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Journal of Experimental Biology 58,627-636 (1973)
Published by Company of Biologists 1973

Sodium Extrusion by a Fish Acclimated to Sea Water: Physiological and Biochemical Description of a Na-FOR-K Exchange System

DAVID H. EVANS ¹, CHARLES H. MALLERY ¹, and LARRY KRAVITZ ¹

¹ Department of Biology and Laboratory for Quantitative Biology, University of Miami, Coral Gables, Florida 33124, U.S.A.

1. The effect of external potassium ions on the extrusion of sodium ions by the seawater-acclimated fat sleeper, Dormitator maculatus, was investigated.

2. Removal of external potassium ions reduced the efflux of sodium from the fish by 22% while addition of 10^-4 M ouabain reduced the efflux of sodium ions by 14%.

3. Addition of potassium ions to distilled-water baths into which fish were rapidly transferred stimulated sodium extrusion in a manner which could be described by the Michaelis-Menten equation. The K_m of this potassium-stimulated sodium efflux was approximately 2 mM-K/1.

4. The calculated rate of sodium extrusion was 10 times the oral ingestion of sodium ions.

5. Biochemical assays of the levels of the enzyme Na-K-activated ATPase extracted from gill tissue determined that seawater-acclimated fish had 3 times the enzymic activity that fish acclimated to freshwater had.

6. In vitro potassium stimulation of the extracted Na-K-activated ATPase showed Michaelis-Menten kinetics with a K_m of approximately 2 mM-K/l.

7. It is concluded that the extrusion of sodium ions by Dormitator maculatus acclimated to sea water is coupled with potassium uptake and is mediated by the enzyme Na-K-activated ATPase.

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