Induced abortion with two prostaglandin F2 alpha analogues in mares: plasma progesterone changes

Three experiments were conducted to test the abortifacient effects of PGF2 alpha analogues on mares during midgestation (average gestation length 141.5 days). The progesterone concentration was measured by radioimmunoassay. In experiment 1. five mares received an injection of PGF2 alpha analogue (fluprostenol: 500 micrograms intramuscularly) and a second injection either at 24, 48, of 72 h. Although the progesterone concentration decreased (P less than 0.05) an average of 44 per cent in 24 h, none of the pregnancies were terminated. In experiment 2, beginning at least 10 days after experiment 1, the same five mares were given PGF2 alpha analogue as follows: 250 micrograms intravaginally and 500 micrograms intramuscularly. The treatment was repeated 48 h later. Progesterone concentrations had not increased since experiment 1 and did not decrease during the 48 h following either injection. In experiment 3, six mares (average gestation length 162 days) were treated every 6 or 12 h with PGF2 alpha analogue (cloprostenol: 375 micrograms) until expulsion of the fetus occurred at 47 +/- 25 h after the initial injection; the mares received an average of 5 treatments. The progesterone concentration averaged 22 +/- 7 ng/ml before the initial PGF2 alpha treatment, decreased (P less than 0.05) to 8.4 +/- 2.7 ng/ml by 12 h before expulsion and 1-8 +/- 0.4 ng/ml 12 h after fetal expulsion. The progesterone concentration remained below 1.0 ng/ml for the next 4 days. However, only one of six mares exhibited estrual behavior after induced abortion.