Human platelet activation is inhibited by the occupancy of glycoprotein IIb/IIIa receptor

We studied the effect of glycoprotein GPIIb/IIIa (integrin alpha IIb beta 3) receptor occupancy by adenosine 5',1-thiotriphosphate (ATP alpha S), a competitive inhibitor of the ADP receptor, by fibrinogen, and by peptides containing the RGD (Arg-Gly-Asp) sequence as RGDW (Arg-Gly-Asp-Trp), RGDS (Arg-Gly-Asp-Ser), or the negative control RGGW (Arg-Gly-Gly-Trp) on human platelet physiological functions: aggregation, ATP secretion, and [Ca2+]in. As the presence of a nucleotide binding site on GPIIb alpha has been demonstrated in platelets [N. J. Greco, N. Yamamoto, B. W. Jackson, N. N. Tandon, M. Moos, and G. A. Jamieson (1991) J. Biol. Chem. 266, 13627-13633], we studied the effect of ATP alpha S, which specifically binds to this site, on platelet activation. We observed that ATP alpha S inhibited aggregation by thrombin, ADP, PMA, and ionophore A23187. Moreover, ATP alpha S dose dependently inhibited ATP secretion by ionophore A23187 and Ca2+ transients by thrombin and vasopressin in both the presence and absence of external Ca2+. Fibrinogen, although induced by a potentiation of platelet aggregation, inhibited ATP secretion and [Ca2+]in elevation induced by low thrombin concentrations or by vasopressin, interfering with both Ca2+ entry and Ca2+ release by the intracellular stores. RGD peptides, which specifically bind to GPIIb/IIIa, inhibited aggregation, secretion, and Ca2+ transients by thrombin, whereas the negative control RGGW did not exert any effect. We conclude that the occupancy of the GPIIb/IIIa receptor binding sites modulates platelet function by giving an inhibitory outside-in signal in platelets, particularly effective in platelets stimulated with low agonist doses. We suggest that ATP alpha S, fibrinogen, or RGD compounds, by interacting with GPIIb/IIIa receptor, prime some intracellular negative feedback mechanisms, which prevent further activation of circulating platelets by low-intensity stimuli and intravascular aggregation.

Physiology and function of platelets from patients with Alzheimer's disease

The discovery that intact Alzheimer amyloid precursor protein is present in platelet granules, has created a great interest in the biochemistry, physiology and function of platelets of patients with Alzheimer disease (AD). In this study we monitored various biochemical and physiological parameters, such as serotonin and adenine nucleotide levels, membrane fluidity, agonist-mediated release of arachidonic acid, thromboxane formation, calcium mobilization, as well as irreversible aggregation and secretion of granule contents. Platelets of patients with AD responded poorly when stirred with weak or potent agonists on a platelet aggregometer. Although capable of agonist-mediated calcium mobilization and synthesis of thromboxanes, the aggregation response of platelets of patients with AD to thrombin and archidonate was considerably compromised. In view of the normal biochemistry and signal transduction capabilities, the compromised response of these cells to potent agonists like thrombin suggested an extrinsic defect. The present study has shown that a plasmatic factor is at least in part responsible for the functional abnormalities of AD platelets.

Retention of glycoprotein Ib/IX receptors on external surfaces of thrombin-activated platelets in suspension

The present study has evaluated the hypothesis stating that glycoprotein (GP) Ib/IX, the receptor for von Willebrand factor (vWF), is downregulated and cleared from exposed surfaces to channels of the open canalicular system (OCS) on platelets activated by thrombin in suspension. Cryosections of resting and thrombin-activated platelets fixed at intervals of 1 to 30 minutes after stimulation by thrombin and stained with antiglycocalicin antibody and protein A gold showed no decrease in the density of GPIb/IX receptors on the platelet surface or increase on linings of the OCS at any interval after stimulation by thrombin. Thin sections of platelets exposed to thrombin in suspension followed by settling onto a plastic chamber for intervals of 1 to 30 minutes revealed retention of GPIb/IX receptors on exposed surfaces detected by vWF, anti-vWF, and protein A gold throughout the 30-minute period of study. Results of this investigation indicate that GPIb/IX receptors remain on the surface of platelets activated by thrombin in suspension, are not cleared to the OCS, and retain the ability to bind vWF for at least 30 minutes.

