ACUTE PHOSPHATE-DEPLETION INHIBITS THE NA+/H+ ANTIPORTER IN A CULTURED RENAL-CELL LINE

We studied the effect of acute P(i) depletion on the regulation of intracellular pH (pH(i)) in the OK opossum kidney cell line by using the pH-sensitive dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Cell recovery from an NH4Cl acid load in HCO3-free buffer disclosed an Na+-dependent component blocked by amiloride and a smaller Na+-independent component that increased on exposure of the cells to a high-K+ buffer. After 24-h incubation of the cells in phosphate-free medium, pH(i) recovery by the Na+/H+ exchanger was markedly inhibited, whereas the Na+-independent pH(i) recovery was not affected. The inhibition of Na+/H+ exchange was reversible on correction of cellular P(i) deficit. A similar phenomenon was observed when cellular P(i) depletion was induced by acute exposure (min) to fructose. P(i) depletion shifted the pH(i) dependence of the exchanger and also reduced its maximal activity. Time-course studies revealed that the effect of P(i) depletion could not be attributed to attenuation of Na+-K+-adenosinetriphosphatase activity and resultant diminution of the transmembrane gradient for the Na+ influx. We conclude that acute P(i) depletion in cultured proximal tubular cells leads to reversible inhibition of the Na+/H+ exchanger. This in vitro finding may relate to the in vivo observation of impaired HCO3 reabsorption and bicarbonaturia in acute P(i) depletion.