Glucocorticoids, thyroid hormones, and iodothyronine deiodinases in embryonic saltwater crocodiles

We investigated the relationship between glucocorticoids, thyroid hormones, and outer ring and inner ring deiodinases (ORD and IRD) during embryonic development in the saltwater crocodile (Crocodylus porosus). We treated the embryos with the synthetic glucocorticoid dexamethasone (Dex), 3,3',5-triiodothyronine (T(3)), and a combination of these two hormones (Dex + T(3)). The effects of these treatments were specific in different tissues and at different stages of development and also brought about changes in plasma concentrations of free thyroid hormones and corticosterone. Administration of Dex to crocodile eggs resulted in a decrease in 3,3',5,5'-tetraiodothyronine (T(4)) ORD activities in liver and kidney microsomes, and a decrease in the high-K(m) rT(3) ORD activity in kidney microsomes, on day 60 of incubation. Dex treatment increased the T(4) ORD activity in liver microsomes, but not kidney microsomes, on day 75 of incubation. Dex administration decreased T(3) IRD activity in liver microsomes. However, this decrease did not change plasma-free T(3) concentrations, which suggests that free thyroid hormone levels are likely to be tightly regulated during development.