Evidence for a NADH/NADPH oxidase in human umbilical vein endothelial cells using electron spin resonance

NADPH oxidase in the vasculature: Expression, regulation and signalling pathways; role in normal cardiovascular physiology and its dysregulation in hypertension

The last 20-25 years have seen an explosion of interest in the role of NADPH oxidase (NOX) in cardiovascular function and disease. In vascular smooth muscle and endothelium, NOX generates reactive oxygen species (ROS) that act as second messengers, contributing to the control of normal vascular function. NOX activity is altered in response to a variety of stimuli, including G-protein coupled receptor agonists, growth-factors, perfusion pressure, flow and hypoxia. NOX-derived ROS are involved in smooth muscle constriction, endothelium-dependent relaxation and smooth muscle growth, proliferation and migration, thus contributing to the fine-tuning of blood flow, arterial wall thickness and vascular resistance. Through reversible oxidative modification of target proteins, ROS regulate the activity of protein tyrosine phosphatases, kinases, G proteins, ion channels, cytoskeletal proteins and transcription factors. There is now considerable, but somewhat contradictory evidence that NOX contributes to the pathogenesis of hypertension through oxidative stress. Specific NOX isoforms have been implicated in endothelial dysfunction, hyper-contractility and vascular remodelling in various animal models of hypertension, pulmonary hypertension and pulmonary arterial hypertension, but also have potential protective effects, particularly NOX4. This review explores the multiplicity of NOX function in the healthy vasculature and the evidence for and against targeting NOX for antihypertensive therapy.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

Toll-like receptor 4 mutation protects obese mice against endothelial dysfunction by decreasing NADPH oxidase isoforms 1 and 4

OBJECTIVE

To analyze the role of toll-like receptor 4 in modulating metabolism and endothelial function.

APPROACH AND RESULTS

Type 2 diabetic mice with mutated toll-like receptor 4 (DWM) were protected from hyperglycemia and hypertension, despite an increased body weight. Isometric tension was measured in arterial rings with endothelium. Relaxations to acetylcholine were blunted in aortae and mesenteric arteries of Lepr(db/db) mice, but not in DWM mice; the endothelial NO synthase dimer/monomer ratio and endothelial NO synthase phosphorylation levels were higher in DWM preparations. These differences were abolished by apocynin. Contractions to acetylcholine (in the presence of L-NAME) were larger in carotid arteries from Lepr(db/db) mice than from DWM mice and were inhibited by indomethacin and SC560, demonstrating involvement of cyclooxygenase-1. The release of 6-ketoprostaglandin F1α was lower in DWM mice arteries, implying lower cyclooxygenase-1 activity. Apocynin, manganese(III) tetrakis(1-methyl-4-pyridyl) porphyrin, catalase, and diethyldithiocarbamate inhibited endothelium-dependent contractions. The mRNA and protein levels of NADPH oxidase isoforms NOX1 and NOX4 were downregulated in DWM mice arteries. The in vivo and in vitro administration of lipopolysaccharide caused endothelial dysfunction in the arteries of wild-type, but not toll-like receptor 4-mutated mice.

CONCLUSIONS

Toll-like receptor 4 plays a key role in obesity and diabetes-associated endothelial dysfunction by increasing oxidative stress.

Cysteine/cystine redox signaling in cardiovascular disease

Extracellular thiol/disulfide redox environments are highly regulated in healthy individuals. The major thiol/disulfide redox couple in human plasma is cysteine (Cys) and its disulfide form, cystine (CySS). Oxidation of this redox couple, measured as a more positive steady-state redox potential (E(h)), is associated with risk factors for cardiovascular disease (CVD), including aging, smoking, obesity, and alcohol abuse. Rodent and vascular cell studies show that the extracellular redox state of Cys/CySS (E(h)CySS) can play a vital role in controlling CVD through proinflammatory signaling. This inflammatory signaling is regulated by cell-surface protein redox state and involves mitochondrial oxidation, nuclear factor-κB activation, and elevated expression of genes for monocyte recruitment to endothelial cells. Gene array and proteomics studies reveal the global nature of redox effects, and different cell types, e.g., endothelial cells, monocytes, fibroblasts, and epithelial cells, show cell-specific redox responses with different phenotypic traits, e.g., proliferation and apoptosis, which can contribute to CVD. The critical nature of the proinflammatory redox signaling and cell biology associated with E(h)CySS supports the use of plasma levels of Cys, CySS, and E(h)CySS as key indicators of vascular health. Plasma redox-state-based pharmacologic interventions to control or improve E(h)CySS may be effective in preventing CVD onset or progression.

