Ca2+ homeostasis in unstimulated platelets

Role of Platelet Mitochondria: Life in a Nucleus-Free Zone

Platelets are abundant, small, anucleate circulating cells, serving many emerging pathophysiological roles beyond hemostasis; including active critical roles in thrombosis, injury response, and immunoregulation. In the absence of genomic DNA transcriptional regulation (no nucleus), platelets require strategic prepackaging of all the needed RNA and organelles from megakaryocytes, to sense stress (e.g., hyperglycemia), to protect themselves from stress (e.g., mitophagy), and to communicate a stress response to other cells (e.g., granule and microparticle release). Distinct from avian thrombocytes that have a nucleus, the absence of a nucleus allows the mammalian platelet to maintain its small size, permits morphological flexibility, and may improve speed and efficiency of protein expression in response to stress. In the absence of a nucleus, platelet lifespan of 7–10 days, is largely determined by the mitochondria. The packaging of 5–8 mitochondria is critical in aerobic respiration and yielding metabolic substrates needed for function and survival. Mitochondria damage or dysfunction, as observed with several disease processes, results in greatly attenuated platelet survival and increased risk for thrombovascular events. Here we provide insights into the emerging roles of platelets despite the lack of a nucleus, and the key role played by mitochondria in platelet function and survival both in health and disease.

Platelet mitochondrial dysfunction and the correlation with human diseases

The platelet is considered as an accessible and valuable tool to study mitochondrial function, owing to its greater content of fully functional mitochondria compared with other metabolically active organelles. Different lines of studies have demonstrated that mitochondria in platelets have function far more than thrombogenesis regulation, and beyond hemostasis, platelet mitochondrial dysfunction has also been used for studying mitochondrial-related diseases. In this review, the interplay between platelet mitochondrial dysfunction and oxidative stress, mitochondrial DNA lesions, electron transfer chain impairments, mitochondrial apoptosis and mitophagy has been outlined. Meanwhile, considerable efforts have been made towards understanding the role of platelet mitochondrial dysfunction in human diseases, such as diabetes mellitus, sepsis and neurodegenerative disorders. Alongside this, we have also articulated our perspectives on the development of potential biomarkers of platelet mitochondrial dysfunction in mitochondrial-related diseases.

Restoration of responsiveness of phospholipase Cγ2-deficient platelets by enforced expression of phospholipase Cγ1

Receptor-mediated platelet activation requires phospholipase C (PLC) activity to elevate intracellular calcium and induce actin cytoskeleton reorganization. PLCs are classified into structurally distinct β, γ, δ, ε, ζ, and η isoforms. There are two PLCγ isoforms (PLCγ1, PLCγ2), which are critical for activation by tyrosine kinase-dependent receptors. Platelets express both PLCγ1 and PLCγ2. Although PLCγ2 has been shown to play a dominant role in platelet activation, the extent to which PLCγ1 contributes has not been evaluated. To ascertain the relative contributions of PLCγ1 and PLCγ2 to platelet activation, we generated conditionally PLCγ1-deficient, wild-type (WT), PLCγ2-deficient, and PLCγ1/PLCγ2 double-deficient mice and measured the ability of platelets to respond to different agonists. We found that PLCγ2 deficiency abrogated αIIbβ3-dependent platelet spreading, GPVI-dependent platelet aggregation, and thrombus formation on collagen-coated surfaces under shear conditions, which is dependent on both GPVI and αIIbβ3. Addition of exogenous ADP overcame defective spreading of PLCγ2-deficient platelets on immobilized fibrinogen, suggesting that PLCγ2 is required for granule secretion in response to αIIbβ3 ligation. Consistently, αIIbβ3-mediated release of granule contents was impaired in the absence of PLCγ2. In contrast, PLCγ1-deficient platelets spread and released granule contents normally on fibrinogen, exhibited normal levels of GPVI-dependent aggregation, and formed thrombi normally on collagen-coated surfaces. Interestingly, enforced expression of PLCγ1 fully restored GPVI-dependent aggregation and αIIbβ3-dependent spreading of PLCγ2-deficient platelets. We conclude that platelet activation through GPVI and αIIbβ3 utilizes PLCγ2 because PLCγ1 levels are insufficient to support responsiveness, but that PLCγ1 can restore responsiveness if expressed at levels normally achieved by PLCγ2.

Nitridergic platelet pathway activation by hementerin, a metalloprotease from the leech Haementeria depressa

Hementerin (HT) is an 80 kDa fibrino(geno)lytic metalloprotease, purified from saliva of the leech Haementeria depressa. In the present report, the effect of HT on several functional parameters of human platelets was assessed. HT inhibited platelet aggregation and ATP release induced by different agonists such as ADP, adrenaline, collagen, thrombin, and arachidonic acid. HT did neither modify the expression of platelet glycoproteins (Ib, IIb-IIIa, Ia-IIa, IV) nor intraplatelet fibrinogen levels, whereas it markedly decreased CD62P and CD63 levels after the stimulation with thrombin. HT significantly increased thrombin-induced platelet Ca2+ intracellular levels, cGMP content and nitric oxide synthase (NOS) activity. The effect of HT on platelet aggregation was reversed by two NOS inhibitors, N(omega)-Nitro-L-arginine methyl ester and 2 N(G)-Nitro-L-arginine. In summary, these results indicate that HT is an effective inhibitor of human platelet aggregation, presumably through activation of the platelet's nitridergic pathway.

The roles of ATP and calcium in morphological changes and cytotoxicity induced by 1,4-benzoquinone in platelets

To understand the mechanism of 1,4-benzoquinone-induced cytotoxicity in platelets, the roles of ATP and calcium in platelet toxicity and morphological changes were investigated. Using scanning electron microscopy, morphological changes including membrane blebbing were observed in rat platelets 5 min after exposure to 1,4-benzoquinone, which were significantly different from shape changes (pseudopod formation) observed in response to physiological agonists. Benzoquinone-induced membrane blebbing of platelets was associated with rapid depletion of intracellular ATP and was independent of the presence of extracellular calcium. Benzoquinone-induced platelet lysis observed between 20 and 30 min was dependent on extracellular calcium and associated with increased cytosolic calcium. Cytotoxicity induced by 1,4-benzoquinone was inhibited by antagonists of calmodulin, suggesting that calmodulin could play an important role in platelet toxicity. These results suggested that the progression of events for benzoquinone-induced cytotoxicity in platelets was as follows: 1,4-benzoquinone depletes intracellular ATP; membrane blebbing occurs; calcium homeostasis is disrupted, activation of calmodulin-dependent processes results; finally cytotoxicity occurs.

