Differential effects of putative inhibitors on cytosolic and membrane associated platelet lipoxygenase

The effects of Indomethacin, Esculetin, ETYA (4, 7, 10, 13-eicosatetraynoic acid, U53119), 3-amino-1-trifluoromethyl-7-phenyl-pyrazoline (BW 755C), Quercetin, Phenidone, and Nordihydroguaretic acid (NDGA) on the synthesis of 12-L hydroxyeicosatetraenoic acid (12-HETE) by human platelet 12-lipoxygenases were investigated. Except Indomethacin and Esculetin, all other drugs demonstrated significant inhibitory effect on 12-lipoxygenase activity. The rank order of potency for the inhibition of lipoxygenase in intact human platelets was ETYA greater than Quercetin greater than NDGA greater than Esculetin greater than Indomethacin. BW755C and Indomethacin were effective against platelet cyclooxygenase also. ETYA (U53119) was the most potent and selective inhibitor of platelet lipoxygenases. Results of our studies suggest that known lipoxygenase inhibitors display differential effects on platelet cyclooxygenase as well as membrane and cytosol associated lipoxygenases.

Interleukin 1 stimulates endothelial cell tissue factor production and expression by a prostaglandin-independent mechanism

Activation of coagulation occurs at inflammatory sites following the ingress of mononuclear cells, and may result from alterations in the vessel wall. Since the monokine, interleukin 1, initiates diverse responses to inflammation, its ability to enhance vascular procoagulant activity was studied. Interleukin 1-treated cultured human endothelial cells acquired elevated levels of the procoagulant, tissue factor. This required de novo protein synthesis, was maximal at 2 h after exposure to interleukin 1, and resulted in persistently elevated cellular procoagulant activity. Tissue factor was later expressed (6-24 h) on the surface of uninjured endothelial cells. Endothelial cell procoagulant production and expression in response to interleukin 1 could be dissociated from endogenous prostaglandin metabolism, being insensitive to hydrocortisone, indomethacin, eicosatetrayionic acid and exogenous arachidonic acid. In addition, no increase in prostaglandin synthesis occurred during the interval in which tissue factor was synthesized. We therefore conclude that interleukin 1 stimulates endothelial synthesis and surface expression of tissue factor by a prostaglandin-independent mechanism.

Inhibition of platelet aggregation by novel triphenylethylene analogs

The present study has evaluated the effect of some newly synthesized triphenylethylene (TPE) analogs on platelet arachidonic acid metabolism and function. All compounds tested inhibited arachidonic acid induced platelet aggregation and several were superior to aspirin in their relative potency. Introduction of a carboxyl function into the alpha-ring, which should enhance binding according to proposed structural models for cyclooxygenase inhibitors, was not found to be beneficial. Increased structural rigidity, which resulted from covalent linkage of two aromatic rings in this series, did not eliminate anti-aggregatory properties.

Epinephrine reverses the inhibitory influence of aspirin on platelet-vessel wall interactions

The effect of in vitro aspirin treatment and alpha adrenergic receptor stimulation on the interaction of platelets with the subendothelium was studied using citrated human blood obtained from normal control donors. Reconstituted blood following drug treatment was circulated through a chamber which housed everted segments of deendothelialized rabbit aorta. The wall shear rate was 800 sec-1. Surface coverage of platelets were morphometrically evaluated. Aspirin treatment significantly reduced platelet thrombi on exposed subendothelium. However, platelet spreading was increased. There was no significant difference in the total percent coverage of platelets in the vascular surface between the aspirin treated group and normal controls. Epinephrine exposure of aspirin treated platelets completely reversed the effect of alterations induced in platelet behavior by aspirin. Epinephrine exposed aspirin treated platelets had as many platelet thrombi on exposed subendothelium as normal control platelets. In addition, epinephrine treatment decreased the spreading of aspirin treated platelets on the vascular surface. Results of the present study and our earlier findings suggest that the epinephrine induced membrane modulation may be a major mechanism for protecting the hemostatic role of platelets after their function is compromised in vivo and in vitro. The failure of aspirin to offer significant protection in many clinical trials may be due to the presence of a salvage pathway in platelets provided by the mechanism of membrane modulation.

