Mucosal Oxygen Research Articles

Overview of the Effect of Citrobacter rodentium Infection on Host Metabolism and the Microbiota.

Citrobacter rodentium is a natural enteric mouse pathogen that models human intestinal diseases, such as pathogenic E. coli infections, ulcerative colitis, and colon cancer. Upon reaching the monolayer of intestinal epithelial cells (IECs) lining the gut, a complex web of interactions between the host, the pathogen, and the microbiota ensues. A number of studies revealed surprisingly rapid changes in IEC bioenergetics upon infection, involving a switch from oxidative phosphorylation to aerobic glycolysis, leading to mucosal oxygenation and subsequent changes in microbiota composition. Microbiome studies have revealed a bloom in Enterobacteriaceae during C. rodentium infection in both resistant (i.e., C57BL/6) and susceptible (i.e., C3H/HeN) strains of mice concomitant with a depletion of butyrate-producing Clostridia. The emerging understanding that dysbiosis of cholesterol metabolism is induced by enteric infection further confirms the pivotal role immunometabolism plays in disease outcome. Inversely, the host and microbiota also impact upon the progression of infection, from the susceptibility of the distal colon to C. rodentium colonization to clearance of the pathogen, both via opsonization from the host adaptive immune system and out competition by the resident microbiota. Further complicating this compendium of interactions, C. rodentium exploits microbiota metabolites to fine-tune virulence gene expression and promote colonization. This chapter summarizes the current knowledge of the myriad of pathogen-host-microbiota interactions that occur during the progression of C. rodentium infection in mice and the broader implications of these findings on our understanding of enteric disease.

The Antioxidants Glutathione, Ascorbic Acid and Uric Acid Maintain Butyrate Production by Human Gut Clostridia in The Presence of Oxygen In Vitro

Uncontrolled oxidative stress, reported in Salmonella and HIV infections, colorectal cancer or severe acute malnutrition, has been associated with anaerobic gut microbiome alteration, impaired butyrate production, mucosal immunity dysregulation and disruption of host-bacterial mutualism. However, the role of major antioxidant molecules in the human body, such as glutathione, ascorbic acid and uric acid, has been neglected in this context. Here, we performed an in vitro metabolomics study of the 3 most odorous anaerobic microbes isolated from the human gut in our laboratory (Clostridium sporogenes, Clostridium subterminale and Romboutsia lituseburensis) when grown in anaerobiosis or in aerobiosis with these 3 antioxidant molecules via gas and liquid chromatography-mass spectrometry (GC/MS and LC/MS). There was no growth or volatile organic compound production in aerobic cultures without the 3 antioxidant molecules. In anaerobiosis, the major metabolic products of the bacteria were thiols, alcohols and short-chain fatty acid esters. The production of alkanes, cycloheptatriene and, paradoxically, increased butyrate production, was observed in the cultures grown in aerobiosis with the 3 antioxidant molecules. The qualitative shift suggests specific molecular mechanisms that remain to be elucidated. The increased production of butyrate, but also isobutyrate and isovalerate in vitro suggests that these 3 antioxidant molecules contributed to the maintenance and active resilience of host-bacterial mutualism against mucosal oxygen and uncontrolled oxidative stress in vivo.

Regional hypothermia improves gastric microcirculatory oxygenation during hemorrhage in dogs.

Mild systemic hypothermia increases gastric mucosal oxygenation (μHbO2) during hemorrhagic shock in dogs. In the context of critical blood loss hypothermia might be fatal due to adverse side effects. Selective regional hypothermia might overcome these limitations. The aim of our study was to analyze the effects of regional gastric and oral mucosal hypothermia on μHbO2 and perfusion (μflow). In a cross-over study six anesthetized dogs were subjected to local oral and gastric mucosal hypothermia (34°C), or maintenance of local normothermia during normovolemia and hemorrhage (-20% blood volume). Macro- and microcirculatory variables were recorded continuously. During normovolemia, local hypothermia increased gastric microcirculatory flow (μflow) without affecting oxygenation (μHbO2) or oral microcirculation. During mild hemorrhagic shock gastric μHbO2 decreased from 72±2% to 38±3% in the normothermic group. This was attenuated by local hypothermia, where μHbO2 was reduced from 74±3% to 52±4%. Local perfusion, oral microcirculation and macrocirculatory variables were not affected. Selective local hypothermia improves gastric μHbO2 during hemorrhagic shock without relevant side effects. In contrast to systemic hypothermia, regional mucosal hypothermia did not affect perfusion and oxygen supply during hemorrhage. Thus, the increased μHbO2 during local hypothermia rather indicates reduced mucosal oxygen demand.

