Responses of isolated canine intermediate auricular arteries to 5-hydroxytryptamine and methysergide

The responses of the isolated canine intermediate auricular arteries to 5-hydroxytryptamine (5-HT), methysergide, norepinephrine (NE) and potassium chloride (KCl) were examined by means of the 'cannula inserting' method. 5-HT produced dose-dependent vasoconstriction more potently than did NE. The effect was blocked by methysergide, i.e. a small dose of methysergide competitively inhibited the 5-HT-induced effect, and a large amount of methysergide inhibited it non-competitively. The 5-HT-induced constriction was slightly but significantly suppressed by phentolamine in doses which markedly blocked NE-induced effects. KCl-induced effects were not significantly affected by methysergide and phentolamine treatment. Methysergide induced vasoconstriction by itself in about half out of all preparations. In cases in which methysergide induced vasoconstriction, the dose-response curves were bell-shaped and the constriction became smaller with large doses. At small doses, methysergide induced reproducible vasoconstriction but at large doses it readily caused tachyphylaxis. The methysergide-induced constriction was not blocked by 1 microgram of phentolamine which inhibited NE-induced constriction. It is concluded that the canine intermediate auricular artery is very sensitive to 5-HT and methysergide, and that the action of 5-HT may partially involve alpha-adrenergic mechanisms.