On the mechanism of histamine action in cardiac muscle.

The effect of histamine on electrical and mechanical properties of guinea-pig ventricular muscle fibers was investigated. It was found that the drug decreased in rate of tension development, the rate of fall of the twitch and decreased the time to peak tension. The height of the plateau of the action potential was increased by histamine and this seems to explain, at least, part of the drug action on heart contractility. The uptake and efflux of 45Ca were both increased by the drug. The positive inotropic action of histamine is largely dependent on the external calcium concentration and temperature and is abolished in low Na solution. It was found that histamine has a marked relaxing action which is probably related to increased Ca sequestration by the sarcotubular system. The drug also produced a decrease in post-extrasystolic potentiation.