Histamine H1- and H2-receptor-mediated gastric microcirculatory effects in the aetiology of stress ulceration in the rat stomach

The role of endogenous ulcerogenic mediators in the pathogenesis of peptic ulcer

The pathogenetic mechanisms of different types of peptic ulcer are still unclear. Extensive investigations have been focused on the identification of potential endogenous ulcerogenic mediators in animals and men. These studies are important in the development of a better therapeutic agent, either to prevent or to cure peptic ulcer in humans. Several endogenous substances have been identified and are reported to be involved in the production of gastrointestinal lesions in animals. Also these substances were increased during inflammatory responses and their actions were reported as vascular dependent and possibly free radicals related. Reports related to the types of these mediators have been extensive. The more important ones include some of the lipid metabolites, neuropeptides, biogenic amines, and also Helicobacter pylori and reactive free radicals. The present study summarizes the ulcerogenic mechanisms of these substances and the types of ulcer involved. More current information may enable us to understand better the etiology of peptic ulcer and possibly its prevention and cure in man. Any particular types of ulceration will not be specifically discussed in this article, because they have been extensively studied and reviewed.

The pharmacological differences and similarities between stress- and ethanol-induced gastric mucosal damage

Stress- and ethanol-induced gastric mucosal damage are the two commonly used ulcer models in animals. They share some of the similarities but also have differences in the etiology of gastric ulceration. This article reviews the influences of various protective drugs on these two types of gastric damage in rats. Verapamil (a calcium antagonist) or N-ethylmaleimide (a sulfhydryl depletor) prevents cold restraint-, but potentiates ethanol-provoked gastric lesion formation. N-Acetylcysteine (a mucolytic agent) and acetaminophen (an antipyretic analgesic) have the opposite actions. Prostaglandins provide a much better antiulcer effect on ethanol-induced lesions. Cimetidine (a histamine H2-receptor antagonist) prevents only stress-induced mucosal damage. These differences in drug actions indicate that stress and ethanol may have dissimilar ulcerogenic mechanisms in rats. On the other hand, carbenoxolone (a mucus inducer), histamine H1-receptor antagonists, leukotriene inhibitors (FPL 55712 and nordihydroguaiaretic acid) and mast cell stabilizers (like zinc compounds, sodium cromoglycate, FPL 52694 and ketotifen), all protect against gastric mucosal damage by stress or ethanol in rats. However, the role of gastric sulfhydryls in both types of gastric lesions is still controversial. These findings imply that the two types of lesion formation share some of the ulcerogenic mechanisms. This communication attempts to analyze the various findings and to relate them to the etiology of stress and ethanol-induced gastric lesions. It also summarizes the uses, and the antiulcer mechanisms, of the drugs that have been studied utilizing these two animal ulcer models, and suggests their possible implications in man.

Paracetamol potentiates stress-induced gastric ulceration in rats

The effect of paracetamol on gastric ulcers produced by restraint at 4 degrees C for 2 h (stress) was studied in rats. Paracetamol treatment s.c. or p.o., with a dose as high as 250 mg kg-1, did not produce any haemorrhagic lesions in the glandular mucosa. Oral administration with 250 mg kg-1, however, significantly reduced the mast cell count in the gastric glandular mucosa and potentiated haemorrhagic ulceration but not mast cell degranulation caused by stress. The potentiating action was maximum when paracetamol was given between 15 and 30 min before stress. Ranitidine, astemizole, dimethylsulphoxide, sucralfate and verapamil did not protect against the adverse action of paracetamol on stress-evoked lesions. This study suggests that paracetamol worsens stress-induced stomach ulceration by an action which appears not to be due to histamine release, free radical production or intracellular calcium disturbance in the gastric mucosa.

