Characterization of 12-O-tetradecanoyl-phorbol-13 acetate mediated ACTH release

Interaction between arginine vasopressin- and raised extracellular potassium-stimulated pathways in adrenocorticotropin secretion

The intracellular regulation of adrenocorticotropin (ACTH) secretion from pituitary corticotroph cells was investigated by simultaneously exposing cultured ovine corticotrophs to arginine vasopressin (AVP) and raised extracellular K+ concentration ([K+]e). Both of these secretagogues activate L-type voltage-sensitive calcium channels (L-VSCC) as part of their respective ACTH secretory responses. When given together at high concentrations, AVP and raised [K+]e caused ACTH responses that were smaller in magnitude than the sum of the individual responses. However, at low agonist concentrations the simultaneous responses were greater in magnitude (i.e., synergistic). Further investigation suggested that activation of protein kinase C (PKC), which is part of the AVP-induced intracellular signalling pathway, is necessary and sufficient for the generation of the synergistic response, although it is not obligatory for AVP-induced ACTH release.

The effects of a chemically diverse range of calcium channel antagonists on the AVP-stimulated ACTH response in ovine corticotrophs

Arginine vasopressin (AVP) stimulates adrenocorticotropin (ACTH) release from pituitary corticotrophs by a mechanism dependent on extracellular calcium (Ca2+e). The involvement, in this response, of Ca2+ influx via L-type voltage sensitive Ca2+ channels (VSCC) was studied using cultured ovine anterior pituitary cells. Representatives of 3 chemically distinct classes of organic antagonists of L-type VSCC (methoxyverapamil (D600), nifedipine and diltiazem) and 2 inorganic blocking ions (Cd2+ and Co2+) were used. All of the blockers reduced AVP-stimulated ACTH release, but none caused complete inhibition. ACTH release in response to raised extracellular K+ concentration was also inhibited by these antagonists, with D600 and Cd2+ completely abolishing the response. These results indicate that there is a considerable, but not total, dependence on Ca2+ influx via L-type VSCC during the ACTH response to AVP in ovine corticotrophs.

The role of intracellular messengers in adrenocorticotropin secretion in vitro

Adrenocorticotropin (ACTH), an opiomelanocortin peptide, is secreted from anterior pituitary corticotrophs upon stimulation with corticotropin-releasing hormone (CRH), arginine vasopressin (AVP) and several other neuropeptides. CRH, the most potent secretagogue of ACTH, stimulates ACTH secretion and biosynthesis by increasing the production of cyclic adenosine 3',5'-monophosphate (cAMP) within corticotrophs. AVP, which is a weak secretagogue of ACTH but strongly potentiates CRH-stimulated ACTH secretion, operates through the phosphatidylinositol (PI) transduction pathway. Both CRH and AVP increase cytosolic free [Ca2+] within normal corticotrophs indicating a role for Ca2+ in ACTH secretion. Glucocorticoids inhibit ACTH synthesis by suppressing transcription of the proopiomelanocortin (POMC) gene and attenuate ACTH release by decreasing cAMP accumulation stimulated by CRH. This review focuses on the roles of these intracellular messengers in ACTH secretion from normal anterior pituitary cells in vitro, and discusses the possible interactions between the cAMP, calcium and PI transduction pathways. Future areas of research are suggested such as identification of protein substrates of cAMP-dependent and Ca2(+)-dependent kinases within normal corticotrophs and evaluation of their role in ACTH biosynthesis and secretion.