11 Antagonist Radioligand Binding to Solubilized Porcine Atrial Al Adenosine Receptors M Leid, P.H Franklin, and T.F. Murray Introduction The negative chronotropic properties of adenosine were initially described in the seminal investigations of Drury and Szent-Gyorgyi (1929). Adenosine and adenine nucleotides are now generally recognized to have physiologically significant and potent cardiovascular actions. In the heart, adenosine exerts a multiplicity of actions including vasodilation of coronary vasculature, depression of sinoatrial and atrioventricular nodal activity, inhibition of atrial contractility, attenuation of the stimulatory effects of catecholamines in the ventricular myocardium, and a depression of ventricular automaticity (Bel ardinell i et al., 1989). The cardioinhibitory responses of adenosine are believed to be mediated through an activation of the At subtype of adenosine receptors (Lohse et al., 1985). The pharmacological profile of adenosine receptors labeled with either an antagonist (Lohse et al., 1987; Martens et al., 1987; Leid et al., 1988) or agonist (Linden et al., 1985; Leid et al., 1988) radioligand suggest that the receptor present in cardiac membranes is of the At subtype. We have previously used the agonist radioligand (_)N 6 _ to characterize adenosine At receptors in porcine atrial membranes (Leid et al., 1988). bound saturably to an apparently homogeneous population of sites with a maximum binding capacity of 35 fmol/mg Erotein and a KD of 2.5 nM. Guanine nucleotides negatively modulated [t I]HPIA binding by enhancing its rate of dissociation. More recently, the porcine atrial At adenosine receptor has been solubilized and labeled with ( eid et al., 1989). This agonist radioligand labeled a single homogeneous population of solubilized receptors with a density of 88 fmol/mg protein and a KQ of 1.4 nM. Thus, solubilization afforded a 2.5 fold enrichment of At adenosine receptor specific activity. The solubilized receptor retained its functional integrity with respect to coupling to guanine nucleotide binding proteins inasmuch as addition of guanine nucleotides to solubilized preparations resulted in a rapid and complete dissociation of (Lied et al., 1989). Thus, the At adenosine receptor-G protein complex solubilized from porcine atria appears to provide an excellent system in which At receptor-G protein interactions can be further studied. This report summarizes our recent characterization of the interactions of the antagonist radioligand [3H]8-cyclopentyl-1,3-dipropylxanthine [3H]DPCPX with the solubilized porcine atrial At receptor. 73 K. A. Jacobson et al. (eds.), Purines in Cellular Signaling © Springer-Verlag New York Inc. 1990