Two types of acetylcholine receptor channels in developing Xenopus muscle cells in culture: further kinetic analyses.

1. Further developmental changes in acetylcholine (ACh) receptor channel function were examined in Xenopus muscle cultures using the cell-attached patch-clamp technique. 2. Two types of acetylcholine (ACh) receptor channel events, low- and high-conductance channel events, were distinguished as reported earlier. Apparent open-time histograms for high-conductance channel events were well fitted by a single exponential but those for low-conductance channel events were sometimes fitted better by two exponentials. 3. In low-conductance channel events when the open-time histogram was well fitted by two exponentials, successive open times were correlated: an event with a long open time tended to be followed after a brief interval (less than 1 ms) by another long-duration event. A short-duration event was less frequently followed by an event within a short interval (1 ms) with a long-duration event. 4. Closed-time histograms for the interval between successive low-conductance channel events and between successive high-conductance channel events were both fitted by two exponentials. The fast time constant was 0.36 ms for the high-conductance channel event and 0.31 ms for the low-conductance channel event. There was an indication that a third and faster component was hidden in the first bin (0-200 microseconds) in the closed-time histogram of both types of channel events. 5. Defining a burst as successive openings separated by closures briefer than 1 ms, the number of gaps per burst was different for the two types of channel events. They were 0.16 for high- and 0.37 for low-conductance channel events. In both types of channels, neither the fast component in the closed-time histogram nor the number of gaps per burst changed with time in culture. 6. The apparent open time of both types of channels increased progressively as ACh concentration was increased, suggesting an increasing number of unresolved closures at higher concentrations. At 100 microM-ACh the apparent open time became shorter probably due to channel blockade by ACh molecules. Closed-time histograms were fitted by two exponentials. The time constant of the fast component remained similar to that at low concentration (0.2 microM) up to 20 microM, but the relative number of closures belonging to this component increased with ACh concentration. In contrast, the slow component shortened its time constant as ACh concentration increased and the relative frequency decreased. Again, there was an indication that another faster component existed in the closed-time histogram.(ABSTRACT TRUNCATED AT 400 WORDS)