Bicarbonate secretion of mouse cholangiocytes involves Na+-HCOmath image cotransport in addition to Na+-independent Cl/HCO3 exchange

Authors

  • Iker Uriarte,

    1. Division of Gene Therapy and Hepatology-Liver Unit, CIMA, CUN, and School of Medicine, University of Navarra, and Ciberehd, Pamplona, Spain
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  • Jesús M. Banales,

    1. Division of Gene Therapy and Hepatology-Liver Unit, CIMA, CUN, and School of Medicine, University of Navarra, and Ciberehd, Pamplona, Spain
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  • Elena Sáez,

    1. Division of Gene Therapy and Hepatology-Liver Unit, CIMA, CUN, and School of Medicine, University of Navarra, and Ciberehd, Pamplona, Spain
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  • Fabián Arenas,

    1. Division of Gene Therapy and Hepatology-Liver Unit, CIMA, CUN, and School of Medicine, University of Navarra, and Ciberehd, Pamplona, Spain
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  • Ronald P. J. Oude Elferink,

    1. Laboratory of Experimental Hepatology, AMC Liver Center, Academic Medical Center, Amsterdam, The Netherlands
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  • Jesús Prieto,

    1. Division of Gene Therapy and Hepatology-Liver Unit, CIMA, CUN, and School of Medicine, University of Navarra, and Ciberehd, Pamplona, Spain
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  • Juan F. Medina

    Corresponding author
    1. Division of Gene Therapy and Hepatology-Liver Unit, CIMA, CUN, and School of Medicine, University of Navarra, and Ciberehd, Pamplona, Spain
    • Division of Gene Therapy and Hepatology, CIMA, CUN and School of Medicine, University of Navarra, Avda. Pío XII 55, E-31008 Pamplona, Spain
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    • fax: +34 948 194717


  • Potential conflict of interest: Nothing to report.

Abstract

Bicarbonate secretion from cholangiocytes is required for appropriate adjustment of primary canalicular bile along the biliary tract. In human and rat cholangiocytes, bicarbonate secretion is mediated by anion exchanger (AE) 2, an electroneutral Na+-independent Cl/HCOmath image AE also involved in intracellular pH (pHi) regulation. In Ae2a,b-deficient mice, pHi is increased in lymphocytes and fibroblasts, whereas it is surprisingly normal in cholangiocytes. Here, we analyze the mechanisms for HCO3 secretion in cultured Ae2math image and Ae2a,b−/− mouse cholangiocytes by microfluorimetric measurement of pHi changes upon established perfusion maneuvers. Cl withdrawal by isethionate-based perfusions showed that Ae2math image but not Ae2a,b−/− mouse cholangiocytes can display Cl/HCOmath image exchange, which is therefore entirely mediated by Ae2. Nevertheless, simultaneous withdrawal of Cl and Na+ revealed that mouse cholangiocytes possess an additional transport activity for HCO3 secretion not observed in control rat cholangiocytes. Propionate-based maneuvers indicated that this supplemental Na+-driven HCOmath image-secreting activity is Cl-independent, consistent with a Na+-HCO3 cotransport (NBC). NBC activity is greater in Ae2math image than Ae2a,b+/+ mouse cholangiocytes, and membrane-depolarization experiments showed that it is electrogenic. Consistent with the potential role of Slc4a4/Nbc1 as the involved transporter, Ae2math image mouse cholangiocytes exhibit up-regulated expression of this electrogenic NBC carrier. Whereas Ae2-mediated Cl/HCO3 exchange in Ae2math image mouse cholangiocytes is stimulated by cyclic adenosine monophosphate (cAMP) and acetylcholine, the NBC activity is down-regulated by cAMP and adenosine triphosphate (ATP) in Ae2a,b−/− mouse cholangiocytes. Polarized Ae2math image mouse cholangiocytes placed in Ussing chambers show decreased (but not abolished) cAMP-dependent Cl current and increased ATP-dependent/Ca2+-activated Cl secretion, which run in parallel with decreased cystic fibrosis transmembrane conductance regulator messenger RNA expression and increased intracellular Ca2+ levels. Conclusion: Bicarbonate secretion in mouse cholangiocytes involves two differentially regulated activities: Ae2-mediated Cl/HCO3 exchange and Na+-HCOmath image cotransport. (HEPATOLOGY 2009.)

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