Hemoglobin-nitric oxide interaction and its implications

The Controversy Persists: Is There a Qualification Criterion to Utilize Inhaled Nitric Oxide in Pre-term Newborns?

Inhaled nitric oxide (iNO) use in premature newborns remains controversial among clinicians. In 2014, the American Academy of Pediatrics, Committee on Fetus and Newborn released a statement that the available data do not support routine iNO use in pre-term newborns. Despite the absence of significant benefits, 2016 California data showed that clinicians continue to utilize iNO in pre-term infants. With studies as recent as January 2017, the Cochrane review confirmed no major advantages of iNO in pre-term newborns. Still, it recognized that a subset of pre-term infants with pulmonary hypertension (PHTN) had not been separately investigated. Furthermore, recent non-randomized controlled trials have suggested that iNO may benefit specific subgroups of pre-term newborns, especially those with PHTN, prolonged rupture of membranes, and antenatal steroid exposure. Those pre-term infants who showed a clinical response to iNO had increased survival without disability. These findings underscore the need for future studies in pre-term newborns with hypoxemic respiratory failure and PHTN. This review will discuss the rationale for using iNO, controversies regarding the diagnosis of PHTN, and additional novel approaches of iNO treatment in perinatal asphyxia and neonatal resuscitation in the pre-term population < 34 weeks gestation.

Enhanced Nitrite Reductase Activity and Its Correlation with Oxygen Affinity in Hemoglobin Bis-Tetramers

The vasoactivity of circulating cross-linked hemoglobin is consistent with the acellular protein penetrating the endothelial lining of blood vessels where hemoglobin can bind nitric oxide, the signal for relaxation of the muscles that surround blood vessels. In an important contrast, derivatives of bis-tetramers that are produced from hemoglobin by chemical coupling do not cause vasoconstriction in animal models. Presumably, they are unable to enter the endothelia where hemoglobin tetramers bind to nitric oxide. In addition, hemoglobin bis-tetramers can produce nitric oxide in circulation through their intrinsic nitrite reductase activity. Examination of this activity for hemoglobin-derived bis-tetramers that are acetylated at lysyl amino groups in their α subunits reveals enhanced activity (k = 2.21 M(-1) s(-1)) compared to that of nonacetylated bis-tetramers (k = 0.70 M(-1) s(-1)). Plots of nitrite reductase activities as a function of the corresponding oxygen affinities of certain allosteric-state-stabilized derivatives reveal a significant correlation, providing a basis for interpretation of the correlated functions.

Oxygen carrier based on hemoglobin/poly(L-lysine)-block-poly(L-phenylalanine) vesicles

An oxygen carrier was prepared by encapsulating carbonylated hemoglobin (CO-Hb) molecules into polypeptide vesicles made from poly(L-lysine)-block-poly(L-phenylalanine) (PLL-b-PPA) diblock copolymers in aqueous medium at pH 5.8. The encapsulation was confirmed by confocal laser scanning microscopy (CLSM). The morphology and size of the vesicles were studied by field-emission scanning electron microscopy (ESEM). They had a spherical shape with a mean diameter of about 4 to 5 microm. The encapsulation efficiency of hemoglobin was 40 wt %, and the hemoglobin content in the vesicles was 32 wt %. The CO-Hb encapsulated in the PLL-b-PPA vesicles was more stable than free CO-Hb under ambient conditions. In the presence of a O(2) atmosphere, the CO-Hb in the vesicle could be converted into oxygen-binding hemoglobin (O(2)-Hb) under irradiation of visible light for 2 h. Therefore, the CO-Hb/PLL-b-PPA vesicles are expected to be used as red blood cell substitutes.

