Differential endocytosis of lipoproteins by capillary endothelial vesicles

Plasma levels of lipoproteins are believed to be controlled largely by lipoprotein receptors on the surfaces of tissue cells which facilitate their internalization and degradation. This presupposes transit of lipoproteins across the walls of blood vessels in order to gain access to the receptor sites. The endothelium of tissue capillaries may therefore constitute an additional regulatory barrier for lipoproteins. In order to test this hypothesis, we have measured the uptake of fluorescent-labeled lipoproteins into endothelial vesicles of capillaries isolated from fat tissue. HDLs are ingested at more than twice the efficiency of LDLs and VLDLs are excluded from vesicular ingestion. This represents a decreased efficiency of ingestion with an increase in molecular diameter of lipoproteins. This phenomenon correlates well with the dimensions of endothelial vesicles and caveolae which may restrict entry of very large serum lipoproteins. Selective transport of lipoproteins by capillary endothelial vesicles on the basis of molecular size may therefore serve to regulate blood-tissue interchanges of lipid.