The effect of cytoplasmic Ca2+ signal on the redox state of mitochondrial pyridine nucleotides

Evodiamine induces ROS-Dependent cytotoxicity in human gastric cancer cells via TRPV1/Ca2+ pathway

Evodiamine (EVO), a key active ingredient of the fruit of Evodiae fructus, is provided with antitumor effects (mainly cytotoxic effect) including proliferation inhibition, cell cycle arrest, apoptosis, and metastasis inhibition. Our study aims to explain the underlying role of TRPV1/Ca²⁺ in EVO-induced cytotoxicity in human gastric cancer cells. Human gastric cancer line BGC-823 was used to study EVO-induced cytotoxicity. Cell viability was examined using CCK-8 assay. Apoptosis was examined using Annexin V-FITC/PI staining assay. Intracellular ROS ([ROS]i) levels were examined using DCFH-DA assay. Mitochondrial morphology was examined using Mitotracker Green staining. Mitochondrial membrane potential (Δψm) were examined using JC-1 assay. Intracellular Ca²⁺ levels ([Ca²⁺]i) were examined using Fluo-4 AM assay. Mitochondrial ROS ([ROS]m)levels were examined using Mitotracker Green/MitoSOX Red staining. Mitochondrial Ca²⁺ ([Ca²⁺]m)levels were examined using Mitotracker Green/Rhod-2 Red staining. The protein levels was detected by Western blot. EVO exposure causes significant ROS generation and apoptotic cell death. Pretreatment of EUK134 significantly ameliorated EVO-induced apoptotic cell death. Furthermore, EVO exposure induced [ROS]i generation and mitochondrial dysfunction, including [ROS]m generation and Δψm dissipation, which can be significantly attenuated by pre-incubation of rotenone indicating that [ROS]m is the main source of EVO-induced intracellular ROS generation. Importantly, EVO-induced cytotoxicity was significantly ameliorated by intracellular Ca²⁺ chelation, confirming that EVO induces cell death through Ca²⁺ overload. Pharmacological and genetic inhibition of TRPV1 could significantly attenuate Ca²⁺ influx, ROS generation and apoptotic cell death induced by EVO exposure, while exogenous TRPV1 overexpression could augment the EVO-induced cytotoxicity. Moreover, genetic inhibition of mitochondrial calcium uniporter (MCU) attenuated EVO-induced cell death and mitochondrial dysfunction. EVO exposure induced endoplasmic reticulum (ER) stress demonstrated by the activation of PERK/CHOP in cells exposed to EVO, and PERK/CHOP activation was depleted by EUK134 pre-treatment. Our results support the concept that EVO induces ROS-dependent cytotoxicity via TRPV1/Ca²⁺ Pathway.

Mitochondrial superoxide anions induced by exogenous oxidative stress determine tumor cell fate: an individual cell-based study

OBJECTIVE

Reactive oxygen species (ROS) are involved in a variety of biological phenomena and serve both deleterious and beneficial roles. ROS quantification and assessment of reaction networks are desirable but difficult because of their short half-life and high reactivity. Here, we describe a pro-oxidative model in a single human lung carcinoma SPC-A-1 cell that was created by application of extracellular H₂O₂ stimuli.

METHODS

Modified microfluidics and imaging techniques were used to determine O₂ ^•- levels and construct an O₂ ^•- reaction network. To elucidate the consequences of increased O₂ ^•- input, the mitochondria were given a central role in the oxidative stress mode, by manipulating mitochondria-interrelated cytosolic Ca²⁺ levels, mitochondrial Ca²⁺ uptake, auto-amplification of intracellular ROS and the intrinsic apoptotic pathway.

RESULTS AND CONCLUSIONS

Results from a modified microchip demonstrated that 1 mmol/L H₂O₂ induced a rapid increase in cellular O₂ ^•- levels (>27 vs. >406 amol in 20 min), leading to increased cellular oxidizing power (evaluated by ROS levels) and decreased reducing power (evaluated by glutathione (GSH) levels). In addition, we examined the dynamics of cytosolic Ca²⁺ and mitochondrial Ca²⁺ by confocal laser scanning microscopy and confirmed that Ca²⁺ stores in the endoplasmic reticulum were the primary source of H₂O₂-induced cytosolic Ca²⁺ bursts. It is clear that mitochondria have pivotal roles in determining how exogenous oxidative stress affects cell fate. The stress response involves the transfer of Ca²⁺ signals between organelles, ROS auto-amplification, mitochondrial dysfunction, and a caspase-dependent apoptotic pathway.

