LATERAL MOTION OF FLUORESCENTLY LABELED ACETYLCHOLINE RECEPTORS IN MEMBRANES OF DEVELOPING MUSCLE-FIBERS

Direct, quantitative measurements of the lateral motion and age-dependent distribution of acetylcholine receptors (AChR) on the surface of rat myotubes in primary culture was made. AChR were fluorescently marked with tetramethylrhodamine-labeled .alpha.-bungarotoxin and AChR lateral motion was measured by the fluoresence photobleaching recovery technique. Two coexisting distinct classes of AChR were found: mobile, uniformly distributed AChR that appeared on all myotubes shortly after fusion from myoblasts and immobile, dense, highly granular AChR in patches of 10-60 .mu.m size that appeared shortly after fusion and disappear after myotubes became extensively interconnected. In addition, evidence of turnover of AChR labeled with tetramethylrhodamine-.alpha.-bungarotoxin was seen in the gradual internalization of surface fluorescence within 36 h after labeling. The membrane dynamics and localization of muscle AChR is discussed.