In vivo nitric oxide detection in the septic rat brain by electron paramagnetic resonance

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

Nitric oxide detection methods in vitro and in vivo

Initially being considered as an environmental pollutant, nitric oxide has gained the momentum of research since its discovery as endothelial derived growth factor in 1987. Extensive researches have revealed the various pathological and physiological roles of nitric oxide such as inflammation, vascular and neurological regulation functions. Hence, the development of methods for quantifying nitric oxide concentration and its metabolites will be beneficial to well know about its biological functions and effects. This review summaries various methods for in vitro and in vivo nitric oxide detection, and introduces their merits and demerits.

Nitric oxide induces vascular endothelial growth factor expression in the rat placenta in vivo and in vitro

We investigated the role of nitric oxide (NO) in vascular endothelial growth factor (VEGF) expression in the rat placenta. A nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine-methyl ester (L-NAME), was constantly infused into pregnant rats 6-24 h before sacrifice on gestational day (GD) 15.5. NO production declined to about 15% of the control level as monitored by NO trapping and electron paramagnetic resonance spectroscopy. VEGF mRNA expression was temporally decreased by L-NAME, but recovered to normal levels after 24 h of treatment, whereas hypoxia inducible factor (HIF)-1α and induced NOS (iNOS) expression increased. VEGF expression decreased significantly in placental explants after 6 h of co-treatment with L-NAME and lipopolysaccharide, an iNOS inducer. Our data indicate that NO induce VEGF expression in vivo and in vitro in the rat placenta, suggesting that peaked NO production was maintained by a reciprocal relationship between NO and VEGF via HIF-1α.

Neuro-oxidative-nitrosative stress in sepsis

Neuro-oxidative-nitrosative stress may prove the molecular basis underlying brain dysfunction in sepsis. In the current review, we describe how sepsis-induced reactive oxygen and nitrogen species (ROS/RNS) trigger lipid peroxidation chain reactions throughout the cerebrovasculature and surrounding brain parenchyma, due to failure of the local antioxidant systems. ROS/RNS cause structural membrane damage, induce inflammation, and scavenge nitric oxide (NO) to yield peroxynitrite (ONOO(-)). This activates the inducible NO synthase, which further compounds ONOO(-) formation. ROS/RNS cause mitochondrial dysfunction by inhibiting the mitochondrial electron transport chain and uncoupling oxidative phosphorylation, which ultimately leads to neuronal bioenergetic failure. Furthermore, in certain 'at risk' areas of the brain, free radicals may induce neuronal apoptosis. In the present review, we define a role for ROS/RNS-mediated neuronal bioenergetic failure and apoptosis as a primary mechanism underlying sepsis-associated encephalopathy and, in sepsis survivors, permanent cognitive deficits.

Early systemic inflammatory response in mice after a single oral gavage with live Escherichia coli is evidenced by increased TNF-alpha and nitric oxide production

Twenty-four female BALB/c mice were orally inoculated with 10(8) CFU Escherichia coli ATCC 25922 and euthanized 2.5, 7, 13 and 25 h post-inoculation. The levels of organ nitric oxide (NO) and plasma endotoxin, TNF-alpha and nitrite/nitrate (NO(x)) were compared to those found in sham-inoculated mice, to evaluate systemic host-response to a low-level oral exposure to Gram-negative bacteria. Organ bacterial culture and immunohistochemistry for iNOS were performed on lungs, liver, kidneys and brain from all mice. Organ NO and plasma TNF-alpha levels were higher in E. coli-inoculated animals, but no differences were detected in plasma endotoxin levels, NO(x) or iNOS immunostaining for any of the animal groups. Single oral gavage with live E. coli stimulates an early systemic immune response in clinically healthy mice as evidenced by increased plasma TNF-alpha and organ NO levels, but bacteremia and endotoxemia are not related to this inflammatory response.

