The Evolution of Vascular Smooth Muscle Responses to Histamine and 5‐Hydroxytryptamine

In an attempt to trace the phylogenetical origin of stimulatory (constrictive) and inhibitory (dilatatory) actions of histamine and 5‐hydroxytryptamine on mammalian vascular smooth muscle, the vascular responses to these drugs were studied in jawless vertebrates and fish. Effects were recorded as blood pressure changes in dorsal and ventral aorta after intravascular administration of drugs, and as resistance changes in artificially perfused branchial and systemic blood vessels after administration into the perfusion circuit. The reliability of experimental procedure and recording technique was tested by observing the presence of responses to adrenaline, noradrenaline or acetylcholine. Pharmacological blocking agents were used to dissociate between specific and unspecific actions. In jawless vertebrates, the vascular effects of histamine and 5‐hydroxytryptamine seemed to reflect unspecific actions on adrenergic receptor mechanisms. The effects in cartilaginous fish were slight. Characteristic effects, produced by specific stimulatory actions on vascular smooth muscle, appeared in higher bony fish. Inhibitory actions were not revealed in any species. By comparing the results obtained in intact animals with those from perfused preparations, it may be concluded that stimulatory actions of 5‐hydroxytryptamine appear as a constrictor response in the branchial blood vessels of teleosts, while the stimulatory actions of both histamine and 5‐hydroxytryptamine are fully developed and produce vasoconstriction in all studied sections of the vascular system in lungfish.