Enhanced Molecular Volume of Conservatively Pegylated Hb: (SP-PEG5K)6-HbA is Non-Hypertensive

Recent studies have suggested that the "pressor effect" of acellular Hb is a consequence of perturbation of the macro-and microcirculatory system in multiple ways, and that PEGylation is an effective approach for controlling the same. In an attempt to confirm this concept, a new and simple thiolation mediated, maleimide chemistry-based conservative PEGylation protocol has been developed to conjugate multiple copies of PEG-chains to Hb. This approach combines the high reactivity of maleimides towards thiols with the propensity of iminothiolane to derivatize the epsilon-amino groups of proteins into reactive thiol groups, with conservation of their positive charge. One of the PEGylated products, namely (SP-PEG5K)6-HbA, that carries on an average six copies of PEG5000 chains per Hb, is non-hypertensive in hamster top load and in rat 50% exchange transfusion models. This hexa-PEGylated-Hb has (i) a hydrodynamic volume corresponding to that of an oligomerized Hb of 256kDa, (ii) a molecular radius of approximately 6.8 nm, (iii) high oxygen affinity, (iv) lowered Bohr effect, and (v) increased viscosity and colloidal osmotic pressure. These properties of (SP-PEG5K)6-HbA are consistent with the emerging new paradigms for the design of Hb based oxygen carriers and confirm the concept that the "pressor effect" of Hb is a multifactorial event. The thiolation mediated maleimide chemistry-based PEGylation protocol described here for the generation of (SP-PEG5K)6-Hb is simple, highly efficient, and is carried out under oxy conditions. The results demonstrate that a non-hypertensive PEG-Hb can be generated by conjugation of a lower number of PEG chains than previously reported.

Role of extension arm in PEG-Hb conjugates on the stability of the tetramer: non-conservative EAF maleimide thio-PEG mediated PEGylation

PEGylation induced increase in colloidal osmotic pressure (COP) of Hb, one of the unique molecular properties of PEG-Hb conjugates, facilitates the neutralization of the vasoconstrictive activity of acellular Hb is a function of chemistry of conjugation of PEG-chains. The dependence of COP with the chemistry is a consequence of PEGylation induced weakening of interdimeric interactions. The conservative Extension Arm Facilitated (EAF) PEGylation exerts least influence on lowering the tetramer stability as compared to direct PEGylation. We have now designed a new, non-conservative EAF PEGylation that uses acylation chemistry to introduce the extension arm onto proteins to delineate the role of the extension arm and of the charge at the site of attachment in PEG-Hb conjugate on tetramer stability. The non-conservative EAF PEGylation does not lower the tetramer stability just as the non-conservative direct PEGylation of Hb. The impact of the extension arm in PEGylated Hb in terms of their structure and potential significance of higher tetramer stability of PEG-Hb conjugates generated by EAF-PEGylation in the in vivo toxicity and in the design of oxygen carrying plasma expander has been discussed.

Molecular mechanisms involved in enhancing HO-1 expression: de-repression by heme and activation by Nrf2, the "one-two" punch

Heme oxygenase (HO)-1 is a stress response protein, which confers cytoprotection against oxidative injury and provides a vital function in maintaining tissue homeostasis. Molecular mechanisms involved in the inducible transcription of ho-1 occurring in response to numerous and diverse stressful conditions have remained elusive. Since the discovery of E1 and E2, the two upstream enhancers regulating induction of ho-1 transcription in 1989, there have been many studies dealing with molecular mechanisms involved in enhancing HO-1 expression. In this commentary, recent advances in our understanding of the mechanisms involved in the induction of HO-1 expression in mammalian cells are summarized with some supportive results reported by others. Currently available data indicate that activation of ho-1 transcription involves both the heme (native substrate)-dependent selective alleviation of repressor and the oxidative stress-dependent activation of transcriptional activator. The stress-released free-heme (HO-1 substrate) from hemoproteins involved in causing oxidative stress itself appears to act as a molecular switch controlling the repressor- activator antagonism on the enhancer sequences of ho-1. Thus, induction of HO-1 appears to operate in a manner like a simple feedback loop. dox Signal. 7, 1674-1687.

