Hyperthermia induces transient changes in intracellular free sodium levels and membrane potential. The possible role of these changes in cell killing by hyperthermia and thermotolerance has been evaluated using Chinese hamster ovary IS1 and HeLa cells. Intracellular sodium was measured with Sodium Green and SBFI, while membrane potential was measured with the oxonol dye <tex-math>${\rm diBAC}_{4}$</tex-math> (3). Heating at either 42.0 or 45.0°C caused nearly the same decrease in free <tex-math>$[{\rm Na}^{+}]_{{\rm i}}$</tex-math> from about 20 mM in unheated cells to 5-7 mM in heated cells. However, survival differed by over two orders of magnitude after heating for 30 min at these two temperatures. In addition, blockage of the heat-induced decrease in <tex-math>$[{\rm Na}^{+}]_{{\rm i}}$</tex-math> using ouabain and/or amiloride did not affect the survival curves for heated cells. Hyperthermia also induced a membrane hyperpolarization of 15 mV after 15 min at 42.0°C or 35 mV after 15 min at 45.0°C which could be blocked with ouabain and amiloride. Both the free <tex-math>$[{\rm Na}^{+}]_{{\rm i}}$</tex-math> and membrane potential recovered to near baseline levels within 30-40 min after heating. Induction of thermotolerance using a 45.0°C, 10-min heat treatment also was not affected by ouabain and/or amiloride. Finally, thermotolerant cells experienced the same heat-induced changes in free <tex-math>$[{\rm Na}^{+}]_{{\rm i}}$</tex-math> and membrane potential as non-thermotolerant cells. We conclude that the heat-induced changes in free <tex-math>$[{\rm Na}^{+}]_{{\rm i}}$</tex-math> and membrane potential are not directly related to cell killing by hyperthermia or thermotolerance.

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