Aldosterone increases T-type calcium currents in human adrenocarcinoma (H295R) cells by inducing channel expression

The mineralocorticoid receptor leads to increased expression of EGFR and T-type calcium channels that support HL-1 cell hypertrophy

The EGF receptor (EGFR) has been extensively studied in tumor biology and recently a role in cardiovascular pathophysiology was suggested. The mineralocorticoid receptor (MR) is an important effector of the renin-angiotensin-aldosterone-system and elicits pathophysiological effects in the cardiovascular system; however, the underlying molecular mechanisms are unclear. Our aim was to investigate the importance of EGFR for MR-mediated cardiovascular pathophysiology because MR is known to induce EGFR expression. We identified a SNP within the EGFR promoter that modulates MR-induced EGFR expression. In RNA-sequencing and qPCR experiments in heart tissue of EGFR KO and WT mice, changes in EGFR abundance led to differential expression of cardiac ion channels, especially of the T-type calcium channel CACNA1H. Accordingly, CACNA1H expression was increased in WT mice after in vivo MR activation by aldosterone but not in respective EGFR KO mice. Aldosterone- and EGF-responsiveness of CACNA1H expression was confirmed in HL-1 cells by Western blot and by measuring peak current density of T-type calcium channels. Aldosterone-induced CACNA1H protein expression could be abrogated by the EGFR inhibitor AG1478. Furthermore, inhibition of T-type calcium channels with mibefradil or ML218 reduced diameter, volume and BNP levels in HL-1 cells. In conclusion the MR regulates EGFR and CACNA1H expression, which has an effect on HL-1 cell diameter, and the extent of this regulation seems to depend on the SNP-216 (G/T) genotype. This suggests that the EGFR may be an intermediate for MR-mediated cardiovascular changes and that SNP analysis can help identify subgroups of patients that will benefit most from MR antagonists.

Ion Channel Function and Electrical Excitability in the Zona Glomerulosa: A Network Perspective on Aldosterone Regulation

Aldosterone excess is a pathogenic factor in many hypertensive disorders. The discovery of numerous somatic and germline mutations in ion channels in primary hyperaldosteronism underscores the importance of plasma membrane conductances in determining the activation state of zona glomerulosa (zG) cells. Electrophysiological recordings describe an electrically quiescent behavior for dispersed zG cells. Yet, emerging data indicate that in native rosette structures in situ, zG cells are electrically excitable, generating slow periodic voltage spikes and coordinated bursts of Ca²⁺ oscillations. We revisit data to understand how a multitude of conductances may underlie voltage/Ca²⁺ oscillations, recognizing that zG layer self-renewal and cell heterogeneity may complicate this task. We review recent data to understand rosette architecture and apply maxims derived from computational network modeling to understand rosette function. The challenge going forward is to uncover how the rosette orchestrates the behavior of a functional network of conditional oscillators to control zG layer performance and aldosterone secretion.

Deficiency of T-type Ca2+ channels Cav3.1 and Cav3.2 has no effect on angiotensin II-induced hypertension but differential effect on plasma aldosterone in mice

T-type Ca²⁺ channel Ca_v3.1 promotes microvessel contraction ex vivo. It was hypothesized that in vivo, functional deletion of Ca_v3.1, but not Ca_v3.2, protects mice against angiotensin II (ANG II)-induced hypertension. Mean arterial blood pressure (MAP) and heart rate were measured continuously with chronically indwelling catheters during infusion of ANG II (30 ng·kg^-1·min^-1, 7 days) in wild-type (WT), Ca_v3.1^-/-, and Ca_v3.2^-/- mice. Plasma aldosterone and renin concentrations were measured by radioimmunoassays. In a separate series, WT mice were infused with ANG II (100 ng·kg^-1·min^-1) with and without the mineralocorticoid receptor blocker canrenoate. Ca_v3.1^-/- and Ca_v3.2^-/- mice exhibited no baseline difference in MAP compared with WT mice, but day-night variation was blunted in both Ca_v3.1 and Ca_v3.2^-/- mice. ANG II increased significantly MAP in WT, Ca_v3.1^-/-, and Ca_v3.2^-/- mice with no differences between genotypes. Heart rate was significantly lower in Ca_v3.1^-/- and Ca_v3.2^-/- mice compared with control mice. After ANG II infusion, plasma aldosterone concentration was significantly lower in Ca_v3.1^-/- compared with Ca_v3.2^-/- mice. In response to ANG II, fibrosis was observed in heart sections from both WT and Ca_v3.1^-/- mice and while cardiac atrial natriuretic peptide mRNA was similar, the brain natriuretic peptide mRNA increase was mitigated in Ca_v3.1^-/- mice ANG II at 100 ng/kg yielded elevated pressure and an increased heart weight-to-body weight ratio in WT mice. Cardiac hypertrophy, but not hypertension, was prevented by the mineralocorticoid receptor blocker canrenoate. In conclusion, T-type channels Ca_v3.1and Ca_v3.2 do not contribute to baseline blood pressure levels and ANG II-induced hypertension. Ca_v3.1, but not Ca_v3.2, contributes to aldosterone secretion. Aldosterone promotes cardiac hypertrophy during hypertension.

