The relationship between intracellular pH and heat sensitivity in a thermoresistant cell line

To explore further the relationship between intracellular pH (pHi) and thermosensitivity, we compared survival and pHi levels in RIF-1 cells and a thermoresistant variant, TR-4 cells, while heating under different conditions of acid or neutral extracellular pH (pHe). We also added 5-(N-ethyl-N-isopropyl) amiloride (EIPA), a potent inhibitor of one of the major membrane regulators of pHi, the Na+/H+ antiport, and/or removed NaHCO3 to inactivate the alternate membrane regulator of pHi, the HCO3-/Cl- exchanger. At pHe 7.3 with NaHCO3, EIPA (15 microM) did not enhance the cytotoxicity of heat in either cell line. At pHe 6.8 with NaHCO3, EIPA enhanced thermal cytotoxicity for RIF-1 cells only, but without NaHCO3, at pHe 6.8 or 6.5, EIPA treatment during heating resulted in a significant decrease in survival of TR-4 cells also. Measurements of pHi levels immediately after heating correlated with the survival data, demonstrating a linear relationship between pHi 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 pHi with heating, and for any given pHi level to demonstrate a higher correlation is different for the two cell lines, suggesting a difference in the relationship between pHi and log surviving fraction between the TR-4 and RIF-1 cells.