The relationship between thermosensitivity and intracellular pH in cells deficient in Na+/H+ antiport function

Background and purpose: We have demonstrated previously the relationship between intracellular pH (pHi) level and heat survival in mammalian cells. To explore this in further detail, we studied thermosensitivity in CCL 39 and their variant PS120 cells, which lack Na+/H+ antiport function. Materials and methods: CCL39 and PS120 cells were heated with or without amiloride, or 5-(N-ethyl(-N-isopropyl) amiloride (EIPA), inhibitors of Na+/H+ antiport function. Antiport activity and pHi measurements were made using the fluorescent dye 2.7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). A clonogenic assay was used to assess survival after heating. Results: Enhanced cytotoxicity was observed when CCL39 cells were heated with either EIPA (15 mu M) or amiloride (2.5 mM) at pH(e)7.3 in the presence of NaHCO3. Under the same conditions, thermal enhancement of PS120 cells was observed only with amiloride at 2.5 mM. When the cells were heated at pH(e) 6.5 in bicarbonate-free medium, both EIPA and amiloride enhanced thermal cytotoxicity in CCL39 cells, but only the higher dose of amiloride sensitized the variant PS120 cells. Surviving fraction was related to pH,, but the data fell into two clusters, depending on whether or not both Na+/H+ antiport and the Na+-dependent HCO3-/Cl- exchangers were functioning. Conclusions: We confirm that Na+/H+ antiport function can mediate thermosensitivity, and corroborate a linear correlation between pHi level and log survival after heating, but suggest that this relationship is complicated by other factors such as membrane exchanger function, and extracellular pH levels during heating.