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 ³²P 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.