Ovarian Gonadotropin-Releasing Hormone Receptors. I. Properties and Inhibition of Luteal Cell Function

Gonadotropin-releasing hormone (GnRH) and its agonist analogs exert paradoxical antifertility effects in several species by causing regression of ovarian function. In collagenase-dispersed rat luteal cells, the agonist analog [D-Ser(tBu)6]des-Gly10-GnRH-N-ethylamide (GnRHa) caused inhibition of hCG-stimulated progesterone production. Epinephrine- stimulated progesterone production in isolated luteal cells was also partially inhibited by the GnRH analog. On the other hand, there was no effect of the analog on the steroid doseresponse curve to dibutyryl cAMP. The effect of the GnRH analog on the steroidogenic response to both hormones was to shift the dose-response curve to the right without changing the maximum production of progesterone. Natural sequence GnRH also inhibited steroid production, with a half-maximal effect occurring at 5.10-9 M in agreement with the binding affinity of GnRH for its gonadal receptors (Ka = 5 × 108 M-1). GnRHa also caused a shift to the right in the dose-response curve for cAMP production stimulated by hCG. However, no direct inhibitory effects of GnRH and its agonist analogs could be detected on hCG- or epinephrine-stimulated adenylate cyclase activity measured in ovarian homogenates. GnRH did not affect the binding affinity or capacity of rat ovarian receptors for hCG. Specific GnRH receptors in luteinized rat ovaries were identified with [l25I]GnRHa and found to have high affinities for the analog (Ka = 5 × 109 M-1). A potent antagonist analog ([D-pGlu1,D-Phe2,DTrp36]GnRH) was also bound with high affinity (Ka = 3 times; 109 M-1) by the ovarian receptors. GTP, guanosine 5′-(β,γ-imido) triphosphate, and ATP as well as prostaglandin F2a and epinephrine did not alter binding of the analog to GnRH receptors. These studies demonstrate that the ovary contains specific, high affinity receptors for GnRH, through which GnRH and its gonist analogs can inhibit steroidogenic responses to hormonal stimulation. Since there was no inhibition of dibutyryl cAMPstimulated progesterone production and no direct action on hCG binding, it is suggested that GnRH interferes with the mechanism of hormonal stimulation of cAMP production.