Thromboxane-induced phosphatidate formation in human platelets. Relationship to receptor occupancy and to changes in cytosolic free calcium

Effect of the dietary intake of melatonin- and hydroxytyrosol-rich wines by healthy female volunteers on the systemic lipidomic-related oxylipins

Oxylipins are lipid mediators involved in the physiopathology of all organs. Moreover, isoprostanes have been established as general and reliable in vivo oxidative stress biomarkers. Red wine has proved to exert several benefits through the maintenance of the oxidative balance of the organism. Antiradical scavenging capacity has been mainly attributed to polyphenols. However, melatonin and hydroxytyrosol should be taken into account as potent antiradical agents. The present research aimed to clarify the situation of enzymatic and oxidative injury and eicosanoid urinary excretion related to the intake of three kinds of red wines and their primary musts. Judging by the reduction in the excretion of isoprostanes, red wine consumption exhibited the highest antioxidant protection against oxidative stress, attributed to its OHTyr content (p < 0.05), and to a lesser extent to its MEL content. Similarly, the intake of red wine leads to the cardioprotective effect due to the reduction in the urinary excretion of the pro-inflammatory prostaglandin 2,3-dinor-11-β-PGF_2α, besides the increase in the vasodilator prostaglandin PGE₁, mediated by the melatonin (p < 0.05) and hydroxytyrosol (p < 0.05) contents. In conclusion, red wine (especially non-aged wine) exerts a higher in vivo antioxidant capacity than must or alcohol.

A review on anti-inflammatory activity of phenylpropanoids found in essential oils

The search for alternative drugs capable of disrupting the inflammatory process has become an important issue in scientific research, especially with reference to the use of natural substances and the reduction of undesirable side effects. Essential oils represent an important source of such substances, since their active constituents often exhibit an array of pharmacological properties, including anti-inflammatory activity. This review presents an overview of the anti-inflammatory action exerted by phenylpropanoids from essential oils and discusses possible mechanisms of action involved in the anti-inflammatory response, assessed through specific experimental models.

Salivary proteins as predictors and controls for oral health

We will provide a translational view of using the recent technological advances in dental research for predicting, monitoring, and preventing the development of oral diseases by investigating the diagnostic and therapeutic role of salivary proteins. New analytical state-of-the-art technologies such as mass spectrometry and atomic force microscopy have revolutionized the field of oral biology. These novel technologies open avenues for a comprehensive characterization of the salivary proteins followed by the evaluation of the physiological functions which could make possible in a near future the development of a new series of synthetic protein for therapeutic propose able to prevent global oral diseases such as periodontal disease and dental caries, the two most prevalent oral diseases in the World.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Involvement of thromboxane a(2) in the modulation of pacemaker activity of interstitial cells of cajal of mouse intestine

Although many studies show that thromboxane A(2) (TXA(2)) has the action of gastrointestinal (GI) motility using GI muscle cells and tissue, there are no reports on the effects of TXA(2) on interstitial cells of Cajal (ICC) that function as pacemaker cells in GI tract. So, we studied the modulation of pacemaker activities by TXA(2) in ICC with whole cell patch-clamp technique. Externally applied TXA(2) (5microM) produced membrane depolarization in current-clamp mode and increased tonic inward pacemaker currents in voltage-clamp mode. The tonic inward currents by TXA(2) were inhibited by intracellular application of GDP-beta-S. The pretreatment of ICC with Ca(2+) free solution and thapsigargin, a Ca(2+)-ATPase inhibitor in endoplasmic reticulum, abolished the generation of pacemaker currents and suppressed the TXA(2)-induced tonic inward currents. However, chelerythrine or calphostin C, protein kinase C inhibitors, did not block the TXA(2)-induced effects on pacemaker currents. These results suggest that TXA(2) can regulate intestinal motility through the modulation of ICC pacemaker activities. This modulation of pacemaker activities by TXA(2) may occur by the activation of G protein and PKC independent pathway via extra and intracellular Ca(2+) modulation.

Preconditioning of coronary artery against vasoconstriction by endothelin-1 and prostaglandin F2alpha during repeated downregulation of epsilon-protein kinase C

The cellular mechanisms of coronary vasospasm are unclear, and a role for protein kinase C (PKC) activation by the endogenous vasoconstrictors endothelin-1 (ET-1) and prostaglandin F2alpha (PGF2alpha) has been suggested. In this study, we developed a phorbol ester-induced PKC downregulation protocol to investigate the relation between the amount and activity of specific PKC isoforms in coronary arterial smooth muscle and coronary vasoconstriction by ET-1 and PGF2alpha. Isometric tension was measured in deendothelialized porcine coronary artery strips, [Ca2+]i was monitored in single coronary smooth muscle cells loaded with fura-2, and the whole tissue, cytosolic, and particulate fractions were examined for PKC activity and reactivity with isoform-specific anti-PKC antibodies using Western blot analysis. In Ca(2+)-free (2 mM EGTA) Krebs solution, ET-1 (10(-7) M), PGF2alpha (10(-5) M) and PKC activator phorbol 12,13-dibutyrate (PDBu) (10(-6) M) caused significant contractions that were completely inhibited by the PKC inhibitors staurosporine and calphostin C, no significant change in [Ca2+]i, and significant activation and translocation of the Ca(2+)-independent epsilon-PKC but not the Ca(2+)-dependent alpha-PKC. In Ca(2+)-free Krebs, a single application of PDBu produced maximal contraction and PKC activity after 30 min, which declined to basal levels in 3 h and remained steady for 24 h, but did not prevent subsequent increases in contraction and PKC activity with a new addition of PDBu and did not significantly decrease the amount of alpha- or epsilon-PKC. Repeated (five to eight) applications of PDBu in Ca(2+)-free Krebs at 3-h intervals completely inhibited subsequent increases in contraction and PKC activity to PDBu, ET-1, or PGF2alpha, and significantly decreased the amount of epsilon-PKC but not that of alpha-PKC. These results provide evidence that a Ca(2+)-independent coronary vasoconstriction induced by ET-1 and PGF2alpha is associated with activation of the epsilon-PKC isoform. The results suggest that, in coronary artery smooth muscle, downregulation of PKC is isoform specific and is more dependent on the frequency rather than the duration of PKC activation. The results also suggest that repeated downregulation of epsilon-PKC might play a role in preconditioning of the coronary artery against vasoconstriction by ET-1 and PGF2alpha.

