Lactational transfer of 3,3',4,4'-tetrachloro- and 2,2',4,4',5,5'-hexachlorobiphenyl induces cytochrome P450IVA1 in neonates. Evidence for a potential synergistic mechanism.

On the first day of lactation, material rats were treated with a single low dose of 5 mg/kg body weight of 3,3',4,4'-tetrachlorobiphenyl (TCB) or 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) or with a combination of both congeners. Lactational transfer of these polychlorinated biphenyls (PCBs) was found in neonates and significant increases in microsomal cytochrome P450, cytochrome b5 and in glutathione-S-transferase activity were observed. Treatment with HCB did not increase neonatal ethoxyresorufin-O-de-ethylation (EROD) activities whereas a more than 26-fold increase in EROD activity was noted in response to exposure to TCB. However, EROD activities were increased more than 65-fold in response to the combined exposure to TCB and HCB. Exposure via milk to TCB caused a significant reduction in the N-demethylation of aminopyrine, but the combined exposure to TCB and HCB produced a significant reduction in the N-demethylation of dimethylnitrosamine. Lactational transfer of either TCB or HCB reduced marginally peroxisomal enzyme activities; however, exposure to a combination of TCB and HCB resulted in the highly significant reduction in KCN-insensitive palmitoyl-CoA oxidation and acetyl-CoA oxidation. Contrary to the reduction of these enzyme activities, the specific concentrations of CYP4A1 were significantly increased when neonates were exposed to either TCB or HCB. The largest induction, however, was observed in response to the combined exposure to both PCBs. Evidence is presented to suggest an induction of CYP4A1 which may be independent of the molecular substitution pattern of the two PCBs used in our studies but on a possible mode of synergistic interaction.