Potentiating effect of methysergide on norepinephrine-induced constriction of the isolated internal carotid artery of the dog.

The stainless steel cannula inserting method was used to examine the effects of methysergide on 5-hydroxytryptamine (5-HT)- and norepinephrine-induced vasoconstriction in the isolated internal carotid artery of the dog. 5-HT, at a dose of 0.3 microgram, induced a marked increase in perfusion pressure, usually over 100 mm Hg. On the other hand, norepinephrine produced a relatively small increase in perfusion pressure (20-40 mm Hg) at a large dose of 10 micrograms. Methysergide inhibited 5-HT-induced vasoconstriction. Norepinephrine-induced vasoconstriction was significantly potentiated by treatment with methysergide and blocked by phentolamine. Methysergide also enhanced the vasoconstrictor response to potassium chloride. Thus, it is suggested that the potentiating effect of methysergide on norepinephrine-induced vasoconstriction may partially be due to activation of the inward calcium channel of the internal carotid artery.