In vitro studies using ion exchange resins as potential phosphate binders for renal failure patients

The uptake of phosphate [expressed as phosphorus (P)] by the anion exchange resins, Dowex 1-X8, Dowex SBR, and Bio-Rex 5, aluminum hydroxide, and sucralfate tablets was evaluated. The maximum uptake capacities (in mg P per gram of "wet" resin or solid) were 56, 49, and 84 mg for Dowex SBR, Dowex 1-X8, and Bio-Rex 5 resins, respectively, and 164-168 mg for aluminum hydroxide and sucralfate. At a concentration of P considered to approximate that encountered in the stomach (0.3 mg/ml), Bio-Rex 5 resin, aluminum hydroxide, and sucralfate bound similar amounts of P. Physiologic concentrations of bicarbonate or chloride and simulated gastric or intestinal fluids caused small changes in P uptake by Bio-Rex 5 resin. The resins bound large quantities of taurocholic (TA) and glycocholic (GA) acids. However, when Bio-Rex 5 was converted to the taurocholate form, it bound the same amount of P as the original chloride-form resin, and the binding of TA was prevented.

Biliary permeability during ethinyl estradiol-induced cholestasis studied by segmented retrograde intrabiliary injections in rats

The influence of ethinyl estradiol (EE) treatment on the permeability of the biliary tree in rats has been assessed by the segmented retrograde intrabiliary injection (SRII) technique. Three pairs of compounds were studied, inert [14C]sucrose and [3H]inulin; [14C]taurocholic acid (TC) and [14C]glycocholic acid (GC) and the non-ionic bile acid derivatives [125I]cholyldiglycylhistamine (CG2H) and [131I]cholyltetraglycylhistamine (CG4H). In control rats recovery in bile after SRII was always greater for the larger of any pair of compounds, confirming that the biliary tree acts as a filter, and that decreased recovery from bile during this technique is an index of greater biliary permeability. After EE treatment recovery of all compounds was significantly reduced, thus confirming that EE increases biliary permeability. Recovery of sucrose and inulin fell from 55-65% of the administered dose in controls to 8-9% in EE rats. Recoveries of TC, GC, CG2H and CG4H was also reduced, but their biliary recovery profiles were consistent with marked re-excretion into bile of that portion which had initially passed out of the biliary system by filtration. During the later phase of the experiment excretion of the negatively charged bile acids TC and GC was greater than that of the non-ionic bile acid derivatives CG2H and CG4H. Although the site at which these permeability changes have occurred is unknown, our results are compatible with previous data implicating increased tight junction permeability as a mechanism of EE-induced cholestasis.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificity of cholylglycine hydrolase for the hydrolysis of bile acid conjugates

The substrate specificity of cholylglycine hydrolase has been investigated using bile acid conjugates with modifications in the steroid ring system, the side chain, or the amino acid moiety. Epimerization at C-3 and C-7 did not affect the activity of the enzyme while oxidation of the three nuclear hydroxyl groups reduced the affinity of the enzyme toward the substrate. Elongation of the side chain by one or three carbons inhibited enzyme activity. Conjugates prepared from C24 bile acids and analogs of taurine and glycine with one or two methylene groups were effectively hydrolyzed, whereas conjugates with a tertiary amide group completely resisted hydrolysis. Increasing the length of the bile acid side chain or using a bile acid conjugate with a tertiary amide group may produce compounds that will resist intestinal bacterial destruction.

[Behavior of conjugated bile acids in chronic liver disease]

In 24 persons with healthy liver and 44 patients with chronic hepatopathies of different degree of severity (23 women and 16 men with chronic active hepatitis as well as 4 women and 1 man with unspecific degenerative damage of the liver parenchyma) in the C-bile conjugated bile acids were qualitatively differentiated. In contrast to persons with healthy liver in chronic hepatopathies apart from conjugated bile acids free bile acids appeared. The conjugation of the bile acids with taurine was proved in greater frequency than the glycine conjugation. Parallel to the degree of severity of the hepatopathy particularly the proportion of the taurine-conjugated dihydroxycholan acids increased, among which the tauro-chenodesoxycholic acid prevailed. The conjugation of the bile acids with glycine also in chronic hepatopathies remained longest in the fraction of the trihydroxycholan acids. The examinations show that the qualitative proof of the conjugated bile acids in the bile gives an additional information for the diagnostic estimation of the chronic hepatopathies.

