Role of blood platelets and prostaglandins in coronary artery disease

Platelet function disorder in patients with coronary slow flow

BACKGROUND

Some patients evaluated for chest pain with angiographically normal coronary arteries show coronary slow flow phenomenon (CSFP) on angiography. Slow flow of dye in epicardial coronary arteries is also not an infrequent finding in patients during routine coronary angiography. The precise pathophysiology of CSFP is not known yet.

HYPOTHESIS

This study investigates the presence of platelet function disorders in patients with CSFP.

METHODS

The patient group included 24 patients with CSFP detected by coronary angiography via the TIMI "frame count" method, and a control group included 23 patients with normal coronary flow. Platelet aggregability induced by use of ristocetin, collagen, and adenosine diphosphate (ADP), was measured from all blood samples in both control and patient groups.

RESULTS

The ratio of platelet aggregability increased significantly in patients with CSFP compared with patients with normal coronary flow (ristocetin 57.6 +/- 15 vs. 45.4 +/- 17.1, collagen 62.9 +/- 16.4 vs. 48.9 +/- 25.3, ADP 59.4 +/- 18 vs. 42.4 +/- 15.2, p < 0.05).

CONCLUSION

Platelet aggregability is increased in patients with CSFP.

Aspirin in the prophylaxis of coronary artery disease

Aspirin has been used for more than 100 years, but its mechanisms of action have only been understood in the past 20 years. Aspirin interferes with arachidonic acid metabolism in platelets and endothelial cells and thereby reduces thromboxane A2 and prostacyclin. It also has other mechanisms of action, including anti-inflammatory roles, protection from oxidative stress, enhancement of fibrinolysis, and suppression of plasma coagulation and platelet-dependent inhibition of thrombin generation. It has been used for primary and secondary prevention of myocardial ischemia, and for primary and secondary prevention of cerebrovascular ischemia. We review the 5 pivotal studies relating to primary prevention for cardiovascular risk and the many studies relating to secondary prevention of myocardial ischemia. We also review the utility of aspirin in primary prevention of myocardial infarction and stroke. We conclude that aspirin is one of the most potent drugs ever discovered and that its effects extend well beyond those of cycloxoxygenase enzyme inhibition. Aspirin treatment does not preclude control of underlying and comorbid conditions such as diabetes mellitus, hypertension, and dyslipidemia. For most patients, a daily dose of 325 mg is optimal. Patients must understand the potential for gastrointestinal upset and hemorrhagic complications. The utility of aspirin is greater in coronary artery disease prevention than in cerebrovascular prevention.

1H-NMR lipid profiles of human blood platelets; links with coronary artery disease

Blood platelets are closely involved in the early development of atherosclerosis and in the events that lead to thrombosis, both of which are dominating factors in coronary artery disease (CAD). The aim of the present study was to evaluate the platelet lipid profiles of patients suffering from CAD and explore the possibility of a link between platelet lipids and CAD, using high-resolution high-field proton nuclear magnetic resonance spectroscopy as the analytical tool. The total platelet lipid profiles of healthy volunteers were compared with those of patients presenting with chest pain requiring coronary angiography. Two lipid groups changed significantly: cholesterol increased by 16.5% and total diacylglycerophospholipids decreased by 15.7%. There was also a significant decrease of the ethanolamine-containing phospholipids, by 4.7%; the extent of unsaturation of the fatty acid chains, by 0.2, and increase of the linoleate content of the fatty acid chains, by 1.9%. Our results suggest that platelet lipid abnormalities occur in patients with CAD and these changes may predate the development of overt atherosclerosis.

Effect of amlodipine on exercise-induced platelet activation in patients affected by chronic stable angina

BACKGROUND

Literature concerning exercise-induced platelet activation in chronic stable angina is somewhat confusing. The reason lies in the type of exercise as well as in methodological problems. A powerful, recently introduced procedure to detect platelet activation is flow cytometry. Platelet response to activating factors is mediated by calcium uptake; however, calcium antagonist effect on platelet activity is still unclear.

HYPOTHESIS

The study was undertaken to investigate exercise-induced platelet activation before and after treatment with amlodipine in chronic stable angina.

METHODS

Twenty patients with chronic stable angina were entered into the study. Each subject underwent a symptom-limited cycloergometer stress test following a washout period of 2 weeks. Blood samples were collected before and immediately after exercise. All subjects were then randomized into two groups of 10 patients each, with Group 1 and Group 2 taking amlodipine 10 mg/day, and placebo for 4 weeks, respectively. They subsequently underwent a second exercise stress test, and blood samples were obtained before and immediately after exercise. Flow-cytometric evaluation of platelet activity was performed in order to recognize GMP-140 expression on platelet membrane.

RESULTS

Strenuous exercise induced a significant increase in platelet activation in all subjects prior to therapy. No significant differences were observed in platelet activity at rest between Groups 1 and 2, whereas a significant decrease in exercise-induced platelet activation was demonstrated in Group 1 compared with Group 2.

CONCLUSION

Our data provide evidence of the favorable effect of amlodipine on exercise-induced platelet activation in patients affected by chronic stable angina.

