Based on the competitive antagonism of adenosine activity by methylxanthines, the existence of adenosine receptors was postulated more than 40 years ago; 20 years later, four receptors were cloned from several mammalian species, including humans1, and identified as members of a large G protein-coupled receptor (GPCR) family.Adenosine is the only important agonist for the three key adenosine receptors — A1, A2A and A2B — and the major, full agonist ligand for the A3 receptor (for which inosine is an incomplete agonist). The concentration of adenosine in the extracellular compartment is the consequence of many biological processes, including extracellular adenosine production, adenosine transport, adenosine formation from intracellular adenosine sources (for example, via the S-adenosylhomocysteine pathway) and adenosine metabolism to inosine or AMP.
Adenosine receptors have been implicated in several key physiological processes, ranging from neuromodulation to immune regulation, and from vascular function to metabolic control. Adenosine has been postulated to have a role as a danger signal involved in homeostasis. Adenosine signalling is not very prominent under physiological conditions in most tissues, but aberrant adenosine signalling has been implicated as a common disease mechanism underlying inflammatory and ischaemic tissue damage.These receptors are widely expressed and have been implicated in several biological functions, both physiological and pathological. These include cardiac rhythm and circulation, lipolysis, renal blood flow, immune function, sleep regulation and angiogenesis, as well as inflammatory diseases, ischaemia-reperfusion and neurodegenerative disorder.

References:

1.Chen JF, et al. Nat Rev Drug Discov. 2013 Apr;12(4):265-86.