Diet, nutrition intake, and metabolism in populations at high and low risk for colon cancer. Binding of bile salts to dietary residues

The binding of glycocholic and glycochenodeoxycholic acids to dried defatted residues of homogenized diets consumed by four dietary groups has been measured. The four groups were true vegetarian Seventh-day Adventists (SDA), lacto-ovo vegetarian SDA, nonvegetarian SDA, and the general population. Glycocholic acid was bound to the same extent (7 to 9%) by all four dietary residues. The true vegetarian SDA residues bound significantly more glycochenodeoxycholic acid than those of either lacto-ovo vegetarian SDA (p less than 0.001) or nonvegetarian SDA (p less than 0.001). The general population residue bound significantly more glycochenodeoxycholic acid than did those of nonvegetarian SDA (p less than 0.001). Binding of glycochenodeoxycholic acid was significantly correlated with dietary neutral detergent fiber (p less than 0.001), hemicellulose (p less than 0.01) and cellulose (p = 0.01). There was no correlation between the binding of glycochenodeoxycholic acid and dietary lignin, pectin, or cutin.

The biliary excretion of sulphated and non-sulphated bile acids and bilirubin in patients with external bile drainage

The biliary excretion of total bilirubin and bile acids, and the fate of tracer doses of radioactive sulphated and non-sulphated bile acids, were studied in patients with percutaneous transhepatic bile drainage. Non-sulphated bile acids were excreted in bile early after biliary decompression, and the serum total 3 alpha-hydroxy bile acid concentrations fell rapidly to normal. Biliary bilirubin excretion was both less than and delayed compared with that of bile acids, and the serum bilirubin concentration fell more slowly. The serum disappearance of [3H]chenodeoxycholate-3-sulphate was slower than that of [14C]glycocholate in all patients with bile drainage, the difference being more marked in the jaundiced patients. The radioactive sulphated bile acids were recovered predominantly in the urine of the jaundiced patients. In contrast [14C]glycocholate was excreted almost exclusively in bile. In an anicteric patient, radioactive sulphated bile acid disappeared from the serum more quickly, and biliary recovery exceeded that in the urine. The studies demonstrate the differences in handling of total bilirubin, and sulphated and non-sulphated bile acids in man after the relief of bile duct obstruction. The biliary excretion of radioactive labelled sulphated bile acids is low for at least 1 week after biliary drainage, but later becomes the predominant route for excretion in the anicteric patient.

Concentrations of glycine- and taurine-conjugated bile acids in portal and systemic venous serum in man

Concentrations of glycine and taurine conjugates of cholic, chenodeoxycholic, and deoxycholic acid in portal and systemic venous serum and in bile were measured in eight subjects undergoing elective cholecystectomy. Mean concentrations in systemic serum ranged from 0.07 to 0.17 mumol/l, in portal serum from 0.49 to 2.09 mumol/l, and in bile from 2.72 to 17.2 mmol/l. The percentage content of trihydroxy-bile acid conjugates in bile (49%) and in portal serum (51%) was higher than in systemic serum (35%) (P less than 0.001). The estimated hepatic fractional uptake of glycocholic acid (mean, 83%) and of taurocholic acid (83%) was higher than the uptakes of the dihydroxy-bile acid conjugates (60-68%). The percentage contents of glycine-conjugated bile acids in systemic serum (mean, 66%), portal serum (62%), and bile (65%) were not significantly different.

Variations in concentrating function of the gallbladder in the conscious monkey

Hepatic bile is concentrated in the gallbladder by absorption of water and electrolytes. The rate of water absorption can be influenced in vitro and in vivo by gastrointestinal hormones and neurogenic transmitters. Recent studies have demonstrated that besides its absorbing activity, the gallbladder has the ability to secrete fluid into its lumen. In the present experiments, we studied the rate of net water absorption in the gallbladders of pig-tailed monkeys sitting in restraining chairs. Bile from the common bile duct and the gallbladder was led out through cannulas to a stream splitter, samples were obtained, and bile was returned to the duodenum. By measuring outflow from the gallbladder and the concentration of bile acids in hepatic bile and bile that had passed through the gallbladder, net water absorption from the gallbladder bile could be calculated. The results demonstrate that during daytime fasting there was a net absorption corresponding to 30% of the fasting gallbladder volume per hour, and after feeding there was a net secretion of fluid into the gallbladder lumen. Compared with the awake fasting state, net water absorption from the gallbladder decreased at night while the animal was fasting and asleep. Physiologically, secretion of fluid after a meal could help to empty the gallbladder, and changes in net water absorption by the gallbladder could influence the kinetics of the enterohepatic circulation of bile acids.

