Gastric mucosal injury and adaptation to oral and rectal administration of naproxen

INTRODUCTION

Oral nonsteroidal anti-inflammatory drugs (NSAIDs) cause acute gastric mucosal injury but the relative importance of systemic and topical effect of NSAIDs to overall gastric damage remains uncertain.

METHODS

Twenty-four healthy volunteers were allocated either oral or rectal naproxen 500 mg b.d. and gastroscoped before and during days 1, 7 and 28 of dosing. Macroscopic gastric damage was assessed using a modified Lanza score, mucosal blood flow recorded using laser Doppler flowmetry and prostaglandin E2 (PGE2) measured in antral mucosal biopsies.

RESULTS

Maximal gastric damage occurred during the first 24 h in the oral naproxen group and was associated with a fall in antral mucosal blood flow (mean +/- S.E.M.) from 58.2 +/- 3.3 to 46.6 +/- 4.1 arbitrary units (a.u.) (P < 0.05). With continued administration of oral naproxen, gastric damage resolved and antral mucosal blood flow returned to baseline (54.2 +/- 3.7 a.u.). No macroscopic damage or significant changes in mucosal blood flow were observed during rectal administration. There was no significant difference between mucosal PGE2 concentrations in those receiving oral or rectal naproxen, falling from an initial level of 335 +/- 29 to 155 +/- 49 pg/mg at day 1 (P = 0.06) in those receiving oral naproxen and from 235 +/- 55 to 107 +/- 31 pg/mg at day 1 (P = 0.1) in those receiving rectal naproxen, and remaining suppressed throughout the study in both groups.

CONCLUSIONS

These observations suggest that acute mucosal damage and changes in mucosal blood flow are caused by the topical rather than systemic actions of naproxen.

Gastric mucosal adaptation to etodolac and naproxen

BACKGROUND

Nonsteroidal anti-inflammatory drugs (NSAIDs) frequently cause damage to the gastroduodenal mucosa, principally by suppressing mucosal prostaglandin synthesis. However, such acute mucosal injury usually resolves, despite continued NSAID administration, by a process known as adaptation. Newer NSAIDs, such as etodolac, have been developed to minimize effects on prostaglandin synthesis.

AIM

To determine whether etodolac causes less acute damage than naproxen, and whether the damage produced resolves with continued NSAID administration.

METHODS

Twenty-four healthy volunteers were given a 28-day course of either etodolac 300 mg b.d. or naproxen 500 mg b.d. Gastroduodenal damage was assessed using a modified Lanza scoring system and mucosal blood flow with laser doppler flowmetry at endoscopy before NSAID administration and during days 1, 7 and 28 of continued intake.

RESULTS

Maximum gastric damage (median grade and interquartile range, IQR) occurred during the first 24 h of administration, being greater with naproxen (2.0, IQR 1.0-3.0) than etodolac (1.0, IQR 1.0-1.5; P = 0.03). Such damage was associated with a fall in antral blood flow in the naproxen group (mean +/- S.E.M.) from 54.5 +/- 3.4 to 43.8 +/- 3.4 arbitrary units (P = 0.07) and a slight increase in mucosal blood flow in the etodolac group from 43.5 +/- 2.24 to 49.5 +/- 3.6 arbitrary units. With continued intake this damage resolved in all subjects taking etodolac and in eight of 14 subjects on naproxen. Resolution in the naproxen group was associated with a return to normal of antral blood flow.

CONCLUSIONS

These observations suggest that etodolac causes less mucosal damage than naproxen and that adaptation occurs to both.

