Transcytotic vesicle fusion is reduced in cholestatic rats: redistribution of phospholipids in the canalicular membrane.

Cholestasis, which affects phospholipid trafficking, therefore would be expected to alter canalicular membrane phospholipid composition and fluidity, as well as fatty acid composition of membrane phospholipid. These alterations may affect transcytotic vesicle fusion and would be expected to cause variety of cholestatic phenomena. The aim of this study was to determine the effect of cholestasis on transcytotic vesicle fusion. Sprague-Dawley rats with extrahepatic and intrahepatic cholestasis were prepared by bile duct ligation (6 hr or three days) and phalloidin infusion (0.4 mg/kg body weight), respectively. Liposomes of phosphatidylserine/phosphatidylcholine were labeled with octadecyl rhodamine B chloride. Fusion of liposomes to canalicular membrane vesicle preparations from cholestatic and control rats was induced by the addition of calcium. The degree of fusion was evaluated by measuring the increase in rhodamine fluorescence. Membrane phospholipid content also was analyzed. Rates of liposomal fusion to membranes from cholestatic rats were decreased compared to controls. The saturated/unsaturated and saturated/polyunsaturated fatty acid ratios were increased in membrane phosphatidylcholine and decreased in membrane sphingomyelin from cholestatic rats. Cholesterol/phospholipid ratios were increased. Thus, in the presence of cholestasis, a redistribution of phospholipid species within canalicular membranes is associated with decreased transcytotic vesicle fusion. Cholestasis likely decreases membrane fluidity. The regulation of phospholipid species within hepatocellular membranes may play an important role in intrahepatic lipid transport.