Hypoxia, red blood cells, and nitrite regulate NO-dependent hypoxic vasodilation

Pharmacology and therapeutic role of inorganic nitrite and nitrate in vasodilatation

Nitrite has emerged as an important bioactive molecule that can be biotransformed to nitric oxide (NO) related metabolites in normoxia and reduced to NO under hypoxic and acidic conditions to exert vasodilatory effects and confer a variety of other benefits to the cardiovascular system. Abundant research is currently underway to understand the mechanisms involved and define the role of nitrite in health and disease. In this review we discuss the impact of nitrite and dietary nitrate on vascular function and the potential therapeutic role of nitrite in acute heart failure.

Hypoxic potentiation of nitrite effects in human vessels and platelets

Previous studies in non-human blood vessels and in platelets have demonstrated that under hypoxic conditions release of NO from nitrite (NO2(-)) is potentiated by deoxyhaemoglobin. In the current study, we characterized hypoxic potentiation of NO2(-) effects in human vasculature and platelets in vitro, addressing underlying mechanisms. The vasodilator efficacy of NO2(-), in comparison with glyceryl trinitrate (GTN), was evaluated in vitro, using segments of human saphenous vein. Under hypoxic conditions, there was a leftward shift of the NO2(-) concentration-response curve (EC50: 22 μM in hyperoxia vs 3.5 μM in hypoxia; p<0.01), but no significant potentiation of GTN effect. In the presence of red blood cells, hypoxic potentiation of NO2(-) vasodilator effect was accentuated. In whole blood samples and platelet-rich plasma (PRP) we assessed inhibition of platelet aggregation by NO2(-) (1mM), in comparison with that of sodium nitroprusside (SNP, 10 μM). In individual subjects (n=37), there was a strong correlation (r=0.75, p<0.0001) between anti-aggregatory effects of NO2(-) and SNP in whole blood, signifying that resultant sGC activation underlies biological effect and responses to NO2(-) are diminished in the presence of NO resistance. In PRP, the effects of NO2(-) were less pronounced than in whole blood (p=0.0001), suggesting an important role of Hb (within RBCs) in the bioconversion of NO2(-) to NO. Inhibition of platelet aggregation by NO2(-) was almost 3-fold greater in venous than in arterial blood (p<0.0001), and deoxyHb concentration directly correlated (r=0.69, p=0.013) with anti-aggregatory response. Incremental hypoxia applied to venous blood samples (in hypoxic chamber) caused a progressive increase in both deoxyHb level and anti-aggregatory effect of NO2(-). When subjects inhaled a 12% O2 mixture for 20 min, there was a 3-fold rise in blood deoxyHb fraction (p<0.01). In PRP, response to NO2(-) also increased under hypoxia, and was further enhanced (p<0.01) by deoxyHb. Furthermore, deoxyHb exerted significant anti-aggregatory effects even in the absence of added NO2(-), suggesting a role for endogenous NO2(-). The results of this work provide further mechanistic insights into hypoxic potentiation of vasodilator and anti-aggregatory actions of NO2(-). In human saphenous veins and blood, the balance of evidence suggests differential rates of NO release from NO2(-) (largely modulated by deoxyHb) as the fundamental mechanism.

Impact of mental and physical stress on blood pressure and pulse pressure under normobaric versus hypoxic conditions

Objective

Hypobaric hypoxia, physical and psychosocial stress may influence key cardiovascular parameters including blood pressure (BP) and pulse pressure (PP). We investigated the effects of mild hypobaric hypoxia exposure on BP and PP reactivity to mental and physical stress and to passive elevation by cable car.

Methods

36 healthy volunteers participated in a defined test procedure consisting of a period of rest 1, mental stress task (KLT-R), period of rest 2, combined mental (KLT-R) and physical task (bicycle ergometry) and a last period of rest both at Graz, Austria (353 m asl) and at the top station Dachstein (2700 m asl). Beat-to-beat heart rate and BP were analysed both during the test procedures at Graz and at Dachstein and during passive 1000 m elevation by cable car (from 1702 m to 2700 m).

Results

A significant interaction of kind of stress (mental vs. combined mental and physical) and study location (Graz vs. Dachstein) was found in the systolic BP (p = .007) and PP (p = .002) changes indicating that during the combined mental and physical stress task sBP was significantly higher under hypoxic conditions whereas sBP and PP were similar during mental stress both under normobaric normoxia (Graz) and under hypobaric hypoxia (Dachstein). During the passive ascent in cable car less trivialization (psychological coping strategy) was associated with an increase in PP (p = .004).

Conclusion

Our data show that combined mental and physical stress causes a significant higher raise in sBP and PP under hypoxic conditions whereas isolated mental stress did not affect sBP and PP under hypoxic conditions. PP-reaction to ascent in healthy subjects is not uniform. BP reactions to ascent that represents an accumulation of physical (mild hypobaric hypoxia) and psychological stressors depend on predetermined psychological traits (stress coping strategies). Thus divergent cardiovascular reactions can be explained by applying the multidimensional aspects of the biopsychosocial concept.

Mechanisms of nitrite bioactivation

It is now accepted that the anion nitrite, once considered an inert oxidation product of nitric oxide (NO), contributes to hypoxic vasodilation, physiological blood pressure control, and redox signaling. As such, its application in therapeutics is being actively tested in pre-clinical models and in human phase I-II clinical trials. Major pathways for nitrite bioactivation involve its reduction to NO by members of the hemoglobin or molybdopterin family of proteins, or catalyzed dysproportionation. These conversions occur preferentially under hypoxic and acidic conditions. A number of enzymatic systems reduce nitrite to NO and their activity and importance are defined by oxygen tension, specific organ system and allosteric and redox effectors. In this work, we review different proposed mechanisms of nitrite bioactivation, focusing on analysis of kinetics and experimental evidence for the relevance of each mechanism under different conditions.

Autofluorescence of liver tissue and bile: organ functionality monitoring during ischemia and reoxygenation

BACKGROUND AND OBJECTIVE

Autofluorescence (AF) based optical biopsy of liver tissue is a powerful approach for the real-time diagnosis of its functionality. Since increasing attention is given to the bile production and composition to monitor the liver metabolic engagement in surgery and transplantation, we have investigated the bile AF properties as a potential, additional diagnostic parameter.

STUDY DESIGN/MATERIALS AND METHODS

Spectrofluorometric analysis has been performed in real time on a rat liver model of warm ischemia and reperfusion-60 minutes partial portal vein and hepatic artery clamping and subsequent restoration of blood circulation-in comparison with sham operated rats. The AF spectra have been recorded through a single fiber optic probe (366 nm excitation) from both liver tissue and bile, collected from the cannulated bile duct, and analyzed by means of curve fitting procedures. Bile composition has been also analyzed through biochemical assays of bilirubin, total bile acids (TBA) and proteins.

RESULTS

Both liver and bile AF signal amplitude and spectral shape undergo changes during induction of ischemia and subsequent reperfusion. The liver tissue response is mainly ascribable to changes in NAD(P)H and flavins and their redox state, largely dependent on oxygen supply, and to the decrease of both vitamin A and fatty acid AF contributions. During comparable times, sham operated rat livers undergo smaller alterations in AF spectral shape, indicating a continuous, slight increase in the oxidized state. Bile AF emission shows a region in the 510-600 nm range ascribable to bilirubin, and resulting from the contribution of two bands, centered at about 515-523 and 570 nm, consistently with its bichromophore nature. Variations in the balance between these two bands depend on the influence of microenvironment on bilirubin intramolecular interchromophore energy transfer efficiency and are likely indicating alteration in a bile composition. This event is supported also by changes observed in the 400-500 nm emission region, ascribable to other bile components.

