Effects of chronic metabolic acidosis on Na(+)-H+ exchangers in LLC-PK1 renal epithelial cells.

Porcine renal epithelial cells (LLC-PK1/clone 4) have Na(+)-H+ exchangers with different kinetic properties in their apical and basolateral membranes. cDNAs encoding the basolateral Na(+)-H+ exchanger were recently cloned. To determine whether expression of the basolateral Na(+)-H+ exchanger was affected by chronic metabolic acidosis, LLC-PK1/clone 4 cells were grown on permeant supports and incubated in control medium (pH 7.4) or acid medium (pH 6.9). After 48 h, Na(+)-H+ exchanger transport activity was measured as N-ethyl-N-isopropylamiloride (EIPA)-sensitive 22Na+ influx. Acidification caused an 84% stimulation of the transport activity of the basolateral Na(+)-H+ exchanger. The apical Na(+)-H+ exchanger was stimulated 72%, and there was no change in the EIPA-insensitive 22Na+ flux across either membrane. Stimulation of Na(+)-H+ exchange was not due to differences in intracellular pH at the time transport was assayed. To determine whether there were corresponding changes in transcript levels, poly(A)+ RNA was isolated from LLC-PK1 cells and hybridized with a cDNA encoding the basolateral Na(+)-H+ exchanger. Levels of transcripts encoding the basolateral Na(+)-H+ exchanger were increased 70% after 48 h of acidification, and there were no changes in transcripts encoding cytoskeletal gamma-actin or glyceraldehyde-3-phosphate dehydrogenase. We conclude that conditions simulating chronic metabolic acidosis coordinately increase the transport activity and transcript levels of the basolateral Na(+)-H+ exchanger in porcine renal epithelial cells.