Osmotic and ionic regulation, and kinetic characteristics of a posterior gill (Na + , K + )-ATPase from the blue crab Callinectes danae on acclimation to salinity challenge

Salt tolerance reflects ecophysiological adaptation, and the wide-ranging distribution of the Brachyura mirrors their ability to adjust body fluid concentrations. The gill (Na + , K + )-ATPase underpins such hyper/hypo-regulatory mechanisms. We evaluate osmotic and chloride regulation in Callinectes danae after 10 days acclimation to a wide salinity range (5–50 ‰S), accompanying alterations in hemolymph osmolality and [Cl − ] during hypo- (15 ‰S) or hyper- (45 ‰S) osmotic challenge. Further, we investigate posterior gill (Na + , K + )-ATPase kinetics, α-subunit immunolocalization and its mRNA and protein expression (15, 30 and 40 or 45 ‰S). The crab is a moderate, asymmetrical hyper/hypo-osmoregulator but is a strong, asymmetrical hyper/hypo-chloride regulator. Hyper-regulation at low salinity is sustained by a threefold increase in (Na + , K + )-ATPase activity, a 3.5-fold increase in α-subunit mRNA expression and 1.6-fold increase in protein expression. α-Subunit signal is highest in 15 ‰S-acclimated crabs, and is uniformly distributed throughout the ionocytes and pillar cells. Activity in 30- and 40% S-acclimated crabs is similar. Affinity for ATP and Na + increases on high salinity acclimation but decreases for ouabain. K + apparent affinity is independent of salinity, while that for Mg 2+ decreases and for NH 4 + increases with increasing salinity. A high-affinity ATP-binding site disappears on acclimation at any salinity. F O F 1 - and Na + - or K + -ATPase activities decrease with increased salinity. Hemolymph chloride hypo-regulation depends little on gill (Na + , K + )-ATPase activity. Hyper-, and hypo-osmotic and ionic regulatory capabilities in C. danae are intricate physiological processes underpinned by multifarious gill (Na + , K + )-ATPase kinetics and altered mRNA and protein expressions.