Antiulcer activity of the calcium antagonist propyl-methylenedioxyindene--II. Effects on acid secretion and gastric emptying in rats

1. Propyl-methylenedioxyindene (pr-MDI) is an intracellularly acting calcium antagonist which protects against cold/restraint-induced stress ulcers in rats. The doses of pr-MDI which produce antiulcer activity (10-30 mg/kg i.p.) are significantly lower than those which exhibit cardiovascular effects. 2. Two potential mechanisms for the antiulcer action of pr-MDI were investigated in this study: the effects on hydrochloric acid secretion and on gastric motility (gastric emptying). 3. Bethanechol-induced hydrochloric acid secretion in acutely pylorus-ligated rats was significantly obtunded by pr-MDI (30 mg/kg i.p.), but the effect was significantly weaker than that produced by verapamil (16 mg/kg i.p.) or cimetidine (10 mg/kg i.p.). Since 30 mg/kg pr-MDI produces greater antiulcer activity than the very high dose of 16 mg/kg verapamil, it is unlikely that inhibition of acid secretion plays more than a contributory role in the antiulcer mechanism of action of pr-MDI. 4. pr-MDI (10-30 mg/kg i.p.) produced a dose-dependent slowing of gastric emptying in rats fed a methylcellulose/Phenol Red test meal, and this effect correlated well with the antiulcer action. Verapamil (16 mg/kg i.p.) did not affect gastric emptying. 5. The results indicate that a reduction of gastric motility plays a major role in the mechanism of the antiulcer action of pr-MDI.

Antiulcer activity of the calcium antagonist propyl-methylenedioxyindene--I. Effect on cold/restraint stress-induced ulcers in rats

1. Propyl-methylenedioxyindene (pr-MDI) is an intracellularly acting calcium antagonist with H2-receptor blocking properties. Stimulus-secretion coupling is inhibited by much lower concentrations of pr-MDI than is excitation-contraction coupling. 2. Since the processes leading to gastric ulceration are calcium-dependent, the aim of this study was to determine if pr-MDI could provide useful antiulcer activity at doses below those required to produce cardiovascular effects. 3. The antiulcer activity of pr-MDI (10-30 mg/kg) was examined in the cold (4 degree C)/restraint (3 hr) stress-induced ulcer model in male rats, and compared with the effects of the H2-blocker cimetidine (10-30 mg/kg) and the calcium channel blocker verapamil (11-32 mg/kg). The drugs were administered intraperitoneally 10 min prior to the cold/restraint stress. 4. All three drugs significantly reduced the number of ulcers and the cumulative length of ulcerated stomach surface in a roughly dose-dependent and equivalent manner. However, whereas the antiulcer doses of verapamil were extremely high, those of pr-MDI were one-sixth to one-half of its antiarrhythmic ED50 in rodents.

Does acidosis contribute to stress-induced ulceration in rat stomachs?

The present study examines the involvement of acidosis in stress ulceration in rat stomachs. Cold restraint stress for 2 hr did not affect the blood lactate level; however, it produced respiratory acidosis, as reflected by the depressed respiratory rate which was associated with increased CO2 tension and a lowered blood pH. Severe hemorrhagic ulceration was found in the glandular mucosa. The effects of stress on blood pH and the stomach were reversed by IV infusion of NaHCO3. Infusion of HCl IV decreased the blood pH and HCO-3 level and produced gastric ulceration. It is concluded that respiratory acidosis could be involved in stress ulceration. The metabolic acidosis evoked by HCl also induced gastric damage, but the effect was much less.

The influence of omeprazole on the protective effects of secretagogues against ethanol-induced gastric ulceration in rats

The influence of omeprazole on the antiulcer effects of histamine, methacholine and pentagastrin on ethanol-induced gastric ulceration was studied. Histamine 5 mg/kg, methacholine 500 micrograms/kg or pentagastrin 100 micrograms/kg pretreatment s.c. markedly reduced ethanol (50%, 10 ml/kg p.o.)-induced gastric ulceration in pylorus-ligated conscious rats, but significantly elevated the gastric secretory volume and acid output. Omeprazole pretreatment 50 mg/kg s.c. reduced gastric secretion and abolished the ulcer-protecting effects of the three secretagogues. The same doses of histamine, methacholine or pentagastrin did not significantly alter the gastric secretory volume but increased the total acid output of ex-vivo stomach chamber preparations in anaesthetised animals. Histamine worsened ethanol-evoked ulceration whereas methacholine and pentagastrin had no effect on the lesions. Omeprazole pretreatment did not prevent ulcer aggravation by histamine. It is concluded that the antiulcer effects of the three secretagogues in conscious animals are probably due to the increased secretory volume which lessens the ulcerogenic action of ethanol by its dilution. Omeprazole reduces the secretory volume and thus prevents the antiulcer effect of these secretagogues. The findings with histamine and omeprazole in the ex-vivo stomach experiments support the suggestion that it is unlikely that increased gastric acid secretion affects the ulcerogenicity of ethanol in rat stomach.

