Gastroduodenal mucosal alkaline secretion and mucosal protection

The electroneutral Na+-HCO3- cotransporter NBCn1 (SLC4A7) modulates colonic enterocyte pHi, proliferation, and migration

NBCn1 (SLC4A7) is one of the two major Na+-HCO3- cotransporters in the human colonic epithelium, expressed predominantly in the highly proliferating colonocytes at the cryptal base. Increased NBCn1 expression levels are reported in tumors, including colorectal cancer. The study explores its importance for maintenance of the intracellular pH (pHi), as well as the proliferative, adhesive, and migratory behavior of the self-differentiating Caco2BBe colonic tumor cell line. In the self-differentiating Caco2BBe cells, NBCn1 mRNA was highly expressed from the proliferative stage until full differentiation. The downregulation of NBCn1 expression by RNA interference affected proliferation and differentiation and decreased intracellular pH (pHi) of the cells in correlation with the degree of knockdown. In addition, a disturbed cell adhesion and reduced migratory speed were associated with NBCn1 knockdown. Murine colonic Nbcn1-/- enteroids also displayed reduced proliferative activity. In the migrating Caco2BBe cells, NBCn1 was found at the leading edge and in colocalization with the focal adhesion markers vinculin and paxillin, which suggests that NBCn1 is involved in the establishment of cell-matrix adhesion. Our data highlight the physiological significance of NBCn1 in modulating epithelial pH homeostasis and cell-matrix interactions in the proliferative region of the colonic epithelium and unravel the molecular mechanism behind pathological overexpression of this transporter in human colorectal cancers.NEW & NOTEWORTHY The transporter NBCn1 plays a central role in maintaining homeostasis within Caco2BBe colonic epithelial cells through its regulation of intracellular pH, matrix adhesion, migration, and proliferation. These observations yield valuable insights into the molecular mechanism of the aberrant upregulation of this transporter in human colorectal cancers.

Role of Serosal TRPV4-Constituted SOCE Mechanism in Secretagogues-Stimulated Intestinal Epithelial Anion Secretion

As little is known about the role of calcium (Ca²⁺) signaling mediating the small intestinal epithelial anion secretion, we aimed to study its regulatory role in secretagogue-stimulated duodenal anion secretion and the underlying molecular mechanisms. Therefore, intestinal anion secretion from native mouse duodenal epithelia was examined with Ussing chambers to monitor PGE₂-, 5-HT-, and CCh-induced short-circuit currents (I_sc). PGE₂ (10 μM) and 5-HT (10 μM) induced mouse duodenal I_sc, markedly attenuated by serosal Ca²⁺-free solution and selective blockers of store-operated Ca²⁺ channels on the serosal side of the duodenum. Furthermore, PGE₂- and 5-HT-induced duodenal I_sc was also inhibited by ER Ca²⁺ chelator TPEN. However, dantrolene, a selective blocker of ryanodine receptors, inhibited PGE₂-induced duodenal I_sc, while LiCl, an inhibitor of IP₃ production, inhibited 5-HT-induced I_sc. Moreover, duodenal I_sc response to the serosal applications of both PGE₂ and 5-HT was significantly attenuated in transient receptor potential vanilloid 4 (TRPV4) knockout mice. Finally, mucosal application of carbachol (100 μM) also induced duodenal I_sc via selective activation of muscarinic receptors, which was significantly inhibited in serosal Ca²⁺-free solution but neither in mucosal Ca²⁺-free solution nor by nifedipine. Therefore, the serosal TRPV4-constituted SOCE mechanism is likely universal for the most common and important secretagogues-induced and Ca²⁺-dependent intestinal anion secretion. These findings will enhance our knowledge about gastrointestinal (G.I.) epithelial physiology and the associated G.I. diseases, such as diarrhea and constipation.

The Role of pHi in Intestinal Epithelial Proliferation-Transport Mechanisms, Regulatory Pathways, and Consequences

During the maturation of intestinal epithelial cells along the crypt/surface axis, a multitude of acid/base transporters are differentially expressed in their apical and basolateral membranes, enabling processes of electrolyte, macromolecule, nutrient, acid/base and fluid secretion, and absorption. An intracellular pH (pHi)-gradient is generated along the epithelial crypt/surface axis, either as a consequence of the sum of the ion transport activities or as a distinctly regulated entity. While the role of pHi on proliferation, migration, and tumorigenesis has been explored in cancer cells for some time, emerging evidence suggests an important role of the pHi in the intestinal stem cells (ISCs) proliferative rate under physiological conditions. The present review highlights the current state of knowledge about the potential regulatory role of pHi on intestinal proliferation and differentiation.

Advances in Ca2+ modulation of gastrointestinal anion secretion and its dysregulation in digestive disorders (Review)

Intracellular calcium (Ca²⁺) is a critical cell signaling component in gastrointestinal (GI) physiology. Cytosolic calcium ([Ca²⁺]_cyt), as a secondary messenger, controls GI epithelial fluid and ion transport, mucus and neuropeptide secretion, as well as synaptic transmission and motility. The key roles of Ca²⁺ signaling in other types of secretory cell (including those in the airways and salivary glands) are well known. However, its action in GI epithelial secretion and the underlying molecular mechanisms have remained to be fully elucidated. The present review focused on the role of [Ca²⁺]_cyt in GI epithelial anion secretion. Ca²⁺ signaling regulates the activities of ion channels and transporters involved in GI epithelial ion and fluid transport, including Cl^- channels, Ca²⁺-activated K⁺ channels, cystic fibrosis (CF) transmembrane conductance regulator and anion/HCO₃^- exchangers. Previous studies by the current researchers have focused on this field over several years, providing solid evidence that Ca²⁺ signaling has an important role in the regulation of GI epithelial anion secretion and uncovering underlying molecular mechanisms. The present review is largely based on previous studies by the current researchers and provides an overview of the currently known molecular mechanisms of GI epithelial anion secretion with an emphasis on Ca²⁺-mediated ion secretion and its dysregulation in GI disorders. In addition, previous studies by the current researchers demonstrated that different regulatory mechanisms are in place for GI epithelial HCO₃^- and Cl^- secretion. An increased understanding of the roles of Ca²⁺ signaling and its targets in GI anion secretion may lead to the development of novel strategies to inhibit GI diseases, including the enhancement of fluid secretion in CF and protection of the GI mucosa in ulcer diseases.

Source of dopamine in gastric juice and luminal dopamine-induced duodenal bicarbonate secretion via apical dopamine D2 receptors

BACKGROUND AND PURPOSE

Dopamine protects the duodenal mucosa. Here we have investigated the source of dopamine in gastric juice and the mechanism underlying the effects of luminal dopamine on duodenal bicarbonate secretion (DBS) in rodents.

EXPERIMENTAL APPROACH

Immunofluorescence, UPLC-MS/MS, gastric incubation and perfusion were used to detect gastric-derived dopamine. Immunofluorescence and RT-PCR were used to examine the expression of dopamine receptors in the duodenal mucosa. Real-time pH titration and pHi measurement were performed to investigate DBS.

KEY RESULTS

H+ -K+ -ATPase was co-localized with tyrosine hydroxylase and dopamine transporters in gastric parietal cells. Dopamine was increased in in vivo gastric perfusate after intravenous infusion of histamine and in gastric mucosa incubated, in vitro, with bethanechol chloride or tyrosine. D2 receptors were the most abundant dopamine receptors in rat duodenum, mainly distributed on the apical membrane of epithelial cells. Luminal dopamine increased DBS in a concentration-dependent manner, an effect mimicked by a D2 receptor agonist quinpirole and inhibited by the D2 receptor antagonist L741,626, in vivo D2 receptor siRNA and in D2 receptor -/- mice. Dopamine and quinpirole raised the duodenal enterocyte pHi . Quinpirole-evoked DBS and PI3K/Akt activity were inhibited by calcium chelator BAPTA-AM or in D2 receptor-/- mice.

CONCLUSION AND IMPLICATIONS

Dopamine in the gastric juice is derived from parietal cells and is secreted along with gastric acid. On arrival in the duodenal lumen, dopamine increased DBS via an apical D2 receptor- and calcium-dependent pathway. Our data provide novel insights into the protective effects of dopamine on the duodenal mucosa.

