Comparative analysis of nitrite uptake and hemoglobin-nitrite reactions in erythrocytes: sorting out uptake mechanisms and oxygenation dependencies

Erythroid anion Exchanger-1 (band 3) transports nitrite for nitric oxide metabolism

Nitrite (NO2-) interacts with hemoglobin (Hb) in various ways to regulate blood flow. During hypoxic vasodilation, nitrite is reduced by deoxyHb to yield nitric oxide (NO). While NO, a hydrophobic gas, could freely diffuse across the cell membrane, how the reactant nitrite anion could permeate through the red blood cell (RBC) membrane remains unclear. We hypothesized that Cl-/HCO3- anion exchanger-1 (AE1; band 3) abundantly embedded in the RBC membrane could transport NO2-, as HCO3- and NO2- exhibit similar hydrated radii. Here, we monitored NO/N2O3 generated from NO2- inside human RBCs by DAF-FM fluorophore. NO2-, not NO3-, increased intraerythrocytic DAF-FM fluorescence. To test the involvement of AE1-mediated transport in intraerythrocytic NO/N2O3 production from nitrite, we lowered Cl- or HCO3- in the RBC-incubating buffer by 20 % and indeed observed slower rise of the DAF-FM fluorescence. Anti-extracellular AE1, but not anti-intracellular AE1 antibodies, reduced the rates of NO formation from nitrite. The AE1 blocker DIDS similarly reduced the rates of NO/N2O3 production from nitrite in a dose-dependent fashion, confirming that nitrite entered RBCs through AE1. Nitrite inside the RBCs reacted with both deoxyHb and oxyHb, as evidenced by 6.1 % decrease in deoxyHb, 14.7 % decrease in oxyHb, and 20.7 % increase in methemoglobin (metHb). Lowering Cl- in the milieu equally delayed metHb production from nitrite-oxyHb and nitrite-deoxyHb reactions. Thus, AE1-mediated NO2- transport facilitates NO2--Hb reactions inside the red cells, supporting NOx metabolism in circulation.

Acute Nitrite Exposure Induces Dysfunction and Oxidative Damage in Grass Carp Isolated Hemocytes

To evaluate the effects of nitrite on the oxidative damage of blood cells of Grass Carp Ctenopharyngodon idella, the isolated hemocytes were exposed to nitrite (0, 1, 10, or 100 mg/L) for up to 24 h. Hemoglobin (Hb) and methemoglobin (MetHb) concentrations, reactive oxygen species (ROS) and malondialdehyde (MDA) levels, mitochondrial membrane potential (∆Ψm), and antioxidant enzyme activity were assayed to assess hematological parameters and the antioxidant defense mechanism. Results showed a remarkable decrease in Hb concentration with increasing nitrite concentration after a 24-h exposure, while the MetHb concentration increased significantly in nitrite exposure groups. The levels of ROS, ∆Ψm, and MDA increased to varying degrees with increases in nitrite exposure concentration and time. The total antioxidant capacity, catalase (CAT) activity, glutathione peroxidase (GPx) activity, and glutathione content showed a trend of rising initially and then decreasing with prolonged exposure time. Superoxide dismutase (SOD) activity was higher in the 1-mg/L nitrite exposure group and lower in the 100-mg/L group than in the control. The relative messenger RNA expression ratios of cat, sod1, and gpx were up-regulated significantly in the 1- and 10-mg/L groups and then declined in the 100-mg/L group. Therefore, it can be concluded that nitrite exposure activates the antioxidant defense mechanism of Grass Carp hemocytes and that the balance of oxidant-antioxidant homeostasis will be undermined by higher nitrite doses or longer exposure periods.

Effects of Nitrite Exposure on the Hematological Properties, Antioxidant and Stress Responses of Juvenile Hybrid Groupers, Epinephelus lanceolatus ♂ × Epinephelus fuscoguttatus ♀

Nitrite concentrations can reach high levels in indoor aquaculture systems, thus it is vital to determine the nitrite tolerance of aquaculture fish species. Here, juvenile hybrid groupers (Epinephelus lanceolatus ♂ × Epinephelus fuscoguttatus ♀, Family: Serranidae) were exposed to waterborne nitrite at 0, 10, 20, 40, and 80 mg NO₂⁻/L for 2 weeks. Nitrite exposure caused significant reductions in hematocrit and hemoglobin levels, significant increases in plasma calcium and plasma ALP levels, but had no significant effects on magnesium and total protein levels. Of the antioxidant responses investigated, SOD activity increased significantly in the liver and gills, but GST activity and GSH levels were significantly inhibited by nitrite exposure. Stress indicators, such as plasma cortisol and HSP 70 levels, were significantly stimulated by nitrite exposure. In brief, nitrite exposure over 20 mg NO₂⁻/L had toxic effects and affected the hematological properties, antioxidant responses, and stress indicators of juvenile hybrid groupers.

