Prostacyclin-dependent differences in TXA2 formation by platelets from normal and atherosclerotic rabbits

Antipsychotic Drugs Inhibit Platelet Aggregation via P2Y 1 and P2Y 12 Receptors

Antipsychotic drugs (APDs) used to treat clinical psychotic syndromes cause a variety of blood dyscrasias. APDs suppress the aggregation of platelets; however, the underlying mechanism remains unknown. We first analyzed platelet aggregation and clot formation in platelets treated with APDs, risperidone, clozapine, or haloperidol, using an aggregometer and rotational thromboelastometry (ROTEM). Our data indicated that platelet aggregation was inhibited, that clot formation time was increased, and that clot firmness was decreased in platelets pretreated with APDs. We also examined the role two major adenosine diphosphate (ADP) receptors, P2Y₁ and P2Y₁₂, play in ADP-mediated platelet activation and APD-mediated suppression of platelet aggregation. Our results show that P2Y₁ receptor stimulation with ADP-induced calcium influx was inhibited by APDs in human and rats' platelets, as assessed by in vitro or ex vivo approach, respectively. In contrast, APDs, risperidone and clozapine, alleviated P2Y₁₂-mediated cAMP suppression, and the release of thromboxane A2 and arachidonic acid by activated platelets decreased after APD treatment in human and rats' platelets. Our data demonstrate that each APD tested significantly suppressed platelet aggregation via different mechanisms.

Involvement of cyclooxygenase 2 in the protective effect of 17beta-estradiol on hypercholesterolemic rabbit aorta

The involvement of cyclooxygenase (COX) in the effects of 17beta-estradiol was investigated on hypercholesterolemic rabbits aorta. Acetylsalycilic acid, nimesulide, or SQ22536 was used as respective antagonist of COX-1, COX-2, or adenylate cyclase using aortic rings precontracted with phenylephrine and exposed to cumulative concentrations of acetylcholine (ACh). The relaxation effect of ACh was impaired by hypercholesterolemia and restored by an 8-week 17beta-estradiol treatment. In the control group treated with estrogen, nimesulide, acetylsalycilic acid, or SQ22536 slightly reduced the response to ACh. In hypercholesterolemic rabbits treated with estrogen, nimesulide significantly reduced the maximal relaxation and shifted to the right the relaxation curve of ACh, whereas acetylsalycilic acid did not modify the maximal response to ACh but displaced slightly the concentration-response curve. SQ22536 reduced the relaxant effect of ACh down to the level obtained in the presence of nimesulide. These results suggest that the protective effect of 17beta-estradiol against hypercholesterolemia involved COX-2/adenylate cyclase pathway.

Analysis of urinary 11-dehydrothromboxane B2 in patients with occluded retinal vein using GC/SIM

We describe variations of 11-dehydrothromboxane B2(11-dehydro-TXB2) levels in human urine samples. Retinal vein occlusion (RVO) is a thrombotic disease in which the retinal vein is blocked by blood aggregations. We considered the possibility that 11-dehydro-TXB2 plays an important role in the formation of RVO. Thus, we determined the 11-dehydro-TXB2 levels in patients with RVO using gas chromatography/selected ion monitoring (GC/SIM) and compared them with those of healthy volunteers. The thromboxane levels in patients with RVO, who did not also have diabetes, were significantly higher than those in healthy volunteers. One cause of RVO may be the variation of thromboxane production. Furthermore, this GC/SIM method can be applied to the prevention and treatment of not only RVO, but also of general thrombosis.

Production of platelet thromboxane A2 and arterial prostacyclin I2 from hypercholesterolemic rats

The plasma cholesterol, plasma malonaldehyde (MDA), platelet thromboxane A2 (TXA2) and vascular prostacyclin (PGI2) were measured in male Sprague-Dawley rats fed diets supplemented with cholesterol (1%) and cholic acid (0.5%). For comparisons, measurements were made in rats fed normal diets. The concentration of cholesterol in the plasma of rats had reached a maximum in 1 week of feeding experimental diets. TXA2 production from collagen and thrombin stimulated platelets was significantly decreased in animals fed experimental diets for 1 week. The production of MDA in the plasma of animals fed experimental diets for 8 weeks was significantly lower compared to the animals fed normal diets. There was a small but significant reduction in the formation of PGI2 in rats fed experimental diets for 8 weeks. These data suggest that feeding cholesterol rich diets to rats alters the platelet membrane properties differently from human and rabbit. Furthermore, cholesterol feeding to rats had some damaging effect on the arterial PGI2 synthesis.

