Lipoproteins inhibit platelet aggregation and arachidonic acid metabolism in experimental hypercholesterolaemia

1. Human plasma contains unidentified components that inhibit arachidonic acid (AA) metabolism. In the present study, we investigated whether plasma from rabbits fed a normal or high-cholesterol diet for 16 weeks also inhibits AA metabolism. Specifically, we studied the effects of plasma on platelet aggregation and on the production of AA metabolites, tri-hydroxyeicosatrienoic acid, 12-hydroxyeicosatetraenoic acid and thromboxane B(2). 2. Haematological and lipid profiles were altered by a high-cholesterol diet. Platelets from hypercholesterolaemic rabbits showed enhanced aggregatory sensitivity to AA and platelet-activating factor. However, plasma from hypercholesterolaemic and control rabbits, when added to the incubation mixture, significantly inhibited platelet aggregation and eicosanoid production. 3. High- and low-density lipoprotein (HDL and LDL, respectively) concentrations increased several-fold in plasma with cholesterol feeding. When added directly to the incubation mixture, both HDL and LDL inhibited platelet aggregation, as well as AA metabolism. 4. Haptoglobin, albumin and Cohn's fraction IV, but not globulins, exhibited antiplatelet and anti-AA metabolism activities. Their concentrations in plasma were not affected by cholesterol feeding. 5. We conclude that LDL and HDL account for at least some of the inhibition of AA metabolism produced by plasma.     

Inhibition of adenosine diphosphate-induced platelet aggregation by alpha-lipoic acid and dihydroquercetin in vitro

Objectives:

To investigate the antiplatelet activity of alpha-lipoic acid (α-LA) and dihydroquercetin (DHQ).

Materials and

Methods: Antiplatelet activity of the α-LA and DHQ was evaluated in rich platelet plasma of rat. The platelet aggregation was induced by adenosine diphosphate (ADP) in concentration of 4 × 10^-5 M.

Results:

α-LA and DHQ inhibited platelet aggregation in concentration-dependent manner. The antiplatelet activity of α-LA was more pronounced than DHQ. DHQ also increased the antiplatelet activity of α-LA by 1.4 times.

Conclusion:

Combined simultaneous use of α-LA and DHQ possessed the high antiplatelet activity, and DHQ potentiated the activity of α-LA.KEY WORDS: Alpha-lipoic acid, antioxidants, antiplatelet activity, dihydroquercetin, platelet aggregation     

Sex difference in the effect of aspirin on rat platelet aggregation and arachidonic acid metabolism

The rat platelet aggregation induced by collagen was stronger in males than in females. The platelet malondialdehyde (MDA) production was more in males than in females, and the platelet cyclooxygenase activity was higher in males than in females. Aspirin at a dose of 10 mg/kg inhibited the collagen-induced aggregation in males, but not in females. Aspirin at a dose of 5 mg/kg blocked the MDA production only in males, but aspirin at a dose of 10 mg/kg inhibited the MDA production in both sexes. The effect of aspirin on the cyclooxygenase activity was only in males, but aspirin at a dose of 10 mg/kg inhibited the MDA production in both sexes. The effect of aspirin on the cyclooxygenase activity was similar to that on the MDA production. In gonadectomized rats, the MDA production and the cyclooxygenase activity were decreased by castration, and they were increased by ovariectomy. Aspirin at a dose of 5 mg/kg failed to inhibit them in castrated rats. Besides, in in vitro experiments, aspirin also inhibited the MDA production and the aggregation. Nevertheless, there was no sex difference in the content of arachidonic acid, a substrate of platelet cyclooxygenase. It is suggested that there is a sex difference in rat platelet cyclooxygenase activity, and it is closely related to the sex difference in the antiplatelet effect of aspirin.     

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.     

Azaprostanoic acid derivatives. Inhibitors of arachidonic acid induced platelet aggregation.

