[The mechanism of action of histamine on the myocardium and the smooth vascular muscles].

Histamine-H1-receptors are only present in the left atrium of guinea-pig hearts, whereas the ventricular myocardium contains histamine-H2-receptors exclusively. Although the stimulation of both receptor types causes positive intropic effects, only the H2-receptor response is closely associated with increases of cyclic AMP. H1- as well as H2-receptors are present in the rabbit mesenteric artery. Cyclic AMP serves as a messenger for H2-receptor mediated relaxation only and not for H1-receptor mediated contraction. Thus, the different effects on the cyclic AMP-system must also be considered in H1- and H2-receptor differentiation.