Synergistic stimulation of luteinizing hormone (LH) release by protein kinase C activators and Ca²⁺-ionophore

When cultured pituitary cells were stimulated with synthetic diacylglycerol such as 1-oleoyl-2-acetylglycerol (OAG), or with a potent tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA), which are known stimulators of Ca²⁺-activated, phospholipid-dependent protein kinase (protein kinase C), enhanced release of luteinizing hormone (LH) was observed. Similarly, LH release was also stimulated by the Ca²⁺-ionophore, A23187. Simultaneous presence of A23187 and OAG or TPA resulted in a synergistic response that mimicked the full physiological response to gonadotropin releasing hormone (GnRH). Removal of extracellular Ca²⁺ only slightly affected the stimulatory action of TPA and OAG on LH release, but completely blocked the effect of GnRH. The results suggest that the stimulatory effect of GnRH on LH release may be mediated by two intracellular pathways involving Ca²⁺ and diacylglycerol as second messengers.