Influx of extracellular calcium and agonist-coupling appear essential for the activation of thromboxane A2-dependent phospholipase A2 in human platelets

The present study demonstrates the existence of a unique mechanism for arachidonic acid (AA)-specific phospholipase A2 (PLA2) activation, which requires both sustained elevation of cytosolic Ca2+ coupled to the influx of extracellular Ca2+ and agonist interaction in platelets. The activation of PLA2 in platelets exposed to thapsigargin was abolished by the inhibition of cyclooxygenase (COX), thus suggesting a requirement of endogenously produced COX metabolite(s) for the activation of this enzyme. A thromboxane A2 (TXA2) analog, U46619, restored the activation of this AA-specific PLA2 activation supporting the requirements of COX metabolite(s) especially TXA2. Our subsequent studies demonstrated that both the effects of TXA2, and U46619 could be mimicked by collagen. Neither the transient cytosolic Ca2+ rise nor the agonists such as U46619 or collagen alone were sufficient to prime the activation of this PLA2 in the absence of thapsigargin. Since collagen behaves very similarly to TXA2, we suggest that this PLA2, is not only responsive to TXA2, but also to other agonists such as collagen, as shown in this study. We suggest that the activation of this distinct TXA2- and collagen-sensitive PLA2 involves two steps: (a) sustained elevation of cytosolic Ca2+ coupled to the influx of extracellular Ca2+; and (b) interaction with agonists such as TXA2 and collagen.

Phospholipase A2 activation in human neutrophils requires influx of extracellular Ca2+ and leukotriene B4

We have demonstrated that phospolipase A2 (PLA2) activation in human neutrophils requires both the influx of extracellular Ca2+ and leukotriene B4 (LTB4). Surprisingly, the eicosanoids (LTB4 and its omega-oxidation products) formed were quantitatively very similar in both thapsigargin (Thap)- and A-23187-stimulated neutrophils. In contrast, Thap had very little effect on the activation of PLA2 when 5-lipoxygenase (5-LO) was blocked by BW755C or MK-886, whereas A-23187 caused a substantial activation. The lack of PLA2 activation in Thap-stimulated neutrophils results from the inhibition of LTB4 formation in the presence of 5-LO inhibitors. It appears that A-23187 activates both LTB4-dependent and -independent PLA2, whereas Thap activates LTB4-dependent PLA2. Experiments with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid demonstrated that activation of Thap-sensitive PLA2 and 5-LO requires the influx of Ca2+. Neither the transient elevation of cytosolic Ca2+ from intracellular stores nor the sustained Ca2+ influx alone without LTB4 appears sufficient to cause the activation of LTB4-dependent PLA2. We suggest that the activation of LTB4-dependent PLA2 involves 1) a sustained elevation of cytosolic Ca2+ coupled to the influx of extracellular Ca2+ and 2) a coupling between LTB4 and its receptor. We conclude that LTB4-dependent PLA2 plays an important role in the poststimulatory formation of lipid mediators such as prostaglandins, leukotrienes, and platelet-activating factor.

Thermosensitization by increasing intracellular acidity with amiloride and its analogs

PURPOSE

The major mechanisms that regulate the intracellular acidity of pHi in mammalian cells are the Na+/H+ exchange and HCO3-/Cl- exchange through the plasma membrane. The purpose of this study was to investigate the feasibility of increasing the thermosensitivity of tumors by increasing intracellular acidity with the use of drugs that inhibit the pHi regulatory mechanisms.

METHODS AND MATERIALS

The pHi of SCK tumor cells in vitro was determined with the fluorescence spectroscopy method. The thermosensitizing effects of the drugs on the cells in neutral (pH 7.2-7.5) and acidic (pH 6.6) media were determined by clonogenic assay. The thermosensitization of SCK tumors in vivo by the drugs was determined with the tumor growth delay and the in vivo-in vitro assay for clonogenic cells.

