Effects of cortisol and thyroid hormone on peripheral outer ring deiodination and osmoregulatory parameters in the Senegalese sole (Solea senegalensis).

The thyroid gland in fish mainly secretes the thyroid prohormone 3,5,3',5'-tetraiodothyronine (T₄), and extrathyroidal outer ring deiodination (ORD) of the prohormone to 3,5,3'-triiodothyronine (T₃) is pivotal in thyroid hormone economy. Despite its importance in thyroid hormone metabolism, factors that regulate ORD are still largely unresolved in fish. In addition, the osmoregulatory role of T₃ is still a controversial issue in teleosts. In this study, we investigated the regulation of the ORD pathway by cortisol and T₃ in different organs (liver, kidney, and gills) of Solea senegalensis and the involvement of T₃ in the control of branchial and renal Na(+), K(+)-ATPase activity, a prime determinant of the hydromineral balance in teleosts. Animals were treated with i.p. slow-release coconut oil implants containing cortisol or T₃. Hepatic and renal ORD activities were up-regulated in cortisol-injected animals. T₃-treated fish showed a prominent decrease in plasma-free T₄ levels, whereas ORD activities did not change significantly. Branchial and renal Na(+), K(+)-ATPase activities were virtually unaffected by T₃, but were transiently up-regulated by cortisol. We conclude that cortisol regulates local T₃ bioavailability in S. senegalensis via ORD in an organ-specific manner. Unlike T₃, cortisol appears to be directly implicated in the up-regulation of branchial and renal Na(+), K(+)-ATPase activities.