cDNA cloning and localization of a band 3-related protein from ileum.

A Cl(-)-HCO3- exchanger in the brush-border membrane mediates active Cl- absorption and regulates intracellular pH in rabbit ileum. The molecular identity of the ileal Cl(-)-HCO3- exchanger has not been established. The best-characterized plasma membrane Cl(-)-HCO3- exchanger is erythroid band 3. Structurally related proteins in nonerythroid tissues comprise an anion exchanger (AE) family. We used the polymerase chain reaction to amplify and clone a cDNA encoding an ileal band 3-related protein (B3RP) from rabbit ileal enterocytes. The composite sequence is 3,909 bp and is predicted to encode a protein of 136 kDa. The deduced amino acid sequence is 95% identical to murine renal AE2, indicating that ileal B3RP is rabbit AE2. Antisera generated against a cytoplasmic fragment of ileal B3RP recognized a 160- to 170-kDa polypeptide in the brush-border membrane, but not the basolateral membrane, of ileal crypt and villus enterocytes. This correlates with previous studies indicating that a Cl(-)-HCO3- exchange is present in brush-border but not basolateral membrane vesicles from rabbit ileal enterocytes. We conclude that ileal B3RP is a product of the AE gene family, and is present in the brush-border of ileal enterocytes, where it may mediate Cl(-)-HCO3- exchange.

Mechanism of intestinal secretion: effect of cyclic AMP on rabbit ileal crypt and villus cells.

Cyclic AMP-dependent secretagogues such as cholera toxin inhibit the coupled absorption of Na+ and Cl- and stimulate the secretion of HCO3- and Cl- in the ileum. Aside from Cl- secretion, little is known about the mechanism of these cyclic AMP-mediated effects. We therefore determined the effect of forskolin, an agent known to increase intracellular cyclic AMP by stimulation of adenylyl cyclase, on Na+/H+ and Cl-/HCO3- exchange in isolated crypt and villus cells from rabbit ileum. Forskolin increased cyclic AMP in the villus cells and decreased intracellular pH. The effect of forskolin on pH in villus cells was HCO3- independent, Na+ dependent, and amiloride sensitive. Further, the rate of recovery from an acid load was decreased by forskolin. These data suggest that increasing cyclic AMP inhibits Na+/H+ exchange in villus cells. In crypt cells also, forskolin increased cyclic AMP; however, forskolin increased intracellular pH in these cells. The effect of forskolin in crypt cells was also HCO3- independent, Na+ dependent, and amiloride sensitive. However, the rate of recovery from an acid load was increased by forskolin, the opposite effect of that seen in villus cells. These data suggest that increasing cyclic AMP in crypt cells stimulates Na+/H+ exchange. Inhibition of Na+/H+ exchange on the brush border membrane in villus cells would be expected to inhibit coupled NaCl absorption (which occurs by coupling of Na+/H+ and Cl-/HCO3- exchange). Stimulation of Na+/H+ exchange in crypt cells, present only on the basolateral membrane, alkalinizes the cell, which would be expected to stimulate HCO3- secretion by stimulating the Cl-/HCO3- exchanger on the brush border membrane. Thus, these results provide a mechanism for some of the previously unexplained in vivo and in vitro effects of cyclic AMP on ileal electrolyte transport.

pH regulation in ileum: Na(+)-H+ and Cl(-)-HCO3- exchange in isolated crypt and villus cells.

