Effect of acetylcholine on calcium efflux from atrial myocardium

The effect of acetylcholine (ACh) on the efflux of 45Ca from atrial myocardial tissue was studied on left guinea-pig auricles. ACh at concentrations of 10(-7)-10(-6) g/ml abolished the twitch responses and caused development of week transient contracture in non-stimulated preparations. The extracellular space (ECS) was not affected. ACh at concentrations of 10(-8)-10(-6) g/ml increased 45Ca efflux from quiescent preparations in a dose-dependent manner. This effect was antagonized by atropine. The contracture developed in Ca2+, Na+-free medium was further enhanced by ACh at concentrations of 10(-7)-10(-6) g/ml. These results suggest that ACh causes the release of calcium from atrial tissue. This effect may contribute to the mechanism of the negative inotropic action of ACh.