The role of intracellular pH and its variance in low pH sensitization of killing by hyperthermia.

Average intracellular pH (pHi) was measured in Chinese hamster ovary (CHO) and murine NG 108-15 neuroblastoma cells by the weak acid, [14C]5,5-dimethyl-2,4-oxazolidinedione-2 [( 14C]-DMO). Intercell variance in pHi in CHO cells was determined by flow cytometry (FCM) and automated image analysis using the pH-sensitive fluorescent dye, 2',7'-bis-(carboxyethyl)-5(and-6)- carboxyfluorescein. Our results are summarized as follows: (i) When extracellular pH (pHe) ranged from 6.1 to 7.9, the relationship between pHe and average pHi was similar for both cell lines, except that pHi of NG108 cells was 0.3 to 0.2 pH units higher than that of CHO cells. (ii) When both cell lines were heated at 43.5 degrees C to give an isosurvival of 0.1 at pHe 7.2, lowering pHe (6.1-6.9) reduced survival more for CHO cells than for NG108-15 cells. However, plots of survival versus average pHi were identical for the two cell lines. (iii) Values of average pHi measured by [14C]DMO agree with those measured by FCM techniques. (iv) A distribution of pHi values was obtained for a population of cells. However, when the cells were sorted on the basis of low or high values of pHi and reanalyzed, the distributions of the sorted populations were almost identical to the distribution of the original population. These results indicated that the distribution of pHi for a cell population is much more homogeneous than that observed by FCM. (v) These observations indicate that pHi at the time of heating and not pHe is responsible for the low pH sensitization of heat killing.