Digoxin reduces beta-adrenergic contractile response in rabbit hearts. Ca(2+)-dependent inhibition of adenylyl cyclase activity via Na+/Ca2+ exchange.

Whereas mobilization of intracellular Ca2+ stimulates neuronal adenylyl cyclase via Ca2+/calmodulin, mobilized Ca2+ directly inhibits adenylyl cyclase in other tissues. To determine the physiologic role of the Ca(2+)-dependent interaction between Na+/Ca2+ exchange and beta-adrenergic signal transduction in the intact heart, digoxin (0.3 mg/kg) was administered intravenously in rabbits. 30 min after the administration, digoxin impaired the peak left ventricular dP/dt response to dobutamine infusions by up to 38% as compared with control rabbits. This impairment was not caused by changes in either beta-adrenergic receptor number or in the functional activity of stimulatory guanine nucleotide-binding protein. It was associated with 33-36% reductions in basal and stimulated adenylyl cyclase activities. Animals treated with calcium gluconate (20 mg/kg/min for 30 min) also demonstrated similar reductions in adenylyl cyclase activities. In addition, increasing the free Ca2+ concentration progressively inhibited adenylyl cyclase activity in the control, digoxin-treated, and calcium gluconate-treated sarcolemma preparations in vitro. Moreover, digoxin and calcium gluconate pretreatment blunted the increase in cAMP in myocardial tissue after dobutamine infusion in vivo. Thus, digoxin rapidly reduces beta-adrenergic contractile response in rabbit hearts. This reduction may reflect an inhibition of adenylyl cyclase by Ca2+ mobilized via Na+/Ca2+ exchange.     

Desensitization of adenylate cyclase and down regulation of beta adrenergic receptors after in vivo administration of beta agonist

In vitro incubation of cells with catecholamines leads to both down regulation of beta adrenergic receptor number and desensitization of agonist-stimulated adenylate cyclase activity. These same parameters, down regulation of beta adrenergic receptor number and desensitization of adenylate cyclase activity were assessed in rat lung membranes after in vivo administration of metaproterenol, a beta-2 selective agonist. In vivo treatment with metaproterenol leads to: 1) reduced beta adrenergic receptor number; 2) reduced isoproterenol-stimulated adenylate cyclase activity; 3) unaffected NaF or 5'-guanylylimidodiphosphate-stimulated adenylate cyclase activity; and 4) reduced affinity of the receptor for isoproterenol similar to the affinity observed in the presence of 5'-guanylylimidodiphosphate. The date suggest that in vivo metaproterenol administration results in an uncoupled receptor-adenylate cyclase complex. The effects of in vivo administration of the glucocorticoid, methylprednisolone, to metaproterenol-pretreated animals were also assessed. Glucocorticoid treatment was associated with 1) increased beta adrenergic receptor number in rats in which the receptors have been down regulated, 2) increased isoproterenol responsiveness in agonist-desensitized rats and 3) no effect on agonist affinity in desensitized animals. These data suggest that the restoration of agonist responsiveness by glucocorticoids in the catecholamine refractive state is not simply a reversal of receptor down regulation or adenylate cyclase desensitization.     

Effects of cyclopentyladenosine on isoproterenol response in adult and senescent cardiac tissue from Fischer 344 rats

To characterize age-related changes in beta-adrenergic responsiveness and to test the hypothesis that an increase in the effects of adenosine contribute to impaired beta-adrenergic responsiveness, Fischer 344 rat right atria (RA), left atria (LA), and left ventricular trabeculae carnae were exposed to the beta-receptor agonist isoproterenol (ISO), followed by four doses of the selective adenosine A(1) receptor agonist cyclopentyladenosine (CPA). Spontaneous contractile rates of adult RA were inhibited more than senescent RA by CPA. Contractility (+dF/dt) of adult LA was reduced more than senescent LA by CPA. Left trabeculae carnae tissue responded weakly to CPA, but senescent tissue was less responsive than adult tissue. Senescent atrial A(1) receptor density was 56% greater than in adult tissue, whereas the density in senescent ventricles was 39% lower than in adult tissue. No significant difference in antagonist affinities (K(d)) of A(1) receptor was observed between adult and senescent atria. In addition, agonist competition curves indicated a significant increase in senescent atrial and a decrease in senescent ventricular tissue in the affinity of agonist for high-affinity A(1) receptors with no difference in dissociation constant (K(i)). No significant age-related differences in atrial or ventricular tissues occurred in either the antagonist affinity (K(d)) or density (B(max)) of the beta-adrenergic receptors. CPA was found to inhibit ISO-stimulated adenylate cyclase activity more in senescent than in adult atrial and ventricular membrane preparations. We conclude that age-related differences in functional response to ISO and CPA, A(1) receptor density, and ISO-stimulated adenylate cyclase activity differ in atrial and ventricular myocardium.     

