Reactions between nitric oxide and haemoglobin under physiological conditions

Interorgan, intraorgan and interplant communication mediated by nitric oxide and related species

Plant survival to a potential plethora of diverse environmental insults is underpinned by coordinated communication amongst organs to help shape effective responses to these environmental challenges at the whole plant level. This interorgan communication is supported by a complex signal network that regulates growth, development and environmental responses. Nitric oxide (NO) has emerged as a key signalling molecule in plants. However, its potential role in interorgan communication has only recently started to come into view. Direct and indirect evidence has emerged supporting that NO and related species (S-nitrosoglutathione, nitro-linolenic acid) are mobile interorgan signals transmitting responses to stresses such as hypoxia and heat. Beyond their role as mobile signals, NO and related species are involved in mediating xylem development, thus contributing to efficient root-shoot communication. Moreover, NO and related species are regulators in intraorgan systemic defence responses aiming an effective, coordinated defence against pathogens. Beyond its in planta signalling role, NO and related species may act as ex planta signals coordinating external leaf-to-leaf, root-to-leaf but also plant-to-plant communication. Here, we discuss these exciting developments and emphasise how their manipulation may provide novel strategies for crop improvement.

Nitric Oxide Ameliorates the Effects of Hypoxia in Mice by Regulating Oxygen Transport by Hemoglobin

Xiaoying Zhou, Wenting Su, Quanwei Bao, Yu Cui, Xiaoxu Li, Yidong Yang, Chengzhong Yang, Chengyuan Wang, Li Jiao, Dewei Chen, and Jian Huang. Nitric oxide ameliorates the effects of hypoxia in mice by regulating oxygen transport by hemoglobin. High Alt Med Biol. 25:174-185, 2024.-Hypoxia is a common pathological and physiological phenomenon in ischemia, cancer, and strenuous exercise. Nitric oxide (NO) acts as an endothelium-derived relaxing factor in hypoxic vasodilation and serves as an allosteric regulator of hemoglobin (Hb). However, the ultimate effects of NO on the hematological system in vivo remain unknown, especially in extreme environmental hypoxia. Whether NO regulation of the structure of Hb improves oxygen transport remains unclear. Hence, we examined whether NO altered the oxygen affinity of Hb (Hb-O2 affinity) to protect extremely hypoxic mice. Mice were exposed to severe hypoxia with various concentrations of NO, and the survival time, exercise capacity, and other physical indexes were recorded. The survival time was prolonged in the 5 ppm NO (6.09 ± 1.29 minutes) and 10 ppm NO (6.39 ± 1.58 minutes) groups compared with the 0 ppm group (4.98 ± 1.23 minutes). Hypoxia of the brain was relieved, and the exercise exhaustion time was prolonged when mice inhaled 20 ppm NO (24.70 ± 6.87 minutes vs. 20.23 ± 6.51 minutes). In addition, the differences in arterial oxygen saturation (SO2%) (49.64 ± 7.29% vs. 42.90 ± 4.30%) and arteriovenous SO2% difference (25.14 ± 8.95% vs. 18.10 ± 6.90%) obviously increased. In ex vivo experiments, the oxygen equilibrium curve (OEC) left shifted as P50 decreased from 43.77 ± 2.49 mmHg (0 ppm NO) to 40.97 ± 1.40 mmHg (100 ppm NO) and 38.36 ± 2.78 mmHg (200 ppm NO). Furthermore, the Bohr effect of Hb was enhanced by the introduction of 200 ppm NO (-0.72 ± 0.062 vs.-0.65 ± 0.051), possibly allowing Hb to more easily offload oxygen in tissue at lower pH. The crystal structure reveals a greater distance between Asp94β-His146β in nitrosyl -Hb(NO-Hb), NO-HbβCSO93, and S-NitrosoHb(SNO-Hb) compared to tense Hb(T-Hb, 3.7 Å, 4.3 Å, and 5.8 Å respectively, versus 3.5 Å for T-Hb). Moreover, hydrogen bonds were less likely to form, representing a key limitation of relaxed Hb (R-Hb). Upon NO interaction with Hb, hydrogen bonds and salt bridges were less favored, facilitating relaxation. We speculated that NO ameliorated the effects of hypoxia in mice by promoting erythrocyte oxygen loading in the lung and offloading in tissues.

Tissue Perfusion and Diffusion and Cellular Respiration: Transport and Utilization of Oxygen

This article provides an overview of the journey of inspired oxygen after its uptake across the alveolar-capillary interface, and the interplay among tissue perfusion, diffusion, and cellular respiration in the transport and utilization of oxygen. The critical interactions between oxygen and its facilitative carriers (hemoglobin in red blood cells and myoglobin in muscle cells), and with other respiratory and vasoactive molecules (carbon dioxide, nitric oxide, and carbon monoxide), are emphasized to illustrate how this versatile system dynamically optimizes regional convective transport and diffusive gas exchange. The rates of reciprocal gas exchange in the lung and the periphery must be well-matched and sufficient for meeting the range of energy demands from rest to maximal stress but not excessive as to become toxic. The mobile red blood cells play a vital role in matching tissue perfusion and gas exchange by dynamically regulating the controlled uptake of oxygen and communicating regional metabolic signals across different organs. Intracellular oxygen diffusion and facilitation via myoglobin into the mitochondria, and utilization via electron transport chain and oxidative phosphorylation, are summarized. Physiological and pathophysiological adaptations are briefly described. Dysfunction of any component across this integrated system affects all other components and elicits corresponding structural and functional adaptation aimed at matching the capacities across the entire system and restoring equilibrium under normal and pathological conditions.

