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J Physiol Vol 188, Issue 2 pp 235-260
Copyright © 1967 by The Physiological Society
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Intracellular sodium concentration and resting sodium fluxes of the frog heart ventricle

 Mary J. Keenan and R. Niedergerke

1. Using 24Na to label the exchangeable sodium in the tissue and either [35S]sulphate or [14C]sucrose to label the extracellular spaces, the intracellular sodium concentration of frog heart ventricles was determined and found to be between about 5 and 10 m-mole/kg cell water.

2. The intracellular potassium concentration, obtained by flame-photometric analysis, was approximately 163 m-mole/kg cell water.

3. Two different methods were employed to study the sodium tracer efflux of resting heart ventricles. One involved a double-tracer technique, using 24Na to indicate the release of the exchangeable tissue sodium, and 35SO4 to indicate, approximately, the release of extra-myocardial sodium. In the other a comparison was made of the sodium release from the tissue when it contained sodium either at the normal concentration or at a concentration enhanced by exchange with intracellular potassium.

4. The magnitude of the sodium efflux from heart fibres as measured by both methods was of the order 50-100 p-mole/cm2 sec. Simultaneously with this cellular efflux a substantial amount of sodium was released from extra-myocardial spaces and tissues.

5. The net efflux of potassium ions from heart cells that occurred when heart ventricles were perfused with potassium-depleted fluids was determined. The results were used to obtain an indirect estimate, of about 2-3 p-mole/cm2 sec, for the resting sodium influx.

6. The significance of these different values of sodium efflux and influx is discussed.






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