Abstract
The intracellular Ca^2+^ content of nontransformed Balb/c3T3 cells is two to three times higher than that of a spontaneously transformed derivative. Depriving either cell type of extracellular Mg^2+^ causes a 2โ to 3โfold increase in their Ca^2+^ content over a 24โhr period. Restoring Mg^2+^ to the medium decreases the Ca^2+^ content of the cells to their original values in about the same time. The increase in Ca^2+^ content is not blocked by cycloheximide suggesting that normal rates of protein synthesis are not required to produce this effect. Mg^2+^ deprivation also decreases the initial rate of Ca^2+^ efflux from the transformed cells and increases the size of the slowly exchanging fraction of Ca^2+^ to the levels found in the nontransformed cells. Since Mg^2+^ deprivation normalizes the appearance and growth behavior of the transformed cells, the possible intermediary role of Ca^2+^ in this normalization was studied. Large changes in extracellular Ca^2+^ produced large changes in the Ca^2+^ content of the transformed cells with little change in appearance or thymidine incorporation rate. Ca^2+^ deprivation did inhibit thymidine incorporation in early passage nontransformed cells; however with repeated passage, this effect decreased, as did the Ca^2+^ content of these cells. The possible role of Mg^2+^ in regulating cellular Ca^2+^ content and distribution is discussed, as is the relation of Ca^2+^ content and distribution to the development of the transformed state.