P-GLYCOPROTEIN IS STRONGLY EXPRESSED IN THE LUMINAL MEMBRANES OF THE ENDOTHELIUM OF BLOOD-VESSELS IN THE BRAIN

Luminal membranes of the vascular endothelium were isolated from brain , heart and lungs by modification of their density. The presence of P- glycoprotein (P-gp) was detected by Western blotting in luminal membra nes from the endothelium of the three tissues. Strong enrichment in br ain capillary luminal membranes, compared with brain capillaries (17-f old) and whole membranes (400-500-fold), indicates that P-gp is mainly located on the luminal side of the brain endothelium. Western blottin g was also performed with antibodies directed against GLUT1, glial fib rillary acidic protein, adaptin, IP3R-3, integrins av and collagen IV as controls to determine whether the preparations were contaminated by other membranes. Strong enrichment of GLUT1 in brain capillary lumina l membranes (9.9-fold) showed that the preparation consisted mainly of endothelial cell plasma membranes. Poor enrichment of glial fibrillar y acidic protein (1.4-fold) and adaptin (2.4-fold) and a decreased lev el of LP3R-3, integrins alpha v and collagen IV excludes the possibili ty of major contamination by astrocytes or internal and anti-luminal m embranes. High levels of P-gp in the luminal membranes of brain capill ary endothelial cells suggests that it may play an important role in l imiting the access of anti-cancer drugs to the brain.