Recent insights into mechanisms of hypoxia-induced vasodilatation in the human brain

The cerebral vasculature manages oxygen delivery by adjusting arterial blood in-flow in the face of reductions in oxygen availability. Hypoxic cerebral vasodilatation, and the associated hypoxic cerebral blood flow reactivity, involve many vascular, erythrocytic and cerebral tissue mechanisms that mediate elevations in cerebral blood flow via micro- and macrovascular dilatation. This contemporary review focuses on in vivo human work - with reference to seminal preclinical work where necessary - on hypoxic cerebrovascular reactivity, particularly where recent advancements have been made. We provide updates with the following information: in humans, hypoxic cerebral vasodilatation is partially mediated via a - likely non-obligatory - combination of: (1) nitric oxide synthases, (2) deoxygenation-coupled S-nitrosothiols, (3) potassium channel-related vascular smooth muscle hyperpolarization, and (4) prostaglandin mechanisms with some contribution from an interrelationship with reactive oxygen species. And finally, we discuss the fact that, due to the engagement of deoxyhaemoglobin-related mechanisms, reductions in O2 content via haemoglobin per se seem to account for ∼50% of that seen with hypoxic cerebral vasodilatation during hypoxaemia. We further highlight the issue that methodological impediments challenge the complete elucidation of hypoxic cerebral reactivity mechanisms in vivo in healthy humans. Future research is needed to confirm recent advancements and to reconcile human and animal findings. Further investigations are also required to extend these findings to address questions of sex-, heredity-, age-, and disease-related differences. The final step is to then ultimately translate understanding of these mechanisms into actionable, targetable pathways for the prevention and treatment of cerebral vascular dysfunction and cerebral hypoxic brain injury.

Local delivery of gaseous signaling molecules for orthopedic disease therapy

Over the past decade, a proliferation of research has used nanoparticles to deliver gaseous signaling molecules for medical purposes. The discovery and revelation of the role of gaseous signaling molecules have been accompanied by nanoparticle therapies for their local delivery. While most of them have been applied in oncology, recent advances have demonstrated their considerable potential in diagnosing and treating orthopedic diseases. Three of the currently recognized gaseous signaling molecules, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S), are highlighted in this review along with their distinctive biological functions and roles in orthopedic diseases. Moreover, this review summarizes the progress in therapeutic development over the past ten years with a deeper discussion of unresolved issues and potential clinical applications.

Cerebral O2 and CO2 transport in isovolumic haemodilution: Compensation of cerebral delivery of O2 and maintenance of cerebrovascular reactivity to CO2

This study investigated the influence of acute reductions in arterial O₂ content (CaO₂) via isovolumic haemodilution on global cerebral blood flow (gCBF) and cerebrovascular CO₂ reactivity (CVR) in 11 healthy males (age; 28 ± 7 years: body mass index; 23 ± 2 kg/m²). Radial artery and internal jugular vein catheters provided measurement of blood pressure and gases, quantification of cerebral metabolism, cerebral CO₂ washout, and trans-cerebral nitrite exchange (ozone based chemiluminescence). Prior to and following haemodilution, the partial pressure of arterial CO₂ (PaCO₂) was elevated with dynamic end-tidal forcing while gCBF was measured with duplex ultrasound. CVR was determined as the slope of the gCBF response and PaCO₂. Replacement of ∼20% of blood volume with an equal volume of 5% human serum albumin (Alburex® 5%) reduced haemoglobin (13.8 ± 0.8 vs. 11.3 ± 0.6 g/dL; P < 0.001) and CaO₂ (18.9 ± 1.0 vs 15.0 ± 0.8 mL/dL P < 0.001), elevated gCBF (+18 ± 11%; P = 0.002), preserved cerebral oxygen delivery (P = 0.49), and elevated CO₂ washout (+11%; P = 0.01). The net cerebral uptake of nitrite (11.6 ± 14.0 nmol/min; P = 0.027) at baseline was abolished following haemodilution (-3.6 ± 17.9 nmol/min; P = 0.54), perhaps underpinning the conservation of CVR (61.7 ± 19.0 vs. 69.0 ± 19.2 mL/min/mmHg; P = 0.23). These findings demonstrate that the cerebrovascular responses to acute anaemia in healthy humans are sufficient to support the maintenance of CVR.

S-Nitroso-l-cysteine and ventilatory drive: A pediatric perspective

Though endogenous S-nitroso-l-cysteine (l-CSNO) signaling at the level of the carotid body increases minute ventilation (v̇E ), neither the background data nor the potential clinical relevance are well-understood by pulmonologists in general, or by pediatric pulmonologists in particular. Here, we first review how regulation of the synthesis, activation, transmembrane transport, target interaction, and degradation of l-CSNO can affect the ventilatory drive. In particular, we review l-CSNO formation by hemoglobin R to T conformational change and by nitric oxide (NO) synthases (NOS), and the downstream effects on v̇E through interaction with voltage-gated K+ (Kv) channel proteins and other targets in the peripheral and central nervous systems. We will review how these effects are independent of-and, in fact may be opposite to-those of NO. Next, we will review evidence that specific elements of this pathway may underlie disorders of respiratory control in childhood. Finally, we will review the potential clinical implications of this pathway in the development of respiratory stimulants, with a particular focus on potential pediatric applications.

The role of globins in cardiovascular physiology

Globin proteins exist in every cell type of the vasculature, from erythrocytes to endothelial cells, vascular smooth muscle cells, and peripheral nerve cells. Many globin subtypes are also expressed in muscle tissues (including cardiac and skeletal muscle), in other organ-specific cell types, and in cells of the central nervous system (CNS). The ability of each of these globins to interact with molecular oxygen (O2) and nitric oxide (NO) is preserved across these contexts. Endothelial α-globin is an example of extraerythrocytic globin expression. Other globins, including myoglobin, cytoglobin, and neuroglobin, are observed in other vascular tissues. Myoglobin is observed primarily in skeletal muscle and smooth muscle cells surrounding the aorta or other large arteries. Cytoglobin is found in vascular smooth muscle but can also be expressed in nonvascular cell types, especially in oxidative stress conditions after ischemic insult. Neuroglobin was first observed in neuronal cells, and its expression appears to be restricted mainly to the CNS and the peripheral nervous system. Brain and CNS neurons expressing neuroglobin are positioned close to many arteries within the brain parenchyma and can control smooth muscle contraction and thus tissue perfusion and vascular reactivity. Overall, reactions between NO and globin heme iron contribute to vascular homeostasis by regulating vasodilatory NO signals and scavenging reactive species in cells of the mammalian vascular system. Here, we discuss how globin proteins affect vascular physiology, with a focus on NO biology, and offer perspectives for future study of these functions.

