Branchial Sodium Transport Mechanisms in Scyliorhinus Canicula: Evidence for Na+/NH4+ And Na+ H+ Exchanges and for A Role of Carbonic Anhydrase

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Abstract
1. Branchial sodium exchanges were measured with the help of 24Na in the marine elasmobranch Scyliorhinus canicula. Handling causes a transient increase of the sodium influx and decrease of the sodium efflux in both intact and hypophysectomized fish. 2. Ammonia-loading (300 µ-equiv./100 g) is followed by an increase of both influx and efflux of sodium resulting in an augmented net sodium uptake lasting for at least 4 h. Ammonia excretion is also increased but only for 2 h. Ammonia-loading results in a metabolic acidosis lasting for at least 4 h. 3. HCl injection (100 µ-equiv./100 g) produces an increase of both influx and efflux of sodium resulting in an augmented net sodium uptake lasting for at least 4 h. Ammonia excretion is not affected. 4. Acetazolamide injection (10 mg/100 g) results in a depression of the sodium influx, while the sodium efflux remains unchanged. This inhibitory effect is observed in control fish as well as in fish treated with HCl or ammonium salt injections. 5. These observations confirm that the gill plays a major role in the maintenance of the pH of the body fluids. The similarities between the sodium transport mechanisms of Scyliorhinus and of the freshwater teleosts are emphasized. These results suggest that living elasmobranchs may have retained branchial mechanisms inherited from their freshwater ancestors.