Lipid peroxidation, reactive oxygen species and antioxidative factors in the pathogenesis of gastric mucosal lesions and mechanism of protection against oxidative stress - induced gastric injury.

The gastric mucosa plays an important role in the physiological function of the stomach. This mucosa acts as gastric barrier, which protects deeper located cells against the detrimental action of the gastric secretory components, such as acid and pepsin. Integrity of the gastric mucosa depends upon a variety of factors, such as maintenance of microcirculation, mucus-alkaline secretion and activity of the antioxidizing factors. The pathogenesis of gastric mucosal damage includes reactive oxygen species (ROS), because of their high chemical reactivity, due to the presence of uncoupled electron within their molecules. Therefore they cause tissue damage, mainly due to enhanced lipid peroxidation. Lipid peroxides are metabolized to malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE). The local increase of MDA and 4-HNE concentration indicates ROS-dependent tissue damage. Superoxide dismutase (SOD) is the main enzyme, which neutralizes ROS into less noxious hydrogen peroxide. A decrease of SOD activity is an indicator of impairment of the protective mechanisms and significantly contributes to cell damage. Hydrogen peroxide is further metabolized to water in the presence of reduced glutathione (GSH). GSH can also work synergetically with SOD to neutralize ROS. The reactions between GSH and ROS yields glutathione free radical (GS(•)), which further reacts with GSH leading to free radical of glutathione disulphide (GSSG(•)). This free radical of GSSG can then donate an electron to the oxygen molecule, producing O2 (•-) Subsequently, O2 (•-) is eliminated by SOD. Adecrease of the GSH level has detrimental consequences for antioxidative defense cellular properties. Gastric mucosa, exposed to stress conditions, exhibits an enhancement of lipid peroxidation (increase of MDA and 4-HNE), as well as a decrease of SOD activity and GSH concentration. This chain reaction of ROS formation triggered by stress, appears to be an essential mechanism for understanding the pathogenesis of stress - induced functional disturbances in the gastric mucosa leading to ulcerogenesis.

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-1ß, TNF-α, superoxide dismutase (SOD) and glutathione peroxidase (GPx) mRNAs in the gastric mucosa and plasma levels of ADMA, IL-1ß 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-1ß 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-1ß 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-1ß 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.