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

Physiological and transcriptional analysis of Chinese soft-shelled turtle (Pelodiscus sinensis) in response to acute nitrite stress

Nitrite is a harmful substance in aquaculture, and has a serious impact on the survival of the Chinese soft-shelled turtle, Pelodiscus sinensis. However, the cellular responses of P. sinensis to nitrite stress have not yet been investigated. The present study showed that nitrite led to a decrease in hemoglobin content and an increase in methemoglobin content in the blood, thus reducing the oxygen-carrying capacity of blood in P. sinensis. Nitrite also affects the antioxidant system of the liver and leads to lipid peroxidation. In addition, nitrite caused immune responses, including a decrease in lysozyme content and an increase in total complement activity, interleukin-6, and heme oxygenase concentrations in the serum. Additionally, the terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) assay showed that apoptosis occurred in liver cells. Finally, a comparative transcriptome analysis was conducted. A total of 573 differentially expressed genes and 15 significantly enriched KEGG pathways were identified. Among them, the glutathione S-transferase omega 1 (GSTO1) gene may relieve nitrite-induced oxidative damage in P. sinensis by participating in a variety of redox-related pathways, while the PPAR signaling pathway has been proposed to play an important regulatory role in lipid metabolism and immune responses. The present study comprehensively explored the cellular responses of P. sinensis to nitrite stress and provided guidance for future studies.

Exposure to nitrite alters thyroid hormone levels and morphology in Takifugu rubripes

Nitrite (NO^2-) can act as a toxic nitrogenous compound with the potential to disrupt endocrine systems in fish. The aim of the present study was to investigate the effects of nitrite on the thyroid endocrine system of Takifugu rubripes. Fish were exposed to 0, 0.5, 1, 3, and 6 mM nitrite concentrations. Blood was collected to assay the concentrations of thyroid-stimulating hormone (TSH), thyroxine (T₄), triiodothyronine (T₃), free thyroxine (FT₄), free triiodothyronine (FT₃), and 3,3,5'-triiodothyronine (rT₃), as well as the activity of iodothyronine deiodinases (Dio1, Dio2, and Dio3,) after 0, 12, 24, 48, and 96 h of exposure to nitrite. The first branchial arch to the third branchial arch of T. rubripes were sampled and fixed, and thyroid morphology was observed. The results showed that exposure to nitrite significantly increased the concentrations of TSH, T₃, FT₃, and reduced the concentrations of T₄, FT₄, and rT₃. The activity of Dio1 and Dio2 increased significantly, whereas Dio3 activity decreased significantly. Additionally, thyroid follicles degenerated and became blurred and most colloid material disappeared 96 h after exposure to high nitrite concentrations. Based on these results, high nitrite concentration exposure can disturb thyroid hormone homeostasis, alter thyroid follicle morphology, and result in thyroid endocrine toxicity.

Impact and tissue metabolism of nitrite at two acclimation temperatures in striped catfish (Pangasianodon hypophthalmus)

Elevated concentrations of nitrite develop occasionally in various aquatic habitats and aquaculture facilities, providing a potential danger for freshwater fish that take up nitrite via the gill chloride uptake mechanism. We studied the uptake, effects and metabolism of nitrite in blood, heart and skeletal muscle at two temperatures in striped catfish Pangasianodon hypophthalmus, a facultative air-breathing fish that is heavily cultivated in Southeast Asia. Exposure to 0.8 mM ambient nitrite increased blood [nitrite] and [methaemoglobin] (metHb) to high values at day 1, but values subsequently decreased towards controls at day 7. Blood [nitrite] and metHb content were unexpectedly higher at 27 °C (∼1.2 mM; 69% at day 1) than at 33 °C (∼0.9 mM; 55%), reflecting a lower nitrite uptake at the highest temperature, possibly via an increased reliance on air-breathing relative to water-breathing with temperature increase. A large fraction of the nitrite taken up was effectively eliminated by being detoxified to nitrate. Further, erythrocyte metHb reductase activity was increased during nitrite exposure, efficiently reducing metHb to functional haemoglobin. The uptake of nitrite into white skeletal musculature (main part of the fish) was much lower than into heart tissue. While heart [nitrite] was close to blood plasma levels, muscle [nitrite] peaked at ∼0.2 mM at day 1 and subsequently declined to ∼0.05 mM at day 7, which is below levels reported in various commercial cured meat products. Nitrite was partly metabolized to iron-nitrosyl, S-nitroso and N-nitroso compounds. The increase in nitros(yl)ated compounds was marginal in skeletal muscle and more pronounced in heart tissue.

Effect of nitrite exposure on the antioxidant enzymes and glutathione system in the liver of bighead carp, Aristichthys nobilis

Nitrite (NO₂^-) can cause oxidative stress in aquatic animal when it accumulates in the organism, resulting in different toxic effects on fish. In the present study, we investigated the effects of nitrite exposure on the antioxidant enzymes and glutathione system in the liver of Bighead carp (Aristichthys nobilis). Fish [Initial average weight: (180.05 ± 0.092) g] were exposed to 48.634 mg/L nitrite for 96 h, and a subsequent 96 h for the recovery test. Fish livers were collected to assay antioxidant enzymes activity, hepatic structure and expression of genes after 0 h, 6 h, 12 h, 24 h, 48 h, 72 h, 96 h of exposure and12 h, 24 h, 48 h, 72 h, 96 h of recovery. The results showed that the activity of glutathione peroxidase (GSH-Px), glutathione S-transferase (GST), and glutathione reductase (GR) increased significantly in the early stages of nitrite exposure. The study also showed that nitrite significantly up-regulated the mRNA levels of glutathione peroxidase (GSH-Px), glutathione S-transferase (GST), and glutathione reductase (GR) after 6, 48, and 72 h of exposure respectively. Nitrite also increased the formation of malondialdehyde (MDA), oxidized glutathione (GSSG), and the activity of catalase (CAT). Nitrite was observed to reduce the activity of superoxide dismutase (SOD) and the level of glutathione (GSH). In the recovery test, GSH and the GSSG recovered but did not return to pre-stress levels. The results suggested that the glutathione system played important roles in nitrite-induced oxidative stress in fish. The bighead carp responds to oxidative stress by enhancing the activity of GSH-Px, GST, GR and up-regulating the expression level of GSH-Px, GST, GR, a whilst simultaneously maintaining the dynamic balance of GSH/GSSG. CAT was also indispensable. They could reduce the degree of lipid peroxidation, and ultimately protect the body from oxidative damage.

