Equivalent inhibition of in vivo platelet function by low dose and high dose aspirin treatment

In vitro platelet function was inhibited in healthy volunteers by two different doses of aspirin, as confirmed by measurement of maximum serum production of thromboxane B2 (TXB2) by platelets. 75 mg aspirin did not fully inhibit serum TXB2 production after 24 hours, whereas 300 mg aspirin did. Inhibition of platelet function in vitro was maintained by both 75 mg/day aspirin or 300 mg/alternate day aspirin. In contrast, in vivo production of TXB2, measured as urinary levels of the 11-keto-TXB2 metabolite, was inhibited similarly by both doses of aspirin throughout the study. These findings suggest that 75 mg/day aspirin may be sufficient adequately to inhibit platelet aggregation in vivo.     

Effects of experimental diabetes on spontaneous contractions, on the output of prostaglandins and on the metabolism of labelled arachidonic acid, in uteri isolated from ovariectomized rats. Influences of estradiol

Spontaneous changes in isometric developed tension (IDT) as a function of time after isolation (contractile constancy) in uteri from control-castrated and castrated chronic streptozotocin-diabetic rats, were explored. The effects of injecting 17-beta estradiol (Eo) were also studied. No differences in the minor changes of contractile constancy, between control and diabetic preparations, during a period of 60 min, were detected, whereas uteri from non-diabetic Eo injected animals (0.5 + 1.0 ug, prior to sacrifice), exhibited a profound reduction of IDT, significantly greater than in tissues obtained from Eo injected-diabetic rats. Moreover, basal generation and outputs into the suspending solution of prostaglandins (PGs) E1, E2 and F2 alpha, were explored in the same groups, at 60 min following tissue isolation. The basal outputs of these three PGs were similar in castrated control rats, but preparations from castrated-diabetics released significantly more PGE1. The administration of Eo to castrated-diabetics, failed to alter the releases of the three PGs explored. In addition, the metabolism of labelled arachidonic acid (AA) into different prostanoids (6-keto-PGF1, PGF2, PGE2 and thromboxane B2-TXB2), was also investigated. The non-diabetic spayed rat uterus converted AA into these four prostanoids, the transformation into 6-keto-PGF1 alpha (as an index of PGI2 formation) being the most prominent. In preparations from diabetic rats the formation) being the most prominent. In preparations from diabetic rats the formation of 6-keto-PGF1 alpha, PGF2 alpha and PGE2, was significantly smaller than in controls, whereas a greater % of TXB2 formation (as an index of TXA2), was detected. On the other hand uterine preparations from non-diabetic spayed rats injected with Eo formed less 6-keto-PGF1 alpha and PGE2 and similar amounts of PGF2 alpha or of TXB2 from AA, than Eo injected controls, whereas uteri from castrated diabetic animals injected with Eo, formed a similar % of 6-keto-PGF1 alpha, PGF2 alpha and PGE2 from AA, than tissue preparations from non-estrogenized controls. However, the enhanced transformation of the labelled fatty acid precursor (AA) into TXB2 in the diabetic group, was significantly reduced by the steroid. The role of the augmented generation and release of PGE1 in uteri from diabetic rats is discussed in terms of precedents indicating the relevance of PGs type E supporting rat uterine motility. In addition the influence of Eo is attractive, because its reducing effect on TX production, in diabetes, a disease known to be accompanied by enhanced synthesis of vasoconstrictor and platelet aggregation TXA2, and by frequent obstructive circulat     

COX-2 is not involved in thromboxane biosynthesis by activated human platelets.

