Acclimation of Solea senegalensis to different ambient temperatures: implications for thyroidal status and osmoregulation

We have investigated the regulation of thyroidal status and osmoregulatory capacities in juveniles from the teleost Solea senegalensis acclimated to different ambient temperatures. Juveniles, raised in seawater at 19A degrees C, were acclimated for 3 weeks to temperatures of 12, 19 and 26A degrees C. Since our preliminary observations showed that at 12A degrees C feed intake was suppressed, our experimental design controlled for this factor. The concentration of branchial Na+,K+-ATPase, estimated by measurements of enzyme activity at the optimum temperature of this enzyme (37A degrees C), did not change. In contrast, an increase in Na+,K+-ATPase activity (measured at 37A degrees C), was observed in the kidney of 12A degrees C-acclimated fish. In fish acclimated to 12A degrees C, the hepatosomatic index had increased, which correlated with increased plasma levels of triglycerides and non-esterified fatty acids. Plasma cortisol levels did not differ significantly between the experimental groups. In liver and gills, the amount of iodothyronine deiodinases that exhibit thyroid hormone outer ring deiodination was up-regulated only when fish did not feed. When assayed at the acclimation temperature, kidney deiodinase activities were similar, indicating a temperature-compensation strategy. 3,5,3'-triiodothyronine (T3) tissue concentrations in gills and kidney did not differ significantly between experimental groups. However, at 12A degrees C, lower T3 tissue levels were measured in plasma and liver. We conclude that S. senegalensis adjusts its osmoregulatory system to compensate for the effects of temperature on electrolyte transport capacity. The organ-specific changes in thyroid hormone metabolism at different temperatures indicate the involvement of thyroid hormones in temperature acclimation.