Platelet function in patients admitted with a diagnosis of myocardial infarction

Platelets as Predictors of Vascular Risk: Is There a Practical Index of Platelet Activity?

Activated platelets play a role in the pathogenesis of coronary heart disease (CHD). Following activation, platelets change shape, aggregate, and release several bioactive substances. The aim of this review is to identify if there is a simple and cost-effective method that indicates platelet activation and predicts the risk of CHD and vascular events. The rationale for identifying high-risk patients is to reduce their risk of vascular events by administering appropriate and effective antiplatelet treatment, like aspirin, clopidogrel, or combination regimens. Many laboratory tests estimating platelet activity have been described. Some are relatively simple, such as spontaneous or agonist-induced platelet aggregation. Other tests include measuring the mean platelet volume (MPV) or plasma soluble P-selectin levels. Some more complex tests include flow cytometry to determine platelet GP Ilb/Illa receptors, platelet surface P-selectin, plateletmonocyte aggregates, and microparticles. Only few prospective studies assessed the predictive value of platelet activation in healthy individuals. Although the MPV seems an 'easy method, there are insufficient data supporting its ability to predict the risk of a vascular event in healthy adults. Platelet aggregation, in whole blood or in platelet-rich plasma was not consistently predictive of vascular risk. Soluble P-selectin measurement is a promising method but it needs further evaluation. Flow cytometry methods are costly, time-consuming, and need specialized equipment. Thus, they are unlikely to be useful in estimating the risk in large numbers of patients. There is as yet no ideal test for the detection of platelet activation. Each currently available test has merits and disadvantages. Simple methods such as the MPV and the determination of platelet release products need further evaluation.

Heparin Versus Bivalirudin in Acute Myocardial Infarction: Unfractionated Heparin Monotherapy Elevated to Primary Treatment in Contemporary Percutaneous Coronary Intervention

Bivalirudin, a direct thrombin inhibitor, was developed as an antithrombin agent for patients undergoing percutaneous coronary interventions (PCI) with the hypothesis that it would reduce bleeding complications without compromising the rate of ischemic events compared to heparin plus GP IIb/IIIa inhibitors. Although the cumulative evidence makes a strong argument for the use of bivalirudin rather than heparin plus systematic GP IIb/IIIa inhibitors for the great majority of patients with acute myocardial infarction (AMI) undergoing PCI, the benefit observed with bivalirudin was achieved because of the major bleeding complications with the use of heparin plus GP IIb/IIIa inhibitors. When bivalirudin was compared with unfractionated heparin alone there was no benefit in ischemic complications with a decrease in major bleeding. However, in a recent large randomized controlled trial comparing bivalirudin with unfractionated heparin alone in AMI patients undergoing primary PCI, bivalirudin did not reduce bleeding complications and was associated with higher rates of stent thrombosis, myocardial reinfarction, and repeat revascularization compared with heparin. Moreover, a very recent meta-analysis shed more insights on the utilization of bivalirudin versus heparin regimens during PCI. Findings from this meta-analysis suggest that routine use of bivalirudin offers little advantage over heparin among PCI patients. In a detailed analysis of some randomized trials and observational studies with bivalirudin in AMI patients done by myself and published almost five years ago in this journal, I rendered some reflections on the future widespread use of bivalirudin. “In the setting of PCI in AMI patients, and in the absence of GP IIb/IIIa inhibitors, bivalirudin did not offer any beneficial effect in the incidence of the composite end points when compared with heparin alone. For now, in real world practice, one would probably choose a well known cheaper drug that has already passed the test of time, heparin. There may be reinforcement in the sole utilization of heparin confining GP IIb/IIIa inhibitors and other intravenous antithrombotics to bailout therapy for periprocedural PCI complications in AMI patients”. Therefore, instead of being the beginning of a new era with bivalirudin, it sure is a welcome back to an old friend, heparin. Indeed, after more than two decades, it is always good to welcome back an old friend, unfractionated heparin, as monotherapy and preferred anticoagulant regimen for contemporary PCI in AMI patients.