Regulation of aldosterone secretion by mineralocorticoid receptor-mediated signaling

We posit the existence of a paracrine/autocrine negative feedback loop, mediated by the mineralocorticoid receptor (MR), regulating aldosterone secretion. To assess this hypothesis, we asked whether altering MR activity in zona glomerulosa (ZG) cells affects aldosterone production. To this end, we studied ex vivo ZG cells isolated from male Wistar rats fed chow containing either high (1.6% Na⁺ (HS)) or low (0.03% Na⁺ (LS)) amount of sodium. Western blot analyses demonstrated that MR was present in both the ZG and zona fasciculata/zona reticularis (ZF/ZR/ZR). In ZG cells isolated from rats on LS chow, MR activation by fludrocortisone produced a 20% and 60% reduction in aldosterone secretion basally and in response to angiotensin II (ANGII) stimulation, respectively. Corticosterone secretion was increased in these cells suggesting that aldosterone synthase activity was being reduced by fludrocortisone. In contrast, canrenoic acid, an MR antagonist, enhanced aldosterone production by up to 30% both basally and in response to ANGII. Similar responses were observed in ZG cells from rats fed HS. Modulating glucocorticoid receptor (GR) activity did not alter aldosterone production by ZG cells; however, altering GR activity did modify corticosterone production from ZF/ZR/ZR cells both basally and in response to adrenocorticotropic hormone (ACTH). Additionally, activating the MR in ZF/ZR/ZR cells strikingly reduced corticosterone secretion. In summary, these data support the hypothesis that negative ultra-short feedback loops regulate adrenal steroidogenesis. In the ZG, aldosterone secretion is regulated by the MR, but not the GR, an effect that appears to be secondary to a change in aldosterone synthase activity.

CACNA1H(M1549V) Mutant Calcium Channel Causes Autonomous Aldosterone Production in HAC15 Cells and Is Inhibited by Mibefradil

We recently demonstrated that a recurrent gain-of-function mutation in a T-type calcium channel, CACNA1H(M1549V), causes a novel Mendelian disorder featuring early-onset primary aldosteronism and hypertension. This variant was found independently in five families. CACNA1H(M1549V) leads to impaired channel inactivation and activation at more hyperpolarized potentials, inferred to cause increased calcium entry. We here aimed to study the effect of this variant on aldosterone production. We heterologously expressed empty vector, CACNA1H(WT) and CACNA1H(M1549V) in the aldosterone-producing adrenocortical cancer cell line H295R and its subclone HAC15. Transfection rates, expression levels, and subcellular distribution of the channel were similar between CACNA1H(WT) and CACNA1H(M1549V). We measured aldosterone production by an ELISA and CYP11B2 (aldosterone synthase) expression by real-time PCR. In unstimulated cells, transfection of CACNA1H(WT) led to a 2-fold increase in aldosterone levels compared with vector-transfected cells. Expression of CACNA1H(M1549V) caused a 7-fold increase in aldosterone levels. Treatment with angiotensin II or increased extracellular potassium levels further stimulated aldosterone production in both CACNA1H(WT)- and CACNA1H(M1549V)-transfected cells. Similar results were obtained for CYP11B2 expression. Inhibition of CACNA1H channels with the T-type calcium channel blocker Mibefradil completely abrogated the effects of CACNA1H(WT) and CACNA1H(M1549V) on CYP11B2 expression. These results directly link CACNA1H(M1549V) to increased aldosterone production. They suggest that calcium channel blockers may be beneficial in the treatment of a subset of patients with primary aldosteronism. Such blockers could target CACNA1H or both CACNA1H and the L-type calcium channel CACNA1D that is also expressed in the adrenal gland and mutated in patients with primary aldosteronism.

Role of voltage-gated calcium channels in the regulation of aldosterone production from zona glomerulosa cells of the adrenal cortex

Zona glomerulosa cells (ZG) of the adrenal gland constantly integrate fluctuating ionic, hormonal and paracrine signals to control the synthesis and secretion of aldosterone. These signals modulate Ca²⁺ levels, which provide the critical second messenger to drive steroid hormone production. Angiotensin II is a hormone known to modulate the activity of voltage-dependent L- and T-type Ca²⁺ channels that are expressed on the plasma membrane of ZG cells in many species. Because the ZG cell maintains a resting membrane voltage of approximately -85 mV and has been considered electrically silent, low voltage-activated T-type Ca²⁺ channels are assumed to provide the primary Ca²⁺ signal that drives aldosterone production. However, this view has recently been challenged by human genetic studies identifying somatic gain-of-function mutations in L-type Ca_V 1.3 channels in aldosterone-producing adenomas of patients with primary hyperaldosteronism. We provide a review of these assumptions and challenges, and update our understanding of the state of the ZG cell in a layer in which native cellular associations are preserved. This updated view of Ca²⁺ signalling in ZG cells provides a unifying mechanism that explains how transiently activating Ca_V 3.2 channels can generate a significant and recurring Ca²⁺ signal, and how Ca_V 1.3 channels may contribute to the Ca²⁺ signal that drives aldosterone production.

