Intrinsic thermal response, thermotolerance development and stepdown heating in murine bone marrow progenitor cells

Thermal response, thermotolerance development and stepdown heating (SDH) in the murine bone marrow granulocyte-macrophage (CFU-GM) progenitors were determined in vitro. Marrow was removed from femora and tibia, heated in McCoy's 5A medium plus 15% FBS and cultured in soft agar in the presence of three different sources of colony stimulating factor. D0's (+/- SE) for survival curves of CFU-GM heated in vitro were 147 +/- 13, 71 +/- 9, 37 +/- 2, 19 +/- 0.7, 11 +/- 1, and 4.3 +/- 0.3 min, for temperatures of 41.8, 42, 42.3, 42.5, 43 and 44 degrees C, respectively. Arrhenius analysis showed inactivation enthalpies of 812 +/- 9 KJoules/mole (193 +/- 2 Kcal/mole) above, and 2142 +/- 157 KJoules/mole (509 +/- 37 Kcal/mole) below, an inflection at 42.5 degrees C. Thermotolerance development was evident during prolonged hyperthermia exposure at temperatures below 42.5 degrees C (chronic hyperthermia) as a change in the slope of the survival curves after approximately 110 min of heating. Thermotolerance development at 37 degrees C after exposure to temperatures of 43 degrees C or greater (acute hyperthermia) was assessed by fractionated heat treatments consisting of an initial heat treatment (15 min at 44 degrees C) followed by incubation at 37 degrees C and challenge with 15 min or 25 min at 44 degrees C. Maximum thermotolerance occurred after 210 and 330 min at 37 degrees C, respectively. The half-time for maximum thermotolerance development was 36 min. Depending on the amount of heat damage and the maximum amount of thermotolerance development, the decay of thermotolerance was complete after approximately 48-72 h at 37 degrees C. An exposure of 10 min at 44 degrees C before incubation at 40 or 41 degrees C (stepdown heating) reduced the slope of the 40 or 41 degrees C survival curves by inhibiting thermotolerance development that would have otherwise occurred. D0's were 100 +/- 19 and 45 +/- 5 min for 40 and 41 degrees C incubation preceded by 10 min at 44 degrees C, respectively. These studies indicate that whole-body or regional hyperthermia protocols designed either to treat solid tumours or to purge leukemic stem cells from marrow ex vivo should avoid inadvertent temperature elevations to large volumes of marrow. Although, marrow progenitors are capable of thermotolerance development during exposure to temperatures up to 42.3 degrees C, results suggest that conditions of stepdown heating may prevent thermotolerance development.