The role of nitric oxide and platelet-activating factor in the inhibition by endotoxin of pentagastrin-stimulated gastric acid secretion

The Role of Gasotransmitters in Gut Peptide Actions

Although gasotransmitters nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H₂S) receive a bad connotation; in low concentrations these play a major governing role in local and systemic blood flow, stomach acid release, smooth muscles relaxations, anti-inflammatory behavior, protective effect and more. Many of these physiological processes are upstream regulated by gut peptides, for instance gastrin, cholecystokinin, secretin, motilin, ghrelin, glucagon-like peptide 1 and 2. The relationship between gasotransmitters and gut hormones is poorly understood. In this review, we discuss the role of NO, CO and H₂S on gut peptide release and functioning, and whether manipulation by gasotransmitter substrates or specific blockers leads to physiological alterations.

The increased gastroprotective effect of pioglitazone in cholestatic rats: role of nitric oxide and tumour necrosis factor alpha

The prevalence of gastric ulcers is high in cholestatic patients, but the exact mechanism of this increased frequency remains uncertain. It has been shown that pioglitazone accelerates the healing of pre-existing gastric ulcers. The present study was designed to investigate the effect of pioglitazone, on the gastric mucosal lesions in cholestatic rats. Cholestasis was induced by surgical ligation of common bile duct and sham-operated rats served as control. Different groups of sham and cholestatic animals received solvent or pioglitazone (5, 15, 30 mg/kg) for 7 days. On the day eight rats were killed after oral ethanol administration and the area of gastric lesions was measured. The serums of rats were also collected to determine serum levels of tumour necrosis factor alpha (TNF-α), IL-1β and bilirubin. The ethanol-induced gastric mucosal damage was significantly more severe in cholestatic rats than sham-operated ones. Pretreatment with pioglitazone dose-dependently attenuated gastric lesions induced by ethanol in both sham and cholestatic rats, but this effect was more prominent in cholestatic ones. The effect of pioglitazone was associated with a significant fall in serum levels of TNF-α in cholestatic rats. L-NAME, a non-selective nitric oxide synthase (NOS) inhibitor, and decreased pioglitazone-induced gastroprotective effect in cholestatic rats, while aminoguanidine, a selective inducible NOS inhibitor, potentiated pioglitazone-induced gastroprotective effect in the cholestatic rats. Chronic treatment with pioglitazone exerts an enhanced gastroprotective effect on the stomach ulcers of cholestatic rats compared to sham rats probably due to constitutive NOS induction and/or inducible NOS inhibition and attenuating release of TNF-α.

Role of nitric oxide in the gastro-protective effect of lithium

BACKGROUND AND AIM

Lithium is widely used for the management of neuropsychiatric symptoms in bipolar disorders. A few studies have shown that lithium has a protective effect against gastric damage with an unknown mechanism. Some of the actions of lithium are mediated through nitric oxide (NO), which has an important role in the regulation of gastric wall blood flow as well as gastric mucosal integrity. The aim of this study was to test the hypothesis if the gastro-protective effect of lithium is mediated through NO.

METHODS

Male Wistar rats were pre-treated with either a non-selective NO synthase inhibitor (N(G)-nitro-l-arginine, 10mg/kg), a selective inducible NO synthase inhibitor (aminoguanidine, 100mg/kg) or saline. Lithium carbonate (10, 20, 50 and 100mg/kg) was then administered intraperitoneally 1h before the induction of gastric mucosal damage. Gastric damage was induced by either water immersion stress or ethanol gavage in rats.

RESULTS

Lithium had a significant protective effect in both stress and ethanol-induced gastric damage, but it needed in ethanol-induced gastric damage a higher dose than in the stress induced lesion. Lithium carbonate doses 20 and 50mg/kg produced plasma concentrations that were in the range of human therapeutic Li levels (0.6-1.0muM). Pre-treatment of animals with N(G)-nitro-l-arginine (20 and 40mg/kg) reduced the protective effect of lithium against ethanol-induced gastric damage, but not in stress-induced damage. Aminoguanidine administration showed no effect on the damage reduction either in control or lithium treated rats.

CONCLUSIONS

The results indicate that NO might play a role in the gastro-protective effect of lithium against ethanol-induced gastric damage in rats.

Direct effects of nitric oxide on histamine release from rat enterochromaffin-like cells

The direct effects of nitric oxide (NO) on enterochromaffin-like (ECL) cells have not yet been demonstrated. In this study we investigated the direct effects of NO donors on rat ECL cells. The NO donor, NOR3 (10 and 100 microM), decreased gastrin-induced histamine release. 100 microM NOR3 increased cGMP levels and reduced gastrin-induced calcium influx. ODQ, an inhibitor of guanylate cyclase, completely blocked NOR3-induced inhibition of histamine release. These results suggest that NO inhibits gastric acid secretion via suppression of gastrin-induced histamine release through a pathway in which NO activates guanylate cyclase, in addition to increasing cGMP levels and reducing gastrin-induced calcium influx. The use of NO as a new type of gastric acid inhibitor that decreases histamine levels in the stomach would be beneficial as increased histamine levels resulting from use of a histamine H2 receptor antagonist or proton pump inhibitor have various effects on tumors and immunological functions.

Effects of lipopolysaccharide and phenylbutazone on gastric contents in the horse

REASONS FOR PERFORMING STUDY

Endotoxaemia causes a disruption of gastrointestinal motility in the horse but there is no information on its effects on gastric secretion. Lipopolysaccharide (LPS) administration is known to affect gastric secretion in other species.

HYPOTHESIS

That LPS, a toxic component of Gram-negative bacteria, would reduce gastric acid secretion and that pretreatment with phenylbutazone (PBZ) would block the effects of LPS.

