Nitrosyl-hemoglobin formation in the blood of animals exposed to nitric oxide

Utility of large-animal models of BPD: chronically ventilated preterm lambs

This paper is focused on unique insights provided by the preterm lamb physiological model of bronchopulmonary dysplasia (BPD). Connections are also made to insights provided by the former preterm baboon model of BPD, as well as to rodent models of lung injury to the immature, postnatal lung. The preterm lamb and baboon models recapitulate the clinical setting of preterm birth and respiratory failure that require prolonged ventilation support for days or weeks with oxygen-rich gas. An advantage of the preterm lamb model is the large size of preterm lambs, which facilitates physiological studies for days or weeks during the evolution of neonatal chronic lung disease (CLD). To this advantage is linked an integrated array of morphological, biochemical, and molecular analyses that are identifying the role of individual genes in the pathogenesis of neonatal CLD. Results indicate that the mode of ventilation, invasive mechanical ventilation vs. less invasive high-frequency nasal ventilation, is related to outcomes. Our approach also includes pharmacological interventions that test causality of specific molecular players, such as vitamin A supplementation in the pathogenesis of neonatal CLD. The new insights that are being gained from our preterm lamb model may have important translational implications about the pathogenesis and treatment of BPD in preterm human infants.

The identification of nitric oxide as endothelium-derived relaxing factor

The identification of endothelium-derived relaxing factor as nitric oxide (NO) dramatically altered the course of vascular biology, as well as other biomedical disciplines. The ubiquity of this natural product of cell metabolism and the complexity of its biochemistry provide a rich source of molecular mediators of phenotype in health and disease.

Brief periods of nitric oxide inhalation protect against myocardial ischemia-reperfusion injury

BACKGROUND

Prolonged breathing of nitric oxide reduces myocardial ischemia-reperfusion injury, but the precise mechanisms responsible for the cardioprotective effects of inhaled nitric oxide are incompletely understood.

METHODS

The authors investigated the fate of inhaled nitric oxide (80 parts per million) in mice and quantified the formation of nitric oxide metabolites in blood and tissues. The authors tested whether the accumulation of nitric oxide metabolites correlated with the ability of inhaled nitric oxide to protect against cardiac ischemia-reperfusion injury.

RESULTS

Mice absorbed nitric oxide in a nearly linear fashion (0.19 +/- 0.02 micromol/g x h). Breathing nitric oxide rapidly increased a broad spectrum of nitric oxide metabolites. Levels of erythrocytic S-nitrosothiols, N-nitrosamines, and nitrosyl-hemes increased dramatically within 30 s of commencing nitric oxide inhalation. Marked increases of lung S-nitrosothiol and liver N-nitrosamine levels were measured, as well as elevated cardiac and brain nitric oxide metabolite levels. Breathing low oxygen concentrations potentiated the ability of inhaled nitric oxide to increase cardiac nitric oxide metabolite levels. Concentrations of each nitric oxide metabolite, except nitrate, rapidly reached a plateau and were similar after 5 and 60 min. In a murine cardiac ischemia-reperfusion injury model, breathing nitric oxide for either 5 or 60 min before reperfusion decreased myocardial infarction size as a fraction of myocardial area at risk by 31% or 32%, respectively.

CONCLUSIONS

Breathing nitric oxide leads to the rapid accumulation of a variety of nitric oxide metabolites in blood and tissues, contributing to the ability of brief periods of nitric oxide inhalation to provide cardioprotection against ischemia-reperfusion injury. The nitric oxide metabolite concentrations achieved in a target tissue may be more important than the absolute amounts of nitric oxide absorbed.

Inhaled NO as a therapeutic agent

In 1991, Frostell and colleagues reported that breathing low concentrations of nitric oxide (NO) decreased pulmonary artery pressure (PAP) in awake lambs with experimental pulmonary hypertension (PH) [Frostell C, Fratacci MD, Wain JC, Jones R, Zapol WM. Inhaled nitric oxide. A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction. Circulation 1991;83:2038-47]. Subsequently, efforts of multiple research groups studying animals and patients led to approval of inhaled NO by the US Food and Drug Administration in 1999 and the European Medicine Evaluation Agency and European Commission in 2001. Inhaled NO is currently indicated for the treatment of term and near-term neonates with hypoxemia and PH. Since regulatory approval, several studies have suggested that NO inhalation can prevent chronic lung disease in premature infants. In addition, unanticipated systemic effects of inhaled NO may lead to treatments for a variety of disorders including ischemia-reperfusion injury. This review summarizes the pharmacology and physiological effects of breathing NO. The application of inhaled NO to hypoxemic neonates with PH is discussed including recent studies exploring the use of inhaled NO to prevent bronchopulmonary dysplasia in premature infants. This review also highlights the application of inhaled NO to treat adults with cardiopulmonary disease, strategies to augment the efficacy of inhaled NO, and potential applications of the systemic effects of the gas.

