Platelet function and biosynthesis of prostacyclin and thromboxane A2 in whole blood after aspirin administration in human subjects

Current controversies in the use of aspirin and ticagrelor for the treatment of thrombotic events

INTRODUCTION

A P2Y₁₂ inhibitor plus aspirin is the most widely used antiplatelet strategy to prevent adverse outcomes in the setting of atherothrombotic vascular disease. Areas covered: A paucity of robust evidence for an optimal dose, gastrointestinal toxicity, ineffectiveness in high-risk patients and interactions with other antiplatelet agents, are major controversies associated with aspirin therapy. Ticagrelor is a reversibly binding oral P2Y₁₂ receptor blocker that mediates potent inhibition of adenosine diphosphate-induced platelet function. It is more effective than clopidogrel in preventing thrombotic events in acute coronary syndrome patients. The absence of a beneficial effect for ticagrelor versus clopidogrel in ACS observed in the North American subgroup of the PLATelet inhibition and patient Outcomes (PLATO) trial has been attributed to a higher concomitant aspirin dose. Expert commentary: Ongoing studies are now investigating the plausibility of removing aspirin therapy in the setting of potent P2Y₁₂ receptor blockade via ticagrelor monotherapy or replacing aspirin with an oral anticoagulant.

Urinary metabolites of prostanoids and risk of recurrent colorectal adenomas in the Aspirin/Folate Polyp Prevention Study (AFPPS)

Aspirin has been shown to protect against colorectal neoplasms; however, the optimal chemopreventive dose and underlying mechanisms are unclear. We aimed to study the relationship between prostanoid metabolites and aspirin's effect on adenoma occurrence. We used data from the Aspirin/Folate Polyp Prevention Study, in which 1,121 participants with a recent adenoma were randomized to placebo or two doses of aspirin (81 or 325 mg/d) to be taken until the next surveillance colonoscopy, anticipated about 3 years later. Urinary metabolites of prostanoids (PGE-M, PGI-M, and dTxB2) were measured using liquid chromatography/mass spectrometry or GC/NICI-MS in 876 participants near the end of treatment follow-up. Poisson regression with a robust error variance was used to calculate relative risks and 95% confidence intervals. PGE-M, PGI-M, and dTxB2 levels were 28%, 37%, and 60% proportionately lower, respectively, in individuals who took 325 mg of aspirin compared with individuals who took placebo (all P < 0.001). Similarly, among individuals who took 81 mg of aspirin, PGE-M, PGI-M, and dTxB2 were, respectively, 18%, 30%, and 57% proportionally lower compared with placebo (all P < 0.005). None of the metabolites or their ratios were statistically significantly associated with the risk of adenoma occurrence. The effect of aspirin in reducing adenoma risk was independent of prostanoid levels. Aspirin use is associated with lower levels of urinary prostanoid metabolites. However, our findings do not support the hypothesis that these metabolites are associated with adenoma occurrence, suggesting that COX-dependent mechanisms may not completely explain the chemopreventive effect of aspirin on colorectal neoplasms.

Impact of aspirin dosing on the effects of P2Y12 inhibition in patients with acute coronary syndromes

The discovery of the antiplatelet effect of low-dose aspirin led to the hugely successful strategy of dual antiplatelet therapy in patients with acute coronary syndromes (ACS). Increasing the dose of aspirin beyond 75-100 mg has never been shown to offer additional efficacy in ACS patients but could possibly increase the risk of bleeding. In the Platelet Inhibition and Patients Outcome (PLATO) study, higher doses of aspirin appeared to neutralise the additional benefit of the potent P2Y12 inhibitor ticagrelor compared to clopidogrel (Circulation 124: 544-554, 2011). However, higher doses of aspirin have not been shown to have an adverse interaction with the potent P2Y12 inhibition provided by prasugrel and double-dose clopidogrel (Journal of the American College of Cardiology, 2013, in press; N Engl J Med 363: 930-942, 2010). This potentially suggests that the mechanism for this interaction is not related to the inhibition of platelet P2Y12 receptors or could simply be a chance finding.

