Inhaled nitric oxide augments nitric oxide transport on sickle cell hemoglobin without affecting oxygen affinity

The Nephroprotective Effect of Nitric Oxide during Extracorporeal Circulation: An Experimental Study

This study aims to determine the effectiveness of administering 80 ppm nitric oxide in reducing kidney injury, mitochondrial dysfunction and regulated cell death in kidneys during experimental perfusion. Twenty-four sheep were randomized into four groups: two groups received 80 ppm NO conditioning with 90 min of cardiopulmonary bypass (CPB + NO) or 90 min of CPB and hypothermic circulatory arrest (CPB + CA + NO), while two groups received sham protocols (CPB and CPB + CA). Kidney injury was assessed using laboratory (neutrophil gelatinase-associated lipocalin, an acute kidney injury biomarker) and morphological methods (morphometric histological changes in kidney biopsy specimens). A kidney biopsy was performed 60 min after weaning from mechanical perfusion. NO did not increase the concentrations of inhaled NO2 and methemoglobin significantly. The NO-conditioning groups showed less severe kidney injury and mitochondrial dysfunction, with statistical significance in the CPB + NO group and reduced tumor necrosis factor-α expression as a trigger of apoptosis and necroptosis in renal tissue in the CPB + CA + NO group compared to the CPB + CA group. The severity of mitochondrial dysfunction in renal tissue was insignificantly lower in the NO-conditioning groups. We conclude that NO administration is safe and effective at reducing kidney injury, mitochondrial dysfunction and regulated cell death in kidneys during experimental CPB.

Determination of Nitric Oxide and Its Metabolites in Biological Tissues Using Ozone-Based Chemiluminescence Detection: A State-of-the-Art Review

Ozone-based chemiluminescence detection (CLD) has been widely applied for determining nitric oxide (•NO) and its derived species in many different fields, such as environmental monitoring and biomedical research. In humans and animals, CLD has been applied to determine exhaled •NO and •NO metabolites in plasma and tissues. The main advantages of CLD are high sensitivity and selectivity for quantitative analysis in a wide dynamic range. Combining CLD with analytical separation techniques like chromatography allows for the analytes to be quantified with less disturbance from matrix components or impurities. Sampling techniques like microdialysis and flow injection analysis may be coupled to CLD with the possibility of real-time monitoring of •NO. However, details and precautions in experimental practice need to be addressed and clarified to avoid wrong estimations. Therefore, using CLD as a detection tool requires a deep understanding of the sample preparation procedure and chemical reactions used for liberating •NO from its derived species. In this review, we discuss the advantages and pitfalls of CLD for determining •NO species, list the different applications and combinations with other analytical techniques, and provide general practical notes for sample preparation. These guidelines are designed to assist researchers in comprehending CLD data and in selecting the most appropriate method for measuring •NO species.

A natural heme deficiency exists in biology that allows nitric oxide to control heme protein functions by regulating cellular heme distribution

A natural heme deficiency that exists in cells outside of the circulation broadly compromises the heme contents and functions of heme proteins in cells and tissues. Recently, we found that the signaling molecule, nitric oxide (NO), can trigger or repress the deployment of intracellular heme in a concentration-dependent hormetic manner. This uncovers a new role for NO and sets the stage for it to shape numerous biological processes by controlling heme deployment and consequent heme protein functions in biology.

Inhaled Nitric Oxide Treatment for Aneurysmal SAH Patients With Delayed Cerebral Ischemia

Background

We demonstrated experimentally that inhaled nitric oxide (iNO) dilates hypoperfused arterioles, increases tissue perfusion, and improves neurological outcome following subarachnoid hemorrhage (SAH) in mice. We performed a prospective pilot study to evaluate iNO in patients with delayed cerebral ischemia after SAH.

Methods

SAH patients with delayed cerebral ischemia and hypoperfusion despite conservative treatment were included. iNO was administered at a maximum dose of 40 ppm. The response to iNO was considered positive if: cerebral artery diameter increased by 10% in digital subtraction angiography (DSA), or tissue oxygen partial pressure (PtiO₂) increased by > 5 mmHg, or transcranial doppler (TCD) values decreased more than 30 cm/sec, or mean transit time (MTT) decreased below 6.5 secs in CT perfusion (CTP). Patient outcome was assessed at 6 months with the modified Rankin Scale (mRS).

Results

Seven patients were enrolled between February 2013 and September 2016. Median duration of iNO administration was 23 h. The primary endpoint was reached in all patients (five out of 17 DSA examinations, 19 out of 29 PtiO₂ time points, nine out of 26 TCD examinations, three out of five CTP examinations). No adverse events necessitating the cessation of iNO were observed. At 6 months, three patients presented with a mRS score of 0, one patient each with an mRS score of 2 and 3, and two patients had died.

Conclusion

Administration of iNO in SAH patients is safe. These results call for a larger prospective evaluation.

Controlling the Direction of S-Nitrosation versus Denitrosation: Reversible Cleavage and Formation of an S-N Bond within a Dicopper Center

Iron and copper enzymes are known to promote reversible S-nitrosation/denitrosation in biology. However, it is unclear how the direction of S-N bond formation/scission is controlled. Herein, we demonstrate the interconversion of metal-S-nitrosothiol adduct M(RSNO) and metal nitrosyl thiolate complex M(NO)(SR), which may regulate the direction of reversible S-(de)nitrosation. Treatment of a dicopper(I,I) complex with RSNO leads to a mixture of two structural isomers: dicopper(I,I) S-nitrosothiol [Cu^ICu^I(RSNO)]²⁺ and dicopper(II,II) nitrosyl thiolate [Cu^IICu^II(NO)(SR)]²⁺. The K_eq between these two structural isomers is sensitive to temperature, the solvent coordination ability, and counterions. Our study illustrates how copper centers can modulate the direction of RS-NO bond formation and cleavage through a minor perturbation of the local environment.

Therapeutic Effects of Inhaled Nitric Oxide Therapy in COVID-19 Patients

The global COVID-19 pandemic has become the largest public health challenge of recent years. The incidence of COVID-19-related acute hypoxemic respiratory failure (AHRF) occurs in up to 15% of hospitalized patients. Antiviral drugs currently available to clinicians have little to no effect on mortality, length of in-hospital stay, the need for mechanical ventilation, or long-term effects. Inhaled nitric oxide (iNO) administration is a promising new non-standard approach to directly treat viral burden while enhancing oxygenation. Along with its putative antiviral affect in COVID-19 patients, iNO can reduce inflammatory cell-mediated lung injury by inhibiting neutrophil activation, lowering pulmonary vascular resistance and decreasing edema in the alveolar spaces, collectively enhancing ventilation/perfusion matching. This narrative review article presents recent literature on the iNO therapy use for COVID-19 patients. The authors suggest that early administration of the iNO therapy may be a safe and promising approach for the treatment of COVID-19 patients. The authors also discuss unconventional approaches to treatment, continuous versus intermittent high-dose iNO therapy, timing of initiation of therapy (early versus late), and novel delivery systems. Future laboratory and clinical research is required to define the role of iNO as an adjunct therapy against bacterial, viral, and fungal infections.

