Tomohisa Ohuchi

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.

The role of capsaicin-sensitive sensory neurons in healing of HCl-lnduced gastric mucosal lesions in rats

BACKGROUND/AIMS Capsaicin-sensitive sensory neurons are important in protecting the stomach against damage. This study investigated the role of these sensory neurons in the healing of gastric lesions. METHODS Rats fasted for 18 hours were given 1 mL of 0.6N HCl orally for induction of gastric lesions; they were then fed normally from 1 hour later. On various days after HCl treatment, the area of lesion, acid secretion, mucosal DNA synthesis, mucosal H+ permeability, and blood flow responses were measured. RESULTS Ablation of sensory neurons by capsaicin pretreatment did not affect the development of gastric lesions in response to HCl but significantly delayed the healing of these lesions. The damaged mucosa secreted less acid and had minimal changes in DNA synthesis but showed a marked increase in H+ permeability, resulting in luminal acid loss accompanied by an increase of mucosal blood flow. Sensory deafferentation did not have any influence on such changes in the damaged stomach, except the mucosal hyperemic response to acid. CONCLUSIONS Capsaicin-sensitive sensory neurons may contribute to the healing of gastric lesions by mediating the mucosal hyperemic responses associated with acid back-diffusion and by facilitating acid disposal in the mucosa.

Induction of gastric lesions and hypoglycemic response by food deprivation in streptozotocin-diabetic rats

Overnight fasting causes hemorrhagic lesions in the stomach of streptozotocin (STZ)-induced diabetic rats, but the pathogenetic mechanism remains unknown. The present study was performed to investigate the pathogenesis of such lesions developed in STZ-diabetic rats after starvation, mainly in relation to blood glucose changes. A single injection of STZ (70 mg/kg, intraperitoneally) induced hyperglycemic conditions one week after the administration, and high blood glucose levels (BGL: >350 mg%) remained up to three weeks later. The STZ-diabetic rats developed gastric lesions with the marked reduction of BGL after 18 hr of fasting, depending upon the duration of diabetes; the lesion score and ΔBGL reduction in the 3-week-old STZ rats were 32.0±7.8 mm and >250 mg/100 ml, respectively. Acid secretion in the pylorus-ligated rats was not significantly changed in the STZ-induced diabetic conditions for the initial two weeks but slightly decreased at three weeks when compared with normal rats. Fasting of normal rats for 18 hr did not cause either BGL reduction or any lesion in the stomach. In the 3-week-old STZ animals, the severity of gastric lesions increased with the duration of fasting (4–18 hr) and was again closely associated with the degree of ΔBGL reduction. These lesions induced by 18 hr of starvation in 3-week-old STZ rats were significantly inhibited by pretreatment with insulin (4 units/rat/day) for the last one week to maintain BGL within normal ranges or by intravenous infusion of 25% glucose during fasting period. Both of these treatments significantly prevented BGL reduction in response to fasting. These results suggest that gastric lesions induced in STZ-diabetic rats by fasting are insulin-sensitive and may be associated with a profound hypoglycemic response to food deprivation.

Diethyldithiocarbamate, a Superoxide Dismutase Inhibitor, Reduces Indomethacin-lnduced Gastric Lesions in Rats

