Isoproterenol enhances rat gastric mucosal barrier function in vivo

Gastric mucosal integrity: gastric mucosal blood flow and microcirculation. An overview

The stomach is in a state of continuous exposure to potentially hazardous agents. Hydrochloric acid together with pepsin constitutes a major and serious threat to the gastric mucosa. Reflux of alkaline duodenal contents containing bile and pancreatic enzymes are additional important injurious factors of endogenous origin. Alcohol, cigarette smoking, drugs and particularly aspirin and aspirin-like drugs, and steroids are among exogenous mucosal irritants that can inflict mucosal injury. The ability of the stomach to defend itself against these noxious agents has been ascribed to a number of factors constituting the gastric mucosal defense. These include mucus and bicarbonate secreted by surface epithelial cells, prostaglandins, sulfhydryl compounds and gastric mucosal blood flow. The latter is considered by several researchers to be of paramount importance in maintaining gastric mucosal integrity. The aim of this paper is to review the experimental and clinical data dealing with the role of mucosal blood flow and in particular the microcirculation in both damage and protection of the gastric mucosa.

New insights into impairment of mucosal defense in portal hypertensive gastric mucosa

Portal hypertension (PHT) increases susceptibility of the gastric mucosa to injury. The aim of this study was to investigate whether PHT affects rat gastric mucosal defense mechanisms in vivo at the pre-epithelial, epithelial, and/or post-epithelial levels. PHT was produced in rats by staged portal vein ligation and sham-operated (SO) rats served as controls. The gastric mucosa was exposed, chambered, and continuously superfused with buffers under in vivo microscopy. We measured gastric mucosal gel layer thickness, surface epithelial cell intracellular pH (pHi), mucosal blood flow, and mucosal/serosal oxygenation. In PHT rats, gastric mucosal gel layer thickness was significantly reduced (88 +/- 16 microm in PHT rats vs. 135 +/- 25 microm in SO rats; P <0.0001), and the surface epithelial cell pHi was significantly decreased (6.80 +/- 0.11 in PHT rats vs. 7.09 +/- 0.21 in SO rats; P <0.01). Although total gastric mucosal blood flow was significantly increased in PHT rats by 72% (P <0.05), the oxygenation of the gastric mucosal surface was decreased by 42% (P <0.05) compared with SO rats. PHT impairs pre-epithelial (mucosal gel layer thickness), epithelial (pHi), and post-epithelial (maldistribution of blood flow) components of the gastric mucosal barrier. These findings can explain the increased susceptibility of portal hypertensive gastric mucosa to injury.

Regulation of intracellular pH and blood flow in rat duodenal epithelium in vivo

Duodenal mucosal defense was assessed by measuring blood flow and epithelial intracellular pH (pHi) of rat proximal duodenum in vivo. Fluorescence microscopy was used to measure epithelial pHi using the trapped, pHi-indicating dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-AM. Blood flow was measured with laser-Doppler flowmetry. The mucosa was briefly superfused with NH4Cl, pH 2.2 buffer, the potent Na+/H+ exchange inhibitor 5-(N,N-dimethyl)-amiloride (DMA), or the anion exchange and Na+-HCO-3 cotransport inhibitor DIDS. Cryostat sections localized dye fluorescence to the villus tip. Steady-state pHi was 7. 02 +/- 0.01, which remained stable for 60 min. Interventions that load the cells with protons without affecting superfusate pH (NH4Cl prepulse, nigericin with low superfusate K+ concentration, DMA, and DIDS) all decreased pHi, supporting our contention that the dye was faithfully measuring pHi. An acid pulse decreased pHi, followed by a DIDS-inhibitable overshoot over baseline. Intracellular acidification increased duodenal blood flow independent of superfusate pH, which was inhibited by DMA, but not by DIDS. We conclude that we have established a novel in vivo microscopy system enabling simultaneous measurements of pHi and blood flow of duodenal epithelium. Na+/H+ exchange and Na+-HCO-3 cotransport regulate baseline duodenal epithelial pHi. Intracellular acidification enhances duodenal blood flow by a unique, amiloride-inhibitable, superfusate pH-independent mechanism.

