Localization of Facilitated Urea Transporters to Tubular Segments in the Bundle and Sinus Zones of the Kidney of the Euryhaline Stingray, Dasyatis sabina

Renal urea reabsorption underpins the osmoregulatory strategy of elasmobranch fishes (sharks, skates and rays). Two urea transporter isoforms (strUT‐1 and strUT‐2) have been identified from the kidney of the Atlantic stingray. strUT‐1 is a carboxy extended form of strUT‐2. We hypothesized that these isoforms play specific roles in urea absorption and are expressed at different tubular sites. To test this hypothesis we used immunohistochemistry to localize strUT expression along the nephron of Atlantic stingrays. Five micron serial paraffin sections were cut from paraformaldehyde fixed kidneys of rays maintained in harbor water (850 mOsmol/kg H 2 O). Sections were incubated with affinity‐purified antibodies that recognized either the C‐terminal of strUT‐1 or a sequence common to both strUT‐1 and strUT‐2. In the presence of the strUT‐1 antibody, positively stained tubular segments were restricted to the last loop in the bundle zone (the intermediate segments). In contrast, numerous positively stained tubular segments were observed in both bundle and sinus zones of sections incubated with the common antibody. We conclude that 1) the strUT‐2 isoform is widely expressed along the nephron of the Atlantic stingray kidney and is the primary isoform involved in urea reabsorption, and 2) the localization of strUT‐1 to intermediate segments in the bundle zone suggests that it plays a distinct role in urea reabsorption.