Influence of the calcium-sensitive fluorophore, Quin 2, on platelet function

Recent investigations using Quin 2, a fluorophore used to monitor cytosolic free calcium shifts, have shown that strong agonists cause a dramatic dose-dependent increase in platelet fluorescence. However, weak agonists stimulated little or no increase in light emission of Quin 2-loaded platelets, suggesting that calcium flux is not involved in activation by these agents. The present study has sought an alternative explanation for the failure of weak stimuli to cause a rise in cytosolic free calcium in platelets containing Quin 2. Conditions used to prepare, wash, load, gel-filter, and evaluate the fluorophore-filled cells were examined for their compatibility with retention of sensitivity to activation by weak agonists. The technique used to measure shifts in cytosolic calcium with Quin 2 requires multiply washed, unstirred platelets. Under these conditions, platelets do not aggregate or secrete in response to weak agonists. Quin 2, at concentrations greater than 40 mumol/L, inhibits the response of platelets to strong agonists, and completely blocks their reaction to weak agonists. Quin 2 inhibition of platelet function appears related to high buffering capacity for free calcium, although other mechanisms cannot be ruled out. This suggestion is supported by the observation that Quin 2-induced blockade can be overcome by membrane modulation, which is a calcium-dependent process. However, since both agonists are weak, significant elevation in cytosolic calcium concurrent with functional restoration could not be demonstrated under the experimental conditions used for monitoring calcium. Thus, the conditions used to prepare platelets for Quin 2 evaluation and Quin 2 itself appear to be responsible for the failure of weak agonists to cause evidence of a calcium shift in fluorophore-loaded cells.

Glutathione levels in human platelets display a circadian rhythm in vitro

Glutathione (GSH) levels were measured in platelet-rich plasma (PRP) and concentrates at 4 hour intervals during storage. The values fell steadily during the first several hours after collection at 9:00 a.m., reaching the lowest level at midnight, 14 hours later. Subsequently, the levels rose to a new peak value at 4:00 a.m. GSH levels continued to show cyclic variation over the 48 hours examined in this study. Change in schedules of light and dark under which the platelets were stored had no effect on GSH periodicity. Platelets from night shift workers also displayed a similar periodicity to that of day workers. Erythrocytes failed to demonstrate a similar time related variation in GSH levels during storage.

Arachidonic acid metabolism in thrombocytes and vascular tissues of turkeys

Turkeys are hypertensive compared to mammals of similar size. In vitro synthesis of thrombocyte thromboxane B2 (TxB2), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT), 12L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE) and aortic prostaglandin (PG) production was studied in one to ten month old domestic white turkeys. Compared to normal human platelets, TxB2 production was increased (55.4 vs. 31.4%) and HETE production was markedly reduced (6.5 vs. 34.6%) in control thrombocytes. Similar to human platelets in which cyclooxygenase inhibition with aspirin results in an increase in HETE production, block of the thrombocyte enzyme with aspirin doubled the production of HETE. In vitro conversion of radiolabeled arachidonic acid (AA) showed that the primary PG produced by turkey aorta was PGE2. A 6-keto immunoreactive PG was present which comigrated with authentic 6-keto PGF1 alpha, but failure of the aortic supernatant to inhibit adenosine diphosphate or AA induced platelet aggregation suggested that PGI2 was not produced. The vasodepressor potency of PGE1, PGE2 and PGI2 was altered in awake turkeys with PGE1 and PGE2 having five times the hypotensive effect as PGI2. In addition, conversion of AA to PGE2 by aorta in one month turkeys was greater (17.3 vs. 9.2%) than in ten month old turkeys. Systemic arterial pressure was increased in the ten month old turkeys (188 mmHg) compared to one month old turkeys (143 mmHg). Thus, both vascular AA metabolism and the vasodepressor potencies of PGE2 and PGI2 are altered and the activity of the lipoxygenase pathway in thrombocytes is limited in the turkey.