Platelet-induced suppression of baroreceptor activity is mediated by a stable diffusible factor

We have demonstrated recently that platelets aggregating in the carotid sinus decrease baroreceptor sensitivity. The goals of the present study were to determine whether platelet-induced suppression of baroreceptor activity is mediated by a diffusible, transferable factor and, if true, whether the factor is short-lived or stable. Baroreceptor activity was recorded from the isolated carotid sinus during slow ramp increases in nonpulsatile pressure in rabbits anesthetized with sodium pentobarbital. Intraluminal exposure of the carotid sinus to washed rabbit platelets resuspended in Krebs buffer (3-5 x 10(8) cells/ml) and activated by thrombin decreased baroreceptor activity significantly (n = 7, P < 0.05). Maximum baroreceptor activity recorded at a pressure of 140 mmHg was reduced to 81 +/- 7% of the control maximum. Injection of cell-free supernatant obtained from filtered thrombin-activated platelets also suppressed baroreceptor activity to a similar extent after 10 min (n = 7) and after 2 h (n = 5) of incubation when maximum baroreceptor activity was reduced to 84 +/- 5 and 82 +/- 5% of the control maximum, respectively. The inhibitory influence of activated platelets and platelet supernatant on baroreceptor activity was still apparent after 10-60 min of heating (95 degrees C) (n = 5) and was reversible upon removal of platelets and supernatant from the sinus. The results indicate that activated platelets release a stable diffusible factor that suppresses baroreceptor activity. We speculate that this 'inhibitory factor' may contribute to impairment of the baroreceptor reflex and neurally-mediated increases in arterial pressure in atherosclerotic and thrombotic states.

Platelet Aggregation, Thromboxane A(2), Prostacyclin Generation and Platelet Sensitivity to Prostacyclin during the First Month after Myocardial Infarction

This study was designed to investigate platelet aggregation, plasma thromboxane A, and prostacyclin concentration and platelet sensitivity to prostacyclin simultaneously during the first month after myocardial infarction (MI). Spontaneous platelet aggregation and aggregation responses to ADP and adrenaline were low on the day of admission, increased rapidly by the 7th day post-MI, remained elevated during the second week post-MI and reached the level of chronic coronary artery disease patients but not healthy persons at the end of the fourth week of illness. An increase in plasma thromboxane B, the spontaneous and stable breakdown product of thromboxane A, level and enhanced prostacyclin production, with a maximum on the third post-MI day, were observed. We also demonstrated a significant platelet resistance to prostacyclin in MI patients. Thrombocyte sensitivity to prostacyclin normalized by the end of the fourth post-MI week. These results indicate the need for therapy with platelet inhibitors in patients with MI.

Platelet activation in stable coronary artery disease

To elucidate the role of physical activity in the pathogenesis of acute ischemic syndromes in patients with coronary artery disease (CAD), we hypothesized that platelet activation occurs when coronary blood flow velocity and shear stress increase across an atherosclerotic vascular bed. We measured platelet aggregation by using angiologic catheterization to obtain simultaneous samples of whole blood from the coronary sinus and the aorta while at rest, 2 minutes after the onset of rapid atrial pacing, and 10 minutes after termination of pacing. Of 82 consecutive patients included in our study, 36 had stenosis of the left coronary artery, 12 had stenosis of the right coronary artery only, and 34 had no evidence of CAD. Samples taken at rest revealed no arteriovenous difference in platelet aggregation between patients with CAD and those without CAD. In patients with significant stenosis (> or = 50%) of the left coronary artery, atrial pacing caused platelet aggregation to increase in samples from the coronary sinus (64 +/- 9% increase; p < 0.01) but not in blood from the aorta (2 +/- 8% decrease; difference not significant). This increase was transient, with aggregation returning almost to resting values 10 minutes after pacing ended. Atrial pacing elicited no change in platelet aggregation in samples from either the coronary sinus or aorta of patients with nonsignificant stenosis (< 50%) of the left coronary artery, patients with significant stenosis of the right coronary artery only, and patients free of CAD. Thus, under resting conditions, no evidence of platelet activation across the coronary bed was seen regardless of CAD status.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiarrhythmic versus antifibrillatory actions: inference from experimental studies

Pathophysiology of the coronary circulation is a major contributor to altering the myocardial substrate, rendering the heart susceptible to the onset of arrhythmias associated with sudden cardiac death. Antiarrhythmic drug therapy for the prevention of sudden cardiac death has been provided primarily on the basis of trial and error and in some instances based on ill-suited preclinical evaluations. The findings of the Cardiac Arrhythmia Suppression Trial (CAST) requires a reexamination of the manner in which antiarrhythmic drugs are developed before entering into clinical testing. The major deficiency in this area of experimental investigation has been the lack of animal models that would permit preclinical studies to identify potentially useful or deleterious therapeutic agents. Further, CAST has emphasized the need to distinguish between pharmacologic interventions that suppresses nonlethal disturbances of cardiac rhythm as opposed to those agents capable of preventing lethal ventricular tachycardia or ventricular fibrillation. Preclinical models for the testing of antifibrillatory agents must consider the fact that the superimposition of transient ischemic events on an underlying pathophysiologic substrate makes the heart susceptible to lethal arrhythmias. Proarrhythmic events, not observed in the normal heart, may become manifest only when the myocardial substrate has been altered. We describe a model of sudden cardiac death that may more closely simulate the clinical state in humans who are at risk. The experimental results show a good correlation with clinical data regarding agents known to reduce the incidence of lethal arrhythmias as well as those showing proarrhythmic actions.

Platelet hyperaggregability across the coronary bed in response to rapid atrial pacing in patients with stable coronary artery disease

BACKGROUND

Platelet aggregation is believed to contribute to the precipitation of acute ischemic syndromes. Because physical activity has been proposed as one possible trigger in converting a patient with chronic coronary artery disease to one with an acute ischemic syndrome, we examined the hypothesis that platelets become activated when coronary blood flow velocities (and thereby shear stress) increase across an atherosclerotic bed.

METHODS AND RESULTS

During catheterization, 82 patients (36 with left coronary artery disease, 12 with only right coronary artery disease, and 34 with normal coronary arteries) had measurement of whole blood platelet aggregation performed on blood samples obtained simultaneously from the coronary sinus and aorta at rest, 2 minutes after onset of rapid atrial pacing, and 10 minutes after pacing was terminated. There was no arteriovenous difference in platelet aggregation under resting conditions in patients with versus those without coronary artery disease. Atrial pacing in patients with left coronary artery disease (greater than or equal to 50% stenosis in a major epicardial vessel) caused an increase in platelet aggregation in the coronary sinus blood (+64 +/- 9%, p less than 0.01) but not in arterial blood (2 +/- 8% decrease, p = NS). This increase was transient and returned nearly to baseline 10 minutes after termination of pacing. Patients with nonsignificant left coronary artery disease, those with normal coronary arteries, and patients with significant disease only in the right coronary artery (venous drainage not into the coronary sinus) did not show any changes in either the coronary sinus or arterial blood with atrial pacing.

