Nitrite as regulator of hypoxic signaling in mammalian physiology

Red Blood Cells and Hemoglobin in Human Atherosclerosis and Related Arterial Diseases

As the main particulate component of the circulating blood, RBCs play major roles in physiological hemodynamics and impact all arterial wall pathologies. RBCs are the main determinant of blood viscosity, defining the frictional forces exerted by the blood on the arterial wall. This function is used in phylogeny and ontogeny of the cardiovascular (CV) system, allowing the acquisition of vasomotricity adapted to local metabolic demands, and systemic arterial pressure after birth. In pathology, RBCs collide with the arterial wall, inducing both local retention of their membranous lipids and local hemolysis, releasing heme-Fe⁺⁺ with a high toxicity for arterial cells: endothelial and smooth muscle cells (SMCs) cardiomyocytes, neurons, etc. Specifically, overloading of cells by Fe⁺⁺ promotes cell death. This local hemolysis is an event associated with early and advanced stages of human atherosclerosis. Similarly, the permanent renewal of mural RBC clotting is the major support of oxidation in abdominal aortic aneurysm. In parallel, calcifications promote intramural hemorrhages, and hemorrhages promote an osteoblastic phenotypic shift of arterial wall cells. Different plasma or tissue systems are able, at least in part, to limit this injury by acting at the different levels of this system.

Effects of Dietary Nitrates on Time Trial Performance in Athletes with Different Training Status: Systematic Review

Much research has been done in sports nutrition in recent years as the demand for performance-enhancing substances increases. Higher intake of nitrates from the diet can increase the bioavailability of nitric oxide (NO) via the nitrate-nitrite-NO pathway. Nevertheless, the increased availability of NO does not always lead to improved performance in some individuals. This review aims to evaluate the relationship between the athlete's training status and the change in time trial performance after increased dietary nitrate intake. Articles indexed by Scopus and PubMed published from 2015 to 2019 were reviewed. Thirteen articles met the eligibility criteria: clinical trial studies on healthy participants with different training status (according to VO_2max), conducting time trial tests after dietary nitrate supplementation. The PRISMA guidelines were followed to process the review. We found a statistically significant relationship between VO_2max and ergogenicity in time trial performance using one-way ANOVA (p = 0.001) in less-trained athletes (VO₂ < 55 mL/kg/min). A strong positive correlation was observed in experimental situations using a chronic supplementation protocol but not in acute protocol situations. In the context of our results and recent histological observations of muscle fibres, there might be a fibre-type specific role in nitric oxide production and, therefore, supplement of ergogenicity.

Tanshinone IIA pretreatment promotes cell survival in human lung epithelial cells under hypoxia via AP-1-Nrf2 transcription factor

Activator protein-1 (AP-1) plays a decisive role in cell proliferation, apoptosis, and inflammation under hypoxia; thus, AP-1 subunits or dimers could be modulated for a desired phenomenon in a cell using a suitable compound of therapeutic potential. Herein, we used Tanshinone-IIA as an AP-1 (subunits) modulator, and the purpose of the study was to investigate the signaling mechanism exhibited by Tan-IIA in facilitating tolerance to hypoxia. A549 cells were pretreated with Tan-IIA and exposed to hypoxia for 6, 12, 24, and 48 h. Biochemical and molecular parameters were assessed in order to trace the signaling pathway. Tan-IIA attenuated hypoxia-induced oxidative stress by modulating the expression of AP-1 subunits (via. MAPK) and Nrf2 transcription factor, which in turn were responsible for maintaining the higher levels of antioxidant enzymes and genes (HO). Tan-IIA increased the cell survival. This could be attributed to an increased NO level via iNOS gene and activated JNK, ERK pathway that induced c-jun/c-fos, c-jun/fosB, junD/c-fos, and junD/fosB heterodimers. This in turn leads to the cell cycle progression by activating cyclins (D and B). This was further confirmed by the lower levels of p53 and their downstream genes (p16, p21, p27). In addition, Tan-IIA decreased pro-inflammatory cytokine levels by inhibiting the formation of junB/fra-1 heterodimer regulated by p38. Tan-IIA increased cell survival to hypoxia by maintaining the higher levels of cellular iNOS, HO-1, jun-D, c-jun, fos B via Nrf2-AP-1.

The Double-Faced Role of Nitric Oxide and Reactive Oxygen Species in Solid Tumors

Disturbed redox homeostasis represents a hallmark of cancer phenotypes, affecting cellular metabolism and redox signaling. Since reactive oxygen and nitrogen species (ROS/RNS) are involved in regulation of proliferation and apoptosis, they may play a double-faced role in cancer, entailing protumorigenic and tumor-suppressing effects in early and later stages, respectively. In addition, ROS and RNS impact the activity and communication of all tumor constituents, mediating their reprogramming from anti- to protumorigenic phenotypes, and vice versa. An important role in this dichotomic action is played by the variable amounts of O₂ in the tumor microenvironment, which dictates the ultimate outcome of the influence of ROS/RNS on carcinogenesis. Moreover, ROS/RNS levels remarkably influence the cancer response to therapy. The relevance of ROS/RNS signaling in solid tumors is witnessed by the emergence of novel targeted treatments of solid tumors with compounds that target ROS/RNS action and production, such as tyrosine kinase inhibitors and monoclonal antibodies, which might contribute to the complexity of redox regulation in cancer. Prospectively, the dual role of ROS/RNS in the different stages of tumorigenesis through different impact on oxidation and nitrosylation may also allow development of tailored diagnostic and therapeutic approaches.

Influence of muscle oxygenation and nitrate-rich beetroot juice supplementation on O2 uptake kinetics and exercise tolerance

We tested the hypothesis that acute supplementation with nitrate (NO₃^-)-rich beetroot juice (BR) would improve quadriceps muscle oxygenation, pulmonary oxygen uptake (V˙O₂) kinetics and exercise tolerance (T_lim) in normoxia and that these improvements would be augmented in hypoxia and attenuated in hyperoxia. In a randomised, double-blind, cross-over study, ten healthy males completed two-step cycle tests to T_lim following acute consumption of 210 mL BR (18.6 mmol NO₃^-) or NO₃^--depleted beetroot juice placebo (PL; 0.12 mmol NO₃^-). These tests were completed in normobaric normoxia [fraction of inspired oxygen (FIO₂): 21%], hypoxia (FIO₂: 15%) and hyperoxia (FIO₂: 40%). Pulmonary V˙O₂ and quadriceps tissue oxygenation index (TOI), derived from multi-channel near-infrared spectroscopy, were measured during all trials. Plasma [nitrite] was higher in all BR compared to all PL trials (P < 0.05). Quadriceps TOI was higher in normoxia compared to hypoxia (P < 0.05) and higher in hyperoxia compared to hypoxia and normoxia (P < 0.05). T_lim was improved after BR compared to PL ingestion in the hypoxic trials (250 ± 44 vs. 231 ± 41 s; P = 0.006; d = 1.13), with the magnitude of improvement being negatively correlated with quadriceps TOI at T_lim (r = -0.78; P < 0.05). T_lim was not improved following BR ingestion in normoxia (BR: 364 ± 98 vs. PL: 344 ± 78 s; P = 0.087, d = 0.61) or hyperoxia (BR: 492 ± 212 vs. PL: 472 ± 196 s; P = 0.273, d = 0.37). BR ingestion increased peak V˙O₂ in hypoxia (P < 0.05), but not normoxia or hyperoxia (P > 0.05). These findings indicate that BR supplementation is more likely to improve T_lim and peak V˙O₂ in situations when skeletal muscle is more hypoxic.

