P-glycoprotein expression in brain capillary endothelial cells after focal ischaemia in the rat.

P-glycoprotein (P-gp) is expressed not only in tumour cells but also in some normal tissues including brain capillaries. We investigated whether or not P-gp was expressed in the capillary endothelial cells of a rat focal ischaemic brain. The brains were immunohistochemically studied for Factor VIII, glial fibrillary acidic protein (GFAP), and P-gp. Endothelial gamma-glutamyl transpeptidase (gamma-GTP) activity, which is thought to be induced by glial cells, was also studied histochemically. The P-gp positive endothelial cells disappeared in the ischaemic lesion by post-ischaemic day 3. Factor VIII-positive regenerating capillaries were first observed on day 3 without P-gp expression. The P-gp positive endothelial cells began to reappear on day 5, and were detected in all the endothelial cells by day 8. The P-gp expression in endothelial cells showed a similar pattern as that of gamma-GTP, and seemed to correlate with GFAP-positive reactive astrocytes. The newly-formed brain capillaries thus appeared to have a potential to express P-gp in abnormal pathogenic conditions as cerebral infarction, and our present study also suggested that P-gp in the brain capillaries might therefore be expressed in conjunction with glial cells.