Alteration of lipid composition of hepatic membranes associated with manganese‐bilirubin induced cholestasis

Summary— One hypothesis concerning the pathogenesis of manganese‐bilirubin (Mn‐BR)‐induced cholestasis is that the molecular organization of the bile canalicular membrane is altered. The purpose of the present study was to evaluate lipid composition and fluidity of hepatic membranes during cholestasis in male Sprague‐Dawley rats. To induce cholestasis, manganese (Mn, 4.5 mg/kg, intravenously [iv]) was given IS min before bilirubin (BR, 25 mg/kg, iv). The rats were killed 30 min after BR injection, at which time bile flow was decreased by approximately 40% compared to control values. Liver cell plasma membranes enriched in canalicular fractions (BCM) and plasma membranes enriched in sinusoidal and lateral fractions (PM), microsomes, mitochondria and cytosol were isolated by differential centrifugation. Total lipids were extracted and measured colorimetrically. To assess fluidity, membranes were incubated in vitro with fluorescent probes [1,6‐diphenyl‐1,3,5‐hexatriene and 1‐(4′‐trimethyl‐ammonium‐phenyl)‐6‐phenyl‐1,3,5‐hexatriene]. After Mn‐BR treatment, BCM cholesterol incorporation increased markedly (about 3‐fold) accompanied by a decrease in fluidity. BCM phospholipid content was unaltered by the cholestatic challenge. In PM‐enriched fractions, the changes in cholesterol and phospholipid content after Mn‐BR treatment were not statistically significant ( P > 0.05) compared to controls. Furthermore, the biochemical alterations in PM were not accompanied by changes in membrane fluidity. These results support the hypothesis that altered lipid composition and fluidity of BCM are involved in the pathogenesis of Mn‐BR cholestasis.