The enzymatic function of the honorary enzyme: S-nitrosylation of hemoglobin in physiology and medicine

The allosteric transition within tetrameric hemoglobin (Hb) that allows both full binding to four oxygen molecules in the lung and full release of four oxygens in hypoxic tissues would earn Hb the moniker of 'honorary enzyme'. However, the allosteric model for oxygen binding in hemoglobin overlooked the essential role of blood flow in tissue oxygenation that is essential for life (aka autoregulation of blood flow). That is, blood flow, not oxygen content of blood, is the principal determinant of oxygen delivery under most conditions. With the discovery that hemoglobin carries a third biologic gas, nitric oxide (NO) in the form of S-nitrosothiol (SNO) at β-globin Cys93 (βCys93), and that formation and export of SNO to dilate blood vessels are linked to hemoglobin allostery through enzymatic activity, this title is honorary no more. This chapter reviews evidence that hemoglobin formation and release of SNO is a critical mediator of hypoxic autoregulation of blood flow in tissues leading to oxygen delivery, considers the physiological implications of a 3-gas respiratory cycle (O2/NO/CO2) and the pathophysiological consequences of its dysfunction. Opportunities for therapeutic intervention to optimize oxygen delivery at the level of tissue blood flow are highlighted.

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

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

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Hypoxic vasodilatory defect and pulmonary hypertension in mice lacking hemoglobin β-cysteine93 S-nitrosylation

Systemic hypoxia is characterized by peripheral vasodilation and pulmonary vasoconstriction. However, the system-wide mechanism for signaling hypoxia remains unknown. Accumulating evidence suggests that hemoglobin (Hb) in RBCs may serve as an O₂ sensor and O₂-responsive NO signal transducer to regulate systemic and pulmonary vascular tone, but this remains unexamined at the integrated system level. One residue invariant in mammalian Hbs, β-globin cysteine93 (βCys93), carries NO as vasorelaxant S-nitrosothiol (SNO) to autoregulate blood flow during O₂ delivery. βCys93Ala mutant mice thus exhibit systemic hypoxia despite transporting O₂ normally. Here, we show that βCys93Ala mutant mice had reduced S-nitrosohemoglobin (SNO-Hb) at baseline and upon targeted SNO repletion and that hypoxic vasodilation by RBCs was impaired in vitro and in vivo, recapitulating hypoxic pathophysiology. Notably, βCys93Ala mutant mice showed marked impairment of hypoxic peripheral vasodilation and developed signs of pulmonary hypertension with age. Mutant mice also died prematurely with cor pulmonale (pulmonary hypertension with right ventricular dysfunction) when living under low O₂. Altogether, we identify a major role for RBC SNO in clinically relevant vasodilatory responses attributed previously to endothelial NO. We conclude that SNO-Hb transduces the integrated, system-wide response to hypoxia in the mammalian respiratory cycle, expanding a core physiological principle.

Airway Thiol-NO Adducts as Determinants of Exhaled NO

Thiol-NO adducts such as S-nitrosoglutathione (GSNO) are endogenous bronchodilators in human airways. Decreased airway S-nitrosothiol concentrations are associated with asthma. Nitric oxide (NO), a breakdown product of GSNO, is measured in exhaled breath as a biomarker in asthma; an elevated fraction of expired NO (F_ENO) is associated with asthmatic airway inflammation. We hypothesized that F_ENO could reflect airway S-nitrosothiol concentrations. To test this hypothesis, we first studied the relationship between mixed expired NO and airway S-nitrosothiols in patients endotracheally intubated for respiratory failure. The inverse (Lineweaver-Burke type) relationship suggested that expired NO could reflect the rate of pulmonary S-nitrosothiol breakdown. We thus studied NO evolution from the lungs of mice (GSNO reductase ^−/−) unable reductively to catabolize GSNO. More NO was produced from GSNO in the ^−/− compared to wild type lungs. Finally, we formally tested the hypothesis that airway GSNO increases F_ENO using an inhalational challenge model in normal human subjects. F_ENO increased in all subjects tested, with a median t_1/2 of 32.0 min. Taken together, these data demonstrate that F_ENO reports, at least in part, GSNO breakdown in the lungs. Unlike GSNO, NO is not present in the lungs in physiologically relevant concentrations. However, F_ENO following a GSNO challenge could be a non-invasive test for airway GSNO catabolism.

Biological Mechanisms of S-Nitrosothiol Formation and Degradation: How Is Specificity of S-Nitrosylation Achieved?

The modification of protein cysteine residues underlies some of the diverse biological functions of nitric oxide (NO) in physiology and disease. The formation of stable nitrosothiols occurs under biologically relevant conditions and time scales. However, the factors that determine the selective nature of this modification remain poorly understood, making it difficult to predict thiol targets and thus construct informatics networks. In this review, the biological chemistry of NO will be considered within the context of nitrosothiol formation and degradation whilst considering how specificity is achieved in this important post-translational modification. Since nitrosothiol formation requires a formal one-electron oxidation, a classification of reaction mechanisms is proposed regarding which species undergoes electron abstraction: NO, thiol or S-NO radical intermediate. Relevant kinetic, thermodynamic and mechanistic considerations will be examined and the impact of sources of NO and the chemical nature of potential reaction targets is also discussed.

Red Blood Cell-Mediated S-Nitrosohemoglobin-Dependent Vasodilation: Lessons Learned from a β-Globin Cys93 Knock-In Mouse

Significance: Red blood cell (RBC)-mediated vasodilation plays an important role in oxygen delivery. This occurs through hemoglobin actions, at least in significant part, to convert heme-bound nitric oxide (NO) (in tense [T]/deoxygenated-state hemoglobin) into vasodilator S-nitrosothiol (SNO) (in relaxed [R]/oxygenated-state hemoglobin), convey SNO through the bloodstream, and release it into tissues to increase blood flow. The coupling of hemoglobin R/T state allostery, both to NO conversion into SNO and to SNO release (along with oxygen), under hypoxia supports the model of a three-gas respiratory cycle (O2/NO/CO2). Recent Advances: Oxygenation of tissues is dependent on a single, strictly conserved Cys residue in hemoglobin (βCys93). Hemoglobin couples SNO formation/release at βCys93 to O2 binding/release at hemes ("thermodynamic linkage"). Mice bearing βCys93Ala hemoglobin that is unable to generate SNO-βCys93 establish that SNO-hemoglobin is important for R/T allostery-regulated vasodilation by RBCs that couple blood flow to tissue oxygenation. Critical Issues: The model for RBC-mediated vasodilation originally proposed by Stamler et al. in 1996 has been largely validated: SNO-βCys93 forms in vivo, dilates blood vessels, and is hypoxia-regulated, and RBCs actuate vasodilation proportionate to hypoxia. Numerous compensations in βCys93Ala animals to alleviate tissue hypoxia (discussed herein) are predicted to preserve vasodilatory responses of RBCs but impair linkage to R/T transition in hemoglobin. This is borne out by loss of responsivity of mutant RBCs to oxygen, impaired blood flow responses to hypoxia, and tissue ischemia in βCys93-mutant animals. Future Directions: SNO-hemoglobin mediates hypoxic vasodilation in the respiratory cycle. This fundamental physiology promises new insights in vascular diseases and blood disorders.