Early microcirculatory stasis in acute gastric mucosal injury in the rat and prevention by 16,16-dimethyl prostaglandin E2 or sodium thiosulfate

We used in vivo microscopy and laser-Doppler velocimetry to examine the effects on the gastric mucosal microcirculation and in gastric mucosal blood flow of agents that induce acute gastric mucosal damage. In vivo microscopic observation of superficial mucosal capillaries revealed vascular stasis within a mean of 54, 81, or 61 s after 100% ethanol, 0.6 N HCl, or 0.2 N NaOH, with the subsequent development of hemorrhagic mucosal lesions. Mucosal blood flow estimated by laser-Doppler velocimetry decreased by 30% at 5 min after luminal application of 100% ethanol, and decreased further to about 40% of basal levels by 15 min. The decreased mucosal blood flow 15 min after application of 50% ethanol correlated with the extent of hemorrhagic mucosal lesions. Examination of the submucosal vessels that supply and drain the mucosa showed moderate dilation of small arterioles 1, 3, and 6 min after exposure to 100% ethanol but there were no consistent changes in venules. Mild vasoconstriction of small- and medium-sized venules could be detected 6, 10, and 15 min after NaOH but not after exposure to HCl. Pretreatment with 16,16-dimethyl prostaglandin E2 or sodium thiosulfate before exposure of the mucosa to ethanol prevented capillary stasis, maintained mucosal blood flow, and prevented the development of hemorrhagic gastric mucosal lesions. Topical mucosal application of 16,16-dimethyl prostaglandin E2 decreased, whereas topical exposure to sodium thiosulfate increased gastric mucosal blood flow, indicating that change in blood flow per se is an unlikely mediator of protection.

Restraint stress in biomedical research: a review

The use of restraint or immobilization for investigations of animal physiology, pathology and pharmacology has an extensive history. The major use of this technique has been as a "stressor" for the induction of stress response syndromes in animals. Many such syndromes have been characterized from the behavioral level to the neurochemical concomitants of stress. As a consequence of this particular use of the restraint procedure, much information concerning drug effects on stress response syndromes has been obtained. Indeed, many researchers in the area of gastrointestinal drugs routinely screen their new compounds in a restraint model of gastric stress ulcer. The purpose of this review is to present for researchers, a summary of the methods for, the parameters of, and known drug effects on, restraint-induced pathology. In our experience, this technique has proven to be a very useful one for the examination of both central and peripheral mechanisms of stress-related disorders, as well as for studying drug effects upon these disorders.

The influence of chronic or acute nicotine pretreatment on ethanol-induced gastric ulceration in the rat

The effects in rats of chronic or acute nicotine pretreatment were studied on three gastric parameters: ethanol-induced ulceration, gastric wall mucus content and gastric acid secretion, under basal or histamine-stimulated conditions. Oral administration of ethanol (40%, 10 ml kg-1) depleted gastric wall mucus and produced ulceration in the gastric glandular mucosa. Ten-day nicotine pretreatment (15 or 25 micrograms ml-1 drinking water) worsened the adverse effects of ethanol on mucosal ulceration and mucus content, potentiated the gastric secretory action of histamine, but did not affect basal acid secretion. Single oral doses of nicotine (2 or 4 mg kg-1, given 1 h beforehand) prevented ulceration and mucus depletion in ethanol-treated animals; however, they did not influence either basal or histamine-stimulated gastric acid output. It is concluded that chronic nicotine administration aggravates ethanol ulceration, possibly by decreasing gastric wall mucus content and sensitizing the stomach to the acid secretory action of histamine. On the other hand, an acute oral dose of nicotine preserves the mucus content and prevents ethanol-induced ulcer formation.

