Regulation of DRA and AE1 in rat colon by dietary Na depletion

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

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.

Effects of Dietary Energy Levels on Rumen Fermentation, Microbial Diversity, and Feed Efficiency of Yaks (Bos grunniens)

The microbial community of the yak (Bos grunniens) rumen plays an important role in surviving the harsh Tibetan environment where seasonal dynamic changes in pasture cause nutrient supply imbalances, resulting in weight loss in yaks during the cold season. A better understanding of rumen microbiota under different feeding regimes is critical for exploiting the microbiota to enhance feed efficiency and growth performance. This study explored the impact of different dietary energy levels on feed efficiency, rumen fermentation, bacterial community, and abundance of volatile fatty acid (VFA) transporter transcripts in the rumen epithelium of yaks. Fifteen healthy castrated male yaks were divided into three groups and fed with low (YL), medium (YM), and high energy (YH) levels diet having different NEg of 5.5, 6.2, and 6.9 MJ/kg, respectively. The increase in feed efficiency was recorded with an increase in dietary energy levels. The increase in dietary energy levels decreased the pH and increased the concentrations of acetate, propionate, butyrate, and valerate in yak rumens. The increase in the mRNA abundance of VFA transporter genes (MCT1, DRA, PAT1, and AE2) in the rumen epithelium of yaks was recorded as dietary energy level increased. High relative abundances of Firmicutes and Bacteroidetes were recorded with the increase in dietary energy levels. Significant population shifts at the genus level were recorded among the three treatments. This study provides new insights into the dietary energy-derived variations in rumen microbial community.

Involvement of butyrate in electrogenic K+ secretion in rat rectal colon

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are synthesized from dietary carbohydrates by colonic bacterial fermentation. These SCFAs supply energy, suppress cancer, and affect ion transport. However, their roles in ion transport and regulation in the intracellular environment remain unknown. In order to elucidate the roles of SCFAs, we measured short-circuit currents (I_SC) and performed RT-PCR and immunohistochemical analyses of ion transporters in rat rectal colon. The application of 30 mM butyrate shifted I_SC in a negative direction, but did not attenuate the activity of epithelial Na⁺ channels (ENaC). The application of bumetanide, a Na⁺-K⁺-2Cl⁻ cotransporter inhibitor, to the basolateral side reduced the negative I_SC shift induced by butyrate. The application of XE991, a KCNQ-type K⁺ channel inhibitor, to the apical side decreased the I_SC shift induced by butyrate in a dose-dependent manner. The I_SC shift was independent of HCO₃⁻ and insensitive to ibuprofen, an SMCT1 inhibitor. The mucosa from rat rectal colon expressed mRNAs of H⁺-coupled monocarboxylate transporters (MCT1, MCT4, and MCT5, also referred to as SLC16A1, SLC16A3, and SLC16A4, respectively). RT-PCR and immunofluorescence analyses demonstrated that KCNQ2 and KCNQ4 localized to the apical membrane of surface cells in rat rectal colon. These results indicate that butyrate, which may be transported by H⁺-coupled monocarboxylate transporters, activates K⁺ secretion through KCNQ-type K⁺ channels on the apical membrane in rat rectal colon. KCNQ-type K⁺ channels may play a role in intestinal secretion and defense mechanisms in the gastrointestinal tract.

Effects of acid-base variables and the role of carbonic anhydrase on oxalate secretion by the mouse intestine in vitro

Hyperoxaluria is a major risk factor for calcium oxalate kidney stones and the intestine is recognized as an important extra-renal pathway for eliminating oxalate. The membrane-bound chloride/bicarbonate (Cl(-)/) exchangers are involved in the transcellular movement of oxalate, but little is understood about how they might be regulated. , CO2, and pH are established modulators of intestinal NaCl cotransport, involving Na(+)/H(+) and Cl(-)/ exchange, but their influence on oxalate transport is unknown. Measuring (14)C-oxalate and (36)Cl fluxes across isolated, short-circuited segments of the mouse distal ileum and distal colon we examined the role of these acid-base variables and carbonic anhydrase (CA) in oxalate and Cl(-) transport. In standard buffer both segments performed net oxalate secretion (and Cl(-) absorption), but only the colon, and the secretory pathway were responsive to and CO2. Ethoxzolamide abolished net oxalate secretion by the distal colon, and when used in tandem with an impermeant CA inhibitor, signaled an intracellular CA isozyme was required for secretion. There was a clear dependence on as their removal eliminated secretion, while at 42 mmol/L was also decreased and eradicated. Independent of pH, raising Pco2 from 28 to 64 mmHg acutely stimulated net oxalate secretion 41%. In summary, oxalate secretion by the distal colon was dependent on , CA and specifically modulated by CO2, whereas the ileum was remarkably unresponsive. These findings highlight the distinct segmental heterogeneity along the intestine, providing new insights into the oxalate transport mechanism and how it might be regulated.

The SLC16A family of monocarboxylate transporters (MCTs)--physiology and function in cellular metabolism, pH homeostasis, and fluid transport

The SLC16A family of monocarboxylate transporters (MCTs) is composed of 14 members. MCT1 through MCT4 (MCTs 1-4) are H(+)-coupled monocarboxylate transporters, MCT8 and MCT10 transport thyroid hormone and aromatic amino acids, while the substrate specificity and function of other MCTs have yet to be determined. The focus of this review is on MCTs 1-4 because their role in lactate transport is intrinsically linked to cellular metabolism in various biological systems, including skeletal muscle, brain, retina, and testis. Although MCTs 1-4 all transport lactate, they differ in their transport kinetics and vary in tissue and subcellular distribution, where they facilitate "lactate-shuttling" between glycolytic and oxidative cells within tissues and across blood-tissue barriers. However, the role of MCTs 1-4 is not confined to cellular metabolism. By interacting with bicarbonate transport proteins and carbonic anhydrases, MCTs participate in the regulation of pH homeostasis and fluid transport in renal proximal tubule and corneal endothelium, respectively. Here, we provide a comprehensive review of MCTs 1-4, linking their cellular distribution to their functions in various parts of the human body, so that we can better understand the physiological roles of MCTs at the systemic level.

