Characterization of a complement-fragment-C5a-stimulated calcium-influx mechanism in U937 monocytic cells

Abstract

The mechanism by which complement fragment C5a elevates intracellular Ca2+ ([Ca2+](i)) levels in two cell types, a monocytic cell line, U937, and neutrophils, has been investigated by the use of fluorometric and radiometric techniques. In U937 cells the influx of extracellular Ca2+ can be distinguished from the release of intracellular Ca2+ stores in terms of dose-responsiveness to C5a and sensitivity to pertussis-toxin poisoning. This suggests that the mechanism of Ca2+ influx in these cells is at least partially independent of both the production of inositol phosphates and elevation of internal Ca2+ concentration. The C5a-stimulated influx of 45Ca2+ into U937 cells is inhibited by a series of metalions (Zn2+ > Co2+ > Mn2+ > Sr2+ ≃ Ni2+ > La3+). The stimulated influx of Ca2+ into neutrophils is inhibited differently (Ni2 ≥ Co2+ > Zn2+ ≃ La3+ > Mn2+ ≃ Sr2+}, is less sensitive to C5a and both the influx of extracellular Ca2+ and the release of intracellular stores are equally sensitive to pertussis toxin treatment. Taken together these results indicate that [Ca2+](i) is controlled in U937 monocytes by mechanisms distinct from those which appear to operate in other myeloid cells, such as neutrophils, stimulated with C5a and formylpeptide.