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 microM) or amiloride (2.5 mM) at pHe7.3 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 pHe 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 pHi, 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.

A stochastic model of hormesis

In order to describe the life-prolonging effect of some agents that are harmful at higher doses, ionizing radiations in particular, a stochastic model is developed in terms of accumulation and progression of intracellular lesions caused by the environment and by the agent itself. The processes of lesion repair, operating at the molecular and cellular level, are assumed to be responsible for this hormesis effect within the framework of the proposed model. Properties of lifetime distributions, derived for analysis of animal experiments with prolonged and acute irradiation, are given special attention. The model provides efficient means of interpreting experimental findings, as evidenced by its application to analysis of some published data on the hormetic effects of prolonged irradiation and of procaine on animal longevity.

Changes in intracellular free calcium during hyperthermia: effects of local anesthetics and induction of thermotolerance

We wished to determine if local anesthetics (LAs) induced changes in intracellular free calcium ([fCa2+i]) that could have an effect on cell killing by hyperthermia. Flow cytometry was used to measure [fCa2+i] of mouse NIH 3T3 cells during heating at 45.5 degrees C. In both non-tolerant and thermotolerant cells, heating caused a rapid increase (within 1 min) in [fCa2+i] of approximately 100 nM, which remained relatively constant during 25 min of continued heating; however, survival was higher in thermotolerant cells. Procaine, lidocaine, and tetracaine had no effect on survival or [fCa2+i] of cells kept at 37 degrees C up to 25 min. Cells heated with procaine and lidocaine showed no difference in [fCa2+i] compared to cells heated without LAs but were greatly sensitized to killing. Cells heated with tetracaine became permeable to trypan blue within 10-15 min of heating. We conclude that heat sensitization by LAs does not involve changes in [fCa2+i]. Furthermore, these studies reject the hypothesis that changes in [fCa2+i] are involved in heat-induced cell killing.

Effects of hyperthermia and membrane-active compounds or low pH on the membrane fluidity of Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells heated in the presence of the membrane-active agents procaine, ethanol, butylated hydroxytoluene (BHT) and glycerol were analysed for changes in fluorescence polarization of the lipid probe (1-[4(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene, an indicator of plasma membrane fluidity). Cells were heated in normal and acid (pH 6.6) medium. Procaine, ethanol, BHT and low pH sensitized cells to heat-killing. Procaine, ethanol and BHT decreased fluorescence polarization (increased plasma membrane fluidity) significantly. Polarization distributions for cells heated with these sensitizers were broadened substantially and were skewed toward lower polarization values. Glycerol, a heat-protector, inhibited changes in fluorescence polarization due to heating at temperatures up to 45.0 degrees C. Heating cells at either 42 or 45 degrees C in pH 6.6 medium had no significant effect on the fluorescence polarization compared with controls, while survival was reduced substantially. Thus, heat-sensitization of low pH cannot be ascribed to changes in membrane fluidity. The changes in membrane fluidity caused by other sensitizers and protector indicate that membrane fluidity changes may be a contributing cause of cell killing by hyperthermia.