Excitation contraction coupling in heart. II. Studies on the role of adenyl cyclase in the calcium transport by dog heart sarcoplasmic reticulum

Differential effects of non-ionic detergents on microsomal and sarcolemmal adenylate cyclase in cardiac muscle

1. About 4 and 23% of the homogenate adenylate cyclase activity was recovered in the microsomal and sarcolemmal fractions isolated from guinea-pig heart ventricles. 2. Cardiac microsomal adenylate cyclase activity [basal as well as p[NH]ppG (guanyl-5'-yl imidodiphosphate)- and NaF-stimulated] was increased over 2-fold in the presence of Lubrol-PX (0.01-0.1%). 3. The sarcolemmal enzyme, however, showed concentration-dependent inhibition caused by the detergent under all assay conditions, except when p[NH]ppG was included in the assay. In the latter case, the detergent (0.01-0.02%) caused a modest increase (30-45%) in enzyme activity. 4. Another non-ionic detergent, Triton X-100, also stimulated the microsomal cyclase and inhibited the sarcolemmal enzyme. 5. With either membrane fraction, Lubrol-PX solubilized the enzyme when the detergent/membrane protein ratio was 2.5 (mumol of detergent/mg of protein). 6. The findings with homogenate and a washed particulate fraction resembled those obtained with sarcolemma, and those with isolated sarcoplasmic reticulum resembled those with microsomal preparations. 7. p[NH]ppG, and to some extent NaF, protected the detergent-induced inactivation of the enzyme observed at higher detergent concentrations (0.5% Lubrol-PX and 0.05-0.5% Triton X-100). 8. In the absence of detergents, p[NH]ppG increased the basal enzyme activity about 2-fold in microsomal fractions, but did not appreciably stimulate the sarcolemmal enzyme. Isoproterenol, on the other hand, increased the sarcolemmal enzyme activity (>2-fold) in the presence of p[NH]ppG and caused only moderate stimulation (31%) of the microsomal enzyme under these conditions. 9. These findings support the view that, although the bulk of adenylate cyclase resides in heart sarcolemma (plasma membrane), the microsomal activity cannot be accounted for solely by contamination of the microsomal fraction with sarcolemma, as has been suggested by others [Besch, Jones & Watanabe (1976) Circ. Res.39, 586-595; Engelhard, Plut & Storm (1976) Biochim. Biophys. Acta451, 48-61]. Further, the results of this study show that cardiac sarcoplasmic-reticulum membranes possess this enzyme.

Effect of phosphodiesterase inhibitors and dBcAMP on the inotropic and relaxing actions of histamine in cardiac muscle

The influence of phosphodiesterase inhibitors, imidazole and dBcAMP on the action of histamine on peak tension, on rate of tension development and rate of relaxation of ventricular muscle of guinea pig's heart was investigated. It was found that theophylline plus dBcAMP enhanced the peak tension, (dF/dt)max and the rate of relaxation and increased the relaxing action of histamine. Imidazole, an activator of phosphodiesterase, also potentiated the action of histamine on peak tension, (dF/dt)max and rate of relaxation. Caffeine, a well-known inhibitor of phosphodiesterase, suppressed the relaxing action of histamine and epinephrine. These discrepant results could be explained by assuming the possibility that these drugs have effects other than those involving inhibition and activation of phosphodiesterase.

Relaxing effects of catecholamines on mammalian heart

1. The effect of catecholamines on the time course and amplitude of contraction and on KCl-induced contractures has been studied in mammalian hearts.2. Marked and reproducible contractures could be obtained in mammalian ventricular trabeculae and papillary muscles after beta-adrenergic block with propanolol or if the hearts were depleted of their catecholamine stores by reserpine or by chemical denervation with 6-hydroxydopamine.3. In neonatal hearts with lower endogenous catecholamine stores and poorly developed sarcoplasmic reticulum KCl contractures are easily produced.4. Catecholamines potentiate twitch tension and relax the contracture tension under all of the above circumstances.5. The relaxant effect of catecholamines is present during the time course of a twitch. This increased relaxation rate as well as the shortening of the time-to-peak of tension is independent of the variation in the duration of the action potential.6. The shortened relaxation time is present when the action potential is shortened with anodal repolarization or prolonged with cathodal depolarization (voltage-clamp).7. The relaxant effect of catecholamines on the twitch is temperature and rate dependent. The effect is observed in the presence of high or low concentrations of calcium.8. The presence of catecholamines is necessary for full relaxation of mammalian heart muscle under high performance conditions or states of calcium overload.9. It is proposed that catecholamines exert their relaxant effect independent of their positive inotropic effect by stimulating the sequestering system (sarcoplasmic reticulum, mitochondria or sarcolemma) for calcium.

Adenyl cyclase and cyclic AMP. Biochemical links in the regulation of myocardial contractility

The actions of many hormones may be related to their capacity to increase adenyl cyclase activity in their target organs. The evidence indicating that catecholamines and glucagon augment myocardial contractility by enhancing the activity of myocardial adenyl cyclase is summarized. Furthermore, data are presented suggesting that the inotropic actions of epinephrine and glucagon ultimately may be due to an increase in sarcotubular calcium stores, an effect that appears to be related to activation of an adenyl cyclase pool localized in the sarcoplasmic reticulum. Other hormones that may alter contractility through actions on the adenyl cyclase-cyclic AMP system are thyroid hormone, histamine, prostaglandin, and acetylcholine. Chronic cardiac decompensation in both the experimental animal and in man diminishes the effectiveness of glucagon as an inotropic agent and interferes with the mechanisms by which glucagon acts to enhance the activity of myocardial adenyl cyclase, changes that may be causally interrelated. Although an understanding of the adenyl cyclase system would seem to be extremely important for an understanding of the basic mechanisms responsible for modulating the intensity of the contractile state of the heart, further investigations are necessary for complete validation of this hypothesis.