Mitochondrial cAMP and Ca2+ metabolism in adrenocortical cells

The biological effects of physiological stimuli of adrenocortical glomerulosa cells are predominantly mediated by the Ca²⁺ and the cAMP signal transduction pathways. The complex interplay between these signalling systems fine-tunes aldosterone secretion. In addition to the well-known cytosolic interactions, a novel intramitochondrial Ca²⁺-cAMP interplay has been recently recognised. The cytosolic Ca²⁺ signal is rapidly transferred into the mitochondrial matrix where it activates Ca²⁺-sensitive dehydrogenases, thus enhancing the formation of NADPH, a cofactor of steroid synthesis. Quite a few cell types, including H295R adrenocortical cells, express the soluble adenylyl cyclase within the mitochondria and the elevation of mitochondrial [Ca²⁺] activates the enzyme, thus resulting in the Ca²⁺-dependent formation of cAMP within the mitochondrial matrix. On the other hand, mitochondrial cAMP (mt-cAMP) potentiates the transfer of cytosolic Ca²⁺ into the mitochondrial matrix. This cAMP-mediated positive feedback control of mitochondrial Ca²⁺ uptake may facilitate the rapid hormonal response to emergency situations since knockdown of soluble adenylyl cyclase attenuates aldosterone production whereas overexpression of the enzyme facilitates steroidogenesis in vitro. Moreover, the mitochondrial Ca²⁺-mt-cAMP-Ca²⁺ uptake feedback loop is not a unique feature of adrenocortical cells; a similar signalling system has been described in HeLa cells as well.

Mitochondrial cAMP exerts positive feedback on mitochondrial Ca2+ uptake via the recruitment of Epac1

We have previously demonstrated in H295R adrenocortical cells that the Ca²⁺-dependent production of mitochondrial cAMP (mt-cAMP) by the matrix soluble adenylyl cyclase (sAC; encoded by ADCY10) is associated with enhanced aldosterone production. Here, we examined whether mitochondrial sAC and mt-cAMP fine tune mitochondrial Ca²⁺ metabolism to support steroidogenesis. Reduction of mt-cAMP formation resulted in decelerated mitochondrial Ca²⁺ accumulation in intact cells during K⁺-induced Ca²⁺ signalling and also in permeabilized cells exposed to elevated perimitochondrial [Ca²⁺]. By contrast, treatment with the membrane-permeable cAMP analogue 8-Br-cAMP, inhibition of phosphodiesterase 2 and overexpression of sAC in the mitochondrial matrix all intensified Ca²⁺ uptake into the organelle. Identical mt-cAMP dependence of mitochondrial Ca²⁺ uptake was also observed in HeLa cells. Importantly, the enhancing effect of mt-cAMP on Ca²⁺ uptake was independent from both the mitochondrial membrane potential and Ca²⁺ efflux, but was reduced by Epac1 (also known as RAPGEF3) blockade both in intact and in permeabilized cells. Finally, overexpression of sAC in the mitochondrial matrix potentiated aldosterone production implying that the observed positive feedback mechanism of mt-cAMP on mitochondrial Ca²⁺ accumulation may have a role in the rapid initiation of steroidogenesis.This article has an associated First Person interview with the first author of the paper.

Supplemental arginine vasopressin during the resuscitation of severe hemorrhagic shock preserves renal mitochondrial function

