Influence of low dose enteric-coated aspirin on platelet function

Little is known about the effect of low dose, enteric-coated aspirin on human blood platelet function. This study was conducted to evaluate the acute effects of a single daily dose of commercially available enteric-coated aspirin on platelet biochemistry, physiology and function. Blood for these studies was obtained from drug-free volunteer donors prior to ingestion of aspirin or following ingestion, either before breakfast or following lunch. Response of platelets to the action of weak agonists was evaluated. In addition, ability of platelets to convert radiolabeled arachidonic acid to thromboxane was monitored. Results of our studies show that a single daily dose of 50 mg of aspirin taken either before breakfast or after lunch effectively prevented the secondary wave aggregation response, as well as secretion of dense body contents when stimulated by agonists such as epinephrine and ADP. Aspirin ingestion caused a dose-dependent inhibition of platelet cyclooxygenase activity as evidenced by the extent of arachidonic acid converted to thromboxane by platelets exposed to aspirin for different time periods. Based on these observations, it is suggested that low dose aspirin may be very useful and desirable to restrain platelet activity in clinical situations in which increased thromboxane formation may initiate vascular hypertension and platelet hyperactivity.

Influence of aspirin and carbacyclin on bovine platelet function

Cows, calves, and sheep are the animals of choice for in vivo studies on total artificial hearts. In this study, the response of human and bovine platelets to agonists and antagonists was followed. Epinephrine and arachidonate failed to cause aggregation of bovine platelets. Exposure of bovine platelets to prostaglandin E1 (PGE1) resulted in inhibition of response to the action of adenosine diphosphate (ADP). Although epinephrine restored the response of PGE1 treated human platelets, it failed to restore the sensitivity of bovine platelets to the action of ADP. Exposure of bovine platelets to aspirin (100.0 mumol/L) or administration to calves intravenously (10.0 mg/kg) inhibited platelet cyclo-oxygenases. Infusion of carbacyclin (U55185) inhibited the ex vivo platelet response to the action of ADP. Results of this study demonstrate that the response of bovine platelets to agonists such as epinephrine, arachidonate, and the endoperoxide mimetic, U46619, is severely compromised. The authors' observations in this study, as well as earlier findings demonstrating the inability of bovine platelets to fully spread on a surface, suggests that the cow may not be an appropriate "model" for evaluating thrombogenicity caused by artificial organs and implants.

Influence of calcium antagonists on thrombin-induced calcium mobilization and platelet-vessel wall interactions

Elevation of cytosolic ionized calcium plays a critical role in human platelet activation. We have evaluated three well-characterized calcium antagonists for their ability to prevent thrombin-induced calcium mobilization in Fura 2 AM-loaded platelets and also their ability to inhibit platelet-vessel wall interactions. Thrombin (0.2 U/ml) caused significant elevation of cytosolic calcium (basal 84 +/- 18, activated 546 +/- 76 nM; n = 3). Verapamil, diltiazem, and nifedipine (100 microM) did not exert any inhibitory effect on thrombin-mediated calcium elevation. Untreated platelets perfused through a Baumgartner chamber containing a rabbit aorta preparation reacted with exposed and denuded subendothelium. The percentage of the total area covered by control platelet thrombi was 39.6 +/- 3.4. Diltiazem and Nifedipine significantly reduced the percentage of area covered by platelet thrombi, but the drugs were not as effective as aspirin (8.2 +/- 1.4). Calcium antagonists studied did not inhibit thrombin-stimulated elevation of cytosolic calcium in blood platelets. Although these drugs have been shown to prevent in vitro platelet aggregation and offer some protection against risks for atherosclerosis and thrombosis, they failed to significantly inhibit platelet-vessel wall interactions leading to formation of spread platelets and aggregates.

