Bile Acid Structure Research Articles

Quantitative Relationship between Bile Acid Structure and Biliary Lipid Secretion in Rats

AbstractA series of unconjugated and taurine conjugated bile acids (BAs) differing in water solubility (SWo), critical micellar concentration (CMC), and hydrophilicity (K′) were infused iv to rats at a tracer dose and a dose of 6 μmol/min/kg over a 1-h period. Bile was collected for 3 h to evaluate the role of BA structure on cholesterol, phospholipids secretions, and bile flow. The BAs studied differ in the number (2–3), position (−3, −6, −7, −12), and orientation of the hydroxyls (α/β); the side chain structure was modified by shortening (C-23, nor-BA) and by lengthening (C-25, homo-BA), while maintaining the same structure of nuclear hydroxyls (3α7β). At a “tracer” dose, all C-24 natural BAs are efficiently recovered in bile when administered in both unconjugated and taurine conjugated forms. At a “high dose”, all taurine conjugated BAs are efficiently recovered in bile (80–100%). However, a variable recovery was observed among unconjugated BAs: trihydroxy BAs are efficiently recovered (85–100%), while dihydroxy BAs are only partially recovered (25–40%). The side chain-modified BAs [i.e., C-23 nor and C-25 homo analogs of ursodeoxycholic acid (UDCA)] are partially recovered at a tracer dose (20–30%), but less at a high dose (10–20%) when administered in the unconjugated form. In contrast, the corresponding taurine conjugates are more efficiently recovered in bile (60–80%). Conjugation with taurine increases total recovery of unconjugated BAs in bile by not more than 30–40%. Highly hydrophilic and water-soluble BAs, such as ursocholic acid (SWo = 1.67 mM) and cholic acid (SWo = 0.27 mM), can also be secreted as unconjugates, and this accounts for their complete recovery. The conjugation step is rate limiting for poorly soluble BAs such as ursodeoxycholic acid (SWo = 0.009 mM) when administered at a high dose, and critical for nor and homo analogs which are poorly soluble and whose side chain modification partially suppresses their conjugation with taurine or glycine and thereby induces alternative pathways such as glucuronidation or sulfation. The induced bile flow is directly related to the hydrophilicity of the natural C-24 bile acid. The C-23 analog of UDCA greatly increased bile flow, which reached a maximum at very low BA secretion, and the effect is drastically reduced when it was administered as taurine conjugates. The phospholipids and, to a lesser extent, the cholesterol secretions are poorly related to both the hydrophilicity and CMC of BAs, and hepatic biotransformations play a major role in modulating the biliary lipid secretion. In addition, specific effects not related to physicochemical properties, such as selective inhibition of cholesterol synthesis, were observed (see chenodeoxycholic acid and hyocholic acid). The quantitative parameters here evaluated by means of a factor analysis could be useful in structure–activity relationship studies and in designing BA analogs; the properties of homo-UDCA as a new synthetic BA are also discussed.

Effective glucuronidation of 6 alpha-hydroxylated bile acids by human hepatic and renal microsomes.

The glucuronidation of bile acids is an established metabolic pathway in different human organs. The hepatic and renal UDP-glucuronyltransferase activities vary according to the bile acids concerned. Thus, hyodeoxycholic acid is clearly differentiated from other bile acids by its high rate of glucuronidation and elevated urinary excretion in man. To determine whether such in vivo observations are related to variations in bile acid structure, human hepatic and renal microsomes were prepared and time courses of bile acid glucuronidation measured with the bile acids possessing hydroxyl groups in different positions. Eleven [24-14C]bile acids were chosen or synthesized in respect of their specific combination of hydroxyl and oxo groups at the 3, 6, 7 and 12 positions and of their alpha or beta hydroxyl configurations. The results clearly demonstrate that bile acids with an hydroxyl group in the 6 alpha position underwent a high degree of glucuronidation. Apparent kinetic Km and Vmax values for UDP-glucuronyltransferase activities ranged over 78-66 microM and 1.8-3.3 nmol.min-1.mg-1 protein in the liver and over 190-19 microM and 0.5-9.2 nmol.min-1.mg-1 protein in the kidney. All the other bile acids tested, each of which possessed a 3 alpha-hydroxyl group and whose second or third hydroxyl was bound at the 6 beta, 7 or 12 positions, were glucuronidated to a degree far below that of the 6 alpha-hydroxylated bile acids. We conclude that an active and highly specific UDP-glucuronyltransferase activity for 6 alpha-hydroxylated bile acids exists in human liver and kidneys. Moreover, this activity results in the linkage of glucuronic acid to the 6 alpha-hydroxyl group and not to the usual 3 alpha-hydroxyl group of bile acids.