The effects of elevated environmental CO2 on nitrite uptake in the air-breathing clown knifefish, Chitala ornata

Nitrite and carbon dioxide are common environmental contaminants in the intensive aquaculture ponds used to farm clown knifefish (Chitala ornata) in the Mekong delta, Vietnam. Here we tested the hypothesis that hypercapnia reduces nitrite uptake across the gills, because pH regulation will reduce chloride uptake and hence nitrite uptake as the two ions compete for the same transport route via the branchial HCO₃^-/Cl^- exchanger. Fish fitted with arterial catheters were exposed to normocapnic/normoxic water (control), nitrite (1 mM), hypercapnia (21 mmHg CO₂), or combined hypercapnia (acclimated hypercapnia) and nitrite for 96 h. Blood was sampled to measure acid-base status, haemoglobin derivatives and plasma ions. Plasma nitrite increased for 48 h, but levels stayed below the exposure concentration, and subsequently decreased as a result of nitrite detoxification to nitrate. The total uptake of nitrite (evaluated as [NO₂^-] + [NO₃^-]) was significantly decreased in hypercapnia, in accordance with the hypothesis. Methemoglobin and nitrosylhemoglobin levels were similarly lower during hypercapnic compared to normocapnic nitrite exposure. The respiratory acidosis induced by hypercapnia was half-compensated by bicarbonate accumulation in 96 h, which was mainly chloride-mediated (i.e. reduced Cl^- influx via the branchial HCO₃^-/Cl^- exchanger). Plasma osmolality and main ions (Na⁺, Cl^-) were significantly decreased by hypercapnia and by nitrite exposure, consistent with inhibition of active transport. We conclude that hypercapnia induces a long-lasting, and mainly chloride-mediated acid-base regulation that reduces the uptake of nitrite across the gills.

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].

Mitigation of nitrite toxicity by increased salinity is associated with multiple physiological responses: A case study using an economically important model species, the juvenile obscure puffer (Takifugu obscurus)

Nitrite is a common pollutant in water and is highly toxic to aquatic animals. To reveal the mechanism of salinity in attenuating nitrite toxicity to fish, we measured the physiological responses of juvenile Takifugu obscurus exposed to nitrite concentrations (0, 10, 20, 50, and 100 mg/L) under different salinity levels (0, 10, and 20 ppt) for 96 h. Salinity increased the survival rates of juvenile T. obscurus exposed to nitrite. Changes in key hematological parameters, antioxidant system, malondialdehyde, Na⁺/K⁺-ATPase, and HSP70 indicated that nitrite induced considerable damage to juveniles; salinity mitigated the harmful effects. This finding reflects similar changing trends in both antioxidants and their gene expressions among different tissues. We applied an overall index, an integrated biomarker response (IBR), that increased under high-nitrite condition but recovered to the normal levels under salinity treatment. Analysis of the selected detection indices and IBR values showed that the overall mitigating effect of salinity on nitrite toxicity seems to be at sub-cellular level and associated with complicated physiological responses.

Acute nitrite exposure alters the metabolism of thyroid hormones in grass carp (Ctenopharyngodon idellus)

Nitrite has the potential to disturb thyroid hormone homeostasis, but little is known about the underlying mechanisms. In the present study, juvenile grass carp (Ctenopharyngodon idellus) were exposed to various concentrations of nitrite (0, 0.5, 1, 4, and 16 mg/L, respectively). Serum concentrations of triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4), 3,3,5'-triiodothyronine (rT3), thyroid-stimulating hormone (TSH), and the activity of iodothyronine deiodinases were assayed at 0, 12, 24, 48, and 96 h after exposure. It was found that acute nitrite exposure significantly altered the TH levels and iodothyronine deiodinase activities. The rT3 levels were significantly increased in the treatment groups, whereas the concentrations of T3, FT3, FT4, and TSH decreased significantly. The concentration of T4 was elevated in the lower-dose exposure group, but was reduced in the higher-dose exposure group. Increases in type I iodothyronine deiodinase (ID1) and type III iodothyronine deiodinase (ID3) activities were observed in the exposure groups. The activity of type II iodothyronine deiodinase (ID2) decreased at 12 and 24 h after exposure. A decrease of colloid in the thyroid follicles was observed in the exposure group. The results indicate that acute nitrite exposure has the potential to disturb the homeostasis of thyroid hormone metabolism, leading to a hypothyroidism state in the juvenile grass carp.