Measurement of ionized calcium in blood platelets with a new generation calcium indicator

Fura 2, a new generation calcium indicator, has a 30 fold brighter fluorescence than Quin 2, shows wavelength shifts upon calcium binding and has a relatively low buffering capacity for free calcium. Quin 2, the most widely used fluorophore, on the other hand, shows no wavelength shifts and has a very high affinity for free calcium. Therefore, we have compared the relative merits of these two fluorophores for monitoring agonist induced alterations in platelet cytosolic calcium. Platelets loaded with Fura 2 showed a significant rise in cytosolic calcium when stirred with agonists such as epinephrine, arachidonate and thrombin, whereas Quin 2 loaded platelets demonstrated a rise in cytosolic calcium only with thrombin stimulation. A rise in agonist induced calcium in Fura 2 loaded platelets was prevented when the cells were exposed first to antagonists such as aspirin or prostaglandin E1. Arachidonate refractory platelets, upon stirring with a single agonist, did not show a significant elevation in cytosolic calcium. However, when refractory platelets were first exposed to epinephrine and then challenged with arachidonate, they revealed a significant elevation in cytosolic calcium. Unlike Quin 2, Fura 2 at the highest concentration tested did not inhibit platelet function. Improved properties of Fura 2 suggest that it may be a useful agent to study agonist induced alterations in cytosolic calcium levels in blood platelets.

Irreversible platelet aggregation does not depend on lipoxygenase metabolites

Previous investigations in our laboratory demonstrated the existence of an intrinsic mechanism, termed membrane modulation, capable of restoring sensitivity to aspirin treated platelets, resulting in irreversible aggregation in response to arachidonic acid (AA). The mechanism underlying correction of aspirin induced inhibition of platelet function, however, was not clear. In the present study we have evaluated the role of lipoxygenase (LO) metabolites of AA in securing irreversible aggregation of drug induced cyclooxygenase (CO) deficient platelets. Platelets treated with aspirin or Ibuprofen did not convert radiolabeled AA to thromboxane, but generated significant quantities of hydroxy acids via the LO pathway. However, drug exposed platelets, when stirred with epinephrine first and then challenged with AA, aggregated irreversibly. Eicosatetraynoic acid (ETYA 1, U53119) inhibited AA conversion by the LO pathway, whereas 5,8,11,14-eicosatetraynoic acid (ETYA 2) inhibited AA conversion by both CO and LO enzymes. Yet, at the inhibitory concentration these fatty acids failed to prevent AA induced irreversible aggregation of CO deficient, alpha adrenergic receptor stimulated platelets. Results of four studies show that the generation of LO metabolites of AA are not essential for securing irreversible aggregation of platelets.

Role of arachidonic acid metabolism in human platelet activation and irreversible aggregation

Previous studies from our laboratory have demonstrated that the aggregation response of platelets inhibited by agents blocking cyclooxygenase activity could be restored to a normal state of sensitivity by prior stimulation of alpha-adrenergic receptors. Since cyclooxygenase activity and thromboxane synthesis are not absolutely required for irreversible platelet aggregation, it is important to define precisely what role this pathway serves in platelet physiology. The present study has evaluated the influence of agents that selectively block arachidonic acid conversion at different steps of synthesis. Inhibition of peroxidase, cyclooxygenase, lipoxygenase, and thromboxane synthetase blocked the second wave response of platelets to several agonists, but did not cause dissociation of aggregates preformed by prior exposure to arachidonate (AA) or adenosine diphosphate. Phospholipase (A2/C) inhibitors, similar to prostaglandin inhibitors, blocked the second wave response of platelets to the action of agonists and, in addition, caused dissociation of aggregates induced by aggregating agents. Results of our study demonstrate that when single agonists are tested at threshold concentrations, products of arachidonate metabolism may play a role in the activation process. However, continued generation of these metabolites does not appear to be essential for the maintenance of irreversible aggregation. When a combination of agents or high concentration of physiological agonists are used, both activation and irreversible aggregation can be secured independent of prostaglandin synthesis or the release reaction.

Disaggregation and reaggregation of 'irreversibly' aggregated platelets: a method for more complete evaluation of anti-platelet drugs

Anti-platelet drugs are generally screened by evaluating their ability to influence initiation and development of irreversible aggregation of platelets. However, to fully characterize the inhibitory effects of an agent, it is essential to determine if it can also disperse irreversibly aggregated cells, whether drug-treated, dispersed platelets are refractory, sensitive to some but not all aggregating agents, as sensitive to all agents as before initial exposure, hypersensitive or whether they can be restored to a sensitive state by modulation of the platelet membrane. We have developed an in vitro system for evaluating the influence of a variety of agents on disaggregation and reaggregation of aggregated blood platelets. Results demonstrate that some agents which inhibit aggregation can also cause disaggregation, while others cannot. The ability to disaggregate is often selective, revealing a dependence on the nature of the agent causing aggregation, and the time after irreversible aggregation that the inhibitor is added. Agents that elevate intracellular cyclic adenosine monophosphate levels were potent inducers of platelet dissociation in aggregates caused by adenosine diphosphate. On the other hand, antimalarials, calmodulin complexing agents and phospholipase inhibitors caused dissociation of platelet aggregates, irrespective of the agonist used. By and large, the dissociated cells were refractory to the action of agonists. Restoration of the sensitivity of disaggregated platelets by treatment with epinephrine demonstrate an ability of inhibitor treated, refractory platelets to recover full functional capacity almost immediately. Thus, careful study of the effects of inhibitors on disaggregation, recovery and reaggregation may reveal features critical to the selection of anti-platelet drugs for clinical utilization.

