Expression of VEGF-A, Flt-1, and Flk-1 in the arterial endothelial cells of the uterine broad ligament throughout the estrous cycle

The aim of the present study was to determine the immunoreactivity of vascular endothelial growth factor (VEGF-A) and its two receptors, viz., Flt-1 (fms-like tyrosine kinase) and Flk-1 (fetal liver kinase), on the surface of endothelial cells of the uterine artery and its branches and of the arcuate arteries in the area of the uterine broad ligament during various phases of the estrous cycle in the pig. We also investigated their expression to determine whether this was phase-related. The highest immunoreactivity for VEGF-A was observed in the uterine artery and arcuate arteries at the early luteal phase and in the branches of the uterine artery during the follicular phase of the estrous cycle. The strongest immunostaining intensity of Flt-1 was found in the uterine artery and its branches at the follicular phase and in arcuate arteries at the mid-luteal phase, whereas Flk-1 immunostaining was at its highest in the uterine artery at the mid-luteal phase and in the branches of the uterine artery and arcuate arteries at the follicular phase. Additionally, VEGF-A expression was assessed by semi-quantitative Western blot analysis, which revealed significantly higher levels of VEGF-A protein during the early luteal and the follicular phase of the estrous cycle (P < 0.001). The phase-related differences in the immunoreactivity and expression of VEGF-A and VEGF receptors suggest that these factors are hormone-dependent during the estrous cycle in the pig.