Ultracytochemical localization of the erythrocyte/HepG2-type glucose transporter (GLUT1) in the ciliary body and iris of the rat eye.

Aqueous humor, with its unique low concentration of proteins, is produced by the ciliary body and isolated by the blood-aqueous barrier from the body fluid. Glucose in aqueous humor is a major source of nutrients for lens and corneal cells, and is maintained near the plasma level, suggesting a specific glucose transport mechanism in the blood-aqueous barrier. Using antibodies against erythrocyte/HepG2-type glucose transporter (GLUT1), one isoform of the facilitated diffusion glucose transporters, the authors found immunocytochemically that GLUT1 localizes in the epithelial cells of ciliary body and iris. GLUT1 is also found in the endothelial cells of blood vessels in the iris, whereas no labeling is seen in the blood vessels in the ciliary body. In the ciliary body epithelium, the plasma membranes of both the pigmented epithelial (PE) and nonpigmented epithelial (NPE) cells are positive for GLUT1. By the colloidal gold particle counting, the basal infoldings of PE cells show approximately two-fold denser labeling than those of NPE cells. Since PE and NPE cells make up a functional syncytium with numerous gap junctions, the authors suggest that glucose transport in the ciliary body occurs in this manner: glucose diffuses out from blood vessels through the pores of fenestrated endothelial cells, is transported into PE cells by GLUT1 in their plasma membrane, enters NPE cells through gap junctions connecting PE and NPE cells, and is finally transported into the aqueous humor by GLUT1 of NPE cells. The higher density of GLUT1 in PE cells may account for the consumption of glucose by PE and NPE cells in addition to the transepithelial transport.