Kinetics and imaging of indium-11-labeled autologous platelets in experimental myocardial infarction

Platelets in cardiac ischaemia/reperfusion injury: a promising therapeutic target

Acute myocardial infarction (AMI) is the single leading cause of mortality and morbidity worldwide. A key component of AMI therapy is the timely reopening of occluded vessels to prevent further ischaemic damage to the myocardium. However, reperfusion of the ischaemic myocardium can itself trigger reperfusion injury causing up to 50% of the overall infarct size. In recent years, considerable research has been devoted to understanding the pathogenesis of ischaemia/reperfusion (I/R) injury and platelets have emerged as a major contributing factor. This review summarizes the role of platelets in the pathogenesis of I/R injury and highlights the potential of platelet-directed therapeutics to minimize cardiac I/R injury. Activated platelets infiltrate specifically into the ischaemic/reperfused myocardium and contribute to I/R injury by the formation of microthrombi, enhanced platelet–leucocyte aggregation, and the release of potent vasoconstrictor and pro-inflammatory molecules. This review demonstrates the benefits of platelet inhibition beyond their well-described anti-thrombotic effect and highlights the direct cardioprotective role of anti-platelet drugs. In particular, the inhibition of COX, the P2Y12 receptor and the GPIIb/IIIa receptor has demonstrated the potential to attenuate I/R injury. Moreover, targeting of drug candidates or regenerative cells to the activated platelets accumulated within the ischaemic/reperfused myocardium shows remarkable potential to protect the myocardium from I/R injury. Overall, activated platelets play a key role in the pathogenesis of I/R injury. Their direct inhibition as well as their use as epitopes for site-directed therapy is a unique and promising therapeutic approach for the prevention of I/R injury and ultimately the preservation of cardiac function.

Platelet deposition in remote cardiac regions after coronary occlusion

BACKGROUND

Activated platelets might contribute to endothelial dysfunction in non-ischaemic territories during acute myocardial infarction. We assessed platelet deposition, coronary flow reserve and contractile function in remote cardiac regions after transient coronary occlusion and their association with systemic platelet activation.

MATERIALS AND METHODS

In 10 pigs (series A) subjected to 48-min occlusion of the left anterior descending coronary artery (LAD), 99mTc-platelet content in the right coronary artery (RCA) and its dependent myocardium was counted after reflow. In 10 pigs (series B) receiving the same occlusion of the RCA, the hyperaemic response at the LAD and systolic shortening in LAD-dependent myocardium were monitored after reperfusion. P-selectin expression on circulating platelets was assessed in both series by flow cytometry.

RESULTS

In series A, platelet counts in the RCA and non-ischaemic myocardium were correlated with platelet content, polymorphonuclear leukocyte infiltration and infarct size in the reperfused zone, as well as with the percentage of P-selectin-positive platelets after reflow. In series B, a transient reduction in peak hyperaemic response in the LAD and sustained contractile dysfunction in non-ischemic myocardium were observed after releasing the RCA occlusion, these changes being also correlated with platelet activation status.

CONCLUSIONS

Ischaemic injury triggers macro- and microvascular platelet deposition and causes an impairment in coronary flow reserve and contractile function in distant regions of the heart, which are related to activation of circulating platelets.

Thrombin generation during reperfusion after coronary artery bypass surgery associates with postoperative myocardial damage

BACKGROUND

Cardiopulmonary bypass and coronary artery bypass grafting (CABG) result in significant thrombin generation and activation of fibrinolysis. Thrombin contributes to myocardial ischemia-reperfusion injury in animal studies, but the role of thrombin in myocardial damage after CABG is unknown.

OBJECTIVES

We measured thrombin generation and fibrin turnover during reperfusion after CABG to evaluate their associations with postoperative hemodynamic changes and myocardial damage.

METHODS

One hundred patients undergoing primary, elective, on-pump CABG were prospectively enrolled. Plasma prothrombin fragment F(1+2) and D-dimer were measured preoperatively and at seven time points thereafter. Mass of the Mb fraction of creatine kinase (Ck-Mbm) and troponin T (TnT) were measured on the first postoperative day.

RESULTS

Reperfusion induced an escalation of thrombin generation and fibrin turnover despite full heparinization. F(1+2) during early reperfusion associated with postoperative pulmonary vascular resistance index. F(1+2) at 6 h after protamine administration correlated with Ck-Mbm (r = 0.40, P < 0.001) and TnT (r = 0.44, P < 0.001) at 18 h postoperatively. Patients with evidence of myocardial damage (highest quintiles of plasma Ck-Mbm and TnT) had significantly higher F(1+2) during reperfusion than others (P < 0.002). Logistic regression models identified F(1+2) during reperfusion to independently associate with postoperative myocardial damage (odds ratios 2.5-4.4, 95% confidence intervals 1.04-15.7).

CONCLUSIONS

Reperfusion caused a burst in thrombin generation and fibrin turnover despite generous heparinization. Thrombin generation during reperfusion after CABG associated with pulmonary vascular resistance and postoperative myocardial damage.