Reactive Oxygen Species–Mediated Signal Transduction in the Endothelium

The endothelium is an important component of vascular homeostasis that is a target for injury in the setting of vascular disease. One means of promoting a maladaptive endothelial cell phenotype such as that seen in atherosclerosis is excess oxidative stress. Although this term once was almost exclusively used to describe low-density lipoprotein (LDL) and lipid oxidation in the vasculature, we now understand that the intracellular oxidant milieu is an important modulator of vascular cell function. Indeed, considerable data indicate that reactive oxygen species (ROS) are an important means of cellular signaling, although the precise mechanisms whereby ROS accomplish this are still under investigation. In this review, the data linking ROS to kinase activation and cell signaling in the endothelium is discussed, with a particular emphasis on the roles of protein thiol modification.

Vascular NAD(P)H oxidases: specific features, expression, and regulation

The importance of reactive oxygen species (ROS) in vascular physiology and pathology is becoming increasingly evident. All cell types in the vascular wall produce ROS derived from superoxide-generating protein complexes similar to the leukocyte NADPH oxidase. Specific features of the vascular enzymes include constitutive and inducible activities, substrate specificity, and intracellular superoxide production. Most phagocyte enzyme subunits are found in vascular cells, including the catalytic gp91phox (aka, nox2), which was the earliest member of the newly discovered nox family. However, smooth muscle frequently expresses nox1 rather than gp91phox, and nox4 is additionally present in all cell types. In cell culture, agonists increase ROS production by activating multiple signals, including protein kinase C and Rac, and by upregulating oxidase subunits. The oxidases are also upregulated in vascular disease and are involved in the development of atherosclerosis and a significant part of angiotensin II-induced hypertension, possibly via nox1 and nox4. Likewise, enhanced vascular oxidase activity is associated with diabetes. Therefore, members of this enzyme family appear to be important in vascular biology and disease and constitute promising targets for future therapeutic interventions.

Interactions of angiotensin II with NAD(P)H oxidase, oxidant stress and cardiovascular disease

An elevation in angiotensin II (Ang II) levels is a common occurrence in a diverse number of cardiovascular diseases including hypertension, hypercholesterolaemia, atherosclerotic coronary artery disease, left ventricular hypertrophy (LVH), heart failure and diabetes. An important effect of Ang II is activation of the NAD(P)H oxidase, a major source of reactive oxygen species (ROS) production by vascular cells. This increase in cellular ROS contributes to the pathogenesis of vascular disease by altering endothelial cell function, enhancing smooth muscle cell growth and proliferation, stimulating inflammatory proteins, including macrophage chemoattractant agents, growth factors and cytokines, and modulating matrix remodelling. Studies of genetically-altered mice have unequivocally shown that activation of the NAD(P)H oxidase by Ang II contributes to hypertension, LVH and atherosclerosis. Furthermore, increasing evidence suggest that the NAD(P)H oxidase contributes to human disease, suggesting that it is a potential target for future therapeutic intervention.

Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension

Tetrahydrobiopterin is a critical cofactor for the NO synthases, and in its absence these enzymes become "uncoupled," producing reactive oxygen species (ROSs) rather than NO. In aortas of mice with deoxycorticosterone acetate-salt (DOCA-salt) hypertension, ROS production from NO synthase is markedly increased, and tetrahydrobiopterin oxidation is evident. Using mice deficient in the NADPH oxidase subunit p47(phox) and mice lacking either the endothelial or neuronal NO synthase, we obtained evidence that hypertension produces a cascade involving production of ROSs from the NADPH oxidase leading to oxidation of tetrahydrobiopterin and uncoupling of endothelial NO synthase (eNOS). This decreases NO production and increases ROS production from eNOS. Treatment of mice with oral tetrahydrobiopterin reduces vascular ROS production, increases NO production as determined by electron spin resonance measurements of nitrosyl hemoglobin, and blunts the increase in blood pressure due to DOCA-salt hypertension. Endothelium-dependent vasodilation is only minimally altered in vessels of mice with DOCA-salt hypertension but seems to be mediated by hydrogen peroxide released from uncoupled eNOS, since it is inhibited by catalase. Tetrahydrobiopterin oxidation may represent an important abnormality in hypertension. Treatment strategies that increase tetrahydrobiopterin or prevent its oxidation may prove useful in preventing vascular complications of this common disease.

Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension

Tetrahydrobiopterin is a critical cofactor for the NO synthases, and in its absence these enzymes become "uncoupled," producing reactive oxygen species (ROSs) rather than NO. In aortas of mice with deoxycorticosterone acetate-salt (DOCA-salt) hypertension, ROS production from NO synthase is markedly increased, and tetrahydrobiopterin oxidation is evident. Using mice deficient in the NADPH oxidase subunit p47(phox) and mice lacking either the endothelial or neuronal NO synthase, we obtained evidence that hypertension produces a cascade involving production of ROSs from the NADPH oxidase leading to oxidation of tetrahydrobiopterin and uncoupling of endothelial NO synthase (eNOS). This decreases NO production and increases ROS production from eNOS. Treatment of mice with oral tetrahydrobiopterin reduces vascular ROS production, increases NO production as determined by electron spin resonance measurements of nitrosyl hemoglobin, and blunts the increase in blood pressure due to DOCA-salt hypertension. Endothelium-dependent vasodilation is only minimally altered in vessels of mice with DOCA-salt hypertension but seems to be mediated by hydrogen peroxide released from uncoupled eNOS, since it is inhibited by catalase. Tetrahydrobiopterin oxidation may represent an important abnormality in hypertension. Treatment strategies that increase tetrahydrobiopterin or prevent its oxidation may prove useful in preventing vascular complications of this common disease.

NAD(P)H oxidase mediates angiotensin II-induced vascular macrophage infiltration and medial hypertrophy

OBJECTIVE

Our preliminary data suggested that angiotensin II (Ang II)-induced reactive oxygen species are involved in intercellular adhesion molecule-1 (ICAM-1) expression and leukocyte infiltration in the rat thoracic aorta. Other reports demonstrating reactive oxygen species-induced cell growth suggested a potential role of NAD(P)H oxidase in vascular hypertrophy. In the present study, we postulate that NAD(P)H oxidase is functionally involved in Ang II-induced ICAM-1 expression, macrophage infiltration, and vascular growth, and that oxidase inhibition attenuates these processes independently of a reduction in blood pressure.

METHODS AND RESULTS

Rats were infused subcutaneously with vehicle or Ang II (750 microg/kg per day) for 1 week in the presence or absence of gp91 docking sequence (gp91ds)-tat peptide (1 mg/kg per day), a cell-permeant inhibitor of NAD(P)H oxidase. Immunohistochemical staining for ICAM-1 and ED1, a marker of monocytes and macrophages, showed that both were markedly increased with Ang II compared with vehicle and were reduced in rats receiving Ang II plus gp91ds-tat. This effect was accompanied by an Ang II-induced increase in medial hypertrophy that was attenuated by coinfusion of gp91ds-tat; however, gp91ds-tat had no effect on blood pressure.

CONCLUSIONS

Ang II-enhanced NAD(P)H oxidase plays a role in the induction of ICAM-1 expression, leukocyte infiltration, and vascular hypertrophy, acting independently of changes in blood pressure.

Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension

Tetrahydrobiopterin is a critical cofactor for the NO synthases, and in its absence these enzymes become "uncoupled," producing reactive oxygen species (ROSs) rather than NO. In aortas of mice with deoxycorticosterone acetate-salt (DOCA-salt) hypertension, ROS production from NO synthase is markedly increased, and tetrahydrobiopterin oxidation is evident. Using mice deficient in the NADPH oxidase subunit p47(phox) and mice lacking either the endothelial or neuronal NO synthase, we obtained evidence that hypertension produces a cascade involving production of ROSs from the NADPH oxidase leading to oxidation of tetrahydrobiopterin and uncoupling of endothelial NO synthase (eNOS). This decreases NO production and increases ROS production from eNOS. Treatment of mice with oral tetrahydrobiopterin reduces vascular ROS production, increases NO production as determined by electron spin resonance measurements of nitrosyl hemoglobin, and blunts the increase in blood pressure due to DOCA-salt hypertension. Endothelium-dependent vasodilation is only minimally altered in vessels of mice with DOCA-salt hypertension but seems to be mediated by hydrogen peroxide released from uncoupled eNOS, since it is inhibited by catalase. Tetrahydrobiopterin oxidation may represent an important abnormality in hypertension. Treatment strategies that increase tetrahydrobiopterin or prevent its oxidation may prove useful in preventing vascular complications of this common disease.

Hyperoxia-induced NAD(P)H oxidase activation and regulation by MAP kinases in human lung endothelial cells

Hyperoxia increases reactive oxygen species (ROS) production in vascular endothelium; however, the mechanisms involved in ROS generation are not well characterized. We determined the role and regulation of NAD(P)H oxidase in hyperoxia-induced ROS formation in human pulmonary artery endothelial cells (HPAECs). Exposure of HPAECs to hyperoxia for 1, 3, and 12 h increased the generation of superoxide anion, which was blocked by diphenyleneiodonium but not by rotenone or oxypurinol. Furthermore, hyperoxia enhanced NADPH- and NADH-dependent and superoxide dismutase- or diphenyleneiodonium-inhibitable ROS production in HPAECs. Immunohistocytochemistry and Western blotting revealed the presence of gp91, p67 phox, p22 phox, and p47 phox subcomponents of NADPH oxidase in HPAECs. Transfection of HPAECs with p22 phox antisense plasmid inhibited hyperoxia-induced ROS production. Exposure of HPAECs to hyperoxia activated p38 MAPK and ERK, and inhibition of p38 MAPK and MEK1/2 attenuated the hyperoxia-induced ROS generation. These results suggest a role for MAPK in regulating hyperoxia-induced NAD(P)H oxidase activation in HPAECs.

Detection of superoxide in vascular tissue

During the past decade, it has become apparent that reactive oxygen species play a critical role in the genesis of many vascular diseases. The superoxide anion is among the most important of these, not only because of its rapid reaction with NO but also because it serves as a progenitor for many other reactive oxygen species. Although there are many approaches to detecting and quantifying superoxide in chemical systems, its detection in intact tissues is more difficult. The validity of the most popular and frequently used assay for this purpose, lucigenin-enhanced chemiluminescence, has been recently questioned. It has been suggested that lucigenin itself, especially at high concentrations (>50 micromol/L), may act as a source for superoxide via redox cycling. Lower lucigenin concentrations (5 micromol/L) do not participate in redox cycling to an important extent in intact tissues and, therefore, provide an accurate assessment of the rate of superoxide production in such samples. Other useful assays for superoxide include those using the fluorescent dye dihydroethidine, 2-methyl-6-phenyl-3,7-dihydroimidazo(1,2-alpha)pyrazin-3-one (CLA), and 2-(p-hydroxybenzyl)-6-(p-hydroxyphenyl) 8-benzylimidazo[1,2-alpha]pyrazin-3-one (coelenterazine). The chemiluminescent compound 5-amino-2,3-dihydroxy-1,4-phthalayineidone (luminol) may also be used to detect various reactive oxygen species and may be made specific for various oxidants, such as hydrogen peroxide, superoxide, and peroxynitrite, by altering the experimental conditions. Although each of these methods may be associated with potential artifacts, the use of > or =2 different techniques that yield similar results provides a reliable approach for the study of reactive oxygen species in intact vascular tissues.

