Mechanisms of Platelet Activation and Inhibition

Fibrinogen, Coagulation, and Ageing

The World Health Organization estimates that the world's population over 60 years of age will nearly double in the next 30 years. This change imposes increasing demands on health and social services with increased disease burden in older people, hereafter defined as people aged 60 years or more. An older population will have a greater incidence of cardiovascular disease partly due to higher levels of blood fibrinogen, increased levels of some coagulation factors, and increased platelet activity. These factors lead to a hypercoagulable state which can alter haemostasis, causing an imbalance in appropriate coagulation, which plays a crucial role in the development of cardiovascular diseases. These changes in haemostasis are not only affected by age but also by gender and the effects of hormones, or lack thereof in menopause for older females, ethnicity, other comorbidities, medication interactions, and overall health as we age. Another confounding factor is how we measure fibrinogen and coagulation through laboratory and point-of-care testing and how our decision-making on disease and treatment (including anticoagulation) is managed. It is known throughout life that in normal healthy individuals the levels of fibrinogen and coagulation factors change, however, reference intervals to guide diagnosis and management are based on only two life stages, paediatric, and adult ranges. There are no specific diagnostic guidelines based on reference intervals for an older population. How ageing relates to alterations in haemostasis and the impact of the disease will be discussed in this chapter. Along with the effect of anticoagulation, laboratory testing of fibrinogen and coagulation, future directions, and implications will be presented.

Familial Mediterranean fever: the molecular pathways from stress exposure to attacks

FMF is an autoinflammatory disease characterized by recurrent attacks and increased IL-1 synthesis owing to activation of the pyrin inflammasome. Although knowledge of the mechanisms leading to the activation of pyrin inflammasome is increasing, it is still unknown why the disease is characterized by attack. The emergence of FMF attacks after emotional stress and the induction of attacks with metaraminol in previous decades suggested that stress-induced sympathoadrenal system activation might play a role in inflammasome activation and triggering attacks. In this review, we will review the possible molecular mechanism of stress mediators on the inflammation pathway and inflammasome activation. Studies on stress mediators and their impact on inflammation pathways will provide a better understanding of stress-related exacerbation mechanisms in both autoinflammatory and autoimmune diseases. This review provides a new perspective on this subject and will contribute to new studies.

Binary agonist surface patterns prime platelets for downstream adhesion in flowing whole blood

As platelets encounter damaged vessels or biomaterials, they interact with a complex milieu of surface-bound agonists, from exposed subendothelium to adsorbed plasma proteins. It has been shown that an upstream, surface-immobilized agonist is capable of priming platelets for enhanced adhesion downstream. In this study, binary agonists were integrated into the upstream position of flow cells and the platelet priming response was measured by downstream adhesion in flowing whole blood. A nonadditive response was observed in which platelets transiently exposed to two agonists exhibited greater activation and downstream adhesion than that from the sum of either agonist alone. Antibody blocking of one of the two upstream agonists eliminated nonadditive activation and downstream adhesion. Crosstalk between platelet activation pathways likely led to a synergistic effect which created an enhanced activation response in the platelet population. The existence of synergy between platelet priming pathways is a concept that has broad implications for the field of biomaterials hemocompatibility and platelet activity testing.

Hemocompatibility of Superhemophobic Titania Surfaces

The hemocompatibility of superhemophobic surfaces is investigated and compared with that of hemophobic surfaces and hemophilic surfaces. This analysis indicates that only those superhemophobic surfaces with a robust Cassie-Baxter state display significantly lower platelet adhesion and activation. It is envisioned that the understanding gained through this work will lead to the fabrication of improved hemocompatible, superhemophobic medical implants.