Neural influences on human esophageal and salivary alkali secretion

Esophageal secretion HCO3- ions occurs in opossum and man and may contribute to mucosal defense. Using a perfusion technique, neuroregulatory influences on esophageal and salivary HCO3- secretion were investigated in 24 healthy human subjects. The sight and smell of food increased median salivary HCO3- output from 424 to 573 mumol/15 min (P = 0.014), without significantly altering esophageal HCO3- secretion (74-105 mumol/15 min, P = 0.24). Atropine reduced both salivary (610 to 68, 17, 10, and 3 mumol/15 min in successive periods; P < 0.028) and esophageal HCO3- output (108 to 78, 35, 18, and 7 mumol/10 cm/15 min; P < 0.028, respectively. Following atropinization, cholinergic stimulation failed to increase salivary secretion but did "unmask" a small rise in esophageal alkali output (7 to 27 mumol/10 cm/15 min, P = 0.036), implicating a noncholinergic mechanism. Cold-induced pain activated sympathetic reflexes and reduced esophageal HCO3- output (91 to 64 mumol/10 cm/15 min, P = 0.041) without influencing salivary secretion. These observations support a role for the autonomic nervous system in modulating human esophageal and salivary HCO3- secretion.

The biosynthesis of threonine by mammalian cells: expression of a complete bacterial biosynthetic pathway in an animal cell

The coding regions for the Escherichia coli gene for aspartokinase I/homoserine dehydrogenase I (thrA) and the Corynebacterium glutamicum gene for aspartic semialdehyde dehydrogenase (asd) have been subcloned into a Simian Virus 40 (SV40)-based mammalian expression vector. Both enzyme activities are expressed in mouse 3T3 cells after transfer of the corresponding chimaeric gene. The kinetic parameters are similar to those of the native bacterial enzymes, and aspartokinase I/homoserine dehydrogenase I retains its allosteric regulation by threonine. An extract of the cells expressing aspartokinase I/homoserine dehydrogenase I, mixed with one from cells expressing aspartic semialdehyde dehydrogenase, produced homoserine when the mixture was incubated with aspartic acid, ATP and NADPH. The thrA and asd expression cassettes were combined into a single plasmid which, when transfected into 3T3 cells, enabled them to produce homoserine from aspartic acid. Homoserine-producing 3T3 cells were transfected with the plasmid pSVthrB/C (homoserine kinase and threonine synthase) and selected for growth on homoserine. Cell lines isolated from these cells expressed the complete bacterial threonine pathway, were independent of threonine for growth and could be maintained in medium which contained no free threonine. The threonine in the proteins of these cells became enriched in 15N when the culture medium contained [15N]aspartic acid. The production of homoserine and the growth of cells was at a maximum when there was more than 2.5 mM aspartate in the medium. Below this concentration the high Km of aspartokinase limited the flux through the pathway. In the presence of additional aspartic acid the new pathway could sustain a cell cycle time close to that of the same cells cultured in threonine-containing medium.

Review article: factors protecting the oesophagus against acid-mediated injury

Reflux of gastric acid and pepsins into the lower oesophagus causes symptoms such as heartburn and nausea, and tissue injury leading to erosive oesophagitis and stricture formation. This article reviews the mechanisms involved in protecting the oesophagus against acid-mediated injury, including the role of the lower oesophageal sphincter, secondary oesophageal peristalsis and swallowed saliva. The oesophageal mucosa has inherent abilities to resist acid damage, and recent data from three laboratories suggest a secretory function with local production of bicarbonate and mucus responsive to local acidification. The evidence for these putative oesophageal defence mechanisms is discussed.

Effect of topical oesophageal acidification on human salivary and oesophageal alkali secretion

Recent human studies suggest that oesophageal HCO3- secretion, in conjunction with salivary HCO3- secretion and secondary oesophageal peristalsis, is important for the protection of oesophageal mucosa from refluxed gastric contents. This study evaluated simultaneously the responsiveness of oesophageal and salivary HCO3- secretion to oesophageal acidification in eight healthy subjects. A 10 cm segment of oesophagus was perfused at a constant rate of 5 ml/min with a specially designed tube assembly. Saline was used initially, and then 10 mM and 100 mM HCl. The perfusates contained 3H-polyethylene glycol (PEG) as a concentration marker to determine volumes. Corrections were applied for a small degree of contamination by swallowed saliva and refluxed gastric alkali. Oesophageal perfusion with 10 mM HCl did not cause symptoms (nausea and heartburn), but tripled the oesophageal HCO3- output from a baseline of 51 mumol/10 cm/10 min (p = 0.021), while doubling the rate of salivary HCO3- secretion from a median basal value of 140 mumol/10 min (p = 0.021). Oesophageal perfusion with 100 mM HCl was associated with symptoms of nausea and heartburn in all subjects. The median oesophageal HCO3- output increased 32 fold to 1659 mumol/10 cm/10 min (interquartile range 569 to 3373; p = 0.036), and salivary HCO3- secretion approximately tripled from basal values (p = 0.036). In conclusion, oesophageal acidification stimulates both salivary and oesophageal HCO3- secretion, responses which may be protective to the oesophageal epithelium.