Mean Platelet Volume and Extent and Complexity of Coronary Artery Disease in Diabetic and Nondiabetic Patients With ST Elevation Myocardial Infarction

We investigated whether the mean platelet volume (MPV) is associated with the extent and complexity of coronary artery disease in patients with ST elevation myocardial infarction (STEMI). We prospectively included 912 STEMI patients (663 male and 249 female; mean age 58.7 ± 12.4 years) who underwent primary percutaneous coronary intervention. The patients were divided into 3 groups according to MPV tertiles. Highest SYNTAX score (SS) was observed in MPVhigh group compared with MPVmid and MPVlow groups ( P < .001 for all). The SS of MPVmid group was higher than MPVlow group ( P = .036). The MPV in diabetic STEMI patients was higher than in nondiabetic STEMI patients ( P < .001). Multivariate linear regression analysis showed that the MPV was associated with diabetes (β = .115, P = .001), troponin level (β = .131, P = .001), platelet count (β = −.241, P < .001), and SS (β = .216, P < .001). The relation between MPV and SS in diabetic STEMI patients was stronger than for nondiabetic STEMI patients ( r = .473, P < .001 vs r = .129, P = .001).

Association of Mean Platelet Volume and Pre- and Postinterventional Flow With Infarct-Related Artery in ST-Segment Elevation Myocardial Infarction

Platelets play a role in the pathogenesis of ST-segment elevation myocardial infarction (STEMI). We assessed the relationship between mean platelet volume (MPV) on admission and pre- and postinterventional flow with the infarct-related artery (IRA) in patients with STEMI. We prospectively included 840 patients with STEMI who underwent primary percutaneous coronary intervention (PCI). The patients were divided into 3 groups according to MPV tertiles. Pre- and post-PCI Thrombolysis In Myocardial Infarction (TIMI) flow grade was determined. Initial TIMI flow grade 3 was accepted as patent IRA. After the primary PCI, normal flow was defined as post-PCI TIMI flow 3. When the MPV was increased, the incidence of pre-PCI patent IRA ( P = .004) and post-PCI normal TIMI flow ( P < .001) was significantly decreased. Multivariate analysis showed that MPV was independently associated with post-PCI TIMI flow grade.

Relationship Between Mean Platelet Volume and Atherosclerosis in Young Patients With ST Elevation Myocardial Infarction

Increased mean platelet volume (MPV) is associated with poor clinical outcome in patients with acute coronary syndrome. We evaluated the predictive role of MPV in young patients with acute myocardial infarction (AMI). This study includes 373 patients who presented to our hospital with AMI (group 1: 134 young patients, males aged <45 years and females aged <55 years; group 2: 239 older patients) and 141 adults with normal coronary angiography as a control group (group 3). In group 1, the levels of MPV and hemoglobin were higher than that in groups 2 and 3. In group 1, blood urea nitrogen levels were lower than that in groups 2 and 3 and creatinine levels were lower than that in group 2. After multivariate analysis, MPV and age were independent predictors of AMI in young patients.

Actual Role of Platelet Glycoprotein IIb/IIIa Receptor Inhibitors as Adjunctive Pharmacological Therapy to Primary Angioplasty in Acute Myocardial Infarction: In the Light of Recent Randomized Trials and Observational Studies with Bivalirudin

