Effect of oral aspirin dose on platelet aggregation and vascular prostacyclin (PGI2) synthesis in humans and rabbits

Aspirin is a safe and effective thromboembolic prophylaxis after total knee arthroplasty: a systematic review and meta-analysis

PURPOSE

Patients undergoing total knee arthroplasty (TKA) are at high risk for thromboembolic events compared to non-surgical patients. Both anticoagulants and antiplatelet agents are used as antithrombotic prophylaxis in TKA. The aim of this review is to understand the role of aspirin in the prevention of thromboembolic events and to compare its efficacy and safety with the main anticoagulants used in antithromboembolic prophylaxis in TKA.

METHODS

A systematic review and meta-analysis was performed according to the PRISMA guidelines. An electronic systematic search was conducted using PubMed, Scopus, and the Cochrane Central Registry to evaluate studies that compared aspirin with other anticoagulants, in terms of deep venous thrombosis and pulmonary embolism after TKA. The meta-analysis compared the rate of complications between aspirin and other anticoagulants.

RESULTS

Thirteen studies were included in the systematic review for a total of 163,983 patients, and 10 studies were included in the meta-analysis. The meta-analysis demonstrated no statistically significant differences between aspirin and other anticoagulants in terms of the rate of deep venous thrombosis (OR 0.93, 95% CI 0.81-1.08, p = 0.35) and pulmonary embolism (OR 0.89, 95% CI 0.56-1.41, p = 0.61).

CONCLUSION

Aspirin is safe, effective, and not inferior to other main anticoagulants in preventing thromboembolic events following TKA.

A Comprehensive Review of the Pleiotropic Effects of Ticagrelor

AIMS

This review summarizes the findings of preclinical studies evaluating the pleiotropic effects of ticagrelor. These include attenuation of ischemia-reperfusion injury (IRI), inflammation, adverse cardiac remodeling, and atherosclerosis. In doing so, it aims to provide novel insights into ticagrelor's mechanisms and benefits over other P2Y12 inhibitors. It also generates viable hypotheses for the results of seminal clinical trials assessing ticagrelor use in acute and chronic coronary syndromes.

METHODS AND RESULTS

A comprehensive review of the preclinical literature demonstrates that ticagrelor protects against IRI in the setting of both an acute myocardial infarction (MI), and when MI occurs while on chronic treatment. Maintenance therapy with ticagrelor also likely mitigates adverse inflammation, cardiac remodeling, and atherosclerosis, while improving stem cell recruitment. These effects are probably mediated by ticagrelor's ability to increase local interstitial adenosine levels which activate downstream cardio-protective molecules. Attenuation and augmentation of these pleiotropic effects by high-dose aspirin and caffeine, and statins respectively may help explain variable outcomes in PLATO and subsequent randomized controlled trials (RCTs).

CONCLUSION

Most RCTs and meta-analyses have not evaluated the pleiotropic effects of ticagrelor. We need further studies comparing cardiovascular outcomes in patients treated with ticagrelor versus other P2Y12 inhibitors that are mindful of the unique pleiotropic advantages afforded by ticagrelor, as well as possible interactions with other therapies (e.g., aspirin, statins, caffeine).

Effect of Suboccipital Release on Pain Perception and Autonomic Reflex Responses to Ischemic and Cold Pain

OBJECTIVE/SUBJECTS

To determine the autonomic effects of suboccipital release (SOR) during experimentally induced pain, 16 healthy subjects (eight women, eight men) experienced ischemic (forearm postexercise muscle ischemia [PEMI]) and cold (cold pressor test [CPT]) pain.

DESIGN

Beat-to-beat heart rate (electrocardiogram), mean arterial blood pressure (finger photoplethysmography), baroreflex sensitivity (transfer function analysis), and pain perception were measured. SOR or a sham (modified yaw; 30 cycles/min) was performed in minute 2 of pain.

