Positive interaction between agonists in the aggregation response of human blood platelets: interation between ADP, adrenaline and vasopressin

Aspirin resistance and platelet turnover: a 25-year old issue

The evidence of an incomplete inhibition of platelet function by aspirin, despite therapeutic doses of the drug proved to be clinically effective are employed, was first reported in the '80s, in the frame of studies devoted to platelet turnover. Because inhibition of platelet aggregation by aspirin is irreversible, the return after an interval of time of the ability to form thromboxane by platelets in circulating blood should reflect the entry into the circulation of platelets whose cyclooxygenase activity has not been affected by aspirin. Based on this concept, the possibility of monitoring the entry of newly formed platelets into the circulation after aspirin ingestion was documented by measuring the return of thromboxane biosynthesis by platelets challenged in vitro by pairs of aggregating agents. The data obtained showed that platelets with intact cyclooxygenase activity could be detected into the circulation of control individuals as early as 4-6 h after aspirin ingestion, and at shorter time intervals in diabetic angiopathy. In the latter setting,the data allowed to conclude that "schedules of aspirin which may suffice in normals are not effective in patients with diabetic angiopathy, presumably because these patients have a high rate of entry of new platelets into the circulation".

Pairwise agonist scanning predicts cellular signaling responses to combinatorial stimuli

Patient-specific prediction of cellular response to multiple stimuli is central to evaluating clinical risk, disease progression, and response to therapy. We deployed Pairwise Agonist Scanning (PAS) to measure calcium signaling of human platelets in EDTA-treated plasma exposed to 6 different agonists (at 0.1, 1, and 10×EC₅₀) used individually or in 135 pairwise combinations. With 154 traces, we trained a neural network (NN) model to accurately predict the entire 6-dimensional response to ADP, convulxin, U46619, SFLLRN, AYPGKF, and PGE₂. The NN successfully predicted calcium responses to sequential agonist additions, all ternary combinations of [ADP]+[convulxin]+[SFLLRN] (R=0.88), and 45 different combinations of 4 to 6 agonists (R=0.88). Furthermore, PAS provided 135 pairwise synergy values that allowed a unique phenotypic scoring and differentiation of 10 donors. Training of NNs with pairs of stimuli across the dose-response regime represents a highly efficient approach to predict integration of multiple, complex signals in a patient-specific disease milieu.

Structural insights in platelet receptor synergism-antiplatelet therapy in post-ischemic cerebrovascular events

Synergy between agonists of platelet aggregation, namely, ADP and epinephrine, has been studied in patients having a history of cerebrovascular ischemic event. There is a significant variability of responsiveness among individuals towards clopidogrel, which is a specific inhibitor of the low-affinity human purinergic receptor (P2Y12). For responders of clopidogrel, simultaneous application of ADP and epinephrine at sub-threshold concentrations (i.e., concentration below the threshold concentration at which aggregation occurs) leads to platelet aggregation, which is followed by deaggregation. For non-responders of the drug, the synergism seems to be stronger, showing no deaggregatory pattern. The inhibition of synergism by yohimbine hydrochloride (YH), a blocker of alpha2A-adrenoreceptors is more pronounced in non-responders. A simple structural model based on receptor-receptor interaction is proposed to explain the synergism. The model explains synergy in terms of cooperative interaction between the low-affinity ADP receptor P2Y12 (Swiss Prot:Q9H244) and the alpha2A-adrenoreceptor (Swiss Prot:P08913). It follows that the synergistic effect can be achieved in only one of the two 3D structures for the alpha2A-adrenoreceptor P08913 permitted by homology modeling, as there is a better docking interface with the Q9H244. The synergism itself and the observed dichotomous phenomenon in relation to inhibition of synergism among responders and non-responders can be accounted for, if the interacting receptors on the dynamic membrane interface compete with the clopidogrel binding.

Pharmacotherapy of cerebral ischemia in cocaine dependence

This paper will examine how cerebral perfusion deficits in cocaine abusers may be a target for pharmacotherapy. The review covers five areas: (1) cerebral ischemia and neuropsychological impairment in cocaine abusers, (2) neuroimaging evidence for cerebral perfusion defects in cocaine abusers, (3) mechanisms for cocaine induced cerebral thrombosis, (4) neurotoxicity from cerebral ischemia and excitatory amino acids, (5) glutamate antagonists as potential treatment agents for cocaine induced neurotoxicity. Several pharmacotherapies are suggested including antiplatelet agents and excitatory amino acid (EAA) antagonists such as lamotrigine. Clinical trials in cocaine abusers with cerebral perfusion defects are indicated and EAA antagonists hold promise as they are developed for stroke treatment.

Effect of low concentration of epinephrine on human platelet aggregation analyzed by particle counting method and confocal microscopy

The effect of a physiologic concentration of epinephrine (Ep) on platelet activation was studied by using a novel method that simultaneously measures platelet aggregation by changes in light transmission and counts particles of various sizes by using light scattering. Detailed morphologic changes associated with activation process were studied by using confocal laser scanning microscopy (CLSM). A low concentration of Ep (20 nmol/L) corresponding to a high physiologic concentration triggered the formation of small platelet aggregates (diameter 7 to 30 microm) without any change in light transmission. The redistribution of filamentous actin (F-actin) and the expression of activated glycoprotein IIb-IIIa complex (GPIIb-IIIa), detected by PAC-1 binding, were also observed in the platelets comprising the small aggregates. Attempts were then made to detect changes in cytoplasmic ionized Ca2+ ((Ca2+)i) in individual platelets involved in the aggregate formation by CLSM with fluo-3, a Ca2+-indicating dye. Ep caused a weak (Ca2+)i increase in some individual platelets involved in the formation of small aggregates. This (Ca2+)i increase was associated with platelet aggregation, because no (Ca2+)i rise was detected in single platelets. Furthermore, platelets stimulated by Ep in the presence of RGDS or Ro 44-9883, a GPIIb-IIIa antagonist, did not form small aggregates or trigger a (Ca2+)i rise. Prior incubation with low concentrations of Ep (20, 100 nmol/L) enhanced the initial formation of small (diameter 7 to 30 microm), medium (diameter 30 to 50 microm), or large (diameter 50 to 70 microm) aggregates induced by a subthreshold concentration of adenosine diphosphate (0.25 micromol/L) as determined by the particle counting method. However, no apparent synergistic (Ca2+)i increase was observed in platelets involved in aggregate formation. From these observations the following conclusions have been reached. (1) A high physiologic concentration of Ep (20 nmol/L) is capable of triggering the formation of small aggregates, resulting in the redistribution of F-actin and the expression of activated GPIIb-IIIa complex. (2) An increase in (Ca2+)i is observed in platelets comprising the small aggregates. This increase is not related to the binding of Ep to its receptor but most likely is triggered by platelet-platelet association. (3) The characteristic potentiating effect of Ep is not due to the synergistic increase in (Ca2+)i.