Actin dynamics in platelets

The human blood platelet circulates in the blood as a non-adherent disk. Upon receiving signals of blood vessel damage, the platelet reorganizes its actin cytoskeleton which transforms it into a spiky dynamic adherent glue. This transformation involves a temporal sequence of four morphologically distinct steps which is reproducible in vitro. The actin dynamics underlying these shape changes depend on a large number of actin-binding proteins. Maintenance of the discoid shape requires actin-binding proteins that inhibit these reorganizations, whereas transformation involves other proteins, some to disassemble old filaments and others to polymerize new ones. F-Actin-affinity chromatography identified a large set of actin-binding proteins including VASP, Arp2 and 2E4/kaptin. Recent discoveries show that VASP inhibits filament disassembly and Arp2/3 is required to polymerize new filaments. Morphological analysis of the distribution of these actin-binding proteins in spread platelets together with biochemical measurements of their interactions with actin lead to a model of interactions with actin that mediate shape change.

Regulation of plasma membrane Ca2+-ATPase by small GTPases and phosphoinositides in human platelets

We have investigated the restoration of [Ca(2+)](i) in human platelets following the discharge of the intracellular Ca(2+) stores. We found that the plasma membrane Ca(2+)-ATPase is the main mechanism involved in Ca(2+) extrusion in human platelets. Treatment of platelets with the farnesylcysteine analogs, farnesylthioacetic acid and N-acetyl-S-geranylgeranyl-l-cysteine, inhibitors of activation of Ras proteins, accelerated the rate of decay of [Ca(2+)](i) to basal levels after activation with thapsigargin combined with a low concentration of ionomycin, indicating that Ras proteins are involved in the negative regulation of Ca(2+) extrusion. Rho A, which is involved in actin polymerization, was not responsible for this effect. Consistent with this, the actin polymerization inhibitors, cytochalasin D and latrunculin A, did not alter the recovery of [Ca(2+)](i). Activation of human platelets with thapsigargin and ionomycin stimulated the tyrosine phosphorylation of the plasma membrane Ca(2+)-ATPase, a mechanism that was inhibited by farnesylcysteine analogs, suggesting that Ras proteins could regulate Ca(2+) extrusion by mediating tyrosine phosphorylation of the plasma membrane Ca(2+)-ATPase. Treatment of platelets with LY294002, a specific inhibitor of phosphatidylinositol 3- and phosphatidylinositol 4-kinase, resulted in a reduction in the rate of recovery of [Ca(2+)](i) to basal levels, suggesting that the products of these kinases are involved in stimulating Ca(2+) extrusion in human platelets.

Physical properties of membrane fractions isolated from human platelets: implications for chilling induced platelet activation

In previous studies, it has been suggested that chilling induced activation of human platelets is related to a lipid phase transition seen in membrane lipids. Those studies showed a single, surprisingly cooperative transition in human platelets, as determined by Fourier transform infrared (FTIR) spectroscopy, findings that are confirmed here with calorimetric measurements. Such transitions have now been studied in membrane fractions obtained from the platelets and it is reported that all fractions and purified phospholipids show similar transitions. In order to obtain these data it was necessary to develop means for separating these fractions. Therefore, a novel method for isolation and separation of dense tubular system (DTS) and plasma membranes in human platelets is described here. Lipid analysis showed that phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were the dominant phospholipids in both fractions, whereas cholesterol and sphingomyelin (SM) were predominantly located in the plasma membranes. Thermotropic phase transitions in the two membrane fractions, determined by differential scanning calorimetry (DSC) and FTIR spectroscopy were found to occur at about 15 degrees C, similar to the Tm of intact human platelets. These data are discussed in relation to the role of the DTS and plasma membranes in the cold-induced activation of human platelets.

Association of localized Ca2+ gradients with redistribution of glycoprotein IIb-IIIa and F-actin in activated human blood platelets

We monitored the intracellular distribution of ionized free Ca2+ concentration ([Ca2+]i) in individual human platelets by digital imaging fluorescence microscopy with fura 2 during platelet activation induced by surface contact or a soluble platelet agonist (thrombin). Contact of platelets with glass resulted in pseudopod formation and spreading, accompanied by a nonuniform rise in [Ca2+]i. The rise in [Ca2+]i was maximal during pseudopod formation. Locally elevated [Ca2+]i was frequently found in pseudopodia and at the edge and core of spread platelets. This pattern was faithfully duplicated by the local pattern of distribution of the cytoskeletal components F-actin, gelsolin, and surface glycoproteins (GP) IIb-IIIa but not by calmodulin. Platelets stimulated by thrombin also showed an inhomogeneous rise in [Ca2+]i, which was well correlated with the staining of F-actin and GPIIb-IIIa. Cytochalasin D, an inhibitor of actin polymerization, inhibited the inhomogeneous increase or redistribution of F-actin and GPIIb-IIIa but did not inhibit the rise in mean [Ca2+]i. These observations suggest that a localized change in [Ca2+]i may be associated with cytoskeletal reorganization and redistribution of GPIIb-IIIa in activated platelets.

Increased calcium stores in platelets from African Americans

Differences in cation transport have been observed between African Americans and whites. These differences may underlie the increased predisposition of African Americans to essential hypertension. To further explore these racial differences, we used platelets as a cellular model for calcium regulation. We measured 45Ca fluxes in platelets from 21 African American and 25 white men. Additionally, using fura 2, we measured cytosolic free calcium levels in resting platelets and platelets treated with ouabain and thrombin. Platelet 45Ca uptake was described by two exchangeable pools: a small, rapidly exchangeable pool and a larger, slowly exchangeable pool. Both pools were larger in platelets from African Americans than from whites (263 versus 185 pmol per 1 x 10(8) platelets for the rapidly exchangeable pool, P < .05; 744 versus 532 pmol per 1 x 10(8) platelets for the slowly exchangeable pool, P < .01). 45Ca washout was described by a rapidly exchangeable pool and a static pool. The former was also higher in platelets from African Americans than from whites (246 versus 202 pmol per 1 x 10(8) platelets, P < .01). The cytosolic free calcium concentrations in resting platelets were lower in African Americans than in whites. After treatment with ouabain and thrombin, the sustained posttransient levels of cytosolic free calcium increased to a greater extent in platelets from African Americans (46.7 nmol/L) than from whites (34.5 nmol/L, P = .033). Platelets from African Americans demonstrate higher intracellular calcium stores than platelets from whites. This racial difference could explain the sensitivity of African Americans to vasoactive agents acting through calcium mobilization from intracellular stores and cytosolic calcium.

