Blood-brain barrier facilitated diffusion

Facilitated diffusion of glucose across the blood-brain barrier and into brain cells is catalyzed by GLUT1,- 2 and -3, products of the SLC2 gene superfamily 90... 

L-Dopa is not bound to plasma proteins. It crosses the blood-brain barrier by stereospeciflc saturable facilitated diffusion and competes with LNAA for transport into the brain. High-dose infusions of phenylalanine and leucine decrease the clinical response to L-dopa without altering L-dopa plasma concentrations. This has led to... 

Lipophilicity. Lipophilicity of a radiotracer facilitates diffusion across membranes, particularly passing the blood-brain barrier, which is required for brain uptake. 

Many nonpolar substances, such as drugs and inert gases, probably diffuse through the endothelial cell membranes. A large number of other compounds are transported through the endothelial capillaries by facilitative transport, whereas others, such as nonessential fatty acids, cannot cross the blood-brain barrier. Essential fatty acids, however, are transported across the barrier. 

As originally formulated (5) the parameter a represents the ratio of unidirectional fluxes of a compoimd across the blood-brain barrier. In the case of a compound like glucose, which passes the blood-brain barrier by carrier mediated facilitated diffusion, (1 + Km/Ct)/(1 + Km/Cb),... 

The blood-CSF barrier is relatively permeable to hydrophilic macromolecules, (i.e., ai-macroglobulin and IgM). In addition, the passage of smaller molecules, which are larger than 500 Da, is facilitated by lipophilicity (i.e., by antibiotics and cytostatic drugs). The composition of the extracellular fluid of the brain parenchyma is unknown. It resembles CSF only in a narrow margin of a few millimeters adjacent to the free CSF space, a zone where a limited diffusion of water-soluble molecules is possible (F2). The composition of CSF is well known because the subarachnoid space can be tapped at its lowest point. Despite the great distance from the site of production, the choroid plexus, it shows all of the characteristics of a filtrate, even in the lumbar sac. 

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