Myocardial platelet trapping after coronary ligation in primates (Papio anubis)

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.

Selective activation of E-type prostanoid(3)-receptors reduces myocardial infarct size. A novel insight into the cardioprotective effects of prostaglandins

Prostaglandins (PGs) and other eicosanoids are members of a large family of lipid mediators (autacoids). In 1978, Lefer and colleagues (Science 200, 52-55 [1978]) reported that prostacyclin reduces the myocardial tissue injury caused by coronary artery occlusion and reperfusion in the cat. Since this discovery, more than 50 papers have reported on the cardioprotective effects of vasodilator PGs, including prostacyclin. The cardioprotective effects of PGs are due in part to (1) a reduction in afterload, (2) an increase in coronary blood flow, (3) the inhibition of platelet function, and (4) the inhibition of the activation and extravasation of polymorphonuclear granulocytes. All of these effects are secondary to the activation of EP (E-type prostanoid)(2)-receptors, which activate G(s)-protein and, hence, adenylate cyclase. In addition, the protection of organs such as the heart by PGs has been attributed to a cytoprotective effect of these agents, the mechanism of which is largely unknown. We recently have discovered that certain E-type PGs, which do not activate EP(2)-receptors, also reduce myocardial infarct size, without causing a fall in blood pressure (EP(2)-receptor-mediated effects). Having provided a brief introduction into the role of eicosanoids in ischaemia-reperfusion injury of the heart, this review focuses on the recent discovery that selective agonists of EP(3)-receptors reduce myocardial infarct size, without causing haemodynamic side effects. The mechanisms of the cardioprotective effects of these agents are discussed, as are the therapeutic implications.

Effects of prostacyclin analogue, OP-2507, on function and metabolism in the ischemic working rat heart

We examined the effects of a new stable prostacyclin analogue, OP-2507, on myocardial function and metabolism in the ischemic working rat heart preparation. The hearts were perfused with Krebs-Henseleit bicarbonate (KHB) buffer, and whole heart ischemia was induced by one-way aortic valve for 15 min follows by reperfusion for 30 min. In the treated hearts, OP-2507, 20 ng.ml(-1), was administered to KHB buffer from the beginning to the end of experiment. During ischemia, coronary flow in the OP-2507 group increased significantly more than that in the control group. The mechanical performance of both groups was impaired after ischemia. However, the recovery of coronary flow, cardiac output, peak systolic pressure and LV dP/dT(max) was significantly higher in the treated group than in the control group. The incidence of ventricular fibrillation during reperfusion was 100% and 25% in the control and the OP-2507 groups, respectively. Myocardial ATP content was significantly higher in the treated hearts than that in the control hearts. These results indicate that this stable prostacyclin analogue is beneficial in myocardial ischemia, even without its well known action of preventing platelet aggregation.

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.

Selective platelet deposition during focal cerebral ischemia in cats

Platelet deposition in the microcirculation may play a role in focal cerebral ischemia. We investigated platelet deposition in selected parts of the cat brain after temporary middle cerebral artery occlusion. Ten anesthesized cats were given autologous indium-111-labeled platelets and chromium-51-labeled erythrocytes. The right middle cerebral artery was occluded with miniature aneurysm clips for 3 hours via a transorbital approach; blood pressure was reduced concomitantly to decrease the collateral circulation. Removal of the clips initiated a 45-minute period of normotensive reperfusion. After sacrifice, the brain was removed and sectioned for comparison of right- versus left-hemisphere platelet deposition. Platelets were selectively deposited in the territory of the occluded right middle cerebral artery. Significant deposition was found in the caudate nucleus, internal capsule, parietal cortex, and the centrum semiovale. Our findings support the evidence that platelets are deposited in the microvasculature during temporary severe focal cerebral ischemia.

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.

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)

Regional myocardial blood flow in experimental myocardial infarction after pretreatment with aspirin

The effects of aspirin on myocardial blood flow in an area of ischemia were studied in 12 baboons. In each, a diagonal branch of the left anterior descending coronary artery was ligated. Six of the baboons received aspirin (2 X 600 mg orally, 12 hours and 1 hour before ligation); the other six did not receive aspirin and served as a control group. The extent of myocardial ischemia was delineated with an electrode wire grid on the surface of the anterior left ventricular wall. The maximal area circumscribed by electrodes with 2 mV or more ST segment elevation was compared with the area of reduced myocardial blood flow. Myocardial blood flow was measured with the radioactive microspheres method using strontium-85-labeled carbonized spheres. Two areas of reduced myocardial blood flow were noted, one with severely reduced flow in the center of the myocardial infarct (0 to 49% of noninfarcted myocardium) and another with mild to moderately reduced myocardial blood flow at the border of the myocardial infarct (50 to 90% of noninfarcted myocardium). Myocardial blood flow in the border area (margins of ST elevation area) for the total wall was 85 +/- 8% of normal in the aspirin-treated animals and 40 +/- 4% in the control group (p less than 0.01); for the epicardium it was 67 +/- 10% of normal in noninfarcted myocardium after aspirin and 37 +/- 5% for the control group (p less than 0.05); and for the endocardium it was 78 +/- 8% of normal in noninfarcted myocardium after aspirin and 39 +/- 6% in the control group (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Platelet serotonin uptake during myocardial ischemia

