The cDNA for the seventh mammalian aquaporin (AQP7) was isolated from rat testis, and its expression was demonstrated at the tail of late spermatids [Ishibashi et al: J Biol Chem 1997;272:20782-20786]. AQP7 is also expressed in the kidney. The localization of AQP7 in the kidney is unknown. We examined the cellular localization of AQP7 in the kidney with Northern blot, reverse transcribed PCR, Western blot and immunohistochemistry in the rat kidney. In Northern blot, AQP7 was expressed in the cortex and the outer medulla but absent in the inner medulla of the kidney. Reverse transcribed PCR of rat nephron segments revealed the selective expression of AQP7 at the proximal straight tubules (PST). Western blot of the membrane fraction of outer medulla revealed a single band of approximately 33 kDa. Immunohistochemistry of the rat kidney showed the selective expression of AQP7 at the brush border membranes of PST (S3 segment). AQP7 is now shown to be localized selectively at the brush border membranes of PST in the rat kidney. The result suggests that AQP7 may function as a pathway for transcellular water transport in PST in concert with more widely expressed AQP1 in proximal tubules. Alternatively, as AQP7 transports urea as well as water, AQP7 may function as a passive urea secretory pathway in this segment and may play a role in the formation and/or maintenance of the medullary urea concentration gradient.
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