Evidence for a PGF_2α auto-amplification system in the endometrium in mares

In mares, prostaglandin F_2α (PGF_2α) secreted from the endometrium is a major luteolysin. Some domestic animals have an autoamplification system in which PGF_2α can stimulate its own production. Here, we investigated whether this is also the case in mares. In an in vivo study, mares at the mid-luteal phase (days 6-8 of estrous cycle) were injected i.m. with cloprostenol (250 ìg) and blood samples were collected at fixed intervals until 72 h after treatment. Progesterone (P4) concentrations started decreasing 45 min after the injection and continued to decrease up to 24 h (P < 0.05). In turn, 13,14-dihydro-15-keto-PGF_2α (PGFM) metabolite started to increase 4h after an injection and continued to increase up to 72 h (P < 0.05). PGF receptor (PTGFR) mRNA expression in the endometrium was significantly higher in the late luteal phase than in the early and regressed luteal phases (P < 0.05). In vitro, PGF_2α significantly stimulated (P < 0.05) PGF_2α production by endometrial tissues and endometrial epithelial and stromal cells and significantly increased (P < 0.05) the mRNA expression of prostaglandin-endoperoxide synthase-2 (PTGS2), an enzyme involved in PGF_2α synthesis in endometrial cell. These findings strongly suggest the existence of an endometrial PGF_2α auto-amplification system in mares.