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

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

Tracing the path of inhaled nitric oxide: Biological consequences of protein nitrosylation

Nitric oxide (NO) is a comprehensive regulator of vascular and airway tone. Endogenous NO produced by nitric oxide synthases regulates multiple signaling cascades, including activation of soluble guanylate cyclase to generate cGMP, relaxing smooth muscle cells. Inhaled NO is an established therapy for pulmonary hypertension in neonates, and has been recently proposed for the treatment of hypoxic respiratory failure and acute respiratory distress syndrome due to COVID-19. In this review, we summarize the effects of endogenous and exogenous NO on protein S-nitrosylation, which is the selective and reversible covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine. This posttranslational modification targets specific cysteines based on the acid/base sequence of surrounding residues, with significant impacts on protein interactions and function. S-nitrosothiol (SNO) formation is tightly compartmentalized and enzymatically controlled, but also propagated by nonenzymatic transnitrosylation of downstream protein targets. Redox-based nitrosylation and denitrosylation pathways dynamically regulate the equilibrium of SNO-proteins. We review the physiological roles of SNO proteins, including nitrosohemoglobin and autoregulation of blood flow through hypoxic vasodilation, and pathological effects of nitrosylation including inhibition of critical vasodilator enzymes; and discuss the intersection of NO source and dose with redox environment, in determining the effects of protein nitrosylation.

Inorganic nitrite bioactivation and role in physiological signaling and therapeutics

The bioactivation of inorganic nitrite refers to the conversion of otherwise 'inert' nitrite to the diatomic signaling molecule nitric oxide (NO), which plays important roles in human physiology and disease, notably in the regulation of vascular tone and blood flow. While the most well-known sources of NO are the nitric oxide synthase (NOS) enzymes, another source of NO is the nitrate-nitrite-NO pathway, whereby nitrite (obtained from reduction of dietary nitrate) is further reduced to form NO. The past few decades have seen extensive study of the mechanisms of NO generation through nitrate and nitrite bioactivation, as well as growing appreciation of the contribution of this pathway to NO signaling in vivo. This review, prepared for the volume 400 celebration issue of Biological Chemistry, summarizes some of the key reactions of the nitrate-nitrite-NO pathway such as reduction, disproportionation, dehydration, and oxidative denitrosylation, as well as current evidence for the contribution of the pathway to human cardiovascular physiology. Finally, ongoing efforts to develop novel medical therapies for multifarious conditions, especially those related to pathologic vasoconstriction and ischemia/reperfusion injury, are also explored.

Nitric oxide: To be or not to be an endocrine hormone?

Nitric oxide (NO), a highly reactive gasotransmitter, is critical for a number of cellular processes and has multiple biological functions. Due to its limited lifetime and diffusion distance, NO has been mainly believed to act in autocrine/paracrine fashion. The increasingly recognized effects of pharmacologically delivered and endogenous NO at a distant site have changed the conventional wisdom and introduced NO as an endocrine signalling molecule. The notion is greatly supported by the detection of a number of NO adducts and their circulatory cycles, which in turn contribute to the transport and delivery of NO bioactivity, remote from the sites of its synthesis. The existence of endocrine sites of synthesis, negative feedback regulation of biosynthesis, integrated storage and transport systems, having an exclusive receptor, that is, soluble guanylyl cyclase (sGC), and organized circadian rhythmicity make NO something beyond a simple autocrine/paracrine signalling molecule that could qualify for being an endocrine signalling molecule. Here, we discuss hormonal features of NO from the classical endocrine point of view and review available knowledge supporting NO as a true endocrine hormone. This new insight can provide a new framework within which to reinterpret NO biology and its clinical applications.

Red Blood Cell Dysfunction in Critical Illness

Oxygen (O2) delivery, which is fundamental to supporting patients with critical illness, is a function of blood O2 content and flow. This article reviews red blood cell (RBC) physiology and dysfunction relevant to disordered O2 delivery in the critically ill. Flow is the focus of O2 delivery regulation: O2 content is relatively fixed, whereas flow fluctuates greatly. Thus, blood flow volume and distribution vary to maintain coupling between O2 delivery and demand. This article reviews conventional RBC physiology influencing O2 delivery and introduces a paradigm for O2 delivery homeostasis based on coordinated gas transport and vascular signaling by RBCs.

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.

Optimization of Ethanol Production using Nitrosative Stress Exposed S.cerevisiae

S.cerevisiae is an industrially important organism known for its ability to produce ethanol as the demand for ethanol is increasing day by day all over the world, the need to find better and alternative ways to increase ethanol production is also rising. In this work we have proposed such alternative but effective method for producing ethanol by S.cerevisiae. Here, we are reporting for the first time the effect of nitrosative stress on ethanol production. Under in vivo condition, nitrosative stress is marked by the modification of macromolecules in the presence of reactive nitrogen species (RNS). Our result showed that treated cells were more capable for ethanol production compared with untreated cells. Our result also showed enhanced alcohol dehydrogenase activity under stressed condition. Further ethanol production was also optimized by using Response Surface Methodology (RSM) with stressed cells. Further, production of ethanol with immobilized beads of stress affected Saccharomyces cerevisiae was also determined. Overall, the obtained data showed that under nitrosative stress, the maximum ethanol production is 34.4 g/l after 24 h and such higher production was observed even after several cycles of fermentation. This is the first report of this kind showing the relation between nitrosative stress and ethanol production in Saccharomyces cerevisiae which may have important industrial application.

[Research progress on mechanism in adaptation of hemoglobin to plateau hypoxia]

Low oxygen partial pressure is the main cause of acute mountain sickness.Hemoglobin plays a crucial physiological role in the binding, utilization, transportation and release of oxygen in the body. To increase the capacity of oxygen binding of hemoglobin or the capacity of oxygen supply in tissues can help alleviate altitude sickness. However, increasing hemoglobin content has certain limitations. Using techniques from molecular biology, researchers are looking for endogenous or exogenous substances that can regulate the conformation of hemoglobin to increase oxygen uptake in the alveoli, or the availability of alveolar oxygen in the tissues. At present, the research on allosteric modulators to improve the affinity of hemoglobin has made some progress, and research on applying this mechanism to plateau hypoxia is also underway. This article reviews the relationship between hemoglobin and hypoxia, the structure of hemoglobin and the role of various allosteric modulators in hypoxia, which would provide information for finding new substances regulating the conformation of hemoglobin.

