DRUG EFFLUX MEDIATED BY THE HUMAN MULTIDRUG-RESISTANCE P-GLYCOPROTEINIS INHIBITED BY CELL SWELLING

P-glycoprotein (P-gp), the product of the human multidrug resistance ( MDR1) gene, confers multidrug resistance on cells by acting as an ATP- dependent drug transporter, A method using confocal microscopy was dev eloped to measure the transport activity of P-gp from the rate of move ment of doxorubicin, a fluorescent substrate of P-gp, across the membr ane of a single cell, Recent work has shown that expression of P-gp en hances the activation of chloride channels in response to cell swellin g, suggesting that membrane stretch might switch P-gp from a drug-tran sporting mode to a mode in which it activates chloride channels, In ag reement with this idea, we find that cell swelling inhibits drug efflu x in cells expressing P-gp but is without effect on the slower backgro und efflux in cells not expressing P-gp and in cells transiently trans fected with a mutated MDR1 in which the ATP hydrolysis sites had been inactivated. The identification of a novel means for inhibiting P-gp-m ediated drug transport may have implications for the reversal of multi drug resistance during chemotherapy.