We investigated the regulation of each intracellular signal transduction system including cyclic AMP (cAMP)-dependent and calcium (Ca
2+) messenger systems in bovine adrenal fasciculo-reticularis cells to clarify the exact mode of action of ACTH. Pretreatment with primaquine and quinacrine, which are phospholipase A
2 inhibitors, significantly inhibited cortisol production activated by both low and high concentrations of ACTH. Therefore, it seems that metabolites induced by phospholipase A
2 are quite essential for cortisol synthesis induced by ACTH, either at low or high concentrations. At low concentrations of ACTH (10
–13–10
–12 M), significant increases of cytosolic calcium ([Ca
2+]
i), but not of cAMP, were observed. Calphostin C, a specific protein kinase C inhibitor, apparently suppressed cortisol production activated by low concentrations of ACTH, while H-89, a specific inhibitor of cAMP-dependent protein kinase, did not. These findings suggest that, at physiologically low concentrations, ACTH activates [Ca
2+]
i and phospholipase A
2 without affecting cAMP formation, resulting in an increased biosynthesis of cortisol, partly via protein kinase C-dependent processes. At high concentrations, ACTH (10
–9–10
–7 M) induced an increase of cAMP and [Ca
2+]
i. The cortisol production induced by high concentrations of ACTH was significantly inhibited by pretreatment with calphostin C, H-89 and H-7, suggesting the participation of cAMP-dependent protein kinase and protein kinase C systems in the regulation of cortisol production in the presence of high concentrations of ACTH. In conclusion, cytosolic calcium is biphasically enhanced by ACTH, although cAMP accumulation is increased only by high concentrations of ACTH. A phospholipase A
2-dependent process may partly play a crucial role in the regulation of cortisol biosynthesis, when stimulated by low and high concentrations of ACTH.
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