A series of 13-azaprostanoic acids (4a-h) and a 15-azaprostanoic acid (11a) have been prepared. Synthesis of the 15-aza derivative is based on a novel transformation of a ketone to an N-substituted ethylenamine using a formylmethylimino phosphate derivative. Several of the azaprostanoic acid derivatives were found to be potent inhibitors of platelet aggregation induced by arachidonic acid, whereas no effect was observed on ADP-induced primary aggregation, indicating blockade of the platelet arachidonic acid cascade. The compounds do not inhibit bovine cyclooxygenase activity and are postulated as acting beyond the synthesis of the prostaglandin endoperoxides. The inhibitory effect of the 13-aza series is highly sensitive to both stereochemistry and length of the amino side chain. Any deviation from the natural prostaglandin skeletal arrangement results in decreased biological activity.     

Inhibition of platelet aggregation by verapamil: quantification by in vivo and in vitro techniques

Platelet aggregation appears to play a prominent role in myocardial ischemia. Verapamil, a slow-channel blocking agent with important antiarrhythmic and vasodilating actions, has been shown to inhibit in vitro platelet aggregation. We used an electronic particle size analyzer to evaluate the effects of verapamil on platelet aggregation in vitro and in vivo in 88 rats. The intravenous injection of verapamil (0.4 mg/kg) did not change the platelet count compared to control animals receiving an equal volume of normal saline (verapamil, 1.1 +/- 0.04 x 10(6)/mm3, vs. control, 1.2 +/- 0.09 x 10(6)/mm3, (p greater than 0.05). The mean size of platelet aggregates induced by adenosine diphosphate (0.2 microM), was reduced by verapamil (verapamil, 15.3 +/- 1.2 x 10(3) micron3 vs. control 24.4 +/- 2.7 x 10(3) micron; p less than 0.01). Platelet aggregates induced in vivo, following a standardized technique of extravasation of right iliac artery blood into the peritoneal cavity, were also smaller following verapamil infusion (verapamil, 12.6 +/- 1.1 x 10(3)micron3, vs control, 17.3 +/- 0.9 x 10(3) micron3 p less than 0.001). We conclude that verapamil exerts and inhibitory effect on platelet aggregation both in vitro and in vivo. This property may add an important new dimension to its potential therapeutic usefulness in ischemic heart disease.     

Modification of platelet function and arachidonic acid metabolism by bioflavonoids. Structure-activity relations

The mechanism of the antiaggregating activity of flavonoids was studied in vitro. The activity of fifteen different compounds was tested on platelet aggregation and arachidonic acid metabolism. The effect of flavonoids on platelet adenosine 3',5'-cyclic monophosphate (cyclic AMP) levels under basal conditions, as well as after stimulation by prostacyclin (PGI2), was also measured. The glycons of flavonoids in general and the flavanone derivatives that we tested did not affect platelet function. On the other hand, flavone, chrysin , apigenin and phloretin inhibited platelet aggregation by depressing the cyclooxygenase pathway. In addition, flavone, chrysin and apigenin reduced the platelet cyclic AMP response to PGI2. This effect was probably mediated by an inhibition of adenylate cyclase. Myricetin and quercetin, however, increased the PGI2-stimulated rise of platelet cyclic AMP. Both of these flavonoids inhibited primarily lipoxygenase activity. Modification of platelet cyclic AMP metabolism through inhibition of phosphodiesterase activity was found to be the probable mechanism of their antiaggregating effect.     

Effects of CI-930, a novel phosphodiesterase III inhibitor, on platelet aggregation and arachidonic acid metabolism

In the platelet-rich plasma of rabbits, 4,5-dihydro-6-[4-(1H-imidazol-1-yl)phenyl]-5-methyl-3(2H)-pyridazinone (CI-930) inhibited platelet aggregation triggered by AA, U-46619, ADP, collagen and PAF, with the IC50 values of 0.91, 0.73, 2.12, 2.35 and 7.15 mumols/L, respectively. The inhibitory effect of CI-930 on AA-induced aggregation was potentiated by PGE1, an adenylate cyclase activator, and antagonized by SQ-22536, an adenylate cyclase inhibitor. The contents of cAMP in washed rabbit platelets were increased by CI-930 5-50 mumols/L. In the concentration range of 0.5-500 mumols/L, CI-930 reduced the synthesis of TXB2 by either washed rat or rabbit platelets or rat pleural neutrophils. At the same time, CI-930 induced a dose-dependent increase of PGE2, PGF2a, and PGD2 biosynthesis by rat platelets and had no significant influence on the formation of 6-keto-PGF1a by the neutrophils. It is showed that CI-930 is an anti-platelet agent with a wide-spectrum activity and its anti-aggregating action may be exerted by dual mechanisms, both increasing cAMP contents and selectively inhibiting TXA2 synthesis in platelets.     

