Regulation of hepatic transport of bile salt. Effect of protein synthesis inhibition on excretion of bile salts and their binding to liver surface membrane fractions.

The overall transport of bile salts across the hepatocyte is characterized as a carrier-mediated process whose rate-limiting step is biliary secretion. Specific bile salt binding proteins have been identified in liver surface membrane fractions and were postulated to represent the initial interaction in bile salt translocation across both the sinusoidal and canalicular membranes. To test this hypothesis, cycloheximide was administered to rats to inhibit hepatic protein synthesis. 16 h after cycloheximide administration [14C]leucine incorporation into hepatic protein was inhibited by 93% at 1 h and 47% at 12 h. However, values of liver function tests were not increased, although serum albumin, serum alanine amino-transferase, and alkaline phosphatase were significantly decreased. Light and electron microscopy did not demonstrate necrosis or fat accumulation. The latter demonstrated minimal disorganization of rough endoplasmic reticulum and occasional lamellar whorls. 16 h after cycloheximide administration bile salt independent bile flow, basal bile salt excretion, and basal bile flow were unaltered, but the maximum bile salt transport capacity was reduced to 62% of control and 24 h later to 38%. Decreased bile salt transport was reversible, for it returned to control values after 48 h, when hepatic protein synthesis was also normal. Maximum bromosulfophthalein (BSP) transport, on the other hand, was reduced after 16 h to only 85% of control. Both bile salt and BPS maximum transport capacities decreased with time during inhibition of protein synthesis, apparently following first order kinetics. It was estimated that their half-lives are 20 h for bile salt transport and 55 h for BSP transport. These different turnover rates suggest that cycloheximide does not decrease active transport through generalized hepatic dysfunction or alteration of high energy sources possibly required for transport. The maximum number of [14C]cholic acid binding sites in liver surface membrane fractions was determined by an ultrafiltration assay. They were reduced to 68% of control after 16 h of cycloheximide and to 25% after 24 h. This reduction in the number of binding sites is apparently selective, for the activities of the liver surface membrane enzymes (Na+-K+)ATPase, Mg++-ATPase, and 5'-nucleotidase were not significantly changed. The associated alterations in bile salt transport and the maximum number of binding sites after cycloheximide administration suggests that these receptors may be the bile salt carriers.     

Direct determination of the driving forces for taurocholate uptake into rat liver plasma membrane vesicles.

To determine directly the driving forces for bile acid entry into the hepatocyte, the uptake of [3H]taurocholic acid into rat liver plasma membrane vesicles was studied. The membrane preparation contained predominantly right-side-out vesicles, and was highly enriched in plasma membrane marker enzymes. The uptake of taurocholate at equilibrium was inversely related to medium osmolarity, indicating transport into an osmotically sensitive space. In the presence of an inwardly directed sodium gradient (NaCl or sodium gluconate), the initial rate of uptake was rapid and taurocholate was transiently accumulated at a concentration twice that at equilibrium (overshoot). Other inwardly directed cation gradients (K+, Li+, choline+) or the presence of sodium in the absence of a gradient (Na+ equilibrated) resulted in a slower initial uptake rate and did not sustain an overshoot. Bile acids inhibited sodium-dependent taurocholate uptake, whereas bromsulphthalein inhibited both sodium-dependent and sodium-independent uptake and D-glucose had no effect on uptake. Uptake was temperature dependent, with maximal overshoots occurring at 25 degrees C. Imposition of a proton gradient across the vesicle (pHo less than pHi) in the absence of a sodium gradient failed to enhance taurocholate uptake, indicating that double ion exchange (Na+-H+, OH- -anion) is unlikely. Creation of a negative intravesicular potential by altering accompanying anions or by valinomycin-induced K+-diffusion potentials did not enhance taurocholate uptake, suggesting an electroneutral transport mechanism. The kinetics of taurocholate uptake demonstrated saturability with a Michaelis constant at 52 microM and maximum velocity of 4.5 nmol X mg-1 X protein X min-1. These studies provide definitive evidence for a sodium gradient-dependent, carrier-mediated, electrically neutral transport mechanism for hepatic taurocholate uptake. These findings are consistent with a model for bile secretion in which the basolateral enzyme Na+,K+-ATPase provides the driving force for "uphill" bile acid transport by establishing a trans-membrane sodium gradient.     

