title: Reversed electrogenic sodium bicarbonate cotransporter 1 is the major acid loader during recovery from cytosolic alkalosis in mouse cortical astrocytes.
abstract: CONCLUSIONS
  The regulation of H(+) i from cytosolic alkalosis has generally been attributed to the activity of Cl(-) -coupled acid loaders/base extruders in most cell types, including brain cells. The present study demonstrates that outwardly-directed sodium bicarbonate cotransport via electrogenic sodium bicarbonate cotransporter 1 (NBCe1) mediates the major fraction of H(+) i regulation from cytosolic alkalosis in mouse cortical astrocytes. Cl(-) -coupled acid-loading transporters play only a minor role in the regulation of H(+) i from alkalosis in mouse cortical astrocytes. NBCe1-mediated H(+) i regulation from alkalosis was dominant, with the support of intracellular carbonic anhydrase II, even when the intra- and extracellular [HCO3 (-) ] was very low (<1mM), as in nominally CO2 /HCO3 (-) free condition. A reversed NBCe1 in astrocytes may also be significant for stabilizing extracellular pH in brain tissue.
  Recovery of intracellular pH from cytosolic alkalosis has been attributed primarily to Cl(-) coupled acid loaders/base extruders such as Cl(-) /HCO3 (-) or Cl(-) /OH(-) exchangers. We have studied this process in cortical astrocytes from wild-type and transgenic mouse models with gene deletion for the electrogenic sodium bicarbonate cotransporter 1 (NBCe1) and for carbonic anhydrase (CA) isoform II. An acute cytosolic alkalosis was induced by the removal of either CO2 /HCO3 (-) or butyric acid, and the subsequent acid loading was analysed by monitoring changes in cytosolic H(+) or Na(+) using ion-sensitive fluorescent dyes. We have identified that NBCe1 reverses during alkalosis and contributes more than 70% to the rate of recovery from alkalosis by extruding Na(+) and HCO3 (-) . After CA inhibition or in CAII-knockout (KO) cells, the rate of recovery was reduced by 40%, and even by 70% in the nominal absence of CO2 /HCO3 (-) . Increasing the extracellular K(+) concentration modulated the rate of acid loading in wild-type cells, but not in NBCe1-KO cells. Removing chloride had only a minor effect on the recovery from alkalosis. Reversal of NBCe1 by reducing pH/[HCO3 (-) ] was demonstrated in astrocytes and in Xenopus oocytes, in which human NBCe1 was heterologously expressed. The results obtained suggest that reversed NBCe1, supported by CAII activity, plays a major role in acid-loading cortical astrocytes to support recovery from cytosolic alkalosis.
authors.affiliation: Department of General Zoology, Faculty of Biology, University of Kaiserslautern, Kaiserslautern, Germany.
authors.forename: Shefeeq M
authors.initials: SM
authors.lastname: Theparambil
authors.affiliation: Department of General Zoology, Faculty of Biology, University of Kaiserslautern, Kaiserslautern, Germany.
authors.forename: Zinnia
authors.initials: Z
authors.lastname: Naoshin
authors.affiliation: Department of General Zoology, Faculty of Biology, University of Kaiserslautern, Kaiserslautern, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Thyssen
authors.affiliation: Department of General Zoology, Faculty of Biology, University of Kaiserslautern, Kaiserslautern, Germany.
authors.forename: Joachim W
authors.initials: JW
authors.lastname: Deitmer
mesh_terms: D000471:Alkalosis; D000818:Animals; D001253:Astrocytes / Q000502:physiology*; D001639:Bicarbonates / Q000378:metabolism; D020148:Butyric Acid / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D005260:Female; D006863:Hydrogen-Ion Concentration; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009865:Oocytes / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D027982:Sodium-Bicarbonate Symporters / Q000502:physiology*; D014982:Xenopus laevis
chemical_list: D001639:Bicarbonates; C498899:Slc4a4 protein, mouse; D027982:Sodium-Bicarbonate Symporters; D020148:Butyric Acid; D002245:Carbon Dioxide; D012964:Sodium