Nitric oxide donors preferentially inhibit neuronally mediated rat gastric acid secretion

Inflammatory cytokines TNFα, IL-1β, and IL-6 are induced in endotoxin- stimulated microglia through different signaling cascades

By using an animal model in which inflammatory cytokines are induced in lipopolysaccharide (LPS)-injected rat brain, we investigated the induction of tumor necrosis factor alpha (TNFα), interleukin-1beta (IL-1β), and IL-6. Immunoblotting and immunohistochemistry revealed that all three cytokines were transiently induced in the cerebral cortex at about 12 h after LPS injection. To clarify which glial cell type induced the cytokines, we examined the respective abilities of astrocytes and microglia in vitro. Primary microglia largely induced TNFα, IL-1β and IL-6 in response to LPS, but primary astrocytes induced only limited levels of TNFα. Thus, we used specific inhibitors to focus on microglia in surveying signaling molecules involved in the induction of TNFα, IL-1β, and IL-6. The experiments using mitogen-activated protein kinases (MAPK) inhibitors revealed that c-Jun N-terminal kinase (JNK)/p38, external signal regulated kinase (ERK)/JNK, and ERK/JNK/p38 are necessary for the induction of TNFα, IL-1β, and IL-6, respectively. The experiments using protein kinase C (PKC) inhibitor clarified that PKCα is required for the induction of all these cytokines in LPS-stimulated microglia. Furthermore, LPS-dependent IL-1β/IL-6 induction was suppressed by pretreatment with a nitric oxide (NO) scavenger, suggesting that NO is involved in the signaling cascade of IL-1β/IL-6 induction. Thus, an inducible NO synthase induced in the LPS-injected cerebral cortex might be related to the induction of IL-1β/IL-6 through the production of NO in vivo. Taken together, these results demonstrated that microglia induce different kinds of inflammatory cytokine through specific combinations of MAPKs and by the presence or absence of NO.

Nesfatin-1 inhibits gastric acid secretion via a central vagal mechanism in rats

Nesfatin-1, a novel hypothalamic peptide, inhibits nocturnal feeding behavior and gastrointestinal motility in rodents. The effects of nesfatin-1 on gastrointestinal secretory function, including gastric acid production, have not been evaluated. Nesfatin-1 was injected into the fourth intracerebral ventricle (4V) of chronically cannulated rats to identify a nesfatin dose sufficient to inhibit food intake. Nesfatin-1 (2 μg) inhibited dark-phase food intake, in a dose-dependent fashion, for >3 h. Gastric acid production was evaluated in urethane-anesthetized rats. Nesfatin-1 (2 μg) was introduced via the 4V following endocrine stimulation of gastric acid secretion by pentagastrin (2 μg·kg(-1)·h(-1) iv), vagal stimulation with 2-deoxy-D-glucose (200 mg/kg sc), or no stimulus. Gastric secretions were collected via gastric cannula and neutralized by titration to determine acid content. Nesfatin-1 did not affect basal and pentagastrin-stimulated gastric acid secretion, whereas 2-deoxy-D-glucose-stimulated gastric acid production was inhibited by nesfatin-1 in a dose-dependent manner. c-Fos immunofluorescence in brain sections was used to evaluate in vivo neuronal activation by nesfatin-1 administered via the 4V. Nesfatin-1 caused activation of efferent vagal neurons, as evidenced by a 16-fold increase in the mean number of c-Fos-positive neurons in the dorsal motor nucleus of the vagus (DMNV) in nesfatin-1-treated animals vs. controls (P < 0.01). Finally, nesfatin-induced Ca(2+) signaling was evaluated in primary cultured DMNV neurons from neonatal rats. Nesfatin-1 caused dose-dependent Ca(2+) increments in 95% of cultured DMNV neurons. These studies demonstrate that central administration of nesfatin-1, at doses sufficient to inhibit food intake, results in inhibition of vagally stimulated secretion of gastric acid. Nesfatin-1 activates DMNV efferent vagal neurons in vivo and triggers Ca(2+) signaling in cultured DMNV neurons.

