Secretion of equine chorionic gonadotropin and its association with supplementary corpus luteum formation and progesterone concentration in Hokkaido native pony recipient mares.

The objectives of this study were to determine the plasma profile of equine chorionic gonadotropin (eCG) and its association with the formation of supplementary corpus luteum (CL) and plasma progesterone concentrations in embryo transfer Hokkaido native pony recipient mares. Blood samples and transrectal ultrasound examination of the reproductive tract were carried out weekly from the day of ovulation until week 32 of gestation (n = 4). Plasma concentrations of eCG and progesterone were measured by enzyme immunoassays. The eCG concentration was first detectable at week 5 for 2 mares and at week 6 for another 2 mares. Immediately after detection, the mean plasma eCG concentrations were observed to rise sharply and reach a peak at week 8. The concentrations then declined dramatically to a baseline (<0.5 IU/mL) by week 21. Plasma progesterone p=p concentrations increased in 2 phases. First, a sharp increase from 0.18 ± 0.05 ng/mL at ovulation to 15.9 ± 4.6 ng/mL at week 1 was observed, then a decrease to 9.69 ± 2.27 ng/mL by week 2, and maintained at this level until week 5 of gestation. The onset of the second rise occurred at week 6 and was observed to peak to 58.3 ± 21.8 ng/mL at week 10, then gradually declined to <10 ng/mL by week 26. The supplementary CLs were first detectable by pregnancy week 6 and 7 for 2 mares each. All supplementary and primary CLs regressed by week 26 for 3 mares and by week 30 for the remaining mare. The mean number of supplementary CL was 4.5 ± 0.8 and their formation in the right ovary (66.7%, 12/18) was higher (P < 0.05) than that in the left ovary (33.3%, 6/18). Among the mares, 1 mare that developed only 2 supplementary CL had 35% lower level of peak eCG and 65% lower concentration of peak progesterone compared with other 3 mares that had 5 or 6 supplementary CL. In conclusion, development of supplementary CL and blood concentrations of progesterone from around day 40 of gestation were associated with eCG concentration. The total number of supplementary CL formation in the present study in embryo transfer Hokkaido native pony recipient mares seemed higher than previously reported supplementary CL number in pregnant mares, with a greater rate in the right ovary than in left.