Intestinal uptake of bile acids: effect of external abdominal irradiation

Abdominal irradiation has recently been shown to influence the uptake of hexoses, amino acids, fatty acids and cholesterol into the jejunum of rats. The present studies were undertaken with a previously validated in vitro technique to determine the effect of abdominal irradiation from a cesium 137 source on the rates of uptake of six bile acids into the jejunum, ileum, and colon. In the ileum of control rats, there were marked differences in the value of the apparent Michaelis constant (Km*), maximal transport rate (Jdm), and apparent passive permeability coefficient (Pd*) between cholic (C), glycocholic (GC), taurocholic (TC), chenodeoxycholic (CDC), and glycochenodeoxycholic (GCDC), and deoxycholic (DC) acid. The Km* for each bile acid except DC was lower three and 14 days after 600 rad, whereas the Jdm for GC fell, but rose for TC, CDC, GCDC and DC and was unchanged for C. The Pd* rose for C, GC, and DC, fell for TC and CDC, but remained unchanged for GCDC 14 days after irradiation. After 600 rad the value of Pd* in the colon was increased at day 3 and 14 for CDC and GCDC, but was unchanged for GC and TC and was decreased for C. The uptake of bile acids was also affected by 300 rad and by 900 rad, but the direction and magnitude of the change was influenced by the intestinal site, the dose of irradiation, and the type of bile acid. The results show that: 1) there likely are multiple ileal carriers for bile acids; 2) abdominal irradiation has a variable effect on these carriers; 3) the passive permeability to bile acids varies with the bile acid and with the site along the intestine; and 4) abdominal irradiation is associated with a rise in the colonic permeability to only some bile acids.

Intracellular bile acid transport in rat liver as visualized by electron microscope autoradiography using a bile acid analogue

The role of hepatocyte organelles in the intracellular transport and secretion of conjugated bile acids has not been defined. Therefore we studied the transport and observed the subcellular localization of the bile acid analogue 125I-cholylglycyltyrosine by electron microscope autoradiography to further understand the possible compartmentation of bile acids within the hepatocyte. 125I-cholylglycyltyrosine, which retains a net negative charge, exhibited transport properties similar to native bile acids. After portal vein injection, the compound was recovered intact from bile, and the pattern of excretion paralleled that of [14C]cholylglycine. In addition, cholylglycyltyrosine uptake by isolated hepatocytes was sodium dependent. For autoradiography the analogue was injected into the portal vein, and the liver was perfusion fixed after 30 or 300 s. Light microscope autoradiography performed 30 s after isotope injection demonstrated a steep periportal-to-centrilobular gradient for 125I-cholylglycyltyrosine uptake. At 30 s quantitative grain analysis of electron microscope autoradiographs showed predominant labeling of the plasma membrane and the smooth endoplasmic reticulum (SER). The grain distribution over the region of the plasma membrane decreased from 15% at 30 s to 7% by 300 s and was associated with a sevenfold increase in labeling of the Golgi apparatus and a sixfold increase in labeling of the pericanalicular region. Grain distribution over the SER at 300 s was the same as that noted at 30 s. The hypothesis is presented that bile acids move from the sinusoidal plasma membrane to bile via a pathway that includes the SER and Golgi apparatus.

Hepatic handling of bile salts and protein in the rat during intrahepatic cholestasis

17 alpha-Ethynyl estradiol-induced cholestasis was used to study the relationship of protein to bile salt transport in liver. The biliary secretion of horseradish peroxidase was unaltered in treated animals despite a 56% reduction in bile flow. Cytochemistry confirmed that estradiol caused no alteration in the handling of tracer. In a second study, the peak biliary secretion of [14C]taurocholate was reduced by approximately 46% in treated animals. The kinetics of 125I-cholyglycylhistamine, a bile salt derivative, were identical to those of taurocholate in control and cholestatic animals. Taurocholate and cholylglycylhistamine secretion were markedly reduced in control animals during competition with unlabeled taurocholate. Quantitative electron microscopic autoradiography with 125I-cholylglycylhistamine revealed a high concentration of grains over the endoplasmic reticulum and the Golgi complex including associated lysosomes and vesicles. These data demonstrate that estradiol markedly inhibits bile salt transport, but not vesicular transport of horseradish peroxidase. Furthermore, estradiol may alter the movement of bile salts through these organelles.

