DRUG-TRANSPORT AND VOLUME-ACTIVATED CHLORIDE CHANNEL FUNCTIONS IN HUMAN ERYTHROLEUKEMIA-CELLS - RELATION TO EXPRESSION LEVEL OF P-GLYCOPROTEIN

The characteristics of volume-activated chloride currents, drug transp ort function and levels of P-glycoprotein (PgP) expression were compar ed between two human chronic erythroleukemia cell lines: a parental (K 562) cell line and a derivative obtained by vinblastine selection (K56 2 VBL400). Parental K562 cells showed no detectable P-glycoprotein exp ression, measured at the protein level (immunofluorescence labeling wi th monoclonal antibodies), and had very low levels of MDR-1 mRNA expre ssion (RT-PCR analysis), when compared with levels measured in K562 VB L400. Differences in Pgp-mediated transport were estimated by comparin g the rates of Fluo3 accumulation. The higher drug-transport function of K562 VBL400 cells (e.g., lower Fluo3 accumulation) correlated with their elevated levels of MDR-1. The rate of dye transport was sensitiv e to verapamil but was not affected by the tonicity of the extracellul ar medium. In contrast to the clear differences in transport function, the characteristics of chloride currents induced by cell swelling wer e indistinguishable between the two cell lines. Currents measured in t he whole-cell configuration were outwardly rectifying, had a higher pe rmeability to iodide than to chloride (SCN- > I- > Cl- > gluconate), w ere potently blocked by NPPB and were unresponsive to verapamil, The p ercentage of responding cells and the mean current density were nearly identical in both cell lines. In addition, activation of the volume-s ensitive current was not prevented during whole-cell recordings obtain ed with pipettes containing high concentration of cytotoxic drugs (vin cristine or vinblastine). These results do not lend support to the pre viously reported association between Pgp expression and volume-sensiti ve chloride channels, and suggest that a different protein is responsi ble for this type of chloride channel in K562 cells.