Changes in the adenylate energy charge in human blood platelets induced by adenosine diphosphate

Platelet mitochondrial dysfunction in critically ill patients: comparison between sepsis and cardiogenic shock

Introduction

Platelet mitochondrial respiratory chain enzymes (that produce energy) are variably inhibited during human sepsis. Whether these changes occur even during other acute critical illness or are associated with impaired platelet aggregation and secretion (that consume energy) is not known. The aims of this study were firstly to compare platelet mitochondrial respiratory chain enzymes activity between patients with sepsis and those with cardiogenic shock, and secondly to study the relationship between platelet mitochondrial respiratory chain enzymes activity and platelet responsiveness to (exogenous) agonists in patients with sepsis.

Methods

This was a prospective, observational, case–control study. Platelets were isolated from venous blood of 16 patients with severe sepsis or septic shock (free from antiplatelet drugs) and 16 others with cardiogenic shock, within 48 hours from admission to Intensive Care. Platelet mitochondrial respiratory chain enzymes activity was measured with spectrophotometry and expressed relative to citrate synthase activity, a marker of mitochondrial density. Platelet aggregation and secretion in response to adenosine di-phosphate (ADP), collagen, U46619 and thrombin receptor activating peptide were measured with lumiaggregometry only in patients with sepsis. In total, 16 healthy volunteers acted as controls for both spectrophotometry and lumiaggregometry.

Results

Platelets of patients with sepsis or cardiogenic shock similarly had lower mitochondrial nicotinamide adenine dinucleotide dehydrogenase (NADH) (P < 0.001), complex I (P = 0.006), complex I and III (P < 0.001) and complex IV (P < 0.001) activity than those of controls. Platelets of patients with sepsis were generally hypo-responsive to exogenous agonists, both in terms of maximal aggregation (P < 0.001) and secretion (P < 0.05). Lower mitochondrial NADH (R² 0.36; P < 0.001), complex I (R² 0.38; P < 0.001), complex I and III (R² 0.27; P = 0.002) and complex IV (R² 0.43; P < 0.001) activity was associated with lower first wave of aggregation with ADP.

Conclusions

Several platelet mitochondrial respiratory chain enzymes are similarly inhibited during human sepsis and cardiogenic shock. In patients with sepsis, mitochondrial dysfunction is associated with general platelet hypo-responsiveness to exogenous agonists.

Trial registration

ClinicalTrials.gov NCT00541827. Registered 8 October 2007.

Intracellular calcium storage and release in the human platelet. Chlorotetracycline as a continuous monitor

The calcium-sensitive fluorescent probe chlorotetracycline was used to monitor calcium movement in human platelets. The chlorotetracycline fluorescence signal is a linear measure of the level of free calcium in the dense tubules and in the mitochondria, with probe sensitivity in the millimolar range. Experiments perturbing the system with the calcium ionophore A23187 shows that the level of free internal calcium in the organelle depends upon the cytoplasmic level, which, in turn, depends upon the passive permeability of the plasma membrane. Chlorotetracycline in the cytoplasmic compartment does not respond to changes in the cytoplasmic calcium concentration, which is held in the micromolar to submicromolar range by an extrusion system. The calcium concentration in the cytoplasmic compartment can be directly manipulated by the calcium ionophore A23187 and is measured in parallel experiments with Quin 2, a high-affinity indicator. The calcium transport systems of the organelles are shown to be less susceptible to short circuit by A23187. Analysis shows that mitochondrial uptake is slow (t 1/2 = 20 minutes), produces a large increase in chlorotetracycline fluorescence, and is inhibited by sodium azide plus oligomycin. Uptake by the dense tubules is more rapid (t 1/2 = 2 minutes), produces a smaller increase in chlorotetracycline fluorescence, is inhibited by trifluoperazine, and is less sensitive to A23187. The Km is estimated as 1 microM or lower. Studies show that the chlorotetracycline technique is useful for the monitoring of calcium uptake and release by the platelet organelles, and suggests that the Quin 2/chlorotetracycline technique will be useful as a diagnostic of both physiological and pathological activation mechanisms.

