Permeability of Brain Capillaries to Hexoses and Pentoses in the Rabbit

The initial loss in the rabbit brain of non‐electrolytes injected into the carotid artery was measured with ‘Indicator Diffusion’ technique, 3–0–methyl‐D‐glucose shared with D‐glucose the ability to pass into the brain by a mediated transport mechanism. The rate of transport of the following substances could not be distinguished: D‐mannose, D‐galactose, D‐arabinose and 1)‐ribose. They all penetrated more slowly than D‐glucose. D‐mannitol was also lost from the blood during the passage through the brain. This observation, together with the known inability of D‐mannitol to pass the blood‐brain barrier, suggests that the loss takes place into the endothelial cytoplasma. Thus the basal plasma membrane of the endothrlial cells together with the basal membrane proper may define blood‐brain barrier chararteristirs.