甲基黄酮醇胺对血小板花生四烯酸代谢通路的影响

甲基黄酮醇胺(MPA)40mg·Kg~(-1)iv,能使花生四烯酸(AA)诱发的小鼠死亡率降低61％.MFA12.5～200μmol·L~(-1)呈剂量依赖性地抑制AA诱导的免血小板聚集.聚集率为49％±10％～4％±4%,对照组为69％±3％.MFA0.1～0.4mmol·L~(-1)呈剂量依赖性抑制AA诱导兔的血小板丙二醛(MDA)的生成,为(nmol·10~(-9)血小板)0.075±0.011～0.111±0.023,对照组为0.170±0.017.MFA0.4 mmol·L~(-1)有效地抑制凝血酶和A A诱导的兔血小板内MDA生成.分别为0.016±0.006.0.080±0.017,对照组分别为0.048±0006,0.160±0.025;普萘洛尔仅抑制凝血酶诱导的MDA生成.对AA诱导的MDA无影响.MFA0.4mmol·L~(-1)不影响血小板内cAMP含量.结果提示MFA抑制血小板AA代谢通路可能是其抑制血小板聚集功能的机理之一.     

Inhibition of thrombin-induced platelet aggregation by uteroglobin

Uteroglobin, a steroid-dependent, small molecular weight (15K) protein in the rabbit, inhibited thrombin-induced aggregation of both rabbit and human gel-filtered platelets (GFP). GFP aggregation by arachidonic acid was not affected by uteroglobin. There were no effects of uteroglobin on thrombin-induced clotting of plasma or purified fibrinogen, or inhibition of thrombin by antithrombin III. Additionally, preliminary results suggest that uteroglobin does not interfere with binding of thrombin to platelets. We suggest that inhibition of platelet aggregation by uteroglobin may function in preventing thrombosis and ensuring free flow of blood through the microvasculature of the uterus and the placenta and may induce some of the antimotility effects of progesterone on the uterus.     

Effects of dilazep on arachidonic acid metabolism by neutrophils and platelet/neutrophil interactions

The effect of dilazep (tetrahydro-1H-1,4-diazepine-1,4(5H)-dipropanolbis(3,4, 5-trimethoxy-benzoate)dihydrochloride monohydrate, Comelian), a coronary and cerebral vasodilator and an anti-platelet agent, on endogenous and exogenous arachidonic acid (AA) metabolism by human neutrophils and platelet/neutrophil interactions was studied in vitro. Neutrophils preincubated with dilazep (up to 300 mumol/l) were incubated with the Ca ionophore A23187 in the absence or presence of AA. Platelet/neutrophil mixtures preincubated with dilazep (up to 100 mumol/l) were incubated with thrombin plus N-formylmethionylleucylphenylalanine (FMLP), FMLP plus AA, and platelet-activating factor (PAF) plus AA. AA metabolites including leukotriene B4 (LTB4), 5-hydroxyeicosatetraenoic acid (5-HETE) and 5S,12S-dihydroxyeicosatetraenoic acid (5S,12S-DiHETE) were analyzed and quantitated by reversed-phase high-performance liquid chromatography. The formation of LTB4 and 5-HETE from endogenous AA by neutrophils was inhibited by dilazep, whereas their production from exogenous AA was enhanced. LTB4 synthesis from endogenous AA by platelet/neutrophil interactions was inhibited by dilazep, while 5S,12S-DiHETE production was increased. The production of 5-lipoxygenase metabolites from exogenous AA by these interactions was increased by this drug. Thus, dilazep inhibits endogenous AA metabolism by neutrophils and by platelet/neutrophil interactions, whereas it stimulates exogenous AA metabolism by these blood cells and their interactions.     

