EFFECT OF AN ANTAGONISTIC ANALOG OF GONADOTROPIN-RELEASING HORMONE UPON OVARIAN GRANULOSA-CELL FUNCTION

We have recently demonstrated that gonadotropin releasing hormone (GnRH) acts directly on ovarian granulosa cells to inhibit the follicle stimulating hormone (FSH)-induced increase in granulosa cell steroidogenesis in vitro. A GnRH antagonist, [D-pGlu1, D-Phe2, D-Trp3,6] GnRH (A), which is known to antagonize GnRH-stimulated gonadotropin release by cultured pituitary cells, was tested in the granulosa cell system. GnRH (10-8M) inhibited estrogen and progesterone production by FSH-treated granulosa cells in vitro, whereas the antagonist A (10-6M) did not affect FSH stimulation of steroidogenesis. Antagonist A, when added together with GnRH and FSH, blocked the GnRH inhibition of FSH-induced steroidogenesis. Estrogen and progesterone production by granulosa cells was increased by 50% at a molar ratio (IDR50) of 20 1 and 12 1 ([antagonist]/[GnRH]), respectively. At 10-6M, antagonist A completely prevented the GnRH (10-8M) inhibition. A similar effect of antagonist A was seen in FSH-induced increase of luteinizing hormone (LH) receptor content. FSH treatment for 2 days in vitro induced an 8-fold increase in LH receptor content in cultured granulosa cells; concomitant treatment with 10-8M GnRH completely inhibited the FSH effect. Antagonist A (10-6M), by itself, had no effect on the FSH action. However, when added together with FSH and GnRH, antagonist A completely abolished the inhibitory effect of GnRH. These results demonstrate that the direct inhibitory effect of GnRH on granulosa cell function can be prevented by a GnRH antagonist and that the GnRH action at the ovarian level may require stringent stereospecific interactions of these peptides with putative GnRH recognition sites. © 1979.