ECCENTRIC CONTRACTION-INDUCED INJURY IN NORMAL AND HINDLIMB-SUSPENDED MOUSE SOLEUS AND EDL MUSCLES

The primary objective of this study was to compare the magnitude of injury in mouse extensor digitorum longus (EDL) and soleus muscles induced by high-force eccentric contractions. A second objective was to study the effect of altering the daily loading of the muscles through hindlimb suspension (HS) on the injury. One of two protocols was performed in vitro: 1) 15 eccentric contractions (n = 20: 10 EDL and 10 soleus muscles) or 2) 15 isometric contractions (n = 20: 10 EDL and 10 soleus muscles). After the protocol, the decrements in contractile performance and lactate dehydrogenase (LDH) release were measured at 15-min intervals over 1 h. Immediately after the eccentric contraction protocol, markedly greater decrements in maximal isometric tetanic force (P-o) occurred in the normal EDL than in the normal soleus muscles (60.7 +/- 4.2 vs. 7.6 +/- 2.1%, P less than or equal to 0.0001). LDH release immediately after the eccentric contraction protocol was 2.7-fold greater in the normal EDL than in the normal soleus muscles. To investigate the role of recent loading of the muscles in the injury, EDL (n = 9) and soleus (n = 10) muscles from mice subjected to HS for 14 days performed the eccentric contraction protocol. HS resulted in greater decrements in contractile performance for the soleus muscles (decreases in P-o immediately after the protocol for HS and normal soleus muscles were 31.0 +/- 1.8 and 7.6 +/- 2.1%, respectively; P less than or equal to 0.0001) but not for the EDL muscles. HS resulted in a 76% greater LDH release in the soleus muscle but had no effect on LDH release by the EDL muscle. HS did not alter the histochemically determined fiber type composition of the soleus muscle. The data demonstrate that EDL muscles are more susceptible than soleus muscles to eccentric contraction-induced injury in normal weight-bearing mice; part of the explanation for the difference appears to be the greater previous loading of the soleus muscle rather than the fiber type composition per se.