Tomoaki Yamazaki

Polychlorinated Biphenyl-Mediated Decrease in Serum Thyroxine Level in Rodents

Effects of Kanechlor-500 (KC500), a commercial polychlorinated biphenyl mixture, on the levels of serum thyroid hormones such as total thyroxine (T4) and triiodothyronine were examined in male mice, hamsters, rats, and guinea pigs. Four days after a single intraperitoneal injection of KC500, significant decreases in the levels of the serum total T4 and free T4 occurred in all the animals examined, whereas a significant decrease in the level of serum triiodothyronine was observed only in guinea pigs among the animals examined. In addition, no significant change in the level of serum thyroid-stimulating hormone was observed in any of the rodents examined. A significant increase in the activity of hepatic T4-UDP-glucuronosyltransferase after the KC500 administration occurred only in guinea pigs, whereas the increase in the amount of biliary [125I]T4 glucuronide after an intravenous injection of [125I]T4 to the KC500-pretreated animals occurred only in rats. On the other hand, in all the rodents examined, KC500-pretreatment promoted the clearance of [125I]T4 from the serum and led to a significant increase in the steady-state distribution volumes of [125I]T4. Likewise, its pretreatment raised the concentration ratio (Kp value) of the liver to serum and the liver distribution of [125I]T4 in all the rodents tested. The present findings indicate that for the first time the KC500-mediated decrease in the serum T4 level in mice, hamsters, rats and guinea pigs occurs mainly through an increase in the accumulation level of T4 in the liver.

THE DECREASE IN LEVEL OF SERUM THYROXINE BY 2,2,4,5,5- PENTACHLOROBIPHENYL IN RATS AND MICE: NO CORRELATION WITH FORMATION OF METHYLSULFONYL METABOLITES

A relationship between formation of methylsulfonyl (MeSO2) metabolites of 2,2′,4,5,5′-pentachlorobiphenyl (PentaCB) and decrease in serum thyroxine (T4) level was examined in the rats and mice after a single i.p. injection of PentaCB (342 μmol/kg body weight). In either rats or mice, levels of the 3- and 4-MeSO2 metabolites of PentaCB in the liver and feces increased in a time-dependent fashion up to 8 days after PentaCB-treatment. However, there was a marked difference between rats and mice in the amount of the metabolites formed, and the cumulative amount of the either MeSO2 metabolite for 8 days after PentaCB treatment in the liver was 4 to 15 times higher in mice than in rats. On the other hand, a 40 to 60% decrease in level of serum total T4 occurred in both rats and mice at 1 day after PentaCB treatment, and the decrease was retained up to 8 days after PentaCB treatment. Thus, there was a marked difference between rats and mice in the formation of MeSO2 metabolites from PentaCB but not a significant difference between rats and mice in PentaCB-induced decrease in the level of serum total T4, indicating that PentaCB-induced decrease in the level of serum total T4 is not necessarily dependent on the MeSO2 metabolites formed.