Chloride transport in human proximal colonic apical membrane vesicles

Mammalian colonocytes possess a carrier-mediated mechanism for uptake of vitamin B3 (niacin): studies utilizing human and mouse colonic preparations

Niacin (vitamin B3; nicotinic acid) plays an important role in maintaining redox state of cells and is obtained from endogenous and exogenous sources. The latter source has generally been assumed to be the dietary niacin, but another exogenous source that has been ignored is the niacin that is produced by the normal microflora of the large intestine. For this source of niacin to be bioavailable, it needs to be absorbed, but little is known about the ability of the large intestine to absorb niacin and the mechanism involved. Here we addressed these issues using the nontransformed human colonic epithelial NCM460 cells, native human colonic apical membrane vesicles (AMV) isolated from organ donors, and mouse colonic loops in vivo as models. Uptake of ³H-nicotinic acid by NCM460 cells was: 1) acidic pH (but not Na⁺) dependent; 2) saturable (apparent Km = 2.5 ± 0.8 μM); 3) inhibited by unlabeled nicotinic acid, nicotinamide, and probenecid; 4) neither affected by other bacterially produced monocarboxylates, monocarboxylate transport inhibitor, or by substrates of the human organic anion transporter-10; 5) affected by modulators of the intracellular protein tyrosine kinase- and Ca²⁺-calmodulin-regulatory pathways; and 6) adaptively regulated by extracellular nicotinate level. Uptake of nicotinic acid by human colonic AMV in vitro and by mouse colonic loops in vivo was also carrier mediated. These findings report, for the first time, that mammalian colonocytes possess a high-affinity carrier-mediated mechanism for nicotinate uptake and show that the process is affected by intracellular and extracellular factors.

A high-affinity and specific carrier-mediated mechanism for uptake of thiamine pyrophosphate by human colonic epithelial cells

All mammals require exogenous sources of thiamine (vitamin B1), as they lack the ability to synthesize the vitamin. These sources are dietary and bacterial (the latter is in reference to the vitamin, which is synthesized by the normal microflora of the large intestine). Bacterially generated thiamine exists in the free, as well as the pyrophosphorylated [thiamine pyrophosphate (TPP)], form. With no (or very little) phosphatase activity in the colon, we hypothesized that the bacterially generated TPP can also be taken up by colonocytes. To test this hypothesis, we examined [(3)H]TPP uptake in the human-derived, nontransformed colonic epithelial NCM460 cells and purified apical membrane vesicles isolated from the colon of human organ donors. Uptake of TPP by NCM460 cells occurred without metabolic alterations in the transported substrate and 1) was pH- and Na(+)-independent, but energy-dependent, 2) was saturable as a function of concentration (apparent K(m) = 0.157 ± 0.028 μM), 3) was highly specific for TPP and not affected by free thiamine (or its analogs) or by thiamine monophosphate and unrelated folate derivatives, 4) was adaptively regulated by extracellular substrate (TPP) level via what appears to be a transcriptionally mediated mechanism(s), and 5) appeared to be influenced by an intracellular Ca(2+)/calmodulin-mediated regulatory pathway. These findings suggest the involvement of a carrier-mediated mechanism for TPP uptake by colonic NCM460 cells, which was further confirmed by results from studies of native human colonic apical membrane vesicles. The results also suggest that the bacterially synthesized TPP in the large intestine is bioavailable and may contribute to overall body homeostasis of vitamin B1 and, especially, to the cellular nutrition of the local colonocytes.

