Adenylyl cyclase and guanine nucleotide-binding proteins in supersensitive guinea pig ventricles

Chronic treatment with reserpine (0.1 mg/kg/day x 7 days) leads to the development of adaptive supersensitivity of ventricular myocardium of guinea pigs. The compensatory increase in sensitivity is associated with a small increase in beta-adrenoreceptor number. However, sensitivity is increased to a number of agonists that do not interact with beta-adrenoceptors. An evaluation of the role of both adenylyl cyclase and guanine nucleotide-binding regulatory proteins in the development of adaptive supersensitivity was carried out using crude membrane fragments from untreated control and chronically reserpine-treated guinea pigs. Quantitative analysis of Gs alpha and Gi protein concentrations was accomplished using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Chronic treatment with reserpine reduced basal levels of adenylyl cyclase activity by nearly 60%. The reduced activity was not the result of a loss of endogenous norepinephrine, because incubation of tissues in the presence of propranolol did not alter the basal level of adenylyl cyclase activity. Incubation in the presence of guanylylimido diphosphate (10(-5) M) also significantly reduced basal adenylyl cyclase activity, by nearly 70%. Chronic treatment with reserpine failed to significantly alter the activation of adenylyl cyclase by isoproterenol, impromidine, NaF, or forskolin. These data suggest that chronic treatment with reserpine does not alter agonist-induced activation of adenylyl cyclase. Furthermore, analysis of Gs alpha and Gi indicated that chronic treatment with reserpine did not affect the levels of these regulatory proteins in ventricular myocardial membranes. The data indicate that the enhanced sensitivity of guinea pig ventricular myocardium is not the result of an alteration in adenylyl cyclase activity or in the concentration of guanine nucleotide regulatory proteins. Therefore, the enhanced responsiveness to widely diverse agonists must be due to an alteration in cellular function beyond the level of adenylyl cyclase.