INVOLVEMENT OF 4 HORMONES IN THYROXINE DEIODINATION IN SEVERAL TISSUES OF IMMATURE YEARLING MASU SALMON, ONCORHYNCHUS-MASOU

Effects of estradiol-17beta, testosterone, cortisol and ovine growth h ormone on 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (r T3) generation (5'-monodeiodinase and 5-monodeiodinase activity) from thyroxine (T4) in liver, gill, kidney and head kidney of immature year ling masu salmon were investigated. Reverse T3 generation was observed in all tissues. Estradiol-17beta significantly depressed both serum T 4 and T3 levels and T3 generation in gill and liver. Ovine growth horm one enhanced serum T3 levels and T3 generation in gill and liver. Alth ough testosterone increased serum T4 levels, it was not effective on e ither T3 or rT3 generation in each tissue. T3 generation was found in the gill following cortisol treatment. However, cortisol showed no eff ects on serum thyroid hormones. Our results suggest two T4 deiodinatio n pathways in the masu salmon. The observed decrease in serum T3 may b e due to depressed 5'-monodeiodinase activity and stimulated 5-monodei odinase activity following estradiol-17beta treatment; and the observe d increase in serum T3 levels may be due to enhanced 5'-monodeiodinase activity following ovine growth hormone treatment.