Impaired vasodilatory responses in the gastric microcirculation of anesthetized rats with secondary biliary cirrhosis

Gastrointestinal Inflammation: A Central Component of Mucosal Defense and Repair

The mucosal layer of the gastrointestinal (GI) tract is able to resist digestion by the endogenous substances that we secrete to digest foodstuffs. So-called “mucosal defense” is multifactorial and can be modulated by a wide range of substances, many of which are classically regarded as inflammatory mediators. Damage to the GI mucosa, and its subsequent repair, are also modulated by various inflammatory mediators. In this article, we provide a review of some of the key Inflammatory mediators that modulate GI mucosal defense, Injury, and repair. Among the mediators discussed are nitric oxide, polyamines, the elcosanolds (prostaglandins and II-poxlns), protease-activated receptors, and cytokines. Many of these endogenous factors, or the enzymes involved in their synthesis, are considered potential therapeutic targets for the treatment of diseases of the digestive tract that are characterized by Inflammation and ulceration.

Endothelin and neonatal capsaicin regulate gastric resistance to injury in BDL rats

AIM: To investigate the relationship between primary afferent neurons, endothelin (ET) and the role of its receptors on ethanol-induced gastric damage in cirrhotic rats.

METHODS: Cirrhosis and portal hypertension were induced in rats by bile duct ligation (BDL) while controls had a sham operation. The association between ET and afferent neurons on the gastric mucosa was evaluated by capsaicin treatment in newborn rats, the use of ET agonists or antagonists, gastric ET-1 and -3 mRNA and synthetic capacity. Ethanol-induced damage was assessed using ex vivo gastric chamber experiments. Gastric blood flow was measured by laser-Doppler flowmetry.

RESULTS: ET-3 and an ET_B receptor antagonist significantly reduced the extent of ethanol-induced gastric damage in BDL rats. Gastric ET-1 and -3 levels were 30% higher in BDL rats compared to control rats. Capsaicin treatment restored the gastric resistance and blood flow responses to topical application of ethanol in BDL rats and ET-1 and -3 production to levels observed in controls.

CONCLUSION: Our results suggest that the reduced resistance of the gastric mucosa of cirrhotic rats to ethanol-induced injury is a phenomenon modulated by ET through the ET_B receptor and by sensory afferent neurons.

Pathogenesis of portal hypertensive gastropathy: translating basic research into clinical practice

Portal hypertensive gastropathy (PHG) is often seen in patients with portal hypertension, and can lead to transfusion-dependent anemia as well as acute, life-threatening bleeding episodes. This Review focuses on the mechanisms that underlie the pathogenesis of PHG that provide reasonable grounds for the treatment of this condition, and ultimately enable translation of basic research into clinical practice. Increased portal pressure associated with cirrhosis and liver dysfunction is critical for the development of clinically significant PHG, and leads to impaired gastric mucosal defense mechanisms that render the stomach susceptible to mucosal injury. The use of pharmacological agents such as beta-blockers reduces the frequency of bleeding episodes in PHG. As a last resort, surgical decompression of the portal system, transjugular intrahepatic stent placement and liver transplantation can resolve this condition. Elimination of known risk factors for gastric injury such as alcohol, aspirin and traditional NSAIDs is critical. The role of Helicobacter pylori colonization of the gastric mucosa in PHG is not clear. Careful and critical interpretation of human and experimental data can be helpful to establish a rationale for the medical management of this important condition.

Prostaglandins, NSAIDs, and gastric mucosal protection: why doesn't the stomach digest itself?

Except in rare cases, the stomach can withstand exposure to highly concentrated hydrochloric acid, refluxed bile salts, alcohol, and foodstuffs with a wide range of temperatures and osmolarity. This is attributed to a number of physiological responses by the mucosal lining to potentially harmful luminal agents, and to an ability to rapidly repair damage when it does occur. Since the discovery in 1971 that prostaglandin synthesis could be blocked by aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), there has been great interest in the contribution of prostaglandins to gastric mucosal defense. Prostaglandins modulate virtually every aspect of mucosal defense, and the importance of this contribution is evident by the increased susceptibility of the stomach to injury following ingestion of an NSAID. With chronic ingestion of these drugs, the development of ulcers in the stomach is a significant clinical concern. Research over the past two decades has helped to identify some of the key events triggered by NSAIDs that contribute to ulcer formation and/or impair ulcer healing. Recent research has also highlighted the fact that the protective functions of prostaglandins in the stomach can be carried out by other mediators, in particular the gaseous mediators nitric oxide and hydrogen sulfide. Better understanding of the mechanisms through which the stomach is able to resist injury in the presence of luminal irritants is helping to drive the development of safer anti-inflammatory drugs, and therapies to accelerate and improve the quality of ulcer healing.

