Flow cytometry was used to measure the fluorescence polarization of the lipid probe trimethylammonium-diphenylhexatriene as an indicator of plasma membrane fluidity of Chinese hamster ovary (CHO) cells heated under various conditions. Fluorescence polarization was measured at room temperature about 25 min after heating. When cells were heated for 45 min at temperatures above 42°C, fluorescence polarization decreased progressively, signifying an increase in plasma membrane fluidity. The fluorescence polarization of cells heated at 42°C for up to 55 h was nearly the same as for unheated control populations, despite a reduction in survival. The fluorescence polarization of cells heated at 45°C decreased progressively with heating time, which indicated a progressive increase in membrane fluidity. The fluorescence polarization distributions broadened and skewed toward lower polarization values for long heating times at 45°C. Thermotolerant cells resisted changes in plasma membrane fluidity when challenged with subsequent 45°C exposures. Heated cells were sorted on the basis of their position in the fluorescence polarization distribution and plated to determine survival. The survival of cells which were subjected to various heat treatments and then sorted from high or low tails of the fluorescence polarization histograms was not significantly different. These results show that hyperthermia causes persistent changes in the membrane fluidity of CHO cells but that membrane fluidity is not directly correlated with cell survival.

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