Bile salt metabolism in the first year of life

Bile salt metabolism was examined in 12 infants (ages 7 weeks to 10 months) who had recovered from protracted infantile diarrhea. Cholic acid kinetics were measured with the isotopic dilution technique. Cholate pool size was 938 +/- 89 mg/m2 (means +/- S.E.), synthetic rate was 478 +/- 52 mg/m2/day, and the fractional turnover rate was 0.537 +/- 0.060 day -1. Estimated chenodeoxycholate pool size was 607 +/- 103 mg/m2. similar cholic acid pool sizes were previously observed in older children; however, the synthetic and turnover rates were significantly lower, p less than 0.05 and 0.025, respectively. A significant inverse relationship between age and fractional turnover rate (r = -0.577, p less than 0.05) was observed in the first 10 months of life. Fasting serum cholylglycine measured by radioimmunoassay was significantly higher (p less than 0.01) in infants than in children. Peak postprandial concentrations were higher in infants than in older children. Accelerated hepatic bile salt synthesis rapidly increases the size of small pools at birth. Intraluminal bile salt concentrations concurrently increase, thereby normalizing fat solubilzation. Slow maturation of intestinal transport mechanisms may result in normal cycling of bile salts by 3 to 7 months of age. Persistent immaturity of hepatic uptake and/or excretion of bile salts leads to intrahepatic retention of a portion of the pool, with resultant regurgitation into the serum during the first months of life.

[Labelled CO2 breath-tests in gastroenterology (author's transl)]

The use of a few microcuries of 14C labelled molecules with very short biological half-life for breath-tests gives no more body irradiation than 10 minutes' exposure to the sun. Yet it provides the gastroenterologist with accurate information. Detection and identification of fat malabsorption are easy when 3 labelled lipids are used. Glycine 1 14C cholate administered orally offers a simple, rapid and effective screening technique for the detection of increased bile salt deconjugation and may provide a valuable diagnostic supplement in the study of steatorrhea after ileal resection, bypass or bacterial overgrowth. The clearance of labelled aminopyrine can be decreased in hepatocellular dysfunction or increased in some alcoholic patients or in patients receiving phenobarbitone. Tracer doses of labelled ethanol were used to study the metabolism of ethanol by the liver in alcoholic patients. Chronic ingestion of alcohol stimulates the activity of microsomal drug-metabolising enzymes in both non-cirrhotic and cirrhotic patients; this response is lost in acute liver damage and in chronic alcoholic disease with severe hepatic dysfunction. The CO2-labelled breath-tests are non-invasive, effective and inexpensive. The forthcoming replacement of 14C by non-radioactive 13C or briefly radioactive 11C will render these tests even more attractive.

On the kinetics of glycocholate uptake and excretion by the normal and diseased liver in man

Serum concentrations of glycocholate and plasma disappearance curves of i.v. injected 14C-glycocholate were measured in 6 controls and 12 patients with liver disease. On the basis of these data, hepatic uptake and biliary excretion of serum glycocholate were calculated using a previously described 3-compartmental model. The results demonstrate that the hepatic uptake increases in correlation with the serum concentrations even if the biliary excretion decreases. As a consequence of reflux of glycocholate into the serum, increases also, indicating an increased bidirectional flux of bile acids in patients with elevated serum concentrations. These findings a) confirm that hepatic uptake and biliary excretion of glycocholate are independent transport mechanisms in man, b) indicate that hepatic uptake the more effective transport mechanism in the diseased liver, and c) suggest that liver uptake and reflux are different transport mechanisms. The possible pathophysiological importance of these findings is discussed.

