THE MULTIDRUG-RESISTANCE MODIFIERS VERAPAMIL, CYCLOSPORINE-A AND TAMOXIFEN INDUCE AN INTRACELLULAR ACIDIFICATION IN COLON-CARCINOMA CELL-LINES IN-VITRO

In this study we have investigated the effects of the multidrug-resist ance (MDR) modifiers verapamil (VPM), cyclosporin A (CsA) and tamoxife n (TMX) an the intracellular pH(pH(i)) of four colon carcinoma-derived cell lines with low P-glycoprotein expression (CaCo-2, HT-29, SW 620 and SW 480). Addition of VPM (1 mu M), CsA (1 mu g/ml) or TMX (2 mu M) in HEPES- or bicarbonate/CO2-buffered Ringer's solution was followed by dose-dependent and reversible decreases of the pH(i) (0.1-0.3 units ) of all cell lines, as measured ratiometrically by the changes in the pH-dependent fluorescence of bis(carboxyethyl)car boxyfluorescein (BC ECF). Testing the effects of the resistance modifiers on the Na+/H+ an tiporter and bicarbonate trans-porters under appropriate buffer condit ions and addition of inhibitors (amiloride, DIDS) revealed that the ch e,momodulator-induced acidification does not interfere with the functi on of these major pH(i)-regulating acid-base transporters. The inducti on of changes in pH(i) shows no correlation with MDR-reversing activit y of the drugs and our data do not support the P-gp-inhibition-mediate d accumulation of acidic substrates as underlying mecha- nism. In addi tion to the P-gp-directed MDR-reversal, chemomodulator -induced intrac ellular acidification may enhance the chemosensitivity of the cells es pecially under alkaline extracellular conditions and contribute to the decreased efficacy of MDR-modifiers in acidic extracellular environme nts and to the chemosensitising effect of VPM in P-gp-negative cell li nes.