Muscarinic cholinergic receptor-mediated attenuation of adenylate cyclase activity in rat heart membranes.

The regulation of adenylate cyclase by muscarinic cholinergic receptors has been studied in rat heart membranes. In the presence of isoproterenol the K0.5 for Mg2+-mediated activation of adenylate cyclase was 0.1 mM; the addition of the muscarinic receptor agonist oxotremorine increased the K0.5 for Mg2+, and activation by Mg2+ no longer followed mass action kinetics for a single site interaction. The extent of oxotremorine-mediated attenuation of adenylate cyclase exhibited a Mg2+ concentration dependence: in short-time assays at 0.25 mM free Mg2+ the attenuation of enzyme activity was 55 percent, whereas at 20mM Mg2+, only 20 percent inhibition was observed. The increase in the apparent K0.5 for Mg2+ in the presence of oxotremorine was less in 20 min assays than in 4 min assays. The effects of oxotremorine on the rates of activation and deactivation of adenylate cyclase also were examined. Oxotremorine increased the rate of deactivation of adenylate cyclase by two-fold. At low free Mg2+ concentrations, oxotremorine also decreased the rate of activation of adenylate cyclase in the presence of either 0.1 uM GTP gamma S or 1 uM GTP. An oxotremorine-mediated decrease in the rate of activation of adenylate cyclase activity was not detectable in the presence of greater than 3 mM Mg2+.