A compartmental model for hepatic transport of taurocholic acid in isolated perfused rat liver

In order to characterize the transport of bile acids through the liver and to study the influence of drugs on these processes, a kinetic model for hepatobiliary transport of taurocholic acid (TC) using the isolated perfused liver was developed. After the system was brought to a steady state by infusing TC at a constant rate, a tracer dose of 14C-TC was injected into the medium. The medium disappearance of 14C-TC followed a first-order kinetic with a single rate constant. The plot of the biliary secretion rate of radioactivity versus time revealed a curve composed of at least three exponential components. From the described results and the present knowledge of hepatobiliary transport of bile acids we proposed a three compartment model, composed of a perfusion medium compartment and two liver compartments. Parameters calculated from the model constants agreed well with model-independent estimations. The influence of bromosulfophthalein (BSP) on the kinetic parameters was studied to compare the result with the known effect of BSP on hepatic transport of taurocholic acid. BSP decreased the constant describing the fractional transfer of taurocholic acid from medium into the liver, which is in agreement with the inhibition of hepatic uptake of taurocholic acid by BSP. Thus a three compartment model may adequately define the hepatobiliary transport of taurocholic acid in the isolated perfused rat liver.

The lack of active bile acid transport in AS-30 D ascites hepatoma cells

The uptake of cholic acid as well as taurocholic acid into AS-30 D ascites hepatoma cells showed linearity with respect to incubation concentrations. It has been suggested that these processes can be described as simple diffusion. In further experiments it could be shown that ascites hepatoma cells were unable to conjugate cholic acid. These results may have significance in the phalloidin action on hepatocytes.

Interaction of fusidates with bile acid uptake by isolated rat hepatocytes

The interaction of fisidic acid and two of its conjugates with carrier-mediated uptake of bile acids was investigated in isolated rat hepatocytes. All three fusidates inhibited the uptake of both cholate and taurocholate competitively suggesting a direct interaction of fusidates with bile acid carrier. The inhibition constants for all three fusidates for the inhibition of cholate uptake were significantly different from the respective inhibition constants for the inhibition of taurocholate uptake. This would indicate that both cholate and taurocholate are transported by more than one carrier into hepatocytes. The results may also indicate that taurine conjugated bile acids may be transported preferentially by one transport system while unconjugated bile acids may be preferentially transported by another transport system.

The interaction of rifamycin SV with hepatic transport of taurocholic acid in the isolated perfused rat liver

The effect of rifamycin SV on hepatic transport of taurocholic acid was investigated using isolated perfused rat liver technique. In all experiments, the perfused liver was maintained at taurocholic acid steady state by infusing constant amount of taurocholic acid. Infusion of rifamycin SV at various rates decreased biliary secretion of bile acids in a dose-dependent manner. Replacement of rifamycin SV by perfusion medium reversed this effect. To determine the site of action of rifamycin SV, kinetic experiments with 14C-taurocholic acid were undertaken. Rifamycin SV elevated the half-life of the medium disappearance of 14C-taurocholic acid. Furthermore, the antibiotic delayed the biliary appearance of 14C-taurocholic acid. The analysis of the results gave indications that the antibiotic interferred with hepatic uptake as well as biliary secretion of taurocholic acid.

Inhibition of hepatic uptake of bile acids by rifamycins

The effect of rifamycin SV and rifampicin on hepatic acid uptake was studied using isolated rat hepatocytes in presence and in absence of albumin. The drugs inhibited cholate uptake more than taurocholate uptake and the inhibition was of non-competitive type. In presence of 3% albumin the inhibitory effect of the drugs was more for cholate and less for taurocholate uptake than in absence of albumin. Neither the binding of bile acids nor that of the drugs to albumin was altered by one another. Thus the effect in presence of albumin cannot be explained by the binding of the drugs and bile acids to albumin alone. It is suggested that albumin interacts with hepatic bile acid uptake process and this interaction with cholate uptake is different from that with taurocholate uptake. This additional and different effect of albumin may explain the effect of the drugs in presence of albumin. The results may be of clinical significance in rifamycins treatments.

Effect of Na on bile acid uptake by isolated rat hepatocytes. Evidence for a heterogeneous system

The effect of Na on cholate and taurocholate uptake was studied in isolated rat hepatocytes. Uptake of both bile acids can be divided into three components: a) a non-saturable component independent of Na, b) a Na-independent saturable and c) a Na-dependent saturable component. The two saturable components were inhibited by KCN, oligomycin and various other bile acids, which suggests carrier-mediated, energy-requiring processes. The relationship between Na concentration and the Na-dependent uptake rate was sigmoidal. The maximal uptake rate, rather than the Km, was affected by Na. These results suggest that more than one Na (probably two) are cosubstrates in the transport of one bile acid molecule and Na exerts a stimulatory effect on the translocation of the bile acid-carrier complex across the membranes. The Na-dependent uptake was inhibited by ouabain, suggesting that this uptake process is linked to the activity of membrane bound (Na-K)ATPase. Inhibition and competition studies suggest that cholate and taurocholate share a common Na-independent transport system while there are at least two Na-dependent transport systems for taurocholate, one of which is shared by cholate.

