MAINTENANCE OF THE CORPUS-LUTEUM OF EARLY-PREGNANCY IN THE EWE .2. PROSTAGLANDIN SECRETION BY THE ENDOMETRIUM INVITRO AND INVIVO

To determine if the synthesis and/or secretion of prostaglandins PGF2(α) and PGE2 differed during the late luteal phase of the cycle and early pregnancy, a combination of in vivo and in vitro parameters were studied. Synthesis of PGF2(α) and PGE2 by ovine endometrial tissue in vitro was studied on Days 13, 15 and 17 of the estrous cycle and pregnancy. The prostaglandins were quantified by radioimmunoassay. There was no difference in the quantity of PGF2(α) synthesized by tissue collected on Day 13 of either the cycle or pregnancy. Synthesis of PGF2(α) was greater on Day 15 (P<0.01) and tended to be greater on Day 17 (P<0.10) of pregnancy than on Day 15 or 17 of the estrous cycle. Synthesis of PGE2 was also significantly greater on Days 15 (P<.001) and 17 (P<0.05) of pregnancy than on the same days of the estrous cycle. Synthesis rates of both PGs in vitro tended to decrease from Days 13-17 in tissue collected from cycling ewes, while increasing from Days 13-15, then falling again on Day 17 of pregnancy. Concentrations of PGF2(α) and PGE2 in endometrial tissue followed the same pattern as in vitro secretion, suggesting that in vitro secretion accurately reflects the in vivo capacity of endometrial tissue to synthesize PGs. Concentrations of PGF2(α) in uteroovarian venous serum were greater on Day 13 (P<0.05) and not different on Days 15 and 17 of pregnancy compared with the same days of the estrous cycle. However, concentrations of PGE2 in uteroovarian venous serum were greater on Days 15 (P<0.01) and 17 (P<0.05) of pregnancy than on the same days of the estrous cycle. Prostaglandin F2(α) was present in uterine flushings in extremely small quantities (0.3-0.6 ng/ml) and PGE2 was nondetectable (<10 pg/ml) on Days 13, 15, and 17 of the estrous cycle. In contrast, concentrations of PGF2(α) and PGE2 were relatively high on all days of pregnancy examined and on Day 15 were at least 50 times greater than on Day 15 of the estrous cycle. These data indicate that the capacity of endometrial tissue to secrete PGF2(α) is increased during early pregnancy. The finding that PGE2 is also secreted at a greater rate during early pregnancy lends support to the hypothesis that PGE2 may be the factor responsible for maintenance of the corpus luteum of early pregnancy.