Adenosine is an endogenous
nucleoside that is released under pathological conditions and interacts with four
G-protein-coupled receptor subtypes. These receptors are widely distributed throughout the body. They are involved in many central and peripheral processes, including immunological and inflammatory responses. In inflammatory and asthmatic conditions, the extracellular concentration of
adenosine increases in the airway tissue. It enhances mast cell degranulation and bronchoconstriction, but may also inhibit eosinophil or lymphocyte function or modulate
reactive oxygen species generation in neutrophils. Despite a large number of studies clearly indicating the effects of
adenosine in vitro, many aspects of the mechanisms involved in the
adenosine-mediated responses are still unclear, and our knowledge is limited in understanding the complex multifactorial interactions occurring in the whole body. The discovery of
adenosine receptor compounds acting with increasing selectivity will bring new approaches to the use of
adenosine receptor agonists and antagonists and may clarify some of the current uncertainties. On the basis of our present knowledge, the development of
adenosine A(2A)- or (A3)-receptor agonists as
antiinflammatory agents or A(2B)-receptor antagonists as inhibitors of mast cell degranulation for the treatment of
asthma holds promise.