Importance of cyclooxygenase-1/prostacyclin in modulating gastric mucosal integrity under stress conditions.

BACKGROUND AND AIM: We investigated the roles of cyclooxygenase (COX) isozymes and prostaglandins (PGs) and their receptors in mucosal defense against cold-restraint stress (CRS)-induced gastric lesions. METHODS: Male C57BL/6 wild-type (WT) mice and those lacking COX-1 or COX-2 as well as those lacking EP1, EP3, or IP receptors were used after 18 h fasting. Animals were restrained in Bollman cages and kept in a cold room at 10°C for 90 min. RESULTS: CRS induced multiple hemorrhagic lesions in WT mouse stomachs. The severity of these lesions was significantly worsened by pretreatment with the nonselective COX inhibitors (indomethacin, loxoprofen) or selective COX-1 inhibitor (SC-560), while neither of the selective COX-2 inhibitors (rofecoxib and celecoxib) had any effect. These lesions were also aggravated in animals lacking COX-1, but not COX-2. The expression of COX-2 mRNA was not detected in the stomach after CRS, while COX-1 expression was observed under normal and stressed conditions. The gastric ulcerogenic response to CRS was similar between EP1 or EP3 knockout mice and WT mice, but was markedly worsened in animals lacking IP receptors. Pretreating WT mice with iloprost (the PGI2 analog) significantly prevented CRS-induced gastric lesions in the presence of indomethacin. PGE2 also reduced the severity of these lesions, and the effect was mimicked by the EP4 agonist, AE1-329. CONCLUSIONS: These results suggest that endogenous PGs derived from COX-1 play a crucial role in gastric mucosal defense during CRS, and this action is mainly mediated by PGI2 /IP receptors and partly by PGE2 /EP4 receptors.

Dexamethasone damages the rat stomach but not small intestine during inhibition of COX-1.

We previously reported that inhibition of both COX-1 and COX-2 is required for the gastrointestinal ulcerogenic properties of nonsteroidal anti-inflammatory drugs (NSAIDs). Inhibition of COX-1 up-regulates COX-2 expression, and the prostaglandins (PGs) produced by COX-2 help to maintain the mucosal integrity during inhibition of COX-1. In the present study we investigated whether dexamethasone damages rat gastrointestinal mucosa during inhibition of COX-1 and further developed the idea that COX-2 expression is a key event in the ulcerogenic actions of NSAIDs. Dexamethasone was given p.o. in the absence or presence of SC-560 (a selective COX-1 inhibitor), and the stomach or intestine was examined 8 or 24 hr later, respectively. Neither dexamethasone nor SC-560 alone damaged the gastrointestinal mucosa. In the presence of SC-560, however, dexamethasone damaged the stomach but not small intestine. SC-560 decreased PGE(2) levels in both tissues, with a gradual recovery accompanying the up-regulation of COX-2 expression, and both the recovery of PGE(2) levels and the expression of COX-2 were inhibited by dexamethasone. In the animals treated with SC-560, iNOS expression was up-regulated in the intestinal but not the gastric mucosa, and this response was also inhibited by dexamethasone. These results suggest a risk from steroid therapy in the stomach when COX-2 expression is up-regulated. Dexamethasone does not provoke damage in the intestine, despite inhibiting the up-regulation of COX-2 expression under conditions of PG deficiency; at least one of the reasons is that this agent prevents the expression of iNOS, a major factor in the pathogenesis of intestinal lesions.

Preconditioning stress prevents cold restraint stress-induced gastric lesions in rats: roles of COX-1, COX-2, and PLA2.