Red Blood Cell and Endothelial eNOS Independently Regulate Circulating Nitric Oxide Metabolites and Blood Pressure

BACKGROUND

Current paradigms suggest that nitric oxide (NO) produced by endothelial cells (ECs) through endothelial nitric oxide synthase (eNOS) in the vessel wall is the primary regulator of blood flow and blood pressure. However, red blood cells (RBCs) also carry a catalytically active eNOS, but its role is controversial and remains undefined. This study aimed to elucidate the functional significance of RBC eNOS compared with EC eNOS for vascular hemodynamics and nitric oxide metabolism.

METHODS

We generated tissue-specific loss- and gain-of-function models for eNOS by using cell-specific Cre-induced gene inactivation or reactivation. We created 2 founder lines carrying a floxed eNOS (eNOSflox/flox) for Cre-inducible knockout (KO), and gene construct with an inactivated floxed/inverted exon (eNOSinv/inv) for a Cre-inducible knock-in (KI), which respectively allow targeted deletion or reactivation of eNOS in erythroid cells (RBC eNOS KO or RBC eNOS KI mice) or in ECs (EC eNOS KO or EC eNOS KI mice). Vascular function, hemodynamics, and nitric oxide metabolism were compared ex vivo and in vivo.

RESULTS

The EC eNOS KOs exhibited significantly impaired aortic dilatory responses to acetylcholine, loss of flow-mediated dilation, and increased systolic and diastolic blood pressure. RBC eNOS KO mice showed no alterations in acetylcholine-mediated dilation or flow-mediated dilation but were hypertensive. Treatment with the nitric oxide synthase inhibitor Nγ-nitro-l-arginine methyl ester further increased blood pressure in RBC eNOS KOs, demonstrating that eNOS in both ECs and RBCs contributes to blood pressure regulation. Although both EC eNOS KOs and RBC eNOS KOs had lower plasma nitrite and nitrate concentrations, the levels of bound NO in RBCs were lower in RBC eNOS KOs than in EC eNOS KOs. Reactivation of eNOS in ECs or RBCs rescues the hypertensive phenotype of the eNOSinv/inv mice, whereas the levels of bound NO were restored only in RBC eNOS KI mice.

CONCLUSIONS

These data reveal that eNOS in ECs and RBCs contribute independently to blood pressure homeostasis.

The oxygen dissociation curve of blood in COVID-19

COVID-19 hinders oxygen transport to the consuming tissues by at least two mechanisms: In the injured lung, saturation of hemoglobin is compromised, and in the tissues, an associated anemia reduces the volume of delivered oxygen. For the first problem, increased hemoglobin oxygen affinity [left shift of the oxygen dissociation curve (ODC)] is of advantage, for the second, however, the contrary is the case. Indeed a right shift of the ODC has been found in former studies for anemia caused by reduced cell production or hemolysis. This resulted from increased 2,3-bisphosphoglycerate (2,3-BPG) concentration. In three investigations in COVID-19, however, no change of hemoglobin affinity was detected in spite of probably high [2,3-BPG]. The most plausible cause for this finding is formation of methemoglobin (MetHb), which increases the oxygen affinity and thus apparently compensates for the 2,3-BPG effect. However, this "useful effect" is cancelled by the concomitant reduction of functional hemoglobin. In the largest study on COVID-19, even a clear left shift of the ODC was detected when calculated from measurements in fresh blood rather than after equilibration with gases outside the body. This additional "in vivo" left shift possibly results from various factors, e.g., concentration changes of Cl-, 2,3-BPG, ATP, lactate, nitrocompounds, glutathione, glutamate, because of time delay between blood sampling and end of equilibration, or enlarged distribution space including interstitial fluid and is useful for O2 uptake in the lungs. Under discussion for therapy are the affinity-increasing 5-hydroxymethyl-2-furfural (5-HMF), erythropoiesis-stimulating substances like erythropoietin, and methylene blue against MetHb formation.

Nitric oxide and the brain. Part 1: Mechanisms of regulation, transport and effects on the developing brain

Apart from its known actions as a pulmonary vasodilator, nitric oxide (NO) is a key signal mediator in the neonatal brain. Despite the extensive use of NO for pulmonary artery hypertension (PAH), its actions in the setting of brain hypoxia and ischemia, which co-exists with PAH in 20-30% of affected infants, are not well established. This review focuses on the mechanisms of actions of NO covering the basic, translational, and clinical evidence of its neuroprotective and neurotoxic properties. In this first part, we present the physiology of transport and delivery of NO to the brain and the regulation of cerebrovascular and systemic circulation by NO, as well the role of NO in the development of the immature brain. IMPACT: NO can be transferred from the site of production to the site of action rapidly and affects the central nervous system. Inhaled NO (iNO), a commonly used medication, can have significant effects on the neonatal brain. NO regulates the cerebrovascular and systemic circulation and plays a role in the development of the immature brain. This review describes the properties of NO under physiologic conditions and under stress. The impact of this review is that it describes the effects of NO, especially regarding the vulnerable neonatal brain, and helps understand the conditions that could contribute to neurotoxicity or neuroprotection.

cGMP modulation therapeutics for sickle cell disease

IMPACT STATEMENT

Sickle cell disease (SCD) is one of the most common inherited diseases and is associated with a reduced life expectancy and acute and chronic complications, including frequent painful vaso-occlusive episodes that often require hospitalization. At present, treatment of SCD is limited to hematopoietic stem cell transplant, transfusion, and limited options for pharmacotherapy, based principally on hydroxyurea therapy. This review highlights the importance of intracellular cGMP-dependent signaling pathways in SCD pathophysiology; modulation of these pathways with soluble guanylate cyclase (sGC) stimulators or phosphodiesterase (PDE) inhibitors could potentially provide vasorelaxation and anti-inflammatory effects, as well as elevate levels of anti-sickling fetal hemoglobin.

Pharmacological preconditioning with inhaled nitric oxide (NO): Organ-specific differences in the lifetime of blood and tissue NO metabolites

BACKGROUND

Endogenous nitric oxide (NO) may contribute to ischemic and anesthetic preconditioning while exogenous NO protects against ischemia-reperfusion (I/R) injury in the heart and other organs. Why those beneficial effects observed in animal models do not always translate into clinical effectiveness remains unclear. To mitigate reperfusion damage a source of NO is required. NO inhalation is known to increase tissue NO metabolites, but little information exists about the lifetime of these species. We therefore sought to investigate the fate of major NO metabolite classes following NO inhalation in mice in vivo.

METHODS

C57BL/6J mice were exposed to 80 ppm NO for 1 h. NO metabolites were measured in blood (plasma and erythrocytes) and tissues (heart, liver, lung, kidney and brain) immediately after NO exposure and up to 48 h thereafter. Concentrations of S-nitrosothiols, N-nitrosamines and NO-heme products as well as nitrite and nitrate were quantified by gas-phase chemiluminescence and ion chromatography. In separate experiments, mice breathed 80 ppm NO for 1 h prior to cardiac I/R injury (induced by coronary arterial ligation for 1 h, followed by recovery). After sacrifice, the size of the myocardial infarction (MI) and the area at risk (AAR) were measured.