Nitric oxide diffusion through cystic fibrosis-relevant media and lung tissue

A simplified diffusion cell methodology was employed to measure the diffusion coefficient of nitric oxide (NO) through phosphate buffered saline (PBS) and artificial sputum medium (ASM)—an in vitro analog for airway mucus. Diffusion through the proteinaceous ASM yielded a significantly lower diffusion coefficient compared to PBS, which is attributed to both the physical obstruction by the mucin mesh and reactive nature of NO radicals towards the biological compounds in ASM. To further confirm that ASM was restricting NO from diffusing freely, a macromolecular propylamine-modified cyclodextrin donor (CD-PA) was employed to release the NO more slowly. The NO diffusion characteristics in ASM via the NO donor were also slower relative to PBS. As NO is likely to interact with lung cells after passing through the mucus barrier, the diffusion of both NO and the CD-PA macromolecular NO donor through differentiated lung tissue was investigated with and without an ASM layer. Comparison of NO diffusion through the three diffusion barriers indicated that the lung tissue significantly impeded NO penetration over the course of the experiment compared to PBS and ASM. In fact, the diffusion of CD-PA through the lung tissue was hindered until after the release of its NO payload, potentially due to the increased net charge of the NO donor structure. Of importance, the viability of the tissue was not influenced by the NO-releasing CD-PA at bactericidal concentrations.

Nitric oxide diffusion monitored through artificial sputum medium using an adaptable diffusion cell and released from donor through human lung tissue.

Role of Nitric Oxide Carried by Hemoglobin in Cardiovascular Physiology: Developments on a Three-Gas Respiratory Cycle

A continuous supply of oxygen is essential for the survival of multicellular organisms. The understanding of how this supply is regulated in the microvasculature has evolved from viewing erythrocytes (red blood cells [RBCs]) as passive carriers of oxygen to recognizing the complex interplay between Hb (hemoglobin) and oxygen, carbon dioxide, and nitric oxide-the three-gas respiratory cycle-that insures adequate oxygen and nutrient delivery to meet local metabolic demand. In this context, it is blood flow and not blood oxygen content that is the main driver of tissue oxygenation by RBCs. Herein, we review the lines of experimentation that led to this understanding of RBC function; from the foundational understanding of allosteric regulation of oxygen binding in Hb in the stereochemical model of Perutz, to blood flow autoregulation (hypoxic vasodilation governing oxygen delivery) observed by Guyton, to current understanding that centers on S-nitrosylation of Hb (ie, S-nitrosohemoglobin; SNO-Hb) as a purveyor of oxygen-dependent vasodilatory activity. Notably, hypoxic vasodilation is recapitulated by native S-nitrosothiol (SNO)-replete RBCs and by SNO-Hb itself, whereby SNO is released from Hb and RBCs during deoxygenation, in proportion to the degree of Hb deoxygenation, to regulate vessels directly. In addition, we discuss how dysregulation of this system through genetic mutation in Hb or through disease is a common factor in oxygenation pathologies resulting from microcirculatory impairment, including sickle cell disease, ischemic heart disease, and heart failure. We then conclude by identifying potential therapeutic interventions to correct deficits in RBC-mediated vasodilation to improve oxygen delivery-steps toward effective microvasculature-targeted therapies. To the extent that diseases of the heart, lungs, and blood are associated with impaired tissue oxygenation, the development of new therapies based on the three-gas respiratory system have the potential to improve the well-being of millions of patients.

Nitric oxide molecular targets: reprogramming plant development upon stress

Plants are sessile organisms that need to complete their life cycle by the integration of different abiotic and biotic environmental signals, tailoring developmental cues and defense concomitantly. Commonly, stress responses are detrimental to plant growth and, despite the fact that intensive efforts have been made to understand both plant development and defense separately, most of the molecular basis of this trade-off remains elusive. To cope with such a diverse range of processes, plants have developed several strategies including the precise balance of key plant growth and stress regulators [i.e. phytohormones, reactive nitrogen species (RNS), and reactive oxygen species (ROS)]. Among RNS, nitric oxide (NO) is a ubiquitous gasotransmitter involved in redox homeostasis that regulates specific checkpoints to control the switch between development and stress, mainly by post-translational protein modifications comprising S-nitrosation of cysteine residues and metals, and nitration of tyrosine residues. In this review, we have sought to compile those known NO molecular targets able to balance the crossroads between plant development and stress, with special emphasis on the metabolism, perception, and signaling of the phytohormones abscisic acid and salicylic acid during abiotic and biotic stress responses.

Alternate and Additional Functions of Erythrocyte Hemoglobin

The review discusses pleiotropic effects of erythrocytic hemoglobin (Hb) and their significance for human health. Hemoglobin is mostly known as an oxygen carrier, but its biochemical functions are not limited to this. The following aspects of Hb functioning are examined: (i) catalytic functions of the heme component (nitrite reductase, NO dioxygenase, monooxygenase, alkylhydroperoxidase) and of the apoprotein (esterase, lipoxygenase); (ii) participation in nitric oxide metabolism; (iii) formation of membrane-bound Hb and its role in the regulation of erythrocyte metabolism; (iv) physiological functions of Hb catabolic products (iron, CO, bilirubin, peptides). Special attention is given to Hb participation in signal transduction in erythrocytes. The relationships between various erythrocyte metabolic parameters, such as oxygen status, ATP formation, pH regulation, redox balance, and state of the cytoskeleton are discussed with regard to Hb. Hb polyfunctionality can be considered as a manifestation of the principle of biochemical economy.

S-Nitrosothiols: Materials, Reactivity and Mechanisms

The article provides a comprehensive view of S-nitrosothiols, chemical behaviour, the pathways leading to their synthesis, their spectral properties, analytical methods of detection and determination, chemical and photochemical reactivity, kinetic aspects and suggested mechanisms. The structure parameters of S-nitrosothiols and the parent thiols are analysed with respect to their effect on the strengthening or weakening the S–NO bond, and in consequence on the S-nitrosothiol stability. This depends also on the ease of S–S bond formation in the product disulphide. These structural features seem to be crucial both to spontaneous as well as to Cu-catalysed decomposition. Principal emphasis is given here to the S-nitrosothiols’ ability to act as ligands and to the effect of coordination on the ligand properties. The chemical and photochemical behaviours of the complexes are described in more detail and their roles in chemical and biochemical systems are discussed.

