Evidence for Two Populations of 5-.Hydroxytryptamine Receptors in Dog Basilar Artery

The aim of the present study was the classification of the receptors that mediate 5-hydroxytryptamine (5-HT)-induced re-sponses in dog basilar artery. Isolated preparations from basilar artery of mongrel dogs denuded of endothelium were contracted by 5-HT in the presence of 6 1M cocaine. In the presence of either ketanserin or spiperone, concentration-effect curves for 5-5-HT became biphasic. The responses to low concentrations of 5-5-HT were resistant to blockade by either antagonist. The responses to high concentrations of 5-HT were antagonized by ketansenn and spiperone in a concentration-dependent and par-tially surmountable manner. The lack of complete surmountability was at least partially due to fade dunng the determination of the cumulative concentration-effect curves. The PKB values for the component of the 5-HT-induced contractions that was antago