Abstract
The enzymatic properties of membrane‐bound Na^+^ + K^+^‐ATPase from gills of killifish acclimated to fresh water, to 16‰ sea water, or to 30‰ sea water appear to be identical, indicating that the same enzyme may function to absorb Na^+^ in low salinities and excrete Na^+^ at the gills in high salinities. Ammonium ion is an effective substitute for K^+^: in the ATPase reaction itself, in blocking phosphorylation of the ATPase protein, and in inhibiting the binding of ouabain to the enzyme. The specific activities of the Na^+^ + K^+^‐ATPase in the three different salinities are consistent with the expected Na^+^ pumping rates: higher in fresh water and 30‰ sea water than in 16‰ sea water. Within one‐half hour after transfer of killifish from one salinity to another, gill Na^+^ + K^+^‐ATPase activities reach equilibrium levels. The rapid increase in Na^+^ + K^+^‐ATPase activity in gill microsomes of fish acclimating from fresh water to 30‰ sea water is accompanied by a slow decrease in the number of binding sites for ouabain, supporting the idea that acclimation to short‐term salinity changes may involve modifications in the catalytic rate rather than the number of Na^+^ + K^+^‐ATPase molecules.