Acetylsalicylic acid inhibits ATP diphosphohydrolase activity by platelets from adult rats

BACKGROUND AND METHODS

The in vitro effect of the nonsteroidal anti-inflammatory drug, acetylsalicylic acid (ASA), on the extracellular adenine nucleotide hydrolysis by intact rat blood platelets was studied.

RESULTS

Our results demonstrate that aspirin, at final concentrations of 2.0 and 3.0 mM, inhibits ATP extracellular hydrolysis in vitro by approximately 17% and 21%, respectively. Aspirin, at a final concentration of 3.0 mM, also inhibited in vitro extracellular ADP hydrolysis by approximately 41%. The same concentrations of this drug, however, did not alter AMP hydrolysis by intact rat blood platelets under similar assay conditions. The kinetic analysis demonstrated that the inhibition of ADP and ATP hydrolysis by aspirin in rat platelets is of the uncompetitive type.

CONCLUSION

In this study, we demonstrated an inhibitory effect of ASA upon E-NTPDase 3 activity of platelets from adult rats and discussed the significance of our findings.

ADP secretion and subsequent P2Y12 receptor signalling play a crucial role in thrombin-induced ERK2 activation in human platelets

Stimulating human platelets with thrombin induces the activation of the extracellular signal-regulated kinase 2 (ERK2). We demonstrate that this effect is highly dependent on ADP secretion and P2Y12 receptor signalling. AR-C69931MX (10 microM), a specific antagonist of the Gi-coupled P2Y12 ADP receptor, inhibits ERK2 activation induced by thrombin. Antagonists of the Gq-coupled P2Y1 ADP receptor, A3P5P (500 microM) and MRS2179 (100 microM), have no effect. ADP and its more potent analogue 2-methylthio-ADP alone (both up to 100 microM) do not induce ERK2 activation. Furthermore, we show that the inhibitory effect of AR-C69931MX on ERK2 activation induced by 0.1 U/ml thrombin as well as on platelet aggregation can be bypassed by epinephrine (1 and 10 microM), whereas epinephrine alone has no effect. Epinephrine acts on platelets mainly via alpha(2A)-adrenergic receptors, which, like P2Y12 receptors, couple to inhibitory G proteins. In addition, 2-methylthio-ADP as well as epinephrine provoke ERK2 activation at a thrombin concentration that alone has no detectable effect (0.05 U/ml). Thromboxane A2 (TXA2), which, like ADP, is released by activated platelets, acts as a positive feedback mediator. Stimulating the Gq-coupled TXA2 -receptor with U46619 (10 microM), which leads to ADP secretion and P2Y12 receptor-dependent platelet aggregation, also induces P2Y12-related ERK2 activation. The inhibition of U46619-induced ERK2 activation and platelet aggregation by AR-C69931MX are also rescued by epinephrine. Pretreatment with aspirin inhibits ERK2 activation induced by 0.1 U/ml thrombin, but has no effect at high concentrations of thrombin. The combination of U46619 and thrombin, at concentrations which alone have no effect, provokes ERK2 activation, suggesting that thrombin and released TXA2 act synergistically. Our data indicate that both primary signalling through Gq, which evokes ADP secretion, as well as subsequent coupling via Gi by the P2Y12 receptor are required for ERK2 activation.

Modulation of adenine nucleotide concentrations in human plasma by erythrocytes and endothelial cells

The regulation of plasma concentrations of adenine nucleotides is unsettled. We tested the possibility of extracellular adenosine triphosphate (ATP) production from adenosine diphosphate (ADP) at physiological low concentrations by erythrocytes and endothelial cells. Filtered erythrocytes and human umbilical vein endothelial cells (HUVEC) were incubated for 15 to 120 s with ADP (10 microM), supplemented with 3H-ADP (2.85 nM) or 14C-ADP (54.6 nM). Enzymatic conversion of ADP to ATP was detected by recovery of the radioactive label in the ATP fraction. ATP was measured in the supernatant using high performance liquid chromatography (HPLC) separation, scintillation techniques, and luminometry. Using etheno (epsilon)-labeled ADP (10 microM), the extracellular localization of the conversion was further corroborated. Following ADP application in plasma, no radioactivity was detected in the ATP fraction. However, in erythrocyte suspensions, 12.9% and 9.7% of the label were recovered in the ATP fraction after application of 3H- and 14C-ADP, respectively. Between 15 and 120 s after 3H-ADP application, the 3H-ATP fraction was found to be stable at around 10%. For the range of ADP concentrations studied (10-40 microM), no saturation of ATP production was achieved. The extracellular localization of conversion was supported by the recovery of the epsilon -label in the epsilon -ATP fraction. In contrast, on HUVEC a conversion of epsilon -ADP to epsilon -ATP was not observed. In conclusion, on erythrocytes there is rapid enzymatic conversion of extracellular ADP to ATP which may play a significant role in adjusting adenine nucleotide concentrations in human plasma. In endothelial cells, extracellular conversion of ADP to ATP is of quantitatively minor importance, if it contributes at all.