News from the end of the gut--how the highly segmental pattern of colonic HCO₃⁻ transport relates to absorptive function and mucosal integrity

A number of transport mechanisms in the colonic epithelium contribute to HCO₃⁻ movement across the apical and basolateral membranes, but this ion has been largely regarded as a by-product of the transport functions it is involved in, such as NaCl or short chain fatty acid (SCFA) absorption. However, emerging data points to several specific roles of HCO₃⁻ for colonic epithelial physiology, including pH control in the colonic surface microenvironment, which is important for transport and immune functions, as well as the secretion and the rheological properties of the mucus gel. Furthermore, recent studies have demonstrated that colonic HCO₃⁻ transporters are expressed in a highly segmental as well as species-specific manner. This review summarizes recently gathered information on the functional anatomy of the colon, the roles of HCO₃⁻ in the colonic epithelium, colonic mucosal integrity, and the expression and function of HCO₃⁻ transporting mechanisms in health and disease.

Mechanisms of lysophosphatidic acid (LPA) mediated stimulation of intestinal apical Cl-/OH- exchange

Lysophosphatidic acid (LPA), a potent bioactive phospholipid, is a natural component of food products like soy and egg yolk. LPA modulates a number of epithelial functions and has been shown to inhibit cholera toxin-induced diarrhea. Antidiarrheal effects of LPA are known to be mediated by inhibiting chloride secretion. However, the effects of LPA on chloride absorption in the mammalian intestine are not known. The present studies examined the effects of LPA on apical Cl(-)/OH(-) exchangers known to be involved in chloride absorption in intestinal epithelial cells. Caco-2 cells were treated with LPA, and Cl(-)/OH(-) exchange activity was measured as DIDS-sensitive (36)Cl(-) uptake. Cell surface biotinylation studies were performed to evaluate the effect of LPA on cell surface levels of apical Cl(-)/OH(-) exchangers, downregulated in adenoma (DRA) (SLC26A3), and putative anion transporter-1 (SLC26A6). Treatment of Caco-2 cells with LPA (100 muM) significantly stimulated Cl(-)/OH(-) exchange activity. Specific agonist for LPA2 receptor mimicked the effects of LPA. LPA-mediated stimulation of Cl(-)/OH(-) exchange activity was dependent on activation of phosphatidylinositol 3-kinase/Akt signaling pathway. Consistent with the functional activity, LPA treatment resulted in increased levels of DRA on the apical membrane. Our results demonstrate that LPA stimulates apical Cl(-)/OH(-) exchange activity and surface levels of DRA in intestinal epithelial cells. This increase in Cl(-)/OH(-) exchange may contribute to the antidiarrheal effects of LPA.

Molecular cloning and promoter analysis of downregulated in adenoma (DRA)

Downregulated in adenoma (DRA), also referred to as SLC26A3, is an intestinal anion transporter essential for intestinal chloride absorption. Mutations in DRA result in congenital chloride diarrhea. DRA expression has been shown to be induced by differentiation and to be modulated by cytokines. However, mechanisms of DRA gene transcription and its tissue-specific targeting have not yet been investigated. In this study, we cloned a 3,765-bp promoter fragment of human DRA gene and characterized its activity in human colonic LS174T and Caco-2 human colon cell lines. Primer extension identified a single transcriptional initiation site that was identical in both colon cancer cell lines and normal colon. Although hepatic nuclear factor HNF-4 is involved in the basal activity of DRA promoter, sodium butyrate induces its activity in LS174T cells via the binding of Yin Yang 1 (YY1) and GATA transcription factors to their respective cis-elements in promoter region. We also demonstrated a reduction in DRA promoter activity in Caco-2 cells by IFN-gamma, suggesting that regulation of DRA promoter by IFN-gamma may contribute to the pathophysiology of intestinal inflammation. Furthermore, we showed that the DRA promoter fragment is sufficient to drive human growth hormone transgene expression specifically in villus epithelial cells of the small intestine and in differentiated upper crypt and surface epithelial cells of the colon. Our studies provide evidence for the involvement of HNF-4, YY1, and GATA transcription factors in DRA expression in intestinal differentiated epithelial cells.

