Effect of GnRH and hCG on progesterone concentration and ovarian and luteal blood flow in diestrous mares.

The research investigates how certain reproductive hormones affect the blood flow in ovarian and luteal bodies in diestrous mares. The study found that while both Gonadotropin-releasing hormone (GnRH) and human chorionic gonadotropin (hCG) induced changes in blood flow, the effects were timed differently, suggesting possible direct influence by GnRH on ovarian blood vessels.

Objective

• The main objective of this study was to examine the impact of reproductive hormones (such as GnRH, hCG, LH, and progesterone) on the regulation of blood flow in the corpus luteum (CL) and ovaries.

Method

• Mares in the stage of their sexual cycle known as diestrus were given a single dose of saline, 100μg gonadorelin (GnRH), or 1500IU hCG, 10 days after ovulation.
• Plasma LH and progesterone concentrations were measured, along with resistance index (RI) for the blood flow in ovarian arteries, and the percentage of CL with blood flow signals. These measurements were taken just before treatment and at nine different time points post-treatment, for six hours.

Findings

• In the GnRH group, LH levels noticeably increased then gradually declined. This change was not observed in the saline and hCG groups.
• The progesterone concentration remained consistent across the groups indicating the changes in progesterone were not contributing to the GnRH and hCG induced increases in ovarian vascular perfusion.
• The GnRH group saw a temporary drop, then a rise in RI, while in the hCG group, two RI decreases occurred before the 2-hour post-treatment mark.
• A greater percentage change in CL with blood flow signals from the start hour was observed at the 30-minutes post-treatment mark in the GnRH group compared to the saline group, with the hCG group showing an intermediate percentage change.

Conclusions

• Although it might appear that the effects are mediated through LH, since hCG also acts in a similar way to LH, the varying timing and degree of blood flow alterations after GnRH or hCG treatment suggest that GnRH may directly affect ovarian blood vessels and vascular control.