Mechanism by which bile salt disrupts the gastric mucosal barrier in the dog.

Bile salts disrupt a functional "gastric mucosal barrier" increasing net forward-diffusion (+) of Na+ and back-diffusion (-) of H+. Studying canine Heidenhain pouches, we attempted to distinguish between two possible mechanisms for this effect: (a) mucosal uptake of bile salt with subsequent cellular injury or (b) dissolution of mucosal lipids by intralumenal bile salt. A 10 mM mixture of six conjugated bile salts simulating the proportions found in human bile induced net Na+ flux of 15.5 +/- 3.2 and net H+ flux of -9.9 +/- 3.3 mueq/min. This change was accompanied by an increase in phospholipid efflux out of gastric mucosa from a base-line value of 13.2 +/- 2.7 to 54.8 +/- 2.8 nmol/min (P < 0.001) and an increase in cholesterol efflux from 11.7 +/- 3.8 to 36.3 +/- 3.2 nmol/min (P < 0.001). Saturation with lecithin (25 mM) and cholesterol (50 mM) blocked disruption of the gastric mucosal barrier by bile salt (Na+ flux - 1.2 +/- 0.9, H+ flux 0.6 +/- 1.8 mueq/min). A 10 mM solution of taurodehydrocholate, a bile salt that does not form micelles, induced no net Na+ (-0.3 +/- 0.8) or H+ flux (-0.7 +/- 1.4) and did not increase efflux of phospholipid (11.3 +/- 1.7) or cholesterol (10.4 +/- 2.0) over base line. Bile salt was absorbed from the mixture of six conjugates at 752 +/- 85 nmol/min. Addition of subsaturation amounts of lecithin (4 mM) reduced bile salt absorption three fold to 252 +/- 57 (P < 0.001), but abnormal Na+ flux (14.1 +/- 3.4) and H+ flux (-15.6 +/- 3.5) persisted. Taurodehydrocholate was absorbed to an intermediate extent (467 +/- 116). Dissolution of mucosal lipids is apparently the mechanism by which bile salt disrupts the gastric mucosal barrier, and presumably at least one mechanism by which bile salt can injure the gastric mucosa.     

Pepsin output after disruption of the human gastric mucosal barrier by taurocholic acid.

Test solutions containing 10 mM taurocholic acid in 160 mM HCl were instilled into the stomachs of healthy volunteers to disrupt the gastric mucosal barrier. Although significant back diffusion of hydrogen ions and exsorption of sodium ions occurred, there was no significant increase in pepsin output compared to control studies with 160 mm HCl alone. Our data suggest that 160 mM HCl produces a many-fold increase in basal pepsin output. In the innervated stomach of man disruption of the gastric mucosal barrier by taurocholic acid in acid solution is not accompanied by increased pepsin secretion compared to that produced by 160 mM HCl alone.     

Protective effects of nonionic triblock copolymers on bile acid-mediated epithelial barrier disruption

Translocation of bacteria and other luminal factors from the intestine following surgical injury can be a major driver of critical illness. Bile acids have been shown to play a key role in the loss of intestinal epithelial barrier function during states of host stress. Experiments to study the ability of nonionic block copolymers to abrogate barrier failure in response to bile acid exposure are described. In vitro experiments were performed with the bile salt sodium deoxycholate on Caco-2 enterocyte monolayers using transepithelial electrical resistance to assay barrier function. A bisphenol A coupled triblock polyethylene glycol (PEG), PEG 15-20, was shown to prevent sodium deoxycholate-induced barrier failure. Enzyme-linked immunosorbent assay, lactate dehydrogenase, and caspase 3-based cell death detection assays demonstrated that bile acid-induced apoptosis and necrosis were prevented with PEG 15-20. Immunofluorescence microscopic visualization of the tight junctional protein zonula occludens 1 (ZO-1) demonstrated that PEG 15-20 prevented significant changes in tight junction organization induced by bile acid exposure. Preliminary transepithelial electrical resistance-based studies examining structure-function correlates of polymer protection against bile acid damage were performed with a small library of PEG-based copolymers. Polymer properties associated with optimal protection against bile acid-induced barrier disruption were PEG-based compounds with a molecular weight greater than 10 kd and amphiphilicity. The data demonstrate that PEG-based copolymer architecture is an important determinant that confers protection against bile acid injury of intestinal epithelia.     