Differential regulation of right and left ventricular beta-adrenergic receptors in newborn lambs with experimental cyanotic heart disease.

To determine whether chronic hypoxemia secondary to an intracardiac right-to-left shunt alters regulation of the myocardial beta-adrenergic receptor/adenylate cyclase system, we produced chronic hypoxemia in nine newborn lambs by creating right ventricular outflow obstruction and an atrial septal defect. Oxygen saturation was reduced to 65-74% for 2 wk. Eight lambs served as normoxemic controls. beta-receptor density (Bmax) and ligand affinity (KD) were determined with the radio-ligand [125I]iodocyanopindolol and adenylate cyclase activity determined during stimulation with isoproterenol, sodium fluoride (NaF), and forskolin. During chronic hypoxemia, Bmax decreased 45% (hypoxemic, 180.6 +/- 31.5 vs. control, 330.5 +/- 60.1 fmol/mg) in the left ventricle (exposed to hypoxemia alone) but was unchanged in the right ventricle (exposed to hypoxemia and pressure overload). KD was not different from control in either ventricle. Left ventricular isoproterenol-stimulated adenylate cyclase activity was decreased by 39% (30.0 +/- 4.3% increase vs. 44.1 +/- 9.5% increase) whereas right ventricular adenylate cyclase activity was unchanged. Stimulation of adenylate cyclase with NaF or forskolin was not different from control in either ventricle. Circulating epinephrine was increased fourfold whereas circulating and myocardial norepinephrine were unchanged. These data demonstrate a down-regulation of the left ventricular beta-adrenergic receptor/adenylate cyclase system during chronic hypoxemia secondary to an intracardiac right-to-left shunt.     

Demonstration of beta adrenoceptor blockade by propafenone hydrochloride: clinical pharmacologic, radioligand binding and adenylate cyclase activation studies

The postulated beta adrenoceptor blocking properties of the new antiarrhythmic drug propafenone were studied by in vivo comparison against placebo and propranolol in the antagonism of both exercise- and isoproterenol-induced tachycardia and by in vitro radioligand binding studies of animal and human left ventricular muscle membrane preparations. Interaction with frog erythrocyte membrane adenylate cyclase was also investigated. In the clinical studies, a double blind crossover comparison of oral propafenone (300 mg), propranolol (40 mg) and placebo indicated significant antagonism of chronotropic response to isoproterenol 2 hr postdose with dose ratios of 4.1 +/- 1.3 (mean +/- S.E.M.) for propafenone and 16.8 +/- 5.1 for propranolol. Chronotropic response to exercise was modestly reduced by propafenone. Analysis of the binding of [125I]iodocyanopindolol to human left ventricular membranes revealed specific beta adrenoceptor competition by propafenone with an EC50 of 111 +/- 13 nM. Propranolol EC50 was 2.4 +/- 0.2 nM in this system. Competitive inhibition of isoproterenol-stimulated frog erythrocyte membrane adenylate cyclase activity was also obtained with propafenone. The ratio of affinities (calculated from the apparent dissociation constant; KD) for propranolol-propafenone was 1:40 for the in vivo study and 1:50 for the in vitro system. Propafenone is a specific antagonist of the human beta adrenoceptor and this action can be demonstrated during in vivo study in human subjects. At clinical dosages it appears likely that it will achieve a modest degree of beta blockade which may contribute to its antiarrhythmic effect.     

