Diffusion of nitric oxide through the gastric wall upon reduction of nitrite by red wine: Physiological impact

Dietary (poly)phenols as modulators of the biophysical properties in endothelial cell membranes: its impact on nitric oxide bioavailability in hypertension

Hypertension is a major contributor to premature death, owing to the associated increased risk of damage to the heart, brain and kidneys. Although hypertension is manageable by medication and lifestyle changes, the risk increases with age. In an increasingly aged society, the incidence of hypertension is escalating, and is expected to increase the prevalence of (cerebro)vascular events and their associated mortality. Adherence to plant-based diets improves blood pressure and vascular markers in individuals with hypertension. Food flavonoids have an inhibitory effect towards angiotensin-converting enzyme (ACE1) and although this effect is greatly diminished upon metabolization, their microbial metabolites have been found to improve endothelial nitric oxide synthase (eNOS) activity. Considering the transmembrane location of ACE1 and eNOS, the ability of (poly)phenols to interact with membrane lipids modulate the cell membrane's biophysical properties and impact on nitric oxide (·NO) synthesis and bioavailability, remain poorly studied. Herein, we provide an overview of the current knowledge on the lipid remodeling of endothelial membranes with age, its impact on the cell membrane's biophysical properties and ·NO permeability across the endothelial barrier. We also discuss the potential of (poly)phenols and other plant-based compounds as key players in hypertension management, and address the caveats and challenges in adopted methodologies.

Do oral and gut microbiota communicate through redox pathways? A novel asset of the nitrate-nitrite-NO pathway

Nitrate may act as a regulator of •NO bioavailability via sequential reduction along the nitrate-nitrite-NO pathway with widespread health benefits, including a eubiotic effect on the oral and gut microbiota. Here, we discuss the molecular mechanisms of microbiota-host communication through redox pathways, via the production of •NO and oxidants by the family of NADPH oxidases, namely hydrogen peroxide (via Duox2), superoxide radical (via Nox1 and Nox2) and peroxynitrite, which leads to downstream activation of stress responses (Nrf2 and NFkB pathways) in the host mucosa. The activation of Nox2 by microbial metabolites is also discussed. Finally, we propose a new perspective in which both oral and gut microbiota communicate through redox pathways, with nitrate as the pivot linking both ecosystems.

The chemical biology of dinitrogen trioxide

Dinitrogen trioxide (N 2 O 3 ) mediates low-molecular weight and protein S- and N-nitrosation, with recent reports suggesting a role in the formation of nitrating intermediates as well as in nitrite-dependent hypoxic vasodilatation. However, the reactivity ofN 2 O 3 in biological systems results in an extremely short half-life that renders this molecule essentially undetectable by currently available technologies. As a result, evidence for in vivoN 2 O 3 formation derives from the detection of nitrosated products as well as from in vitro kinetic determinations, isotopic labeling studies, and spectroscopic analyses. This review will discuss mechanisms ofN 2 O 3 formation, reactivity and decomposition, as well as address the role of sub-cellular localization as a key determinant of its actions. Finally, evidence will be discussed supporting different roles forN 2 O 3 as a biologically relevant signaling molecule.

(Poly)phenols and nitrolipids: Relevant participants in nitric oxide metabolism

Nitric oxide (^•NO) is an essential molecule able to control and regulate many biological functions. Additionally, ^•NO bears a potential toxicity or damaging effects under conditions of uncontrolled production, and because of its participation in redox-sensitive pathways and oxidizing reactions. Several plant (poly)phenols present in the diet are able to regulate the enzymes producing ^•NO (NOSs). In addition, (poly)phenols are implicated in defining ^•NO bioavailability, especially by regulating NADPH oxidases (NOXs), and the subsequent generation of superoxide and ^•NO depletion. Nitrolipids are compounds that are present in animal tissues because of dietary consumption, e.g. of olive oil, and/or as result of endogenous production. This endogenous production of nitrolipids is dependent on the nitrate/nitrite presence in the diet. Select nitrolipids, e.g. the nitroalkenes, are able to exert ^•NO-like signaling actions, and act as ^•NO reservoirs, becoming relevant for systemic ^•NO bioavailability. Furthermore, the presence of (poly)phenols in the stomach reduces dietary nitrite to ^•NO favoring nitrolipids formation. In this review we focus on the capacity of molecules representing these two groups of bioactives, i.e. (poly)phenols and nitrolipids, as relevant participants in ^•NO metabolism and bioavailability. This participation acquires especial relevance when human homeostasis is lost, for example under inflammatory conditions, in which the protective actions of (poly)phenols and/or nitrolipids have been associated with local and systemic ^•NO bioavailability.