RESULTS

After NO inhalation, elevated nitroso/nitrosyl levels returned to baseline over the next 24 h, with distinct multi-phasic decay profiles in each compartment. S/N-nitroso compounds and NO-hemoglobin in blood decreased exponentially, but remained above baseline for up to 30min, whereas nitrate was elevated for up to 3hrs after discontinuing NO breathing. Hepatic S/N-nitroso species concentrations remained steady for 30min before dropping exponentially. Nitrate only rose in blood, liver and kidney; nitrite tended to be lower in all organs immediately after NO inhalation but fluctuated considerably in concentration thereafter. NO inhalation before myocardial ischemia decreased the ratio of MI/AAR by 30% vs controls (p = 0.002); only cardiac S-nitrosothiols and NO-hemes were elevated at time of reperfusion onset.

CONCLUSIONS

Metabolites in blood do not reflect NO metabolite status of any organ. Although NO is rapidly inactivated by hemoglobin-mediated oxidation in the circulation, long-lived tissue metabolites may account for the myocardial preconditioning effects of inhaled NO. NO inhalation may afford similar protection in other organs.

Hydroxyurea therapy modulates sickle cell anemia red blood cell physiology: Impact on RBC deformability, oxidative stress, nitrite levels and nitric oxide synthase signalling pathway

Hydroxyurea (HU) has been suggested to act as a nitric oxide (NO) donor in sickle cell anemia (SCA). However, little is known about the HU NO-related effects on red blood cell (RBC) physiology and NO signalling pathway. Thirty-four patients with SCA (22 under HU treatment (HU+) and 12 without (HU-)) and 17 healthy subjects (AA) were included. RBC nitrite content, deformability and reactive oxygen species (ROS) levels were measured. RBC NO-synthase (RBC-NOS) signalling pathway was assessed by the measurement of RBC-NOS serine¹¹⁷⁷ and RBC-AKT serine⁴⁷³ phosphorylation. We also investigated the in vitro effects of Sodium Nitroprusside (SNP), a NO donor, on the same parameters in SCA RBC. RBC nitrite content was higher in HU+ than in HU- and AA. RBC deformability was decreased in SCA patients compared to AA but the decrease was more pronounced in HU-. RBC ROS level was increased in SCA compared to AA but the level was higher in HU- than in HU+. RBC-NOS serine¹¹⁷⁷ and RBC-AKT serine⁴⁷³ phosphorylation were decreased in HU+ compared to HU- and AA. SCA RBC treated with SNP showed increased deformability, reduced ROS content and a decrease in AKT and RBC-NOS phosphorylation. Our study suggests that HU, through its effects on foetal hemoglobin and possibly on NO delivery, would modulate RBC NO signalling pathway, RBC rheology and oxidative stress.

Simple chronic transfusion therapy, a crucial therapeutic option for sickle cell disease, improves but does not normalize blood rheology: What should be our goals for transfusion therapy?

Sickle cell anemia is characterized by a mutation resulting in the formation of an abnormal beta-hemoglobin called hemoglobin S. Hemoglobin S polymerizes upon deoxygenation, causing impaired red blood cell deformability and increased blood viscosity at equivalent hematocrits. Thus, sickle cell disease is a hemorheologic disease that results in various pathologic processes involving multiple organ systems including the lungs, heart, kidneys and brain. Red blood cell mechanics and the perturbations on blood flow-endothelial interaction underlie much of the pathology found in sickle cell disease. Transfusion therapy is one of the few therapeutic options available to patients, acting as both primary and secondary prevention of stroke. Transfusion therapy, both simple and exchange, is also used for unremitting and frequent pain crises and pulmonary hypertension. Therefore, understanding basic rheologic changes following transfusion inform other therapeutic options that aim to mitigate this diffuse pathologic process. This review will aim to highlight transfusion effects on blood rheology.

Nitric oxide pathology and therapeutics in sickle cell disease

Sickle cell disease is caused by a mutant form of hemoglobin that polymerizes under hypoxic conditions which leads to red blood cell (RBC) distortion, calcium-influx mediated RBC dehydration, increased RBC adhesivity, reduced RBC deformability, increased RBC fragility, and hemolysis. These impairments in RBC structure and function result in multifaceted downstream pathology including inflammation, endothelial cell activation, platelet and leukocyte activation and adhesion, and thrombosis, all of which contribute vascular occlusion and substantial morbidity and mortality. Hemoglobin released upon RBC hemolysis scavenges nitric oxide (NO) and generates reactive oxygen species (ROS) and thereby decreases bioavailability of this important signaling molecule. As the endothelium-derived relaxing factor, NO acts as a vasodilator and also decreases platelet, leukocyte, and endothelial cell activation. Thus, low NO bioavailability contributes to pathology in sickle cell disease and its restoration could serve as an effective treatment. Despite its promise, clinical trials based on restoring NO bioavailability have so far been mainly disappointing. However, particular "NO donating" agents such as nitrite, which unlike some other NO donors can improve sickle RBC properties, may yet prove effective.

Brain venular pattern by 7T MRI correlates with memory and haemoglobin in sickle cell anaemia

Sickle cell anaemia (SCA) is a hereditary hemoglobinopathy characterised by extensive vascular dysfunction that stems from inflammation, thrombosis and occlusion of post-capillary venules. Cognitive impairment is a neurological complication of SCA whose pathogenesis is unknown. We hypothesised that cerebral venular abnormalities are linked to cognitive impairment in SCA. Thus, we employed 7T magnetic resonance imaging (MRI) to examine the association between venular density and cognitive function in homozygous SCA. We quantified the density of total, long, and short venules in pre-defined regions of interest between the frontal and occipital cornu on each hemisphere. Cognitive function was assessed using the Hopkins Verbal Learning Test - Revised (HVLT-R) test of learning and memory. Patients (n=11) were compared with race, age and gender-equated controls (n=7). Compared to controls, patients had an overall venular rarefaction, with significantly lower density of long venules and greater density of short venules which was inversely related to HVLT-R performance and haemoglobin. To our knowledge, this is the first 7T MRI study in SCA and first report of associations between cerebral venular patterns and cognitive performance and haemoglobin. Future studies should examine whether these novel neuroimaging markers predict cognitive impairment longitudinally and are mechanistically linked to severity of anaemia.

High red blood cell nitric oxide synthase activation is not associated with improved vascular function and red blood cell deformability in sickle cell anaemia

Human red blood cells (RBC) express an active and functional endothelial-like nitric oxide (NO) synthase (RBC-NOS). We report studies on RBC-NOS activity in sickle cell anaemia (SCA), a genetic disease characterized by decreased RBC deformability and vascular dysfunction. Total RBC-NOS content was not significantly different in SCA patients compared to healthy controls; however, using phosphorylated RBC-NOS-Ser(1177) as a marker, RBC-NOS activation was higher in SCA patients as a consequence of the greater activation of Akt (phosphorylated Akt-Ser(473) ). The higher RBC-NOS activation in SCA led to higher levels of S-nitrosylated α- and β-spectrins, and greater RBC nitrite and nitrotyrosine levels compared to healthy controls. Plasma nitrite content was not different between the two groups. Laser Doppler flowmetric experiments demonstrated blunted microcirculatory NO-dependent response under hyperthermia in SCA patients. RBC deformability, measured by ektacytometry, was reduced in SCA in contrast to healthy individuals, and pre-shearing RBC in vitro did not improve deformability despite an increase of RBC-NOS activation. RBC-NOS activation is high in freshly drawn blood from SCA patients, resulting in high amounts of NO produced by RBC. However, this does not result in improved RBC deformability and vascular function: higher RBC-NO is not sufficient to counterbalance the enhanced oxidative stress in SCA.

