The Basis for Active Transport at the Blood-Brain Barrier
Certain low molecular weight compounds have been shown to enter the brain by specific carrier-mediated transport systems in the endothelial cell membranes (1–4). At least eight such carriers capable of moving solutes from blood into brain have been described (4). Most of the blood-brain barrier (BBB) transport systems that have been described are not able to move solutes against concentration gradients, but instead, facilitate the equilibration of brain and blood concentrations for their particular substrates. Although the BBB acts as an impermeable wall for some solutes and a passive sieve for others, it is generally agreed that an important function is active regulation of the internal milieu of the central nervous system (CNS) (3,5,6). Several reports have suggested that the BBB can mediate solute efflux from the brain against a concentration gradient (7–13), but these studies utilized intact animals and, consequently, their interpretation is made difficult by the presence of neurons, glia and choroid plexus.
KeywordsActive Transport Neutral Amino Acid Brain Capillary Brain Capillary Endothelial Cell Large Neutral Amino Acid
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