Repair of rat gastric mucosa: effect of 16,16-dimethyl prostaglandin E2.

Prostaglandins protect the gastric mucosa against a variety of injurious agents and may accelerate the recovery of the gastric mucosa following damage. In previous studies prostaglandins were given prior to the injurious agent, so it was not possible to distinguish their potential effects on accelerating repair or reducing initial damage. We have investigated the effect of 16,16-dimethyl prostaglandin E2 (dmPGE2) on the repair of the gastric mucosa after injury induced by several injurious agents. dmPGE2 was given orally 15 min prior to aspirin or sodium salicylate, or 30 min after aspirin, sodium salicylate, or ethanol. dmPGE2 delivered prior to injury reduced the aspirin-induced fall in mucosal potential difference (PD), but had no effect on that induced by sodium salicylate. dmPGE2 administered after ASA injury significantly increased recovery of PD (P < 0.05), but did not alter the rate of recovery of PD with other damaging agents. Histological damage was decreased in rats treated with dmPGE2 after aspirin compared to aspirin-only-treated rats (P < 0.02). Exogenous dmPGE2 protects and restores gastric mucosal integrity after aspirin damage but has no effect on the repair of sodium salicylate and ethanol injured mucosa, suggesting that repair of the gastric mucosa after aspirin damage is enhanced by dmPGE2 due to its ability to prevent ongoing damage, rather than directly enhancing repair processes.

Indomethacin damage to rat gastric mucosa is markedly dependent on luminal pH.

1. There is good evidence that acid is a prerequisite for aspirin induced gastric mucosal damage; however, there is inconsistent information available for non-salicylate NSAID. The present study examines the effect of gastric luminal pH on indomethacin-induced gastric mucosal damage. 2. Macroscopic gastric mucosal damage induced by indomethacin (40 mg/kg) or vehicle, administered intraduodenally to male pylorus-ligated rats (n = 5-10/group), was assessed at four different levels of luminal pH (2,4,5.5 and 7) by means of digital planimetry. 3. There was a marked difference in the extent of damage induced by indomethacin at the different luminal pH levels (P = 0.001). There was no difference between the percentage of haemorrhagic lesions at pH 2 and 4 (P > 0.05), nor between pH 5.5 and 7 (P > 0.05). However, the damage at the high levels of luminal acidity (pH 2 and 4) was strikingly different from that at pH 5.5 and 7 (P < 0.05). 4. Gastric mucosal damage induced by indomethacin, a non-salicylate NSAID, is augmented by the presence of high concentration of acid in the gastric lumen. The main finding, that indomethacin injury is markedly less above pH 4, may have clinical implications in the prevention of NSAID-induced mucosal injury.

Adaptation of the gastric epithelium to injury is maintained in vitro and is associated with increased TGF-alpha expression.

Adaptation is the name given to the progressive decrease in gastric mucosal damage following repeated dosing with damaging agents. This study aimed to determine whether adaptation is an intrinsic property of the gastric epithelium and the role in the development of this process of TGF alpha. Rats were given either one or six daily doses of 10 mg/kg diclofenac or six daily doses of vehicle only (1% methylcellulose). On the 7th day, antral mucosa was taken for organ culture and loaded with [51Cr]. Explants were challenged with ethanol and damage quantified by [51Cr] release. In a separate experiment, rats were dosed as above and the gastric mucosa was extracted and TGF alpha quantified by RIA. The rate of [51Cr] release was significantly lower after ethanol injury in explants from rats previously adapted to diclofenac (9.2 +/- 2.5%) compared with those exposed to a single damaging dose of diclofenac (25.9 +/- 3.5%) or vehicle only (26.4 +/- 3.3%; P < 0.01; ANOVA). The concentration of TGF alpha was significantly higher in the gastric epithelium of rats adapted to diclofenac than other groups (P < 0.05; t-test). Cross adaptation of the gastric mucosa to injury has therefore been demonstrated in antral mucosal explants in organ culture while TGF alpha peptide expression is elevated in the adapted gastric mucosa. These findings suggest that adaptation is an inherent property of the gastric epithelium and it is likely that TGF alpha may play a role in its maintenance.

