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

Oxalate secretion is stimulated by a cAMP-dependent pathway in the mouse cecum

Elevated levels of the intracellular second messenger cAMP can stimulate intestinal oxalate secretion however the membrane transporters responsible are unclear. Oxalate transport by the chloride/bicarbonate (Cl-/HCO3-) exchanger Slc26a6 or PAT-1 (Putative Anion Transporter 1), is regulated via cAMP when expressed in Xenopus oocytes and cultured cells but whether this translates to the native epithelia is unknown. This study investigated the regulation of oxalate transport by the mouse intestine focusing on transport at the apical membrane hypothesizing PAT-1 is the target of a cAMP-dependent signaling pathway. Adopting the Ussing chamber technique we measured unidirectional 14C-oxalate and 36Cl- flux ([Formula: see text] and [Formula: see text]) across distal ileum, cecum and distal colon, employing forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX) to trigger cAMP production. FSK/IBMX initiated a robust secretory response by all segments but the stimulation of net oxalate secretion was confined to the cecum only involving activation of [Formula: see text] and distinct from net Cl- secretion produced by inhibiting [Formula: see text]. Using the PAT-1 knockout (KO) mouse we determined cAMP-stimulated [Formula: see text] was not directly dependent on PAT-1, but it was sensitive to mucosal DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid), although unlikely to be another Cl-/HCO3- exchanger given the lack of trans-stimulation or cis-inhibition by luminal Cl- or HCO3-. The cAMP-activated oxalate efflux was reliant on CFTR (Cystic Fibrosis Transmembrane conductance Regulator) activity, but only in the presence of PAT-1, leading to speculation on the involvement of a multi-transporter regulatory complex. Further investigations at the cellular and molecular level are necessary to define the mechanism and transporter(s) responsible.

Physiological and Pathological Functions of SLC26A6

Solute Carrier Family 26 (SLC26) is a conserved anion transporter family with 10 members in human (SLC26A1-A11, A10 being a pseudogene). All SLC26 genes except for SLC26A5 (prestin) are versatile anion exchangers with notable ability to transport a variety of anions. SLC26A6 has the most extensive exchange functions in the SLC26 family and is widely expressed in various organs and tissues of mammals. SLC26A6 has some special properties that make it play a particularly important role in ion homeostasis and acid-base balance. In the past few years, the function of SLC26A6 in the diseases has received increasing attention. SLC26A6 not only participates in the development of intestinal and pancreatic diseases but also serves a significant role in mediating nephrolithiasis, fetal skeletal dysplasia and arrhythmia. This review aims to explore the role of SLC26A6 in physiology and pathophysiology of relative mammalian organs to guide in-depth studies about related diseases of human.

Calcium-sensing receptor regulates intestinal dipeptide absorption via Ca2+ signaling and IKCa activation

Although absorption of di- and tripeptides into intestinal epithelial cells occurs via the peptide transporter 1 (PEPT1, also called solute carrier family 15 member 1 (SLC15A1)), the detailed regulatory mechanisms are not fully understood. We examined: (a) whether dipeptide absorption in villous enterocytes is associated with a rise in cytosolic Ca2+ ([Ca2+ ]cyt ), (b) whether the calcium sensing receptor (CaSR) is involved in dipeptide-elicited [Ca2+ ]cyt signaling, and (c) what potential consequences of [Ca2+ ]cyt signaling may enhance enterocyte dipeptide absorption. Dipeptide Gly-Sar and CaSR agonist spermine markedly raised [Ca2+ ]cyt in villous enterocytes, which was abolished by NPS-2143, a selective CaSR antagonist and U73122, an phospholipase C (PLC) inhibitor. Apical application of Gly-Sar induced a jejunal short-circuit current (Isc), which was reduced by NPS-2143. CaSR expression was identified in the lamina propria and on the basal enterocyte membrane of mouse jejunal mucosa in both WT and Slc15a1-/- animals, but Gly-Sar-induced [Ca2+ ]cyt signaling was significantly decreased in Slc15a1-/- villi. Clotrimazole and TRM-34, two selective blockers of the intermediate conductance Ca2+ -activated K+ channel (IKCa ), but not iberiotoxin, a selective blocker of the large-conductance K+ channel (BKCa ) and apamin, a selective blocker of the small-conductance K+ channel (SKCa ), significantly inhibited Gly-Sar-induced Isc in native tissues. We reveal a novel CaSR-PLC-Ca2+ -IKCa pathway in the regulation of small intestinal dipeptide absorption, which may be exploited as a target for future drug development in human nutritional disorders.

