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

Abstract

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.

Ca2+-independent phospholipase A2 enhances store-operated Ca2+ entry in dystrophic skeletal muscle fibers

“…Peak values of cytosolic Ca 2+ transients triggered by KCl depolarization were also not significantly different between C57BL/6J and mdx 5cv fibers in both normal and Ca 2+ -free solution, suggesting that mechanisms involved in Ca 2+ release are not altered in dystrophic fibers, as previously reported for electrically-evoked Ca 2+ transients (Collet et al, 1999;Gillis, 1996;Plant and Lynch, 2003;Tutdibi et al, 1999).…”

Skinned single fibers from normal and dystrophin‐deficient dogs incur comparable stretch‐induced force deficits

“…Our findings are consistent with data reported by Lynch et al, 15 who examined stretch-induced force deficits in single skinned fibers from mdx and normal mice. 6,16 Recent findings in mdx mice 5,12,17,21 and our previous reports 3,4 in GRMD dogs support the idea that dystrophin confers protection to the contractile apparatus from mechanical damage assessed by force deficits either immediately following a single stretch or after 3 days of repeated in vivo lengthening activations. Taken together, the findings suggest that the extent of stretch-induced force deficits following repeated stretch-activations in the GRMD dog may be a useful method to assess future therapeutic interventions (such as gene therapy) aimed at replacing dystrophin in the sarcolemmal membrane.…”

Upregulation of store-operated Ca2+ entry in dystrophic mdx mouse muscle

“…This result indicates the Ca 2ϩ release machinery of the SR is affected in mdx muscle. Importantly, the lack of difference between the SR Ca 2ϩ loading properties in WT and mdx fibers (35) shows that the SR Ca 2ϩ pump is not affected by the absence of dystrophin.…”