Characterization of Na/K-ATPase in Macrobrachium rosenbergii and the effects of changing salinity on enzymatic activity

A ouabain-sensitive Na/K-ATPase kinetic assay system based on the hydrolysis of ATP and the oxidation of NADH was adapted in order to characterize enzymatic activity in gills and examine the effects of changing salinity in Macrobrachium rosenbergii. Maximum inhibition by ouabain occurred at a concentration of 1.4 mM, and the K(m) of the reaction was 0.2 mM. In a first experiment, animals were acclimated to freshwater, 1/3 seawater, 2/3 seawater and full seawater for up to 1 week. Na/K-ATPase activity in front gills was 1. 62+/-0.19 micromol ADP/mg protein per h in freshwater, and was seen to increase slightly in 1/3 seawater (1.88+/-0.19 micromol ADP/mg protein per h) and 2/3 seawater (2.09+/-0.24 micromol ADP/mg protein per h), decreasing slightly in full seawater (1.92+/-0.43 micromol ADP/mg protein per h); however, differences were not significant. Back gills showed slightly higher levels, and a similar pattern of Na/K-ATPase activity. In a second experiment, animals were acclimated to 1/3 seawater and 2/3 seawater, and then transferred to freshwater. However, no changes in activity were seen, indicating that exposure to dilute media did not effect enzymatic activity. Whereas Na/K-ATPase is important in osmoregulatory function in marine euryhaline crustaceans, it may not play a significant role in adaptation in freshwater crustaceans that inhabit a more narrow range of salinities.