Effects of Cyclic Nucleotides and Protein Kinases on Platelet Calcium Homeostasis and Mobilization

A systematic review of randomized controlled trials related to the effects of garlic supplementation on platelet aggregation

The increment of platelet aggregation factors has been considered a key phenomenon in atherosclerosis. Studies have shown that garlic (Allium sativum) is associated with a reduction in platelet aggregation and thrombosis. Hence, the present systematic review was conducted to evaluate the effect of garlic on platelet aggregation. All randomized controlled trials (RCTs) with keywords related to garlic and platelet aggregation were thoroughly searched in electronic databases including PubMed, Scopus, ISI Web of Science, and Google Scholar up to January 2021. Moreover, the references of all related articles were screened to discover more relevant studies. The quality of each study was reported based on Cochrane Collaboration's tool. In total, 12 studies met the inclusion criteria from 18,235 identified articles (including 595 participants). Most of the studies assessed platelet aggregation in response to different inducers. Of the 12 clinical trials, six studies depicted the beneficial effect of garlic on reducing platelet aggregation. The summary of the quality assessment indicated that most of the studies had high-quality scores. Regarding the small number of RCTs and heterogeneity between studies, it is impossible to make a proper conclusion about the impacts of garlic on platelet aggregation. Therefore, further precise trials with a standard design are necessary to validate the anti-thrombotic effect of garlic.

Effects of acute and chronic psychological stress on platelet aggregation in mice

Although psychological stress has long been known to alter cardiovascular function, there have been few studies on the effect of psychological stress on platelets, which play a pivotal role in cardiovascular disease. In the present study, we investigated the effects of acute and chronic psychological stress on the aggregation of platelets and platelet cytosolic free calcium concentration ([Ca(2+)]i). Mice were subjected to both transportation stress (exposure to novel environment, psychological stress) and restraint stress (psychological stress) for 2 h (acute stress) or 3 weeks (2 h/day) (chronic stress). In addition, adrenalectomized mice were subjected to similar chronic stress (both transportation and restraint stress for 3 weeks). The aggregation of platelets from mice and [Ca(2+)]i was determined by light transmission assay and fura-2 fluorescence assay, respectively. Although acute stress had no effect on agonist-induced platelet aggregation, chronic stress enhanced the ability of the platelet agonists thrombin and ADP to stimulate platelet aggregation. However, chronic stress failed to enhance agonist-induced increase in [Ca(2+)]i. Adrenalectomy blocked chronic stress-induced enhancement of platelet aggregation. These results suggest that chronic, but not acute, psychological stress enhances agonist-stimulated platelet aggregation independently of [Ca(2+)]i increase, and the enhancement may be mediated by stress hormones secreted from the adrenal glands.

Concentration dependent effect of (-)-Epicatechin on Na(+) /K(+) -ATPase and Ca(2+) -ATPase inhibition induced by free radicals in hypertensive patients: comparison with L-ascorbic acid

Although the antioxidant properties of flavonoids are well documented, it is still unclear whether these effects are dependent on radical scavenging or iron chelating activities. Oxidative stress, a state of excessive reactive oxygen species (ROS) activity, is associated with vascular disease conditions such as hypertension. Both the anti- and pro-oxidant effects of tea catechins have been implicated in the alterations of cellular functions that determine their chemoprotective and therapeutic potentials in health and diseases. The present study examined the concentration dependent (10(-7) to 10(-4)  m) effects of (-)-epicatechin and L-ascorbic acid on Na(+) /K(+) -ATPase and Ca(2+) -ATPase activity in hypertensive patients and normal subjects. L-ascorbic acid has been used as a positive control to compare the effect of (-)-epicatechin. A significant (p < 0.0001) decrease in the activities of Na(+) /K(+) -ATPase and Ca(2+) -ATPase was observed in hypertensive patients compared with normal subjects. We report that (-)-epicatechin shows a significant (p < 0.001) dose-dependent protective effect against oxidative stress induced by tert-butyl hydroperoxide (t-BHP), which is manisfested as a decrease in the activity of erythrocyte Na(+) /K(+) -ATPase and Ca(2+) -ATPase, in hypertensive patients as well as normal subjects. The effect of L-ascorbic acid was also significant (p < 0.001) and was comparable with that of (-)-epicatechin.

