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1 Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, Miami, Florida 33136; and 2 Department of Physiology, University of Texas Southwestern Medical Center, Dallas, Texas 75390
The role of phosphorylation of the
myosin regulatory light chains (RLC) is well established in smooth
muscle contraction, but in striated (skeletal and cardiac) muscle its
role is still controversial. We have studied the effects of RLC
phosphorylation in reconstituted myosin and in skinned skeletal muscle
fibers where Ca2+ sensitivity and the kinetics of
steady-state force development were measured. Skeletal muscle myosin
reconstituted with phosphorylated RLC produced a much higher
Ca2+ sensitivity of thin filament-regulated ATPase activity
than nonphosphorylated RLC (change in 
log of the Ca2+
concentration producing half-maximal activation = ~0.25). The same was true for the Ca2+ sensitivity of force in skinned
skeletal muscle fibers, which increased on reconstitution of the fibers
with the phosphorylated RLC. In addition, we have shown that the level
of endogenous RLC phosphorylation is a crucial determinant of the
Ca2+ sensitivity of force development. Studies of the
effects of RLC phosphorylation on the kinetics of force activation with
the caged Ca2+, DM-nitrophen, showed a slight
increase in the rates of force development with low statistical
significance. However, an increase from 69 to 84% of the initial
steady-state force was observed when nonphosphorylated
RLC-reconstituted fibers were subsequently phosphorylated with
exogenous myosin light chain kinase. In conclusion, our results suggest
that, although Ca2+ binding to the troponin-tropomyosin
complex is the primary regulator of skeletal muscle contraction, RLC
play an important modulatory role in this process.
steady-state force; calcium regulation; regulatory light chain depletion; myosin light chain kinase
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