Effects of an alkylating derivative of oxotremorine (BM 123A) on heart rate, muscarinic receptors and adenylate cyclase activity in the rabbit myocardium.

The effects of the aziridinium ion (BM 123A) of a 2-chloroethylamine derivative of oxotremorine, N-[4-(2-chloroethylmethylamino)-2-butynyl]-2-pyrrolidone (BM 123), on heart rate, muscarinic receptor binding properties and adenylate cyclase activity were investigated in the rabbit myocardium. BM 123A was a potent inhibitor of heart rate in spontaneously beating rabbit hearts. Perfusion of hearts with BM 123A for 30 min caused a lack of sensitivity of the heart to the muscarinic agonist oxotremorine-M and an 80% alkylation of muscarinic receptors as measured by the reduction in the binding capacity of N-[3H]methylscopolamine. Alkylation of muscarinic receptors in the heart with BM 123A had no significant effect on adenylate cyclase activity by itself but caused a functional blockade of muscarinic receptor-mediated inhibition of adenylate cyclase activity. The reversible binding characteristics of both BM 123 and BM 123A were investigated in competitive binding experiments with N-[3H]methylscopolamine at 0 degree C, conditions under which little or no receptor alkylation occurs. The concentration of BM 123A required for half-maximal receptor occupancy was similar to that of oxotremorine-M whereas the parent mustard (BM 123) was 200-fold less potent. GTP (10 microM) had no significant effect on the binding of N-methylscopolamine but caused a 16-fold increase in the concentration of BM 123A required for half-maximal receptor occupancy.