SELECTION FOR MDR1 P-GLYCOPROTEIN ENHANCES SWELLING-ACTIVATED K+ AND CL- CURRENTS IN NIH/3T3 CELLS

The relationship between multidrug resistance (MDR) P-glycoprotein expression and swelling activated Cl- and K+ conductance was investigated in mouse NIH/3T3 fibroblasts and their colchicine-selected counterparts (COL1000, high P-glycoprotein). Whole cell patch-clamp and isotopic flux experiments confirmed that swelling activated Cl- currents were induced by 20-30% bath dilution only in the MDR-expressing cell Line. However, at bath dilutions >30%, both cell lines developed Cl- currents that reached similar large magnitudes at higher dilution levels. Thus the apparent absolute difference in cell lines at lower dilutions is due to a shift in the response curve relating hypotonicity to Cl- conductance. At all dilutions and in both cell Lines, the swelling activated Cl- currents were outwardly rectifying, active at negative cell voltages, and inactivated at positive voltages. Verapamil (100 mu M) and 1,9-dideoxyforskolin (100 mu M), which inhibit P-glycoprotein drug transport, did not significantly inhibit the swelling-activated Cl- conductance in COL1000 cells. Unexpectedly, swelling-activated K+ efflux in the COL1000 cells also showed a leftward shift in the response curve to hypotonicity. These results indicate that colchicine-selection for increased P-glycoprotein expression did not lend to the expression of swelling-activated Cl- channels, but instead enhanced a step in the pathway from bath dilution to regulatory volume decrease that is common to both K+ and Cl- channels.