Malondialdehyde Formation in Rat Platelet-Rich Plasma

Malondialdehyde (MDA) is a stable product of arachidonic acid metabolism, catalyzed by the enzyme cyclo-oxygenase. The experimental conditions for measuring the kinetics of MDA formation in rat citrated platelet-rich plasma were defined. Platelets were stimulated with either arachidonic acid, the substrate of MDA, or thrombin, an enzyme which induces release of free arachidonic acid from platelet membrane phospholipids. MDA formation was almost linear for a limited period of time (between 0 and 2 min with arachidonic acid and between 1 and 3 min with thrombin) and was concentration-dependent with saturation kinetics.

The hyperbolic curves obtained could be recast in linear plots (according to Woolf transformation S/V versus S) when arachidonic acid was used. With thrombin, in contrast, the highest concentration at which no MDA production could be detected (3 NIH u/ml) had to be subtracted from each concentration of the enzyme used to obtain Woolf plots. The apparent Km value of arachidonic acid was 0.49 ± 0.09 mM and Vmax was 1.44 ± 0.06 nmoles MDA/1.4 × 109 platelets/min. The corresponding values in experiments with thrombin were 6.5 ± 1.5 NIH u/ml and 0.233 ± 0.012 nmoles MDA/1.4 × 109 platelets/min.

Malondialdehyde Formation in Rat Platelet-Rich Plasma

The type of inhibition and the relative potency of aspirin and indomethacin on rat platelet malondialdehyde (MDA) formation were investigated in an in vitro system. Both drugs inhibited the production of MDA after platelet stimulation with either thrombin (10 or 25 NIH u/ml) or sodium arachidonate (0.5–2.25 mM).

Inhibition by both drugs was concentration-dependent, was partially removed when platelet-rich plasma was diluted with platelet-poor plasma, was much stronger when either drug was preincubated with platelets for 10 minutes than for 1 minute and was apparently competitive when analysed by Dixon plots (1/V versus inhibitor concentrations).

It is suggested that both aspirin and indomethacin may inhibit in vitro cyclo-oxygenase activity in citrated platelet-rich plasma by a similar, if not identical, partially reversible mechanism, not involving – in a first step – covalent binding of either drug to enzyme.

A scheme of the interaction of both drugs with cyclo-oxygenase is presented which also takes into account the irreversible acetylation of the enzyme occurring after longer incubation times with aspirin.

A single sub-kilometre Kuiper belt object from a stellar occultation in archival data

The Kuiper belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper belt objects. Small, sub-kilometre-sized, Kuiper belt objects elude direct detection, but the signature of their occultations of background stars should be detectable. Observations at both optical and X-ray wavelengths claim to have detected such occultations, but their implied abundances are inconsistent with each other and far exceed theoretical expectations. Here we report an analysis of archival data that reveals an occultation by a body with an approximately 500-metre radius at a distance of 45 astronomical units. The probability of this event arising from random statistical fluctuations within our data set is about two per cent. Our survey yields a surface density of Kuiper belt objects with radii exceeding 250 metres of 2.1(-1.7)(+4.8) x 10(7) deg(-2), ruling out inferred surface densities from previous claimed detections by more than 5sigma. The detection of only one event reveals a deficit of sub-kilometre-sized Kuiper belt objects compared to a population extrapolated from objects with radii exceeding 50 kilometres. This implies that sub-kilometre-sized objects are undergoing collisional erosion, just like debris disks observed around other stars.

[Ticks bite in foresters]

The objective of the study is evalutation of the risk for ticks strings on foresters. The sample constituted by 325 foresters belong to Messina province as been submitted to medical examination venous tests. Whole sample had to answer to a questionnaire to consider. The prevalence of systemic and skin reactions and we have dose Immunoglobulines versus Brucella Melitensis, Rickettsie Conorii e Borrelia Burgdorferi. The results showed that the 19% has declared past stings of tick, and 4.9% reported symptoms probably deriving to a past infections determined by inquired microorganisms. The serum tests showed that 70% was positive for all microorganisms, instead only 31%. Was never infected by inquired microorganisms. In conclusion our study shows that zoonos is risk linked to stings of tick is relatively high in foresters.

Long gamma-ray bursts and core-collapse supernovae have different environments

When massive stars exhaust their fuel, they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even more brilliant relativistic explosion known as a long-duration gamma-ray burst. One would then expect that these long gamma-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the gamma-ray bursts are far more concentrated in the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long gamma-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration gamma-ray bursts are associated with the most extremely massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long gamma-ray bursts are relatively rare in galaxies such as our own Milky Way.

Gravitational microlensing by low-mass objects in the globular cluster M22

Gravitational microlensing offers a means of determining directly the masses of objects ranging from planets to stars, provided that the distances and motions of the lenses and sources can be determined. A globular cluster observed against the dense stellar field of the Galactic bulge presents ideal conditions for such observations because the probability of lensing is high and the distances and kinematics of the lenses and sources are well constrained. The abundance of low-mass objects in a globular cluster is of particular interest, because it may be representative of the very early stages of star formation in the Universe, and therefore indicative of the amount of dark baryonic matter in such clusters. Here we report a microlensing event associated with the globular cluster M22. We determine the mass of the lens to be 0.13(+0.03)(-0.02) solar masses. We have also detected six events that are unresolved in time. If these are also microlensing events, they imply that a non-negligible fraction of the cluster mass resides in the form of free-floating planetary-mass objects.

