Calcium influx evoked by Ca2+ store depletion in human platelets is more susceptible to cytochrome P-450 inhibitors than receptor-mediated calcium entry

High-throughput assessment identifying major platelet Ca2+ entry pathways via tyrosine kinase-linked and G protein-coupled receptors

In platelets, elevated cytosolic Ca²⁺ is a crucial second messenger, involved in most functional responses, including shape change, secretion, aggregation and procoagulant activity. The platelet Ca²⁺ response consists of Ca²⁺ mobilization from endoplasmic reticulum stores, complemented with store-operated or receptor-operated Ca²⁺ entry pathways. Several channels can contribute to the Ca²⁺ entry, but their relative contribution is unclear upon stimulation of ITAM-linked receptors such as glycoprotein VI (GPVI) and G-protein coupled receptors such as the protease-activated receptors (PAR) for thrombin. We employed a 96-well plate high-throughput assay with Fura-2-loaded human platelets to perform parallel [Ca²⁺]_i measurements in the presence of EGTA or CaCl₂. Per agonist condition, this resulted in sets of EGTA, CaCl₂ and Ca²⁺ entry ratio curves, defined by six parameters, reflecting different Ca²⁺ ion fluxes. We report that threshold stimulation of GPVI or PAR, with a variable contribution of secondary mediators, induces a maximal Ca²⁺ entry ratio of 3-7. Strikingly, in combination with Ca²⁺-ATPase inhibition by thapsigargin, the maximal Ca²⁺ entry ratio increased to 400 (GPVI) or 40 (PAR), pointing to a strong receptor-dependent enhancement of store-operated Ca²⁺ entry. By pharmacological blockage of specific Ca²⁺ channels in platelets, we found that, regardless of GPVI or PAR stimulation, the Ca²⁺ entry ratio was strongest affected by inhibition of ORAI1 (2-APB, Synta66) > Na⁺/Ca²⁺ exchange (NCE) > P2×₁ (only initial). In contrast, inhibition of TRPC6, Piezo1/2 or STIM1 was without effect. Together, these data reveal ORAI1 and NCE as dominating Ca²⁺ carriers regulating GPVI- and PAR-induced Ca²⁺ entry in human platelets.

Effects of saikosaponin-d on CYP3A4 in HepaRG cell and protein-ligand docking study

Saikosaponin-d (SSd) is a major bioactive triterpenoid saponin extracted from Bupleurum, which has anti-inflammatory, anticancer, antioxidative and anti-hepatic fibrosis effects. Due to the effects of Bupleurum-related formulations on cytochrome P450 (CYPs) expression still remain unclear, the combination therapies involved formulations containing Bupleurum may sometimes lead to unexpected drug-drug interactions in clinical practice. These interactions can limit the clinical applications of related formulations. In this study, we tried to explore the effects of SSd on CYP3A4 mRNA, protein expression and the enzyme activity in HepaRG cells by real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR), Western blot (WB) and HPLC method, respectively. The interaction between SSd and CYP3A4 was analysed by molecular docking. HepaRG cells were cultured with different concentrations of SSd (0.5, 1, 5 and 10 μmol/L) for 72 hours. It is revealed that SSd can inhibit CYP3A4 mRNA and its protein expression, and also the enzyme activity. Molecular docking study demonstrated that SSd can bind to several key active sites of amino acid residues of CYP3A4 protein with hydrogen bonds and hydrophobic interactions. Thus, drug-drug interactions resulted by SSd inhibiting CYP3A4 need attention when formulations containing SSd or Bupleurum are co-administrated with drugs metabolized by CYP3A4.

A novel role for carbon monoxide as a potent regulator of intracellular Ca2+and nitric oxide in rat pancreatic acinar cells

Carbon monoxide (CO) is known as an essential gaseous messenger that regulates a wide array of physiological and pathological processes, similar to nitric oxide (NO) and hydrogen sulfide. The aim of the present study was to elucidate the potential role of CO in Ca2+homeostasis and to explore the underlying mechanisms in pancreatic acinar cells. The exogenous application of a CO-releasing molecule dose-dependently increased intracellular Ca2+concentration ([Ca2+]i). A heme oxygenase (HO) inducer increased [Ca2+]iin a concentration-dependent manner, and the increase was diminished by an HO inhibitor. The CO-induced [Ca2+]iincrease persisted in the absence of extracellular Ca2+, indicating that Ca2+release is the initial source for the increase. The inhibition of G protein, phospholipase C (PLC), and inositol 1,4,5-trisphosphate (IP3) receptor diminished the CO-induced [Ca2+]iincrease. CO upregulated endothelial nitric oxide synthase (eNOS) expression and stimulated NO production, and NOS inhibitor, calmodulin inhibitor, or the absence of extracellular Ca2+eliminated the latter response. Blocking the phosphatidylinositol 3-kinase (PI3K)-Akt/protein kinase B (PKB) pathway abolished CO-induced NO production. Pretreatment with an NOS inhibitor, NO scavenger, or soluble guanylate cyclase inhibitor, did not affect the CO-induced [Ca2+]iincrease, indicating that NO, soluble guanylate cyclase, and cyclic guanosine 5′-monophosphate are not involved in the CO-induced [Ca2+]iincrease. CO inhibited the secretory responses to CCK-octapeptide or carbachol. We conclude that CO acts as a regulator not only for [Ca2+]ihomeostasis via a PLC-IP3-IP3receptor cascade but also for NO production via the calmodulin and PI3K-Akt/PKB pathway, and both CO and NO interact. Moreover, CO may provide potential therapy to ameliorate acute pancreatitis by inhibiting amylase secretion.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

The unique contribution of ion channels to platelet and megakaryocyte function

Ion channels are transmembrane proteins that play ubiquitous roles in cellular homeostasis and activation. In addition to their recognized role in the regulation of ionic permeability and thus membrane potential, some channel proteins possess intrinsic kinase activity, directly interact with integrins or are permeable to molecules up to ≈1000 Da. The small size and anuclear nature of the platelet has often hindered progress in understanding the role of specific ion channels in hemostasis, thrombosis and other platelet-dependent events. However, with the aid of transgenic mice and 'surrogate' patch clamp recordings from primary megakaryocytes, important unique contributions to platelet function have been identified for several classes of ion channel. Examples include ATP-gated P2X1 channels, Orai1 store-operated Ca2+ channels, voltage-gated Kv1.3 channels, AMPA and kainate glutamate receptors and connexin gap junction channels. Furthermore, evidence exists that some ion channels, such as NMDA glutamate receptors, contribute to megakaryocyte development. This review examines the evidence for expression of a range of ion channels in the platelet and its progenitor cell, and highlights the distinct roles that these proteins may play in health and disease.

Cytochrome P450-derived epoxyeicosatrienoic acids and pulmonary hypertension: central role of transient receptor potential C6 channels

Hypoxia induces the constriction of pulmonary resistance arteries, which results in the redistribution of blood from poor to better ventilated areas, thus optimizing its oxygenation. Many different oxygen-sensing mechanisms have been proposed to regulate this process, including cytochrome P450 enzymes. These enzymes, which convert substrates such as arachidonic acid into bioactive epoxides (the epoxyeicosatrienoic acids [EETs]), are highly expressed in the lung as is the soluble epoxide hydrolase which metabolizes the epoxides to their less active diols. The EETs play a well-documented role as endothelium-derived vasodilators in the systemic vasculature, but in the pulmonary circulation, they are generated in vascular smooth muscle cells and potentiate vasoconstriction. Preventing the breakdown of 11,12-EET by the inhibition or genetic deletion of the soluble epoxide hydrolase strongly augments the response to hypoxia. Mechanistically, 11,12-EET potentiates the contractile response by recruiting transient receptor potential C6 channels to caveolae. Indeed, neither 11,12-EET nor hypoxia is able to elicit pulmonary vasoconstriction in TRPC6 knockout mice. The cytochrome and soluble epoxide hydrolase enzymes are also implicated in the vascular remodeling associated with chronic hypoxia and pulmonary hypertension. Thus, targeting this pathway may be in an attractive new therapeutic approach to treat this incapacitating disease.

