Involvement of protein phosphatases in gonadotropin releasing hormone regulated gonadotropin secretion

The role of persistent protein phosphorylation upon gonadotropin releasing hormone (GnRH) stimulated luteinizing hormone (LH) release was investigated by the use of the selective inhibitors of protein phosphatase 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 calyculin A, respectively. The inactive OA analog norokadone and the protein tyrosine phosphatase inhibitor vanadyl hydroperoxide had no significant effect on GnRH-induced LH release. The stimulatory effects of the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol 13-acetate (TPA, 50 ng/ml) or the Ca²⁺ ionophore, ionomycin (1 μm), upon LH release were also abolished by pretreatment with OA (10-20 nM) or calyculin A (30nM). Stimulation of LH release by high K⁺ (28 mM) or residual LH release stimulated by GnRH in Ca²⁺-free medium were also blocked 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.