Strategies for preventing ischemic complications during percutaneous coronary interventions (PCI) in the setting of acute myocardial infarction (AMI) have focused on the platelet surface-membrane glycoprotein (GP) IIb/IIIa receptor. The platelet GP IIb/IIIa receptor inhibitors, by blocking the final common pathway of platelet aggregation, have become a breakthrough in the management of acute coronary syndromes. Current adjuvant pharmacological therapy of AMI with aspirin, clopidogrel, unfractionated heparin (UH), and platelet GP IIb/IIIa inhibitors provides useful therapeutic benefits. Although the use of more potent antithrombin and antiplatelet agents during PCI in AMI has reduced the rate of ischemic complications, in parallel, the rate of bleeding has increased. Several studies have reported an association between bleeding after PCI and an increase in morbidity and mortality. Therefore, investigational studies have focused in pharmacological agents that would reduce bleeding complications without compromising the rate of major adverse cardiovascular events. Based on the results of several randomized trials, abciximab with UH, aspirin and clopidogrel have become a standard adjunctive therapy with primary PCI for AMI. However, some of the trials were done before the use of stents and the widespread use of thienopyridines. In addition, GP IIb/IIIa inhibitors use have been associated with thrombocytopenia, high rates of bleeding, and the need for transfusions, which increase costs, length of hospital stay, and mortality. On the other hand, in the stent era, bivalirudin, a semi-synthetic direct thrombin inhibitor, has recently been shown to provide similar efficacy with less bleeding compared with unfractionated heparin plus platelet GP IIb/IIIa inhibitors in AMI patients treated with primary PCI. The impressive results of this recent randomized trial and other observational studies make a strong argument for the use of bivalirudin rather than heparin plus GP IIb/IIIa inhibitors for the great majority of patients with AMI treated with primary PCI. However, some controversial results and limitations in the studies with bivalirudin exert some doubts in the future widespread use of this drug.

Markers of thrombosis and hemostasis in acute coronary syndromes: relationship to increased heart rate and reduced heart-rate variability

BACKGROUND

Acute coronary syndromes (ACS) are characterized by abnormal heart-rate variability (HRV) and biomarkers of endothelial damage and thrombosis.

HYPOTHESIS

We hypothesized an association between these factors in patients with ACS.

METHODS

We studied 99 patients with ACS measuring HRV and plasma markers of endothelial damage/dysfunction (von Willebrand factor, vWF) and thrombosis/hemostasis (soluble P-selectin (s-Psel); CD(40)-ligand (CD(40)-L); D-dimer). HRV and plasma indices were compared to age- and gender-matched controls. Measures were repeated at 4 months in a subset. vWF, s-Psel and D-Dimer levels were raised compared to control.

RESULTS

HRV indices were reduced (mean RR, SDNN, SDNNi, RMSSD, Triangular index, LF and HF). There were weak correlations between mean RR and s-Psel (R = - 0.234, p = 0.023) and D-dimer (R = - 0.219, p = 0.041). At 4-month follow-up, significant correlations were between mean RR and CD(40)L (R = - 0.414, p = 0.008) and D-dimer (R = - 0.363, p = 0.012). On multivariate logistic regression analysis statin use (p = 0.046) was the only independent predictor of acute s-Psel levels. Age (p = 0.004) and mean RR interval (p = 0.01) were independent predictors of D-dimer levels at follow-up.

CONCLUSIONS

Abnormal HRV is associated with markers of hemostasis and thrombosis in ACS, and present both in the acute and rehabilitation phases.

Platelet thromboxane A2 secretion in patients with major depression responsive to electroconvulsive therapy

OBJECTIVE

To determine a) whether clinical response to electroconvulsive therapy (ECT) is associated with decreased platelet activation in patients with major depressive disorder (MDD) and b) if any medical/demographic characteristics predict response to ECT or changes in platelet activation. Increased platelet activation may underlie the increased risk of coronary artery disease (CAD) in patients with MDD.

METHODS

Before their first and sixth ECT treatments, study patients (n = 44) completed the Beck Depression Inventory (BDI) to assess the severity of depressive symptoms. Activity of the platelet thromboxane (TBX) A(2) pathway was assessed by measuring the morning spot urinary concentrations of 11-dehydroxy-thromboxane B(2) (11-D-TBX B(2)), a major metabolite of platelet-derived TBX A(2).

