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 carri er for Na+ and by altering the availability of divalent and monovalent phos phate. As the transport of monovalent phosphate would be expected to acidif y, and that of divalent phosphate to alkalinize the cell interior, the effe ct of phosphate transport on cytosolic pH was studied using ion selective m icroelectrodes in Xenopus oocytes expressing NaPi-3 or NaPi-5. At an alkali ne extracellular pH (pH,) of 8.0, addition of phosphate elicited a strong i nward current, depolarization of the cell membrane and cytosolic alkaliniza tion. At pH, 6.0 the phosphate-induced inward current and depolarization we re reduced and the alkalinization completely abolished. In conclusion, at a lkaline pH, phosphate transport is enhanced and the transport of divalent p hosphate prevails. At pH, 6.0, phosphate transport is attenuated and is acc omplished by transport of both divalent and monovalent phosphate.