Does injection of prostaglandin F 2α (PGF 2α) cause ovulation in anestrous Western White Face ewes?

Abstract

In a previous study in our laboratory, treatment of non-prolific Western White Face (WWF) ewes with PGF 2α and intravaginal sponges containing medroxyprogesterone acetate (MAP) on ∼Day 8 of a cycle (Day 0 = first ovulation of the interovulatory interval) resulted in ovulations during the subsequent 6 days when MAP sponges were in place. Two experiments were performed on WWF ewes during anestrus to allow us to independently examine if such ovulations were due to the direct effects of PGF 2α on the ovary or to the effects of a rapid decrease in serum concentrations of progesterone at PGF 2α-induced luteolysis. Experiment 1: ewes fitted with MAP sponges for 6 days ( n = 12) were injected with PGF 2α ( n = 6; 15 mg im), or saline ( n = 6) on the day of sponge insertion. Experiment 2: ewes received progesterone-releasing subcutaneous implants ( n = 6) or empty implants ( n = 5) for 5 days. Six hours prior to implant removal, all ewes received a MAP sponge, which remained in place for 6 days. Ewes from both experiments underwent ovarian ultrasonography and blood sampling once daily for 6 days before and twice daily for 6 days after sponge insertion. Additional blood samples were collected every 4 h during sponge treatment. Experiment 1: 4–6 (67%) PGF 2α-treated ewes ovulated ∼1.5 days after PGF 2α injection; these ovulations were not preceded by estrus or a preovulatory surge release of LH, and resulted in transient corpora hemorrhagica (CH). The growth phase was longer ( P < 0.05) and the growth rate slower ( P < 0.05) in ovulating versus non-ovulating follicles in PGF 2α-treated ewes. Experiment 2: in ewes given progesterone implants, serum progesterone concentrations reached a peak (1.72 ng/mL; P < 0.001) on the day of implant removal and decreased to basal concentrations (<0.17 ng/mL; P < 0.001) within 24 h of implant removal. No ovulations occurred in either the treated or the control ewes. We concluded that ovulations occurring after PGF 2α injection, in the presence of a MAP sponge, could be due to a direct effect of PGF 2α at the ovarian level, rather than a sudden decline in circulating progesterone concentrations.