METHODS

The effects of LPS and PBZ on gastric contents were investigated in fasted, mature horses, with permanent gastric cannulae. Horses were pretreated with either saline or PBZ 15 mins before a 60 min infusion of either LPS or saline. Gastric contents were collected at 15 min intervals for 3 h, beginning 15 mins after the start of the LPS or saline infusion.

RESULTS

Lipopolysaccharide significantly decreased gastric acid output, [K+] and potassium output and increased [Na+] and sodium output. Phenylbutazone did not affect basal gastric acid secretion but decreased LPS-induced changes in the secreted volume, [Na+] and sodium output.

CONCLUSIONS

This study provides evidence that LPS affects gastric acid secretion in the horse and that these LPS-induced changes are mediated, in part, by prostaglandins.

POTENTIAL RELEVANCE

Lipopolysaccharide administration can induce changes in the composition of gastric contents in the horse but further work is needed to determine the source of these changes.

The role of nitric oxide in the gastric acid secretion induced by ischemia-reperfusion in the pylorus-ligated rat

In a rat model of the ischemia-reperfusion with pylorus ligation, gastric ulcer was formed, although gastric acid secretion was reduced. When the polymorphonuclear leukocytes were inactivated in advance, gastric ulcer was not formed, but acid secretion was increased, indicating that gastric acid is not a cause of the ulcer formation in this model. The mechanism of gastric acid suppression accompanied by ischemia-reperfusion was examined in relation to the role of oxygen-free radicals in this rat model. Prior administration of superoxide dismutase did not modulate acid secretion, but N-nitro-L-arginine methyl ester (L-NAME) increased acid secretion. The action of L-NAME was antagonized specifically by L-arginine, but not by D-arginine. S-nitroso-N-acetylpenicillamine did not inhibit basal acid secretion but antagonized the action of L-NAME. Aminoguanidine increased significantly the gastric acid output that was suppressed by ischemia-reperfusion. When polymorphonuclear leukocytes were inactivated by treatment with their antibody, the gastric acid output recovered to the level in the pylorus-ligated rat without ischemia-reperfusion. These results suggested that nitric oxide (NO) produced by the infiltrated polymorphonuclear leukocytes plays an important role in the suppression of acid secretion induced by ischemia-reperfusion.

Protective role of nitric oxide in indomethacin-induced gastric ulceration by a mechanism independent of gastric acid secretion

Gastric ulceration was induced in rats by i.p. injection of the non-steroidal anti-inflammatory drug (NSAID), indomethacin (IND) (30 mg kg(-1)). Pyloric ligation was carried out in each animal before injection to enable collection of the gastric juice. Three hours later, the animals were killed and their stomachs were removed. In the gastric juice, the amounts of mucin, pepsin and HCl were assessed. Gastric mucosa were scrapped for the determination of nitric oxide (NO) (as nitrite) after evaluation of the gastric ulcer index. The influence of arginine (ARG) (300 mg kg(-1)), a NO precursor, N(G)-nitro- l -arginine methyl ester (l -NAME) (50 mg kg(-1)), a non-selective constitutive nitric oxide synthase/inducible nitric oxide synthase (cNOS/iNOS) inhibitor, and the selective iNOS inhibitor aminoguanidine (AMG) (50 mg kg(-1)) were studied. Each NO modulator was injected i.p. 30 min before IND administration. Results indicated that IND elevated gastric acidity by 80% of the normal group, decreased non-significantly mucosal nitrite by 22% and exhibited a remarkably high ulcer index (chi = 17). Neither mucin nor pepsin levels were significantly altered. In comparison with the IND group, pretreatment with l -NAME caused a significant decrease in gastric HCl, further decrease in mucosal nitrite (50% of normal) and a two-fold increase in the ulcer index score (chi = 34), despite the decrease in HCl. AMG did not alter gastric acidity, decreased mucosal nitrite by 38% of the normal value and failed to alter significantly the ulcer index of IND. On the other hand, pretreatment with ARG did not alter the gastric acidity and raised mucosal nitrite by 10% above normal. Surprisingly, ARG improved the gastric ulcer score (chi = 1) almost similar to the normal score (chi = zero). Therefore, this study creates a new pathway for the potential treatment of NSAID gastric ulceration through modulation of NO synthesis, regardless of the effect on gastric acidity.

Role of central glutamate receptors, nitric oxide and soluble guanylyl cyclase in the inhibition by endotoxin of rat gastric acid secretion

1. This study examines the role of a central pathway involving glutamate receptors, nitric oxide (NO) and cyclic GMP in the acute inhibitory effects of low doses of peripheral endotoxin on pentagastrin-stimulated acid production. 2. Vagotomy or intracisternal (i.c.) microinjections of the NO-inhibitor, N(G)-nitro-L-arginine methyl esther (L-NAME; 200 microg rat(-1)) restored acid secretory responses in endotoxin (10 microg kg(-1), i.v.)-treated rats. 3. The acid-inhibitory effect of i.v. endotoxin (10 microg kg(-1), i.v.) was prevented by prior i.c. administration of the NMDA receptor antagonists, dizocilpine maleate (MK-801; 10 nmol rat(-1)) and D-2-amino-5-phosphono-valeric acid (AP-5; 20 nmol rat(-1)), or the AMPA/kainate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 nmol rat(-1)). However, the competitive metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 20 - 1000 nmol rat(-1)) did not antagonize the effects of endotoxin. 4. I.c. administration of L-glutamate (0.1 nmol rat(-1)) inhibited pentagastrin-stimulated gastric acid secretion. Coadministration with L-NAME (200 microg rat(-1)) prevented the inhibition of gastric acid secretion by the aminoacid. 5. I.c. administration of 1H-[1,2, 4]Oxazodiolo[4,3-a]quinoxalin-1-one (ODQ; 100 nmol rat(-1)), a soluble guanylyl cyclase (sGC) blocker, reversed the hyposecretory effect of endotoxin. 6. I.c. administration of the cyclic GMP analogue 8-Bromoguanosine-3,5-cyclic monophosphate (8-Br-cGMP; 100 - 300 nmol rat(-1)) reduced gastric acid production in a dose-dependent manner. 7. We conclude that central NMDA and AMPA/kainate receptors are involved in the acid inhibitory effect of peripherally administered endotoxin. This central pathway involves synthesis of NO, which acts on the enzyme sGC.