RESULTS

The pHi of SCK tumor cells in pH 7.2-7.5 media was similar to the media pH, while the pHi of the cells in pH 6.6 media was about 7.0. The pHi declined and the thermosensitivity of the tumor cells increased when the Na+/H+ exchange was inhibited with amiloride (3,5 diamino-6-chloro-N-(diaminomethylene) pyrazinecarboxamide) and its analogs, HMA (3-amino-6-chloro-5-(1-homopiperidyl)-N-(diaminomethylene) pyrazinecarboxamide) or EIPA (3-amino-6-chloro-5-(N-ethyl-N-isopropylamino)-N-diaminomethylene) pyrazinecarboxamide), especially in acidic medium. The potencies of HMA and EIPA to decrease the pHi and increase the thermosensitivity in vitro were more than 50 times greater than that of amiloride. DIDS (4,4-diiosothiocyanatostilbene-2,2'-disulfonic acid), an inhibitor of the Na(+)-dependent HCO3-/Cl- exchange, exerted little effect on the pHi and thermosensitivity of SCK cells in vitro, but it enhanced the effects of amiloride and its analogs. Amiloride and HMA also significantly enhanced the thermal effect on tumors in vivo, as judged by the tumor growth delay and also by the in vitro-in vivo assay for clonogenic cells. Combinations of DIDS with amiloride or HMA were more effective than either of them alone in increasing the thermal damage in vivo. As in vitro, HMA was far more potent than amiloride in increasing the thermosensitivity of tumor cells in vivo. However, EIPA was not effective in vivo, probably due to a rapid metabolic breakdown of the drug.

CONCLUSION

The drugs that interfere with the pHi regulatory mechanism significantly thermosensitized the tumor cells in vitro, particularly those in acidic media. The drugs were also effective in increasing the thermosensitivity of tumors. Because the interstitial environment in tumors is acidic relative to that in normal tissues, the thermosensitization by the drugs may be greater in tumors than that in normal tissues.

1,3-Dioctanoylglycerol (1,3-DiC8) is as effective as 1,2-dioctanoylglycerol (1,2-DiC8) in priming phospholipase A2 activation in human platelets and neutrophils

In the present study, we investigated the effects of different diacylglycerols in comparison with phorbol 12-myristate 13-acetate (PMA) on eicosanoid-independent phospholipase A2 (PLA2) activation in human platelets and neutrophils. Eicosanoid-independent PLA2 activation was measured under conditions where both cyclooxygenase and lipoxygenases were blocked by BW755C. In the presence of PMA (50 nM), the amount of mass arachidonic acid (AA) released represented 400 and 257% of control (without PMA) in A23187-stimulated platelets and neutrophils, respectively, while 1,2-dioctanoylglycerol (1,2-DiC8) and 1-oleoyl-2-acetyl-sn-glycerol (OAG) had increased the eicosanoid-independent AA release by 150 and 117-134% of control, in platelets and neutrophils, respectively. Our results further demonstrate that 1,3-dioctanoylglycerol (1,3-DiC8), a poor activator of protein kinase C (PKC), is nearly as effective as diacylglycerols, such as OAG and 1,2-DiC8 (activators of PKC) in priming PLA2 activation, but is less effective than PMA as a priming agent. However, all three diacylglycerols were less effective than PMA as priming agents. Furthermore, diacylglycerols including 1,3-DiC8 exerted a much greater effect on PLA2 activation in platelets than in neutrophils. Neither 1,3-DiC8 nor 1,2-DiC8 and OAG had any significant priming effect on the accumulation of palmitic and stearic acids, while PMA caused a substantial accumulation of these fatty acids in platelets, but not in neutrophils. We also found that exogenously added OAG underwent significant hydrolysis even in unstimulated platelets, but not in neutrophils, suggesting that exogenously added OAG may be readily accessible for diacylglycerol (DAG) lipase/PLA1 in platelets.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of various doses of aspirin (in vivo) on platelet arachidonic acid metabolism (ex vivo) and function

The influence of various doses of enteric-coated aspirin was evaluated for its in vivo effect on ex vivo platelet arachidonic acid metabolism and function. 24 h after ingestion of the drugs compromised platelet response to the action of agonists such as epinephrine and arachidonate could be demonstrated with as low a dose as 50 mgs aspirin. However, platelets with compromised function were still capable of producing significant quantities of thromboxane. On the other hand, platelets with almost total inhibition of cyclooxygenase activity were capable of aggregating irreversibly when challenged with epinephrine and arachidonate.

