Effect of salinity on modulation by ATP, protein kinases and FXYD2 peptide of gill (Na+, K+)-ATPase activity in the swamp ghost crab Ucides cordatus (Brachyura, Ocypodidae)

Abstract

The gill (Na+, K+)-ATPase is the main enzyme that underpins osmoregulatory ability in crustaceans that occupy biotopes like mangroves, characterized by salinity variation. We evaluated osmotic and ionic regulatory ability in the semi-terrestrial mangrove crab Ucides cordatus after 10-days acclimation to different salinities. We also analyzed modulation by exogenous FXYD2 peptide and by endogenous protein kinases A and C, and Ca2+- calmodulin-dependent kinase of (Na+, K+)-ATPase activity. Hemolymph osmolality was strongly hyper-/hypo-regulated in crabs acclimated at 2 to 35 ‰S. Cl- was well hyper-/hypo- regulated although Na+ much less so, becoming iso-natremic at high salinity. (Na+, K+)- ATPase activity was greatest in isosmotic crabs (26 ‰S), diminishing progressively from 18 and 8 ‰S (≈0.5 fold) to 2 ‰S (0.04-fold), and decreasing notably at 35 ‰S (0.07-fold). At low salinity, the (Na+, K+)-ATPase exhibited a low affinity ATP-binding site that showed Michaelis-Menten behavior. Above 18 ‰S, an additional, high affinity ATP-binding site, corresponding to 10-20% of total (Na+, K+)-ATPase activity appeared. Activity is stimulated by exogenous pig kidney FXYD2 peptide, while endogenous protein kinases A and C and Ca2+/calmodulin-dependent kinase all inhibit activity. This is the first demonstration of inhibitory phosphorylation of a crustacean (Na+, K+)-ATPase by Ca2+/calmodulin-dependent kinase. Curiously, hyper-osmoregulation in U. cordatus shows little dependence on gill (Na+, K+)-ATPase activity, suggesting a role for other ion transporters. These findings reveal that the salinity acclimation response in U. cordatus consists of a suite of osmoregulatory and enzymatic adjustments that maintain its osmotic homeostasis in a challenging, mangrove forest environment.