Effect of molecular charge on choroid-plexus permeability: Tracer studies with cationized ferritins.

The permeability of the choroid plexus and renal glomerulus to intravenously injected native, anionic ferritin and various cationic ferritin derivatives was studied in normal rats by electron microscopy. In both structures, anionic, native ferritin was largely confined to the circulatory compartment while the cationic forms penetrated and accumulated within the filtration barriers. In the choroid plexus, cationic ferritin concentrated in relationship to the endothelial fenestrations and the subendothelial basal lamina region. In the glomerulus, there was also an inner concentration of cationic tracer and, in addition, an aggregation of tracer along the outer, subepithelial portion of the basal lamina. The results indicate that the localization of tracer within the filtration barrier of both the choroid plexus and renal glomerulus is directly related to the tracer's isoelectric point. The findings suggest that the choroid plexus, like the glomerulus, contains fixed anionic groups within the capillary wall which influence its permeability.