EFFECTS OF INSULIN-LIKE GROWTH-FACTOR-I AND INSULIN ON PROLIFERATION AND ON BASAL AND LUTEINIZING HORMONE-INDUCED STEROIDOGENESIS OF BOVINETHECAL CELLS - INVOLVEMENT OF GLUCOSE AND RECEPTORS FOR INSULIN-LIKE GROWTH-FACTOR-I AND LUTEINIZING-HORMONE

The objective of the present study was to determine the effects of IGF -I and insulin on cell proliferation, LH receptors, and basal and LH-i nduced progesterone and androstenedione production by bovine thecal ce lls. Cells from large (greater than or equal to 8 mm) bovine follicles were cultured for 1 or 2 d in medium containing 10% fetal calf serum (FCS) and treated for 1 or 2 d in serum-free medium with IGF-I, insuli n, and(or) LH. Treatment with 30 and 100 ng/mL of IGF-I for 1 or 2 d i ncreased thecal cell numbers in the absence of LH regardless of whethe r treatments were initiated after 1 or 2 d of exposure to 10% FCS. Cot reatment with LH reduced the stimulatory effect of IGF-I on thecal cel l numbers. Insulin at 10 and 100 ng/mL increased cell numbers in the p resence of LH. Both IGF-I and insulin were ineffective at stimulating thecal cell progesterone or androstenedione production in the absence of LH. However, IGF-I and insulin increased(P < .05) androstenedione a nd progesterone production in the presence of LH. Alone, LH had little or no effect on androstenedione and progesterone production, whereas in the presence of 30 and 100 ngl mt IGF-I or 1 to 100 ng/mL insulin, LH stimulated(P < .05) androstenedione production. The stimulatory eff ects of IGF-I on cell proliferation and progesterone production were n ot detected in the presence of 100 ng/mL insulin. However, co-treatmen t with various doses of IGF-I and 100 ng/mL insulin further increased androstenedione production above that seen with insulin alone. In gluc ose-deficient medium, 25 to 75 mg/dL of glucose increased (P < .05) th ecal cell proliferation, progesterone production, and androstenedione production. In the absence or presence of glucose, insulin (100 ng/mL) increased (P < .05) thecal cell proliferation, progesterone productio n, and androstenedione production. Treatment with 3, 10, or 100 ng/mL LH had no effect(P > .10) on the numbers of IGF-I binding sites on the cal cells but increased (P < .05) androstenedione production. Treatmen t with 10 and 100 ng/mL IGF-I increased (P < .01) numbers of LH/hCG bi nding sites. These results indicate that IGF-I and insulin may each pl ay a significant role in thecal cell mitogenesis and LH-induced thecal cell steroidogenesis during follicular development in cattle and that glucose enhances these effects. Furthermore, the synergism between IG F-I and LH on increasing steroidogenesis does not seem to be mediated through increased binding sites for IGF-I in bovine thecal cells but r ather, in part, through increased binding sites for LH.