Functional Role of Basolateral ClC-2 Channels in the Regulation of Duodenal Anion Secretion in Mice

BACKGROUND

Although ClC-2 channels are important in colonic Cl^- secretion, it is unclear about their roles in small intestinal anion secretion. Therefore, we sought to examine whether ClC-2 channels play important roles in anion secretion, particularly duodenal bicarbonate secretion (DBS).

METHODS

Duodenal mucosae from mice were stripped of seromuscular layers and mounted in Ussing chambers. Both duodenal short-circuit current (I_sc) and HCO₃^- secretion in vitro were simultaneously recorded. DBS in vivo was measured by a CO₂-sensitive electrode.

RESULTS

Lubiprostone, a selective ClC-2 activator, concentration-dependently increased both duodenal I_sc and DBS only when applied basolaterally, but not when applied apically. Removal of extracellular Cl^- abolished lubiprostone-induced duodenal I_sc, but did not alter HCO₃^- secretion even in the presence of DIDS, a Cl^-/HCO₃^- exchanger inhibitor. However, further addition of glibenclamide, a CFTR channel blocker, abolished lubiprostone-evoked HCO₃^- secretion. Moreover, lubiprostone-induced HCO₃^- secretion was impaired in CFTR^-/- mice compared to wild-type littermates. Luminal perfusion of duodenal lumen with lubiprostone did not alter basal DBS in vivo, but lubiprostone (i.p.) was able to induce DBS, which was also significantly inhibited by Cd²⁺, a ClC-2 channel blocker. [Ca²⁺]_cyt level, Ca²⁺-activated K⁺ channel- and cAMP-mediated duodenal I_sc, and HCO₃^- secretion were unchanged by lubiprostone.

CONCLUSIONS

We have provided the first evidence for the novel functional role of basolateral ClC-2 channels in the regulation of duodenal anion secretion.

Extracellular pH and lung infections in cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disease caused by CFTR mutations. It is characterized by high NaCl concentration in sweat and the production of a thick and sticky mucus, occluding secretory ducts, intestine and airways, accompanied by chronic inflammation and infections of the lungs. This causes a progressive and lethal decline in lung function. Therefore, finding the mechanisms driving the high susceptibility to lung infections has been a key issue. For decades the prevalent hypothesis was that a reduced airway surface liquid (ASL) volume and composition, and the consequent increased mucus concentration (dehydration), create an environment favoring infections. However, a few years ago, in a pig model of CF, the Na⁺/K⁺ concentrations and the ASL volume were found intact. Immediately a different hypothesis arose, postulating a reduced ASL pH as the cause for the increased susceptibility to infections, due to a diminished bicarbonate secretion through CFTR. Noteworthy, a recent report found normal ASL pH values in CF children and in cultured primary airway cells, challenging the ASL pH hypothesis. On the other hand, recent evidences revitalized the hypothesis of a reduced ASL secretion. Thus, the role of the ASL pH in the CF is still a controversial matter. In this review we discuss the basis that sustain the role of CFTR in modulating the extracellular pH, and the recent results sustaining the different points of view. Finding the mechanisms of CFTR signaling that determine the susceptibility to infections is crucial to understand the pathophysiology of CF and related lung diseases.

Bicarbonate in cystic fibrosis

BACKGROUND

Cystic fibrosis (CF, mucoviscidosis) is caused by mutations in the gene encoding CF transmembrane conductance regulator (CFTR), which is a chloride and bicarbonate channel necessary for fluid secretion and extracellular alkalization. For a long time, research concentrated on abnormal Cl^- and Na⁺ transport, but neglected bicarbonate as a crucial factor in CF.

METHODS

The present short review reports early findings as well as recent insights into the role of CFTR for bicarbonate transport and its defects in CF.

RESULTS

The available data indicate impaired bicarbonate transport not only in pancreas, intestine, airways, and reproductive organs, but also in salivary glands, sweat duct and renal tubular epithelial cells. Defective bicarbonate transport is closely related to the impaired mucus properties and mucus blocking in secretory organs of CF patients, causing the life threatening lung disease.

CONCLUSIONS

Apart from the devastating lung disease, abrogated bicarbonate transport also leads to many other organ dysfunctions, which are outlined in the present review.

