Basolateral Na⁺‐dependent HCO₃⁻ transporter NBCn1‐mediated HCO₃⁻ influx in rat medullary thick ascending limb

Abstract

The electroneutral Na + -dependent HCO 3 − transporter NBCn1 is strongly expressed in the basolateral membrane of rat medullary thick ascending limb cells (mTAL) and is up-regulated during NH 4 + -induced metabolic acidosis. Here we used in vitro perfusion and BCECF video-imaging of mTAL tubules to investigate functional localization and regulation of Na + -dependent HCO 3 − influx during NH 4 + -induced metabolic acidosis. Tubule acidification was induced by removing luminal Na + (∆pH i : 0.88 ± 0.11 pH units, n = 10). Subsequently the basolateral perfusion solution was changed to CO 2 /HCO 3 − buffer with and without Na + . Basolateral Na + -H + exchange function was inhibited with amiloride. Na + -dependent HCO 3 − influx was determined by calculating initial base flux of Na + -mediated re-alkalinization. In untreated animals base flux was 8.4 ± 0.9 pmol min −1 mm −1 . A 2.4-fold increase of base flux to 21.8 ± 3.2 pmol min −1 mm −1 was measured in NH 4 + -treated animals (11 days, n = 11). Na + -dependent re-alkalinization was significantly larger when compared to control animals (0.38 ± 0.03 versus 0.22 ± 0.02 pH units, n = 10). In addition, Na + -dependent HCO 3 − influx was of similar magnitude in chloride-free medium and also up-regulated after NH 4 + loading. Na + -dependent HCO 3 − influx was not inhibited by 400 µm DIDS. A strong up-regulation of NBCn1 staining was confirmed in immunolabelling experiments. RT-PCR analysis revealed no evidence for the Na + -dependent HCO 3 − transporter NBC4 or the two Na + -dependent CI − /HCO 3 − exchangers NCBE and NDCBE. These data strongly indicate that rat mTAL tubules functionally express basolateral DIDS-insensitive NBCn1. Function and protein are strongly up-regulated during NH 4 + -induced metabolic acidosis. We suggest that NBCn1-mediated basolateral HCO 3 − influx is important for basolateral NH 3 exit and thus NH 4 + excretion by means of setting pH i to a more alkaline value.

The electroneutral sodium/bicarbonate cotransporter containing an amino terminal 123-amino-acid cassette is expressed predominantly in the heart

“…In particular, the NBCn1 mRNA was almost absent in spleen and testis, kidney, and brain, where a probe to the transmembrane domains of NBCn1 detected moderate to strong signals [3]. The absence of signals in the brain and kidney is consistent with recent reports that the transporter in these tissues lacks cassette II [6,7]. Our data are also similar to the data from human northern blot, except that, in human, NBCn1 signals are strongly detected in both heart and skeletal muscles [4].…”

“…[5]). While these provide useful information on physiological properties of NBCn1, recent reports suggest that the transporters in many tissues appear different from the initially identified cardiovascular clone; NBCn1 in adult brain and kidney lacks, in the N-terminal domain, a region of 123 amino acids (cassette II) that comprise $10% of the total amino acids in the protein [6,7]. Identification of this deletion variant led us to reinvestigate the cardiac tissue specificity of NBCn1.…”

Calcium-sensing Receptor Stimulates Luminal K+-dependent H+ Excretion in Medullary Thick Ascending Limbs of Henle's Loop of Mouse Kidney

“…In the mTAL, apical Na + -H + exchanger (NHE3) is the most important transport mechanism responsible for bicarbonate resorption, although a small but significant contribution of H + -ATPase-mediated bicarbonate resorption has also been identified (Brown et al 1988). The basolateral membrane contains at least three different acid base transporters: NHE1, NHE4 and electroneutral Na + -HCO 3 -cotransporter (NBCn1) (Good et al 1995Chambrey et al 2001Odgaard et al 2004). NHE1 has recently been identified to play a novel role in HCO 3 -absorption.…”

Characterization of Human SLC4A10 as an Electroneutral Na/HCO3 Cotransporter (NBCn2) with Cl– Self-exchange Activity

“…NBCn1 additionally mediates a Na ϩ conductance that is independent of its Na/HCO 3 cotransport activity (13). The poor sensitivity of the transporter to DIDS (12,13) matches the pharmacological profile of stilbene-insensitive NCBT evident in the basolateral membrane of medullary thick ascending limb epithelia (14,15) where NBCn1 is known to be expressed.…”