Current evidence suggests that intestinal crypt and villus cells have different functions in electrolyte transport. To study the regulation of transporters, we isolated and separated these two cell types. This was accomplished by sequential collection of enterocytes from rabbit ileal loops incubated with buffered solutions of calcium chelators. Alkaline phosphatase and thymidine kinase activity, sodium-glucose cotransport, and morphological criteria were used to determine cell separation. Cell viability was evaluated with trypan blue exclusion, leucine incorporation into protein, and morphological features. The role of Na(+)-H+ and Cl(-)-HCO3- exchange in the regulation of intracellular pH was analyzed using an intracellular pH sensitive dye, BCECF. Removal of external Na+ or the addition of amiloride resulted in acidification of both crypt and villus cells. Removal of Cl- or the addition of DIDS resulted in alkalinization of both cell types. The cells could be acidified with NH4Cl, and recovery from this acid load was dependent on Na+ and inhibited by amiloride. Similarly, the cells could be alkalinized with propionate and recovery was Cl- dependent and DIDS sensitive. These data are consistent with the presence of Na(+)-H+ and Cl(-)-HCO3- exchange in both crypt and villus cells. Both exchanges appear to be involved in the regulation of basal pH as well as in recovery from alterations in intracellular pH. Having demonstrated the presence of Na(+)-H+ and Cl(-)-HCO3- exchange activity in both crypt and villus cells, we can now use these cells to determine the regulation of these exchangers by intracellular second messengers.

Mechanism of intestinal secretion. Effect of serotonin on rabbit ileal crypt and villus cells.

To determine the mechanism of action of an intestinal secretagogue, serotonin, we have isolated crypt and villus cells and demonstrated Na:H and Cl:HCO3 exchange activity using the intracellular pH-sensitive fluorescent dye, 2,7-bis (carboxy-ethyl)-5,6-carboxy-fluorescein. Serotonin alkalinized both crypt and villus cells. Alkalinization in villus cells was HCO3 dependent and Na independent. In contrast, alkalinization in crypt cells was HCO3 independent and Na dependent. In villus cells, recovery from an alkaline load induced by Cl removal, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid or propionate pulse, known to occur via the Cl:HCO3 exchange, is inhibited by serotonin. In contrast, in crypt cells, recovery from an acid load induced by Na removal, amiloride and NH4Cl pulse, known to occur via Na:H exchange, is stimulated by serotonin. These data suggest that serotonin is inhibiting Cl:HCO3 exchange in villus cells and stimulating Na:H exchange in crypt cells. These effects of serotonin would be expected to inhibit coupled Na and Cl absorption by villus cells and stimulate HCO3 secretion by crypt cells in the intact ileum.

Sulfate and oxalate exchange for bicarbonate across the basolateral membrane of rabbit ileum.

The purpose of these studies was to further define the transport of SO4(2-) and oxalate across the basolateral membrane (BLM) of rabbit ileum. We previously found evidence for Cl-(-)SO4(2-) exchange but no evidence for carrier-mediated oxalate transport. A HCO3- gradient (but not a pH gradient) was found to stimulate SO4(2-) and oxalate uptake in BLM vesicles; uptake was inhibited by DIDS. Oxalate cis-inhibited HCO3- gradient SO4(2-) uptake and trans-stimulated SO4(2-) uptake, suggesting SO4(2-) and oxalate used the same carrier (SO4(2-)- and oxalate-HCO3-exchange). Cl- had no effect on HCO3- gradient-stimulated SO4(2-) uptake, indicating that Cl(-)-SO4(2-) exchange was a different carrier. Other substrates found to be transported on the HCO3- exchanger were oxaloacetate and S2O3(2-)-.SO4(2-), S2O3(2-). and oxalate were found to also use the SO4(2-)-Cl- exchanger, whereas oxalate was only transported on the HCO3- exchanger. SO4(2-)-HCO3- exchange was found on villus, but not crypt cell, BLM. These results indicate that there is a SO4(2-)-HCO3- exchanger on the BLM of villus cells that also transports oxalate, and along with our previous studies, the results provide evidence for the transepithelial transport of oxalate.

Characterization of Na(+)-H+ exchangers on villus cells in rabbit ileum.