Hemoglobin oxygen affinity in patients with cystic fibrosis

In patients with cystic fibrosis lung damages cause arterial hypoxia. As a typical compensatory reaction one might expect changes in oxygen affinity of hemoglobin. Therefore position (standard half saturation pressure P₅₀st) and slope (Hill’s n) of the O₂ dissociation curve as well as the Bohr coefficients (BC) for CO₂ and lactic acid were determined in blood of 14 adult patients (8 males, 6 females) and 14 healthy controls (6 males, 8 females). While Hill’s n amounted to approximately 2.6 in all subjects, P₅₀st was slightly increased by 1mmHg in both patient groups (controls male 26.7±0.2, controls female 27.0±0.1, patients male 27.7±0.5, patients female 28.0±0.3 mmHg; mean and standard error, overall p<0.01). Main cause was a rise of 1–2 µmol/g hemoglobin in erythrocytic 2,3-biphosphoglycerate concentration. One patient only, clearly identified as an outlier and with the mutation G551D, showed a reduction of both P₅₀st (24.5 mmHg) and [2,3-biphosphoglycerate] (9.8 µmol/g hemoglobin). There were no differences in BCCO₂, but small sex differences in the BC for lactic acid in the controls which were not detectable in the patients. Causes for the right shift of the O₂ dissociation curve might be hypoxic stimulation of erythrocytic glycolysis and an increased red cell turnover both causing increased [2,3-biphosphoglycerate]. However, for situations with additional hypercapnia as observed in exercising patients a left shift seems to be a more favourable adaptation in cystic fibrosis. Additionally when in vivo PO₂ values were corrected to the standard conditions they mostly lay left of the in vitro O₂ dissociation curve in both patients and controls. This hints to unknown fugitive factors influencing oxygen affinity.

The significant blood resistance to lung nitric oxide transfer lies within the red cell

The lung nitric oxide (NO) diffusing capacity (DlNO) mainly reflects alveolar-capillary membrane conductance (Dm). However, blood resistance has been shown in vitro and in vivo. To explore whether this resistance lies in the plasma, the red blood cell (RBC) membrane, or in the RBC interior, we measured the NO diffusing capacity (Dno) in a membrane oxygenator circuit containing ∼1 liter of horse or human blood exposed to 14 parts per million NO under physiological conditions on 7 separate days. We compared results across a 1,000-fold change in extracellular diffusivity using dextrans, plasma, and physiological salt solution. We halved RBC surface area by comparing horse and human RBCs. We altered the diffusive resistance of the RBC interior by adding sodium nitrite converting oxyhemoglobin to methemoglobin. Neither increased viscosity nor reduced RBC size reduced Dno. Adding sodium nitrite increased methemoglobin and was associated with a steady fall in Dno ( P < 0.001). Similar results were obtained at NO concentrations found in vivo. The RBC interior appears to be the site of the blood resistance.

Oxidative stress in sickle cell disease: an overview of erythrocyte redox metabolism and current antioxidant therapeutic strategies

Erythrocytes have an environment of continuous pro-oxidant generation due to the presence of hemoglobin (Hb), which represents an additional and quantitatively significant source of superoxide (O2(-)) generation in biological systems. To counteract oxidative stress, erythrocytes have a self-sustaining antioxidant defense system. Thus, red blood cells uniquely function to protect Hb via a selective barrier allowing gaseous and other ligand transport as well as providing antioxidant protection not only to themselves but also to other tissues and organs in the body. Sickle hemoglobin molecules suffer repeated polymerization/depolymerization generating greater amounts of reactive oxygen species, which can lead to a cyclic cascade characterized by blood cell adhesion, hemolysis, vaso-occlusion, and ischemia-reperfusion injury. In other words, sickle cell disease is intimately linked to a pathophysiologic condition of multiple sources of pro-oxidant processes with consequent chronic and systemic oxidative stress. For this reason, newer therapeutic agents that can target oxidative stress may constitute a valuable means for preventing or delaying the development of organ complications.

Nitric oxide reduces sickle hemoglobin polymerization: potential role of nitric oxide-induced charge alteration in depolymerization

We previously demonstrated that inhaling nitric oxide (NO) increases the oxygen affinity of sickle red blood cells (RBCs) in patients with sickle cell disease (SCD). Our recent studies found that NO lowered the P(50) values of sickle hemoglobin (HbS) hemolysates but did not increase methemoglobin (metHb) levels, supporting the role of NO, but not metHb, in the oxygen affinity of HbS. Here we examine the mechanism by which NO increases HbS oxygen affinity. Because anti-sickling agents increase sickle RBC oxygen affinity, we first determined whether NO exhibits anti-sickling properties. The viscosity of HbS hemolysates, measured by falling ball assays, increased upon deoxygenation; NO treatment reduced the increment. Multiphoton microscopic analyses showed smaller HbS polymers in deoxygenated sickle RBCs and HbS hemolysates exposed to NO. These results suggest that NO inhibits HbS polymer formation and has anti-sickling properties. Furthermore, we found that HbS treated with NO exhibits an isoelectric point similar to that of HbA, suggesting that NO alters the electric charge of HbS. NO-HbS adducts had the same elution time as HbA upon high performance liquid chromatography analysis. This study demonstrates that NO may disrupt HbS polymers by abolishing the excess positive charge of HbS, resulting in increased oxygen affinity.

Nitric oxide for inhalation in the acute treatment of sickle cell pain crisis: a randomized controlled trial

CONTEXT

Inhaled nitric oxide has shown evidence of efficacy in mouse models of sickle cell disease (SCD), case series of patients with acute chest syndrome, and 2 small placebo-controlled trials for treatment of vaso-occlusive pain crisis (VOC).

OBJECTIVE

To determine whether inhaled nitric oxide gas reduces the duration of painful crisis in patients with SCD who present to the emergency department or hospital for care.

DESIGN, SETTING, AND PARTICIPANTS

Prospective, multicenter, double-blind, randomized, placebo-controlled clinical trial for up to 72 hours of inhaled nitric oxide gas vs inhaled nitrogen placebo in 150 participants presenting with VOC of SCD at 11 centers between October 5, 2004, and December 22, 2008. Intervention Inhaled nitric oxide gas vs inhaled nitrogen placebo.

MAIN OUTCOME MEASURES

The primary end point was the time to resolution of painful crisis, defined by (1) freedom from parenteral opioid use for 5 hours; (2) pain relief as assessed by visual analog pain scale scores of 6 cm or lower (on 0-10 scale); (3) ability to walk; and (4) patient's and family's decision, with physician consensus, that the remaining pain could be managed at home.

RESULTS

There was no significant change in the primary end point between the nitric oxide and placebo groups, with a median time to resolution of crisis of 73.0 hours (95% confidence interval [CI], 46.0-91.0) and 65.5 hours (95% CI, 48.1-84.0), respectively (P = .87). There were no significant differences in secondary outcome measures, including length of hospitalization, visual analog pain scale scores, cumulative opioid usage, and rate of acute chest syndrome. Inhaled nitric oxide was well tolerated, with no increase in serious adverse events. Increases in venous methemoglobin concentration confirmed adherence and randomization but did not exceed 5% in any study participant. Significant increases in plasma nitrate occurred in the treatment group, but there were no observed increases in plasma or whole blood nitrite.

