Cystic fibrosis gene mutation reduces epithelial cell acidification and injury in acid-perfused mouse duodenum

Duodenal chemosensory system: enterocytes, enteroendocrine cells, and tuft cells

PURPOSE OF REVIEW

The gut barrier serves as the primary interface between the environment and host in terms of surface area and complexity. Luminal chemosensing is a term used to describe how small molecules in the gut lumen interact with the host through surface receptors or via transport into the subepithelial space. In this review, we have summarized recent advances in the understanding of the luminal chemosensory system in the gastroduodenal epithelium consisting of enterocytes, enteroendocrine, and tuft cells, with particular emphasis on how chemosensing affects mucosal protective responses and the metabolic syndrome.

RECENT FINDINGS

Recent single-cell RNA sequencing provides detailed cell type-specific expression of chemosensory receptors and other bioactive molecules as well as cell lineages; some are similar to lingual taste cells whereas some are gut specific. Gut luminal chemosensing is not only important for the local or remote regulation of gut function, but also contributes to the systemic regulation of metabolism, energy balance, and food intake. We will discuss the chemosensory mechanisms of the proximal intestine, in particular to gastric acid, with a focus on the cell types and receptors involved in chemosensing, with emphasis on the rare chemosensory cells termed tuft cells. We will also discuss the chemosensory functions of intestinal ectoenzymes and bacterial components (e.g., lipopolysaccharide) as well as how they affect mucosal function through altering the gut-hormonal-neural axis.

SUMMARY

Recent updates in luminal chemosensing by different chemosensory cells have provided new possibilities for identifying novel molecular targets for the treatment of mucosal injury, metabolic disorders, and abnormal visceral sensation.

Lipopolysaccharides transport during fat absorption in rodent small intestine

Lipopolysaccharides (LPS) are potent pro-inflammatory molecules that enter the systemic circulation from the intestinal lumen by uncertain mechanisms. We investigated these mechanisms and the effect of exogenous glucagon-like peptide-2 (GLP-2) on LPS transport in the rodent small intestine. Transmucosal LPS transport was measured in Ussing-chambered rat jejunal mucosa. In anesthetized rats, the appearance of fluorescein isothiocyanate (FITC)-LPS into the portal vein (PV) and the mesenteric lymph was simultaneously monitored after intraduodenal perfusion of FITC-LPS with oleic acid and taurocholate (OA/TCA). In vitro, luminally applied LPS rapidly appeared in the serosal solution only with luminal OA/TCA present, inhibited by the lipid raft inhibitor methyl-β-cyclodextrin (MβCD) and the CD36 inhibitor sulfosuccinimidyl oleate (SSO), or by serosal GLP-2. In vivo, perfusion of FITC-LPS with OA/TCA rapidly increased FITC-LPS appearance into the PV, followed by a gradual increase of FITC-LPS into the lymph. Rapid PV transport was inhibited by the addition of MβCD or by SSO, whereas transport into the lymph was inhibited by chylomicron synthesis inhibition. Intraveous injection of the stable GLP-2 analog teduglutide acutely inhibited FITC-LPS transport into the PV, yet accelerated FITC-LPS transport into the lymph via Nω-nitro-l-arginine methyl ester (l-NAME)- and PG97-269-sensitive mechanisms. In vivo confocal microscopy in mouse jejunum confirmed intracellular FITC-LPS uptake with no evidence of paracellular localization. This is the first direct demonstration in vivo that luminal LPS may cross the small intestinal barrier physiologically during fat absorption via lipid raft- and CD36-mediated mechanisms, followed by predominant transport into the PV, and that teduglutide inhibits LPS uptake into the PV in vivo.NEW & NOTEWORTHY We report direct in vivo confirmation of transcellular lipopolysaccharides (LPS) uptake from the intestine into the portal vein (PV) involving CD36 and lipid rafts, with minor uptake via the canonical chylomicron pathway. The gut hormone glucagon-like peptide-2 (GLP-2) inhibited uptake into the PV. These data suggest that the bulk of LPS absorption is via the PV to the liver, helping clarify the mechanism of LPS transport into the PV as part of the "gut-liver" axis. These data do not support the paracellular transport of LPS, which has been implicated in the pathogenesis of the "leaky gut" syndrome.