Pharmacological profile of the novel inotropic agent (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744)

The novel Na(+)/K(+)-ATPase inhibitor (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744) was characterized for its inotropic and toxic properties. Inhibition potency on dog kidney Na(+)/K(+)-ATPase was comparable (0.43 microM) to that of digoxin (0.45 microM). PST2744 concentration-dependently increased force of contraction in guinea pig atria and twitch amplitude in isolated guinea pig myocytes; in the latter, aftercontractions developed significantly less than with digoxin. Intravenous infusion of 0.2 mg/kg/min PST2744 in anesthetized guinea pigs exerted an immediate and long-lasting inotropic effect (ED(80) of 1.89 +/- 0.37 mg/kg) without causing lethal arrhythmias up to a cumulative dose of 18 mg/kg. Conversely, an equieffective infusion of digoxin (0.016 mg/kg/min; ED(80) of 0.32 mg/kg) caused lethal arrhythmias at a cumulative dose of 0.81 mg/kg. At a higher rate (0.4 mg/kg/min), PST2744 induced lethal arrhythmias, with a lethal dose/ED(80) ratio significantly greater than digoxin (20.2 +/- 6.3 versus 3.23 +/- 0.55, p < 0.05). Decay of the inotropic effect (t(1/2), min) was significantly faster for PST2744 (6.0 +/- 0.39) than for digoxin (18.3 +/- 4.5, p < 0.05). In anesthetized dogs, PST2744 dose-dependently increased maximum velocity of pressure rise (+dP/dt(max)) in the range 32 to 500 microg/kg i.v. and was safer than digoxin. In conscious dogs with a healed myocardial infarction, PST2744 significantly increased resting values of +dP/dt(max), left ventricular pressure, and SPB, and increased +dP/dt(max) throughout treadmill exercise while reverting the increase in left ventricular end diastolic pressure seen in control animals. Digoxin significantly decreased basal heart rate, while not affecting the hemodynamic response to exercise. Thus, PST2744 represents a new class of Na(+)/K(+)-ATPase inhibitors endowed with inotropic activity comparable with that of digitalis but having greater safety.     

Drug-drug interactions mediated through P-glycoprotein: clinical relevance and in vitro-in vivo correlation using digoxin as a probe drug

The clinical pharmacokinetics and in vitro inhibition of digoxin were examined to predict the P-glycoprotein (P-gp) component of drug-drug interactions. Coadministered drugs (co-meds) in clinical trials (N = 123) resulted in a small, 0.1 is predictive of clinical digoxin interactions (AUC and C(max)).     

Dose-dependent Hemodynamic Effect of Digoxin Therapy in Severe Verapamil Toxicity

Calcium chloride (CaCl(2)) alone is an ineffective antidote in severe calcium channel antagonist overdoses. Digoxin has been evaluated as a therapy to increase the effectiveness of calcium in severe calcium channel antagonist overdoses. OBJECTIVE: To determine if there is a dose-dependent hemodynamic effect of digoxin in the setting of severe verapamil toxicity treated with high-dose CaCl(2). METHODS: Eight dogs were instrumented to measure systolic and diastolic blood pressure, cardiac output, pulmonary artery pressures, and left ventricular pressures. Verapamil toxicity (50% decrease in mean arterial pressure) was induced with verapamil 6 mg/kg/hr and maintained for 30 minutes by titrating the verapamil rate. Following verapamil toxicity, each dog received one dose of digoxin equivalent to 0, 1, 1.5, 2, 3, 4, 6, or 8 times the loading dose of digoxin (0.009 mg/kg). The verapamil rate was changed to 4 mg/kg/hr and continued for the next five hours. CaCl(2) boluses were given (0.5 g immediately following verapamil toxicity and 1 g at one, two, and three hours). Measurements were compared with the loading dose of digoxin using linear regression analysis. RESULTS: Digoxin resulted in a dose-dependent increase in systolic blood pressure at 4 hours (10.23 mm Hg/loading dose of digoxin, 95% CI = 2.74 to 17.73), 4 hours, 15 minutes (13.9 mm Hg/loading dose of digoxin, 95% CI = 8.75 to 19.01), and 5 hours (17.04 mm Hg/loading dose of digoxin, 95% CI = 1.76 to 32.32). Digoxin resulted in a dose-dependent increase in maximal ventricular pressure at the end of hour 3 (8.55 mm Hg/loading dose of digoxin, 95% CI = 3.41 to 13.69), 3 hours, 15 minutes (11.81 mm Hg/loading dose of digoxin, 95% CI = 4.89 to 18.73), hour 4 (8.26 mm Hg/loading dose of digoxin, 95% CI = 1.03 to 15.48), and 4 hours, 15 minutes (9.74 mm Hg/loading dose of digoxin, 95% CI = 4.47 to 15.00). The authors were unable to detect a dose-dependent increase in other parameters, including diastolic relaxation (diastolic change in pressure over time) and time to onset of death. No ventricular arrhythmias developed in any dogs. CONCLUSIONS: There is a dose-dependent effect of digoxin on systolic blood pressure and maximal ventricular pressure in the setting of severe verapamil toxicity treated with high-dose CaCl(2).     