A phase I study of paclitaxel and continuous daily CAI in patients with refractory solid tumors

BACKGROUND

Carboxyamido-triazole (CAI) is a calcium influx inhibitor with anti-angiogenic and anti-invasive properties and stabilizes tumor progression in patients. We hypothesized daily oral micronized CAI with q3 week paclitaxel would be well-tolerated and active.

RESULTS

Twenty-nine heavily pretreated patients [median 3 [0-7]] were enrolled on five dose levels. No additive or cumulative toxicity was observed, and grade III nonhematological toxicity was rare. Neutropenia was the most common hematologic toxicity, seen in 79% of patients, with a trend towards increasing grade with higher paclitaxel doses. The recommended phase II dose defined by the maximum tolerated dose (MTD) was CAI 250 mg daily and paclitaxel 200 mg/m(2) q3weeks. Pharmacokinetic analysis revealed paclitaxel increases CAI trough concentration at all dose levels by over 100% (p < 0.0001). A trend towards higher steady-state CAI trough concentrations was found in patients with a partial response (PR; p = 0.09). Six patients had confirmed PR (24%; 4-67 cycles, median 10); two patients had minor responses.

PATIENTS AND METHODS

Eligible patients with solid tumors received micronized CAI daily (150-250 mg PO) and paclitaxel intravenously q3weeks (175-250 mg/m(2)), sequentially escalating each drug. CAI preceded paclitaxel by one week to permit pharmacokinetic analysis. Patients were assessed for toxicity, pharmacokinetics and disease outcome.

CONCLUSIONS

The MTD of the combination of CAI and paclitaxel is 250 mg daily and 200 mg/m(2) q3weeks, respectively. The combination is tolerable and has potential antitumor activity.

Defining targets for investigating the pharmacogenomics of adverse drug reactions to antifungal agents

Adverse drug reactions (ADRs) associated with antifungal therapy are major problems in patients with invasive fungal infections. Whether by clinical history or patterns of genetic variation, the identification of patients at risk for ADRs should result in improved outcomes while minimizing deleterious side effects. A major contributing factor to ADRs with antifungal agents relates to drug distribution, metabolism and excretion. Genetic variation in key genes can alter the structure and expression of genes and gene products (e.g., proteins). Thus far, the effort has focused on identifying polymorphisms with either empirical or predicted in silico functional consequences; the best candidate genes encode phase I and II drug-metabolizing enzymes (e.g., CYP2C19 and N-acetyltransferase), plasma proteins (albumin and lipoproteins) and drug transporters (P-glycoprotein and multidrug resistance proteins), which can affect the disposition of antifungal agents, eventually leading to dose-dependent (type A) toxicity. Less is known regarding the key genes that interact with antifungal agents, resulting in idiosyncratic (type B) ADRs. The possible role of certain gene products and genetic polymorphisms in the toxicities of antifungal agents are discussed in this review. The preliminary data address the following: low-density lipoproteins and cholesteryl ester transfer protein in amphotericin B renal toxicity; toll-like receptor 1 and 2 in amphotericin B infusion-related ADRs; phosphodiesterase 6 in voriconazole visual adverse events; flavin-containing monooxygenase, glutathione transferases and multidrug resistance proteins 1 and 2 in ketoconazole and terbinafine hepatotoxicity; CYP enzymes and P-glycoprotein in drug interactions between azoles and coadministered medications; multidrug resistance proteins 8 and 9 on 5-flucytosine bone marrow toxicity; and mast cell activation in caspofungin histamine release. This will focus on high-priority candidate genes, which could provide a starting point for molecular studies to elucidate the potential mechanisms for understanding toxicity associated with antifungal drugs as well as identifying candidate genes for large population prospective genetic association studies.

Electrophysiological properties of cardiomyocytes isolated from CYP2J2 transgenic mice

CYP2J2 is abundant in cardiac tissue and active in the biosynthesis of eicosanoids such as epoxyeicosatrienoic acids (EETs). To determine the effects of CYP2J2 and its eicosanoid products in the heart, we characterized the electrophysiology of single cardiomyocytes isolated from adult transgenic (Tr) mice with cardiac-specific overexpression of CYP2J2. CYP2J2 Tr cardiomyocytes had a shortened action potential. At 90% repolarization, the action potential duration (APD) was 30.6 +/- 3.0 ms (n = 22) in wild-type (Wt) cells and 20.2 +/- 2.3 ms (n = 19) in CYP2J2 Tr cells (p < 0.005). This shortening was probably due to enhanced maximal peak transient outward K(+) currents (I(to,peak)), which were 38.6 +/- 2.8 and 54.4 +/- 4.9 pA/pF in Wt and CYP2J2 Tr cells, respectively (p < 0.05). In contrast, the late portion of the transient outward K(+) current (I(to,280ms)), the slowly inactivating outward K(+) current (I(K,slow)), and the voltage-gated Na(+) current (I(Na)) were not significantly altered in CYP2J2 Tr cells. N-Methylsulphonyl-6-(2-proparglyloxy-phenyl)hexanamide (MS-PPOH), a specific inhibitor of EET biosynthesis, significantly reduced I(to,peak) and increased APD in CYP2J2 Tr cardiomyocytes but not in Wt cells. Intracellular dialysis with a monoclonal antibody against CYP2J2 also significantly reduced I(to,peak) and increased APD in CYP2J2 Tr cardiomyocytes. Addition of 11,12-EET or 8-bromo-cAMP significantly reversed the MS-PPOH- or monoclonal antibody-induced changes in I(to,peak) and APD in CYP2J2 Tr cells. Together, our data demonstrate that shortening of the action potential in CYP2J2 Tr cardiomyocytes is associated with enhanced I(to,peak) via an EET-dependent, cAMP-mediated mechanism.

Cyclic AMP-dependent modulation of cardiac L-type Ca2+ and transient outward K+ channel activities by epoxyeicosatrienoic acids

The three major enzyme systems, cyclo-oxygenase, lipoxygenase, and cytochrome P450 (P450/CYP), metabolize arachidonic acid (AA) to biologically active compounds. P450 and its associated monooxygenase activities have been identified in mammalian cardiac tissue, including humans. The four regioisomeric eicosanoids, 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs) of AA metabolites derived by P450 epoxygenases have shown to possess potent biological effects in numerous tissues. In the coronary circulation the EETs are leading candidates for endothelial-derived hyperpolarizing factors that hyperpolarize vascular smooth muscle cells by opening Ca2+-activated K+ channels. Recently, the effects of the CYP pathways and their metabolites on cardiac ischemia-reperfusion injury have been evaluated in animal models. Some of these AA metabolites are cardioprotective and some are detrimental. However, EETs appear to be cardioprotective in CYP2J2 transgenic mice and in a canine ischemic model. Multiple effects of EETs on cardiac ion channels have been observed, such as activation of ATP-sensitive K+ channels and L-type Ca2+ channels in cardiomyocytes and inhibition of cardiac Na+ channels and L-type Ca2+ channels reconstructed in planar lipid bilayers. This brief review summarizes EET-induced modulation of cardiac ion channels.

Enhancement of cardiac L-type Ca2+ currents in transgenic mice with cardiac-specific overexpression of CYP2J2

CYP2J2 is abundant in cardiomyocytes and is involved in the metabolism of arachidonic acid (AA) to epoxyeicosatrienoic acids (EETs), which affect multiple cell functions. In this study, we investigated the effect of overexpression of CYP2J2 on cardiac L-type Ca2+ currents (ICa) in adult transgenic mice. Cardiac-specific overexpression of CYP2J2 was achieved using the alpha-myosin heavy chain promoter. ICa was recorded from isolated ventricular cardiomyocytes. Compared with the wild-type cardiomyocytes (n = 60), the density of ICa was significantly increased by 40 +/- 9% in the CYP2J2 transgenic cardiomyocytes (n = 71; P < 0.001). N-Methylsulfonyl-6-(2-proparglyloxyphenyl)hexanamide (MS-PPOH), a specific inhibitor of EET biosynthesis, and clotrimazole, a cytochrome P450 inhibitor, significantly reduced ICa in both wild-type and transgenic cardiomyocytes; however, MS-PPOH inhibited ICa to a greater extent in the CYP2J2 transgenic cells (n = 10) than in the wild-type cells (n = 10; P < 0.01). Addition of 11,12-EET significantly restored ICa in MS-PPOH-treated cells. Intracellular dialysis with either of two inhibitory monoclonal antibodies against CYP2J2 significantly reduced ICa in both wild-type and transgenic mice. Membrane-permeable 8-bromo-cAMP and the beta-adrenergic agonist isoproterenol significantly reversed the monoclonal antibody-induced inhibition of ICa. In addition, the total protein level of the alpha1 subunit of the Cav1.2 L-type Ca2+ channel was not altered in CYP2J2 transgenic hearts, but the phosphorylated portion was markedly increased. In conclusion, overexpression of CYP2J2 increases ICa in CYP2J2 transgenic cardiomyocytes via a mechanism that involves cAMP-protein kinase A-dependent phosphorylation of the L-type Ca2+ channel.