Nitrates/Nitrites in Food-Risk for Nitrosative Stress and Benefits

In the context of impact on human health, nitrite/nitrate and related nitrogen species such as nitric oxide (NO) are a matter of increasing scientific controversy. An increase in the content of reactive nitrogen species may result in nitrosative stress-a deleterious process, which can be an important mediator of damage to cell structures, including lipids, membranes, proteins and DNA. Nitrates and nitrites are widespread in the environment and occur naturally in foods of plant origin as a part of the nitrogen cycle. Additionally, these compounds are used as additives to improve food quality and protect against microbial contamination and chemical changes. Some vegetables such as raw spinach, beets, celery and lettuce are considered to contain high concentrations of nitrates. Due to the high consumption of vegetables, they have been identified as the primary source of nitrates in the human diet. Processed meats are another source of nitrites in our diet because the meat industry uses nitrates/nitrites as additives in the meat curing process. Although the vast majority of consumed nitrates and nitrites come from natural vegetables and fruits rather than food additives, there is currently a great deal of consumer pressure for the production of meat products free of or with reduced quantities of these compounds. This is because, for years, the cancer risks of nitrates/nitrites have been considered, since they potentially convert into the nitrosamines that have carcinogenic effects. This has resulted in the development and rapid expansion of meat products processed with plant-derived nitrates as nitrite alternatives in meat products. On the other hand, recently, these two ions have been discussed as essential nutrients which allow nitric oxide production and thus help cardiovascular health. Thus, this manuscript reviews the main sources of dietary exposure to nitrates and nitrites, metabolism of nitrites/nitrates, and health concerns related to dietary nitrites/nitrates, with particular emphasis on the effect on nitrosative stress, the role of nitrites/nitrates in meat products and alternatives to these additives used in meat products.

Mechanisms of neuroinflammation and inflammatory mediators involved in brain injury following subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a devastating cerebrovascular disorder. Neuroinflammation is a critical cause of brain injury following SAH in both acute and chronic phases. While accumulating evidence has shown that therapies targeting neuroinflammation exerted beneficial effects in experimental SAH, there is little clinical evidence. One of the factors making neuroinflammation complicated is that inflammatory signaling pathways and mediators act as protective or detrimental responses at different phases. In addition, biomarkers to detect neuroinflammation are little known in clinical settings. In this review, first, we discuss how the inflammatory signaling pathways contribute to brain injury and other secondary pathophysiological changes in SAH. Damage-associated molecular patterns arising from mechanical stress, transient global cerebral ischemia, red blood cell breakdown and delayed cerebral ischemia following SAH trigger to activate pattern recognition receptors (PRRs) such as Toll-like receptors, nucleotide-binding oligomerization domain-like receptors, and receptors for advanced glycation end products. Most of PRRs activate common downstream signaling transcriptional factor nuclear factor-κΒ and mitogen-activated protein kinases, releasing pro-inflammatory mediators and cytokines. Next, we focus on how pro-inflammatory substances play a role during the course of SAH. Finally, we highlight an important inducer of neuroinflammation, matricellular protein (MCP). MCPs are a component of extracellular matrix and exert beneficial and harmful effects through binding to receptors, other matrix proteins, growth factors, and cytokines. Treatment targeting MCPs is being proved efficacious in pre-clinical models for preventing brain injury including neuroinflammation in SAH. In addition, MCPs may be a candidate of biomarkers predicting brain injury following SAH in clinical settings.

Tyrosine nitration as a key event of signal transduction that regulates functional state of the cell

This review is dedicated to the role of nitration of proteins by tyrosine residues in physiological and pathological conditions. First of all, we analyze the biochemical evidence of peroxynitrite formation and reactions that lead to its formation, types of posttranslational modifications (PTMs) induced by reactive nitrogen species, as well as three biological pathways of tyrosine nitration. Then, we describe two possible mechanisms of protein nitration that are involved in intracellular signal transduction, as well as its interconnection with phosphorylation/dephosphorylation of tyrosine. Next part of the review is dedicated to the role of proteins nitration in different pathological conditions. In this section, special attention is devoted to the role of nitration in changes of functional properties of actin-protein that undergoes PTMs both in normal and pathological conditions. Overall, this review is devoted to the main features of protein nitration by tyrosine residue and the role of this process in intracellular signal transduction in basal and pathological conditions.

Revisiting the nitrite reductase activity of hemoglobin with differential pulse voltammetry

Nitric oxide (NO) is an omnipresent signalling molecule in all vertebrates. NO modulates blood flow and neural activity. Nitrite anion is one of the most important sources of NO. Nitrite is reduced to NO by various physiological mechanisms including reduction by hemoglobin in vascular system. In this study, nitrite reductase activity (NRA) of hemoglobin is reported using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in a wide potential window from +0.3 V to -1.3 V (vs. Ag/AgCl). To the best of our knowledge, a detailed look into NRA of hemoglobin is proposed here for the first time. Our results indicated two different regimes for reduction of nitrite by hemoglobin in its Fe(II) and Fe(I) states. Both reactions showed a reversible behaviour in the time scale of the experiments. The first reduction displayed a normal redox behaviour, while the latter one had the characteristics of a catalytic electro-reduction/oxidation. The reduction in Fe(II) state was selected as a tool for comparing the NRA of hemoglobin (Hb) and hemoglobin-S (Hb-S) under native-like conditions in a didodecyldimethyl ammonium bromide (DDAB) liquid crystal film. These investigations lay the prospects and guidelines for understanding the direct electrochemistry of hemoglobin utilizing a simplified mediator-free platform.

Physiological Effects of Beetroot in Athletes and Patients

Dietary supplementation with beetroot juice (BRJ), a naturally rich source of nitrate, is an area of considerable interest to elite athletes as well as recreational exercisers. Nitrate and nitrite have previously been thought of as mainly final elimination products of nitric oxide (NO), but this view has been challenged and evidence indicates that these compounds can be converted to NO in vivo. We conducted a narrative review summarizing the literature regarding evidence of beetroot used as dietary supplement and its effects on training physiology and athletic performance in healthy and diseased populations. The databases PubMed and Web of Science were used to obtain articles. It was evident that BRJ supplementation had an effect on oxygen cost and consumption during exercise by more efficient adenosine triphosphate (ATP) production in combination with lower ATP consumption. However, the effect seems to be dependent on dose and duration. Effect on exercise performance is conflicting, time to exhaustion seems to increase but its effect on time-trial performance needs further elucidation. Ergogenic benefits might depend on individual aerobic fitness level, where individuals with lower fitness level may gain higher benefits regarding athletic performance. Dietary nitrate supplementation appears to have some effect on training performance in patients with peripheral artery disease, heart failure, and chronic pulmonary obstructive disease. However, larger randomized controlled trials are necessary to determine the overall utility of beetroot as a dietary supplement.