Platelets activated by transient coronary occlusion exacerbate ischemia-reperfusion injury in rat hearts

Platelets (Plt) accumulate in reperfused myocardium but their effect on myocardial necrosis has not been established. We tested the hypothesis that the effect of Plt depends on their activation status. Pig Plt were obtained before 48 min of coronary occlusion (pre-CO-Plt), 10 min after reperfusion (R-Plt), or after a 60-min sham operation (sham-Plt). Plt were infused into isolated rat hearts (n = 124) and subsequently submitted to 60 min of ischemia and 60 min of reperfusion. P-selectin expression was higher (P = 0.02) in R-Plt than in pre-CO-Plt or sham-Plt. Lactate dehydrogenase (LDH) release during reperfusion was similar in hearts receiving pre-CO-Plt, sham-Plt, or no Plt, but R-Plt increased LDH release by 60% (P = 0.004). Activation of pre-CO-Plt with thrombin increased P-selectin expression and LDH release (P < 0.001), and these results were unaffected by tirofiban. There was a close correlation between P-selectin expression and LDH release (r = 0.84; P < 0.001), and myocardial Plt accumulation (r = 0.85; P < 0.001). We conclude that the deleterious effect of Plt on reperfused myocardium depends on their activation status as represented by P-selectin expression, which is enhanced by ischemia-reperfusion.

Lack of effect of glycoprotein IIb/IIIa blockade on myocardial platelet or polymorphonuclear leukocyte accumulation and on infarct size after transient coronary occlusion in pigs

OBJECTIVES

We sought to assess the effect of glycoprotein (GP) IIb/IIIa blockade on myocardial platelet and polymorphonuclear leukocyte accumulation and on infarct size after coronary injury and transient coronary occlusion (CO) in pigs.

BACKGROUND

It has been suggested that platelet GP IIb/IIIa blockade might reduce the severity of microvascular damage after reperfusion.

METHODS

Sixteen thiopental-anesthetized, open-chest pigs, in whom platelets had been labeled with technetium-99m (99mTc) on the previous day, were submitted to catheter-induced left anterior descending coronary artery (LAD) injury followed by 55 min of CO and 5 h of reperfusion. Five minutes before reflow, the animals were blindly allocated to receive lamifiban (intravenous bolus of 250 microg/kg body weight and continuous infusion of 3 microg/kg per min) or saline.

RESULTS

Lamifiban had a rapid and potent platelet anti-aggregatory effect, as demonstrated by significant prolongation of the bleeding time and profound (approximately 90%) inhibition of ex vivo platelet aggregation, and completely prevented the development of cyclic flow reductions of the LAD (0 vs. 5 +/- 1, one of them followed by re-occlusion, in control animals, p = 0.005). However, compared with animals receiving placebo, those treated with lamifiban had a similar (p = NS) content of (99m)Tc platelets in the reperfused myocardium (288 +/- 40% vs. 205 +/- 27% of the value in the control region, respectively) and similar myeloperoxidase activity (0.50 +/- 0.17 U/g vs. 0.47 +/- 0.17 U/g, respectively) and infarct size (46.8 +/- 12.0% vs. 49.8 +/- 10.5% of the area at risk, respectively). Arteriolar platelet thromboemboli were very rarely seen on histologic analysis. Lamifiban did not modify platelet P-selectin expression in additional studies.

CONCLUSIONS

Platelet GP IIb/IIIa blockade has a potent antithrombotic effect at the culprit lesion, but does not significantly reduce the magnitude of microvascular platelet accumulation or myocardial damage after transient CO.

The major components of human energy balance during chronic beta-adrenergic blockade

PURPOSE

The authors compare the major components of energy balance--dietary food intake, resting metabolic rate, and physical energy expenditure--in patients receiving beta-blocking medications and healthy controls.

METHODS

The authors recruited subjects who were receiving beta-blocking agents (n = 22) from local hospitals and clinics. Twenty-two healthy controls were matched to each patient based on age, weight, and gender. Resting metabolic rates were determined using indirect calorimetry. Three-day dietary intakes and physical activity assessments were analyzed.

RESULTS

There were no between-group differences in calories ingested or self-reported physical activity patterns. However, resting metabolic rates were lower in the beta-blocked subjects (beta-blocked = 1,541 +/- 464 kcals/day; controls = 1,817 +/- 426 kcals/day; P < 0.05).

CONCLUSIONS

beta-adrenergic blockade causes a reduction in resting metabolic rate. Therefore, beta-blocked individuals should increase physical activity, decrease dietary intake, or both to maintain daily energy balance and prevent weight gain that accompanies a reduction in metabolic rate.

Potential interactions between iloprost and SIN-1 on platelet aggregation and myocardial infarct size in vivo

Nitric oxide and prostacyclin are endothelial-derived vasodilators which inhibit platelet aggregation in a synergistic manner. Experiments were designed to examine whether 3-morpholino-sydnonimine (SIN-1) and iloprost have synergistic cardioprotective actions and whether their effects on infarct size are related to inhibition of platelet aggregation. Anaesthetized rabbits (n = 9-10 per group) were subject to 40 min myocardial ischaemia followed by 3 h reperfusion. Infarct size (percentage of area at risk) was not altered significantly by 3 microg kg(-1) min(-1) SIN-1 (29.7 +/- 1.9%), but was reduced by 0.03 microg kg(-1) min(-1) iloprost (24.6 +/- 1.6%) and to a greater extent by the combination of SIN-1 and iloprost (18.8 +/- 1.7%), compared to controls (33.6 +/- 4.7%). In control rabbits, there were reductions in the ex vivo aggregation of platelets in response to ADP or collagen after ischaemia and reperfusion. SIN-1 and iloprost caused some alterations in platelet responses, but combined administration of both drugs did not produce greater effects. Although the reduction in myocardial infarct size was greatest with both drugs, this did not appear to be a synergistic interaction and was not dependent on the effects of the drugs on haemodynamics or platelet aggregation.

