Calcium‐dependent activation of lymphocytes by ionophore, A23187, and a phorbol ester tumor promoter

The phorbol ester tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) and the calcium ionophore, A23187, have similar effects on many different cells. For example, both show mitogenic and comitogenic activities for lymphocytes. It had been suggested that some of TPA's effects are due to its ability to act as a calcium ionophore. In order to test this idea, we compared the ability of TPA and ionophore to synergize with concanavalin A (Con A) in a two‐phase system of lymphocyte mitogenesis. We found that ionophore was most comitogenic with Con A when present in the early phase of stimulation. TPA was only comitogenic when present in the late phase, lonophore and TPA could not replace one another in the system. However, both ionophore and TPA together could replace Con A and stimulate DNA synthesis when they were presented to the cells in the sequential order of ionophore followed by TPA. Both compounds required the presence of external calcium to be effective.