Endocytosis of cationic and anionic iron colloid particles by rat macrophages

By using anionic and cationic iron colloid particles, which are Prussian blue reaction positive and 5-250 nm in diameter, the surface charge-related endocytosis of rat macrophages was observed. The anionic particles were solely adhered to the surface of peritoneal and liver macrophages by exposing the cells for a short time period to the particles in vitro as well as in vivo by perfusion. The particles then were taken up selectively by macrophages, but not by neutrophils, eosinophils, lymphocytes, mast cells, fibroblasts and vascular endothelial cells, as observed by light and electron microscopy after intraperitoneal, subcutaneous and intravenous injections of the particles. The anionic iron colloid particles were taken up by macrophages, delivered to lysosomes and disintegrated quickly. Ferritin was first synthesized there and then dispersed into the cytoplasmic matrix. In contrast, cationic iron colloid particles were adhered to the surfaces of all kind of somatic cells and internalized by the process similar to adsorptive endocytosis. The ingested particles were found in thin canalicular structure or tiny vesicles being adhered to their luminal surfaces. The morphology of these compartment was much different from the phagosomes of macrophages formed by ingesting anionic iron colloid particles, and showed no deliverly of the particles to lysosomes as long as 4 hr after the incubation. Some particles were found in lysosomes by another 5 hr of chase incubation. The results indicate that the “non-specific receptor mediated phagocytosis” by macrophages is induced by the binding of anionic macromolecules to the cationic receptor on the cell surface. And the adsorptive endocytosis will induce two kinds of different mechanism, one is currently accepted concept, the receptor mediated endocytosis and another one is the surface charge-mediated endocytosis.