Induction of thermotolerance by multiple heat fractions in Chinese hamster ovary cells.

A heat-conditioning treatment of 10 mm at 45°followed by 370 incubation resulted in maximal thermotolerance 8 hr later. The D@of the heat survival curve increased from 4.5 mm (control) to 19.8 mm at 45°.Two heat-conditioning treatments (10 mm at 45°+ 4 hr at 37°+ 15 mm at 45°)resulted in a higher thermotolerance maximum (D@= 26.7 mm) 8 hr after the second • conditioning treatment. In comparison, a single conditioning treatment of 25 mm at 45°increased thermotoler ance by a factor of approximately 6 to a D0 of 32 mm at 45°, 24 hr after heat conditioning. Multiple, equal heat treatments of either 10 or 20 mm at 45°separated by intervals of 2 or 8 hr resulted in biphasic survival curves. The resistant phase was characterized by a D0 of 32 mm, and this was independent of both the size of the heat fractions and the fractionation interval. However, both the fraction of the total cell population which was rendered tolerant and the time required for the develop ment of this maximal thermotolerance were a function of the fraction size and the fractionation interval. Multiple heating with 24-hr intervals separating treatments of either 10 or 20 mm at 45°resulted in a telatively constant level of thermotolerance. This level was predicted from single conditioning treatments and was a function of both the size of the heat fraction and the fractionation interval as long as the conditioning treatments remained constant. However, increasing the size of the heat conditioning fractions from 10 to 20 mm at 45°in the course of multiple fractionation also increased the level of thermo tolerance. These data suggest that an absolute maximum in thermotolerance can be expected in Chinese hamster ovary cells with a D@of approximately 30 mm at 45°and that the kinetics and the magnitude of thermotolerance, developed after a single conditioning treatment, can be defined in simple math ematical relationships.