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, 100μg gonadorelin (GnRH), or 1500IU hCG 10days 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