The aim of the article is to demonstrate that the contribution of S-nitrosothiols to chemical and biochemical processes is more diverse than supposed hitherto. Nevertheless, their role is predictable and, based on the correlation between structure and reactivity, many important mechanisms of biochemical processes can be interpreted and various applications designed.

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.

LDL dinitrosyl iron complex: A new transferrin-independent route for iron delivery in hepatocytes

In view of the interrelations between NO, Fe, and LDL in the cardiovascular system it appears interesting to find out, if the lipoprotein particles undergo the process of iron-nitrosylation, commonly observed for other proteins and what is the biological fate of iron-nitrosylated LDL particles. Iron-nitrosylated LDL preparation containing Fe(NO)₂ motif (DNICLDL) was obtained and characterized for the first time. In order to test its interactions with potential target cells, DNICLDL was administered to the hepatoma HepG2 cells. The effects were referred to those induced by native LDL (nLDL) and oxidized LDL (oxLDL) particles. DNICLDL administration considerably increased total iron content in the studied cell line, but did not influence the level of calcein-chelatable ions. DNICLDL was found to be low toxic to cells. The study suggests that DNICLDL might be a potential transducer of iron. © 2017 BioFactors, 44(2):192-201, 2018.

Kinetic Analysis and Structural Interpretation of Competitive Ligand Binding for NO Dioxygenation in Truncated Hemoglobin N

The conversion of nitric oxide (NO) into nitrate (NO₃^- ) by dioxygenation protects cells from lethal NO. Starting from NO-bound heme, the first step in converting NO into benign NO₃^- is the ligand exchange reaction FeNO+O₂ →FeO₂ +NO, which is still poorly understood at a molecular level. For wild-type (WT) truncated hemoglobin N (trHbN) and its Y33A mutant, the calculated barriers for the exchange reaction differ by 1.5 kcal mol^-1 , compared with 1.7 kcal mol^-1 from experiment. It is directly confirmed that the ligand exchange reaction is rate-limiting in trHbN and that entropic contributions account for 75 % of the difference between the WT and the mutant. Residues Tyr 33, Phe 46, Val 80, His 81, and Gln 82 surrounding the active site are expected to control the reaction path. By comparison with electronic structure calculations, the transition state separating the two ligand-bound states was assigned to a ² A state.

Pulmonary Delivery of Therapeutic and Diagnostic Gases

The 21st Congress for the International Society for Aerosols in Medicine included, for the first time, a session on Pulmonary Delivery of Therapeutic and Diagnostic Gases. The rationale for such a session within ISAM is that the pulmonary delivery of gaseous drugs in many cases targets the same therapeutic areas as aerosol drug delivery, and is in many scientific and technical aspects similar to aerosol drug delivery. This article serves as a report on the recent ISAM congress session providing a synopsis of each of the presentations. The topics covered are the conception, testing, and development of the use of nitric oxide to treat pulmonary hypertension; the use of realistic adult nasal replicas to evaluate the performance of pulsed oxygen delivery devices; an overview of several diagnostic gas modalities; and the use of inhaled oxygen as a proton magnetic resonance imaging (MRI) contrast agent for imaging temporal changes in the distribution of specific ventilation during recovery from bronchoconstriction. Themes common to these diverse applications of inhaled gases in medicine are discussed, along with future perspectives on development of therapeutic and diagnostic gases.

A Multiplex Enzymatic Machinery for Cellular Protein S-nitrosylation

S-nitrosylation, the oxidative modification of Cys residues by nitric oxide (NO) to form S-nitrosothiols (SNOs), modifies all main classes of proteins and provides a fundamental redox-based cellular signaling mechanism. However, in contrast to other post-translational protein modifications, S-nitrosylation is generally considered to be non-enzymatic, involving multiple chemical routes. We report here that endogenous protein S-nitrosylation in the model organism E. coli depends principally upon the enzymatic activity of the hybrid cluster protein Hcp, employing NO produced by nitrate reductase. Anaerobiosis on nitrate induces both Hcp and nitrate reductase, thereby resulting in the S-nitrosylation-dependent assembly of a large interactome including enzymes that generate NO (NO synthase), synthesize SNO-proteins (SNO synthase), and propagate SNO-based signaling (trans-nitrosylases) to regulate cell motility and metabolism. Thus, protein S-nitrosylation by NO in E. coli is essentially enzymatic, and the potential generality of the multiplex enzymatic mechanism that we describe may support a re-conceptualization of NO-based cellular signaling.

Pharmacologic Targeting of Red Blood Cells to Improve Tissue Oxygenation

Disruption of microvascular blood flow is a common cause of tissue hypoxia in disease, yet no therapies are available that directly target the microvasculature to improve tissue oxygenation. Red blood cells (RBCs) autoregulate blood flow through S-nitroso-hemoglobin (SNO-Hb)-mediated export of nitric oxide (NO) bioactivity. We therefore tested the idea that pharmacological enhancement of RBCs using the S-nitrosylating agent ethyl nitrite (ENO) may provide a novel approach to improve tissue oxygenation. Serial ENO dosing was carried out in sheep (1-400 ppm) and humans (1-100 ppm) at normoxia and at reduced fraction of inspired oxygen (FiO₂ ). ENO increased RBC SNO-Hb levels, corrected hypoxia-induced deficits in tissue oxygenation, and improved measures of oxygen utilization in both species. No adverse effects or safety concerns were identified. Inasmuch as impaired oxygenation is a major cause of morbidity and mortality, ENO may have widespread therapeutic utility, providing a first-in-class agent targeting the microvasculature.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

MANAGEMENT OF NITROGEN OXIDE EMISSIONS AS A FACTOR OF REDUCING THE LEVEL OF CONTAMINATION AND INCREASING THE SAFETY OF HUMAN LIFE

The issues related to the reduction of the level of pollution and the increase in the safety of human life through the control of the operation of coke batteries used in heavy industry are investigated. Data on sources of emissions of harmful substances, including nitrogen oxides, are presented. Different modes of operation of coke batteries are analyzed and mathematical models are obtained, which allow choosing rational operating regimes based on the criterion for minimizing emissions of nitrogen oxides.