CONCLUSIONS

There is no evidence of platelet activation across a normal or an atherosclerotic coronary bed at rest. When coronary blood flow increases in the presence of significant (greater than or equal to 50%) narrowing of epicardial coronary arteries, however, platelets are activated and aggregate more easily. This mechanism may play a role in the precipitation of acute ischemic syndromes in patients with coronary artery disease.

Gender differences in platelet count and its association with cigarette smoking in a large cohort in Israel

Cigarette smoking has an effect on platelet function and aggregation although the sensitivity of platelet count in reflecting this phenomenon is not known. The association of platelet count with smoking habits was examined in a cohort of 5017 Israeli industrial workers aged 20-64 years. Males had a significantly lower age-adjusted mean platelet count than females (225,600 vs 247,800/microliters; p less than 0.001). Female smokers had lower platelet counts than non-smokers (231,000 for heavy vs 252,000 for never smokers) with a strong dose-response relationship (p less than 0.0001), whereas among males platelet count was slightly higher in smokers (224,000 for non-smokers vs 227,000 for heavy smokers; p = 0.243). The difference in platelet count between the sexes remained almost identical after controlling for smoking status and hematocrit. In multiple regression analysis, the negative association between smoking and platelet count in women remained highly significant (p less than 0.001) after controlling for ethnic origin, alcohol consumption, body mass, hematocrit, cholesterol and HDL-cholesterol, whereas for males the slight positive association was not significant. The reduced platelet count observed in males compared with females and in female smokers, suggests that platelet count may reflect sex differences in hemostasis and the effects of smoking on the hemostatic system. This may have implications for the mechanisms underlying the pathogenesis of ischemic heart disease and should be explored further.

Platelet response to the aggregatory effect of platelet activating factor (PAF) ex vivo in patients with acute myocardial infarction

Platelets from patients with acute myocardial infarction exhibit an increased sensitivity to the aggregatory effect of PAF, in vitro, the first 48 h after the onset of the symptoms. This sensitivity, expressed as PAF EC50 values, seems to be transient after the 2 day period. Also, a remarkable decreased sensitivity to the inhibitory effect of PGI2 against the aggregation induced by PAF appears to the platelets of those patients the first hours after the onset of the symptoms, and persists for at least 14 days. Treatment of patients by drugs with a known inhibitory effect on platelet aggregation in vivo and in vitro (aspirin, nifedipine, indomethacin), does not influence the increase in platelet sensitivity to PAF, but inhibits the secondary aggregation induced by the released aggregating factors from the PAF activated platelets. The increase in platelet sensitivity to PAF is not unique to the AMI since it is also observed in patients with acute bacterial pneumonia. However, we cannot support the theory that it is a general phenomenon of acute tissue injury since it is general phenomenon of acute tissue injury since it is not observed in patients with acute muscular injury.

Vascular protection from atherosclerosis: potential of calcium antagonists

Atherosclerosis is the result of complex, interrelated processes, many of which involve calcium. Interventions that interfere with calcium uptake by cells retard lesion development in experimental models of atherosclerosis, underscoring calcium's critical role in atherogenesis. A wide variety of calcium antagonists, including nifedipine, verapamil and diltiazem, have been shown to protect against atherosclerosis in animal models. While these drugs are quite different from each other pharmacologically, they all block intracellular calcium influx. This common property is thus the proposed mechanism for their antiatherosclerotic effects. The effectiveness of the calcium antagonists against the development of atherosclerosis in experimental models may be relevant in the selection of antihypertensive therapy--provided that their protective effects can be demonstrated in future clinical trials as well.

Prostacyclin-to-thromboxane A2 ratio in arteriogenic impotence

It has been suggested that penile hypercoagulability predisposes to aging penile vascular changes and impotence, and that elevated thromboxane A2 during erection may contribute to hypercoagulability and atherosclerosis. Since the ratio of the prostacyclin concentration to the thromboxane A2 concentration is constantly maintained in normal hemostatic responses, an imbalance between thromboxane A2 and prostacyclin may be a factor to initiate vascular diseases and decrease blood flow. We assess the usefulness of the prostacyclin-to-thromboxane A2 ratio in penile blood during erection for diagnosis of arteriogenic impotence. The ratio in the arteriogenic impotence group was significantly lower (p less than 0.01) than in the psychogenic and venogenic impotence groups. Therefore, the prostacyclin-to-thromboxane A2 ratio seems to be useful to diagnose arteriogenic impotence.

Suppression of silent ischemia by metoprolol without alteration of morning increase of platelet aggregability in patients with stable coronary artery disease

To determine the effect of metoprolol on silent ischemia and platelet aggregability, 10 patients with coronary artery disease were studied with a randomized, double-blind, placebo-controlled, crossover trial. Patients were treated with metoprolol (200 mg b.i.d.) or placebo for 1 week and then received the alternate therapy. Two days before the end of each treatment period, platelet aggregability was studied for 24 hours, and a 48-hour ambulatory electrocardiogram was obtained. Compared with placebo, metoprolol significantly decreased the total number (from 26 to 4, p less than 0.1) and duration (from 735 to 84 minutes, p less than 0.01) of silent ischemic episodes. This decrease was accompanied by a decrease in the mean blood pressure (from 127/81 to 118/71 mm Hg, p less than 0.01) and the mean heart rate (from 70 to 54 beats/min, p less than 0.01). The incidence of silent ischemic episodes in the morning was significantly higher in untreated patients than in treated patients. The few episodes observed during metoprolol treatment occurred at the same time as the peak incidence observed during placebo treatment. During placebo treatment, platelet aggregability increased from 6:00 to 9:00 AM as reflected by a decrease in the threshold concentrations of ADP and epinephrine required to induce biphasic platelet aggregation (from 4.8 +/- 0.8 to 2.6 +/- 0.4 microM, p less than 0.02; and from 7.3 +/- 2.3 to 1.8 +/- 0.9 microM, respectively, p less than 0.02). Metoprolol did not alter the basal level nor blunt the morning increase of platelet aggregability.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcoronary platelet activation and consumption in coronary artery disease: studies at rest

