Elevated beta thromboglobulin in peripheral venous blood of patients with acute myocardial ischemia: direct evidence for enhanced platelet reactivity in vivo

Pathophysiology of acute coronary syndromes

The natural history of coronary artery disease is punctuated by clinical manifestations of unstable angina, acute myocardial infarction, and ischemic sudden death. These acute coronary syndromes share common pathophysiologic mechanisms that include fissuring of a plaque followed by varying degrees of dynamic coronary obstruction, which is due to vasoconstriction and coronary thrombosis. The response to plaque fissure is likely to be modulated by local and/or systemic procoagulant and anticoagulant-fibrinolytic activities. The key role of coronary thrombosis in acute coronary syndromes has substantial implications for prevention and treatment of complications of coronary atherosclerosis.

Plasma levels of platelet-derived growth factor in normal subjects and patients with ischemic heart disease

Plasma levels of platelet-derived growth factor (PDGF) were measured in 24 normal control subjects, 31 patients with stable angina pectoris, 25 patients with unstable angina pectoris, and 31 patients with acute myocardial infarction (AMI) by a sensitive direct radioimmunoassay. The plasma PDGF level in normal control subjects was 273 +/- 25 pg/ml; there was no significant correlation between the plasma PDGF level and age. Plasma PDGF levels in patients with unstable angina pectoris and acute myocardial infarction were significantly lower than those in normal control subjects and patients with stable angina pectoris (p less than 0.05). In patients with acute myocardial infarction the plasma PDGF level in the chronic phase was significantly higher than that in the acute phase (p less than 0.05). These observations raise the possibility that PDGF is involved in the pathophysiology of ischemic heart disease.

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.

Restoration by insulin of impaired prostaglandin E1/I2 receptor activity of platelets in acute ischemic heart disease

Treatment of normal platelet-rich plasma with a physiological amount of insulin (100 microunits/ml, optimum concentration) for 3 hours at 23 degrees C stimulated the binding of prostaglandin E1 by more than twofold (3,940 +/- 250 sites/10(8) platelets) compared with the nontreated, control platelet-rich plasma (1,590 +/- 265 sites/10(8) platelets). After platelet-rich plasma from patients with acute ischemic heart disease (n = 43), whose platelets showed impaired prostaglandin E1/I2 receptor activity (850 +/- 100 sites/10(8) platelets), was incubated with insulin (optimum amounts varied from 100 to 200 microunits/ml), the binding of the prostanoid was restored to normal levels (1,790 +/- 140 sites/10(8) platelets) in 75% of the cases. Twenty-five percent of the patients did not respond to the stimulatory effect of insulin. The increased binding of the prostanoid to the insulin-treated platelets also resulted in increased cyclic AMP levels both in normal subjects (44.14 +/- 3.1 pmol/10(8) [insulin-treated] platelets versus 16.35 +/- 2.91 pmol/10(8) [control] platelets) and in patients with acute ischemic heart disease (23.87 +/- 4.1 pmol/10(8) [insulin-treated] platelets versus 7.70 +/- 2.0 pmol/10(8) [control] platelets) by the prostanoid (1.0 microM). The treatment of platelet-rich plasma with the hormone decreased the minimum inhibitory concentration of the prostanoid from 34 +/- 14 to 15 +/- 9 nM (p less than 0.001) in the case of normal volunteers and from 49 +/- 15 to 32 +/- 11 nM (p = 0.002) in the case of "responder" patients. Insulin did not produce any effect on the inhibition of platelet aggregation by the prostaglandin in "nonresponder" patients. In the follow-up study, although the stimulatory effects of insulin on platelets from responder patients were improved to normal levels, the platelets from the nonresponder patients remained persistently unresponsive to the effect of the hormone.

Increased plasma beta-thromboglobulin in patients with coronary artery vein graft occlusion: response to low dose aspirin

The therapeutic effect of aspirin on vein graft patency was studied in patients undergoing coronary artery bypass graft surgery. The study design enabled the prospective evaluation of the relation of platelet activation, as measured by plasma beta-thromboglobulin concentration, to subsequent coronary vein graft occlusion. Serial beta-thromboglobulin levels were measured in 105 patients randomized to receive aspirin (324 mg/day) or placebo beginning within 1 h after surgery. Graft patency was assessed angiographically at 1 week and 1 year after surgery. Of 49 patients receiving placebo, 17 (34.7%) had one or more graft occlusions, 6 early, 10 late and 1 with both early and late occlusion. Of 56 patients receiving aspirin, 7 (12.5%) had one or more occlusions, 3 early and 4 late (p less than 0.01). Preoperatively, the beta-thromboglobulin level in surgical patients (29 +/- 13.5 ng/ml) was significantly higher than that of 51 control subjects (22.6 +/- 11.1 ng/ml) (p less than 0.004). Plasma beta-thromboglobulin levels remained comparatively constant at 3 and 12 months after surgery in the 43 patients who had both samples available (p less than 0.001, r = 0.65). The reduction in beta-thromboglobulin concentration from the preoperative level to 12 months postoperatively was greater in the aspirin-treated group (p less than 0.001). Multivariate logistic regression analysis demonstrated a significant association between preoperative beta-thromboglobulin concentration and graft occlusion (p less than 0.02), and aspirin treatment was effective in preventing occlusion when adjusted for the preoperative beta-thromboglobulin level (p less than 0.005). Plasma beta-thromboglobulin concentrations are elevated in patients with coronary artery disease, suggesting ongoing platelet activation.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired prostaglandin E1/I2 receptor activity of human blood platelets in acute ischemic heart disease