Platelet function in adolescent idiopathic scoliosis

Recent studies have reported abnormal platelet morphology and function in patients with adolescent idiopathic scoliosis. These abnormalities include increased platelet size and dense body numbers, abnormal aggregation, thromboxane A2 synthesis, serotonin release to adenosine diphosphate and epinephrine stimulus, and decreased myosin-adenosine-triphosphatase-specific activity. It was postulated that a membrane-specific defect in calcium transport may be partially responsible for the abnormalities found. In response to a suggestion in the literature that platelet screening could be clinically useful in scoliosis evaluation as well as in basic research of its pathophysiology, a study was performed to evaluate platelet morphology, biochemistry, and function in patients with adolescent idiopathic scoliosis. Platelets from nine volunteers with adolescent idiopathic scoliosis were compared with cells from a control group of nine patients. No significant differences in measured platelet parameters were noted between adolescent idiopathic scoliosis patients and control groups. Platelets from both groups demonstrated normal aggregation and release patterns with all agents except for a mild decreased aggregation and secretion response to epinephrine. No significant differences were noted in serotonin or adenine nucleotide levels. No significant ultrastructural differences were noted. Earlier findings of an abnormal aggregation and secretion response to adenosine diphosphate, increased numbers of dense bodies, or increased intracellular calcium could not be confirmed. On the contrary, we found normal, if not slightly decreased, numbers of dense bodies per platelet and calcium levels that were not different from controls.

Influence of thrombolytic agents on human platelet function

In this study, we have evaluated the effects of four different thrombolytic agents, including Streptokinase from Hoechst and from Kabivitrum, Urokinase from Abbott and tissue plasminogen activator (t-PA) from Genetech, on platelet-rich plasma clots and platelet aggregation. At concentrations lower than 50 ugs/ml, t-PA had no inhibitory effect on clot retraction or platelet aggregation induced by weak or potent agonist. At a higher concentration (greater than 100 ugs/ml), t-PA specifically antagonized the action of thrombin on clot formation and platelet aggregation. Streptokinase (Kabivitrum) potentiated the action of weak agonists on platelet aggregation, but the same agent from Hoechst had no negative or positive influence. None of the drugs tested had an adverse effect on platelet function at suggested therapeutic levels. None of the thrombolytic agents were capable of dissociating preformed clots made from platelet-rich plasma. However, all of them caused lysis of whole blood clots. Also, prior incubation of plasma alone or platelet-rich plasma with any of the agents prevented subsequent clot formation. The studies demonstrate that thrombolytic drugs at therapeutic concentrations do not affect platelet function adversely. They have a potent effect on whole blood clots, but not on clots from platelet-rich plasma. Therefore, platelets may play a critical role in determining the degree of reperfusion and the frequency of reocclusion following treatment with thrombolytic agents in vivo.

Influence of heparins on inositol 1,4,5-trisphosphate-induced calcium mobilization in permeabilized human platelets

Heparin has been shown to prevent inositol 1,4,5-trisphosphate (IP3) binding to its receptor and to inhibit IP3-induced calcium mobilization in a variety of cells. Heparin added to whole blood at a concentration of 1 U/ml prevented thrombin-induced secretion of granule contents and irreversible aggregation of platelets. Heparin (2-15 kDa) had no inhibitory effect on IP3-induced calcium mobilization in Fura 2-loaded, saponin (10-15 micrograms/ml)-permeabilized platelets. None of the commercially available heparin preparations can induce inhibition of agonist-induced calcium mobilization in intact platelets because they are not cell permeant. Mild saponin treatment makes the membrane permeable to IP3, but restricts the action of heparins. Recent observations suggesting heparin's affinity to IP3 binding sites will be of clinical interest if effective cell permeant analogs can be developed.