β2GP1, Anti-β2GP1 Antibodies and Platelets: Key Players in the Antiphospholipid Syndrome

Anti-beta 2 glycoprotein 1 (anti-β2GP1) antibodies are commonly found in patients with autoimmune diseases such as the antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE). Their presence is highly associated with increased risk of vascular thrombosis and/or recurrent pregnancy-related complications. Although they are a subtype of anti-phospholipid (APL) antibody, anti-β2GP1 antibodies form complexes with β2GP1 before binding to different receptors associated with anionic phospholipids on structures such as platelets and endothelial cells. β2GP1 consists of five short consensus repeat termed "sushi" domains. It has three interchangeable conformations with a cryptic epitope at domain 1 within the molecule. Anti-β2GP1 antibodies against this cryptic epitope are referred to as 'type A' antibodies, and have been suggested to be more strongly associated with both vascular and obstetric complications. In contrast, 'type B' antibodies, directed against other domains of β2GP1, are more likely to be benign antibodies found in asymptomatic patients and healthy individuals. Although the interactions between anti-β2GP1 antibodies, β2GP1, and platelets have been investigated, the actual targeted metabolic pathway(s) and/or receptor(s) involved remain to be clearly elucidated. This review will discuss the current understanding of the interaction between anti-β2GP1 antibodies and β2GP1, with platelet receptors and associated signalling pathways.

Progress in the research of GPIIb/IIIa antagonists

This review covers the recent advances in the development of small RGD (Arg-Gly-Asp sequence) containing peptides and their mimetics as potential antithrombotic agents. Glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists include monoclonal antibodies, RGD peptides, peptide hybrids and nonpeptide mimetics. The current trend in the development of nonpeptide mimetics is clearly directed toward orally active and safe antithrombotic drug candidates. But several nonpeptide mimetics, being evaluated for their oral activity in human clinical trials, are currently not approved for clinical use due to poor safety profile. It is expected that newer and more effective nonpeptide mimetics will be developed in the near future.

Overview of the coagulation system

Coagulation is a dynamic process and the understanding of the blood coagulation system has evolved over the recent years in anaesthetic practice. Although the traditional classification of the coagulation system into extrinsic and intrinsic pathway is still valid, the newer insights into coagulation provide more authentic description of the same. Normal coagulation pathway represents a balance between the pro coagulant pathway that is responsible for clot formation and the mechanisms that inhibit the same beyond the injury site. Imbalance of the coagulation system may occur in the perioperative period or during critical illness, which may be secondary to numerous factors leading to a tendency of either thrombosis or bleeding. A systematic search of literature on PubMed with MeSH terms ‘coagulation system, haemostasis and anaesthesia revealed twenty eight related clinical trials and review articles in last 10 years. Since the balance of the coagulation system may tilt towards bleeding and thrombosis in many situations, it is mandatory for the clinicians to understand physiologic basis of haemostasis in order to diagnose and manage the abnormalities of the coagulation process and to interpret the diagnostic tests done for the same.

The PPAR-Platelet Connection: Modulators of Inflammation and Potential Cardiovascular Effects

Historically, platelets were viewed as simple anucleate cells responsible for initiating thrombosis and maintaining hemostasis, but clearly they are also key mediators of inflammation and immune cell activation. An emerging body of evidence links platelet function and thrombosis to vascular inflammation. peroxisome proliferator-activated receptors (PPARs) play a major role in modulating inflammation and, interestingly, PPARs (PPARβ/δ and PPARγ) were recently identified in platelets. Additionally, PPAR agonists attenuate platelet activation; an important discovery for two reasons. First, activated platelets are formidable antagonists that initiate and prolong a cascade of events that contribute to cardiovascular disease (CVD) progression. Dampening platelet release of proinflammatory mediators, including CD40 ligand (CD40L, CD154), is essential to hinder this cascade. Second, understanding the biologic importance of platelet PPARs and the mechanism(s) by which PPARs regulate platelet activation will be imperative in designing therapeutic strategies lacking the deleterious or unwanted side effects of current treatment options.

