Mechanism of inhibition of Na+-bile acid cotransport during chronic ileal inflammation in rabbits

U. Sundaram1, S. Wisel1, S. Stengelin2, W. Kramer2, and V. Rajendran3

1 Division of Digestive Diseases, Departments of Medicine and Physiology, Ohio State University School of Medicine, Columbus, Ohio 43210; 2 Hoechst Marion Roussel, D-65926 Frankfurt, Germany; and 3 Yale University School of Medicine, New Haven, Connecticut 06520

    ABSTRACT
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Abstract
Introduction
Methods
Results
Discussion
References

In the chronically inflamed ileum, unique mechanisms of alteration of similar transport processes suggest regulation by different immune-inflammatory mediator pathways. In a rabbit model of chronic ileitis, we previously demonstrated that Na+-glucose cotransport was inhibited by a decrease in the cotransporter numbers, whereas Na+-amino acid cotransport was inhibited by a decrease in the affinity for the amino acid. In this study, we demonstrated that Na+-bile acid cotransport was reduced in villus cells from the chronically inflamed ileum. In villus cell brush-border membrane vesicles from the chronically inflamed ileum, Na+-bile acid cotransport was reduced as well, suggesting a direct effect at the cotransporter itself. Kinetic studies demonstrated that Na+-bile acid cotransport was inhibited by both a decrease in the affinity as well as a decrease in the maximal rate of uptake of the bile acid. Analysis of steady-state mRNA and immunoreactive protein levels of the Na+-bile acid cotransporter also demonstrated some reduction during chronic ileitis. Thus, in the chronically inflamed ileum, the mechanisms of inhibition of Na+-glucose, Na+-amino acid, and Na+-bile acid cotransport are different. These data suggest that different cotransporters are uniquely altered either secondary to their intrinsic differences or by different immune-inflammatory mediators during chronic ileitis.

inflammatory bowel disease; intestinal nutrient absorption; immune regulation of nutrient transport; sodium-potassium-adenosinetriphosphatase; bile acid absorption

    INTRODUCTION
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Abstract
Introduction
Methods
Results
Discussion
References

IN DISEASES CHARACTERIZED by chronic inflammation of the terminal ileum (e.g., chronic inflammatory bowel disease) bile acid malabsorption is commonly seen (6, 9, 21). The wide variety of immune-inflammatory mediators known to be endogenously produced in the chronically inflamed ileum may, at least in part, have an effect on transport pathways (2, 11, 13). At present, it is not known whether a given immune-inflammatory mediator pathway is responsible for alterations seen with a specific transport pathway during chronic ileitis. Unique mechanisms of alteration of each member of a related family of transport pathways in the chronically inflamed ileum would suggest that different immune-inflammatory pathways may regulate different transport pathways.

In the normal ileum, a group of similar cotransporters exist that are not only important for Na+ absorption but also for solute assimilation (14). It is not currently known whether the different Na+-dependent solute cotransporters that transport different solutes are altered by different mechanisms in the chronically inflamed ileum. This is undoubtedly secondary to the lack of animal models of chronic ileitis and the inability to isolate viable enterocytes suitable for the study of electrolyte transport from the chronically inflamed intestine.

In a rabbit model of chronic ileal inflammation, we have previously demonstrated that two of the Na+-dependent nutrient cotransport pathways, Na+-glucose cotransport and Na+-amino acid cotransport, were inhibited by different mechanisms. The mechanism of inhibition of Na+-glucose cotransport during chronic ileal inflammation was due to a decrease in the number of cotransporters and not secondary to an alteration in the affinity for glucose (20). In contrast, the mechanism of inhibition of Na+-alanine cotransport during chronic ileal inflammation was secondary to a decrease in the affinity for alanine rather than a decrease in the number of cotransporters (19).

Another important Na+-dependent solute cotransport process in the normal ileum is Na+-bile acid cotransport (5, 22). This cotransport process is primarily responsible for bile acid assimilation in the intestine. Although bile acid malabsorption is well recognized during chronic ileal inflammation (6, 9, 21), the mechanism of this alteration is unknown. Given this background, as a representative of Na+-bile acid cotransport, we studied Na+-dependent taurocholate uptake in this model of chronic ileitis. Na+-taurocholate cotransport has previously been demonstrated in the rabbit ileum (8). The favorable Na+ gradient for this cotransport is provided by Na+-K+-ATPase (5, 22). Thus, during chronic ileal inflammation, cellular alterations in Na+-taurocholate cotransport may be at the level of the cotransporter and/or secondary to an alteration in Na+-K+-ATPase.