Role of p47(phox) in vascular oxidative stress and hypertension caused by angiotensin II

Hypertension caused by angiotensin II is dependent on vascular superoxide (O2*-) production. The nicotinamide adenine dinucleotide phosphate (NAD[P]H) oxidase is a major source of vascular O2*- and is activated by angiotensin II in vitro. However, its role in angiotensin II-induced hypertension in vivo is less clear. In the present studies, we used mice deficient in p47(phox), a cytosolic subunit of the NADPH oxidase, to study the role of this enzyme system in vivo. In vivo, angiotensin II infusion (0.7 mg/kg per day for 7 days) increased systolic blood pressure from 105+/-2 to 151+/-6 mm Hg and increased vascular O2*- formation 2- to 3-fold in wild-type (WT) mice. In contrast, in p47(phox-/-) mice the hypertensive response to angiotensin II infusion (122+/-4 mm Hg; P<0.05) was markedly blunted, and there was no increase of vascular O2*- production. In situ staining for O2*- using dihydroethidium revealed a marked increase of O2*-production in both endothelial and vascular smooth muscle cells of angiotensin II-treated WT mice, but not in those of p47(phox-/-) mice. To directly examine the role of the NAD(P)H oxidase in endothelial production of O2*-, endothelial cells from WT and p47(phox-/-) mice were cultured. Western blotting confirmed the absence of p47(phox) in p47(phox-/-) mice. Angiotensin II increased O2*- production in endothelial cells from WT mice, but not in those from p47(phox-/-) mice, as determined by electron spin resonance spectroscopy. These results suggest a pivotal role of the NAD(P)H oxidase and its subunit p47(phox) in the vascular oxidant stress and the blood pressure response to angiotensin II in vivo.

Reactive oxygen species, mitochondria, and NAD(P)H oxidases in the development and progression of heart failure

Reactive oxygen species (ROS) released acutely in large amounts have been traditionally implicated in the cell death associated with myocardial infarction or reperfusion injury. These ROS can be released from the cardiac myocyte mitochondria, xanthine oxidase, and the phagocytic nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase. Interestingly, the chronic release of ROS has been recently linked to the development of left ventricular hypertrophy and heart failure progression. The chronic release of ROS appears to derive from the nonphagocytic NAD(P)H oxidase and mitochondria. Experimental data are accumulating suggesting that the release of ROS is required for the normal, physiologic activity of cardiac cells, but abnormal activation of the nonphagocytic NAD(P)H oxidase in response to neurohormones (angiotensin II, norepinephrine, tumor necrosis factor-a) has been shown to contribute to cardiac myocyte hypertrophy. Furthermore, the fibrosis, collagen deposition, and metalloproteinase activation involved in the remodeling of the failing myocardium are dependent on ROS released during the phenotypic transformation of fibroblasts to myofibroblasts associated with progression of end-stage heart failure. Future studies are necessary to identify the sources, mechanisms of activation of NAD(P)H oxidases, and downstream signaling targets implicated in the progression of chronic heart failure.

Overestimation of NADH-driven vascular oxidase activity due to lucigenin artifacts

Several limitations have recently been described for lucigenin, a probe frequently used to assess the activity of vascular NAD(P)H oxidase, a major superoxide source. The preferential reducing substrate of such oxidase remains unclear. We assessed whether lucigenin artifacts could affect detection of NAD(P)H oxidase activity. Initial chemiluminescence assays were performed with vascular rings or homogenates at 5, 50, or 250 microM concentrations. Results showed preferential signals with NADPH (vs. NADH) with 5 and 50 microM lucigenin, which were blocked by diphenylene iodonium (DPI), superoxide dismutase (SOD), or its cell-permeable mimetic MnTBAP. With 250 microM lucigenin, the relative signal with NADH became larger than with NADPH, and was poorly inhibited by all three antagonists above. All SOD/DPI-resistant signals were effectively blocked by the electron acceptor nitrobluetetrazolium. Spin trapping with DMPO showed an approximate doubling of DMPO-OH radical adduct signal upon addition of 5 microM lucigenin to homogenates incubated with either NADPH or NADH. With 50 or 250 microM lucigenin, much larger increases were observed with NADH, as opposed to NADPH. Furthermore, oxygen consumption measurements showed analogous results. In summary, our data suggest that: (i) Lucigenin redox-cycling is detectable in vascular tissue even at 5 microM concentrations, while at 250 microM redox-cycling becomes predominant and is markedly increased when NADH is the assayed substrate; and (ii) With 250 microM lucigenin, preferentially with NADH, signals are further overestimated by direct, oxidase-dependent, superoxide-independent two-electron transfer. Therefore, previous reports of preferential NADH affinity of the vascular oxidase may have been due to these artifacts.