Signal transduction system in epinephrine stimulated platelets; comparison between epinephrine sensitive and insensitive platelets

We recently reported the high prevalence of impaired platelet responsiveness only to epinephrine in healthy Japanese. This abnormality was associated with a 50% decrease in the number of alpha 2-adrenergic receptors. Platelets from non-responders (NR) do not undergo secondary platelet aggregation even after exposure to 100 microM epinephrine, but they can potentiate the effect of ADP to provoke platelet aggregation. To further define the nature of the defect and to delineate controversial steps of epinephrine stimulated signal transduction, a signaling pathway of epinephrine was investigated in platelets from NR and R(normal responder to epinephrine). In a unique particle counting apparatus, epinephrine initially triggered the formation of small platelet aggregates composing of 10-1000 cells from both R and NR, but the aggregates became larger (4600 > cells) only in platelets from R. Thus, platelets from NR lack the ability to form larger aggregates. A similar defect was reproduced by treating normal platelets with aspirin. In the presence of fibrinogen, platelets from NR lacked phospholipase A2 activation, determined by arachidonic acid liberation in the presence of inhibitors to cyclooxygenase and lipoxygenase. In the absence of fibrinogen, aggregation and phospholipase A2 activation were not evident in R and NR. The surface expression of GPIIb/IIIa was markedly decreased in platelets from NR after stimulation by epinephrine, in comparison with those from R. The resting level and epinephrine stimulated increase in cAMP were not significantly different between NR and R. Incubating R platelets with a half saturating dose of yohimbine rendered them insensitive to epinephrine. These results indicated that the impaired platelet aggregation induced by epinephrine was due to the impaired surface exposure of glycoproteins GPIIbIIIa integral to the activation of phospholipase A2, which requires the full and normal occupancy of the alpha 2-adrenergic receptor by epinephrine.

Factors affecting the observation of a synergistic response in platelet aggregation induced by pairs of excitatory agonists

When aggregation is measured as the disappearance of single platelets synergistic interaction between excitatory agonist pairs can be observed using washed platelets in a modified Tyrode's medium or platelet-rich plasma anticoagulated with hirudin; but not using citrated platelet-rich plasma. For aggregation induced by the ADP/adrenaline agonist pair, both the observation of synergistic interaction and the sensitivity of the platelets to these agonists, is a function of extracellular [Ca(2+)]. Synergistic interaction and reduced sensitivity to the individual agonists, especially adrenaline, is observed when extracellular [Ca(2+)] > 100 µM. The data suggest that lower affinity binding of Ca(2+) to the glycoprotein IIb/IIIa complex may modulate platelet sensitivity to these excitatory agonists. The conditions used to resuspend the platelets also influences the nature of the response to the ADP/adrenaline agonist pair and the sensitivity of the platelets to these agonists. A synergistic response and/or reduced sensitivity to ADP is observed on resuspension in modified Tyrode's medium but does not occur on resuspension in citrated plasma or in plasma anticoagulated with hirudin. The factor responsible for enhancing sensitivity, and hence abolishing the synergistic response, is a species of low molecular weight (M(r) less than 25 KDa). It is neither citrate nor Ca(2+).

Studies on the effect of dopamine on the human platelet response

1. The present study investigated the in vitro effect of dopamine on platelet responses in healthy subjects. 2. Dopamine concentrations over 5 mumol/L induced a primary aggregating response and a slight release of alpha-granule proteins, beta-thromboglobulin and platelet factor-4 in all subjects. In 25% of investigated subjects a delayed secondary aggregation was observed with dopamine concentrations over 100 mumol/L. 3. Low dopamine concentrations (5-7.5 nmol/L) increased the platelet sensitivity to other aggregating agents (adenosine diphosphate, collagen and sodium arachidonate). The effect of subaggregating concentrations of serotonin was potentiated by dopamine. 4. The effect of dopamine on platelet responses was prevented by low concentrations of alpha-adrenoceptor antagonists (phentolamine and yohimbine); antagonists of dopamine receptors (haloperidol and domperidone) were able to decrease the extent of the dopamine-induced secondary aggregating wave in the responders, but they failed to prevent the primary aggregation and the effects on platelet response to other aggregating agents. 5. The present data demonstrated that the effects of dopamine on human platelets are mainly mediated by interactions with alpha-adrenoceptors.

Calcium-channel blocking agents verapamil and diltiazem are inhibitors of vasopressin-induced human platelet activation

1. This study investigated the influences of calcium-channel blocking agents verapamil and diltiazem on platelet responses induced by arginine vasopressin (AVP) and lysine vasopressin (LVP). 2. The substances inhibited platelet aggregation induced by both low and high AVP concentrations, LVP and adrenaline plus AVP. IC50 values of each drug are lower than those determined for ADP- and collagen-elicited aggregation. Verapamil and diltiazem also decreased AVP-induced thromboxane B2 synthesis. 3. Other series of experiments showed that the addition of ethyleneglycol-bis-(beta-amino-ethyl ether) N, N'-tetra-acetic acid to platelet-rich plasma samples also prevented the platelet response to vasopressin polypeptides. 4. Our data provide evidence that the effects of verapamil and diltiazem on vasopressin-induced platelet responses may be directly related to inhibition of extracellular calcium entry.