Guanylin peptides regulate electrolyte and fluid transport in the Gulf toadfish (Opsanus beta) posterior intestine

The physiological effects of guanylin (GN) and uroguanylin (UGN) on fluid and electrolyte transport in the teleost fish intestine have yet to be thoroughly investigated. In the present study, the effects of GN, UGN, and renoguanylin (RGN; a GN and UGN homolog) on short-circuit current ( Isc) and the transport of Cl−, Na+, bicarbonate (HCO3−), and fluid in the Gulf toadfish ( Opsanus beta) intestine were determined using Ussing chambers, pH-stat titration, and intestinal sac experiments. GN, UGN, and RGN reversed the Isc of the posterior intestine (absorptive-to-secretory), but not of the anterior intestine. RGN decreased baseline HCO3− secretion, but increased Cl− and fluid secretion in the posterior intestine. The secretory response of the posterior intestine coincides with the presence of basolateral NKCC1 and apical cystic fibrosis transmembrane conductance regulator (CFTR), the latter of which is lacking in the anterior intestine and is not permeable to HCO3− in the posterior intestine. However, the response to RGN by the posterior intestine is counterintuitive given the known role of the marine teleost intestine as a salt- and water-absorbing organ. These data demonstrate that marine teleosts possess a tissue-specific secretory response, apparently associated with seawater adaptation, the exact role of which remains to be determined.

Loss of Slc26a9 anion transporter alters intestinal electrolyte and HCO3(-) transport and reduces survival in CFTR-deficient mice

Slc26a9 is an anion transporter that is strongly expressed in the stomach and lung. Slc26a9 variants were recently found associated with a higher incidence of meconium ileus in cystic fibrosis (CF) infants, raising the question whether Slc26a9 is expressed in the intestine and what its functional role is. Slc26a9 messenger RNA (mRNA) was found highly expressed in the mucosae of the murine and human upper gastrointestinal tract, with an abrupt decrease in expression levels beyond the duodenum. Absence of SLC26a9 expression strongly increased the intestinally related mortality in cystic fibrosis transmembrane conductance regulator (CFTR)-deficient mice. Proximal duodenal JHCO3(-) and fluid secretion were reduced in the absence of Slc26a9 expression. In the proximal duodenum of young Slc26a9 KO mice, the glands and villi/crypts were elongated and proliferation was enhanced. This difference was lost with ageing, as were the alterations in fluid movement, whereas the reduction in JHCO3(-) remained. Laser dissection followed by qPCR suggested Slc26a9 expression to be crypt-predominant in the duodenum. In summary, deletion of Slc26a9 caused bicarbonate secretory and fluid absorptive changes in the proximal duodenal mucosa and increased the postweaning death rates in CFTR-deficient mice. Functional alterations in the duodenum were most prominent at young ages. We assume that the association of meconium ileus and Slc26a9 variants may be related to maldigestion and impaired downstream signaling caused by loss of upper GI tract digestive functions, aggravating the situation of lack of secretion and sticky mucus at the site of obstruction in CF intestine.

Rescue of epithelial HCO3- secretion in murine intestine by apical membrane expression of the cystic fibrosis transmembrane conductance regulator mutant F508del

This study investigated whether expression of the common cystic fibrosis transmembrane conductance regulator (CFTR) mutant F508del in the apical membrane of enterocytes confers increased bicarbonate secretory capacity on the intestinal epithelium of F508del mutant mice compared to that of CFTR knockout (KO) mice. CFTR KO mice, F508del mutant mice (F508del) and wild-type (WT) littermates were bred on the FVB/N background. F508del isolated brush border membrane (BBM) contained approximately 5-10% fully glycosylated band C protein compared to WT BBM. Similarly, the forskolin (FSK)-induced, CFTR-dependent short-circuit current (I(sc)) of F508del mucosa was approximately 5-10% of WT, whereas the HCO(3)(-) secretory response ( ) was almost half that of WT in both duodenum and mid-colon studied in vitro and in vivo. While WT intestine retained full FSK-induced in the absence of luminal Cl(-), the markedly higher than I(sc) in F508del intestine was dependent on the presence of luminal Cl(-), and was blocked by CFTR inhibitors. The Ste20-related proline-alanine-rich kinases (SPAK/OSR1), which are downstream of the with-no-lysine (K) protein kinases (WNK), were rapidly phosphorylated by FSK in WT and F508del, but significantly more slowly in CFTR KO intestine. In conclusion, the data demonstrate that low levels of F508del membrane expression in the intestine of F508del mice significantly increased FSK-induced HCO(3)(-) secretion mediated by Cl(-)/HCO(3)(-) exchange. However, in WT mucosa FSK elicited strong SPAK/OSR1 phosphorylation and Cl(-)-independent HCO(3)(-) efflux. This suggests that therapeutic strategies which deliver F508del to the apical membrane have the potential to significantly enhance epithelial HCO(3)(-) secretion.

