Molecular and functional characterization of a urea transporter from the kidney of the Atlantic stingray

In general, marine elasmobranch fishes ( sharks, skates, and rays) maintain body fluid osmolality above seawater, principally by retaining large amounts of urea. Maintenance of the high urea concentration is due in large part to efficient renal urea reabsorption. Regulation of renal urea reabsorption also appears to play a role in maintenance of fluid homeostasis of elasmobranchs that move between habitats of different salinities. We identified and cloned a novel 2.7- kb cDNA from the kidney of the euryhaline Atlantic stingray Dasyatis sabina ( GenBank accession no. AF443781). This cDNA putatively encoded a 431- amino acid protein ( strUT- 1) that had a high degree of sequence identity ( 71%) to the shark kidney facilitated urea transporter ( UT). However, the predicted COOH- terminal region of strUT- 1 appears to contain an additional sequence that is unique among cloned renal UTs. Injection of strUT- 1 cRNA into Xenopus oocytes induced a 33- fold increase in [ C-14] urea uptake that was inhibited by phloretin. Four mRNA bands were detected in kidney by Northern blot: a transcript at 2.8 kb corresponding to the expected size of strUT- 1 mRNA and bands at 3.8, 4.5, and 5.5 kb. Identification of a facilitated UT in the kidney of the Atlantic stingray provides further support for the proposal that passive mechanisms contribute to urea reabsorption by elasmobranch kidney.