Taurodeoxycholate modulates apical Cl-/OH- exchange activity in Caco2 cells

Bile acid malabsorption has been shown to be associated with diarrhea in cases such as ileal resection Crohn's disease of the ileum, and radiation enteritis. The mechanisms of bile acid-induced diarrhea are not fully understood. Although the induction of colonic chloride secretion in response to bile acids has been extensively investigated, to date the direct effect of bile acids on intestinal chloride absorption has not been well defined. Therefore, the current studies were undertaken to investigate the effect of bile acids on the apical Cl(-)/OH(-) exchange process utilizing Caco2 monolayers as an in vitro cellular model. Cl(-)/OH(-) exchange activity was measured as DIDS-sensitive pH gradient-driven (36)Cl uptake. The results are summarized as follows: (i) short-term exposure (20 min) of Caco2 cells to taurodeoxycholate (TDC; 200 microM) and glycochenodeoxycholate (GCDC; 200 microM) acids significantly inhibited apical Cl(-)/OH(-) exchange (by approximately 60-70%); (ii) the Ca(2+) chelator BAPTA-AM blocked the inhibition by TDC; (iii) the reduction in Cl(-)/OH(-) exchange by TDC was reversed by the PKC inhibitor, chelerythrine chloride; (iv) functional and inhibitor studies indicated that TDC induced inhibition of Cl(-)/OH(-) exchange was mediated via the activation of the PKC beta I isoform; (v) the effect of TDC on apical Cl(-)/OH(-) exchange was completely blocked by the PI3 kinase inhibitor LY294002 (5 microM); and (vi) the PKA inhibitor, RpcAMP, had no effect on TDC induced inhibition of Cl(-)/OH(-) exchange. In conclusion, our studies provide direct evidence for inhibition of human intestinal apical Cl(-)/OH(-) exchange activity by bile acids via Ca(2+)-, PI3 kinase-, and PKC beta I-dependent pathways in Caco2 cells.

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.

The CLCA gene locus as a modulator of the gastrointestinal basic defect in cystic fibrosis

To determine whether the CLCA gene family of calcium-activated chloride channels is a modulator of the basic defect of cystic fibrosis (CF), an association study was performed with polymorphic microsatellite markers covering a 40-Mbp region spanning the CLCA gene locus on human chromosome 1p in CF patients displaying CF transmembrane conductance regulator (CFTR)-independent residual chloride conductance in gastrointestinal epithelia. Statistically significant association of the electrophysiological phenotype with the allele distribution of markers 5' of and within the CLCA locus was observed. Transmission disequilibrium and the significance of the association decreased within the locus from hCLCA2 towards hCLCA4. Expression of hCLCA1 and hCLCA4 in human rectal mucosa was proven by microarray analysis. The CLCA gene region was identified to encode mediators of DIDS-sensitive anion conductance in the human gastrointestinal tract that modulate the CF basic defect.

Bicarbonate transport proteins

Bicarbonate is not freely permeable to membranes. Yet, bicarbonate must be moved across membranes, as part of CO2 metabolism and to regulate cell pH. Mammalian cells ubiquitously express bicarbonate transport proteins to facilitate the transmembrane bicarbonate flux. These bicarbonate transporters, which function by different transport mechanisms, together catalyse transmembrane bicarbonate movement. Recent advances have allowed the identification of several new bicarbonate transporter genes. Bicarbonate transporters cluster into two separate families: (i) the anion exachanger (AE) family of Cl-/HCO3- exchangers is related in sequence to the NBC family of Na+/HCO3- cotransporters and the Na(+)-dependent Cl/HCO3- exchangers and (ii) some members of the SLC26a family of sulfate transporters will also transport bicarbonate but are not related in sequence to the AE/NBC family of transporters. This review summarizes our understanding of the mammalian bicarbonate transporter superfamily.