CONCLUSION

Among patients with SCD hospitalized with VOC, the use of inhaled nitric oxide compared with placebo did not improve time to crisis resolution.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00094887.

Dependence of Golgi apparatus integrity on nitric oxide in vascular cells: implications in pulmonary arterial hypertension

Although reduced bioavailability of nitric oxide (NO) has been implicated in the pathogenesis of pulmonary arterial hypertension (PAH), its consequences on organellar structure and function within vascular cells is largely unexplored. We investigated the effect of reduced NO on the structure of the Golgi apparatus as assayed by giantin or GM130 immunofluorescence in human pulmonary arterial endothelial (HPAECs) and smooth muscle (HPASMCs) cells, bovine PAECs, and human EA.hy926 endothelial cells. Golgi structure was also investigated in cells in tissue sections of pulmonary vascular lesions in idiopathic PAH (IPAH) and in macaques infected with a chimeric simian immunodeficiency virus containing the human immunodeficiency virus (HIV)-nef gene (SHIV-nef) with subcellular three-dimensional (3D) immunoimaging. Compounds with NO scavenging activity including 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), methylene blue, N-acetylcysteine, and hemoglobin markedly fragmented the Golgi in all cell types evaluated as did monocrotaline pyrrole, while LY-83583, sildenafil, fasudil, Y-27632, Tiron, Tempol, or H(2)O(2) did not. Golgi fragmentation by NO scavengers was inhibited by diethylamine NONOate, was evident in HPAECs after selective knockdown of endothelial nitric oxide synthase using small interfering RNA (siRNA), was independent of microtubule organization, required the GTPase dynamin 2, and was accompanied by depletion of α-soluble N-ethylmaleimide-sensitive factor (NSF) acceptor protein (α-SNAP) from Golgi membranes and codispersal of the SNAP receptor (SNARE) Vti1a with giantin. Golgi fragmentation was confirmed in endothelial and smooth muscle cells in pulmonary arterial lesions in IPAH and the SHIV-nef-infected macaque with subcellular 3D immunoimaging. In SHIV-nef-infected macaques Golgi fragmentation was observed in cells containing HIV-nef-bearing endosomes. The observed Golgi fragmentation suggests that NO plays a significant role in modulating global protein trafficking patterns that contribute to changes in the cell surface landscape and functional signaling in vascular cells.

Fast measurement of blood T1 in the human jugular vein at 3 Tesla

Current T(1) values for blood at 3T largely came from in vitro studies on animal blood or freshly drawn human blood. Measurement of blood T(1) in vivo could provide more specific information, e.g., for individuals with abnormal blood composition. Here, blood T(1) at 3T was measured rapidly (<1 min) in the internal jugular vein using a fast inversion-recovery technique in which multiple inversion time can be acquired rapidly due to constant refreshing of blood. Multishot EPI acquisition with flow compensation yielded high resolution images with minimum partial volume effect. Results showed T(1) = 1852 ± 104 msec among 24 healthy adults, a value higher than for bovine blood phantoms (1584 msec at Hct of 42%). A second finding was that of a significant difference (P < 0.01) between men and women, namely T(1) = 1780 ± 89 msec (n = 12) and T(1) = 1924 ± 58 msec (n = 12), respectively. This difference in normal subjects is tentatively explained by the difference in Hct between genders. Interestingly, however, studies done on sickle cell anemia patients with much lower Hct (23 ± 3%, n = 10) revealed similar venous blood T(1) = 1924 ± 82 msec, indicating other possible physical influences affecting blood T(1).

The Clinical Application of Ozonetherapy

The reader may be eager to examine in which diseases ozonetherapy can be proficiently used and she/he will be amazed by the versatility of this complementary approach (Table 9 1). The fact that the medical applications are numerous exposes the ozonetherapist to medical derision because superficial observers or sarcastic sceptics consider ozonetherapy as the modern panacea. This seems so because ozone, like oxygen, is a molecule able to act simultaneously on several blood components with different functions but, as we shall discuss, ozonetherapy is not a panacea. The ozone messengers ROS and LOPs can act either locally or systemically in practically all cells of an organism. In contrast to the dogma that “ozone is always toxic”, three decades of clinical experience, although mostly acquired in private clinics in millions of patients, have shown that ozone can act as a disinfectant, an oxygen donor, an immunomodulator, a paradoxical inducer of antioxidant enzymes, a metabolic enhancer, an inducer of endothelial nitric oxide synthase and possibly an activator of stem cells with consequent neovascularization and tissue reconstruction.

Hemoglobin, nitric oxide and molecular mechanisms of hypoxic vasodilation

The protected transport of nitric oxide (NO) by hemoglobin (Hb) links the metabolic activity of working tissue to the regulation of its local blood supply through hypoxic vasodilation. This physiologic mechanism is allosterically coupled to the O(2) saturation of Hb and involves the covalent binding of NO to a cysteine residue in the beta-chain of Hb (Cys beta93) to form S-nitrosohemoglobin (SNO-Hb). Subsequent S-transnitrosation, the transfer of NO groups to thiols on the RBC membrane and then in the plasma, preserves NO vasodilator activity for delivery to the vascular endothelium. This SNO-Hb paradigm provides insight into the respiratory cycle and a new therapeutic focus for diseases involving abnormal microcirculatory perfusion. In addition, the formation of S-nitrosothiols in other proteins may regulate an array of physiological functions.

Regulation of nitric oxide signalling by thrombospondin 1: implications for anti-angiogenic therapies

In addition to long-term regulation of angiogenesis, angiogenic growth factor signalling through nitric oxide (NO) acutely controls blood flow and haemostasis. Inhibition of this pathway may account for the hypertensive and pro-thrombotic side effects of the vascular endothelial growth factor antagonists that are currently used for cancer treatment. The first identified endogenous angiogenesis inhibitor, thrombospondin 1, also controls tissue perfusion, haemostasis and radiosensitivity by antagonizing NO signalling. We examine the role of these and other emerging activities of thrombospondin 1 in cancer. Clarifying how endogenous and therapeutic angiogenesis inhibitors regulate vascular NO signalling could facilitate development of more selective inhibitors.

Nitric oxide metabolism and the acute chest syndrome of sickle cell anemia

OBJECTIVE

To review the role of endothelial dysfunction and nitric oxide metabolism in the pathogenesis of the acute chest syndrome.

DATA SOURCE

A thorough literature search of PubMed for publications relevant to acute chest syndrome and nitric oxide metabolism in sickle cell disease was performed using search terms that included acute chest syndrome, sickle cell disease, nitric oxide metabolism, arginine, nitrite, nitrate, exhaled nitric oxide, nitric oxide synthase, and oxidant injury. We identified randomized controlled trials, case reports, editorials, and review articles from English-language and non-English-language studies of adult, pediatric, animal, and human subjects that describe the pathophysiology of acute chest syndrome, the biology of nitric oxide relevant to the pathophysiology of sickle cell disease, and the evidence for the role of endothelial dysfunction and abnormal nitric oxide metabolism in acute chest syndrome. We identified and reviewed 350 publications by the initial search and subsequent bibliography review. The articles most pertinent to the topic of this article were selected to support the discussion.

RESULTS

Acute chest syndrome is the leading cause of acute respiratory system dysfunction and a leading cause of morbidity and mortality among patients with sickle cell disease. Evidence is available to support decreased nitric oxide production, increased nitric oxide consumption, and abnormal metabolism of nitric oxide in patients with acute chest syndrome. Moreover, substrate availability is disturbed, and alternate pathways for substrate and nitric oxide metabolism exist.

