Characterization of vascular histamine receptors in the rat.

1 In the rat the decrease in blood pressure caused by histamine involves activation of both H1- and H2-receptors. Since arterial pressure measurements alone do not permit the separation of responses into cardiac and vascular components, the following experiments were undertaken to study vascular histamine receptors. 2 Vascular responses were studied in the autoperfused hindquarters of anaesthetized rats. Intra-arterial histamine caused vasodilatation which was only partially attenuated by treatment with mepyramine, an H1-receptor antagonist. Treatment with metiamide, the H2-receptor antagonist, did not affect vasodilatation caused by histamine but did attenuate vasodilatation which persisted after mepyramine. 3 Intra-arterial 4-methylhistamine, an H2-receptor agonist, caused vasodilatation which was reduced by metiamide. The H1-receptor agonist, 2-(2-pyridyl)ethylamine also caused vasodilatation which was blocked by mepyramine. 4 It is concluded that in the rat, histamine causes vasodilatation mediated by both H1- and H2-receptors.