Nitric oxide as a mediator of gastrointestinal mucosal injury?-Say it ain't so

Studies of the Impact of the Bifidobacterium Species on Inducible Nitric Oxide Synthase Expression and Nitric Oxide Production in Murine Macrophages of the BMDM Cell Line

Bifidobacterium species are one of the most important probiotic microorganisms which are present in both, infants and adults. Nowadays, growing data describing their healthy properties arise, indicating they could act at the cellular and molecular level. However, still little is known about the specific mechanisms promoting their beneficial effects. Nitric oxide (NO), produced by inducible nitric oxide synthase (iNOS), is involved in the protective mechanisms in the gastrointestinal tract, where it can be provided by epithelial cells, macrophages, or bacteria. The present study explored whether induction of iNOS-dependent NO synthesis in macrophages stems from the cellular action of Bifidobacterium species. The ability of ten Bifidobacterium strains belonging to 3 different species (Bifidobacterium longum, Bifidobacterium adolescentis, and Bifidobacterium animalis) to activate MAP kinases, NF-κB factor, and iNOS expression in a murine bone-marrow-derived macrophages cell line was determined by Western blotting. Changes in NO production were determined by the Griess reaction. It was performed that the Bifidobacterium strains were able to induce NF-қB-dependent iNOS expression and NO production; however, the efficacy depends on the strain. The highest stimulatory activity was observed for Bifidobacterium animalis subsp. animals CCDM 366, whereas the lowest was noted for strains Bifidobacterium adolescentis CCDM 371 and Bifidobacterium longum subsp. longum CCDM 372. Both TLR2 and TLR4 receptors are involved in Bifidobacterium-induced macrophage activation and NO production. We showed that the impact of Bifidobacterium on the regulation of iNOS expression is determined by MAPK kinase activity. Using pharmaceutical inhibitors of ERK 1/2 and JNK, we confirmed that Bifidobacterium strains can activate these kinases to control iNOS mRNA expression. Concluding, the induction of iNOS and NO production may be involved in the protective mechanism of action observed for Bifidobacterium in the intestine, and the efficacy is strain-dependent.

Influence and Implications of the Molecular Paradigm of Nitric Oxide Underlying Inflammatory Reactions of the Gastrointestinal Tract of Dog: A Major Hallmark of Inflammatory Bowel Disease

Nitric oxide (NO), a pleiotropic free radical messenger molecule, is responsible for the various cellular function of the gastrointestinal mucosa. It plays a major role in the maintenance of perfusion, regulation of microvascular, epithelial permeability, and immune functions. Nitric oxide exerts its beneficial effect on the initiation and maintenance of inflammation in human inflammatory bowel disease (IBD). But the accelerated production of NO triggers activation of the inducible form of the NO synthase enzyme (iNOS) that leads to damages of the intestinal membrane. Nitric oxide synthase enzyme is responsible for the higher production of NO from l-arginine and causes an inflammatory condition in the intestinal epithelium. Nitric oxide induces nitrative DNA damage and oxidative DNA damage in the cellular system. Accelerated production of NO enhances iNOS activity that is associated with cytotoxicity and apoptosis of gastrointestinal epithelial cells in the dog. Chronic inflammation leads to angiogenesis that is modulated by the immune system in IBD. Chronic inflammation is a major risk factor for the development of gastrointestinal malignancies. Nitric oxide participates in mucosal inflammation in the intestine through invigoration of NO synthase enzyme. The intrinsic complex mechanism is correlated with the inflammation in the gastrointestinal tract and is also correlated with the expression of iNOS, enzymatic activity and NO production. Nitric oxide employs a significant role in modulating epithelial permeability with accelerated immune response in acute colitis. But the enormous generation of NO causes adverse effects on the mucosal cell during the inflammatory process in IBD. In this review, a complex episode of NO generation with altered biochemical pathways was assessed for the regulation of mucosal inflammation in inflammatory bowel disease of dogs. This review is a unique compilation of the role of NO in the pathogenesis of inflammatory bowel disease of dogs. Nitric oxide plays a key role in modulating cancer in the gastrointestinal tract. This review seeks to explore the characteristics of NO as a major hallmark of canine inflammatory bowel diseases.

Effect of Acrylamide Supplementation on the CART-, VAChT-, and nNOS-Immunoreactive Nervous Structures in the Porcine Stomach

Acrylamide is found in food products manufactured with high-temperature processing, and exposure to acrylamide contained in food products may cause a potential risk to human health. The aim of this investigation was to demonstrate the changes in the population of CART-, nNOS-, and VAChT-immunoreactive enteric neurons in the porcine stomach in response to supplementation of low and high acrylamide doses. The study was carried out with 15 Danish landrace gilts divided into three experimental groups: the control group-animals were administered empty gelatine capsules; the low-dose group-animals were administrated a tolerable daily intake (TDI) dose (0.5 µg/kg of body weight (b.w.)/day) of acrylamide capsules, and the high-dose group-animals were administrated high-dose (ten times higher than TDI: 5 µg/kg b.w./day) acrylamide capsules for 28 days. Using the double immunofluorescence staining method, it was established that supplementation with low and high doses of acrylamide resulted in alterations of the porcine stomach neuron phenotype, which was reflected in an increased number of CART-, VAChT-, and nNOS-immunoreactive neurons. These changes were accompanied by an increased density of CART-, VAChT-, and nNOS-positive fibres. The results suggest that the enteric nervous system plays an important role in protecting the gastrointestinal tract during acrylamide intoxication.

Drug-Mediated Intracellular Donation of Nitric Oxide Potently Inhibits 5-Lipoxygenase: A Possible Key to Future Antileukotriene Therapy

AIMS

5-Lipoxygenase (5-LO) is the key enzyme of leukotriene (LT) biosynthesis and is critically involved in a number of inflammatory diseases such as arthritis, gout, bronchial asthma, atherosclerosis, and cancer. Because 5-LO contains critical nucleophilic amino acids, which are sensitive to electrophilic modifications, we determined the consequences of a drug-mediated intracellular release of nitric oxide (NO) on 5-LO product formation by human granulocytes and on 5-LO-dependent pulmonary inflammation in vivo.

RESULTS

Clinically relevant concentrations of NO-releasing nonsteroidal anti-inflammatory drugs and other agents releasing NO intracellularly suppress 5-LO product synthesis in isolated human granulocytes via direct S-nitrosylation of 5-LO at the catalytically important cysteines 416 and 418. Furthermore, suppression of 5-LO product formation was observed in ionophore-stimulated human whole blood and in an animal model of pulmonary inflammation.