RESULTS

Multivariate logistic regression analyses revealed that improvement on the BDI was significantly more likely in patients without a history of hypertension (p = .02) and in patients who were prescribed a greater number of "platelet-altering" medications (p = .03). During a course of ECT, a decrease in urinary 11-D-TBX B(2) was significantly more likely to occur in ECT nonresponders (p = .01) and younger patients (p = .02).

CONCLUSIONS

Clinical response to ECT coadministered may not be associated with decreases in platelet-derived TBX. Future studies will confirm which somatic "antidepression" treatments offer optimal thrombovascular benefits for depressed patients with multiple risk factors for, or clinically evident, cerebral disease or CAD.

Homocysteine activates platelets in vitro

The mechanism of thrombogenicity in hyperhomocysteinemia remains controversial. The authors investigated the association between elevated plasma homocysteine levels, platelet function, and blood coagulation. Blood was collected from healthy subjects and patients with critical limb ischemia. Basal platelet counts and platelet aggregation as well as flow cytometry were performed to assess spontaneous- and agonist-induced platelet aggregation as well as P-selectin and Glycoprotein IIb/IIIa expression at different homocysteine concentrations. Thromboelastography was performed, and platelet shape change was assessed, using a channelyzer, by measuring median platelet volume. Lactate dehydrogenase was measured, to indirectly assess red blood cell membrane integrity, after homocysteine exposure. The study results suggest that platelet activation and hypercoagulability occur after exposure to homocysteine, especially in patients with critical limb ischemia. Homocysteine concentrations of approximately 50 micromol/L appear to be the level at which these changes occur in vitro, and this effect on platelets appears to be indirect.

Platelets and white blood cell subpopulations among patients with myocardial infarction and unstable angina

Platelet (PLT)-endothelial cell and PLT-leukocyte interactions at lesion-prone sites might trigger a local inflammatory response early in the genesis of atherosclerosis and contribute to plaque destabilization leading to acute coronary syndromes (ACS). The aim of this study was to assess the PLT count, mean PLT volume (MPV), PLT mass, white blood cell (WBC; including eosinophils) and plasma interleukin (IL)-5, in patients with ACS and controls. PLT count, MPV, PLT mass, WBC and eosinophil percentage were determined in 167 consecutive patients with ACS (86 with acute myocardial infarction, AMI, and 81 unstable angina, UA) and 83 controls. Plasma IL-5 was measured in some patients and controls. Patients were considered in subgroups depending on smoking status and if they had or did not have diabetes mellitus (DM). The PLT count was lower in the UA and AMI groups although this did not always achieve significance. The MPV was significantly raised in all patient groups except in DM non-smokers with UA or AMI. All AMI patients had significantly higher WBC counts compared with controls. The percentage of eosinophils was lower in the UA and AMI groups although this did not always achieve significance. Plasma IL-5 levels were significantly increased in the UA and AMI groups. In conclusion, patients with ACS present with changes in the count of several cell types. These cells may become therapeutic targets and these changes may also act as markers of myocardial damage or prognosis.

The Antihypertensive Efficacy and Safety of a Chronotherapeutic Formulation of Propranolol in Patients With Hypertension

This study evaluated the antihypertensive efficacy and tolerability of a chronotherapeutic formulation of propranolol designed for nighttime dosing (propranolol controlled release [CR]). A total of 434 patients with mild-to-moderate hypertension were randomized to placebo or to one of four doses of propranolol CR (80, 120, 160, or 640 mg/d). At baseline, the mean morning blood pressures were similar in each treatment group and averaged 152/101 mm Hg. After 8 weeks of treatment, morning diastolic blood pressure, the primary efficacy measurement, was significantly reduced from baseline in placebo (-6.98 mm Hg) and all propranolol groups (p<0.001). The decreases ranged from 10.1 mm Hg in the 80-mg/d group to 11.0 mm Hg in the 120-mg/d group and were significantly larger than placebo in the 120-, 160-, and 640-mg/d groups (p<0.05). Blood pressure measured in the evening (trough) demonstrated similar antihypertensive efficacy. Heart rate and rate-pressure product were reduced in a dose-related manner by propranolol CR. The formulation was well tolerated with only fatigue and dizziness being reported more frequently than in the placebo group. Propranolol CR is an effective antihypertensive formulation that may reduce blood pressure during the morning period of maximum cardiovascular risk.