The effect of enteric-coated aspirin on the morning increase in platelet activity

In vitro platelet aggregability to adenosine diphosphate (ADP) and epinephrine increases in the morning, as does the frequency of myocardial infarction. A single-blind, randomized, cross-over study of 15 healthy males was conducted to determine: (1) if other measures of platelet activity show a morning increases and (2) if aspirin eliminates any increases in platelet activity detected. Subjects received 325 mg of enteric-coated aspirin (ECA) or placebo. During placebo therapy,platelet thromboxane A2 production (following collagen stimulation) increased significantly after the subjects got up, as did platelet aggregability to ADP, epinephrine, and collagen. ECA markedly reduced baseline platelet thromboxane A2 production and eliminated the increase after the subjects got up. It also abolished biphasic aggregation in response to epinephrine and ADP (thereby eliminating the morning increase in aggregability to these agents), lengthened collagen lag time, reduced synergistic aggregation to combined agonists, was effective on day 2, and did not alter increases of tissue plasminogen activator that occurred following the subjects' arising. If aspirin prevents myocardial infarction by its antiplatelet action, as seems likely, the preferential reduction of morning infarction observed in the Physicians' Health Study, and the demonstration that aspirin eliminates the morning increase in platelet activity, suggest that the morning increase in myocardial infarction is due in part to a concurrent relatively modest increase in platelet activity.

Platelet responses promoted by the activation of protein kinase C or the increase of cytosolic Ca2+ are potentiated by adrenaline. Effects of cAMP and staurosporine

We studied the action of the alpha 2 adrenergic agonist adrenaline on the platelet responses evoked by the activation of protein kinase C or by the ionophore induced increase of cytosolic Ca2+. Both the phorbol ester and ionomycin-induced aggregation are strongly potentiated by adrenaline which per se does not behave as an activating agonist. The potentiation by adrenaline is observed both when added before and after the aggregating agent; in the latter case the effect increases on increasing the delay of adrenaline addition. Adrenaline also reverses the inhibition by cAMP of the PMA (or ionomycin) induced aggregation. It also has a strong potentiating effect (over 100%) on the phorbol ester induced ATP secretion and a weaker effect on the secretion induced by ionomycin. The effect on secretion is visible only when adrenaline is added prior to the stimulus. The inhibition by cAMP of the PMA or ionomycin induced secretion is also counteracted by adrenaline. In no case adrenaline modifies the pattern of platelet phosphoproteins. Ionomycin induces some platelet aggregation also in the presence of the protein kinase inhibitor staurosporine; also this phosphoprotein independent aggregation is strongly stimulated by adrenaline.

Cooperativity between platelet-activating factor and collagen in aggregation of bovine platelets III

In cooperative aggregation of bovine platelets induced by simultaneous addition of PAF and collagen at subthreshold concentrations the following observations were made. (i) Formation of phosphatidic acid and arachidonic acid metabolites, which characterize PAF and collagen alone-induced aggregation, respectively, was observed very obviously. (ii) A thromboxane antagonist did not completely block the cooperative aggregation. The extent of residual aggregation activity was dependent on concentration of collagen used in the simultaneous administration with PAF. These results suggest that both signal transduction pathways activated by PAF and collagen alone at high concentrations are attained by simultaneous addition of both agonists at subthreshold concentrations through unknown mechanisms.

Desensitization of the platelet aggregation response to adrenaline during insulin-induced hypoglycaemia in man

The aim of the present study was to investigate the influence of insulin-induced hypoglycaemia on platelet sensitivity to adrenaline and non-adrenergic agonists in man. Twenty-five healthy male subjects volunteered for the study. To evaluate the effects on platelets of different insulin-adrenaline interrelationships, two experimental models were used. In the first, hypoglycaemia was induced by a 60-min IV infusion of human insulin at the rate of 64 mU m-2 min-1, whereas in the other the same insulin dose was administered as an IV bolus (3.84 U m-2). Throughout the studies, plasma glucose, insulin, and adrenaline were measured together with platelet sensitivity to adrenaline, ADP, platelet activating factor, collagen, and sodium arachidonate. In both studies, hypoglycaemia induced a reduction of platelet sensitivity to adrenaline (p = 0.006 in infusion and p = 0.045 in injection study). In particular, maximal aggregation to adrenaline fell from 47.9 +/- 9.9 (+/- SE) to 31.1 +/- 11.3% at the hypoglycaemic nadir in the infusion study, and from 64.6 +/- 8.2 to 34.6 +/- 10.3% at the hypoglycaemic nadir in the injection study. In the injection study an increase of platelet sensitivity to ADP (p = 0.05), platelet activating factor (p = 0.018), sodium arachidonate (p = 0.035), and collagen (p = 0.027) was also found, in agreement with observations already published using the infusion protocol. Thus, insulin-induced hypoglycaemia increases platelet sensitivity to non-adrenergic agonists and decreases platelet response to adrenaline.

Simultaneous administration of thromboxane A2- and serotonin S2-receptor antagonists markedly enhances thrombolysis and prevents or delays reocclusion after tissue-type plasminogen activator in a canine model of coronary thrombosis

Dynamic changes of the thrombus after its formation due to platelet activation may affect the speed of thrombolysis. In the present study, we wanted to evaluate the role played by thromboxane A2 (TXA2) and serotonin (5HT) in mediating platelet activation during lysis of intracoronary thrombi with human recombinant tissue-type plasminogen activator (t-PA). Coronary thrombi were induced in 26 anesthetized, open-chest dogs by inserting a copper coil into the left anterior descending coronary artery (LAD). LAD blood flow was monitored throughout the experiment by means of a Doppler flow probe placed proximally to the coil. Presence of the thrombus was documented for 30 minutes. The dogs were then assigned to one of four groups as follows: group 1 dogs (n = 8), serving as controls, received a bolus of heparin (200 units/kg) and a bolus of t-PA (80 micrograms/kg) followed by a continuous infusion (8 micrograms/kg/min) for up to 90 minutes or until reperfusion was achieved; group 2 dogs (n = 10) received, immediately before heparin and t-PA, an intravenous bolus of SQ29548 (SQ) (0.4 mg/kg, a selective TXA2-receptor antagonist) and LY53857 (LY) (0.2 mg/kg, a selective serotonin S2-receptor antagonist); group 3 dogs (n = 7) received, before heparin and t-PA, an intravenous bolus of SQ alone (0.4 mg/kg); and group 4 dogs (n = 7) received, before heparin and t-PA, an intravenous bolus of LY alone (0.2 mg/kg). After thrombolysis, all dogs were monitored for 90 minutes or until a persistent reocclusion occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Epinephrine potentiates calcium mobilization and activation of protein kinases in platelets stimulated by ADP through a mechanism unrelated to phospholipase C

