Intracellular actions of 5‐hydroxytryptamine on the bivalve myocardium. I. Adenylate and guanylate cyclases

The effects of the neurotransmitters 5‐hydroxytryptamine (5HT) and acetylcholine (ACh) on the adenylate and guanylate cyclase activities in the ventricles of two species of bivalve molluscs, Mercenaria mercenaria and Macrocallista nimbosa, were examined. 5HT increased molluscan adenylate cyclase activity and intracellular cyclic AMP; ACh had no effect. Neither of these agents significantly altered ventricular cyclic GMP. A positive correlation between the excitatory action of 5HT and the simultaneous increase in intracellular cyclic AMP strongly suggests that this nucleotide mediates 5HT excitation. This proposal is further supported by experiments on ventricles of Modiolus demissus granosissimus that are depressed by 5HT and exhibit very low levels of adenyl cyclase activity. However, classical pharmacological evidence is lacking; cyclic AMP, dibutyryl cyclic AMP, and phosphodiesterase inhibitors have no effect on the mechanical activity of isolated bivalve ventricles.