Intracellular redox status affects transplasma membrane electron transport in pulmonary arterial endothelial cells

Pulmonary arterial endothelial cells possess transplasma membrane electron transport (TPMET) systems that transfer intracellular reducing equivalents to extracellular electron acceptors. As one aspect of determining cellular mechanisms involved in one such TPMET system in pulmonary arterial endothelial cells in culture, glycolysis was inhibited by treatment with iodoacetate (IOA) or by replacing the glucose in the cell medium with 2-deoxy-D-glucose (2-DG). TPMET activity was measured as the rate of reduction of the extracellular electron acceptor polymer toluidine blue O polyacrylamide. Intracellular concentrations of NADH, NAD(+), NADPH, and NADP(+) were determined by high-performance liquid chromatography of KOH cell extracts. IOA decreased TPMET activity to 47% of control activity concomitant with a decrease in the NADH/NAD(+) ratio to 34% of the control level, without a significant change in the NADPH/NADP(+) ratio. 2-DG decreased TPMET activity to 53% of control and decreased both NADH/NAD(+) and NADPH/NADP(+) ratios to 51% and 55%, respectively, of control levels. When lactate was included in the medium along with the inhibitors, the effects of IOA and 2-DG on both TPMET activity and the NADPH/NADP(+) ratios were prevented. The results suggest that cellular redox status is a determinant of pulmonary arterial endothelial cell TPMET activity, with TPMET activity more highly correlated with the poise of the NADH/NAD(+) redox pair.

Electron spin resonance characterization of the NAD(P)H oxidase in vascular smooth muscle cells

Endogenously produced reactive oxygen species are important for intracellular signaling mechanisms leading to vascular smooth muscle cell (VSMC) growth. It is therefore critical to define the potential enzymatic sources of ROS and their regulation by agonists in VSMCs. Previous studies have investigated O2*- production using lucigenin-enhanced chemiluminescence. However, lucigenin has been recently criticized for its ability to redox cycle and its propensity to measure cellular reductase activity independent from O2*-. To perform a definitive characterization of VSMC oxidase activity, we used electron spin resonance trapping of O2*- with DEPMPO. We confirmed that the main source of O2*- from VSMC membranes is an NAD(P)H oxidase and that the O2*- formation from mitochondria, xanthine oxidase, arachidonate-derived enzymes, and nitric oxide synthases in VSMC membranes was minor. The VSMC NAD(P)H oxidase(s) are able to produce more O2*- when NADPH is used as the substrate compared to NADH (the maximal NADPH signal is 2.4- +/- 0.4-fold higher than the NADH signal). The two substrates had similar EC(50)'s ( approximately 10-50 microM). Stimulation with angiotensin II and platelet-derived growth factor also predominantly increased the NADPH-driven signal (101 +/- 8% and 83 +/- 1% increase above control, respectively), with less of an effect on NADH-dependent O2*- (17 +/- 3% and 36 +/- 5% increase, respectively). Moreover, incubation of the cells with diphenylene iodonium inhibited predominantly NADPH-stimulated O2*-. In conclusion, electron spin resonance characterization of VSMC oxidase activity supports a major role for an NAD(P)H oxidase in O2*- production in VSMCs, and provides new evidence concerning the substrate dependency and agonist-stimulated activity of this key enzyme.