Intrafollicular effect of IGF1 on development of follicle dominance in mares

The effect of an injection of a supraphysiologic dose of rhIGF1 into the second-largest ovarian follicle (F2) at the expected beginning of deviation (F1, ≥20 mm; Day 0) on development of dominance by F2 was studied in mares (n = 16; controls, n = 8). F1 became dominant (≥28 mm) in 8 of 8 and 15 of 16 follicles in the controls and treated groups, respectively. The incidence of dominance (P < 0.001) and ovulation (P < 0.02) for F2 was greater for the IGF1 group (13 of 16 and 10 of 16) than for the controls (1 of 8 and 1 of 8). There were day effects but no group effects or group-by-day interactions for systemic FSH, LH, estradiol, or ir-inhibin during the 4 days after treatment. In another experiment, treatment of every follicle, excluding F1, when it reached ≥20 mm after the expected beginning of deviation resulted in dominance by 8 of 12 follicles treated with rhIGF1 on Days 1–3 (n = 8 mares). Results demonstrated that the IGF1 system plays a pivotal intrafollicular role in the deviation mechanism without altering systemic concentrations of the gonadotropins and ovarian follicular hormones.