INNOVATION

Here, we report for the first time that drugs releasing NO intracellularly are efficient 5-LO inhibitors in vitro and in vivo at least equivalent to approved 5-LO inhibitors.

CONCLUSION

Our findings provide a novel mechanistic strategy for the development of a new class of drugs suppressing LT biosynthesis by site-directed nitrosylation. The results may also help to better understand the well-recognized anti-inflammatory clinically relevant actions of NO-releasing drugs. Furthermore, our study describes in detail a novel molecular mode of action of NO. Rebound Track: This work was rejected during standard peer review and rescued by Rebound Peer Review (Antioxid Redox Signal 16: 293-296, 2012) with the following serving as open reviewers: Angel Lanas, Hartmut Kühn, Joan Clària, Orina Belton. Antioxid. Redox Signal. 28, 1265-1285.

Aminoguanidine pretreatment prevents methotrexate-induced small intestinal injury in the rat by attenuating nitrosative stress and restoring the activities of vital mitochondrial enzymes

BACKGROUND

One of the major toxic side effects of methotrexate (MTX) is enterocolitis, for which there is no efficient standard treatment. Nitric oxide overproduction has been reported to play an important role in MTX-induced mucositis. This study was designed to investigate whether pretreatment with aminoguanidine (AG) - a selective iNOS inhibitor - prevents MTX-induced mucositis in rats.

METHODS

Rats were pretreated with AG (30 and 50 mg/kg body weight) i.p. daily 1 h before MTX (7 mg/kg body weight) administration for 3 consecutive days. After the final dose of MTX, the rats were killed, and the small intestines were used for analysis.

RESULTS

The small intestines of MTX-treated rats showed moderate to severe injury. Pretreatment with AG had a dose-dependent protective effect on MTX-induced mucositis. AG pretreatment reduced iNOS protein levels, mucosal nitric oxide levels, and protein tyrosine nitration. AG pretreatment also restored the activities of electron transport chain (ETC) complexes, vital tricarboxylic acid (TCA cycle) enzymes, and mitochondrial antioxidant enzymes.

CONCLUSIONS

These findings suggest that AG is beneficial in ameliorating MTX-induced enteritis in rats.

Neurochemical Plasticity of the Coeliac-Superior Mesenteric Ganglion Complex Neurons Projecting to the Prepyloric Area of the Porcine Stomach following Hyperacidity

This study was designed to determine neurochemical properties of the coeliac-superior mesenteric ganglion (CSMG) neurons supplying the prepyloric area of the porcine stomach in physiological state and following experimentally induced hyperacidity. To localize sympathetic neurons innervating the studied area of stomach, the neuronal retrograde tracer Fast Blue (FB) was applied to control animals and hydrochloric acid infusion (HCl) groups. After 23 days, animals of the HCl group were reintroduced into a state of general anesthesia and intragastrically given 5 mL/kg of body weight of 0.25 M aqueous solution of hydrochloric acid. On the 28th day, all animals were sacrificed. The CSMG complexes were then collected and processed for double-labeling immunofluorescence. In the control animals, FB-positive perikarya displayed immunoreactivity to tyrosine hydroxylase (TH), dopamine β-hydroxylase (DβH), neuropeptide Y (NPY), and galanin (GAL). Experimentally induced gastric hyperacidity changed the neurochemical phenotype of the studied neurons. An upregulated expression of GAL and NPY and the de novo synthesis of neuronal nitric oxide synthase (nNOS) and leu5-enkephalin (LENK) as well as downregulated expression of TH and DβH in the stomach-projecting neurons were observed. These findings enrich existing knowledge about the participation of these active substances in adaptive mechanism(s) of the sympathetic neurons during pathological processes within the gastrointestinal tract.

The Influence of Prolonged Acetylsalicylic Acid Supplementation-Induced Gastritis on the Neurochemistry of the Sympathetic Neurons Supplying Prepyloric Region of the Porcine Stomach

This experiment was designed to establish the localization and neurochemical phenotyping of sympathetic neurons supplying prepyloric area of the porcine stomach in a physiological state and during acetylsalicylic acid (ASA) induced gastritis. In order to localize the sympathetic perikarya the stomachs of both control and acetylsalicylic acid treated (ASA group) animals were injected with neuronal retrograde tracer Fast Blue (FB). Seven days post FB injection, animals were divided into a control and ASA supplementation group. The ASA group was given 100 mg/kg of b.w. ASA orally for 21 days. On the 28^th day all pigs were euthanized with gradual overdose of anesthetic. Then fourteen-micrometer-thick cryostat sections were processed for routine double-labeling immunofluorescence, using primary antisera directed towards tyrosine hydroxylase (TH), dopamine β-hydroxylase (DβH), neuropeptide Y (NPY), galanin (GAL), neuronal nitric oxide synthase (nNOS), leu 5-enkephalin (LENK), cocaine- and amphetamine- regulated transcript peptide (CART), calcitonin gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP). The data obtained in this study indicate that postganglionic sympathetic nerve fibers supplying prepyloric area of the porcine stomach originate from the coeliac-cranial mesenteric ganglion complex (CCMG). In control animals, the FB-labelled neurons expressed TH (94.85 ± 1.01%), DβH (97.10 ± 0.97%), NPY (46.88 ± 2.53%) and GAL (8.40 ± 0.53%). In ASA group, TH- and DβH- positive nerve cells were reduced (85.78 ± 2.65% and 88.82 ± 1.63% respectively). Moreover, ASA- induced gastritis resulted in increased expression of NPY (76.59 ± 3.02%) and GAL (26.45 ± 2.75%) as well as the novo-synthesis of nNOS (6.13 ± 1.11%) and LENK (4.77 ± 0.42%) in traced CCMG neurons. Additionally, a network of CART-, CGRP-, SP-, VIP-, LENK-, nNOS- immunoreactive (IR) nerve fibers encircling the FB-positive perikarya were observed in both intact and ASA-treated animals. The results of this study indicate involvement of these neuropeptides in the development or presumably counteraction of gastric inflammation.