Group I secreted PLA2 and arachidonic acid metabolites in the maintenance of cat LES tone

Spontaneous tone of in vitro lower esophageal sphincter (LES) circular muscle is associated with elevated levels of arachidonic acid (AA), PGF(2alpha), and increased [35S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding to Gq-, Gi3-, and G(i1/i2)-like G proteins. Tone and AA levels were reduced by inhibitors of a pancreatic-like (group I) secreted phospholipase A2 (sPLA2), by the cyclooxygenase inhibitor indomethacin, and by the thromboxane A2 antagonist SQ-29548. In addition, pertussis toxin (PTX) reduced LES tone, confirming a role of PTX-sensitive G proteins in maintenance of LES tone. PGF(2alpha) contracted LES smooth muscle (strips and cells) and increased [35S]GTPgammaS binding to Gq and Gi3 in solubilized LES circular muscle membranes. PGF(2alpha)-induced contraction of LES permeable muscle cells was inhibited by Gq and Gi3 but not by G(i1/i2) and Go antibodies. The thromboxane A2 analog U-46619 contracted LES smooth muscle and increased Gq binding. U-46619-induced contraction was inhibited by Gq but not by Gi3, G(i1/i2), and Go antibodies. LES tone and [(35)S]GTPgammaS binding were significantly reduced by indomethacin. We conclude that group I sPLA2 may mediate "spontaneous" LES tone by producing AA, which is metabolized to PGF(2alpha) and thromboxane A2. These AA metabolites activate receptors linked to Gi3 and Gq to maintain LES contraction.

Differential effects of glibenclamide on responses to thromboxane A2 mimic, U46619, in the pulmonary and hindquarters vascular beds of the cat

The inhibitory effects of the oral sulfonylurea, glibenclamide, on vasoconstrictor responses to the thromboxane A2 mimic, U46619, were investigated in the pulmonary and hindquarters vascular beds of the cat under constant flow conditions. When lobar arterial tone was at resting conditions (14 +/- 2 mm Hg), intralobar injections of U46619, prostaglandin F2alpha, prostaglandin D2, angiotensin II, norepinephrine, and BAY K 8644 caused dose-related increases in lobar arterial pressure without altering left atrial pressure. Following an intralobar infusion of glibenclamide (5 mg/kg), vasoconstrictor responses to U46619, prostaglandin F2alpha and prostaglandin D2 were significantly reduced, whereas vasoconstrictor responses to norepinephrine and angiotensin II were not altered and responses to BAY K 8644 were significantly enhanced. When tone in the pulmonary vascular bed was raised to a high steady level (36 +/- 3 mm Hg), glibenclamide in a dose of 5 mg/kg i.a. markedly attenuated responses to injections of U46619 and reduced the vasodilator responses to the K+-ATP channel opener, levcromakalim, whereas responses to acetylcholine and S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide donor, were not changed. In the hindquarters vascular bed of the cat, administration of glibenclamide in a dose of 5 mg/kg i.a. had no significant effect on vasoconstrictor responses to U46619, norepinephrine or angiotensin II. Hindquarters vasodilator responses to levcromakalim, but not to nitric oxide, were decreased significantly following administration of glibenclamide. These data suggest that glibenclamide, in addition to inhibiting K+-ATP channels, has thromboxane A2 receptor blocking activity in the pulmonary vascular bed of the cat. These data also suggest that vasoconstrictor responses to U46619 may be mediated by different thromboxane A2 receptors with different binding affinities in the pulmonary and in the hindquarters vascular beds of the cat.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

Thromboxane inhibition potentiates antihypertensive effects of alpha 1 adrenoceptor antagonists in the rat

We investigated the influence of the vascular and renal thromboxane system on the antihypertensive effects of the alpha 1 adrenoceptor antagonist (alpha 1 blocker) bunazosin in spontaneously hypertensive rats (SHR). SHR were treated for 2 weeks with the alpha 1, blocker bunazosin (0.5 mg/kg body weight/day). The systolic blood pressure immediately declined with bunazosin treatment, and then rose toward the level observed in untreated SHR. This antihypertensive effect was accompanied by a decrease in the ratio of prostacyclin to thromboxane A2 in the vascular wall and the kidney. A subdepressor dose of the thromboxane synthase inhibitor OKY-046 lessened the thromboxane generation during bunazosin treatment, and synergistically potentiated the antihypertensive action of the alpha 1 blocker. Such synergy was also observed between OKY-046 and prazosin, an alternative alpha 1 blocker, but not with amosulalol, an alpha 1 blocker having no quinazoline moiety. alpha 1 blockers with a quinazoline moiety dose-dependently stimulate thromboxane generation in cultured smooth muscle cells from SHR. These data indicate that alpha 1 blockers enhance thromboxane generation in the arterial wall and kidney, thereby contributing to the lessening of the antihypertensive effects observed during alpha 1 blocker treatment.

Thromboxane A2 receptor mediation of calcium and calcium transients in rat cardiomyocytes

We have examined the effect of the selective thromboxane A2 (TxA2) receptor agonist U46,619 on intracellular ionized Ca ([Ca2+]i) and the calcium transient rate (CATR) in cultured neonatal rat cardiomyocytes using the Ca-sensitive probe fura 2 and ratiometric microfluoroscopy. U46,619, 10(-6)-10(-8)M, increased basal diastolic Ca fluorescence and 10(-6) and 10(-7) M increased CATR. These effects were completely blocked by the highly selective TxA2 receptor antagonist SQ-29,548 (p > 0.5, n = 4 compared to baseline), confirming this response is a specific receptor-mediated event in the cardiomyocytes. TxA2 blockade did not diminish the Angiotensin (Ang II)-mediated [Ca2+]i and calcium transient rate response from that observed in non-blocked cells (p = 0.18 and 0.21 respectively, n = 4). The TxA2-mediated changes in Ca2+ fluorescence did not exhibit homologous desensitization as does Ang II, they did not exhibit heterologous desensitization, and maximally stimulating concentrations were additive in their effect on peak [Ca2+]i. These data support the hypothesis that TxA2 secretion or release following ischemia or other pathophysiologic events could alter cardiac calcium homeostasis. Although Ang II is reported to stimulate the release of TxA2 in a variety of tissues, including the heart, the Ca2+ and CATR response to Ang II are not diminished when TxA2 receptors are blocked. This study cannot rule out the possibility that Ang II-mediated increases in TxA2 may have an additive effect on Ca homeostasis.

