Present concepts on the mechanisms of platelet aggregation

TXA2-antagonistic properties of agents affecting prostaglandin synthesis or the cyclic nucleotide system in human platelets

Prostaglandins (PG) E1 and E2 as well as 3-isobutyl-1-methylxanthine, nitroprusside, dibutyryl cyclic AMP and N-0164 inhibited platelet aggregation induced by the thromboxane (TX) A2-mimetic prostaglandin endoperoxide analogue U46619. Non-steroidal anti-inflammatory agents - acetylsalicylic acid, indomethacin, tolfenamic acid, flumizole, nictindole and proquazone - did not demonstrate any antagonistic actions on U46619-induced aggregation at concentrations causing inhibition of prostaglandin/thromboxane synthesis-dependent forms of platelet aggregation. Comparing with the effects of the different test substances on ADP-or arachidonic acid-induced platelet aggregation, it can be suggested that PGE2 as well as 3-isobutyl-1-methylxanthine, nitroprusside, and dibutyryl cyclic AMP are functional antagonists and N-0164 is a receptor level antagonist of TXA2 in platelets.

Effects of PGI2 on platelet aggregation and adenylate cyclase activity in human type IIa hypercholesterolemia

The sensitivity to PGI2 of platelets of 20 selected type IIa hypercholesterolemic patients was studied and compared to that of platelets of 14 normocholesterolemic subjects. Type IIa subjects required higher concentrations of PGI2 to inhibit platelet aggregation elicited by 1 microM ADP, 1 microgram/ml collagen and 1.4 microM epinephrine. Adenylate cyclase activity was also measured in washed platelet membranes from the two groups of subjects. Adenylate cyclase activity, both basal and PGI2-stimulated, was not statistically different in the two groups examined. Therefore changes at the level of PGI2 receptors coupled to adenylate cyclase are not likely to be responsible for the different platelet sensitivity to prostacyclin.

The influence of phospholipase A2 and glutathione peroxidase on the elimination of membrane lipid peroxides

The relationship between release of membrane lipid peroxidation products and phospholipase action was examined. Rat liver microsomes and phosphatidylcholine liposome-phospholipase A2 preparations were subjected to iron ascorbate-induced lipid peroxidation. Peroxidation products were characterized by measurement of malondialdehyde and lipid peroxides. Experiments were designed to demonstrate phospholipase dependent removal of peroxidation products origination in the membrane. Increased lysophosphatidylcholine formation was evident following lipid peroxidation in phospholipase A2-containing liposomes which was inhibited by p-bromophenacyl bromide and mepacrine. Lipoxygenase-dependent oxygen consumption, as well as peroxide transfer from microsomes to the incubation medium, was largely dependent on phospholipase and could be diminished by phospholipase inhibitors. Furthermore, lipid hydroperoxides formed by subjecting phosphatidylcholine liposomes to iron ascorbate-induced peroxidation, or those present in aged liposomes, were effectively reduced by glutathione peroxidase when phospholipase A2 was present in the assay. Low level glutathione peroxidase activity was observed in the absence of phospholipase A2.

Platelet-activating factor stimulates phosphatidylinositol turnover in human platelets

Platelet-activating factor stimulates phosphatidylinositol turnover in human platelets as indicated by [32P]phosphatidate accumulation in platelets pre-labelled with [32P]Pi, and by [3H]phosphatidate accumulation and [3H]phosphatidylinositol loss in platelets pre-labelled with [3H]arachidonate. These effects of platelet-activating factor are direct and are independent of the production and/or release of endogenous platelet agonists such as ADP, 5-hydroxytryptamine and thromboxane A2.

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.

Stimulation of calcium uptake by 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor) in rabbit platelets: possible involvement of the lipoxygenase pathway

1-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor) induces an increase of Ca2+ uptake in rabbit platelets. This process depends upon the extracellular concentration of Ca2+ with the maximum stimulation occurring at 1-3 mM; uptake under these conditions is blocked by verapamil, a calcium-entry blocker. Increase of calcium uptake by the bioactive phospholipid was independent of ADP-induced platelet responses and of metabolites of arachidonic acid metabolism formed through the cyclooxygenase pathway. However, mepacrine, p-bromophenacyl bromide, eicosatetraynoic acid, and nordihydroguaiaretic acid significantly or totally inhibited the stimulation of Ca2+ uptake by 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine. When arachidonic acid was given sufficient time to be metabolized to other products by the platelets, stimulation of Ca2+ uptake also occurred. Arachidonic acid and platelet-activating factor did not produce an additive or synergistic effect. Our data suggest that a metabolite(s) generated from arachidonic acid through the lipoxygenase pathway may be the mediator(s) responsible for the action of platelet-activating factor in the induction of increased Ca2+ uptake in rabbit platelets.