Systemic use of selective iNOS inhibitor 1400W or non-selective NOS inhibitor l-NAME differently affects systemic nitric oxide formation after oral Porphyromonas gingivalis inoculation in mice

OBJECTIVE

Nitric oxide synthase (NOS) inhibitors are reported to protect against the local tissue damage in gingivitis and periodontal disease by reducing nitroxidative stress during inflammation, but their systemic effects are not well investigated.

DESIGN

NOS inhibitors systemic effects were investigated in a murine chronic oral inoculation model using live Porphyromonas gingivalis ATCC 33277 (0.3 ml; 10(9)cfu/ml) or sterile broth (0.3 ml). Organ nitric oxide (NO) and plasma nitrite/nitrate (NOx) were determined in mice treated with non-selective NOS inhibitor l-NAME (50mg/kg/24h i.p.) or selective iNOS inhibitor 1400W (10mg/kg/6h i.p.) for the last 5 days of the experiment. Differences between groups were evaluated by nonparametric Wilcoxon's rank-sum one-sided two-sample test and the results compared to those obtained from sham-treated (sterile broth) sham-inoculated animals (water for injection i.p./6h).

RESULTS

Repeated ingestion of P. gingivalis resulted in generalized production of NO in organs and NOx in plasma, the levels of both typically being reduced in P. gingivalis-inoculated-1400W-treated mice, whilst the use of l-NAME was largerly ineffective, even promoting NO/NOx formation. Application of either inhibitor to sham-inoculated animals enhanced NO/NOx formation, due only in part to the repeated i.p. injections.

CONCLUSIONS

The systemic use of 1400W or l-NAME differently affects systemic nitric oxide formation in mice orally challenged with P. gingivalis, but the sequelae of such an intervention should be evaluated further.

Apoptosis caused by an inhibitor of NO production in the decidua of rat from mid-gestation

We previously reported that nitric oxide (NO) is first detected in the uterus of a pregnant rat on gestational day 13.5 (GD13.5) and that NO levels peak on GD17.5. In addition, NO production in the uterus is mainly derived from the decidua and not the myometrium. The aim of the present study was to reveal the role of NO that peaked on GD17.5 of gestation in the decidua. To inhibit NO production, pregnant rats were continuously administered by an nitric oxide synthase inhibitor, NG-nitro-l-arginine-methyl ester (l-NAME) for 48 h. In the control group, saline was infused instead of l-NAME. After treatment, the decidua were obtained from GD13.5, GD17.5 and GD21.5 rats. Apoptosis and activated caspase-3-positive cells were observed by transferase-mediated dUTP nick-end labeling (TUNEL) assay and immunohistochemistry, respectively. The caspase-3 enzyme activity was also measured in the cell lysate from the decidua. The numbers of TUNEL-positive cells and activated caspase-3-positive cells each increased and the amount of caspase-3 activity also increased significantly in rats on GD17.5 than in rats in the control group, but no changes were observed in rats on GD13.5 and GD21.5. Furthermore, enzyme activity regarding the initiator caspases, caspase-8 and -9, upstream factors for caspase-3 in the caspase cascade, was measured simultaneously on GD17.5 under the same treatment. Caspase-8 and -9 enzyme activities increased significantly in the control group; an increment of caspase-8 activity was especially prominent. The present results indicate that an inhibitor of NO production caused apoptosis through typical apoptotic signals in the decidua on GD17.5, suggesting that an NO peak in the decidua is essential to cell survival and the maintenance of uterine formation.

