Calcium Dependence of the Inhibitory Effect of Gonadotropin-Releasing Hormone on Luteinizing Hormone-Induced Cyclic Amp Production in Rat Granulosa Cells

The role of calcium in the inhibitory action of the GnRH superagonist, [D-Ala6]des-Gly10-GnRH N-ethylamide (GnRHa), on LH-stimulated cAMP production was studied in cultured granulosa cells obtained from immature hypophysectomized diethylstilbestrol (DES)-implanted rats. After culture for 48 h with FSH to induce LH receptors, cells were incubated for 2 h with LH or LH plus GnRHa. In this system, the cAMP response to LH was independent of extracellular calcium. In the presence of 0.25 to 1 mM extracellular calcium, 10(-8) M GnRHa caused a 15 to 40% inhibition of LH-stimulated cAMP production. Omission of extracellular calcium completely abolished the inhibitory effect of GnRHa upon LH-induced cAMP production. The inhibitory effects of GnRHa on prostaglandin E2 (PGE2) and isoproterenol-induced cAMP productions were also markedly reduced in the absence of extracellular calcium. Addition of the phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (MIX), reversed the inhibitory action of GnRHa on LH-induced cAMP production. These results demonstrate that extracellular calcium is necessary for the acute inhibitory action of GnRHa upon LH-induced cAMP production in cultured rat granulosa cells, and indicate that increased degradation of cAMP may be a contributing factor for this effect of GnRHa.