Salinity dependence of the ouabain-insensitive Mg2+-dependent Na+-ATPase in gills of rainbow trout (Oncorhynchus mykiss Walbaum) adapted to fresh and brackish water

1. 1. Rainbow trout adapated to freshwater (FW) and brackish water (BW) exhibit an ouabain-insensitive gill Na+-ATPase activity that roughly reproduces the enzyme features previously described in osmoregulatory organs from euryhaline teleosts. 2. 2. The Na+-ATPase activity is higher in FW than in BW, at odds with the previously characterized coexistent (Na+ + K+)-ATPase. 3. 3. The enzyme response to the assay pH, MgATP and Na+ concentrations is quite similar in the two habitats. The Na+-ATPase is maximally activated at pH 5.2, 6 mM MgATP and 40–60 mM Na+, in FW and BW, respectively, with Michaelian activation kinetics by MgATP and Na+ in both habitats. 4. 4. The possible role of the Na+-ATPase in the active salt uptake from the environment is discussed and related to teleost osmoregulation under different salinity conditions.