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

- The mare is a valuable model for studying the mechanisms regulating ovarian blood flow.
- Progesterone does not seem to have a role in acute regulation of ovarian vascular perfusion during diestrus.
- LH and hCG might have direct effects on of ovarian vascular perfusion during diestrus.
- GnRH effects on ovarian blood flow might be mediated by LH or by direct effects on ovarian blood vessels.

The objective of the present study was to investigate the effect of reproductive hormones (GnRH, hCG, LH and progesterone) on the regulation of corpus luteum (CL) and ovarian blood flow. Diestrous mares received a single treatment of saline, 100 μg gonadorelin (GnRH), or 1500 IU hCG 10 days after ovulation. Plasma LH and progesterone concentrations, resistance index (RI) for ovarian artery blood-flow, and percentage of corpus luteum (CL) with color-Doppler signals of blood flow were determined immediately before treatment (hour 0) and at hours 0.25, 0.5, 1, 1.5, 2, 3, 4, 5, and 6. In the GnRH group, LH increased (P < 0.0001) between hours 0 and 0.25 and then progressively decreased; concentration of LH was not affected in the saline and hCG groups. Progesterone concentration was not different among groups. In the GnRH group, RI tended (P < 0.07) to decrease between hours 0 and 1.5 and increased (P < 0.01) between hours 1.5 and 4. In the hCG group, two transient RI decreases (P < 0.05) occurred before hour 2. The percentage change from hour 0 in the percentage of CL with blood-flow signals was greater at hour 0.5 in the GnRH group than in the saline group and was intermediate in the hCG group. The similarity among groups in progesterone concentration indicated that changes in progesterone were not involved in the GnRH and hCG stimulation of ovarian vascular perfusion. Effects of treatment might have been mediated through LH; however, since hCG biological activity is primarily LH-like, the differences in timing and degree of ovarian and luteal blood flow changes after GnRH or hCG administration in the present study suggest that GnRH might have a direct effect on ovarian blood vessels and vascular control.