T-Type Calcium Channel: A Privileged Gate for Calcium Entry and Control of Adrenal Steroidogenesis

Intracellular calcium plays a crucial role in modulating a variety of functions such as muscle contraction, hormone secretion, gene expression, or cell growth. Calcium signaling has been however shown to be more complex than initially thought. Indeed, it is confined within cell microdomains, and different calcium channels are associated with different functions, as shown by various channelopathies. Sporadic mutations on voltage-operated L-type calcium channels in adrenal glomerulosa cells have been shown recently to be the second most prevalent genetic abnormalities present in human aldosterone-producing adenoma. The observed modification of the threshold of activation of the mutated channels not only provides an explanation for this gain of function but also reminds us on the importance of maintaining adequate electrophysiological characteristics to make channels able to exert specific cellular functions. Indeed, the contribution to steroid production of the various calcium channels expressed in adrenocortical cells is not equal, and the reason has been investigated for a long time. Given the very negative resting potential of these cells, and the small membrane depolarization induced by their physiological agonists, low threshold T-type calcium channels are particularly well suited for responding under these conditions and conveying calcium into the cell, at the right place for controlling steroidogenesis. In contrast, high threshold L-type channels are normally activated by much stronger cell depolarizations. The fact that dihydropyridine calcium antagonists, specific for L-type channels, are poorly efficient for reducing aldosterone secretion either in vivo or in vitro, strongly supports the view that these two types of channels differently affect steroid biosynthesis. Whether a similar analysis is transposable to fasciculata cells and cortisol secretion is one of the questions addressed in the present review. No similar mutations on L-type or T-type channels have been described yet to affect cortisol secretion or to be linked to the development of Cushing syndrome, but several evidences suggest that the function of T channels is also crucial in fasciculata cells. Putative molecular mechanisms and cellular structural organization making T channels a privileged entry for the "steroidogenic calcium" are also discussed.

Helicobacter pylori CagA protein induces factors involved in the epithelial to mesenchymal transition (EMT) in infected gastric epithelial cells in an EPIYA- phosphorylation-dependent manner

As a result of Helicobacter pylori adhesion to gastric epithelial cells, the bacterial effector cytotoxin-associated gene A (CagA) is translocated intracellularly, and after hierarchical tyrosine phosphorylation on multiple EPIYA motifs, de-regulates cellular polarity and contributes to induction of an elongation and scattering phenotype that resembles the epithelial to mesenchymal transition (EMT). Stromelysin-1/matrix metalloproteinase-3 (MMP-3) has been reported to induce a sequence of molecular alterations leading to stable EMT transition and carcinogenesis in epithelial cells. To identify the putative role of CagA protein in MMP-3 induction, we exploited an experimental H. pylori infection system in gastric epithelial cell lines. We utilized isogenic mutants expressing CagA protein with variable numbers of EPIYA and phosphorylation-deficient EPIFA motifs, as well as cagA knockout and translocation-deficient cagE knockout strains. Increased levels of MMP-3 transcriptional activation were demonstrated by quantitative real time-PCR for strains with more than two terminal EPIYA phosphorylation motifs in CagA. MMP-3 expression in total cell lysates and the corresponding culture supernatants was associated with CagA expression and translocation and was dependent on CagA phosphorylation. A CagA EPIYA phosphorylation-dependent increase in gelatinase and caseinolytic activity was also detected in culture supernatants by zymography. A significant increase in the transcriptional activity of the mesenchymal markers Vimentin, Snail and ZEB1 and the stem cell marker CD44 was observed in the case of CagA containing phosphorylation-functional EPIYA motifs. Our data suggest that CagA protein induces EMT through EPIYA phosphorylation-dependent up-regulation of MMP-3. Moreover, no significant increase in EMT and stem cell markers was observed following infection with H. pylori strains that cannot effectively translocate CagA protein.

Presence of terminal EPIYA phosphorylation motifs in Helicobacter pylori CagA contributes to IL-8 secretion, irrespective of the number of repeats

CagA protein contributes to pro-inflammatory responses during H. pylori infection, following its intracellular delivery to gastric epithelial cells. Here, we report for the first time in an isogenic background, on the subtle role of CagA phosphorylation on terminal EPIYA-C motifs in the transcriptional activation and expression of IL-8. We utilized isogenic H. pylori mutants of P12 reference strain, expressing CagA with varying number of EPIYA-C motifs and the corresponding phosphorylation defective EPIFA-C motifs while preserving intact the CM multimerization motifs. These mutants had been previously closely scrutinized in terms of type IV secretion system functionality, CagA translocation and its subsequent phosphorylation. Following infection of gastric epithelial cell lines, transcriptional activation of IL-8 gene and secreted IL-8 levels were found to be strictly dependent upon the functionality of the EPIYA-C phosphorylation motifs, as EPIFA-C phosphorylation-deficient CagA expression failed to induce full IL-8 transcriptional activity. Interestingly, levels of IL-8 gene activation and of secreted IL-8 were the same, irrespective of the number of EPIYA-C terminal repeats. We monitored IkBα phosphorylation and confirmed CagA involvement in NF-kB activation. Furthermore, we observed that presence of EPIYA-C functional phosphorylation motifs contributed to NF-kB activation. NF-kB upstream signaling events, such as early ERK1/2 and AKT activation were confirmed to be independent of EPIYA-C phosphorylation. On the contrary, use of TAK1 specific inhibitor 5Z-7-Oxozeaenol resulted in complete arrest of IL-8 secretion, in a dose-dependent manner, irrespective of CagA status. H. pylori-infected TAK1^-/- mouse embryonic fibroblasts (MEFs) failed to induce NF-kB activity, unlike the respective control MEFs. CagA and TAK1 were found to immunoprecipitate together, irrespective of CagA EPIYA-C status, thus confirming earlier reports of TAK1 and CagA protein interaction. Our data suggest that CagA may potentially interfere with TAK1 activity during NF-kB activation for IL-8 induction in early H. pylori infection.

