HEPATIC TRANSPORT OF BILIRUBIN

Similarities in maximal biliary bilirubin output in the normal rat after administration of unconjugated bilirubin or bilirubin diglucuronide

The rate-limiting step in the overall plasma-to-bile transport of a saturating load of bilirubin is still a matter of controversy. We reassessed the apparent maximal biliary bilirubin excretion following i.v. infusion of unconjugated bilirubin and--for the first time--of highly purified bilirubin diglucuronide in the rat. The bilirubin diglucuronide preparation could be kept in a stable form at -20 degrees C for at least 2 months after addition of 3 mM sodium ascorbate. The biliary bilirubin excretion rates in animals with and without bile depletion in order to induce different flow rates were comparable after infusion of unconjugated bilirubin and of bilirubin diglucuronide. No significant hydrolysis of bilirubin diglucuronide seemed to occur during the hepatic transport of the pigment. Injection of bilirubin diglucuronide into rats which were already being infused with saturating doses of unconjugated bilirubin did not result in increased biliary bilirubin excretion. In contrast, a reversible inhibition of bilirubin output and bile acid-dependent bile flow was observed. If unconjugated and diglucuronidated bilirubin follow the same intracellular routes, the present results would suggest that conjugation did not restrict maximal biliary excretion. However, if exogenously administered diglucuronide utilizes a separate pathway, as was recently proposed, the biliary secretion of this exogenous conjugate might be restricted, presumably due to a toxic effect of the high local concentration of diglucuronide. The pathways utilized by the unconjugated pigment, on the other hand, could be primarily determined by the conjugating capacity.

Effect of increased bile flow and bile acid secretion on the hepatic elimination of sodium valproate in the cat

The effect of increased bile flow and hepatic bile acid flux on the systemic clearance and hepatic elimination of intravenously administered sodium valproate was studied in the bile fistula cat. Taurochenodeoxycholic acid (TCDC), tauro-3 alpha, 7 beta-dihydroxy-12-keto-5 beta-cholanoic acid (T12K), SC-2644, and secretin were infused intravenously to vary bile flow and biliary bile acid secretion. Control animals were infused with 0.15 mol/l NaCl. Less than 1% of the drug administered to controls appeared as unchanged valproate in the bile over 6 h. Although SC-2644, T12K, and secretin significantly increased biliary excretion of valproate, it did not exceed 2% of the intravenous dose. Biliary clearance was directly related to the rate of bile flow, but not to the bile acid flux. By contrast, 18-19% of the dose appeared in bile as metabolite in controls, and none of the choleretic agents significantly increased this percentage. As a result, systemic clearance of valproate was unaltered. We conclude that the movement of unchanged valproate into bile is consistent with a process of simple diffusion. The fact that choleretic agents do not increase metabolite excretion into bile suggests that metabolite formation may be the rate-limiting step in the hepatic elimination of valproate, rather than transport and excretion of metabolites into bile.

Intrahepatic cholestasis and hyperbilirubinemia in ethynyl estradiol and chlorpromazine-treated rats

Intrahepatic cholestasis associated with hyperbilirubinemia was induced by the simultaneous administration of ethynyl estradiol (EE) and chlorpromazine hydrochloride (CPZ) for 7 days to female Sprague-Dawley rats. Increases in direct serum bilirubin levels and alkaline phosphatase activities were observed concomitantly with diminished bile flow and bile acid excretion. However, the bilirubin output in the bile remained unchanged. [14C]Erythritol clearance decreased in parallel with the diminished bile flow, while [14C]sucrose biliary clearance increased, suggesting that the decrease in bile flow was of canalicular origin and also due to increased permeability in the biliary tract. There was a prominent decrease in the bile acid-independent flow, and the bile acid-dependent flow also decreased concomitantly with the diminished bile acid excretion. Slight increases in cytochrome P-450 and anilin hydroxylase activities in liver microsomes were observed, and bilirubin UDP-glucuronyl transferase activity increased significantly. Indirect bilirubin clearances determined by a bilirubin load test were markedly reduced in the icteric rats. The bilirubin load test also suggested that bilirubin flowed back into the circulating blood and that gluculonidation of bilirubin mono-gluculonide (BMG) to bilirubin diglucuronide (BDG) was impaired. Light microscopic examinations of the liver revealed marked proliferation of the bile ductules and numerous vacuoles in the hepatocytes. Dilatation of the bile canaliculi with diminished microvilli was also detected by scanning electron microscopy.