The platelets from 74 patients with acute myocardial infarction or with unstable angina showed decreased prostaglandin E1/I2 receptor activity when compared with that of 56 normal volunteers by using [3H]prostaglandin E1 as a probe. In normals, Scatchard analyses showed the presence of one high-affinity-low-capacity (Kd1 = 9.0 +/- 1.2 nM [mean +/- SD]; n1 = 120 +/- 30 sites/cell) and one low-affinity-high-capacity (Kd2 = 1.1 +/- 0.5 microM; n2 = 1,460 +/- 250 sites/cell) prostaglandin E1/I2 receptor population in platelets. In contrast (p less than 0.01), platelets from patients showed decreased ligand binding (n1 = 40 +/- 20 sites/cell; n2 = 800 +/- 210 sites/cell) with little change in the affinity of the receptors (Kd1 = 7.50 +/- 1.6 nM; Kd2 = 0.68 +/- 0.24 microM). On the other hand, the platelets from the patients with dilated cardiomyopathy (n = 7) who were hospitalized for acute chest pain but had normal coronary arteries did not show any impairment of the receptor activity. The plasma prostacyclin level of the patients with acute ischemic heart disease was similar to that of normal volunteers; this finding indicated that the defective receptor function was not related to the prostaglandin receptors occupancy in vivo. The impaired receptor activity was temporary in nature. The follow-up studies showed that the prostaglandin receptor activity of the patients' platelets improved to "normal" levels within 2-8 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Dipyridamole for coronary artery bypass surgery

A randomized trial to compare the effects of oral and intravenous dipyridamole was conducted in 58 patients undergoing coronary artery bypass graft (CABG) surgery. Preoperative oral administration of dipyridamole resulted in lower plasma drug concentrations in the early postoperative period than perioperative intravenous administration. Postoperative platelet counts were highest in the patients receiving intravenous dipyridamole, intermediate in those receiving oral dipyridamole and lowest in the control group. Postoperative blood loss was significantly reduced with both oral and intravenous dipyridamole. A second randomized trial was conducted in an additional 40 patients undergoing CABG surgery to evaluate the effects of dipyridamole on myocardial platelet and leukocyte deposition and the cardiac release of thromboxane. Twenty patients received intravenous dipyridamole perioperatively. Autologous platelets and leukocytes were labeled with 111In and 99mTc respectively and were infused before release of the crossclamp. Myocardial biopsies were obtained after aortic declamping and indicated that platelets and leukocytes were deposited in the myocardium during reperfusion. Dipyridamole reduced both platelet and leukocyte deposition. Cardiac release of thromboxane B2 occurred in the early postoperative period and was reduced by dipyridamole. In conclusion, dipyridamole preserved platelets and reduced postoperative bleeding and blood product transfusions in patients undergoing CABG surgery. Dipyridamole also reduced cardiac platelet deposition and thromboxane release and may reduce perioperative ischemic injury.

Increased platelet thromboxane A2/prostaglandin H2 receptors in patients with acute myocardial infarction

Platelets have been implicated in the formation of occlusive intracoronary thrombi leading to unstable angina pectoris and acute myocardial infarction. Evidence of platelet involvement in these syndromes includes increased thromboxane A2 synthesis during ischemic events and enhanced in vitro sensitivity to agonists. To determine the density and affinity of platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors in patients with acute myocardial infarction and unstable angina pectoris, the maximum number of binding sites (Bmax) per platelet and the dissociation constant (Kd) of the TXA2/PGH2 receptor antagonist, [125I]-PTA-OH, was determined at equilibrium in washed platelets. Patients with acute myocardial infarction had a significantly (p = 0.006) higher Bmax (4,468 +/- 672 sites/platelet, n = 9) compared with controls (2,206 +/- 203 sites/platelet, n = 8). Restudied at a time when the patients' coronary artery disease was clinically stable, Bmax values for the myocardial infarction group had returned to within normal limits. The dissociation constant for [125I]-PTA-OH was not significantly different in the acute myocardial infarction patients compared with controls. In patients with acute myocardial infarction, the duration of chest pain was positively correlated (r = 0.71, p less than 0.02) with the number of [125I]-PTA-OH binding sites (Bmax). In vitro platelet sensitivity to the TXA2/PGH2 mimetic, U46619, was assessed in aggregation studies. The maximal velocity of aggregation (slope) correlated with platelet TXA2/PGH2 receptor number (r = 0.67, p less than 0.001) and was significantly higher (p less than 0.02) in the acute myocardial infarction patients compared with the other study groups. There was no significant difference in the aggregation EC50 values for the thromboxane mimetic U46619 between unstable angina, acute myocardial infarction, and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian variation of plasma fibrinopeptide A level in patients with variant angina

Plasma levels of fibrinopeptide A (FPA), beta-thromboglobulin (BTG), and platelet factor 4 (PF4) were examined on venous plasma samples taken every 4 hours for 24 hours in 20 patients with variant angina and 20 patients with stable exertional angina together with 24-hour Holter recordings. The mean plasma FPA levels (ng/ml) at 2:00 PM, 6:00 PM, 10:00 PM, 2:00 AM, 6:00 AM, and 10:00 AM were 4.6 +/- 1.0, 3.1 +/- 0.5, 6.1 +/- 1.6, 9.9 +/- 2.4, 8.7 +/- 1.4, and 4.2 +/- 0.8 in patients with variant angina (p less than 0.01) and 1.8 +/- 0.2, 2.3 +/- 0.3, 1.9 +/- 0.3, and 2.3 +/- 0.2 in those with stable exertional angina. In seven patients with variant angina, we also examined the effects of heparin (3,000 units), given subcutaneously at 6:00 PM, 10:00 PM, and 2:00 AM, on the plasma FPA levels and the anginal attacks. Although heparin suppressed the elevation and circadian variation of plasma FPA levels, it did not suppress the attacks and their circadian variation in these patients. Plasma FPA levels increased significantly from 3.7 +/- 0.5 to 12.5 +/- 2.7 ng/ml during or immediately after an attack in the seven patients with no heparin. On the other hand, the plasma levels of BTG and PF4 were increased in patients with variant angina as compared with those with stable exertional angina but did not show a significant circadian variation in both groups. We conclude that 1) plasma levels of FPA, BTG, and PF4 were increased in patients with variant angina as compared with those with stable exertional angina; 2) there was a significant circadian variation in the plasma levels of FPA in parallel with that of the frequency of the attacks with the peak level occurring from midnight to early morning in patients with variant angina; and 3) elevated levels of plasma FPA are the result and not the cause of coronary spasm.

