Effects of glucagon on adenyl cyclase activity in the left and right ventricles and liver in experimentally-produced isolated right ventricular failure

The positive inotropic effect of glucagon in the chronically failed right ventricle as demonstrated in the isolated cat heart

Previous in vitro studies had provided evidence to show that papillary muscles obtained from cats with chronic right ventricular failure had lost their ability to develop a positive inotropic response to glucagon. Since it is difficult to extrapolate from the isolated papillary muscle to the intact heart, studies were done to assess the effects of glucagon in the perfused isovolumically beating heart obtained from cats four months after surgical banding of the pulmonary artery for the experimental production of chronic right ventricular failure (CRVF). At the peak of the dose-response curve, glucagon increased right ventricular isovolumic pressure 25% (39.00 +/- 4.37 to 49.67 +/- 5.15 mm Hg; p less than 0.001) and right ventricular dP/dt 63% (522.2 +/- 93.9 to 852.6 +/- 159.9 mm Hg/sec; p less than 0.001) in 6 normal hearts. Similar dose related increases in right ventricular isovolumic pressure and dP/dt were obtained in 6 hearts taken from cats with chronic right ventricular failure. The respective increases in right ventricular isovolumic pressure and dP/dt were 43% (30.33 +/- 4.01 to 43.67 +/- 6.25 mm Hg; p less than 0.025) and 73% (317.50 +/- 30.29 to 550.83 +/- 89.04 mm Hg/sec; p less than 0.025). These results provide evidence that glucagon possesses the capacity to augment myocardial contractility in the heart with experimentally induced chronic right ventricular failure.

The glucagon receptor and adenylate cyclase

Acidic phospholipids play a critical role in the hormone activation of adenylate cyclase. Solubilized myocardial adenylate cyclase is unresponsive to glucagon and the catecholamines, two of the hormones which activate the membrane-bound enzyme. Phosphatidylserine, purified from bovine brain restored glucagon responsiveness of the solubilized adenylate cyclase. Monophosphatidylinositol, also purified from bovine brain, restored catecholamine responsiveness. Solubilized preparations of myocardial adenylate cyclase bind 125-I-glucagon either in the presence of added phosphatidylserine, thereby providing a clear separation of the processes of activation and binding. Solubilized myocardial adenylate cyclase has a molecular weight of about 160,000. Sephadex G-100 chromatography of the solubilized enzyme following the binding of 125-I-glucagon to its myocardial receptor reveals two distinct peaks; one, having catalytic activity and a molecular weight greater than 100,000 and two, the binding material having no catalytic activity and a molecular weight of 24,000-28,000. These data are consistent with the presence of a dissociable glucagon receptor site. The role of this dissociation in the activation-inactivation of the enzyme remains to be explored. It is postulated that phospholipids induce the required configurational change in the catalytic site following the binding of hormone to its receptor, and by this means couples the receptor to the catalytic site. This model may be applicable to certain clinical situations. Cardiac adenylate cyclase is unresponsive to glucagon in chronic congestive heart failure. The defect may reside either in the binding of glucagon to its receptor site or in the metabolism of a specific acidic phospholipid such as phosphatidylserine.