Calcium‐dependent stimulation by a phorbol ester (PMA) of thymic lymphoblast DNA synthesis and proliferation

PMA (phorbol myristate acetate, i.e., 12‐0‐tetradecanoyl‐phorbol‐13‐acetate) a tumor‐promoting ester from croton oil, at its most effective concentration of 0.05 μg per milliliter, rapidly (within one hour) induces a large fraction of the lymphoblasts in suspended thymic lymphocyte populations to start making DNA, and these stimulated cells later progress into mitosis. This stimulatory PMA action is probably mediated by calcium because it disappears when calcium is omitted from the medium, and PMA strikingly increases the sensitivity of the lymphoblasts to calcium's stimulatory action.