Evidence for receptor-mediated hepatic uptake of pullulan in rats

Fluorescein-labeled pullulan (FP-60; MW 58,200) was prepared by reaction with FITC according to the method of de Belder and Granath. The hepatic distribution of FP-60 was examined using a specific high-performance size-exclusion chromatography. Intravenously administered FP-60 was rapidly eliminated from the blood circulation followed by an appreciable distribution to the liver. A marked dose-dependency was seen in the hepatic uptake of FP-60 which was markedly reduced by the coadministration of both asialofetuin and arabinogalactan. Measurement of the hepatocellular localization demonstrated the overwhelming distribution of FP-60 in the parenchymal liver cell fraction. Furthermore, microscopic examination revealed that FP-60 was effectively endocytosed by the parenchymal liver cells. Radiolabeled pullulan ([(125)I]P-60) was prepared by (125)I-labeling the tyramine derivative of pullulan which was synthesized by the cyano-transfer method. [(125)I]P-60 was predominantly accumulated in sliced rat liver tissue at 37 degrees C, which was drastically inhibited by the addition of both asialofetuin and arabinogalactan. The kinetic parameters of the specific binding of [(125)I]P-60 to monolayered hepatocytes at 0 degrees C were almost identical to those for asialofetuin. The binding of [(125)I]P-60 to isolated parenchymal cells was significantly inhibited by arabinogalactan and asialofetuin, however dextran, the same glucan as pullulan, did not affect the binding of [(125)I]P-60. It was found that pullulan, which is bound to the asialoglycoprotein receptor with high affinity, is subsequently internalized to the hepatocyte via receptor-mediated endocytosis.