Ablation of primary afferent neurons by neonatal capsaicin treatment reduces the susceptibility of the portal hypertensive gastric mucosa to ethanol-induced injury in cirrhotic rats

Primary sensory afferent neurons modulate the hyperdynamic circulation in cirrhotic rats with portal hypertension. The stomach of cirrhotic rats is prone to damage induced by ethanol, a phenomenon associated with reduced gastric hyperemic response to acid-back diffusion. The aim of this study was to examine the impact of ablation of capsaicin-sensitive neurons and the tachykinin NK(1) receptor antagonist A5330 on the susceptibility of the portal hypertensive gastric mucosa to ethanol-induced injury and its effects on gastric cyclooxygenase (COX) and nitric oxide synthase (NOS) mRNA expression. Capsaicin was administered to neonatal, male, Wistar rats and the animals were allowed to grow. Cirrhosis was then induced by bile duct ligation in adult rats while controls had sham operation. Ethanol-induced gastric damage was assessed using ex vivo gastric chamber experiments. Gastric blood flow was measured as well as COX/NOS mRNA expression. Topical application of ethanol produced significant gastric damage in cirrhotic rats compared to controls, which was reversed in capsaicin- and A5330-treated animals. Mean arterial and portal pressure was normalized in capsaicin-treated cirrhotic rats. Capsaicin and A5330 administration restored gastric blood flow responses to topical application of ethanol followed by acid in cirrhotic rats. Differential COX and NOS mRNA expression was noted in bile duct ligated rats relative to controls. Capsaicin treatment significantly modified gastric eNOS/iNOS/COX-2 mRNA expression in cirrhotic rats. Capsaicin-sensitive neurons modulate the susceptibility of the portal hypertensive gastric mucosa to injury induced by ethanol via tachykinin NK(1) receptors and signalling of prostaglandin and NO production/release.

Sildenafil prevents indomethacin-induced gastropathy in rats: role of leukocyte adherence and gastric blood flow

Nitric oxide (NO) is an important mediator of gastric mucosal defense. Sildenafil (SILD), a cyclic GMP-specific phosphodiesterase inhibitor, promotes an increase in cGMP concentrations in the gastrointestinal tract. cGMP mediates many of the biological actions of NO. We tested the hypothesis that SILD could increase mucosal defense against indomethacin-induced gastropathy in rats. SILD (1, 4 or 10 mg kg(-1), p.o.) pretreatment significantly reduced (P < 0.01) the gastric damage and the increase in gastric myeloperoxidase (MPO) activity elicited by indomethacin (20 mg kg(-1) p.o.), with the maximal effect at the dose of 10 mg kg(-1). L-NAME (3, 10 or 20 mg kg(-1), i.p.) dose dependently reversed the protective effects of SILD, an effect not seen when L-arginine (L-ARG) (200 mg kg(-1), i.p.) was co-administered with L-NAME. Indomethacin-induced leukocyte adhesion, assessed by intravital microscopy, was decreased (P < 0.01) by SILD, and this effect was reversed by L-NAME cotreatment. Indomethacin elicited a decrease in gastric blood flow and in gastric PGE2 levels. SILD was able to prevent the decrease in gastric blood flow (P < 0.01), without diminishing the inhibitory effect of indomethacin on prostaglandin synthesis. These results indicate that SILD, acting via NO-dependent mechanisms, prevents indomethacin-induced gastropathy, possibly through a reduction of leukocyte adhesion and maintenance of gastric blood flow.

Zinc L-carnosine protects against mucosal injury in portal hypertensive gastropathy through induction of heat shock protein 72

BACKGROUND AND AIMS

Increased susceptibility to gastric mucosal injury is observed in portal hypertensive gastropathy (PHG). In this study, the effects of zinc L-carnosine, an anti-ulcer drug, were evaluated on expression of heat shock protein (hsp) 72 and cytoprotection in gastric mucosa in a rat model of PHG.

METHODS

Portal hypertensive gastropathy with liver cirrhosis was induced by bile duct ligation for 4 weeks in male Sprague-Dawley rats. Expression of gastric mucosal hsp72 was evaluated by Western blotting at 6 h after intragastric administration of L-carnosine, zinc sulfate, or zinc L-carnosine. Blood was also collected for determination of serum zinc level. Mucosal protective abilities against hydrochloric acid (HCl) (0.6N) followed by pretreatment with L-carnosine, zinc sulfate or zinc L-carnosine were also studied.

