ENDOGENOUS PROSTAGLANDIN SECRETION DURING CLOPROSTENOL-INDUCED ABORTION IN MARES

Repeated administration of prostaglandin is the treatment of choice fo r the termination of pregnancy in mares more than 40 days pregnant. Ev en though it is well documented that PGF-2 alpha or analogue needs to be administered every 12 - 24 h for successful induction of abortion, little is known about the underlying endocrine changes and the mechani sm by which abortion occurs. The aim of this study was to characterize the changes in PGF-2 alpha, progesterone and estrogen secretion durin g prostaglandin-induced abortion. Six mares, 82 - 102 days pregnant, w ere treated daily with 250 mu g cloprostenol, blood was collected at 1 -h intervals until fetal expulsion and pregnancy examination was perfo rmed daily. Four mares, 92 - 97 days pregnant, received no treatment b ut were subjected to the same hourly blood collections and daily genit al examinations described for cloprostenol-treated mares for 3 days. M ean time from first cloprostenol administration until fetal expulsion was 48.6 +/- 5.6 h and required 2.8 +/- 0.2 cloprostenol administratio ns. In all mares, progesterone concentrations decreased in a near line ar manner after the first cloprostenol administration and were invaria bly low (1.3 +/- 0.2 ng ml(-1), mean +/- SEM) at the time of fetal exp ulsion. Mean estrogen secretion remained unchanged until 5 h before fe tal expulsion and then decreased rapidly to non-pregnant levels. Endog enous PGF-2 alpha secretion rate increased with each cloprostenol admi nistration and culminated in sustained PGF-2 alpha secretion which per sisted until fetal expulsion was completed. From these results we conc lude that cloprostenol-induced abortion is associated with endogenous PGF-2 alpha secretion, fetal expulsion coincides with sustained PGF-2 alpha secretion and low progesterone concentrations and plasma estroge n concentrations remain unchanged until hours before fetal expulsion.