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 functional features of excitation-contraction (E-C) coupling in mouse skeletal myotubes. In high-serum-treated (10%) myotubes, depolarization elicited Ca2+ release which continued for tens of milliseconds following the end of the pulse, after which [Ca2+]i decayed slowly. In low-serum-treated (0.5%) myotubes, the Ca2+ transient caused by depolarization had an increased rate of rise and peak amplitude, and [Ca2+]i began to decay rapidly upon repolarization. When a depolarizing pulse (0.5-1.0 s) was applied to low-serum-treated myotubes during a Ca2+ transient induced by 5-10 mM caffeine, repolarization usually caused the caffeine transient to terminate rapidly (RISC; repolarization-induced stop of caffeine-induced Ca2+ release). The RISC was less prominent in high-serum-treated myotubes. These results suggest that low serum promotes the maturation of myotubes so that Ca(2+)-release and Ca(2+)-removal activities are accelerated. Additionally, the essential features of the communication between the voltage sensor and the Ca(2+)-release channel are shared by myotubes and adult muscle fibers.