DIFFERENTIAL STEROIDOGENIC RESPONSE OF SUBPOPULATIONS OF PORCINE GRANULOSA-CELLS TO INSULIN-LIKE GROWTH-FACTOR-I (IGF-1) OR IGF-1 ANALOGS

Two experiments were conducted to examine responses of porcine granulosa cells to insulin-like growth factor-1 (IGF-1) or IGF-1 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 medium-sized 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-I 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-1 or its analogs. We conclude that I) 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.