Morphological effects of aspirin and prostaglandin on the canine gastric mucosal surface. Analysis with a phospholipid-selective cytochemical stain

Advent of novel phosphatidylcholine-associated nonsteroidal anti-inflammatory drugs with improved gastrointestinal safety

The mucosa of the gastrointestinal (GI) tract exhibits hydrophobic, nonwettable properties that protect the underlying epithelium from gastric acid and other luminal toxins. These biophysical characteristics appear to be attributable to the presence of an extracellular lining of surfactant-like phospholipids on the luminal aspects of the mucus gel layer. Phosphatidylcholine (PC) represents the most abundant and surface-active form of gastric phospholipids. PC protected experimental rats from a number of ulcerogenic agents and/or conditions including nonsteroidal anti-inflammatory drugs (NSAIDs), which are chemically associated with PC. Moreover, preassociating a number of the NSAIDs with exogenous PC prevented a decrease in the hydrophobic characteristics of the mucus gel layer and protected rats against the injurious GI side effects of NSAIDs while enhancing and/or maintaining their therapeutic activity. Bile plays an important role in the ability of NSAIDs to induce small intestinal injury. NSAIDs are rapidly absorbed from the GI tract and, in many cases, undergo enterohepatic circulation. Thus, NSAIDs with extensive enterohepatic cycling are more toxic to the GI tract and are capable of attenuating the surface hydrophobic properties of the mucosa of the lower GI tract. Biliary PC plays an essential role in the detoxification of bile salt micelles. NSAIDs that are secreted into the bile injure the intestinal mucosa via their ability to chemically associate with PC, which forms toxic mixed micelles and limits the concentration of biliary PC available to interact with and detoxify bile salts. We have worked to develop a family of PC-associated NSAIDs that appear to have improved GI safety profiles with equivalent or better therapeutic efficacy in both rodent model systems and pilot clinical trials.

Phosphatidylcholine as a constituent in the colonic mucosal barrier--physiological and clinical relevance

Phosphatidylcholine (PC) is an important constituent of the gastrointestinal tract. PC molecules are not only important in intestinal cell membranes but also receiving increasing attention as protective agents in the gastrointestinal barrier. They are largely responsible for establishing the hydrophobic surface of the colon. Decreased phospholipids in colonic mucus could be linked to the pathogenesis of ulcerative colitis, a chronic inflammatory bowel disease. Clinical studies revealed that therapeutic addition of PC to the colonic mucus of these patients alleviated the inflammatory activity. This positive role is still elusive, however, we hypothesized that luminal PC has two possible functions: first, it is essential for surface hydrophobicity, and second, it is integrated into the plasma membrane of enterocytes and it modulates the signaling state of the mucosa. The membrane structure and lipid composition of cells is a regulatory component of the inflammatory signaling pathways. In this perspective, we will shortly summarize what is known about the localization and protective properties of PC in the colonic mucosa before turning to its evident medical importance. We will discuss how PC contributes to our understanding of the pathogenesis of ulcerative colitis and how reinforcing the luminal phospholipid monolayer can be used as a therapeutic concept in humans.

Effect of indomethacin on bile acid-phospholipid interactions: implication for small intestinal injury induced by nonsteroidal anti-inflammatory drugs

The injurious effect of nonsteroidal anti-inflammatory drugs (NSAIDs) in the small intestine was not appreciated until the widespread use of capsule endoscopy. Animal studies found that NSAID-induced small intestinal injury depends on the ability of these drugs to be secreted into the bile. Because the individual toxicity of amphiphilic bile acids and NSAIDs directly correlates with their interactions with phospholipid membranes, we propose that the presence of both NSAIDs and bile acids alters their individual physicochemical properties and enhances the disruptive effect on cell membranes and overall cytotoxicity. We utilized in vitro gastric AGS and intestinal IEC-6 cells and found that combinations of bile acid, deoxycholic acid (DC), taurodeoxycholic acid, glycodeoxycholic acid, and the NSAID indomethacin (Indo) significantly increased cell plasma membrane permeability and became more cytotoxic than these agents alone. We confirmed this finding by measuring liposome permeability and intramembrane packing in synthetic model membranes exposed to DC, Indo, or combinations of both agents. By measuring physicochemical parameters, such as fluorescence resonance energy transfer and membrane surface charge, we found that Indo associated with phosphatidylcholine and promoted the molecular aggregation of DC and potential formation of larger and isolated bile acid complexes within either biomembranes or bile acid-lipid mixed micelles, which leads to membrane disruption. In this study, we demonstrated increased cytotoxicity of combinations of bile acid and NSAID and provided a molecular mechanism for the observed toxicity. This mechanism potentially contributes to the NSAID-induced injury in the small bowel.

