Gonadotropin-releasing hormone analogs inhibit primate granulosa cell steroidogenesis via a mechanism distinct from that in the rat

Gonadotropin-releasing hormone (GnRH) and related peptides are implicated in the local control of rat ovarian function, but evidence to date for direct effects of such peptides on primate ovarian cells is equivocal. In contrast to rat ovaries, where GnRH action is mediated through specific, high-affinity GnRH receptors, no such binding sites have been identified in primate tissue. Using undifferentiated granulosa cells from immature follicles in cyclic (luteal phase) marmoset ovaries, we have observed direct suppression of human (h) FSH-induced steroidogenesis by GnRH analogs in vitro. Granulosa cells from immature (less than 1 mm diameter) follicles were incubated for 4 days in the presence of hFSH and testosterone (aromatase substrate) to stimulate cyclic AMP (cAMP) production and steroidogenesis. The additional presence of GnRH alone (up to 10 microM) had no effect on FSH action. However, the GnRH agonist, [D-Ser(But)6]GnRH 1-9)-ethylamide (Buserelin, 0.1 microM-10 microM), caused time- and dose-dependent inhibition of estradiol (maximum inhibition = 79%; ED50 = 0.55 microM) and progesterone production (maximum inhibition = 93%; ED50 = 0.1 microM). Accumulation of cAMP was also inhibited by up to 54%. Paradoxically, a GnRH antagonist [( N-Ac-D-Nal(2)1,D-pCl-Phe2, D-Trp3, D-hArg(Et2)6, D-Ala10]-GnRH; 10 microM) alone also inhibited hFSH-stimulated cAMP and steroid production by 40% and 70%, respectively. Moreover, the suppressive effects of the GnRH agonist on granulosa cell functions were augmented by the presence of the GnRH antagonist (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)