Effects of phenylalanine and phenylpyruvate on ATP-ADP hydrolysis by rat blood serum

The nucleotide (ATP-ADP)/nucleoside (adenosine) ratio in the circulation can modulate the processes of vasoconstriction, vasodilatation and platelet aggregation. The main objective of the present study with rat blood serum was to evaluate the possibility of changes in nucleotide hydrolysis by phenylalanine (Phe) and phenylpyruvate (PP), the levels of which could increase in the circulation of individuals with phenylketonuria. Results demonstrated that Phe in the range 1.0-5.0 mM inhibited the ADP hydrolysis by rat serum. The effect of inhibition by Phe on ATP hydrolysis appeared only at a concentration of 5.0 mM. PP had no significant effect upon nucleotide hydrolysis. Kinetic analysis indicated that the inhibition of ADP and ATP hydrolysis by Phe in rat blood serum is uncompetitive. Conversely, Phe and PP did not affect the hydrolysis of p-nitrophenyl-5'-TMP by rat serum.

Measurement of adenine nucleotides in plasma

The goal of this study was to identify the most important variables affecting bioluminescent ATP, ADP and AMP measurements in plasma and to develop an assay that takes these variables into account. Blood samples were drawn from conscious dogs. A 'stop solution' containing EDTA was prepared, which greatly retarded plasma ATP degradation by chelating Mg(+2) and Ca(+2) that are co-factors for many ATPases. Stop solution and blood were mixed using a two-syringe withdrawal system. Samples were centrifuged twice in order to remove red blood cells, and ATP was measured in the supernatant using the firefly luciferase assay. Sample pH was adjusted to the optimal range (7.75-7.95) and Mg(2+) (necessary for the luciferase reaction) was added back to the sample within the luminometer 2 s prior to luciferase addition. Four assay tubes were prepared for each plasma sample, containing standard additions of 0-15 pmol added ATP, in order to quantify native plasma ATP content. In separate plasma/stop solution samples ADP + ATP was measured after converting ADP to ATP via the pyruvate kinase reaction, and AMP + ADP + ATP was measured after addition of both myokinase and pyruvate kinase. Addition of forskolin and isobutylmethylxanthine (IBMX) to the stop solution to inhibit platelets resulted in lower ATP concentrations. Measurement of ATP and haemoglobin from lysed erythrocytes revealed that haemolysis exerts a strong influence on plasma ATP concentration that must be taken into account.

Leptin-dependent platelet aggregation in healthy, overweight and obese subjects

OBJECTIVE

To investigate the effects of leptin on platelet aggregation and platelet free calcium (Ca(2+)) concentrations, and the role of the long form of leptin receptor (ObRb) and the phospholipase C (PLC) in mediating leptin effects on platelet function.

DESIGN

Cross-sectional, clinical study.

SETTING

Outpatient's Service for Prevention and Treatment of Obesity at the University Hospital of Messina, Italy.

SUBJECTS

A total of 19 healthy, 14 overweight, and 16 obese male subjects.

MEASUREMENTS

ADP-induced platelet aggregation and platelet Ca(2+) were measured after incubation of platelet-rich plasma with leptin alone 5-200 ng/ml, leptin 200 ng/ml and anti-human leptin receptor long-form antibody (ObRb-Ab) 5-10 microl, or leptin 200 ng/ml and PLC inhibitor U73122 0.5-1 nmol/l.

RESULTS

Platelet stimulation with leptin lead to a significant and dose-dependent increase in platelet aggregation in healthy subjects. This effect was blunted in overweight, and strongly reduced in obese subjects. Similarly, the incubation with leptin induced a significant and dose-dependent increase in platelet free calcium, which was blunted in overweight and obese patients. The effect of leptin on platelet aggregation and platelet Ca(2+) was completely abated by the anti-ObRb-Ab and the PLC inhibitor U73122.

CONCLUSIONS

Leptin produces a dose-dependent enhancement of ADP-induced platelet aggregation in humans. Platelet aggregation response to leptin is blunted, but not completely abolished in overweight/obese subjects, thus suggesting that platelet may represent a site of leptin resistance in human obesity. Leptin increases platelet free calcium in a dose-dependent manner. The inhibition of PLC completely abates the effect of leptin on both platelet aggregation and Ca(2+) levels. These findings suggest that signaling pathway other than JAK-STAT tyrosine phosphorylation (ie PLC and calcium) may be involved in mediating the prothrombotic action of leptin.