SLC26A3 mutations in congenital chloride diarrhea

Congenital chloride diarrhea (CLD) is an autosomal recessive disorder of intestinal electrolyte absorption. It is characterized by persistent secretory diarrhea resulting in polyhydramnios and prematurity prenatally, and dehydration, hypoelectrolytemia, hyperbilirubinemia, abdominal distention, and failure to thrive immediately after birth. CLD is caused by mutations in the solute carrier family 26, member 3 gene (SLC26A3, alias CLD or DRA), which encodes a Na+-independent Cl-/HCO3- (or OH-) exchanger. SLC26A3 is a member of the SLC26 sulfate permease/anion transporter family and it is expressed mainly in the apical brush border of intestinal epithelium. The only extraintestinal tissues showing SLC26A3 expression are eccrine sweat glands and seminal vesicles. A wide variety of different mutations in the SLC26A3 gene have been associated with CLD with no apparent evidence of phenotype-genotype correlation. The clinical course of CLD, however, is variable and may rather depend on environmental factors and compensatory mechanisms than mutations. In this report, we present a summary of all published and two novel SLC26A3 mutations and polymorphisms, and review them in the context of their functional consequences and clinical implications.

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.

Mechanism of n-butyrate uptake in the human proximal colonic basolateral membranes

Current studies were undertaken to characterize the mechanism of short-chain fatty acid (SCFA) transport in isolated human proximal colonic basolateral membrane vesicles (BLMV) utilizing a rapid-filtration n-[(14)C]butyrate uptake technique. Human colonic tissues were obtained from mucosal scrapings from organ donor proximal colons. Our results, consistent with the existence of a HCO(3)(-)/SCFA exchanger in these membranes, are summarized as follows: 1) n-[(14)C]butyrate influx was significantly stimulated into the vesicles in the presence of an outwardly directed HCO(3)(-) and an inwardly directed pH gradient; 2) n-[(14)C]butyrate uptake was markedly inhibited (approximately 40%) by anion exchange inhibitor niflumic acid (1 mM), but SITS and DIDS (5 mM) had no effect; 3) structural analogs e.g., acetate and propionate, significantly inhibited uptake of HCO(3)(-) and pH-gradient-driven n-[(14)C]butyrate; 4) n-[(14)C]butyrate uptake was saturable with a K(m) for butyrate of 17.5 +/- 4.5 mM and a V(max) of 20.9 +/- 1.2 nmol x mg protein(-1) x 5 s(-1); 5) n-[(14)C]butyrate influx into the vesicles demonstrated a transstimulation phenomenon; and 6) intravesicular or extravesicular Cl(-) did not alter the anion-stimulated n-[(14)C]butyrate uptake. Our results indicate the presence of a carrier-mediated HCO(3)(-)/SCFA exchanger on the human colonic basolateral membrane, which appears to be distinct from the previously described anion exchangers in the membranes of colonic epithelia.

Down-regulated in adenoma mediates apical Cl-/HCO3- exchange in rabbit, rat, and human duodenum

BACKGROUND & AIMS

Duodenal bicarbonate secretion is in part mediated by an apical Cl-/HCO3- exchanger of unknown molecular nature. The recently discovered dra (down-regulated in adenoma) gene encodes a transport protein (DRA) for SO4(2-), Cl-, and HCO3-. The aim of this study was to investigate whether DRA may be the duodenal apical Cl-/HCO3- exchanger.

METHODS

DRA, Na+/H+ exchanger (NHE) isoform 3, and anion exchanger isoform (AE) 2 messenger RNA expression levels were studied in rat, rabbit, and human gastrointestinal tract by semiquantitative reverse-transcription polymerase chain reaction and in situ hybridization (DRA in human intestine). The subcellular localization of DRA was determined by Western analysis and immunohistochemistry. Using rabbit and rat duodenal brush border membrane vesicles, anion exchange characteristics were investigated.

