ENZYMATIC DIGESTION INCREASES PERMEABILITY OF THE OUTER BLOOD-RETINALBARRIER FOR HIGH-MOLECULAR-WEIGHT SUBSTANCES

Background: The purpose of the study was to investigate whether lysoso mal enzymes can participate in damaging the outer blood-retinal barrie r and to examine the role of glycosaminoglycans in maintaining the bar rier function for high-molecular-weight substances. Methods: The cilia ry artery was cannulated in freshly enucleated pig eyes. Perfusion was performed with buffer (controls), with heparinase (substrate: heparan sulfate), or with lysosomal enzymes freshly prepared from pig retinal pigment epithelium at 36-degrees-C, followed by perfusion with the tr acer native ferritin (NF) or the marker cationized ferritin (CF). The eyes were examined by electron microscopy. Results: In controls treate d with buffer alone, NF was found in high concentration in the lumina of the choroidal capillaries; however, little NF was found in Bruch's membrane (BsM). The tracer did not penetrate to any extent beyond BsM. In eyes digested with heparinase or lysosomal enzymes, significantly higher numbers of tracer molecules were found in BsM. Furthermore, NF penetrated BsM and was apparent in the subretinal space and also insid e retinal pigment epithelial cells, probably due to pinocytosis. Concl usions: The results indicate that heparan sulfate proteoglycan is impo rtant for the maintenance of the outer blood-retinal barrier and that lysosomal proteases may participate in damaging this barrier, causing increased permeability to high-molecular-weight substances.