Defective platelet aggregation in response to platelet-activating factor in uremia associated with low platelet thromboxane A2 generation

The bleeding tendency associated with uremia is likely due to a qualitative platelet dysfunction. So far the data available on platelet aggregation are conflicting. Since platelet-activating factor (PAF) plays a role in primary hemostasis, we studied platelet aggregation in response to PAF in 40 patients with chronic uremia on regular hemodialysis and 12 control subjects. Our results showed that in 28 of 40 uremics, platelet aggregation response to PAF was normal, whereas in the remaining 12 it was defective in that no second wave of aggregation was elicited even if the PAF concentrations were increased by a factor of 10,000. This abnormal response was peculiar to PAF and only partially related to factor(s) of plasma origin. The number of platelet PAF receptors and their affinity for the agonist were comparable in controls and "PAF-unresponsive" patients. The defective platelet aggregation in response to PAF was associated with a statistically significant reduction (P less than 0.01) in thromboxane A2 (TxA2) generation in platelet-rich plasma (PRP) challenged with PAF (10 and 100 nmol/L). When PRPs from PAF-unresponsive patients were preincubated with a stable analogue of prostaglandin endoperoxides/TxA2 U-46619, an irreversible platelet aggregation in response to PAF was obtained. Thus in a subpopulation of uremics, platelet aggregation in response to PAF is selectively abnormal as a consequence of a reduced TxA2 generation.

Enhanced glomerular thromboxane A2 mediates some pathophysiologic effect of platelet-activating factor in rabbit nephrotoxic nephritis: evidence from biochemical measurements and inhibitor trials

Previous studies have shown that platelet-activating factor (PAF) receptor blocking has a protective effect on rabbit nephrotoxic nephritis (NTN). We examined whether arachidonic acid (AA) metabolism is altered in NTN and whether a PAF receptor antagonist has any influence on such changes. Rabbits injected with anti-glomerular basement membrane antiserum in the heterologous phase had a markedly increased glomerular thromboxane B2 (TxB2) production level, whereas no changes have been detected in glomerular 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and prostaglandin E2 (PGE2). During the autologous phase of the disease, the glomerular TxB2 level was even higher than in the heterologous phase. The level of 6-keto-PGF1 alpha was significantly lower than normal, and the level of PGE2 was unchanged in respect to the basal values. The use of L-652,731 (a specific PAF receptor antagonist) reversed the abnormal generation of AA metabolites at glomerular level both in the heterologous and autologous phase of the disease. The effect of L-652,731 on AA metabolism is likely to be an indirect result of the PAF receptor blocking, because L-652,731 given to normal rabbits had no direct effect on glomerular AA metabolism. To assess whether the beneficial effect of L-652,731 in NTN is at least in part mediated by its capability of suppressing the excessive intrarenal synthesis of thromboxane A2 (TxA2), we compared the effect of L-652,731 with that of a selective TxA2-synthase inhibitor (FCE-22178). FCE-22178 ameliorated the morphologic expression of rabbit NTN and reduced function deterioration. The protective effect of L-652,731 on proteinuria in the autologous phase and on glomerular filtration rate in both phases was superior to that of FCE-22178. We conclude that an excessive intraglomerular synthesis of TxA2 occurs in rabbit NTN that can play a role in renal function deterioration. Both a specific PAF receptor antagonist and a TxA2-synthase inhibitor reduced the exaggerated TxA2 synthesis and favorably influenced the evolution of the disease.

Selective inhibition of platelet thromboxane generation with low-dose aspirin does not protect rats with reduced renal mass from the development of progressive disease

Rats with extensive renal mass reduction develop hypertension, proteinuria and progressive glomerulosclerosis. Previous studies have demonstrated that these changes are associated with an increased urinary excretion of thromboxane compared with normal rats and that the administration of a thromboxane synthetase inhibitor prevents glomerulosclerosis and progressive renal function deterioration. On this basis it has been speculated that the thromboxane synthetase inhibitor, by inhibiting platelet thromboxane, reduces platelet aggregation and prevents the generation of substances that can influence glomerular functional properties. Because the thromboxane synthetase inhibitor also inhibits thromboxane synthesis by resident glomerular cells and lowers blood pressure in these animals, the question of whether platelet thromboxane is indeed the factor implicated in the development of renal disease after renal ablation remains unanswered. To address this issue the authors administered at different time intervals from the surgical procedure a low-dose of oral aspirin (ASA) to rats with remnant kidney. This approach resulted in selective inhibition of platelet cyclooxygenase leading to an almost complete prevention of platelet thromboxane generation. Low-dose ASA spared renal cyclooxygenase as documented by a lack of significant inhibition of glomerular and urinary 6-keto-PGF1 alpha and did not lower blood pressure. Renal function studies showed that low-dose ASA, despite inhibiting platelet aggregation, had no effect on proteinuria and progressive renal insufficiency irrespectively if administered late (ie, 80 days after surgery) and given daily for all the observation period (ie, 20 days) or earlier in the course of the disease (ie, 40 and 10 days after surgery). Histologic data showed that the degree of glomerulosclerosis and tubulo-interstitial damage was not significantly different in rats with reduction of renal mass alone compared with rats with remnant kidney given low-dose ASA. In conclusion, the present findings indicate that inhibition of platelet aggregation and thromboxane formation does not prevent the progressive glomerulosclerosis that develops in rats with surgical reduction of renal mass. It is suggested that the beneficial results obtained previously in the same model by the use of a thromboxane synthesis inhibitor must be attributed either to an effect on resident glomerular cell thromboxane synthesis or to lowering systemic blood pressure.

