Methoxychlor accelerates embryo transport through the rat reproductive tract.

The estrogenic pesticide methoxychlor (MXC) is known to reduce implantation, and, in our previous work, this reduction has been attributed to a direct effect on uterine function. The present study was designed to investigate the effect of MXC on embryo transport rate, another phenomenon that is vulnerable to estrogenic effects. MXC was administered by gavage, at 0, 100, 200, and 500 mg/kg/day, to groups of rats on Days 1-3 of pregnancy (Day 0 = sperm positive), and the distribution of embryos in the oviducts and uteri of animals was assessed at five time intervals prior to implantation. No effect of MXC was detected by the afternoon of Day 1. On Days 2 and 3 of pregnancy, 200 and 500 mg/kg/day MXC were found to accelerate embryo transport into the uterus; the 500 mg/kg/day dosage also reduced the total number of embryos recovered from the tract. On the third day, 100 mg/kg/day MXC also accelerated embryo transport to the uterus and 200 mg/kg/day MXC reduced total embryo recovery. Until the afternoon of Day 3, most control embryos remained in the oviduct. These data demonstrate that MXC produces a dose-dependent acceleration of embryo transport through the female reproductive tract. When compared with previous work, the current data indicate that such an acceleration is the primary cause of MXC-induced preimplantation embryonic loss when exposure occurs after fertilization.