Inhibition of platelet activation and aggregation

It has recently been established that platelets are involved at all stages of atherosclerotic disease. A major platelet mediated process is the acute vessel closure at the site of atherosclerotic plaque rupture and there is emerging evidence for platelet adhesion to endothelial cells in the early stage of atherosclerotic disease. This, through engagement of other cells, leads to the development of the atherosclerotic plaque. Beside dietary, cholesterol- and lipid-lowering, and other pharmaceutical approaches antiplatelet therapy plays an important part in the treatment of atherosclerosis and its multifarious clinical manifestations. Antiplatelet therapy and the currently approved substances for oral (acetylsalicylic acid, dipyridamole, cilostazol, ticlopidin and clopidogrel) and parenteral (acetylsalicylic acid, abciximab, eptifibatide and tirofiban) administration are discussed in the following section. Attention is given to each single agent and its mechanism of action. Differences in pharmacodynamic and pharmacokinetic properties are elucidated and outlook on future antiplatelet strategies is discussed.     

Inhibition of platelet aggregation by adenosine receptor agonists

2-(Ar)alkoxyadenosines, which are agonists selective for the A_2AAR in PC 12 cell and rat striatum membranes, are also agonists at the A₂AR coupled to adenylate cyclase (AC) that mediates the inhibition of platelet aggregation. A panel of twelve well-characterized adenosine analogues stimulated human platelet AC and inhibited ADP-induced platelet aggregation at sub- to low-micromolar concentrations with a potency ranking CGS 21680 < adenosine < R-PIA. There were significant correlations between the ECso of anti-aggregatory activity and either the ECso of stimulation of platelet and PC 12 cell AC (r ² = 0.66 and 0.67, respectively) or the K₁ of inhibition of [³H]NECA binding to the rat striatum membranes (r ² = 0.75). Likewise, platelet AC stimulation correlated well with stimulation of PC 12 cell AC and with [³H]NECA binding (r ² = 0.94 and 0.91, respectively). Ten 2-(ar)alkoxyadenosines stimulated platelet AC at EC₅₀s ranging between 0.16 and 2.3 μM and inhibited platelet aggregation at EC₅₀s ranging between 2 and 30 μM. There were no correlations between the EC₅₀s of anti-aggregatory activity and either the EC₅₀s of the stimulation of platelet or PC 12 AC (r ² = 0.08 and 0.06, respectively) or with the K₁ of the inhibition of [³H]NECA binding to the A_2aAR in rat striatum (r ² = 0.02). The EC₅₀s of the stimulation of platelet AC correlated with those of the stimulation of PC 12 AC (r ² = 0.48), and also with the K₁ of [³H]NECA binding (r ² = 0.71). Each of the 23 adenosines completely inhibited platelet aggregation and thus, functionally, all behaved as full agonists. As stimulants of PC 12 cell AC, Group A and B analogues were equally efficacious. As stimulants of platelet AC, however, the efficacy relative to NECA ( = 1.0) of Group B analogues was significantly less than that of Group A analogues, 0.49 ± 0.2 vs. 0.72 ± 0.05, P±0.01. The partial agonist activity of Group B analogues at the platelet A2AR but full agonist activity at the PC 12 cell A_2aAR, as well as the relatively low correlations between platelet AC stimulation and other indices of A_2aAR agonist actlVlty, suggest the platelet receptor is not a typical A_2aAR. Further, the lack of a correlation between the platelet anti-aggregatory and AC stimulatory activity suggests that (a) the 2-(ar)alkoxyadenosines might affect platelet aggregation by mechanisms other than AC stimulation or (b) that the stimulation of the platelet membrane AC by 2-(ar)alkoxy-adenosines does not correspond to the accumulation of cyclic AMP in intact platelets.     