Therefore, the aim of this study was to test the hypothesis that chronic inflammation uniquely alters Na+-taurocholate cotransport in the rabbit model of chronic ileitis and to determine the cellular mechanisms of this alteration.

    METHODS
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Abstract
Introduction
Methods
Results
Discussion
References

Induction of chronic inflammation. Chronic ileal inflammation was produced in rabbits as previously reported (18). Pathogen-free rabbits were intragastrically inoculated with 10,000 Eimeria magna oocytes or sham inoculated with 0.9% NaCl (control animals). None of the sham inoculations and ~80% of inoculations with coccidia resulted in chronic ileal inflammation during days 13-15. Only enterocytes from those animals that had histologically confirmed chronic ileal inflammation were utilized for experiments.

Cell isolation. Villus and crypt cells were isolated from the normal and the chronically inflamed ileum by a Ca2+ chelation technique, as previously described (17, 18, 20). Previously established criteria were utilized to validate good separation and viability of villus and crypt cells (17-20). The cells were maintained in short-term culture for up to 6-8 h. Cells utilized for brush-border membrane vesicle (BBMV) preparation were frozen immediately in liquid nitrogen and stored at -70°C until required.

BBMV preparation. BBMV from rabbit ileal villus cells were prepared by CaCl2 precipitation and differential centrifugation as previously described (20). BBMV were resuspended in a medium appropriate to each experiment. BBMV purity was assured with marker enzyme enrichment (17-20).

Uptake studies in villus and crypt cells. Uptake studies were performed by rapid filtration technique, as previously described (20). In brief, villus or crypt cells (100 mg wet wt) were washed and resuspended in HEPES buffer containing (in mM) 0.1 taurocholate, 4.5 KCl, 1.2 KH2PO4, 1.0 MgSO4, 1.25 CaCl2, 20 HEPES, and 130 sodium chloride or choline chloride and gassed with 100% O2 (pH 7.4 at 37°C). [3H]taurocholate (10 µCi) was added to a 1-ml cell suspension in the HEPES buffer, and 100-µl aliquots were removed at desired time intervals. The uptake was arrested by mixing with 3 ml ice-cold stop solution (choline-HEPES buffer). The mixture was filtered on 0.65-µm Millipore (HAWP) filters. The filters were washed with ice-cold stop solution with 1 mM taurocholate. Then the filters were dissolved in 4 ml Optifluor, and the radioactivity was determined in a Beckman LS-5 scintillation counter.

BBMV uptake studies. Uptake studies were performed by rapid filtration technique, as previously described (20). In brief, 10 µl of BBMV resuspended in 100 mM choline chloride, 0.10 mM MgSO4, 50 mM HEPES-Tris (pH 7.5), 50 mM mannitol, and 50 mM KCl were incubated in 90 µl reaction medium that contained 50 mM HEPES-Tris buffer (pH 7.5), 0.1 mM taurocholate, 20 µCi [3H]taurocholate, 0.10 mM MgSO4, 50 mM KCl, 50 mM mannitol, and 100 mM of either NaCl or choline chloride. The vesicles were voltage clamped with 10 µM valinomycin and 100 µM FCCP. At desired times, uptake was arrested by mixing with ice-cold stop solution [50 mM HEPES-Tris buffer (pH 7.5), 0.10 mM MgSO4, 75 mM KCl, and 100 mM choline chloride]. The mixture was filtered on 0.45-µm Millipore (HAWP) filters and washed twice with 3 ml ice-cold stop solution containing 1 mM taurocholate. Filters with BBMV were dissolved in Optifluor, and radioactivity was determined in a Beckman LS-5 scintillation counter.

Na+-K+-ATPase measurement. Na+-K+-ATPase was measured as Pi liberated by the method of Forbush (4) in cellular homogenates from the same amount of cells from normal or inflamed ileum as previously described (20). Enzyme specific activity was expressed as nanomoles of Pi released per milligram of protein per minute.

Northern blot studies. Total RNA was extracted from rabbit ileal villus cells by the guanidinium isothiocyanate-cesium chloride method, as previously reported (3, 20). mRNA was isolated from total RNA utilizing oligo(dT)-cellulose chromatography (1). After denaturation, mRNA was electrophoresed on 1.8% agarose-formaldehyde gel, transferred to a nylon membrane (Schleicher & Schuell, Keene, NH), and incubated with prehybridization solution. Membranes were hybridized with 32P-labeled cDNA. Hybridized membrane was exposed to autoradiography film (NEN Research Products, Boston, MA). Human beta -actin was utilized to ensure equal loading of mRNA onto the electrophoresis gels. Rabbit ileal specific Na+-bile acid cotransporter cDNA and beta -actin DNA were random labeled with [32P]CTP with Klenow polymerase. Densitometric analysis of the Northern blots was done using Pharmacia LKB Ultrascan XL with Alpha Imager 2000.

