Salinity-dependent modulation by protein kinases and the FXYD2 peptide of gill (Na+, K+)-ATPase activity in the freshwater shrimp Macrobrachium amazonicum (Decapoda, Palaemonidae)

The geographical distribution of aquatic crustaceans is determined by ambient factors like salinity that modulate their biochemistry, physiology, behavior, reproduction, development and growth. We investigated the effects of exogenous pig FXYD2 peptide and endogenous protein kinases A and C on gill (Na+, K+)-ATPase activity, and characterized enzyme kinetic properties in a freshwater population of Macrobrachium amazonicum in fresh water (<0.5 parts per thousand salinity) or acclimated to 21 parts per thousand S. Stimulation by FXYD2 peptide and inhibition by endogenous kinase phosphorylation are salinity-dependent. While without effect in shrimps in fresh water, the FXYD2 peptide stimulated activity in salinity-acclimated shrimps by approximate to 50 %. PKA-mediated phosphorylation inhibited gill (Na+, K+)-ATPase activity by 85 % in acclimated shrimps while PKC phosphorylation markedly inhibited enzyme activity in freshwater- and salinity-acclimated shrimps. The (Na+, K+)-ATPase in salinity-acclimated shrimp gills hydrolyzed ATP at a V-max of 54.9 +/- 1.8 nmol min(-1) mg(-1) protein, corresponding to approximate to 60 % that of freshwater shrimps. Mg2+ affinity increased with salinity acclimation while K+ affinity decreased. (Ca2+, Mg2+)-ATPase activity increased while V(H+)- and Na+- or K+-stimulated activities decreased on salinity acclimation. The 120kDa immunoreactive band expressed in salinity-acclimated shrimps suggests nonspecific alpha-subunit phosphorylation by PKA and/or PKC. These alterations in (Na+, K+)-ATPase kinetics in salinity-acclimated M. amazonicum may result from regulatory mechanisms mediated by phosphorylation via protein kinases A and C and the FXYD2 peptide rather than through the expression of a different alpha-subunit isoform. This is the first demonstration of gill (Na+, K+)-ATPase regulation by protein kinases in freshwater shrimps during salinity challenge.

Automated summary

This study investigated the regulation of gill (Na+, K+)-ATPase activity in freshwater crustaceans, finding that protein kinases A and C, as well as FXYD2 peptide, play a role in modulating enzyme activity through phosphorylation mechanisms instead of through the expression of different isoforms.