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

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.