Atherogenesis and atherothrombosis--focus on diabetes mellitus

Diabetes mellitus represents a major cause of cardiovascular morbidity and mortality in developed countries, and atherothrombosis accounts for most deaths among patients with diabetes mellitus. Atherothrombosis is defined as atherosclerotic lesion disruption with superimposed thrombus formation. As a long-term, progressive disease process, atherosclerosis often results in an acute atherothrombotic event through plaque rupture and formation of a platelet-rich thrombus. The principal clinical manifestations of atherothrombosis are sudden cardiac death, myocardial infarction, ischaemic stroke, and peripheral arterial ischaemia comprising both intermittent claudication and critical limb ischaemia. Atherosclerosis is the leading cause of morbidity and mortality in the industrialized world, and diabetes mellitus magnifies the risk of cardiovascular events. In addition to the well-known microvascular complications of diabetes mellitus - such as nephropathy, retinopathy and neuropathy - the risk of macrovascular complications affecting the large conduit arteries markedly increases in patients with diabetes mellitus.

Thiazolidinediones as anti-inflammatory and anti-atherogenic agents

In the last few years, there has been increasing focus on the impact of interventions on cardiovascular outcomes in patients with type 2 diabetes. Insulin resistance and hyperglycaemia often co-exist with a cluster of risk factors for coronary artery disease, but the underlying mechanisms leading to the development of such vascular complications are complex. The over-production of free radicals in patients suffering from diabetes results in a state of oxidative stress, which leads to endothelial dysfunction and a greater risk of atherosclerosis. Moreover, inflammatory factors which play a critical role in atherothrombosis and plaque rupture are often found to be at elevated levels in this patient population. Thiazolidinediones (TZDs) are now routinely used to manage glucose levels, and have been suggested to influence other cardiovascular risk factors and therefore the pathways leading to macrovascular events. Consequently, recent studies have investigated the anti-inflammatory and anti-atherogenic properties of TZDs. The data available up to the present time, in the context of the emerging cardiovascular outcome profiles of rosiglitazone and pioglitazone, will be discussed here.

Reduced platelet serotonergic responsivity as assessed by dense granule secretion in first-episode psychosis

OBJECTIVES

To determine whether blunted serotonergic responsivity, indicated by decreased platelet dense granule secretion (DGS), occurs in neuroleptic-naïve patients with schizophrenia, as observed previously in chronic schizophrenia.

DESIGN AND METHODS

Serotonin (5-HT)-amplified DGS was examined in 40 first-episode neuroleptic-naïve patients (24 with schizophrenia and 16 with mood disorders) and 24 healthy subjects.

RESULTS

Healthy controls showed robustly increased DGS. Schizophrenic patients showed very modest DGS increases; mood disorder patients showed intermediate response.

CONCLUSIONS

Blunted DGS appears to a characteristic of schizophrenia that is observed in the treatment-naïve condition.

Causes and consequences of increased sympathetic activity in renal disease

Much evidence indicates increased sympathetic nervous activity (SNA) in renal disease. Renal ischemia is probably a primary event leading to increased SNA. Increased SNA often occurs in association with hypertension. However, the deleterious effect of increased SNA on the diseased kidney is not only caused by hypertension. Another characteristic of renal disease is unbalanced nitric oxide (NO) and angiotensin (Ang) activity. Increased SNA in renal disease may be sustained because a state of NO-Ang II unbalance is also present in the hypothalamus. Very few studies have directly compared the efficacy of adrenergic blockade with other renoprotective measures. Third-generation beta-blockers seem to have more protective effects than traditional beta-blockers, possibly via stimulation of NO release. Although it has been extensively documented that muscle SNA is increased in chronic renal failure, data on renal SNA and cardiac SNA are not available for these patients before end-stage renal disease. It is also unknown whether additional treatment with third-generation beta-blockers can delay the progression of renal injury and prevent cardiac injury in chronic renal failure more efficiently than conventional treatment with angiotensin-converting enzyme inhibitors only.