Chronic norepinephrine elicits desensitization by uncoupling the beta-receptor.

The goal of this study was to determine the mechanism of beta-adrenergic receptor desensitization after chronic elevation of circulating NE levels. Osmotic minipumps containing either NE or saline were implanted subcutaneously in dogs for 3-4 wk. Physiologic desensitization to isoproterenol was confirmed in conscious dogs, i.e., left ventricular dP/dt increased in response to isoproterenol (0.4 micrograms/kg per min) by 5,625 +/- 731 mmHg/s in control dogs with saline pumps, and significantly less, P less than 0.01, by 2,093 +/- 263 mmHg/s in dogs with NE pumps. Myocardial beta-adrenergic receptor density as determined with 125I-cyanopindolol binding was 49% higher (p less than 0.05) in the NE pump group. However, beta-adrenergic receptor agonist binding with isoproterenol demonstrated a significant shift into the low affinity state for the animals with NE pumps. Basal, GTP plus isoproterenol, 5'-guanylylimidodiphosphate, sodium fluoride, and forskolin-stimulated adenylate cyclase activity in the NE pump group were significantly depressed (P less than 0.05) by amounts ranging from 20 to 40%. The functional activity of the guanine nucleotide binding protein Gs was also reduced (P less than 0.05) in animals with NE pumps. Thus, the process of desensitization in response to chronic elevation of NE levels in intact, normal dogs does not involve a decrease in beta-adrenergic receptor density. Rather, it is characterized by reduced adenylate cyclase activation and uncoupling of the beta-adrenergic receptor in association with decreased activity of the GTP-coupling protein Gs.     

维持性血液透析患者血清甲状旁腺激素水平与心功能评价的相关性研究

目的 探讨维持性血液透析（MHD）患者全段甲状旁腺激素（iPTH）水平的变化与左心室结构和功能的相关性,评估iPTH对MHD合并心力衰竭早期诊断、治疗及判定预后的临床价值。方法 随机选取沈阳军区总医院血液净化科186名MHD患者。入选标准为：血液透析3次／周,每次4h;年龄在18～75岁;透析年限6个月以上,10年以下;无急性心血管事件。利用放免法检测患者透析前iPTH水平、电化学发光免疫分析法检测透析血浆B型钠尿肽前体（pro—BNP）水平。左心室肥厚（LVH）可由心脏超声检查判断。超声心动图检测患者左心房内径（LAD）、左心室舒张末内径（LVDd）、室间隔厚度（IVST）、左心室后壁厚度（LVPwT）、左心室射血分数（LVEF）等,计算左心室重量（LVM）,左心室心肌重量指数（LVMI）。根据2009年KDIGO指南iPTH水平的目标范围,按iPTH正常参考值上限水平2～9倍为界将患者分为3组,探讨iPTH与MHD患者左心功能的关系。结果 iPTH≥549．9pg／ml组患者的LAD、LVMI、LVDd、IVST、LVPwT显著高于iPTH≤122．2pg／ml,122．2～549．9pg／ml组（P〈0．05）。随着iPTH的增长,患者pro—BNP水平升高,有统计学差异（P〈0．05）。随着iPTH的增长,iPTH与NYHA心功能分级呈正相关,血清iPTH水平与LAD、LVMI、LVDd、LVDs、IVST、LVPwT、左心室收缩功能、pro—BNP呈正相关（P〈0．05）,与LVEF呈负相关（P〈0．05）。结论 MHD患者iPTH水平与左心室功能密切相关,可能成为诊断及评估患者心功能的一个重要标志物。     

Desensitization of platelet alpha 2-adrenoceptors after short term infusions of adrenoceptor agonist in man

The effect of intravenous infusion of catecholamines and related drugs on human platelet alpha 2-adrenoceptor number and function was investigated. Short (60-120 min) infusions of catecholamines with alpha 2 agonist activity in vivo produced attenuation of the platelet responses to adrenaline in vitro. This desensitization was specific for the adrenaline induced aggregatory response. The maximum number of [3H]yohimbine binding sites on platelets was not altered by adrenaline infusion. The ability of adrenaline to reduce platelet cyclic AMP levels was significantly reduced after the infusions. Acute infusions of alpha 2-adrenoceptor agonists may alter the coupling of the platelet alpha 2-adrenoceptor to adenylate cyclase.     

