EFFECTS OF GESTATIONAL AND LACTATIONAL EXPOSURE TO COPLANAR POLYCHLORINATED BIPHENYL (PCB) CONGENERS OR 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN(TCDD) ON THYROID-HORMONE CONCENTRATIONS IN WEANLING RATS

Perinatal exposure to polychlorinated biphenyl (PCB) mixtures or to ce rtain ortho-substituted PCB congeners dramatically reduces circulating thyroxine (T4) concentrations, It is not clear whether perinatal expo sure to coplanar PCBs or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) ha s a similar effect. In this study, time-mated Sprague-Dawley rats were dosed with 2 or 8 mg/kg/day PCB 77 (3,3',4,4' -tetrachlorobiphenyl), 0.25 or 1.00 mu g/kg/day PCB 126 (3,3',4,4',5-pentachlorobiphenyl), 0. 025 or 0.10 mu g/kg/day TCDD, or corn oil vehicle orally on gestation days 10-16, At weaning, plasma total T4 concentrations in PCB 77 and T CDD high-dose female pups were significantly depressed, but the change s were modest (84.4 and 79.6% of control, respectively). T4 concentrat ions in PCB 126 high-dose females and all high-dose males were also de pressed slightly, but the changes were not statistically significant. UDP-Glucuronosyl transferase (UDP-GT) activity towards 4-nitrophenol w as increased in all high-dose groups. Thus, the modest decreases in T4 could be due in part to increased T4 glucuronidation by UDP-GT. Triio dothyronine (T3) and thyroid stimulating hormone (TSH) concentrations were unchanged in all groups, In contrast to the minor changes in thyr oid hormone status, liver microsomal ethoxyresorufin-O-deethylase (ERO D) was markedly induced in all exposure groups and thymus weights were depressed in the high-dose groups. Because doses of coplanar PCBs or TCDD that caused marked induction of EROD activity had only minor effe cts on T4, we conclude that changes in thyroid hormone status at weani ng are not among the more sensitive effects of perinatal exposure to t hese compounds.