The obligatory role of host microbiota in bioactivation of dietary nitrate

Nitric oxide (NO) is a key signalling molecule in the regulation of cardiometabolic function and impaired bioactivity is considered to play an important role in the onset and progression of cardiovascular and metabolic disease. Research has revealed an alternative NO-generating pathway, independent of NO synthase (NOS), in which the inorganic anions nitrate (NO₃^-) and nitrite (NO₂^-) are serially reduced to form NO. This work specifically aimed at investigating the role of commensal bacteria in bioactivation of dietary nitrate and its protective effects in a model of cardiovascular and metabolic disease. In a two-hit model, germ-free and conventional male mice were fed a western diet and the NOS inhibitor l-NAME in combination with sodium nitrate (NaNO₃) or placebo (NaCl) in the drinking water. Cardiometabolic parameters including blood pressure, glucose tolerance and body composition were measured after six weeks treatment. Mice in both placebo groups showed increased body weight and fat mass, reduced lean mass, impaired glucose tolerance and elevated blood pressure. In conventional mice, nitrate treatment partly prevented the cardiometabolic disturbances induced by a western diet and l-NAME. In contrast, in germ-free mice nitrate had no such beneficial effects. In separate cardiovascular experiments, using conventional and germ-free animals, we assessed NO-like signalling downstream of nitrate by administration of sodium nitrite (NaNO₂) via gavage. In this acute experimental setting, nitrite lowered blood pressure to a similar degree in both groups. Likewise, isolated vessels from germ-free mice robustly dilated in response to the NO donor sodium nitroprusside. In conclusion, our findings demonstrate the obligatory role of host-microbiota in bioactivation of dietary nitrate, thus contributing to its favourable cardiometabolic effects.

Therapeutic Implications of Nitrite in Hypertension

Nitrite, an anion produced from the oxidative breakdown of nitric oxide (NO), has traditionally been viewed as an inert molecule. However, this dogma has been challenged with the findings that nitrite can be readily reduced to NO under pathological conditions, hence representing a physiologically relevant storage reservoir of NO either in the blood or tissues. Nitrite administration has been demonstrated to improve myocardial function in subjects with heart failure and to lower the blood pressure in hypertensive subjects. Thus, extensive amount of work has since been carried out to investigate the therapeutic potential of nitrite in treating cardiovascular diseases, especially hypertension. Studies done on several animal models of hypertension have demonstrated the efficacy of nitrite in preventing and ameliorating the pathological changes associated with the disease. This brief review of the current findings aims to re-evaluate the use of nitrite for the treatment of hypertension and in particular to highlight its role in improving endothelial function.

Role of Heme Oxygenase as a Modulator of Heme-Mediated Pathways

The heme oxygenase (HO) system is essential for heme and iron homeostasis and necessary for adaptation to cell stress. HO degrades heme to biliverdin (BV), carbon monoxide (CO) and ferrous iron. Although mostly beneficial, the HO reaction can also produce deleterious effects, predominantly attributed to excessive product formation. Underrated so far is, however, that HO may exert effects additionally via modulation of the cellular heme levels. Heme, besides being an often-quoted generator of oxidative stress, plays also an important role as a signaling molecule. Heme controls the anti-oxidative defense, circadian rhythms, activity of ion channels, glucose utilization, erythropoiesis, and macrophage function. This broad spectrum of effects depends on its interaction with proteins ranging from transcription factors to enzymes. In degrading heme, HO has the potential to exert effects also via modulation of heme-mediated pathways. In this review, we will discuss the multitude of pathways regulated by heme to enlarge the view on HO and its role in cell physiology. We will further highlight the contribution of HO to pathophysiology, which results from a dysregulated balance between heme and the degradation products formed by HO.

Effects of inorganic nitrate in a rat model of monocrotaline-induced pulmonary arterial hypertension

The nitrate-nitrite-nitric oxide (NO) pathway represents an alternative source of NO generation, which is independent of NO synthase and potentiated by hypoxia. Augmentation of this pathway by dietary nitrate has proven favourable effects in several cardiovascular disease models. However, less is known regarding its potential value in pulmonary arterial hypertension (PAH). The aim of this study was to assess the effects of oral inorganic nitrate administration in monocrotaline (MCT)-induced PAH. Male 12-week-old Wistar rats were injected subcutaneously with monocrotaline (MCT, 60 mg/kg). Nitrate treatment (0.3 or 1 mmol/kg/d; drinking water) commenced on day 12 following the MCT injection and continued for 16 days. Nitrate administration did not attenuate right ventricular (RV) hypertrophy, increased lung weight and up-regulated mRNA expression of brain natriuretic peptide. Plasma nitrate and nitrite levels were significantly increased as well as lung nitrate level, whereas nitrite lung level was decreased following nitrate treatment (1 mmol/kg/d). MCT-induced PAH resulted in an increased MnSOD protein level, which was not observed following nitrate treatment. MCT-associated up-regulation of nNOS in the lung appeared to be dose-dependently prevented by nitrate treatment. Western blot analysis did not reveal any differences in eNOS, iNOS, XO or gp91phox expression in the lungs among the groups. In conclusion, nitrate treatment did not significantly attenuate pathological RV and lung remodelling in the rat MCT model of PAH. The suppression of MnSOD and nNOS expression by nitrate could be interpreted as reduced demand of endogenous antioxidant defence in this model.

Human skeletal muscle nitrate store: influence of dietary nitrate supplementation and exercise

Rodent skeletal muscle contains a large store of nitrate that can be augmented by the consumption of dietary nitrate. This muscle nitrate reservoir has been found to be an important source of nitrite and nitric oxide (NO) via its reduction by tissue xanthine oxidoreductase. To explore if this pathway is also active in human skeletal muscle during exercise, and if it is sensitive to local nitrate availability, we assessed exercise-induced changes in muscle nitrate and nitrite concentrations in young healthy humans, under baseline conditions and following dietary nitrate consumption. We found that baseline nitrate and nitrite concentrations were far higher in muscle than in plasma (~4-fold and ~29-fold, respectively), and that the consumption of a single bolus of dietary nitrate (12.8 mmol) significantly elevated nitrate concentration in both plasma (~19-fold) and muscle (~5-fold). Consistent with these observations, and with previous suggestions of active muscle nitrate transport, we present western blot data to show significant expression of the active nitrate/nitrite transporter sialin in human skeletal muscle. Furthermore, we report an exercise-induced reduction in human muscle nitrate concentration (by ~39%), but only in the presence of an increased muscle nitrate store. Our results indicate that human skeletal muscle nitrate stores are sensitive to dietary nitrate intake and may contribute to NO generation during exercise. Together, these findings suggest that skeletal muscle plays an important role in the transport, storage and metabolism of nitrate in humans.