We examined the effect of diethyldithiocarbamate (DDC), the superoxide dismutase (SOD) inhibitor, on the development of gastric lesions induced by indomethacin in rats. Indomethacin (25 mg/kg) was given subcutaneously, and gastric acid secretion, motility, lipid peroxidation, vascular permeability, and myeloperoxidase as well as gastric lesions were measured. Indomethacin produced high-amplitude contractions of the stomach and caused hemorrhagic lesions in the corpus mucosa with significant increase in neutrophil-related processes such as myeloperoxidase activity, vascular permeability and lipid peroxidation. These changes caused by indomethacin were all significantly inhibited by prior administration of atropine (3 mg/kg s.c.). Pretreatment of the animals with DDC (75-1,000 mg/kg s.c.) prevented these lesions induced by indomethacin in the corpus mucosa in a dose-dependent manner (> 100 mg/kg), though at high doses (> 750 mg/kg) some damage was found in both the antrum and duodenum. DDC showed a significant inhibition against the gastric mucosal SOD activity (> 400 mg/kg), yet potently suppressed the increase of lipid peroxidation, vascular permeability, and myeloperoxidase activity caused by indomethacin. DDC dose-dependently (> 75 mg/kg) inhibited the enhancement of gastric motility caused by indomethacin and showed a weak antisecretory effect at high doses (> 750 mg/kg). These results showed that DDC reduced indomethacin-induced gastric lesions by suppressing gastric motility, despite inhibiting SOD activity. This study also indicates the prime importance of gastric hypercontraction in the pathogenesis of this lesion model and suggests that other events including the neutrophil-related processes may be secondary to gastric hypercontraction caused by indomethacin.

Effects of nitric oxide synthase inhibitors on duodenal alkaline secretion in anesthetized rats

We examined the effects of NG-nitro-L-arginine methyl ester (L-NAME), the nitric oxide (NO) synthase inhibitor, on duodenal HCO3- secretion in anesthetized rats. L-NAME (1-5 mg/kg i.v.), given as a single injection, increased HCO3- secretion in a dose-dependent manner. This effect of L-NAME was mimicked by NG-monomethyl-L-arginine (50 mg/kg i.v.) and was significantly antagonized by the simultaneous administration of L-arginine (200 mg/kg i.v.) but not D-arginine. The increased HCO3- response to L-NAME was also significantly reduced in vagotomized animals. These findings suggest that the inhibition of NO biosynthesis leads to an increase of duodenal HCO3- secretion, partly mediated by the vagus nerves.

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 withl-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 byl-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 ofl-arginine (200 mg/kg, intravenously) but notd-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 inhibitorl-NAME increases duodenal HCO3− secretion and protects the duodenal mucosa against acid injury.

Irritant and protective action of urea-urease ammonia in rat gastric mucosa : different effects of ammonia and ammonium ion

The effects of urea-urease-ammonia on the rat gastric mucosa were examined and compared with those of NH4OH and NH4Cl. The mucosal application of urea with urease produced a reduction in potential difference (PD) in a dose-related manner for urea, and a significant drop was observed by >0.1% urea in the presence of 100 units urease. Such PD reduction was also observed when the mucosa was exposed to either NH4OH (>0.03%) or NH4Cl (>1%); ΔPD (20 mV) caused by 0.3% NH4OH and 3% NH4Cl was equivalent to that induced by 0.5% urea + urease (100 units). The combined oral administration of urea (∼6%) and urease (100 units) did not induce any macroscopic damage in the gastric mucosa. NH4Cl given orally had no or little effect on the mucosa at any dose levels even at 10%, while NH4OH given orally caused hemorrhagic lesions in the mucosa at the dose of >0.3%. In contrast, both urea + urease and NH4Cl given prior to HCl/ethanol protected the gastric mucosa against damage in a dose-related manner, and a significant effect was obtained by urea at >0.5% and by NH4Cl at >1%. NH4OH was also effective in reducing the severity of HCl/ethanol-induced gastric lesions at lower dose (0.3%). The protective effect of urea + urease was attenuated significantly by prior administration of indomethacin or coadministration of hydroxyurea, while that of NH4Cl or NH4OH was mitigated by indomethacin. These results suggest that the urea-urease-ammonia system in the stomach may be beneficial in normal conditions; ammonia produced by the hydrolysis of urea yields NH4OH and NH4Cl in the lumen, both of which act on the stomach as mild irritant, resulting in adaptive cytoprotection mediated by endogeneous prostaglandins.