Albinism and Hermansky-Pudlak syndrome in Puerto Rico

Five types of oculocutaneous albinism and two types of ocular albinism were found among 349 Puerto Rican albinos. The most prevalent type of albinism was the Hermansky-Pudlak syndrome (HPS). HPS was observed in five of every six albinos in Puerto Rico. The prevalence of HPS was highest in the northwestern quarter of the island, affecting approximately one in 1,800 persons, and approximately one in 22 are carriers of the gene. HPS is an autosomal recessively inherited triad of a tyrosinase-positive type of albinism, a hemorrhagic diathesis due to storage pool deficient platelets and accumulation of ceroid in tissues. The pigmentary phenotype of HPS albinos resembled that of any other type of oculocutaneous or ocular albinism. The most reliable method of diagnosing HPS is by a deficiency of platelet dense bodies observed by electron microscopy. The accumulation of ceroid in the tissues is associated with fibrotic restrictive lung disease and granulomatous enteropathic disease. The enteropathic disorder resembles Crohn's disease and with few exceptions, had its onset after 13 years of age. The major causes of death were fibrotic restrictive pulmonary disease, hemorrhagic episodes and sequelae of granulomatous enteropathic disease. Menometrorrhagia was common in women with HPS. No immune deficiency was found in HPS patients. The majority of patients with HPS had visual acuities of 20/200 or worse and consequently were legally blind. Albinos of all types, including HPS, lacked binocular vision due to nearly complete crossing of the optic tracts.

Influence of nitric oxide on agonist-mediated calcium mobilization in platelets

Recent studies have characterized endothelium-derived relaxing factor as nitric oxide. It appears to exert its effect by elevating intracellular levels of cyclic GMP. In this study we confirm that nitric oxide is a potent inhibitor of agonist-induced irreversible aggregation. At the concentrations tested nitric oxide effectively blocked thrombin-stimulated mobilization of cytosolic-free calcium in Fura 2-loaded platelets. In addition, nitric oxide prevented the inositol 1,4,5-trisphosphate-stimulated calcium rise in cytosolic calcium in saponin-permeabilized Fura 2-loaded platelets. Similar to the action of adenylate cyclase stimulators, nitric oxide facilitated lowering of calcium levels raised by the action of agonists. The specific mechanism by which it exerts its effect on intracellular levels of calcium is not clear.

Aspirin, prostaglandin E1 and Quin-2 AM-induced platelet dysfunction: restoration of function by noradrenalin

Studies from our laboratory have demonstrated that adrenalin can restore the function of drug-induced refractory platelets to the action of physiological agonists via a novel mechanism (membrane modulation). In various disease states and clinical conditions the circulating levels of noradrenalin (NA) increase several fold more than adrenalin. Therefore, in this study the influence of NA on three well characterized platelet refractory models has been evaluated. Aspirin-exposed platelets were obtained for these studies from blood of donors who had taken one baby aspirin (80 mgs) per day for four consecutive days. Prostaglandin-exposed platelets were obtained by disaggregating ADP-induced aggregates through addition of prostaglandin E1 (1 microM). Finally, low calcium platelets were obtained by buffering cytosolic free calcium with a calcium specific cell permeant fluorophore, Quin-2 AM (60 microM). Drug-exposed platelets did not aggregate irreversibly when stirred with arachidonate (0.45 mM) or NA 5 microM). However, when treated with NA first, drug-treated platelets regained their sensitivity to the action of arachidonate and aggregated irreversibly. The ability of NA to restore the sensitivity of drug-induced refractory platelets was effectively blocked by yohimbine (10 microM), an alpha 2 adrenoceptor antagonist. Results of these studies suggest that NA, similar to the action of adrenalin, can activate membrane modulation and restore the sensitivity of platelets to the action of physiological agonists under a variety of experimental conditions.

Surface-activated bovine platelets do not spread, they unfold

The present study has examined the response of bovine platelets to surface activation and compared it to the reaction of human cells. Human platelets react to surfaces by losing their discoid shape, extending pseudopods, converting to dendritic forms, and finally, spreading into thin films resembling pancakes. Bovine platelets do not spread, they unfold. Surface activation causes them to transform from discs to irregular, flattened shapes resembling dendritic platelets, but they are unable to fill in spaces between pseudopods, a step required for spreading. Bovine platelets lack the surface-connected open canalicular system (OCS), which serves as a reservoir of membrane for human platelet spreading. Its absence may be the major factor in the failure of bovine platelet spreading, but there are other possible factors. Circumferential microtubules are more resistant to disassembly in surface-activated bovine than human cells, and their stability as rings or fractured bundles may limit spreading. Actin filament assembly is similar in human and bovine platelets, but the organization is different. Human platelets form a peripheral weave of actin that expands the membrane between pseudopods. A peripheral weave does not form in surface-activated bovine platelets. The absence of the OCS and differences in cytoskeletal organization in bovine platelets may also affect spreading of the surface membrane. Fibrinogen-gold (Fgn-Au) probes added to spread human platelet move from pseudopods and the cell margin toward the center and concentrate in the OCS. Fgn-Au particles bind to surface-activated bovine cells, but move very little, or not at all. All of these factors may contribute to the inability of bovine platelets to react to surfaces by spreading like human cells, but absence of the OCS appears to be the major cause.