Clinical trial: comparison of ibuprofen-phosphatidylcholine and ibuprofen on the gastrointestinal safety and analgesic efficacy in osteoarthritic patients

BACKGROUND

Chronic use of NSAIDs is associated with gastrointestinal (GI) toxicity that increases with age.

AIM

To evaluate the GI safety and therapeutic efficacy of ibuprofen chemically associated with phosphatidylcholine (PC) in osteoarthritic (OA) patients.

METHODS

A randomized, double-blind trial of 125 patients was performed. A dose of 2400 mg/day of ibuprofen or an equivalent dose of ibuprofen-PC was administered for 6 weeks. GI safety was assessed by endoscopy. Efficacy was assessed by scores of analgesia and anti-inflammatory activity. Bioavailability of ibuprofen was pharmacokinetically assessed.

RESULTS

Ibuprofen-PC and ibuprofen provided similar bioavailability/therapeutic efficacy. In the evaluable subjects, a trend for improved GI safety in the ibuprofen-PC group compared with ibuprofen that did not reach statistical significance was observed. However, in patients aged >55 years, a statistically significant advantage for ibuprofen-PC treatment vs. ibuprofen in the prevention of NSAID-induced gut injury was observed with increases in both mean Lanza scores and the risk of developing >2 erosions or an ulcer. Ibuprofen-PC was well tolerated with no major adverse events observed.

CONCLUSION

Ibuprofen-PC is an effective osteoarthritic agent with an improved GI safety profile compared with ibuprofen in older OA patients, who are most susceptible to NSAID-induced gastroduodenal injury.

Effect of a Hydrophobic Phospholipid Lining of the Gastric Mucosa in Bioadhesion

PURPOSE

The role of a model hydrophobic phospholipid simulating lining of the gastric mucosa, as to adhesion of polymers with different surface functional groups and surface hydrophobicities, was evaluated using an in vitro gastric mucus model.

MATERIALS AND METHOD

Front-faced fluorescence measurement was used to determine adhesion of fluorescent polystyrene microspheres with different surface functional groups. Contact angle measurements and sticking bubble technique were used to measure relative surface hydrophobicity of the polymers.

RESULTS

Adhesion of fluorescent polystyrene microspheres using front-faced fluorescence measurement revealed the hydrophobic phospholipid lining of the in vitro gastric mucus model did not allow adhesion of microspheres with -COOH and -NH(2) functional groups, whereas it did allow adhesion of microspheres with hydrophobic attributes. In addition, in vitro adhesive force studies using diblock copolymers of polystyrene and polyacrylate showed that the in vitro adhesive force between the hydrophobic phospholipid lining of the in vitro gastric mucus model and the polymer increased when the surface hydrophobicity of the polymer increased.

CONCLUSION

The hydrophobic phospholipid acts as an adhesion barrier to hydrophilic bioadhesive polymers and polymers with surface functional groups of carboxylic acid and amine. The hydrophobic phospholipid lining of the gastric mucosa should be taken into considerations for screening and designing of a new gastric bioadhesive polymer.

Effect of salicylate on the elasticity, bending stiffness, and strength of SOPC membranes

Salicylate is a small amphiphilic molecule which has diverse effects on membranes and membrane-mediated processes. We have utilized micropipette aspiration of giant unilamellar vesicles to determine salicylate's effects on lecithin membrane elasticity, bending rigidity, and strength. Salicylate effectively reduces the apparent area compressibility modulus and bending modulus of membranes in a dose-dependent manner at concentrations above 1 mM, but does not greatly alter the actual elastic compressibility modulus at the maximal tested concentration of 10 mM. The effect of salicylate on membrane strength was investigated using dynamic tension spectroscopy, which revealed that salicylate increases the frequency of spontaneous defect formation and lowers the energy barrier for unstable hole formation. The mechanical and dynamic tension experiments are consistent and support a picture in which salicylate disrupts membrane stability by decreasing membrane stiffness and membrane thickness. The tension-dependent partitioning of salicylate was utilized to calculate the molecular volume of salicylate in the membrane. The free energy of transfer for salicylate insertion into the membrane and the corresponding partition coefficient were also estimated, and indicated favorable salicylate-membrane interactions. The mechanical changes induced by salicylate may affect several biological processes, especially those associated with membrane curvature and permeability.