Circadian regulation of electrolyte absorption in the rat colon

The intestinal transport of nutrients exhibits distinct diurnal rhythmicity, and the enterocytes harbor a circadian clock. However, temporal regulation of the genes involved in colonic ion transport, i.e., ion transporters and channels operating in absorption and secretion, remains poorly understood. To address this issue, we assessed the 24-h profiles of expression of genes encoding the sodium pump (subunits Atp1a1 and Atp1b1), channels (α-, β-, and γ-subunits of Enac and Cftr), transporters (Dra, Ae1, Nkcc1, Kcc1, and Nhe3), and the Na(+)/H(+) exchanger (NHE) regulatory factor (Nherf1) in rat colonic mucosa. Furthermore, we investigated temporal changes in the spatial localization of the clock genes Per1, Per2, and Bmal1 and the genes encoding ion transporters and channels along the crypt axis. In rats fed ad libitum, the expression of Atp1a1, γEnac, Dra, Ae1, Nhe3, and Nherf1 showed circadian variation with maximal expression at circadian time 12, i.e., at the beginning of the subjective night. The peak γEnac expression coincided with the rise in plasma aldosterone. Restricted feeding phase advanced the expression of Dra, Ae1, Nherf, and γEnac and decreased expression of Atp1a1. The genes Atp1b1, Cftr, αEnac, βEnac, Nkcc1, and Kcc1 did not show any diurnal variations in mRNA levels. A low-salt diet upregulated the expression of βEnac and γEnac during the subjective night but did not affect expression of αEnac. Similarly, colonic electrogenic Na(+) transport was much higher during the subjective night than the subjective day. These findings indicate that the transporters and channels operating in NaCl absorption undergo diurnal regulation and suggest a role of an intestinal clock in the coordination of colonic NaCl absorption.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

Segregation of Na/H exchanger-3 and Cl/HCO3 exchanger SLC26A3 (DRA) in rodent cecum and colon

The colon is believed to absorb NaCl via the coupled operation of apical Na/H exchanger-3 (NHE3) and Cl/HCO(3) exchanger SLC26A3 (DRA). Efficient coupling requires that NHE3 and DRA operate in close proximity within common luminal and cytosolic microenvironments. Thus we examined whether these proteins coexist along the apical margin of surface enterocytes by quantitative immunofluorescence microscopy in consecutive colon segments from nonfasted mice and rats. The cecocolonic profiles of NHE3 and DRA expression were roughly inverse; NHE3 was highest in proximal colon (PC) and negligible in distal colon, whereas DRA was absent in early PC and highest in the late midcolon, and DRA was prominent in the cecum whereas NHE3 was not. NHE3 and DRA coexisted only in the middle third of the colon. The consequences of unpaired NHE3/DRA expression on mucosal surface (subscript MS) pH and Na(+) concentration ([Na(+)]) were assessed in nonfasted rats in situ using miniature electrodes. In the cecum, where only DRA is expressed, pH(MS) was approximately 7.5, markedly higher than underlaying stool (6.3), consistent with net HCO(3)(-) secretion. In the early PC, where NHE3 is not expressed with DRA, pH(MS) was acidic (6.2), consistent with unopposed H(+) secretion. [Na(+)](MS) was approximately 60 mM in the cecum, decreased along the PC to approximately 20 mM, and declined further to approximately 10 mM distally. Cl(-) was secreted into the PC, then reabsorbed distally. Our results suggest a model in which 1) unpaired DRA activity in the cecum maintains an alkaline mucosal surface that could neutralize fermentative H(+); 2) unpaired NHE3 activity in the early PC preserves an acidic mucosal surface that could energize short-chain fatty acid absorption; and 3) coupled NHE3/DRA activities in the midcolon allow for vigorous NaCl absorption at a neutral pH(MS).

Characteristics of Cl- uptake in rat alveolar type I cells

Although Cl- transport in fetal lung is important for fluid secretion and normal lung development, the role of Cl- transport in adult lung is not well understood. In physiological studies, the cystic fibrosis transmembrane regulator (CFTR) plays a role in fluid absorption in the distal air spaces of adult lung, and alveolar type II cells cultured for 5 days have the capacity to transport Cl-. Although both alveolar type I and type II cells express CFTR, it has previously not been known whether type I cells transport Cl-. We studied Cl- uptake in isolated type I cells directly, using either radioisotopic tracers or halide-sensitive fluorescent indicators. By both methods, type I cells take up Cl-. In the presence of beta-adrenergic agonist stimulation, Cl- uptake can be inhibited by CFTR antagonists. Type I cells express both the Cl-/HCO3- anion exchanger AE2 and the voltage-gated Cl- channels CLC5 and CLC2. Inhibitors of AE2 also block Cl- uptake in type I cells. Together, these results demonstrate that type I cells are capable of Cl- uptake and suggest that the effects seen in whole lung studies establishing the importance of Cl- movement in alveolar fluid clearance may be, in part, the result of Cl- transport across type I cells.

