Local Autoamplification System for Prostaglandin F2 Alpha in Bovine Endometrium.

Abstract

Prostaglandin (PG) F2a is the main luteolytic factor in ruminants. It has been generally accepted that there is a positive feedback loop between endometrial PGF2a and ovarian oxytocin (OT) in ruminants. On the other hand, since administration of OT antagonist together with PGF2a does not suppress uterine PGF2a release, OT may not be essential for PGF2a stimulation in cow. Based on above evidence, we hypothesized the presence of a local autoamplification system for PGF2a and that uterine PGF2a is stimulated by itself in cow. In the present study, to test the above hypothesis, we determined 1) mRNA and protein expressions of F-series-prostanoid receptor (FPr) throughout the estrous cycle and 2) effect of PGF2a on PGF2a production in bovine endometrial tissue. Endometrial tissues were collected at estrus (Day 0), the early I (Days 2-3), early II (Days 5-6), mid (Days 8-12), late (Days 15-17) luteal stages and the follicular stage (Days 19-21). FPr mRNA and protein expressions were determined by real time RT-PCR and western blotting analysis. FPr mRNA expression was higher at the follicular stage than at the estrus and early I luteal stages whereas FPr protein expression increased after the developing luteal stage and it became the highest level at the late luteal stage (P<0.05). Since the highest level of FPr protein was expressed at late luteal stage, endometrial tissue strips of the late luteal stage were used to assess the dose-dependent effects of PGF2a on the PGF2a production by the tissue. The tissues were exposed to PGF2a (0.01, 0.1 and 1 microM), or PGF2a together with cyclooxygenase (COX) inhibitor (indomethacin: 10 microM) for 4 h, media were exchanged twice to remove the treated PGF2a then the tissues were cultured for an additional 4 h. PGF2a was determined by enzyme immunoassay (EIA). PGF2a stimulated PGF2a production in a dose-dependent manner. This effect was significant at 1 microM (P<0.05). In addition, endometrial tissues from the early I and mid luteal stages were also exposed to PGF2a (1 microM) to analyze the stimulatory effect of PGF2a at different stages of the estrous cycle (early I, mid and late luteal stages). In the both stages, PGF2a stimulated PGF2a production (P<0.05). PGF2a stimulation was higher at the late luteal stage compared to the early and mid luteal stage (P<0.05). Indomethacin suppressed PGF2a-stimulated PGF2a production in endometrial tissues of the all stages studied. This result suggests that the increase in PGF2a detected in the medium is not the PGF2a added to the medium but the endometrial product, and that PGF2a is increased by stimulation of cyclooxygenase activity. The overall results suggest the presence of a local autoamplification system for PGF2a in bovine endometrium. This autoamplification system together with the positive feedback loop between OT and PGF2a may induce the drastic increase in endometrial PGF2a secretion at the late luteal stage to ensure luteolysis in cow. (poster)