Evaluation of the Clot Signature Analyzer as a hemostasis test in healthy volunteers exposed to low doses of aspirin

Several variables affect bleeding time that make it difficult to obtain consistent measurements. The Clot Signature Analyzer (CSA) has been developed to assess in vitro hemostasis using well-controlled flow chambers. In this study, the equivalencies in the CSA parameters with the conventional bleeding time or platelet aggregation methods were evaluated in subjects exposed to aspirin. The CSA parameters, platelet hemostasis time (PHT) and collagen-induced thrombus formation (CITF), were compared to bleeding time (Surgicutt2) and collagen-induced platelet aggregation, respectively. Fifty-three healthy volunteers were given two doses of aspirin (81 and 243 mg) in one day. Following the baseline period, the volunteers took 81 mg of aspirin and then took 243 mg 2 hours later. The changes in each value from the baseline to that at either aspirin dose (2 hours after dosing) were evaluated. Platelet hemostasis time and CITF correlated well with bleeding time and aggregation, respectively, but PHT was not significantly increased after 81 mg of aspirin, whereas bleeding time was significantly increased. The variation in PHT was slightly higher than that of bleeding time. At 81 mg, CITF was significantly increased but aggregation was not, even though the variation was comparable. This suggests that PHT and CITF can simulate the changes in bleeding time and aggregation, respectively, but the sensitivity of PHT for detecting the changes in bleeding time was no better than the conventional method. Also, CITF was more sensitive than aggregation in detecting platelet response to collagen. In conclusion, the proposed CSA is not always suitable for detecting hemostatic abnormalities.

Comparison of enteric-coated aspirin and uncoated aspirin effect on bleeding time

Aspirin therapy is an essential part of the drug regimen for patients with acute myocardial infarction (MI), unstable angina, or after coronary angioplasty and coronary stenting. Recognizing this importance, this study sought to compare the bleeding time in two groups of 10 normal volunteers 4 hr after ingestion of either an enteric-coated aspirin or an uncoated aspirin, assuming that a difference between the two groups could be clinically significant. Defining < or = 8 min as normal, 80% of the uncoated group developed abnormal bleeding times, compared to 10% of the enteric-coated group (P < 0.01). The study demonstrates a significant difference between the two types of aspirin preparations on bleeding times in normal individuals. This strongly suggests that some enteric-coated aspirin preparations may not be as effective as uncoated aspirin in acutely decreasing platelet aggregation. Therefore, uncoated aspirin is recommended in the setting of acute MI, unstable angina, or after percutaneous transluminal coronary angioplasty.

Effect of aspirin dosage and enteric coating on platelet reactivity

Although aspirin is effective in the prevention and treatment of cardiovascular diseases, the optimal dose remains uncertain. The purpose of this study was to compare the platelet inhibitory and prostacyclin-sparing effects of 2 doses (81 and 325 mg) and forms (enteric-coated and regular) of aspirin. Since platelet reactivity has been reported to increase after strenuous exercise, a known trigger of myocardial infarction, subjects were studied following maximal treadmill exercise as well as at rest. Forty male healthy subjects were evaluated using a randomized, double-blind, parallel study design. Blood samples were obtained before and after maximal treadmill exercise at baseline and after 7 days on aspirin therapy. Both enteric and regular aspirin in 81- and 325-mg dosages markedly inhibited adenosine diphosphate and epinephrine-induced aggregation at rest and after exercise. Aspirin also inhibited the platelet response to collagen as assessed by a longer lag time to aggregation. The prolongation of lag time was greater for 325 mg than for 81 mg (100 +/- 7 vs 91 +/- 7; p = 0.04, after exercise). There were no significant dose-related differences in plasma 6-keto-prostaglandin F1alpha level; however, enteric-coated aspirin inhibited the exercise-induced increase in 6-keto-prostaglandin F1alpha to a lesser extent than regular aspirin. Although both doses (81 and 325 mg) and types (regular and enteric-coated) of aspirin inhibited adenosine diphosphate and epinephrine-induced aggregation equally, the 325-mg dose inhibited collagen-induced aggregation to a greater extent than 81 mg. The greater platelet inhibition observed with 325 mg may be clinically relevant in acute coronary syndromes characterized by plaque rupture with extensive collagen exposure and platelet activation.

