Duchenne muscular dystrophy results from the lack of dystrophin, a cytoskeletal protein associated with the inner surface membrane, in skeletal muscle. The absence of dystrophin induces an abnormal increase of sarcolemmal calcium influx through cationic channels in adult skeletal muscle fibers from dystrophic (mdx) mice. We observed that the activity of these channels was increased after depletion of the stores of calcium with thapsigargin or caffeine. By analogy with the situation observed in nonexcitable cells, we therefore hypothesized that these store-operated channels could belong to the transient receptor potential channel (TRPC) family. We measured the expression of TRPC isoforms in normal and mdx adult skeletal muscles fibers, and among the seven known isoforms, five were detected (TRPC1, 2, 3, 4, and 6) by RT-PCR. Western blot analysis and immunocytochemistry of normal and mdx muscle fibers demonstrated the localization of TRPC1, 4, and 6 proteins at the plasma membrane. Therefore, an antisense strategy was used to repress these TRPC isoforms. In parallel with the repression of the TRPCs, we observed that the occurrence of calcium leak channels was decreased to one tenth of its control value (patch-clamp technique), showing the involvement of TRPC in the abnormal calcium influx observed in dystrophic fibers.

The effects of tetanus duration on the relaxation rate of extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles were studied in normal (wild‐type, WT) and parvalbumin‐deficient (PVKO) mice, at 20 °C. In EDL of PVKO, the relaxation rate was low and unaffected by tetanus duration (< 3.2 s). In contrast, the relaxation rate of WT muscles decreased when tetanus duration increased from 0.2 to 3.2 s. In WT muscles, fast relaxation recovered as the rest interval increased. Specific effect of parvalbumin was asserted by calculating the difference in relaxation rate between WT and PVKO muscles. For EDL, the rate constant of relaxation slowing was 1.10 s−1 of tetanization; the rate constant of relaxation recovery was 0.05 s−1 of rest. In FDB, the effects of tetanus duration on WT and PVKO muscles were qualitatively similar to those observed in EDL. Relaxation slowing as tetanus duration increases, reflects the progressive saturation of parvalbumin by Ca2+, while recovery as rest interval increases reflects the return to Ca2+‐free parvalbumin. At all tetanus durations, relaxation rate still remained slightly faster in WT muscles. This suggested that parvalbumin facilitates calcium traffic from myofibrils to the SR. No difference was found between WT and PVKO muscles for: (i) the expression of the fast isoforms of myosin heavy chains, (ii) the force‐velocity relationship and maximal shortening velocity and (iii) the Ca2+‐activated ATPase activity from isolated preparations of the sarcoplasmic reticulum (SR).

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Rat triceps surae was minced and orthotopically autografted. Twitch time to peak, maxima tetanic tension, lactate dehydrogenase activity and total creatine concentration were measured in muscles regenerating for 30, 60 and 90 days. If the minces were frozen and thawed before grafting, muscle regeneration was suppressed. If they were further heated before grafting, muscle regeneration was also suppressed. If one half of the mince was either frozen and thawed or frozen, thawed and heated, and then recombined with the remaining half, muscle regeneration was delayed. However, at 90 days, 'intensive properties' (twitch time to peak, maximum tetanic tension, total creatine concentration and lactate dehydrogenase activity) of regenerates obtained from such partially treated minces were similar to those of regenerates obtained from untreated minces although their 'extensive properties' (weight and maximal tetanic force) were approximately halved. The extent of regeneration depends on the mass of untreated mince autografted and thus, presumably, on the number of viable muscle stem cells initially present in the mince.

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