EVIDENCE FOR AN INHIBITORY EFFECT OF INSULIN-LIKE GROWTH FACTOR-I ANDFACTOR-II ON INSULIN-STIMULATED STEROIDOGENESIS BY NONTRANSFORMED OVARIAN GRANULOSA-CELLS

The objective of the present studies was to determine the interaction between insulin and ICE-I and -II on FSH-induced estrogen production b y bovine granulosa cells. FSH-induced estradiol production by granulos a cells from large (greater than or equal to 8mm) follicles (expressed as pg estradiol/10(5) cells/24 h) was maximally increased (P<0.05) by 10-1000 ng/ml of bovine insulin. In comparison, 0.1-10 ng/ml of insul in maximally enhanced (P<0.05) FSH-induced estradiol production in cel ls from small (1-5 mm) follicles. When treated concomitantly, 100 ng/m l of ICE-I or IGF-II inhibited (P<0.05) insulin-induced estradiol prod uction by 74-88% and 44-50% in cultures of cells from small and large follicles, respectively. This inhibition was evident at greater than o r equal to 30 ng/ml of IGF-I. Binding of [I-125]insulin was also inhib ited by IGF-I. In summary, IGF-I and -II have inhibitory effects on in sulin-induced estradiol production by bovine granulosa cells of both s mall and large follicles. Thus, it appears that IGF-I and -II may act as insulin antagonists of hormone-induced aromatase activity in bovine granulosa cells.