Hyperaggregation of platelets in intestinal tuberculosis: role of platelets in chronic inflammation

Eighty-eight percent (38/43) patients of intestinal tuberculosis showed significant hyperaggregation of platelets (P < 0.001). Serum and plasma from 15 patients when incubated with normal platelets caused hyperaggregation. Gel filtered platelets from 2 patients suspended in normal plasma showed hyperaggregation of platelets with arachidonic acid. Tubercular protein had no effect on platelet aggregation. A role of hyperactive platelets in chronic inflammatory response is discussed.

Ca2+ efflux from platelets. Control by protein kinase C and the filling state of the intracellular Ca2+ stores

Large amounts of Ca2+ (almost 20 nmol/10(8) cells) are released from platelets by exocytosis. This secretory-granule-associated Ca2+ does not contribute to the cytosolic free Ca2+ ([Ca2+]i), which is controlled by the much smaller agonist-sensitive Ca2+ pool, unless high (1 microM), but not low (0.04 microM) concentrations of ionomycin are present. Low concentrations of ionomycin release Ca2+ almost exclusively from the agonist-sensitive stores. In aspirinated platelets incubated in the presence of 0.5 mM EGTA the extensive depletion of the agonist-sensitive stores is obtained by the combined action of low ionomycin and the endomembrane Ca(2+)-ATPase inhibitor thapsigargin (which individually promote only a partial depletion). The subsequent decay of [Ca2+]i is increased by phorbol-myristate acetate, confirming that Ca2+ efflux from platelets is potentiated by the activation of protein kinase C [Pollock, W. K., Sage, S. O. & Rink, T. J. (1987) FEBS Lett. 210, 132-140]. A novel type of control of Ca2+ efflux appears to be exerted by the filling state of the stores. Treatment with low ionomycin or thapsigargin determines the release of a fraction of the stores-associated Ca2+; the subsequent decay of [Ca2+]i is slow. The decay rate of [Ca2+]i accelerates after extensive depletion of the stores following the addition of thapsigargin or ionomycin. If the depletion of the stores is induced by thrombin, added alone or in combination with thapsigargin, the increases of [Ca2+]i are the same and the subsequent decay rates are largely superimposable; however a large fraction of [Ca2+]i is reaccumulated into the stores in the absence, but not in the presence of thapsigargin, indicating that Ca2+ efflux is activated when the stores are empty. Ca2+ efflux can proceed against a concentration gradient. In 45Ca-loaded platelets, the thrombin-promoted 45Ca efflux is potentiated by thapsigargin. The protein-kinase-C-dependent and store-depletion-dependent stimulations of 45Ca efflux are additive. These observations indicate that, in addition to being activated by protein kinase C, Ca2+ efflux from platelets is activated by the depletion of the stores. The two activations appear to be additive.

Calcium mobilisation controls tyrosine protein phosphorylation independently of the activation of protein kinase C in human platelets

We have investigated the regulation of tyrosine proteins phosphorylation by intracellular Ca2+ level ([Ca2+]i) and protein kinase C (PKC) during platelet stimulation. We found that chelation of extracellular calcium completely prevented phosphorylation of tyrosine proteins induced by thapsigargin and phorbol 12-myristate 13-acetate (PMA), whereas, when induced by thrombin, it prevented a subset of tyrosine proteins. The selective inhibition of PKC by GF 109203X did not abolish tyrosine protein phosphorylation when induced by thrombin and thapsigargin. The results suggest that in human platelets tyrosine protein phosphorylation is dependent on [Ca2+]i, although direct PKC activation can also induce phosphorylation of tyrosine proteins.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Kinetics of calcium transport across the lymphocyte plasma membrane

We have investigated plasma membrane Ca2+ transport by monitoring the fluorescence of human peripheral T-lymphocytes loaded with fura 2. Thapsigargin (TG) was utilized the block the Ca(2+)-ATPase of the endoplasmic reticulum and elevate the cytosolic Ca2+ (Ca2+i). Ca2+ influx was inhibited by chelating extracellular Ca2+ with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The rate of decline in the Ca2+i signal of TG-treated lymphocytes after exposure to EGTA was used to assess Ca2+ extrusion across the plasma membrane. Initial rates of Ca2+i decline were examined in cells suspended in Na(+)-containing and Na(+)-free solutions; initial rates were linearly related to the [Ca2+]i at the onset of the Ca2+i decline and were unaffected by varying the extracellular Ca2+. Extracellular Na+ increased the rate of Ca2+ extrusion and decreased the threshold [Ca2+]i for extrusion, indicating a substantial role for the Na(+)-Ca2+ exchange in Ca2+i homeostasis. Both decreased temperature and calmodulin inhibition significantly slowed the Ca2+i decline in Na(+)-free HEPES-buffered solution, suggesting Ca2+ extrusion under these conditions was mediated by the Ca2+ pump. Protein kinase C (PKC) activation or inhibition did not affect the Ca2+i decline parameters. However, Ca2+ accumulation and Mn2+ (a Ca2+ surrogate) uptake were significantly and Mn2+ (a Ca2+ surrogate) uptake were significantly inhibited by activators of PKC. Cyclic nucleotides altered neither the parameters of the Ca2+i decline nor Mn2+ uptake. Thus human T-lymphocytes exhibit Na(+)- and Ca(2+)-dependent transporters characterized as the Na(+)-Ca2+ exchanger and Ca2+ pump. The main effect of PKC in these cells is the modulation of Ca2+ entry across the lymphocyte plasma membrane.

