Stimulation of the largest subordinate follicle by intrafollicular treatment with insulin-like growth factor 1 is associated with inhibition of the dominant follicle in heifers

The effect of intrafollicular treatment of the second-largest follicle (F2) with insulin-like growth factor (IGF) 1 on the largest follicle (F1) and F2 was studied in heifers. Treatment of F2 was done when F1 reached ≥8.2 mm (expected beginning of follicle deviation; Day 0 or Hour 0). In each of two experiments, three groups (n = 6 or 7 heifers/group) were used: controls, F2 treated with vehicle and F2 treated with IGF1. The IGF1 treatment consisted of 200 μg of recombinant human IGF1 (pharmacological dose) in 20 μL of vehicle. In Experiment 1, the hypothesis that treatment of F2 with IGF1 has a stimulatory effect on F2 was supported by a greater (P < 0.05) incidence of F2 dominance (≥10 mm) in the IGF1 group (71%) than in the other two groups (8%), and a greater (P < 0.02) growth rate of F2 on Days 0–2. Unexpectedly, treatment of F2 with IGF1 had an inhibitory effect on F1, as indicated by a reduced (P < 0.03) growth rate of F1 during Days 0–1 and Days 0–4 and a lesser (P < 0.05) maximum diameter of F1 in the IGF1 group. In Experiment 2, the hypothesis of an inhibitory effect on F1 when F2 was treated with IGF1 was supported by a lesser (P < 0.04) increase in diameter of F1 and a lesser (P < 0.04) percentage of follicle wall with power-Doppler signals of blood flow between Hours 0 and 14 in the IGF1 group. Circulating concentrations of FSH and LH were not altered significantly in either experiment. In conclusion, treatment of F2 with IGF1 at the expected beginning of deviation had a stimulatory effect on F2, but an inhibitory effect on F1.