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)
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 {per thousand}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 {per thousand}S), diminishing progressively from 18 and 8 {per thousand}S ({approx}0.5 fold) to 2 {per thousand}S (0.04-fold), and decreasing notably at 35 {per thousand}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 {per thousand}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. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=138 HEIGHT=200 SRC="FIGDIR/small/058297v1_ufig1.gif" ALT="Figure 1"> View larger version (69K): org.highwire.dtl.DTLVardef@19a35f2org.highwire.dtl.DTLVardef@14bd0d3org.highwire.dtl.DTLVardef@7adaa5org.highwire.dtl.DTLVardef@1a88e10_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIGill (Na+, K+)-ATPase activity is greatest in isosmotic crabs, diminishing in lower and higher salinities. C_LIO_LIA high affinity ATP-binding site (10-20% of total activity) is exposed above 18 {per thousand}S. C_LIO_LIExogenous FXYD2 peptide stimulates activity; endogenous PKA, PKC and CaMK inhibit activity. C_LIO_LIFirst demonstration of inhibitory phosphorylation of crustacean (Na+, K+)-ATPase by CaMK. C_LIO_LIHyper-osmoregulation shows little dependence on (Na+, K+)-ATPase activity. C_LI