Biliary excretion of copper, manganese, and horseradish peroxidase in Eisai hyperbilirubinemic mutant rats (EHBRs) with defective biliary excretion of glutathione

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

Manganese deposition in the globus pallidus in patients with biliary atresia

BACKGROUND

Chronic liver diseases may alter trace element contents in the brain. Among these trace elements, manganese is a ubiquitous transition metal excreted by the liver into the bile. Blood concentrations of manganese are elevated in patients with biliary atresia who have undergone hepatic portoenterostomy. The present study investigated the effects of liver transplantation on manganese deposition in the brain in such patients.

METHODS

The signal intensity of the globus pallidus was calculated as an index defined as the percentile ratio of signal intensity in the globus pallidus to the subcortical frontal white-matter in sagittal T1-weighted magnetic resonance imaging planes.

RESULTS

Brain magnetic resonance imaging revealed hyperintense signals in the globus pallidus due to manganese deposition in biliary atresia patients. Few neurologic symptoms related to manganese intoxication were observed. However, one 23-year-old female with biliary atresia had depressive symptoms and dyskinesia; she improved after oral administration of the dopamine precursor, L-DOPA. Manganese deposition disappeared in two patients after living-related reduced-size hepatic transplantation.

CONCLUSIONS

Manganese accumulates in the brain during cholestasis associated with biliary atresia and disappears after hepatic transplantation. Manganese deposition is likely to be subclinical and reversible but may be associated with some age-related neurologic symptoms.

Outputs of hepatic copper and cadmium stimulated by tetrathiomolybdate (TTM) injection in Long-Evans Cinnamon (LEC) rats pretreated with cadmium, and in Fischer rats pretreated with copper and cadmium

The Long-Evans Cinnamon (LEC) rat, an inbred mutant rat derived from the Long-Evans strain, is characterized by spontaneous hepatitis due to gross accumulation of hepatic Cu. The accumulation, accompanied by marked induction of metallothionein (MT), is believed to be due to the inherent lack of output of Cu into the bile duct and blood vessels. In this study, the acute effect of tetrathiomolybdate (TTM), a chelator for output of hepatic Cu and Cd in LEC rats treated with Cd, was investigated. Female LEC rats were injected subcutaneously with Cd (Cd; 1.0 mg/kg) to induce Cd, Cu-MT. Fischer rats were treated with Cd (Cd; 1.0 mg/kg) and Cu (Cu; 3.0 mg/kg). Forty-eight hours after the injections of metals, TTM (5 mg/kg bw) was injected intravenously under anesthesia. The TTM injection rapidly stimulated biliary excretions of Cu (at a microgram/ml level) and Cd (at a ng/ml level). Furthermore, Cu and Cd concentrations were increased in serum sampled 60 min after the TTM injection. The increase of biliary Cu excretion was not accompanied by increased biliary excretion of MT. The TTM injection caused the hepatic Cu concentrations to decrease from 306 +/- 2 to 262 +/- 12 and from 43 +/- 6 to 20 +/- 5 micrograms/g in LEC and Fischer rats, respectively. The hepatic Cd concentration was not decreased by TTM treatment. Hepatic MT and Cu, but not Cd, concentrations in the MT fraction were also reduced by TTM injection. Our results showed that TTM can rapidly remove Cu from MT to increase bile and blood Cu levels. The output of Cd stimulated by TTM injection may be related to MT reduction resulting from removal of MT-bound Cu. Our results indicate that to avoid the toxic effect of Cu, TTM injection is an effective initial treatment, although it remains to be established how metals, including Cu, are finally metabolized.

Biliary excretion of exogenous cadmium, and endogenous copper and zinc in the Eisai hyperbilirubinuric (EHB) rat with a near absence of biliary glutathione

Mutant Eisai hyperbilirubinuric (EHB) rats derived from an inbred strain of Sprague-Dawley (SD) rats are characterized by a near absence of biliary excretion of glutathione (GSH) due to inherently impaired ATP-driven organic anion transport. Cd (0.1 mg/kg bw from CdCl2) was injected intravenously into EHB rats and control SD rats. Output of biliary excretion of Cd was followed over 15-min intervals up to 60 min. Cd was excreted rapidly and reached the maximum level (73.2 ng/15 min) in the period from 15 to 30 min in SD rats. Its excretion in EHB rats, however was one-fortieth (only 1.8 ng/15 min) of that in SD rats. Biliary concentrations of two endogenous metals, Cu and Zn were also measured. The output of Cu in EHB rat bile (50 ng/15 min before Cd injection) was about one-fifth of that in SD rat bile (270 ng/15 min). The output was not influenced by the Cd injection in the two groups. There was a slight difference of Zn output between the two groups. The biliary excretion of GSH was 500 to 700 micrograms/15 min and only 1 to 2 micrograms/15 min in SD and EHB rats, respectively. Sixty min after Cd injection, the Cd concentrations in the serum, liver and kidney were slightly higher in EHB rats than in SD rats. There was no difference in the hepatic metallothionein (MT-I and-II) concentration between SD (34 micrograms/g liver) and EHB (33 micrograms/g liver) rats. The renal Cu concentration was about four times in the higher in the EHB rat than in the SD rat. These results suggest that reduced biliary excretion of Cd is mainly, but that of Cu is only partly, based in reduced canalicular transport of GSH due to lack of an ATP-driven organic anion transport system, not MT induction in EHB rats. It seems likely that biliary excretion of Cd is regulated mainly by the canalicular anion transport in rats.