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Tetanus relaxation of fast skeletal muscles of the mouse made parvalbumin deficient by gene inactivation

1 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. 2 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. 3 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. 4 In FDB, the effects of tetanus duration on WT and PVKO muscles were qualitatively similar to those observed in EDL. 5 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. 6 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. 7 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).