RESULTS

DRA expression was high in duodenum and colon of all species, whereas NHE3 messenger RNA expression was low in duodenum and high in colon. Western analysis and immunohistochemistry showed an apical localization for DRA. Rabbit and rat duodenal brush border membrane vesicles showed 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive Cl-/Cl-, HCO3-/Cl-, SO4(2-)/Cl-, and Cl-/SO4(2-) exchange, with evidence for one major brush border membrane Cl-/anion exchanger, an affinity for Cl- > HCO3-, and a much higher affinity for SO4(2-) in rat than rabbit. The strong predominance of DRA over NHE3 and NHE2 expression in duodenum was paralleled by much higher Cl-/HCO3- than Na+/H+ exchange rates in brush border membrane vesicles and likely explains the high duodenal HCO3- secretory rates.

CONCLUSIONS

These data suggest that DRA is the major apical anion exchanger in the duodenum as well as the colon and the likely transport protein for duodenal electroneutral HCO3- secretion.

Evidence for the existence of a distinct SO(4)(--)-OH(-) exchange mechanism in the human proximal colonic apical membrane vesicles and its possible role in chloride transport

Recent studies have demonstrated that mutations in human downregulated in adenoma gene (DRA) result in congenital chloride diarrhea (CLD), and that DRA may be involved in chloride transport across the intestinal apical domains. DRA is highly homologous to sulfate transporters, but not to any member of the anion exchanger gene family (AEs). Our previous studies have characterized the existence of a distinct Cl(-)-OH(-) (HCO(3)(-)) exchanger, with minimal affinity for sulfate in the human colonic apical membrane vesicles (AMV). However, the mechanism(s) of sulfate movement across the colonocyte plasma membranes in the human colon is not well understood. Current studies were undertaken to elucidate sulfate transport pathways in AMVs of human proximal colon. Purified AMV and rapid filtration (35)SO(4)(--) uptake techniques were used. Our results demonstrate the presence of a pH gradient-driven carrier-mediated SO(4)(--)-OH(-) exchange process in the human proximal colonic luminal membranes based on the following: a marked increase in the SO(4)(--) uptake in the presence of an outwardly directed OH(-) gradient; a significant inhibition of SO(4)(--) uptake by the membrane anion transport inhibitor, DIDS; demonstration of saturation kinetics (K(m) for SO(4)(--): 0.80 +/- 0.17 mM and Vmax 649 +/- 74 pmol/mg protein/10 sec); competitive inhibition of SO(4)(--)-OH(-) exchange by oxalate; SO(4)(--) uptake was insensitive to alterations in the membrane potential; and inwardly directed Na(+) gradient under non-pH gradient conditions did not stimulate SO(4)(--) uptake. SO(4)(--) uptake was significantly inhibited by increasing concentrations of chloride (1-10 mM) in the incubation media with a K(i) for Cl(-) of 9.3 +/- 1.4 mM. In contrast, OH(-)/HCO(3)(-) gradient-driven (36)Cl(-) uptake into these vesicles was unaffected by increasing concentrations of sulfate (10-50 mM). The above data indicate that two distinct transporters may be involved in SO(4)(--) and Cl(-) transport in the human intestinal apical membranes: an anion exchanger with high affinity for SO(4)(--) and oxalate but low affinity for Cl(-), and a distinct Cl(-)-OH(-) (HCO(3)(-)) exchanger with low affinity for SO(4)(--).

Sulfate and chloride transport in Caco-2 cells: differential regulation by thyroxine and the possible role of DRA gene