RESULTS

PEMI increased blood pressure by 23 ± 2 and 20 ± 2 mmHg; no differences occurred between SOR or yaw. PEMI modestly elevated heart rate during ischemia, followed by significant reduction from baseline with SOR (-3 ± 2 bpm) and yaw (-4 ± 2 bpm); no differences were observed between treatments. CPT increased blood pressure (SOR = 11 ± 1, yaw = 9 ± 2 mmHg) and heart rate (SOR = 10 ± 2, yaw = 8 ± 3 bpm) before SOR and yaw. Neither treatment nor sham blunted blood pressure increases (SOR = 25 ± 2, yaw = 22 ± 2 mmHg) during CPT; both decreased heart rate (SOR = -3 ± 2, yaw = -2 ± 2 bpm) from baseline. PEMI and CPT caused increased pain without treatment modulation. Following pain and manual intervention, SOR increased baroreflex sensitivity in the 0.15-0.35 Hz range and decreased R-R interval power spectral density in the 0.03-0.5 Hz range compared with yaw. To probe potential mechanisms and interactions between manual treatment and a prototypic analgesic, oral aspirin (967 mg) was given 60 minutes before testing to reduce prostaglandin synthesis. Aspirin slightly attenuated pain but neither altered cardiovascular changes to PEMI nor interacted with SOR or yaw.

CONCLUSIONS

SOR has the capacity to modulate pain-induced autonomic control and regulation.

Relationship of arachidonic acid concentration to cyclooxygenase-dependent human platelet aggregation

Inhibition of ex vivo arachidonic acid (AA)-induced aggregation is a biomarker for the isotype selectivity of cyclooxygenase (COX) inhibitors since platelets express COX-1 but not COX-2. At low concentrations, there is broad inter- and intrasubject variability in AA-induced aggregation of platelets ex vivo. This study defined a concentration that reliably induces aggregation without overcoming inhibition by therapeutic aspirin therapy (ASA, 81-mg) treatment. Logistic regression analysis of ex vivo aggregation, induced with increasing concentrations of AA in platelet-rich plasma (PRP), estimated that platelets from > or = 90% of subjects would aggregate at > or = 1.5 mM AA (95% confidence interval [CI], 1.1, 2.1). A concentration of 1.6 mM AA failed to aggregate platelets from 26 healthy volunteers, who had previously aggregated at this concentration, following six daily oral doses of 81 mg of ASA. These data demonstrate that 1.6 mM AA reproducibly induces platelet aggregation in PRP from healthy volunteers without overcoming the antiplatelet effect of daily low-dose aspirin therapy.

Effects of acetyl salicylic acid therapy on an experimental thrombosis induced by laser beam

Aspirin inhibits the synthesis of both platelet and vascular arachidonic acid metabolism which have opposite effects on platelet functions. The rationale for its clinical use as an antithrombotic drug has therefore been questioned. Therefore, we investigated the effects of acetylsalicylic acid (ASA) at 100 mg/kg on an experimental thrombosis induced by laser beams using different groups of rats that were previously treated with the same dose (100 mg/kg), according to the delay between the first and second injections. A partial occlusion was induced by laser beams in the rat mesenteric microvessels (15-25 m). The thrombus formed within seconds after the laser lesion; both it and the embolization which began within minutes after, were continuously accounted. Experiments were done on 11 groups of 5 animals each: 45 rats received a first injection of ASA at j(0) and a second injection 30 minutes before thrombosis induction at j(0)+x (x=2, 4, 6, 8, 9, 10, 12, 14 and 16 days). Different groups are defined according to the x value. The rats receiving NaCl 0.9% or a single injection of ASA at 100 mg/kg 30 minutes before thrombosis induction were used as control (Group I) and reference group (Group II) respectively. In this study, ASA treatment showed two types of results. The administration of ASA (100 mg/kg) 30 minutes before laser-induced thrombosis prevented thrombus formation. In the same way, ASA injected to rats already treated with the same dose 2 or 4 day later also demonstrated a potent antithrombotic effect. The same trends were observed with animals receiving the second injection (100 mg ASA) at j(0+8), j(0+12), j(0+14), and j(0+16). However, when injected to rats at j(0+6) and at j(0+10), ASA did not shown any effects on thrombus formation compared to the control (p>/=0.05). The same phases of ASA action were observed on the induced hemorrhagic time. The antithrombotic effects of the later second injection of ASA (100 mg/kg) were neutralized in rats previously receiving the same dose of this drug. This phenomenon seems to be periodic and is of great importance for the observance of ASA treatment.

