Preconditioning ischemia attenuates molecular indices of platelet activation‐aggregation

Abstract

Summary. Background: Previous studies have shown that ischemic preconditioning (PC) not only limits infarct size, but also improves arterial patency in models of recurrent thrombosis. We hypothesize that this enhanced patency is presumably because of a PC-induced attenuation of platelet-mediated thrombosis. However, there is, at present, no direct evidence that PC acts on the platelets per se and favorably downregulates platelet reactivity. Objectives: Our goal was to test the concept that PC ischemia attenuates molecular indices of platelet activation-aggregation. Methods: Anesthetized dogs were randomly assigned to receive 10 min of PC ischemia followed by 10 min of reperfusion or a time-matched control period. Spontaneous recurrent coronary thrombosis was then initiated in all dogs by injury + stenosis of the left anterior descending coronary artery. Coronary flow was monitored for 3 h poststenosis, and molecular indices of platelet activationaggregation were quantified by whole blood flow cytometry. Results: Coronary patency was, as expected, better-maintained following injury + stenosis in the PC group vs. controls (53% ± 5%* vs. 23% ± 5% of baseline flow, respectively; *P < 0.05). Moreover, PC was accompanied by: (i) a significant down-regulation of platelet-fibrinogen binding and formation of neutrophil-platelet aggregates (112% ± 14%* vs. 177% ± 21% and 107% ± 8%* vs. 155% ± 19% of baseline values in PC vs. control groups); and (ii) a trend towards a reduction in platelet P-selectin expression (148% ± 12% vs. 190% ± 21% of baseline; *P < 0.05 and P ¼ 0.09 vs. control). Conclusion: These data provide novel, direct evidence in support of the concept that ischemic PC attenuates molecular indices of platelet activation-aggregation.

Effect of adenosine A2 receptor stimulation on platelet activation–aggregation: Differences between canine and human models

“…The CGS-treated cohort was part of a larger, randomized 3-group study design, with the remaining dogs assigned to receive 10 min of preconditioning ischemia followed by 10 min of reperfusion (n=10), or a matched no-intervention control period (n=12). In vivo data for the control and preconditioned groups have been reported previously [8].…”

“…A left lateral thoracotomy was performed and two adjacent segments of the left anterior descending coronary artery (LAD) were isolated: the distal segment was instrumented with a Doppler flow probe for continuous measurement of mean coronary blood flow (CBF), while the proximal segment served as the site of later thrombosis (methods described in [8]). …”

“…Following the 20 min treatment phase, spontaneous recurrent thrombosis was initiated by squeezing the isolated LAD segment with forceps and applying a stenosis at the site of vessel damage [1,8,18,19,22]. CBF was monitored for 3 hours without further intervention.…”

“…Moreover, release of adenosine from ischemic-reperfused cardiomyocytes has been hypothesized to contribute to the favorable attenuation of recurrent thrombosis evoked by brief antecedent preconditioning ischemia [1][2][3]7,8]. These improvements in patency have been proposed to be a consequence of the welldocumented, platelet inhibitory effects of adenosine, initiated via stimulation of adenosine A 2 receptors on the platelets' surface [1][2][3][4][5][6][8][9][10][11][12]. However, there is at present no direct evidence to substantiate this premise, and the specific site of action of adenosine/adenosine agonists (i.e., stimulation of A 2 receptors on platelets versus other blood-borne elements and/or vascular smooth muscle) has not been established.…”

“…In addition, given the recognized differences among species in platelet responsiveness and signaling [13][14][15][16][17], it is possible that the site and mechanisms of A 2 receptor stimulation may be model-dependent and, most notably, may differ in humans. Accordingly, in the current study, we apply an integrated approach, employing a classic in vivo canine model of recurrent coronary thrombosis (the 'Folts' model [1,8,18,19]), in vitro analysis of canine blood samples and in vitro analysis of blood samples obtained from healthy human volunteers to test the hypothesis that the anti-thrombotic effects of adenosine A 2 receptor agonist treatment involve stimulation of A 2 receptors on platelets and a resultant down-regulation in one or more molecular markers of platelet activation-aggregation.…”