Intracellular acidification inhibits opposum kidney cell phosphate uptake

The aim of our study is to examine the effect of intracellular pH (pHi) on inorganic phosphate (Pi) uptake by a proximal tubular cell line, the opposum kidney (OK) cells. The OK cell pHi (7.48 +/- 0.02; n = 12) was altered to levels between 6.5 and 8.5 by the high-K+ nigericin method, and cell uptakes were measured at 7.5 extracellular pH. It was found that pHi acidification suppressed Pi uptake with a decrease in maximal reaction rate, whereas alkalinization had no significant effect. Other Na(+)-dependent transport systems for glucose and amino acid were not affected by these pHi changes. The inhibition of cell Pi uptake by pHi acidification was not prevented by protein synthesis inhibitors (actinomycin D or cycloheximide) or by Na+/H+ exchange inhibitor [5-(N-ethyl-N-isopropyl)-amiloride]. pHi acidification caused a significant decrease in cellular adenosine 3',5'-cyclic monophosphate (cAMP) content, and cAMP-dependent protein kinase inhibitor (H-89) also did not prevent inhibition of cell Pi uptake by pHi acidification. However, pHi acidification stimulated protein kinase C (PKC) activity and inhibition of PKC by PKC inhibitors (bisindolylmaleimide, calphostin C, or staurosporine) or prolonged exposure to phorbol ester abrogated the inhibitory effect of pHi acidification on cell Pi uptake. In summary, these studies showed that pHi acidification inhibits Pi uptake in OK cells, probably through PKC activation. These effects of pHi acidification may thus contribute to increase acid excretion in systemic acidosis.