Nitric oxide production and its contribution to hepatocyte proliferation in normal juvenile rats

Nitric oxide (NO) has been reported as a key mediator in enhancing hepatocyte proliferation during liver regeneration. Juvenile hepatocytes have a strong ability to proliferate while still in their undifferentiated state but the mechanism of NO production and its contribution to hepatocyte proliferation are not yet fully understood. The present study was designed to investigate NO production in the normal liver and its contribution to hepatocyte proliferation in juvenile rats. Endogenous NO production was evaluated quantitatively using a spin trap followed by electron paramagnetic resonance spectroscopy with the Fe-N, N-diethyldithiocarbamate complex as an NO-trapping reagent in the rat liver. NO production in the liver significantly peaked at 3 weeks after birth, but NO synthase (NOS) 3 expression did not change between 2 to 5 weeks after birth, while NOS 1 and NOS 2 mRNA were not detected. Hepatocyte proliferation, measured by the incorporation of 5-bromo-2'-deoxyuridine into the DNA, was found to decline significantly when endogenous NO production was inhibited by the administration of the NOS inhibitor N(G)-nitro- (L)-arginine methyl ester. These findings indicate that endogenous NO production peaked at 3 weeks after birth and hepatocyte proliferation declined significantly when NO production was inhibited. Thus, this study provides a novel insight into the contribution of NO to hepatic growth and liver maturation in juveniles.

NOS2 gene deficiency protects from sepsis-induced long-term cognitive deficits

To date, long-term consequences of septic encephalopathy on cerebral metabolism, cognition, learning, and memory capabilities and factors involved are poorly understood. In this study, we used a murine sepsis model to demonstrate that bacterial lipopolysaccharide (LPS) causes long-term cognitive deficits in mice. Two months after LPS treatment, wild-type mice committed more working and reference memory errors than controls. The behavioral impairment was independent of the cerebral glucose uptake as evidenced by (18)F-Fluordeoxyglucose small animal positron emission tomography. In contrast, mice deficient for the inducible nitric oxide synthase gene (NOS2-/-) did not show any cognitive changes when challenged with LPS. Immunohistochemical analysis demonstrated that LPS did not lead to neuronal cell death but caused sustained microglial activation in wild-type as compared to NOS2-/- mice. Expression analysis showed that LPS-treated NOS2-/- mice had lower brain mRNA levels for proinflammatory factors compared with wild-type mice. Expression analysis demonstrated distinct changes in the content of synaptic proteins in wild-type mice, which were not observed in the NOS2-/- mice. Together, this data set outlines the importance of the NOS2 activation for long-term cerebral changes after severe sepsis.

Single oral inoculation with Escherichia coli (ATCC 25922) stimulates generalised production of nitric oxide in mice

Nitric oxide (NO) production was investigated in the lungs, thoracic aorta, heart, liver, spleen, kidneys and brain of mice inoculated orally with Escherichia coli ATCC 25922. Detection of NO was performed by electron paramagnetic resonance (EPR) using diethyldithiocarbamate (DETC) spin trap. Nitric oxide synthase (NOS) inhibitors [nonselective: L-NAME and inducible NOS (iNOS) selective: 1400W] were used to determine the source of NO. Spin-trap only and untreated mice were included as controls. Within 2.5 hours (h) of a single oral inoculation with E. coli half of the animals had increased NO levels in all investigated organs. Thereafter the signals dropped before increasing again to reach maximal median values by 25 h in all organs of all inoculated mice. The most intense response occurred in livers, followed by aorta and lungs. Early (2.5 h) inhibition of the signal was achieved using both NOS inhibitors. L-NAME was also effective at 25 h, while 1400W-treated mice had increased NO levels beyond 7 h. The generalised increase in NO production in the short and longer term indicates a host response to E. coli administered by the oral route of infection.

Release and hemodynamic influence of nitro-glycerine-derived nitric oxide in endotoxemic rats

BACKGROUND AND AIM

Nitric oxide released from nitro-glycerine (NG) has been considered to improve the microcirculation. Septic conditions are, however, associated with excessive formation of nitric oxide (NO), which is formed from l-arginine by the inducible NO synthase (iNOS) activity. Since the characteristics and influence of NG-derived NO in sepsis remains unclear, the major aims of the present study were to quantify the release and to determine the effects of NO formed from NG on systemic blood pressure under endotoxemic conditions.