Inhibitory effect of unconjugated bile acids on the enterohepatic circulation of methotrexate

The effect of unconjugated cholic and deoxycholic acids on intestinal and hepatic transport and bile secretion of methotrexate was studied using everted sacs of rat proximal jejunum and isolated perfused rat liver. Cholic and deoxycholic acids competitively inhibit the mucosal-to-serosal transport of methotrexate (Ki, 0.08 and 0.06 mM, respectively). Cholic and deoxycholic acids also decrease intestinal tissue content of methotrexate in a concentration-dependent manner. Structural and functional damage to the intestinal mucosa does not occur in tissue treated with 0.1 mM and lower concentration of deoxycholic acid as assessed by histological studies, transmural potential difference measurements and the release of the cytoplasmic marker enzyme, lactate dehydrogenase. In the isolated liver, cholic and deoxycholic acids inhibit the uptake, retention and biliary secretion of methotrexate. At 1 mM cholic and deoxycholic acids, 72 and 80% inhibition in liver uptake and 93 and 99% inhibition in bile secretion of 1 microM methotrexate are observed, respectively. These studies demonstrate that unconjugated bile acids inhibit the enterohepatic circulation of methotrexate by impairing its intestinal transport and hepatic uptake and retention and biliary secretion.     

Role of farnesoid X receptor in the enhancement of canalicular bile acid output and excretion of unconjugated bile acids: a mechanism for protection against cholic acid-induced liver toxicity

Mice lacking the farnesoid X receptor (FXR) involved in the maintenance of hepatic bile acid levels are highly sensitive to cholic acid-induced liver toxicity. Serum aspartate aminotransferase (AST) activity was elevated 15.7-fold after feeding a 0.25% cholic acid diet, whereas only slight increases in serum AST (1.7- and 2.5-fold) were observed in wild-type mice fed 0.25 and 1% cholic acid diet, respectively. Bile salt export pump mRNA and protein levels were increased in wild-type mice fed 1% cholic acid diet (2.1- and 3.0-fold) but were decreased in FXR-null mice fed 0.25% cholic acid diet. The bile acid output rate was 2.0- and 3.7-fold higher after feeding of 0.25 and 1.0% cholic acid diet in wild-type mice, respectively. On the other hand, no significant increase in bile acid output rate was observed in FXR-null mice fed 0.25% cholic acid diet in contrast to a significant decrease observed in mice fed a 1.0% cholic acid diet in spite of the markedly higher levels of hepatic tauro-conjugated bile acids. Unconjugated cholic acid was not detected in the bile of wild-type mice fed a control diet, but it was readily detected in wild-type mice fed 1% cholic acid diet. The ratio of biliary unconjugated cholic acid to total cholic acid (unconjugated cholic acid and tauro-conjugated cholic acid) reached 30% under conditions of hepatic taurine depletion. These results suggest that the cholic acid-induced enhancement of canalicular bile acid output rates and excretion of unconjugated bile acids are involved in adaptive responses for prevention of cholic acid-induced toxicity.     

Regulation of Bile Salt Transport in Rat Liver: EVIDENCE THAT INCREASED MAXIMUM BILE SALT SECRETORY CAPACITY IS DUE TO INCREASED CHOLIC ACID RECEPTORS

