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

Abstract

The kidney maintains systemic acid-base balance by reclaiming from the renal tubule lumen virtually all HCO₃⁻ filtered 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⁻ 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⁻ 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⁻ reclamation: the conventional indirect pathway, in which NHE₃ and the proton pump secrete H⁺ to titrate luminal HCO₃⁻, and the novel direct pathway, in which NBCn2 removes HCO₃⁻ from the lumen. The reciprocal regulation of NBCn2 and NHE₃ under different physiologic conditions is consistent with our mathematical simulations, which suggest that HCO₃⁻ uptake and H⁺ secretion have reciprocal efficiencies for HCO₃⁻ reclamation versus titration of luminal buffers.