The electroneutral Na⁺:HCO₃⁻ cotransporter NBCn1 is a major pHi regulator in murine duodenum

Duodenocyte pHi control and HCO3 − secretion protects the proximal duodenum against damage by gastric acid. The molecular details of duodenocyte pH control are not well understood. A selective duodenal expression (within the upper GI tract) has been reported for the electroneutral Na+:HCO3 − cotransporter NBCn1 (Slc4a7). We aimed to determine the role of NBCn1 and NBCe2 in duodenocyte intracellular pH regulation as well as basal and agonist-stimulated duodenal bicarbonate secretion (JHCO3 −), exploiting mouse models of genetic slc4a7 and slc4a5 disruption. Basal and forskolin (FSK)-stimulated JHCO3 − was measured by single-pass perfusion in the duodenum of slc4a7−/− and slc4a7+/+ as well as slc4a5−/− and slc4a5+/+ mice in vivo, and by pH-stat titration in isolated duodenal mucosa in vitro. Duodenocyte HCO3 − uptake rates were fluorometrically assessed after acidification of intact villi and of isolated duodenocytes. Slc4a7−/− mice displayed significantly lower basal and FSK-stimulated duodenal HCO3 − secretion than slc4a7+/+ littermates in vivo. FSK-stimulated HCO3 − secretion was significantly reduced in slc4a7−/− isolated duodenal mucosa. Na+- and HCO3 −-dependent base uptake rates were significantly decreased in slc4a7−/− compared with slc4a7+/+ villus duodenocytes when measured in intact villi. Carbonic anhydrase (CA)-mediated CO2 hydration played no apparent role as a HCO3 − supply mechanism for basal or FSK-stimulated secretion in the slc4a7+/+ duodenum, but was an important alternative HCO3 − supply mechanism in the slc4a7−/− duodenum. NBCe2 (Slc4a5) displayed markedly lower duodenal mRNA expression levels, and its disruption did not interfere with duodenal HCO3 − secretion. The electroneutral Na+:HCO3 − cotransporter NBCn1 (slc4a7) is a major duodenal HCO3 − importer that supplies HCO3 − during basal and FSK-stimulated HCO3 − secretion.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

The impact of cyclooxygenase inhibition on duodenal motility and mucosal alkaline secretion in anaesthetized rats

AIM

Non-steroidal anti-inflammatory drugs (NSAIDs) are effective in the treatment of various human diseases. However, these drugs also have serious adverse effects in the gastrointestinal tract. In the duodenum NSAIDs inhibit mucosal alkaline secretion (DMAS), an important protective mechanism against the acid emptied from the stomach in most species, including humans. Surprisingly, NSAIDs have been shown to stimulate DMAS in an anaesthetized rat model. The aim of this review was to summarize the effects of NSAIDs and selective cyclooxygenase-2 (COX-2) inhibition on duodenal function in the rat and provide an explanation for why these drugs stimulate DMAS. Included are new data examining the effect of α-adrenergic drugs on duodenal motility and DMAS.

METHODS

Experiments were performed in anaesthetized rats. The proximal duodenum was perfused luminally with an isotonic NaCl solution. DMAS, motility, fluid flux and epithelial permeability were assessed in the absence and presence of various COX inhibitors.

RESULTS

COX inhibition induced duodenal motility, increased DMAS and augmented the sensitivity as well as the maximal response of the duodenal mucosa to lidocaine- or hypotonicity-induced increases in mucosal permeability. Furthermore, the ability of the duodenum to absorb water and to adjust osmolality in response to luminal hypotonicity was improved in COX-inhibited animals. These improvements are mediated predominately via inhibition of COX-2.

CONCLUSIONS

Inhibition of COX-2 in rats with postoperative duodenal ileus induces muscle contractions, which in turn activate a nicotinic receptor-dependent intramural reflex that stimulates duodenocytes to increase the activity of apical Cl⁻/HCO₃⁻ exchangers, resulting in a rise in DMAS.

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.

Motility-induced but not vasoactive intestinal peptide-induced increase in luminal alkalinization in rat duodenum is dependent on luminal Cl(-)

AIM

to investigate whether the motility- and the vasoactive intestinal peptide (VIP)-induced increase in luminal alkalinization in the duodenum is dependent on luminal Cl(-).

