Acetylcholine receptor channel properties during development of Xenopus muscle cells in culture.

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Abstract
Developmental changes in acetylcholine (ACh) receptor channel function on aneural cultures of embryonic myotonal muscle cells were examined using the patch-clamp technique. At all stages of differentiation, 2 different unitary-event amplitudes were observed, corresponding to high-.gamma. (single-channel conductance) (64 pS [Siemen]) and low-.gamma. (46 pS) channel types. No change in conductance occurred for either channel type during the 6-day in vitro period examined. At resting membrane potential (-85 mV), the low-.gamma. channel exhibited a mean open time of .apprx. 2 ms, which, on the average, was 2- to 3-fold longer than that measured for the high-.gamma. channel. Neither the estimated mean channel open time nor the voltage dependence of the open state measured for either channel type changed during development. In recordings with low ACh concentration (0.1-0.25 .mu.M), both high-.gamma. and low-.gamma. channel types exhibited non-stationary opening probabilities over the recording period. Usually the opening rate of both channel types decreased with time following seal formation; however, the drop-out rate was faster for the low-.gamma. channel. A developmental increase in the proportion of high-.gamma. events occurred between day 1 (16%) and day 5 (56%) in culture, paralleling the time-dependent changes in the channel kinetics based on ACh-activated membrane noise. The development of nonjunctional muscle membrane evidently is associated with increased expression of high-.gamma. channels. This process was primarily responsible for the previously reported developmental alterations in macroscopic ACh receptor channel currents.