Regulation of beta-adrenoceptor-mediated relaxation of the rat aorta is modulated by endogenous ovarian hormones

The aim of this study was to assess the influence of the endogenous status of ovarian hormones on the relaxation induced by the beta-adrenoceptor agonists isoprenaline (isoproterenol) and dobutamine in thoracic aorta segments, precontracted with noradrenaline, from age-matched (13-week-old) virgin (oestrus) and ovariectomized (OVX) prepubertal female Wistar rats. Isoprenaline-induced relaxation was decreased in intact aortic segments from OVX rats compared with that in segments from oestrus rats. Relaxation was significantly reduced by endothelium removal, 1 micromol/l propranolol or 100 micromol/l N(G)-nitro-L-arginine methyl ester (L-NAME). The beta(1)-adrenoceptor agonist dobutamine induced less relaxation in intact arteries from oestrus rats than did isoprenaline, and dobutamine-induced relaxation was markedly less in intact segments from OVX compared with oestrus rats. This dobutamine-induced relaxation was abolished by endothelium removal, and reduced by 1 micromol/l propranolol, 100 micromol/l L-NAME or 1 micromol/l yohimbine. Cholera toxin (an activator of the stimulatory G-protein G(s)) caused relaxation in intact arteries from oestrus rats; this relaxation was decreased by both deprivation of ovarian hormones and endothelium removal. Forskolin (a direct activator of the catalytic subunit of adenylate cyclase) and sodium nitroprusside (a nitric oxide donor and cGMP-dependent vasodilator agonist) induced similar endothelium-independent relaxation in arteries from both oestrus and OVX rats. These results suggest that the relaxation elicited by endothelial beta-adrenoceptor activation in the rat thoracic aorta is impaired by deprivation of female ovarian hormones; this impairment is caused, at least in part, by decreases in both the endothelial release of NO and G(s) function.     

Effects of dietary-induced hyperparathyroidism on the parathyroid hormone-receptor-adenylate cyclase system of canine kidney. Evidence for postreceptor mechanism of desensitization.

The present studies were designed to examine the consequences of chronic mild elevations of endogenous parathyroid hormone (PTH) in vivo on the PTH receptor-adenylate cyclase system of canine kidney cortex. Hyperparathyroidism was induced in normal dogs by feeding a diet low in calcium, high in phosphorus to the animals for a period of 6-9 wk. This maneuver resulted in a two to threefold increase in the plasma levels of carboxy-terminal immunoreactive PTH. This degree of hyperparathyroidism is similar to that seen in patients with hyperparathyroidism and normal renal function. After 6-9 wk on the diet the animals were killed and basolateral renal cortical membranes prepared for the study of the PTH receptor-adenylate cyclase system in vitro. The dietary hyperparathyroidism resulted in desensitization of the PTH-responsive adenylate cyclase (Vmax 3,648 +/- 654 pmol cyclic (c)AMP/mg protein per 30 min in hyperparathyroid animals vs. 5,303 +/- 348 in normal controls). The Kact (concentration of PTH required for half-maximal enzyme activation) was unchanged. However, PTH receptor binding (125I-norleucyl8-norleucyl18-tyrosinyl34, 125I[Nle8, Nle18, Tyr34] bPTH (1-34) NH2 as radioligand) was not different in the two groups of animals. Thus, dietary hyperparathyroidism resulted in an uncoupling of the PTH receptor-adenylate cyclase system. This defect was not corrected by guanyl nucleotides in vitro, and the effects of guanyl nucleotides on PTH binding and enzyme activation appeared normal. NaF-stimulated enzyme activity was reduced in the hyperparathyroid animals (8,285 +/- 607 pmol cAMP/mg protein per 30 min vs. 10,851 +/- 247 in controls). These data indicate that desensitization of the PTH-responsive adenylate cyclase system of canine kidney as a result of mild chronic elevations of endogenous PTH is due to a postreceptor defect, demonstrable by NaF activation, not corrected by guanyl nucleotides, leading to abnormal PTH-receptor adenylate cyclase coupling.     

