Concanavalin A and the initiation of thymic lymphoblast DNA synthesis and proliferation by a calcium‐dependent increase in cyclic GMP level

Exposure of a thymic lymphocyte population (suspended in serum‐free synthetic medium) to the phytomitogen concanavalin A (Con A) causes brief (within the first 8 to 12 minutes) rises in the cellular contents of cyclic AMP and cyclic GMP. However, the rise in the cyclic GMP level is calcium (extracellular)‐dependent, but the cyclic AMP rise is not. These changes are followed during the next hour by the initiation of DNA synthesis by a large fraction of the lymphoblast subpopulation which, like the preceding cyclic GMP rise, is calcium‐dependent. The stimulated lymphoblasts eventually progress into mitosis. Additional observations indicate that Con A operates by sensitizing lymphoblasts to calcium ions which, in turn, cause the initiation of DNA synthesis by a process mediated by cyclic GMP, but not cyclic AMP.