NAADP/SERCA3-Dependent Ca2+ Stores Pathway Specifically Controls Early Autocrine ADP Secretion Potentiating Platelet Activation.

RATIONALE: Ca2+ signaling is a key and ubiquitous actor of cell organization and its modulation controls many cellular responses. SERCAs (sarco-endoplasmic reticulum Ca2+-ATPases) pump Ca2+ into internal stores that play a major role in the cytosolic Ca2+ concentration rise upon cell activation. Platelets exhibit 2 types of SERCAs, SERCA2b and SERCA3 (SERCA3 deficient mice), which may exert specific roles, yet ill-defined. We have recently shown that Ca2+ mobilization from SERCA3-dependent stores was required for full platelet activation in weak stimulation conditions. OBJECTIVE: To uncover the signaling mechanisms associated with Ca2+ mobilization from SERCA3-dependent stores leading to ADP secretion. METHODS AND RESULTS: Using platelets from wild-type or Serca3-deficient mice, we demonstrated that an early (within 5-10 s following stimulation) secretion of ADP specifically dependent on SERCA3 stored Ca2+ is exclusively mobilized by nicotinic acid adenosine dinucleotide-phosphate (NAADP): both Ca2+ mobilization from SERCA3-dependent stores and primary ADP secretion are blocked by the NAADP receptor antagonist Ned-19, and reciprocally both are stimulated by permeant NAADP. In contrast, Ca2+ mobilization from SERCA3-dependent stores and primary ADP secretion were unaffected by inhibition of the production of IP3 (inositol-1,4,5-trisphosphate) by phospholipase-C and accordingly were not stimulated by permeant IP3. CONCLUSIONS: Upon activation, an NAADP/SERCA3 Ca2+ mobilization pathway initiates an early ADP secretion, potentiating platelet activation, and a secondary wave of ADP secretion driven by both an IP3/SERCA2b-dependent Ca2+ stores pathway and the NAADP/SERCA3 pathway. This does not exclude that Ca2+ mobilized from SERCA3 stores may also enhance platelet global reactivity to agonists. Because of its modulating effect on platelet activation, this NAADP-SERCA3 pathway may be a relevant target for anti-thrombotic therapy. Graphic Abstract: A graphic abstract is available for this article.

PDK1 Determines Collagen-Dependent Platelet Ca2+ Signaling and Is Critical to Development of Ischemic Stroke In Vivo.

OBJECTIVE: Activation of platelets by subendothelial collagen results in an increase of cytosolic Ca(2+) concentration ([Ca(2+)]i) and is followed by platelet activation and thrombus formation that may lead to vascular occlusion. The present study determined the role of phosphoinositide-dependent protein kinase 1 (PDK1) in collagen-dependent platelet Ca(2+) signaling and ischemic stroke in vivo. APPROACH AND RESULTS: Platelet activation with collagen receptor glycoprotein VI agonists collagen-related peptide or convulxin resulted in a significant increase in PDK1 activity independent of second-wave signaling. PDK1 deficiency was associated with reduced platelet phospholipase Cγ2-dependent inositol-1,4,5-trisphosphate production and intracellular [Ca(2+)]i in response to stimulation with collagen-related peptide or convulxin. The defective increase of [Ca(2+)]i resulted in a substantial defect in activation-dependent platelet secretion and aggregation on collagen-related peptide stimulation. Furthermore, Rac1 activation and spreading, adhesion to collagen, and thrombus formation under high arterial shear rates were significantly diminished in PDK1-deficient platelets. Mice with PDK1-deficient platelets were protected against arterial thrombotic occlusion after FeCl3-induced mesenteric arterioles injury and ischemic stroke in vivo. These mice had significantly reduced brain infarct volumes, with a significantly increased survival of 7 days after transient middle cerebral artery occlusion without increase of intracerebral hemorrhage. Tail bleeding time was prolonged in pdk1(-/-) mice, reflecting an important role of PDK1 in primary hemostasis. CONCLUSIONS: PDK1 is required for Ca(2+)-dependent platelet activation on stimulation of collagen receptor glycoprotein VI, arterial thrombotic occlusion, and ischemic stroke in vivo.