Reduction of bacterial titers by low-dose aspirin in experimental aortic valve endocarditis

Using a rabbit model of Staphylococcus aureus endocarditis, we studied the effects of aspirin on the natural progression of this infection. Compared with untreated animals, the aspirin-treated animals showed a 30% (P = 0.11) reduction in the weight of the vegetations and an 84% (P = 0.03) reduction in the bacterial titer of the vegetations.

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.

Dose-dependent aspirin hydrolysis and platelet aggregation in patients with atherosclerosis

In a double blind, randomized trial, the effects of aspirin (1, 5, and 15 mg/kg) were compared with the changes in platelet aggregation at 6 and 24 hours after dosage. It is found that there is a negative correlation between aspirin hydrolysis velocity in blood and capability of aspirin to decrease platelet aggregation with ADP and collagen in patients with atherosclerosis. Relationship between these parameters depends on aspirin dosage. The correlation was more marked for low doses of aspirin. It is suggested that the effect of aspirin in low dosage on platelet aggregation might be ineffective in many patients without control of aspirin hydrolysis velocity in blood.

Enhancement of antithrombotic effect of aspirin by metoclopramide in rats

The potential antithrombotic effect of aspirin and metoclopramide were studied in a model of arterial thrombosis in rats. Thrombosis was produced in the abdominal aorta by the combination of local partial obstruction and intravenous administration of hypotonic saline containing 5-HT. The resulting aortic occlusion and the effects of drugs were quantified by measuring rectal temperature. Metoclopramide as well as ketanserine effectively reversed while zacopride failed to alter thrombotic effect. Metoclopramide or ketanserine when combined with aspirin enhanced the antithrombotic effect of the latter. On the other hand, co-administration of metoclopramide with ketanserine failed to show any synergistic effect. As with ketanserine, the antithrombotic effect of metoclopramide appeared to be mediated through the blockade of 5-HT2 receptors in the platelets.

Brain kininogen following experimental brain injury: evidence for a secondary event

Previous studies have shown that following experimental brain injury cerebral arterioles dilate and display endothelial lesions and reduced responsiveness to hypocapnia. These abnormalities are caused by cyclo-oxygenase-dependent free radical generation. There is evidence that the kallikrein-kinin system may in part stimulate the cyclooxygenase-dependent damage since bradykinin is a powerful stimulator of prostaglandin formation and it has recently been shown that a specific kinin receptor blocker decreases the arteriolar abnormalities caused by injury. In order to further examine the hypothesis that the kallikrein-kinin system is important in inducing damage, rat brain tissue was examined for kininogen, the precursor of kinins, at 10 minutes and 1, 3, 6, 15, 24, 48, and 72 hours after injury. A fluid-percussion brain injury device was attached over the right cerebral cortex of rats and a 1.6-atmosphere pressure injury was administered. The kininogen content was determined by a radioimmunoassay procedure in tissues which were free of intravascular blood. After injury, bleeding was confined mainly to the right hemisphere. The kininogen content in the right hemisphere was significantly elevated by one hour after injury, continued to rise until 15 hours after injury, then was significantly decreased by 2 days after injury. In the left hemisphere, kininogen was significantly elevated at 1 hour postinjury, returned toward control levels over the 3- to 6-hour period after injury, then was again elevated at 15 hours after injury. These studies also show that brain water and cerebrovascular permeability were greater at 15 hours postinjury than at earlier time points. The data further support a role for the kallikrein-kinin system in brain injury and, when considered with the results of other studies, suggest that a secondary event is occurring in the 12- to 24-hour period after neural injury. The authors hypothesize that this secondary event is related to endothelial and vascular repair and may be important for the return of normal cerebrovascular function.

