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

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Abstract
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, 0.3 .mu.g, induced a marked increase in perfusion pressure, usually over 100 mmHg. Norepinephrine produced a relatively small increase in perfusion pressure (20-40 mmHg) at a large dose of 10 .mu.g. 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 KCl. The potentiating effect of methysergide on norepinephrine-induced vasoconstriction may be due partially to activation of the inward Ca2+ channel of the internal carotid artery.