Superoxide anion contributes to the induction of tumor necrosis factor alpha (TNFα) through activation of the MKK3/6-p38 MAPK cascade in rat microglia

Stimulation of rat microglia with lipopolysaccharide (LPS) in vitro induces production of the inflammatory/cytotoxic cytokine tumor necrosis factor alpha (TNFα) along with superoxide anion (O(2)(-)) and nitric oxide (NO). In this study, we investigated the role of O(2)(-) and NO in the induction of TNFα in microglia. The LPS-inducible TNFα was significantly suppressed by pretreatment with the O(2)(-) scavenger N-acetyl cysteine (NAC), but not by the NO scavenger 2-(4-Carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide, suggesting the close association of O(2)(-) with TNFα induction. NAC strongly depressed phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which is necessary for inducing TNFα in microglia. On the other hand, an O(2)(-) donor, 3-(4-Morpholinyl)sydnonimine (SIN-1), induced TNFα in microglia, and the effects of SIN-1 were completely abolished in the presence of superoxide dismutase. There is little likelihood that the NO produced in SIN-1 degradation induces TNFα in microglia, because TNFα was not induced in microglia exposed to the NO-donor S-nitroso-N-acetyl-dl-penicillamine. Moreover, the addition of SIN-1 to microglia resulted in activation of p38 MAPK and its upstream kinase MKK3/6. Taken together, these results showed that O(2)(-) is an important signaling molecule for activating the MKK3/6-p38 cascade, which is requisite for inducing TNFα in microglia.

Dual role of nitric oxide in gastric hypersecretion in the distended stomach: inhibition of acid secretion and stimulation of pepsinongen secretion

AIMS

We investigate the role of nitric oxide (NO) in the hypersecretion of acid and pepsinogen induced by stomach distension.

MAIN METHOD

The rat stomach was distended by instillation of saline through an acute fistula under urethane anesthesia.

KEY FINDINGS

Both secretions of acid and pepsinogen were increased by the distension depending on the volume of saline introduced, and responses were attenuated by bilateral cervical vagotomy or prior administration of atropine. N(G)-nitro-l-arginine methyl ester (L-NAME) had a dual effect on these responses, causing an increase in the acid response and a decrease in the pepsin response, both in an l-arginine-sensitive manner. Distension of the stomach increased the luminal NO release; this response was suppressed by vagotomy and L-NAME. Intragastric application of FK409, a NO donor, dose-dependently increased pepsinogen secretion while decreasing acid secretion in the stomach without distension. However, serosal application of both FK409 and 8-bromo-guanosine cyclic 3', 5'-monophosphate (8-Br-cGMP) stimulated the secretion of pepsinogen in isolated mouse stomachs in vitro. The stimulatory effect of FK409 on pepsinogen secretion was totally abolished by LY83583, a guanylate cyclase inhibitor.

SIGNIFICANCE

Distension of the stomach increases both acid and pepsinogen secretion through a vagal-cholinergic pathway in addition to the luminal release of NO, and NO affects these responses in opposite ways, suppressing the acid response while enhancing the pepsin response, both mediated by a guanylate cyclase/cGMP pathway.

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.

Stimulatory effects of endogenous and exogenous nitric oxide on gastric acid secretion in anesthetized rats

We previously reported the stimulatory effect of endogenous nitric oxide (NO) on gastric acid secretion in the isolated mouse whole stomach and histamine release from gastric histamine-containing cells. In the present study, we investigated the effects of endogenous and exogenous NO on gastric acid secretion in urethane-anesthetized rats. Acid secretion was studied in gastric-cannulated rats stimulated with several secretagogues under urethane anesthesia. The acid secretory response to the muscarinic receptor agonist bethanechol (2 mg/kg, s.c.), the cholecystokinin(2) receptor agonist pentagastrin (20 microg/kg, s.c.) or the centrally acting secretagogue 2-deoxy-D-glucose (200 mg/kg, i.v.) was dose-dependently inhibited by the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA, 10 or 50 mg/kg, i.v.). This inhibitory effect of L-NNA was reversed by a substrate of NO synthase, L-arginine (200 mg/kg, i.v.), but not by D-arginine. The histamine H(2) receptor antagonist famotidine (1 mg/kg, i.v.) completely inhibited the acid secretory response to bethanechol, pentagastrin or 2-deoxy-D-glucose, showing that all of these secretagogues induced gastric acid secretion mainly through histamine release from gastric enterochromaffin-like cells (ECL cells). On the other hand, histamine (10 mg/kg, s.c.)-induced gastric acid secretion was not inhibited by pretreatment with L-NNA. The NO donor sodium nitroprusside (0.3-3 mg/kg, i.v.) also dose-dependently induced an increase in acid secretion. The sodium nitroprusside-induced gastric acid secretion was significantly inhibited by famotidine or by the soluble guanylate cyclase inhibitor methylene blue (50 mg/kg, i.v.). These results suggest that NO is involved in the gastric acid secretion mediated by histamine release from gastric ECL cells.