Role of glutathione and glutathione peroxidase in human platelet arachidonic acid metabolism

Selective removal of intracellular glutathione (GSH) and inhibition of the GSH-dependent peroxidase (GSH-Px) by 1-chloro-2,4-dinitrobenzene (CDNB) was used to evaluate the role of GSH and GSH-Px in arachidonic acid (AA) metabolism in human platelets. Although total conversion of AA through the lipoxygenase pathway is lowered by GSH depletion, significant 12-HETE formation was observed suggesting that GSH and GSH-Px are not required for the generation of 12-HETE in human platelets. Prolonged treatment of platelets with CDNB (2 h) completely destroyed GSH-Px activity creating a model in which the effects of GSH alone could be determined. Platelet homogenates replenished with GSH, but lacking GSH-Px activity converted significantly higher amounts of AA to 12-HPETE and 12-HETE than control. Platelet cytosolic metabolism of 15-HPETE to 15-HETE decreased after CDNB, while the membrane metabolism remained similar to control due to high GSH-independent peroxidase activity associated with the membranes. These results indicate that GSH and GSH-Px function to enhance lipoxygenase activity, rather than catalyse the reduction of 12-HPETE to 12-HETE.

The influence of glutathione depleting agents on human platelet function

Human platelets were dose- and time-dependently depleted of intracellular glutathione (GSH) by treatment with the chemical oxidizing agents diamide and N-ethylmaleimide (NEM), and by formation of chemical conjugates with 1-chloro-2,4-dinitrobenzene (CDNB) catalyzed by GSH-S-transferase. In addition to effects upon GSH, these agents also inhibited platelet GSH-peroxidase activity. The inhibitory effect of CDNB was selective for GSH-peroxidase, while diamide and NEM treatment caused inhibition of several other cytosolic enzymes tested. Arachidonic acid (AA) induced aggregation and secretion responses measured in platelets depleted of GSH by diamide and NEM were attenuated. In contrast, these platelet functions remained identical to control following GSH depletion by CDNB treatment, suggesting that GSH is not required for normal platelet aggregation or secretion. Effects of diamide and NEM apart from their action on GSH may account for the platelet dysfunction induced by these compounds.

Influence of phospholipase A2 on human blood platelet alpha adrenergic receptor function

Exposure of platelets to phospholipase A2 selectively obliterates the aggregation response of human platelets to the action of epinephrine. This treatment does not impair the affinity of the ligand binding although it reduces the number of sites available on platelets. Reduction in the number of alpha receptors caused by phospholipase treatment in no way affected the epinephrine-induced restoration of sensitivity of refractory platelets to the action of agonists.

Platelet hypersensitivity induced by 1-chloro-2, 4-dinitrobenzene, hydroperoxides and inhibition of lipoxygenase

Platelets administered 1-chloro-2,4-dinitrobenzene to deplete intracellular glutathione (GSH) and inhibit GSH-peroxidase responded with irreversible aggregation to low doses of arachidonic acid (AA) more rapidly than control cells. This increase in sensitivity was correlated to inhibition of GSH-peroxidase, and not with the depletion of GSH. Addition of hydrogen peroxide, 15-hydroperoxyeicosatetraenoic acid, or inhibition of the lipoxygenase metabolic pathway by 4,7,10,13-eicosatetraynoic acid also induced a hypersensitive aggregation response to AA. These results suggest that the three modes of treatment share a common mechanism of increasing AA metabolism to biologically active prostaglandins and thromboxane A2 through alterations in cyclooxygenase kinetics and available enzyme substrate.