The expression of Escherichia coli diaminopimelate decarboxylase in mouse 3T3 cells

We have subcloned the coding sequence for the Escherichia coli lysA gene coding for diaminopimelic acid decarboxylase (DAP decarboxylase) into a eukaryotic expression vector based on the SV40 early promoter. The activities of a series of constructs with different lengths of non-coding DNA at the 5' and 3' ends of the coding region have been compared by measuring the synthesis of lysine from diaminopimelic acid (DAP) in mouse 3T3 cells. A short non-coding sequence at the 3' end reduced the expression of enzyme activity. Stable lines of 3T3 cells have been produced by co-transfection of the chimeric gene with a plasmid coding for G-418 resistance. Cells were grown in medium containing G-418 and resistant clones were screened for an ability to synthesise lysine from DAP. [3H]Lysine produced from [3H]DAP was incorporated into cell proteins. An enzyme extract from a cell line which had incorporated two copies of the gene synthesised 0.082 nmol of lysine/min per mg protein. In the intact cell the rate of lysine synthesis is limited by the uptake of DAP which is taken up at only 5% of the rate of lysine. lysA has a potential as a reporter gene in studies of gene expression in mammalian cells.

Threonine synthesis from homoserine as a selectable marker in mammalian cells

The plasmid pSVthrBC expresses the Escherichia coli thrB (homoserine kinase) and thrC (threonine synthase) genes in mouse cells and enables them to synthesize threonine from homoserine. After transfection with pSVthrBC and culture in medium containing homoserine, only cells that have incorporated pSVthrBC survive. Homoserine at concentrations greater than 1 mM is toxic to mammalian cells. Mouse cells selected from medium containing 5 mM homoserine had incorporated 20-100 copies of the plasmid per cell and had homoserine kinase activities of 0.001-0.012 nmol/min per mg of protein per copy. Cells selected from medium containing 10 mM homoserine had incorporated one or two copies of the plasmid per cell and had homoserine kinase activities of 0.06-0.39 nmol/min per mg of protein per copy. By using high concentrations of homoserine, it is possible to use pSVthrBC to select and isolate cell lines that have one or two copies of the plasmid incorporated into an active region of chromatin. CHO and HeLa cells have also been successfully transfected with pSVthrBC. COS-7 cells are naturally resistant to homoserine as they are able to metabolize homoserine.

Measurement of bicarbonate output from the intact human oesophagus

Injury of the oesophageal mucosa can result from exposure to refluxed gastric acid and pepsin. Competence of the lower oesophageal sphincter and peristaltic activity serve to reduce contract time between luminal acid and oesophageal mucosa, but intraluminal neutralisation of residual acid by bicarbonate may also be important in preventing oesophageal mucosal injury. Whereas swallowed saliva contains bicarbonate, recent experiments have also demonstrated alkali secretion from the mammalian oesophagus. Bicarbonate secretion from the human oesophagus was therefore examined with an intubation technique and perfusion of the oesophagus with a non-absorbable marker. Saliva, gastric, and oesophageal aspirates were collected and bicarbonate concentrations determined by measurements of pH and pCO2 or by back titration. In 32 normal subjects (17 women, 15 men) median basal oesophageal bicarbonate secretion determined by a pH/pCO2 method was 416 (range 139-1050) mumol/hour/10 cm. In a subgroup of 15 experiments median oesophageal bicarbonate output was 489 (range 157-1033) mumol/hour/10 cm (pH/pCO2 method) compared with a median alkali output of 563 (range 135-799) mumol/hour/10 cm as determined by back titration. The difference was not significant. Salivary contamination of the oesophagus accounted for 25% of all bicarbonate measured within the oesophagus and refluxed gastric bicarbonate accounted for 2.5%. Bicarbonate secretion from the normal human oesophagus may, in combination with swallowed salivary bicarbonate, play a part in preventing oesophageal mucosal damage due to refluxed gastric acid and pepsin.

