Nitrite as a vascular endocrine nitric oxide reservoir that contributes to hypoxic signaling, cytoprotection, and vasodilation

The enigma of nitroglycerin bioactivation and nitrate tolerance: news, views and troubles

Nitroglycerin (glyceryl trinitrate; GTN) is the most prominent representative of the organic nitrates or nitrovasodilators, a class of compounds that have been used clinically since the late nineteenth century for the treatment of coronary artery disease (angina pectoris), congestive heart failure and myocardial infarction. Medline lists more than 15 000 publications on GTN and other organic nitrates, but the mode of action of these drugs is still largely a mystery. In the first part of this article, we give an overview on the molecular mechanisms of GTN biotransformation resulting in vascular cyclic GMP accumulation and vasodilation with focus on the role of mitochondrial aldehyde dehydrogenase (ALDH2) and the link between the ALDH2 reaction and activation of vascular soluble guanylate cyclase (sGC). In particular, we address the identity of the bioactive species that activates sGC and the potential involvement of nitrite as an intermediate, describe our recent findings suggesting that ALDH2 catalyses direct 3-electron reduction of GTN to NO and discuss possible reaction mechanisms. In the second part, we discuss contingent processes leading to markedly reduced sensitivity of blood vessels to GTN, referred to as vascular nitrate tolerance. Again, we focus on ALDH2 and describe the current controversy on the role of ALDH2 inactivation in tolerance development. Finally, we emphasize some of the most intriguing, in our opinion, unresolved puzzles of GTN pharmacology that urgently need to be addressed in future studies.

Nitric oxide promotes distant organ protection: evidence for an endocrine role of nitric oxide

Endothelial NOS (eNOS)-derived NO has long been considered a paracrine signaling molecule only capable of affecting nearby cells because of its short half-life in blood and relatively limited diffusion distance in tissues. To date, no studies have demonstrated that endogenously generated NO possesses a clearly defined endocrine function. Therefore, we evaluated whether enzymatic generation of NO in the heart is capable of modulating remote physiological actions and cell signaling. Mice with cardiac-specific overexpression of the human eNOS gene (CS-eNOS-Tg) were used to address this hypothesis. Cardiac-specific eNOS overexpression resulted in significant increases in nitrite, nitrate, and nitrosothiols in the heart, plasma, and liver. To examine whether the increase in hepatic NO metabolites could modulate cytoprotection, we subjected CS-eNOS-Tg mice to hepatic ischemia-reperfusion (I/R) injury. CS-eNOS-Tg mice displayed a significant reduction in hepatic I/R injury (4.2-fold reduction in the aminotransferase and a 3.5-fold reduction in aspartate aminotransferase) compared with WT littermates. These findings demonstrate that endogenously derived NO is transported in the blood, metabolized in remote organs, and mediates cytoprotection in the setting of I/R injury. This study presents clear evidence for an endocrine role of NO generated endogenously from eNOS and provides additional evidence for the profound cytoprotective actions of NO in the setting of I/R injury.

The functional nitrite reductase activity of the heme-globins

Hemoglobin and myoglobin are among the most extensively studied proteins, and nitrite is one of the most studied small molecules. Recently, multiple physiologic studies have surprisingly revealed that nitrite represents a biologic reservoir of NO that can regulate hypoxic vasodilation, cellular respiration, and signaling. These studies suggest a vital role for deoxyhemoglobin- and deoxymyoglobin-dependent nitrite reduction. Biophysical and chemical analysis of the nitrite-deoxyhemoglobin reaction has revealed unexpected chemistries between nitrite and deoxyhemoglobin that may contribute to and facilitate hypoxic NO generation and signaling. The first is that hemoglobin is an allosterically regulated nitrite reductase, such that oxygen binding increases the rate of nitrite conversion to NO, a process termed R-state catalysis. The second chemical property is oxidative denitrosylation, a process by which the NO formed in the deoxyhemoglobin-nitrite reaction that binds to other deoxyhemes can be released due to heme oxidation, releasing free NO. Third, the reaction undergoes a nitrite reductase/anhydrase redox cycle that catalyzes the anaerobic conversion of 2 molecules of nitrite into dinitrogen trioxide (N(2)O(3)), an uncharged molecule that may be exported from the erythrocyte. We will review these reactions in the biologic framework of hypoxic signaling in blood and the heart.