Expansion of the bile salt pool size in rats increases maximum excretory capacity for taurocholate. We examined whether increased bile salt transport is due to recruitment of centrolobular transport units or rather to adaptive changes in the hepatocyte. Daily sodium cholate (100 mg/100 g body wt) was administered orally to rats. This treatment was well tolerated for at least 4 d and produced an 8.2-fold expansion of the bile salt pool. This expanded pool consisted predominently (99%) of cholic and deoxycholic acids. Significantly increased bile salt transport was not observed until 16 h after bile acid loading, and maximum elevations of transport capacity to 2.3-fold of control required ~2 d. In contrast, maximum sulfobromophthalein excretion rates increased 2.2-fold as early as 4 h and actually fell to 1.5-fold increase at 4 d. We studied the possibility that this adaptive increase in bile salt secretory transport was due to changes in canalicular surface membrane area, lipid composition, or increased number of putative carriers. Canalicular membrane protein recovery and the specific activities of leucine aminopeptidase, Mg⁺⁺-ATPase and 5′-nucleotidase activities were unaltered by bile salt pool expansion. The content of free and esterified cholesterol and total phospholipids was unchanged in liver surface membrane fractions compared with control values. In contrast, sodium cholate administration selectively increased specific [¹⁴C]cholic acid binding sites twofold in liver surface membrane fractions. Increased numbers of [¹⁴C]cholic acid receptors (a) was associated with the time-dependent increase in bile salt transport, and (b) was selective for the taurine conjugate of cholate and (c) was reduced by chenodeoxycholate. Changes in bile acid binding sites 16 h following taurocholate and chenodeoxycholate and the lack of change with glycocholate was associated with comparable changes in bile salt transport. In conclusion, selective bile salts increase bile salt transport in the liver through an adaptive increase in the density of putative bile acid carriers in liver surface membrane.     

Cyclical oxidation-reduction of the C3 position on bile acids catalyzed by 3 alpha-hydroxysteroid dehydrogenase. II. Studies in the prograde and retrograde single-pass, perfused rat liver and inhibition by indomethacin.

[3 beta-3H, 24-14C]Lithocholic, chenodeoxycholic, and cholic acids were administered in tracer bolus doses either prograde or retrograde in the isolated perfused rat liver. Little 3H loss from cholic acid was observed, whereas with the other bile acids, 20-40% of the administered 3H was lost in a single pass from perfusate to bile. Most of the 3H loss occurred rapidly (5 min) and was recovered as [3H]water in perfusate. Excretion of bile acids was delayed with retrograde administration, and 3H loss was more extensive. In both prograde and retrograde studies, indomethacin markedly inhibited the excretion of the bolus of bile acid into bile. Indomethacin inhibited the extraction of glycocholate (50 microM) during steady state perfusion without affecting transport maximum for excretion. At lower glycocholate concentration (5 microM), indomethacin inhibited both extraction and excretion. A greater effect was seen on excretion in the latter case, which suggests that displacement of bile acid from the cytosolic protein lead to redistribution in the hepatocyte as well as reflux into the sinusoid. These data suggest that binding of bile acids to cytosolic 3 alpha-hydroxysteroid dehydrogenases occurs extensively during hepatic transit and is important in mediating the translocation of bile acids from the sinusoidal to canalicular pole of the cell.     

The binding of cholic acid and its taurine conjugate to serum proteins

By the method of steady-state gel filtration, cholic and taurocholic acids have been shown to be strongly bound by plasma albumin. Cholic acid is more avidly bound than its taurine conjugate. There is probably one primary binding site per albumin molecule, and approximately 7 secondary sites of weaker affinity. Plasma has a very high binding capacity for bile acids and the binding shows rapid equilibration.     

Characterization and effect of phospholipid on bile acid absorption by villi isolated from hamster small intestine

The effects of phospholipid on absorption of bile acids by hamster small intestine were studied to determine if this compound inhibits absorption of bile acids. Absorption of taurocholic and cholic acids was studied using a new in vitro technique that relates uptake rates to the weight of the villi present on the intestinal sample rather than to the weight of the entire segment of intestine used for the study. This procedure removes from consideration various components of the intestinal wall that are not directly involved with the absorptive process. Using radioactive techniques absorption of each type of bile acid was determined over a broad range of concentrations both in the presence and absence of phospholipid in the incubation medium. Absorption of taurocholic acid by villi from jejunum was determined to be a passive process, as previously reported by others. Villi from ileum absorbed both bile acids by an apparent active process when initial concentrations of bile acids were below 2.0 mM. Above this concentration bile acid absorption by the ileum appeared to be mainly passive. Phospholipid was found to inhibit bile acid absorption by ileum when initial bile acid concentrations were moderately high. However, at low substrate concentration, phospholipid has no appreciable effect on bile acid transport.     

