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Unformatted text preview: V1
V2 receptor action Aquaporin Side view of AQP showing the pore profile (turquoise dots) and the residues that line the pore (opaque ball-and-stick structures). The extracellular vestibule is above; cytoplasmic, below. The pinched-in area with the highest concentration of turquoise dots is the constriction region. More than ten different aquaporins have been found in human body. They form tetramers in the cell membrane, and facilitate the transport of water and, in some cases, other small solutes across the membrane.
However, the water pores are completely impermeable to charged ions such as protons.
A remarkable property that is critical for the conservation of membrane's electrochemical potential, but paradoxical at the same time, since protons can usually be transferred readily through water molecules.
Water molecules passing the channel are forced, by the protein's electrostatic forces, to flip at the center of the channel, thereby breaking the alternative donor-acceptor arrangement that is necessary for proton translocation. Aquaporin-2 in the absence of vasopressin, resides in membrane vesicles in the cytoplasm. Binding of vasopressin to its V2 receptor stimulates transcription of the aquaporin-2 gene, and insertion of the intracellular pool of aquaporin-2 into the apical membrane. The cell is now able to efficiently take up water from the lumen of the duct. Aquaporin-3 is constitutively expressed in the basolateral membrane. When water floods into the cell through aquaporin-2 channels, it can rapidly exit the cell through the aquaporin-3 channels and flow into blood. SagiKal
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- Spring '09