title: Basolateral Na+-dependent HCO3- transporter NBCn1-mediated HCO3- influx in rat medullary thick ascending limb.
abstract: The electroneutral Na(+)-dependent HCO3- transporter NBCn1 is strongly expressed in the basolateral membrane of rat medullary thick ascending limb cells (mTAL) and is up-regulated during NH4(+)-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 HCO3- influx during NH4(+)-induced metabolic acidosis. Tubule acidification was induced by removing luminal Na+ (DeltapHi: 0.88 +/- 0.11 pH units, n = 10). Subsequently the basolateral perfusion solution was changed to CO2/HCO3- buffer with and without Na+. Basolateral Na(+)-H+ exchange function was inhibited with amiloride. Na(+)-dependent HCO3- 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 NH4(+)-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 HCO3- influx was of similar magnitude in chloride-free medium and also up-regulated after NH4+ loading. Na(+)-dependent HCO3- influx was not inhibited by 400 microm DIDS. A strong up-regulation of NBCn1 staining was confirmed in immunolabelling experiments. RT-PCR analysis revealed no evidence for the Na(+)-dependent HCO3- transporter NBC4 or the two Na(+)-dependent CI-/HCO3- 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 NH4(+)-induced metabolic acidosis. We suggest that NBCn1-mediated basolateral HCO3- influx is important for basolateral NH3 exit and thus NH4+ excretion by means of setting pHi to a more alkaline value.
authors.affiliation: Institute of Physiology, Univeristy of Aarhus, Denmark.
authors.forename: Elvin
authors.initials: E
authors.lastname: Odgaard
authors.forename: Jakob K
authors.initials: JK
authors.lastname: Jakobsen
authors.forename: Sebastian
authors.initials: S
authors.lastname: Frische
authors.forename: Jeppe
authors.initials: J
authors.lastname: Praetorius
authors.forename: Søren
authors.initials: S
authors.lastname: Nielsen
authors.forename: Christian
authors.initials: C
authors.lastname: Aalkjaer
authors.forename: Jens
authors.initials: J
authors.lastname: Leipziger
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D005260:Female; D066298:In Vitro Techniques; D007679:Kidney Medulla / Q000378:metabolism*; D008138:Loop of Henle / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D027982:Sodium-Bicarbonate Symporters / Q000378:metabolism*
chemical_list: D001639:Bicarbonates; D000644:Quaternary Ammonium Compounds; C422004:Slc4a4 protein, rat; D027982:Sodium-Bicarbonate Symporters