Unheralded sudden cardiac death: do autonomic tone and thrombosis interact as key factors in aetiology?

Unheralded sudden cardiac death is a personal and family tragedy that continues to elude research-based progress on aetiology or prevention. Instinctive links between autonomic imbalance, sympathetic activation and serious arrhythmia are longstanding and backed by many observational reports. However the role of the more familiar mechanisms of coronary occlusion and thrombus formation are underplayed. Sympathetic overactivity may also mediate sudden death through precipitation of vasospasm; platelet activation and inhibition of endogenous fibrinolysis as well as the propagation of arrhythmia. The integration of autonomic, thrombotic and vascular tone may be the key to better understanding of the individual process of unheralded sudden cardiac death. In this review we analyse the evidence for this hypothesis.

Platelet hyperaggregability to platelet activating factor (PAF) in non-ST elevation acute coronary syndromes

It is known that myocardial ischaemia increases platelet aggregatory response to various agonists, ex vivo. We investigated the platelet aggregatory response to platelet activating factor (PAF), ex vivo, in patients with non-ST elevation acute coronary syndromes and determined the specificity and sensitivity of this response. Thirty-two consecutive patients with non-ST elevation acute coronary syndromes and 20 healthy volunteers were studied. Platelet aggregation in platelet-rich plasma was studied on the day of admission. The maximal aggregation achieved within 2 min after the addition of PAF (100 nM) was expressed as a percentage of 100% light transmission. PAF EC50 values were defined as the concentration that induces 50% of maximal aggregation. The PAF EC50 values of the non-ST elevation acute coronary syndromes patients were significantly lower compared to those of the controls (p < 0.0001). The maximal percentage of aggregation was also significantly higher (p < 0.0005). Ninety-one per cent of the patients were correctly classified using PAF EC50 values (specificity 90.0% and sensitivity 91.2%); the corresponding results using the maximal percentage of aggregation were 80% (specificity 70.0% and sensitivity 87.5%). The estimated values used as thresholds were 22.47 nM and 17.97 for the PAF EC50 and the maximal percentage of aggregation, respectively. The results of the present study suggest that platelet hyperaggregability to PAF, ex vivo, in non-ST elevation acute coronary syndromes is characterised by a high specificity and sensitivity, and thus it may represent a mechanism contributing to the pathophysiology of acute coronary syndromes.

Effects of nisoldipine on cytosolic calcium, platelet aggregation, and coagulation/fibrinolysis in patients with coronary artery disease

The effect of nisoldipine, a dihydropyridine Ca2+ antagonist, on the platelet cytosolic Ca2+ concentration ([Ca2+]i), platelet aggregation, and various coagulation and fibrinolysis parameters was assessed in normotensive patients with coronary artery disease (CAD). Eleven patients with angiographically confirmed CAD (4 men, 7 women aged 67.3 +/- 5.4 years) were administered nisoldipine at 10 mg/day for two weeks. The [Ca2+]i was determined by use of fura2-loaded platelets, platelet aggregation was measured with an aggregometer, and coagulation/fibrinolysis parameters were measured by standard methods. Nisoldipine did not significantly affect blood pressure or heart rate. However, the [Ca2+]i decreased significantly (P<0.05) and platelet aggregation was also significantly inhibited. Plasma D-dimer levels decreased significantly (P<0.01). Thus, nisoldipine not only suppressed platelet activation but also affected the coagulation system, suggesting that it is not only a vasodilator and platelet inhibitor but also an antithrombotic agent.

