimulate adenylate cyclase activity in heart membranes. Propranolol inhibited the isoproterenol-stimulated activity with a potency that was one log less in BAT compared with heart. Propranolol fully blocked the high-affinity component but only weakly blocked the low-affinity component of CGP-12177A-stimulated activity in BAT. Pindolol was also less potent in BAT but inhibited the CGP-12177A-stimulated activity in a manner similar to the inhibition of the isoproterenol-stimulated activity, suggesting the CGP-12177A activation was beta-receptor mediated. Binding curves of [125I]iodocyanopindolol ([125I]ICYP) in competition with CGP-12177A demonstrated a shift to lower affinity in the presence of beta,gamma-imidoguanosine 5'-triphosphate, indicating that CGP-12177A has agonist properties with respect to the [125I]ICYP binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-activity relationships of histamine H2-agonists, a new class of positive inotropic drugs

The cimetidine-like moiety in impromidine was replaced by either alternative partial structures known from H2-antagonists or by H2-nonspecific lipophilic groups. The most potent H2-agonists were found in a series of compounds structurally derived from the H1-antagonist pheniramine. Arpromidine (N1-[3-(4-fluorophenyl)-3-(2-pyridinyl)propyl]-N2-[3-(1H-imidazol-4- yl)propyl]guanidine) may be considered a new lead for the development of "cardiohistaminergics". This guanidine combines both about 100 times the potency of histamine in the isolated guinea-pig atrium (pD2 = 8.0) and H1-antagonistic activity (pA2 = 7.65) in the range of pheniramine. Analogues difluorinated in 3,4-(BU-E-75) or 3,5-position (BU-E-76) or chlorinated in 3,4-position (BU-E-64) are up to 160 times more potent H2-agonists than histamine. In contrast to other types of guanidines, in the arpromidine series the order of potency found in guinea-pig atria was in good agreement with the results from isolated perfused guinea-pig hearts. In particular, the two-fold halogenated arpromidine analogues proved to be more potent positive inotropic agents than impromidine with lower stimulating effects on heart rate and reduced arrhythmogenic properties.

Effects of chronic alpha 2-adrenoceptor blockade on platelet and lymphocyte adrenoceptor binding in normal volunteers

Platelet and lymphocyte adrenoceptor binding was measured in 12 healthy male volunteers before and after 22 days treatment with the alpha 2-adrenoceptor antagonist idazoxan 40 mg tds. Platelet alpha 2-adrenoceptor number assessed by the agonist 3H-UK 14304 [correction of UK 14303] was significantly increased following idazoxan, with a smaller increase in antagonist binding (3H-rauwolscine). Lymphocyte beta-adrenoceptor number was unaltered by idazoxan, although the variance within the sample was significantly increased. Plasma MHPG levels were significantly reduced by chronic idazoxan. These data indicate upregulation of the platelet alpha 2-adrenoceptor in response to chronic blockade and suggest that this may reflect a similar change in presynaptic alpha 2-adrenoceptors which regulate norepinephrine release.

Catecholamine- and endotoxin-influenced cutaneous basophil hypersensitivity in chickens

1. Catecholamines suppress cutaneous basophil hypersensitivity (CBH) to intradermal phytohemagglutinin (PHA-P) challenge in chickens. 2. E. coli 0127:B8 endotoxin, reserpine, and propranolol prevented a norepinephrine-induced suppression of CBH. 3. PHA-P induced CBH appeared to be mediated through a beta-adrenergic receptor mechanism associated with plasma corticosterone in chickens.

Decreased Gs alpha mRNA levels accompany the fall in Gs and adenylyl cyclase activities in compensated left ventricular hypertrophy. In heart failure, only the impairment in adenylyl cyclase activation progresses

We have previously reported that there is a global reduction in adenylyl cyclase associated with a decrement in Gs functional activity in cardiac sarcolemma from animals with pressure overload-induced hypertrophy and heart failure. This study was performed to determine whether hypertrophy alone in the absence of heart failure is sufficient to promote these changes and whether the superimposition of heart failure intensified these changes. Basal and stimulated adenylyl cyclase and Gs activity, as determined in the S49 cyc- reconstitution assay, were measured in sarcolemma from normal (NL), left ventricular hypertrophy (LVH) and heart failure (HF) animals. Simultaneously, we measured the mRNA level encoding for the Gs alpha subunit. These studies indicate that Gs activity and Gs alpha mRNA are decreased by approximately 30% both in the failing heart and even in the heart with compensated hypertrophy before heart failure develops (Gs activity, pmol cyclic AMP/10 min per microgram, NL 4.2 +/- 0.4, LVH 3.0 +/- 0.2, HF 3.2 +/- 0.3; Gs alpha mRNA, pg/10 micrograms RNA, NL 131 +/- 9.0, LVH 104 +/- 7.4, HF 97.4 +/- 9.1; P less than 0.05 as compared with NL for LVH and HF). Accompanying this decrement in Gs activity is a fall in adenylyl cyclase, both basal and stimulated. However, we also identified a further decrease in adenylyl cyclase without any additional change in Gs or in its alpha subunit mRNA level. This is seen only in the sarcolemma from animals with heart failure as compared with those with compensated LV hypertrophy (e.g., NaF-stimulated activity, pmol cyclic AMP/min per mg, NL 420.2 +/- 17.5, LVH 347.1 +/- 29.6, HF 244.2 +/- 27.3; P less than 0.05 compared with NL for LVH and HF, P less than 0.05 compared with LVH for HF). In summary, these studies indicate that both Gs and adenylyl cyclase activities fall in parallel with the development of LV hypertrophy followed by a further decrement in adenylyl cyclase, independent of Gs, in the setting of heart failure.