Low baseline serotonin-2A receptors predict clinical response to olanzapine in first-episode schizophrenia patients.

The purpose of this study was to determine whether platelet serotonin-2A (5-HT2A) binding sites and inositol 1,4,5 trisphosphate (IP3) concentrations before treatment can identify olanzapine-responsive patients. The study included 21 never medicated, first-episode schizophrenia patients (antipsychotic-naïve) and 21 patients with a DSM-IV-TR diagnosis of paranoid schizophrenia who had not received depot antipsychotic treatment in the previous 6 months or oral antipsychotic or antidepressant treatment in the previous 2 months (antipsychotic-free). In the antipsychotic-naïve group, olanzapine responders had a significantly lower number of 5-HT2A receptors and lower IP3 concentrations at baseline than non-responders. The combination of baseline 5-HT2A and IP3 values significantly predicted an improvement in negative symptomatology after 6 weeks of treatment with olanzapine. In the antipsychotic-free group, responders had significantly higher positive and lower negative symptomatology at baseline, together with a reduced number of 5-HT2A receptors. However, basal 5-HT2A receptors or IP3 concentrations did not significantly predict positive, negative or general clinical response. The reported results suggest that platelet 5-HT2A binding might be a trait marker that could help to identify those patients likely to show greater improvement in negative symptomatology after olanzapine treatment.

Inositol 1,4,5-triphosphate-mediated shuttling between intracellular stores and the cytosol contributes to the sustained elevation in cytosolic calcium in FMLP-activated human neutrophils.

The current study was designed to probe Ca2+ shuttling between intracellular stores and the cytosol as a potential mechanism contributing to the prolongation of elevated Ca2+ transients in N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP)-activated human neutrophils. Cytosolic Ca2+ concentrations and transmembrane fluxes of the cation were measured using spectrofluorimetric and radiometric procedures, respectively, while inositol 1,4,5-triphosphate (IP3) was measured using a radioreceptor assay. The Ca2+-chelating agent, ethylene glycol-bis (beta-aminoethyl ether) N,N,N'N'-tetraacetic acid (EGTA; 10mM), was used to exclude store-operated influx of Ca2+ into neutrophils, while the IP3 receptor antagonist, 2-aminoethoxydiphenyl borate (2-APB, 100 microM), added to the cells 10s after FMLP (0.01 and 1 microM), at which time the increases in IP3 and cytosolic Ca2+ were maximal, was used to eliminate both sustained release from stores and influx of Ca2+. Addition of FMLP at 0.01 or 1 microM resulted in equivalent peak increases in cytosolic Ca2+, while the increase in IP3 was greater and the rate of clearance of Ca2+ from the cytosol slower, in cells activated with 1 microM FMLP. Treatment of the cells with either EGTA or 2-APB following addition of 1 microM FMLP, completely (EGTA) or almost completely (2-APB) abolished the influx of Ca2+ and accelerated the rate of clearance of the cation from the cytosol. Post-peak cytosolic Ca2+ concentrations were lower, and the Ca2+ content of the stores higher, in cells treated with 2-APB. The involvement of IP3 was confirmed by similar findings in cells treated with U-73122 (1 microM), a selective inhibitor of phospholipase C. Taken together, these observations are compatible with IP3-mediated Ca2+ shuttling in neutrophils activated with FMLP.

Platelet serotonergic markers as endophenotypes for obsessive-compulsive disorder.

Although compelling evidence has shown that obsessive-compulsive disorder (OCD) has a strong genetic component, its genetic basis remains to be elucidated. Identifying biological abnormalities in nonaffected relatives is one of the strategies advocated to isolate genetic vulnerability factors in complex disorders. Since peripheral serotonergic disturbances are frequently observed in OCD patients, the aim of this study was to investigate if they could represent endophenotypes, by searching for similar abnormalities in the unaffected parents of OCD patients. We assessed whole blood serotonin (5-HT) concentration, platelet 5-HT transporter (5-HTT) and 5-HT2A receptor-binding characteristics, and platelet inositol trisphosphate (IP3) content in a sample of OCD probands (n = 48) and their unaffected parents (n = 65), and compared them with sex- and age-matched controls (n = 113). Lower whole blood 5-HT concentration, fewer platelet 5-HTT-binding sites, and higher platelet IP3 content were found in OCD probands and their unaffected parents compared to controls. Whole blood 5-HT concentration showed a strong correlation within families (p < 0.001). The only parameter that appeared to discriminate affected and unaffected subjects was 5-HT2A receptor-binding characteristics, with increased receptor number and affinity in parents and no change in OCD probands. The presence of peripheral serotonergic abnormalities in OCD patients and their unaffected parents supports a familial origin of these disturbances. These alterations may serve as endophenotypic markers in OCD, and could contribute to the study of the biological mechanisms and genetic underpinnings of the disorder.

