Effect of embryos on luteolysis and termination of early pregnancy in sheep with cloprostenol.

An investigation of the control of the function of the corpus luteum of early pregnancy in sheep has been carried out by assessing the luteolytic effects of a dose of 100 micrograms of the synthetic prostaglandin, cloprostenol. Cloprostenol injected into 21-day pregnant sheep carrying single or multiple embryos caused luteolysis and termination of pregnancy within 2--5 days in 8 of 23 sheep. The mean numbers of corpora lutea in these ewes differed significantly from those which carried their pregnancy (1.8 +/- 0.3, SEM, vs 3.2 +/- 0.4; P less than 0.05). It appeared that the greater number of embryos, or a greater mass of embryonic tissue, afforded increased protection to the corpora lutea through unidentified antiluteolytic or luteotrophic factors. The number of corpora lutea present did not appear to be an operative factor, as the same dose of cloprostenol was luteolytic in 43 of 46 nonpregnant ewes with 1--6 corpora lutea; the 3 refractive ewes each having only 1 corpus luteum. The progesterone concentration in the plasma of pregnant ewes which underwent luteolysis following cloprostenol treatment was 0.5 +/- 0.1 ng/ml 48 h after injection, while that of the ewes remaining pregnant was significantly higher at 2.1 +/- 0.3 ng/ml. Seven of these refractive ewes which were given a second injection of cloprostenol on Day 28 of pregnancy subsequently experienced luteolysis and abortion. The corpora lutea of the remaining 8 ewes were not affected by saline injections given on Day 28, but 4 of these ewes later aborted after Day 44. It is concluded that pregnancies on ewes carrying 1 embryo can be terminated by a single dose of 100 micrograms cloprostenol given around Day 21 of pregnancy, but 2 injections given about 7 days apart may be needed when multiple embryos are present. Hormonal control of the sensitivity of the corpus luteum to luteolytic prostaglandins appears to reside in the conceptus in such a way that a decrease in sensitivity is related to the number of embryos present.