Na+/K+–ATPase activity and gill ultrastructure in the hyper-hypo-regulating crab Chasmagnathus granulatus acclimated to dilute, normal, and concentrated seawater

We studied Na⁺/K⁺ –ATPase activity and ultrastructure in gills of the hyper-hypo-regulating crab Chasmagnathus granulatus Dana, 1851 acclimated to different salinities: 10, 30 and 45‰, known to be hypo-, iso-, and hyper-osmotic to the hemolymph, respectively. After centrifugation of homogenates at 11,000 g, Na⁺/K⁺–ATPase activity was almost entirely found in the pellets from the posterior (6–8) and anterior (3–5) gills, whereas very little was detected in the supernatant liquid. Specific activity of gill 6 was 41.3, 30.2, and 28.2 µmol Pi h–¹ mg prot–¹ for crabs acclimated to 10, 30, and 45‰, respectively, the result for 10‰ being significantly higher than those at 30 and 45‰. Although the concentration of sodium at which the reaction rate is half-maximal (K M) was similar in the three acclimation salinities, only the enzyme from crabs acclimated to 10‰ was inhibited by high sodium concentration. Specific activity of gill 5 increased with the increment in external salinity (10.1, 15, and 18.1 µmol Pi h–¹ mg prot–¹ for 10, 30, and 45‰, respectively), the only significant difference being that between the extreme salinities. The epithelium thickness of the dorsal portion of gill 6 showed a variation among salinities: 21.7, 15.8 and 17.2 µm for 10, 30 and 45‰, respectively. There were significant differences in epithelium thickness between the 10‰ and the other salinities. In all three salinities, the ultrastructure of gill 6 epithelium showed a high density of mitochondria, estimated by their volume fraction (Vv ₘ=0.307–0.355). These mitochondria were packed between extensive basolateral membrane interdigitations in ionocytes and pillar cells. Gill 5 showed three cell types: pillars which possess mitochondria packed between membrane folds only in their interdigitations with neighbouring cells; type-I cells 8.0 µm thick with low density of mitochondria (Vv ₘ=0.088), and type-II cells, 9.9 µm thick and rich in mitochondria (Vv ₘ=0.423), but lacking basolateral interdigitations. Vv ₘ of type-I cells of gill 5 was significantly lower than those of type-II cells of the same gill and the ionocytes of gill 6. No significant difference in Vv ₘ was detected between the latter cell types.