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Developmental changes in acetylcholine (ACh) receptor channel function on aneural cultures of embryonic myotomal muscle cells were examined using the patch-clamp technique. At all stages of differentiation two different unitary-event amplitudes were observed, corresponding to high-gamma (single-channel conductance) (64 pS) 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 approximately 2 ms which, on the average, was 2-3-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 microM) 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. We conclude that the development of non-junctional muscle membrane is associated with increased expression of high-gamma channels and that this process is primarily responsible for the previously reported developmental alterations in macroscopic ACh receptor channel currents.

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