SummaryWe investigated acute bronchoconstriction and changes in airway responsiveness to methacholine following the inhalation of platelet activating factor (PAF) in an open study of 12 non‐asthmatic subjects. Ventilatory function was monitored using a flow rate at 30 of vital capacity (V̇30) and airway responsiveness was measured as PD40V̇30, i.e. the dose of metacholine causing a 40 fall in V̇30. PAF (3–422 μg) resulted in dose‐related acute bronchoconstriction in 10 of the 12 subjects. There was no association between the airway responsiveness to PAF and to methacholine. Ten subjects showed some increase in airway responsiveness to methacholine 1 or 3 days following PAF. Overall, these changes were statistically significant (P<0·05) but were of small magnitude (geometric mean PD40V̇30pre‐PAF=457 μg; 24 hr after PAF = 259 μg; 72 hr after PAF =258 μg) and variable: only seven subjects showing increased airway responsiveness on both day 1 and day 3 after PAF. Six subjects who appeared to show increases in airway responsiveness following PAF were re‐studied with the inhaled PAF pre‐medicated by either placebo or a specific thromboxane receptor antagonist (GR32191B) in a double‐blind fashion. GR32191B did not reduce the acute bronchoconstriction due to PAF. In this part of the study, these six subjects did not show significant increases in airway responsiveness following the placebo pre‐medicated PAF challenge and so no effect of the drug on airway responsiveness could be shown. We conclude that PAF‐induced bronchoconstriction in normal individuals is not mediated by thromboxane and that any increases in airway responsiveness following PAF are frequently poorly sustained, are poorly reproducible, and
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