Book Description

The renal reabsorption of urea and electrolytes is the primary mechanism underlying the osmoregulatory strategies of marine elasmobranchs. However, the sites and mechanisms by which these solutes are reabsorbed have yet to be elucidated. Based on the finding that the fractional reabsorption of urea is greater than 90%, we hypothesized that solute reabsorption occurs at multiple sites along the nephron of elasmobranchs. Histological techniques were utilized to map the tubular segments that comprise the nephron of the stingray, Dasyatis sabina. Immunohistochemistry was then utilized to localize facilitated urea transporter isoforms as well as Na-K+-ATPase within the kidney. Since exposure to low salinity has been shown to induce an increase in absolute reabsorption, the effect of exposure to low salinity on the abundance and localization of these membrane transporters was also determined. Kidneys were obtained from stingrays either maintained at control salinity (850 mOsmol/kg H2O) or subjected to a 50% decrease in salinity over 3 days. Immunohistochemical localization of strUT-1 was limited the second loop in the bundle zone. In contrast, staining for strUT-2 was observed in the Proximal-II and Distal-I segments within the bundle zone and in all segments of the sinus zone except Proximal IV. Na+-K+-ATPase was localized to Neck-II, Proximal-II, Intermediate-x, and Distal-I segments within the bundle zone and to Intermediate-I in the sinus zone. The abundance of the membrane transporters in whole tissue homogenates was not different between rays in control or low salinity. In contrast, the abundance of these transporters in the membrane fraction was significantly higher in rays subjected to low salinity. The findings suggest that in D. sabina, urea reabsorption occurs through facilitated urea transporters at multiple sites along the nephron. The increase in absolute solute reabsorption in low salinity may be due to the shuttling of membrane transport proteins to the plasma membrane.