Figure 1.

Classification of purinergic receptors. P1Rs consist of four subtypes and respond to the endogenous agonist adenosine. P2Rs consist of two subtypes, the ligand-gated cationic channels P2X (P2X1–7) responding to ATP only, and the G protein-coupled receptors P2Y (P2Y_{1,2,4,6,11,12,13,14}). The principal agonists of P2YRs are the following: ATP/ADP (P2Y_1,11,12,13), UTP/UDP (P2Y_2,4,6), and UDP-glucose (P2Y₁₄). Purinergic receptors were classified by the late Geoffrey Burnstock (see photo), together with his colleague Maria Pia Abbracchio [3].