An in vivo model for gastric physiological and pathophysiological studies in the mouse

Ca2+ signaling in HCO3- secretion and protection of upper GI tract

The cytosolic calcium ([Ca²⁺]cyt) is one of the most important cell signaling that can modulate gastrointestinal (GI) epithelial secretion and promote GI mucosal wound repair. The GI mucosal bicarbonate secretion is the main mechanism of mucosal protection. Our research team has been working in this field and provided solid evidence for the important role of Ca²⁺ signaling in the regulation of GI epithelial secretion and the underlying molecular mechanisms. In this review, we attempt to systemically review the current status of our knowledge on the role of Ca²⁺ signaling in the regulation of intestinal bicarbonate secretion and in the upper GI epithelial protection. We expect that novel targets could be identified for drug development to better protect GI mucosa and treat mucosal injury with the advance in this filed.

Dietary nitrite induces nitrosation of the gastric mucosa: the protective action of the mucus and the modulatory effect of red wine

The stomach chemical environment promotes the production of new molecules that can induce post-translational modifications of endogenous proteins with physiological impact. The nitrate-nitrite-nitric oxide pathway is relevant in this process via production of nitric oxide ((•)NO) and nitric oxide-derived nitrogen oxides (NOx) at high concentrations. Using a highly sensitive and selective chemiluminescence approach, we found that exposure the stomach of rats to nitrite yielded S- and N-nitroso derivatives in gastric mucus cysteine-rich glycoproteins (mucins). To lesser extent, the underlying epithelial cell layers also suffered nitrite-driven S- and N-nitroso modifications which increased upon mucus removal, indicating that, under normal nitrite load, (•)NO and NOx can reach inner layers of the stomach wall and locally modify proteins. S-nitrosation was by large the predominant modification. In vitro and ex vivo experiments indicated that the gastric nitrosation pattern is triggered by dietary nitrite in a concentration dependent manner, encompassing the intermediary formation of (•)NO and is susceptible to modulation by dietary reductants, notably red wine polyphenols. Collectively, these results suggest a protective action of the mucus and potential (•)NO-dependent biochemical effects at deeper cells layers of the mucosa.

Importance of Ca(2+) in gastric epithelial restitution-new views revealed by real-time in vivo measurements

It has been a few decades since Ca(2+) was identified as one of the important factors that can accelerate gastric wound repair as well as contribute to epithelial homeostasis and regulation of gastric secretions. The mechanistic basis has remained largely unexplored in vivo because it was not possible to track in real time either intracellular Ca(2+) mobilization or wound repair in living tissues. Recent advances in technology, such as combining high resolution light microscopy and genetically encoded Ca(2+) reporters in mice, now allow the monitoring of Ca(2+) mobilization during gastric epithelial cell restitution. Ca(2+) is a ubiquitous second messenger that influences numerous cellular processes, including gastric acid/bicarbonate secretion, mucus secretion, and cell migration. We have demonstrated that cytosolic Ca(2+) mobilization within the restituting gastric epithelial cells is a central signal driving small wound repair. However, extracellular Ca(2+) is also mobilized in the juxtamucosal luminal space above a wound, and evidence suggests extracellular Ca(2+) is a third messenger that also promotes gastric epithelial restitution. Interplay between intracellular and extracellular Ca(2+) is necessary for efficient gastric epithelial restitution.

Mucus properties and goblet cell quantification in mouse, rat and human ileal Peyer's patches

Peyer's patches (PPs) are collections of lymphoid follicles in the small intestine, responsible for scanning the intestinal content for foreign antigens such as soluble molecules, particulate matter as well as intact bacteria and viruses. The immune cells of the patch are separated from the intestinal lumen by a single layer of epithelial cells, the follicle-associated epithelium (FAE). This epithelium covers the dome of the follicle and contains enterocyte-like cells and M cells, which are particularly specialized in taking up antigens from the gut. However, the presence and number of goblet cells as well as the presence of mucus on top of the FAE is controversial. When mouse ileal PPs were mounted in a horizontal Ussing-type chamber, we could observe a continuous mucus layer at mounting and new, easily removable mucus was released from the villi on the patch upon stimulation. Confocal imaging using fluorescent beads revealed a penetrable mucus layer covering the domes. Furthermore, immunostaining of FAE from mice, rats and humans with a specific antibody against the main component of intestinal mucus, the MUC2 mucin, clearly identify mucin-containing goblet cells. Transmission electron micrographs further support the identification of mucus releasing goblet cells on the domes of PPs in these species.

Assessment of mucus thickness and production in situ

The nature of the mucus gel layer covering the gastrointestinal tract makes it difficult to study outside its natural site attached to the mucosa. Here, we describe a technique for intravital microscopy studies of the mucus gel layer from the stomach down to the colon in anesthetized rats and mice. Mucus thickness and accumulation rate in each segment of the gastrointestinal tract is measured with a micropipette technique under observation through a stereomicroscope. In this way, the nature of the mucus gel in vivo is readily studied, and effects of interventions or disease on the mucus can be determined in longitudinal studies or by comparing animals. Using this technique, we have been able to demonstrate that there are two forms of mucus gel adherent to the stomach and colon mucosa: one layer which is removable by suction and an underlying firm adherent gel layer, while in the small intestine, all mucus adhering to the mucosa can easily be removed.

