Biochemical mechanisms of myocardial adenylate cyclase subsensitivity to isoproterenol in cardiac hypertrophy of spontaneously hypertensive rats

The responsiveness of the myocardial adenylate cyclase (AC) system in generating cAMP was studied using isoproterenol (a beta-adrenergic receptor agonist), cholera toxin (a guanosinetriphosphatase inhibitor) and forskolin (a catalytic unit activator) in isolated myocytes of age-matched, 14-17 weeks old Wistar Kyoto normotensive rates (WKYs) and spontaneously hypertensive rats (SHRs). We found a reduction in isoproterenol-stimulated cAMP formation in myocytes of SHRs compared with WKYs. This reduction was not due to changes in isoproterenol-receptor interactions. Scatchard plot analysis of [3H]CGP 12177 binding to beta-adrenergic receptors in isolated myocytes of WKYs and SHRs revealed to significant differences in the maximum number of binding sites or dissociation constant. There were no significant differences in Ki and IC50 calculated from the competitive displacement of [3H]CGP 12177 binding by (-) isoproterenol, suggesting no change in the affinity of the beta-adrenergic receptors for isoproterenol. We found no significant differences in forskolin-stimulated cAMP formation between the two groups. This suggest that the reduction in isoproterenol-stimulated cAMP formation observed in myocytes of SHRs is not due to changes in the ability of catalytic unit to convert ATP to cAMP. Interestingly, cholera toxin-stimulated cAMP formation was increased in myocytes of SHRs. One possible explanation for these observations may be increased guanosinetriphosphatase (GTPase) activation by isoproterenol in myocytes of SHRs. The activation of GTPase by isoproterenol in myocytes of SHRs. The activation of GTPase by isoproterenol was measured as the release of Pi from (γ-32P)GTP. There was an increase in isoproterenol-stimulated GTPase activity in myocytes of SHRs compared with WKYs