ALTERED CHOLESTEROL-SYNTHESIS AS A MECHANISM INVOLVED IN METHYL ISOBUTYL KETONE-POTENTIATED EXPERIMENTAL CHOLESTASIS

Mechanisms by which ketones potentiate manganese-bilirubin (Mn-BR)-ind uced cholestasis are unknown. The purpose of the present study was to investigate the effect of methyl isobutyl ketone (MiBK), a widely used ketonic solvent, at the level of the bile canalicular membrane (BCM) and to verify if altered membrane lipid dynamics could be involved in MiBK-potentiated Mn-BR cholestasis. Male Sprague-Dawley rats were expo sed 4 hr/day for 3 days to MiBK vapors (200 or 600 ppm). Eighteen hour s after the last exposure, manganese (Mn, 4.5 mg/kg) was given iv foll owed 15 min later by bilirubin (BR, 25 mg/kg). Rats were killed 30 min after BR; Liver cell plasma membranes (bile canalicular and sinusoida l), microsomes, mitochondria, and cytosol were isolated by differentia l centrifugation. Lipids were extracted and cholesterol was measured i n each fraction. After Mn-BR and MiBK exposure (600 ppm), results indi cated a marked increase in BCM cholesterol content compared to rats ex posed to air only. This increase was greater than that due to Mn-BR or MiBK given alone. Also, results indicated that cholesterol increased in a dose-related fashion in BCM after MiBK exposure, whereas PM chole sterol remained unaltered. To identify the source of the increased BCM cholesterol and to permit distinction between de novo cholesterol syn thesis and subcellular shifts, the hepatic lipid pool was labeled in v ivo with [H-3]-cholesterol and [2-C-14]-mevalonic acid, a cholesterol synthesis precursor. Results showed that after 600 ppm MiBK exposure, C-14-labeled cholesterol was greater than H-3-labeled cholesterol, ind icating that the contribution of de novo cholesterol synthesis to the total cholesterol content of the various isolated hepatocellular fract ions was more important than the contribution of intracellular pools. Therefore, increased BCM cholesterol content and enhanced accumulation of newly synthesized cholesterol appear to be involved in MiBK potent iation of Mn-BR-induced cholestasis. (C) 1997 Academic Press.