title: Bicarbonate sensing in mouse cortical astrocytes during extracellular acid/base disturbances.
abstract: The present study suggests that the electrogenic sodium-bicarbonate cotransporter, NBCe1, supported by carbonic anhydrase II, CAII, provides an efficient mechanism of bicarbonate sensing in cortical astrocytes. This mechanism is proposed to play a major role in setting the pHiresponses to extracellular acid/base challenges in astrocytes. A decrease in extracellular [HCO3-] during isocapnic acidosis and isohydric hypocapnia, or an increase in intracellular [HCO3-] during hypercapnic acidosis, was effectively sensed by NBCe1, which carried bicarbonate out of the cells under these conditions, and caused an acidification and sodium fall in WT astrocytes, but not in NBCe1-knockout astrocytes. Isocapnic acidosis, hypercapnic acidosis and isohydric hypocapnia evoked inward currents in NBCe1- and CAII-expressing Xenopus laevis oocytes, but not in native oocytes, suggesting that NBCe1 operates in the outwardly directed mode under these conditions consistent with our findings in astrocytes. We propose that bicarbonate sensing of astrocytes may have functional significance during extracellular acid/base disturbances in the brain, as it not only alters intracellular pH/[HCO3-]-dependent functions of astrocytes, but also modulates the extracellular pH/[HCO3-] in brain tissue.
  
  
  
  Extracellular acid/base status of the mammalian brain undergoes dynamic changes during many physiological and pathological events. Although intracellular pH (pHi) of astrocytes responds to extracellular acid/base changes, the mechanisms mediating these changes have remained unresolved. We have previously shown that the electrogenic sodium-bicarbonate cotransporter, NBCe1, is a high-affinity bicarbonate carrier in cortical astrocytes. In the present study, we investigated whether NBCe1 plays a role in bicarbonate sensing in astrocytes, and in determining the pHiresponses to extracellular acid/base challenges. We measured changes in intracellular H+and Na+in astrocytes from wild-type (WT) and from NBCe1-knockout (KO) mice, using ion-selective dyes, during isocapnic acidosis, hypercapnic acidosis and hypocapnia. We also analysed NBCe1-mediated membrane currents in Xenopus laevis oocytes under similar conditions. Comparing WT and NBCe1-KO astrocytes, we could dissect the contribution of NBCe1, of diffusion of CO2across the cell membrane and, after blocking carbonic anhydrase (CA) activity with ethoxyzolamide, of the role of CA, for the amplitude and rate of acid/base fluxes. Our results suggest that NBCe1 transport activity in astrocytes, supported by CA activity, renders astrocytes bicarbonate sensors in the mouse cortex. NBCe1 carried bicarbonate into and out of the cell by sensing the variations of transmembrane [HCO3-], irrespective of the changes in intra- and extracellular pH, and played a major role in setting pHiresponses to the extracellular acid/base challenges. We propose that bicarbonate sensing of astrocytes may have potential functional significance during extracellular acid/base alterations in the brain.
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Shefeeq M
authors.initials: SM
authors.lastname: Theparambil
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Zinnia
authors.initials: Z
authors.lastname: Naoshin
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Defren
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Jana
authors.initials: J
authors.lastname: Schmaelzle
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Weber
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Schneider
authors.affiliation: Abteilung für Allgemeine Zoologie, FB Biologie, University of Kaiserslautern, Pb 3049, D-67653, Kaiserslautern, Germany.
authors.forename: Joachim W
authors.identifier: 0000-0001-8763-7650
authors.initials: JW
authors.lastname: Deitmer
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D001639:Bicarbonates / Q000378:metabolism* / Q000494:pharmacology; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D027963:Chloride-Bicarbonate Antiporters / Q000378:metabolism*; D045604:Extracellular Fluid / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D027982:Sodium-Bicarbonate Symporters / Q000378:metabolism*; D029867:Xenopus Proteins / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D001639:Bicarbonates; D027963:Chloride-Bicarbonate Antiporters; C522009:SLC4A4 protein, Xenopus; D027982:Sodium-Bicarbonate Symporters; D029867:Xenopus Proteins
keywords: bicarbonate secretion; hypercapnic acidosis; isocapnic acidosis; pH; sodium-bicarbonate cotransporter NBCe1