Pulmonary vascular dysfunction in preterm lambs with chronic lung disease

Chronic lung injury from prolonged mechanical ventilation after premature birth inhibits the normal postnatal decrease in pulmonary vascular resistance (PVR) and leads to structural abnormalities of the lung circulation in newborn sheep. Compared with normal lambs born at term, chronically ventilated preterm lambs have increased pulmonary arterial smooth muscle and elastin, fewer lung microvessels, and reduced abundance of endothelial nitric oxide synthase. These abnormalities may contribute to impaired respiratory gas exchange that often exists in infants with chronic lung disease (CLD). Nitric oxide inhalation (iNO) reduces PVR in human infants and lambs with persistent pulmonary hypertension. We wondered whether iNO might have a similar effect in lambs with CLD. We therefore studied the effect of iNO on PVR in lambs that were delivered prematurely at approximately 125 days of gestation (term = 147 days) and mechanically ventilated for 3 wk. All of the lambs had chronically implanted catheters for measurement of pulmonary vascular pressures and blood flow. During week 2 of mechanical ventilation, iNO at 15 parts/million for 1 h decreased PVR by approximately 20% in 12 lambs with evolving CLD. When the same study was repeated in eight lambs at the end of week 3, iNO had no significant effect on PVR. To see whether this loss of iNO effect on PVR might reflect dysfunction of lung vascular smooth muscle, we infused 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP; 150 microg. kg(-1). min(-1) iv) for 15-30 min in four of these lambs at the end of week 3. PVR consistently decreased by 30-35%. Lung immunohistochemistry and immunoblot analysis of excised pulmonary arteries from lambs with CLD, compared with control term lambs, showed decreased soluble guanylate cyclase (sGC). These results suggest that loss of pulmonary vascular responsiveness to iNO in preterm lambs with CLD results from impaired signaling, possibly related to deficient or defective activation of sGC, the intermediary enzyme through which iNO induces increased vascular smooth muscle cell cGMP and resultant vasodilation.

Inhaled nitric oxide, oxygen, and alkalosis: dose-response interactions in a lamb model of pulmonary hypertension

Inhaled nitric oxide (NO) is currently used as an adjuvant therapy for a variety of pulmonary hypertensive disorders. In both animal and human studies, inhaled NO induces selective, dose-dependent pulmonary vasodilation. However, its potential interactions with other simultaneously used pulmonary vasodilator therapies have not been studied. Therefore, the objective of this study was to determine the potential dose-response interactions of inhaled NO, oxygen, and alkalosis therapies. Fourteen newborn lambs (age 1-6 days) were instrumented to measure vascular pressures and left pulmonary artery blood flow. After recovery, the lambs were sedated and mechanically ventilated. During steady-state pulmonary hypertension induced by U46619 (a thromboxane A2 mimic), the lambs were exposed to the following conditions: Protocol A, inhaled NO (0, 5, 40, and 80 ppm) and inspired oxygen concentrations (FiO2) of 0.21, 0.50, and 1.00; and Protocol B, inhaled NO (0, 5, 40, and 80 ppm) and arterial pH levels of 7.30, 7.40, 7.50, and 7.60. Each condition (in randomly chosen order) was maintained for 10 min, and all variables were allowed to return to baseline between conditions. Inhaled NO, oxygen, and alkalosis produced dose-dependent decreases in mean pulmonary arterial pressures (P < 0.05). Systemic arterial pressure remained unchanged. At 5 ppm of inhaled NO, alkalosis and oxygen induced further dose-dependent decreases in mean pulmonary arterial pressures (P < 0.05). At inhaled NO doses > 5 ppm, alkalosis induced further dose-independent decreases in mean pulmonary arterial pressure, while oxygen did not. We conclude that in this animal model, oxygen, alkalosis, and inhaled NO induced selective, dose-dependent pulmonary vasodilation. However, when combined, a systemic arterial pH > 7.40 augmented inhaled NO-induced pulmonary vasodilation, while an FiO2 > 0.5 did not. Therefore, weaning high FiO2 during inhaled NO therapy should be considered, since it may not diminish the pulmonary vasodilating effects. Further studies are warranted to guide the clinical weaning strategies of these pulmonary vasodilator therapies.

Nitric oxide metabolism and breakdown

The steady-state concentration and thus the biological effects of NO are critically determined not only by its rate of formation, but also by its rate of decomposition. Bioreactivity of NO at physiological concentrations may differ substantially from that suggested by in vitro experiments. The charge neutrality and its high diffusion capacity are hallmarks that characterize NO bioactivity. Reactive oxygen derived species are major determinants of NO breakdown. Biotransformation of NO and its related N-oxides occurs via different metabolic routes within the body. S-Nitrosothiols formed upon reaction of NO with redox-activated thiols represent an active storage pool for NO. The major oxidative metabolites represent nitrite and nitrate, the ratio of both is determined by the microenvironmental redox conditions. In humans, circulating nitrite represents an attractive estimate of regional endothelial NO formation, whereas nitrate, with some caution, appears useful in estimating overall nitrogen/NO turnover. Within the near future, more specific biochemical tools for diagnosis of reduced NO bioactivity will become available. Increasing knowledge on the complex metabolism of NO in vivo will lead to the development of new therapeutic strategies to enhance bioactivity of NO via modulation of its metabolism.

Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produces low-spin ferric heme hemichromes which have been characterized by electron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2.21, and 2.97. Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59. The formation of these species goes undetected by UV-vis spectroscopy, but the oxygenation of myoglobin to metmyoglobin is correlated with complete conversion of nitric oxide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.