The presence of Na(+)-H+ exchange activity is demonstrated on both the brush-border membrane (BBM) and the basolateral membrane (BLM) of villus cells from rabbit ileum. The possibility that the Na(+)-H+ exchange activity on the BLM represents HCO3- cotransport is excluded. The two Na(+)-H+ exchangers are then compared in terms of kinetics and substrate and inhibitor specificity. The most striking difference between the two exchangers was sensitivity to amiloride and K+. The IC50 for amiloride on the BLM was 10-fold lower than the BBM (11.2 +/- 2.1 vs. 103 +/- 20.9 microM; P less than 0.02). External K+, in concentrations as low as 10 mM, inhibited Na(+)-H+ exchange on the BBM but not on the BLM. The Na+ Km and proton Km were twice as high on the BLM exchanger (46.3 +/- 3.4 vs. 28.8 +/- 2.3 mM and 468 +/- 9 vs. 232 +/- 45 nM, respectively). Proton Vmax was similar, whereas Na+ Vmax was higher on the BLM. Inhibition by Li+ was similar on both membranes. These results indicate distinct differences between the two Na(+)-H+ exchangers. Whether these differences are due to the two different gene products or are the result of posttranslational modification of a single gene product remains to be determined.

Rabbit ileal brush-border membrane Cl-HCO3 exchanger is activated by an internal pH-sensitive modifier site.

The purpose of these studies was to look for evidence of a pH-sensitive modifier site on the Cl-HCO3 exchanger on the brush-border membrane in rabbit ileum utilizing membrane vesicles. When internal pH and HCO3 were increased, Cl uptake was stimulated in a sigmoidal fashion consistent with a modifier effect. Increasing internal HCO3 alone did not have a similar effect, and increasing pH alone in the absence of HCO3 resulted in very little uptake of Cl. These results suggested that OH was a poor substrate for the exchanger but that it "activated" the transport of HCO3. Further evidence for this hypothesis was provided by the observation that increasing internal pH also stimulated Cl-Cl exchange. Altering the membrane potential with K and valinomycin had no effect on Cl uptake at high internal pH, suggesting no change in the 1:1 coupling ratio for Cl-HCO3 exchange at high internal pH. These studies provide evidence that there is an internal pH-sensitive modifier site on the Cl-HCO3 exchanger.

Sodium-adenosine cotransport in brush-border membranes from rabbit ileum.

To determine the mechanism(s) of transcellular adenosine transport in epithelial tissues that possess an adenosine receptor response, we studied [3H]adenosine uptake using vesicles prepared from isolated brush-border and basolateral membranes of the rabbit ileum. In the presence of the adenosine deaminase inhibitor deoxycoformycin uptake of [3H]adenosine into brush-border membrane vesicles is stimulated fivefold by an inwardly directed Na gradient. Na-dependent [3H]adenosine uptake is enhanced and concentrative under conditions that increase inside negativity of vesicles, thus providing evidence for an electrogenic carrier. Na-dependent adenosine uptake is a saturable function of adenosine concentration with a Michaelis-Menten constant of 17.3 +/- 7.1 microM and maximum transport rate of 216.9 +/- 20.2 pmol.min-1.mg protein-1. Both uridine and inosine inhibit [3H]adenosine uptake, suggesting that the Na-dependent transporter has broad substrate specificity for both purine and pyrimidine ribonucleosides. Na-dependent adenosine uptake is inhibited by dipyridamole but is insensitive to 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine. We conclude that adenosine is transported across ileal brush-border membranes by a Na-ribonucleoside cotransport system. In contrast, adenosine uptake in basolateral membranes is not stimulated by a Na gradient. These studies show asymmetry in the distribution of transport systems for adenosine in polarized intestinal epithelia.

Pulmonary manifestations of gastrointestinal disease.

The respiratory and gastrointestinal systems share several functional similarities. In health, the two systems remain structurally distinct and functionally integrated, so as to maintain physiologic homeostasis. However, in the setting of disease, pathophysiologic alterations in one system may be reflected in the other. This article focuses upon the nonmalignant gastrointestinal causes of respiratory illness. Using an anatomic approach, the pulmonary manifestations of selected gastrointestinal diseases are outlined. Emphasis is placed on the distinguishing features, pathophysiology, and therapy of the pulmonary sequelae of digestive diseases.

Evaluation of the patient with suspected malabsorption.

The term "malabsorption" is generally used to indicate any defect in absorption; strictly speaking, however, it is a defect in the mucosal phase of absorption. Defects in the intraluminal phase are termed "maldigestion." This distinction is essential when considering the pathophysiology to apply diagnostic tests in the evaluation of malabsorption. This article first discusses the signs and symptoms associated with malabsorption, the disorders associated with malabsorption, normal intestinal absorption, and, finally, the diagnostic test used to investigate absorption.

