The effects of 5-hydroxytryptamine on the isolated and intact circulatory system of the pinto abalone,Haliotis kamtschatkana,and its presence in cardiac and non-cardiac tissues

1. The in vivo and in vitro effects of 5-hydroxytryptamine (5-HT) on the circulatory system of the pinto abalone, Haliotis kamtschatkana, were examined.2. In the isolated heart, 5-HT caused a dose-dependent increase in beat frequency.3. In the intact animal, however, this effect was accompanied by increases in aortic systolic, diastolic, and pulse pressures and, at very high doses, efferent ctenidial pressure.4. The effects of 5-HT were inhibited by several antagonists.5. Furthermore, high levels of 5-HT were found in the tissue samples taken from the auricles, and ventricle and in the hemolymph, and pericardial fluid.6. Therefore, 5-HT may have a role as an excitatory neurotransmitter in the control of circulation in H. kamtschatkana.