ADP, added to suspensions of aspirinized 32P-prelabelled washed platelets, induced reversible platelet aggregation, the rapid elevation of cytosolic Ca2+ (maximum at 2 s), 20 kDa myosin light chain phosphorylation (maximum faster than 3 s), 40 kDa protein phosphorylation (maximum at 3-10 s) and phosphatidic acid formation (maximum at 30 s). Prior addition of epinephrine potentiated platelet aggregation, cytosolic Ca2(+)-elevation, 20 and 40 kDa protein phosphorylation evoked by ADP, but it did not enhance phosphatidic acid formation induced by ADP. The potentiating effect of epinephrine on aggregation, cytosolic Ca2(+)-increase and 20 and 40 kDa protein phosphorylation induced by ADP was also observed in the presence of EGTA. Ethylisopropylamiloride, an inhibitor of Na+/H(+)-exchange, did not affect the potentiation of ADP-induced platelet aggregation by epinephrine. We conclude that epinephrine primes platelets to increase Ca2(+)-influx and Ca2(+)-mobilization in response to ADP. The potentiation of cytosolic Ca2(+)-elevation by epinephrine leads to further stimulation of myosin light chain phosphorylation and protein kinase C activation and ultimately to enhanced platelet aggregation. These effects of epinephrine do not seem to take place at the level of phospholipase C.

Exercise-induced changes in platelet aggregation; a comparison of whole blood and platelet rich plasma techniques

Studies have been performed to assess the effect of exercise on spontaneous platelet aggregation in shaken whole blood, and on agonist-induced platelet aggregation in whole blood and platelet rich plasma (PRP). Spontaneous platelet aggregation in shaken whole blood was increased following exercise compared to pre-exercise values. The increase in spontaneous aggregation after exercise correlated inversely with the increase in white cell count in whole blood. Platelet sensitivity in whole blood to adrenaline, collagen and adenosine diphosphate (ADP) was increased following exercise. Changes in platelet sensitivity to adrenaline following exercise correlated with increases in plasma noradrenaline levels but not with changes in blood cell counts. In PRP, platelet sensitivity to ADP and to collagen was increased following exercise when the pre and post-exercise PRP platelet counts were not corrected to allow for the increase in platelet count which occurred with exercise. When the PRP platelet counts were corrected, no changes in platelet sensitivity to any agonist after exercise were observed.

Specific correction of impaired acid hydrolase secretion in storage pool-deficient platelets by adenosine diphosphate

Storage pool-deficient (SPD) platelets, which have decreased amounts of dense-granule and/or alpha-granule constituents, contain normal amounts of lysosomal acid hydrolases, but in some cases exhibit impaired secretion of these enzymes. We examined this impaired secretion response in SPD patients with varying extents of granule deficiencies, and determined the effects of added dense-granule constituents. Acid hydrolase secretion was impaired in patients with severe dense-granule deficiencies, but not in patients with lesser dense-granule deficiencies, including those with alpha-granule deficiencies as well. When dense-granule constituents (ADP, ATP, serotonin, Ca+2, pyrophosphate) were added to gel-filtered platelets, ADP, but none of the other constituents, completely corrected the impairment of thrombin and A23187-induced secretion in SPD platelets. The concentration of ADP required to normalize thrombin-induced secretion varied markedly, from 0.01 to 10 microM, among the individual patients. Fixation of platelets with formaldehyde before centrifugation did not prevent the enhancement of secretion by ADP. Excess ATP, which acts as a specific antagonist of ADP-mediated responses, completely blocked this enhancement of secretion in SPD platelets by ADP, and partially inhibited acid hydrolase secretion induced by low, but not high, concentrations of thrombin in normal platelets as well. Treatment of normal platelets with acetylsalicylic acid in vivo, but not in vitro, produced an impairment of acid hydrolase secretion similar in extent to that in SPD platelets, but which could not be completely corrected by added ADP. One possible explanation of these results is that the impairment of acid hydrolase secretion may be secondary to the dense-granule deficiency in SPD platelets, and that secreted ADP may potentiate the lysosomal secretion response in normal platelets as well.

The Na+/H+ antiporter is not involved in potentiation of thrombin-induced responses by epinephrine

Stimulation of platelets with thrombin, ADP and epinephrine has recently been shown to activate a Na+/H+ antiporter, with a resulting alkalinization of the cytoplasm. Unlike thrombin, however, epinephrine is incapable of directly activating phospholipase C, but is well known to potentiate the effects of thrombin on this enzyme and other subsequent steps of platelet activation. Therefore, we have studied the involvement of the Na+/H+ antiporter in this aspect of epinephrine action to see whether alkalinization of platelet cytosol could be a requirement for agonists to stimulate inositol phospholipid hydrolysis and mobilize cellular Ca2+ stores. Alpha-thrombin induced the rapid formation of inositol trisphosphate with a parallel mobilization of intracellular Ca2+ stores. Epinephrine alone had no effect on either of these parameters. The response to thrombin desensitized over a period of minutes, and after this had occurred, epinephrine was able to activate phospholipase C and induce the release of intracellular Ca2+. This showed that epinephrine was able to recouple thrombin receptors to phospholipase C, and this appeared to be mediated by the same mechanism which is involved in potentiation by epinephrine of thrombin-stimulation of phospholipase C. These effects of epinephrine were not altered by inhibition of the Na+/H+ antiporter with ethylisopropylamiloride or by use of the Na+/H+ ionophore, monensin. We conclude that epinephrine potentiates thrombin-induced responses by a mechanism which is unrelated to its effects on the Na+/H+ antiporter, and this is not a requirement for thrombin-induced phospholipase C activation.