Plasma drug and antiplatelet profiles of the original acetylsalicylic acid preparations used in the AMIS, PARIS and German-Austrian trials for secondary prevention of myocardial infarction

In a cross-over study 6 healthy male subjects were given for 9 days the acetylsalicylic acid (ASA) preparations used in the Aspirin Myocardial Infarction Study (AMIS), Persantine-Aspirin Reinfarction Study (PARIS) and German-Austrian secondary heart attack prevention trials, exactly according to the original study protocols. Plasma concentrations of ASA and its main metabolites salicylic acid (SA) and salicyluric acid (SUA), as well as platelet function (collagen-induced platelet aggregation; tissue extract-induced change in platelet shape) were studied repeatedly on the first day of each medication period and were again examined on the sixth and ninth days. Differences in the plasma concentrations of ASA and its metabolites were found only on the first day, probably as a result of different absorption rates. Collagen-induced platelet aggregation was more rapidly inhibited the faster the preparation was absorbed. Each ASA preparation inhibited tissue extract-induced platelet shape change from the first dose, although statistically significant inhibition was seen only with the AMIS preparation. It is concluded that differences in the antithrombotic efficiency of ASA cannot be explained by differences in the pharmacokinetic and antiplatelet profiles of the various ASA preparations tested.

Plasma concentrations and anti-platelet effects after low dose acetylsalicylic acid

The present study has investigated whether low-dose acetylsalicylic acid (ASA) can inhibit platelet aggregation locally at its site of gastrointestinal absorption without concentrations in the systemic circulation high enough for inhibition of cyclooxygenase. For this purpose platelet aggregation, thromboxane formation as well as ASA plasma concentrations were measured in 8 volunteers before oral intake of 100 mg ASA as well as 20 to 300 minutes thereafter. At each time 5 ml of blood were mixed with 5 ml of blood obtained from a second, untreated volunteer. Aggregation and thromboxane formation were also determined in these mixed blood samples. The same protocol was performed with 4 volunteers after administration of 1500 mg ASA as well as after no drug intake. In a separate experiment the concentration-effect-relationship of ASA was assessed in vitro. One hundred and forty minutes after administration of 100 mg ASA aggregation and thromboxane formation were significantly decreased to 49.4 and 4.5% of the initial values, respectively, whereas in the mixed blood sample aggregation was not impaired. Inhibition of thromboxane formation was constantly 73% of the inhibition observed in the unmixed sample throughout the study period and thus most probably was caused by dilution of the platelets of the untreated volunteer by the inactivated platelets of the ASA-treated volunteer. These data suggest the absence of pharmacologically active drug concentrations in the peripheral blood. ASA plasma concentration was highest after 40 minutes (2.2 +/- 1.6 microgram/ml; n = 5). After the 1500 mg ASA dose platelet function and thromboxane formation decreased to 29.8 and 2.0% of the initial values, respectively. Furthermore, aggregation and thromboxane formation in the mixed blood sample were markedly reduced. Thus, after the high dose of ASA effective plasma concentrations were present in the peripheral circulation. Highest ASA plasma concentrations were 21.1 +/- 8.9 micrograms/ml. IC50 values were 1.00 +/- 0.36 and 0.30 +/- 0.05 microgram/ml for aggregation and thromboxane formation in vitro, respectively. It is concluded that low dose ASA can effectively inhibit platelet function without producing pharmacologically active concentrations in the peripheral circulation.

Prostaglandins, other eicosanoids and endothelial cells

Endothelial cells are an important source of eicosanoid formation in the cardiovascular systems. All major pathways of eicosanoid production have been demonstrated in endothelial cells, yielding significant amounts of prostacyclin (PGI2), PGE2, PGF2 alpha, thromboxane A2, leukotrienes and a number of hydroxy fatty acids. The regulation of eicosanoid formation by endothelial cells is poorly understood. There is evidence that precursors, such as arachidonic acid or prostaglandin endoperoxides, may also be provided by other cell types. Endothelial cell-derived eicosanoids are involved in the regulation of local vessel tone, intravascular platelet activation, cell locomotion and, eventually, cell proliferation. Most of the available information considers PGI2. This compound is the quantitatively dominating eicosanoid in endothelial cells. Major actions of PGI2 include inhibition of platelet activation and aggregation, relaxation of arterial vessels and inhibition of growth-factor release. There is probably a tight interaction with other biologically active mediators which needs further evaluation. This also applies to the clinical significance of eicosanoid-related pathways for the mechanism of action of cardiovascular drugs, such as organic nitrates or acetylsalicylic acid. The unique property of the eicosanoid system to become activated only in response to stimulation, the local nature of this reaction, the multiplicity of products formed and the short half-time of most of them are currently the most significant obstacles to define the role of endothelial cell-derived eicosanoids in clinical practice.