Effects of oxidized low density lipoprotein, lipid mediators and statins on vascular cell interactions

The integrin heterodimer CDllb/CD18 (alphaMbeta2, Mac-1, CR3) expressed on monocytes or polymorphonuclear leukocytes (PMN) is a receptor for iC3b, fibrinogen, heparin, and for intercellular adhesion molecule (ICAM)-1 on endothelium, crucially contributing to vascular cell interactions in inflammation and atherosclerosis. In this report, we summarize our findings on the effects of lipid mediators and lipid-lowering drugs. Exposure of endothelial cells to oxidized low density lipoprotein (oxLDL) induces upregulation of ICAM-1 and increases adhesion of monocytic cells expressing Mac-1. Inhibition experiments show that monocytes use distinct ligands, i.e. ICAM-1 and heparan sulfate proteoglycans for adhesion to oxLDL-treated endothelium. An albumin-transferable oxLDL activity is inhibited by the antioxidant pyrrolidine dithiocarbamate (PDTC), while 8-epi-prostaglandin F2alpha (8-epi-PGF2alpha) or lysophosphatidylcholine had no effect, implicating yet unidentified radicals. Sequential adhesive and signaling events lead to the firm adhesion of rolling PMN on activated and adherent platelets, which may occupy areas of endothelial denudation. Shear-resistant arrest of PMN on thrombin-stimulated platelets in flow conditions requires distinct regions of Mac-1, involving its interactions with fibrinogen bound to platelet alphallbbeta3, and with other platelet ligands. Both arrest and adhesion strengthening under flow are stimulated by platelet-activating factor and leukotriene B4, but not by the chemokine receptor CXCR2. We tested whether Mac-1-dependent monocyte adhesiveness is affected by inhibitors of hydroxy-methylglutaryl-Coenzyme A reductase (statins) which improve morbidity and survival of patients with coronary heart disease. As compared to controls, adhesion of isolated monocytes to endothelium ex vivo was increased in patients with hypercholesterolemia. Treatment with statins decreased total and low density lipoprotein (LDL) cholesterol plasma levels, surface expression of Mac-1, and resulted in a dramatic reduction of Mac-1-mediated monocyte adhesion to endothelium. The inhibition of monocyte adhesion was reversed by mevalonate but not LDL in vitro, indicating that isoprenoid precursors are crucial for adhesiveness of Mac-1. Such effects may crucially contribute to the clinical benefit of statins, independent of cholesterol-lowering, and may represent a paradigm for novel, anti-inflammatory mechanisms of action by this class of drugs.

Neutrophil accumulation on activated, surface-adherent platelets in flow is mediated by interaction of Mac-1 with fibrinogen bound to alphaIIbbeta3 and stimulated by platelet-activating factor

We have studied the pathways that lead to arrest and firm adhesion of rolling PMN on activated, surface-adherent platelets. Stable arrest and adhesion strengthening of PMN on thrombin-stimulated, surface-adherent platelets in flow required distinct Ca2+- and Mg2+-dependent regions of Mac-1 (alphaMbeta2), and involved interactions of Mac-1 with fibrinogen, which was bound to platelets via alphaIIbbeta3. Mac-1 also bound to other unidentified ligands on platelets, which were not intracellular adhesion molecule-2 (ICAM-2), heparin, or heparan-sulfate proteoglycans. This was shown by inhibition with mAbs or peptides, by treatment of platelets with heparitinase, and by using platelets with defective alphaIIbbeta3 from a patient with Glanzmann thrombasthenia. Tethering of PMN on platelet ICAM-2 via LFA-1 (alphaLbeta2) was observed, which may facilitate the transition between rolling on selectins and Mac-1-dependent arrest. Arrest and adhesion strengthening was pertussis toxin sensitive and in flow was mainly induced by platelet-activating factor but not through activation of the chemokine receptor CXCR2. In stasis, spreading occurred and the CXCR2 contributed to firm adhesion.

Role of platelets in myocardial ischemia-reperfusion injury in dogs

We investigated the involvement of circulating platelets in myocardial ischemia-reperfusion injury in canine autoperfused heart-lung preparations using filters to deplete platelets and/or leukocytes. The left anterior descending (LAD) coronary artery was occluded for 40 min, followed by 40 min reperfusion, in 3 groups: a leukocyte-platelet-depleted (LPD) group, in which both leukocytes and platelets were depleted; a leukocyte-depleted (LD) group, in which leukocytes alone were depleted; and a control group. There were no differences in hemodynamics or arrhythmias among groups before or during coronary occlusion. After reperfusion, the maximum rate of change in left ventricular pressure during systole and diastole was significantly higher in the LPD group than in the control and LD groups. The LPD group also showed gradual recovery of regional myocardial function and a decrease in the frequency of premature ventricular contractions. The LD group showed a slight improvement in cardiac function and arrhythmias compared with the control group. Although there was no significant difference in the pulmonary arterial plasma level of thromboxane B2 at any stage among groups, the control group showed an increase after reperfusion. These results suggest that platelets are important in reperfusion injury and that the depletion of both leukocytes and platelets effectively protects against reperfusion injury.

Close correlation of the cardioprotective effect of FK409, a spontaneous NO releaser, with an increase in plasma cyclic GMP level

FK409 ((+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexeneamide) , which has been reported by us to be a new spontaneous nitric oxide (NO) releaser, prevented myocardial infarction following occlusion and reperfusion in rat coronary artery and increased plasma cyclic GMP level in rats, dose-dependently and significantly at 32 mg kg-1. Isosorbide dinitrate (ISDN), which is the most popular orally active NO donor used in the treatment of ischaemic cardiovascular diseases, did not show significant effects at 32 mg kg-1 in either experiment. Therefore, it is suggested that FK409 can attenuate myocardial injury during ischaemia and reperfusion in contrast to ISDN and a change in plasma cyclic GMP level may serve as an indicator of the cardioprotective effect of NO-releasing drugs.