Functional chromaffin cell plasticity in response to stress: focus on nicotinic, gap junction, and voltage-gated Ca2+ channels

An increase in circulating catecholamines constitutes one of the mechanisms whereby human body responds to stress. In response to chronic stressful situations, the adrenal medullary tissue exhibits crucial morphological and functional changes that are consistent with an improvement of chromaffin cell stimulus-secretion coupling efficiency. Stimulus-secretion coupling encompasses multiple intracellular (chromaffin cell excitability, Ca(2+) signaling, exocytosis, endocytosis) and intercellular pathways (splanchnic nerve-mediated synaptic transmission, paracrine and endocrine communication, gap junctional coupling), each of them being potentially subjected to functional remodeling upon stress. This review focuses on three chromaffin cell incontrovertible actors, the cholinergic nicotinic receptors and the voltage-dependent T-type Ca(2+) channels that are directly involved in Ca(2+)-dependent events controlling catecholamine secretion and electrical activity, and the gap junctional communication involved in the modulation of catecholamine secretion. We show here that these three actors react differently to various stressors, sometimes independently, sometimes in concert or in opposition.

Direct Corticosteroid Modulation of GABAergic Neurons in the Anterior Hypothalamic Area of GAD65-eGFP Mice

Corticosterone is known to modulate GABAergic synaptic transmission in the hypothalamic paraventricular nucleus. However, the underlying receptor mechanisms are largely unknown. In the anterior hypothalamic area (AHA), the sympathoinhibitory center that project GABAergic neurons onto the PVN, we examined the expression of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) of GABAergic neurons using intact GAD65-eGFP transgenic mice, and the effects of corticosterone on the burst firing using adrenalectomized transgenic mice. GR or MR immunoreactivity was detected from the subpopulations of GABAergic neurons in the AHA. The AHA GABAergic neurons expressed mRNA of GR (42%), MR (38%) or both (8%). In addition, in brain slices incubated with corticosterone together with RU486 (MR-dominant group), the proportion of neurons showing a burst firing pattern was significantly higher than those in the slices incubated with vehicle, corticosterone, or corticosterone with spironolactone (GR-dominant group; 64 vs. 11~14%, p< 0.01 by χ(2)-test). Taken together, the results show that the corticosteroid receptors are expressed on the GABAergic neurons in the AHA, and can mediate the corticosteroid-induced plasticity in the firing pattern of these neurons. This study newly provides the experimental evidence for the direct glucocorticoid modulation of GABAergic neurons in the AHA in the vicinity of the PVN.

A calcium-induced calcium release mechanism supports luteinizing hormone-induced testosterone secretion in mouse Leydig cells

Leydig cells are responsible for the synthesis and secretion of testosterone, processes controlled by luteinizing hormone (LH). Binding of LH to a G protein-coupled receptor in the plasma membrane results in an increase in cAMP and in intracellular Ca2+ concentration ([Ca2+]i). Here we show, using immunofluorescence, that Leydig cells express ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3Rs). Measurements of intracellular calcium changes using the fluorescent calcium-sensitive dye fluo-3 and confocal microscopy show that both types of receptors are involved in a calcium-induced calcium release (CICR) mechanism, which amplifies the initial Ca2+ influx through plasma membrane T-type calcium channels (CaV3). The RyRs and IP3Rs are functional, as judged from both their activation by caffeine and IP3 and block by ryanodine and 2-aminoethoxydiphenyl borate (2-APB), respectively. RyRs are the principal players involved in the release of Ca2+ from the endoplasmic reticulum, as evidenced by the fact that global Ca2+ changes evoked by LH are readily blocked by 100 μM ryanodine but not by 2-APB or xestospongin C. Finally, steroid production by Leydig cells is inhibited by ryanodine but not by 2-APB. These results not only broaden our understanding of the role played by calcium in Leydig cells but also show, for the first time, that RyRs have an important role in determining testosterone secretion by the testis.

Neurohormonal regulation of cardiac ion channels in chronic heart failure

Alteration of neurohormonal homeostasis is a hallmark of the pathophysiology of chronic heart failure (CHF). In particular, overactivation of the renin-angiotensin-aldosterone system and the sympathetic catecholaminergic system is consistently observed. Chronic overactivation of these hormonal pathways leads to a detrimental arrhythmogenic remodeling of cardiac tissue due to dysregulation of cardiac ion channels. Sudden cardiac death resulting from ventricular arrhythmias is a major cause of mortality in patients with CHF. All the drug classes known to reduce mortality in patients with CHF are neurohormonal blockers. The aim of this review was to provide an overview of how cardiac ion channels are regulated by hormones known to play a central role in the pathogenesis of CHF.

