Inhaled adenosine and its parent nucleotide, adenosine 5'-monophosphate (AMP) provoke bronchoconstriction in atopic and asthmatic individuals but not in normal subjects. 2. In clinical studies, histamine H1-receptor antagonists, cyclo-oxygenase inhibitors and the mast cell 'stabilising' drugs, sodium cromoglycate and nedocromil, protect against the effects of adenosine bronchoprovocation suggesting the involvement of secondary mast cell mediator release. 3. Adenosine and its analogues potentiate histamine and leukotriene release from mast cells activated by other stimuli in vitro, and may also increase net mediator release from mast cells by counteracting the inhibitory effect of circulating adrenaline. 4. Although adenosine fulfils many of the criteria required for a mediator in asthma, its importance is not fully understood, and the mechanisms by which it provokes bronchoconstriction in asthmatic subjects is far from concluded. 5. Two possibilities are that either adenosine acts directly on luminal mast cells to upregulate histamine secretion, or it acts to initiate neuronal reflexes which stimulate histamine release indirectly and possibly activate peptidergic and/or cholinergic pathways.