Role of bacteria in experimental colitis

Epidemiology suggests some relationship between the establishment of the gut flora and the risk of developing inflammatory bowel disease. Unrestrained activation of the immune system against commensal bacteria appears to be responsible for the chronicity of these diseases. In animal models, broad-spectrum antibiotics reduce the bacterial load and militate against intestinal inflammation. Several bacterial species found in of the common microflora, including anaerobes, are able to invade the colonic wall when there is dysfunction of the colonic mucosal barrier. Most aerobes provoke focal areas of acute inflammation, but some anaerobes in the predominant flora induce diffuse a fibrogenic transmural response. Current research aims to identify the probiotics that might act against these bacteria. Colonization with specific probiotic strains, including a bacterium genetically engineered to secrete interleukin-10, prevents spontaneous colitis in susceptible mice. Certain lactobacilli exhibit anti-inflammatory properties naturally, i.e. without previous genetic manipulation. Prebiotics may increase colonization by lactobacilli and can prevent mucosal inflammation. Modulation of the gut flora with probiotics may prove useful in the prevention and control of inflammatory bowel diseases.

Epidermal growth factor increases surface hydrophobicity and resistance to acid in the rat duodenum

Epidermal growth factor (EGF) is produced in Brunner's glands and plays a role in healing and repair of duodenal ulcers. We examined the participation of zwitterionic phospholipids of mucus in the effects of EGF. Under anesthesia, groups of rats received an intraduodenal bolus of either saline or EGF. Some rats received subcutaneous indomethacin followed by EGF or EGF followed by a detergent (5% Brij 35, a nonionic detergent that solubilizes luminal phospholipids). Thirty minutes after treatment, mucosal surface hydrophobicity and phospholipid concentration in the mucus layer were measured. Matched groups of rats were challenged with 0.5 M HCl, instilled intraduodenally 30 min after treatment, and mucosal damage was assessed 1 h after acid challenge. Exogenous EGF significantly increased surface hydrophobicity and phosphatidylcholine concentration in the mucus layer. EGF treatment also reduced mucosal damage induced by acid. However, indomethacin pretreatment or detergent administration after EGF abolished both protection against acid and changes in the mucus layer. These data suggest that EGF increases duodenal resistance to luminal acid via stimulation of mucosal zwitterionic phospholipids.

Evidence for surfactant contributing to the gastric mucosal barrier of the horse

This study was undertaken to determine the hydrophobicity of the luminal surface of the equine stomach and to elucidate the ultrastructure of the lining imparting that property. Gastric and duodenal mucosal samples from 5 horses were collected immediately after euthanasia and subjected to surface contact angle measurement using a goniometer. Gastric mucosal samples from 4 horses and a foal were examined by electron microscopy following a fixation procedure known to preserve phospholipids and oligolamellar structures. Contact angles for the equine gastric glandular mucosal surface (mean +/- s.e. 78.0 +/- 11.0 degrees) were greater than for the duodenum (33.4 +/- 8.7 degrees), (P = 0.003). The contact angles for gastric squamous tissue (50.4 +/- 4.5 degrees) tended to be greater than for duodenum (P = 0.15). Electron microscopy revealed the existence of surfactant as abundant osmiophilic phospholipid material within both squamous and glandular gastric mucosae. These results indicate the hydrophobic nature of the equine gastric mucosae. We propose that the water-repellent nature of the stomach contributes to the 'gastric mucosal barrier' and is imparted by surface-active phospholipid adsorbed to the surface. Phospholipids may also be utilised as a physical barrier to back-diffusion of acid by lining intracellular canaliculi and oxyntic ducts where other defence mechanisms are absent.

Profile and mechanisms of gastrointestinal and other side effects of nonsteroidal anti-inflammatory drugs (NSAIDs)

The popular view that all nonsteroidal anti-inflammatory drugs (NSAIDs) act by inhibiting the production of prostaglandins has been challenged by the discovery that they also affect a wide variety of cellular processes that are important for their therapeutic actions and side effects. Although recognition of the differential activities of new and established NSAIDs on the activities of the cyclooxygenases (COXs) affecting production of inflammatory prostaglandins (from COX-2) and those that are physiologically important (from COX-1) may have significance for the prostaglandin components of the underlying inflammatory and physiologic processes, there are important features of their chemical structures that determine the various cellular and biochemical actions of these agents. Several established NSAIDs have low propensity to cause gastrointestinal (GI) ulceration and bleeding that may relate to these drugs having unique pharmacokinetic characteristics (pro-drugs, protein binding, etc). They also have weak effects on the production of GI mucosal prostaglandins and have specific physicochemical characteristics such that they cause minimal damage to mucosal membranes or effects on nonprostaglandin-related cellular mechanisms important in mucosal defenses. Some of the new COX-2-selective drugs with methyl or amino-sulfonyl moieties have relatively high pKa values and other properties that are similar to established NSAIDs with low GI ulcerogenicity. These physicochemical properties may contribute to the low irritancy of the new COX-2-selective drugs quite apart from their sparing of COX-1 in the GI mucosa. With concerns that some established NSAIDs may accelerate cartilage destruction in osteoarthritis (OA), interest is now focusing on whether the COX-2-selective drugs may have a lower potential for this adverse effect by avoiding the inhibitory effects on cartilage proteoglycan metabolism seen with such drugs as indomethacin and the salicylates. Meloxicam appears to be without inhibitory effects on proteoglycan metabolism, but it remains to be seen if this translates into any beneficial actions on the progression of joint changes in OA observed radiologically or from magnetic resonance imaging.

