P receptors are divided into two major classes termed adenosine or P1 receptors, at which adenosine is the principal natural ligand, and P2 receptors, at which ATP, ADP, UTP and UDP are the principal natural ligands.

• P1受体的内源性配体主要是腺苷(ADO),P2受体的内源性配体主要有ATP、二磷酸腺苷(ADP)、三磷酸尿苷(UTP)和二磷酸尿苷(UDP)等。