Nitrite reductase activity of hemoglobin as a systemic nitric oxide generator mechanism to detoxify plasma hemoglobin produced during hemolysis

Hemoglobin (Hb) potently inactivates the nitric oxide (NO) radical via a dioxygenation reaction forming nitrate (NO(3)(-)). This inactivation produces endothelial dysfunction during hemolytic conditions and may contribute to the vascular complications of Hb-based blood substitutes. Hb also functions as a nitrite (NO(2)(-)) reductase, converting nitrite into NO as it deoxygenates. We hypothesized that during intravascular hemolysis, nitrite infusions would limit the vasoconstrictive properties of plasma Hb. In a canine model of low- and high-intensity hypotonic intravascular hemolysis, we characterized hemodynamic responses to nitrite infusions. Hemolysis increased systemic and pulmonary arterial pressures and systemic vascular resistance. Hemolysis also inhibited NO-dependent pulmonary and systemic vasodilation by the NO donor sodium nitroprusside. Compared with nitroprusside, nitrite demonstrated unique effects by not only inhibiting hemolysis-associated vasoconstriction but also by potentiating vasodilation at plasma Hb concentrations of <25 muM. We also observed an interaction between plasma Hb levels and nitrite to augment nitroprusside-induced vasodilation of the pulmonary and systemic circulation. This nitrite reductase activity of Hb in vivo was recapitulated in vitro using a mitochondrial NO sensor system. Nitrite infusions may promote NO generation from Hb while maintaining oxygen delivery; this effect could be harnessed to treat hemolytic conditions and to detoxify Hb-based blood substitutes.

SNO-hemoglobin is not essential for red blood cell-dependent hypoxic vasodilation

The coupling of hemoglobin sensing of physiological oxygen gradients to stimulation of nitric oxide (NO) bioactivity is an established principle of hypoxic blood flow. One mechanism proposed to explain this oxygen-sensing-NO bioactivity linkage postulates an essential role for the conserved Cys93 residue of the hemoglobin beta-chain (betaCys93) and, specifically, for S-nitrosation of betaCys93 to form S-nitrosohemoglobin (SNO-Hb). The SNO-Hb hypothesis, which conceptually links hemoglobin and NO biology, has been debated intensely in recent years. This debate has precluded a consensus on physiological mechanisms and on assessment of the potential role of SNO-Hb in pathology. Here we describe new mouse models that exclusively express either human wild-type hemoglobin or human hemoglobin in which the betaCys93 residue is replaced with alanine to assess the role of SNO-Hb in red blood cell-mediated hypoxic vasodilation. Substitution of this residue, precluding hemoglobin S-nitrosation, did not change total red blood cell S-nitrosothiol abundance but did shift S-nitrosothiol distribution to lower molecular weight species, consistent with the loss of SNO-Hb. Loss of betaCys93 resulted in no deficits in systemic or pulmonary hemodynamics under basal conditions and, notably, did not affect isolated red blood cell-dependent hypoxic vasodilation. These results demonstrate that SNO-Hb is not essential for the physiologic coupling of erythrocyte deoxygenation with increased NO bioactivity in vivo.

Chronic sodium nitrite therapy augments ischemia-induced angiogenesis and arteriogenesis

Chronic tissue ischemia due to defective vascular perfusion is a hallmark feature of peripheral artery disease for which minimal therapeutic options exist. We have reported that sodium nitrite therapy exerts cytoprotective effects against acute ischemia/reperfusion injury in both heart and liver, consistent with the model of bioactive NO formation from nitrite during ischemic stress. Here, we test the hypothesis that chronic sodium nitrite therapy can selectively augment angiogenic activity and tissue perfusion in the murine hind-limb ischemia model. Various therapeutic doses (8.25-3,300 mug/kg) of sodium nitrite or PBS were administered. Sodium nitrite significantly restored ischemic hind-limb blood flow in a time-dependent manner, with low-dose sodium nitrite being most effective. Nitrite therapy significantly increased ischemic limb vascular density and stimulated endothelial cell proliferation. Remarkably, the effects of sodium nitrite therapy were evident within 3 days of the ischemic insult demonstrating the potency and efficacy of chronic sodium nitrite therapy. Sodium nitrite therapy also increased ischemic tissue nitrite and NO metabolites compared to nonischemic limbs. Use of the NO scavenger carboxy PTIO completely abolished sodium nitrite-dependent ischemic tissue blood flow and angiogenic activity consistent with nitrite reduction to NO being the proangiogenic mechanism. These data demonstrate that chronic sodium nitrite therapy is a recently discovered therapeutic treatment for peripheral artery disease and critical limb ischemia.

