Implications of a rise in cytosolic free calcium in the activation of RAW-264 macrophages for tumor cell killing.

The concentration of cytosolic-free calcium (Ca2+i) was determined with aequorin in RAW-264 macrophage-like cells activated in vitro for tumor cell killing with lymphokine (LK) and lipopolysaccharide (LPS). Treatments of these cells with optimal doses of stimulants, which evoked the development of cytolytic activity, also induced a rise in their Ca2+i. No rise in Ca2+i could be observed under treatments which failed to activate cells. The presence of both stimulants was an absolute requirement for evoking cytolytic activity and also a rise in Ca2+i. There was an apparent parallelism between the rate of activation and the rate of rise in Ca2+i. Cells which slowly developed their cytolytic activity exhibited a slow rise in Ca2+i, while macrophages which acquired their cytolytic activity at the faster rate also showed a more rapid increase in Ca2+i. The development of cytolytic activity in RAW-264 macrophages was inhibited by two intracellular calcium antagonists, TMB-8 and ruthenium red. This inhibition could be reversed by high concentrations of extracellular calcium. TMB-8, at the concentrations which were effective in inhibiting the activation process, also completely blocked the associated rise in Ca2+i. These results suggested that Ca2+i might play a role in the mechanism of tumoricidal transformation of RAW-264 macrophages.