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.

Involvement of vagal nerves in the pancreatostimulatory effects of luminal melatonin, or its precursor L-tryptophan. Study in the rats.

UNLABELLED: Melatonin, known as a product of pineal gland is also produced in the digestive system. Melatonin receptors have been detected on pancreatic beta cells and this indoloamine influences the endocrine pancreatic function but the role of melatonin on pancreatic exocrine secretion is not known. AIM: To evaluate the effects of intraduodenal administration of melatonin or its precursor L-tryptophan on pancreatic protein output under basal conditions or following the stimulation of exocrine pancreas with diversion of pancreato-bliliary juice (DBPJ) and to assess the involvement of vagal nerves, and CCK in this process. METHODS: Under pentobarbiturate anesthesia the Wistar rats weighting 300g were surgically equipped with silicone catheters, one of them was inserted into pancreato-biliary duct, the other one--into duodenum. Melatonin (1, 5 or 25 mg/kg) or L-tryptophan (10, 50 or 250 mg/kg) were administered to the rats as intraduodenal (i.d.) bolus injection. Bilateral vagotomy was performed in the group of animals 7 days before the experiment. To assess the role of CCK in the melatonin or L-tryptophan-induced pancreatic secretory functions, lorglumide, the CCK(1) receptor antagonist was administered at dose of 1 mg/kg i.d. 15 minutes before the application of examine substances. During the study samples of pancreato-biliary juice were collected in 15 minutes aliquots to measure the protein outputs. RESULTS: Melatonin (1, 5, or 25 mg/kg ) or L-tryptophan (10, 50 or 250 mg/kg) produced significant and dose-dependent increases in pancreatic protein secretion under basal conditions or following the stimulation of this secretion by DBPJ. This was accompanied by a dose-dependent rise in CCK plasma level. Stimulation of pancreatic protein outputs caused by melatonin or L-tryptophan was completely abolished by vagotomy, or pretreatment with lorglumide. We conclude that melatonin as well as its precursor L-tryptophan, stimulates pancreatic exocrine function via mechanisms involving enteropancreatic reflexes and CCK.

Modulation of pancreatic enzyme secretion by melatonin and its precursor; L-tryptophan. Role of CCK and afferent nerves.

Melatonin, a pineal hormone, is also produced in the gastrointestinal tract. Melatonin receptors have been detected in the stomach, intestine and pancreas. This indole inhibits insulin secretion but its role in the physiological modulation of exocrine pancreatic function is yet unknown. The aim of this study was to evaluate the pancreatic secretory effect of melatonin and its precursor; L-tryptophan given intraduodenally (i.d.) to the conscious rats with intact or capsaicin deactivated sensory nerves. CCK(1) receptor antagonist; tarazepide, was used in the part of the study to determine the involvement of CCK in the secretory effects of melatonin. The secretory studies were performed on awaken rats surgically equipped with silicone catheters, one of them was inserted into pancreato-biliary duct, the other one--into duodenum. Melatonin (1, 5 or 25 mg/kg) or L-tryptophan (10, 50 or 250 mg/kg) were administered i.d. Samples of pancreatic juice were collected in 15 minutes aliquots. Tarazepide (2,5 mg/kg i.p.) was given to the rats 15 min prior to the administration of melatonin or L-tryptophan. Neurotoxic dose of capsaicin (100 mg/kg s.c.) was used to deactivate afferent nerves and thus to assess the role of these nerves in the melatonin-induced pancreatic enzyme secretion. Administration of melatonin (1, 5 or 25 mg/kg i.d.) or L-tryptophan (10, 50 or 250 mg/kg i.d.) significantly increased pancreatic amylase outputs. Deactivation of sensory nerves by capsaicin or administration of CCK(1) - receptor antagonist; tarazepide, reversed the stimulatory effects of melatonin or L-tryptophan on pancreatic secretory function. Administration of melatonin or its amino-acid precursor to the rats resulted in the significant and dose-dependent rises of melatonin and CCK plasma levels. We conclude that melatonin or its precursor; L-tryptophan stimulates pancreatic enzyme secretion via stimulation of CCK release and activation of duodeno-pancreatic reflexes.

Comparison of nitric oxide-releasing NSAID and vitamin C with classic NSAID in healing of chronic gastric ulcers; involvement of reactive oxygen species.