Differential inhibition by aspirin of platelet thromboxane and renal prostaglandins in the rat

Aspirin (ASA) beside inhibiting platelet thromboxane A2 (TxA2) can suppress the formation of renal prostacyclin (PGI2) and prostaglandin E2 (PGE2) which play a crucial role in the control of renal hemodynamics. Previous studies based on urinary PG measurements have suggested that p.o. ASA can spare renal cyclooxygenase. We wanted to establish by direct measurement whether p.o. ASA has a renal sparing effect and to establish to which extent changes in renal cyclooxygenase activity can be predicted measuring urinary excretion of 6-keto-PGF1 alpha and PGE2. Our results showed that in normal rats 10 mg/kg of ASA given p.o. partially inhibits platelet TxA2 formation (measured as serum TxB2) and does not inhibit glomerular and medullary PGI2 and PGE2 synthesis. Higher doses of ASA (30-200 mg/kg) effectively and completely inhibit platelet TxA2 independently if given p.o. or i.v., and also inhibit glomerular and medullary PG synthesis. The kinetics of the effect of ASA on platelet vs. renal cyclooxygenase is different: the inhibition being irreversible in platelets, but rapidly reversible in glomeruli and medulla. Six hours after the administration of 10 and 30 mg/kg i.v. and 30 mg/kg p.o., kidney cyclooxygenase activity recovers completely. This transient inhibition of renal cyclooxygenase is not reflected by urinary excretion of 6-keto-PGF1 alpha and PGE2 (6- and 24-hr collection periods). In conclusion our present results indicate that doses of ASA enough to inhibit platelet TxA2, transiently inhibit glomerular and medullary PGI2 and PGE2. Although the inhibitory effect on platelets is long lasting, the effect on renal cyclooxygenase is transient and rapidly reversible.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of arachidonic acid in isolated glomeruli from pig kidney

Arachidonic acid metabolism in isolated glomeruli from pig kidney was investigated. Arachidonic acid metabolism via cyclooxygenase was studied by three different methodological approaches: radioimmunoassay (RIA), high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). By all these techniques, the major prostaglandins (PG) formed by pig glomeruli appeared to be 6-keto-PGF1 alpha and PGF2 alpha, the former being the most abundant. RIA and GC-MS also detected lower amounts of thromboxane B2 (TxB2) and PGE2. This emphasises the similarity with human glomeruli, in which the main cyclooxygenase product has indeed been reported to be 6-keto-PGF1 alpha. The lipoxygenase activity in isolated pig glomeruli, as studied by HPLC, generated 15-HETE, 12-HETE and 5-HETE. These data demonstrate that isolated glomeruli from pig kidney possess cyclooxygenase as well as lipoxygenase activity. Since a marked functional similarity exists between human and pig kidney, the pig can be regarded as a good model for studying the influence of arachidonic acid metabolites on glomerular pathophysiology.

Role of platelet-activating factor in primary hemostasis

To determine whether platelet-activating factor (PAF) has a physiological role in the process of primary hemostasis in the rabbit, we measured skin bleeding times in animals given orally a specific PAF-receptor antagonist L-652,731. One hour after the administration of L-652,731 (20 or 40 mg/kg), a significant prolongation of bleeding time was observed. Parameters known to interfere with the process of primary hemostasis were not altered by PAF-receptor antagonist. In addition, no changes were observed in platelet count and vessel wall arachidonic acid metabolism, as revealed by serum thromboxane B2 levels, vascular 6-ketoprostaglandin F1 alpha generation, and by aspirin experiment. The fact that aspirin alone did not induce prolongation of bleeding time in rabbits is probably due to the simultaneous inhibition of platelet thromboxane A2 and vascular prostacyclin. [3H]PAF binding to washed platelets of rabbits given L-652,731 showed a significant reduction in maximum number of detectable binding sites in comparison with values found in the same animals before the L-652,731 administration. The results suggest that the bleeding-time prolongation observed after L-652,731 administration results from its ability to selectively inhibit PAF bioactivity. Thus locally released PAF appears to exert a physiological role in the process of platelet plug formation that follows a skin incision in the rabbit.

Aspirin prolongs bleeding time in uremia by a mechanism distinct from platelet cyclooxygenase inhibition

We reported that aspirin (ASA) abnormally prolongs bleeding time (BT) in uremia. The present study was designed to investigate whether the abnormally prolonged post-ASA BT in uremia is due to different ASA pharmacokinetics and bioavailability that might be a consequence of uremic condition, platelet cyclooxygenase is peculiarly sensitive to ASA in uremia, and ASA affects primary hemostasis in uremia by a mechanism independent of cyclooxygenase inhibition. Our results showed that in patients with uremia, but not in normal subjects, ASA markedly prolongs the BT. This effect is transient and depends on the presence of ASA in the blood. The observed differences in ASA kinetic parameters are not an explanation of the exaggerated effect of ASA on primary hemostasis in uremia. The sensitivity of platelet cyclooxygenase to ASA inhibition is comparable in uremics and in normal subjects. The temporal dissociation between ASA-induced prolongation of BT and the effect on platelet thromboxane A2 generation suggests that ASA inhibits platelet function in uremia by a mechanism distinct from cyclooxygenase blocking. This possibility is strengthened by the observation that ibuprofen at a dose that fully inhibits platelet cyclooxygenase activity does not significantly prolong BT.