Effects of hormone replacement therapy on blood platelets

BACKGROUND

Hormone replacement therapy (HRT) increases the risk of cardiovascular morbidity in postmenopausal women under certain circumstances. Part of this effect may be the result of the influence of HRT on blood platelets. We studied the effect of short-term oral hormone replacement therapy (unopposed oestradiol or sequentially combined oestradiol and trimegestone or dydrogesterone) on platelet activation parameters in healthy postmenopausal women.

DESIGN

We designed a prospective, randomised, placebo-controlled 12-week study. Sixty healthy, normotensive, nonhysterectomised, postmenopausal women received daily micronised oestradiol (E2) 2 mg (n = 16), or 2 mg E2 daily sequentially combined with either trimegestone 0.5 mg daily (n = 14) or dydrogesterone 10 mg daily (n = 14), or placebo (n = 16). Data on platelet activation were collected at baseline and after 12 weeks of treatment using flow cytometry.

RESULTS

Twelve weeks of treatment with combined HRT was associated with an increase in platelet activation parameters P-selectin and glycoprotein 53 (by 17% and 14%, respectively, P = 0.04 vs. the placebo group for both comparisons), suggesting alpha granule and lysosome degranulation. E2 replacement therapy was associated with an increase in P-selectin labelling by 22% (P = 0.04 vs. the placebo group).

CONCLUSION

Short-term treatment with oestradiol or combined HRT increases the amount of circulating activated platelets as measured by flow cytometry. This could be a mechanism by which short-term HRT might increase the risk of thrombosis.

Diabetes and macrovascular disease

Cardiovascular disease (CVD) is the major cause of morbidity and mortality in patients with diabetes. Macrovascular events, including stroke, myocardial infarction (MI), and peripheral arterial disease (PAD), occur earlier than in nondiabetics and the underlying pathologies are often more diffuse and severe. Diabetic arteriopathy, which encompasses endothelial dysfunction, hypercoagulability, changes in blood flow, and platelet abnormalities, contributes to the early evolution of these events. Tight glucose and blood pressure control improves the vascular status of these patients by varying degrees. Antiplatelet agents have also been shown to be effective in the secondary prevention of cardiovascular events. In the ideal world, every risk factor would be addressed and each diabetic would have excellent glycemic control, a low normal blood pressure, a low LDL, and be prescribed an ACE inhibitor, together with aspirin and clopidogrel. If this is done, this emerging epidemic of macrovascular disease will be contained.

Platelet dysfunction in type 2 diabetes

Insulin resistance is a uniform finding in type 2 diabetes, as are abnormalities in the microvascular and macrovascular circulations. These complications are associated with dysfunction of platelets and the neurovascular unit. Platelets are essential for hemostasis, and knowledge of their function is basic to understanding the pathophysiology of vascular disease in diabetes. Intact healthy vascular endothelium is central to the normal functioning of smooth muscle contractility as well as its normal interaction with platelets. What is not clear is the role of hyperglycemia in the functional and organic microvascular deficiencies and platelet hyperactivity in individuals with diabetes. The entire coagulation cascade is dysfunctional in diabetes. Increased levels of fibrinogen and plasminogen activator inhibitor 1 favor both thrombosis and defective dissolution of clots once formed. Platelets in type 2 diabetic individuals adhere to vascular endothelium and aggregate more readily than those in healthy people. Loss of sensitivity to the normal restraints exercised by prostacyclin (PGI(2)) and nitric oxide (NO) generated by the vascular endothelium presents as the major defect in platelet function. Insulin is a natural antagonist of platelet hyperactivity. It sensitizes the platelet to PGI(2) and enhances endothelial generation of PGI(2) and NO. Thus, the defects in insulin action in diabetes create a milieu of disordered platelet activity conducive to macrovascular and microvascular events.

