Expression of the Na-K-2Cl Cotransporter in Branchial MitochondrionRich Cells of Mozambique Tilapia (Oreochromis mossambicus) Subjected to Varying Chloride Conditions

The Na/K/2Cl cotransporter (NKCC) plays a critical role of active Cl- transport through the basolateral membranes of mitochondrion-rich (MR) cells in seawater teleosts. Recently, the NKCC was also found in apical membranes of MR cells of freshwater (FW)-acclimated tilapia (Oreochromis mossambicus). In this study, immunocytochemistry and immunoblotting were used to analyze the expression levels of the NKCC in
FW-acclimated tilapia in response to different ambient chloride levels. The amount of branchial NKCC from lowCl-
- (0.002-0.006 mM)-acclimated tilapia was about 3- and 5.5-fold higher than that from normal- (0.49-0.51 mM)
and high-Cl- (9.8-10.2 mM)-acclimated tilapia. With confocal microscopy, the NKCC was localized mainly in the apical membranes of MR cells with Cl-dependent abundances. The staining intensity of the NKCC in individual MR cells was significantly higher in the low-Cl- group than in the normal and high-Cl- groups. Results suggest that the apically localized NKCC is induced by a Cl- deficiency and is probably also involved in the Cl- uptake mechanism of MR cells.