Effect of NaPi-mediated phosphate transport on intracellular pH

Extracellular pH has been shown previously to influence transport via type-II Na⁺/phosphate (NaPi) transporters by modifying the affinity of the carrier for Na⁺ and by altering the availability of divalent and monovalent phosphate. As the transport of monovalent phosphate would be expected to acidify, and that of divalent phosphate to alkalinize the cell interior, the effect of phosphate transport on cytosolic pH was studied using ion selective microelectrodes in Xenopus oocytes expressing NaPi-3 or NaPi-5. At an alkaline extracellular pH (pH_e) of 8.0, addition of phosphate elicited a strong inward current, depolarization of the cell membrane and cytosolic alkalinization. At pH_e 6.0 the phosphate-induced inward current and depolarization were reduced and the alkalinization completely abolished. In conclusion, at alkaline pH_e phosphate transport is enhanced and the transport of divalent phosphate prevails. At pH_e 6.0, phosphate transport is attenuated and is accomplished by transport of both divalent and monovalent phosphate.