Histamine H1- and H2-receptors in the cat and their roles during alveolar hypoxia.

We sought to define the roles of H1-and H2-receptors in the cat and to evaluate the roles of these receptors during alveolar hypoxia. In pentobarbital anesthetized cats, we found that histamine infusion (1.1 microgram/kg/min for 3 min) increased cardiac output and decreased pulmonary and systemic vascular resistances. However, when cardiac output was held constant, histamine infusion induced pulmonary vasoconstriction. Histamine infusions after H1-and H2-receptor blockade (chlorpheniramine and metiamde, respctively) indicated that H2-receptors mediated systemic vasodilatation. In the lung, H1-receptors mediated vasoconstriction, and H2-receptors mediated vasodilatation. Hypoxia (10% O2) caused large increases in pulmonary vascular resistance which were not blocked by H1-, H2-, or combined H1- and H2-receptor blockade. In the intact cat, histamine does not appear to mediate hypoxic pulmonary hypertension.