Effects of intestinal ischemia/reperfusion injury on gastric acid secretion

BACKGROUND

The mechanism responsible for gastric colonization in critically injured ICU patients remains to be fully elucidated. Moreover, the effects of gut ischemia/reperfusion (I/R) injury on gastric function are unclear. It was our hypothesis that gut I/R injury would cause gastric dysfunction.

MATERIALS AND METHODS

Rats were anesthetized and, via laparotomy, the superior mesenteric artery (SMA) was clamped at its aortic origin for 45 min followed by clamp removal. Rats were allowed to awaken and then killed after 6 h of reperfusion. Control rats underwent laparotomy with SMA isolation. Stomachs were removed, gastric fluid was aspirated, and the volume, pH, and protein, bicarbonate, and glucose contents were determined. Serum and antral mucosa were prepared for gastrin radioimmunoassay and the glandular mucosa was assessed for morphologic injury.

RESULTS

SMA I/R injury caused significant accumulation of gastric luminal fluid that was alkaline and rich in protein, glucose, and bicarbonate content when compared with sham controls. SMA I/R injury also caused gastric surface epithelial cell injury and significantly increased serum and antral gastrin levels. In additional rats, gut I/R injury inhibited basal acid secretion and blunted the acid secretory response to pentagastrin.

CONCLUSIONS

This study demonstrated for the first time that small intestinal I/R injury causes significant gastric dysfunction. The findings suggest that this type of injury, a frequent occurrence in critically injured ICU patients, may predispose patients to gastric colonization due to stasis and loss of the natural bactericidal effects of gastric acid.

Investigation on the mechanism involved in the effects of agmatine on ethanol-induced gastric mucosal injury in rats

Effects of agmatine, an endogenous metabolite formed by decarboxylation of L-arginine, on ethanol-induced gastric mucosal injury were investigated in rats. Agmatine at 1 and 10 mg/kg i.p doses significantly increased ethanol-induced gastric mucosal injury. This effect of agmatine was abolished completely by pretreatment with idazoxan, an imidazoline receptor-antagonist and alpha2 receptor- antagonist, (0.5 mg/kg i.p), partly by yohimbine, an alpha2 receptor- antagonist, (1 mg/kg i.p) but not by L-arginine, a precursor of nitric oxide, (500 mg/kg i.p). Our results suggest that agmatine had a potent ulcerogenic effect mediated, at least in part, by both alpha2-adrenoceptors and imidazoline receptors.

N(G)-nitro-L-arginine methylester is protective against ethanol-induced gastric damage in cholestatic rats

In this study the effect of nitric oxide (NO) synthesis inhibition on ethanol-induced gastric damage was evaluated in bile duct-ligated, sham-operated and unoperated rats. The animals were injected intraperitoneally with saline, L-arginine (200 mg/kg) or N(G)-nitro-L-arginine methylester (L-NAME) in doses of 5, 15 and 30 mg/kg, 30 min before ethanol administration. The animals were killed 1 h after ethanol administration and their stomachs were removed for measurement of gastric mucosal damage. The results showed that L-NAME significantly enhanced the development of gastric mucosal lesions in sham-operated and unoperated rats, while in bile duct-ligated animals, L-NAME decreased and L-arginine enhanced the potentiation of ethanol-induced gastric mucosal damage. The plasma level of nitrite and nitrate was also measured and was significantly higher in bile duct-ligated rats than in control groups. The results suggest that inhibition of NO synthase with L-NAME has different effects on ethanol-induced gastric damage in cholestatic groups and in normal rats and that these effects can be explained by overproduction of NO in bile duct-ligated animals.

Modulation by nitric oxide of gastric acid secretion in toads

Nitric oxide (NO) is a novel chemical messenger that mediates a variety of biological actions. This study was undertaken to investigate the effects of NO on parietal cell function. The rate of [3H]arginine conversion to [3H]citrulline, a parameter of NO synthase activity, and NO formation (as NO2-), were inhibited by the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), in a concentration-dependent manner in the non-stimulated toad gastric mucosa. This range of concentrations of L-NAME provoked stimulation of H+ secretion in a similar fashion, which was blocked by L-arginine but not by D-arginine. Pre-treatment with carbachol plus ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetra-acetic acid (EGTA) prevented the effect of L-NAME on H+ secretion and drastically reduced NO synthase activity. L-arginine had an inhibitory effect on H+ secretion in non-stimulated and carbachol-stimulated gastric mucosa, which was reversed by L-NAME. Carbachol and pentagastrin, but not histamine, significantly increased NO formation in the toad gastric mucosa. The results suggest that changes in NO synthesis in the gastric mucosa may modulate parietal cell function and that a calcium-dependent mechanism may be involved.

Changes in gastric mucosal permeability induced by haemorrhagic shock in the anaesthetized rat: modulation by acid

Gastric mucosal damage induced by haemorrhagic shock in the anaesthetized rat has been evaluated by studying changes in capillary-to-lumen clearance of fluorescein isothiocyanate (FITC)-labelled dextran. Haemorrhagic shock (20 min ischaemia + 20 min reperfusion) induced a significant increase in blood-to-lumen permeability to FITC-dextran of different molecular weight (10,000, 40,000 and 70,000) without modifying the macroscopic integrity of the gastric mucosa. The increase in vascular permeability was dependent on the time of administration of the tracer and was correlated with an elevation of the protein content of the gastric lumen. Intravenous administration of the secretagogue pentagastrin (20 or 50 micrograms kg-1 h-1) did not significantly modify the vascular permeability to dextran in control animals or in animals subjected to haemorrhagic shock. When the intraluminal pH was reduced by intragastric administration of acidic saline solution, only pH 1, which itself induced the appearance of macroscopic mucosal lesions, significantly increased vascular permeability to dextran, both in control animals and in animals subjected to haemorrhagic shock. These findings suggest that stress induced by haemorrhagic shock increases vascular gastric permeability to dextran, by an acid-independent mechanism, without affecting the macroscopic integrity of the gastric mucosa.