Nitric oxide formation from the reaction of nitrite with carp and rabbit hemoglobin at intermediate oxygen saturations

The nitrite reductase activity of deoxyhemoglobin has received much recent interest because the nitric oxide produced in this reaction may participate in blood flow regulation during hypoxia. The present study used spectral deconvolution to characterize the reaction of nitrite with carp and rabbit hemoglobin at different constant oxygen tensions that generate the full range of physiological relevant oxygen saturations. Carp is a hypoxia-tolerant species with very high hemoglobin oxygen affinity, and the high R-state character and low redox potential of the hemoglobin is hypothesized to promote NO generation from nitrite. The reaction of nitrite with deoxyhemoglobin leads to a 1 : 1 formation of nitrosylhemoglobin and methemoglobin in both species. At intermediate oxygen saturations, the reaction with deoxyhemoglobin is clearly favored over that with oxyhemoglobin, and the oxyhemoglobin reaction and its autocatalysis are inhibited by nitrosylhemoglobin from the deoxyhemoglobin reaction. The production of NO and nitrosylhemoglobin is faster and higher in carp hemoglobin with high O(2) affinity than in rabbit hemoglobin with lower O(2) affinity, and it correlates inversely with oxygen saturation. In carp, NO formation remains substantial even at high oxygen saturations. When oxygen affinity is decreased by T-state stabilization of carp hemoglobin with ATP, the reaction rates decrease and NO production is lowered, but the deoxyhemoglobin reaction continues to dominate. The data show that the reaction of nitrite with hemoglobin is dynamically influenced by oxygen affinity and the allosteric equilibrium between the T and R states, and that a high O(2) affinity increases the nitrite reductase capability of hemoglobin.

Inhaled nitric oxide enables artificial blood transfusion without hypertension

BACKGROUND

One of the major obstacles hindering the clinical development of a cell-free, hemoglobin-based oxygen carrier (HBOC) is systemic vasoconstriction.

METHODS AND RESULTS

Experiments were performed in healthy mice and lambs by infusion of either murine tetrameric hemoglobin (0.48 g/kg) or glutaraldehyde-polymerized bovine hemoglobin (HBOC-201, 1.44 g/kg). We observed that intravenous infusion of either murine tetrameric hemoglobin or HBOC-201 induced prolonged systemic vasoconstriction in wild-type mice but not in mice congenitally deficient in endothelial nitric oxide (NO) synthase (NOS3). Treatment of wild-type mice by breathing NO at 80 ppm in air for 15 or 60 minutes or with 200 ppm NO for 7 minutes prevented the systemic hypertension induced by subsequent intravenous administration of murine tetrameric hemoglobin or HBOC-201 and did not result in conversion of plasma hemoglobin to methemoglobin. Intravenous administration of sodium nitrite (48 nmol) 5 minutes before infusion of murine tetrameric hemoglobin also prevented the development of systemic hypertension. In awake lambs, breathing NO at 80 ppm for 1 hour prevented the systemic hypertension caused by subsequent infusion of HBOC-201.

CONCLUSIONS

These findings demonstrate that HBOC can cause systemic vasoconstriction by scavenging NO produced by NOS3. Moreover, in 2 species, inhaled NO administered before the intravenous infusion of HBOC can prevent systemic vasoconstriction without causing methemoglobinemia.