Platelet activating factor (PAF) as a mediator of injury in nephrotoxic nephritis

Release of acetyl glyceryl ether phosphorylcholine, platelet-activating factor (PAF), has been demonstrated to be associated with glomerular inflammatory damage in acute serum sickness. Moreover, PAF can increase glomerular permeability to proteins and induce mesangial contraction. Thus PAF might be a good candidate as a mediator of glomerular damage. However the in vivo evidence that PAF might cause glomerular injury is lacking. To evaluate if PAF has a major role in promoting glomerular inflammatory reaction and fibrin deposition, we studied the effect of a molecule, L-652,731, which blocks the PAF receptor, on the evolution of an experimental model of anti-glomerular basement membrane (anti-GBM) glomerulonephritis (GN). GN was initiated by sheep-anti-rabbit nephrotoxic serum. A proliferative GN regularly occurred in which heavy proteinuria, intra and extracapillary proliferation of resident and inflammatory cells and fibrin deposition in Bowman's capsule were the prominent findings. Our results showed that the PAF receptor antagonist reduces the glomerular damage in anti-GBM GN, supporting the hypothesis that PAF is indeed a mediator of glomerular inflammatory reaction. PAF receptor blocking prevented renal function deterioration in the early phase of the disease, probably preserving glomerular hemodynamics. In the delayed phase of the disease the PAF receptor antagonist reduced proteinuria and prevented renal function deterioration and fibrin deposition. These effects appear to be mediated by an inhibitory action of PAF receptor blocking on macrophage accumulation and activation.

Conjugated estrogens for the management of bleeding associated with renal failure

Bleeding is a major complication of uremia. Both cryoprecipitate and desmopressin effectively shorten the prolonged bleeding time and favorably influence clinical bleeding, but the former carries the risk of transmitting blood-borne infectious diseases, and both cryoprecipitate and desmopressin have a short duration of action. Preliminary evidence has suggested that estrogens may be useful, and we therefore performed a randomized, double-blind, crossover trial comparing the effect of conjugated estrogens with that of placebo on hemorrhagic tendencies and the bleeding time in six patients with uremia who were on maintenance hemodialysis. Five daily infusions of placebo or conjugated estrogens were administered at the beginning of one-month trial periods. Estrogen shortened the bleeding time in all six patients. The effect was detectable six hours after the first infusion, reached its maximum in all patients between days 5 and 7, and lasted for 14 days. By day 16 after the last infusion, the bleeding time had returned to base line in four of the six patients. No side effects were noted during or after estrogen infusion. Estrogens did not influence the circulating level of von Willebrand factor or change its multimeric structure. Moreover, the defective platelet aggregation and thromboxane formation observed in the patients were not corrected by estrogens. We conclude that conjugated estrogens are an adequate alternative to cryoprecipitate or desmopressin for the treatment of bleeding associated with renal failure, especially when a longer duration of action is needed and immediate onset of the effect is not essential. The mechanism of action of estrogens remains to be clarified.

Moderate doses of aspirin and risk of bleeding in renal failure

Uraemic patients have a bleeding tendency thought to be due to platelet functional abnormalities, but haemodialysis paradoxically exposes patients to the thrombotic complications of arteriovenous shunts. Possible treatments of the latter have been debated. The effect of 100 mg/m2 aspirin on haemostatic function was studied in 29 uraemic patients on chronic haemodialysis who had normal or only slightly prolonged bleeding times. Aspirin did not significantly affect bleeding time in healthy controls but prolonged it in uraemic patients. In 12 of the 29 uraemic patients, the bleeding time after aspirin was longer than 15 min. Aspirin completely abolished thromboxane A2 generation by both control and uraemic platelets, indicating that its effect in uraemic patients is not due to differential inhibition of platelet cyclo-oxygenase. Products of lipoxygenase enzyme and factor VIII von Willebrand factor did not seem to have a role. A careful risk-benefit evaluation is necessary before giving aspirin to uraemic patients on haemodialysis to prevent thrombosis of the arteriovenous shunt.

Acetyl glycerylphosphorylcholine aggregates human platelets through two distinct pathways, both dependent on arachidonic acid metabolism

Acetyl glyceryl ether phosphorylcholine (AGEPC) is a mediator of inflammation and anaphylaxis, released from IgE-sensitized basophils when exposed to antigens. Convincing evidence shows that AGEPC is synthesized by inflammatory cells, and its mechanism of action has been extensively studied with particular interest focusing on platelet stimulation, the key event initiating the inflammatory cascade. However, widely different results have been reported so that the mode of action of AGEPC in activating platelets is still largely obscure. The objective of this investigation was to elucidate whether AGEPC stimulates human platelets independently from arachidonate metabolism, as previously suggested, and to define the role of nonsteroidal antiinflammatory drugs in inhibiting AGEPC-mediated platelet activation. Irreversible platelet aggregation in vitro occurred at low AGEPC concentrations, was dose dependent, and was accompanied by thromboxane A2 formation. Aspirin inhibited irreversible aggregation induced by low AGEPC concentrations. A thromboxane inhibitor, UK-37, 248-01, failed to affect AGEPC aggregation, suggesting a crucial role for cyclic endoperoxides in AGEPC's mechanism of action. A thromboxane/endoperoxide receptor antagonist 13-azaprostanoic acid prevented irreversible platelet aggregation induced by low AGEPC concentrations. The inhibiting effect of aspirin on AGEPC aggregation could be overcome by increasing the concentration of the agonist by a factor of 10, unless a lipoxygenase inhibitor, nordihydroguaiaretic acid, or 5,8,11,14-eicosatetraynoic acid was previously added to platelet-rich plasmas. The phospholipase inhibitor, mepacrine, completely suppressed human platelet aggregation induced by AGEPC. These results indicate that AGEPC induces irreversible aggregation in human platelets by two distinct pathways, both dependent upon arachidonic acid metabolism: a cyclooxygenase-dependent one at low AGEPC concentrations and a cyclooxygenase-independent one, possibly mediated through the formation of lipoxygenase products, at high AGEPC concentrations.