Protective actions of a thromboxane receptor antagonist, SQ 29548 on the ischemic myocardium: morphologic and hemodynamic effects

The effects of thromboxane A2 (TXA2)/prostaglandin endoperoxide receptor blockade on myocardial infarct size and cardiac dynamics were determined in a canine model of 24 h acute myocardial infarction. Anesthetized open-chest dogs were subjected to left anterior descending (LAD) coronary artery occlusion. Twenty minutes post-occlusion the dogs were given i.v. saline (0.9% NaCl solution) (n = 12) or the TXA2 receptor antagonist SQ 29548 (0.2 mg/kg i.v. loading dose +0.2 mg/kg/h i.v. for 4 h) (n = 10). SQ 29548 treatment resulted in a significant (P < 0.01) reduction in infarct size. Heart rate (HR) and systolic blood pressure (SAP) were not markedly affected by the drug. The sharp rise in the left ventricular end diastolic pressure (LVEDP) in the saline-treated animals was significantly lowered by SQ 29548 treatment and the correction of this variable was maintained till 24 h post-occlusion. The lowered maximal rate of rise of left ventricular pressure (LVdP/dt max) in the saline-treated animals was corrected albeit non-significantly by the drug treatment. Thus, SQ 29548 treatment resulted in a significant salvage of myocardial tissue and marked alterations in left ventricular dynamics. The study suggests a deleterious role for thromboxane A2 in ischemia; indicating that TXA2 blockade may have potential as a mode of therapy for ischemic heart disease.

Ticlopidine and aspirin pretreatment reduces coagulation and platelet activation during coronary dilation procedures

OBJECTIVES

It is unknown whether a therapeutic combination of aspirin (ASA) and ticlopidine might effectively decrease activation of hemostasis.

BACKGROUND

Percutaneous transluminal coronary angioplasty (PTCA), rotational atherectomy and stent implantation are procedures that fracture or ablate endothelium and plaque, a situation that activates hemostasis.

METHODS

In 85 patients undergoing PTCA for a 77.8 +/- 1% stenosis, we measured markers of coagulation and platelet activation (thrombin-antithrombin complexes [TAT], prothrombin fragment 1 + 2 [F1 + 2] serotonin and the presence of circulating activated platelets reacting with monoclonal antibodies against glycoproteins exposed on platelet membranes). Blood samples were drawn from a peripheral vein and from the coronary ostium before the procedures. Both immediately and 10 min after angioplasty, and 10 min afterward, samples were collected from a probing catheter (0.018 in, [0.46 cm]) positioned beyond the stenosis. All patients were being treated with antianginal drugs and ASA, 250 mg/day. Seventy of them had taken ticlopidine, 250 mg, twice daily for < or = 1 day (< or = 24 h) (n = 28) or for > or = 3 days (> or = 72 h) (n = 42). Heparin (150 U/kg) was administered before angioplasty. Thirty patients underwent PTCA; 15 of them were not treated with ticlopidine and 15 were given ticlopidine (> or = 72 h). Thirty-five patients had stent implantation, 20 rotational atherectomy.

RESULTS

Before and during the procedures, there was greater thrombin generation (expressed by higher TAT and F1 + 2 plasma levels) in patients not taking ticlopidine or taking it for < or = 24 h (p < 0.05). Platelet activation and plasma serotonin levels were also significantly higher in the no ticlopidine or < or = 24-h ticlopidine groups.

CONCLUSIONS

The combined use of ticlopidine, ASA and heparin effectively controls activation of coagulation in patients with stable or unstable angina undergoing coronary dilation.