Labeling of human platelet plasma membrane thromboxane A2/prostaglandin H2 receptors using SQB, a novel biotinylated receptor probe

This study reports the synthesis, biological evaluation, and application of a new biotinylated derivative 1-[[1S-[1 alpha, 2 alpha (Z),3 alpha, 4 alpha]]-7-[3-[[[[(1-oxocyclohexylpropyl)amino]acetyl]amino] methyl]-7-oxabicyclo [2.2.1]hept-2-yl]-5-heptenoyl]-2-[hexahydro-2'-oxo-1H-thieno[3',4' d] imidazole-4'-pentanoyl]hydrazine (SQB) of the thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor antagonist [1S-[1 alpha,2 alpha(Z),3 alpha,4 alpha]]-7-[3-[[[[(1-oxocyclohexylpropyl)amino]acetyl] amino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid (SQ31,491). SQB was synthesized by reacting SQ31,491 with biotin hydrazide, and the product was purified by flash chromatography. It was found that SQB specifically inhibited platelet aggregation in response to U46619 with an IC50 of 275 nM. On the other hand, SQB did not inhibit adenosine diphosphate or A23187-induced aggregation. Competition binding studies revealed that SQB produced a concentration-dependent inhibition of [3H]-[1S-[1 alpha, 2 beta (5Z),3 beta, 4 alpha]]-7-[3-[[2[(phenylamino) carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid ([3H]SQ29,548) specific binding in 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate (CHAPS)-solubilized platelet membranes, with a Ki of 220 nM. The shape of the SQB inhibition binding curve was indistinguishable from that produced by the TXA2/PGH2 receptor antagonist BM13.177. Finally, incubation of gel-filtered platelets or platelet-rich plasma with SQB and fluorescein isothiocyanate (FITC)-avidin demonstrated fluorescent labeling of platelet plasma membrane TXA2/PGH2 receptors. Furthermore, this SQB-FITC fluorescent labeling was reduced significantly by co-incubation of the platelets with the TXA2/PGH2 antagonist SQ29,548. Based on the ability of SQB-FITC-avidin to label intact platelets, it can be concluded: (1) that a pool of platelet TXA2/PGH2 receptors resides in the plasma membrane; and (2) that the binding domains for these receptors are oriented at or near the external membrane surface. Collectively, these data demonstrate that SQB is a highly specific probe for TXA2/PGH2 receptors, which should be of significant value for receptor localization studies in platelets and other tissues.

Antiplatelet and calcium inhibitory properties of eugenol and sodium eugenol acetate

1. Eugenol (3-methoxy-4-hydroxy-propenylbenzene) or sodium eugenol acetate (4-O-acetic acid sodium-3-methoxy-1-propenylbenzene) (0.25, 0.5, 1 mM) concentration-dependently inhibited arachidonic acid (AA)-, collagen-, epinephrine- and ADP-induced platelet aggregation. 2. Eugenol or sodium eugenol acetate inhibited collagen-induced aggregation of washed rabbit platelets synergistically with creatine phosphate/creatine phosphokinase (CP/CPK, 5 mM/10 U/ml) or p-bromophenacyl bromide (p-BPB, 10 microM), and they also potentiated the inhibitory action of imidazole (0.5 mM) on AA-induced aggregation. 3. Eugenol or sodium eugenol acetate (0.25, 0.5, 1 mM) concentration-dependently inhibited AA-induced thromboxane B2 and prostaglandin E2 formation. 4. The rise of intracellular Ca2+ caused by collagen, epinephrine, ADP, and AA were inhibited by eugenol or sodium eugenol acetate (1 mM).

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

Mechanisms of acquired thymic tolerance in experimental autoimmune encephalomyelitis: thymic dendritic-enriched cells induce specific peripheral T cell unresponsiveness in vivo

Experimental autoimmune encephalomyelitis (EAE), an experimental model for the study of multiple sclerosis, is an autoimmune disease of the central nervous system that can be induced in a number of species by immunization with myelin basic protein (MBP). MBP-reactive CD4+ T cells, predominantly expressing the V beta 8.2 T cell receptor (TCR), migrate from the peripheral lymphoid organs and initiate the inflammatory response in the brain. We have previously shown that a single intrathymic injection of MBP or its major encephalitogenic peptide (p71-90), but not a nonencephalitogenic peptide (p21-40), induces antigen-specific systemic tolerance and inhibits the induction of EAE in Lewis rats. In this study, we investigated the mechanisms of induction and maintenance of acquired thymic tolerance in this model. First, we investigated which thymic cell is responsible for "induction" of systemic tolerance. Thymic dendritic-enriched cells, isolated by plastic adherence, when incubated in vitro with p71-90 and injected intravenously into Lewis rats, were capable of preventing the development of EAE, but his protection was lost in thymectomized recipients. In addition, intravenous injection of thymic dendritic cells isolated from animals that had been previously injected intrathymically with p71-90 but not p21-40 also prevented the development of EAE. Second, to determine the "effector" mechanisms involved in acquired thymic tolerance, we compared TCR expression in the brains of animals with actively induced EAE with TCR expression in animals that received intrathymic injection of p71-90 or p21-40. Using a semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) technique, we found increased expression of CD4 and V beta 8.2 message in brains of immunized animals compared with those of naive animals. In animals intrathymically injected with p71-90 but not p21-40, CD4 and V beta 8.2 transcript levels were significantly reduced compared with immunized controls. Immunohistologic studies of brain tissue and spleens with specific V beta 8.2 and control V beta 10 monoclonal antibodies confirmed these observations in vivo. These findings, taken together with recent data demonstrating that activated T cells circulate through the thymus, suggest that interaction of thymic dendritic cells with specific TCR of activated peripheral T cells can lead to inactivation of these antigen-specific cells and confirm the role of V beta 8.2-expressing T cells in EAE.

Involvement of protein-tyrosine kinase p72syk in collagen-induced signal transduction in platelets

Previous studies have demonstrated that activation of platelets by collagen results in a dramatic increase in tyrosine phosphorylation of several cellular proteins, including pp125FAK, through the interaction of collagen with integrin alpha 2 beta 1 (GP Ia-IIa). In this study, we report that p72syk is a potential candidate for the protein-tyrosine phosphorylation event following collagen stimulation in porcine platelets. Washed platelets were stimulated with collagen and the activation of p72syk was assessed in an immunoprecipitation kinase assay. The activity of p72syk increased within 1 min, reached a maximum at 5 min after stimulation by collagen, and the phosphorylation at tyrosine residues of p72syk in platelets also occurred in the same time course as the activation of p72syk. Prior treatment of platelets with cytochalasin D to inhibit actin polymerization, or with aspirin and apyrase to inhibit the secondary reaction, or EGTA and the acetoxymethyl ester of 5,5'-dimethyl-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid to chelate both extracellular and intracellular Ca2+, did not affect the activation of p72syk induced by collagen. Furthermore, herbimycin A, a potent protein-tyrosine-kinase inhibitor, was capable of reducing collagen-evoked p72syk activation, Ca2+ mobilization and platelet aggregation. These results suggest that upon stimulation by collagen p72syk is physically activated by a process that is independent of the effects of Ca2+, ADP, and actin polymerization, and may participate in the regulation of Ca2+ mobilization mediated by collagen in platelets.