Comparative pharmacology of cyclooxygenase inhibitors on platelet function

Previous studies from our laboratory have demonstrated a critical role for ferrous heme in prostaglandin synthesis. Based upon these studies, we proposed a model for heme-arachidonic acid interaction and demonstrated that compounds which interfere with this interaction inhibit arachidonic oxidation by ferrous heme. In this study, we have examined the effect of four different inhibitors for their effect on platelet arachidonic acid metabolism and function. The compounds studied were an iron chelator, 2,2'-dipyridyl, the cyclooxygenase inhibitors, Ibuprofen and aspirin, and a polyenoic acid, docosahexaenoic acid. All four compounds at approximately 100 microM concentration blocked the second wave of platelet aggregation in response to epinephrine or adenosine diphosphate. They were equally potent in inhibiting 14C-arachidonic conversion by platelets to thromboxane. However, inhibition of platelet thromboxane production and function by dipyridyl and DHA was reversible. Removal of these compounds from the medium restored platelets ability to respond to agonists and generate products through the cyclooxygenase pathway. The inhibitory effect of Ibuprofen and aspirin on cyclooxygenase activity could not be reversed by washing the platelets. However, Ibuprofen treated platelets aggregated when stirred with arachidonate in a normal way. No such response could be elicited from aspirin treated platelets. All compounds (except DHA) interfered with heme-arachidonic acid interaction in a cell-free system and prevented arachidonic acid oxidation. Results of our studies suggest a common mechanism of action for these different classes of compounds. In spite of the common mechanism, each class of drug seems to have a relatively different effect upon platelet cyclooxygenase and function.

Enteric-coated aspirin, platelet cyclooxygenase activity and function

Recent studies evaluating the effect of slow releasing enteric-coated aspirin formulations have reported contradictory findings regarding the bioavailability of the active molecule in the circulating blood and the length of duration of the inhibitory effect on platelet function. In the present study, we have evaluated the effect of a single dose of two commercially available enteric-coated aspirins on platelet arachidonic acid metabolism and function. A single dose of slow releasing aspirin was as effective as fast acting regular aspirin in its effect on cyclooxygenase activity and platelet function in both human and canine platelets. However, in view of its slow releasing property, the onset of inhibition was considerably delayed compared to the action of fast acting aspirin in those subjects who ingested enteric-coated aspirins.

Recycling of platelet phosphorylation and cytoskeletal assembly

The shape change and aggregation of washed platelets induced by 10 microM arachidonic acid (AA) can be reversed by 20 ng/ml prostacyclin (PGI2), but these platelets can be reactivated by treatment with 30 microM epinephrine and subsequent addition of 10 microM AA mixture. These events may be modulated by cAMP since 2 mM dibutyryl cAMP also reversed activation without reactivation by epinephrine and AA. We examined protein phosphorylation and formation of cytoskeletal cores resistant to 1% Triton X-100 extraction of these platelets and correlated these processes with aggregation, fibrinogen binding, and changes in ultrastructure. Unactivated platelet cores contained less than 15% of the total actin and no detectable myosin or actin-binding protein. AA-induced cytoskeletal cores, which contained 60-80% of the total actin, myosin, and actin-binding protein as the major components, were disassembled back to unactivated levels by PGI2 and then fully reassembled by epinephrine and AA. Phosphorylation of myosin light chain and a 40,000-dalton protein triggered by AA (two- to fivefold) was reversed to basal levels by PGI2 but was completely restored to peak levels upon addition of the epinephrine and AA mixture. The reversibility of actin-binding protein phosphorylation could not be established clearly because both PGI2 and dibutyryl cAMP caused its phosphorylation independent of activation. With this possible exception, cytoskeletal assembly with associated protein phosphorylation, aggregation, fibrinogen binding, and changes in ultrastructure triggered by activation are readily and concertedly recyclable.