Effects of low-dose aspirin on in vitro platelet aggregation in the early minutes after ingestion in normal subjects

Aspirin interferes with platelet aggregation by inhibiting the metabolism of arachidonic acid to thromboxane A2. Although both high- and low-dose aspirin therapies are effective for secondary prophylaxis in patients with atherosclerotic vascular disease, the acute response to low-dose aspirin therapy is controversial. Eighteen volunteer subjects ingested 81, 162, or 324 mg of aspirin in a longitudinal crossover study design. Initial doses were randomly assigned and dosing intervals were separated by 2 weeks. Platelet aggregation in response to 0.9 mM arachidonic acid was measured at baseline, 15, 30, 60, and 90 minutes after ingestion. Thromboxane B2 production was assayed on simultaneously obtained samples after stimulation with arachidonic acid. The median inhibition of aggregation was 97%, 97%, and 97% 15 minutes after ingestion of 81, 162, and 324 mg, respectively. Four subjects had < 20% inhibition 15 minutes after ingesting 81 mg, but all 4 had > 90% inhibition after 30 minutes. Thromboxane B2 production declined by > 93% in all subjects at each dose. There was no difference between doses in inhibition of thromboxane B2 production.

High dose intravenous aspirin, not low dose intravenous or oral aspirin, inhibits thrombus formation and stabilizes blood flow in experimental coronary vascular injury

OBJECTIVES

The purpose of this study was to assess the anti-thrombotic potential of various forms of aspirin administration.

BACKGROUND

Platelet activation in response to endothelial injury has been implicated in acute coronary syndromes.

METHODS

Delivering 100-microA anodal direct current to the intima of the left circumflex coronary artery in dogs at a site of moderate external stenosis provides a thrombogenic model of vascular injury. Animals were treated with aspirin (Group I, 20 mg/kg intravenously [n = 11]; Group II, 4.6 mg/kg intravenously [n = 6]; Group III, 4.6 mg/kg orally 18 h before the experiment [n = 7]) or vehicle (Group IV, control [n = 11]).

RESULTS

The time required for thrombotic occlusion to occur was longer and the incidence of thrombosis was lower in Group I (Group I, 238 +/- 7 min [n = 2]; Group II, 127 +/- 25 min [n = 3]; Group III, 156 +/- 35 min [n = 6]; Group IV, 90 +/- 11 min [n = 11]) (p < 0.05). Thrombus mass was smaller in Group I (Group I, 5.0 +/- 0.8 mg; Group II, 12.2 +/- 2.6 mg; Group III, 11.6 +/- 3.9 mg; Group IV, 9.1 +/- 1.6 mg) (p < 0.05). Initial hemodynamic variables did not differ among groups. An increase in mean arterial pressure was noted for several hours after intravenous aspirin administration in Group I (99 +/- 5 to 110 +/- 4 mm Hg) (p < 0.05). Left circumflex coronary artery blood flow was stable for 5 h in Group I (Group I, 31 +/- 2 to 26 +/- 4 ml/min) but decreased in all the other groups (Group II, 26 +/- 4 to 10 +/- 5 ml/min; Group III, 27 +/- 5 to 7 +/- 7 ml/min; Group IV, 29 +/- 4 to 0 ml/min) (p < or = 0.05). The in vivo area of left ventricle perfused by the left circumflex coronary artery was not different among groups. Platelet counts were similar and did not change over the course of the protocol. Ex vivo arachidonic acid-induced platelet aggregation decreased in all groups after aspirin (p < or = 0.001). Indium-111-labeled platelet adherence to the coronary vasculature was decreased in distal vessel segments after all doses of aspirin (p < 0.05). Platelet deposition in thrombi was similar for all treatment groups.

CONCLUSIONS

High dose intravenous aspirin has salutary effects. It stabilizes left circumflex coronary artery blood flow, prolongs the time to thrombosis, reduces the incidence of thrombotic occlusion, reduces thrombus mass and limits platelet adherence to sites of arterial injury. Low dose aspirin given intravenously or orally was ineffective. When persistent intracoronary thrombi precipitate unstable coronary syndromes, high dose intravenous aspirin may be useful in the acute period even though platelets continue to interact with injured vascular segments through aspirin-insensitive mechanisms.

