Myosin light chain phosphorylation is required for peak power output of mouse fast skeletal muscle in vitro

The skeletal myosin light chain kinase (skMLCK) catalyzed phosphorylation of the myosin regulatory light chain (RLC) is associated with potentiation of force, work, and power in rodent fast twitch muscle. The purpose of this study was to compare concentric responses of EDL from wild-type (WT) and skMLCK devoid (skMLCK^-/-) muscles at a range of shortening speeds (0.05 to 0.70 V _max) around that expected to produce maximal power (in vitro, 25 °C) both before (unpotentiated) and after (potentiated) a potentiating stimulus (PS). When collapsed across all speeds tested, neither unpotentiated force, work, or power differed between genotypes (all data n = 10, P < 0.05). In contrast, although both genotypes displayed speed-dependent increases, these increases were greater for WT than skMLCK^-/- muscles. For example, when collapsed across the six fastest speeds we tested, both concentric force and power were increased 30-34 % in WT but only 15-17 % in skMLCK^-/- muscles. In contrast, at the two slowest speeds, these parameters were increased in WT but decreased in skMLCK^-/- muscles (8-10 and 7-9 %, respectively). Intriguingly, potentiation of concentric force and power was optimal near speeds producing maximal power in both genotypes. Because the PS elevated RLC phosphorylation above resting levels in WT but not in skMLCK^-/- muscles, our data suggest that skMLCK-catalyzed phosphorylation of the RLC is required for maximal concentric power output of mouse EDL muscle stimulated at high frequency in vitro.