Bacteria rapidly colonize and modulate healing of gastric ulcers in rats

The stomach is generally regarded as an environment that is not conducive to bacterial colonization. In this study, we examined the possibility that this changes significantly when an ulcer has formed and that colonization of ulcers interferes with the normal healing process. Gastric ulcers were induced by serosal application of acetic acid. The relationship between ulcer healing and bacterial colonization was examined. The effects of antibiotics, induction of Lactobacillus colonization, and selective colonization with an antibiotic resistant strain of Escherichia coli on ulcer healing were examined. Within 6-12 h of their induction, gastric ulcers were colonized by a variety of bacteria, with gram-negative bacteria predominating. Suppression of colonization with antibiotics resulted in marked acceleration of healing. Induction of Lactobacillus colonization also accelerated ulcer healing. The beneficial effects of antibiotics were reversed through selective colonization with antibiotic-resistant E. coli. Bacterial colonization occurred irrespective of the method used to induce the ulcer. This study demonstrates that colonization of gastric ulcers in rats occurs rapidly and significantly impairs ulcer healing. This effect appeared to be primarily attributable to gram-negative bacteria.

Role of nitric oxide in regulation of gastric acid secretion in rats: effects of NO donors and NO synthase inhibitor

1. The role of nitric oxide (NO) in the regulation of acid secretion was examined in the anaesthetized rat. 2. A rat stomach was mounted in an ex vivo chamber, instilled with 2 ml of saline every 15 min, and the recovered sample was titrated at pH 7.0 against 0.1 N NaOH by use of an automatic titrator for acid secretion. Gastric mucosal blood flow (GMBF) was measured simultaneously by laser Doppler flowmeter. 3. Intragastric application of NO donors such as FK409 (3 and 6 mg ml[-1]) and sodium nitroprusside (SNP; 6 and 12 mg ml[-1]) as well as i.p. administration of cimetidine (60 mg kg[-1]), a histamine H2-receptor antagonist, significantly inhibited the increase in acid secretion in response to pentagastrin (60 microg kg(-1) h(-1), i.v.), in doses that increased gastric mucosal blood flow (GMBF). 4. Intragastric application of FK409 (6 mg ml[-1]) increased both basal and stimulated acid secretion induced by YM-14673 (0.3 mg kg(-1), i.v.), an analogue of thyrotropin-releasing hormone (TRH), but had no effect on the acid secretory response induced by histamine (4 mg kg(-1) h(-1), i.v.). 5. Pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME; 10 mg kg(-1), i.v.) did not affect basal acid secretion, but significantly potentiated the increase in acid secretion induced by YM-14673 and slightly augmented the acid secretory response to pentagastrin. 6. Both pentagastrin and YM-14673 increased the release of nitrite plus nitrate (NOx), stable NO metabolites, into the gastric lumen, and these changes were completely inhibited by prior administration of L-NAME (10 mg kg(-1), i.v.). 7. Pentagastrin caused an increase in luminal release of histamine and this response was significantly suppressed by intragastric application of FK409 (6 mg ml[-1]). 8. These results suggest that either exogenous or endogenous NO has an inhibitory action on gastric acid secretion through suppression of histamine release from enterochromaffin-like (ECL) cells.

Effects of endotoxin on neurally-mediated gastric acid secretion in the rat

The effects of a peripheral administration of E. coli endotoxin on neurally-mediated gastric acid secretion and the role of endogenous opioids or PAF receptors in endotoxin effects have been evaluated in the continuously perfused stomach of the anaesthetized rat. Gastric acid secretion stimulated by distension (20 cm H2O) was reduced dose-dependently by single intravenous bolus injection of endotoxin (0.1-10 microg kg(-1)). Doses of 5 microg kg(-1) induced a peak reduction of distension-stimulated acid output and significantly reduced the secretory response induced by an intravenous bolus of 2-deoxy-D-glucose (150 mg kg(-1)). This dose of endotoxin did not significantly modify mean systemic arterial blood pressure throughout the experimental period. Pretreatment with the opioid receptor antagonist naloxone (1 mg kg(-1) , i.v.) or the platelet-activating factor (PAF) receptor antagonist WEB 2086 (2 mg kg(-1), i.v.) did not reverse the inhibitory effects of endotoxin (5 microg kg(-1) , i.v.) on acid secretion stimulated by both distension and 2-deoxy-D-glucose. These findings suggest that endotoxin-induced acute inhibition of neurally-mediated acid responses, stimulated by gastric distension or administration of 2-deoxy-D-glucose, do not involve the activation of endogenous opioids or PAF receptors.

Nitric oxide modulates the acute increase of gastrointestinal transit induced by endotoxin in rats: a possible role for tachykinins

Because of the evidence that endogenous nitric oxide (NO) plays an essential role in the physiological regulation of gastrointestinal motility we have investigated, by use of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), the role of endogenous NO in the acute endotoxin-induced changes of gastrointestinal transit. Pre-treatment with E. coli endotoxin (100 micrograms kg-1, i.v.) induced a significant increase in the gastrointestinal transit of a charcoal suspension in anaesthetized rats. Previous administration of the NO synthase inhibitor, L-NAME (10 mg kg-1, i.v.) significantly prevented the effects of endotoxin. L-arginine (200 mg kg-1, i.v.) and the substance P antagonist [D-Pro2, D-Trp7,9]-substance P (SPA), significantly reversed the effects of L-NAME on gastrointestinal transit in rats treated with endotoxin. Pre-treatment with dexamethasone (5 mg kg-1, s.c., twice), an inhibitor of the expression of inducible NO synthase, did not affect the increase in the gastrointestinal transit through constitutive NO synthesis. The results suggest that constitutive nitric oxide is involved in the increase of gastrointestinal transit induced by endotoxin and that the reduction in transit induced by L-NAME in endotoxin-treated rats is mediated by endogenous tachykinins.