Platelet activation during myocardial ischaemia: a contributory arrhythmogenic mechanism

Experimental and clinical observations of the involvement of platelets in the pathophysiology of myocardial ischaemia indicate the importance of interactions between these formed elements and the heart. The aim of this review is to outline evidence linking platelet activation, myocardial ischaemia and infarction, and to present evidence for a link between platelet activation, arrhythmogenesis and sudden death. A brief review of platelet physiology and pharmacology is provided, with a review of the cardiac electrophysiological effects of ischaemia and the electrophysiological effects of platelet-derived substances. The concept that platelet activation during myocardial ischaemia is a contributory arrhythmogenic mechanism is discussed.

Platelet Aggregation, Thromboxane A(2), Prostacyclin Generation and Platelet Sensitivity to Prostacyclin during the First Month after Myocardial Infarction

This study was designed to investigate platelet aggregation, plasma thromboxane A, and prostacyclin concentration and platelet sensitivity to prostacyclin simultaneously during the first month after myocardial infarction (MI). Spontaneous platelet aggregation and aggregation responses to ADP and adrenaline were low on the day of admission, increased rapidly by the 7th day post-MI, remained elevated during the second week post-MI and reached the level of chronic coronary artery disease patients but not healthy persons at the end of the fourth week of illness. An increase in plasma thromboxane B, the spontaneous and stable breakdown product of thromboxane A, level and enhanced prostacyclin production, with a maximum on the third post-MI day, were observed. We also demonstrated a significant platelet resistance to prostacyclin in MI patients. Thrombocyte sensitivity to prostacyclin normalized by the end of the fourth post-MI week. These results indicate the need for therapy with platelet inhibitors in patients with MI.

Platelet glycoprotein IIb-IIIa and size are increased in acute myocardial infarction

Platelet aggregation, which plays an important role in acute myocardial infarction (AMI), is mediated by fibrinogen binding to the platelet membrane glycoprotein (GP)IIb-IIIa (CD41). This study measured the relative number of GPIIb-IIIa complexes on platelets from patients immediately following AMI (n = 14) compared with those from controls (n = 14). Flow cytometry was used to demonstrate that there were, on average, 20% more GPIIb-IIIa complexes on platelets after AMI compared with controls. Platelet size was also 7% greater in AMI and it is known that larger platelets are more reactive. Since platelet size and protein content are determined at thrombopoiesis the majority of these platelets must have been circulating prior to AMI. Larger platelets, with more GPIIb-IIIa may, therefore, be causally related to AMI.

Effect of milrinone on thromboxane A2 synthesis, cAMP phosphodiesterase and 45Ca2+ uptake by human platelets

The phosphodiesterase inhibitors milrinone and isobutylmethylxanthine (IBMX) inhibited the conversion of [3H]cAMP to [3H]AMP by washed human platelets in concentration-dependent manners (IC50: milrinone, 2.6 x 10(-6) M; IBMX, 4.6 x 10(-6) M). Milrinone and IBMX increased cAMP levels when stimulated by a single concentration (0.3 microM) of iloprost. EC50:milrinone, 5.6 x 10(-5) M; IBMX, 3 x 10(-5) M. Milrinone was a potent inhibitor of platelet thromboxane A2 (TXA2) synthesis when stimulated by median stimulatory doses of collagen (IC50: 3 x 10(-7) M), sodium fluoride (NaF) (a non-specific G protein activator; IC50: 3 x 10(-7) M) and phorbol ester myristate acetate (PMA) (a protein kinase C activator; IC50: 2.2 x 10(-7) M). In contrast, at median stimulatory doses of A23187 and arachidonate there was a marked decrease in the potency of milrinone in inhibiting TXA2 synthesis. Milrinone had a weak inhibitory effect on TXA2 synthesis when elicited by freeze fracturing. In all experiments IBMX was a weaker inhibitor of TXA2 synthesis, although the general pattern of effects was similar to milrinone. Milrinone inhibited both collagen- and adrenaline-stimulated 45Ca2+ uptake by human platelets in dose-dependent manners. Since platelet TXA2 synthesis is dependent on Ca2+, and milrinone inhibited 45Ca2+ uptake, it is concluded that milrinone exerts its inhibitory effect on platelet activity, principally through an action on Ca2+ mobilisation/binding to effector proteins (protein kinase C and/or phospholipase A2).