MATERIAL AND METHODS

Four hours following endotoxin challenge (10 mg/kg intraperitoneally), rats received an infusion of NG (0.5 or 5.0 micromol/kg/h) over 45 min. We determined the changes in blood pressure and the NO concentrations generated in brain, heart, intestine, kidney, liver, and lung by means of NO trapping and EPR technique.

RESULTS

NG infusion in control rats and endotoxin challenge decreased systemic blood pressure to the same extent. However, in rats subjected to endotoxin challenge NG infusion did not affect the blood pressure. The endotoxin-induced increase in tissue NO concentrations were found to be 15-folds higher than tissue levels of NO following NG infusion.

CONCLUSION

Our results suggest that under endotoxic shock conditions in rats NG may not additionally affect the systemic blood pressure. This may relate to the excessive tissue NO levels induced by endotoxin that are not further increased by NG.

Spin trapping of nitric oxide with the iron-dithiocarbamate complex: chemistry and biology

This brief review describes chemical and biological aspects concerning spin trapping of nitric oxide (NO) with the iron-dithiocarbamate (Fe-DTC) complex as a spin trap. Knowledge on basic properties of the Fe-DTC complex would help in understanding the applicability and limitation of the Fe-DTC-based NO spin-trapping method when it is employed in viable biological systems.

In vivo spin trapping of nitric oxide

The measurement of nitric oxide (NO) in biological samples has normally required destructive chemical techniques. The ability to detect NO non-invasively in living animals or excised organs has great potential using specialized electron paramagnetic resonance (EPR) methods. Although NO is paramagnetic, it cannot be observed directly unless it is complexed with ferrous iron-dithiocarbamate ligand spin trap complexes. Despite the minimally invasive nature of the technique, highly sensitive localized concentrations of NO may be observed ("trapped") in vivo by both L-band EPR and magnetic resonance imaging.

Progress in clinical neurosciences: sepsis-associated encephalopathy: evolving concepts

Systemic sepsis commonly produces brain dysfunction, sepsis-associated encephalopathy, which can vary from a transient, reversible encephalopathy to irreversible brain damage. The encephalopathy in the acute phase clinically resembles many metabolic encephalopathies: a diffuse disturbance in cerebral function with sparing of the brain stem. The severity of the encephalopathy, as reflected in progressive EEG abnormalities, often precedes then parallels dysfunction in other organs. Recent research has revealed a number of potentially important, non-mutually exclusive, mechanisms that have therapeutic implications.

Nitric oxide synthase inhibitors affect nitric oxide synthesis in normoxic but not in ischemic organs during intestinal ischemia and early reperfusion

Inhibition of endogenous nitric oxide (NO) synthesis during early intestinal ischemia/reperfusion (I/R(i)) enhances remote organ damage related to I/R(i). However, the effects of NO synthase (NOS) inhibitors on NO formation in various organs have not yet been specified. We therefore investigated the effects of N-G-monomethyl-L-arginine (L-NMMA), a nonspecific NOS inhibitor, and L-arginine, the NOS substrate, on NO formed in ischemic intestine versus normoxic remote organs (lung and liver). We used electron paramagnetic resonance spectroscopy and a specific NO trap to assay NO in blood, intestine, lung, and liver of rats subjected to local I/R(i), with and without L-NMMA and L-arginine supplementation. We found that I/R(i) increased NO levels in the intestine and blood, but not in the remote organs lung and liver. Administration of L-NMMA before I/R(i) decreased I/R(i)-independent basal NO levels in normoxic lung and liver without influencing I/R(i)-induced increase in NO levels in intestinal tissue or in blood. L-arginine supplementation increased circulating levels of NO, with sensitivity to L-NMMA, without affecting NO levels in normoxic or ischemic tissue. Our data suggest that NOS activity controls the NO generated in normally perfused remote organs during early I/R(i). Hence NOS inhibitors, when administered during I/R(i), decrease physiological NO levels in normoxic remote organs without affecting increased NO levels originating from ischemic intestine. This may explain the harmful effect of nonspecific NOS inhibitors during early I/R(i). In addition, the generation of NO in remote organs is not limited by tissue L-arginine concentrations and, therefore, not influenced by exogenous L-arginine. The protective effect of L-arginine supplementation during I/R(i) is probably related to increasing intravascular NO formation.

