Intracellular sodium concentration and resting sodium fluxes of the frog heart ventricle

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Abstract
Using Na24 to label the exchangeable sodium in the tissue and either [S35]sulphate or [C14]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. The. intracellular K concentration, obtained by flame-photometric analysis, was approximately 163 m-mole/kg cell water. Two different methods were employed to study the Na tracer efflux of resting heart ventricles. One involved a double-tracer technique, using Na24 to indicate the release of the exchangeable tissue Na, and S35O4 to indicate, approximately, the release of extra- myocardial Na. In the other a comparison was made of the Na release from the tissue when it contained Na either at the normal concentration or at a concentration enhanced by exchange with intracellular K. The magnitude of the Na efflux from heart fibers as measured by both methods was of the order 50-100 p-mole/cm2 sec. Simultaneously with this cellular efflux a substantial amount of Na was released from extra-myocardial spaces and tissues. The net efflux of K ions from heart cells that occurred when heart ventricles were perfused with K depleted fluids was determined. The results were used to obtain an indirect estimate, of about 2-3 p-mole/cm.2 sec, for the resting Na influx. The significance of these different values of Na efflux and influx is discussed.