Arginine vasopressin (AVP), a hormone secreted by the posterior pituitary, plays a vital role in maintaining vasomotor tone during acute blood loss. We hypothesized that decompensated hemorrhagic shock is associated with decreased AVP stores and supplementation during resuscitation would improve both blood pressure and renal function. Using a decompensated hemorrhagic shock model, male Long-Evans rats were bled to mean arterial blood pressure (MAP) of 40mmHg and maintained until the MAP could not be sustained without fluid. Once 40% of the shed volume was returned in lactated Ringer’s (Severe Shock), animals were resuscitated over 60 minutes with 4x the shed volume in lactated Ringer’s (LR) or the same fluids with AVP (0.5 units/kg+ 0.03 units/kg/min). Animals (n = 6-9/group) were sacrificed before hemorrhage (Sham), at Severe Shock, following resuscitation (60R, 60R with AVP) or 18 hours post-resuscitation (18hr, 18hr with AVP). Blood samples were taken to measure AVP levels and renal function. Pituitaries were harvested and assayed for AVP. Kidney samples were taken to assess mitochondrial function, histology, and oxidative damage. Baseline pituitary AVP stores (30,364 ± 5311 pg/mg) decreased with severe shock and were significantly depressed post-resuscitation (13,910 ± 3016 pg/ml. p<0.05) and at 18hr (15,592 ±1169 pg/ml, p<0.05). Resuscitation with LR+AVP led to higher serum AVP levels at 60R (31±8 vs 79±12; p<0.01) with an improved MAP both at 60R (125±3 vs 77±7mmHg; p<0.01) and 18hr (82±6 vs 69±5mmHg;p<0.05). AVP supplementation preserved complex I respiratory capacity at 60R and both complex I and II function at 18hr (p<0.05). AVP was also associated with decreased reactive oxygen species at 60R (856±67 vs 622±48F RFU) and significantly decreased oxidative damage as measured by mitochondrial lipid peroxidation (0.9±0.1 vs 1.7±0.1 fold change, p<0.01) and nitrosylation (0.9±0.1 vs 1.4±0.2 fold change, p<0.05). With AVP, renal damage was mitigated at 60R and histologic architecture was conserved at 18 hours. In conclusion, pituitary and serum AVP levels decrease during severe hemorrhage and may contribute to the development of decompensated hemorrhagic shock. Supplementing exogenous AVP during resuscitation improves blood pressure, preserves renal mitochondrial function, and mitigates acute kidney injury.

Functional TASK-3-Like Channels in Mitochondria of Aldosterone-Producing Zona Glomerulosa Cells

Ca²⁺ drives aldosterone synthesis in the cytosolic and mitochondrial compartments of the adrenal zona glomerulosa cell. Membrane potential across each of these compartments regulates the amplitude of the Ca²⁺ signal; yet, only plasma membrane ion channels and their role in regulating cell membrane potential have garnered investigative attention as pathological causes of human hyperaldosteronism. Previously, we reported that genetic deletion of TASK-3 channels (tandem pore domain acid-sensitive K⁺ channels) from mice produces aldosterone excess in the absence of a change in the cell membrane potential of zona glomerulosa cells. Here, we report using yeast 2-hybrid, immunoprecipitation, and electron microscopic analyses that TASK-3 channels are resident in mitochondria, where they regulate mitochondrial morphology, mitochondrial membrane potential, and aldosterone production. This study provides proof of principle that mitochondrial K⁺ channels, by modulating inner mitochondrial membrane morphology and mitochondrial membrane potential, have the ability to play a pathological role in aldosterone dysregulation in steroidogenic cells.

Signaling Interactions in the Adrenal Cortex

The major physiological stimuli of aldosterone secretion are angiotensin II (AII) and extracellular K(+), whereas cortisol production is primarily regulated by corticotropin (ACTH) in fasciculata cells. AII triggers Ca(2+) release from internal stores that is followed by store-operated and voltage-dependent Ca(2+) entry, whereas K(+)-evoked depolarization activates voltage-dependent Ca(2+) channels. ACTH acts primarily through the formation of cAMP and subsequent protein phosphorylation by protein kinase A. Both Ca(2+) and cAMP facilitate the transfer of cholesterol to mitochondrial inner membrane. The cytosolic Ca(2+) signal is transferred into the mitochondrial matrix and enhances pyridine nucleotide reduction. Increased formation of NADH results in increased ATP production, whereas that of NADPH supports steroid production. In reality, the control of adrenocortical function is a lot more sophisticated with second messengers crosstalking and mutually modifying each other's pathways. Cytosolic Ca(2+) and cGMP are both capable of modifying cAMP metabolism, while cAMP may enhance Ca(2+) release and voltage-activated Ca(2+) channel activity. Besides, mitochondrial Ca(2+) signal brings about cAMP formation within the organelle and this further enhances aldosterone production. Maintained aldosterone and cortisol secretion are optimized by the concurrent actions of Ca(2+) and cAMP, as exemplified by the apparent synergism of Ca(2+) influx (inducing cAMP formation) and Ca(2+) release during response to AII. Thus, cross-actions of parallel signal transducing pathways are not mere intracellular curiosities but rather substantial phenomena, which fine-tune the biological response. Our review focuses on these functionally relevant interactions between the Ca(2+) and the cyclic nucleotide signal transducing pathways hitherto described in the adrenal cortex.

