Summary
Measurements of isometric tension were performed in single twitch skeletal muscle fibres and the effect of extracellular Na+ removal on contraction was investigated. Na+ withdrawal brought about an increase in the amplitude of K+ contractures and their time course became faster. The potentiation of K+ contractures depended strongly on extracellular Ca2+ and developed slowly following an exponential time course with a time constant of approximately 8 min. Removal of extracellular Na+ greatly increased the amplitude of caffeine contractures and lowered its threshold: caffeine (0.5 mM) had no effect on resting tension in Ringer's but produced contractures in Na+-free solutions. Intramembrane charge movement (charge 1) was monitored in contracting voltage-clamped segments of frog skeletal muscle fibres using the triple-Vaseline-gap technique. Movement of charge 1 did not depend on the presence of extracellular Na+. However, the mechanical threshold decreased by approximately 10 mV at several pulse durations and the charge which produced just detectable contractions decreased by approximately 5 nC μF−1 in the absence of extracellular Na+. Intracellular heparin (40 mg ml−1) increased the mechanical threshold by approximately 20 mV without affecting the movement of charge 1. The effect of Na+-free solutions on the mechanical threshold was additive to that of heparin. It is concluded that the effects of Na+-withdrawal on contraction take place at a location beyond the voltage sensor of excitation-contraction coupling.
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Garcia, M.C., Diaz, A.F., Godinez, R. et al. Effect of sodium deprivation on contraction and charge movement in frog skeletal muscle fibres. J Muscle Res Cell Motil 13, 354–365 (1992). https://doi.org/10.1007/BF01766463
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DOI: https://doi.org/10.1007/BF01766463