Promotion of human platelet adhesion and aggregation by a synthetic, triple-helical "mini-collagen"

Platelet activation and aggregation by fibrillar collagens are based on substrate primary, secondary, tertiary, and quaternary structure. Although several peptides incorporating sequences from the triple-helical domains of types I and III collagen inhibit collagen-mediated platelet aggregation, none independently promote platelet activation and aggregation. It is believed that the absence of these platelet activities is due to the lack of proper substrate tertiary and quaternary structures. We have utilized a synthetic, triple-helical "mini-collagen" that incorporates a known cell adhesion site (alpha 1(IV)1263-1277) to better understand the relationship between substrate primary, secondary, tertiary, and quaternary structure and platelet activation and aggregation. The promotion of platelet adhesion, activation, and aggregation was compared for this triple-helical polypeptide (THP), fibrillar and type IV collagens, and a single-stranded peptide (SSP) incorporating the alpha 1(IV)1263-1277 sequence. Glass-coated fibrillar and type IV collagens and the THP supported platelet adhesion at substrate concentrations of 0.33 nM, 0.20 nM, and 0.89 microM, respectively. When platelets were stirred with 10 micrograms/ml of fibrillar (0.33 nM) and type IV (0.20 nM) collagen, SSP (2.1 microM), and THP (0.89 microM), only the fibrillar collagen caused 3H-labeled arachidonic acid release, elevation of cytosolic calcium, irreversible aggregation, and secretion of granule contents. The THP (0.45-1.8 microM) effectively inhibited fibrillar collagen-mediated platelet aggregation, while the SSP did not. At a substrate concentration of 40 micrograms/ml, the THP (3.6 microM) was as effective as fibrillar collagen (1.3 nM) at inducing 3H-labeled oleic acid-labeled platelet activation and microaggregate formation, while the SSP and type IV collagen were relatively ineffective. Rotary shadowing images indicated that aggregates of the THP could form distinct quaternary structures, while the type IV collagen used here could not. These results are the first demonstrations of a synthetic peptide promoting platelet adhesion, activation, and aggregation and suggest that the combination of THP primary, secondary, tertiary, and quaternary structural features are required for platelet aggregation.

Differential effects of phorbol 12-myristate 13-acetate and diacylglycerols on thromboxane A2-independent phospholipase A2 activation in collage-stimulated human platelets

We investigated the priming effects of protein kinase C (PKC) activators such as phorbol 12-myristate 13-acetate (PMA), 1,2-DiC8 and OAG, and 1,3-DiC8 (a poor activator of PKC) on thromboxane A2 (TxA2)-independent phospholipase A2 (PLA2) activation in human platelets using collagen and A23187 as agonists. We measured PLA2 activation in collagen-stimulated platelets in the presence of BW755C, which abolished TxA2 synthesis, rise in cytosolic Ca2+, and aggregation. In the presence of PMA (50 nM), the amount of arachidonic acid (AA) released in platelets stimulated with collagen and A23187 represented 300% (13.85 nmol versus 4.5 nmol) and 400% (28 nmol versus 7 nmol) of controls (without PMA), respectively, while 1,2-DiC8, OAG, and 1,3-DiC8 increased TxA2-independent AA release by 50% in A23187-stimulated platelets and had no effect on the release of AA in collagen-stimulated platelets. Interestingly, 1,3-DiC8, which is a poor activator of PKC, was as effective as the other two DAGs (OAG and 1,2-DiC8) in priming TxA2-independent PLA2 activation, but was less effective than PMA in platelets stimulated with A23187. These results suggest that the TXA2-dependent IP3-mediated rise in cytosolic Ca2+ may not be obligatory for priming PLA2 activation in the presence of PMA in collagen-stimulated platelets. In contrast, 1,2-DiC8, OAG, and 1,3-DiC8 likely enhanced PLA2 activation via intracellular Ca2+ as they selectively affect this enzyme only in A23187-stimulated platelets. We also observed a significant increase in both saturated (palmitic and stearic acids) and unsaturated fatty acids (oleic and linoleic acids) in platelets stimulated by collagen or A23187 in the presence of PMA (50 nM), but not in the presence of DAGs. These findings imply that PMA may also affect the activation of DAG/MAG lipases, PLA1, or nonspecific PLA2. Since both 1,2-DiC8 and OAG exert no significant effect on the release of these fatty acids, the effects observed with PMA on DAG lipase/PLA1 may not involve a PKC-dependent mechanism. We, therefore, conclude that the mechanisms by which PMA and DAGs prime PLA2 activation are different and that the priming mechanism by DAGs may not involve PKC, but may require a rise in intracellular Ca2+.

