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.