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

Publisher Website
Google Scholar
Abstract
The intracellular pH (pH i ) 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 pH i of cells placed in an acidic medium (pH o below 7.15) decreases and reaches a steady-state value that is more alkaline than the outside. For example when pH o is acutely reduced to 5.5, pH i 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 · min) andK m is 0.74 ± 0.09 × 10−6 m. Steady-state cells with pH i above 6.8 continuously extrude H+ by a process that is not dependent on ATP but is inhibited by anaerobiosis. Acid-loaded cells (pH i 6.3) when returned to pH o 7.3 medium respond by transporting H+, resulting in a rapid rise in pH i . 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 pH i to its steady-state value.