Thromboxane A2 receptor blocking abrogates donor-specific unresponsiveness to renal allografts induced by thymic recognition of major histocompatibility allopeptides

Recent in vitro studies have documented that thromboxane (Tx)A2 induces thymocyte apoptosis by acting on specific receptors abundantly expressed on the surface of immature T lymphocytes. No information is available on the in vivo relevance of this observation in development of self- or acquired tolerance. We and others have previously documented that injection of donor cells into adult thymus of experimental animals induced specific systemic unresponsiveness to allografts in the rat and mouse models. More recently, we have shown that intrathymic injection of synthetic class II major histocompatibility complex (MHC) allopeptides resulted in donor-specific unresponsiveness to renal allografts. The induction of unresponsiveness was abrogated by recipient thymectomy within the first week. We now report the effect of TxA2 blockade on acquired thymic tolerance to renal allografts induced by intrathymic injection of synthetic class II MHC allopeptides in the Wistar-Furth (WF) to Lewis rat strain combination. Administration of the TxA2 receptor blocker prior to transplantation or 2 wk postengraftment completely abrogated the unresponsive state. In addition, inhibiting the TxA2-forming enzyme by aspirin or dexamethasone also abolished the induction of acquired thymic tolerance. Evidence is also provided for a critical "dose" of peptides to be injected into the thymus to induce systemic unresponsiveness to renal allografts. These data, coupled with observations that activated peripheral T cells can circulate through the thymus, provide evidence that TxA2/TxA2 receptor interaction in the thymic microenvironment, leading to anergy/programmed cell death of activated T cells, may play an important role in the development of acquired unresponsiveness in vivo.

Thromboxane A2 receptor is highly expressed in mouse immature thymocytes and mediates DNA fragmentation and apoptosis

We have recently revealed that the thymus is the organ showing the highest expression of thromboxane (TX) A2 receptor in mice. In this study, thymic cell populations expressing the receptor were identified, and the effects of a TXA2 agonist on these cells were examined. Radioligand binding using a TXA2 receptor-specific radioligand revealed a single class of binding sites in the thymocytes with an affinity and specificity identical to those reported for the TXA2 receptor. The receptor density in these cells was comparable to that seen in blood platelets. This receptor is most highly expressed in CD4-8- and CD4+8+ immature thymocytes, followed by CD4+8- and CD4-8+ cells. The receptor density in splenic T cells was less than one fifth of that in CD4+8+ cells and no binding activity was detectable in splenic B cells. The addition of a TXA2 agonist, STA2, to thymocytes induced the disappearance of the CD4+8+ cells in a time- and concentration-dependent manner and caused DNA fragmentation. These changes were blocked by a specific TXA2 antagonist, S-145. These results demonstrate that TXA2 induces apoptotic cell death in immature thymocytes by acting on the TXA2 receptor on their cell surface and suggest a role for the TXA2/TXA2 receptor system in the thymic micro-environment.

Eicosanoid/thromboxane A2-independent and -dependent generation of lysoplasmenylethanolamine via phospholipase A2 in collagen-stimulated human platelets

Collagen-induced human platelet stimulation is dependent on the release of arachidonic acid (AA) from membrane phospholipid and the formation of thromboxane A2 (TxA2) for TxA2-induced platelet activation. Since plasmenylethanolamine represents the single major phospholipid reservoir of AA in resting human platelets, we assessed its hydrolysis via phospholipase A2 upon platelet stimulation with low levels of collagen by determining the generation of [3H]lysoplasmenylethanolamine via eicosanoid/TxA2-independent and -dependent processes. Ethanolamine phospholipids in platelets were prelabelled with [3H]ethanolamine before stimulation with either collagen or the TxA2 mimetic U46619, in the presence or absence of BW755C, a dual inhibitor of the cyclooxygenase/lipoxygenase activities, or GR32191B, a TxA2-receptor antagonist. Collagen stimulation promoted a marked generation of [3H]lysoplasmenylethanolamine, which was only moderately decreased when TxA2 synthesis or TxA2 receptors were blocked by BW755C or GR32191B respectively. The moderate rise in [3H]lysoplasmenylethanolamine formation with U46619 as the agonist was only slightly affected by BW755C and blocked by GR32191B. Evidence for eicosanoid/TxA2-independent and -dependent generation of [3H]lysophosphatidylethanolamine was also obtained. A significant quantitative loss of AA from plasmenylethanolamine was also demonstrated in collagen-stimulated platelets. The present findings indicate the activation of plasmenylethanolamine cleavage via phospholipase A2 in collagen-stimulated human platelets, which, to a considerable extent, does not depend on eicosanoid/TxA2 synthesis. This may represent an important source of releasable AA for TxA2 generation and the promotion of further liberation of AA and phospholipid-mediated signalling pathways.

Inhibition of platelet aggregation by the cAMP-phosphodiesterase inhibitor, cilostamide, may not be associated with activation of cAMP-dependent protein kinase

We examined the involvement of cAMP-dependent protein kinase (A kinase)2 in the inhibition by cilostamide, a specific inhibitor of the low Km cAMP-phosphodiesterase (PDE), on 9,11-epithio-11,12-methanothromboxane A2 (STA2)-induced platelet aggregation. For comparative purposes, the PGE1 analogue, 17S-20-dimethyl-trans-delta 2-PGE1 (OP-1206) was used. OP-1206 (IC50 = 18 +/- 0.55 nM) and cilostamide (IC50 = 40 +/- 4.5 nM) were both potent inhibitors of the platelet aggregation induced by STA2 (1 microM). OP-1206 and cilostamide dose-dependently inhibited elevations in intracellular free Ca2+ ([Ca2+]i) caused by STA2. OP-1206 caused an almost complete inhibition of Ca2+ mobilization, but cilostamide did not prevent the STA2-induced elevation in [Ca2+]i to the same extent as OP-1206, even at a high concentration (greater than 200 nM). Cilostamide did not increase the cAMP level at concentrations (5-100 nm) which affected STA2-induced aggregation. OP-1206 significantly increased cAMP contents in platelets, and the degree of aggregation inhibition by OP-1206 appears to be related to the size of increase in cAMP. OP-1206 increased phosphorylation of the 50,000 mol. wt vasodilator-stimulated phosphoprotein, at concentrations of 7.9-79 nM, which inhibited aggregation induced by STA2. Cilostamide treatment resulted in a marginal increase in the 50,000 mol. wt phosphorylation at concentrations (10-100 nM) which completely inhibited the STA2-induced aggregation. (8R*, 9S*, 11S*)-(-)-9-Hydroxy-9-n-hexyloxy-8-methyl-2,3,9,10- tetrahydro-8,11-epoxy-1H, 8H, 11H-2, 7b, 11a-triazadibenzo(a,g)-cycloocta(c,d,e)trinden-1-one (KT-5720), a specific inhibitor of A kinase, not only reversed the inhibition by OP-1206 of STA2-induced platelet aggregation, but also inhibited the OP-1206-induced protein phosphorylation. However, the inhibition by cilostamide of STA2-induced aggregation was not prevented by pretreatment with KT-5720. Inhibition of the STA2-induced aggregation by OP-1206 may be associated with cAMP-dependent protein phosphorylation, while cilostamide may have inhibitory effects on STA2-induced platelet activation through mechanisms other than the activation of A kinase.