Extreme nitrite tolerance in the clown knifefish Chitala ornata is linked to up-regulation of methaemoglobin reductase activity

The clown knifefish is a facultative air breather, which is widely farmed in freshwater ponds in Vietnam. Here we report a very high nitrite tolerance (96h LC₅₀ of 7.82mM) in this species and examine the effects of 1mM (LC₅) and 2.5mM (LC₁₀) ambient nitrite on haemoglobin (Hb) derivatives, electrolyte levels, acid-base status, and total body water content during 7days of exposure. Furthermore, we tested the hypothesis that erythrocyte methaemoglobin (metHb) reductase activity is upregulated by nitrite exposure. Plasma nitrite levels increased for 2-3days but stayed below environmental levels and fell towards control values during the last half of the exposure period. Plasma nitrate, in contrast, rose continuously, reflecting detoxification of nitrite to nitrate. MetHb generated from the reaction between nitrite and erythrocyte Hb reached 38% at day 2, but then decreased to 17% by the end of experiment. The first order rate constant for metHb reduction by erythrocyte metHb reductase increased from 0.01 in controls to 0.046min^-1 after 6days of nitrite exposure, showing up-regulation of this enzyme. While such upregulation has been suggested in nitrite-exposed fish species, this study provides the first experimental evidence.

Interspecific variation and plasticity in hemoglobin nitrite reductase activity and its correlation with oxygen affinity in vertebrates

Deoxygenated hemoglobin (Hb) is a nitrite reductase that reduces naturally occurring nitrite to nitric oxide (NO), supplying physiological relevant NO under hypoxic conditions. The nitrite reductase activity is modulated by the allosteric equilibrium between the R and T structures of Hb that also determines oxygen affinity. In the present study we investigated nitrite reductase activity and O₂ affinity in Hbs from ten different vertebrate species under identical conditions to disclose interspecific variations and allow an extended test for a correlation between the rate constant for nitrite reduction and O₂ affinity. We also tested plastic changes in Hb properties via addition of T-structure-stabilizing organic phosphates (ATP and GTP). The decay in deoxyHb during its reaction with nitrite was exponential-like in ectotherms (Atlantic hagfish, carp, crucian carp, brown trout, rainbow trout, cane toad, Indian python and red-eared slider turtle), while it was sigmoid in mammals (harbor porpoise and rabbit). Typically, hypoxia-tolerant species showed a faster reaction than intolerant species. Addition of ATP and GTP decreased O₂ affinity and slowed the rate of nitrite reduction in a concentration-dependent manner. The initial second order rate constant of the deoxyHb-mediated nitrite reduction showed a strong curvilinear correlation with oxygen affinity among all ectothermic vertebrates, and the relationship also applied to plastic variations of Hb properties via organic phosphates. The relationship predicts high nitrite reductase activity in hypoxic tolerant species with high Hb-O₂ affinity and reveals that the decrease in erythrocyte ATP and/or GTP during acclimation to hypoxia in ectotherms increases the erythrocyte NO generating capacity.

The effect of environmental hypercapnia and size on nitrite toxicity in the striped catfish (Pangasianodon hypophthalmus)

Striped catfish (Pangasianodon hypophthalmus) are farmed intensively at high stocking densities in Vietnam where they are likely to encounter environmental hypercapnia as well as occasional high levels of aquatic nitrite. Nitrite competes with Cl(-) for uptake at the branchial HCO3(-)/Cl(-) exchanger, causing a drastic reduction in the blood oxygen carrying capacity through the formation of methaemoglobin and nitrosylhaemoglobin. Environmental hypercapnia induces a respiratory acidosis where the branchial HCO3(-)/Cl(-) exchange activity is reduced in order to retain HCO3(-) for pH recovery, which should lead to a reduced nitrite uptake. To assess the effect of hypercapnia on nitrite uptake, fish were cannulated in the dorsal aorta, allowing repeated blood sampling for measurements of haemoglobin derivatives, plasma ions and acid-base status during exposure to 0.9mM nitrite alone and in combination with acute and 48h acclimated hypercapnia over a period of 72h. Nitrite uptake was initially reduced during the hypercapnia-induced acidosis, but after pH recovery the situation was reversed, resulting in higher plasma nitrite concentrations and lower functional haemoglobin levels that eventually caused mortality. This suggests that branchial HCO3(-)/Cl(-) exchange activity is reduced only during the initial acid-base compensation, but subsequently increases with the greater availability of internal HCO3(-) counter-ions as pH is compensated. The data further suggest that branchial Na(+)/H(+) exchange plays a significant role in the initial phase of acid-base compensation. Overall, longer term environmental hypercapnia does not protect against nitrite uptake in P. hypophthalmus, but instead enhances it. In addition, we observed a significant size effect in nitrite accumulation, where large fish attained plasma [nitrite] above the ambient concentration, while small fish did not. Small P. hypophthalmus instead had significantly higher plasma [nitrate], and haemoglobin concentrations, revealing greater capacity for detoxifying nitrite by oxidising it to nitrate.

Hypoxia tolerance, nitric oxide, and nitrite: lessons from extreme animals

Among vertebrates able to tolerate periods of oxygen deprivation, the painted and red-eared slider turtles (Chrysemys picta and Trachemys scripta) and the crucian carp (Carassius carassius) are the most extreme and can survive even months of total lack of oxygen during winter. The key to hypoxia survival resides in concerted physiological responses, including strong metabolic depression, protection against oxidative damage and-in air-breathing animals-redistribution of blood flow. Each of these responses is known to be tightly regulated by nitric oxide (NO) and during hypoxia by its metabolite nitrite. The aim of this review is to highlight recent work illustrating the widespread roles of NO and nitrite in the tolerance to extreme oxygen deprivation, in particular in the red-eared slider turtle and crucian carp, but also in diving marine mammals. The emerging picture underscores the importance of NO and nitrite signaling in the adaptive response to hypoxia in vertebrate animals.