Inhibition of human platelet aggregation by parathyroid hormone. Is cyclic AMP implicated?

Parathyroid hormone (PTH) is a polypeptide which in different in vitro systems raises intracellular cyclic AMP (cAMP) levels via adenyl cyclase activation and stimulates Ca2+ transport across cell membranes. We tested whether, on the basis of this mechanism, PTH would inhibit human platelet aggregation. The latter was tested in vitro by a photometric technique. Platelet aggregation induced by the calcium ionophore A 23187 was inhibited by PTH at concentrations (0.5-3 USP U/ml) similar to those effective in other in vitro systems. Higher concentrations of PTH were required to prevent aggregation initiated by adenosine-5'-diphosphate, arachidonic acid, or platelet-aggregating factor. The terminal synthetic fragment 1-34 b PTH was ineffective against all aggregation stimuli. The antiaggregating effect of PTH was potentiated by verapamil and theophylline and was additive to that of PGI2. However, PTH did not appear to increase platelet cAMP levels and was not counteracted by an inhibitor of platelet adenyl cyclase. It is therefore unlikely that PTH inhibits platelet aggregation through an adenyl cyclase stimulated increase of cAMP. Since PTH levels are markedly increased in uremic plasma, it might contribute to the defective platelet function and the bleeding tendency frequently occurring in uremic patients.

Inhibition of human platelet cyclo-oxygenase activity by sulfinpyrazone and three of its metabolites

Sulfinpyrazone and three of its major metabolites were compared in vitro for their inhibitory effect on human platelet cyclo-oxygenase activity. Sulfinpyrazone appeared to be about 15-20 times less potent than its sulfide metabolite (G25671) and 6-7 times less potent than the other two compounds, the sulfone metabolite (G31442) and p-hydroxysulfide (G33378). All four compounds were apparently competitive inhibitors of platelet cyclo-oxygenase activity. Comparison of the potency of sulfinpyrazone and its metabolites, as determined in the present study and the plasma levels previously measured in man, indicates that sulfinpyrazone and G33378 were not potent enough to be effective in man. G31442 showed inhibitory potency slightly lower than its corresponding plasma levels, whereas G25671 was effective at concentrations well below those found in human plasma. This study supports the hypothesis that sulfinpyrazone metabolites (in particular the sulfide) rather than the drug itself affect platelet function when administered therapeutically.

The metabolism of arachidonic acid by platelets in nephrotic syndrome

The production of malondialdehyde (MDA) and thromboxane B2 (TxB2) by platelets following an arachidonic acid (AA) challenge was greater in nephrotic platelet rich plasma (PRP) than in normal PRP. The uptake of 14C-AA, and its subsequent conversion to 14C-TxB2 following a thrombin stimulus, was also greater in nephrotic than normal PRP. Normal plasma diminished the MDA production by nephrotic platelets. The addition of albumin to nephrotic PRP, or, the intravenous infusion of albumin in quantities sufficient to correct hypoalbuminemia also diminished the excessive production of prostaglandin metabolites by nephrotic platelets. The platelet aggregate ratio (PAR), which measures circulating platelet aggregates, was abnormal during the acute phase of nephrotic syndrome but reverted to normal following remission. These data indicate that hypoalbuminemia is associated with increased AA metabolism by platelets and suggest that platelet "hyperactivity" may contribute to the proclivity toward thrombosis observed in nephrotic syndrome.

Salicylate fails to prevent the inhibitory effect of 5,8,11,14-eicosatetraynoic acid on human platelet cyclo-oxygenase and lipoxygenase activities

Sodium salicylate is inactive both on cyclo-oxygenase and lipoxygenase prepared from human platelets. It prevents the inhibition of cyclo-oxygenase induced by aspirin, but does not counteract the inhibitory effect of 5,8,11,14-eicosatetraynoic acid on both enzymes. It also fails to interfere with the inhibitory activity of nordihydroguaiaretic acid on lipoxygenase. These data indicate that, unlike eicosatetraynoic acid, non-steroidal anti-inflammatory drugs interact with a site on cyclo-oxygenase distinct from the catalytic site, although related to it. Such a supplementary binding site is lacking on lipoxygenase.

In vivo platelet activation with in vitro hyperaggregability to arachidonic acid in renal allograft recipients

Renal allograft recipients were investigated to determine the extent and possible nature of in vivo platelet activation. In 92 allografted patients stable for more than 4 months' duration, intraplatelet serotonin in circulating platelets was depleted significantly. In a further 16 patients studied serially for 12 to 16 weeks following transplantation, intraplatelet serotonin fell abruptly within 4 days from transplantation to very low levels, and remained thus for 10 weeks, rising toward normal at about 12 weeks. Although some patients showed abrupt falls in intraplatelet serotonin coincident with acute rejection episodes, there was no difference in intraplatelet serotonin in seven patients whose grafts functioned well immediately and remained stable, and seven in whom repeated rejection led to graft loss within 3 months. Thus, these tests of platelet function do not permit diagnosis of rejection or prediction of graft outcome. Plasma platelet factor 4 (PF4) concentrations, in contrast, were normal in most patients during the first 6 weeks after grafting, then rose and remained abnormal up to 13 years following the allograft in the long-term stable graft recipients. This discrepancy suggests a different mode of platelet activation in the first few weeks after grafting from subsequent months. Despite universal depletion of intraplatelet amines and alpha-granule contents only four out of 14 early allograft recipients had an abnormal bleeding time, and platelet aggregation thresholds with adenosine-5'-diphosphate and collagen were not different from controls. However, thresholds for platelet aggregation with arachidonic acid were reduced significantly (P less than 0.01) and thromboxane B2 generation was increased in vitro. There was no correlation between depletion of intraplatelet serotonin and circulating platelet-agglutinating material, but nine of 17 biopsy specimens from rejecting allografts taken during the first 3 months showed extensive glomerular localization of platelet membrane antigens and PF4.