Stimulatory effects of nitric oxide donors on histamine release in isolated rat gastric mucosal cells

We previously reported stimulatory effects of endogenous and exogenous nitric oxide (NO) on gastric acid secretion. In the present study, we investigated effects of NO donors on release of histamine, which is related to acid secretion, in isolated rat gastric mucosal cells. NO donors such as (+/-)-(E)-4-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexanamide (NOR 1) and sodium nitroprusside significantly augmented the histamine release. It was inhibited by 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-amide (carboxy-PTIO), a NO scavenger, and 6-(phenylamino)-5,8-quinolinedione (LY83583), a soluble guanylate cyclase inhibitor. Dibutyryl cyclic GMP also stimulated histamine release. These results suggest that NO donors act on cyclic GMP pathway in isolated gastric mucosal cells, resulting in facilitation of histamine release. NO may stimulate gastric acid secretion through histamine release from the histamine-containing cells, possibly enterochromaffin-like cells.

Cephalic phase of acid secretion involves activation of medullary TRH receptor subtype 1 in rats

Mechanisms involved in the cephalic phase of gastric acid secretion were studied in awake fasted rats with chronic gastric fistula and exposed to the sight and smell of chow for 30 min. Acid secretion was monitored using constant intragastric perfusion and automatic titration. Sham feeding induced a peak acid response reaching 82 +/- 7 micromol/10 min within 20 min compared with the average 22 +/- 2 micromol/10 min in controls. The sham-feeding response was abolished by intracisternal pretreatment with the TRH(1)-receptor antisense oligodeoxynucleotides or subcutaneous injection of atropine, whereas TRH(1) mismatch oligodeoxynucleotides had no effect. Serum gastrin was not altered by the sham feeding and increased by refeeding. Gastrin antibody did not block the rise in acid during sham feeding, although the net acid response was reduced by 47% compared with the control group. Glycine-gastrin antibody, indomethacin and nitro-l-arginine methyl ester had no effect. Atropine and gastrin antibody decreased basal acid secretion by 98 and 75%, respectively, whereas all other pretreatments did not. These results indicate that the cholinergic-dependent acid response to sham feeding is mediated by brain medullary TRH(1) receptors in rats.

Stimulatory effects of centrally injected nitric oxide donors on gastric acid secretion in anesthetized rats

The effects of centrally injected nitric oxide (NO) donors on gastric acid secretion were investigated in continuously perfused stomach of anesthetized rats. The lateral cerebroventricular (LV) injection of NOC5 (30 - 100 microg) and NOC12 (10 - 100 microg) dose-dependently stimulated gastric acid secretion. The LV injection of NOC18 (30 microg) also stimulated gastric acid secretion. The other type of NO donor, sodium nitroprusside (3 - 30 microg, LV), also dose-dependently stimulated gastric acid secretion. The effect of NOC5 at 100 microg was blocked by carboxy-PTIO, an NO scavenger, and by cervical vagotomy. Furthermore, NOC12 (30, 100 microg) dose-dependently stimulated gastric acid secretion in pylorus-ligated conscious rats. These results suggest that centrally injected NO donors stimulate gastric acid secretion in both conscious and anesthetized rats through vagus activation.

Morphological support for paracrine inhibition of gastric acid secretion by nitric oxide in humans

BACKGROUND

Functional studies have shown that nitric oxide (NO) inhibits gastric acid secretion in a variety of species, including man. We have performed a morphological study with the intention of localizing the endothelial NO synthase (eNOS) in the human gastric mucosa.

METHODS

Fifteen healthy subjects voluntarily participated in the study, and mucosal biopsies were obtained from the cardia, corpus and antrum. The presence and localization of eNOS were studied using immunohistochemical techniques.

RESULTS

eNOS-immunoreactivity (eNOS-IR) is found in surface mucous cells of cardia, corpus and antrum. Unique to the oxyntic mucosa is the presence of eNOS-IR in 'endocrine-like' cells, found in close contact with parietal cells.

CONCLUSIONS

eNOS-IR cells in close apposition to parietal cells provide morphological support for paracrine inhibition of gastric acid secretion by NO.