Inhibitory effects of nitrite on the reactions of bovine carbonic anhydrase II with CO2 and bicarbonate consistent with zinc-bound nitrite

Carbonic anhydrase (CA) is a zinc enzyme that catalyzes hydration of carbon dioxide (CO2) and dehydration of bicarbonate in red blood cells, thus facilitating CO2 transport and excretion. Bovine CA II may also react with nitrite to generate nitric oxide, although nitrite is a known inhibitor of the CO2 hydration reaction. To address the potential in vivo interference of these reactions and the nature of nitrite binding to the enzyme, we here investigate the inhibitory effect of 10-30 mM nitrite on Michaelis-Menten kinetics of CO2 hydration and bicarbonate dehydration by stopped-flow spectroscopy. Our data show that nitrite significantly affects the apparent dissociation constant KM for CO2 (11 mM) and bicarbonate (221 mM), and the turnover number kcat for the CO2 hydration (1.467 × 10(6) s(-1)) but not for the bicarbonate dehydration (7.927 × 10(5) s(-1)). These effects demonstrate mixed and competitive inhibition for the reaction with CO2 and bicarbonate, respectively, and are consistent with nitrite binding to the active site zinc. The high apparent dissociation constant found here for CO2, bicarbonate and nitrite (16-120 mM) are all overall consistent with published data and reveal a large capacity of free enzyme available for binding each of the three substrates at their in vivo levels, with little or no significant interference among reactions. The low affinity of the enzyme for nitrite suggests that the in vivo interaction between red blood cell CA II and nitrite requires compartmentalization at the anion exchanger protein of the red cell membrane to be physiologically relevant.

Validation of a method to directly and specifically measure nitrite in biological matrices

The bioactivity of nitric oxide (NO) is influenced by chemical species generated through reactions with proteins, lipids, metals, and its conversion to nitrite and nitrate. A better understanding of the functions played by each of these species could be achieved by developing selective assays able of distinguishing nitrite from other NO species. Nagababu and Rifkind developed a method using acetic and ascorbic acids to measure nitrite-derived NO in plasma. Here, we adapted, optimized, and validated this method to assay nitrite in tissues. The method yielded linear measurements over 1-300 pmol of nitrite and was validated for tissue preserved in a nitrite stabilization solution composed of potassium ferricyanide, N-ethylmaleimide and NP-40. When samples were processed with chloroform, but not with methanol, ethanol, acetic acid or acetonitrile, reliable and reproducible nitrite measurements in up to 20 sample replicates were obtained. The method's accuracy in tissue was ≈ 90% and in plasma 99.9%. In mice, during basal conditions, brain, heart, lung, liver, spleen and kidney cortex had similar nitrite levels. In addition, nitrite tissue levels were similar regardless of when organs were processed: immediately upon collection, kept in stabilization solution for later analysis or frozen and later processed. After ip nitrite injections, rapidly changing nitrite concentrations in tissue and plasma could be measured and were shown to change in significantly distinct patterns. This validated method could be valuable for investigations of nitrite biology in conditions such as sickle cell disease, cardiovascular disease, and diabetes, where nitrite is thought to play a role.

The capacity of red blood cells to reduce nitrite determines nitric oxide generation under hypoxic conditions

Nitric oxide (NO) is a key regulator of vascular tone. Endothelial nitric oxide synthase (eNOS) is responsible for NO generation under normoxic conditions. Under hypoxia however, eNOS is inactive and red blood cells (RBC) provide an alternative NO generation pathway from nitrite to regulate hypoxic vasodilation. While nitrite reductase activity of hemoglobin is well acknowledged, little is known about generation of NO by intact RBC with physiological hemoglobin concentrations. We aimed to develop and apply a new approach to provide insights in the ability of RBC to convert nitrite into NO under hypoxic conditions. We established a novel experimental setup to evaluate nitrite uptake and the release of NO from RBC into the gas-phase under different conditions. NO measurements were similar to well-established clinical measurements of exhaled NO. Nitrite uptake was rapid, and after an initial lag phase NO release from RBC was constant in time under hypoxic conditions. The presence of oxygen greatly reduced NO release, whereas inhibition of eNOS and xanthine oxidoreductase (XOR) did not affect NO release. A decreased pH increased NO release under hypoxic conditions. Hypothermia lowered NO release, while hyperthermia increased NO release. Whereas fetal hemoglobin did not alter NO release compared to adult hemoglobin, sickle RBC showed an increased ability to release NO. Under all conditions nitrite uptake by RBC was similar. This study shows that nitrite uptake into RBC is rapid and release of NO into the gas-phase continues for prolonged periods of time under hypoxic conditions. Changes in the RBC environment such as pH, temperature or hemoglobin type, affect NO release.

Tissue oxygen demand in regulation of the behavior of the cells in the vasculature

The control of arteriolar diameters in microvasculature has been in the focus of studies on mechanisms matching oxygen demand and supply at the tissue level. Functionally, important vascular elements include EC, VSMC, and RBC. Integration of these different cell types into functional units aimed at matching tissue oxygen supply with tissue oxygen demand is only achieved when all these cells can respond to the signals of tissue oxygen demand. Many vasoactive agents that serve as signals of tissue oxygen demand have their receptors on all these types of cells (VSMC, EC, and RBC) implying that there can be a coordinated regulation of their behavior by the tissue oxygen demand. Such functions of RBC as oxygen carrying by Hb, rheology, and release of vasoactive agents are considered. Several common extra- and intracellular signaling pathways that link tissue oxygen demand with control of VSMC contractility, EC permeability, and RBC functioning are discussed.

