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Background & Aims: Dihydroxy bile acids induce a bicarbonate-rich hypercholeresis when secreted into canalicular bile in unconjugated form; the mechanism is cholehepatic shunting. The aim of this study was to identify a xenobiotic that induces hypercholeresis by a similar mechanism. Methods: Five organic acids (sulindac, ibuprofen, ketoprofen, diclofenac, and norfloxacin) were infused into rats with biliary fistulas. Biliary recovery, bile flow, and biliary bicarbonate were analyzed. Sulindac transport was further characterized using Tr^- rats (deficient in mrp2, a canalicular transporter for organic anions), the isolated perfused rat liver, and hepatocyte membrane fractions. Results: In biliary fistula rats, sulindac was recovered in bile in unconjugated form and induced hypercholeresis of canalicular origin. Other compounds underwent glucuronidation and were not hypercholeretic. In the isolated liver, sulindac had delayed biliary recovery and induced prolonged choleresis, consistent with a cholehepatic circulation. Sulindac was secreted normally in Tr^- rats, indicating that its canalicular transport did not require mrp2. In the perfused liver, sulindac inhibited cholyltaurine uptake, and when coinfused with cholyltaurine, induced acute cholestasis. With both basolateral and canalicular membrane fractions, sulindac inhibited cholyltaurine transport competitively. Conclusions: Sulindac is secreted into bile in unconjugated form by a canalicular bile acid transporter and is absorbed by cholangiocytes, inducing hypercholeresis. At high flux rates, sulindac competitively inhibits canalicular bile salt transport; such inhibition may contribute to the propensity of sulindac to induce cholestasis in patients. GASTROENTEROLOGY 1999;117:962-971

The 37° cholesterol solubilities in over 50 solvents, including the homologousn‐alkanols through dodecanol and homologous ethyl carboxylates through the undecanoate, and the 37° β‐sitosterol solubilities in then‐alkanols through decanol are reported. Additionally, solubility data for cholesterol at 7, 17, and 27° in the alcohol series were obtained. These measurements allowed the calculation of heats of solution for cholesterol in the alkanols, which range from 7.5 kcal for methanol to 4.3 kcal for decanol and which tend to decrease, although irregularly, with increasing alkanol chain length. A solubility maximum in all of these series for both solutes was observed between a chain length of six and seven. A surprisingly irregular, odd‐even alternating solubility pattern was noted for cholesterol in the alkanols at all four temperatures. Experimental evidence indicated that this pattern was due to solvent‐induced crystalline changes, presumably solvate formation, in each alkanol solvent through C10. Overall, the solubility studies screened solvents for their utility in dissolving cholesterol and, thus, cholesterol gallstones. To these ends, some limited dissolution experiments were performed, which indicated that the solution rate is directly related to the measured solubility in organic solvents. The dissolution behavior is thus different from micellar bile salt solutions, in which a significant interfacial barrier controls kinetics.

Bile acids are important endogenous compounds involved in many pathophysiological processes. The synthetic production of natural bile acids, as well as that of new ones not present in nature, is of increasing interest as those compounds are becoming used as drugs. Among the novel bile acids, nor-deoxycholate (nor-DC) presents an unespected high choleretic effect that, in the authors opinion, could be related to a cholehepatic recirculation.

The metabolism and enterohepatic circulation of deoxycholic acid (DCA), a major secondary bile acid in humans, was simulated using a linear multicompartmental physiologic pharmacokinetic model. The model was similar to that previously reported and used to simulate the metabolism of cholic acid and chenodeoxycholic acid, but differed in two respects: (a) the input of newly formed DCA molecules originated from colonic absorption rather than from de novo hepatic biosynthesis and (b) a new type of transfer coefficient was proposed to describe the movement of DCA molecules from an insoluble, bound compartment to a soluble compartment. Simulations were performed to define the effect of varying fractional colonic absorption (from 0.1 to 0.6) as well as varying fractional formation of DCA from cholic acid (from 0.3 to 1). The simulations indicated that the exchangeable total DCA pool expanded up to 12-fold as fractional colonic absorption was increased from 0.1 to 0.6. The fractional turnover rate of the DCA pool showed a corresponding decrease. Increased conversion of cholic acid to DCA had an effect on DCA pool size that was similar to that resulting from increased colonic fractional absorption. So long as ileal absorption was efficient, the “soluble” colonic pool of DCA remained small relative to other organ pools, and the absorption of unconjugated DCA from the colon was <10% of the total DCA absorption from the ileum. It is proposed that the relatively large proportion of DCA in the biliary bile acids of white adults in the Western world as compared with that of most other mammals is attributable to (a) a high fractional absorption of DCA because of a diet relatively low in fiber, (b) the absence of hepatic 7-hydroxylation of DCA, and (c) effective competition by DCA conjugates for active transport by the terminal ileum.