Anti-platelet action of isoliquiritigenin, an aldose reductase inhibitor in licorice

The mechanism was studied by which isoliquiritigenin, a new aldose reductase inhibitor purified from licorice (Glycyrrhizae radix), inhibits platelet aggregation. This new agent significantly inhibited the phosphorylation of 40,000- and 20,000-dalton proteins, and inhibited the formation of 12 (S)-hydroxy-5,8,10-heptadecatrienoic acid, 12-hydroxyeicosatetraenoic acid and thromboxane B2. The inhibitory effect of isoliquiritigenin on platelet aggregation in vitro was comparable to that of aspirin. Our findings may indicate that isoliquiritigenin elicits an anti-platelet action by inhibiting not only cyclooxygenase but also lipoxygenase or peroxidase activity in platelets. Isoliquiritigenin also showed an anti-platelet action in vivo. Isoliquiritigenin appears to be the only aldose reductase inhibitor with a significant anti-platelet action. Since the hyperaggregability of platelets has been implicated in the pathogenesis of diabetic complications, isoliquiritigenin may offer a unique benefit as an aldose reductase inhibitor.

Purification of the human blood platelet thromboxane A2/prostaglandin H2 receptor protein

The human platelet thromboxane A2/prostaglandin H2 receptor has been purified 6100-fold to apparent homogeneity by a three-step chromatographic procedure with an overall yield of 6%. A 6-fold purification of the receptor was first achieved by chromatography of 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate (CHAPS)-solubilized membrane proteins from human platelets on a diethylaminoethyl (DEAE)-Sepharose column. The DEAE eluate fractions containing receptor activity were then applied to a newly developed affinity column using the cyclohexyl derivative of SQ30,741 (SQ31,491) as the immobilized ligand. Elution of the receptor from the affinity column with BM13.177 yielded a further purification of 1700-fold. An additional 4-fold receptor purification from the affinity column eluate was achieved by HPLC using GPC 500 and GPC 100 columns connected in tandem. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining of the HPLC eluate containing purified receptor revealed a single, distinct band with a molecular weight of 55,000. The receptor binding activity was detected with [3H]SQ29,548 using a newly developed binding assay which involved immobilization of the receptor on polyethyleneimine-treated glass fiber filters. The binding of [3H]SQ29,548 to the purified receptor was time dependent, saturable, reversible and highly specific. Unlabeled SQ29,548, BM13.505, and U46619 (but not thromboxane B2 or 6-keto prostaglandin F1 alpha) competed for [3H]SQ29,548 binding to the purified receptor in a concentration-dependent manner. Scatchard analysis of [3H]SQ29,548 binding to the purified receptor revealed the presence of a single class of high-affinity binding sites, with a Kd of 4 nM and a Bmax of 17 nmol/mg protein.

Two-step mobilization of arachidonic acid in platelet activation induced by low concentrations of TP 82, a monoclonal antibody against CD9 antigen

Arachidonic acid mobilization in platelets activated by low concentrations (less than or equal to 1.6 micrograms/ml) of TP 82, a monoclonal antibody against CD9, appears to consist of two distinct phases. In the first phase, limited arachidonic acid release occurs concomitantly with a shape change induced by TP 82. This appears to be dependent upon phospholipase A2 activation, since it is well preserved in the presence of aspirin, which completely blocked both intracellular Ca2+ elevation and phosphatidic acid formation which would indicate phospholipase C activation. The Na+ Exchange was also found to participate in the first phase of arachidonic acid mobilization, since extracellular Na+ depletion and ethylisopropylamiloride, a specific inhibitor of the Na+/H+ exchanger, effectively blocked this limited mobilization of arachidonic acid. The second, much larger, phase of arachidonic acid mobilization occurs with the beginning of platelet aggregation. A limited amount of thromboxane A2 formed during the first phase of arachidonic acid release plays an important role in induction of the massive arachidonic mobilization in the second phase. Factors, as yet unidentified, also appear to work synergistically with thromboxane A2 to induce the full picture of arachidonic acid mobilization.

Platelet and vascular thromboxane A2/prostaglandin H2 receptors. Evidence for different subclasses in the rat

Thromboxane A2 (TXA2) and its precursor prostaglandin H2 (PGH2) induce platelet aggregation and vascular contraction through shared cell surface receptors commonly referred to as TXA2 or TXA2/PGH2 receptors. Whether different subclasses of TXA2/PGH2 receptors exist in platelets and vascular smooth muscle cells is controversial. In this study, TXA2 receptors on washed rat and human platelets and cultured rat aortic smooth muscle cells (RASMC) were characterized using radioligand competition binding assays with the 125I-labeled TXA2/PGH2 receptor agonist [1S-(1 alpha,2 beta(5Z),3 alpha(1E,3R*),4 alpha)] -7- [3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl) -7- oxabicyclo-[2.2.1]- heptan-2-yl] -5- heptenoic acid (I-BOP) and various agonists and antagonists. Scatchard analyses of equilibrium binding data revealed Kd values of 205 +/- 68 pM (N = 6), 2.2 +/- 0.3 nM (N = 9) and 310 +/- 60 pM (N = 7) and Bmax values of 1.3 +/- 0.45 fmol/10(6) platelets, 2.8 +/- 0.2 fmol/10(6) platelets and 20.9 +/- 2.2 fmol/10(6) cells for rat and human platelets and RASMC, respectively. Concentration-dependent increases in intracellular free Ca2+ concentrations induced by I-BOP were observed in RASMC loaded with the calcium sensitive dye fura-2. The IC50 values for various TXA2/PGH2 analogues in competition binding assays with 125I-BOP were determined. Based on their IC50 values, the rank orders were I-BOP less than L657925 less than ONO11113 less than or equal to SQ29548 less than PTA-TPO less than PTA-NO less than or equal to L657926 less than or equal to I-PTA-OH less than PTA-OH[2] = meta-I-PTA-PO less than or equal to ONO11120[2] = ONO11120[1] less than PTA-OH[1] in rat platelets. I-BOP less than SQ29548 less than PTA-TPO = L657925 less than or equal to ONO11113 less than I-PTA-OH less than PTA-NO less than or equal to meta-I-PTA-PO less than or equal to PTA-OH[2] less than ONO11120[2] less than or equal to ONO11120[1] less than L657926 less than or equal to PTA-OH[1] in human platelets, and I-BOP less than L657925 less than ONO11113 less than or equal to SQ29548 less than ONO11120[2] less than or equal to L657926 less than or equal to PTA-OH[2] less than PTA-TPO less than ONO11120[1] less than I-PTA-OH less than meta-I-PTA-PO less than PTA-NO less than PTA-OH[1] in RASMC.(ABSTRACT TRUNCATED AT 400 WORDS)

Increase in the cytosolic concentration of calcium in platelets of diabetics type II

The intracellular concentration of calcium (Cai) was measured in platelets of healthy control subjects and diabetics type II. Cai was elevated in platelets of diabetics type II under basal conditions, after stimulation by collagen, in the presence and absence of calcium in the incubation medium. The increase in Cai after stimulation by collagen was inhibited by nitrendipine in both kinds of platelets, however Cai remained elevated in platelets of diabetics. Our data and findings of others may be understood as indications of disturbances in the handling of Cai in diabetes.

