Granulosa cells from diethylstilbestrol-treated immature rats were cultured in a defined medium on collagen-coated plates. Thymidine incorporation was significantly increased by insulin (ED50, 656 ± 110 ng/ml) and insulin-like growth factor (IGF-I; ED50, 95 ± 10 ng/ml). Insulin and IGF-I stimulations were amplified by methylisobutylxanthine an inhibitor of phosphodiesterase activity. The effect of both peptides were also enhanced by low doses of (Bu)2cAMP (0.2-1 mM). In contrast, higher concentrations were inhibitory. Similarly, FSH produced a biphasic enhancement of the insulin- and IGF-I-stimulated DNA synthesis. Maximal effects (2- to 6-fold increases) were observed with the lower doses (2-20 ng/ml) of the gonadotropin. FSH enhancement of IGF-I-stimulated DNA synthesis was dependent on cell density. Plating densities of 3-5 × 105 cells/cm2 were required for a maximal interaction. It is concluded that FSH, acting through a cAMP-mediated pathway, may regulate granulosa cell proliferation by modulating the mitogenic effects of insulin and/or IGF-I.
