Dissociation of force production from MHC and actin contents in muscles injured by eccentric contractions.

The primary purpose of this study was to determine the relationship between myosin heavy chain (MHC) and actin contents and maximum isometric tetanic force (Po) in mouse extensor digitorum longus (EDL) muscles following eccentric contraction-induced injury. Po and protein contents were measured in injured (n = 80) and contralateral control (n = 80) EDL muscles at the following time points after in vivo injury: sham, 0, 0.25, 1, 3, 5, 14, and 28 days. Po was reduced by 37 +/- 2.3% to 49 +/- 3.8% (p < or = 0.05), while MHC and actin contents were unaltered from 0 to 3 days after injury. Whereas Po partially recovered between 3 and 5 days (from -49 +/- 3.8% to -35 +/- 3.6%), MHC and actin contents in the injured muscles declined by 19 +/- 4.9% and 20 +/- 5.3%, respectively, by 5 days compared with control muscles. Decrements in Po were similar to the reductions in MHC and actin contents at 14 (approximately 24%) and 28 (approximately 11%) days. Evaluation of myofibrillar and soluble protein fractions indicated significant reductions in the content of major proteins at 5 and 14 days. Immunoblots of heat shock protein 72 revealed elevations starting at 0.25 days, peaking during 1-3 days, and declining after 5 days. These findings indicate that decreased contractile protein content is not related to the initial decrease in Po. However, decreased MHC and actin contents could account for 58% of the Po reduction at 5 days, and for nearly all the decrements in Po from 14 to 28 days.