Figure 1.

Adenosine and its receptors. Schematic representation of adenosine metabolism, transport and metabotropic receptors. The ectonucleotidases CD39 and CD73 metabolize ATP and ADP to AMP, and AMP to adenosine. The equilibrative nucleoside transporter (ENT) or concentrative nucleoside transporter (CNT) families mediate adenosine reuptake. Adenosine metabotropic receptors (A₁, A_2A, A_2B and A₃ receptors: A₁R, A_2AR, A_2BR and A₃R) are differently coupled to adenylyl cyclase (AC) inhibition or stimulation. Adenosine deaminase (ADA) deactivates extracellular adenosine by converting it into inosine.