Regulation of intestinal Cl−/HCO3− exchanger SLC26A3 by intracellular pH

SLC26A3, a Cl−/HCO3− exchanger, is highly expressed in intestinal epithelial cells, and its mutations cause congenital chloride diarrhea. This suggests that SLC26A3 plays a key role in NaCl absorption in the intestine. Electroneutral NaCl absorption in the intestine is mediated by functional coupling of the Na+/H+ exchanger and Cl−/HCO3− exchanger. It is proposed that the coupling of these exchangers may occur as a result of indirect linkage by changes of intracellular pH (pHi). We therefore investigated whether SLC26A3 is regulated by pHi. We generated a hemagglutinin epitope-tagged human SLC26A3 construct and expressed it in Chinese hamster ovary cells. Transport activities were measured with a fluorescent chloride-sensitive dye dihydro-6-methoxy- N-ethylquinolinium iodide (diH-MEQ). pHi was clamped at a range of values from 6.0 to 7.4. We monitored the transport activity of SLC26A3 by reverse mode of Cl−/HCO3− and Cl−/NO3− exchange. None of these exchange modes induced membrane potential changes. At constant external pH 7.4, Cl−/HCO3− exchange was steeply inhibited with pHi decrease between 7.3 and 6.8 as opposed to thermodynamic prediction. In contrast, however, Cl−/NO3− exchange was essentially insensitive to pHi within physiological ranges. We also characterized the pHi dependency of COOH-terminal truncation mutants. Removal of the entire COOH-terminal resulted in decrease of the transport activity but did not noticeably affect pHi sensitivity. These results suggest that Cl−/HCO3− exchange mode of human SLC26A3 is controlled by a pH-sensitive intracellular modifier site, which is likely in the transmembrane domain. These observations raise the possibility that SLC26A3 activity may be regulated via Na+/H+ exchanger 3 (NHE3) through the alteration of pHi under physiological conditions.

Functional coupling of the downregulated in adenoma Cl-/base exchanger DRA and the apical Na+/H+ exchangers NHE2 and NHE3

Non-nutrient-dependent salt absorption across the brush-border membrane of intestinal epithelial cells is primarily mediated by coupled apical Na(+)/H(+) (aNHE) and anion exchange transport, with the latter suspected of being mediated by DRA (downregulated in adenoma; SLC26A3) that is defective in congenital chloridorrhea. To investigate DRA in greater detail and determine whether DRA and NHE activities can be coupled, we measured (22)Na(+) and (36)Cl(-) uptake in Caco2BBE colon cells infected with the tet-off-inducible DRA transgene. Under basal conditions, DRA activity was low in normal and infected Caco2BBE cells in the presence of tetracycline, whereas NHE activities could be easily detected. When apical NHE activity was increased by transfection or serum-induced expression of the aNHE isoforms NHE2 and NHE3, increased (36)Cl(-) uptake was observed. Inhibition of DRA activity by niflumic acid was greater than that by DIDS as well as by the NHE inhibitor dimethylamiloride and the carbonic anhydrase inhibitor methazolamide. DRA activity was largely aNHE-dependent, whereas a component of DRA-independent aNHE uptake continued to be observed. Coupled aNHE and DRA activities were inhibited by increased cellular cAMP and calcium and were associated with synaptotagmin I-dependent, clathrin-mediated endocytosis. In summary, these data support the role of DRA in electroneutral NaCl absorption involving functional coupling of Cl(-)/base exchange and apical NHE.

Map of differential transcript expression in the normal human large intestine

While there is considerable research related to using differential gene expression to predict disease phenotype classification, e.g., neoplastic tissue from nonneoplastic controls, there is little understanding of the range of expression in normal tissues. Understanding patterns of gene expression in nonneoplastic tissue, including regional anatomic expression changes within an organ, is vital to understanding gene expression changes in diseased tissue. To explore the gene expression change along the proximal-distal axis of the large intestine, we analyzed microarray data in 184 normal human specimens using univariate and multivariate techniques. We found 219 probe sets that were differentially expressed between the proximal and distal colorectal regions and 115 probe sets that were differentially expressed between the terminal segments, i.e., the cecum and rectum. We did not observe any probe sets that were statistically different between any two contiguous colorectal segments. The dominant expression pattern (65 probe sets) follows a dichotomous expression pattern consistent with the midgut-hindgut embryonic origins of the gut while a second pattern (50 probe sets) depicts a gradual change in transcript levels from the cecum to the rectum. While the dichotomous pattern includes roughly equal numbers of probe sets that are elevated proximally and distally, nearly all probe sets that show a gradual change demonstrate increasing expression levels moving from proximal to distal segments. These patterns describe an expression map of individual transcript variation as well as multigene expression patterns along the large intestine. This is the first gene expression map of an entire human organ.

Dopamine stimulates Cl− absorption coupled with HCO3− secretion in rat late distal colon

Freshly isolated rat colonic mucosa close to anus (the late distal colonic mucosa) was used to investigate the effect of dopamine on the rat late distal colonic ion transport by means of short-circuit current (I(SC)) recording and reverse transcription PCR (RT-PCR) analysis. The results showed that the basolateral addition of dopamine (0.1-1000 micromol/l) produced a concentration-dependent downward deflection in I(SC) with an apparent EC(50) of 20.06 micromol/l in the late distal colon. The dopamine-induced I(SC) decrease was abolished by bilateral removal of Cl(-) or HCO(3)(-), apical Cl(-) replacement and apical pretreatment with non-specific Cl(-) channel blocker/transporter inhibitor, DPC (1 mmol/l) or glibenclamide (1 mmol/l), and reversed by subsequent addition of glibenclamide. Removal of basolateral Na(+) or reducing basolateral HCO(3)(-) (3 mmol/l) as well as basolateral pretreatment with DIDS (4,4'-didsothio- cyanostilbene-2, 2'-disulfonic acid) (250 micromol/l), an inhibitor of NBC or AE, could also inhibit the dopamine-induced I(SC) response. However, apical pretreatment with epithelial Na(+) channel blocker, amiloride (10 micromol/l), Ca(2+)-dependent Cl(-) channel blocker/anion exchanger, DIDS (100 micromol/l), or putative K(+) blockers such as Ba(2+) (5 mmol/l), TEA (tetraethylammonium) (5 mmol/l) or 293B (trans-6-cyano-4- (N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl-chromane) (10 micromol/l) did not significantly affect the dopamine-induced I(SC) response. RT-PCR results showed the expression of anion exchanger, SLC26A3, but not SLC26A6, in rat late distal colon. In conclusion, the present results suggest that dopamine may promote rat late distal colonic epithelial Cl(-) absorption coupled with HCO(3)(-) secretion, which may be mediated by apical electrogenic anion exchanger, SLC26A3, and require basolateral entry of HCO(3)(-) through Na(+)-HCO(3)(-) cotransporter. The present findings reveal a previously unreported dopamine-regulated anion transport process in rat late distal colon, which may have implication in Parkinson's disease.