Deciphering the plasma membrane hallmarks of apoptotic cells: phosphatidylserine transverse redistribution and calcium entry

BACKGROUND

During apoptosis, Ca2+-dependent events participate in the regulation of intracellular and morphological changes including phosphatidylserine exposure in the exoplasmic leaflet of the cell plasma membrane. The occurrence of phosphatidylserine at the surface of specialized cells, such as platelets, is also essential for the assembly of the enzyme complexes of the blood coagulation cascade, as demonstrated by hemorrhages in Scott syndrome, an extremely rare genetic deficiency of phosphatidylserine externalization, without other apparent pathophysiologic consequences. We have recently reported a reduced capacitative Ca2+ entry in Scott cells which may be part of the Scott phenotype.

RESULTS

Taking advantage of these mutant lymphoblastoid B cells, we have studied the relationship between this mode of Ca2+ entry and phosphatidylserine redistribution during apoptosis. Ca2+ ionophore induced apoptosis in Scott but not in control cells. However, inhibition of store-operated Ca2+ channels led to caspase-independent DNA fragmentation and decrease of mitochondrial membrane potential in both control and Scott cells. Inhibition of cytochrome P450 also reduced capacitative Ca2+ entry and induced apoptosis at comparable extents in control and Scott cells. During the apoptotic process, both control and more markedly Scott cells externalized phosphatidylserine, but in the latter, this membrane feature was however dissociated from several other intracellular changes.

CONCLUSIONS

The present results suggest that different mechanisms account for phosphatidylserine transmembrane migration in cells undergoing stimulation and programmed death. These observations testify to the plasticity of the plasma membrane remodeling process, allowing normal apoptosis even when less fundamental functions are defective.

Ragged spiking of free calcium in ADP-stimulated human platelets: regulation of puff-like calcium signals in vitro and ex vivo

1. Human platelets respond to agonists of G protein (G(q))-coupled receptors by generating an irregular pattern of spiking changes in cytosolic Ca2+ ([Ca2+]i). We have investigated the ADP-induced Ca2+ responses of single, Fluo-3-loaded platelets in the presence or absence of autologous plasma or whole blood under flow conditions. 2. In plasma-free platelets, incubated in buffer medium, baseline separated [Ca2+]i peaks always consisted of a rapid rising phase (median time 0.8 s) which was abruptly followed by a slower, mono-exponential decay phase. The decay constant differed from platelet to platelet, ranging from 0.23 +/- 0.02 to 0.63 +/- 0.03 s(-1) (mean +/- S.E.M., n = 3-5), and was used to identify individual Ca2+ release events and to determine the Ca2+ fluxes of the events. 3. Confocal, high-frequency measurements of adherent, spread platelets (diameter 3-5 microm) indicated that different optical regions had simultaneous patterns of both low- and high-amplitude Ca2+ release events. 4. With or without plasma or flowing blood, the ADP-induced Ca2+ signals in platelets had the characteristics of irregular Ca2+ puffs as well as more regular Ca2+ oscillations. Individual [Ca2+]i peaks varied in amplitude and peak-to-peak interval, as observed for separated Ca2+ puffs within larger cells. On the other hand, the peaks appeared to group into periods of ragged, shorter-interval Ca2+ release events with little integration, which were alternated with longer-interval events. 5. We conclude that the spiking Ca2+ signal generated in these small cells has the characteristics of a 'poor' oscillator with an irregular frequency being reactivated from period to period. This platelet signal appears to be similar in an environment of non-physiological buffer medium and in flowing, whole blood.

Fibrinogen binding to the integrin alpha(IIb)beta(3) modulates store-mediated calcium entry in human platelets

Effects of the occupation of integrin alpha(IIb)beta(3) by fibrinogen on Ca(++) signaling in fura-2-loaded human platelets were investigated. Adding fibrinogen to washed platelet suspensions inhibited increases in cytosolic [Ca(++)] concentrations ([Ca(++)](i)) evoked by adenosine diphosphate (ADP) and thrombin in a concentration-dependent manner in the presence of external Ca(++) but not in the absence of external Ca(++) or in the presence of the nonselective cation channel blocker SKF96365, indicating selective inhibition of Ca(++) entry. Fibrinogen also inhibited store-mediated Ca(++) entry (SMCE) activated after Ca(++) store depletion using thapsigargin. The inhibitory effect of fibrinogen was reversed if fibrinogen binding to alpha(IIb)beta(3) was blocked using RDGS or abciximab and was absent in platelets from patients homozygous for Glanzmann thrombasthenia. Fibrinogen was without effect on SMCE once activated. Activation of SMCE in platelets occurs through conformational coupling between the intracellular stores and the plasma membrane and requires remodeling of the actin cytoskeleton. Fibrinogen inhibited actin polymerization evoked by ADP or thapsigargin in control cells and in cells loaded with the Ca(++) chelator dimethyl BAPTA. It also inhibited the translocation of the tyrosine kinase p60(src) to the cytoskeleton. These results indicate that the binding of fibrinogen to integrin alpha(IIb)beta(3) inhibits the activation of SMCE in platelets by a mechanism that may involve modulation of the reorganization of the actin cytoskeleton and the cytoskeletal association of p60(src). This action may be important in intrinsic negative feedback to prevent the further activation of platelets subjected.

Human platelet calcium mobilisation in response to beta-amyloid (25-35): buffer dependency and unchanged response in Alzheimer's disease

In the present study, the effects of beta-amyloid (25-35) (Abeta (25-35)) upon calcium signalling by the human platelet has been investigated. When assays were conducted using HEPES buffers, Abeta (25-35), but not the inactive peptide Abeta (35-25), produced a robust increase in intracellular calcium that remained after removal of extracellular calcium but was abolished by the phospholipase C inhibitor U-73122. There was no significant difference between the calcium response to Abeta (25-35) in platelets from patients with Alzheimer's disease and from age-matched controls. In contrast to the robust effects on calcium mobilisation in HEPES buffers, very little calcium response to Abeta (25-35) was seen when Krebs (pH 7.8) buffer was used.

Altered coronary dilation in deoxycorticosterone acetate-salt hypertension

OBJECTIVE

To compare coronary dilation in uninephrectomized hypertensive deoxycorticosterone acetate (DOCA)-salt rats (HTRs), treated for 2 or 4 weeks, with age-matched uninephrectomized normotensive rats (NTRs). DESIGN AND METHODS Coronary perfusion pressure was recorded in isolated hearts perfused at a constant flow rate to evaluate coronary resistance.

RESULT

A decreased vasoconstriction due to NG-nitro-Larginine (NNLA, 30 pmol/I) in hearts from HTRs suggested a reduced basal nitric oxide (NO) release. In contrast, coronary vasodilation due to the NO donor, sodium nitroprusside (3 pmol/I), remained unaffected in 2-week HTRs, and was enhanced in 4-week HTRs. Cumulative dose-response curves to bradykinin induced an important vasodilation in NTRs, with a maximal response that remained unaffected in the presence of either NNLA (30 pmol/I), indomethacin (10 pmol/l) or the two combined. In contrast, hearts from HTRs showed a diminished maximal relaxation to bradykinin, suggesting an altered endothelium-dependent relaxation. The presence of NNLA or indomethacin had no effect on the weak relaxation observed in HTRs. However, NNLA and indomethacin combined unmasked an important relaxation due to bradykinin in HTRs. The addition of clotrimazole (1 pmol/I) to NNLA and indomethacin blunted the relaxation due to bradykinin in both NTRs and HTRs. Perfusion with superoxide dismutase (120 IU/ml) restored most of the coronary relaxation due to bradykinin in hearts from HTRs. Bradykinin-induced prostaglandin 12 (PGI2) and E2 (PGE2) production was unaffected by hypertension. No increase in thromboxane A2 (TXA2) due to bradykinin was detected. Finally, reduced reactivity to papaverine and forskolin was observed in hearts from HTRs.