Dissociation of the anti-ischaemic effects of cloricromene from its anti-platelet activity

1. Cloricromene is a non-anticoagulant coumarin derivative with anti-platelet and anti-leukocyte properties, which has beneficial effects in various models of ischaemia and shock. 2. We have assessed the effects of cloricromene on (a) ex vivo platelet aggregation, and (b) infarct size using a model of myocardial ischaemia in the anaesthetized rabbit. 3. Cloricromene (1-1000 micrograms kg-1 min-1 for 15 min) induced a dose-dependent inhibition of ex vivo platelet aggregation, causing only a minimal increase in heart rate and no change in mean arterial blood pressure. The inhibitory activity was considerably stronger when platelet aggregation was induced by collagen than by ADP. 4. Cloricromene inhibited ex vivo platelet aggregation in rabbits pretreated with indomethacin (5 mg kg-1) and this inhibition persisted for 30-60 min. 5. The model of myocardial ischaemia involved 1 h occlusion of the first antero-lateral branch of the left coronary artery followed by 2 h of reperfusion. Infusion of cloricromene (30 or 300 micrograms kg-1 min-1), ibuprofen (80 micrograms kg-1 min-1) or vehicle began 15 min prior to occlusion, and continued throughout the experiment. 6. While area at risk was similar for all groups studied, cloricromene (30 or 300 micrograms kg-1 min-1) or ibuprofen caused a reduction in infarct size, and decreased myeloperoxidase activity in the tissue of the infarcted myocardium. 7. Cloricromene at 300 micrograms kg-1 min-1 also reduced the occlusion-induced elevation of the ST-segment of the rabbit electrocardiogram, and inhibited platelet aggregation ex vivo. Ibuprofen or cloricromene at 30 fg kg-1 min-1 had no effect on either the ST-elevation or platelet reactivity.8. Thus, cloricromene exhibits a cardioprotective activity via an inhibition of leukocyte infiltration, in the presence (300 microg kg-l min-1) or absence (30 microg kg-1 min-1) of inhibition of platelet activity ex vivo.The anti-aggregatory activity of cloricromene acts via a mechanism that is either different from, or in addition to, inhibition of cyclo-oxygenase, and is of long duration.

Coronary cyclic flow variations "precondition" ischemic myocardium

BACKGROUND

Repeated brief episodes of myocardial ischemia performed by mechanical clamping of a coronary artery "precondition" the heart and reduce infarct size after a subsequent sustained ischemia. It is not known, however, whether spontaneous episodes of transient ischemia caused by formation of platelet thrombi, which may occur in unstable angina, have a similar cardioprotective effect.

METHODS AND RESULTS

Therefore, our objective was to determine whether brief spontaneous thrombotic episodes of ischemia/reperfusion could limit infarct size and preserve contractile function following 60 minutes (protocol 1) or 90 minutes (protocol 2) of sustained ischemia and 4-4.5 hours of reperfusion in the canine model. Before the sustained coronary occlusion, dogs underwent a 30-minute "treatment" period consisting of: no intervention (control group), four repeated episodes of 3-minute mechanical occlusion plus 5-minute reperfusion (preconditioned group), or coronary artery stenosis and endothelial injury, resulting in a mean of four spontaneous episodes of cyclic flow variations (CFV group) caused by formation and dislodgment of platelet thrombi. In protocol 1 (60-minute sustained ischemia plus 4.5-hour reperfusion), infarct size was significantly smaller in both the preconditioned and CFV groups compared with controls (3.5 +/- 1.4%,* 3.4 +/- 2.1%,* and 9.9 +/- 2.7% of the myocardium at risk, respectively; *p less than 0.05 versus control). In contrast, neither preconditioning nor CFV preserved contractile function: Segment shortening during sustained occlusion was equally depressed at -15% to -20% of baseline values among the three groups and equally stunned at +12% to +18% of baseline during the 4.5 hours of reflow. In protocol 2 (90-minute sustained ischemia plus 4-hour reperfusion), only CFV continued to exert a cardioprotective effect: Infarct size averaged 15.0 +/- 4.1%, 7.4 +/- 2.5%,* and 16.5 +/- 4.4% of the region at risk in the preconditioned, CFV, and control groups, respectively (*p less than 0.05 versus control). Contractile function, however, was similar among all three groups both during 90 minutes of sustained occlusion and throughout 4 hours of reperfusion.

CONCLUSIONS

We therefore conclude that repeated coronary thrombus formation preconditions the ischemic myocardium: In fact, in contrast to mechanical preconditioning, cardioprotection provided by CFV persisted following 90 minutes of sustained coronary occlusion. However, preconditioning by thrombotic or mechanical occlusion neither preserved myocardial contractile function during sustained coronary occlusion nor prevented stunning after reperfusion. These data raise the possibility that clinical episodes of unstable angina prior to acute myocardial infarction may precondition the ischemic myocardium.

Protein C activation following coronary artery occlusion in the in situ porcine heart

BACKGROUND

Activated protein C, the body's natural anticoagulant, may play a role in protecting the heart during and following an occlusion of the left anterior descending coronary artery (LAD) in the porcine heart.