Different functional roles for K+ channel subtypes in regulating small intestinal glucose and ion transport

Although K⁺ channels are important in mediating the driving force for colonic ion transport, their role in small intestinal transport is poorly understood. To investigate this, small intestinal short circuit currents (I_sc ) and HCO₃ ^- secretion were measured in mice, and intracellular pH (pH_i) was measured in small intestinal epithelial SCBN cells. The expression and location of Kv subtypes were verified by RT-PCR, western blotting and immunohistochemistry. Diabetic mice were also used to investigate the role of Kv subtypes in regulating intestinal glucose absorption. We found that K_V7.1 is not involved in duodenal ion transport, while K_Ca3.1 selectively regulates duodenal I_sc and HCO₃ ^- secretion in a Ca²⁺-mediated but not cAMP-mediated manner. Blockade of K_Ca3.1 increased the rate of HCO₃ ^- fluxes via cystic fibrosis transmembrane conductance regulator (CFTR) channels in SCBN cells. Jejunal I_sc was significantly stimulated by glucose, but markedly inhibited by 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Moreover, both Kv1.1 and Kv1.3 were expressed in jejunal mucosae. Finally, 4-AP significantly attenuated weight gain of normal and diabetic mice, and both 4-AP and TEA significantly lowered blood glucose of diabetic mice. This study not only examines the contribution of various K⁺ channel subtypes to small intestinal epithelial ion transport and glucose absorption, but also proposes a novel concept for developing specific K⁺ channel blockers to reduce weight gain and lower blood glucose in diabetes mellitus.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Background and Purpose:

Although Ca²⁺ signaling may stimulate small intestinal ion secretion, little is known about its critical role and the molecular mechanisms of Ca²⁺-mediated biological action.

Key Results

Activation of muscarinic receptors by carbachol(CCh) stimulated mouse duodenal I_sc, which was significantly inhibited in Ca²⁺-free serosal solution and by several selective store-operated Ca²⁺ channels(SOC) blockers added to the serosal side of duodenal tissues. Furthermore, we found that CRAC/Orai channels may represent the molecular candidate of SOC in intestinal epithelium. CCh increased intracellular Ca²⁺ but not cAMP, and Ca²⁺ signaling mediated duodenal Cl^- and HCO₃^- secretion in wild type mice but not in CFTR knockout mice. CCh induced duodenal ion secretion and stimulated PI3K/Akt activity in duodenal epithelium, all of which were inhibited by selective PI3K inhibitors with different structures. CCh-induced Ca²⁺ signaling also stimulated the phosphorylation of CFTR proteins and their trafficking to the plasma membrane of duodenal epithelial cells, which were inhibited again by selective PI3K inhibitors.

Materials and Methods

Functional, biochemical and morphological experiments were performed to examine ion secretion, PI3K/Akt and CFTR activity of mouse duodenal epithelium. Ca²⁺ imaging was performed on HT-29 cells.

Conclusions and Implications

Ca²⁺ signaling plays a critical role in intestinal ion secretion via CRAC/Orai-mediated SOCE mechanism on the serosal side of epithelium. We also demonstrated the molecular mechanisms of Ca²⁺ signaling in CFTR-mediated secretion via novel PI3K/Akt pathway. Our findings suggest new perspectives for drug targets to protect the upper GI tract and control liquid homeostasis in the small intestine.

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.

Gastroprotective Effects of PMK-S005 against Ethanol-Induced Acute Gastric Damage in Rats

Background/Aims

This study aimed to examine the gastroprotective effects of PMK-S005, which is a synthetic S-allyl-l-cysteine (SAC; a sulfur-containing amino acid), against acute ethanol-induced gastric damage in rats.

Methods

Sprague-Dawley rats were divided into six groups, including a nonethanol group, groups treated with absolute ethanol 1 hour after pretreatment with various doses of PMK-S005 (1, 5, and 10 mg/kg) or rebamipide (50 mg/kg), and an absolute ethanol-only group. Ethanol-induced gross ulcer and mucus levels were measured. Myeloperoxidase, tumor necrosis factor α, interleukin 1β, PGE₂, LTB₄, cPLA₂, COX-1, and COX-2 levels were estimated by enzyme-linked immunosorbent assay or Western blot analysis. Furthermore, the protein expression levels of antioxidant enzymes, including heme oxygenase-1 (HO-1), NAD(P)H:quinine oxidoreductase 1 (NQO-1), GCLC, and GCLM, were assessed.

Results

PMK-S005 significantly attenuated the ethanol-induced gastric damage; it reduced mucosal inflammatory cytokine production and increased mucus levels. The expression levels of cPLA₂, COX-1, and COX-2 were decreased by PMK-S005. PMK-S005 did not affect PGE₂ synthesis, but LTB₄ production was significantly suppressed. In addition, long-term administration of PMK-S005 significantly increased the expression of HO-1, NQO-1, GCLC, and GCLM.

Conclusions

These results strongly suggest that PMK-S005 prevents gastric mucosal damage and that these gastroprotective activities are due to anti-inflammatory effects and enhancement of the gastric defense system, including antioxidant enzymes.

Estrogen and estrogen receptors in the modulation of gastrointestinal epithelial secretion

Gastrointestinal (GI) epithelial ion transport is physiologically important in many aspects of humans, such as in maintaining fluid balance of whole body, and also plays a role in the development and progression of common GI disease. Estrogen and estrogen receptors have been shown to modulate the activity of epithelial ion secretion in GI tract. This review aims to address the current state of knowledge about the role of estrogen and estrogen receptors in modulation of GI epithelial secretion and to elucidate the underlying mechanisms. We highlight the recent findings regarding the importance of estrogen and estrogen receptors in GI epithelia protection and body fluid balance by modulation of gastrointestinal epithelial HCO₃^- and Cl^- secretion, especially current information about the regulatory mechanisms of duodenal HCO₃^- secretion based on our study in this field. Since there are no reviews on this topic but only few papers to address the main issues, we hope to timely provide new perspectives for the association between estrogen and GI disease.

Advances with Neonatal Aerodigestive Science in the Pursuit of Safe Swallowing in Infants: Invited Review

Feeding, swallowing, and airway protection are three distinct entities. Feeding involves a process of sequential, neurosensory, and neuromotor interactions of reflexes and behaviors facilitating ingestion. Swallowing involves anterograde bolus movement during oral-, pharyngeal-, and esophageal phases of peristalsis into stomach. During these events, coordination with airway protection is vital for homeostasis in clearing any material away from airway vicinity. Neurological-airway-digestive inter-relationships are critical to the continuum of successful feeding patterns during infancy, either in health or disease. Neonatal feeding difficulties encompass a heterogeneous group of neurological, pulmonary, and aerodigestive disorders that present with multiple signs posing as clinical conundrums. Significant research breakthroughs permitted understanding of vagal neural pathways and functional aerodigestive connectivity involved in regulating swallowing and aerodigestive functions either directly or indirectly by influencing the supra-nuclear regulatory centers and peripheral effector organs. These neurosensory and neuromotor pathways are influenced by pathologies during perinatal events, prematurity, inflammatory states, and coexisting medical and surgical conditions. Approaches to clarify pathophysiologic mapping of aerodigestive interactions, as well as translating these discoveries into the development of personalized and simplified feeding strategies to advance child health are discussed in this review article.

