Potential mechanisms of cytosolic calcium modulation in interferon-gamma treated U937 cells

Interferon-gamma (IFN-gamma) at a concentration of 50 U/ml increased internal Ca2+ in the monocyte-like cell line U937 by about 100% within 3 min of addition, as determined by indo-1 fluorescence. This IFN-gamma-induced increase was reduced to 30-40% of basal (Ca2+) by the addition of diltiazem (1 microM) or incubation in Ca2+-free buffer. Ai crude membrane preparation obtained by differential centrifugation of sonicated U937 cells possessed Ca2+-ATPase activity (10 nmol ATP hydrolyzed/min/mg protein at 30 C) and sequestered Ca2+ to a level of 8 nmol/mg protein in 30 min. Addition of inositol trisphophate (IP3) (10 microM) after accumulation of Ca2+ resulted in release of a portion of the sequestered Ca2+ within 30 s, which was then resequestered. Although mitochondrial contamination was indicated by partial inhibition of Ca2+ uptake by oligomycin A, this mitochondrial inhibitor had no effect on the IP3-induced Ca2+ release. These results suggest that the increase in U937 cell cytoplasmic Ca2+ induced by IFN-gamma results from both intracellular redistribution of Ca2+, probably via polyphosphoinositide metabolism, and the entry of extracellular Ca2+ through slow channels.