Arrhenius analysis of single-heated and step-down heated V79 and L cellsin vitro

The response to single heat treatment and Step-down heat (SDH) treatmentin vitro of V₇₉ and L cells was studied. Colony-forming ability was assayed in medium after treatmentin vitro. Time-response curves were established and subjected to Arrhenius analysis. The Arrhenius curves showed inflection points at 43° C for V₇₉ cells and at 42° C for L cells. The activation energies were 145 kcal/mole and 400 kcal/mole above and below 43° C (P<0.05), respectively, for V₇₉ cells, while 160 kcal/mole and 300 kcal/mole above and below 42° C (P<0.05), respectively, for L cells. Thermosensitivity of L cells are markedly higher than V₇₉ cells. Both V₇₉ and L cells were sensitized by SDH. The SDH effect was characterized by a reduction in shoulder (an addition effect to sublethal damage), an increase in slope (thermosensitization), and the delay and disappearance of thermotolerant “tail” for V₇₉ and L cells at 45° C to 40° C and 44° C to 42° C SDH treatment respectively. Particularly, 42° C to 39° C or 42° C to 40° C SDH for L cells resulted in thermosensitization effect up to a factor of 7.1 or 2.7, respectively. The effect was quantified by thermorsensitization ratio (TSR), defined as T₀ single heated/T₀SDH-heated. The relative ratio was much higher for V₇₉ than for L cells. Heat killing with SDH characterized by Arrhenius analysis showed that Step-down heating reduced the activation energy for heat killing more than single heating. The decrease of activation energy for L cells was markedly greater than for V₇₉. These data suggest that greater cellular sensitivity under step-down heating conditions may reflect a different mechanism for cell killing.