The synergistic effect of serotonin and epinephrine on the human platelet at the level of signal transduction

Simultaneous addition to platelets of submaximal amounts of excitatory agonists acts synergistically in provoking secretory and aggregatory responses. By measuring changes in intracellular free Ca2+ concentration, inositol phospholipid metabolism and protein phosphorylation, we verified whether synergism could be evidenced at the level of signal transduction. Challenging platelets with epinephrine only induced minor changes on the measured parameters. However, when added together with serotonin, epinephrine amplified mobilisation of intracellular Ca2+, PA formation, PIP formation, protein kinase C and myosin light chain kinase activity as compared to the alterations induced by serotonin alone. It is concluded that synergistic effects on simultaneous addition of serotonin and epinephrine might originate at the level of signal transduction.

Synergism of platelet-aggregating agents. Role of elevation of cytoplasmic calcium

When a pair of platelet agonists, each in subthreshold concentration, is added together or in sequence to a platelet suspension, the platelet response is enhanced. Addition of two agonists to platelets loaded with aequorin also enhanced the observed rise in cytoplasmic ionized calcium ([Ca2+]i) in response to the second agonist if the agonists were added within 20 s of each other. Enhancement of aggregation and secretion required that an increase in [Ca2+]i (as indicated by aequorin but not necessarily indo-1) followed the first agonist, but not that the [Ca2+]i remain elevated until addition of the second agonist. Enhancement was not prevented by aspirin, ADP scavengers, or chelators of extracellular Ca2+. We conclude that a rise in [Ca2+]i induced by a first agonist "primes" platelets for an augmented functional response to a second agonist, which is not, however, determined by the [Ca2+]i at the time of addition of the second agonist.

Activation of human platelets through gp140, the C3d/EBV receptor (CR2)

gp140, the C3d/EBV receptor (CR2), previously isolated and characterized from human B lymphocytes, was identified on human platelets: by measuring the specific binding of either polyclonal anti-gp140 IgG and monoclonal anti-C3d/EBVR antibodies, as OKB-7 and HB-5, or human C3d; by isolating gp140 from solubilized platelet components with polyclonal anti-gp140 IgG or monoclonal OKB-7, using immunoprecipitation and electro-immunoblotting assays; by inducing specific activation of human platelets. Cross-linking of this receptor by polyclonal anti-gp140 IgG induced aggregation of human platelets and stimulated ATP release. Absence of lactate dehydrogenase release and inhibition by EDTA and prostacyclin of anti-gp140-induced aggregation, support strongly active aggregation and absence of lysis. Platelet aggregation by anti-gp140 required metabolic activities and was modulated by fibrinogen, paf-acether or thrombin. OKB-7 triggered human platelet aggregation when cross-linked by anti-mouse second-step antibodies. In the same way, platelet activation by C3d fragment was detected, in presence of fibrinogen, only when C3d was cross-linked on the cell surface by anti-C3d F(ab')2 fragments.

Possible mechanisms of the potentiation of blood-platelet activation by adrenaline

Vasopressin and adrenaline in combination exert synergistic effects on platelet activity. This study investigated the effects of sub-threshold concentrations of adrenaline (0.1-1 microM) on vasopressin (10 nM-1 microM)-induced platelet aggregation, ATP secretion, elevation of cytosolic free Ca2+ concentration ([Ca2+]i) and hydrolysis of inositol phospholipids, monitored as [32P]phosphatidic acid formation. Potentiation of vasopressin-induced aggregation and ATP secretion by adrenaline was accompanied by enhanced elevation of [Ca2+]i and [32P]phosphatidic acid formation. The stimulatory effects of adrenaline on vasopressin-induced platelet activation were mimicked by the combination of the Ca2+ ionophore, ionomycin, and the protein kinase C activator, phorbol 12-myristate 13-acetate, but not by either of these agents alone. These results suggest that the potentiation of vasopressin-induced platelet activation by adrenaline is mediated via enhancement of inositol phospholipid hydrolysis and elevation of [Ca2+]i.

Activation of phospholipases A and C in human platelets exposed to epinephrine: role of glycoproteins IIb/IIIa and dual role of epinephrine

Human platelets stimulated by epinephrine undergo enhanced turnover of phosphatidylinositol 4,5-bisphosphate, accumulate inositol trisphosphate, diacylglycerol, and phosphatidic acid, and phosphorylate a 47-kDa protein. All of these phenomena indicate stimulation of phospholipase C. These responses are blocked completely by inhibitors of alpha 2-adrenergic receptors (yohimbine), cyclooxygenase (aspirin or indomethacin), phospholipase A [2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (ONO-RS-082)], Na+/H+ exchange [ethylisopropylamiloride (EIPA)], fibrinogen binding to glycoprotein IIb/IIIa (antibody A2A9), Ca2+/Mg+ binding (EDTA), or removal of fibrinogen. Epinephrine evokes (i) an increased turnover of ester-linked arachidonic acid in aspirin treated platelets that is inhibited by ONO-RS-082, EDTA, yohimbine, or the absence of fibrinogen and (ii) a rapid cytoplasmic alkalinization that is inhibited partially by blockage of cyclooxygenase activity and completely by A2A9 or EIPA. In contrast, when incubated with subaggregatory concentrations of the prostaglandin H2/thromboxane A2 analogue [(15S)-hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic acid (U46619) and epinephrine, aspirin-treated platelets show a potentiation of phospholipase C activation that is unaffected by the above inhibitors. We propose that epinephrine, in promoting exposure of glycoprotein IIb/IIIa sites for fibrinogen binding, leads to a cytoplasmic alkalinization, which, in conjunction with local shifts in Ca2+, promotes low-level activation of phospholipase A. The resulting free arachidonic acid is converted to cyclooxygenase products, which, potentiated by epinephrine, activate phospholipase C. This further amplifies the initial stimulatory response.