The potential role of milk-derived peptides in cardiovascular disease

Bioactive peptides derived from milk proteins are of particular interest to the food industry due to the potential functional and physiological roles that they demonstrate, particularly in relation to cardiovascular disease (CVD). By 2020 it is estimated that heart disease and stroke will become the leading cause of death and disability worldwide. Acute and chronic cardiovascular events may result from alterations in the activity of the renin-angiotensin aldosterone system and activation of the coagulation cascade and of platelets. Medications that inhibit angiotensin converting enzyme (ACE) are widely prescribed in the treatment and prevention of cardiovascular disease. ACE inhibitory peptides are of particular interest due to the presence of encrypted inhibitory peptide sequences. In particular, Ile-Pro-Pro and Val-Pro-Pro are fore runners in ACE inhibition, and have been incorporated into commercial products. Additionally, studies to identify additional novel peptides with similar bio-activity and the ability to withstand digestion during transit through the gastrointestinal tract are ongoing. The potential sources of such peptides in cheese and other dairy products are discussed. Challenges to the bio-availability of such peptides in the gastro intestinal tract are also reviewed. Activation of platelets and the coagulation cascade play a central role in the progression of cardiovascular disease. Platelets from such patients show spontaneous aggregation and an increased sensitivity to agonists which results in vascular damage and endothelial dysfunction associated with CVD. Peptide sequences exhibiting anti-thrombotic activity have been identified from fermented milk products. Studies on such peptides are reviewed and their effects on platelet function are discussed. Finally the ability of food derived peptides to decrease the formation of blood clots (thrombi) is reviewed. In conclusion, due to the widespread nature of cardiovascular disease, the identification of food derived compounds that exhibit a beneficial effect in such widespread areas of CVD regulation will have strong clinical potential. Due to the perception that food derived products have an acceptable risk profile they have the potential for widespread acceptance by the public. In this review, selected biological effects relating to CVD are discussed with a view to providing essential information to researchers.

Analysis of functionalized polyethylene terephthalate with immobilized NTPDase and cysteine

Polyethylene terephthalate (PET) was functionalized to introduce carboxyl groups onto its surface by a carboxylation technique. Surface and bulk properties, such as possible surface deterioration, surface roughness and the mechanical strength of the carboxylated polymers, were studied and compared with those of aminolyzed and hydrolyzed PET. Atomic force microscopy studies showed that unlike aminolysis and hydrolysis, which increased the surface roughness significantly due to cracking and pitting, the surface roughness of unmodified and carboxylated PET were comparable. While hydrolysis and aminolysis of PET resulted in significant loss of strength, tensile testing revealed that unmodified and carboxylated polymers had similar strength. The development of mechanically stable, functionalized PET would vastly improve the biomedical applications of this polymer. To understand the potential for improving biomedical applications, biologically active molecules, namely nucleoside triphosphate diphosphohydrolase (NTPDase) and cysteine, were immobilized on the carboxylated PET using amide bonds. NTPDase was also immobilized to aminolyzed PET using imine bonds, while cysteine was immobilized on aminolyzed PET using both imine and amide bonds. Attachment of NTPDase and cysteine was verified by analyzing the NTPDase activity and the cysteine surface concentration. The stability of these immobilizations was also studied.

Regulation of plasma membrane Ca2+-ATPase in human platelets by calpain

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low cytosolic Ca(2+) in resting human platelets by extruding Ca(2+) from the cytoplasm across the plasma membrane. Since PMCA is the main agent of Ca(2+) efflux in platelets, it is a key point for regulation of platelet Ca(2+) metabolism. PMCA has been shown to be an excellent substrate for the Ca(2+)-activated cysteine protease calpain, a major platelet protein that is turned on during platelet activation. The objectives of the present work were to determine if PMCA is degraded during thrombin- and collagen-mediated platelet activation, and if calpain is responsible. The kinetics of PMCA degradation during platelet activation were analysed using SDS polyacrylamide gel electrophoresis and immunoblotting. The role of calpain was tested using the calpain inhibitors calpeptin and ALLN. Platelet activation mediated by both collagen and thrombin resulted in degradation of 60% of platelet PMCA within 18 minutes. Calpeptin and ALLN significantly inhibited the rate and extent of PMCA degradation. We conclude that calpain-mediated degradation of PMCA during platelet activation likely contributes significantly to Ca(2+) regulation and, therefore, to platelet function.

Haematological effects of anaesthetics and anaesthesia

Halothane is still unique in its ability to inhibit platelet aggregation and to increase bleeding time in vivo at clinical concentrations, although sevoflurane inhibits platelet aggregation induced by weak agonists. Propofol itself, but not its fat emulsion, inhibits platelet aggregation and suppresses calcium mobilization. Extradural anaesthesia has been shown to prevent hypercoagulability during the perioperative period. Aprotinin reduces both blood loss and the incidence of blood transfusion during major orthopaedic and cardiac surgery.

Effects of plasma membrane Ca(2+) -ATPase tyrosine phosphorylation on human platelet function

BACKGROUND

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low intracellular Ca(2+) ([Ca(2+)](i)) in resting platelets. Earlier studies demonstrated that platelet activation by thrombin results in tyrosine phosphorylation of PMCA, which inhibits pump activity.

OBJECTIVES

The objective was to determine the functional consequences of PMCA tyrosine phosphorylation.

METHODS

A decapeptide including the tyrosine phosphorylation site of PMCA and a scrambled version were synthesized and introduced into human platelets using saponin. Fura-2 calcium monitoring and aggregometry were used to characterize the effects of inhibition of tyrosine phosphorylation.