Current concepts for a drug-induced inhibition of formation and action of thromboxane A2

Urinary and plasma metabolites of thromboxane A2 (TxA2) indicate an increased TxA2 synthesis in a number of diseases, whereby TxA2 is assumed to contribute to the underlying pathomechanisms by its profound effects on platelet aggregation and smooth muscle contraction. In some clinical situations the increment in TxA2 biosynthesis is accompanied by an increased formation of prostacyclin (PGI2) which is one of the most potent inhibitors of platelet activation and smooth muscle contraction. Therefore, drugs are being developed which suppress the formation or action of TxA2 without interfering with its functional antagonist PGI2. Low doses of acetylsalicyclic acid (ASA) preferentially inhibit cyclooxygenase activity in platelets and the synthesis of TxA2 in vivo. However, neither low doses (approximately 300 mg/day) nor very low doses spare the formation of PGI2 completely. Despite its limited selectivity, very low dose ASA (approximately 40 mg/day) provides an attractive perspective in TxA2 pharmacology. Although thromboxane synthase inhibitors selectively suppress TxA2 biosynthesis PGH2 can accumulate instead of TxA2 and substitute for TxA2 at their common TxA2/PGH2 receptors. Thromboxane synthase inhibitors can only exert platelet-inhibiting and vasodilating effects if PGH2 rapidly isomerizes to functional antagonists like PGI2 that can be formed from platelet-derived PGH2 by the vessel wall. TxA2/PGH2 receptor antagonists provide a specific and effective approach for inhibition of TxA2. These inhibitors do not interfere with the synthesis of PGI2 and other prostanoids but prevent TxA2 and PGH2 from activating platelets and inducing smooth muscle contractions. Most of the available TxA2/PGH2 receptor antagonists produce a competitive antagonism that can be overcome by high agonist concentrations. Since in certain disease states very high local TxA2 concentrations are to be antagonized, non-competitive receptor antagonists may be of particular interest. Some recent TxA2/PGH2 receptor antagonists produce such a non-competitive type of inhibition due to their low dissociation rate constant. As a consequence, agonists like TxA2 or PGH2 only reach a hemiequilibrium state at their receptors, previously occupied by those antagonists. A combination of a thromboxane synthase inhibitor with a TxA2/PGH2 receptor antagonist presents a very high inhibitory potential that utilizes the dual activities of the synthase inhibitor to increase PGI2 formation and of the receptor antagonist to antagonize PGH2 and TxA2. Such combinations or dual inhibitors, combining both moieties in one compound, prolong the skin bleeding time to a greater extent than thromboxane synthase inhibitors and even more than low dose ASA or TxA2/PGH2 receptor antagonists.

Effects of aspirin and prostaglandin E1 on in vitro thrombolysis with urokinase. Evidence for a possible role of inhibiting platelet activity in thrombolysis

The formation of thrombi in vivo includes the activation of both platelets and the coagulation cascade. Conventional thrombolytic therapy is primarily directed toward the dissolution of fibrin. To evaluate the possibility that platelet activity impairs the lysis of thrombi, we studied the effects of aspirin and platelet-deaggregating prostaglandin E1 on thrombolysis with urokinase. Combined platelet and fibrin thrombi were produced in vitro by adding CaCl2 and collagen (1 microgram/ml) to citrated platelet-rich plasma (250,000 platelets per microliters). Urokinase (500-10,000 units/ml) caused a dose-dependent weight loss of the thrombi that was maximal at 2,000 units/ml. The addition of aspirin (10-200 micrograms/ml) to platelet-rich plasma before thrombus formation markedly enhanced thrombolysis with urokinase. This effect was most pronounced at 20 micrograms/ml aspirin. However, when aspirin was added after completion of thrombus formation, no significant effect on thrombolysis was noted. Prostaglandin E1 (1-100 mumol/l) improved the lysis with urokinase of the combined platelet and fibrin thrombi. This effect was maximal at 20 mumol/l prostaglandin E1. When pure fibrin thrombi were produced in platelet-free plasma, prostaglandin E1 was without effect on lysis. Thus, in vitro lysis with urokinase of combined platelet and fibrin thrombi was enhanced by the addition of platelet-deaggregating prostaglandin E1 and by pretreatment with aspirin.