AlphaIIbbeta3-mediated outside-in signaling induced by the agonist peptide LSARLAF utilizes ADP and thromboxane A2 receptors to cause alpha-granule secretion by platelets

The peptide LSARLAF (LSA) causes alphaIIbbeta3-dependent platelet activation that results in alpha-granule secretion and aggregation. LSARLAF-induced, alphaIIbbeta3-mediated outside-in signaling causing alpha-granule secretion and platelet aggregation was studied using washed mouse platelets. ADP receptor antagonists, enzyme inhibitors, normal platelets and platelets from mice that lack either Galphaq or thromboxane (Tx) A2 receptors were used for this investigation. The results demonstrate that LSA-induced alphaIIbbeta3-mediated signaling producing aggregation of washed platelets is mediated through the release of ADP and thromboxane, which cause alpha-granule release by mediating their effects though Galphaq and/or Gi depending on the level of LSA used to activate the platelets. Specifically, alphaIIbbeta3 elicited aggregation of washed platelets in response to a low level of LSA requires signaling through the ADP receptor P2Y1 and Galphaq, and the ADP receptor P2Y12 and Gi as well as TxA2 receptors. However, this aggregation is independent of Galphaq and TxA2 signaling in response to high LSA concentrations, but is dependent on ADP signaling through its receptor P2Y12, and therefore presumably Gi, regardless of the level of LSA used to activate the platelets. PKC function is required for ADP secretion and the subsequent signaling through P2Y12 regardless of the level of LSA used to activate the platelets. The end point of the LSA-induced alphaIIbbeta3-mediated signaling characterized in this study is alpha-granule secretion, which provides the fibrinogen required for aggregation of washed platelets.

EDTA inhibits collagen-induced ATP+ADP secretion and tyrosine phosphorylation in platelets independently of Mg2+ chelation and decrease in pH

The effects of Mg2+ and EDTA on collagen-induced platelet dense granule secretion and protein tyrosine phosphorylation were studied in the presence and absence of various inhibitors of autocrine agonists that are released from secretory granules. Addition of EDTA to gel-filtered platelets in Tyrode's solution caused a decrease in pH of 0.7-0.9 pH units, due to chelation of Mg2+, and a marked inhibition of cATP+ADP secretion induced by collagen (25 (micro)g/ml). Lowering pH of the platelet suspension to the same extent by HCl had no effect on secretion which also was the same in the absence of exogenous Mg2+ as in its presence. Similarly, secretion induced by collagen per se, i.e., isolated from autocrine agonists by inhibitors, which was about 50% less than with autocrine agonists, was not affected by the presence or absence of exogenous Mg2+. The level of tyrosine phosphorylation of several proteins in resting platelets was higher in the absence of Mg2+ than in its presence, but the onset of collagen-induced phosphorylation and dephosphorylation was more rapid in the absence of Mg2+. Tyrosine phosphorylation of p38 was specifically enhanced in the presence of inhibitors of autocrine phosphorylation and even more enhanced in the absence of Mg2+. EDTA inhibited the protein phosphorylation to the same extent in the presence as in the absence of exogenous Mg2+. These results show that EDTA inhibits collagen-induced dense granule secretion neither through chelation of Mg2+ nor lowering of pH, and thus probably through other effects on the platelet by the EDTA2- species that are reflected in protein tyrosine phosphorylation signalling.

Elevated expression of integrin alpha(IIb) beta(IIIa) in drug-naïve, first-episode schizophrenic patients

BACKGROUND

Patients with schizophrenia have an increased risk over the general public of developing cardiovascular illness. It is unknown if there are functional changes in platelet surface receptors in schizophrenia. We therefore analyzed the surface expression of glycoprotein (GP)Ib, the integrin receptor alpha(IIb)beta(IIIa), CD62 (P-selectin), and CD63, and investigated platelet function in schizophrenic patients compared with healthy volunteers.

METHODS

Nineteen drug-naive, first-episode patients with a DSM IV diagnosis of paranoid schizophrenia were compared with matched healthy controls. Flow cytometry was used to assess platelet surface expression levels of GPIb, alpha(IIb)beta(IIIa), CD62, and CD63. Adenosine diphosphate-induced platelet aggregation was assayed.

RESULTS

The schizophrenic patients had a significantly (p < .0001) increased number of 68,145 +/- 8,260.1 alpha(IIb)beta(IIIa) receptors, platelet compared with 56,235 +/- 8,079.4 receptors, platelet in healthy controls.

