Acetylcholine receptor clustering in rat myotubes: requirement for CA2+ and effects of drugs which depolymerize microtubules.

The acetylcholine receptor (AChR) clusters which form in the plasma membranes of cultured rat myotubes disappear when the myotubes are exposed to medium depleted of Ca2+. This loss of receptor clusters is reversible and depends both on extracellular Ca2+ concentrations and on temperature. Other divalent cations (Mg2+, Ba2+, Sr2+, Co2+, Mn2+) do not maintain receptor clusters when Ca2+ is absent. Extracellular Na+, which promotes Ca2+ efflux from myotube cultures, is necessary for cluster loss to occur in Ca2+-depleted medium. Experiments using colchicine and other drugs which depolymerize microtubules suggest that the cytoskeleton is involved in AChR clustering. At submicromolar concentrations, these drugs afford partial protection against cluster loss caused by Ca2+ depletion. Colchicine also alters the organization of vinculin in the vicinity of AChR clusters. It is concluded that receptor clustering probably depends on intracellular Ca2+ and on a structure or mechanism indirectly affected by colchicine.