Acetylcholine receptor kinetics. A description from single-channel currents at snake neuromuscular junctions.

Single-channel currents from acetylcholine receptor channels of garter snake neuromuscular junctions were recorded using the patch-clamp technique. Low concentrations of acetylcholine or carbamylcholine induced populations of single current events whose amplitudes and durations had unimodal distributions. The probability with which channel opening transitions occurred was time dependent, so that it was more probable for channels to open during the several hundred microseconds following a closing transition than during any later equivalent interval. The time-dependent distributions of duration and opening-transition probability were fitted by a sequential, reversible kinetic model in which the agonist binding steps occur before, and separately from, channel activation. This description allowed estimates to be obtained of both the opening (approximately 750s-1) and closing (approximately 500s-1) transition rates of these channels and of the mean lifetimes of the open- (approximately 2 ms) and the closed-channel state (approximately 200 mus) to which the open state was reversibly related.