Conjugation of Multiple Copies of Polyethylene Glycol to Hemoglobin Facilitated Through Thiolation: Influence on Hemoglobin Structure and Function

PEGylation induced changes in molecular volume and solution properties of HbA have been implicated as potential modulators of its vasoconstrictive activity. However, our recent studies with PEGylated Hbs carrying two PEG chains/Hb, have demonstrated that the modulation of the vasoconstrictive activity of Hb is not a direct correlate of the molecular volume and solution properties of the PEGylated Hb and implicated a role for the surface charge and/or the pattern of surface decoration of Hb with PEG. HbA has now been modified by thiolation mediated maleimide chemistry based PEGylation that does not alter its surface charge and conjugates multiple copies of PEG5K chains. This protocol has been optimized to generate a PEGylated Hb, (SP-PEG5K)(6)-Hb, that carries approximately six PEG5K chains/Hb - HexaPEGylated Hb. PEGylation increased the O(2) affinity of Hb and desensitized the molecule for the influence of ionic strength, pH, and allosteric effectors, presumably a consequence of the hydrated PEG-shell generated around the protein. The total PEG mass in (SP-PEG5K)(6)-Hb, its molecular volume, O(2) affinity and solution properties are similar to that of another PEGylated Hb, (SP-PEG20K)(2)-Hb, that carries two PEG20K chains/Hb. However, (SP-PEG5K)(6)-Hb exhibited significantly reduced vasoconstriction mediated response than (SP-PEG20K)(2)-Hb. These results demonstrate that the enhanced molecular size and solution properties achieved through the conjugation of multiple copies of small PEG chains to Hb is more effective in decreasing its vasoconstrictive activity than that achieved through the conjugation of a comparable PEG mass using a small number of large PEG chains.

Effects of cell-free hemoglobin on hypoxia-inducible factor (HIF-1alpha) and heme oxygenase (HO-1) expressions in endothelial cells subjected to hypoxia

We have investigated the impact of diaspirin cross-linked hemoglobin (DBBF-Hb), a blood substitute, on cell signaling pathways that are modulated in part by biological peroxides (i.e., hydrogen peroxide, lipid peroxide, and peroxynitrite). Bovine aortic endothelial cells (BAECs) subjected to hypoxia expressed hypoxia-inducible factor (HIF-1alpha) in a time course that paralleled the expressions of heme oxygenase (HO-1). Co-incubation of the oxy form (HbFe(2+)) with hypoxic BAECs resulted in an increase in the expression of HIF-1alpha in a manner that corresponded linearly with the decay of HbFe(2+) and accumulation of the ferric form (HbFe(3+)). Inclusion of HbFe(3+) with hypoxic BAECs produced twice as much expression in the HIF-1alpha and HO-1 proteins as opposed to HbFe(2+) alone, or HbFe(2+) plus hypoxia. In addition, higher and more persistent levels of the ferryl form (HbFe(4+)), due to the consumption of endogenous peroxides, were found in the hypoxic media containing hemoglobin. Nitric oxide (NO) released from an NO donor reduced the levels of HIF-1alpha in the hypoxic cells treated with either HbFe(2+) or HbFe(3+), but had little or no effect on the levels of HO-1. DBBF-Hb modulates key cell-signaling pathways by competing with peroxides required for the deactivation of HIF-1alpha, which may modulate important physiological mediators.

Increased tissue PO2 and decreased O2 delivery and consumption after 80% exchange transfusion with polymerized hemoglobin

