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 an high incidence of defective CL unless animals are primed with progesterone, which completely eliminates luteal dysfunction. Progesterone priming upregulates luteal vascularization; however, its effect on follicular angiogenesis 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 vascular endothelial growth factor (VEGF), VEGF receptor-2, angiopoietin (ANG)-1, ANG-2, and TIE-2 during early and late preovulatory follicle development. Ten seasonally anoestrous ewes were given 20 mg of progesterone im 3 days before the start of GnRH treatment; 10 other animals served as controls. Intravenous injections of 500 ng GnRH were given to all animals every 2 hours for 28 hours, followed at 30 hours with a 300-μg GnRH bolus injection to synchronize the preovulatory LH surge. Ovaries were collected at 24 and 46 hours after the start of GnRH treatment. Small (2-2.5 mm) and large (>2.5 mm) follicles were analyzed for protein and mRNA expression of the angiogenic factors using immunohistochemistry and in situ hybridization assays. Progesterone priming did not have an 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 with nonprimed ones. These data suggest that progesterone priming alters the expression of angiogenic factors in large preovulatory follicles, ensuring adequate luteal development and function.