Ultrastructure of the hydrophobic gastric surfactant barrier in the dog

OBJECTIVES

To confirm the hydrophobicity of the luminal surface of the canine stomach and to elucidate the ultrastructure of the lining imparting that property.

DESIGN AND PROCEDURES

Gastric and duodenal mucosal samples from eight dogs were collected immediately after euthanasia and subjected to contact angle measurement using a goniometer. Other samples were examined by electron microscopy following a fixation procedure known to preserve phospholipids and oligolamellar structures.

RESULTS

Contact angles for the canine gastric mucosal surface (85.1 +/- 5.5) were significantly greater (P < 0.0001) than for the duodenum (24.0 +/- 1.7). Electron microscopy revealed the existence of surfactant as abundant osmiophilic phospholipid material within the gastric and duodenal mucosae.

CONCLUSION

We have confirmed the hydrophobic nature of the canine gastric mucosa whereas the luminal surface of the duodenum is hydrophilic. We propose that the water-repellent nature of the canine gastric lining contributes to the 'gastric mucosal barrier' and is imparted by an oligolamellar layer of surface-active phospholipid ('gastric surfactant') adsorbed to the surface. Both gastric and duodenal mucosae may also utilise phospholipids as an intercellular defense mechanism in the event that tight junctions are breached by acid. It is tempting to speculate that a deficiency of gastric phospholipids may predispose dogs to ulcers. Further, exogenous administration of phospholipids may be useful in preventing gastric ulceration.

Effect of Helicobacter pylori infection on gastric mucosal phospholipid contents and their fatty acid composition

To investigate the effect of Helicobacter pylori infection on the 'gastric mucosal barrier', phospholipid contents and the fatty acid composition of endoscopic biopsy specimens of the gastric mucosa were analysed in healthy volunteers with and without H. pylori infection. The gastric corporeal phosphatidylcholine (PC) content of H. pylori-positive healthy volunteers was less than that of H. pylori-negative healthy volunteers (P < 0.05). Moreover, H. pylori-positive healthy volunteers had a decrease in linoleic acid composition (P < 0.0001) and an increase in arachidonic acid composition (P < 0.0001) and in the arachidonic acid/linoleic acid ratio (P < 0.0001) of antral and corporeal PC compared with H. pylori-negative healthy volunteers. These findings suggest that H. pylori infection enhances production of various eicosanoids, resulting in changes in the gastric mucosal phospholipid contents and their fatty acid composition, that may consequently cause the gastric mucosal barrier to be weakened.

Gastroprotective effect of ranitidine bismuth citrate is associated with increased mucus bismuth concentration in rats

BACKGROUND

Antisecretory and bismuth compounds protect the gastric mucosa from injury resulting from non-steroidal anti-inflammatory drugs.

AIM

To study the mechanism underlying the gastroprotective effects of ranitidine bismuth citrate (GG311) in rats.

METHODS

Indomethacin rat injury model and in vivo microscopy in which acid output, surface cell intracellular pH (pHi), gastric mucus gel thickness, and mucosal blood flow were measured simultaneously.

RESULTS

In injury studies, GG311 dose dependently protected against severe injury induced by indomethacin (60 mg/kg subcutaneously). In in vivo microscopic studies, indomethacin significantly decreased mucus gel thickness and increased the initial rate of acidification of gastric surface cells when the superfusate pH was lowered from 7.4 to 1.0, and impaired pHi during acid exposure. Indomethacin had no effect on mucosal blood flow or acid output. GG311 alone had no effect on gel thickness, blood flow, or pHi homeostasis during acid exposure, but improved the initial acidification rate and pHi during superfusion with pH 1.0 solutions in the presence of indomethacin. In separate experiments, indomethacin pretreatment considerably increased gastric mucus bismuth concentrations in rats given GG311.

CONCLUSIONS

The gastroprotective effect of GG311 against indomethacin induced gastric injury is associated with high and prolonged gastric mucus bismuth concentrations, which may impair proton permeation across the mucus gel.

