[Sodium flux as a function of the salinity of the medium in the Isopod crustacean Sphaeroma serratum (Fabricius)].

The crustacean isopod Sphaeroma serratum, in steady state towards its medium, shows intensive sodium fluxes which have been evaluated by several different technics. In normal sea water (SW) the Na influx reaches 37.9 +/- 9.8 and the outflux represents 35.1 +/- 9.3 microEq h-1 100 mg-1. Renewable internal Na constitute three kinetic compartments whose turnover rate are 228.9; 63.2 +/- 14.6; 13.0 +/- 4.3%. h-1 respectively. The volume of the faster kinetic compartment is equal to 32.0 +/- 2.7% of the wet weight and it is likely equivalent to the extracellular space. In half diluted sea water (SW 1/2) the Na influx is 9.6 +/- 1.5 while outflux is found to be 9.3 +/- 2.2 microEq h-1 100 mg-1. The potential difference between internal and external media is equal to -2.0 +/- 0.4 mV in SW and -5.1 +/- 1.0 mV in SW 1/2. The ratio of the unidirectional fluxes in SW matches the electrochemical gradient, however in SW 1/2 the existence of an active Na influx is plausible.