One milligramme of acetylsalicylic acid daily inhibits platelet thromboxane A2 production

To seek the lowest dose of acetylsalicylic acid (ASA) capable of inhibiting platelet thromboxane A2 (TxA)2 production, 18 healthy volunteers ingested 9 mg, 3 mg or 1 mg of ASA/day for twenty days and the release of TxB2 (a metabolite of TxA2) during the spontaneous clotting of blood was measured by radioimmunoassay. In addition, the production of prostacyclin (epoprostenol, PGI2) was investigated by measuring the urinary excretion of its break-down product, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) by radioimmunoassay. Significant inhibition of platelet TxA2 production was seen from the 15th day of treatment onwards with 1 mg of ASA (maximally 15%), from the 4th day of treatment onwards with 3 mg of ASA (maximally 40%), from the 1st day of treatment onwards with 9 mg of ASA (maximally 67%). No ASA dose changed platelet counts or urinary 6-keto-PGF1 alpha excretion. One mg of ASA daily, is the lowest dose ever shown to inhibit platelet TxA2 production.

Effects of aspirin dosage and time of administration on arterial prostacyclin production and platelet aggregation in rats

Previously we reported that electrically-induced carotid artery thrombosis in anesthetized rats was prevented by 3.3 or 10 mg/kg aspirin (ASA) given i.v. 10 min before injury but not by 1.7, 20 or 100 mg/kg and protection was lost by delaying injury to 20 min (Haemostasis 13:42, 1983). Here, collagen-induced platelet aggregation and arterial prostacyclin-generating activity, measured by RIA for 6 keto-PGF1 alpha and by human platelet aggregation bioassay, were studied ex vivo after i.v. ASA to anesthetized rats. In all cases where platelet aggregation was inhibited less than 50%, no protection had been observed (1.7 mg/kg at 10 min, 3.3 at 20 min, 20 at 10 min). In the two cases where protection had been observed, platelet aggregation was inhibited by about 75% or more and in one, prostacyclin activity was about 50% of normal (3.3 mg/kg at 10 min). Thus in five of six dose-time combinations tested, antithrombotic protection could be explained by a requirement for about 50% of normal prostacyclin activity and about 75% of inhibition of collagen aggregation. Aberrant findings are discussed in the light of knowledge of salicylate/aspirin competition for cyclooxygenase.

Sex difference in the inhibitory effect of aspirin on prostacyclin production of rat aortae

The effect of aspirin on the prostacyclin (PGI2) production of rat aorta was investigated, and the influence of sex hormones on the effect of aspirin was studied by the treatments of hormone administration, ovariectomy and castration. There was no significant sex difference in the arterial production of PGI2 between male and female rats. However, the PGI2 production was decreased with aspirin treatment, and the effect of aspirin was more efficient in male rats. The inhibitory effect of aspirin was reduced in the rats treated with estradiol and the castrated male rats, but it was potentiated in the rats treated with testosterone and the ovariectomized female rats. These results suggest that sex hormones may regulate the effect of aspirin on the PGI2 production in the aorta.