CONCLUSION

DOCA-salt hypertension is associated with alterations in coronary reactivity. Basal NO formation appears to be reduced in HTRs, but the intact relaxation to exogenous NO suggests a preserved guanylate cyclase pathway. In addition, alteration in adenylate cyclase activity, and not in prostaglandin production, may explain the blunted cAMP-mediated responses in HTRs. The combined nitric-oxide synthase (NOS) and cyclo-oxygenase (COX) inhibition unmasked an endothelium-derived hyperpolarizing factor (EDHF) involvement in the coronary dilation due to bradykinin in hearts from HTRs, suggesting that endothelial NO and PGI2, although unable to induce coronary smooth-muscle relaxation, can inhibit EDHF production in HTRs. Impairment in the adenylate cyclase pathway and the suppression of NO by free radicals may explain the blunted vasodilation in DOCA-salt hypertension.

Protein kinase C activates non-capacitative calcium entry in human platelets

1. In many non-excitable cells Ca2+ influx is mainly controlled by the filling state of the intracellular Ca2+ stores. It has been suggested that this store-mediated or capacitative Ca2+ entry is brought about by a physical and reversible coupling of the endoplasmic reticulum with the plasma membrane. Here we provide evidence for an additional, non-capacitative Ca2+ entry mechanism in human platelets. 2. Changes in cytosolic Ca2+ and Sr2+ were measured in human platelets loaded with the fluorescent indicator fura-2. 3. Depletion of the internal Ca2+ stores with thapsigargin plus a low concentration of ionomycin stimulated store-mediated cation entry, as demonstrated upon Ca2+ or Sr2+ addition. Subsequent treatment with thrombin stimulated further divalent cation entry in a concentration-dependent manner. 4. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol also stimulated divalent cation entry, without evoking the release of Ca2+ from intracellular stores. Cation entry evoked by thrombin or activators of PKC was abolished by the PKC inhibitor Ro-31-8220. 5. Unlike store-mediated Ca2+ entry, jasplakinolide, which reorganises actin filaments into a tight cortical layer adjacent to the plasma membrane, did not inhibit divalent cation influx evoked by thrombin when applied after Ca2+ store depletion, or by activators of PKC. Thrombin also activated Ca2+ entry in platelets in which the release from intracellular stores and store-mediated Ca2+ entry were blocked by xestospongin C. 6. These results indicate that the non-capacitative divalent cation entry pathway is regulated independently of store-mediated entry and does not require coupling of the endoplasmic reticulum and the plasma membrane. These results support the existence of a mechanism for receptor-evoked Ca2+ entry in human platelets that is independent of Ca2+ store depletion. This Ca2+ entry mechanism may be activated by occupation of G-protein-coupled receptors, which activate PKC, or by direct activation of PKC, thus generating non-capacitative Ca2+ entry alongside that evoked following the release of Ca2+ from the intracellular stores.

Rapid inhibition by sodium azide of the phosphoinositide-mediated calcium response to serotonin stimulation in human platelets: preservation in Alzheimer's disease

The effect of sodium azide (NaN(3)) upon platelet Ca(2+) signalling has been investigated. A 60 s preincubation with 1 mM NaN(3) reduced the Ca(2+) response to 1 microM serotonin without a corresponding reduction in the responses to 52 mU/ml thrombin or 70 microM beta-amyloid(25-35) (A beta(25-35)). The effect of NaN(3) upon the response to serotonin, which was not blocked by either glutathione ethyl ester (GTEE) or dithiothreitol (DTT), was similar in platelets obtained from patients with Alzheimer's disease and from age- and gender-matched controls. After a preincubation time of 5 min was used, the Ca(2+) response to thrombin was greatly reduced by 1 mM NaN(3), but not by 50 microM 4-hydroxynonenal (HNE, 50 microM). Platelet levels of HNE and malondialdehyde were not significantly affected by up to 30 min of incubation with NaN(3) at room temperature. It is concluded that the rapid effect of NaN(3) upon the Ca(2+) response to serotonin in human platelets is not mediated by an inhibition of cytochrome c oxidase, and is due to an action proximal to phosphoinositide-specific phospholipase C.

The roles of P(2X1)and P(2T AC)receptors in ADP-evoked calcium signalling in human platelets

The roles of P(2X1)and P(2T AC)receptors in ADP-evoked Ca(2+)signalling were investigated in fura-2-loaded human platelets. Desensitization of the P(2X1)receptor with the selective agonist, alphabeta-methylene ATP, reduced the integral of the ADP-evoked rise in [Ca(2+)](i)to about 90% of control; a reduction equivalent to the integral of the P(2X1)-evoked response alone. After elevating cAMP or cGMP levels using prostaglandin E(1)or sodium nitroprusside, prior P(2X1)desensitization reduced the integral of the ADP-evoked response to about 70% of control. This reduction was greater than the integral of the P(2X1)-evoked response alone under the same conditions, suggesting rapidly activated Ca(2+)entry via the P(2X1)receptor potentiates Ca(2+)responses evoked via the phospholipase C-coupled P(2Y1)receptor. The P(2T AC)receptor antagonist, AR-C69931MX, at a concentration completely inhibiting aggregation, did not significantly affect the initial peaks but caused a significant reduction in the integrals of the ADP-evoked rises in [Ca(2+)](i)to about 71% or 77% of controls in the presence or absence of external Ca(2+)respectively. This suggests that the main effect of lowering cAMP levels after inhibition of adenylyl cyclase via P(2T AC)receptors may be reduced Ca(2+)removal from the cytosol. These results indicate that both the P(2X1)and P(2T AC)receptors play a significant role in ADP-evoked Ca(2+)signalling in human platelets.

Clotrimazole inhibits the recombinant human cardiac L-type Ca2+ channel alpha 1C subunit

1. Clotrimazole (CLT) is an antimycotic agent with a potential role in the treatment of cancer. Whole-cell patch clamp recordings and Fura-2 AM fluorescence measurements were used to investigate the inhibition by CLT of recombinant human cardiac L-type Ca2+ channel alpha 1C subunits, stably expressed in human embryonic kidney (HEK 293) cells. 2. CLT (100 nmol l-1 to 25 mumol l-1) reduced Ca2+ channel currents in a concentration-dependent manner. Inhibition was neither use- or voltage-dependent. The effects of CLT were rapid and maximal effects were attained within 3 min. Application of CLT also caused an acceleration of apparent Ca2+ channel current inactivation. 3. Basal current density and the degree of inhibition due to CLT were not significantly altered by pretreating cells with 3 mmol l-1 1-aminobenzotriazole for 1 h, or by dialysing cells for 10 min with 2 mmol l-1 alpha-napthoflavone via the patch pipette, suggesting that the inhibitory action of CLT was not due to inhibition of cytochrome P-450. 4. CLT (10 mumol l-1) did not influence [Ca2+]i, as determined by Fura-2 AM fluorescence measurements. 5. Dialysing cells for 10 min with the non-specific serine/threonine kinase inhibitor H-7 (10 mumol l-1) was without effect on basal current density or on the inhibitory response to 10 mumol l-1 CLT, indicating that CLT is not acting via an indirect effect on these kinases. 6. These data suggest that CLT exerts a direct blocking effect on the alpha 1C subunit at therapeutic concentrations. This effect may explain the abbreviation of the action potential duration by CLT observed in cardiac myocytes.