RESULTS

Western blot analysis of immunoprecipitates showed that introduction of the inhibitory peptide decreased tyrosine phosphorylation of PMCA by nearly 60% in saponin-permeabilized, thrombin-treated platelets as compared with the scrambled control peptide. Concomitant with inhibition of PMCA tyrosine phosphorylation was a significant decrease in [Ca(2+)](i) during thrombin-mediated platelet activation. The functional consequence of reduced PMCA tyrosine phosphorylation and decreased [Ca(2+)](i) was a significant delay in the onset of thrombin-mediated platelet aggregation.

CONCLUSIONS

The results demonstrate that PMCA tyrosine phosphorylation regulates [Ca(2+)](i) during platelet activation, which affects downstream events in the activation process. Moreover, PMCA tyrosine phosphorylation and resultant inhibition of PMCA activity produces a positive feedback loop mechanism by enhancing the increase in [Ca(2+)](i) accompanying platelet activation.

Improved hemocompatibility of poly(ethylene terephthalate) modified with various thiol-containing groups

Thiol groups were attached to polyethylene terephthalate (PET) to promote the transfer of a known platelet inhibitor, nitric oxide (NO), from nitrosated thiols naturally found in the body to PET, followed by the release of NO from PET to prevent platelet adhesion. In order to immobilize the most thiols on the modified polymer, the processing parameters used to attach the following three thiol containing groups were assessed: L-cysteine, 2-iminothiolane, and a cysteine polypeptide. When comparing the immobilized concentrations of thiol groups from each of the optimized processes the amount of immobilized thiol groups increased in order with the following groups: cysteine polypeptide <2-iminothiolane <L-cysteine. The effect of each optimized polymer on platelet adhesion was studied by in vitro experiments utilizing a parallel plate perfusion chamber. Platelets in the following solutions were tested: Tyrode's buffer, 7 microm nitrosated bovine serum albumin in Tyrode's buffer, 50% plasma in Tyrode's buffer, and 50% whole blood in Tyrode's buffer. All of the polymers demonstrated a significant decrease in platelet adhesion compared to controls when exposed to the BSANO, plasma and whole blood solutions. The most significant decrease was for the L-cysteine modified polymer in the plasma solution with a 65% decrease.

New horizons in the analysis of circulating cell-derived microparticles

Analysis of circulating cell-derived microparticles (MP) is becoming more refined and clinically useful. This review, stemming from lectures given at Tokyo late 2003, does not repeat prior reviews but focuses on new horizons. A major theme is the rising recognition of platelets and their MP (PMP) as key mediators of inflammation/immunity. Among the major concepts developed are that (i) many so-called soluble markers of inflammation are in reality MP-bound; (ii) PMP and other MP appear to serve important signaling and immune functions including antigen presentation. In conclusion, MP analysis is poised to enter the mainstream of clinical testing, measuring specific antigens rather than gross levels. However, more research is needed to decisively establish their functions, and international standards are needed to allow comparing results from different laboratories.

Inhibition of ADP-induced intracellular Ca2+ responses and platelet aggregation by the P2Y12 receptor antagonists AR-C69931MX and clopidogrel is enhanced by prostaglandin E1

P2Y(12) antagonists such as clopidogrel and AR-C69931MX inhibit aggregation by antagonizing the effects of ADP at P2Y(12) receptors on platelets. Agents such as PGE(1) also inhibit aggregation by stimulating adenylate cyclase to produce cAMP, which interferes with Ca(2+) mobilization within the cell. Since one facet of P2Y(12) receptors is that they mediate inhibition of adenylate cyclase by ADP, it might be expected that P2Y(12) antagonists would interact with PGE(1). We have explored the effects of PGE(1) and AR-C69931MX singly and in combination on ADP-induced intracellular Ca(2+) ([Ca(2+)](i)) responses and aggregation. PGE(1) alone caused parallel dose-dependent inhibition of [Ca(2+)](i) and aggregation responses. AR-C66931MX alone caused only partial inhibition of [Ca(2+)](i) despite a marked inhibitory effect on aggregation. Combinations of PGE(1) with AR-C66931MX were found to act in synergy to reduce both [Ca(2+)](i) and aggregation. This effect was confirmed in patients with acute coronary syndromes by studying the inhibitory effects of PGE(1) on [Ca(2+)](i) and aggregation before and after clopidogrel. In summary, we have shown that P2Y(12) antagonists interact with natural agents such as PGE(1) to provide more effective inhibition of [Ca(2+)](i) and platelet aggregation. This would contribute to the effectiveness of P2Y(12) antagonists as antithrombotic agents in man.