In vitro recovery of canine gastric mucosal surface hydrophobicity and potential difference after aspirin damage

This study determined how the luminal surface hydrophobicity and transmucosal potential difference (PD) of canine gastric mucosa changed during the recovery period after the tissue was challenged with acidified aspirin. Luminal aspirin reduced both the contact angle and PD of mucosae incubated in Ussing chambers. After the removal of aspirin, surface hydrophobicity was found to recover before PD, and nutrient 16,16-dimethyl prostaglandin E2 accelerated the recovery of both parameters. Restoration of luminal surface hydrophobicity may be an important component of how the stomach reestablishes its barrier properties after exposure to a luminal damaging agent.

Non-steroidal anti-inflammatory drugs (NSAIDs) associate with zwitterionic phospholipids: insight into the mechanism and reversal of NSAID-induced gastrointestinal injury

The molecular basis of the injurious actions of non-steroidal anti-inflammatory drugs (NSAIDs) on the gastrointestinal (GI) tract is only partly understood. In this study we have obtained evidence, employing both in vitro and in vivo systems, that five NSAIDs have the ability to form a chemical association with zwitterionic phospholipids. Since this same class of phospholipids line the luminal aspects of the mucus gel layer to provide it with non-wettable properties, this intermolecular association may be the mechanism by which NSAIDs attenuate the hydrophobic barrier properties of the upper GI tract. Preassociating a number of NSAIDs with exogenous zwitterionic phospholipids prevented this increase in surface wettability of the mucus gel layer and protected rats against the injurious GI side-effects of these drugs, while enhancing their lipid permeability, antipyretic and anti-inflammatory activity.

Mechanisms of mucosal injury and healing: the role of non-steroidal anti-inflammatory drugs

Biological insights into injurious effects of non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin (ASA), on mucosal protection and repair, are largely from studies of acute injury. That chronic ulceration is similar is not established. NSAIDs directly injure tissues, including endothelia, and at the same time impair the operation of many of the processes that normally contribute to mucosal protection, whatever the injurious agent. Many protective processes are mediated through prostaglandins, whose synthesis is abolished by inhibition of the constitutive isoenzyme, cyclooxygenase I (COX I) or Prostaglandin H-Synthase1 (PGHS1). The aims of therapy are aimed at inhibiting the inducible isozyme cyclooxygenase II (COX II) or prostaglandin-H Synthase2 (PGHS2), which contributes to prostanoid synthesis at sites of inflammation. Newer NSAIDs, selectively inhibiting COX II, promise to revolutionize the treatment of inflammatory disease while reducing mucosal injury. Meanwhile, there is increasing evidence that direct injury to both mucosae and endothelia is mediated by free-radical species, exacerbated by reduced blood flow, and by the local release of inflammatory and other mediators, which accentuate vascular leakage and hemorrhage, and cause intravascular aggregation of blood elements, stasis, hypoxia, and additional free-radical injury. The NSAIDs also inhibit epithelial cell division and the angiogenesis critical to healing and repair.

Phosphatidylcholines as mediators of adaptive cytoprotection of the rat duodenum

BACKGROUND/AIMS

Surfactant phospholipids impede diffusion of acid through the gastric mucus, but their relevance in the defense of the duodenum against luminal acid is not known.

METHODS

Duodenal resistance to acid was tested in anesthetized rats by instillation of HCl using a tube implanted in the proximal duodenum. The effects of a detergent (Brij 35; Sigma, St. Louis, MO) and a lipid mixture flushed through the luminal surface on duodenal resistance to acid were studied. The lipid content in the mucus and the effects of acid, prostaglandins, and indomethacin on the lipid layer were also analyzed.

RESULTS

Instillation of 100 mumol HCl or 5 micrograms/kg 16,16-dimethyl prostaglandin E2 increased resistance to acid, preventing duodenal lesions induced by 500 mumol HCl. However, 100 mumol HCl or 16,16-dimethyl prostaglandin E2 did not prevent lesions induced by 500 mumol HCl in rats undergoing perfusions with 5% Brij 35. Indomethacin suppressed acid-induced protection. A mixture of tripalmitin and dipalmitoyl-phosphatidylcholine protected against 500 mumol HCl, and the effect was also observed in rats receiving indomethacin. Finally, 100 mumol HCl increased the phosphatidylcholine content in the duodenal mucus but not in rats receiving 5% Brij 35 or indomethacin.

CONCLUSIONS

Surface-active phospholipids are critical for adaptive cytoprotection to acid in the rat duodenum.

