Evidence for an indirect component of the action of histamine on canine portal vein.

Histamine caused contraction of spiral strips of the canine portal vein. This is an H1 response and no evidence of any H2 receptor activity was observed. The response to histamine was blocked by the alpha-adrenoceptor antagonists phentolamine and prazosin, but not by the alpha 2-receptor blocker yohimbine. Pretreatment of the tissue with 6-hydroxydopamine significantly reduced the response to histamine and tyramine while the response to noradrenaline was unchanged. A marked tachyphylaxis developed to tyramine and histamine, but not to noradrenaline. We conclude that a component of the contractile activity of histamine on the canine portal vein involves release of endogenous catecholamines and that this response is mediated by H1 receptors.