Lipid peroxidation and protective enzymes during myocardial infarction

Fasting blood taken from 34 patients with myocardial infarction, 19 with unstable angina and 40 healthy controls, was analysed for malondialdehyde and erythrocyte detoxification enzymes, superoxide dismutase and glutathione peroxidase. Malondialdehyde concentration was raised in the patients with myocardial infarction during the initial 48 h after an attack, and correlated with the severity of the attack. 12 days after the infarct, malondialdehyde concentrations were lower but still raised. Superoxide dismutase activity was below normal during the initial 48 h post infarct and raised twelve days after. Glutathione peroxidase was reduced after 12 days. Similar, but less marked changes were seen in the patients unstable angina.

Increased prostacyclin and thromboxane A2 biosynthesis in atherosclerosis

It has been proposed that atherosclerotic arteries produce less prostacyclin (PGI2) than nonatherosclerotic arteries do, thereby predisposing arteries to vasospasm and thrombosis in vivo. We reexamined this concept by measuring spontaneous as well as arachidonate-induced PGI2 biosynthesis in aortic segments from nonatherosclerotic and cholesterol-fed atherosclerotic New Zealand White rabbits. Thromboxane A2 (TXA2) generation was also measured. Formation of PGI2, as well as TXA2, as measured by radioimmunoassay (RIA) of their metabolites, was increased in atherosclerotic aortic segments relative to nonatherosclerotic segments (P less than or equal to 0.05) at 0, 5, 10, 15, and 30 min of incubation with arachidonate. Pretreatment of arterial segments with indomethacin inhibited PGI2 as well as TXA2 formation, whereas pretreatment with the selective TXA2 inhibitor OKY-046 inhibited only TXA2 release, thus confirming the identity of icosanoids. To confirm the RIA data, aortic segments were incubated with [14C]arachidonate prior to stimulation with unlabeled arachidonate. The uptake of arachidonate was similar, but the release of incorporated [14C]arachidonate was significantly (P less than or equal to 0.05) greater in atherosclerotic segments than in nonatherosclerotic ones. Conversions of released [14C]arachidonate to 6-keto[14C]prostaglandin F1 alpha and [14C]thromboxane B2 were similar in the two types of aortic segments. Thus, synthesis of PGI2 as well as TXA2 is increased in atherosclerosis, and this alteration in arachidonate metabolism is related to increased release of arachidonate.

Study of fatty acids in atheroma induced in rabbits by an atherogenic diet with or without silicon i.v. treatment

Fifty-two rabbits were submitted for two months to an atherogenic diet with or without addition of silicon in the form of an I.V. silicon organic compound and compared to a control group of 21 rabbits. Out of the 26 rabbits receiving cholesterol alone, 23 showed atheromatous lesions; out of the 26 rabbits receiving cholesterol + silicon, only 8 had lesions. Free fatty acids, total fatty acids and esters were studied in the plasma and in the aorta. During atheroma, saturated fatty acids decrease, in particular 18:0, unsaturated fatty acids increase, in particular 18:1, 18:2, 20:4; with added silicon the variations are less important: in free fatty acids in plasma, there is a decrease of 20:4; in cholesterol esters in plasma and aorta an increase of 18:0 and a decrease of 18:2. There is a negative correlation between atheromatous lesions and myristic and stearic acids, and a positive correlation between oleic, linoleic and arachidonic acids and atheroma. Arachidonic acid, involved in phenomena of lipid peroxidation, decreased in the silicon treated rabbits.

[Thromboxane concentrations in the arterial blood of cigarette smokers]

Thromboxane concentrations were investigated in the arterial plasma of 44 patients (25 smokers, 19 nonsmokers) with chronic obstructive arterial disease (COAD) of the lower extremities, documented by angiography. The thromboxane levels were measured by radioimmunoassay. The results indicate that smokers with COAD have significantly higher thromboxane levels than nonsmokers with COAD. The potential effect of nicotine on thromboxane is discussed.

Differences in platelet aggregation kinetics in normal and hyperlipidemic rabbits

The difference in the aggregation mechanism between normal and hyperlipidemic rabbits was studied by kinetic analysis of changes in the number of residual single platelets in adenosine diphosphate (ADP)-induced platelet aggregation. When ADP was added to platelet rich plasma (PRP) obtained from normal rabbit, the number of single platelets decreased exponentially, but with PRP from hyperlipidemic rabbit, it decreased hyperbolically. However, the aggregation of platelets isolated from plasma of hyperlipidemic rabbit and resuspended in normal plasma showed an exponential decay, while that of normal rabbit platelets resuspended in hyperlipidemic rabbit plasma showed a hyperbolic decay. The results suggested that these aggregation mechanisms are altered mainly due to changes in the plasma components, such as the cholesterol levels.