Effect of ammonium ion on the hydrophobic and barrier properties of the gastric mucus gel layer: implications on the role of ammonium in H. pylori-induced gastritis

Infection with the bacterium Helicobacter pylori is associated with both the development of gastritis and an attenuation in the hydrophobic properties of the stomach. In order to better understand the effect of ammonium, one of the major products of H. pylori urease on these properties, a series of in vivo and in vitro experiments was performed. In the in vivo studies rats were intragastrically administered NH4Cl alone and in combination with the mucolytic agent, Muco-Mist, in various dosing strategies and concentrations. It was determined that the intragastric administration of four consecutive doses of a NH4Cl/Muco-Mist mixture (20 mmol/L/5%) was capable of converting the stomach from a hydrophobic to hydrophilic state as determined by contact angle analysis. Further, the treated rats became more susceptible to the injurious effect of luminal acid as determined by measuring the haemoglobin concentration of a collected gastric perfusate. In the in vitro studies it was determined that exposure of the hydrophobic surface of a synthetic mucus gel layer to increasing concentrations of NH4Cl (0-20 mmol/L) resulted in a rapid transition to a hydrophilic state and an associated increase in the flux of H+ across its surface. Helicobacter pylori may induce an attenuation in both mucosal hydrophobicity and barrier properties by producing high concentrations of NH4+ in the mucus gel layer. The molecular mechanism of this action may be related to the chemical similarities of NH4+ and choline-based phospholipids which contribute to the stomach's hydrophobic surface.

Demonstration of a phospholipid-rich zone in the human gastric epithelium damaged by Helicobacter pylori

BACKGROUND

Recently, a hydrophobic layer has been shown by the contact angle method in human gastric biopsy specimens. The aim of this study was to show the existence of a phospholipid-rich layer in the human gastric epithelium and to investigate the influence of Helicobacter pylori on these structures.

METHODS

Biopsy specimens from the gastric antrum were obtained during gastroscopy from patients with normal gastric mucosa as well as from patients with H. pylori-positive gastritis for electron microscopical and histological examination. Structures reacting with a phospholipid-selective stain, iodoplatinate were analyzed by electron microscopy and electron-dispersive x-ray microanalysis.

RESULTS

Both methods revealed the morphological existence of a phospholipid-rich zone covering the human gastric epithelial layer. Reaction products could be localized within the cells, at the epithelium closely associated with the surface, and in connection with the mucus. In infected tissue, H. pylori affects iodoplatinate-reactive material within the mucous layer and material covering the epithelium.

CONCLUSIONS

The phospholipid-rich zone in the apical region of surface mucous cells is likely to represent an important factor of the gastric protective system in humans. The destruction of this hydrophobic layer may result in a reduction of hydrophobicity giving access to gastric acid.

Phospholipase activity of Helicobacter pylori and its inhibition by bismuth salts. Biochemical and biophysical studies

In this study we measured phospholipase A (PLA) and C (PLC) activity of media filtrates and French Press lysates of the gastritis-inducing bacteria Helicobacter pylori. We report here that both H. pylori lysates and filtrates contain PLA1, PLA2, and C enzymes, which readily hydrolyze a radiolabeled dipalmitoylphosphatidylcholine (DPPC) and phosphorylcholine substrates, respectively. The specific activity of both PLA and C enzymes were greatest in the 6.5-7.0 and 8.4-8.8 pH ranges, respectively. Colloidal bismuth subcitrate (CBS) induced a dose-dependent inhibition of PLA2 and C activity of both H. pylori lysates and filtrates. This inhibitory effect of CBS on PLA2 was antagonized in a dose-dependent fashion by the addition of CaCl2 to the incubation mixture, suggesting that calcium and bismuth may be competing for the same site on the enzyme. In contrast, the ability of bismuth salts to inhibit PLC activity of H. pylori lysates was not antagonized by CaCl2. Employing a biophysical assay system for surface wettability, it was determined that H. pylori lysates had the capacity to remove a synthetic phospholipid monolayer off a glass in a dose-dependent fashion. This ability of the bacterial lysates to catalyze the transformation of a hydrophobic surface to a wettable state was significantly attenuated in the presence of bismuth salts. Our experimental results are, therefore, consistent with the possibility that H. pylori colonization compromises the stomach's barrier to acid by eroding a phospholipid lining, possibly a monolayer, on the surface of the gastric mucus gel and that this process is blocked in response to bismuth therapy.

