Mobility of extrajunctional acetylcholine receptors on denervated adult muscle fibers.

We report the lateral mobility of extrajunctional acetylcholine receptors (AChR), marked with fluorescently labeled alpha-bungarotoxin, on rat flexor digitorum brevis single muscle fibers maintained in cell culture. Mobility is measured by a modification of the fluorescence photobleaching recovery technique. The denervated rat flexor digitorum brevis muscle fibers exhibit a nonuniform distribution of AChR on their sarcolemmas. Diffusely distributed AChR have an average diffusion coefficient of about 2.5 X 10(-11) cm2/sec, somewhat lower than that of diffusely distributed AChR on embryonic rat myotubes in culture. Extrajunctional AChR aggregated into small clusters (less than 20 microns) have an average diffusion coefficient of about 5 X 10(-12) cm2/sec. Both the clustered and nonclustered extrajunctional AChR have lateral mobilities several orders of magnitude greater than that of junctional AChR.