Cftr Modulates Wnt/β-Catenin Signaling and Stem Cell Proliferation in Murine Intestine

BACKGROUND & AIMS

Cystic fibrosis (CF) patients and CF mouse models have increased risk for gastrointestinal tumors. CF mice show augmented intestinal proliferation of unknown etiology and an altered intestinal environment. We examined the role of the cystic fibrosis transmembrane conductance regulator (Cftr) in Wnt/β-catenin signaling, stem cell proliferation, and its functional expression in the active intestinal stem cell (ISC) population. Dysregulation of intracellular pH (pH_i) in CF ISCs was investigated for facilitation of Wnt/β-catenin signaling.

METHODS

Crypt epithelia from wild-type (WT) and CF mice were compared ex vivo and in intestinal organoids (enteroids) for proliferation and Wnt/β-catenin signaling by standard assays. Cftr in ISCs was assessed by immunoblot of sorted Sox9 ^{enhanced green fluorescent protein(EGFP)} intestinal epithelia and pH_i regulation by confocal microfluorimetry of leucine-rich G-protein-coupled receptor 5 ISCs. Plasma membrane association of the Wnt transducer Dishevelled 2 (Dvl2) was assessed by fluorescence imaging of live enteroids from WT and CF mice crossed with Dvl2-EGFP/ACTB-tdTomato,-EGFP)Luo/J (Rosa^mT/mG) mice.

RESULTS

Relative to WT, CF intestinal crypts showed an ∼30% increase in epithelial and Lgr5+ ISC proliferation and increased Wnt/β-catenin signaling. Cftr was expressed in Sox9^EGFPLo ISCs and loss of Cftr induced an alkaline pH_i in ISCs. CF crypt-base columnar cells showed a generalized increase in plasma membrane Dvl2-EGFP association as compared with WT. Dvl2-EGFP membrane association was charge- and pH-dependent and increased in WT crypt-base columnar cells by Cftr inhibition.

CONCLUSIONS

CF intestine shows increased ISC proliferation and Wnt/β-catenin signaling. Loss of Cftr increases pH_i in ISCs, which stabilizes the plasma membrane association of the Wnt transducer Dvl, likely facilitating Wnt/β-catenin signaling. Absence of Cftr-dependent suppression of ISC proliferation in the CF intestine may contribute to increased risk for intestinal tumors.

The cystic fibrosis of exocrine pancreas

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is highly expressed in the pancreatic duct epithelia and permits anions and water to enter the ductal lumen. This results in an increased volume of alkaline fluid allowing the highly concentrated proteins secreted by the acinar cells to remain in a soluble state. This work will expound on the pathophysiology and pathology caused by the malfunctioning CFTR protein with special reference to ion transport and acid-base abnormalities both in humans and animal models. We will also discuss the relationship between cystic fibrosis (CF) and pancreatitis, and outline present and potential therapeutic approaches in CF treatment relevant to the pancreas.

Neuronal cGMP kinase I is essential for stimulation of duodenal bicarbonate secretion by luminal acid

Brief contact of the duodenal mucosa with luminal acid elicits a long-lasting bicarbonate (HCO(3)(-)) secretory response, which is believed to be the primary protective mechanism against mucosal damage. Here, we show that cGMP-dependent protein kinase type I-knockout (cGKI(-/-)) mice are unable to respond to a physiological H(+) stimulus with a HCO(3)(-) secretory response and spontaneously develop duodenal ulcerations. Smooth muscle-selective cGKI knock-in rescued the motility disturbance but not the defective HCO(3)(-) secretion. Proton-induced HCO(3)(-) secretion was not attenuated by selective inactivation of the cGKI gene in interstitial cells of Cajal or in enterocytes, but was abolished by inactivation of cGKI in neurons (ncGKI(-/-)). cGKI was expressed in the brainstem nucleus tractus solitarius that connects the afferent with the efferent N. vagus. Accordingly, truncation of the subdiaphragmal N. vagus significantly diminished proton-induced HCO(3)(-) secretion in wild-type mice, whereas stimulation of the subdiaphragmal N. vagus elicited a similar HCO(3)(-) secretory response in cGKI(-/-), ncGKI(-/-) and wild-type mice. These findings show that protection of the duodenum from acid injury requires neuronal cGKI.

