Postconditioning reduces infarct size via adenosine receptor activation by endogenous adenosine
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Objective This study tested the hypothesis that brief cycles of iterative ischemia-reperfusion at onset of reperfusion (termed "postconditioning", post-con) delays washout of intravascular adenosine and thereby increases endogenous adenosine receptor (AR) activation during the early moments of reperfusion (R). Methods Isolated mouse hearts were subjected to 20 min global ischemia (I) and 30 min R with or without post-con (3 or 6 cycles of 10 s R&I). Intravascular purines in coronary effluent were analyzed by HPLC. To assess the functional role of endogenous AR activation in post-con, an open-chest rat model of myocardial infarction was employed. Rats were randomly divided into 11 groups: control, no intervention at R; post-con, three cycles of 10 s R followed by 10 s LCA re-occlusion immediately upon R. In the following interventions, drugs (or vehicle) were administered 5 min before R in the absence or presence (᩠of post-con. Vehicle (DMSO <300 嬯kg); 8-SPT (non-selective AR antagonist, 10 mg/kg) ᠰost-con; DPCPX (A1AR antagonist, 0.1 mg/kg) ᠰost-con; ZM241385 (A2AAR antagonist, 0.2 mg/kg) ᠰost-con; MRS1523 (A3AR antagonist, 2 mg/kg) ᠰost-con. Results In isolated mouse hearts, post-con reduced diastolic pressure during both early (26 ᠳ* vs. 37 ᠳ mmHg at 5 min) and late (22 ᠳ* vs. 34 ᠳ mmHg at 30 min) R. Post-con also hastened the early recovery of contractile function (developed pressure 39 ᠶ* vs. 16 ᠲ mmHg at 5 min R), although differences did not persist at 30 min R. Importantly, post-con was associated with reduced adenosine washout (58 ᠵ* vs. 155 ᠱ6 nM/min/g) at 2 min R suggesting greater retention time of intravascular adenosine. In rats, post-con significantly attenuated infarct size compared to control (40 ᠳ% vs. 53 ᠲ%* in control), an effect that was unaltered by DPCPX (42 ᠲ%) but was abrogated by 8-SPT (50 ᠲ%), ZM241385 (49 ᠳ%) or MRS1523 (52 ᠱ%) (P<0.02). Conclusion These data suggest that post-con involves endogenous activation of A2A and A3 but not A1AR subtypes. This activation may be linked to the delay in the washout of intravascular adenosine during the early minutes of R during which post-con is applied.
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