Parathyroid hormone raises cytosolic calcium in pancreatic islets: study on mechanisms

The pancreatic islets of Langerhans are targets for PTH and the action of the hormone on the islet is most likely mediated through the ability of PTH to increase cytosolic calcium ([Ca2+]i) of the islet cells. Although direct evidence for such an effect has been clearly demonstrated, the mechanisms through which the hormone exerts such an action are not elucidated. The present study examined these questions using pancreatic islets isolated from normal rats. Both 1-34 and 1-84 PTH produced a dose dependent increase in [Ca2+]i of the islets but the effect of the latter was significantly (P < 0.01) greater than that of the former. This action of PTH was significantly (P < 0.01) decreased by the use of PTH antagonist or by verapamil. The G protein activator (GTP gamma S) mimicked the effect of PTH while pertussis toxin and the G protein inhibitor (GDP beta S) significantly reduced the PTH-induced rise in [Ca2+]i. Dibutyryl cAMP, and phorbol ester 12-myristate 13 acetate increased [Ca2+]i of pancreatic islets in a dose dependent manner and the effect was inhibited (P < 0.01) by verapamil. Staurosporine inhibited the effect of TPA as well as of 1-84 PTH on [Ca2+]i of the islets. These data indicate that: (1) PTH increases [Ca2+]i of pancreatic islets, (2) this action is partly receptor mediated and is produced by activation of L-type calcium channels through stimulation of G protein(s), and (3) the rise in [Ca2+]i is due to both stimulation of cAMP generation and activation of protein kinase C.

Relationships between serotonin induced elevation of intracellular Ca2+ concentration and stimulation of Ca2+ influx in blood platelets

Serotonin (5-HT) caused immediate elevation of intracellular Ca2+ concentration ([Ca2+]i) in blood platelets, and it was completely inhibited by 1 mM EGTA. In Ca2+ replenished platelets, however, 2 mM EGTA did not affect the 5-HT induced elevation of [Ca2+]i when EGTA was applied just before or during the stimulation by 5-HT. At the same concentration 5-HT was also found to enhance Ca2+ influx through the activation of 5-HT2 receptor, but with rather longer latent time. From these results it is suggested that 5-HT induced elevation of [Ca2+]i is caused by mobilization of Ca2+ from intracellular Ca2+ storage sites, but not by direct stimulation of Ca2+ influx. Depletion of such Ca2+ stores might impair the effect of 5-HT on [Ca2+]i. Thus, 5-HT induced augmentation of Ca2+ influx might be secondary to replenishment of the depleted Ca2+ stores which was caused by 5-HT induced internal release of Ca2+. It is concluded that the effects of 5-HT on [Ca2+]i and Ca2+ influx in platelets are manifested sequentially or independently.

Fluorescence digital image analysis of thrombin and ADP induced rise in intracellular Ca2+ concentration of single blood platelets

Cytoplasmic free Ca2+ concentration, [Ca2+]i, was estimated in single rabbit blood platelets by digital imaging microscopy with the use of the specific Ca(2+)-indicator dye Fura-2. Uneven distribution and low level of [Ca2+]i was found in the resting platelet even in the presence of extracellular 1 mM Ca2+. Thrombin at 1 unit/ml immediately caused a transient increase in [Ca2+]i, which was followed by a secondary and sustained increase in [Ca2+]i. The distribution of increased levels of [Ca2+]i was also shown to be uneven within the cell. The presence of 1 mM EGTA in the medium only slightly decreased the initial rise in [Ca2+]i, but completely inhibited the latter phase, a sustained rise in [Ca2+]i. This result shows that the initial rise of [Ca2+]i might not be caused by Ca2+ influx, but might be induced by mobilization of Ca2+ from intracellular Ca2+ storage sites. This speculation is further supported by the fact that the elevated [Ca2+]i induced by thrombin immediately decreased to the base line value when 3 mM EGTA was applied. Thus, thrombin induced elevation of [Ca2+]i is suggested to consist of two different processes, namely the mobilization of Ca2+ from the intracellular storage sites and the successive Ca2+ influx through the receptor activated Ca2+ channels. Stimulation with ADP also caused a rapid elevation of platelet [Ca2+]i, but this effect of ADP was different form that of thrombin. Thus, the ADP induced rise in [Ca2+]i was accompanied by oscillation and was inhibited by extracellular EGTA. Our present experiment is the first report that clearly and directly reveals the differences between the effects of thrombin and ADP on [Ca2+]i of platelets.

Stimulation of dense tubular Ca2+ uptake in human platelets by cAMP

Elevation of intracellular cAMP is shown to increase the rate (V) and maximal extent of Ca2+ uptake by the dense tubules in intact human platelets. Elevation of [cAMP] was accomplished by preincubation with the adenylate cyclase activator forskolin or with dibutyryl-cAMP (Bt2-cAMP). The free concentration of Ca2+ in the dense tubular lumen ([Ca2+]dt) was monitored using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. The free cytoplasmic Ca2+ concentration ([Ca2+]cyt) was monitored in parallel experiments with quin2. Both [Ca2+]cyt and [Ca2+]dt were analyzed in terms of competition between pump and leak mechanisms in the plasma membrane (PM) and dense tubular membrane (DT). When platelets are incubated in media with approx. 1 microM external Ca2+, [Ca2+]cyt is approx. 50 nM and [Ca2+]dt is very low. When 2 mM external Ca2+ is added, [Ca2+]cyt rises to approx. 100 nM and the process of dense tubular Ca2+ uptake can be resolved. Forskolin (10 microM) and Bt2-cAMP increase the rate of dense tubular Ca2+ uptake (V) to 2.1 +/- 0.60 and 1.70 +/- 40 times control values (respectively). The agents also increase the final [Ca2+]dt to 1.70 +/- 0.21 and 1.72 +/- 0.60 times control values (respectively). Titrations with ionomycin (Iono) showed that the increase was due to an increase in the Vm of the dense tubular Ca2+ pump. With [Iono] = 500 nM, [Ca2+]cyt was raised to greater than or equal to 1.0 microM and Vm of the dense tubular pump was elicited. (At [Iono] = 1.0 microM, the final [Ca2+]dt values were degraded 15% due to shunting of Ca2+ uptake.) Analysis showed that forskolin (10 microM) and Bt2-cAMP (1 mM) increase the Vm by a factors of 1.56 +/- 40 and 1.56 +/- 40, respectively. Analysis showed that neither agent changed the Km of the pump significantly from its control value of 180 nM. Neither agent changed the rate constant for passive leakage of Ca2+ across the DT membrane (1.7 min-1).

