Competitive role of calcium in sodium transport in the crab carcinus mediterraneus Acclimated to low salinities

Influence of calcium on sodium fluxes was investigated in the brackish-water crab Carcinus mediterraneus Csrn., after activation of sodium regulatory mechanisms, during longterm acclimation in diluted (15.9 S) sea water. The ²²Na outflux constants measured in whole crabs are noticeably lower (0.188 to 0.374h^-1) in diluted sea water enriched by calcium (5.8 to 10.4 mM Ca²⁺/l), than in ordinary diluted sea water (0.545 h^-1). The sodium-outflux constants in hemolymph, gills and muscle show the same trend of slower exchange of ²²Na in calcium-enriched sea water. In ordinary sea water, the total sodium-outflux rate from the hemolymph amounts to 46.31 M Na/g/h, while in calcium-enriched sea water (8.23 mM Ca²⁺/l) it is inhibited, amounting to 13.86 M Na/g/h. Sodium and potassium concentrations of intracellular muscles in diluted sea water enriched with calcium and control diluted sea water are similar. The outflux of intracellular sodium from the muscle amounts to 2.84 M Na/g/h in crabs acclimated to diluted sea water.