The occurrence of aspirin resistance has been inferred by the assessment of platelet aggregation ex vivo in patients with ischemic vascular syndromes taking aspirin. Since aspirin is a weak inhibitor of the inducible isoform of prostaglandin H synthase (COX-2), it was suggested that COX-2 may play a role in aspirin resistance. However the cellular source(s) of COX-2 possibly responsible for aspirin resistance remains unknown. Recently, the expression of the inducible isoform of COX-2 in circulating human platelets was reported. To investigate the possible contribution of COX-2 expression in platelet thromboxane (TX) biosynthesis, we have compared the inhibitory effects of NS-398 and aspirin, selective inhibitors of COX-2 and COX-1, respectively, on prostanoid biosynthesis by thrombin-stimulated platelets vs lipopolysaccharide (LPS)stimulated monocytes (expressing high levels of COX-2) isolated from whole blood of healthy subjects. NS-398 was 180-fold more potent in inhibiting monocyte COX-2 activity than platelet TXB2 production. In contrast, aspirin (55 micromol/L) largely suppressed platelet TXB2 production without affecting monocyte COX-2 activity. By using specific Western blot techniques, we failed to detect COX-2 in platelets while COX-1 was readily detectable. Our results argue against the involvement of COX-2 in TX biosynthesis by activated platelets and consequently dispute platelet COX-2 expression as an important mechanism of aspirin resistance.     

Differential salicylate-aspirin interaction on vascular prostacyclin and platelet thromboxane synthesis in patients undergoing saphenectomy

We studied the ability of salicylate to counteract the effect of aspirin on platelet thromboxane synthesis and prostacyclin formation in venous tissue in patients undergoing saphenectomy. A single intravenous dose of 40 mg aspirin completely blocked thromboxane formation and reduced prostacyclin to about 43% of the control values. When salicylate (1000 mg po) corresponding in anesthetized subjects to blood levels of 25.9 +/- 5 micrograms/ml was administered before aspirin, vascular prostacyclin was no longer inhibited, whereas platelet thromboxane was still significantly blocked. These results suggest that the combination of salicylate with aspirin at an appropriate dose ratio may result in almost complete dissociation of the drug's effect on platelets and vessels in man.     

Thromboxane B2, 6-keto-prostaglandin F1 alpha, and prostaglandin F2 alpha production by contracting pregnant rate uteri in vitro

While prostaglandin production by uterine tissue has been shown to be involved in the contractile mechanism of this tissue, less attention has focused upon the involvement of other prostanoids. We have simultaneously measured in vitro isometric contractility of pregnant rat uteri with the release of prostaglandin F2 alpha (PGF), 6-keto-prostaglandin F1 alpha (6-k-PGF1 alpha) and thromboxane B2 (TXB2) into the bathing medium under various conditions. Frequency of uterine contractions and integrated contractile force (ICF) increased from 15 days of gestation and peaked at the time of parturition. Activity was generally greatest during the first 15 min of incubation except during parturition and on Day 1 postpartum when the uterine segment remained active for 1 h experimental period. Indomethacin (INDO) significantly reduced contractile activity regardless of gestational stage. PGF, TXB2, and 6-k-PGF1 alpha increased with gestational age, peaking at the time of parturition. Production was greatest during the first 15 min of incubation and INDO inhibited production of each prostanoid regardless of gestational stage. Imidazole (100 micrograms/ml) inhibited TXB2 production without affecting PGF or 6-k-PGF1 alpha levels. Frequency of contraction and ICF were not affected by imidazole treatment despite TXB2 reduction. These data demonstrate that the in vitro uterus from pregnant rats is capable of producing prostanoids other than prostaglandins and their production generally parallels uterine contractile activity. Thus, the possibility that these prostanoids are involved in physiologic changes during parturition warrants further investigation.     

High glucose levels modulate eicosanoid production in uterine and placental tissue from non-insulin-dependent diabetic rats during late pregnancy