Coagulation and Bleeding Disorders: Review and Update

Hemostasis is initiated by injury to the vascular wall, leading to the deposition of platelets adhering to components of the subendothelium. Platelet adhesion requires the presence of von Willebrand factor and platelet receptors (IIb/IIIa and Ib/IX). Additional platelets are recruited to the site of injury by release of platelet granular contents, including ADP. The “platelet plug” is stabilized by interaction with fibrinogen. In this review, I consider laboratory tests used to evaluate coagulation, including prothrombin time, activated partial thromboplastin time, thrombin time, and platelet count. I discuss hereditary disorders of platelets and/or coagulation proteins that lead to clinical bleeding as well as acquired disorders, including disseminated intravascular coagulation and acquired circulating anticoagulants.

Comparative in vitro efficacy of different platelet glycoprotein IIb/IIIa antagonists on platelet-mediated clot strength induced by tissue factor with use of thromboelastography: differentiation among glycoprotein IIb/IIIa antagonists

In the present study, the in vitro efficacy of different platelet glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists on platelet-fibrin-mediated clot strength under shear was compared with the antiaggregatory efficacy by using tissue factor (TF) thromboelastography (TEG). The ability of platelets to augment the elastic properties of blood clots under shear conditions was measured by computerized TEG under conditions of maximal platelet activation accelerated by recombinant TF. Under these conditions, platelets significantly enhance clot strength 8-fold (relative to platelet-free fibrin clots). This effect was inhibited to a different extent by various platelet GPIIb/IIIa receptor antagonists; this inhibition appears to be dependent on the transmission of platelet contractile force to fibrin via the GPIIb/IIIa receptors. The GPIIb/IIIa antagonists with high binding affinity for resting and activated platelets and slow platelet dissociation rates (class I) but not those with fast platelet dissociation rates (class II) demonstrated potent and comparable inhibition of platelet aggregation and TF-TEG clot strength. Platelet GPIIb/IIIa antagonists of class I, such as XV459 (free-acid form of roxifiban), DMP802, XV454, and c7E3, demonstrated comparable inhibitory dose responses of TF-TEG clot strength and platelet aggregation, with an IC(50) of 50 to 70 nmol/L. In contrast, platelet GPIIb/IIIa antagonists from class II, with comparable antiaggregatory efficacy, such as DMP728, YZ202 (free-acid form of orbofiban), YZ211 (free-acid form of sibrafiban), YZ751, and other antagonists, have a much lower efficacy in altering the strength of TF-mediated clot formation (IC(50) >1.0 micromol/L). These data suggest differential efficacy among different GPIIb/IIIa antagonists in inhibiting platelet-fibrin clot retraction despite of equivalent antiaggregatory potency.

Prevention of carotid artery thrombosis by oral platelet GPIIb/IIIa antagonist in dogs

BACKGROUND AND PURPOSE

Current antithrombotic therapy in acute ischemic stroke and myocardial infarction in which a combination of antiplatelet agents (aspirin) and anticoagulants (heparin) was used led to partial reduction of acute thrombotic complications. Recent advances in antiplatelet research led to the discovery of the platelet glycoprotein IIb/IIIa complex (GPIIb/IIIa), the final common pathway for platelet aggregation. The present study was undertaken to determine the oral antithrombotic efficacy of a potent and specific platelet GPIIb/ IIIa antagonist, DMP728, in an electrically induced carotid artery thrombosis model in dogs. Based on the powerful antiplatelet efficacy of this mechanism in inhibiting all agonist-induced platelet aggregation as well as in inhibiting platelet procoagulant activity (thrombin generation and hence fibrin formation), an orally active antagonist for this integrin receptor might have potential benefits in stroke.

METHODS

Anesthetized dogs were instrumented for monitoring of arterial blood pressure, heart rate, and carotid artery flow velocity. Animals were treated with saline or DMP728 (0.1 to 1.0 mg/kg PO). Thrombus formation (platelet-rich aggregate with fibrous coating and a few erythrocytes) by anodal electrolytic stimulation (300 microA) to the intimal surface of the right carotid artery was initiated 120 minutes after oral DMP728 administration and continued for 180 minutes. Whole blood cell counts, ex vivo platelet aggregation, and template bleeding time were determined at different time points throughout the study.

