The effects of energy metabolism inhibitors on the distribution of acetylcholine receptors (AChRs) in the surface membranes of non-innervated, cultured rat myotubes were studied by visualizing the AChRs with monotetramethylrhodamine-a-bungarotoxin . Incubation of myotubes with inhibitors of energy metabolism causes a large decrease in the fraction of myotubes displaying clusters of AChR . This decrease is reversible, and is dependent on temperature, the concentration of inhibitor, and the duration of treatment . Cluster dispersal is probably not the result of secondary effects on Ca" or cyclic nucleotide metabolism, membrane potential, cytoskeletal elements, or protein synthesis . Sequential observations of identified cells treated with sodium azide showed that clusters appear to disperse by movement of receptors within the sarcolemma without accompanying changes in cell shape .AChR clusters dispersed by pretreating cells with sodium azide rapidly reform upon removal of the inhibitor. Reclustering involves the formation of small aggregates of AChR, which act as foci for further aggregation and which appear to be precursors of large AChR clusters . Small AChR aggregates also appear to be precursors of clusters which form on myotubes never exposed to azide. Reclustering after azide treatment does not necessarily occur at the same sites occupied by clusters before dispersal, nor does it employ only receptors which had previously been in clusters . Cluster reformation can be blocked by cycloheximide, colchicine, and drugs which alter the intracellular cation compositon .
KEY WORDS acetylcholine receptor clusters "iological relevance of capping, but have encounpatching and capping . muscle membrane trred the difficulty that caps or clusters rarely form energy metabolism inhibitors -fluorescent spontaneously or under physiological conditions. a-bungarotoxin -cultured rat muscleOne of the few surface components known to do Ever since capping was discovered in lymphocytes, so is the acetylcholine receptor (AChR)' of culclustering of plasmalemmal components has been extensively studied (for a review, see reference 36) . 'Abbreviations used in this paper: AChR, acetylcholine Some researchers have tried to determine the physreceptor ; aBT, a-bungarotoxin ; CCCP, carbonyl cyanide 626 J. CELL BIOLOGY