Characteristics of collagen-induced Ca2+ mobilization in bovine platelets

We characterized the collagen-induced increase in cytosolic Ca2+ ([Ca2+]i) of bovine platelets loaded with the Ca2+ indicator Fura-PE3/AM. Collagen (10 micrograms/ml)-induced increase in [Ca2+]i was only partially inhibited by aspirin, a cyclooxygenase inhibitor, or adenosine 3'-phosphate 5'-phosphosulfate (A3P5PS, a P2Y1 receptor antagonist), while in human platelets it was almost completely suppressed by aspirin. Collagen-induced increase in [Ca2+]i of bovine platelets was inhibited by U73122 (0.3-5 microM), a phospholipase C inhibitor. Collagen (10 micrograms/ml) increased production of inositol 1,4,5-trisphosphate, which was prevented by pretreatment with U73122 (5 microM). Collagen (10 micrograms/ml) accelerated Mn2+ entry, since the rate of Fura-PE3 quenching by Mn2+ was enhanced by 13-fold following stimulation with collagen. U73122 inhibited the acceleration of Mn2+ entry induced by collagen. PGE1 (2.5 microM) partially inhibited the collagen (50 micrograms/ml)-induced increase in [Ca2+]i in bovine platelets but not in human platelets. The data suggest that collagen-induced Ca2+ mobilization in bovine platelets is mediated by phospholipase C. The Ca2+ mobilization in bovine platelets is different from that in human ones as to the dependency on arachidonic acid metabolites and sensitivity to PGE1.

trans-Resveratrol inhibits calcium influx in thrombin-stimulated human platelets

1. The phytoestrogenic compound trans-resveratrol (trans-3,5, 4'-trihydroxystilbene) is found in appreciable quantities in grape skins and wine. It has been shown that both products rich in trans-resveratrol and pure trans-resveratrol inhibit platelet aggregation both in vivo and in vitro. However the mechanism of this action still remains unknown. 2. An essential component of the aggregation process in platelets is an increase in intracellular free Ca2+ ([Ca2+]i). Ca2+ must enter the cell from the external media through specific and tightly regulated Ca2+ channels in the plasma membrane. The objective of this study was to characterize what effect trans-resveratrol had on the Ca2+ channels in thrombin stimulated platelets. 3. In this study we showed that trans-resveratrol immediately inhibited Ca2+ influx in thrombin-stimulated platelets with an IC50 of 0.5 microM. trans-Resveratrol at 0.1, 1.0 and 10.0 microM produced 20+/-6, 37+/-6 and 57+/-4% inhibition respectively of the effect of thrombin (0.01 u ml(-1)) to increase [Ca2+]i. 4. trans-Resveratrol also inhibited spontaneous Ba2+ entry into Fura-2 loaded platelets, with 0.1, 1.0 and 10.0 microM trans-resveratrol producing 10+/-5, 30+/-5 and 50+/-7% inhibition respectively. This indicated that trans-resveratrol directly inhibited Ca2+ channel activity in the platelets in the absence of agonist stimulation. 5. trans-Resveratrol also inhibited thapsigargin-mediated Ca2+ influx into platelets. This suggests that the store-operated Ca2+ channels are one of the possible targets of trans-resveratrol. These channels rely on the emptying of the internal Ca2+ stores to initiate influx of Ca2+ into the cell. 6. The phytoestrogens genistein, daidzein, apigenin and genistein-glucoside (genistin) produced inhibitory effects against thrombin similar to those seen with trans-resveratrol. 7. We conclude that trans-resveratrol is an inhibitor of store-operated Ca2+ channels in human platelets. This accounts for the ability of trans-resveratrol to inhibit platelet aggregation induced by thrombin.

Serotonin stimulation of calcium mobilisation in human platelets: choice of units of measurement, effects of age and tobacco use, and correlation with serotonin2A receptor density

The calcium responses to serotonin and thrombin, as assessed using the fluorescent indicator Fura-2, have been investigated in platelets taken from 59 non-smokers and 17 smokers. The peak responses above baseline, calculated either as fluorescence ratios or calibrated calcium concentrations, were in the order of magnitude thrombin 520 mU/ml > thrombin 52 mU/ml >> serotonin 1 micromol/l approximately serotonin 100 micromol/l. Multiple regression analyses indicated that the responses to 1 micromol/l serotonin, but not the responses to thrombin, were significantly correlated with the serotonin2A receptor density measured using [3H]LSD as radioligand. No effects of age, gender, smoking habit or the time of year of sampling were seen on the calcium responses to serotonin and thrombin. It is concluded that cellular processes distal to the serotonin2A receptor recognition site may compensate to some extent for the large differences in recognition site expression, thus underlining the importance of providing a functional correlate in addition to [3H]LSD binding site densities when studying platelet serotonin2A receptors in neuropsychiatric disorders. The measurement of Ca2+ responses to serotonin provides a useful such functional correlate.

Mechanism of rise and decay of thapsigargin-evoked calcium signals in MDCK cells

We studied the effect of thapsigargin on intracellular calcium levels ([Ca2+]i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. Thapsigargin elevated [Ca2+]i dose dependently with an EC50 of approximately 0.15 microM. The Ca2+ signal consisted of a slow rise, a gradual decay and a plateau. Depletion of the endoplasmic reticulum Ca2+ store with thapsigargin for 7 min abolished the [Ca2+]i increases evoked by bradykinin. Removal of extracellular Ca2+ reduced the thapsigargin response by approximately 50%. The Ca2+ signal was initiated by Ca2+ release from the internal store followed by capacitative Ca2+ entry (CCE). The thapsigargin-evoked CCE was abolished by La3 and Gd3+, and was partly inhibited by SKF 96365 and econazole. After depletion of the internal Ca2+ store for 30 min with another inhibitor of the internal Ca2+ pump, cyclopiazonic acid, thapsigargin failed to increase [Ca2+]i, thus suggesting that the thapsigargin-evoked Ca2+ influx was solely due to CCE. We investigated the mechanism of decay of the thapsigargin response. Pretreatment with La3+ (or Gd3+) or alkalization of extracellular medium to pH 8 significantly potentiated the Ca2+ signal; whereas pretreatment with carbonylcyanide m-chlorophynylhydrozone (CCCP) or removal of extracellular Na+ had no effect. Collectively, our results imply that thapsigargin increased [Ca2+]i in MDCK cells by depleting the internal Ca2+ store followed by CCE, with both pathways contributing equally. The decay of the thapsigargin response might be significantly governed by efflux via the plasmalemmal Ca2+ pump.

Multiple effects of econazole on calcium signaling: depletion of thapsigargin-sensitive calcium store, activation of extracellular calcium influx, and inhibition of capacitative calcium entry

The effect of econazole on intracellular calcium levels ([Ca2+]i) in Madin Darby canine kidney cells was investigated using fura-2 fluorimetry. Econazole increased [Ca2+]i dose-dependently at 5-50 microM. The Ca2+ signal consisted of an initial rise, a gradual decay and a sustained plateau. Extracellular Ca2+ removal partially reduced the econazole response. Mn2+ quench of fura-2 fluorescence confirmed econazole-induced Ca2+ influx. The econazole-sensitive intracellular Ca2+ store overlaps with that sensitive to thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+ pump, because 25 microM econazole depleted the thapsigargin-sensitive store, and conversely, thapsigargin abolished the econazole response. Econazole (25-50 microM) partially inhibited capacitative Ca2+ entry induced by cyclopiazonic acid, another endoplasmic reticulum Ca2+ pump inhibitor, measured by depleting internal Ca2+ store in Ca(2+)-free medium followed by adding 10 mM CaCl2. Econazole induced capacitative Ca2+ entry itself. Pretreatment with La3+ (100 microM) partially inhibited 25 microM econazole-induced Mn2+ quench of fura-2 fluorescence, and La3+ immediately reduced 20 microM econazole-induced Ca2+ signal when added at the peak of the signal, suggesting that econazole induced Ca2+ influx via two separate pathways: one is sensitive to La3+, the other is not. La3+ enlarged 25 microM econazole-induced [Ca2+]i transient during the decay phase. The econazole response was not altered when the cytosolic level of inositol 1,4,5-trisphosphate was inhibited by the phospholipase C inhibitor U73122.