RESULTS

DMP728 administered at 0.1 to 1.0 mg/kg PO exhibited dose-dependent antithrombotic efficacy in this model. DMP728 was shown to be significantly effective in inhibiting ex vivo platelet aggregation and in inhibiting thrombosis at 0.3 to 1.0 mg/kg PO. The antiplatelet, antithrombotic effects of DMP728 were demonstrated without any significant changes in the different hemodynamic or coagulation parameters. These data demonstrated the oral antithrombotic efficacy of DMP728 in dogs.

CONCLUSIONS

Platelet GPIIb/IIIa blockade with an orally active antagonist was shown to be safe and effective in the prevention of carotid artery occlusive thrombosis.

Calcium channel antagonists in the modern era of coronary thrombolysis: benefit or detriment?

Calcium channel antagonists are among the world's most widely prescribed class of drugs and are used most often in patients with hypertension and coronary artery disease. However, in the recent past serious questions have been raised concerning their potentially detrimental effects. One area of considerable clinical importance that deserves close inspection is the role of calcium channel antagonists following coronary reperfusion. Specifically, is there benefit or detriment?

Na(+)-independent Active H(+) Extrusion and HCO(3)(-)/C1(-) Exchange Contribute to Cytoplasmic pH Regulation in the Human Platelet

Homeostatic regulation of cytoplasmic pH (pH(eyt)) against acid and alkaline challenges was studied in the human platelet using the intracellular indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Activation of a Na(+)/H(+) exchanger in the plasma membrane is a known mechanism by which the platelet resists cytoplasmic acidification. The present study demonstrates an additional Na(+)-independent H extrusion mechanism which persists at low external Na(+) concentration in the presence of 100 µM ouabain. This mechanism is inhibited by rotenone, oligomycin and an inhibitor of glycolysis, and is tentatively identified as a plasmalemmal H(+)-ATPase. The Na(+)-independent H extrusion mechanism partially restores cytoplasmic pH (pH(eyt)) after the cytoplasm is acidified by addition of 1 µM nigericin. The restoration process is energy-dependent and has a t(1/2) of 7-21 min. The Na(+)-independent H(+) extrusion mechanism is also shown capable of maintaining pH(eyt) ≊ 6.0 against an acidic pH, of 5.3 in an energy-dependent manner. The present study also revealed a Na(+)-independent, DIDS-inhibitable Cl(-)/HCO(3)(-) exchange activity. It removes alkaline equivalents from the cytoplasm with a half-time of 2.0 ± 0.4 min after alkaline loading with 25 mM NH(4)Cl. Its activity was also revealed in chloride to gluconate and gluconate to chloride perturbations of the external medium which raised or lowered pH(eyt) by 0.17 ± 0.05 or 0.14 ± 0.04 units, respectively. The activity of the anion exchanger allows the platelet to maintain pH(ext) = 7.00 ± 0.11 at the alkaline pH(ext) of 8.25. The combined activities of the Na(+)-independent H(+) extrusion and Cl(-)/HCO(3)(-) exchange mechanisms make the platelet cytoplasm very resistant to changes in external pH. For variation of pH(ext) between 5.0 and 8.5, pH(eyt) varies between 6.0 and 7.0, or roughly one-third as much.

The role of protein phosphorylation in the regulation of cyclic nucleotide phosphodiesterases

The cyclic nucleotide phosphodiesterases constitute a complex superfamily of enzymes responsible for catalyzing the hydrolysis of cyclic nucleotides. Regulation of cyclic nucleotide phosphodiesterases is one of the two major mechanisms by which intracellular cyclic nucleotide levels are controlled. In many cases the fluctuations in cyclic nucleotide levels in response to hormones is due to the hormone responsiveness of the phosphodiesterase. Isozymes of the cGMP-inhibited, cAMP-specific, calmodulin-stimulated and cGMP-binding phosphodiesterases have been demonstrated to be substrates for protein kinases. Here we review the evidence that hormonally responsive phosphorylation acts to regulate cyclic nucleotide phosphodiesterases. In particular, the cGMP-inhibited phosphodiesterases, which can be phosphorylated by at least two different protein kinases, are activated as a result of phosphorylation. In contrast, phosphorylation of the calmodulin-stimulated phosphodiesterases, which coincides with a decreased sensitivity to activation by calmodulin, results in decreased phosphodiesterase activity.