Duodenal epithelial sensing of luminal acid: role of carbonic anhydrases

Sensing the luminal contents is a prerequisite to activate appropriate gastrointestinal functions. A major task of the duodenal epithelium is to resist the repeated challenges of hydrochloric acid expelled from the stomach. Although extensive research in this field, the complete mechanisms providing this defence remain to be revealed. The duodenal epithelium exports bicarbonate into a submillimetre-thick mucus gel on top of the mucosal surface. Despite the very low pH of the luminal contents, the duodenal mucus-bicarbonate barrier provides a means of maintaining a virtually neutral pH at the epithelial surface. Instead of pH, CO₂ generated by the mixing of acid and bicarbonate at levels not found elsewhere in the body serves as the mediator for sensing the luminal acid. Carbonic anhydrases (CAs) catalyse the reversible hydration of CO₂ and are heavily expressed in the duodenal segment. Accumulating data support the key function of CAs in sensing luminal acid and CO₂. Recent advances demonstrate that the presence of CA II in upper villus plays a crucial role in enterocyte intracellular acidification preceding the secretory increase in response to luminal acid. However, CAs only have a minor role in the bicarbonate supply destined for duodenal bicarbonate secretion into the lumen. The purpose of this review is to summarize the current knowledge of how intraluminal acid is sensed by the duodenal mucosa, with a focus on the role of CAs.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Endogenous luminal surface adenosine signaling regulates duodenal bicarbonate secretion in rats

Luminal ATP increases duodenal bicarbonate secretion (DBS) via brush border P2Y receptors. Because ATP is sequentially dephosphorylated to adenosine (ADO) and the brush border highly expresses adenosine deaminase (ADA), we hypothesized that luminal [ADO] regulators and sensors, including P1 receptors, ADA, and nucleoside transporters (NTs) regulate DBS. We measured DBS with pH and CO(2) electrodes, perfusing ADO ± adenosine receptor agonists or antagonists or the cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTR(inh)-172 on DBS. Furthermore, we examined the effect of inhibitors of ADA or NT on DBS. Perfusion of AMP or ADO (0.1 mM) uniformly increased DBS, whereas inosine had no effect. The A(1/2) receptor agonist 5'-(N-ethylcarboxamido)-adenosine (0.1 mM) increased DBS, whereas ADO-augmented DBS was inhibited by the potent A(2B) receptor antagonist N-(4-cyanophenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]-acetamide (MRS1754) (10 μM). Other selective adenosine receptor agonists or antagonists had no effect. The A(2B) receptor was immunolocalized to the brush border membrane of duodenal villi, whereas the A(2A) receptor was immunolocalized primarily to the vascular endothelium. Furthermore, ADO-induced DBS was enhanced by 2'-deoxycoformycin (1 μM) and formycin B (0.1 mM), but not by S-(4-nitrobenzyl)-6-thioinosine (0.1 mM), and it was abolished by CFTR(inh)-172 pretreatment (1 mg/kg i.p). Moreover, ATP (0.1 mM)-induced DBS was partially reduced by (1R,2S,4S,5S)-4-2-iodo-6-(methylamino)-9H-purin-9-yl]-2-(phosphonooxy)bicyclo[3.1.0]hexane-1-methanol dihydrogen phosphate ester tetraammonium salt (MRS2500) or 8-[4-[4-(4-chlorophenzyl)piperazide-1-sulfonyl)phenyl]]-1-propylxanthine (PSB603) and abolished by both, suggesting that ATP is sequentially degraded to ADO. Luminal ADO stimulates DBS via A(2B) receptors and CFTR. ATP release, ecto-phosphohydrolases, ADA, and concentrative NT may coordinately regulate luminal surface ADO concentration to modulate ADO-P1 receptor signaling in rat duodenum.

Gastroduodenal mucosal defense

PURPOSE OF REVIEW

The gastroduodenum has multiple means by which it resists injury from intrinsic and extrinsic factors, including gastric acid, nonsteroidal anti-inflammatory drugs, and Helicobacter pylori. We review recent insights into the mechanisms by which the gastroduodenum resists injury and discuss factors contributing to defensive failure.

RECENT FINDINGS

Duodenal bicarbonate secretion, a primary defensive mechanism, is mediated by the downregulated in adenoma anion exchanger and is stimulated by estrogens. Nonsteroidal anti-inflammatory drug gastric damage is dependent on toll-like receptor signaling. Portal hypertensive gastropathy impairs extracellular signal-regulated kinase 1/2 phosphorylation, increasing oxidative stress. H. pylori-induced peptic ulcer disease is associated with inadequate regulatory T cell responses.