Western blot studies. BBMV (4 mg) were diluted in SDS reducing buffer, boiled, and electrophoresed on a 12% SDS-PAGE gel. The gel was electroblotted onto polyvinyldifluoride membrane (NEN Research Products) and blocked for 2 h in 5% BSA at room temperature. The membrane was incubated at room temperature with 1:3,000 antibody against rabbit ileal specific Na+-bile acid cotransporter (anti-KIBMAL3) followed by goat anti-rabbit IgG coupled to horseradish peroxidase (1:10,000, Pierce, Rockford, IL). Anti-KIBMAL3 is an antibody directed against the COOH-terminal 26 residues from rabbit Na+-bile acid cotransporter. After each incubation, the membrane was washed extensively with PBS and 0.2% Tween 20. The signal was developed with the chemiluminescence Western blot kit (NEN Research Products). Densitometric analysis of the Western blots was performed using Pharmacia LKB Ultrascan XL with Alpha Imager 2000.

Data presentation. When data are averaged, means ± SE are shown, except when error bars are inclusive within the symbol. All uptakes were done in triplicates, and n for any set of experiments refers to vesicle or isolated cell preparations from different animals. Preparations in which cell viability was <85% were excluded from analysis. Student's t-test was used for statistical analysis.

    RESULTS
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Abstract
Introduction
Methods
Results
Discussion
References

Initially, the distribution of Na+-taurocholate cotransport along the crypt-villus axis of the normal rabbit ileum was determined. In villus cells from the normal ileum (Fig. 1A), taurocholate uptake was significantly stimulated by extracellular Na+ for up to 30 min. However, Na+-stimulated taurocholate uptake was not observed in crypt cells from the normal ileum (Fig. 1B).


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Fig. 1.   Effect of extracellular Na+ on [3H]taurocholate uptake as a function of time in villus and crypt cells from normal rabbit ileum. Only least significant P values are shown. A: villus cells. Extracellular Na+ significantly stimulated [3H]taurocholate uptake at all time points. B: crypt cells. Extracellular Na+ does not stimulate [3H]taurocholate uptake.

To confirm these findings, we measured steady-state mRNA levels for the rabbit ileal specific Na+-bile acid cotransporter in villus and crypt cells from the normal rabbit ileum. The cDNA probe identified a band at the expected size of 4.6 kb in villus cells (Fig. 2A). Minimal expression (<5% of that in villus cells) is seen in crypt cells, which may reflect contamination of crypt cell function with villus cells. These results suggest that the message for Na+-bile acid cotransport is primarily present in villus cells from the normal rabbit ileum.


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Fig. 2.   A: crypt-villus distribution of steady-state mRNA levels of Na+-bile acid cotransporter in normal ileum. Data in A are representative of 4 experiments. Northern blot analysis demonstrated that steady-state levels of mRNA for Na+-bile acid cotransporter are present primarily in villus cells from normal ileum. Human beta -actin was utilized to ensure equal loading of mRNA in gel. B: Western blot analysis demonstrated the presence of immunoreactive rabbit ileal specific Na+-bile acid cotransporter primarily in villus cell brush-border membrane from normal ileum as well. Data in B are representative of 4 experiments each with different animals.

Because steady-state mRNA levels may not directly correlate with functional protein levels in the BBM, we determined immunoreactive protein levels for the ileal Na+-bile acid cotransporter. Western blot analysis of BBMV prepared from villus and crypt cells showed that the anti-KIBMAL3 antibody recognized the major immunoreactive protein of the Na+-bile acid cotransporter at the expected size of 48 kDa in villus cells (Fig. 2B). Minimal expression (<8% of that in villus cells) is seen in crypt cells. Thus these results suggest that the message as well as the functional protein for ileal Na+-bile acid cotransport is present predominantly in villus cells in the normal rabbit ileum.

Na+-stimulated taurocholate uptake was then determined in intact villus and crypt cells from the chronically inflamed ileum. Extracellular Na+ stimulated taurocholate uptake was present for up to 30 min in villus cells from the chronically inflamed ileum (Fig. 3A). However, similar to in the normal ileum, Na+-stimulated taurocholate uptake was also not present in crypt cells from the chronically inflamed ileum (Fig. 3B).