Stimulation of non-selective cation channels providing Ca2+ influx into platelets by platelet-activating factor and other aggregation inducers

To elucidate the mechanism of the receptor-stimulated Ca2+ entry into human platelets, the influence of Ca(2+)-mobilizing agonists on plasma membrane potential (Em) has been studied. Em changes were registered using potentiometric probe 3,3'-dipropyl-2,2'-thiadicarbocyanine iodide. The agonist effect on Em varied from hyperpolarization to slight and slow rise. On the contrary, after loading of platelets with intracellular Ca2+ indicator quin2, platelet-activating factor (PAF), thrombin, vasopressin, ADP and thromboxane-A2-mimetic U46619 cause substantial transient membrane depolarization. Similar effects were observed after platelet loading with other Ca2+ chelators fura-2 and indo-1. Agonist-induced depolarization considerably reduced if quin2-loaded platelets were suspended in isoosmotic choline-containing medium. Using Ba2+ as a substitute of Ca2+, we have demonstrated that in choline-containing medium PAF-induced Ba2+ entry into platelets results in membrane depolarization. Dependence on Ba2+ concentration and depolarization kinetics correlates with the dose dependence and kinetics of Ba2+ entry detected by quin2 fluorescence. The agonists also stimulate considerable Na+, Li+ and Cs+ inward currents into platelets. Na(+)-dependent depolarization is 2-5-fold suppressed by extracellular Ca2+ [median inhibitory concentration (IC50) approximately 0.3 mM]. Ni2+ and Cd2+ at similar concentrations block Ca2+ entry and agonist-induced Na2+ current (IC50 for both cations approximately 50 microM). Agonist-induced depolarization is blocked by the adenylate cyclase stimulator prostaglandin E1 and the protein kinase C stimulator phorbol ester. It is concluded that agonists stimulate Ca2+ entry into human platelets via receptor-operated channels which are not strictly selective toward divalent cations and are permeable to Na+, Li+ and Cs+.

Eicosanoid-dependent and -independent formation of individual [14C]stearoyl-labelled lysophospholipids in collagen-stimulated human platelets

Low level collagen activation of platelets is mediated via the release of arachidonic acid (AA) from membrane phospholipids and the formation of thromboxane A2 (TxA2). To assess the specific phospholipids undergoing deacylation via phospholipase A2 thereby providing source(s) of releasable AA, we have measured the individual lysophospholipid formations in platelets prelabelled with [14C]stearic acid and incubated with a low level (2 micrograms/ml) or a high level (10 micrograms/ml) of collagen in the absence or presence of BW755C, a dual inhibitor of cyclooxygenase and lipoxygenase activities. Collagen activation resulted in the generation of [14C]stearoyl-labelled lysophosphatidylinositol (lysoPI), lysophosphatidylcholine (lysoPC), lysophosphatidylethanolamine (lysoPE) and lysophosphatidylserine. BW755C significantly inhibited these collagen-induced changes, suggesting that much of the lysophospholipid, and therefore AA release, was eicosanoid-mediated. At the lower level of collagen, considerable generation of [14C]lysoPE was maintained even in the presence of BW755C, suggesting an eicosanoid-independent degradation of phosphatidyl-ethanolamine. The TxA2-dependent release of AA was also investigated in U-46619-stimulated platelets. This TxA2 mimetic induced considerable formation of the 14C-labelled lysophospholipids, including lysoPI and lysoPC, but not lysoPE. These results suggest that an eicosanoid-independent degradation of phosphatidylethanolamine via phospholipase A2 at lower collagen levels may provide a source of the initial AA for conversion to TxA2 and the subsequent deacylation of phosphatidylinositol, phosphatidylcholine, and also phosphatidylserine.

Effects of various inhibitors on platelet activation induced by TP 82, a CD 9 monoclonal antibody

TP 82, a monoclonal antibody against CD 9 antigen, induced human platelet activation at concentrations higher than 0.4 microgram/mL in terms of aggregation, release of intracellular granule contents, production of arachidonic acid metabolites, and elevation of the intracellular Ca2+ concentration. The effects of a competitive inhibitor of ADP, acetylsalicylic acid, EGTA, and GRGDSP which blocks fibrinogen binding to IIb/IIIa complex suggested that each of released ADP, thromboxane A2, extracellular Ca2+, and close cell contact acts together to potentiate platelet activation induced by TP 82. While each of these inhibitors severely suppressed platelet activation induced by lower concentrations of the antibody (less than or equal to 0.8 microgram/mL), that induced by higher concentrations (greater than or equal to 3.2 micrograms/mL) was only partially blocked. Intracellular Ca2+ elevation was totally dependent upon the production of thromboxane A2, regardless of the antibody concentrations.

Collagen-induced platelet activation mainly involves the protein kinase C pathway

This study analyses early biochemical events in collagen-induced platelet activation. An early metabolic event occurring during the lag phase was the activation of PtdIns(4,5)P2-specific phospholipase C. Phosphatidic acid (PtdOH) formation, phosphorylation of P43 and P20, thromboxane B2 (TXB2) synthesis and platelet secretion began after the lag phase, and were similarly time-dependent, except for TXB2 synthesis, which was delayed. Collagen induced extensive P43 phosphorylation, whereas P20 phosphorylation was weak and always lower than with thrombin. The dose-response curves of P43 phosphorylation and granule secretion were similar, and both reached a peak at 7.5 micrograms of collagen/ml, a dose which induced half-maximal PtdOH and TXB2 formation. Sphingosine, assumed to inhibit protein kinase C, inhibited P43 phosphorylation and secretion in parallel. However, sphingosine was not specific for protein kinase C, since a 15 microM concentration, which did not inhibit P43 phosphorylation, blocked TXB2 synthesis by 50%. Sphingosine did not affect PtdOH formation at all, even at 100 microM, suggesting that collagen itself induced this PtdOH formation, independently of TXB2 generation. The absence of external Ca2+ allowed the cleavage of polyphosphoinositides and the accumulation of InsP3 to occur, but impaired P43 phosphorylation, PtdOH and TXB2 formation, and secretion; these were only restored by adding 0.11 microM-Ca2+. In conclusion, stimulation of platelet membrane receptors for collagen initiates a PtdInsP2-specific phospholipase C activation, which is independent of external Ca2+, and might be the immediate receptor-linked response. A Ca2+ influx is indispensable to the triggering of subsequent platelet responses. This stimulation predominantly involves the protein kinase C pathway associated with secretion, and appears not to be mediated by TXB2, at least during its initial stage.