Effects of troleandomycin administration on cholesterol 7 alpha-hydroxylase activity and bile secretion in rats

Repeated administration of troleandomycin increased bile flow but decreased the biliary secretion of bile acids in rats. The increased bile flow was associated with a parallel increase in the biliary clearance of [14C]erythritol. Analysis of the relationship between bile flow and bile acid secretion indicated that, for any given rate of bile acid secretin, bile flow was higher in troleandomycin-treated rats than in control rats. The increased bile flow was associated with an increased activity of Na+,K+-adenosine triphosphatase in liver plasma membranes. The decreased bile acid secretion into bile was associated with a similar decrease in the bile acid pool size, a decreased bile acid synthesis rate and a decreased activity of microsomal cholesterol 7 alpha-hydroxylase. The concentration of bile acids in serum, the hepatic extraction ratio of [3H]taurocholate and its biliary transport maximum were not modified. It is concluded that repeated administration of troleandomycin increases the canalicular bile acid-independent flow but decreases the activity of cholesterol 7 alpha-hydroxylase, the synthesis, the pool size and the biliary secretion rate of bile acid in rats.     

Bile acid metabolism in cirrhotic liver tissue--altered synthesis and impaired hepatic secretion

Bile acid analysis of mild and severe cirrhotic liver showed that with the advancement of cirrhosis the concentration of chenodeoxycholic acid in liver tissue becomes higher, resulting in the lower ratio of cholic to chenodeoxycholic acid probably due to the progressive alteration of cholic and chenodeoxycholic acid synthesis with the advancement of liver cirrhosis. Bile acid analysis of paired liver and bile of severe cirrhosis showed that the ratio of cholic to chenodeoxycholic acid in liver was lower than that in bile or even with that in bile. This can be explained by postulating the impaired hepatic secretion of bile acids, especially chenodeoxycholic acid. The impaired secretion together with the relatively well preserved chenodeoxycholic acid synthesis results in the accumulation of chenodeoxycholic acid in liver tissue with cirrhosis.     

Determination of cholic acid and chenodeoxycholic acid pool sizes and fractional turnover rates by means of stable isotope dilution technique, making use of deuterated cholic acid and chenodeoxycholic acid

A procedure is described for the simultaneous determination of cholic acid and chenodeoxycholic acid pool sizes and fractional turnover rates. After oral administration of known amounts of 11,12-dideuterated chenodeoxycholic acid and 2,2,4,4-tetradeuterated cholic acid, the ratios of chenodeoxycholic acid-D2/chenodeoxycholic acid and cholic acid-D4/cholic acid are measured in consecutive serum samples, after which fractional turnover rates and pool sizes of chenodeoxycholic acid and cholic acid are determined arithmetically. In 7 healthy volunteers pool sizes for chenodeoxycholic acid and cholic acid were 22.9 +/- 7.8 and 24.1 +/- 11.7 mumol/kg, respectively. The corresponding values for the fractional turnover rates were 0.23 +/- 0.10 and 0.29 +/- 0.12/day. After oral administration of the labelled bile acids in capsule, the obtained pool sizes were significantly higher than after administration in a bicarbonate solution. Bile acid kinetics were also performed in a patient suffering from a cholesterol synthesis deficiency and in a patient very likely suffering from a bile acid synthesis deficiency. Furthermore, the kinetics of the intestinal absorption and hepatic clearance of unconjugated bile acids have been investigated in 2 healthy subjects.     

Separate transport systems for biliary secretion of sulfated and unsulfated bile acids in the rat.

