Na+/NH4+, Na+/H+ Exchanges and NH3 Movement Across the Gill of Carassius Auratus

1. Sodium exchange, ammonia excretion and H+ ion movement across the gill were measured in sodium-depleted Carassius auratus. Sodium uptake and ammonia excretion are considerably enhanced by sodium depletion. In this respect sodium-depleted fish behave as ammonia-loaded fish. 2. A decrease of the external pH by one unit results in a 50-60% decrease in both influx and efflux of sodium. A transient augmentation of the rate of ammonia excretion is observed upon acidification, suggesting an increased permeability of the gill to the free-base form of ammonia. 3. The ionic balance sheet of the movements of the monovalent ions across the gill was drawn up when a permeant co-ion for sodium was absent from the external medium. In both acid and alkaline media the ionic balance was achieved in accordance with the law of electroneutrality of external and internal solutions. 4. Net Na+ uptake was only found to be correlated to NH4+ excretion if H+ ion movements across the gill were taken into account. 5. Ammonia-gradient experiments were performed to test the ability of the gill to excrete ammonia against a gradient of ammonia partial pressure. Ammonia excretion is independent of this gradient in alkaline media when sodium ions are present. In these conditions ammonia moves in the ionic form. In acid media, with Na present in the external medium, ammonia movement is correlated with the pNH3 gradient but uphill movement still occurs. Ammonia moves in both the free-base and the ionized forms. In the absence of external sodium, movements of ammonia obey the law for passive transfer and ammonia moves exclusively in the free-base form, even though the external medium is alkaline.