The Effects of Oral l-Arginine and l-Citrulline Supplementation on Blood Pressure

Nitric oxide (NO) is a well-known vasodilator produced by the vascular endothelium via the enzyme endothelial nitric oxide synthase (eNOS). The inadequate production of NO has been linked to elevated blood pressure (BP) in both human and animal studies, and might be due to substrate inaccessibility. This review aimed to investigate whether oral administration of the amino acids l-arginine (Arg) and l-citrulline (Cit), which are potential substrates for eNOS, could effectively reduce BP by increasing NO production. Both Arg and Cit are effective at increasing plasma Arg. Cit is approximately twice as potent, which is most likely due to a lower first-pass metabolism. The current data suggest that oral Arg supplementation can lower BP by 5.39/2.66 mmHg, which is an effect that is comparable with diet changes and exercise implementation. The antihypertensive properties of Cit are more questionable, but are likely in the range of 4.1/2.08 to 7.54/3.77 mmHg. The exact mechanism by which Cit and Arg exert their effect is not fully understood, as normal plasma Arg concentration greatly exceeds the Michaelis constant (K_m) of eNOS. Thus, elevated plasma Arg concentrations would not be expected to increase endogenous NO production significantly, but have nonetheless been observed in other studies. This phenomenon is known as the "l-arginine paradox".

Inhaled nebulized nitrite and nitrate therapy in a canine model of hypoxia-induced pulmonary hypertension

Dysfunction in the nitric oxide (NO) signaling pathway can lead to the development of pulmonary hypertension (PH) in mammals. Discovery of an alternative pathway to NO generation involving reduction from nitrate to nitrite and to NO has motivated the evaluation of nitrite as an alternative to inhaled NO for PH. In contrast, inhaled nitrate has not been evaluated to date, and potential benefits include a prolonged half-life and decreased risk of methemoglobinemia. In a canine model of acute hypoxia-induced PH we evaluated the effects of inhaled nitrate to reduce pulmonary arterial pressure (PAP). In a randomized controlled trial, inhaled nitrate was compared to inhaled nitrite and inhaled saline. Exhaled NO, PAP and systemic blood pressures were continuously monitored. Inhaled nitrite significantly decreased PAP and increased exhaled NO. In contrast, inhaled nitrate and inhaled saline did not decrease PAP or increase exhaled NO. Unexpectedly, we found that inhaled nitrite resulted in prolonged (>5 h) exhaled NO release, increase in nitrate venous/arterial levels and a late surge in venous nitrite levels. These findings do not support a therapeutic role for inhaled nitrate in PH but may have therapeutic implications for inhaled nitrite in various disease states.

Dysregulation of stress systems and nitric oxide signaling underlies neuronal dysfunction in Alzheimer's disease

Stress is a multimodal response involving the coordination of numerous body systems in order to maximize the chance of survival. However, long term activation of the stress response results in neuronal oxidative stress via reactive oxygen and nitrogen species generation, contributing to the development of depression. Stress-induced depression shares a high comorbidity with other neurological conditions including Alzheimer's disease (AD) and dementia, often appearing as one of the earliest observable symptoms in these diseases. Furthermore, stress and/or depression appear to exacerbate cognitive impairment in the context of AD associated with dysfunctional catecholaminergic signaling. Given there are a number of homologous pathways involved in the pathophysiology of depression and AD, this article will highlight the mechanisms by which stress-induced perturbations in oxidative stress, and particularly NO signaling, contribute to neurodegeneration.

Hypoxically Induced Nitric Oxide: Potential Role as a Vasodilator in Mytilus edulis Gills

Intertidal Mytilus edulis experience rapid transgression to hypoxia when they close their valves during low tide. This induces a physiological stress response aiming to stabilize tissue perfusion against declining oxygen partial pressure in shell water. We hypothesized that nitric oxide (NO) accumulation supports blood vessel opening in hypoxia and used live imaging techniques to measure NO and superoxide anion (O2∙-) formation in hypoxia-exposed gill filaments. Thirty minutes of moderate (7 kPa pO₂) and severe hypoxia (1 kPa pO₂) caused 1.6- and 2.4-fold increase, respectively, of NO accumulation in the endothelial muscle cells of the hemolymphatic vessels of the gill filaments. This led to a dilatation of blood vessel diameter by 43% (7 kPa) and 56% (1 kPa), which facilitates blood flow. Experiments in which we applied the chemical NO-donor Spermine NONOate (concentrations ranging from 1 to 6 mM) under normoxic conditions corroborate the dilatational effect of NO on the blood vessel. The formation of O2∙- within the filament epithelial cells increased 1.5 (7 kPa) and 2-fold (1 kPa) upon treatment. Biochemical analysis of mitochondrial electron transport complexes in hypoxia-exposed gill tissue indicates decreased activity of complexes I and III in both hypoxic conditions; whereas complex IV (cytochrome-c oxidase) activity increased at 7 kPa and decreased at 1 kPa compared to normoxic exposure conditions. This corresponds to the pattern of pO₂-dependent gill respiration rates recorded in ex-vivo experiments. Severe hypoxia (1 kPa) appears to have a stabilizing effect on NO accumulation in gill cells, since less O₂ is available for NO oxidation to nitrite/nitrate. Hypoxia thus supports the NO dependent inhibition of complex IV activity, a mechanism that could fine tune mitochondrial respiration to the local O₂ availability in a tissue. Our study highlights a basal function of NO in improving perfusion of hypoxic invertebrate tissues, which could be a key mechanism of tolerance toward environmental O₂ variations.

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

Photodissociation dynamics of nitric oxide from roussin’s red ester probed by time-resolved infrared spectroscopy

To explore nitric oxide (NO)-donating capability of Roussin’s red ester [Fe2(μ-RS)2(NO)4] (RRE), photodissociation of NO from RRE and its subsequent reaction dynamics were probed by monitoring the N–O stretching mode after excitation with a 400-nm photon. As a result of the experiment, the NO generation quantum yield was obtained.

Controlled nitrite anion encapsulation and release in the molecular cavity of decamethylcucurbit[5]uril: solution and solid state studies

Nitrite anion encapsulation was realized using molecular cavitands of decamethylcucurbit[5]urils as molecular receptors.

AMP-activated protein kinase activation and NADPH oxidase inhibition by inorganic nitrate and nitrite prevent liver steatosis

Liver steatosis, or fatty liver, is the most common liver disease in the world, affecting up to 25% of all Americans. There is currently no approved drug available for this condition, which may progress to serious disease, including steatohepatitis, fibrosis, and cirrhosis. Here, we show in rodent and human models of metabolic syndrome that steatosis can be prevented by a simple dietary approach. Inorganic nitrate, present in green leafy vegetables, is converted in vivo to nitric oxide (NO) in a process involving symbiotic host bacteria. NO then induces key metabolic regulatory pathways to ultimately reduce oxidative stress and improve cardiometabolic functions. Clinical trials would be helpful to tell if dietary nitrate is useful in treatment and prevention of fatty liver disease.

