Killing of Hypoxic Cells by Lowering the Intracellular pH in Combination with Hyperthermia

The acidic intracellular environment or low intracellular pH ( pH _i) increases the thermosensitivity of mammalian cells. The cells in a hypoxic environment produce a greater amount of acidic metabolites than those in an oxygenated environment. However, the hypoxic cells are not more thermosensitive than oxygenated cells since the decrease in pH _i is minimized by the mechanisms that regulate the pH _i such as Na⁺/ H⁺ antiport. We hypothesized, therefore, that blocking the regulation of pH _i might greatly reduce the pH _i and increase the thermosensitivity of hypoxic cells. We tested our hypothesis by heating SCK tumor cells under oxygenated and hypoxic conditions in pH 7.5 or 6.6 medium with or without amiloride, an inhibitor of Na⁺/ H⁺ antiport. We observed that amiloride increased the thermosensitivity of hypoxic cells markedly. Such a thermosensitization of hypoxic cells by amiloride was more pronounced in an acidic environment, with an enhancement ratio of 1.8, than in a neutral environment, with an enhancement ratio of 1.5. We concluded that lowering the pH _i by blocking the regulation of pH _i in combination with hyperthermia may be a useful way to eliminate the radioresistant hypoxic cells.