We investigated the protective effect of mild stress on gastric lesions induced by cold-restraint stress, especially concerning prostaglandins (PGs)/cyclo-oxygenase (COX) isozymes. Rats were exposed to severe stress (cold-restraint stress at 10 degrees C for 6 hr) or mild stress (cold-restraint stress at 10 degrees C for 30 min and kept at room temperature for 60 min) followed by severe stress. Severe stress induced gastric lesions, with a concomitant decrease in body temperature (BT). The ulcerogenic response was inhibited by atropine but worsened by indomethacin and SC-560 but not rofecoxib, although none of these agents had any effect on the change in BT. Mild stress suppressed the gastric ulceration and the decrease in BT induced by severe stress, and these effects were reversed by both COX-1 and COX-2 inhibitors. The expression of COX-2 in the stomach was up-regulated from 4 hr after severe stress and this response was slightly expedited by mild stress. COX-2 was also expressed in the hypothalamus under normal and stressed conditions. Quinacrine (phospholipase A(2) inhibitor) attenuated the protective effect of mild stress on the ulceration and decrease in BT caused by severe stress. TA-0910 (TRH analogue) at a low dose also prevented the gastric ulceration and the decrease in BT induced by severe stress. These results suggest that mild stress protects against cold-restraint stress-induced gastric ulceration, and the effect is peripherally and centrally mediated by PGs derived from both COX-1 and COX-2 through the activation of phospholipase A(2). TRH may also be involved in the protective effect of mild stress, probably through regulation of the thermogenic system.

Factors involved in upregulation of inducible nitric oxide synthase in rat small intestine following administration of nonsteroidal anti-inflammatory drugs.

We investigated the functional mechanisms underlying the expression of inducible nitric oxide (NO) synthase (iNOS) in the rat small intestine following the administration of nonsteroidal anti-inflammatory drugs (NSAIDs) and found a correlation with the intestinal ulcerogenic properties of NSAIDs. Conventional NSAIDs (indomethacin, diclofenac, naproxen, and flurbiprofen), a selective cyclooxygenase (COX)-1 inhibitor (SC-560) and a selective COX-2 inhibitor (rofecoxib) were administered p.o., and the intestinal mucosa was examined 24 hours later. Indomethacin decreased prostaglandin E2 (PGE2) production in the intestinal mucosa and caused intestinal hypermotility and bacterial invasion as well as the upregulation of iNOS expression and NO production, resulting in hemorrhagic lesions. Other NSAIDs similarly inhibited PGE2 production and caused hemorrhagic lesions with intestinal hypermotility as well as iNOS expression. Hypermotility in response to indomethacin was prevented by both PGE2 and atropine but not ampicillin, yet all these agents inhibited not only bacterial invasion but also expression of iNOS as well, resulting in prevention of intestinal lesions. SC-560, but not rofecoxib, caused a decrease in PGE2 production, intestinal hypermotility, bacterial invasion, and iNOS expression, yet this agent neither increased iNOS activity nor provoked intestinal damage because of the recovery of PGE2 production owing to COX-2 expression. Food deprivation totally attenuated both iNOS expression and lesion formation in response to indomethacin. In conclusion, the expression of iNOS in the small intestine following administration of NSAIDs results from COX-1 inhibition and is functionally associated with intestinal hypermotility and bacterial invasion. This process plays a major pathogenic role in the intestinal ulcerogenic response to NSAIDs.

Rofecoxib produces intestinal but not gastric damage in the presence of a low dose of indomethacin in rats.

Indomethacin in small doses is known to inhibit prostaglandin (PG) production, yet it does not damage the gastrointestinal mucosa. We examined whether a cyclooxygenase (COX)-2 inhibitor induces gastrointestinal damage in the presence of a low dose of indomethacin and investigated the ulcerogenic mechanism in relation to COX-2 expression. Rats with or without 18-h fasting were administered rofecoxib (a selective COX-2 inhibitor; 10 or 30 mg/kg p.o.) in the absence or presence of indomethacin (3 mg/kg p.o.), and the gastric or intestinal mucosa was examined 8 and 24 h later, respectively. Neither indomethacin nor rofecoxib alone caused damage in the stomach or small intestine. However, indomethacin damaged the small intestine in the presence of rofecoxib, yet the same treatment did not damage the stomach. Indomethacin reduced the mucosal PGE2 content in both tissues, whereas rofecoxib did not. The COX-2 mRNA was up-regulated in the intestine but not the stomach after indomethacin treatment, and the reduced PGE2 content was significantly recovered later only in the small intestine, in a rofecoxib-inhibitable manner. Indomethacin produced hypermotility in the small intestine but not the stomach, whereas rofecoxib had no effect. These results suggest that the PG deficiency caused by a low dose of indomethacin produces hypermotility and COX-2 expression in the small intestine but not the stomach, resulting in damage when COX-2 is inhibited. It is assumed that the hypermotility response is a key event in the expression of COX-2 and thereby important in the development of mucosal damage in the gastrointestinal tract.