Charge effect on binding, uptake and transport of ferritin through fenestrated endothelium

The binding, uptake and transport of hydrosoluble macromolecules as a function of their electric charge have been investigated in mouse pancreatic exocrine fenestrated endothelium. After intravenous injection of ferritin derivatives with different isoelectric points (3.7, 4.5, 7.0 and 8.4) specimens of pancreas were collected at 5, 25, 45 and 60 min and processed for electron microscopy. Although the concentrations (10 mg/ml) and injected volumes (1 ml/100 g body weight) were identical, the tracers with similar molecular weight (Mr approximately 960,000), but with different net electric charge, followed different routes in endocytotic and transcytotic processes. The endothelial cell internalizes ferritins into multivesicular bodies and lysosomes. While anionic ferritins are endocytosed via plasmalemmal vesicles (fluid phase endocytosis), the cationic derivatives are taken up exclusively by coated pits (adsorptive endocytosis). The anionic and neutral macromolecules seem to be transcytosed via plasmalemmal vesicles. The circulating polycations are aggregated by plasma proteins and transcytosed as such either in fluid phase by large vacuoles, or by adsorption on the coated structures. The first mechanism is physiological, the latter probably appears in unusual conditions by which the endothelium participates in the plasma clearance. Our findings indicate that, for the transport of macromolecules, the capillary endothelium can be accounted not only as a size barrier but also as a selective-differentiated charge barrier with complex and multiple mechanisms for the modulation of its transport systems. The endocytotic and transcytotic processes undergo the effect of net electric charge of the plasma components.