We studied activation of the nicotinic acetylcholine (ACh) receptor on cells of a mouse clonal muscle cell line (BC3H1). We analyzed single-channel currents through outside-out patches elicited with various concentrations of acetylcholine (ACh), carbamylcholine (Carb) and suberyldicholine (Sub). Our goal is to determine a likely reaction scheme for receptor activation by agonist and to determine values of rate constants for transitions in that scheme. Over a wide range of agonist concentrations the open-time duration histograms are not described by single exponential functions, but are well-described by the sum of two exponentials, a brief-duration and a long-duration component. At high concentration, channel openings occur in groups and these groups contain an excess number of brief openings. We conclude that there are two open states of the ACh receptor with different mean open times and that a single receptor may open to either open state. The concentration dependence of the numbers of brief and long openings indicates that brief openings do not result from the opening of channels of receptors which have only one agonist molecule bound to them. Closed-time duration histograms exhibit a major brief component at low concentrations. We have used the method proposed by Colquhoun and Sakmann (1981) to analyze these brief closings and to extract estimates for the rates of channel opening (beta) and agonist dissociation (k-2). We find that this estimate of beta does not predict our closed-time histograms at high agonist concentration (ACh: 30–300 microM; Carb: 300–1,000 microM). We conclude that brief closings at low agonist concentrations do not result solely from transitions between the doubly-liganded open and the doubly-liganded closed states. Instead, we postulate the existence of a second closed-channel state coupled to the open state.
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