Microbiome and potential targets for chemoprevention of esophageal adenocarcinoma

Esophageal cancer is one of the deadliest cancers, with a dismal prognosis. It is increasingly recognized that esophageal cancer is a heterogeneous disease. It can be subdivided into two distinct groups: squamous cell carcinoma and adenocarcinoma, based on histological appearance. In the Western world, the incidence of squamous cell carcinoma was considerably higher than esophageal adenocarcinoma (EA) until the 1990s when, due to a dramatic increase, the incidence of EA surpassed that of squamous cell carcinoma. EA typically follows a well-established stepwise evolution from chronic inflammation due to reflux esophagitis (RE) that progresses to metaplasia (Barrett's esophagus [BE]) to dysplasia, which often culminates in EA. The pathophysiology of EA is complex and involves diverse factors, including gastroesophageal reflux, gastric acid secretion, dysfunction of the antireflux barrier, gastric emptying disturbances, and abnormalities in esophageal defense mechanisms. The current understanding of the etiology of EA is mainly derived from epidemiological studies of risk factors such as cigarette smoking, obesity, gastroesophageal reflux disorders (GERD), and low fruit and vegetable consumption. Numerous studies have been done, but the factors that drive the dynamic increase in the incidence of EA remain elusive. The advent of widespread antibiotic use occurred in the 1950s, preceding the surge of EA. Based on this temporal sequence, it has been hypothesized that antibiotics alter the microbiome to which the esophagus is exposed in patients who have GERD and that chronic exposure to this abnormal microbiome (ie, changes in species diversity or abundance) accounts for the increase in EA. If changes in the proposed factors alter the stepwise progression (RE-BE-dysplasia-EA), they may represent potential targets for chemoprevention. New discoveries will help improve our understanding of the biology and pathogenesis of these cancers, and aid in finding novel therapeutic targets.

"Gastric cytoprotection" is still relevant

Although Andre Robert's historic article on "gastric cytoprotection" in 1979 introduced this new name and concept, gastroprotective drugs (e.g. sofalcone, sucralfate), which prevent and/or accelerate healing of gastric ulcers without inhibiting acid secretion, were known in Japan before or around that time. But since Robert's studies were solely focused on prostaglandins (PG), they became the center of gastrointestinal research for more than 30 years. As endogenous products, PG were implicated in mediating the gastroprotective effect of other drugs such as sofalcone and sucralfate, despite that the cyclooxygenase inhibitor indomethacin diminished but never abolished gastroprotection by other drugs. Another group of endogenous substances, that is, sulfhydryls (SH), investigated in parallel with PG, also seem to play a mechanistic role in gastroprotection, especially since SH alkylators like N-ethylmaleimide counteract virtually any form of gastroprotection. In Robert's terms of "prevention of chemically induced acute mucosal lesions," so far no single mechanism could explain the beneficial effects of diverse protective agents, but I argue that these two endogenous substances (i.e. PG, SH), in addition to histamine, are the main mechanistic mediators of acute gastroprotection: PG and histamine, because as mediators of acute inflammation, they increase vascular permeability (VP), and SH scavenge free radicals. This is contrary to the search for a single mechanism of action, long focused on enhanced secretion of mucus and/or bicarbonate that may contribute but cannot explain all forms of gastroprotection. Nevertheless, based on research work of the last 30 years, in part from our lab, a new mechanistic explanation of gastroprotection may be formulated: it's a complex but orderly and evolution-based physiologic response of the gastric mucosa under pathologic conditions. Namely, one of the first physiologic defense responses of any organ is inflammation that starts with rapid vascular changes (e.g. increased VP and blood flow), followed by cellular events (e.g. infiltration by acute and chronic inflammatory cells). Thus, PG and histamine, by increasing VP create a perivascular edema that dilutes and delays toxic agents reaching the subepithelial capillaries. Otherwise, damaging chemicals may induce severe early vascular injury resulting in blood flow stasis, hypoxia, and necrosis of surrounding epithelial and mesenchymal cells. In this complex response, increased mucus and/or bicarbonate secretion seem to cause luminal dilution of gastrotoxic chemicals that is further reinforced by a perivascular, histodilutional component. This mechanistic explanation would encompass the protective actions of diverse agents as PG, small doses of histamine, motility stimulants, and dilute irritants (i.e. "adaptive cytoprotection"). Thus, although markedly increased VP is pathologic, slight increase in VP seems to be protective, that is, a key element in the complex pathophysiologic response during acute gastroprotection. Over the years, "gastroprotection" was also applied to accelerated healing of chronic gastroduodenal ulcers without reduction of acid secretion. The likely main mechanism here is the binding of angiogenic growth factors (e.g. basic fibroblast growth factor, vascular endothelial growth factor) to the heparin-like structures of sucralfate and sofalcone. Thus, despite intensive research of the last 30 years, gastroprotection is incompletely understood, and we are still far away from effectively treating Helicobacter pylori-negative ulcers and preventing nonsteroidal anti-inflammatory drugs-caused erosions and ulcers in the upper and lower gastrointestinal tract; hence "gastric cytoprotection" research is still relevant.

Prolonged acetylsalicylic-acid-supplementation-induced gastritis affects the chemical coding of the stomach innervating vagal efferent neurons in the porcine dorsal motor vagal nucleus (DMX)

The main goal of our research was to study the possible alterations of the chemical coding of the dorsal motor vagal nucleus (DMX) neurons projecting to the porcine stomach prepyloric region following prolonged acetylsalicylic acid supplementation. Fast Blue (FB) was injected into the studied area of the stomach. Since the seventh day following the FB injection, acetylsalicylic acid (ASA) was given orally to the experimental gilts. All animals were euthanized on the 28th day after FB injection. Medulla oblongata sections were then processed for double-labeling immunofluorescence for choline acetyltransferase (ChAT), pituitary adenylate cyclase-activating peptide (PACAP), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), galanin (GAL), substance P (SP), leu enkephalin (LENK), and cocaine- and amphetamine-regulated transcript (CART). In the control DMX, only PACAP was observed in 30.08 ± 1.97 % of the FB-positive neurons, while VIP, NOS, GAL, SP, LENK, and CART were found exclusively in neuronal processes running between FB-labeled perikarya. In the ASA DMX, PACAP was revealed in 49.53 ± 5.73 % of traced vagal perikarya. Moreover, we found de novo expression of VIP in 40.32 ± 7.84 %, NOS in 25.02 ± 6.08 %, and GAL in 3.37 ± 0.85 % of the FB-labeled neurons. Our results suggest that neuronal PACAP, VIP, NOS, and GAL are mediators of neural response to aspirin-induced stomach inflammatory state.