Raised concentrations of glucose and adrenaline and increased in vivo platelet activation after myocardial infarction

Plasma concentration of beta thromboglobulin was used as an index of in vivo platelet activation in 36 patients after acute myocardial infarction. Twelve patients had diabetes, seven had pulmonary oedema or cardiogenic shock (pump failure) or both, and 17 had uncomplicated infarcts. On the first day of admission, concentrations of beta thromboglobulin were higher in the patients with diabetes and those with pump failure than in those with uncomplicated infarcts. Concentrations of beta thromboglobulin in the non-diabetic patients were studied by multiple regression analysis and were significantly associated with plasma concentrations of adrenaline, pump failure, and glucose but not with noradrenaline or infarct size. When all subjects were considered together, glucose, adrenaline, and pump failure were associated with the beta thromboglobulin concentration but diabetes was without significant effect. Hyperglycaemia and raised plasma adrenaline concentration after myocardial infarction may activate platelets, and this could contribute to poor outcome in such patients.

Effect of pacing-induced myocardial ischemia on platelet activation and fibrin formation in the coronary circulation

The effect of pacing-induced myocardial ischemia on platelet activation and fibrin formation was investigated in seven patients with severe proximal lesions of the left anterior descending coronary artery to determine if acute ischemia activates the coagulation system. Fibrin formation was assessed from plasma levels of fibrinopeptide A. Platelet activation was assessed by levels of platelet factor 4, beta-thromboglobulin and thromboxane B2. Plasma levels were measured before, during and after acute myocardial ischemia induced by rapid atrial pacing. Blood samples were collected from the ascending aorta and from the great cardiac vein through heparin-bonded catheters. The occurrence of anterior myocardial ischemia was established by electrocardiography and by myocardial lactate extraction. No significant transmyocardial gradients in the levels of fibrinopeptide A, platelet factor 4, beta-thromboglobulin or thromboxane B2 were found at rest, during ischemia or in the recovery period, and levels in the great cardiac vein did not change in response to ischemia. These data indicate that pacing-induced myocardial ischemia does not result in release of fibrinopeptide A, platelet factor 4, beta-thromboglobulin or thromboxane B2 into the coronary circulation, and imply that acute ischemia does not induce platelet activation or fibrin formation in the coronary circulation.

Vasoconstrictor activity of coronary sinus plasma from patients with coronary artery disease

Vasoactive properties of plasma samples taken from the coronary sinus, a systemic artery and the superior vena cava of 13 patients with angiographically proven coronary artery disease and from 5 patients with normal coronary arteriograms were assayed in vitro by measurement of the changes in tension of rings of isolated canine coronary arteries. Addition of 1 ml samples of platelet-rich plasma from the coronary sinus of patients with coronary artery disease into a 20 ml organ bath induced an initial relaxation followed by sustained constriction in bioassay coronary artery rings with endothelium and only vasoconstriction in rings without endothelium. The vasoconstrictor activity of the coronary sinus plasma showed positive correlation with the severity and extent of coronary artery narrowing. Systemic arterial and venous plasma samples from patients with coronary artery disease and the coronary sinus plasma from patients with no coronary artery disease evoked only endothelium-dependent relaxations. These vasoactive properties of the various plasma samples were similar whether the samples were taken during rest or during supine bicycle exercise. The serotoninergic receptor antagonist methiothepin prevented the vasoconstriction induced by the coronary sinus plasma samples. These data demonstrate that the coronary sinus blood of patients with atheromatous coronary artery disease exhibits vasoconstrictor activity that may be associated with 5-hydroxytryptamine (serotonin), presumably released from platelets.

Fibrinopeptide A and platelet factor levels in unstable angina pectoris

Fibrinopeptide A, platelet factor 4, beta-thromboglobulin, thromboxane B2, and 6-keto-prostaglandin F1 alpha were estimated by radioimmunoassay on venous plasma samples taken within 48 hr of admission from 16 consecutive patients with unstable angina and 15 patients with stable angina matched for clinical variables. The ratio of circulating platelet aggregates, platelet aggregation to increasing concentrations of ADP (0.455 to 1.82 micrograms/ml), and platelet thromboxane B2 production in vitro were also tested. The two groups of patients were statistically similar in terms of sex distribution, age, presence of risk factors, use of medication, extent of coronary artery disease and history of previous myocardial infarction. Mean plasma levels of fibrinopeptide A were 2.7 +/- 0.4 ng/ml (geometric means +/- SEM, range 1.5 to 5.5) in patients with stable angina vs 5.5 +/- 1.8 ng/ml (range 2.4 to 32; p less than .001) in those with unstable angina. In the latter group, after 6 to 8 days, fibrinopeptide A levels decreased to 3.6 +/- 0.5 ng/ml (range 1.5 to 9.3; p less than .04 vs admission). All other variables measured were statistically identical in the two groups. We conclude that plasma fibrinopeptide A levels, as opposed to platelet factors, discriminate between patients with unstable and stable angina, indicating an activation of the coagulation system in unstable angina.