SUMMARY

Enhanced understanding of the mechanisms of gastroduodenal defense and injury provides new insight into potential therapeutic targets, which contributes towards the development of more well tolerated and more effective therapies.

Luminal L-glutamate enhances duodenal mucosal defense mechanisms via multiple glutamate receptors in rats

Presence of taste receptor families in the gastrointestinal mucosa suggests a physiological basis for local and early detection of a meal. We hypothesized that luminal L-glutamate, which is the primary nutrient conferring fundamental umami or proteinaceous taste, influences mucosal defense mechanisms in rat duodenum. We perfused the duodenal mucosa of anesthetized rats with L-glutamate (0.1-10 mM). Intracellular pH (pH(i)) of the epithelial cells, blood flow, and mucus gel thickness (MGT) were simultaneously and continuously measured in vivo. Some rats were pretreated with indomethacin or capsaicin. Duodenal bicarbonate secretion (DBS) was measured with flow-through pH and CO(2) electrodes. We tested the effects of agonists or antagonists for metabotropic glutamate receptor (mGluR) 1 or 4 or calcium-sensing receptor (CaSR) on defense factors. Luminal L-glutamate dose dependently increased pH(i) and MGT but had no effect on blood flow in the duodenum. L-glutamate (10 mM)-induced cellular alkalinization and mucus secretion were inhibited by pretreatment with indomethacin or capsaicin. L-glutamate effects on pH(i) and MGT were mimicked by mGluR4 agonists and inhibited by an mGluR4 antagonist. CaSR agonists acidified cells with increased MGT and DBS, unlike L-glutamate. Perfusion of L-glutamate with inosinate (inosine 5'-monophosphate, 0.1 mM) enhanced DBS only in combination, suggesting synergistic activation of the L-glutamate receptor, typical of taste receptor type 1. L-leucine or L-aspartate had similar effects on DBS without any effect on pH(i) and MGT. Preperfusion of L-glutamate prevented acid-induced cellular injury, suggesting that L-glutamate protects the mucosa by enhancing mucosal defenses. Luminal L-glutamate may activate multiple receptors and afferent nerves and locally enhance mucosal defenses to prevent subsequent injury attributable to acid exposure in the duodenum.

Luminal chemosensing and upper gastrointestinal mucosal defenses

The upper gastrointestinal mucosa is exposed to endogenous and exogenous substances, including gastric acid, carbon dioxide, and foodstuffs. Physiologic processes such as secretion, digestion, absorption, and motility occur in the gastrointestinal tract in response to ingested substances, which implies the presence of mucosal sensors. We hypothesize that mucosal acid sensors and tastelike receptors are important components of the mucosal chemosensing system. We have shown that luminal acid/carbon dioxide is sensed via ecto- and cytosolic carbonic anhydrases and ion transporters in the epithelial cells and via acid sensors on the afferent nerves in the duodenum and esophagus. Furthermore, a luminal l-glutamate signal is mediated via mucosal l-glutamate receptors with activation of afferent nerves and cyclooxygenase in the duodenum, which suggests the presence of luminal l-glutamate sensing. These luminal chemosensors help to activate mucosal defense mechanisms to maintain the mucosal integrity and physiologic responses of the upper gastrointestinal tract. Because neural pathways are components of the luminal chemosensory system, investigation of these pathways may help to identify novel molecular targets in the treatment and prevention of mucosal injury and visceral sensation.

Pass the bicarb: the importance of HCO3- for mucin release

Accumulation of thick, sticky mucus is a hallmark of the genetic disease cystic fibrosis (CF) and has a central role in CF pathophysiology. Mutations in the CF transmembrane regulator (CFTR) ion channel are known to result in abnormally thick and sticky mucus; however, why mucus accumulates in CF is still not completely understood. In this issue of the JCI, Garcia and colleagues show that mucin--the heavily glycosylated protein contained within mucus--requires CFTR and bicarbonate in order to be released from mouse intestine (see the related article beginning on page 2613). The authors propose a model whereby CFTR-mediated bicarbonate secretion must be concurrent with mucin exocytosis for proper mucin release.