Correlation between transmucosal potential difference and morphological damage during aspirin injury of gastric mucosa in rats.

The potential difference (PD) that is maintained across healthy gastric mucosa is thought to be due to asymmetric ion pumping combined with resistance to back-diffusion of the separated charge. However, the structures that are responsible for this have not been clearly defined. This study examined the temporal changes in PD in rat stomach after injury by a single dose of aspirin. Multiple linear regression was used to compare this with the time course of several parameters of histological damage: (i) the per cent mucosal length showing superficial (confined to surface and gastric pits), deep (involving the isthmus or deeper in oxyntic glands) and total damage; (ii) the number of discrete erosions; and (ii) the total area of erosions per cm sectioned. Mucosal PD fell during the first 30-60 min after aspirin. Superficial damage appeared early and was already recovering by this time. The time course of deep damage more closely matched the alterations in PD and stepwise regression analysis showed that this could be predicted by the amount of deep damage alone (P < 0.001). Changes in transmucosal PD after acute aspirin injury probably reflect damage to structures in the oxyntic glands and not just the breaking of the surface and pit cell 'barrier'.

Gastric mucosal adaptation to diclofenac injury.

Adaptation occurs to the gastric injury produced by nonsteroidal antiinflammatory drugs during continued dosing. The aim of this study was to identify characteristics of this phenomenon that might help in the search for underlying mechanisms. The time frame for onset and offset of adaptation of diclofenac (damage assessed planimetrically) was examined in rats. Adaptation to oral diclofenac took three to five days to develop, and persisted for up to five days after the last dose. It was also demonstrable after subcutaneous dosing or when injury was measured by a change in mucosal potential difference. Diclofenac-adapted rats were protected against injury induced by subsequent exposure to ethanol, indomethacin, aspirin, or piroxicam, indicating that adaptation is not specific to injury by the adapting agent. This cross-adaptation was dose-dependent and characterized histologically by a reduction in deep damage. In conclusion, gastric adaptation to diclofenac is mediated by mechanisms that take several days to develop and be lost. The route of administration appears to be unimportant, but the development of both adaptation and cross-adaptation is influenced by dosage size.

Gastric mucosal defensive factors: the therapeutic strategy.

There are several interesting approaches to augmenting defence or repair mechanisms that can be used already or may find a place in therapy for ulcer disease. Factors such as epidermal growth factor and basic fibroblast growth factor show potential. Alternative strategies might be to stimulate mucosal blood flow with agents that release nitric oxide (NO), or to scavenge free radicals in the inflamed or ischaemic mucosa. If such approaches are to find a role in therapy, it is likely that it will be restricted: perhaps for the treatment of refractory ulcers, or for prophylaxis of stress ulceration. This is because most ulcers in future are likely to be healed with tolerable and high efficacy acid-inhibiting drugs then have their recurrence prevented by regimens that eradicate Helicobacter pylori. The most important current indication for concentrating on enhancing mucosal defences is for managing non-steroidal anti-inflammatory drug (NSAID)-induced ulcers. There is no clear advantage in using a defence-enhancing agent (rather than an acid suppressant) to heal an NSAID ulcer if the NSAID can be stopped. The main value of prostaglandins is for prophylaxis of NSAID ulcers in those patients who need ongoing treatment with NSAID. For cost-benefit reasons, prostaglandins should probably be used mainly for those at high risk of NSAID complications, and there has been progress in identifying these. Another interesting approach is aimed at clarifying mechanisms of gastric adaptation to NSAID, so that we might be able to design drugs and dosing regimens to maximize this phenomenon.

Gastric mucosal damage induced by nonsalicylate nonsteroidal antiinflammatory drugs in rats is mediated systemically.