The Nitrate-Nitrite-Nitric Oxide Pathway on Healthy Ageing: A Review of Pre-clinical and Clinical Data on the Impact of Dietary Nitrate in the Elderly

We are living longer. Are we living healthier? As we age, cellular and molecular damage reshape our physiological responses towards environmental and endogenous stimuli. The free radical theory of ageing has been proposed long before ageing has been considered a “scientific discipline” and, since then, has been discussed and upgraded as a major contributor to aberrant ageing. Assuming that ageing results merely from the accumulation of oxidative modifications of biomolecules is not only a simplistic and reductive view of such a complex and dynamic process, but also free radicals and related oxidants are now considered pivotal signalling molecules. The fine modulation of critical signalling pathways by redox compounds demands a novel approach to tackle the role of free radicals in ageing. Nitric oxide (^⋅NO) is a paradigmatic example given its biological functions in cardiovascular, neurologic and immune systems. In addition to the canonical ^⋅NO synthesis by a family of enzymes, nitrate from green leafy vegetables, is reduced to nitrite in the oral cavity which is further reduced to ^⋅NO in the stomach. Boosting this nitrate-nitrite-NO pathway has been shown to improve gastrointestinal, cardiovascular, metabolic and cognitive performance both in humans and in animal models of disease. In the elderly, nitrate-derived ^⋅NO has been shown improve several physiological functions that typically decline during ageing. In this paper, the role of nitrate and derived nitrogen oxides will be discussed while reviewing pre-clinical and clinical data on the cardiovascular, neuronal, musculoskeletal and metabolic effects of nitrate during healthy ageing.

The Peculiar Facets of Nitric Oxide as a Cellular Messenger: From Disease-Associated Signaling to the Regulation of Brain Bioenergetics and Neurovascular Coupling

In this review, we address the regulatory and toxic role of ^·NO along several pathways, from the gut to the brain. Initially, we address the role on ^·NO in the regulation of mitochondrial respiration with emphasis on the possible contribution to Parkinson's disease via mechanisms that involve its interaction with a major dopamine metabolite, DOPAC. In parallel with initial discoveries of the inhibition of mitochondrial respiration by ^·NO, it became clear the potential for toxic ^·NO-mediated mechanisms involving the production of more reactive species and the post-translational modification of mitochondrial proteins. Accordingly, we have proposed a novel mechanism potentially leading to dopaminergic cell death, providing evidence that NO synergistically interact with DOPAC in promoting cell death via mechanisms that involve GSH depletion. The modulatory role of NO will be then briefly discussed as a master regulator on brain energy metabolism. The energy metabolism in the brain is central to the understanding of brain function and disease. The core role of ^·NO in the regulation of brain metabolism and vascular responses is further substantiated by discussing its role as a mediator of neurovascular coupling, the increase in local microvessels blood flow in response to spatially restricted increase of neuronal activity. The many facets of NO as intracellular and intercellular messenger, conveying information associated with its spatial and temporal concentration dynamics, involve not only the discussion of its reactions and potential targets on a defined biological environment but also the regulation of its synthesis by the family of nitric oxide synthases. More recently, a novel pathway, out of control of NOS, has been the subject of a great deal of controversy, the nitrate:nitrite:NO pathway, adding new perspectives to ^·NO biology. Thus, finally, this novel pathway will be addressed in connection with nitrate consumption in the diet and the beneficial effects of protein nitration by reactive nitrogen species.

