In this paper, we 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 were applied to outside-out patches of cell membrane by means of a fast perfusion system at concentrations from 100 nM to 10 mM. We obtained current records with intracellular potentials of -60 and +40 mV. The goal of this study was to estimate the channel opening rate. 2. By fitting sums of exponentials 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 orders of magnitude, from around 100 s-1 (fetal) at low concentrations ACh to over 100,000 s-1 (fetal and adult) at the highest concentrations. 3. We establish that our measurement of the on-rate is not limited by technical constraints, and can therefore be related to the rate constants of a kinetic scheme. Our observations are consistent with a model having a rate-limiting channel opening step with a forward rate 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 ratio (beta/alpha) 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 errors of measurement, but indicates patch to patch variation.
