Lipoxin A4 elevates cytosolic calcium in human neutrophils

Lipoxin A4 (LXA4), a lipoxygenase-derived metabolite of arachidonic acid, stimulated a dose-dependent elevation in cytosolic free Ca2+ concentration, [Ca2+]i, in fura-2-loaded human neutrophils, with an EC50 of 0.4-0.5 microM. The time for [Ca2+]i to peak was also dose-dependent. In the presence of extracellular Ca2+ (CaDT-PA added), the rise in [Ca2+]i was due to a combination of Ca2+ release from internal stores and influx of extracellular Ca2+. In the absence of extracellular Ca2+, the rise in [Ca2+]i was due to release from internal stores, which then became depleted. No response to LXA4 was seen in the absence of divalent cation chelators (EGTA or DTPA); this is presumably because LXA4 forms an inactive complex with heavy metal cations. In the presence of extracellular Ca2+, LXA4 had no effect on the subsequent response of neutrophils to the chemotactic peptide fmetleu-phe (fmlp). In the absence of extracellular Ca2+, LXA4 dose-dependently reduced the subsequent response of neutrophils to fmlp; this is presumably because LXA4 discharges the store, and so reduces the amount of Ca2+ available for subsequent release by fmlp.