Endocytosis of serum albumin-gold conjugates by microvascular endothelial cells in rat adrenal gland: regional differences between cortex and medulla

Abstract

We investigated the distribution of serum albumin (bovine) modified by adsorption to 15 nm diameter colloidal gold (Au15BSA) in rat adrenal vasculature after perfusion of Au15BSA in situ for 6-20 min. A striking difference in permeability to and uptake of Au15BSA by adrenal cortical and medullary endothelial cells was found. The fenestrated capillary endothelial cells in the cortex were permeable to Au15BSA particles, but showed no significant endocytosis of these conjugates. Endothelial cells of the medullary capillaries and venous tributaries of the central vein were markedly less fenestrated than cortical capillaries, and resembled morphologically the sinusoidal endothelial cells of bone marrow. These endothelial cells, unlike those in the cortex, were not permeable to the perfused Au15BSA, and showed endocytosis of the gold-conjugated protein by means of coated pits and vesicles. No evidence of transcytosis of Au15BSA by means of non-coated, plasmalemmal vesicles was found. Uptake of Au15BSA by medullary endothelia was not appreciably diminished by the presence of one hundred-fold excess, unmodified, monomeric BSA. One hundred-fold excess of serum albumin which was modified by treatment with formaldehyde (FmBSA), on the other hand, did effectively compete for uptake of Au15BSA. These results indicate that adrenal medullary endothelial cells, in contrast to those of the cortex, selectively take up serum albumin modified by adsorption to colloidal gold beads, or modified by treatment with formaldehyde. The endocytosis appears to involve selective adsorption of the Au-modified albumin to sites, perhaps receptors, at coated regions of the luminal endothelial surface.