COMPARISONS OF P-GLYCOPROTEIN EXPRESSION IN ISOLATED RAT-BRAIN MICROVESSELS AND IN PRIMARY CULTURES OF ENDOTHELIAL-CELLS DERIVED FROM MICROVASCULATURE OF RAT-BRAIN, EPIDIDYMAL FAT PAD AND FROM AORTA

In vivo expression of P-glycoprotein in isolated rat brain microvessel s is compared with that in vitro in primary cultures of brain endothel ial cells, More P-glycoprotein is detected by Western immunoblotting i n microvessels than in cultured endothelium. RT-PCR with isoform-speci fic primers and immunoblotting with a mdr1b-specific antibody reveals only mdr1a in vivo but both mdr1a and mdr1b in vitro, Thus mdr1a decre ases whereas mdr1b increases during culture, P-Glycoprotein activity i s evident in vitro, with resistance modulators, e.g. verapamil, produc ing increases in intracellular [H-3]vincristine accumulation, Endothel ial cells cultured from epididymal fat pad microvasculature and aorta contain little or no P-glycoprotein. Here, resistance modulators are l ess effective.