RESULTS

L-carnosine, zinc sulfate, and zinc L-carnosine induced hsp72 in gastric mucosa of rats with bile duct ligation. Zinc sulfate and zinc L-carnosine suppressed HCl-induced mucosal injury. However, L-carnosine could not suppress HCl-induced mucosal injury. Serum zinc levels were significantly elevated after zinc L-carnosine administration. Furthermore, pretreatment with zinc L-carnosine (30-300 mg/kg) increased the expression of hsp72 in gastric mucosa and prevented HCl-induced mucosal injury in rats with bile duct ligation in a dose-dependent manner.

CONCLUSIONS

Zinc derivatives, especially zinc L-carnosine, protected portal hypertensive gastric mucosa with increased hsp72 expression in cirrhotic rats. It is postulated that zinc L-carnosine may be beneficial to the mucosal protection in PHG as a 'chaperone inducer'.

Capsaicin and gastric ulcers

In recent years, infection of the stomach with the organism Helicobacter Pylori has been found to be the main cause of gastric ulcers, one of the common ailments afflicting humans. Excessive acid secretion in the stomach, reduction in gastric mucosal blood flow, constant intake of non-steroid anti-inflammatory drugs (NSAIDS), ethanol, smoking, stress etc. are also considered responsible for ulcer formation. The prevalent notion among sections of population in this country and perhaps in others is that "red pepper" popularly known as "Chilli," a common spice consumed in excessive amounts leads to "gastric ulcers" in view of its irritant and likely acid secreting nature. Persons with ulcers are advised either to limit or avoid its use. However, investigations carried out in recent years have revealed that chilli or its active principle "capsaicin" is not the cause for ulcer formation but a "benefactor." Capsaicin does not stimulate but inhibits acid secretion, stimulates alkali, mucus secretions and particularly gastric mucosal blood flow which help in prevention and healing of ulcers. Capsaicin acts by stimulating afferent neurons in the stomach and signals for protection against injury causing agents. Epidemiologic surveys in Singapore have shown that gastric ulcers are three times more common in the "Chinese" than among Malaysians and Indians who are in the habit of consuming more chillis. Ulcers are common among people who are in the habit of taking NSAIDS and are infected with the organism "Helicobacter Pylori," responsible for excessive acid secretion and erosion of the mucosal layer. Eradication of the bacteria by antibiotic treatment and avoiding the NSAIDS eliminates ulcers and restores normal acid secretion.

Resveratrol, a red wine constituent polyphenol, protects gastric tissue against the oxidative stress in cholestatic rats

This experimental study was designed to determine the effects of resveratrol on the level of malondialdehyde (MDA), reduced glutathione (GSH), and nitric oxide (NO) in gastric tissue after bile duct ligation (BDL). Swiss albino rats were divided into three groups: Group 1, sham (n = 7); Group 2, BDL (BDL only group; n = 7); and Group 3, BDL plus resveratrol (n = 7). Animals in the resveratrol group were treated with 10 mg/kg resveratrol (i.p.) once a day throughout 28 days. In the resveratrol group, levels of MDA and NO in gastric tissue were significantly lower than in the BDL-only group (P < 0.001). The level of GSH in the resveratrol group was significantly higher than in the BDL-only group (P < 0.001). The present study demonstrates that intraperitoneal administration of resveratrol maintains antioxidant defenses and reduces oxidative gastric damage. This effect of resveratrol may be useful to preserve gastric tissue under oxidative stress due to cholestasis.

COX-1 and COX-2 conversely promote and suppress ischemia-reperfusion gastric injury in mice

OBJECTIVE

Neutrophil activation followed by free radical production is a feature that is common to the various forms of gastric injury. However, the roles of cyclooxygenase (COX)-1 and -2 in neutrophil activation have yet to be clarified in the gastric mucosa. We examined the roles of both COX-1 and COX-2 in neutrophil activation and free radical production in ischemia-reperfusion (IR) injury in the gastric mucosa of mice.

