Long-term exposure of the freshwater shrimp Macrobrachium olfersii to elevated salinity: Effects on gill (Na+,K+)-ATPase α-subunit expression and K+-phosphatase activity

The kinetic properties of a microsomal gill (Na+,K+)-ATPase from the freshwater shrimp, Macrobrachium olfersii, acclimated to 21‰ salinity for 10 days were investigated using the substrate p-nitrophenylphosphate. The enzyme hydrolyzed this substrate obeying cooperative kinetics at a rate of 123.6 ± 4.9 U mg− 1 and K0.5 = 1.31 ± 0.05 mmol L− 1. Stimulation of K+-phosphatase activity by magnesium (Vmax = 125.3 ± 7.5 U mg− 1; K0.5 = 2.09 ± 0.06 mmol L− 1), potassium (Vmax = 134.2 ± 6.7 U mg− 1; K0.5 = 1.33 ± 0.06 mmol L− 1) and ammonium ions (Vmax = 130.1 ± 5.9 U mg− 1; K0.5 = 11.4 ± 0.5 mmol L− 1) was also cooperative. While orthovanadate abolished p-nitrophenylphosphatase activity, ouabain inhibition reached 80% (KI = 304.9 ± 18.3 μmol L− 1). The kinetic parameters estimated differ significantly from those for freshwater-acclimated shrimps, suggesting expression of different isoenzymes during salinity adaptation. Despite the ≈2-fold reduction in K+-phosphatase specific activity, Western blotting analysis revealed similar α-subunit expression in gill tissue from shrimps acclimated to 21‰ salinity or fresh water, although expression of phosphate-hydrolyzing enzymes other than (Na+,K+)-ATPase was stimulated by high salinity acclimation.