Muscarinic receptor regulation of cardiac adenylate cyclase activity

Stimulation and inhibition of adenylate cyclase activity are mediated by the guanine nucleotide regulatory proteins Gs and Gi, respectively. Two general mechanisms have been proposed for the inhibition of activated adenylate cyclase: direct inhibition of the catalyst by Gi, and indirect inhibition of the activated catalyst mediated by Gi inhibition of Gs. We have assessed direct inhibition of adenylate cyclase by evaluating the ability of Gpp(NH)p to inhibit the forskolin-stimulated enzyme in the presence of various concentrations of magnesium ions and the guanine nucleotide. Gpp(NH)p inhibition of adenylate cyclase activity was only observed in the presence of forskolin and low concentrations of MgCl2. Muscarinic agonists did not increase Gpp(NH)p inhibition of the forskolin-stimulated enzyme, even in the presence of low concentrations of MgCl2 and guanine nucleotide (near the respective Kact or Ki). Whether in the absence or presence of muscarinic agonists, no concentration of Gpp(NH)p was found to inhibit basal adenylate cyclase activity in the absence of forskolin. In addition, muscarinic agonists had no effect on the rate constant (kon) for Gpp(NH)p activation of the enzyme. In contrast to these data, the muscarinic agonist methacholine stimulated the inactivation rate constant (koff) for isoproterenol plus GTP-activated adenylate cyclase activity 15-fold, and the increase in koff was blocked by atropine. Moreover, the sarcolemma displayed specific, high affinity GTP hydrolytic activity which was stimulated by methacholine activation of muscarinic receptors. These data further support our original hypothesis, indicating that although direct inhibition of the catalyst by Gi may occur in cardiac sarcolemma, physiologically relevant attenuation of adenylate cyclase activity by muscarinic agonists occurs by a mechanism linked to GTP hydrolysis.