Effects of depletion of K + , Na + , or Ca 2+ on DNA synthesis and cell cation content in chick embryo fibroblasts

Decreasing the K + concentration of the medium from 5 mM to 0.59 mM decreased the K + content of chick embryo fibroblasts to 22% of control values and increased the Na + content to 820% of control values. The alteration of monovalent cation content occurred within two hours but had no effect on the rate of DNA synthesis, as measured by 3 H‐thymidine incorporation, for at least 16 hours. By decreasing the Na + concentration in the medium, a 50% reduction in cellular Na + could be obtained with no effect on thymidine incorporation. Since these changes in cellular Na + and K + are much larger than any known to occur under physiological conditions but have no effect on thymidine incorporation, we conclude that Na + and K + do not play a critical role in determining multiplication rate. Addition of 1.8 mM EGTA to cells in media containing 1.7 mM Ca 2+ and 0.8 mM Mg 2+ inhibited thymidine incorporation and sharply decreased cellular K + and increased cellular Na + content. However, there was no reduction in total cellular Ca 2+ levels. Likewise, decreasing the Ca 2+ concentration of the medium below 0.01 mM inhibited thymidine incorporation, decreased cellular K + and Mg 2+ , and increased cellular Na + but did not affect total cellular Ca 2+ levels. Inhibition of DNA synthesis, therefore, could not be correlated with changes in cellular Ca 2+ levels.