Expression and Localization of the Water Channel, Aquaporin, in the Rat Urinary Tract

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Water is filtered from the circulation into urine in the glomerulus and is reabsorbed mostly in the proximal tubules and descending loop of Henle and partially in the collecting ducts through several types of water channels, the aquaporin (AQP1, AQP2, AQP3, AQP4 and AQP7). Since AQP2, AQP3, and AQP4 have been identified in the collecting ducts, which originate from urinary buds during the development of the kidney as the ureter, bladder, and urethra, it is presumable for these aquaporins to locate in the urothelium covering the luminal surface of these tissues. However, it has been suggested that the urothelium is impermeable to water and/or solutes. Hence, we examined the expression and localization of aquaporin family members (AQP1, AQP2, AQP3 and AQP4) in the urinary tract in rats. As a consequence, the expression of AQP1 and AQP3 mRNA were demonstrated in the ureter, bladder, and urethra by ribonuclease protection assay. Western blotting showed distinct bands for both AQP1 and AQP3 in the ureter, bladder, and urethra. By immunohistochemistry, AQP1 was localized in the endothelial cells of blood vessels and AQP3 in intermediate and basal cell layers of the urothelium in the ureter, bladder and urethra. No AQP2 or AQP4 mRNA expression or immunolocalization of AQP2 and AQP4 were detected in these tissues. AQP2 expression was enhanced hi the kidney after dehydration; however, the expression was unchanged in the ureter, bladder, and urethra. The current findings suggest that water crosses the urothelium by utilizing AQP3 water channels (transcellular pathway) in the ureter, bladder, and urethra, but the expression of AQP3 is not regulated by a dehydrated condition.
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Expression and Localization of the Water Channel, Aquaporin, in the Rat Urinary Tract

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