Desensitization of the muscarinic receptor in the mammalian atrial myocardium.

Action potentials of constantly driven isolated trabeculae of the rabbit's atrial myocardium, being depressed by acetylcholine (ACh) at concentrations between 0.2 and 20 mumol/l, recovered during a constant drug application. The sensitivity of the atrial myocardium to ACh after a prolonged drug application was decreased but found to be restored about 7 min after switching off the ACh superfusion. The recovery during drug application was accelerated in Ca++ excess solutions but decelerated in K+ excess solutions. All the effects of ACh were blocked by atropine (1 mumol/l) completely. The results suggest that the muscarinic ACh receptor in the mammalian myocardium, responsible for the electrotropic effects, shows desensitization to ACh similar to the nicotinic receptor in other tissues.