CONCLUSIONS

In parallel with the intratissue AF properties, mainly reflecting redox metabolic activities, the bile AF analysis can provide additional information to assess alterations and recovery in the balance of liver metabolic activities.

Hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) continues to fascinate cardiopulmonary physiologists and clinicians since its definitive description in 1946. Hypoxic vasoconstriction exists in all vertebrate gas exchanging organs. This fundamental response of the pulmonary vasculature in air breathing animals has relevance to successful fetal transition to air breathing at birth and as a mechanism of ventilation-perfusion matching in health and disease. It is a complex process intrinsic to the vascular smooth muscle, but with in vivo modulation by a host of factors including the vascular endothelium, erythrocytes, pulmonary innervation, circulating hormones and acid-base status to name only a few. This review will provide a broad overview of HPV and its mechansms and discuss the advantages and disadvantages of HPV in normal physiology, disease and high altitude.

Role of nitrite in regulation of fetal cephalic circulation in sheep

Nitrite has been postulated to provide a reservoir for conversion to nitric oxide (NO), especially in tissues with reduced oxygen levels as in the fetus. Nitrite would thus provide local vasodilatation and restore a balance between oxygen supply and need, a putative mechanism of importance especially in the brain. The current experiments test the hypothesis that exogenous nitrite acts as a vasodilator in the cephalic vasculature of the intact, near term fetal sheep. Fetuses were first instrumented to measure arterial blood pressure and carotid artery blood flow and then studied 4-5 days later while in utero without anaesthesia. Initially l-nitro-arginine (LNNA) was given to block endogenous NO production. Carotid resistance to flow increased 2-fold from 0.54 ± 0.01 (SEM) to 1.20 ± 0.08 mmHg min ml(-1) (in 13 fetuses, P < 0.001), indicating NO tonically reduces cerebral vascular tone. Sodium nitrite (or saline as control) was then infused in increasing step-doses from 0.01 to 33 μm in half-log increments over a period of 2 h. Carotid artery pressure, blood flow and vascular resistance did not change compared to fetuses receiving saline, even at plasma nitrite concentrations two orders of magnitude above the physiological range. The results indicate that while cephalic vascular tone is controlled by endogenous nitric oxide synthase activity, exogenously administered nitrite is not a vasodilator at physiological concentrations in the vasculature served by the carotid artery of fetal sheep.

Metabolomics of ADSOL (AS-1) red blood cell storage

Population-based investigations suggest that red blood cells (RBCs) are therapeutically effective when collected, processed, and stored for up to 42 days under validated conditions before transfusion. However, some retrospective clinical studies have shown worse patient outcomes when transfused RBCs have been stored for the longest times. Furthermore, studies of RBC persistence in the circulation after transfusion have suggested that considerable donor-to-donor variability exists and may affect transfusion efficacy. To understand the limitations of current blood storage technologies and to develop approaches to improve RBC storage and transfusion efficacy, we investigated the global metabolic alterations that occur when RBCs are stored in AS-1 (AS1-RBC). Leukoreduced AS1-RBC units prepared from 9 volunteer research donors (12 total donated units) were serially sampled for metabolomics analysis over 42 days of refrigerated storage. Samples were tested by gas chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry, and specific biochemical compounds were identified by comparison to a library of purified standards. Over 3 experiments, 185 to 264 defined metabolites were quantified in stored RBC samples. Kinetic changes in these biochemicals confirmed known alterations in glycolysis and other pathways previously identified in RBCs stored in saline, adenine, glucose and mannitol solution (SAGM-RBC). Furthermore, we identified additional alterations not previously seen in SAGM-RBCs (eg, stable pentose phosphate pathway flux, progressive decreases in oxidized glutathione), and we delineated changes occurring in other metabolic pathways not previously studied (eg, S-adenosyl methionine cycle). These data are presented in the context of a detailed comparison with previous studies of SAGM-RBCs from human donors and murine AS1-RBCs. Global metabolic profiling of AS1-RBCs revealed a number of biochemical alterations in stored blood that may affect RBC viability during storage as well as therapeutic effectiveness of stored RBCs in transfusion recipients. These results provide future opportunities to more clearly pinpoint the metabolic defects during RBC storage, to identify biomarkers for donor screening and prerelease RBC testing, and to develop improved RBC storage solutions and methodologies.

Nitrite reduction and cardiovascular protection

Inorganic nitrite, a metabolite of endogenously produced nitric oxide (NO) from NO synthases (NOS), provides the largest endocrine source of directly bioavailable NO. The conversion of nitrite to NO occurs mainly through enzymatic reduction, mediated by a range of proteins, including haem-globins, molybdo-flavoproteins, mitochondrial proteins, cytochrome P450 enzymes, and NOS. Such nitrite reduction is particularly favoured under hypoxia, when endogenous formation of NO from NOS is impaired. Under normoxic conditions, the majority of these nitrite reductases also scavenge NO, or diminish its bioavailability via reactive oxygen species (ROS) production, suggesting an intricate balance. Moreover, nitrite, whether produced endogenously, or derived from exogenous nitrite or nitrate administration (including dietary sources via the Nitrate-Nitrite-NO pathway) beneficially modulates many key cardiovascular pathological processes. In this review, we highlight the landmark studies which revealed nitrite's function in biological systems, and inspect its evolving role in cardiovascular protection. Whilst these effects have mainly been ascribed to the activity of one or more nitrite reductases, we also discuss newly-identified mechanisms, including nitrite anhydration, the involvement of s-nitrosothiols, nitro-fatty acids, and direct nitrite normoxic signalling, involving modification of mitochondrial structure and function, and ROS production. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Endothelial nitric oxide synthase in red blood cells: key to a new erythrocrine function?

Red blood cells (RBC) have been considered almost exclusively as a transporter of metabolic gases and nutrients for the tissues. It is an accepted dogma that RBCs take up and inactivate endothelium-derived NO via rapid reaction with oxyhemoglobin to form methemoglobin and nitrate, thereby limiting NO available for vasodilatation. Yet it has also been shown that RBCs not only act as "NO sinks", but exert an erythrocrine function - i.e an endocrine function of RBC - by synthesizing, transporting and releasing NO metabolic products and ATP, thereby potentially controlling systemic NO bioavailability and vascular tone. Recent work from our and others laboratory demonstrated that human RBCs carry an active type 3, endothelial NO synthase (eNOS), constitutively producing NO under normoxic conditions, the activity of which is compromised in patients with coronary artery disease. In this review we aim to discuss the potential role of red cell eNOS in RBC signaling and function, and to critically revise evidence to this date showing a role of non-endothelial circulating eNOS in cardiovascular pathophysiology.

Continuous monitoring of changes in plasma nitrite following cerebral ischemia in a rabbit embolic stroke model

In this proof-of-concept study, we investigated direct, continuous monitoring of plasma nitrite as an indicator of cerebral ischemia following clot embolization of rabbits via an indwelling carotid catheter. Two groups of rabbits were studied to compare the effects of embolization on nitrite levels. In the control group, blood was continuously obtained from a jugular venous catheter. The blood was immediately passed through an ultrafiltration filter; the filtrate was chemically reduced to convert free nitrite to nitric oxide (NO) and then measured using a NO-specific electrode. In the embolized group, after a baseline nitrite level was achieved, blood clots were injected into the brain via the carotid artery catheter, and then nitrite levels were continuously measured from jugular venous blood. The stroke group showed a significantly greater increase in nitrite as compared to controls (p=0.017). Using the area-under-the-curve (AUC) method, results reached statistical significance (p<0.05) within 3 min of embolization. In embolized rabbits, NO(2) levels increased 424±256% compared to baseline. This study shows that nitrite can be measured immediately following a stroke and that our system measures nitrite independent of the extent of the stroke. This study provides evidence for the feasibility of using nitrite as a marker of ischemic stroke.

