The role of the vagus nerve in the protective action of acid inhibitors on ethanol-induced gastric mucosal damage in rats

Gastric mucosal integrity: gastric mucosal blood flow and microcirculation. An overview

The stomach is in a state of continuous exposure to potentially hazardous agents. Hydrochloric acid together with pepsin constitutes a major and serious threat to the gastric mucosa. Reflux of alkaline duodenal contents containing bile and pancreatic enzymes are additional important injurious factors of endogenous origin. Alcohol, cigarette smoking, drugs and particularly aspirin and aspirin-like drugs, and steroids are among exogenous mucosal irritants that can inflict mucosal injury. The ability of the stomach to defend itself against these noxious agents has been ascribed to a number of factors constituting the gastric mucosal defense. These include mucus and bicarbonate secreted by surface epithelial cells, prostaglandins, sulfhydryl compounds and gastric mucosal blood flow. The latter is considered by several researchers to be of paramount importance in maintaining gastric mucosal integrity. The aim of this paper is to review the experimental and clinical data dealing with the role of mucosal blood flow and in particular the microcirculation in both damage and protection of the gastric mucosa.

H2-blocker modulates heart rate variability

The use of H2-blockers in the treatment of patients with peptic ulcer has become popular. However, this treatment has adverse cardiovascular effects. The aim of this study was to investigate proarrhythmic rhythm and autonomic nervous activity by analyzing heart rate variability in patients treated with omeprazole, ranitidine, and plaunotol. Nineteen patients (mean age 67.5 +/- 2.7 years) with active gastric ulcer were treated with omeprazole (20 mg/day) for 8 weeks, then ranitidine (300 mg/day) for the next 4 months, and finally plaunotol (240 mg/day). At each stage of the treatment, Holter electrocardiography was performed, and heart rate variability and arrhythmias analyzed. Heart rate variability yielded power in the low- (0.04-0.15 Hz) and high-frequency components (0.15-0.4 Hz). Although both ranitidine and omeprazole induced little change in cardiac rhythm, the high-frequency power was higher (10.3 +/- 0.8 vs 8.6 +/- 0.6 ms, P < 0.05) and the ratio of low-to-high frequency power was lower (1.41 +/-0.10 vs 1.59 +/- 0.09. P < 0.05) during ranitidine than during plaunotol treatment. Cosinor analysis of heart rate variability revealed a decreased amplitude of low-frequency power during omeprazole compared with during ranitidine and plaunotol treatment. Ranitidine modulated high-frequency power which may be related to the adverse cardiovascular effects of H2-blocker.

Intracisternal PYY increases gastric mucosal resistance: role of cholinergic, CGRP, and NO pathways

The influence of intracisternal injection of peptide YY (PYY) on gastric lesions induced by ethanol was studied in urethan-anesthetized rats. Gastric lesions covered 15-22% of the corpus as monitored 1 h after intragastric administration of 45% ethanol (5 ml/kg) in intracisternal vehicle control groups. PYY, at doses of 23, 47, or 117 pmol 30 min before ethanol, decreased gastric lesions by 27%, 63%, and 59%, respectively. Thyrotropin-releasing hormone (TRH) receptor antisense oligodeoxynucleotide pretreatment (intracisternally, 48 and 24 h before intracisternal PYY) did not influence the gastroprotective effect of intracisternal PYY (47 pmol) but abolished that of intracisternal TRH analog RX-77368 (4 pmol). RX-77368 (2.6 pmol) and PYY (6 pmol) were ineffective when injected intracisternally alone but reduced ethanol lesions by 44% when injected simultaneously. Atropine (subcutaneously), the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP-(8-37) (intravenously), or the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, intravenously) completely abolished the gastroprotective effect of intracisternal PYY (47 pmol), whereas indomethacin (intraperitoneally) had no effect. The L-NAME action was reversed by L-arginine but not by D-arginine (intravenously). These results suggest that intracisternal PYY acts independently of medullary TRH to decrease ethanol-induced gastric lesions. The PYY action involves vagal cholinergic-mediated CGRP/NO protective mechanisms.

