PROSTAGLANDINS Patrick M. McCue DVM, PhD, Diplomate American College of Theriogenologists

Prostaglandins are in a class known as fatty- program or after having missed a breeding acid hormones. They are produced by many opportunity. It should be noted that tissues in the body, but in equine reproduction prostaglandins are only effective in bringing we are concerned primarily about the type mares back into estrus if a mature corpus produced by the uterine lining or luteum is present. A can be . In the non-pregnant, cycling effectively destroyed by prostaglandins mare, prostaglandins are produced and beginning approximately 5 days after secreted in pulses by the endometrium from ovulation. Administration of a single approximately day 13 to 15 post-ovulation. intramuscular dose of prostaglandins is Uterine prostaglandins enter the blood stream usually sufficient to cause complete luteolysis. and are eventually transported to the An important concept to be remembered is where they cause destruction or lysis of the that prostaglandins do not directly cause a corpus luteum. production mare to come into heat. Prostaglandins cause decreases and the mare returns to estrus in 3 luteolysis and a rapid decline in progesterone to 4 days. In the pregnant mare, prosta- production. Return to estrus is dependent on glandin production is prevented and subsequent follicular development and consequently the corpus luteum is spared and estrogen production. Prostaglandins will have it continues to produce the progesterone no effect in indirectly bringing a mare into required for maintenance of . estrus when administered in the absence of a corpus luteum (i.e. during seasonal anestrus, Indications for administration of the spring transition period or to non-cycling prostaglandins in broodmare management mares during the breeding season). are primarily to cause destruction of a corpus luteum (luteolysis) or to stimulate The interval from administration of uterine contractions. The products most prostaglandins to ovulation is dependent on commonly used in horses are Lutalyse® and the size of the existing follicles at the time of Estrumate®. While both products are treatment and ranges from 6 to 10 days. It is effective in inducing luteolysis, Estrumate® not an exact science, but if you want to try to has less of a tendency to cause side effects avoid a weekend semen collection, administer such as mild sweating and abdominal prostaglandins on a Wednesday to a mare discomfort. with a mature follicle and tentatively plan on breeding the mare between Tuesday and Prostaglandins are commonly administered to Friday the following week. ‘short-cycle’ mares in a timed breeding

1 Synchronization of estrus in 2 or more mares Several recent studies have confirmed that for the purpose of embryo transfer or timed prostaglandins should not be administered to breeding may be accomplished by mares after ovulation in an effort to clear administration of 2 injections of prosta- uterine fluid. Treatment of mares after glandins 14 days apart. Prostaglandins may ovulation will adversely affect development also be used to terminate an unwanted of the corpus luteum, reduce progesterone pregnancy or after a mismating. Inadvertent production and potentially decrease administration of prostaglandins to a pregnant pregnancy rates. Treatment with mare is highly likely to cause loss of the prostaglandins prior to ovulation does not pregnancy. However, if the mistake is affect function of the subsequent corpus recognized early, the pregnancy can often be luteum. does not have any adverse saved by daily administration of progesterone. effects on corpus luteum development when administered either prior to or after In the past several years, prostaglandins ovulation. Consequently, prostaglandins or have been used as an alternative to oxytocin oxytocin may be used to evacuate uterine to expel fluid from the uterus of mares. fluid prior to ovulation, but oxytocin appears Prostaglandins have been reported to to be a better choice for treatment of uterine stimulate uterine contractions for 2 to 4 fluid after ovulation is detected. hours or more. In contrast, oxytocin will cause uterine contractions for 30 to 45 Prostaglandins are one of the most widely minutes. It is now relatively common to used hormones in equine reproduction. utilize a combination of uterine lavage and Adherence to the guidelines provided here either oxytocin or prostaglandins to evacuate and by your equine veterinarian can help retained uterine fluid. Uterine fluid ensure the safe effective use of this product. retention is especially common in older mares after breeding and is referred to as persistent mating-induced endometritis. Affected mares are unable to physically clear fluid from their uterus and fluid retention may be associated with a reduction in fertility.

2