Central and peripheral haemodynamic effects of non-steroidal anti-inflammatory drugs in man

The haemodynamic effects of non-steroidal anti-inflammatory (NSAI) drugs can be attributed either to their common property of inhibiting the formation of prostaglandins (PG) in the cardiovascular system, or to direct actions on the tone and sensitivity of the resistance vessels in various regions. Indomethacin (IND) is the most frequently studied NSAI drug, in animals and in man. Its cardiovascular effects differ somewhat from those of other NSAI, due to the fact that, besides inhibiting PG formation, IND acts as a direct vasoconstrictor. The stimulatory effect of IND in vascular smooth muscle results in an increased systemic vascular resistance which, although partially compensated by a decreased cardiac output, gives rise to a moderate increase in systemic blood pressure. The vasoconstrictor effect of IND is of particular interest in patients with ischemic heart disease, since it lowers their already decreased coronary flow, and may thereby accentuate the risk of myocardial infarction. Administration of IND also leads to a decreased blood flow in the splanchnic region, the kidneys, and the brain. The cerebral blood flow is lowered by 25-35%; in addition, IND almost entirely erases the hyperemic flow response to hypercapnia. Of other NSAI drugs, at least aspirin and naproxen are completely devoid of such actions on the cerebral circulation. A common vascular effect of all NSAI drugs is a diminution of reactive hyperemia, the local hyperemia that develops in a tissue subjected to a short period of arterial occlusion. Part of this hyperemic response is dependent on an intact vascular PG formation and consequently it is inhibited when PG formation is blocked. In contrast, NSAI drugs do not affect the functional increase in the blood flow in working skeletal muscle.

Manipulation of the local thromboxane and prostacyclin balance in vivo by the antithrombotic compounds dazoxiben, acetylsalicylic acid and nafazatrom

We have developed an experimental model for the study of local prostaglandin production by platelets and the vessel wall following stimulation 'in vivo'. A nylon thread was inserted into the external jugular vein of rabbits; its presence did not induce an occluding thrombus. Thromboxane (TXB2) values in the blood, sampled through the facial vein, immediately distal to the stimulus, rose and remained high for at least 4 hr, while 6-keto prostaglandin (PG) F1 alpha levels, after a first increase, gradually returned to normal ('exhaustion' of the endothelial cells?). No changes were observed in the contralateral jugular vein without thread. After infusion via the femoral vein of 10 mg/kg dazoxiben, a thromboxane synthetase inhibitor, local TXB2 production was completely abolished, whereas 6-keto PGF1 alpha formation no longer returned to basal values, but tended to increase. This leads to the conclusion that upon inhibition of TXB2 formation endoperoxide metabolism is reoriented 'in vivo' towards prostacyclin, and this mainly at the site where platelets are activated. Injection of 100 mg/kg lysine acetylsalicylic acid resulted in complete inhibition of TXB2 and 6-keto PGF1 alpha formation, the latter, however, slowly recovering with time. The administration of nafazatrom to the animals did not influence the local TXB2 changes, but partially prevented the decline of 6-keto PGF1 alpha with time. The antithrombotic properties of this drug thus could be related to protection of the endothelial cells from 'exhaustion'.

Anti-thrombotic effect of very low doses of acetyl salicylic acid in rats

The anti-thrombotic effect of acetyl salicylic acid (ASA) was studied using the following experimental model: a silk thread was inserted in an extracorporeal shunt between the right carotid artery and the opposite jugular vein. A thrombus developed around the thread which was weighed at 10 minutes intervals. ASA inhibits the development of the thrombus at a dose of 3.1 mg/kg. When the dose was increased the anti-thrombotic effect disappeared. These results suggest that ASA may be an effective antithrombotic agent in man if the dosage is based on pharmacological data.

Acute arterial thrombosis in rabbits: reduced platelet accumulation after treatment with dazoxiben hydrochloride (UK 37,248-01)

1 Acute thrombosis was induced in the carotid arteries of anaesthetized rabbits by local electrical stimulation (1 mA for 2 min) of the vessel wall. Histological findings confirmed the platelet-rich composition of the thrombus. Platelet accumulation at the stimulus site was quantitated with 111Indium-labelling of autologous platelets. 2 In rabbits injected intravenously with either the thromboxane synthetase inhibitor dazoxiben 2 mg/kg or aspirin 10 mg/kg, accumulation of labelled platelets was considerably reduced. Animals which received vehicle injection only, showed no such reduced thrombus formation. 3 In separate experiments in anaesthetized rabbits, the levels of thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha in clotting blood were measured in blood samples taken from animals which had received the above drug treatments. Aspirin markedly reduced the production of both arachidonate metabolites. In contrast, dazoxiben almost totally inhibited TXB2 production but caused a 3.5-fold increase in the levels of 6-keto PGF1 alpha. 4 These findings demonstrate an antithrombotic effect and confirm the mechanistic selectivity of a thromboxane synthetase inhibitor.