The O2-carrying blood substitute based on polymerized bovine hemoglobin (PBH) was used to determine efficacy in maintaining tissue Po2 after an 80% isovolemic blood exchange leading to a hematocrit of 19% [5.4 g Hb/dl from red blood cells (RBCs) and 6.3 g Hb/dl from PBH]. Effects were studied in terms of O2 delivery, O2 extraction, and tissue Po2 at the microcirculatory level at 1, 12, and 24 h after exchange transfusion in awake hamsters prepared with a window chamber model. At 1 h after exchange, arteriolar and venular diameters were decreased compared with baseline. Arteriolar diameter did not fully recover at 12 h after exchange, but venular diameter returned to normal. At 24 h after exchange, arteriolar and venular diameters were not different from baseline. Combining diameter and flow velocity data allowed us to calculate arteriolar and venular flows. At 1 h after exchange, arteriolar and venular flow was reduced compared with baseline. Arteriolar flow was lower at 12 h after exchange and recovered after 24 h. The number of capillaries with RBC passage [functional capillary density (FCD)] at 1 h after exchange with PBH was significantly lower than baseline. FCD remained decreased at 12 h; at 24 h after exchange transfusion, FCD was fully recovered. Tissue Po2 was maximal at 1 h after exchange and decreased progressively at 12 and 24 h after exchange. O2 release to the tissue was minimal at 1 h and increased at 12 and 24 h after exchange. These results suggest the impairment of tissue O2 metabolism after introduction of PBH into the circulation, which is mitigated as PBH concentration declines.

Antagonism of nitrous oxide-induced anxiolytic-like behavior in the mouse light/dark exploration procedure by pharmacologic disruption of endogenous nitric oxide function

RATIONALE

Previous studies have shown the anxiolytic-like effects of nitrous oxide (N(2)O) to be sensitive to antagonism by non-specific inhibitors of nitric oxide synthase (NOS).

OBJECTIVES

The present study was conducted to demonstrate further the involvement of nitric oxide (NO) and ascertain whether a specific isoform of NOS is involved in N(2)O-induced behavior in mice.

METHODS

Male NIH Swiss mice were tested in the light/dark exploration test to determine how N(2)O-induced behavior was affected by the following pretreatments: the NO scavenger hemoglobin (Hb); the selective nNOS-inhibitor S-methyl- l-thiocitrulline (SMTC); the selective eNOS-inhibitor N(5)-(1-iminoethyl)- l-ornithine ( l-NIO); and the selective iNOS-inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT). Furthermore, NOS activity was assessed in the whole brain as well as five brain areas of N(2)O- versus room air-exposed mice to determine the effects of N(2)O on NOS activity.

RESULTS

The behavioral effects of N(2)O in the light/dark exploration test were significantly attenuated following pretreatment with Hb (2.0 nmol, i.c.v.), SMTC (0.3 micro g and 1.0 micro g per mouse, i.c.v.) and the higher dose of l-NIO (30 mg/kg, s.c.). However, the N(2)O-induced behavioral effect was unaltered by pretreatment with either the lower dose of l-NIO (10 mg/kg, s.c.) or AMT (1.0 mg/kg and 3.0 mg/kg, s.c.). Finally exposure to 50% N(2)O for 15 min significantly increased NOS activity in the cerebellum and corpus striatum but not in other brain regions or whole brain.

CONCLUSION

These findings provide further support for the hypothesis that NO is involved in N(2)O-induced anxiolytic-like behavior and that this NO is the product of nNOS enzyme activity.

Site-specific PEGylation of hemoglobin at Cys-93(beta): correlation between the colligative properties of the PEGylated protein and the length of the conjugated PEG chain