To define whether side-chain length influences the physiologic properties of bile acids, nor-ursodeoxycholate (nor-UDC), the C23-nor derivative of ursodeoxycholate (UDC), was synthesized in both nonradioactive and radioactive forms (23-14C). Its hepatic translocation, hepatic biotransformation, and effect on bile flow, biliary bicarbonate, and biliary lipid secretion were compared with that of UDC and those of their respective glycine and taurine conjugates in anesthetized biliary fistula hamsters, rats, and guinea pigs, as well as the isolated perfused hamster liver. Hepatic uptake and biliary output of nor-UDC was slower than that of UDC or cholyltaurine in the isolated perfused hamster liver. In biliary fistula animals, nor-UDC was secreted only in bile. Biliary recovery of nor-UDC as compared to that of UDC was prolonged in the rat and hamster, although not in the guinea pig. Hepatic biotransformation, assessed by chromatography of bile, showed that conjugation of nor-UDC was inefficient, as unconjugated nor-UDC was present in bile; there was little amidation with glycine or taurine in any species, but sulfates and glucuronides, as well as other metabolites, were formed, with the pattern of biotransformation varying among species. When infused over a dosage range of 0.2–30 μmol/kg · min, nor-UDC induced a striking choleresis of canalicular origin. The bile acid-dependent flow was increased threefold in hamsters, ninefold in rats, and nearly twofold in guinea pigs when compared to that induced by UDC. The choleresis was associated with a linear increase in bicarbonate output and concentration in bile, and little phospholipid or cholesterol secretion was induced. A competition experiment in the bile fistula hamster indicated that nor-UDC or its metabolites, or both, appeared to compete for canalicular transport of ursocholyltaurine (a cholyltaurine epimer) when the latter was secreted under its Vmaxconditions. Conjugates of nor-UDC and UDC were promptly and almost completely recovered in bile without appreciable hepatic biotransformation; the conjugates did not induce a hypercholeresis or increase biliary bicarbonate concentration. It is proposed that a fraction of nor-UDC is secreted into canalicular bile in the unconjugated form and is protonated by a hydrogen ion derived from carbonic acid that was generated by the hydration of luminal CO2by carbonic anhydrase present in biliary ductular cells.

A physical model approach was utilized to investigate cholesterol monohydrate dissolution kinetics in simulated bile. The static pellet method and the Berthoud theory were employed to assess the contributions of the diffusion-convection mass transfer resistance and those of the interfacial resistance to the overall kinetics. For almost all situations studied, the interfacial resistance was the dominant rate- determining factor. The effects of four bile acids and their concentrations, the bile acid-lecithin ratio, and the added electrolytes and their concentrations on the interfacial resistance were examined. The results were correlated with those obtained with human bile samples, and the indications were that the kinetics of cholesterol dissolution in bile may be explainable on the basis of the principal bile acids, lecithin, and the electrolytes in the bile.

Cholesterol monohydrate dissolution kinetics in human gallbladder bile were studied to determine the magnitudes of the in vitrodissolution rates, the rate resistances in human gallbladder bile, and the extent that the interfacial resistance is the rate-determining factor. Dissolution rate studies also were conducted using human duodenal bile and animal bile for comparison. The dissolution rate resistance, R, ranged from 104sec/cm for chicken bile to 104–106sec/cm for human bile. Interfacial resistance was the rate-determining factor for essentially all results. Where chemical composition data were obtained, the Rvalues for the human bile samples were consistent with predictions made from the simulated bile studies. In two human gallbladder specimens having low bile acid-lecithin molar ratios (i.e., 2.9 and 2.3), very high Rvalues of 1.9×105and 4.1×106sec/cm were found. These values were in good agreement with the findings in the simulated bile studies and suggest that stone dissolution in patients with low bile acid–lecithin ratios may proceed very slowly, even when the bile is highly undersaturated with respect to cholesterol.

