To explore further the relationship between intracellular pH ( pH i) and thermosensitivity, we compared survival and pH i levels in RIF-1 cells and a thermoresistant variant, TR-4 cells, while heating under different conditions of acid or neutral extracellular pH ( pH e). We also added 5-(N-ethyl-N-isopropyl) amiloride (EIPA), a potent inhibitor of one of the major membrane regulators of pH i, the Na+/ H+ antiport, and/or removed NaHCO3 to inactivate the alternate membrane regulator of pH i, the${\rm HCO}_{3}^{-}/{\rm Cl}^{-}$ exchanger. At pH e 7.3 with NaHCO3, EIPA (15 μM) did not enhance the cytotoxicity of heat in either cell line. At pH e 6.8 with NaHCO3, EIPA enhanced thermal cytotoxicity for RIF-1 cells only, but without NaHCO3, at pH e 6.8 or 6.5, EIPA treatment during heating resulted in a significant decrease in survival of TR-4 cells also. Measurements of pH i levels immediately after heating correlated with the survival data, demonstrating a linear relationship between pH i and log surviving fraction for both cell lines. This relationship, however, is different between the two cell lines in that the TR-4 cells are more resistant to reduction in pH i with heating, and for any given pH i level demonstrate a higher surviving fraction compared to the RIF-1 cells. In addition, this correlation is different for the two cell lines, suggesting a difference in the relationship between pH i and log surviving fraction between the TR-4 and RIF-1 cells.

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