The Gastrointestinal Circulation: Physiology and Pathophysiology

The gastrointestinal (GI) circulation receives a large fraction of cardiac output and this increases following ingestion of a meal. While blood flow regulation is not the intense phenomenon noted in other vascular beds, the combined responses of blood flow, and capillary oxygen exchange help ensure a level of tissue oxygenation that is commensurate with organ metabolism and function. This is evidenced in the vascular responses of the stomach to increased acid production and in intestine during periods of enhanced nutrient absorption. Complimenting the metabolic vasoregulation is a strong myogenic response that contributes to basal vascular tone and to the responses elicited by changes in intravascular pressure. The GI circulation also contributes to a mucosal defense mechanism that protects against excessive damage to the epithelial lining following ingestion of toxins and/or noxious agents. Profound reductions in GI blood flow are evidenced in certain physiological (strenuous exercise) and pathological (hemorrhage) conditions, while some disease states (e.g., chronic portal hypertension) are associated with a hyperdynamic circulation. The sacrificial nature of GI blood flow is essential for ensuring adequate perfusion of vital organs during periods of whole body stress. The restoration of blood flow (reperfusion) to GI organs following ischemia elicits an exaggerated tissue injury response that reflects the potential of this organ system to generate reactive oxygen species and to mount an inflammatory response. Human and animal studies of inflammatory bowel disease have also revealed a contribution of the vasculature to the initiation and perpetuation of the tissue inflammation and associated injury response.

Assessment of nitric oxide (NO) redox reactions contribution to nitrous oxide (N2 O) formation during nitrification using a multispecies metabolic network model

Over the coming decades nitrous oxide (N2O) is expected to become a dominant greenhouse gas and atmospheric ozone depleting substance. In wastewater treatment systems, N2O is majorly produced by nitrifying microbes through biochemical reduction of nitrite (NO2(-)) and nitric oxide (NO). However it is unknown if the amount of N2O formed is affected by alternative NO redox reactions catalyzed by oxidative nitrite oxidoreductase (NirK), cytochromes (i.e., P460 [CytP460] and 554 [Cyt554 ]) and flavohemoglobins (Hmp) in ammonia- and nitrite-oxidizing bacteria (AOB and NOB, respectively). In this study, a mathematical model is developed to assess how N2O formation is affected by such alternative nitrogen redox transformations. The developed multispecies metabolic network model captures the nitrogen respiratory pathways inferred from genomes of eight AOB and NOB species. The performance of model variants, obtained as different combinations of active NO redox reactions, was assessed against nine experimental datasets for nitrifying cultures producing N2O at different concentration of electron donor and acceptor. Model predicted metabolic fluxes show that only variants that included NO oxidation to NO2(-) by CytP460 and Hmp in AOB gave statistically similar estimates to observed production rates of N2O, NO, NO2(-) and nitrate (NO3(-)), together with fractions of AOB and NOB species in biomass. Simulations showed that NO oxidation to NO2(-) decreased N2O formation by 60% without changing culture's NO2(-) production rate. Model variants including NO reduction to N2O by Cyt554 and cNor in NOB did not improve the accuracy of experimental datasets estimates, suggesting null N2O production by NOB during nitrification. Finally, the analysis shows that in nitrifying cultures transitioning from dissolved oxygen levels above 3.8 ± 0.38 to <1.5 ± 0.8 mg/L, NOB cells can oxidize the NO produced by AOB through reactions catalyzed by oxidative NirK.

