Low serum promotes maturation of excitation-contraction coupling in myotubes

Patch-clamping and the simultaneous fluorescence measurement of cytoplasmic Ca2+ ([Ca2+](i)) were used to analyze the effect of serum on the functiona l features of excitation-contraction (E-C) coupling in mouse skeletal myotu bes. In high-serum-treated (10%) myotubes, depolarization elicited Ca2+ rel ease which continued for tens of milliseconds following the end of the puls e, after which [Ca2+](i) decayed slowly. In low-serum-treated (0.5%) myotub es, the Ca2+ transient caused by depolarization had an increased rate of ri se and peak amplitude, and [Ca2+](i) began to decay rapidly upon repolariza tion. When a depolarizing pulse (0.5-1.0 s) was applied to low-serum-treate d myotubes during a Ca2+ transient induced by 5-10 mM caffeine, repolarizat ion usually caused the caffeine transient to terminate rapidly (RISC; repol arization-induced stop of caffeine-induced Ca2+ release). The RISC was less prominent in high-serum-treated myotubes. These results suggest that low s erum promotes the maturation of myotubes so that Ca2+-release and Ca2+-remo val activities are accelerated. Additionally, the essential features of the communication between the voltage sensor and the Ca2+-release channel are shared by myotubes and adult muscle fibers.