Unstable angina pectoris: the first half century: natural history, pathophysiology, and treatment

Unstable angina pectoris as a distinct syndrome intermediate between chronic stable angina and acute myocardial infarction was first described about a half century ago. The incidence of death or myocardial infarction rises in the first few months after destabilization of angina. Hemodynamic, scintigraphic, and arteriographic studies in the last 15 years have shown that unstable angina is chiefly due to "dynamic" coronary stenoses, transient reversible limitations in coronary blood flow caused by a complex interaction between coronary vasoconstriction, transient platelet plugging, and transient thrombosis. The trigger for the onset of dynamic coronary stenoses is probably acute changes in coronary arterial morphology in or near atherosclerotic plaques making those areas more thrombogenic. A large fraction of patients with unstable angina restabilize initially with medical management. The role of beta blockers is unclear, but they may protect against development of coronary events for patients with unstable angina similar to that reported for patients with myocardial infarction. Nitrates and calcium blockers are probably superior to beta blockers in restabilization of angina, but protection against coronary events has not yet been demonstrated clearly. Further investigation is needed to distinguish the relative benefits of a two-drug (heart rate-limiting calcium blocker plus nitrates) regimen vs. a three-drug regimen including beta blocker. There is no basis for emergency coronary bypass surgery to prevent myocardial infarction or death. Urgent surgery should be limited to patients who do not stabilize readily with medical therapy. One third or more of the patients who initially restabilize with medical therapy will require coronary revascularization in the year after unstable angina because of severe angina. An antithrombotic regimen of aspirin (or possibly heparin) reduces the incidence of progression to death or myocardial infarction. Two important future directions for research should be promising: development of better antithrombotic regimens other than aspirin alone for protection against coronary events; and improved ability to distinguish the patients who initially respond to medical therapy who are at low risk for later severe angina from those at higher risk.

Abnormal diurnal changes in in-vivo platelet activation in patients with atherosclerotic diseases

We measured the urinary excretion of beta-thromboglobulin in timed urine samples collected by 2 groups of healthy volunteers, (group I, n = 20, mean age 34 years, group II, n = 15, mean age 64 years) and by patients (n = 40) with symptomatic atherosclerotic diseases. Older healthy subjects were found to excrete high amounts of BTG in comparison to young subjects (302.25 +/- 50.61 vs 219.65 +/- 59.31 ng/day, P less than 0.05). Higher (P less than 0.01) levels of urinary BTG were observed in patients with coronary (427.61 +/- 179.96 ng/day), cerebral (422.13 +/- 223.2 ng/day) and peripheral (454.16 +/- 269.05 ng/day) arterial diseases and in diabetic patients with diffuse vascular complications (613.71 +/- 253.07 ng/day). The diurnal variability of BTG excretion, measured as coefficient of variation (C.V. %) of the mean daily excretion rate, was higher (P less than 0.001) in atherosclerotic patients (70.59 +/- 26.57) as compared with the similar values observed in the control groups of young (32.05 +/- 14.54) and older subjects (26.38 +/- 8.4). Comparable diurnal variabilities of the creatinine excretion rate were observed in the control groups and in patients. These data indicated that in vivo platelet activation may occur in atherosclerotic patients with a distinctive high fluctuation rate.

Factors determining the activity of ischemic heart disease

Transient regional myocardial ischemia appears to underlie symptoms such as angina pectoris and represents a key pathophysiologic step, since it is an objective marker of disease activity and is capable of causing disabling symptoms and damage to left ventricular myocardium. A study of the characteristics of transient ischemia in and out of the hospital has shown that symptoms are an inconsistent underestimation of these events. Ischemia is generally prolonged, mostly asymptomatic, and usually accompanied by a regional decrease in myocardial perfusion. Studies out of the hospital have also shown that these episodes are frequently triggered by a wide range of ordinary everyday activities. These new features of transient ischemia are worth noting when searching for relevant causes that are present during everyday life and when trying to choose more rational therapy. More detailed studies of patient activity have shown that different levels of mental arousal are the most common triggering mechanism causing ischemia out of the hospital. In addition, the occurrence of transient ischemia during everyday life displays a circadian rhythm, with an increase and peak occurrence between 6:00 A.M. and 12 noon each day. The day-to-day variability of ischemia is marked, indicating functional disturbances of coronary stenoses against a background of a severe reduction in cross-sectional area. The examination of proximal stenoses has shown that the reduction in cross-sectional area is usually underestimated by conventional angiography; pressure gradients across coronary stenoses are common and, with reduced poststenotic blood pressure, can jeopardize perfusion; disturbances of vessel caliber and antegrade flow can accompany many of the ordinary everyday activities known to trigger ischemia detected in Holter tapes studied out of the hospital; and there is clear-cut evidence of endothelial dysfunction in these patients, with reversal of the normal dilator response to acetylcholine and paradoxical constriction of stenoses. This evidence of endothelial dysfunction in humans could be central to the problems of atheromatous narrowing, thrombus, and disturbed vasomotion.

