Exogenous insulin and additional energy affect follicular distribution, follicular steroid concentrations, and granulosa cell human chorionic gonadotropin binding in swine

The objective was to determine whether exogenous insulin and dietary energy interact to affect follicular development in gilts. In a 2 x 2 x 2 completely randomized design, main effects were level of dietary energy (5771 or 9960 kcal metabolizable energy/day beginning on Day 12 of the estrous cycle), insulin dosage (0 or 0.4 IU/kg twice daily beginning on Day 15 of the cycle), and day of cycle at ovary removal (Day 17 or Day 19). Percentage of follicles designated small (≤ 3 mm diameter) decreased from Day 17 to Day 19 of the cycle, and the percentage of large follicles (≥7 mm) increased (p < 0.05). Insulin interacted with day of the cycle (P < 0.05) to affect distribution of medium (4-6 mm) and macroscopically atretic follicles. Percentage of atretic follicles increased from Day 17 to Day 19 in saline-treated (from 15.5% to 38.2%) but not in insulin-treated animals (6.3% to 10.7%). Percentage of medium (4-6 mm) follicles decreased from Day 17 to Day 19 in saline-treated gilts (from 41.7 to 16.6%) but not in insulin-treated gilts (39.8% to 35.1%). Intrafollicular testosterone and progesterone concentrations were not affected by treatments. In medium follicles, the ratio of estradiol to progesterone was greater (p < 0.05) for insulin-treated gilts on Day 17 than for the other treatment combinations. Binding of ¹²⁵I-human chorionic gonadotropin (hCG) by granulosa cells of medium follicles was similar on Days 17 and 19 in saline-treated animals but decreased over the same period in insulin-treated gilts (p < 0.05). For large follicles, the ratio of estradiol to progesterone was greater for gilts receiving high energy than for the other three treatment combinations. Steroidogenic effects of insulin on medium follicles and of dietary energy on large follicles were similar. Both insulin and dietary energy appeared to alter steroidogenesis and follicular development. The unique effect of exogenous insulin was to maintain medium-sized follicles by reducing atresia.