Changes in kinetics of acetylcholine receptor channels after initial expression in Xenopus myocyte culture.

1. Developmental changes in single acetylcholine (ACh) receptor channel properties were analysed in cultured Xenopus myocytes at low temperatures (11-16 degrees C) using the cell-attached patch clamp technique. Single-channel recordings were done at fourteen stages of development ranging from several hours (stage 21) to 5 days (stage 47) after ACh receptors first appear in the muscle membrane. 2. Two types of channels, low conductance and high conductance, which have been described previously, were observed at all stages. At low concentrations of ACh, channel events often occurred as bursts of openings separated by closures briefer than 1 ms. Such bursts were treated as one event. Many brief, isolated channel openings, which were described in the preceding paper, were also observed. Developmental changes in burst duration, brief openings and brief closures were assessed for the period studied in culture. 3. Throughout development, most of the burst duration histograms for the low-conductance channel were not well fitted by a single exponential, having an excess of brief openings. The brief component could be largely accounted for by singly liganded openings, as described in the preceding paper. The burst durations reported here represent the main component of the distribution. At 150 nM-ACh, high-conductance channel burst duration histograms were well fitted by single exponentials. 4. There was a developmental increase in the percentage of single-channel events belonging to the high-conductance class. The percentage of high-conductance events remained low (less than 10%) for the first day after ACh receptors appeared (stages 20-34), and increased to 60% at stage 47. 5. In addition to a shift in channel population, there was a decrease in both low- and high-conductance channel burst durations during early stages of development (mostly within the first day in culture). The effect was more dramatic for the low-conductance channel: overall, low-conductance channel burst duration at 50 mV hyperpolarization over the resting potential decreased from 24 ms at stage 24/25 to 6 ms at stage 47, a 4-fold decrease. During the same period, high-conductance burst duration decreased from 10.6 to 4 ms. a 2.5-fold decrease. In contrast to the burst duration, the mean open time of 60 microseconds for the brief isolated class of opening in the low-conductance channel did not change with development. 6. Neither the single-channel conductance nor the voltage dependence of burst duration changed with development.(ABSTRACT TRUNCATED AT 400 WORDS)