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 parts per thousand or 33 parts per thousand salinity. C ornatus is isosmotic after acclimation to 21 parts per thousand but is hyposmotic at 33 parts per thousand salinity; hemolymph ions do not recover initial levels on acclimation to 21 parts per thousand 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 parts per thousand) and 76.2 +/- 3.5 nmol Pi min(-1) mg(-1) (33 parts per thousand), with little change in K-M values (approximate to 50 mu mol L-1). K+ together with NH4+ synergistically stimulated activity to maximum rates of approximate to 240 nmol Pi min(-1) mg(-1). K, values for ouabain inhibition (approximate to 110 mu mol L-1) decreased to 44.9 +/- 1.0 mu mol L-1 (21 parts per thousand) and 28.8 +/- 1.3 mu mol L-1 (33 parts per thousand) 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. (C) 2009 Elsevier Inc. All rights reserved.