THE CHANNEL OPENING RATE OF ADULT-TYPE AND FETAL-TYPE MOUSE MUSCLE NICOTINIC RECEPTORS ACTIVATED BY ACETYLCHOLINE

1. In this paper, eve examine acetylcholine (ACh)-induced currents in quail fibroblast cell lines expressing either the Fetal (Q-F18) or the adult (Q-A33) complement of nicotinic acetylcholine receptor subunits derived from mouse skeletal muscle. Pulses of,ACh mere applied to out side-out patches of cell membrane by means of a fast perfusion system, at concentrations from 100 nM to 10 mM. We obtained current records w ith intracellular potentials of -60 and +40 mV. The goal of this study was to estimate the channel opening rate. 2. By fitting sums of expon entials to averaged responses, we estimated the rate of development of the current on the application of acetylcholine. The rate constant of the predominant exponential component (the on-rate) ranges over 3 ord ers of magnitude, from around 100 s(-1) (fetal) at low concentrations of ACh to over 100 000 s(-1) (fetal and adult) at the highest concentr ations. 3. We establish that our measurement of the on-rate is not lim ited by technical constraints, and can therefore be related to the rat e constants of a kinetic scheme. Our observations are consistent with a model having a rate-limiting channel opening step with a forwards ra te constant (beta) of 80 000 s(-1) on average for adult receptors and 60 000 s(-1) for fetal receptors, and a minimum opening to closing rat io (Pla) of around 33 (adult) or 50 (fetal). The channel opening rate, beta, varies from around 30 000 s(-1) to well over 100 000 s(-1) for different patches. The large variation cannot all be ascribed to error s of measurement, but indicates patch to patch variation.