cDNA cloning of a functional water channel from toad urinary bladder epithelium

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Abstract
A cDNA was cloned from the epithelium of toad (Bufo marinas) urinary bladder, based on homology to the mammalian aquaporins (AQP). The cDNA [947 base pairs (bp), identified as AQP-t1] encoded a 272-amino acid protein with 76% identity to mammalian aquaporin-1 (AQP-1) and 88% identity to frog water channel FA-CHIP. AQP-t1 cDNA was nearly identical to a fragment of a nonfunctional cDNA cloned recently from toad bladder ["AQP-TB"; J. Siner, A. Paredes, C. Hosselet, T. Hammond, K. Strange, and H.W. Harris, Am. J. Physiol. 270 (Cell Physiol. 39): C372-C381, 1996], except for reading frame shifts at bp 253, 264, and 682, two single amino acid deletions, a different 3'-coding sequence downstream from bp 786, and a different 5' sequence upstream from bp 9. Water permeability (Pf) in Xenopus laevis oocytes expressing AQP-t1 cRNA was strongly increased from (0.83 +/- 0.06) x 10(-3) cm/s (water-injected control) to (17 +/- 4) x 10(-3) cm/s, with 80% inhibition by 0.3 mM HgCl2; glycerol and urea permeabilities were not increased. Northern blot analysis showed a single AQP-t1 mRNA of 2.8 kb in eye > lung > urinary bladder > skin > stomach approximately heart, brain, and intestine. AQP-t1 mRNA expression was not changed by a 3-day dehydration of toads or an 8-h stimulation of Pf in isolated bladders by forskolin. These results indicate that the epithelium of toad urinary bladder expresses a functional homologue of AQP-1 and FA-CHIP that is probably not vasopressin regulated.

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