PD 81,723, an Allosteric Enhancer of the A 1 Adenosine Receptor, Lowers the Threshold for Ischemic Preconditioning in Dogs

Abstract

PD 81,723 (PD) acts allosterically to increase agonist binding to A 1 adenosine receptors and to enhance functional A 1 receptor–mediated responses in the heart and other tissues. To determine if PD lowers the threshold for ischemic preconditioning (PC), pentobarbital-anesthetized dogs were subjected to 60 minutes of left anterior descending coronary artery (LAD) occlusion and 3 hours of reperfusion. Ischemic PC was produced by either 2.5 or 5 minutes of LAD occlusion 10 minutes before the 60-minute occlusion. PD (100 μg/kg total dose, 5 to 50 μmol/L in coronary arterial blood) or vehicle was infused intracoronarily for 17.5 minutes before the 60-minute occlusion period in non-PC dogs or in dogs preconditioned with 2.5 minutes of ischemia. Myocardial infarct size was determined by triphenyltetrazolium staining and expressed as a percentage of the area at risk. Compared with the control group (26.3±3.6%, mean±SEM), infarct size was not significantly affected by 2.5 minutes of PC alone (23.4±4.2%) or by PD alone (26.5±1.7%) but was decreased by PD+PC (14.6±1.7%, P <.05) or by a longer period (5 minutes) of PC alone (12.5±3.3%). The intravenous administration of the selective antagonist of A 1 adenosine receptors, 8-cyclopentyl-1,3-dipropylxanthine (1 mg/kg), or the ATP-sensitive K + channel blocker, glibenclamide (0.3 mg/kg), for 15 minutes before PD+PC blocked the protection (23.6±2.3% or 25.9±3.3%, respectively). None of the compounds studied affected systemic hemodynamics, collateral blood flow, or AAR. To determine which subtypes of canine adenosine receptors were affected by 10 μmol/L PD, radioligand binding studies were conducted using membranes derived from COS-7 cells expressing recombinant canine receptors and agonist radioligands. PD enhanced the binding of [ 125 I] N 6 -4-amino-3-iodobenzyladenosine ( 125 I-ABA) to A 1 receptors by increasing the t 1/2 for dissociation by 2.18-fold, but PD had no effect on the dissociation kinetics of 125 I-ABA from A 3 receptors or [ 125 I]-[2-(4-amino-3-iodo-phenyl)ethylamino]adenosine from A 2A receptors. Glibenclamide at concentrations up to 10 μmol/L had no effect on the binding of radioligands to recombinant canine A 1 , A 2A , or A 3 receptors. These data suggest that PD reduces the amount of time required for ischemia to produce preconditioning by enhancing adenosine binding to its A 1 receptor. Glibenclamide prevents the protection afforded by A 1 receptor activation by a mechanism not involving adenosine receptor blockade.