Myocardial histamine(H)2 receptor stimulation has been studied in six normal men. Since histamine is a potent vasodilator, the haemodynamic effects of histamine infusion were compared with those of nitroprusside at equihypotensive doses, to identify changes in myocardial contractility attributable to vasodilatation. After H1 receptor blockade with mepyramine, subjects received, in single blind crossover fashion, either histamine alone and with the H2 receptor antagonist cimetidine, or nitroprusside alone and with cimetidine. Echocardiographic left ventricular dimensions, plasma catecholamines, blood pressure, and heart rate were measured. The rise in catecholamines suggested similar baroreflex activation by both histamine and nitroprusside. Echo ejection phase indices did not alter significantly after nitroprusside, but histamine caused an increase in percentage fractional shortening from 38.2 +/- 4.1 to 53.5 +/- 3-6% and in mean fibre shortening velocity from 1.31 +/- 0.19 to 1.99 +/- 0.22 cm/s. These changes were both greatly reduced by cimetidine and suggest that H2 receptor stimulation in man causes a direct positive inotropic response.
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