Membrane distribution of sodium-hydrogen and chloride-bicarbonate exchangers in crypt and villus cell membranes from rabbit ileum.

Present evidence suggests that in the small intestine, villus cells are primarily absorptive and crypt cells are primarily secretory. In order to further confirm that there are differences in transport properties between villus and crypt cells, we have separated villus from crypt cells, using calcium chelations techniques, and determined the distribution of Na:H and Cl:HCO3 exchange activity on brush border membrane and basolateral membrane preparations from these two cell populations. Separation of cells was determined utilizing alkaline phosphatase and maltase activity as a marker of villus cells and thymidine kinase activity as a marker of crypt cells. Utilizing these techniques, we were able to sequentially collect cells along the villus-crypt axis. Na-stimulated glucose and alanine uptake in brush border membrane vesicles diminished from the villus to the crypt region in the sequentially collected cells fractions, further suggesting separation of these cells. Brush border and basolateral membranes were then prepared from cells from the villus and crypt areas, utilizing a continuous sucrose gradient. In the villus cells, Na:H exchange activity was found associated with both the brush border and basolateral membrane, whereas, in crypt cells, Na:H exchange activity was only found on the basolateral membrane. Cl:HCO3 exchange activity was found only on the brush border membrane, in both villus and crypt cells. These studies suggest functional heterogeneity in ion transport between villus and crypt cells.

Evidence for carrier-mediated Cl-SO4 exchange in rabbit ileal basolateral membrane vesicles.

In rabbit ileal basolateral membrane (BLM) vesicles, an outwardly directed Cl gradient ([Cl] in/out = 60/6 mM) stimulated the initial velocity of SO4 uptake compared with uptake in the absence of Cl. Under Cl gradient conditions, SO4 was transiently accumulated at a concentration twice that found at equilibrium ("overshoot"). Chloride gradient-stimulated SO4 uptake was markedly reduced by inhibitors of anion exchange (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid) and was saturable (SO4 Km = 0.302 +/- 0.064 mM; Vmax = 1.59 +/- 0.22 nmol SO4 . mg protein-1 . min-1). SO4 uptake by BLM vesicles was not stimulated by imposition of an inside-positive electrical potential, suggesting that the stimulation by a Cl gradient was not due to an induced electrical potential. Oxalate, nitrate, iodide, and bromide inhibited the initial velocity of Cl gradient-stimulated SO4 uptake, whereas phosphate, beta-hydroxybutyrate, lactate, and p-aminohippurate had no effect. When SO4 uptake by BLM vesicles was compared with that of brush-border membrane vesicles, Cl gradient-stimulated SO4 uptake was found predominantly in the BLM preparation. In conclusion, these findings provide evidence for a carrier on the ileal basolateral membrane that mediates Cl-SO4 exchange.

Augmentation of neutral sodium chloride absorption by increased flow rate in rat ileum in vivo.

Studies in intact animals have shown that intestinal solute absorption is enhanced with increasing flow rates; the mechanism of this phenomenon has not been explored in detail. We used single pass perfusions of rat ileum to study the effect of higher flow rate on electrolyte absorption. Augmenting perfusion rate from 0.5 to 5.0 ml/min resulted in increased rates of sodium (11.0 +/- 0.9 vs. 23.5 +/- 2.7 mueq/min X g) and chloride (12.1 +/- 0.8 vs. 25.0 +/- 2.2 mueq/min X g) absorption, reduction in the estimated unstirred layer thickness (668 +/- 31 vs. 433 +/- 28 micron), minimal changes in intraluminal pressure and transmural potential difference, and a small, though significant, increase in intraluminal volume (19.4 +/- 8.4%). Removal of sodium from the perfusion medium abolished the effect of increased flow rate on chloride absorption as did removal of chloride on sodium absorption; addition of furosemide or acetazolamide to Ringer's solution also inhibited this effect. In separate experiments, stepwise increases in intraluminal volume were induced by elevating the outflow tubing; no effect on electrolyte transport was observed. These studies demonstrate that neutral sodium chloride absorption is enhanced in rat ileum at higher flow rates, perhaps as a result of a decrease in the thickness of unstirred layers.

