We analyzed the modulation by exogenous FXYD2 peptide and by endogenous protein kinases A and C, and Ca2+-calmodulin-dependent kinase, of gill (Na+, K+)-ATPase activity in the semi-terrestrial mangrove crab Ucides cordatus after 10-days acclimation to different salinities. Osmotic and ionic regulatory ability and gill (Na+, K+)-ATPase activity also were evaluated. (Na+, K+)-ATPase activity is stimulated by exogenous pig kidney FXYD2 peptide, while phosphorylation by endogenous protein kinases A and C and Ca2+/calmodulin-dependent kinase inhibits activity. Stimulation by FXYD2 and inhibition by protein kinase C and Ca2+/calmodulin-dependent kinase are salinity-dependent. This is the first demonstration of inhibitory phosphorylation of a crustacean (Na+, K+)-ATPase by Ca2+/calmodulin-dependent kinase. At low salinities, the (Na+, K+)-ATPase exhibited a single, low affinity ATP-binding site that showed Michaelis-Menten behavior. Above 18‰S, a second, cooperative, high affinity ATP-binding site appeared, corresponding to 10-20% of total (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 isonatremic at elevated salinity. (Na+, K+)-ATPase activity was greatest in isosmotic crabs (26‰S), decreasing notably at 35‰S and also diminishing progressively from 18to 2‰S. Hyper-osmoregulation in U. cordatus showed 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 enzymatic and osmoregulatory adjustments that maintain its osmotic homeostasis in a challenging, mangrove forest environment.
Keywords: Ca(2+)/calmodulin dependent protein kinase; Crab gill microsomal (Na(+), K(+))-ATPase; FXYD2 peptide; Intracellular signaling cascade; Protein kinases a and C; Salinity acclimation.
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