A physiological and pharmacological study of 5-hydroxytryptamine on hearts of mollusca bivalvia

Abstract

The question under examination is whether there are various 5-HT receptor types present in the hearts of different species of molluscan bivalves. In order to examine this question, the actions of 5-HT and some analogues on the hearts of several species of bivalve molluscs were investigated in two different preparations. The effects of 5-HT and analogues were examined on isolated whole ventricle preparations and on the unitary currents of patch-clamped myocytes from some of the same species. The 5-HT receptor activated in Mya arenaria and Mercenaria mercenaria myocytes produced a decrease in unitary current activity across the membrane, which correlated with the effect seen by elevation of cAMP intracellularly. This fits well with the excitation of the whole hearts by 5-HT, and the earlier literature. In Geukensia demissa the presence of another 5-HT receptor was observed which when activated also causes an increase in K+ unitary current activity; which was not mimicked by elevation of intracellular cAMP. This also correlated with some of the mixed effects of 5-HT on the whole hearts. However, 5-CT and 5-MEOT caused a decrease in K+ unitary current activity; which also correlated with the inhibition of the whole hearts by 5-CT and 5-MEOT. The effects observed with 5-HT, 5-CT, 5-MEOT in Geukensia demissa appear to be linked to the activation of an intracellular pathway independent of cAMP. The results described from the whole heart and the myocytes experiments suggest that there are in fact more than one conformation of 5-HT receptor present in the hearts of bivalves.