METHODS

  experiments were performed in anaesthetized rats in vivo. The proximal duodenum was perfused luminally with an isotonic solution, containing zero or low Cl(-) and the effects on luminal alkalinization, motility, fluid flux and epithelial permeability were determined. Parecoxib, a COX-2 inhibitor, was used to induce duodenal contractions.

RESULTS

control rats lacked duodenal wall contractions while parecoxib-treated ones exhibited contractions throughout the experiment. Most animals had a net fluid absorption during the perfusion with isotonic NaCl. Luminal alkalinization was about 100% higher in parecoxib-treated rats than in controls. Cl(-) -free solutions did not affect epithelial permeability or motility but decreased luminal alkalinization by ≥50% and decreased net fluid absorption in both control and parecoxib-treated animals. Reduction in luminal Cl(-) decreased alkalinization in a concentration-dependent manner. The parecoxib-induced increase in alkalinization was markedly reduced in the absence of luminal Cl(-) . VIP increased luminal alkalinization and induced fluid secretion. The lack of luminal Cl(-) did not affect the VIP-induced increase in alkalinization but reduced fluid secretion.

CONCLUSIONS

the parecoxib-induced increase in luminal alkalinization is highly dependent on luminal Cl(-) and it is proposed that COX-2 inhibition, via induction of duodenal motility, enhances HCO(3) (-) efflux through stimulation of apical Cl(-) /HCO(3) (-) exchange in duodenal epithelial cells. Although the VIP-induced stimulation of fluid secretion is partly dependent on luminal Cl(-) , the VIP-induced increase in luminal alkalinization is not.

The switch of intestinal Slc26 exchangers from anion absorptive to HCOFormula secretory mode is dependent on CFTR anion channel function

CFTR has been recognized to function as both an anion channel and a key regulator of Slc26 anion transporters in heterologous expression systems. Whether this regulatory relationship between CFTR and Slc26 transporters is seen in native intestine, and whether this effect is coupled to CFTR transport function or other features of this protein, has not been studied. The duodena of anesthetized CFTR-, NHE3-, Slc26a6-, and Scl26a3-deficient mice and wild-type (WT) littermates were perfused, and duodenal bicarbonate (HCO(3)(-)) secretion (DBS) and fluid absorptive or secretory rates were measured. The selective NHE3 inhibitor S1611 or genetic ablation of NHE3 significantly reduced fluid absorptive rates and increased DBS. Slc26a6 (PAT1) or Slc26a3 (DRA) ablation reduced the S1611-induced DBS increase and reduced fluid absorptive rates, suggesting that the effect of S1611 or NHE3 ablation on HCO(3)(-) secretion may be an unmasking of Slc26a6- and Slc26a3-mediated Cl(-)/HCO(3)(-) exchange activity. In the absence of CFTR expression or after application of the CFTR(inh)-172, fluid absorptive rates were similar to those of WT, but S1611 induced virtually no increase in DBS, demonstrating that CFTR transport activity, and not just its presence, is required for Slc26-mediated duodenal HCO(3)(-) secretion. A functionally active CFTR is an absolute requirement for Slc26-mediated duodenal HCO(3)(-) secretion, but not for Slc26-mediated fluid absorption, in which these transporters operate in conjunction with the Na(+)/H(+) exchanger NHE3. This suggests that Slc26a6 and Slc26a3 need proton recycling via NHE3 to operate in the Cl(-) absorptive mode and Cl(-) exit via CFTR to operate in the HCO(3)(-) secretory mode.

Secretagogues stimulate electrogenic HCO3- secretion in the ileum of the brushtail possum, Trichosurus vulpecula: evidence for the role of a Na+/HCO3- cotransporter

Fluid secretion is essential for intestinal function and, in eutherian mammals, is driven by electrogenic Cl(-) transport, which is dependent upon a bumetanide-sensitive, basolateral Na(+)/K(+)/2 Cl(-) cotransporter, NKCC1. However, ileal secretion in the brushtail possum, a marsupial, involves a fundamentally different process, since NKCC1 expression is low in this tissue and the secretagogue-induced short circuit current (I(sc)) is insensitive to bumetanide. In view of these differences we have investigated the basis of the secretory response of the possum ileum. In the Ussing chamber the secretory I(sc) is independent of Cl(-) but dependent upon Na(+) and serosal HCO(3)(-)/CO(2), suggesting that secretagogues stimulate electrogenic HCO(3)(-) secretion. In agreement with this, serosal DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulfonate; 1 mmol l(-1)) inhibited the secretory response. However, acetazolamide (1 mmol l(-1)) and serosal amiloride (1 mmol l(-1)) had little effect, indicating that HCO(3)(-) secretion is driven by HCO(3)(-) transport from the serosal solution into the cell, rather than hydration of CO(2) by carbonic anhydrase. Consistent with this the pancreatic variant of the electrogenic Na(+)/HCO(3)(-) cotransporter (pNBC) is highly expressed in the ileal epithelium and is located in the basolateral membrane of the epithelial cells, predominantly in the mid region of the villi, with lower levels of expression in the crypts and no expression in the villous tips. We conclude that the secretory response of the possum ileum involves electrogenic HCO(3)(-) secretion driven by a basolateral pNBC and that the ileal HCO(3)(-) secretion is associated with a specialised function of the possum ileum, most probably related to hindgut fermentation.

