Changes in contractile activation characteristics of rat fast and slow skeletal muscle fibres during regeneration

Damaged skeletal muscle fibres are replaced with new contractile units via muscle regeneration. Regenerating muscle fibres synthesize functionally distinct isoforms of contractile and regulatory proteins but little is known of their functional properties during the regeneration process. An advantage of utilizing single muscle fibre preparations is that assessment of their function is based on the overall characteristics of the contractile apparatus and regulatory system and as such, these preparations are sensitive in revealing not only coarse, but also subtle functional differences between muscle fibres. We examined the Ca²⁺- and Sr²⁺-activated contractile characteristics of permeabilized fibres from rat fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles at 7, 14 and 21 days following myotoxic injury, to test the hypothesis that fibres from regenerating fast and slow muscles have different functional characteristics to fibres from uninjured muscles. Regenerating muscle fibres had ∼10% of the maximal force producing capacity (P_o) of control (uninjured) fibres, and an altered sensitivity to Ca²⁺ and Sr²⁺ at 7 days post-injury. Increased force production and a shift in Ca²⁺ sensitivity consistent with fibre maturation were observed during regeneration such that P_o was restored to 36–45% of that in control fibres by 21 days, and sensitivity to Ca²⁺ and Sr²⁺ was similar to that of control (uninjured) fibres. The findings support the hypothesis that regenerating muscle fibres have different contractile activation characteristics compared with mature fibres, and that they adopt properties of mature fast- or slow-twitch muscle fibres in a progressive manner as the regeneration process is completed.

The calcineurin signal transduction pathway is essential for successful muscle regeneration in mdx dystrophic mice

Although mdx mice share the same genetic defect and lack dystrophin expression as in Duchenne muscular dystrophy (DMD), their limb muscles have a high regenerative capacity that ensures a more benign phenotype and essentially normal function. The cellular pathways responsible for this enhanced regenerative capacity are unknown. We tested the hypothesis that the calcineurin signal transduction pathway is essential for the successful regeneration following severe degeneration observed in the limb muscles of young mdx mice (2–4 weeks old) and that inhibition of this pathway using cyclosporine A (CsA) would exacerbate the dystrophic pathology. Eighteen-day-old mdx and C57BL/10 mice were treated with CsA for 16 days. CsA administration severely disrupted muscle regeneration in mdx mice, but had minimal effect in C57BL/10 mice. Muscles from CsA-treated mdx mice had fewer centrally nucleated fibers and extensive collagen, connective tissue, and mononuclear cell infiltration than muscles from vehicle-treated littermates. The deleterious effects of CsA on muscle morphology were accompanied by a 30–35% decrease in maximal force producing capacity. Taken together, these observations indicate that the calcineurin signal transduction pathway is a significant determinant of successful skeletal muscle regeneration in young mdx mice. Up-regulating this pathway may have clinical significance for DMD.