Purification and properties of three kinds of α-hydroxysteriod dehydrogenases from Brevibacterium fuscum DC33

The cell-free extract of Brevibacterium fuscum DC33 contained three kinds of hydroxysteriod dehydrogenase (3 a-, 7 a-, and 12 a-hydroxysteriod dehydrogenases). 7 a-Hydroxysteroid dehydrogenase (EC 1.1.1.59) was purified to electrophoretical homogeneity by ion exchange chromatography, affinity chromatography, and preparative electrophoresis. Its molecular weight was 104, 000 and the enzyme was composed of four identical subunits. The enzyme had an optimum pH of 5.3 for dehydrocholic acid reduction, and around 10 for cholic acid oxidation. It was stable in a pH range of 5.7 to 10.5 at 5°C overnight. The enzyme was most active at 25° to 30°C. The activity was not affected by incubation at 30°C for 30 min, but it was lost at 40°C for 30 min. Withe the assumption of two-substrate kinetics, we calculated various kinetic constants for dehydrocholic acid, 7, 12-diketolithocholic acid, 12-ketochenodeoxycholic acid, and 3, 12-diketolithocholic acid (for the structure of bile acids, see Table 2) together with NAD + or NADH. The enzyme was active only toward hydroxysteroids with a 7 a-hydroxyl group. The production of 7-ketochenodeoxycholic acid from cholic acid and of 3, 12-diketolithocholic acid from dehydrocholic acid by the purified 7 a-hydroxysteroid dehydrogenase was confirmed by thin-layer chromatography. 12 a-Hydroxysteroid dehydrogenase was purified by a similar method. It was active toward hydroxysteroids with a 12 a-hydroxyl group. 3 a-Hydroxysteroid dehydrogenase was purified by preparative electrophoresis. It was active toward hydroxysteroids with a 3 a-hydroxyl group.

Effect of bile acids on hepatic protoporphyrin metabolism in perfused rat liver

To determine the effect of bile acids on hepatic protoporphyrin metabolism, balance studies were performed in isolated perfused rat livers. Hepatic protoporphyrin metabolism was found to increase linearly as a function of protoporphyrin dose in livers infused with and without taurocholate (0.7 μmol/min), but their rates differed significantly. Employing a standard 1500-nmol protoporphyrin bolus dose, infusions (0.7 μmol/min) of taurocholate, glycocholate, deoxycholate, and chenodeoxycholate, but not tauroursodeoxycholate or ursodeoxycholate, increased protoporphyrin metabolism 1.7-to 2.7-fold over control (0 bile acid) values. Bile acid infusion ranging from 0.175 to 1.4 μmol/min confirmed that both taurocholate and chenodeoxycholate increased protoporphyrin disposal significantly more than ursodeoxycholate. For all bile acids, the increase of protoporphyrin metabolism was most pronounced between biliary bile acid excretion rates of 10–50 nmol/min · g liver. These data indicate that (a) bile acids facilitated protoporphyrin metabolism, (b) bile acid structure influenced the effect, and (c) ursodeoxycholate may not be a prime candidate to study the role of bile acids in the treatment of protoporphyria.

