AJP - Cell Physiology, Vol 269, Issue 3 C713-C724, Copyright © 1995 by American Physiological Society

ARTICLES

Myosin phosphorylation augments force-displacement and force-velocity relationships of mouse fast muscle

R. W. Grange, C. R. Cory, R. Vandenboom and M. E. Houston
Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, Ontario, Canada.

Two studies were conducted to examine the effect of myosin regulatory light chain (R-LC) phosphorylation on the rate and extent of shortening in submaximally activated mouse extensor digitorum longus muscles in vitro at 25 degrees C. For each study, R-LC phosphate content was increased 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 between 10 and 40 s after the CS. Maximum isometric twitch force and the maximum rate of force development (+dF/dtmax) were potentiated in the range 13-17% and 9-17% (P < 0.05), respectively, after the CS. In study 1, the maximal rate and extent of shortening were significantly enhanced by 10 and 21% (P < 0.001), respectively, when measured using a twitch zero-load clamp technique. In study 2, the force-velocity and force-displacement relationships were both augmented when determined with the twitch afterload technique. Displacement was enhanced between 20 and 82% for loads that ranged from 3 to 75% of active peak twitch force, whereas velocity was increased 6-8% over the same range (P < 0.05), including the predicted maximum velocity (Vmax; 5.08 vs. 4.69 muscle length/s). In both studies the increase in velocity likely represents a shift along the force-velocity relationship toward true Vmax that reflects a decrease in relative load due to force potentiation. Furthermore, with the decrease in relative load, displacement at a given load was also increased. Potentiated displacement and extent of R-LC phosphorylation also decreased in parallel when studied for 5 min after the CS. The increase in muscle shortening is a novel finding and suggests a function for R-LC phosphorylation with respect to movement because both peak work and power were also enhanced by up to 22%. These effects are consistent with an R-LC phosphorylation-induced increase in fapp, the apparent rate constant that describes the cross-bridge transition from the non-force-generating to the force-generating state.

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