Effects of the PAF antagonists BN50726 and BN50739 on arrhythmogenesis and extent of necrosis during myocardial ischaemia/reperfusion in rabbits

1. The effects of two novel platelet activating factor (PAF) antagonists BN50726 and BN50739 on arrhythmias, haemodynamics and extent of necrosis during myocardial ischaemia and reperfusion were investigated in anaesthetized rabbits subjected to coronary artery ligation. 2. BN50739 reduced heart rate prior to coronary artery occlusion (P < 0.005) but had no other significant haemodynamic effects at this time. BN50739 and BN50726 did not significantly alter heart rate or blood pressure during 30 min of ischaemia or 30 min of reperfusion, compared to control hearts. 3. BN50739 and BN50726 had no effect on the incidence of arrhythmias during ischaemia. BN50726 significantly reduced the incidence of reperfusion ventricular fibrillation compared to controls (0% v 40%, P < 0.05), and improved survival (80% v 39%, P < 0.05). Similar trends were observed with BN50739. 4. BN50726 reduced the extent of necrosis compared to control hearts (18 +/- 2% v 30 +/- 3%, P < 0.01). A similar trend was observed with BN50739. 5. These results demonstrate that PAF antagonism with BN50726 attenuates reperfusion-induced arrhythmias and preserves myocardium in the early phase of ischaemia, independently of haemodynamic effects.

Lisinopril and nifedipine administration inhibits the ex vivo uptake of [45Ca2+] by platelets from hypertensive diabetic patients

1. The effect of administration of the angiotensin converting enzyme inhibitor (ACEI), lisinopril (Carace; 10-40 mg twice daily) and the calcium channel blocker, nifedipine (Adalat Retard; 20-40 mg twice daily) on ex vivo [45Ca2+] uptake by platelets from hypertensive diabetic (type 1 and 2) patients was investigated. 2. At the end of at least 3 months treatment, blood was collected prior to the patient taking the morning dose of medication and washed platelets prepared. [45Ca2+] uptake was monitored following the addition of adrenaline, isoprenaline and dibutyryl cAMP (dbcAMP), as well as in unstimulated (zero) platelets. 3. Both nifedipine and lisinopril significantly inhibited the ex vivo uptake of [45Ca2+] by platelets when this process was stimulated by adrenaline, isoprenaline and dibutyryl cAMP. Basal uptake was also inhibited in both groups. 4. These data consolidate the hypothesis that ACE inhibitors may possess calcium channel/calcium mobilisation blocking properties. Apart from its hypertensive action, lisinopril may also reduce platelet activity via modulation of calcium dynamics, thereby reducing the incidence of vascular complications associated with diabetes mellitus.

Lack of significant effect of therapeutic propranolol on measurable platelet function in healthy subjects

Propranolol, a non-selective beta blocker, was administered orally in therapeutic doses. The effects of a single dose (160 mg) and one week's treatment (80 mg twice a day) on platelet function were compared in healthy young subjects. There were no significant changes in circulating platelet aggregates, template bleeding time, platelet factor 3 availability and thromboxane beta 2 (TX beta 2) generation. Platelet aggregation responses as assessed by angle of slope and maximal percentage aggregation (all agonists) and lag phase (collagen) showed no changes of biological importance, although minor changes reaching significance were observed with some agonists. These findings suggest that propranolol does not significantly affect platelet function when taken at doses commonly encountered in clinical practice.