Effect of bile acid structure on the feed-back regulation of bile acid synthesis in man: Studies on cholesterol 7 α-hydroxylation

Effect of bile acid structure on the feed-back regulation of bile acid synthesis in man: Studies on cholesterol 7 α-hydroxylation

Bile acid structure and bile formation in the guinea pig

The effects of intravenous infusions (1–4 μmol/min/kg) of 14 bile acids, cholic, deoxycholic, ursodeoxycholic, chenodeoxycholic, dehydrocholic, and their glycine and taurine conjugates, on bile flow and composition and on the biliary permeation of inert carbohydrates have been studied in the guinea pig bile fistula. Hydroxy bile acids were eliminated in bile without major transformation, except for conjugation (over 90%) when unconjugated bile acids were infused. During infusion of dehydrocholate and taurodehydrocholate, 77–100% of the administered dose was recovered in bile as 3-hydroxy bile acids, thus indicating that reduction of the keto group in position 3 was virtually complete. All bile acids produced choleresis at the doses employed: the strongest choleretic was deoxycholate (81.78 μ/μmol), the weakest was taurodehydrocholate (10.2 μ/μmol). Choleretic activity was directly and linearly related to bile acid hydrophobicity, as inferred by HPLC., both for similarly conjugated bile acids, and for bile acids having the same number, position, or configuration of the hydroxyl groups. In all instances, the rank ordering was: deoxycholate ⪢ chenodeoxycholate ⪢ cholate ⪢ ursodeoxycholate. During choleresis produced by any of the bile acids tested, bicarbonate concentration in bile slightly declined, but the calculated concentration in bile-acid-stimulated bile (45–57 mmol/1) was always higher than that measured in plasma (23–26 mmol/l). Biliary concentrations of cholesterol (20–68 μmol/1) and phospholipid (14–63 μmol/1) were very low during spontaneous secretion, and declined even further following bile acid choleresis. None of the infused bile acids consistently modified biliary excretion of cholesterol and phospholipid. Consistent with a previous observation from this laboratory, all hydroxy bile acids reversibly diminished [ 14C]erythritol and [ 14C]mannitol biliary entry during choleresis, while they increased or failed to modify that of [ 3H]sucrose and [ 3H]inulin. The rank ordering for the inhibitory effect on [ 14C]erythritol and [ 14C]mannitol permeation was: 3α,7α,12α-trihydroxy ⪢ 3α,7α-dihydroxy ⪢ 3α,7β-dihydroxy ⪢ 3α,12α-dihydroxy bile acids. These results are interpreted to suggest that, in the guinea pig, (i) the differing choleretic effects of bile acids reflect differences in their cholehepatic cycling and/or in their stimulation of a bicarbonate pump; (ii) bile acids do not regulate cholesterol and phospholipid biliary excretion; and (iii) hydroxy bile acids diminish the canalicular membrane permeability to [ 14C]erythritol and [ 14C]mannitol by rearranging the architecture of the membrane ‘aqueous pores’ through which these solutes enter the canalicular lumen.

Chemical properties of bile acids: III. Bile acid structure and solubility in water

The temperature dependence of the solubilities in water at pH 3.00 of a representative series of ten cholanoic acids (bile acids) were measured over the temperature range 10 to 50°C. The solubilities ranged from 5×10−8M for 3α-hydroxy-5β-cholanoic acid (lithocholic acid) to 2×10−3M for 3α-7β-12α-trihydroxy-5β-cholanoic acid (ursocholic acid). The solubilities increased with temperature and were found to be dependent on the number, position, and orientation of the hydroxyl groups. The hydroxyl groups affect solubility both by forming hydrogen bonds with the solvent and by reducing the hydrophobic area of the steroid nucleus. The enthalpic ΔH0 and entropic ΔS0 contributions to Δμ0 associated with the dissolution process were obtained from the temperature dependence of the solubilities. A qualitative interpretation of the thermodynamic parameters in relation to bile acid structure is also presented.