CONCLUSIONS

Patients with schizophrenia have increased platelet expression of alpha(IIb)beta(IIIa), which may contribute to their increased risk of cardiovascular illness compared with the general population.

[Effect of 1,2,4-trimethylbenzene inhalation on energy metabolism in experimental animals]

The authors' investigations show that red cells and tissues of animals exposed to a single 4-h inhalation of vapors of 1,2,4-trimethylbenzene in concentration 10 mg/m3 contained reduced amount of ATP, accumulated amounts of ADP, AMP in active inclusion of 32-P in the structure of these compounds. Activation of ATPase is accompanied with a rise of Na and P concentrations and fall in K, Mg and Ca ions concentrations. In blood plasm there were lower levels of Na and Ca and higher levels of K, Mg and P ions. In exposure to ecotoxicant in concentration 0.01-0.1 and 1.0 mg/m3 the changes were insignificant.

Rho-GTPase-dependent platelet-neutrophil interaction affected by HMG-CoA reductase inhibition with altered adenosine nucleotide release and function

Platelet activation and aggregation is considered a crucial step in the initiation and aggravation of arterial thrombosis. ADP from activated platelets is recognized as major factor in thrombus formation and is a potent stimulator of oxygen-free radical release from neutrophils. The aim of the present investigation was to determine in vitro the direct effects of statins on ATP and ADP secretion by platelets and its impact on subsequent oxidative burst activity in neutrophils. Human neutrophils and platelets were isolated from peripheral blood. Levels of platelet-derived ATP and ADP were measured by high-performance liquid chromatography, oxygen-free radical release of neutrophils was measured fluorometrically, and chemotaxis experiments were performed. Rho-GTPases were studied by Western blot analysis. Thrombin-activated platelets primed neutrophils for enhanced oxygen-free radical release on triggering with formyl-Met-Leu-Phe, reduced by cerivastatin and simvastatin treatment of platelets. The two statins decreased the amount of adenosine-derivative release in these cells. Rho-GTPases, required for the thrombin signaling in platelets and neutrophils, were decreased after coincubation with statins. Data demonstrate that inhibition of Rho-GTPases by statins inhibit platelet ADP and ATP release and the consecutive augmentation of neutrophil oxygen-free radical release. Statins affect platelet-neutrophil interactions by altering Rho-GTPase-dependent adenosine nucleotide function.

Role of the Src family kinase Lyn in TxA2 production, adenosine diphosphate secretion, Akt phosphorylation, and irreversible aggregation in platelets stimulated with gamma-thrombin

Members of the Src family of kinases are abundant in platelets. Although their localization is known, their role(s) in platelet function are not well understood. Lyn is a Src-family kinase that participates in signal transduction pathways elicited by collagen-related peptide; it has also been implicated through biochemical studies in the regulation of von Willebrand factor signaling. Here, we provide evidence that Lyn plays a role in gamma-thrombin activation of platelets. Unlike the wild-type platelets, platelets from Lyn-deficient mice do not undergo irreversible aggregation, produce thromboxane A2, or secrete adenosine diphosphate in response to submaximal gamma-thrombin concentrations that cause secretion-dependent irreversible aggregation. Phosphorylation of Akt, a downstream effector of phosphatidylinositol 3-kinase, also requires a higher concentration of gamma-thrombin in Lyn-deficient platelets than in wild-type platelets. These findings demonstrate that Lyn signaling is required for thrombin induction of secretion-dependent platelet aggregation. Specifically, Lyn is required under these conditions to enable thrombin-induced TxA2 production and adenosine diphosphate secretion, necessary steps in secretion-dependent platelet aggregation.

Mechanistic model of myocardial energy metabolism under normal and ischemic conditions

A moderate reduction in coronary blood flow results in decreased myocardial oxygen consumption, accelerated glycolysis, decreased pyruvate oxidation, and lactate accumulation. To quantitatively understand cardiac metabolism during ischemia, we have developed a mechanistic, mathematical model based on biochemical mass balances and reaction kinetics in cardiac cells. By numerical solution of model equations, computer simulations showed the dynamic responses in glucose, fatty acid, glucose-6-phosphate, glycogen, triglyceride, pyruvate, lactate, acetyl-CoA, and free-CoA as well as CO2, O2, phosphocreatine/creatine, nicotinamide adenine dinucleotide (reduced form)/nicotinamide adenine dinucleotide (oxidized form) (NADH/NAD+), and adenosine diphosphate/adenosine triphosphate (ADP/ATP). When myocardial ischemia was simulated by a 60% reduction in coronary blood flow, the model generated myocardial concentrations, uptakes, and fluxes that were consistent with experimental data from in vivo pig studies. After 60 min of ischemia the concentrations of glycogen, phosphocreatine, and ATP were decreased by 60%, 75%, and 50%, respectively. With the onset of ischemia, myocardial lactate concentration increased and the myocardium switched from net consumer to net producer of lactate. Our model predicted a rapid 13-fold increase in NADH/NAD+, but only a twofold increase in the ratio of acetyl-CoA to free-CoA. These findings are consistent with the concept that pyruvate oxidation is inhibited during ischemia partially by the rise in NADH/NAD+.

