Calcium transport and cellular distribution in quiescent and serum‐stimulated primary cultures of bone cells and skin fibroblasts

Primary cultures of bone cells and skin fibroblasts were examined for their Ca ++ content, intracellular distribution and Ca ++ fluxes. Kinetic analysis of 45 Ca ++ efflux curves indicated the presence of three exchangeable Ca ++ compartments which turned over at different rates: a “very fast turnover” (S 1 ), a “fast turnover” (S 2 ), and a “slow turnover” Ca ++ pool (S 3 ). S 1 was taken to represent extracellular membrane‐bound Ca ++ , S 2 represented cytosolic Ca ++ , and S 3 was taken to represent Ca ++ sequestered in some intracellular organelles, probably the mitochondria. Bone cells contained about twice the amount of Ca ++ as compared with cultured fibroblasts. Most of this extra Ca ++ was localized in the “slow turnover” intracellular Ca ++ pool (S 3 ). Serum activation caused the following changes in the amount, distribution, and fluxes of Ca ++ : (1) In both types of cells serum caused an increase in the amount of Ca ++ in the “very fast turnover” Ca ++ pool, and an increase in the rate constant of 45 Ca ++ efflux from this pool, indicating a decrease in the strength of Ca ++ binding to ligands on cell membranes. (2) In fibroblasts, serum activation also caused a marked decrease in the content of Ca ++ in the “slow turnover” Ca ++ pool (S 3 ), an increase in the rates of Ca ++ efflux from the cells to the medium, and from S 3 to S 2 , as well as a decrease in the rate of influx into S 3 . (3) In bone cells the amount of Ca ++ in S 3 remained high in “serum activated” cells, the rate of efflux from S 3 to S 2 increased, and the rate of influx into S 3 also increased. The rate of efflux from the cells to the medium did not change. The results suggest specific properties of bone cells with regard to cell Ca ++ presumably connected with their differentiation. Following serum activation we investigated the time course of changes in the amount of exchangeable Ca ++ in bone cells and fibroblasts, in parallel with measurements of 3 H‐thymidine incorporation and cell numbers. Serum activation caused a rapid decrease in the content of cell Ca ++ which was followed by a biphasic increase lasting until cell division.