The micellar sink: a quantitative assessment of the association of organic anions with mixed micelles and other macromolecular aggregates in rat bile

Although the importance of mixed micelles in the solubilization and biliary excretion of lipids is established, little is known about a possible role of mixed micelles in the excretion of other biliary solutes. Ultrafiltration and ultracentrifugation techniques were used to investigate the interaction between substances that are excreted in bile and biliary mixed micelles. Substances (urea, erythritol, sucrose) excreted in bile at concentrations equal to, or less than, that in plasma did not show an association with mixed micelles, whereas substances (indocyanine green, iopanoic acid, rose bengal, unconjugated and conjugated sulfobromophthalein, and conjugated bilirubin) excreted in bile at high concentration relative to plasma did. The percentage of these latter substances in bile associated with micelles varied from 26 to 93% and was relatively independent of concentration. In addition to their association with mixed micelles, these test solutes formed self-aggregates that were stabilized primarily by ionic bonds, and only a small percentage (range = 0-5%) of these solutes were present in bile in the form of monomer or complexes small enough to pass a 5,000-mol wt membrane.These findings offer a possible explanation for the increase in sulfobromophthalein, bilirubin, and indocyanine green maximal biliary excretory rate produced by bile salt infusion, and suggest that the concentrative transport into bile of endogenous compounds and xenobiotics may result from their incorporation into mixed micelles and other macromolecular complexes.

Hepatic organic anion uptake in the rat

The hepatic uptake of bilirubin (BR), indocyanine green (ICG), and sulfobromophthalein (BSP) was studied in 350 anesthetized Sprague-Dawley rats by determining the initial plasma disappearance rate (V) of various doses of unlabeled ICG, or of tracer quantities of [3H]BR or [35S]BSP injected into the jugular vein simultaneously with varying amounts of unlabeled BR or BSP. Similar studies were also performed involving the simultaneous injection of potential inhibitors of hepatic uptake. The results indicate that: (a) hepatic uptake determined by direct tissue measurement could be accurately estimated from the plasma disappearance data; (b) saturation of hepatic uptake with increasing dose was readily demonstrated for each of these three organic anions, and in each instance a plot of V versus dose took the form of a rectangular hyperbola analyzable in terms of Michaelis-Menten kinetics; (c) for BR, the saturable uptake process showed a Vmax more than 100 times the normal net transfer rate from plasma to bile; (d) hepatic uptake of BR, BSP, and ICG showed relatively selective, mutually competitive inhibition; glycoholic acid did not inhibit hepatic uptake of any of these substances; and (e) "counter-transport" could be demonstrated for each of the three test substances. These data are compatible with the existence of a carrier-mediated transport process for hepatic uptake of each of these three organic anions and clarify the relationship of hepatic BR uptake to its overall transport from plasma to bile.

Biliary excretion of xenobiotics

The biliary route is very important for the elimination of some foreign compounds from the body. For many of these compounds, an increase in the rate at which they are excreted into the bile will decrease their toxicity and vice versa. A number of factors which are known to alter the biliary excretion of xenobiotics, as well as the current concepts of the physiological mechanisms responsible for the excretion of foreign compounds, have been enumerated. However, much remains still to be understood; essentially nothing is known at the subcellular level about the biliary excretion of foreign compounds. It has recently been concluded that our knowledge of the biliary excretion of compounds is about 40 years behind that of the renal excretion mechanism.

Breast milk jaundice. Effect of inhibitory breast milk and 3 alpha, 20 abeta-pregnanediol on glucuronyl transferase

The effect of breast milk and 3α, 20β-pregnanediol on the conjugation of bilirubin and o-aminophenol by rat liver was compared. Breast milk inhibited glucuronyl transferase, whereas 3α, 20β-pregnanediol inhibited the secretion of bilirubin glucuronide from rat liver slices. In the inhibitory breast milk examined the steroid was probably not the sole inhibitory factor.

Two hepatic cytoplasmic protein fractions, Y and Z, and their possible role in the hepatic uptake of bilirubin, sulfobromophthalein, and other anions

Two hepatic cytoplasmic protein fractions, designated Y and Z, which bind sulfobromophthalein (BSP), bilirubin, and other organic anions, have been separated by G75 Sephadex gel filtration. The physiologic role of these protein fractions has been investigated. They are present in the 110,000 g supernatant fraction from the livers of all the species tested (rats, mice, guinea pigs, Rhesus monkeys, sheep, and man). Tissues which do not preferentially extract BSP or bilirubin from plasma do not contain these fractions, with the exception of small intestinal mucosa which contains Z. Anion binding by Y and Z fractions is not due to contamination with albumin. These fractions are responsible for the cytoplasmic localization of bilirubin in Gunn rats, and the fractions bind bilirubin, BSP, or indocyanine green (ICG), whether given in vivo or added in vitro to liver supernate from normal rats. Flavaspidic acid-N-methylglucaminate, bunamiodyl, and iodipamide, drugs known to interfere with the hepatic uptake mechanism, compete with bilirubin and BSP for binding to Z. These proteins appear to be important in the transfer of organic anions from plasma into the liver and provide a tool for the investigation of hepatic uptake mechanisms.