The Extracellular Calcium-Sensing Receptor in the Intestine: Evidence for Regulation of Colonic Absorption, Secretion, Motility, and Immunity

Different from other epithelia, the intestinal epithelium has the complex task of providing a barrier impeding the entry of toxins, food antigens, and microbes, while at the same time allowing for the transfer of nutrients, electrolytes, water, and microbial metabolites. These molecules/organisms are transported either transcellularly, crossing the apical and basolateral membranes of enterocytes, or paracellularly, passing through the space between enterocytes. Accordingly, the intestinal epithelium can affect energy metabolism, fluid balance, as well as immune response and tolerance. To help accomplish these complex tasks, the intestinal epithelium has evolved many sensing receptor mechanisms. Yet, their roles and functions are only now beginning to be elucidated. This article explores one such sensing receptor mechanism, carried out by the extracellular calcium-sensing receptor (CaSR). In addition to its established function as a nutrient sensor, coordinating food digestion, nutrient absorption, and regulating energy metabolism, we present evidence for the emerging role of CaSR in the control of intestinal fluid homeostasis and immune balance. An additional role in the modulation of the enteric nerve activity and motility is also discussed. Clearly, CaSR has profound effects on many aspects of intestinal function. Nevertheless, more work is needed to fully understand all functions of CaSR in the intestine, including detailed mechanisms of action and specific pathways involved. Considering the essential roles CaSR plays in gastrointestinal physiology and immunology, research may lead to a translational opportunity for the development of novel therapies that are based on CaSR's unique property of using simple nutrients such as calcium, polyamines, and certain amino acids/oligopeptides as activators. It is possible that, through targeting of intestinal CaSR with a combination of specific nutrients, oral solutions that are both inexpensive and practical may be developed to help in conditioning the gut microenvironment and in maintaining digestive health.

Dysphagia in the high-risk infant: potential factors and mechanisms

Neonatal dysphagia, or abnormalities of swallowing, represent a major global problem, and consequences of dysfunctional feeding patterns carry over into infancy and toddler age groups. Growth, development, and independent feeding skills are all delayed among high-risk infants. Such a group comprises premature birth, low-birth-weight, congenital anomalies, perinatal asphyxia, postsurgical, and sepsis categories. The conflict between pathophysiologic and pragmatic feeding strategies remains a major conundrum and is largely due to a lack of validated diagnostic approaches amid heterogeneity of the patient phenotype. Thus, well-tested feeding management strategies that can be generalizable are lacking. Furthermore, the aerodigestive symptoms and signs, potential risk factors, and contributory etiologies remain nonspecific. This article presents mechanistic evidence related to the pathophysiologic basis of neonatal dysphagia as well as potential opportunities to improve feeding abilities and long-term development.

Intestinal Colonization Dynamics of Vibrio cholerae

To cause the diarrheal disease cholera, Vibrio cholerae must effectively colonize the small intestine. In order to do so, the bacterium needs to successfully travel through the stomach and withstand the presence of agents such as bile and antimicrobial peptides in the intestinal lumen and mucus. The bacterial cells penetrate the viscous mucus layer covering the epithelium and attach and proliferate on its surface. In this review, we discuss recent developments and known aspects of the early stages of V. cholerae intestinal colonization and highlight areas that remain to be fully understood. We propose mechanisms and postulate a model that covers some of the steps that are required in order for the bacterium to efficiently colonize the human host. A deeper understanding of the colonization dynamics of V. cholerae and other intestinal pathogens will provide us with a variety of novel targets and strategies to avoid the diseases caused by these organisms.

Loss of NHE8 expression impairs ocular surface function in mice

Sodium/hydrogen exchanger (NHE) 8 is expressed at the apical membrane of the epithelial cells and plays important roles in neutral sodium absorption in the gastrointestinal tract and the kidney. It also has an important role in epithelial mucosal protection in the gastric gland and the intestine. Although NHE8 has broad tissue distribution, the precise location and the physiological role of NHE8 in the eye remain unknown. In the present study, we successfully detected the expression of NHE8 in the ocular surface by PCR and Western blot in human and mouse eyes. Immunohistochemistry staining located NHE8 protein at the plasma membrane of the epithelial cells in the conjunctiva, the cornea, and the lacrimal gland both in human and mouse. We also detected the expression of downregulated-in-adenoma (DRA, a Cl(-)/HCO3 (-) transporter) in the ocular surface epithelial cells. Using NHE8-/- mouse model, we found that loss of NHE8 function resulted in reduced tear production and increased corneal staining. These NHE8-/- mice also showed increased expression of TNF-α and matrix metalloproteinase 9 (MMP9) genes. The expression of epithelial keratinization marker genes, small proline-rich protein 2h (Sprr2h) and transglutaminase 1 (Tgm1), were also increased in NHE8-/- eyes. Furthermore, DRA expression in NHE8-/- mice was reduced in the conjunctiva, the cornea, and the lacrimal glands in association with a reduction in conjunctival mucosal pH. Altered ocular surface function and reduced epithelial DRA expression in NHE8-/- mice suggest that the role of NHE8 in ocular surface tissue involve in tear production and ocular epithelial protection. This study reveals a potential novel mechanism of dry eye condition involving abnormal NHE8 function.

Molecular mechanisms of calcium-sensing receptor-mediated calcium signaling in the modulation of epithelial ion transport and bicarbonate secretion

Epithelial ion transport is mainly under the control of intracellular cAMP and Ca(2+) signaling. Although the molecular mechanisms of cAMP-induced epithelial ion secretion are well defined, those induced by Ca(2+) signaling remain poorly understood. Because calcium-sensing receptor (CaSR) activation results in an increase in cytosolic Ca(2+) ([Ca(2+)]cyt) but a decrease in cAMP levels, it is a suitable receptor for elucidating the mechanisms of [Ca(2+)]cyt-mediated epithelial ion transport and duodenal bicarbonate secretion (DBS). CaSR proteins have been detected in mouse duodenal mucosae and human intestinal epithelial cells. Spermine and Gd(3+), two CaSR activators, markedly stimulated DBS without altering duodenal short circuit currents in wild-type mice but did not affect DBS and duodenal short circuit currents in cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice. Clotrimazole, a selective blocker of intermediate conductance Ca(2+)-activated K(+) channels but not chromanol 293B, a selective blocker of cAMP-activated K(+) channels (KCNQ1), significantly inhibited CaSR activator-induced DBS, which was similar in wild-type and KCNQ1 knockout mice. HCO3 (-) fluxes across epithelial cells were activated by a CFTR activator, but blocked by a CFTR inhibitor. CaSR activators induced HCO3 (-) fluxes, which were inhibited by a receptor-operated channel (ROC) blocker. Moreover, CaSR activators dose-dependently raised cellular [Ca(2+)]cyt, which was abolished in Ca(2+)-free solutions and inhibited markedly by selective CaSR antagonist calhex 231, and ROC blocker in both animal and human intestinal epithelial cells. Taken together, CaSR activation triggers Ca(2+)-dependent DBS, likely through the ROC, intermediate conductance Ca(2+)-activated K(+) channels, and CFTR channels. This study not only reveals that [Ca(2+)]cyt signaling is critical to modulate DBS but also provides novel insights into the molecular mechanisms of CaSR-mediated Ca(2+)-induced DBS.

The ethanol-induced stimulation of rat duodenal mucosal bicarbonate secretion in vivo is critically dependent on luminal Cl-

Alcohol may induce metabolic and functional changes in gastrointestinal epithelial cells, contributing to impaired mucosal barrier function. Duodenal mucosal bicarbonate secretion (DBS) is a primary epithelial defense against gastric acid and also has an important function in maintaining the homeostasis of the juxtamucosal microenvironment. The aim in this study was to investigate the effects of the luminal perfusion of moderate concentrations of ethanol in vivo on epithelial DBS, fluid secretion and paracellular permeability. Under thiobarbiturate anesthesia, a ∼30-mm segment of the proximal duodenum with an intact blood supply was perfused in situ in rats. The effects on DBS, duodenal transepithelial net fluid flux and the blood-to-lumen clearance of ⁵¹Cr-EDTA were investigated. Perfusing the duodenum with isotonic solutions of 10% or 15% ethanol-by-volume for 30 min increased DBS in a concentration-dependent manner, while the net fluid flux did not change. Pre-treatment with the CFTR inhibitor CFTR_inh172 (i.p. or i.v.) did not change the secretory response to ethanol, while removing Cl⁻ from the luminal perfusate abolished the ethanol-induced increase in DBS. The administration of hexamethonium (i.v.) but not capsazepine significantly reduced the basal net fluid flux and the ethanol-induced increase in DBS. Perfusing the duodenum with a combination of 1.0 mM HCl and 15% ethanol induced significantly greater increases in DBS than 15% ethanol or 1.0 mM HCl alone but did not influence fluid flux. Our data demonstrate that ethanol induces increases in DBS through a mechanism that is critically dependent on luminal Cl⁻ and partly dependent on enteric neural pathways involving nicotinic receptors. Ethanol and HCl appears to stimulate DBS via the activation of different bicarbonate transporting mechanisms.

