Time Dependency of a Critical Effect of EGTA on DNA Synthesis in Cultures of Swiss 3T3 Cells

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Abstract
In a temporal analysis of the mitogenic response to serum, a critical period has been demonstrated just prior to the onset of replicative DNA synthesis during which transient calcium depletion blocks the subsequent entry of the cells into the S phase of the mitotic cycle. Transient washing of monolayer cultures of 3T3 cells with 2.5 mM EGTA between 6 and 8 h after serum-stimulated initiation of DNA synthesis was found to reduce cell-associated calcium levels and to inhibit thymidine incorporation, whereas similar treatment before (1–5 h) and after (8–9 h) had no detectable effect on either of these parameters when estimated after 21 h incubation. The effects during the chelation-sensitive period were reversed by the subsequent addition of fresh serum.