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Articles in PresS, published online ahead of print December 21, 2001
J Appl Physiol, 10.1152/jap.00858.2001
Submitted on August 15, 2001
Accepted on December 17, 2001
1 Molecular and Cellular Pharmacology, University of Miami School of Medicine, Miami, Florida, USA
2 Physiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA
* To whom correspondence should be addressed. E-mail: JPotter{at}med.miami.edu.
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 the 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 non-phosphorylated RLC (
pCa50 ~ 0.25). The same was true for the Ca2+-sensitivity of force in skinned skeletal muscle fibers, which increased upon 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 non-phosphorylated RLC-reconstituted fibers were subsequently phosphorylated with exogenous MLCK. In conclusion, our results suggest that although Ca2+ binding to the troponin-tropomyosin complex is the primaryregulator of skeletal muscle contraction, the RLC play an important modulatory role in this process.
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