Increasing the molecular size of acellular hemoglobin (Hb) has been proposed as an approach to reduce its undesirable vasoactive properties. The finding that bovine Hb surface decorated with about 10 copies of PEG5K per tetramer is vasoactive provides support for this concept. The PEGylated bovine Hb has a strikingly larger molecular radius than HbA (1). The colligative properties of the PEGylated bovine Hb are distinct from those of HbA and even polymerized Hb, suggesting a role for the colligative properties of PEGylated Hb in neutralizing the vasoactivity of acellular Hb. To correlate the colligative properties of surface-decorated Hb with the mass of the PEG attached and also its vasoactivity, we have developed a new maleimide-based protocol for the site-specific conjugation of PEG to Hb, taking advantage of the unusually high reactivity of Cys-93(beta) of oxy HbA and the high reactivity of the maleimide to protein thiols. PEG chains of 5, 10, and 20 kDa have been functionalized at one of their hydroxyl groups with a maleidophenyl moiety through a carbamate linkage and used to conjugate the PEG chains at the beta-93 Cys of HbA to generate PEGylated Hbs carrying two copies of PEG (of varying chain length) per tetramer. Homogeneous preparations of (SP-PEG5K)(2)-HbA, (SP-PEG10K)(2)-HbA, and (SP-PEG20K)(2)-HbA have been isolated by ion exchange chromatography. The oxygen affinity of Hb is increased slightly on PEGylation, but the length of the PEG-chain had very little additional influence on the O(2) affinity. Both the hydrodynamic volume and the molecular radius of the Hb increased on surface decoration with PEG and exhibited a linear correlation with the mass of the PEG chain attached. On the other hand, both the viscosity and the colloidal osmotic pressure (COP) of the PEGylated Hbs exhibited an exponential increase with the increase in PEG chain length. In contrast to the molecular volume, viscosity, and COP, the vasoactivity of the PEGylated Hbs was not a direct correlate of the PEG chain length. There appeared to be a threshold for the PEG chain length beyond which the protection against vasoactivity is decreased. These results suggest that the modulation of the vasoactivity of Hb by PEG could be a function of the surface shielding afforded by the PEG, the latter being a function of the disposition of the PEG chain on the protein surface, which in turn is a function of the length of the PEG chain. Thus, the biochemically homogeneous PEGylated Hbs described in the present study, surface-decorated with PEG chains of appropriate size, could serve as potential candidates for Hb-based oxygen carriers.

Hemolink, an o-raffinose cross-linked haemoglobin-based oxygen carrier, does not affect activation and function of human platelets in whole blood in vitro

Haemoglobin-based oxygen carriers (HBOCs) are anticipated to be safe and efficient alternatives to RBC transfusions. Haemoglobin (Hb) raffimer (Hemolink; Hemosol, Toronto, ON, Canada) is polymerized human Hb, cross-linked with o-raffinose. As administration of cell-free Hb may affect blood cells and tissues, this study was focused on evaluating effects of Hb raffimer on human platelets in whole blood in vitro. Citrated blood from healthy donors was incubated with Hb raffimer to achieve raffimer concentrations of 2-50 vol percentage (2-50 g/l). Platelet activation, phosphatidylserine exposure and microparticle generation were measured by flow cytometry. Aperture closure time on collagen/ADP- and collagen/epinephrine-coated membranes was determined by a platelet function analyser (PFA-100). We found that addition of Hb raffimer to blood samples up to 50 vol % did not affect human platelets as measured by various markers of platelet activation (CD42b, CD41, PAC-1, CD62, CD63), procoagulant activity (annexin V) and microparticle formation; differences between Hb raffimer- and lactated Ringer's-diluted blood were not significant. Similarly, no adverse effect of Hb raffimer on closure time was observed at concentrations up to 50 vol %, in comparison with Ringer's solution. These data indicate that exposure of human blood to high concentrations of Hb raffimer in vitro did not cause platelet activation nor affect platelet function.

2001 Lemieux Award Lecture Organic chemistry and hemoglobin: Benefits from controlled alteration

Hemoglobin carries oxygen in circulation within red cells but does not function outside the cells because it fails not only to release oxygen but also dissociates into dimers that make up the tetrameric protein. Bifunctional anionic acylating agents that contain a structurally rigid bridge introduce cross-links that stabilize hemoglobin and alter its oxygen affinity so that it could be used to carry oxygen outside cells. Nitric oxide binds to hemoglobin and in circulation this causes undesirable increases in blood pressure. It had been reported that higher weight collections of hemoglobin do not cause vasoconstriction. Reagents with two pairs of reaction sites joined by a rigid link connect and cross-link two hemoglobins. The resulting bis-tetramers lack the cooperativity of the native protein and bind oxygen too tightly to be useful; occupation by oxygen blocks the sites from nitric oxide. Nitric oxide may be delivered from thionitrosyl groups, which occur in hemoglobin in the red cell. Cross-linked hemoglobin can be specifically nitrosylated. These species can then serve as circulating sources of nitric oxide resulting from an internal electron transfer.Key words: proteins, hemoglobin, cross-link, red cells, cooperativity, connecting.

