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 microvessels is compared with that in vitro in primary cultures of brain endothelial cells. More P-glycoprotein is detected by Western immunoblotting in microvessels than in cultured endothelium. RT-PCR with isoform-specific primers and immunoblotting with a mdr1b-specific antibody reveals only mdr1a in vivo but both mdr1a and mdr1b in vitro. Thus mdr1a decreases whereas mdr1b increases during culture. P-Glycoprotein activity is evident in vitro, with resistance modulators, e.g. verapamil, producing increases in intracellular [3H]vincristine accumulation. Endothelial cells cultured from epididymal fat pad microvasculature and aorta contain little or no P-glycoprotein. Here, resistance modulators are less effective.