Phytohemagglutinin from red kidney bean (Phaseolus vulgaris) inhibits sodium and chloride absorption in the rabbit ileum.

Phytohemagglutinin (PHA), derived from red kidney bean (Phaseolus vulgaris), can induce malabsorption and diarrhea when fed to rats. In this study, we determined the effect of PHA on ion transport in the rabbit ileum in vitro. Compared with control tissues, PHA (1 mg/ml) added to the mucosal solution increased short-circuit current (1.1 +/- 0.2 microEq/cm2 X h, p less than 0.001), decreased net Na (-1.0 +/- 0.5 microEq/cm2 X h, p less than 0.02) and Cl (-1.2 +/- 0.6 microEq/cm2 X h, p less than 0.025) absorption, and decreased tissue conductance (-1.8 +/- 0.5 mS/cm2, p less than 0.001). Serosal addition of PHA had no effect on the short-circuit current or tissue conductance. Mucosal PHA did not increase mucosal levels of cyclic adenosine monophosphate or cyclic guanosine monophosphate. Removal of serosal calcium did not affect the increase in short-circuit current induced by mucosal PHA. Utilizing fluorescent microscopy, rhodamine-labeled PHA was found to bind to the luminal border of villus cells, but not to crypt cells, in the ileum. In the descending rabbit colon, PHA did not affect either the short-circuit current or conductance, and rhodaminated PHA did not bind to the epithelial surface. Using the increase in short-circuit current as an indicator of absorption, PHA did not affect Na-coupled glucose or amino acid absorption in the ileum. This study suggests that dietary lectins may play a role in regulating intestinal fluid and electrolyte transport.

Oxalate transport by anion exchange across rabbit ileal brush border.

This study demonstrates the presence of oxalate transporters on the brush border membrane of rabbit ileum. We found that an inside alkaline (pH = 8.5 inside, 6.5 outside) pH gradient stimulated [14C]oxalate uptake 10-fold at 1 min with a fourfold accumulation above equilibrated uptake at 5 min. 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonate (disodium salt; DIDS) profoundly inhibited the pH-gradient stimulated oxalate uptake. Using an inwardly directed K+ gradient and valinomycin, we found no evidence for potential sensitive oxalate uptake. In contrast to Cl:HCO3 exchange, HCO3 did not stimulate oxalate uptake more than was seen with a pH gradient in the absence of HCO3. An outwardly directed Cl gradient (50 mM inside, 5 mM outside) stimulated oxalate uptake 10-fold at 1 min with a fivefold accumulation above equilibrated uptake. Cl-stimulated oxalate uptake was largely inhibited by DIDS. Addition of K+ and nigericin only slightly decreased the Cl gradient-stimulated oxalate uptake, which indicates that this stimulation was not primarily due to the Cl gradient generating an inside alkaline pH gradient via Cl:OH exchange. Further, an outwardly directed oxalate gradient stimulated 36Cl uptake. These results suggested that both oxalate:OH and oxalate:Cl exchange occur on the brush border membrane. To determine if one or both of these exchanges were on contaminating basolateral membrane, the vesicle preparation was further fractionated into a brush border and basolateral component using sucrose density gradient centrifugation. Both exchangers localized to the brush border component. A number of organic anions were examined (outwardly directed gradient) to determine if they could stimulate oxalate and Cl uptake. Only formate and oxaloacetate were found to stimulate oxalate and Cl uptake. An inwardly directed Na gradient only slightly stimulated oxalate uptake, which was inhibited by DIDS.

pH gradient-stimulated sulfate transport by rabbit ileal brush-border membrane vesicles: evidence for SO4-OH exchange.

