The possibility may remain that the decreases in serum progesterone levels were caused by suppression of deciduoma growth. It appears that inhibition of the decidual response may be related to an increase in the incidence of postimplantation embryonic loss and implantation failure. In order to determine the effect of significantly low food consumption and body weight on uterine weight and serum progesterone levels, the pseudopregnant rats in the pair-fed group were given an amount of feed equal to the feed consumption of rats in the highest dose group of each dosing period. None of the parameters measured in the pair-fed group was significantly different from the control value excluding maternal weight gain and food intake. This indicates that suppression of the decidual cell response observed in the TBTCl-treated groups was due to the effects of TBTCl and not due to malnutrition or reduced body weight.
Further work is necessary to determine the effect of TBTCl on ovarian progesterone production.
In summary, the present study demonstrated that TBTCl induced suppression of the decidual cell response in pseudopregnant rats, concurrent with a decrease in serum progesterone levels, suggesting that TBTCl administered during early pregnancy adversely affects uterine decidualization, mediated by a decrease in serum progesterone levels. This inhibition of the decidual cell response may be responsible for the blockade of implantation and induction of embryonic resorption in pregnant rats given TBTCl during early pregnancy. This study was supported in part by a grant from the Environment Agency in Japan.