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Fig. 3.   Effect of chronic ileal inflammation on [3H]taurocholate uptake in villus and crypt cells. A: villus cells. Extracellular Na+ significantly stimulated [3H]taurocholate uptake at all time points. B: crypt cells. Extracellular Na+ does not stimulate [3H]taurocholate uptake. Only least significant P values are shown.

Figure 4 compares the Na+-dependent uptake of taurocholate in villus cells from the normal and chronically inflamed ileum. Na+-dependent taurocholate uptake was significantly reduced in intact villus cells from the chronically inflamed ileum. These data suggested that Na+-taurocholate cotransport was inhibited in intact villus cells from the chronically inflamed ileum.


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Fig. 4.   Effect of chronic inflammation on Na+-dependent acid uptake. Na+-dependent taurocholate uptake, defined as [3H]taurocholate uptake in the presence minus that in the absence of Na+, was significantly reduced in villus cells from chronically inflamed ileum at all times measured. Only least significant P values are shown.

Inhibition of Na+-taurocholate cotransport at the cellular level may represent a direct effect on the cotransporter on the BBM and/or may be secondary to an inhibition of Na+-K+-ATPase on the basolateral membrane (BLM), which provides the favorable Na+-electrochemical gradient for this cotransport process. Thus Na+-K+-ATPase activity was measured in homogenates of villus cells from normal and chronically inflamed ileum. Na+-K+-ATPase activity was reduced ~50% in villus cells from the inflamed ileum (11.3 ± 1.9 and 5.9 ± 1.2 nmol · mg protein-1 · min-1 in normal and inflamed ileum, respectively; n = 6, P < 0.05). These data suggested that the inhibition of Na+-taurocholate cotransport in inflamed ileal villus cells may, at least in part, be due to reduced electrochemical gradients of Na+ across the BBM of these cells.

To determine whether chronic inflammation has a direct effect on the Na+-taurocholate cotransporter itself, we determined taurocholate uptake in BBMV prepared from villus cells from the normal and chronically inflamed ileum. Extravesicular Na+ significantly stimulated taurocholate uptake in villus cell BBMV from the normal ileum and the chronically inflamed ileum. However, Na+-dependent taurocholate uptake was significantly reduced in villus cell BBMV from the chronically inflamed ileum (Fig. 5). Thus these data indicated that the Na+-taurocholate cotransporter itself was directly affected during chronic ileal inflammation.


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Fig. 5.   Effect of extravesicular Na+ on [3H]taurocholate uptake as a function of time in villus cell brush-border membrane vesicles (BBMV) from normal and chronically inflamed rabbit ileum. Vesicle uptake was carried out in triplicate. [3H]taurocholate uptake in villus cell BBMV from normal and inflamed ileum was significantly stimulated by extravesicular Na+. However, Na+-dependent taurocholate uptake, defined as [3H]taurocholate uptake in the presence minus that in the absence of Na+, was significantly reduced in villus cell BBMV from chronically inflamed ileum. Only least significant P values are shown.

To determine whether the inhibition of Na+-taurocholate cotransport during chronic ileal inflammation was due to an alteration in the affinity for taurocholate and/or in the rate of uptake of taurocholate, we performed kinetic studies. Because Na+-dependent taurocholate uptake in BBMV was linear for at least 10 s in the normal as well as in the inflamed ileum, uptake for all the concentrations was carried out at 5 s (data not shown). Figure 6 demonstrates the kinetics of taurocholate uptake in villus cell BBMV from the normal and chronically inflamed ileum. Figure 6 shows the uptake of taurocholate as a function of varying concentrations of extravesicular taurocholate. As the extravesicular concentration of taurocholate was increased, the uptake of taurocholate was stimulated and subsequently became saturated in the normal as well as in the chronically inflamed ileum. Using Enzfitter, a Lineweaver-Burk plot of these data was generated and is shown in Fig. 6. Kinetic parameters derived from three such experiments demonstrate that the maximal rate of uptake (Vmax) of taurocholate was diminished in the chronically inflamed ileum (Vmax for taurocholate uptake in BBMV was 9.37 ± 2.2 and 3.62 ± 0.6 nmol/mg protein at 1 min in the normal and inflamed ileum, respectively; n = 3, P < 0.05). In addition, the apparent Michaelis constant (Km) for taurocholate uptake was significantly increased in the inflamed ileum (41.7 ± 4.1 and 73.7 ± 12.2 µM for normal and inflamed ileum, respectively; n = 3, P < 0.05). These data suggested that Na+-taurocholate cotransport was inhibited in the chronically inflamed ileum secondary to both a decrease in the affinity for taurocholate as well as a diminished number of cotransporters.


