IMAGING BILIARY LIPID SECRETION IN THE RAT - ULTRASTRUCTURAL EVIDENCEFOR VESICULATION OF THE HEPATOCYTE CANALICULAR MEMBRANE

Physical-chemical and biological studies of hepatic bile suggest that biliary phospholipid molecules are secreted as unilamellar vesicles. S ystematic ultrastructural studies of bile canaliculi were undertaken t o visualize this event. Liver tissue was obtained from normal adult ma le rats (control), from bile salt-depleted rats (by overnight biliary diversion), and from depleted rats infused intravenously with a hydrop hilic-hydrophobic congener series of common taurine-conjugated bile sa lts. Livers were fixed in situ either by modified chemical methods or by ultrarapid cryofixation. In control rats, chemical fixation reveale d unilamellar vesicles 63 +/- 17 (+/- SD) nm in diameter, mostly free within canalicular lumena. Vesicles were infrequent in canaliculi of b ile salt-depleted rats, but were present in canaliculi of rats infused with taurocholate. In cryofixed liver tissue, vesicles 67 +/- 13 nm i n diameter were observed in canaliculi of control rats and bile-salt d epleted rats infused with common bile salts. The majority of these ves icles were affixed to the luminal side of the canalicular membrane. Th e average number of vesicles per bile canaliculus was in agreement wit h that estimated on the basis of biliary phospholipid secretion rates, mean vesicle size, and area of close-packed phosphatidylcholine molec ules. By immunoelectron microscopy, canalicular vesicles were free of actin and of a 100 kDa canalicular membrane protein. We conclude that biliary phospholipid molecules are secreted from hepatocytes into bile canalicular lumena as unilamellar vesicles -63-67 nm in average diame ter. We postulate that this secretion mechanism involves lumenal bile salt-induced vesiculation of lipid microdomains in the exoplasmic hemi leaflet of the canalicular membrane.