Duodenal acidity "sensing" but not epithelial HCO3- supply is critically dependent on carbonic anhydrase II expression

Carbonic anhydrase (CA) is strongly expressed in the duodenum and has been implicated in a variety of physiological functions including enterocyte HCO(3)(-) supply for secretion and the "sensing" of luminal acid and CO(2). Here, we report the physiological role of the intracellular CAII isoform involvement in acid-, PGE(2,) and forskolin-induced murine duodenal bicarbonate secretion (DBS) in vivo. CAII-deficient and WT littermates were studied in vivo during isoflurane anesthesia. An approximate 10-mm segment of the proximal duodenum with intact blood supply was perfused under different experimental conditions and DBS was titrated by pH immediately. Two-photon confocal microscopy using the pH-sensitive dye SNARF-1F was used to assess duodenocyte pH(i) in vivo. After correction of systemic acidosis by infusion of isotonic Na(2)CO(3), basal DBS was not significantly different in CAII-deficient mice and WT littermates. The duodenal bicarbonate secretory response to acid was almost abolished in CAII-deficient mice, but normal to forskolin- or 16,16-dimethyl PGE(2) stimulation. The complete inhibition of tissue CAs by luminal methazolamide and i.v. acetazolamide completely blocked the response to acid, but did not significantly alter the response to forskolin. While duodenocytes acidified upon luminal perfusion with acid, no significant pH(i) change occurred in CAII-deficient duodenum in vivo. The results suggest that CA II is important for duodenocyte acidification by low luminal pH and for eliciting the acid-mediated HCO(3)(-) secretory response, but is not important in the generation of the secreted HCO(3)(-) ions.

Intestinal alkaline phosphatase regulates protective surface microclimate pH in rat duodenum

Regulation of localized extracellular pH (pH(o)) maintains normal organ function. An alkaline microclimate overlying the duodenal enterocyte brush border protects the mucosa from luminal acid. We hypothesized that intestinal alkaline phosphatase (IAP) regulates pH(o) due to pH-sensitive ATP hydrolysis as part of an ecto-purinergic pH regulatory system, comprised of cell-surface P2Y receptors and ATP-stimulated duodenal bicarbonate secretion (DBS). To test this hypothesis, we measured DBS in a perfused rat duodenal loop, examining the effect of the competitive alkaline phosphatase inhibitor glycerol phosphate (GP), the ecto-nucleoside triphosphate diphosphohydrolase inhibitor ARL67156, and exogenous nucleotides or P2 receptor agonists on DBS. Furthermore, we measured perfusate ATP concentration with a luciferin-luciferase bioassay. IAP inhibition increased DBS and luminal ATP output. Increased luminal ATP output was partially CFTR dependent, but was not due to cellular injury. Immunofluorescence localized the P2Y(1) receptor to the brush border membrane of duodenal villi. The P2Y(1) agonist 2-methylthio-ADP increased DBS, whereas the P2Y(1) antagonist MRS2179 reduced ATP- or GP-induced DBS. Acid perfusion augmented DBS and ATP release, further enhanced by the IAP inhibitor l-cysteine, and reduced by the exogenous ATPase apyrase. Furthermore, MRS2179 or the highly selective P2Y(1) antagonist MRS2500 co-perfused with acid induced epithelial injury, suggesting that IAP/ATP/P2Y signalling protects the mucosa from acid injury. Increased DBS augments IAP activity presumably by raising pH(o), increasing the rate of ATP degradation, decreasing ATP-mediated DBS, forming a negative feedback loop. The duodenal epithelial brush border IAP-P2Y-HCO(3-) surface microclimate pH regulatory system effectively protects the mucosa from acid injury.

CFTR inhibition augments NHE3 activity during luminal high CO2 exposure in rat duodenal mucosa

