Increased capacity for store regulated calcium influx in U937 cells differentiated by treatment with dibutyryl cAMP

We have investigated the mechanisms underlying calcium signalling evoked by cross-linking of the high affinity IgG receptor (Fc gamma RI) in populations of the human monocyte-like cell line, U937, following activation of the cells by cytokine treatment, or differentiation to a more macrophage-like state by treatment with dibutyryl cAMP (Bt2cAMP). We have shown previously that a larger and more prolonged entry of external calcium occurs in Bt2cAMP-differentiated cells, although there is a smaller initial release from internal stores in these cells than in those activated by IFN-gamma treatment. In this paper we demonstrate, by use of the endomembrane Ca(2+)-ATPase inhibitor, thapsigargin, that this effect is explained (at least in part) by an enhanced capacity for store regulated entry of calcium in Bt2cAMP-differentiated cells.