A shortcut to wide-ranging biological actions of dietary polyphenols: modulation of the nitrate-nitrite-nitric oxide pathway in the gut

Dietary polyphenols are complex, natural compounds with recognized health benefits. Initially attractive to the biomedical area due to their in vitro antioxidant properties, the biological implications of polyphenols are now known to be far from their acute ability to scavenge free radicals but rather to modulate redox signaling pathways. Actually, it is now recognized that dietary polyphenols are extensively metabolized in vivo and that the chemical, biophysical and biological properties of their metabolites are, in most cases, quite different from the ones of the parent molecules. Hence, the study of the metabolic, absorptive and signaling pathways of both phenolics and derivatives has become a major issue. In this paper we propose a short-cut for the systemic effects of polyphenols in connection with nitric oxide (˙NO) biology. This free radical is a ubiquitous signaling molecule with pivotal functions in vivo. It is produced through an enzymatic pathway and also through the reduction of dietary nitrate and nitrite in the human stomach. At acidic gastric pH, dietary polyphenols, in the form they are conveyed in foods and at high concentration, not only promote nitrite reduction to ˙NO but also embark in a complex network of chemical reactions to produce higher nitrogen oxides with signaling functions, namely by inducing post-translational modifications. Modified endogenous molecules, such as nitrated proteins and lipids, acquire important physiological functions. Thus, local and systemic effects of ˙NO such as modulation of vascular tone, mucus production in the gut and protection against ischemia-reperfusion injury are, in this sense, triggered by dietary polyphenols. Evidence to support the signaling and biological effects of polyphenols by modulation of the nitrate-nitrite-NO pathway will be herein provided and discussed. General actions of polyphenols encompassing absorption and metabolism in the intestine/liver are short-cut via the production of diffusible species in the stomach that have not only a local but also a general impact.

Dietary flavonoids and nitrate: effects on nitric oxide and vascular function

Emerging evidence highlights dietary flavonoids and nitrate as candidates that may explain at least part of the cardioprotective effect of a fruit and vegetable diet. Nitric oxide plays a pivotal role in cardiovascular health. Components of a fruit and vegetable diet that are cardioprotective, in part through effects on nitric oxide status, could substantially reduce the cardiovascular risk profile of the general population with increased intake of such a diet. Epidemiological evidence suggests that dietary flavonoids and nitrate have a cardioprotective effect. Clinical trials with flavonoid- and nitrate-rich foods have shown benefits on measures of vascular health. While the molecular mechanisms by which flavonoids and nitrate are cardioprotective are not completely understood, recent evidence suggests both nonspecific and specific effects through nitric oxide pathways. This review presents an overview of nitric oxide and its key role in cardiovascular health and discusses the possible vascular benefits of flavonoids and nitrate, individually and in combination, through effects on nitric oxide status.

Interactions between cocoa flavanols and inorganic nitrate: additive effects on endothelial function at achievable dietary amounts

Dietary intervention studies have shown that flavanols and inorganic nitrate can improve vascular function, suggesting that these two bioactives may be responsible for beneficial health effects of diets rich in fruits and vegetables. We aimed to study interactions between cocoa flavanols (CF) and nitrate, focusing on absorption, bioavailability, excretion, and efficacy to increase endothelial function. In a double-blind randomized, dose-response crossover study, flow-mediated dilation (FMD) was measured in 15 healthy subjects before and at 1, 2, 3, and 4 h after consumption of CF (1.4-10.9 mg/kg bw) or nitrate (0.1-10 mg/kg bw). To study flavanol-nitrate interactions, an additional intervention trial was performed with nitrate and CF taken in sequence at low and high amounts. FMD was measured before (0 h) and at 1h after ingestion of nitrate (3 or 8.5 mg/kg bw) or water. Then subjects received a CF drink (2.7 or 10.9 mg/kg bw) or a micro- and macronutrient-matched CF-free drink. FMD was measured at 1, 2, and 4 h thereafter. Blood and urine samples were collected and assessed for CF and nitric oxide (NO) metabolites with HPLC and gas-phase reductive chemiluminescence. Finally, intragastric formation of NO after CF and nitrate consumption was investigated. Both CF and nitrate induced similar intake-dependent increases in FMD. Maximal values were achieved at 1 h postingestion and gradually decreased to reach baseline values at 4 h. These effects were additive at low intake levels, whereas CF did not further increase FMD after high nitrate intake. Nitrate did not affect flavanol absorption, bioavailability, or excretion, but CF enhanced nitrate-related gastric NO formation and attenuated the increase in plasma nitrite after nitrate intake. Both flavanols and inorganic nitrate can improve endothelial function in healthy subjects at intake amounts that are achievable with a normal diet. Even low dietary intake of these bioactives may exert relevant effects on endothelial function when ingested together.