Studies on the stimulation of human blood platelets by semi-synthetic platelet-activating factor

"Saturated" and "unsaturated" platelet-activating factor (PAF) obtained from ratfish liver oil were proved to exert potent stimulation on human blood platelets. Using 0.025 to 1.0 mumol/1 PAF a dose-dependent platelet aggregation in platelet-rich plasma was observed. During PAF-induced irreversible aggregation a 9 to 40% release of platelet bound serotonin occurred. The specific effect of PAF, however, seems to be limited to induce reversible aggregation since second wave of aggregation and serotonin release were suppressed by a combination of acetylsalicylic acid and an ADP scavenging system. Incubation of PAF for 30 min in plasma resulted in a 90% loss of its platelet aggregating power. Subthreshold concentrations of PAF enhanced the platelet aggregation triggered by suboptimal concentrations of ADP, epinephrine, or collagen. Vice versa non-aggregating concentrations of ADP, epinephrine, collagen, Ca-ionophore A 23,187, or arachidonic acid amplified PAF-induced platelet aggregation. The synergistic effect of PAF and other stimuli of blood platelet activation can be partly interpreted as a stimulating effect of PAF on the metabolization of arachidonic acid.

Inhibition of tumor cell metastasis by modulation of the vascular prostacyclin/thromboxane A2 system

The interaction between metastasizing tumor cells and the hemostatic system of the host has been implicated in successful tumor cell dissemination. Prostacyclin (PGI2) decreases metastasis from tail vein injected B16 amelanotic melanoma (B16a) cells when administered 15 min prior to tumor cells. This effect is potentiated by a phosphodiesterase inhibitor. Initial trapping of 125I Udr labelled tumor cells in pulmonary vascular beds is unaltered by PGI2 but retention time is decreased. PGI2 decreases retention time even when administered 60 min post tumor cells. Structurally unrelated thromboxane (TX) synthetase inhibitors and a TXA2 receptor antagonist also reduce metastasis from tail vein injected B16a cells. Furthermore, one inhibitor, 1-(7-carboxyheptyl)imidazole, when injected intraperitoneally reduced spontaneous metastasis from subcutaneous B16a and Lewis lung carcinoma tumors. These results suggest that selective manipulation of PGI2 and TXA2 can reduce the hematogenous spread of tumor cells.

Serotonin content of platelets in inflammatory rheumatic diseases. Correlation with clinical activity

Significantly decreased platelet serotonin contents were measured in rheumatoid arthritis, systemic lupus erythematosus (SLE), progressive systemic sclerosis, and mixed connective tissue disease. An inverse relationship between platelet serotonin levels and clinical disease activity was observed in both rheumatoid arthritis and systemic lupus erythematosus. SLE patients with multiple organ involvement showed the lowest platelet serotonin values. No correlation was observed between platelet serotonin contents and nonsteroidal antiinflammatory drug treatment, presence of circulating platelet reactive IgG, or the amount of circulating immune complexes. The results are interpreted as indicating platelet release occurring in vivo during inflammatory episodes of the rheumatic disorders investigated.

Effects of propranolol in vitro and in vivo on platelet function and thromboxane formation in normal volunteers

In vitro and ex vivo effects of propranolol on platelet aggregation, formation of thromboxane B2 (TXB2) and platelet sensitivity to prostacyclin were studied in healthy men. Propranolol, added in vitro to platelet rich plasma (PRP) inhibited platelet aggregation and TXB2 formation induced by ADP, 1-epinephrine, collagen and arachidonic acid. Concentrations of 20-100 microM propranolol were effective when ADP, 1-epinephrine and collagen were used as stimuli. Higher concentrations (250-500 microM) were needed to inhibit aggregation induced by arachidonic acid. Oral administration of propranolol either as a single dose (120 mg) or for one week (3 x 40 mg/day) did, however, not affect platelet aggregation, thromboxane formation and platelet sensitivity to prostacyclin. In addition, withdrawal of propranolol was without effect on these parameters. Although propranolol has potent effects on platelet function in vitro, it seems that the blood levels achievable by oral administration of propranolol are too low to affect platelet aggregation and TXB2 formation.

