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L5178Y cells were cultured in vitro at various temperatures. When the cells were in the exponential growth phase, the cells were in the "steady state of growth," i.e., the fraction of cells in the G1, S, G2, and M stages and the durations of each stage were constant. The life cycle analysis of the cells in the steady state of growth demonstrated that the G1 stage and the S stage were affected the most by variation of temperature, and suggested that these two stages have considerable influence on the growth rate of the L5178Y cells. The calculated activation energies were positive in each stage of the life cycle, whereas the entropies of activation were negative throughout. The possible significance of these findings in our search for the regulatory mechanisms of cell growth is discussed.Although the effect of temperature on growth rate has been studied in detail in bacteria for many years (15,16,19,36), relatively little work of this nature has been carried out with mammalian cells. If combined with modern techniques of life cycle analysis, such studies in mammalian cells might shed light on the mechanisms regulating cell growth. For example, temperature change could modify cell growth in one of the following ways: (a) the rate of progress through all four stages of the life cycle (G1, S, G2, and M (14, 18)) could be altered to the same degree; (b) the rate of progress through only one or two stages could be affected; and (c) the rates through all four stages could be modified to different degrees resulting in a situation somewhat between (a) and (b). By pinpointing the exact stages in the life cycle where change in temperature or other environmental factors affect the growth rate, it might be possible to identify molecular events occurring simultaneously as the mechanisms involved in the regulation of growth rate.To date, the limited observations of the effect of temperature on cultured mammalian cells are contradictory. In human amnion cells, Sisken (37,38) reported that the main effect of temperature on growth rate is to change the rate of cell passage through the G1 stage. In contrast, Rao and Engelberg (30) observed that temperature change affects the growth rate of HeLa $3 cells by modifying the rate of progress through G1, S, and G2 stages to a similar degree. Although Paul (24) studied the effect of temperature on the growth rate of L5178Y cells, no experimental work combining temperature effect and life cycle analysis has been reported in this cell line.In the present paper, mouse leukemic cells (L5178Y) were incubated at various temperatures and life cycle analyses of cells growing in the exponential growth phase were carried out. The thermodynamics of the rates of progress through the four stages of the life cycle were considered to determine the "energetics" involved in cell growth. An attempt was made to relate these observations to growth regulatory mechanisms.

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The addition of alcohols (methanol, ethanol, t-butanol, ethylene glycol, and glycerol), SH compounds (cysteamine, cysteine and mercaptoethanol) and cystamine protected DNA molecules of mammalian cells from radiation-induced single-strand scissions . The protection afforded with these scavengers increased as their concentrations were increased, but always only up to a certain maximum . The maximum protection was the same for all the alcohols and cystamine, but another maximum was found for all the SH compounds . The extent of radiation-induced single-strand breaks can therefore be grouped into protectable and non-protectable fractions .In the protectable fraction, there is a linear relationship between the scavenger concentrations for half maximum protection and the reaction rate constants for the OH radical reacting with the scavengers . No such linear relationship was demonstrated with rate constants of H and e aq -. This leads us to the following suggestions : (1) that the protectable fraction is mostly of indirect action ; (2) that the OH radical plays a major role in radiation-induced single-strand breaks of DNA in cultured mammalian cells ; (3) that a major protection mechanism in indirect action is radical scavenging (or a competitive reaction) ; and (4) that the protection mechanism by SH compounds may consist of radical scavenging as well as some other type of reaction(s) resulting in radiation protection .