5-Hydroxytryptamine-induced tachyphylaxis of the molluscan heart and concomitant desensitization of adenylate cyclase.

High concentrations of 5-hydroxytryptamine (5HT) first excite the isolated ventricle of Mercenaria mercenaria and then specifically desensitize it to further additions of the neuropeptide. This 5HT-induced tachyphylaxis is paralleled by a 5HT-specific desensitization of the myocardial adenylate cyclase and a decrease in intracellular cyclic AMP. However, FMRF-NH2, a cardioexcitatory tetrapeptide, can still increase contractility, cyclic AMP, and the adenylate cyclase activity of a tachyphylactic ventricle. These results are consistent with the hypothesis that 5HT augments molluscan myocardial contractility by elevating intracellular cyclic AMP.