Modification of Intracellular pH and Thermosensitivity

The effects of amiloride (an inhibitor of Na⁺/ H⁺ antiport), DIDS (an inhibitor of Na⁺-coupled and Na⁺-independent <tex-math>${\rm HCO}_{3}^{-}/{\rm Cl}-$</tex-math> exchange) and nigericin (<tex-math>${\rm K}^{+}/{\rm H}^{+}$</tex-math> ionophore) alone and in various combinations on the intracellular pH ( pH _i) and thermosensitivity of SCK tumor cells were studied. Hyperthermia alone at 43°C for 2 h decreased pH _i of SCK cells by 0.15-0.20 pH units, as measured fluorometrically using the pH-sensitive dye BCECF. When the cells were treated with 0.5 mM amiloride at 37°C, the pH _i declined by 0.10-0.15 pH units at an extracellular pH ( pH _e) of both 7.2 and 6.6. Amiloride at 0.5 mM enhanced the thermal damage to SCK cells at pH _e 6.6 but not at pH _e 7.2. DIDS alone at 0.1 mM exerted no effect on pH _i or cellular thermosensitivity. DIDS, however, enhanced the effects of amiloride in decreasing pH _i and in increasing the thermoresponse of SCK cells, particularly at pH _e 6.6. Treatment of the cells with nigericin at 0.1-1.0 μg/ml lowered the pH _i and enhanced the thermosensitivity of the cells in a dose-dependent manner. Reductions in pH _i and increases in thermosensitivity by nigericin at the lower concentration at pH _e 6.6 were far greater than at pH _e 7.2. When a mixture of 1.0 μg/ml nigericin, 0.5 mM amiloride, and 0.1 mM DIDS was present in the medium, the pH _i rapidly decreased by about 0.3 and 0.4 pH units at pH _e 7.2 and 6.6, respectively. This drug combination was also extremely effective in sensitizing SCK cells to heat, particularly at pH _e 6.6. The fact that the thermosensitization by these drugs at pH _e 6.6 is more pronounced than at pH _e 7.2 and that intratumor environments are known to be acidic strongly suggested that it may then be possible to enhance the thermal damage with such drugs preferentially in tumors relative to normal tissues.