SOME EXPERIMENTS ON OUABAIN-SENSITIVENESS OF ²²Na EFFLUX AND AVAILABILITY OF EXOGENOUS ADENOSINE TRIPHOSPHATE IN THE MUSCLE FIBRE MEMBRANE

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In the isolated sartorius muscles, some characteristics of active Na efflux were investigated by washout experiments of radioactive Na, Ca or P with special reference to ouabain action, Ca kinetics and ATP utilization .1) Ouabain affected an accelerated Na-K exchange in the state of the membrane depolarized with Ca⁺⁺ removal, high K⁺ or azide. These depolarized states were different in membrane macromolecular conformation from each other when considered from the effect of ouabain on ²²Na efflux in every case, and the change in the efflux was not directly interpreted in terms of withdrawal of membrane Ca⁺⁺2) External Ca ions antagonized the action of ouabain, the relationship between them being probably indirect. The effective concentration of ouabain in suppressing ²²Na efflux was higher in Na-loaded muscles than in normal muscles, and it was assumed that an addition of external K⁺ and ouabain could result in the change in Na⁺ content by influencing separately on Na pump activity. ²²Na net efflux under Na free condition was affected by ouabain and also by externally applied ATP.3) ATP disturbed the effect of ouabain on ²²Na efflux but not on its effect on ⁴⁵Ca dissociation. ATP may behave as an energy source at lower concentrations and may form certain complex to prevent further release of membrane Ca⁺⁺ at higher concentrations. Labile state of membrane Ca⁺⁺ caused by ouabain was facilitated by trypsinization but not by lipolysis. Ouabain mobilized transiently membrane Ca⁺⁺. Ouabain may cause the effect on active Na extrusion by influencing on protein portion of the membrane and this change may simultaneously make membrane Ca⁺⁺ labile. Moreover, ouabain rendered the phosphorous component of the membrane unstable but this fact did not characterize the effect of this drug on ²²Na efflux.