Prostaglandin release and luteolysis associated with physiological and pathological conditions of the reproductive cycle of the mare: a review

Summary The advent of radioimmunoassay for the major metabolite of prostaglandin F 2α (PGF 2α ), 15‐keto‐13, 14‐dihydro‐PGF 2α (15‐ketodihydro‐PGF 2α ), has allowed the dynamics of PGF 2α , secretion to be studied in a variety of physiological and pathological situations in domestic animals, including the mare. In the non‐pregnant cyclic mare, PGF 2α is responsible for the regression of the corpus luteum. In the pregnant mare, pulsatile secretion of PGF 2α is blocked by the presence of the conceptus and the corpus luteum continues to secrete progesterone, essential to the support of pregnancy. At term, the massive increase in PGF 2α synthesis is the major stimulus for uterine contractions during delivery. The endometrium of the non‐pregnant mare secretes PGF 2α in response to inflammatory changes caused by bacterial infection, intra‐uterine infusion of saline (low pH) or other irritants; the amount of PGF 2α released is often sufficient to initiate luteolysis. In the pregnant mare, stimuli that can initiate PGF 2α secretion by the endometrium include uterine and cervical manipulation, chronic uterine inflammation, surgical manipulation of the uterus in conjunction with removal of one of twin conceptuses and manual rupture of one embryonic vesicle per rectum in mares carrying twins. Prostaglandin F 2α can be released by tissues other than the uterus. Systemic administration of endotoxin will induce the synthesis and release of PGF 2α . The secretion of PGF 2α in this situation can result in a decrease of luteal function or complete luteolysis in both the non‐pregnant and pregnant mare. In mares less than 55 days pregnant, compromised luteal activity following the administration of endotoxin results in death of the conceptus. Ongoing studies indicate that flunixin meglumine, a non‐steroidal anti‐inflammatory drug, effectively blocks PGF 2α secretion and can be used to prevent embryonic death in the event of endotoxaemia. Altrenogest can be used as an alternative treatment for the prevention of embryonic death. To be effective, flunixin meglumine must be administered soon after the onset of endotoxaemia whereas altrenogest treatment can be initiated up to 12 h later and still prevent pregnancy loss.