Effects of arachidonic acid and oxytocin on equine endometrial PGF2α during normal cycles and pseudopregnancy

The effect of excess arachidonic acid or oxytocin on equine endometrial prostaglandin F_2α(PGF_2α) synthesis was measured in vitro under physiologic and pathophysiologic conditions. Endometrial tissues obtained by uterine biopsy at 5, 10, 12, 14, 16 and 20 days post-ovulation from cycling mares, after 0, 2, 4, 6, 8, 10, 12, 14 or 16 days of progesterone (P₄) in ovariectomized mares and at 30 days postovulation in mares undergoing spontaneously prolonged corpus luteum (SPCL) activity were incubated in vitro with and without added arachidonic acid or oxytocin. Endometrial PGF_2α content and synthetic capacities were determined by radioimmunoassay. PGF_2α production increased significantly at Days 12–16. Arachidonic acid did not alter this effect. Oxytocin stimulated additional PGF_2α production on Days 5, 16, and 20. SPCL tissues had minimal PGF_2α production which was increased significantly by arachidonic acid but not oxytocin. PGF_2α synthesis in ovariectomized P₄ treated mares was minimal and did not vary with length of progesterone exposure or addition of arachidonic acid. These results suggest that a) oxytocin may play a role in luteolysis in the equine, b) although arachidonic acid appears not to be limiting to PGF₂α production under normal physiological conditions, its absence may play a role in pathophysiological conditions, c) factors in addition to progesterone and arachidonic acid are required to initiate PGF_2α synthesis in the mare.