Early development of two types of nicotinic acetylcholine receptors.

Functional changes of acetylcholine receptor (AChR) channels in embryonic Xenopus myotomal muscle cells were examined during their development in culture. Single-channel currents evoked by 50 or 500 nM ACh were measured using the patch-clamp technique. In Xenopus myocytes the first emergence of AChRs takes place at about stage 20 (Nieuwkoop and Faber). Myotomes were dissociated at very early stages and plated in culture. Single-channel currents through AChRs were recorded at times ranging from a few hours (stage 21) to several days (stage 47) after the first emergence of AChRs. Two classes of AChR channel were recorded: One class had a low conductance with a long burst duration (low-conductance channel), and the other had a high conductance with a short burst duration (high-conductance channel). Both of these classes were active from the earliest time recorded (stages 21-24). One effect of development was a shift in the relative activity of the low- and high-conductance channels. Initially (stages 21-24), the low conductance channels predominated, accounting for over 95% of the observed events. After 3 d in culture, however, high- and low-conductance events occurred with roughly equal frequency. The other effect of development was a 4-fold decrease in the mean burst length of the low-conductance channel. The decrease in burst length took place rapidly, with about 60% of the change occurring within 24 hr in culture. The burst length of the high-conductance channel remained virtually constant during development, as did the unitary conductance of both channels and the voltage dependence of their mean burst lengths. The developmental change in the proportion of low- and high-conductance channels is likely due to the increased insertion of new high-conductance channels. However, the molecular mechanism of the shortening of burst length of the low-conductance channel is unknown.