Influence of 3-methylether of ethinylestradiol (Mestranol) on oviductal egg transport in rats.

The development of fertilized eggs and their transport through the oviduct were studied in rats treated orally with 3-methylether of ethinylestradiol (mestranol) at a single administration in different doses. The ED50 values were in the same range (0.11--0.34 mg mestranol/kg b.w.) as well in postcoital pregnancy inhibition test as in the experiments on tubal egg contents on day 3 as in implantation sites in niagara blue test on day 6. This suggests that the uniform cause of pregnancy inhibition of postcoital mestranol treatment on day 1 in rats is the acceleration of tubal egg transport. The ED50 of mestranol given on day 1 shortened the stay of eggs in tubes to 24 hours. Mestranol dosage above the ED50 reduced the tubal stay towards two days in comparison to five days in control animals. The number of eggs prematurely expelled from the tubes into the uterus was relatively high on day 2 and 3 in animals treated with mestranol in high dosage and with ligated cervix, compared with animals without ligation. But on day 4 on blastocysts were found. This result shows that the prevention of pregnancy is caused by the expulsion of blastocysts from uterus and/or the degeneration of zygotes in utero. Blastocysts or morulae recovered from treated rats in high dosage were transferred to the uteri of pseudopregnant recipients: 29% developed into normal term foetuses compared to 29 or 30% for both untreated control groups.