Severe uterine and placental disturbances have been described in diabetes pathology. The relative severity of these changes appears to correlate with high glucose levels in the plasma and incubating environment. In order to characterize changes in eicosanoid production we compared uterine and placental arachidonic acid conversion from control and non-insulin-dependent diabetes mellitus (NIDDM) rats on day 21 of pregnancy, into different prostanoids, namely PGE2, PGF22alpha, TXB2 (indicating the production of TXA2) and 6-keto-PGF1 (indicating the generation of PGI2). PGE2, PGF2alpha and TXB2 production was higher and 6-keto-PGF1alpha was similar in diabetic compared to control uteri. PLA2 activity was found diminished in the NIDDM uteri in comparison to control. A role for PLA2 diminution as a protective mechanism to avoid prostaglandin overproduction in uterine tissue from NIDDM rats is discussed. Placental tissues showed an increment in TXB2 generation and a decrease in 6-keto PGF1alpha level in diabetic rats when compared to control animals. Moreover, when control uterine tissue was incubated in the presence of elevated glucose concentrations (22 mM), similar generation of 6-keto PGF1alpha and elevated production of PGE2, PGF2alpha and TXB2 were found when compared to those incubated with glucose 11 mM. Placental TXB2 production was higher and 6-keto PGF1alpha was lower when control tissues were incubated in the presence of high glucose concentrations. However, high glucose was unable to modify uterine or placental prostanoid production in diabetic rats. We conclude that elevated glucose levels induced an abnormal prostanoid profile in control uteri and placenta, similar to those observed in non-insulin-dependent diabetic tissues.     

Inhibition of platelet aggregation and arachidonate metabolism in platelets by procyanidins

The effects of procyanidins on platelet aggregation and arachidonate metabolism in platelets were studied. Nine procyanidins were used in this investigation. Procyanidins B-2-S, EEC and C-1 significantly induced the inhibition of platelet aggregation, and the potency of inhibition was comparable with aspirin. Procyanidin B-2-S was used as a representative of procyanidins for further studies on the effect on arachidonate metabolism. In arachidonate metabolism by fatty acid cyclooxygenase pathway, B-2-S inhibited TXB2 and HHT formation by intact platelets treated with exogenous arachidonic acid. It also inhibited TXB2 formation measured by a specific radioimmunoassay when the cells were challenged with calcium ionophore A23187. In cell-free system, B-2-S inhibited both TXB2 and 12-HETE bioxynthesis in platelet microsome and cytosol, respectively. The inhibitory effect on thromboxane biosynthesis might explain the inhibitory effect of procyanidins on platelet aggregation.     

The relationship between thromboxane and cytosolic calcium modifiers in rat platelets.

Platelet activity is controlled, in part, by cytosolic free ionized calcium concentration ([Ca++]i). Regulation of platelet thromboxane (TXB2) synthesis may be by regulation of [Ca++]i. Dietary linoleate is a regulator of TXB2 synthesis, therefore, it may act by influencing [Ca++]i. Aspirin is a regulator of TXB2 synthesis by inhibition of cyclooxygenase; ouabain and nifedipine are regulators of [Ca++]i. This study was conducted to determine whether these affectors of TXB2 synthesis and [Ca++]i cause associated responses. Male nonobese Zucker rats were fed diets supplying 30% of energy (en%) as fat. Dietary fat was a mixture of corn oil and beef tallow to provide 3.0, 4.5, 6.0, or 7.5 en% linoleic acid, with cholesterol added to provide equal cholesterol in all diets. Rats were fed for 30 days with 6 rats/diet. Isolated rat platelets were assayed for FA composition; the percentage of linoleic acid in platelet FA rose linearly with increasing dietary linoleate (r = 0.76, P less than 0.0001). Resting and thrombin-stimulated platelet [Ca++]i and TXB2 synthesis were measured in the presence or absence of extracellular calcium and aspirin, ouabain, or nifedipine. Aspirin caused reductions in both parameters; nifedipine blocked [Ca++]i, but did not affect TXB2; ouabain increased both. Changes induced by those modifiers of TXB2 and platelet [Ca++]i caused changes that were in the same direction for both. CaCl2 caused an increase in both and the [Ca++]i was correlated with the square root of the TXB2; without CaCl2 the two were negatively correlated; aspirin, ouabain, and nifedipine treatments resulted in no significant correlations. The results suggest that there is a common modifier of [Ca++]i and TXB2 synthesis.     

