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The purpose of this investigation was to determine the direct effects of glucagon on adenyl cyclase activity in cat and human heart particles and to elucidate the role of the beta receptor in any glucagon-mediated activation of adenyl cyclase. At the peak of its dose-response curve, crystalline glucagon increased the conversion of AT32P to cyclic 3′, 5′-AM32P in particulate fractions of both cat and human heart homogenates by approximately 70%. The activation of adenyl cyclase was dose-related over a concentration ranging from 1 × 10−7M to 1 × 10−5M. Half maximal activity was observed at 8 × 10−7M. DL-propranolol, 1 × 10−5M, did not block the activation of adenyl cyclase produced by glucagon, 1 × 10−6M or 1 × 10−5M. However, the same concentration of propranolol blocked adenyl cyclase activation induced by norepinephrine, 1 × 10−6M and 1 × 10−5M. Combined maximal doses of glucagon and norepinephrine did not produce additive effects on the activation of adenyl cyclase. The failure of DL-propranolol to block the glucagon-mediated activation of adenyl cyclase suggests that there are at least two receptor sites in myocardial tissue responsible for the activation of adenyl cyclase, one responsive to glucagon and one to norepinephrine. Moreover, since combined maximal doses of glucagon and norepinephrine failed to produce an additive response, it appears that in the heart there is probably only a single adenyl cyclase enzyme responsive to these hormones.

Activation of Adenyl Cyclase by Glucagon in Cat and Human Heart