Ovary

In Vivo Effects of an Intrafollicular Injection of Insulin-Like Growth Factor-1 on the Mechanism of Follicle Deviation in Heifers and Mares

Abstract
In cattle and mares, free IGF-1 is higher in the future dominant follicle (F1) than in the future largest subordinate follicle (F2) before deviation in diameter or follicle selection is manifested between the two follicles. The effect of IGF-1 on other follicular-fluid factors and on the destiny of F2 were studied in two experiments in each species, using a total of 40 heifers and 42 mares. An injection of IGF-1 was made into F2 at the expected beginning of deviation (heifers, F1 8.5 mm; mares, F1 20.0 mm; Hour 0). In heifers, follicular-fluid was taken from F2 at Hours 3, 6, 12, or 24; each heifer was sampled only once. In mares, sequential F2 samples were taken from each mare at Hours 0, 6, and 24 or at Hours 12 and 24. Transvaginal ultrasound guiding was used for treating and sampling. In heifers, IGF-1 treatment of F2 stimulated the secretion of estradiol (P < 0.05) between Hours 3 and 6 and androstenedione (P < 0.05) between Hours 3 and 12. In F2 of control heifers, estradiol decreased (P < 0.05) and androstenedione did not change significantly. In mares, IGF-1 treatment of F2 did not affect the concentrations of estradiol during the 24 h post-treatment period; androstenedione decreased (P < 0.04) in the IGF-1 group and increased (P < 0.006) in the controls. Compared to control mares, the IGF-1 group had higher (P < 0.04) activin-A at Hours 12 and 24 and higher (P < 0.0006) inhibin-A at Hour 24. After ablating F1 at Hour 24 in mares, F2 became dominant and ovulated in more mares (P < 0.0002) in the IGF-1 group (12/14) than in the controls (2/14). These results are consistent with reported temporal relationships among follicular factors during deviation in both species and indicate that IGF-1 plays a key role in controlling the temporal relationships; however, no indication was found that IGF-1 stimulated estradiol production in mares during the 24 h after treatment.

Key words: Ovary • Estradiol • Follicular development • Growth factors • Steroid hormones

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