Effects of insulin on vasoconstriction induced by thromboxane A2 in porcine coronary artery

To elucidate the role of insulin in the control of coronary artery tone, its effects on porcine coronary artery contraction evoked by thromboxane A2 (TXA2) were studied in vitro. Ring preparations of porcine proximal coronary artery were suspended in a Magnus apparatus filled with Tyrode's solution at 37 degrees C and aerated with 100% O2, and the isometric tension of the contractions was measured. Insulin itself caused neither contraction nor relaxation. Insulin had no significant effect on the coronary artery contractions evoked by 20 mM K+, norepinephrine, histamine, and serotonin; however, 120 minutes of preincubation with a physiological concentration of insulin (30-300 muunits/ml) significantly accentuated coronary artery contractions evoked by STA2 (10(-11) to 10(-7) M), a stable analogue of TXA2 that is known to act on TXA2/prostaglandin H2 receptors (141.4 +/- 10.9% of the control at 10(-7) M STA2 in the presence of 300 muunits/ml insulin; p less than 0.01). The enhancing effects of insulin on the STA2-induced contractions were affected by extracellular glucose or magnesium ion concentrations. The enhancing effects of insulin were observed only at the glucose concentrations of 100-300 mg/dl and magnesium concentrations of 0.5-1.5 mM. Therefore, insulin was suspected of enhancing TXA2-induced contraction through a process that depends on extracellular glucose and Mg2+.

Dexamethasone selectively attenuates prostanoid-induced vasoconstrictor responses in vitro

Glucocorticoids bind to specific vascular receptors resulting in a variety of functional consequences that may affect vascular smooth muscle behavior. We, therefore, examined in rabbits the effect of treatment with dexamethasone (2.5 mg/kg) for 6 days on vascular responses to pressor prostanoids in aortic and carotid arterial rings and in the isolated perfused kidney. Isometric tension development to prostaglandin F2 alpha and U46619, a thromboxane/prostaglandin endoperoxide mimetic, was markedly reduced in vessels from dexamethasone-treated rabbits. The inhibitory effect of dexamethasone on vascular reactivity was manifested by an increase in the concentration of agonist for threshold tension development and a reduction in the maximal response to prostaglandin F2 alpha and U46619. In contrast, reactivity to phenylephrine, potassium, histamine, or endothelin was not affected by dexamethasone treatment. In addition, pressor responses to prostaglandin F2 alpha and U46619 in Krebs'-perfused kidneys from dexamethasone-treated rabbits were also diminished. These data suggest that dexamethasone selectively interferes with the expression of receptor-mediated contractile responses to eicosanoids.

Extracellular Na+, but not Na+/H+ exchange, is necessary for receptor-mediated arachidonate release in platelets

The effect of extracellular Na+ removal and replacement with other cations on receptor-mediated arachidonate release in platelets was studied to investigate the role of Na+/H+ exchange in this process. Replacement with choline+, K+, N-methylglucamine+ (which abolished the thrombin-induced pHi rise) or Li+ (which allowed a normal thrombin-induced pHi rise) significantly decreased arachidonate release in response to all concentrations (threshold to supra-maximal) of thrombin and collagen. This inhibition was not reversed by NH4Cl (10 mM) addition, which raised the pHi in the absence of Na+, but, on the contrary, NH4Cl addition further decreased the extent of thrombin- and collagen-induced arachidonate release, as well as decreasing 'weak'-agonist (ADP, adrenaline)-induced release and granule secretion in platelet-rich plasma. No detectable pHi rises were seen with collagen (1-20 micrograms/ml) and ADP (10 microM) in bis-(carboxyethyl)carboxyfluorescein-loaded platelets. Inhibition of thrombin-induced pHi rises was seen with 0.5-5 microM-5-NN-ethylisopropylamiloride (EIPA), but at these concentrations EIPA had little effect on thrombin-induced arachidonate release. At higher concentrations such as those used in previous studies (20-50 microM), EIPA inhibited aggregation/release induced by collagen and ADP in Na+ buffer as well as in choline+ buffer (where there was no detectable exchanger activity), suggesting that these concentrations of EIPA exert 'non-specific' effects at the membrane level. The results suggest that (i) Na+/H+ exchange and pHi elevations are not only necessary, but are probably inhibitory, to receptor-mediated arachidonate release in platelets, (ii) inhibition of receptor-mediated release in the absence of Na+ is most likely due to the absent Na+ ion itself, and (iii) caution should be exercised in the use of compounds such as EIPA, which, apart from inhibiting the Na+/H+ exchanger, have other undesirable and misleading effects in platelets.

Diets rich in n-9, n-6 and n-3 fatty acids differentially affect the generation of inositol phosphates and of thromboxane by stimulated platelets, in the rabbit

We have studied the effects of semi-synthetic diets rich in either n-9 (olive oil, OO) or n-6 (corn oil, CO), or n-3 (fish oil, FO, as MaxEPA) fatty acids on the levels of major PUFA in platelet lipids, on the generation of inositol phosphates by [3H]inositol labelled platelets after stimulation with thrombin and of thromboxane B2 (TxB2) by platelet rich plasma (PRP) after stimulation with collagen. The predicted elevations of oleic (OA), linoleic (LA) and eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids were observed in platelet lipids of each animal group, but in the MaxEPA fed group accumulation of EPA was associated with depletion of linoleic acid (LA) rather than of arachidonic acid (AA). Basal levels of inositol-tris-phosphate (IP3) in platelets were lowest in the OO group and highest in the CO group, whereas the increment after thrombin stimulation (1 unit/ml NIH) was maximal in the OO group and minimal in the FO group. Instead, when generation of TxB2 by stimulated platelets was evaluated, no appreciable difference among the various groups could be detected, in accordance with the limited modifications of platelet AA content induced by the diets. The overall data indicate that dietary fatty acids modulate the pathway of inositol phosphate generation in rabbit platelets, independently of modifications of TxB2 production.