The normal acid-base status of mice

Rodent models are commonly used for various physiological studies including acid-base regulation. Despite the widespread use of especially genetic modified mice, little attention have been made to characterise the normal acid-base status in these animals in order to reveal proper control values. Furthermore, several studies report blood gas values obtained in anaesthetised animals. We, therefore, decided to characterise blood CO(2) binding characteristic of mouse blood in vitro and to characterise normal acid-base status in conscious BALBc mice. In vitro CO(2) dissociation curves, performed on whole blood equilibrated to various PCO₂ levels in rotating tonometers, revealed a typical mammalian pK' (pK'=7.816-0.234 × pH (r=0.34)) and a non-bicarbonate buffer capacity (16.1 ± 2.6 slyke). To measure arterial acid-base status, small blood samples were taken from undisturbed mice with indwelling catheters in the carotid artery. In these animals, pH was 7.391 ± 0.026, plasma [HCO(3)(-)] 18.4 ± 0.83 mM, PCO₂ 30.3 ± 2.1 mm Hg and lactate concentration 4.6 ± 0.7 mM. Our study, therefore, shows that mice have an arterial pH that resembles other mammals, although arterial PCO₂ tends to be lower than in larger mammals. However, pH from arterial blood sampled from mice anaesthetised with isoflurane was significantly lower (pH 7.239 ± 0.021), while plasma [HCO(3)(-)] was 18.5 ± 1.4 mM, PCO₂ 41.9 ± 2.9 mm Hg and lactate concentration 4.48 ± 0.67 mM. Furthermore, we measured metabolism and ventilation (V(E)) in order to determine the ventilation requirements (VE/VO₂) to answer whether small mammals tend to hyperventilate. We recommend, therefore, that studies on acid-base regulation in mice should be based on samples taken for indwelling catheters rather than cardiac puncture of terminally anaesthetised mice.

Acute effects of Helicobacter pylori extracts on gastric mucosal blood flow in the mouse

AIM: To investigate the mechanisms underlying the reduction in gastric blood flow induced by a luminal water extract of Helicobacter pylori (HPE).

METHODS: The stomachs of isoflurane-anesthetized mice were exteriorized, and the mucosal surface exposed. Blood flow was measured with the laser-Doppler technique, and systemic arterial blood pressure monitored. C57BL/6 mice were exposed to water extract produced from H pylori strain 88-23. To investigate the role of a nerve- or iNOS-mediated pathway, we used intraluminal lidocaine and iNOS-/- mice. Blood flow response to the endogenous nitric oxide synthase inhibitor asymmetric dimethyl arginine (ADMA) was also assessed.

RESULTS: In wild-type mice, HPE decreased mucosal blood flow by approximately 30%. This reduction was abolished in iNOS-deficient mice, and by pre-treatment with lidocaine. Luminally applied ADMA resulted in reduction in blood flow similar to that observed in wild-type mice exposed to HPE.

CONCLUSION: A H pylori water extract reduces gastric mucosal blood flow acutely through iNOS- and nerve-mediated pathways.

The gastric mucus layers: constituents and regulation of accumulation

The mucus layer continuously covering the gastric mucosa consists of a loosely adherent layer that can be easily removed by suction, leaving a firmly adherent mucus layer attached to the epithelium. These two layers exhibit different gastroprotective roles; therefore, individual regulation of thickness and mucin composition were studied. Mucus thickness was measured in vivo with micropipettes in anesthetized mice [isoflurane; C57BL/6, Muc1-/-, inducible nitric oxide synthase (iNOS)-/-, and neuronal NOS (nNOS)-/-] and rats (inactin) after surgical exposure of the gastric mucosa. The two mucus layers covering the gastric mucosa were differently regulated. Luminal administration of PGE(2) increased the thickness of both layers, whereas luminal NO stimulated only firmly adherent mucus accumulation. A new gastroprotective role for iNOS was indicated since iNOS-deficient mice had thinner firmly adherent mucus layers and a lower mucus accumulation rate, whereas nNOS did not appear to be involved in mucus secretion. Downregulation of gastric mucus accumulation was observed in Muc1-/- mice. Both the firmly and loosely adherent mucus layers consisted of Muc5ac mucins. In conclusion, this study showed that, even though both the two mucus layers covering the gastric mucosa consist of Muc5ac, they are differently regulated by luminal PGE(2) and NO. A new gastroprotective role for iNOS was indicated since iNOS-/- mice had a thinner firmly adherent mucus layer. In addition, a regulatory role of Muc1 was demonstrated since downregulation of gastric mucus accumulation was observed in Muc1-/- mice.

