A class of histamine receptors discriminated by their pharmacology and mode of action. Histamine H2 receptors act via G-proteins to stimulate adenylate cyclase. Among the many responses mediated by these receptors are gastric acid secretion, smooth muscle relaxation, inotropic and chronotropic effects on heart muscle, and inhibition of lymphocyte function. (From Biochem Soc Trans 1992 Feb;20(1):122-5).