Su2062 – Measurement of Mucosal Mitochondrial Oxygen During Upper Endoscopy As a Potential Novel Test for the Diagnosis of Mesenteric Ischemia

Su2062 – Measurement of Mucosal Mitochondrial Oxygen During Upper Endoscopy As a Potential Novel Test for the Diagnosis of Mesenteric Ischemia

Evaluation of endoscopic visible light spectroscopy: comparison with microvascular oxygen tension measurements in a porcine model

BackgroundVisible light spectroscopy (VLS) is a technique used to measure the mucosal oxygen saturation during upper gastrointestinal endoscopy to evaluate mucosal ischemia, however in vivo validation is lacking. We aimed to compare VLS measurements with a validated quantitative microvascular oxygen tension (μPO2) measurement technique.MethodsSimultaneous VLS measurements and μPO2 measurements were performed on the small intestine of five pigs. First, simultaneous measurements were performed at different FiO2 values (18%–100%). Thereafter, the influence of bile was assessed by comparing VLS measurements in the presence of bile and without bile. Finally, simultaneous VLS and μPO2 measurements were performed from the moment a lethal dose potassium chloride intravenously was injected.ResultsIn contrast to μPO2 values that increased with increasing FiO2, VLS values decreased. Both measurements correlated poorly with R2 = 0.39, intercept 18.5, slope 0.41 and a bias of − 16%. Furthermore, the presence of bile influenced VLS values significantly (median (IQR)) before bile application 57.5% (54.8–59.0%) versus median with bile mixture of the stomach 73.5% (66.8–85.8), p = < 2.2 * 10−16; median with bile mixture of small bowel 47.6% (41.8–50.8) versus median after bile removal 57.0% (54.7–58.6%), p = < 2.2 * 10−16). Finally, the VLS mucosal oxygen saturation values did not decrease towards a value of 0 in the first 25 min of asystole in contrast to the μPO2 values.ConclusionsThese results suggest that VLS measures the mixed venous oxygen saturation rather than mucosal capillary hemoglobin oxygen saturation. Further research is needed to establish if the mixed venous compartment is optimal to assess gastrointestinal ischemia.

Detection of mesenteric ischemia by means of endoscopic visible light spectroscopy after luminal feeding

Endoscopic visible light spectroscopy (VLS) enables measurement of mucosal oxygen saturation during upper GI endoscopy and is used in the diagnostic work-up of chronic mesenteric ischemia (CMI). Currently, VLS is performed when the patient has fasted. We aimed to determine whether food challenge improves the diagnostic performance of VLS measurements for the diagnosis of CMI. This was a single-center prospective study of healthy controls and consecutive patients suspected of having CMI and referred to a Dutch specialized CMI center for standardized diagnostic CMI work-up. Immediately after conventional fasting, VLS measurements were taken, luminal feeding was administered, and VLS measurements were repeated 45 minutes later. Patients were classified as CMI if a multidisciplinary expert-based consensus diagnosis of CMI was established and successful revascularization therapy resulted in symptom relief. Patients were classified as no-CMI when consensus diagnosis was not reached or when symptom relief did not occur after technically successful treatment. We included 60 patients with suspected CMI and 16 healthy controls. Duodenal oxygen saturation was significantly higher postprandially compared with the fasting state: healthy controls: median (interquartile range) pre 54% (49%-56%), post 56% (53%-58%), P= .02), no-CMI patients (pre 55% (51%-57%), post 57% (53%-59%), P> .01); CMI patients: pre 51% (48%-53%), post 54% (50%-58%), P= .01. Mucosal oxygen saturation did notsignificantly increase postprandially in the duodenal bulb or antrum of the stomach. Absolute postprandial oxygen measurements and the absolute or relative difference between preprandial versus postprandial oxygen measurements did not provide additional discriminative ability for the diagnosis of CMI. Postprandial VLS measurements have no added benefit for the diagnosis of CMI.