Cytoprotective effect of bismuth subsalicylate in indomethacin-treated rats is associated with enhanced mucus bismuth concentration

BACKGROUND

Bismuth compounds prevent gastric injury from the short-term administration of nonsteroidal anti-inflammatory drugs. We studied the mechanisms underlying the gastroprotective actions of bismuth subsalicylate against indomethacin-induced injury in rats.

METHODS

An in vivo microscopic technique was used in which acid output, surface cell intracellular pH (pHi), gastric mucus gell thickness and mucosal blood flow were measured simultaneously. Concentrations of bismuth in mucus were measured by atomic absorption.

RESULTS

Indomethacin (60 mg/kg) significantly thinned the mucus gel layer and augmented the decrease of pHi during luminal acid superfusion, consistent with a weakened gastric mucosal barrier to acid. Bismuth subsalicylate partially reversed this effect of indomethacin on pHi, consistent with gastroprotection. Neither a prostaglandin-inhibiting but non-injurious dose of indomethacin (5 mg/kg), bismuth subsalicylate, or their combination affected mucus gel thickness or pHi homeostasis. In separate experiments, indomethacin (60 mg/kg) significantly increased gastric mucus bismuth concentration in rats given bismuth subsalicylate.

CONCLUSION

Bismuth accumulation in the gastric mucus during the evolution of mucosal injury may play an important role in the gastroprotective effect of bismuth subsalicylate against indomethacin injury.

Primary afferent nerves mediate in part beta-adrenoceptor stimulation-induced mesenteric hyperemia in rats

We tested the hypothesis that in the rat duodenum and intestine the mesenteric hyperemia due to beta-adrenoceptor stimulation is mediated by capsaicin-sensitive afferent nerves. Superior mesenteric artery blood flow was measured by pulsed Doppler flowmetry in the anesthetized rat. Functional ablation of afferent nerves was accomplished by subcutaneous 125 mg/kg capsaicin pretreatment 10 to 14 days before blood flow studies. Blockade of capsaicin-sensitive cation-selective ion channels of the duodenal and intestinal mucosal afferent nerves was achieved by intraduodenal 0.1% ruthenium red given 15 min prior to the intraduodenal administration of 5 mg/kg isoproterenol. Functional ablation of the afferent nerves and blockade of the capsaicin-sensitive cation-selective ion channels alone or in combination resulted in a significant reduction of mesenteric hyperemia induced by intraduodenal isoproterenol. These data support the hypothesis that beta-adrenoceptor stimulation by intraduodenal isoproterenol induces mesenteric hyperemia in part through afferent nerves in the rat duodenal and intestinal mucosa. The results suggest for the first time a link between beta-adrenoceptor function and peripheral capsaicin-sensitive afferent nerve-mediated mechanism in the rat gut.

Evaluation of frog gastric mucous secretion by 7T magnetic resonance imaging

This work describes a new method to evaluate the action of drugs on gastric mucus secretion by 7 T MRI. A polyethylene probe was inserted into the stomach of frogs, a water washing cycle was performed, air was inflated into the stomach, and the probe was removed. Drugs were administered topically by mixing them with the water used for stomach washing, and a series of images was collected. In untreated animals, a mucous layer emitting a high intensity signal covered the gastric mucosa. Atropine administration strongly reduced the mucous layer thickness. After pilocarpine administration, MRI images showed that the mucous layer was thicker than in controls. After metacholine administration, the thickness of the mucous layer was increased in comparison with both controls and pilocarpine-treated animals, in accord with the expected drug-induced increase of mucous secretion. Quantitative analysis of the mucous layer thickness on MRI images confirmed the above findings. In conclusion, these results indicate that MRI at 7 T is a sensitive in vivo screening test to evaluate the effect of drugs on gastric mucus secretion in absence of surgical manipulation, thereby enabling morphofunctional studies that cannot be performed by conventional methods.