The role of calcium influx pathways in phospholipase D activation in bovine adrenal glomerulosa cells

The steroid hormone aldosterone maintains sodium homeostasis and is therefore important in the control of blood volume and pressure. Angiotensin II (AngII) and elevated extracellular potassium concentrations ([K(+)](e)), the prime physiologic regulators of aldosterone secretion from adrenal glomerulosa cells, activate phospholipase D (PLD) in these cells. The role of Ca(2+) in the activation by these agents is unknown, although nitrendipine, a voltage-dependent Ca(2+) channel antagonist, does not inhibit AngII-elicited PLD activation, despite the fact that this compound blocked elevated [K(+)](e)-stimulated PLD activity. PLD activation triggered by AngII was also unaffected by the T-type calcium channel inhibitor nickel. Nevertheless, Ca(2+) influx was required for AngII-induced PLD activation in both primary cultures of bovine adrenal glomerulosa cells and a glomerulosa cell model, the NCI H295R adrenocortical carcinoma cell line. The involvement of store-operated Ca(2+) (SOC) influx and Ca(2+) release-activated Ca(2+) (CRAC) influx pathways in PLD activation was investigated using thapsigargin, an endoplasmic reticulum Ca(2+) pump inhibitor that empties the store to induce SOC influx, and the SOC inhibitor YM-58483 (BTP2), as well as a CRAC inhibitor, tyrphostin A9. In bovine glomerulosa cells, tyrphostin A9 inhibited AngII-induced PLD activation without affecting elevated [K(+)](e)-stimulated enzyme activity. On the other hand, differences were observed between the bovine adrenal glomerulosa and H295R cells in the involvement of Ca(2+) influx pathways in PLD activation, with the involvement of the SOC pathway suggested in the H295R cells. In summary, our results indicate that Ca(2+) entry only through certain Ca(2+) influx pathways is linked to PLD activation.

Calcium signaling and T-type calcium channels in cancer cell cycling

Regulation of intracellular calcium is an important signaling mechanism for cell proliferation in both normal and cancerous cells. In normal epithelial cells, free calcium concentration is essential for cells to enter and accomplish the S phase and the M phase of the cell cycle. In contrast, cancerous cells can pass these phases of the cell cycle with much lower cytoplasmic free calcium concentrations, indicating an alternative mechanism has developed for fulfilling the intracellular calcium requirement for an increased rate of DNA synthesis and mitosis of fast replicating cancerous cells. The detailed mechanism underlying the altered calcium loading pathway remains unclear; however, there is a growing body of evidence that suggests the T-type Ca²⁺ channel is abnormally expressed in cancerous cells and that blockade of these channels may reduce cell proliferation in addition to inducing apoptosis. Recent studies also show that the expression of T-type Ca²⁺ channels in breast cancer cells is proliferation state dependent, i.e. the channels are expressed at higher levels during the fast-replication period, and once the cells are in a non-proliferation state, expression of this channel is minimal. Therefore, selectively blocking calcium entry into cancerous cells may be a valuable approach for preventing tumor growth. Since T-type Ca²⁺ channels are not expressed in epithelial cells, selective T-type Ca²⁺ channel blockers may be useful in the treatment of certain types of cancers.

Blockade of T-type voltage-dependent Ca2+ channels by benidipine, a dihydropyridine calcium channel blocker, inhibits aldosterone production in human adrenocortical cell line NCI-H295R

Benidipine, a long-lasting dihydropyridine calcium channel blocker, is used for treatment of hypertension and angina. Benidipine exerts pleiotropic pharmacological features, such as renoprotective and cardioprotective effects. In pathophysiological conditions, the antidiuretic hormone aldosterone causes development of renal and cardiovascular diseases. In adrenal glomerulosa cells, aldosterone is produced in response to extracellular potassium, which is mainly mediated by T-type voltage-dependent Ca2+ channels. More recently, it has been demonstrated that benidipine inhibits T-type Ca2+ channels in addition to L-type Ca2+ channels. Therefore, effect of calcium channel blockers, including benidipine, on aldosterone production and T-type Ca2+ channels using human adrenocortical cell line NCI-H295R was investigated. Benidipine efficiently inhibited KCl-induced aldosterone production at low concentration (3 and 10 nM), with inhibitory activity more potent than other calcium channel blockers. Patch clamp analysis indicated that benidipine concentration-dependently inhibited T-type Ca2+ currents at 10, 100 and 1000 nM. As for examined calcium channel blockers, inhibitory activity for T-type Ca2+ currents was well correlated with aldosterone production. L-type specific calcium channel blockers calciseptine and nifedipine showed no effect in both assays. These results indicate that inhibition of T-type Ca2+ channels is responsible for inhibition of aldosterone production in NCI-H295R cells. Benidipine efficiently inhibited KCl-induced upregulation of 11-beta-hydroxylase mRNA and aldosterone synthase mRNA as well as KCl-induced Ca2+ influx, indicating it as the most likely inhibition mechanism. Benidipine partially inhibited angiotensin II-induced aldosterone production, plus showed additive effects when used in combination with the angiotensin II type I receptor blocker valsartan. Benidipine also partially inhibited angiotensin II-induced upregulation of the above mRNAs and Ca2+ influx inhibitory activities of benidipine for aldosterone production. T-type Ca2+ channels may contribute to additional benefits of this drug for treating renal and cardiovascular diseases, beyond its primary anti-hypertensive effects from blocking L-type Ca2+ channels.