Quantification of energy consumption in platelets during thrombin-induced aggregation and secretion. Tight coupling between platelet responses and the increment in energy consumption

The involvement of metabolic energy in platelet responses was investigated by measuring the energy consumption during aggregation and secretion from dense, alpha- and acid-hydrolase-containing granules. Gel-filtered human platelets were stimulated with different amounts of thrombin (0.05-5.0 units X ml-1). At various stages during aggregation and secretion the energy consumption was measured from the changes in metabolic ATP and ADP following abrupt arrest of ATP resynthesis. Stimulation with 5 units of thrombin X ml-1 increased the energy consumption from 6.2 +/- 0.9 to 17.8 +/- 0.4 mumol of ATPeq. X min-1 X (10(11) platelets)-1 during the first 15 s. It decreased thereafter and returned to values found in resting cells after about 30 s. With 0.05 unit of thrombin X ml-1, the energy consumption accelerated more slowly and took at least 3 min before it normalized. A strong positive correlation was found between the velocities of the three secretion responses and the concurrent energy consumption (a) at different stages of the responses induced by a given dose of thrombin, and (b) at different secretion velocities initiated by different amounts of thrombin. When, at different stages of the responses, the extent of secretion was compared with the amount of energy that had been consumed, a strong linear correlation was found with the increment in energy consumption but not with the total energy consumption. This correlation was independent of the concentration of thrombin and indicated that complete secretion from dense, alpha- and acid-hydrolase-containing granules was paralleled by an increment of 4.0, 6.5 and 6.7 mumol of ATPeq. X (10(11) platelets)-1, respectively. An energy cost of 0.7 mumol of ATPeq. X (10(11) platelets)-1 was calculated for separate dense-granule secretion, whereas the combined alpha- and acid-hydrolase granule secretion required 5.3 mumol of ATPeq. X (10(11) platelets)-1. There was no correlation between energy consumption and optical aggregation. In contrast, the rate of single platelet disappearance, which is a measure for the early formation of small aggregates, correlated closely with the rate of energy consumption. Compared with secretion, however, the energy requirement of single platelet disappearance was minor, since 2mM-EDTA completely prevented this response but decreased the energy consumption only slightly. An increase of 0.5-1.0 mumol of ATPeq. X (10(11) platelets)-1 was seen before single platelet disappearance and the three secretion responses were initiated, indicating an increase in energy consuming processes that preceded these responses.(ABSTRACT TRUNCATED AT 400 WORDS)

The fibrinogen-dependent pathway of platelet aggregation

Fibrinogen participates in platelet aggregation induced by ADP and this participation is receptor mediated. The role of fibrinogen in platelet function is not restricted to the ADP stimulus as the molecule may regulate aggregation induced by the direct interaction of the protein with a single, specific receptor system. Thus, a common fibrinogen-dependent mechanism leading to platelet aggregation is hypothesized.

A novel technique for rapid determination of energy consumption in platelets. Demonstration of different energy consumption associated with three secretory responses

A novel method has been developed for rapid and quantitative determination of the rate of energy consumption in platelets. In platelets suspended in a cyanide-containing medium. ATP resynthesis is abruptly blocked by addition of 2-deoxyglucose and D-glucono-1,5-lactone. We demonstrate that the subsequent changes in the levels of cytoplasmic ATP and ADP reflect the velocity of energy consumption in the platelets immediately before addition of the inhibitors. Despite the arrest in ATP resynthesis the platelets remain responsive to stimulation by thrombin (5 units x ml-1) which triggers the secretion of the contents of dense, alpha- and acid hydrolase granules. Unstimulated platelets were found to consume about 3.5 and 0.5 mumol of ATP equivalents x min-1 x (10(11) cells)-1 at 37 degrees C and 15 degrees C, respectively; the thrombin-treated platelets consumed respectively 16 and 2 mumol of ATP equivalents x min-1 x (10(11) cells)-1 at these temperatures. When the velocity of energy consumption was varied by (a) changing the temperature and (b) preincubation with glyco(geno)lytic inhibitors, it was found to be linearly related to the initial rate of secretion from the three types of granules. The precise nature of this relationship differed between the three types of secretion responses and indicated an increasing requirement for metabolic energy for secretion from the three types of granules in the order: dense granule less than alpha-granule less than acid hydrolase granule. The results obtained with changes in temperature were superimposable on those obtained with the glyco(geno)lytic inhibitors for dense granule secretion and alpha-granule secretion, suggesting an apparent coupling between energy consumption and the rate of these secretion responses. The rate of secretion of acid hydrolase was always higher when energy consumption was varied by temperature changes than when glyco(geno)lytic inhibitors were used, probably as a result of metabolic changes prior to induction of secretion. On the basis of these experiments, we calculated an incremental energy consumption during complete secretion of dense, alpha- and acid hydrolase granule contents of 2.5, 4.2 and 6.7 mumol of ATP equivalents x (10(11) platelets)-1, respectively.