The gastric toxicities of an enteric-coated formulation and conventional indomethacin were compared in rats. Both formulations were equally damaging to the mucosa, suggesting that topical damage was not the major route of injury. The importance of systemically mediated damage was further determined by gastrotoxicity dose-response curves and pyloric ligation experiments in which indomethacin was administered either orally or parenterally, or into stomach or duodenum with the pylorus occluded. Gastric damage was significantly higher in those groups that had received the drug parenterally or intraduodenally. The extent of deeper mucosal damage, assessed histologically, was greater in parenterally dosed rats. In further experiments, oral and parenteral routes of administration of two other nonsalicylate NSAIDs, naproxen and sodium diclofenac, were found to be equally damaging to the mucosa. Our results show that indomethacin-induced gastric damage, unlike aspirin injury, is mediated mainly systemically. Enteric-coating may not be a useful strategy in reducing gastric injury by nonsalicylate, nonsteroidal antiinflammatory drugs.

Adaptation of rat gastric mucosa to repeated doses of non-salicylate non-steroidal anti-inflammatory drugs.

To determine whether gastric mucosal adaptation occurs to the damaging effects of repeated non-steroidal anti-inflammatory drug (NSAID) administration, we compared the extent of gastric damage in rats after single or repeated doses of four non-salicylate NSAID. With daily dosing, adaptation occurred only to repeated doses of the short-acting NSAID ibuprofen and diclofenac but not to indomethacin and naproxen, both of which have considerably longer half-lives. Adaptation to indomethacin was demonstrable if the dosage interval was lengthened and the drug was given on alternate days rather than daily. Histological examination of the gastric mucosa of diclofenac-treated rats showed a similar degree of superficial damage in the single and repeatedly dosed groups. However there was a highly significant reduction in the amount of deeper mucosal damage in the repeatedly dosed rats. Our findings show that under certain conditions of dosage, adaptation to non-salicylate NSAID is demonstrable. The pharmacokinetics of individual NSAID appear to be important in determining whether or not adaptation occurs. Histological examination showed that adaptation to one of the shorter acting NSAID, diclofenac, was characterized by a significant reduction in deeper mucosal damage.

The role of acid regulation in the treatment of NSAID-induced mucosal damage.

Gastric acid probably exacerbates mucosal injury caused by non-steroidal anti-inflammatory drugs (NSAIDs) in two ways: first, by increasing absorption of NSAIDs, which are weak acids and predominantly in their undissociated form at low pH--this is probably mainly relevant for salicylates; then, by a 'second wave' of injury that leads to deeper erosions, as gastric acid accesses the partially denuded mucosa. Regulation of gastric acid secretion by acid-inhibitory drugs has been shown to decrease acute NSAID-induced injury to varying extents, depending on the drug used and the method of assessing mucosal damage. Healing of chronic NSAID-induced ulcers is slow if NSAID therapy is continued, but is facilitated by treatment with H2-receptor antagonists or prostaglandins. One study of the acid pump inhibitor omeprazole has shown a high rate of healing, even though NSAID therapy was continued.

Repair and healing of established gastric mucosal injury.

Superficial injury of the gastric mucosa, such as occurs after ethanol or aspirin ingestion, is usually resurfaced in 1 or 2 h. This is accomplished by rapid migration of surface mucous cells to cover the defect, followed by a later increase in cellular proliferation to restore the pits to their original length. This rapid restitution has been documented with electron microscopy, and is also reflected in the speedy return to baseline of the transmucosal potential difference and ionic barrier function after superficial injury. Deeper mucosal injury--which occurs focally after injury by aspirin, other NSAIDs, and restraint stress--is evident as macroscopically visible erosions. These heal more slowly, and are resurfaced after several hours with apparently undifferentiated cells. These are probably derived from surviving surface mucous cells that rapidly dedifferentiate. Cell proliferation peaks after about 12 h, but too late to contribute to the resurfacing process. Specialized cells reappear 1 or 2 days later, and glandular remodeling is needed in the deeper lesions. Ulcers heal by processes analogous to the deeper erosions, but the time needed is much longer (weeks). Additional tasks are the re-formation of the muscularis mucosae, and the creation of pits and glands from the initially single sheet of migrating cells.