A defect in collagen receptor-Ca2+ signaling system in platelets from cattle with Chediak-Higashi syndrome.

Decreased platelet aggregation to collagen is a cause for bleeding diathesis of Chediak-Higashi syndrome (CHS). We investigated whether the collagen receptor-Ca2+ signaling system was impaired in platelets from cattle affected with CHS. A collagen-induced increase in cytosolic Ca2+ ([Ca2+]i) was depressed in CHS platelets, which was accompanied by a decrease in the production of inositol 1,4,5-trisphosphate. When the influences of endogenous arachidonic acid metabolites and ADP were excluded, convulxin or collagen-related peptide, which are specific agonists for the collagen receptor GPVI, increased [Ca2+]i in both normal and CHS platelets. In contrast, rhodocytin, which was thought to activate another collagen receptor GPIa/IIa, increased [Ca2+]i in CHS platelets to a lesser extent than in normal ones. Cytochalasin D, an actin polymerization inhibitor, depressed the response to collagen or rhodocytin but not the response to convulxin. Adhesion of CHS platelets to acid soluble type I collagen, which was mediated by GPIa/IIa, was similar to that of normal platelets. These results suggest that a defect in the rhodocytin-sensitive pathway is responsible for decreasing the response to collagen in CHS platelets. It remains to be determined which receptor is associated with the mechanism.

Sevoflurane does not inhibit human platelet aggregation induced by thrombin.

BACKGROUND: Sevoflurane reportedly inhibits adenosine diphosphate-induced platelet aggregation by suppressing thromboxane A2 formation. The increase in intracellular calcium concentration that fosters platelet aggregation, however, is also induced by other cell signaling pathways, such as activation of the production of inositol 1,4,5-triphosphate by thrombin. The current study aimed to clarify the net influence of sevoflurane on thrombin-induced platelet aggregation. METHODS: Washed platelets were stimulated by thrombin after incubation with 0.5, 1.0, or 1.5 mM sevoflurane, halothane, or isoflurane. Aggregation curves were measured by an aggregometer. Intracellular calcium concentration was measured fluorometrically using fura-2. Calcium mobilization via plasma membrane calcium channels and the dense tubular system was assessed differentially. Intracellular inositol 1,4,5-triphosphate was measured by radioimmunoassay. RESULTS: Halothane significantly suppressed aggregation ratios at 5 min compared with those in controls (89 +/- 7%) to 71 +/- 10% (1.0 mM) and 60 +/- 11% (1.5 mM) and the increase in intracellular calcium concentration (controls, 821 +/- 95 nM vs. 440 +/- 124 nM [1.0 mM] or 410 +/- 74 nM [1.5 mM]). Halothane also significantly inhibited release of calcium from the dense tubular system (controls, 220 +/- 48 nM vs. 142 +/- 31 nM [1.0 mM]). Neither sevoflurane nor isoflurane produced a net change in aggregation ratios, intracellular calcium concentration, or calcium mobilization. Halothane (1 mM) significantly suppressed inositol 1,4,5-triphosphate concentrations, whereas neither 1 mM isoflurane nor 1 mM sevoflurane had any effect. CONCLUSIONS: Although sevoflurane has been reported to inhibit human platelet aggregation induced by weak agonists such as adenosine diphosphate, it does not inhibit human platelet aggregation induced by strong agonists such as thrombin.

Decreased platelet serotonin transporter sites and increased platelet inositol triphosphate levels in patients with unipolar depression: effects of clomipramine and fluoxetine.