Platelets are believed to play a role in the pathologic sequelae of acute myocardial ischemia. Several of the compounds generated and released by activated platelets during cardiac ischemia may contribute to the production of cellular necrosis (free oxygen radicals), coronary vasoconstriction (thromboxane, serotonin, catecholamines), and perpetuation of the platelet aggregatory reaction (thromboxane, serotonin, adenosine diphosphate). In the present study, it was demonstrated that platelet serotonin uptake function is markedly depressed following the induction of myocardial ischemia in cats. The possible mechanism through which the depression occurs and the resulting pathologic sequelae are discussed. The results of the present study illustrate another mechanism by which platelets can potentially mediate the severity and perpetuation of cellular events during acute myocardial ischemic insult.

Coronary artery ligation, early arrhythmias, and determination of the ischemic area in conscious rats

A method for studying the acute phase of myocardial infarction in conscious rats has been developed. In preliminary surgery, a loose ligature of atraumatic silk was understitched around the left coronary artery. Its ends were pulled through a polyethylene tube placed within the thorax and fixed under the skin. Seven days later, coronary occlusion was performed by tightening the ligature in conscious animal. Lidocaine and pindolol pretreatment increased the survival rate and attenuated the life-threatening arrhythmias during the first 20 min, but did not influence the infarct size 16 hrs later. An ex vivo perfusion technique for determining the ischemic area has also been developed. 3, 6, and 20 min after coronary ligation, the hearts were excised and perfused with 4% formaldehyde. The ischemic area could not be perfused and remained dark red with a sharp border-line. At the 3rd and 20th min its size was as same as that of the 16-hr infarcted area; however at the 6th min it increased by 50%. Lidocaine and pindolol eliminated this transitory increase. These methods appear to be valuable for large-scale determination of drug effects on the acute phase of experimental myocardial infarction in conscious rats, and for estimating their action on the size of ischemic area very early after coronary occlusion.

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

The kinetics of accumulation and the external imaging patterns of indium-111-labeled platelets infused in a dog model of left anterior descending coronary artery occlusion with reperfusion were studied. The effects of infarct age and regional residual myocardial blood flow upon platelet accumulation were quantified, and the capacity of indium-111 platelets to image the experimental infarction was evaluated qualitatively. The endocardial accumulation of indium-111 platelets occurred primarily in infarct zones with residual blood flow less than 0.6 times normal and was maximal (24.98 +/- 2.76 times normal) in the lowest blood flow zone (less than 0.1 times normal). Indium-111 platelet accumulation in the epicardium occurred in the regions with blood flow less than 0.6 times normal and was maximal (17.83 +/- 1.20 times normal) in the lowest blood flow zone (less than 0.1 times normal). The maximal endocardial and epicardial platelet accumulation occurred 24 hours after reperfusion and was significantly decreased at 48 hours. In vivo cardiac images revealed discrete areas of increased myocardial radioactivity uptake in the anterior wall of dogs 24 hours after reperfusion. All images 48 hours after reperfusion were negative. Thus, in the experimental setting, indium-111 platelets allow quantification of platelet accumulation after myocardial infarction at a tissue level and provide a noninvasive means of in vivo imaging of reperfused infarcted myocardium.

Myocardial infarction without coronary occlusion. A morphologic study in sheep

The macroscopic, histologic and enzyme-histochemical characteristics of the myocardial lesion obtained after heating of a thermoprobe in a branch of the left coronary artery in sheep is reported. In 13 sheep such myocardial lesions were produced distal to the location of the thermoprobe. Alterations were observed in accordance with generally accepted morphologic criteria for myocardial infarction. The coronary artery branch in which the thermoprobe was located showed erythrocyte and platelet aggregates immediately after the heating episode, which disappeared within a few min, as demonstrated by coronary arteriography. Injection of radiolabelled microspheres into the coronary circulation after induction of the myocardial lesion, cryosectioning of the heart and autoradiography revealed a lack of blood flow in the damaged myocardial region. We consider this new method a suitable tool for further studies on the complex pathology involved in the development of myocardial infarction.

Cause and course of acute myocardial infarction

A complete discussion of factors involved in causing acute myocardial infarcts is contained in ths review, and a detailed discussion of various complications of acute myocardial infarction is also provided. Interventions that are capable of reducing infarct size in animal models with experimentally produced acute myocardial infarcts are discussed, and factors that might alter infarct size in patients are reviewed. Prognostic factors that allow one to identify high risk patients with acute myocardial infarction following their hospital admission and in the follow-up period after hospital discharge are identified. The need to develop more accurate measurements of the extent of myocardial infarction and myocardial ischemia, and to develop additional prognostic markers that allow one to separate patients most at risk for sudden death and new myocardial infarction from those with ongoing chronic ischemic heart disease and those without additional complications from their ischemic heart disease is also emphasized in this review.