Characterization of 12–0–Tetradecanoyl-Phorl-13 Acetate Mediated Acth Release

Abstract

Activation of calcium-activated, phospholipid-dependent protein kinase C by phorbol esters such as 12–0–tetradecanoyl-phorbol-13–acetate (TPA) has been shown to mediate release of hormones in many systems. To investigate the role of protein kinase C in the mechanism of pituitarv ACTH release, we studied the effect of the following conditions on TPA mediated ACTH release in primary rat pituitary cultures; corticotropin releasing hormone (CRH), different concentrations of extracellular calcium (Ca⁺²), nifedipine (a calcium channel blocker), PGE₂ and cortisol (regulators of ACTH secretions). TPA induced ACTH release in a dose dependent fashion with an ED₅₀ of 4.2 × 10^–10M. The maximal amount of ACTH release individually induced by TPA (10^–8M) and CRH (10^–8) were identical. TPA (10_–8) potentiated the amount of ACTH release from cells already maximally stimulated with CRH (4 × 10^–8M). TPA mediated ACTH release was dependent on extracellular calcium and inhibited by nifedipine, to a maximum of 35%. Cortisol decreased the amount of ACTH individually released by TPA and CRH in a similar and dose dependent fashion with maximal inhibition of 47% occurring at 10^–7M. PGE₂ caused a dose dependent reduction of TPA mediated ACTH release. In conclusion, the pathways of ACTH release induced by CRH and TPA are not entirely the same but may share a common regulatory pathway. Extracellular calcium and calcium cell influx may be important for maximal ACTH release induced by TPA. Protein kinase C activation may play an important role in CRH stimulated ACTH release.