Gonadotropin-releasing hormone activates a rapid Ca2+-independent phosphodiester hydrolysis of polyphosphoinositides in pituitary gonadotrophs.

Addition of gonadotropin releasing hormone (GnRH) to pituitary cells prelabeled with [32P]Pi or with myo-[2-3H]inositol, resulted in a rapid decrease in the level of [32P]phosphatidylinositol 4,5-bisphosphate (approximately 10 s), and in [32P]phosphatidylinositol 4-phosphate (approximately 1 min), followed by increased labeling of [32P]phosphatidylinositol and [32P]phosphatidic acid (1 min). GnRH stimulated the appearance of [3H]myo-inositol 1,4,5-trisphosphate (10 s), [3H]myo-inositol 1,4-bisphosphate (15 s), and [3H]myo-inositol 1-phosphate (1 min) in the presence of Li+ (10 mM). Li+ alone stimulated the accumulation of [3H]myo-inositol 1-phosphate and [3H]myo-inositol 1,4-bisphosphate but not [3H]myo-inositol 1,4,5-trisphosphate, but had no effect on luteinizing hormone release. The effect of GnRH on inositol phosphates (Ins-P) production was dose-related (ED50 = 1-5 nM), and was blocked by a potent antagonist [D-pGlu,pClPhe,D-Trp]GnRH. Elevation of cytosolic free Ca2+ levels ([Ca2+]i), by ionomycin and A23187 from intracellular or extracellular Ca2+ pools, respectively, had no significant effect on [3H]Ins-P production. GnRH-induced [3H]Ins-P production was not dependent on extracellular Ca2+ and was noticed also after extracellular or intracellular Ca2+ mobilization by A23187 or ionomycin, respectively. The effect of GnRH on [3H]Ins-P accumulation was not affected by prior treatment of the cells with the tumor promoter phorbol ester 12-O-tetradecanoylphorbol-13-acetate or with islet-activating protein pertussis toxin. These results indicate that GnRH stimulates a rapid phosphodiester hydrolysis of polyphosphoinositides. The stimulatory effect is not mediated via an islet-activating protein-substrate, is not dependent on elevation of [Ca2+]i, neither is it negatively regulated by 12-O-tetradecanoylphorbol-13-acetate which activates Ca2+/phospholipid-dependent protein C kinase. The results are consistent with the hypothesis that GnRH-induced phosphoinositide turnover is responsible for Ca2+ mobilization followed by gonadotropin release.