Nitric oxide: relation to integrity, injury, and healing of the gastric mucosa

Nitric oxide (NO) plays a multifaceted role in mucosal integrity. The numerous functions of NO and the double-edged role played by NO in most of them provide a great complexity to the NO action. The three enzymatic sources of NO, neuronal NO-synthase (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS), have been characterised in the gastrointestinal tract. The protective properties of the NO derived from constitutive NO-synthases (eNOS and nNOS) have already been well established. Less clear is the role assigned to iNOS. The simplistic initial view of low levels of NO synthesised by constitutive NOS being protective while exaggerated NO levels after iNOS induction leading irremediably to cytotoxicity is being questioned by new evidence. As initially reported for constitutive NOS, iNOS activity may be associated to reduced leukocyte-endothelium interaction and platelet aggregation as well as protection of mucosal microcirculation. Moreover, iNOS activity may be important to resolve inflammation by increasing apoptosis in inflammatory cells. It is entirely possible that a low level of expression of iNOS will reflect a positive host-defense response to challenge, but that exaggerated or uncontrolled expression of iNOS itself becomes detrimental. There is no doubt about the protective role of NO in physiological conditions. However, when the mucosa is threatened, the role of NO becomes multiple and the final effect will probably depend on the nature of the insult, the environment involved, and the interaction with other mediators.

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 effect of L-arginine against stress-induced gastric mucosal lesions in rats and its relation to nitric oxide-mediated inhibition of neutrophil infiltration

Pretreatment with l -arginine (150-600 mg kg(-1), i.p.), but not d -arginine (600 mg kg(-1), i.p.), protected against gastric mucosal lesions in rats with water immersion restraint stress over a 6-h period. This protective effect occurred in a dose-dependent manner. Increases in the activities of inducible nitric oxide synthase (iNOS) and myeloperoxidase (MPO), an index of tissue neutrophil infiltration, and the concentration of nitrite/nitrate, breakdown products of nitric oxide, and a decrease in the activity of constitutive nitric oxide synthase (cNOS) occurred in the gastric mucosal tissue with the development of gastric mucosal lesions. The l -arginine pretreatment attenuated the increases in iNOS and MPO activities and nitrite/nitrate concentration and the decrease in cNOS activity in the gastric mucosal tissue in a dose-dependent manner, while the d -arginine pretreatment did not. Both the protective effect of l -arginine (300 mg kg(-1)) against stress-induced gastric mucosal lesions and the attenuating effect of the amino acid on the increases in gastric mucosal iNOS and MPO activities and the decrease in gastric mucosal cNOS activity with the lesion development were counteracted by pretreatment with N(G)-monomethyl- l -arginine (100 mg kg(-1), s.c.), a nitric oxide synthase inhibitor, but not its d -isomer (100 mg kg(-1), s.c.). These results suggest that the protective effect of exogenously administered l -arginine against stress-induced gastric mucosal lesions in rats is, at least in part, due to nitric oxide-mediated inhibition of neutrophil infiltration into the gastric mucosal tissue.

Stimulatory effects of nitric oxide donors on gastric acid secretion in isolated mouse stomach

We previously reported on the stimulatory role of endogenous nitric oxide (NO) in gastric acid secretion. In the present study, we investigated the effects of NO donors on acid secretion in isolated mouse stomach. Nitroprusside (100 microM(-1) x mM) inhibited the gastric acid secretion induced by histamine (500 microM) in a concentration-dependent manner. In addition, nitroprusside abolished the acid secretion induced by bethanechol (100 microM) and by electrical stimulation (10 Hz) of the vagus nerve. On the other hand, nitroprusside, 75 microM, which did not affect the acid secretion induced by histamine, itself elicited an increase in acid secretion. The acid secretion induced by 75 microM nitroprusside was inhibited by 10 microM famotidine, a histamine H2 receptor antagonist. These results suggest that NO donors at high doses act on gastric parietal cells, resulting in inhibition of the stimulated acid secretion, and, at lower doses, facilitate histamine release from histamine-containing cells, leading to the increased acid secretion.

