Organization of acetylcholine receptor clusters in cultured rat myotubes is calcium dependent

The effect of extracellular Ca²⁺ concentration and myasthenic globulin on the distribution and appearance of acetylcholine receptor (AChR) clusters on rat myotubes was studied with tetramethyl-rhodamine-labeled αBTX. Low Ca²⁺ medium (2.5 x 10^-5 M) caused a time-dependent loss of AChR clusters, and a concomitant increase in small punctate areas of fluorescence. High Ca²⁺ concentrations (1.5 x 10^-2 M) increased the size of AChR clusters without altering AChR synthesis. These changes were not observed with other divalent ions. In the presence of myasthenic globulin, the rate of AChR turnover increases, and AChR clusters are rapidly dispersed. High Ca²⁺ concentration partially protects the AChR clusters from dispersal and decreases the rate of receptor turnover.