Calcium mobilization in platelets from schizophrenic and healthy subjects. Regulation by lithium and neuroleptics

Intracellular free calcium concentrations ([Ca(2+)](1)) were measured in platelets from healthy volunteers before and after adding thrombin, chlorpromazine, haloperidol and/or lithium, and in platelets from DSM-III-R diagnosed schizophrenic patients receiving neuroleptic medication. Thrombin increased [Ca(2+)]( 1) in a dose- dependent fashion. Chlorpromazine and haloperidol also mobilized Ca(2+) in a dose-dependent fashion, and augmented the response to low doses of thrombin without changing the maximal response to thrombin. The effects of all three drugs were not additive, suggesting that they affected the same intraplatelet calcium pool; most likely the dense tubular system. Lithium also increased [Ca(2+) ] but without affecting the response to thrombin, chlorpromazine or haloperidol. The effects of the latter three drugs were additive to that of lithium, suggesting that lithium was acting on a different calcium pool. The response to thrombin was significantly lower in platelets from schizophrenic patients than in platelets from healthy volunteers. Further studies are required to explore potential causes for this observation. Such causes include schizophrenia per se and chronic neuroleptic treatment.

Halysin, an antiplatelet Arg-Gly-Asp-containing snake venom peptide, as fibrinogen receptor antagonist

By means of Sephadex G-75 and CM-Sephadex C-50 column chromatography and reverse-phase HPLC, a low molecular weight (Mr = 7500), cysteine-rich peptide, halysin, was purified from Agkistrodon halys (mamushi) snake venom. Halysin is a potent platelet aggregation inhibitor that concentration-dependently inhibited human platelet aggregation stimulated by ADP, thrombin and collagen (IC50 = 0.16 to 0.36 microM) without affecting platelet secretion. It was active in inhibiting platelet aggregation of platelet-rich plasma and whole blood. Halysin had no effect on thromboxane B2 formation of platelets or intracellular Ca2+ mobilization of Quin 2-AM loaded platelets stimulated by thrombin. It inhibited the fibrinogen-induced aggregation of elastase-treated platelets. Halysin concentration-dependently inhibited the 125I-fibrinogen binding to ADP-stimulated platelets in a competitive manner (IC50 = 0.16 microM). 125I-Halysin bound to resting platelets (Kd = 1.6 x 10(-7) M) and to ADP-stimulated platelets (Kd = 3.4 x 10(-8) M) in a saturable manner. EDTA, the Arg-Gly-Asp (RGD)-containing snake venom peptides trigamin and rhodostomin, Arg-Gly-Asp-Ser (RGDS), and Gly-Gln-Gln-His-His-Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val blocked both 125I-fibrinogen binding and 125I-halysin binding to ADP-stimulated platelets. The monoclonal antibody, 7E3, raised against glycoprotein IIb-IIIa complex blocked both 125I-fibrinogen and 125I-halysin binding, whereas 10E5 had no significant effect on halysin binding to ADP-stimulated platelets, indicating that 7E3 and halysin bind to an epitope which is different from that of 10E5. RGDS concentration-dependently inhibited 125I-halysin binding in a competitive manner. We determined the primary structure of halysin which is a single peptide chain of 71 amino acid residues. An RGD sequence appeared in the carboxy-terminal domain of halysin. Halysin showed about an 85% identical sequence with trigamin which is a specific antagonist of fibrinogen receptor associated with glycoprotein IIb-IIIa complex. In conclusion, halysin inhibited platelet aggregation by interfering with fibrinogen binding to the fibrinogen receptor of the activated platelets. The RGD sequence of halysin plays an important role in the expression of its biological activity.

Parathyroid hormone, platelet calcium, and blood pressure in normotensive subjects

Relations between platelet cytosolic calcium, parathyroid hormone, and blood pressure were investigated in 91 normotensive subjects: 47 men and 44 women ranging in age from 24 to 70 years. The men had higher mean arterial blood pressure, serum creatinine, and body mass index than the women. Serum total calcium, plasma ionized calcium, and parathyroid hormone (measured as both intact hormone and mid-molecule fragment) were not different between men and women; however, serum phosphate was higher in women than in men. Basal platelet cytosolic calcium was higher in men than in women (113.7 +/- 1.9 versus 105.9 +/- 1.7, respectively; p less than 0.01), but there was no difference in the peak platelet cytosolic calcium responses to thrombin between the two groups. In the combined group of male and female subjects, platelet cytosolic calcium correlated with diastolic blood pressure and mean arterial pressure (r = 0.37, p less than 0.001 and r = 0.32, p less than 0.01, respectively). Intact parathyroid hormone correlated with systolic and mean arterial blood pressure (r = 0.41, p less than 0.001 for both). Age correlated with both systolic blood pressure (r = 0.40, p less than 0.001) and intact parathyroid hormone (r = 0.51, p less than 0.001). When multiple regression analysis was performed using mean arterial pressure as the dependent variable, platelet cytosolic calcium and intact parathyroid hormone maintained significant correlations with mean arterial pressure. Platelet cytosolic calcium did not correlate with intact parathyroid hormone. These results suggest that both platelet cytosolic calcium and intact parathyroid hormone are associated with blood pressure regulation in normotensive subjects. However, the influences of these two factors on blood pressure are not interrelated.

Effect of tumour-promoting phorbol ester on calcium homeostasis in human platelets

The more interesting features of the effects or PMA on [Ca2+]i and ATP release were the following: 1. preincubation with PMA inhibited thrombin-evoked calcium transients; 2. PMA stimulated slightly the release of calcium and ATP whereas inhibited calcium and ATP pools sensitive to thrombin; 3. A23187 reversed the inhibitory effect of PMA; 4. subsaturating thrombin concentrations gave results similar to PMA on thrombin-induced calcium and ATP release but not on [Ca2+]i.

Calcium sequestration in human platelets: is it stimulated by protein kinase C?

Sequestration of calcium into an intracellular storage site is an important mechanism in helping to maintain a low cytoplasmic Ca2+ level in many cells. In platelets, increasing cytoplasmic cAMP lowers the free calcium level in correlation with the phosphorylation of a 22 kD protein. This protein has been thought to enhance uptake of calcium into a platelet membrane bound storage site by activating a calcium-ATPase activity by analogy with phospholamban in cardiac muscle. The evidence for an analogue of phospholamban in platelets is unclear. A pathway involving cAMP dependent kinase also seems unlikely to account for the transience of the calcium signal following agonists in platelets, some of which inhibit the cAMP dependent kinase. Here we discuss the issue of whether activation of protein kinase C, which follows agonist action, leads to enhanced calcium sequestration in platelets and if so, what indications there are for a mechanism. The evidence from our experiments with phorbol myristate acetate treated platelets shows that such an enhancement can be produced by activating protein kinase C. Phosphorylation studies suggest the involvement of a polypeptide or polypeptides distinct from the 22 kD polypeptide. Further work to test this idea is necessary. A brief overview of research on the role of phosphoproteins in calcium regulation in platelets and comparison with their role in cardiac muscle is also presented.

