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. Increas ed 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 giv en ip H-3-cholesterol, followed 18 h later by 2-C-14-mevalonic acid (a prec ursor of cholesterol synthesis). To induce cholestasis, manganese (Mn, 4.5 mg/kg) and bilirubin (BR, 25 mg/kg) were injected iv; animals were killed 3 0 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 radiol abeled techniques. In Mn-BR cholestasis, the total cholesterol concentratio ns of BCM and cytosol were significantly increased. Also, the contribution of C-14-labeled cholesterol (newly synthesized cholesterol) was enhanced in all isolated cellular fractions. The results are consistent with the hypot hesis that accumulation of newly synthesized cholesterol in BCM is involved in Mn-BR cholestasis. An enhanced rate of synthesis of cholesterol, howeve r, does not appear to be the causal event, as the activity of HMG-CoA reduc tase (rate-limiting enzyme in cholesterol synthesis), assessed in vitro, wa s decreased following Mn-BR treatment. Treatment with the Mn-BR combination may affect other aspects of intracellular cholesterol dynamics.