NADPH-diaphorase in antral gastritis of childhood

BACKGROUND

Nitric oxide has an important role in the pathophysiology of the gastric mucosa. However, to date, it is not clear if nitric oxide plays a cytoprotective or cytotoxic role in the pathogenesis of mucosal lesion.

METHODS

We have used the NADPH-diaphorase histochemistry that selectively stains cells containing nitric oxide synthase, the enzyme that catalyzes the production of nitric oxide on the antral gastric mucosa of children with antral gastritis.

RESULTS

In the lamina propria of the mucosa, the presence of the enzymatic activity was found in perivascular round cells and nerve fibers. In the epithelium, focal positivity to NADPH-diaphorase was found in superficial cells, mainly located in the extrusive zones. The epithelial cells in the pits and glands were negative, in the mucous layer, Helicobacter pylori were also stained by NADPH-diaphorase. A single H. pylori-infected child who was also examined after eradication of the H. pylori showed during the control examination absence of microorganisms and reduction of the NADPH-diaphorase-positive cells in the mucosa.

CONCLUSIONS

Our results demonstrate that, in gastric mucosa, endogenous and exogenous structures express a NADPH-diaphorase activity.

The role of platelet-activating factor in conscious, normotensive endotoxemia

The effects of endotoxin have been postulated to be mediated in large part by release of endogenous platelet-activating factor (PAF) due to the similarity of their hemodynamic and gastric effects in anesthetized animals, and to the ability of PAF inhibitors to ameliorate endotoxin's effects. We chose to examine the relationship with doses that would not produce circulatory shock, in unrestrained conscious animals, in order to mimic clinical situations. Adult male rats were prepared with vascular access, hemodynamic and temperature monitors, and gastric strain gauge transducers. After an overnight fast, rats received a 4-hr infusion of saline (0.5 ml/hr), endotoxin (12.5 mg/kg/hr), PAF (36 micrograms/kg/hr, or 600 ng/kg/min), or endotoxin plus the PAF inhibitor CV 3988 (1 mg/kg/hr, after an initial pretreatment of 1 mg/kg). Rats were killed, stomachs were harvested, and contents were analyzed at the end of the infusions. Blood pressure was not affected by any treatment, but all treated groups developed diarrhea and vasodilatation. Endotoxin and PAF infusions decreased heart rate and body temperature to a similar extent, although the PAF effect on temperature was delayed. The PAF inhibitor did not prevent the body temperature effect, but did reverse it. Gastric secretions were affected by PAF to a lesser extent than by endotoxin, and the PAF inhibitor did not decrease endotoxin's gastric secretory effects. PAF has similar systemic and gastric effects to endotoxin in conscious, unrestrained, normotensive animals. The systemic effects of endotoxin at 12.5 mg/kg/hr were prevented or reversed by the PAF inhibitor CV-3988 at 1 mg/kg/hr, but not the gastric secretory effects.

Inhibition of gastric acid secretion by stress: a protective reflex mediated by cerebral nitric oxide

Moderate somatic stress inhibits gastric acid secretion. We have investigated the role of endogenously released NO in this phenomenon. Elevation of body temperature by 3 degrees C or a reduction of 35 mmHg (1 mmHg = 133 Pa) in blood pressure for 10 min produced a rapid and long-lasting reduction of distension-stimulated acid secretion in the rat perfused stomach in vivo. A similar inhibitory effect on acid secretion was produced by the intracisternal (i.c.) administration of oxytocin, a peptide known to be released during stress. Intracisternal administration of the NO-synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) reversed the antisecretory effect induced by all these stimuli, an action prevented by intracisternal coadministration of the NO precursor, L-arginine. Furthermore, microinjection of L-NAME into the dorsal motor nucleus of the vagus nerve reversed the acid inhibitory effects of mild hyperthermia, i.v. endotoxin, or i.c. oxytocin, an action prevented by prior microinjection of L-arginine. By contrast, microinjection of L-NAME into the nucleus tractus solitarius failed to affect the inhibitory effects of hyperthermia, i.v. endotoxin, or i.c. oxytocin. Immunohistochemical techniques demonstrated that following hyperthermia there was a significant increase in immunoreactivity to neuronal NO synthase in different areas of the brain, including the dorsal motor nucleus of the vagus. Thus, our results suggest that the inhibition of gastric acid secretion, a defense mechanism during stress, is mediated by a nervous reflex involving a neuronal pathway that includes NO synthesis in the brain, specifically in the dorsal motor nucleus of the vagus.