Advanced age and unhealthy dietary habits contribute to the increasing incidence of obesity and type 2 diabetes. These metabolic disorders, which are often accompanied by oxidative stress and compromised nitric oxide (NO) signaling, increase the risk of adverse cardiovascular complications and development of fatty liver disease. Here, we investigated the therapeutic effects of dietary nitrate, which is found in high levels in green leafy vegetables, on liver steatosis associated with metabolic syndrome. Dietary nitrate fuels a nitrate–nitrite–NO signaling pathway, which prevented many features of metabolic syndrome and liver steatosis that developed in mice fed a high-fat diet, with or without combination with an inhibitor of NOS (l-NAME). These favorable effects of nitrate were absent in germ-free mice, demonstrating the central importance of host microbiota in bioactivation of nitrate. In a human liver cell line (HepG2) and in a validated hepatic 3D model with primary human hepatocyte spheroids, nitrite treatment reduced the degree of metabolically induced steatosis (i.e., high glucose, insulin, and free fatty acids), as well as drug-induced steatosis (i.e., amiodarone). Mechanistically, the salutary metabolic effects of nitrate and nitrite can be ascribed to nitrite-derived formation of NO species and activation of soluble guanylyl cyclase, where xanthine oxidoreductase is proposed to mediate the reduction of nitrite. Boosting this nitrate–nitrite–NO pathway results in attenuation of NADPH oxidase-derived oxidative stress and stimulation of AMP-activated protein kinase and downstream signaling pathways regulating lipogenesis, fatty acid oxidation, and glucose homeostasis. These findings may have implications for novel nutrition-based preventive and therapeutic strategies against liver steatosis associated with metabolic dysfunction.

Sodium nitrite augments lung S-nitrosylation and reverses chronic hypoxic pulmonary hypertension in juvenile rats

Deficient nitric oxide (NO) signaling plays a critical role in the pathogenesis of chronic neonatal pulmonary hypertension (PHT). Physiological NO signaling is regulated by S-nitrosothiols (SNOs), which act both as a reservoir for NO and as a reversible modulator of protein function. We have previously reported that therapy with inhaled NO (iNO) increased peroxynitrite-mediated nitration in the juvenile rat lung, although having minimal reversing effects on vascular remodeling. We hypothesized that sodium nitrite (NaNO2) would be superior to iNO in enhancing lung SNOs, thereby contributing to reversal of chronic hypoxic PHT. Rat pups were exposed to air or hypoxia (13% O2) from postnatal days 1 to 21. Dose-response prevention studies were conducted from days 1–21 to determine the optimal dose of NaNO2. Animals then received rescue therapy with daily subcutaneous NaNO2 (20 mg/kg), vehicle, or were continuously exposed to iNO (20 ppm) from days 14–21. Chronic PHT secondary to hypoxia was both prevented and reversed by treatment with NaNO2. Rescue NaNO2 increased lung NO and SNO contents to a greater extent than iNO, without causing nitration. Seven lung SNO proteins upregulated by treatment with NaNO2 were identified by multiplex tandem mass tag spectrometry, one of which was leukotriene A4 hydrolase (LTA4H). Rescue therapy with a LTA4H inhibitor, SC57461A (10 mg·kg−1·day−1 sc), partially reversed chronic hypoxic PHT. We conclude that NaNO2 was superior to iNO in increasing tissue NO and SNO generation and reversing chronic PHT, in part via upregulated SNO-LTA4H.

Oxygen Tension Strongly Influences Metabolic Parameters and the Release of Interleukin-6 of Human Amniotic Mesenchymal Stromal Cells In Vitro

The human amniotic membrane (hAM) has been used for tissue regeneration for over a century. In vivo (in utero), cells of the hAM are exposed to low oxygen tension (1-4% oxygen), while the hAM is usually cultured in atmospheric, meaning high, oxygen tension (20% oxygen). We tested the influence of oxygen tensions on mitochondrial and inflammatory parameters of human amniotic mesenchymal stromal cells (hAMSCs). Freshly isolated hAMSCs were incubated for 4 days at 5% and 20% oxygen. We found 20% oxygen to strongly increase mitochondrial oxidative phosphorylation, especially in placental amniotic cells. Oxygen tension did not impact levels of reactive oxygen species (ROS); however, placental amniotic cells showed lower levels of ROS, independent of oxygen tension. In contrast, the release of nitric oxide was independent of the amniotic region but dependent on oxygen tension. Furthermore, IL-6 was significantly increased at 20% oxygen. To conclude, short-time cultivation at 20% oxygen of freshly isolated hAMSCs induced significant changes in mitochondrial function and release of IL-6. Depending on the therapeutic purpose, cultivation conditions of the cells should be chosen carefully for providing the best possible quality of cell therapy.

Sex differences in the nitrate-nitrite-NO• pathway: Role of oral nitrate-reducing bacteria

Oral reduction of nitrate to nitrite is dependent on the oral microbiome and is the first step of an alternative mammalian pathway to produce nitric oxide in humans. Preliminary evidence suggests important sex differences in this pathway. We prospectively investigated sex-differences following inorganic nitrate supplementation on nitrate/nitrite levels and vascular function, and separately examined sex differences in oral nitrate reduction, and oral microbiota by 16S rRNA profiling. At baseline, females exhibit higher nitrite levels in all biological matrices despite similar nitrate levels to males. Following inorganic nitrate supplementation, plasma nitrite was increased to a significantly greater extent in females than in males and pulse wave velocity was only reduced in females. Females exhibited higher oral bacterial nitrate-reducing activity at baseline and after nitrate supplementation. Despite these differences, there were no differences in the composition of either the total salivary microbiota or those oral taxa with nitrate reductase genes. Our results demonstrate that females have augmented oral nitrate reduction that contributes to higher nitrite levels at baseline and also after inorganic nitrate supplementation, however this was not associated with differences in microbial composition (clinicaltrials.gov: NCT01583803).

Effects of Oral Supplementation With Nitrate-Rich Beetroot Juice in Patients With Pulmonary Arterial Hypertension-Results From BEET-PAH, an Exploratory Randomized, Double-Blind, Placebo-Controlled, Crossover Study

BACKGROUND

The nitrate-nitrite-nitric oxide (NO) pathway may represent a potential therapeutic target in patients with pulmonary arterial hypertension (PAH). We explored the effects of dietary nitrate supplementation, with the use of nitrate-rich beetroot juice (BRJ), in patients with PAH.

