H+ transport and the regulation of intracellular pH in Ehrlich ascites tumor cells.

The intracellular pH (pHi) of Ehrlich ascites tumor cells, both in the steady state and under conditions of acid loading or recovery from acid loading, was investigated by measuring the transmembrane flux of H+ equivalents and correlating this with changes in the distribution ratio of dimethyloxazolidine-2,4-dione (DMO). The pHi of cells placed in an acidic medium (pHo below 7.15) decreases and reaches a steady-state value that is more alkaline than the outside. For example when pHo is acutely reduced to 5.5, pHi falls exponentially from 7.20 +/- 0.06 to 6.29 +/- 0.04 with a halftime of 5.92 +/- 1.37 min, suggesting a rapid influx of H+. The unidirectional influx of H+ exhibits saturation kinetics with respect to extracellular [H+]; the maximal flux is 15.8 +/- 0.05 mmol/(kg dry wt X min) and Km is 0.74 +/- 0.09 X 10(-6) M. Steady-state cells with pHi above 6.8 continuously extrude H+ by a process that is not dependent on ATP but is inhibited by anaerobiosis. Acid-loaded cells (pHi 6.3) when returned to pHo 7.3 medium respond by transporting H+, resulting in a rapid rise in pHi. The halftime for this process is 1.09 +/- 0.22 min. The H+ efflux measured under similar conditions increases as the intracellular acid load increases. An ATP-independent as well as an ATP-dependent efflux contributes to the restoration of pHi to its steady-state value.