= S. Nielsen
{c}
<S. Nielsen> is the first author of a 1995 study connecting <vasopressin>-regulated water permeability to the movement of water-channel <proteins>. The investigators perfused isolated rat <collecting ducts>, measured their permeability to <water>, and used labeled <electron microscopy> to locate the channels in the same tubules. During <vasopressin> exposure, permeability and channel labeling at the exposed <cell membrane> increased; removing <vasopressin> reversed both changes.<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, perfusion, immunoelectron microscopy and withdrawal experiment]{ref}
The study supported redistribution of channels from internal <vesicles> to the <cell membrane>, rather than treating a physiological response as evidence of new <protein> synthesis. The channel is known as <aquaporin-2>. For a conceptual example, a fixed pool of 100 channels can change from 20 at the surface to 40 at the surface by moving channels from internal stores. The total is unchanged even though the surface pool doubles. This distinction between total abundance, subcellular location and measured function makes trafficking a testable explanation for a rapid hormonal response.
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