Ionic Control of Postsynaptic Differentiation in Muscle

The formation of acetylcholine receptor (AChR) clusters in cultured Xenopus muscle cells can be effected by polycation-coated latex beads. In a manner resembling innervation, the beads induce AChR clustering discretely at the bead-muscle contacts. In this study, we examined the role of intracellular Ca²⁺ and pH on the development of this postsynaptic-type specialization. A transient increase in intracellular Ca²⁺ was immediately elicited when the muscle cell was stimulated by the beads. This Ca²⁺ transient is a necessary condition for the activation of the clustering process. In addition, the polycations on the beads caused a cytoplasmic alkalinization which also appeared to be necessary for the AChR clustering to take place. These studies indicate that the signaling of postsynaptic differentiation at the neuromuscular junction may be mediated by an ionic mechanism.