Ammonia and Urea Permeability of Mammalian Aquaporins
The humanaquaporins,AQP3,AQP7, AQP8,AQP9, and possibly AQP10, are permeable to ammonia, and AQP7, AQP9, and possibly AQP3, are permeable to urea. In humans, these aquaporins supplement the ammonia transport of the Rhesus (Rh) proteins and the urea transpo
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Models of Ammonia Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Transport Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 How to Distinguish between Transport Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Experimental Evidence for Ammonia Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Permeability of NH3 , Quantitative Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Permeability of NH3 , Qualitative Estimates from Reflection Coefficients . . . . . . . . 3.3 In which Form does Ammonia Permeate? Evidence from Electrophysiology . . . . . 4 Molecular Structure and Ammonia Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 The Molecular Signature of Ammonia-Permeable Aquaporins . . . . . . . . . . . . . . . . . 4.2 Single Mutations can Abolish or Induce Ammonia Permeability in Aquaporins . . 5 Physiological Relevance of Ammonia Permeability in Aquaporins . . . . . . . . . . . . . . . . . . . 5.1 Lessons from Simpler Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Tissue Distribution of Ammonia Permeable Aquaporins in Mammals . . . . . . . . . . . 5.3 Colocalization of Aquaporins, Rh Proteins, and Urea Transporters . . . . . . . . . . . . . 6 General Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Abstract The human aquaporins, AQP3, AQP7, AQP8, AQP9, and possibly AQP10, are permeable to ammonia, and AQP7, AQP9, and possibly AQP3, are permeable to urea. In humans, these aquaporins supplement the ammonia transport of the Rhesus (Rh) proteins and the urea transporters (UTs). The mechanism by which ammonium is transported by aquaporins is not fully resolved. A comparison of transport equations, models, and experimental data shows that ammonia is transported in its neutral form, NH3 . In the presence of NH3 , the aquaporin stimulates H+ transport. Consequently, this transport of H+ is only significant at alkaline pH. It is debated whether the T. Zeuthen (¬) Nordic Center for Water Imbalance Related Disorders. Institute of Cellular and Molecular Medicine, The Panum Institute, Blegdamsvej 3C, University of Copenhagen, DK-2200N, Denmark [email protected] E. Beitz (ed.), Aquaporins, Handbook of Experimental Pharmacology 190, c Springer-Verlag Berlin Heidelberg 2009
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H+ ion passes via the aquaporin or by some external route; the investigation of this problem requires the aquapo
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