Platelet hyperaggregability in patients with chest pain and angiographically normal coronary arteries

Forty-one patients with chest pain and angiographically normal coronary arteries were studied for platelet abnormalities. Patients with conditions known or suspected to be associated with chest pain or platelet dysfunction were excluded. After coronary angiography and 2-week withdrawal from all medications, platelet aggregometry was performed using peripheral venous plasma samples and 3 concentrations of adenosine diphosphate, 2.34, 1.17 and 0.58 microM, and epinephrine, 11, 1.1 and 0.55 microM, as stimuli. Platelet morphology in response to surface contact (adhesion) was evaluated by transmission electron microscopy to determine the percentage of platelets in the round/abortive (inactive), dendritic (intermediate) and spread (activated) forms. Plasma specimens obtained from healthy volunteers of similar age and sex were analyzed in parallel and served as control subjects. Compared with control subjects, patients had increased aggregation at all concentrations of both adenosine diphosphate and epinephrine (p less than 0.001). Patients also had fewer platelets in the dendritic form and more in the round/abortive and spread forms. Thus, patients with chest pain and normal coronary arteries have platelet hyperaggregability in vitro, although the clinical relevance of this finding is unclear.

Study on the platelet factor and beta-thromboglobulin in the patients with ischemic heart disease

The platelet factor 4 (PF₄) and β-thromboglobulin (β-TG) were measured to evaluate the platelet activation in vivo in patients with ischemic heart disease. The results are summarized as follows: 1. In patients with acute myocardial infarction, the PF₄ and the β-TG levels were significantly higher compared to those of normal controls (P<0.005). 2. In patients with angina pectoris, the PF₄ and the β-TG levels were not signivicantly different from those of normal controls. 3. A pattern of decline in the PF₄ and the β-TG level was shown in patients with acute myocardial infarction by day 10. In conclusion, the measurement of PF₄ and β-TG is a useful method to detect the platect activation in vivo in the patients with ischemic heart disease.

Plasma levels of platelet secretory proteins

Platelets contain three types of secretory organelles: the dense granules, the alpha granules, and the lysosomes. Most of the proteins secreted from platelets are stored in the alpha granules, whereas the dense granules contain substances such as adenine nucleotides, serotonin, Ca++, and inorganic pyrophosphate types as well as a heparatinase. Three of the secreted alpha granule proteins have been measured by radioimmunoassay and it has been suggested that levels of these proteins in patient plasmas provide an index of in vivo platelet activation and secretion. These three are beta-thromboglobulin, platelet factor 4, and thrombospondin. In this chapter the chemistry of these proteins will be considered briefly, as will their clearance from the circulation, and then the clinical studies will be reviewed critically. Since radioimmunoassays were developed for these proteins (the first was reported in 1975), there has been a profusion of reports on levels of one or another of these proteins in a wide range of disease states, and these reports have indicated secreted platelet protein levels ranging from normal to grossly elevated in a given disease state. Possible reasons for such variability will be discussed.

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)

Histamine-induced spasm not significantly modulated by prostanoids in a swine model of coronary artery spasm

The role of prostanoids in a swine model of coronary artery spasm was examined. Eighteen miniature pigs underwent endothelial denudation of the left coronary artery (left circumflex branch in 14 pigs and left anterior descending branch in 4 pigs) followed by high cholesterol feeding. Three months after the denudation, when coronary artery spasm was repeatedly provoked along the denuded portion of the coronary artery by histamine, the vasoconstrictive effect of thromboxane A2 and the preventive effects of indomethacin and prostacyclin against histamine-induced coronary artery spasm were examined. Intracoronary administration of thiothromboxane A2, 200 micrograms, a stable thromboxane A2 analog, failed to provoke coronary artery spasm (seven of seven cases) but nonselectively constricted the coronary artery by 33%. Intravenous administration of indomethacin, 2 mg/kg, or continuous intravenous infusion of prostacyclin, 50 ng/kg per min, failed to prevent histamine-induced coronary artery spasm (four of four and eight of eight cases, respectively), yet the spasm was all but prevented by intravenous pretreatment with diphenhydramine at a dose of 1 mg/kg. Thus, in this swine model, prostanoids may not play a primary role in the occurrence of coronary artery spasm.

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.

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.

Calcium antagonists for acute ischemic heart disease

The pharmacologic and physiologic effects of the slow-channel calcium antagonists verapamil, nifedipine and diltiazem are reviewed. Relevant pathophysiologic features of various acute ischemic heart disease syndromes are also reviewed and the clinical applications of the slow-channel calcium antagonists to acute ischemic heart disease syndromes in patients are discussed.

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.

Effect of heparin bonding on catheter-induced fibrin formation and platelet activation

Pathologic and experimental evidence indicates that platelet activation and fibrin formation contribute to the pathogenesis of angina pectoris, coronary vasospasm and myocardial infarction. Detection of localized intravascular platelet activation and fibrin formation in vivo by selective blood sampling requires catheters that do not induce coagulation ex vivo. We studied the effect of heparin bonding of catheter surfaces on activation of the coagulation system by cardiovascular catheters. Woven Dacron, polyvinylchloride, and polyurethane catheters were tested and compared with identical catheters with heparin-bonded surfaces in 47 patients undergoing percutaneous cardiac catheterization. Platelet activation was measured by radioimmunoassay of plasma platelet factor 4 (PF4), beta-thromboglobulin (BTG), and thromboxane B2 (TXB2) in blood samples withdrawn through catheters, and fibrin formation was assessed by determination of fibrinopeptide A (FPA) levels. In blood samples collected through conventional catheters, FPA, PF4, BTG, and TXB2 levels were markedly elevated; blood sampling through heparin-bonded catheters had no significant effect on FPA, PF4, BTG, or TXB2 levels. Scanning electron microscopy disclosed extensive platelet aggregates and fibrin strands adherent to the surface of conventional catheters but not to heparin-bonded catheter surfaces. This study demonstrates that (1) collection of blood samples through cardiovascular catheters causes artifactual elevation of FPA, PF4, BTG, and TXB2 levels, and (2) heparin-bonded catheter surfaces effectively prevent catheter-induced platelet alpha-granule release and fibrin formation on catheter surfaces. Heparin-bonded catheters will facilitate investigation of the role of intravascular coagulation in coronary artery disease by eliminating catheter-induced fibrin formation and platelet activation.

