We have developed an immunoadsorption technique for quantitating EGTA-resistant gelsolin/actin complexes in macrophages extracted with Triton X-100. We report here that the proportion of gelsolin complexed irreversibly to actin is low in freshly harvested macrophages. The amount of the EGTA-resistant complex increases spontaneously during incubation of the cells in suspension at 37 degrees C, or after exposure to the Ca2+ ionophore ionomycin. On the other hand, exposure of suspended cells to the chemotactic oligopeptide, FMLP, or plating of the cells onto tissue culture dishes causes the EGTA-resistant complex to dissociate rapidly. Plating even prevents Ca2+ ionomycin-treated cells with elevated intracellular Ca2+ from inducing this complex. Therefore, our results suggest that macrophages possess a mechanism, not directly involving Ca2+, for dissociating actin/gelsolin EGTA-resistant complexes. This mechanism may be a Ca2+-independent signal for leukocyte activation.

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