ROLE OF CA2+ IN THE ACTION OF ADRENOCORTICOTROPIN IN CULTURED HUMAN ADRENAL GLOMERULOSA CELLS

The present report details the role of Ca2+ in the early events of ACT H action in human adrenal glomerulosa cells. Threshold stimulations of both aldosterone and cAMP production were obtained with a concentrati on of 10 pM ACTH, an ED(50) of 0.1 nM, and maximal aldosterone stimula tion (5.5-fold increase over control) at 10 nM ACTH. ACTH also induced a sustained increase of intracellular calcium ([Ca2+](i)) with maxima l stimulation of 1.6 +/- 0.1-fold over control values. This increase d oes not involve mobilization of calcium from intracellular pools since no response was observed in Ca2+-free medium or in the presence of ni fedipine, suggesting the involvement of Ca2+ influx by L-type Ca2+ cha nnels. This was confirmed by patch clamp studies that demonstrated tha t ACTH stimulates L-type Ca2+ channels. Moreover, the Ca2+ ion is not required for ACTH binding to its receptor, but is essential for sustai ned cAMP production and aldosterone secretion after ACTH stimulation. These results indicate that, in human adrenal glomerulosa cells, a pos itive feedback loop between adenylyl cyclase-protein kinase A-Ca2+ cha nnels ensures a slow but sustained [Ca2+](i) increase that is responsi ble for sustained cAMP production and aldosterone secretion.