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.

The role of platelet hyperfunction in thrombus formation in hyperlipidemia

The mechanism of thrombus formation in hyperlipidemia was studied. Attempts at artificial creation of an arterial thrombus in control rabbits stenosing the femoral artery by ligature were not successful unless ellagic acid was administered by injection. However, in rabbits with hyperlipidemia, mere creation of stenosis in the femoral artery resulted in a high percentage of thrombus formation. In rabbits with hyperlipidemia, both thromboxane (Tx) A2 biosynthesis in platelets and prostacyclin (PGI2) biosynthesis in the aorta were increased and these changes were noted at the level of cyclooxygenase in the arachidonic acid metabolic pathway. Therefore, these results suggest that thrombi are likely to be formed in hyperlipidemia and that such thrombus formation is due largely to platelet hyperfunction.

Platelet aggregation induced by arachidonic acid and thromboxane generation in patients with hypertension or cerebrovascular disease

The aggregability of platelets to arachidonic acid (AA) was investigated in 26 control subjects, 40 patients with essential hypertension, 20 patients with ischemic cerebrovascular diseases (CVD) not taking aspirin and 11 patients with CVD taking aspirin. The aggregability of platelets was evaluated on the basis of threshold concentrations of AA to induce irreversible platelet aggregation. The enhanced sensitivity of platelets to AA was observed more frequently in hypertensives and/or CVD patients not taking aspirin than in the controls. The relationship between platelet aggregation induced by AA and thromboxane B2 (TXB2) formation from AA or prostaglandin H2 (PGH2) in platelets was also studied in the subjects taking or not taking aspirin. It was proposed that the assessment of platelet aggregability with AA could provide a tool for identifying a subgroup of patients who might substantially benefit from the secondary preventive treatment with aspirin or other anti-platelet drugs. The clinical usefulness of this aggregation test for the management of the patients taking aspirin was also discussed.

Prostaglandins and ischemic heart disease

There is an abundance of information suggesting that prostaglandins are involved in the development and clinical expression of atherosclerosis. Many studies demonstrate a relationship between prostaglandins and the risk factors for peripheral and coronary artery disease. Thus, part of the mechanism by which hyperlipidemia, diabetes mellitus, smoking, hypertension, sex hormones, age, heredity, emotional stress and diet contribute to the development and progression of atherosclerosis may be through an imbalance between thromboxane A2 and prostaglandin I2. Recent studies show a temporal relationship between acute ischemic events (specifically, unstable angina) and a transcardiac increase in thromboxane B2, while others demonstrate a salutary effect of disaggregatory and vasodilatory prostaglandins in such patients. If prostaglandins and thromboxane prove important in ischemic vascular disease, attention will be directed at the correction of their pathologic imbalance. This may be accomplished by dietary manipulation as well as by the development of prostaglandin receptor antagonists or inhibitors of specific prostaglandin pathways.

OKY-1581: a selective inhibitor of thromboxane synthesis in vivo and in vitro

OKY-1581 is an effective inhibitor of thromboxane synthesis in vivo and in vitro. The generation of thromboxane B2 (TxB2), prostaglandin E (PGE) and prostaglandin F (PGF) was measured following clotting and during platelet aggregation induced by collagen. The presence of OKY 1581 either in vivo or in vitro caused a reduction in TxB2 generation during clotting and platelet aggregation with a concomitant increase in PGE and PGF. The effect could be observed two hours after oral or subcutaneous administration of 5 to 100 mg per rabbit and lasted for 24 to 48 hours. The reduction in TxB2 was not accompanied by an inhibition of clotting or platelet aggregation. OKY-1581 appears to be a suitable agent for studying the role of TxB2 in atherosclerosis.