Determinants of biliary calcium (Ca) secretion: Effect of bile acid (BA) structure

Determinants of biliary calcium (Ca) secretion: Effect of bile acid (BA) structure

Effect of bile acid structure on liver uptake and secretion in rat

Effect of bile acid structure on liver uptake and secretion in rat

Influence of bile acid structure on bile flow and biliary lipid secretion in the hamster.

A comprehensive study of the influence of bile acid structure on bile flow and biliary lipid secretion was carried out by infusing pure bile acids at a physiological rate into the proximal small intestine of a bile fistula hamster. Twelve individual bile acids, cholate (C), ursocholate (UC), chenodeoxycholate (CDC), and ursodeoxycholate (UDC) as their glycine (G), taurine (T), or unconjugated form, were studied so that influence of the hydroxy substituents as well as side-chain structure could be defined. The pattern of bile acid output was dependent on bile acid structure and reflected the site and rate of intestinal absorption. Conjugated bile acid output was delayed because of late ileal absorption, and TUC was poorly absorbed. Unconjugated trihydroxy bile acids, C and UC, also exhibited a delay in absorption, while CDC and UDC were absorbed immediately and achieved the highest bile acid output. Unconjugated bile acids were conjugated initially mostly with taurine and then mostly with glycine. The effect of glycine conjugates of each bile acid on bile flow and biliary lipid secretion was similar to that of their corresponding taurine conjugates. All conjugated bile acids induced a similar rate of bile flow (9-15 microliter bile/mumol bile acid), but unconjugated bile acids other than C induced more flow (20-25 microliter bile/mumol bile acid) than their corresponding conjugates. Conjugates of the dihydroxy bile acids induced a greater secretion of phospholipid and cholesterol than cholyl conjugates, whereas conjugates of UC were unique in inducing extremely low phospholipid and cholesterol secretion. For an increase of 1 mumol X min-1 X kg-1 in bile acid output, the increase in phospholipid secretion was 0.072 mumol X min X kg for GCDC and TCDC; 0.051 mumol X min-1 X kg-1 for GUDC and TUDC; and 0.030 mumol X min-1 X kg-1 for GC and TC. Increase in cholesterol output per mumol X min-1 X kg-1 of bile acid output was 0.013 mumol X min-1 X kg-1 for GCDC and TCDC, 0.011 mumol X min-1 X kg-1 for GUDC and TUDC, and 0.005 mumol X min-1 X kg-1 for GC and TC. In general, unconjugated bile acids induced more phospholipid and cholesterol than their corresponding conjugates; however, the rank-order effect of the steroid nucleus substituents was similar to that observed for the respective conjugates. These results indicate that both nuclear and side-chain structure influence the enterohepatic circulation and biliary secretory properties of bile acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of four taurine-conjugated bile acids on mucosal uptake and lymphatic absorption of cholesterol in the rat.

The importance of the bile acid structure on mucosal uptake and lymphatic absorption of cholesterol was investigated using four different taurine-conjugated bile acids. Pure synthetic conjugates of a trihydroxy bile acid, taurocholate, and three dihydroxy bile acids, tauroursodeoxycholate, taurochenodeoxycholate, and taurodeoxycholate were used to completely solubilize [14C]cholesterol and polar lipids for steady rate intraduodenal infusion for 8 hr in bile fistula rats. Lymph output and esterification of [14C]cholesterol and endogenous cholesterol were measured in hourly samples. A second group of bile fistula rats was given the same bile acids as the first group but without added cholesterol or other lipid, i.e., fasting lymph fistula group. Mucosal uptake of [14C]cholesterol was studied using recovery of [14C]cholesterol from lumen and mucosa after 1-hr infusions in conscious bile fistula rats. Lymph output of [14C]cholesterol was promoted more rapidly with taurocholate than with the dihydroxy conjugates and [14C]cholesterol output differed for the three groups given dihydroxy bile acids. The mass of cholesterol in lymph, measured chemically, varied in parallel with [14C]cholesterol absorption. For fasting lymph, infusion of dihydroxy bile acids failed to produce a significant change in endogenous cholesterol output when compared with rats given saline only. Taurocholate infusion markedly increased endogenous cholesterol in lymph of fasted rats. Under all conditions where cholesterol output was stimulated, the increase could be accounted for mainly as esterified cholesterol. Mucosal uptake of [14C]cholesterol during 1-hr infusions in conscious bile fistula rats was slower with the dihydroxy bile acids than with taurocholate. The results indicate the marked effect of the number and configuration of the hydroxyl groups on the solubilizing bile acid for cholesterol absorption.

