RECEPTORS MEDIATING THE EFFECTS OF 5-HYDROXYTRYPTAMINE ON THE TRACHEAL VASCULATURE AND SMOOTH-MUSCLE OF SHEEP

The effects of 5-hydroxytryptamine (5-HT) and the selective 5-HT3 receptor agonist 2-methyl-5-HT were examined on the tracheal vasculature in anaesthetised, paralysed and artificially ventilated sheep. The cranial tracheal arteries were perfused with blood at constant flow and perfusion pressure was measured. 5-HT and 2-methyl-5-HT were injected into the arterial perfusate. Changes in tracheal smooth muscle tone were also measured. 5-HT contracted the tracheal smooth muscle. This contraction was not affected by the 5-HT2 receptor antagonist ketanserin, but was blocked by the combined '5-HT1-like' and 5-HT2 receptor antagonist methysergide. 2-methyl-5-HT had no effect on the tracheal smooth muscle. 5-HT had a complex action on the tracheal vasculature producing either a biphasic change (vasoconstriction followed by dilatation) or just a constriction. 2-methyl-5-HT had a weak vasoconstrictor effect. The vasoconstrictions produced by 5-HT and 2-methyl-5-HT were blocked by ketanserin. The vasodilatation produced by 5-HT was resistant to ketanserin but was antagonised by methysergide. Thus 5-HT contracts the tracheal smooth muscle of sheep by stimulating '5-HT1-like' receptors. 5-HT constricts the tracheal vascular bed by stimulating 5-HT2 receptors or dilates the vascular bed by stimulating '5-HT1-like' receptors. 5-HT3 receptors appear not to be involved in the responses of the tracheal vasculature or smooth muscle to 5-HT.