METHODS AND RESULTS

Infusion of 1 unit thrombin over 30 seconds into the LAD of juvenile pigs produced a prolongation of the Xa clotting time (153 +/- 14%) in blood drawn from the anterior interventricular vein (AIV). The action of the anticoagulant was blocked by a polyclonal immunoglobulin G antibody directed against porcine activated protein C. A brief (30 seconds) occlusion of the LAD induced a similar prolongation of the Xa clotting time (138 +/- 11%), which was also blocked by the polyclonal antibody. To determine whether activated protein C helps sustain the heart during and following a 2-minute occlusion, three groups of pigs were studied: 11 controls, six receiving activated porcine protein C, and nine receiving a monoclonal antibody (HPC4) that blocks protein C activation. Relative to the controls, HPC4-treated animals recovered function, as measured by the maximum time derivative of left ventricular pressure and segmental shortening, more slowly and were not able to sustain this recovery. Animals receiving activated protein C recovered more quickly and sustained this recovery.

CONCLUSIONS

These data indicate that an ischemic insult induces rapid activation of protein C in the coronary microcirulation and that blocking this activation impairs recovery.

Reduction in reperfusion injury by blood-free reperfusion after experimental myocardial infarction

Because myocardial reperfusion injury may be caused by various blood constituents, a transient period of blood-free reperfusion was evaluated in closed chest dogs subjected to a 90 min angioplasty balloon occlusion of the left anterior descending coronary artery. In the treated group (n = 13), the balloon remained inflated for an additional 15 min while the infarct vessel was perfused with an acellular oxygenated perfluorochemical emulsion (Fluosol). The balloon was then deflated, permitting blood reperfusion. In the control group (n = 13), the balloon was deflated after 90 min of coronary occlusion. One week after infarction, the area at risk was defined in vivo by monastral blue dye staining, and the area of myocardial necrosis was assessed using triphenyltetrazolium chloride staining with histologic confirmation. Major determinants of infarct size, including rate-pressure product, area at risk and severity of myocardial ischemia (assessed by the extent of ST segment elevation during coronary occlusion), were not significantly different in the two groups. Treated dogs demonstrated a 47% reduction in infarct size expressed as a percent of the area at risk compared with control dogs (27.0 +/- 4.4% versus 50.8 +/- 4.4%, p less than 0.01). Treated dogs also demonstrated a superior global left ventricular ejection fraction (57.5 +/- 2.5% versus 51.0 +/- 2.2%, p less than 0.05) and anterolateral (regional) ejection fraction (32.6 +/- 3.6% versus 19.8 +/- 3.9%, p less than 0.05) compared with values in control dogs assessed by contrast ventriculography after 1 week of reperfusion. It is concluded that a transient period of blood-free reperfusion with an oxygenated perfluorochemical reduces reperfusion injury in a canine model of myocardial infarction.

Kinetics of pulmonary platelet deposition and clearance during thrombin-induced microembolism in rabbits

Using 111In-labeled autologous platelets, we studied the kinetics of pulmonary platelet deposition and clearance in relation to hemodynamic and structural events during thrombin-induced pulmonary microembolism in rabbits. Autologous platelets were radiolabeled and returned to animals prior to infusion of thrombin (100 units/kg over 15 min) (n = 20) or saline (n = 6). All animals were pretreated with tranexamic acid, an inhibitor of fibrinolysis. Thrombin-treated animals manifested progressive increases in mean pulmonary platelet activity, reaching a maximum of 38% above baseline (p less than .0001), whereas no change was observed in saline-treated controls. Animals that died during, or immediately following, thrombin infusion manifested significantly greater increases in pulmonary platelet uptake (mean 1.55 +/- 0.47 times baseline), compared to surviving animals (1.14 +/- 0.16; p less than .05 survivors vs. nonsurvivors). In surviving animals, following cessation of thrombin, pulmonary platelet activity cleared gradually, with a half-time of approximately 12 min. Thrombin reduced circulating platelet counts (p less than .001), increased mean pulmonary artery pressure (13 +/- 3 mm Hg to 18 +/- 6 mm Hg; p less than .0001), and reduced mean systemic arterial pressure (55 +/- 10 mm Hg to 44 +/- 7 mm Hg; p less than .001). The time courses of these events approximated that of thrombin-induced pulmonary platelet uptake. Furthermore, the increase in pulmonary artery pressure occurred predominantly in the group of animals in which the increase in pulmonary radiolabeled platelet activity exceeded the median value of 20%. Postmortem histology showed extensive pulmonary thrombus extending from small arterial to capillary levels in animals that died during, or immediately following, thrombin infusion, but not in surviving animals. Our findings suggest that platelet aggregation plays an important role in the pathogenesis of hemodynamic change following thrombin-induced pulmonary embolization.

Thromboxane synthetase inhibitors reduce infarct size by a platelet-dependent, aspirin-sensitive mechanism

Platelets are suggested to exacerbate ischemia-induced myocardial injury, which has led to the study of various antiplatelet therapies including thromboxane synthetase inhibitors (TXSI). Two such agents, benzylimidazole and OKY-046, reduce infarct size commensurate with a diminution in serum thromboxane B2 formation in anesthetized dogs subjected to 90 minutes of coronary artery occlusion followed by 5 hours of reperfusion. In contrast, platelet depletion with specific antiserum does not reduce infarct size but prevents the cardioprotection afforded by the TXSI. Platelet-derived prostaglandin endoperoxides (PGG2 and PGH2), which cannot be converted to thromboxane A2 in the inhibited platelet, can be transformed to PGE2 and PGD2 in plasma and to PGI2 by the blood vessel wall. These prostaglandins are considered "cardioprotective." Consequently, a low dose of aspirin (3-5 mg/kg) given 24 hours before coronary occlusion was used to selectively block the platelet cyclooxygenase enzyme. Aspirin, by itself, does not reduce infarct size, but it suppresses the myocardial salvage induced by OKY-046. Thus, TXSI reduce infarct size by platelet-dependent, aspirin-sensitive mechanism that depends on the redirection of platelet-derived PGG2 and PGH2 to protective metabolites, rather than inhibition of thromboxane A2 per se. Moreover, myocardial salvage induced by the TXSI is accompanied by a reduction in neutrophil accumulation in the myocardium, as indicated by the levels of the neutrophil-specific myeloperoxidase enzyme. Platelet depletion or pretreatment with aspirin prevents the TXSI-induced suppression of neutrophil accumulation. Consequently, it is proposed that the prostaglandin-mediated protective effects of TXSI can be resolved, at least in part, in terms of a braking action on neutrophil activation to prevent leukocyte-dependent tissue injury.

