Because a d16 embryo can be successfully transferred to a cycling cow at d16 (the day that luteolysis is initiated), it has been suggested that IFN-т also has a quick, direct effect on endometrial synthesis of PGs. In the present study, the expression of COX-2 is strongly enhanced in the LE during the pregnancy recognition period and after IFN-т treatment. COX-2 might be involved in opposed physiological events, forcing the female reproductive tract either to return to the cycle or to accommodate the embryo, because both luteolysis/OT treatment and pregnancy recognition/ IFN-т treatment are accompanied by an elevated expression of COX-2 in the LE.
Indeed, COX-2 generates PGH-2, the common precursor for PGF2a, PGE2, and PGI2—tonameonlythosePGswelllmowntoparticipate at the materno-fetal interface—which rnerespei:;tivelylu-teolytic, antiluteolytic/luteotropic, and required during decidualization and implantation in mice. It can thus be expected that the fate of the corpus luteum depends in part upon the balance between luteotro-pic PGE2 and luteolytic PGF2a. Even though an elevated COX-2 expression during the periods of luteolysis and pregnancy recognition would result in the simultaneous synthesis of both PGs, PGE2 is not sufficient to prevent luteolysis during a natural cycle and endogenous PGF2a is not sufficient to trigger luteolysis during early pregnancy. Results therefore suggest that the corpus luteum is differentially resistant to both types of PGs.