Differential Steroidogenic Response of Subpopulations of Porcine Granulosa Cells to Insulin-Like Growth Factor-1 (IGF-1) or IGF-1 Analogs1

Two experiments were conducted to examine responses of porcine granulosa cells to insulin-like growth factor-1 (IGF-1) or IGF-I analogs (des [1–3] and Long R3) that have reduced affinity for IGF-binding proteins (IGFBP). Both experiments evaluated estradiol and IGFBP production by granulosa cells after separation of cells into subpopulations that maintain long-term estradiol production in vitro (tightly bound) and those that do not (weakly associated). Granulosa cells were obtained from mediumsized follicles (4–6 mm) at random stages of the estrous cycle in experiment 1 and from the 10 largest follicles per ovary at 0 or 48 h after weaning in experiment 2. Follicle diameter and follicular fluid estradiol concentrations increased with time after weaning (p < 0.05). Tightly bound cells produced more estradiol than weakly associated cells at 24–120 h of culture in experiment 1 and from 0 to 48 h in experiment 2 (p < 0.05). In tightly bound but not weakly associated cells, IGF-1 stimulated estradiol production. The IGF analogs were more potent stimulators than IGF-1 in experiment 1 (p < 0.05); and in experiment 2, this response was restricted to cells collected at 48 h after weaning. Conversely, tightly bound cells obtained at 0 h after weaning responded similarly to IGF-1 and des (1–3). During the final 48 h of culture, weakly associated cells produced greater quantities of 28–30-kDa IGFBP than did tightly bound cells in response to IGF-1 or analogs (both experiments; p < 0.05). Concentration of 40–44-kDa IGFBP was influenced minimally by IGF-I or its analogs. We conclude that 1) tightly bound granulosa cells produce greater amounts of estradiol in vitro than weakly associated cells when sampled from medium-sized follicles; 2) tightly bound cells from follicles of sows at random stages of the cycle or at 48 h after weaning produce more estradiol in response to IGF analogs than to IGF-1; and 3) tightly bound cells secrete less 20–30-kDa IGFBP in long-term culture than weakly associated cells. These results indicate that IGFBP modulate secretion of estradiol in porcine granulosa cells in vitro and implicate their potential for in vivo regulation.