Increase in intracellular calcium induced by the polycation-coated latex bead, a stimulus that causes postsynaptic-type differentiation in cultured Xenopus muscle cells.

The polycation-coated latex bead is a potent stimulus for the induction of postsynaptic-type differentiation in cultured Xenopus myotomal muscle cells. Specializations characteristic of the neuromuscular junction, such as clusters of acetylcholine receptors and other postsynaptic-specific proteins, develop at the bead-muscle contact. Previous studies have shown that a deprivation of extracellular calcium inhibits the effect of the beads in causing the development of these specializations. This suggests that an increase in intracellular Ca2+ is a necessary condition for the development of this specialization. In this study, we tested whether an increase in intracellular calcium is observable upon the bead-muscle contact. The measurement was carried out on cells loaded with the fluorescent calcium indicator fura-2 AM by digitized video microscopy. When polycation-coated beads were added to cells, an increase in intracellular calcium concentration in the range of 5 to 57% of the resting level was observed within 10 sec after bead-muscle contact. Afterward, the calcium level gradually returned to the resting level with a time course of about 3 min. Uncoated beads, which do not induce the formation of acetylcholine receptor clustering, failed to elicit this calcium transient. Removal of extracellular calcium as well as blocking calcium channels with 50 microM verapamil also suppressed this transient induced by the polycation-coated beads. Both treatments are known to suppress the formation of receptor clusters by these beads. These results suggest that the polycation-coated beads cause an influx of calcium by increasing the membrane conductance to this ion. This process may underlie the signaling of the postsynaptic differentiation.