Abstract
Oxytocin is a major peptide product of the ruminant corpus luteum, and the release of oxytocin from serum-free cultures of bovine granulosa cells is stimulated by insulin and insulin-like growth factor-I (IGF-I). Here we have assessed the effects of insulin and IGF-I on oxytocin gene expression in bovine granulosa cells and the dependence of these effects on the developmental status of the cells. When cells from individual follicles were cultured, the estradiol concentration of the follicular fluid was highly correlated with insulin-stimulated oxytocin release. Subsequently, cells were pooled from follicles selected on the basis of estradiol content, and two subsets of cells were distinguished. The first contained highly differential cells, as judged by the high estradiol (HE-cells) concentration of the follicular fluid (> 40 ng/ml), high levels of LH receptors, and high hCG-stimulated cAMP accumulation. The second subset contained cells from follicles with low estradiol (< 1 ng/ml; LE-cells) which have fewer LH receptors and low hCG-stimulated cAMP accumulation. Oxytocin production was increased more than 50-fold by insulin (EC50, 230 .+-. 57 ng/ml) and IGF-I (EC50, > 10 ng/ml), but only in the HE-cells. Oxytocin mRNA was also greatly increased by insulin and IGF-I in the HE-cells only. In contrast, insulin and IGF-I stimulated progesterone release from both HE- and LE-cells. Since oxytocin production is a characteristic of bovine luteal cells, our results support possible roles for IGF-I and insulin in regulation of leuteinization or luteal activity. The data suggest that effects of insulin and IGF-I on oxytocin production reflect their effects on oxytocin gene transcription, and that granulosa cells must be appropiately primed (presumably by the in vivo hormonal environment) before they are able to produce oxytocin in response to these polypeptides.