Epidermal growth factor stimulates chloride transport in primary cultures of weanling and adult rabbit colonocytes

OBJECTIVES

We have shown that Ca2+-dependent regulation of Cl- secretion in the mammalian colon exhibits age dependence. Because epidermal growth factor (EGF) has a well-established role in growth and can increase intracellular calcium [Ca2+]i, it is conceivable that its developmental influence may extend to the regulation of intestinal ion transport. In this study, we examined the role of EGF in the regulation of Cl- transport in the developing rabbit distal colon.

MATERIALS AND METHODS

Because serum contains growth factors, which could have confounded our studies, we first established an optimal milieu for testing EGF in primary cultures of adult rabbit distal colonocytes by culturing them for 24 h in media containing 0%, 1%, 5%, and 20% serum. Chloride transport (millimoles per second) and [Ca2+]i were measured with use of the fluorescent indicator N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) and Fura-2AM, respectively.

RESULTS

Serum depletion had no effect on cell number, DNA content, or basal Cl- transport, but it significantly affected cell viability. In media with 0%, 1%, or 20% serum, bethanechol, 8BrcAMP, taurodeoxycholate, and EGF stimulated Cl- transport to a similar extent. EGF maximally stimulated Cl- transport at 16.3 nmol/L and 20 minutes. Bethanechol, but not EGF, increased [Ca2+]i. EGF did not alter bethanechol-stimulated Cl- transport or [Ca2+]i. EGF acts via an EGF-receptor and mitogen activated protein kinase (MAPK) signaling pathway, since stimulation of Cl- transport was abolished by genistein, AG1478, and PD98059. Weanling and adult colonocytes, cultured in 1% serum, showed similar basal and EGF-stimulated Cl- transport.

CONCLUSIONS

EGF stimulates rabbit colonic Cl- transport via a Ca2+-independent, tyrosine kinase- and MAPK-dependent pathway, and its effects are not age dependent.

Characteristics of rat downregulated in adenoma (rDRA) expressed in HEK 293 cells

Studies with apical membrane vesicles have shown that two distinct and separate anion exchange processes are present in rat distal colon, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS)-sensitive CL(-)-HCO(3)(-) exchange, and DIDS-resistant Cl(-)-OH(-) exchange. These studies proposed that anion exchanger (AE)-1 isoform encodes the former as both apical membrane DIDS-sensitive CL(-)-HCO(3)(-) exchange, and AE1 specific mRNA are present only in surface cells and are downregulated in Na-depleted rats, whereas downregulated in adenoma (DRA) encodes the latter as both DIDS-resistant Cl(-)-OH(-) exchange, and DRA-specific proteins are present in apical membranes of both surface and crypt cells and are not altered in Na(+)-depleted rats. Studies were, therefore, initiated to identify the function of rat DRA (rDRA) in vitro. rDRA cDNA isolated from rat distal colon encodes a 757-amino-acid protein which has 96 and 81% homology with mDRA and hDRA, respectively. rDRA-specific mRNA expression was detectable only in specific segments of the digestive tract (duodenum, ileum, cecum, proximal colon, and distal colon) but not in the stomach, jejunum, or in the kidney, brain, heart, and lung. HEK 293 cells stably transfected with rDRA exhibited DIDS-insensitive and intracellular acid pH (pH(i) 6.5)-sensitive Cl uptake that: (1) was significantly stimulated by outward Cl(-), HCO(3)(-), isobutyrate, and possibly OH(-) gradients; (2) was saturated as a function of increasing extracellular Cl concentrations with an apparent K (m) for Cl of 2.9 +/- 0.3 mM; and (3) was inhibited competitively by extracellular oxalate but not by SO(4)(2-). A high rate of DIDS-insensitive Cl influx at pH 6.5 was also present under physiological Cl(-) concentration. Our observations that rDRA mediates DIDS-insensitive, acid pH-dependent Cl(-) uptake are consistent with prior observations that rDRA does not mediate DIDS-sensitive Cl(-)-HCO(3)(-) exchange in rat distal colon. We speculate that, in addition to mediating pH-sensitive Cl(-) uptake, rDRA may function as a modifier of other anion transport proteins.

Low carbon dioxide permeability of the apical epithelial membrane of guinea-pig colon

We have investigated the apical membrane permeability for CO2 of intact epithelia of proximal and distal colon of the guinea pig. The method used was the mass spectrometric 18O-exchange technique previously described. In a first step, we determined the intraepithelial carbonic anhydrase (CA) activity by studying vital isolated colonocytes before and after lysis with Triton X-100. Intraepithelial CA activity was found to be 41,000 and 900 for proximal and distal colon, respectively. Then 18O-exchange measurements were done with stripped intact epithelial layers, which on their apical side were exposed to the reaction solution containing 18O-labelled CO2 and HCO3-. The mass spectrometric signals in these measurements are determined by the intracellular epithelial CA activity, and by the apical membrane permeabilities for CO2 and HCO3-, P(CO2) and P(HCO3). From the signals, we calculated the two permeabilities while inserting the CA activities obtained from isolated colonocytes. From layers of intact colon epithelium, the apical P(CO2) was determined to be 1.5 x 10(-3) cm s(-1) for proximal and 0.77 x 10(-3) cm s(-1) for distal colon. These values are > or =200 times lower than the P(CO2) of the human red cell membrane as studied with the same technique (0.3 cm s(-1)). We conclude that the apical membrane offers a significant resistance towards CO2 diffusion, which implies that a major drop in CO2 partial pressure (pCO2) will occur across the apical membrane when luminal pCO2 is higher than basolateral or capillary pCO2. In view of the very high pCO2 that can occur in the colonic lumen, this property of the apical membrane constitutes a significant protection of the cell against the high acid load associated with high pCO2.

