A(3) adenosine receptors: Physiological characterization of their role in modulating neutrophil-mediated myocardial reperfusion injury

Reperfusion of ischemic myocardium is necessary to salvage tissue from death. However, reperfusion after even brief periods of ischemia is associated with changes that represent either an acceleration of ischemic processes, or new pathophysiological changes initiated after reperfusion. Neutrophils contribute significantly to this "reperfusion injury" by releasing oxygen free radicals, cytokines and pro-inflammatory mediators, and by adhering to the coronary vascular endothelium. Adenosine exhibits a broad spectrum of effects against neutrophil-mediated events, and therefore may be a useful therapeutic agent against ischemia-reperfusion injury. Many of the protective properties of adenosine have been attributed to the well described A 1 and A2a adenosine receptor subtypes, both of which produce substantial side effects that limit their use clinically. However, the recent discovery of a new adenosine receptor subtype, termed the A3 receptor, may lead to more promising clinical applications of adenosine-related therapy without the untoward effects exhibited by activation of other adenosine receptor subtypes. This dissertation explores the physiologic actions of A3 adenosine receptors on neutrophil functions, and how the modulation of these functions translates to myocardial protection from ischemia-reperfusion injury.; Study 1. This study tested the hypothesis that Cl-IB-MECA, a highly selective A3 adenosine agonist, reduces superoxide production and degranulation from isolated neutrophils, and prevents the adherence of neutrophils to coronary artery endothelium. These experiments demonstrated that Cl-IB-MECA has no direct modulatory effect on superoxide production or release of enzymes (degranulation) from neutrophils. However, there was a potent decrease (back to basal levels) in the adherence of neutrophils to coronary endothelium. Furthermore, this effect was concentration-dependent and blocked by a selective A3 antagonist, but not by A1- or A2a-selective antagonists. When the endothelium was selectively pre-treated with Cl-IB-MECA and activated with thrombin, neutrophil adherence was significantly reduced. Hence, the effect of Cl-IB-MECA may be the result of activation of A3 receptors on the endothelium. A role of A 3 receptors on neutrophils, however, has not been ruled out.; Study 2. This study addressed the ability of Cl-IB-MECA to reduce the effects of post-ischemic neutrophil-mediated myocardial injury. Exposure of globally ischemic, buffer perfused hearts to an infusion of neutrophils at reperfusion significantly reduced left ventricular performance compared to buffer controls. Cl-IB-MECA (10 nM, initiated at reperfusion) significantly improved contractile performance over untreated ischemic-reperfused hearts exposed to neutrophils, while eliciting no additional benefit in the absence of neutrophils. Cl-IB-MECA markedly reduced post-ischemic neutrophil accumulation assessed by both myeloperoxidase activity and by immunohistochemical staining, suggesting that neutrophil inhibition was the mechanism by which post-ischemic functional recovery was improved.; Overall, these studies suggest that activation of A3 adenosine receptors at reperfusion with Cl-IB-MECA has the capacity to protect the heart from neutrophil-mediated reperfusion injury. This protection is likely mediated by an inhibition of neutrophil adherence to the coronary endothelium and subsequent accumulation within the myocardium. Since Cl-IB-MECA can protect the heart from ischemia-reperfusion injury without the side effects seen with A 1 and A2a receptor activation, Cl-IB-MECA and other selective A3 adenosine receptor agonists may represent a clinically relevant therapeutic strategy for the treatment of myocardial ischemia-reperfusion.