Oral phosphatidylcholine preserves the gastrointestinal mucosal barrier during LPS-induced inflammation

The hydrophobic surface layer of the gastrointestinal (GI) tract, which has been attributed to the presence of phosphatidylcholine (PC) in the mucus gel, protects the mucosa of the GI tract and is disrupted by parenteral LPS treatment. We investigated the potential for repletion of this layer as a means to prevent LPS-induced GI injury. Rats were treated orally with PC 1 h before LPS (i.p.). Gastric and ileal tissues were assessed for changes in permeability 5 h later, and gastric fluid was analyzed for signs of GI-related LPS effects (bile acid reflux, increased volume, and pH) and gastric injury (bleeding). Serum TNF-alpha was assessed as a measure of a non-GI, LPS response. Radiolabeled PC was tracked through the GI tract to verify the extent of luminal exposure during the time of the study. Pretreatment with oral PC significantly blocked permeability increases in gastric and ileal tissue due to LPS. A portion of orally administered PC gained access to the entire GI tract in 1 h. Exogenous PC did not prevent the increase in serum TNF-alpha or gastric fluid volume or pH induced by LPS, nor did it prevent the duodenogastric reflux of bile acid. There was a tendency for PC to reduce gastric bleeding after LPS. Orally administered PC seems to act directly on the mucosa to prevent GI permeability disturbances due to LPS. Under the conditions studied, oral PC does not block systemic effects of LPS. However, enteral formulations containing PC may be useful adjuncts in the prevention of GI injury from endotoxemia.     

MRI引导与监测聚焦超声靶向开放血脑屏障兔脑给药

目的 探讨MRI引导与监测聚焦超声靶向开放血脑屏障(BBB)兔脑给药的效果.方法 25只新西兰大白兔随机均分为5组(0 h,2 h,4 h,8 h,24 h组),MRI引导聚焦超声辐照各组兔左脑,取靶点组织作为辐照组,取右脑相应解剖组织为对照组,分别于辐照后不同时间点静注欧乃影和甲氨喋呤(MTX),行T1加权相增强扫描,通过比较辐照组与对照组MRI信号强度增强率观察靶点BBB开放情况,并以靶点伊文氏蓝(EB)染色结果检验其准确性;以高效液相色谱法测定、比较辐照组与对照组MTX的浓度,计算靶点脑组织信号强度增强率与MTX浓度之间的线性相关系数.结果 兔脑靶点BBB开放处T1WI表现为点状或斑片状异常高信号影,与辐照前MRI定位时预设靶点位置吻合良好,并与EB蓝染结果一致,同时靶点脑组织信号强度增强率与MTX浓度之间的相关系数为0.96(P<0.01),两者有良好的相关性.结论 MRI能够引导与监测聚焦超声靶向开放BBB脑内给药,有望成为临床个体化治疗中枢神经系统疾病的理想监测手段.     

Systemic delivery of antagomirs during blood-brain barrier disruption is disease-modifying in experimental epilepsy

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Lipid metabolism in bile acid malabsorption

Malabsorption of bile acid increases cholesterol synthesis and activates hepatic LDL receptors which leads to enhanced elimination of cholesterol from the body. Interruption of enterohepatic circulation of bile acids may lead to a smaller bile acid pool, which, in turn, impairs cholesterol and fat absorption by reduced micellar solubilization. Together with reduced cholesterol absorption, the increased cholesterol loss as bile acids also reduces plasma cholesterol concentrations and the biliary cholesterol excretion, too. Diminished biliary cholesterol in bile acid malabsorption may contribute to the increased incidence of gallstones associated with ileal dysfunction. Malabsorption of bile acid leads to a fall in LDL-cholesterol concentration, and an increase of HDL-cholesterol concentration has been reported. VLDL-triglyceride concentrations are almost invariably raised. Enhanced cholesterol and bile acid synthesis in ileal dysfunction is reflected by raised concentrations of plasma cholesterol precursors, especially lathosterols, which can be used as an indicator of increased bile acid loss to faeces. Cholesterol absorption, in turn, correlates positively with plasma plant sterol concentrations levels and the ratio of lathosterols to campesterols can be used as a screening measurement for ileal dysfunction. Plasma fatty acid composition is also altered as a response to fat malabsorption associated with ileal dysfunction. The proportion of essential fatty acids is inversely correlated with faecal fat excretion and endogenous fatty acid synthesis is activated.     