Human platelet response to three salicylate dosage forms

Acetylsalicylic acid (ASA) inhibition of platelet aggregation as evaluated by collagen-induced 14C-serotonin release, has been measured in 12 healthy male subjects. Each subject received a single oral dose (650 mg) of enteric-coated ASA (ecASA) and compressed ASA tablets (cASA), or ecASA and sodium salicylate (578 mg) separated by a minimum of 5 weeks. The platelet response was related to plasma ASA and salicyclic acid determined by high-pressure liquid chromatography. Both ecASA and cASA inhibited 14C-serotonin release; no significant difference was observed in the maximum effect between these two products (p less than 0.05). No relationship was found between the maximum observed plasma ASA level and the maximum effect. Further, no correlation was found between the maximum inhibition of 14C-serotonin release in vivo and the release predicted from in vitro experiments wherein the effect was measured after incubating plasma containing specified ASA concentrations.

Effects of aspirin and prostacyclin on arrhythmias resulting from coronary artery ligation and on infarct size

1 The effects of pretreatment with aspirin, and of prostacyclin (PGI(2)) infusions, on responses to myocardial ischaemia and infarction produced by ligation of a coronary artery were investigated in conscious rats.2 Surgical preparation, under halothane anaesthesia, consisted of implanting exteriorized aortic and jugular cannulae, ECG leads and a polypropylene/polyethylene occluder for the left anterior descending coronary artery. Ligation of the coronary artery was performed six to nine days after surgery.3 Aspirin pretreatment consisted of 100 mg/kg given intravenously 1 or 36 h before ligation. PGI(2) infusions (10-400 ng kg(-1) min(-1), i.v.) were begun 2 min before ligation and continued for 4 h afterwards.4 ECG, blood pressure, heart rate and arrhythmias were recorded starting 30 min before, and continuing for 4 h after, ligation. Twenty-four hours after ligation, in surviving animals, the heart was removed for estimation of occluded and infarcted zones.5 Some treatments provided antiarrhythmic and other protection in the first 30 min post-ligation. By 4 and 24 h post-ligation, protective effects were lost.6 Both aspirin pretreatment and low doses of prostacyclin reduced arrhythmias occurring within 30 min of ligation. The highest dose of prostacylin (400 ng kg(-1) min(-1)) was arrhythmogenic.7 None of the treatments influenced the amount of cardiac tissue occluded or infarcted by ligation.8 The conclusions from this study in conscious rats were that acute aspirin pretreatment and low doses of infused prostacyclin have limited beneficial actions which are mainly confined to the earliest post-ligation period.

Effect of acetylsalicylic acid on plasma thromboxane B2 and platelet aggregation in man

The effect of acetylsalicylic acid (ASA) on plasma thromboxane A2 (TXA2) and platelet aggregation was studied in 12 healthy, non-smoking, male students, in a double-blind, cross-over study, after single doses and 14-days on ASA 50, 100, 250 and 1000 mg/day. Platelet production of TXA2 was assessed by measuring the thromboxane B2 (TXB2) content of clotted venous blood by RIA. Platelet aggregation induced by ADP and adrenaline was studied by the method of Born. All doses of ASA completely suppressed the production of TXB2 within 3 h, with the exception of the 50 mg dose, which effected only 61% suppression (p less than 0.001). After administration for 14 days the suppression was complete, even including the lowest dose. At that time ASA had blocked the secondary phase of adrenaline- and ADP-induced platelet aggregation. It is concluded that the maximal antithromboxane and antiaggregatory effects, which last for at least 24 h, can be achieved by continuous daily administration of ASA 50 mg.