Aspirin inhibits rat megakaryocyte thromboxane synthesis

This study examines the question of whether the aspirin-induced delay in the recovery of platelet cyclooxygenase pathway activity, as measured by RIA of thromboxane B₂, results from a direct effect on megakaryocyte cyclooxygenase. From our measurement of recovery of TXB₂ and information on megakaryocyte transit time in rats, we propose that thromboxane synthesis may represent a relatively late step in the differentiation of megakaryocytes. Megakaryocyte thromboxane production was depressed by 70% and that of platelets by 85% at two hr after 20 mg/kg oral aspirin dissolved in DMSO. Full megakaryocyte thromboxane recovery occurred by 72 hr and preceded complete platelet thromboxane recovery by 24 hr. Whereas megakaryocyte thromboxane synthesis showed substantial recovery by 36 hr after aspirin, platelet recovery did not begin for 24 hr and achieved a maximal recovery rate over the following 12 hr. This finding is consistent with predictions based upon human data for both megakaryocyte labeling studies and post-aspirin platelet recovery. We conclude from our data and from estimates of megakaryocyte maturation times in marrow, that thromboxane synthesis develops in rat megakaryocytes after approximately 48 hr of cytoplasmic differentiation toward platelet shedding. This metabolic capacity therefore serves as a marker of megakaryocyte differentiation.     

Effects of lipid-esterified conjugated linoleic acid isomers on platelet function: evidence for stimulation of platelet phospholipase activity

The effects of four conjugated linoleic acid (CLA) isomers on in vitro collagen-induced human platelet aggregation and thromboxane (TXB(2), the inactive metabolite of the proaggregatory TXA(2)) production were examined. As the free fatty acid (FFA), 9t, 11t-CLA was the most effective inhibitor of these two processes (I(50)s of 2.2 and 4 microM, respectively) and the 9c, 11c-CLA was the least effective (I(50)s of 8.3 and 37 microM) of the isomers tested. When platelets were preesterified with either 25 microM 9t, 11t-CLA or 9c, 11c-CLA, CLA incorporation in total platelet lipids increased from 0.24% to 0.31% and 0.38%, and most of this increase was found to be in the phosphatidyl choline and phosphatidyl ethanolamine subclasses. The decrease in arachidonic acid (AA) content in total fatty acids or phospholipids was an order of magnitude greater. Furthermore, no significant differences between platelets prelabeled with either 9t, 11t- or 9c, 11c-CLA in the inhibition of collagen-induced aggregation and TXB(2) formation were observed. However, platelets prelabeled with 9c, 11c-CLA stimulated basal TXB(2) production (4-fold) which was not observed with platelets pretreated with either 9t, 11t-CLA, linoleic acid or stearic acid. This enhancement was associated with a 2.4-5-fold increase in the release of endogenous AA. Our results suggest that the presence of a conjugated cis, cis double bond appears to change the lipid environment sufficiently to stimulate the basal platelet phospholipase activity, which in turn increases the formation of TXB(2).     

Antithrombotic effect of onion in streptozotocin-induced diabetic rat

In the present study, we investigated whether onion has antithrombotic effect in streptozotocin (STZ)-induced diabetic rats. In diabetic rats, serum thromboxane B(2) (TXB(2)) level was elevated compared to that in normal, and this elevation in diabetes was significantly inhibited by treatment with onion (0.5 g/ml/kg/day, i.p.) for 4 weeks. In normal rats, the serum TXB(2) level remained unaltered after the treatment with onion. To investigate in vitro effect of onion, we examined its effect on TXB(2) formation, platelet aggregation and arachidonic acid (AA)-release in platelets from diabetic and normal rats. Onion showed a significant inhibitory effect on collagen- or AA-induced TXB(2) formation with greater potency in diabetic platelets than in normal. Similarly, more potent inhibitory effects of onion in diabetes were observed in collagen- or AA-induced platelet aggregation and collagen-induced AA release response. In conclusion, these results suggest that onion can produce more beneficial antithrombotic effect in diabetes.     