Reduced platelet thromboxane formation in uremia. Evidence for a functional cyclooxygenase defect

A qualitative platelet abnormality and a bleeding tendency are frequently associated with renal failure and uremia. We demonstrated previously that uremic patients display an abnormal platelet aggregation to arachidonic acid and reduced malondialdehyde production in response to thrombin and arachidonic acid. The objectives of this investigation were: (a) to compare platelet prostaglandin (PG) and thromboxane (TX) production in whole blood and in platelet-rich plasma (PRP) of 21 uremic patients and 22 healthy subjects; (b) to evaluate the concentration and activity of platelet PG- and TX-forming enzymes; (c) to assess the functional responsiveness of the platelet TXA(2)/PGH(2) receptor; (d) to explore the hemostatic consequences of partially reduced TXA(2) production.Platelet immunoreactive TXB(2) production during whole blood clotting was significantly reduced, by approximately 60%, in uremic patients as compared to age- and sex-matched controls. Exogenous thrombin (5-30 IU/ml) failed to restore normal TXB(2) production in uremic platelets. Uremic PRP produced comparable or slightly higher amounts of TXB(2) than normal PRP at arachidonate concentrations 0.25-1 mM. However, when exposed to substrate concentrations >2 mM, uremic PRP produced significantly less TXB(2) than normal PRP. To discriminate between reduced arachidonic acid oxygenation and altered endoperoxide metabolism, the time course of immunoreactive TXB(2) and PGE(2) production was measured during whole blood clotting. The synthesis and release of both cyclooxygenase-derived products was slower and significantly reduced, at all time intervals considered. Furthermore, PGI(2) production in whole blood, as reflected by serum immunoreactive 6-keto-PGF(1alpha) concentrations, was significantly reduced in uremic patients as compared with healthy subjects. PGH synthase levels, as determined by an immunoradiometric assay, were not significantly different in platelets from uremic patients as compared to control platelets. A single 40-mg dose of aspirin given to five healthy volunteers reduced their serum TXB(2) to levels found in uremic patients. This was associated with a significant increase of threshold aggregating concentrations of ADP and arachidonic acid and prolongation of bleeding time. Substantially similar threshold concentrations of U46619, a TXA(2) agonist, induced aggregation of normal and uremic platelets. Prostacyclin induced a significant elevation of uremic platelet cyclic AMP, which was suppressed by U46619, further suggesting normal responsiveness of the TXA(2)/PGH(2) receptor. WE CONCLUDE THAT: (a) an abnormality of platelet arachidonic acid metabolism exists in uremia, leading to a reduced TXA(2) production; (b) the characteristics of this abnormality are consistent with a functional cyclooxygenase defect; (c) reduced TXA(2) production may partially explain the previously described abnormality of platelet function in uremia.

Indomethacin prevents the long-lasting inhibitory effect of aspirin on human platelet cyclo-oxygenase activity

Aspirin and indomethacin do interact with the same site on cyclo-oxygenase. This suggestion is based on in vitro studies on ram seminal vesicles and in vivo drug interaction studies on rat platelets. The purpose of the present study was to ascertain whether the same interaction also occurred after administration of both drugs to human volunteers. Platelet aggregation induced by sodium arachidonate or by collagen, and formation of platelet MDA and TxB2 were measured before, two and 48 hours after ingestion of either indomethacin (50 mg) or aspirin (500 mg) or of both drugs (30 minutes apart). While the inhibitory effect of indomethacin on these parameters was short lasting, that of aspirin persisted for at least 48 hours. However, when both drugs were given concurrently, the long-lasting effect of aspirin was no longer detectable. Since competition at levels other than platelets was unlikely, this study indicates that indomethacin and aspirin inhibit human platelet cyclo-oxygenase by the same basic mechanism. Acetylation of the enzyme appears to be a secondary mechanism which makes the inhibitory effect of aspirin persistent.

Platelet function in patients on maintenance hemodialysis: depressed or enhanced?

Uremic patients on long-term hemodialysis show a paradoxical association of hemorrhages on the one hand and thrombotic complications or atherosclerosis on the other. Platelet function has been found to be depressed in some cases but enhanced in others. In 19 patients, both platelet aggregation and prostaglandin formation appeared to be significantly enhanced in response to low concentrations of arachidonic acid but significantly reduced with high concentrations. It is suggested that this double functional abnormality of uremic platelets may contribute to the complex vascular disturbances of hemodialyzed patients.

Non-steroidal anti-inflammatory drugs react with two sites on platelet cyclo-oxygenase. Evidence from "in vivo" drug interaction studies in rats

Non-steroidal anti-inflammatory drugs inhibit platelet cyclo-oxygenase activity. This study shows that salicylate, diflunisal, sulphinpyrazone and indomethacin prevention vivo aspirin inhibitory effect on cyclo-oxygenase activity as measured by the formation of malondialdehyde and thromboxane B2, two products of platelet arachidonic acid metabolism. Salicylate also prevents the inhibitory effect of indomethacin. All these drugs therefore appear to interact with the same site on platelet cyclo-oxygenase. Since salicylate is inactive by itself on this platelet enzyme and diflunisal and sulphinpyrazone were used at ineffective doses, it is suggested that their interaction with aspirin (or indomethacin) occurs at the level of a supplementary binding site, rather than directly on the substrate active site. Interaction with this putative supplementary site is necessary but not sufficient for the efficacy of these drugs on cyclo-oxygenase inhibitors. Acetylation by aspirin of the active site appears to be a phenomenon secondary to the binding of salicylate moiety to the supplementary site.