BACKGROUND: The central serotonergic system has been implicated in the pathophysiology of depression and in the mechanism of the action of antidepressant drugs. The human platelet has been proposed as a peripheral model of central serotonergic neurons. METHODS: Six peripheral serotonergic parameters were determined simultaneously in 27 patients with unipolar depression before and after 2, 4, and 12 weeks of clomipramine or fluoxetine treatment according to the psychiatrist. RESULTS: In patients with depression versus matched control subjects, platelet [3H]paroxetine binding sites were found to be significantly decreased (2.10 +/- 0.70 versus 3.88 +/- 0.77 fmol/10(9) platelets; P = .0001), platelet serotonin (5-HT) content was found to be significantly decreased (1.90 +/- 1.52 versus 2.74 +/- 1.12 nmol/10(9) platelets; P = .001), and platelet inositol triphosphate levels were found to be significantly increased (2.85 +/- 0.70 versus 1.85 +/- 0.77 fmol/10(9) platelets; P = .0001). No significant difference between patients and control subjects was found for platelet [3H]-lysergic acid diethylamide ([3H]LSD) binding sites, aggregation tests with 5-HT or adenosine diphosphate and plasma 5-HT levels. Treatment with both clomipramine and fluoxetine gradually further reduced the density of platelet [3H]paroxetine binding sites and induced a dramatic decrease in platelet and plasma 5-HT levels. With clomipramine, the decreased blood 5-HT levels are associated with increased platelet [3H]LSD binding sites and aggregation responses. After 12 weeks, nonresponders to both treatments had platelet inositol triphosphate levels that were still increased (2.81 +/- 0.75 fmol/10(9) platelets) when responders levels were not different from those of control subjects (1.41 +/- 0.45 versus 1.70 +/- 0.25 fmol/10(9) platelets). CONCLUSIONS: Drug-free patients with depression had simultaneously decreased 5-HT transporter (5-HTT) sites and overstimulated phosphoinositide signaling systems. Clomipramine and fluoxetine treatments, which further decreased the density of 5-HTT sites, allowed platelet inositol triphosphate levels to return to normal values only in responders.

Propofol has both enhancing and suppressing effects on human platelet aggregation in vitro.

BACKGROUND: Volatile anesthetics are known to suppress platelet aggregation. In contrast, there is conflicting information regarding the effect of propofol on platelet function. The present study was designed to clarify the effects of propofol on platelet function and the mechanisms underlying these effects. METHODS: Propofol or an equivalent volume of 10% Intralipos (as a control) was added to test tubes 5 min before the induction of each reaction. Platelet aggregation induced by epinephrine, arachidonic acid (AA), prostaglandin G2 (PGG2), or STA2 (a thromboxane A2 [TXA2] analog) was measured using an eight-channel aggregometer. To determine type 1 cyclooxygenase activity, AA (0.5 mM) was added to an assay mixture containing type 1 cyclooxygenase, and the concentration of the final product, malonaldehyde, was measured by spectrophotometry. Epinephrine-, adenosine diphosphate-, AA-, and PGG2-induced TXA2 formation was measured using a commercially available radioimmunoassay kit. To estimate TXA2 receptor-binding affinity, 3H-S145, a specific TXA2 receptor antagonist, was added, and the radioactivity of receptor-bound 3H-S145 was determined using a liquid scintillation analyzer. Inositol 1,4,5-triphosphate formation was measured in STA2-stimulated platelets using a commercially available inositol 1,4,5-triphosphate assay kit. RESULTS: Propofol (40 microM) enhanced, whereas 100 microM suppressed, adenosine diphosphate- and epinephrine-induced secondary aggregation without affecting primary aggregation. Propofol (40 microM) also enhanced, but 100 microM suppressed, AA-induced aggregation. Propofol (100 microM) enhanced PGG2- and STA2-induced aggregation. Propofol (100 microM) suppressed AA-induced TXA2 formation but did not alter that induced by PGG2. Propofol (30-100 microM) suppressed AA-induced malonaldehyde formation, indicating inhibition of type 1 cyclooxygenase activity. Propofol did not alter TXA2 receptor-binding affinity. Propofol (30 and 100 microM) augmented inositol 1,4,5-triphosphate formation in STA2-stimulated platelets. CONCLUSIONS: The present findings clearly indicate that high concentrations of propofol suppress the activity of type 1 cyclooxygenase, the enzyme that converts AA to PGG2. Furthermore, propofol also enhanced STA2-induced inositol 1,4,5-triphosphate formation. These results may explain the inconsistent findings of previous investigators.