The phospholipase C inhibitor U73122 increases cytosolic calcium in MDCK cells by activating calcium influx and releasing stored calcium

The effects of the phospholipase C (PLC) inhibitor U73122 on intracellular calcium levels ([Ca2+]i) were studied in MDCK cells. U73122 elevated [Ca2+]i dose-dependently. Ca2+ influx contributed to 75% of 20 microM U73122-induced Ca2+ signals. U73122 pretreatment abolished the [Ca2+]i transients evoked by ATP and bradykinin, suggesting that U73122 inhibited PLC. The Ca2+ signals among individual cells varied considerably. The internal Ca2+ source for the U73122 response was the endoplasmic reticulum (ER) since the response was abolished by thapsigargin. The depletion of the ER Ca2+ store triggered a La3+-sensitive capacitative Ca2+ entry. Independently of the internal release and capacitative Ca2 entry, U73122 directly evoked Ca2+ influx through a La3+-insensitive pathway. The U73122 response was augmented by pretreatment of carbonylcyanide m-chlorophynylhydrozone (CCCP), but not by Na+ removal, implicating that mitochondria contributed significantly in buffering the Ca2+ signal, and that efflux via Na+/Ca2+ exchange was insignificant.

Serotonin-stimulated calcium responses in human platelets: assay buffer dependency

In Fura-2 labelled human platelets, the observed calcium response to serotonin stimulation is increased if the assay is conducted in a Krebs buffer compared with the HEPES buffer system usually used. The serotonin response in the Krebs buffer system was concentration dependent with an EC50 value of approximately 0.3 microM and was blocked by nanomolar concentrations of the serotonin2-receptor antagonist methiothepin. In a series of samples collected from 11 persons, a reasonable correlation was seen between the response to 1 microM serotonin and the serotonin2A-receptor density measured in corresponding membrane preparations. The calcium response to serotonin was reduced by treatment with hydrogen peroxide. It is concluded that the use of the Krebs buffer gives a more sensitive response of the Fura-2 loaded human platelets to serotonin stimulation than does the use of HEPES buffer and that the increased sensitivity is achieved without affecting the pharmacological properties of the response. For the response to thrombin, the response intensity is affected by the pH of the buffer used but is not sensitive to the buffer composition.

Inhibition of platelet serotonin uptake by cytochrome P450 inhibitors miconazole and econazole

Serotonin uptake in human platelets was inhibited by cytochrome P450 inhibitors such as miconazole and econazole but not clotrimazole. There was a correlation between inhibition of serotonin uptake and inhibition of imipramine binding, suggesting that these P450 inhibitors may inhibit serotonin uptake via direct binding to the transporter. P450 inhibitor effects on serotonin uptake did not seem to be related to the effects of these compounds on intracellular calcium mobilization. Additionally, nitric oxide pathway stimulation does not appear to be involved.

The sulphydryl oxidizing reagent diamide affects phosphoinositide-mediated signal transduction: implications for the pathogenesis of Alzheimer's disease

In fura-2-labelled human platelets, the thiol oxidising agent diamide decreases the intracellular calcium response to thrombin and serotonin without affecting the basal calcium levels. The effect of diamide on the thrombin response could be prevented by pre-treatment with dithiothreitol (DTT) and reduced when DTT was added 60 s after diamide. The effects of diamide and hydrogen peroxide on the thrombin response were additive. Hydrogen peroxide also produced a calcium response per se, but this response was not affected by diamide. Hydrogen peroxide increased rat brain phosphoinositide hydrolysis and reduced the response to carbachol and noradrenaline, whereas diamide was without effect. The binding of [3H]inositol-1,4,5-trisphosphate to human platelet membranes was inhibited by diamide but not by hydrogen peroxide. Thus diamide affects the phosphoinositide signal transduction pathway in a qualitatively different manner from that found with hydrogen peroxide. It is suggested that oxidative stress may contribute to the disturbances in the phosphoinositide transduction pathway that are found in Alzheimer's disease.

The role of calcium in the etiology of the affective disorders

Calcium abnormalities are some of the more consistent findings in platelets of affective disorder patients. While medication status does not correlate with this finding, antidepressants do modulate intracellular calcium. This, in combination with reports that calcium channel inhibitors may have antidepressant potential, suggests that calcium may play an important role in this disorder. This paper reviews the specificity of calcium abnormalities for the affective disorders and also discusses possible mechanisms of action.

Cytochrome P450: a novel system modulating Ca2+ channels and contraction in mammalian heart cells

1. Cytochrome P450 (P450) is a ubiquitous enzyme system that catalyses oxidative reactions of numerous endogenous and exogenous compounds. The modulatory effects of P450 on the L-type Ca2+ current (ICa), intracellular free Ca2+ signals and cell shortening were assessed in adult rat single ventricular myocytes. 2. Bath administration of the imidazole antimycotics, clotrimazole, econazole and miconazole, which are potent P450 inhibitors, significantly suppressed cardiac ICa. While the Ca2+ channel antagonist nifedipine blocked ICa within 30 s, clotrimazole-induced suppression of ICa required 5.1 +/- 0.4 min (n = 14) to reach a steady low level. The suppression of ICa was dose dependent and recovered after washout of clotrimazole. Intracellular dialysis with the P450 antibody anti-rat CYP1A2 also significantly reduced cardiac ICa. 3. Additional administration of the beta-adrenergic agonist isoprenaline (1 microM) or the membrane-permeable 8-bromo-cAMP (2 mM) completely reversed the suppressant effects of clotrimazole and NaCN on ICa. In addition, intracellular dialysis with 2 mM cAMP abolished the P450 inhibitor-induced suppression of ICa. Phosphorylation of the channel with hydrolysis-resistant ATPgammaS prevented the suppressant effect of clotrimazole on ICa. Furthermore, dephosphorylation of the Ca2+ channel with intracellular dialysis with phosphatase types I and II reduced ICa by 85 +/- 3 % and abolished clotrimazole-induced suppression of ICa. 4. Extracellular administration of the phospholipase A2 inhibitors mepacrine and 4-bromophenacyl bromide significantly suppressed ICa. 5. Clotrimazole, econazole, miconazole and CN- also significantly inhibited intracellular free Ca2+ signals and cell shortening in rat single ventricular myocytes. 6. Intracellular cAMP content was significantly reduced in isolated ventricular myocytes incubated with clotrimazole or CN-. Extracellular administration of 11, 12-epoxyeicosatrienoic acid, one of the P450-mediated metabolites of arachidonic acid, enhanced ICa and intracellular cAMP content. The epoxyeicosatrienoic acid also restored the amplitude of the reduced ICa in P450 antibody-dialysed myocytes. 7. The present data suggest that cytochrome P450 modulates cardiac ICa and cell contraction, and the modulation may result from changes in intracellular levels of cAMP by P450- mediated metabolites of arachidonic acid.

FcgammaRI coupling to phospholipase D initiates sphingosine kinase-mediated calcium mobilization and vesicular trafficking

Aggregation of receptors specific for the constant region of immunoglobulin G activates a repertoire of monocyte responses that can lead ultimately to targeted cell killing via antibody-directed cellular cytotoxicity. The high affinity receptor, FcgammaRI, contains no recognized signaling motif in its cytoplasmic tail but rather utilizes the gamma-chain of FcepsilonRI as an accessory molecule to recruit tyrosine kinases for signal transduction. We show here that, in a human monocytic cell line primed with interferon-gamma, FcgammaRI mobilizes intracellular calcium stores using a novel pathway that involves tyrosine kinase coupling to phospholipase D and resultant downstream activation of sphingosine kinase. Moreover, FcgammaRI is not coupled to phospholipase C; hence, calcium release from intracellular stores occurred in the absence of any measurable rise in inositol triphosphate. Finally, as this novel activation pathway is also shown to be responsible for mediating the vesicular trafficking of internalized immune complexes for degradation, it is likely to play a key role in controlling intracellular events triggered by FcgammaRI.

Activation of platelets in bronchial asthma

STUDY OBJECTIVES

To investigate whether platelets are activated in asthmatics with increased release of preformed mediators and to investigate the influence of oral administration of theophylline on them.

DESIGN

Comparison of the intracellular free calcium concentration ([Ca2+]i) in platelets as an indicator of platelet activation, CD62P expression on platelets, and the chemokine regulated upon activation in normal T cells expressed and presumably secreted (RANTES) level in platelet-rich buffer supernatants between asthmatics and normal subjects.

SETTING

The respiratory outpatient clinics, Hiroshima University, Japan.

