Background: Inhalation of adenosine-5'-monophosphate (AMP) causes bronchoconstriction in asthma but not in healthy subjects. Bronchoconstriction upon AMP inhalation is thought to occur by histamine release and subsequent binding to receptors on airway smooth muscle cells.
Methods: To explain enhanced sensitivity to AMP in asthma, mast cell expression of the adenosine A2A and A2B receptors and histamine release were measured after incubation of human mast cell line 1 (HMC-1) cells with AMP and the non-specific adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) for 1.5 and 6 h. To establish a Thelper-2 environment resembling the asthma phenotype, HMC-1 cells were additionally cultured with IL-4 and IL-13 alone or stimulated with the combination of both cytokines and AMP and NECA. To study effects of prolonged presence of the inflammatory environment, the cells were pre-incubated overnight (18 h) with IL-4 and IL-13 and additionally stimulated with AMP and NECA for 1.5 or 6 h.
Results: AMP and NECA hardly affected adenosine receptor expression but increased IL-8 secretion. Incubation with IL-4 and IL-13 for 6 h increased adenosine A2A receptor expression and histamine secretion, but decreased IL-8 secretion. The combination of IL-4, IL-13, and AMP/NECA for 6 h increased A2B receptor expression and IL-8 secretion. Overnight stimulation with IL-4, IL-13 and subsequent stimulation with AMP/NECA for 1.5 h decreased A2AR expression which was accompanied by increased histamine secretion.
Conclusion: These results suggest a role for decreased A(2A)R expression in enhanced adenosine responsiveness as observed in asthma.