MYOBLAST FUSION IS NOT A PREREQUISITE FOR THE APPEARANCE OF CALCIUM CURRENT, CALCIUM-RELEASE, AND CONTRACTION IN RAT SKELETAL-MUSCLE CELLS DEVELOPING IN CULTURE

During in vitro development of rat skeletal muscle cells, contraction and calcium currents progressively appear after fusion of myoblasts. To investigate whether muscle-specific functions are expressed in the absence of myoblast fusion, rat neonatal muscle cells were cultured in a differentiation medium under conditions that are well known to inhibit fusion: prolonged culture in a low-calcium medium or treatment with cytochalasin B. We have demonstrated that the fusion-arrested cells expressed differentiative properties in L-type calcium current, transient release of calcium ions from internal stores in response to caffeine and depolarizing agents, and contraction elicited by depolarization. Properties and potential-dependence of L-type calcium currents were similar to that in control fused cells, but T-type calcium currents were not observed, while both types coexist in myotubes. Properties of calcium transients and voltage dependence of contraction suggested that the excitation-contraction mechanisms were well established. However, comparing to well-developed myotubes at the same time of culture, the characteristics of calcium transients and contraction of fusion-arrested cells were closer to those of younger myotubes, which can be interpreted in terms of a delay in maturation of excitation-contraction coupling and contractile machinery. All these observations demonstrate that myoblast fusion is not necessary for triggering the establishment of calcium transport and release and contractile functions of rat muscle cells developing in culture. The appearance of muscle-specific functions is consistent with previous results demonstrating that the fusion-arrested cells express muscle-specific proteins and structures. (C) 1995 Academic Press, Inc.