Acid increases NHE8 surface expression and activity in NRK cells

We previously demonstrated that there is a paucity of brush-border membrane NHE3 in neonates, the predominant Na⁺/H⁺ exchanger in the adult proximal tubule, while NHE8 is relatively highly expressed in neonates compared with adults. We recently showed that metabolic acidosis in neonatal rodents can increase brush-border membrane NHE8 protein expression and Na⁺/H⁺ exchange activity. To further examine the regulation of NHE8 by acid, we incubated NRK cells, which express NHE8 but not NHE3, with either acid or control media (6.6 vs. 7.4). There was an increase in Na⁺/H⁺ exchanger activity within 6 h of incubation with acid media assessed as the rate of sodium-dependent recovery of pH from an acid load (dpH_;i;/dt). The acid stimulation persisted for at least 24 h. The increase in Na⁺/H⁺ exchange activity was paralleled by an increase in surface expression of NHE8, assessed by surface biotinylation and streptavidin precipitation. The increase in both apical membrane NHE8 protein expression and Na⁺/H⁺ exchange activity with pH 6.6 media compared with 7.4 media was not affected by actinomycin D or cycloheximide consistent with an increase in surface expression independent of mRNA or protein synthesis. Furthermore, there was no increase in total cellular NHE8 protein abundance or mRNA abundance with acid media. Finally, we demonstrate that the increase in surface expression of NHE8 with acid media was blocked by colchicine and cytochalasin D and mediated by acid increasing the rate of exocytosis. In conclusion, NHE8 surface expression and activity are regulated by acid media by increasing the rate of trafficking to the apical membrane.