Pflügers Archiv

, Volume 450, Issue 6, pp 415–428 | Cite as

NH3 and NH4+ permeability in aquaporin-expressing Xenopus oocytes

  • Lars M. Holm
  • Thomas P. Jahn
  • Anders L. B. Møller
  • Jan K. Schjoerring
  • Domenico Ferri
  • Dan A. Klaerke
  • Thomas Zeuthen
Ion Channels, Transporters


We have shown recently, in a yeast expression system, that some aquaporins are permeable to ammonia. In the present study, we expressed the mammalian aquaporins AQP8, AQP9, AQP3, AQP1 and a plant aquaporin TIP2;1 in Xenopus oocytes to study the transport of ammonia (NH3) and ammonium (NH4+) under open-circuit and voltage-clamped conditions. TIP2;1 was tested as the wild-type and in a mutated version (tip2;1) in which the water permeability is intact. When AQP8-, AQP9-, AQP3- and TIP2;1-expressing oocytes were placed in a well-stirred bathing medium of low buffer capacity, NH3 permeability was evident from the acidification of the bathing medium; the effects observed with AQP1 and tip2;1 did not exceed that of native oocytes. AQP8, AQP9, AQP3, and TIP2;1 were permeable to larger amides, while AQP1 was not. Under voltage-clamp conditions, given sufficient NH3, AQP8, AQP9, AQP3, and TIP2;1 supported inwards currents carried by NH4+. This conductivity increased as a sigmoid function of external [NH3]: for AQP8 at a bath pH (pHe) of 6.5, the conductance was abolished, at pHe 7.4 it was half maximal and at pHe 7.8 it saturated. NH4+ influx was associated with oocyte swelling. In comparison, native oocytes as well as AQP1 and tip2;1-expressing oocytes showed small currents that were associated with small and even negative volume changes. We conclude that AQP8, AQP9, AQP3, and TIP2;1, apart from being water channels, also support significant fluxes of NH3. These aquaporins could support NH4+ transport and have physiological implications for liver and kidney function.


Ammonia Ammonium Aquaporins Conduction Oocytes Mitochondria Liver Kidney Plant 


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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Lars M. Holm
    • 1
  • Thomas P. Jahn
    • 2
  • Anders L. B. Møller
    • 2
  • Jan K. Schjoerring
    • 2
  • Domenico Ferri
    • 3
  • Dan A. Klaerke
    • 1
  • Thomas Zeuthen
    • 1
  1. 1.Nordic Centre for Water Imbalance Related Disorders. Department of Medical PhysiologyPanum Institute, University of CopenhagenDenmark
  2. 2.Plant Nutrition Laboratory, Department of Agricultural SciencesRoyal Veterinary and Agricultural UniversityDenmark
  3. 3.Department of Zoology, Laboratory of Histology and Comparative AnatomyUniversity of BariItaly

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