Sulphasalazine and experimental stress ulcers

The effects of sulphasalazine on gastric ulceration induced by restraint at 4 degrees C (stress) for 2 h were studied in rats. Doses of 63 or 125 mg/kg s.c., which had no effect on stomach wall prostaglandin E2 (PGE2) levels, prevented stress ulceration but not the lesions produced by indomethacin. Stress significantly increased gastric glandular mucosal PGE2 levels. Indomethacin pretreatment (20 mg/kg, p.o.) markedly reduced PGE2 levels in the same region of the stomachs, and worsened stress-induced lesion formation. Pretreatment with sulphasalazine of animals given indomethacin and then subjected to stress did not appear to affect the indomethacin component of indomethacin-stress ulceration. Oral administration of PGE2 200 micrograms/kg significantly elevated gastric PGE2 levels, but had no effect on stress ulceration. It appears that neither the antiulcer activity of sulphasalazine nor stress-induced ulceration is associated with gastric tissue PGE2 increase or decrease, respectively. The protective mechanism may result from the ability of sulphasalazine to inhibit lipoxygenase activity.

The influence of verapamil on the gastric effects of stress in rats

The influence of verapamil on stress- or bethanechol-induced gastric effects was investigated in rats. Intraperitoneally injected verapamil (1, 2 or 4 mg/kg), given 30 min beforehand, dose-dependently prevented gastric glandular ulceration, mast cell degranulation and the increased stomach wall contractions evoked by restraint at 4 degrees C for 1 h. Gastric acid secretion, as well as ulceration in both the forestomach and glandular segment, produced by subcutaneously-injected bethanechol (3.2 mg/kg) were also inhibited. It is concluded that decreased amine release from the mast cells, stomach wall relaxation and reduced gastric acid were responsible for the ulcer-antagonising effects of the calcium-entry blocker. The possible antiulcer actions of verapamil are discussed in the light of present knowledge regarding calcium involvement in the various mechanisms thought to contribute to the pathophysiology of stress ulceration in rat stomachs.

Gastrointestinal ulceration in the guinea pig in response to dimaprit, histamine, and H1- and H2-blocking agents

Histamine is known to induce gastric ulcers in guinea pigs after intraperitoneal administration and duodenal ulcers after repeated intramuscular administrations. This study was undertaken to clarify further the differential role of H1 and H2 receptor sites in respect to gastric and duodenal ulcer in the guinea pig. Groups of guinea pigs were treated with histamine, intraperitoneal (1.81 mg/kg intraperitoneal) or intramuscular (0.09 mg/kg intramuscular X 8 doses); the selective H2 agonist dimaprit (0.09-0.18 mg/kg X 8 doses intramuscular or 1.81-3.62 mg/kg intraperitoneal); NaCl, 154 mM (control); and the selective H1 and H2 antagonists, diphenhydramine (125 mg/kg X 2 doses, intramuscular) or cimetidine (50 mg/kg X 3 doses, intramuscular). Gastric and duodenal lesions were evaluated and residual gastric contents were analyzed. The selective induction of gastric or duodenal ulceration by histamine was confirmed, and the H2 agonist, dimaprit, has been shown to be ulcerogenic to the guinea pig duodenum by intraperitoneal or intramuscular administration. Diphenhydramine produced considerably more protection against histamine-induced gastric ulceration (62% decrease in incidence), while cimetidine was particularly effective in the prevention of histamine-induced duodenal ulcer (64% decrease in incidence). A differential role of histamine in the pathogenesis of gastric as opposed to duodenal ulcer is suggested by the present findings.

