Action of acetylcholine on the atropinised frog heart during the winter months

During the winter months, high doses of acetylcholine produced positive inotropic action without any chronotropic action on the perfused atropinised frog heart, in 5 out of 24 preparations. In the remaining preparations acetylcholine failed to produce any action and positive inotropic effect of acetylcholine on these preparations was noticed if they were perfused with the medium containing excess of calcium. However, the rate remained unchanged. The positive inotropic action was blocked by the local anaesthetic amethocaine and thus may be due to increased penetration of calcium into the cardiac cell.