Progesterone, 17β-estradiol, and IGF-I plasma concentrations during the estrus cycle in the mare

The physiology of reproduction in the mare is regulated by the action of several hormones. Among these, progesterone (P4), 17 beta-estradiol (E2) and the insulin-growth factor 1 (IGF-I) play major roles. The aim of this study is to give a complete picture about the hormonal variation during the estrus cycle in the mare, evaluating P4, E2, and IGF-I plasma levels between two subsequent ovulations. Ten mares were enrolled and were ultrasonographically examined every 12 hours starting from the beginning of estrus until ovulation; then ultrasound examination was performed every day before the blood sampling and again every 12 hours from the beginning of the following estrus until the second ovulation. Blood samples were collected from each mare every morning between 8 and 10 a.m. from the day of the first ovulation until the following ovulation. P4, E2 and IGF-I were analyzed by RIA. The mean estrus cycle length was 22,1 days and the mean preovulatory follicle diameter was 41,3 mm. The mean P4 value at ovulation was 0,15 ng/ml; the following days P4 progressively increased, reaching values higher than 10 ng/ml from 5 to 13 days after ovulation. The mean plasmatic concentration of E2 in the day of ovulation was 6,5 pg/ml. The decrease of E2 concentrations progressively continued until 9 days post-ovulation, then a rapid increase of plasmatic concentrations could be observed. Considering both the ovulations, IGF-I ranged between 40,1 and 80 ng/ml. The higher value was recorded at day 19, corresponding at the first day of estrus, even if values continue to jump during the whole estrus cycle.