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. T o investigate whether muscle-specific functions are expressed in the a bsence of myoblast fusion, rat neonatal muscle cells were cultured in a differentiation medium under conditions that are well known to inhib it fusion: prolonged culture in a low-calcium medium or treatment with cytochalasin B. We have demonstrated that the fusion-arrested cells e xpressed differentiative properties in L-type calcium current, transie nt release of calcium ions from internal stores in response to caffein e and depolarizing agents, and contraction elicited by depolarization. Properties and potential-dependence of L-type calcium currents were s imilar to that in control fused cells, but T-type calcium currents wer e not observed, while both types coexist in myotubes. Properties of ca lcium 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-arrest ed cells were closer to those of younger myotubes, which can be interp reted in terms of a delay in maturation of excitation-contraction coup ling and contractile machinery. All these observations demonstrate tha t 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 functi ons is consistent with previous results demonstrating that the fusion- arrested cells express muscle-specific proteins and structures. (C) 19 95 Academic Press, Inc.