Protective effect of taurine against ammonia-induced gastric mucosal lesions in rats

Role of taurine on acid secretion in the rat stomach

Background

Taurine has chemical structure similar to an inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Previous studies on GABA in the stomach suggest GABAergic neuron is involved in acid secretion, but the effects of taurine are poor understood.

Methods

The effects of taurine on acid secretion, signal transduction, and localization of taurinergic neurons were determined in the rat stomach using everted whole stomach, RIA kit and immunohistochemical methods.

Results

We used antibodies against taurine-synthesizing enzyme, cysteine sulfuric acid decarboxylase (CSAD), and taurine. CSAD- and taurine-positive cells were found in the muscle and mucosal layers. Distributions of CSAD- and taurine-positive cells in both mucosal and muscle layers were heterogeneous in the stomach. Taurine at 10^-9~10^-4 M induced acid secretion, and the maximum secretion was at 10^-5 M, 1.6-fold higher than the spontaneous secretion. Taurine-induced acid secretion was completely inhibited by bicuculline and atropine but not by cimetidine, proglumide, or strychnine. Atropine and tetrodotoxin (TTX) completely inhibited the acid secretion induced by low concentrations of taurine and partially inhibited induced by high concentrations. Verapamil, a calcium blocker agent, inhibited acid output elicited by taurine. We assumed all Ca²⁺ channels involved in the response to these secretagogues were equally affected by verapamil. Intracellular cAMP (adenosine 3', 5'-monophosphat) in the stomach significantly increased with taurine treatment in a dose-dependent manner. High correlation (r=0.859, p < 0.001) of taurine concentrations with cAMP was observed.

Conclusions

Our results demonstrated for the first time in taurine-induced acid secretion due to increase intracellular calcium may act through the A type of GABA receptors, which are mainly located on cholinergic neurons though cAMP pathway and partially on nonneuronal cells in the rat stomach.

Pharmacological profile of novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methyl cyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526)

The pharmacological profiles of the novel acid pump antagonist 7-(4-fluorobenzyloxy)-2,3-dimethyl-1-{[(1S,2S)-2-methylcyclopropyl]methyl}-1H-pyrrolo[2,3-d]pyridazine (CS-526) were investigated in terms of hog gastric H+,K+-ATPase activity, gastric acid secretion, and acute gastroesophageal lesions in comparison with other proton pump inhibitors (PPIs). CS-526 inhibited H+,K+-ATPase activity in a concentration-dependent manner, with an IC50 value of 61 nM. The inhibitory effect of CS-526 on H+,K+-ATPase activity was more potent than that of any of the other PPIs examined. The inhibitory mechanism of CS-526 on H+,K+-ATPase was a competitive antagonism to the K+ binding site of H+,K+-ATPase, and it was also a reversible inhibition. In pylorus-ligated rats, intraduodenal or oral administration of CS-526 inhibited gastric acid secretion in a dose-dependent manner, and the ID50 values were 2.8 or 0.7 mg/kg, respectively. In Heidenhain pouch dogs, intrapouch administration of CS-526 inhibited histamine-stimulated gastric acid secretion in a dose- and retention time-dependent manner. In a reflux esophagitis model, intraduodenal and oral administration of CS-526 prevented esophageal lesions with ID50 values of 5.4 and 1.9 mg/kg, respectively. Lansoprazole prevented esophagitis only by intraduodenal administration (ID50 = 2.2 mg/kg). Furthermore, CS-526 inhibited acute gastric mucosal lesions. These data demonstrate that the novel acid pump antagonist CS-526 has potent antisecretory and antiulcer effects. These findings indicate that CS-526 would have a curative effect on gastroesophageal reflux disease via its potent antisecretory and antiulcer actions.

Effects of misoprostol and taurine on monochloramine ulcerogenesis in rats

We compared gastroprotective characteristics of synthetic prostaglandin E1 misoprostol and amino acid taurine on rat model of monochloramine injury to the gastric mucosa. Both substances exhibited a pronounced gastroprotective effect.

Suppression of Helicobacter pylori-induced interleukin-8 production in gastric cancer cell lines by an anti-ulcer drug, geranylgeranylacetone

BACKGROUND

Geranylgeranylacetone (GGA) is an antigastritis and anti-ulcer agent, with as yet an unknown mechanism of action. In this study, we investigated the effect of GGA on Helicobacter pylori-induced interleukin (IL)-8 production and IL-8 mRNA expression in KATOIII cells, an established gastric cell line.

METHODS

Interleukin-8 production in H. pylori-infected KATOIII cells was measured by using enzyme-linked immunoassay. The cytotoxicity of H. pylori on KATOIII cells was measured by a 51Cr release assay. The effect of GGA on H. pylori-induced IL-8 mRNA expression was measured by using northern blotting.

