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 responses 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 microM 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 ketanserin and spiperone in a concentration-dependent and partially surmountable manner. The lack of complete surmountability was at least partially due to fade during the determination of the cumulative concentration-effect curves. The pKB values for the component of the 5-HT-induced contractions that was antagonized by ketanserin and spiperone were 9.4 and 10.2 (-log M), respectively. The findings are consistent with the assumption of an interaction of ketanserin and spiperone with 5-HT at 5-HT2 receptors. On the other hand, the response to low concentrations of 5-HT is not mediated through 5-HT2 receptors. This response is antagonized by phentolamine with an affinity approximately 10 times lower than its affinity to 5-HT2 receptors, but not by prazosin or benextramine. It is conceivable that the receptor that mediates the response to low concentrations of 5-HT belongs to the 5-HT1A-subpopulation as suggested recently.