The platelet count(PC), plasma platelet factor 4 (PF4) and plasma beta-thromboglobulin(beta TG) have been measured in blood obtained from a peripheral vein, the aortic root and the coronary sinus in 7 patients with normal coronary arteries, 9 patients with lesser degrees of coronary artery disease(CAD) and in 13 patients with severe CAD under resting conditions. In each patient group values obtained in the peripheral venous blood were similar to those obtained in normal subjects. In each group values obtained in blood from the coronary sinus were similar to those obtained in blood from the coronary aortic root and in most instances these were similar to values obtained in peripheral venous blood. for example, in the 13 subjects with hemodynamically significant 3-vessel or 2-vessel CAD the mean values in blood from a peripheral vein, the aorta and the coronary sinus respectively were: PC-194, 205, and 208 x 10(9)/1; PF4-3.3, 3.7, and 3.5 ng/ml; and beta TG-15.5, 23.0 and 18.6 ng/ml. These findings provide no support for the occurrence of continuous platelet activation or platelet consumption in the coronary vessels or elsewhere in patients with stable CAD, under resting conditions, regardless of its severity.

Decrease in frequency of anginal episodes by control of thrombin generation with low-dose heparin: a controlled cross-over randomized study

Increased thrombin generation is frequently associated with an increase in anginal activity. A cross-over, single-blind, completely randomized study was planned in order to evaluate whether the control of thrombin generation affected the increase in anginal activity. After discharge from the hospital, 24 patients (18 men and 6 women, aged 40 to 69 years) suffering from spontaneous angina were followed up to 12 months and were alternatively treated during two consecutive 6-month periods with calcium heparin, 12,500 IU by the subcutaneous route, or with placebo by the intramuscular route, in addition to the usual antianginal medications. Thrombin generation and clinical activity of angina were assessed every 15 days by measuring fibrinopeptide A (FPA) plasma levels and by grading in three classes (symptomless, mildly symptomatic, and severely symptomatic) the anginal activity on the basis of the number and the time concentration of the ischemic attacks and ECG changes. Low-dose heparin treatment significantly reduced both the FPA plasma level (from 4.1 +/- 3.7 to 2.3 +/- 1.8 ng/ml, p less than 0.001) and the clinical activity of angina. During heparin treatment, the frequency of the observations in the severely and mildly symptomatic classes decreased, respectively, by 53% and by 30%, whereas that in the symptomless class increased by 23% (p less than 0.001) in comparison with the period on placebo. Present results indicate that the control of thrombin generation obtained by low-dose heparin treatment favorably affects the degree of anginal activity in patients with spontaneous angina.

Influence of anti-platelet drugs on platelet-vessel wall interactions

The influence of in vitro treatment of platelets with antiplatelet drugs on the interaction of these cells with the subendothelium was studied using citrated human blood obtained from normal control donors. Reconstituted blood following drug treatment was circulated through a special chamber which housed everted segments of de-endothelialized rabbit aorta. The wall shear rate used in these studies was 800 sec-1. Surface coverage of platelets on the subendothelium were morphometrically evaluated. Aspirin, Ibuprofen, Prostaglandin E1 and 13-Azaprostanoic acid significantly reduced platelet thrombi on exposed subendothelium. The calcium antagonists, Quin 2 and Diltiazem, exerted similar inhibitory effects, whereas Verapamil was a poor inhibitor. Aspirin treatment significantly enhanced platelet adhesion to the exposed vascular surface. Salicylate and Salicylamide did not enhance platelet adherence. Only Aspirin enhanced the formation of lipoxygenase metabolites of radiolabeled arachidonate. Results suggest that drugs which inhibit platelet aggregation and secretion of granule contents reduce formation of platelet thrombi. However, these drugs may or may not have a similar influence on platelet interaction with the subendothelium leading to spreading, adherence or formation of aggregates.

Altered intraplatelet arachidonic acid metabolism during the acute state of unstable angina

Thromboxane A2 (TxA2) generation and 1-14C arachidonic acid (AA) metabolism by platelets (stimulated with thrombin) were studied in vitro in 16 patients with unstable angina both during the acute and chronic inactive phase of the angina. Eight patients with stable effort angina and 21 controls were also investigated. In acute unstable angina 1-14C AA metabolism was significantly increased through cyclooxygenase pathway resulting in a higher selective TxA2 generation than in stable effort angina and in controls (p less than 0.01). No differences were found between patients with stable effort angina and controls. The alterations in AA metabolism were no longer found when patients reverted to the inactive phase of angina. TxA2 generation by platelets was independent of the number of the daily ischemic attacks (r = 0.17, ns) in patients with unstable angina. Present results indicate that an altered intraplatelet AA metabolism leading to the increased TxA2 synthesis occurs simultaneously with the conversion of angina from the chronic to the acute phase.