MATERIAL AND METHODS

Ischemia was induced by clamping the celiac artery for 30 min, then removing the clamp for 90 min. SC-560, a selective COX-1 inhibitor; NS-398, a selective COX-2 inhibitor; or rebamipide, a mucoprotective agent, was administered to mice 60 min before ischemia. Gastric damage was evaluated histologically and by measuring myeloperoxidase (MPO) activity. Expressions of COX protein and intercellular adhesion molecule (ICAM)-1 were evaluated by Western blot analysis and ELISA, respectively. Effects of these drugs on thiobarbituric acid reactive substances (TBARS) and gastric blood flow were also evaluated.

RESULTS

COX-2 expression was induced in gastric mucosa 60 min after reperfusion, whereas COX-1 expression remained unaltered. Localization of COX-1 and ICAM-1 in IR-injured mucosa was observed mainly in endothelial cells, while COX-2 expression was detected in mesenchymal cells such as mononuclear cells, spindle-like cells and endothelial cells. SC-560 significantly decreased gastric blood flow at the reperfusion point and reduced gastric mucosal injury in IR mice. Furthermore, SC-560 pretreatment significantly reduced MPO activity, TBARS levels and ICAM-1 expression. In contrast, NS-398 significantly increased ICAM-1 expression, MPO activity and TBARS levels, and aggravated gastric damage in IR mice. Rebamipide pretreatment reduced both COX-2 expression and IR injury.

CONCLUSIONS

In IR mice, COX-2 protects the gastric mucosa by down-regulating ICAM-1 expression, whereas COX-1 is involved in up-regulating reperfusion flow, thereby aggravating the mucosa.

Role of acute elevation of portal venous pressure by exogenous glucagon on gastric mucosal injury in rats with portal hypertension

Time-course studies revealed the increased susceptibility of the gastric mucosa to noxious injury in portal hypertension correlates with the level of elevated portal venous pressure and hyperglucagonemia. Whether acute elevation of portal venous pressure by exogenous glucagon aggravates such injury is not known. We tested the hypothesis that glucagon in a dose sufficient to acutely elevate portal venous pressure aggravates noxious injury of the gastric mucosa in rats with portal hypertension. Infusion of a portal hypotensive dose of somatostatin should reverse these changes. In anesthetized rats with portal vein ligation, glucagon, somatostatin or the combination was administered intravenously in a randomized, coded fashion. Acidified ethanol-induced gastric mucosal injury was determined. Portal venous pressure and gastric mucosal perfusion and oxygenation (reflectance spectrophotometry) were monitored to confirm the effects of the respective intravenous treatments. Exogenous glucagon exacerbated acidified ethanol-induced gastric mucosal injury. The exacerbation was attenuated by somatostatin. These changes paralleled the portal hypertensive and hypotensive effects of glucagon and somatostatin, respectively. Our data suggest that a unique mechanism is triggered with the onset of portal hypertension. In an antagonistic manner, glucagon and somatostatin modulate this novel mechanism that controls portal venous pressure and susceptibility of the gastric mucosa to noxious injury.

Inflammatory mediators in gastrointestinal defense and injury

Inflammation of the mucosal layer of the gastrointestinal (GI) tract is not only a feature almost always associated with ulceration of those tissues, but it also plays an important role in both the production and healing of the lesions. The mediators that coordinate inflammatory responses also have the capability to alter the resistance of the mucosa to injury induced by noxious substances, while others render the mucosa more susceptible to injury. In this article, we provide a review of the inflammatory mediators that modulate GI mucosal defense. Among the mediators discussed are nitric oxide, the eicosanoids (prostaglandins, leukotrienes, and thromboxanes), neuropeptides, cytokines, and proteinases. Many of these mediators are considered potential therapeutic targets for the treatment of ulcerative diseases of the digestive tract.

Pathogenesis of NSAID-induced gastroduodenal mucosal injury

The use of non-steroidal anti-inflammatory drugs (NSAIDs), even in the era of selective COX-2 inhibitors, remains limited by the ability of these agents to cause gastroduodenal ulceration and bleeding. This damage is caused mainly through the ability of these agents to inhibit prostaglandin synthesis, which has a negative impact on several components of mucosal defence. Many NSAIDs also have topical irritant effects on the epithelium which may be particularly important in the production of small intestinal injury. While the presence of acid in the lumen of the stomach may not be a primary factor in the pathogenesis of NSAID-induced gastroenteropathy it can make an important contribution to the chronicity of these lesions and to bleeding by impairing the restitution process, interfering with haemostasis and inactivating several growth factors that are important in mucosal defence and repair. Through better understanding of the pathogenesis of ulcers induced by NSAIDs, some new approaches to the development of more effective and safer anti-inflammatory drugs have been taken in recent years.