Lack of elevation of platelet factor IV in plasma from patients with myocardial infarction

Platelet factor IV and beta-thromboglobulin are protein constituents of platelet granules. Elevated levels of these proteins in plasma have been used as sensitive indicators of platelet degranulation. Clearance of platelet factor IV is much faster than that of beta-thromboglobulin after release of the proteins in vivo. Although increases of platelet factor IV have been observed in patients with infarction, the implication that they reflect pathogenetic phenomena such as coronary thrombosis has not been assessed explicitly. Accordingly, plasma samples obtained serially from 52 patients with acute myocardial infarction under rigorous conditions verified to minimize platelet degranulation in vitro were evaluated prospectively. Correlative studies were performed to detect left ventricular mural thrombus, and coronary thrombosis was assessed independently in selected patients with indium-111 platelet scintigraphy. Platelet factor IV was normal at the time of admission in patients with infarction, averaging 6.3 +/- 3.3 ng/ml, similar to values in 44 other patients with chest pain without subsequent infarction (5.7 +/- 2.7 ng/ml) and in 25 normal subjects (4.3 +/- 1.6 ng/ml). Platelet factor IV generally did not increase during hospitalization in patients with infarction despite recurrent chest pain, development of left ventricular thrombus or documented recurrent infarction. However, platelet factor IV increased consistently after invasive procedures, accounting for 104 of the total of 110 increases due to platelet activation in vivo as reflected by persistence of elevated levels of beta-thromboglobulin. Thus, platelet factor IV values generally remain normal despite acute myocardial infarction. Rare increases that occur reflect platelet degranulation in vitro due to sampling artifact or perturbations of platelets in vivo due to invasive procedures.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet factor 4 release during exercise in patients with coronary artery disease

Many recent studies provide evidence that increased platelet activation occurs in a significant number of patients with atherosclerotic coronary artery disease. The mechanisms responsible for this activation are unknown, although there have been studies suggesting a correlation with abnormal lipoproteinemia, acute myocardial infarction, unstable angina, and exercise-induced myocardial ischemia. We studied 84 patients undergoing standardized treadmill exercise using either a Bruce [N = 63] or symptom-limited Naughton protocol [N = 21]. In contrast to ten healthy volunteer subjects, the patient group demonstrated a significant increase in plasma concentrations of platelet factor 4 [PF4] between pre- and postexercise blood samples confirming earlier reports of exercise-induced platelet activation and secretion. As with previous studies, however, only a subset of patients demonstrated this response. When the entire group was analyzed for the presence or absence of electrocardiographic ischemic changes and the presence of documented versus suspected coronary artery occlusions, there were no differences noted between groups that explained the variable responses measured. However, there was a significant difference between patient groups when analyzed by whether or not they were being treated with beta-blocking agents. Patients who were being treated with propranolol or one of the longer-acting beta-blocking agents did not have a significant increase in plasma PF4 following exercise, in contrast to patients who were not beta-blocked. Plasma concentrations of epinephrine, norepinephrine, and lactic acid were measured in 49 patients and all normal subjects. There was no correlation between the changes in plasma PF4 concentrations and any of these three variables, suggesting that platelet activation was not occurring through direct platelet activation by circulating catecholamines. This study provides further evidence that there is a subset of CAD patients with platelet hyperactivity. This is the first time that beta-blockade has been demonstrated to modify this platelet response. The effectiveness of beta-blocking agents in CAD may be in part related to their antiplatelet effect.

Coagulopathy and platelet activation in Kawasaki syndrome: identification of patients at high risk for development of coronary artery aneurysms

Prospective evaluation of platelet activation and hypercoagulability was performed in 31 patients with Kawasaki syndrome. Most patients had elevated acute-phase reactants when studied during the first 3 weeks of their illness; 17 of 25 (68%) patients had factor VIII activity greater than 150%, 18 of 24 (75%) had fibrinogen greater than 400 mg/dl, and 17 of 31 (55%) had a platelet count greater than 450,000/mm3. Antithrombin III was depressed initially in 17 of 25 (68%) patients. Depleted fibrinolytic activity, as measured by a euglobulin lysis time greater than 300 minutes, was documented in nine of 20 (45%) patients. Plasma beta-thromboglobulin (BTG) measured at 0 to 3 weeks was elevated (greater than 43 ng/ml) in seven of 24 (29%) patients. All patients with coronary artery aneurysms had elevated BTG values. The mean BTG in the group with aneurysms was 72.3 ng/ml when measured during the first 3 weeks after onset of fever, and 87.7 ng/ml at 4 to 7 weeks. The group without aneurysms had mean BTG values of 29.4 and 28.3 ng/ml at 0 to 3 and 4 to 7 weeks, respectively. The difference between the two groups was significant (P less than 0.002) for both the initial and later values. An elevated BTG during the first 3 weeks after onset of fever was highly associated with aneurysm formation in our patients (P less than 0.007). No aneurysms occurred in patients with a normal BTG value.