Bacterial lipopolysaccharide-induced changes in FOS protein expression in the rat brain: correlation with thermoregulatory changes and plasma corticosterone

In the present study the regions of the brain showing an increase in the number of FOS protein stained cells 180 min following intravenous saline or bacterial lipopolysaccharide (LPS) treatment were investigated and correlated with changes in body temperature and plasma corticosterone levels. Particular attention was given to the possible involvement of the circumventricular organs and regions of the brainstem containing central noradrenergic neurones. LPS at doses of 0.35, 3.5 and 50 micrograms caused highly significant increases in FOS protein expression in the organum vasculosum of lamina terminalis, the area postrema and the subfornical organ compared with saline controls. Marked increases in bacterial lipopolysaccharide-induced FOS protein expression were observed in the ventrolateral medulla, the nucleus of the solitary tract and the locus coeruleus which contain the A1, A2 and A6 noradrenergic neurones respectively. The changes in body temperature induced by LPS were found to be dependent upon the dose of LPS administered; the lowest dose employed (0.35 micrograms) induced an immediate and sustained fever, 3.5 micrograms LPS caused a biphasic response consisting of a hypothermic response followed by a febrile response, whereas 50 micrograms LPS induced a hypothermic response which then normalised by 160 min post-injection. Intravenous saline injection had no significant effect on body temperature. The occurance of LPS-induced hypothermia was coincident with increased FOS expression in the bed nucleus of stria terminalis, which houses vasopressinergic neurones involved in antipyresis, whereas in animals showing an LPS-induced febrile response there was no significant difference in the number of FOS stained cells in the bed nucleus of stria terminalis compared with saline treated animals. LPS also caused marked increases in FOS protein expression in the parvocellular regions of the paraventricular nucleus (pPVN) of the hypothalamus, the central nucleus of the amygdala and the ventral septal area. Plasma corticosterone was unaffected by the lowest dose of LPS (0.35 micrograms), however the higher doses employed (3.5 and 50 micrograms) caused significant increases in plasma corticosterone which correlated with the increases in the number of FOS stained cells in the pPVN. The results of the present study suggest that, in addition to the organum vasculosum of lamina terminalis, the area postrema and subfornical organ may be important in the responses to antigenic challenge that are mediated by the central nervous system. They also add support to the possible involvement of the bed nucleus of stria terminalis in LPS-induced hypothermia and of the involvement of the of the major noradrenergic cell groups (A1, A2 & A6) and a number of hypothalamic and extrahypothalamic forebrain regions in the interaction of immune and central nervous systems.

Nitric oxide and sensory afferent neurones modulate the protective effects of low-dose endotoxin on rat gastric mucosal damage

Pretreatment (1 h) with low doses (5-40 micrograms/kg i.p.) of Escherichia coli endotoxin dose dependently reduced the gastric mucosal damage induced by a 10 min challenge with 1 ml ethanol (50% and 100%) in conscious rats. Treatment with the nitric oxide synthesis inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 5 and 10 mg/kg i.p.), significantly inhibited the protective effects of endotoxin (40 micrograms/kg i.p.). The actions of L-NAME were reversed by the prior administration of L-arginine (100 mg/kg i.p.). The protective effects of endotoxin were not influenced by pretreatment with dexamethasone (5 mg/kg s.c. twice) or indomethacin (5 mg/kg s.c.). However, ablation of sensory afferent neurons by capsaicin pretreatment (20, 30 and 50 mg/kg s.c.) abolished the mucosa protective effects of endotoxin (40 micrograms/kg). These findings suggest that the protection elicited by low doses of endotoxin against ethanol-induced mucosal damage involves synthesis of nitric oxide and activation of sensory neurones.

Mechanism of gastric alkaline response in the stomach after damage. Roles of nitric oxide and prostaglandins

The gastric mucosa responds to hypertonic NaCl by significantly decreasing acid secretion. We examined the role of nitric oxide (NO) in this phenomenon in comparison with endogenous prostaglandins (PGs). A rat stomach was mounted in an ex vivo chamber, perfused with saline, and the potential difference (PD), pH, and acid/alkaline responses were measured before and after the application of hypertonic NaCl (1 mol/liter) with or without pretreatment with NG-nitro-L-arginine methyl ester (L-NAME; an inhibitor of NO biosynthesis) or indomethacin (a cyclooxygenase inhibitor). NaCl at 1 M caused a PD reduction, a decrease in acid secretion, and an increase in luminal HCO3-. Prior administration of L-NAME (5 mg/kg, intravenously) as well as indomethacin (5 mg/kg, subcutaneously) did not affect PD and HCO3- responses, but significantly attenuated the inhibitory effect of 1 M NaCl on acid secretion, although the effect of L-NAME was more potent when compared to indomethacin. This effect of L-NAME was antagonized by the simultaneous administration of L-arginine but not by D-arginine (200 mg/kg, intravenously), whereas the effect of indomethacin was completely reversed by PGE2 (100 micrograms/kg, intravenously). The histamine-stimulated acid secretion in the normal stomach was significantly decreased by nitroprusside (the exogenous NO donor; 4 mg/kg, intravenously) and PGE2, but not by either L-NAME or indomethacin. These results suggest that in addition to PGs, NO is involved in the mechanism of the gastric alkaline response after damage with 1 M NaCl. Irritation of the gastric mucosa by hypertonic NaCl may release endogenous NO and PGs, both of which in turn inhibit acid secretion and unmask luminal alkalinization due to HCO3- flux in the damaged portion.

Effects of nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester on duodenal alkaline secretory and ulcerogenic responses induced by mepirizole in rats

The inhibition of nitric oxide (NO) production by NO synthase inhibitors stimulates HCO3- secretion in the rat duodenal mucosa. Therefore, we examined the effects of NG-nitro-L-arginine methyl ester (L-NAME, the NO synthase inhibitor) and nitroprusside (the exogenous NO donor) on the duodenal HCO3- and ulcerogenic responses in anesthetized rats. Animals were administered mepirizole (200 mg/kg, subcutaneously) for induction of duodenal ulcers, and gastric acid and duodenal HCO3- secretions were measured with or without pretreatment with L-NAME (5 mg/kg, intravenously) or nitroprusside (4 mg/kg, intravenously). Mepirizole increased acid secretion, decreased the acid-induced duodenal HCO3- secretion, and induced hemorrhagic lesions in the proximal duodenum. The inhibition of NO production by L-NAME potentiated the acid secretory response, increased the duodenal HCO3- secretion, and prevented the duodenal lesions, and these changes were all antagonized by simultaneous administration of L-arginine (200 mg/kg, intravenously) but not D-arginine. On the other hand, nitroprusside slightly reduced the acid response but further decreased the HCO3- output, resulting in aggravation of duodenal lesions induced by mepirizole. These data suggest that the inhibition of endogenous NO production by the NO synthase inhibitor L-NAME increases duodenal HCO3- secretion and protects the duodenal mucosa against acid injury.