Effect of cation type and concentration of nitrates on neurological disorders during experimental cerebral ischemia

Experiments were performed on the model of ischemic stroke due to bilateral occlusion of the carotid arteries. Nitrates had various effects on the dynamics of neurological disorders and mortality rate of Wistar rats, which depended on the cation type and concentration.

Nitric oxide scavenging by cell-free hemoglobin may be a primary factor determining hypertension in polycythemic patients

We tested the hypothesis that hypertension associated with polycythemia vera (PV) may be related to hemoglobin released from erythrocytes (cell-free hemoglobin, fHb). We assessed hematocrit, mean arterial pressure (MAP), blood viscosity, and the level of fHb and nitrite/nitrate (NOx) in the plasma of 73 PV patients and 38 healthy controls. The effect of isovolemic erythrocytapheresis (ECP) on the considered parameters was also studied. From the whole group of PV patients a subset of subjects with normal (normotensive patients, n = 16) and elevated MAP (hypertensive patients, n = 57) can be subtracted. It was found that in comparison with healthy controls, PV patients have significantly (p ≤ 0.01) elevated Hct (0.567 vs. 0.422), blood viscosity (5.45 vs. 3.56 cP), MAP (106.8 vs. 93.8 mmHg), plasma fHb (9.7 vs. 2.8 mg/dL), and NOx levels (34.1 vs. 27.5 μM). Compared with normotensive patients, hypertensive PV patients demonstrated a higher rise in fHb (10.2 vs. 8.0) and plasma NOx levels (35.8 vs. 31.0). In PV patients, fHb positively correlates with MAP (r = 0.489), NOx levels (r = 0.461), hematocrit (r = 0.428), and viscosity (r = 0.393). Blood viscosity positively correlated with hematocrit (r = 0.894), but not with other considered parameters. In PV patients MAP poorly correlated with hematocrit, whereas the correlation between MAP and NOx altered from - 0.325 (healthy control) to + 0.268 (PV patients). ECP procedure was associated with a significant (p < 0.01) reduction of hematocrit, fHb, blood viscosity, and MAP. In the normotensive subgroup of PV patients the ECP procedure did not affect MAP. It can be concluded that accelerated scavenging of nitric oxide by fHb rather than high Hct may be a key factor determining the development of hypertension in PV patients.

Red blood cells in sports: effects of exercise and training on oxygen supply by red blood cells

During exercise the cardiovascular system has to warrant substrate supply to working muscle. The main function of red blood cells in exercise is the transport of O₂ from the lungs to the tissues and the delivery of metabolically produced CO₂ to the lungs for expiration. Hemoglobin also contributes to the blood's buffering capacity, and ATP and NO release from red blood cells contributes to vasodilation and improved blood flow to working muscle. These functions require adequate amounts of red blood cells in circulation. Trained athletes, particularly in endurance sports, have a decreased hematocrit, which is sometimes called “sports anemia.” This is not anemia in a clinical sense, because athletes have in fact an increased total mass of red blood cells and hemoglobin in circulation relative to sedentary individuals. The slight decrease in hematocrit by training is brought about by an increased plasma volume (PV). The mechanisms that increase total red blood cell mass by training are not understood fully. Despite stimulated erythropoiesis, exercise can decrease the red blood cell mass by intravascular hemolysis mainly of senescent red blood cells, which is caused by mechanical rupture when red blood cells pass through capillaries in contracting muscles, and by compression of red cells e.g., in foot soles during running or in hand palms in weightlifters. Together, these adjustments cause a decrease in the average age of the population of circulating red blood cells in trained athletes. These younger red cells are characterized by improved oxygen release and deformability, both of which also improve tissue oxygen supply during exercise.

The key role of nitric oxide in hypoxia: hypoxic vasodilation and energy supply-demand matching

SIGNIFICANCE

A mismatch between energy supply and demand induces tissue hypoxia with the potential to cause cell death and organ failure. Whenever arterial oxygen concentration is reduced, increases in blood flow--hypoxic vasodilation--occur in an attempt to restore oxygen supply. Nitric oxide (NO) is a major signaling and effector molecule mediating the body's response to hypoxia, given its unique characteristics of vasodilation (improving blood flow and oxygen supply) and modulation of energetic metabolism (reducing oxygen consumption and promoting utilization of alternative pathways).

RECENT ADVANCES

This review covers the role of oxygen in metabolism and responses to hypoxia, the hemodynamic and metabolic effects of NO, and mechanisms underlying the involvement of NO in hypoxic vasodilation. Recent insights into NO metabolism will be discussed, including the role for dietary intake of nitrate, endogenous nitrite (NO₂⁻) reductases, and release of NO from storage pools. The processes through which NO levels are elevated during hypoxia are presented, namely, (i) increased synthesis from NO synthases, increased reduction of NO₂⁻ to NO by heme- or pterin-based enzymes and increased release from NO stores, and (ii) reduced deactivation by mitochondrial cytochrome c oxidase.

CRITICAL ISSUES

Several reviews covered modulation of energetic metabolism by NO, while here we highlight the crucial role NO plays in achieving cardiocirculatory homeostasis during acute hypoxia through both vasodilation and metabolic suppression.

FUTURE DIRECTIONS

We identify a key position for NO in the body's adaptation to an acute energy supply-demand mismatch.

Nitric oxide transport in blood: a third gas in the respiratory cycle

The trapping, processing, and delivery of nitric oxide (NO) bioactivity by red blood cells (RBCs) have emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency. We present here an expanded paradigm for the human respiratory cycle based on the coordinated transport of three gases: NO, O₂, and CO₂. By linking O₂ and NO flux, RBCs couple vessel caliber (and thus blood flow) to O₂ availability in the lung and to O₂ need in the periphery. The elements required for regulated O₂-based signal transduction via controlled NO processing within RBCs are presented herein, including S-nitrosothiol (SNO) synthesis by hemoglobin and O₂-regulated delivery of NO bioactivity (capture, activation, and delivery of NO groups at sites remote from NO synthesis by NO synthase). The role of NO transport in the respiratory cycle at molecular, microcirculatory, and system levels is reviewed. We elucidate the mechanism through which regulated NO transport in blood supports O₂ homeostasis, not only through adaptive regulation of regional systemic blood flow but also by optimizing ventilation-perfusion matching in the lung. Furthermore, we discuss the role of NO transport in the central control of breathing and in baroreceptor control of blood pressure, which subserve O₂ supply to tissue. Additionally, malfunctions of this transport and signaling system that are implicated in a wide array of human pathophysiologies are described. Understanding the (dys)function of NO processing in blood is a prerequisite for the development of novel therapies that target the vasoactive capacities of RBCs.

Effects of erythrocyte aging on nitric oxide and nitrite metabolism

AIMS

Recent studies have suggested that in addition to oxygen transport, red blood cells (RBC) are key regulators of vascular function by both inhibiting and promoting nitric oxide (NO)-mediated vasodilation. Most studies assume that RBC are homogenous, but, in fact, they comprise cells of differing morphology and biochemical composition which are dependent on their age, parameters that control NO reactions. We tested the hypothesis that distinct RBC populations will have differential effects on NO signaling.

RESULTS

Young and old RBC were separated by density gradient centrifugation. Consistent with previous reports, old RBC had decreased levels of surface N-acetyl neuraminic acid and increased oxygen binding affinities. Competition kinetic experiments showed that older RBCs scavenged NO∼2-fold faster compared with younger RBC, which translated to a more potent inhibition of both acetylcholine and NO-donor dependent vasodilation of isolated aortic rings. Moreover, nitrite oxidation kinetics was faster with older RBC compared with younger RBC; whereas no differences in nitrite-reduction kinetics were observed. This translated to increased inhibitory effect of older RBC to nitrite-dependent vasodilation under oxygenated and deoxygenated conditions. Finally, leukodepleted RBC storage also resulted in more dense RBC, which may contribute to the greater NO-inhibitory potential of stored RBC.

