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 γ-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.