An expanded model of bilirubin kinetics: effect of feeding, fasting, and phenobarbital in Gilbert's syndrome

Gilbert's syndrome

While Gilbert's syndrome is extremely common and benign, its pathogenesis may not be as straightforward as once believed. It has been used as a model to examine aberrations of virtually every step in bilirubin metabolism. The clinical hallmarks are of a hereditary, chronic, mild unconjugated hyperbilirubinaemia. Not infrequently subclinical haemolysis may coexist. Liver histology is normal although some minor ultrastructural abnormalities may be evident. The universal defect appears to be a reduction in hepatic bilirubin-GT activity. However, other associated abnormalities in bilirubin metabolism, which occur less consistently, suggest that this may not be the sole defect in all patients. The syndrome is almost certainly part of a spectrum which includes the Crigler-Najjar syndromes; molecular biology data suggests that there is an absence of one (or even more) GT isoenzymes in these disorders. Whether one or more genes is consistently culpable remains open to speculation. Despite the complicated pathogenesis of Gilbert's syndrome, management remains simply reassurance alone.

Evaluation of hepatobiliary disorders in the horse

This article addresses clinical problems that present in equine liver disease. It also discusses the variety of laboratory tests available to the clinician that can differentiate the type and degree of liver dysfunction. This is followed by a more specific discussion regarding unique features of equine bilirubin and bile acid metabolism.

Hepatic disposition and biliary excretion of bilirubin and bilirubin glucuronides in intact rats. Differential processing of pigments derived from intra- and extrahepatic sources

Mechanisms for transport of bilirubin and its conjugates in hepatocytes have not been defined. We investigated the hepatic processing of bilirubin glucuronides and their precursors, and characterized the disposition of bile pigments arising from intraversus extrahepatic sources. Tracer doses of purified radiolabeled biliverdin, bilirubin, bilirubin monoglucuronide (BMG) or diglucuronide (BDG) were administered intravenously to intact normal or jaundiced homozygous Gunn rats. Rapid sequential analysis of radiolabeled BMG and BDG in bile revealed comparable excretion patterns following biliverdin and bilirubin injection, with BDG as the major pigment. Biliary excretion of radiolabeled conjugates from injected BMG was more rapid, with BMG predominating. Excretion of injected BDG in normal rats and BMG or BDG in Gunn rats was virtually identical to that of unaltered BMG in normal rats. Model independent analysis by deconvolution provided objective comparison of the disposition of radiolabeled pigments from the different sources. These findings indicate that bilirubin glucuronides formed in the liver from endogenous (hepatic) and exogenous (extrahepatic) sources of bilirubin follow a similar excretory pathway. BMG formed endogenously is converted preferentially to BDG, whereas circulating BMG is excreted predominantly unchanged. Exogenous conjugated bilirubins are excreted more rapidly than those generated intrahepatically, by a transcellular pathway that is largely independent of the conjugation system.

Pharmacokinetics of antipyrine, acetaminophen and lidocaine in fed and fasted horses

Previous studies demonstrated that plasma clearance of organic anions such as bilirubin, bile acid, sulfobromophthalein (BSP) and indocyanine green (ICG), was reduced from 36% (bile acid) to 55% (ICG) in fasted (3 days) horses. It is believed that a general decline in carrier-mediated hepatic uptake may have accounted for those changes. However, fasting may also affect hepatic blood flow, thereby contributing to reduced clearance of these compounds. In order to test this hypothesis, plasma clearance of antipyrine, acetaminophen and lidocaine, drugs known to be cleared by the liver yet not suspected of undergoing carrier-mediated hepatic uptake, were investigated in nine healthy adult mares (three horses/drug group) before and following a 3-day fast. Results demonstrate that fasting decreased clearance of organic anions from previous studies more than clearance of drugs used in these studies. In addition, clearance of lidocaine, the drug with the highest plasma clearance and therefore the drug most likely to be affected by reduced hepatic blood flow, was affected least by fasting. Therefore, reductions in clearance of these compounds due to fasting must not be due entirely to reductions in hepatic blood flow, but must also involve reductions in intrinsic hepatic clearance.

