Effect of PGF2alpha and 13,14-dihydro-15-keto-PGF2alpha (PGFM) on corpora luteal function in nonpregnant mares

The objective of this study was to determine if the primary circulating metabolite of PGF2alpha, 13,14-dihydro-15-keto-PGF2alpha (PGFM), is biologically active and would induce luteolysis in nonpregnant mares. On Day 9 after ovulation, mares (n = 7/group) were randomly assigned to receive: 1) saline control, 2) 10 mg PGF2alpha or 3) 10 mg PGFM in 5 mL 0.9% sterile saline i.m. On Days 0 through 16, blood was collected for progesterone analysis. In addition, blood was collected immediately prior to treatment, hourly for 6 h, and then at 12 and 24 h after treatment for progesterone and PGFM analysis; PGFM was measured to verify that equivalent amounts of hormone were administered to PGF2alpha- and PGFM-treated mares. Mares were considered to have undergone luteolysis if progesterone decreased toor = 1.0 ng/mL within 24 h following treatment. Luteolysis was induced in 0/7 control, 7/7 PGF2alpha-treated, and 0/7 PGFM-treated mares. There was no difference (P0.1) in the occurrence of luteolysis in control and PGFM-treated mares. More (P0.001) PGF2alpha-treated mares underwent luteolysis than control or PGFM-treated mares. There was no difference (P0.1) in progesterone concentrations between control and PGFM-treated mares on Days 10 through 16. Progesterone concentrations were lower (P0.01) on Days 10 through 14 in PGF2alpha-treated compared with control and PGFM-treated mares. There was no difference (P0.05) in PGFM concentrations between PGF2alpha- and PGFM-treated mares; PGFM concentrations in both groups were higher (P0.001) than in control mares. These results do not support the hypothesis that PGFM is biologically active in the mare, since there was no difference in corpora luteal function between PGFM-treated and control mares.