Bicarbonate exporting transporters in the ovine ruminal epithelium

In order to stabilize the intraruminal pH, bicarbonate secretion by the ruminal epithelium seems to be an important prerequisite. The present study therefore focussed on the characterization of bicarbonate exporting systems in ruminal epithelial cells. Intracellular pH (pH(i)) was measured spectrofluorometrically in primary cultured ruminal epithelial cells loaded with the pH-sensitive fluorescent dye, 2,7-bis(carboxyethyl)-5(6')-carboxyfluorescein acetomethyl ester. Switching from CO2/HCO3- -buffered to HEPES-buffered solution caused a rapid intracellular alkalinization followed by a counter-regulation towards initial pH(i). The recovery of pH(i) was dependent upon extracellular chloride, but independent of extracellular sodium. Adding 500 microM H2DIDS significantly reduced the increase of pH(i). For further characterization of the bicarbonate exporting systems, we tested the ability to reverse the direction from HCO3- export to import in the absence of sodium and chloride. Under sodium and chloride-free conditions, counter-regulation after CO2-induced pH(i) decrease did not differ from pH(i) recovery in the presence of sodium and chloride. Existence of bicarbonate exporting systems in cultured ruminal epithelial cells and intact ruminal epithelium was verified by reverse transcription polymerase chain reaction (RT-PCR). Using RT-PCR and subsequent sequencing, expression of mRNA encoding for AE2, DRA and PAT1 could be found. Bicarbonate exporting systems could therefore be detected both on the functional and structural level.

SECRETION AND ABSORPTION BY COLONIC CRYPTS

The intestines play an important role in the absorption and secretion of nutrients. The colon is the final area for recapturing electrolytes and water prior to excretion, and in order to maintain this electrolyte homeostasis, a complex interaction between secretory and absorptive processes is necessary. Until recently it was thought that secretion and absorption were two distinct processes associated with either crypts or surface cells, respectively. Recently it was demonstrated that both the surface and crypt cells can perform secretory and absorptive functions and that, in fact, these functions can be going on simultaneously. This issue is important in the complexities associated with secretory diarrhea and also in attempting to develop treatment strategies for intestinal disorders. Here, we update the model of colonic secretion and absorption, discuss new issues of transporter activation, and identify some important new receptor pathways that are important modulators of the secretory and absorptive functions of the colon.

Intestinal transport of an obdurate anion: oxalate

In this review, we focus on the role of gastrointestinal transport of oxalate primarily from a contemporary physiological standpoint with an emphasis on those aspects that we believe may be most important in efforts to mitigate the untoward effects of oxalate. Included in this review is a general discussion of intestinal solute transport as it relates to oxalate, considering cellular and paracellular avenues, the transport mechanisms, and the molecular identities of oxalate transporters. In addition, we review the role of the intestine in oxalate disease states and various factors affecting oxalate absorption.

The SLC26 gene family of multifunctional anion exchangers

The ten-member SLC26 gene family encodes anion exchangers capable of transporting a wide variety of monovalent and divalent anions. The physiological role(s) of individual paralogs is evidently due to variation in both anion specificity and expression pattern. Three members of the gene family are involved in genetic disease; SLC26A2 in chondrodysplasias, SLC26A3 in chloride-losing diarrhea, and SLC26A4 in Pendred syndrome and hereditary deafness (DFNB4). The analysis of Slc26a4-null mice has significantly enhanced the understanding of the roles of this gene in both health and disease. Targeted deletion of Slc26a5 has in turn revealed that this paralog is essential for electromotor activity of cochlear outer hair cells and thus for cochlear amplification. Anions transported by the SLC26 family, with variable specificity, include the chloride, sulfate, bicarbonate, formate, oxalate and hydroxyl ions. The functional versatility of SLC26A6 identifies it as the primary candidate for the apical Cl(-)-formate/oxalate and Cl(-)-base exchanger of brush border membranes in the renal proximal tubule, with a central role in the reabsorption of Na(+)-Cl(-) from the glomerular ultrafiltrate. At least three of the SLC26 exchangers mediate electrogenic Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange; the stoichiometry of Cl(-)-HCO(3)(-) exchange appears to differ between SLC26 paralogs, such that SLC26A3 transports >/=2 Cl(-) ions per HCO(3)(-) ion, whereas SLC26A6 transports >/=2 HCO(3)(-) ions per Cl(-) ion. SLC26 Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange is activated by the cystic fibrosis transmembrane regulator (CFTR), implicating defective regulation of these exchangers in the reduced HCO(3)(-) transport seen in cystic fibrosis and related disorders; CFTR-independent activation of these exchangers is thus an important and novel goal for the future therapy of cystic fibrosis.