Platelet activation by endogenous 5-hydroxytryptamine and histamine released by mast cell degranulation with compound 48/80 in the rat

The intravenous injection of the mast cell degranulator C 48/80 (1 mg/kg) in rats did not produce thrombocytopenia nor circulating platelet aggregates but sensitized the platelets to aggregate upon turbulence challenge. Such turbulence-induced platelet aggregation was not accompanied by formation of thromboxane B2. Electron microscopy revealed absence of platelet degranulation. Turbulence-induced platelet aggregation was completely prevented by pre-treatment of the rats with cyproheptadine, dipyridamole and VK 774, partially with ketanserin (5HT2-receptor antagonist), but not with methysergide (antiserotonergic drug), pyrilamine (antihistaminic drug), suprofen, aspirin (cyclo-oxygenase inhibitors), phentolamine, propranolol, flunarizine, lidoflazine, oxycoumarin or Trasylol. Combined treatment with the anti-histaminic drug pyrilamine and the 5HT2-receptor antagonist ketanserin resulted in a dose-related inhibition for ketanserin of the turbulence-induced platelet aggregation. These experiments point to an interaction between histamine and 5-hydroxytryptamine in the platelet activation by mast cell released mediators.

Selective inhibition of thromboxane B2 accumulation and metabolism in perfused guinea-pig lung

1 U46619, a prostaglandin H2 endoperoxide analogue and thromboxane A2 agonist, dose-dependently inhibited accumulation and metabolism of thromboxane B2 in the isolated perfused lung of the guinea-pig. At similar doses prostaglandins E1, E2, F1 alpha, F2 alpha, I2, 5, 6-trans-PGE2 and 8-iso-PGE1 were ineffective. 2 U46619 did not affect accumulation and metabolism of prostaglandin F2 alpha under similar conditions. 3 The pulmonary disposition of thromboxane B2, which occurs by uptake into pulmonary cells or binding to a specific macromolecular component, is mediated by a mechanism distinct from that handling prostaglandin F2 alpha. The possible relevance of these findings to the pulmonary disposition of thromboxane A2 is discussed.

Comparison of effects of ethanol on platelet function and synaptic transmission

Ethanol, at concentrations tolerated by man, is generally inhibitory to platelet function in vitro, producing significant inhibition of aggregation in response to most chemical aggregating agents. The calcium ionophore, A23187, is the agent which is most inhibited by ethanol, whereas aggregation induced by arachidonic acid is not inhibited even by concentrations of ethanol far in excess of the lethal range. This spectrum of inhibition found suggests that ethanol inhibits the platelet release reaction by a mechanism involving inhibition of Ca2+-activated phospholipase A2. These effects can be observed in superfused human platelets as well as in those suspended in buffer or in plasma. In superfused rat brain slices however, ethanol does not inhibit the A23187-induced release of radiolabelled neurotransmitter, although it can be shown to inhibit Ca2+-activated phospholipase A2 activity in rat synaptosomal preparations. It is concluded that, although there are many similarities between the effects of ethanol on the platelet and the synapse there may be differences between the way in which ethanol modifies release of intracellular contents in the two situations.

Platelet-activating factor (PAF-acether): thromboxane-independent synergism with adrenaline on human platelets and recent insights into its mode of action

The mode of action of PAF-acether on human and rabbit plasma-free platelets is reviewed. PAF-acether and adrenaline synergize to trigger aggregation of human platelets, and this synergism is refractory to aspirin. When degranulated rabbit platelets are stimulated with PAF-acether, with thrombin or with the snake venom component convulxin, aggregation is obtained in the absence of detectable secretion. Collagen-induced aggregation is reduced, and is suppressed when aspirin is applied to the degranulated platelets. The formation of PAF-acether by platelets, and their stimulation by PAF-acether itself, should be added to the newly recognized pathways for platelet stimulation.