Abnormal membrane composition and membrane-dependent transduction mechanisms in cluster headache

Previous studies have indicated that membrane structure and function may be abnormal in cluster headache. This has been further investigated by analysis of membrane phosphatidylcholine, total phospholipids, and cholesterol in erythrocytes and by assay of receptor-mediated transduction. The stimulation of lymphocyte adenylate cyclase with isoprenaline and prostacyclin was used as the test system. A significant increase in the ratio of membrane phosphatidylcholine to cholesterol without change in cholesterol was found in cluster headache patients as compared with control subjects. This indicated a reduced turnover of phosphatidylcholine, since erythrocyte choline is significantly reduced in this condition. Abnormal membrane function was also indicated from the significant depression of high-affinity prostaglandin receptor stimulation of lymphocyte adenylate cyclase and the similar trend in the beta-adrenoceptor response. Since no change in agonist affinity and beta-adrenoceptor density occurred, this depression indicates a generalized defect in coupling of receptors to adenylate cyclase. It is hypothesized that the impaired function that would result might contribute to the aetiology of cluster headache.     

高盐诱导左心室舒张功能障碍大鼠心肌Zip13表达增高

目的 探讨锌及锌转运体Zip13在高盐诱导的左心室舒张功能障碍（LVDD）大鼠心肌中的表达及作用机制。方法 将36只Dahl盐敏感大鼠（Dahl salt sensitive rat,DSS大鼠）随机分为模型组（8% 氯化钠高盐饮食,n=22）和对照组（0.3%氯化钠饮食,n=14）。用Vevo 2100小动物超声仪评价各组大鼠心功能;电感耦合等离子体发射光谱法（inductively coupled plasma optical emission spectroscopy, ICP DES）测定血清和心肌组织中锌离子浓度;分别用Western blot和荧光实时定量PCR技术检测心肌中Zip13的蛋白和mRNA表达水平。结果 与对照组相比,模型组左心室后壁舒张末期厚度（LVPWd）、左心室舒张末期内径（LVDd）、舒张末期室间隔厚度（IVSd）、左心室后壁收缩末期厚度（LVPWs）、左心室质量（LVM）以及左心室体重比（LVM/Weight）均显著升高（P＜0.05）。心肌组织中锌离子浓度明显升高,而血清中锌离子浓度明显降低（P＜0.05）;模型组心肌组织中Zip13蛋白和mRNA表达水平均显著升高（P＜0.05）。结论 LVDD大鼠出现了心肌锌离子浓度升高的锌稳态失衡,同时Zip13蛋白和mRNA表达均上调,提示Zip13可能参与了调控和纠正锌稳态失衡,从而实现抑制LVDD心肌重构作用,具体机制有待进一步研究。     

Prostacyclin production and mediation of adenylate cyclase activity in the pulmonary artery. Alterations after prolonged hypoxia in the rat.

Prostacyclin is a critical mediator of structure and function in the pulmonary circulation, causing both the inhibition of vascular smooth muscle growth and vasodilation via the stimulation of adenylate cyclase. To examine the potential role of alterations in prostacyclin production or mechanism of action in chronic hypoxic pulmonary hypertension, we determined the effects of prolonged (7 d) in vivo hypoxia on in vitro prostacyclin synthesis and mediation of adenylate cyclase activity in rat main pulmonary arteries. In control arteries prostacyclin production exceeded that of prostaglandin (PG) E2 by 25-fold, with 42% originating from the endothelium. Studies utilizing indomethacin revealed that endogenous prostaglandins mediate at least 69% of basal adenylate cyclase activity. Prostacyclin-stimulated enzyme activity was enhanced by exogenous GTP, indicating that this is a receptor-mediated process involving G protein amplification. Comparable dose-related responses to prostacyclin and PGE2 suggest that these agents may activate a common receptor. After 7 d of in vivo hypoxia there was a 2.7-fold increase in in vitro prostacyclin production, with equivalent increases in synthesis in the endothelium and vascular smooth muscle. However, despite this increase there was no change in basal adenylate cyclase activity, and this was associated with attenuated sensitivity of the enzyme to prostacyclin stimulation. Concomitant diminution of the response to beta-adrenergic stimulation, with previously-demonstrated beta receptor downregulation and unaltered postreceptor-mediated activity, suggests that the blunted response to prostacyclin is due to receptor downregulation. Parallel studies of the thoracic aorta indicated that these changes are specific to the pulmonary artery. It is postulated that attenuation of the response of adenylate cyclase to prostacyclin may contribute to the structural changes and hypertension observed in the pulmonary vasculature of the rat with chronic hypoxia.     

