Fish Physiology and Biochemistry vol:20 issue:3 pages:201-207
In the present study, we examined the effects of experimentally-induced increases or decreases in plasma concentrations of thyroid hormones on iodothyronine deiodinases in tilapia, Oreochromis niloticus. To obtain hyperthyroid tilapia, fish were injected with porcine follicle stimulating hormone (pFSH) 36 hours before sampling or fed on demand for 11 days with tilapia pellets containing 12 ppm T-3. Tilapias were made hypothyroid by providing them food containing 0.2% methimazole for 11 days. Plasma T-4 and T-3 and the in vitro deiodinase activity in liver, kidney, brain and gill were measured at the end of the treatment period. Injection with pFSH caused an increase in plasma T-4 but had no influence on plasma T-3 levels. A small increase in plasma T-3 was observed in T-3-fed fish. Plasma levels of both T-4 and T-3 were decreased by methimazole treatment. We observed no changes in kidney type I deiodinase (D1), whereas liver type II deiodinase (D2) was increased during hypothyroidism and decreased during hyperthyroidism. Hypothyroidism resulted in a significant decrease in brain, gill and liver type III deiodinase (D3). An pFSH-induced increase in T-4 stimulated brain and gill D3 but not liver D3, whereas the opposite was true in T-3-fed fish. We conclude that the regulation of D1 and D3 in tilapia is probably different compared to mammals.