Thromboxane B2, 6-keto-PGF1 alpha, PGE2, PGF2 alpha, and PGA1 plasma levels in arteriosclerosis obliterans: relationship to clinical manifestations, risk factors, and arterial pathoanatomy

Current concepts of atherogenesis based on animal and human investigations indicate prostaglandins as a key factor in atherosclerotic lesions. The plasma profiles of thromboxane B2 (TXB2), 6-keto-PGF1 alpha, PGE2, PGF2 alpha, and PGA1 were investigated by means of a sensitive radioimmunoassay technique in 40 patients with arteriosclerosis obliterans and in 30 healthy control subjects. Abnormally high levels of TXB2 and PGE2 (222.97 +/- 320.86 pg/ml, mean +/- SD, vs 20 +/- 2.1 and 352.66 +/- 235.54 vs 24.4 +/- 3, p less than 0.01) were detected in arteriosclerosis obliterans patients. The ratio between TXB2 and 6-keto-PGF1 alpha was increased from 1.2 in control subjects to 6.0 in patients. In arteriosclerosis obliterans TXB2 increased in relation to clinical manifestations and to the extension of the vascular damage. In addition, TXB2 was positively related to serum triglyceride content (r = 0.562, p less than 0.05) and inversely related to platelet count (r = 0.727, p less than 0.001). The marked imbalance between the stable metabolites of thromboxane and prostacyclin in arteriosclerosis obliterans patients provides biologic evidence which fits well with the thrombogenic theory of atherosclerosis. These results further support the theory that prostaglandins may be heavily involved in atherosclerosis.

Are platelet factor 4 and beta-thromboglobulin markers of cardiovascular disorders?

Beta-thromboglobulin and platelet factor 4 are the two best characterized platelet-specific proteins. They are stored in the platelet alpha-granules and released during platelet activation. Their physiological function is unknown. PF4 has high anti-heparin activity, whilst beta-TG does not. Certain factors can affect the plasma level of one or both of these two proteins and these must be borne in mind whenever the evaluation of beta-TG and PF4 are thought to represent true in vivo platelet activation: their artificial release due to sample collection and processing, the in vivo release of PF4 induced by heparin, and the elevation of beta-TG due to renal failure. What really represents an abnormal level of beta-TG and PF4 is unknown, since we do not know their pathophysiology. At present, however, the platelet-specific proteins, even if they are considered as 'markers' of platelet activation, do not necessarily reflect the severity of the cardiovascular disorders nor do they signal thrombus formation, as thrombosis is a consequence of several interacting factors.

Fibrinopeptide A and beta thromboglobulin in patients with angina pectoris and acute myocardial infarction

The purpose of this study was to investigate the degree of platelet activation and thrombin generation in 40 patients with stable angina pectoris and in 20 patients with acute myocardial infarction (AMI) by determining the plasma beta thromboglobulin (BTG) and fibrinopeptide A (FPA) concentrations. In patients with angina pectoris increased platelet activation correlated with extensive coronary pathology; the activation, however, was not influenced by a previous myocardial infarction, use of oral anticoagulants, beta-blocking agents, or hyperlipidemia. The plasma beta thromboglobulin concentration predicted more accurately the extent of the coronary artery disease than the functional angina pectoris classification. Thrombin generation was within the normal range. In patients with acute myocardial infarction increased platelet activation and enhanced thrombin generation were found, which were not related to the infarct localization, infarct size, or the presence of complications. Consequently, in these patients determination of plasma beta thromboglobulin and fibrinopeptide A concentrations is useless for the diagnosis of venous thromboembolism.

Protective effects of aspirin against acute myocardial infarction and death in men with unstable angina. Results of a Veterans Administration Cooperative Study

We conducted a multicenter, double-blind, placebo-controlled randomized trial of aspirin treatment (324 mg in buffered solution daily) for 12 weeks in 1266 men with unstable angina (625 taking aspirin and 641 placebo). The principal end points were death and acute myocardial infarction diagnosed by the presence of creatine kinase MB or pathologic Q-wave changes on electrocardiograms. The incidence of death or acute myocardial infarction was 51 per cent lower in the aspirin group than in the placebo group: 31 patients (5.0 per cent) as compared with 65 (10.1 per cent); P = 0.0005. Nonfatal acute myocardial infarction was 51 per cent lower in the aspirin group: 21 patients (3.4 per cent) as compared with 44 (6.9 per cent); P = 0.005. The reduction in mortality in the aspirin group was also 51 per cent--10 patients (1.6 per cent) as compared with 21 (3.3 per cent)--although it was not statistically significant; P = 0.054. There was no difference in gastrointestinal symptoms or evidence of blood loss between the treatment and control groups. Our data show that aspirin has a protective effect against acute myocardial infarction in men with unstable angina, and they suggest a similar effect on mortality.

Effects of provocation on transcardiac thromboxane in patients with coronary artery disease

Thromboxane A2 exerts powerful effects on vascular smooth muscle tone and platelet aggregability. Previous studies have demonstrated increases in transcardiac thromboxane B2 (a stable thromboxane A2 metabolite) in patients with unstable angina and recent chest pain. To determine whether these increases in transcardiac thromboxane B2 are unique to the unstable anginal syndrome or are merely a consequence of ongoing myocardial ischemia, simultaneous ascending aortic and coronary sinus blood samples were obtained for quantitation of thromboxane B2 in 52 patients with a history of chest pain. Provocation was performed with (1) rapid cardiac pacing in 23 patients, (2) cold pressor stress in 19 patients, and (3) sustained isometric exertion in 10 patients. Of the 52 patients, only 5 had a substantial (greater than 3-fold) increase in coronary sinus thromboxane B2 in response to provocation: 1 had unstable angina and chest pain during the previous 48 hours and 4 had a myocardial infarction within the previous 6 weeks. Similarly, only 7 had a greater than 3-fold increase in the coronary sinus/aortic thromboxane B2 ratio in response to provocation: 1 had unstable angina and recent chest pain, 5 had a recent myocardial infarction, and 1 had both of these. There were no other clinical features unique to these patients. The remaining patients with similar diagnoses did not develop a marked increase in coronary sinus thromboxane B2 or the coronary sinus/aortic thromboxane B2 ratio with provocation. None of the 35 patients with stable ischemic heart disease or nonischemic chest pain syndromes had a substantial increase in coronary sinus thromboxane B2 or the coronary sinus/aortic thromboxane B2 ratio (p less than 0.001 for both coronary sinus thromboxane B2 and the coronary sinus/aortic thromboxane B2 ratio in comparison with the 17 patients with recent unstable angina or myocardial infarction). Thus, generous amounts of thromboxane B2 are released into the coronary circulation after provocation in some patients with unstable angina or recent myocardial infarction but not in those with stable ischemic heart disease or nonischemic chest pain syndromes.