Change in cytoplasmic free Ca2+ concentration in rabbit platelets contacting with albumin coated and uncoated polystyrene surfaces

Direct measurement of cytoplasmic free Ca2+ concentration in rabbit platelets in contact with polystyrene latexes was examined by monitoring an intracellular trapped fluorescent indicator dye, Fura 2, as a quantitative evaluation of platelet activation on material surfaces. It was found that an increase in cytoplasmic free calcium levels in platelets in contacting latexes was strongly reduced by the adsorption of albumin on the latex surfaces. Further, such an elevation of cytoplasmic free calcium levels was found to require a calcium influx from an extracellular medium across the platelet-plasma membrane.

Reversible electropermeabilisation of human and rat blood platelets: evaluation of morphological and functional integrity 'in vitro' and 'in vivo'

A high-voltage discharge procedure has been developed for permeabilising the plasma membranes of both human and rat blood platelets. The cells can be resealed by incubation at 37 degrees C, show less than 4% loss of lactate dehydrogenase (LDH) implying minimal cell lysis and also have well maintained morphological and functional integrity. The prototype apparatus used at field strengths between 6 and 8 kV/cm produces membrane pores which allow free diffusion of low molecular weight substances such as adenine nucleotides, inositol phosphate and fluorescent dyes. Two properties, namely Ca2+-induced secretion of granule stored 5-hydroxytryptamine (5HT) and inositol 1,4,5-trisphosphate (IP3)-induced release of intracellularly sequestered 45Ca, which are both well expressed immediately after permeabilisation, are essentially abolished after resealing. The efficiency of permeabilisation and resealing can be simply monitored by shifts in 'apparent platelet volume' using a resistive particle counter (Coulter). Permeabilised platelets show a shift in modal volumes from a control range 4-7 fl to 10-15 fl. Resealing restores these modal volumes to the original control range. Encapsulation of the fluorochrome, Lucifer yellow (Mr 550), during permeabilisation revealed that after resealing greater than 85% of rat platelets, and close to 100% human platelets, contained the encapsulated dye. The initial rates and % aggregation responses of both human and rat platelets to collagen, thrombin and the thromboxane A2-mimetic U46619 remained essentially normal after permeabilisation and resealing further illustrating the maintenance of functional competence following treatment. Resealed rat platelets reinfused into the circulation after labelling with [111In]indium oxine gave survival curves similar to those of control platelets. Therefore, this reversible permeabilisation procedure may allow the use of autologous or heterologous platelets as carrier vehicles for the delivery of drugs and other agents 'in vivo'.

Stimulation of polymorphonuclear leukocytes by dicyclohexylcarbodiimide: calcium homeostasis

The involvement of calcium in N,N'-dicyclohexylcarbodiimide (DCCD)-mediated stimulation of guinea pig neutrophils was investigated. Exposure to DCCD resulted in a fast though moderate elevation of cytosolic calcium concentration. Exchange experiments indicated that DCCD enhanced 45Ca2+ efflux without affecting uptake of the radioisotope from the medium. Plasma membranes isolated from DCCD-stimulated cells failed to support ATP-dependent 45Ca2+ uptake indicating inhibition of their Ca-ATPase. The finding that the enhanced efflux of 45Ca2+ depended on the presence of Na+ ions in the medium implicated a Na+/Ca2+ exchanger in efflux of the ion observed in DCCD-stimulated neutrophils. This is the first indication for the participation of this carrier in calcium homeostasis in stimulated neutrophils. Experiments carried out with 14C-DCCD indicated covalent binding of the reagent to 20 and 150 Kd membrane proteins.

Platelet free calcium concentrations measured with fura-2 are influenced by the transmembrane sodium gradient

The influence of the transmembrane Na+ gradient on the intracellular free calcium concentration, [Ca2+]i, was studied in Sepharose gel-filtered platelets from healthy human subjects, using the Ca-sensitive fluorescent dye, fura-2. Raising the internal Na+ concentration, [Na+]i, by Na+ pump inhibition with 0.05 mM ouabain, without changing external Na+ did not cause a significant increase in [Ca2+]i. Substitution of extracellular Na+ by iso-osmolar sucrose induced a rapid (half-time about 2 min) and significant rise in [Ca2+]i; this effect was amplified in Na-loaded platelets. Partial restitution of external Na+ in these cells with increased [Ca2+]i promoted a significant and rapid Na+ concentration-dependent fall in [Ca2+]i; little decline in [Ca2+]i was observed if K+ was used instead of Na+. These observations represent in vitro evidence for the existence of a Na/Ca exchange mechanism in human platelets that may, in vivo, participate in the control of [Ca2+]i.

Abnormal Ca2+ homeostasis in platelets of patients with myeloproliferative disorders: low levels of Ca2+ influx and efflux across the plasma membrane and increased Ca2+ accumulation into the dense tubular system

Ca2+ influx and efflux in unstimulated platelets and Ca2+ uptake by simultaneously isolated two membrane fractions of platelets from patients with myeloproliferative disorders (MPD) have been investigated. In MPD, Ca2+ influx and efflux across the plasma membrane in unstimulated platelets were in equilibrium at significantly lower levels than in normals. Ca2+ uptake by external membrane fraction isolated from MPD platelets was lower, whereas, uptake by internal membrane fraction enriched with dense tubular system (DTS) from MPD platelets was significantly higher than that from normal platelets. This corresponded with the membrane associated Ca2+-activated ATPase activity. These abnormalities of calcium ion movement in plasma membrane and dense tubular system illustrates one of the mechanisms of qualitative abnormalities of MPD platelets.

