NH3 and NH4+ permeability in aquaporin-expressing Xenopus oocytes.
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.
Holm
Lars M
LM
Nordic Centre for Water Imbalance Related Disorders. Department of Medical Physiology, Panum Institute, University of Copenhagen, 2200N, Denmark.
Jahn
Thomas P
TP
Møller
Anders L B
AL
Schjoerring
Jan K
JK
Ferri
Domenico
D
Klaerke
Dan A
DA
Zeuthen
Thomas
T
D000641:Ammonia / Q000378:metabolism*; D000643:Ammonium Chloride / Q000378:metabolism*; D000818:Animals; D020346:Aquaporins / Q000096:biosynthesis*; D005260:Female; D005559:Formamides / Q000378:metabolism / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010539:Permeability / Q000187:drug effects; D014981:Xenopus
D020346:Aquaporins; D005559:Formamides; D000643:Ammonium Chloride; C031066:formamide; D000641:Ammonia