Modification of acetylcholine action by bivalent cations on the perfused heart of the frog.

When frog heart is perfused with high calcium Ringer solution it may be stimulated by a low dose of acetylcholine. This stimulant action is mediated through atropine-sensitive receptors and is blocked by certain calcium antagonists such as local anesthetics, cobalt, magnesium, and phenoxybenzamine. However, the nicotinic action of high doses of acetylcholine on atropinized heart is not blocked by cobalt and magnesium ions.