Mechanisms of diarrhea in the interleukin-2-deficient mouse model of colonic inflammation

Colitis in interleukin-2-deficient (IL-2(-/-)) mice resembles ulcerative colitis in humans. We studied epithelial transport and barrier function in IL-2(-/-) mice and used this model to characterize mechanisms of diarrhea during intestinal inflammation. (22)Na(+) and (36)Cl(-) fluxes were measured in proximal colon. Net Na(+) flux was reduced from 4.0 +/- 0.5 to 0.8 +/- 0.5 micromol.h(-1).cm(-2), which was paralleled by diminished mRNA and protein expression of the Na(+)/H(+) exchanger NHE3. Net Cl(-) flux was also decreased from 2.2 +/- 1.6 to -2.7 +/- 0.6 micromol.h(-1).cm(-2), indicating impaired Na(+)-Cl(-) absorption. In distal colon, aldosterone-induced electrogenic Na(+) absorption was 6.1 +/- 0.9 micromol.h(-1).cm(-2) in controls and was abolished in IL-2(-/-) mice. Concomitantly, mRNA expression of beta- and gamma-subunits of the epithelial sodium channel (ENaC) was reduced. Epithelial barrier was studied in proximal colon by impedance technique and mannitol fluxes. In contrast to ulcerative colitis, epithelial resistance was increased and mannitol fluxes were decreased in IL-2(-/-) mice. This was in accord with the findings of reduced ion transport as well as increased expression of tight junction proteins occludin and claudin-1, -2, -3, and -5. In conclusion, the IL-2(-/-) mucosa exhibits impaired electroneutral Na(+)-Cl(-) absorption and electrogenic Na(+) transport due to reduced mRNA and protein expression of NHE3 and ENaC beta- and gamma-subunit mRNA. This represents a model of early intestinal inflammation with absorptive dysfunction due to impaired transport protein expression/function while epithelial barrier is still intact. Therefore, this model is ideal to study regulation of transporter expression independent of barrier defects.

Acid-base effects on intestinal Cl- absorption and vesicular trafficking

In rat ileum and colon, apical membrane Cl(-)/HCO(3)(-) exchange and net Cl(-) absorption are stimulated by increases in Pco(2) or [HCO(3)(-)]. Because changes in Pco(2) stimulate colonic Na(+) absorption, in part, by modulating vesicular trafficking of the Na(+)/H(+) exchanger type 3 isoform to and from the apical membrane, we examined whether changes in Pco(2) affect net Cl(-) absorption by modulating vesicular trafficking of the Cl(-)/HCO(3)(-) exchanger anion exchanger (AE)1. Cl(-) transport across rat distal ileum and colon was measured in the Ussing chamber, and apical membrane protein biotinylation of these segments and Western blots of recovered proteins were performed. In colonic epithelial apical membranes, AE1 protein content was greater at Pco(2) 70 mmHg than at Pco(2) 21 mmHg but was not affected by pH changes in the absence of CO(2). AE1 was internalized when Pco(2) was reduced and exocytosed when Pco(2) was increased, and both mucosal wortmannin and methazolamide inhibited exocytosis. Wortmannin also inhibited the increase in colonic Cl(-) absorption caused by an increase in Pco(2). Increases in Pco(2) stimulated ileal Cl(-) absorption, but wortmannin was without effect. Ileal epithelial apical membrane AE1 content was not affected by Pco(2). We conclude that CO(2) modulation of colonic, but not ileal, Cl(-) absorption involves effects on vesicular trafficking of AE1.

Unique regulation of anion/HCO3- exchangers by constitutive nitric oxide in rabbit small intestine

In the rabbit small intestine, there are three functionally different brush-border membrane (BBM) anion/HCO3- exchangers: 1) Cl/HCO3- exchange on the BBM of villus cells responsible for coupled NaCl absorption; 2) Cl/HCO3- exchange on the BBM of crypt cells possibly involved in HCO3- secretion; and 3) short-chain fatty acid (SCFA)/HCO3- exchange on the BBM of villus cells, which facilitates SCFA absorption. Although constitutive nitric oxide (cNO) has been postulated to alter many gastrointestinal tract functions, how cNO may specifically alter these three transporters is unknown. Inhibition of cNO synthase with NG-nitro-L-arginine methyl ester (L-NAME) 1) did not affect villus cell BBM Cl/HCO3 change, 2) stimulated crypt cell BBM Cl/HCO3- exchange, and 3) inhibited villus cell BBM SCFA/HCO3- exchange. D-NAME, an inactive analog of L-NAME, and L-N6-(1-iminoethyl)lysine, a more selective inhibitor of inducible NO, did not affect these transport processes. Kinetic studies demonstrated that 1) the mechanism of inhibition of crypt cell BBM Cl/HCO3- exchange is secondary to a decrease in the maximal rate of uptake of Cl, without an alteration in the affinity of the transporter for Cl, and 2) the mechanism of stimulation of villus cell BBM SCFA/HCO3- exchange is secondary to an increase in the affinity of the transporter for SCFA without an alteration in the maximal rate of uptake of SCFA. These results indicate that cNO uniquely regulates the three BBM anion/HCO3- transporters in the rabbit small intestine.