Stimulation of ornithine decarboxylase activity and DNA synthesis by phorbol esters or bile acids in rat colon.

The changes of colonic epithelial ornithine decarboxylase (ODC) activity and DNA synthesis following intrarectal administration of a tumor-promoting agent, 12-O-tetradecanoylphorbol-13-acetate (TPA), or various bile acids to male noninbred rats were studied. A single instillation of TPA, at a dose as low as 16 nmol, led to a significant (about 10-fold) increase in colonic ODC activity. Peak ODC activity was observed at 4 hr, and the enzyme activity returned to the control level about 24 hr after intrarectal TPA. This pattern was almost the same as that observed after sodium deoxycholate treatment. TPA showed more potent induction of ODC activity than deoxycholate, although the maximal induction was greater in the case of deoxycholate treatment. Both TPA and deoxycholate stimulated DNA synthesis at 2 days after intrarectal instillation, after an initial depression at 4-12 hr. A structure-activity study of 26 bile acids revealed that 5 beta-cholanoic acid with alpha-hydroxy groups in two of the 3 alpha, 7 alpha, 12 alpha positions and 5 beta-cholanoic acid with a 3 alpha-hydroxy group induced colonic ODC activity significantly, while the 3 alpha, 6 alpha-dihydroxy acid did not. Replacement of hydroxy groups by keto groups or a change from alpha to beta configuration decreased the ODC-inducing activities. Tri-substituted 5 beta-cholanoic acid derivatives, whether hydroxy or keto, did not stimulate ODC. These data indicate that a specific bile acid structure with a definite spatial relationship of the hydroxy groups is required for induction of colonic ODC activity.

Tauroursodeoxycholic acid is less damaging than taurochenodeoxycholic acid to the gastric and esophageal mucosa.

Bile acids are capable of disrupting the gastric and esophageal mucosal barriers and are known to differ in their ability to injure these mucosae. Two bile acids, chenodeoxycholic and its 7-B epimer, ursodeoxycholic, that are being used to dissolve gallbladder stones were evaluated for their damaging effects on experimental preparations of the esophageal (rabbit) and gastric (dog) mucosa. Damage was assessed by measuring indices of mucosal barrier function, including net acid flux, potential difference, and tissue resistance, before and after exposure to the taurine conjugates of these bile acids. In both the esophageal and gastric mucosa, tauroursodeoxycholic acid caused significantly less disruption of barrier function than taurochenodeoxycholic acid. These results demonstrate that minor differences in conjugated bile acid structure can cause major changes in the effects of bile acids on the upper gastrointestinal mucosa and that ursodeoxycholic acid may be the preferred bile acid for oral ingestion to dissolve gallbladder stones.

Bile acid transport in cultured rat hepatocytes.