Protective effect of the specific thromboxane receptor antagonist, BM-13505, in reperfusion injury following acute myocardial ischemia in cats

The ability of BM-13505, 4-[2-(4-chlorobenzenesulfonylamino) ethyl]-benzene acetic acid), a specific thromboxane/endoperoxide receptor antagonist, to protect the myocardium against ischemia and reperfusion injury, was assessed in an anesthetized cat model. Cats were rendered ischemic by left anterior descending (LAD) coronary artery ligation for 1 1/2 hours followed by reperfusion for 4 1/2 hours. BM-13505 or its vehicle (i.e., Na2CO3) was administered intravenously 30 minutes before reperfusion at a rate of 1 mg/kg followed by 1 mg/kg/hr. BM-13505 significantly (p less than 0.001) reduced the area of ischemic tissue as a percent of total left ventricular mass and total area at risk, without altering basic hemodynamics (i.e., arterial blood pressure, heart rate, or their product) and thereby not influencing myocardial oxygen demand. The mechanism of the protective effect of the thromboxane receptor antagonist appears to be cytoprotective but may involve the prevention of neutrophil-induced cellular damage.

Reduced myocardial neutrophil accumulation and infarct size following thromboxane synthetase inhibitor or receptor antagonist

Since thromboxane A2 (TXA2) release may relate to the extension of myocardial injury following coronary ligation, the authors examined the effects of pretreatment with a selective TXA2 synthetase inhibitor U-63,557A, or a TXA2 receptor antagonist SQ-29,548, on myocardial infarct size forty-eight hours following left coronary ligation in rats. Myocardial infarct size (as percent of left ventricle, LV) was decreased from 44 +/- 3% in saline-treated control animals to 34 +/- 4% (P less than 0.05) in U-63,557A-treated animals and to 32 +/- 4% (P less than 0.05) in SQ-29,548 treated animals (U-63,557A-treated vs SQ-29,548-treated, P = NS). LV creatine kinase (CK) was 5.08 +/- 0.42 IU/mg protein in noninfarcted untreated rats and 1.79 +/- 0.21 IU/mg protein in saline-treated infarcted rats. LV CK was 2.86 +/- 0.40 IU/mg protein in U-63,557A-treated rats and 3.11 +/- 0.51 IU/mg protein in SQ-29,548-treated infarcted rats (both P less than 0.05 compared with saline-treated rats). The beneficial effects of U-63,557A and of SQ-29,548 were not accompanied by reduction in indices of myocardial oxygen demand (heart rate and arterial pressure). However, neutrophil accumulation in the infarcted myocardium was markedly decreased by U-63,557A and SQ-29,548 pretreatment. Myocardial myeloperoxidase activity, a specific marker of neutrophil infiltration, was also decreased (P less than 0.02) in U-63,557A- and SQ-29,548-treated animals (0.09 +/- 0.03 and 0.07 +/- 0.02 units/100 mg, respectively) compared with saline-treated infarcted rats (0.19 +/- 0.04 units/100 mg). In vitro incubation of U-63,557A and SQ-29,548 caused a significant and similar reduction in f-MLP-induced neutrophil chemotaxis, and U-63,557A increased prostacyclin formation in whole blood. These data suggest that reduction in the extent of myocardial injury by TXA2 synthetase or receptor inhibitors may, in part, relate to a decrease in neutrophil accumulation in the infarcted tissue. In spite of differences in mechanisms of action of U-63,557A and SQ-29,548, both agents exert a similar protective effect on the extent of myocardial injury following coronary ligation. Reduction in neutrophil accumulation in the infarcted zone, as well as in f-MLP-directed chemotaxis in vitro, suggests that TXA2 inhibition may modulate neutrophil migration.

Impaired canine coronary vasodilator response to acetylcholine and bradykinin after occlusion-reperfusion