The current studies were undertaken to establish an in vitro cellular model to study the transport of SO and Cl(-) and hormonal regulation and to define the possible function of the downregulated in adenoma (DRA) gene. Utilizing a postconfluent Caco-2 cell line, we studied the OH(-) gradient-driven (35)SO and (36)Cl(-) uptake. Our findings consistent with the presence of an apical carrier-mediated (35)SO/OH(-) exchange process in Caco-2 cells include: 1) demonstration of saturation kinetics [Michaelis-Menten constant (K(m)) of 0.2 +/- 0.08 mM for SO and maximum velocity of 1.1 +/- 0.2 pmol x mg protein(-1) x 2 min(-1)]; 2) sensitivity to inhibition by DIDS (K(i) = 0.9 +/- 0.3 microM); and 3) competitive inhibition by oxalate and Cl(-) but not by nitrate and short chain fatty acids, with a higher K(i) (5.95 +/- 1 mM) for Cl(-) compared with oxalate (K(i) = 0.2 +/- 0.03 mM). Our results also suggested that the SO/OH(-) and Cl(-)/OH(-) exchange processes in Caco-2 cells are distinct based on the following: 1) the SO/OH(-) exchange was highly sensitive to inhibition by DIDS compared with Cl(-)/OH(-) exchange activity (K(i) for DIDS of 0.3 +/- 0.1 mM); 2) Cl(-) competitively inhibited the SO/OH(-) exchange activity with a high K(i) compared with the K(m) for SO, indicating a lower affinity for Cl(-); 3) DIDS competitively inhibited the Cl(-)/OH(-) exchange process, whereas it inhibited the SO/OH(-) exchange activity in a mixed-type manner; and 4) utilizing the RNase protection assay, our results showed that 24-h incubation with 100 nM of thyroxine significantly decreased the relative abundance of DRA mRNA along with the SO/OH(-) exchange activity but without any change in Cl(-)/OH(-) exchange process. In summary, these studies demonstrated the feasibility of utilizing Caco-2 cell line as a model to study the apical SO/OH(-) and Cl(-)/OH(-) exchange processes in the human intestine and indicated that the two transporters are distinct and that DRA may be predominantly a SO transporter with a capacity to transport Cl(-) as well.

Human intestinal anion exchanger isoforms: expression, distribution, and membrane localization

A family of anion exchangers (AEs) including AE1, AE2 and AE3 has been described. AE3 gene has been shown to encode two alternatively spliced isoforms termed as bAE3 (brain subtype) and cAE3 (cardiac subtype). The identity of the AE(s) involved in the human intestinal NaCl absorption is not fully understood. Current studies were undertaken to identify the AE isoforms expressed in the human intestine, to define their regional and vertical axis (crypt vs. surface cells) distribution, and to elucidate their membrane localization in the epithelial cells along the entire length of the human intestine. Our studies utilizing reverse transcription (RT)-PCR with total RNA extracted from pinch biopsies from various regions of the human intestine demonstrate that AE2 and bAE3 but not AE1 or cAE3 were expressed in all the regions of the human intestine. Utilizing in situ RT-PCR, we demonstrated that the message of AE2 was expressed throughout the vertical surface--crypt axis of the colon. Our Western blotting studies demonstrated that AE2 and bAE3 are localized to the basolateral but not the apical membranes of the intestinal epithelial cells from the human ileum and colon. In conclusion, our results demonstrated that in the human intestine, AE2 and bAE3, but not AE1 or cAE3, are expressed throughout the tract with the highest expression in the colon compared to the ileum and jejunum. Both the isoforms were found to be localized to the basolateral but not the apical membranes of the epithelial cells. We speculate that, in the human intestine, AE2 and bAE3 may be the 'housekeeping' isoforms, and the apical AE, the potential candidate for chloride absorption, remains to be identified.

Residual chloride secretion in intestinal tissue of deltaF508 homozygous twins and siblings with cystic fibrosis. The European CF Twin and Sibling Study Consortium

BACKGROUND & AIMS

Cholinergic stimulation of chloride secretion is impaired in the intestines of patients with cystic fibrosis (CF). However, intestinal chloride secretion has been observed in patients with mild CF mutations. The aim of this study was to investigate residual Cl(-) secretion in the intestine of DeltaF508 homozygous CF patients, and examine the contribution of cystic fibrosis transmembrane conductance regulator (CFTR) and alternative Cl(-) conductances. Twins and siblings with identical CFTR genotypes were investigated to determine the impact of factors other than CFTR on chloride secretion.

METHODS

Chloride secretion in rectal tissue was investigated by applying Ca(2+) and adenosine 3',5'-cyclic monophosphate (cAMP)-linked agonists before and after the inhibition of alternative Cl(-) conductances with 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS).

