ADMINISTRATION OF PROSTAGLANDIN-F2-ALPHA TO HEIFERS WITH PERSISTENT CORPORA-LUTEA FOLLOWING PROSTAGLANDIN-F2-ALPHA IMMUNIZATION - ESTRUS AND OVARIAN RESPONSES

The objectives were to determine (1) if persistent corpora lutea (CL), formed following prostaglandin F-2 alpha (PGF) immunization, would un dergo luteolysis after PGF administration, and (2) the fate of the sub sequently formed CL. Thirty-one heifers were immunized against PGF con jugated to human serum albumin, with diethylaminoethyl (DEAE)-dextran adjuvant, and were confirmed to have persistent CL 172 days after firs t immunization (i.e. day of PGF/saline treatment). Heifers were blocke d by antibody titre and, within block, randomly assigned to one of thr ee treatments: (1) 5 ml intramuscular (i.m.) saline injection (control s; n = 10); (2) 2 ml i.m. PGF analogue (500 mu g cloprostenol) injecti on (n = 11); (3) 5 ml i.m. synthetic PGF (25 mg dinoprost) injection ( n = 10). Blood samples were collected for progesterone assay on a dail y basis from 2 days before PGF injection until 10 days after injection , and at 2-4 day intervals thereafter. Heifers were checked for oestro us behaviour twice daily with the aid of vasectomized bulls. Within 6 days of injection, persistent CL regressed in all (21/21) PGF-treated heifers compared with 1/10 (P < 0.01) of the control heifers. The mean intervals from PGF injection until progesterone concentrations decrea sed below 0.5 ng ml(-1) (2.6 +/- 0.28 and 3.2 +/- 0.29 days) or from P GF 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 (11/11) than dinoprost (7/10) injection . Following PGF-induced luteolysis, the newly formed CL either became persistent(n = 7; mean +/- SEM titre, 36 +/- 6.6% binding) or underwen t normal luteolysis (n = 14; mean +/- SEM titre, 12 +/- 1.1% binding; P < 0.01) within the 32 day period following PGF injection. In conclus ion, persistent CL induced by PGF immunization can be induced to regre ss using either synthetic PGF or an analogue; the fate of the subseque nt CL that forms is dependent on the level of PGF antibody titres pres ent.