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1 Physiology, Michigan State University, East Lansing, MI, USA
2 Physiology, Michigan State University, East Lansing, MI, USA; Radiology, Michigan State University, East Lansing, MI, USA
* To whom correspondence should be addressed. E-mail: rwiseman{at}msu.edu.
Mitogen-activated protein kinases (MAPK), in particular p38 MAPK, are phosphorylated in response to contractile activity yet the mechanism for this is not understood. We tested the hypothesis the force of contraction is responsible for p38 MAPK phosphorylation in skeletal muscle. Extensor digitorum longus (EDL) muscles, isolated from adult male Swiss Webster mice, were stimulated at fixed length at 10 Hz for 15 min and then analyzed by Western blot for the phosphorylation of p38 MAPK and ERK(1/2). Contralateral muscles were fixed at resting length and not stimulated. Stimulated muscles showed a 2.5-fold increase in phosphorylated p38 MAPK relative to non-stimulated contralateral controls; there was no change in the phosphorylation of ERK(1/2). When contractile activity was inhibited with N-benzyl-p-toluene sulphonamide (BTS), a specific inhibitor of actomyosin ATPase, force production decreased in both a time and concentration dependent manner. Preincubation with 25, 75 and 150 µM BTS caused 78+/-4, 97+/-0.2 and 99+/-0.2 % inhibition in contractile force respectively and was stable after 30 minutes of treatment. Fluorescence measurements demonstrated that Ca2+ cycling was unaffected by BTS treatment. Surprisingly, BTS did not suppress the level of p38 MAPK phosphorylation in stimulated muscles. These data do not support the view that force generation per se activates p38 MAPK and suggest that other events associated with contraction must be responsible.
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