Hemorrhagic potential of combined diltiazem and recombinant tissue-type plasminogen activator administration

In the Thrombolysis in Myocardial Infarction (TIMI) phase II study, use of calcium channel antagonists at study entry was associated with an increased risk of intracerebral hemorrhage. Whether the observed association was due solely to chance, underlying cerebrovascular disease, or an effect of calcium channel antagonists themselves was not determined. Accordingly, blood loss from standardized ear incisions was measured in six groups of anesthetized New Zealand white rabbits: (1) saline control, (2) intravenous diltiazem (20 micrograms/kg/min x 60 minutes), (3) intravenous recombinant tissue-type plasminogen activator (rTPA) (1.0 mg/kg over 60 minutes, 10% bolus), (4) diltiazem plus rTPA, (5) diltiazem daily for 3 consecutive days, and (6) diltiazem (3 days) plus rTPA given on day 3. The combination of rTPA plus diltiazem (3 days) resulted in significantly more blood loss than rTPA alone, diltiazem (60-minute infusion), or rTPA plus diltiazem (60-minute infusion) (p = 0.003). Similarly, diltiazem (3 days) resulted in more blood loss than either agent alone or rTPA plus diltiazem (60-minute infusion) (p < 0.05). Thus, in this animal model, prolonged exposure to diltiazem with or without rTPA was associated with increased bleeding. The potential for chronic use of oral calcium channel antagonist to increase hemorrhagic risk after rTPA administration requires further investigation.

Evidence on the role of three calcium pools in Ca-ionophore A23187-stimulated rat blood platelet aggregation

The effect of Ca-ionophore A23187 on activation of rat blood platelets was investigated to elucidate the involvement of extracellular and intracellular Ca2+ ions. Platelet aggregation induced by 10 concentrations of the stimulus was studied in Ca-free medium as well as in the presence of EGTA and/or calcium. In Ca-free medium, A23187 induced platelet aggregation in a dose-dependent way; the mean effective concentration was 1.43 +/- 0.08 mumol/l. The stimulatory effect of ionophore was potentiated by addition of 0.01 and 0.1 mM calcium and inhibited when the calcium concentration was increased to 1 mmol/l. In the presence of EGTA, A23187-stimulated aggregation of isolated rat platelets was recorded only at a 10-times higher ionophore concentration and was then reduced to 30% in comparison with aggregation in Ca-free medium. The inhibitory effect of 1 mM EGTA was abolished by addition of 2 mM calcium. We suggest the participation of at least three calcium pools in the stimulation of rat platelets by A23187, i.e. the extracellular pool, the membrane-associated pool and the pool displacing calcium intracellularly.

Platelet antiaggregate activity

Calcium ions act as a second messenger to platelet agonists, with increases in intracellular calcium bringing about changes in shape, aggregation, and release reactions. Changes in platelet function have been reported previously in migraine sufferers and there is evidence that hyperaggregability occurs during a migraine attack. It was decided to assess platelet aggregation with platelet-rich plasma (PRP) from nicardipine-treated migraine sufferers because dihydropyridine derivatives are known to inhibit adenosine diphosphate (ADP)- and epinephrine-induced aggregation. Aggregation induced by 1.4 mumol/L arachidonic acid was similar in PRP from control subjects and untreated migraine sufferers, whereas 1 or 2 mumol/L ADP-induced aggregation was lower in PRP from migraine sufferers. Treatment with 20 mg of nicardipine three times daily for 2 months significantly (p less than 0.05) increased 2 mumol/L ADP-induced aggregation. It is concluded that nicardipine was acting either on migraine pathogenic mechanisms or directly on the platelets.