Functional relationships among intrafollicular insulin-like growth factor 1, circulatory gonadotropins, and development of the dominant follicle in mares

The functional relationships among intrafollicular free insulin-like growth factor 1 (IGF1), circulatory gonadotropins, and development of the dominant follicle were studied in 40 mares in two experiments. A GnRH antagonist (Acyline) was given i.m. at the expected beginning of follicular deviation (largest follicle or F1 ≥ 20 mm; Day 0) alone (Acyline group) or in combination with intrafollicular treatment of F1 with rhIGF1 (Acyline/IGF1 group). In Experiment 1, blood samples, follicular-fluid samples, and diameter of F1 were taken on Days 1 and 2. In Experiment 2, daily follicular diameter and blood samples were taken from Day 0 to ovulation. The GnRH antagonist induced a 50% decrease in circulatory FSH concentrations for 1 d and in LH for 2 d. In Experiment 1, control and Acyline/IGF1 groups had greater intrafollicular free IGF1 (P < 0.05) and inhibin-A concentrations (P < 0.08) than the Acyline group. The intrafollicular concentration of estradiol on Day 2 was greater (P < 0.05) in the control group than in the Acyline and the Acyline/IGF1 groups. In Experiment 2, a decrease in diameter of F1 in the Acyline group was followed by a new follicular wave. All IGF-treated follicles grew and ovulated. Results indicated that the increase in intrafollicular free IGF1 observed in F1 in association with deviation is gonadotropin dependent. During the period of lesser gonadotropin concentrations from Acyline treatment, intrafollicular IGF1 stimulated follicular growth and inhibin concentrations, but not intrafollicular estradiol production.