Electron paramagnetic resonance and oxygen binding studies of alpha-Nitrosyl hemoglobin. A novel oxygen carrier having no-assisted allosteric functions

alpha-Nitrosyl hemoglobin, alpha(Fe-NO)2beta(Fe)2, which is frequently observed upon reaction of deoxy hemoglobin with limited quantities of NO in vitro as well as in vivo, has been synthetically prepared, and its reaction with O2 has been investigation by EPR and thermodynamic equilibrium measurements. alpha-Nitrosyl hemoglobin is relatively stable under aerobic conditions and undergoes reversible O2 binding at the heme sites of its beta-subunits. Its O2 binding is coupled to the structural/functional transition between T- (low affinity extreme) and R- (high affinity) states. This transition is linked to the reversible cleavage of the heme Fe-proximal His bonds in the alpha(Fe-NO) subunits and is sensitive to allosteric effectors, such as protons, 2,3-biphosphoglycerate, and inositol hexaphosphate. In fact, alpha(Fe-NO)2beta(Fe)2 is exceptionally sensitive to protons, as it exhibits a highly enhanced Bohr effect. The total Bohr effect of alpha-nitrosyl hemoglobin is comparable to that of normal hemoglobin, despite the fact that the oxygenation process involves only two ligation steps. All of these structural and functional evidences have been further confirmed by examining the reactivity of the sulfhydryl group of the Cysbeta93 toward 4, 4'-dipyridyl disulfide of several alpha-nitrosyl hemoglobin derivatives over a wide pH range, as a probe for quaternary structure. Despite the halved O2-carrying capacity, alpha-nitrosyl hemoglobin is fully functional (cooperative and allosterically sensitive) and could represent a versatile low affinity O2 carrier with improved features that could deliver O2 to tissues effectively even after NO is sequestered at the heme sites of the alpha-subunits. It is concluded that the NO bound to the heme sites of the alpha-subunits of hemoglobin acts as a negative allosteric effector of Hb and thus might play a role in O2/CO2 transport in the blood under physiological conditions.

[Inhaled nitric oxide in anesthesia and intensive care]

The role of endothelium in vascular relaxation is linked to the existence of endothelium derived relaxing factors (EDRF) known since 1980. In 1987, nitric oxide (NO) was identified as one of these factors. NO acts in many physiologic and pathophysiologic events. Atmospheric NO is a pollutant. Inhaled NO allows selective pulmonary vasodilation and is used to treat pulmonary artery hypertension (PAH). As inhaled NO is inactivated immediately in the blood by linking to haemoglobin, systemic vasodilation does not occur and right ventricular coronary perfusion pressure does not decrease. This is particularly important in the treatment of right ventricular failure due to PAH following cardiothoracic surgery. In patients with an acute respiratory distress syndrome (ARDS), inhaled NO improves the perfusion of adequately ventilated pulmonary territories. Very low concentrations of NO, such as two parts per million, decrease intrapulmonary venous admixture and may reverse hypoxaemia. However its long term benefits in ARDS must be assessed more accurately with multicentre controlled studies. Inhaled NO also improves refractory hypoxaemia in neonates. Its bronchodilatory effect, demonstrated experimentally, does not occur in acute obstructive bronchopulmonatory disease. The toxicity of NO, and overall of its oxidated derivative NO2 requires precise conditions of administration and close monitoring of inhaled fractions. In that case, the risk of NO toxicity seems very low when compared to its therapeutic benefits in selected patients.

Nitrosyl hemoglobin in blood of normoxic and hypoxic sheep during nitric oxide inhalation

During nitric oxide (NO) inhalation therapy, NO combines with deoxyhemoglobin to form nitrosyl hemoglobin (HbNO). We used electron spin resonance (ESR) spectroscopy to measure HbNO in arterial and mixed venous blood of normoxic and hypoxic sheep during NO inhalation. Our aim was to quantitatively measure HbNO levels in the blood during NO inhalation, because large amounts of HbNO reduce the oxygen capacity of blood, particularly in hypoxia. Another aim was to investigate the transfer of exogenous NO to the alpha-heme iron of hemoglobin. Thirteen sheep were anesthetized with pentobarbital sodium, and 60 parts per million (ppm) NO were administered for 1 h in the presence of normoxia and hypoxia. Two-way analysis of variance revealed that the HbNO level was dependent on the oxygen level (normoxia vs. hypoxia) and NO inhalation, and there was a significant negative correlation between the HbNO level and arterial O2 saturation (SaO2). Although the HbNO level increased during NO inhalation in hypoxia, the HbNO level at SaO2 > 60% was < 11 mumol/l monomer hemoglobin (0.11% of total 10 mmol/l monomer hemoglobin). The peak of the HbNO ESR spectrum in arterial blood is located in almost the same position in mixed venous blood with an asymmetric HbNO signal, indicating that the NO in beta-heme HbNO molecules had been transferred to alpha-heme molecules. The three-line hyperfine structure of HbNO on ESR spectra was distinct in venous blood in hypoxia during NO inhalation, indicating pentacoordinate alpha-NO heme formation in hypoxic blood. In conclusion, the amount of HbNO during 60 ppm NO inhalation did not considerably reduce the oxygen capacity of the blood even in the presence of hypoxia, and the NO of HbNO was transferred to the alpha-heme iron of hemoglobin, forming pentacoordinate alpha-NO heme in mixed venous blood in hypoxia.

In vitro and in vivo cytotoxic effects of nitric oxide on metastatic cells

It has been reported that nitric oxide (NO) is tumoricidal in vitro and inhalation of NO is effective for the therapy of pulmonary hypertension. However, little attention has been addressed to the effects of inhaled NO on tumors in the lung. In the present study cytotoxic effects of NO have been investigated both in vitro and in vivo using metastatic cell lines. Viabilities of both B16 melanoma and Lewis lung carcinoma cells were decreased in the presence of S-nitroso-N-acetyl-DL-penicillamine (SNAP) in vitro and the cytotoxicity of SNAP was reduced dose-dependently by NO radical scavenger, oxyhemoglobin. To examine in vivo tumoricidal activity of NO, mice were exposed to 10-80 ppm NO gas after intravenous injection of both tumor cell lines. Intravenous injection of both cell lines produced metastatic tumor colonies in the mouse lung. However, inhaled NO did not reduce the tumor colony formation in the lung. The increase in NO concentration was accompanied by elevation of concomitant nitric dioxide concentration in exposure chambers and exposure to higher concentration of NO appeared to enhance tumor colony formation in the lung.