Synergistic responses in human platelets. Comparison between aggregation, secretion and cytosolic Ca2+ concentration

Synergistic interaction between ADP, adrenaline, 5-hydroxytryptamine (5HT) and [8-arginine]vasopressin is not observed for the aggregatory response of aspirin-treated human platelets when this response is estimated directly from the decrease in the number of single platelets in the suspension. This finding is in marked contrast with prior reports of synergistic interaction between these agonists when the rate and extent of the aggregometer response is estimated from the increase in the light transmittance of the suspension, using a platelet aggregometer. We propose that the apparent synergistic response detected using the aggregometer results from the inability of this instrument to respond during the initial phase of aggregation. Significant synergistic interaction is observed for the increase in cytosolic [Ca2+] induced by addition of the ADP/5HT and, to a lesser extent, of the ADP/vasopressin agonist pairs as compared with that caused by addition of the individual agonists. This effect is not, however, typical of the system since increases in cytosolic [Ca2+] induced by addition of the ADP/thrombin or 5HT/vasopressin agonist pairs are no greater than the sum of the responses to these agonists added separately. Addition of collagen prior to ADP or 11,9-epoxymethanoprostaglandin H2 (U46619) fails to enhance the increase in cytosolic [Ca2+] induced by these latter agonists. Adrenaline, when added prior to non-saturating concentrations of U46619, thrombin, vasopressin or ADP, significantly enhances the increase in cytosolic [Ca2+] induced by these agonists in platelets suspended in media containing less than 0.1 microM or 1 mM Ca2+. However, adrenaline fails to enhance the increase in cytosolic [Ca2+] induced by the divalent cation ionophore, ionomycin. Enhancement by adrenaline of Ca2+ influx induced by U46619, thrombin and ADP has been shown by using Mn2+ as probe. Adrenaline also enhances the extent of [3H]5HT secretion induced by U46619, thrombin and vasopressin but fails to increase that induced by ADP in this aspirin-treated preparation. These results are in part consistent with the postulate that adrenaline, acting via an alpha 2-adrenoceptor, modulates receptor--phospholipase-C coupling. However, such modulation does not appear to involve inhibition of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of ADP and epinephrine on macaque and human platelets. Implications for studies on human atherosclerosis

The ranges for near-threshold ADP concentrations for the aggregation of macaque and human citrated platelets overlapped. The minimum concentrations of epinephrine, 0.05 microM to 1.0 microM, that at least doubled the aggregation response at threshold ADP concentrations were comparable for macaque and human citrated platelets. Epinephrine (1.0 microM to 10 mM) alone never aggregated macaque citrated platelets. Biphasic aggregation occurred with both macaque and human citrated platelets. The addition of heparin to a final concentration of 2.2 units/ml had no effect on the threshold ADP concentrations or the sensitivity of macaque or human citrated platelets to epinephrine. One microM phentolamine eliminated the potentiating effect of 1 microM epinephrine on ADP-induced aggregation of macaque and human citrated platelets. The threshold concentrations of ADP for macaque platelets were sharply reduced when heparin was used as an anticoagulant rather than citrate. However, epinephrine induced a similar increase in aggregability with both citrated and heparinized platelets, 0.55 +/- 0.09 SEM% and 0.44 +/- 0.09 SEM%, respectively. These data indicate that macaque and human platelets behave in a similar manner in response to ADP and that epinephrine potentiates the ADP-induced aggregation of macaque and human platelets equally well.

Human platelet arginine vasopressin

Approximately 90% of arginine vasopressin circulating in blood is associated with the platelets. Studies in six patients with cranial diabetes insipidus indicate that the platelet vasopressin originates in the hypothalamus and is taken up from plasma. Extracted platelet vasopressin is chromatographically identical to the posterior pituitary peptide and biologically active. Infusion of low-doses of exogenous vasopressin into 10 normal male subjects caused a marked rise in both plasma and platelet hormone concentrations. Stimulation of endogenous vasopressin release by hypertonic saline (0.51 mol/l) infusion for 135 min in five male subjects also caused a rise in platelet and plasma peptide concentrations. In seven male smokers, the deep inhalation of two consecutively smoked unfiltered cigarettes caused a significant rise in both platelet and plasma vasopressin. Baseline platelet vasopressin concentrations in the smokers were higher than in the non-smoking males even after 10 hours abstinence.

Physiological concentrations of epinephrine potentiate thromboxane A2 release from platelets in the isolated rat heart

The isolated rat heart perfused with washed platelets was used as a model to examine platelet-vessel wall interactions. Release of prostacyclin and thromboxane A2 was measured, using a cascade of smooth muscle bioassay tissues or radioimmunoassays of the stable hydration products. In hearts perfused with rabbit or human platelets, injection of sodium arachidonate caused release of both prostacyclin and thromboxane A2. In hearts perfused with aspirin-pretreated platelets, arachidonate released only prostacyclin indicating that thromboxane A2 originates largely in the platelets. Infusion of epinephrine (0.6-6 nmol/liter) through the heart potentiated arachidonate-induced release of thromboxane A2. Similar potentiation of thromboxane A2 release was observed in rat hearts perfused with either rabbit or human platelets, and in rabbit hearts perfused with rabbit platelets. In contrast, when rabbit platelets were infused through an incubation coil of tubing in place of the heart, epinephrine did not alter thromboxane A2 release. There was no significant loss of rabbit platelets on perfusion through rat hearts, and no aggregates were observed in the effluent either before or immediately after arachidonate injections, even in the presence of epinephrine. Thus, potentiation of thromboxane A2 production could not be explained by aggregation. However, it is clear from these studies that physiological concentrations of epinephrine can potentiate thromboxane A2 release from platelets when they are stimulated by arachidonic acid within the heart. This could result from a redirection of arachidonate metabolism to a local potentiating factor in the vessel wall. Potentiation of thromboxane A2 release might contribute to myocardial ischemia associated with platelet activation.

Effect on human platelets of catecholamines at levels achieved in the circulation

Epinephrine at concentrations varying between 3.3 and 12.5 nM had no effect on blood platelets when added alone, but augmented the in vitro platelet response to collagen and thrombin. Both aggregation and secretion responses were enhanced. Norepinephrine produced similar effects but was 50-60% less active than epinephrine. The minimum concentration of epinephrine or norepinephrine to achieve potentiation of platelet responses was even lower in the presence of 5-hydroxy- tryptamine. In contrast to the effects observed with higher concentrations of catecholamines, the synergistic interaction of these low concentrations of catecholamines with other agonists was not transient. The augmented response to catecholamines was mediated by platelet alpha 2-adrenoceptors. The response was inhibited by aspirin indicating that metabolism of arachidonic acid contributes to the synergy between low concentrations of catecholamines and other agonists. These studies show that the levels of the hormones epinephrine and norepinephrine obtained in circulating blood in humans, can be sufficient to enhance platelet responses. The action of catecholamines on platelets may be important in hemostasis and could provide an explanation for the association between certain risk factors and cardiovascular disease.

