Recently, Inbe et al. adenosine re-uptake by dipyridamole increased the bronchoconstrictor response to inhaled AMP, indicating that accumulation of extracellular adenosine was closely associated with the asthmatic airway response (Cushley of increasing extracellular adenosine levels (Griffiths studies confirmed that adenosine and A2 receptor analogues (e.g. 5-mast cell activation as suggested by our early studies was pursued in several ways. Firstly, AMP provocation of asthmatic airways was accompanied by a rise in circulating histamine levels (Phillips (Holgate pharmacology available at the time, it had been assumed that adenosine was active through a single A2 receptor linked to adenylate cyclase AZD7986 and that was quite distinct from the other purinergic receptors that responded more selectively to ATP and UTP (e.g. P2Yand P2X). However, a paradox that could not be explained was how an agent which increased cyclic AMP within mast cells and basophils could augment rather than inhibit mediator release, as would be expected since increases in cyclic 35-AMP produced by other agonists, for example, with 2-adrenoceptor agonists BLIMP1 (Okayama & Church, 1992) or PG E2 (Peters et al., 1982) were strongly inhibitory for mediator release. Further clarity came with the discovery that adenosine A2 receptors existed as two subtypes C A2A linked to adenylate cyclase and involving Gs coupling, and A2B linked to both adenylate cyclase and the phosphatidyl trisphosphate (PI3)-calcium signalling pathway involving both Gs and Gq coupling (Feoktistov & Biaggioni, 1995; Feoktistov et al., 1998). Thus, while exhibiting no AZD7986 antagonist properties against adenosine A2A receptors, enprofylline was shown to be a highly selective, albeit weak, antagonist of A2B receptors (Feoktistov & Biaggioni, 1995; Kim et al., 2002; Fan et al., 2003). This critical observation helped explain our finding of a preferential inhibitory effect of intravenous emprofylline on AMP-induced bronchoconstriction (Clarke et al., 1989). The identification of the A2B receptor subtype revitalised interest in adenosine as a mediator of asthma and becoming a new therapeutic target for this disease (Feoktistov et al., 1998). Although most of the work identifying A2B receptors on human mast cells was conducted on the HMC-1 mastocytosis derived cell line, recently A2B receptors mediating enhanced mediator release have also been found on mast cells dispersed from human lung tissue (Zhong H, personal communication). In addition to causing mast cell mediator release, activation of A2B receptors on HMC-1 cells cultured with human B cells results in Ig isotype, switching to IgE involving costimulation utilising CD40 and enhanced IL-4 and IL-13 secretion (Ryzhov et al., 2004). With the identification of this new subclass of A2 receptors, the ease with which repeated exposure to adenosine (and AMP) results in tolerance and cross-tolerance became of the target of further study. The A2B receptor appears to be regulated differently from many other G-protein-coupled receptors. Mundell and co-workers have shown that agonist activation of A2B receptors results in arrestin-dependent internalisation of the receptor complex with antisense neutralisation of arrestin, resulting in loss of desensitisation AZD7986 (Mundell et al., 2000; Matharu et al., 2001). Recent work has shown that human A2B receptors associate with intracellular signalling proteins other than G proteins such as those containing PDZ (PSD-95, Dig 20-1) domains, and more specifically with the PDZ domain-containing protein E3KARP (Sitaraman et al., 2002). This is known to interact with ezrin/radixin/moesin (ERM) proteins which in turn interact with the actin cytoskeleton that control A2B receptor trafficking. This molecular-based work provides a good explanation for the ease with which A2B receptor stimulation results in rapid and profound tachyphylaxis, and also for cross-desensitisation between A2B and other G-protein-coupled receptors (Sitaraman et al., 2000). The first observation that inhaled corticosteroids were highly active in rapidly suppressing AMP-induced bronchoconstriction (Doull et al., 1997; Holgate et al., 2000) and the recent demonstration that AMP challenge induces eosinophil influx into the airways (van den Berge et al., 2004) further strengthened interest of the role of A2B receptor in asthma. The rapidity with which this occurs (Wilson et al., 2003) suggests that a unique effect of corticosteroids on the A2B receptor internalisation mechanisms possibly involving the recently described rapid steroid response receptor (Long et al., 2005). Observation on the role of adenosine in AZD7986 animal models Adenosine receptors are also involved in mediating bronchoconstriction in a number of animal models, but between animal species there is heterogeneity of the receptors involved. In the rabbit the airway response is mediated through A1 receptors (Nyce &.