Deborah L. MacLatchy

The influence of short-term 17β-estradiol treatment on plasma T3 levels and in vitro hepatic T4 5′-monodeiodinase activity in immature rainbow trout, Salmo gairdneri

To determine the effects of 17β-estradiol (E2) 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 E2 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 (T3) were significantly depressed on Day 7 but returned to control levels by Day 12. Plasma thyroxine (T4) levels were not altered consistently by E2 treatment. Hepatic microsomal T4 5′-monodeiodinase (5′D) activity responsible for conversion of T4 to T3 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 Vmax, indicating a lower enzyme concentration. A slight reduction in Km was also observed. These results confirm that high E2 levels, comparable to those in vitellogenesis, depress plasma T3 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.

The relationship between T3 production and energy balance in salmonids and other teleosts.

Extrathyroidal T4 5′-monodeiodination, demonstrated in several teleost species, generates T3 which binds more effectively than T4 to putative nuclear receptors and is probably the active thyroid hormone. T4 to T3 conversion is sensitive to the physiological state and provides a pivotal regulatory link between the environment and thyroid hormone action. T3 generation is enhanced in anabolic states (positive energy balance or conditions favoring somatic growth; food intake or treatment with androgens or growth hormone) and is suppressed in catabolic states (negative energy balance or conditions not favoring somatic growth; starvation, stress, or high estradiol levels associated with vitellogenesis). In fish, as in mammals, thyroidal status may be finely tuned to energy balance and through T3 production regulate energy-demanding processes, which in fish include somatic growth, development and early gonadal maturation.