The mechanisms of bile acid uptake have been studied with primary monolayer cultures of rat hepatocytes. Hepatocytes were incubated with taurocholic acid (TC), glycocholic acid (GC), cholic acid (CA), glycochenodeoxycholic acid (GCDC), chenodeoxycholic acid (CDCA), deoxycholic acid (DOCA), lithocholic acid (LCA), or cholylglycylhistamine (CCH), a neutral bile acid derivative for 10 s to 60 min in medium containing sodium chloride, sodium chloride with 1 mM ouabain, or choline chloride. Cells were washed free of radioactive tracer, cell-associated radioactivity was quantitated, and bile acid uptake rates, kinetic parameters of uptake, and steady-state bile acid content were calculated. Two mechanisms for bile acid uptake were identified. Uptake of TC, GC, CA, and GCDC occurred predominantly via a sodium-dependent, ouabain-suppressible saturable mechanism, presumably sodium-coupled transport. Estimates of apparent Km and Vmax for these bile acids were TC, 33 micro M and 0.36 nmol . min-1 . mg prot-1; GC, 18 micro M and 0.22 nmol . min-1 . mg prot-1; CA, 13 micro M and 0.10 nmol . min-1 . mg prot; and GCDC, 6 micro M and 0.21 nmol . min-1 . mg prot, respectively. Uptake via this sodium-coupled mechanism exhibited considerable substrate selectivity. It was enhanced by increased ring hydroxylation and amino acid conjugation and decreased by further conjugation with a neutral histamine group (CGH). In contrast, uptake of CDCA, DOCA, LCA, and CGH occurred primarily via a nonsaturable sodium-independent mechanism, possibly simple diffusion. This mechanism accounted for only a small portion of uptake of TC, GC, CA, and GCDC at low bile acid concentrations. Nonsaturable bile acid uptake rates appeared to correlate with decane-buffer partition coefficients and to be related to bile acid structure.

Hepatic bile acid uptake: effect of conjugation, hydroxyl and keto groups, and albumin binding

Hepatic extraction of trihydroxy (free, glyco- and tauro-conjugated) dihydroxy, and monohydroxy bile acids has been evaluated in single pass liver perfusion experiments in rats. The percentage of each bile acid bound to albumin was also evaluated by equilibrium dialysis. Conjugation increased bile acid liver extraction, without relevant differences in the percentage of bile acid bound to albumin. Among the free bile acids, trihydroxy bile acids were more efficiently cleared by the liver than the dihydroxy acids, and the latter more than monohydroxy bile acids. 7-Ketolithocholic acid uptake was slightly less than that of cholic acid. Conversely, among dihydroxy bile acids, the percentage of the bile acid bound to albumin decreased from lithocholic acid to cholic acid. Decreasing the albumin concentration in the medium, and hence the fraction of the bile acid bound to albumin, resulted in an increase in bile acid liver extraction. Therefore besides differences in the chemical structure of bile acids, the extent of bile acid-albumin binding may be a determinant in bile acid liver uptake.

The effect of bile acid structure on the activity of bile acid-CoA:glycine/taurine-N-acetyltransferase.

Bile acid-CoA:glycine/taurine N-acetyltransferase from bovine liver was analyzed for its ability to conjugate a variety of different bile acid-CoA analogues. A complete steady state bisubstrate kinetic analysis was conducted for each analogue. The enzyme demonstrated strict specificity for the normal 4-substituted pentanoic acid side chain; shortening the side chain by 1 methylene group (norcholyl-CoA) completely eliminates enzymatic activity, and extending the side chain by 1 methylene group (homocholyl-CoA) causes a 30-fold decrease in activity at Vmax. These effects of side chain modification were not related to decreased binding affinity as much as to decreases in the rates of the bond-breaking and bond-making steps. Bile acid-CoA analogues with a variety of ring substitutions involving keto and hydroxyl groups were also examined. Varying the position of substitution and the nature of the substituent had major effects on both the Km and Vmax terms. The analogues with the highest activities at Vmax were 7-dehydrocholyl-CoA and the "allo" bile acid 5 alpha,6-ketolithocholyl-CoA. However, in both cases, the high activity is obtained at the expense of binding energy. The most efficient substrates were 7-ketolithocholyl-CoA and 3-dehydrocholyl-CoA. The more common analogue, chenodeoxycholyl-CoA, was a surprisingly inefficient substrate. The relative rates of formation of glycine versus taurine conjugates were also found to vary with changes in structure. This indicates that certain bile acids are more likely to be conjugated with taurine, and others with glycine.