Intraobserver and interobserver reliability of visible light spectroscopy during upper gastrointestinal endoscopy

ABSTRACTBackground: Visible light spectroscopy (VLS) performed during upper gastrointestinal endoscopy allows measuring mucosal oxygen saturation levels to determine gastrointestinal ischemia. We aimed to determine the observer variability of VLS.Methods: This is a single-center prospective study of 24 patients planned for usual care upper endoscopy. To test intraobserver variability, VLS measurements were performed in duplicate by a single endoscopist in 12 patients. For interobserver variability analysis, in another 12 patients VLS measurements were repeatedly and independently performed by two endoscopists in the same patient during the same endoscopy session. Observer variability was assessed with intraclass correlation coefficient (ICC) and clinical disagreement defined as >5% difference between first and second set of VLS measurements.Results: The intraobserver reliability was excellent (ICC antrum 0.77, duodenal bulb 0.81 and duodenum 0.84) with clinical disagreement only in antrum (3% of all intraobserver measurements). The interobserver reliability was good for the duodenal bulb (ICC 0.70) without clinical disagreement; however, interobserver reliability was fair for duodenum (ICC 0.49) and antrum (ICC 0.56) with clinical disagreement occurring in 11% of all interobserver measurements.Conclusions: The observer reliability of VLS is fair to good with intraobserver reliability being better than interobserver reliability. This supports the use of VLS for detection of gastrointestinal ischemia.

Optical techniques for perfusion monitoring of the gastric tube after esophagectomy: a review of technologies and thresholds.

Anastomotic leakage is one of the most severe complications after esophageal resection with gastric tube reconstruction. Impaired perfusion of the gastric fundus is seen as the main contributing factor for this complication. Optical modalities show potential in recognizing compromised perfusion in real time, when ischemia is still reversible. This review provides an overview of optical techniques with the aim to evaluate the (1) quantitative measurement of change in perfusion in gastric tube reconstruction and (2) to test which parameters are the most predictive for anastomotic leakage.A Pubmed, MEDLINE, and Embase search was performed and articles on laser Doppler flowmetry (LDF), near-infrared spectroscopy (NIRS), laser speckle contrast imaging (LSCI), fluorescence imaging (FI), sidestream darkfield microscopy (SDF), and optical coherence tomography (OCT) regarding blood flow in gastric tube surgery were reviewed. Two independent reviewers critically appraised articles and extracted the data: Primary outcome was quantitative measure of perfusion change; secondary outcome was successful prediction of necrosis or anastomotic leakage by measured perfusion parameters.Thirty-three articles (including 973 patients and 73 animals) were selected for data extraction, quality assessment, and risk of bias (QUADAS-2). LDF, NIRS, LSCI, and FI were investigated in gastric tube surgery; all had a medium level of evidence. IDEAL stage ranges from 1 to 3. Most articles were found on LDF (n=12), which is able to measure perfusion in arbitrary perfusion units with a significant lower amount in tissue with necrosis development and on FI (n=12). With FI blood flow routes could be observed and flow was qualitative evaluated in rapid, slow, or low flow. NIRS uses mucosal oxygen saturation and hemoglobin concentration as perfusion parameters. With LSCI, a decrease of perfusion units is observed toward the gastric fundus intraoperatively. The perfusion units (LDF, LSCI), although arbitrary and not absolute values, and low flow or length of demarcation to the anastomosis (FI) both seem predictive values for necrosis intraoperatively. SDF and OCT are able to measure microvascular flow, intraoperative prediction of necrosis is not yet described.Optical techniques aim to improve perfusion monitoring by real-time, high-resolution, and high-contrast measurements and could therefore be valuable in intraoperative perfusion mapping. LDF and LSCI use perfusion units, and are therefore subjective in interpretation. FI visualizes influx directly, but needs a quantitative parameter for interpretation during surgery.

