The acetylcholine channel open time in chick muscle is not decreased following innervation.

1. The mean channel open time (tau) of ACh receptors was measured in chick muscles at various stages of development. Tau was estimated by analysing ACh induced current fluctuations recorded extracellularly from small (ca. 20 micrometers2) membrane patches. 2. At random sites on uninnervated, embryonic chick muscle fibres in vitro, tau was relatively long--4 msec at 23 degrees C. 3. Estimates of tau at synaptic sites on embryonic myotubes innervated in vitro were identical to estimates at extrasynaptic sites on the same fibres. Both were comparable to estimates on uninnervated myotubes. 4. Synaptic currents at cultured junctions decayed slowly as simple exponentials. The decay time constants were never shorter than the mean channel open time measured by fluctuation analysis. 5. In anterior latissimus dorsi and intercostal muscle fibres of 4- to 18-week posthatched chicks, fluctuation analysis and synaptic current decays indicate that the channel open time of mature chick endplate receptors is as long as that of embryonic synaptic receptors in vitro. Apparently, tau remains prolonged throughout the maturation of chick neuromuscular junctions.