The effect of systemic hypoxia on interstitial and blood adenosine, AMP, ADP and ATP in dog skeletal muscle

1. We investigated the effect of moderate systemic hypoxia on the arterial, venous and interstitial concentration of adenosine and adenine nucleotides in the neurally and vascularly isolated, constant-flow perfused gracilis muscles of anaesthetized dogs. 2. Systemic hypoxia reduced arterial PO2 from 129 to 28 mmHg, venous PO2 from 63 to 23 mmHg, arterial pH from 7.43 to 7.36 and venous pH from 7.38 to 7.32. Neither arterial nor venous PCO2 were changed. Arterial perfusion pressure remained at 109 +/- 8 mmHg for the first 5 min of hypoxia, then increased to 131 +/- 11 mmHg by 9 min, and then decreased again throughout the rest of the hypoxic period. 3. Arterial adenosine (427 +/- 98 nM) did not change during hypoxia, but venous adenosine increased from 350 +/- 52 to 518 +/- 107 nM. Interstitial adenosine concentration did not increase (339 +/- 154 nM in normoxia and 262 +/- 97 nM in hypoxia). Neither arterial nor venous nor interstitial concentrations of adenine nucleotides changed significantly in hypoxia. 4. Interstitial adenosine, AMP, ADP and ATP increased from 194 +/- 40, 351 +/- 19, 52 +/- 7 and 113 +/- 36 to 764 +/- 140, 793 +/- 119, 403 +/- 67 and 574 +/- 122 nM, respectively, during 2 Hz muscle contractions. 5. Adenosine, AMP, ADP and ATP infused into the arterial blood did not elevate the interstitial concentration until the arterial concentration exceeded 10 microM. 6. We conclude that the increased adenosine in skeletal muscle during systemic hypoxia is formed by the vascular tissue or the blood cells, and that adenosine is formed intracellularly by these tissues. On the other hand, adenosine formation takes place extracellularly in the interstitial space during muscle contractions.

The aerobic physiology of the air-breathing blue gourami, Trichogaster trichopterus, necessitates behavioural regulation of breath-hold limits during hypoxic stress and predatory challenge

Physiological characteristics of the blood oxygen transport system and muscle metabolism indicate a high dependence on aerobic pathways in the blue gourami, Trichogaster trichopterus. Haemoglobin concentration and haematocrit were modest and the blood oxygen affinity (P50=2.31 kPa at pH 7.4 and 28 degrees C) and its sensitivity to pH (Bohr factor, phi=-0.34) favour oxygen unloading at a relatively high oxygen pressure (PO2). The intracellular buffering capacity (44.0 slykes) and lactate dehydrogenase (LDH) activity (154.3 iu g(-1)) do not support exceptional anaerobic capabilities. Air-breathing frequency in the blue gourami is expected to increase when aquatic oxygen tensions decline. Under threat of predation, however, this behaviour must be modified at a potential cost to aerobic metabolism. We therefore tested the hypothesis that metabolic responses to predatory challenge and aquatic hypoxia are subject to behavioural modulation. Computer-generated visual stimuli consistently reduced air-breathing frequency at 19.95, 6.65 and 3.33 kPa PO2. Bi-directional rates of spontaneous activity were similarly reduced. The metabolic cost of this behaviour was estimated and positively correlated with PO2 but not with visual stimulation thus indicating down-regulation of spontaneous activity rather than breath-holding behaviour. Neither PO2 nor visual stimulation resulted in significant change to muscle lactate and ATP concentrations and confirm that aerobic breath-hold limits were maintained following behavioural modulation of metabolic demands.

Blood purine and energy status in rats with colitis

Colitis reduces the blood and tissue levels of adenosine deaminase and adenylate deaminase. Whether this has any effect on blood purines remains to be determined. The aim of this study was to measure the adenylate pool, substrates of the above enzymes, and energy status in blood from rats with colitis. Colitis was induced by intrarectal administration of acetic acid and followed over a period of seven days. The levels of ATP, ADP, AMP, adenosine, inosine, and uric acid were analyzed by HPLC, and energy status was estimated. Myeloperoxidase was used as a marker of colitis. Concentrations of ATP, ADP, AMP and adenosine decreased during days 1-5, whereas energy status decreased on day 2. The concentrations of inosine, uric acid, and hemoglobin remained unaltered, whereas colonic myeloperoxidase activity increased. These, findings demonstrate colitis-induced reduction of the circulating purines, which may be due to their enhanced usage for the repair of the inflamed colon.