Dietary nitrite induces nitrosation of the gastric mucosa: the protective action of the mucus and the modulatory effect of red wine

The stomach chemical environment promotes the production of new molecules that can induce post-translational modifications of endogenous proteins with physiological impact. The nitrate-nitrite-nitric oxide pathway is relevant in this process via production of nitric oxide ((•)NO) and nitric oxide-derived nitrogen oxides (NOx) at high concentrations. Using a highly sensitive and selective chemiluminescence approach, we found that exposure the stomach of rats to nitrite yielded S- and N-nitroso derivatives in gastric mucus cysteine-rich glycoproteins (mucins). To lesser extent, the underlying epithelial cell layers also suffered nitrite-driven S- and N-nitroso modifications which increased upon mucus removal, indicating that, under normal nitrite load, (•)NO and NOx can reach inner layers of the stomach wall and locally modify proteins. S-nitrosation was by large the predominant modification. In vitro and ex vivo experiments indicated that the gastric nitrosation pattern is triggered by dietary nitrite in a concentration dependent manner, encompassing the intermediary formation of (•)NO and is susceptible to modulation by dietary reductants, notably red wine polyphenols. Collectively, these results suggest a protective action of the mucus and potential (•)NO-dependent biochemical effects at deeper cells layers of the mucosa.

The acute effect of flavonoid-rich apples and nitrate-rich spinach on cognitive performance and mood in healthy men and women

Flavonoids and nitrate in a fruit and vegetable diet may be protective against cardiovascular disease and cognitive decline through effects on nitric oxide (NO) status. The circulating NO pool is increased via distinct pathways by dietary flavonoids and nitrate. Our aim was to investigate the acute effects of apples, rich in flavonoids, and spinach, rich in nitrate, independently and in combination on NO status, cognitive function and mood in a randomised, controlled, cross-over trial with healthy men and women (n = 30). The acute effects of four energy-matched treatments (control, apple, spinach and apple + spinach) were compared. Endpoints included plasma nitric oxide status (determined by measuring S-nitrosothiols + other nitroso species (RXNO)), plasma nitrate and nitrite, salivary nitrate and nitrite, urinary nitrate and nitrite as well as cognitive function (determined using the Cognitive Drug Research (CDR) computerized cognitive assessment battery) and mood. Relative to control, all treatments resulted in higher plasma RXNO. A significant increase in plasma nitrate and nitrite, salivary nitrate and nitrite as well as urinary nitrate and nitrite was observed with spinach and apple + spinach compared to control. No significant effect was observed on cognitive function or mood. In conclusion, flavonoid-rich apples and nitrate-rich spinach augmented NO status acutely with no concomitant improvements or deterioration in cognitive function and mood.

The redox interplay between nitrite and nitric oxide: From the gut to the brain

The reversible redox conversion of nitrite and nitric oxide ((•)NO) in a physiological setting is now widely accepted. Nitrite has long been identified as a stable intermediate of (•)NO oxidation but several lines of evidence support the reduction of nitrite to nitric oxide in vivo. In the gut, this notion implies that nitrate from dietary sources fuels the longstanding production of nitrite in the oral cavity followed by univalent reduction to (•)NO in the stomach. Once formed, (•)NO boosts a network of reactions, including the production of higher nitrogen oxides that may have a physiological impact via the post-translational modification of proteins and lipids. Dietary compounds, such as polyphenols, and different prandial states (secreting specific gastric mediators) modulate the outcome of these reactions. The gut has unusual characteristics that modulate nitrite and (•)NO redox interplay: (1) wide range of pH (neutral vs acidic) and oxygen tension (c.a. 70 Torr in the stomach and nearly anoxic in the colon), (2) variable lumen content and (3) highly developed enteric nervous system (sensitive to (•)NO and dietary compounds, such as glutamate). The redox interplay of nitrite and (•)NO might also participate in the regulation of brain homeostasis upon neuronal glutamatergic stimulation in a process facilitated by ascorbate and a localized and transient decrease of oxygen tension. In a way reminiscent of that occurring in the stomach, a nitrite/(•)NO/ascorbate redox interplay in the brain at glutamatergic synapses, contributing to local (•)NO increase, may impact on (•)NO-mediated process. We here discuss the implications of the redox conversion of nitrite to (•)NO in the gut, how nitrite-derived (•)NO may signal from the digestive to the central nervous system, influencing brain function, as well as a putative ascorbate-driven nitrite/NO pathway occurring in the brain.

