Dual role of CFTR in cAMP-stimulated HCO3- secretion across murine duodenum

Bicarbonate secretion and acid/base sensing by the intestine

The transport of bicarbonate across the enterocyte cell membrane regulates the intracellular as well as the luminal pH and is an essential part of directional fluid movement in the gut. Since the first description of "active" transport of HCO3- ions against a concentration gradient in the 1970s, the fundamental role of HCO3- transport for multiple intestinal functions has been recognized. The ion transport proteins have been identified and molecularly characterized, and knockout mouse models have given insight into their individual role in a variety of functions. This review describes the progress made in the last decade regarding novel techniques and new findings in the molecular regulation of intestinal HCO3- transport in the different segments of the gut. We discuss human diseases with defects in intestinal HCO3- secretion and potential treatment strategies to increase luminal alkalinity. In the last part of the review, the cellular and organismal mechanisms for acid/base sensing in the intestinal tract are highlighted.

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.

Case Report: Irreversible Watery Diarrhea, Severe Metabolic Acidosis, Hypokalemia and Achloridria Syndrome Related to Vasoactive Intestinal Peptide Secreting Malignant Pheochromocytoma

BACKGROUND

Pheochromocytoma (PHEO) clinical manifestations generally mirror excessive catecholamines secretion; rarely the clinical picture may reflect secretion of other hormones. Watery diarrhea, hypokalemia and achlorhydria (WDHA) is a rare syndrome related to excessive secretion of vasoactive intestinal peptide (VIP).

CLINICAL CASE

A 73-year-old hypotensive man affected by adrenal PHEO presented with weight loss and watery diarrhea associated with hypokalemia, hyperchloremic metabolic acidosis (anion gap 15 mmol/l) and a negative urinary anion gap. Abdominal computed tomography scan showed a right adrenal PHEO, 8.1 cm in maximum diameter, with tracer uptake on ⁶⁸GaDOTA-octreotate positron emission tomography. Metastasis in lumbar region and lung were present. Both chromogranin A and VIP levels were high (more than10 times the normal value) with slightly elevated urine normetanephrine and metanephrine excretion. Right adrenalectomy was performed and a somatostatin analogue therapy with lanreotide started. Immunostaining showed chromogranin A and VIP co-expression, with weak somatostatin-receptor-2A positivity. In two months, patient clinical conditions deteriorated with severe WDHA and multiple liver and lung metastasis. Metabolic acidosis and hypokalemia worsened, leading to hemodynamic shock and exitus.

CONCLUSIONS

A rare case of WDHA syndrome caused by malignant VIP-secreting PHEO was diagnosed. High levels of circulating VIP were responsible of the rapidly evolving clinical picture with massive dehydration and weight loss along with severe hyperchloremic metabolic acidosis and hypokalemia due to the profuse untreatable diarrhea. The rescue treatment with lanreotide was unsuccessful because of the paucity of somatostatin-receptor-2A on VIP-secreting PHEO chromaffin cells.

Characterization of intracellular buffering power in human induced pluripotent stem cells and the loss of pluripotency is delayed by acidic stimulation and increase of NHE1 activity

The homeostasis of intracellular pH (pH_i ) affects many cellular functions. Our previous study has established a functional and molecular model of the active pH_i regulators in human induced pluripotent stem cells (hiPSCs). The aims of the present study were to further quantify passive pH_i buffering power (β) and to investigate the effects of extracellular pH and Na⁺ -H⁺ exchanger 1 (NHE1) activity on pluripotency in hiPSCs. pH_i was detected by microspectrofluorimetry with pH-sensitive dye-BCECF. Western blot, immunofluorescence staining, and flow cytometry were used to detect protein expression and pluripotency. Our study in hiPSCs showed that (a) the value of total (β_tot ), intrinsic (β_i ), and CO₂ -dependent ( β C O 2 ) buffering power all increased while pH_i increased; (b) during the spontaneous differentiation for 4 days, the β values of β_tot and β C O 2 changed in a tendency of decrease, despite the absence of statistical significance; (c) an acidic cultured environment retained pluripotency and further upregulated expression and activity of NHE1 during spontaneous differentiation; (d) inhibition on NHE1 activity promoted the loss of pluripotency. In conclusion, we, for the first time, established a quantitative model of passive β during differentiation and demonstrated that maintenance of NHE1 at a higher level was of critical importance for pluripotency retention in hiPSCs.

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.

Huqi San-Evoked Rat Colonic Anion Secretion through Increasing CFTR Expression

Huqi San (HQS) is a Chinese herbal preparation of eight medicinal herbs that promote diuresis, detoxification, blood circulation, and cholestasis. Defects in transporter expression and function can cause cholestasis and jaundice. However, the mechanism of the cholestasis underlying HQS effects, especially on the gastrointestinal tract ion secretion, has not been elucidated. Real-time RT-PCR and Western blotting were used to study the expression and localization of cystic fibrosis transmembrane conductance regulator (CFTR) and α-ENaC in rat alimentary tract, and then the effect of HQS on the ion transport in rat distal colon mucosa was investigated using the short-circuit current (I_SC) technique. The results showed that pretreatment with HQS significantly enhanced mRNA transcripts and protein content of CFTR in liver and distal colon but not α-ENaC in alimentary organs. HQS increases I_SC and decreases the transepithelial resistance. Pretreatment with epithelial Na⁺ channel blocker did not affect the I_SC responses elicited by HQS, but removal of extracellular Cl⁻ or pretreatment with Cl⁻ channel or Na⁺-K⁺-2Cl⁻ cotransporter blocker inhibited HQS-elicited I_SC responses. These findings demonstrated that HQS, RA, and RP can stimulate Cl⁻ secretion in the distal colon by increasing the mRNA transcripts and protein content of CFTR in liver and distal colon.

Effect of genistein on basal jejunal chloride secretion in R117H CF mice is sex and route specific

Cystic fibrosis (CF) results from the loss or reduction in function of the CFTR (cystic fibrosis transmembrane conductance regulatory protein) chloride channel. The third most common CFTR mutation seen clinically is R117H. Genistein, a naturally occurring phytoestrogen, is known to stimulate CFTR function in vitro. We aimed to determine whether route of administration of genistein could mediate differential effects in R117H male and female CF mice. Mice were fed (4 weeks) or injected subcutaneously (1 week) with the following: genistein 600 mg/kg diet (600Gd); genistein-free diet (0Gd); genistein injection 600 mg/kg body weight (600Gi); dimethyl sulfoxide control (0Gi). In male R117H mice fed 600Gd, basal short circuit current (Isc) was unchanged. In 600Gd-fed female mice, there was a subgroup that demonstrated a significant increase in basal Isc (53.14±7.92 μA/cm(2), n=6, P<0.05) and a subgroup of nonresponders (12.05±6.59 μA/cm(2), n=4), compared to 0Gd controls (29.3±6.5 μA/cm(2), n=7). In R117H mice injected with 600Gi, basal Isc was unchanged in both male and female mice compared to 0Gi controls. Isc was measured in response to the following: the adenylate cyclase activator forskolin (10 μM, bilateral), bumetanide (100 μM, basolateral) to indicate the Cl(-) secretory component, and acetazolamide (100 μM, bilateral) to indicate the HCO3 (-) secretory component; however, there was no effect of genistein (diet or injection) on any of these parameters. Jejunal morphology (ie, villi length, number of goblet cells per villus, crypt depth, and number of goblet cells per crypt) in R117H mice suggested no genistein-mediated difference among the groups. Serum levels of genistein were significantly elevated, compared to respective controls, by either 600Gd (equally elevated in males and females) or 600Gi (elevated more in females versus males). These data suggest a sex-dependent increase in basal Isc of R117H mice and that the increase is also specific for route of administration.

