The impact of asymmetric dimethylarginine (ADAMA), the endogenous nitric oxide (NO) synthase inhibitor, to the pathogenesis of gastric mucosal damage

This review was designed to provide an update on the role of asymmetric arginine (ADMA), the endogenous inhibitor of nitric oxide (NO) synthase in the pathophysiology of the upper gastrointestinal (GI) tract. Numerous studies in the past confirmed that NO is a multifunctional endogenous gas molecule involved in most of the body organs' functional and metabolic processes including the regulation of gastrointestinal (GI) secretory functions, motility, maintenance of GI integrity, gastroprotection and ulcer healing. NO is metabolized from L-arginine by enzymatic reaction in the presence of constitutive NO synthase. In upper GI tract, NO acts as a potent vasodilator known to increase gastric mucosa blood flow, regulates the secretion of mucus and bicarbonate, inhibits the gastric secretion and protects the gastric mucosa against the damage induced by a variety of damaging agents and corrosive substances. In contrast, ADMA first time described by Vallance and coworkers in 1992, is synthesized by the hydrolysis of proteins containing methylated arginine amino acids located predominantly within the nucleus of cells. This molecule has been shown to competitively inhibit NO synthase suggesting its regulatory role in the functions of vascular endothelial cells and systemic circulation in humans and experimental animals. Nowadays, ADMA is a potentially important risk factor for coronary artery diseases and a marker of cardiovascular risk. Increased plasma levels of ADMA have been documented in several conditions that are characterized by endothelial dysfunction, including hypertension, hypercholesterolemia, hyperglycemia, renal failure and tobacco exposure. The role of ADMA in other systems including GI-tract has been so far less documented. Nevertheless, ADMA was shown to directly induce oxidative stress and cell apoptosis in gastric mucosal cells in vitro and to contribute to the inflammatory reaction associated with major human pathogen to gastric mucosa, Helicobacter pylori (H.pylori). Infection of gastric mucosa with this germ or H. pylori water extract led to marked increase in the plasma concentration of ADMA and significantly inhibited bicarbonate secretion, considered as one of the important components of upper GI-tract defense system. When administered to rodents, ADMA aggravated gastric mucosal lesions injury induced by cold stress, ethanol and indomethacin and this worsening effect on gastric lesions was accompanied by the significant increase in the plasma level of ADMA. This exaggeration of gastric lesions by ADMA was coincided with the inhibition of NO, the suppression of gastric blood flow and excessive release of proinflammatory cytokine TNF-α. This metabolic analog of L-arginine applied to rats was exposed to water immersion and restraint stress and ischemia-reperfusion, causing an elevation of plasma levels of ADMA and gastric MDA content, which is the marker of lipid peroxidation. These effects, including the rise in the plasma levels of ADMA in rats with stress and ischemia-reperfusion-induced gastric lesions, were attenuated by concomitant treatment with L-arginine, the substrate for NO-synthase, and superoxide dismutase (SOD), a reactive oxygen metabolite scavenger added to ADMA. We conclude that ADMA could be considered as an important factor contributing to the pathogenesis of gastric mucosal damage and inflammatory reaction in H. pylori-infected stomach due to inhibition of NO, suppression of GI microcirculation, and the proinflammatory and proapoptotic actions of this arginine analog.

[Role of hydrogen sulfide in the physiology of gastrointestinal tract and in the mechanism of gastroprotection]

Hydrogen sulfide (H2S) is commonly known as a toxic gas with an unpleasant odor. However, in the human body it plays a role as a gaseous transmitter involved in the control of physiological processes. Studies published so far have shown that H2S increased synaptic long-term potentiation in the central nervous system and exerted the inflammatory and anti-inflammatory effects on vascular endothelium. These effects clearly depend on the concentration of this gaseous molecule. H2S exerts vasodilatory effect in the cardiovascular system similar to those exhibited by carbon monoxide or nitric oxide. It is believed that H2S may play a potential role in the physiology of the gastrointestinal tract including the mechanism of gastroprotection of gastric mucosa and possibly exerts a protective effect in other parts of the digestive system. The administration of L-cysteine, the precursor of H2S or NaHS, the exogenous donor of this gaseous molecule, significantly reduced gastric damage induced by ethanol, an agent that is known to induce acute gastric damage and hemorrhagic necrosis to the gastric mucosa. The administration of H2S results in increased secretion of protective bicarbonate and mucus secretions and these effects could, in part, explain the H2S-induced protection of duodenal mucosa against the damage induced by gastric acid. Despite these promising results, little is known about the therapeutic efficacy of H2S in relation to two other important gases, nitric oxide and carbon monoxide, and future studies are definitely needed to assess its usefulness in the treatment of upper and lower gastrointestinal tract disorders. 