Nitric oxide as an anterograde neurotransmitter in the trigeminal motor pool

We demonstrate the presence of nitric oxide synthase containing fibers within the guinea pig trigeminal motor nucleus and describe the effects of nitric oxide (NO) on trigeminal motoneurons. Using immunohistochemical techniques, we observed nitrergic fibers displaying varicosities and giving rise to bouton-like structures in apposition to retrogradely labeled motoneuron processes, most of which were dendrites. NO-donors evoked a membrane depolarization (mean 7.5 mV) and a decrease in rheobase (mean 38%). These substances also evoked an apparent increase in an hyperpolarization-activated cationic current (I(H)). These changes were not accompanied by any modification of the motoneurons' input resistance or time constant. The effects were suppressed by blocking the cytosolic guanlyate cyclase. A membrane-permeant cyclic guanosine 3,5'-monophosphate (cGMP) analogue mimicked the effects of NO. There was a considerable increase in synaptic activity following NO-donors or db-cGMP application. Tetrodotoxin supressed the increase in synaptic activity evoked by NO-donors. The histological and electrophysiological evidence, taken together, indicates the existence of a nitrergic system able to modulate trigeminal motoneurons under yet unknown physiological conditions.

Sepsis inhibits reduction of dehydroascorbic acid and accumulation of ascorbate in astroglial cultures: intracellular ascorbate depletion increases nitric oxide synthase induction and glutamate uptake inhibition

Sepsis is associated with oxidative stress and impaired glutamatergic transmission in brain. We investigated whether sepsis impairs accumulation of the antioxidant, ascorbate, and uptake of glutamate by astrocytes. Bacterial endotoxin (Escherichia coli lipopolysaccharide, LPS) and the inflammatory cytokine, interferon-gamma (IFNgamma), were applied to primary astrocyte cultures to model sepsis. In the absence of ascorbate, the combination of LPS and IFNgamma (LPS + IFNgammay) up-regulated inducible nitric oxide synthase (iNOS) and decreased the initial rate of glutamate uptake by 50% within 24 h. Cell viability and facilitated glucose transport activity were not affected at 24 h. Pre-treatment with ascorbate-2-O-phosphate increased intracellular ascorbate concentration and attenuated the induction of iNOS and inhibition of glutamate uptake caused by LPS + IFNgamma. Subsequent experiments examined the mechanisms by which cells accumulate ascorbate. LPS + IFNy decreased slightly the initial rate of uptake of ascorbate and inhibited markedly the rate with which intracellular dehydroascorbic acid (DHAA) was reduced to ascorbate. We conclude that septic insult impairs astrocytic clearance of DHAA from the extracellular fluid and decreases intracellular ascorbate concentration. Furthermore, sepsis induces iNOS and inhibits glutamate uptake by astrocytes through mechanisms that can be modulated by intracellular ascorbate. These results indicate treatments that increase intracellular ascorbate concentration may be beneficial for patients at risk for neurologic complication in sepsis.

Expression of nitric oxide synthase isoforms and detection of nitric oxide in rat placenta