Action of guanosine 5'-[beta-thio]diphosphate on thrombin-induced activation and Ca2+ mobilization in saponin-permeabilized and intact human platelets

The non-hydrolysable guanine analogues guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta-thio]diphosphate (GDP[S]) have been used extensively (as promoters and inhibitors respectively) to probe the importance of G-protein function. We report on the use of GDP[S] in permeabilized and intact platelets. The stimulatory analogue GTP[S] (9-60 microM) induces shape change, aggregation and 5-hydroxy[14C]-tryptamine secretion when added to saponin (12-14 micrograms/ml)-permeabilized platelets, but not to intact platelets. In line with the activation responses in permeabilized cells, GTP[S] induces an increase in [32P]-phosphatidic acid, which is indicative of phospholipase C activity. GDP[S] (greater than 400 microM) totally inhibits GTP[S] (90 microM)-stimulated phospholipase C activity and functional responses in saponized platelets. GDP[S] (1 mM) was also effective at inhibiting low-dose thrombin (0.1 unit/ml)-induced aggregation and secretion responses (without affecting shape change) in permeabilized platelets with inhibition of [32P]-phosphatidic acid formation. At higher doses of thrombin (greater than 0.5 unit/ml), both functional responses and [32P]phosphatidic acid formation are restored in the presence of GDP[S]. Studies on intact cells revealed that GDP[S] was as effective at inhibiting low-dose thrombin-induced functional responses as in the permeabilized cells, but there was no inhibition of [32P]phosphatidic acid formation, indicating that the agent is nonmembrane-penetrating. This reflected the fact that GDP[S] has additional inhibitory sites on the surface of platelets. In Fura-2-loaded cells GDP[S] inhibited thrombin-induced Ca2+ mobilization, as measured by Fura-2 fluorescence, in a dose-dependent manner. In studies with and without Ca2+ present on the outside, the effect of GDP[S] was to block Ca2+ influx. These studies indicate that, although GDP[S] is a valuable tool in studying G-protein function in permeabilized cells, it also has inhibitory activities on the surface of platelets, and one of these has been identified as an effect on the Ca2+-influx channel after agonist stimulation.

Epinephrine-induced reversal of aspirin effects on platelet deformability

The present study has evaluated the influence of epinephrine on the resistance of platelets to aspiration into micropipettes, and the effect of epinephrine on the altered deformability of aspirin treated platelets. Unlike other platelet agonists previously studied, epinephrine stimulation did not alter platelet deformability at a concentration capable of causing platelet aggregation. Aspirin caused a dramatic decrease in the resistance of platelets to aspiration. Pretreatment of platelets with epinephrine prevented aspirin from altering platelet deformability, and exposure of platelets to epinephrine after treatment with aspirin reversed the increased deformability produced by the drug. Blockade of alpha-2 adrenergic receptors with yohimbine or clonidine prevented epinephrine antagonism of the mechanical effects of aspirin. The studies provide further evidence of the novel antagonism between epinephrine and aspirin on platelet structure and function.

Measurement of ionized calcium in normal human blood platelets

To measure the concentrations of cytosolic ionized calcium in platelets we used a calcium-sensitive fluorophor (Fura-2) with two different spectrofluorometers (Perkin-Elmer LS-5 and Fluorolog 222). Values obtained by these two instruments for the basal cytosolic ionized calcium concentration of resting platelets and those of agonist-activated platelets did not differ significantly. Both instruments were capable of monitoring the shifts in wavelengths induced by the dye-calcium complex, the ratio between absorbances at the two wavelengths (340/380 nm), and calcium concentrations continuously during agonist-induced platelet activation. We conclude that a relatively inexpensive instrument may be adequate for measuring ionized calcium in cells by this method, although sophisticated kinetic studies may require analytical or research-grade instruments.