Proteinase-activated receptor-2 and hyperalgesia: A novel pain pathway

Using a combined pharmacological and gene-deletion approach, we have delineated a novel mechanism of neurokinin-1 (NK-1) receptor-dependent hyperalgesia induced by proteinase-activated receptor-2 (PAR2), a G-protein-coupled receptor expressed on nociceptive primary afferent neurons. Injections into the paw of sub-inflammatory doses of PAR2 agonists in rats and mice induced a prolonged thermal and mechanical hyperalgesia and elevated spinal Fos protein expression. This hyperalgesia was markedly diminished or absent in mice lacking the NK-1 receptor, preprotachykinin-A or PAR2 genes, or in rats treated with a centrally acting cyclooxygenase inhibitor or treated by spinal cord injection of NK-1 antagonists. Here we identify a previously unrecognized nociceptive pathway with important therapeutic implications, and our results point to a direct role for proteinases and their receptors in pain transmission.

NSAID-induced gastric damage in rats: requirement for inhibition of both cyclooxygenase 1 and 2

BACKGROUND & AIMS

Selective cyclooxygenase (COX)-2 inhibitors produce less gastric damage than conventional nonsteroidal anti-inflammatory drugs (NSAIDs), suggesting that NSAIDs cause damage by inhibiting COX-1. We tested this hypothesis in rats by using a selective COX-1 inhibitor (SC-560).

METHODS

The effects of SC-560, celecoxib (selective COX-2 inhibitor), or a combination of both inhibitors on gastric damage and prostaglandin synthesis were determined. Selectivity of the drugs for COX-1 vs. COX-2 was assessed in the carrageenan-airpouch model. A COX-1-preferential inhibitor, ketorolac, was also evaluated. The effects of these inhibitors on leukocyte adherence to vascular endothelium and on gastric blood flow were assessed.

RESULTS

SC-560 markedly reduced gastric prostaglandin synthesis and platelet COX-1 activity, but spared COX-2 and did not cause gastric damage. Celecoxib did not affect gastric prostaglandin E(2) synthesis and did not cause gastric damage. However, the combination of SC-560 and celecoxib invariably caused hemorrhagic erosion formation, comparable to that seen with indomethacin. Ketorolac caused damage only at doses that inhibited both COX isoforms, or when given with a COX-2 inhibitor. Celecoxib, but not SC-560, significantly increased leukocyte adherence, whereas SC-560, but not celecoxib, reduced gastric blood flow.

CONCLUSIONS

Inhibition of both COX-1 and COX-2 is required for NSAID-induced gastric injury in the rat.

The gastroduodenal toxicity of nonsteroidal anti-inflammatory drugs: a review of the literature

Nonsteroidal anti-inflammatory drugs (NSAIDs) are popular and important for the treatment of inflammation and pain. However, conventional NSAIDs are intrinsically toxic to the gastroduodenal (GD) mucosa. The literature can, and should, guide us towards safer prescribing of NSAIDs. Factors known to increase the risk of GD toxicity include: history of peptic ulcer disease; advanced age; high doses; and coadministration of aspirin, anticoagulants or corticosteroids. Patients with any one of these risk factors, with the possible exception of age alone, should receive gastroprotective prophylaxis with proton pump inhibitors or misoprostol. Standard dose H2 antagonists do not protect against NSAID-induced gastric ulcers and are unsuitable for prophylaxis. Awareness of risk factors and appropriate prophylactic agents will minimize the risk to patients. Whether the new generation of highly selective COX-2 inhibitors and nitric oxide-donating NSAIDs are safer drugs in long-term use be remains to be proven, though initial clinical trial data are positive.

Nitric oxide in mucosal defense: a little goes a long way

Nitric oxide (NO) is now recognized as an important modulator of an enormous number of physiological processes, ranging from blood pressure regulation to neuronal transmission to penile erectile function. In the gastrointestinal tract, NO also participates in many physiological and pathophysiological processes. In this review, we summarize the contribution made by NO to the ability of the gastrointestinal mucosa to resist injury induced by luminal toxins and to defend against microbial invasion. We also review some of the main features of NO chemistry and the potential of NO as a target for new drugs to treat gastrointestinal disorders.

N-bisphosphonates cause gastric epithelial injury independent of effects on the microcirculation

BACKGROUND

Nitrogen-containing bisphosphonates have been shown to be effective for the treatment of osteoporosis and Paget's disease of bone. Unfortunately, these drugs also have the capacity to irritate the upper gastrointestinal mucosa. In this study we investigated the ability of alendronate and pamidronate to directly damage the gastric epithelium and attempted to determine whether these drugs caused injury through gastric microcirculatory alterations.

