Effects of thermal adaptation at 40 degrees C on membrane viscosity and the sodium-potassium pump in Chinese hamster ovary cells.

We have demonstrated increased heat resistance in Chinese hamster ovary cells grown to confluence at 37 degrees C and thermally adapted at the nonlethal temperature of 40 degrees C for 24 h. Membrane viscosity, estimated by fluorescence anisotropy, was inversely related to temperature, from 5 degrees C to 45 degrees C. For a given temperature viscosity was consistently higher in thermally adapted cells than in native cells. Having demonstrated a change in membrane structure associated with thermal adaptation, we carried out a study of the Na+-K+ pump in native and thermally adapted cells as an example of a vital active transport process known to be sensitive to membrane viscosity. 86Rb uptake measured from 31 degrees C to 50 degrees C increased steadily to 46 degrees C and then decreased rapidly in both native and thermally adapted cells. Detailed measurement of ouabain-sensitive 86Rb influx demonstrated an increase in both Km and Vmax between 37 degrees C and 45 degrees C, but there was no difference between native and thermally adapted cells. We have thus demonstrated an adaptive structural change in the cell membrane of mammalian cells which may be related to the induction of thermal resistance at 40 degrees C but which is not associated with any change in this active transport system.