Sensitization to Hyperthermia (45°C) of Normal and Thermotolerant CHO Cells by Anisotonic Media

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Abstract
Asynchronous CHO [Chinese hamster ovary] cells heated in the presence of hypertonic medium (600 mosM [milliosmolar]) showed an increased sensitivity (lowered D0 [mean lethal dose]) to 45.degree. C hyperthermia. The extent varied as a function of heating time with respect to the hypertonic shock. Heating 1 min after hypertonic shock reduced the D0 from 4.4 .+-. 0.4 to 2.1 .+-. 0.2 min and the extrapolation number, n, from 5.9 to 5.2. Hyperthermia initiated 30 min after the hypertonic shock did not reduce the D0 significantly but did reduce n to 1.7. Hypotonic medium (150 mosM) reduced n without significantly reducing the D0. In contrast to hypertonic medium, this effect was independent of the temporal relationship between the osmotic shock and heating. Sensitization of non-thermotolerant cells by hypotonic shock was possible only when cells were heated immediately upon return to isotonic medium following a 1-h equilibration in hypotonic medium. Thermotolerant CHO cells were sensitized slightly more than non-thermotolerant cells (a factor of 2.2 vs. 1.7) when heated immediately after the hypertonic shock. Heat conditioning in the presence of hypertonic medium sensitized the cells but not as dramatically as when thermotolerant cells were reheated in the presence of hypertonic medium (thermotolerant control D0 = 16.8 .+-. 1.3 min vs. 11.4 .+-. 2.4 and 7.8 .+-. 0.2, respectively, for the hypertonic studies). In contrast to the studies with non-thermotolerant cells, thermotolerant cells were sensitized when heated immediately after the hypotonic shock (D0 = 10.7 .+-. 1.6 min). Incubating the heat-conditioned cells in hypertonic medium impaired the development of thermotolerance. Anisotonic-media reduction of the extrapolation number apparently is associated with osmotically induced membrane stress. The difference in sensitization between thermotolerant and non-thermotolerant cells is apparently one of quantity and not quality. In contrast to bacterial data, these mammalian data do not suggest a possible role for the altered intracellular ion concentrations in the development of thermotolerance.