METHODS AND RESULTS

We prospectively studied 15 patients with PAH in an exploratory randomized, double-blind, placebo-controlled, crossover trial. The patients received nitrate-rich beetroot juice (∼16 mmol nitrate per day) and placebo in 2 treatment periods of 7 days each. The assessments included; exhaled NO and NO flow-independent parameters (alveolar NO and bronchial NO flux), plasma and salivary nitrate and nitrite, biomarkers and metabolites of the NO-system, N-terminal pro-B-type natriuretic peptide, echocardiography, ergospirometry, diffusing capacity of the lung for carbon monoxide, and the 6-minute walk test. Compared with placebo ingestion of BRJ resulted in increases in; fractional exhaled NO at all flow-rates, alveolar NO concentrations and bronchial NO flux, and plasma and salivary levels of nitrate and nitrite. Plasma ornithine levels decreased and indices of relative arginine availability increased after BRJ compared to placebo. A decrease in breathing frequency was observed during ergospirometry after BRJ. A tendency for an improvement in right ventricular function was observed after ingestion of BRJ. In addition a tendency for an increase in the peak power output to peak oxygen consumption ratio (W peak/VO₂ peak) was observed, which became significant in patients reaching an increase of plasma nitrite >30% (responders).

CONCLUSIONS

BRJ administered for 1 week increases pulmonary NO production and the relative arginine bioavailability in patients with PAH, compared with placebo. An increase in the W peak/VO₂ peak ratio was observed after BRJ ingestion in plasma nitrite responders. These findings indicate that supplementation with inorganic nitrate increase NO synthase-independent NO production from the nitrate-nitrite-NO pathway.

Nitric oxide inhibits hypoxia-induced impairment of human RBC deformability through reducing the cross-linking of membrane protein band 3

AIM

Nitric oxide (NO) prevents the decline of RBC deformability under high altitude and other ischemic and hypoxic conditions, but the clear mechanisms remain unknown. Here, we have carried out a systematic study to find the mechanisms of NO-induced regulation of RBC deformability under hypoxia.

METHODS

NO levels, RBCs membrane elongation index (EI), membrane protein band 3 methemoglobin (MetHb) were determined during hypoxia (0 to 120 minutes). To validate the role of NO in regulating RBC deformability, tests were also performed with a NO donor (sodium nitroprusside) or a NO synthase inhibitor (l-nitro-arginine methylester) under 60 minutes hypoxia.

RESULTS

Hypoxia for 45 minutes increased NO levels from 25.65 ± 1.95 to 35.26 ± 2.01 μmol/L, and there was a plateau after 60 minutes hypoxia. The EI did not change before 45 minutes hypoxia, but decreased from 0.567 ± 0.019 to 0.409 ± 0.042 (30 Pa) after 60 minutes hypoxia. The cross-linking of band 3 and phosphotyrosine increased after 45 minutes hypoxia. All can be alleviated by supplement NO and aggregated by inhibiting NOS. However, the MetHb was not present this trend.

CONCLUSION

NO may prevent decreased of RBCs deformability through reducing the cross-linking of membrane band 3 under hypoxia; this helps microvascular perfusion of RBCs during ischemic and hypoxic disease states.

Nitro-fatty acid formation and metabolism

Nitro-fatty acids (NO₂-FA) are pleiotropic modulators of redox signaling pathways. Their effects on inflammatory signaling have been studied in great detail in cell, animal and clinical models primarily using exogenously administered nitro-oleic acid. While we know a considerable amount regarding NO₂-FA signaling, endogenous formation and metabolism is relatively unexplored. This review will cover what is currently known regarding the proposed mechanisms of NO₂-FA formation, dietary modulation of endogenous NO₂-FA levels, pathways of NO₂-FA metabolism and the detection of NO₂-FA and corresponding metabolites.

Metabolic Effects of Dietary Nitrate in Health and Disease

Nitric oxide (NO), generated from L-arginine and oxygen by NO synthases, is a pleiotropic signaling molecule involved in cardiovascular and metabolic regulation. More recently, an alternative pathway for the formation of this free radical has been explored. The inorganic anions nitrate (NO₃^-) and nitrite (NO₂^-), originating from dietary and endogenous sources, generate NO bioactivity in a process involving seemingly symbiotic oral bacteria and host enzymes in blood and tissues. The described cardio-metabolic effects of dietary nitrate from experimental and clinical studies include lowering of blood pressure, improved endothelial function, increased exercise performance, and reversal of metabolic syndrome, as well as antidiabetic effects. The mechanisms underlying the salutary metabolic effects of nitrate are being revealed and include interaction with mitochondrial respiration, activation of key metabolic regulatory pathways, and reduction of oxidative stress. Here we review the recent advances in the nitrate-nitrite-NO pathway, focusing on metabolic effects in health and disease.

Nitric oxide in red blood cell adaptation to hypoxia

Nitric oxide (NO) appears to be involved in virtually every aspect of cardiovascular biology. Most attention has been focused on the role of endothelial-derived NO in basal blood flow regulation by relaxing vascular smooth muscle; however, it is now known that NO derived from red blood cells (RBCs) plays a fundamental role in vascular homeostasis by enhancing oxygen (O2) release at the cellular and physiological level. Hypoxia is an often seen problem in diverse conditions; systemic adaptations to hypoxia permit people to adjust to the hypoxic environment at high altitudes and to disease processes. In addition to the cardiopulmonary and hematologic adaptations that support systemic O2 delivery in hypoxia, RBCs assist through newly described NO-based mechanisms, in line with their vital role in O2 transport and delivery. Furthermore, to increase the local blood flow in proportion to metabolic demand, NO regulates membrane mechanical properties thereby modulating RBC deformability and O2 carrying-releasing function. In this review article, we focus on the effect of NO bioactivity on RBC-based mechanisms that regulate blood flow and RBC deformability. RBC adaptations to hypoxia are summarized, with particular attention to NO-dependent S-nitrosylation of membrane proteins and hemoglobin (S-nitrosohemoglobin). The NO/S-nitrosylation/RBC vasoregulatory cascade contributes fundamentally to the molecular understanding of the role of NO in human adaptation to hypoxia and may inform novel therapeutic strategies.

Carbonic anhydrase is not a relevant nitrite reductase or nitrous anhydrase in the lung

KEY POINTS

Carbonic anhydrase (CA) inhibitors such as acetazolamide inhibit hypoxic pulmonary vasoconstriction (HPV) in humans and other mammals, but the mechanism of this action remains unknown. It has been postulated that carbonic anhydrase may act as a nitrous anhydrase in vivo to generate nitric oxide (NO) from nitrite and that this formation is increased in the presence of acetazolamide. Acetazolamide reduces HPV in pigs without evidence of any NO generation, whereas nebulized sodium nitrite reduces HPV by NO formation; however; combined infusion of acetazolamide with sodium nitrite inhalation did not further increase exhaled NO concentration over inhaled nitrite alone in pigs exposed to alveolar hypoxia. We conclude that acetazolamide does not function as either a nitrous anhydrase or a nitrite reductase in the lungs of pigs, and probably other mammals, to explain its vasodilating actions in the pulmonary or systemic circulations.