Acute arterial thrombosis in rabbits: reduced platelet accumulation after treatment with thromboxane synthetase inhibitor dazoxiben hydrochloride, (UK-37, 248-01)

Acute thrombosis was induced in the carotid arteries of anaesthetised rabbits by local electrical stimulation (1mA for 2 min) of the vessel wall. Histological findings confirmed the platelet-rich composition of the thrombus. Platelet accumulation at the stimulus site was quantitated with "'Indium-labelling of autologous platelets. In rabbits injected intravenously with either 2 mg/kg dazoxiben or 10 mg/kg aspirin, accumulation of labelled platelets was considerably reduced. Animals which received vehicle injection only, showed no such reduced thrombus formation. In separate experiments in anaesthetised rabbits, the levels of TxB2 and 6KPGF1 alpha in clotting blood were measured in blood samples taken from animals which had received the above drug treatments. Aspirin markedly reduced the production of both arachidonate metabolites. In contrast, dazoxiben almost totally inhibited TxB2 production but caused a 3.5 fold increase in the levels of 6KPGF1 alpha. These findings demonstrate an anti-thrombotic effect and confirm the mechanistic selectivity of a thromboxane synthetase inhibitor.

Toxoplasmosis and the wild rabbit Oryctolagus cuniculus in Victoria, Australia with suggested mechanisms for dissemination of oocysts

Sera or serum eluates from 1697 wild rabbits [Oryctolagus cuniculus (L)], collected over the past 10 years from 24 sites in 5 geographic regions of Victoria, were examined for antibodies to Toxoplasma gondii. Sera generally were classified into three broad groups, negative (titre less than 10), weakly reactive (titre 10 to 100) and strongly positive (titre greater than 1000). Strongly positive sera came from rabbits collected in a small number of localized areas, most significantly the Melbourne and Metropolitan Board of Works (MMBW) Sewerage Farm at Werribee and Mud Island in Port Phillip Bay.

Differential inhibition by low-dose aspirin of human venous prostacyclin synthesis and platelet thromboxane synthesis

The capacity of venous tissue for prostacyclin synthesis was determined in 68 patients undergoing surgery for removal of varicose veins. A single dose of aspirin (81 mg or 300 mg) taken 14 h preoperatively strongly inhibited its synthesis, and the effect of 300 mg was still evident 48 h after ingestion. A single dose of 40 mg aspirin taken 14 h preoperatively had no effect on prostacyclin synthesis. The capacity of blood platelets to synthesise thromboxane (measured as malondialdehyde) was determined in volunteers before and at various times after ingestion of 300 mg or 40 mg aspirin. Both doses had an inhibitory effect that lasted for at least 96 h. The length of time for which the amount of thromboxane synthesised was insufficient to support platelet aggregation and the platelet release reaction depended on both the donor and the dose of aspirin. If prostacyclin and thromboxane are important in the pathogenesis of thrombosis, then doses of aspirin much lower than those used previously should be tested. The long-lasting effect of 300 mg aspirin on both venous tissue and platelets indicates that this dose is unlikely to produce a favourable prostacyclin/thromboxane balance.

Low dose aspirin, platelet function and prostaglandin synthesis: influence of epinephrine and alpha adrenergic blockade

The present investigation has evaluated the effects of low doses of oral aspirin on platelet prostaglandin synthesis and function. Whole (80 mg) or half (40 mg) tablets of baby aspirin given to adults had no effect on the response of their platelets to thrombin, ADP and epinephrine, but selectively inhibited aggregation induced by threshold concentrations of arachidonate 16-20 hours after ingestion. Larger amounts of arachidonate overcame the inhibition imposed by low dose aspirin, but not by adult aspirin tablets (600 mg). Epinephrine, in concentrations too low to cause aggregation, restored the sensitivity of aspirin-treated platelets to arachidonate. Studies with alpha-adrenergic agonists, antagonists and calcium channel blockers demonstrated that the corrective effect of epinephrine was mediated by an alpha-adrenergic receptor influence on calcium modulation of the platelet membrane.