Comparative effects of insulin and insulin-like growth factor-1 on follicle-stimulating hormone-induced responses in porcine granulosa cells

The effect of insulin and insulin-like growth factor-1 (IGF-1) on progesterone secretion by porcine granulosa cells and their modulatory effect on follicle-stimulating hormone (FSH)-induced responses were examined. For comparative purposes, growth hormone (GH), previously shown to stimulate IGF-1 secretion, was also included. Granulosa cells from ovarian follicles (3 to 5 mm) were cultured in multiwell plates for the first 48 hours, either in the presence or absence of 1% fetal bovine serum (FBS). Following plating, all cultures were maintained in serum-free media. The addition of only insulin, but not IGF-1 or GH, enhanced progesterone secretion under both culture conditions. When low-density lipoprotein was provided as steroid substrate, a stimulatory effect of insulin on progesterone accumulation was observed with a minimum dose of 10 ng/ml. Granulosa cells cultured in serum-free media from the time of plating secreted less progesterone and were less responsive to FSH compared with cultures plated with 1% FBS. Only insulin, but not IGF-1, enhanced FSH responses to threefold in cells cultured with 1% FBS. However, when cells were cultured in serum-free media from the time of plating, both insulin and IGF-1, but not GH, potentiated the responses to FSH, but insulin was more potent than IGF-1. Insulin-like growth-factor-1 binding studies with granulosa cells indicate the presence of specific high-affinity binding sites (Kd 3.96 nM). A dose of 100 ng/ml of insulin had negligible cross-reactivity with IGF-1 receptors.