1. Pretreatment of Carassius auratus in artificial media (de-ionized water, sodium sulphate or choline chloride solutions), resulted in simultaneous loss of internal Na and Cl, variations in the total CO2 and NH3 plasma content; blood pH shifts towards alkalosis in fish treated with sodium sulphate and towards acidosis in fish treated with de-ionized water or choline chloride.

2. Fish kept in de-ionized water and in sodium sulphate solution were compared with control (tap water) fish. Branchial chloride fluxes were studied as a function of external chloride concentration, the accompanying cation being impermeant (choline) or permeant (sodium). For chloride influx saturation kinetics prevails, the maximal rate of uptake being about 3 times faster in sodium sulphate than in fish kept in de ionized water, the apparent affinity of the uptake mechanisms (Km) remaining the same.

3. Substitution of choline by sodium increases the maximal rate of influx without changing the Km or efflux in all types of fish.

4. The quantity of base excreted is correlated to the quantity of Cl- absorbed confirming the Cl-/HCO3- exchange process.

5. The degree of linkage between Na and Cl absorption is discussed in relation to the maintenance of acid-base balance by the gill.

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