1. Capillaries were isolated from the brains of 1‐ to 45‐day‐old rats in order to study the development of metabolic and transport aspects of the blood‐brain barrier. 2. The hydroxyproline content of capillary hydrolysates increased nearly threefold between 5 and 45 days of age. This finding is consistent with histological studies showing thickening of capillary basement membrane during development. 3. The activities of L‐DOPA decarboxylase and monoamine oxidase were greatest in capillaries from 10‐day‐old rat brain. Thus, the metabolic blood‐brain barrier for amine precursors is present during early development. 4. Capillaries from all ages were able to metabolize glucose, beta‐hydroxybutyrate and palmitate. The rate of glucose oxidation more than doubled between 21 and 30 days of age but subsequently decreased. In contrast, beta‐hydroxybutyrate and palmitate oxidation increased throughout development. These data suggest a sparing effect by alternate fuels on glucose metabolism. 5. Capillary glucose uptake was similar at 10 and 30 days of age and activity of the ouabain‐sensitive K+ pump (measured using 86Rb+) was relatively constant at all ages. In contrast, Na+‐dependent neutral amino acid transport was not present until after 21 days of age. Since this transport system may be responsible for the active efflux of neutral amino acids from brain to blood, it is likely that this process does not occur at the immature blood‐brain barrier. 6. We conclude that various aspects of brain capillary functions show distinct developmental patterns which may be related to changes in blood‐brain barrier permeability during development.
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