The age dependence of bile acid metabolism in rats

The age dependence of bile acid metabolism in rats was investigated and the following results were obtained. 1) Synthesis rate, pool size and total secretion decreased in old rats (24 month) significantly compared to young rats (6 weeks). 2) The quotient of taurine to glycine conjugates is is significant lower in old rats compared to young rats. 3) The lithogenic index remains unchanged in all age groups. 4) Old rats show a significantly higher Km for cholic acid uptake, whereas the diffusion coefficient is decreased. The possible influence of humoral factors on age dependent regulations of bile metabolism is discussed.

Characterization of methylumbilliferone (mendiaxon r)-induced choleresis in the isolated perfused rat liver

The influence of the choleretic drug methylumbilliferone on bile formation in the isolated perfused rat liver is characterized. The compound induces rapidly an elevation of bile flow, bile acid secretion and soium excretion. The increased production of bile is of canalicular origin. The choleretic effect was defined as "bile acid like" choleresis due to excretion of the drug into the bile. It is discussed that the excretion of methylumbilliferone can influence the transport of bile in form of a positive cooperation on transport mechanism.

Cholic acid binding to isolated rat liver plasma membranes

Cholic acid binding to isolated rat liver plasma membranes was studied using a centrifugal filtration technique which allowed independent determination of free and membrane-bound cholic acid. Binding of cholic acid was very rapid and reversible. Scatchard analysis revealed at least three binding sites with high, medium and low affinity. The high affinity binding a) displayed saturability and isotope replacement, b) was not present in rat liver mitochondria and red blood cell ghosts and c) was temperature dependent. This binding has a very low capacity with a dissociation constant in the physiological range of plasma cholic acid concentration and has an affinity for other common bile acids. Cholic acid binding to the high affinity binding site was not inhibited by estrone, beta-estradiol or cholesterol. These results would suggest that the high affinity binding site represents a specific binding site for cholic acid and may also be specific for other common bile acids. This binding was not dependent on Na and was inhibited by bromosulfophthalein. Cholic acid binding to the high affinity site has some features in common with cholic acid uptake by isolated rat hepatocytes, and this would suggest that the high affinity binding site could be the postulated carrier for hepatic uptake of cholic acid.

Cholic acid uptake and isolated rat hepatocytes

Cholic acid uptake was studied in isolated rat hepatocytes using a centrifugal filtration technique to allow rapid sampling. Hepatocytes were found to adsorb as well as to transport cholic acid. The adsorption was characterized by a capacity of 24 nmol X mg cell protein-1 and an association constant of 0.59 X 103 M-1. Cholic acid uptake was linear with respect to concentration at or below 10 degree C, suggesting a unsaturable uptake process which was considered to represent simple diffusion and is quantitated by a diffusion coefficient of 1.76 pmol cholic acid X min-1 X mg protein-1 X muM-1. Above 10 degrees C the uptake curve was biphasic. After subtracting the unsaturable component from uptake rates at higher temperatures, a curve showing saturable kinetics resulted. The apparent Km and V values at 37 degrees C were calculated to be 31muM and 0.8 nmol X min-1 X mg protein-1 respectively. This saturable uptake process was temperature-dependent with an activation energy of 13 kcal X mol-1 (5.44 X 104 J X mol-1) and was inhibited by oligomycin and KCN. Countertransport was demonstrated with cholic, taurocholic and chenodeoxycholic acids. The results suggest that cholic acid is transported by an energy-dependent carrier-mediated process in addition to simple diffusion by hepatocytes, and that the postulated carrier has affinity for other bile acids.

A compartmental model for bilirubin kinetics in isolated perfused rat liver

Bilirubin kinetics were studied in an isolated, perfused rat liver system using unconjugated (14C) bilirubin (UC(14C)B) and delta-amino (4-14 C) levulinic acid (A(14 C)LA) to derive a suitable compartmental model. Plasma disappearance of UC(14C)B, plasma appearance of conjugated (14c) bilirubin (C(14C)B) and biliary excretion of C(14C)B were followed for 90-120 min following injection of UC(14C)B. Hepatic content of labeled bilirubin 12 min after the injection of UC(14C)B was determined directly in five separate perfusion experiments. UCB was found to reflux back to plasma from liver in two experiments using A(14C)LA. Bilirubin binding to red blood cells (6-8% of the perfusate level) and the components of the perfusion apparatus (4-6% of perfusate level) was estimated by performing a control experiment without the liver. A six compartment model was necessary and adequate to explain the experimental data and current knowledge of bilirubin metabolism: (1) UCB bound to red blood cells and the perfusion apparatus, (2) plasma UCB, (3) liver UCB, (4) liver CB, (5) plasma CB, and (6) bile CB. The proposed model could serve as a reference point for studies of bilirubin kinetics in whole animals for normal and abnormal states.

