Inhaled platelet-activating factor increases airway sensitivity but not maximal airway narrowing to methacholine in normal subjects.

To examine the effect of inhaled platelet-activating factor (PAF) on airway sensitivity and on maximal airway narrowing, we measured airway response to doubling concentrations of methacholine (MCh) 48 h before and 48 h after inhalation of 10, 50 and 100 micrograms of PAF in six nonatopic, nonasthmatic subjects. The forced expiratory volume in one second (FEV1) and airflow at 30 percent of vital capacity (V30) from partial forced expiration were used to assess changes in airway calibre. Inhalation of PAF caused only minor changes in FEV1. In contrast, inhalation of 100 micrograms of PAF caused a significant fall in V30 from 2.64 +/- 0.35 to 1.35 +/- 0.43 l.min-1 (p < 0.05). Two days after PAF inhalation a leftward shift of the concentration-response curve to MCh was observed. The MCh concentration causing a 20% fall in FEV1 (PC20FEV1) was 11.25 +/- 1.78 and 2.38 +/- 1.29 mg.ml-1 (geometric mean +/- GSEM; p < 0.05) before and after PAF inhalation, respectively. PAF did not affect the maximal airway response to MCh. The maximum percentage fall in FEV1 was 36.2 +/- 1.9% at baseline and 37.6 +/- 1.8% after PAF inhalation. Likewise, maximum percentage change in V30 was 72.8 +/- 3.7% at baseline and 73.6 +/- 3.4% after PAF inhalation. The results of this study show that PAF inhalation increases airway sensitivity without altering the maximal bronchoconstrictive response to MCh in normal subjects.