Nitric oxide (NO) production in the rat placenta was monitored and quantified by electron paramagnetic resonance (EPR) spectroscopy with hemoglobin and an Fe-N-(dithiocarboxy)sarcosine (DTCS) complex as NO-trapping reagents. Expression of nitric oxide synthase (NOS) isoforms was also examined by quantitative RT-PCR analysis. The EPR spectrum of the placenta with hemoglobin trapping showed a three-line hyperfine structure (g = 2.008 and a = 1.66-mT). The EPR signal was diminished after the placenta was homogenized or the NOS inhibitor L-NAME was administered to pregnant rats. Therefore, the specific signal was definitely identified as being derived from endogenous NO spin-trapped by hemoglobin, and the EPR spectrum showed that the NO adduct existed as a pentacoordinate alpha-NO heme species. The EPR spectrum of the placenta with Fe-DTCS trapping showed a triplet signal (g = 2.038) derived from an NO-Fe-DTCS complex. The height of the triplet signal did not vary significantly with gestational stage during the last few days of gestation. At the gestational stages examined, the level of NOS II mRNA expression was significantly higher than that of NOS III mRNA. NOS II expression in term (day 21.5) placenta was significantly increased compared with that in preterm (day 19.5) placenta (P < 0.01, n = 4 or 5). These results suggest that NOS II is the predominant producer of NO in the placenta and that NOS II-generated NO plays significant roles in the maintenance of placental functions immediately before birth.

Can we model nitric oxide biotransport? A survey of mathematical models for a simple diatomic molecule with surprisingly complex biological activities

Nitric oxide (NO) is a remarkable free radical gas whose presence in biological systems and whose astonishing breadth of physiological and pathophysiological activities have only recently been recognized. Mathematical models for NO biotransport, just beginning to emerge in the literature, are examined in this review. Some puzzling and paradoxical properties of NO may be understood by modeling proposed mechanisms with known parameters. For example, it is not obvious how NO can survive strong scavenging by hemoglobin and still be a potent vasodilator. Recent models do not completely explain how tissue NO can reach effective levels in the vascular wall, and they point toward mechanisms that need further investigation. Models help to make sense of extremely low partial pressures of NO exhaled from the lung and may provide diagnostic information. The role of NO as a gaseous neurotransmitter is also being understood through modeling. Studies on the effects of NO on O2 transport and metabolism, also reviewed, suggest that previous mathematical models of transport of O2 to tissue need to be revised, taking the biological activity of NO into account.

Spin trapping agent, phenyl N-tert-butylnitrone, reduces nitric oxide production in the rat brain during experimental meningitis

Phenyl N-tert-butylnitrone (PBN) is a spin trapping agent previously shown to exert a neuroprotective effect in infant rat brain during bacterial meningitis. In the present study, we investigated the effect of systemic PBN administration on nitric oxide (NO) production in a rat model of experimental meningitis induced by lipopolysaccharide (LPS). We assessed the NO concentration in rat brain tissues with an electron paramagnetic resonance (EPR) NO trapping technique. In this model, rats receiving intracisternal LPS administration showed symptoms of meningitis and cerebrospinal fluid (CSF) pleocytosis. The time course study indicated that the concentration of NO in the brain reached the maximum level 8.5 h after injection of LPS, and returned to the control level 24 h after the injection. When various doses of PBN (125-400 mg/kg) were injected intraperitoneally 30 min prior to LPS, NO production in the brain was reduced with increasing PBN dose (250 mg/kg suppressed 80% at 8.5 h after LPS injection), and white blood cells (WBC) in CSF were significantly decreased. We concluded that reduction of NO generation during bacterial meningitis contributes to the neuroprotective effect of PBN in addition to its possible direct scavenging of reactive oxygen intermediate (ROI).

Nitric oxide-forming reaction between the iron-N-methyl-D-glucamine dithiocarbamate complex and nitrite