We hypothesized that the function of duodenocyte apical membrane acid-base transporters are essential for H(+) absorption from the lumen. We thus examined the effect of inhibition of Na(+)/H(+) exchanger-3 (NHE3), cystic fibrosis transmembrane regulator (CFTR), or apical anion exchangers on transmucosal CO(2) diffusion and HCO(3)(-) secretion in rat duodenum. Duodena were perfused with a pH 6.4 high CO(2) solution or pH 2.2 low CO(2) solution with the NHE3 inhibitor, S3226, the anion transport inhibitor, DIDS, or pretreatment with the potent CFTR inhibitor, CFTR(inh)-172, with simultaneous measurements of luminal and portal venous (PV) pH and carbon dioxide concentration ([CO(2)]). Luminal high CO(2) solution increased CO(2) absorption and HCO(3)(-) secretion, accompanied by PV acidification and PV Pco(2) increase. During CO(2) challenge, CFTR(inh)-172 induced HCO(3)(-) absorption, while inhibiting PV acidification. S3226 reversed CFTR(inh)-associated HCO(3)(-) absorption. Luminal pH 2.2 challenge increased H(+) and CO(2) absorption and acidified the PV, inhibited by CFTR(inh)-172 and DIDS, but not by S3226. CFTR inhibition and DIDS reversed HCO(3)(-) secretion to absorption and inhibited PV acidification during CO(2) challenge, suggesting that HCO(3)(-) secretion helps facilitate CO(2)/H(+) absorption. Furthermore, CFTR inhibition prevented CO(2)-induced cellular acidification reversed by S3226. Reversal of increased HCO(3)(-) loss by NHE3 inhibition and reduced intracellular acidification during CFTR inhibition is consistent with activation or unmasking of NHE3 activity by CFTR inhibition, increasing cell surface H(+) available to neutralize luminal HCO(3)(-) with consequent CO(2) absorption. NHE3, by secreting H(+) into the luminal microclimate, facilitates net transmucosal HCO(3)(-) absorption with a mechanism similar to proximal tubular HCO(3)(-) absorption.

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.

Review article: duodenal bicarbonate - mucosal protection, luminal chemosensing and acid-base balance

The duodenum serves as a buffer zone between the stomach and the jejunum. Over a length of only 25 cm, large volumes of strong acid secreted by the stomach must be converted to the neutral-alkaline chyme of the hindgut lumen, generating large volumes of CO(2). The duodenal mucosa consists of epithelial cells connected by low-resistance tight junctions, forming a leaky epithelial barrier. Despite this permeability, the epithelial cells, under intense stress from luminal mineral acid and highly elevated Pco(2), maintain normal functioning. Bicarbonate ion uniquely protects the duodenal epithelial cells from acid-related injury. The specific protective mechanisms likely involve luminal bicarbonate secretion, intracellular pH buffering and interstitial buffering. Furthermore, the duodenum plays an active role in foregut acid-base homeostasis, absorbing large amounts of H(+) and CO(2). We have studied mucosal protection and acid-base balance using live-animal fluorescence ratio microimaging and by performing H(+) and CO(2) balance studies on duodenal perfusates. On the basis of these data, we have formulated novel hypotheses with regard to mucosal protection.

Epithelial carbonic anhydrases facilitate PCO2 and pH regulation in rat duodenal mucosa

The duodenum is the site of mixing of massive amounts of gastric H+ with secreted HCO3-, generating CO2 and H2O accompanied by the neutralization of H+. We examined the role of membrane-bound and soluble carbonic anhydrases (CA) by which H+ is neutralized, CO2 is absorbed, and HCO3- is secreted. Rat duodena were perfused with solutions of different pH and PCO2 with or without a cell-permeant CA inhibitor methazolamide (MTZ) or impermeant CA inhibitors. Flow-through pH and PCO2 electrodes simultaneously measured perfusate and effluent pH and PCO2. High CO2 (34.7 kPa) perfusion increased net CO2 loss from the perfusate compared with controls (pH 6.4 saline, PCO2 approximately 0) accompanied by portal venous (PV) acidification and PCO2 increase. Impermeant CA inhibitors abolished net perfusate CO2 loss and increased net HCO3- gain, whereas all CA inhibitors inhibited PV acidification and PCO2 increase. The changes in luminal and PV pH and [CO2] were also inhibited by the Na+-H+ exchanger-1 (NHE1) inhibitor dimethylamiloride, but not by the NHE3 inhibitor S3226. Luminal acid decreased total CO2 output and increased H+ loss with PV acidification and PCO2 increase, all inhibited by all CA inhibitors. During perfusion of a 30% CO2 buffer, loss of CO2 from the lumen was CA dependent as was transepithelial transport of perfused 13CO2. H+ and CO2 loss from the perfusate were accompanied by increases of PV H+ and tracer CO2, but unchanged PV total CO2, consistent with CA-dependent transmucosal H+ and CO2 movement. Inhibition of membrane-bound CAs augments the apparent rate of net basal HCO3- secretion. Luminal H+ traverses the apical membrane as CO2, is converted back to cytosolic H+, which is extruded via NHE1. Membrane-bound and cytosolic CAs cooperatively facilitate secretion of HCO3- into the lumen and CO2 diffusion into duodenal mucosa, serving as important acid-base regulators.