Soluble adenylyl cyclase in the eye

Adenylyl cyclases (ACs) are a family of enzymes which convert ATP to cAMP, an essential intermediate in many signal transduction pathways. Of the 10 AC genes in man, 9 fall into the category of transmembrane ACs (tmACs), which associate with G-protein coupled receptors (GPCRs) and are activated by forskolin. The 10th AC, termed soluble AC (sAC) is neither activated by forskolin nor does it interact with GPCRs. Rather, sAC can be found in many compartments within the cell and is activated by bicarbonate. As such, sAC is considered a major sensor of bicarbonate in many tissues. The pathways involving sAC vary in different tissues and organ systems, and are as diverse as facilitating sperm capacitation and regulating pressure in the eye. The role of sAC in the eye has only recently begun to receive significant attention. Here we summarize what is known about the roles of sAC in the eye. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

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.

Oxygen in the regulation of intestinal epithelial transport

The transport of fluid, nutrients and electrolytes to and from the intestinal lumen is a primary function of epithelial cells. Normally, the intestine absorbs approximately 9 l of fluid and 1 kg of nutrients daily, driven by epithelial transport processes that consume large amounts of cellular energy and O2. The epithelium exists at the interface of the richly vascularised mucosa, and the anoxic luminal environment and this steep O2 gradient play a key role in determining the expression pattern of proteins involved in fluid, nutrient and electrolyte transport. However, the dynamic nature of the splanchnic circulation necessitates that the epithelium can evoke co-ordinated responses to fluctuations in O2 availability, which occur either as a part of the normal digestive process or as a consequence of several pathophysiological conditions. While it is known that hypoxia-responsive signals, such as reactive oxygen species, AMP-activated kinase, hypoxia-inducible factors, and prolyl hydroxylases are all important in regulating epithelial responses to altered O2 supply, our understanding of the molecular mechanisms involved is still limited. Here, we aim to review the current literature regarding the role that O2 plays in regulating intestinal transport processes and to highlight areas of research that still need to be addressed.

The cystic fibrosis intestine

The clinical manifestations of cystic fibrosis (CF) result from dysfunction of the cystic fibrosis transmembrane regulator protein (CFTR). The majority of people with CF have a limited life span as a consequence of CFTR dysfunction in the respiratory tract. However, CFTR dysfunction in the gastrointestinal (GI) tract occurs earlier in ontogeny and is present in all patients, regardless of genotype. The same pathophysiologic triad of obstruction, infection, and inflammation that causes disease in the airways also causes disease in the intestines. This article describes the effects of CFTR dysfunction on the intestinal tissues and the intraluminal environment. Mouse models of CF have greatly advanced our understanding of the GI manifestations of CF, which can be directly applied to understanding CF disease in humans.

Neurogenic mucosal bicarbonate secretion in guinea pig duodenum

BACKGROUND AND PURPOSE

To test a hypothesis that: (i) duodenal pH and osmolarity are individually controlled at constant set points by negative feedback control centred in the enteric nervous system (ENS); (ii) the purinergic P2Y(1) receptor subtype is expressed by non-cholinergic secretomotor/vasodilator neurons, which represent the final common excitatory pathway from the ENS to the bicarbonate secretory glands.

EXPERIMENTAL APPROACH

Ussing chamber and pH-stat methods investigated involvement of the P2Y(1) receptor in neurogenic stimulation of mucosal bicarbonate (HCO(3)(-)) secretion in guinea pig duodenum.

KEY RESULTS

ATP increased HCO(3)(-) secretion with an EC(50) of 160 nM. MRS2179, a selective P2Y(1) purinergic receptor antagonist, suppressed ATP-evoked HCO(3)(-) secretion by 47% and Cl(-) secretion by 63%. Enteric neuronal blockade by tetrodotoxin or exposure to a selective vasoactive intestinal peptide (VIP, VPAC(1)) receptor antagonist suppressed ATP-evoked HCO(3)(-) secretion by 61 and 41%, respectively, and Cl- by 97 and 70% respectively. Pretreatment with the muscarinic antagonist, scopolamine did not alter ATP-evoked HCO3(-) or Cl(-) secretion.

CONCLUSION AND IMPLICATIONS

Whereas acid directly stimulates the mucosa to release ATP and stimulate HCO(3)(-) secretion in a cytoprotective manner, neurogenically evoked HCO(3)(-) secretion accounts for feedback control of optimal luminal pH for digestion. ATP stimulates duodenal HCO(3)(-) secretion through an excitatory action at purinergic P2Y(1) receptors on neurons in the submucosal division of the ENS. Stimulation of the VIPergic non-cholinergic secretomotor/vasodilator neurons, which are one of three classes of secretomotor neurons, accounts for most, if not all, of the neurogenic secretory response evoked by ATP.

Normal mucus formation requires cAMP-dependent HCO3- secretion and Ca2+-mediated mucin exocytosis

  Evidence from the pathology in cystic fibrosis (CF) and recent results in vitro indicate that HCO3- is required for gel-forming mucins to form the mucus that protects epithelial surfaces. Mucus formation and release is a complex process that begins with an initial intracellular phase of synthesis, packaging and apical granule exocytosis that is followed by an extracellular phase of mucin swelling, transport and discharge into a lumen. Exactly where HCO3- becomes crucial in these processes is unknown, but we observed that in the presence of HCO3-, stimulating dissected segments of native mouse intestine with 5-hydroxytryptamine (5-HT) and prostaglandin E2 (PGE2) induced goblet cell exocytosis followed by normal mucin discharge in wild-type (WT) intestines. CF intestines that inherently lack cystic fibrosis transmembrane conductance regulator (CFTR)-dependent HCO3- secretion also demonstrated apparently normal goblet cell exocytosis, but in contrast, this was not followed by similar mucin discharge. Moreover, we found that even in the presence of HCO3-, when WT intestines were stimulated only with a Ca2+-mediated agonist (carbachol), exocytosis was followed by poor discharge as with CF intestines. However, when the Ca2+-mediated agonist was combined with a cAMP-mediated agonist (isoproterenol (isoprenaline) or vasoactive intestinal peptide) in the presence of HCO3- both normal exocytosis and normal discharge was observed. These results indicate that normal mucus formation requires concurrent activation of a Ca2+-mediated exocytosis of mucin granules and an independent cAMP-mediated, CFTR-dependent, HCO3- secretion that appears to mainly enhance the extracellular phases of mucus excretion.