P022 Severe disease activity in UC is associated with mucosal hypoxia measured endoscopically

Tissue hypoxia has been shown to be a key microenvironmental regulator of mucosal barrier function and inflammation in pre-clinical models of colitis. However, the role of mucosal hypoxia in the pathogenesis of UC has yet to be extensively studied in IBD patients. Visible light spectroscopy can be utilised to measure the haemoglobin (Hb) oxygen saturations (%) colonic mucosa during endoscopy. Carbonic anhydrase 9 (CA-9) is a hypoxia-inducible protein and endogenous marker of hypoxia. The aim of this study was to compare mucosal oxygen saturations in normal and inflamed colonic mucosa in UC patients and assess relationship with disease activity. Patients undergoing colonoscopy were prospectively recruited in a single academic centre, with >3800 IBD patients. Endoscopies were performed using CO2 insufflation. Measurements of colonic mucosal Hb saturations were performed segmentally and from zones of transition from inflamed to normal mucosa, using an endoscopic probe (applied to intact mucosa) and tissue oximeter (T-Stat System, Spectros).Endoscopic severity was graded using the endoscopic Mayo score. Mucosal biopsies were obtained to grade histologic activity and for protein analysis. 38 patients were included in this study; n = 7 controls, n = 31 UC; male=25 (65.7%). The median age of UC patients was 39 years (IQR 34.6- 55.9). n = 5 had Mayo 0, n = 9 Mayo 1, n = 11 Mayo 2 and n = 6 Mayo 3 activity. At the time of the procedure, five patients were on biologic therapy, five immunomodulators and six on steroids. The median rectal saturation in the control group was 76.2% (IQR 73–80),compared with 77% (IQR 73.8–80.6) in the UC cohort (Mayo 0–3 inclusive) (p = 1.00). There was no significant difference across any colonic segment between these two cohorts. The median rectal mucosal O2 saturation in Mayo 0 was 77.6% (77.4–81.2),Mayo 1: 80.6% (77–83.8), Mayo 2: 75.4% (73.5–79.2) and Mayo 3:73.3% (68.7–74.8) (p = 0.02). Mucosal Hb Saturations (%) by disease severity. CA9 protein expression was significantly higher in biopsies with Mayo 2–3 activity compared with Mayo 0-1 disease. CA-9 protein expression in rectal biopsies. Mucosal O2 saturation is significantly decreased in severely active UC and oxygen signalling pathways are activated suggesting that mucosal hypoxia is an important determinant of disease severity in UC. These observations suggest the potential for oxygen signalling pathways as novel therapeutic targets.

Intestinal microcirculation and mucosal oxygenation during hemorrhagic shock and resuscitation at different inspired oxygen concentrations.