[Melatonin as a therapeutic factor in gastric ulcer healing under experimental diabetes]

Melatonin (N-acetyl-5-methoxytryptamine) is a hormon secreted mostly by the pineal gland in the brain which maintains the body's circadian rhythm. Interestingly, this indol derivative is produced by enterochromaffin-like cells (ECL) in the gastrointestinal tract (GIT) in amount about 400 fold greater than detected in the pinealocytes. Previous studies revealed that melatonin exerts beneficial action against acute gastric damage induced by stress ethanol, aspirin and ischemia-reperfusion. Hyperglycemia, which is the main symptom of diabetes mellitus, is known to induce mitochondrial dysfunction and endoplasmic reticulum stress, both promoting the generation of reactive oxygen species (ROS). ROS were shown to exhibit higher activity than molecular oxygen under basal conditions due to unpaired electron in its outermost shell of electrons. ROS lead to damage of cellular proteins, nucleic acids and membrane polyunsaturated fatty lipids. In this study, we induced diabetes mellitus by the application of strep. tozocin in presence of gastric ulcers. Male Wistar rats were used in this model. 9 days after gastric ulcers and diabetes mellitus induction, groups of rats were treated with saline or melatonin (20 mg/kg i.g.). At the termination of the experiment, rats were anesthetized, abdomen was opened and gastric blood flow (GBF) was measured. Stomachs were removed for determination of gastric ulcers area by planimetry. Tissue samples were collected for biochemical assays. We demonstrated that melatonin significantly accelerates gastric ulcers healing with and without coexistence of diabetes mellitus. This effect was accompanied by increase of GBF level. Moreover, we observed an increase in superoxide dismutase (SOD) activity and an decrease in lipid peroxidation products concentration within gastric tissue homogenates of animals treated with melatonin, as compared with control group. Melatonin application accelerates gastric ulcers healing with and without presence of diabetes mellitus. We conclude that melatonin can physiologically regulate anti-oxidative enzymes activity and increase GBF level.

Asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase, interacts with gastric oxidative metabolism and enhances stress-induced gastric lesions

Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide (NO) synthase known to exert vasoconstriction of vascular bed. The elevation of ADMA has been considered as the cardiovascular risk factor associated with hyperlipidemia, hypercholesterolemia and metabolic syndrome. ADMA is produced by the action of dimethylarginine dimethylaminohydrolase (DDAH), which hydrolyzes ADMA to L-citrulline and dimethylamine. Previous studies have shown that endogenous NO plays an important role in the mechanism of gastric mucosal defense, but the role of ADMA in the pathogenesis of serious clinical entity, such as the acute gastric mucosal injury induced by stress has been little studied. In present study, we determined the effect of intragastric (i.g.) pretreatment with ADMA applied in graded doses ranging from 0.1 up to 20 mg/kg on gastric mucosal lesions induced by 3.5 h of water immersion and restraint stress (WRS). The number of gastric lesions was determined by planimetry and the gastric blood flow (GBF) was assessed by laser Doppler technique. The malondialdehyde and 4-hydroxynonenal (MDA+4-HNE) concentration, as an index of oxygen radical-lipid peroxidation was assessed in the gastric mucosa in rats exposed to WRS with or without ADMA administration. Proinflammatory cytokines IL-1β, TNF-α, superoxide dismutase (SOD) and glutathione peroxidase (GPx) mRNAs in the gastric mucosa and plasma levels of ADMA, IL-1β and TNF-α were analyzed by RT-PCR and ELISA, respectively. The exposure of rats to WRS for 3.5 h produced acute gastric lesions accompanied by a significant rise in the plasma ADMA levels and a significant fall in the GBF, an increase in MDA+4-HNE concentrations and the significant increase in the expression and release of IL-1β and TNF-α. The pretreatment with ADMA, applied i.g. 30 min before WRS dose-dependently, aggravated WRS damage and this effect was accompanied by a further significant fall in the GBF. The ADMA induced exacerbation of WRS lesions and the accompanying rise in the plasma ADMA levels and the fall in GBF were significantly attenuated by concurrent treatment with glyceryl trinitrate (GTN) (10 mg/kg i.g.) in the presence of ADMA. Administration of ADMA resulted in a significant decrease in the expression of SOD and GPx mRNAs and the up-regulation of mRNA for IL-1β and TNF-α followed by an increase in these plasma cytokine levels as compared to respective values observed in vehicle-pretreated animals. We conclude that 1) ADMA could be implicated in the mechanism of WRS-induced ulcerogenesis, 2) ADMA exacerbates WRS-induced gastric lesions due to enhancement in neutrophil dependent lipid peroxidation and overexpression and release of proinflammatory cytokines IL-1β and TNF-α and a potent depletion of antioxidative enzymes SOD and GPx expression and activity.