Role of nitrite in regulation of fetal cephalic circulation in sheep

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

Quantitative Systems Pharmacology Model of NO Metabolome and Methemoglobin Following Long-Term Infusion of Sodium Nitrite in Humans

A long-term sodium nitrite infusion is intended for the treatment of vascular disorders. Phase I data demonstrated a significant nonlinear dose-exposure-toxicity relationship within the therapeutic dosage range. This study aims to develop a quantitative systems pharmacology model characterizing nitric oxide (NO) metabolome and methemoglobin after sodium nitrite infusion. Nitrite, nitrate, and methemoglobin concentration–time profiles in plasma and RBC were used for model development. Following intravenous sodium nitrite administration, nitrite undergoes conversion in RBC and tissue. Nitrite sequestered by RBC interacts more extensively with deoxyhemoglobin, which contributes greatly to methemoglobin formation. Methemoglobin is formed less-than-proportionally at higher nitrite doses as characterized with facilitated methemoglobin removal. Nitrate-to-nitrite reduction occurs in tissue and via entero-salivary recirculation. The less-than-proportional increase in nitrite and nitrate exposure at higher nitrite doses is modeled with a dose-dependent increase in clearance. The model provides direct insight into NO metabolome disposition and is valuable for nitrite dosing selection in clinical trials.

Comparison of the relative propensities of isoamyl nitrite and sodium nitrite to ameliorate acute cyanide poisoning in mice and a novel antidotal effect arising from anesthetics

Isoamyl nitrite has previously been considered acceptable as an inhaled cyanide antidote; therefore, the antidotal utility of this organic nitrite compared with sodium nitrite was investigated. To facilitate a quantitative comparison, doses of both sodium nitrite and isoamyl nitrite were given intraperitoneally in equimolar amounts to sublethally cyanide-challenged mice. Righting recovery from the knockdown state was clearly compromised in the isoamyl nitrite-treated animals, the effect being attributable to the toxicity of the isoamyl alchol produced during hydrolysis of the isoamyl nitrite to release nitrite anion. Subsequently, inhaled aqueous sodium nitrite aerosol was demonstrated to ameliorate sublethal cyanide toxicity, when provided to mice after the toxic dose, by the more rapid recovery of righting ability compared to that of the control animals given only the toxicant. Aerosolized sodium nitrite has thus been shown by these experiments to have promise as a better alternative to organic nitrites for development as an inhaled cyanide antidote. The inhaled sodium nitrite led to the production of NO in the bloodstream as determined by the appearance of EPR signals attributable to nitrosylhemoglobin and methemoglobin. The aerosol delivery was performed in an unmetered inhalation chamber, and in this study, no attempt was made to optimize the procedure. It is argued that administration of an effective inhaled aqueous sodium nitrite dose in humans is possible, though just beyond the capability of current individual metered-dose inhaler designs, such as those used for asthma. Finally, working at slightly greater than LD50 NaCN doses, it was fortuitously discovered that (i) anesthesia leads to significantly prolonged survival compared to that of unanesthetized animals and that (ii) the antidotal activity of nitrite anion was completely abolished under anesthesia. Plausible explanations for these effects in mice and their practical consequences in relation to testing putative cyanide antidotes are discussed.

Nitrite and nitrite reductases: from molecular mechanisms to significance in human health and disease

Nitrite, previously considered physiologically irrelevant and a simple end product of endogenous nitric oxide (NO) metabolism, is now envisaged as a reservoir of NO to be activated in response to oxygen (O(2)) depletion. In the first part of this review, we summarize and compare the mechanisms of nitrite-dependent production of NO in selected bacteria and in eukaryotes. Bacterial nitrite reductases, which are copper or heme-containing enzymes, play an important role in the adaptation of pathogens to O(2) limitation and enable microrganisms to survive in the human body. In mammals, reduction of nitrite to NO under hypoxic conditions is carried out in tissues and blood by an array of metalloproteins, including heme-containing proteins and molybdenum enzymes. In humans, tissues play a more important role in nitrite reduction, not only because most tissues produce more NO than blood, but also because deoxyhemoglobin efficiently scavenges NO in blood. In the second part of the review, we outline the significance of nitrite in human health and disease and describe the recent advances and pitfalls of nitrite-based therapy, with special attention to its application in cardiovascular disorders, inflammation, and anti-bacterial defence. It can be concluded that nitrite (as well as nitrate-rich diet for long-term applications) may hold promise as therapeutic agent in vascular dysfunction and ischemic injury, as well as an effective compound able to promote angiogenesis.

Nitric oxide formation versus scavenging: the red blood cell balancing act

Nitric oxide (NO) is a key modulator of vascular homeostasis controlling critical functions related to blood flow, respiration, cell death and proliferation, and protecting the vasculature from pro-inflammatory and coagulative stresses. Inhibition of NO formation, and/or diversion of NO away from its physiological signalling targets lead to dysregulated NO bioavailability, a hallmark of numerous vascular and pulmonary diseases. Current concepts suggest that the balance between NO formation and NO scavenging is critical in disease development, with the corollary being that redressing the balance offers a target for therapeutic intervention. Evidence presented over the last two decades has seen red blood cells (RBCs) and haemoglobin specifically emerge as prominent effectors in this paradigm. In this symposium review article, we discuss recent insights into the mechanisms by which RBCs may modulate the balance between NO-formation and inhibition. We discuss how these mechanisms may become dysfunctional to cause disease, highlight key questions that remain, and discuss the potential impact of these insights on therapeutic opportunities.

