Receptor-Mediated Actions of Growth Hormone Releasing Factor on Granulosa Cell Differentiation

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Abstract
GRF promotes follicular maturation and ovulation when administered with FSH in the treatment of infertility. Such actions could be mediated by stimulation of GH secretion and insulin-like growth factor I production, but the known actions of the structurally related hormone, vasoactive intestinal peptide (VIP), on granulosa cell function suggested that GRF may also act directly on the ovary to stimulate follicular development. Radioligand binding and activation studies, performed in granulosa cells from immature estrogen-treated rats, revealed a common receptor for VIP and rat (r) GRF in the ovary. Specific binding of [125I]VIP to granulosa cells was saturable and dependent on time and temperature. The relative potencies of VIP-related peptides for inhibition of radioligand binding were: VIP > GRF > peptide histidine isoleucinamide > [His1, Nle27] human GRF (1-32)NH2 > secretin. In binding studies with the potent GRF agonist, [125][His1, Nle27]GRF(1-32)NH2, relative potencies were: rGRF(1-43)OH > [His1,Nle27]human GRF(1-32)NH2 > VIP > peptide histidine isoleucinamide > secretin. Glucagon and gastric inhibitory peptide, other peptides of the glucagon superfamily, and unrelated peptides including CRF and .beta.-endorphin, did not inhibit binding of either radioligand to ovarian receptors. In cultured granulosa cells, rGRF and VIP stimulated cAMP formation, consistent with coupling of their receptors to the adenylate cyclase system, and potentiated FSH-induced cAMP production. Both peptides also amplified FSH-induced progesterone biosynthesis, aromatase activity, and LH receptor formation. These observations demonstrated that rGRF is a potent cAMP-mediated agonist in the rat ovary and acts on a common VIP/GRF receptor in maturing granulosa cells. It is likely that the potentiating effect of administered GRF on gonadotropin-stimulated follicular development in vivo is in part mediated by direct actions of the peptide on the VIP/GRF receptor. Also, since GRF is present in the gonads, it is possible that the locally-produced peptide promotes follicular maturation by paracrine modulation of the stimulatory action of FSH on granulosa cell function.

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