Involvement of alpha 2-adrenoceptors in mechanism of intragastric nicotine protection against ethanol injury in rat stomach

Involvement of alpha(2)-adrenoceptors in the gastric protective effect of nitroglycerin against acidified ethanol-induced mucosal injury

Clinical observations reveal that alcohol intake is associated with an increase in upper gastrointestinal hemorrhage requiring hospitalization and that nitroglycerin or long-acting nitrates lower this risk. Nitroglycerin, a gastric vasodilator that can increase gastric fluid volume, protects the rodent stomach against damage, including that caused by 70% ethanol. Blockade of alpha(2)-adrenoceoptors attenuates gastric protection by intragastric nicotine against 40% ethanol. We tested the hypothesis that the protective effect of nitroglycerin is mediated by an increase in gastric fluid volume and alpha(2)-adrenoceoptors. Nitroglycerin, 5 mg/kg, vehicle, or acidified ethanol was administered intragastrically. In study 1 acidified ethanol-induced mucosal injury was measured. In study 2 the effect of increasing gastric volume (1 ml/kg) on mucosal injury was assessed. In study 3 the effect of yohimbine (alpha(2)-adrenoceoptor antagonist), 5 mg/kg subcutaneously, on the nitroglycerein-mediated protective effect was determined. Results showed that nitroglycerin significantly attenuated the number and length of mucosal lesions induced by acidified ethanol. Increase in gastric fluid volume by exogenously administered saline did not alter the protective effect. Yohimbine blocked the nitroglycerin-mediated protection. These experimental data are consistent with the observation that nitrates lower the risk of ethanol-induced gastrointestinal complications. alpha(2)-Adrenoceoptors are responsible in part for the protective effect of nitroglycerin.

Intragastric nitroglycerin at a vasodilatory dose attenuates acidified aspirin-induced gastric mucosal injury

Clinical studies reveal that aspirin intake to prevent myocardial and cerebral ischemia is associated with a significant increase in upper gastrointestinal hemorrhage requiring hospitalization and that nitroglycerin or long-acting nitrates significantly lower this risk. Nitroglycerin can increase gastric blood flow and slow gastric emptying. We hypothesized that both features contribute to its gastroprotective property. Fasted anesthetized rats (Study 1) and conscious mice (Studies 2 to 4) received intragastric nitroglycerin or vehicle. The effects of these two treatments on various parameters were assessed in Study 1, on blood pressure and gastric blood flow; Study 2, on acidified aspirin-induced gastric mucosal lesions; and Study 3, on the weight of gastric content. In Study 4, the effect of nitroglycerin, vehicle, or vehicle plus saline, on acidified aspirin-induced gastric mucosal lesion was assessed. Compared with vehicle, nitroglycerin decreased blood pressure and produced a mild but significant increase in gastric vascular conductance, blood flow, and volume of gastric content. The number and length of gastric mucosal lesions induced by acidified aspirin were significantly attenuated by intragastric nitroglycerin in a vasodilatory dose. Exogenous saline in a volume equivalent to the increase produced by nitroglycerin, however, did not attenuate the lesions. These experimental data are consistent with the clinical observation that nitrates lower the risk of aspirin-induced gastrointestinal complications. Confirmation of the efficacy of nitroglycerin and nitrates in preventing such aspirin-induced complications in controlled trials is worthy of consideration by clinical investigators.

Protection by intragastric polyethylene glycol 400 in rat stomach against ethanol damage involves alpha2-adrenoceptors

The aim of this study was to investigate the role of alpha2-adrenoceptors in the mechanism of intragastric polyethylene glycol 400 (PEG-400) protection against ethanol-induced gastric mucosal damage. In the injury study, 0.5 hr after subcutaneous control or yohimbine (5 mg/kg), a selective alpha2-adrenoceptor antagonist, rats were treated with intragastric vehicle or PEG-400 (5500 mg/kg). One hour later animals received 96% ethanol (gavage needle), 5 ml/kg, and the rats were killed after another hour. Total lengths of the gastric mucosal lesions were measured by an unbiased observer in a blinded fashion using a binocular magnifier having 5x magnification. In a separate set of experiments, 0.5 hr after subcutaneous control or yohimbine (5 mg/kg) rats received intragastric vehicle or PEG-400 (5500 mg/kg). One hour later gastric mucus volume, gastric juice volume, and gastric acid output in the gastric juice were measured. The protective effect offered by intragastric PEG-400 against ethanol-induced gastric mucosal damage was significantly diminished although not completely abolished by a selective alpha2-adrenoceptor antagonist (yohimbine). Yohimbine also significantly diminished both the basal and PEG-400-stimulated increase in gastric mucus volume. These findings suggest that the protective effect afforded by intragastric PEG-400 against ethanol-induced gastric mucosal damage is partially mediated by alpha2-adrenoceptors, and a mucus-dependent mechanism may be involved.

Is smoking still important in the pathogenesis of peptic ulcer disease?

The pathogenesis of peptic ulcer disease is multifactorial, including the effects of Helicobacter pylori, gastric acid, pepsin, gastroduodenal motility, smoking and nicotine, and the complex interaction of an array of other so-called aggressive and protective factors. Since the discovery and acceptance of H. pylori as a major etiologic agent in peptic ulcer disease, the role of smoking has received less attention. Smokers are more likely to develop ulcers, ulcers in smokers are more difficult to heal, and ulcer relapse is more likely in smokers. These clinical observations may be explained by the adverse effects that smoking has on mucosal aggressive and protective factors. Of the aggressive factors, smoking appears to have no consistent effect on acid secretion. However, smoking impairs the therapeutic effects of histamine-2 antagonists, may stimulate pepsin secretion, promotes reflux of duodenal contents into the stomach, increases the risk for and harmful effects of H. pylori, and increases production of free radicals, vasopressin, secretion by the pituitary, secretion of endothelin by the gastric mucosa, and production of platelet activating factor. Smoking also affects the mucosal protective mechanisms. It decreases gastric mucosal blood flow and inhibits gastric mucous secretion, gastric prostaglandin generation, salivary epidermal growth factor secretion, duodenal mucosal bicarbonate secretion, and pancreatic bicarbonate secretion. These adverse effects of smoking on aggressive and protective factors quality it as an important contributor to the pathogenesis of peptic ulcer disease and indicate that smoking plays a significant facilitative role in the development and maintenance of peptic ulcer disease.

