PAF-induced airway responses in sheep: Effects of a PAF antagonist and nedocromil sodium

Platelet-activating factor (PAF) is a potent inflammatory mediator that can cause bronchoconstriction and airway hyperresponsiveness in selected human subjects and animals. The mechanism by which PAF induces these changes is not clearly understood. We therefore studied the effects of intratracheal instillation of PAF (30 μg/kg) on airway resistance and airway responsiveness in allergic sheep (n = 7) and attempted to modulate these effects with the specific PAF antagonist, WEB-2086, and the antiasthmatic agent, nedocromil sodium (NED). Specific lung resistance (SR_L) was measured to assess bronchial responses to PAF, and airway responsiveness was determined by deriving a provocative dose of carbachol in breath units causing an increase in SR_L to 4 L times centimeters of H₂O per liters per second (PD₄) from carbachol dose-response curves. PAF instillation increased mean ± SD SR_L to 228 ± 134% above baseline. Two to 4 hours after PAF instillation, PD₄ decreased by 55 ± 9% from a baseline of 39 ± 9 breath units (p < 0.05). Airway responsiveness remained increased at 24 hours but returned to baseline by 48 hours. Pretreatment with WEB-2086 (3 mg/kg, intravenously) or NED (1 mg/kg, nebulized) blocked (p < 0.05,) PAF-induced bronchoconstriction and PAF-induced airway hyperresponsiveness. Instillation of lyso-PAF (30 μg/kg) did not cause bronchoconstriction or airway hyperresponsiveness. Thus, instilled PAF causes bronchoconstriction and airway hyperresponsiveness in allergic sheep by a receptor-mediated mechanism that likely involves the release of secondary mediators, the latter process being sensitive to NED.