CONCLUSIONS

Abnormalities of nitric oxide metabolism are prevalent during acute illness and baseline health in patients with sickle cell disease. Further investigation is needed to understand the clinical significance of aberrant nitric oxide metabolism as well as the potential for therapeutic manipulation of the arginine-nitric oxide pathway in patients with sickle cell disease.

Reductive gas-phase chemiluminescence and flow injection analysis for measurement of the nitric oxide pool in biological matrices

There is growing evidence for nitric oxide (NO.) being involved in cell signaling and pathology. Much effort has been made to elucidate and characterize the different biochemical reaction pathways of NO.in vivo. However, a major obstacle in assessing the significance of nitrosated species and oxidized metabolites often remains: a reliable analytical technique for the detection of NO. in complex biological matrices. This chapter presents refined methodologies, such as chemiluminescence detection and flow injection analysis, compared with adequate sample processing procedures to reliably quantify and assess the circulating and resident NO(.) pool, consisting of nitrite, nitrate, nitroso, and nitrosylated species.

Sickle cell disease: role of reactive oxygen and nitrogen metabolites

1. Sickle cell disease (SCD) is an inherited disorder of haemoglobin synthesis that is associated with significant morbidity and mortality due to sequelae of episodic vaso-occlusive events: pain crises and multiorgan damage. The microvascular responses to the initiation, progression and resolution of vaso-occlusive events are consistent with an inflammatory phenotype as suggested by activation of multiple cell types, an oxidatively stressed environment and endothelial cell dysfunction. 2. Decreased anti-oxidant defences in SCD patients and mice are accompanied by activation of enzymatic (NADPH oxidase, xanthine oxidase) and non-enzymatic (sickle haemoglobin auto-oxidation) sources of reactive oxygen species. The resultant oxidative stress leads to dysfunction/activation of arteriolar and venular endothelial cells, resulting in impaired vasomotor function and blood cell-endothelial cell adhesion. 3. Changes in substrate and cofactor availability for endothelial cell nitric oxide synthase may underlie reactive oxygen- and nitrogen-induced events that contribute to SCD-induced vasculopathy. 4. The emerging role of reactive oxygen and nitrogen species in the pathogenesis of SCD provides a platform for the development of novel agents to treat this painful and lethal disease.

S-Nitrosothiol measurements in biological systems

S-Nitrosothiol (SNO) cysteine modifications are regulated signaling reactions that dramatically affect, and are affected by, protein conformation. The lability of the SNO bond can make SNO-modified proteins cumbersome to measure accurately. Here, we review methodologies for detecting SNO modifications in biology. There are three caveats. (1) Many assays for biological SNOs are used near the limit of detection: standard curves must be in the biologically relevant concentration range. (2) The assays that are most reliable are those that modify SNO protein or peptide chemistry the least. (3) Each result should be quantitatively validated using more than one assay. Improved assays are needed and are in development.

The role of beta93 Cys in the inhibition of Hb S fiber formation

Recent studies have suggested that nitric oxide (NO) binding to hemoglobin (Hb) may lead to the inhibition of sickle cell fiber formation and the dissolution of sickle cell fibers. NO can react with Hb in at least 3 ways: 1) formation of Hb(II)NO, 2) formation of methemoglobin, and 3) formation of S-nitrosohemoglobin, through nitrosylation of the beta93 Cys residue. In this study, the role of beta93 Cys in the mechanism of sickle cell fiber inhibition is investigated through chemical modification with N-ethylmaleimide. UV resonance Raman, FT-IR and electrospray ionization mass spectroscopic methods in conjunction with equilibrium solubility and kinetic studies are used to characterize the effect of beta93 Cys modification on Hb S fiber formation. Both FT-IR spectroscopy and electrospray mass spectrometry results demonstrate that modification can occur at both the beta93 and alpha104 Cys residues under relatively mild reaction conditions. Equilibrium solubility measurements reveal that singly-modified Hb at the beta93 position leads to increased amounts of fiber formation relative to unmodified or doubly-modified Hb S. Kinetic studies confirm that modification of only the beta93 residue leads to a faster onset of polymerization. UV resonance Raman results indicate that modification of the alpha104 residue in addition to the beta93 residue significantly perturbs the alpha(1)beta(2) interface, while modification of only beta93 does not. These results in conjunction with the equilibrium solubility and kinetic measurements are suggestive that modification of the alpha104 Cys residue and not the beta93 Cys residue leads to T-state destabilization and inhibition of fiber formation. These findings have implications for understanding the mechanism of NO binding to Hb and NO inhibition of Hb S fiber formation.

Decrease in lung nitric oxide production after peritonitis in mice with sickle cell disease*

OBJECTIVE

Nitric oxide bioavailability may limit the occurrence or severity of acute vaso-occlusive episodes in patients with sickle cell disease. Because sepsis is frequently involved in the initiation of vaso-occlusive crisis and acute chest syndrome, we designed the present study in transgenic (SAD) sickle cell mice to investigate whether acute infectious peritonitis affects the enzymatic balance (nitric oxide synthases/arginases) that governs lung nitric oxide production.

DESIGN

Controlled animal study.

SETTING

Research laboratory of an academic institution.

SUBJECTS

Transgenic Hbbsingle/single SAD1 (SAD) mice and nontransgenic wild-type littermates (C57/Black mice, control group).

INTERVENTIONS

Cecal ligation and puncture-induced peritonitis.

MEASUREMENTS AND MAIN RESULTS

We found that 24 hrs after peritonitis, control littermate mice showed an increase in inducible and endothelial nitric oxide synthase messenger RNA and proteins, together with an increase in exhaled nitric oxide (shift of the balance toward nitric oxide synthesis). In contrast, SAD mice, which showed elevated inducible and endothelial nitric oxide synthase protein expression at baseline, showed a marked decrease in nitric oxide synthase proteins, lung nitric oxide end-products, and exhaled nitric oxide after peritonitis, reflecting a shift of the enzymatic balance toward inhibition of nitric oxide synthesis. Peritonitis increased messenger RNA levels of arginase I and arginase II in controls and SAD mice but with a greater increase in arginase I in SAD than in control mice. Peritonitis was associated with a higher mortality rate at 24 hrs in SAD mice. Inhalation of nitric oxide (40 ppm in air) abolished the mortality rate induced by acute peritonitis in SAD mice.

CONCLUSIONS

Acute peritonitis in SAD mice is associated with a defect in lung nitric oxide production and bioavailability that may participate in the acute systemic and lung vaso-occlusive complications of sickle cell disease.

Clinical indication for use and outcomes after inhaled nitric oxide therapy

BACKGROUND

Inhaled nitric oxide (iNO) use is widespread, but the long-term outcomes after therapy in adult patients remain unknown.

METHODS

All 376 patients receiving perioperative iNO (excluding pediatric and interventional cardiology procedures) at Columbia University Medical Center were prospectively followed from 2000 to 2003. Survival data were collected from chart review.

