Bile canalicular membrane fluidity is modulated by
phospholipid molecular species within
membrane lipid bilayers. Thus, organellar
membrane lipid composition is a determinant of canalicular function. In this study, the effect of
phalloidin-induced
cholestasis on bile
lipid composition and liver subcellular membrane fraction composition in rats was examined to clarify the relationship between
cholestasis and hepatic lipid metabolism.
METHODS AND RESULTS: Each rat received one
phalloidin dose (400 microg/kg, i.v.). After the bile was collected, liver microsomes and canalicular membranes were analysed. The bile flow rate decreased by 50% 3.5 h after
phalloidin administration. Although the
bile acid output remained almost the same, the
phospholipid and
cholesterol output were significantly decreased (by 40.3+/-5.97% and 76.9+/-5.56%, respectively). Thus, the
cholesterol:
phospholipid (C:P) ratio in bile was significantly decreased by 80.4+/-10.1%.
Phalloidin administration also increased the saturated:
unsaturated fatty acid ratio (S:U) in bile for
phosphatidylcholine by 25.5+/-3.2%. In the canalicular membrane, the C:P and S:U ratios for
phosphatidylcholine were increased (24.8+/-4.2% and 34.4+/-6.9%, respectively), while the S:U for
sphingomyelin was decreased by 61.0+/-6.2%. In microsomes, the C:P was decreased by 41.0+/-6.0%, but the S:U for both
phosphatidylcholine and
sphingomyelin were unaffected. Canalicular membrane fluidity, assayed by
1,6-diphenyl-1,3,5-hexatriene fluorescence depolarization, decreased significantly. Therefore, increased secretion of hydrophobic
phosphatidylcholine into bile was associated with more hydrophobic canalicular membrane
phosphatidylcholine, while
sphingomyelin in the canalicular membrane was less hydrophobic.
CONCLUSIONS: These results indicate that
phalloidin uncouples secretion of
cholesterol and
phospholipids, which causes a redistribution of fatty acyl chain species among canalicular membrane
phospholipids that alters membrane fluidity. These changes may be a homeostatic response mediated by the
phospholipid translocator in the canalicular membrane, although direct evidence for this is unavailable.