Duodenal Carbonic Anhydrase: Mucosal Protection, Luminal Chemosensing, and Gastric Acid Disposal

The duodenum serves as a buffer zone between the stomach and jejunum. Over a length of only 25 cm, large volumes of strong acid secreted by the stomach must be converted to the neutral-alkaline chyme of the hindgut lumen, generating large volumes of CO2, which the duodenum then absorbs. The duodenal mucosa consists of epithelial cells connected by low-resistance tight junctions, forming a leaky epithelial barrier. Despite this high permeability, the epithelial cells, under intense stress from luminal mineral acid and highly elevated P(CO2), maintain normal functioning. Furthermore, the duodenum plays an active role in foregut acid-base homeostasis, absorbing large amounts of H+ and CO2 that are recycled by the gastric parietal cells. Prompted by the high expression of cytosolic and membrane carbonic anhydrase (CAs) in duodenal epithelial cells, and the intriguing observation that CA activity appears to augment cellular acid stress, we formulated a novel hypothesis regarding the role of CA in duodenal acid absorption, epithelial protection, and chemosensing. In this review, we will describe how luminal CO2/H+ traverses the duodenal epithelial cell brush border membrane, acidifies the cytoplasm, and is sensed in the subepithelium.

Carbonic anhydrase isozyme-II-deficient mice lack the duodenal bicarbonate secretory response to prostaglandin E2

Duodenal bicarbonate secretion (DBS) is accepted as the primary mucosal defense against acid discharged from the stomach and is impaired in patients with duodenal ulcer disease. The secretory response to luminal acid is the main physiological stimulus for DBS and involves mediation by PGE2 produced by mucosal cells. The aim of this investigation is to elucidate the role of carbonic anhydrases (CAs) II and IX in PGE2-mediated bicarbonate secretion in the murine duodenum. CA II- and IX-deficient mice and different combinations of their heterozygous and WT counterparts were studied. A 10-mm segment of the proximal duodenum with intact blood supply was isolated, and DBS was titrated by pH-stat (TitroLine-easy, Schott, Mainz, Germany). Mean arterial blood pressure (MAP) was continuously recorded, and blood acid/base balance and gastrointestinal morphology were analyzed. The duodenal segment spontaneously secreted HCO3(-) at a steady basal rate of 5.3 +/- 0.6 micromol x cm(-1) x h(-1). Perfusing the duodenal lumen for 20 min with 47 microM PGE2 caused a significant increase in DBS to 13.0 +/- 2.9 micromol x cm(-1) x h(-1), P < 0.0001. The DBS response to PGE2 was completely absent in Car2-/- mice, whereas basal DBS was normal. The CA IX-deficient mice with normal Car2 alleles showed a slight increase in DBS. Histological abnormalities were observed in the gastroduodenal epithelium in both CA II- and IX-deficient mice. Our data demonstrate a gastrointestinal phenotypic abnormality associated with CA II deficiency. The results show that the stimulatory effect of the duodenal secretagogue PGE2 completely depends on CA II.

A novel small molecule CFTR inhibitor attenuates HCO3- secretion and duodenal ulcer formation in rats

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) plays a crucial role in mediating duodenal bicarbonate (HCO(3)(-)) secretion (DBS). Although impaired DBS is observed in CF mutant mice and in CF patients, which would predict increased ulcer susceptibility, duodenal injury is rarely observed in CF patients and is reduced in CF mutant mice. To explain this apparent paradox, we hypothesized that CFTR dysfunction increases cellular [HCO(3)(-)] and buffering power. To further test this hypothesis, we examined the effect of a novel, potent, and highly selective CFTR inhibitor, CFTR(inh)-172, on DBS and duodenal ulceration in rats. DBS was measured in situ using a standard loop perfusion model with a pH stat under isoflurane anesthesia. Duodenal ulcers were induced in rats by cysteamine with or without CFTR(inh)-172 pretreatment 1 h before cysteamine. Superfusion of CFTR(inh)-172 (0.1-10 microM) over the duodenal mucosa had no effect on basal DBS but at 10 microM inhibited acid-induced DBS, suggesting that its effect was limited to CFTR activation. Acid-induced DBS was abolished at 1 and 3 h and was reduced 24 h after treatment with CFTR(inh)-172, although basal DBS was increased at 24 h. CFTR(inh)-172 treatment had no effect on gastric acid or HCO(3)(-) secretion. Duodenal ulcers were observed 24 h after cysteamine treatment but were reduced in CFTR(inh)-172-pretreated rats. CFTR(inh)-172 acutely produces CFTR dysfunction in rodents for up to 24 h. CFTR inhibition reduces acid-induced DBS but also prevents duodenal ulcer formation, supporting our hypothesis that intracellular HCO(3)(-) may be an important protective mechanism for duodenal epithelial cells.

