Distribution of D and L l-C-14-glucose radioactivity was measured in retina, brain and other tissues by a combustion technic at short intervals following an intravenous injection in rat, rabbit and guinea pigs. Five minutes after injection, the ratios of D/L radioactivity in the retina of the three species were 23.7, 11.6 and 8.3, respectively. Similar high ratios were found in the brain but not in voluntary muscle or liver. The explanation may lie in the special characteristics of capillaries in brain and retina, which have tight intracellular junctions. Substances that enter fhe brain and retina must therefore cross the endothelial cells and cannot diffuse between them as in other tissues. The results suggest the presence of a stereospecific carrier for glucose in the endothelial cells which controls entry of glucose into the retina, as it appears to in the brain.

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