The expression of Escherichia coli threonine synthase and the production of threonine from homoserine in mouse 3T3 cells

We have subcloned the coding sequence for the Escherichia coli threonine synthase gene into a eukaryotic expression vector based on the simian-virus-40 early promoter. When mouse 3T3 cells which already expressed homoserine kinase were transfected with the new plasmid, the cells were able to incorporate radioactivity from [14C]homoserine into their cell proteins. Stable cell lines were established by co-transfecting 3T3 cells with the plasmid coding for threonine synthase and another coding for homoserine kinase and G-418 (Geneticin) resistance. Cells were selected for G-418 resistance and then screened for an ability to synthesize threonine from homoserine and incorporate it into the cell protein. A cell line which expressed both the homoserine kinase and threonine synthase genes was capable of growth in a threonine-deficient medium containing homoserine.

Expression of Escherichia coli homoserine kinase in mouse 3T3 cells

The Escherichia coli gene for homoserine kinase (thrB) has been cloned into a simian-virus-40-based eukaryotic expression vector which also includes a neomycin-resistance gene. Mouse 3T3 cells transfected with this plasmid were selected for resistance and screened for homoserine kinase activity. It has thus been possible to isolate clones which are capable of accumulating homoserine O-phosphate when supplied with homoserine. In broken-cell preparations the kinetic constants for the production of homoserine O-phosphate were similar to those of the wild-type E. coli enzyme. These experiments demonstrate that E. coli homoserine kinase can be expressed in an animal cell and that it can successfully phosphorylate L-homoserine in the intact cell utilizing endogenous ATP.

Mucosal adaptation to indomethacin induced gastric damage in man--studies on morphology, blood flow, and prostaglandin E2 metabolism

The effect of 28 days' continuous administration of oral indomethacin on gastroduodenal morphology, gastric mucosal blood flow, and gastric mucosal prostaglandin E2 (PGE2) metabolism in man was studied to define further the mechanisms of mucosal injury induced by indomethacin. Indomethacin caused acute gastroduodenal damage in all cases, which was maximal at 24 hours of administration. With continued intake, mucosal adaptation occurs resulting in resolution of endoscopic mucosal damage. At the time of maximal mucosal damage, gastric mucosal blood flow was significantly reduced compared with values before treatment (p less than 0.001 in fundus and p less than 0.002 in antrum), with good correlation between the severity of damage and the magnitude of the reduction in blood flow (r = 0.76). Mucosal recovery was associated with a return of the blood flow to normal. PGE2 in mucosal homogenate was significantly reduced by indomethacin in both the fundus (p less than 0.01) and antrum (p less than 0.01) after 24 hours but there was no correlation between the magnitude of this reduction and the severity of mucosal damage (r = -0.34). Despite mucosal recovery by 28 days, PGE2 values remained significantly below those before treatment in both the fundus (p less than 0.01) and antrum (p less than 0.01). The PGE2 degradation capacity was not influenced by indomethacin. In conclusion, mucosal adaptation to acute damage by indomethacin occurs in man and seems independent of local PGE2 metabolism.

