PD-81,723, AN ALLOSTERIC ENHANCER OF THE A(1) ADENOSINE RECEPTOR, LOWERS THE THRESHOLD FOR ISCHEMIC PRECONDITIONING IN DOGS

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 th e threshold for ischemic preconditioning (PC), pentobarbital-anestheti zed dogs were subjected to 60 minutes of left anterior descending coro nary artery (LAD) occlusion and 3 hours of reperfusion. Ischemic PC wa s produced by either 2.5 or 5 minutes of LAD occlusion 10 minutes befo re the 60-minute occlusion. PD (100 mu g/kg total dose, 5 to 50 mu mol /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 inf arct size was determined by triphenyltetrazolium staining and expresse d as a percentage of the area at risk. Compared with the control group (26.3+/-3.6%, mean+/-SEM), infarct size was not significantly affecte d 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 perio d (5 minutes) of PC alone (12.5+/-3.3%). The intravenous administratio n of the selective antagonist of Al adenosine receptors, 8-cyclopentyl -1,3-dipropylxanthine (1 mg/kg), or the ATP-sensitive K+ channel block er, 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 co mpounds studied affected systemic hemodynamics, collateral blood flow, or AAR. To determine which subtypes of canine adenosine receptors wer e affected by 10 mu mol/L PD, radioligand binding studies were conduct ed using membranes derived from COS-7 cells expressing recombinant can ine receptors and agonist radioligands. PD enhanced the binding of [I- 125]N-6-4-amino-3-iodobenzyladenosine (I-125-ABA) to A(1) receptors by increasing the t(1/2) for dissociation by 2.18-fold, but PD had no ef fect on the dissociation kinetics of I-125-ABA from A, receptors or [I -125]-[2-(4-amino-3-iodo-phenyl)ethylamino] adenosine from Az, recepto rs. Glibenclamide at concentrations up to 10 mu mol/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 req uired for ischemia to produce preconditioning by enhancing adenosine b inding to its A(1) receptor. Glibenclamide prevents the protection aff orded by A(1) receptor activation by a mechanism not involving adenosi ne receptor blockade.