LOWER ELECTRICAL MEMBRANE-POTENTIAL AND ALTERED PH(I) HOMEOSTASIS IN MULTIDRUG-RESISTANT (MDR) CELLS - FURTHER CHARACTERIZATION OF A SERIESOF MDR CELL-LINES EXPRESSING DIFFERENT LEVELS OF P-GLYCOPROTEIN

Recently [Roepe, P. D. (1992) Biochemistry 31, 12555-12564], increased steady-state levels of chemotherapeutic drug efflux from multidrug-re sistant (MDR) myeloma cells were correlated with intracellular alkalin ization. To better understand elevated pH(i) in MDR cells, Na+- and Cl --dependent recovery of pH(i) upon intracellular acid or alkaline shoc k has been examined for this same series of MDR cell lines. In agreeme nt with another recent report [Boscoboinik, D., Gupta, R. S., & Epand, R. M. (1990) Br. J. Cancer 61, 568-572], we find that the rate of Na-induced alkalinization after an intracellular acid shock is increased in the MDR cells, relative to the drug-sensitive parent. Interestingl y, we also now find that mRNA encoding the human Na+/H+ exchanger (NHE ) is overexpressed in these MDR cells, but the level of overexpression does not correlate with the relative drug resistance or steady-state pH(i). It is also found that the efficiency of Cl--dependent reacidifi cation of pH(i) after an intracellular alkaline shock is reduced in th e MDR cells. This effect appears to correlate with the relative expres sion of MDR protein, but not the relative expression of Cl-/HCO3- exch anger (AE), which we now find is also altered in the series of cells. Since elevated pH(i) will increase DELTApH across the plasma membrane, we have also measured the electrical potential for these cells using three different methods. Most interestingly, the magnitude of the plas ma membrane electrical potential (DELTAPSI) decreases concomitant with increased expression of the MDR protein. Energy provided by increased DELTApH compensates for the lowered DELTApsi such that the total elec trochemical membrane potential (DELTAmu(H+)) remains similar among the cells in this series (DELTAmu(H+) = DELTAPSI - ZDELTApH). These data, along with other recent experiments that associated an increased Cl- conductance with the expression of MDR protein [Valverde, M., Diaz, M. , Sepulveda, F. V., Gill, D. R., Hyde, S. C., & Higgins, C. F. (1992) Nature 355, 830-833], are consistent with a model for MDR protein-medi ated multidrug resistance that does not entail direct active transport of lipophilic drugs by the MDR protein.