Coupling of muscarinic receptors to adenylate cyclase in the rabbit myocardium: effects of receptor inactivation.

The relationship between muscarinic receptor occupancy and adenylate cyclase inhibition was investigated in homogenates of the rabbit myocardium. The highly efficacious muscarinic agonist oxotremorine-M caused half-maximal inhibition of adenylate cyclase activity at a concentration (Ki) that was 10-fold smaller than that required for half-maximal receptor occupancy in the presence of 0.1 mM GTP (D50-GTP) as measured by competitive displacement of the binding [3H]N-methylscopolamine. In contrast, there was much closer agreement between the Ki and D50-GTP of the less efficacious oxotremorine analog BM5 [N-methyl-N-(1-methyl-4-pyrrolidino-2-butynyl)acetamide]. By comparing equal levels of adenylate cyclase inhibition before and after partial inactivation of muscarinic receptors with benzilycholine mustard, it was possible to estimate the dissociation constants (KA) of the oxotremorine analogs. There was good agreement between KA and D50-GTP and also between the degree of receptor inactivation determined pharmacologyically and that estimated by measurements of the binding of [3H]N-methylscopolamine.