Adrenaline and PAF-acether synergize to trigger cyclooxygenase-independent activation of plasma-free human platelets

Platelet-activating factor (PAF-acether, 1-0-octadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine), a potent aggregating agent for the platelets in plasma, induced only a moderate aggregation of plasma-free suspensions of human platelets. By contrast, addition of PAF-acether plus adrenaline or ADP to the platelet suspension was followed by full aggregation, accompanied by a moderate secretion of ATP. This synergism was observed better in the presence of fibrinogen, but was also seen in its absence when using the combination of PAF-acether with adrenaline. Aspirin failed to interfere with the synergized aggregation, ruling out a role for cyclooxygenase. The order of addition of adrenaline and of PAF-acether to the platelets was critical. When the former was added first to the platelets suspension, aggregation was induced by the latter even if added after one hour. Conversely, aggregation was only induced by adrenaline added after PAF-acether, if the interval between both was of around one minute. Removal of ADP with the scavenging system creatine phosphate/creatine phosphokinase prevented the synergism between PAF-acether plus ADP, but failed to interfere with that induced by PAF-acether plus adrenaline. The synergized aggregation induced by adrenaline or ADP and PAF-acether may represent a novel mechanism, accounting for the increased aggregability under various physiopathological conditions.

Role of cyclic AMP in the inhibition of human platelet aggregation by quercetin, a flavonoid that potentiates the effect of prostacyclin

Quercetin (3.3',4',5,7-pentahydroxyflavone) has previously been shown to inhibit cyclic nucleotide phosphodiesterases prepared from various cell homogenates and the function of intact human platelets. We now report that (1) high concentrations of quercetin raise platelet cAMP levels; and (2) quercetin potentiates the inhibitory effect of prostacyclin (PGI2) on ADP-induced washed human platelet aggregation and the elevation of platelet cAMP levels elicited by PGI2. These results suggest a role for cAMP in the mechanism of action of quercetin on blood platelets.

Platelet activating factor (PAF) causes human platelet aggregation through the mechanism of membrane modulation

Recent investigations have shown that human and canine platelets possess a new pathway for securing irreversible platelet aggregation independent of ADP secretion and prostaglandin synthesis. Platelet activating factor (PAF) is a potent aggregating agent for rabbit platelets and is also reputed to stimulate the cells by a completely independent pathway. Since the newly recognized mechanism in human and dog cells, termed membrane modulation, might be mediated by PAF, we evaluated the influence of this agent on human cells. Results of that study demonstrated that PAF did not activate platelets through an independent pathway and was not involved in the mechanism of membrane modulation. In the present investigation we have examined the possibility that PAF might act on human platelets through the mechanism of membrane modulation. Human platelets were made refractory to amounts of PAF routinely causing irreversible aggregation by prolonged incubation of C-PRP at 37 degrees C, by exposure to PGI2 or aspirin, or by aggregation-disaggregation of control or aspirin treated cells. Epinephrine at concentrations which did not cause aggregation turned on the mechanism of membrane modulation in refractory platelets and restored their sensitivity to PAF. Thus, PAF does not appear to activate human platelets through a unique, independent pathway. Rather, it usually causes irreversible aggregation of human cells through the mechanisms of ADP secretion and thromboxane synthesis or, as shown in this study, can be dependent on membrane modulation.

The effect of sulphinpyrazone on whole blood thromboxane and prostacyclin generation in man

A new method, based on the observation that leukocytes contain PGI2-synthetase activity, was used to measure the effect of pharmacological levels of sulphinpyrazone on the TXA2 and PGI2 production in whole blood. Four human volunteers took 400 mg sulphinpyrazone twice daily for 5 1/2 days. Blood was drawn before the study started, 3-4 h after the initial dosage, 12 h after the 10th dosage on the 5th day, and 3-4 h after the 11th (final) dosage on the 6th day. Collagen was added to the citrated blood samples to stimulate prostaglandin production. Aliquots were removed at regular intervals and TXB2 and 6-keto-PGF1 alpha measured by radioimmunoassay. The production of PGI2 was significantly inhibited in all the samples collected after sulphinpyrazone intake (p less than 0.01). The production of TXA2 was significantly inhibited only in the samples collected 3-4 h after sulphinpyrazone intake on the 6th day (p less than 0.01). These results confirmed the cyclooxygenase inhibitory action of sulphinpyrazone and also showed that in the whole blood system, PGI2 production was more effectively inhibited than TXA2 production.

