Prostaglandin F2(alpha) regulates distinct physiological changes in early and mid-cycle bovine corpora lutea

Prostaglandin (PG) F2(alpha) is the primary luteolysin in most species. A single treatment with PGF2(alpha) will cause regression of the mid-cycle but not the early-cycle (Days 1-5 after estrus) bovine corpus luteum (CL) despite the presence of similar concentrations of high-affinity PGF2(alpha) receptors (FP receptors). This study was designed to determine whether PGF2(alpha) activated similar intracellular processes in early- and mid-cycle CL. Cows received saline or 25 mg PGF2(alpha) injection (i.m.; n = 6/group) on Day 4 or 11 after onset of the LH surge (induced by GnRH injection), and CL were collected at 4 h after treatment. As expected, CL volumes and luteal weights were not different at 4 h after PGF2(alpha) treatment. Luteal vitamin C concentration and steady-state concentrations of mRNA for 3beta- hydroxysteroid dehydrogenase and for FP receptor were decreased by 4 h in both Day 4 and 11 CL treated with PGF2(alpha) (p < 0.05). These results demonstrate clear actions of PGF2(alpha) in the early CL. In contrast, steady-state concentrations of mRNA encoding PG G/H synthase- 2 (PGHS-2) were increased by treatment with PGF2(alpha) in mid-cycle CL but decreased by PGF2(alpha) in early-cycle CL (p < 0.05). In addition, treatment of mid-cycle but not early-cycle cows with PGF2(alpha) decreased luteal and serum progesterone concentrations by 4 h (p < 0.05). In summary, PGF2(alpha) clearly exerts actions in both early- and mid-cycle bovine CL. The lack of PGF2(alpha)-induced luteolysis in the early CL may be due to specific changes in gene expression, especially PGHS-2, that may prevent intraluteal PGF2(alpha) production and possibly other key luteolytic processes.