Rapid degradation of "new" acetylcholine receptors at neuromuscular junctions.

Acetylcholine receptors at innervated neuromuscular junctions are very stable, with half-lives reported to be 6 to 13 days. Their turnover is described as a first-order process, implying a single population of receptors. In this study, two subpopulations of acetylcholine receptors at normally innervated junctions have been identified. One has a rapid turnover rate with a half-life of 18.7 hours, similar to that of extrajunctional receptors, and the other has a slow turnover rate with a half-life of 12.4 days. The rapidly turned over subpopulation represents approximately 20 percent of the total junctional receptors. This finding may account for the discrepancies in previous reports of turnover rates and may explain the rapid reversibility in vivo of agents that "irreversibly" block acetylcholine receptors. This finding also implies that the synthesis rate of junctional acetylcholine receptors may be higher than previous estimates. The rapidly turned-over subpopulation may represent receptors that were newly inserted into the neuromuscular junction and that were not yet stabilized by an influence of the motor nerve.