Employing the in situ perfused rat liver, we examined the origins and mechanisms of transport of proteins into bile. First, utilizing polyacrylamide gels, we noted that many biliary proteins co-migrated with dominant serum proteins. Upon liver perfusion with serum-free medium, most proteins disappeared from the biliary profile; one major biliary protein that was not present in serum, identified as secretory component, remained. Kinetic analysis of the disappearance half-lives of the biliary proteins suggested that some serum proteins enter bile by a slow (20 to 30 min; transcellular) route, while others utilize both slow and rapid (5 min; paracellular) routes. In biosynthetic labeling experiments, secretion of newly synthesized proteins into bile was delayed about 20 min when compared with secretion of proteins into the perfusion medium and comprised less than 1% of the total secreted proteins. When a new liver was inserted into the perfusion medium containing newly synthesbd secreted proteins, only two proteins, hemopexin and an unidentiried protein, were transported into the bile from the perfusion medium; other biliary proteins were presumed to come directly from the hepatocyte. Tbis latter group included some proteins that were secreted into the perfusion medium as well as into bile, and others, e.g., secretory component, that were secreted only into bile. Based on our results we have defined six pathways for entry of proteins into bile.
Hepatocytes synthesize and secrete proteins into both blood and bile. Those proteins that are secreted into blood travel a fairly well-characterized pathway similar to that described for exocrine glands (1). Much less is known about the proteins in bile and the mechanism(s) for their entry into bile. Many of the biliary proteins are immunologically similar to serum proteins and are thought to be transported from serum into bile (2, 3). Such serum-derived proteins might enter the bile either by diffusion across the tight junctions between hepato-