Hypotensive resuscitation is the standard of care of hemorrhagic shock resuscitation. The optimal level of arterial pressure is debated and there is a lack of data on relationships between arterial pressure, microcirculation and tissue oxygenation. We investigated the relationship between mean arterial pressure, intestinal microcirculation and mucosal oxygen tension during hemorrhagic shock and resuscitation at different inspired oxygen fraction concentration. The study was divided into two phases: 32 mice were progressively exsanguinated and then transfused in mean arterial pressure (MAP)-titrated steps of 10 mm Hg. Mice were randomized to four experimental groups: a control group in which sham mice underwent a laparotomy and three interventional groups with a common phase of exsanguination followed by progressive resuscitation at three different inspired oxygen concentrations (FIO2) (15%, 30%, and 100%). Intestinal mucosal oxygenation (intestinal PO2) and microcirculatory parameters were recorded at each 10 mm Hg MAP step. During exsanguination, intestinal PO2 decreased linearly with MAP levels. Microcirculatory parameters decreased nonlinearly with MAP levels while they had a linear relationship with intestinal PO2. Intestinal mucosal hypoxia (PO2 ≤ 20 mm Hg) began at a MAP of 60 mm Hg and MAP < 60 mm Hg was associated with a high percentage of animal with intestinal hypoxia (≥32%). Combination of MAP and microcirculatory parameters was superior to MAP alone at predicting mucosal oxygenation. Inversely, during resuscitation with FIO2 = 30%, the microcirculatory parameters increased linearly with MAP levels while they had a nonlinear relationship with intestinal PO2. Hypoxia (FIO2 = 15%) was poorly tolerated. In hyperoxic group (FIO2 = 100%) intestinal PO2 became significantly higher than baseline values as soon as 50 mm Hg MAP. During hemorrhagic shock, intestinal PO2 decreased linearly with MAP levels and microcirculatory parameters. Associating MAP and microcirculatory parameters allowed a better prediction of intestinal PO2 than MAP alone. A MAP < 60 mm Hg was associated with a high percentage of animal with intestinal hypoxia. Normoxic resuscitation (FIO2 = 30%) was sufficient to restore intestinal PO2.

Intestinal hypoxia and hypoxia-induced signalling as therapeutic targets for IBD.

Tissue hypoxia occurs when local oxygen demand exceeds oxygen supply. In chronic inflammatory conditions such as IBD, the increased oxygen demand by resident and gut-infiltrating immune cells coupled with vascular dysfunction brings about a marked reduction in mucosal oxygen concentrations. To counter the hypoxic challenge and ensure their survival, mucosal cells induce adaptive responses, including the activation of hypoxia-inducible factors (HIFs) and modulation of nuclear factor-κB (NF-κB). Both pathways are tightly regulated by oxygen-sensitive prolyl hydroxylases (PHDs), which therefore represent promising therapeutic targets for IBD. In this Review, we discuss the involvement of mucosal hypoxia and hypoxia-induced signalling in the pathogenesis of IBD and elaborate in detail on the role of HIFs, NF-κB and PHDs in different cell types during intestinal inflammation. We also provide an update on the development of PHD inhibitors and discuss their therapeutic potential in IBD.

Effect of Topical Iloprost and Nitroglycerin on Gastric Microcirculation and Barrier Function during Hemorrhagic Shock in Dogs

Background: Topical drug application is used to avoid systemic side effects. The aim of this study was to analyze whether locally applied iloprost or nitroglycerin influence gastric mucosal perfusion, oxygenation, and barrier function during physiological and hemorrhagic conditions. Methods: In repeated experiments, 5 anesthetized dogs received iloprost, nitroglycerin, or normal saline during physiological and hemorrhagic (-20% blood volume) conditions. Macro- and microcirculatory variables were recorded continuously. Gastric barrier function was assessed via translocation of sucrose into the blood. Results: During hemorrhage, gastric mucosal oxygenation decreased from 77 ± 4 to 37 ± 7%. This effect was attenuated by nitroglycerin (78 ± 6 to 47 ± 13%) and iloprost (82 ± 4 to 54 ± 9%). Sucrose plasma levels increased during hemorrhage from 7 ± 4 to 55 ± 15 relative amounts. This was alleviated by nitroglycerin (5 ± 8 to 29 ± 38 relative amounts). These effects were independent of systemic hemodynamic variables. Conclusions: During hemorrhage, topical nitroglycerin and iloprost improve regional gastric oxygenation without affecting perfusion. Nitroglycerin attenuated the shock-induced impairment of the mucosal barrier integrity. Thus, local drug application improves gastric microcirculation without compromising systemic hemodynamic variables, and it may also protect mucosal barrier function.

Quantitative analysis of mucosal oxygenation using ex vivo imaging of healthy and inflamed mammalian colon tissue.