A safe clinical system for nitric oxide inhalation therapy for pediatric patients

A safe clinical system for nitric oxide (NO) inhalation therapy was developed. The system consists of three parts: a NO controller, a NO monitor, and a patient circuit. NO gas flow and carrier gas flow are controlled by a special rust-proof thermal mass flowmeter. Standard gas quality NO gas (10,000 ppm, balance nitrogen) is used. The outlet of the NO gas tank is connected to the distal end of a heated humidifer that is very close (12 mL) to the patient, to decrease acidic water precipitation and decrease contact time between NO and oxygen (O2). Fail-safe mechanisms to prevent the delivery of a hypoxic mixture or excessive NO concentration are incorporated. Inspiratory NO concentration is continuously monitored by a modified electrochemical NO meter. The patient circuit consists of a breathing circuit and a ventilator with a scavenging unit. A modified Mapleson D type circuit is used. Fresh gas, humidified and mixed with NO, is introduced to the patient connection port. A mechanical ventilator, either of conventional or of high-frequency oscillation type, is connected to the expiratory limb of the Mapleson D circuit. A coaxial scavenging unit including activated charcoal is placed in between the expiratory limb and the ventilator. The adjustment of inspiratory NO concentration (y) was accurate over a wide range (1-80 ppm) of concentrations (x) (y = 0.36 + 0.96x, R2 = 0.999, n = 45) and showed good agreement with the chemiluminescence method. Inspiratory nitrous oxide (NO2) concentration was less than 0.3 ppm, and acidic water accumulation as measured by NO2- and NO3- was less than 5 ppm, even at an extremely high NO concentration of 80 ppm with an FiO2 of 1.0 and 10 L/min of fresh gas flow. Environmental NO and NO2 concentrations in the ICU remained below 0.005 and 0.05 ppm, respectively. This system was used clinically on 214 pediatric patients and proved to be accurate, safe, and useful.

Inhaled nitric oxide: therapeutic applications in cardiothoracic surgery

Hypoxemia and increased pulmonary vascular resistance can greatly complicate the management of cardiothoracic surgical patients. These complications are commonly found in the setting of thoracic organ transplantation, adult and pediatric cardiac surgical procedures, and general thoracic surgical procedures. Inhaled nitric oxide is a new therapy that promises to be extremely valuable to the cardiothoracic surgeon. It has been shown to improve oxygenation in the setting of acute lung injury and to selectively lower pulmonary vascular resistance, without producing unwanted systemic vasodilation. The purpose of this review is to discuss the biochemistry, toxicity, experimental studies, and therapeutic applications of inhaled nitric oxide administration in cardiothoracic surgical patients.

Generation of nitric oxide and superoxide during reperfusion after focal cerebral ischemia in rats

We investigated the levels of nitrosyl hemoglobin (HbNO) in rat jugular blood by electron spin resonance (ESR) spectroscopy during and after middle cerebral artery occlusion. The levels of plasma nitric oxide (NO) end products, nitrate plus nitrate, were compared with the levels of HbNO. Small amounts of HbNO were detected in sham-operated rats (n=4) and those subjected to 2 h of occlusion (n=4), whereas nitrite plus nitrate was increased only in the latter (P<0.01; vs.sham). Upon reperfusion after 2 h of occlusion both HbNO and nitrite plus nitrate clearly increased after 15 min (n=4) and 30 min (n=6) reperfusion (P<0.01; vs.occlusion). Administration of superoxide dismutase (5 mg/kg) significantly increased HbNO (P<0.05) but not plasma nitrate plus nitrate (n=5). The increase in HbNO suppressed by administration of NG-nitro-L-arginine methyl ester (20mg/kg; n=4,P<0.01), and this suppression could be reversed by L-arginine (200 mg/kg) (n=4). The present study clearly showed that the L-arginine-NO synthase pathway was activated during reperfusion after focal cerebral ischemia and indicated the involvement of a reaction between NO and superoxide during early reperfusion.

Inhaled nitric oxide therapy for persistent pulmonary hypertension of the newborn

Increasing evidence suggests that the pulmonary vascular endothelium is an important mediator of resting pulmonary vascular tone through the synthesis and release of a variety of vasoactive substances including nitric oxide (NO). In addition, pulmonary endothelial dysfunction (such as impairment of NO synthesis) is present in lung injury and may contribute to the pathophysiology of pulmonary hypertensive disorders. Recently, exogenously administered NO gas has been utilized to treat infants with persistent pulmonary hypertension of the newborn (PPHN). These preliminary studies suggest that inhaled NO is a promising new therapy for the treatment of infants with PPHN. Controlled clinical trials must now be performed to determine if the use of inhaled NO improves the long-term outcome of patients with PPHN. Long-term exposure must be monitored closely for potential toxicity which includes methemoglobinemia and lung injury secondary to peroxynitrite and nitrogen dioxide production.

Cerebral hemodynamics and distribution of left ventricular output during inhalation of nitric oxide

OBJECTIVES

Inhaled nitric oxide is being utilized as a selective pulmonary vasodilator in the treatment of persistent pulmonary hypertension of the newborn. However, the effects of inhaled nitric oxide on cerebral hemodynamics and distribution of left ventricular output in newborn subjects have not been studied. This study was designed to measure quantitatively the effect of inhaled nitric oxide on the distribution of left ventricular output and on cerebral hemodynamics in a perinatal animal model.

DESIGN

Prospective, controlled, experimental study.

SETTING

Research laboratory.

SUBJECTS

Eight fetal sheep.

