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 transport function and levels of P-glycoprotein (PgP) expression were compared between two human chronic erythroleukemia cell lines: a parental (K562) cell line and a derivative obtained by vinblastine selection (K562 VBL400). Parental K562 cells showed no detectable P-glycoprotein expression, measured at the protein level (immunofluorescence labeling with monoclonal antibodies), and had very low levels of MDR-1 mRNA expression (RT-PCR analysis), when compared with levels measured in K562 VBL400. Differences in Pgp-mediated transport were estimated by comparing 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 sensitive to verapamil but was not affected by the tonicity of the extracellular medium. In contrast to the clear differences in transport function, the characteristics of chloride currents induced by cell swelling were indistinguishable between the two cell lines. Currents measured in the whole-cell configuration were outwardly rectifying, had a higher permeability to iodide than to chloride (SCN- > I- > Cl- > gluconate), were potently blocked by NPPB and were unresponsive to verapamil. The percentage of responding cells and the mean current density were nearly identical in both cell lines. In addition, activation of the volume-sensitive current was not prevented during whole-cell recordings obtained with pipettes containing high concentration of cytotoxic drugs (vincristine or vinblastine). These results do not lend support to the previously reported association between Pgp expression and volume-sensitive chloride channels, and suggest that a different protein is responsible for this type of chloride channel in K562 cells.