Cytosolic calcium elevation in response to Fc receptor cross-linking in undifferentiated and differentiated U937 cells.

We have used the calcium indicator, Fura-2, to investigate cytosolic calcium responses to cross-linking of monomeric IgG-occupied surface Fc gamma receptors (Fc gamma R), using populations of the human monocyte-like cell line, U937. The magnitude and duration of the calcium response observed, and the relative contribution to the response of internal stores and external calcium, are found to depend on the state of differentiation of these cells. Initial release of calcium from stores following Fc gamma R cross-linking is enhanced by prior treatment of U937 cells with both interferon-gamma, and, to a lesser extent, with dibutyryl cAMP. A large and prolonged entry of external calcium is observed in dibutyryl cAMP treated cells; this may be due to direct regulation of calcium channels by the low affinity receptor, Fc gamma RII (whose expression is up-regulated in these cells), since the smaller and more transient entry observed in interferon-gamma treated cells, (where the high affinity receptor, Fc gamma RI, is up-regulated) argues against a common pathway of store-mediated calcium entry.