Phosphoinositide signalling system in platelets of schizophrenic patients and the effect of neuroleptic therapy.

Alterations in the phosphoinositide signalling system have been proposed as a possible biological marker of schizophrenia. We studied the levels of inositol 1,4,5-trisphosphate (IP3), cytosolic Ca2+ concentrations ([Ca2+]i), and the incorporation of [32P]-orthophosphate into inositol phospholipids and phosphatidic acid (PA) in blood platelets of neuroleptic-treated schizophrenics in comparison with controls. The [Ca2+]i was significantly higher in platelets of one month neuroleptic-treated patients (155+/-5.8 nM) in comparison with controls (95+/-5.4 nM). Neuroleptic therapy decreased the [Ca2+]i, but even after long-term therapy it remained significantly higher (114+/-5.7 nM) than in controls. Differences were also found in the level of IP3 between controls (30+/-4.0 pmol/10(9) platelets), drug-free schizophrenics (52+/-9.0 pmol/10(9) platelets) and treated patients (50+/-6.0 pmol/10(9) platelets). The increased turnover of PA was observed in platelets of neuroleptic-treated schizophrenic patients. The study suggests that the regulation of calcium homeostasis and pathways involved in the phosphoinositide signalling system are altered in the platelets of schizophrenics. Neuroleptic therapy did not remove the observed changes in [Ca2+]i and IP3 levels.

Evidence for separate effects of U73122 on phospholipase C and calcium channels in human platelets.

U73122 ((1-[6-(( 17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)exyl]-1H-p yrrole-2,5-dione)) is generally used as a selective inhibitor of phospholipase C (PLC) and the related rise in cytosolic Ca2+. Recently, by using hepatocytes, it was suggested that its action sites are different for PLC activation and increase in Ca2+ concentration. To verify whether U73122 has different sites for inhibiting PLC activation and calcium responses in human platelets, aggregation, Mn2+ influx, cytosolic Ca2+ increase and PLC activation were studied in response to thrombin and the synthetic agonist of the thromboxane receptor U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxyprostaglandin F2alpha). With both agonists, U73122 inhibited aggregation, Mn2+ influx and the enhancement of cytosolic calcium at concentrations of 2 microM or lower, while 10 microM was necessary to inhibit PLC activation. Our results suggested that U73122 is much more active in antagonizing Ca2+ channels, both the intracellular ones, which are activated by formation of inositol 1,4,5 P3 and those present on plasma membrane, than in reducing the activation of PLC.

Involvement of cyclic AMP generation in the inhibition of respiratory burst by 2-phenyl-4-quinolone (YT-1) in rat neutrophils.

The inhibitory effect of 2-phenyl-4-quinolone (YT-1) on respiratory burst in rat neutrophils was investigated, and the underlying mechanism of action was assessed. YT-1 caused a concentration-dependent inhibition of the rate of O2.- release from rat neutrophils in response to formylmethionyl-leucyl-phenylalanine (fMLP), but not to phorbol 12-myristate 13-acetate (PMA), with an IC50 value of 60.7+/-8.2 microM. A comparable effect was also demonstrated in the inhibition of O2 consumption. Unlike superoxide dismutase, YT-1 had no effect on O2.- generation in the xanthine-xanthine oxidase system and during dihydroxyfumaric acid autoxidation. The fMLP-induced inositol trisphosphate (IP3) formation was unaffected by YT-1. In addition, YT-1 did not affect the initial spike of [Ca2+]i, but it accelerated the rate of [Ca2+]i decline in cells in response to fMLP. YT-1 was found to have little effect on the activity of neutrophil cytosolic protein kinase C (PKC). YT-1 increased the cellular cyclic AMP level, while having no effect on the cyclic GMP level. In addition, YT-1 increased neutrophil cytosolic protein kinase A (PKA) activity, but had no direct effect on the enzyme activity of pure porcine heart PKA. When neutrophils were treated with (8R,9S,11S)-(-)-9-hydroxy-9-hexoxycarbonyl-8-methyl-2,3,9,10-tetra hydro-8,11-epoxy- 1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[cde]trinde n-1-one, (KT 5720), a PKA inhibitor, the inhibition of O2.- generation by YT-1, as well as by prostaglandin E1 (PGE1) and dibutyryl cyclic AMP, was attenuated effectively. YT-1 did not activate the adenylate cyclase associated with neutrophil particulate fraction but inhibited the cytosolic phosphodiesterase (PDE) activity in a concentration-dependent manner. Neutrophils treated with YT-1 had a more pronounced increase in cellular cyclic AMP level by PGE1. Moreover, the ability of PGE1 to inhibit the respiratory burst in neutrophils was greatly enhanced by YT-1. These results suggest that the increase in cellular cyclic AMP levels by YT-1 through the inhibition of PDE (probably PDE4 isoenzyme) activity is involved in its inhibition of fMLP-induced respiratory burst in rat neutrophils.