Chloride conductance of CFTR facilitates basal Cl-/HCO3- exchange in the villous epithelium of intact murine duodenum

Villi of the proximal duodenum are situated for direct exposure to gastric acid chyme. However, little is known about active bicarbonate secretion across villi that maintains the protective alkaline mucus barrier, a process that may be compromised in cystic fibrosis (CF), i.e., in the absence of a functional CF transmembrane conductance regulator (CFTR) anion channel. We investigated Cl(-)/HCO(3)(-) exchange activity across the apical membrane of epithelial cells located at the midregion of villi in intact duodenal mucosa from wild-type (WT) and CF mice using the pH-sensitive dye BCECF. Under basal conditions, the Cl(-)/HCO(3)(-) exchange rate was reduced by approximately 35% in CF compared with WT villous epithelium. Cl(-)/HCO(3)(-) exchange in WT and CF villi responded similarly to inhibitors of anion exchange, and membrane depolarization enhanced rates of Cl(-)(out)/HCO(3)(-)(in) exchange in both epithelia. In anion substitution studies, anion(in)/HCO(3)(-)(out) exchange rates were greater in WT epithelium using Cl(-) or NO(3)(-), but decreased to the level of the CF epithelium using the CFTR-impermeant anion, SO(4)(2-). Similarly, treatment of WT epithelium with the CFTR-selective blocker glybenclamide decreased the Cl(-)/HCO(3)(-) exchange rate to the level of CF epithelium. The mRNA expression of Slc26a3 (downregulated in adenoma) and Slc26a6 (putative anion exchanger-1) was similar between WT and CF duodena. From these studies of murine duodenum, we conclude 1) characteristics of Cl(-)/HCO(3)(-) exchange in the villous epithelium are most consistent with Slc26a6 activity, and 2) Cl(-) channel activity of CFTR facilitates apical membrane Cl(-)(in)/HCO(3)(-)(out) exchange by providing a Cl(-) "leak" under basal conditions.

Mechanism of augmented duodenal HCO(3)(-) secretion after elevation of luminal CO(2)

The proximal duodenum is exposed to extreme elevations of P(CO(2)) because of the continuous mixture of secreted HCO(3)(-) with gastric acid. These elevations (up to 80 kPa) are likely to place the mucosal cells under severe acid stress. Furthermore, we hypothesized that, unlike most other cells, the principal source of CO(2) for duodenal epithelial cells is from the lumen. We hence examined the effect of elevated luminal P(CO(2)) on duodenal HCO(3)(-) secretion (DBS) in the rat. DBS was measured by the pH-stat method. For CO(2) challenge, the duodenum was superfused with a high Pco(2) solution. Intracellular pH (pH(i)) of duodenal epithelial cells was measured by ratio microfluorometry. CO(2) challenge, but not isohydric solutions, strongly increased DBS to approximately two times basal for up to 1 h. Preperfusion of the membrane-permeant carbonic anhydrase inhibitor methazolamide, or continuous exposure with indomethacin, fully inhibited CO(2)-augmented DBS. Dimethyl amiloride (0.1 mM), an inhibitor of the basolateral sodium-hydrogen exchanger 1, also inhibited CO(2)-augumented DBS, although S-3226, a specific inhibitor of apical sodium-hydrogen exchanger 3, did not. DIDS, an inhibitor of basolateral sodium-HCO(3)(-) cotransporter, also inhibited CO(2)-augemented DBS, as did the anion channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid. CO(2) decreased epithelial cell pH(i), followed by an overshoot after removal of the CO(2) solution. We conclude that luminal CO(2) diffused in the duodenal epithelial cells and was converted to H(+) and HCO(3)(-) by carbonic anhydrase. H(+) initially exited the cell, followed by secretion of HCO(3)(-). Secretion was dependent on a functioning basolateral sodium/proton exchanger, a functioning basolateral HCO(3)(-) uptake mechanism, and submucosal prostaglandin generation and facilitated hydration of CO(2) into HCO(3)(-) and H(+).