Formation of prostacyclin-sensitive platelet aggregates in human whole blood in vitro. Part II. The occurrence of the phenomenon in males suffering from acute myocardial infarction

The de-aggregatory effect of prostacyclin (PGI2) and the rate of spontaneous platelet aggregation (SPA) were studied in vitro in whole blood of 24 males with acute myocardial infarction (MI) and 18 males, patient controls (PC). The de-aggregatory effect of PGI2 and the rate of SPA (measured as a percentage of changes in free platelet number in whole blood) were higher (p less than 0.01) in MI than PC. The de-aggregatory effect of PGI2 in whole blood was higher (p less than 0.05) on the first day of MI than on day 14 following MI. The highest de-aggregatory effect of PGI2 was found in whole blood of patients with MI complicated by ventricular fibrillation. In neither of the groups did the de-aggregatory effect of PGI2 correlate with patients' age, haematocrit, erythrocyte and leucocyte counts, triglycerides, HDL, LDL or total cholesterol levels. In the MI group, de-aggregatory effect of PGI2 was correlated with free platelet concentration (r = -0.59, p less than 0.05), elevation of glutamic oxalacetic transaminase (r = 0.53, p less than 0.05) and creatinine phosphokinase (r = 0.69, p less than 0.001). The de-aggregatory effect of PGI2 in blood of patients with evolving MI did not differ from that in PC. It is concluded that the increased rate of SPA and formation of PGI2-sensitive platelet aggregates in vitro in whole blood of MI patients are secondary to myocardial necrosis.

Effect of single dose aspirin on the development of penile hypercoagulability during erection

The effect of aspirin on the development of hypercoagulability in the penile blood during erection was studied in five Chacma baboons. Aspirin prevented the generation of hypercoagulability and may be of importance in delaying the development of penile atherosclerosis and ageing impotence.

Spontaneous platelet aggregation: observations on potential mechanisms

We identified SPA in three young apparently healthy women. SPA was associated with release of TXA2 and was only partially inhibited by ADP-inhibitor apyrase and alpha 2-adrenoceptor blocker yohimbine. In vitro incubation of aspirin (90 micrograms/ml) or selective TXA2 synthetase inhibitor OKY-046 (0.1 uM) with platelet rich plasma (PRP) did not abolish SPA, although platelet generation of TXA2 was markedly inhibited. In contrast, oral administration of large amounts of aspirin in one subject or in vitro incubation of PRP with TXA2 -endoperoxide receptor blocker SQ 29,548 (20-100 nM) significantly inhibited SPA. These studies suggest that SPA is associated with TXA2 release. Since TXA2 -endoperoxide receptor blocker completely abolishes the secondary wave, agents like this may be of therapeutic value in individuals with SPA and evidence of tissue ischemia.

Effect of complement depletion on O2 supply and consumption in ischemic dog myocardium

The purpose of this study was to determine whether depletion of serum complement can decrease the severity of an ischemic episode by improving regional O2 supply and consumption parameters in the ischemic region of the heart. Fourteen anesthetized dogs with serum complement intact or depleted (100 U/kg cobra venom factor given 8 hrs before) were subjected to left anterior descending coronary artery (LAD) occlusion for 6 hrs. Myocardial blood flows were determined before and 6 hrs after LAD occlusion using radioactive microspheres. Regional arterial and venous O2 saturations were determined using microspectrophotometry. In control animals, flow decreased from 122 +/- 42 to 13 +/- 14 ml/min/100 g (mean +/- SD) in the occluded LAD region. With complement depletion, LAD occlusion resulted in a flow reduction in the ischemic region (38 +/- 29 ml/min/100 g), but to a lesser degree than seen in the same region in control animals, especially in the subendocardium. O2 consumption was decreased in the ischemic region of both treatment groups, though O2 consumption was higher in this region in complement depleted animals compared to the values in control animals. The O2 supply/consumption ratio was decreased similarly in the ischemic region of control and complement depleted groups. Thus, with complement depletion, flow to the ischemic zone was improved but this region was still flow restricted. The flow increase during complement depletion was sufficient to allow an increased O2 utilization in the ischemic region.

Regulation of large coronary arteries

The majority of studies on the control of coronary artery vasoactivity have examined changes in coronary blood flow and coronary vascular resistance, indices that primarily reflect regulation of small arterioles and precapillary vessels. With the emergence of coronary artery vasospasm as a significant cause of angina pectoris, myocardial infarction, and sudden death, the control of large coronary artery caliber has assumed more significance. It is clear that resistance coronary vessels and large coronary arteries differ in response to both pharmacologic and physiologic stimuli. Vasodilation of large coronary arteries may occur by direct action of agents on the arterial smooth muscle or by the indirect action of receptor occupation, changes in blood flow, or liberation of endothelial factors. These indirect factors appear to contribute also to responses to agents that constrict coronary smooth muscle directly or through the autonomic nervous system. Furthermore, the mechanisms responsible for control of large coronary vessels in the normal circulation are likely to be profoundly different from those in the presence of diseased vessels. For example, several factors associated with coronary artery disease--elevated plasma cholesterol levels, endothelial disruption, atherosclerosis, vascular stenosis, and aggregated platelets--all have important actions on the control of large coronary arteries.

Acute myocardial infarction: an episodic event of several coronary spasms followed by dilatation?

Acute myocardial infarction (AMI) can no longer be considered as a single event, but as a series of episodes. In most of the cases, the initial event may be the induction of a severe spasm of the coronary artery by vasoconstrictive substance released from aggregated platelets. These spasms are followed by dilatation, which is caused by substances set free from the ischemic tissue. Dilatation then results in a washing-out of vasoactive mediators (as well as myoglobin) and platelets, which is reflected as a peak in the blood myoglobin concentration-time curve. The local depletion of vasodilative metabolites allows a further contraction of the coronary vessel. A new accumulation of platelets then stimulates another spasm. This vicious circle (thrombo-ischemic re-entry-mechanism) is repeated several times and can be interrupted by the systemic administration of thrombolytic drugs (streptokinase).

