RELAXANT RESPONSE OF GOAT TRACHEA TO 5‐HYDROXYTRYPTAMINE MEDIATED BY D‐TRYPTAMINE RECEPTORS

Abstract

1 Goat isolated trachea contracted in response to carbachol, histamine and 2-pyridylethylamine (an H₁-receptor agonist) and relaxed after application of isoprenaline, 5-hydroxytryptamine (5-HT) and phenylephrine. 2 Mepyramine, a selective H₁-receptor antagonist, blocked histamine- and 2-pyridylethylamine-induced contractions. In high doses it also exhibited some nonspecific antagonism to carbachol. After H₁-receptor blockade, 4-methylhistamine and dimaprit (specific H₂-agonists) relaxed the carbachol-contracted trachea. 3 Propranolol, a β-adrenoceptor blocker, antagonized relaxation in response to isoprenaline and phenylephrine. In high doses, it produced a reversal of the phenylephrine response. 4 Indomethacin enhanced contractions in response to carbachol and histamine. 5 Relaxation to 5-HT was not affected by propranolol, indomethacin, metiamide or cimetidine (H₂-blockers). These findings appear to exclude the involvement of adrenergic, prostaglandinergic and H₂-histaminergic mechanisms in the mediation of this response. 6 Atropine potentiated 5-HT-induced relaxations. This suggests the participation of a ‘masked’ excitatory cholinergic mechanism. 7 Methysergide, dibenamine and dibenzyline selectively antagonized or reversed 5-HT-induced relaxation. Dibenamine and dibenzyline enhanced relaxations to isoprenaline. 8 This investigation showed (i) a relaxant response of goat trachea to 5-HT, mediated via D-muscular tryptamine receptors; (ii) a small population of excitatory M-neuronal tryptamine and α-adrenoceptors; and (iii) predominance of H₁-histamine receptors in the goat trachea.