Effects of Temperature on Branchial Sodium-Exchange and Extrusion Mechanisms in the Seawater-Adapted Flounder Platichthys Flesus L

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Abstract
1. The effects of short-term and long-term temperature changes on the branchial components of sodium balance have been studied in the seawater-adapted flounder Platichthys flesus. 2. When fish adapted to 6°C are compared with fish adapted to 16°C a disturbance of sodium balance is observed; while the plasma level of sodium remains constant, an increased internal sodium space can be demonstrated isotopically. Increase of the muscle sodium content accounts for only a small part of the extra sodium content of the fish. The increased sodium load is the result of an impairment of the sodiumextrusion mechanism in the gills, and is demonstrated by the disappearance of the Na/K exchange activity of the gills. The passive sodium fluxes (by simple diffusion or exchange-diffusion) decrease only twofold. 3. Abrupt temperature changes in the 6-21°C temperature range are followed by instantaneous and reversible changes of the total sodium efflux (Q10 = 2), of the sodium leak observed after transfer into fresh water (Q10 = 1.7) and of the Na/Na exchange (Q10 = 2). The Na/K exchange, which corresponds presumably to the active sodium extrusion mechanism, shows in contrast a much greater temperature dependence (Q10 = 6). The total sodium efflux follows the Arrhenius relation between 6 and 21°C. 4. Abrupt transfer to higher temperatures (23-26 °C) produces irreversible damage to the transport system. 5. No compensatory acclimatization of the flux rates is observed during adaptation in the 6 °C environment. 6. The biological, physiological, ecological as well as biophysical implications of these findings are discussed. Particular emphasis is given to the problem of gill haemo-dynamics.