Pepsin is nitrated in the rat stomach, acquiring antiulcerogenic activity: a novel interaction between dietary nitrate and gut proteins

Dietary nitrate is reduced to nitrite and nitric oxide ((•)NO) in the gut, producing reactive species able to nitrate proteins and lipids. We investigated intragastric production of (•)NO and nitrating agents in vivo by examining selective nitration of pepsinogen and pepsin. We further addressed the functional impact of nitration on peptic activity by evaluating the progression of secretagogue-induced ulcers. Pepsinogen nitration was assessed in healthy and diclofenac-induced ulcerated rat stomachs. Both groups were fed nitrite or water by oral gavage. Protein nitration was studied by immunofluorescence and immunoprecipitation. In parallel experiments, pentagastrin was administered to rats and nitrite was then instilled intragastrically. (•)NO levels were measured before and after nitrite administration by chemiluminescence. Macroscopic damage was assessed and nitrated pepsin was examined in the margin of ulcers. Protein nitration was detected physiologically in the stomach of healthy animals. Nitrite had a dual effect on intragastric nitration: overall nitration was decreased under physiological conditions but enhanced by acute inflammation. Pepsin and pepsinogen were also nitrated via a nitrite-dependent pathway. Nitration of both pepsin and its zymogen led to decreased peptic activity in response to classical substrates (e.g., collagen). Under conditions of acute ulceration, nitrite-dependent pepsin nitration prevented the development of gastric ulcers. Dietary nitrite generates nitrating agents in the stomach in vivo, markedly decreasing peptic activity. Under inflammatory and ulcerogenic conditions pepsin nitration attenuates the progression of gastric ulceration. These results suggest that dietary nitrite-dependent nitration of pepsin may have a novel antiulcerogenic effect in vivo.

Dietary nitrate and reductive polyphenols may potentiate the vascular benefit and alleviate the ulcerative risk of low-dose aspirin

The recent revelation that daily low-dose aspirin not only lowers risk for vascular events, but also can notably decrease risk for a range of adenocarcinomas, decreasing total cancer mortality by about 20%, makes it highly desirable to implement this protective strategy on a population-wide basis. Nonetheless, the fact that low-dose aspirin approximately doubles risk for serious gastrointestinal bleeding may impede health authorities from recommending its use by people judged to be at low cardiovascular risk. Nitric oxide (NO) exerts gastroprotective effects by boosting blood flow and mucus production in the gastric mucosa - effects which demonstrably oppose the pro-ulcerative impact of aspirin and other NSAIDs. A nitrate-rich diet, as well as ingestion of reductive catechol-bearing polyphenols, can collaborate in promoting NO generation in gastric juice, and they are protective in rodent models of gastric ulceration. Moreover, a high-nitrate diet, as well as certain reductive polyphenols such as epicatechin and quercetin, can exert platelet-stabilizing effects complementary to those of aspirin, and act in other ways to preserve vascular health. Hence, diets rich in nitrate and reductive polyphenols have the potential to amplify the vascular-protective benefits of low-dose aspirin, while diminishing its pro-ulcerative risk. Low-dose aspirin may be more unequivocally recommendable within the context of such a dietary strategy.