In the presence of a pH gradient (7.7 inside, 5.5 outside), the initial velocity of SO4 uptake by rabbit ileal brush-border membrane (BBM) vesicles was markedly stimulated compared with uptake in the absence of a pH gradient. Under pH gradient conditions, SO4 was transiently accumulated at a concentration 13-fold higher than at equilibrium ("overshoot"). Superimposition of a HCO3 gradient did not further stimulate the initial velocity of SO4 uptake compared with a pH gradient alone. Evidence that this pH gradient-stimulated SO4 uptake represented SO4-OH exchange included lack of sensitivity of SO4 transport to alterations of the membrane potential; 85-95% inhibition of SO4 uptake by the anion exchange inhibitors DIDS and SITS; and saturation kinetics (Km for SO4 = 0.475 +/- 0.054 mM; Vmax = 4.1 +/- 0.1 nmol SO4 X mg prot-1 X min-1). Sulfate did not inhibit pH gradient-stimulated 36Cl uptake, indicating that SO4-OH and Cl-HCO3(OH) are different exchangers. When BBM vesicles were compared with basolateral membrane (BLM) vesicles, pH gradient-stimulated SO4 uptake was found predominantly in the BBM preparation. Brush-border SO4-OH exchange was further localized by demonstrating Na-stimulated SO4 efflux from vesicles loaded under pH gradient conditions, suggesting that Na-SO4 cotransport and SO4-OH exchange are on the same BBM vesicles. In conclusion, a SO4-OH exchanger (or H-SO4 cotransporter) exists on the brush border of rabbit ileum which is distinct from the brush-border Cl-HCO3(OH) exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cations on pH gradient-stimulated sulfate transport in rabbit ileal brush-border membrane vesicles.

In brush-border membrane vesicles from rabbit ileum, we previously reported pH gradient-stimulated SO4 uptake and presented evidence that this represents carrier-mediated SO4-OH exchange. In the present study inhibitors of SO4-OH exchange (H-SO4 cotransport) were shown not to inhibit Na-SO4 cotransport, suggesting that these are two separate carrier-mediated transport mechanisms. While pH gradient-stimulated SO4 uptake was inhibited 87% by 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, disodium salt (DIDS) and 79% by 1.0 mM furosemide, Na+-stimulated SO4 uptake was only inhibited 11 and 0%, respectively. K+ (20 mM), Cl (5 mM), and oxalate (0.25 mM) inhibited pH gradient-stimulated SO4 uptake (38-65%) but had no effect on Na+-stimulated SO4 uptake. Finally, at Na+ concentrations (10 mM) significantly less than that required for Na+-stimulated SO4 uptake (60-100 mM), external Na+ inhibited pH gradient-stimulated SO4 uptake, suggesting two independent effects of this cation. SO4 uptake was also inhibited by external K+ both in the presence and absence of a pH gradient. A Dixon plot of the DIDS-sensitive SO4 uptake under pH gradient conditions yielded a straight line, indicating a single site of interaction between external K+ and the SO4-OH carrier (apparent Ki = 7.2 mM). In contrast to the inhibition by external K+, internal K+ stimulated SO4 uptake. This effect was DIDS sensitive and not enhanced by valinomycin, suggesting an interaction of internal K+ with the SO4-OH exchanger independent of a K+-induced electrical potential. SO4 uptake and the effects of K+ were pH modulated with less SO4 uptake and less K+ effect at higher pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium and chloride transport across rabbit ileal brush border. II. Evidence for Cl-HCO3 exchange and mechanism of coupling.