INTERVENTIONS

Each animal was exposed to three separate study periods: a) mechanical ventilation with low FIO2 (maintaining fetal levels of PaO2); b) inhalation of nitric oxide (20 parts per million) during mechanical ventilation and low FIO2; and c) mechanical ventilation with an FIO2 of 1.0.

MEASUREMENTS AND MAIN RESULTS

Left ventricular output and cerebral blood flow were measured with radiolabeled microspheres. Cerebral oxygen delivery and consumption variables were calculated using measurements of arterial and cerebral venous (sagittal sinus) oxygen content. Total left ventricular output did not differ among the three treatment groups: 235 +/- 16 mL/min/kg with hypoxic ventilation; 283 +/- 13 mL/min/kg with nitric oxide inhalation; and 242 +/- 17 mL/min/kg with an FIO2 of 1.0. Lung blood flow increased 2.7-fold with inhaled nitric oxide and 1.6-fold during mechanical ventilation with an FIO2 of 1.0. With a left ventricle microsphere injection, increased lung blood flow is indicative of increased systemic-to-pulmonary shunt across the ductus arteriosus. Whole brain blood flow did not differ between the three groups: 49.6 +/- 6.7 mL/min/100 g with hypoxic ventilation; 46.4 +/- 7.4 mL/min/100 g with nitric oxide inhalation; and 36.4 +/- 3.8 mL/min/100 g with an FIO2 of 1.0. Cerebral oxygen delivery increased during inhalation of an FIO2 of 1.0 when compared with nitric oxide inhalation (p < .007); fractional extraction of oxygen decreased (p < .004 compared with hypoxic ventilation, p < .0005 compared with nitric oxide inhalation). Cerebral oxygen consumption did not differ between the three groups (1.11 +/- 0.12 microns/min/100 g with hypoxic ventilation, 0.95 +/- 0.12 microns/min/100 g with nitric oxide inhalation, and 0.96 +/- 0.08 microns/min/100 g with an FIO2 of 1.0).

CONCLUSION

Acute pulmonary vasodilation caused by inhalation of nitric oxide does not change left ventricular output, cerebral blood flow, or cerebral oxygen consumption, despite an increased systemic-to-pulmonary shunt across the ductus arteriosus.

Inhaled nitric oxide therapy

OBJECTIVE

To review the basic science, physiology, toxicity, and delivery of inhaled nitric oxide (NO).

DESIGN

A literature review of inhaled NO is presented, and a brief discussion of current clinical applications is included.

RESULTS

Inhaled NO is a new investigational drug used for selective vasodilation of the pulmonary vasculature. It mimics the effects of endogenously produced endothelium-derived relaxing factor. In addition to selective pulmonary vasodilation, inhaled NO can improve hypoxemia by improving ventilation-perfusion relationships within the lung. The doses of inhaled NO that produce improvements in oxygenation are lower than those needed to produce maximal vasodilation. Inhaled NO is being used in the intensive-care unit to treat critically ill patients with pulmonary hypertension or hypoxemia associated with ventilation-perfusion imbalance. It is also being used in the cardiac catheterization laboratory as a diagnostic tool. Few adverse effects have been associated with the use of inhaled NO.

CONCLUSION

Despite a lack of randomized, controlled studies that show improved outcome in comparison with traditional treatments, inhaled NO seems to be an effective new therapy for patients with pulmonary vasospasm or hypoxemia associated with ventilation-perfusion imbalance. It may also prove to be a valuable diagnostic tool in the cardiac catheterization laboratory.

The pharmacology and toxicology of three new biologic agents used in pulmonary medicine

Biological agents have played an important role in the evolution of modern medical therapeutics. Recent advances in biologicals have in part been stimulated by the biotechnology revolution seen over the last several years. Toxicologists need to be aware of the proposed mechanisms and approved and experimental uses of these new biologic agents. Further, controversies about their use, efficacy, cost issues and potential toxicities should be known. Often these drugs are designed for small patient populations thus limiting the availability of human toxicological data bases. This paper reviews the pharmacology and toxicology of three new biologics (recombinant human DNase I, alpha 1-protease inhibitor, and nitric oxide). These agents appear to have important roles in treating specific diseases or disease states seen in pulmonary medicine.

Nitrosyl hemoglobin production during reperfusion after focal cerebral ischemia in rats

We first detected a definite nitrosyl hemoglobin (HbNO) signal in the jugular blood by electron spin resonance spectroscopy during early reperfusion after cerebral ischemia. A distinct three-line hyperfine structure, characteristic to HbNO, was demonstrated at 30 min of recirculation after 2 h of middle cerebral artery occlusion in rats. Only a weak HbNO signal was observed in animals with 2 h sustained ischemia or with sham operation. The present findings suggest that reperfusion after cerebral ischemia facilitates nitric oxide generation in the brain, which leads to the increased nitrosylation of erythrocyte hemoglobin in the cerebral circulating blood.

