Salinity-dependent expression of a Na+, K+, 2Cl- cotransporter in gills of the brackish medaka Oryzias dancena: a molecular correlate for hyposmoregulatory endurance.

This study used the brackish medaka (Oryzias dancena) to characterize Na+, K+, 2Cl- cotransporter (NKCC) expression from the genetic to cellular level in gills. Using RT-PCR to survey tissue distribution of nkcc1a, 1b, and 2, we report that gills of brackish medaka prominently express Odnkcc1a. The full-length cDNA of Odnkcc1a was cloned from gill tissue. In situ hybridization indicates that Odnkcc1a was localized to mitochondrion-rich (MR) cells. Higher mRNA levels of Odnkcc1a were found in gills from seawater (SW) and brackish water (BW) medaka when compared to freshwater (FW) fish. Furthermore, higher amounts of NKCC1a-like protein were detected by the monoclonal antibody in gills of SW and BW medaka compared to FW medaka. Double immunofluorescence staining revealed that NKCC1a-like protein colocalizes with Na+, K+-ATPase on the basolateral membrane of MR cells in BW and SW fish. In addition, transfer of brackish medaka from SW to FW revealed that expression of NKCC1a-like protein in gills was retained until 7days, which is a likely mechanism for maintaining hyposmoregulatory endurance. The study illustrates salinity-dependent expression of NKCC1a in branchial MR cells from brackish medaka and suggests a critical role for NKCC1a in hyposmoregulatory endurance of this fish.