Defects in the cappuccino (cno) gene on mouse chromosome 5 and human 4p cause Hermansky-Pudlak syndrome by an AP-3-independent mechanism

Defects in a triad of organelles (melanosomes, platelet granules, and lysosomes) result in albinism, prolonged bleeding, and lysosome abnormalities in Hermansky-Pudlak syndrome (HPS). Defects in HPS1, a protein of unknown function, and in components of the AP-3 complex cause some, but not all, cases of HPS in humans. There have been 15 inherited models of HPS described in the mouse, underscoring its marked genetic heterogeneity. Here we characterize a new spontaneous mutation in the mouse, cappuccino (cno), that maps to mouse chromosome 5 in a region conserved with human 4p15-p16. Melanosomes of cno/cno mice are immature and dramatically decreased in number in the eye and skin, resulting in severe oculocutaneous albinism. Platelet dense body contents (adenosine triphosphate, serotonin) are markedly deficient, leading to defective aggregation and prolonged bleeding. Lysosomal enzyme concentrations are significantly elevated in the kidney and liver. Genetic, immunofluorescence microscopy, and lysosomal protein trafficking studies indicate that the AP-3 complex is intact in cno/cno mice. It was concluded that the cappuccino gene encodes a product involved in an AP-3-independent mechanism critical to the biogenesis of lysosome-related organelles. (Blood. 2000;96:4227-4235)

[Aggregation of human blood platelets by remnant-like lipoprotein particles]

The action of lipoprotein lipase on chylomicrons (CM) and very low density lipoproteins (VLDL) produces remnant lipoproteins (RLP) that are rich in triglycerides, cholesterol and apolipoprotein E (apo E). Apo E serves as a ligand for the LDL receptor and mediates uptake of RLP by macrophages, vascular wall and other cells that express the LDL receptor. Uptake of RLP can profoundly alter the physiology of cells and promote atherosclerosis and thrombosis. Like RLP, blood platelets also have roles in atherosclerosis and thrombosis; hence it is likely that RLP can influence platelet activity as well. Platelet aggregation was assessed by measuring the loss of single platelets. Apo E3/3-rich RLP derived from normal human plasma VLDL and CM were prepared by an immunoseparation method. At 2.5 to 10 microliters, RLP induced platelet aggregation that increased with the dose of RLP, but decreased it at 25 to 200 microliters. Unlike apo E3/3-rich RLP, apo E4/3 (heterozygous phenotype) rich RLP caused platelet aggregation in a dose-dependent manner, without producing a bell-shape dose-response relationship. Scanning electron microscopy revealed that activated platelets had adhered to and formed aggregates on the red cell membrane. The platelet response was unaffected by aspirin, but was inhibited by apyrase (an ADP scavenger), 2-chloroadenosine (a platelet ADP-receptor antagonist) and cilostazol, a phosphodiesterase type III inhibitor. It is thought that RLP cause leakage of ADP from red cells, which then mediates platelet aggregation.

Platelets from a patient heterozygous for the defect of P2CYC receptors for ADP have a secretion defect despite normal thromboxane A2 production and normal granule stores: further evidence that some cases of platelet 'primary secretion defect' are heterozygous for a defect of P2CYC receptors

Two unrelated patients with a congenital bleeding diathesis associated with a severe defect of the platelet ADP receptor coupled to adenylate cyclase (P2(CYC)) have been described so far. In one of them, platelet secretion was shown to be abnormal. We recently showed that platelets with the primary secretion defect (PSD; characterized by abnormal secretion but normal granule stores, thromboxane A(2) production, and ADP-induced primary wave of aggregation) have a moderate defect of P2(CYC). Therefore, the interaction of ADP with the full complement of its receptors seems to be essential for normal platelet secretion, and PSD patients may be heterozygotes for the congenital severe defect of P2(CYC). In this study, we describe 2 new related patients with a severe defect of P2(CYC) and the son of one of them, who is to be considered an obligate heterozygote for the defect. The 2 patients with the severe defect had lifelong histories of abnormal bleeding, prolonged bleeding times, abnormalities of platelet aggregation and secretion, lack of inhibition of adenylate cyclase by ADP, and a deficiency of platelet-binding sites for [(33)P]2 MeS-ADP (240 and 225 sites per platelet; normal range, 530 to 1102). The son of one of them had a mildly prolonged bleeding time and abnormalities of platelet aggregation and secretion similar to those found in patients with PSD. In addition, his platelets showed a moderate defect of binding sites for [(33)P]2 MeS-ADP (430 sites per platelet) and of adenylate cyclase inhibition by ADP. This study of a family with the platelet disorder characterized by a defect of the platelet P2(CYC) receptor supports our hypothesis that the full complement of the platelet ADP receptors is essential for normal platelet secretion and that some patients with the common, ill-defined diagnosis of PSD are actually heterozygous for the defect.