Bile acids : LXIII. Relationship betweeb the mobility on reversed-phase high-performance liquid chromatography and the structure of bile acids

The mobility of bile acids, as defined by the term “relative capacity factor”, was determined on a μBondapak C 18 column, eluted with a mixture of 2-propanol and 10m M potassium phosphate (pH 7.0). The contributions to mobility due to substituents such as hydroxyl, oxo, olefin, and side-chain methylene and amino acid substituents were calculated. From these values, the relative capacity factors of polyfunctionalized bile acids were computed. The agreements were good between the theoretical and observed values, provided that only hydroxyl substituents were present on the a-surface and that they were spaced sufficiently apart. When β-hydroxyl or oxo groups were also attached to the steroid nucleus, substantial disagreements were noted which could be rationalized on the basis of conformational analysis. Thus, the mobility of a compound is not necessarily correlated to the sum of the polarity contributions of each substituent.

Bile acid structure and bile formation: a comparison of hydroxy and keto bile acids

The effect of four bile acids, taurocholic (TC), taurochenodeoxycholic (TCDC), tauro-3 alpha,7 alpha-dihydroxy-12-keto-5 beta-cholanoic (T12K), and cholic (C), on bile lipid and water secretion was studied in the bile fistula cat over a 20-fold range of bile acid secretion rates. The critical micellar concentration (CMC) and lecithin-solubilizing capacity of TC, TCDC, and T12K were measured in vitro. T12K and C produced a greater choleresis than TC or TCDC; phospholipid secretion was strikingly reduced with T12K; and cholesterol secretion strikingly increased with TCDC, with a high cholesterol-phospholipid ratio. The estimated CMCs of TC, TCDC, and T12K were 3.3, 0.7, and 6.2 mM respectively; the relative lecithin-solubilizing capacities were 4.4:2.6:1. We conclude that the micellar characteristics of the bile acids studied do not explain the differences in water and lipid secretion observed, and that effects within the hepatocyte may be equally important.

Proceedings of the Australian Society for Medical Research.

Proceedings of the Australian Society for Medical Research.

Structure of Bile Acids Associated with Secretion in the Rat Cecum

An isolated in vivo rat cecal loop technique was utilized to determine what structure of bile acids is required to stimulate net colonic secretion of water and sodium. A dose response curve for water and sodium movement was determined for deoxycholic acid (1-6 mM) and chenodeoxycholic acid (3-6 mM). Both of these bile acids were associated with significant secretion of water and sodium at 4 mM concentration. Therefore, this concentration was used for all test bile acids studied. Test solutions included the tri- and di-substituted bile acids: cholic acid, hyocholic acid, 3 alpha, 7 beta, 12 alpha-trihydroxycholanic acid, 3 alpha, 12 alpha-dihydroxy-7-ketocholanic acid, ursodeoxycholic acid, 3 alpha-hydroxy-7-ketocholanic acid, 7 alpha, 12 alpha-dihydroxycholanic acid, and hyodeoxycholic acid. Only three bile acids, deoxycholic acid, chenodeoxycholic acid, and 7 alpha, 12 alpha-dihydroxycholanic acid were associated with net secretion of water and sodium. Cecal histology after incubation with bile acids revealed mucosal alterations in those sacs in which secretion occurred, but not where absorption was noted. These data indicate that bile acid-associated water and sodium secretion in the rat cecum requires a specific bile acid structure with a definite spatial relationship of the hydroxyl groups. Secretion occurred only with two alpha-hydroxyl groups in either the 3, 7, or 12 positions.