Reduction in infarct size by the prostacyclin analogue iloprost (ZK 36374) after experimental coronary artery occlusion-reperfusion

In this study we attempted to determine whether administration of iloprost (ZK 36374), a chemically stable prostacyclin analogue, would reduce infarct size after experimental coronary artery occlusion and reperfusion. One hour of coronary artery occlusion was performed in 28 open-chest, anesthetized rabbits++, followed by 5 hours of reperfusion. Two minutes after occlusion, 99mTc-labeled albumin microspheres were injected into the left atrium for later assessment of the area at risk of infarction. Fifteen minutes after occlusion animals were randomly assigned to either the treatment group (iloprost, 1.2 micrograms/kg/min intravenously for 6 hours; n = 14) or the control group (n = 14). In vitro platelet aggregation was inhibited in rabbits receiving iloprost. In 10 rabbits (five treated and five control) regional myocardial blood flow was also measured by means of differentially labeled radioactive microspheres. Infarct size was significantly smaller in treated rabbits (53.6 +/- 4.1% of the risk zone vs 89.4 +/- 3.8% in control rabbits; p less than 0.001). Flow to the nonischemic myocardium was higher in treated animals, that is, 1.87 +/- 0.20 ml/min/gm of tissue 50 minutes after occlusion and 1.90 +/- 0.20 ml/min/gm of tissue 4 hours after reperfusion, compared with 1.54 +/- 0.20 and 1.64 +/- 0.30 ml/min/gm of tissue, respectively, in control rabbits (p less than 0.01). Collateral flow to the ischemic region was not affected by the drug. Mean arterial blood pressure, heart rate, and pressure-rate product in treated rabbits were not significantly different from values in control rabbits. In conclusion, administration of iloprost reduced myocardial infarct size in this model of myocardial ischemia and reperfusion in absence of major hemodynamic effects.

Efficacy of platelet depletion in counteracting the detrimental effect of acute hypercholesterolemia on infarct size and the no-reflow phenomenon in rabbits undergoing coronary artery occlusion-reperfusion

It has recently been demonstrated that acute hypercholesterolemia per se, independently of its atherogenic effect, increases the extent of myocardial injury in rabbits undergoing coronary artery occlusion-reperfusion. Estimation of myocardial blood flow after reperfusion indicated that this deleterious effect was due to a vascular obstruction that limited the efficacy of reperfusion. The goal of this study was to evaluate the role played by platelets in contributing to the occurrence of this deleterious effect. Accordingly, New Zealand White rabbits were fed a standard laboratory chow diet (plasma cholesterol 67 +/- 12 mg/dl) or a 2% cholesterol-enriched diet for 3 days (plasma cholesterol 329 +/- 70 mg/dl). In a first series of experiments autologous platelets were labeled with 111In-oxine. After labeling, platelets were reinjected in the same animal and 30 min later coronary artery occlusion (CAO) was induced. CAO was maintained for 30 min followed by 5.5 hr of reperfusion. The animals were then killed, their hearts were excised, and each left ventricle was divided into ischemic and normally perfused samples. Myocardial samples were then counted in a gamma counter. Platelet accumulation ratio, i.e., 111In activity in the ischemic myocardium per gram of tissue divided by 111In activity in the normal myocardium per gram of tissue, was calculated. The ratio was 2.4 +/- 0.2 (mean +/- SEM) in controls (n = 7) and 10.3 +/- 1.0 in the cholesterol-fed group (n = 6, p less than .001), indicating that a marked accumulation of platelets occurs in the ischemic myocardium of hypercholesterolemic rabbits. To evaluate the importance of this phenomenon, another series of experiments was performed.(ABSTRACT TRUNCATED AT 250 WORDS)

Indium-111 platelet scintigraphy and two-dimensional echocardiography for detection of left ventricular thrombus: influence of clot size and age

Two-dimensional echocardiography and indium-111 platelet scintigraphy were performed on 50 dogs to determine the influence of clot age and size on the detection of experimentally induced left ventricular mural thrombus. Thrombus was induced by apical infarction and injection of a sclerosing agent and thrombin. The animals were classified into four groups according to the time of indium-111 platelet injection after thrombus induction: Group I (17 dogs, 1/2 hour after induction; 3 dogs, before induction), Group II (12 dogs, 24 hours after induction) and Group III (12 dogs, 1 week after induction). In Group IV (six control dogs) apical infarction was produced, but thrombin was not injected; indium-111 platelets were injected 1/2 to 1 hour after infarction. The dogs were studied by indium-111 platelet scintigraphy and by two-dimensional echocardiography 1/2 to 5 hours (Group I) and 1 to 5 and up to 72 hours (Groups II to IV) after platelet administration and before death was induced. Two-dimensional echocardiography showed the best overall sensitivity for detection of acute thrombus (97%; 29 of 30). The sensitivity of indium-111 platelet scintigraphy was 86% (18 of 21) for clots greater than or equal to 0.08 ml in size, and 67% (20 of 30) for detection of all clots. Thrombus did not form in 14 dogs of Groups I to III and in 6 of 6 control dogs. The specificity of scintigraphy was 100% (20 of 20) compared with 80% (16 of 20) for echocardiography. Echocardiography was more sensitive than scintigraphy for detecting very small clots in this experimental model.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostacyclin reduction of regional ischemic injury in the canine myocardium

