Summary Background Intravital studies performed in the mouse microcirculation show that hemostatic thrombi formed after penetrating injuries develop a characteristic architecture in which a core of fully-activated, densely-packed platelets is overlaid with a shell of less activated platelets. Objective Large differences in hemodynamics and vessel wall biology distinguish arteries from arterioles. Here we asked whether these differences affect the hemostatic response and alter the impact of anticoagulants and antiplatelet agents. Methods Approaches previously developed for intravital imaging in the mouse microcirculation were adapted to the femoral artery, enabling real time fluorescence imaging despite the markedly thicker vessel wall. Results Arterial thrombi initiated by penetrating injuries developed the core-and-shell architecture previously observed in the microcirculation. However, although platelet accumulation was greater in arterial thrombi, the kinetics of platelet activation were slower. Inhibiting platelet ADP P2Y12 receptors destabilized the shell and reduced thrombus size without affecting the core. Inhibiting thrombin with hirudin suppressed fibrin accumulation, but had little impact on thrombus size. Removing the platelet collagen receptor, glycoprotein VI, had no effect. Conclusions These results 1) demonstrate the feasibility of performing high speed fluorescence imaging in larger vessels and 2) highlight differences as well as similarities in the hemostatic response in the macro- and microcirculation. Similarities include the overall core-and-shell architecture. Differences include the slower kinetics of platelet activation and a smaller contribution from thrombin, which may be due in part to the greater thickness of the arterial wall and the correspondingly greater separation of tissue factor from the vessel lumen.

Platelets play a central role in the pathogenesis of coronary artery disease (CAD). Mean platelet volume (MPV) is an indicator of platelet activation, and has been demonstrated to be correlated with platelet reactivity. Diabetic patients have been shown to have larger MPV, that may contribute to higher platelet reactivity and atherothrombotic complications observed in these patients. Therefore, the aim of the current study was to investigate whether MPV is associated with platelet reactivity and the extent of CAD among diabetic patients. We performed a cohort study including 1016 consecutive diabetic patients undergoing coronary angiography at the University Hospital 'Maggiore della Carita', Novara, Italy. CAD is defined as stenosis above 50% in at least one coronary vessel at coronary angiography. Platelet reactivity was evaluated in 50 diabetic patients without history of CAD and who were free (in the past month) from medications which may affect platelet aggregation. Platelet aggregation was evaluated by light transmission aggregometry after stimulation with 1 μg/ml collagen type I. We additionally evaluated platelet surface expression of P-selectin after stimulation with U46619 (a stable synthetic analogue of the prostaglandin PGH2) and plasma concentration of thromboxane B2 (TxB2). Patients were grouped according to tertile values of MPV (<10.6 fl, group 1; 10.6-11.3 fl, group 2; >11.4 fl, group 3). MPV was associated with age (P=0.011), baseline fasting glucose (P=0.044), glycosylated haemoglobin (P=0.005), creatinine (P=0.052) and haemoglobin (P=0.003), but inversely related to platelet count (P<0.001) and triglycerides (P=0.031). Larger MPV was associated with therapy with statins (P=0.012) and diuretics (P=0.021). CAD was observed in 826 patients (81.3%). MPV was not associated with the prevalence of CAD [odds ratio (OR), 0.85 (0.7-1.03), P=0.11]. The results were confirmed in terms of severe CAD [OR, 1.03 (0.88-1.21), P=0.7]. The absence of any significant relationship between MPV and CAD was confirmed after correction for baseline confounding factors [OR, 0.9 (0.75-1.08), P=0.19]. Finally, MPV was not related to platelet reactivity. This is the first study showing that in diabetic patients MPV is not related to platelet reactivity and the prevalence and extent of CAD. Therefore, MPV may not be considered a risk factor for CAD among diabetic patients.

Key Points• Coordinated thromboxane A 2 and ADP/P2Y 12 signaling is required for platelet accumulation in the outer shell region of hemostatic plugs.• Platelet activation within the hemostatic plug core region is predominantly mediated by thrombin.The local microenvironment within an evolving hemostatic plug shapes the distribution of soluble platelet agonists, resulting in a gradient of platelet activation. We previously showed that thrombin activity at a site of vascular injury is spatially restricted, resulting in robust activation of a subpopulation of platelets in the hemostatic plug core. In contrast, adenosine 59-diphosphate (ADP)/P2Y 12 signaling contributes to the accumulation of partially activated, loosely packed platelets in a shell overlying the core. The contribution of the additional platelet agonists thromboxane A 2 (TxA 2 ) and epinephrine to this hierarchical organization was not previously shown. Using a combination of genetic and pharmacologic approaches coupled with real-time intravital imaging, we show that TxA 2 signaling is critical and nonredundant with ADP/P2Y 12 for platelet accumulation in the shell region but not required for full platelet activation in the hemostatic plug core, where thrombin activity is highest. In contrast, epinephrine signaling is dispensable even in the presence of a P2Y 12 antagonist. Finally, dual P2Y 12 and thrombin inhibition does not substantially inhibit hemostatic plug core formation any more than thrombin inhibition alone, providing further evidence that thrombin is the primary driver of platelet activation in this region. Taken together, these studies show for the first time how thrombin, P2Y 12 , and TxA 2 signaling are coordinated during development of a hierarchical organization of hemostatic plugs in vivo and provide novel insights into the impact of dual antiplatelet therapy on hemostasis and thrombosis.