Electrogenic Cl(-) secretion does not occur in the ileum of the Australian common brushtail possum, Trichosurus vulpecula, due to low levels of expression of the NaK2Cl cotransporter, NKCC1

The colon of the brushtail possum does not have an electrogenic secretory response. Given the functional significance of electrogenic Cl(-) secretion in the intestine of eutherian mammals, we have investigated the secretory response in the small intestine of this marsupial. In the Ussing chamber cAMP-dependent secretagogues stimulated a sustained increase in ileal short-circuit current (Isc), whereas Ca(2+)-dependent secretagogues induced a transient increase. Both the responses were inhibited by mucosal addition of the anion channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (100 mciromol l(-1)), consistent with an anion secretory response. However, the responses were not inhibited by serosal bumetanide (10 mciromol l(-1)) and were independent of bath Cl(-), indicating that the stimulated ileal Isc does not involve electrogenic Cl(-) secretion driven by the NaK2Cl cotransporter, NKCC1. Consistent with this, there were low levels of NKCC1 expression in the ileal epithelium. In particular, NKCC1 expression in the ileal crypt cells was comparable to that of the villous cells. This differs from eutherian mammals where high levels of NKCC1 expression in the ileal crypt cells are associated with their role in Cl(-) secretion. The cAMP- and Ca(2+)-dependent secretory responses were inhibited by the removal of HCO(3) (-) suggesting that these responses were due to electrogenic HCO(3) (-) secretion. We conclude that the ileum of the possum does not secrete Cl(-) due to low levels of NKCC1 expression. It does however appear to secrete HCO(3) (-). These results are further significant examples of differences in the transport function of the possum intestinal epithelium compared with eutherian mammals.

Bicarbonate transport in cell physiology and disease

The family of mammalian bicarbonate transport proteins are involved in a wide-range of physiological processes. The importance of bicarbonate transport follows from the biochemistry of HCO(3)(-) itself. Bicarbonate is the waste product of mitochondrial respiration. HCO(3)(-) undergoes pH-dependent conversion into CO(2) and in doing so converts from a membrane impermeant anion into a gas that can diffuse across membranes. The CO(2)-HCO(3)(-) equilibrium forms the most important pH buffering system of our bodies. Bicarbonate transport proteins facilitate the movement of membrane-impermeant HCO(3)(-) across membranes to accelerate disposal of waste CO(2), control cellular and whole-body pH, and to regulate fluid movement and acid/base secretion. Defects of bicarbonate transport proteins manifest in diseases of most organ systems. Fourteen gene products facilitate mammalian bicarbonate transport, whose physiology and pathophysiology is discussed in the present review.

PAT-1 (Slc26a6) is the predominant apical membrane Cl-/HCO3- exchanger in the upper villous epithelium of the murine duodenum

