Inhalation of adenosine causes bronchoconstriction in asthmatic subjects, but the effect of this purine nucleotide on airway vascular permeability is unknown.In order to determine whether adenosine produces airway microvascular leakage and, if so, to examine the effect of cromolyn (sodium cromoglycate (SCG)) on this extravasation of Evans blue was measured in the airways of ovalbumin-sensitized Brown Norway rats.Inhaled adenosine caused microvascular leakage in sensitized but not in nonsensitized rats, and the response was abolished by capsaicin pretreatment or the tachykinin neurokinin-1 receptor antagonist FK888. Adenosine-induced vascular leakage became apparent in nonsensitized rats when treated with phosphoramidon, and airway neutral endopeptidase activity was lower in sensitized than in nonsensitized animals. The extravasation induced by adenosine in sensitized rats was dose dependently inhibited by SCG aerosols. SCG likewise inhibited microvascular responses to substance P, but had no effect on those to platelet-activating factor.These results suggest that: 1) adenosine induces airway microvascular leakage in sensitized rats through stimulation of neurokinin-1 receptors; 2) this effect is associated with a sensitization-induced decrease in neutral endopeptidase activity; and 3) sodium cromoglycate inhibits adenosine-induced extravasation, presumably via functional antagonism of tachykinins. Eur Respir J 1999; 14: 1082±1087. Adenosine is a naturally occurring purine nucleotide formed by the cleavage of adenosine 5'-monophosphate by 5'-nucleotidase or by the catabolism of S-adenosylhomocysteine. Based on the findings that adenosine can be released from mast cells in both early-and late-phase asthmatic responses via antigen/immunoglobulin E (IgE) interaction after allergen bronchoprovocation [1] and that inhaled adenosine causes bronchoconstriction in asthmatics but not in normal subjects [2], this purine nucleotide is thought to play a role in the pathophysiology of asthma. Increased microvascular permeability and plasma protein extravasation in the airways might contribute to mucosal oedema, inflammatory cell infiltration and epithelial cell desquamation, which are characteristic features of asthma [3]. Previous studies have shown that adenosine induces plasma extravasation in rat skin [4] and hamster cheek pouch [5], but its effect on airway microvascular permeability is unknown.Cromolyn (sodium cromoglycate (SCG)) is an antiallergic drug and has been widely used in the treatment of asthma [6,7]. Although SCG has long been recognized as a mast cell-stabilizing agent [8], its mechanism of action in asthma is still unclear. CROSSMAN et al. [9] showed that oedema formation in the human skin produced by substance P was reduced by SCG, suggesting that SCG may modify tachykinin actions. Therefore, the purposes of the present study were: 1) to determine whether adenosine produces airway vascular leakage; 2) to elucidate the possible involvement of tachykinins in the effect of adenosine; and 3) to deter...