Role of gill Na+ + K+ ‐dependent ATPase in acclimation of blue crabs (Callinectes sapidus) to low salinity

Male blue crabs (Callinectes sapidus Rathbun) were acclimated to salinities of 5 and 34 o/oo. Microsomal preparations of gills from these animals possessed Na+ + K+‐dependent ATPase specific activities of 2.33 ± 0.11 and 1.29 ± 0.11 μmoles Pi min per mg protein respectively, implicating the Na+ + K+‐dependent ATPase in the previously recognized ability of the blue crab to maintain high hemolymph sodium levels in low salinity environments. The specific ATPase activity of gill microsomes increased significantly within two and one‐half hours after transfer of crabs from high to low salinity, prior to stabilization of hemolymph sodium levels. The ATPase isolated from crabs acclimated to the two different salinities were both highly sensitive to ouabain, and required either K+ or NH4+ as well as Na+. Sodium‐dependent phosphorylation of both enzymes was blocked by K+ or NH4+. Polyacrylamide gel electrophoresis of the sodium dodecyl sulfate‐solubilized ATPases revealed one band of radioactivity in each case, with an apparent molecular weight of 104,000. The ability of NH4+ to serve as an effective counterion supports the hypothesis that, in blue crabs acclimated to low salinity, Na+ uptake may be coupled to NH4+ excretion.