Broad-spectrum cardioprotection with adenosine.

Ischemia-reperfusion results in contractile dysfunction, necrosis, and vascular injury. This postischemic injury is mediated in part by superoxide radical production, neutrophils, dysfunction to ionic pumps, and edema formation. Adenosine is an autacoid released tonically by myocytes, endothelium, and neutrophils; the release of adenosine from the myocyte compartment into the interstitium is increased during ischemia. The major effects of adenosine are mediated by specific receptors identified as A1, A2a, A2b, and A3. Each receptor subtype contributes to physiological responses that influence ischemia-reperfusion injury. Adenosine has potent cardioprotective properties exerted during three major windows of opportunity: pretreatment, ischemia, and reperfusion. The cardioprotective effects exerted during pretreatment and ischemia may involve metabolic changes and hyperpolarization via K(ATP)-channel activation, mediated through A1 receptor mechanisms. The cardioprotective mechanisms exerted during reperfusion involve inhibition of neutrophils directly (superoxide anion generation, expression of adhesion molecules), and by inhibiting activation of the endothelium through A2 receptor-mediated mechanisms, thereby preventing neutrophil-endothelial cell interactions, which initiate the inflammatory-like component of reperfusion injury. Activation of the newly identified A3 receptor has been shown to be cardioprotective partially by inhibition of neutrophil adherence to endothelium and by neutrophil-independent mechanisms. These mechanisms of cardioprotection have been suggested to play major roles in the reduction of infarction and apoptosis after myocardial ischemia, cardioplegic arrest, and subsequent reperfusion. Adenosine has been used as an adjunct to both crystalloid and blood cardioplegia, but its potential as a cardioprotective agent has not been fully explored.