Gastric mucosal barrier: evidence for Helicobacter pylori ingesting gastric surfactant and deriving protection from it

Ultrastructural examination by electron microscopy has been undertaken on human oxyntic mucosa from biopsy specimens obtained during diagnostic endoscopy from patients in whom infection by Helicobacter pylori was subsequently confirmed. A novel fixation procedure was used that avoided conventional fixatives based upon glutaraldehyde, which can destroy the hydrophobic lining of surfaces such as gastric mucosa. The resulting electron micrographs show densely osmiophilic inclusions of varying sizes in Helicobacter, some of which can be resolved and identified as lamellar bodies and their partially digested states. This finding indicates that Helicobacter may act as an aggressive agent by ingesting a gastric mucosal barrier of gastric surfactant, exposing the surface to attack by acid while simultaneously rendering it less hydrophobic. There is also evidence that Helicobacter pylori avoid their own digestion by coating themselves with essentially the same barrier of gastric surfactant, probably derived from the host. This is a possible explanation for the apparent absence of these bacteria in the duodenum.

Gastroprotective capability of exogenous phosphatidylcholine in experimentally induced chronic gastric ulcers in rats

Phosphatidylcholine (PC) is a main component of the hydrophobic gastric mucosal barrier. Exogenously administered, it prevents acute lesions. We evaluated the gastroprotective capacity of exogenous PC in both acute (ethanol- and indomethacin-induced) and chronic (indomethacin-induced) lesions in rats. Polyunsaturated (PPC) or hydrogenated PC in different concentrations were given intragastrically, before or after the injury factor, in single or repeated doses. Mucosal lesions were significantly reduced by a single dose of PPC, given before or after the injury factor, in both acute models. In the chronic model a single dose of PPC or hydrogenated PC significantly reduced lesions evaluated 6 h after ulcer induction, whereas after 72 h no protective effect was noticed. Repeated doses of PC were ineffective. In conclusion, in acute models exogenous PC reduces lesions in a dose-dependent manner and contributes to the mucosal defense. In chronic models an incomplete and temporary protection might be due to complex pathogenesis that requires activation of all levels in the mucosal defense. Strengthening of only one level was insufficient to restrict injury.

Effect of 16,16-dimethyl prostaglandin E2 on lipidic organelles of rat gastric surface mucous cells

BACKGROUND

We examined the effect of 16,16-dimethyl prostaglandin E2 (dmPGE2) on the subcellular distribution of phospholipids in rat gastric surface mucous cells (SMCs) using the cytochemical stain, iodoplatinate (IP).

METHODS

The volume of a number of subcellular organelles and the density of IP reactivity within these organelles was determined by modified vertical section stereology.

RESULTS

The volume occupied by most of the subcellular organelles was not affected by dmPGE2 treatment, with the exception that the volume fraction of two classes of large infranuclear inclusion bodies (LIIB1 and LIIB2, organelles we have previously shown to contain hydrophobic lipids), were significantly expanded by a factor of 3-4-fold. The distribution of IP reactivity among the various subcellular compartments appeared to undergo a shift in response to dmPGE2 treatment. Once again the major prostanoid effect was seen in the infranuclear inclusion bodies, as the volume density of IP reactivity was increased 2-24-fold in LIIB1 and 9-10-fold in LIIB2 in comparison to control values.

CONCLUSION

dmPGE2 administration to rats induces an increase in the volume and IP reactivity of a family of lipid-containing organelles, which may underlie its ability to increase the hydrophobic surface properties of the stomach.

Regulation of canine gastric mucin synthesis and phospholipid secretion by acid secretagogues

Key components of the mucous gel include the glycoprotein mucin and surface-active phospholipids. In the present study, mucin production and release of the surface-active phospholipid phosphatidylcholine (PC) into the medium were measured with an isolated canine mucous cell culture system. Stimulation of glycoprotein synthesis in response to 10(-4) mol/L histamine (160% +/- 9% of control, P < 0.01), 10(-6) mol/L gastrin (129% +/- 7%, P < 0.01), and 10(-6) mol/L carbamylcholine (129% +/- 7%, P < 0.01) was observed by metabolic labeling, whereas prostaglandin E2 (PGE2) had no effect. The effect of histamine was blocked by the H2 receptor antagonist cimetidine but not the H1 receptor antagonist diphenhydramine (P < 0.01). Activators of adenylate cyclase and cyclic adenosine monophosphate analogs significantly stimulated mucin synthesis (P < 0.05). A 7.8% +/- 1.7% increase in mucin above basal levels after 24 hours was observed with a solid-phase immunoassay in control wells, whereas histamine, gastrin, and carbamylcholine increased total mucin by 14% +/- 0.7%, 17% +/- 4.3%, and 20.4% +/- 4%, respectively (all P < 0.01), and PGE2 had no significant effect. PC release was stimulated by the administration of histamine, carbamylcholine, gastrin (108%-110% of control, P < or = 0.05), and PGE2 (120% of control, P < 0.01). The acid secretagogues histamine, gastrin, and carbamylcholine stimulated mucin synthesis and PC release. PGE2 has no direct role in the synthesis of canine gastric mucin but stimulates release of surface-active phospholipids. The mechanisms responsible for acid secretion provide for the coordinated production of the primary layer of defense against the injurious effects of low pH.