Inhibition of NO-synthase and degranulation of rat omental mast cells in vitro

Mast cell amines, platelet-activating factor (PAF), thromboxanes and leukotrienes have been shown to be released during nitric oxide-synthase inhibition in the rat intestine. Mast cells in rat isolated omentum (OMCs) or isolated from the rat peritoneal cavity (PMCs) have been used here to investigate the relationship(s) between these agents. N-nitro-L-arginine methyl ester (L-NAME, 100 μM) caused some degranulation of OMCs, but no enhancement of histamine release from PMCs. PAF (5 μM) and U46619 (1 μM) degranulated OMCs and enhanced histamine release from PMCs. Pre-treatment of the omentum with BN52021 (10 μM) inhibited degranulation of OMCs in response to L-NAME, PAF or U46619. Pretreatment with 1-benzylimidazole (5 or 50 μM) inhibited the effect of L-NAME but not that of PAF. Indomethacin (1 μM) or sodium nitroprusside (10 μM) also inhibited the effects of L-NAME, but nordihydroguaiaretic acid (30 μM) did not. In PMCs BN52021 inhibited PAF-induced, but not U46619-induced, release of histamine. These results suggest that inhibition of nitric oxidesynthase in the omentum by L-NAME allows thromboxanes to release PAF, which in turn degranulates and releases histamine from OMCs.

Nitric oxide decreases intestinal haemorrhagic lesions in rat anaphylaxis independently of mast cell activation

The purpose of this study is to assess the role of nitric oxide (NO) in the intestinal lesions of passive anaphylaxis, since this experimental model resembles necrotizing enterocolitis. Sprague-Dawley rats were sensitized with IgE anti-dinitrophenol monoclonal antibody. Extravasation of protein-rich plasma and haemorrhagia were measured in the small intestine. Plasma histamine was measured to assess mast cell activation. The effect of exogenous NO on the lesions was assessed by using two structurally unrelated NO-donors: sodium nitroprusside and S-nitroso-Nacetyl-penicillamine (SNAP). An increased basal production of NO was observed in cells taken after anaphylaxis, associated with a reduced response to platelet-activating factor, interleukin 1beta, and IgE/DNP-bovine serum albumin complexes. The response to bacterial lipopolysaccharide and dibutyryl cyclic adenosine monophosphate (AMP) was enhanced 24 h after challenge, but at earlier times was not significantly different from that observed in controls. Treatment with either sodium nitroprusside or SNAP produced a significant reduction of the haemorrhagic lesions, which are a hallmark of rat anaphylaxis. The extravasation of protein-rich plasma was not influenced by NO-donors. The increase of plasma histamine elicited by the anaphylactic challenge was not influenced by SNAP treatment. NO-donors protect intestinal haemorrhagic lesions of rat anaphylaxis by a mechanism apparently independent of mast cell histamine release.

Oxidative stress, nitric oxide and prostaglandin E2 levels in the gastrointestinal tract of aging rats

Objectives

To evaluate the presence of oxidative stress and alterations in the levels of two cytoprotective agents, prostaglandin E2 and nitric oxide, in the gastrointestinal tract of aging rats.

Methods

The production of superoxide anion, lipid peroxides, levels of superoxide dismutase and catalase, and production of prostaglandin E2 and nitric oxide in the stomach and duodenum of rats were determined at 1.5, 3, 12, 18 and 24 months of age.

Key findings

Oxidative stress was present in the stomach of the old rats (24 months), whereas prostaglandin E2 and nitric oxide production remained stable at 18 and 24 months. In the duodenum, no oxidative stress was observed at 24 months, but at 18 months, an increase in superoxide anion levels was detected. Prostaglandin E2 remained constant in the aged rats but nitric oxide decreased significantly at 24 months.

Conclusions

The absence of macroscopic gastric injury throughout the gastrointestinal tract indicates that the oxidative stress in the stomach and the significant decrease of nitric oxide in the duodenum in the old rats are not sufficient to disrupt the mucosal defence network. The results support the notion that the disruption of the mucosal network is essentially regulated by the cytoprotective agents prostaglandin E2 and nitric oxide, and that injury appears only when both substances are concurrently reduced.

Nitric oxide releasing derivatives of tolfenamic acid with anti-inflammatory activity and safe gastrointestinal profile

Tolfenamic acid esters with nitrooxyalcohols are synthesized. They are anti-inflammatory agents reducing carrageenan rat paw edema, with low gastrointestinal and general toxicity. In vitro, they are nitric oxide donors, inhibitors of lipoxygenase and cyclooxygenases. A two to three carbon chain between carboxylic and nitric ester groups seems optimal for activity.

Synthesis and biological evaluation of 3,4-diphenyl-1,2,5-oxadiazole-2-oxides and 3,4-diphenyl-1,2,5-oxadiazoles as potential hybrid COX-2 inhibitor/nitric oxide donor agents

A group of 3,4-diphenyl-1,2,5-oxadiazole-2-oxides (3,4-diphenylfuroxans) and the corresponding N-desoxy 3,4-diphenyl-1,2,5-oxadiazoles (3,4-diphenylfurazans) analogs, were synthesized for in vitro evaluation as hybrid cyclooxygenase (COX) inhibitor/nitric oxide donor agents. Reaction of 1-[4-(methylsulfonyl)phenyl]-2-phenylethene with an aqueous sodium nitrite solution in acetic acid afforded a mixture (3:1 ratio) of the inseparable 4-[4-(methylsulfonyl)phenyl]-3-phenyl-1,2,5-oxadiazole-2-oxide (13a) and 3-[4-(methylsulfonyl)phenyl]-4-phenyl-1,2,5-oxadiazole-2-oxide (13b) regioisomers. A group of related regioisomers possessing either a p-aminosulfonylphenyl (16) or a p-azidosulfonylphenyl (17), moiety were obtained by chlorosulfonation of the unsubstituted 3,4-diphenylfuroxan (10) and subsequent reaction with either ammonium hydroxide or sodium azide, respectively. The methanesulfonyl regioisomers 13a,b [COX-1 IC50=11.6 microM; COX-2 IC50=0.12 microM; COX-2 selectivity index (SI)=97] and aminosulfonyl regioisomers 16 (COX-1 IC50=9.8 microM; COX-2 IC50=0.78 microM; COX-2 SI=12), like the reference drug celecoxib (COX-1 IC50=33.1 microM; COX-2 IC50=0.07 microM; COX-2 SI=472), were potent in vitro COX-2 inhibitors with a good COX-2 selectivity index. Release of nitric oxide (NO) from the 3,4-diphenylfuroxan compounds (10, 13a,b, 16, 17) was thiol-dependent since the % NO released was higher upon incubation in the presence of l-cysteine (0.57-3.18%) compared to that in phosphate buffer solution at pH7.4 (0.06-0.15%). Molecular modeling (docking) studies show that the methanesulfonyl (MeSO2) COX-2 pharmacophore present in regioisomers 13a,b is positioned in the vicinity of the COX-2 secondary pocket. The in vitro NO release data, COX-1/COX-2 inhibition and COX-2 SI structure-activity relationships acquired, and molecular modeling docking studies suggest that the 1,2,5-oxadiazole-2-oxide (furoxan) ring possesses beneficial features that should be present in a suitable central ring template (bioisostere) pertinent to the design novel hybrid COX-2 inhibitor/nitric oxide donor agents with a low ulcerogenicity profile that may be free from adverse cardiovascular effects.