Nitric oxide signaling in the microcirculation

Several apparent paradoxes are evident when one compares mathematical predictions from models of nitric oxide (NO) diffusion and convection in vasculature structures with experimental measurements of NO (or related metabolites) in animal and human studies. Values for NO predicted from mathematical models are generally much lower than in vivo NO values reported in the literature for experiments, specifically with NO microelectrodes positioned at perivascular locations next to different sizes of blood vessels in the microcirculation and NO electrodes inserted into a wide range of tissues supplied by the microcirculation of each specific organ system under investigation. There continues to be uncertainty about the roles of NO scavenging by hemoglobin versus a storage function that may conserve NO, and other signaling targets for NO need to be considered. This review describes model predictions and relevant experimental data with respect to several signaling pathways in the microcirculation that involve NO.

Platelet inhibition by nitrite is dependent on erythrocytes and deoxygenation

Background

Nitrite is a nitric oxide (NO) metabolite in tissues and blood, which can be converted to NO under hypoxia to facilitate tissue perfusion. Although nitrite is known to cause vasodilation following its reduction to NO, the effect of nitrite on platelet activity remains unclear. In this study, the effect of nitrite and nitrite+erythrocytes, with and without deoxygenation, on platelet activity was investigated.

Methodology/Finding

Platelet aggregation was studied in platelet-rich plasma (PRP) and PRP+erythrocytes by turbidimetric and impedance aggregometry, respectively. In PRP, DEANONOate inhibited platelet aggregation induced by ADP while nitrite had no effect on platelets. In PRP+erythrocytes, the inhibitory effect of DEANONOate on platelets decreased whereas nitrite at physiologic concentration (0.1 µM) inhibited platelet aggregation and ATP release. The effect of nitrite+erythrocytes on platelets was abrogated by C-PTIO (a membrane-impermeable NO scavenger), suggesting an NO-mediated action. Furthermore, deoxygenation enhanced the effect of nitrite as observed from a decrease of P-selectin expression and increase of the cGMP levels in platelets. The ADP-induced platelet aggregation in whole blood showed inverse correlations with the nitrite levels in whole blood and erythrocytes.

Conclusion

Nitrite alone at physiological levels has no effect on platelets in plasma. Nitrite in the presence of erythrocytes inhibits platelets through its reduction to NO, which is promoted by deoxygenation. Nitrite may have role in modulating platelet activity in the circulation, especially during hypoxia.

Dramatic increase of nitrite levels in hearts of anoxia-exposed crucian carp supporting a role in cardioprotection

Nitrite (NO(2)(-)) functions as an important nitric oxide (NO) donor under hypoxic conditions. Both nitrite and NO have been found to protect the mammalian heart and other tissues against ischemia (anoxia)-reoxygenation injury by interacting with mitochondrial electron transport complexes and limiting the generation of reactive oxygen species upon reoxygenation. The crucian carp naturally survives extended periods without oxygen in an active state, which has made it a model for studying how evolution has solved the problems of anoxic survival. We investigated the role of nitrite and NO in the anoxia tolerance of this fish by measuring NO metabolites in normoxic, anoxic, and reoxygenated crucian carp. We also cloned and sequenced crucian carp NO synthase variants and quantified their mRNA levels in several tissues in normoxia and anoxia. Despite falling levels of blood plasma nitrite, the crucian carp showed massive increases in nitrite, S-nitrosothiols (SNO), and iron-nitrosyl (FeNO) compounds in anoxic heart tissue. NO(2)(-) levels were maintained in anoxic brain, liver, and gill tissues, whereas SNO and FeNO increased in a tissue-specific manner. Reoxygenation reestablished normoxic values. We conclude that NO(2)(-) is shifted into the tissues where it acts as NO donor during anoxia, inducing cytoprotection under anoxia/reoxygenation. This can be especially important in the crucian carp heart, which maintains output in anoxia. NO(2)(-) is currently tested as a therapeutic drug against reperfusion damage of ischemic hearts, and the present study provides evolutionary precedent for such an approach.

Current perspectives and challenges in understanding the role of nitrite as an integral player in nitric oxide biology and therapy

Beyond an inert oxidation product of nitric oxide (NO) metabolism, current thinking posits a key role for nitrite as a mediator of NO signaling, especially during hypoxia. This concept has been discussed in the context of nitrite serving a role as an endogenous modulator of NO homeostasis, but also from a novel clinical perspective whereby nitrite therapy may replenish NO signaling and prevent ischemic tissue injury. Indeed, the relatively rapid translation of studies delineating mechanisms of action to ongoing and planned clinical trials has been critical in fuelling interest in nitrite biology, and several excellent reviews have been written on this topic. In this article we limit our discussions to current concepts and what we feel are questions that remain unanswered within the paradigm of nitrite being a mediator of NO biology.