Prevention of peptic ulcer relapse by sucralfate: mechanisms of action

Sucralfate has a complex effect on the luminal and mucosal environment of the stomach and duodenum. Some of the actions are important in ulcer healing whilst others are important in preventing subsequent ulcer relapse. Although sucralfate has little direct effect on acid secretion, there is evidence that after ulcer healing with this drug, parietal cell responsiveness is reduced. This may in part be mediated by increased somatostatin release from gastric D cells and may be important in reducing ulcer relapse. Sucralfate has been shown to increase mucosal resistance to damaging agents, such as ethanol and aspirin. Studies have shown that this protective action may be related to the drugs effect on various protective zones such as the 'mucous-bicarbonate' barrier, mucosal hydrophobicity, epithelial cell function and morphology, and mucosal blood flow. These complex actions of sucralfate are in part related to direct interaction between the drug or its components and gastroduodenal tissues, and in part related to effects on various mediators of tissue injury and repair.

Mechanisms of gastroduodenal protection by sucralfate

Over the past 5-10 years, a number of studies have shown that topical sucralfate enhances a number of gastric and duodenal mechanisms, e.g., the "mucus-bicarbonate barrier," mucosal hydrophobicity, mucosal blood flow, cell viability, and local production of prostaglandins, as well as endogenous mediators of tissue injury and repair. It seems likely that the complex actions of sucralfate are in part related to direct interaction between the drug or its components (aluminum, sucrose, and sulfate) and gastric mucosal tissues, and in part related to effects of the drug on the various mucosal mediators of tissue injury and repair. Local actions may play a role in accelerating healing of ulcer-damaged mucosa, but this does not explain the protective actions of sucralfate on normal mucosa. Thus sucralfate appears to enhance the protective function of the "mucus-bicarbonate" barrier by actions on both components. This may depend in part on an interaction with the unstirred layer overlying gastric epithelium. Sucralfate has also been shown to increase the hydrophobicity of mucus gel. There is little doubt that sucralfate increases local production and release of protective prostaglandins (PGs), but the precise role played by these agents in mediating mucosal protection and in chronic ulcer healing remains uncertain. Currently, the mechanism of action of sucralfate on vascular integrity remains unknown and the role of PGs in this protective function is unclear. There is little evidence that epidermal growth factor plays any role in mediating mucosal protection by sucralfate, but it may be important in its ulcer-healing action. Sucralfate has been shown to be truly "cytoprotective" in that it protects isolated epithelial cells from damage by noxious agents. In animals treated with sucralfate, the surface epithelial cells were disrupted, but necrotic lesions in the deep proliferative zone were virtually absent. It seems likely that investigations of the actions of sucralfate and its components will move ever closer to defining the target cells, the intracellular events, and the mediators that bring about its protective and ulcer-healing activity.

Sarcosine kinetics in pigs by infusion of [1-14C]sarcosine: use for refining estimates of glycine and threonine kinetics

To investigate in vivo the interconversion between glycine (Gly) and its N-methyl product sarcosine (Sar), [1-13C]Gly and [1-14C]Sar were infused into hourly fed pigs receiving diets with low- and high-threonine levels. An open two-pool model was developed to calculate Sar demethylation (DM) and Gly methylation (GM). During [1-14C]Sar infusion, intracellular Gly specific radioactivities (SA) in the liver and kidney were higher than plasma Gly SA, suggesting that demethylation of Sar occurred in those tissues. DM estimated by using hippuric acid (HA) as the production pool had a mean value of 1.55 mumol.kg-1.h-1, similar to the Sar production rate (mean 1.85 mumol.kg-1.h-1). GM was undetectable (less than 0.5 mumol.kg-1.h-1). These results suggest that, in fed pigs, Sar is produced mainly from choline catabolism and is degraded only to Gly in liver and kidney. On the assumption that Sar degradation gave rise only to Gly, the production rate of Gly (Gly PR) was calculated from [1-13C]Gly and [1-14C]Sar infusions using either the primary pools (plasma Gly and HA, respectively) or the secondary pools (HA and plasma Gly, respectively). The results were explained by a liver-plasma Gly exchange model. The whole body Gly irreversible loss, i.e., direct loss from plasma and liver, was calculated from this model to be 832 +/- 58 mumol.kg-1.h-1, showing that the estimation of Gly PR with [1-13C]Gly infusion and plasma Gly enrichment (599 +/- 56 mumol.kg-1.h-1) was a significant underestimate of the true value.(ABSTRACT TRUNCATED AT 250 WORDS)