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Fig. 6.   Kinetics of [3H]taurocholate uptake in villus cell BBMV from normal and chronically inflamed ileum. Data are representative of 3 experiments. A: uptake of [3H]taurocholate as a function of varying concentrations of extravesicular taurocholate. Uptakes for all concentrations were carried out at 5 s. Isosmolarity was maintained by adjusting mannitol concentration. As extravesicular taurocholate concentration was increased, uptake of [3H]taurocholate was stimulated and subsequently became saturated in normal and inflamed ileum. B: analysis of these data with Lineweaver-Burk plot provided kinetic parameters. Maximal rate of uptake of taurocholate was reduced during chronic ileal inflammation. Also, affinity (1/Michaelis constant) for taurocholate uptake was significantly diminished in chronically inflamed ileum.

To confirm these findings, we determined the steady-state levels of mRNA for the Na+-dependent bile acid cotransporter in villus and crypt cells from the normal and the chronically inflamed ileum (Fig. 7A). The probe recognized a band at the expected size of 4.6 kb in villus cells from the normal and chronically inflamed ileum. Densitometric analysis of the Na+-bile acid cotransporter band demonstrates a ~50% reduction in villus cells from the chronically inflamed ileum. Furthermore, similar to the normal ileum, the message for this cotransporter is present predominantly in villus rather than crypt cells in the chronically inflamed ileum.


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Fig. 7.   A: steady-state mRNA levels of Na+-bile acid cotransporters in chronically inflamed ileum. Data are representative of 4 experiments. Northern blot analysis demonstrated that mRNA for the Na+-taurocholate acid cotransporter is present primarily in villus cells from chronically inflamed ileum. Furthermore, steady-state levels of mRNA are diminished in villus cells from chronically inflamed ileum. Human beta -actin was utilized to ensure equal loading of mRNA in the gel. B: Western blot analysis also demonstrated the presence of immunoreactive rabbit ileal specific Na+-bile acid cotransporter primarily in villus cell BBMV from chronically inflamed ileum. Furthermore, it demonstrates a reduction in cotransporter protein levels in villus cell BBMV from chronically inflamed ileum. Data are representative of 4 experiments each with different animals.

Finally, immunoreactive protein levels of this cotransporter were measured in BBM of villus cells from the normal and inflamed ileum (Fig. 7B). The anti-KIBMAL3 antibody recognized one major immunoreactive protein band at the expected size of 48 kDa in villus cells from the normal and chronically inflamed ileum. Densitometric analysis of the Western blots demonstrates ~60% reduction in villus cells from the chronically inflamed ileum. Furthermore, similar to in the normal ileum, the immunoreactive protein of this cotransporter is present predominantly in villus rather than crypt cells in the chronically inflamed ileum.

    DISCUSSION
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Abstract
Introduction
Methods
Results
Discussion
References

This study demonstrates that, in the normal rabbit ileum, Na+-dependent bile acid cotransport is present in villus cells but not in crypt cells. This has previously been demonstrated in the guinea pig and rat ileum as well (7, 15). Furthermore, this crypt-villus distribution is preserved in the chronically inflamed ileum. However, Na+-taurocholate cotransport is significantly inhibited in intact villus cells from the chronically inflamed ileum. Inhibition of Na+-taurocholate cotransport in villus cells from the chronically inflamed ileum may occur at the level of the cotransporter and/or secondary to an alteration in Na+ extrusion from the cell facilitated by Na+-K+-ATPase. This study indicates that during chronic ileal inflammation the mechanism of inhibition of Na+-taurocholate cotransport is at the level of the cotransporter and not exclusively secondary to an alteration in the Na+ extrusion capacity of the villus cell.

At the level of the cotransporter, the mechanism of inhibition of Na+-bile acid cotransport is secondary to a reduction in Vmax as well as the affinity (1/Km) for taurocholate in the chronically inflamed ileum. Analyses of the steady-state mRNA levels and immunoreactive protein levels of this cotransporter also confirm the decrease in cotransporter numbers in the chronically inflamed ileum.

