Gill Na+, K+-ATPase in a series of hyper-regulating gammarid amphipods. Enzyme characterisation and the effects of salinity acclimation

The enzyme Na+, K+-ATPase was investigated in the gills of selected hyper-regulating gammarid amphipods. Gill Na+, K+-ATPase was characterised with respect to the main cation and co-factor concentrations for the freshwater amphipod Gammarus pulex. The optimum cation and co-factor concentrations for maximal gill Na+, K+-ATPase activity in G. pulex were 100 mM Na+, 15 mM K+, 15 mM Mg2+ and 5 mM ATP, at pH 7.2. The effects of salinity acclimation on gill Na+, K+-ATPase activity and haemolymph sodium concentrations was investigated in selected gammarid amphipods from different salinity environments. Maximal enzyme activity occurred in all gammarids when acclimated to the most dilute media. This maximal activity coincided with the largest sodium gradient between the haemolymph and the external media. As the haemolymph/medium sodium gradient decreased, a concomitant reduction in gill Na+, K+-ATPase activity occurred. This implicates the involvement of gill Na+, K+-ATPase in the active uptake of sodium from dilute media in hyper-regulating gammarids.