Calcium uptake and release characteristics of the dense tubules of digitonin-permeabilized human platelets

The kinetics of ATP-driven Ca2+ uptake by the dense tubules were studied in digitonin-permeabilized human blood platelets. Digitonin at 3 micrograms/ml was shown capable of permeabilizing the plasma membrane to lactate dehydrogenase and the cytoplasmic Ca2+ indicator Quin2 without increasing the passive permeability of the dense tubular membrane for Ca2+. Experimentation was carried out with platelets treated with 3 micrograms/ml digitonin reisolated and resuspended in detergent-free medium ('digitonin-permeabilized' platelets). Active Ca2+ accumulation, which occurs over a period of minutes, was monitored by the increase in the fluorescence of chlorotetracycline after the addition of Mg-ATP (37 degrees C). The active uptake is inhibited by 15 microM trifluoperazine. The process is saturable with respect to external [Ca2+], with a Km of 180 +/- 5 nM and a Hill coefficient (n) of 1.40 +/- 0.05. Analysis of the maximal uptake in steady state gave similar results (Km = 160 +/- 5 nM, n = 1.50 +/- 0.05). The rate of uptake at [Ca2+] approximately Km is increased when the digitonin-permeabilized platelets are preincubated with 100 nM phorbol 12-myristate 13-acetate. Actively accumulated Ca2+ is rapidly released (less than 1 min) by addition of D-myo-inositol trisphosphate (IP3). The maximal extent of release is 50%; the EC50 for IP3 is approx. 12 microM. The data are compared with findings for fractionated dense tubular membrane vesicles and for the intact platelet.

Na+-Ca2+ exchange and calcium permeability in canine basolateral membrane vesicles: the effects of dibutyryl cAMP and specific inhibitors

The role of dibutyryl 3',5'-cyclic adenosine monophosphate (dibutyryl cAMP) as putative second messenger for parathyroid hormone (PTH) in regulating canine proximal tubular basolateral membrane Na+-Ca2+ exchange and passive calcium permeability was assessed, as was the nature of this passive calcium permeability. Dibutyryl cAMP (50 mg) infused in vivo over 30 min increased fractional phosphate excretion from 4.9 +/- 1.8% to 20.5 +/- 4.6%, P less than 0.05, n = 6, but had no effect on either passive Ca2+ efflux or sodium-stimulated Ca2+ efflux from Ca2+-preloaded basolateral membrane vesicles (BLMV). Both of these mechanisms have been previously shown to be stimulated by PTH. Further studies were performed to investigate the mechanism of the passive calcium flux. Calcium uptake by BLMV was blocked by lanthanum (La3+) but not by the calcium-channel blocker verapamil. La3+ blocked efflux of Ca2+ from preloaded vesicles when it was placed in the external solution. This La3+-blockable efflux was larger in potassium equivalent BLMV prepared from normal dogs than in BLMV prepared from thyroparathyroidectomized dogs. Benzamil produced 50% inhibition of sodium-stimulated Ca2+ uptake at 250 microM whereas neither amiloride nor diltiazem achieved 50% inhibition at the maximal doses studied. Benzamil, 1 mM, had no effect on passive calcium efflux and neither did the substitution of sucrose for potassium, which has been shown to affect Ca2+-Ca2+ exchange by the Na+-Ca2+ exchanger. This suggests that the calcium flux under potassium equivalent conditions was not mediated by Ca2+-Ca2+ exchange by the Na+-Ca2+ exchanger. These results demonstrate that the basolateral membrane of proximal tubular cells possesses both a Na+-Ca2+ exchanger inhibitable by benzamil and a passive calcium permeability not inhibited by benzamil nor by verapamil but by La3+. Neither of these two mechanisms of calcium flux was affected by dibutyryl cAMP whereas both have been shown to be stimulated by PTH.

Deliberate quin2 overload as a method for in situ characterization of active calcium extrusion systems and cytoplasmic calcium binding: application to the human platelet

The objectives of the title were accomplished by a four-step experimental procedure followed by a simple graphical and mathematical analysis. Platelets are (i) overloaded with the indicator quin2 to cytoplasmic concentrations of 2.9 mM and (ii) are exposed to 2 mM external Ca2+ and 1.0 microM ionomycin to rapidly achieve cytoplasmic Ca2+ ([Ca2+]cyt) of ca. 1.5 microM. (iii) The external Ca2+ is removed by EGTA addition, and (iv) the active Ca2+ extrusion process is then monitored as a function of time. Control experiments show that the ionophore shunts dense tubular uptake and does not contribute to the Ca2+ efflux process during phases iii-iv and that the extrusion process is sensitive to metabolic inhibitors. The progress curves for the decline of quin2 fluorescence (resulting from active Ca2+ extrusion) were analyzed as a function of [Ca2+]cyt using a mathematical model involving the probability that an exported Ca2+ was removed from a quin2 complex (vs. a cytoplasmic binding element). The observed rates of decline of quin2 fluorescence at a particular [Ca2+]cyt are dependent upon (i) the absolute rate of the extrusion system (a function of its Km, Vm and Hill coefficient (n)), (ii) the intrinsic Ca2+ buffer capacity of the cytoplasm (a function of the total site concentration ([B]T) and its Kd) and (iii) the buffer capacity of the intracytoplasmic quin2 (a function of its concentration and Kd). The contribution of (iii) was known and varied and was used to determine (ii) and (i) as a function of [Ca2+]cyt. The Ca2+ binding data were verified by 45Ca2+ experimentation.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in both calcium pool size and morphology of human platelets incubated in various concentrations of calcium ion. Calcium-specific bleb formation on platelet-membrane surface

In this study, the response of gel-filtered human platelets to extracellular Ca2+ at Ca2+ concentrations [Ca2+]o of 1-10 mM was investigated. The distribution of Ca2+ among various pools was studied using: (1) quin2, to estimate the cytosolic free Ca2+ concentration ([Ca2+]i); and (2) 45CaCl2 plus EGTA, to quantitate the sizes of the EGTA-releasable, EGTA-nonreleasable and surface-bound Ca2+ pools. The morphological changes were revealed by scanning electron-microscopy (scanning EM), and the effect on thrombin-stimulated aggregation was examined using an aggregometer. Platelets continuously sequestered Ca2+ into both EGTA-releasable and EGTA-nonreleasable pools to maintain a low [Ca2+]i level. The rate of sequestration to the EGTA-releasable pool was independent of [Ca2+]o, while that of the EGTA-nonreleasable pool exhibited first-order kinetics. The cell morphology changed gradually from discoid to the tadpole-like type, and finally to irregular forms. This morphological change correlated with the gradual increase in [Ca2+]i. The EGTA-nonreleasable pool saturated at about 3000 pmol/10(8) cells. This saturation resulted in a drastic increase in the EGTA-releasable pool size, and the cell was lysed concomitantly. The maximum safety capacity of the EGTA-releasable pool was estimated to be 1100 pmol/10(8) cells. The contribution of the cellular compartments to these two pool sizes is extensively discussed. The surface-bound pool size also increased continuously. When two different capacities were reached, i.e., 160 and 600 pmol/10(8) cells, the binding rate increased above the initial rate by 7- and 11-fold, respectively. Hence, the surface-binding capacity might be a critical factor which alters the membrane structure and exposes more binding sites. The cell surface appeared to have blebs, after the binding size had reached more than 600 pmol/10(8) cells. Bleb formation resulted in the inhibition of platelet function. Divalent cations, such as Mg2+, Sr2+ and Ba2+ did not cause bleb formation, which could mean that this formation is a Ca2+-specific phenomenon.

