Administration of prostaglandin F2α to heifers with persistent corpora lutea following prostaglandin F2α immunization: oestrus and ovarian responses

The objectives were to determine (1) if persistent corpora lutea (CL), formed following prostaglandin F2α (PGF) immunization, would undergo luteolysis after PGF administration, and (2) the fate of the subsequently formed CL. Thirty-one heifers were immunized against PGF conjugated to human serum albumin, with diethylaminoethyl (DEAE)-dextran adjuvant, and were confirmed to have persistent CL 172 days after first immunization (i.e. day of PGF/saline treatment). Heifers were blocked by antibody titre and, within block, randomly assigned to one of three treatments: (1) 5 ml intramuscular (i.m.) saline injection (controls; n = 10); (2) 2 ml i.m. PGF analogue (500 μg cloprostenol) injection (n = 11); (3) 5 ml i.m. synthetic PGF (25 mg dinoprost) injection (n = 10). Blood samples were collected for progesterone assay on a daily basis from 2 days before PGF injection until 10 days after injection, and at 2–4 day intervals thereafter. Heifers were checked for oestrous behaviour twice daily with the aid of vasectomized bulls. Within 6 days of injection, persistent CL regressed in all (2121) PGF-treated heifers compared with 110 (P < 0.01) of the control heifers. The mean intervals from PGF injection until progesterone concentrations decreased below 0.5 ng ml−1 (2.6 ± 0.28 and 3.2 ± 0.29 days) or from PGF to expression of oestrus (4.0 ± 0.39 and 3.7 ± 0.36 days) were not different between heifers treated with cloprostenol or dinoprost, respectively. A greater (P < 0.05) number of heifers were detected in oestrus following cloprostenol (1111) than dinoprost (710) injection. Following PGF-induced luteolysis, the newly formed CL either became persistent (n = 7; mean ± SEM titre, 36 ± 6.6% binding) or underwent normal luteolysis (n = 14; mean ± SEM titre, 12 ± 1.1% binding; P < 0.01) within the 32 day period following PGF injection. In conclusion, persistent CL induced by PGF immunization can be induced to regress using either synthetic PGF or an analogue; the fate of the subsequent CL that forms is dependent on the level of PGF antibody titres present.