RESPONSES OF RABBIT PORTAL VEIN TO HISTAMINE

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Abstract
Histamine produced a dose‐dependent contraction of the isolated portal vein of the rabbit. This contraction was not antagonized by atropine, methysergide, indomethacin, cocaine or 6‐hydroxydopamine, nor by pretreatment of the rabbit with reserpine. The response to histamine was blocked by H1‐receptor antagonists only when the blocking agent was used in very high concentrations, and was not antagonized by the H2‐receptor blocking agent, metiamide. H1‐receptor antagonists did not block the effects of 5‐hydroxytryptamine. The contractions elicited by histamine, 5‐hydroxytryptamine and noradrenaline were blocked by phentolamine. Desensitization to high doses of 5‐hydroxytryptamine caused a concomitant depression in the response to histamine but not to noradrenaline or acetylcholine. The results suggest that the contractions of rabbit portal vein elicited by histamine are not mediated by receptors of the H1‐ or the H2‐type, but may involve an action of histamine at a receptor which is also involved in the action of 5‐hydroxytryptamine.