Although it is clear that inhibition of bile acid absorption occurs in diarrheal illnesses characterized by chronic inflammation of the terminal ileum (e.g., Crohn's disease), the cellular mechanism of these alterations is poorly understood (6, 9). Undoubtedly, this is secondary to a lack of good animal models of chronic ileal inflammation. Two other models of chronic small intestinal inflammation, peptidoglycan polysaccharide-induced enterocolitis in rats (12) and alloimmunization-induced enterocolitis in guinea pigs (10), have not yet been utilized for transport studies.

In this rabbit model of chronic ileal inflammation, we have previously demonstrated that two other Na+-dependent solute cotransport processes are inhibited. At the cellular level Na+-glucose, Na+-amino acid, and, in this study, Na+-bile acid cotransport are all inhibited secondary to an effect at the level of the cotransporter as well as a reduction in Na+-K+-ATPase in the chronically inflamed ileum. However, at the level of the cotransporter each system is uniquely altered in the chronically inflamed ileum (Table 1). Na+-glucose cotransport was inhibited by a decrease in the number of cotransporters without a change in the affinity for glucose in the chronically inflamed ileum (20). In contrast, Na+-amino acid cotransport was inhibited by a change in the affinity for the amino acid without a change in the number of cotransporters during chronic ileitis (19). Unlike these two Na+-solute cotransport processes, as demonstrated in this study, Na+-bile acid cotransport was inhibited by both a decrease in the affinity as well as in cotransporter numbers in the chronically inflamed ileum. Thus these three types of Na+-dependent solute uptake pathways are inhibited by different mechanisms in the chronically inflamed ileum. One explanation for these unique alterations may be the intrinsic differences between these transporters that cause them to respond differently to a given immune-inflammatory mediator. However, a more likely explanation is that, since a variety of immune-inflammatory mediators are known to be released in the chronically inflamed ileum, it is hypothesized that different immune-inflammatory mediators may regulate these three transport pathways in the chronically inflamed ileum.

                              
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Table 1.   Na+-bile acid cotransport in rabbit ileum

This hypothesis is further supported by other specific transport pathway alterations in villus and crypt cells previously reported in this model of chronic ileal inflammation (18). It was demonstrated that coupled NaCl absorption that occurs by the dual operation of Na+/H+ and Cl-/HCO-3 exchange on the BBM of villus cells (18) was inhibited due to a reduction in Cl-/HCO-3 but not Na+/H+ exchange. Unlike the villus cells, in the crypt cells, Na+/H+ exchange known to be present only on the BLM (18) was stimulated in the chronically inflamed ileum. This stimulation of basolateral Na+/H+ exchange alkalinizes the crypt cell, which may subsequently stimulate the BBM Cl-/HCO-3 exchange, resulting in the secretion of HCO-3 by these cells (18).

Taken together, these studies illustrate that specific transport pathways are altered in villus and crypt cells during chronic ileal inflammation. Furthermore, a related family of Na+-dependent nutrient cotransport processes is uniquely altered during chronic ileitis. These transporters are uniquely altered either secondary to their intrinsic differences or by different agents during chronic ileitis. Given the numerous immune-inflammatory mediators that are produced in the chronically inflamed ileum and that at least some of them are capable of altering transport pathways (2, 11, 13), it is reasonable to postulate that different immune-inflammatory mediators may regulate different transport pathways during chronic ileitis. It has yet to be delineated which of these agents is responsible for the transport abnormalities observed in this rabbit model of chronic ileal inflammation

In conclusion, Na+-taurocholate cotransport is inhibited during chronic ileal inflammation. The inhibition is not entirely a consequence of a reduction in the Na+ extrusion capacity of the cell. At the level of the cotransporter, the mechanism of inhibition of Na+-taurocholate cotransport is secondary to a decrease in the affinity for taurocholate as well as a decrease in the transporter numbers. This mechanism of inhibition of Na+-bile acid cotransport differs from that of Na+-glucose or Na+-amino acid cotransport during chronic ileal inflammation.

    ACKNOWLEDGEMENTS

We thank V. Annapurna for technical assistance with some of the uptake studies.

    FOOTNOTES

This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Research Grant DK-45062, an American Gastroenterological Association Research/Industry Scholar Award, and a Davis Medical Center Grant (U. Sundaram).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests: U. Sundaram, Division of Digestive Diseases, Ohio State Univ. School of Medicine, N-214 Doan Hall, 410 W. Tenth Ave., Columbus, OH 43210.

Received 18 May 1998; accepted in final form 26 August 1998.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

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Am J Physiol Gastroint Liver Physiol 275(6):G1259-G1265
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