Indomethacin disrupts the protective effect of phosphatidylcholine against bile salt-induced ileal mucosa injury

BACKGROUND: Indomethacin (Indo) exerts local toxic effects on small intestinal mucosa, possibly in association with hydrophobic bile salts. We investigated the potential toxic effects of Indo on ileal mucosa and the role of phosphatidylcholine (PC). MATERIALS AND METHODS: Transmucosal resistance and Na-fluorescein permeability of ileal mucosa segments from female Wistar rats were determined in Ussing chambers during a 30-min incubation with model systems containing: control-buffer, taurodeoxycholate (TDC), Indo, TDC-Indo, TDC-PC, or TDC-PC-Indo. Decrease of resistance and increase of permeability were considered as parameters for mucosal injury. After incubation in Ussing chambers, the histopathology was examined to quantify the extent of mucosal injury. Also, in CaCo-2 cells, LDH-release was determined as a measure of cytotoxicity, after incubation with various model systems. RESULTS: Decrease of resistance and increase of permeability were highest in systems containing TDC-Indo (P < 0.01). Phosphatidylcholine protected against the cytotoxic effects of TDC in absence of Indo only. Extent of mucosal injury by histological examination was also highest in systems containing TDC-Indo (P = 0.006). Again, PC exhibited protective effects in absence of Indo only. The LDH-release by CaCo2-cells was strongest in TDC-Indo systems (P < 0.001). CONCLUSIONS: Indomethacin disrupts protective effects of PC against bile salt-induced ileal mucosa injury. This finding is relevant for small intestinal injury induced by non-steroidal anti-inflammatory drugs.     

Membrane structural changes support the involvement of mitochondria in the bile salt-induced apoptosis of rat hepatocytes

The accumulation of toxic bile salts within the hepatocyte plays a key role in organ injury during liver disease. Deoxycholate (DC) and glycochenodeoxycholate (GCDC) induce apoptosis in vitro and in vivo, perhaps through direct perturbation of mitochondrial membrane structure and function. In contrast, ursodeoxycholate (UDC) and its taurine-conjugated form (TUDC) appear to be protective. We show here that hydrophobic bile salts induced apoptosis in cultured rat hepatocytes, without modulating the expression of pro-apoptotic Bax protein, and caused cytochrome c release in isolated mitochondria. Co-incubation with UDC and TUDC prevented cell death and efflux of mitochondrial factors. Using spin-labelling techniques and EPR spectroscopy analysis of isolated rat liver mitochondria, we found significant structural changes at the membrane-water surface in mitochondria exposed to hydrophobic bile salts, including modified lipid polarity and fluidity, altered protein order and increased oxidative injury. UDC, TUDC and cyclosporin A almost completely abrogated DC- and GCDC-induced membrane perturbations. We conclude that the toxicity of hydrophobic bile salts to hepatocytes is mediated by cytochrome c release, through a mechanism associated with marked direct effects on mitochondrial membrane lipid polarity and fluidity, protein order and redox status, without modulation of pro-apoptotic Bax expression. UDC and TUDC can directly suppress disruption of mitochondrial membrane structure, which may represent an important mechanism of hepatoprotection by these bile salts.     

急性重症大鼠胰炎大鼠肠黏膜超微结构的改变

目的 探讨肠黏膜超微结构改变在急性重症胰腺炎时肠膜屏障功能障碍中的意义。方法 对急性重症胰腺炎（ASP）模型大鼠及假手术组大鼠（SO）于术后6、12h取回肠黏膜组织进行光镜及电镜（利用辣根过氧化酶作为黏膜通透性改变的示踪剂）。同时对血、肠系膜淋巴结（LMN）、肝、脾、肺组织进行了细菌培养及鉴定,测定门静脉血中内毒素水平。结果 ASP大鼠6及12h时相点肠黏膜组织结构基本正常;电镜观察制膜12h肠上皮细胞紧密连接间有高密度电子物质沉着,细胞紧密连接开放;假手术组肠黏膜上皮细胞紧密连接完整,细胞间隙无示踪剂沉着。12h时相点ASP组大鼠远隔部位细菌移位率较假手术组显著增加,培养出的细菌为大肠杆菌、绿脓杆菌、变形杆菌、芳香杆菌、枯草杆菌及肺炎克雷伯杆菌,几乎均为肠源性细菌;12h时相点ASP组大鼠血浆内毒水平较SO组显著增加。结论 肠黏膜上皮细胞间紧密连接开放,通透性增加是ASP早期肠道细菌、内毒素移位,肠黏膜屏障功能障碍的形态学基础。     

