Impromidine is a partial histamine H2-receptor agonist on human ventricular myocardium

The inotropic effects of impromidine have been studied and compared with those of histamine on human isolated left ventricular preparations stimulated at 1 Hz. Both drugs caused concentration-related increases in force of contraction and were of similar potency, although the maximum response to impromidine was markedly and significantly less than that to histamine. The positive inotropic responses of impromidine were inhibited by cimetidine 1 X 10(-5) M, consistent with histamine H2-receptor involvement. Impromidine 1 X 10(-4) M inhibited maximal responses to histamine to a level equal to the maximal impromidine response; however, impromidine did not inhibit responses to isoprenaline. Positive inotropic activity and inhibition of maximal responses to histamine occurred over a similar impromidine concentration-range. Impromidine displaced histamine concentration-response curves to the right, whereas mepyramine had no effect on responses to histamine. It is concluded that impromidine has positive inotropic activity on the human ventricle, that the response is mediated via histamine H2-receptors, and that impromidine is a partial agonist compared with histamine.