Intestinal CFTR expression alleviates meconium ileus in cystic fibrosis pigs

Cystic fibrosis (CF) pigs develop disease with features remarkably similar to those in people with CF, including exocrine pancreatic destruction, focal biliary cirrhosis, micro-gallbladder, vas deferens loss, airway disease, and meconium ileus. Whereas meconium ileus occurs in 15% of babies with CF, the penetrance is 100% in newborn CF pigs. We hypothesized that transgenic expression of porcine CF transmembrane conductance regulator (pCFTR) cDNA under control of the intestinal fatty acid-binding protein (iFABP) promoter would alleviate the meconium ileus. We produced 5 CFTR-/-;TgFABP>pCFTR lines. In 3 lines, intestinal expression of CFTR at least partially restored CFTR-mediated anion transport and improved the intestinal phenotype. In contrast, these pigs still had pancreatic destruction, liver disease, and reduced weight gain, and within weeks of birth, they developed sinus and lung disease, the severity of which varied over time. These data indicate that expressing CFTR in intestine without pancreatic or hepatic correction is sufficient to rescue meconium ileus. Comparing CFTR expression in different lines revealed that approximately 20% of wild-type CFTR mRNA largely prevented meconium ileus. This model may be of value for understanding CF pathophysiology and testing new preventions and therapies.

Mucociliary transport in porcine trachea: differential effects of inhibiting chloride and bicarbonate secretion

This study was designed to assess the relative importance of Cl(-) and HCO(3)(-) secretion to mucociliary transport rate (MCT) in ex vivo porcine tracheas. MCT was measured in one group of tissues that was exposed to adventitial HCO(3)(-)-free solution while a parallel group was exposed to adventitial HCO(3)(-)-replete solution. After measurement of baseline MCT rates, acetylcholine (ACh) was added to stimulate submucosal gland mucous liquid secretion, and MCT rates were again measured. Before ACh addition, the mean MCT was higher in the HCO(3)(-)-free group (4.2 ± 0.9 mm/min) than in the HCO(3)(-)-replete group (2.3 ± 0.3 mm/min), but this difference was not statistically significant. ACh addition significantly increased MCT in both groups, but ACh-stimulated MCT was significantly lower in the HCO(3)(-)-free group (11.0 ± 1.5 mm/min) than in the HCO(3)(-)-replete group (17.0 ± 2.0 mm/min). A second series of experiments examined the effect on MCT of blocking Cl(-) secretion with 100 μM bumetanide. Before adding ACh, MCT in the bumetanide-treated group (1.0 ± 0.2 mm/min) was significantly lower than in the control group (3.8 ± 1.1 mm/min). ACh addition significantly increased MCT in both groups, but there was no significant difference between the bumetanide-treated group (21.4 ± 1.7 mm/min) and control group (19.5 ± 3.4 mm/min). These results indicate that ACh-stimulated MCT has greater dependence on HCO(3)(-) secretion, whereas the basal MCT rate has greater dependence on Cl(-) secretion.

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.

Basolateral chloride loading by the anion exchanger type 2: role in fluid secretion by the human airway epithelial cell line Calu-3

Anion exchanger type 2 (AE2 or SLC4A2) is an electroneutral Cl(-)/HCO(3)(-) exchanger expressed at the basolateral membrane of many epithelia. It is thought to participate in fluid secretion by airway epithelia. However, the role of AE2 in fluid secretion remains uncertain, due to the lack of specific pharmacological inhibitors, and because it is electrically silent and therefore does not contribute directly to short-circuit current (I(sc)). We have studied the role of AE2 in Cl(-) and fluid secretion by the airway epithelial cell line Calu-3. After confirming expression of its mRNA and protein, a knock-down cell line called AE2-KD was generated by lentivirus-mediated RNA interference in which AE2 mRNA and protein levels were reduced 90%. Suppressing AE2 increased the expression of the cystic fibrosis transmembrane conductance regulator (CFTR) by ∼70% without affecting the levels of NKCC1 (Na(+)-K(+)-2Cl(-) cotransporter) or NBCe1 (Na(+)-nHCO(3)(-) cotransporter). cAMP agonists stimulated fluid secretion by parental Calu-3 and scrambled shRNA cells >6.5-fold. In AE2-KD cells this response was reduced by ∼70%, and the secreted fluid exhibited elevated pH and [HCO(3)(-)] as compared with the control lines. Unstimulated equivalent short-circuit current (I(eq)) was elevated in AE2-KD cells, but the incremental response to forskolin was unaffected. The modest bumetanide-induced reductions in both I(eq) and fluid secretion were more pronounced in AE2-KD cells. Basolateral Cl(-)/HCO(3)(-) exchange measured by basolateral pH-stat in cells with permeabilized apical membranes was abolished in AE2-KD monolayers, and the intracellular alkalinization resulting from basolateral Cl(-) removal was reduced by ∼80% in AE2-KD cells. These results identify AE2 as a major pathway for basolateral Cl(-) loading during cAMP-stimulated secretion of Cl(-) and fluid by Calu-3 cells, and help explain the large bumetanide-insensitive component of fluid secretion reported previously in airway submucosal glands and some other epithelia.

Water absorption and bicarbonate secretion in the intestine of the sea bream are regulated by transmembrane and soluble adenylyl cyclase stimulation

In the marine fish intestine luminal, HCO₃⁻ can remove divalent ions (calcium and magnesium) by precipitation in the form of carbonate aggregates. The process of epithelial HCO₃⁻ secretion is under endocrine control, therefore, in this study we aimed to characterize the involvement of transmembrane (tmACs) and soluble (sACs) adenylyl cyclases on the regulation of bicarbonate secretion (BCS) and water absorption in the intestine of the sea bream (Sparus aurata). We observed that all sections of sea bream intestine are able to secrete bicarbonate as measured by pH-Stat in Ussing chambers. In addition, gut sac preparations reveal net water absorption in all segments of the intestine, with significantly higher absorption rates in the anterior intestine that in the rectum. BCS and water absorption are positively correlated in all regions of the sea bream intestinal tract. Furthermore, stimulation of tmACs (10 μM FK + 500 μM IBMX) causes a significant decrease in BCS, bulk water absorption and short circuit current (Isc) in a region dependent manner. In turn, stimulation of sACs with elevated HCO₃⁻ results in a significant increase in BCS, and bulk water absorption in the anterior intestine, an action completely reversed by the sAC inhibitor KH7 (200 μM). Overall, the results reveal a functional relationship between BCS and water absorption in marine fish intestine and modulation by tmACs and sAC. In light of the present observations, it is hypothesized that the endocrine effects on intestinal BCS and water absorption mediated by tmACs are locally and reciprocally modulated by the action of sACs in the fish enterocyte, thus fine-tuning the process of carbonate aggregate production in the intestinal lumen.