Central and peripheral vagal mechanisms involved in gastric protection against ethanol injury

Activation of medullary thyrotropin-releasing hormone (TRH), at a dose subthreshold to increase gastric acid secretion, protects the gastric mucosa against ethanol injury through vagal cholinergic pathways in urethane-anaesthetized rats. Peripheral mediators involve the efferent function of capsaicin-sensitive splanchnic afferents leading to calcitonin gene-related peptide (CGRP)- and nitric oxide (NO)-dependent gastric vasodilatory mechanisms. In addition, gastric prostaglandins participate in gastric protection through mechanisms independent of the stimulation of gastric mucosal blood flow and mucus secretion. Medullary TRH has physiological relevance in the vagal-dependent adaptive gastric protection induced by mild (acid or ethanol), followed by strong, irritants. Additional neuropeptides, namely peptide YY (PYY), somatostatin analogues, CGRP and adrenomedullin, also act in the brainstem to induce a vagal-dependent gastric protection against ethanol through interactions with their specific receptors in the medulla. Central PYY and adrenomedullin act through vagal cholinergic prostaglandins and NO pathways, while somatostatin analogue acts through vagal non-adrenergic, non-cholinergic vasoactive intestinal peptide and NO mechanisms. Although their biological relevance is still to be established, these peptides provide additional tools to investigate the multiple vagal-dependent mechanisms which increase the resistance of the gastric mucosa to injury.

Protective role of vagal afferents in experimentally-induced colitis in rats

The aim of this study was to evaluate the regulatory role of vagal afferents in the development of colonic inflammation induced by trinitrobenzenesulfonic acid (TNBS) in rats. Groups of Wistar rats were treated with capsaicin or its vehicle applied perivagally (sham treatment). Colonic transit time was evaluated, and, two days later, one half of the animals received an intracolonic instillation of TNBS/ethanol (40 mg/kg), and the other received saline. Inflammation was evaluated functionally (gut permeability), biochemically (myeloperoxydase activity) and histologically. Vagal capsaicin deafferentation did not modify colonic transit time. In TNBS treated groups, inflammation was enhanced by capsaicin pretreatment, as determined by an increased gut permeability, MPO activity, and histological damage score. These results suggest that vagal afferents have a protective role in TNBS-induced colitis in rats, unrelated to changes in colonic transit time.

Can misoprostol and omeprazole reduce nicotine and ethanol induced gastric mucosal injury? A quantitative macroscopic and microscopic analysis in rats

We compared the effects of misoprostol, omeprazole and methylcellulose (control) on gastric mucosal injury induced by nicotine and/or ethanol. The results demonstrate that misoprostol and omeprazole each significantly reduce macroscopic injury and deep injury at a microscopic level (P < 0.05) induced by nicotine alone, ethanol alone or a combination of ethanol and nicotine. Misoprostol and omeprazole each reduced the leakage of fluorescein isothiocyanate-albumin into the interstitium in the gastric mucosa. Misoprostol and omeprazole are each effective in preventing injury induced by nicotine and ethanol and vascular factors are involved.

Role of dorsal motor nucleus of vagus in gastric function and mucosal damage induced by ethanol in rats

