High- and Low-Affinity Transport of D-Glucose from Blood to Brain

Abstract: Measurements of the unidirectional blood-brain glucose flux in rat were incompatible with a single set of kinetic constants for transendothelial transport. At least two transfer mechanisms were present: a high-affinity, low-capacity system, and a low-affinity, high-capacity system. The low-affinity system did not represent passive diffusion because it distinguished between D-and L-glucose. The T_max and K _m, for the high-affinity system were 0.16 mmol 100 g⁻¹ min⁻¹ and 1 mM; for the low-affinity system, ∼ 5 mmol 100 g⁻¹ min⁻¹ and ∼ 1 M. With these values, physiological glucose concentrations were not sufficient to saturate the low-affinity system. In normoglycemia, therefore, three independent pathways of glucose transport from blood to brain appear to exist: a high-affinity facilitated diffusion pathway of apparent permeability 235·10⁻⁷ cm s⁻¹, a specific but nonsaturable diffusion pathway of permeability 85·10⁻⁷ cm s^−l, and a nonspecifc passive diffusion pathway of permeability 2·10⁻⁷ cm s⁻¹.