The membrane water channel aquaporin (AQP) family is composed of 13 isoforms in mammals, eight of which are reportedly expressed in the kidney: AQP1,2,3,4,6, 7,8, and 11. These isoforms are differentially expressed along the renal tubules and collecting ducts. AQP1 and 7 are distributed in the proximal tubules, whereas AQP2,3, and 4 occur in the collecting duct system. They play important roles in the reabsorption of water and some solutes across the plasma membrane. In contrast to other aquaporins found in the kidney, AQP6,8, and 11 are localized to the cytoplasm rather than to the apical or basolateral membranes. It is therefore doubtful that these isoforms are directly involved in water or solute reabsorption.
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