Lyso-platelet-activating factor (PAF), the precursor and metabolite of PAF, is considered inactive, although it may be converted to PAF by airway cells. We have investigated the effects of inhaled lyso-PAF on bronchoconstriction and airway microvascular leakage in anesthetized guinea pigs. Lung resistance (RL) was measured for 6 min after inhalation of lyso-PAF (0.3, 1, and 3 mM; 30 breaths) followed by measurement of extravasation of intravenous Evans blue dye into airway tissues, which was used as an index of airway microvascular leakage. Inhaled lyso-PAF caused an increase in RL and leakage of dye at all airway levels in a dose-dependent fashion, but intravenous lyso-PAF (0.25 mg/kg) had no airway effect. The maximum dose of inhaled lyso-PAF increased RL significantly by approximately 200%. The amount of extravasation of dye induced was 96 +/- 4 (SE) ng/mg of tissue in trachea, 77 +/- 8 ng/mg in main bronchi, and 65 +/- 7 and 25 +/- 1 ng/mg in proximal and distal intrapulmonary airways respectively; these values were all significantly higher (P < 0.01) than control values. These responses were completely abolished by a specific PAF-receptor antagonist WEB-2086 (2 mg/kg iv). Our results show that inhaled lyso-PAF is potent in increasing airway microvascular leakage. The effects of lyso-PAF may result from its metabolic transformation to PAF by lyso-PAF:acetyl-CoA acetyltransferase in the airway.