Gill-specific (Na+, K+)-ATPase activity and α-subunit mRNA expression during low-salinity acclimation of the ornate blue crab Callinectes ornatus (Decapoda, Brachyura)

We evaluate (Na+, K+)-ATPase activity, and protein and gene expression of the α-subunit in posterior gills 6 and 7 of Callinectes ornatus, a euryhaline crab, during a 10-day acclimation period from seawater (33‰ S) to low salinity (21‰ S). (Na+, K+)-ATPase activity decreased within 1 h after transfer to 21‰ S, values recovering by 24 h and attaining a maximum of ≈ 180 nmol Pi min− 1 mg− 1 after 10 days (≈ 2.5-fold increase). (Na+, K+)-ATPase activity is ≈ 1.5-fold greater in gill 6 than in gill 7, independently of salinity. Relative expression of (Na+, K+)-ATPase α-subunit mRNA increased in both gills within 1- to 2-h exposure to low salinity, reaching an ≈ 8-fold maximum after 24-h exposure, decreasing slightly by 10 days acclimation to low salinity. This increase in α-subunit mRNA expression may underpin the increased (Na+, K+)-ATPase activity seen after 10 days acclimation to low salinity. Enzyme affinity for ATP was greater in gill 6 than in gill 7, in contrast to ouabain affinity that was greater in gill 7. Western blotting analysis identified a single immunoreactive band against the (Na+, K+)-ATPase α-subunit with an Mr of ≈ 105 kDa, independently of gill number and low salinity acclimation. Despite these differences, gills 6 and 7 appear to perform similar functions in salt uptake from the dilute medium. The partial cDNA sequence obtained for the gill (Na+, K+)-ATPase of C. ornatus (GenBank deposit KF056804) showed 97 to 91% identities with similar sequences from other portunid crab gills. The regulation of gill (Na+, K+)-ATPase activity during acclimation to low salinity is discussed.