Salicylate-aspirin interaction in the rat. Evidence that salicylate accumulating during aspirin administration may protect vascular prostacyclin from aspirin-induced inhibition

Aspirin inhibits cyclooxygenase, thus preventing thromboxane A2 production in blood platelets and prostacyclin in vascular cells. Aspirin is rapidly hydrolyzed to salicylate in the circulation. The objectives of this study were (a) to evaluate whether administration of salicylate, though ineffective by itself, prevents the inhibitory effect of aspirin on platelet and/or vascular cyclooxygenase activity; (b) to verify whether salicylate accumulating in blood after aspirin administration interferes with the pharmacological activity of further doses of aspirin. Pretreatment of rats with sodium salicylate (25-100 mg/kg i.p.) resulted in dose-related prevention of the effect of a subsequent dose of aspirin (2.5-10 mg/kg i.v.) on both platelet and vascular cells. Sodium salicylate appeared to amplify the greater response of platelets to aspirin compared with vessel wall. Pretreatment of rats with repeated high doses of aspirin (200 mg/kg) resulted after 24 h in blood salicylate levels (150-200 microgram/ml) that significantly prevented the inhibitory effect of a subsequent dose of aspirin on newly synthesized vascular prostacyclin. Blood salicylate levels obtained after 36 or 48 h (less than 50 microgram/ml) were too low to blunt aspirin's effect. The interference with aspirin of its major endogenous metabolite should be borne in mind when interpreting results obtained with high dose aspirin or during repeated administration of this drug.

Platelet and vascular prostaglandins in uraemia, thrombotic microangiopathy and pre-eclampsia

The metabolism of arachidonic acid in platelets and vascular cells is often altered in clinical conditions associated with haemorrhagic or thromboembolic complications. We have focused on the one hand on uraemia as a condition frequently complicated by bleeding episodes and, on the other, on thrombotic microangiopathy (thrombotic thrombocytopenic purpura, t.t.p.; haemolytic-uraemic syndrome, h.u.s.) and pre-eclampsia as conditions characterized by uncontrolled intravascular platelet activation. The observation that prostaglandin synthesis may be regulated by factors present in normal human plasma (Saeed et al . 1977; MacIntyre et al . 1978) prompted us to investigate whether such plasmatic control was altered in the clinical situations mentioned. Venous specimens removed from uraemic patients during the institution of an artero-venous shunt for haemodialysis generated significantly more prostacyclin (prostaglandin I 2 , PGI 2 ) than control vessels (Remuzzi et al . 1977). Similar findings were subsequently reported in aortic tissue from nephrectomized rats and in arterial tissue from uraemic patients (Leithner et al . 1978). Uraemic plasma showed a greater capacity for stimulating PGI 2 synthesis by vascular rings or endothelial cultured cells (Remuzzi et al . 1978; Defreyn et al . 1980).

Differential effects of itanoxone--a new hypolipidemic and hypouricemic drug--on platelet and vascular prostaglandin generation in rats

Itanoxone ((chloro-2'-diphenyl)-4-oxo-4 methylene 2-butyric acid), a newly developed, hypolipidemic and hypouricemic compound with moderate anti-inflammatory activity, showed a short-lived, dose-dependent (20--200 mg/kg, orally), apparently competitive inhibition of platelet malondialdehyde (MDA), stimulated by either thrombin or arachidonic acid. Repeated doses did not result in any cumulative effect. At doses which completely blocked MDA production, itanoxone also inhibited thrombin-stimulated thromboxane B2 production in platelets but had no measurable effect on vascular prostacyclin generation. Pretreatment with itanoxone partially prevented the inhibitory effect of aspirin on both platelet and vascular prostaglandin synthesis. This suggests that itanoxone--like aspirin--acts at the level of cyclo-oxygenase but with greater selectivity on the platelet enzyme. This pharmacological activity is of great theoretical interest for the potential use of this compound as an antithrombic drug.

Impaired thromboxane production by newly formed platelets after aspirin administration to thrombocytopenic rats

The in vivo inhibitory effect of aspirin on platelet cyclo-oxygenase is irreversible and lasts for the entire platelet life-span. Reappearance of cyclo-oxygenase activity in blood after aspirin has been proposed as a measure of the formation of new platelets and as an indirect indicator of platelet survival. A delay of 24--72 h in recovery, however, has been observed and it has been suggested that aspirin might also inhibit megakaryocyte cyclo-oxygenase. To test this possibility, aspirin (100 mg/kg) or saline were administered i.p. to rats made thrombocytopenic 2 h later (platelet count less than 5% of basal value) by a specific antiplatelet antiserum. Malondialdehyde (MDA) and thromboxane B2 (TxB2) production by platelets was measured by spectrophotometry and radioimmunoassay respectively, during the period of platelet count restoration. By 24 h after thrombocytopenia was induced, platelet count was about 15% of basal values in control and aspirin-treated rats. However, while in controls MDA and TxB2 production was restored to about 20% of basal values, in aspirin-treated rats less than 5% return of activity was detected. A marked difference between the two groups was still found 96 h after induction of thrombocytopenia, when platelet count restoration was similar. Since aspirin disappeared very rapidly from the circulation, the delay in recovery of cyclo-oxygenase activity supports the hypothesis of a megakaryocyte effect of this drug.