Previous studies indicate impairment of coronary arterial ring relaxation in response to acetylcholine (ACh) following coronary reperfusion, mediated via loss of endothelium-derived relaxing factor (EDRF). To examine if coronary vasodilator reserve is reduced following coronary occlusion-reperfusion in intact animals, 16 open-chest mongrel dogs were subjected to 1 hour of total left circumflex (Cx) coronary artery occlusion followed by reperfusion for 1 hour. Prior to Cx occlusion, coronary blood flow increased and vascular resistance decreased (both p less than or equal to 0.01) in response to ACh and bradykinin (BK). Following reperfusion, increase in Cx coronary flow in response to both vasodilators was significantly (p less than or equal to 0.01) impaired. Myocardial histology showed extensive neutrophil infiltration and capillary plugging by neutrophils in the Cx compared with the left anterior descending coronary artery-supplied myocardium. Myocardial myeloperoxidase activity was also increased in the Cx compared with the left anterior descending region (p less than or equal to 0.02). Pretreatment of four dogs with indomethacin partially reduced the vasodilator response to BK but not to ACh. However, indomethacin did not affect reperfusion-induced attenuation of BK or ACh's coronary vasodilator effects. To determine if calcium blocker verapamil would modify reperfusion-induced impairment in coronary vasodilator reserve, six dogs were treated with verapamil. Although verapamil enhanced coronary vasodilator effects of ACh and BK, it did not modify reperfusion-induced attenuation of coronary vasodilator reserve. Myocardial neutrophil accumulation and myeloperoxidase activity was also similar in control, indomethacin, and verapamil-treated dogs. These observations suggest that coronary reperfusion impairs coronary vasodilator reserve in intact dogs. This impairment is not modified by prostaglandin inhibition or by calcium blockade. Besides loss of EDRF, capillary plugging by neutrophils may contribute to the altered coronary flow reserve observed in the immediate post-reperfusion period. Furthermore, indomethacin or verapamil are not effective in modifying the reperfusion-related impairment of coronary vasodilator reserve.

Alterations in platelet alpha 2-adrenoceptors by aspirin

Epinephrine-induced platelet aggregation (mediated through interaction with alpha 2-adrenoceptors) is inhibited by aspirin. To determine if aspirin modulates alpha 2-adrenoceptors, we quantitated dissociation constant (KD) and maximum number of binding sites (Bmax) on isolated platelet membranes using alpha 2-antagonist 3H-yohimbine in normal subjects given 650 mg of aspirin orally. Alpha 2-receptor KD increased from 3.20 +/- 1.80 to 7.32 +/- 3.32 nM (p less than 0.02) and Bmax from 115 +/- 77 to 190 +/- 140 fmol/mg protein. To determine if these alterations in alpha 2-receptors by aspirin were mediated through circulatory or intracellular effects, intact platelets or isolated platelet membranes were incubated with aspirin for 30 minutes in vitro. In these in vitro experiments, alpha 2-receptor KD increased from 2.92 +/- 1.76 to 9.83 +/- 8.55 nM and Bmax from 140 +/- 81 to 191 +/- 129 fmol/mg protein (p less than 0.05). Oral ingestion of aspirin or incubation of aspirin with intact platelets or lysates increased (3 to 10 fold) the concentration of 1-epinephrine required for inhibition of 3H-yohimbine binding by 50% (p less than 0.05). Basal platelet cyclic AMP as well as its elevation with PGE1 or PGI2 and decrease with catecholamines were not influenced by aspirin treatment of platelets. These data indicate that aspirin decreases platelet alpha 2-receptor affinity for agonist as well as antagonist. These effects of aspirin are independent of circulatory or dynamic intraplatelet changes.

Dietary supplementation with omega-3 polyunsaturated fatty acids in patients with stable coronary heart disease. Effects on indices of platelet and neutrophil function and exercise performance

