Nucleic acid metabolism in the gastrointestinal tract of the mouse during fasting and restraint-stress

Importance of nitric oxide and capsaicin-sensitive afferent nerves in healing of stress lesions induced by epidermal growth factor

Epidermal growth factor (EGF) is a potent mitogen implicated in gastroprotection and ulcer healing, but its possible interaction with nitric oxide (NO) and sensory nerves on healing after acute gastric damage has not been assessed. We examined the effects of topical application of a small dose (0.5 mg/kg) of capsaicin to stimulate sensory nerves and a larger parenteral dose of capsaicin (125 mg/kg s.c.) to deactivate these neurons or the effect of systemic administration of NG-nitro-L-arginine methyl ester (L-NAME) (20 mg/kg i.v.) to suppress NO synthase on healing of gastric lesions induced by 3.5 h of water immersion and restraint stress (WRS) in rats without or with EGF administration. Rats were sacrificed at 0, 6, 12, or 24 h after WRS and the gastric blood flow (GBF) was measured by the H2 gas clearance technique. Exposure to WRS produced many gastric lesions, with a marked decrease in GBF, but at 12 h these lesions started to heal and the lesion number was reduced by 75% after 24 h. This was accompanied by progressive increase in the GBF and an increase in expression of EGF mRNA in gastric mucosa, as detected by RT-PCR. Pretreatment with L-NAME or functional ablation of sensory nerves by capsaicin significantly delayed the healing of WRS lesions and accompanying hyperemia. In contrast, pretreatment with EGF (100 micrograms/kg s.c.) or glyceryl trinitrate (10 mg/kg i.g.), a donor of NO, or stimulation of sensory nerves by topical capsaicin significantly enhanced the healing of these lesions and increased the GBF. The acceleration of the healing and accompanying hyperemia induced by EGF at 12 h after WRS were completely reversed in rats pretreated with L-NAME or in those with capsaicin denervation. Addition of L-arginine but not D-arginine to L-NAME restored the healing of stress lesions and gastric hyperemia induced by this peptide. Removal of salivary glands, which reduced luminal content of EGF and DNA synthesis by about fourfold compared to rats with intact glands, produced a significant delay in healing, and this was further aggravated by capsaicin denervation. We conclude that EGF, sensory nerves, and NO play an important role in the healing of gastric mucosa from lesions induced by stress and that sensory nerves and NO appear to interact with EGF in the mechanism of mucosal recovery from stress lesions.

Expression of epidermal growth factor and transforming growth factor-alpha after exposure of rat gastric mucosa to stress

BACKGROUND

This study was designed to determine whether transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), and their common receptor (EGFR) are involved in the recovery of the gastric mucosa after exposure to water immersion and restraint stress.

METHODS

Wistar rats were exposed to a standard period (3.5 h) of water immersion and restraint stress. Animals were killed immediately or 2 h, 4 h, 6 h, or 12 h after the stress. Tissues were removed, the area of the ulcerations was measured planimetrically, half of the stomach was taken for measurement of DNA synthesis, and the other half was embedded in paraffin. Sections were stained immunohistochemically for proliferating nuclear antigen (an index of cellular proliferation) and TGF-alpha, EGF, and EGFR.

RESULTS

A single stress insult resulted in numerous haemorrhagic erosions in the oxyntic mucosa and a significant drop in DNA synthesis. During the recovery phase a marked increase in the expression of EGF peaked at 4 h, whereas the expression of EGFR peaked 6 h after stress. Thereafter the labelling indices for EGF and EGFR decreased, whereas DNA synthesis showed a gradual increase starting after about 6 h and peaking 12 h after the stress. In contrast, immunohistochemical expression of TGF-alpha showed a constant increase for up to 12 h after stress. Cell proliferation reached a maximum after 6 h and returned to normal values 12 h after the stress.

CONCLUSIONS

EFG and TGF-alpha and their receptors are involved in the mucosal recovery from stress, and this is followed by enhanced DNA synthesis and mucosal cell proliferation.