Colonic inflammation is associated with decreased tissue oxygenation, significantly affecting gut homeostasis. However, the crosstalk between O2 consumption and supply in the inflamed tissue are not fully understood. Using a murine model of colitis, we analysed O2 in freshly prepared samples of healthy and inflamed colon tissue. We developed protocols for efficient ex vivo staining of mouse distal colon mucosa with a cell-penetrating O2 sensitive probe Pt-Glc and high-resolution imaging of O2 concentration in live tissue by confocal phosphorescence lifetime-imaging microscopy (PLIM). Microscopy analysis revealed that Pt-Glc stained mostly the top 50-60μm layer of the mucosa, with high phosphorescence intensity in epithelial cells. Measured O2 values in normal mouse tissue ranged between 5 and 35μM (4-28Torr), tending to decrease in the deeper tissue areas. Four-day treatment with dextran sulphate sodium (DSS) triggered colon inflammation, as evidenced by an increase in local IL6 and mKC mRNA levels, but did not affect the gross architecture of colonic epithelium. We further observed an increase in oxygenation, partial activation of hypoxia inducible factor (HIF) 1 signalling, and negative trends in pyruvate dehydrogenase activity and O2 consumption rate in the colitis mucosa, suggesting a decrease in mitochondrial respiration, which is known to be regulated via HIF-1 signalling and pyruvate oxidation rate. These results along with efficient staining with Pt-Glc of rat and human colonic mucosa reveal high potential of PLIM platform as a powerful tool for the high-resolution analysis of the intestinal tissue oxygenation in patients with inflammatory bowel disease and other pathologies, affecting tissue respiration.

Vasopressin V1A receptors mediate the stabilization of intestinal mucosal oxygenation during hypercapnia in septic rats

BackgroundMicrovascular oxygen saturation (μHBO2) plays an essential role in the development and outcome of sepsis. Hypercapnia (HC) improves the microvascular oxygenation of the mucosa in both healthy and septic animals. Vasopressin V1A receptor blockade prevents this positive effect under otherwise physiological conditions. The aim of this study was to investigate the effects and mechanisms of the vasopressin system during hypercapnia under septic conditions. Methods80 rats were randomized into 8 groups (N=10). Colon ascendens stent peritonitis (CASP) or sham surgery was performed on 40 animals each to establish a moderate polymicrobial sepsis or sham control, respectively. 24h after sepsis induction the animals were subjected to 120min of volume-controlled and pressure-limited ventilation with either normocapnic (pCO2 35–45mmHg) or moderate hypercapnic (pCO2 of 65–75mmHg) ventilation targets. Animals received either vasopressin V1A receptor blockade (SR 49059, 1mgkg−1 i.v.) or vehicle solution (dimethyl sulfoxide, 1%). Blood pressure, heart rate, pO2 and pCO2 were measured and microcirculatory oxygenation (μHBO2) and microcirculatory flow (μflow) were recorded using tissue reflectance spectrophotometry. Oxygen supply (μDO2) and consumption (μVO2) were calculated from intermittent blood gas analysis. ResultsIn septic animals, μHBO2 declined during normocapnia (−11±10.3) but remained unchanged during hypercapnia. μHBO2 declined with vasopressin V1A receptor blockade both during normocapnia (−7.4±10.6) and hypercapnia (−9.2±9.8). Microcirculatory oxygen consumption was significantly reduced by hypercapnia in septic animals (−2.4·105 [AU]±2.4·105 [AU]). In sham animals, μHBO2 and μVO2 did not change. ConclusionVasopressin V1A receptors mediate the beneficial effects of hypercapnia on microcirculatory oxygenation during sepsis. The effects of vasopressin on μHBO2 might be related to decreased oxygen consumption during hypercapnia.