Interaction between selective cyclooxygenase inhibitors and capsaicin-sensitive afferent sensory nerves in pathogenesis of stress-induced gastric lesions. Role of oxidative stress

Gastric microcirculation plays an important role in the maintenance of the mucosal gastric integrity and the mechanism of injury as well as providing protection to the gastric mucosa. Disturbances in the blood perfusion, through the microcapillaries within the gastric mucosa may result in the formation of mucosal damage. Acute gastric mucosal lesions constitute an important clinical problem. Originally, one of the essential component of maintaining the gastric mucosal integrity was the biosynthesis of prostaglandins (PGs), an issue that has captured the attention of numerous investigations. PGs form due to the activity of cyclooxygenase (COX), an enzyme which is divided into 2 isoforms: constitutive (COX-1) and inducible (COX-2) ones. The inhibition of COX-1 by SC-560, or COX-2 by rofecoxib, reduces gastric blood flow (GBF) and impairs gastric mucosal integrity. Another detrimental effect on the gastric mucosal barrier results from the ablation of sensory afferent nerves by neurotoxic doses of capsaicin. Functional ablation of the sensory afferent nerves by capsaicin attenuates GBF and also renders the gastric mucosa more susceptible to gastric mucosal damage induced by ethanol, aspirin and stress. However, the role of reactive oxygen species (ROS) in the interaction between COX specific inhibitors and afferent sensory nerves has not been extensively studied. The aim of our present study was to determine the participation of ROS in pathogenesis of stress-induced gastric lesions in rats administered with SC-560 or rofecoxib, with or without ablation of the sensory afferent nerves. ROS were estimated by measuring the gastric mucosal tissue level of MDA and 4-HNE, the products of lipid peroxidation by ROS as well as the SOD activity and reduced glutathione (GSH) levels, both considered to be scavengers of ROS. It was demonstrated that exposure to 3.5 h of WRS resulted in gastric lesions, causing a significant increase of MDA and 4-HNE in the gastric mucosa, accompanied by a decrease of SOD activity and mucosal GSH level. Pretreatment with COX-1 and COX-2 inhibitors (SC-560 and rofecoxib, respectively) aggravated the number of gastric lesions, decreased GBF, attenuated GSH level without further significant changes in MDA and 4-HNE tissue levels and SOD activity. Furthermore, the capsaicin--nactivation of sensory nerves resulted in exaggeration of gastric mucosal damage induced by WRS and this was further augmented by rofecoxib. We conclude that oxidative stress, as reflected by an increase of MDA and 4-HNE tissue concentrations (an index of lipid peroxidation), as well as decrease of SOD activity and the fall in GSH tissue level, may play an important role in the mechanism of interaction between the inhibition of COX activity and afferent sensory nerves releasing vasoactive neuropeptides. This is supported by the fact that the addition of specific COX-1 or COX-2 inhibitors to animals with capsaicin denervation led to exacerbation of gastric lesions, and further fall in the antioxidizing status of gastric mucosa exposed to stress.