Natural progression of the lower esophageal mucosal ring

Among 390 patients with endoscopically verified lower esophageal mucosal ring (LEMR), 22 cases were identified with previous or subsequent radiologic examinations of the esophagogastric region. Among these, it was found that 2 cases of LEMR had developed from a normal esophagus. In 3 patients, there was increasing stenosis of the LEMR. In 8 cases, the LEMR was transformed into an esophageal stricture. In 10 of the 13 cases, esophagitis of varying degree was present endoscopically. In the 9 patients exhibiting no change in the LEMR, only 1 patient had esophagitis. The data suggest that there is a potential progression from normal esophagus to lower esophageal ring to esophageal stricture that occurs in association with reflux esophagitis.     

Disruption of the mucosal barrier during gut ischemia allows entry of digestive enzymes into the intestinal wall

Intestinal ischemia is associated with high morbidity and mortality, but the underlying mechanisms are uncertain. We hypothesize that during ischemia the intestinal mucosal barrier becomes disrupted, allowing digestive enzymes access into the intestinal wall initiating autodigestion. We used a rat model of splanchnic ischemia by occlusion of the superior mesenteric and celiac arteries up to 30 min with and without luminal injection of tranexamic acid as a trypsin inhibitor. We determined the location and activity of digestive proteases on intestinal sections with in situ zymography, and we examined the disruption of two components of the mucosal barrier: mucin isoforms and the extracellular and intracellular domains of E cadherin with immunohistochemistry and Western blot techniques. The results indicate that nonischemic intestine has low levels of protease activity in its wall. After 15-min ischemia, protease activity was visible at the tip of the villi, and after 30 min, enhanced activity was seen across the full thickness of the intestinal wall. This activity was accompanied by disruption of the mucin layer and loss of both intracellular and extracellular domains of E cadherin. Digestive protease inhibition in the intestinal lumen with tranexamic acid reduced morphological damage and entry of digestive enzymes into the intestinal wall. This study demonstrates that disruption of the mucosal epithelial barrier within minutes of intestinal ischemia allows entry of fully activated pancreatic digestive proteases across the intestinal barrier triggering autodigestion.     

Astrocyte-derived VEGF-A drives blood-brain barrier disruption in CNS inflammatory disease

In inflammatory CNS conditions such as multiple sclerosis (MS), current options to treat clinical relapse are limited, and more selective agents are needed. Disruption of the blood-brain barrier (BBB) is an early feature of lesion formation that correlates with clinical exacerbation, leading to edema, excitotoxicity, and entry of serum proteins and inflammatory cells. Here, we identify astrocytic expression of VEGF-A as a key driver of BBB permeability in mice. Inactivation of astrocytic Vegfa expression reduced BBB breakdown, decreased lymphocyte infiltration and neuropathology in inflammatory and demyelinating lesions, and reduced paralysis in a mouse model of MS. Knockdown studies in CNS endothelium indicated activation of the downstream effector eNOS as the principal mechanism underlying the effects of VEGF-A on the BBB. Systemic administration of the selective eNOS inhibitor cavtratin in mice abrogated VEGF-A-induced BBB disruption and pathology and protected against neurologic deficit in the MS model system. Collectively, these data identify blockade of VEGF-A signaling as a protective strategy to treat inflammatory CNS disease.     

Effect of cimetidine on gastric mucosal barrier in dogs

The effect of cimetidine on the gastric mucosal barrier (GMB) was investigated in adult mongrel dogs with a Heidenhain pouch. A linear correlation was found between the net flux of Na+ and H+, and transmucosal electrical potential difference (PD). The PD was thus considered to be a good indicator for the degree of impairment of the GMB which can be determined easily with high reproducibility. When irrigated with HCl alone, there was a slight, though not significant, upward tendency of the PD following intravenous cimetidine. The destruction of the GMB during perfusion with HCl plus sodium taurocholate resulted in a marked reduction of the PD. This response of the PD was significantly reduced by the injection of cimetidine. These results indicate that cimetidine protects the GMB from impairments by bile acid.