Gill Na+-K+-2Cl- cotransporter abundance and location in Atlantic salmon: Effects of seawater and smolting

Na⁺-K⁺-2Cl⁻cotransporter abundance and location was examined in the gills of Atlantic salmon (Salmo salar) during seawater acclimation and smolting. Western blots revealed three bands centered at 285, 160, and 120 kDa. The Na⁺-K⁺-2Cl⁻cotransporter was colocalized with Na⁺-K⁺-ATPase to chloride cells on both the primary filament and secondary lamellae. Parr acclimated to 30 parts per thousand seawater had increased gill Na⁺-K⁺-2Cl⁻ cotransporter abundance, large and numerous Na⁺-K⁺-2Cl⁻ cotransporter immunoreactive chloride cells on the primary filament, and reduced numbers on the secondary lamellae. Gill Na⁺-K⁺-2Cl⁻ cotransporter levels were low in presmolts (February) and increased 3.3-fold in smolts (May), coincident with elevated seawater tolerance. Cotransporter levels decreased below presmolt values in postsmolts in freshwater (June). The size and number of immunoreactive chloride cells on the primary filament increased threefold during smolting and decreased in postsmolts. Gill Na⁺-K⁺-ATPase activity and Na⁺-K⁺-2Cl⁻ cotransporter abundance increased in parallel during both seawater acclimation and smolting. These data indicate a direct role of the Na⁺-K⁺-2Cl⁻ cotransporter in salt secretion by gill chloride cells of teleost fish.