Effect of docosahexaenoic acid (DHA) on arachidonic acid metabolism and platelet function

Studies from our laboratory have suggested a role for ferrous iron in the metabolism of arachidonic acid and demonstrated that inhibitors of prostaglandin synthesis exert their effect by complexing with the heme group of cyclooxygenase. Docosahexaenoic acid (DHA) is a potent competitive inhibitor of arachidonic acid metabolism by sheep vesicular gland prostaglandin synthetase. In this study we have evaluated the effect of exogenously added DHA on platelet function and arachidonic acid metabolism. DHA at 150 microM concentration inhibited aggregation of platelets to 450 microM arachidonic acid. At this concentration DHA also inhibited the second wave of the platelet response to the action of agonists such as epinephrine, adenosine diphosphate and thrombin. Inhibition induced by this fatty acid could be overcome by the agonists at higher concentrations. DHA inhibited the conversion of labeled arachidonic acid to thromboxane by intact, washed platelet suspensions. However, platelets in plasma incubated first with DHA then washed and stirred with labeled arachidonate generated as much thromboxane as control platelets. These results suggest that the polyenoic acids, if released in sufficient quantities in the vicinity of cyclooxygenase, could effectively compete for the heme site and inhibit the conversion of arachidonic acid.

Influence of a microtubule stabilizing agent on platelet structural physiology

Unstimulated blood platelets have a characteristic discoid form supported by a circumferential band of microtubules. After exposure to aggregating agents, platelets lose their lentiform shape and become irregularly convoluted, with many pseudopods. The changes in surface contour are associated with a process of internal transformation. Randomly dispersed granules are concentrated in cell centers and enclosed within tight-fitting rings of microtubules and microfilaments. The mechanism involved in the shift of circumferential microtubules into rings encircling clumped granules is uncertain. Recent studies have suggested that microtubules are disassembled shortly after stimulation and reassemble in new locations a few minutes later. Taxol, a microtubule stabilizing agent, has been used in the present study to evaluate the disassembly-reassembly hypothesis of internal transformation in activated platelets. Stabilization of microtubules by taxol did not injure platelet biochemistry or structure. Concentrations of taxol that protected platelet microtubules from dissociation by cold or vincristine did not inhibit platelet shape change, pseudopod formation, internal transformation, secretion, aggregation, or clot retraction. The number of microtubules wrapped around centrally clumped granules, present in pseudopods or spread through the cytoplasm, was equal to or greater than that found in untreated platelets following activation and aggregation. Though it is possible that microtubules dissolve in platelets following activation, the results of this study demonstrate that such an event is not essential for any aspect of the physiologic response of platelets in hemostasis.

Ibuprofen protects platelet cyclooxygenase from irreversible inhibition by aspirin

Previous investigations have shown that ibuprofen inhibits the second wave of platelet aggregation and blocks the conversion of 14C-arachidonic acid to thromboxane. However, the influence of the drug on platelet function and cyclooxygenase is transitory, lasting only 24 hours. The present study has taken advantage of the short-lived influence of ibuprofen to study its interaction with the long-term effects of aspirin. As expected, both aspirin and ibuprofen suppressed platelet cyclooxygenase activity and function, but addition of aspirin to ibuprofen-treated platelets did not increase the degree of inhibition in vitro. Platelet function and prostaglandin synthesis recovered completely 26 hours following ingestion of ibuprofen, but remained compromised 26 hours after taking aspirin. When 650 mg of aspirin was administered after ibuprofen, platelet function and cyclooxygenase activity recovered as completely at 26 hours as did platelets which had been exposed to ibuprofen alone. Thus, prior exposure to ibuprofen in vivo completely protected cyclooxygenase from the irreversible effects of aspirin. Our findings indicate that ibuprofen-like indomethacin and other nonsteroidal antiinflammatory drugs react with the heme group of cyclooxygenase to prevent arachidonic acid conversion. Since ibuprofen completely blocks the effects of aspirin in platelets in vitro and in vivo, aspirin's primary influence on inhibition of cyclooxygenase must also be through action on the heme portion of the enzyme, rather than acetylation of the protein.

