Heat sensitization of G1and S phase cells by procaine HCl. II: toxicity and probability of dividing following treatment

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Abstract
Extended studies of the procaine sensitization of synchronous populations of CHO cells heated in monolayers were performed to help explain the mechanism for this sensitization. Cells were treated with 43.0, 45.5 and 43.0.degree. C + procaine, clonal survival of cells was determined, and time-lapse cinematography was used to monitor the time-dependent morphological changes and division delay of the various populations for up to 72 h following treatment. Procaine sensitized G1 and S phase populations to killing by 43.0.degree. C with thermal enhancement ratios of 7.4 .+-. 0.5 and 7.9 .+-. 1.1 (mean .+-. SE), respectively. The division delays (39.2 .+-. 3.5 h) of G1 cells exposed to 45.5 or 43.0.degree. C .+-. procaine were longer than those (21.9 .+-. 2.3 h) of S phase cells treated to comparable surviving fractions (SF). In these experiments the SF of the G1 populations was 0.50 .+-. 0.17 versus 0.33 .+-. 0.08 for the S phase populations. G1 populations heated with 43.0.degree. C alone (SF, 0.34 .+-. 0.05) had division delays of 19.4 .+-. 2.4 h. Colony formation correlated well with both G1 and S phase cells that had regular divisions subsequent to treatments; the heat-induced death of G1 and S phase cells correlated with interphase lysis and irregular divisions (including bipolar divisions yielding micronucleated progeny). Deaths due to both categories of lesions are increased by treatment with procaine; however, death due to interphase lysis is enhanced more by procaine. In summary, procaine HC1 sensitized G1 and S phase cells comparably to killing by 43.0.degree. C.