Mitochondrial dysfunction and oxidative stress in the pathogenesis of alcohol- and obesity-induced fatty liver diseases

Fatty liver disease associated with chronic alcohol consumption or obesity/type 2 diabetes has emerged as a serious public health problem. Steatosis, accumulation of triglyceride in hepatocytes, is now recognized as a critical "first-hit" in the pathogenesis of liver disease. It is proposed that steatosis "primes" the liver to progress to more severe liver pathologies when individuals are exposed to subsequent metabolic and/or environmental stressors or "second-hits." Genetic risk factors can also influence the susceptibility to and severity of fatty liver disease. Furthermore, oxidative stress, disrupted nitric oxide (NO) signaling, and mitochondrial dysfunction are proposed to be key molecular events that accelerate or worsen steatosis and initiate progression to steatohepatitis and fibrosis. This review article will discuss the following topics regarding the pathobiology and molecular mechanisms responsible for fatty liver disease: (1) the "two-hit" or "multi-hit" hypothesis, (2) the role of mitochondrial bioenergetic defects and oxidant stress, (3) the interplay between NO and mitochondria in fatty liver disease, (4) genetic risk factors and oxidative stress-responsive genes, and (5) the feasibility of antioxidants for treatment.

Thrombospondin-1: a physiological regulator of nitric oxide signaling

Thrombospondin-1 is a secreted protein that modulates vascular cell behavior via several cell surface receptors. In vitro, nanomolar concentrations of thrombospondin-1 are required to alter endothelial and vascular smooth muscle cell adhesion, proliferation, motility, and survival. Yet, much lower levels of thrombospondin-1 are clearly functional in vivo. This discrepancy was explained with the discovery that the potency of thrombospondin-1 increases more than 100-fold in the presence of physiological levels of nitric oxide (NO). Thrombospondin-1 binding to CD47 inhibits NO signaling by preventing cGMP synthesis and activation of its target cGMP-dependent protein kinase. This potent antagonism of NO signaling allows thrombospondin-1 to acutely constrict blood vessels, accelerate platelet aggregation, and if sustained, inhibit angiogenic responses. Acute antagonism of NO signaling by thrombospondin-1 is important for hemostasis but becomes detrimental for tissue survival of ischemic injuries. New therapeutic approaches targeting thrombospondin-1 or CD47 can improve recovery from ischemic injuries and overcome a deficit in NO-responsiveness in aging. (Part of a Multi-author Review).

Hypoxic modulation of exogenous nitrite-induced vasodilation in humans

BACKGROUND

It has been proposed that under hypoxic conditions, nitrite may release nitric oxide, which causes potent vasodilation. We hypothesized that nitrite would have a greater dilator effect in capacitance than in resistance vessels because of lower oxygen tension and that resistance-vessel dilation should become more pronounced during hypoxemia. The effect of intra-arterial infusion of nitrite on forearm blood flow and forearm venous volumes was assessed during normoxia and hypoxia.

METHODS AND RESULTS

Forty healthy volunteers were studied. After baseline infusion of 0.9% saline, sodium nitrite was infused at incremental doses from 40 nmol/min to 7.84 mumol/min. At each stage, forearm blood flow was measured by strain-gauge plethysmography. Forearm venous volume was assessed by radionuclide plethysmography. Changes in forearm blood flow and forearm venous volume in the infused arm were corrected for those in the control arm. The peak percentage of venodilation during normoxia was 35.8+/-3.4% (mean+/-SEM) at 7.84 micromol/min (P<0.001) and was similar during hypoxia. In normoxia, arterial blood flow, assessed by the forearm blood flow ratio, increased from 1.04+/-0.09 (baseline) to 1.62+/-0.18 (nitrite; P<0.05) versus 1.07+/-0.09 (baseline) to 2.37+/-0.15 (nitrite; P<0.005) during hypoxia. This result was recapitulated in vitro in vascular rings.

CONCLUSIONS

Nitrite is a potent venodilator in normoxia and hypoxia. Arteries are modestly affected in normoxia but potently dilated in hypoxia, which suggests the important phenomenon of hypoxic augmentation of nitrite-mediated vasodilation in vivo. The use of nitrite as a selective arterial vasodilator in ischemic territories and as a potent venodilator in heart failure has therapeutic implications.

Cytochrome c oxidase regulates endogenous nitric oxide availability in respiring cells: a possible explanation for hypoxic vasodilation

One of the many routes proposed for the cellular inactivation of endogenous nitric oxide (NO) is by the cytochrome c oxidase of the mitochondrial respiratory chain. We have studied this possibility in human embryonic kidney cells engineered to generate controlled amounts of NO. We have used visible light spectroscopy to monitor continuously the redox state of cytochrome c oxidase in an oxygen-tight chamber, at the same time as which we measure cell respiration and the concentrations of oxygen and NO. Pharmacological manipulation of cytochrome c oxidase indicates that this enzyme, when it is in turnover and in its oxidized state, inactivates physiological amounts of NO, thus regulating its intra- and extracellular concentrations. This inactivation is prevented by blocking the enzyme with inhibitors, including NO. Furthermore, when cells generating low concentrations of NO respire toward hypoxia, the redox state of cytochrome c oxidase changes from oxidized to reduced, leading to a decrease in NO inactivation. The resultant increase in NO concentration could explain hypoxic vasodilation.