Oral glutamine attenuates surgical manipulation-induced alterations in the intestinal brush border membrane

BACKGROUND

Our earlier work has shown that surgical manipulation of the intestine results in oxidative stress and mucosal damage along with alterations in the brush border membrane (BBM). Glutamine feeding is known to offer protection against damage to mucosa under various conditions and this study looked at the effect of oral supplementation of glutamine or glutamic acid in the intestinal BBM alterations after surgical manipulation.

MATERIALS AND METHODS

Control and rats pretreated for 7 days with 2% glutamine or glutamic acid or isonitrogenous amino acids, glycine, or alanine were subjected to surgical manipulation of the intestine. BBMs were isolated from the intestine and functional and structural alterations to these membranes were assessed and compared.

RESULTS

Surgical manipulation resulted in oxidative stress in the enterocyte BBM and these changes included a decrease in alkaline phosphatase activity and alpha-tocopherol content along with an increase in lipid peroxidation parameters. A decrease in glucose transport by the isolated BBM vesicles suggested functional impairment. Surgical manipulation also resulted in phospholipid degradation possibly mediated by PLA(2) and membrane protease activation. Glutamine or glutamic acid supplementation prevented these changes but not by glycine or alanine.

CONCLUSION

This study suggests that oral glutamine or glutamic acid supplementation prior to surgery can offer protection to the intestine and this might prevent postsurgical complications.

Effect of dietary nitric oxide on gastric mucosal mast cells in absence or presence of an experimental gastritis in rats

Synthetic nitric oxide donors are known to protect the gastric mucosa from damage and dietary nitrate is known to release NO in the stomach. Mast cells have been found to be involved in gastric mucosal damage in humans or in rodents, and recent studies have pointed out the possibility of nitric oxide from endogenous or exogenous origin to modulate mast cell reactivity. This study aimed to determine whether the protective effect afforded by dietary nitrate against gastric mucosal damage was linked to mast cell stabilization. Mast cell involvement in iodoacetamide-induced gastritis was investigated in rats receiving oral administration of iodoacetamide together with the mast cell stabilizer doxantrazole (ip) or its solvent. The effects of dietary nitrate on mast cells during gastritis were investigated in rats receiving iodoacetamide orally, associated or not with KNO3. Control groups were given water instead of iodoacetamide either with or without KNO3, doxantrazole or its solvent. After sacrifice, blood samples were taken to determine RMCP II serum level and the stomach was resected in order to determine myeloperoxidase (MPO) activity and mucosal mast cell (MMC) number. Iodoacetamide significantly increased gastric MPO activity but did not modify RMCP II serum level or MMC number. Doxantrazole and KNO3 significantly reduced iodoacetamide-induced increase in gastric MPO activity, increased MMC number, and decreased RMCP II serum level in basal conditions. Only doxantrazole was able to modify all parameters under inflammatory conditions. These results suggest that nitric oxide released by dietary nitrate in the stomach stabilizes mast cells in basal conditions but exerts its protective effect against experimental gastritis through other pathways.

Protective effect of dietary nitrate on experimental gastritis in rats

Nitrates have long been considered as harmful dietary components and judged responsible for deleterious effects on human health, leading to stringent regulations concerning their levels in food and water. However, recent studies demonstrate that dietary nitrate may have a major role in human health as a non-immune mechanism for host defence, through its metabolism to NO in the stomach. NO is a versatile molecule and although evidence exists showing that administration of low doses of exogenous NO protects against gastrointestinal inflammation, higher NO doses have been shown to exacerbate injury. So, the effect of an ingestion of nitrates in doses corresponding to a normal diet in human consumers on an experimental gastritis induced by iodoacetamide in rats was investigated. During gastritis one of the following compounds was given orally: water; KNO3; the NO donor sodium nitroprusside; the NO scavenger haemoglobin given with either water or KNO3. N(G)-nitro-l-arginine methyl ester (l-NAME), a non-specific NO synthase inhibitor, was administered with either water, iodoacetamide alone, or combined with KNO3. After killing, the stomach was resected and microscopic damage scores, myeloperoxidase and NO synthase activities were determined. Iodoacetamide-induced gastritis was significantly reduced by KNO3 administration, an effect which was reproduced by sodium nitroprusside and reversed by haemoglobin. l-NAME induced gastric mucosal damage in itself, and KNO3 did not prevent the gastritis induced by iodoacetamide associated with l-NAME. In conclusion, dietary nitrate exerts a protective effect against an experimental gastritis in rats by releasing NO in the stomach but such an effect requires the production of endogenous NO.

The therapeutic potential of NO-NSAIDs

NSAIDs, including those that are selective for cyclooxygenase-2, are among the most widely used drugs. However, these drugs produce significant side effects in the gastrointestinal and cardiorenal systems, which greatly limit their utility. In recent years, a new type of anti-inflammatory agent has been developed that appears to offer significant advantages over conventional and Cox-2-selective NSAIDs. No-NSAIDs are derivatives of conventional NSAIDs, which are able to release nitric oxide over prolonged periods of time. The combination of balanced inhibition of the two main isoforms of COX with controlled release of nitric oxide yields a series of drugs that exert anti-inflammatory and analgesic activities in a wide range of settings, and have markedly reduced gastrointestinal and cardiorenal toxicity. Recent clinical trials of NO-NSAIDs have provided a 'proof of concept' that is completely consistent with pre-clinical characterization of these compounds.

Heat preconditioning attenuates oxygen free radical-mediated alterations in the intestinal brush border membrane induced by surgical manipulation

BACKGROUND

The intestine is highly susceptible to free radical-induced damage and our earlier work has shown that surgical manipulation of the intestine results in generation of oxygen free radicals and mucosal damage along with alterations in the brush border membrane (BBM). Heat preconditioning is known to offer protection against various stresses including oxidative stress and this study looked at the effect of heat preconditioning on the intestinal BBM alterations following surgical manipulation.

METHODS

Control and rats heat preconditioned were subjected to surgical manipulation by opening the abdominal wall and handling the intestine. BBM were isolated from the intestine and structural and functional alterations to these membranes were assessed.