Biliary secretion of 3 alpha-sulfated bile acids has been studied in Wistar rats with an autosomal recessive defect in the hepatic transport of bilirubin. Liver function, established by measurement of various enzymes in plasma, by enzyme histochemical methods, and by electron microscopy, appeared to be normal in these rats. Serum levels of unconjugated, monoglucuronidated, and diglucuronidated bilirubin were 0.62, 1.62, and 6.16 mumol/liter, respectively, compared with 0.17, 0.08, and 0.02 mumol/liter in control rats. Biliary bilirubin secretion was strongly reduced in the mutant animals: 0.21 +/- 0.03 vs. 0.39 +/- 0.03 nmol/min per 100 g body wt in control rats. Despite normal biliary bile acid output, bile flow was markedly impaired in the mutant animals, due to a 53% reduction of the bile acid-independent fraction of bile flow. The transport maximum for biliary secretion of dibromosulphthalein (DBSP) was also drastically reduced (-53%). Biliary secretion of intravenously administered trace amounts of the 3 alpha-sulfate esters of 14C-labeled taurocholic acid (-14%), taurochenodeoxycholic acid (-39%), taurolithocholic acid (-73%), and glycolithocholic acid (-91%) was impaired in the jaundiced rats compared with controls, in contrast to the biliary secretion of the unsulfated parent compounds. Hepatic uptake of sulfated glycolithocholic acid was not affected in the jaundiced animals. Preadministration of DBSP (15 mumol/100 g body wt) to normal Wistar rats significantly impaired the biliary secretion of sulfated glycolithocholic acid, but did not affect taurocholic acid secretion. We conclude that separate transport systems in the rat liver exist for biliary secretion of sulfated and unsulfated bile acids; the sulfates probably share secretory pathways with the organic anions bilirubin and DBSP. The described genetic defect in hepatic transport function is associated with a reduced capacity to secrete sulfated bile acids into bile; this becomes more pronounced with a decreasing number of hydroxyl groups on the sulfated bile acid's molecule.     

Active and Passive Bile Acid Absorption in Man. PERFUSION STUDIES OF THE ILEUM AND JEJUNUM

Absorption of the major human bile acids was studied in 12 healthy volunteers by steady state perfusion of the ileum in 112 experiments and of the jejunum in 48 experiments. Use of a randomized order of four perfusions on 1 day of study and use of up to 4 consecutive days of study in a subject allowed important comparisons of data from the same individuals. That there is active ileal absorption of chenodeoxycholic, glycochenodeoxycholic, and taurocholic acids in man was supported by the finding of saturation kinetics and of competition for absorption among conjugated bile acids. Values for apparent kinetic constants (apparent maximal transport velocity [(*)V(max)] and apparent Michaelis constant) in man are similar to those in other species. The ileum absorbed chenodeoxycholic acid more rapidly than its glycine conjugate, due mainly to a ninefold greater permeability for the free acid. Taurocholate had the highest (*)V(max) and was absorbed more rapidly than glycochenodeoxycholate. Passive permeability of the jejunum to bile acids was twice that of the ileum, and the permeabilities to free and glycine-conjugated chenodeoxycholate were in the same ratio as in the ileum (9: 1). Jejunal permeability to chenodeoxycholic acid was three times that to cholic acid. Variation of intraluminal pH by up to 1.4 units did not influence jejunal uptake of free bile acids. These results, which are comparable with those from animal experiments, provide a basis for estimation of intestinal reabsorption of bile acids in intact man.     

Biliary lipids, bile acids, and gallbladder function in the human female. Effects of pregnancy and the ovulatory cycle.

To study the events that might lead to an increased risk of cholesterol gallstones, we examined biliary lipid composition and secretion and bile acid composition and kinetics at different stages of pregnancy or ovulation in young, nonobese, healthy women. Lipid composition and bile acid distribution were determined in duodenal fluid obtained in the fasting state and after stimulation of the gallbladder. Biliary lipid secretion was measured by the marker-perfusion technique. Bile acid kinetics were determined with cholic and chenodeoxycholic acids labeled with carbon13, by measuring the relative abundance of 13C in duodenal bile acids for 4--5 d. In a subset of patients we measured gallbladder storage and emptying during the kinetic study. The phase of the ovulatory cycle had no effects, but there were significant changes during pregnancy. The lithogenic or cholesterol saturation index of fasting hepatic and gallbladder bile increased during the second and third trimesters. The mean secretion rate of biliary lipids was not altered, but in the last two-thirds of pregnancy, cholesterol secretion increased in relation to bile acid and phospholipid secretion. There was a progressive decrease in the percentage of chenodeoxycholic acid and a similar increase in the percentage of cholic acid. The pool size of each major bile acid increased in the first trimester. Chenodeoxycholic acid and deoxycholic acid pools, but not cholic acid pools, subsequently decreased. The fractional turnover rate of both primary bile acids was slower during pregnancy. The synthesis rate of chenodeoxycholic but not cholic acid decreased in a linear manner during the first 20 wk of pregnancy. The rate of enterohepatic cycling of the bile acid pool was reduced throughout pregnancy. The volume of the fasting gallbladder and the residual volume after a physiologically stimulated contraction were directly correlated with bile acid pool size. The residual volume was also directly related to total bile acid synthesis.     