Intracellular calcium changes in mice Leydig cells are dependent on calcium entry through T-type calcium channels

Luteinizing hormone (LH) regulates testosterone synthesis in Leydig cells by inducing an intracellular increase in cAMP concentration. LH also increases the intracellular calcium concentration ([Ca2+]i), dependent on the presence of Ca2+ in the extracellular medium ([Ca2+]e) for its effect. Despite these evidences, the identity of a pathway for calcium entry has remained elusive and the relationship between cAMP and [Ca2+]i has been questioned. Here we show that mice Leydig cells do have an inward Ca2+ current carried by T-type Ca2+ channels. In 10 mm [Ca2+]e, the currents start to be activated at -60 mV, reaching maximal amplitude of 1.8 +/- 0.3 pA pF(-1) at -20 mV. Currents were not modified by Ba2+ or Sr2+, were suppressed in Ca2+-free external solution, and were blocked by 100 microm nickel or 100 microm cadmium. The Ki for Ni2+ is 2.6 microm and concentrations of Cd2+ smaller than 50 microm have a very small effect on the currents. The calcium currents displayed a window centred at -40 mV. The half-voltage (V0.5) of activation is -30.3 mV, whereas the half-voltage steady-state inactivation is -51.1 mV. The deactivation time constant (taudeactivation) is around 3 ms at -35 mV. Confocal microscopy experiments with Fluo-3 loaded cells reveal that both LH and dibutyryl-cAMP (db-cAMP) increase [Ca2+]i. The db-cAMP induced calcium increase was dependent on Ca2+ influx since it was abolished by removal of extracellular Ca2+ and by 400 microm Ni2+. [Ca2+]i increases in regions close to the plasma membrane and in the cell nucleus. Similar effects are seen when Leydig cells are depolarized by withdrawing K+ from the extracellular solution. Altogether, our studies show that Ca2+ influx through T-type Ca2+ channels in the plasma membrane of Leydig cells plays a crucial role in the intracellular calcium concentration changes that follow binding of LH to its receptor.

Blocking T-type Ca2+ channels with efonidipine decreased plasma aldosterone concentration in healthy volunteers

Efonidipine can block both L- and T- type Ca2+ channels. In a previous in vitro study, we clarified that efonidipine dramatically suppresses aldosterone secretion from human adrenocortical tumor cells during angiotensin II (Ang II)- and K+-stimulation, whereas nifedipine, a dominant L-type Ca2+ channel antagonist, does not. This study was conducted to assess the in vivo effects of efonidipine and nilvadipine on the plasma aldosterone concentration. Placebo, 40 mg of efonidipine, or 2 mg of nilvadipine was administered to five healthy male volunteers. Hemodynamic parameters (pulse rate [PR] and blood pressure [BP]), plasma concentrations of neurohormonal factors (plasma renin activity, Ang II, aldosterone, and adrenocorticotropic hormone [ACTH]), and serum concentrations of Na+ and K+ were measured before and 6 h after administration of the agents. All three agents had little effect on PR and BP. Efonidipine and nilvadipine significantly increased plasma renin activity and Ang II. Both had little effect on ACTH, Na+, and K+. The plasma aldosterone concentration was significantly decreased after efonidipine treatment (88.3 +/- 21.3 to 81.6 +/- 24.9 pg/ml, p = 0.0407), whereas it was significantly increased after nilvadipine treatment (66.5 +/- 12.2 to 82.17 +/- 16.6 pg/ml, p = 0.0049). Placebo had little effect on neurohormonal factors. Efonidipine decreased plasma aldosterone concentration despite the increase in plasma renin activity and Ang II, suggesting that T-type Ca2+ channels may also play an essential role in the secretion of aldosterone in healthy human volunteers.

Estrogen upregulates T-type calcium channels in the hypothalamus and pituitary

Low voltage-activated (T-type) Ca2+ channels are responsible for generating low-threshold spikes (LTS) that facilitate burst firing and transmitter release in neurons. The T-type Ca2+ channels contain a regulatory alpha1 subunit, and several isoforms of the alpha1 subunit (Cav3.1, 3.2, 3.3) have been cloned. The Cav 3.1 alpha1 subunit is abundantly expressed in the hypothalamus. Previously, we found that 17 beta-estradiol (E2) increased the number of arcuate neurons expressing LTS. Therefore, we used an ovariectomized female guinea pig model to measure the distribution and regulation of Cav3.1 mRNA expression by E2. Guinea pig Cav3.1 alpha1 subunit sequences, which were cloned by PCR, were used in ribonuclease protection (RPA) and in situ hybridization assays to evaluate mRNA expression. Based on a RPA, E2 significantly increased the mRNA expression of Cav3.1 alpha1 subunit in the mediobasal hypothalamus and the pituitary. In situ hybridization analysis revealed that E2 significantly increased Cav 3.1 mRNA expression in medial preoptic nuclei, bed nuclei stria terminalis, and the arcuate nucleus. Whole-cell patch recordings in arcuate neurons revealed that E2 treatment significantly increased the peak T-type Ca2+ current density by twofold without affecting the activation/inactivation characteristics and augmented the rebound excitation by threefold to fourfold. These results suggest that estrogen regulates the mRNA expression of T-type calcium channels, which leads to increased functional expression of the channel. Increased expression of T-type channels could be one mechanism by which estrogen augments burst firing and transmitter release in hypothalamic neurons.