Calcium-dependent mitochondrial cAMP production enhances aldosterone secretion

Glomerulosa cells secrete aldosterone in response to agonists coupled to Ca(2+) increases such as angiotensin II and corticotrophin, coupled to a cAMP dependent pathway. A recently recognized interaction between Ca(2+) and cAMP is the Ca(2+)-induced cAMP formation in the mitochondrial matrix. Here we describe that soluble adenylyl cyclase (sAC) is expressed in H295R adrenocortical cells. Mitochondrial cAMP formation, monitored with a mitochondria-targeted fluorescent sensor (4mtH30), is enhanced by HCO3(-) and the Ca(2+) mobilizing agonist angiotensin II. The effect of angiotensin II is inhibited by 2-OHE, an inhibitor of sAC, and by RNA interference of sAC, but enhanced by an inhibitor of phosphodiesterase PDE2A. Heterologous expression of the Ca(2+) binding protein S100G within the mitochondrial matrix attenuates angiotensin II-induced mitochondrial cAMP formation. Inhibition and knockdown of sAC significantly reduce angiotensin II-induced aldosterone production. These data provide the first evidence for a cell-specific functional role of mitochondrial cAMP.

Separating NADH and NADPH fluorescence in live cells and tissues using FLIM

NAD is a key determinant of cellular energy metabolism. In contrast, its phosphorylated form, NADP, plays a central role in biosynthetic pathways and antioxidant defence. The reduced forms of both pyridine nucleotides are fluorescent in living cells but they cannot be distinguished, as they are spectrally identical. Here, using genetic and pharmacological approaches to perturb NAD(P)H metabolism, we find that fluorescence lifetime imaging (FLIM) differentiates quantitatively between the two cofactors. Systematic manipulations to change the balance between oxidative and glycolytic metabolism suggest that these states do not directly impact NAD(P)H fluorescence decay rates. The lifetime changes observed in cancers thus likely reflect shifts in the NADPH/NADH balance. Using a mathematical model, we use these experimental data to quantify the relative levels of NADH and NADPH in different cell types of a complex tissue, the mammalian cochlea. This reveals NADPH-enriched populations of cells, raising questions about their distinct metabolic roles.

NAD and NADP play fundamentally different roles in cellular metabolism, and yet these pyridine nucleotides cannot be distinguished spectroscopically in living cells. Blacker et al. demonstrate that fluorescence lifetime imaging can be used to quantify NADPH/NADH balance in cultured cells and in the mammalian cochlea.

Loss-of-function mutations in MICU1 cause a brain and muscle disorder linked to primary alterations in mitochondrial calcium signaling

Mitochondrial Ca(2+) uptake has key roles in cell life and death. Physiological Ca(2+) signaling regulates aerobic metabolism, whereas pathological Ca(2+) overload triggers cell death. Mitochondrial Ca(2+) uptake is mediated by the Ca(2+) uniporter complex in the inner mitochondrial membrane, which comprises MCU, a Ca(2+)-selective ion channel, and its regulator, MICU1. Here we report mutations of MICU1 in individuals with a disease phenotype characterized by proximal myopathy, learning difficulties and a progressive extrapyramidal movement disorder. In fibroblasts from subjects with MICU1 mutations, agonist-induced mitochondrial Ca(2+) uptake at low cytosolic Ca(2+) concentrations was increased, and cytosolic Ca(2+) signals were reduced. Although resting mitochondrial membrane potential was unchanged in MICU1-deficient cells, the mitochondrial network was severely fragmented. Whereas the pathophysiology of muscular dystrophy and the core myopathies involves abnormal mitochondrial Ca(2+) handling, the phenotype associated with MICU1 deficiency is caused by a primary defect in mitochondrial Ca(2+) signaling, demonstrating the crucial role of mitochondrial Ca(2+) uptake in humans.

