Antagonism by ketanserin of 5-HT-induced vasoconstriction unmasks a 5-HT-induced vasodilation

The effect of ketanserin on the action of 5-hydroxytryptamine (5-HT) and noradrenaline has been studied using the rat isolated perfused mesentery preparation. Both 5-HT and noradrenaline produced constriction in the mesenteric vessels. The responses to 5-HT and noradrenaline were greater in tissues from hypertensive animals. Responses to 5-HT were abolished by 1 nM ketanserin. Ketanserin was found to be a competitive antagonist of the noradrenaline-induced constrictions (pA2 = 7.5). In partially constricted tissues, 5-HT induced vasoconstriction and vasodilation. In the presence of ketanserin the vasoconstriction was reduced and the 5-HT-induced vasodilation was potentiated. The amount of potentiation of the 5-HT-induced vasodilation by ketanserin was similar in tissues from normotensive and hypertensive rats. It is suggested that the 5-HT-induced vasoconstriction is mediated by activation of 5-HT2 receptors. In the presence of ketanserin, the 5-HT2 receptor mediated vasoconstriction is antagonized, unmasking a 5-HT-induced vasodilation.