Aldosterone increases voltage-gated sodium current in ventricular myocytes

The role of aldosterone in the pathogenesis of heart failure (HF) is still poorly understood. Recently, aldosterone has been shown to modulate the function of cardiac Ca2+and K+channels, thus playing a role in the electrical remodeling process. The goal of this work was to investigate the role of aldosterone on the cardiac Na+current ( INa). We analyzed the effects of aldosterone on INain isolated adult mouse ventricular myocytes, using the whole cell patch-clamp technique. After 24 h incubation with 1 μM aldosterone, the INadensity was significantly increased (+55%), without alteration of the biophysical properties and the cell membrane capacitance. Aldosterone (10 nM) increased the INaby 23%. In 24-h coincubation experiments, with the use of actinomycin D, cycloheximide, or brefeldin A, the effect of aldosterone on INawas abolished. Spironolactone (mineralocorticoid receptor antagonist, 10 μM) prevented the 1 μM aldosterone-dependent INaincrease, whereas RU-38486 (glucocorticoid receptor antagonist, 10 μM) did not. The action potential duration (APD) was longer in aldosterone-treated (APD90: +53%) than in control myocytes. In addition, the L-type Ca2+current was also upregulated (+48%). We performed quantitative RT-PCR measurements and Western blots to quantify the mRNA and protein levels of Nav1.5 and Cav1.2 (main channels mediating cardiac INaand ICa), but no significant difference was found. In conclusion, this study shows that aldosterone upregulates the cardiac INaand suggest that this phenomenon may contribute to the HF-induced electrical remodeling process that may be reversed by spironolactone.

Inhibitory Effect of Efonidipine on Aldosterone Synthesis and Secretion in Human Adrenocarcinoma (H295R) Cells

Targeting aldosterone synthesis and/or release represents a potentially useful approach to the prevention of cardiovascular disease. Aldosterone production is stimulated by angiotensin II (Ang II) or extracellular K+ and is mediated mainly by Ca2+ influx into adrenal glomerulosa cells through T-type calcium channels. We therefore examined the effects of efonidipine, a dual T-type/L-type Ca2+ channel blocker, on aldosterone secretion in the H295R human adrenocarcinoma cell line; 100 nmol/L Ang II and 10 mmol/L K+ respectively increased aldosterone secretion from H295R cells 12-fold and 9-fold over baseline. Efonidipine dose-dependently inhibited both Ang II- and K+-induced aldosterone secretion, and nifedipine, an L-type Ca2+ channel blocker, and mibefradil, a relatively selective T-type channel blocker, similarly inhibited Ang II- and K+-induced aldosterone secretion, but were much less potent than efonidipine. Efonidipine also lowered cortisol secretion most potently among these drugs. Notably, efonidipine and mibefradil also significantly suppressed Ang II- and K+-induced mRNA expression of 11-beta-hydroxylase and aldosterone synthase, which catalyze the final two steps in the aldosterone synthesis, whereas nifedipine reduced only K+-induced enzyme expression. These findings suggest that efonidipine acts via T-type Ca2+ channel blockade to significantly reduce aldosterone secretion, and that this effect is mediated, at least in part, by suppression of 11-beta-hydroxylase and aldosterone synthase expression.

Differential expression of T-type calcium channels in P/Q-type calcium channel mutant mice with ataxia and absence epilepsy

Mutations in P/Q-type calcium channels generate common phenotypes in mice and humans, which are characterized by ataxia, paroxysmal dyskinesia, and absence seizures. Subsequent functional changes of T-type calcium channels in thalamus are observed in P/Q-type calcium channel mutant mice and these changes play important roles in generation of absence seizures. However, the changes in T-type calcium channel function and/or expression in the cerebellum, which may be related to movement disorders, are still unknown. The leaner mouse exhibits severe ataxia, paroxysmal dyskinesia, and absence epilepsy due to a P/Q-type calcium channel mutation. We investigated changes in T-type calcium channel expression in the leaner mouse thalamus and cerebellum using quantitative real-time polymerase chain reaction (qRT-PCR) and quantitative in situ hybridization histochemistry (ISHH). qRT-PCR analysis showed no change in T-type calcium channel alpha 1G subunit (Cav3.1) expression in the leaner thalamus, but a significant decrease in alpha 1G expression in the whole leaner mouse cerebellum. Interestingly, quantitative ISHH revealed differential changes in alpha 1G expression in the leaner cerebellum, where the granule cell layer showed decreased alpha 1G expression while Purkinje cells showed increased alpha 1G expression. To confirm these observations, the granule cell layer and the Purkinje cell layer were laser capture microdissected separately, then analyzed with qRT-PCR. Similar to the observation obtained by ISHH, the leaner granule cell layer showed decreased alpha 1G expression and the leaner Purkinje cell layer showed increased alpha 1G expression. These results suggest that differential expression of T-type calcium channels in the leaner cerebellum may be involved in the observed movement disorders.

Aldosteronism and a Proinflammatory Vascular Phenotype

Background— Chronic, inappropriate (relative to dietary Na + ) elevations in circulating aldosterone, such as occur in congestive heart failure, are accompanied by a proinflammatory vascular phenotype involving the coronary and systemic vasculature. An immunostimulatory state with activated peripheral blood mononuclear cells (PBMCs) precedes this phenotype and is induced by a fall in cytosolic free [Mg 2+ ] i and subsequent Ca 2+ loading of these cells and transduced by oxidative/nitrosative stress.

Methods and Results— We sought to further validate this hypothesis in rats with aldosterone/1%NaCl treatment (ALDOST) by using several interventions as cotreatment: a Mg 2+ -supplemented diet; amlodipine, a CCB; and N -acetylcysteine, an antioxidant. Blood samples were obtained at weeks 1 to 4 of ALDOST to monitor [Mg 2+ ] i , [Ca 2+ ] I , and H 2 O 2 production in PBMCs. Coronal ventricular sections were examined for invading inflammatory cells and 3-nitrotyrosine labeling, a marker of oxidative/nitrosative stress. In response to ALDOST and compared with untreated controls, we found an early and persistent reduction in [Mg 2+ ] i with a subsequent rise in [Ca 2+ ] i and H 2 O 2 production, each of which was either attenuated or abrogated by the Mg 2+ -supplemented diet and by N -acetylcysteine, whereas amlodipine prevented Ca 2+ loading and an altered redox state. Cotreatment with these interventions either markedly attenuated or prevented the appearance of the proinflammatory coronary vascular phenotype and the presence of 3-nitrotyrosine in invading inflammatory cells.