The objective of this study was to elucidate the origin of the nitric oxide-forming reactions from nitrite in the presence of the iron-N-methyl-D-glucamine dithiocarbamate complex ((MGD)(2)Fe(2+)). The (MGD)(2)Fe(2+) complex is commonly used in electron paramagnetic resonance (EPR) spectroscopic detection of NO both in vivo and in vitro. Although it is widely believed that only NO can react with (MGD)(2)Fe(2+) complex to form the (MGD)(2)Fe(2+).NO complex, a recent article reported that the (MGD)(2)Fe(2+) complex can react not only with NO, but also with nitrite to produce the characteristic triplet EPR signal of (MGD)(2)Fe(2+).NO (Hiramoto, K., Tomiyama, S., and Kikugawa, K. (1997) Free Radical Res. 27, 505-509). However, no detailed reaction mechanisms were given. Alternatively, nitrite is considered to be a spontaneous NO donor, especially at acidic pH values (Samouilov, A., Kuppusamy, P., and Zweier, J. L. (1998) Arch Biochem. Biophys. 357, 1-7). However, its production of nitric oxide at physiological pH is unclear. In this report, we demonstrate that the (MGD)(2)Fe(2+) complex and nitrite reacted to form NO as follows: 1) (MGD)(2)Fe(2).NO complex was produced at pH 7.4; 2) concomitantly, the (MGD)(3)Fe(3+) complex, which is the oxidized form of (MGD)(2)Fe(2+), was formed; 3) the rate of formation of the (MGD)(2)Fe(2+).NO complex was a function of the concentration of [Fe(2+)](2), [MGD], [H(+)] and [nitrite].

Detection and imaging of endogenously produced nitric oxide with electron paramagnetic resonance spectroscopy

Nitric oxide (NO) represents a new paradigm for second messengers in regulation. Despite the numerous physiological and pathophysiological functions of NO, its importance as an endogenous second messenger and a cytostatic and/or cytotoxic agent was unknown until 1987. Recent developments in detection methods for endogenous NO produced directly or indirectly from NO synthases (NOSs) have enabled major advances in our understanding of the role of NO in biological systems. The spin-trapping technique combined with electron paramagnetic resonance (EPR) spectroscopy is a method for analyzing NO production directly both in vivo and in vitro. Iron complexes with dithiocarbamate derivatives are noteworthy among the spin-trapping reagents for NO because NO has a high affinity for iron complexes. The resultant stable nitrosyl iron complexes exhibit an intense three-line signal at room temperature and an axial signal at low temperature. Besides the facility and wide applicability of this method, its outstanding feature is that noninvasive in vivo measurements are available by using a low-frequency EPR spectrometer. In this article, we review on previous and recent developments of in vitro, in vivo, and ex vivo EPR detection and imaging of endogenously produced NO.

Mechanistic studies of the oxidation of pyridoxalated hemoglobin polyoxyethylene conjugate by nitrogen monoxide

Pyridoxalated hemoglobin polyoxyethylene conjugate (PHP), a modified human-derived hemoglobin, is currently in clinical trials as a nitrogen monoxide scavenger for the treatment of shock. Stopped-flow spectroscopy studies of the reaction between nitrogen monoxide and PHP indicate that at pH 7 the second-order rate constant is (88 +/- 3) x 10(6) M(-1) s(-1), a value very similar to that for the unmodified human hemoglobin. At alkaline pH the reaction proceeds via the intermediate peroxynitrito complex PHP-Fe(III)OONO, which rapidly decomposes to nitrate and the iron(III) form of PHP. The rate of decay of PHP-Fe(III)OONO increases significantly with decreasing pH such that it does not accumulate in concentrations large enough to be observed spectroscopically under neutral or acidic conditions. Ion chromatographic analysis of the nitrogen-containing products of the NO(*)-mediated reaction of PHP shows that nitrate is formed quantitatively at both pH 7 and pH 9.

Direct evidence of in vivo nitric oxide production and inducible nitric oxide synthase mRNA expression in the brain of living rat during experimental meningitis

To detect endogenous nitric oxide (NO) produced in a rat bacterial meningitis model, the authors applied an electron paramagnetic resonance (EPR) NO-trapping technique. Iron complex with N,N-diethyldithiocarbamate were used as a trapping agent. Experimental meningitis was induced by a mixture of lipopolysaccharide and interferon-gamma. Sequential changes of NO formation under meningitis were observed in rat brain tissue by using X-band (9 GHz) EPR spectroscopy, and endogenous NO was detected in the head of a living rat with a 700-MHz EPR system. Inducible NO synthase mRNA expression in the brain tissues also was proven by using a reverse transcriptase-polymerase chain reaction technique.

