MYOSIN PHOSPHORYLATION AUGMENTS FORCE-DISPLACEMENT AND FORCE-VELOCITYRELATIONSHIPS OF MOUSE FAST MUSCLE

Two studies were conducted to examine the effect of myosin regulatory light chain (R-LC) phosphorylation on the rate and extent of shortenin g in submaximally activated mouse extensor digitorum longus muscles in vitro at 25 degrees C. For each study, R-LC phosphate content was inc reased fivefold by application of a 5-Hz, 20-s conditioning stimulus ( CS) to 0.65-0.68 mol phosphate/mol R-LC; this level was sustained betw een 10 and 40 s after the CS. Maximum isometric twitch force and the m aximum rate of force development (+ dF/dt(max)) were potentiated in th e range 13-17% and 9-17% (P < 0.05), respectively, after the CS. In st udy 1, the maximal rate and extent of shortening were significantly en hanced by 10 and 21% (P < 0.001), respectively, when measured using a twitch zero-load clamp technique. In study 2, the force-velocity and f orce-displacement relationships were both augmented when determined wi th the twitch afterload technique. Displacement was enhanced between 2 0 and 82% for loads that ranged from 3 to 75% of active peak twitch fo rce, whereas velocity was increased 6-8% over the same range (P < 0.05 ), including the predicted maximum velocity (V-max; 5.08 vs. 4.69 musc le length/s). In both studies the increase in velocity likely represen ts a shift along the force-velocity relationship toward true V-max tha t reflects a decrease in relative load due to force potentiation. Furt hermore, with the decrease in relative load, displacement at a given l oad was also increased. Potentiated displacement and extent of R-LC ph osphorylation also decreased in parallel when studied for 5 min after the CS. The increase in muscle shortening is a novel finding and sugge sts a function for R-LC phosphorylation with respect to movement becau se both peak work and power were also enhanced by up to 22%. These eff ects are consistent with an R-LC phosphorylation-induced increase in f (app), the apparent rate constant that describes the cross-bridge tran sition from the non-force-generating to the force-generating state.