RESULTS

Surgical manipulation resulted in oxidative stress suggested by a decrease in alkaline phosphatase activity and alpha-tocopherol content, accompanied by an increase in lipid peroxidation. A decrease in glucose transport by the isolated BBM vesicles suggested functional impairment. Surgical manipulation resulted in phospholipid degradation with generation of arachidonic acid along with appearance of cPLA(2) in the membrane. These changes were prevented by heat preconditioning of the animal prior to surgical manipulation.

CONCLUSION

These results suggest that heat preconditioning offers protection from damage to the intestinal BBM following surgical manipulation and mild whole body hyperthermia might prevent postsurgical complications.

Nutritional aspects of nitric oxide: human health implications and therapeutic opportunities

Nitric oxide (NO), the most potent natural vasorelaxant known, has close historical ties to cardiovascular physiology, despite NO's rich physiologic chemistry as an ubiquitous, signal-transducing radical. Aspects of NO biology critical to gastrointestinal health and, consequently, nutritional status are increasingly being recognized. Attempts are underway to exploit the gastrointestinal actions of NO for therapeutic gain. Cross-talk between NO and micronutrients within and outside the gastrointestinal system affects the establishment or progression of several diseases with pressing medical needs. These concepts imply that NO biology can influence nutrition and be nutritionally modulated to affect mammalian (patho)physiology. At least four nutritional facets of NO biology are at the forefront of contemporary biomedical research: 1) NO as modulator of feeding behavior and mediator of gastrointestinal homeostasis; 2) NO supplementation as a therapeutic modality for preserving gastrointestinal health; 3) interactions among elemental micronutrients (e.g., zinc), NO, and inflammation as potential contributors to diarrheal disease; and 4) vitamin micronutrients (e.g., vitamins E and C) as protectors of NO-dependent vascular function. Discussion of extant data on these topics prompts speculation that future research will broaden NO's nutritional role as an integrative signaling molecule supporting gastrointestinal and nutritional well-being.

Nitric oxide protects the intestine from the damage induced by laparotomy and gut manipulation

BACKGROUND

The intestine is highly susceptible to free radical-induced damage, and our earlier work has shown that surgical stress induces the generation of oxygen free radicals in enterocytes, resulting in intestinal damage along with ultrastructural changes. Since nitric oxide (NO) is an important mediator of gastrointestinal function, this study looked at the effect of NO on surgical stress-induced intestinal alterations.

MATERIALS AND METHODS

Control rats and rats pretreated with the NO donor l-arginine were subjected to surgical stress by opening the abdominal wall and handling the intestine as done during laparotomy. Enterocytes were isolated and homogenate prepared, and the protection offered by l-arginine against damage due to surgical stress was determined and compared with normal controls. Protection to structural as well as functional aspects of the intestine was also examined.

RESULTS

Intestinal manipulation affected intestinal structure as assessed by electron microscopy. Functional impairment of the enterocyte was also evident, with increased xanthine oxidase activity resulting in production of superoxide anion. This impairment is more dramatic in the crypt cells. Increased protease activity was also seen following laparotomy and handling. Pretreatment with the NO synthase substrate l-arginine prevented these damaging effects. Arginine protection was abolished in the presence of the NO synthase inhibitor NG-nitro-l-arginine methyl ester, indicating the role of NO.

CONCLUSION

Stress in the small intestine due to any surgery can affect enterocyte structure and function. These damaging effects can be prevented by NO, an important modulator of cellular function.

Antiproliferative effects of nitrosulindac on human colon adenocarcinoma cell lines

Nonsteroidal anti-inflammatory drugs (NSAIDs) reduce the incidence of colon cancer, but their use is limited by toxicity in the gastrointestinal tract. The coupling of a nitric oxide-releasing moiety to NSAIDs strongly reduces these side effects. We demonstrated that the NO-releasing sulindac (nitrosulindac) has much more potent effects on colon adenocarcinoma cell lines compared to sulindac. Moreover, it could inhibit the growth of cells in soft agar experiments, demonstrating the antineoplastic activity at low concentration of nitrosulindac. However, this reduction in the growth of colon cancer cells seemed to be independent of the classical apoptosis pathway and could be explained by a cytostatic effect. Nitrosulindac caused a light perturbation of the cell cycle parameters not linked to a modification of the levels of p21 or the proliferating cell nuclear antigen. Moreover, neither sulindac, nor nitrosulindac, were able to inhibit the NF-kappa B pathway. These data suggested that nitrosulindac could be a better solution compared to other NSAIDs in the treatment of colon cancer.

Redox control of mitochondrial functions

Redox reactions and electron flow through the respiratory chain are the hallmarks of mitochondria. By supporting oxidative phosphorylation and metabolite transport, mitochondrial redox reactions are of central importance for cellular energy conversion. In the present review, we will discuss two other aspects of the mitochondrial redox state: (i) its control of mitochondrial Ca2+ homeostasis, and (ii) the intramitochondrial formation of reactive oxygen or nitrogen species that strongly influence electron flow of the respiratory chain.

Increased nitrite and nitrate concentrations in sera and urine of patients with cholera or shigellosis

OBJECTIVES

Nitric oxide (NO) is an important regulator of cell function. In the intestine, NO regulates blood flow, peristalsis, secretion, and is associated with inflammation and tissue injury. The objectives of this study were to assess and compare the role of NO in cholera, a noninflammatory enteric infection, and in shigellosis, a bacterial inflammation of the colon.

METHODS

We determined serum and urinary concentrations of nitrite and nitrate during acute illness and early convalescence in 45 hospitalized children: 24 with cholera and 21 with shigellosis; 18 healthy children served as controls. Nitrite and nitrate concentrations were determined spectrophotometrically using Greiss reaction-dependent enzyme assay.

RESULTS

Serum nitrite and nitrate concentrations were significantly (p < 0.05) increased during acute illness compared to the early convalescence in both cholera and shigellosis. Urinary nitrite and nitrate excretions were significantly (p < 0.01) increased during acute disease in shigellosis, but not in cholera. Nitrite concentrations correlated with stool volume (r2 = 0.851) in cholera and with leukocytosis (r2 = 0.923) in shigellosis.

CONCLUSIONS

Both cholera and shigellosis are associated with increased production of NO, suggesting its pathophysiologic roles in these diseases.