PARTICIPANTS

Twenty-five normal volunteers, 19 asthmatics taking no oral drugs associated with asthma treatment (group A), and 18 asthmatics taking oral theophylline (group B).

MEASUREMENTS AND RESULTS

While the resting [Ca2+]is in platelets were similar among the three groups, the [Ca2+]is in group A were significantly higher than those in normal subjects (p<0.05) and group B (p<0.01) after thrombin or 9,11-epithia-11,12-methano-thromboxane A2 (STA2) stimulation in the absence of external Ca2+. The CD62P expression level and RANTES level in group A after STA2 stimulation were significantly higher than those in normal subjects and group B (p<0.05).

CONCLUSIONS

We conclude that agonist-mediated activation of platelets is augmented in asthmatics resulting in enhanced release of chemokine such as RANTES, which could be suppressed by oral administration of theophylline.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Kinetics of store-operated Ca2+ influx evoked by endomembrane Ca2+-ATPase inhibitors in human platelets

Influx of Ca2+ from the extracellular medium into the platelet cytosol is regulated by the Ca2+ content of intracellular Ca2+ stores. In this paper, we show that activation of this pathway of store-operated Ca2+ influx (SOCI) by the endomembrane Ca2+-ATPase inhibitors, thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone, is mediated by an intracellular factor that accumulates in time. We also describe that SOCI is stimulated by treatment of the platelets with low doses of primaquine, a compound known to inhibit intracellular vesicular transport at higher concentrations. The protein tyrosine kinase inhibitor, genistein, completely antagonized the stimulating and accelerating effects on SOCI of primaquine treatment. These results suggest that SOCI is controlled by a time-dependent factor, whose generation is stimulated by primaquine and protein tyrosine kinase activation.

Purinoceptor-evoked calcium signalling in human platelets

ADP evokes a rise in platelet cytosolic Ca2+ concentration by stimulating Ca2+ entry and releasing Ca2+ from intracellular stores. Single cell studies indicate that the response consists of a series of spikes in cytosolic Ca2+. The release of stored Ca2+ is mediated by the generation of inositol 1,4,5-trisphosphate. Store depletion in turn leads to activation of a store-regulated Ca2+ entry pathway via a mechanism which appears to involve a protein tyrosine phosphorylation step. Preceding these events, ADP activates a receptor-operated non-selective cation channel, which mediates the entry of Ca2+ and Na+ with a latency of just a few milliseconds. Recent studies indicate that this channel is activated via a P2X1 purinoceptor at which ATP and diadenosine tetraphosphate are agonists. This receptor is distinct from that leading to the release of stored Ca2+ and to store-regulated Ca2+ entry.

Calibration of Fura-2 signals introduces errors into measurement of thrombin-stimulated calcium mobilisation in human platelets

The intracellular calcium indicator dye Fura-2 has been widely used for the study of human platelet thrombin receptor-mediated calcium mobilisation in disease states. In general, authors (a) use a Fura-2/AM concentration of 2-3 mumol/l and (b) calibrate fluorescence signals on the basis of maximum and minimum ratios of fluorescence (Rmax and Rmin) and the ratio of the calcium-free and calcium-bound fluorescence at an excitation wavelength of 380 nm ("C2/B2"). In the present study, it is found (a) that a greater peak response to thrombin is seen when 1 mumol/l Fura-2/AM rather than 2 or 3 mumol/l is used; and (b) that calibration leads to a poorer test-retest reliability and in general a greater variability of the obtained calcium signal than when the simple measurement of the 340 nm/380 nm fluorescence ratio is used. It is suggested that this poor variability is due to the presence of an extracellular factor that can quench the Fura-2 signal once the platelets have been permeabilised by detergent treatment. Consistent with this, addition of bovine serum albumin to the assay medium has no significant effect on the fluorescence ratio response to thrombin, but greatly increases the observed calibrated calcium signal.

Activation of microsomal cytochrome P450 mono-oxygenase by Ca2+ store depletion and its contribution to Ca2+ entry in porcine aortic endothelial cells

1. We investigated how microsomal cytochrome P450 mono-oxygenase (Cyp450 MO) is regulated in cultured porcine aortic endothelial cells. The hypothesis that a Cyp450 MO-derived metabolite links Ca2+ store depletion and Ca2+ entry was studied further. 2. Microsomal Cyp450 MO was monitored fluorometrically by dealkylation of 1-ethoxypyrene-3,6,8-tris-(dimethyl-sulphonamide; EPSA) in saponin permeabilized cells or in subcellular compartments. Endothelial Ca2+ signalling was measured by a standard fura-2 technique, membrane potential was determined with the potential-sensitive fluorescence dye, bis-(1,3-dibutylbarbituric acid) pentamethine oxonol (DiBAC4(5)) and tyrosine kinase was quantified by measuring the phosphorylation of a immobilized substrate with a horseradish peroxidase labelled phosphotyrosine specific antibody. 3. Depletion of cellular Ca2+ pools with inositol 1,4,5-trisphosphate (IP3), thapsigargin or cyclopiazonic acid activated microsomal Cyp450 MO. Similar to direct Ca2+ store depletion, chelating of intramicrosomal Ca2+ with oxalate stimulated Cyp450 MO activity, while changing cytosolic free Ca2+ failed to influence Cyp450 MO activity. These data indicate that microsomal Cyp450 MO is activated by depletion of IP3-sensitive stores. 4. Besides the common cytochrome P450 inhibitors, econazole, proadifen and miconazole, thiopentone sodium and methohexitone inhibited Cyp450 MO in a concentration-dependent manner. The physiological substrate of Cyp450 MO, arachidonic acid, inhibited EPSA dealkylation. In contrast to most other cytochrome P450 inhibitors used in this study, thiopentone sodium did not directly interfere with Ca2+ entry pathways, membrane hyperpolarization due to K+ channel activation or tyrosine kinase activity. 5. Inhibition of Cyp450 MO by thiopentone sodium diminished Ca2+/Mn2+ entry to Ca2+ store depletion by 43%, while it did not interfere with intracellular Ca2+ release by IP3 or thapsigargin. 6. Cyp450 MO inhibition with thiopentone sodium diminished autacoid-induced membrane hyperpolarization. 7. Induction of Cyp450 MO with dexamethasone/clofibrate for 72 h yielded increases in thapsigargin-induced Cyp450 MO activity (by 35%), Ca2+/Mn2+ entry (by 105%) and membrane hyperpolarization (by 40%). 8. The Cyp450 MO-derived compounds, 11,12 and 5,6-epoxyeicosatrienoic acids (EETs) yielded membrane hyperpolarization, insensitive to thiopentone sodium. 9. These data demonstrate that endothelial Cyp450 MO is activated by Ca2+ store depletion and Cyp450 MO produced compounds that hyperpolarize endothelial cells. 10. The data presented and our previous findings indicate that Cyp450 MO plays a crucial role in the regulation of store-operated Ca2+ influx. We propose that Cyp450 MO-derived EETs constitute a signal for Ca2+ entry activation and increase the driving force for Ca2+ entry by membrane hyperpolarization in porcine aortic endothelial cells.

SK&F 96365 inhibits intracellular Ca2+ pumps and raises cytosolic Ca2+ concentration without production of nitric oxide and von Willebrand factor

The effects of the imidazole compound SK&F 96365 on Ca2+ movements and production of nitric oxide (NO) and von Willebrand factor (vWF) have been investigated in human endothelial cells. Changes in cytosolic Ca2+ concentration ([Ca2+]i) were measured with Fura-2. Real-time production of NO was monitored with a porphyrinic microsensor and the release of vWF with an enzyme-linked immunosorbent assay. Irrespective of the transmembrane Ca2+ gradient, 30 microM SK&F 96365 doubled [Ca2+]i suggesting a Ca2+ release from intracellular stores. The SK&F 96365-induced [Ca2+]i rise was not accompanied by detectable NO and vWF production, while 1 microM thapsigargin enhanced [Ca2+]i 2.5 times, doubled the secretion of vWF and increased the NO production to 10 +/- 4 nM (n = 5). Pretreatment with SK&F 96365 prevented thapsigargin from increasing [Ca2+]i, NO production and vWF secretion. To investigate the mechanism by which SK&F 96365 released Ca2+ from internal pools, its effect and that of thapsigargin on the ATP-dependent 45Ca2+ uptake into platelet membrane vesicles were compared. SK&F 96365 as thapsigargin, dose-dependently reduced the initial rate of 45Ca2+ uptake. In conclusion, we demonstrate that, in the absence of Ca2+ entry from the extracellular space, the [Ca2+]i increase elicited by SK&F 96365 or thapsigargin is not sufficient to initiate NO synthesis and vWF secretion. This confirms the important role of Ca2+ influx in endothelial secretion processes.