RESULTS

cAMP-mediated Cl(-) secretion was observed in 73% of patients, and 20% showed DIDS-sensitive Ca(2+)-activated Cl(-) secretion. This DIDS-sensitive alternative chloride conductance was seen only in CF patients who also responded to cAMP agonists. Chloride secretion was more concordant within monozygous twins than within dizygous pairs.

CONCLUSIONS

These results suggest the presence of CFTR-mediated Cl(-) secretion in a subgroup of patients, implying that a portion of deltaF508 CFTR can be processed in vivo and function as a chloride channel in the apical membrane of intestinal cells. Moreover, a considerable number of deltaF508 homozygous patients express chloride conductances other than CFTR in their intestinal epithelia.

Basolateral K+ channel involvement in forskolin-activated chloride secretion in human colon

1. In this study we investigated the role of basolateral potassium transport in maintaining cAMP-activated chloride secretion in human colonic epithelium. 2. Ion transport was quantified in isolated human colonic epithelium using the short-circuit current technique. Basolateral potassium transport was studied using nystatin permeabilization. Intracellular calcium measurements were obtained from isolated human colonic crypts using fura-2 spectrofluorescence imaging. 3. In intact isolated colonic strips, forskolin and prostaglandin E2 (PGE2) activated an inward transmembrane current (ISC) consistent with anion secretion (for forskolin DeltaISC = 63.8+/-6.2 microA cm(-2), n = 6; for PGE2 DeltaISC = 34.3+/-5.2 microA cm(-2), n = 6). This current was inhibited in chloride-free Krebs solution or by inhibiting basolateral chloride uptake with bumetanide and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid DIDS). 4. The forskolin- and PGE2-induced chloride secretion was inhibited by basolateral exposure to barium (5 mM), tetrapentylammonium (10 microM) and tetraethylammonium (10 mM). 5. The transepithelial current produced under an apical to serosal K+ gradient in nystatin-perforated colon is generated at the basolateral membrane by K+ transport. Forskolin failed to activate this current under conditions of high or low calcium and failed to increase the levels of intracellular calcium in isolated crypts 6. In conclusion, we propose that potassium recycling through basolateral K+ channels is essential for cAMP-activated chloride secretion.

Mouse down-regulated in adenoma (DRA) is an intestinal Cl(-)/HCO(3)(-) exchanger and is up-regulated in colon of mice lacking the NHE3 Na(+)/H(+) exchanger

Mutations in human DRA cause congenital chloride diarrhea, thereby raising the possibility that it functions as a Cl(-)/HCO(3)(-) exchanger. To test this hypothesis we cloned a cDNA encoding mouse DRA (mDRA) and analyzed its activity in cultured mammalian cells. When expressed in HEK 293 cells, mDRA conferred Na(+)-independent, electroneutral Cl(-)/CHO(3)(-) exchange activity. Removal of extracellular Cl(-) from medium containing HCO(3)(-) caused a rapid intracellular alkalinization, whereas the intracellular pH increase following Cl(-) removal from HCO(3)(-)-free medium was reduced greater than 7-fold. The intracellular alkalinization in Cl(-)-free, HCO(3)(-)-containing medium was unaffected by removal of extracellular Na(+) or by depolarization of the membrane by addition of 75 mM K(+) to the medium. Like human DRA mRNA, mDRA transcripts were expressed at high levels in cecum and colon and at lower levels in small intestine. The expression of mDRA mRNA was modestly up-regulated in the colon of mice lacking the NHE3 Na(+)/H(+) exchanger. These results show that DRA is a Cl(-)/HCO(3)(-) exchanger and suggest that it normally acts in concert with NHE3 to absorb NaCl and that in NHE3-deficient mice its activity is coupled with those of the sharply up-regulated colonic H(+),K(+)-ATPase and epithelial Na(+) channel to mediate electrolyte and fluid absorption.