Gonadotropin-induced expression of receptors for growth hormone releasing factor in cultured granulosa cells

The hypothalamic neuropeptide, GRF, is formed in the ovary and acts via specific receptors in granulosa cells to enhance cAMP production and steroidogenic responses to the pituitary gonadotropin, FSH. Granulosa cells cultured without hormonal treatment displayed low levels of binding sites for GRF and the related neuropeptide, vasoactive intestinal peptide. However, treatment with increasing concentrations (50-500 ng/ml) of FSH caused dose-dependent increases in cAMP production and expression of binding sites measured with radioiodinated [His¹, Nle²⁷]human GRF(1-32)NH₂, with no change in binding affinity. The maximum increase in GRF binding sites (2.2-fold) was elicited by 250 ng/ml FSH after 72 h incubation. GRF binding sites were also increased by agents that elevate intracellular cAMP, including choleragen, vasoactive intestinal peptide, dibutyryl cAMP, and forskolin. Low doses of forskolin that did not alone increase [¹²⁵I][His¹,Nle²⁷] human GRF(1-32)NH₂ binding potentiated the action of FSH on GRF binding sites, but the effects of maximal stimulatory doses of both agents were not additive. These findings demonstrate that FSH promotes the expression of GRF receptors in maturing granulosa cells through cAMP-dependent mechanisms. Since GRF enhances the actions of FSH on cAMP production and granulosa cell differentiation, and GRF receptors are increased by the cAMP-mediated actions of FSH, locally produced GRF could exert a positive autoregulatory action to accelerate follicular maturation by amplifying the granulosa cell response to FSH.