Uptake of free and liposome-entrapped 125I-labelled PVP by rat intestinal sacs in vitro: evidence for endocytosis.

The uptake of free and liposome-entrapped 125I-labelled PVP was measured in and intestinal-sac preparation from adult rats. Uptake of the free macromolecule was directly proportional to the substrate concentration and was reduced by colchicine and sodium azide. Uptake of the liposome-entrapped macromolecule was greater than that of the free macromolecule, was not directly proportional to substrate concentration, showed signs of saturation at high liposome concentrations, and was also reduced by sodium azide and colchicine. These results suggest that gut epithelial cells take up the free macromolecule by fluid-phase endocytosis and the liposome-entrapped macromolecule by adsorptive endocytosis.