In vivo interaction of cationised ferritin with the surface coat and endocytosis by pulmonary intravascular macrophages: a tracer kinetic study.

The pulmonary intravascular macrophages (PIMs) of goat lung contain a unique electron-dense coat consisting of globular units arranged in a linear fashion along the cell surface. The globules showed a high affinity for cationised ferritin (CF) within 2-5 min of its intravenous injection, whereas native ferritin did not bind with the globules of the coat. The CF-labelled (hybrid) globules were subsequently internalised via receptor-mediated endocytosis. During 2-5 min of CF treatment, hybrid globules rapidly reached endosomes, multivesicular bodies and lysosomes, which acquired a high visibility in the form of interconnected tubular structures in the area of the Golgi complex. It is suggested that globular units of the surface coat, by showing high affinity for CF, resemble negatively charged large low density lipoprotein molecules. It supports our earlier postulate that heparin and lipolytic lipase sensitive globules of the surface coat of PIMs may be composed of lipoprotein-like substances. The ubiquity of the surface coat as a differentiated surface structure perhaps enables PIMs to cope with a barrage of stimulatory and suppressive signals within the microcirculatory units of the lungs of these animals.