Functional Cftr in crypt epithelium of organotypic enteroid cultures from murine small intestine

Physiological studies of intact crypt epithelium have been limited by problems of accessibility in vivo and dedifferentiation in standard primary culture. Investigations of murine intestinal stem cells have recently yielded a primary intestinal culture in three-dimensional gel suspension that recapitulates crypt structure and epithelial differentiation (Sato T, Vries RG, Snippert HJ, van de Wetering M, Barker N, Stange DE, Van Es JH, Abo A, Kujala P, Peters PJ, Clevers H. Nature 459: 262-265, 2009). We investigated the utility of murine intestinal crypt cultures (termed "enteroids") for physiological studies of crypt epithelium by focusing on the transport activity of the cystic fibrosis transmembrane conductance regulator Cftr. Enteroids had multiple crypts with well-differentiated goblet and Paneth cells that degranulated on exposure to the muscarinic agonist carbachol. Modified growth medium provided a crypt proliferation rate, as measured by 5-ethynyl-2'-deoxyuridine labeling, which was similar to proliferation in vivo. Immunoblots demonstrated equivalent Cftr expression in comparisons of freshly isolated crypts with primary and passage 1 enteroids. Apparent enteroid differences in mRNA expression of other transporters were primarily associated with villous epithelial contamination of freshly isolated crypts. Microelectrode analysis revealed cAMP-stimulated membrane depolarization in enteroid epithelium from wild-type (WT) but not Cftr knockout (KO) mice. Morphological and microfluorimetric studies, respectively, demonstrated Cftr-dependent cell shrinkage and lower intracellular pH in WT enteroid epithelium in contrast to Cftr KO epithelium or WT epithelium treated with Cftr inhibitor 172. We conclude that crypt epithelium of murine enteroids exhibit Cftr expression and activity that recapitulates crypt epithelium in vivo. Enteroids provide a primary culture model that is suitable for physiological studies of regenerating crypt epithelium.

Stimulation of murine intestinal secretion by daily genistein injections: gender-dependent differences

BACKGROUND/AIMS

The effect of daily injections with genistein (naturally occurring phytoestrogen) on intestinal chloride (Cl(-)) secretion was measured with Ussing chamber short circuit current (I(sc), μA/cm(2)), in C57BL/6J male and female mice, using 600 mg/kg genistein/day (600G), 300 mg/kg genistein/day (300G), 150 mg/kg genistein/day (150G) or genistein-free vehicle control (0G) for 1- or 2-weeks.

METHODS AND RESULTS

Injecting with 600G elicited significant increases in basal I(sc) in females after 1-week (ñ70 μA/cm(2), n=15, p < 0.05) and in males after 2-weeks (ñ80 μA/cm(2), n=5, p < 0.05) compared to their 0G counterparts. Chloride-free ringer significantly reduced basal I(sc) by 65% in 600G males and 72% in 600G females, suggesting that Cl(-) was the major anion comprising the genistein-stimulated secretion. The forskolin-stimulated (10 μM) I(sc) was significantly inhibited by the CFTR chloride channel inhibitors, glibenclamide (500 μM) and CFTR(inh)-172 (100 μM) in 600G males and females, suggesting some contribution by genistein-dependent CFTR-mediated Cl(-) secretion. We found no associated changes in intestinal morphology, nor change in total CFTR protein with 600G. There was a 5% increase in apical/subapical ratio in 600G males compared to controls (no change in females).

CONCLUSION

These data suggest that male and female mice both exhibit increased Cl- secretion with 600G, however, the mechanisms mediating this are gender-dependent.

Functional activity of Pat-1 (Slc26a6) Cl(−)/HCO₃(−) exchange in the lower villus epithelium of murine duodenum

AIMS

The apical membrane anion exchanger putative anion transporter-1 (Pat-1) is expressed at significant levels in the lower villus epithelium of murine duodenum. However, previous studies of Cl(−)/HCO₃(−) exchange in the lower villus have failed to demonstrate Pat-1 function. Those studies routinely included luminal glucose which induces Na(+) -coupled glucose transport and acidifies the villus epithelium. Since Pat-1 has been proposed to be an electrogenic 1Cl(−)/2HCO₃(−) exchanger, membrane depolarization or cell acidification during glucose transport may obscure Pat-1 activity. Therefore, we investigated the effects of luminal glucose on Cl(−)(IN)/HCO₃(−) (OUT) exchange activity in the lower villus epithelium.

METHODS

Cl(−)(IN) /HCO (−) (OUT) exchange of villus epithelium in duodenal mucosa from Pat-1 knockout (KO), Slc26a3 [down-regulated in adenoma (Dra)] KO, cystic fibrosis transmembrane conductance regulator (Cftr) KO and wild-type (WT) littermate mice was measured using the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein. Short-circuit current (I(sc) ) was measured in Ussing chambers.

RESULTS

During glucose absorption, Cl(−)(IN)/HCO₃(−) (OUT) exchange in the lower villus epithelium was abolished in the Dra KO and unaffected in the Pat-1 KO relative to WT. However, during electroneutral mannose absorption or electrogenic α-D-methyl glucoside absorption, Cl(−)(IN) /HCO₃(−) (OUT) exchange was reduced in both Pat-1 KO and Dra KO villi. Exposure to high [K(+)] abolished Cl(−)(IN) /HCO₃(−) (OUT) exchange in the Dra KO but not the Dra/Cftr double KO epithelium, suggesting that Pat-1 activity is little affected by membrane depolarization except in the presence of Cftr.

CONCLUSIONS

The metabolic and electrogenic activity of glucose transport obscures Cl(−)(IN) /HCO₃(−) (OUT) exchange activity of Pat-1 in the lower villus. The inhibitory effects of membrane depolarization on Pat-1 Cl(−)(IN) /HCO₃(−) (OUT) exchange may require concurrent membrane association with Cftr.

