Investigation into the positive inotropic effect of clondine in isolated hearts

Summary Clonidine in doses 2.5–40 g injected into the aortic cannula of isolated, perfused hearts of guinea pigs produced an increase in contraction amplitude. This positive inotropic effect was not antagonised by infusions of doberol (10 g/min), phentolamine (10 g/min) or pheniramine (50 g/min), it was antagonised by burimamide (30 g/min), a histamine H₂ receptor blocking agent. Clonidine showed some similarities to histamine such as parallel dose-response curves and during infusions of burimamide the curve of clonidine was shifted in a way which indicated competitive inhibition. It is suggested therefore that the action of clonidine is due to stimulation of histamine H₂ receptors.