INNOVATION

These results suggest that a key element in vascular NO-homeostasis mechanisms is the distribution of RBC ages across the physiological spectrum (0-120 days) and suggest a novel mechanism for inhibited NO bioavailability in diseases which are characterized by a shift to an older RBC phenotype.

CONCLUSION

Older RBC inhibit NO bioavailability by increasing NO- and nitrite scavenging.

The eel heart: multilevel insights into functional organ plasticity

The remarkable functional homogeneity of the heart as an organ requires a well-coordinated myocardial heterogeneity. An example is represented by the selective sensitivity of the different cardiac cells to physical (i.e. shear stress and/or stretch) or chemical stimuli (e.g. catecholamines, angiotensin II, natriuretic peptides, etc.), and the cell-specific synthesis and release of these substances. The biological significance of the cardiac heterogeneity has recently received great attention in attempts to dissect the complexity of the mechanisms that control the cardiac form and function. A useful approach in this regard is to identify natural models of cardiac plasticity. Among fishes, eels (genus Anguilla), for their adaptive and acclimatory abilities, represent a group of animals so far largely used to explore the structural and ultrastructural myoarchitecture organization, as well as the complex molecular networks involved in the modulation of the heart function, such as those converting environmental signals into physiological responses. However, an overview on the existing current knowledge of eel cardiac form and function is not yet available. In this context, this review will illustrate major features of eel cardiac organization and pumping performance. Aspects of autocrine–paracrine modulation and the influence of factors such as body growth, exercise, hypoxia and temperature will highlight the power of the eel heart as an experimental model useful to decipher how the cardiac morpho-functional heterogeneities may support the uniformity of the whole-organ mechanics.

Oxygen transport in the microcirculation and its regulation

OBJECTIVE

Cells require energy to carry out their functions and they typically use oxidative phosphorylation to generate the needed ATP. Thus, cells have a continuous need for oxygen, which they receive by diffusion from the blood through the interstitial fluid. The circulatory system pumps oxygen-rich blood through a network of increasingly minute vessels, the microcirculation. The structure of the microcirculation is such that all cells have at least one nearby capillary for diffusive exchange of oxygen and red blood cells release the oxygen bound to hemoglobin as they traverse capillaries.

METHODS

This review focuses first on the historical development of techniques to measure oxygen at various sites in the microcirculation, including the blood, interstitium, and cells.

RESULTS

Next, approaches are described as to how these techniques have been employed to make discoveries about different aspects of oxygen transport. Finally, ways in which oxygen might participate in the regulation of blood flow toward matching oxygen supply to oxygen demand is discussed.

CONCLUSIONS

Overall, the transport of oxygen to the cells of the body is one of the most critical functions of the cardiovascular system and it is in the microcirculation where the final local determinants of oxygen supply, oxygen demand, and their regulation are decided.

Hemoglobin-mediated nitric oxide signaling

The rate that hemoglobin reacts with nitric oxide (NO) is limited by how fast NO can diffuse into the heme pocket. The reaction is as fast as any ligand/protein reaction can be and the result, when hemoglobin is in its oxygenated form, is formation of nitrate in what is known as the dioxygenation reaction. As nitrate, at the concentrations made through the dioxygenation reaction, is biologically inert, the only role hemoglobin was once thought to play in NO signaling was to inhibit it. However, there are now several mechanisms that have been discovered by which hemoglobin may preserve, control, and even create NO activity. These mechanisms involve compartmentalization of reacting species and conversion of NO from or into other species such as nitrosothiols or nitrite which could transport NO activity. Despite the tremendous amount of work devoted to this field, major questions concerning precise mechanisms of NO activity preservation as well as if and how Hb creates NO activity remain unanswered.

Mechanisms for cellular NO oxidation and nitrite formation in lung epithelial cells

Airway lining fluid contains relatively high concentrations of nitrite, and arterial blood levels of nitrite are higher than venous levels, suggesting the lung epithelium may represent an important source of nitrite in vivo. To investigate whether lung epithelial cells possess the ability to convert NO to nitrite by oxidation, and the effect of oxygen reactions on nitrite formation, the NO donor DETA NONOate was incubated with or without A549 cells or primary human bronchial epithelial (HBE) cells for 24 h under normoxic (21% O2) and hypoxic (1% O2) conditions. Nitrite production was significantly increased under all conditions in the presence of A549 or HBE cells, suggesting that both A549 and HBE cells have the capacity to oxidize NO to nitrite even under low-oxygen conditions. The addition of oxyhemoglobin to the A549 cell medium decreased the production of nitrite, consistent with NO scavenging limiting nitrite formation. Heat-denatured A549 cells produced much lower nitrite and nitrate, suggesting an enzymatic activity is required. This NO oxidation activity was highest in membrane-bound proteins with molecular size <100kDa. In addition, 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one and cyanide inhibited formation of nitrite in A549 cells. It has been shown that ceruloplasmin (Cp) possesses an NO oxidase and nitrite synthase activity in plasma based on NO oxidation to nitrosonium cation. We observed that Cp is expressed intracellularly in lung epithelial A549 cells and secreted into the medium under basal conditions and during cytokine stimulation. However, an analysis of Cp expression level and activity measured via p-phenylenediamine oxidase activity assay revealed very low activity compared with plasma, suggesting that there is insufficient Cp to contribute to detectable NO oxidation to nitrite in A549 cells. Additionally, Cp levels were knocked down using siRNA by more than 75% in A549 cells, with no significant change in either nitrite or cellular S-nitrosothiol formation compared to scrambled siRNA control under basal conditions or cytokine stimulation. These data suggest that lung epithelial cells possess NO oxidase activity, which is enhanced in cell-membrane-associated proteins and not regulated by intracellular or secreted Cp, indicating that alternative NO oxidases determine hypoxic and normoxic nitrite formation from NO in human lung epithelial cells.

Protein disulfide isomerase may facilitate the efflux of nitrite derived S-nitrosothiols from red blood cells

Protein disulfide isomerase (PDI) is an abundant protein primarily found in the endoplasmic reticulum and also secreted into the blood by a variety of vascular cells. The evidence obtained here, suggests that PDI could directly participate in the efflux of NO⁺ from red blood cells (RBC). PDI was detected both in RBC membranes and in the cytosol. PDI was S-nitrosylated when RBCs were exposed to nitrite under ∼50% oxygen saturation but not under ∼100% oxygen saturation. Furthermore, it was observed that hemoglobin (Hb) could promote PDI S-nitrosylation in the presence of ∼600 nM nitrite. In addition, three lines of evidence were obtained for PDI–Hb interactions: (1) Hb co-immunoprecipitated with PDI; (2) Hb quenched the intrinsic PDI fluorescence in a saturable manner; and (3) Hb–Fe(II)–NO absorption spectrum decreased in a [PDI]-dependent manner. Finally, PDI was detected on the surface RBC under ∼100% oxygen saturation and released as soluble under ∼50% oxygen saturation. The soluble PDI detected under ∼50% oxygen saturation was S-nitrosylated. Based on these data it is proposed that PDI is taken up by RBC and forms a complex with Hb. Hb–Fe(II)–NO that is formed from nitrite reduction under ∼50% O₂, then transfers NO⁺ to either Hb–Cys β93 or directly to PDI resulting in S-nitroso-PDI which transverses the RBC membrane and attaches to the RBC surface. When RBCs enter tissues the S-nitroso-PDI is released from the RBC-surface into the blood where its NO⁺ is transferred into the endothelium thereby inducing vasodilation, suggesting local oxygen-dependent dynamic interplays between nitrite, NO and S-nitrosylation.