Penicillin induced human platelet dysfunction and its reversal by epinephrine

Antibiotics administered orally can impair platelet function and cause a bleeding diathesis. Platelet function impairment induced by antibiotics such as penicillin, carbenicillin and tetracycline could be corrected by exposing the refractory platelets to epinephrine first and then challenging with other agonists. Penicillin used in this study inhibited the platelet aggregation response and release reaction without blocking the ability of these platelets to convert arachidonic acid to thromboxane. The thromboxane generated in the presence of penicillin could not activate the platelets when threshold concentrations of arachidonate was used as stimulus. Epinephrine stimulation restored the platelet membrane sensitivity and normalized the platelet response to the action of agonists. These studies demonstrate that both by in vitro as well as in vivo routes penicillin induced temporary impairment of platelet function. The refractoriness was not due to the interference of membrane receptors for various agonists or due to the depletion of serotonin, adenine nucleotides or inhibition of cyclooxygenase activity. Penicillin inhibits some essential mechanism involved in platelet activation by agonists and this process may be calcium dependent. Epinephrine through alpha adrenergic stimulation restores the mechanism by which agonists initiate platelet activation.

Vessel wall arachidonate metabolism after angioplasty: possible mediators of postangioplasty vasospasm

The mechanism of postangioplasty vasospasm is uncertain. It was postulated that a reduction in vasodilator prostaglandin I2 (PGI2) or prostaglandin E2 (PGE2) or an increase in vasoconstrictor hydroperoxy acids might contribute to spasm of a dilated artery. Twelve mongrel dogs were anesthetized, intubated, and mechanically ventilated. Heart rate and aortic pressures were continuously monitored and arterial blood gases maintained within physiologic limits. A single carotid artery was dilated in each animal (4 atm X 1 minute X 3) using nonexpandable polyethylene balloon catheters with inflated balloon diameters 50 to 100% larger than the internal arterial lumen. The opposite carotid artery served as a control. In 4 animals, aspirin (10 mg/kg, intravenously) was injected 30 minutes before dilation. Sixty minutes after dilation, animals were heparinized and the carotid arteries carefully removed. The in vitro conversion of carbon-14(14C)-arachidonic acid (AA) to 6-keto PGF1 alpha (PGI2), PGE2, and 12L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE) was determined using thin-layer radiochromatography. Angioplasty caused a 70% decrease in vessel wall PGI2 production and a 44% decrease in PGE2 production (n = 4, p less than 0.05). Reductions in in vitro conversion of 14C-AA to PGI2 and PGE2 induced by angioplasty were comparable to that produced by cyclooxygenase inhibition with aspirin. Angioplasty, in addition, caused a 104% increase in vessel wall HETE production (n = 4, p less than 0.05). Therefore, angioplasty results in a local derangement of AA metabolism characterized by decreases in vasodilator prostaglandins and increases in vasoconstrictor hydroperoxy acids. These local changes may contribute, in part, to sudden arterial occlusion after angioplasty.

Platelet activating factor (PAF) causes human platelet aggregation through the mechanism of membrane modulation

Recent investigations have shown that human and canine platelets possess a new pathway for securing irreversible platelet aggregation independent of ADP secretion and prostaglandin synthesis. Platelet activating factor (PAF) is a potent aggregating agent for rabbit platelets and is also reputed to stimulate the cells by a completely independent pathway. Since the newly recognized mechanism in human and dog cells, termed membrane modulation, might be mediated by PAF, we evaluated the influence of this agent on human cells. Results of that study demonstrated that PAF did not activate platelets through an independent pathway and was not involved in the mechanism of membrane modulation. In the present investigation we have examined the possibility that PAF might act on human platelets through the mechanism of membrane modulation. Human platelets were made refractory to amounts of PAF routinely causing irreversible aggregation by prolonged incubation of C-PRP at 37 degrees C, by exposure to PGI2 or aspirin, or by aggregation-disaggregation of control or aspirin treated cells. Epinephrine at concentrations which did not cause aggregation turned on the mechanism of membrane modulation in refractory platelets and restored their sensitivity to PAF. Thus, PAF does not appear to activate human platelets through a unique, independent pathway. Rather, it usually causes irreversible aggregation of human cells through the mechanisms of ADP secretion and thromboxane synthesis or, as shown in this study, can be dependent on membrane modulation.

