Myosin phosphorylation potentiated steady state work output without altering contractile economy of mouse fast skeletal muscles

Skeletal myosin light chain kinase (skMLCK) catalyzed phosphorylation of the myosin regulatory light chain (RLC) increases (i.e. potentiates) mechanical work output of fast skeletal muscle. The influence of this event on contractile economy (i.e. energy cost/work performed) remains controversial, however. Our purpose was to quantify contractile economy of potentiated extensor digitorum longus (EDL) muscles from mouse skeletal muscles with (wildtype, WT) and without (skMLCK ablated, skMLCK−/−) the ability to phosphorylate the RLC. Contractile economy was calculated as the ratio of total work performed to high-energy phosphate consumption (HEPC) during a period of repeated isovelocity contractions that followed a potentiating stimulus (PS). Consistent with genotype, the PS increased RLC phosphorylation measured during before and after isovelocity contractions in WT but not skMLCK−/− muscles (i.e. 0.65 and 0.05 mol phos mol RLC, respectively). In addition, although the PS enhanced work during repeated isovelocity contractions in both genotypes the increase was significantly greater in WT than in skMLCK−/− muscles (1.51±0.03 vs. 1.10±0.05, respectively) (all data P<0.05, n=8). Interestingly, the HEPC determined during repeated isovelocity contractions was statistically similar between genotypes at 19.03±3.37 and 16.02±3.41 µmol∼P; respectively (P<0.27). As a result, despite performing significantly more work, the contractile economy calculated for WT muscles was similar to that calculated for skMLCK−/− muscles (i.e. 5.74±0.67 and 4.61±0.71 J•kg−1μmol∼P−1; respectively (P<0.27). In conclusion, our results support the notion that myosin RLC phosphorylation enhances dynamic contractile function of mouse fast skeletal muscle but does so without decreasing contractile economy.