ENDOTHELIAL THROMBOXANE PRODUCTION PLAYS A ROLE IN THE CONTRACTION CAUSED BY 5-HYDROXYTRYPTAMINE IN RAT BASILAR ARTERIES

The goal of the present study was to characterize the role of the endothelium in the 5-hydroxytryptamine (5-HT)-induced contraction of the rat basilar artery. Rat basilar artery segments were mounted in myographs to study their isometric tension development. 5-HT caused dose-dependent contractions that were minimally affected by endothelium removal. The dose-response curve obtained with the 5-HT1 receptor agonist, 5-carboxamidotryptamine (5-CT), was biphasic in arteries with endothelium; removal of the endothelium eliminated the first phase of the contraction. The 5-HT2 receptor antagonist, ketanserin (30 nM), shifted the dose-response curve to 5-HT to the right; in arteries with endothelium, the curve became biphasic. Ketanserin inhibited the second phase of the dose-response curve to 5-CT. The mixed 5-HT1/5-HT2 receptor antagonist, metergoline (30 nM), shifted the dose-response curve to 5-HT non-competitively to the right and depressed both phases of the dose-response curve to 5-CT. In basilar arteries with endothelium and treated with ketanserin, the thromboxane A2 receptor antagonist, ICI 192605 (1 muM), significantly decreased the responsiveness to 5-HT and the dose-response curve for 5-HT became monophasic. ICI 192605 and the thromboxane A 2 synthase inhibitor, ridogrel (10 muM), both suppressed the first phase of the dose-response curve to 5-CT. These data indicate that both endothelial 5-HT1 and smooth muscle 5-HT2 receptors participate in the contractions caused by 5-HT in the rat basilar artery. Activation of the endothelial 5-HT1 receptors causes the release of a substance that subsequently activates thromboxane A2 rcceptors on the smooth muscle cells; this substance most likely is thromboxane A2 itself.