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

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Abstract
PD 81,723 (PD) acts allosterically to increase agonist binding to A1 adenosine receptors and to enhance functional A1 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 A1 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 [125I]N6-4-amino-3-iodobenzyladenosine (125I-ABA) to A1 receptors by increasing the t1/2 for dissociation by 2.18-fold, but PD had no effect on the dissociation kinetics of 125I-ABA from A3 receptors or [125I]-[2-(4-amino-3-iodo-phenyl)ethylamino]adenosine from A2A receptors. Glibenclamide at concentrations up to 10 μmol/L had no effect on the binding of radioligands to recombinant canine A1, A2A, or A3 receptors. These data suggest that PD reduces the amount of time required for ischemia to produce preconditioning by enhancing adenosine binding to its A1 receptor. Glibenclamide prevents the protection afforded by A1 receptor activation by a mechanism not involving adenosine receptor blockade.

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