Cloned receptors and cardiovascular responses to adenosine

Four subtypes of adenosine receptors have recently been cloned from thyroid, brain and testis. In this review we have summarised properties of these purinergic receptors. The cloned A1 and A2 subtypes are probably similar or identical to receptors that exist on cardiac and vascular tissues, respectively. A comparison of the amino acid sequences of A1, A2a, and A2b receptors reveals several stretches of conserved amino acids that are unique to adenosine receptors, primarily in the membrane spanning regions. Species differences in A1 receptors indicate that minor changes in receptor structure can produce marked changes in ligand binding properties and may facilitate the identification of amino acids involved in ligand recognition. A confusing A1 receptor subclassification system of putative A1a, A1b, and A3 subtypes has emerged based on subtle rank order potency differences for various ligands among tissues. cDNAs corresponding to these A1 subtypes have not yet been isolated. Atrial A1 receptors activate K+ channels and inhibit adenylyl cyclase. These two pathways appear to be independently up and down regulated, suggesting the existence either of atrial A1 receptor subtypes or of differential regulation of the coupling of a single receptor to distinct GTP binding proteins. An adenosine receptor distinct from A1 and A2 receptors has been cloned from testis and designated TGPCR, or A3, although it differs from the pharmacologically defined A3 receptor. We suggest that the current A1/A3 receptor subtype nomenclature be abandoned and superseded by a nomenclature based solely on receptor cDNAs. In addition to the cloned adenosine receptors, a novel A4 subtype has been proposed based on pharmacological and electrophysiological criteria.(ABSTRACT TRUNCATED AT 250 WORDS)