INVOLVEMENT OF PROTEIN PHOSPHATASES IN GONADOTROPIN-RELEASING-HORMONEREGULATED GONADOTROPIN-SECRETION

The role of persistent protein phosphorylation upon gonadotropin relea sing hormone (GnRH) stimulated luteinizing hormone (LH) release was in vestigated by the use of the selective inhibitors of protein phosphata se type 1 (PP1) and 2A (PP2A), okadaic acid (OA) and calyculin A. Pre- incubation of cultured rat pituitary cells with OA (24 h) or calyculin A (30 min) resulted in inhibition of GnRH-stimulated LH release with significant inhibition being detected at 10 nM and 30 nM for OA and ca lyculin A, respectively. The inactive OA analog norokadone and the pro tein tyrosine phosphatase inhibitor vanadyl hydroperoxide had no signi ficant effect on GnRH-induced LH release. The stimulatory effects of t he protein kinase C (PKC) activator 12-O-tetradecanoylphorbol 13-aceta te (TPA, 50 ng/ml) or the Ca2+ ionophore, ionomycin (1 mu m), upon LH release were also abolished by pretreatment with OA (10-20 nM) or caly culin A (30 nM). Stimulation of LH release by high K+ (28 mM) or resid ual LH release stimulated by GnRH in Ca2+-free medium were also blocke d by OA. These observations indicate that protein dephosphorylation is involved positively in GnRH-stimulated LH release. The site of action of the protein phosphatases PPI and PP2A is most likely downstream to Ca2+ elevation and PKC activation by GnRH.