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Structural Specificity of Sugar Transport at the Blood‐Nerve Barrier

Abstract: The major component of D‐glucose transfer across the membranous sites of the blood‐nerve barrier (BNB) occurs via a facilitative mechanism at a rate greater than twice the rate of D‐glucose metabolism by nerve. To characterize further properties of monosaccharide transport at the BNB, unidirectional transfer constant (K) values were determined in vivo in tibial nerve of anesthetized rats for radiolabeled mannitol, L‐glucose, and a series of D‐glucose analogs. K values (× 10−4 ml s−1 g−l) equaled 4.8 for 2‐deoxy‐D‐glucose, 3.7 for D‐glucose, 2.3 for 3‐O‐methyl‐D‐glucose, 1.4 for D‐man‐nose, 0.6 for D‐galactose, 0.2 for mannitol, and 0.19 for L‐glucose. The rank order of ratios between K values of a D‐hexose and D‐glucose, which reflects the rank order of affinity of the system for individual sugars, was 2‐deoxy‐D‐glucose > D‐glucose > 3‐O‐methyl‐D‐glucose > D‐mannose > D‐galactose. The results demonstrate that the order of substrate affinity of the monosaccharide carrier at the BNB is similar to that at cerebral capillaries and at erythrocytes. At normal concentrations of plasma D‐glucose, the contribution of simple passive diffusion to unidirectional D‐glucose influx across the BNB equals 5%, which is greater than that at cerebral capillaries and reflects the greater permeability to hydrophilic nonelectrolytes of the endoneurial vasculature.