The role of platelet-activating factor (PAF) in the late asthmatic responses was studied. The concentrations of lyso-form of PAF in plasma were measured at 0 and 20 min, and 6 and 24 h after the antigen inhalation challenge among patients with bronchial asthma. PAF activities were measured by their aggregating ability of washed rabbit platelets after acetylation of lyso-PAF into the biological active form of PAF, when there were no detectable amounts of PAF in plasma. The concentrations of lyso-PAF were found to be significantly increased in patients with the late asthmatic response compared with patients with the single immediate response at 6 h after the antigen challenge. In contrast, lyso-PAF levels were not significantly different at 20 min after the antigen challenge between these two groups. PAF inactivator activity in plasma increased when there was a decrease in the lyso-PAF level. These results suggest that PAF may participate in the late asthmatic response and may provide a new insight into the pathogenesis and the treatment of bronchial asthma.
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