The effect of glucagon on rat cardiac cyclic AMP, phosphorylase a and force of contraction.

Glucagon produced dose-dependent increases in cyclic AMP, contractile force and % phosphorylase a when injected into isolated perfused rat hearts. Time-response experiments demonstrated that the increase in cyclic AMP occurred prior to changes in the other two parameters. All glucagon effects were enhanced by perfusion of the hearts with 1.0 mM theophylline although contractile force was affected less than the other 2 events. The data lend support to the concept that increases in cyclic AMP may be involved in the inotropic effect of glucagon. It should be noted, however, that the cyclic AMP increases produced by glucagon were considerably less than those previously noted with either the catecholamines or histamine.