RT Journal Article SR Electronic T1 Characterization of agonist radioligand interactions with porcine atrial A1 adenosine receptors. JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 334 OP 339 VO 34 IS 3 A1 M Leid A1 M I Schimerlik A1 T F Murray YR 1988 UL http://molpharm.aspetjournals.org/content/34/3/334.abstract AB The agonist radioligand (-)-N6-[125I]-p-hydroxyphenylisopropyl-adenosine ([ 125I]HPIA) was used to characterize adenosine recognition sites in porcine atrial membranes. [125I]HPIA showed saturable binding to an apparently homogeneous population of sites with a maximum binding capacity of 35 +/- 3 fmol/mg of protein and an equilibrium dissociation constant of 2.5 +/- 0.4 nM. Kinetic experiments were performed to address the molecular mechanism of [125I]HPIA binding in porcine atrial membranes. [125I]HPIA apparently interacts with the cardiac adenosine receptor in a simple bimolecular reaction. A kinetically derived [125I] HPIA dissociation constant (2.4 nM) was in good agreement with that parameter measured at equilibrium. Guanyl nucleotides negatively modulated [125I]HPIA binding by increasing its rate of dissociation. This finding is consonant with the formation of a ternary complex in porcine atrial membranes, consisting of ligand, receptor, and guanyl nucleotide-binding protein. Prototypic adenosine receptor agonists and antagonists inhibited specific binding in a manner consistent with the labeling of an A1 adenosine receptor. The results of these experiments suggest that the adenosine receptor present in porcine atrial membranes, as labeled by [125I]HPIA, is of the A1 subtype.