METHODS

An ex vivo gastric chamber model was used. Effects of topically applied alendronate and pamidronate on transmucosal potential difference and epithelial integrity (histology) were determined. Also, the effects of agents capable of preventing microvascular injury in the stomach (PGE2 and two nitric oxide donors) were examined for their ability to prevent gastric injury induced by the two N-bisphosphonates.

RESULTS

Alendronate and pamidronate caused a concentration-dependent decrease in transmucosal potential difference, widespread epithelial injury and infiltration of neutrophils into the mucosa. PGE2 and the two nitric oxide donors did not prevent the changes in potential difference or the epithelial injury, but did reduce neutrophil infiltration. Significant release of PGE2 into the lumen was observed following application of the two bisphosphonates, but neither drug altered mucosal blood flow.

CONCLUSIONS

These results suggest that these N-bis- phosphonates directly damage the gastric epithelium independent of actions on the microvasculature.

Does upregulation of inducible nitric oxide synthase (iNOS) render the stomach more susceptible to damage?

Nitric oxide from constitutive nitric oxide synthase (NOS) augments gastric mucosal blood flow and is important in mucosal defense. However, the function of the inducible isoform of NOS (iNOS) in the gastric mucosa remains to be fully elucidated. This study was done to examine the role of iNOS in gastric mucosal blood flow and gastric injury following endotoxemia. Conscious rats were given intraperitoneal saline or lipopolysaccharide (LPS, 5 or 20 mg/kg). Five hours later, rats were anesthetized, a laparotomy made, gastric fluid aspirated, and 3 ml of 20% ethanol introduced into the forestomach. Rats were sacrificed 10 min later for assessment of macroscopic injury (mm2) to the gastric mucosa. Other rats did not receive 20% ethanol, but instead, gastric mucosal blood flow was determined with laser Doppler, followed by sacrifice and removal of stomachs for determination of gastric mucosal iNOS immunoreactivity (Western immunoblot). Lipopolysaccharide dose dependently increased gastric injury, decreased gastric mucosal blood flow, and increased gastric mucosal iNOS immunoreactivity compared to rats receiving saline. In additional experiments and using a similar protocol, intraperitoneal administration of aminoguanidine (45 mg/kg), an iNOS inhibitor, reversed lipopolysaccharide-induced gastric injury and restored gastric mucosal blood flow to baseline, whereas the nonselective NOS inhibitor, NG-nitro-l-arginine methyl ester (5 mg/kg) did not. Taken together, these data suggest that upregulation of iNOS is in part responsible for the detrimental effects of LPS on the gastric mucosa, possibly from a reduction in gastric mucosal blood flow.

Reduction of acute and reactivated colitis in rats by an inhibitor of neutrophil activation

Neutrophils have been implicated as major contributors to tissue injury in inflammatory bowel disease. In this study, we have assessed the effects of an inhibitor of neutrophil activation and adherence, NPC-18915 (4-¿2-[2-(2-benzofuranyl)phenyl]-(E)-ethenyl¿benzoic acid sodium salt), in models of both acute and reactivated colitis. Acute colitis was induced by intracolonic administration of a hapten. In other rats, colitis was reactivated 6 wk after a bout of acute colitis by subcutaneous administration of the hapten. NPC-18915 given during the first 4 days after induction of acute colitis significantly reduced tissue injury and the incidence of diarrhea and adhesions. When treatment of NPC-18915 was initiated after colitis was firmly established (48 h posthapten), it did not produce a significant effect. NPC-18915 was effective at significantly reducing colonic injury and granulocyte infiltration in the reactivated colitis model, and a similar effect could be observed in rats treated with antineutrophil serum. These results demonstrate that an inhibitor of neutrophil activation is effective in both acute and reactivated colitis, although in the former case, effectiveness is only seen when the drug is given before full establishment of colitis. These results also suggest that neutrophils, are a critical effector cell of hapten-induced colitis in the rat, particularly in the case of reactivated colitis.