Inhaled NO accelerates restoration of liver function in adults following orthotopic liver transplantation

Ischemia/reperfusion (IR) injury in transplanted livers contributes to organ dysfunction and failure and is characterized in part by loss of NO bioavailability. Inhalation of NO is nontoxic and at high concentrations (80 ppm) inhibits IR injury in extrapulmonary tissues. In this prospective, blinded, placebo-controlled study, we evaluated the hypothesis that administration of inhaled NO (iNO; 80 ppm) to patients undergoing orthotopic liver transplantation inhibits hepatic IR injury, resulting in improved liver function. Patients were randomized to receive either placebo or iNO (n = 10 per group) during the operative period only. When results were adjusted for cold ischemia time and sex, iNO significantly decreased hospital length of stay, and evaluation of serum transaminases (alanine transaminase, aspartate aminotransferase) and coagulation times (prothrombin time, partial thromboplastin time) indicated that iNO improved the rate at which liver function was restored after transplantation. iNO did not significantly affect changes in inflammatory markers in liver tissue 1 hour after reperfusion but significantly lowered hepatocyte apoptosis. Evaluation of circulating NO metabolites indicated that the most likely candidate transducer of extrapulmonary effects of iNO was nitrite. In summary, this study supports the clinical use of iNO as an extrapulmonary therapeutic to improve organ function following transplantation.

Characterization of the deoxyhemoglobin binding site on human erythrocyte band 3: implications for O2 regulation of erythrocyte properties

Band 3, the major protein of the human erythrocyte membrane, associates with multiple metabolic, ion transport, and structural proteins. Functional studies demonstrate that the oxygenation state of the erythrocyte regulates cellular properties performed by these and/or related proteins. Because deoxyhemoglobin, but not oxyhemoglobin, binds band 3 reversibly with high affinity, these observations raise the hypothesis that hemoglobin might regulate erythrocyte properties through its reversible, oxygenation-dependent association with band 3. To explore this hypothesis, we have characterized the binding site of deoxyHb on human erythrocyte band 3. We report that (1) deoxyHb binds to residues 12-23 of band 3; (2) mutation of residues on either side of this sequence greatly enhances affinity of deoxyHb for band 3, suggesting that evolution of a higher affinity interaction would have been possible had it been beneficial for survival; (3) Hb does not bind to 2 other sequences in band 3 despite their high sequence homology to residues 12-23, and (4) the Hb binding site on band 3 lies proximal to binding sites for glycolytic enzymes, band 4.1 and ankyrin, suggesting possible mechanisms through which multifarious erythrocyte properties might be regulated by the oxygenation state of the cell.

An electron paramagnetic resonance investigation of the oxygen dependence of the arterial-venous gradient of nitrosyl hemoglobin in blood circulation

Whether there is a nitrosyl hemoglobin (HbNO) gradient between the venous and the arterial parts of the circulatory system is a very controversial issue in nitric oxide research. We have carefully evaluated the measurement of HbNO concentration in blood using EPR generated in vivo by the NO donor DEANO under various oxygen tensions. We found that the absolute concentrations of HbNO in venous and arterial blood were the same within experimental error, independent of hemoglobin saturation; only the ratios of 5-coordinate and 6-coordinate HbNO differed. The HbNO concentration increased when the oxygen concentration breathed by the rats decreased in a manner that was linear in hemoglobin saturation. These results do not support the existence of an arterial-venous gradient of HbNO under our experimental conditions.

