Histamine directly acts on beta-adrenoceptors as well as H1-histaminergic receptors, and causes positive inotropic effects in isolated ventricular muscles of carp heart (Cyprinus carpio).

1. The mechanism for positive and negative inotropic effects of histamine was studied in electrically stimulated ventricular strips of carp heart. 2. A high concentration of histamine (1 mM) caused a transient negative, and subsequent positive inotropic effects. The positive effect was significantly reduced by pyrilamine, diphenhydramine or dl-propranolol, but was not affected by cimetidine or d-propranolol. 3. Prior treatment with reserpine significantly decreased epinephrine and norepinephrine contents in ventricular muscles, and also almost completely abolished the positive inotropic effect caused by tyramine; however, this treatment failed to affect the positive inotropic effect of histamine. 4. The transient negative inotropic effect was reduced by neither atropine, diphenhydramine, pyrilamine nor cimetidine, and potentiated by pyrilamine. 5. These results suggest that the positive inotropic effect of histamine observed in the ventricular muscle of carp heart is mediated by a direct stimulation of both H1-receptors and beta-adrenoceptors. The negative inotropic effect is unrelated to either cholinergic or histaminergic receptor stimulation.