The effect of prostacyclin (PGI2) on the myocardium of the awake dog subjected to coronary artery occlusion was examined. Animals were randomly administered PGI2 200 ng/kg/min (n = 6), PGI2 100 ng/kg/min (n = 6), or the vehicle control (n = 6), beginning 30 min prior to coronary artery occlusion. Radiolabeled microspheres (15 microns) were used to measure myocardial blood flow. The myocardial region at risk was determined by fluorescein injection, and infarct size was assessed by triphenyl tetrazolium staining. Segmental myocardial function was evaluated from the systolic ejection shortening (SES) by subendocardial ultrasonic dimension crystals in normal, ischemic, and border zones. PGI2 200 ng/kg/min produced significant decreases in aortic pressure and systemic vascular resistance. PGI2 100 ng/kg/min, which achieves 95% platelet inhibition, had no significant hemodynamic effects. Animals receiving PGI2 200 ng/kg/min had significantly higher blood flow to the ischemic region, better border zones SES, and a smaller infarct. PGI2 ameliorates myocardial injury and reduces functional impairment produced by ischemia in doses that elicit vasodilation. This beneficial effect of PGI2 does not appear to be mediated solely by an antiplatelet mechanism.

Usefulness of defibrotide in protecting ischemic myocardium from early reperfusion damage

Defibrotide, a partially depolymerized polydeoxyribonucleotide obtained from mammalian lungs, was found to stimulate prostacyclin (PGI2) production and to possess significant antithrombotic and fibrinolytic activities. The present study was designed to evaluate the actions of defibrotide on feline myocardial ischemia, produced by 3 hours of occlusion of the left anterior descending coronary artery (LAD) and followed by 2 hours of reperfusion. Intravenous administration of defibrotide (32 mg/kg/hour subsequent to a 32 mg/kg bolus injection), beginning 30 minutes after LAD occlusion, resulted in a 60% reduction in loss of CK specific activity from ischemic myocardium at 5 hours, while the nonischemic myocardium remained unaffected. Defibrotide largely antagonized the increase in ST segment during LAD occlusion and prevented the appearance of a Q wave during early reperfusion, which was found in all vehicle-treated cats. Although 2 of 8 vehicle-treated cats died from ventricular fibrillation and another had severe ventricular tachyarrhythmia, none of the defibrotide-treated cats had similar severe changes in the electrocardiogram and all 7 cats survived the 5-hour observation period. Defibrotide had no direct action on general hemodynamic functions. In a separate set of experiments, defibrotide (0.1 mg/ml) produced an 8- to 10-fold stimulation of PGI2 release. The data suggest a remarkable protective potential of defibrotide on reperfusion damage of the ischemic myocardium, which may be associated with a PGI2-related mechanism.

Nafazatrom-induced salvage of ischemic myocardium in anesthetized dogs is mediated through inhibition of neutrophil function

The effects of nafazatrom on leukocyte function in vitro and in vivo were related to its ability to salvage ischemic myocardium in an occlusion-reperfusion model of myocardial injury in the anesthetized dog. Nafazatrom (0.4-75 microM) produced dose-related inhibition in vitro of neutrophil aggregation, superoxide anion generation, arachidonic acid metabolism, and, to a lesser extent, the release of beta-glucuronidase. In contrast, nafazatrom (0.4-37.5 microM) did not substantially influence platelet aggregation or the platelet metabolism of arachidonic acid. In vivo nafazatrom (10 mg/kg, po) reduced infarct size from 58 +/- 3% of the risk area (mean +/- SEM, n = 9) in control dogs to 23 +/- 2% of the risk area (n = 9, P less than 0.01). Nafazatrom also reduced the incidence of accompanying arrhythmias. Nafazatrom-induced myocardial salvage was not associated with any hemodynamic changes; moreover, it was independent of platelets, since thrombocytopenia did not prevent nafazatrom from exerting a protective effect. Measurements of the neutrophil-specific myeloperoxidase enzyme in ischemic myocardium indicate that the smaller infarct size in dogs treated with nafazatrom is accompanied by diminished leukocyte infiltration. Thus, the ability of nafazatrom to inhibit neutrophil function in vitro and cell infiltration in vivo may underly its myocardial-protective effects.

Platelet depletion in experimental myocardial infarction

Accumulation of platelets in the microvasculature after acute myocardial ischemia may exacerbate tissue injury through the formation of microthrombi and by the release of vasoactive substances. To assess the role of platelets in myocardial ischemic injury and infarction, circulating platelets were reduced by 94 +/- 2% (mean +/- S.E.M.) with sheep antiserum to canine platelets. Regional myocardial ischemia was produced by occlusion of the left circumflex coronary artery (LCCA) for 90 min followed by reperfusion for 5 hours. Infarct size did not differ significantly between antiplatelet serum and nonimmune serum groups: 36 +/- 8 vs. 43 +/- 4% of the area at risk, determined by a post-mortem dual staining technique (p greater than 0.05). A second occlusion-reperfusion control group, sacrificed at 24 hours, did not differ from 5 hr reperfused groups with regard to infarct size. Coronary sinus thromboxane B2 (TXB2) concentrations were not altered significantly by platelet depletion. Histopathologic examination confirmed the presence of necrosis in the infarcted myocardium and revealed substantial leukocytic infiltration in both groups. The results suggest that circulating platelets are not required for the full expression of myocardial ischemic injury resulting from temporary coronary artery occlusion followed by reperfusion.