Purinergic regulation of duodenal surface pH and ATP concentration: implications for mucosal defence, lipid uptake and cystic fibrosis

The duodenum secretes HCO₃⁻ as part of a multi-layered series of defence mechanisms against damage from luminal acid. In the 1980s, an alkaline surface layer was measured over the mucosa which correlated with the rate of HCO₃⁻ secretion. As all biological processes are regulated, we investigated how the alkaline pH of the surface layer was maintained. As the ecto-phosphorylase alkaline phosphatase (AP) is highly expressed in the duodenal brush border, we hypothesized that its extreme alkaline pH optimum (∼pH 8-9) combined with its ability to hydrolyse regulatory purines such as ATP was part of an ecto-purinergic signalling system, consisting also of brush border P2Y receptors and cystic fibrosis transmembrane regulator-mediated HCO₃⁻ secretion. Extracellular ATP increases the rate of HCO₃⁻ secretion through this purinergic system. At high surface pH (pH(s)), AP activity is increased, which then increases the rate of ATP hydrolysis, decreasing surface ATP concentration ([ATP](s)), with a resultant decrease in the rate of HCO₃⁻ secretion, which subsequently decreases pH(s) . This feedback loop is thus hypothesized to regulate pH(s) over the duodenal mucosa, and in several other HCO₃⁻ secretory organs. As AP activity is directly related to pH(s) , and as AP hydrolyses ATP, [ATP](s) and pH(s) are co-regulated. As many essential tissue functions such as ciliary motility and lipid uptake are dependent on [ATP](s) , dysregulation of pH(s) and [ATP](s) may help explain the tissue dysfunction characteristic of diseases such as cystic fibrosis.

Role of epithelial HCO3⁻ transport in mucin secretion: lessons from cystic fibrosis

The invitation to present the 2010 Hans Ussing lecture for the Epithelial Transport Group of the American Physiological Society offered me a unique, special, and very surprising opportunity to join in saluting a man whom I met only once, but whose work was the basis, not only for my career, but also for finding the molecular defect in the inherited disease cystic fibrosis (CF). In this context, I will venture to make the tribute with a new explanation of why a mutation in a single gene that codes for an anion channel can cause devastation of multiple epithelial systems with pathogenic mucus. In so doing, I hope to raise awareness of a new role for that peculiar anion around which so much physiology revolves, HCO(3)(-). I begin by introducing CF pathology as I question the name of the disease as well as the prevalent view of the basis of its pathology by considering: 1) mucus, 2) salt, and 3) HCO(3)(-). I then present recent data showing that HCO(3)(-) is required for normal mucus discharge, and I will close with conjecture as to how HCO(3)(-) may support mucus discharge and why the failure to transport this electrolyte is pathogenic in CF.

A new role for bicarbonate secretion in cervico-uterine mucus release

Cervical mucus thinning and release during the female reproductive cycle is thought to rely mainly on fluid secretion. However, we now find that mucus released from the murine reproductive tract critically depends upon concurrent bicarbonate (HCO(3)(-)) secretion. Prostaglandin E(2) (PGE(2))- and carbachol-stimulated mucus release was severely inhibited in the absence of serosal HCO(3)(-), HCO(3)(-) transport, or functional cystic fibrosis transmembrane conductance regulator (CFTR). In contrast to mucus release, PGE(2)- and carbachol-stimulated fluid secretion was not dependent on bicarbonate or on CFTR, but was completely blocked by niflumic acid. We found stimulated mucus release was severely impaired in the cystic fibrosis F508 reproductive tract, even though stimulated fluid secretion was preserved. Thus, CFTR mutations and/or poor bicarbonate secretion may be associated with reduced female fertility associated with abnormal mucus and specifically, may account for the increased viscosity and lack of cyclical changes in cervical mucus long noted in women with cystic fibrosis.

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.

AE2 Cl-/HCO3- exchanger is required for normal cAMP-stimulated anion secretion in murine proximal colon

Anion secretion by colonic epithelium is dependent on apical CFTR-mediated anion conductance and basolateral ion transport. In many tissues, the NKCC1 Na(+)-K(+)-2Cl(-) cotransporter mediates basolateral Cl(-) uptake. However, additional evidence suggests that the AE2 Cl(-)/HCO(3)(-) exchanger, when coupled with the NHE1 Na(+)/H(+) exchanger or a Na(+)-HCO(3)(-) cotransporter (NBC), contributes to HCO(3)(-) and/or Cl(-) uptake. To analyze the secretory functions of AE2 in proximal colon, short-circuit current (I(sc)) responses to cAMP and inhibitors of basolateral anion transporters were measured in muscle-stripped wild-type (WT) and AE2-null (AE2(-/-)) proximal colon. In physiological Ringer, the magnitude of cAMP-stimulated I(sc) was the same in WT and AE2(-/-) colon. However, the I(sc) response in AE2(-/-) colon exhibited increased sensitivity to the NKCC1 inhibitor bumetanide and decreased sensitivity to the distilbene derivative SITS (which inhibits AE2 and some NBCs), indicating that loss of AE2 results in a switch to increased NKCC1-supported anion secretion. Removal of HCO(3)(-) resulted in robust cAMP-stimulated I(sc) in both AE2(-/-) and WT colon that was largely mediated by NKCC1, whereas removal of Cl(-) resulted in sharply decreased cAMP-stimulated I(sc) in AE2(-/-) colon relative to WT controls. Inhibition of NHE1 had no effect on cAMP-stimulated I(sc) in AE2(-/-) colon but caused a switch to NKCC1-supported secretion in WT colon. Thus, in AE2(-/-) colon, Cl(-) secretion supported by basolateral NKCC1 is enhanced, whereas HCO(3)(-) secretion is diminished. These results show that AE2 is a component of the basolateral ion transport mechanisms that support anion secretion in the proximal colon.

Knockout mouse models for intestinal electrolyte transporters and regulatory PDZ adaptors: new insights into cystic fibrosis, secretory diarrhoea and fructose-induced hypertension

Knockout mouse models have provided key insights into the physiological significance of many intestinal electrolyte transporters. This review has selected three examples to highlight the importance of knockout mouse technology in unravelling complex regulatory relationships important for the understanding of human diseases. Genetic ablation of the cystic fibrosis transmembrane conductance regulator (CFTR) has created one of the most useful mouse models for understanding intestinal transport. Recent work has provided an understanding of the key role of the CFTR anion channel in the regulation of HCO(3)(-) secretion, and the important consequences that a defect in HCO(3)(-) output may have on the viscoelastic properties of mucus, on lipid absorption and on male and female reproductive function. The regulation of CFTR activity, and also that of the intestinal salt absorptive transporter NHE3, occurs via the formation of PSD95-Drosophila homologue Discs-large-tight junction protein ZO-1 (PDZ) adaptor protein-mediated multiprotein complexes. The recent generation of knockout mice for three members of the sodium-hydrogen regulatory factor (NHERF) family of PDZ adaptor proteins, namely NHERF1 (EBP50), NHERF2 (E3KARP) and NHERF3 (PDZK1), has helped to explain why NHERF1 is essential for both normal and mutant CFTR function. In addition, they have provided new insight into the molecular mechanisms of secretory diarrhoeas. Genetic ablation of members of the recently discovered Slc26 anion transporter gene family not only reproduced the phenotype of the genetic diseases that led to the discovery of the gene family, but also resulted in new insights into complex human diseases such as secretory diarrhoea, fructose-induced hypertension and urolithiasis.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Diarrhea as a cause of mortality in a mouse model of infectious colitis

Analysis of gene expression in the colons of Citrobacter rodentium-infected susceptible and resistant mice suggests that mortality is associated with impaired intestinal ion transport.

