Discussion: session 1. Detection of health effects of exposure to low doses of agents—scientific constraints*

-The postulate that low intracellular pH acts as a preconditioner for the destructive effects of hyperthermia (42°C) was examined, using a heat-sensitive line of malignant cells derived from rat mammary gland (SDB). Intracellular pH (pHi) was measured indirectly, from the distribution of the weak, non-metabolizable organic acid 5,5-dimethyl-2,4-oxazolidinedione (DMO) between intraand extracellular water. Respiration, aerobic and anaerobic glycolysis of the cells were studied at normal pHi (pH 7.0-7.4) or at low pHi (pH 6.2-6.6) and at 38°C or 42°C over 6 h in Warburg manometers; the ability of the cells to replicate in culture was examined after 3 h or 6 h incubation in the flasks. The relationship between pHi and extracellular pH (pHe) depended upon the buffer system used and the exact pH in question; no assumption regarding pHi based only on pHe measurement could be made. At 38°C and low pHi, the Pasteur effect became negative due to a relatively greater inhibition of anaerobic than aerobic glycolysis. Respiration was unaffected and cell replicative ability unimpaired. At 42°C and normal pHi, respiration was totally inhibited after 4 h and the Pasteur effect was decreased, in this case due to a compensatory increase in aerobic glycolysis without alteration in anaerobic CO2 production. Low pHi in the presence of hyperthermia enabled cell respiration to continue at a reduced level with no further change in glycolysis. There was delayed cell replication after 3 h at 42°C and inability to multiply following 6 h hyperthermia: low pHi did not influence these results. It is concluded that with these cancer cells, pHi values maintained in the region of 1.0 pH unit below normal for 6 h had no deleterious effect on the cells. No sensitizing effect of the low pHi for the destructive effect of hyperthermia on the cells was observed.