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Isogenic carp Cyprinus carpio L. were acclimated to water temperatures of 15, 22 and 29 degrees C for at least 8 weeks. The acclimations consistently resulted in slightly, but significantly, different plasma osmolality, sodium, potassium and chloride concentrations between the groups studied. Plasma total and ionic calcium levels were unaffected, indicating successful adaptation. The apparent changes in set point for plasma ion levels are explained by altered sodium pump activity and hormonal control of branchial permeability to water and ions. It appears that in 15 degrees C-acclimated fish, a lower apparent Na(+)/K(+)-ATPase activity is compensated by strongly enhanced Na(+)/K(+)-ATPase expression (determined biochemically and immunohistochemically). In 29 degrees C-acclimated fish, the higher ambient temperature activates the enzyme. Arrhenius plots for branchial Na(+)/K(+)-ATPase preparations of the three groups of fish suggest the occurrence of different enzyme isoforms or protein (in)stability as explanations for differences in apparent enzyme activities, rather than temperature-dependent changes in membrane fluidity. As for hormonal control over permeability, prolactin mRNA expression (and anticipated production and release) is lower in fish kept at 29 degrees C, suggesting that control over branchial permeability to water and ions needs to be downregulated at higher temperatures. In so doing, enhanced sodium pump activity is balanced by a controlled passive ion loss to fine-tune plasma sodium levels. Basal plasma cortisol levels did not correlate positively with Na(+)/K(+)-ATPase expression, but doubling plasma cortisol levels in control fish by administering exogenous cortisol (for 7 days, using implanted minipumps and thus stress-free) enhanced Na(+)/K(+)-ATPase expression. This effect must be the result of a glucocorticoid action of the steroid: in fish, mineralocorticoid receptors have higher affinity for cortisol than glucocorticoid receptors. At a lower ambient temperature, branchial Na(+)/K(+)-ATPase expression is upregulated to counteract the temperature-inhibited activity of the sodium pump, perhaps via a mineralocorticoid receptor.

Regulation of branchial Na+/K+-ATPase in common carpCyprinus carpioL. acclimated to different temperatures