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Received April 8, 2004
Revised May 6, 2004
Accepted after revision June 1, 2004
1 The University of Melbourne
2 University of Melbourne
* To whom correspondence should be addressed. E-mail: gsl{at}unimelb.edu.au.
Damaged skeletal muscle fibres are replaced with new contractile units via muscle regeneration. Regenerating muscle fibres synthesise functionally distinct isoforms of contractile and regulatory proteins but little is known of their functional properties during the regeneration process. An advantage of utilising 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 2+-activated contractile characteristics of permeabilized fibres from rat fast (extensor digitorum longus) and slow (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 than fibres from uninjured muscles. Regenerating muscle fibres had ~10% of the maximal force producing capacity (Po) 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 Po was restored to 36-45% of 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 muscle fibres in a progressive manner as the regeneration process is completed.
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