Antisecretory and gastroprotective activities of compounds endowed with H2 antagonistic and nitric oxide (NO) donor properties

In spite of the well recognized gastric antisecretory activity, the gastroprotective potential of histamine H2 receptor antagonists is controversial. Most clinical studies in fact indicate that these drugs do not substantially protect the gastric mucosa from aggressive factors. Nitric oxide (NO) has been recently recognized as a fundamental mediator in gastric defence mechanisms, due to its ability to increase gastric mucosal blood flow and mucus production and to inhibit neutrophils adherence to endothelial cells. The aim of this study was to investigate the gastroprotective and H2 receptor antagonistic activity of a series of lamtidine analogues which contain different NO-releasing moieties (furoxan, nitroxy and nitrosothiol). These compounds were tested, in comparison with related H2 antagonists devoid of NO-donor structures, in different H2 receptor assays and in the conscious rat against 0.6 N HCl-induced gastric lesions. All the compounds tested were able to antagonize histamine-mediated responses at cardiac and gastric H2 receptors; however, furoxan and nitroxy derivatives were 10-fold less potent than the analogues devoid of NO-donor properties. By contrast, NO-donor compounds were more active than reference H2 antagonists as gastroprotective agents against mucosal injury induced by 0.6 N HCl. Among the different NO-donor moieties, the furoxan group conferred to the H2 antagonist molecule the highest gastroprotective potential; this finding closely correlates with the characteristics of NO release. In conclusions, lamtidine-analogue H2 antagonists combined with NO-donor moieties are endowed with gastric antisecretory and protective activity and could be the prototypes of a new class of anti-ulcer drugs. Finally, the furoxan NO donor group seems to be the most favourable among the different moieties tested.

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.

Distribution of nitric oxide synthase in the esophagus of the cat and monkey

The distribution of nitrergic neurons and processes in the esophagus of the cat and monkey was studied by light microscopic immunocytochemistry using a specific antibody against purified rat brain nitric oxide synthase and immunoperoxidase procedures. Immunoreactive nerve fibers were found pervading the myenteric plexus, submucous plexus and plexus of the muscularis mucosae, and particularly in the lower esophagus a few immunoreactive fibers entered the epithelium as free nerve endings, some of which derived from perivascular fibers. In the upper esophagus immunoreactive motor end-plates were found in the striated muscle. Thirty-forty-five percent of neuronal cell bodies found in the intramural ganglia and along the course of nerve fiber bundles were immunoreactive and were of the three morphological types earlier described. In the intramural ganglia immunoreactive nerve fibers formed a plexus in which varicose nerve terminals were in close relation to immunoreactive and non-immunoreactive neurons. The intramural blood vessels that crossed the different layers of the esophageal wall were surrounded by paravascular and perivascular plexuses containing immunoreactive nerve fibers. The anatomical findings suggest that nitric oxide is involved in neural communication and in the control of peristalsis and vascular tone in the esophagus. In the lower esophagus a few nitrergic nerve fibers are anatomically disposed to subserve a sensory-motor function.

Inhibitory effect of N(omega)-nitro-L-arginine on gastric secretion induced by secretagogues and vagal stimulation in the isolated stomach

The involvement of endogenous nitric oxide (NO) in the control of gastric acid secretion induced by some secretagogues was studied in the mouse isolated whole stomach. The gastric acid secretion induced by McNeil A-343 [4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N-trimethyl-2-butyn- 1-aminium chloride], a muscarinic M1 receptor agonist, pentagastrin or electrical vagus nerve stimulation was markedly inhibited by pretreatment with the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA). This inhibitory effect of L-NNA was reversed by L-arginine, but not by D-arginine. Histamine-induced gastric acid secretion was not influenced by treatment with L-NNA. Famotidine completely inhibited the gastric acid secretion induced by McNeil A-343, pentagastrin or electrical vagus nerve stimulation, showing that these stimulations induced gastric acid secretion mainly through histamine release from histamine-containing cells in the gastric mucosa. Moreover, the pentagastrin- and bethanechol-induced histamine release from gastric mucosal cells was significantly inhibited by L-NNA. The NO donor, sodium nitroprusside, at a concentration not affecting histamine-induced gastric acid secretion, increased the acid secretory response, and this response was inhibited by famotidine. These results suggest that endogenous NO is involved in the gastric acid secretion via histamine release from histamine-containing cells.

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.

Impaired insulin but normal pentagastrin effect on gastric acid secretion in diabetic rats: a role for nitric oxide