Functional hepatocellular heterogeneity

Hepatocytes from the periportal (afferent) and perivenous (efferent) zones of the liver parenchyma differ in their enzyme content and subcellular structures. Therefore, different functions are proposed for the two zones. (a) Oxidative energy metabolism, beta-oxidation, amino acid catabolism, ureagenesis from amino acids, gluconeogenesis, bile acid, and bilirubin excretion and oxidation protection are preferentially located in the periportal zone. (b) Glycolysis, liponeogenesis, ureagenesis from ammonia, and biotransformation are predominantly situated in the perivenous zone. Heterogeneity in the synthesis of plasma proteins also appears to exist. The heterogeneous expression of the genome in hepatocytes is apparently caused by the periportal to perivenous gradient in oxygen and hormone concentrations, and by a different autonomic innervation of the parenchymal zones.

Differentiation of cholestatic jaundice in infants. Utility of diethyl-IDA

Time-activity curves of technetium-labeled diethyl-IDA were used to evaluate 31 patients in an effort to differentiate infants with cholestatic jaundice. Results can be obtained rapidly with minimal radiation exposure and no known secondary effects. The combination of diethyl-IDA time-activity curves and gamma glutamyl transferase activity was found to discriminate biliary atresia from neonatal hepatitis. The group with neonatal hepatitis was characterized by abnormal, yet lower, gamma glutamyl transferase activities and diethyl-IDA time-activity curves which peaked within 1 minute of injection, yet decayed in a manner similar to cardiac activity. The group with biliary atresia was characterized by elevated gamma glutamyl transferase activities, hepatic peaks at 8 minutes and markedly delayed decay. In that early operative intervention is associated with increased survival in biliary atresia, prompt differentiation should improve prognosis.

Bilirubin kinetics in intact rats and isolated perfused liver. Evidence for hepatic deconjugation of bilirubin glucuronides

Most previous compartmental models describing bilirubin transport and metabolism in the liver have been validated solely by analysis of the plasma disappearance of radiolabeled bilirubin in human subjects. We now have determined the transport kinetics of a bilirubin tracer pulse by analysis of plasma, liver, and bile radioactivity data from 30 intact rats. Plasma [3H]bilirubin disappearance was best described by the sum of three exponentials, and a six-compartment model, derived by simulation analysis, was necessary and adequate to describe all experimental data. Examination of the injected radiolabeled bilirubin by extraction with hexadecyltrimethylammonium bromide and thin-layer chromatography revealed that 6.6% (mean) of the original pigment had been degraded to labeled nonbilirubin derivatives during preparation of the tracer dose. This material exhibited a significantly longer half-life (mean 50.6 min) of the plasma terminal exponential than that of authentic radiobilirubin (20.6 min). In isolated perfused rat liver, the kinetics of [3H]bilirubin in perfusate and bile readily fitted the proposed model. Compatibility of the model with the data obtained, both in the isolated liver and in vivo, required that a fraction of bilirubin conjugated in the liver be deconjugated and returned to the plasma. Deconjugation of bilirubin glucuronides was evaluated directly by infusion of bilirubin monoglucuronides, containing 14C in the glucuronosyl group, into rats with an external bile fistula. Since metabolic degradation of hydrolyzed 14C-labeled glucuronic acid yields 14CO2, this was measured in expired air. Whereas 86% of the administered labeled pigment was recovered in bile, 7% of the label appeared in 14CO2. These findings directly validate a portion of the proposed kinetic model and suggest that hepatic deconjugation of a small fraction of bilirubin glucuronides is a physiological event. Deconjugation may also account, at least in part, for the presence of increased concentrations of unconjugated bilirubin in the plasma of patients with cholestasis.

Degradation of endogenous hepatic heme by pathways not yielding carbon monoxide. Studies in normal rat liver and in primary hepatocyte culture

The conversion of endogenous hepatic heme to bilirubin and CO is established. However, it is unknown whether this process is quantitative or whether heme may be degraded to other products as well. To study this question, we administered the heme precursor, delta-amino-[5-14C]levulinic acid to rats in vivo. In liver, [14C]heme was predominately associated with microsomal cytochromes, and its degradation was examined over a period of 12--14 h; concurrently, excretion of labeled carbon monoxide 14CO by the animal was measured. After correction for 14CO derived from the breakdown of renal [14C]heme, the rate of heme degradation calculated from the 14CO excreted was substantially less than the rate of disappearance of hepatic [14C]heme measured directly. The discrepancy between actual loss of labeled heme from the liver and generation of labeled CO was confirmed by direct study of endogenous [14C]heme degradation in primary hepatocyte culture, in which only 25% of the labeled heme disappearing during the incubation was converted to 14CO. By contrast, cultured cells converted exogenous [14C]heme nearly quantitatively to 14CO. We conclude that heme associated with microsomal cytochromes in normal rat liver is degraded substantially by non-CO forming processes.