Ex vivo EPR detection of nitric oxide in brain tissue

The concentration of nitric oxide (NO) was measured in the brain of septic-shock animals by electron paramagnetic resonance spectrometry (EPR). NO was spin trapped and quantitated in several regions of the brain (cortex, hippocampus, hypothalamus, cerebellum, and olfactory bulb) as well as other organs (liver, kidney, and heart) of rats induced with lipopolysaccharide (LPS) using Fe(II)/dithiocarbamate complexes containing diethyldithiocarbamate (DETC) or N-methyl-D-glucamine (MGD). The spin trap, (DETC)(2)-Fe(II), complexed NO generated in all tissues examined, but (MGD)(2)-Fe(II) complex was ineffective in detecting NO in the brain of septic-shock rats, although identical amounts of NO were detected in the liver with either spin trap. A triplet EPR spectrum of (DETC)(2)-Fe(II)-NO with a(N) = 12.8 gauss and g = 2.04 was observed in the cortex, hippocampus, hypothalamus, cerebellum, but not the olfactory bulb. The amount of NO in the brain was about 20% of that found in the liver. The (DETC)(2)-Fe(II)-NO signal in all the tissues of septic-shock rats was markedly suppressed by preadministration of the nitric oxide synthase (NOS) inhibitors, N(G)-monomethyl-L-arginine (L-NMMA) or 3-bromo-7-nitroindazole, suggesting that the NO detected from brain tissue was produced enzymatically by NOS. In contrast to previous studies on the liver and other organs, phenyl-N-tert-butyl nitrone (PBN), did not suppress iNOS expression in brain tissue of LPS-treated rats. This could be due to a totally different regulation system for iNOS in liver versus brain tissue. Magn Reson Med 42:599-602, 1999.

Characterization of a novel monoclonal antibody that senses nitric oxide-dependent activation of soluble guanylate cyclase

Two monoclonal antibodies (mAbs) against bovine lung soluble guanylate cyclase (sGC) were prepared and characterized. mAb 3221 recognized both the alpha- and beta-subunits of sGC and had greater binding affinity to the enzyme in the presence of NO. mAb 28131 recognized only the beta-subunit and its affinity did not change with NO. Neither mAb cross-reacted with particulate GC. Cultured Purkinje cells from rats were treated with S-nitroso-N-acetylpenicillamine, an NO donor, and examined by immunocytochemical methods. The immunoreactivity associated with mAb 3221 increased with the cGMP content in a crude extract of cerebellum and the NO2 generated in the culture medium increased.

Continuous monitoring of in vivo nitric oxide formation using EPR analysis in biliary flow

A method to continuously monitor the nitric oxide (NO) level in anesthetized rats, using an in vivo trapping reaction of NO by iron-dithiocarbamate complex, is reported. Previously, we developed a method of monitoring NO in bile samples containing an NO complex excreted from the liver (Anal. Biochem. 243, 8-14, 1996). In the present study, we modified the method so that the bile flows directly through the EPR sample cell. Rats were injected with low doses of lipopolysaccharide (LPS) to induce NO formation and were later anesthetized. After cannulation, the bile duct was connected to the inlet of the EPR sample cell and the trapping agent iron complex of D-N-methylglucamine dithiocarbamate (MGD-Fe) was administered. The EPR signal level from NO complex of MGD-Fe in the flowing bile was continuously monitored. Using this method, immediate changes in in vivo NO level in rats were observed following administration of drugs that can affect NO formation. In addition, a continuous intravenous saline containing MGD-Fe made the EPR signal level stable and improved animal condition as well as survival time. Therefore, this method has two merits; (1) one can continuously monitor NO formation until it reaches the maximum level; (2) a rapid change in NO level after intervention can be followed. Using this method, we tested the effect of the substrate L-arginine and inhibitors for NO synthase activity and NO synthase induction. The sensitivity of the present method was tested by monitoring NO formation in rats after exposure to ionizing radiation.