ANGIOTENSIN-II POTENTIATES ADRENOCORTICOTROPIC HORMONE-INDUCED CAMP FORMATION IN BOVINE ADRENAL GLOMERULOSA CELLS THROUGH A CAPACITATIVE CALCIUM INFLUX

Angiotensin II (AngII) plays a crucial role in the control of aldoster one biosynthesis in adrenal glomerulosa cells through the stimulation of two distinct Ca2+ entry pathways: (1) opening of voltage-operated c alcium channels, and (2) activation of a capacitative Ca2+ entry that is dependent on calcium release from intracellular pools. Adrenocortic otrophic hormone (ACTH), on the other hand, a major hormonal regulator of steroidogenesis, induces an increase in intracellular cAMP through the activation of a G-protein-coupled adenylyl cyclase. Recent studie s have demonstrated that the rise in cAMP induced by ACTH can be poten tiated by AngII in bovine glomerulosa cells. The aim of the present st udy was to investigate the mechanism of AngII action on ACTH-induced c AMP production. In primary cultures of bovine glomerulosa cells, we fo und that AngII (100 nM), which had no effect by itself on cAMP product ion, significantly potentiated maximal ACTH-induced cAMP formation in the presence of extracellular calcium (1.2 mM). In contrast, in the ab sence of extracellular calcium, AngII did not affect ACTH-induced cAMP production. These results suggest that calcium entry into the cell pl ays an important role in the activation of the cyclase by AngII. The i nhibition of voltage-operated calcium channels by nicardipine, a dihyd ropyridine calcium antagonist blocking both low-threshold (T-type) and high-threshold (L-type) Ca2+ channels, did not significantly affect t he potentiating effect of AngII. Moreover, the cAMP response to ACTH w as insensitive to activation of these Ca2+ channels induced by potassi um ions and, even when cytosolic free-calcium concentration ([Ca2+](e) ) was kept elevated with the Ca2+ ionophore, ionomycin, no stimulation of adenylyl cyclase was observed at concentrations of [Ca2+](e) up to 640 nM. In contrast, thapsigargin, an activator of capacitative Ca2influx, mimicked the potentiating effect-of AngII on ACTH-induced cAMP formation. In agreement with the characteristics of cAMP modulation b y Ca2+ in these cells, the presence of type III adenylyl cyclase was o bserved by immunodetection in bovine glomerulosa cell membranes. In co nclusion, these data suggest a tight coupling between the capacitative Ca2+ influx induced upon stimulation by either AngII or thapsigargin and a calcium-sensitive isoform of adenylyl cyclase, probably type III , in bovine glomerulosa cells.