The Mechanism of Sodium and Chloride Uptake by the Gills of a Fresh-Water Fish, Carassius auratus i. Evidence for an independent uptake of sodium and chloride ions

Carassius auratus placed in a dilute sodium chloride solution (400 gM) is able to absorb sodium and chloride ions at very different rates, or to absorb one ion and to lose the other. This is the case not only for fish which have been previously kept in choline chloride or sodium sulfate solutions or deionized water, in order to stimulate their absorption processes, but also in control fish which have not been deprived of sodium or chloride. The absorption of sodium or chloride appears to be unaffected by the presence of a nonpermeant co-ion such as choline or sulfate. Conductivity measurements of the external medium show that during ion uptake the conductivity is constant or increases slowly. This suggests the existence of exchange processes between the ions absorbed and endogenous ions excreted. It is unlikely that potassium or calcium is exchanged for sodium, because of the low permeability of the gills to these ions. Finally, the flux ratios observed for both sodium and chloride ions in the present investigation can only be explained, in relation to their electrochemical gradients across the gills, in terms of active transport. Fresh water organisms maintain a concentration gradient between the electrolytes of their internal medium and those of their very dilute external medium. Invertebrates, fish, and amphibians have been the objects of numerous investigations designed to elucidate the processes by which they maintain this gradient. Special attention has been paid to sodiumand chloride-concentrating mechanisms. In this field diverse experimental techniques on various taxonomic groups yield results which may be classified into two categories: (a) those which indicate the presence of a special transport mechanism for the sodium (active transport) while the chloride follows passively (Ussing, 1954)