Interactions of endogenous lipoproteins with capillary endothelium in spontaneously hyperlipoproteinemic rats

In spontaneously hyperlipoproteinemic old Sprague-Dawley rats, endogenous lipoproteins (LP) in the size range of 15 to 40 nm were directly visualized within the blood vessels due to specimen mordanting with tannic acid. LP morphometric analysis at the level of the endothelium of diaphragm capillaries revealed that particles of the dimensions of low-density lipoproteins, high-density lipoproteins (HDL1), and very low density lipoproteins occur in endothelial structures involved in receptor-mediated endocytosis coated pits-vesicles, endosomes, lysosomes) and transcytosis (plasmalemmal vesicles and transendothelial channels). No such particles could be detected in the intercellular junctions. Intravenously injected cationized ferritin (CF) of pI 8.4 bound uniformly to LP forming an CF-LP complex. Examined at 5, 20, and 60 min after CF administration, the CF-LP complex was found to be taken up by endothelium only by endocytosis (adsorptive via coated pits-vesicles, and fluid phase through a fraction of plasmalemmal vesicles). CF-LP complexes are progressively accumulated within lysosomes. These findings reveal the importance of the net surface charge of macromolecular complexes for their intracellular sorting and fate.