Pathways of transcytosis in the fenestrated endothelium of pancreatic capillaries.

A procedure which covalently attaches an electron dense tracer to cell surface sialoglycoconjugates was used in order to label the luminal surface of capillary endothelium in rat pancreas. Mild oxidation of the vasculature by perfusion with 1 mM sodium periodate, followed by a 30-90 min incubation with ferritin hydrazide (FH) at 37 degrees C, led to a characteristic decoration of the endothelial surface. FH was taken up by the endothelium in coated vesicles and, to a much lesser extent, in plasmalemmal vesicles or vacuoles. Though rarely, FH was found at the abluminal front of the endothelium in coated vesicles opened to the extracellular space either directly or via a smooth-surfaced vesicle, thus suggesting the involvement of coated structures in transendothelial transport (transcytosis). A conjugate of wheat germ agglutinin with ferritin, which binds noncovalently to cell surface glycoconjugates containing N-acetylneuraminyl and N-acetylglucosaminyl residues, was transported transcellularly in monomeric form, mainly through smooth-surfaced (plasmalemmal) vesicles and transendothelial channels. Cationized ferritin (CF), pI approximately 8.4, which binds to cell surface acidic sites via an electrostatic noncovalent interaction, was rapidly internalized by the oxidized endothelium in coated structures and in vacuoles. Big aggregates of CF were transported and delivered to the abluminal front of the endothelium via large vacuoles, vesicles, and through coated structures and multivesicular bodies. These results show that the capillary endothelial cell possesses multiple mechanisms for transporting macromolecules across the cell, involving vacuoles, smooth or coated vesicles, channels, and lysosomal-related compartments. They also indicate that, besides molecular weight and electric charge of macromolecules, other characteristics of the ligands (e.g., type of bond with cell surface components, state of aggregation) are factors influencing the transendothelial transport.