Continuous inhalation of nitric oxide protects against development of pulmonary hypertension in chronically hypoxic rats

Exposure to hypoxia and subsequent development of pulmonary hypertension is associated with an impairment of the nitric oxide (NO) mediated response to endothelium-dependent vasodilators. Inhaled NO may reach resistive pulmonary vessels through an abluminal route. The aim of this study was to investigate if continuous inhalation of NO would attenuate the development of pulmonary hypertension in rats exposed to chronic hypoxia. In conscious rats previously exposed to 10% O2 for 3 wk, short-term inhalation of NO caused a dose-dependent decrease in pulmonary artery pressure (PAP) from 44 +/- 1 to 32 +/- 1 mmHg at 40 ppm with no changes in systemic arterial pressure, cardiac output, or heart rate. In normoxic rats, acute NO inhalation did not cause changes in PAP. In rats simultaneously exposed to 10% O2 and 10 ppm NO during 2 wk, right ventricular hypertrophy was less severe (P < 0.01), and the degree of muscularization of pulmonary vessels at both alveolar duct and alveolar wall levels was lower (P < 0.01) than in rats exposed to hypoxia alone. Tolerance to the pulmonary vasodilator effect of NO did not develop after prolonged inhalation. Brief discontinuation of NO after 2 wk of hypoxia plus NO caused a rapid increase in PAP. These data demonstrate that prolonged inhalation of low concentrations of NO induces sustained pulmonary vasodilation and reduces pulmonary vascular remodeling in response to chronic hypoxia.

Methemoglobin production by nitric oxide in fresh sheep blood

As nitric oxide (NO) inhalation is used therapeutically, we studied the production of methemoglobin (metHb) by NO in fresh adult sheep blood. NO solutions were prepared by bubbling a 10% NO-90% N2 gas mixture in phosphate-buffered saline (pH = 7.41 at 20 degrees C) for at least 60 min. Fresh blood samples were obtained from catheterized femoral arteries or veins just prior to mixing with NO solution. Measurements of metHb were made at times 0, 30 sec, 2 min and 5 min after mixing of NO-containing buffer and blood using a Radiometer OSM-3 hemoximeter. Mixing was performed using two syringes connected via a stopcock. The reaction of NO with blood occurred rapidly after mixing since data values for each of the time points after 30 sec were unchanged for all mixtures. The mixing volume ratio of NO-containing buffer to blood was either 1:1 (protocol A) for comparisons of arterial vs venous blood, or the ratios were randomized (protocol B) to investigate effects of Hb oxygenation. Protocol A elicited only slight increases of metHb in arterial and venous blood which did not differ significantly. In protocol B, an increase of metHb was associated with a relative decline of deoxyhemoglobin (deoxyHb). A higher molar ratio of NO/deoxyHb yielded greater amounts of metHb. Therefore, in fresh sheep blood, deoxyHb is converted to metHb in the presence of NO. This reaction is not affected by the presence of oxyhemoglobin.

ESR spectral transition by arteriovenous cycle in nitric oxide hemoglobin of cytokine-treated rats

Nitric oxide (NO) generation was induced in rats by Escherichia coli lipopolysaccharide (LPS) as detected by electron spin resonance (ESR) signals of NO hemoglobin (HbNO). However, there were inconsistencies in ESR spectral shape among them. We have therefore carried out a systematic study to clarify the in vivo spectral changes. First, the spectra of the alpha-NO heme species had the distinct three-line hyperfine structure in venous blood but not in arterial blood in all rats treated with tumor necrosis factor, interleukin-1, and/or LPS, and methemoglobin was not detected at the g = 6 (high-spin methemoglobin) region. Second, when the treated rats died, the three-line hyperfine structure was very distinct even in arterial blood. Third, even if HbNO was formed by injection of nitrite to rats, the three-line hyperfine structure of HbNO in venous blood was more marked than that in arterial blood, independent of the appearance of the methemoglobin signal. Fourth, an ex vivo study using whole blood demonstrated that the three-line hyperfine structure intensified lineally when O2 saturation of hemoglobin decreased but disappeared on reoxygenation of hemoglobin. These results directly demonstrate in vivo quaternary structural transition of the hemoglobin tetramer from the high-affinity state in the arterial cycle to the low-affinity state in the venous cycle. The transition makes the diverse ESR spectra of HbNO in vivo.

The role of nitric oxide (formerly endothelium-derived relaxing factor-EDRF) in vasodilatation and vasodilator therapy

Nitric oxide is widely distributed in the body. It has an important role in the regulation of the circulation and as yet, ill-defined roles in nervous and immune systems. It is derived from L-arginine from a reaction catalysed by a constitutive intracellular enzyme, nitric oxide synthase. It is recognised as the endogenous nitrovasodilator whose action is mimicked by all exogenous nitrovasodilators. After production in the vascular endothelial cell, it diffuses to the smooth muscle cell where it activates the enzyme guanylate cyclase which leads to an increase in cyclic GMP and thence to muscle relaxation. The duration of its action is brief, a few seconds. Disorders of NO metabolism underlie many disease states including endotoxic shock in which prolonged production of nitric oxide may be induced by cytokines. Deficiencies in endogenous production may account for hypertension in various disease states including atherosclerosis and chronic renal failure. NO therapy been used experimentally to successfully treat idiopathic pulmonary hypertension and pulmonary hypertension associated with cardiac and respiratory diseases. However, the long-term benefits have yet to be studied. Administration of NO requires the use of a device to monitor the concentrations of both NO and of NO2. The latter is a noxious agent and a time-related product of the reaction between NO and O2 and is a possible contaminant of preparations of NO. Precautions must be taken to prevent contamination of the work-place atmosphere with NO and NO2. These include gas scavenging and the use of a leak-free system for spontaneous and mechanical ventilation. Using NO in its gaseous form, clinicians have at long last been provided with the means to treat pulmonary hypertension without adversely causing systemic hypotension. The therapy is most suited to short-term use in mechanically ventilated patients. Safe practical long-term NO therapy must await the development of agents which release NO from aerosol preparations.

Advances in the treatment of persistent pulmonary hypertension of the newborn

Despite an association with meconium and blood aspiration, pneumonia, sepsis, pneumothorax, prematurity, and congenital diaphragmatic hernia, no cause for persistent pulmonary hypertension of the newborn can be found in many cases. This article discusses the advances in current therapies including the promising new therapy of inhaled nitric oxide.