Experimental evidence indicates that the autonomic nervous system, especially the cholinergic pathway modulates the mucosal defensive mechanism and affects mucosal damage in the stomach. The present study investigated the role of the dorsal motor nucleus of vagus (DMV) in gastric function and its influences on ethanol-induced mucosal damage in pentobarbitone-anesthetized rats. Electrolytic lesion of the DMV as compared with sham operation and lesions of other brain areas, eg, nucleus reticular gigantocellularis and cuneate nucleus, reduced the basal gastric mucosal blood flow (GMBF) and also the blood flow after ethanol administration. The same operation did not affect the acid secretion either in the basal state or during the ethanol treatment period. Lesions at the caudal half of the DMV produced a bigger depression of GMBF when compared with lesion at the rostral half. In the sham-operated rats, ethanol induced severe hemorrhagic lesions in the gastric glandular mucosa, and this was significantly potentiated by lesions at the DMV, especially in the caudal half. The present findings indicate that acute DMV damage at the caudal half markedly affects the GMBF but not the acid secretion. The action on GMBF may contribute to the aggravation of ethanol-induced gastric damage in rats. These data reinforce the idea that the central vagal pathway, especially the caudal half of the DMV, plays a significant role in the modulation of GMBF, which in turn affects the integrity of gastric mucosal barrier.

Medullary thyrotropin-releasing hormone mediates vagal-dependent adaptive gastric protection induced by mild acid in rats

BACKGROUND & AIMS

Adaptive gastric protection is dependent on vagal pathways in rats. It is hypothesized that medullary thyrotropin-releasing hormone (TRH), known to regulate vagal function, is part of the brain mechanisms mediating adaptive gastric protection.

METHODS

Urethane-anesthetized rats were pretreated with either acute bilateral subdiaphragmatic vagotomy, sham operation, or intracisternal injection of purified control, TRH, or peptide YY antibody. Gastric lesions were assessed 75 minutes after orogastric administration of 1 mL of either vehicle or 0.35N HCl followed 15 minutes later by 0.6N or 1.0N HCl.

RESULTS

Injection of 0.6N and 1.0N HCl induced gastric lesions covering 23.1% +/- 2.7% and 37.8% +/- 3.3% of the corpus mucosa, respectively. Pretreatment with 0.35N HCl resulted in 67.3% and 50.5% reductions in gastric lesions induced by 0.6N and 1.0N HCl, respectively. Subdiaphragmatic vagotomy or intracisternal injection of TRH antibody increased gastric lesions induced by 0.6N HCl to 32.2% +/- 2.2% and 42.9% +/- 5.6%, respectively, and completely abolished the protective effect of 0.35N HCl pretreatment. Control or peptide YY antibody injected intracisternally did not alter the gastric protection induced by mild acid.

CONCLUSIONS

These results indicate that medullary TRH plays a role in the vagally mediated adaptive gastric protection induced by mild acid.

Intracisternal thyrotropin-releasing hormone-induced vagally mediated gastric protection against ethanol lesions: central and peripheral mechanisms

The vagus is involved in mediating gastric cytoprotection and adaptive cytoprotection. However, the central and peripheral mechanisms through which the vagus expresses its action are still poorly known. Medullary thyrotropin-releasing hormone (TRH) plays an important role in the vagal regulation of gastric function. The stable TRH analogue, RX 77368, micro-injected into the cisterna magna or the dorsal motor nucleus (DMN) of the vagus at a dose that did not influence gastric acid secretion prevented gastric injury induced by intragastric administration of 60% ethanol in conscious or urethane-anaesthetized rats. The cytoprotective action of TRH is mediated through vagal cholinergic release of prostaglandin E2 (PGE2). Prostaglandin E2 action is unrelated to changes in gastric mucosal blood flow (GMBF). In addition, other peripheral mechanisms involve calcitonin gene-related peptide (CGRP) contained in capsaicin sensitive afferent fibres and nitric oxide, both of which mediate the associated increase in GMBF induced by intracisternal injection of RX 77368. These data indicate that medullary TRH induces vagally mediated gastric protection against ethanol lesions. Its action is expressed through the muscarinic dependent release of PGE2 and nitric oxide, and efferent function of capsaicin-sensitive afferent fibres releasing CGRP.