The potential of flavanol and procyanidin intake to influence age-related vascular disease

Advancing age is an independent major risk factor for cardiovascular disease (CVD). Age-associated impairments in the control of inflammation, excessive oxidative stress, and reduced cellular repair can all contribute to the development and progression of CVD. Current recommendations for both the primary and secondary prevention of CVD promote lifestyle modifications that include the adoption of healthy dietary patterns, such as the consumption of diets rich in plant foods, as these have been associated with a lower lifetime risk for the development of CVD. The potential for a diet rich in plant foods to be cardiovascular protective is also supported by prospective studies that suggest the intake of foods providing high amounts of certain phytochemicals, in particular flavanols and procyanidins, reduce the risk for CVD. These observations are further supported by a number of dietary intervention trials that show improvements in vascular function and reduced platelet reactivity following the consumption of high flavanol foods. In the current article we review a selection of these studies, and comment on some of the potential mechanisms that have been postulated to underlie the health effects of flavanol and procyanidin-rich foods.

Increase in gastric pH reduces hypotensive effect of oral sodium nitrite in rats

The new pathway nitrate-nitrite-nitric oxide (NO) has emerged as a physiological alternative to the classical enzymatic pathway for NO formation from l-arginine. Nitrate is converted to nitrite by commensal bacteria in the oral cavity and the nitrite formed is then swallowed and reduced to NO under the acidic conditions of the stomach. In this study, we tested the hypothesis that increases in gastric pH caused by omeprazole could decrease the hypotensive effect of oral sodium nitrite. We assessed the effects of omeprazole treatment on the acute hypotensive effects produced by sodium nitrite in normotensive and L-NAME-hypertensive free-moving rats. In addition, we assessed the changes in gastric pH and plasma levels of nitrite, NO(x) (nitrate+nitrite), and S-nitrosothiols caused by treatments. We found that the increases in gastric pH induced by omeprazole significantly reduced the hypotensive effects of sodium nitrite in both normotensive and L-NAME-hypertensive rats. This effect of omeprazole was associated with no significant differences in plasma nitrite, NO(x), or S-nitrosothiol levels. Our results suggest that part of the hypotensive effects of oral sodium nitrite may be due to its conversion to NO in the acidified environment of the stomach. The increase in gastric pH induced by treatment with omeprazole blunts part of the beneficial cardiovascular effects of dietary nitrate and nitrite.

Contributions of nitric oxide synthases, dietary nitrite/nitrate, and other sources to the formation of NO signaling products

Mice lacking all three nitric oxide synthase (NOS) genes remain viable even though deletion of the major downstream target of NO, soluble guanylyl cyclase, is associated with a dramatically shortened life expectancy. Moreover, findings of relatively normal flow responses in eNOS knockouts are generally attributed to compensatory mechanisms including upregulation of remaining NOS isoforms, but the alternative possibility that dietary nitrite/nitrate (NOx) may contribute to basal levels of NO signaling has never been investigated.

AIM

The aim of the present study was to examine how NO signaling products (nitrosated and nitrosylated proteins) and NO metabolites (nitrite, nitrate) are affected by single NOS deletions and whether dietary NOx plays a compensatory role in any deficiency. Specifically, we sought to ascertain whether profound alterations of these products arise upon genetic deletion of either NOS isoform, inhibition of all NOS activity, NOx restriction, or all of the above.

RESULTS

Our results indicate that while some significant changes do indeed occur, they are surprisingly moderate and compartmentalized to specific tissues. Unexpectedly, even after pharmacological inhibition of all NOSs and restriction of dietary NOx intake in eNOS knockout mice significant levels of NO-related products remain. Innovation/Conclusion: These findings suggest that a yet unidentified source of NO, unrelated to NOSs or dietary NOx, may be sustaining basal NO signaling in tissues. Given the significance of NO for redox regulation in health and disease, it would seem to be important to identify the nature of this additional source of NO products as it may offer new therapeutic avenues for correcting NO deficiencies.