The effects of intravenous and oral nifedipine on ex vivo platelet function

We have studied the possible anti-platelet effects of an intravenous formulation of nifedipine (0.75 mg as a bolus and an infusion of 1.2 mg.h-1 for 2 h), or equivalent volumes of the vehicle alone, or normal saline, in a double-blind crossover fashion in six healthy subjects. The effects of a standard oral formulation (20 mg sustained-release) compared to identical placebo were also studied in twelve other subjects. Platelet function was assessed by the addition of collagen, adenosine diphosphate, or adrenaline to whole blood followed by single platelet counting. Intravenous nifedipine had no effect on aggregation in response to any of the agonists, but oral nifedipine reduced aggregation caused by collagen by approximately 15%, despite similar plasma nifedipine concentrations after both formulations (18.5 ng.ml-1 after intravenous and 21.5 ng.ml-1 after oral administration). The lack of effect of intravenous nifedipine may be due to endothelial irritation caused by the vehicle. Intravenous nifedipine is unlikely to have a useful anti-platelet effect in patients who have acute coronary insufficiency.

Cyclosporine A enhances platelet aggregation

In view of the reported increase in thromboembolic episodes following cyclosporine A (CyA) therapy, the effect of this drug on platelet aggregation and thromboxane A2 release was investigated. The addition of CyA, at therapeutic concentrations to platelet rich plasma from normal subjects in vitro was found to increase aggregation in response to adrenaline, collagen and ADP. Ingestion of CyA by healthy volunteers was also associated with enhanced platelet aggregation. The CyA-mediated enhancement of aggregation was further enhanced by the addition in vitro of therapeutic concentrations of heparin. Platelets from renal allograft recipients treated with CyA also showed hyperaggregability and increased thromboxane A2 release, which were most marked at "peak" plasma CyA concentration and less so at "trough" concentrations. Platelet hyperaggregability in renal allograft patients on long-term CyA therapy tended to revert towards normal following the replacement of CyA with azathioprine. Hypertensive patients with renal allografts on nifedipine therapy had normal platelet function and thromboxane release in spite of CyA therapy. These observations suggest that CyA-mediated platelet activation may contribute to the pathogenesis of the thromboembolic phenomena associated with the use of this drug. The increased release of thromboxane A2 (a vasoconstrictor) may also play a role in mediating CyA-related nephrotoxicity.

Platelet activation following intravenous injection of a conventional heparin: absence of effect with a low molecular weight heparinoid (Org 10172)

1. The effects of an intravenous injection of a conventional high molecular weight heparin (HMWH) were compared with those of a low molecular weight heparinoid (Org 10172). A bolus injection of HMWH (5000 iu) was associated with: (a) a small but significant prolongation of bleeding time (BT); (b) a significant fall in PRP platelet count; (c) significantly enhanced platelet aggregation; and (d) significantly increased platelet thromboxane A2 (TXA2) release. These changes were not observed following the intravenous injection of Org 10172 (3200 anti Xa U). 2. These experiments were repeated following the oral administration of acetylsalicylic acid (ASA). HMWH again caused some enhancement of platelet aggregation despite the ASA-mediated inhibition of platelet aggregation and TXA2 release. Administration of Org 10172 to subjects taking ASA did not alter any of the platelet function indices. 3. In additional control experiments the injection of 5000 iu of HMWH was associated with a significant fall in PRP but not whole blood platelet counts. This finding suggests that the fall in PRP platelet count is a methodological artefact. 4. The HMWH also induced a significantly greater increase in serum non-esterified fatty acid (NEFA) concentrations than Org 10172. 5. The present findings indicate that Org 10172 is a less potent stimulator of platelet aggregation and lipolysis than HMWH. 6. The minor prolongation of the BT after HMWH is not compatible with enhanced aggregation but may be a consequence of alterations in the activity of coagulation factors and vascular-platelet interactions or of ongoing platelet activation accompanied by granule depletion. 7. The different effects of the two anticoagulants assessed suggests a therapeutic advantage in favour of Org 10172, especially in patients with hyperactive platelets.