Abstract

Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A₁, A_2A, A_2B, and A₃. The A_2A and A_2B receptors preferably interact with members of the G_s family of G proteins and the A₁ and A₃ receptors with G_i/oproteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A₃ receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A₁, A_2A, and A₃ expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A₁, A_2A, and A₃ receptors but few drugs that interact selectively with A_2B receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.