Inhaled nitric oxide reverses pulmonary vasoconstriction in the hypoxic and acidotic newborn lamb

We determined whether inhaling low levels of nitric oxide (NO) gas could selectively reverse hypoxic pulmonary vasoconstriction in the near-term newborn lamb and whether vasodilation would be attenuated by respiratory acidosis. To examine the mechanism of air and NO-induced pulmonary vasodilation soon after birth, we measured plasma and lung cGMP levels in the newly ventilated fetal lamb. Breathing at FIO2 0.10 nearly doubled the pulmonary vascular resistance index in newborn lambs and decreased pulmonary blood flow primarily by reducing left-to-right blood flow through the ductus arteriosus. Inhaling 20 ppm NO at FIO2 0.10 completely reversed hypoxic pulmonary vasoconstriction within minutes. Maximum pulmonary vasodilation occurred during inhalation of > or = 80 ppm NO. Breathing 8% CO2 at FIO2 0.10 elevated the pulmonary vascular resistance index to a level similar to breathing at FIO2 0.10 without added CO2. Respiratory acidosis did not attenuate pulmonary vasodilation by inhaled NO. In none of our studies did inhaling NO produce systemic hypotension or elevate methemoglobin levels. Four minutes after initiating ventilation with air in the fetal lamb lung, cGMP concentration nearly doubled without changing preductal plasma cGMP concentration. Ventilation with 80 ppm NO at FIO2 0.21 increased both lung and preductal plasma cGMP concentration threefold. Our data suggest that inhaled NO gas is a rapid and potent selective vasodilator of the newborn pulmonary circulation with an elevated vascular tone due to hypoxia and respiratory acidosis that acts by increasing lung cGMP concentration.

Bronchodilator action of inhaled nitric oxide in guinea pigs

The effects of inhaling nitric oxide (NO) on airway mechanics were studied in anesthetized and mechanically ventilated guinea pigs. In animals without induced bronchoconstriction, breathing 300 ppm NO decreased baseline pulmonary resistance (RL) from 0.138 +/- 0.004 (mean +/- SE) to 0.125 +/- 0.002 cmH2O/ml.s (P less than 0.05). When an intravenous infusion of methacholine (3.5-12 micrograms/kg.min) was used to increase RL from 0.143 +/- 0.008 to 0.474 +/- 0.041 cmH2O/ml.s (P less than 0.05), inhalation of 5-300 ppm NO-containing gas mixtures produced a dose-related, rapid, consistent, and reversible reduction of RL and an increase of dynamic lung compliance. The onset of bronchodilation was rapid, beginning within 30 s after commencing inhalation. An inhaled NO concentration of 15.0 +/- 2.1 ppm was required to reduce RL by 50% of the induced bronchoconstriction. Inhalation of 100 ppm NO for 1 h did not produce tolerance to its bronchodilator effect nor did it induce substantial methemoglobinemia (less than 2%). The bronchodilating effects of NO were additive with the effects of inhaled terbutaline, irrespective of the sequence of NO and terbutaline administration. Inhaling aerosol generated from S-nitroso-N-acetylpenicillamine also induced a rapid and profound decrease of RL from 0.453 +/- 0.022 to 0.287 +/- 0.022 cmH2O/ml.s, which lasted for over 15 min in guinea pigs broncho-constricted with methacholine. Our results indicate that low levels of inhaled gaseous NO, or an aerosolized NO-releasing compound are potent bronchodilators in guinea pigs.

The nitric oxide/heme protein complex as a biologic marker of exposure to nitrogen dioxide in humans, rats, and in vitro models

In an effort to develop a biologic marker of exposure to nitrogen dioxide (NO2), we investigated the in vivo formation of a complex between heme proteins and nitric oxide (NO). In aqueous solution, NO2 disproportionates to NO and nitrate. The NO binds to the iron of heme proteins to form an electron spin resonance (ESR)-detectable complex. We have shown that when rat liver, lung, or nasal microsomes are exposed to 20 ppm NO2 in vitro, an ESR signal attributable to an NO/heme protein complex is detected. After inhalation exposure of rats to 20 ppm NO2 for 6 h, this same ESR signal was detected in microsomes prepared from the exposed rats' lungs or liver; microsomes prepared from the nasal tissue failed to yield any detectable signal. When we lavaged the lungs of rats exposed for 6 h to 0, 5, 10, 20, or 30 ppm NO2 and isolated the bronchoalveolar cell pellets, the NO/heme protein complex was detected in the cell pellets. We were able to demonstrate a dose-dependent relationship between the ESR signal intensity of the NO/heme protein complex and the NO2 exposure concentration. Finally, we used ESR to examine bronchoalveolar lavage cell pellets obtained from human volunteers exposed to either 1.5 or 4 ppm NO2, for 20 min every other day, for six exposures. No signal was found in any of the samples taken 3 wk prior to NO2 exposure, but an ESR signal attributable to the NO/heme protein complex was detected in every sample obtained after the 4 ppm NO2 exposure and in five of eight samples obtained after the 1.5 ppm NO2 exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhaled nitric oxide. A selective pulmonary vasodilator reversing hypoxic pulmonary vasoconstriction

Background. The gas nitric oxide (NO) is an important endothelium-derived relaxing factor, inactivated by rapid combination with heme in hemoglobin. Methods and Results. Awake spontaneously breathing lambs inhaled 5-80 ppm NO with an acutely constricted pulmonary circulation due to either infusion of the stable thromboxane endoperoxide analogue U46619 or breathing a hypoxic gas mixture. Within 3 minutes after adding 40 ppm NO or more to inspired gas, pulmonary hypertension was reversed. Systemic vasodilation did not occur. Pulmonary hypertension resumed within 3-6 minutes of ceasing NO inhalation. During U46619 infusion pulmonary vasodilation was maintained up to 1 hour without tolerance. In the normal lamb, NO inhalation produced no hemodynamic changes. Breathing 80 ppm NO for 3 hours did not increase either methemoglobin or extravascular lung water levels nor modify lung histology compared with control lambs. Conclusions. Low dose inhaled NO (5-80 ppm) is a selective pulmonary vasodilator reversing both hypoxia- and thromboxane-induced pulmonary hypertension in the awake lamb [corrected].