A molecular biological approach to reducing dietary amino acid needs

Rapid developments in transgenic animal technology make it possible to consider introducing new metabolic capabilities into animals, using genes from other species. Lysine and threonine are both essential amino acids in mammals, and are commonly the first and second limiting amino acids, respectively, for protein accretion in pigs and poultry fed cereal based diets. Here we consider the potential for transgenic animals with microbial biosynthetic pathways for these amino acids.

Quantitative partition of threonine oxidation in pigs: effect of dietary threonine

Kinetic aspects of threonine (Thr) metabolism were examined in eight pigs fed hourly with a diet containing either 0.68% (LT group) or 0.81% (HT group) of Thr (wt/wt), corresponding to 10 and 30% Thr excess, respectively, compared with an "ideal" diet. Primary production (PR) and disposal (DR) rates were obtained for Thr, glycine (Gly), and 2-keto-butyrate (KB) after a 12-h continuous infusion of L-[U-14C]-Thr together with [1-13C]Gly and a 6-h continuous infusion of [1-14C]KB. Transfer of Thr into secondary pools was also monitored, and from these the rates of Thr oxidation through the catabolic pathways of L-Thr 3-dehydrogenase (DR(Thr-Gly)) and threonine dehydratase (DR(Thr-KB)) were estimated. For the LT group the results were (mumol.kg-1.h-1) PR(Thr) 314 +/- 3, PR(Gly) 551 +/- 24, PR(KB) 41 +/- 3, DR(Thr-Gly) 22 +/- 2, and DR(Thr-KB) 7 +/- 1. For the HT group they were PR(Thr) 301 +/- 23, PR(Gly) 598 +/- 55, PR(KB) 39 +/- 4, DR(Thr-Gly) 32 +/- 2, and DR(Thr-KB) 8 +/- 1. The increase in Thr intake (14 mumol.kg-1.h-1, P less than 0.01) induced a commensurate increase in the sum of DR(Thr-Gly) and DR(Thr-KB) (14 mumol.kg-1.h-1, P less than 0.001) when liver was used as the precursor pool. This was mainly due to the increased DR(Thr-Gly) (13 mumol.kg-1.h-1, P less than 0.01); the change in DR(Thr-KB) was not statistically significant. By comparison of intracellular-to-plasma ratios of specific activities (or enrichments) for different tissues with each type of infusion, liver was shown to be the major site of production of Gly and KB from Thr. These data suggest that in fed growing pigs a 30% excess of Thr in the diet does not alter the partition of Thr oxidation, since 80% of Thr oxidation occurs through the L-Thr 3-dehydrogenase pathway for both LT and HT groups.

The effects of indomethacin on gastroduodenal morphology and mucosal pH gradient in the healthy human stomach

To define further the injury and the mechanisms of mucosal injury induced by indomethacin, the effect of 28-day continuous administration of oral indomethacin on gastroduodenal morphology, gastric histology, and the protective mucus-bicarbonate barrier overlying gastroduodenal mucosa in humans was studied. In the studies, indomethacin caused acute gastroduodenal damage in 100% of cases, with maximal damage at 24 hours of administration. With continued intake this damage resolves, although a minority (two study subjects) progressed to discrete ulceration. Why these two subjects failed to adapt is unknown. Biopsy specimens taken during the studies showed no significant changes in inflammatory or regenerative features, and thus failed to shed any light on this process of adaptation to damage. Mucosal pH gradient studies showed a significant increase in juxtamucosal pH at the time of maximal damage (24 hours); this is thought to represent passive diffusion of alkali from damaged mucosa. In conclusion, mucosal adaptation to acute damage by indomethacin occurs in humans. The mechanisms through which the mucosa adapts in this intriguing way remain unknown.