Failure of tritiated thymidine incorporation into DNA to reflect the autoradiographically demonstrable calcium-induced increase in thymic lymphoblast DNA synthesis.

Increasing the extracellular calcium concentration in thymic lymphocyte suspension from 0.6 to 1.8 mM stimulated the proliferation of the lymphoblast subpopulation as measured by increases in the proportion of cells autoradiographically labeled with 3H-TdR and in mitotic activity. However it was not possible to show this increased DNA synthesis by scintillometric measurement of the amount of 3H-TdR incorporated into extracted DNA. On the other hand, calcium did raise the incorporation of 14C-formate into the thymine residues of DNA, and increased the activity of isolated thymocyte thymidylate synthetase. In contrast to the mitogenic calcium ion, a thymidylate synthetase inhibitor, methotrexate, actually increased the incorporation of 3H-TdR into DNA. It is concluded that calcium increases the endogenous synthesis of thymidylate which in turn prevents the amount of incorporation of exogenous 3H-TdR from accurately reflecting the true level of DNA synthesis.