Systemic and transcardiac platelet activity in acute myocardial infarction in man: resistance to prostacyclin

There is increasing evidence that platelets play an important role in the pathogenesis of acute ischemic heart disease. Therefore an understanding of factors that influence platelet performance is important. This study was undertaken (1) to characterize during evolving myocardial infarction platelet activity in the peripheral circulation and across the ischemic/infarcting myocardial compartment, the locus of presumed platelet hyperactivity, and (2) to evaluate the effects of prostacyclin (PGI2), a most potent antiplatelet agent and vasodilator. A total of 59 patients with evolving myocardial infarction were studied. Twenty-two patients were instrumented with arterial and coronary sinus catheters and received intravenous infusion of PGI2, 13 +/- 4.5 ng/kg/min (mean +/- SD), for 90 min. In 15 patients with anterior myocardial infarction, transcardiac platelet function and response to PGI2 were studied. Plasma levels of beta-thromboglobulin (beta-TG) and of thromboxane B2 (TxB2), in vivo measures of platelet activity, were elevated three- and 10-fold. 6-Keto-prostaglandin F 1 alpha, the stable end product of PGI2, was less than 10 pg/ml, reflecting a leftward shift of the TxB2/PGI2 ratio. Platelets circulating during evolving myocardial infarction ("ischemic platelets") were hyperaggregable in response to ADP and relatively resistant to PGI2, both in vivo and in vitro. Concentrations of platelet cyclic AMP and the cyclic AMP response to PGI2 were diminished. The platelet hyperreactivity, expressed by plasma beta-TG, platelet aggregation, and PGI2-induced inhibition of aggregation, was most intense early during infarct evolution and decreased with time. The increased platelet performance resulted in "platelet fatigue," indicated by decreased contents of beta-TG of the ischemic platelet and decreased TxA2 production in response to collagen. However, the ischemic platelet produced twice normal TxA2 in response to arachidonic acid (stimulus and substrate), demonstrating a heightened metabolic capacity. TxA2 was produced across the ischemic/infarcting compartment in 10 of 15 patients with anterior myocardial infarction. The antiplatelet effect of PGI2 was greatly diminished. In summary, the data define an abnormal pattern of platelet behavior during evolving myocardial infarction, characterized by a proaggregatory environment, heightened platelet reactivity in both the peripheral and coronary circulation, and relative resistance to PGI2. The clinical consequences of the data are that the patient in the acute phase of myocardial infarction may benefit from suppression of platelet function and requires significantly greater doses of PGI2 than normal subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Synergistic effects of platelet-activating factor and other platelet agonists in human platelet aggregation and release: the role of ADP and products of the cyclooxygenase pathway

The synergistic effects of platelet-activating factor (PAF) with ADP, collagen, thrombin, A23187, adrenaline, sodium arachidonate and ristocetin in human platelet aggregation and beta-thromboglobulin (beta-TG) release were investigated in citrated platelet-rich plasma (PRP). Synergism in both aggregation and release was present with all agonists except ristocetin. Upon oral intake of aspirin (ASA) the PAF-induced irreversible aggregation as well as the synergistic irreversible aggregation became reversible. Both prior to and after ASA ingestion ADP removal by creatine phosphate/creatine phosphokinase (CP/CPK) resulted in a reduced, reversible platelet aggregation induced by PAF alone or in combination with the other agonists. The ADP-removal and ASA-ingestion also strongly inhibited the beta-TG release. The synergistic aggregation and release were also inhibited by ASA and indomethacin in vitro as well as by the competitive ADP-inhibitor ATP. It is concluded that not only the activation of human platelets by low doses of PAF itself, but also the synergism of PAF and other platelet agonists is highly dependent upon ADP and products of the cyclooxygenase pathway.

Agonist-specific refractory states of bovine platelets-disappearance of synergism in aggregation

The specificity of refractory state of bovine platelets developed by various agonists was examined. When bovine unwashed platelets were preincubated with 5-hydroxytryptamine (5HT) (or ADP), they became non responding to that agonist (refractory) but they responded quite normally to collagen, thrombin and ADP (or 5HT). When platelets became refractory to 5HT, the synergistic aggregation normally obtained with this agonist plus ADP was lost, but that with ADP plus thrombin was not changed. Platelets made refractory to ADP (or 5HT) after the aggregation and disaggregation cycle by the agonist responded normally to 5HT (or ADP). These results strongly indicate that platelets become refractory to one agonist without impairing the sensitivity to any other agonists.

Synergistic interaction and platelet inhibitory agonists

Marked synergistic interaction is observed between two platelet excitatory agonists neither of which cause inhibition of adenylate cyclase. No synergistic interaction is observed between the platelet inhibitory agonists PGI2, PGD2 and adenosine nor between ADP and adrenaline when acting as inhibitory agonists for adenylate cyclase. The studies suggest that synergistic interaction between platelet agonists may be restricted to the excitatory responses.

