Methoctramine, a cardioselective antagonist: muscarinic receptor mediating prostaglandin synthesis in isolated rabbit heart

The antimuscarinic properties of the methoctramine with high selectivity for cardiac muscarinic M2 receptors were investigated on cholinergically induced changes in prostaglandin (PG) synthesis and mechanical function in the isolated perfused rabbit heart. Acetylcholine (ACh)- and arecaidine propargyl ester (APE)-induced increases in PG synthesis were significantly attenuated by methoctramine in a concentration-dependent manner. Methoctramine at a low concentration of 0.1 microM potentiated ACh-induced PG synthesis, which was blocked by simultaneous infusion of hexahydro-sila-difenidol (HHSiD), a M3 receptor antagonist. Methoctramine produced an additive effect with HHSiD in diminishing the ACh- or APE-induced PG synthesis. Methoctramine displayed a potent antagonistic activity at M2 receptors that mediate the decrease in heart rate and increase in coronary perfusion pressure in isolated perfused rabbit heart. Methoctramine also minimized ACh- and APE-induced decrease in developed tension. In contrast, at 0.1-0.75 microM it exhibited no antagonistic activity at vascular muscarinic receptors (M3) mediating vasodilation in response to ACh or APE. These data suggest that methoctramine has a high affinity for cardiac M2 receptors mediating increases in PG output and coronary perfusion pressure as well as decrease in heart rate and developed tension and has a low affinity for M3 receptors mediating coronary vasodilator response.