Cardioprotection Induced by Adenosine A1 Receptor Agonists in a Cardiac Cell Ischemia Model Involves Cooperative Activation of Adenosine A2A and A2B Receptors by Endogenous Adenosine

Extracellular adenosine concentrations increase within the heart during ischemia, and any exogenous adenosine receptor agonists therefore work in the context of significant local agonist concentrations. We evaluated the interactions between A(1), A(2A), A(2B), and A(3) receptors in the presence and absence of adenosine deaminase (ADA, which is used to remove endogenous adenosine) in a cardiac cell ischemia model. Simulated ischemia (SI) was induced by incubating H9c2(2-1) cells in SI medium for 12 hours in 100% N-2 gas before assessment of necrosis using propidium iodide (5 mu M) or apoptosis using AnnexinV-PE flow cytometry. N-6- Cyclopentyladenosine (CPA; 10(-7)M) and N-6-(3-iodobenzyl) adenosine-5'- N-methyluronamide (IB-MECA; 10(-7)M) reduced the proportion of nonviable cells to 30.87 +/- 2.49% and 35.18 +/- 10.30%, respectively (% of SI group). In the presence of ADA, the protective effect of CPA was reduced (62.82 +/- 3.52% nonviable), whereas the efficacy of IB-MECA was unchanged (35.81 +/- 3.84% nonviable; P < 0.05, n = 3-5, SI vs. SI + ADA). The protective effects of CPA and IB-MECA were abrogated in the presence of their respective antagonists DPCPX (8-cyclopentyl-1,3-dipropylxanthine) and MRS1191 [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine3,5-dicarboxylate], whereas A(2A) and A(2B) agonists had no significant effect. CPA-mediated protection was abrogated in the presence of both A(2A) (ZM241385, 4-(2-[7- amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-lamino]ethyl)phenol; 50 nM) and A(2B) (MRS1754, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxylphenyl]-1,3-di(n-propyl) xanthine; 200 nM) antagonists (n = 3-5, P < 0.05). In the absence of endogenous adenosine, significant protection was observed with CPA in presence of CGS21680 (4-[2-[[6-amino-9-(N-ethyl-b-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid) or LUF5834 [2-amino4-(4-hydroxyphenyl)-6-(1H-imidazol-2-ylmethylsulfanyl)pyridine-3,5-dicarbonitrile] (P < 0.05 vs. SI + ADA + CPA). Apoptosis (14.35 +/- 0.15% of cells in SI + ADA group; P < 0.05 vs. control) was not significantly reduced by CPA or IB-MECA. In conclusion, endogenous adenosine makes a significant contribution to A, agonist mediated prevention of necrosis in this SI model by cooperative interactions with both A(2A) and A(2B) receptors, but does not play a role in A(3) agonist-mediated protection.