Permeability of ocular vessels and transport across the blood-retinal-barrier.

This paper reviews quantitative studies on the permeability of retinal and choroidal vessels and the exchange of nutrients over the blood retinal barrier (BRB). The fenestrated capillaries in the choroid are very permeable to low molecular weight substances; sodium permeability in the choroid is probably 50 times that in skeletal muscle. This results in high concentrations and rapid turnover of nutrients in the extra-vascular compartment of the choroid. Free diffusion is restricted by the pigment epithelium barrier. Also the retinal capillaries, with tight junctions between the endothelial cells, have very low permeability even to sodium. The uptake index technique has provided evidence for several carrier systems in the BRB; hexoses, neutral and basic amino acids, and monocarboxylic acids, very similar to those found in the brain. At least for glucose and lactate these carriers operate at both levels of the BRB; the RPE and the endothelium of the retinal capillaries, and in both directions; i.e. inwards and outwards.