An intravenous infusion of platelet activating factor (Paf) in the guinea‐pig elicits an increase in bronchial responsiveness to the spasmogens, histamine and bombesin. Airways obstruction induced by bombesin in Paf‐treated animals is poorly reversed by isoprenaline compared to comparable airways obstruction induced by bombesin in vehicle‐treated animals. Isoprenaline induced a comparable dose‐related relaxation in vitro of tracheal smooth muscle isolated from Paf‐ and vehicle‐treated animals. No change in β‐adrenoceptor numbers or binding affinity was observed in lungs removed from Paf‐treated animals in comparison with those from vehicle‐treated animals, or after direct incubation with Paf in vitro. The reduced bronchodilator responsiveness to isoprenaline in Paf‐treated animals is not related to changes in pulmonary β‐adrenoceptor function. These results suggest that non‐spasmogenic elements may contribute to airways obstruction induced in hyper‐responsive animals.
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