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1. The effects of replacement or concentration change of a constituent ion of Ringer's solution on the twitch tension and action potential of frog sartorius muscle were investigated.2. The Na removal within the limit of 75% improved the excitation-contraction coupling.3. The excess K facilitated the excitation-contraction coupling under the limit of 9 mEq.4. In the depolarized muscle, the excess Ca facilitated the coupling, but in the non-depolarized muscle, the twitch tension was depressed by the excess Ca, while the spike height increased.5. In Ca-free Ringer's solution, none of significant change in mechanical response was observed at single or low frequency stimulation. But the maintenance tension at high frequency stimulation fell down far quickly than the control.6. The Cl removal improved the excitation-contraction coupling.7. TEA, substituted for Na, and anomalous anions, substituted for Cl, prolonged the duration of spike potential and increased the negative after-potential as well as the twitch tension. The relation between electrical change and mechanicalresponse, however, was not uniform. Because the specific effect of each ion was superposed on the general effect of Na and/or Cl removal.8. The significance of Ca-entry to the excitation-contraction coupling as related to the depolarization and Na-permeability, and the dual (general and specific) effects of ion substitution were discussed.

ROLES OF EXTERNAL IONS IN THE EXCITATIONCONTRACTIONCOUPLING OF FROG SKELETAL MUSCLE