Comparison of the effects of hydrogen peroxide, 4-hydroxy-2-nonenal and beta-amyloid (25-35) upon calcium signalling.

The neurotoxic beta-amyloid (Abeta) peptide fragment Abeta(25-35) has been suggested to exert its deleterious effects on cells via production of hydrogen peroxide. In human platelets and in the presence of DMSO to prevent production of hydroxyl radicals from hydrogen peroxide, both Abeta(25-35) and hydrogen peroxide were found to increase intracellular calcium levels. Hydrogen peroxide in addition reduced the calcium response to thrombin, whereas this was not seen with Abeta(25-35). A similar pattern of effects to those seen with hydrogen peroxide were also seen with the neurotoxic aldehyde lipid peroxidation product 4-hydroxy-2-nonenal (HNE). The initial increase in calcium produced by hydrogen peroxide was not affected by EGTA, but was partially prevented by dithiothreitol. The calcium response to Abeta(25-35) [which was also seen with Abeta(1-40) and Abeta(1-42) but not with the inactive peptide Abeta(40-1)] consisted of an EGTA-sensitive and an EGTA-resistant component, of which the latter was also sensitive to DTT. Hydrogen peroxide increased basal phosphoinositide breakdown in rat brain miniprisms and decreased the responses to noradrenaline, carbachol and veratrine. The specific binding of [3H]inositol-1,4,5-trisphosphate ([3H]Ins(1,4,5)P3) to its receptor recognition site in human platelet membranes was increased by Abeta(25-35) but remained unchanged following hydrogen peroxide treatment. It is concluded that under conditions where production of hydroxyl radicals from hydrogen peroxide is blocked, hydrogen peroxide and Abeta(25-35) produce their effects on calcium by affecting the mobilisation of intracellular calcium. The qualitative differences in the calcium responses of these two agents can be explained (a) by an additional effect of Abeta(25-35) upon calcium entry and (b) by differences in their effects upon the Ins(1,4,5)P3 receptor.

Effect of PP1D-1, a synthetic antiplatelet compound, on rabbit platelets.

The antiplatelet mechanism of a synthetic compound, 2-chloro-3-methoxycarbonylpropionamido-1,4-naphthoquinone (PP1D-1), was studied by employing washed rabbit platelets in vitro. PP1D-1 concentration-dependently inhibited thrombin (0.1 U/ml)-, platelet-activating factor (2 ng/ml)-, collagen (10 microg/ml)-, arachidonic acid (100 microM)- and U46619 (1 microM)-induced aggregation and ATP release in washed rabbit platelets. The IC50 values of PP1D-1 for aggregation induced by the above inducers are 17.9+/-1.7, 9.8+/-1.1, 3.9+/-0.4, 1.8+/-0.3 and 1.7+/-0.3 microM, respectively. PP1D-1 did not affect platelet thromboxane B2 or prostaglandin D2 formation induced by arachidonic acid, indicating that it did not affect cyclooxygenase and thromboxane synthase activities. PP1D-1 significantly inhibited the formation of inositol 1,4,5-trisphosphate caused by these five platelet stimulators. Moreover, PP1D-1 inhibited the increase in intracellular calcium concentration induced by these agents. On the contrary, PP1D-1 did not inhibit thapsigargin-elevated intracellular calcium concentration in indomethacin-pretreated platelets, indicating it did not influence the effect of thapsigargin. According to these data, PP1D-1 exerts antiplatelet effects mainly by inhibiting phosphoinositide turnover.