RESULTS

Interleukin-8 production increased with time and H. pylori dose; the most significant increase was seen within 6-24 h of coculture with H. pylori. A dose of 0.1 mmol GGA suppressed IL-8 production (P = 0.0077) and inhibited H. pylori-induced IL-8 mRNA expression (P = 0.0019). Furthermore, H. pylori-induced gastric mucosal cell injury associated with IL-8 and neutrophil activation was enhanced by NH3, and this enhancement was suppressed by GGA (P = 0.0043).

CONCLUSIONS

Helicobacter pylori-infected gastric mucosal cells produce IL-8, which can promote neutrophil activation, thus contributing to mucosal tissue injury associated with H. pylori infection. Agents like GGA, which can suppress IL-8 production may have a protective role in the treatment of mucosal tissue damage seen in H. pylori infection.

Effect of taurine on ulcerogenic response and impaired ulcer healing induced by monochloramine in rat stomachs

BACKGROUND

It is well known that neutrophil-derived hypochlorous acid interacts with ammonia (NH4OH) to generate monochloramine (NH2Cl) and that NH2Cl irritates the gastric mucosa and impairs ulcer healing.

AIM

To examine the effect of taurine, a hypochlorous acid scavenger, on the mucosal ulcerogenic and the impaired healing response induced by NH2Cl in rat stomachs, in comparison with those of methionine and glycine.

METHODS AND RESULTS

Under anaesthesia, oral administration of NH2Cl (120 mmol/L) produced severe lesions in male Sprague-Dawley rat stomachs. Taurine (10-100 mg/kg) given p.o. 30 min prior to NH2Cl dose-dependently prevented these lesions in response to NH2Cl. This action was mimicked by methionine (3-30 mg/kg) but not by glycine (10-100 mg/kg). Under urethane anaesthesia, mucosal exposure to NH4OH (120 mmol/L) caused a marked reduction of potential difference (PD) in the ex vivo chambered stomachs after induction of ischaemia, resulting in severe lesions. These ulcerogenic and PD responses by NH4OH plus ischaemia were also mitigated by taurine and methionine, but not glycine, applied to the chamber 20 min before the onset of NH4OH plus ischaemia. Moreover, oral administration of 100% ethanol produced severe haemorrhagic lesions in rat stomachs, all of which rapidly healed within 7 days after lesion induction. Daily administration of NH2Cl (20 mmol/L) significantly delayed the healing of these lesions, but recovery of this impaired healing response was obtained by concurrent administration of taurine. Both taurine and methionine showed a potent scavenging effect against NH2Cl in vitro.

CONCLUSIONS

(1) NH2Cl generated either exogenously or endogenously damages the gastric mucosa and impairs the healing response; (2) taurine exerts a prophylactic effect against the deleterious effects of NH2Cl, mainly due to its scavenging action against NH2Cl; and (3) this effect of taurine may be useful for treatment of gastritis associated with Helicobacter pylori infection.

Protective role of vitamin E, 2-deoxy-D-glucose, and taurine on perchloroethylene induced alterations in ATPases

Perchloroethylene (PER) administered by oral gavage for 15 consecutive days, at a dose of 3000 mg/kg body wt. decreased the activities of Na+, K(+)-ATPase and Mg(2+)-ATPase with an increase in the activity of Ca(2+)-ATPase. It also decreased RBC and platelet counts but the WBC count was found to be increased. An investigation of the relative importance of the modulators, vitamin E, 2-deoxy-D-glucose (2DG) and taurine in rendering protection to tissues against PER induced membrane damage was performed. PER administered mice were subjected to vitamin E (400 mg/kg body wt/day), 2DG (500 mg/kg body wt/day by i.p.) and taurine (100 mg/kg body wt/day) administration for 15 days to study their individual effect on ATPase and on certain hematological parameters. Vitamin E, 2DG and taurine treated mice showed a marked reversal of these metabolic changes related to membrane damage caused by PER. These results suggest that PER induced membrane damage may be associated with energy metabolism and hemolysis, which can be effectively prevented by these modulators.

Attenuation by methionine of monochloramine-enhanced gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in Wistar rats

Helicobacter pylori appears to play a major role in the development of gastric cancer in humans. The mechanism behind the carcinogenic or co-carcinogenic effects of H. pylori has not been established. Ammonia, generated by urea from H. pylori, has been studied as a possible cause. However, the ammonia-monochloramine system has been shown to play a more important role in H. pylori-associated mucosal injury. Therefore, the effects of combined administration of monochloramine and methionine, singly or together, on the development of gastric cancers induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were investigated in inbred Wistar rats. After receiving oral MNNG and regular chow pellet for 25 weeks, rats received regular chow pellets or chow pellets containing 20% ammonium acetate, and normal tap water or water containing 30 mM sodium hypochlorite, with or without a subcutaneous injection of methionine, until the end of the experiment (week 52). Treatment with both ammonium acetate and sodium hypochlorite, which produce monochloramine, significantly increased the incidence of gastric cancers in week 52, whereas the concomitant administration of methionine with ammonium acetate and sodium hypochlorite significantly attenuated such enhanced gastric carcinogenesis. Spectrophotometric examination revealed that methionine scavenged monochloramine. Our findings suggest that H. pylori-associated gastric carcinogenesis may be mediated by monochloramine.