Special features of mitochondrial Ca²⁺ signalling in adrenal glomerulosa cells

Aldosterone, secreted by adrenal glomerulosa cells, allows the adaptation of the vertebrate organism to a wide range of physiological and pathological stimuli including acute haemodynamic challenges and long-term changes in dietary sodium and potassium intake. Most of the extracellular signals are mediated by cytosolic Ca²⁺ signal deriving from Ca²⁺ release, store-operated and/or voltage-gated Ca²⁺ influx. Mitochondria in glomerulosa cells play a fundamental role in generating and modulating the final biological response. These organelles not only house several enzymes of aldosterone biosynthesis but also-in a Ca²⁺-dependent manner-provide NADPH for the function of these enzymes. Moreover, mitochondria, constituting a high portion of cytoplasmic volume and displaying a uniquely low-threshold Ca²⁺ sequestering ability, shape and thus modulate the decoding of the complex cytosolic Ca²⁺ response. The unusual features of mitochondrial Ca²⁺ signalling that permit such an integrative function in adrenal glomerulosa cells are hereby described.

The role of mitochondrial Ca(2+) and NAD(P)H in the control of aldosterone secretion

The mineralocorticoid hormone aldosterone is synthesized in the zona glomerulosa of the adrenal cortex. Glomerulosa cells respond to the physiological stimuli, elevated extracellular [K(+)] and angiotensin II, with an intracellular Ca(2+) signal. Cytosolic Ca(2+) facilitates the transport of the steroid-precursor cholesterol to mitochondria and, after a few hours, it also induces the transcription of aldosterone synthase. Therefore, the cytosolic Ca(2+) signal is regarded as the most important short and long-term mediator of aldosterone secretion. However, cytosolic Ca(2+) is also taken up by mitochondria and, in turn, the mitochondrial Ca(2+) response activates mitochondrial dehydrogenases resulting in stimulation of respiration and increase in reduced pyridine nucleotides. Since both cholesterol side-chain cleavage and all of the hydroxylation steps of steroid synthesis require NADPH as a cofactor, the importance of cytosolic Ca(2+) - mitochondrial Ca(2+) coupling and of appropriate NADPH supply in respect to hormone production can be assumed. However, the importance of the mitochondrial factors has been neglected so far. Here, after summarizing earlier findings we provide new results obtained through modifying mitochondrial Ca(2+) uptake by knocking down p38 MAPK or OPA1 and overexpressing S100G, supporting the notion that mitochondrial Ca(2+) and reduced pyridine nucleotides are facilitating factors for both basal and stimulated steroid production.

Anti-proliferative effects of the novel squamosamide derivative (FLZ) on HepG2 human hepatoma cells by regulating the cell cycle-related proteins are associated with decreased Ca(2+)/ROS levels

FLZ is a synthetic novel squamosamide derivative and has previously been proved to be a potential drug for Parkinson's disease and Alzheimer's disease. FLZ has strong antioxidant activity, which implies that FLZ could eliminate excessive intracellular reactive oxygen species (ROS) in tumor cells and induce a pathway related to low cellular ROS levels, thereby inhibiting tumor cells proliferation. However, few reports have focused on the antitumor effects of FLZ. In this study, we investigated the antitumor efficacy of FLZ in HepG2 cells and the mechanism of cell growth inhibition. FLZ effectively inhibited HepG2 cell proliferation in a dose- and time-dependent manner; meanwhile, it was minimally toxic to normal cells. FLZ induced a significant decrease in oxidative stress through elimination of excessive intracellular ROS and strengthening of the glutathione antioxidant system. In addition, FLZ can effectively attenuate redundant [Ca(2+)](i), thereby avoiding uncontrolled amplification by Ca(2+)/ROS positive feedback. Furthermore, Western blot showed that FLZ inhibited phosphorylation of Akt and retinoblastoma protein (Rb), down-regulated the expressions of cyclin D1, cyclin E, cyclin-dependent kinase 2 (CDK2), and enhanced the expression of CDK inhibitor p27(kip1), while did not affect CDK4 expression. These results suggest that FLZ has potent anti-proliferative activity against malignant human hepatoma cells via modulation of the expression or activation of cell-cycle regulatory proteins, which are associated with decreased Ca(2+)/ROS levels, and indicate that FLZ is a potential liver cancer drug worthy of further research and development.

When is high-Ca+ microdomain required for mitochondrial Ca+ uptake?