o-Raffinose cross-linked hemoglobin improves the hemostatic defect associated with anemia and thrombocytopenia in rabbits

Several different preparations of cross-linked hemoglobin (CLHb) are being evaluated for their efficacy and safety as red cell substitutes in a variety of preclinical and clinical settings. Because CLHb is known to sequester nitric oxide (NO) and inhibit NO-mediated processes, we hypothesized that CLHb would have a hemostatic effect by enhancing platelet reactivity, inducing vasoconstriction, or both. Infusion of o-raffinose CLHb shortened the prolonged microvascular bleeding time and decreased blood loss from ear incisions in rabbits rendered anemic and thrombocytopenic. Moreover, this hemostatic effect persisted for at least 24 hours after infusion. Phenylephrine induced a degree of vasoconstriction similar to that induced by CLHb but did not shorten the bleeding time or decrease blood loss, suggesting that vasoconstriction alone cannot account for the hemostatic effect of CLHb. There was no evidence of CLHb-induced activation of coagulation in vivo, since infusion of CLHb did not increase circulating levels of thrombin-antithrombin complex. In vitro, CLHb abolished the inhibitory effect of the NO donor 3-morpholinosydnonimine on platelet aggregation and enhanced the aggregation of stimulated but not resting platelets. This potentiating effect was not attenuated by the addition of superoxide dismutase or catalase. To evaluate the potential arterial thrombogenicity of CLHb, a model of carotid artery thrombosis was developed in rabbits without thrombocytopenia or anemia. Compared with albumin infusion, CLHb infusion shortened the time to complete carotid occlusion. These data suggest that CLHb may shift the thromboregulatory balance toward clot formation, resulting in decreased bleeding in anemic and thrombocytopenic rabbits and possibly increasing arterial thrombogenicity in normal rabbits.

o-Raffinose cross-linked hemoglobin improves the hemostatic defect associated with anemia and thrombocytopenia in rabbits

Several different preparations of cross-linked hemoglobin (CLHb) are being evaluated for their efficacy and safety as red cell substitutes in a variety of preclinical and clinical settings. Because CLHb is known to sequester nitric oxide (NO) and inhibit NO-mediated processes, we hypothesized that CLHb would have a hemostatic effect by enhancing platelet reactivity, inducing vasoconstriction, or both. Infusion of o-raffinose CLHb shortened the prolonged microvascular bleeding time and decreased blood loss from ear incisions in rabbits rendered anemic and thrombocytopenic. Moreover, this hemostatic effect persisted for at least 24 hours after infusion. Phenylephrine induced a degree of vasoconstriction similar to that induced by CLHb but did not shorten the bleeding time or decrease blood loss, suggesting that vasoconstriction alone cannot account for the hemostatic effect of CLHb. There was no evidence of CLHb-induced activation of coagulation in vivo, since infusion of CLHb did not increase circulating levels of thrombin-antithrombin complex. In vitro, CLHb abolished the inhibitory effect of the NO donor 3-morpholinosydnonimine on platelet aggregation and enhanced the aggregation of stimulated but not resting platelets. This potentiating effect was not attenuated by the addition of superoxide dismutase or catalase. To evaluate the potential arterial thrombogenicity of CLHb, a model of carotid artery thrombosis was developed in rabbits without thrombocytopenia or anemia. Compared with albumin infusion, CLHb infusion shortened the time to complete carotid occlusion. These data suggest that CLHb may shift the thromboregulatory balance toward clot formation, resulting in decreased bleeding in anemic and thrombocytopenic rabbits and possibly increasing arterial thrombogenicity in normal rabbits.