In vitro and in vivo kinetic handling of nitrite in blood: effects of varying hemoglobin oxygen saturation

Growing evidence suggests that nitrite, acting via reduction to nitric oxide by deoxyhemoglobin, may play an important role in local control of blood flow during hypoxia. To investigate the effect of hypoxia (65 Torr arterial Po2) on the kinetic properties of nitrite, a bolus injection of sodium nitrite (10 mg/kg iv) was given to normoxic or hypoxic newborn lambs, and the time course of plasma nitrite and methemoglobin (MetHb) concentrations was measured. The in vivo kinetics of nitrite disappearance from plasma were biphasic and were not affected by hypoxia. Changes in MetHb, a product of the nitrite-hemoglobin reaction, also did not differ with the level of oxygenation. Hypoxia potentiated the hypotensive effects of nitrite on pulmonary and systemic arterial pressures. The disappearance of nitrite from plasma was equivalent to the increase in MetHb on a molar basis. In contrast, nitrite metabolism in sheep blood in vitro resulted in more than one MetHb per nitrite equivalent under mid- and high-oxygenation conditions: oxyhemoglobin (HbO2) saturation = 50.3 ± 1.7% and 97.0 ± 1.3%, respectively. Under the low-oxygenation condition (HbO2 saturation = 5.2 ± 0.9%), significantly less than 1 mol of MetHb was produced per nitrite equivalent, indicating that a significant portion of nitrite is metabolized through pathways that do not produce MetHb. These data support the idea that the vasodilating effects of nitrite are potentiated under hypoxic conditions due to the reduction of nitrite to nitric oxide by deoxyhemoglobin.

Hemoglobin oxygen fractional saturation regulates nitrite-dependent vasodilation of aortic ring bioassays

Nitrite reacts with deoxyhemoglobin to generate nitric oxide (NO). This reaction has been proposed to contribute to nitrite-dependent vasodilation in vivo and potentially regulate physiological hypoxic vasodilation. Paradoxically, while deoxyhemoglobin can generate NO via nitrite reduction, both oxyhemoglobin and deoxyhemoglobin potently scavenge NO. Furthermore, at the very low O(2) tensions required to deoxygenate cell-free hemoglobin solutions in aortic ring bioassays, surprisingly low doses of nitrite can be reduced to NO directly by the blood vessel, independent of the presence of hemoglobin; this makes assessments of the role of hemoglobin in the bioactivation of nitrite difficult to characterize in these systems. Therefore, to study the O(2) dependence and ability of deoxhemoglobin to generate vasodilatory NO from nitrite, we performed full factorial experiments of oxyhemoglobin, deoxyhemoglobin, and nitrite and found a highly significant interaction between hemoglobin deoxygenation and nitrite-dependent vasodilation (P < or = 0.0002). Furthermore, we compared the effect of hemoglobin oxygenation on authentic NO-dependent vasodilation using a NONOate NO donor and found that there was no such interaction, i.e., both oxyhemoglobin and deoxyhemoglobin inhibited NO-mediated vasodilation. Finally, we showed that another NO scavenger, 2-carboxyphenyl-4,4-5,5-tetramethylimidazoline-1-oxyl-3-oxide, inhibits nitrite-dependent vasodilation under normoxia and hypoxia, illustrating the uniqueness of the interaction of nitrite with deoxyhemoglobin. While both oxyhemoglobin and deoxyhemoglobin potently inhibit NO, deoxyhemoglobin exhibits unique functional duality as an NO scavenger and nitrite-dependent NO generator, suggesting a model in which intravascular NO homeostasis is regulated by a balance between NO scavenging and NO generation that is dynamically regulated by hemoglobin's O(2) fractional saturation and allosteric nitrite reductase activity.

Nitrite augments tolerance to ischemia/reperfusion injury via the modulation of mitochondrial electron transfer

Nitrite (NO(2)(-)) is an intrinsic signaling molecule that is reduced to NO during ischemia and limits apoptosis and cytotoxicity at reperfusion in the mammalian heart, liver, and brain. Although the mechanism of nitrite-mediated cytoprotection is unknown, NO is a mediator of the ischemic preconditioning cell-survival program. Analogous to the temporally distinct acute and delayed ischemic preconditioning cytoprotective phenotypes, we report that both acute and delayed (24 h before ischemia) exposure to physiological concentrations of nitrite, given both systemically or orally, potently limits cardiac and hepatic reperfusion injury. This cytoprotection is associated with increases in mitochondrial oxidative phosphorylation. Remarkably, isolated mitochondria subjected to 30 min of anoxia followed by reoxygenation were directly protected by nitrite administered both in vitro during anoxia or in vivo 24 h before mitochondrial isolation. Mechanistically, nitrite dose-dependently modifies and inhibits complex I by posttranslational S-nitrosation; this dampens electron transfer and effectively reduces reperfusion reactive oxygen species generation and ameliorates oxidative inactivation of complexes II-IV and aconitase, thus preventing mitochondrial permeability transition pore opening and cytochrome c release. These data suggest that nitrite dynamically modulates mitochondrial resilience to reperfusion injury and may represent an effector of the cell-survival program of ischemic preconditioning and the Mediterranean diet.

