INTRACELLULAR ACIDIFICATION INHIBITS OPOSSUM KIDNEY-CELL PHOSPHATE-UPTAKE

The aim of our study is to examine the effect of intracellular pH (pH( i)) on inorganic phosphate (P-i) uptake by a proximal tubular cell lin e, the opposum kidney (OK) cells. The OK cell pH(i) (7.48 +/- 0.02, n = 12) was altered to levels between 6.5 and 8.5 by the high-K+ nigeric in method, and cell uptakes were measured at 7.5 extracellular pH. It was found that pH(i) acidification suppressed P-i uptake with a decrea se in maximal reaction rate, whereas alkalinization had no significant effect. Other Na+-dependent transport systems for glucose and amino a cid were not affected by these pH(i) changes. The inhibition of cell P -i uptake by pH(i) acidification was not prevented by protein synthesi s inhibitors (actinomycin D or cycloheximide) or by Na+/H+ exchange in hibitor [5-(N-ethyl-N-isopropyl)-amiloride]. pH(i) acidification cause d a significant decrease in cellular adenosine 3',5'-cyclic monophosph ate (cAMP) content, and cAMP-dependent protein kinase inhibitor (H-89) also did not prevent inhibition of cell P-i uptake by pH(i) acidifica tion. However, pH(i) acidification stimulated protein kinase C (PKC) a ctivity and inhibition of PKC by PKC inhibitors (bisindolylmaleimide, calphostin C, or staurosporine) or prolonged exposure to phorbol ester abrogated the inhibitory effect of pH(i) acidification on cell P-i up take. In summary these studies showed that pH(i) acidification inhibit s P-i uptake in OK cells, probably through PKC activation. These effec ts of pH(i) acidification may thus contribute to increase acid excreti on in systemic acidosis.