Myocardial ischemia: platelet and thromboxane concentrations in cardiac lymph and the effects of ibuprofen and prostacyclin

Blood platelets have been implicated in several mechanisms leading to and/or modifying myocardial ischemia. Cardiac lymph examination allows insight into the extracellular fluid that is in equilibrium with the capillary blood. In order to obtain an index of platelet activation during coronary artery events in the awake chronic animal, we wished to ascertain whether evaluation of cardiac lymph would detect changes in platelet activation resulting from a vascular occlusion. The study used conscious dogs in which cardiac lymph vessels had been previously cannulated by open-chest surgical protocol. The concentrations of immunoreactive thromboxane B2 and platelet counts were assessed in the cardiac lymph during the control period, the 10-60 minute occlusions, and the reperfusion periods. The same protocols were effected on another series of dogs after infusion of ibuprofen or prostacyclin. Initially, immunoreactive thromboxane B2 concentrations in the systemic blood and cardiac lymph were identical. A three-fold increase in immunoreactive thromboxane B2 concentrations occurred in untreated animals and was accompanied by a fall in platelet count in the lymph. The infusion of ibuprofen or prostacyclin, which inhibit platelet aggregation by different mechanisms, prevented both the decrease in platelets and the increase in immunoreactive thromboxane B2. In this study, intravascular events resulting from coronary occlusion invoke a rapid rise of immunoreactive thromboxane B2 in the extravascular fluid. A decrease in platelet escape into the extravascular compartment is interpreted as a result of intravascular aggregation promoting decreased platelet numbers. Thus, examination of continuously flowing cardiac lymph allows rapid detection of intravascular activation of platelets in the awake animal in the absence of surgical trauma.

Biological actions of leukotrienes. State of the art lecture

Leukotrienes are novel mediators derived from arachidonic acid through the 5-lipoxygenase enzyme system. Leukotriene B4 has potent effects on leukocyte function and in vivo induces leukocyte accumulation and changes in vascular permeability and modulates pain responses. Peptido-lipid leukotrienes are potent smooth muscle--contracting agents. They may have important cardiovascular actions through mechanisms involving either vasoconstriction or indirect vasodilatation. Evidence for leukotriene production has been found in subjects with allergic conditions and psoriasis, indicating a putative role for these substances in human disease.

Leukotrienes potentiate the effects of epinephrine and thrombin on human platelet aggregation

A cooperation between leukocytes and platelets relative to metabolism of arachidonic acid has been observed in animal studies. To determine potential stimulatory effects of leukotrienes (LTs) on human platelets, LTs were incubated with platelet rich plasma followed by addition of subthreshold concentration of aggregatory stimulus. LTs (LTE4 LTD4 LTC4) alone had no direct effect on platelet aggregation, but potentiated the effects of subthreshold concentrations of epinephrine and thrombin and caused complete platelet aggregation. This potentiation was similar in citrated or heparinized blood and was unaffected by exogenous CaCl2. LTs did not induce secondary wave of aggregation in aspirin or selective TXA2-synthetase blocker OKY-046-treated platelets. In addition, LTs stimulated TXA2 biosynthesis by platelets in the presence of subaggregatory concentrations of epinephrine, but not when platelets had been pretreated with OKY-046. These data indicate that LTs potentiate epinephrine-induced platelet aggregation by modulating TXA2 synthetase activity.

Biphasic effects of platelet-activating factor on coronary blood flow in anesthetized dog

Platelet-activating factor (PAF), modulates vascular tone by influencing prostaglandin release and vascular permeability. To determine its coronary effects we administered PAF (0.3 to 10 ug) into the left main coronary artery of anesthetized dogs with patent left circumflex (LCx) and narrowed left anterior descending (LAD) coronary arteries. PAF produced an initial increase followed by a decrease in coronary blood flow (CBF). The CBF increase was greater in the patent LCx than in the narrowed LAD, but the decrease was similar in both. These effects of PAF on CBF were dose-dependent, and associated with an increase in prostacyclin and thromboxane A2 metabolites. To examine the contribution of prostaglandin release in coronary effects of PAF, dogs were pretreated with indomethacin (5 mg/kg) followed by administration of PAF. In indomethacin-pretreated animals, the coronary effects of PAF were significantly attenuated. This study shows that PAF has biphasic effects on CBF in the normal coronary artery, but the major effect in the narrowed coronary is decrease in CBF. These effects of PAF can be attenuated by prior treatment of dogs with indomethacin.

Thromboxane release during percutaneous transluminal coronary angioplasty

The reason for coronary artery occlusion following percutaneous transluminal angioplasty (PTCA) remains an enigma. We postulated that alterations in arachidonic acid metabolism might contribute to coronary artery occlusion, particularly if platelets are perturbed and release thromboxane because of mechanical stimuli during PTCA. We serially monitored coronary sinus and peripheral arterial plasma thromboxane (TX) and prostacyclin (by standard radioimmunoassay of the metabolites TXB2 and 6-keto-PFG1 alpha) during PTCA in 10 patients. TX and prostacyclin were unchanged from control in seven uncomplicated procedures. In one patient with vasospasm, no changes were found. In two patients with occlusion, marked increases were measured in coronary sinus plasma TX. Patient No. 1 increased from 390 to 1375 pg/ml. Patient No. 2 increased from 155 to 1425 pg/ml. Both required emergency bypass grafting. No change in 6-keto-PGF1 alpha was found. Uncomplicated PTCA does not alter arachidonic acid metabolism through cyclooxygenase. Vasospasm need not be associated with TX release, but coronary artery occlusion is. TX may play a role in coronary artery occlusion during PTCA because of (1) increased release and (2) unopposed physiologic effects because increases were not found in the physiologic antagonist prostacyclin.

