Heterogeneous kinetic properties of acetylcholine receptor channels in Xenopus myocytes

We have used the cell-attached patch-clamp technique to examine the kinetic and conductance properties of acetylcholine receptor channel currents in cultured Xenopus myocytes. At high agonist concentrations (5-100 microM) the currents occurred in bursts of openings. The probability that a channel existed in an ion-conducting conformation during a burst (Po) was adopted as an empirical measure of channel kinetic behaviour. All openings within a given burst were to the same mean current amplitude. However, different bursts could have openings with a mean conductance of either 46 pS (gamma 40) or 64 pS (gamma 60). For gamma 40 bursts there were three predominant populations which could be distinguished by their mean Po values (approximately 0.9, approximately 0.3, and less than 0.01 at 20 microM-acetylcholine). Po values increased as the acetylcholine concentration within the micropipette was increased. About 80% of bursts were from the highest Po population. gamma 60 bursts also occurred in three predominant modes. The highest Po population accounted for greater than 80% of all bursts and had a mean Po of approximately 0.6 at 20 microM-acetylcholine. For both gamma 40 and gamma 60 channels, bursts from the highest Po population had open-interval durations which were approximately 4 times longer than those from bursts from the medium Po population. Closed intervals from gamma 40, high Po bursts were approximately 4 times shorter than those from medium Po bursts. Occasional examples of switching between different kinetic modes were observed, suggesting that the Po populations may represent different activity patterns of a homogeneous channel population.