Transfer of very low density lipoprotein-associated phospholipids to activated human platelets

LDL-associated phospholipids (PLs) may be transferred into platelets. In this work, we characterized the role of VLDLs as PL donors. VLDL transferred radiolabeled PLs to platelets in a temperature- and concentration-dependent manner. LPL stimulated this process through its action on VLDL lipolysis, because it was abolished by tetrahydrolipstatin. LPL also stimulated the platelet production of thromboxane B2 (TXB2). Both LPL actions were inhibited in the presence of fatty acid-free albumin, suggesting that they were attributable to fatty acids generated during VLDL lipolysis. To study the relationship between PL transfers and platelet activation, we performed incubations in the presence of HDL, a physiological acceptor of PL released from VLDL. HDL antagonized the transfer of PL from VLDL to platelets but had no effect on the production of TXB2, suggesting that PL transfers were driven by platelet activation. Confirming this idea, thrombin stimulated both the production of TXB2 and the transfers of PL. In conclusion, VLDL can transfer PL to platelets. These transfers are stimulated by LPL and thrombin through their action on platelet activation. They might be enhanced in pathologies characterized by increased VLDL concentrations.     

Fractional conversion of thromboxane A2 and B2 to urinary 2,3-dinor-thromboxane B2 and 11-dehydrothromboxane B2 in the cynomolgus monkey

Following the intravenous administration of thromboxane (TX) B2, the stable hydration product of TXA2, to human and nonhuman primates the most abundant urinary metabolites are 2,3-dinor-TXB2 and 11-dehydro-TXB2. However, it is not known whether fractional conversion of TXB2 to its enzymatic metabolites is an accurate representation of TXA2 metabolism. Thus, we have compared the metabolic disposition of synthetic TXA2 and TXB2 via the beta-oxidation and 11-OH-dehydrogenase pathways in vivo in the monkey. TXA2 or TXB2 (20 ng/kg) was intravenously administered to four cynomolgus monkeys pretreated with aspirin in order to suppress endogenous TXA2 production. Urinary TXB2, 2,3-dinor-TXB2 and 11-dehydro-TXB2 were measured before, during and up to 24 h after thromboxane administration by means of reversed-phase high-performance liquid chromatography radioimmunoassay. Aspirin treatment suppressed urinary 2,3-dinor-TXB2 and 11-dehydro-TXB2 by approx. 75%. A similar fractional conversion of TXA2 and TXB2 into 2,3-dinor-TXB2 and 11-dehydro-TXB2 was found. These results suggest that TXA2 is hydrolyzed to TXB2 prior to enzymatic degradation and that metabolites of the latter represent reliable indices of TXA2 biosynthesis. Due to the variability in the conversion of thromboxanes into 2,3-dinor-TXB2 and 11-dehydro-TXB2, the measurement of both metabolites seems to represent a more reliable index of acute changes in TXA2 production.     

Effect of OKY-046, a selective thromboxane inhibitor,on prostaglandin production and function of pregnant rat uteri and platelets

The effect of OKY-046 (ONO, Japan), a selective TX inhibitor, was studied for its effect on uterine and platelet activity. On day 21 of pregnancy, rats were injected with either 0, 1, or 5 mg/kg OKY-046 via the tail vein. One hour following injections, in vitro activity of uteri and platelets was assessed. A decrease (P<.01) in uterine TXB₂ production (measured by RIA) occurred with increasing OKY-046 dose ( 104 ± 31 vs 44 ± 6 vs 24 ± 2 ng TXB₂/g tissue/45 min). OKY-046 treatment had no effect on other prostanoids. Contractile activity was not affected by OKY-046. The amount of TXB₂ produced in platelets from OKY-046 (5 mg/Kg) treated rats was 45.5% less than that from controls (P<.001). Likewise, arachidonate-induced aggregation of platelets from OKY-046 treated rats was 46.1% less (P<.05) than that of controls. In summary, in vivo administration of OKY-046 selectively reduced uterine TXB₂ without altering other prostanoids, or affecting uterine contractions. In contrast, both platelet TXB₂ production and platelet function (aggregation) was decreased.     

