Role of the capacitative calcium influx in the activation of steroidogenesis by angiotensin-II in adrenal glomerulosa cells.

Abstract

Angiotensin-II (AngII)-induced Ca2+ influx in adrenal glomerulosa cells, a signal necessary for the stimulation of steroidogenesis by the hormone, is believed to involve two distinct mechanisms: 1) opening of voltage-operated Ca2+ channels, and 2) activation of a capacitative Ca2+ entry pathway that is dependent on calcium release from intracellular stores. Nicardipine, a dihydropyridine calcium antagonist, has been used to investigate the role of these Ca2+ entry mechanisms in the steroidogenic response to AngII. As demonstrated with the patch-clamp technique, micromolar concentrations of nicardipine completely blocked voltage-operated Ca2+ channel activity of both T- and L-types. This agent similarly inhibited the rise of cytosolic free calcium concentration induced by potassium, but did not significantly affect the response to thapsigargin, an activator of the capacitative pathway. Nicardipine reduced by only 22% the calcium influx stimulated by AngII, and the nicardipine-insensitive part of this response was abolished after exhausting the intracellular Ca2+ stores with thapsigargin. Similarly, aldosterone secretion induced by AngII was only partially inhibited (40%) by nicardipine at concentrations that completely abolished the steroidogenic response to potassium. Thapsigargin by itself was able to stimulate aldosterone production, an action highly potentiated by physiological concentrations of extracellular potassium. These data strongly suggest that the major part of the calcium influx response to AngII, leading to aldosterone formation, involves a capacitative calcium entry pathway activated by the release of calcium from intracellular stores. This mechanism of calcium influx could be responsible for some features of aldosterone response to the hormone, such as its poor sensitivity to dihydropyridines or its potentiation by potassium.

Janus Kinase 2 and Calcium Are Required for Angiotensin II-dependent Activation of Steroidogenic Acute Regulatory Protein Transcription in H295R Human Adrenocortical Cells

“…Together these data indicate that Ang II-stimulated StAR promoter activity is not dependent upon reaching a threshold level of [Ca 2ϩ ] i . Previous studies reported Tg increased aldosterone production in both primary cultures of bovine and rat adrenal glomerulosa cells (5,39,40). However, a significant Tg response in bovine adrenal cells required 8 mM K ϩ in the medium, and our culture conditions had 4 mM K ϩ , which may account for the difference.…”

“…Depletion of the intracellular calcium stores activates capacitative calcium entry, thus Ang II mediates an influx of external Ca 2ϩ via calcium release activated calcium (CRAC) channels (3,4). Although Ang II has been shown to activate voltage-gated calcium channels, this pathway is not considered a major contributor to the steroidogenic response (5). K ϩ activation of aldosterone synthesis, on the other hand, is associated with an influx of external Ca 2ϩ via voltage-gated Ca 2ϩ channels (6).…”

“…For both of these regulators it has been well established that aldosterone synthesis requires influx of external Ca 2ϩ (2,6). However, while both agents activate Ca 2ϩ signaling, the mechanisms leading to aldosterone synthesis are distinct, and the Ang II response cannot be mimicked by K ϩ depolarization of the plasma membrane (5,7,8).…”

Potassium‐angiotensin interplay in the regulation of aldosterone biosynthesis

“…The emptying of calcium stores triggers 'capacitative' Ca2+ entry by a yet unknown mechanism. Yet the calcium signal in response to angiotensin I1 is larger than that induced by thapsigargin, suggesting an additional calcium entry pathway (Burnay et al, 1994).…”

“…, 1991), which appears to be mediated by protein kinase C (PKC) and to lead to stimulation of Ca2+ influx through T channels. Nicardipine, at concentrations which block both T-and L-calcium channels, reduces only to a minor degree the calcium and aldosterone responses to angiotensin 11, again pointing to a dual mechanism of intracellular calcium elevation in response to angiotensin I1 (Burnay et al, 1994). Finally, angiotensin I1 in single bovine glomerulosa cells has been found to induce cytosolic free calcium oscillations which are dependent on the presence of extracellular calcium (Johnson et al, 1989).…”