Both A2a and A2b adenosine receptors at reperfusion are necessary to reduce infarct size in mouse hearts

Abstract

Pre- and postconditioning depend on the activation of adenosine receptors (ARs) at the end of the index ischemia. The aim of this study was to determine which receptor subtypes must be activated. In situ mouse hearts underwent 30 min of regional ischemia, followed by 2 h of reperfusion. As expected, either ischemic postconditioning (6 cycles of 10 s of reperfusion and 10 s of coronary occlusion) or infusion of the selective A(2b) adenosine receptor (A(2b)AR) agonist BAY60-6583 (BAY60) for 60 min, starting 5 min before reperfusion reduced infarct size in wild-type C57Bl/6N mice. Protection from either was abolished by the selective A(2b)AR antagonist MRS-1754, confirming a role for A(2b)AR. Additionally, the coadministration of ischemic postconditioning and a selective A(2a)AR antagonist led to the loss of protection as well. 5'-Ectonucleotidase (CD73) is thought to be necessary for the production of adenosine during ischemia. As predicted, ischemic postconditioning did not protect CD73 knockout mice. Selective agonists of either A(2b)AR (BAY60) or A(2a)AR (CGS-21680), as well as the coadministration of ischemic postconditioning and BAY60, also failed to protect hearts of the CD73 knockout mice. But the nonselective A(1)/A(2)AR agonist 5'-(N-ethylcarboxamido)adenosine (NECA) was protective, suggesting that the activation of multiple AR subtypes might be required. The coadministration of CGS-21680 and BAY60 also elicited profound protection, indicating that two AR subtypes, A(2a) and A(2b), must be simultaneously activated for protection to occur.

Fig. 1.

Results of open chest experiments in C57Bl/6N wild-type mice. Postconditioning (PostC) as well as the A_2b adenosine receptor (A_2bAR) agonist BAY60-6583 (BAY60) given at reperfusion reduced infarct size compared with untreated control hearts. The protective effect of PostC could be abolished by the selective A_2bAR antagonist MRS-1754 and the selective A_2aAR antagonist SCH-442416. While MRS-1754 blocked the protective effect of BAY60, it had no effect when given alone. Open circles represent infarct size as percentage of risk zone for individual hearts, and closed circles show the group means ± SE. ***P < 0.001 vs. control.

Fig. 2.

Results of open-chest experiments in CD73^−/− knockout mice. PostC as well as activation of a single AR subtype with either BAY60 or CGS-21680 and coadministration of PostC and BAY60 failed to protect. In contrast, the nonselective adenosine agonist 5′-(N-ethylcarboxamido)adenosine (NECA) or cotreatment with A_2aAR and A_2bAR agonists produced profound protection, indicating a requirement for both AR subtypes. **P < 0.01 vs. control.