Endotoxin inhibition of distension-stimulated gastric acid secretion in rat: mediation by NO in the central nervous system

1. The involvement of nitric oxide in the acute inhibitory effects of low doses of endotoxin, following intracerebroventricular (i.c.v.) or intravenous (i.v.) administration, on gastric acid secretion stimulated by distension or i.v. infusion of pentagastrin has been investigated in the continuously perfused stomach of the anaesthetized rat. 2. The i.c.v. administration of E. coli endotoxin (800 ng kg-1) abolished the acid secretory response induced by gastric distension (20 cm water intragastric pressure) within 30 min of administration. 3. By contrast, submaximal rates of acid secretion induced by i.v. infusion of pentagastrin (8 micrograms kg-1 h-1) were not inhibited by i.c.v. administration of endotoxin (800 ng kg-1). 4. Prior i.c.v. administration of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 800 micrograms kg-1) restored the acid secretory responses to distension in rats treated with endotoxin (i.c.v.). 5. Likewise, i.v. administration of endotoxin (5 micrograms kg-1) abolished the acid secretory response induced by gastric distension within 30 min of administration. Prior i.c.v. injection of L-NAME (800 micrograms kg-1) or its i.v. administration (10 mg kg-1) restored acid secretory responses in rats receiving i.v. endotoxin. 6. The reversal by L-NAME (i.v.) of the acid inhibitory effects of endotoxin (i.v.) was prevented by L-arginine (12 mg kg-1, i.c.v. or 100 mg kg-1, i.v.), but not by its enantiomer D-arginine. 7. The present results imply the existence of an acute response to endotoxin involving NO synthesis in the brain. NO may act as a neuromodulator or neurotransmitter in a nervous reflex leading to the inhibition of acid secretion stimulated by gastric distension.

Interactions of constitutive nitric oxide with PAF and thromboxane on rat intestinal vascular integrity in acute endotoxaemia

1. The involvement of endogenous platelet activating factor (PAF) and thromboxane A2 in the acute microvascular damage in the ileum and colon induced by the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) following endotoxin administration was investigated in the rat over a 1 h period. 2. Administration of L-NAME (1-10 mg kg-1, s.c.) concurrently with E. coli lipopolysaccharide (LPS; 3 mg kg-1, i.v.) dose-dependently increased vascular permeability in the ileum and colon, as determined by the leakage of radiolabelled albumin, and caused macroscopic mucosal damage in the ileum determined 1 h later. Neither LPS administration nor L-NAME (5 mg kg-1) alone affected resting vascular permeability. 3. Infusion of phenylephrine (10 micrograms kg-1 min-1, i.v. for 1 h) caused an elevation in blood pressure similar to that found following L-NAME administration (5 mg kg-1, i.v. or s.c.), but did not increase intestinal vascular permeability, when administered with LPS (3 mg kg-1, i.v.). 4. The increased vascular permeability in the ileum and colon and macroscopic damage in the ileum, induced by L-NAME (5 mg kg-1, s.c.) and LPS (3 mg kg-1, i.v.) was dose-dependently inhibited following s.c. pretreatment (15 min before challenge) with the thromboxane synthase inhibitors, OKY 1581 (5-25 mg kg-1) or 1-benzyl-imidazole (1-50 mg kg-1), or with the thromboxane receptor antagonist, BM 13177 (0.2-2 mg kg-1). 5. Pretreatment with the cyclo-oxygenase inhibitor, indomethacin (2-5 mg kg-', s.c., 15 min before challenge) reduced the microvascular injury in the ileum and colon and macroscopic lesions in the ileum,observed after the concurrent administration of L-NAME and LPS.6. Pretreatment (15 min) with the PAF-receptor antagonists, WEB 2086 (0.5-1 mg kg-', s.c.) or BN52021 (2.5-10 mg kg-', s.c.) likewise attenuated this intestinal vascular injury.7. Combined administration of low doses of l-benzyl-imidazole (1 mg kg-') with WEB 2086(0.5 mg kg-')15 min before L-NAME and LPS challenge, abolished this vascular damage and macroscopic injury.8. These results suggest that PAF and thromboxane A2 are released acutely following challenge with a low dose of endotoxin. However, these mediators do not appear to injure the intestinal micro vascular bed unless NO synthase is concurrently inhibited. Such findings support the protective role of constitutively-formed NO, counteracting the injurious vascular actions of cytotoxic mediators released under pathological conditions.

Role of endogenous nitric oxide in the control of gastric acid secretion, blood flow and gastrin release in conscious dogs

Nitric oxide (NO) was shown to mediate gastric hyperemia following secretory stimulation but its role in the control of gastric secretion has not been clarified. Secretory studies were carried out on conscious dogs with chronic gastric fistula, Heidenhain pouch and esophageal fistula, while changes in gastric blood flow were measured in the mucosa of Heidenhain pouuch by laser Doppler flowmetry. Plasma gastrin was determined by radioimmunoassay. Infusion of NG-nitro-L-arginine (L-NNA) (bolus i.v. injection of 2.5 mg/kg followed by infusion of 0.5 mg/kg/h), a potent inhibitor of nitric oxide synthase, failed to affect basal gastric secretion or plasma gastrin level but suppressed an increase of this secretion induced by sham-feeding, ordinary meat feeding or i.v. infusion of bombesin (0.5 microgram/kg/h), pentagastrin (4 micrograms/kg/h) or histamine (40 micrograms/kg/h). In tests with feeding and bombesin infusion, L-NNA caused a significant and dose-dependent reduction in plasma gastrin levels. The inhibition by L-NNA of gastric acid secretory response to pentagastrin, histamine or feeding was accompanied by a decline in blood flow. Addition of L-arginine (bolus i.v. dose of 50 mg/kg followed by infusion of 5 mg/kg/h) significantly attenuated the L-NNA induced inhibition of gastric secretion and the reduction in plasma gastrin response as well as in the fall of gastric blood flow. We conclude that endogenous nitric oxide affects the gastric secretion and that this effect is mediated, at least in part, by the changes in the gastrin release and gastric blood flow.

