Effects of Amiloride on Hyperthermic Cell Killing of Normal and Thermotolerant Mouse Fibroblast LM Cells

The effect of amiloride on hyperthermic cell killing of normal and thermotolerant mouse fibroblast LM cells was investigated under normal (pH 7·4) and acidic (pH 6·8) conditions. Amiloride is known to inhibit the Na+/H+ exchanger in the plasma membrane, the main pH regulating mechanism in mammalian cells. The effects of low pH and amiloride on the mouse fibroblasts were qualitatively similar. For normal cells, mainly a reduction of the shoulder of the survival curve was observed, while an increase of the slope of the exponential part of the survival curve was found in thermotolerant cells. When a combination of 3 mmol dm−3 amiloride and low pH was used the effect on the hyperthermic sensitivity of normal and thermotolerant cells was not additive. This may be explained by a similarity in the mechanism of action of the two treatments, viz. inhibition of the Na+/H+ exchange, which is probably complete when 3 mmol dm−3 amiloride is used. The amiloride sensitivity of normal and thermotolerant fibroblasts is dose dependent in the range of 0·1 to 3 mmol dm−3. Because the D0 of control cells is almost independent of the amount of amiloride, a concentration-dependent reduction of the thermotolerance ratio is found, especially at higher concentrations of amilorid