Accelerated degradation of adenine nucleotide in erythrocytes of patients with chronic renal failure

Recently, we have shown that erythrocytes obtained from patients with chronic renal failure (CRF) exhibited an increased rate of ATP formation from adenine as a substrate. Thus, we concluded that this process was in part responsible for the increase of adenine nucleotide concentration in uremic erythrocytes. There cannot be excluded however, that a decreased rate of adenylate degradation is an additional mechanism responsible for the elevated ATP concentration. To test this hypothesis, in this paper we compared the rate of adenine nucleotide breakdown in the erythrocytes obtained from patients with CRF and from healthy subjects. Using HPLC technique, we evaluated: (1) hypoxanthine production by uremic RBC incubated in incubation medium: (a) pH 7.4 containing 1.2 mM phosphate (which mimics physiological conditions) and (b) pH 7.1 containing 2.4 mM phosphate (which mimics uremic conditions); (2) adenine nucleotide degradation (IMP, inosine, adenosine, hypoxanthine production) by uremic RBC incubated in the presence of iodoacetate (glycolysis inhibitor) and EHNA (adenosine deaminase inhibitor). The erythrocytes of healthy volunteers served as control. The obtained results indicate that adenine nucleotide catabolism measured as a hypoxanthine formation was much faster in erythrocytes of patients with CRF than in the cells of healthy subjects. This phenomenon was observed both in the erythrocytes incubated at pH 7.4 in the medium containing 1.2 mM inorganic phosphate and in the medium which mimics hyperphosphatemia (2.4 mM) and metabolic acidosis (pH 7.1). The experiments with EHNA indicated that adenine nucleotide degradation proceeded via AMP-IMP-Inosine-Hypoxanthine pathway in erythrocytes of both patients with CRF and healthy subjects. Iodoacetate caused a several fold stimulation of adenylate breakdown. Under these conditions: (a) the rate of AMP catabolites (IMP + inosine + adenosine + hypoxanthine) formation was substantially higher in the erythrocytes from patients with CRF; (b) in erythrocytes of healthy subjects degradation of AMP proceeded via IMP and via adenosine essentially at the same rate; (c) in erythrocytes of patients with CRF the rate of AMP degradation via IMP was about 2 fold greater than via adenosine. The results presented in this paper suggest that adenine nucleotide degradation is markedly accelerated in erythrocytes of patients with CRF.

[The concentration of adenine nucleotides in platelets of patients with primary chronic glomerulonephritis]

Nephrotic syndrome (n.s.) is associated with numerous blood coagulation abnormalities and a marked predisposition to thromboembolism. Increased aggregation and activation of platelets in patients with glomerulonephritis (g.l.n. p.t.s.) may partly explain this status. The aim of this study was to measure the platelets adenine nucleotides concentration. The study was performed in 57 patients with a renal biopsy confirmed primary glomerulonephritis and 24 sex and age matched healthy volunteers which served as a control group. The patients were divided into two subgroups: subgroup I/A--36 patients with the symptoms of the nephrotic syndrome and subgroup I/B--21 patients with chronic glomerulonephritis and proteinuria but without the symptoms of nephrotic syndrome. Concentration of adenine nucleotides in platelets was measured using HPLC. In the subgroup I/A significantly lower levels of ATP, ADP and AMP concentrations in platelets were observed comparing to control subjects. Simultaneously significant correlation between both ATP and ADP concentration and plasma levels of albumin, total cholesterol, LDL-cholesterol, triglycerides and fibrinogen were found in g.l.n. p.t.s.

SUMMARY AND CONCLUSIONS

1. Significantly lower concentrations of adenine nucleotides in platelets of gln pts with the nephrotic syndrome may result from their activation. 2. Protein and lipid metabolism as well as fibrinogen seem to influence ATP and ADP concentrations in platelets of g.l.n. p.t.s.

