COMPARISON OF PAF-INDUCED AND FMLP-INDUCED [CA2+](I) TRANSIENTS IN HUMAN POLYMORPHONUCLEAR LEUKOCYTES

Changes of [Ca2+](i) in human polymorphonuclear leukocytes (PMNL) were studied. PMNL suspension was activated three times every 5 min with 1 0(-7) M PAF and fMLP. Both PAF and fMLP, induced three consecutive [Ca 2+](i) transients in PMNL suspended in medium with 1 mM Ca2+. The firs t Ca2+ response was a result of Ca2+ release from internal stores and the extracellular Ca2+ influx, while the second and third responses we re completely dependent on Ca2+ influx from extracellular space. The c ontribution of Ca2+ from intracellular stores to the first PAF-induced Ca2+ response was about 1.4-fold lower in comparison with the first f MLP induced Ca2+ response (27 +/- 1 vs 37 +/- 6% (p < 0.05). Previous addition of PAF enhanced 3-fold (p < 0.001) the PMNL response to fMLP while cells pretreated with fMLP failed to increase their [Ca2+](i) af ter challenge with PAF. PMNL from 40% of donors did not respond to PAF in the presence of 100 nM Ca2+. However, the cells responding to PAF as the cells treated with fMLP or cyclopiazonic acid released almost t he entire Ca2+ from intracellular stores after challenge. Subtraction of mean [Ca2+](i) transients in the presence of 100 nM Ca2+ from that obtained in medium with 1 mM Ca2+ showed that, in PMNL stimulated with PAF in contrast to the cells treated with fMLP, the onset of Ca2+ inf lux from extracellular space precedes Ca2+ release from intracellular stores. These results suggest that PAF-induced Ca2+ influx from extrac ellular space is at least partly independent of Ca2+ release from intr acellular stores.