Protein kinase A mediates inhibition of the thrombin-induced platelet shape change by nitric oxide

The thrombin-induced platelet shape change was blocked by nitric oxide (NO), as revealed by scanning electron microscopy, light transmission, and resistive-particle volume determination. The inhibitory effect of NO was accompanied by an increase in levels of both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) and phosphorylation of the vasodilator-stimulated phosphoprotein (VASP). However, the inhibition of the shape change was only mimicked by cAMP analogs (Sp-5,6-DClcBIMPS, 8-AHA-cAMP, and 8-CPT-cAMP) and not by cGMP analogs (8-Br-PET-cGMP, 8-Br-cGMP, and 8-pCPT-cGMP). The effect of NO on the thrombin-induced shape change was prevented by the protein kinase A (PKA) antagonists Rp-8-Br-cAMPS and Rp-cAMPS. The protein kinase G (PKG) antagonist Rp-8-CPT-cGMPS strongly inhibited PKG-mediated 46-kDa VASP Ser239 phosphorylation, but did not inhibit the thrombin-induced shape change or the PKA-mediated VASP Ser157 phosphorylation. Whereas an inhibitor of cyclic nucleotide phosphodiesterase (PDE) 3A (milrinone) mimicked the effect of NO, inhibitors of PDE2 (erythro-9-(2-hydroxy-3-nonyl)adenine) and PDE5 (dipyridamole) were poorly effective. We concluded that (1) NO was a potent and reversible inhibitor of the platelet shape change, (2) the shape change was reversible, (3) the inhibitory effect of NO was mediated through activation of PKA, (4) the onset of the NO effect coincided with VASP Ser157 phosphorylation, and (5) removal of NO and platelet shape change coincided with VASP Ser157 dephosphorylation. These findings are compatible with elevation of cGMP by NO in a compartment close to PDE3A, PKA, and VASP, leading to a local increase of cAMP able to block thrombin-induced shape change.

Anti-thrombotic effect of milrinone is caused by inhibition of calcium release from the dense tubular system in human platelets

AIM

Milrinone, a phosphodiesterase III inhibitor, exerts positive inotropic effects which induce an increase in the intracellular calcium concentration by raising the cyclic adenosine monophosphate level in cardiac muscle. Milrinone was also reported to inhibit platelet aggregation, however, its mechanism remains unknown. Therefore, we investigated the effects of milrinone on intracellular calcium mobilization when platelets were activated.

METHODS

Washed platelets, obtained from six healthy volunteers, were preincubated with milrinone (0.9 micro M) for 1 min and then exposed to 0.015 i micro ml-1 thrombin for 5 min. The effect of milrinone on changes in the intracellular calcium level using a fluorescent dye, fura-2, was also observed. Calcium mobilizations via plasma membrane calcium channels and the dense tubular system were assessed differentially.

RESULTS

Milrinone (0.9 micro M) significantly suppressed the aggregation ratios at 5 min compared with those in controls (86+/-5%) to 75+/-8%. The increase in the intracellular calcium concentration was also significantly suppressed (controls, 915+/-293 nM vs. 405+/-240 nM) when stimulated by thrombin. Milrinone also significantly inhibited the release of calcium from the dense tubular system (controls, 284+/-111 nM vs. 158+/-51 nM). Calcium influx through the plasma membrane was suppressed by milrinone 2.4 micro M.

CONCLUSION

Milrinone (0.9 micro M) inhibited thrombin-induced platelet aggregation. This inhibitory effect was mainly mediated by suppressing calcium release from the dense tubular system.

The role of protein nitration in the inhibition of platelet activation by peroxynitrite

Peroxynitrite at low concentrations (3-10 microM) inhibited agonist-induced platelet aggregation by a mechanism not dependent on the formation of cyclic guanosine monophosphate. Platelets recovered completely from peroxynitrite-induced inhibition within 30 min. Peroxynitrite induced nitration of cytosolic proteins, but this diminished to near basal levels within 60 min of exposure to the oxidant. During this period there was a reduction in tyrosine phosphorylation of specific proteins such as syk, but this was not due to direct nitration of these same proteins. The inhibition of phosphorylation was reversible with platelet proteins recovering the ability to be phosphorylated within 15 min of exposure to peroxynitrite. Conversely, peroxynitrite increased phosphorylation of other proteins, but again these events were not directly linked to nitration. Nitration may affect the phosphorylation of tyrosine residues in a number of proteins, but by an indirect route, possibly by acting on proteins upstream in the signalling cascades. We suggest that low concentrations of peroxynitrite reversibly inhibit platelet aggregation by preventing the phosphorylation of key signalling proteins.

