DOXORUBICIN SELECTION FOR MDR1 P-GLYCOPROTEIN REDUCES SWELLING-ACTIVATED K+ AND CL- CURRENTS IN MES-SA CELLS/

To test the hypothesis that P-glycoprotein enhances swelling currents through regulation of volume-sensitive Cl- channels [recently termed V SOAC (volume-sensitive osmolyte and anion channel)], a human uterine s arcoma cell line (MES-SA) and its doxorubicin-selected counterpart (Dx 5) were studied. P-glycoprotein mRNA and protein levels were detected only in Dx5 cells. However, whole cell patch-clamp experiments showed that swollen Dx5 cells (n = 5) produced smaller VSOAC currents than ME S-SA cells (n = 4; 106 +/- 26 pA/pF vs. 232 +/- 76 pA/pF at 90 mV). In radioisotopic efflux experiments, both swelling-activated I-125 (Cl-) currents (n = 15) and Rb-86 (K+) currents (n = 8) were found to be tw o-to fourfold smaller in the Dx5 (high P-glycoprotein) cells. Inhibito rs of P-glycoprotein showed no specificity for the doxorubicin-selecte d cells (Dx5). Dideoxyforskolin (100 mu M) blocked swelling-activated I-125 efflux equally in both cell lines, whereas 100 mu M verapamil ha d no effect. Thus, in this cell line, selection for P-glycoprotein exp ression is associated with reduced swelling currents. These findings s uggest that P-glycoprotein expression does not directly facilitate VSO AC.