In the rat hepatoctye, the internalization and degradation of asialoglycoproteins and the secretion of plasma and biliary proteins require specific intracellular sorting of vesicles. To aid in the biochemical characterization of these different vesicular pathways, we examined the effects of the ionophore monensin on the uptake and degradation of '251-asialoorosomucoid (ASOR) and on the secretion of plasma and biliary proteins by the in situ perfused rat liver. In control livers, 77 % of injected '251-ASOR was extracted on first pass; 93% of the extracted radioactivity was released back into the circulation (totally degraded and some intact ASOR was found); and approximately 2% was recovered in the bile some of which was intact. Monensin treatment decreased first pass uptake of '*51-ASOR to 57 % and abruptly blocked the release of radioactivity into the rfusate and the ensin treatment dramatically reduced and delayed the secretion of newly synthesized proteins into both the perfusate and the bile. In contrast with control livers, in which secretion of protein into the perfusate preceded secretion of protein into the bile, TCA-precipitable 3H-protein appeared in bile about 20 min before TCAprecipitable 3H-protein appeared in the perfusate in monensin-treated livers. Thus, monensin treatment in the perfused liver blocked the degradation of asialoglycoproteins and inhibited the secretion of plasma proteins but had less effect on biliary protein secretion. These data document physiologic effects of monensin in an intact organ and suggest that biochemical distinctions between different vesicular pathways exist in the rat hepatocyte. bile. When hepatic proteins were biosynthetically labeled with p" H-leucine, mon-Key words: monesin, vesicular transport pathways, liver perfusion, asialoorosomucoid
The processes that regulate and direct the movement and sorting of intracellular vesicles are unexplained. Endocytic vesicles that internalize extracellular ligands via Abbreviations used; ASOR, asialoorosomucoid; pIgA, polymeric immunoglobulin A; SC, secretory component; SDS-PAGE, sodium dodecylsulfate-polyacrylamide gel electrophoresis; TCA, trichloroacetic acid.