In vivo development of nicotinic acetylcholine receptor channels in Xenopus myotomal muscle.

The development of acetylcholine receptor (AChR) channel function in Xenopus myotomal muscle was studied by single-channel recordings from cell-attached patches of nonjunctional membrane in vivo. AChR channels were studied from the time of their first appearance on the muscle membrane until the time of full maturity of the muscle. Two predominant amplitude classes of AChR channels were observed with slope conductances of about 40 and 60 pS. During the first day after their initial appearance on the membrane, the small-conductance channels were the most numerous class on the muscle membrane. The large-conductance channels then began to be expressed in significant numbers and, over the next 2 d, became the predominant channel type. The large-conductance class had an apparent mean open time of approximately 0.7 msec at resting potential, which remained constant throughout development. The small channel initially had an apparent open time of approximately 3 msec at resting potential, which decreased during development by about 50%. The decrease in open time of the small channel was correlated in time with the increased expression of the large-conductance channels. Openings of the large-conductance channels were generally separated by closed intervals of more than 1 msec, whereas openings of the small-conductance channels were commonly interrupted by brief gaps of about 0.2 msec duration. The duration of the brief gaps did not change during development.