Studies of relationship among bile-acid uptake, Na+, K+-ATPase, and Na+ gradient in isolated cells from rat ileum

Studies were performed to determine relationships among Na+, K+-ATPase, the transmucosal Na+ gradient, and bile-acid transport in metabolically viable cells isolated from rat ileum. Incubation of cells with 0, 10(-6), 10(-5), 10(-4) and 10(-3) M ouabain resulted, respectively, in a 0, 10.3, 42.1, 97.0, and 100% decrease in glycocholate uptake and a 0, 10.7, 46.4, 76.8, and 100% decrease in Na+, K+-ATPase activity. Thus, one-half maximal inhibition of glycocholate uptake and Na+, K+-ATPase activity occurred at 5.5 x 10(-5) M and 1.7 x 10(-5) M ouabain, respectively. A change in glycocholate uptake was correlated with a change in Na+, K+-ATPase activity after daily injections of methylprednisolone. After 4 days treated animals showed a 26% and 36% increase in glycocholate uptake and Na+, K+-ATPase activity, respectively, over pair-fed saline-treated controls (p less than 0.001). Methylprednisolone did not significantly alter the activity of (Mg++)-ATPase when compared with controls (p greater than 0.05). Glycocholate uptake was reduced by the omission of Na+ from the incubation medium. Preincubation of cells at 37 degrees C with gramicidin D, 10 micrograms/ml, to alter membrane permeability to Na+, resulted in a significant rise in cell Na+ (p less than 0.01) and a significant fall in glycocholate uptake from values in untreated cells (p less than 0.01) to approach values for glycocholate uptake at 0 degrees C. These data suggest that Na+, K+-ATPase may play a role in a bile-acid uptake into ileal cells possibly by maintaining a Na+ electrochemical potential gradient for coupled Na+-bile-acid transport.

A comparison of bile salt binding to lymph and plasma albumin in the rat

The binding of bile salts to proteins in rat plasma and rat lymph has been investigated. Under the non-equilibrium conditions of gel chromatography no binding of glycochenodeoxycholate or glycocholate to any of the lymph proteins was observed. In contrast, plasma bound a proportion of both bile salts. When lymph was treated with charcoal, binding of glycochenodeoxycholate to a protein with a molecular weight identical to albumin occurred. Equilibrium binding studies showed that the binding of glycocholate to partially purified plasma albumin exhibited saturation kinetics with a dissociation constant of 2 x 10(-4) M. In contrast, the binding of glycocholate to lymph albumin was non-saturable. Potassium oleate, when added to plasma in a free fatty acid : albumin molar ratio of 3.8 : 1, almost completely inhibited the binding of chenodeoxycholate to plasma albumin. The endogenous free fatty acid : albumin ratios found in systemic plasma and lymph were 0.6 : 1 and 9.2 : 1, respectively. It is suggested that the high free fatty acid concentrations found in lymph inhibit the binding of bile salts to albumin.

[Pruritus and bile acids. Determination of sulfalithoglycocholate and glycocholate in suction blister fluid and in serum (author's transl)]

Until now is not clear, whether bile acids play a role in hepatogenic pruritus in patients with cholestasis. A correlation between serum levels and itching does not exist, and therefore an accumulation of bile acids in the skin was made responsible for the etiology of pruritus. However, this could not be proved in skin homogenisates whereas bile acids were found accumulated in the surface lipids of the skin. For this reason, the investigations presented here deal with the determination of glycocholate and sulfalithoglycocholate in suction blister fluid and in serum. The levels of these bile acids were remarkably lower in the fluid of the subepidermally located blisters than in the serum. No significant difference was found between the group of patients with pruritus and cholestasis and the group without cholestasis. These results show that an accumulation of bile acids in the skin does not exist. A correlation between bile acids in the skin and pruritus cannot be proved.

Fecal 13C analysis for the detection and quantitation of intestinal malabsorption. Limits of detection and application to disorders of intestinal cholylglycine metabolism

The use of 14CO2 breath tests and fecal analyses for the detection and quantitation of intestinal malabsorption has been extensively documented in adult subjects. The use of stable isotopes has extended the range of breath test applications to include pediatric and obstetric subjects. Here we report a fecal 13C analysis that can be used in conjunction with 13CO2 breath tests. Twenty-four-hour fecal samples were collected before and after the administration of a labeled substrate. The samples were homogenized and combusted to CO2, and the 13C abundance was determined by high-precision, differential isotope ratio mass spectrometry. The isotopic variation between successive 24 hr fecal samples was 0.6 0/00 (0.0006 atom percent). This variation limited the sensitivity of the fecal analysis to 13 mumol of 13C label per mold of fecal carbon. Simultaneous cholyglycine 13CO2 breath tests and fecal assays were performed in five children. One child with bacterial overgrowth had an abnormal breath test and a normal fecal test. Of three children with ileal dysfunction, only one had an abnormal breath test, whereas the fecal test was abnormal in all three, Both the breath test and fecal test were abnormal for a child who had undergone an ileal resection. Both tests were normal for a child with ulcerative colitis.