The mechanistic pathway of gastric adaptive cytoprotection: a study on different components of the autonomic nervous system

1. The involvement of different components of the autonomic nervous system in the pathogenesis of ethanol-induced damage and the adaptive cytoprotection of mild irritants were studied in the gastric mucosa of male rats. 2. Capsaicin, yohimbine, and domperidone aggravated the 100% ethanol-induced gastric mucosal damage and attenuated the cytoprotective action of 20% ethanol, but not of 5% NaCl and 0.3 M HCl. Butoxamine and prazosin blocked the adverse actions of yohimbine and domperidone respectively. 3. Atropine, pirenzepine, and lidocaine lessened the severity of 100% ethanol-induced mucosal injury and further increased the cytoprotective action of 5% NaCl and 0.3 M HCl, but not of 20% ethanol. 4. Our results demonstrated that sensory afferent neurones, alpha 2-adrenoceptors and D2-dopaminergic receptors all play a significant role in the defensive mechanism of the gastric mucosa and the adaptive cytoprotection of 20% ethanol, while the M1- and M2-muscarinic receptors and sensory chemoreceptors on the gastric mucosa contribute only to the former action. The adverse effect of yohimbine and domperidone on lesion formation is probably mediated through the release of catecholamines, which subsequently act on the beta 2- and alpha 1-adrenoceptors respectively.

Topical nicotine protects rat gastric mucosa against ASA-induced damage. A role for mucosal fluid secretion in cytoprotection

Acute intragastric nicotine administration has previously been shown to protect against ethanol-induced gastric mucosal damage. The aim of this study was to examine the effects of acute nicotine exposure on ASA-induced gastric mucosal damage and to determine if nicotine's protective effect is secondary to an increase in mucosal blood flow or in mucosal fluid secretion, as reflected by changes in the juxtamucosal pH gradient and volume of intragastric fluid. Mucosal blood flow, using a laser Doppler flowmeter, juxtamucosal pH gradient (depth, magnitude, and surface pH), using antimony microelectrodes, and changes in volume of luminal bathing solutions were measured in rat ex vivo gastric chamber preparations prior to and after a 10-min exposure to topical nicotine (1 mg in 8 ml of 0.2 M mannitol in 50 mM HCl), or to mannitol-HCl solution (vehicle). This was followed by application of acidified ASA (80 mM in 160 mM HCl) to the chambered mucosae for 10 min. Lesion area, expressed as the percentage of total glandular mucosa which was damaged, was significantly (P < 0.05) reduced by nicotine pretreatment. Blood flow decreased with nicotine exposure by 18.4%, compared to 13.6% in the control group (NS). Both gradient depth and gastric fluid volume increased significantly in the nicotine group (P < 0.05) compared to controls. Yohimbine pretreatment prevented both the increase in juxtamucosal pH gradient depth and the protective effect of nicotine. These results suggest that acute intragastric nicotine exposure protects against ASA-induced gastric damage in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of smoking and nicotine on the gastric mucosa: a review of clinical and experimental evidence

Epidemiological and experimental evidence have shown that nicotine has harmful effects on the gastric mucosa. The mechanisms by which cigarette smoking or nicotine adversely affect the gastric mucosa have not been fully elucidated. In this report, clinical and experimental data are reviewed. The effects of nicotine from smoking on gastric aggressive or defensive factors are discussed. Nicotine potentiates gastric aggressive factors and attenuates defensive factors; it also increases acid and pepsin secretions, gastric motility, duodenogastric reflux of bile salts, the risk of Helicobacter pylori infection, levels of free radicals, and platelet-activating factor, endothelin generation, and vasopressin secretion. Additionally, nicotine impairs the therapeutic effect of H2-receptor antagonists and decreases prostaglandin synthesis, gastric mucosal blood flow, mucus secretion, and epidermal growth factor secretion. Although many of the studies provide conflicting results, the bulk of the evidence supports the hypothesis that nicotine is harmful to the gastric mucosa.

Intragastric nicotine protects against 40% ethanol-induced gastric injury despite pretreatment with NG-nitro-L-arginine methyl ester or adrenal medullectomy in rats

We tested the hypotheses that the protective effect of intragastric nicotine against ethanol-induced gastric mucosal injury is dependent on endogenous nitric oxide or peripheral sympathoadrenal mechanisms. Rats were pretreated with NG-nitro-L-arginine methyl ester (3 mg/kg subcutaneous, 1 h prior to study) to block endogenous nitric oxide synthesis or with adrenal medullectomy (three weeks prior to study) to ablate the effect of the adrenal medulla. At 1-h intervals, vehicle or nicotine (4 mg/kg) and 40% ethanol were then given intragastrically. The total lengths of the linear gastric corpus mucosal lesions were measured unbiasedly. The protective effect of intragastric nicotine was not modified by either pretreatment. We conclude that the mechanism mediating intragastric nicotine protection against 40% ethanol-induced gastric mucosal injury is independent of endogenous nitric oxide or the adrenal medulla.