Dihydropyridine agonist Bay K 8644 inhibits platelet activation by competitive antagonism of thromboxane A2-prostaglandin H2 receptor

The dihydropyridine calcium channel antagonists, such as nifedipine, inhibit platelet aggregation in vitro and ex vivo, but the mechanism by which this occurs is uncertain. Bay K 8644 (BAY) is a substituted dihydropyridine that has effects on voltage-dependent calcium channels in cardiac and smooth muscle that are opposite the effects of nifedipine. To evaluate the mechanism responsible for dihydropyridine-induced inhibition of platelet function, we studied the in vitro effects of BAY on human platelet aggregation and secretion plus several related biochemical parameters, including cytoplasmic ionized calcium ([Ca2+]i). BAY exerted concentration-dependent effects on platelet aggregation and secretion of [14C]serotonin. BAY (1-10 microns) inhibited the second wave of platelet aggregation and secretion stimulated by adenosine diphosphate or epinephrine and blocked shape change, aggregation, and secretion induced by the thromboxane A2 (TXA2) mimic, U46619. BAY also inhibited U46619-induced phosphorylation of the approximately 40,000-dalton cytoplasmic protein substrate of protein kinase C (40K protein), formation of TXA2, and rise in [Ca2+]i, all biochemical consequences of platelet activation. The (+)-(R) enantiomer of BAY [BAY(+)] was predominantly responsible for the inhibitory effects of racemic BAY. Nifedipine had the same inhibitory effects on platelet function and biochemistry, except it was approximately 10 times less potent than BAY. Since these results suggested inhibition of the TXA2-prostaglandin H2 (PGH2) receptor, we measured binding of [3H]U46619 to intact platelets. BAY, BAY(+), and nifedipine all functioned as competitive antagonists of [3H]U46619 binding (BAY Ki = 1.47 microM). They did not inhibit binding of [3H]yohimbine to platelet alpha 2-adrenergic receptors. At 1-10 nM BAY, BAY(+) and the (-)-(S) enantiomer of BAY [BAY(-)] all resulted in slight stimulation of platelet function and biochemical events. No significant increase in [3H]U46619 binding was demonstrable, however. Therefore, dihydropyridines that function as either calcium channel agonists or antagonists in cardiac or smooth muscle exert concentration-dependent effects on platelet function. In nanomolar concentrations, they augment, and in micromolar concentrations, they inhibit platelet activation induced by TXA2 or U46619. These data indicate that dihydropyridines do not inhibit TXA2-induced platelet activation by an effect on voltage-dependent calcium channels; they define the mechanism of inhibition as competitive antagonism of the TXA2-PGH2 receptor. The mechanism responsible for augmentation of platelet activation is uncertain.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on calcium exchange in platelets in human diabetes

Calcium has a key role in platelet aggregation. In order to assess the role of calcium metabolism in the platelet in diabetes mellitus, calcium exchange into and out of the cytosol was investigated in platelets from eight patients with insulin-treated diabetes with a mean duration of 10 years without any clinical complications. Their mean HbAIc (glycosylated haemoglobin) was 9% (normal range 3.4-5.4%). Influx rate of 45Ca2+ was significantly increased by 35% in diabetic platelets compared with controls. The efflux rate was significantly reduced during the first 15 min but total efflux measured over 2 h was equal in diabetic and control platelets. Access to sequestered non-mitochondrial calcium in the dense tubular system was gained by permeabilization of the cells with saponin. Into cells loaded with 45Ca2+, Inositol 1, 4, 5 triphosphate (IP3) was introduced and release of the sequestered ion would be determined. In control platelets calcium release was prompt and amounted to 43%. In diabetic platelets the response to IP3 was blunted and was only 17%. The nutritional and hormonal status influenced the response to IP3 in diabetes and, to a lesser extent, in controls. In the fasting state and without insulin the release was reduced but in the fed state and after insulin the release of calcium from diabetic platelets was equal to those of controls. It is concluded that abnormalities exist in platelet calcium handling in diabetes that can be influenced by insulin and the nutritional status.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by the antilipogenic antibiotic cerulenin of thrombin-induced activation of human platelets

Upon incubation with an antibiotic cerulenin, human platelets lost their abilities to aggregate and to release serotonin or ATP in response to various stimuli such as thrombin, ADP, collagen and platelet activating factor. The loss of activities was dependent on both incubation time and cerulenin concentrations. As judged by 14C-serotonin release, concentrations of cerulenin required for the half-maximal inhibition of thrombin-induced activation were 5-10 and 70 micrograms/ml in the incubation for 120 min at 37 degrees C for washed platelets and those in platelet rich plasma, respectively. The cerulenin treatment also resulted in a significant inhibition of 14C-acetate incorporation into the lipid fraction of platelets, suggesting that de novo synthesis of fatty acids was inhibited by the treatment. No release of lactate dehydrogenase activity nor morphological changes in platelet structure was detected upon cerulenin treatment. When effects of cerulenin on intracellular Ca2+ concentration were examined, mobilization of intracellular Ca2+ by thrombin was significantly depressed in the cerulenin-treated platelets as judged by Fura2, Quin2 or chlortetracycline fluorescence. Since the influx of external Ca2+ is not essential to the thrombin-induced platelet activation (Rink, T. J. et al. FEBS Lett. 148 21-26, 1982), the results suggest that cerulenin-treatments affect the platelet function through the inhibition of intracellular Ca2+ mobilization.

Blood elements at surfaces: platelets

This brief review is designed to highlight the limitations of our knowledge given the difficulties in simulating the in vivo condition in models established in vitro and ex vivo. Despite this uncertainty, mounting evidence indicates that the GPIIb-IIIa receptor mediates crucial platelet functions and that control over its interactions may offer the potential for altering platelet behavior in vivo. Since this receptor appears to be part of a generalized family of adhesive protein receptors that utilize the tripeptide RGD as a recognition signal, insights derived from platelet function may have broad biological implications.