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Red blood cells (RBC) contain protein disulfide isomerase (PDI) that can associate with hemoglobin.

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Formation of S-nitroso-PDI is an oxygen- and Hb-dependent process.

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S-nitroso-PDI associates with RBC surface in an oxygen dependent manner that facilitates its release under hypoxia.

Blood substitutes: evolution from noncarrying to oxygen- and gas-carrying fluids

The development of oxygen (O2)-carrying blood substitutes has evolved from the goal of replicating blood O2 transport properties to that of preserving microvascular and organ function, reducing the inherent or potential toxicity of the material used to carry O2, and treating pathologies initiated by anemia and hypoxia. Furthermore, the emphasis has shifted from blood replacement fluid to "O2 therapeutics" that restore tissue oxygenation to specific tissues regions. This review covers the different alternatives, potential and limitations of hemoglobin-based O2 carriers (HBOCs) and perfluorocarbon-based O2 carriers (PFCOCs), with emphasis on the physiologic conditions disturbed in the situation that they will be used. It describes how concepts learned from plasma expanders without O2-carrying capacity can be applied to maintain O2 delivery and summarizes the microvascular responses due to HBOCs and PFCOCs. This review also presents alternative applications of HBOCs and PFCOCs namely: 1) How HBOC O2 affinity can be engineered to target O2 delivery to hypoxic tissues; and 2) How the high gas solubility of PFCOCs provides new opportunities for carrying, dissolving, and delivering gases with biological activity. It is concluded that the development of current blood substitutes has amplified their applications horizon by devising therapeutic functions for O2 carriers requiring limited O2 delivery capacity restoration. Conversely, full, blood-like O2-carrying capacity reestablishment awaits the control of O2 carrier toxicity.

Generating S-nitrosothiols from hemoglobin: mechanisms, conformational dependence, and physiological relevance

In vitro, ferrous deoxy-hemes in hemoglobin (Hb) react with nitrite to generate nitric oxide (NO) through a nitrite reductase reaction. In vivo studies indicate Hb with nitrite can be a source of NO bioactivity. The nitrite reductase reaction does not appear to account fully for this activity because free NO is short lived especially within the red blood cell. Thus, the exporting of NO bioactivity both out of the RBC and over a large distance requires an additional mechanism. A nitrite anhydrase (NA) reaction in which N2O3, a potent S-nitrosating agent, is produced through the reaction of NO with ferric heme-bound nitrite has been proposed (Basu, S., Grubina, R., Huang, J., Conradie, J., Huang, Z., Jeffers, A., Jiang, A., He, X., Azarov, I., Seibert, R., Mehta, A., Patel, R., King, S. B., Hogg, N., Ghosh, A., Gladwin, M. T., and Kim-Shapiro, D. B. (2007) Nat. Chem. Biol. 3, 785-794) as a possible mechanism. Legitimate concerns, including physiological relevance and the nature of the mechanism, have been raised concerning the NA reaction. This study addresses these concerns demonstrating NO and nitrite with ferric hemes under near physiological conditions yield an intermediate having the properties of the purported NA heme-bound N2O3 intermediate. The results indicate that ferric heme sites, traditionally viewed as a source of potential toxicity, can be functionally significant, especially for partially oxygenated/partially met-R state Hb that arises from the NO dioxygenation reaction. In the presence of low levels of nitrite and either NO or a suitable reductant such as L-cysteine, these ferric heme sites can function as a generator for the formation of S-nitrosothiols such as S-nitrosoglutathione and, as such, should be considered as a source of RBC-derived and exportable bioactive NO.

HBOC vasoactivity: interplay between nitric oxide scavenging and capacity to generate bioactive nitric oxide species

SIGNIFICANCE

Despite many advances in blood substitute research, the development of materials that are effective in maintaining blood volume and oxygen delivery remains a priority for emergency care and trauma. Clinical trials on hemoglobin (Hb)-based oxygen carriers (HBOCs) have not provided information on the mechanism of toxicity, although all commercial formulations have safety concerns. Specifically, it is important to reconcile the different hypotheses of Hb toxicity, such as nitric oxide (NO) depletion and oxidative reactions, to provide a coherent molecular basis for designing a safe HBOC.

RECENT ADVANCES

HBOCs with different sizes often exhibit differences in the degree of HBOC-induced vasoactivity. This has been attributed to differences in the degree of NO scavenging and in the extent of Hb extravasation. Additionally, it is appears that Hb can undergo reactions that compensate for NO scavenging by generating bioactive forms of NO.

CRITICAL ISSUES

Engineering modifications to enhance bioactive NO production can result in diminished oxygen delivery by virtue of increased oxygen affinity. This strategy can prevent the HBOC from fulfilling the intended goal on preserving oxygenation; however, the NO production effects will increase perfusion and oxygen transport.

FUTURE DIRECTIONS

Hb modifications influence NO scavenging and the capacity of certain HBOCs to compensate for NO scavenging through nitrite-mediated reactions that generate bioactive NO. Based on the current understanding of these NO-related factors, possible synthetic strategies are presented that address how HBOC formulations can be prepared that: (i) effectively deliver oxygen, (ii) maintain tissue perfusion, and (iii) limit/reverse underlying inflammation within the vasculature.

Xuezhikang, extract of red yeast rice, improved abnormal hemorheology, suppressed caveolin-1 and increased eNOS expression in atherosclerotic rats

Background

Xuezhikang is the extract of red yeast rice, which has been widely used for the management of atherosclerotic disease, but the molecular basis of its antiatherosclerotic effects has not yet been fully identified. Here we investigated the changes of eNOS in vascular endothelia and RBCs, eNOS regulatory factor Caveolin-1 in endothelia, and hemorheological parameters in atherosclerotic rats to explore the protective effects of Xuezhikang.

Methodology/Principal Findings

Wistar rats were divided into 4 groups (n = 12/group) group C, controls; group M, high-cholesterol diet (HCD) induced atherosclerotic models; group X, HCD+Xuezhikang; and group L, HCD +Lovastatin. In group X, Xuezhikang inhibited oxidative stress, down-regulated caveolin-1 in aorta wall (P<0.05), up-regulated eNOS expression in vascular endothelia and erythrocytes (P<0.05), increased NOx (nitrite and nitrate) in plasma and cGMP in erythrocyte plasma and aorta wall (P<0.05), increased erythrocyte deformation index (EDI), and decreased whole blood viscosity and plasma viscosity (P<0.05), with the improvement of arterial pathology.

Conclusions/Significance

Xuezhikang up-regulated eNOS expression in vascular endothelia and RBCs, increased plasma NOx and improved abnormal hemorheology in high cholesterol diet induced atherosclerotic rats. The elevated eNOS/NO and improved hemorheology may be beneficial to atherosclerotic disease.

Sodium nitrate supplementation does not enhance performance of endurance athletes

PURPOSE

Supplementation with inorganic nitrate has been suggested to be an ergogenic aid for athletes as nitric oxide donor. The purpose of this study was to determine whether ingestion of inorganic sodium nitrate benefits well-trained athletes performing a 40-min exercise test in laboratory conditions. In addition, we investigated the effect of this supplement on plasma levels of endothelin-1 (ET-1) and in nitrated proteins.

METHODS

Thirteen trained athletes participated in this randomized, double-blind, crossover study. They performed a 40-min cycle ergometer distance-trial test after two 3-d periods of dietary supplementation with sodium nitrate (10 mg·kg of body mass) or placebo.

