Selective activation of adenosine A3 receptors with N6-(3-chlorobenzyl)5′-N-methylcarboxamidoadenosine (CB-MECA) provides cardioprotection via KATP channel activation

Objective: The aim of this study was to characterize the adenosine A, receptor agonist, N-6-( 3-chlorobenzyl)-5'-N-methylcarboxamidoadenosine (CB-MECA), evaluate its ability to reduce myocardial ischemia/reperfusion injury and determine the role of K-ATP-channel activation in A(3) receptor-mediated cardioprotection. Methods: Binding affinities and adenylate cyclase inhibition were examined in CHO cells expressing rabbit recombinant adenosine A(1) or A(3) receptors. Infarct size (normalized for area-at-risk; % IA/AAR) was measured in buffer-perfused rabbit hearts exposed to 30-min regional ischemia and 120 min of reperfusion. Results: CB-MECA was 100-fold selective for A(3) vs. A(1) receptors (A(3) K-i: 1 nM; A(1) K-i: 105 nM). Five-min perfusion with CB-MECA before ischemia/reperfusion elicited a concentration-dependent reduction in infarct size (EC50: 0.3 nM). The CB-MECA-dependent cardioprotection (control: 58+/-2; CB-MECA: 21+/-3% IA/AAR) was unchanged by an A(1)-selective concentration of the antagonist, BWA1433, but was completely prevented (P<0.05) by a nonselective (A(1)/A(3)) concentration (55+/-6% IA/AAR). The K-ATP channel inhibitors, glibenclamide and 5-HD, had no effect on control infarct size, yet significantly (P<0.05) blunted the CB-MECA-dependent cardioprotection (glibenclamide: 49+/-6; 5-HD: 58+/-4% IA/AAR). Conclusions: CB-MECA is a novel 100-fold A(3) receptor-selective agonist which should prove useful for elucidating A(3)-dependent mechanisms in the rabbit heart. Selective stimulation of adenosine A(3) receptors with CB-MECA reduces myocardial ischemia/reperfusion injury via a mechanism which involves activation of K-ATP channels. (C) 1998 Elsevier Science B.V. All rights reserved.