Characterization of Cl-HCO3 exchange in basolateral membrane of rat distal colon

Sodium-independent Cl movement (i.e., Cl-anion exchange) has not previously been identified in the basolateral membranes of rat colonic epithelial cells. The present study demonstrates Cl-HCO3 exchange as the mechanism for 36Cl uptake in basolateral membrane vesicles (BLMV) prepared in the presence of a protease inhibitor cocktail from rat distal colon. Studies of 36Cl uptake performed with BLMV prepared with different types of protease inhibitors indicate that preventing the cleavage of the COOH-terminal end of AE2 protein by serine-type proteases was responsible for the demonstration of Cl-HCO3 exchange. In the absence of voltage clamping, both outward OH gradient (pHout/pHin: 7.5/5.5) and outward HCO3 gradient stimulated transient 36Cl uptake accumulation. However, voltage clamping with K-ionophore, valinomycin, almost completely (87%) inhibited the OH gradient-driven 36Cl uptake, whereas HCO3 gradient-driven 36Cl uptake was only partially inhibited (38%). Both electroneutral HCO3 and OH gradient-driven 36Cl uptake were 1) completely inhibited by DIDS, an anion exchange inhibitor, with a half-maximal inhibitory constant (Ki) of approximately 26.9 and 30.6 microM, respectively, 2) not inhibited by 5-nitro-2-(3-phenylpropylamino)benzoic acid(NPPB), a Cl channel blocker, 3) saturated by increasing extravesicular Cl concentration with a Km for Cl of approximately 12.6 and 14.2 mM, respectively, and 4) present in both surface and crypt cells. Intracellular pH (pHi) was also determined with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-acetomethylester (BCECF-AM) in an isolated superfused crypt preparation. Removal of Cl resulted in a DIDS-inhibitable increase in pHi both in HCO3-buffered and in the nominally HCO3-free buffered solutions (0.28 +/- 0.02 and 0.11 +/- 0.02 pH units, respectively). We conclude that a carrier-mediated electroneutral Cl-HCO3 exchange is present in basolateral membranes and that, in the absence of HCO3, Cl-HCO3 exchange can function as a Cl-OH exchange and regulate pHi across basolateral membranes of rat distal colon.

Identification of a basolateral Cl-/HCO3- exchanger specific to gastric parietal cells

The basolateral Cl(-)/HCO(3)(-) exchanger in parietal cells plays an essential role in gastric acid secretion mediated via the apical gastric H(+)-K(+)-ATPase. Here, we report the identification of a new Cl(-)/HCO(3)(-) exchanger, which shows exclusive expression in mouse stomach and kidney, with expression in the stomach limited to the basolateral membrane of gastric parietal cells. Tissue distribution studies by RT-PCR and Northern hybridizations demonstrated the exclusive expression of this transporter, also known as SLC26A7, to stomach and kidney, with the stomach expression significantly more abundant. No expression was detected in the intestine. Cellular distribution studies by RT-PCR and Northern hybridizations demonstrated predominant localization of SLC26A7 in gastric parietal cells. Immunofluorescence labeling localized this exchanger exclusively to the basolateral membrane of gastric parietal cells, and functional studies in oocytes indicated that SLC26A7 is a DIDS-sensitive Cl(-)/HCO(3)(-) exchanger that is active in both acidic and alkaline pH(i). On the basis of its unique expression pattern and function, we propose that SLC26A7 is a basolateral Cl(-)/HCO(3)(-) exchanger in gastric parietal cells and plays a major role in gastric acid secretion.

Regulation of the apical Cl-/HCO-3 exchanger pendrin in rat cortical collecting duct in metabolic acidosis

Pendrin is an apical Cl(-)/OH(-)/HCO(3)(-) exchanger in beta-intercalated cells (beta-ICs) of rat and mouse cortical collecting duct (CCD). However, little is known about its regulation in acid-base disorders. Here, we examined the regulation of pendrin in metabolic acidosis, a condition known to decrease HCO(3)(-) secretion in CCD. Rats were subjected to NH(4)Cl loading for 4 days, which resulted in metabolic acidosis. Apical Cl(-)/HCO(3)(-) exchanger activity in beta-ICs was determined as amplitude and rate of intracellular pH change when Cl was removed in isolated, microperfused CCDs. Intracellular pH was measured by single-cell digital ratiometric imaging using fluorescent pH-sensitive dye 2',7'-bis-(3-carboxypropyl)-5-(and-6)-carboxyfluorescein-AM. Pendrin mRNA expression in kidney cortex was examined by Northern blot hybridizations. Expression of pendrin protein was assessed by indirect immunofluorescence. Microperfused CCDs isolated from acidotic rats demonstrated approximately 60% reduction in apical Cl(-)/HCO(3)(-) exchanger activity in beta-ICs (P < 0.001 vs. control). Northern blot hybridizations indicated that the mRNA expression of pendrin in kidney cortex decreased by 68% in acidotic animals (P < 0.02 vs. control). Immunofluorescence labeling demonstrated significant reduction in pendrin expression in CCDs of acidotic rats. We conclude that metabolic acidosis decreases the activity of the apical Cl(-)/HCO(3)(-) exchanger in beta-ICs of the rat CCD by reducing the expression of pendrin. Adaptive downregulation of pendrin in metabolic acidosis indicates the important role of this exchanger in acid-base regulation in the CCD.

Inhibition of apical Cl-/OH- exchange activity in Caco-2 cells by phorbol esters is mediated by PKCepsilon

The present studies were undertaken to examine the possible regulation of apical membrane Cl-/OH- exchanger in Caco-2 cells by protein kinase C (PKC). The effect of the phorbol ester phorbol 12-myristate 13-acetate (PMA), an in vitro PKC agonist, on OH- gradient-driven 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl uptake in Caco-2 cells was assessed. The results demonstrated that PMA decreased apical Cl-/OH- exchanger activity via phosphatidylinositol 3-kinase (PI3-kinase)-mediated activation of PKCepsilon. The data consistent with these conclusions are as follows: 1) short-term treatment of cells for 1-2 h with PMA (100 nM) significantly decreased Cl-/OH- exchange activity compared with control (4alpha-PMA); 2) pretreatment of cells with specific PKC inhibitors chelerythrine chloride, calphostin C, and GF-109203X completely blocked the inhibition of Cl-/OH- exchange activity by PMA; 3) specific inhibitors for PKCepsilon (Ro-318220) but not PKCalpha (Go-6976) significantly blocked the PMA-mediated inhibition; 4) specific PI3-kinase inhibitors wortmannin and LY-294002 significantly attenuated the inhibitory effect of PMA; and 5) PI3-kinase activators IRS-1 peptide and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] mimicked the effects of PMA. These findings provide the first evidence for PKCepsilon-mediated inhibition of Cl-/OH- exchange activity in Caco-2 cells and indicate the involvement of the PI3-kinase-mediated pathways in the regulation of Cl- absorption in intestinal epithelial cells.

