Adenosine elicits a wide variety of physiological responses' via interactions with two major subtypes of extracellular receptors, designated as Al and A2. The two receptor subtypes were originally defined in terms of different effects on adenylate cyclase. Al receptors exhibit relatively high affinity to adenosine in binding studies (nM) and some are coupled to and inhibit adenylate cyclase. In contrast, A2 receptors exhibit low affinity to adenosine (pM) ...