Pharmacology of platelet activation-inhibitory drugs

The role of blood platelets in the pathogenesis of atherosclerosis, thrombosis, thromboembolism and stroke (hemorrhagic/thrombotic) is well established. In view of this recognized role played by platelets in the complications associated with coronary artery disease and cerebrovascular disease, there is considerable interest in the pharmacology of platelet activation inhibitory drugs. These drugs exert their effect by blocking several different activation signalling mechanisms. Some of the known compounds that modulate platelet function include: inhibitors of arachidonic acid metabolism (nonsteroidal anti-inflammatory drugs and thromboxane synthetase inhibitors), drugs that alter membrane phospholipid composition (omega 3 fatty acids), stimulators of adenylyl cyclase and guanylyl cyclase (PGE1, PGI2, PGD2/ERRF [nitric oxide], nitroglycerin, nitroprusside), phosphodiesterase inhibitors (dipyridamole and methylxanthines) and calcium antagonists (verapamil, nifedipine, diltiazem). Current research on the pharmacology of platelet activation inhibitory drugs is focused on the development of specific receptor antagonists (antibodies, peptides, receptor antagonists). Since platelets have multiple mechanisms for achieving activation, and the process of thrombosis involves multicellular modulation of platelet activity, it will be rather difficult to develop a compound that is capable of causing complete inhibition of activation mechanisms. Therefore, future research will be devoted to development of designer drugs that will be used for preventing discrete platelet responses. This approach may be useful as total inhibition of platelet activation, although it may prevent thrombotic events, may possibly precipitate hemorrhagic conditions. A better understanding of cell signalling pathways and the mechanisms involved in the pathogenesis of cardiovascular cerebrovascular disease will facilitate the development of efficient antiplatelet drugs.

Influence of antioxidants on arachidonic acid metabolism and platelet function

Studies from our laboratory demonstrated that the free radical scavenger, nitro blue tetrazolium, and iron chelators, such as dypyrydil, are potent inhibitors of arachidonic acid oxidation and platelet function. In the present study, we have evaluated the effects of known antioxidants, such as butylated hydroxyanisol (BHA), butylated hydroxytoluene (BHT), and diphenylamine, on arachidonic acid metabolism and platelet function. Diphenylamine, a common dye intermediate used in hair color formulations, was the most potent inhibitor of arachidonic acid metabolism by platelet cyclooxygenases. Diphenyl and BHA were also potent inhibitors of arachidonic acid oxidation. Other diphenyl analogues and BHT were relatively poor inhibitors of arachidonic-mediated platelet activation. Results of this study, as well as those of our earlier studies, suggest that antioxidants and iron chelators prevent arachidonic acid metabolism and alter platelet function by interfering with the heme/arachidonic acid interaction and blocking cyclooxygenase metabolites essential for the formation of thromboxane A2, a potent platelet agonist.

Possible mechanisms of epinephrine actions in quin-2-loaded platelets refractory to arachidonic acid

