The antiplatelet activity of PMC, a potent alpha-tocopherol analogue, is mediated through inhibition of cyclo-oxygenase

Inhibitory Effect of Propolis on Platelet Aggregation In Vitro

Platelet hyperactivity plays an important role in arterial thrombosis and atherosclerosis. The present study was aimed to investigate the effects of different extracts of propolis and components of flavonoids on platelet aggregation. Platelet-rich plasma was prepared and incubated in vitro with different concentrations of the tested extracts and components of flavonoids. Platelets aggregation was induced by different agonists including adenosine diphosphate (ADP, 10 μM), thrombin receptor activator peptide (TRAP, 50 μM), and collagen (5 μg/mL). At 25 mg/L to 300 mg/mL, the water extract propolis (WEP) inhibited three agonists-induced platelet aggregations in a dose-dependent manner. The flavonoids isolated from the propolis also showed markedly inhibited platelet aggregation induced by collagen, ADP, and TRAP, respectively. The components including caffeic acid phenethyl ester (CAPE), galangin, apigenin, quercetin, kaempferol, ferulic acid, rutin, chrysin, pinostrobin, and pinocembrin and their abilities of inhibiting platelet aggregation were studied. It was concluded that propolis had an antiplatelet action in which flavonoids were mainly implicated.

Multi-conformer molecules in solutions: an NMR-based DFT/MP2 conformational study of two glucopyranosides of a vitamin E model compound

Overall geometries of both glucosyl derivatives of PMC were found on the basis of their NMR spectra in CDCl₃and relatedδ_H,C/ⁿJ_HHIEF-PCM(UFF,CHCl₃)/DFT calculational results.

In Vitro Susceptibility of Wistar Rat Platelets to Hydrogen Peroxide and AAPH-Induced Oxidative Stress

Hydroxyl and peroxyl radicals are biologically active species because of their likelihood to damage cellular constituents. An in vitro study on Wistar rats was conducted to investigate the influence of hydrogen peroxide (H2O2) and 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) on platelets and compare the vulnerability of platelets to oxidative stress (OS) induced by these two free radical initiators. Isolated platelets were divided into controls (without free radical initiators; n = 5) and experimentals (with free radical initiators; n = 5). Different concentrations (0.5, 1.0 and 2.0) of free radical initiators H2O2 and AAPH were used to treat the platelets and incubated for 5, 15 and 30 min. Biomarkers such as platelet aggregation, superoxide generation, lipid peroxidation (thiobarbituric acid reactive substances, conjugate dienes), protein oxidation (protein carbonyls, sulfhydryls) and antioxidant enzymes were assessed. In H2O2 and AAPH treated platelets, though OS was observed at concentrations of 0.5 and 1.0 mM, platelets could tolerate the oxidative insult. Treatment of platelets with 2.0 mM H2O2 demonstrated the onset of irreversible changes in platelets as observed in the results of increased superoxide generation and lipid peroxidation products. In 2.0 mM AAPH platelets, the oxidative damage was evident as indicated through increased aggregation, superoxide generation and conjugate dienes and lower protein sulfhydryls. Platelets were more susceptible to AAPH than H2O2, as AAPH acted on both lipids and proteins whereas H2O2 acted only on lipids. This study gives insight on platelet survival under different OS situations.

Inhibitory effect of PMC, a potent hydrophilic α-tocopherol derivative, on vascular smooth muscle cell proliferation: the pivotal role of PKC-α translocation

CONTENT

Vascular smooth muscle cells (VSMCs) play a major role in the pathogenesis of atherosclerosis and restenosis, and thus the excessive proliferation of VSMCs contributes to neointimal thickening during atherosclerosis and restenosis. PMC (2,2,5,7,8-pentamethyl-6-hydroxychromane) is the most potent hydrophilic derivative of the alpha-tocopherols; it acts as a potent anti-inflammatory and free-radical scavenger.

OBJECTIVE

The present study was designed to examine the inhibitory mechanisms of PMC in VSMC proliferation.

MATERIALS AND METHODS

VSMC proliferation and cytotoxicity were measured by MTT and LDH assays, respectively. The cell cycle and translocation of PKC-alpha in VSMCs were used by flow cytometry and confocal microscope, respectively. To detect PKC-alpha translocation and activation in VSMCs, immunoblotting was performed in the present study.

RESULTS

In this study, we demonstrate an anti-proliferative effect of PMC in VSMCs. Concentration-dependent inhibition of serum-induced VSMC proliferation was observed in PMC (20 and 50 muM)-treated cells. PMC pretreatment also arrested VSMC cell cycle progression at the G2/M phase. Furthermore, PMC exhibited obvious inhibitory effects on phorbol 12-myristate 13-acetate (PMA)-induced protein kinase C (PKC)-alpha translocation and phospho-(Ser/Thr) substrate phosphorylation.

DISCUSSION AND CONCLUSION

The inhibitory mechanisms of PMC on VSMC proliferation is mediated, at least in part, by inhibition of PKC-alpha translocation and causes cell cycle arrest in the G2/M phase. PMC treatment may represent a novel approach for lowering the risk of or improving function in abnormal VSMC proliferation-related vascular diseases.

Comparison of the relative activities of inducing platelet apoptosis stimulated by various platelet-activating agents

Apoptosis-like events are known to occur in anuclear platelets. Although the mechanisms responsible for these events are still not completely understood, studies suggested that some platelet agonists can activate platelet apoptosis. However, the relative activities of various platelet agonists in inducing apoptosis have not yet been investigated. In the present study we explored this issue, and attempted to identify the correlation between platelet activation and apoptosis. In a platelet aggregation study, there were no significant differences respectively stimulated by arachidonic acid (AA; 100 microM), ADP (20 microM), collagen (10 microg/mL), thrombin (0.1 U/mL), U46619 (10 microM), and A23187 (5 microM). In a subsequent study, we fixed these concentrations of agonists to further compare their relative activities in inducing platelet apoptosis. Our results found that thrombin, U46619, and A23187 possess stronger activities than the other agonists in inducing platelet apoptosis (i.e., phosphatidylserine exposure, mitochondrial membrane potential depolarization, eukaryotic initiation factor (eIF)2alpha, and caspase activation). On the other hand, AA induced no apoptotic events in platelets. Based on this approach, we demonstrated for the first time that thrombin, U46619, and A23187, but not AA, possess stronger activity in inducing platelet apoptosis. In addition, we also found that platelet activation might not necessarily be associated with the occurrence of platelet apoptosis. The in vivo physiological function of the apoptotic machinery in platelets is not yet clearly understood, and needs to be further investigated in the future.

Inhibitory mechanisms of activated matrix metalloproteinase-9 on platelet activation

The intracellular mechanisms underlying the signaling pathways of activated matrix metalloproteinase-9 (MMP-9) in platelets are not yet completely understood. Therefore, the aim of this study was to further examine the effects of activated MMP-9 in preventing platelet aggregation. In this study, activated MMP-9 time-dependently (3-60 min) inhibited platelet aggregation in washed human platelet suspensions stimulated by agonists. However, activated MMP-9 had no significant effect on the binding of FITC-triflavin to the platelet glycoprotein IIb/IIIa complex. Triflavin is a specific antagonist of the glycoprotein IIb/IIIa complex purified from snake venom. Moreover, activated MMP-9 (21 and 90 ng/ml) markedly decreased the fluorescence intensity of platelet membranes tagged with diphenylhexatriene. The thrombin-evoked increase in pHi was inhibited in the presence of activated MMP-9 (21 and 90 ng/ml). In addition, activated MMP-9 (21 and 90 ng/ml) markedly reduced the electron spin resonance (ESR) signal intensity of hydroxyl radicals in collagen (1 mug/ml)-activated platelets. These results indicate that the antiplatelet activity of activated MMP-9 may involve the following pathways: (1) activated MMP-9 may initially induce conformational changes in platelet membranes and hydroxyl radical formation, leading to inhibition of platelet aggregation; and (2) activated MMP-9 also inhibits the Na(+)/H(+) exchanger, leading to reduced intracellular Ca(2+) mobilization, and ultimately to inhibition of platelet aggregation. This study further provides new insights concerning the effects of activated MMP-9 on platelet aggregation.

Inhibitory effects of lycopene on in vitro platelet activation and in vivo prevention of thrombus formation

Lycopene is a natural carotenoid antioxidant that is present in tomatoes and tomato products. The pharmacologic function of lycopene in platelets is not yet understood. Therefore, in this study we sought to systematically examine the effects of lycopene in the prevention of platelet aggregation and thrombus formation. We found that lycopene concentration-dependently (2-12 micromol/L) inhibited platelet aggregation in human platelets stimulated by agonists. Lycopene (6 and 12 micromol/L) inhibited phosphoinositide breakdown in platelets labeled with tritiated inositol, intracellular Ca+2 mobilization in Fura-2 AM-loaded platelets, and thromboxane B2 formation stimulated by collagen. In addition, lycopene (6 and 12 micromol/L) significantly increased the formations of cyclic GMP and nitrate but not cyclic AMP in human platelets. Rapid phosphorylation of a protein of 47,000 Da (P47), a marker of protein kinase C activation, was triggered by PDBu (60 nmol/L). This phosphorylation was markedly inhibited by lycopene (12 micromol/L) in phosphorus-32-labeled platelets. In an in vivo study, thrombus formation was induced by irradiation of mesenteric venules in mice pretreated with fluorescein sodium. Lycopene (5, 10, and 20 mg/kg) significantly prolonged the latency period for the induction of platelet-plug formation in mesenteric venules. These results indicate that the antiplatelet activity of lycopene may involve the following pathways: (1) Lycopene may inhibit the activation of phospholipase C, followed by inhibition of phosphoinositide breakdown and thromboxane B2 formation, thereby leading to inhibition of intracellular Ca+2 mobilization. (2) Lycopene also activated the formations of cyclic GMP/nitrate in human platelets, resulting in the inhibition of platelet aggregation. The results may imply that tomato-based foods are especially beneficial in the prevention of platelet aggregation and thrombosis.

Protective mechanisms of inosine in platelet activation and cerebral ischemic damage

OBJECTIVE

Inosine is a naturally occurring nucleoside degraded from adenosine. Recent studies have demonstrated that inosine has potent immunomodulatory and neuroprotective effects. In the present study, we further investigated the inhibitory effects of inosine on platelet activation in vitro and in vivo, as well as in attenuating middle cerebral artery occlusion (MCAO)-induced focal cerebral ischemia in rats.

METHODS AND RESULTS

Inosine concentration-dependently (0.5 to 6.0 mmol/L) inhibited platelet aggregation stimulated by agonists. Inosine (1.5 and 3.0 mmol/L) inhibited phosphoinositide breakdown, [Ca+2]i, and TxA2 formation in human platelets stimulated by collagen (1 microg/mL). In addition, inosine (1.5 and 3.0 mmol/L) markedly increased levels of cyclic guanylate monophosphate (GMP) and cyclic GMP-induced vasodilator-stimulated phosphoprotein Ser157 phosphorylation. Rapid phosphorylation of a platelet protein of molecular weight 47,000 (P47), a marker of protein kinase C activation, was triggered by collagen (1 microg/mL). This phosphorylation was markedly inhibited by inosine (3.0 mmol/L). Inosine (1.5 and 3.0 mmol/L) markedly reduced hydroxyl radical in collagen (1 microg/mL)-activated platelets. In in vivo studies, inosine (400 mg/kg) significantly prolonged the latency period of inducing platelet plug formation in mesenteric venules of mice, and administration of 2 doses (100 mg/kg) or a single dose (150 mg/kg) of inosine significantly attenuated MCAO-induced focal cerebral ischemia in rats.

CONCLUSIONS

Platelet aggregation contributes significantly to MCAO-induced focal cerebral ischemia. The most important findings of this study suggest that inosine markedly inhibited platelet activation in vitro and in vivo, as well as cerebral ischemia. Thus, inosine treatment may represent a novel approach to lowering the risk of or improving function in thromboembolic-related disorders and ischemia-reperfusion brain injury.

Low concentration of oxidized low density lipoprotein suppresses platelet reactivity in vitro: an intracellular study

The intracellular mechanisms underlying oxidized low density lipoprotein (oxLDL)-signaling pathways in platelets remain obscure and findings have been controversial. Therefore, we examined the influence of oxLDL in washed human platelets. In this study, oxLDL concentration-dependently (20-100 microg/mL) inhibited platelet aggregation in human platelets stimulated by collagen (1 microg/mL) and arachidonic acid (60 microM), but not by thrombin (0.02 U/mL). The activity of oxLDL was greater at 24 h in inhibiting platelet aggregation than at 12 h. At 24 h, oxLDL concentration-dependently inhibited intracellular Ca2+ mobilization and thromboxane B2 formation in human platelets stimulated by collagen. In addition, at 24 h oxLDL (40 and 80 microg/mL) significantly increased the formation of cyclic AMP, but not cyclic GMP or nitrate. In an ESR study, 24 h-oxLDL (40 microg/mL) markedly reduced the ESR signal intensity of hydroxyl radicals (OH(-)) in both collagen (2 microg/mL)-activated platelets and Fenton reaction (H2O2 + Fe2+). The inhibitory effect of oxLDL may induce radical-radical termination reactions by oxLDL-derived lipid radical interactions with free radicals (such as hydroxyl radicals) released from activated platelets, with a resultant lowering of intracellular Ca2+ mobilization, followed by inhibition of thromboxane A2 formation, thereby leading to increased cyclic AMP formation and finally inhibited platelet aggregation. This study provides new insights concerning the effect of oxLDL in platelet aggregation.

Expression of matrix metalloproteinase-9 in human platelets: regulation of platelet activation in in vitro and in vivo studies

1. The aim of this study was to identify the presence of matrix metalloproteinase-9 (MMP-9) in human platelets and systematically examine its inhibitory mechanisms of platelet activation. 2. In this study, we report on an efficient method for the quantitative analysis of pro-MMP-9 in human platelets using capillary zone electrophoresis (CZE). To elucidate subcellular localization of MMP-9 in human platelets, we investigated intraplatelet MMP-9 by immunogold labeling and visualized it using electron microscopy. In an in vivo thrombotic study, platelet thrombus formation was induced by irradiation of mesenteric venules with filtered light in mice pretreated with fluorescein sodium. 3. MMP-9-gold labeling was observed on the plasma membrane, alpha-granules, open canalicular system, and within the cytoplasma both in resting and activated platelets. Furthermore, activated MMP-9 concentration-dependently (15-90 ng ml(-1)) inhibited platelet aggregation stimulated by agonists. Activated MMP-9 (21 and 90 ng ml(-1)) inhibited phosphoinositide breakdown, intracellular Ca(2+) mobilization, and thromboxane A(2) formation in human platelets stimulated by collagen (1 microg ml(-1)). In addition, activated MMP-9 (21 and 90 ng ml(-1)) significantly increased the formation of nitric oxide/cyclic GMP. 4. Rapid phosphorylation of a platelet protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by phorbol-12, 13-dibutyrate (PDBu) (60 nm). This phosphorylation was markedly inhibited by activated MMP-9 (21 and 90 ng ml(-1)). Activated MMP-9 (1 microg g(-1)) significantly prolonged the latency period of inducing platelet plug formation in mesenteric venules. 5. These results indicate that the antiplatelet activity of activated MMP-9 may be involved in the following pathways. (1) Activated MMP-9 may inhibit the activation of phospholipase C, followed by inhibition of phosphoinositide breakdown, protein kinase C activation, and thromboxane A(2) formation, thereby leading to inhibition of intracellular Ca(2+) mobilization. (2) Activated MMP-9 also activated the formation of nitric oxide/cyclic GMP, resulting in inhibition of platelet aggregation. These results strongly indicate that MMP-9 is a potent inhibitor of aggregation. It may play an important role as a negative feedback regulator during platelet activation.

Inhibitory mechanisms of YC-1 and PMC in the induction of iNOS expression by lipoteichoic acid in RAW 264.7 macrophages

In the present study, the signal pathways involved in NO formation and iNOS expression in RAW 264.7 macrophages stimulated by LTA were investigated. We also compared the relative inhibitory activities and mechanisms of PMC, a novel potent antioxidant of alpha-tocopherol derivatives, with those of YC-1, an sGC activator, on the induction of iNOS expression by LTA in cultured macrophages in vitro and LTA-induced hypotension in vivo. LTA induced concentration (0.1-50 microg/mL)- and time (4-24 hr)-dependent increases in nitrite (an indicator of NO biosynthesis) in macrophages. Both PMC (50 microM) and YC-1 (10 microM) inhibited NO production, iNOS protein, mRNA expression, and IkappaBalpha degradation upon stimulation by LTA (20 microg/mL) in macrophages. On the other hand, PMC (50 microM) almost completely suppressed JNK/SAPK activation, whereas YC-1 (10 microM) only partially inhibited its activation in LTA-stimulated macrophages. Moreover, PMC (10 mg/kg, i.v.) and YC-1 (5 mg/kg, i.v.) significantly inhibited the fall in MAP stimulated by LTA (10 mg/kg, i.v.) in rats. In conclusion, we demonstrate that YC-1 shows more-potent activity than PMC at abrogating the expression of iNOS in macrophages in vitro and reversing delayed hypotension in rats with endotoxic shock stimulated by LTA. The inhibitory mechanisms of PMC may be due to its antioxidative properties, with a resulting influence on JNK/SAPK and NF-kappaB activations. YC-1 may be mediated by increasing cyclic GMP, followed by, at least partly, inhibition of JNK/SAPK and NF-kappaB activations, thereby leading to inhibition of iNOS expression.

Inhibitory mechanisms of metallothionein on platelet aggregation in in vitro and platelet plug formation in in vivo experiments

Metallothionein (MT) is a low-molecular-weight, cysteine-rich protein that contains heavy metals such as cadmium and zinc. The biological function of MT in platelets is not yet understood. Therefore, the aim of this study was to systematically examine the inhibitory mechanisms of metallothionein in platelet aggregation. In this study, metallothionein concentration-dependently (1-8 microM) inhibited platelet aggregation in human platelets stimulated by agonists. Metallothionein (4 and 8 microM) inhibited phosphoinositide breakdown in [3H]-inositol-labeled platelets, intracellular Ca+2 mobilization in Fura-2 AM-loaded platelets, and thromboxane A2 formation stimulated by collagen. In addition, metallothionein (4 and 8 microM) significantly increased the formation of cyclic GMP but not cyclic AMP in human platelets. Rapid phosphorylation of a protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by PDBu (100 nM). This phosphorylation was markedly inhibited by metallothionein (4 and 8 microM) in phosphorus-32-labeled platelets. In an in vivo thrombotic study, platelet thrombus formation was induced by irradiation of mesenteric venules in mice pretreated with fluorescein sodium. Metallothionein (6 microg/g) significantly prolonged the latency period for inducing platelet plug formation in mesenteric venules. These results indicate that the antiplatelet activity of metallothionein may involve the following pathways: (1) metallothionein may inhibit the activation of phospholipase C, followed by inhibition of phosphoinositide breakdown and thromboxane A2 formation, thereby leading to inhibition of intracellular Ca+2 mobilization; (ii) Metallothionein also activated the formation of cyclic GMP in human platelets, resulting in inhibition of platelet aggregation. The results strongly indicate that metallothionein provides protection against thromboembolism.

Involvement of the antiplatelet activity of magnesium sulfate in suppression of protein kinase C and the Na+/H+ exchanger

Magnesium sulfate is widely used to prevent seizures in pregnant women with hypertension. The aim of this study was to examine the inhibitory mechanisms of magnesium sulfate in platelet aggregation in vitro. In this study, magnesium sulfate concentration-dependently (0.6-3.0 mM) inhibited platelet aggregation in human platelets stimulated by agonists. Magnesium sulfate (1.5 and 3.0 mM) also concentration-dependently inhibited phosphoinositide breakdown and intracellular Ca+2 mobilization in human platelets stimulated by thrombin. Rapid phosphorylation of a platelet protein of M(r) 47,000 (P47), a marker of protein kinase C activation, was triggered by phorbol-12-13-dibutyrate (PDBu, 50 nM). This phosphorylation was markedly inhibited by magnesium sulfate (3.0 mM). Magnesium sulfate (1.5 and 3.0 mM) further inhibited PDBu-stimulated platelet aggregation in human platelets. The thrombin-evoked increase in pHi was markedly inhibited in the presence of magnesium sulfate (3.0 mM). In conclusion, these results indicate that the antiplatelet activity of magnesium sulfate may be involved in the following two pathways: (1) Magnesium sulfate may inhibit the activation of protein kinase C, followed by inhibition of phosphoinositide breakdown and intracellular Ca+2 mobilization, thereby leading to inhibition of the phosphorylation of P47. (2) On the other hand, magnesium sulfate inhibits the Na+/H+ exchanger, leading to reduced intracellular Ca+2 mobilization, and ultimately to inhibition of platelet aggregation and the ATP-release reaction.

Morphine-potentiated platelet aggregation in in vitro and platelet plug formation in in vivo experiments

The detailed mechanisms underlying morphine-signaling pathways in platelets remain obscure. Therefore, we systematically examined the influence of morphine on washed human platelets. In this study, washed human platelet suspensions were used for in vitro studies. Furthermore, platelet thrombus formation induced by irradiation of mesenteric venules with filtered light in mice pretreated with fluorescein sodium was used for an in vivo thrombotic study. Morphine concentration dependently (0.6, 1, and 5 microM) potentiated platelet aggregation and the ATP release reaction stimulated by agonists (i.e., collagen and U46619) in washed human platelets. Yohimbine (0.1 microM), a specific alpha(2)-adrenoceptor antagonist, markedly abolished the potentiation of morphine in platelet aggregation stimulated by agonists. Morphine also potentiated phosphoinositide breakdown and intracellular Ca(2+) mobilization in human platelets stimulated by collagen (1 microg/ml). Moreover, morphine (0.6-5 microM) markedly inhibited prostaglandin E(1) (10 microM)-induced cyclic AMP formation in human platelets, while yohimbine (0.1 microM) significantly reversed the inhibition of cyclic AMP by morphine (0.6 and 1 microM) in this study. The thrombin-evoked increase in pH(i) was markedly potentiated in the presence of morphine (1 and 5 microM). Morphine (2 and 5 mg/g) significantly shortened the time require to induce platelet plug formation in mesenteric venules. We concluded that morphine may exert its potentiation in platelet aggregation by binding to alpha(2)-adrenoceptors in human platelets, with a resulting inhibition of adenylate cyclase, thereby reducing intracellular cyclic AMP formation followed by increased activation of phospholipase C and the Na(+)/H(+) exchanger. This leads to increased intracellular Ca(2+) mobilization, and finally potentiation of platelet aggregation and of the ATP release reaction.

Morphine-potentiated agonist-induced platelet aggregation through alpha2-adrenoceptors in human platelets

Morphine dose-dependently (0.6, 1, and 5 microM) potentiated platelet aggregation and ATP release stimulated by agonists (i.e., collagen and U46619) in washed human platelets. Furthermore, morphine (1 and 5 microM) markedly potentiated collagen (1 microg/ml) evoked an increase of intracellular Ca2+ mobilization in fura 2-AM loading human platelets. Morphine (1 and 5 microM) did not influence the binding of fluorescein isothiocyanate-triflavin to platelet glycoprotein IIb/IIIa complex. Yohimbine (0.1 microM), a specific alpha2-adrenoceptor antagonist, markedly abolished the potentiation of morphine in platelet aggregation stimulated by collagen. Moreover, morphine (0.6-5 microM) markedly inhibited prostaglandin E1 (10 microM)-induced cAMP formation in human platelets, and yohimbine (0.1 microM) significantly reversed the inhibition of cAMP by morphine (0.6 and 1 microM) in this study. Morphine (1 and 5 microM) significantly potentiated thromboxane B2 formation stimulated by collagen in human platelets, and yohimbine also reversed this effect of morphine in this study. In addition, morphine (1 and 5 microM) did not significantly affect nitrate production in human platelets. Morphine may exert its potentiation in platelet aggregation by binding to alpha2-adrenoceptors in human platelets, which leads to reduced cAMP formation and subsequently to increased intracellular Ca2+ mobilization; this, in turn, is followed by increased thromboxane A formation and finally potentiates platelet aggregation and ATP release.

Antithrombotic effect of PMC, a potent alpha-tocopherol analogue on platelet plug formation in vivo

Platelet thrombi formation was induced by irradiation of mesenteric venules with filtered light in mice pretreated intravenously with fluorescein sodium. PMC (2, 2, 5, 7, 8-pentamethyl-6-hydroxychromane; 20 microg/g, i.v.) significantly prolonged the latent period of inducing platelet plug formation in mesenteric venules. When fluorescein sodium was given at 10 microg/kg, PMC (20 microg/g) delayed occlusion time by about 1.7-fold. Furthermore, aspirin (250 microg/g) also showed similar activity in delaying the occlusion time. On a molar basis, PMC was about 14-fold more potent than aspirin at delaying the occlusion time. PMC was also effective in reducing the mortality of ADP-induced acute pulmonary thromboembolism in mice when administered intravenously at doses of 5 and 10 microg/g. In addition, intravenous injection of PMC (5 microg/g) significantly prolonged bleeding time by about 1.6-fold compared with normal saline in severed mesenteric arteries of rats. Continuous infusion of PMC (1 microg/g/min) significantly increased the bleeding time by about 1.6-fold and the bleeding time was also significantly prolonged for up to 90 min after cessation of PMC infusion. These results suggest that PMC has an effective antiplatelet effect in vivo and may be a potential therapeutic agent for arterial thrombosis, but must be assessed further for toxicity.

Mechanisms involved in agonist-induced hyperaggregability of platelets from normal pregnancy

There is substantial evidence of increased platelet reactivity in vivo and in vitro during pregnancy. Platelet activation occurs in pregnancy with a risk of the development of preeclampsia. In this study, platelet behavior was studied during 28-40 weeks of gestation in a group of women who remained normotensive and a group of nonpregnant female controls. Platelet aggregation and ATP release stimulated by agonists (i.e. collagen and adenosine 5'-diphosphate) were markedly enhanced in washed platelets from pregnant subjects. Furthermore, the collagen-evoked increase in intracellular Ca(2+) ([Ca(2+)](i)) mobilization in fura-2-AM-loaded platelets was also enhanced in pregnant subjects. Moreover, the binding activity of fluorescein isothiocyanate-triflavin toward the platelet glycoprotein IIb/IIIa complex did not significantly differ between the nonpregnant and pregnant groups. In addition, the amount of thromboxane A(2) (TxA(2)) formation from pregnant subjects was significantly greater than that from nonpregnant subjects in both resting and collagen-activated platelets. On the other hand, prostaglandin E(2) formation in the presence of imidazole in either resting or arachidonic acid (100 microM)-treated platelets did not significantly differ between these two groups. The levels of cyclic AMP formation in both resting and prostaglandin E(1) (10 microM)-treated platelets from pregnant subjects were significantly lower than those in nonpregnant subjects. Nitric oxide production was measured by a chemiluminescence detection method in this study. The extent of nitrate production in either resting or collagen-stimulated platelets from pregnant subjects did not significantly differ from that of platelets from the nonpregnant group. We conclude that the agonist-induced hyperaggregability of platelets from normal pregnancy may be due, at least partly, to an increase in TxA(2) formation and a lowering of the level of cyclic AMP formation, which leads to increased [Ca(2+)](i) mobilization and finally to enhanced platelet aggregation and ATP release.

The protective effects of PMC against chronic carbon tetrachloride-induced hepatotoxicity in vivo

In this study, PMC (2,2,5,7,8-pentamethyl-6-hydroxychromane), a derivative of alpha-tocopherol, dose-dependently (1-10 mg/kg) ameliorated the increase in plasma aspartate aminotransferase (GOT) and alanine aminotransferase (GPT) levels caused by chronic repeated carbon tetrachloride (CCl4) intoxication in mice. Moreover, PMC significantly improved the CCl4-induced increase of hepatic glutathione peroxidase, reductase, and superoxide dismutase activities. PMC also restored the decrement in the glutathione content of hepatic tissues in CCl4-intoxicated mice. Furthermore, it also dose-dependently inhibited the formation of lipid peroxidative products during carbon tetrachloride treatment. Histopathological changes of hepatic lesions induced by carbon tetrachloride were significantly improved by treatment with PMC in a dose-dependent manner. These results suggest that PMC exerts effective protection in chronic chemical-induced hepatic injury in vivo.