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MUSCLE CELL BIOLOGY AND CELL MOTILITY

Depolarization-induced contraction and SR function in mechanically skinned muscle fibers from dystrophic mdx mice

Department of Physiology, The University of Melbourne, Victoria 3010, Australia

Submitted 15 August 2002 ; accepted in final form 21 April 2003

Dystrophin is absent in muscle fibers of patients with Duchenne muscular dystrophy (DMD) and in muscle fibers from the mdx mouse, an animal model of DMD. Disrupted excitation-contraction (E-C) coupling has been postulated to be a functional consequence of the lack of dystrophin, although the evidence for this is not entirely clear. We used mechanically skinned fibers (with a sealed transverse tubular system) prepared from fast extensor digitorum longus muscles of wild-type control and dystrophic mdx mice to test the hypothesis that dystrophin deficiency would affect the depolarization-induced contractile response (DICR) and sarcoplasmic reticulum (SR) function. DICR was similar in muscle fibers from mdx and control mice, indicating normal voltage regulation of Ca²⁺ release. Nevertheless, rundown of DICR (<50% of initial) was reached more rapidly in fibers from mdx than control mice [control: 32 ± 5 depolarizations (n = 14 fibers) vs. mdx: 18 ± 1 depolarizations (n = 7) before rundown, P < 0.05]. The repriming rate for DICRs was decreased in fibers from mdx mice, with lower submaximal DICR observed after 5, 10, and 20 s of repriming compared with fibers from control mice (P < 0.05). SR Ca²⁺ reloading was not different in fibers from control and mdx mice, and no difference was observed in SR Ca²⁺ leak. Caffeine (2–7 mM)-induced contraction was diminished in fibers from mdx mice compared with control (P < 0.05), indicating depressed SR Ca²⁺ release channel activity. Our findings indicate that fast fibers from mdx mice exhibit some impairment in the events mediating E-C coupling and SR Ca²⁺ release channel activity.

muscular dystrophy; depolarization-induced force; T-system; calcium release; sarcoplasmic reticulum; mdx mouse

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