Presence of bile acid metabolites in serum, urine, and faeces in cirrhosis.

Studies have been made of the presence of bile acid metabolites in ten patients with liver cirrhosis as a consequence of alcohol abuse. Eight of the patients were categorized as Child group A, indicating only mild impairment of liver function, whereas the remaining two patients comprised Child group C. A complex mixture of bile acids was isolated from serum, urine, and faeces, and 26 bile acids were identified by gas-liquid chromatography coupled to mass spectrometry. Identification was made of the primary bile acids, cholic (C) and chenodeoxycholic (CDC) acid, and metabolites of these bile acids converted through 7-dehydroxylation, keto-formation, 6-hydroxylation, 1 beta-hydroxylation, allo-formation or nor-formation. All of the bile acids have previously been described either in healthy humans or patients with hepatobiliary disease. With the exception of C, CDC, and deoxycholic acid, all of the bile acids were present only infrequently, and none of the bile acids was pathognomonic for liver cirrhosis. The proportion of metabolites of the primary bile acids C and CDC was similar to that previously reported in healthy humans, the lowest proportion being recorded in the Child group C patients. Repeated determinations of the metabolite pattern in two patients showed large variations, indicating that the bile acid metabolism varies from time to time. We conclude that in mild cirrhosis, no significant alterations in microbial or hepatic transformation of bile acids seem to occur.     

Effects of rifampin on biliary lipids in humans.

Because the action of rifampin induces hepatic microsomal enzymes, a study was carried out in four patients to determine whether this drug alters the composition of biliary lipids. Several different measurements were made while patients were both on and off rifampin therapy for various infective processes. These measurements included multiple determinations of lipid composition of gallbladder bile, the relative proportions f individual bile acids, and kinetics of cholic acid and chenodeoxycholic acid. In all four patients, the saturation of gallbladder bile increased during rifampin treatment, and the bile consistently became supersaturated. The relative portions of chenodeoxycholic acid and cholic acid were essentially unchanged by treatment, but total synthesis of bile acids increased in three tested patients with rifampin therapy. These results indicate that rifampin increases saturation of bile with cholesterol, but this increase is not due to a reduction in bile acid production.     

Bile acid therapies applied to patients suffering from cerebrotendinous xanthomatosis

Patients suffering from cerebrotendinous xanthomatosis, an inborn error of metabolism in bile acid synthesis, were given oral treatment with chenodeoxycholic acid, ursodeoxycholic acid, cholic acid and taurocholic acid. The effectiveness of the different therapies was evaluated by measuring the urinary excretion of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,23,25-pentol, which should decrease, when the administered bile acid is able to suppress endogenous bile acid synthesis. From the results it is concluded that chenodeoxycholic acid and cholic acid activate the bile acid negative feedback mechanism, contrary to ursodeoxycholic acid and taurine conjugated cholic acid. Either cholic acid or chenodeoxycholic acid are the therapies of choice for the treatment of cerebrotendinous xanthomatosis. For various reasons the use of cholic acid is especially recommended.     

Regulation of the fasting enterohepatic circulation of bile acids by the migrating myoelectric complex in dogs.