Anti-ulcer activity of essential oil constituents

Essential oils have attracted considerable worldwide attention over the last few decades. These natural products have wide-ranging pharmacological activities and biotechnological applications. Faced with the need to find new anti-ulcer agents and the great effort on the development of drugs for the treatment of ulcers, in this review, the anti-ulcer activities of 21 bioactive compounds found in essential oils are discussed.

Regulated traffic of anion transporters in mammalian Brunner's glands: a role for water and fluid transport

The Brunner's glands of the proximal duodenum exert barrier functions through secretion of glycoproteins and antimicrobial peptides. However, ion transporter localization, function, and regulation in the glands are less clear. Mapping the subcellular distribution of transporters is an important step toward elucidating trafficking mechanisms of fluid transport in the gland. The present study examined 1) changes in the distribution of intestinal anion transporters and the aquaporin 5 (AQP5) water channel in rat Brunner's glands following second messenger activation and 2) anion transporter distribution in Brunner's glands from healthy and disease-affected human tissues. Cystic fibrosis transmembrane conductance regulator (CFTR), AQP5, sodium-potassium-coupled chloride cotransporter 1 (NKCC1), sodium-bicarbonate cotransporter (NBCe1), and the proton pump vacuolar ATPase (V-ATPase) were localized to distinct membrane domains and in endosomes at steady state. Carbachol and cAMP redistributed CFTR to the apical membrane. cAMP-dependent recruitment of CFTR to the apical membrane was accompanied by recruitment of AQP5 that was reversed by a PKA inhibitor. cAMP also induced apical trafficking of V-ATPase and redistribution of NKCC1 and NBCe1 to the basolateral membranes. The steady-state distribution of AQP5, CFTR, NBCe1, NKCC1, and V-ATPase in human Brunner's glands from healthy controls, cystic fibrosis, and celiac disease resembled that of rat; however, the distribution profiles were markedly attenuated in the disease-affected duodenum. These data support functional transport of chloride, bicarbonate, water, and protons by second messenger-regulated traffic in mammalian Brunner's glands under physiological and pathophysiological conditions.

Putative interaction of brush cells with bicarbonate secreting cells in the proximal corpus mucosa

The gastric epithelium is protected from the highly acidic luminal content by alkaline mucus which is secreted from specialized epithelial cells. In the stomach of mice strong secretion of alkaline fluid was observed at the “gastric groove,” the border between corpus and fundus mucosa. Since this region is characterized by numerous brush cells it was proposed that these cells might secrete alkaline solution as suggested for brush cells in the bile duct. In fact, it was found that in this region multiple cells express elements which are relevant for the secretion of bicarbonate, including carbonic anhydrase (CAII), the cystic fibrosis transmembrane conductance regulator (CFTR) and the Na⁺/H⁺ exchanger (NHE1). However, this cell population was distinct from brush cells which express the TRP-channel TRPM5 and are considered as putative sensory cells. The location of both cell populations in close proximity implies the possibility for a paracrine interaction. This view was substantiated by the finding that brush cells express prostaglandin synthase-1 (COX-1) and the neighboring cells a specific receptor type for prostaglandins. The notion that brush cells may be able to sense a local acidification was supported by the observation that they express the channel PKD1L3 which contributes to the acid responsiveness of gustatory sensory cells. The results support the concept that brush cells may sense the luminal content and influence via prostaglandins the secretion of alkaline solution.

Pathophysiology of aerodigestive pulmonary disorders in the neonate

No test can provide a definitive diagnosis of aerodigestive disease. When interpreting tests, one should weigh the benefits and weaknesses of different technologies and methods, scientific appropriateness of the testing conditions, clinicopathologic correlation, and pharmacologic approaches. Gastroesophageal reflux disease (GERD) symptoms and airway symptoms can coexist, and they cannot be distinguished without specific testing and direct observations. Important aerodigestive disorders include dysphagia, GERD, and aggravation of airway injury due to malfunctions of swallowing or airway protection mechanisms. Objective evaluation of aerodigestive reflexes and symptom correlation may provide support for evidence-based personalized management of feeding and airway protection strategies.

Impaired mucin synthesis and bicarbonate secretion in the colon of NHE8 knockout mice

Sodium/hydrogen exchanger 8 (NHE8), the newest member of the SLC9 family, is expressed at the apical membrane of the epithelial cells in the intestine and the kidney. Although NHE8 has been shown to be an important player for intestinal sodium absorption early in development, its physiological role in the intestine remains unclear. Here, we successfully created a NHE8 knockout (NHE8(-/-)) mouse model to study the function of this transporter in the intestinal tract. Embryonic stem cells containing interrupted NHE8 gene were injected into mouse blastocyst to produce NHE8(+/-) chimeras. NHE8(-/-) mice showed no lethality during embryonic and fetal development. These mice had normal serum sodium levels and no signs of diarrhea. Apically expressed NHE2 and NHE3 were increased in the small intestine of the NHE8(-/-) mice in compensation. The number of goblet cells and mucin (MUC)-positive cells in the colon was reduced in NHE8(-/-) mice along with mucosal pH, MUC2 expression as well as downregulated in adenoma (DRA) expression. Therefore, the role of NHE8 in the intestine involves both sodium absorption and bicarbonate secretion.

Expression of acid-sensing ion channels in intestinal epithelial cells and their role in the regulation of duodenal mucosal bicarbonate secretion

AIMS

As little is currently known about acid-sensing ion channels (ASICs) in intestinal epithelial cells, the aims of the present study were to investigate the expression and function of ASICs in intestinal epithelial cells, particularly their physiological role in the acid-stimulated duodenal mucosal bicarbonate secretion (DMBS).

METHODS

RT-PCR and digital Ca²(+) imaging were used to determine the expression and function of ASICs in HT29 cells and SCBN cells, intestinal epithelial crypt cell lines. The acid-stimulated DMBS was measured in C57 black mice in vivo to study the role of ASICs in this physiological process.

RESULTS

ASIC1a mRNA expression was detected in the duodenal mucosa stripped from mice and epithelial cell lines, in which cytoplasmic free Ca²(+) ([Ca²(+) ](cyt)) in response to extracellular acidosis was also increased. In Ca²(+) -containing solutions, acidosis (pH 6.0-5.0) raised [Ca²(+) ](cyt) in both HT29 cells and SCBN cells in a similar pH-dependent manner. Acidosis-induced increase in [Ca²(+) ](cyt) was markedly inhibited by amiloride (an ASICs blocker), SK&F96365 (a blocker for non-selective cation channels), or in Ca²(+) -free solutions; but was abolished by amiloride in Ca²(+) -free solutions. However, acidosis-induced increase in [Ca²(+) ](cyt) was slightly affected by U73122 (a PLC inhibitor), or nifedipine (a voltage-gated Ca²(+) channel blocker). After acidosis raised [Ca²(+) ](cyt) , stimulation of purinergic receptors with ATP further increased [Ca²(+) ](cyt) , but acidosis-induced increase in [Ca²(+) ](cyt) was not altered by suramin. Moreover, acid-stimulated murine DMBS was significantly attenuated by amiloride.

CONCLUSION

Therefore, ASICs are functionally expressed in intestinal epithelial cells, and may play a role in acid-stimulated DMBS through a Ca²(+) signalling pathway.

Differential activation of the HCO(3)(-) conductance through the cystic fibrosis transmembrane conductance regulator anion channel by genistein and forskolin in murine duodenum

BACKGROUND AND PURPOSE

Many cystic fibrosis (CF)-associated mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels affect CFTR-activated HCO(3)(-) transport more than Cl(-) transport. Targeting the CFTR HCO(3)(-) conductance, if possible, may therefore be of major therapeutic benefit. In the present study, we examined the effects of genistein and forskolin on duodenal mucosal HCO(3)(-) and Cl(-) secretion.

EXPERIMENTAL APPROACH

Murine duodenal mucosal HCO(3)(-) and Cl(-) secretions were examined in vitro in Ussing chambers by the pH stat and short circuit current (I(sc)) techniques.