We evaluated the effect of Quin-2 loading (> 20 microM) on platelet responses such as phosphoinositide turnover, elevation of cytosolic Ca2+, phosphorylation of myosin light chain (MLC) and a 47-kDa protein, and aggregation in human platelets stimulated with arachidonic acid (AA) and epinephrine. The formation of inositol phosphates (IP, IP2, and IP3) in platelets stimulated with AA was inhibited by 50.4, 59.5, and 61%, respectively, in the presence of Quin-2 (40 microM). A similar degree of inhibition was observed in platelets stimulated with epinephrine (50 microM) and thrombin (0.1 U/ml). Even though Quin-2-induced inhibition of aggregation in response to AA was reversed by epinephrine, its effect on phosphoinositide turnover remained unaffected. Monitoring of cytosolic Ca2+ changes further indicates that the ability of epinephrine to restore aggregation in Quin-2-loaded (40 microM) and AA-stimulated platelets is not coupled to an increase in cytosolic Ca2+. Quin-2 loading (40 microM) caused a significant inhibition of MLC phosphorylation (20 kDa) in platelets stimulated by AA. However, it had no effect on the phosphorylation of the 47-kDa protein induced by AA. Furthermore, Quin-2 loading (40 microM) exerted no significant effect on shape change, actin filament assembly, and spreading, but caused a significant inhibition of secretion and clot retraction. We conclude that the formation of inositol phosphates, increases in cytosolic Ca2+, and phosphorylation of MLC affected by Quin-2 are not coupled to the mechanisms by which platelets develop stickiness, undergo shape change, spreading, and aggregation in response to epinephrine and AA. It appears that the effect of epinephrine in restoring the aggregation response of refractory platelets is coupled to a calcium-mediated alpha-adrenergic receptor, and it may serve as a critical salvage pathway in platelets with compromised functions.

Physiology of blood platelet activation

Blood platelets interact with a variety of soluble agonists such as epinephrine and adenosine diphosphate (ADP); many insoluble cell matrix components, including collagen and laminin, and biomaterials used for construction of invasive medical devices. These interactions stimulate specific receptors and glycoprotein-rich domains (integrins and nonintegrin) on the plasma membrane and lead to the activation of intracellular effector enzymes. The majority of regulatory events appear to require free calcium. Ionized calcium is the primary bioregulator, and a variety of biochemical mechanisms modulate the level and availability of free cytosolic calcium. Major enzymes that regulate the free calcium levels via second messengers include phospholipase C, phospholipase A2, and phospholipase D, together with adenylyl and guanylyl cyclases. Activation of phospholipase C results in the hydrolysis of phosphatidyl inositol 4,5-bisphosphate and formation of second messengers 1,2-diacylglycerol and inositol 1,4,5-trisphosphate (IP3). Diglyceride induces activation of protein kinase C, whereas IP3 mobilizes calcium from internal membrane stores. Elevation of cytosolic calcium stimulates phospholipase A2 and liberates arachidonic acid. Free arachidonic acid is transformed to a novel metabolite, thromboxane A2, by fatty acid synthetases. Thromboxane A2 is the major metabolite of this pathway and plays a critical role in platelet recruitment, granule mobilization and secretion. Up-regulation in signalling pathways will increase the risk for clinical complications associated with thromboembolic episodes. Down-regulation of signal transduction mechanisms may precipitate bleeding diathesis or stroke.

Influence of heat on platelet biochemistry, structure, and function

The present investigation has evaluated the influence of temperatures ranging from 37 degrees C to 45 degrees C for intervals of 30, 60, and 90 minutes on the biochemistry, morphology, and function of human platelets. Exposure to temperatures up to 43 degrees C for an hour did not significantly alter platelet morphology or physiologic response to aggregating agents. Samples of platelets heated at 43 degrees for 60 minutes lost their ability to aggregate in response to arachidonate, but sensitivity was restored by pretreatment with epinephrine. Platelets heated to 45 degrees C for 90 minutes were converted from discs to spheres and failed to aggregate in response to all agonists, whether or not they were pretreated with epinephrine. The platelets heated at 45 degrees C for 90 minutes could adhere to formvar or denuded subendothelium but were unable to extend pseudopods or to spread. They maintained normal levels of adenine nucleotides and serotonin but failed to secrete these products on stimulation. Studies with fibrinogen coupled to gold (Fgn/Au) revealed only a few particles bound to glycoprotein IIb-IIIa (GPIIb-IIIa) on platelets heated at 43 degrees C for 90 minutes. Ligands that did bind to GPIIb-IIIa were transported to the open canalicular system. Biochemical studies demonstrated normal synthesis of thromboxane B2 and calcium flux by platelets heated at 45 degrees C for 90 minutes. Polyacrylamide gels of platelets heated at 45 degrees C for 90 minutes showed an increase in talin incorporation into heated platelet cytoskeletons but no increase in filamentous actin. The findings indicate that impaired function of heated platelets is due to the influence of heat on cytoskeletal proteins important for pseudopod extension, shape change, expression of GPIIb-IIIa, or other surface membrane receptors and secretion, but that impaired function is not due to inhibition of biochemical systems involved in activation events.