Effects of a microtubule stabilizing agent on the response of platelets to vincristine

The discoid shape of blood platelets is supported by a circumferential bundle of microtubules. Removal of the microtubules by an antimitotic drug, vincristine, is associated with loss of lentiform appearance, formation of tubulin paracrystals, a depressed response to aggregating agents, and impaired secretory activity. Recent studies have suggested that the action of vincristine on platelet secretion and aggregation is directly related to its action on microtubules, while other work had indicated that the antimitotic drug prevents the release reaction by inhibiting prostaglandin synthesis. The present study has examined the influence of taxol, a microtubule stabilizing agent, on the response of platelets to vincristine. Taxol completely prevented vincristine-induced shape change, microtubule disassembly, and tubulin paracrystal formation, even at concentrations one-tenth that of the antimitotic drug. Pretreatment with vincristine to dissociate microtubules and convert tubulin to crystals before exposure to taxol did not affect altered shape or tubulin paracrystals, but did cause assembly of free pools of tubulin into tubular polymers. Studies of physiology confirmed that vincristine, in amounts that remove microtubules, depresses platelet aggregation and secretion, effects that could be overcome by increasing agonist concentration. Although completely preventing microtubule dissociation, taxol had no corrective influence on vincristine-induced inhibition of platelet function. Biochemical studies revealed that vincristine concentrations that disassembled microtubules and blocked secretion did not inhibit conversion of 14C-arachidonic acid to thromboxane B2. The findings suggest that vincristine inhibits platelet function through some mechanism other than disassembling microtubules, but the other mechanism does not involve inhibition of prostaglandin synthesis.

Effect of acetaminophen and salicylate on aspirin-induced inhibition of human platelet cyclo-oxygenase

Recent studies have shown that salicylic acid, a metabolite of aspirin, effectively competes for the same site on the platelet cyclo-oxygenase enzyme. In the present investigation we have evaluated the effect of salicylate and acetaminophen on aspirin induced inhibition of cyclo-oxygenase and platelet function. Results of our studies show that both drugs at equimolar concentrations had no inhibitory effect on aspirin induced blockage of cyclo-oxygenase or platelet function. Even at higher concentrations acetaminophen failed to protect cyclo-oxygenase or prevent inhibition of platelet function by aspirin. Salicylate at concentrations above 5 mM effectively blocked the inhibition of cyclo-oxygenase activity and platelet aggregation in response to arachidonate.

Acute effects of amrinone on regional myocardial and systemic blood flow distribution in the dog

The effect of bolus intravenous injections of amrinone (1-2 mg/kg) on abdominal organ, central nervous system, and myocardial blood flow distribution was examined in 15 anesthetized dogs. Blood flows were measured during control conditions and 5 and 60 min following drug administration using left atrial injection of 15-micrometers radionuclide-labeled spheres. Analysis of variance revealed that blood flow changes were similar in dogs receiving either drug dose (P greater than 0.10). Five minutes following injection, blood flow was increased (all P less than 0.05) in the renal cortex (+20.4%), spleen (+40.4%), and liver (+47.1%); flow was unchanged in other abdominal organs (pancreas, gallbladder, small and large intestine, and fundic and antral gastric mucosa) and the central nervous system (cervical spinal cord, pons, medulla, dorsal thalamus, cerebellum, caudate nucleus, and cerebral cortical gray and white matter); and flow was reduced in the triceps muscle (-23.7%). At this time, left ventricular flow was increased (+25.0%) and the left ventricular subendocardial/subepicardial (Endo/Epi) flow ratio was reduced (1.09 +/- 0.02 (SE) vs. 0.90 +/- 0.02, P less than 0.001). Sixty minutes following injection, renal and hepatic flows had returned to control values while splenic flow remained increased (+61.6%); intestinal, gastric mucosal, gallbladder, and triceps flows were reduced by values ranging from 26.7 to 38.9% and central nervous system perfusion was reduced by values ranging from 11.8 to 19.4% in all regions except the caudate nucleus. Although left ventricular flow had returned to control values, the Endo/Epi ratio (1.02 +/- 0.02) remained minimally reduced at this time (P less than 0.001). These results suggest that vascular responsiveness to intravenous amrinone is not uniform in different circulatory beds and that relative subendocardial under-perfusion of the left ventricular myocardium occurs following bolus intravenous amrinone injections in the dog.