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.

New insights into the classification and mechanisms of hereditary, chronic, non-haemolytic hyperbilirubinaemias

Gilbert's syndrome is typically associated with a deficiency in hepatic bilirubin UDP-glucuronosyltransferase activity (B-GTA). The overproduction of bilirubin that is often found in this condition could be a fortuitous coincidence that leads to the unmasking of the disease, which otherwise often remains latent. Some cases of chronic unconjugated hyperbilirubinaemia could, however, be related to a defect in hepatic uptake, as reflected by alterations in BSP kinetics. Severe deficiencies of hepatic B-GTA exist in all types of Crigler-Najjar disease. An increased proportion of bilirubin monoglucuronide is always found in bile when a B-GTA deficiency is present. This observation strongly suggests a common biochemical defect in Gilbert's syndrome and in Crigler-Najjar disease, and thus renders the suggestion that the latter condition may be separated into two groups somewhat inappropriate. There is, however, no doubt that further knowledge of the conjugating enzyme, or enzymes, is required: such information may lead to the characterisation of several types of enzymic defects. Whereas little is new as far as the Dubin-Johnson syndrome is concerned, Rotor's syndrome can no longer be considered to be a variant of the former. The transport defect which is involved in most cases of Rotor's syndrome, if not in all, is an impairment of hepatic storage, thus distinguishing it from the impairment of excretion which is involved in the Dubin-Johnson syndrome. The distinct patterns of urinary coproporphyrin excretion, which were recently reported in Dubin-Johnson and Rotor's syndromes, offer additional evidence for a clear differentiation between these two entities.

The mechanism of haem catabolism. Bilirubin formation in living rats by [18O]oxygen labelling

1. The pathway of haem breakdown in living rats was studied by using 18O in the oxygen that the animals consumed. By cannulation of the common bile duct and collection of bile, labelled bilirubin was isolated and its mass spectrum determined. One set of results was obtained for a rat to which haemoglobin had been intravenously administered and another set obtained for a rat that was not given exogenous haem. Isomerization of bilirubin IXalpha to the XIIIalpha and IIIalpha isomers did not occur to any significant extent. The 18O-labelling pattern obtained in the bilirubin was consistent with a Two-Molecule Mechanism, whereby the terminal lactam oxygen atoms of bilirubin are derived from different oxygen molecules. The consequences of this mechanism are discussed in terms of the possible intermediates of the catabolic pathway. 2. 18O-labelled bilirubin appeared in the bile in less than 10 min after exposure of the animals to labelled oxygen. This result suggests that all of the chemical transformations involving production of biliverdin, reduction to bilirubin and conjugation of the bilirubin are fast processes. 3. The quantitative recovery of label obtained in the experiments suggests that there is little or no exchange of newly synthesized bilirubin with existing bilirubin pools in the animal.

Bilirubin dynamics in the Gunn rat during phototherapy

Bilirubin dynamics were studied in homozygous Gunn rats under normal room lighting conditions and under conditions simulating phototherapy. A kinetic model was developed for the formation, distribution, and elimination of bilirubin. The decrease in plasma bilirubin concentration during illumination with low intensity [300 footcandles (fc)] and high intensity (1000-1100 fc) light was studied. The plasma bilirubin concentration in the rats decreased under phototherapy until a new steady-state concentration was reached, the decline being more rapid under high intensity light conditions. Gunn rats were also injected with a tracer dose of 14C-bilirubin following a period during which the rats were illuminated with low or high intensity light. The distribution and elimination of the labeled bilirubin were followed under continuous illumination. The chosen kinetic model, when adapted to the set of data under investigation, fit all of the data concerning bilirubin kinetics in Gunn rats under continuous illumination.