Progesterone exposure of seasonally anoestrous ewes alters the expression of angiogenic growth factors in preovulatory follicles

Small dose multiple injections of GnRH given to seasonally anoestrous ewes induce final stages of the preovulatory follicle development but result in high incidence of defective corpora lutea unless animals are primed with progesterone, which completely eliminates luteal dysfunction. Progesterone priming upregulates luteal vascularization, however, its effect on follicular angiogenisis is poorly understood. This study tested the hypothesis that progesterone priming of seasonally anoestrous ewes treated with dose multiple injections of GnRH eliminates defective luteal function by altering the expression of VEGF, VEGFR-2, ANG-1, ANG2 and TIE-2 during early and late preovulatory follicle development. Ten seasonally anoestrous ewes were given 20 mg of progesterone i.m. 3 days before the start of GnRH treatment, while 10 other animals served as controls. Intravenous injections of 500ng GnRH were given to all animals every 2 h for 28 h, followed at 30 h with a 300μg GnRH bolus injection to synchronize the preovulatory luteinizing hormone surge. Ovaries were collected at 24 and 46 h after the start of GnRH treatment. Small (2-2.5 mm) and large (>2.5 mm) follicles were analysed for protein and mRNA expression of the angiogenic factors using immunohistochemistry and in situ hybridization assays. Progesterone priming did not have any significant influence on angiogenic factor levels in small follicles. However, progesterone primed animals showed significantly (P≤0.05) higher levels of VEGF, VEGFR-2, ANG-1 and ANG-2 in large follicles compared to non-primed ones. These data suggest that progesterone priming alters the expression of angiogenic factors in large preovulatory follicles ensuring adequate luteal development and function.