Study of the relationship between thyroid hormones and lipid metabolism during KClO4-induced metamorphosis of landlocked lamprey, Petromyzon marinus.
This study examines the role of thyroid hormones (TH) (thyroxine and triiodothyronine) in regulating lipid metabolism of landlocked larval sea lampreys, Petromyzon marinus. Larvae were treated with either thyroxine (0.5 or 1 mg l(-1) water) or triiodothyronine (0.25 or 1 mg l(-1) water) in the presence or absence of the goitrogen, potassium perchlorate (KClO4) (0.05% w/v), for 4, 8, and 16 weeks. Treatment with KClO4 alone, which induced metamorphosis after 8 weeks and lowered plasma TH levels, reduced hepatic and renal total lipid content after 8 weeks of treatment. KClO4-induced lipid depletion after the 8-week treatment was supported by an increased rate of hepatic lipolysis, as indicated by increased triacylglycerol lipase activity. Furthermore, reduced lipogenesis in the liver was indicated by decreased hepatic acetyl-CoA carboxylase and diacylglycerol acyltransferase (DGAT) activities, and by decreased renal DGAT activity following 8 weeks of KClO4 treatment. Treatment of larvae for 4 weeks with TH alone resulted in either no change or a slight increase of lipid in the liver and kidney. TH treatments in combination with KClO4 failed to induce metamorphosis and, after up to 8 weeks, several TH treatments blocked changes in lipid content and enzyme activity associated with KClO4-induced metamorphosis. These experimental results suggest that TH deficiency during metamorphosis may promote lipid catabolism, while the presence of TH tends to protect/promote lipid reserves, perhaps favoring the larval condition. The actions of TH and KClO4 on metamorphosis-associated lipid metabolism in sea lampreys may be direct, permissive, and/or indirect via other factors.