Enhancement of ADP-induced aggregation by 5-HT in rabbit platelets.

AIM: To study the enhanced effects of 5-hydroxytryptamine (5-HT) on ADP-induced aggregation. METHODS: Platelet aggregation was quantified by the light transmission, the cytosolic-free calcium ([Ca2+]i) was measured by digital fluorescent microscopy, and inositol 1,4,5-triphosphate (IP3) was determined by receptor binding assay. RESULTS: In rabbit platelet-rich plasma (PRP), 5-HT 0.03-3 mumol.L-1 induced a decrease in light transmission (DLT) in a concentration-dependent manner with centralization of granules, as revealed by electron microscopy. The DLT was accompanied with neither platelet aggregation nor a release reaction. In single washed platelets loaded with Fura-2, 5-HT caused a concentration-dependent elevation of [Ca2+]i, and IP3 level was also transiently increased in washed platelets at 15 s after stimulation by 5-HT. Adenosine diphosphate (ADP) also caused DLT transiently in PRP before its own aggregation without a release reaction. Pretreatment of PRP or washed platelets with 5-HT, the DLT by ADP was reduced concentration-dependently and ADP-induced aggregation and [Ca2+]i mobilization were enhanced. CONCLUSION: The enhancement of ADP-induced aggregation was attributed to the superimposition of the calcium release from the storage sites and calcium influx induced by ADP over the calcium release from the storage sites by 5-HT.

Propionic acid stimulates superoxide generation in human neutrophils.

Short-chain carboxylic acids are the metabolic by-products of pathogenic anaerobic bacteria and are found at sites of infection in millimolar quantities. We previously reported that propionic acid, one of the short-chain carboxylic acids, induces an increase in intracellular Ca2+ ([Ca2+]i) in human neutrophils. Here we investigate the effect of propionic acid on superoxide generation in human neutrophils. Propionic acid (10 mm) induced inositol 1,4, 5-trisphosphate (IP3) formation and a rapidly transient increase in [Ca2+]i, but not superoxide generation, whereas 1 microm formylmethionyl-leucyl-phenylalanine (fMLP), a widely used neutrophil-stimulating bacterial peptide, stimulated not only IP3 formation and Ca2+ mobilization but also superoxide generation. The IP3 level induced by propionic acid was slightly lower than that induced by fMLP. The transient increase in [Ca2+]i induced by propionic acid immediately returned to the basal level, whereas a sustained increase in [Ca2+]i, which was higher than the basal level, following a transient increase in [Ca2+]i was induced by fMLP. The peak level induced by propionic acid was lower than that with fMLP. In the absence of extracellular Ca2+, thapsigargin, a potent inhibitor of endoplasmic reticulum Ca2+-ATPase, induced an increase in [Ca2+]i even after propionic acid stimulation, but not after fMLP. The Ca2+ ionophore A23187 and thapsigargin induced superoxide generation by themselves. Propionic acid enhanced the superoxide generating effect of A23187 and thapsigargin. These results suggest that Ca2+ mobilization induced by propionic acid is much weaker than that with fMLP, and propionic acid is able to generate superoxide in the presence of a Ca2+ ionophore and a Ca2+ influx activator.

Stimulation of intracellular Ca2+ levels in human neutrophils by soluble immune complexes. Functional activation of FcgammaRIIIb during priming.