To find a possible explanation for the selective hepatic conjugation of bile acids with glycine or taurine, the N-acyl amidates of cholic acid and a number of amino acids and amino acid analogues were synthesized, and their susceptibility to hydrolysis by pancreatic juice, gastric juice, serum, or small intestinal mucosal enzymes was measured. Deconjugation by pure carboxypeptidase A and B was also examined, and hydrolysis by these tissue fluids and enzymes was compared with that mediated by a bacterial cholylglycine hydrolase. Human pancreatic juice efficiently hydrolyzed cholyl conjugates of all neutral-l-amino acids (cholyl-l-alanine, cholyl-l-valine, cholyl-l-leucine, and cholyl-l-tyrosine), except cholylglycine. The net hourly rate of hydrolysis (in micromales per milligram protein per hour) increased when the terminal residue was aromatic or branched aliphatic, and appeared to be specific for l-α-amino acids as cholyl-β-alanine and cholyl-d-valine were not cleaved. From cholyl glycylglycine, only the terminal glycine was efficiently removed. Cholyltaurine and cholyl conjugates with the methyl and propyl analogues of taurine were resistant to hydrolysis. Two basic amino acid conjugates (cholyl-l-lysine and cholyl-l-arginine) were cleaved, whereas conjugates of acidic amino acids (cholyl-aspartate and cholyl-cysteate) were not cleaved. Studies using pure enzymes showed that bovine carboxypeptidase A hydrolyzed the cholyl conjugates of the neutral l-α-amino acids with similar specificity as observed for the human pancreatic juice, whereas bovine carboxypeptidase B cleaved the basic amino acid conjugates. Cholyl-l-lysine and cholyl-l-arginine were also cleaved by serum and plasma, which are known to possess carboxypeptidase activity. Cholyl conjugates were not cleaved by gastric juice, by trypsin, or by homogenates of rat small intestinal mucosa. In contrast, all cholyl conjugates were cleaved by a bacterial cholylglycine hydrolase. These experiments indicate that glycine and taurine amidates of cholic acid differ from a number of other conjugates with neutral and basic amino acid in being resistant to hydrolysis by pancreatic and plasma carboxypeptidases. These data, together with other data indicating that bile acid conjugation greatly decreases passive intestinal absorption, indicate that a physiologic function of bile acid conjugation with glycine or taurine is to form surfactants that remain indigestible and rather nonabsorbable during digestion in the proximal small intestine.

Various compounds that might function as cholesterol gallstone dissolution accelerators were studied. The dissolution rates of cholesterol monohydrate pellets in synthetic bile (116mMsodium cholate–32mMlecithin) containing the agent at various concentration levels were determined. In the absence of any dissolution rate accelerator, the dissolution kinetics for cholesterol previously were found to be interfacial resistance controlled, and the rates were around 20 times less than the diffusion-controlled rates in the present experiments. Primary, secondary, and tertiary amines and quaternary ammonium compounds were effective accelerators. When the alkyl chain lengths were long enough and/or when the agent concentrations were high enough, the dissolution rates generally approached diffusion-controlled rates. Steroidal amines generally had good activity. Anionic and nonionic surfactants had little or negative activity.

Normal human gallbladder bile and gallbladder bile of patients undergoing chenodeoxycholic acid therapy were simulated by using appropriate combinations of taurine and glycine conjugates of cholic, chenodeoxycholic, and deoxycholic acids. Also, the total bile acid concentration and the total bile acid to lecithin ratio were varied over physiological ranges. Dissolution rates of cholesterol monohydrate pellets (model gallstone) in these solutions were 90–99% interfacially controlled. Even under conditions favorable for dissolution, i.e.,high bile acid concentration and high bile acid to lecithin ratio, the interfacial resistances were extremely large. These results are of the same order of magnitude as those found in the limited studies with actual gallbladder bile and suggest that the bile acids, lecithin, and the electrolytes are the primary determinants of the interfacial resistance for cholesterol dissolution. Furthermore, the kinetics of dissolution were always much faster with the chenodeoxycholic acid-rich compositions than with the corresponding normal compositions. This finding suggests, therefore, that in addition to desaturating bile with respect to cholesterol, the feeding of chenodeoxycholic acid further facilitates cholesterol gallstone dissolution by reducing the interfacial resistance of the process.

Recent studies suggested that in situdissolution of gallstones in man might be dissolution rate controlled. In this work, several physical models for the dissolution rate of cholesterol-type stones were developed. One is based upon simple diffusion-controlled dissolution and another upon a leaching process. Two others relate to interfacial coat-type barriers. Calculations were carried out employing models with reasonable input parameters. The results show, for example, that the theoretical dissolution rate based on the simple dissolution model would predict that a 2.5-mm. stone should dissolve in several days into an undersaturated bile solution. Both clinical and in vitroexperiments show that much longer times, e.g., months, are needed.