Non-steroidal anti-inflammatory drugs if combined with anti-histamine and anti-serotonin agents interfere with the bronchial and platelet effects of "platelet-activating factor" (PAF-acether)

The non-steroidal anti-inflammatory drugs (NSAID) indomethacin, aspirin and salicylic acid, as well as the anti-histamine, mepyramine, and the anti-serotonin, methysergide, fail to interfere with bronchoconstriction, thrombo-cytopenia and hypotension induced by platelet-activating factor (PAF-acether, 1-O-octadecyl-2-acetyl-sn-3-glycerylphosphorylcholine) in the guinea-pig. When one of the NSAID was given combined with mepyramine and with methysergide, bronchoconstriction was suppressed, but thrombocytopenia and hypotension persisted. Platelets prepared from blood collected from animals treated with the NSAID or with mepyramine and/or methysergide, aggregated to a similar extent to PAF-acether; however, the accompanying release of ATP was inhibited in those animals which had been treated with the drug combination effective in vivo against bronchoconstriction. In vitro application to platelets of the drugs effective in vivo and ex vivo was ineffective in blocking the platelet-release reaction. This suggests the existence of in vivo sites of action for the synergistic inhibitory activity of NSAID and anti-histamine/anti-serotonin drugs on bronchoconstriction and on platelet secretion. PAF-acether possible releases from the platelets a bronchoconstrictor component, distinguishable from thromboxane A2 and depleted by reserpine administration to the animals, which could account for the in vivo effects on the bronchopulmonary system.

Thromboxane B2 uptake and metabolism in isolated perfused lung. Identification and comparison with prostaglandin F2 alpha

Thromboxane B2 was metabolised in isolated perfused guinea-pig lungs to a product identified by negative ion chemical ionisation mass spectrometry as 13,14-dihydro-15-ketothromboxane B2. Conversion was measured by radio TLC and was greater in guinea-pig than rat lungs (29.1 vs. 13.8% at 10 ng/ml), but similar in lungs from normal and sensitized guinea-pigs. Thromboxane B2 metabolism was less than that of prostaglandin F2 alpha but, like it, was prevented at 5 degrees C and reduced by cycloheximide pretreatment. Tissue to medium ratio in perfused guinea-pig lungs was 3.4 for thromboxane B2, but 0.2 for insulin (showing that thromboxane B2 is accumulated within the lung) and was altered after experimental manipulations. Neither lung slices, crude homogenates, cytosolic and microsomal fractions nor purified prostaglandin 15-hydroxydehydrogenase metabolised thromboxane B2 in vitro, although prostaglandin F2 alpha was extensively inactivated. Quantitative partition coefficient and albumin-binding data confirm that thromboxane B2 lacks prominent lipophilicity, implying that cellular uptake in lung must be carrier-mediated. We conclude that thromboxane B2 is a substrate for pulmonary degradation which may form a route for the biological inactivation of thromboxane A2. Its resistance to prostaglandin 15-hydroxydehydrogenase as conventionally tested remains paradoxical and is discussed.

Inhibition of 5-hydroxytryptamine-induced and -amplified human platelet aggregation by ketanserin (R 41 468), a selective 5-HT2-receptor antagonist

Ketanserin, a selective 5-HT2-receptor antagonist, inhibits the reversible aggregation induced by 5-hydroxytryptamine (5-HT) in human platelet-rich plasma (PRP). In this respect, the compound is equipotent to cyproheptadine and more active than methysergide (IC50: 1.66 x 10(-8) M, 1.44 x 10(-8) M and 5.62 x 10(-8) M respectively). Ketanserin is active against 5-HT-induced platelet aggregation after both in vitro and oral administration to human volunteers. At concentrations up to 500 times in excess of the IC50 for 5-HT-induced platelet reactions, ketanserin does not affect the aggregation induced by ADP, epinephrine, collagen or Thrombofax, the prostaglandin biosynthesis of thrombin-stimulated platelets, nor the active uptake of 14C-5-HT by platelets. 5-Hydroxytryptamine amplifies the human platelet aggregation induced by threshold concentrations of ADP, collagen, epinephrine, norepinephrine and induced irreversible aggregation of platelets pre-sensitized with Thrombofax. This amplification by 5-hydroxytryptamine results in a platelet response typical for the potentiated agonist; for the combination of the monoamine with collagen, the serotonergic amplification results in enhanced aggregation, release of beta-TG and PF4 and excessive formation of TXB2. Ketanserin, after both in vitro and oral administration to man reduces the amplified response to the level of the potentiated agonist. The present evidence suggests the presence of functional 5-HT2 receptors on the human platelet, different from those involved in the uptake of the monoamine.