Helicobacter pylori factors involved in the development of gastroduodenal mucosal damage and ulceration

Many putative virulence determinants of Helicobacter pylori are believed to trigger and worsen the gastroduodenal mucosa damage observed in infected patients. H. pylori urease reacts with the gastric urea and generates ammonia; ammonia combines with water and yields ammonium hydroxide, which is cytotoxic. Ammonia may also inhibit cell proliferation and cause indirect mucosal injury by stimulating neutrophils. Phospholipases may damage the gastric mucosa by degrading phospholipids and generating precursors of ulcerogenic components. Other enzymes, such as protease, neuraminidase, fucosidase, and alcohol dehydrogenase, can contribute to damage of the gastric epithelium by destroying the integrity of mucus or by inducing lipid peroxidation. Infection by vacuolating cytotoxic (VacA+) H. pylori strains is considered to constitute increased risk for development of peptic ulcer and gastric cancer. Exploration of the vacA gene structure has shown the existence of strongly toxigenic strains, and has confirmed at the molecular level the increased ulcerogenic potential of VacA+ H. pylori strains. A pathogenicity island called cag has been recently described in Type 1 H. pylori strains (VacA+/CagA+).cag contains the cagA gene (whose expression is associated with toxigenicity) and many genes, some of which are highly homologous to virulence genes of other virulent bacteria, that account for the enhanced pathogenic potential of CagA+ organisms.

Relationship between interleukin-8 levels and myeloperoxidase activity in human gastric mucosa

Although interleukin (IL)-8 is well known as a chemotactic agent for neutrophil migration in vitro, the relationship between IL-8 activity and the degree of neutrophil infiltration in gastric mucosa is still unclear. In the present study, we investigated IL-8 and myeloperoxidase activity, a marker of neutrophil infiltration, in gastric antral mucosa using biopsy samples in 23 patients with no gastric lesions. The results indicate that there is a good correlation between IL-8 and myeloperoxidase activity (y = 0.173x + 13.9; r = 0.49, P < 0.01). Furthermore, IL-8 and myeloperoxidase activity are significantly higher in Helicobacter pylori-positive patients than in H. pylori-negative patients. In conclusion, an increase of IL-8 activity in the gastric mucosa causes increased neutrophil infiltration in human gastric mucosa and H. pylori infection accelerates these reactions in the mucosa.

Mechanism of Helicobacter pylori-associated gastric mucosal injury

Helicobacter pylori infection is associated with gastric mucosal damage and the infiltration of neutrophils. Myeloperoxidase from neutrophils produces hypochlorous acid, which yields monochloramine in the presence of ammonia produced by urease enzyme of Helicobacter pylori. The target cells of gastric mucosal damage are gastric mucosal cells and endothelial cells. We therefore tested the hypothesis that ammonium, hypochlorous acid, and monochloramine damage the target cells. We studied the in vitro cytotoxic effects of ammonium chloride, sodium hypochlorite, monochloramine, and activated neutrophils on the target cells. Cytotoxicity was measured by a 51Cr-release assay. Ammonium chloride, sodium hypochlorite, and monochloramine were toxic to labeled cells in a concentration dependent manner. The toxicity of these agents was in the order monochloramine > sodium hypochlorite >> ammonium chloride. Incubation of labeled cells with activated neutrophils, Helicobacter pylori, and urea resulted in cytolysis. These cytotoxicities were significantly inhibited by the scavenger of hypochlorous acid, taurine. Monochloramine is more toxic to the target cells than ammonium chloride. Although ammonium chloride at neutral pH by itself has little direct damaging effect on the gastric mucosa, it is damaging to the gastric mucosa through a reaction with hypochlorous acid, suggesting that it plays a role in Helicobacter pylori-associated gastric damage.

Products of neutrophil metabolism increase ammonia-induced gastric mucosal damage

Recent studies have indicated that ammonia is involved in the pathophysiology of Helicobacter pylori-associated gastric mucosal damage. Helicobacter pylori-associated chronic active gastritis is characterized by an invasion of neutrophils. We investigated the interrelationship among hypochlorous acid (oxidant produced by neutrophil), ammonia (product of Helicobacter pylori urease), and monochloramine (product of ammonia and hypochlorous acid) in the development of gastric mucosal damage in rats. Gastric mucosal lesions were produced by exposure of the gastric mucosa to ammonia, urea with urease, or urea with Helicobacter pylori in rats subjected to ischemia. Pretreatment with taurine (scavenger of hypochlorous acid) or antineutrophil serum significantly attenuated gastric mucosal lesions induced by the above test agents. Ammonia-induced gastric mucosal lesions were exacerbated in the presence of hypochlorous acid with concomitant generation of monochloramine. These results suggest that the ammonia, hypochlorous acid, and monochloramine triad may be important in Helicobacter pylori-mediated gastric mucosal damage.