Release of prostaglandins and thromboxane into the coronary circulation in patients with ischemic heart disease

Endogenous modulators of platelet aggregability and vascular tone may play a part in coronary-artery disease. We therefore measured the release of prostaglandins and thromboxane into the coronary circulation in patients with various kinds of cardiac disease. Simultaneous coronary-sinus (CS) and ascending-aortic (AO) blood samples were obtained from 60 patients for measurement of 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha, a prostaglandin I2 metabolite) and of thromboxane B2 (TxB2). Samples from 45 of these patients were also tested for prostaglandin E2 (PGE2) and lactate. Patients with unstable angina pectoris who reported chest pain within 24 hours of study had higher TxB2 CS/AO ratios (5.8 +/- 2.8, mean +/- S.D.) than patients whose most recent anginal pain was more than 96 hours before study (1.3 +/- 0.6; P less than 0.05), than those with nonischemic chest pain (1.2 +/- 0.4; P less than 0.05), or with valvular or congenital nonischemic heart disease (1.2 +/- 0.6; P less than 0.05). Those whose most recent anginal pain occurred 24 to 96 hours before study were distributed bimodally: the majority had low TxB2 CS/AO ratios (range, 0.5 to 2.1) like the patients in the three aforementioned groups, whereas a few had markedly elevated values (range, 10.5 to 46.6). The 6-keto-PGF1 alpha and PGE2 CS/AO ratios and myocardial lactate extraction were not significantly different among the five groups. These data suggest that local thromboxane release is associated with recent episodes of angina in patients with unstable angina pectoris, but whether this release is a cause or an effect is not yet known.

Decreased sensitivity of human platelets to PGI2 during long-term intraarterial prostacyclin infusion in patients with peripheral vascular disease--a rebound phenomenon?

During successful treatment of peripheral vascular disease with synthetic prostacyclin no alteration in platelet function was reported (1). In 8 patients infused with synthetic prostacyclin continuously for 7 days intraarterially, the platelet function was monitored. Special attention was drawn to the platelet sensitivity in vitro for PGI2, which is discussed as an important factor maintaining the hemostatic balance. In all the patients with peripheral vascular disease between 24 and 48 hours after the beginning of the infusion a sudden decrease in platelet sensitivity accompanied by an increase in platelet count could be seen. These dramatic alterations representing probably a rebound phenomenon occurring during long-term PGI2-treatment might be an explanation for a non-beneficial effect of the treatment and in some cases a limiting factor for the continuation of the infusion itself. It is not clear, if this rebound phenomenon is due to a stimulation of an endogenous inhibitor, lowering the synthesis of a naturally occurring substance acting against this inhibitor or tachyphylaxia.

Dietary modification of the vascular effect of prostacyclin in hypertensive rats

Hypertension increased the responsiveness of spontaneously hypertensive rats (SHR) to exogenously infused prostacyclin (PGI2) at rates of 300 ng/kg/min and 1 microgram/kg/min. This did not occur for angiotensin infusion. Substitution of a diet containing alpha-linolenic acid for the control diet to SHR abolished this increased responsiveness to PGI2. Accompanying the abolition of the increased responsiveness to PGI2 was a significant increase in specific activity of two lysosomal hydrolases (N-acetylglucosaminadase and cathepsin D) within the aorta of SHR. These changes may reflect compensatory responses of the vasculature to protect the animal.

Effects of dipyridamole in experimental atherosclerosis. Action on PGI2, platelet aggregation and atherosclerotic plaque formation

Experimental atherosclerosis in rabbits induced by feeding a standard atherogenic diet for 4 months resulted in an increased sensitivity of platelets to the proaggregatory action of collagen and ADP. Treatment with dipyridamole (3 x 10 mg/day i.m.) for 4 weeks normalized platelet loss in atherosclerotic rabbits and abolished the increased sensitivity to proaggregatory collagen, but not to ADP. Dipyridamole treatment lowered basal as well as PGI2-induced cAMP levels below values seen in platelets from normal rabbits, but the stimulation by PGI2 relative to basal cAMP levels was not affected or even increased by dipyridamole treatment. Dipyridamole did not affect the increased sensitivity of platelets from atherosclerotic rabbits to the antiaggregatory action of PGI2, indicating that dipyridamole decreased absolute cAMP levels, probably due to reduction of the adenine nucleotide pool in platelets without affecting the adenylate cyclase function. Dipyridamole enhanced atherosclerotic plaque formation in arterial walls. Basal as well as PGI2-stimulated cAMP content was lower in homogenates from atherosclerotic than from normal aortic tissue. Dipyridamole-treated animals showed a further decrease in basal as well as PGI2-stimulated cAMP content of the aortic tissue, suggesting that this decrease in cAMP content may be linked to the enhanced proliferative activity seen in artherosclerotic plaque formation.