The effects of HCG, indomethacin, flufenamic acid and aspirin in the immature female rat.

Immature female rats (21 to 23 days old, 35 to 45 g) were injected subcutaneously with 2-5 i. u. HCG 18 hr before autopsy. Ovaries and uteri were removed; wet weight, dry weight and uterine protein content were determined. Ovarian and uterine weights, ovarian blood volume and uterine protein content were increased after HCG treatment. When immature female rats were pretreated with indomethacin, flufenamic acid or aspirin, the ovarian effects of HCG were inhibited: only slight increases in ovarian weight and blood volume were observed. Indomethacin attenuated the increases in uterine weight, and protein content, but neither flufenamic acid nor aspirin were effective in inhibiting these responses. The possible role of prostaglandins and of oestrogen as mediators of these responses is discussed.     

Time-dependent inhibition of platelet cyclo-oxygenase by indomethacin is slowly reversible

Indomethacin has been charterized as a time-dependent, irreversible inhibitor of cyclo-oxygenase, yet its effects on human platelets have been found to be reversible . To understand this apparent contradiction, we have investigated the kinetics of recovery of platelet thromboxane production after a single dose of indomethacin. The inhibition of platelet thromboxane production was greater than would be expected from the levels of indomethacin found in the plasma suggesting that the time-dependent inhibition occurs . Yet recovery of platelet thromboxane production was faster than expected for the irreversible inhibitor, with 50% of control values being regained within 24 hours after ingestion of the drug. When platelets were isolated and resuspended in homologous drug-free plasma, slow recovery of thromboxane production was seen to occur with 50% of control activity regained in 100 minutes. This recovery was much slower than that seen from a competitive inhibitor of cyclo-oxygenase, ibuprofen. Ibuprofen-treated platelets recovered nearly completely immediately on being resuspended in drug-free plasma. When microsomes were isolated from platelets, then treated with indomethacin, no time-dependent recovery of activity was seen. The recovery of cyclo-oxygenase after indomethacin inhibition appears to be limited to the unperturbed enzyme in this natural milieu.     

Time-dependent inhibition of platelet cyclooxygenase by indomethacin is slowly reversible

Indomethacin has been characterized in vitro as a time-dependent, irreversible inhibitor of cyclo-oxygenase, yet its effects on human platelets have been found to be reversible in vivo. To understand this apparent contradiction, we have investigated the kinetics of recovery of platelet thromboxane production after a single dose of indomethacin. The inhibition of platelet thromboxane production was greater than would be expected from the levels of indomethacin found in the plasma suggesting that the time-dependent inhibition occurs in vivo. Yet recovery of platelet thromboxane production was faster than expected for an irreversible inhibitor, with 50% of control values being regained within 24 hours after ingestion of the drug. When platelets were isolated and resuspended in homologous drug-free plasma, slow recovery of thromboxane production was seen to occur with 50% of control activity regained in 100 minutes. This recovery was much slower than that seen from a competitive inhibitor of cyclo-oxygenase, ibuprofen. Ibuprofen-treated platelets recovered nearly completely immediately on being resuspended in drug-free plasma. When microsomes were isolated from platelets, then treated with indomethacin, no time-dependent recovery of activity was seen. The recovery of cyclo-oxygenase after indomethacin inhibition appears to be limited to the unperturbed enzyme in its natural milieu.     

A critical period in the onset of parturition in rats and uterine sensitivity to estradiol and progesterone