BACKGROUND: Nonsteroidal anti-inflammatory drugs such as aspirin (ASA) are known to induce gastric mucosal damage including bleeding, ulceration and perforation in humans and experimental animals. These adverse effects of ASA were originally attributed to the inhibition of cyclooxygenase and the deficiency of endogenous prostaglandins induced by this drug but the role of reactive oxygen species (ROS), lipid peroxidation and antioxidizing mechanism in the pathogenesis of ASA damage has been little studied. New class of nitric oxide (NO)-releasing nonsteroidal anti-inflammatory drugs was shown to inhibit cyclooxygenase and prostaglandin generation without causing mucosal damage but it remains unknown whether these agents affect the healing process of chronic gastric ulcers. MATERIAL AND METHODS: In this study the effect of NO-releasing aspirin (NO-ASA) and was compared with that of native aspirin applied with or without vitamin C on the healing of acetic acid ulcers. The area of gastric ulcer was determined by planimetry, the gastric blood flow (GBF) at ulcer margin was measured by H2 gas clearance method and mucosal release of ROS was quantified by measuring the chemiluminescence before and after the treatment with ASA or NO-ASA alone and ASA combined with vitamin C. The plasma antiinflammatory cytokine such as IL-1b and oxygen radical-mediated lipid peroxidation was measured in the ulcerated gastric mucosa of ASA and NO-ASA-treated animals. RESULTS: ASA delayed significantly ulcer healing and this effect was accompanied by a marked increase in the chemiluminescence, lipid peroxidation and the fall in the GBF at ulcer margin. Vitamin C attenuated significantly both the ASA-induced gastric damage and accompanying fall in the GBF at ulcer margin and the rise in the chemiluminescence and reversed the ASA-induced lipid peroxidation. In contrast, NO-ASA failed to affect healing of gastric ulcers and failed to produce the rise in the plasma IL-1b levels and the increase of lipid peroxidation as compared to those recorded in ASA-treated animals. CONCLUSIONS: 1) ROS-induced enhancement in lipid peroxidation plays an important role in the mechanism of gastric damage induced by ASA, 2) vitamin C attenuates the deleterious effect of ASA on ulcer healing due to its antioxidizing activity by mechanism involving preservation of gastric microcirculation and attenuation of lipid peroxidation and cytokine release and 3) coupling of NO to aspirin fails to delay the ulcer healing suggesting that NO might compensate for prostaglandin deficiency induced by NSAID.

Gastroprotective effect of histamine and acid secretion on ammonia-induced gastric lesions in rats.

BACKGROUND: Previous studies have shown that ammonia produced by Helicobacter pylori urease or administrated intragastrically exhibits a toxic effect on the gastric mucosa. In the present study we investigated the influence of histamine and gastric acid secretion on ammonia (NH4OH)-induced gastric lesions. METHODS: The gastric mucosa in rats was exposed to NH4OH (1.5 ml of 250 mM solution) under basal conditions, after administration of histamine (1 mg/kg), urea with urease, and ranitidine (40 mg/kg subcutaneously) given alone or in combination. We measured the area of gastric lesions, gastric blood flow (GBF), plasma gastrin concentration, DNA synthesis, gastric acid secretion and gastric luminal concentration of PGE2. RESULTS: Application of NH4OH resulted in the formation of acute gastric lesions. This effect was accompanied by a fall in GBF, a rise in gastric pH, and a reduction in mucosal DNA synthesis. Administration of histamine 30 min prior to NH4OH reduced the area of gastric lesions. This was accompanied by an increase in GBF, DNA synthesis, and prostaglandin E2 (PGE2) production. Ranitidine given prior to NH4OH enhanced gastric mucosa damage, and reduced GBF and DNA synthesis. This effect was accompanied by a reduction in gastric acid secretion. Ranitidine given prior to histamine abolished gastric acid secretion and the protective effect of histamine against NH4OH-induced damage; these effects were accompanied by a decrease in GBF, DNA synthesis, and concentration of PGE2. Pretreatment with 2% urea with urease given prior to NH4OH reduced NH4OH lesions. This effect was associated with an increase in gastric acid secretion, gastric generation of PGE2, GBF, and DNA synthesis. Ranitidine given prior to urea with urease inhibited gastric acid secretion and the gastroprotective effect of urea-urease gastroprotection. CONCLUSIONS: Histamine and gastric secretion exhibit a protective effect against ammonia-induced gastric lesions. This effect appears to depend upon the stimulation of gastric acid secretion and PGE2 production, and the improvement of gastric microcirculation.

Pancreatic secretion and cyclic AMP in pancreatic tissue after intravenous infusion of secretin and vasoactive intestinal peptide in cats.

This study demonstrates that secretin and vasoactive intestinal peptide increased dose-dependently pancreatic bicarbonate secretion and cyclic AMP content in pancreatic tissue homogenates in cats. It is concluded that both hormones may act on the same receptor site in the pancreas and that their action is based on stimulation of the adenylate cyclase-cyclic AMP system.