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Intracellular calcium release is more efficient than calcium influx in stimulating mitochondrial NAD(P)H formation in adrenal glomerulosa cells.

ABSTRACT: We compared the effect on mitochondrial NAD(P)H formation of calcium release from intracellular stores with that of calcium influx from the extracellular space. Simultaneous measurements of cytoplasmic free calcium ion concentration and mitochondrial NAD(P)H were performed on fura-PE3-loaded single rat adrenal glomerulosa cells. The effects of equipotent stimuli in terms of the evoked Ca2+ response were compared. Angiotensin II (AII; 1 nM) induced a higher amplitude NAD(P)H response than K+ (5.6-7.6 mM). Vasopressin (1 microM) also induced a greater initial NAD(P)H formation than K+, although the Ca2+ signal evoked by the two agonists had similar amplitude. To examine the effect of Ca2+ release from internal stores we applied AII in Ca2+-free medium. We compared the effect on NAD(P)H formation of Ca2+ release with Ca2+ influx induced by K+, and with capacitative Ca2+ influx induced by AII. NAD(P)H formation in response to Ca2+ release was greater than that induced by Ca2+ influx, irrespective of whether induced by K+ or AII. Our results indicate that Ca2+, presumably released in the vicinity of mitochondria, activates mitochondrial dehydrogenases more efficiently than Ca2+ entering through the plasma membrane. These data confirm the biological significance of previous observations showing that Ca2+ released from inositol 1,4, 5-trisphosphate-sensitive internal stores increases mitochondrial matrix [Ca2+] to a greater extent than extracellular Ca2+.

SUBMITTER: Rohacs T

PROVIDER: S-EPMC1218951 | biostudies-other | 1997 Dec

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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Intracellular calcium release is more efficient than calcium influx in stimulating mitochondrial NAD(P)H formation in adrenal glomerulosa cells.

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