Systemic and transcardiac platelet activity in acute myocardial infarction in man: resistance to prostacyclin

There is increasing evidence that platelets play an important role in the pathogenesis of acute ischemic heart disease. Therefore an understanding of factors that influence platelet performance is important. This study was undertaken (1) to characterize during evolving myocardial infarction platelet activity in the peripheral circulation and across the ischemic/infarcting myocardial compartment, the locus of presumed platelet hyperactivity, and (2) to evaluate the effects of prostacyclin (PGI2), a most potent antiplatelet agent and vasodilator. A total of 59 patients with evolving myocardial infarction were studied. Twenty-two patients were instrumented with arterial and coronary sinus catheters and received intravenous infusion of PGI2, 13 +/- 4.5 ng/kg/min (mean +/- SD), for 90 min. In 15 patients with anterior myocardial infarction, transcardiac platelet function and response to PGI2 were studied. Plasma levels of beta-thromboglobulin (beta-TG) and of thromboxane B2 (TxB2), in vivo measures of platelet activity, were elevated three- and 10-fold. 6-Keto-prostaglandin F 1 alpha, the stable end product of PGI2, was less than 10 pg/ml, reflecting a leftward shift of the TxB2/PGI2 ratio. Platelets circulating during evolving myocardial infarction ("ischemic platelets") were hyperaggregable in response to ADP and relatively resistant to PGI2, both in vivo and in vitro. Concentrations of platelet cyclic AMP and the cyclic AMP response to PGI2 were diminished. The platelet hyperreactivity, expressed by plasma beta-TG, platelet aggregation, and PGI2-induced inhibition of aggregation, was most intense early during infarct evolution and decreased with time. The increased platelet performance resulted in "platelet fatigue," indicated by decreased contents of beta-TG of the ischemic platelet and decreased TxA2 production in response to collagen. However, the ischemic platelet produced twice normal TxA2 in response to arachidonic acid (stimulus and substrate), demonstrating a heightened metabolic capacity. TxA2 was produced across the ischemic/infarcting compartment in 10 of 15 patients with anterior myocardial infarction. The antiplatelet effect of PGI2 was greatly diminished. In summary, the data define an abnormal pattern of platelet behavior during evolving myocardial infarction, characterized by a proaggregatory environment, heightened platelet reactivity in both the peripheral and coronary circulation, and relative resistance to PGI2. The clinical consequences of the data are that the patient in the acute phase of myocardial infarction may benefit from suppression of platelet function and requires significantly greater doses of PGI2 than normal subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Sub-aggregatory doses of catecholamines prevent prostacyclin-induced inhibition of platelet aggregation

To assess the effect of subaggregatory concentrations of catecholamines on the antiaggregatory effect of prostacyclin (PGI2), platelets from normal human volunteers were exposed sequentially in vitro to epinephrine (less than or equal to 50 nM)- or norepinephrine (less than or equal to 1 microM) followed by PGI2 and adenosine diphosphate (ADP). Platelets thus pretreated did not manifest the normal inhibitory response to PGI2, aggregating to a similar extent as platelets exposed to ADP alone. This effect was unaffected by aspirin but was abolished by exposure to phentolamine. Catecholamine pretreatment similarly blocked the PGI2-induced increase in intracellular cyclic AMP, an effect which was also reversed by phentolamine. These data suggest that platelets exposed in vivo to elevated catecholamine concentrations, such as are seen clinically during myocardial infarction, might be similarly unresponsive to endogenous PGI2.

Actions of synthetic leukotrienes on platelets and blood vessels in the anesthetised pig: the release of a platelet derived vasodilator

The actions of leukotrienes (LT's) C4, D4, E4 and F4 have been investigated in the perfused hind-limb of the anesthetized pig. In the blood perfused hind limb LTC4, D4 and E4 increased the perfusion pressure in a dose-dependent fashion whereas LTF4 decreased perfusion pressure. In the Tyrode perfused hind limb all LT's increased perfusion pressure (rank order potency LTC4 = LTD4 much greater than LTF4). The actions of LTF4 were not affected by a wide variety of pharmacological treatments, including indomethacin, methysergide and FPL-55712. The LT's aggregated porcine platelets (rank order potency LTC4 greater than LTF4 greater than LTD4) and induced the release of a platelet-derived vasodilatory mediator. The results provide pharmacological evidence of specific leukotriene receptors in vivo and that leukotrienes can independently modulate blood flow. These data suggest that important interactions may occur between platelets, the arachidonate lipoxygenase products and platelet-derived substances in response to inflammatory stimuli in the cardiovascular system.

Lack of efficacy of nafazatrom, a novel anti-thrombotic compound, in patients with coronary artery disease

Nafazatrom (Bay G 6575) is a novel antithrombotic compound, which acts by stimulation of prostacyclin as well as by inhibition of lipoxygenase enzymes. To determine its effects on exercise performance in coronary artery disease patients, a double-blind study was conducted. Twenty patients with coronary artery disease underwent an exercise stress test before and 2 hours after administration of placebo or nafazatrom (1.2 gm). Before the drug administration, there was evidence of enhanced platelet activity, as reflected by elevated resting plasma beta thromboglobulin and thromboxane B2 concentrations. Plasma 6-keto-PGF1 alpha levels were undetectable in most patients. All coagulation tests were in the normal range. None of these parameters changed with exercise. Administration of placebo or nafazatrom before the exercise stress test did not significantly influence any of the coagulation or platelet function parameters or plasma concentrations of thromboxane B2 and 6-keto-PGF1 alpha. This lack of effect was evident both at rest and upon exercise. Compared to placebo, nafazatrom did not significantly increase exercise tolerance time or exercise-induced symptoms. In conclusion, nafazatrom did not influence exercise performance in patients with coronary disease.