Exercise-induced myocardial ischemia in patients with coronary artery disease: lack of evidence for platelet activation or fibrin formation in peripheral venous blood

The hypothesis that exercise-induced myocardial ischemia is associated with abnormal platelet activation and fibrin formation or dissolution was tested in patients with coronary artery disease undergoing upright bicycle stress testing. In vivo platelet activation was assessed by radioimmunoassay of platelet factor 4, beta-thrombo-globulin and thromboxane B2. In vivo fibrin formation was assessed by radioimmunoassay of fibrinopeptide A, and fibrinolysis was assessed by radioimmunoassay of thrombin-increasable fibrinopeptide B which reflects plasmin cleavage of fibrin I. Peripheral venous concentrations of these substances were measured in 10 normal subjects and 13 patients with coronary artery disease at rest and during symptom-limited peak exercise. Platelet factor 4, beta-thromboglobulin and thromboxane B2 concentrations were correlated with rest and exercise catecholamine concentrations to determine if exercise-induced elevation of norepinephrine and epinephrine enhances platelet activation. Left ventricular end-diastolic and end-systolic volumes, ejection fraction and segmental wall motion were measured at rest and during peak exercise by first pass radionuclide angiography. All patients with coronary artery disease had documented exercise-induced myocardial ischemia manifested by angina pectoris, ischemic electrocardiographic changes, left ventricular segmental dyssynergy and a reduction in ejection fraction. Rest and peak exercise plasma concentrations were not significantly different for platelet factor 4, beta-thromboglobulin, thromboxane B2, fibrinopeptide A and thrombin-increasable fibrinopeptide B. Peripheral venous concentrations of norepinephrine and epinephrine increased significantly (p less than 0.001) in both groups of patients. The elevated catecholamine levels did not lead to detectable platelet activation. This study demonstrates that enhanced platelet activation, thromboxane release and fibrin formation or dissolution are not detectable in peripheral venous blood of patients with coronary disease during exercise-induced myocardial ischemia.

The effect of native and single stranded DNA on the platelet release reaction. Enhancement of aggregated IgG-induced serotonin release

Native (n) but not single stranded (ss) DNA was found to induce release of 3H-serotonin (5-HT) from platelets of the majority of normal individuals. However, ssDNA markedly enhanced 5-HT release induced by heat-aggregated IgG (aggIgG), while less enhancement was seen using nDNA. Similar enhancement was produced by polyinosinic acid but not by polyinosinic:polycytidylic acid. The ability of ssDNA to potentiate aggIgG-induced 5-HT release seemed specific for aggIgG, since no effect on ADP or epinephrine-induced release was observed and thrombin-induced release was inhibited. In contrast, nDNA in high concentrations (100 micrograms/ml) potentiated ADP, epinephrine, and thrombin-induced 5-HT release. These results suggest that ss-and nDNA may interact with platelets by different mechanisms and provide a means by which DNA, released at sites of tissue injury, could modulate the role of platelets in the inflammatory response. The ability of DNA to enhance the aggIgG-induced platelet release reaction may be important in immune complex diseases such as systemic lupus erythematosus.

Technical considerations for platelet aggregation and related problems

Platelet aggregation generally is ordered by the physician to evaluate platelet function in hemorrhagic or thrombotic disorders. Malfunction of the platelet may be the result of an intrinsic congenital defect or an acquired problem induced by drugs or certain circulating plasma factors. It is necessary to obtain information from the patient with respect to family history, drug ingestion, physical or mental stress. In addition, other laboratory studies should be obtained to rule out general coagulation disorders affecting the plasma factors. A bleeding time will be helpful in establishing the severity of any platelet dysfunction. Technical considerations with regard to the preparation of the samples are of primary importance in determining platelet aggregation. Aggregating studies require the use of a variety of binding agents. (Studies on shape change, adhesion of platelets, release of platelet granule substance, and or lysis with extrusion of cytoplasmic constituents may be helpful in certain cases.) Instrumentation for platelet aggregation presently is available in many hospitals. The technical factors to be considered for routine aggregation studies include the type and strength of anticoagulant, centrifugation technique used in preparing the platelet-rich and platelet-poor plasma, platelet concentration, time of storage of the sample after venipuncture and after centrifugation, temperature, and the mixing of the sample. In general, critical concentrations of each reagent should be employed to improve the discrimination capability of the assay. Small differences in response may be obliterated by using excessive concentrations of a given reagent. Comparison in response to the test platelets with control platelets is best done at the same time by performing the aggregation in a dual instrument so that handling procedures will be identical and artifactual differences eliminated.

Effects of N-acetylglucosamine and platelet inhibitors on the synergistic interaction of platelets and aggregating agents in the presence of wheat germ agglutinin

Low concentrations of wheat germ agglutinin (4 micrograms/ml) have been shown to act synergistically to induce platelet aggregation with epinephrine, collagen, arachidonate and ionophore A23187. Aggregation ceased on the addition of the haptenic sugar N-acetylglucosamine at any time following the onset of aggregation with these agonists and a small degree of disaggregation was observed during the reversible first wave with the biphasic aggregating agents epinephrine and ADP. Cyclooxygenase inhibitors such as indomethacin and aspirin blocked the second wave of aggregation with the biphasic aggregating agents epinephrine and ADP but a synergistic response continued to be shown with the first wave in the presence of these inhibitors. Release of [14C]serotonin and the mobilization of [3H]arachidonate by epinephrine and collagen were markedly stimulated in the presence of wheat germ agglutinin but there was no increase of either radiolabel in the case of ADP. Platelet shape change, but not aggregation, occurred with low levels of wheat germ agglutinin and the synergistic response with ADP, collagen or ionophore A23187 occurred without further shape change. Wheat germ agglutinin did not affect the basal or stimulated levels of cyclic AMP. The membrane fluidity of platelets was not affected by the lectin or by thrombin as shown by the lack of change in fluorescence polarization with diphenylhexatriene. It is suggested that the binding of wheat germ agglutinin to the platelet surface induces platelet activation by mechanisms similar to those of other agonists and that it may affect the distribution of membrane-bound Ca2+ by a reversible perturbation of the platelet membrane.

Synergistic responses and receptor occupancy in rabbit blood platelets

Several observations indicate that simultaneous receptor occupancy is necessary for generation of a synergistic response of rabbit platelets to two excitatory agonists. First, an aggregatory response to adrenaline induced by prior addition of 5HT reverses rapidly unless stabilised by inhibition of 5HT uptake. The extent of response correlates with the extracellular 5HT concentration. Second, addition of an ADPase following adrenaline enhances the rate of disaggregation if the response to this agonist has been induced by prior addition of ADP. Third, disaggregation is induced, or enhanced, if an antagonist selective to the inducing agonist is added following completion of the induced aggregatory response. These data suggest marked lability in the mechanisms responsible for generation of synergistic responses.