Basal HCO(3)(-) secretion across the duodenum has been shown in several species to principally involve the activity of apical membrane Cl(-)/HCO(3)(-) exchanger(s). To investigate the identity of relevant anion exchanger(s), experiments were performed using wild-type (WT) mice and mice with gene-targeted deletion of the following Cl(-)/HCO(3)(-) exchangers localized to the apical membrane of murine duodenal villi: Slc26a3 [down-regulated in adenoma (DRA)], Slc26a6 [putative anion transporter 1 (PAT-1)], and Slc4a9 [anion exchanger 4 (AE4)]. RT-PCR of the isolated villous epithelium demonstrated PAT-1, DRA, and AE4 mRNA expression. Using the pH-sensitive dye BCECF, anion exchange rates were measured across the apical membrane of epithelial cells in the upper villus of the intact duodenal mucosa. Under basal conditions, Cl(-)/HCO(3)(-) exchange activity was reduced by 65-80% in the PAT-1(-) duodenum, 30-40% in the DRA(-) duodenum, and <5% in the AE4(-) duodenum compared with the WT duodenum. SO(4)(2-)/HCO(3)(-) exchange was eliminated in the PAT-1(-) duodenum but was not affected in the DRA(-) and AE4(-) duodenum relative to the WT duodenum. Intracellular pH (pH(i)) was reduced in the PAT-1(-) villous epithelium but increased to WT levels in the absence of CO(2)/HCO(3)(-) or during methazolamide treatment. Further experiments under physiological conditions indicated active pH(i) compensation in the PAT-1(-) villous epithelium by combined activities of Na(+)/H(+) exchanger 1 and Cl(-)-dependent transport processes at the basolateral membrane. We conclude that 1) PAT-1 is the major contributor to basal Cl(-)/HCO(3)(-) and SO(4)(2-)/HCO(3)(-) exchange across the apical membrane and 2) PAT-1 plays a role in pH(i) regulation in the upper villous epithelium of the murine duodenum.

Dopamine D1-like receptor-mediated inhibition of Cl/HCO3- exchanger activity in rat intestinal epithelial IEC-6 cells is regulated by G protein-coupled receptor kinase 6 (GRK 6)

The present study investigated the effect of dopamine D1-like receptor stimulation on the Cl-/HCO3- exchange activity in rat intestinal epithelial IEC-6 cells. The Cl-/HCO3- exchange activity was found to be a chloride-dependent, DIDS-sensitive and niflumate-insensitive process. The presence of the SLC26A6 anion exchanger was detected by both RT-PCR and immunoblotting analysis in IEC-6 cells, in which three different small interfering RNAs (siRNAs) targeting SLC26A6 markedly inhibited Cl-/HCO3- exchange. Activation of dopamine D1-like receptors with SKF 38393 inhibited Cl-/HCO3- exchanger activity, this being antagonized by the D1 selective antagonist SKF 83566. However, effects of SKF 38393 were maximal at 5 min of exposure to the agonist and rapidly diminished with no effect at 15 min, suggestive of agonist-induced desensitization of D1-like receptors. Pretreatment of cells with heparin, a non-selective inhibitor of G protein-coupled receptor kinases (GRKs), prevented the observed attenuation of SKF 38393-induced inhibition of Cl-/HCO3- exchange. Overnight pretreatment with anti-GRK6A and anti-GRK6B, but not with anti-GRK4 antibodies, prevented the loss of SKF 38393-mediated effects. Both PKA and PKC signaling pathways participate in SKF 38393-mediated inhibition of Cl-/HCO3- exchange. These findings suggest that SLC26A6 is at least one of the anion exchanger's family members responsible for Cl-/HCO3- exchange in IEC-6 cells. Dopamine D1 receptors in IEC-6 rapidly desensitize to D1-like agonist stimulation and GRK 6, but not GRK 4, appear to be involved in agonist-mediated responsiveness and desensitization.

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.

The emerging role of PDZ adapter proteins for regulation of intestinal ion transport

In the gastrointestinal tract, CFTR, in conjunction with one or several members of the SLC26 anion exchanger family, mediates electrogenic Cl- and HCO3- secretion. Na+/H+ exchanger isoform NHE3, on the other hand, coupled to one or several of the SLC26 isoforms, mediates electroneutral NaCl absorption. The agonist-induced activation of anion secretion and inhibition of salt absorption causes secretory diarrhea. Current dogma sees the formation of a multiprotein complex of transport proteins, postsynaptic density-95/discs large/zonula occludens-1 (PDZ) adapter proteins, anchoring proteins, the cytoskeleton, and the involved protein kinases as one crucial step in the regulation of these transport processes. Data obtained in heterologous expression studies suggest an important role of these PDZ adapter proteins in trafficking, endocytic recycling, and membrane retention of the respective transmembrane proteins. This article reviews recent advances in our understanding of the role of the PDZ adapter proteins NHERF, E3KARP, PDZK1, IKEPP (NHERF-1 to NHERF-4), CAL, and Shank-2 that bind to CFTR, NHE3, and the intestinal SLC26 members in the regulation of intestinal fluid transport. Current concepts are mostly derived from heterologous expression studies and studies on their role in organ physiology are still in infancy. Recently, however, PDZ adapter protein-deficient mice and organ-specific cell lines have become available, and the first results suggest a more cell-type and possibly signal-specific role of these adapter proteins. This opens the potential for drug development targeted to PDZ domain interactions, which is, in theory, one of the most efficient antidiarrheal strategies.