Coronary artery disease: an overview of risk factors

Coronary artery disease (CAD) is an important cause of morbidity and mortality in most industrialized nations, and is gaining in importance as a major disease in developing countries as well. Several risk factors, such as cholesterol, smoking, hypertension, obesity, diabetes, stress and physical activity, have been identified as contributors to the pathogenesis of this disease. Studies done in many countries on South Asian immigrants clearly demonstrate the increased risk for coronary heart disease in this population compared to that of local ethnic groups. Higher prevalence of diabetes, blood pressure, glucose intolerance, insulin resistance, low levels of HDL-cholesterol, high levels of LDL-cholesterol, increased plasma triglycerides and obesity are some of the risk factors identified as contributing to CAD in South Asians. The present report will review briefly the available data on CAD and its pathogenesis with particular emphasis on the problems unique to South Asians.

Aspirin in ischemic heart disease--an overview

Aspirin is one of the oldest and most commonly used nonprescription drugs in the world. Although commonly it is used for relief from common headache and muscular pain, its use in the prevention and treatment of platelet related complications in cardiovascular diseases (CVD) and cerebrovascular disease (CBVD) is quite controversial. A brief review of the major aspirin trials indicated that a full strength aspirin taken daily had no significant beneficial effect in reducing mortality of patients with CVD/CBVD. However, two major trials (ISIS-2, PHS) in which either low dose aspirin (160 mg) or one aspirin administered every other day, have demonstrated significant reduction in fatal and nonfatal cardiovascular events. Even a dose as low as 1 mg aspirin per day significantly lowers platelet thromboxane synthesis. As a result of these studies, low dose aspirin should be the choice of prophylactic therapy aimed at the inhibition of platelet cyclooxygenase activity. Controlled-release low dose aspirin may favorably reduce platelet thromboxane production and spare vascular prostacyclin synthesis. At least 100 mgs of aspirin per day are essential to completely inhibit steady state thromboxane formation. Low dose aspirin (160 mgs) has been shown to be as effective as the full strength aspirin (325 mgs) in reducing clinical complications related to platelet activation. The antithrombotic effect of aspirin is well established and improved formulations, well thought out therapeutic protocols, customized dosage, appropriate timing of delivery, a better understanding of platelet function and pathophysiology of CUD/CBUD will facilitate maximization of the beneficial effects of aspirin.

Detergent-resistant cytoskeleton of the surface-activated platelet differs from the suspension-activated platelet cytoskeleton

This study contrasts the protein composition of the detergent-resistant cytoskeleton of platelets fully spread on glass with the cytoskeletal composition of resting platelets and platelets aggregated in suspension with thrombin. Complete Triton X-100-insoluble cytoskeletons were isolated from spread, resting, and suspension-activated platelets in the presence of protease inhibitors, solubilized in sodium dodecyl sulfate/EDTA and analyzed on reduced, one-dimensional polyacrylamide gels. The protein composition of the cytoskeletons differed both qualitatively and quantitatively. Most notable were more extensive incorporation of total protein, talin, and vinculin into the cytoskeleton of spread platelets than the cytoskeleton of suspension-activated platelets. Varying the concentration and time of exposure to thrombin during suspension activation did not mimic the cytoskeletal changes of surface activation. Scanning electron microscopy, measurement of lipid phosphorus content, and varying the duration of Triton extraction did not show incomplete solubilization or nonspecific trapping of constituents in the spread platelet cytoskeleton. Proteolysis of talin was minimal in suspension-activated platelets and in platelets spread for 50 minutes. The differences in the detergent-resistant cytoskeletons of surface- and suspension-activated platelets indicate significant divergence in the physiologies of platelet spreading on surfaces and platelet activation in suspension.