Myosin phosphorylation augments the force-displacement and force-
velocity relationships of mouse fast muscle.
Grange, R. W., C. R. Cory, R. Vandenboom, and M. E. Houston.
Department of Kinesiology, Faculty of Applied Health Sciences,
University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
APStracts 2:0139C, 1995.
Two studies were conducted to examine the effect of myosin regulatory
light chain (R-LC) phosphorylation on the rate and extent of
shortening in sub-maximally activated mouse extensor digitorum longus
(EDL) muscles in vitro at 25 degrees C. For each study, R-LC
phosphate content was increased 5 fold by application of a 5-Hz 20-s
conditioning stimulus (CS) to 0.65-0.68 mol phosphate per mol R-LC;
this level was sustained between 10 and 40 s following 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 following the CS. In study one, 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 (ZLC) technique. In study two, 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 3-75% of active peak
twitch force, while velocity was increased 6-8% over the same range
(P < 0.05) including the predicted Vmax (5.08 vs. 4.69 Lm/s). In
both studies the increase in velocity likely represents a shift along
the force-velocity relationship towards 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 minutes
following the CS. The increase in muscle shortening is a novel
finding and suggests a function for R-LC phosphorylation with respect
to movement since 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 rate constant that describes the cross
bridge transition from the non-force generating to the force
generating state.
Received 4 October 1994; accepted in final form 28 February 1995.
APS Manuscript Number C592-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 21 March 1995.