A3 Adenosine Receptor Ligands: History and Perspectives

Adenosine regulates many physiological functions through specific cell membrane receptors. On the basis of pharmacological studies and molecular cloning, four different adenosine receptors have been identified and classified as A1, A2A, A2B, and A3. These adenosine receptors are members of the G-protein-coupled receptor family. While adenosine A1 and A2A receptor subtypes have been pharmacologically characterized through the use of selective ligands, the A3 adenosine receptor subtype is presently under study in order to better understand its physio-pathological functions. Activation of adenosine A3 receptors has been shown to stimulate phospholipase C and D and to inhibit adenylate cyclase. Activation of A3 adenosine receptors also causes the release of inflammatory mediators such as histamine from mast cells. These mediators are responsible for processes such as inflammation and hypotension. It has also been suggested that the A3 receptor plays an important role in brain ischemia, immunosuppression, and bronchospasm in several animal models. Based on these results, highly selective A3 adenosine receptor agonists and/or antagonists have been indicated as potential drugs for the treatment of asthma and inflammation, while highly selective agonists have been shown to possess cardioprotective effects. The updated material related to this field of research has been rationalized and arranged in order to offer an overview of the topic. © 2000 John Wiley & Sons, Inc. Med Res Rev, 20, No. 2, 103–128, 2000