KEY RESULTS

Genistein markedly stimulated duodenal HCO(3)(-) secretion and I(sc) in a dose-dependent manner in CFTR wild-type mice, but not in CFTR null mice. CFTR(inh)-172, a highly specific CFTR inhibitor, inhibited genistein-stimulated duodenal HCO(3)(-) secretion and I(sc) in wild-type mice. Genistein induced 59% net HCO(3)(-) increase and 123% net I(sc) increase over basal value, whereas forskolin, an activator of adenylate cyclase, induced 94% net HCO(3)(-) increase and 507% net I(sc) increase, indicating that, compared with forskolin, genistein induced a relatively high HCO(3)(-)/I(sc) ratio. Further data showed that CFTR HCO(3)(-)/Cl(-) conductance ratio was 1.05 after genistein stimulation, whereas after forskolin stimulation, the CFTR HCO(3)(-)/Cl(-) conductance ratio was 0.27.

CONCLUSIONS AND IMPLICATIONS

Genistein stimulates duodenal HCO(3)(-) and Cl(-) secretion through CFTR, and has a relatively high selectivity for the CFTR HCO(3)(-) conductance, compared with forskolin. This may indicate the feasibility of selective targeting of the HCO(3)(-) conductance of the CFTR channels.

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.

Protective effects of DIDS against ethanol-induced gastric mucosal injury in rats

The compound 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) is an efficient anion exchanger inhibitor that can block the activities of anion exchanger 2 (AE2), which plays an indispensable role in gastric acid secretion. DIDS also has potent anti-oxidative and antiapoptosis activities. This study aimed to investigate the effect of DIDS on ethanol-induced mucosal damage in rats and to evaluate the underlying mechanisms that mediate the action of the compound. The rats received 1 ml of absolute ethanol or saline orally. DIDS [50 mg/kg intravenous (i.v.)] was given 5 min before ethanol administration. Gastric lesions were evaluated macroscopically, microscopically, and electron microscopically at 60 min after ethanol challenge. Gastric myeloperoxidase (MPO) activity, malonyldialdehyde (MDA) level, prostaglandin E2 (PGE2) synthesis, and cyclooxygenase-2 (COX-2) expression were assessed. For the evaluation of the effect of DIDS on gastric acid secretion, histamine-stimulatory gastric acid secretion was examined with or without pretreatment of DIDS (50 mg/kg; i.v.). Ethanol-induced gastric lesions were characterized by increasing gastric MDA level, MPO activity, and COX-2 expression, and decreasing PGE2 synthesis. It was found that DIDS significantly reduced the extent of gastric mucosal damage and reversed tissue MDA level and MPO activity. DIDS further enhanced the expression of COX-2 and reversed the decrease of PGE2. Our results suggested that DIDS is beneficial in rat model of gastric injury through mechanisms that involve inhibiting inflammatory cell infiltration and lipid peroxidation and up-regulating the COX-2/PGE2 pathway.

P2Y receptors mediate Ca2+ signaling in duodenocytes and contribute to duodenal mucosal bicarbonate secretion

Since little is known about the role of P2Y receptors (purinoceptors) in duodenal mucosal bicarbonate secretion (DMBS), we sought to investigate the expression and function of these receptors in duodenal epithelium. Expression of P2Y(2) receptors was detected by RT-PCR in mouse duodenal epithelium and SCBN cells, a duodenal epithelial cell line. UTP, a P2Y(2)-receptor agonist, but not ADP (10 microM), significantly induced murine duodenal short-circuit current and DMBS in vitro; these responses were abolished by suramin (300 microM), a P2Y-receptor antagonist, or 2-aminoethoxydiphenyl borate (2-APB; 100 microM), a store-operated channel blocker. Mucosal or serosal addition of UTP induced a comparable DMBS in wild-type mice, but markedly impaired response occurred in P2Y(2) knockout mice. Acid-stimulated DMBS in vivo was significantly inhibited by suramin (1 mM) or PPADS (30 microM). Both ATP and UTP, but not ADP (1 microM), raised cytoplasmic-free Ca(2+) concentrations ([Ca(2+)](cyt)) with similar potencies in SCBN cells. ATP-induced [Ca(2+)](cyt) was attenuated by U-73122 (10 microM), La(3+) (30 microM), or 2-APB (10 microM), but was not significantly affected by nifedipine (10 microM). UTP (1 microM) induced a [Ca(2+)](cyt) transient in Ca(2+)-free solutions, and restoration of external Ca(2+) (2 mM) raised [Ca(2+)](cyt) due to capacitative Ca(2+) entry. La(3+) (30 microM), SK&F96365 (30 microM), and 2-APB (10 microM) inhibited UTP-induced Ca(2+) entry by 92, 87, and 94%, respectively. Taken together, our results imply that activation of P2Y(2) receptors enhances DMBS via elevation of [Ca(2+)](cyt) that likely results from an initial increase in intracellular Ca(2+) release followed by extracellular Ca(2+) entry via store-operated channel.

Gender-specific protection of estrogen against gastric acid-induced duodenal injury: stimulation of duodenal mucosal bicarbonate secretion

Because human duodenal mucosal bicarbonate secretion (DMBS) protects duodenum against acid-peptic injury, we hypothesize that estrogen stimulates DMBS, thereby attributing to the clinically observed lower incidence of duodenal ulcer in premenopausal women than the age-matched men. We found that basal and acid-stimulated DMBS responses were 1.5 and 2.4-fold higher in female than male mice in vivo, respectively. Acid-stimulated DMBS in both genders was abolished by ICI 182,780 and tamoxifen. Estradiol-17beta (E2) and the selective estrogen receptor (ER) agonists of ERalpha [1,3,5-Tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole] and ERbeta [2,3-bis(4-hydroxyphenyl) propionitrile], but not progesterone, rapidly stimulated ER-dependent murine DMBS in vivo. E2 dose dependently stimulated murine DMBS, which was attenuated by a Cl(-)/HCO3(-) anion exchanger inhibitor 4,4'-didsothio- cyanostilbene-2, 2'-disulfonic acid, removal of extracellular Cl(-), and in cystic fibrosis transmembrane conductance regulator knockout female mice. E2 stimulated murine DMBS in vitro in both genders with significantly greater response in female than male mice (female to male ratio = 4.3). ERalpha and ERbeta mRNAs and proteins were detected in murine duodenal epithelium of both genders; however, neither ERalpha nor ERbeta mRNA and protein expression levels differed according to gender. E2 rapidly mobilized intracellular calcium in a duodenal epithelial SCBN cell line that expresses ERalpha and ERbeta, whereas BAPTA-AM abolished E2-stimulated murine DMBS. Thus, our data show that E2 stimulates DMBS via ER dependent mechanisms linked to intracellular calcium, cystic fibrosis transmembrane conductance regulator, and Cl(-)/HCO3(-) anion exchanger. Gender-associated differences in basal, acid- and E2-stimulated DMBS may have offered a reasonable explanation for the clinically observed lower incidence of duodenal ulcer in premenopausal women than age-matched men.

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.

Food-induced expression of orexin receptors in rat duodenal mucosa regulates the bicarbonate secretory response to orexin-A

Presence of appetite-regulating peptides orexin-A and orexin-B in mucosal endocrine cells suggests a role in physiological control of the intestine. Our aim was to characterize orexin-induced stimulation of duodenal bicarbonate secretion and modulation of secretory responses and mucosal orexin receptors by overnight food deprivation. Lewis x Dark Agouti rats were anesthetized and proximal duodenum cannulated in situ. Mucosal bicarbonate secretion (pH stat) and mean arterial blood pressure were continuously recorded. Orexin-A was administered intra-arterially close to the duodenum, intraluminally, or into the brain ventricles. Total RNA was extracted from mucosal specimens, reverse transcribed to cDNA and expression of orexin receptors 1 and 2 (OX1 and OX2) measured by quantitative real-time PCR. OX1 protein was measured by Western blot. Intra-arterial orexin-A (60-600 nmol.h(-1).kg(-1)) increased (P < 0.01) the duodenal secretion in fed but not in fasted animals. The OX1 receptor antagonist SB-334867, which was also found to have a partial agonist action, abolished the orexin-induced secretory response but did not affect secretion induced by the muscarinic agonist bethanechol. Atropine, in contrast, inhibited bethanechol but not orexin-induced secretion. Orexin-A infused into the brain ventricles (2-20 nmol.kg(-1).h(-1)) or added to luminal perfusate (1.0-100 nM) did not affect secretion, indicating that orexin-A acts peripherally and at basolateral receptors. Overnight fasting decreased mucosal OX1 and OX2 mRNA expression (P < 0.01) as well as OX1 protein expression (P < 0.05). We conclude that stimulation of secretion by orexin-A may involve both receptor types and is independent of cholinergic pathways. Intestinal OX receptors and secretory responses are markedly related to food intake.

