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 in skeletal muscle. Its physiological role remains uncertain, although its absence is known to cause muscular dystrophy. In this study, the function 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 nondegenerated, 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 at 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 differences in mitochondria, since cytochrome-c oxidase activity was similar in mdx and control muscle. Contractile properties of control and mdx 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 half-relaxation times did not differ. Mass action potential area (M wave), an index of muscle excitability, was not significantly different between 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 dystrophin has only a moderate impact in modifying muscle function relative to contractile properties, fatigability, or excitability.(ABSTRACT TRUNCATED AT 250 WORDS)