Specificity of the requirements for magnesium and calcium in the growth and metabolism of chick embryo fibroblasts

The rate of entry of chick embryo fibroblasts (CEF) into the S‐period of the cell cycle is reduced by lowering the external supply of Mg 2+ below 0.2 mM. This slowdown, which is measured by the rate of incorporation of 3 H thymidine into DNA, can be largely reversed by doubling or tripling the concentration of Ca 2+ in the medium, normally 1.7 mM. The Ca 2+ ‐induced stimulation is shown not to depend on contaminating traces of Mg 2+ in the added Ca 2+ . The increase in cell number in the Ca 2+ ‐stimulated cultures is delayed, possibly due to cell detachment. The effect of Ca 2+ on thymidine incorporation can be simulated almost quantitatively by Sr 2+ . Ba 2+ does not produce the effect, nor do any of the other cations tested. As little as 0.2 mM Mg 2+ produces a full stimulation of thymidine incorporation in the absence of added Ca 2+ , and no substitute was found that is effective in the same concentration range. In short term experiments, i.e., 16 hours, even 5.0 mM Ca 2+ cannot stimulate thymidine incorporation to the extent achieved with 0.2 mM Mg 2+ . Large amounts of Ca 2+ or Sr 2+ can accelerate the uptake of 2‐dGlc in Mg 2+ ‐deprived cultures, but they are much less efficient than Mg 2+ in this regard also. It is suggested that Mg 2+ is the direct intracellular effector in controlling the diverse reactions of the coordinate response, and that Ca 2+ and Sr 2+ act indirectly by making Mg 2+ available to participate in these reactions.