The purpose of this study was to correlate the fasting enterohepatic circulation (EHC) of bile acids with the migrating myoelectric complex. Four dogs were surgically provided with a functional cholecystectomy, a duodenal cannula for direct vision cannulation of the common bile duct, and 12 bipolar electrodes implanted from stomach to terminal ileum. Bile was collected in equal-volume, timed aliquots over 6 to 10 h. Aliquots were sampled and either returned to the duodenum for study of the intact EHC, or collected and retained in order to study the time course of the bile acid pool washout. In the washout experiments boluses of radiolabeled taurocholic acid were instilled into the duodenum before and after duodenal phase III of the migrating motor or myoelectric complex (MMC). In another group of experiments the bile acid pool was washed out and during a continuous duodenal infusion of taurocholic acid bile was collected to study the pattern of hepatic secretion. Results: (a) In all experiments, a single broad peak of bile flow and bile acid secretion occurred at 35-55% of the MMC migration time. At this time the MMC had migrated to a point 70-85% of the distance along the small intestine. (b) During bile acid pool washout the peak of bile flow and bile acid secretion occurred with the distal migration of the first MMC and then bile flow and bile acid secretion rates decreased to a minimum and stabilized. (c) In bile acid pool washout experiments the radiolabeled bile acids instilled into the duodenum prior to duodenal phase III were secreted and peaked with peak endogenous bile acid secretion. The secretion of radiolabeled bile acids instilled into the duodenum after duodenal phase III was delayed until the subsequent cycle of the MMC. 88% of the bile acid pool collected over 6 h was secreted during the distal migration of the first MMC (2.4 +/- 0.4 h). (d) After bile acid pool washout and during continuous duodenal infusion of taurocholic acid, hepatic bile flow and bile acid secretion continued to fluctuate with the same pattern observed with the EHC intact. Conclusions: (a) In the fasting state, the transport of intestinal bile acids to the liver is pulsatile rather than continuous and is determined by the MMC. Maximum hepatic secretion occurs when phase III of the MMC propels the intraluminal bile acid pool to its site of absorption in the distal small bowel. (b) The "housekeeping" action of the MMC is very efficient and clears 88% of the 6-h washout bile acid pool in one pass.     

Hepatic disposition of the acyl glucuronide 1-O-gemfibrozil-beta-D-glucuronide: effects of clofibric acid, acetaminophen, and acetaminophen glucuronide

Glucuronidation of carboxylic acid compounds results in the formation of electrophilic acyl glucuronides. Because of their polarity, carrier-mediated hepatic transport systems play an important role in determining both intra- and extrahepatic exposure to these reactive conjugates. We have previously shown that the hepatic membrane transport of 1-O-gemfibrozil-beta-D-glucuronide (GG) is carrier-mediated and inhibited by the organic anion dibromosulfophthalein. In this study, we examined the influence of 200 microM acetaminophen, acetaminophen glucuronide, and clofibric acid on the disposition of GG (3 microM) in the recirculating isolated perfused rat liver preparation. GG was taken up by the liver, excreted into bile, and hydrolyzed within the liver to gemfibrozil, which appeared in perfusate but not in bile. Mean +/- S. D. hepatic clearance, apparent intrinsic clearance, hepatic extraction ratio, and biliary excretion half-life of GG were 10.4 +/- 1.4 ml/min, 94.1 +/- 17.9 ml/min, 0.346 +/- 0.046, and 30.9 +/- 4.9 min, respectively, and approximately 73% of GG was excreted into bile. At the termination of the experiment (t = 90 min), the ratio of GG concentrations in perfusate, liver, and bile was 1:35:3136. Acetaminophen and acetaminophen glucuronide had no effect on the hepatic disposition of GG, suggesting relatively low affinities of acetaminophen conjugates for hepatic transport systems or the involvement of multiple transport systems for glucuronide conjugates. In contrast, clofibric acid increased the hepatic clearance, extraction ratio, and apparent intrinsic clearance of GG (P <.05) while decreasing its biliary excretion half-life (P <.05), suggesting an interaction between GG and hepatically generated clofibric acid glucuronide at the level of hepatic transport. However, the transporter protein(s) involved remains to be identified.