Nitric oxide donors preferentially inhibit neuronally mediated rat gastric acid secretion

Continuous i.v. infusion of the nitric oxide (NO) donors, S-nitroso-glutathione (10-50 micrograms kg-1 min-1) and S-nitroso-N-acetyl-penicillamine (10 micrograms kg-1 min-1) inhibited neuronally mediated gastric acid secretion, as induced by gastric distension (20 cm water) or i.v. bolus administration of 2-deoxy-D-glucose (150 mg kg-1) in the anaesthetized rat. By contrast, gastric acid responses to i.v. infusion of submaximal doses of pentagastrin (8 micrograms kg-1 h-1) or histamine (1 mg kg-1 h-1) were not influenced by these NO donors. These findings suggest that NO does not directly influence acid secretion in vivo but could play an inhibitory modulator role in neuronally mediated acid responses.

Involvement of neuronal processes and nitric oxide in the inhibition by endotoxin of pentagastrin-stimulated gastric acid secretion

Administration of E. coli endotoxin (1 mg kg-1, i.v.) abolished the acid response induced by the i.v. infusion of pentagastrin (8 micrograms kg-1 h-1) in the continuously perfused stomach of the anaesthetized rat. Local serosal application of tetrodotoxin (36 ng per rat) completely restored acid responses to pentagastrin in endotoxin-treated rats. However, pretreatment with atropine (0.5 mg kg-1, s.c.), capsaicin (20, 30, and 50 mg kg-1, s.c. 2 weeks before the study) or guanethidine (16 mg kg-1, s.c. 3 and 16h before) did not influence the inhibitory effects of endotoxin. Continuous i.v. infusion with NG-nitro arginine methyl ester (L-NAME, 10 mg kg-1 h-1) restored the secretory responses to pentagastrin in endotoxin treated rats. The effects of L-NAME were reversed by L-arginine (100 mg kg-1 h-1, i.v.), but not by its enantiomer D-arginine (100 mg kg-1 h-1, i.v.). The secretory responses elicited by pentagastrin (10(-10)-10(-6) M) in the isolated lumen perfused stomach of the rat were not influenced by incubation (100 min) with endotoxin (10 micrograms ml-1). These observations with tetrodotoxin indicate that inhibition of acid secretion by endotoxin in vivo involves neuronal activity, while inhibition of NO synthesis had a comparable inhibitory action. Activation of a systemic non-adrenergic non-cholinergic neuronal pathway involving NO could thus mediate the acute acid inhibitory effects of endotoxin.

Endogenous nitric oxide in gastric alkaline response in the rat stomach after damage

BACKGROUND/AIMS

The gastric mucosa responds to hypertonic NaCl by significantly decreasing acid secretion. This study examined the role of nitric oxide in this phenomenon.

METHODS

A rat stomach was mounted in an ex vivo chamber and perfused with saline; then the potential difference (PD), pH, and acid and/or alkaline responses were measured before and after the application of 1 mol/L NaCl with or without pretreatment with NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO biosynthesis.

RESULTS

A dose of 1 mol/L NaCl caused a reduction in PD, a decrease in basal and histamine-stimulated acid secretion, and an increase in luminal HCO3-. Prior administration of L-NAME did not affect either the PD or the HCO3- response but significantly attenuated the inhibitory effect of 1 mol/L NaCl on acid secretion. This effect of L-NAME was antagonized by the simultaneous administration with L-arginine but not D-arginine. Histamine-stimulated acid secretion in the normal stomach was significantly reduced by the exogenous NO donor nitroprusside but not by L-NAME.

CONCLUSIONS

NO is involved in the mechanism of the gastric alkaline response after damage with 1 mol/L NaCl. Irritation of the gastric mucosa by hypertonic NaCl may release endogenous NO, which in turn inhibits acid secretion and unmasks luminal alkalinization caused by HCO3- flux in the damaged portion.

Involvement of endogenous nitric oxide in the inhibition by endotoxin and interleukin-1 beta of gastric acid secretion

Administration of Escherichia coli endotoxin abolished the acid secretory response induced by a bolus injection of pentagastrin in the continuously perfused stomach of the anaesthetized rat. Likewise, acid secretion stimulated by the continuous intravenous perfusion of pentagastrin was inhibited by administration of interleukin-1 beta (IL-1 beta). In both cases pretreatment with NG-nitro-L-arginine methyl ester (L-NAME) but not dexamethasone or indomethacin substantially restored the secretory responses to pentagastrin. The actions of L-NAME were reversed by the prior administration of L-arginine but not by its enantiomer D-arginine. Even though L-NAME increased blood pressure, this does not seem to be the mechanism by which endotoxin-induced acid inhibition was prevented, since similar systemic pressor responses induced by phenylephrine had no such effect. The secretory response elicited by pentagastrin in the isolated lumen perfused stomach of the rat was not influenced by incubation (100 min) with IL-1 beta. These observations suggest that the acute inhibition of acid responses to pentagastrin by endotoxin and IL-1 beta involves nitric oxide (NO) synthesis from L-arginine.

Thirteenth Gaddum Memorial Lecture. Neuronal and endothelium-derived mediators in the modulation of the gastric microcirculation: integrity in the balance

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