PERFORMANCE AND EXCITABILITY OF MDX MOUSE MUSCLE AT 2, 5, AND 13 WK OF AGE

Dystrophin is a 427-kDa protein localized adjacent to the sarcolemma i n skeletal muscle. Its physiological role remains uncertain, although its absence is known to cause muscular dystrophy. In this study, the f unction of dystrophin was investigated using the dystrophin-deficient mdx mouse. Control and mdx animals at 2, 5, and 13 wk of age (n = 8-11 /age) were compared to evaluate in situ gastrocnemius-plantaris-soleus muscle contractile, endurance, and excitability properties at nondege nerated, degenerated, and regenerated stages, respectively. Twitch and tetanic tensions expressed per gram of muscle mass were lower in mdx muscle only at 5 wk. Fatigue produced during successive contractions a t 2, 10, and 20 Hz did not differ between the two groups at 2 and 5 wk but was lower in mdx muscle at 13 wk. This was not attributed to diff erences in mitochondria, since cytochrome-c oxidase activity was simil ar in mdx and control muscle. Contractile properties of control and md x muscle became faster with age, and at 13 wk the time to peak twitch tension was shorter in mdx muscle relative to control, whereas the hal f-relaxation times did not differ. Mass action potential area (M wave) , an index of muscle excitability, was not significantly different bet ween mdx and control muscle at 2 or 5 wk but was greater in mdx muscle at 13 wk. Thus, in this weight-bearing muscle group, the lack of dyst rophin has only a moderate impact in modifying muscle function relativ e to contractile properties, fatigability, or excitability. The differ ences noted between mdx and control muscle are age specific and are li kely related to the extent of degeneration and regeneration processes that occur secondary to the absence of dystrophin in mdx muscle.