Effect of sucralfate on components of mucosal barrier produced by cultured canine epithelial cells in vitro

The mucous gel maintains a neutral microclimate at the epithelial cell surface, which may play a role in both the prevention of gastroduodenal injury and the provision of an environment essential for epithelial restitution and regeneration after injury. Enhancement of the components of the mucous barrier by sucralfate may explain its therapeutic efficacy for upper gastrointestinal tract protection, repair, and healing. We studied the effect of sucralfate and its major soluble component, sucrose octasulfate (SOS), on the synthesis and release of gastric mucin and surface active phospholipid, utilizing an isolated canine gastric mucous cells in culture. We correlated these results with the effect of the agents on mucin synthesis and secretion utilizing explants of canine fundus in vitro. Sucralfate and SOS significantly stimulated phospholipid secretion by isolated canine mucous cells in culture (123% and 112% of control, respectively). Indomethacin pretreatment significantly inhibited the effect of sucralfate, but not SOS, on the stimulation of phospholipid release. Administration of either sucralfate or SOS to the isolated canine mucous cells had no effect upon mucin synthesis or secretion using a sensitive immunoassay. Sucralfate and SOS did not stimulate mucin release in the canine explants; sucralfate significantly stimulated the synthesis of mucin, but only to 108% of that observed in untreated explants. No increase in PGE2 release was observed after sucralfate or SOS exposure to the isolated canine mucous cells. Our results suggest sucralfate affects the mucous barrier largely in a qualitative manner. No increase in mucin secretion or major effect on synthesis was noted, although a significant increase in surface active phospholipid release was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathogenesis of gastroduodenal injury due to nonsteroidal antiinflammatory drugs: implications for prevention and therapy

Nonsteroidal antiinflammatory drugs (NSAIDs) initiate gastroduodenal ulceration and promote complications such as bleeding and perforation by interfering with the ability of the proximal gastrointestinal tract to maintain its defensive capabilities. Mucosal defense is composed of three critical components: preepithelial, epithelial, and postepithelial. Preepithelial defense is composed of the mucous gel containing mucin, bicarbonate, and surface-active phospholipids. The epithelial component includes the surface cells, their apical tight junctions, and membrane transporters. Postepithelial defense is maintained by mucosal blood flow, which is essential for both defense and repair. NSAIDs interfere with each component of mucosal defense via direct toxic effects along with cyclooxygenase inhibition and depletion of endogenous prostaglandins. Although NSAID injury is dependent on luminal acid, attempts to prevent NSAID injury by acid suppression using H2-receptor antagonists in humans have had limited success, whereas complete inhibition of acid secretion with proton pump inhibition may have promise. Prostaglandins appear most effective for prevention of NSAID injury, sucralfate appears ineffective, and bismuth compounds have not been studied extensively. Recent evidence suggests that NSAID ulcers heal quickly with proton pump inhibitors compared with H2-receptor antagonists in patients who continue NSAID therapy.

Phospholipid- and neutral lipid-containing organelles of rat gastroduodenal mucous cells. Possible origin of the hydrophobic mucosal lining

In this study, an attempt was made to localize the cell types and subcellular organelles that synthesize and secrete surface-active lipids from the rat gastroduodenal mucosa. Two fluorescent hydrophobic probes--Nile Red and 1-anilinonaphthalene-8-sulfonic acid--and a phospholipid-selective cytochemical stain--iodoplatinate--were used for fluorescence and electron microscopic studies, respectively. The results showed the presence of phospholipid and neutral lipid fluorescent hot spots in the mucous cells of both the gastric mucosa and the submucosal Brunner's glands of the duodenum. In contrast, other cell types of the stomach (parietal, chief, and endocrine cells) or the duodenum (goblet cells and villous and crypt enterocytes) were either unreactive or weakly stained with the dyes. Ultrastructurally, two classes of large infranuclear inclusion bodies were observed in these two types of mucous cells. The location and size of these inclusion bodies appeared to be in agreement with the position and size of the large hydrophobic fluorescent hot spots detected at the light microscopic level. The ultrastructural appearance, localization, and histochemical staining pattern of these lipid-containing organelles suggest that they are specific for these cell types and not a general feature of degenerative epithelium undergoing lysosome-induced autolysis. It was concluded that the mucous cells of both the stomach and the Brunner's glands contain a subcellular organelle that stores hydrophobic material constituting both neutral lipids and phospholipids. These lipidic substances may be secreted into the gastroduodenal mucous gel and provide its surface with a nonwettable lining to repel luminal acid.