GI ischemia in patients with portal vein thrombosis: a prospective cohort study

GI ischemia is a concerning adverse event of portal vein thrombosis (PVT). Minimally invasive techniques, such as visible light spectroscopy (VLS), have greatly improved the ability to diagnose GI ischemia. The aim of this study was to assess the clinical presentation and characteristics of GI ischemia in patients with PVT. Patients with noncirrhotic, nonmalignant PVT were included in this prospective cohort study. Clinical symptoms of GI ischemia were assessed by a structured questionnaire, VLS, and radiologic evaluation of the mesenteric vasculature. VLS measurements were compared with those in patients with cirrhosis and with a reference population. We included 15 patients with chronic PVT and 1 patient with acute PVT (median age 46.1 years [interquartile range [IQR], 30.9-53.7]; 44% male). Decreased mucosal oxygenation in at least 1 location of the GI tract was found in 12 patients (75%). Compared with the reference population (median 60.0 [IQR, 56.2-61.7]), VLS measurements were mostly decreased in the descending duodenum in patients with PVT (median 55.5 [IQR, 52.3-58.8]; P = .02) and patients with cirrhosis (median 52.0 [IQR, 46.5-54.0], P = .003). Symptoms typical for GI ischemia, such as postprandial pain and exercise-induced pain, were reported in 10 patients (63%) with PVT. In patients with extension of thrombosis into the superior mesenteric vein and splenic vein and/or presence of hypercoagulability, decreased VLS measurements were observed compared with historical control subjects. In patients with chronic PVT, GI ischemia is frequent. VLS enables objective and quantitative determination of GI mucosal ischemia. Onset of abdominal symptoms such as postprandial pain should prompt the physician to re-evaluate extent, cause, and treatment of PVT.

Physiologic hypoxia and oxygen homeostasis in the healthy intestine. A Review in the Theme: Cellular Responses to Hypoxia

In recent years, the intestinal mucosa has proven to be an intriguing organ to study tissue oxygenation. The highly vascularized lamina propria juxtaposed to an anaerobic lumen containing trillions of metabolically active microbes results in one of the most austere tissue microenvironments in the body. Studies to date have determined that a healthy mucosa contains a steep oxygen gradient along the length of the intestine and from the lumen to the serosa. Advances in technology have allowed multiple independent measures and indicate that, in the healthy mucosa of the small and large intestine, the lumen-apposed epithelia experience Po2 conditions of <10 mmHg, so-called physiologic hypoxia. This unique physiology results from a combination of factors, including countercurrent exchange blood flow, fluctuating oxygen demands, epithelial metabolism, and oxygen diffusion into the lumen. Such conditions result in the activation of a number of hypoxia-related signaling processes, including stabilization of the transcription factor hypoxia-inducible factor. Here, we review the principles of mucosal oxygen delivery, metabolism, and end-point functional responses that result from this unique oxygenation profile.

Hypothermia improves oral and gastric mucosal oxygenation during hypoxic challenges

Therapeutic hypothermia, used primarily for protective effects after hypoxia, improves oral and gastric mucosal microvascular oxygenation (μHbO₂) during additional haemorrhage. Therefore, we questioned whether hypothermia likewise improves μHbO₂ during hypoxic challenges. Since both hypothermia and hypoxia reduce cardiac output (e.g. by myofilament Ca(2+) desensitization), and modulate vasomotor tone via K(+) ATP channels, we hypothesized that the Ca(2+) sensitizer levosimendan and K(+) ATP channel blocker glibenclamide would support the cardiovascular system. The effects of mild hypothermia (34°C) on μHbO₂ during hypoxia [Formula: see text] were analysed in a cross-over study on five anaesthetized dogs and compared with normothermia (37.5°C) and hypoxia. During hypothermia, but before hypoxia, glibenclamide (0.2 mg kg(-1)) or levosimendan (20 µg kg(-1)+0.25 µg kg(-1) min(-1)) was administered. Systemic haemodynamic variables, gastric and oral mucosal microvascular oxygenation (reflectance spectrophotometry), and perfusion (laser Doppler flowmetry) were recorded continuously. Data are presented as mean (sem), P<0.05. Hypoxia during normothermia reduced gastric μHbO₂ by 27 (3)% and oral μHbO₂ by 28 (3)% (absolute change). During hypothermia, this reduction was attenuated to 16 (3)% and 13 (1)% (absolute change). This effect was independent of microvascular flow that did not change during hypoxia and hypothermia. Additional administration of levosimendan during hypothermia restored reduced cardiac output but did not change flow or μHbO₂ compared with hypothermia alone. Glibenclamide did not exert any additional effects during hypothermia. Hypothermia attenuates the decrease in μHbO₂ during additional hypoxic challenges independent of systemic or regional flow changes. A reduction in cardiac output during hypothermia is prevented by Ca(2+) sensitization with levosimendan but not by K(+) ATP channel blockade with glibenclamide.