Malondialdehyde formation in rat platelet-rich plasma. I.A kinetic approach

Malondialdehyde (MDA) is a stable produce of arachidonic acid metabolism, catalyzed by the enzyme cyclo-oxygenase. The experimental conditions for measuring the kinetics of MDA formation in rat citrated platelet-rich plasma were defined. Platelets were stimulated with either arachidonic acid, the substrate of MDA, or thrombin, an enzyme which induces release of free arachidonic acid from platelet membrane phospholipids. MDA formation was almost linear for a limited period of time (between 0 and 2 min with arachidonic acid and between 1 and 3 min with thrombin) and was concentration-dependent with saturation kinetics. The hyperbolic curves obtained could be recast in linear plots (according to Woolf transformation S/V versus S) when arachidonic acid was used. With thrombin, in contrast, the highest concentration at which no MDA production could be detected (3 NIH u/ml) had to be subtracted from each concentration of the enzyme used to obtain Woolf plots. The apparent Km value of arachidonic acid was 0.49 /+- 0.09 mM and Vmax was 1.44 /+- 0.06 nmoles MDA/1.4 Z 10(9) platelets/min. The corresponding values in experiments with thrombin were 6.5 /+- 1.5 NIH u/ml and 0.233 /+- 0.012 nmoles MDA/1.4 X 10(9) platelets/min.

Malondialdehyde formation in rat platelet-rich plasma. II. Modification of the reaction kinetics by aspirin and indomethacin in vitro

The type of inhibition and the relative potency of aspirin and indomethacin on rat platelet malondialdehyde (MDA) formation were investigated in an in vitro system. Both drugs inhibited the production of MDA after platelet stimulation with either thrombin (10 or 25 NIH u/ml) or sodium arachidonate (0.5-2.25 mM). Inhibition by both drugs was concentration-dependent, was partially removed when platelet-rich plasma was diluted with platelet-poor plasma, was much stronger when either drug was preincubated with platelets for 10 minutes than for 1 minute and was apparently competitive when analysed by Dixon plots (1/V versus inhibitor concentrations). It is suggested that both aspirin and indomethacin may inhibit in vitro cyclo-oxygenase activity in citrated platelet-rich plasma by a similar, if not identical, partially reversible mechanism, not involving - in a first step - covalent binding of either drug to enzyme. A scheme of the interaction of both drugs with cyclo-oxygenase is presented which also takes into account the irreversible acetylation of the enzyme occurring after longer incubation times with aspirin.

Arachidonic acid-induced malondialdehyde formation in rat platelets. Kinetic aspects and inhibition by acetylsalicylic acid and indomethacin

Malondialdehyde (MDA) is a stable product of arachidonic acid metabolism, catalyzed by the enzyme cyclo-oxygenase. The experimental conditions for measuring the kinetic of MDA formation in rat platelet-rich plasma were defined. In platelets stimulated with arachidonic acid MDA formation was almost linear for a limited period of time (between 0 and 2 min) and was concentration-dependent with saturation kinetics. The hyperbolic curves obtained were recast in a linear plot (according to transformation S/V versus S) and straight lines fitting all experimental points were obtained. The apparent Km value of arachidonic acid was 0.49 +/- 0.09 mM and Vmax 1.44 +/- 0.06 nmoles MDA/1.4 x 10(9) platelets/min. The apparent type of inhibition and the relative potency of acetylsalicylic acid and indomethacin on MDA formation were also investigated. Inhibition by both drugs was concentration-dependent, and was much stronger when either drug was preincubated with platelets for 10 min than for 1 min. Analysis of the data by Dixon plots (1/V versus inhibitor concentrations) revealed an apparently competitive type of inhibition. It is suggested that both acetylsalicylic acid and indomethacin inhibit cyclo-oxygenase in platelet-rich plasma by a similar mechanism, not involving covalent binding of either drug to the enzyme.

Treatment of the hemolytic uremic syndrome with plasma

Two patients with the hemolytic uremic syndrome were treated with plasma exchange an infusion: in both cases, the reduced platelet count reverted to normal values and the microangiopathic anemia ceased within a few days. Systemic blood pressure and requirement for antihypertensive drug therapy were also markedly reduced following treatment with plasma. Venousprostacyclin (antiplatelet aggregating) activity was undetectable in both patients before but was restored after treatment with plasma. The plasma samples collected before, but not those collected at various intervals after replacement therapy, had decreased capacity to stimulate prostacyclin activity in rat aortic rings. It is suggested that in patients with the hemolytic uremic syndrome or with other clinical conditions which can be included under this rubric (such as thrombotic thrombocytopenic purpura) a plasma factor is lacking which stimulates prostacyclin activity. Plasma would supply such a missing factor, thus representing a rational treatment for some of the life-threatening manifestations (thrombocytopenia, hemolytic anemia, hypertension) of this severe syndrome.

Prostacyclin and human foetal circulation

Tissues from human umbilical cord arteries and placental veins generated much greater prostacyclin activity than vessels from normal adults. High prostacyclin generation could contribute to maintaining the low peripheral vascular resistance typical of foetal circulation in which blood pressure is low despite very high cardiac output.

Bleeding in renal failure: a possible role of vascular prostacyclin (PGI2)

Specimens of venous tissues from a group of 25 patients with chronic uremia and 7 patients with acute renal failure generated significantly higher PGI2-like (platelet aggregation inhibiting) activity than venous tissues from 30 normal subjects. After repeated washings, when this activity could barely be detected in the controls, pronounced inhibitory activity was still evident in samples containing venous tissues from uremic patients. Both prolonged bleeding times and increased PGI2-like activity returned to normal in 4 acute uremic patients on restoration of their renal function. These findings may be relevant to the pathogenesis of bleeding in renal failure.