Isoflurane-induced acidosis depresses basal and PGE(2)-stimulated duodenal bicarbonate secretion in mice

When running in vivo experiments, it is imperative to keep arterial blood pressure and acid-base parameters within the normal physiological range. The aim of this investigation was to explore the consequences of anesthesia-induced acidosis on basal and PGE(2)-stimulated duodenal bicarbonate secretion. Mice (strain C57bl/6J) were kept anesthetized by a spontaneous inhalation of isoflurane. Mean arterial blood pressure (MAP), arterial acid-base balance, and duodenal mucosal bicarbonate secretion (DMBS) were studied. Two intra-arterial fluid support strategies were used: a standard Ringer solution and an isotonic Na(2)CO(3) solution. Duodenal single perfusion was used, and DMBS was assessed by back titration of the effluent. PGE(2) was used to stimulate DMBS. In Ringer solution-infused mice, isoflurane-induced acidosis became worse with time. The blood pH was 7.15-7.21 and the base excess was about -8 mM at the end of experiments. The continuous infusion of Na(2)CO(3) solution completely compensated for the acidosis. The blood pH was 7.36-7.37 and base excess was about 1 mM at the end of the experiment. Basal and PGE(2)-stimulated DMBS were markedly greater in animals treated with Na(2)CO(3) solution than in those treated with Ringer solution. MAP was slightly higher after Na(2)CO(3) solution infusion than after Ringer solution infusion. We concluded that isoflurane-induced acidosis markedly depresses basal and PGE(2)-stimulated DMBS as well as the responsiveness to PGE(2), effects prevented by a continuous infusion of Na(2)CO(3). When performing in vivo experiments in isoflurane-anesthetized mice, it is recommended to supplement with a Na(2)CO(3) infusion to maintain a normal acid-base balance.

Dietary nitrate increases gastric mucosal blood flow and mucosal defense

Salivary nitrate from dietary or endogenous sources is reduced to nitrite by oral bacteria. In the acidic stomach, nitrite is further reduced to bioactive nitrogen oxides, including nitric oxide (NO). In this study, we investigated the gastroprotective role of nitrate intake and of luminally applied nitrite against provocation with diclofenac and taurocholate. Mucosal permeability ((51)Cr-EDTA clearance) and gastric mucosal blood flow (laser-Doppler flowmetry) were measured in anesthetized rats, either pretreated with nitrate in the drinking water or given acidified nitrite luminally. Diclofenac was given intravenously and taurocholate luminally to challenge the gastric mucosa. Luminal NO content and nitrite content in the gastric mucus were determined by chemiluminescence. The effect of luminal administration of acidified nitrite on the mucosal blood flow was also investigated in endothelial nitric oxide synthase-deficient mice. Rats pretreated with nitrate or given nitrite luminally had higher gastric mucosal blood flow than controls. Permeability increased more during the provocation in the controls than in the nitrate- and nitrite-treated animals. Dietary nitrate increased luminal NO levels 50 times compared with controls. Nitrate intake also resulted in nitrite accumulation in the loosely adherent mucous layer; after removal of this mucous layer, blood flow was reduced. Nitrite administrated luminally in endothelial nitric oxide synthase-deficient mice increased mucosal blood flow. We conclude that dietary nitrate and direct luminal application of acidified nitrite decrease diclofenac- and taurocholate-induced mucosal damage. The gastroprotective effect likely involves a higher mucosal blood flow caused by nonenzymatic NO production. These data suggest an important physiological role of nitrate in the diet.

Impaired mucus-bicarbonate barrier in Helicobacter pylori-infected mice

To resist the harsh intrinsic milieu, several lines of defense exist in the stomach. The aim of this study was to investigate the effect of the gastric pathogen Helicobacter pylori on these mechanisms in vivo. We used FVB/N mice expressing human alpha-1,3/4-fucosyl transferase (producing Lewis b epitopes) and inoculated with H. pylori 1. Mice were anesthetized with isoflurane or Hypnorm-midazolam, the stomach was exteriorized, and the surface of the corpus mucosa was exposed. Mucus thickness was measured with micropipettes, juxtamucosal pH (pH(jm)) was measured with pH-sensitive microelectrodes, blood flow was measured with laser-Doppler flowmetry, and mRNA levels of the bicarbonate transporter SLC26A9 were quantified with real-time PCR. The increase in mucosal blood flow seen in response to luminal acid (pH 1.5) in control animals (140 +/- 9% of control) was abolished in infected mice. The firmly adherent mucus layer was significantly thinner in infected mice (31 +/- 2 microm) than in control mice (46 +/- 5 microm), and no mucus accumulation occurred in infected mice. pH(jm) decreased significantly more on exposure to luminal acid in infected mice (luminal pH 1.5, pH(jm) 2.4 +/- 0.7) than in control mice (pH(jm) 6.4 +/- 0.5). Despite reduced pH(jm), SLC26A9 mRNA expression was significantly, by increased 1.9-fold, in infected mice. The reduction in pH(jm) by infection with H. pylori might be due to a reduced firmly adherent mucus layer, increased mucus permeability to H(+), and/or inhibition of bicarbonate transport. The upregulation of SLC26A9 in H. pylori-infected epithelium might be a result of continuous inhibition of the transporter, e.g., by ammonium, a H. pylori product, which has been previously shown to inhibit SLC26A9.