Influence of anti-platelet drugs on platelet-vessel wall interactions

The influence of in vitro treatment of platelets with antiplatelet drugs on the interaction of these cells with the subendothelium was studied using citrated human blood obtained from normal control donors. Reconstituted blood following drug treatment was circulated through a special chamber which housed everted segments of de-endothelialized rabbit aorta. The wall shear rate used in these studies was 800 sec-1. Surface coverage of platelets on the subendothelium were morphometrically evaluated. Aspirin, Ibuprofen, Prostaglandin E1 and 13-Azaprostanoic acid significantly reduced platelet thrombi on exposed subendothelium. The calcium antagonists, Quin 2 and Diltiazem, exerted similar inhibitory effects, whereas Verapamil was a poor inhibitor. Aspirin treatment significantly enhanced platelet adhesion to the exposed vascular surface. Salicylate and Salicylamide did not enhance platelet adherence. Only Aspirin enhanced the formation of lipoxygenase metabolites of radiolabeled arachidonate. Results suggest that drugs which inhibit platelet aggregation and secretion of granule contents reduce formation of platelet thrombi. However, these drugs may or may not have a similar influence on platelet interaction with the subendothelium leading to spreading, adherence or formation of aggregates.

Influence of calmodulin antagonist (stelazine) on agonist-induced calcium mobilization and platelet activation

Phenothiazines at high concentrations inhibit platelet aggregation and the secretion of granule contents. In this study we have evaluated the influence of stelazine on platelet function at low concentrations. Stelazine alone had no influence on resting calcium levels in platelets but facilitated agonist-induced elevation of cytosolic calcium. Platelets combined with low concentrations of stelazine (10 microM) and stimulated with subthreshold concentrations of thrombin (0.05 mu/ml) aggregated irreversibly and released significant quantities of ATP. Results of these studies suggest a new role for the calmodulin antagonist stelazine in platelet activation.

Influence of a calcium dependent protease inhibitor on platelet activation and secretion

Continuous proteolysis resulting in consumption of major cytoskeletal proteins may be essential for platelet activation and aggregation. In this study we evaluated the effect of a known protease inhibitor, Leupeptin, on agonist induced platelet aggregation and secretion. Platelets exposed to 10 ugs/ml of Leupeptin did not aggregate in response to the action of thrombin (0.2 u/ml). However, a concentration of Leupeptin as high as 250 ugs/ml did not prevent arachidonate induced aggregation and secretion. Leupeptin (100 ugs/ml) effectively blocked thrombin (0.2 u/ml) induced elevation of cytosolic calcium, but did not affect arachidonate induced elevation of platelet intracellular calcium levels. At a concentration of 100 ug/ml, Leupeptin effectively blocked thrombin (0.5 u/ml) induced clot formation of platelet poor plasma, suggesting that it can exert its effect on thrombin by preventing fibrinogen degradation. Effective Ki for the competitive inhibition of thrombin induced hydrolysis of a chromogenic substrate, S2238, by Leupeptin was 2.4 uM. Leupeptin inhibition of platelet function was reversible by washing platelets free of the polypeptide. Results of our study demonstrate that Leupeptin inhibits thrombin induced platelet activation, probably by interfering with its proteolytic activity on the platelet surface membrane. However, inhibition of platelet surface membrane associated proteases did not prevent activation of platelets by other agonists.

Aspirin treatment reduces platelet resistance to deformation

The present investigation has evaluated the influence of aspirin, its constituents, and other nonsteroidal anti-inflammatory agents on the resistance of human platelets to aspiration into micropipettes. Aspirin increased the length of platelet extensions into the micropipette over the entire negative tension range of 0.04 to 0.40 dynes/cm after exposure to the drug in vitro or after ingestion of the agent. Other cyclooxygenase inhibitors, ibuprofen and indomethacin, did not increase platelet deformability. The influence of aspirin was mimicked to some degree by high concentrations of salicylic acid, but acetylation of platelets with acetic anhydride had little influence on platelet deformability. Incubation of platelets with both salicylic acid and acetic anhydride had no more effect than salicylic acid alone. Benzoic acid, chemically similar to salicylic acid, had a minimal effect. The studies demonstrate that aspirin makes platelets more deformable, while components of the drug or other nonsteroidal antiinflammatory agents and cyclooxygenase inhibitors do not have the same influence on resistance to deformation.