Differentiation of the human monocytic cell line U937 results in an upregulation of the calcium release-activated current, I-CRAC

1. Single cell fura-2 fluorescence measurements and whole-cell patch clamp recordings were used to investigate the effects of macrophage-like differentiation, induced by dibutyryl Ca2+ influx triggered by Ca2+ store depletion in the human monocytic cell line, U937. 2. In differentiated cells, the rise in intracellular [Ca2+] following store depletion by thapsigargin (TG) in nominally Ca2+-free solution was 94% greater and the [Ca2+], rise on subsequent re-addition of external Ca2+ (2 mM) was 292% greater than in undifferentiated cells. 3. Under conditions where [Ca2+], was buffered by BAPTA, TG-induced store depletion failed to activate a detectable inward Ca2+ current in undifferentiated U937 cells. Under identical conditions, store depletion of differentiated U937 cells generated an inwardly rectifying Ca2+-selective current which showed no reversal from -140 to +30 mV and was blocked by 1 mu M external La3+; characteristics of the calcium release-activated Ca2+ current (I-CRAC) identified in other cells. 4. We conclude that U937 cells show a differentiation-dependent upregulation of a store mediated Ca2+ entry pathway, identified as I-CRAC, which is not correlated with the small associated increase in the size of TG-sensitive Ca2+ pools.