Effects of polychlorinated biphenyls on thyroid hormones and liver type I monodeiodinase in the chick embryo

Polychlorinated biphenyls (PCBs) are widespread environmental contaminants which can biomagnify to higher tropic level organisms including birds. Circ ulating thyroid hormones (TH) and growth are decreased by PCB exposure. The first set of studies investigated the effects of PCBs on an enzyme respons ible for TH homeostasis, hepatic type I monodeiodinase (MDI) in chicken emb roys, Fertile chicken eggs were injected with Aroclor 1242, Aroclor 1254, 2 ,2',6,6'-tetrachlorobiphenyl (TCB), 3,3',4,4'-TCB, or 3,3',5,5'-TCB on Day 0 and studies were terminated on Incubation Day 21, Hepatic MDI activity wa s reduced in embryos treated with the Aroclor mixtures. No effects on MDI a ctivities were observed after PCB isomer treatment. Liver weights from embr yos treated with Aroclor 1242 were decreased. In the second study, chick em bryos were exposed to these same PCBs in order to evaluate their effect on circulating THs and growth. Treatment with PCBs had no effect on body weigh t, Femur length mere decreased with Arcolor 1242 treatment. A decrease in p lasma concentration of thyroxine was observed after treatment with Aroclor 1242 and Aroclor 1254, Based on these findings, it is evident that PCBs alt er the thyroid axis, Bird circulating TH levels, which are generally report ed, may not be a good biomarker for low-dose exposure to PCBs, However, the reduction in MDI activity was more sensitive to PCB mixture exposure and m ay be a useful biomarker, (C) 1999 Academic Press.