Plasma membrane Ca(2+)-ATPase associates with the cytoskeleton in activated platelets through a PDZ-binding domain

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low cytosolic Ca(2+) in resting platelets. During platelet activation PMCA is phosphorylated transiently on tyrosine residues resulting in inhibition of the pump that enhances elevation of Ca(2+). Tyrosine phosphorylation of many proteins during platelet activation results in their association with the cytoskeleton. Consequently, in the present study we asked if PMCA interacts with the platelet cytoskeleton. We observed that very little PMCA is associated with the cytoskeleton in resting platelets but that approximately 80% of total PMCA (PMCA1b + PMCA4b) is redistributed to the cytoskeleton upon activation with thrombin. Tyrosine phosphorylation of PMCA during activation was not associated with the redistribution because tyrosine-phosphorylated PMCA was not translocated specifically to the cytoskeleton. Because PMCA b-splice isoforms have C-terminal PSD-95/Dlg/ZO-1 homology domain (PDZ)-binding domains, a C-terminal peptide was used to disrupt potential PDZ domain interactions. Activation of saponin-permeabilized platelets in the presence of the peptide led to a significant decrease of PMCA in the cytoskeleton. PMCA associated with the cytoskeleton retained Ca(2+)-ATPase activity. These results suggest that during activation active PMCA is recruited to the cytoskeleton by interaction with PDZ domains and that this association provides a microenvironment with a reduced Ca(2+) concentration.

Inhibition by diallyl trisulfide, a garlic component, of intracellular Ca(2+) mobilization without affecting inositol-1,4, 5-trisphosphate (IP(3)) formation in activated platelets

Garlic has been used in herbal medicine for thousands of years. Some reports have shown that garlic has protective effects against atherosclerosis and inhibits platelet function. In this study, we investigated the mechanism by which diallyl trisulfide (DT), a component of garlic, inhibits platelet function. DT inhibited platelet aggregation and Ca(2+) mobilization in a concentration-dependent manner without increasing intracellular cyclic AMP and cyclic GMP. DT also had no inhibitory effects on thromboxane A(2) (TXA(2)) production in cell-free systems. Collagen-related peptide (CRP)-induced Ca(2+) mobilization is regulated by phospholipase C-gamma2 (PLC-gamma2) activation. We evaluated the effect of DT on tyrosine phosphorylation of PLC-gamma2 and the production of inositol-1,4,5-trisphosphate (IP(3)). DT at concentrations that inhibited platelet aggregation and Ca(2+) mobilization had no effects on tyrosine phosphorylation of PLC-gamma2 or on the formation of IP(3) induced by CRP. Similar results were obtained with thrombin-induced platelet activation. DT inhibited platelet aggregation and Ca(2+) mobilization induced by thrombin without affecting the production of IP(3.) We then evaluated the effect of DT on the binding of IP(3) to its receptor. DT at high concentrations partially blocked the binding of IP(3) to its receptor. Taken together, our findings suggest that the agent suppresses Ca(2+) mobilization at a step distal to IP(3) formation. DT may provide a good tool for investigating Ca(2+) mobilization.

Chloroquine: a multipotent inhibitor of human platelets in vitro

Chloroquine inhibited human platelet aggregation in vitro both at receptor- and nonreceptor-operated stimuli. The inhibition was dose-dependent, recorded on isolated platelets as well as in platelet-rich plasma, and followed the rank order of stimuli: adrenaline (second phase)>phorbol 12-myristate 13 acetate>adenosine diphosphate>adrenaline (first phase)>thrombin>calcium ionophore A23187. In thrombin-activated platelets, chloroquine decreased in a dose-dependent manner phospholipase A(2)-induced arachidonic acid liberation from membrane phospholipids, malondialdehyde formation (a marker of membrane phospholipid peroxidation), and thromboxane generation, considered the most potent autoaggregatory agent. Chloroquine only slightly altered the arachidonic acid cascade of platelets stimulated with A23187 and phorbol 12-myristate 13 acetate. Histamine formation and liberation induced with thrombin and A23187 were not affected by chloroquine. On the other hand, thrombin-stimulated serotonin secretion was significantly decreased with chloroquine in the concentration of 10 micromol/L. This indicated that chloroquine might interfere with stimulated secretion from platelets. The results suggest that chloroquine inhibited activated platelets: first, intracellularly; second, in a close relationship to the intraplatelet Ca(2+) mobile pool; and third, most probably at the site of platelet phospholipase A(2) activation.

Effects of hypochlorite-modified low-density and high-density lipoproteins on intracellular Ca2+ and plasma membrane Ca(2+)-ATPase activity of human platelets

The presence of hypochlorite-modified lipoproteins in atherosclerotic lesions suggests that HOCl, a naturally occurring oxidant formed by the myeloperoxidase-catalyzed reaction of H2O2 and Cl-, is a candidate for generation of modified lipoproteins in vivo. We have previously demonstrated that Cu(2+)-oxidized LDL inhibits platelet plasma membrane Ca(2+)-ATPase (PMCA) in isolated membranes and causes an increase in cytosolic Ca2+ in resting whole platelets. However, Cu(2+)-oxidized LDL may not be identical in structure and function to the physiologically modified lipoprotein. Since platelet function may be affected by native and modified lipoproteins, the effect of HOCl-modified LDL and HDL3 on platelet PMCA and on the free intracellular Ca2+ concentration ([Ca2+]i) of whole platelets has been investigated. We demonstrate that in contrast to Cu(2+)-oxidized LDL, HOCl-modified LDL and HDL3 stimulate platelet PMCA activity in isolated membranes and that this effect results in a decrease of [Ca2+]i in vivo. Thus, HOCl-oxidation produces modified lipoproteins with the potential for altering platelet function and with properties different from those of the Cu(2+)-oxidized counterparts.