Platelet factor 4 (PF4) and heparin-released platelet factor 4 (HR-PF4) in patients with cardiovascular disorders

The recent introduction of the determinations of platelet factor 4 (PF4) and beta-thromboglobulin (beta TG) by radioimmunoassay provided a new tool to obtain knowledge of in vivo platelet activation. We evaluated the plasma level of PF4 and beta TG in 14 normal subjects (mean PF4 7.7 ng/ml; beta TG 28.8 ng/ml), in 29 patients with chronic stable cardiovascular disorders (mean PF4 9.8 ng/ml; beta TG 32.6 ng/ml) and in 15 diabetics with vascular disease (mean PF4 14.5 ng/ml; beta TG 41.8 ng/ml). The great majority had normal values and no statistical differences were noted among the three groups (p greater than 0.05). Fifteen days of treatment with 150 mg daily of dipyridamole produced a significant reduction in the levels of both proteins (p less than 0.01), in contrast of the daily administration of 650 mg of aspirin, which failed to produce any significant change (p greater than 0.5). The patients and the normal subjects were also administered 3,000 USP units intravenously of porcine heparin. The values of the heparin released-platelet factor 4 (HR-PF4), evaluated 5 minutes after the injection, showed a good correlation between platelet concentration and HR-PF4 levels (z = 2.37, p less than 0.02) in the patients. The determination of standard residual following linear regression analysis of HR-PF4 indicated the presence of two distinct patient populations. One group, including the vast majority of patients, did not differ from the control (patients mean HR-PF4 111.1 ng/ml; controls: mean HF-PF4 136 ng/ml). The other group, with severe cardiovascular disease, but with normal levels of PF4 and beta TG in almost all patients and similar platelet concentrations, showed a significantly higher HR-PF4 (219 ng/ml). Neither aspirin nor dipyridamole had any effect on the level of HR-PF4. This HR-PF4 could represent a possible marker of the interaction of platelets with a seriously damaged atherosclerotic vessel wall.

Effects of dazoxiben on transcardiac thromboxane levels and haemodynamics in coronary heart disease

1 Plasma thromboxane levels were obtained from both the coronary sinus and aorta in patients with stable angina pectoris paced to angina, and in unstable angina patients before and after dazoxiben 100 mg. 2 Although there was a wide range of values in the different groups, dazoxiben significantly reduced plasma thromboxane levels in all patients. 3 Dazoxiben had no adverse effect on coronary and systemic haemodynamics, and atrial pacing time to angina was increased from 245 +/- 41 to 308 +/- 48s (P less than 0.01).

Plasma beta-thromboglobulin as a measure of platelet activity. Effect of risk factors and findings in ischemic heart disease and after acute myocardial infarction

The plasma concentration of beta-thromboglobulin (BTG), a platelet-specific protein released during platelet aggregation, is considered a sensitive marker of in vivo platelet activity. The mean plasma level in 133 asymptomatic individuals was 32.3 +/- 1.1 ng/ml, and there was no difference between those with no risk factors (32.2 +/- 1.2 ng/ml, n = 56), those who smoked (31.8 +/- 1.8 ng/ml, n = 45), those with hyperlipidemia (32.8 +/- 1.7 ng/ml, n = 15), and those exposed to both of these risk factors (34.1 +/- 2.7 ng/ml, n = 17). The mean plasma BTG level in 104 patients with symptomatic ischemic heart disease was significantly elevated (40.9 +/- 1.4 ng/ml, p less than 0.01), but there was considerable overlap with normal levels. Although no difference was found between patients with no risk factors (38.1 +/- 4.0 ng/ml, n = 13) and those with only 1 risk factor (37.0 +/- 1.8 ng/ml, n = 44), patients with 2 or more risk factors ahd a significantly elevated plasma BTG level (45.2 +/- 2.2 ng/nl, n = 47, p less than 0.01). It is concluded that risk factors themselves do not increase platelet activity, but that patients with vascular disease have activated platelets that may contribute to the progression of the disease. Plasma BTG was also measured serially for 10 days in 29 patients after hospitalization with acute ischemic cardiac pain. Although the median plasma level was elevated above normal there were no acute changes in plasma BTG after either acute infarction (n = 22) or acute ischemia (n = 7), except in 2 patients in whom pericardial friction rubs developed. Thus, measurement of systemic plasma BTG did not detect platelet involvement in acute coronary occlusion or acute ischemia.

The interactions of platelet-derived mediators on isolated canine coronary arteries

The interactions of 5-hydroxytryptamine (5-HT) and a thromboxane A2 mimetic, U44069, were investigated on rings of canine coronary arteries. 5-HT (10(-8) - 10(-5) M) and U44069 (10(-9) - 10(-6) M) produce coronary vasoconstriction which is potentiated by indomethacin (3 X 10(-6) M). In the presence of 10(-9) M 5-HT, U44069-induced vasoconstriction is enhanced. Similarly, 10(-9) M U44069 enhances the effects of 5-HT. In both cases this potentiation is greatest in the presence of indomethacin. Synergism between the platelet-derived mediators could contribute to coronary vasospasm and myocardial ischaemia.