Na+/HCO3–Cotransporter NBCn2 Mediates HCO3− Reclamation in the Apical Membrane of Renal Proximal Tubules

The kidneymaintains systemic acid-base balance by reclaiming from the renal tubule lumen virtually all HCO3 2filtered in glomeruli and by secreting additional H+ to titrate luminal buffers. For proximal tubules, which are responsible for about 80% of this activity, it is believed that HCO3 2 reclamation depends solely on H+ secretion, mediated by the apical Na+ /H+ exchanger NHE3 and the vacuolar proton pump. However, NHE3 and the proton pump cannot account for all HCO3 2 reclamation. Here, we investigated the potential contribution of two variants of the electroneutral Na+ /HCO3 – cotransporter NBCn2, the amino termini of which start with the amino acids MCDL (MCDL-NBCn2) and MEIK (MEIK-NBCn2). Western blot analysis and immunocytochemistry revealed that MEIK-NBCn2 predominantly localizes at the basolateral membrane of medullary thick ascending limbs in the rat kidney, whereas MCDL-NBCn2 localizes at the apical membrane of proximal tubules. Notably, NH4Cl-induced systemic metabolic acidosis or hypokalemic alkalosis downregulated the abundance of MCDL-NBCn2 and reciprocally upregulated NHE3. Conversely, NaHCO3-induced metabolic alkalosis upregulated MCDL-NBCn2 and reciprocally downregulated NHE3. We propose that the apical membrane of the proximal tubules has two distinct strategies for HCO3 2 reclamation: the conventional indirect pathway, in which NHE3 and the proton pump secrete H+ to titrate luminal HCO3 2, and the novel direct pathway, in which NBCn2 removes HCO3 2 from the lumen. The reciprocal regulation of NBCn2 and NHE3 under different physiologic conditions is consistent with our mathematical simulations, which suggest that HCO3 2uptake andH+ secretion have reciprocal efficiencies forHCO3 2reclamation versus titration of luminal buffers.