Cellular mechanisms of impaired adrenergic responsiveness in neonatal dogs.

The myocardial responsiveness of conscious, instrumental dogs to exogenously administered isoproterenol and norepinephrine was investigated in neonatal, 6-wk-old, and adult animals. Comparable base-line values for peak left ventricular derivative of pressure with respect to time were observed in all age categories. However, when compared with adult responses, the sympathomimetic amine-induced increases in neonatal left ventricular dP/dt were significantly blunted at each concentration of adrenergic agonist examined, whereas the 6-wk-old puppies displayed an intermediate inotropic response. To investigate the cellular mechanisms of this blunted neonatal response, we correlated physiologic and biochemical measurements of the myocardial responses to catecholamines in each age category. When compared with adult myocardial membrane preparations, neonatal cardiac membranes were characterized in vitro by an increased density of beta-adrenergic binding sites, comparable affinity for adrenergic agonists and antagonists, and an enhanced coupling of adenylate cyclase activation to receptor occupancy. Simultaneous changes in either the serum catecholamine concentration or the membrane content of other intrinsic proteins failed to account for the observed neonatal increase in beta-adrenergic receptor density. These findings are most consistent with a compensatory mechanism of the cardiac cell membrane, whereby an inherent depression in the adrenergic responsiveness of the immature myocardium appears to induce the increase in receptor density and activation of adenylate cyclase.     

Influence of beta1-adrenoceptor blockade on the gene expression of adenylate cyclase subtypes and beta-adrenoceptor kinase in human atrium

There is accumulating evidence of cross-regulation between stimulatory G-protein (G(s))-coupled receptors in human atrium. Chronic beta(1)-adrenoceptor antagonist treatment can sensitize beta(2)-adrenoreceptors, 5-HT(4) receptors, histamine H(2) receptors and possibly beta(4)-adrenoreceptors. To investigate the mechanism of such cross-talk between G(s)-coupled receptors, we have measured the mRNA expression of the cardiac adenylate cyclases (types IV, V, VI and VII) and of beta-adrenoceptor kinase 1 in human atria using TaqMan quantitative PCR, and compared expression between patients chronically receiving a beta(1)-adrenoceptor antagonist and non-treated patients. The results showed no difference in gene expression between the two groups of patients; however, significant positive correlations of gene expression between adenylate cyclase subtypes were found. We conclude that beta(1)-adrenoceptor antagonists do not change the absolute levels of gene expression of adenylate cyclase subtypes in human atrium. The co-ordinate regulation among adenylate cyclase subtypes and the influence of beta(1)-adrenoceptor antagonists need to be investigated further.     

Effects of angiotensin-converting enzyme inhibition and angiotensin II type 1 receptor blockade on beta-adrenoceptor signaling in heart failure produced by myocardial Infarction in rabbits: reversal of altered expression of beta-adrenoceptor kinase and G i alpha

Both angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 (AT1) receptor blockers have been demonstrated to improve symptoms and prognosis in heart failure (HF). We compared the effects of ACE inhibition and AT1 receptor blockade on myocardial beta-adrenoceptor desensitization in rabbits with HF established 3 weeks after myocardial infarction (MI) with left circumflex coronary artery ligation. Rabbits with MI were randomized to no treatment, the ACE inhibitor temocapril (0.5 mg/kg/day) or AT1 receptor blocker valsartan (3 mg/kg/day). Echocardiographic examinations showed that, relative to rabbits with untreated MI, rabbits receiving temocapril or valsartan had a limitation of cardiac remodeling and prevention of the development of systolic dysfunction. Circulating plasma norepinephrine levels that were markedly elevated in MI animals were strongly inhibited by temocapril or valsartan therapy. beta-Adrenoceptor density, beta-adrenoceptor proportion showing high-affinity agonist binding, and basal and isoproterenol-stimulated adenylate cyclase activities were significantly reduced in MI rabbits. These defects were similarly reversed by temocapril or valsartan. Importantly, as found in human HF, myocardial protein levels of beta-adrenoceptor kinase 1 and G(i alpha) were significantly elevated in MI rabbits, suggesting that these molecules are contributing to the defects in myocardial beta-adrenoceptor signaling. The expression levels of these molecules were normalized equally by both treatments. The results suggest that pharmacologically different interventions in the renin-angiotensin system can equivalently improve the derangements in the beta-adrenoceptor signaling system in the failing heart. This may be important for the beneficial effects of these agents in HF.