牛心内膜腺苷三磷酸双磷酸酶对血小板聚集的抑制作用(英文)

目的:研究牛心内膜内皮细胞(EEC)腺苷三磷酸双磷酸酶的抗血小板聚集效应。方法:培养牛EEC。以高效液相色谱法测定ADP,用比浊法测血小板聚集。结果:ADP 500μmol·L~(-1)与EEC温孵后,引起ADP浓度进行性地降低。与此相适应的是,未代谢的ADP诱导血小板聚集的能力也降低。在阿司匹林处理过的EEC 1×10~9cells·L~(-1)存在下,凝血酶500U·L~(-1)及PAF 1nmol·L~(-1)诱导的经阿司匹林1mmol·L~(-1)及亚甲基蓝10μmol·L~(-1)处理过的血小板聚集明显受到抑制,且聚集是可逆的;EEC的此种作用类似于腺苷三磷酸双磷酸酶的作用。EEC明显抑制ADP 5μmol·L~(-1)诱导的血小板聚集,但不能抑制ADP-β-S 15μmol·L~(-1)诱导的血小板聚集。结论:腺苷三磷酸双磷酸酶水解ADP是牛EEC重要的抗血栓机制。     

Triphenyltin fluoride in vitro inhibition of rabbit platelet collagen-induced aggregation and ATP secretion and blockade of arachidonic acid mobilization from membrane phospholipids

Recent studies have demonstrated that triphenyltin fluoride (TPTF) inhibits collagen-induced aggregation and ATP secretion of rabbit platelets in vivo [S. Manabe and O. Wada, J. Toxic. Sci. 6, 236 (1981)]. The aim of the present investigation was to test the effects in vitro of TPTF on platelet aggregation and to elucidate the mechanism of the inhibitory action by studying the release and metabolism of arachidonic acid and the cyclic AMP contents of rabbit platelets treated in vitro with TPTF. Although no inhibitory effect of TPTF was found on sodium arachidonate-induced platelet aggregation and ATP secretion, TPTF inhibited both reactions induced by collagen. Triphenylarsine and triphenylantimony did not inhibit, even at a concentration of 10(-3) M. The anti-aggregating concentration (IC50) of TPTF was 6.0 x 10(-6) M against collagen. TPTF had no inhibitory effect on the conversion of exogenous arachidonic acid to malondialdehyde (MDA) by platelets, while the collagen-induced production of arachidonate metabolites [MDA, 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and thromboxane B2] was remarkably inhibited by TPTF. Furthermore, TPTF apparently inhibited the collagen-induced release of arachidonic acid from platelets, although the formation of phosphatidic acid was not inhibited. Total cyclic AMP content after TPTF exposure was not changed significantly. These results indicate that TPTF inhibited the collagen-induced arachidonic acid release from platelet phospholipids, presumably by acting on phospholipase A2. Furthermore, it seems unlikely that the inhibition of arachidonic acid release by TPTF can be explained by the level of cyclic AMP in platelets.     

The distribution and metabolism of arachidonic acid in rabbit platelets during aggregation and its modification by drugs.

1 Gas chromatographic and radio-isotope labelling techniques have been used to establish the origin of the arachindonic acid used by the platelet cyclo-oxygenase for the synthesis of pro-aggregatory prostaglandin endoperoxide derivatives. 2 Measurements of total platelet arachidonate content indicated that more than 95% is esterified in the phosphatide fraction of the cells. 3 During aggregation by collagen or thrombin as much as 80% of the total platelet arachidonate may be liberated and transformed by the platelet enzymes into hydroxyacids and other more polar compounds. 4 The phosphatidylethanolamine, phosphatidylcholine and phosphatidylinositol fractions are major sources of the arachidonate thus used. 5 Indomethacin, which prevents platelet aggregation by inhibiting the cyclo-oxygenase, did not affect this release of arachidonate from the phosphatides but did prevent the transformation of arachidonate to endoperoxide derivatives. 6 Mepacrine, a drug which possesses weak anti-phospholipase activity in platelets, also prevents aggregation by collagen or thrombin, but seems to do so by preventing substrate release from the phosphatide fraction. 7 It is suggested that phospholipase A2 plays a key role in the initial events during platelet aggregation induced by collagen.