Differential energy requirements for platelet responses. A simultaneous study of aggregation, three secretory processes, arachidonate liberation, phosphatidylinositol breakdown and phosphatidate production

Previous studies have indicated different energy requirements for some platelet responses; these differences could, however, be due to inadequate methodology and differences in platelet preparation. The present study describes the effect of decreasing ATP availability on seven platelet responses measured in gel-filtered human platelets. The cells, prelabelled with 5-hydroxy[(3)H]tryptamine, [(3)H]- or [(14)C]adenine, [(32)P]P(i) or [(3)H]arachidonate, were incubated with antimycin A and 2-deoxy-d-glucose. Platelet responses induced by thrombin and collagen (secretion only), level of metabolic ATP and the adenylate energy charge (AEC) were determined at various times during incubation. Platelet aggregation was rapidly inhibited after a lag of 5-15 min and with 50% inhibition at AEC = 0.55-0.60. Secretion of 5-hydroxy[(14)C]tryptamine and ATP + ADP from dense granules and of fibrinogen and beta-thromboglobin from alpha-granules were inhibited in parallel, without a lag and with 50% inhibition at AEC = 0.65-0.70. The inhibition of secretion of platelet factor 4 from the alpha-granules followed another pattern with 50% inhibition at AEC = 0.70-0.80. Breakdown of [(3)H]-phosphatidylinositol, formation of [(3)H]- and [(32)P]-phosphatidate, liberation of [(3)H]arachidonate and secretion of acid hydrolases were inhibited in parallel and inhibition was present at the start of incubation with 50% inhibition at AEC = 0.80-0.87. These results suggest that the responses have different energy requirements, increasing in the order: aggregation < dense granule and alpha-granule secretion < acid hydrolase secretion, phosphatidylinositol breakdown, phosphatidate formation and arachidonate liberation. The powerful inhibition of phosphatidylinositol breakdown by metabolic inhibitors suggests that energy-requiring steps are involved in the activation of phospholipase C.

Metabolic aspects of the secretion of stored compounds from blood platelets. The effect of NaF at different pH on nucleotide metabolism and function of washed platelets

The purpose of this study was to investigate the response of human blood platelets to fluoride at different pH. The results were as follows. (1) Fluoride induced secretion faster and at a lower concentration when pH was lowered. (2) Platelets exposed to 2 mM-fluoride at 0 degrees C at pH 5.3 underwent secretion when first pH and then temperature was raised, although no secretion was seen at 2 mM-fluoride concentration in the absence of the preincubation at low pH. (3) The concentration of [14C]ATP in platelets decreased steeply in response to fluoride before induction of secretion. Addition of antimycin blocked or partly inhibited secretion. Fluoride thus exerts an inhibitory effect on platelet glycolysis before induction of secretion. (4) Fluoride accumulated in the platelet pellet by a time course that preceded secretion. The accumulation was faster and greater at pH 6 than at 7.4. These four points are taken as indirect evidence that fluoride has to penetrate to the interior of the platelet to induce secretion. The activation takes place over a wide range of acid pH in contrast with induction of platelet function via the outside of the plasma membrane. In addition evidence is presented that the salvage pathway may under special circumstances play an important role in the re-synthesis of platelet adenine nucleotides.

Adenine in blood preservation

The recent Food and Drug Administration (U.S.) approval of a new blood preservative (CPDA-1) which contains adenine not only introduces a new blood product into the American blood banking system, but also heralds the advent of novel approaches to blood product preservation. The use of adenine to effect maintenance of red cell adenosine triphosphate (ATP), and hence to prolong storability, has a well-founded biochemical rationale. Effects of adenine on red cell metabolism are generally well understood, but effects on other blood components have not been fully delineated. The efficacy of adenine preservatives in enhancing the duration of red cell storage appears to outweigh the small risk of toxicity from free adenine. Clinical use of millions of units of adenine-preserved blood in Europe during more than a decade has resulted in only one report of possible adenine toxicity. Marginal acceptability of 24-hr 51Cr red cell recovery of packed red cells stored for 35 days in CPDA-1 has stimulated development and evaluation of an improved preservative (CPDA-2) which may extend blood storability beyond 35 days. A heightened awareness of the hematological consequences of prolonged storage has come with the extension of blood storage beyond 21 days. The concepts of component-specific preservation systems and optimal preservation systems have emerged as a result of experimentation on adenine preservatives. While the influence of adenine preservatives on American blood banking is yet to become manifest, the ultimte impact of adenine on blood preservation may be the development of novel systems which optimally preserve specific blood components at the option of the user.