Temporal changes in 12-HETE formation in two models of canine myocardial infarction

Arachidonic acid (AA) metabolism by infarcted canine myocardium was studied and correlated with matched histologic analyses following permanently occluded or reperfused infarction. Histologic analysis of tissues from reperfused infarcts showed a marked acceleration of inflammatory cell invasion and of granulation tissue formation when compared to the occlusive infarct. In the reperfused infarct, polymorphonuclear leukocytes (PMNs) were very prominent at one day after infarction while in the occlusive infarcts the neutrophilic invasion was less intense but more sustained. At one day following reperfused infarction the major arachidonate product, which co-migrated by thin layer chromatography with the mono-hydroxyeicosatetraenoic acids (HETEs), was significantly elevated (254 +/- 49 pmoles/gm wet weight, n = 3) when compared to normal tissue (48 +/- 6 pmoles/gm n = 19). This occurred at a time when the number of PMNs was maximal in the infarcted tissue. Addition of the calcium ionophore A23187 caused a further marked stimulation in HETE production in the one day reperfused infarct but not at the other time points studied. The production of HETE was not significantly different in the infarcted tissue than in the normal tissue at three and seven days following reperfused infarction or at one, three, or seven days after occlusive infarction. The identity of this HETE product was investigated using reverse phase high performance liquid chromatography (RP-HPLC) and gas chromatography-mass spectrometry (GC-MS) and found to be predominantly 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) with a small amount of 15-HETE. Thus the production of 12-HETE parallels the number of neutrophils invading the infarcted area of the heart.

Scintigraphic detection of coronary artery thrombi in patients with acute myocardial infarction

To determine whether coronary thrombi can be detected scintigraphically after acute myocardial infarction, 24 patients were studied with a new method employing indium-111-labeled platelets and technetium-99m-labeled red blood cells. Nine patients with suspected infarction were evaluated initially within 9 hours of the onset of symptoms and again 18 to 24 hours after onset. Eight patients with neurologic symptoms but without overt cardiac disease and seven patients with angina but without infarction served as unmatched control subjects. Foci of net indium accumulation were detected after image processing that incorporated subtraction of blood pool activity. Carotid and pulmonary artery reference regions, in which blood pool activity is high and active platelet deposition unlikely, were used to correct digitized cardiac scintigrams for indium-111 platelet activity in the blood pool. In patients with infarction, distinct foci of net indium accumulation were present in regions corresponding to the coronary artery supplying ischemic zones. This occurred in seven of eight patients at the time of the earliest evaluation (5.6 +/- 3.3 hours [mean +/- SD] after the onset of symptoms) and in eight of nine patients at the time of subsequent imaging (23.6 +/- 1.9 hours after onset). Only 1 of the 15 control patients exhibited a cardiac focus of net indium accumulation. The percent of indium excess (100 [total indium-111 activity-blood pool indium-111 activity]/blood pool indium-111 activity) within the cardiac region measured (+/- SD) 16.8 +/- 11.6% in all patients with myocardial infarction (19.1 +/- 11.2% in those with visually identified foci) compared with 0.4 +/- 4.3% in control patients (p less than 0.001). This method permits early detection and sequential assessment of coronary artery thrombi. It should permit improved characterization of the role of platelets in the pathogenesis of acute manifestations of coronary vascular disease and improved evaluation of interventions designed to prevent or lyse coronary thrombi.

Cardiac and vascular imaging with labeled platelets and leukocytes

The contribution of platelets in atherosclerosis and thrombosis in animal models and in clinical studies has been quantified with 111In-platelet scintigraphy. New in vitro quantitative techniques have been developed using 111In-labeled platelets to determine the number of adherent platelets on deendothelialized surfaces of damaged vessel walls and synthetic vascular grafts. In vivo imaging techniques are semi-quantitative in nature; in these studies 111In radioactivity on thrombotic vessels or graft surfaces of iliac, femoral, or popliteal arteries is compared with contralateral vessels. Background 111In radioactivity in the circulating blood pool of venous and capillary networks and radioactivity in marrow decreases the sensitivity of these techniques. Despite these limitations, the dynamic process of platelet deposition in most of the denuded, atherosclerotic vessels and prostheses in the circulatory system can be recorded. This ongoing thrombosis and embolization has been observed in 5-10-year-old vascular grafts of Teflon and Dacron biomaterials. Currently used platelet function inhibitor drugs, eg, aspirin, Persantine, sulfinpyrazone, and Motrin, have a demonstrable effect on platelet deposition. Slight changes in reduction of platelet deposition on these surfaces due to medical intervention have been observed by noninvasive imaging with 111In-platelets. Subtraction of blood pool radioactivity with 99mTc-labeled autologous red cells and calculation of 111In radioactivity associated with platelet thrombus on vessel walls also have been performed for coronary, carotid, and femoral arteries. Although platelet concentrates are used frequently after open heart surgery (one to six per patient), consumption of platelets in the artificial lung or oxygenator, lysis of platelets during pumping, and suction of blood only recently have been quantified with the use of 111In-labeled platelets. These studies also demonstrated far less trauma to platelets with the use of a membrane rather than a bubble oxygenator. Further reduction in platelet consumption and trauma was observed with the use of prostacyclin, a short-acting drug with significant beneficial effect on platelet thrombus reduction and disaggregation of aggregated platelets. The role of polymorphonuclear leukocytes in inflammation, infection and myocardial infarction, and in vivo evaluation with 111In-leukocyte scintigraphy in animals and humans has been described.(ABSTRACT TRUNCATED AT 400 WORDS)