5-HT1-like receptors requiring functional cyclo-oxygenase and 5-HT2 receptors independent of cyclo-oxygenase mediate contraction of the human umbilical artery.

1. The interactions between 5-hydroxytryptamine (5-HT) and the antagonists ketanserin, methysergide and phentolamine were studied in isolated preparations of human umbilical artery (HUA) at physiological oxygen tension (Po2 approximately 15 mmHg) and at high PO2 (approximately 120 mmHg). 2. At physiological Po2 ketanserin, methysergide and phentolamine behaved as silent competitive antagonists of the 5-HT-induced contraction of HUA. pA2 values calculated by Schild analysis were 8.92, 8.52 and 6.37, respectively. 3. At high Po2, 5-HT-induced contractions were antagonised in a biphasic manner by ketanserin (0.1 microM); the response to low but not to high concentrations of 5-HT was resistant to blockade by ketanserin. The ketanserin-resistant component was abolished following cyclo-oxygenase inhibition by indomethacin (1 microM). 4. At high Po2, methysergide behaved as a partial agonist. Methysergide-induced contractions were inhibited but not abolished by indomethacin, and resistant to 5-HT2 receptor and alpha 1-adrenoceptor blockade. 5. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was mimicked by the selective 5-HT1-like receptor agonist 5-carboxamidotryptamine (5-CT): 5-CT was 7 fold more potent than 5-HT. 6. At high Po2 the component of the response to 5-HT mediated by the ketanserin-resistant receptor was antagonised by phentolamine and the selective alpha 2-adrenoceptor antagonist Wy 26703. 7. These results suggest that (i) at physiological Po2 5-HT2 receptors almost exclusively mediate contractions induced by 5-HT, and (ii) at high Po2 the agonist potency order of 5-CT greater than 5-HT greater than methysergide suggests that ketanserin-resistant responses are mediated by 5-HT1-like receptors which require functional cyclo-oxygenase.