The influence of cimetidine, a histamine H2-receptor antagonist, on the gastric effects of reserpine in rats

The effects of graded doses of cimetidine on both resting and reserpine-evoked gastric acid secretion were examined in relation to their influence on reserpine-induced ulceration, mast cell degranulation and mucosal microcirculatory changes in rat stomachs. Cimetidine 10 mg/kg or above reduced resting or reserpine-provoked gastric acid secretion as well as rumenal and glandular ulceration. However, non-acid-inhibiting doses, 5 mg/kg or below, continued to prevent glandular ulceration. Reserpine-evoked gastric glandular mucosal mast cell degranulation was unaffected by both acid-inhibiting and non-acid-inhibiting doses of cimetidine which dose-dependently blocked the superficial glandular mucosal microcirculatory volume changes. These results suggest that cimetidine prevents reserpine-induced glandular ulceration largely by blocking the ulcerogenic effect of histamine H2-receptor-mediated mucosal microcirculatory congestion, in contrast to antagonising rumenal lesions through acid inhibition; they also support the idea tha reserpine may release histamine mainly from the glandular mucosal mast cells. The possibility of another antiulcer mechanism, due to cytoprotection, is discussed.

Effects of cimetidine upon gastric secretion and mucosal blood flow in the rat stressed by restraint. A dose-response and prophylaxis trial

The effect of the histamine-H2-receptor blocking agent cimetidine (CT) on gastric secretion, mucosal blood flow (MBF), and gastric mucosal stress ulcer index was studied in rats (n = 96) during control conditions (3.9, 15.6, 62.4 microMol/kg/h CT i.p. over 8 h) with calculation of kinetic constants (Vmax; Km) for gastric secretion, MBF and ulcer index, it is shown that CT actions become considerably modified by severe stress. Under mild stress the reduction of MBF and acid secretion by increasing CT doses is parallel. Severe stress alone reduces MBF more than gastric acid (65 vs. 30%). This phenomenon is partly reversed by a dose of 62.4 microMol/kg/h (= 16 mg/kg/h), which is associated with the lowest ulcer index observed. There is no significant correlation between the latter and either acid concentration, acid output, or MBF. The concentrations of pepsin in gastric juice or of serum gastrin neither change with severe stress alone or with additional CT. It is concluded that CT has anti-stress ulcer properties coinciding with reduced acidity in the gastric lumen, but that in addition this beneficial effect may be mediated by factors other than acid reduction.

A comparative study of the gastric ulcerogenic effects of stress and reserpine in rats with decreased stomach wall mast cell populations

Stress-induced gastric glandular ulcers in rats appeared less severe than those evoked by reserpine, although glandular mucosal mast cell counts were equally decreased. Prior depletion of the glandular mucosal mast cell population confirmed the hypothesis that an additional mechanism contributes to reserpine ulceration.

Disodium cromoglycate: a novel gastric antiulcer agent?

The influence of oral pretreatment with disodium cromoglycate (2.5 or 5 mg/kg X 9 doses) on the 4-h gastric effects of i.p. injected reserpine (5 mg/kg) was examined in rats. Disodium cromoglycate markedly prevented reserpine-induced ulceration, mucosal mast cell degranulation and superficial mucosal microcirculatory changes in the glandular portion of the stomach wall. These interesting results points to the possibility that disodium cromoglycate may also have gastric antiulcer effects in man.

Cholinergic-mediated gastric mast cell degranulation with subsequent histamine H1-and H2-receptor activation in stress ulceration in rats

The effects of atropine, mepyramine, metiamide or NaHCO3 on gastric ulceration, gastric secretion and gastric mast cell degranulation were studied in stressed pylorus-occluded rats. The influence of dexamethasone pretreatment on stress ulcers in animals without pylorus occlusion (intact rats) was also examined. Stress produced a high glandular lesion incidence and ulcer index, and markedly lowered gastric secretion and glandular wall mast cell counts. Injected 0.5 h before stress, atropine, mepyramine or metiamide strongly antagonised ulceration. Atropine or metiamide, but not mepyramine, reduced gastric secretion. Only atropine prevented stress-induced mast cell changes. NaHCO3, given intragastrically before stress, did not prevent ulceration or mast cell degranulation despite complete neutralisation of gastric acid. Dexamethasone-induced gastric mucosal mast cell depletion could reduce stress ulceration. The findings show that stress degranulates stomach mast cells via a cholinergic pathway; released histamine from this source is largely responsbile for gastric ulceration through H1- and H2-receptor effects. Histamine H2-receptor-mediated gastric acid may play only a small contributory role in stress ulcers in rats. The antiulcer mechanisms of histamine H1- and H2-receptor blockade are discussed.