Mapping the blood vessels with paracellular permeability in the retinas of diabetic rats

Diabetic retinopathy increases the permeability of the blood-retinal barrier, but the specific vessels that become permeable have not been identified. Both transcellular and paracellular pathways of vascular solute flux have been proposed. This study was conducted to test the hypothesis that paracellular flux contributes to increased retinal vascular permeability after VEGF treatment or diabetes, and to map the types of vessels that became permeable.

Regions of paracellular flux were identified by perfusion with fluorescent concanavalin A (ConA). Rats were injected intravitreally with VEGF or made diabetic with streptozotocin (STZ). After specified times, the rats were perfused with fixative followed by ConA, which binds to the basement membrane but not the luminal surface of endothelial cells. With this approach, ConA labels only blood vessels with paracellular permeability. Retinas were also labeled by immunofluorescence for the tight junction proteins occludin and claudin-5 and examined by confocal microscopy.

ConA labeling increased in the superficial arterioles and postcapillary venules, 2 weeks after the onset of diabetes. After 1 month, ConA labeling dramatically increased and extended to the capillaries of the outer plexiform layer. There was an inverse relationship between occludin immunoreactivity and ConA binding, but no change in claudin-5 immunoreactivity was detected. Injection of VEGF gave similar results.

Diabetes and VEGF increase paracellular vascular permeability in the retina, associated with redistribution of occludin. This permeability begins in the superficial arterioles and postcapillary venules and progresses to the capillary bed.