The influence of short-term 17β-estradiol treatment on plasma T 3 levels and in vitro hepatic T 4 5′-monodeiodinase activity in immature rainbow trout, Salmo gairdneri

Abstract

To determine the effects of 17β-estradiol (E 2) on aspects of thyroid function, immature rainbow trout were intraperitoneally injected with estradiol benzoate (0.5 mg/100 g) on Days 0 and 3 and sampled on Days 7 and 12. This protocol created plasma E 2 concentrations during the first 7 days comparable to those during naturally induced vitellogenesis. Control trout received peanut oil alone. Plasma levels of 3,5,3′-triiodothyronine (T 3) were significantly depressed on Day 7 but returned to control levels by Day 12. Plasma thyroxine (T 4) levels were not altered consistently by E 2 treatment. Hepatic microsomal T 4 5′-monodeiodinase (5′D) activity responsible for conversion of T 4 to T 3 was significantly depressed on Day 7 but returned to control levels by Day 12. Lineweaver-Burke plots showed that the lower hepatic 5′D resulted from a 10-fold decrease in V max, indicating a lower enzyme concentration. A slight reduction in K m was also observed. These results confirm that high E 2 levels, comparable to those in vitellogenesis, depress plasma T 3 levels in trout and show that, at least in part, this effect is the result of a decrease in the amount of functional hepatic 5′D.