Functional regeneration in the hindlimb skeletal muscle of the mdx mouse

The pattern of spontaneous skeletal muscle degeneration and clinical recovery hindlimb muscles of the mdx mutant mouse was examined for functional and metabolic confirmation of apparent structural regeneration. The contractile properties, histochemical staining and myosin light chain and parvalbumin contents of extensor digitorum longus (EDL) and soleus (Sol) muscles of mdx and age-matched control mice were studied at 3-4 and 32 weeks. Histochemical staining (myofibrillar ATPase and NADH-tetrazolium reductase) revealed no significant change in slow-twitch-oxidative (SO) or fast-twitch-oxidative-glycolytic (FOG) fibre type proportions in mdx Sol apart from the normal age-related increase in SO fibres. At 32 weeks mdx EDL, however, showed significantly smaller fast-twitch-glycolytic (FG) and larger FOG proportions than those in control EDL. These fibre type distributions were confirmed by differential staining with antibodies to myosin slow-twitch and fast-twitch heavy chain isozymes. Frequency distribution of cross-sectional area for each fibre type showed a wider than normal range of areas especially in FOG fibres of mdx Sol, and FG fibres of mdx EDL, supporting previous observations using autoradiography of myofibre regeneration. Isometric twitch and tetanic tensions in Sol were significantly less than in controls at 4 weeks, but by 32 weeks, values were not different from age-matched controls. In mdx EDL at 3 weeks, twitch and tetanus tensions were significantly less, and time-to-peak twitch tensions were significantly faster than in control EDL. By 32 weeks, mdx EDL twitch and tetanus tensions expressed relative to muscle weight continued to be significantly lower than in age-matched controls, despite normal absolute tensions. The maximum velocity of shortening in 32-week mdx EDL was significantly lower than in control EDL. Myosin light chain distribution in mdx Sol exhibited significantly less light chain 2-slow (LC2s) and more light chain 1b-slow(LC1bs) at 32 weeks than age-matched control Sol. Gels of EDL from 32-week-old mdx mice showed significantly less light chain 2-fast-phosphorylated (LC2f-P) and light chain 3-fast (LC3f) and significantly more light chain 1-fast (LC1f) and light chain 2-fast (LC2f), but normal parvalbumin content compared to age-matched controls. These observations suggest that mdx hindlimb muscles are differentially affected by the disease process as it occurs in murine models of dystrophy. However, the uniqueness of mdx Sol and to a lesser extent EDL is that they also undergo an important degree of functional regeneration which is able to compensate spontaneously for degenerative influences of genetic origin.(ABSTRACT TRUNCATED AT 400 WORDS)