Phosphatidylinositol 3-kinase is involved in prostaglandin E2-mediated murine duodenal bicarbonate secretion

Prostaglandin E(2) (PGE(2)) plays an important role in the regulation of duodenal bicarbonate (HCO(3)(-)) secretion, but its signaling pathway(s) are not fully understood. In the present study, we investigated the signaling pathways involved in PGE(2)-mediated duodenal HCO(3)(-) secretion. Murine duodenal mucosal HCO(3)(-) secretion was examined in vitro in Ussing chambers by pH-stat titration in the presence of a variety of signal transduction modulators. Phosphatidylinositol 3-kinase (PI3K) activity was measured by immunoprecipitation of PI3K and ELISA, and Akt phosphorylation was measured by Western analysis with anti-phospho-Akt and anti-Akt antibodies. PGE(2)-stimulated duodenal HCO(3)(-) secretion was reduced by the cAMP-dependent signaling pathway inhibitors MDL-12330A and KT-5720 by 23% and 20%, respectively; the Ca(2+)-influx inhibitor verapamil by 26%; and the calmodulin antagonist W-13 by 24%; whereas the PI3K inhibitors wortmannin and LY-294002 reduced PGE(2)-stimulated HCO(3)(-) secretion by 51% and 47%, respectively. Neither the MAPK inhibitor PD-98059 nor the tyrosine kinase inhibitor genistein altered PGE(2)-stimulated HCO(3)(-) secretion. PGE(2) application caused a rapid and concentration-dependent increase in duodenal mucosal PI3K activity and Akt phosphorylation. These results demonstrated that PGE(2) activates PI3K in duodenal mucosa and stimulates duodenal HCO(3)(-) secretion via cAMP-, Ca(2+)-, and PI3K-dependent signaling pathways.

Role of Ca2+-activated K+ channels in duodenal mucosal ion transport and bicarbonate secretion

Stimulation of muscarinic receptors in the duodenal mucosa raises cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)), thereby regulating duodenal epithelial ion transport. However, little is known about the downstream molecular targets that account for this Ca(2+)-mediated biological action. Ca(2+)-activated K(+) (K(Ca)) channels are candidates, but the expression and function of duodenal K(Ca) channels are poorly understood. Therefore, we determined whether K(Ca) channels are expressed in the duodenal mucosa and investigated their involvement in Ca(2+)-mediated duodenal epithelial ion transport. Two selective blockers of intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels, clotrimazole (30 muM) and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34; 10 muM), significantly inhibited carbachol (CCh)-induced duodenal short-circuit current (I(sc)) and duodenal mucosal bicarbonate secretion (DMBS) in mice but did not affect responses to forskolin and heat-stable enterotoxin of Escherichia coli. Tetraethylammonium, 4-aminopyridine, and BaCl(2) failed to inhibit CCh-induced I(sc) and DMBS. A-23187 (10 muM), a Ca(2+) ionophore, and 1-ethyl-2-benzimidazolinone (1-EBIO; 1 mM), a selective opener of K(Ca) channels, increased both I(sc) and DMBS. The effect of 1-EBIO was more pronounced with serosal than mucosal addition. Again, both clotrimazole and TRAM-34 significantly reduced A23187- or 1-EBIO-induced I(sc) and DMBS. Moreover, clotrimazole (20 mg/kg ip) significantly attenuated acid-stimulated DMBS of mice in vivo. Finally, the molecular identity of IK(Ca) channels was verified as KCNN4 (SK4) in freshly isolated murine duodenal mucosae by RT-PCR and Western blotting. Together, our results suggest that the IK(Ca) channel is one of the downstream molecular targets for [Ca(2+)](cyt) to mediate duodenal epithelial ion transport.

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.

Heat-stable enterotoxin of Escherichia coli stimulates a non-CFTR-mediated duodenal bicarbonate secretory pathway

The cystic fibrosis (CF) transmembrane conductance regulator (CFTR) is an important pathway for duodenal mucosal bicarbonate secretion. Duodenal biopsies from CF patients secrete bicarbonate in response to heat-stable enterotoxin from Escherichia coli (STa) but not cAMP. To explore the mechanism of STa-induced bicarbonate secretion in CF more fully, we examined the role of CFTR in STa-stimulated duodenal bicarbonate secretion in mice. In vivo, the duodenum of CFTR (-/-) or control mice was perfused with forskolin (10(-4) M), STa (10(-7) M), uroguanylin (10(-7) M), 8-bromoguanosine 3',5'-cGMP (8-Br-cGMP) (10(-3) M), genistein (10(-6) M) plus STa, or herbimycin A (10(-6) M) plus STa. In vitro, duodenal mucosae were voltage-clamped in Ussing chambers, and bicarbonate secretion was measured by pH-stat. The effect of genistein, DIDS (10(-4) M), and chloride removal was also studied in vitro. Control, but not CF, mice produced a significant increase in duodenal bicarbonate secretion after perfusion with forskolin, uroguanylin, or 8-Br-cGMP. However, both control and CF animals responded to STa with significant increases in bicarbonate output. Genistein and herbimycin A abolished this response in CF mice but not in controls. In vitro, STa-stimulated bicarbonate secretion in CF tissues was inhibited by genistein, DIDS, and chloride-free conditions, whereas bicarbonate secretion persisted in control mice. In the CF duodenum, STa can stimulate bicarbonate secretion via tyrosine kinase activity resulting in apical Cl(-)/HCO(3)(-) exchange. Further studies elucidating the intracellular mechanisms responsible for such non-CFTR mediated bicarbonate secretion may lead to important therapies for CF.

Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin

Secretion of bicarbonate into the adherent layer of mucus gel creates a pH gradient with a near-neutral pH at the epithelial surfaces in stomach and duodenum, providing the first line of mucosal protection against luminal acid. The continuous adherent mucus layer is also a barrier to luminal pepsin, thereby protecting the underlying mucosa from proteolytic digestion. In this article we review the present state of the gastroduodenal mucus bicarbonate barrier two decades after the first supporting experimental evidence appeared. The primary function of the adherent mucus gel layer is a structural one to create a stable, unstirred layer to support surface neutralization of acid and act as a protective physical barrier against luminal pepsin. Therefore, the emphasis on mucus in this review is on the form and role of the adherent mucus gel layer. The primary function of the mucosal bicarbonate secretion is to neutralize acid diffusing into the mucus gel layer and to be quantitatively sufficient to maintain a near-neutral pH at the mucus-mucosal surface interface. The emphasis on mucosal bicarbonate in this review is on the mechanisms and control of its secretion and the establishment of a surface pH gradient. Evidence suggests that under normal physiological conditions, the mucus bicarbonate barrier is sufficient for protection of the gastric mucosa against acid and pepsin and is even more so for the duodenum.

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(+).