Nitric oxide and platelet aggregation

Platelets are small cells, 1/14th the volume of erythrocytes, and about 1000 billion circulate in human blood as smooth anucleate disks. Their job is to survey the lining of our blood vessels, the endothelium. In acute damage and extravasation, platelets are activated by contact with exposed collagen and aggregate together at the wound sites to initiate clotting and stop bleeding. Forming a physical plug to seal a hemorrhaging vessel is the key role of blood platelets. However, milder injury to the endothelium, perhaps a result of high blood pressure, raised plasma cholesterol, or smoking, also causes platelets to adhere to the internal walls of arteries. Such precipitate adhesion and activation of platelets initiates an inflammatory response of the vessel wall and predisposes to vascular complications, including thrombosis, premature heart disease, myocardial infarcts or strokes, and diabetes. It is essential, therefore, that during normal vascular hemostasis platelet activation is tightly controlled. Indeed, both platelets and endothelial cells produce and secrete chemicals that directly inhibit platelet aggregation. A key agent is the free radical gas nitric oxide (NO). Here, we review how this 30-Da molecular messenger is synthesized by a catalytic cassette 10,000 times larger and how it functions to suppress platelet "stickiness." We also present new evidence that directly links plasma lipoproteins with platelet activation: we describe at the molecular level how apoE, a protein with a prominent role in cholesterol transport, interacts with the platelet surface to stimulate NO production and hence attenuate platelet activation.

Platelet cytosolic calcium hyperresponsivity to serotonin in patients with hypertension and depressive symptoms

BACKGROUND

Data from recent studies indicate that the presence of depression is an independent risk factor for cardiovascular and cerebrovascular events. The mechanism by which depression increases the morbidity and mortality risks in patients with comorbid vascular disease is currently the object of considerable research interest. Platelets may be involved in this pathological process. Although many investigators have extensively evaluated platelet biochemistry in depressed patients, there currently exists very little information regarding how the biochemical alterations might relate to an increased risk of cardiovascular events. In this study, we examined the responsivity of platelet cytosolic calcium concentrations ([Ca++]i) to serotonin stimulation in populations of hypertensive patients with or without comorbid depressive symptoms.

METHODS

We utilized Fura-2 loaded platelets to compare changes in intracellular calcium levels (delta [Ca++]i) following serotonin stimulation among 48 patients with hypertension and varying degrees of depressive symptomatology.

RESULTS

We found that those patients with higher scores on standardized depression rating scales showed significantly greater [Ca++]i (82.82 +/- 15.88 mmol/L) increase compared with [Ca++]i (60.10 +/- 22.65 mmol/L) patients with lower depression scores.

CONCLUSIONS

The results of this study support the hypothesis that the enhanced platelet reactivity seen in patients with depressive symptoms may mediate the deleterious effects of depression on cardiovascular disease.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Platelet cytosolic calcium responses to serotonin in depressed patients and controls: relationship to symptomatology and medication

BACKGROUND

Serotonin produces an exaggerated rise in platelet cytosolic calcium (delta [Ca++]i) in patients with mood disorders. Studies on patients with bipolar disorder consistently demonstrate calcium abnormalities. By comparison, data on patients with major depression are more variable.

METHODS

To determine causes of variability, we utilized Fura-2 loaded platelets to compare changes in platelet intracellular calcium levels (delta [Ca++]i) following serotonin stimulation in 24 patients with major depression and in 20 controls. We also sought relationships between the delta [Ca++]i responses and scores on clinical depression and anxiety scales.

RESULTS

We found positive correlations between delta [Ca++]i responses and the clinical scales across all subjects. Furthermore, depressed patients with high anxiety had significantly increased delta [Ca++]i responses compared to depressed patients with low anxiety. In addition, patients receiving selective-serotonin reuptake inhibitors (SSRIs) demonstrated reduced delta [Ca++]i responses compared to patients not on SSRIs.

CONCLUSIONS

Since elevations in [Ca++]i mediate platelet aggregation and secretion cascades, the enhanced responsivity observed in depressed, and in particular anxious, depressed patients may contribute to their increased risk for vascular disease.

