SODIUM SPACE IN FRESH-WATER AND SEA-WATER EELS

1. 1. The sodium space was measured in fresh-water- and sea-water-adapted eels after intraperitoneal injection or cardiac infusion of labelled sodium. The volume is defined as that which the total exchangeable sodium would occupy when homogeneously distributed at the concentration of that of the plasma, and is calculated from Vint = Qt [Naint*]t, where Qt in the total quantity of radiosodium present in the fish at time t and [Naint*] the corresponding radioactivity per unit volume of plasma 2. 2. The space so measured at equilibrium is independent of the site of introduction of the isotope but depends on the adaptation medium. It is 32 ml/ 100 g in sea water and 24 ml/100 g in fresh water. 3. 3. The kinetics of the filling up were studied and found to be a simple exponential function, about 2 hr being necessary for 95 per cent of the space to be filled, irrespective of the adaptation medium. 4. 4. The measurement of the sodium space when the isotope is introduced in the external medium is also considered, and the dimensions of the sodium space and kinetics of its filling up discussed. © 1969.