Na(+)/Ca(2+) exchange regulates Ca(2+)-dependent duodenal mucosal ion transport and HCO(3)(-) secretion in mice

Stimulation of muscarinic receptors in duodenal mucosa raises intracellular Ca(2+), which regulates ion transport, including HCO(3)(-) secretion. However, the underlying Ca(2+) handling mechanisms are poorly understood. The aim of the present study was to determine whether Na(+)/Ca(2+) exchanger (NCX) plays a role in the regulation of duodenal mucosal ion transport and HCO(3)(-) secretion by controlling Ca(2+) homeostasis. Mouse duodenal mucosa was mounted in Ussing chambers. Net ion transport was assessed as short-circuit current (I(sc)), and HCO(3)(-) secretion was determined by pH-stat. Expression of NCX in duodenal mucosae was analyzed by Western blot, and cytosolic Ca(2+) in duodenocytes was measured by fura 2. Carbachol (100 muM) increased I(sc) in a biphasic manner: an initial transient peak within 2 min and a later sustained plateau starting at 10 min. Carbachol-induced HCO(3)(-) secretion peaked at 10 min. 2-Aminoethoxydiphenylborate (2-APB, 100 muM) or LiCl (30 mM) significantly reduced the initial peak in I(sc) by 51 or 47%, respectively, and abolished the plateau phase of I(sc) without affecting HCO(3)(-) secretion induced by carbachol. Ryanodine (100 muM), caffeine (10 mM), and nifedipine (10 muM) had no effect on either response to carbachol. In contrast, nickel (5 mM) and KB-R7943 (10-30 muM) significantly inhibited carbachol-induced increases in duodenal mucosal I(sc) and HCO(3)(-) secretion. Western blot analysis showed expression of NCX1 proteins in duodenal mucosae, and functional NCX in duodenocytes was demonstrated in Ca(2+) imaging experiments where Na(+) depletion elicited Ca(2+) entry via the reversed mode of NCX. These results indicate that NCX contributes to the regulation of Ca(2+)-dependent duodenal mucosal ion transport and HCO(3)(-) secretion that results from stimulation of muscarinic receptors.

Carbonic anhydrases in normal gastrointestinal tract and gastrointestinal tumours

Carbonic anhydrases (CAs) catalyse the hydration of CO₂ to bicarbonate at physiological pH. This chemical interconversion is crucial since HCO₃^- is the substrate for several biosynthetic reactions. This review is focused on the distribution and role of CA isoenzymes in both normal and pathological gastrointestinal (GI) tract tissues. It has been known for many years that CAs are widely present in the GI tract and play important roles in several physiological functions such as production of saliva, gastric acid, bile, and pancreatic juice as well as in absorption of salt and water in intestine. New information suggests that these enzymes participate in several processes that were not envisioned earlier. Especially, the recent reports on plasma membrane-bound isoenzymes IX and XII have raised considerable interest since they were reported to participate in cancer invasion and spread. They are induced by tumour hypoxia and may also play a role in von Hippel-Lindau (VHL)-mediated carcinogenesis.

Extracellular calcium acts as a "third messenger" to regulate enzyme and alkaline secretion

It is generally assumed that the functional consequences of stimulation with Ca²⁺-mobilizing agonists are derived exclusively from the second messenger action of intracellular Ca²⁺, acting on targets inside the cells. However, during Ca²⁺ signaling events, Ca²⁺ moves in and out of the cell, causing changes not only in intracellular Ca²⁺, but also in local extracellular Ca²⁺. The fact that numerous cell types possess an extracellular Ca²⁺ “sensor” raises the question of whether these dynamic changes in external [Ca²⁺] may serve some sort of messenger function. We found that in intact gastric mucosa, the changes in extracellular [Ca²⁺] secondary to carbachol-induced increases in intracellular [Ca²⁺] were sufficient and necessary to elicit alkaline secretion and pepsinogen secretion, independent of intracellular [Ca²⁺] changes. These findings suggest that extracellular Ca²⁺ can act as a “third messenger” via Ca²⁺ sensor(s) to regulate specific subsets of tissue function previously assumed to be under the direct control of intracellular Ca²⁺.

Low-dose PGE2 mimics the duodenal secretory response to luminal acid in mice

Luminal exposure to concentrated acid, the most accepted physiological stimulus for duodenal bicarbonate secretion (DBS), cannot be used with in vitro preparations due to potential tissue damage. We thus examined whether exposure to PGE(2), a well-characterized physiological duodenal secretagogue, could mimic the effects of acid perfusion. DBS was measured in C57/BL mice by pH-stat/back-titration and measurement of total dissolved CO(2) concentration ([CO(2)](t)). Anion transport inhibitor DIDS, anion channel inhibitor 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), carbonic anhydrase inhibitor methazolamide, and nonselective cyclooxygenase inhibitor indomethacin were used to inhibit separate components of HCO(3)(-) secretory pathway. Baseline DBS was not altered by exposure to methazolamide (0.1 mM) but was slightly reduced by DIDS (0.5 mM). DBS and [CO(2)](t) increased after acid and PGE(2) exposure. DIDS (0.5 mM) and NPPB (0.2 mM) abolished acid-induced DBS increase. Methazolamide (0.1 mM) and DIDS inhibited acid-induced [CO(2)](t) increase. DIDS, NPPB, or methazolamide had little effect on DBS in response to high concentration PGE(2) (100 microg/ml). Low concentration PGE(2) (1 microg/ml) increased DBS that was inhibited by DIDS, NPPB, and methazolamide. Pretreatment with indomethacin (5 mg/kg) inhibited DBS induced by acid exposure but not by PGE(2). High-dose PGE(2) substantially increases DBS by a mechanism that appears to be different than secretory response to luminal acid perfusion. Secretory response to low-dose PGE(2), at least in terms of inhibitor profile, closely resembles secretion in response to perfusion of physiological acid concentrations and may be a useful stimulus for in vitro study of DBS in isolated mouse duodenum.

Protein kinase C potentiates cAMP-stimulated mouse duodenal mucosal bicarbonate secretion in vitro

PKC has been shown to regulate epithelial Cl(-) secretion in a variety of models. However, the role of PKC in duodenal mucosal bicarbonate secretion is less clear. We aimed to investigate the role of PKC in regulation of duodenal mucosal bicarbonate secretion. Bicarbonate secretion by murine duodenal mucosa was examined in vitro in Ussing chambers using a pH-stat technique. PKC isoform expression and activity were assessed by Western blotting and in vitro kinase assays, respectively. PMA (an activator of PKC) alone had no effect on duodenal bicarbonate secretion or short-circuit current (I(sc)). When PMA and dibutyryl-cAMP (db-cAMP) were added simultaneously, PMA failed to alter db-cAMP-stimulated duodenal bicarbonate secretion or I(sc) (P > 0.05). However, a 1-h preincubation with PMA potentiated db-cAMP-stimulated duodenal bicarbonate secretion and I(sc) in a concentration-dependent manner (from 10(-8) to 10(-5)M) (P < 0.05). PMA preincubation had no effects on carbachol- or heat-stable toxin-stimulated bicarbonate secretion. Western blot analysis revealed that PKCalpha, -gamma, -epsilon, -, -micro, and -iota/lambda were expressed in murine duodenal mucosa. Ro 31-8220 (an inhibitor active against PKCepsilon, -alpha, -beta, and -gamma), but not Gö 6983 (an inhibitor active against PKCalpha, -gamma, -beta, and -delta), reversed the potentiating effect of PMA on db-cAMP-stimulated bicarbonate secretion. PMA also time- and concentration-dependently increased the activity of PKCepsilon, an effect that was prevented by Ro 31-8220 but not Gö 6983. These results demonstrate that activation of PKC potentiates cAMP-stimulated duodenal bicarbonate secretion, whereas it does not modify basal secretion. The effect of PKC on cAMP-stimulated bicarbonate secretion is mediated by the PKCepsilon isoform.

