Hemolymph ionic regulation and adjustments in gill (Na+, K+)-ATPase activity during salinity acclimation in the swimming crab Callinectes ornatus (Decapoda, Brachyura)

We evaluate hemolymph osmotic and ionic regulatory abilities and characterize a posterior gill microsomal (Na+, K+)-ATPase from the marine swimming crab, Callinectes ornatus, acclimated to 21‰ or 33‰ salinity. C. ornatus is isosmotic after acclimation to 21‰ but is hyposmotic at 33‰ salinity; hemolymph ions do not recover initial levels on acclimation to 21‰ salinity but are anisoionic compared to ambient concentrations, revealing modest regulatory ability. NH4+ modulates enzyme affinity for K+, which increases 187-fold in crabs acclimated to 33‰ salinity. The (Na+, K+)-ATPase redistributes into membrane fractions of different densities, suggesting that altered membrane composition results from salinity acclimation. ATP was hydrolyzed at maximum rates of 182.6 ± 7.1 nmol Pi min− 1 mg− 1 (21‰) and 76.2 ± 3.5 nmol Pi min− 1 mg− 1 (33‰), with little change in KM values (≈ 50 µmol L− 1). K+ together with NH4+ synergistically stimulated activity to maximum rates of ≈ 240 nmol Pi min− 1 mg− 1. KI values for ouabain inhibition (≈ 110 µmol L− 1) decreased to 44.9 ± 1.0 µmol L− 1 (21‰) and 28.8 ± 1.3 µmol L− 1 (33‰) in the presence of both K+ and NH4+. Assays employing various inhibitors suggest the presence of mitochondrial F0F1-, and K+- and V-ATPase activities in the gill microsomes.