Differential uptake and metabolism of nitrite in normoxic and hypoxic goldfish

Nitrite is a physiologically important nitric oxide donor at low concentrations but becomes toxic at high concentrations, as develops in freshwater fish exposed to environmental nitrite. We hypothesized that nitrite uptake across the gills differs between normoxic and hypoxic fish and that nitrite accumulation causes excess nitric oxide formation and nitrosative stress. Nitrite and its metabolites were measured via chemiluminescence in normoxic and hypoxic goldfish in control conditions and after 1 day of nitrite exposure. Exposure to nitrite produced much higher nitrite levels in plasma, red blood cells (RBCs) and muscle tissue of normoxic than hypoxic goldfish, suggesting that nitrite uptake was augmented by normoxia in spite of a predictable lower gill surface area. Elevation of nitrite was associated with increased concentrations of S-nitroso, N-nitroso and Fe-nitrosyl compounds in both extracellular and intracellular compartments, revealing nitrosative stress with extensive nitros(yl)ation of thiols, amines and heme groups. The degree of nitrosative stress correlated with nitrite load. Nitrate levels increased in all compartments, reflecting that a significant fraction of the nitrite taken up was converted to non-toxic nitrate. The generation of methemoglobin and nitrosylhemoglobin (assessed by spectral deconvolution) was more pronounced during normoxic nitrite exposure than during hypoxic nitrite exposure, in agreement with the higher nitrite load in normoxic fish. However, at any given nitrite load inside RBCs, the formation of S-nitroso compounds was augmented by hypoxia. We conclude that ambient oxygen conditions have profound influence on branchial nitrite uptake and that nitrosative stress is an integral part of nitrite toxicity at high nitrite concentrations.

Nitric oxide metabolites in goldfish under normoxic and hypoxic conditions

Nitric oxide (NO), produced by nitric oxide synthases (NOS enzymes), regulates multiple physiological functions in animals. NO exerts its effects by binding to iron (Fe) of heme groups (exemplified by the activation of soluble guanylyl cyclase) and by S-nitrosylation of proteins – and it is metabolized to nitrite and nitrate. Nitrite is used as a marker for NOS activity but it is also a NO donor that can be activated by various cellular proteins under hypoxic conditions. Here, we report the first systematic study of NO metabolites (nitrite, nitrate, S-nitroso, N-nitroso and Fe-nitrosyl compounds) in multiple tissues of a non-mammalian vertebrate (goldfish) under normoxic and hypoxic conditions. NO metabolites were measured in blood (plasma and red cells) and heart, brain, gill, liver, kidney and skeletal muscle, using highly sensitive reductive chemiluminescence. The severity of the chosen hypoxia levels was assessed from metabolic and respiratory variables. In normoxic goldfish, the concentrations of NO metabolites in plasma and tissues were comparable with values reported in mammals, indicative of similar NOS activity. Exposure to hypoxia [at PO2 (partial pressure of O2) values close to and below the critical PO2] for two days caused large decreases in plasma nitrite and nitrate, which suggests reduced NOS activity and increased nitrite/nitrate utilization or loss. Tissue NO metabolites were largely maintained at their tissue-specific values under hypoxia, pointing at nitrite transfer from extracellular to intracellular compartments and cellular NO generation from nitrite. The data highlights the preference of goldfish to defend intracellular NO homeostasis during hypoxia.

The potential role of the red blood cell in nitrite-dependent regulation of blood flow

Nitrite was once thought to have little physiological relevance. However, nitrite is now being increasingly recognized as a therapeutic or possibly even physiological precursor of nitric oxide (NO) that is utilized when needed to increase blood flow. It is likely that different mechanisms for nitrite bioconversion occur in different tissues, but in the vascular system, there is evidence that erythrocyte haemoglobin (Hb) is responsible for the oxygen-dependent reduction of nitrite to modulate blood flow. Here, we review the complex chemical interactions of Hb and nitrite and discuss evidence supporting its role in vasodilation. We also discuss ongoing work focused on defining the precise mechanisms for export of NO activity from red blood cells and of other pathways that may mediate nitrite-dependent vasodilation.

Erythrocyte-dependent regulation of human skeletal muscle blood flow: role of varied oxyhemoglobin and exercise on nitrite, S-nitrosohemoglobin, and ATP

The erythrocyte is proposed to play a key role in the control of local tissue perfusion via three O(2)-dependent signaling mechanisms: 1) reduction of circulating nitrite to vasoactive NO, 2) S-nitrosohemoglobin (SNO-Hb)-dependent vasodilatation, and 3) release of the vasodilator and sympatholytic ATP; however, their relative roles in vivo remain unclear. Here we evaluated each mechanism to gain insight into their roles in the regulation of human skeletal muscle blood flow during hypoxia and hyperoxia at rest and during exercise. Arterial and femoral venous hemoglobin O(2) saturation (O(2)Hb), plasma and erythrocyte NO and ATP metabolites, and leg and systemic hemodynamics were measured in 10 healthy males exposed to graded hypoxia, normoxia, and graded hyperoxia both at rest and during submaximal one-legged knee-extensor exercise. At rest, leg blood flow and NO and ATP metabolites in plasma and erythrocytes remained unchanged despite large alterations in O(2)Hb. During exercise, however, leg and systemic perfusion and vascular conductance increased in direct proportion to decreases in arterial and venous O(2)Hb (r(2) = 0.86-0.98; P = 0.01), decreases in venous plasma nitrite (r(2) = 0.93; P < 0.01), increases in venous erythrocyte nitroso species (r(2) = 0.74; P < 0.05), and to a lesser extent increases in erythrocyte SNO (r(2) = 0.59; P = 0.07). No relationship was observed with plasma ATP (r(2) = 0.01; P = 0.99) or its degradation compounds. These in vivo data indicate that, during low-intensity exercise and hypoxic stress, but not hypoxic stress alone, plasma nitrite consumption and formation of erythrocyte nitroso species are associated with limb vasodilatation and increased blood flow in the human skeletal muscle vasculature.