Evidence from knockout mice against physiologically significant aquaporin 8-facilitated ammonia transport

Abstract

Aquaporin (AQP)8-facilitated transport of NH3 has been suggested recently by increased NH3 permeability in Xenopus oocytes and yeast expressing human or rat AQP8. We tested the proposed roles of AQP8-facilitated NH3 transport in mammalian physiology by comparative phenotype studies in wild-type vs. AQP8-null mice. AQP8-facilitated NH3 transport was confirmed in mammalian cell cultures expressing rat or mouse AQP8, in which the fluorescence of a pH-sensing yellow fluorescent protein was measured in response to ammonia (NH3/NH4+) gradients. Relative AQP8 single-channel NH3-to-water permeability was ∼0.03. AQP8-facilitated NH3 and water permeability in a native tissue was confirmed in membrane vesicles isolated from testes of wild-type vs. AQP8-null mice, in which BCECF was used as an intravesicular pH indicator. A series of in vivo studies were done in mice, including 1 ) serum ammonia measurements before and after ammonia infusion, 2 ) renal ammonia clearance, 3 ) colonic ammonia absorption, and 4 ) liver ammonia accumulation and renal ammonia excretion after acute and chronic ammonia loading. Except for a small reduction in hepatic ammonia accumulation and increase in ammonia excretion in AQP8-null mice loaded with large amounts of ammonia, there were no significant differences in wild-type vs. AQP8-null mice. Our results support the conclusion that AQP8 can facilitate NH3 transport but provide evidence against physiologically significant AQP8-facilitated NH3 transport in mice.