The differential mechanisms of mild irritants on adaptive cytoprotection

The protective action of mild irritants has been established. However, the mechanisms as to how they antagonize the injurious action produced by the subsequent challenge with an ulcerogenic stimulus are still unclear. The present study examined the different protective mechanisms of an oral administration of the three mild irritants, 20% ethanol, 0.3 mol/L HCl or 5% NaCl against the gastric injurious actions of absolute ethanol in rats. In an attempt to clarify the pathways and mediators involved in the adaptive cytoprotection, [D-Pro2, D-Trp7,9]-substance P (substance P antagonist), Nw-nitro-L-arginine methyl ester (L-NAME), indomethacin, capsaicin, lidocaine, atropine or hexamethonium was given. The protective action of 20% ethanol but not the other two mild irritants, was antagonized by L-NAME, indomethacin and capsaicin, which are the inhibitors of nitric oxide (NO) and prostaglandins (PG) synthesis, and afferent sensory neuron blocker, respectively. Substance P antagonist, lidocaine or atropine given alone, prevented mucosal damage; however, only substance P antagonist enhanced the anti-lesion action of 20% ethanol, while atropine and lidocaine increased the protective effect of NaCl and HCl. The three mild irritants increased the residual gastric secretion. Only 20% ethanol and 5% NaCl but not 0.3% HCl significantly increased the basal adherent mucus and also attenuated the mucus depletion by absolute ethanol. It is concluded that the cytoprotective action of either ethanol or NaCl seems to be mediated through the increase of residual gastric secretion and adherent mucus. In the ethanol-treated group, these actions could act through the afferent sensory fibres, with NO and PG as the possible mediators.(ABSTRACT TRUNCATED AT 250 WORDS)

The vagus nerve and its non-cholinergic mechanism in the modulation of ethanol-induced gastric mucosal damage in rats

The role of the cholinergic pathway in the vagus nerve in modulating gastric lesion formation by ethanol was examined, using an ex-vivo stomach chamber preparation. Subdiaphragmatic vagotomy significantly increased the lesion areas but lowered acid secretion and gastric mucosal blood flow (GMBF). Atropine had no effect, whereas pirenzepine antagonized ethanol-induced mucosal damage. All three procedures showed similar potencies in depressing acid secretion, but only pirenzepine reversed the fall in the GMBF produced by ethanol. These differential effects of vagotomy, atropine and pirenzepine on gastric function suggest that the cholinergic component in the vagus nerve may not be important in the formation of ethanol-induced gastric damage. The persistent protective action as well as the restoration of ethanol-induced GMBF drop by pirenzepine in vagotomized animals further support this hypothesis. The worsening effect of vagotomy is probably modulated by a non-cholinergic mechanism, the abolition of which makes the gastric mucosa more susceptible to damage by ethanol. The acid-independent protective action of pirenzepine and its influence on the GMBF, which were not exhibited by atropine, are indeed unique and perhaps may be attributed to this non-cholinergic pathway.

Portal hypertension. Its effects on gastric function and ethanol-induced gastric mucosal damage in rats

The time-course effects of portal hypertension on gastric secretory function, mucosal blood flow, vascular permeability, and ethanol-induced gastric mucosal damage were examined in anesthetized rats. Partial ligation of the portal vein effectively produced portal hypertension one to three days later but the raised pressure returned to normal on the sixth day after ligation. This time-course effect coincided with reduced pepsin secretion and mucosal blood flow and also with potentiated ethanol-induced mucosal damage during the first to third days. These effects started to tail off on the sixth day. However, gastric acid output was significantly reduced on the third day, and this was strongest on the sixth day after operation. Portal vein ligation also reduced basal vascular permeability, which was markedly potentiated after ethanol treatment. It is concluded that: (1) portal vein blood pressure changes are a time-dependent process following ligation; (2) changes in gastric mucosal blood flow (GMBF) and lesion formation are closely related to portal hypertension; (3) gastric mucosal injury is associated with vascular damage, as evidenced by increased in vascular permeability; and (4) pepsin but not acid secretion is closely related to the state of the GMBF.