It has been suggested that omega-3 polyunsaturated fatty acids (PUFAs) may alter the course of coronary artery disease by influencing platelet and neutrophil function, arachidonic acid metabolism, and circulating lipid concentrations. To examine this hypothesis, placebo or omega-3 PUFAs as Max-EPA (equivalent to 3.2 g of eicosapentaenoic acid and 2.2 g of docosahexaenoic acid daily) was administered to eight patients with stable coronary artery disease and positive exercise stress test results in a randomized, double-blind, crossover fashion over a 12-week period. With Max-EPA administration, platelet aggregation threshold to epinephrine was increased in only two patients, but neutrophil aggregation and chemotaxic functions decreased consistently (both p less than or equal to 0.01 compared with preceding placebo phase) in all eight. Serum and platelet-rich plasma thromboxane B2 concentrations decreased 40 percent and 28 percent, respectively (both p less than or equal to 0.05). Neutrophil leukotriene B4 formation decreased 23 percent (p less than or equal to 0.01) and synthesis of leukotriene B5 became apparent in all subjects. Serum triglyceride concentrations fell 52 percent (p less than or equal to 0.05) without significant change in total cholesterol, high-density lipoprotein-cholesterol, or low-density lipoprotein-cholesterol concentrations. Systolic blood pressure and the double product (heart rate X systolic blood pressure) were lower (p less than or equal to 0.05) at the end of the Max-EPA phase than in the preceding placebo phase. Heart rate, systolic blood pressure, and the double product were also lower (p less than or equal to 0.05) at three as well as at six minutes of an exercise stress test, indicating a significant reduction in myocardial oxygen demand. Despite these alterations in platelet and neutrophil function, arachidonic acid metabolism, serum triglyceride concentrations, and myocardial oxygen demand, there were no significant changes in subjective parameters of coronary artery disease during the Max-EPA phase (angina frequency 3.7 versus 2.8 episodes per week, nitroglycerin consumption 3.0 versus 1.9 tablets per week, both p = NS). Similarly, exercise times to ST-segment depression (6.5 versus 4.1 minutes) and to onset of angina (5.4 versus 5.0 minutes) were not altered by administration of Max-EPA. Thus, short-term dietary supplementation with omega-3 PUFAs to patients with stable coronary artery disease does not appear to alter subjective or objective parameters of myocardial ischemia.

Prevention of recurrent myocardial infarction and sudden death with aspirin therapy

In October 1985, the Food and Drug Administration approved a new indication of aspirin for the secondary prevention of recurrent myocardial infarction (MI) and death in patients with MI or unstable angina. Clinical trials have demonstrated the efficacy of this drug, especially when treatment is begun soon after the initial event. The antiplatelet actions of aspirin seem to be the most plausible explanation for its efficacy in reducing mortality and the rate of reinfarction. A single daily 325-mg tablet is effective and produces side-effect incidences of only zero to two percent above those produced by placebo. This article assesses the current state of knowledge regarding the value of aspirin therapy in survivors of acute MI and the implications for clinical practice.

Reduction in myocardial neutrophil accumulation and infarct size following administration of thromboxane inhibitor U-63,557A

We examined the effects of a new selective thromboxane A2 (TXA2) synthetase inhibitor, U-63,557A, on myocardial infarct size 48 hours following left coronary ligation in rats. With a single 8 mg/kg dose of U-63,557A (furegrelate) administered prior to coronary ligation, platelet aggregation and serum TXA2 formation declined significantly (p less than 0.02) for up to 48 hours. Myocardial infarct size, as measured by planimetry of serial left ventricular sections, was decreased from 44 +/- 3% (saline-treated control rats) to 34 +/- 4% (p less than 0.05). Left ventricular creatine kinase (CK) following coronary ligation was also preserved in U-63,557A vs saline-treated control animals (p less than 0.05). These beneficial effects of U-63,557A were not accompanied by reduction in the indices of myocardial oxygen demand (heart rate and arterial pressure). Furthermore, neutrophil accumulation in the infarcted myocardium was significantly decreased by U-63,557A (26 +/- 6 vs 96 +/- 3/high-power field [p less than 0.01]). These data suggest that administration of a single dose of selective TXA2 synthetase inhibitor prior to coronary ligation modulates platelet function for up to 48 hours and reduces the extent of myocardial injury, which may, in part, relate to decrease in neutrophil accumulation.

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.

Aspirin does not inhibit human megakaryocyte thromboxane synthesis in vivo

Cyclooxygenase activity in human platelets reappears after ingestion of aspirin as a function of the platelet production rate. In different studies the activity reappeared without delay or with an interval of at least 48 h after stopping the drug. Because inhibition of megakaryocyte cyclooxygenase is the sole likely explanation for a delay we determined thromboxane B2 in human megakaryocytes obtained under local anaesthesia. We found that aspirin does not inhibit human megakaryocytes in vivo but does so in vitro. Therefore, it does not seem likely that there is actually a delay in platelet cyclooxygenase resurgence after aspirin intake. In order to suppress platelet cyclooxygenase constantly, aspirin should be given once or twice a day and not once or twice a week.