Indomethacin does not alter the effect of pentagastrin on rat gastric defense mechanisms

We studied the effect of the prostaglandin synthesis inhibitor, indomethacin, on pentagastrin-associated enhancements of gastric mucosal defense mechanisms, using a microfluorometric technique in which mucus gel thickness, intracellular pH (pHi), gastric mucosal blood flow, and acid secretion were measured simultaneously in vivo. Intravenous infusion with pentagastrin (80 micrograms/kg/h) increased mucus gel thickness, induced a hyperemic response to luminal acid, and enhanced pHi homeostasis during acid superfusion. Indomethacin, (5 mg/kg, IP) did not alter the effects of pentagastrin on acid output, mucus gel thickness, mucosal blood flow, and pHi homeostasis. Indomethacin alone did not affect any of these measures. We conclude that the enhancement of gastric defense mechanisms due to pentagastrin is independent of prostaglandin synthesis.

In vivo morpho-functional study of rat gastric mucus secretion

BACKGROUND

Morpho-functional studies of gastric mucosa are hampered by the lack of a technique allowing direct in situ visualization of the mucus in small living laboratory animals.

METHODS

The material covering the gastric surface was studied in vivo in rats by magnetic resonance imaging (MRI) at 4.7 Tesla, and modification of its secretion was evaluated after pharmacological treatment.

RESULTS

In unstimulated animals, the glandular portion of the stomach was lined by a layer of material emitting a signal of high intensity. Administration of 16,16-dimethyl prostaglandin E2 caused an accumulation of this material within a maximum 30 min after the administration of the drug. At 45 min, gastric emptying occurred and at 60 min, the lumen was almost free of material emitting a signal of high intensity. An increase in the intensity of the signal emitted from the material filling the gastric lumen was found after pentagastrin injection. After 45 min, the intensity of the signal emitted from the material in the gastric lumen decreased. 1H localized spectroscopy showed that after injection of pentagastrin there was an increase in the water proton peak within the gastric lumen. About one hour after stimulation, the water proton peak returned to the basal value.

CONCLUSIONS

This study demonstrates that MRI displays gastric mucus in living small rodents and represents a sensitive screening test for pharmacological action on this structure, enabling morpho-functional studies.

Prostaglandin E2 enhances gastric defense mechanisms against acid injury in uremic rats

BACKGROUND/AIMS

Uremia increases gastric mucosal H+ permeability and acid back-diffusion-related injury in rats. The aim of this study was to examine the effect of the synthetic gastroprotective compound 16,16-dimethyl prostaglandin E2 (16,16-dm PGE2) on the gastric barrier to acid injury in uremic rats.

METHODS

Chronic renal failure was induced by subtotal nephrectomy. Acid back-diffusion injury was induced by superfusion with 15% ethanol in 0.15N HCl and was assessed by image analysis. Intracellular pH, initial surface cell acidification rate, and thickness of mucous gel layer were measured with in vivo microscopy. Gastric mucosal blood flow was measured in separate experiments by laser-Doppler flowmetry.

RESULTS

Pretreatment with 16,16-dm PGE2 attenuated H+ back-diffusion and prevented the production of gross lesions. 16,16-dm PGE2 increased gastric mucous gel thickness, decreased initial acidification rate, and maintained intracellular pH homeostasis during exposure to luminal acid. Gastric mucosal blood flow was not changed during superfusion with a neutral buffer but increased during acid exposure in rats treated with 16,16-dm PGE2.

CONCLUSIONS

16,16-dm PGE2 attenuated H+ back-diffusion injury in uremic rats. This effect was associated with blunting of the initial decrease of intracellular pH and enhanced surface cell intracellular pH homeostasis during acid exposure. These effects were associated with an increased mucous gel layer thickness and an acid-related increase in blood flow.