NH3 and NH4+ permeability in aquaporin-expressing Xenopus oocytes

• Published in: Pflügers Archiv 2005-09, Vol.450 (6), p.415-428
• Main Authors: Holm, Lars M, Jahn, Thomas P, Møller, Anders L B, Schjoerring, Jan K, Ferri, Domenico, Klaerke, Dan A, Zeuthen, Thomas
• Format: Article
• Language: English
• Subjects: Ammonia - metabolism; Ammonium Chloride - metabolism; Animals; Aquaporins - biosynthesis; Female; Formamides - metabolism; Formamides - pharmacology; Hydrogen-Ion Concentration; Oocytes - drug effects; Oocytes - physiology; Patch-Clamp Techniques; Permeability; Permeability - drug effects; Xenopus
• ISSN: 0031-6768; 1432-2013
• DOI: 10.1007/s00424-005-1399-1

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 (pH(e)) of 6.5, the conductance was abolished, at pH(e) 7.4 it was half maximal and at pH(e) 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.