Ovarian hyperstimulation (OH) is a technique used during human fertility treatments, such as in vitro fertilisation to stimulate the growth of several follicles resulting in superovulation. However, this treatment alters the normal hormone levels, which has negative effects on the endometrium. Vascular endothelial growth factor (VEGF) is an important protein involved in successful implantation; however the altered hormone profile in OH may impact VEGF and VEGF receptor levels.
The rat OH model is a useful technique in studying changes in the endometrium in response to fertility drugs. Female rats with regular oestrous cycles were IP injected with 20 IU of equine serum gonadotropin followed by 20 IU of human chorionic gonadotropin 24-hours later. The rats were then mated overnight and sacrificed on day 6 (time of implantation) of OH and normal pregnancy. The uterine tissue was then collected and processed for immunohistochemistry, western blot and qPCR analysis.
Bilateral ovariectomies were performed on a second group of rats to determine the hormonal regulation of VEGF. The rats received a subcutaneous injection of either a vehicle control, progesterone, oestrogen, or a combination of progesterone and oestrogen.
The major isoform in the rat uterus, VEGF188 is reduced at the time of implantation in OH compared to normal pregnancy, whereas there is no change in VEGF164 mRNA. VEGFR2 protein is also reduced at the time of implantation in OH. Our ovariectomy studies show that both VEGF188 and VEGF164 are significantly decreased by oestrogen, and to a lesser extent, progesterone, compared to control.
The altered progesterone:oestrogen ratio seen in OH, taken together with our ovariectomy studies explains the changes to VEGF mRNA in OH at the time of implantation. Since VEGF is important during implantation, the changes to VEGF and VEGFR-2 levels in the endometrium may help explain the observed lower endometrial receptivity following OH.