Adaptation of the gastric mucosa to stress. Role of prostaglandin and epidermal growth factor

This study was designed to determine whether repeated exposures to stress lead to the adaptation of the gastric mucosa to stress ulcerogenesis. Wistar rats with intact or resected salivary glands were exposed to a standard period (3.5 h) of water-immersion and restraint stress every other day up to 8 days. The significant reduction in the severity of gastric lesions was first noticed after the second exposure to stress and was maximal after 6-day exposures to stress. This tolerance to stress ulcerogenesis disappeared after a 6-day rest during which animals were not exposed to stress. Histologically, the hemorrhages and edema seen after a single stress were less frequent during adaptation; instead the mucosa regenerated in spite of continuation of exposure to stress. During adaptation, the mucosal blood flow (MBF) and mucosal biosynthesis of PG were markedly increased. Administration of indomethacin (5 mg/kg i.p.) completely abolished gastric adaptation to stress and this was accompanied by about 85% reduction in mucosal generation of PG and significant decrease in the MBF. Salivectomy, which significantly reduced the luminal contents of epidermal growth factor (EGF) in the stomach, delayed and reduced the adaptation. We conclude that the stomach has the ability to adapt to repeated exposures to stress and that this adaptation is mediated, at least in part, by endogenous PG and EGF.

Role of salivary glands and epidermal growth factor (EGF) in gastric secretion and mucosal integrity in rats exposed to stress

EGF, produced mainly by salivary glands, inhibits gastric acid secretion, stimulates the proliferation of gastric mucosal cells and protects the mucosa against various ulcerogens, but its role in the pathogenesis of stress ulcerations is unknown. In this study, rats with intact or resected salivary glands were exposed to water immersion and restraint stress (WRS) without and with pretreatment with exogenous EGF or dimethyl PGE2 (dmPGE2) at doses which were shown previously to protect the mucosa against topical irritants. During 1.5-12 h of WRS, the formation of gastric ulcerations increased progressively with the duration of stress reaching peak after 6 h of stress and being significantly higher in rats with removed salivary glands than in intact animals. Gastric acid secretion and DNA synthesis in oxyntic mucosa declined with the duration of WRS, but after sialoadenectomy a significant increase in gastric acid secretion and a further decline in DNA synthesis were observed after WRS. EGF contents in the gastric lumen and the gastric mucosa were several times higher in rats subjected to stress than in control unstressed animals, indicating that stress causes an extensive release of EGF. Both exogenous EGF (17 nmol/kg/h) and dmPGE2 (143 nmol/kg) prevented, in part, the formation of gastric lesions, while inhibiting gastric acid secretion both in rats with intact or resected salivary glands. We conclude that water immersion and restraint stress is accompanied by an excessive release of EGF, which appears to attenuate gastric secretion, enhances the DNA synthesis and may limit the formation of stress-induced gastric ulcerations.

Role of epidermal growth factor, prostaglandin, and sulfhydryls in stress-induced gastric lesions

Epidermal growth factor promotes the growth of and protects gastric mucosa against various ulcerogens, including stress, but little is known about its role in the pathogenesis of stress ulcerations. In this study, Wistar rats with intact and resected salivary glands were exposed to water-immersion and restraint stress. During 2-14 hours of water-immersion restraint stress, the formation of gastric ulcerations increased progressively and the duration of stress was accompanied by a decrease in DNA synthesis in the gastric mucosa. Following sialoadenectomy, a significant increase in the number of stress ulcerations and further reduction in DNA synthesis were observed. Exogenous epidermal growth factor and dimethyl prostaglandin E2 significantly reduced the ulcerations in the stressed rats with intact salivary glands, but this reduction was significantly less pronounced after sialoadenectomy. Water-immersion restraint stress also resulted in about 50% reduction in mucosal prostaglandin E2 generation, and the pretreatment with indomethacin, which suppressed prostaglandin E2 by about 90%, almost doubled the number of stress ulcerations and abolished the gastro-protective effect of exogenous epidermal growth factor (but not dimethyl prostaglandin E2) against the stress lesions. An inhibition of ornithine decarboxylase activity by difluoromethyl ornithine also augmented stress-induced ulcerogenesis and abolished the protective action of epidermal growth factor while the administration of spermine almost completely prevented stress ulcerations in rats both without and with pretreatment with difluoromethylornithine. Water-immersion restraint stress also significantly reduced mucosal content of glutathione. Cysteamine increased tissue glutathione and reduced stress ulcerations but N-ethylmaleimide, an sulfhydryl blocker, decreased mucosal content of glutathione without affecting the stress ulcerations. This study indicates that the stress ulcers are accompanied by the reduction in mucosal synthesis of DNA, prostaglandin, and glutathione and that the presence of salivary glands attenuates the stress ulcerogenesis probably by releasing epidermal growth factor which acts, in part, by enhancing ornithine decarboxylase activity, mucosal growth, and prostaglandin and glutathione formation.

