Abstract 181: β-Adrenergic Preconditioning Is Adenosine Mediated

This study investigated the roles of adenosine, oxygen free radicals (ROS), the K + ATP channels and the PI 3 K/PKB/Akt and ERK signal transduction pathways during the triggering and mediation phases of beta-adrenergic preconditioning (BPC) Methods : Using the isolated working rat heart, BPC was elicited by administration of denopamine (B 1 receptor agonist, 10 -7 M), isoproterenol (B 1 /B 2 receptor agonist, 10 -7 M) or formoterol (B 2 receptor agonist, 10 -9 M) for 5 min followed by 5 min washout. Index ischemia was 35 min regional ischemia followed by 2h reperfusion and infarct size was determined with the tetrazolium method. The role of adenosine was studied using selective adenosine receptor antagonists. Involvement of ROS, PKC, the mitochondrial K + ATP channels, endogenous opioids and bradykinin was studied by using N-acetyl cysteine (NAC), bisindolylmaleimide, 5-HD, naloxone or HOE140, respectively. Wortmannin and PD98,059 were used to inhibit PI 3 K and ERK respectively. Phosphorylation of these kinases was determined by Western blot. Results: Preconditioning with all three B-adrenergic receptor agonists caused a significant (40-60%) reduction in infarct size and improvement in postischemic function. BPC with isoproterenol and denopamine was abolished by the adenosine A 3 receptor antagonist MRS1191 during both the triggering and mediation phases, while being effective during reperfusion only in formoterol preconditioned heart s. B 1 /B 2 PC was attenuated by MRS1754 (adenosine A 2B receptor antagonist) during the triggering and mediation phases. The mediation phase of B 1 /B 2 PC was also abolished by ZM241385 (A 2A receptor blocker). The adenosine A 1 receptor blocker, DPCPX, was without effect. The ROS scavenger NAC caused a significant attenuation of cardioprotection induced by all three B-adrenergic receptor agonists. 5-HD, naloxone and Hoe140 were without effect on BPC. Activation of ERK and PI3K/PKB/Akt during both triggering and reperfusion phases are required for cardioprotection. Conclusion: BPC in rat hearts is dependent on activation of the A 3 adenosine receptors by endogenous adenosine and production of free radicals during the triggering and mediation phases while the A 2A and A 2B receptors participate mainly during reperfusion.