Conclusions— We suggest that the immunostimulatory state that appears during aldosteronism and leads to a proinflammatory coronary vascular phenotype is induced by a fall in [Mg 2+ ] i with Ca 2+ loading of PBMCs and is transduced by H 2 O 2 production in these cells.

Disruption of growth hormone secretion alters Ca2+ current density and expression of Ca2+ channel and insulin-like growth factor genes in rat atria

The influence of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis on expression of low-voltage-activated (LVA) Ca2+ current in atrial tissue was investigated using spontaneous dwarf (SpDwf) rats, a mutant strain that lacks GH. Atrial myocytes from SpDwf rats express LVA and high-voltage-activated (HVA) Ca2+ currents and the Ca2+ channel alpha1-subunit genes CaV1.2, CaV2.3, CaV3.1, and CaV3.2. LVA current density decreases significantly beginning at, or shortly after, birth in normal animals; however, its density is maintained in SpDwf rats at 1 pA/pF for > or =12 wk after birth. The abundance of mRNAs encoding CaV2.3 and CaV3.2 declines with advancing age in normal atrial development, yet expression of CaV2.3 mRNA remains significantly elevated in older SpDwf animals. Quantitation of local transcript levels for mRNAs encoding IGF-I and IGF-I receptor (IGF-IR) also reveals significant differences in expression of these transcripts in atrial tissue of SpDwf animals compared with controls. In SpDwf rats, the abundance of IGF-IR mRNA remains elevated at many postnatal ages, whereas mRNA encoding IGF-I is maintained only in older animals. Physiological concentrations of IGF-I cause two- to threefold increases in LVA current density in primary cultures of atrial myocytes, and this effect is blocked by an antisense oligonucleotide targeting the IGF-IR. Thus disruption of GH production in SpDwf animals alters expression of atrial LVA Ca2+ channel and IGF genes as well as postnatal regulation of LVA Ca2+ current density, most likely acting through compensatory mechanisms via the local IGF-IR.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Intracellular transport of calcium from plasma membrane to mitochondria in adrenal H295R cells: implication for steroidogenesis

Angiotensin II and extracellular potassium stimulate aldosterone production in adrenal glomerulosa cells by mobilizing the calcium messenger system. This response requires calcium influx across the plasma membrane, followed by calcium uptake into the mitochondria. It has been proposed that calcium is transported to the mitochondria via the lumen of the endoplasmic reticulum, acting as a kind of intracellular calcium pipeline. This hypothesis has been tested in the present study by measuring intramitochondrial calcium variations in H295R cells with a new fluorescent calcium probe, ratiometric pericam. Calyculin A, a protein phosphatase inhibitor, induced the formation of a large cortical layer of actin filaments, removing the peripheral endoplasmic reticulum away from the plasma membrane and thereby physically uncoupling the calcium channels from the pipeline. The mitochondrial calcium response to potassium was markedly reduced after calyculin treatment, but that of AngII was unaffected. Under the same conditions, potassium-stimulated pregnenolone and aldosterone production was significantly reduced, whereas the steroidogenic response to AngII remained unchanged. The inhibitory action of calyculin A on the responses to potassium was not mediated by a modification of the calcium channel activity and was not accompanied by a reduction of the cytosolic calcium response. It therefore appears that, in H295R cells, the organization of the actin cytoskeleton at the cell periphery influences the steroidogenic action of potassium, but not the response to angiotensin II. The response to potassium is proposed to be dependent on the endoplasmic reticulum-mediated transfer of calcium entering through plasma membrane calcium channels to the mitochondria.

Overexpression of an alpha 1H (Cav3.2) T-type calcium channel during neuroendocrine differentiation of human prostate cancer cells

Neuroendocrine differentiation of prostate epithelial cells is usually associated with an increased aggressivity and invasiveness of prostate tumors and a poor prognosis. However, the molecular mechanisms involved in this process remain poorly understood. We have investigated the possible expression of voltage-gated calcium channels in human prostate cancer epithelial LNCaP cells and their modulation during neuroendocrine differentiation. A small proportion of undifferentiated LNCaP cells displayed a voltage-dependent calcium current. This proportion and the calcium current density were significantly increased during neuroendocrine differentiation induced by long-term treatments with cyclic AMP permeant analogs or with a steroid-reduced culture medium. Biophysical and pharmacological properties of this calcium current suggest that it is carried by low-voltage activated T-type calcium channels. Reverse transcriptase-PCR experiments demonstrated that only a single type of LVA calcium channel mRNA, an alpha(1H) calcium channel mRNA, is expressed in LNCaP cells. Quantitative real-time reverse transcriptase-PCR revealed that alpha(1H) mRNA was overexpressed during neuroendocrine differentiation. Finally, we show that this calcium channel promotes basal calcium entry at resting membrane potential and may facilitate neurite lengthening. This voltage-dependent calcium channel could be involved in the stimulation of mitogenic factor secretion and could therefore be a target for future therapeutic strategies.