Surgical stress induces phospholipid degradation in the intestinal brush border membrane

BACKGROUND

Surgical stress can lead to translocation of bacteria from the intestine into the systemic circulation. The intestinal brush border membrane (BBM) plays an important role in defense against such invasion by luminal bacteria and endotoxin. Our earlier work has shown the development of oxidative stress in the intestine after surgical stress and since the BBM is sensitive to free radical attack, this study examined the effect of surgical stress on the structure and function of intestinal BBM.

METHODS

Intestinal BBM were isolated from control and after surgical stress and compared for structural and functional alterations. Surgical stress was also carried out following pretreatment with the xanthine oxidase inhibitor allopurinol or the nitric oxide donor l-arginine, to study the protection offered by these compounds.

RESULTS

Surgical stress affected intestinal BBM structure as well as function. A decrease in alkaline phosphatase activity and alpha-tocopherol content, accompanied by an increase in lipid peroxidation, was seen. Surgical stress induced phospholipid degradation with generation of arachidonic acid. Functional impairment with a decrease in glucose transport ability was also seen. These changes are prevented by inhibition of xanthine oxidase by allopurinol pretreatment but not by NO.

CONCLUSION

Surgical stress in the small intestine causes structural and functional alterations in the BBM through oxidative stress. This damage could affect gut barrier integrity and generation of arachidonic acid might mediate distal organ dysfunction.

Effects of known and potential antioxidants on animal models of pathological processes (diabetes, gastric lesions, allergic bronchospasm)

Highly reactive oxygen-free radicals are implicated in the pathogenic process of various diseases. Using an animal model of diabetes (alloxan-induced hyperglycaemia in mice), a model of gastric ulcer (indomethacin-induced gastric lesion in rats), and a model of bronchial asthma (ovalbumin-induced allergic bronchospasm in guinea pigs), a potential therapeutic effect was tested in known antioxidant drugs (alpha-tocopherol, ubiquinone), the thio-compound mesna, and drugs with a possible antioxidant effect (substances derived from the ergoline structure: 6-hydroxynicotinic acid and 4-hydroxypyridine). The pre-treatment with ubiquinone and 6-hydroxynicotinic acid almost completely prevented alloxan-induced hyperglycaemia (94 and 93% inhibition of hyperglycaemia, respectively). A weaker effect was shown by alpha-tocopherol and 4-hydroxypyridine (31 and 27% inhibition of hyperglycaemia, respectively). Mesna negligibly increased hyperglycaemia. 32% and 21% inhibitions of the number of gastric lesions were shown after administration of 6-hydroxynicotinic acid and alpha-tocopherol, respectively. Other drugs, most markedly mesna, aggravated gastric lesions. The most marked protective effect on ovalbumin-induced bronchospasm was exerted by 6-hydroxynicotinic acid (the pulmonary ventilation was increased by 84% in comparison with control group), while mesna and (alpha-tocopherol had a weaker effect (amelioration by 50 and 51 %, respectively). Ubiquinone and 4-hydroxypyridine aggravated pulmonary ventilation. The most marked protective effect in the animal models used was shown by 6-hydroxynicotinic acid.

Histaminergic effect on apoptosis of rat small intestinal mucosa after ischemia-reperfusion

This study aimed to examine the relationship between a harmful effect of histamine and apoptosis following ischemia-reperfusion in the rat intestine. The superior mesenteric artery was occluded for 60 min followed by reperfusion for 60 min. Rats were infused with H1-receptor antagonist (chlorpheniramine maleate) or H2-receptor antagonist (cimetidine). Additional rats were pretreated with aminoguanidine (100 mg/kg). Percent apoptosis in the intestinal mucosa increased after reperfusion, but neither H1 nor H2 antagonists had any effect on apoptosis. Aminoguanidine pretreatment inhibited activity of diamine oxidase and increased the plasma histamine concentration. Aminoguanidine attenuated the increase in mucosal apoptosis following reperfusion. Apoptosis induced by an ischemic insult to the intestinal mucosa was not related to an undesirable effect of histamine. Attenuation of increased intestinal apoptosis might be due to increased plasma histamine level and/or other pharmacological action of aminoguanidine, including inhibition of inducible nitric oxide synthase.

COX-2 selective nonsteroidal anti-Inflammatory drugs: do they really offer any advantages?

Nonsteroidal anti-inflammatory drugs (NSAIDs) are responsible for substantial morbidity and mortality as a result of the complications associated with gastroduodenal ulcers, such as perforation and bleeding. The central mechanism leading to the gastroduodenal toxicity of NSAIDs is their ability to inhibit mucosal prostaglandin synthesis. Recent recognition that there are 2 isoforms of the enzyme cyclooxygenase (COX) responsible for prostaglandin synthesis has enabled the development of drugs capable of sparing the gastric mucosa. The inducible COX-2 enzyme is responsible for some aspects of pain and inflammation in arthritis while the constitutive COX-1 enzyme appears responsible for most of the gastro-protective prostaglandin synthesis in the stomach and duodenum. Drugs selective for COX-2 probably act by binding to a pocket in the enzyme that is present in COX-2 but not in COX-1. As a result, drugs that have little or no COX-1 activity across their therapeutic dosage range have been developed. Two drugs that are claimed to be highly selective or specific in their ability to inhibit COX-2, rofecoxib and celecoxib, are now available on prescription in the US and rofecoxib is available in Europe. Short term volunteer studies of 7 days' duration and patient studies of 6 months' duration have shown these drugs to have a level of gastroduodenal injury that is similar or equivalent to that seen with placebo, whereas high rates of damage and ulceration are seen with nonselective NSAIDs. In addition, there appear to have been fewer perforations, clinical ulcers and bleeds in the phase III clinical trials of these agents, compared with nonselective NSAIDS. However, more experience will be needed before this promise can be confirmed. In addition, COX-2 inhibitors share the adverse effects of NSAIDs outside the gastrointestinal tract that are dependent on COX-2 inhibition.

Nitric Oxide. V. therapeutic potential of nitric oxide donors and inhibitors

Nitric oxide is a crucial mediator of gastrointestinal mucosal defense, but, paradoxically, it also contributes to mucosal injury in several situations. Inhibitors of nitric oxide synthesis and compounds that release nitric oxide have been useful pharmacological tools for evaluating the role of nitric oxide in gastrointestinal physiology and pathophysiology. Newer inhibitors with selectivity for one of the isoforms of nitric oxide synthase are even more powerful tools and may have utility as therapeutic agents. Also, agents that can scavenge nitric oxide or peroxynitrite are promising as drugs to prevent nitric oxide-associated tissue injury. Compounds that release nitric oxide in small amounts over a prolonged period of time may also be very useful for prevention of gastrointestinal injury associated with shock and with the use of drugs that have ulcerogenic effects. Indeed, the coupling of a nitric oxide-releasing moiety to nonsteroidal anti-inflammatory drugs has proven to be a valid means of substantially reducing the gastrointestinal toxicity of these drugs without decreasing their efficacy.