Gastric mucosal damage in sepsis--effects of pretreatment with a synthetic prostaglandin E1 analogue

An experimental model was used which includes intragastric instillation of 80 mM HCl and 0.6 ml bile/kg followed by intravenous infusion of live E coli in cats for up to three hours. This procedure regularly induces gastric mucosal ulcerations. Mucosal blood flow was measured by microspheres before, early, and late in sepsis. Total gastric blood flow was recorded electromagnetically. Mucosal regeneration capacity as reflected by the RNA/DNA ratio was measured. Misoprostol (a PGE1 analogue) was infused iv (5 micrograms/kg X h) or given locally in the stomach (10 micrograms/kg) before bacteriemia. Misoprostol did not influence the haemodynamic response to bacteria. The gastric mucosal damage was assessed either as an index representative for the entire corpus-fundus region or as the number of areas with intact surface epithelium within the series. Misoprostol iv protected the mucosa from ulceration compared with untreated septic controls while misoprostol intragastrically significantly reduced the number of damaged areas only. Topical misoprostol increased total gastric and mucosal blood flows early in sepsis compared to iv or no pretreatment while no difference was seen during late sepsis. The protective effect of misoprostol was thus not dependent on increased gastric mucosal blood flow. Nor was it mediated through effects on mucosal nucleic acid concentrations or ratio.

Prostaglandin prevents alterations in DNA, RNA, and protein in damaged gastric mucosa

Fasted rats were given either 16,16-dimethyl-PGE2 (dmPGE2) (1 microgram/kg) or normal saline subcutaneously followed by the oral administration of 1 ml of 100% ethanol or saline 30 min later. At 1, 3, 6, and 24 hr later, animals were sacrificed, their stomachs examined for necrotic ulcerations, and the incorporation of [3H]thymidine into DNA as well as tissue levels of DNA, RNA, and protein content of glandular mucosa determined. Compared with control animals, severe ulcerations of 70-80% of the glandular mucosa were observed in rats given 100% ethanol at all time periods. Accompanying these ulcerations were marked depressions in tissue levels of DNA and RNA at 1, 3, 6, and 24 hr after exposure to ethanol, and protein at 1, 3, and 6 hr following ethanol. In rats pretreated with dmPGE2 before ethanol administration, these alterations in tissue levels of DNA, RNA, and protein were prevented as were ulcerations of the glandular stomach at each time period. Synthesis of mucosal DNA was not significantly different from control rats in any of the groups studied. It is concluded that (1) gastric mucosal damage by alcohol is associated with a decrease in tissue levels of DNA, RNA, and protein; (2) dmPGE2 maintains normal tissue levels of DNA, RNA, and protein by preventing the shedding of mucosal cells by alcohol; and (3) the ability of dmPGE2 to prevent gastric damage by alcohol is not mediated through stimulation of DNA synthesis.

Evidence for an enterotropic hormone: compensatory hyperplasia in defunctioned bowel

The relative contributions of luminal and systemic factors to the development of compensatory intestinal hyperplasia were assessed by comparing the adaptive response to jejunal resection in functioning and defunctioned ileum. In Thiry--Vella loops of upper ileum, mucosal contents of RNA and DNA increased by 16--21 per cent 2--7 days after jejunectomy (P = 0.05--0.01), but DNA specific activity was unchanged. In lower ileum remaining in continuity with the nutrient stream, nucleic acids were elevated by 27--86 per cent (P less than 0.001) and specific activity by 34--36 per cent (P = 0.05--0.005). Proximal to the site of resection, duodenal hyperplasia was equally rapid but less intense than in the distal bowel. Full adaptation requires both intraluminal nutrients and enterotropic hormones to be present.