Characterization and distribution of histamine H1- and H2-receptors in precapillary vessels.

Selective H1- and H2-receptor agonists and antagonists are available to characterize histaminergic receptors and their distribution in the vasculature. Histamine can contract isolated vessels by acting on H1-receptors (i.e., mepyramine-sensitive) and induce relaxation (after inducing tone by, e.g., an alpha-adrenergic agonist) through H2-receptors (i.e., antagonized by burimamide, metiamide, or cimetidine) and H1-receptors. There are relatively few studies in which histamine receptors have been characterized using selective agonists to minimize problems arising from the coexistence of both types of receptor in the same tissue. The histamine H1- and H2-receptors in precapillary vessels may be similar to those in the guinea pig ileum and atria, respectively, but there are many results that appear to be at variance with this conclusion. The effect of histamine in vivo may be complicated by several factors, including distribution (e.g., leading to modification of transmission at sympathetic ganglia and post-ganglionic nerve terminals) and the method of administration. Intravenous injection of histamine produces hypotension, mediated predominantly by H1-receptors. After histamine infusion, the fall in blood pressure is mediated initially through H1-receptors, but there is also a sustained hypotensive effect (with a slower rate of onset) mediated by H2-receptors. In contrast to the systemic circulation, in the pulmonary circulation, histamine generally elicits vasoconstriction mediated by H1-receptors and vasodilatation by H2-receptors.