Acetyl glycerylphosphorylcholine inhibition of prostaglandin I2-stimulated adenosine 3',5'-cyclic monophosphate levels in human platelets. Evidence for thromboxane A2 dependence

Previous studies with AGEPC (1-O-hexadecyl/octadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine) stress the independence of the proaggregatory activity of AGEPC from the platelet cyclooxygenase. However, our dose response analyses in human platelet-rich plasma show distinct primary and secondary waves of aggregation in response to AGEPC. Second wave aggregation is inhibited completely by either 10 micro M indomethacin, a cyclooxygenase inhibitor, or 5.6 micro M 9,11-azoprosta-5,13-dienoic acid, a thromboxane A2 synthetase inhibitor. Simultaneous addition of AGEPC and prostaglandin I2 to platelet-rich plasma results in a marked increase in platelet cyclic AMP, which is not different from the prostaglandin I2 response alone. However, if prostaglandin I2 is added to AGEPC-stimulated platelets at a point where secondary aggregation is just beginning, AGEPC can attenuate prostaglandin I2-stimulated cyclic AMP accumulation. The inhibition by AGEPC is blocked by either cyclooxygenase or thromboxane A2 synthetase inhibitors, and radioimmunoassay of thromboxane B2 confirmed that the inhibition of prostaglandin I2-stimulated cyclic AMP accumulation is due to thromboxane A2 synthesis, and that AGEPC-stimulated secondary aggregation does not start until thromboxane A2 is synthesized. These data suggest that much of the bioactivity of AGEPC is attributable to thromboxane A2.

Biosynthesis of platelet-activating factor (PAF-ACETHER). II. Involvement of phospholipase A2 in the formation of PAF-ACETHER and lyso-PAF-ACETHER from rabbit platelets

The role of phospholipase A2 (PLA2) in the formation of platelet-activating factor (PAF-acether) by rabbit platelets is supported by several pieces of evidence. First, the release of PAF-acether was accompanied by that of its deacetylated analog, lyso-PAF-acether. Second, EDTA, EGTA, db-AMPc, p'-bromophenacylbromide and 874 CB, which, in spite of their structural diversity, are all PLA2 blockers, inhibited the release of both PAF-acether and of the lyso-compound. Third, addition of hog pancreas PLA2 to platelets as well as platelet lysis resulted in the release of lyso-PAF-acether, thus mimicking the metabolic events initiating formation of PAF-acether. These results indicate that PLA2 activation triggers both the second and the third pathway of platelet activation.

Inhibition by sulphinpyrazone of the platelet-dependent bronchoconstriction due to platelet-activating factor (PAF-acether) in the guinea pig

Platelet-activating factor (PAF-acether) injected into guinea-pigs induced platelet-dependent bronchoconstriction and thrombocytopenia. Treatment of the animals with sulphinpyrazone suppressed bronchoconstriction without affecting thrombocytopenia. When tested ex vivo and in vitro, sulphinpyrazone suppressed the PAF-acether-induced platelet release reaction, as measured by the release of ATP, more efficiently than platelet aggregation. In contrast, bronchoconstriction and thrombocytopenia in vivo, as well as platelet aggregation and the release reaction ex vivo and in vitro, induced by arachidonic acid (AA), were suppressed by sulphinpyrazone. This effect is accounted for by the known anti-arachidonate cyclooxygenase activity of sulphinpyrazone. Finally, aggregation by ADP was only marginally inhibited by sulphinpyrazone, and the inhibition was easily surmounted when the amounts of ADP added were increased. Sulphinpyrazone exerts a specific and cyclooxygenase-independent protective effect towards platelet activation by PAF-acether, which results in inhibition of platelet-dependent bronchoconstriction even though aggregation, and consequently in vivo thrombocytopenia, may persist.

Pharmacokinetics of oral dipyridamole (Persantine) and its effect on platelet adenosine uptake in man

Two preparations of dipyridamole have been studied by oral administration to 11 normal volunteers. The plasma levels of dipyridamole and its glucuronide were determined simultaneously by high performance liquid chromatography. The instant form (I.F., 100 mg) was administered four times daily and the slow release preparation (SRP, 200 mg) twice daily, for 3 days. Multiple blood samples were collected on Days 1-4 to provide plasma for assay, and simultaneously, platelet rich plasma was prepared for ex vivo study of the effect of dipyridamole on platelet uptake of adenosine. The pharmacokinetics of absorption and distribution of dipyridamole were described using a two compartment model with lag time and prolonged absorption. Strong inhibition of the platelet adenosine uptake was observed at therapeutic plasma levels. The inhibition of platelet adenosine uptake may be related to some of the pharmacological properties of dipyridamole.