Effect of nitric oxide to axonal degeneration in multiple sclerosis via downregulating monocarboxylate transporter 1 in oligodendrocytes

Multiple sclerosis (MS) is a neurodegenerative disease of the central nervous system (CNS). Axonal degeneration, one of the main pathological characteristics of MS, is affected by nitric oxide (NO). In turn, NO induces mitochondrial dysfunction of neurons and glial cells. Inadequate glucose causes monocarboxylate transporter 1 (MCT1) to transfer lactate from oligodendrocytes (OLs) to neurons, which decreases MCT1 and results in energy substrate deficit (mainly lactate) in axons. The condition gradually leads to axonal degeneration. This study proposes that NO-induced MCT1 down-regulation in OLs may be involved in the pathological process of axonal degeneration, which eventually leads to MS.

Pulmonary and Systemic Vascular Resistances After Cardiopulmonary Bypass: Role of Hemolysis

OBJECTIVES

Prolonged cardiopulmonary bypass (CPB) is associated with hemolysis, resulting in increased plasma oxyhemoglobin and vascular nitric oxide depletion. The authors hypothesized that hemolysis associated with CPB would reduce nitric oxide bioavailability, resulting in high pulmonary and systemic vascular resistances that after CPB would normalize gradually over time, due to clearance of plasma oxyhemoglobin. The authors also investigated whether prolonged CPB (≥140 min) produced increased levels of hemolysis and greater pulmonary and systemic vasoconstriction.

DESIGN

Prospective cohort study.

SETTING

Single-center university hospital.

PATIENTS

The study comprised 50 patients undergoing elective cardiac surgery requiring CPB.

INTERVENTIONS

Plasma hemoglobin and plasma nitric oxide consumption were measured before surgery and after CPB. Pulmonary and systemic hemodynamics were measured after CPB. The effects of short (<140 min) and prolonged (≥140 min) CPB on these parameters were considered.

MEASUREMENTS AND MAIN RESULTS

Pulmonary and systemic vascular resistances and plasma hemoglobin and nitric oxide consumption were highest at 15 minutes after CPB and then decreased over time. Pulmonary and systemic vascular resistances and plasma hemoglobin and plasma nitric oxide consumption were higher in patients requiring prolonged CPB. The reduction in plasma nitric oxide consumption from 15 minutes to 4 hours after CPB was correlated independently with the reductions in pulmonary and systemic vascular resistances.

CONCLUSIONS

Prolonged CPB was associated with increased plasma hemoglobin and plasma nitric oxide consumption and pulmonary and systemic vascular resistances. The reduction in plasma nitric oxide consumption at 4 hours after CPB was an independent predictor of the concomitant reductions in pulmonary and systemic vascular resistances.

Thermography Improves Clinical Assessment in Patients with Systemic Sclerosis Treated with Ozone Therapy

Objective. Treatment of scleroderma is challenging and limited. The aim of our study was to evaluate the usefulness of thermography in assessment of the clinical condition (joints movability and skin thickness) in clinically advanced patients with systemic sclerosis before and after ozone therapy. Method. The study included 42 patients aged 32 to 73 years with advanced systemic sclerosis hospitalized in the university clinic between 2003 and 2006. Thermography and clinical examinations were conducted at baseline and after two series of bath in water with ozone. Results. The comparison of results showed significant increase in skin temperature by 2.5°C, significant increase in interphalangeal joints movability by 18 degrees, and significant decrease in skin score by 14.7 points. The skin temperature was correlated with skin score (r = -0.59) and joints movability (r = +0.8). Conclusions. Ozone therapy shows positive effect on clinical parameters and skin temperature as measured with thermography. The study indicated possibility of introducing ozonotherapy as an independent therapy in cases with low level of progression or during remission periods and as additional treatment in patients with advanced disease requiring immunosuppressive treatment. Thermography is useful in assessment of skin condition showing strong correlation between skin temperature and clinical parameters.

Normalization of hemoglobin-based oxygen carrier-201 induced vasoconstriction: targeting nitric oxide and endothelin

Hemoglobin-based oxygen carrier (HBOC)-201 is a cell-free modified hemoglobin solution potentially facilitating oxygen uptake and delivery in cardiovascular disorders and hemorrhagic shock. Clinical use has been hampered by vasoconstriction in the systemic and pulmonary beds. Therefore, we aimed to 1) determine the possibility of counteracting HBOC-201-induced pressor effects with either adenosine (ADO) or nitroglycerin (NTG); 2) assess the potential roles of nitric oxide (NO) scavenging, reactive oxygen species (ROS), and endothelin (ET) in mediating the observed vasoconstriction; and 3) compare these effects in resting and exercising swine. Chronically instrumented swine were studied at rest and during exercise after administration of HBOC-201 alone or in combination with ADO. The role of NO was assessed by supplementation with NTG or administration of the eNOS inhibitor N^ω-nitro-l-arginine. Alternative vasoactive pathways were investigated via intravenous administration of the ET_A/ET_B receptor blocker tezosentan or a mixture of ROS scavengers. The systemic and to a lesser extent the pulmonary pressor effects of HBOC-201 could be counteracted by ADO; however, dosage titration was very important to avoid systemic hypotension. Similarly, supplementation of NO with NTG negated the pressor effects but also required titration of the dose. The pressor response to HBOC-201 was reduced after eNOS inhibition and abolished by simultaneous ET_A/ET_B receptor blockade, while ROS scavenging had no effect. In conclusion, the pressor response to HBOC-201 is mediated by vasoconstriction due to NO scavenging and production of ET. Further research should explore the effect of longer-acting ET receptor blockers to counteract the side effect of hemoglobin-based oxygen carriers.NEW & NOTEWORTHY Hemoglobin-based oxygen carrier (HBOC)-201 can disrupt hemodynamic homeostasis, mimicking some aspects of endothelial dysfunction, resulting in elevated systemic and pulmonary blood pressures. HBOC-201-induced vasoconstriction is mediated by scavenging nitric oxide (NO) and by upregulating endothelin (ET) production. Pressor effects can be prevented by adjuvant treatment with NO donors or direct vasodilators, such as nitroglycerin or adenosine, but dosages must be carefully monitored to avoid hypotension. However, hemodynamic normalization is more easily achieved via administration of an ET receptor blocker.