Background

Comparative characterization of genome-wide transcriptional changes during infection can help elucidate the mechanisms underlying host susceptibility. In this study, transcriptional profiling of the mouse colon was carried out in two cognate lines of mice that differ in their response to Citrobacter rodentium infection; susceptible inbred FVB/N and resistant outbred Swiss Webster mice. Gene expression in the distal colon was determined prior to infection, and at four and nine days post-inoculation using a whole mouse genome Affymetrix array.

Results

Computational analysis identified 462 probe sets more than 2-fold differentially expressed between uninoculated resistant and susceptible mice. In response to C. rodentium infection, 5,123 probe sets were differentially expressed in one or both lines of mice. Microarray data were validated by quantitative real-time RT-PCR for 35 selected genes and were found to have a 94% concordance rate. Transcripts represented by 1,547 probe sets were differentially expressed between susceptible and resistant mice regardless of infection status, a host effect. Genes associated with transport were over-represented to a greater extent than even immune response-related genes. Electrolyte analysis revealed reduction in serum levels of chloride and sodium in susceptible animals.

Conclusion

The results support the hypothesis that mortality in C. rodentium-infected susceptible mice is associated with impaired intestinal ion transport and development of fatal fluid loss and dehydration. These studies contribute to our understanding of the pathogenesis of C. rodentium and suggest novel strategies for the prevention and treatment of diarrhea associated with intestinal bacterial infections.

Platelet-activating factor-induced chloride channel activation is associated with intracellular acidosis and apoptosis of intestinal epithelial cells

Platelet-activating factor (PAF) is a phospholipid inter- and intracellular mediator implicated in intestinal injury primarily via induction of an inflammatory cascade. We find that PAF also has direct pathological effects on intestinal epithelial cells (IEC). PAF induces Cl(-) channel activation, which is associated with intracellular acidosis and apoptosis. Using the rat small IEC line IEC-6, electrophysiological experiments demonstrated that PAF induces Cl(-) channel activation. This PAF-activated Cl(-) current was inhibited by Ca(2+) chelation and a calcium calmodulin kinase II inhibitor, suggesting PAF activation of a Ca(2+)-activated Cl(-) channel. To determine the pathological consequences of Cl(-) channel activation, microfluorimetry experiments were performed, which revealed PAF-induced intracellular acidosis, which is also inhibited by the Cl(-) channel inhibitor 4,4'diisothiocyanostilbene-2,2'disulfonic acid and Ca(2+) chelation. PAF-induced intracellular acidosis is associated with caspase 3 activation and DNA fragmentation. PAF-induced caspase activation was abolished in cells transfected with a pH compensatory Na/H exchanger construct to enhance H(+) extruding ability and prevent intracellular acidosis. As ClC-3 is a known intestinal Cl(-) channel dependent on both Ca(2+) and calcium calmodulin kinase II phosphorylation, we generated ClC-3 knockdown cells using short hairpin RNA. PAF induced Cl(-) current; acidosis and apoptosis were all significantly decreased in ClC-3 knockdown cells. Our data suggest a novel mechanism of PAF-induced injury by which PAF induces intracellular acidosis via activation of the Ca(2+)-dependent Cl(-) channel ClC-3, resulting in apoptosis of IEC.

Duodenal brush border intestinal alkaline phosphatase activity affects bicarbonate secretion in rats

We hypothesized that duodenal HCO(3)(-) secretion alkalinizes the microclimate surrounding intestinal alkaline phosphatase (IAP), increasing its activity. We measured AP activity in rat duodenum in situ in frozen sections with the fluorogenic substrate ELF-97 phosphate and measured duodenal HCO(3)(-) secretion with a pH-stat in perfused duodenal loops. We examined the effects of the IAP inhibitors L-cysteine or L-phenylalanine (0.1-10 mM) or the tissue nonspecific AP inhibitor levamisole (0.1-10 mM) on AP activity in vitro and on acid-induced duodenal HCO(3)(-) secretion in vivo. AP activity was the highest in the duodenal brush border, decreasing longitudinally to the large intestine with no activity in stomach. Villous surface AP activity measured in vivo was enhanced by PGE(2) intravenously and inhibited by luminal L-cysteine. Furthermore, incubation with a pH 2.2 solution reduced AP activity in vivo, whereas pretreatment with the cystic fibrosis transmembrane regulator (CFTR) inhibitor CFTR(inh)-172 abolished AP activity at pH 2.2. L-Cysteine and L-phenylalanine enhanced acid-augmented duodenal HCO(3)(-) secretion. The nonselective P2 receptor antagonist suramin (1 mM) reduced acid-induced HCO(3)(-) secretion. Moreover, L-cysteine or the competitive AP inhibitor glycerol phosphate (10 mM) increased HCO(3)(-) secretion, inhibited by suramin. In conclusion, enhancement of the duodenal HCO(3)(-) secretory rate increased AP activity, whereas inhibition of AP activity increased the HCO(3)(-) secretory rate. These data support our hypothesis that HCO(3)(-) secretion increases AP activity by increasing local pH at its catalytic site and that AP hydrolyzes endogenous luminal phosphates, presumably ATP, which increases HCO(3)(-) secretion via activation of P2 receptors.

Duodenal bicarbonate secretion in rats: stimulation by intra-arterial and luminal guanylin and uroguanylin

AIM

Uroguanylin and guanylin are endogenous ligands for guanylate cyclase C, an upstream regulator of the cystic fibrosis transmembrane resistance (CFTR) anion channel, and both peptides increase intestinal anion export in vitro. We have compared the effects of close intra-arterial and luminal administration of uroguanylin and guanylin on duodenal bicarbonate secretion in vivo and studied the interactions with melatonin and cholinergic stimulation.

METHODS

Lewis x Dark Agouti rats were anaesthetized and a segment of the proximal duodenum with intact blood supply was cannulated in situ. Mucosal bicarbonate secretion (pH stat) was continuously recorded and peptides were infused intra-arterially or added to the luminal perfusate.

RESULTS

Intra-arterial (50-1000 pmol kg(-1) h(-1)) as well as luminal administration (50-500 nmol L(-1)) of guanylin or uroguanylin caused dose-dependent increases in the duodenal secretion. Luminal administration induced more rapidly appearing rises in secretion and the two peptides induced secretory responses of similar shape and magnitude. The melatonin MT(2)-selective antagonist luzindole (600 nmol kg(-1)) significantly depressed the response to intra-arterial guanylins but did not affect secretion induced by luminal guanylins. Similarly, the muscarinic antagonist atropine (0.75 micromol kg(-1) followed by 0.15 micromol kg(-1) h(-1)) abolished the response to intra-arterial uroguanylin but caused only slight suppression of the response to luminal uroguanylin.

CONCLUSIONS

Intra-arterial as well as luminal uroguanylin and guanylin are potent stimuli of duodenal mucosal bicarbonate secretion in vivo. The response to luminal guanylins reflects an action at apical receptors. Stimulation by parenteral guanylins, in contrast, is under cholinergic influence and interacts with melatonin produced by mucosal enteroendocrine cells.

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.

Slc26a6 regulates CFTR activity in vivo to determine pancreatic duct HCO3- secretion: relevance to cystic fibrosis

Fluid and HCO(3)(-) secretion are vital functions of the pancreatic duct and other secretory epithelia. CFTR and Cl(-)/HCO(3)(-) exchange activity at the luminal membrane are required for these functions. The molecular identity of the Cl(-)/HCO(3)(-) exchangers and their relationship with CFTR in determining fluid and HCO(3)(-) secretion are not known. We show here that the Cl(-)/HCO(3)(-) exchanger slc26a6 controls CFTR activity and ductal fluid and HCO(3)(-) secretion. Unexpectedly, deletion of slc26a6 in mice and measurement of fluid and HCO(3)(-) secretion into sealed intralobular pancreatic ducts revealed that deletion of slc26a6 enhanced spontaneous and decreased stimulated secretion. Remarkably, inhibition of CFTR activity with CFTR(inh)-172, knock-down of CFTR by siRNA and measurement of CFTR current in WT and slc26a6(-/-) duct cells revealed that deletion of slc26a6 resulted in dis-regulation of CFTR activity by removal of tonic inhibition of CFTR by slc26a6. These findings reveal the intricate regulation of CFTR activity by slc26a6 in both the resting and stimulated states and the essential role of slc26a6 in pancreatic HCO(3)(-) secretion in vivo.

Ouabain-sensitive bicarbonate secretion and acid absorption by the marine teleost fish intestine play a role in osmoregulation

The gulf toadfish (Opsanus beta) intestine secretes base mainly in the form of HCO3- via apical anion exchange to serve Cl- and water absorption for osmoregulatory purposes. Luminal HCO3- secretion rates measured by pH-stat techniques in Ussing chambers rely on oxidative energy metabolism and are highly temperature sensitive. At 25 degrees C under in vivo-like conditions, secretion rates averaged 0.45 micromol x cm(-2) x h(-1), of which 0.25 micromol x cm(-2) x h(-1) can be accounted for by hydration of endogenous CO2 partly catalyzed by carbonic anhydrase. Complete polarity of secretion of HCO3- and H+ arising from the CO2 hydration reaction is evident from equal rates of luminal HCO3- secretion via anion exchange and basolateral H+ extrusion. When basolateral H+ extrusion is partly inhibited by reduction of serosal pH, luminal HCO3- secretion is reduced. Basolateral H+ secretion occurs in exchange for Na+ via an ethylisopropylamiloride-insensitive mechanism and is ultimately fueled by the activity of the basolateral Na+-K+-ATPase. Fluid absorption by the toadfish intestine to oppose diffusive water loss to the concentrated marine environment is accompanied by a substantial basolateral H+ extrusion, intimately linking osmoregulation and acid-base balance.

Lysophosphatidic acid inhibits cholera toxin-induced secretory diarrhea through CFTR-dependent protein interactions

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells that line the airway, gut, and exocrine glands; it is well established that CFTR plays a pivotal role in cholera toxin (CTX)-induced secretory diarrhea. Lysophosphatidic acid (LPA), a naturally occurring phospholipid present in blood and foods, has been reported to play a vital role in a variety of conditions involving gastrointestinal wound repair, apoptosis, inflammatory bowel disease, and diarrhea. Here we show, for the first time, that type 2 LPA receptors (LPA₂) are expressed at the apical surface of intestinal epithelial cells, where they form a macromolecular complex with Na⁺/H⁺ exchanger regulatory factor–2 and CFTR through a PSD95/Dlg/ZO-1–based interaction. LPA inhibited CFTR-dependent iodide efflux through LPA₂-mediated G_i pathway, and LPA inhibited CFTR-mediated short-circuit currents in a compartmentalized fashion. CFTR-dependent intestinal fluid secretion induced by CTX in mice was reduced substantially by LPA administration; disruption of this complex using a cell-permeant LPA₂-specific peptide reversed LPA₂-mediated inhibition. Thus, LPA-rich foods may represent an alternative method of treating certain forms of diarrhea.

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

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

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

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

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

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

Innate host defense of the lung: effects of lung-lining fluid pH

Lung-lining fluid (LLF) is a primary constituent of the pulmonary host defense system. It is distributed continuously throughout the respiratory tract but is heterogeneous regarding its chemistry and physiology between the conducting airways and alveoli. The conducting airways are lined with airway surface liquid (ASL), a mucus gel-aqueous sol complex that interacts functionally with epithelial cilia as the mucociliary escalator. The alveoli are lined with alveolar subphase fluid (AVSF) and pulmonary surfactant. AVSF sterility is maintained in part by the phagocytic activity of resident alveolar macrophages. Normal ASL and AVSF are both more acidic than blood plasma. However, the details of acid-base regulation differ between the two media. Appreciable transepithelial acid-base flux is possible across the airway epithelium, whereas the alveolar epithelium is relatively impermeable to transepithelial acid-base flux. Moreover, one must consider the influence of resident macrophages on AVSF pH. Resident macrophages occupy a sizable fraction of AVSF by volume and are a substantial source of metabolic H+. The buffering capacities of ASL and AVSF probably are largely due to secreted peptides (e.g., ASL mucins and AVSF surfactant proteins). Acid-base exchange between the extracellular hydrophase and intracellular buffering systems of resident macrophages represents an additional buffer pool for AVSF. The pH of ASL and AVSF can be depressed by disease or inflammation. Low pH is predicted to suppress microbe clearance from the airways and alveoli, increase pathogen survival in both regions, and alter mediator release by resident macrophages and recruited leukocytes thereby increasing the propensity for bystander cell injury. Overall, ASL/AVSF pH is expected to be a major determinant of lung host defense responses.

Tetramethylpyrazine stimulates cystic fibrosis transmembrane conductance regulator-mediated anion secretion in distal colon of rodents

AIM: To investigate the effect of tetramethylpyrazine (TMP), an active compound from Ligustium Wollichii Franchat, on electrolyte transport across the distal colon of rodents and the mechanism involved

METHODS: The short-circuit current (I_SC) technique in conjunction with pharmacological agents and specific inhibitors were used in analyzing the electrolyte transport across the distal colon of rodents. The underlying cellular signaling mechanism was investigated by radioimmunoassay analysis (RIA) and a special mouse model of cystic fibrosis.