RESULTS

Concentration of plasma nitrate (256 ± 35 μM) and nitrite (334 ± 86 nM) increased significantly (P < 0.05) after nitrate supplementation compared with placebo (nitrate: 44 ± 11 μM; nitrite: 187 ± 43 nM). In terms of exercise performance, there were no differences in either the mean distance (nitrate: 26.4 ± 1.1 km; placebo: 26.3 ± 1.2 km; P = 0.61) or mean power output (nitrate: 258 ± 28 W; placebo: 257 ± 28 W; P = 0.89) between treatments. Plasma ET-1 increased significantly (P < 0.05) just after exercise in nitrate (4.0 ± 0.8 pg·mL) and placebo (2.4 ± 0.4 pg·mL) conditions. This increase was significantly greater (P < 0.05) in the nitrate group. Levels of nitrated proteins did not differ between treatments (nitrate: preexercise, 91% ± 23%; postexercise, 81% ± 23%; placebo: preexercise, 95% ± 20%; postexercise, 99% ± 19%).

CONCLUSION

Sodium nitrate supplementation did not improve a 40-min distance-trial performance in endurance athletes. In addition, concentration of plasma ET-1 increased significantly after exercise after supplementation with sodium nitrate.

Oxygen binding to partially nitrosylated hemoglobin

Reactions of nitric oxide (NO) with hemoglobin (Hb) are important elements in protection against nitrosative damage. NO in the vasculature is depleted by the oxidative reaction with oxy Hb or by binding to deoxy Hb to generate partially nitrosylated Hb (Hb-NO). Many aspects of the formation and persistence of Hb-NO are yet to be clarified. In this study, we used a combination of EPR and visible absorption spectroscopy to investigate the interactions of partially nitrosylated Hb with O2. Partially nitrosylated Hb samples had predominantly hexacoordinate NO-heme geometry and resisted oxidation when exposed to O2 in the absence of anionic allosteric effectors. Faster oxidation occurred in the presence of 2,3-diphosphoglycerate (DPG) or inositol hexaphosphate (IHP), where the NO-heme derivatives had higher levels of pentacoordinate heme geometry. The anion-dependence of the NO-heme geometry also affected O2 binding equilibria. O2-binding curves of partially nitrosylated Hb in the absence of anions were left-shifted at low saturations, indicating destabilization of the low O2 affinity T-state of the Hb by increasing percentages of NO-heme, much as occurs with increasing levels of CO-heme. Samples containing IHP showed small decreases in O2 affinity, indicating shifts toward the low-affinity T-state and formation of inert α-NO/β-met tetramers. Most remarkably, O2-equilibria in the presence of the physiological effector DPG were essentially unchanged by up to 30% NO-heme in the samples. As will be discussed, under physiological conditions the interactions of Hb with NO provide protection against nitrosative damage without impairing O2 transport by Hb's unoccupied heme sites. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Nitroreductase detection and hypoxic tumor cell imaging by a designed sensitive and selective fluorescent probe, 7-[(5-nitrofuran-2-yl)methoxy]-3H-phenoxazin-3-one

A highly selective and sensitive fluorescence probe, 7-[(5-nitrofuran-2-yl)methoxy]-3H-phenoxazin-3-one (1), is developed for imaging the hypoxic status of tumor cells via the indirect detection of nitroreductase. The detection mechanism is based on the fact that nitroreductase can selectively catalyze the reduction of the nitro group in 1 to a hydroxylamine or amino group in the presence of reduced nicotinamide adenine dinucleotide as an electron donor that is indispensable, followed by the 1,6-rearrangement-elimination and the release of resorufin. As a result, the reaction produces a distinct color and fluorescence change from almost colorless and nonfluorescent to pink and strong red fluorescence. The fluorescence increase of probe 1 at λ(550/585 nm) is directly proportional to the concentration of nitroreductase in the range of 15-300 ng/mL, with a detection limit of 0.27 ng/mL. The ready reduction of the nitro group in 1 under hypoxic conditions leads to the establishment of a sensitive and selective fluorescence method for imaging the hypoxic status of tumor cells, and with this method Hela and A549 cells under normoxic and hypoxic conditions (even for different extents of hypoxia) can be differentiated successfully. This method is simple and may be useful for the imaging of disease-relevant hypoxia.

Red blood cells stored for increasing periods produce progressive impairments in nitric oxide-mediated vasodilation

BACKGROUND

Clinical outcomes in transfused patients may be affected by the duration of blood storage, possibly due to red blood cell (RBC)-mediated disruption of nitric oxide (NO) signaling, a key regulator of vascular tone and blood flow.

STUDY DESIGN AND METHODS

AS-1 RBC units stored up to 42 days were sampled at selected storage times. Samples were added to aortic rings ex vivo, a system where NO-mediated vasodilation could be experimentally controlled.

RESULTS

RBC units showed storage-dependent changes in plasma hemoglobin (Hb), RBC 2,3-diphosphoglycerate acid, and RBC adenosine triphosphate conforming to expected profiles. When freshly collected (Day 0) blood was added to rat aortic rings, methacholine (MCh) stimulated substantial NO-mediated vasodilation. In contrast, MCh produced no vasodilation in the presence of blood stored for 42 days. Surprisingly, the vasoinhibitory effects of stored RBCs were almost totally mediated by RBCs themselves: removal of the supernatant did not attenuate the inhibitory effects, while addition of supernatant alone to the aortic rings only minimally inhibited MCh-stimulated relaxation. Stored RBCs did not inhibit vasodilation by a direct NO donor, demonstrating that the RBC-mediated vasoinhibitory mechanism did not work by NO scavenging.

CONCLUSIONS

These studies have revealed a previously unrecognized vasoinhibitory activity of stored RBCs, which is more potent than the described effects of free Hb and works through a different mechanism that does not involve NO scavenging but may function by reducing endothelial NO production. Through this novel mechanism, transfusion of small volumes of stored blood may be able to disrupt physiologic vasodilatory responses and thereby possibly cause adverse clinical outcomes.

Red blood cells stored for increasing periods produce progressive impairments in nitric oxide-mediated vasodilation

BACKGROUND

Clinical outcomes in transfused patients may be affected by the duration of blood storage, possibly due to red blood cell (RBC)-mediated disruption of nitric oxide (NO) signaling, a key regulator of vascular tone and blood flow.

STUDY DESIGN AND METHODS

AS-1 RBC units stored up to 42 days were sampled at selected storage times. Samples were added to aortic rings ex vivo, a system where NO-mediated vasodilation could be experimentally controlled.

RESULTS

RBC units showed storage-dependent changes in plasma hemoglobin (Hb), RBC 2,3-diphosphoglycerate acid, and RBC adenosine triphosphate conforming to expected profiles. When freshly collected (Day 0) blood was added to rat aortic rings, methacholine (MCh) stimulated substantial NO-mediated vasodilation. In contrast, MCh produced no vasodilation in the presence of blood stored for 42 days. Surprisingly, the vasoinhibitory effects of stored RBCs were almost totally mediated by RBCs themselves: removal of the supernatant did not attenuate the inhibitory effects, while addition of supernatant alone to the aortic rings only minimally inhibited MCh-stimulated relaxation. Stored RBCs did not inhibit vasodilation by a direct NO donor, demonstrating that the RBC-mediated vasoinhibitory mechanism did not work by NO scavenging.

CONCLUSIONS

These studies have revealed a previously unrecognized vasoinhibitory activity of stored RBCs, which is more potent than the described effects of free Hb and works through a different mechanism that does not involve NO scavenging but may function by reducing endothelial NO production. Through this novel mechanism, transfusion of small volumes of stored blood may be able to disrupt physiologic vasodilatory responses and thereby possibly cause adverse clinical outcomes.

