Desensitization at the frog neuromuscular junction: a biphasic process

Abstract

The desensitization of the cholinergic receptor was investigated at the frog neuromuscular junction. The agonist was either perfused or applied by ionophoresis. In all situations, desensitization develops in 2 phases: a fast one, experimentally in the 2nd range but likely to be briefer, and a slower one, which extends over tens of seconds. When the presence of the agonist is prolonged, desensitiziation approaches a steady state, estimated through the amplitude of a test response. In steady-state conditions, this amplitude depends upon the desensitizing agonist concentration. The dose-response curve for desensitization induced by carbachol (CCh) indicates that half of the receptors can be desensitized at room temperature in the presence of 2.3 .mu.M-CCh. The shape of the curve suggests that 1 desensitized receptor can bind in 2 CCh molecules. The recovery from desensitization, estimated with a repetitive test pulse, displays 2 exponential phases. The time constant of the fast phase is 11-12 s, and 4-5 min for the slow phase, regardless of the concentration or the nature of the agonist (acetylcholine or carbachol). The factor which most strikingly affects the relative amplitudes of the fast and slow phases of recovery is the duration of the (desensitizing) agonist application. Desensitizations lasting a few seconds are followed by a fast recovery; the slow phase of recovery is prominent then when the agonist was applied for more than 2 min. The fast and slow phases of desensitization onset and offset are not due to independent causes but are coupled; the onset can be essentially fast, and the recovery slow. All the findings can fit in a cyclic scheme of desensitization, derived from one of the Katz and Thesleff with 2 modifications: whether activatable or desensitized, 1 receptor molecule would have 2 agonist binding sites; moreover, the desensitized receptor would exist in 2 distinct and interconverting conformations: D1, giving rise to the fast phases of onset and offset, and D2, responsible for the existence of the slow components of desensitization.