Figure 1.

Adenosine receptors and the main transduction pathways involved in their activation. Schematic representation of G protein-coupled A₁, A_2A, A_2B and A₃ receptor (A₁R, A_2AR, A_2BR and A₃R) subtypes activated by extracellular adenosine (ADO), and the main intracellular pathways involved. A_2ARs and A_2BRs are coupled to the Gs protein, which leads to adenylyl cyclase (AC) activation and cyclic AMP (cAMP) increase. On the other hand, A₁R and A₃R are coupled to the G_i protein that inhibits AC and reduces cAMP. In some districts, A_2BRs are also coupled with G_q proteins and A₁R with Go, which stimulate Ca²⁺ release from intracellular stores. All adenosine receptors are coupled to mitogen-activated protein kinase (MAPK) pathways, including extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAPK and Jc-Jun-NH2 terminal Kinase (JNK).