Significant changes in gastrointestinal function, decreased gastric secretion and motility in particular, are often observed in patients with chronic diabetes. The mechanisms leading to those remain unclear. In these studies we evaluated the gastric acid secretory response to insulin and pentagastrin in normal Wistar and streptozotocin diabetic rats. We also sought to determine the role of nitric oxide (NO) in this process. The animals were anesthetized with sodium pentobarbital. Warm saline was perfused through a polyethylene tube placed in the oesophagus and collected from the duodenum at 10 min intervals. Following a 50 min equilibration period, a bolus intra-jugular infusion of insulin (4.0 U/kg), 2-deoxyglucose (200 mg/kg) or pentagastrin 4.0 (ug/kg) was started and samples of the gastrointestinal perfusate were collected for an additional 80 min. Insulin-stimulated acid secretion peaked 60 min after bolus infusion in normal animals; a response that was significantly decreased in the diabetic rats. Similarly, 2-deoxyglucose-induced glucopenia increased gastric acid secretion to a lower extent in diabetic versus normal rats. The stimulatory response to pentagastrin was prompt and essentially equal in normal and diabetic animals. However, when hypoglycemia was prevented by glucose infusion, insulin did not stimulate gastric acid secretion in normal rats. Further, glucose infusion in these animals actually enhanced the secretory response to pentagastrin. Nitro-L-arginine methyl ester (L-NAME 20 mg/kg i.v.), an inhibitor of NO synthetase, also prevented the secretory response to insulin but not to pentagastrin. Preinfusion of arginine (100 mg/kg i.v.) in diabetic rats restored the gastric secretory response to insulin toward that of normal animals. We conclude that the gastric acid secretory response to insulin, but not to pentagastrin, is decreased in diabetic animals, that this response may operate through a NO mediated mechanism possibly set in motion by central nervous system glucopenia and that this NO-mediated mechanism is attenuated in diabetes.

Nitrosoglutathione suppresses cochlear potentials and DPOAEs but not outer hair cell currents or voltage-dependent capacitance

Biochemical and pharmacological evidence support a role for nitric oxide (NO) and glutathione (GSH) in the cochlea. GSH combines with NO in tissue to form nitrosoglutathione (GSNO) that can act as a storage form for GSH and NO. Therefore, we tested GSNO on sound-evoked responses of the cochlea (cochlear microphonic, CM; summating potential, SP; compound action potential, CAP; cubic distortion product otoacoustic emission, DPOAE), on the endocochlear potential (EP), on isolated outer hair cell (OHC) currents and voltage-dependent capacitance, and on Deiters' cell currents. In vivo application of GSNO in increasing concentrations reversibly reduced low-intensity sound-evoked CAP, SP and DPOAEs starting at about 1 mM (CAP) and 3.3 mM (SP, DPOAE). However, even at 10 mM, GSNO had little effect on the EP. In vitro, salicylate (10 mM) but not GSNO (3 and 10 mM) suppressed the early capacitative transients of OHCs. GSNO (3 and 10 mM) had no effect on the whole cell currents of OHCs or Deiters' cells. Results show that GSNO suppresses cochlear function. This suppression may be due to an effect of GSNO on the cochlear amplifier. The actions of GSNO were different from those of other NO donors; therefore, the effects of GSNO may not be mediated by NO. The mechanisms underlying GSNO effects seem to be different from those of salicylate.

The involvement of nitric oxide (NO) in the CGRP-induced behavior of rats

The possible role of nitric oxide (NO) in CGRP-induced passive avoidance, active avoidance, and open field behavior was tested in rats. A specific NO synthase inhibitor, N omega-nitro-L-arginine (L-NA), was used to disrupt NO synthesis. ICV administration of 5 micrograms of L-NA reversed the action of CGRP in passive and active avoidance tests. In an open field, L-NA prevented the action of CGRP on locomotion and grooming. The inactive isomer D-NA had no effect on behavior of animals. The data suggest that NO might contribute to CGRP-induced behavior in rats.

Pancreatic polypeptide, microinjected into the dorsal vagal complex, potentiates glucose-stimulated insulin secretion in the rat

Specific binding sites for circulating pancreatic polypeptide (PP) have been found within the dorsal vagal complex (DVC) in the caudal medulla oblongata. Therefore, the effects of rat PP on pancreatic hormone secretion upon its microinjection into the DVC in halothane-anesthetized rats at doses of 0.4-40 pmol were investigated. At this range of doses, the changes in plasma concentrations of insulin, glucagon and glucose over basal levels did not differ from those after vehicle microinjection. In a separate series of experiments, vehicle and PP at doses of 0.4 and 4 pmol were microinjected into the right DVC 40 min after the continuous infusion of D-glucose had been started. In animals receiving continuous infusion of D-glucose, PP microinjected into the DVC (4 pmol), resulted in markedly higher insulin levels at corresponding time points compared to those with vehicle microinjected into the DVC. These data indicate, for the first time, that microinjection of PP into the DVC may potentiate glucose-stimulated insulin secretion in halothane-anesthetized rats.

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.