Treatment of intact hepatocytes with the calcium ionophore A23187 perturbs both the synthesis and the degradation of the second messenger cyclic AMP. Actions on adenylate cyclase and cyclic AMP phosphodiesterase activities.

The presence of the calcium ionophore A23187 augmented glucagon's ability to elevate intracellular cyclic AMP concentrations in intact hepatocytes. However, when the cyclic AMP phosphodiesterase inhibitor 1-isobutyl-3-methylxanthine (IBMX) was added to prevent the degradation of cyclic AMP then the presence of A23187 attenuated the ability of glucagon to increase intracellular cyclic AMP concentrations. Treatment of intact hepatocytes with A23187 led to a dose-dependent persistent inhibition of the glucagon-stimulated adenylate cyclase activity expressed by a membrane fraction isolated from such ionophore-treated hepatocytes. In hepatocytes where glucagon-stimulated adenylate cyclase activity was desensitized then A23187-treatment of hepatocytes failed to exert any inhibitory action on adenylate cyclase. Treatment of isolated membranes directly with A23187 did not elicit any changes in glucagon-stimulated adenylate cyclase activity. Such actions of A23187 were blunted when Ca2+ (2.5 mM) was not added to the extracellular medium. It is suggested that treatment of hepatocytes with A23187 leads to the functional uncoupling of glucagon-stimulated adenylate cyclase activity in a manner which appears to mimic the desensitization process. A23187-treatment also exerted an overall inhibitory effect on the cyclic AMP phosphodiesterase activity displayed by intact hepatocytes. Thus treatment of hepatocytes with A23187 exerted a profound effect on cyclic AMP metabolism in these cells.