Regulation of platelet plasma membrane Ca2+-ATPase by cAMP-dependent and tyrosine phosphorylation

As a consequence of its central role in the regulation of calcium metabolism in the platelet, the plasma membrane Ca2+-ATPase (PMCA) was assessed for cAMP-dependent and tyrosine phosphorylation. Addition of forskolin or prostaglandin E1, agents known to elevate platelet cAMP and calcium efflux, to platelets pre-labeled with [32P]PO4 resulted in the direct phosphorylation of platelet PMCA. Similarly, addition of the catalytic subunit of protein kinase A to platelet plasma membranes resulted in a 1.4-fold stimulation of activity. Thus, the previously reported inhibition of platelet activation by elevated intracellular cAMP may be accomplished in part by stimulation of PMCA, likely resulting in a decrease in intracellular calcium. Treatment with thrombin evoked tyrosine phosphorylation of platelet PMCA, while PMCA from resting platelets exhibited little tyrosine phosphorylation. Phosphorylation of platelet plasma membranes by pp60(src) resulted in 75% inhibition of PMCA activity within 15 min. Similarly, membranes isolated from thrombin-treated platelets exhibited 40% lower PMCA activity than those from resting platelets. Phosphorylation of erythrocyte ghosts and purified PMCA by pp60(src) also resulted in up to 75% inhibition of Ca2+-ATPase activity, and inhibition was correlated with tyrosine phosphorylation. Sequencing of a peptide obtained after 32P labeling of purified erythrocyte PMCA in vitro showed that tyrosine 1176 of PMCA4b is phosphorylated by pp60(src). These results indicate that tyrosine phosphorylation of platelet PMCA may serve as positive feedback to inhibit PMCA and increase intracellular calcium during platelet activation.

Apolipoprotein E inhibits platelet aggregation through the L-arginine:nitric oxide pathway. Implications for vascular disease

We have previously reported that plasma apolipoprotein (apo) E-containing high density lipoprotein particles have a potent anti-platelet action, apparently by occupying saturable binding sites in the cell surface. Here we show that purified apoE (10-50 microg/ml), complexed with phospholipid vesicles (dimyristoylphosphatidylcholine, DMPC), suppresses platelet aggregation induced by ADP, epinephrine, or collagen. This effect was not due to sequestration of cholesterol from platelet membranes; apoE x DMPC chemically modified with cyclohexanedione (cyclohexanedione-apoE x DMPC) did not inhibit aggregation but nevertheless removed similar amounts of cholesterol as untreated complexes, about 2% during the aggregation period. Rather we found that apoE influenced intracellular platelet signaling. Thus, apoE x DMPC markedly increased cGMP in ADP-stimulated platelets which correlated with the resulting inhibition of aggregation (r = 0.85; p < 0.01, n = 10), whereas cyclohexanedione-apoE x DMPC vesicles had no effect. One important cellular mechanism for up-regulation of cGMP is through stimulation of nitric oxide (NO) synthase, the NO generated by conversion of L-arginine to L-citrulline, binds to and activates guanylate cyclase. This signal transduction pathway was implicated by the finding that NO synthase inhibitors of distinct structural and functional types all reversed the anti-platelet action of apoE, whereas a selective inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (100 nM), had a similar reversing action. Direct confirmation that apoE stimulates NO synthase was obtained by use of L-[3H]arginine; platelets pretreated with apoE x DMPC produced markedly more L-[3H]citrulline (0.71 +/- 0.1 pmol/h/10(9) platelets) than controls (0.18 +/- 0.03; p < 0.05). In addition, hemoglobin which avidly binds NO also suppressed the anti-aggregatory effect, indicating that apoE stimulated sufficient production of NO by platelets for extracellular release to occur. We conclude that apoE inhibits platelet aggregation through the L-arginine:NO signal transduction pathway.

Calcium influx is a determining factor of calpain activation and microparticle formation in platelets

We have related the release of procoagulant microparticles from platelets to calcium movement and the activation of the Ca(2+)-dependent protease calpain. The effects of the Ca(2+)-ATPase inhibitors thapsigargin, cyclopiazonic acid and 2.5-di-(t-butyl)-1,4-benzohydroquinone were compared with those of the Ca2+ ionophore A23187. Whereas all three Ca(2+)-ATPase inhibitors induced aminophospholipid exposure on platelets, only thapsigargin and cyclopiazonic acid promoted microparticle formation and only when strong Ca2+ influx, calpain activation and proteolysis of cytoskeletal proteins occurred concomitantly. Preincubation with dibutylbenzohydroquinone inhibited the responses to thapsigargin and cyclopiazonic acid but not to A23187. When platelets were suspended in a Ca(2+)-free medium, calpain activation and microparticle formation were not observed, even with maximum mobilisation of internal Ca2+ stores by A23187. Incubation of fluo-3-loaded plateters with A23187 in 0.1 mM EGTA followed by the sequential addition of 25 microM Ca2+ increments to the medium showed that calpain activation occurred when the intraplatelet [Ca2+] reached 3-8 microM. To assess the physiologic significance of these results, the subpopulation of platelets that expressed procoagulant activity after stimulation by a thrombin/collagen mixture was isolated by means of annexin-V-coupled magnetic beads. Subsequent western blotting experiments confirmed that this subpopulation contained activated calpain. Overall, our results provide evidence that microparticle formation and calpain activation require an elevated intraplatelet [Ca2+] that is brought about by influx across the plasma membrane.