Intracoronary platelet activation in ischemic heart disease: effects of ticlopidine

Plasma levels of platelet factor 4 have been measured in the aortic and coronary sinus blood of 35 patients: group I (n = 12) with normal coronary arteriograms; group II (n = 15) with angiographically proven coronary artery disease; and group III (n = 8) composed of patients with ischemic heart disease who were being treated with the antiaggregant agent ticlopidine at the time of cardiac catheterization. The mean increase in platelet factor 4 levels through the coronary circulation was 27.4 +/- 21.9 ng/ml (mean +/- standard deviation) in group II, compared with -1 +/- 4.5 ng/ml in group I (p less than 0.01). In group III plasma levels of platelet factor 4 in aortic and coronary sinus samples were all within the normal range. Thus, we conclude that platelet activation constantly occurs in the coronary circulation of patients with stable coronary artery disease, and can be prevented with ticlopidine.

The prostacyclin synthesis stimulating plasma factor is unchanged during acute angina pectoris

Earlier studies have demonstrated that the levels of prostacyclin synthesis stimulating plasma factor are changed during various clinical situations thus leading to disturbances in hemostatic balance. Therefore, we studied the plasma factor in 20 patients undergoing acute anginal attacks in order to see whether there is any influence or a timedependent change after the event. The patients were subdivided into males or females as well as into those with or without maturity onset diabetes and who were smokers or no-smokers. As in-vitro test systems the rat abdominal aorta, human coronary artery and cultured endothelial and smooth muscle cells obtained from minipigs were used. Our findings demonstrate that incubating the different tissue samples in plasma leads to a significant increase of prostacyclin formation or of its stable breakdown 6-oxo-PGF1 respectively, in comparison to buffer control incubation. However, in none of the groups was a change observed during an anginal attack or in rate 60 and 120 minutes thereafter. These findings suggest that the prostacyclin synthesis stimulating plasma factor is not involved in hemostatic dysregulation, which has been observed to occur during and immediately after a coronary anginal attack.

Verapamil decreases the formation of thromboxane from exogenous 14C-arachidonic acid in human platelets in vitro

The effects of verapamil on the metabolism of exogenous 14C-arachidonic acid were studied in human platelets in vitro. 1 mM verapamil decreased the formation of TXB2 and HHT and increased that of PGE2, PGD2 and PGF2 alpha. The radioactivity at the area of 12-HPETE on the thin layer chromatography plate was also increased by 1 mM verapamil. In addition, 10 microM and 1 mM verapamil caused a slight decreasing trend in the amount of free unmetabolized arachidonic acid. The results suggest that high concentrations of verapamil may decrease the formation of the aggregatory thromboxane and increase that of anti-aggregatory compounds, i.e. PGD2 and 12-HPETE in human platelets in vitro. However, as lower concentrations of verapamil (10 and 100 microM) had no significant effect on the metabolism of exogenous arachidonic acid, the anti-platelet effects of the drug at therapeutic concentrations are more likely to be mediated via other mechanisms, possibly via the inhibition of arachidonate release from the platelet membrane phospholipids.

Failure of percutaneous transluminal coronary angioplasty to stimulate platelet and prostaglandin activity

Platelet function and prostaglandin activity were evaluated in nine patients with coronary artery disease undergoing percutaneous left anterior descending coronary artery angioplasty (PTCA) and compared to nine normal controls. Transcoronary measurements (arterial-coronary sinus) of platelet counts, mean platelet volume, platelet factor 4 (PF4), beta thromboglobulin, thromboxane (B2), and 6-keto-PGF 1 alpha were made. When compared to normal controls, the patients with coronary artery disease had higher circulating baseline levels of PF4 in the coronary sinus. There was no transcardiac production of any factor at baseline or immediately after infusion of nitroglycerin or performance of PTCA. These results suggest that PTCA does not grossly alter arachidonic acid metabolism or platelet activity.

Increased vasoconstrictor activity of proximal coronary arteries with endothelial damage in intact dogs

In this study we examined the hypothesis that endothelial damage increases proximal coronary arterial vasomotor tone and sensitivity to vasoconstrictor stimulation. The response of the left anterior descending coronary artery (LAD) (% area change) to serotonin and nitroglycerin were examined in eight anesthetized (Innovar + nitrous oxide), closed-chest dogs by means of quantitative coronary angiography. Dose-response curves of percent change in arterial cross-sectional area for three doses of intracoronary serotonin were examined before and after endothelial damage produced by a balloon catheter in the LAD. Endothelial damage was verified by postmortem scanning electron microscopic examination. Intracoronary injection of 133Xe provided coronary flow data. The damaged segment of LAD showed spontaneous vasoconstriction and further constriction in response to serotonin (33 +/- 5% before and 52 +/- 6% area reduction after damage; p less than .05). Nitroglycerin reversed serotonin-induced vasoconstriction in LAD segments without damage but not in the LAD segment with endothelial damage. No significant changes were observed in aortic pressure, and heart rate was kept constant by pacing. Blood flow in the LAD was not affected by endothelial damage itself (control, 2.44 +/- 0.09 ml/min/g; damage, 2.53 +/- 0.22 ml/min/g). Endothelial damage induced spontaneous proximal coronary constriction and diminished the relaxant response to nitroglycerin in the presence of serotonin. These results suggest that focal coronary narrowing that occurs in some patients after provocation with vasoconstrictor agents may be caused by local areas of damaged endothelium.

Effects of BM 13.177, a new antiplatelet drug in patients with atherosclerotic disease

Seven males with atherosclerotic disease received daily 1.6 g of the thromboxane antagonist BM 13.177 in two separate oral dosages over a period of four days. The drug significantly reduced elevated plasma levels of thromboxane B2, beta-thromboglobulin and platelet factor 4, whereas thromboxane B2 generation was only slightly depressed. BM 13.177 inhibited platelet aggregation by collagen, and to a minor degree the second wave of ADP induced aggregation. Platelet sensitivity to prostacyclin and aggregation by ristocetin were not altered. Bleeding time was prolonged. All effects disappeared within 24 hours after the last application of the drug. No side effects were noted. Thus BM 13.177 appears to be a safe and effective new antiplatelet drug.