Characterization of Luteal Blood Flow and Secretion of Progesterone in Mares Treated With Human Chorionic Gonadotropin for Ovulation Induction or During Early Diestrus

• Luteal function is not improved by hCG therapy during early diestrus in mares.
• Luteal hemodynamic post-treatment with subdoses of hCG is described for the first time.
• Low doses of hCG are efficient to induce ovulations in mares.

Human chorionic gonadotropin (hCG) has been used to induce ovulation and as a luteotrophic agent in cattle. However, the effect of hCG therapy on the functional status of the equine corpus luteum (CL) is unclear. This study aimed to characterize the hemodynamic and secretory function of early CL of mares treated with different doses of hCG at distinct stages of the estrous cycle. Mares were assigned to nine experimental groups (n = 6 mares/group) according to dose of hCG and time of treatment. A single injection of one of three different doses of hCG (750, 1,500, or 2,500 IU) was performed in one of three distinct stages of the estrous cycle: preovulatory follicle ≥35 mm, day of ovulation (D0), or 48 hours after ovulation (D2). In addition, a control group treated with NaCl 0.9% was included in the study. The end points evaluated daily from D0 to D8 were area of the CL, luteal vascularity, number of colored pixels and total pixel intensity, and concentrations of plasma progesterone (P4). No effect (P > .1) of dose or time of treatment was observed for any end point, within each day. Luteal area did not differ throughout the days (P > .1), whereas Doppler parameters and concentrations of plasma P4 presented a progressive increase (P < .05) after ovulation in all groups. Secretory function and luteal hemodynamic were not affected (P > .1) by hCG dose and time of treatment. In conclusion, hCG therapy during estrus or early diestrus, at the doses tested, did not improve P4 secretion or luteal blood flow.