Blocking of KClO-Induced Metamorphosis in Premetamorphic Sea Lampreys by Exogenous Thyroid Hormones (TH); Effects of KClO and TH on Serum TH Concentrations and Intestinal Thyroxine Outer-Ring Deiodination

Immediately premetamorphic larval sea lampreys (Petromyzon marinus) ( greater than or equal to 120 mm in length) were treated for 4, 8, or 16 weeks with one of two concentrations of either exogenous thyroxine (T sub(4); 1 or 0.5 mg/L) or 3,5,3'-triiodothyronine (T sub(3); 1 or 0.25 mg/L) in the presence or absence of the goitrogen potassium perchlorate (KClO sub(4); 0.05%) as well as with KClO sub(4) alone. Larvae from all treatments were examined for signs of metamorphosis, changes in serum T sub(4) and T sub(3) concentrations (serum T sub(4) and serum T sub(3)), and changes in intestinal T sub(4) outer-ring (5') deiodination to T sub(3) (T sub(4)ORD). KClO sub(4) depressed both serum T sub(4) and T sub(3) and induced metamorphosis in 80% of larvae treated for 8 weeks or longer. However, neither effect was observed in larvae exposed to KClO sub(4) combined with either thyroid hormone (TH). These data confirm previous suggestions that exogenous TH blocks KClO sub(4)-induced metamorphosis by elevating serum TH concentrations, and provide evidence that declines in serum TH concentrations are mandatory for precocious metamorphosis.