Adenosines/Purinergic Agonists and Antagonists

Adenosine receptors, also known as P₁ receptors, are classified as A₁, A₂a, A₂b, and A₃. Adenosine is an endogenous nucleoside released from metabolically active cells. It acts as a potent biological mediator that modulates the activity of numerous cell types including platelets, neutrophils and mast cells. Adenosine receptors are linked to adenylate cyclase and other effector systems via the GTP-binding proteins. Pharmacological modulation of these receptors by methylxanthines has gained significance due to their involvement in the pathophysiology of asthma. The P₂ purinergic receptors mediate the action of ATP and exhibit a far greater sensitivity to ATP than to adenosine. They are also known as ATP receptors. They are not blocked by treatment with methylxanthines. In the CNS, the ATP receptors are divided into two major classes, ionotropic (P₂X_n) and G-protein coupled (P₂Y_n) receptors. Seven subtypes of each group, P₂X and P₂Y, are reported in the literature. The overall sequence homology between P₁ and P₂ purinoreceptors is reported to be less than 20%.