Desensitization of the cholinergic receptor at the sinoatrial cell of the kitten

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Abstract
The hyperpolarizing effects of long periods of vagal stimulation were studied in kitten sinoatrial node-vagus nerve preparations. Verapamil (2.2 .times. 10-6 M) was used to arrest spontaneous pacemaker activity thus permitting uninterrupted observation of the time course of cholinergically mediated hyperpolarizations. With progressively longer vagal trains the hyperpolarization was not maintained but decreased, rapidly at first, and then more gradually despite continuous vagal stimulation. Similar decay of the cholinergic effect was observed during continuous iontophoretic application of acetylcholine (ACh) or carbamylcholine (CCh). The decay of the hyperpolarizing response apparently cannot be due to fatigue of nerve terminals, to a gradual reduction in the driving force for K+ or to hydrolysis of ACh by cholinesterase. The development of desensitization of the cholinergic receptor at the sinoatrial cell membrane is demonstrated. The data fit the cyclic reaction model proposed for the neuromuscular junction.