Muscles which are stretched during contraction are liable to a mild form of muscle damage including immediate weakness and delayed stiffness, soreness and swelling. In muscular dystrophy, which is characterized by profound muscle degeneration, stretch-induced damage is more severe and recovery incomplete. We have studied the mechanisms involved by measuring ionic changes in single fibres following a short series of stretched contractions. Intracellular sodium shows a slow increase after stretched contractions, not seen after isometric contractions, which can be prevented by either Gd3+ or streptomycin (Yeung et al., 2003) . Both Gd3+ and streptomycin block a class of mechanosensitive channels (MSCs), permeable to both Na+ and K+>, present in mammalian muscle fibres (Franco-Obregon & Lansman, 2002) . In mdx mice, which lack dystrophin, muscles showed an increased resting [Na+]i and a greater increase in [Na+]i following stretched contractions which was also blocked by the above agents. We have previously shown that stretched contraction increase resting [Ca2+]i but reduced tetanic [Ca2+]i (Balnave & Allen, 1995) . Recently we found that Gd3+, streptomycin or a potent blocker of MSCs, GsMTx4, could prevent the rise in resting [Ca2+]i and simultaneously increased the tetanic [Ca2+]i from the reduced level observed after stretched contractions. In both wild-type and mdx muscles the reduced force following stretched contractions was partly reversed by MSC blockers, presumably as a consequence of the increased tetanic [Ca2+]i noted above.
These data suggest that MSCs are present in both wild-type and mdx muscle and are persistently opened by stretched contractions. The subsequent entry of Na+ and Ca2+ may have roles in the reduced force and perhaps other aspects of muscle damage.
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(2) Franco-Obregon A & Lansman JB (2002). Changes in mechanosensitive channel activity following mechanical stimulation in skeletal muscle myotubes from the mdx mouse. J Physiol 539, 391-407.
(3) Yeung EW, Head SI & Allen DG (2003). Gadolinium reduces short-term stretch-induced muscle damage in isolated mdx mouse muscle fibres. J Physiol 552, 449-458.