Variability of the thrombin- and ADP-induced Ca2+ response among human platelets measured using fluo-3 and fluorescent videomicroscopy

The intracellular free Ca2+ concentration ([Ca2+]cyt) of individual human platelets localized between siliconized glass cover slips was determined at rest and after stimulation with thrombin and ADP using the Ca2+ indicator fluo-3 (0.97 +/- 0.30 mmol/l cell volume) with fluorescence video microscopy. Resting [Ca2+]cyt in the presence of 2 mM external Ca2+ showed only small inter-platelet variability ([Ca2+]cyt = 86 +/- 30 (S.D.) nM). Resting [Ca2+]cyt of individual fluo-3-loaded platelets measured as a function of time had a S.D. of 10 nM or 12% (S.D./mean). Individual platelets showed no affinity for the siliconized support and their [Ca2+]cyt showed no tendency to oscillate in either the resting or in the activated state. When 0.2 U/ml thrombin or 20 microM ADP were added, all platelets showed a characteristic Ca2+ transient whereby [Ca2+]cyt increased to peak values within 8-12 sec and then declined. The Ca2+ transients measured with fluo-3 were in approximate synchrony but peak [Ca2+]cyt values showed large inter-platelet variability. The ensemble average peak [Ca2+]cyt for thrombin and ADP were 672 +/- 619 (S.D.) nM and 640 +/- 642 (S.D.) nM, respectively. Thus inter-platelet variations (S.D./mean) were 92% or 100% as large as the average measured values. Mathematically-constructed averages of the single platelet experiments agreed reasonably well with platelet-averaged values obtained in parallel experiments with stirred platelet suspensions in a plastic cuvette, measured with a conventional spectrofluorometer. Peak [Ca2+]cyt values reflecting dense tubular Ca2+ release alone (external Ca2+ removed) also showed large interplatelet variation (171 +/- 105 (S.D.) nM with thrombin and 183 +/- 134 (S.D.) nM with ADP). Dense tubular Ca2+ release induced by cyclopiazonic acid (a dense tubular Ca2+-ATPase inhibitor) gave peak [Ca2+]cyt of 289 +/- 170 nM. Thus the size of the dense tubular Ca2+ pool has an inter-platelet variation of 59% (S.D./mean). Variability of the dense tubular pool size accounts for some, but not all, of the large interplatelet variation in peak (Ca2+]cyt seen with thrombin and ADP activation.

Oxidized low density lipoprotein inhibits platelet plasma membrane Ca(2+)-ATPase

Oxidized low density lipoprotein (LDL) has been shown to enhance platelet activation. Since platelet activation is accompanied by an increase in cytosolic calcium, the effects of oxidized LDL on plasma membrane Ca(2+)-ATPase, plasma membrane fluidity and cytoplasmic calcium were studied in human platelets and purified platelet plasma membranes. Our results demonstrate that oxidized LDL, but not native LDL, inhibits the activity of Ca(2+)-ATPase in purified platelet plasma membranes (P < 0.01). Addition of the free radical scavenger alpha-tocopherol had no effect on the ability of oxidized LDL to inhibit the Ca(2+)-ATPase. An increased cytoplasmic calcium level in whole platelets was induced by oxidized LDL (P < 0.01), indicating that the plasma membrane Ca(2+)-extrusion pump may also be inhibited in vivo by oxidized LDL, although other mechanisms for the increase in cytoplasmic calcium are possible. Since no change in membrane fluidity was observed in platelet plasma membranes exposed to oxidized or native LDL as estimated by steady state trimethylammonium diphenylhexatriene (TMA-DPH) anisotropy, oxidized LDL does not affect the Ca(2+)-ATPase by grossly changing the membrane environment. The present results suggest that exposure of platelets to oxidized LDL causes inhibition of the plasma membrane Ca(2+)-ATPase which contributes to the observed increase in cytoplasmic calcium and increased sensitivity to agonists.

Distribution of plasma membrane Ca(2+)-ATPase and inositol 1,4,5-trisphosphate receptor in human platelet membranes

Human platelet plasma membranes were prepared by the glycerol lysis method of Harmon et al. [Harmon JT. Greco NJ. Jamieson GA. (1992) Isolation of human platelet plasma membranes by glycerol lysis. Meth. Enzymol., 215, 32-36]. The membranes were observed to contain a Ca(2+)-ATPase with different properties than those of internal membranes. The specific activity of Ca(2+)-ATPase was lower in plasma membranes (10-40 nmol ATP hydrolyzed/min/mg), but the ATPase was less sensitive to thapsigargin (41% inhibition at 500 nM) and more sensitive to vanadate (50% inhibition at 4 microM) than the Ca(2+)-ATPase in internal platelet membranes. The plasma membranes contained a Ca(2+)-ATPase detectable by monoclonal and polyclonal antibodies against erythrocyte Ca(2+)-ATPase that had a molecular mass of 144 kD. However, an anti-peptide antibody against an N-terminal sequence of the inositol 1,4,5-trisphosphate receptor recognized this protein in internal membranes, but not plasma membranes.