Blockade of ADP-induced Ca2+-signal and platelet aggregation by lipoxygenase inhibitors

Stimulation of platelets results in the liberation of arachidonic acid (AA) which is further metabolized via the cyclooxygenase or lipoxygenase (LPG) pathway. We have examined the effect of inhibition of LPG on (i) the ADP-induced increase of cytoplasmic Ca2+ concentration and (ii) platelet aggregation. Lipoxygenase inhibitors, nordigidroguaiaretic acid (NDGA) and BW-755C, both suppressed ADP-induced Ca2+-signals and aggregation in a dose-dependent manner, with an IC50 value of 1 2 microM for NDGA. Qualitatively the same effect was obtained with 4-bromophenylacyl bromide, the inhibitor of phospholipases A2 and C. By contrast, cyclooxygenase inhibitor indomethacin had only a negligible effect on Ca2+-signals and suppressed only the second phase of ADP-induced aggregation. It is concluded that the LPG pathway of AA metabolism in platelets might play a crucial role in ADP-induced Ca2+-signal generation and platelet aggregation.

Aggregation and/or oxygenated products of arachidonic acid are not required for collagen-induced deacylation of phosphatidylcholine in human platelets

In the present study the effects of collagen on platelet aggregation and arachidonic acid (AA) mobilization, specifically from phosphatidylcholine (PC), were investigated in the presence and absence of BW755C, a selective inhibitor of cyclo-oxygenase and lipoxygenases. The inhibition of cyclo-oxygenase and lipoxygenase(s) by BW755C (75 microM) resulted in severe impairment in collagen-induced platelet aggregation. In the presence of BW755C, the aggregation response amounted to 14, 26, 37 and 49% of the corresponding controls (without BW755C) at 10, 25, 50 and 100 micrograms of collagen respectively. On the contrary, the amount of AA released from PC, which ranged from 3.5 to 8.6 nmol/10(9) platelets, in response to the action of collagen (10-100 micrograms) remained unaffected by the presence of BW755C. Substantial amounts of non-esterified AA were detected in the free fatty acid fractions obtained from collagen-stimulated platelets in the presence as well as in the absence of BW755C. However, the presence of BW755C caused a greater accumulation of free AA (mass) and this ranged from 4 to 16 nmol, depending upon the amount of collagen. In addition, small increases in free stearic and oleic acids were observed in collagen-stimulated platelets as compared with unstimulated platelets. The amount of AA lost from PC represented 67, 80, 49 and 52% of the free AA obtained at 10, 25, 50 and 100 micrograms of collagen respectively. Our results adhesion of platelets to collagen fibres may be responsible for much of the AA release from PC Furthermore, these results demonstrate that aggregation and/or cyclo-oxygenase/lipoxygenase metabolites are not obligatory for the release of AA from PC in collagen-stimulated human platelets.

Adenine nucleotides mobilize cellular Ca2+ and inhibit parathyroid hormone secretion

Measurements of the concentration of intracellular free calcium [( Ca2+]i) were used to screen for the presence of Ca2+-mobilizing receptors on dissociated and purified bovine parathyroid cells loaded with fura-2. Among a wide variety of agents known to mobilize cellular Ca2+ in other cells, only ATP and certain other nucleotides were capable of altering [Ca2+]i in parathyroid cells. The addition of ATP or adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) (10-200 microM) to parathyroid cells evoked a rapid and transient increase that was followed by a small, steady-state increase in [Ca2+]i. Cytosolic Ca2+ transients elicited by ATP or ATP gamma S persisted in the absence of extracellular Ca2+ and presence of a mitochondrial uncoupler but were blocked by pretreatment with ionomycin or fluoride. Cytosolic Ca2+ transients elicited by ATP were inhibited by increased concentrations of extracellular Ca2+, Mg2+, or Sr2+. Conversely, ATP depressed increases in [Ca2+]i elicited by these extracellular divalent cations. Parathyroid hormone (PTH) secretion was inhibited by ATP gamma S but not by those nucleotides that were without effect on [Ca2+]i. Loading cells with 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and fura-2 blocked cytosolic Ca2+ transients elicited by ATP gamma S but did not block the inhibitory effects of ATP gamma S on PTH secretion. The results show that the activation of a calcium-mobilizing receptor, in this case by ATP gamma S, is sufficient to inhibit PTH secretion. This favors the view that extracellular Ca2+ acts via a Ca2+-mobilizing receptor to regulate PTH secretion.

Fish oil affects phosphoinositide turnover and thromboxane A metabolism in cultured vascular muscle cells

Fish oil has been reported as having beneficial effects on cardiovascular diseases. Elevated serum lipoproteins, prostaglandins and intracellular free calcium concentrations [( Ca2+]i) of the vasculature and thus the phosphoinositide (PI) turnover may be involved in the pathogenesis of these disorders. Therefore, the effect of fish oil on the potency of both low-density lipoprotein (LDL) and angiotensin II (AII) to stimulate the PI turnover in cultured rat vascular smooth muscle cells (VSMC) has been studied. Furthermore, a possible link between PI turnover activity and thromboxane A2 (TXA2) metabolism in these cells has been investigated. In VSMC cultured for up to 7 weeks with either fish oil or n-3 eicosapentaenoic acid (EPA) a decrease to 5-48% of the LDL-induced inositol trisphosphate (IP3) formation (= 100%) was found. A similar range of decreased IP3 synthesis was observed, when AII was used instead of LDL. Both LDL- and AII-stimulated TXA2 synthesis was suppressed concomitantly within the range 34-60%. Blockade of VSMC TXA2 biosynthesis with either indomethacin or TXA2 synthetase blocker (SQ-80338) inhibited LDL-induced formation of IP3 in a dose-dependent manner. Similar results were obtained, when TXA2 receptor coupling antagonists (SQ-27427 or BM-13177) were used. However, blockers of TXA2 synthesis and of TXA2 receptor binding failed to affect AII-induced formation of IP3.

Association of phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase activities with the cytoskeleton in human platelets

The inositol lipid kinases were investigated in the cytoskeletons of human platelets. In the absence of added lipids the kinases were only barely detectable in the Triton-soluble fractions and undetectable in cytoskeletons of resting cells. However at least 30% of the total phosphatidylinositol kinase was present in the cytoskeleton as revealed by saturation of the enzyme. Phosphatidylinositol 4-phosphate kinase was also found in significant amounts in the cytoskeletons. On the other hand, both enzymes being only recovered in the particulate fraction of the cells, we suggest that inositol lipid kinases may be present near the anchoring points of the cytoskeletons at the membranes.

Cyclic nucleotide elevating vasodilators inhibit platelet aggregation at an early step of the activation cascade

The mechanism of vasodilator-induced inhibition of platelet aggregation was investigated in human platelets. Cyclic nucleotide-elevating vasodilators stimulated cAMP- or cGMP-dependent protein phosphorylation, inhibited the activation of both protein kinase C and myosin light chain kinase, and inhibited the thrombin-induced hydrolysis of phosphatidylinositol-4,5-bisphosphate without affecting its resynthesis. The results suggest that cAMP- and cGMP-elevating vasodilators both inhibit platelet aggregation at an early step of the activation cascade, presumably at the level of phospholipase C.