Shortening Speed Dependence of Concentric Force Potentiation in the Absence of Myosin Phosphorylation

Abstract

Contraction‐induced elevation of myoplasmic Ca2+ leads to sequential activation of skeletal muscle myosin light chain kinase (skMLCK), an enzyme responsible for phosphorylating the myosin regulatory light chain (RLC). In wildtype (WT) mouse skeletal muscle containing skMLCK, force potentiation correlates with RLC phosphate content; although, these muscles may contain an RLC phosphorylation‐independent component. In this study, extensor digitorum longus (EDL) muscles from knockout (KO) C57BL/6 mice, devoid of skMLCK, were activated in vitro (25°C) across a range of stimulation frequencies (10, 25, 45, 70 and 100 Hz) and subjected to shortening ramps at 0.1, 0.3 or 0.5 of maximal shortening velocity (Vmax). This was performed before and after a standard conditioning stimulus (CS) that potentiated mean concentric force at 0.3 and 0.5 Vmax to 1.20 ± 0.03 and 1.30 ± 0.02 of unpotentiated (pre‐CS) values, respectively; however, mean forces were depressed to 0.84 ± 0.03 of pre‐CS values at 0.1 Vmax (n = 3, P < 0.05). Increasing shortening speed augmented the frequency at which concentric force potentiation was maximal: 25 and 45 Hz at 0.3 and 0.5 Vmax, respectively. These results mirrored previous results from WT muscles (Gittings et al., 2012), except that concentric force potentiation was attenuated by ~50% across all frequencies at 0.3 and 0.5 Vmax, while force at 0.1 Vmax was reduced rather than unchanged. Thus, our data indicates the presence of an RLC phosphorylation‐independent mechanism for potentiation in skMLCK KO muscles that may complement the intact RLC phosphorylation mechanism in WT muscles.Supported by NSERC (2014‐05122).