Role of the b93cys, ATP and adenosine in red cell dependent hypoxic vasorelaxation

Two of the proposed mechanisms by which red blood cells (RBC) mediate hypoxic vasorelaxation by coupling hemoglobin deoxygenation to the activation of nitric oxide signaling involve ATP-release from RBC and S-nitrosohemoglobin (b93C(SNO)Hb) dependent bioactivity. However, different studies have reached opposite conclusions regarding the aforementioned mechanisms. Using isolated vessels, hypoxic vasorelaxation induced by human, C57BL/6 or mouse RBC which exclusively express either native human hemoglobin (HbC93) or human hemoglobin in which the conserved b93cys was replaced with Ala (HbC93A) were compared. All RBCs stimulated hypoxic vasodilation to similar extents suggesting the b93cys is not required for this RBC-mediated function. Hypoxic vasorelaxation was inhibited by co-incubation of ATP-pathway blockers including L-NAME (eNOS inhibitor) and Apyrase. Moreover, we tested if modulation of adenosine-dependent signaling affected RBC-dependent vasorelaxation using pan- or subtype specific adenosine receptor blockers, or adenosine deaminase (ADA). Interestingly, ADA and adenosine A2 receptor blockade, but not A1 receptor blockade, inhibited HbC93, HbC93A dependent hypoxic vasorelaxation. Equivalent results were obtained with human RBC. These data suggest that using isolated vessels, RBC do not require the presence of the b93cys to elicit hypoxic vasorelaxation and mediate this response via ATP- and a novel adenosine-dependent mechanism.

An improved mass spectrometry-based measurement of NO metabolites in biological fluids

Assessment of NO metabolism in vivo relies on the accurate measurement of its metabolites nitrite (NO(2)(-)), nitrate (NO(3)(-)), and nitrosothiols (RSNOs) in biological fluids. We report a sensitive method to simultaneously determine NO(2)(-) and NO(3)(-) in biological matrixes. Tetraoctylammonium was used to catalyze the complete conversion of NO(2)(-) and NO(3)(-) to stable pentafluorobenzyl (PFB) derivatives directly from aqueous acetone medium before gas chromatography and negative-ion chemical ionization mass spectrometry (GC/NICI/MS). This catalyst dramatically improved the yield of PFB derivatives for NO(2)(-) (4.5 times) and NO(3)(-) (55 times) compared to noncatalyzed derivatization methods. Analysis was performed using (15)N-labeled internal standards by selected-ion monitoring at m/z 46 for fragment NO(2)(-) and m/z 47 for its isotope analogue, (15)NO(2)(-), and m/z 62 for NO(3)(-) and m/z 63 for (15)NO(3)(-). This method allowed specific detection of both PFB derivatives over a wide dynamic range with a limit of detection below 4.5 pg for NO(2)(-) and 2.5 pg for NO(3)(-). After the specific conversion of RSNOs by HgCl(2) to NO(2)(-), this GC/NICI/MS analysis was used to measure RSNOs in plasma. A further comparison with the widely used tri-iodide chemiluminescence (I(3)(-)-CL) assay indicated that the GC/MS assay validated the lower physiological RSNO and nitrite levels reported using I(3)(-)-CL detection compared with values obtained using UV-photolysis methods. Plasma levels of RSNOs determined by GC/MS and I(3)(-)-CL were well correlated (r = 0.8). The improved GC/MS method was successfully used to determine the changes in plasma, urinary, and salivary NO(2)(-) and NO(3)(-) as well as plasma RSNOs in humans after either a low-NO(3)(-) or a high-NO(3)(-) meal.

Oxygen mediates vascular smooth muscle relaxation in hypoxia

The activation of soluble guanylate cyclase (sGC) by nitric oxide (NO) and other ligands has been extensively investigated for many years. In the present study we considered the effect of molecular oxygen (O₂) on sGC both as a direct ligand and its affect on other ligands by measuring cyclic guanosine monophosphate (cGMP) production, as an index of activity, as well as investigating smooth muscle relaxation under hypoxic conditions. Our isolated enzyme studies confirm the function of sGC is impaired under hypoxic conditions and produces cGMP in the presence of O₂, importantly in the absence of NO. We also show that while O₂ could partially affect the magnitude of sGC stimulation by NO when the latter was present in excess, activation by the NO independent, haem-dependent sGC stimulator 3-(5′-hydroxymethyl-2′-furyl)-1-benzylindazole (YC-1) was unaffected. Our in vitro investigation of smooth muscle relaxation confirmed that O₂ alone in the form of a buffer bolus (equilibrated at 95% O₂/5% CO₂) had the ability to dilate vessels under hypoxic conditions and that this was dependent upon sGC and independent of eNOS. Our studies confirm that O₂ can be a direct and important mediator of vasodilation through an increase in cGMP production. In the wider context, these observations are key to understanding the relative roles of O₂ versus NO-induced sGC activation.

RBC-NOS-dependent S-nitrosylation of cytoskeletal proteins improves RBC deformability

Background

Red blood cells (RBC) possess a nitric oxide synthase (RBC-NOS) whose activation depends on the PI3-kinase/Akt kinase pathway. RBC-NOS-produced NO exhibits important biological functions like maintaining RBC deformability. Until now, the cellular target structure for NO, to exert its influence on RBC deformability, remains unknown. In the present study we analyzed the modification of RBC-NOS activity by pharmacological treatments, the resulting influence on RBC deformability and provide first evidence for possible target proteins of RBC-NOS-produced NO in the RBC cytoskeletal scaffold.

Methods/Findings

Blood from fifteen male subjects was incubated with the NOS substrate L-arginine to directly stimulate enzyme activity. Direct inhibition of enzyme activity was induced by L-N5-(1-Iminoethyl)-ornithin (L-NIO). Indirect stimulation and inhibition of RBC-NOS were achieved by applying insulin and wortmannin, respectively, substances known to affect PI3-kinase/Akt kinase pathway. The NO donor sodium nitroprusside (SNP) and the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) were additionally applied as NO positive and negative controls, respectively. Immunohistochemical staining was used to determine phosphorylation and thus activation of RBC-NOS. As a marker for NO synthesis nitrite was measured in plasma and RBCs using chemiluminescence detection. S-nitrosylation of erythrocyte proteins was determined by biotin switch assay and modified proteins were identified using LC-MS. RBC deformability was determined by ektacytometry. The data reveal that activated RBC-NOS leads to increased NO production, S-nitrosylation of RBC proteins and RBC deformability, whereas RBC-NOS inhibition resulted in contrary effects.

Conclusion/Significance

This study first-time provides strong evidence that RBC-NOS-produced NO modifies RBC deformability through direct S-nitrosylation of cytoskeleton proteins, most likely α- and β-spectrins. Our data, therefore, gain novel insights into biological functions of RBC-NOS by connecting impaired RBC deformability abilities to specific posttranslational modifications of RBC proteins. By identifying likely NO-target proteins in RBC, our results will stimulate new therapeutic approaches for patients with microvascular disorders.

Bilateral spontaneous hyphaema: case report and review of literature

To report a case of bilateral spontaneous hyphaema in a patient on warfarin sodium for atrial fibrillation and COPD. A case report and literature review. A 76-year-old man presented with bilateral spontaneous hyphaema. There was no anterior chamber pathology known to predispose for spontaneous bleeding except for a history of paroxysmal atrial fibrillation treated with a daily dose of 3 mg of warfarin sodium. In addition, he was also suffering from severe COPD and was on oxygen supplementation. This is a rare case of a bilateral spontaneous hyphaema. Although the patient was on warfarin sodium, his INR was only 2.6 at the onset of his symptoms. It may be possible that the combined action of anti-coagulant properties of warfarin sodium and hypoxic vasodilatation of iris vessels may be responsible for bilateral hyphaema in this case.

