The effect of platelet-activating factor on the generation of superoxide anion in human eosinophils and neutrophils.

The precise role of platelet-activating factor (PAF) in asthma has yet to be established. Nonetheless, the potential relationship between PAF and asthma appears to include the eosinophil (EOS) as an important link. Thus, to evaluate the effect of PAF on leukocyte-dependent inflammation, purified populations of human blood EOSs and neutrophils were isolated from the same subject. The two granulocyte populations were then incubated with PAF, and superoxide anion (O2-) generation was measured by reduction of cytochrome c in a microassay system. Both granulocyte cell types generated O2- when they were incubated with PAF. However, the generation of O2- was 3.4 times greater with EOSs (9.8 +/- 1.5 nmole of cytochrome c reduced per 5 x 10(5) cells) than neutrophils (2.9 +/- 0.4 nmole of cytochrome c reduced per 5 x 10(5) cells; p less than 0.0001). When the effect of PAF on [Ca++]i was measured with the fluorescent label, Indo-1, PAF caused similar increases in cellular fluorescence in both neutrophils and EOSs, but the increase in [Ca++]i of neutrophils occurred with lower concentrations of PAF. Furthermore, when similar experiments were conducted in the presence of an extracellular calcium chelator, ethylene glycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraacetic acid, there was partial suppression in both the cellular fluorescence and O2- generation to PAF; this suggests that full expression of EOS generation of O2- by PAF requires both intracellular mobilization and a transmembrane influx of Ca++. Our data indicate that PAF can stimulate leukocyte O2- generation, but this response is greater in the EOS than the neutrophil. Therefore, our findings support the observation that the EOS is more responsive to PAF activation than other granulocytes and that this difference may contribute to participation of PAF in asthma.