Phosphate transport in Ehrlich ascites tumor cells: Inhibition by H +

The effect of changes in extracellular pH (pH o ) and intracellular pH (pH i ) on Na + ‐dependent and Na + ‐independent inorganic phosphate (P i ) transport in Ehrlich cells was investigated. In the presence of Na + , acutely reducing pH o from 7.30 to 5.50 results first in a transient (∼7 min) stimulation of P i transport. The enhanced rate of transport is a saturable function of the extracellular [H + ]; the Ks equals 2.3 × 10 −6 M (pH o 6.68). However, P i transport is progressively inhibited as pH i falls below 6.50. The effect of pH i on P i transport measured at various intracellular [Na + ] suggests that inhibition develops as a consequence of H + interaction with an intracellular Na + site(s) on the Na + ‐dependent carrier. At pH o 7.4, about 15% of the steady state P i flux persists in the absence of Na + . However, when pH o is reduced, transport is stimulated to the same extent and with the same time course and kinetic characteristics as in the presence of Na + . Thus, H + stimulated P i transport does not require Na + , raising the possibility that the Na + ‐independent component is mediated by the anion (CI − ) exchanger.