Colorectal Carcinogenesis: A Cellular Response to Sustained Risk Environment

The current models for colorectal cancer (CRC) are essentially linear in nature with a sequential progression from adenoma through to carcinoma. However, these views of CRC development do not explain the full body of published knowledge and tend to discount environmental influences. This paper proposes that CRC is a cellular response to prolonged exposure to cytotoxic agents (e.g., free ammonia) as key events within a sustained high-risk colonic luminal environment. This environment is low in substrate for the colonocytes (short chain fatty acids, SCFA) and consequently of higher pH with higher levels of free ammonia and decreased mucosal oxygen supply as a result of lower visceral blood flow. All of these lead to greater and prolonged exposure of the colonic epithelium to a cytotoxic agent with diminished aerobic energy availability. Normal colonocytes faced with this unfavourable environment can transform into CRC cells for survival through epigenetic reprogramming to express genes which increase mobility to allow migration and proliferation. Recent data with high protein diets confirm that genetic damage can be increased, consistent with greater CRC risk. However, this damage can be reversed by increasing SCFA supply by feeding fermentable fibre as resistant starch or arabinoxylan. High protein, low carbohydrate diets have been shown to alter the colonic environment with lower butyrate levels and apparently greater mucosal exposure to ammonia, consistent with our hypothesis. Evidence is drawn from in vivo and in vitro genomic and biochemical studies to frame experiments to test this proposition.

Hypercapnia counteracts captopril-induced depression of gastric mucosal oxygenation

Hypercapnia (HC) increases systemic oxygen delivery (DO2) and gastric mucosal oxygenation. However, it activates the renin-angiotensin-aldosterone system (RAAS), which conversely reduces mesenteric perfusion. The aims of this study were to evaluate the effect of RAAS inhibition during normocapnia and HC on oral and gastric mucosal oxygenation (μHbO2) and to assess the effect of blood pressure under these circumstances. Five dogs were repeatedly anesthetized to study the effects of ACE inhibition (ACE-I; 5 mg/kg captopril, followed by 0.25 mg/kg per h) on μHbO2 (reflectance spectrophotometry) and hemodynamic variables during normocapnia (end-tidal CO2=35 mmHg) and HC (end-expiratory carbon dioxide (etCO2)=70 mmHg). In the control group, the dogs were subjected to HC alone. To exclude the effects of reduced blood pressure, in one group, blood pressure was maintained at baseline values via titrated phenylephrine (PHE) infusion during HC and additional captopril infusion. ACE-I strongly reduced gastric μHbO2 from 72±2 to 65±2% and mean arterial pressure (MAP) from 64±2 to 48±4 mmHg, while DO2 remained unchanged. This effect was counteracted in the presence of HC, which increased gastric μHbO2 from 73±3 to 79±6% and DO2 from 15±2 to 22±4 ml/kg per min during ACE-I without differences during HC alone. However, MAP decreased similar to that observed during ACE-I alone from 66±3 to 47±5 mmHg, while left ventricular contractility (dPmax) increased from 492±63 to 758±119 mmHg/s. Titrated infusion of PHE had no additional effects on μHbO2. In summary, our data suggest that RAAS inhibition reduces gastric mucosal oxygenation in healthy dogs. HC not only abolishes this effect, but also increases μHbO2, DO2, and dPmax. The increase in μHbO2 during ACE-I under HC is in accordance with our results independent of blood pressure.

The sympathetic nervous system mediates protective effects of hypothermia on gastric mucosal oxygenation during haemorrhagic shock

The sympathetic nervous system mediates protective effects of hypothermia on gastric mucosal oxygenation during haemorrhagic shock