Hemoglobin substitute and cardiopulmonary bypass

The effects of diaspirin crosslinked hemoglobin (DCLHb, Baxter Health Care Corp., Round Lake, IL) on oxygen exchange in the setting of cardiopulmonary bypass (CPB) are unknown. Six calves (71.2 +/- 1.3 kg) were connected to CPB by jugular venous and carotid arterial cannulation for 5 hours. Each 1 hour period included 45 min of partial CPB (mean flow rate of 50 ml/kg per min) followed by 15 min without CPB, at the end of which 500 ml of blood were substituted for with either 500 ml of hydroxyethyl starch (Haes; n = 3) or 500 ml of DCLHb (n = 3). A total of 2 liters of blood was, thus, exchanged (28 ml/kg of blood substitute). Values are expressed as mean +/- 1 SD. Analysis of variance for repeated measurements was used. The cardiac output (CO) values at 1 h, 3 h, and 5 h were in the Haes group: 5.7 +/- 2, 6.7 +/- 2.5, and 7.7 +/- 2.5L/min, and in the DCLHb group: 5.7 +/- 0.6, 4 +/- 1, and 4.7 +/- 1.2 L/min, respectively. The arteriovenous oxygen content difference (Ca-Cvo2) values at 1 h, 3 h, and 5 h were in the Haes group: 4.6 +/- 1, 3.3 +/- 1.5, and 3.5 +/- 1.5 ml/dl, and in the DCLHb group: 4.9 +/- 0.6, 7.4 +/- 0.7, and 6.6 +/- 0.6 ml/dl, respectively. The oxygen consumption (Vo2) values at 1 h, 3 h, and 5 h were in the Haes group: 244 +/- 29, 198 +/- 58, and 249 +/- 42 ml/min, and in the DCLHb group: 273 +/- 28, 296 +/- 75, and 306 +/- 65 ml/min, respectively. CO and Ca-Cvo2 showed a significant difference (p < 0.01), whereas Vo2 did not (p = 0.52). In the DCLHb group of this CPB animal model, the cardiac output is lower and the arteriovenous oxygen content difference higher than in the Haes group, allowing for preserved oxygen consumption.

Effects of the RBC membrane and increased perfusate viscosity on hypoxic pulmonary vasoconstriction

Red blood cells (RBCs) augment hypoxic pulmonary vasoconstriction (HPV) in part by scavenging of nitric oxide (NO) by Hb (Deem S, Swenson ER, Alberts MK, Hedges RG, and Bishop MJ, Am J Respir Crit Care Med 157: 1181-1186, 1998). We studied the contribution of the RBC compartmentalization of Hb to augmentation of HPV and scavenging of NO in isolated perfused rabbit lungs. Lungs were initially perfused with buffer; HPV was provoked by a 5-min challenge with hypoxic gas (inspired O(2) fraction 0.05). Expired NO was measured continuously. Addition of free Hb to the perfusate (0.25 mg/ml) resulted in augmentation of HPV and a fall in expired NO that were similar in magnitude to those associated with a hematocrit of 30% (intracellular Hb of 100 mg/ml). Addition of dextran resulted in a blunting of HPV after free Hb but no change in expired NO. Blunting of HPV by dextran was not prevented by NO synthase inhibition with N(omega)-nitro-L-arginine and/or cyclooxygenase inhibition. RBC ghosts had a mild inhibitory effect on HPV but caused a small reduction in expired NO. In conclusion, the RBC membrane provides a barrier to NO scavenging and augmentation of HPV by Hb. Increased perfusate viscosity inhibits HPV by an undetermined mechanism.