Relative sensitivity of soluble guanylate cyclase and mitochondrial respiration to endogenous nitric oxide at physiological oxygen concentration

Nitric oxide (NO) is a widespread biological messenger that has many physiological and pathophysiological roles. Most of the physiological actions of NO are mediated through the activation of sGC (soluble guanylate cyclase) and the subsequent production of cGMP. NO also binds to the binuclear centre of COX (cytochrome c oxidase) and inhibits mitochondrial respiration in competition with oxygen and in a reversible manner. Although sGC is more sensitive to endogenous NO than COX at atmospheric oxygen tension, the more relevant question is which enzyme is more sensitive at physiological oxygen concentration. Using a system in which NO is generated inside the cells in a finely controlled manner, we determined cGMP accumulation by immunoassay and mitochondrial oxygen consumption by high-resolution respirometry at 30 microM oxygen. In the present paper, we report that the NO EC50 of sGC was approx. 2.9 nM, whereas that required to achieve IC50 of respiration was 141 nM (the basal oxygen consumption in the absence of NO was 14+/-0.8 pmol of O2/s per 10(6) cells). In accordance with this, the NO-cGMP signalling transduction pathway was activated at lower NO concentrations than the AMPKs (AMP-activated protein kinase) pathway. We conclude that sGC is approx. 50-fold more sensitive than cellular respiration to endogenous NO under our experimental conditions. The implications of these results for cell physiology are discussed.

A novel method of measuring reduction of nitrite-induced methemoglobin applied to fetal and adult blood of humans and sheep

The reaction of nitrite with deoxyhemoglobin results in the production of nitric oxide and methemoglobin, a reaction recently proposed as an important oxygen-sensitive source of vasoactive nitric oxide during hypoxic and anoxic stress, with several animal studies suggesting that nitrite may have therapeutic potential. Accumulation of toxic levels of methemoglobin is suppressed by reductase enzymes present within the erythrocyte. Using a novel method of measuring methemoglobin reductase activity in intact erythrocytes, we compared fetal and adult sheep and human blood. After nitrite-induced production of 20% methemoglobin, the blood was equilibrated with carbon monoxide, which effectively stopped further production. Methemoglobin disappearance was first order in nature with specific rate constants (k x 1,000) of 12.9 +/- 1.3 min(-1) for fetal sheep, 5.88 +/- 0.26 min(-1) for adult sheep, 4.27 +/- 0.34 for adult humans, and 3.30 +/- 0.15 for newborn cord blood, all statistically different from one another. The effects of oxygen tensions, pH, hemolysis, and methylene blue are reported. Studies of temperature dependence indicated an activation energy of 8,620 +/- 1,060 calories/mol (2.06 kJ/mol), appreciably higher than would be characteristic of processes limited by passive membrane diffusion. In conclusion, the novel methodology permits absolute quantification of the reduction of nitrite-induced methemoglobin in whole blood.

Nitric oxide regulation of microvascular oxygen

The role of nitric oxide (NO) as a highly diffusible free radical gaseous vasodilator is intrinsically linked to the control of blood flow and oxygen (O(2)) delivery to tissue. NO also is involved in regulating mitochondrial O(2) metabolism, growth of new blood vessels, and blood oxygenation through control of respiratory ventilation. Hemoglobin and myoglobin may help to conserve NO for subsequent release of a NO-related vasoactive species under hypoxic conditions. NO has a major role in regulating microvascular O(2), and dysfunctional NO signaling is associated with the pathogenesis of metabolic and cardiovascular diseases.