RESULTS

Inhaled nitric oxide was used to treat pulmonary and right ventricular failure in patients undergoing orthotopic heart transplantation (OHT, n = 67), orthotopic lung transplantation (n = 45), cardiac surgery (n = 105), and ventricular assist device placement (n = 66), and for hypoxemia in other surgery (n = 34) and medical patients (n = 59). Average follow-up was 2.9 +/- 1.0 years. Overall mortality was lowest when iNO was used after OHT (25.4%) and orthotopic lung transplantation (37.8%), intermediately after cardiac surgery (61%), ventricular assist device (62%), and other surgery patients (75%), and highest among medical patients (90%; all p < 0.005). The cost of iNO therapy was lower in transplantation versus medical patients, with a trend toward shorter duration of use. In multivariate analysis, respiratory failure and use in non-OHT were independent predictors of mortality (both p = 0.001). A risk score greater than 1 (score = non-OHT use 1, plus right ventricular failure 1) predicted a mortality of 76.5% versus 37.2% (p < 0.001).

CONCLUSIONS

Use of iNO for pulmonary hypertension in patients undergoing OHT and orthotopic lung transplantation was associated with a significantly lower overall mortality rate compared with its use after cardiac surgery or for hypoxemia in medical patients. Inhaled nitric oxide does not appear to be cost effective when treating hypoxemia in medical patients with high-risk scores and irreversible disease.

Extrapulmonary effects of inhaled nitric oxide: role of reversible S-nitrosylation of erythrocytic hemoglobin

Early applications of inhaled nitric oxide (iNO), typically in the treatment of diseases marked by acute pulmonary hypertension, were met by great enthusiasm regarding the purported specificity of iNO: vasodilation by iNO was specific to the lung (without a change in systemic vascular resistance), and within the lung, NO activity was said to be confined spatially and temporally by Hb within the vascular lumen. Underlying these claims were classical views of NO as a short-lived paracrine hormone that acts largely through the heme groups of soluble guanylate cyclase, and whose potential activity is terminated on encountering the hemes of red blood cell (RBC) Hb. These classical views are yielding to a broader paradigm, in which NO-related signaling is achieved through redox-related NO adducts that endow NO synthase products with the ability to act at a distance in space and time from NO synthase itself. Evidence supporting the biological importance of such stable NO adducts is probably strongest for S-nitrosothiols (SNOs), in which NO binds to critical cysteine residues in proteins or peptides. The circulating RBC is a major SNO reservoir, and RBC Hb releases SNO-related bioactivity peripherally on O2 desaturation. These new paradigms describing NO transport also provide a plausible mechanistic understanding of the increasingly recognized peripheral effects of inhaled NO. An explanation for the peripheral actions of inhaled NO is discussed here, and the rationale and results of attempts to exploit the "NO delivery" function of the RBC are reviewed.

Pathophysiology and treatment of stroke in sickle-cell disease: present and future

Sickle-cell anaemia is the most common cause of stroke in children, and stroke is one of the most devastating complications of sickle-cell disease. Overt strokes are typically due to large-artery vasculopathy affecting the intracranial internal carotid arteries and proximal middle cerebral arteries, whereas silent strokes typically occur in the territory of penetrating arteries. The sickled red blood cell can contribute to the pathogenesis of stroke via abnormal adherence to the vascular endothelium and by haemolysis, which results in endothelial cell activation, a hypercoaguable state, and alterations in vasomotor tone. Red-blood-cell transfusion, the most common preventive measure for stroke in sickle-cell disease, is associated with iron overload in chronic disease. Therefore, interventions directed towards the potential mechanisms that promote vasculopathy and occlusion in sickle-cell anaemia should be investigated. Here we review the epidemiology, clinical spectrum, and pathophysiology of stroke in sickle-cell disease to identify potential therapeutic targets.

Nitric oxide scavenging by red blood cells as a function of hematocrit and oxygenation

The reaction rate between nitric oxide and intraerythrocytic hemoglobin plays a major role in nitric oxide bioavailability and modulates homeostatic vascular function. It has previously been demonstrated that the encapsulation of hemoglobin in red blood cells restricts its ability to scavenge nitric oxide. This effect has been attributed to either factors intrinsic to the red blood cell such as a physical membrane barrier or factors external to the red blood cell such as the formation of an unstirred layer around the cell. We have performed measurements of the uptake rate of nitric oxide by red blood cells under oxygenated and deoxygenated conditions at different hematocrit percentages. Our studies include stopped-flow measurements where both the unstirred layer and physical barrier potentially participate, as well as competition experiments where the potential contribution of the unstirred layer is limited. We find that deoxygenated erythrocytes scavenge nitric oxide faster than oxygenated cells and that the rate of nitric oxide scavenging for oxygenated red blood cells increases as the hematocrit is raised from 15% to 50%. Our results 1) confirm the critical biological phenomenon that hemoglobin compartmentalization within the erythrocyte reduces reaction rates with nitric oxide, 2) show that extra-erythocytic diffusional barriers mediate most of this effect, and 3) provide novel evidence that an oxygen-dependent intrinsic property of the red blood cell contributes to this barrier activity, albeit to a lesser extent. These observations may have important physiological implications within the microvasculature and for pathophysiological disruption of nitric oxide homeostasis in diseases.

Detection of human red blood cell-bound nitric oxide

Major disparities in reported levels of basal human nitric oxide metabolites have resulted in a recent literature focusing almost exclusively on methods. We chose to analyze triiodide chemiluminescence, drawn by the prospect of identifying why the most commonly employed assay in nitric oxide biology typically yielded lower metabolite values, compared with several other techniques. We found that the sensitivity of triiodide was greatly affected by the auto-capture of nitric oxide by deoxygenated cell-free heme in the reaction chamber. Potential contaminants and signal losses were also associated with standard sample purification procedures and the chemistry involved in nitrite removal. To inhibit heme nitric oxide auto-capture, we added potassium ferricyanide to the triiodide reagent, reasoning this would provide a more complete detection of any liberated nitric oxide. From human venous blood samples, we established nitric oxide levels ranging from 0.000178 to 0.00024 mol nitric oxide/mol hemoglobin. We went on to find significantly elevated nitric oxide levels in venous blood taken from diabetic patients in comparison to healthy controls (p < 0.0001). We concluded that the lack of signals reported of late by several groups using triiodide chemiluminescence for the detection of hemoglobin-bound nitric oxide may not represent levels on the border of assay sensitivity but rather underestimated values because of methodological limitations. We therefore stress the need for assay systems to be developed that differentiate between individual nitric oxide metabolite species and overcome the limitations we outline, allowing accurate conclusions to be drawn regarding physiological nitric oxide metabolite levels.

Chronic sickle cell lung disease: new insights into the diagnosis, pathogenesis and treatment of pulmonary hypertension

Pulmonary hypertension is a common complication of sickle cell disease (SCD). In spite of the mild elevations in pulmonary artery pressures in these patients, the associated morbidity and mortality is high. In fact, in adult patients with SCD, pulmonary hypertension is emerging as the major independent risk factor for death. The aetiology of pulmonary hypertension is probably multifactorial, including haemolysis, impaired nitric oxide bioavailability, chronic hypoxaemia, thromboembolism, parenchymal and vascular injury because of sequestration of sickle erythrocytes, chronic liver disease and asplenia. Interestingly, pulmonary hypertension is emerging as a common, and probably, invariant sequella of lifelong haemolytic anaemia in other hereditary and acquired haemolytic diseases, such as thalassaemia, stomatocytosis and spherocytosis. There are currently limited specific data on the effects of any treatment modality for pulmonary hypertension in patients with SCD. It is likely that maximization of SCD therapy, in all patients, and treatment with selective pulmonary vasodilators and antiproliferative agents, in patients with severe disease, would be beneficial. A large trial evaluating the effects of therapy for pulmonary hypertension in the SCD population is clearly indicated.

