Manganese-bilirubin effect on cholesterol accumulation in rat bile canalicular membranes.

Manganese-bilirubin (Mn-BR)-induced cholestasis in rats is associated with altered lipid composition of various hepatic subcellular fractions. Increased bile canalicular (BCM) cholesterol content in Mn-BR cholestasis and the intracellular source of the accumulating cholesterol were investigated. To label the total hepatic cholesterol pool, male Sprague-Dawley rats were given ip 3H-cholesterol, followed 18 h later by 2-14C-mevalonic acid (a precursor of cholesterol synthesis). To induce cholestasis, manganese (Mn, 4.5 mg/kg) and bilirubin (BR, 25 mg/kg) were injected iv; animals were killed 30 min after BR injection; canalicular and sinusoidal membranes, microsomes, mitochondria, and cytosol were isolated. Total cholesterol content of each fraction was determined by spectrophotometric techniques as well as radiolabeled techniques. In Mn-BR cholestasis, the total cholesterol concentrations of BCM and cytosol were significantly increased. Also, the contribution of 14C-labeled cholesterol (newly synthesized cholesterol) was enhanced in all isolated cellular fractions. The results are consistent with the hypothesis that accumulation of newly synthesized cholesterol in BCM is involved in Mn-BR cholestasis. An enhanced rate of synthesis of cholesterol, however, does not appear to be the causal event, as the activity of HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis), assessed in vitro, was decreased following Mn-BR treatment. Treatment with the Mn-BR combination may affect other aspects of intracellular cholesterol dynamics.

Ketone potentiation of intrahepatic cholestasis: effect of two aliphatic isomers.

Occupational exposure to methyl isobutyl ketone (MiBK) or methyl n-butyl ketone (MnBK) normally occurs by inhalation. The present study reports that exposure to both ketones can potentiate cholestasis experimentally induced by taurolithocholic acid (TLC, 30 mumol/kg) or by a combination of manganese (Mn, 4.5 mg/kg) and bilirubin (BR, 25 mg/kg). Male Sprague-Dawley rats were exposed for 3 d, 4 h/d, to MiBK or MnBK vapors using 0.5, 1, 1.5, or 2 times the minimal effective concentration (MEC). The estimated MiBK or MnBK MEC for potentiating TLC- or Mn-BR-induced cholestasis were 400 and 150 ppm, respectively. Eighteen hours after ketone exposure, rats were injected i.v. with TLC or Mn-BR. Bile flow was measured from 15 to 150 min after the cholestatic regimen. Rats exposed to MiBK or MnBK exhibited an enhanced diminution in bile flow compared to controls that was dose-dependent with the inhaled ketone dose. The dose-effect characteristics of the potentiation phenomenon were established. Results indicate that MiBK or MnBK inhalation potentiated both TLC and Mn-BR cholestasis in a dose-related fashion. Quantitative differences, however, exist between both ketones with respect to their ability to potentiate both models. Comparison between the two isomers was established, and MnBK was found to be more potent than MiBK.

Tissue concentrations of methyl isobutyl ketone, methyl n-butyl ketone and their metabolites after oral or inhalation exposure.

Quantitative relationships between plasma, liver and lung methyl isobutyl ketone (MiBK) and methyl n-butyl ketone (MnBK) concentrations after oral or inhalation exposure were established. Their respective metabolites (4-methyl-2-pentanol, 4-hydroxy-methyl isobutyl ketone, 2-hexanol, and 2,5-hexanedione) were also quantified. Male Sprague-Dawley rats were exposed for 3 days to MiBK or MnBK vapors (4 h/day) or treated orally for 3 days with a MiBK- or MnBK-corn oil solution. Both ketones and their respective metabolites in plasma or tissue concentrations were determined by gas chromatography. MiBK and MnBK plasma and tissue concentrations increased in a dose-related manner with the administered dose irrespective of the route of administration. Metabolite concentrations, however, were influenced by the route of administration.

Plasma concentrations in methyl isobutyl ketone-potentiated experimental cholestasis after inhalation or oral administration.

In studies of methyl isobutyl ketone (MiBK)-potentiated cholestasis induced by taurolithocholic acid (TLC) or manganese-bilirubin (Mn-BR) combinations, MiBK is usually given by gavage whereas industrial exposure to MiBK normally occurs by inhalation. The present study was conducted to verify if the route of administration could influence the potentiation. Male Sprague-Dawley rats were treated with MiBK for 3 days orally or by inhalation (4 hr/day). The minimal effective doses (MED) for potentiating both models of intrahepatic induced cholestasis were estimated to be 3 mmol/kg or 400 ppm for the oral or inhalation route, respectively. Groups of rats were treated with 0.5, 1, or 2 times the MED. Half of each group was sacrificed after the last MiBK administration to determine plasma concentrations of MiBK and its metabolites by gas-liquid chromatography. The other half was challenged 18 hr later with TLC (30 mumol/kg) or a combination of manganese (4.5 mg/kg) and bilirubin (15 mg/kg). Bile flow was measured from 15 to 135 min after the cholestatic challenge. Rats exposed to MiBK orally or by inhalation exhibited an enhanced diminution in bile flow that was dose-dependent. With dosages of 3 mmol/kg po or 400 ppm by inhalation or more, diminution in bile flow was significantly different from control values. Comparisons between maximal bile flow decrease and MiBK plasma concentration showed that the severity of the hepatotoxic response was dependent on the plasma MiBK concentration, irrespective of the route of administration.