= Aquaporin-2
<Aquaporin-2> is a water-channel <membrane protein> in <kidney> <collecting duct> cells. <Vasopressin> regulates how much of it is present in the apical <cell membrane>, changing the pathway available for <water> reabsorption. <Aquaporin-2> supports passive <water> movement driven by the surrounding osmotic conditions.<Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane by S. Nielsen and others>[1995, abstract: collecting-duct water channels, membrane localization and osmotic permeability]{ref}
In isolated perfused rat <collecting ducts>, researchers measured <water> permeability and then fixed the same tubules for <electron microscopy> with antibody labelling. <Vasopressin> increased apical channel labelling and measured permeability; withdrawal reversed both. Coupling a functional measurement to localization supported regulated transfer of channel-bearing <vesicles> to and from the surface.<Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane by S. Nielsen and others>[Abstract: before, during and after hormone exposure]{ref}
A toy redistribution makes the distinction clear: of 100 channel units, move 20 from <vesicles> into a surface pool initially containing 20. Surface abundance doubles to 40 while total abundance stays 100. This illustrates how trafficking can produce a rapid response; it is not a measured fold change in channel number or <water> flux.
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