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Jerry Lewis Neuromuscular Research Center, Department of Physiology, UCLA School of Medicine 90024.

1. Single acetylcholine (ACh) receptor channel currents were analysed in Xenopus myocyte culture. Channel events activated by ACh were recorded using the cell-attached patch clamp technique at low temperatures (11.2-15.8 degrees C) and at 100 mV hyperpolarization over the average resting membrane potential of -75 mV. There were two types of channels, low- and high-conductance channels, as reported previously, with unitary current amplitudes 4.7 +/- 0.4 pA (mean +/- S.D., n = 17) and 6.6 +/- 1.2 pA (n = 14), respectively. 2. At low concentrations of ACh many brief openings (less than 1 ms in duration) were observed. Almost all of them occurred in isolation. The mean open time, which was not dependent on ACh concentration, was 63.1 +/- 18.1 microseconds (n = 15) for the low-conductance channel and 80.3 +/- 18.7 microseconds (n = 2) for the high-conductance channel. 3. Brief openings were frequently observed at 20 nM-ACh, at which concentration the frequency of longer openings (longer than 1 ms) was rare. The frequency of brief openings reached maximum at 50 nM-ACh and declined at high concentrations. The occurrence ratio between the long and brief openings increased linearly with ACh concentration. This finding suggests strongly that the brief openings are due to singly liganded receptor channels, since the long openings are most likely due to doubly liganded receptor channels. 4. A burst of openings was defined as a group of openings separated by brief closures (gaps) lasting less than 1 ms. The main component in the closed time histogram had a time constant of 25.0 +/- 5.1 microseconds (n = 16) for the low-conductance channel and 32.0 +/- 10.2 microseconds (n = 10) for the high-conductance channel. These values were not dependent on ACh concentration. The number of gaps per burst was 2.22 +/- 0.69 (n = 16) for the low-conductance channel and 0.85 +/- 0.23 (n = 9) for the high-conductance channel. The former is significantly greater than the latter. 5. During the recording from a single patch, the frequency of low-conductance channel events declined with time at all ACh concentrations, and the rate of decline was slower in the high-conductance channel in the same patch. The rate of decline was dependent linearly on the ACh concentration. It is most likely that this decline is due to desensitization of the receptors.(ABSTRACT TRUNCATED AT 400 WORDS)