Decay of thermal resistance following acute heating is independent of the G1- to S-phase transition.

The phenomena of heat-induced G1 delay and thermal resistance were compared in synchronous populations of CHO cells. Mildly toxic induction doses of 5 min (Single cell survival, (SCS) = 0.90 +/- 0.06) and 10 min (SCS = 0.69 +/- 0.12) at 45 degrees C resulted in G1 delays of 4.3 and 11.3 h, respectively. Thermal resistance was tested (30 min, 45 degrees C) for up to 32-92 h following the induction dose. Thermal resistance did not start to decay prior to 26 h following the induction dose. These data confirm reports by R. R. Read, M. H. Fox, and J. S. Bedford [Radiat. Res. 98, 491-505 (1984)] and G. L. Rice, J. W. Gray, P. N. Dean, and W. C. Dewey [Cancer Res. 44, 2368-2376 (1984)] that acutely heated G1 populations of CHO cells progress into S phase without a concurrent loss of thermal resistance, using 45 degrees C induction doses even less toxic than used by other workers.