Newly synthesized hepatocyte plasma membrane proteins are transported in transcytotic vesicles in the bile duct-ligated rat.
BACKGROUND
Newly synthesized apical membrane proteins in hepatocytes go first to the basolateral membrane, from which they are retrieved and delivered to the apical domain. The goal of the present study was to identify the vesicular carriers of these molecules.


METHODS
The common bile duct of rats was ligated for 10-72 hours, and then various plasma membrane proteins were localized using immunofluorescence and quantitative immuno-electron microscopy of fixed liver tissue.


RESULTS
By immunofluorescence, we found intracellular punctate staining near the bile canalicular membrane of polymeric immunoglobulin A (IgA) receptor and several apical membrane proteins, but not basolateral proteins. This compartment was membrane bounded and pleiomorphic by immunoelectron microscopy. Colocalization at the electron microscopic level showed that the apical protein, dipeptidyl peptidase IV, was in the same structures as aminopeptidase N, polymeric IgA receptor, or intravenously injected horseradish peroxidase. This intracellular immunolabeling decreased after cycloheximide treatment (t1/2 = 2-2.5 hours) or reversal of the ligation for 1 hour. In the latter case, bile canalicular labeling increased. Furthermore, polymeric IgA receptor was delivered to the bile canaliculi.


CONCLUSIONS
Bile duct ligation leads to an intracellular accumulation of vesicles carrying polymeric IgA receptor, several apical membrane proteins, and a fluid phase marker. These vesicles continue to fuse with the apical membrane, even during ligation.
Barr
V A
VA
Department of Cell Biology and Anatomy, Johns Hopkins University School of Medicine, Baltimore, Maryland.
Hubbard
A L
AL
D000818:Animals; D001652:Bile Ducts; D001692:Biological Transport; D003513:Cycloheximide / Q000494:pharmacology; D005455:Fluorescent Antibody Technique; D006735:Horseradish Peroxidase / Q000378:metabolism; D008026:Ligation; D008099:Liver / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism*; D011108:Polymers; D051381:Rats; D017207:Rats, Sprague-Dawley; D011961:Receptors, Fc; D011971:Receptors, Immunologic / Q000378:metabolism; D014018:Tissue Distribution
C045281:IgA receptor; D008565:Membrane Proteins; D011108:Polymers; D011961:Receptors, Fc; D011971:Receptors, Immunologic; D003513:Cycloheximide; D006735:Horseradish Peroxidase