Phosphate concentration and transport in Ehrlich ascites tumor cells: Effect of sodium

The effects of extracellular P i and Na + on cellular P i concentration and transport were studied. Steady‐state P i exchange flux was measured by 32 P uptake in the presence and absence of Na + . Model experiments were also conducted to assess the possibility that hydrolysis of organic phosphate esters contributes to the chemically measured intracellular P i concentration of Ehrlich ascites tumor cells. The results of these experiments indicate that hydroloysis of labile organic phosphate esters does not contribute to the measured intracellular pool of P i . The P i transport system exhibits an apparent K s of 0.115 mM P i and a maximal flux of 1.73 mmole min −1 (kg dry wt) −1 . When incubated in a phosphate‐buffered choline chloride medium (5 mM P i ) the intracellular P i and the P i influx fall by 65 and 88%, respectively. At 5 mM extracellular P i , the Na + ‐dependent component of P i transport fits Michaelis‐Menten kinetics with the maximal flux equal to 2.46 mmole min −1 (kg dry wt) −1 and an apparent K s of 35.4 mM Na + . In addition, a Na + ‐independent component of P i transport, comprising about 12% of the total P i flux, was identified. The data support the hypothesis that a P i transport system, dependent on Na + , plays a principal role in the maintenance of intracellular P i concentration.