Na(+)-Ca2+ exchange induces low Na+ contracture in frog skeletal muscle fibers after partial inhibition of sarcoplasmic reticulum Ca(2+)-ATPase.

Abstract : Contractile responses due to reduction in external sodium concentration ([Na+]o) were investigated in twitch skeletal muscle fibers of frog semitendinosus. Experiments were conducted after partial inhibition of sarcoplasmic reticulum Ca(2+)-ATPase by cyclopiazonic acid (CPA). In the absence of CPA, Na+ withdrawal failed to produce any change in resting tension. In the presence of CPA (2-10 microM), [Na+]o reduction induced a transient contracture without a significant change in the resting membrane potential. The amplitude of the contracture displayed a step dependence on [Na+]o, was increased by K(+)-free medium and was prevented in Ca(2+)-free medium. This contracture was inhibited by various blockers of the Na(+)-Ca2+ exchange but was little affected by inhibitors of sarcolemmal Ca(2+)-ATPase or mitochondria. When sarcoplasmic reticulum function was impaired, low-Na+ solutions caused no contracture. These results provide evidence that skeletal muscle fibers possess a functional Na(+)-Ca2+ exchange which can mediate sufficient Ca2+ entry to activate contraction by triggering Ca2+ release from sarcoplasmic reticulum when the sodium electrochemical gradient is reduced, and sarcoplasmic reticulum Ca(2+)-ATPase is partially inhibited. This indicates that when the sarcoplasmic reticulum Ca(2+)-ATPase is working (no CPA), Ca2+ fluxes produced by the exchanger are buffered by the sarcoplasmic reticulum. Thus the Na(+)-Ca2+ exchange may be one of the factors determining sarcoplasmic reticulum Ca2+ content and thence the magnitude of the release of Ca2+ from the sarcoplasmic reticulum.
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Submitted on : Thursday, June 6, 2019 - 10:16:37 AM
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William Même, C Léoty. Na(+)-Ca2+ exchange induces low Na+ contracture in frog skeletal muscle fibers after partial inhibition of sarcoplasmic reticulum Ca(2+)-ATPase.. Pflügers Archiv European Journal of Physiology, Springer Verlag (Germany), 1999, 438 (6), pp.851-859. ⟨10.1007/s004249900142⟩. ⟨hal-02149088⟩

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