ABSTRACT

The carbonic anhydrase (CA) inhibitors acetazolamide and its structurally similar analogue methazolamide prevent or reduce hypoxic pulmonary vasoconstriction (HPV) in dogs and humans in vivo, by a mechanism unrelated to CA inhibition. In rodent blood and isolated blood vessels, it has been reported that inhibition of CA leads to increased generation of nitric oxide (NO) from nitrite and vascular relaxation in vitro. We tested the physiological relevance of augmented NO generation by CA from nitrite with acetazolamide in anaesthetized pigs during alveolar hypoxia in vivo. We found that acetazolamide prevents HPV in anaesthetized pigs, as in other mammalian species. A single nebulization of sodium nitrite reduces HPV, but this action wanes in the succeeding 3 h of hypoxia as nitrite is metabolized and excreted. Pulmonary artery pressure reduction and NO formation as measured by exhaled gas concentration from inhaled sodium nitrite were not increased by acetazolamide during alveolar hypoxia. Thus, our data argue against a physiological role of carbonic anhydrase as a nitrous anhydrase or nitrite reductase as a mechanism for its inhibition of HPV in the lung and blood in vivo.

Upregulation of microRNA‑24 causes vasospasm following subarachnoid hemorrhage by suppressing the expression of endothelial nitric oxide synthase

MicroRNA (miR)‑24 has been reported to associate with various diseases by acting on different signaling pathways. The present study aimed to elucidate the association between miR‑24 expression levels and vasospasm following subarachnoid hemorrhage (SAH), and its underlying mechanism. An miR online database was searched, identifying endothelial nitric oxide synthase (NOS3) as a potential target gene of miR‑24. A luciferase reporter assay performed to investigate the regulatory association between miR‑24 and NOS3 revealed that miR‑24 bound to the NOS3 3' untranslated region and inhibited NOS3 expression. Reverse transcription‑quantitative polymerase chain reaction and western blot analysis were performed to investigate the miR‑24 and NOS3 expression levels in samples from patients with SAH, and demonstrated a negative correlation between the two. In addition, miR‑24 expression levels were increased in SAH patients with vasospasm compared with those without, whereas the opposite results were observed for NOS3. Vascular smooth muscle cells (VSMCs) transfected with an miR‑24 inhibitor exhibited increased expression levels of NOS3, whereas those transfected with an miR‑24 mimic or NOS3 small interfering RNA exhibited reduced expression levels of NOS3, compared with the control. These results indicated a negative regulatory association between miR‑24 and NOS3. Downregulation of NOS3 may induce vasospasm following SAH, which may be due to the upregualtion of miR‑24 in VSMCs.

Hypoxia Tolerance in Teleosts: Implications of Cardiac Nitrosative Signals

Changes in environmental oxygen (O₂) are naturally occurring phenomena which ectotherms have to face on. Many species exhibit a striking capacity to survive and remain active for long periods under hypoxia, even tolerating anoxia. Some fundamental adaptations contribute to this capacity: metabolic suppression, tolerance of pH and ionic unbalance, avoidance and/or repair of free-radical-induced cell injury during reoxygenation. A remarkable feature of these species is their ability to preserve a normal cardiovascular performance during hypoxia/anoxia to match peripheral (tissue pO₂) requirements. In this review, we will refer to paradigms of hypoxia- and anoxia-tolerant teleost fish to illustrate cardiac physiological strategies that, by involving nitric oxide and its metabolites, play a critical role in the adaptive responses to O₂ limitation. The information here reported may contribute to clarify the molecular and cellular mechanisms underlying heart vulnerability vs. resistance in relation to O₂ availability.

Nitrate, the oral microbiome, and cardiovascular health: a systematic literature review of human and animal studies

Background

Dietary nitrate is an important source of nitric oxide (NO), a molecule critical for cardiovascular health. Nitrate is sequentially reduced to NO through an enterosalivary nitrate-nitrite-NO pathway that involves the oral microbiome. This pathway is considered an important adjunct pathway to the classical l-arginine-NO synthase pathway.

Objective

The objective of this study was to systematically assess the evidence for dietary nitrate intake and improved cardiovascular health from both human and animal studies.

Design

A systematic literature search was performed according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines by using key search terms in Medline and EMBASE databases and defined inclusion and exclusion criteria.

Results

Thirty-seven articles on humans and 14 articles on animals were included from 12,541 screened references. Data on the effects of dietary nitrate on blood pressure, endothelial function, ischemic reperfusion injury, arterial stiffness, platelet function, and cerebral blood flow in both human and animal models were identified. Beneficial effects of nitrate on vascular health have predominantly been observed in healthy human populations, whereas effects in populations at risk of cardiovascular disease are less clear. Few studies have investigated the long-term effects of dietary nitrate on cardiovascular disease clinical endpoints. In animal studies, there is evidence that nitrate improves blood pressure and endothelial function, particularly in animal models with reduced NO bioavailability. Nitrate dose seems to be a critical factor because there is evidence of cross-talk between the 2 pathways of NO production.

Conclusions

Evidence for a beneficial effect in humans at risk of cardiovascular disease is limited. Furthermore, there is a need to investigate the long-term effects of dietary nitrate on cardiovascular disease clinical endpoints. Further animal studies are required to elucidate the mechanisms behind the observed effects.

Potential benefits of dietary nitrate ingestion in healthy and clinical populations: A brief review

This article provides an overview of the current literature relating to the efficacy of dietary nitrate (NO₃^-) ingestion in altering aspects of cardiovascular and metabolic health and exercise capacity in healthy and diseased individuals. The consumption of NO₃^--rich vegetables, such as spinach and beetroot, have been variously shown to promote nitric oxide bioavailability, reduce systemic blood pressure, enhance tissue blood flow, modulate muscle O₂ utilisation and improve exercise tolerance both in normoxia and in hypoxia, as is commonly observed in a number of disease states. NO₃^- ingestion may, therefore, act as a natural means for augmenting performance and attenuating complications associated with limited O₂ availability or transport, hypertension and the metabolic syndrome. Recent studies indicate that dietary NO₃^- might also augment intrinsic skeletal muscle contractility and improve the speed and power of muscle contraction. Moreover, several investigations suggest that NO₃^- supplementation may improve aspects of cognitive performance both at rest and during exercise. Collectively, these observations position NO₃^- as more than a putative ergogenic aid and suggest that increasing natural dietary NO₃^- intake may act as a prophylactic in countering the predations of senescence and certain cardiovascular-metabolic diseases.