The functional and physical relationship between the DRA bicarbonate transporter and carbonic anhydrase II

COOH-terminal cytoplasmic tails of chloride/bicarbonate anion exchangers (AE) bind cytosolic carbonic anhydrase II (CAII) to form a bicarbonate transport metabolon, a membrane protein complex that accelerates transmembrane bicarbonate flux. To determine whether interaction with CAII affects the downregulated in adenoma (DRA) chloride/bicarbonate exchanger, anion exchange activity of DRA-transfected HEK-293 cells was monitored by following changes in intracellular pH associated with bicarbonate transport. DRA-mediated bicarbonate transport activity of 18 +/- 1 mM H+ equivalents/min was inhibited 53 +/- 2% by 100 mM of the CAII inhibitor, acetazolamide, but was unaffected by the membrane-impermeant carbonic anhydrase inhibitor, 1-[5-sulfamoyl-1,3,4-thiadiazol-2-yl-(aminosulfonyl-4-phenyl)]-2,6-dimethyl-4-phenyl-pyridinium perchlorate. Compared with AE1, the COOH-terminal tail of DRA interacted weakly with CAII. Overexpression of a functionally inactive CAII mutant, V143Y, reduced AE1 transport activity by 61 +/- 4% without effect on DRA transport activity (105 +/- 7% transport activity relative to DRA alone). We conclude that cytosolic CAII is required for full DRA-mediated bicarbonate transport. However, DRA differs from other bicarbonate transport proteins because its transport activity is not stimulated by direct interaction with CAII.

Identification of an apical Cl(-)/HCO3(-) exchanger in the small intestine

HCO3(-) secretion is the most important defense mechanism against acid injury in the duodenum. However, the identity of the transporter(s) mediating apical HCO3(-) secretion in the duodenum remains unknown. A family of anion exchangers, which include downregulated in adenoma (DRA or SLC26A3), pendrin (PDS or SLC26A4), and the putative anion transporter (PAT1 or SLC26A6) has recently been identified. DRA and pendrin mediate Cl(-)/base exchange; however, the functional identity and distribution of PAT1 (SLC26A6) is not known. In these studies, we investigated the functional identity, tissue distribution, and membrane localization of PAT1. Expression studies in Xenopus oocytes demonstrated that PAT1 functions in Cl(-)/HCO3(-) exchange mode. Tissue distribution studies indicated that the expression of PAT1 is highly abundant in the small intestine but is low in the colon, a pattern opposite that of DRA. PAT1 was also abundantly detected in stomach and heart. Immunoblot analysis studies identified PAT1 as a approximately 90 kDa protein in the duodenum. Immunohistochemical studies localized PAT1 to the brush border membranes of the villus cells of the duodenum. We propose that PAT1 is an apical Cl(-)/HCO3(-) exchanger in the small intestine.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

A transport metabolon. Functional interaction of carbonic anhydrase II and chloride/bicarbonate exchangers

The cytoplasmic carboxyl-terminal domain of AE1, the plasma membrane chloride/bicarbonate exchanger of erythrocytes, contains a binding site for carbonic anhydrase II (CAII). To examine the physiological role of the AE1/CAII interaction, anion exchange activity of transfected HEK293 cells was monitored by following the changes in intracellular pH associated with AE1-mediated bicarbonate transport. AE1-mediated chloride/bicarbonate exchange was reduced 50-60% by inhibition of endogenous carbonic anhydrase with acetazolamide, which indicates that CAII activity is required for full anion transport activity. AE1 mutants, unable to bind CAII, had significantly lower transport activity than wild-type AE1 (10% of wild-type activity), suggesting that a direct interaction was required. To determine the effect of displacement of endogenous wild-type CAII from its binding site on AE1, AE1-transfected HEK293 cells were co-transfected with cDNA for a functionally inactive CAII mutant, V143Y. AE1 activity was maximally inhibited 61 +/- 4% in the presence of V143Y CAII. A similar effect of V143Y CAII was found for AE2 and AE3cardiac anion exchanger isoforms. We conclude that the binding of CAII to the AE1 carboxyl-terminus potentiates anion transport activity and allows for maximal transport. The interaction of CAII with AE1 forms a transport metabolon, a membrane protein complex involved in regulation of bicarbonate metabolism and transport.

Downregulated in adenoma and putative anion transporter are regulated by CFTR in cultured pancreatic duct cells

The mechanism of the pancreatic ductal HCO secretion defect in cystic fibrosis (CF) is not well defined. However, a lack of apical Cl(-)/HCO exchange may exist in CF. To test this hypothesis, we examined the expression of Cl(-)/HCO exchangers in cultured pancreatic duct epithelial cells with physiological features prototypical of CF [CFPAC-1 cells lacking a functional CF transmembrane conductance regulator (CFTR)] or normal duct cells (CFPAC-1 cells transfected with functional wild-type CFTR, CFPAC-WT). Cl(-)/HCO exchange activity, assayed with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in cells grown on coverslips, increased about twofold in cells transfected with functional CFTR. This correlated with increased apical (36)Cl influx in cells expressing functional CFTR and grown on permeable support. Northern hybridizations indicated the induction of downregulated in adenoma (DRA) in cells expressing functional CFTR. The expression of putative anion transporter PAT1 also increased significantly in cells expressing functional CFTR. DRA was detected at high levels in native mouse pancreas by Northern hybridization and localized to the apical domain of the duct cells by immunohistochemical studies. In conclusion, CFTR upregulates DRA and PAT1 expression in cultured pancreatic duct cells. We propose that the pancreatic HCO secretion defect in CF patients is partly due to the downregulation of apical Cl(-)/HCO exchange activity mediated by DRA (and possibly PAT1).