Expression of sodium/hydrogen exchanger 3 and cation-chloride cotransporters in the kidney of Japanese eel acclimated to a wide range of salinities

Reabsorption of monovalent ions in the kidney is essential for adaptation to freshwater and seawater in teleosts. To assess a possible role of Na+/H+ exchanger 3 (NHE3) in renal osmoregulation, we first identified a partial sequence of cDNA encoding NHE3 from the Japanese eel kidney. For comparison, we also identified cDNAs encoding kidney specific Na+-K+-2Cl(-) cotransporter 2 (NKCC2 alpha) and Na+-Cl- cotransporter (NCC alpha). In eels acclimated to a wide range of salinities from deionized freshwater to full-strength seawater, the expression of NHE3 in the kidney was the highest in eel acclimated to full-strength seawater. Meanwhile, the NCC alpha expression exhibited a tendency to increase as the environmental salinity decreased, whereas the NKCC2 alpha expression was not significantly different among the experimental groups. Immunohistochemical studies showed that NHE3 was localized to the apical membrane of epithelial cells composing the second segments of the proximal renal tubule in seawater-acclimated eel. Meanwhile, the apical membranes of epithelial cells in the distal renal tubule and collecting duct showed more intense immunoreactions of NKCC2 alpha and NCC alpha, respectively, in freshwater eel than in seawater eel. These findings suggest that renal monovalent-ion reabsorption is mainly mediated by NKCC2 alpha. and NCC alpha in freshwater eel and by NHE3 in seawater eel. (C) 2012 Elsevier Inc. All rights reserved.