Mechanisms of L-NG nitroarginine/indomethacin-resistant relaxation in bovine and porcine coronary arteries

1. Coronary arteries from bovines (BCA) and pigs (PCA) were used for measuring endothelium-dependent relaxation in the presence of L-NG nitroarginine and indomethacin. As some compounds tested have been found to have an inhibitory effect on autacoid-activated endothelial Ca2+ signalling, endothelium-dependent relaxation was initiated with the Ca2+ ionophore A23187. 2. The common compounds for modulating arachidonic acid release/pathway, mepacrine and econazole only inhibited L-NG nitroarginine-resistant relaxation in BCA not in PCA. In contrast, proadifen (SKF 525A) diminished relaxation in BCA and PCA. Mepacrine and proadifen inhibited Hoe-234-initiated relaxation in BCA and PCA, while econazole only inhibited Hoe 234-induced relaxation in PCA. Due to the multiple effects of these compounds, caution is necessary in the interpretation of results obtained with these compounds. 3. The inhibitor of Ca(2+)-activated K+ channels, apamin, strongly attenuated A23187-induced L-NG nitroarginine-resistant relaxation in BCA while apamin did not affect L-NG nitroarginine-resistant relaxation in PCA. 4. Pertussis toxin blunted L-NG nitroarginine-resistant relaxation in BCA, while relaxation of PCA was not affected by pertussis toxin. 5. Thiopentone sodium inhibited endothelial cytochrome P450 epoxygenase (EPO) in PCA but not in BCA, while L-NG nitroarginine-resistant relaxation of BCA and PCA were unchanged. Protoporphyrine IX inhibited EPO in BCA and PCA and abolished L-NG nitroarginine-resistant relaxation of BCA not PCA. 6. An EPO-derived compound, 11,12-epoxy-eicosatrienoic acid (11,12-EET) yielded significant relaxation in BCA and PCA in three out of six experiments. 7. These findings suggest that L-NG nitroarginine-resistant relaxation in BCA and PCA constitutes two distinct pathways. In BCA, activation of Ca(2+)-activated K+ channels via a pertussis-toxin-sensitive G protein and EPO-derived compounds might be involved. In PCA, no selective inhibition of L-NG nitroarginine-resistant relaxation was found.

Extracellular site for econazole-mediated block of Ca2+ release-activated Ca2+ current (Icrac) in T lymphocytes

1. Standard whole cell patch clamp recording techniques were used to study the pharmacological characteristics and site of econazole-mediated inhibition of calcium release-activated calcium current (Icrac) in the human leukaemic T cell line, Jurkat. 2. Extracellularly applied econazole blocked Icrac in a concentration-dependent manner (IC50 approximately 14 microM). Block developed over a relatively slow timecourse of 30-60 s (10 microM), and only partially reversed over minutes. 3. Econazole dialysed from the pipette into the cytosol at concentrations ranging from 0.1 to 30 microM did not reduce Icrac, or quantitatively affect Icrac block by extracellularly applied econazole. 4. A less lipophilic quaternary iodide derivative of econazole was synthesized to retard absorption through the cell membrane. When applied extracellularly, this compound blocked Icrac in a concentration-dependent manner with onset kinetics comparable to econazole. 5. Results with intracellularly dialysed econazole and the quaternary econazole derivative provide convergent evidence that econazole blocks Icrac via an extracellular interaction. 6. The inability of intracellularly applied econazole to inhibit Icrac argues against the notion that econazole inhibits capacitative Ca2+ entry pathways secondary to its known inhibitory effects on cytochrome P-450.

Interdependence of calcium signaling and protein tyrosine phosphorylation in human endothelial cells

The signal transduction cascade which initiates transmembraneous influx of Ca2+ into endothelial cells in response to the discharge of intracellular Ca2+ stores is thought to involve a step sensitive to tyrosine kinase inhibition. We investigated the interrelationship between Ca2+ signaling and protein tyrosine phosphorylation following cell stimulation with either the receptor-dependent agonist, bradykinin, or the protein-tyrosine phosphatase inhibitor, phenylarsine oxide. In cultured human endothelial cells phenylarsine oxide instigated a concentration-dependent increase in the intracellular concentration of free Ca2+ ([Ca2+]i). This increase in [Ca2+]i was not associated with the tyrosine phosphorylation of phospholipase C gamma, enhanced formation of inositol 1,4,5-trisphosphate, or the rapid depletion of intracellularly stored Ca2+ but was coincident with the enhanced and prolonged tyrosine phosphorylation of a number of cytoskeletal proteins. In bradykinin-stimulated cells the tyrosine phosphorylation of the same cytoskeletal proteins (most notably 85- and 100-kDa proteins) was transient when cells were stimulated in the presence of extracellular Ca2+, was maintained under Ca2+-free conditions, and was reversed following readdition of extracellular Ca2+. These data suggest that the tyrosine phosphorylation of 2 cytoskeletal proteins is determined by the level of Ca2+ present in intracellular stores thus indicating a critical role for tyrosine phosphorylation in the control of capacitative Ca2+ entry in endothelial cells.

Pharmacological and functional properties of voltage-independent Ca2+ channels

During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.

Prostacyclin and sodium nitroprusside inhibit the activity of the platelet inositol 1,4,5-trisphosphate receptor and promote its phosphorylation

Prostaglandin I2 (PGI2) and sodium nitroprusside (SNP) induce a rapid decay of the thrombin-promoted increase of [Ca2+]i in aspirin-treated platelets incubated in the absence of external Ca2+. The mechanism of their effect was studied with a new method which utilizes ionomycin to increase [Ca2+]i, followed by bovine serum albumin (BSA) to remove the Ca2+ ionophore. The rapid decay of [Ca2+]i after BSA is mostly due to the reuptake into the stores, since it is strongly inhibited by the endomembrane Ca2+-ATPase inhibitor thapsigargin. PGI2 and SNP are without effect on the BSA-promoted decay both with and without thapsigargin, showing that they do not affect the activity of the Ca2+-ATPases. The fast decay of [Ca2+]i after BSA is decreased by thrombin which produces the Ca2+ releaser inositol 1,4,5-trisphosphate (InsP3), thus counteracting the activity of the endomembrane Ca2+ pump. When added after thrombin, PGI2 and SNP accelerate the BSA-activated decay of [Ca2+]i. However, under the same conditions, they do not decrease the concentration of InsP3. In saponin-permeabilized platelets, cAMP and cGMP counteract the Ca2+ release induced by exogenous InsP3. Their inhibitory effect disappears at high InsP3 concentrations. This demonstrates that PGI2 and SNP potentiate Ca2+ reuptake by inhibiting the InsP3 receptor. Two bands of approximately 260 kDa are recognized by a monoclonal antibody recognizing the C-terminal region of the InsP3 receptor. Both are phosphorylated rapidly, the heavier more intensely, in the presence of PGI2 and SNP. The phosphorylation of the InsP3 receptor is fast enough to be compatible with its involvement in the inhibition of the receptor by cyclic nucleotides.

Wortmannin inhibits store-mediated calcium entry and protein tyrosine phosphorylation in human platelets

The effects of the WT on store-mediated Ca2+ entry and protein tyrosine phosphorylation were investigated in fura-2-loaded human platelets. Wortmannin (2 microM) attenuated the rise in [Ca2+]i caused by Ca2+ entry while having no effect on the mobilisation of Ca2+ from internal stores. It also reduced store-depletion-evoked protein tyrosine phosphorylation. These findings demonstrate that WT is an inhibitor of tyrosine phosphorylation and store-mediated calcium entry and provide further evidence for the involvement of a tyrosine phosphorylation step in the link between Ca2+ store depletion and Ca2+ influx in human platelets.