| Abstract: | When the dose-response curve of adrenocorticotropin (ACTH)-induced aldosterone secretion is compared to that of ACTH-induced intracellular cAMP, the ED50 for intracellular cAMP is more than 10 times as high as that for aldosterone production. In contrast, the dose-response curve of forskolin-induced aldosterone secretion correlates well with that for forskolin-induced intracellular cAMP. ACTH, but not forskolin, increases calcium influx into glomerulosa cells without inducing the mobilization of calcium from an intracellular pool. The effect of ACTH on calcium influx is dose-dependent and ED50 is 3.5 X 10(-11) M. In a perifusion system, the effect of 1 nM ACTH on aldosterone secretion is much greater than that of 1 microM forskolin, even though these two stimulators induce identical increases in the intracellular cAMP. Perifusion with combined A23187 (50 nM) and forskolin (1 microM) stimulates aldosterone secretion to a value comparable to that induced by 1 nM ACTH. Likewise, BAY K 8644 (1 nM), which induces a comparable increase in calcium influx, potentiates the effect of 1 microM forskolin. When the intracellular [Ca2+] is fixed at either 100 or 300 nM, forskolin-stimulated intracellular cAMP content is identical, but ACTH-stimulated intracellular cAMP content at 100 nM [Ca2+]i is 60% of that at 300 nM [Ca2+]i. Both the ACTH- and forskolin-induced aldosterone secretion rate is higher at 300 nM than at 100 nM [Ca2+]i. These results indicate that ACTH stimulates calcium influx, that calcium potentiates ACTH-induced but not forskolin-induced cAMP generation, and that Ca2+ and cAMP act as synarchic messengers in ACTH-mediated aldosterone secretion. |