Influences of urethane anesthesia on indomethacin-induced gastric mucosal lesions in rats : relation to blood glucose levels

Effects of urethane on gastric motility and mucosal ulcerogenic responses induced by indomethacin were investigated in the rat in relation to blood glucose levels (BGL) and compared with those of pentobarbital Na. Urethane (1.25 g/kg) given intraperitoneally, caused a progressive and significant rise in BGL, while pentobarbital (30 mg/kg) given intraperitoneally did not affect BGL. Subcutaneous administration of indomethacin (25 mg/kg) caused high-amplitude gastric contractions and induced hemorrhagic lesions in the stomachs of conscious rats. These lesions were significantly inhibited by urethane but not pentobarbital. Administration of urethane abolished basal gastric motility and almost completely suppressed the motility responses induced by indomethacin, while pentobarbital did not have much effect on gastric motility under basal and indomethacin-stimulated conditions. Acid secretion was significantly decreased by urethane and increased by pentobarbital. Pretreatment of the animals with yohimbine (5 mg/kg, subcutaneously) but not prazosin (0.5 mg/kg) inhibited the elevation in BGL seen after administration of urethane and allowed resumption both gastric motility and ulcerogenic responses induced by indomethacin, with less change in acid secretion. These results suggest that intraperitoneal administration of urethane prevented indomethacin-induced gastric lesions, probably by inhibiting the enhanced gastric motility response, and this effect may relate to its hyperglycemic action mediated by α2-adrenoceptors. These findings also provide further evidence to support the importance of gastric motility in the pathogenesis of these lesions.

Capsaicin-sensitive sensory neurons in healing gastric lesions induced by HCl in rats

The role of capsaicin-sensitive sensory neurons in the healing of HCl-induced gastric lesions was investigated in rats. Rats fasted for 18 hr were given 0.6 N HCl orally for induction of gastric lesions, and they were fed normally from 1 hr later. On various days after HCl treatment, area of lesions, acid secretion, mucosal H+ permeability, and blood flow responses were measured. Functional ablation of capsaicin-sensitive sensory neurons was performed two weeks before the experiment by subcutaneous injections of high-dose capsaicin. Sensory deafferentation did not affect the development of gastric damage in response to HCl but significantly delayed the healing of these lesions. The mucosa damaged by HCl secreted less acid but showed significant rise in H+ permeability, resulting in acid back-diffusion accompanied by an increase of mucosal blood flow. Sensory deafferentation had no effect on acid secretion and mucosal permeability changes in the damaged stomach but completely blocked the hyperemic response caused by acid back-diffusion. Capsaicin-sensitive sensory neurons may contribute to healing of gastric lesions, probably by mediating the mucosal hyperemic responses associated with acid back-diffusion and by facilitating acid disposal in the mucosa.

Regulation of gastroduodenal bicarbonate secretion by capsaicin-sensitive sensory neurons in rats.

We investigated the role of capsaicin-sensitive sensory neurons in regulation of gastroduodenal HCO3- secretion in anesthetized rats. The stomach (under acid inhibition by omeprazole 60 mg/kg i.p.) or the duodenum was perfused with saline (pH 4.5) and HCO3- output was determined by pH change in the perfusate. Both the duodenum and stomach responded to prostaglandin E2 (PGE2; 300 micrograms/kg i.v.) or luminal acid by a significant increase in pH and HCO3- output. These tissues also responded to luminal application of capsaicin (0.3-6 mg/ml for 30 min), resulting in a significant increase of pH and HCO3- output in a concentration-related manner. The HCO3- stimulatory action of capsaicin was markedly attenuated by functional ablation of capsaicin-sensitive sensory neurons, significantly mitigated by indomethacin, and exhibited tachyphylaxis after repeated application at a high concentration. The acid-induced pH and HCO3- responses were also significantly mitigated by sensory deafferentation and by indomethacin, whereas those induced by PGE2 remained unaffected. In addition, defunctionalization of these sensory nerves resulted in macroscopically visible damage in the duodenum when acid secretion was concomitantly stimulated by histamine. We conclude that capsaicin-sensitive sensory neurons may be involved in the regulatory mechanism of gastroduodenal HCO3- secretion and contribute to protection of the mucosa against acid. Endogenous PGs may be involved in the HCO3- stimulatory action mediated by capsaicin-sensitive sensory neurons.