Phorbol esters and thyroliberin have distinct actions regarding stimulation of prolactin secretion and activation of adenylate cyclase in rat pituitary tumour cells (GH4C1 cells).

The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) enhances the effects of TRH on phase II of prolactin secretion as well as on hormone synthesis at both low and high TPA receptor occupancy. Furthermore TPA, but not the biologically inactive substance 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), stimulates the particulate bound adenylate cyclase with a time course paralleling that of TRH activation. However, the combined additions of TRH and TPA activate this cyclase in an additive manner while the Gpp(NH)p- and the forskolin-sensitive enzyme are unaffected by TPA addition. Polymyxin B, which inhibits protein kinase C, abolishes activation of adenylate cyclase by TPA without interfering with the stimulatory action of TRH. Also, when phosphatase activity is preferentially inhibited by pretreatment of the cells with sodium vanadate, the TRH-sensitive cyclase is unaltered, while TPA activation is obliterated. Maximal stimulation of adenylate cyclase by cholera toxin pretreatment, obliterated the actions of TRH and TPA. Cells pretreated with pertussis toxin retained their TRH-sensitive cyclase, however, TPA-responsiveness was lost. We therefore suggest that the action of TPA as it relates to activation of adenylate cyclase, is probably mediated via the Gi component of the adenylate cyclase complex, while TRH stimulates the enzyme via the classical pathway involving the stimulatory GTP binding protein (Gs).