An inside-alkaline pH gradient (pH 7.7 inside, 5.5 outside) stimulated Cl uptake in brush-border vesicles from rabbit ileum. The addition of HCO3 without changing the pH gradient further stimulated Cl uptake to a level fourfold greater than equilibrated Cl uptake. Although a K diffusion potential stimulated Cl uptake, this was insensitive to inhibition by 4,4-diisothiocyanostilbene-2,2'-disulfonate (DIDS), whereas pH and HCO3 gradient-stimulated Cl uptake was inhibited by DIDS. pH and HCO3 gradient-stimulated Cl uptake was found to be a saturable function of the Cl concentration with a Km of 3.5 mM and a Vmax of 49 nmol X mg prot-1 X min-1. To distinguish between coupling of Na and Cl transport by cotransport or dual exchange (Na-H and Cl-HCO3 exchange), we determined uptake with high (134 mM Tris-HEPES-MES) internal buffer and low (1.34 mM Tris-HEPES-MES) internal buffer concentrations. Inwardly directed gradients of 50 mM Na, 50 mM K, or 50 mM Na and K did not stimulate Cl uptake, and 50 mM Cl, 50 mM K, or 50 mM KCl did not stimulate Na uptake, with high internal buffer, excluding cotransport. In contrast, 50 mM Na stimulated Cl uptake (inhibited by 1 mM DIDS) and 50 mM Cl stimulated Na uptake (inhibited by 1 mM amiloride) in low buffer media. To determine a role for carbonic anhydrase, Na-stimulated Cl uptake was determined in low buffer media, equilibrated with either 100% N2 or 95% N2-5% CO2. Na stimulated Cl uptake 80% (compared with trimethylammonium control) with CO2 but only 30% with N2 (P less than 0.05). Acetazolamide partially inhibited (P less than 0.025) the stimulation of Cl uptake with CO2 but not with N2. Carbonic anhydrase activity was measured in homogenate and brush-border membrane and was enriched 7.9 +/- 0.4-fold, whereas sucrase was enriched 14.0 +/- 1.1-fold. We conclude that coupled Na and Cl transport occurs by dual exchange (Na-H and Cl-HCO3) and carbonic anhydrase, apparently located on the brush-border membrane, facilitates dual exchange by providing HCO3.

Evidence for carrier-mediated chloride/bicarbonate exchange in canalicular rat liver plasma membrane vesicles.

To determine whether anion exchangers might play a role in hepatic bile formation, we looked for the presence of Cl-:OH- and Cl-:HCO3- exchange in highly purified canalicular (c) and basolateral (bl) rat liver plasma membrane (LPM) vesicles. In cLPM vesicles, a pH gradient (7.7 in/6.0 out) stimulated 36Cl- uptake twofold above values obtained during pH-equilibrated conditions (7.7 in = out). When 50 mM HCO3- was also present inside the vesicles, the same pH gradient (7.7 in/6.0 out) resulted in Cl- uptake to levels fourfold above pH- and HCO3--equilibrated controls and two- to threefold above Cl- equilibrium (overshoot). Initial rates of both pH and HCO3- gradient-stimulated Cl- uptake were completely inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS). A valinomycin-induced K+ diffusion potential (inside positive) also stimulated Cl- uptake in cLPM, but this conductive Cl- pathway was insensitive to DIDS. The DIDS-sensitive, pH and HCO3- gradient-stimulated Cl- uptake demonstrated: saturation with Cl- (Km approximately 6.3 mM; Vmax approximately 51 nmol X mg-1 X min-1); partial inhibition by bumetanide (26%), furosemide (33%), probenecid (37%), and 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (49%); cis-inhibition by chloride and nitrate but not by sulfate and various organic anions, and independence from the membrane potential. These data demonstrate the presence of an electroneutral Cl-:OH- and Cl-:HCO3- exchanger in rat liver canalicular membranes that favors Cl-:HCO3- exchange. In contrast, no evidence was found for the presence of a Cl-:HCO3- (OH-) exchange system in blLPM vesicles. Furthermore, neither blLPM nor cLPM vesicles exhibited Na+-stimulatable Cl- uptake, indicating the absence of a NaCl co-transport system in either LPM subfraction. These findings are consistent with a functional role for a Cl-:HCO3- (OH-) exchanger in canalicular bile formation.

Nephrolithiasis and intestinal disease.

Kidney stones in patients with inflammatory bowel disease are usually composed of calcium oxalate. Two factors are important in the increased absorption of dietary oxalate which is responsible for those stones: 1) increased absorption of oxalate in the presence of steatorrhea, and 2) increased permeability of the colon to oxalate. Fortunately, some of the physiologic abnormalities can be corrected. A therapeutic approach is detailed.