Soluble immune complexes bind to unprimed neutrophils and generate intracellular Ca2+ transients but fail to activate the NADPH oxidase. Following priming of the neutrophils with either tumor necrosis factor alpha or granulocyte-macrophage colony-stimulating factor, stimulation of the cells with the soluble immune complexes leads to an enhanced Ca2+ signal and significant secretion of reactive oxidants. The enhanced Ca2+ signal observed in primed neutrophils results from the influx of Ca2+ from the external environment and is partly sensitive to tyrosine kinase inhibitors. This is in contrast to the Ca2+ signal observed in unprimed neutrophils, which arises from the mobilization of intracellular stores. When the surface expression of FcgammaRIIIb on primed neutrophils was decreased either through incubation with Pronase or phosphoinositide-specific phospholipase C, the extra enhanced Ca2+ mobilization seen in primed cells was significantly lowered, while the initial rise in intracellular Ca2+ was unaffected. Depletion of FcgammaRIIIb had no significant effect on the Ca2+ transients in unprimed neutrophils. Cross-linking FcgammaRII, but not FcgammaRIIIb, induced increases in intracellular Ca2+ in unprimed neutrophils, while cross-linking either of these receptors increased Ca2+ levels in primed neutrophils. The FcgammaRII-dependent intracellular Ca2+ rise in primed cells was unaffected by incubation in Ca2+-free medium, whereas the FcgammaRIIIb-dependent transient was significantly decreased when Ca2+ influx was prevented in Ca2+-free medium supplemented with EGTA. Cross-linking either FcgammaRII or FcgammaRIIIb in primed or unprimed cells failed to stimulate substantial levels of inositol 1,4,5-trisphosphate production. These results indicate that following stimulation of primed neutrophils with soluble immune complexes the enhanced Ca2+ mobilization observed is the result of a functional activation of the glycosylphosphatidylinositol-linked FcgammaRIIIb.

Defective signal transduction through the thromboxane A2 receptor in a patient with a mild bleeding disorder: deficiency of the inositol 1,4,5-triphosphate formation despite normal G-protein activation.

We describe an 11-year-old girl with a mild bleeding disorder since early childhood. The disorder was characterized by a prolonged bleeding time, and the patient's platelets showed defective aggregation responses to thromboxane A2 (TXA2) mimetic U46619 and arachidonic acid. In contrast, the platelets showed normal responses to thrombin and Ca ionophore A23187. When the platelet TXA2 receptor was examined with the [3H]-labeled TXA2 agonist U46619, the equilibrium dissociation rate constants (kd) and the maximal concentration of binding sites (Bmax) of the patient's platelets were within normal ranges. Normal GTPase activity was also induced in the patient's platelets by stimulation with U46619, however, inositol 1,4,5-triphosphate (IP3) formation was not induced by U46619. These results suggests that the patient's platelets had a defect in phospholipase C activation beyond TXA2 receptors.

Inositol 1,4,5-trisphosphate in blood and skeletal muscle in human malignant hyperthermia.

The in vitro contracture test (IVCT) is the only available diagnostic method at present for evaluation of malignant hyperthermia (MH) susceptibility. However, the disadvantage of the IVCT is that it is invasive. Several studies suggest that an altered inositol phosphate system is involved in the development of MH. A greater concentration of inositol 1,4,5-trisphosphate (1,4,5-IP3) was found in MH susceptible (MHS) than in normal (MHN) skeletal muscles. In this study the concentrations of 1,4,5-IP3 in blood samples and skeletal muscle specimens of identical patients were measured in an attempt to define susceptibility to MH. Muscle biopsies were obtained from 34 patients with clinical suspicion of MH. Patients were first classified as MHS (n = 19), MHN (n = 8) or MH equivocal (MHE; n = 7) by the standard IVCT. For detection of 1,4,5-IP3 concentrations, blood samples were obtained and an additional muscle specimen was excised. After sample preparation, concentrations of 1,4,5-IP3 were measured using radioimmunoassay. In blood samples, concentrations of 1,4,5-IP3 were similar in all individuals tested for MH susceptibility and in control patients not tested for MH susceptibility (n = 44). In skeletal muscle, 1,4,5-IP3 concentrations were significantly higher in MHS than in MHE or MHN patients, respectively. Each MHS sample contained more 1,4,5-IP3 than the highest concentration measured in MHN muscle. Defining arbitrary thresholds for 1,4,5-IP3 concentration in skeletal muscles in order to discriminate between MHS and MHN status, it was possible to assign three MHE patients to MHS and four to MHN. This study supports the hypothesis that an altered inositol phosphate system might be involved in MH. However, measurement of 1,4,5-IP3 concentration in a simple blood sample preparation is not reliable for MH susceptibility screening.