NH3 and NH 4 + 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.