5-HT induces duodenal mucosal bicarbonate secretion via cAMP- and Ca2+-dependent signaling pathways and 5-HT4 receptors in mice

In previous studies, we have found that 5-hydroxytryptamine (5-HT) is a potent stimulant of duodenal mucosal bicarbonate secretion (DMBS) in mice. The aim of the present study was to determine the intracellular signaling pathways and 5-HT receptor subtypes involved in 5-HT-induced DMBS. Bicarbonate secretion by murine duodenal mucosa was examined in vitro in Ussing chambers. 5-HT receptor involvement in DMBS was inferred from pharmacological studies by using selective 5-HT receptor antagonists and agonists. The expression of 5-HT(4) receptor mRNA in duodenal mucosa and epithelial cells was analyzed by RT-PCR. cAMP-dependent signaling pathway inhibitors MDL-12330A, Rp-cAMP, and H-89 and Ca(2+)-dependent signaling pathway inhibitors verapamil and W-13 markedly reduced 5-HT-stimulated duodenal bicarbonate secretion and short-circuit current (I(sc)), whereas cGMP-dependent signaling pathway inhibitors NS-2028 and KT-5823 failed to alter these responses. Both SB-204070 and high-dose ICS-205930 (selective 5-HT(4) receptor antagonists) markedly inhibited 5-HT-stimulated bicarbonate secretion and I(sc), whereas methiothepine (5-HT(1) receptor antagonist), ketanserin (5-HT(2) receptor antagonist), and a low concentration of ICS-205930 (5-HT(3) receptor antagonist) had no effect. RS-67506 (partial 5-HT(4) receptor agonist) concentration-dependently increased bicarbonate secretion and I(sc), whereas 5-carboxamidotryptamine (5-HT(1) receptor agonist), alpha-methyl-5-HT (5-HT(2) receptor agonist), and phenylbiguanide (5-HT(3) receptor agonist) did not significantly increase bicarbonate secretion or I(sc). RT-PCR analysis confirmed the expression of 5-HT(4) receptor mRNA in murine duodenal mucosa and epithelial cells. These results demonstrate that 5-HT regulates DMBS via both cAMP- and Ca(2+)-dependent signaling pathways and 5-HT(4) receptors located in the duodenal mucosa and/or epithelial cells.

Short fasting dramatically decreases rat duodenal secretory responsiveness to orexin A but not to VIP or melatonin

Orexins are involved in the central nervous control of appetite and behavior, and in addition, they are present in endocrine cells and/or neurons in the intestine. The role of these peptides in peripheral regulation of intestinal secretion has not been investigated. We thus compared the effects of orexin A and some established secretagogues on duodenal HCO3- secretion in fed rats with effects in rats exposed to short (overnight) food deprivation. Rats were anesthetized with thiobarbiturate, a 12-mm segment of proximal duodenum with intact blood supply was cannulated in situ, and the alkaline secretion was titrated by pH stat. Secretagogues were supplied specifically to the duodenum by close intra-arterial infusion. Orexin A (60-600 pmol x kg(-1) x h(-1)) caused marked and dose-dependent stimulation of the duodenal secretion in fed animals but did not affect secretion in overnight food-deprived animals. Similarly, short fasting caused a 100-fold increase in the amount of the muscarinic agonist bethanechol (from 50 to 5,000 nmol x kg(-1) x h(-1)) required for stimulation of the secretion. In contrast, the secretory responses to VIP (50-1,000 pmol x kg(-1) x h(-1)) and melatonin (20-200 nmol x kg(-1) x h(-1)) were not affected. The appetite-regulating peptide orexin A is thus a stimulant of intestinal secretion, but the response to this peptide as well as the muscarinic agonist bethanechol is markedly dependent on previous intake of food. Overnight fasting is a standard experimental procedure in studies of gastrointestinal function and pathophysiology in humans and animals. Studies made on neuroendocrine control of intestinal secretion may require reevaluation with respect to feeding status.

Independence of apical Cl-/HCO3- exchange and anion conductance in duodenal HCO3- secretion

Reduced gastrointestinal HCO3- secretion contributes to malabsorption and obstructive syndromes in cystic fibrosis. The apical HCO3- transport pathways in these organs have not been defined. We therefore assessed the involvement of apical Cl-/HCO3- exchangers and anion conductances in basal and cAMP-stimulated duodenal HCO3- secretion. Muscle-stripped rat and rabbit proximal duodena were mounted in Ussing chambers, and electrical parameters, HCO3- secretion rates, and 36Cl-, 22Na+, and 3H+ mannitol fluxes were assessed. mRNA expression levels were measured by a quantitative PCR technique. Removal of Cl- from or addition of 1 mM DIDS to the luminal perfusate markedly decreased basal HCO3- secretion but did not influence the HCO3- secretory response to 8-bromo-cAMP, which was inhibited by luminal 5-nitro-2-(3-phenylpropylamino)-benzoate. Bidirectional 22Na+ and 36Cl- flux measurements demonstrated an inhibition rather than a stimulation of apical anion exchange during cAMP-stimulated HCO3- secretion. The ratio of Cl- to HCO3- in the anion secretory response was compatible with both Cl- and HCO3- being secreted via the CFTR anion channel. CFTR expression was very high in the duodenal mucosa of both species. We conclude that in rat and rabbit duodena, an apical Cl-/HCO3- exchanger mediates a significant part of basal HCO3- secretion but is not involved in the HCO3- secretory response to cAMP analogs. The inhibitor profile, the strong predominance of Cl- over HCO3- in the anion secretory response, and the high duodenal CFTR expression levels suggest that a major portion of cAMP-stimulated duodenal HCO3- secretion is directly mediated by CFTR.

Endogenous cyclo-oxygenase activity regulates mouse gastric surface pH

In the stomach, production of prostaglandins by cyclo-oxygenase (COX) is believed to be important in mucosal defence. We tested the hypothesis that endogenous COX activity is required for protective gastric surface pH control. Intact stomachs of anaesthetized mice were perfused with a weakly buffered solution (150 mM NaCl + 4 mM Homopipes) at pH values from 2.5 to 7.0. Gastric effluents were collected to measure pH and estimate amounts of acid or alkali secretion in nanomoles secreted per minute. A switch from net acid to net alkali secretion was seen in response to acidifying luminal pH with an apparent 'set point' between pH 4 and 5. At luminal pH 3, the net alkali secretion (12.7 +/- 2.8 nmol OH(-) equivalents min(-1)) was abolished (2.2 +/- 1.7 nmol OH(-) min(-1)) by the non-specific COX inhibitor indomethacin (5 mg kg(-1) I.P.). Similar inhibition was observed using a COX-1 inhibitor (SC-560; 10 mg kg(-1) I.P.), but not a COX-2 inhibitor (NS-398; 10 mg kg(-1) I.P.). Subsequent treatment with 16,16-dimethyl prostaglandin E(2) (dm-PGE(2); 1 mg kg(-1) I.P.) rescued the alkali secretion (21.8 +/- 2.7 nmol OH(-) min(-1)). In either the absence or presence of the H(+),K(+)-ATPase inhibitor omeprazole (60 mg kg(-1) I.P.), indomethacin blocked similar amounts of net alkali secretion (10.5 +/- 2.7 and 16.4 +/- 3.4 nmol OH(-) min(-1), respectively). We also used in vivo confocal microscopy to examine pH near the mucosal surface. The gastric mucosal surface of anaesthetized mice was exposed and mucosal surface pH was imaged using the fluorescence intensity ratio of Cl-NERF as a pH indicator. Results showed a switch from a continuous net acid to net alkali secretion by the stomach in response to changing superfusate pH from 5 to 3. At luminal pH 3, the relatively alkaline surface pH (4.3 +/- 0.1) was acidified (3.6 +/- 0.2) by indomethacin, and subsequent dm-PGE(2) restored surface pH (4.2 +/- 0.2). We conclude that the pre-epithelial alkaline layer is regulated by endogenous COX activity.

Cellular bicarbonate protects rat duodenal mucosa from acid-induced injury

Secretion of bicarbonate from epithelial cells is considered to be the primary mechanism by which the duodenal mucosa is protected from acid-related injury. Against this view is the finding that patients with cystic fibrosis, who have impaired duodenal bicarbonate secretion, are paradoxically protected from developing duodenal ulcers. Therefore, we hypothesized that epithelial cell intracellular pH regulation, rather than secreted extracellular bicarbonate, was the principal means by which duodenal epithelial cells are protected from acidification and injury. Using a novel in vivo microscopic method, we have measured bicarbonate secretion and epithelial cell intracellular pH (pH(i)), and we have followed cell injury in the presence of the anion transport inhibitor DIDS and the Cl(-) channel inhibitor, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB). DIDS and NPPB abolished the increase of duodenal bicarbonate secretion following luminal acid perfusion. DIDS decreased basal pH(i), whereas NPPB increased pH(i); DIDS further decreased pH(i) during acid challenge and abolished the pH(i) overshoot over baseline observed after acid challenge, whereas NPPB attenuated the fall of pH(i) and exaggerated the overshoot. Finally, acid-induced epithelial injury was enhanced by DIDS and decreased by NPPB. The results support the role of intracellular bicarbonate in the protection of duodenal epithelial cells from luminal gastric acid.