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

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Abstract
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.

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