Recent developments in nitric oxide donor drugs

During the 1980s, the free radical, nitric oxide (NO), was discovered to be a crucial signalling molecule, with wide-ranging functions in the cardiovascular, nervous and immune systems. Aside from providing a credible explanation for the actions of organic nitrates and sodium nitroprusside that have long been used in the treatment of angina and hypertensive crises respectively, the discovery generated great hopes for new NO-based treatments for a wide variety of ailments. Decades later, however, we are still awaiting novel licensed agents in this arena, despite an enormous research effort to this end. This review explores some of the most promising recent advances in NO donor drug development and addresses the challenges associated with NO as a therapeutic agent.

Inhaled NO as a therapeutic agent

In 1991, Frostell and colleagues reported that breathing low concentrations of nitric oxide (NO) decreased pulmonary artery pressure (PAP) in awake lambs with experimental pulmonary hypertension (PH) [Frostell C, Fratacci MD, Wain JC, Jones R, Zapol WM. Inhaled nitric oxide. A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction. Circulation 1991;83:2038-47]. Subsequently, efforts of multiple research groups studying animals and patients led to approval of inhaled NO by the US Food and Drug Administration in 1999 and the European Medicine Evaluation Agency and European Commission in 2001. Inhaled NO is currently indicated for the treatment of term and near-term neonates with hypoxemia and PH. Since regulatory approval, several studies have suggested that NO inhalation can prevent chronic lung disease in premature infants. In addition, unanticipated systemic effects of inhaled NO may lead to treatments for a variety of disorders including ischemia-reperfusion injury. This review summarizes the pharmacology and physiological effects of breathing NO. The application of inhaled NO to hypoxemic neonates with PH is discussed including recent studies exploring the use of inhaled NO to prevent bronchopulmonary dysplasia in premature infants. This review also highlights the application of inhaled NO to treat adults with cardiopulmonary disease, strategies to augment the efficacy of inhaled NO, and potential applications of the systemic effects of the gas.

Nitrite-dependent vasodilation is facilitated by hypoxia and is independent of known NO-generating nitrite reductase activities

The reduction of circulating nitrite to nitric oxide (NO) has emerged as an important physiological reaction aimed to increase vasodilation during tissue hypoxia. Although hemoglobin, xanthine oxidase, endothelial NO synthase, and the bc(1) complex of the mitochondria are known to reduce nitrite anaerobically in vitro, their relative contribution to the hypoxic vasodilatory response has remained unsolved. Using a wire myograph, we have investigated how the nitrite-dependent vasodilation in rat aortic rings is controlled by oxygen tension, norepinephrine concentration, soluble guanylate cyclase (the target for vasoactive NO), and known nitrite reductase activities under hypoxia. Vasodilation followed overall first-order dependency on nitrite concentration and, at low oxygenation and norepinephrine levels, was induced by low-nitrite concentrations, comparable to those found in vivo. The vasoactive effect of nitrite during hypoxia was abolished on inhibition of soluble guanylate cyclase and was unaffected by removal of the endothelium or by inhibition of xanthine oxidase and of the mitochondrial bc(1) complex. In the presence of hemoglobin and inositol hexaphosphate (which increases the fraction of deoxygenated heme), the effect of nitrite was not different from that observed with inositol hexaphosphate alone, indicating that under the conditions investigated here deoxygenated hemoglobin did not enhance nitrite vasoactivity. Together, our results indicate that the mechanism for nitrite vasorelaxation is largely intrinsic to the vessel and that under hypoxia physiological nitrite concentrations are sufficient to induce NO-mediated vasodilation independently of the nitrite reductase activities investigated here. Possible reaction mechanisms for nitrite vasoactivity, including formation of S-nitrosothiols within the arterial smooth muscle, are discussed.

Fluorometric measurement of nitrite/nitrate by 2,3-diaminonaphthalene

We describe a step-by-step protocol for measuring the stable products of the nitric oxide (NO) pathway: nitrite, nitrite plus nitrate and nitrate. This described protocol is easy to apply and is about 50 times more sensitive than the commonly used Griess reaction or commercially available assay kits based on the Griess reaction. It also allows the study of minimal changes in the NO pathway. With this method, it takes about 3 h to analyze the above-mentioned stable products in culture supernatants or in various body fluids, and the method has a sensitive linear range of 0.02-10.0 microM. This restricted linear range suggests that the technique is useful for studying small changes of nitrite and nitrate, rather than for routine diagnostic measurements.