In vivo use of neutral radiolabelled molecular probes to evaluate blood-ocular barrier integrity in normal and streptozotocin-diabetic rats.

Evidence suggests that the consequences of diabetes mellitus are numerous and that net changes in ocular barrier permeability are necessarily complex functions of changes at specific anatomical loci. In this study we explore changes in blood-aqueous and vitreous permeability in streptozotocin-diabetic rats using five stable radiolabelled probes. Three probes, (3H)-L-glucose, (14C)-sucrose and (14C)-carboxylinulin are relatively large molecules and are expected to move into ocular humours via paracellular routes. Two probes, (14C)-urea and (14C)-glycerol, are small and likely have a trans-cellular component to permeability. Pulse-chase kinetic studies follow the appearance of test molecules into ocular humours with rate constants estimated via linear modelling. Larger neutral probes L-glucose, sucrose and carboxylinulin entered the aqueous humour of control rats slowly via routes that presumably circumvent tight-junctioned barriers. These slow-entry rates were found to increase in diabetic animals suggesting an increase in passive paracellular permeability with significant variation among animals. In contrast, aqueous entry rates of smaller probes urea and glycerol were decreased in diabetic animals suggesting that these probes cross membranes and cells less efficiently in diabetic animals. The magnitude of these changes increased with the length of exposure to diabetes. Paralleling the aqueous humour studies, we found a significant but variable increase in vitreous entry rate with L-glucose, sucrose and carboxylinulin, but a decrease in entry rates with small probes urea and glycerol. These results suggests that diabetes-related blood-ocular permeability changes are complex and depend on the size and properties of the probe as well as the degree of diabetes exposure.