Flavonoid-rich apples and nitrate-rich spinach augment nitric oxide status and improve endothelial function in healthy men and women: a randomized controlled trial

Flavonoids and nitrates in fruits and vegetables may protect against cardiovascular disease. Dietary flavonoids and nitrates can augment nitric oxide status via distinct pathways, which may improve endothelial function and lower blood pressure. Recent studies suggest that the combination of flavonoids and nitrates can enhance nitric oxide production in the stomach. Their combined effect in the circulation is unclear. Here, our objective was to investigate the independent and additive effects of flavonoid-rich apples and nitrate-rich spinach on nitric oxide status, endothelial function, and blood pressure. A randomized, controlled, crossover trial with healthy men and women (n=30) was conducted. The acute effects of four energy-matched treatments (control, apple, spinach, and apple+spinach), administered in random order, were compared. Measurements included plasma nitric oxide status, assessed by measuring S-nitrosothiols+other nitrosylated species (RXNO) and nitrite, blood pressure, and endothelial function, measured as flow-mediated dilatation of the brachial artery. Results are means and 95% CI. Relative to control, all treatments resulted in higher RXNO (control, 33 nmol/L, 26, 42; apple, 51 nmol/L, 40, 65; spinach, 86 nmol/L, 68, 110; apple+spinach, 69 nmol/L, 54, 88; P<0.01) and higher nitrite (control, 35 nmol/L, 27, 46; apple, 69 nmol/L, 53, 90; spinach, 99 nmol/L, 76, 129; apple+spinach, 80 nmol/L, 61, 104; P<0.01). Compared to control, all treatments resulted in higher flow-mediated dilatation (P<0.05) and lower pulse pressure (P<0.05), and apple and spinach resulted in lower systolic blood pressure (P<0.05). No significant effect was observed on diastolic blood pressure. The combination of apple and spinach did not result in additive effects on nitric oxide status, endothelial function, or blood pressure. In conclusion, flavonoid-rich apples and nitrate-rich spinach can independently augment nitric oxide status, enhance endothelial function, and lower blood pressure acutely, outcomes that may benefit cardiovascular health.

Stimulation of acidic reduction of nitrite to nitric oxide by soybean phenolics: possible relevance to gastrointestinal host defense

This study aimed to evaluate the potential of soybean-promoted acidic nitrite reduction and to correlate this activity with the content of phenolics and with the bactericidal activity against Escherichia coli O157:H7. Extracts of embrionary axes and cotyledons enriched in phenolics increased •NO formation at acidic pH at values that were 7.1 and 4.5 times higher, respectively, when compared to the reduction of the nonenriched extracts. Among the various phenolics accumulated in the soybean extracts, five stimulated nitrite reduction in the following decreasing order of potency: epicatechin gallate, chlorogenic acid, caffeic acid, galic acid and p-coumaric acid. Extracts of embrionary axes presented higher contents of epicatechin gallate and caffeic acid, compared to that of cotyledons, indicating a positive correlation between activity of the extracts and content of phenolics with regard to nitrite reducing activity. Soybean extracts enriched in phenolics interacted synergistically with acidified nitrite to prevent E. coli O157:H7 growth. The results suggest that soybean phenolics may interfere with the metabolism of •NO in an acidic environment by accelerating the reduction of nitrite, with a potential antimicrobial effect in the stomach.

Basic biochemical mechanisms behind the health benefits of polyphenols

Polyphenols and consequently many flavonoids have several beneficial actions on human health. However, the actual molecular interactions of polyphenols with biological systems remain mostly speculative. This review addresses the potential mechanisms of action that have been so far identified, as well as the feasibility that they could occur in vivo. Those mechanisms include: i) non specific actions, based on chemical features common to most polyphenols, e.g. the presence of a phenol group to scavenge free radicals; and ii) specific mechanisms; based on particular structural and conformational characteristics of select polyphenols and the biological target, e.g. proteins, or defined membrane domains. A better knowledge about the nature and biological consequences of polyphenol interactions with cell components will certainly contribute to develop nutritional and pharmacological strategies oriented to prevent the onset and/or the consequences of human disease.