Transendothelial transport (transcytosis) of iron-transferrin complex in the rat liver.

To determine the transport pathway of iron-transferrin complex (Fe-TF) into the hepatocyte, we labeled Fe-TF with colloidal gold and perfused rat liver through the portal vein with this probe under different conditions. The tissue was then studied by transmission electron microscopy. At a cold temperature (approximately 4 degrees C), the probe bound to the luminal surface of sinusoidal endothelium without internalization. The binding was limited to the endothelium and there was no binding to Kupffer cells or hepatocytes. The binding was inhibitable in the presence of excess soluble Fe-TF, indicating the specificity of the bindings. At 37 degrees C the probe was internalized via a system of coated pits and vesicles. Morphometric analyses of the temporal sequence of events suggested that the probe was transported, in part, across the endothelium via a system of tubules and vesicles and externalized on the abluminal side via a system of coated pits. The probe was then taken up by hepatocytes. Only minimal uptake was noted when gold-labeled bovine serum albumin was used either at the low temperature or at 37 degrees C. The findings suggest that the liver uptake of Fe-TF complex by hepatocytes is a transendothelial phenomenon (transcytosis).