BLASTOCYST SURVIVAL AND NIDATION IN RATS TREATED WITH OESTROGEN ANTAGONISTS

Abstract

U-ll,100A and U-11,555A prevent pregnancy in rats when administered prior to the expected time of blastocyst implantation. When given to Provera-treated rats at effective antifertility doses, they do not interfere with survival of the blastocysts, with subsequent estrone induced implantation or with the capacity of blastocysts to implant in new hosts. Implantation is delayed in Provera-treated rats until such time as an estrogen, such as estrone, is administered. At pregnancy inhibiting doses, neither U-ll,100A nor U-11,555A are capable of inducing implantation in the Provera-treated rat. When administered concomitantly with estrone they can, however, prevent the expected estrogen-induced implantation. In fact the combinations of U-11,555A and estrone compatible with further embryonic development are very restricted. The nidatory process is more sensitive to changes in this balance than are the normal ovarian or vaginal functions associated with the estrous cycle. The antifertility efficacy of these agents is ascribed to an alteration of estrogen-sensitive implantation processes rather than to changes in tubal transport or to a direct effect on the blastocyst.