Role of neuronal and inducible nitric oxide synthases in the guinea pig ileum myenteric plexus during in vitro ischemia and reperfusion

BACKGROUND

Intestinal ischemia and reperfusion (I/R) injury leads to abnormalities in motility, namely delay of transit, caused by damage to myenteric neurons. Alterations of the nitrergic transmission may occur in these conditions. This study investigated whether an in vitro I/R injury may affect nitric oxide (NO) production from the myenteric plexus of the guinea pig ileum and which NO synthase (NOS) isoform is involved.

METHODS

The distribution of the neuronal (n) and inducible (i) NOS was determined by immunohistochemistry during 60 min of glucose/oxygen deprivation (in vitro ischemia) followed by 60 min of reperfusion. The protein and mRNA levels of nNOS and iNOS were investigated by Western-immunoblotting and real time RT-PCR, respectively. NO levels were quantified as nitrite/nitrate.

KEY RESULTS

After in vitro I/R the proportion of nNOS-expressing neurons and protein levels remained unchanged. nNOS mRNA levels increased 60 min after inducing ischemia and in the following 5 min of reperfusion. iNOS-immunoreactive neurons, protein and mRNA levels were up-regulated during the whole I/R period. A significant increase of nitrite/nitrate levels was observed in the first 5 min after inducing I/R and was significantly reduced by N(ω) -propyl-l-arginine and 1400 W, selective inhibitors of nNOS and iNOS, respectively.

CONCLUSIONS & INFERENCES

Our data demonstrate that both iNOS and nNOS represent sources for NO overproduction in ileal myenteric plexus during I/R, although iNOS undergoes more consistent changes suggesting a more relevant role for this isoform in the alterations occurring in myenteric neurons following I/R.

Is methemoglobin an inert bystander, biomarker or a mediator of oxidative stress--The example of anemia?

Acute anemia increases the risk for perioperative morbidity and mortality in critically ill patients who experience blood loss and fluid resuscitation (hemodilution). Animal models of acute anemia suggest that neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) is adaptive and protects against anemia-induced mortality. During acute anemia, we have observed a small but consistent increase in methemoglobin (MetHb) levels that is inversely proportional to the acute reduction in Hb observed during hemodilution in animals and humans. We hypothesize that this increase in MetHb may be a biomarker of anemia-induced tissue hypoxia. The increase in MetHb may occur by at least two mechanisms: (1) direct hemoglobin oxidation by increased nNOS-derived NO within the perivascular tissue and (2) by increased deoxyhemoglobin (DeoxyHb) nitrite reductase activity within the vascular compartment. Both mechanisms reflect a potential increase in NO signaling from the tissue and vascular compartments during anemia. These responses are thought to be adaptive; as deletion of nNOS results in increased mortality in a model of acute anemia. Finally, it is possible that prolonged activation of these mechanisms may lead to maladaptive changes in redox signaling. We hypothesize, increased MetHb in the vascular compartment during acute anemia may reflect activation of adaptive mechanisms which augment NO signaling. Understanding the link between anemia, MetHb and its treatments (transfusion of stored blood) may help us to develop novel treatment strategies to reduce the risk of anemia-induced morbidity and mortality.

S-nitrosylation: integrator of cardiovascular performance and oxygen delivery

Delivery of oxygen to tissues is the primary function of the cardiovascular system. NO, a gasotransmitter that signals predominantly through protein S-nitrosylation to form S-nitrosothiols (SNOs) in target proteins, operates coordinately with oxygen in mammalian cellular systems. From this perspective, SNO-based signaling may have evolved as a major transducer of the cellular oxygen-sensing machinery that underlies global cardiovascular function. Here we review mechanisms that regulate S-nitrosylation in the context of its essential role in "systems-level" control of oxygen sensing, delivery, and utilization in the cardiovascular system, and we highlight examples of aberrant S-nitrosylation that may lead to altered oxygen homeostasis in cardiovascular diseases. Thus, through a bird's-eye view of S-nitrosylation in the cardiovascular system, we provide a conceptual framework that may be broadly applicable to the functioning of other cellular systems and physiological processes and that illuminates new therapeutic promise in cardiovascular medicine.

Erythrocyte storage increases rates of NO and nitrite scavenging: implications for transfusion-related toxicity

Storage of erythrocytes in blood banks is associated with biochemical and morphological changes to RBCs (red blood cells). It has been suggested that these changes have potential negative clinical effects characterized by inflammation and microcirculatory dysfunction which add to other transfusion-related toxicities. However, the mechanisms linking RBC storage and toxicity remain unclear. In the present study we tested the hypothesis that storage of leucodepleted RBCs results in cells that inhibit NO (nitric oxide) signalling more so than younger cells. Using competition kinetic analyses and protocols that minimized contributions from haemolysis or microparticles, our data indicate that the consumption rates of NO increased ~40-fold and NO-dependent vasodilation was inhibited 2-4-fold comparing 42-day-old with 0-day-old RBCs. These results are probably due to the formation of smaller RBCs with increased surface area: volume as a consequence of membrane loss during storage. The potential for older RBCs to affect NO formation via deoxygenated RBC-mediated nitrite reduction was also tested. RBC storage did not affect deoxygenated RBC-dependent stimulation of nitrite-induced vasodilation. However, stored RBCs did increase the rates of nitrite oxidation to nitrate in vitro. Significant loss of whole-blood nitrite was also observed in stable trauma patients after transfusion with 1 RBC unit, with the decrease in nitrite occurring after transfusion with RBCs stored for >25 days, but not with younger RBCs. Collectively, these data suggest that increased rates of reactions between intact RBCs and NO and nitrite may contribute to mechanisms that lead to storage-lesion-related transfusion risk.

Contribution of membrane permeability and unstirred layer diffusion to nitric oxide-red blood cell interaction

Nitric oxide (NO) consumption by red blood cell (RBC) hemoglobin (Hb) in vasculature is critical in regulating the vascular tone. The paradox of NO production at endothelium in close proximity of an effective NO scavenger Hb in RBCs is mitigated by lower NO consumption by RBCs compared to that of free Hb due to transport resistances including membrane resistance, extra- and intra-cellular resistances for NO biotransport to the RBC. Relative contribution of each transport resistance on NO-RBC interactions is still not clear. We developed a mathematical model of NO transport to a single RBC to quantify the contributions from individual transport barriers by analyzing the effect of RBC membrane permeability (P(m)), hematocrit (Hct) and NO-Hb reaction rate constants on NO-RBC interactions. Our results indicated that intracellular diffusion of NO was not a rate limiting step for NO-RBC interactions. The extracellular diffusion contributed 70-90% of total transport resistance for P(m)>1 cm s(-1) whereas membrane resistance accounts for 50-75% of total transport resistance for P(m)<0.1 cm s(-1). We propose a narrow P(m) range of 0.21-0.44 cm s(-1) for 10-45% Hct, respectively, below which membrane resistance is more significant and above which extracellular diffusion is a dominating transport resistance for NO-RBC interactions.

Nitric oxide-mediated protein modification in cardiovascular physiology and pathology

Nitric oxide (NO) is a key regulator of cardiovascular functions including the control of vascular tone, anti-inflammatory properties of the endothelium, cardiac contractility, and thrombocyte activation and aggregation. Numerous experimental data support the view that NO not only acts via cyclic guanosine monophosphate (cGMP)-dependent mechanisms but also modulates protein function by nitrosation, nitrosylation, glutathiolation, and nitration, respectively. To understand how NO regulates all of these diverse biological processes on the molecular level a comprehensive assessment of NO-mediated cGMP-dependent and independent targets is required. Novel proteomic approaches allow the simultaneous identification of large quantities of proteins modified in an NO-dependent manner and thereby will considerably deepen our understanding of the role NO plays in cardiovascular physiology and pathophysiology.