Neural emergency system in the stomach

The maintenance of gastric mucosal integrity depends on the rapid alarm of protective mechanisms in the face of pending injury. Afferent neurons of extrinsic origin constitute an emergency system that is called into operation when the gastric mucosa is endangered by acid and other noxious chemicals. The function of these chemoceptive afferents can be manipulated selectively and explored with the excitotoxin capsaicin. Most of the homeostatic actions of capsaicin-sensitive afferents are brought about by peptides released from their peripheral endings in the gastric wall. When stimulated, chemoceptive afferents enhance gastric blood flow and activate hyperemia-dependent and hyperemia-independent mechanisms of protection and repair. In the rodent stomach, these local regulatory roles of sensory neurons are mediated by calcitonin gene-related peptide acting via calcitonin gene-related peptide 1 receptors and neurokinin A acting via neurokinin 2 receptors, with both peptides using nitric oxide as their common messenger. In addition, capsaicin-sensitive neurons form the afferent arc of autonomic reflexes that control secretory and motor functions of the stomach. The pathophysiological potential of the neural emergency system is best portrayed by the gastric hyperemic response to acid backdiffusion, which is signaled by afferent nerve fibers. This mechanism limits damage to the surface of the mucosa and creates favorable conditions for rapid restitution and healing of the wounded mucosa.

Underlying mechanisms of portal hypertensive gastropathy

Gastric mucosal lesions are frequently observed in patients with liver cirrhosis and portal hypertension. Similar lesions can be observed in experimental portal hypertension. This review summarizes our current knowledge of the pathophysiology of portal hypertensive gastropathy, with a particular focus on the microcirculatory disturbances that characterize this condition. The stomach of cirrhotic patients exhibits an increased susceptibility to injury induced by several irritants. Similarly, the stomach of portal hypertensive animals is less resistant to injury. One of the most important factors contributing to the increased susceptibility to damage is an impaired hyperemic response when the epithelium is exposed to irritants. This appears to be related to a reduction in mucosal prostaglandin production and to altered microcirculatory responsiveness to nitric oxide. Nitric oxide overproduction in portal hypertension may have direct effects on gastric blood flow regulation. Elevated production of tumor necrosis factor-alpha by gastric mucosa in portal hypertensive rats has also been shown to contribute to mucosal injury. A better understanding of the pathogenesis of portal hypertensive gastropathy may lead to development of specific therapeutic interventions for this condition.

Alendronate induces gastric injury and delays ulcer healing in rodents

Gastric ulceration associated with the use of NSAIDs is most frequently observed in elderly women, the same sector of society most likely to be receiving therapy for osteoporosis. As some anti-osteoporosis medications have been suggested to irritate the upper gastrointestinal mucosa, we evaluated the ability of one such drug, alendronate, to damage the gastric mucosa and to influence the severity and healing of gastric ulcers in rodents. The effects of alendronate on indomethacin-induced antral ulceration was evaluated in the rabbit, while effects on ulcer healing and on the formation of gastric erosions was evaluated in the rat. Effects of alendronate on gastric acid secretion, blood flow and prostaglandin synthesis were also evaluated. Alendronate caused erosions in the rabbit stomach, but not antral ulceration. However, at the highest doses tested (80 mg) alendronate increased the incidence and size of indomethacin-induced antral ulcers. Alendronate also enhanced indomethacin-induced gastric damage in the rat, and delayed gastric ulcer healing. These effects of alendronate were not attributable to changes in gastric acid secretion, blood flow, prostaglandin synthesis or the pharmacokinetics of indomethacin. The damaging effects of alendronate on the stomach were due to topical irritant effects and could be observed at concentrations as low as 4 mg/ml within 30 min of oral administration or topical superfusion. These results support preliminary clinical evidence that alendronate can damage the gastric mucosa. While gastric injury may be a rare occurrence in patients taking this drug, concomitant use of alendronate and NSAIDs may increase the incidence or severity of ulceration.

Reduction of shock-induced gastric damage by a nitric oxide-releasing aspirin derivative: role of neutrophils

The gastric damage associated with hemorrhagic shock appears to occur, at least in part, through neutrophil-dependent mechanisms. Nitric oxide (NO)-releasing derivatives of aspirin have been shown to spare the gastrointestinal tract of injury. As NO can inhibit neutrophil adherence, it is possible that such a derivative of aspirin (NCX-4016) would exert inhibitory effects on neutrophil adherence and therefore be capable of protecting the stomach against shock-induced gastric damage. This hypothesis was tested in this study. Oral administration of NCX-4016 or glyceryl trinitrate or depletion of circulating neutrophils with antineutrophil serum significantly reduced the extent of gastric damage induced by hemorrhagic shock, whereas aspirin had no effect. NCX-4016 and antineutrophil serum pretreatment resulted in significant preservation of gastric blood flow during the shock period. Moreover, NCX-4016, but not aspirin, was capable of inhibiting N-formyl-Met-Leu-Phe-induced leukocyte adherence to postcapillary mesenteric venules. These results suggest that an NO-releasing aspirin derivative reduces the susceptibility of the stomach to shock-induced damage through inhibitory effects on neutrophil adherence to the vascular endothelium.