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.

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.

Mechanisms of bicarbonate secretion in the pancreatic duct

In many species the pancreatic duct epithelium secretes HCO3- ions at a concentration of around 140 mM by a mechanism that is only partially understood. We know that HCO3- uptake at the basolateral membrane is achieved by Na+-HCO3- cotransport and also by a H+-ATPase and Na+/H+ exchanger operating together with carbonic anhydrase. At the apical membrane, the secretion of moderate concentrations of HCO3- can be explained by the parallel activity of a Cl-/HCO3- exchanger and a Cl- conductance, either the cystic fibrosis transmembrane conductance regulator (CFTR) or a Ca2+-activated Cl- channel (CaCC). However, the sustained secretion of HCO3- into a HCO- -rich luminal fluid cannot be explained by conventional Cl-/HCO3- exchange. HCO3- efflux across the apical membrane is an electrogenic process that is facilitated by the depletion of intracellular Cl-, but it remains to be seen whether it is mediated predominantly by CFTR or by an electrogenic SLC26 anion exchanger.

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.

Chloride channels in the small intestinal cell line IEC-18

Small intestinal crypt cells play a critical role in modulating Cl- secretion during digestion. The types of Cl- channels mediating Cl- secretion in the small intestine was investigated using the intestinal epithelial cell line, IEC-18, which was derived from rat small intestine crypt cells. In initial radioisotope efflux studies, exposure to forskolin, ionomycin or a decrease in extracellular osmolarity significantly increased 36Cl efflux as compared to control cells. Whole cell patch clamp techniques were subsequently used to examine in more detail the swelling-, Ca2+-, and cAMP-activated Cl- conductance. Decreasing the extracellular osmolarity from 290 to 200 mOsm activated a large outwardly rectifying Cl- current that was voltage-independent and had an anion selectivity of I- > Cl-. Increasing cytosolic Ca2+ by ionomycin activated whole cell Cl- currents, which were also outwardly rectifying but were voltage-dependent. The increase in intracellular Ca2+ levels with ionomycin was confirmed with fura-2 loaded IEC-18 cells. A third type of whole cell Cl- current was observed after increases in intracellular cAMP induced by forskolin. These cAMP-activated Cl- currents have properties consistent with cystic fibrosis transmembrane regulator (CFTR) Cl- channels, as the currents were blocked by glibenclamide or NPPB but insensitive to DIDS. In addition, the current-voltage relationship was linear and had an anion selectivity of Cl- > I-. Confocal immunofluorescence studies and Western blots with two different anti-CFTR antibodies confirmed the expression of CFTR. These results suggest that small intestinal crypt cells express multiple types of Cl- channels, which may all contribute to net Cl- secretion.

Renal and intestinal transport defects in Slc26a6-null mice

SLC26A6 (PAT1, CFEX) is an anion exchanger that is expressed on the apical membrane of the kidney proximal tubule and the small intestine. Modes of transport mediated by SLC26A6 include Cl-/formate exchange, Cl-/HCO3- exchange, and Cl-/oxalate exchange. To study its role in kidney and intestinal physiology, gene targeting was used to prepare mice lacking Slc26a6. Homozygous mutant Slc26a6-/- mice appeared healthy and exhibited a normal blood pressure, kidney function, and plasma electrolyte profile. In proximal tubules microperfused with a low-HCO3-/high-Cl- solution, the baseline rate of fluid absorption (Jv), an index of NaCl transport under these conditions, was the same in wild-type and null mice. However, the stimulation of Jv by oxalate observed in wild-type mice was completely abolished in Slc26a6-null mice (P<0.05). Formate stimulation of Jv was partially reduced in null mice, but the difference from the response in wild-type mice did not reach statistical significance. Apical membrane Cl-/base exchange activity, assayed with the pH-sensitive dye BCPCF in microperfused proximal tubules, was decreased by 58% in Slc26a6-/- animals (P<0.001 vs. wild types). In the duodenum, the baseline rate of HCO3- secretion measured in mucosal tissue mounted in Ussing chambers was decreased by approximately 30% (P<0.03), whereas the forskolin-stimulated component of HCO3- secretion was the same in wild-type and Slc26a6-/- mice. We conclude that Slc26a6 mediates oxalate-stimulated NaCl absorption, contributes to apical membrane Cl-/base exchange in the kidney proximal tubule, and also plays an important role in HCO3- secretion in the duodenum.

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.