Susceptibility to sarcomere injury induced by single stretches of maximally activated muscles of mdx mice

The purpose was to investigate the contribution of mechanical damage to sarcomeres to the greater susceptibility of dystrophic muscle fibers to contraction- induced injury. Single stretches provide an effective method for studying mechanical factors that contribute to the initiation of contraction- induced injury. We hypothesized that, after single stretches, the deficits in isometric force would be greater for muscles of mdx than C57BL/10 mice, whereas membrane damage would be minimal for all muscles. Extensor digitorum longus (EDL) and soleus muscles of mice were removed under anesthesia with Avertin ( tribromoethanol). During the plateau of a maximum isometric contraction in vitro, muscles were stretched through single strains of 20 - 60% fiber length. Isometric force was remeasured 1 min later, and muscles were then incubated in procion orange dye to identify fibers with membrane damage. Force deficits at 1 min were two- to threefold greater for EDL muscles of mdx compared with C57BL/10 mice, whereas no significant differences were observed between soleus muscles of mdx and C57BL/10 mice. For all muscles, membrane damage was minimal and not significantly increased by single stretches for either strain of mice. These data support a critical role of dystrophin maintaining sarcomere stability in EDL muscles, whereas soleus muscles retain abilities, in the absence of dystrophin, not different from control muscles to resist sarcomere damage.