Ischemic preconditioning (PC) is the well-described phenomenon whereby brief episodes of myocardial ischemia render cardiomyocytes resistant to a later, sustained ischemic insult. Evidence from our group has shown that the benefits of PC extend beyond the myocyte per se: i.e., antecedent PC ischemia attenuates platelet activation-aggregation and improves vessel patency in damaged and stenotic arteries - a favorable effect that is triggered, at least in part, by stimulation of adenosine A2 receptors on the platelets' surface. These data suggest that adenosine A2-mediated inhibition of platelet activation-aggregation may, in future, be used in the design of new clinical therapies to attenuate thrombotic events in patients with acute ischemic syndromes. However: (1) all studies to date have been conducted in adult cohorts; and (2) recent preliminary in vitro results suggest that the beneficial anti-thrombotic effect of A2 receptor stimulation may be lost in blood samples obtained from aging populations. Accordingly, our current aims are to: (1) establish the physiologic relevance of our preliminary findings to the in vivo setting of recurrent thrombosis; and (2) obtain initial insight into the mechanisms that underlie the apparently diminished responsiveness of platelet adenosine A2 receptors in old cohorts. To address Aim 1 (and test our primary hypothesis that the anti- thrombotic effects of PC/A2 stimulation wane with age), we will utilize novel rat and mouse models of spontaneous, recurrent arterial (femoral and coronary) thrombosis - models that mimic the key pathophysiologic features of recurrent ischemia seen in unstable angina. We will, via measurement of blood flow and tissue perfusion, compare the effects of prophylactic treatment with PC ischemia and CGS 21680 (a potent A2 receptor agonist) on arterial patency in senescent 2-year old animals versus adults. To explore Aim 2, we will harvest platelets from 2-year old versus adult rats to test our second ancillary hypothesis: that the age-associated loss of the anti-thrombotic effects of PC/A2 stimulation is a consequence of: (1) a reduction in A2 receptor protein; (2) diminished A2 receptor responsiveness, and/or (3) defects in distal signaling. Resolution of these issues is crucial for any future clinical application of A2 receptor stimulation, as the middle-aged and elderly are precisely the population in whom acute coronary syndromes are most common and thus anti-thrombotic treatment strategies are most relevant. ? ? ?