Eight weeks of nitrate supplementation improves blood flow and reduces the exaggerated pressor response during forearm exercise in peripheral artery disease

Peripheral artery disease (PAD) is characterized by a reduced blood flow (BF) and an elevated blood pressure (pressor) response during lower extremity exercise. Although PAD is evident in the upper extremities, no studies have determined BF and pressor responses during upper extremity exercise in PAD. Emerging evidence suggests that inorganic nitrate supplementation may serve as an alternative dietary strategy to boost nitric oxide bioavailability, improving exercising BF and pressor responses during exercise. The present study investigated 1) BF and pressor responses to forearm exercise in patients with PAD ( n = 21) relative to healthy age-matched control subjects ( n = 16) and 2) whether 8 wk of NaNO₃ supplementation influenced BF and pressor responses to forearm exercise in patients with PAD. Patients with moderate to severe PAD were randomly assigned to a NaNO₃ (1 g/day, n = 13)-treated group or a placebo (microcrystalline cellulose, n = 8)-treated group. Brachial artery forearm BF (FBF; via Doppler) and blood pressure (via finger plethysmography) were measured during mild-intensity (~3.5-kg) and moderate-intensity (~7-kg) handgrip exercise. The absolute change (from baseline) in FBF was reduced (except in the 3.5-kg condition) and BP responses were increased in patients with PAD compared with healthy control subjects in 3.5- and 7-kg conditions (all P < 0.05). Plasma nitrate and nitrite were elevated, exercising (7-kg) ΔFBF was improved (from 141 ± 17 to 172 ± 20 ml/min), and mean arterial pressure response was reduced (from 13 ± 1 to 9 ± 1 mmHg, P < 0.05) in patients with PAD that received NaNO₃ supplementation for 8 wk relative to those that received placebo. These results suggest that the BF limitation and exaggerated pressor response to moderate-intensity forearm exercise in patients with PAD are improved with 8 wk of NaNO₃ supplementation. NEW & NOTEWORTHY Peripheral artery disease (PAD) results in an exaggerated pressor response and reduced blood flow during lower limb exercise; however, the effect of PAD in the upper limbs has remained unknown. These results suggest that 8 wk of inorganic nitrate supplementation improves the blood flow limitation and exaggerated pressor response to moderate-intensity forearm exercise in PAD.

Erythrocytes and Vascular Function: Oxygen and Nitric Oxide

Erythrocytes regulate vascular function through the modulation of oxygen delivery and the scavenging and generation of nitric oxide (NO). First, hemoglobin inside the red blood cell binds oxygen in the lungs and delivers it to tissues throughout the body in an allosterically regulated process, modulated by oxygen, carbon dioxide and proton concentrations. The vasculature responds to low oxygen tensions through vasodilation, further recruiting blood flow and oxygen carrying erythrocytes. Research has shown multiple mechanisms are at play in this classical hypoxic vasodilatory response, with a potential role of red cell derived vasodilatory molecules, such as nitrite derived nitric oxide and red blood cell ATP, considered in the last 20 years. According to these hypotheses, red blood cells release vasodilatory molecules under low oxygen pressures. Candidate molecules released by erythrocytes and responsible for hypoxic vasodilation are nitric oxide, adenosine triphosphate and S-nitrosothiols. Our research group has characterized the biochemistry and physiological effects of the electron and proton transfer reactions from hemoglobin and other ferrous heme globins with nitrite to form NO. In addition to NO generation from nitrite during deoxygenation, hemoglobin has a high affinity for NO. Scavenging of NO by hemoglobin can cause vasoconstriction, which is greatly enhanced by cell free hemoglobin outside of the red cell. Therefore, compartmentalization of hemoglobin inside red blood cells and localization of red blood cells in the blood stream are important for healthy vascular function. Conditions where erythrocyte lysis leads to cell free hemoglobin or where erythrocytes adhere to the endothelium can result in hypertension and vaso constriction. These studies support a model where hemoglobin serves as an oxido-reductase, inhibiting NO and promoting higher vessel tone when oxygenated and reducing nitrite to form NO and vasodilate when deoxygenated.

Vasoactivity of nitrite in the iliac artery of the toad Rhinella marina

Nitrite ([Formula: see text]) causes vasodilation in mammals due to the formation of (nitric oxide) NO by endogenous [Formula: see text] reduction in the vascular wall. In this study, we determined if a similar mechanism operates in amphibians. Dual-wire myography of the iliac artery from Rhinella marina showed that applied [Formula: see text] caused a concentration-dependent vasodilation in normoxia (21% O₂; EC₅₀: 438 µM). Hypoxia (0.63% O₂) significantly increased the maximal dilation to [Formula: see text] by 5% ( P = 0.0398). The addition of oxyhemoglobin significantly increased the EC₅₀ ( P = 0.0144; EC₅₀: 2,236 µM) but did not affect the maximal vasodilation. In contrast, partially deoxygenated hemoglobin (90% desaturation) did not affect the EC₅₀ ( P = 0.1189) but significantly ( P = 0.0012) increased the maximal dilation to [Formula: see text] by 11%. The soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) completely abolished the response to [Formula: see text] ( P < 0.0001)_, and of the nitric oxide synthase inhibitors, only N⁵-(1-imino-3-butenyl)-l-ornithine (vinyl-l-NIO; P = 0.0028) significantly reduced the [Formula: see text] vasodilation. The xanthine oxidoreductase inhibitor allopurinol ( P = 0.927), the nitric oxide-scavenger 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (C-PTIO; P = 0.478), and disruption of the endothelium ( P = 0.094) did not affect the [Formula: see text] vasodilation. Incubation of iliac arteries with 1 mM [Formula: see text] did not a cause a change in the cGMP concentration (P = 0.407). Plasma [Formula: see text] was found to be 0.86 ± 0.20 µmol/l, while nitrate ([Formula: see text]) was 19.55 ± 2.55 µmol/l. Both cygb and ngb mRNAs were expressed in the iliac artery, and it is possible that these globins facilitate [Formula: see text] reduction in hypoxia. In addition, [Formula: see text] intracellular disproportionation processes could be important in the generation of NO from [Formula: see text].

Effect of dietary nitrate levels on nitrate fluxes in rat skeletal muscle and liver

Rodent skeletal muscle has high levels of nitrate ions and this endogenous nitrate reservoir can supply nitrite/nitric oxide (NO) for functional hyperemia and/or for other physiological processes in muscle during exercise. Mice with a NOS1 knockout have markedly reduced muscle nitrate levels, suggesting NO production by NOS and its reaction with oxymyoglobin as a source of nitrate. However, oxygen levels are normally low in most internal organs, which raises the possibility that nitrate-derived NO pathway is physiologically important even at "normoxia", and muscle nitrate reservoir is the main endogenous NO backup when exogeneous (dietary) nitrate intake is low. Using dietary nitrate manipulations, we explore the importance of diet for maintaining and renewal of muscle nitrate reservoir and its levels in other tissues. We found that skeletal muscle nitrate is extensively used when nitrate in diet is low. One week of nitrate starvation leads to dramatic nitrate depletion in skeletal muscle and a substantial decrease in liver. Nitrate depleted from skeletal muscle during starvation is quickly recovered from new dietary sources, with an unexpected significant "overload" compared with animals not subjected to nitrate starvation. Our results suggest the importance of dietary nitrate for nitrate reserves in muscle and in other tissues, when compared with endogenous NOS-derived sources. This requires an active transport mechanism for sequestering nitrate into cells, stimulated by lack of dietary nitrate or other enzymatic changes. These results confirm the hypothesis that muscle is a major storage site for nitrate in mammals.