The heme oxygenase pathway and its interaction with nitric oxide in the control of cellular homeostasis

Heme oxygenase is the rate limiting enzyme in heme degradation to carbon monoxide (CO), iron and bilirubin. The inducible isoform of the protein, heme oxygenase-1 (HO-1), is susceptible to up-regulation by a diverse variety of conditions and agents in mammalian tissue, leading to the common conception that HO-1 is a stress related enzyme. However, as attempts are made to unravel the mechanisms by which HO-1 is induced and as we discover that CO, iron and bilirubin may be important effector molecules, we are learning to appreciate that heme oxygenases may be central to the regulation of many physiological and pathophysiological processes besides their established function in heme catabolism. One such process may be closely linked to nitric oxide (NO). It has been demonstrated that NO and NO donors are capable of inducing HO-1 protein expression, in a mechanism depending on the de novo synthesis of RNA and protein. Thus, it is postulated that NO may serve as a signaling molecule in the modulation of the tissue stress response. This review will highlight the current ideas on the role of CO-heme oxygenase and NO-nitric oxide synthase in cell signaling and discuss how the two systems are interrelated.

Heme oxygenase-1-derived carbon monoxide contributes to the suppression of acute hypertensive responses in vivo

The enzyme heme oxygenase, which exists in inducible (HO-1) and constitutive (HO-2) isoforms, catalyzes the degradation of heme to biliverdin and CO in mammalian tissues. CO has been implicated in the control of vascular tone in a manner similar to that for NO. In the present study, we investigated the contribution of the heme oxygenase/CO pathway to the modulation of acute hypertensive responses in vivo induced by (1) alphaalphaHb, a chemically modified hemoglobin known to scavenge NO, and (2) NG-nitro-L-arginine methyl ester (L-NAME), a competitive NOS inhibitor. Experiments were carried out in conscious rats in which femoral arteries and veins were surgically catheterized 1 or 5 days before treatment with the vasoconstrictor agents. Intravenous infusion of alphaalphaHb (8% solution) or L-NAME (30 micromol/kg) [corrected] produced an acute and significant increase in mean arterial pressure (P<0.05) in rats at 5 days after catheter implantation. In contrast, no change in blood pressure was observed when alphaalphaHb or L-NAME was infused 1 day after the surgical intervention. The suppression of the hypertensive response observed at 1 day after surgery correlated with a significant (P<0.05) HO-1 expression in aorta, heart, and liver as well as increased aortic CO production and cGMP levels. At 1 day after surgery, pretreatment of animals with the heme oxygenase inhibitor zinc protoporphyrin IX (50 micromol/kg IP) markedly decreased aortic CO and cGMP levels and completely restored the vasoconstrictor effects of both alphaalphaHb and L-NAME. These results provide evidence for a crucial role of the heme oxygenase/CO pathway in the regulation of blood pressure under stress conditions in vivo.

Reactions between nitric oxide and haemoglobin under physiological conditions

The tenet of high-affinity nitric oxide (NO) binding to a haemoglobin (Hb) has shaped our view of haem proteins and of small diffusible signaling molecules. Specifically, NO binds rapidly to haem iron in Hb (k approximately 10[7] M[-1] s[-1]) and once bound, the NO activity is largely irretrievable (Kd approximately 10[-5] s[-1]); the binding is purportedly so tight as to be unaffected by O2 or CO. However, these general principles do not consider the allosteric state of Hb or the nature of the allosteric effector, and they mostly derive from the functional behaviour of fully nitrosylated Hb, whereas Hb is only partially nitrosylated in vivo. Here we show that oxygen drives the conversion of nitrosylhaemoglobin in the 'tense' T (or partially nitrosylated, deoxy) structure to S-nitrosohaemoglobin in the 'relaxed' R (or ligand-bound, oxy) structure. In the absence of oxygen, nitroxyl anion (NO-) is liberated in a reaction producing methaemoglobin. The yields of both S-nitrosohaemoglobin and methaemoglobin are dependent on the NO/Hb ratio. These newly discovered reactions elucidate mechanisms underlying NO function in the respiratory cycle, and provide insight into the aetiology of S-nitrosothiols, methaemoglobin and its related valency hybrids. Mechanistic reexamination of NO interactions with other haem proteins containing allosteric-site thiols may be warranted.