Evidence for a Role for Prostaglandins in the Initiation of Blastocyst Implantation in the Rat1

Abstract

To investigate the possible role of prostaglandins [PG] in implantation in rats, indomethacin [INDO], an inhibitor of PG synthesis, was given to rats at 0800 and 1300 h on day 5 of pregnancy. The uterine dye site reaction, indicative of the initiation of implantation, was present on the evening of day 5 in 10/10 and 1/10 control and INDO-treated rats, respectively. INDO treatment on day 5 did not terminate pregnancy; on day 8 there was no significant difference between control and treated rats in the proportions pregnant or in the numbers of implantation sites. Implantation sites weighed significantly less and the length of pregnancy was slightly longer in treated rats, suggesting that INDO treatment had delayed implantation. Implantation in treated rats was not initiated without an increase in endometrial capillary permeability; rats treated with INDO on day 5 and killed on the morning of day 6 had uterine dye sites which were smaller and less evenly spaced than those of control animals killed at the same time. For rats ovariectomized on day 3, treated with progesterone [P] and given estradiol-17.beta. to induce implantation, the uterine dye site reaction was present in 9/9 and 1/9 control and INDO-treated rats, respectively, suggesting that the INDO effect was not mediated by inhibition of ovarian steroidogenesis. Serum P levels on day 5 were not influenced by INDO treatment. The PG concentrations of both the E and F series were significantly greater in the dye site areas than elsewhere in the uterus on the evening of day 5. PG probably are involved at the uterine level in the initiation of implantation in the rat.