The purpose of this paper was to impair normal parturition in rats in order to measure tissue levels of progestins and estrogens and compare these results with those of normal parturition in rats. Abnormal parturition was obtained by injection of isotonic saline into the uterine lumen of pregnant rats at the end of pregnancy or by handling the uterus. After each of these treatments on day 21 of pregnancy, parturition was impaired in 70 to 98% of the rats. When the treatments were carried out earlier or later in pregnancy, there was little or no impairment. Our results indicate transient discrepancies in plasma and tissue levels of steroids 6 h after treatment on day 21: 20 alpha-HP concentrations increased in treated rats compared to controls (uteri: 470%; p less than 0.01; ovaries: 89%; p less than 0.001); concomitantly, there was a sharp rise in P concentrations in uteri (+ 74% : p less than 0.05) and ovaries (+ 52%; p less than 0.05). Inversely, uterine concentrations of E2 decreased 6 h after treatment compared to controls (- 30%; p less than 0.05), although there was a transient rise of E2 in the ovaries (+ 30%; p less than 0.05). Twenty-four hours later, E2 concentrations were always lower in the uteri (- 30%; p less than 0.01). No change in E2 levels was noted in the uteri or ovaries of either the control or treated rats. The physiological significance of these changes and their consequences on uterine reactivity at term have been discussed. The data demonstrate that day 21 was a critical period in the parturient activity of the rat uterus which appears to be primarily affected by uterine levels of E2 between days 21 and 22 of pregnancy.     

Effect of single oral administrations of non steroidal antiinflammatory drugs to healthy volunteers on arachidonic acid metabolism in peripheral polymorphonuclear and mononuclear leukocytes

The effects of a single oral administration of acetylsalicylic acid (500 mg), indomethacin (50 mg) and piroxicam (40 mg) to healthy volunteers on functional and biochemical parameters of platelets, polymorphonuclear (PMN) and mononuclear (MNL) leukocytes were evaluated. Blood was collected before and two hours after the drug intake and blood cells separated according to conventional techniques. The considered drugs almost completely suppressed the aggregation of platelets, whereas they did not affect either PMN and MNL aggregation. Superoxide anion generation by leukocytes was (PMN), or no effect (MNL) was observed after piroxicam and indomethacin respectively. The formation of arachidonate metabolites via the 5-lipoxygenase pathway by PMN and MNL challenged with 10 microM A23187 was unchanged following aspirin and indomethacin. In this respect a selective increase of 5-HETE and LTC4 synthesis by MNL only was detected after piroxicam administration. The three drugs similarly reduced TXB2 synthesis by platelets and PMN (-80% for aspirin and indomethacin, and -40% for piroxicam). As far as MNL is concerned, aspirin inhibited this metabolite by 80%, while indomethacin reduced it by 40% only. In contrast piroxicam increased TXB2 synthesis by stimulated MNL. It can be concluded that the considered antiinflammatory drugs 1) differently affect the cyclooxygenase enzyme in platelets and leukocytes; 2) at variance with the situation in platelets, the inhibition of thromboxane formation by leukocytes is not related to modifications of cellular function; 3) the formation of arachidonate metabolites via the 5-lipoxygenase pathway is affected by piroxicam only.     

Slow intravenous administration of low dose aspirin inhibits both vascular and platelet cyclooxygenase activity: an experimental study in the rat

Aspirin irreversibly inhibits cyclooxygenase, thus preventing thromboxane (Tx)A2 production in platelets and prostacyclin in vascular cells. While it is generally accepted that the inhibitory effect of low dose aspirin is cumulative on platelet cyclooxygenase, it is still a matter of debate whether a similar phenomenon also occurs on vascular cyclooxygenase. We have measured in anesthetized rats the inhibitory effect of two doses of aspirin (2.5 and 5.0 mg/kg), given intravenously either as a bolus or as a continuous infusion (for 30 min), on platelet TxB2 and 6-ketoprostaglandin F1 alpha generation by different vascular segments. Aspirin significantly inhibited both platelet and vascular cyclooxygenase independently of the rate of drug administration. The aspirin peak plasma levels at the end of bolus injection was about 170 times higher than the average level measured during the slow infusion (1.21 +/- 0.15 micrograms/ml). At this concentration aspirin did not affect in vitro either platelet or vascular cyclooxygenase activity. Thus the inhibitory effect of aspirin on both platelet and vascular cyclooxygenase seems to be related to total exposure of the enzyme to the drug rather than to the maximal drug concentration attainable in the systemic circulation. These findings may be relevant to the current debate on optimal conditions for the biochemical selectivity of aspirin as an antithrombotic drug.