The interaction between nitrogen oxides and hemoglobin and endothelium-derived relaxing factor

Among nitrogen oxides, NO and NO2 are free radicals and show a variety of biological effects. NO2 is a strongly oxidizing toxicant, although NO, not oxidizing as NO2, is toxic in that it interacts with hemoglobin to form nitrosyl- and methemoglobin. Nitrosylhemoglobin shows a characteristic electron spin resonance (ESR) signal due to an odd electron localized on the nitrogen atom of NO and reacts with oxygen to yield nitrate and methemoglobin, which is rapidly reduced by methemoglobin reductase in red cells. NO was found to inhibit the reductase activity. Part of NO inhaled in the body is oxidized by oxygen to NO2, which easily dissolves in water and converts to nitrite and nitrate. The nitrite oxidizes oxyhemoglobin autocatalytically after a lag. The mechanism of the oxidation, particularly the involvement of superoxide, was controversial. The stoichiometry of the reaction has now been established using nitrate ion electrode and a methemoglobin free radical was detected by ESR during the oxidation. Complete inhibition of the autocatalysis by aniline or aminopyrine suggests that the radical catalyzes conversion of nitrite to NO2, which oxidizes oxyhemoglobin. Recently NO was shown to be one of endothelium-derived relaxing factors and the relaxation induced by the factor was inhibited by hemoglobin and potentiated by superoxide dismutase.

Generation of valency hybrids and nitrosylated species of hemoglobin in mice by nitric oxide vasodilators

The hemoglobin fraction of blood samples from C57BL/6 male mice was purified by column chromatography and subjected to isolectric focusing (IEF) across a pH gradient. Densitometric scanning of the IEF gel showed the presence of a single peak corresponding to the fully reduced tetramer, H, or (alpha 2+ beta 2+)2. Twenty minutes after an ip injection of 1.1 mmol/kg NaNO2 blood samples treated the same way showed four peaks corresponding to the species: H = 43%, X or (alpha 2+ beta 3+)2 = 10%, Y or (alpha 3+ beta 2+)2 = 33%, and M or (alpha 3+ beta 3+)2 = 14%. In contrast blood samples from control CD-1 male mice showed the presence of three IEF distinct peaks which were all believed to be H valency forms, and six distinct peaks were seen after treatment in vivo with NaNO2. Thus, the C57BL/6 mice yield patterns similar to those observed after in vitro treatment of human red cells with NaNO2 (H. Kruszyna, R. Kruszyna, R. P. Smith, and D. E. Wilcox, 1987b, Toxicol. Appl. Pharmacol. 91, 429-438), and the CD-1 mice are a much less satisfactory model. The appearance and disappearance of the species X, Y, and M over time after ip injection of 1.1 mmol/kg NaNO2 or hydroxylamine HCl were followed in C57BL/6 mice by the technique of IEF. In each case the patterns were consistent with previously established patterns for the respective methemoglobinemias as determined by absorption spectrophotometry, and they were consistent with the suggestion that two pathways exist for the oxidation of H and for the reduction of M which proceed through X and Y, respectively. By using electron paramagnetic resonance (EPR) spectroscopy, we were also able to follow with time the concentration of nitrosylated heme (NO-heme) on reduced subunits in both mouse strains. The peak for the NO-heme coincided in time with the peak methemoglobinemia as determined by either IEF or absorption spectrophotometry. EPR was also used to determine NO-heme in CD-1 mice after injection of a series of NO-vasodilators with and without methylene blue (MB). Low, but clearly detectable amounts of NO-heme were found in the blood of animals given all xenobiotics tested including NaNO2, hydroxylamine HCl, glyceryl trinitrate, hydralazine, sodium nitroprusside, and sodium azide. MB has little effect on the response, and no NO-heme could be detected in control mice.(ABSTRACT TRUNCATED AT 400 WORDS)

Uptake and distribution of 14C during and following exposure to [14C]methyl isocyanate

Guinea pigs were exposed to [14C]methyl isocyanate (14CH3-NCO, 14C MIC) for periods of 1 to 6 hr at concentrations of 0.5 to 15 ppm. Arterial blood samples taken during exposure revealed immediate and rapid uptake of 14C. Clearance of 14C was then gradual over a period of 3 days. Similarly 14C was present in urine and bile immediately following exposure, and clearance paralleled that observed in blood. Guinea pigs fitted with a tracheal cannula and exposed while under anesthesia showed a reduced 14C uptake in blood indicating that most of the 14C MIC uptake in normal guinea pigs occurred from retention of this agent in the upper respiratory tract passages. In exposed guinea pigs 14C was distributed to all examined tissues. In pregnant female mice similarly exposed to 14C MIC, 14C was observed in all tissues examined following exposure including the uterus, placenta, and fetus. While the form of 14C distributed in blood and tissues has not yet been identified, these findings may help to explain the toxicity of MIC or MIC reaction products on organs other than the respiratory tract, as noted by several investigators.