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Angiotensin II inhibits K(+)-induced Ca2+ signal generation in rat adrenal glomerulosa cells.

ABSTRACT: The Ca2(+)-mobilizing hormone angiotensin II (AII) dose-dependently inhibited the K(+)-induced sustained increase of cytoplasmic Ca2+ concentration in adrenal glomerulosa cells and caused a rapid decrease of cytoplasmic Ca2+ when added to cells already stimulated with K+. These effects of AII on the K(+)-induced Ca2+ signal were mimicked, although less effectively, by other Ca2(+)-mobilizing agonists such as [Arg8]vasopressin (AVP) and thapsigargin. Phorbol esters did not show such effects, nor did corticotropin (ACTH), a secretagogue acting via cyclic AMP. The K(+)-stimulated initial 45Ca2+ uptake, a measure of Ca2+ entry into glomerulosa cells, was also prevented by AII pretreatment, and was inhibited by AVP, but not by ACTH. The stimulatory effect of K+ on aldosterone production, however, was not inhibited by AII, and the AII-induced aldosterone production was further increased by increasing K+. These data indicate that AII is able to inhibit static increases in cytoplasmic Ca2+ by inhibiting Ca2+ entry through voltage-sensitive Ca2+ channels and, possibly, by activating Ca2+ extrusion from the cells. It is also concluded that the Ca2+ signal evoked by AII is very efficient in stimulating hormone secretion, and the secretory response of the cells becomes more sensitive to any further increase of Ca2+ entry through voltage-sensitive Ca2+ channels.

SUBMITTER: Balla T

PROVIDER: S-EPMC1149859 | biostudies-other | 1991 Jan

REPOSITORIES: biostudies-other

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Project description:Angiotensin II (AngII) plays a crucial role in the control of aldosterone biosynthesis in adrenal glomerulosa cells through the stimulation of two distinct Ca2+ entry pathways: (1) opening of voltage-operated calcium channels, and (2) activation of a capacitative Ca2+ entry that is dependent on calcium release from intracellular pools. Adrenocorticotrophic 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 studies have demonstrated that the rise in cAMP induced by ACTH can be potentiated by AngII in bovine glomerulosa cells. The aim of the present study was to investigate the mechanism of AngII action on ACTH-induced cAMP production. In primary cultures of bovine glomerulosa cells, we found that AngII (100 nM), which had no effect by itself on cAMP production, significantly potentiated maximal ACTH-induced cAMP formation in the presence of extracellular calcium (1.2 mM). In contrast, in the absence of extracellular calcium, AngII did not affect ACTH-induced cAMP production. These results suggest that calcium entry into the cell plays an important role in the activation of the cyclase by AngII. The inhibition of voltage-operated calcium channels by nicardipine, a dihydropyridine calcium antagonist blocking both low-threshold (T-type) and high-threshold (L-type) Ca2+ channels, did not significantly affect the potentiating effect of AngII. Moreover, the cAMP response to ACTH was insensitive to activation of these Ca2+ channels induced by potassium ions and, even when cytosolic free-calcium concentration ([Ca2+]c) was kept elevated with the Ca2+ ionophore, ionomycin, no stimulation of adenylyl cyclase was observed at concentrations of [Ca2+]c up to 640 nM. In contrast, thapsigargin, an activator of capacitative Ca2+ influx, mimicked the potentiating effect of AngII on ACTH-induced cAMP formation. In agreement with the characteristics of cAMP modulation by Ca2+ in these cells, the presence of type III adenylyl cyclase was observed by immunodetection in bovine glomerulosa cell membranes. In conclusion, 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.

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Angiotensin II inhibits K(+)-induced Ca2+ signal generation in rat adrenal glomerulosa cells.

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