Platelet dysfunction detected at high shear in patients with heart valve disease

Whether patients with valvular heart disease have a defect of platelet function has been unclear. Despite evidence that these individuals have an abnormality detectable only under conditions of high shear stress, no methods have been widely available to adequately assess platelet function under such conditions. The Platelet Function Analyzer (PFA)-100 measures platelet function in a high shear environment and is well suited to the detection of platelet dysfunction in the clinical laboratory. The instrument records the time for platelets to occlude a membrane coated with collagen and either epinephrine (CEPI) or ADP (CADP). We studied the PFA-100 in 398 patients before open heart surgery; 308 for coronary artery bypass grafting (CABG) and 90 for aortic or mitral valve replacement (VR). Patients were classified as normal (CEPI < or = 153 s); 'aspirin effect' (CEPI > 153 s but CADP < or = 109 s) or abnormal (CEPI > 153 s and CADP > 109 s). In the CABG group, 41.2% were classified as normal, 43.2% as 'aspirin effect' and 15.6% as abnormal. In contrast, in patients undergoing VR, these values were 6.7, 11.1 and 82.4%, respectively. Patients with valvular disease had significantly longer closure times for both CEPI and CADP tests (P < 0.001). In addition, the valvular disease group had a significantly higher proportion of patients with markedly prolonged (> 150 s) closure times in the CADP cartridge (43.3 vs. 3.6%, respectively). Only one (0.3%) patient in the CABG group had non-closure (> 300 s) in the CADP test compared to seven (7.8%) in the valvular disease group. Three of six patients in the latter group bled excessively during surgery. We conclude that abnormal CADP closure is much more frequent among patients with aortic or mitral valve disease compared to those with coronary artery disease. This may reflect pre-existing high-shear damage to platelets that renders them refractory to subsequent shear activation and aggregation in the PFA-100 system. Further studies are needed to more precisely define the platelet defect in these patients. Markedly prolonged CADP closure in patients with valvular disease may indicate an increased likelihood of intra-operative bleeding, although an appropriately designed prospective study is needed to adequately address this hypothesis.

Adrenergic regulation of Rana balcanica erythrocyte pyruvate kinase

The regulation of pyruvate kinase activity by noradrenaline was investigated in Rana balcanica red cells. Thirty minutes of noradrenaline incubation induced a significant increase in the Vo/Vmax ratio of pyruvate kinase. The S0.5 for phosphoenolpyruvate of the enzyme significantly increased in the presence of noradrenaline while the Km for ADP decreased. In response to hormonal stimulation the Na +/H+ exchange was activated as was shown by the increase in Na+ and cyclic adenosine monophosphate from the 3rd min of incubation. All these effects were specific to alpha1 and beta antagonists. High concentrations of fructose diphosphate significantly activated the enzyme in the presence of noradrenaline but not in its absence. Furthermore, the presence of noradrenaline partially released the inhibition of the enzyme by adenosine triphosphate, inorganic phosphate and 2,3-diphosphoglycerate. The results suggest that noradrenaline stimulates glycolysis through pyruvate kinase activation. The mechanism of stimulation may is through Na+/H+ exchange activation, cyclic adenosine monophosphate concentration and Na(+)-K(+)-ATPase activation.

Storage pool defect in pooled buffy coat platelet concentrates within the shelf-life period

Platelet concentrate (PC) transfusions are useful for maintaining haemostasis in a variety of clinical situations. The function of transfused platelets is of critical importance, and changes on storage of buffy coat-prepared PC may influence their haemostatic potential. Total platelet adenine nucleotide content and platelet aggregation responses were studied, serially, in pooled buffy coat-derived PCs (n = 7), stored under UK recommended blood bank conditions, over the stipulated shelf-life of 5 days. Mean platelet volume (MPV), platelet counts and platelet distribution width (PDW) were also quantified. Total platelet ADP content decreased from 4.45+/-0.78 to 3.71+/-0.69 nmol/108 platelets (P<0.01, day 1 versus day 5, mean +/- SEM) over the shelf-life period. This was associated with reduced aggregatory responses: responses (expressed as percentage of maximum height) to 5 and 10 microM ADP decreased from 10.8+/-2.8% to 1.0+/-1.0% (P<0.005, 5 microM, day 1 versus day 5) and from 18.0+/-5.4% to 4.7+/-2.2% (P<0.02, 10 microM, day 1 versus day 5) while the decreased responsiveness was more pronounced for 4 microg/ml of collagen: 49.0+/-13.3% to 7.2+/-7.1% (P<0.01, day 1 versus day 4) and 49.9 +/-13.3% to 2.1+/-1.9% (P<0.001, day 1 versus day 5). These data indicate an acquired storage pool defect that is maximal by day 4 or 5 and accompanied by decreased platelet function, characterized by significant decreases in platelet aggregation responses. Addition of freeze-thawed plasma (autologous day 1) to PCs on days 2, 3, 4 and 5 did not alter the responses to ADP and collagen.