Myosin phosphorylation improves contractile economy of mouse fast skeletal muscle during staircase potentiation

Phosphorylation of the myosin regulatory light chain (RLC) by skeletal myosin light chain kinase (skMLCK) potentiates rodent fast twitch muscle but is an ATP requiring process. Our objective was to investigate the effect of skMLCK-catalyzed RLC phosphorylation on the energetic cost of contraction and the contractile economy (ratio of mechanical output to metabolic input) of mouse fast muscle in vitro (25° C). To this end, extensor digitorum longus (EDL) muscles from Wildtype and from skMLCK devoid (skMLCK−/−) mice were subjected to repetitive low-frequency stimulation (10 Hz for 15 s) to produce staircase potentiation of isometric twitch force after which muscles were quick frozen for determination of high-energy phosphate consumption (HEPC). During stimulation, Wildtype muscles displayed significant potentiation of isometric twitch force while skMLCK−/− muscles did not (i.e. 23 vs 5% change, respectively). Consistent with this, RLC phosphorylation was increased∼3.5-fold from the unstimulated control value in Wildtype, but not in skMLCK−/− muscles. Despite these differences, the HEPC of Wildtype muscles was not greater than for skMLCK−/− muscles. As a result of the increased contractile output relative to HEPC, the calculated contractile economy of Wildtype muscles was greater than for skMLCK−/− muscles. Thus, our results suggest that skMLCK-catalyzed phosphorylation of the myosin RLC increases the contractile economy of Wildtype mouse EDL muscle compared to skMLCK−/− muscles without RLC phosphorylation.