Plasma bile acid elevation following CCI4 induced liver damage in dogs, sheep, calves and ponies

Plasma bile acid concentration was determined in normal dogs,sheep, calves and ponies for three days before and six days after liver damage, induced by carbon tetrachloride. In all species, a significant increase in plasma bile acid concentration was associated with a concomitant significant increase in plasma sorbitol dehydrogenase and transferase activity. Plasma bilirubin also significantly increased in all animals except the dogs. Results suggested that plasma bile acid levels could be used to test liver function in domestic animals.

Glucose utilization and recycling in ponies

Variables of glucose metabolism determined by the use of [U-14C]glucose were compared in fed and fasted ponies. Relative recycling of glucose carbon with respect to tritium in fed animals was negligible for 6-T and 3-T and 16% for 2-T studies; in fasted animals relative recycling was 12 and 14% for 6-T and 3-T studies, respectively. Minimal mass of total-body glucose decreased significantly in the fasted ponies. Based on relative recycling of carbon to tritium, a negligible fraction of plasma glucose was produced via the Cori cycle or from glycerol in fed ponies; recycled tricarbon units contributed 12% of glucose produciton in ponies fasted 72 h. In fed ponies, 16% of plasma glucose carbon was recycled via a futile cycle at the glucose 6-phosphate stage. Glucose utilization was best estimated with the use of [6-T]glucose (or 3-T).

Effect of bile acid on hepatic excretion and storage of bilirubin in ponies

Endogenous bilirubin uptake from plasma and biliary bilirubin excretion were determined in ponies with chronic biliary T-tube fistulas. Excreted bile was quantitatively recovered. Uptake was calculated from the plasma disappearance of 14C-labeled bilirubin. Biliary bilirubin excretion was determined directly in excreted bile. When bile acid excretion was low (during continuous drainage without bile acid replacment), bilirubin excretion was 37% less than uptake. Uptake and excretion were essentially identical when taurocholic acid was infused to replace bile acids. After depletion of the bile acid pool, replacement of bile acids (by taurocholic acid infusion) greatly increased both bilirubin excretion and its biliary concentration for approximately 1 hour. After this initial increase, bilirubin excretion was maintained at a rate approximately 30% greater than the preinfusion rate. Bile acid excretion was found to be essential for normal, endogenous bilirubin excretion.

Leucocidin from Pseudomonas aeruginosa and membrane functions

Leucocidin from Pseudomonas aeruginosa (strain 158) induced loss of potassium from isolated hepatocytes. The (Na+-K+) stimulated ATPase activity of isolated rat liver plasma membranes showed dose-dependent activation up to 56%. Electron-spin-resonance (ESR) measurements gave no indication of toxin-induced changes in membrane fluidity. On isolated guinea pig heart auricles the toxin produced an increase in frequency from 180/min to about 300/min, with arrhythmia and transitory flutter. On isolated nerve-diaphragm preparations the toxin caused a contracture and a decline in twitch tension, with a loss of potassium into the bathing solution. The action potential of the electrically stimulated N. ischiadicus of rat or frog was not affected when leucocidin was added to the bathing solution up to a concentration of 10 micrograms/ml.

Bile acid kinetics and bile secretion in the pony

Bile acid pool size and synthesis rate were determined by both isotope-dilution and washout methods in ponies with chronic external biliary fistulas. Bile acid pool size (10.9 mumol/kg) and synthesis rate (11.2 mumol/day per kg) estimated by the isotope-dilution method did not differ significantly from pool size (9.4 mumol/kg) and synthesis rate (9.5 mumol/day per kg) estimated by washout method. Bile acid-dependent and -independent fractions of bile flow, determined by a method that circumvents any inevitable correlation of flow to bile acid secretion due to common factors in both parameters, did not differ from those values obtained by linear regression of bile flow versus bile acid secretion. The choleretic effect of infused chenodeoxycholic acid was higher than that of both endogenous bile acid and infused taurocholic acid.

Bile secretion in ponies with biliary fistuals

Surgically placed bile duct cannulas allowed collection of secreted bile from nonanesthetized ponies. UNINTERRUPTED ENTEROPHEPATIC CIRCULATION WAS PERMITTED BETWEEN COLLECTIONS. Deleterious effects of cannulation were not observed. Average bile flow was 18.6 plus or minus 1.72 (standard error) mul/minute/kg, bile acid excretion was 0.179 plus or minus 0.0212 mumole/minute/kg, and bilirubin excretion averaged 1.22 plus or minus 0.136 mug/minute/kg.