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Volumes of mitochondria in capillary endothelial cells were determined stereologically from electron micrographs of rat cerebellum, cerebral cortex, spinal cord, cauda equina, choroid plexus, anterior pituitary, median eminence of the hypothalamus, renal proximal tubules, skin, cardiac and skeletal muscle, lung, and renal glomerulus. The capillaries of the first four of these tissue types exhibit blood-brain barrier (BBB) characteristics of permeability and capillary ultrastructure and were found to have mitochondrial contents amounting to 8 to 11% of the endothelial cytoplasmic volume. Tissues from non-BBB regions were determined to have mitochondrial volumes of 2 to 5% of their respective cytoplasmic volumes, with a variety of capillary ultrastructures. The apparent excess metabloic work capability of the BBB suggested by this greater number of mitochondria may be related to maintenance of ion differentials between blood plasma and brain extracellular fluid, to extrachoroidal cerebrospinal fluid formation, or to maintaining the unique structural characteristics of central nervous system capillaries.

The percentages of 22 14C-labeled amino acids remaining in rat brain 15 s after carotid injection were measured relative to a simultaneously injected diffusible internal standard, 3HOH. The injected solution also contained a nondiffusible internal standard, [113mIn]EDTA to correct for incomplete brain blood compartment washout. Self-inhibition and cross-inhibition was demonstrated by inclusion of unlabeled amino acids and carboxylic acids. All amino acids tested, excluding proline, alanine, and glycine, could be assigned to one, and only one, blood-brain barrier carrier system. The neutral carrier system transported phenylalanine, leucine, tyrosine, isoleucine, methionine, tryptophane, valine, DOPA, cysteine, histidine, threonine, glutamine, asparagine, and serine. Affinity for a basic amino acid carrier system was demonstrated for arginine, ornithine, and lysine. A third, low-capacity independent carrier system transporting aspartic and glutamic acids was demonstrated.

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