= Vasopressin
<Vasopressin>, also called antidiuretic hormone, is a <peptide hormone> made by <neurons> in the <hypothalamus> and released into <blood> through the <posterior pituitary>. An important action is to conserve <water> by increasing its reabsorption in the <kidney>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§17.3, Posterior Pituitary and Antidiuretic Hormone]{ref}
In the <collecting duct>, <vasopressin> regulates the abundance of <aquaporin-2> water channels in the <cell membrane> facing the fluid that will become urine. More channels increase <membrane permeability> to <water>, allowing more <water> to return toward the <blood> by <osmosis>. This changes a route through the membrane rather than making the channel an active water pump.<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, water reabsorption and reversible channel redistribution]{ref}
A revealing 1995 experiment perfused isolated rat <collecting ducts> and combined measured water flow with <electron microscopy> locating labeled channels. During <vasopressin> exposure, water permeability and channel abundance at the surface increased together. Withdrawal reversed both changes. Measuring function and channel location in the same tubules connected the physiological response to movement of channels from internal <vesicles> to the <cell membrane>.<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, experimental design and exposure/withdrawal results]{ref}
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