Ca(2+) release from IP(3)-sensitive stores in the endoplasmic reticulum (ER) induced by Ca(2+)-mobilizing agonists generates high-Ca(2+) microdomains between ER vesicles and neighbouring mitochondria. Here we present a model that describes when such microdomains are required and when submicromolar [Ca(2+)] is sufficient for mitochondrial Ca(2+) uptake. Mitochondrial Ca(2+) uptake rate in angiotensin II-stimulated H295R adrenocortical cells correlates with the proximity between ER vesicles and the mitochondrion, reflecting the uptake promoting effect of high-Ca(2+) peri-mitochondrial microdomains. Silencing or inhibition of p38 mitogen-activated protein kinase (MAPK) or inhibition of the novel isoforms of protein kinase C enhances mitochondrial Ca(2+) uptake and abolishes the positive correlation between Ca(2+) uptake and ER-mitochondrion proximity. Inhibition of protein phosphatases attenuates mitochondrial Ca(2+) uptake and also abolishes its positive correlation with ER-mitochondrion proximity. We postulate that during IP(3)-induced Ca(2+) release, Ca(2+) uptake is confined to ER-close mitochondria, because of the simultaneous activation of the protein kinases. Attenuation of Ca(2+) uptake prevents Ca(2+) overload of mitochondria and thus protects the cell against apoptosis. On the other hand, all the mitochondria accumulate Ca(2+) at a non-inhibited rate during physiological Ca(2+) influx through the plasma membrane. Membrane potential is higher in ER-distant mitochondria, providing a bigger driving force for Ca(2+) uptake. Our model explains why comparable mitochondrial Ca(2+) signals are formed in response to K(+) and angiotensin II (equipotent in respect to global cytosolic Ca(2+) signals), although only the latter generates high-Ca(2+) microdomains.

Antioxidant Protective Mechanisms against Reactive Oxygen Species (ROS) Generated by Mitochondrial P450 Systems in Steroidogenic Cells

Mitochondrial P450 type enzymes catalyze central steps in steroid biosynthesis, including cholesterol conversion to pregnenolone, 11beta and 18 hydroxylation in glucocorticoid and mineralocorticoid synthesis, C-27 hydroxylation of bile acids, and 1alpha and 24 hydroxylation of 25-OH-vitamin D. These monooxygenase reactions depend on electron transfer from NADPH via FAD adrenodoxin reductase and 2Fe-2S adrenodoxin. These systems can function as a futile NADPH oxidase, oxidizing NADPH in absence of substrate, and leak electrons via adrenodoxin and P450 to O(2), producing superoxide and other reactive oxygen species (ROS). The degree of uncoupling depends on the P450 and steroid substrate. Studies with purified proteins and overexpression in cultured cells show consistently that adrenodoxin, but not reductase, is responsible for ROS production that can lead to apoptosis. In the ovary and corpus luteum, antioxidant enzyme activities superoxide dismutase, catalase, and glutathione peroxidase parallel steroidogenesis. Antioxidant beta-carotene, alpha-tocopherol, and ascorbate can protect against oxidative damages of P450 systems. In testis Leydig cells, steroidogenesis is associated with aging of the steroidogenic capacity.

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.

Mitochondrial reactive oxygen species and Ca2+ signaling

Mitochondria are an important source of reactive oxygen species (ROS) formed as a side product of oxidative phosphorylation. The main sites of oxidant production are complex I and complex III, where electrons flowing from reduced substrates are occasionally transferred to oxygen to form superoxide anion and derived products. These highly reactive compounds have a well-known role in pathological states and in some cellular responses. However, although their link with Ca(2+) is well studied in cell death, it has been hardly investigated in normal cytosolic calcium concentration ([Ca(2+)](i)) signals. Several Ca(2+) transport systems are modulated by oxidation. Oxidation increases the activity of inositol 1,4,5-trisphosphate and ryanodine receptors, the main channels releasing Ca(2+) from intracellular stores in response to cellular stimulation. On the other hand, mitochondria are known to control [Ca(2+)](i) signals by Ca(2+) uptake and release during cytosolic calcium mobilization, specially in mitochondria situated close to Ca(2+) release channels. Mitochondrial inhibitors modify calcium signals in numerous cell types, including oscillations evoked by physiological stimulus. Although these inhibitors reduce mitochondrial Ca(2+) uptake, they also impair ROS production in several systems. In keeping with this effect, recent reports show that antioxidants or oxidant scavengers also inhibit physiological calcium signals. Furthermore, there is evidence that mitochondria generate ROS in response to cell stimulation, an effect suppressed by mitochondrial inhibitors that simultaneously block [Ca(2+)](i) signals. Together, the data reviewed here indicate that Ca(2+)-mobilizing stimulus generates mitochondrial ROS, which, in turn, facilitate [Ca(2+)](i) signals, a new aspect in the biology of mitochondria. Finally, the potential implications for biological modeling are discussed.