S-Nitrosohemoglobin: an allosteric mediator of NO group function in mammalian vasculature

Since the discovery of NO as the endothelium-derived relaxing factor, there has been considerable interest in how NO interacts with hemoglobin (Hb). Numerous investigations have highlighted the possibility that rather than operating as a sink to consume NO, the vasculature can operate as a delivery mechanism for NO. The principal hypothesis proposed to explain this phenomenon is that Hb can transport NO on the conserved cysteine residue beta93 and deliver that NO to the tissues in an allosterically dependent manner. This proposal has been termed the S-Nitrosohemoglobin (SNO-Hb) Hypothesis. This review addresses the experimental evidence that led to development of this hypothesis and examines much of the research that resulted from its generation. Specifically it covers the evidence concerning NO in the vasculature, the SNO-Hb Hypothesis itself, the biochemical and biophysical data relating to NO and Hb interactions, SNO-Hb in human physiology, and alternative vascular forms of NO. Finally a model of NO in the vasculature in which SNO-Hb forms the central core is proposed.

The use of nitric oxide in neonatal care

Knowledge of NO and its role in the human body currently is limited. Further scientific research involving this unique molecule will expand its clinical usefulness. It is an exciting era in research,involving numerous body processes and systems. The initial work on pulmonary vascular response in newborns who have PPHN has opened the door to seemingly endless possibilities involving many aspects of health.

Hypoxia-induced acute lung injury in murine models of sickle cell disease

Vaso-occlusive events are the major source of morbidity and mortality in sickle cell disease (SCD); however, the pathogenic mechanisms driving these events remain unclear. Using hypoxia to induce pulmonary injury, we investigated mechanisms by which sickle hemoglobin increases susceptibility to lung injury in a murine model of SCD, where mice either exclusively express the human alpha/sickle beta-globin (halphabetaS) transgene (SCD mice) or are heterozygous for the normal murine beta-globin gene and express the halphabetaS transgene (mbeta+/-, halphabetaS+/-; heterozygote SCD mice). Under normoxia, lungs from the SCD mice contained higher levels of xanthine oxidase (XO), nitrotyrosine, and cGMP than controls (C57BL/6 mice). Hypoxia increased XO and nitrotyrosine and decreased cGMP content in the lungs of all mice. After hypoxia, vascular congestion was increased in lungs with a greater content of XO and nitrotyrosine. Under normoxia, the association of heat shock protein 90 (HSP90) with endothelial nitric oxide synthase (eNOS) in lungs of SCD and heterozygote SCD mice was decreased compared with the levels of association in lungs of controls. Hypoxia further decreased association of HSP90 with eNOS in lungs of SCD and heterozygote SCD mice, but not in the control lungs. Pretreatment of rat pulmonary microvascular endothelial cells in vitro with xanthine/XO decreased A-23187-stimulated nitrite + nitrate production and HSP90 interactions with eNOS. These data support the hypotheses that hypoxia increases XO release from ischemic tissues and that the local increase in XO-induced oxidative stress can then inhibit HSP90 interactions with eNOS, decreasing *NO generation and predisposing the lung to vaso-occlusion.

Vasorelaxation by red blood cells and impairment in diabetes: reduced nitric oxide and oxygen delivery by glycated hemoglobin

Vascular dysfunction in diabetes is attributed to lack of bioavailable nitric oxide (NO) and is postulated as a primary cause of small vessel complications as a result of poor glycemic control. Although it has been proposed that NO is bound by red blood cells (RBCs) and can induce relaxation of blood vessels distal to its site of production in the normal circulation, the effect of RBC glycation on NO binding and relaxation of hypoxic vessels is unknown. We confirm RBC-induced vessel relaxation is inversely related to tissue oxygenation and is proportional to RBC S-nitrosohemoglobin (HbSNO) content (but not nitrosylhemoglobin content). We show more total NO bound inside highly glycated RBCs (0.0134 versus 0.0119 NO/Hb, respectively; P<0.05) although proportionally less HbSNO (0.0053 versus 0.0088 NO/Hb, respectively; P<0.05). We also show glycosylation impairs the vasodilator function of RBCs within a physiological range of tissue oxygenation. These findings may represent an important contribution to reduced NO bioavailability in the microvasculature in diabetes.

Imaging hemoglobin oxygen saturation in sickle cell disease patients using noninvasive visible reflectance hyperspectral techniques: effects of nitric oxide

Sickle cell disease is characterized by microvascular occlusion and hemolytic anemia, factors that impair tissue oxygen delivery. We use visible reflectance hyperspectral imaging to quantitate skin tissue hemoglobin oxygen saturation (HbO2) and to determine whether changes in blood flow during nitric oxide (NO) stimulation or gas administration (therapies proposed for this disease) improve skin tissue oxygen saturation in five patients with sickle cell disease. Compared with six healthy African-American subjects, sickle cell patients exhibited higher forearm blood flows (7.4 +/- 1.8 vs. 3.2 +/- 0.4 ml.min-1.100 ml tissue-1, P = 0.037) but significantly reduced percentages of skin HbO2 (61.0 +/- 0.2 vs. 77.5 +/- 0.2%, P < 0.001). Administration of acetylcholine to patients increased blood flow by 15.1 +/- 3.8 ml.min-1.100 ml tissue-1 and the percentage of skin HbO2 by 4.1 +/- 0.3% (P = 0.02, P < 0.001, respectively, from baseline values). Sodium nitroprusside, a direct NO donor, increased blood flow by 3.9 +/- 1.1 ml/min and the percentage of skin HbO2 by 2.9 +/- 0.3% (P = 0.02, P < 0.001, respectively). NO inhalation had no effect on forearm blood flow, yet increased the percentage of skin HbO2 by 2.3 +/- 0.3% (P < 0.001). Percentages of skin HbO2 were exponentially related to blood flow (R = 0.97, P < 0.001), indicating a limit to skin tissue oxygen saturation at high blood flows. Thus, for acetylcholine infusion leading to blood flows sevenfold greater than those of healthy resting African-American subjects, patients still exhibited lower percentages of skin HbO2 (65.2 +/- 0.2 vs. 77.5 +/- 0.2%, P < 0.001). Visible reflectance hyperspectral imaging demonstrates that either the stimulation or the administration of NO pharmacologically or by gas inhalation improves, but does not normalize, skin tissue oxygen saturation in patients with sickle cell disease.

The physiology of S-nitrosothiols: carrier molecules for nitric oxide

Recent work has demonstrated that inhalation of nitric oxide (NO) can impact the peripheral vasculature, suggesting that an NO-stabilizing moiety may exist in vivo. One possibility is the formation of S-nitrosothiols, which extend the half-life of NO manyfold. In this review, we provide evidence that S-nitrosothiols exist in the vasculature, particularly during NO inhalation. The potential biochemical pathways that have been proposed for the formation of these products are also summarized. Finally, we highlight the limited evidence for the role that these potent vasodilating molecules may play as physiologically and therapeutically important regulators of the vascular system.