Phosphate Transport in Capillaries of the Blood-Brain Barrier
Endothelial cells of cerebral capillaries form a continuous barrier between blood and brain interstitium. These endothelial cells, sealed together by tight junctions, do not possess fenestrations or transendothelial channels (1,2). Brain capillaries also contain a large number of mitochondria and are able to metabolize a variety of substrates (3, 4). This specialized endothelium regulates the movement of solutes between blood and brain. This is accomplished through carrier-mediated transport systems for hexoses, monocarboxylic acids, amino acids, nucleosides, purines and amines (5). Active transport pumps in the endothelial cells of brain capillaries appear to maintain the volume and composition of brain’s interstitial fluid (6). This extracellular fluid differs from plasma in that it is almost free of protein and differs from an ultrafiltrate of plasma by maintaining the concentration of various ions at distinct levels. In fact, the concentration of inorganic phosphate in the interstitial fluid of the brain is held between 0.5 and 1.0 mM, and the plasma level is maintained between 1.5 and 1.8 mM (7). Maintenance of this transcellular phosphate concentration gradient could be a result of transport mechanisms at the luminal and anti-luminal sites of the membranes of cerebral capillaries. The relationship of inorganic phosphate to the phosphorylation processes could account for a regulation of the concentration of this ion by a membrane transport mechanism. Furthermore, capillary cells require phosphate for energy demands, as reflected by the large number of mitochondria, and for synthesis of nucleic acids and complex lipids (3). Phosphate transport in brain capillaries was studied with metabolically active capillaries isolated from bovine cortex (8).
KeywordsPermeability Glycerol Filtration Adenosine Luminal
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