Direct measurement of hepatic extraction of bile acids in subjects with and without liver disease

1. The hepatic extraction ratio of 14C-labelled bile acids has been measured directly by hepatic vein catheterization in five patients without liver disease (glycocholic acid, three; cholic acid, two) and in 16 patients with histologically confirmed liver disease (glycocholic acid, seven; cholic acid, nine). 2. After intravenous administration of [14C]-glycocholic acid by bolus injection (two control subjects) or constant infusion (one control subject), directly measured hepatic extraction ratio was 0.91, 0.84 and 0.88, greater than that for indocyanine green. The extraction ratio of [14C]cholic acid in two subjects was 0.72 and 0.70, confirming a lower extraction of the unconjugated bile acid. 3. The hepatic extraction ratio of both bile acids was reduced in patients with chronic liver disease (range 0.07-0.69), although the extraction ratio of glycocholic acid remained normal in one patient with viral hepatitis. 4. Estimates of liver flow calculated from the extraction of [14C]glycocholic acid, but not cholic acid, correlated with those calculated from indocyanine green kinetics, although numbers were small. 5. Measurement of the hepatic extraction of individual bile acids, not previously reported in man, allows a more accurate description of the enterohepatic circulation.

Effects of ethinylestradiol on enzymes catalyzing bile acid conjugation and sulfation

Administration of large doses of estrogens has been shown to result in decreased conjugation and increased sulfation of bile acids as well as cholestasis. There have been no previous studies on the effects of low doses of estrogens on these parameters of bile acid metabolism. Therefore, rats were given ethinylestradiol, 0.06 to 600 micrograms/kg/day subcutaneously for up to 21 days and the in vitro activity of the hepatic conjugation and sulfation enzymes was measured. Conjugating enzyme activity was unchanged at doses below 600 micrograms/kg/day. In contrast, hepatic sulfation of conjugated bile acids increased significantly after 21 days treatment with 0.6 micrograms/kg/day. The magnitude of the increase was both time- and dose-dependent. Increased sulfotransferase activity was noted only in the liver and was specific for conjugated bile acids; there was no change in the rate of sulfation of the unconjugated bile acids or in renal sulfotransferase activity. Increased hepatic bile acid sulfotransferase activity occurred in the presence of normal bile flow and bile acid secretion. These data indicate that treatment with doses of ethinylestradiol comparable to those present in oral contraceptives may lead to readily detectable time- and dose-dependent changes in bile acid sulfation without producing cholestasis. The data also suggest that there may be significant differences in the enzymatic sulfation of conjugated and unconjugated bile acids in the liver.

Autoradiographic evidence for hepatic lobular concentration gradient of bile acid derivative

Using an iodinated bile-acid analog with hepatic uptake and transport characteristics similar to conventional bile acids, the hepatic lobular gradient concept of Goresky was studied utilizing autoradiography. 125I-labeled cholylglycylhistamine (125I-CGH) was infused into the portal veins of male Sprague-Dawley rats and the livers were fixed for light microscopic autoradiography at 1 and 5 min after infusion. In two animals, sequential samples of bile were collected to assess the transport characteristics of 125I-CGH. By 1 min, virtually all (98%) of the injected 125I-CGH was taken up and retained by hepatocytes after perfusion fixation. Less than 15% of the label was lost during subsequent tissue processing. 125I-CGH appeared in bile within minutes, reaching maximum levels at 7 min. Quantitative autoradiography demonstrated that the first six to nine periportal hepatocytes were, by far, the most active (P less than 0.0005) in sequestering the bile-acid analog than were the remaining cells in the lobule. This study, therefore, provides the first autoradiographic evidence of a hepatic lobular concentration gradient for the uptake of a bile-acid analog.