Metabolism and function of human platelets washed by albumin density gradient separation

A method for washing platelets by albumin density gradient separation, originally designed for the study of platelet coagulant activities, has been modified for platelet aggregation and metabolic studies. Platelets are sedimented into a continuous density gradient of isosmolar albumin containing apyrase to protect them from clumping and physical injury and are resuspended in calcium-free Tyrode's solution. The mean recovery of platelets after two separations relative to platelet-rich plasma (PRP) was 90.3%. When small amounts of plasma were added to washed platelet suspensions, aggregation and release of [14C]5-hydroxytryptamine (5HT) in response to adenosine diphosphate (adp) or 5HT were similar to results obtained with PRP. When fibrinogen was substituted for plasma, ADP-induced aggregation occurred but was feeble. Without added plasma or fibrinogen, platelets were refractory to ADP and insensitive to the cyclic endoperoxide analogue U44619. When both ADP and U44619 were added simultaneously, in low concentrations, to washed platelets without added plasma or fibrinogen, aggregation occurred immediately. Washed platelets were not aggregated by adrenaline, which potentiated ADP-induced aggregation. Several biochemical measurements which are sensitive indicators of cellular damage were normal in washed platelets, including [14C]adenine uptake, adenylate energy charge, hypoxanthine formation and the response of adenylate cyclase to stimulation by PGE1 or PGD2. Platelet coagulant activities were not made available and heparin-neutralizing activity (HNA) was not spontaneously released by the washing procedure, but the washed platelets responded normally to appropriate agents by developing coagulant activities and releasing HNA. The ultrastructure of washed platelets was similar to those in control PRP. Inclusion of apyrase in the first albumin gradient had a beneficial effect on platelet morphology, aggregation and metabolism, but washing at 37degreesC compared with 25degreesC did not. Albumin density gradient separation is a useful method for isolating platelets for aggregation and metabolic studies.

Hydrogen peroxide, an inhibitor of platelet function: effect on adenine nucleotide metabolism, and the release reaction

The in vitro effects of H2O2 on platelet adenine nucleotide metabolism and on the ADP-induced platelet release reaction were examined. All studies were performed on human platelet-rich plasma (PRP) preincubated with (3H)-adenine. Within 3 min of incubation with PRP, H2O2 (100-500 micronM) caused an irreversible reduction in the (3H)-ATP level with a concomitant increase in (3H)-IMP and (3H)-inosine and hypoxanthine levels. The adenylate energy charge (AEC) initially decreased while the ATP level fell. The AEC, however, returned to levels slightly lower than the control during further incubation. No leakage of ATP and ADP to plasma was observed. The fall in the steady-state levels of (3H)-ATP increased with increase of the H2O2 concentration (decrease of 8.7-40% at H2O2 concentrations from 5 to 600 micronM). H2O2 pretreatment of PRP caused absence of ADP-induced biphasic aggregation, partial inhibition of the primary wave, and complete inhibition of release of platelet nonmetabolic ATP and ADP. Our in vitro findings support the view that part of the inhibitory effect of H2O2 may be related to the lowering of metabolic ATP levels in platelets.

Human platelet 6-phosphofructokinase. Relation between inhibition by Mg-ATP2-and cooperativity towards fructose 6-phosphate and investigations on the formation of a ternary complex

Human platelet 6-phosphofructokinase (EC 2.7.1.11) shows cooperativity towards Fru-6-P and is allosterically inhibited by high Mg-ATP2- concentrations. No relation could be demonstrated between the cooperativity towards Fru-6-P and the inhibition by Mg-ATP2-. Increasing the concentrations of Mg-ATP2- only raised the apparent Km values for Fru-6-P, but did not change the Hill constants. A possible formation of a Mg-ATP2--enzyme-Fru-6-P complex during catalysis was investigated. Our calculations suggest that such a ternary complex is indeed formed during the reaction.