Portal Hypertensive gastropathy

The term portal hypertensive gastropathy (PHG) defines a wide spectrum of diffuse macroscopic lesions that appear in the gastric mucosa of patients with portal hypertension. Histologically, these lesions correspond to dilated vessels in the mucosa and submucosa in the absence of erosions or inflammation. Endoscopically, the lesions are classified as mild when mosaic pattern or superficial reddening are present, and severe when gastric mucosa appear with diffuse cherry red spots. Mild lesions are highly prevalent (65-90%), whereas severe lesions are present in only 10-25% of cirrhotic patients. The pathogenesis of PHG is not well known, but both venous congestion related with raised portal pressure and increased gastric blood flow seem to be crucial factors for its development. Variceal sclerosis may contribute to the development or aggravation of the lesions. Bleeding is the unique clinical manifestation of PHG, and occurs only in those patients with severe lesions. During a 5-year follow-up, the risk of overt bleeding or chronic bleeding, which induces anaemia, is 60 % and 90%, respectively, for patients with severe PHG. Propranolol is the only pharmacological treatment that has been proven useful in preventing bleeding from PHG. Porto-systemic shunts and liver transplantation are also effective.

Vascular hyporesponsiveness to vasodilators in rats with cirrhosis

BACKGROUND/AIMS

In cirrhosis, the activation of nitric oxide and prostacyclin contributes to vasodilation, and ATP-sensitive K+ (KATP) channel activation or L-type calcium (Ca2+) channel inhibition may also play a role in this process. At the same time in cirrhosis, certain endogenous mechanisms may be stimulated which limit the influence of vasodilator mechanisms on vascular tone, thus altering vascular responses to exogenous substances such as nitric oxide donors, exogenous prostacyclin, KATP channel openers or L-type Ca2+ channel blockers. The aim of the present study was to examine the arterial depressor to these exogenous substances in normal rats and in rats with secondary biliary cirrhosis.

METHODS

Arterial depressor dose-response curves to nitroprusside (a nitric oxide donor, 5-60 micrograms.kg-1.min-1), prostacyclin (0.5-5 micrograms.kg-1) and aprikalim (a KATP channel opener, 10-200 micrograms.kg-1) were obtained in both groups. The effects of different L-type Ca2+ channel blockers, i.e. nicardipine (a dihydropyridine, 0.02-0.5 mg.kg-1), diltiazem (a benzothiazepine, 0.5-5 mg.kg-1) and verapamil (a phenylalkylamine, 0.02-0.2 mg.kg-1. min-1), were also studied.

RESULTS

Cirrhosis produced hyporeactivity to the arterial depressor effect of all doses of nitroprusside, the lowest dose of prostacyclin and the highest doses of aprikalim or diltiazem. Cirrhosis did not significantly change depressor responses to nicardipine or verapamil.

CONCLUSIONS

Rats with cirrhosis are hyporeactive to exogenous nitric oxide, prostacyclin, KATP channel opener and benzothiazepine (an L-type Ca2+ channel blocker). Therefore, cirrhosis-induced mechanisms seem to limit the decrease in vascular tone by most vasodilators. However, these mechanisms appear to be more marked in nitric oxide-mediated vasodilation than in other vasorelaxation mechanisms.

Review article: new insights into prostaglandins and mucosal defence

A role for prostaglandins in maintaining mucosal integrity in the gastrointestinal tract is well documented. While traditionally the effects of prostaglandins on mucosal blood flow and epithelial function have been regarded as critical in the mechanism of action of these fatty acids, recent evidence that mucosal ulceration is almost invariably associated with mucosal inflammation has caused a re-evaluation of the role of prostaglandins in mucosal defence. This review focuses on the ability of prostaglandins to exert anti-inflammatory, and therefore anti-ulcerogenic, effects in the gastrointestinal tract. These effects of prostaglandins are attributable to their ability to suppress the release of inflammatory mediators and reactive oxygen metabolites from a number of immunocytes, stromal cells and inflammatory cells. There is emerging evidence for cooperative interactions between prostaglandins and nitric oxide in maintaining mucosal integrity. Recent work on the inducible isoform of prostaglandin synthase as it pertains to mucosal defence is also reviewed.