RESULTS: TMP stimulated a concentration-dependent rise in I_SC, which was dependent on both Cl^- and HCO₃^-, and inhibited by apical application of diphenylamine-2,2’-dicarboxylic acid (DPC) and glibenclamide, but resistant to 4,4’-diisothiocyanatostilbene-2,2’-disulfonic acid disodium salt hydrate (DIDS). Removal of Na⁺ from basolateral solution almost completely abolished the I_SC response to TMP, but it was insensitive to apical Na⁺ replacement or apical Na⁺ channel blocker, amiloride. Pretreatment of colonic mucosa with BAPTA-AM, a membrane-permeable selective Ca²⁺ chelator, did not significantly alter the TMP-induced I_SC. No additive effect of forskolin and 3-isobutyl-1-methylxanthine (IBMX) was observed on the TMP-induced I_SC, but it was significantly reduced by a protein kinase A inhibitor, H₈₉. RIA results showed that TMP (1 mmol/L) elicited a significant increase in cellular cAMP production, which was similar to that elicited by the adenylate cyclase activator, forskolin (10 µmol/L). The TMP-elicited I_SC as well as forskolin- or IBMX-induced I_SC were abolished in mice with homozygous mutation of the cystic fibrosis transmembrane conductance regulator (CFTR) presenting defective CFTR functions and secretions.

CONCLUSION: TMP may stimulate cAMP-dependent and CFTR-mediated Cl^- and HCO₃^- secretion. This may have implications in the future development of alternative treatment for constipation.

Role of short-chain fatty acids in colonic HCO(3) secretion

Luminal isobutyrate, a relatively poor metabolized short-chain fatty acid (SCFA), induces HCO(3) secretion via a Cl-independent, DIDS-insensitive, carrier-mediated process as well as inhibiting both Cl-dependent and cAMP-induced HCO(3) secretion. The mechanism(s) responsible for these processes have not been well characterized. HCO(3) secretion was measured in isolated colonic mucosa mounted in Lucite chambers using pH stat technique and during microperfusion of isolated colonic crypts. (14)C-labeled butyrate, (14)C-labeled isobutyrate, and (36)Cl uptake were also determined by apical membrane vesicles (AMV) isolated from surface and/or crypt cells. Butyrate stimulation of Cl-independent, DIDS-insensitive 5-nitro-3-(3-phenylpropyl-amino)benzoic acid-insensitive HCO(3) secretion is greater than that by isobutyrate, suggesting that both SCFA transport and metabolism are critical for HCO(3) secretion. Both lumen and serosal 25 mM butyrate inhibit cAMP-induced HCO(3) secretion to a comparable degree (98 vs. 90%). In contrast, Cl-dependent HCO(3) secretion is downregulated by lumen 25 mM butyrate considerably more than by serosal butyrate (98 vs. 37%). Butyrate did not induce HCO(3) secretion in isolated microperfused crypts, whereas an outward-directed HCO(3) gradient-driven induced (14)C-butyrate uptake by surface but not crypt cell AMV. Both (36)Cl/HCO(3) exchange and potential-dependent (36)Cl movement in AMV were inhibited by 96-98% by 20 mM butyrate. We conclude that 1) SCFA-dependent HCO(3) secretion is the result of SCFA transport across the apical membrane via a SCFA/HCO(3) exchange more than intracellular SCFA metabolism; 2) SCFA-dependent HCO(3) secretion is most likely a result of an apical membrane SCFA/HCO(3) exchange in surface epithelial cells; 3) SCFA downregulates Cl-dependent and cAMP-induced HCO(3) secretion secondary to SCFA inhibition of apical membrane Cl/HCO(3) exchange and anion channel activity, respectively.

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.

Potential genetic modifiers of the cystic fibrosis intestinal inflammatory phenotype on mouse chromosomes 1, 9, and 10

Background

Although cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, the severity of disease is highly variable indicating the influence of modifier genes. The intestines of Cftr deficient mice (CF mice: Cftr^tm1Unc) are prone to obstruction by excessive mucus accumulation and are used as a model of meconium ileus and distal intestinal obstruction syndrome. This phenotype is strongly dependent on the genetic background of the mice. On the C57Bl/6 background, the majority of CF mice cannot survive on solid mouse chow, have inflammation of the small intestine, and are about 30% smaller than wild type littermates. In this work potential modifier loci of the CF intestinal phenotype were identified.

Results

CF mice on a mixed genetic background (95% C57Bl/6 and 5% 129Sv) were compared to CF mice congenic on the C57Bl/6 background for several parameters of the intestinal CF phenotype. CF mice on the mixed background exhibit significantly greater survival when fed dry mouse chow, have reduced intestinal inflammation as measured by quantitative RT-PCR for marker genes, have near normal body weight gain, and have reduced mucus accumulation in the intestinal crypts. There was an indication of a gender effect for body weight gain: males did not show a significant improvement at 4 weeks of age, but were of normal weight at 8 weeks, while females showed improvement at both 4 and 8 weeks. By a preliminary genome-wide PCR allele scanning, three regions were found to be potentially associated with the milder phenotype. One on chr.1, defined by marker D1Mit36, one on chr. 9 defined by marker D9Mit90, and one on chr. 10, defined by marker D10Mit14.

Conclusion

Potential modifier regions were found that have a positive impact on the inflammatory phenotype of the CF mouse small intestine and animal survival. Identification of polymorphisms in specific genes in these regions should provide important new information about genetic modifiers of the CF intestinal phenotype.

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(+).

The duodenal mucosal bicarbonate secretion

The duodenal lumen is exposed to aggressive factors with a high potential to cause damage to the mucosa. Bicarbonate secretion by the duodenal mucosa is accepted as the primary important defense mechanism against the hydrochloric acid intermittently expelled from the stomach. The present work concerns both the influence of the central nervous system and the effects of the hormone melatonin on duodenal bicarbonate secretion in anesthetized rats in vivo as well as effects of melatonin on intracellular calcium signaling by duodenal enterocyte in vitro examined in tissues of both human and rat origin. The main findings were as follows: Melatonin is a potent stimulant of duodenal mucosal bicarbonate secretion and also seems to be involved in the acid-induced stimulation of the secretion. Stimulation elicited in the central nervous system by the alpha1-adrenoceptor agonist phenylephrine induced release of melatonin from the intestinal mucosa and a four-fold increase in alkaline secretion. The melatonin antagonist luzindole abolished the duodenal secretory response to administered melatonin and to central nervous phenylephrine but did not influence the release of intestinal melatonin. Central nervous stimulation was also abolished by synchronous ligation of the vagal trunks and the sympathetic chains at the sub-laryngeal level. Melatonin induced release of calcium from intracellular stores and also influx of extracellular calcium in isolated duodenal enterocytes. Enterocytes in clusters functioned as a syncytium. Overnight fasting rapidly and profoundly down-regulated the responses to the duodenal secretagogue orexin-A and the muscarinic agonist bethanechol but not those to melatonin or vasoactive intestinal polypeptide.

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.