Copper-indomethacinate associated with zwitterionic phospholipids prevents enteropathy in rats: effect on inducible NO synthase

Intestinal toxicity exerted by indomethacin was compared to that induced by copper-indomethacinate, free or associated to zwitterionic phospholipids. A single high dose of indomethacin (15 or 20 mg/kg), copper-indomethacinate (15 or 20 mg/kg), or copper-indomethacinate liposomes or nanocapsules (15 mg/kg) was orally administered. Then 24 hr later jejunoileal tissue was taken for macroscopic observation, ex vivo nitrite production, and determination of myeloperoxydase and iNOS activities. Antiinflammatory activity of the drugs was investigated using the carrageenan-induced paw edema model. Indomethacin induced penetrating ulcerations of the intestine that were maximal at hour 24. Copper-indomethacinate induced significantly less ulceration than indomethacin with no significant difference in MPO and iNOS activities. The injurious action of indomethacin on the small intestine was further reduced when copper-indomethacinate was administered as the phospholipid-associated state while similar anti-inflammatory action was observed on rat paw edema. The antiulcerogen effect of copper-indomethacinate seems to be linked to its free radical scavenging effect without any modification of nitric oxide release.

Nitric Oxide. III. A molecular prelude to intestinal inflammation

Nitric oxide (NO) synthesis is markedly augmented in states of inflammation, largely due to the expression of inducible nitric oxide synthase (iNOS). Although NO has anti-inflammatory consequences under basal conditions, it remains enigmatic as to why NO displays proinflammatory characteristics in chronic inflammation. Either the anti-inflammatory actions are weak and of little consequence or, alternatively, other factors influence the role of NO in chronic inflammation. We propose that the answer to this enigma lies in the conversion of NO to other higher oxides of nitrogen (NO2, nitrogen dioxide; N2O3, dinitrogen trioxide; and ONOO-, peroxynitrite). Emerging therapeutic strategies may be independent of NO synthesis; e.g., antioxidants have no direct interaction with NO but attenuate the levels and activity of higher nitrogen oxides. Thus, whereas iNOS may be a marker for the proinflammatory actions of NO, the species that mediate tissue injury/dysfunction in inflammation are likely to be nitrogen oxides other than NO.

Nitrotyrosine in plasma of celiac disease patients as detected by a new sandwich ELISA

Inflammation is characterized by increased nitric oxide production. Nitrotyrosine has recently been suggested to be useful as a marker for NO-mediated tissue damage. In context of the development of an ELISA for detection of nitrotyrosine in plasma, monoclonal anti-nitrotyrosine antibodies were developed by injecting mice with nitrated keyhole limpet hemocyanin. The specificity of the antibodies was determined by binding to nitrated BSA, lack of binding to unmodified BSA, tyrosine, 3-chlorotyrosine or phenylalanine and inhibition of binding by nitrotyrosine. The antibodies developed are useful for Western blot analysis and immunohistochemical staining. Using these antibodies a nitrotyrosine sandwich ELISA was developed with a lower detection limit of approximately 0.2 nM. The intra- and interassay variance were 2.4% and 11.9%, respectively. Using this newly developed ELISA, 1.27 +/- 1.03 microM nitrotyrosine was detected in plasma samples of celiac disease patients whereas nitrotyrosine was undetectable in control samples. Elevated nitrotyrosine levels were paralleled by an increase in plasma concentrations of NO-oxidation products (NOx), nitrite and nitrate from 15.1 +/- 6.1 microM in controls to 61.0 +/- 28.2 microM in celiac disease patients. Both nitrotyrosine and NOx levels declined when the patients were on a gluten-free diet, suggesting a relation between intestinal inflammation and plasma nitrotyrosine and NOx levels.

Nitrite and nitrate analyses: a clinical biochemistry perspective

OBJECTIVE

To review the assays available for measurement of nitrite and nitrate ions in body fluids and their clinical applications.

DESIGN AND METHODS

Literature searches were done of Medline and Current Contents to November 1997.

RESULTS

The influence of dietary nitrite and nitrate on the concentrations of these ions in various body fluids is reviewed. An overview is presented of the metabolism of nitric oxide (which is converted to nitrite and nitrate). Methods for measurement of the ions are reviewed. Reference values are summarized and the changes reported in various clinical conditions. These include: infection, gastroenterological conditions, hypertension, renal and cardiac disease, inflammatory diseases, transplant rejection, diseases of the central nervous system, and others. Possible effects of environmental nitrite and nitrate on disease incidence are reviewed.

CONCLUSIONS

Most studies of changes in human disease have been descriptive. Diagnostic utility is limited because the concentrations in a significant proportion of affected individuals overlap with those in controls. Changes in concentration may also be caused by diet, outside the clinical investigational setting. The role of nitrite and nitrate assays (alongside direct measurements of nitric oxide in breath) may be restricted to the monitoring of disease progression, or response to therapy in individual patients or subgroups. Associations between disease incidence and drinking water nitrate content are controversial (except for methemoglobinemia in infants).

Failure of L-NAME to cause inhibition of nitric oxide synthesis: role of inducible nitric oxide synthase

We addressed the hypothesis that administration of nitric oxide synthase inhibitor, NG -nitro-L-arginine methyl ester (L-NAME) does not result in a sustained suppression of nitric oxide (NO) synthesis, because of a compensatory expression of inducible nitric oxide synthase (iNOS). L-NAME was administered in the drinking water (0.1-1.0 mg/ml) for 7 days to guinea pigs and rats. Nitric oxide synthesis was assessed by [1] ex vivo formation of nitrite in blood vessels and intestine [2] tissue levels of cGMP [3] iNOS gene expression by RT-PCR [4] NADPH diaphorase staining [5] direct assessment of NO release in tissue explants using a microelectrode/electrochemical detection system. Chronic L-NAME administration elevated intestinal cGMP and nitrite levels in guinea pigs (p < 0.05). In rats, intestinal nitrite levels were comparable in control and L-NAME treatment groups, whereas direct assessment of NO release defined a marked increase in the L-NAME group. Chronic L-NAME resulted in an induction of iNOS gene expression in rats and guinea pigs and novel sites of NADPH diaphorase staining in the intestine. We conclude that iNOS expression is responsible for a compensatory increase or normalization of NO synthesis during sustained administration of L-NAME.