The interaction of bovine factor VIII with human platelets

Treatment of human platelets with purified bovine Factor VIII caused three types of aggregation: (a) primary agglutination; (b) secondary aggregation involving the platelet release reaction; and (c) super-aggregation, in which the platelets were gathered into only a few large clumps. Removal of calcium ions or treatment with p-hydroxymercuiriphenyl sulfonate blocked the release reaction, but not primary agglutination or super-aggregation. Platelets treated with formalin were not aggregated by ADP, thrombin, or collagen, but were agglutinated by bovine Factor VIII, although they did not show super-aggregation. For malin-treated platelets were agglutinated by phytohemagglutinin P less extensively and less rapidly than by bovine Factor VIII. Treatment of platelets and Factor VIII with neuraminidase released 60 and 53%, respectively, of the sialic acid residues without affecting the agglutination reaction or the procoagulant activity of the Factor VIII. Agglutination was inhibited by high salt concentrations, dextran sulfate, and heparin. During agglutination, both the procoagulant and platelet-agglutinating activities of Factor VIII became bound to the platelet surface.

Hereditary giant platelet syndrome. Absence of collagen-induced coagulant activity and deficiency of factor-XI binding to platelets

The platelets from two related patients with the hereditary giant platelet syndrome were examined. They were larger than normal but otherwise ultrastructurally normal; they contained increased storage pools of adenine nucleotides and heparin-neutralizing activity and took up serotonin at an increased rate. They aggregated normally with ADP and collagen but failed to aggregate with bovine factor VIII and Ristocetin. Some change in shape occurred with ADP, and the reduction in adenylate energy change after addition of ADP to platelet-rich plasma was smaller than normal. Platelet coagulant activities including contact product forming activity, intrinsic factor-Xa forming activity and platelet factor 3 activity were normal or increased, but collagen-induced coagulant activity was absent whether tested in washed platelet suspensions or platelet-rich plasma. Platelet washing experiments showed decreased binding of factors V and VIII to hereditary giant platelets and no detectable factor XI in washed platelet suspensions. It is concluded that (1) the hereditary giant platelets studied lacked a binding mechanism for factors, V, VIII and XI; (2) the normal development of collagen-induced coagulant activity apparently depends upon the binding of factor XI to the platelet membrane; and (3) the defective prothrombin consumption observed in these patients may have resulted from the failure of their platelets to bind factor XI.

Effects of antimycin and 2-deoxyglucose on adenine nucleotides in human platelets. Role of metabolic adenosine triphosphate in primary aggregation, secondary aggregation and shape change of platetets

1. Human platelet-rich plasma prelabelled with [(3)H]adenine was incubated at 37 degrees C with antimycin A and 2-deoxy-d-glucose. Variations in the amounts of ATP, ADP and P(i), and in the radioactivity of ATP, ADP, AMP, IMP, hypoxanthine+inosine and adenine were determined during incubation. Adrenaline- and ADP-induced platelet aggregation and the ADP-induced shape change of the platelets were determined concurrently. 2. 2-Deoxyglucose caused conversion of [(3)H]ATP to [(3)H]hypoxanthine+inosine. The rate of this conversion increased with increasing 2-deoxyglucose concentration and was markedly stimulated by addition of antimycin, which had no effect alone. At maximal ATP-hypoxanthine conversion rates, the IMP radioactivity remained at values tenfold higher than control, whereas [(3)H]ADP and [(3)H]AMP radioactivity gave variations typical for product/substrates in consecutive reactions. The specific radioactivityof ethanol-soluble platelet ATP decreased during incubation to less than one-tenth of its original value. The amounts and radioactivity of ethanol-insoluble ADP did not vary during incubation with the metabolic inhibitors. 3. The rate of ADP- and adrenaline-induced primary aggregation decreased as the amount of radioactive ATP declined, and complete inhibition of aggregation was obtained at a certain ATP concentration (metabolic ATP threshold). This threshold decreased with increasing concentration of inducer ADP. 4. Secondary platelet aggregation (release reaction) had a metabolic ATP threshold markedly higher than that of primary aggregation. 5. Shape change was gradually inhibited as the ATP radioactivity decreased, and had a metabolic ATP threshold distinctly lower than that of primary aggregation, and which decreased with increasing concentration of ADP. 6. A small but distinct fraction of [(3)H]ATP disappeared rapidly during the combined shape change-aggregation process induced by ADP in platelets incubated with metabolic inhibitors, whereas no ATP disappearance occurred during aggregation in their absence.