Nitrite and S -Nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation

Background:

The mechanisms underlying red blood cell (RBC)–mediated hypoxic vasodilation remain controversial, with separate roles for nitrite ( ) and S -nitrosohemoglobin (SNO-Hb) widely contested given their ability to transduce nitric oxide bioactivity within the microcirculation. To establish their relative contribution in vivo, we quantified arterial-venous concentration gradients across the human cerebral and femoral circulation at rest and during exercise, an ideal model system characterized by physiological extremes of O 2 tension and blood flow.

Methods:

Ten healthy participants (5 men, 5 women) aged 24±4 (mean±SD) years old were randomly assigned to a normoxic (21% O 2 ) and hypoxic (10% O 2 ) trial with measurements performed at rest and after 30 minutes of cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled simultaneously from the brachial artery and internal jugular and femoral veins with plasma and RBC nitric oxide metabolites measured by tri-iodide reductive chemiluminescence. Blood flow was determined by transcranial Doppler ultrasound (cerebral blood flow) and constant infusion thermodilution (femoral blood flow) with net exchange calculated via the Fick principle.

Results:

Hypoxia was associated with a mild increase in both cerebral blood flow and femoral blood flow ( P <0.05 versus normoxia) with further, more pronounced increases observed in femoral blood flow during exercise ( P <0.05 versus rest) in proportion to the reduction in RBC oxygenation ( r =0.680–0.769, P <0.001). Plasma gradients reflecting consumption (arterial>venous; P <0.05) were accompanied by RBC iron nitrosylhemoglobin formation (venous>arterial; P <0.05) at rest in normoxia, during hypoxia ( P <0.05 versus normoxia), and especially during exercise ( P <0.05 versus rest), with the most pronounced gradients observed across the bioenergetically more active, hypoxemic, and acidotic femoral circulation ( P <0.05 versus cerebral). In contrast, we failed to observe any gradients consistent with RBC SNO-Hb consumption and corresponding delivery of plasma S -nitrosothiols ( P >0.05).

Conclusions:

These findings suggest that hypoxia and, to a far greater extent, exercise independently promote arterial-venous delivery gradients of intravascular nitric oxide, with deoxyhemoglobin-mediated reduction identified as the dominant mechanism underlying hypoxic vasodilation.

Alterations in nitric oxide homeostasis during traumatic brain injury

Changes in nitric oxide (NO) levels have been often associated with various forms of trauma, including secondary damage after traumatic brain injury (TBI). Several studies demonstrate the upregulation of NO synthase (NOS) enzymes, and concomitant increases in brain NO levels, which contribute to the TBI-associated glutamate cytotoxicity, including the pathogenesis of mitochondrial dysfunction. TBI is also associated with elevated NO levels in remote organs, indicating that TBI can induce systemic changes in NO regulation, which can be either beneficial or detrimental. Here we review the possible mechanisms responsible for changes in NO metabolism during TBI. Better understanding of the changes in NO homeostasis in TBI will be necessary to design rational therapeutic approaches for TBI. This article is part of a Special Issue entitled: Immune and Metabolic Alterations in Trauma and Sepsis edited by Dr. Raghavan Raju.

Nitric Oxide and Hydrogen Sulfide Regulation of Ischemic Vascular Remodeling

Blockage or restriction of blood flow through conduit arteries results in tissue ischemia downstream of the disturbed area. Local tissues can adapt to this challenge by stimulating vascular remodeling through angiogenesis and arteriogenesis thereby restoring blood perfusion and removal of wastes. Multiple molecular mechanisms of vascular remodeling during ischemia have been identified and extensively studied. However, therapeutic benefits from these findings and insights are limited due to the complexity of various signaling networks and a lack of understanding central metabolic regulators governing these responses. The gasotransmitters NO and H2 S have emerged as master regulators that influence multiple molecular targets necessary for ischemic vascular remodeling. In this review, we discuss how NO and H2 S are individually regulated under ischemia, what their roles are in angiogenesis and arteriogenesis, and how their interaction controls ischemic vascular remodeling.

Nitric Oxide Signaling and Clinical Alternatives to Nitric Oxide

Nitric oxide has been implicated in numerous biological processes, particularly those involved with the cardiovascular system. Nitric oxide production is closely regulated and influenced by a number of factors in both health and disease. Nitric oxide is involved in maintaining the vascular system in its healthy, nondiseased state by producing vasorelaxation which enhances blood flow and prevents both leukocyte and platelet adhesion to the vascular wall. Dysfunctional endothelial cell nitric oxide production has been implicated in a number of disease states, including hypertension and atherosclerosis, and has been associated with adverse cardiac events. Various recent therapies may exert their beneficial effects in part by enhancing endothelial nitric oxide bloavallability. Nitric oxide has been used therapeutically in a number of cardiorespiratory disease states. An improved understanding of the pathologic processes underlying these diseases has resulted in several alternative agents being investigated and used clinically.

EPR analysis of extra- and intracellular nitric oxide in liver biopsies

PURPOSE

To develop an assay that can enable the quantification of intra- and extracellular nitric oxide (NO) levels in liver biopsies without application of potentially harmful exogenous NO traps.

THEORY

Electron paramagnetic resonance (EPR) spectroscopy is currently the most appropriate method of measuring NO in biological samples due to the outstanding specificity resulting from the interaction of NO with exogenous NO traps. Because such traps are not allowed in clinical settings, we tested the reliability of endogenous NO traps for the determination of NO levels in blood and liver compartments.

METHODS

Rats were injected with 0-8 mg/kg lipopolysaccharide (LPS) to gradually induce a systemic inflammatory response. Specific features of NO-hemoglobin and NO-Fe EPR signals were quantified using a specifically developed calibration procedure.

RESULTS

Whereas both NO-hemoglobin (NO-Hb^{LIVER BLOOD} ) and NO-Fe (NO-Fe^LIVER ) complexes were detected in nonperfused liver tissue, only NO-Fe complexes were detected in perfused tissue and only NO-Hb complexes were detected in blood (NO-Hb^BLOOD ). The NO concentrations increased in the sequence NO-Hb^BLOOD  < NO-Fe^LIVER  < NO-Hb^{LIVER BLOOD} (9.4, 18.5, 27.9 nmol/cm³ , respectively at 2.5 mg/kg LPS). The detection limit of the method was 0.61 nmol/cm³ for NO-Hb and 0.52 nmol/cm³ for NO-Fe.

CONCLUSION

The assay reported here does not influence natural NO pathways and enables the quantification of NO distribution in two liver compartments using a single liver biopsy. Magn Reson Med 77:2372-2380, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints.