The protective effects of paclitaxel on platelet aggregation through the inhibition of thromboxane A<Subscript>2</Subscript> synthase

Paclitaxel is an anticancer drug used in the treatment of ovarian, breast, head and neck, lung, and prostate cancer. We investigated the anti-platelet activity of paclitaxel in vitro as well as a possible anti-platelet mechanism. Paclitaxel inhibited washed rabbit platelet aggregation induced by collagen in a concentration-dependent manner, with an IC₅₀ of 59.7 ± 3.5. However, it had little effect on platelet aggregation mediated by arachidonic acid, U46619, a thromboxane (TX) A₂ mimic, or thrombin, suggesting that paclitaxel may strongly inhibit collagenmediated signal transduction. In accordance with these findings, paclitaxel blocked collageninduced cytosolic calcium mobilization, arachidonic acid liberation, and serotonin secretion. In addition, it inhibited arachidonic acid-mediated platelet aggregation by about 37% by interfering with TXA₂ synthase as measured by the formation of arachidonic acid-mediated TXA₂ and prostaglandin D₂, as well as cyclooxygenase-1 and TXA₂ synthase activity assays. Taken together, these results point to a cellular mechanism for the anti-platelet activity of paclitaxel through the inhibition of TXA₂ synthase and cytosolic calcium mobilization. This may contribute to the beneficial effects of paclitaxel on the cardiovascular system.     

Antiplatelet activity of carnosic acid, a phenolic diterpene from Rosmarinus officinalis

Carnosic acid is a major phenolic diterpene derived from Rosmarinus officinalis and has been reported to have antioxidant, antibacterial, anticancer, antiobese and photoprotective activities. This study investigated the antiplatelet activity of carnosic acid. carnosic acid significantly inhibited collagen-, arachidonic acid-, U46619- and thrombin-induced washed rabbit platelet aggregation in a concentration-dependent manner, with IC50 values of 39+/-0.3, 34+/-1.8, 29+/-0.8 and 48+/-2.9 microM, respectively, while it failed to inhibit PMA-(a direct PKC activator) and ADP-induced platelet aggregation. In agreement with its antiplatelet activity, carnosic acid blocked collagen-, arachidonic acid-, U46619- and thrombin-mediated cytosolic calcium mobilization. accordingly, serotonin secretion and arachidonic acid liberation were also inhibited in a similar concentration-dependent manner. However, in contrast to the inhibition of arachidonic acid-induced platelet aggregation, carnosic acid had no effect on the formation of arachidonic acid-mediated thromboxane A2 and prostaglandin D2, thus indicating that carnosic acid has no effect on the cyclooxygenase and thromboxane A2 synthase activity. Overall, these results suggest that the antiplatelet activity of carnosic acid is mediated by the inhibition of cytosolic calcium mobilization and that carnosic acid has the potential of being developed as a novel antiplatelet agent.     

Antiplatelet activity of [5-(2-methoxy-5-chlorophenyl)furan-2-ylcarbonyl]guanidine (KR-32570), a novel sodium/hydrogen exchanger-1 and its mechanism of action

The antiplatelet effects of a novel guanidine derivative, KR-32570 ([5-(2-methoxy-5-chlorophenyl) furan-2-ylcarbonyl]guanidine), were investigated with an emphasis on the mechanisms underlying its inhibition of collagen-induced platelet aggregation. KR-32570 significantly inhibited the aggregation of washed rabbit platelets induced by collagen (10 microg/mL), thrombin (0.05 U/mL), arachidonic acid (100 microM), a thromboxane (TX) A2 mimetic agent U46619 (9,11-dideoxy-9,11-methanoepoxy-prostaglandin F2, 1 microM) and a Ca2+ ATPase inhibitor thapsigargin (0.5 microM) (IC50 values: 13.8 +/- 1.8, 26.3 +/- 1.2, 8.5 +/- 0.9, 4.3 +/- 1.7 and 49.8 +/- 1.4 microM, respectively). KR-32570 inhibited the collagen-induced liberation of [3H]arachidonic acid from the platelets in a concentration dependent manner with complete inhibition being observed at 50 microM. The TXA2 synthase assay showed that KR-32570 also inhibited the conversion of the substrate PGH2 to TXB2 at all concentrations. Furthermore, KR-32570 significantly inhibited the [Ca2+]i mobilization induced by collagen at 50 microM, which is the concentration that completely inhibits platelet aggregation. KR-32570 also decreased the level of collagen (10 microg/mL)-induced secretion of serotonin from the dense-granule contents of platelets, and inhibited the NHE-1-mediated rabbit platelet swelling induced by intracellular acidification. These results suggest that the antiplatelet activity of KR-32570 against collagen-induced platelet aggregation is mediated mainly by inhibiting the release of arachidonic acid, TXA2 synthase, the mobilization of cytosolic Ca2+ and NHE-1.     

Antiplatelet activity of β-carboline alkaloids from Perganum harmala: A possible mechanism through inhibiting PLCγ2 phosphorylation

Beta-carboline alkaloids including harmalol, harmaline, norharmane, harmol, harmine and harmane are important constituents of the medicinal plant, Perganum harmala L. (Zygophylaceae), which has been used in traditional medicine. In the present study, the antiplatelet activities of six β-carboline alkaloid compounds were investigated in vitro. At a concentration of 200 μM, these compounds have no effect on arachidonic acid (AA)-, thrombin- and U46619 (a thromboxane A₂ mimic)-stimulated platelet aggregation. On the contrary, it was revealed that collagen-induced platelet aggregation could be inhibited by these compounds with different potencies (harmane and harmine were most potent, harmol had medium potency, and harmol, norharmane, harmalol and harmaline had a weak, non significant effect), indicating a selective inhibition on collagen-mediated platelet activation. Consistently, further study revealed that collagen-mediated phospholipase (PL) Cγ2 and protein tyrosine phosphorylation, cytosolic calcium mobilization and arachidonic acid liberation were completely inhibited by harmane and harmine in a concentration-dependent manner, while the other compounds were only partially or not effective at all. Taken together, these results indicate that three of these six β-carboline alkaloids can selectively affect collagen-induced platelet aggregation with different potencies; in particular, harmane and harmine were most potent, and their antiplatelet activities may be mediated by inhibiting PLCγ2 and protein tyrosine phosphorylation with sequential suppression of cytosolic calcium mobilization and arachidonic acid liberation, indicating that harmane and harmine have a potential to be developed as a novel agent for atherothrombotic diseases.     

Antiplatelet activity of epigallocatechin gallate is mediated by the inhibition of PLCgamma2 phosphorylation, elevation of PGD2 production, and maintaining calcium-ATPase activity

We have previously reported that green tea catechins displayed a potent antithrombotic effect by inhibition of platelet aggregation. In the present study, the antiplatelet and antithrombotic activities of epigallocatechin gallate (EGCG), the major catechin derived from green tea, were extensively investigated. EGCG inhibited arterial thrombus formation and U46619-, collagen-, and arachidonic acid (AA)-induced washed rabbit platelet aggregation in a concentration-dependent manner, with IC50 values of 61 +/- 3, 85 +/- 4, and 99 +/- 4 microM, respectively. In line with the inhibition of collagen-induced platelet aggregation, EGCG revealed blocking of the collagen-mediated phospholipase (PL) Cgamma2 and protein tyrosine phosphorylation, and it caused concentration-dependent decreases of cytosolic calcium mobilization, AA liberation, and serotonin secretion. In addition, the platelet aggregation, intracellular Ca2+ mobilization, and protein tyrosine phosphorylation induced by thapsigargin, a Ca2(+)-ATPase pump inhibitor, were completely blocked by EGCG. Contrary to the inhibition of AA-induced platelet aggregation, EGCG failed to inhibit cyclooxygenase and thromboxane (TX) A2 synthase activities, but it concentration-dependently elevated AA-mediated PGD2 formation. In contrast, epigallocatechin (EGC), a structural analogue of EGCG lacking a galloyl group in the 3' position, slightly inhibited collagen-stimulated cytosolic calcium mobilization, but failed to affect other signal transductions as did EGCG in activated platelets and arterial thrombus formation. These results suggest that antiplatelet activity of EGCG may be attributable to its modulation of multiple cellular targets, such as inhibitions of PLCgamma2, protein tyrosine phosphorylation and AA liberation, and elevation of cellular PGD2 levels, as well as maintaining Ca2(+)-ATPase activity, which may underlie its beneficial effect on the atherothrombotic diseases.     

Antiplatelet activity of carnosol is mediated by the inhibition of TXA2 receptor and cytosolic calcium mobilization

Carnosol, a naturally occurring phenolic diterpene found in rosemary, has been reported to exhibit antioxidant, anticancer and hepatoprotective effects. In the present study, the antiplatelet activity of carnosol was investigated. Carnosol concentration-dependently inhibited washed rabbit platelet aggregation induced by collagen and arachidonic acid (AA), with IC(50) values of 5.5+/-0.3 and 42.5+/-0.9 microM, respectively, while failed to inhibit that induced by, ADP and thrombin. Consist with inhibition of collagen-induced platelet aggregation, carnosol revealed blocking of collagen-mediated cytosolic calcium mobilization, serotonin secretion and arachidonic acid liberation. However, contrary to the inhibition of AA-induced platelet aggregation, carnosol has no effect on AA-mediated TXA(2) and PGD(2) formation, indicating carnosol may directly inhibit TXA(2) receptor, which was supported by the finding that carnosol potently inhibited U46619 (a TXA(2) mimic)-induced platelet aggregation, with an IC(50) value of 22.0+/-2.5 microM. In addition, the U46619-induced concentration-response curve was downward shifted by the application of carnosol at concentrations of 22 and 50 microM, indicating a typical non-competitive antagonism on TXA(2) receptor. Taken together, these results suggest that antiplatelet activity of carnosol may be mediated by the inhibition of TXA(2) receptor and cytosolic calcium mobilization, and carnosol has a potential to be developed as a novel-antiplatelet agent.     

Anti-platelet activity of KR-32560, a novel sodium/hydrogen exchanger-1 inhibitor.

We investigated the anti-platelet effect of a newly synthesized guanidine derivative KR-32560, a sodium/hydrogen exchanger-1 (NHE-1) inhibitor, together with the elucidation of the possible mode of action. KR-32560 concentration dependently inhibited the aggregation of washed rabbit platelets induced by collagen (10 microg mL(-1)) and arachidonic acid (AA; 100 microM), with IC50 values of 25 and 46 microM, respectively. Whereas, KR-32560 showed weaker potency against aggregation induced by thrombin (0.05 UmL(-1)) and U46619 (1 microM), and had no effect on thapsigargin (0.5 microM)- or A23187 (5 microM)-induced platelet aggregation up to 50 microM. KR-32560 inhibited the collagen-induced [3H]AA liberation in a concentration-dependent manner. In addition, KR-32560 significantly suppressed TXB2 formation in AA-exposed platelets, but had no effect on production of PGD2, indicating an inhibitory effect on TXA2 synthase. This finding was supported by a TXA2 synthase assay that KR-32560 inhibited the conversion of PGH2 into TXB2 with a similar magnitude to suppression of TXB2 formation. Furthermore, KR-32560 significantly inhibited the collagen-induced [Ca2+]i mobilization and serotonin secretion. Taken together, these observations suggest that the anti-platelet activity of KR-32560 may be mediated by the inhibition of cytoplasmic Ca2+ mobilization and AA liberation.     

In vitro metabolism of obovatol and its effect on cytochrome P450 enzyme activities in human liver microsomes

Obovatol, a major constituent of the leaves of Magnolia obovata Thunb, is known to inhibit nuclear factor‐κB activity and arachidonic acid‐induced platelet aggregation. This study was performed to identify the metabolites of obovatol in human liver microsomes. Human liver microsomes incubated with obovatol in the presence of NADPH and/or UDPGA resulted in the formation of six metabolites, M1–M6. M1 and M2 were identified as hydroxyobovatol, on the basis of liquid chromatography/tandem mass spectrometric (LC‐MS/MS) analysis. M1, M2 and obovatol were further metabolized to their glucuronide conjugates, obovatol‐glucuronide (M3), obovatol‐diglucuronide (M4) and hydroxyobovatol‐glucuronide (M5 and M6). The inhibitory potency of obovatol on eight major human P450s was also investigated in human liver microsomes. In these experiments, obovatol strongly inhibited CYP2C19‐mediated S‐mephenytoin hydroxylase activity with an IC₅₀ value of 0.8 µm , which could have implications for drug–drug interactions. Copyright © 2013 John Wiley & Sons, Ltd.     

15,16-Dihydrotanshinone I, a major component from Salvia miltiorrhiza Bunge (Dansham), inhibits rabbit platelet aggregation by suppressing intracellular calcium mobilization

Radix Salviae miltiorrhiza (RSM, ‘Dansham’ in Korea, ‘Danshen’ in Chinese), the root of Salviae miltiorrhiza Bunge (Labiate) has been used as Chinese fork medicine for the treatment of cardiovascular diseases such as angina pectoris, coronary heart disease, myocardial infarction, and hypertension. In the present study, we evaluated the inhibitory effects of 15, 16-Dihydrotanshinone I, one of the major ingredients of Salvia miltiorrhiza Bunge, on platelet aggregation, with elucidation of its mechanisms of action. 15,16-Dihydrotanshinone I concentration-dependently inhibited collagen-induced aggregation of rabbit washed platelets with IC₅₀ of 8.7±5.6 μM, the potency being about seven-fold greater than EGCG, an active Green tea catechin component (IC₅₀: 56.6±48.7 μM). 15,16-Dihydrotanshinone I significantly inhibited the intracellular calcium ([Ca²⁺]_i) mobilization in a concentration-dependent manner. 15,16-dihdydrotanshinone I also significantly suppressed collagen (50 μg/mL)-induced liberation of [³H]Arachidonic acid from [³H]Arachidonic acid-incorporated rabbit platelet. In addition, 15,16-Dihydrotanshinone I at 50 μM slightly but significantly inhibited collagen-induced production of thromboxane B₂. These results indicate that 15,16-Dihydrotanshinone I exert potent anti-platelet activity via suppression of [Ca²⁺]_i mobilization and arachidonic acid liberation.     

Antiplatelet and antithrombotic effects of the diterpene spiramine Q from Spiraea japonica var. incisa

Spiramine Q, a diterpene, was isolated from a Chinese herbal plant Spiraea japonica var. incisa Yu. Born's and Wan HY's methods were used to investigate effects of spiramine Q on rabbit platelet aggregation and serotonin release, respectively. Its antithrombotic effect in mice was also evaluated by Myers' method. Spiramine Q selectively inhibited arachidonic acid-induced platelet aggregation in vitro or ex vivo, and decreased serotonin secretion from rabbit platelets. Spiramine Q (5 mg/kg) decreased the mouse mortality caused by injection of 80 mg/kg arachidonic acid in the tail vein. The results suggested that spiramine Q showed potent antiplatelet and antithrombotic activites.     

Oleanolic acid,a pentacyclic triterpenoid,induces rabbit platelet aggregation through a phospholipase C-calcium dependent signaling pathway

Oleanolic acid (3beta-hydroxy-olea-12-en-28-oic), a pentacyclic triterpenoid, exists widely in the plant kingdom and has a wide variety of pharmacological effects such as antitumor, antifungal, insecticidal, hepatoprotective and anti-HIV activities. This paper reports that oleanolic acid induces the aggregation of rabbit platelets, a mechanism was also investigated. Oleanolic acid at concentrations of 25, 50, 100 and 200 microM induced the aggregation of washed rabbit platelets in a concentration-dependent manner. Pretreating the platelets with U73122, a phospholipase C (PLC) inhibitor, blocked the oleanolic acid induced-aggregation, whereas acetylsalicylic acid (ASA), a cyclooxygenase (COX) inhibitor, had no effect. In addition, the effect of oleanolic acid on serotonin secretion, which is a marker for dense granule secretion, was determined. Oleanolic acid induced serotonin secretion in a similar concentration-dependent manner as observed with platelet aggregation. Pretreating the platelets with U73122 blocked the oleanolic acid-induced serotonin secretion and cytosolic calcium mobilization. Overall, these results suggest that oleanolic acid can induce platelet aggregation, which may be mediated by the stimulation of PLC-mediated cytosolic calcium mobilization.     

Acetoxy benzene derivatives: in vitro antiaggregant activity

A dozen acetoxy benzene derivatives (mono- or diacetylated diphenols) were tested for inhibitory activity on platelet aggregation in human platelet rich plasma induced with collagen, ADP and arachidonic acid. All the compounds tested showed an activity, and the results emphasize the antiaggregant properties of diacetylated derivatives which are more potent than aspirin. Unlike aspirin, their action is not prevented by salicylate. However, the large inhibition of serotonin release and thromboxane synthesis localizes their activity in arachidonic acid metabolism.     

Effects of clofibrate and 6-substituted chroman analogs on human platelet function:: Mechanism of inhibitory action

The effects of cloflbrate (CPIB) and two related cyclic analogs, 6-chlorochroman-2-carboxylic acid (CCCA) and 6-phenylchroman-2-carboxylic acid (PCCA), on human platelet function were evaluated. CPIB, CCCA and PCCA all inhibited platelet activation, i.e. aggregation and secretion of [¹⁴C]serotonin induced by ADP, epinephrine, collagen and thrombin, in a concentration-dependent manner. PCCA was at least fifty-two times more effective as an inhibitor of ADP-, epinephrine- and collagen-induced platelet activation and only 2-fold more effective as an inhibitor of thrombin-induced platelet activation when compared with CPIB or CCCA. Only PCCA inhibited platelet aggregation and [¹⁴C]serotonin secretion induced by arachidonic acid (AA) in a concentration-dependent manner. CPIB and CCCA did not inhibit AA-induced platelet activation. In fact, both of these agents had a potentiating effect on the onset of platelet aggregation by AA. All three compounds inhibited thrombin-induced release of [³H]arachidonic acid ([³H]AA) from platelet phospholipids and thrombin-mediated malondialdehyde (MDA) production. Only PCCA, however, inhibited AA-induced MDA production. These results indicate that CPIB, CCCA and PCCA all inhibit platelet activation by inhibiting prostaglandin biosynthesis. PCCA blocked AA-induced platelet activation, and this additional inhibitory action of PCCA appears to be responsible for its comparatively higher inhibitory potency. A comparison of the structure-activity relationship of the inhibitors indicated that replacement of the chloro group by a phenyl group produced a compound (PCCA) that was a potent inhibitor of prostaglandin biosynthesis and was thereby a more effective antiaggregatory agent than either CPIB or CCCA.     

Antiplatelet Mechanism of 2‐Chloro‐3‐(4‐hexylphenyl)‐amino‐1,4‐naphthoquinone (NQ304), an Antithrombotic Agent

Abstract: The effects of 2‐chloro‐3‐(4‐hexylphenyl)‐amino‐1,4‐naphthoquinone (NQ304), an antithrombotic agent, on aggregation, binding of fibrinogen to glycoprotein IIb/IIIa and intracellular signals were investigated using human platelets. NQ304 inhibited thrombin‐, arachidonic acid‐ and thapsigargin‐induced aggregation of washed human platelets with the IC₅₀ values of 22.2±0.7, 6.5±0.2, and 7.6±0.1 μM, respectively. NQ304 significantly inhibited fluorescein isothiocyanate‐conjugated fibrinogen binding to human platelet surface glycoprotein IIb/IIIa receptor by 75%, but failed to inhibit the fibrinogen binding to purified glycoprotein IIb/IIIa receptor. This result suggests that NQ304 inhibit platelet aggregation by suppression of an intracellular pathway that involves exposure of the glycoprotein IIb/IIIa receptor, rather than by direct inhibition of fibrinogen‐glycoprotein IIb/IIIa binding. NQ304 significantly inhibited thrombin‐induced increase in intracellular Ca²⁺ mobilization at the dose of 30 μM and ATP secretion in a dose‐dependent manner. It also inhibited thrombin‐ and arachidonic acid‐induced thromboxane A₂ formation in human platelet dose‐dependently. In conclusion, the antiplatelet mechanism of NQ304 may be due to the reduction of the thromboxane A₂ formation, inhibition of adenosine triphosphate release and intracellular calcium mobilization.     

Mechanism of Action of p-Chlorobiphenyl on the Inhibition of Platelet Aggregation

p-Chlorobiphenyl (1–50 μm ) concentration-dependently inhibited the aggregation and release reaction of rabbit washed platelets induced by arachidonic acid and collagen, but not those induced by platelet-activating factor (PAF), U46619 and thrombin. The IC50 values of p-chlorobiphenyl on the arachidonic acid and collagen-induced platelet aggregation were 2.9 ± 0.5 and 12.8 ± 2.3 μm , respectively. The formation of both platelet thromboxane B₂ and prostaglandin D₂ caused by arachidonic acid was inhibited by p-chlorobiphenyl concentration-dependently. In myo-[³H]inositol-labeled and fura-2-loaded platelets, [³H]inositol monophosphate generation and the rise in intracellular Ca²⁺ stimulated by arachidonic acid were inhibited by p-chlorobiphenyl. In human platelet-rich plasma, p-chlorobiphenyl and indomethacin prevented the secondary aggregation and blocked ATP release from platelets induced by adenosine 5′-diphosphate and adrenaline without affecting the primary aggregation. It is concluded that p-chlorobiphenyl may be a cyclo-oxygenase inhibitor and its antiplatelet action is mainly due to the inhibition of thromboxane formation.     

Miconazole inhibition of platelet aggregation by inhibiting cyclooxygenase

Platelet dysfunction was found in rabbits to which a dose of miconazole nitrate (1.6 mg/kg body wt) therapeutic for human subjects had been given intravenously. The present experiments were conducted to elucidate the mechanism of inhibitory effects of miconazole on platelet function. After administration of a single dose of miconazole, rabbit platelet aggregation induced by collagen and sodium arachidonate was inhibited significantly for approximately 24 hr. On the other hand, hypertriglycemia, one of the major side effects of this drug, was not seen during 2 days of observations, nor were any other outstanding manifestations observed. In in vitro experiments, miconazole nitrate (10 μm) also significantly inhibited rabbit and human platelet aggregation (P < 0.01). Biochemical analyses revealed that the stimulant-induced formation of prostaglandin E₂ (PGE₂) and thromboxane B₂ (TXB₂), metabolites via cyclooxygenase, was inhibited by miconazole nitrate in both human and rabbit platelets in vitro. PGE₂ production was decreased dose-dependently with the increase of micronazole concentration (10 to 100 μM), and the decrease was in parallel with a decrease of TXB₂ production. In addition, malondialdehyde (MDA) production of human and rabbit platelets induced by exogenous arachidonate and collagen was also inhibited significantly by miconazole. Chromatographic studies showed that the amount of 12-l-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE), a metabolite via lipoxygenase, was increased markedly in accordance with the miconazole-induced decrease of TXB₂ and 12-l-hydroxy-5,8,10-heptadecatrienoic acid (HHT) formation in both human and rabbit platelets. These results indicate that miconazole nitrate inhibits platelet cyclooxygenase, without affecting the stimulant-induced release of arachidonic acid from platelet phospholipids. Use of this drug in the treatment of sytemic fungal infection appears to be increasing. Careful attention should be paid to the inhibitory effects of miconazole on platelet function, especially in the case of intravenous treatment.     

Cimetidine and ketotifen inhibit stimulated aggregation and histamine liberation from rabbit blood platelets

The effect of cimetidine and ketotifen was studied on thrombin-stimulated rabbit platelet aggregation as well as on histamine and serotonin liberation. Both drugs inhibited the stimulation of platelets in a dose-dependent way. The effect of ketotifen on stimulated aggregation was about twice that of cimetidine. Stimulated histamine and serotonin liberation was more inhibited by ketotifen than by cimetidine. Both drugs inhibited the liberation of serotonin to a greater extent than that of histamine.     

Effects of cryptolepine on collagen-induced aggregation and on the mobilization and metabolism of arachidonic acid by rabbit platelets

1. We examined the effect of cryptolepine on collagen-induced aggregation and on the mobilization, and metabolism of arachidonic acid in rabbit platelets.2. Preincubation of platelets with cryptolepine (50–100 μM) did not affect the primary wave of aggregation but resulted in a dose-dependent, surmountable inhibition of the secondary wave of aggregation induced by collagen (5 μg/ml). The inhibition by cryptolepine was greater when cryptolepine was incubated with the platelets after the peak of the primary wave of aggregation.3. Cryptolepine (50–100 μM) dose-dependently inhibited thrombin (1.5 U/ml) and A23187 (2.5 μM)-induced release of ¹⁴C[AA] from platelet membrane phospholipid pools. The percentage inhibition of A23187-induced ¹⁴C[AA] release was 31.3 ± 4.3% (50 μM) and 79.3 ± 5.4% (100 μM), while thrombin-induced release was inhibited by 39.2 ± 2.4% (50 μM) and 68.2 ± 3.6% (100 μM).4. At near maximal concentration (100 μM) which significantly inhibited secondary aggregatory response and ¹⁴C[AA] release, cryptolepine had no effect on the platelet metabolism of ¹⁴C[AA] to thromboxane B₂, HHT and 12 HETE.5. The present findings suggest that cryptolepine inhibited collagen-induced secondary aggregation through a selective antiphospholipase-like activity. There was not effect on platelet cyclooxygenase and lipoxygenase activities of platelets.     

Inhibitory effect of 8-bromo cyclic GMP on an extracellular Ca2+-dependent arachidonic acid liberation in collagen-stimulated rabbit platelets

The inhibitory effect of cyclic GMP on collagen-induced platelet activation was studied using 8-bromo cyclic GMP (8brcGMP) in washed rabbit platelets. Addition of collagen (1 micrograms/ml) to platelet suspension caused shape change and aggregation associated with thromboxane (TX) A2 formation. 8brcGMP (10-1000 microM) inhibited collagen-induced platelet aggregation and TXA2 formation in a concentration-dependent manner. 8brcGMP did not affect platelet cyclooxygenase pathways, but markedly inhibited collagen-induced arachidonic acid (AA) liberation from membrane phospholipids in [3H]AA-prelabeled platelets, indicating that the inhibitory effect of 8brcGMP on collagen-induced aggregation is due to an inhibition of AA liberation. In [32P]orthophosphate-labeled platelets, collagen stimulated phosphorylation of a 20,000 dalton (20-kD) and 40-kD proteins. 8BrcGMP stimulated phosphorylation of a specific protein having molecular weight of 46-kD and inhibited collagen-induced both 20- and 40-kD protein phosphorylation. Collagen could stimulate the AA liberation without activation of phospholipase C or Na+-H+ exchange, but could not in the absence of extracellular Ca2+. These findings suggest that cyclic GMP inhibits collagen-induced AA liberation which is mediated by an extracellular Ca2+-dependent phospholipase A2. However, cyclic GMP seems to inhibit the Ca2+-activated phospholipase A2 indirectly, since 8brcGMP had no effect on Ca2+ ionophore A23187-induced platelet aggregation or AA liberation. It is therefore suggested that cyclic GMP may regulate collagen-induced increase in an availability of extracellular Ca2+ which is responsible for phospholipase A2 activation in rabbit platelets.     

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.