Size reduction-induced chain breaking in Haldane-chain compounds SrNi2-x Mg x V2O8 (x = 0 and 0.1)

We report size reduction-induced chain breaking in the spin-1 Haldane-chain SrNi_2-x Mg _x V₂O₈ (x  =  0 and 0.1) by magnetization and electron spin resonance measurements. For x  =  0.0, the magnetic susceptibility of all samples can be well described by a temperature-independent term, a Curie-Weiss term and a Haldane-gap term. This implies that a reduced sample grain size breaks the long chain and creates a considerable number of S  =  1/2 edge spins, resulting in the enhancement of magnetization and the decrease of Haldane gap in the samples. These edge spins as well as the other paramagnetic ions at grain boundary and surface might be weakly coupled with each other. For the Mg-doped sample with x  =  0.1, there are more S  =  1/2 spins creased in relative to x  =  0.0 because of a combined effect of lattice defects, Mg-doping and reduced size. In addition, the antiferromagnetic resonance of x  =  0.1 is also presented.

Phase diagram of the Nd2Fe14B–Sm2Fe14B pseudo-binary system

The phase relations in the (1−x)Nd2Fe14B–xSm2Fe14B system over the whole concentration range have been studied by means of X-ray powder diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Crystal structure parameters for all studied compositions of (Nd1−xSmx)2Fe14B have been determined by full-profile Rietveld refinements. These results revealed that all intermediate alloys of (Nd1−xSmx)2Fe14B are similar to the end member of the investigated system, Nd2Fe14B, with a tetragonal structure (space groupP42/mnm). The formation of continuous solid solutions has been found in this system. The normalized lattice parameters and unit-cell volumes of (Nd1−xSmx)2Fe14B solid solutions decrease linearly with increasing Sm content. The DTA measurements show that the melting temperature of (Nd1−xSmx)2Fe14B increases linearly with increasing Sm content and no metastable phases were detected. Based on the DTA data and XRD results, a tentative phase diagram for the pseudo-binary system Nd2Fe14B–Sm2Fe14B has been constructed.

Fast and clean functionalization of MWCNTs by DBD plasma and its influence on mechanical properties of C–epoxy composites

TPD profile of various MWCNTs samples before and after air plasma treatment: (a) CO₂ evolution, (b) CO evolution.

Disappearance of Ising nature in Ca3ZnMnO6 studied by high-field ESR

High-field electron spin resonance measurements of an antiferromagnet Ca3ZnMnO6 isostructure, with the Ising-chain multiferroic Ca3CoMnO6, have been carried out. Two distinct resonance modes were observed below TN = 25 K, which is well explained by conventional antiferromagnetic resonance theory with easy-plane anisotropy. The zero-field spin gap is derived to be about 166 GHz, originating from the easy-plane anisotropy and exchange interaction. Our result suggests that the Dzyaloshinsky-Moriya interaction, which may induce spin canting, is absent. Disappearance of Ising anisotropy in Ca3ZnMnO6 suggests that the Co(4+) ion, as well as the Co-Mn superexchange, plays an important role for the Ising nature in Ca3CoMnO6.

Influence of Adrenergic Receptor Blockade on Aspirin-induced Inhibition of Platelet Function

Studies from our laboratory have demonstrated that epinephrine via alpha(2)-adrenoreceptor modulation can restore the sensitivity of aspirin-treated platelets to the action of agonists independent of secretion of granule contents or synthesis of prostanoids. The present study has evaluated the effects of full strength aspirin (take on alternate days) and low dose aspirin (taken daily) and sought to determine whether agents which antagonise alpha or beta adrenergic receptors can prevent the corrective influence of epinephrine on aspirin-treated platelets. Adult volunteers were given 325 mg aspirin every other day or 80 mg aspirin daily. After confirming the inhibitory effect of aspirin, these individuals were given yohimbine (5 mg), atenolol (100 mg), verapamil (160 mg) or ethanol (white wine 8 oz). Two hours later, blood was drawn for platelet studies. Cells exposed to aspirin did not aggregate when stirred with arachidonate (0.45 mM) or epinephrine (5 M) alone. Addition of epinephrine to aspirin treated platelets restored the sensitivity to the action of arachidonate and resulted in irreversible aggregation. Ethanol and verapamil at the concentrations tested did not potentiate the action of aspirin. Ingestion of yohimbine, an alpha(2)-adrenergic antagonist, or atenolol, a beta blocker, prevented the corrective influence of epinephrine on aspirin-treated refractory platelets. Results suggest that alpha and beta blockers may serve a useful role in reinforcing the antithrombotic influence of aspirin in vivo.

Differential response of human and bovine platelets to bovine von Willebrand factor and vascular subendothelium

Human platelets undergo agglutination when stirred with bovine plasma (BP), but bovine platelets do not. The present study has shown that exposure of washed bovine platelets to subthreshold concentrations of adenosine diphosphate or thrombin before stirring restores their sensitivity to BP, and the cells undergo rapid agglutination. This agglutination was prevented by a monoclonal antibody, to glycoprotein GPIb. Flow cytometry studies revealed that exposure of bovine platelets to thrombin caused an increase in their ability to bind antibodies known to react with human GPIb or GPIIb-IIIa receptors. Interaction of bovine and human platelets with vascular subendothelium revealed additional differences in reactivity. Bovine platelets in citrate anticoagulant reacted poorly with subendothelium under flow conditions compared with human platelets. In contrast, bovine platelets in blood with low molecular weight heparin as anticoagulant adhered more readily than human cells. These findings suggest that different mechanisms are involved in hemostasis in human and bovine species.

Development of poly(Lactic acid)/chitosan co-matrix microspheres: controlled release of taxol-heparin for preventing restenosis

Smooth muscle cell proliferation plays a major role in the genesis of restenosis after angioplasty or vascular injury. Controlled release of appropriate drugs alone and in combinations is one approach for treating coronary obstructions, balloon angioplasty, restenosis associated with thrombosis, and calcification. We demonstrated the possibility of encapsulating taxol-loaded polylactic acid (PLA) microspheres within heparin-chitosan spheres to develop a prolonged release co-matrix form. The in vitro release profile of taxol and heparin from this co-matrix system was monitored in phosphate buffered saline pH 7.4, using an ultraviolet spectrophotometer. The amount of taxol/heparin release was initially much higher, followed by a constant slow release profile for a prolonged period. The initial burst release of taxol (15.8%) and heparin (32.7%) from the co-matrix was modified with polyethylene glycol coatings (13.5% and 25.4%, respectively, for 24 hr). From scanning electron microscopy studies, it appears that these drugs diffuse out slowly to the dissolution medium through the micropores of the co-matrix. However, the surface micropores were modified with polyethylene glycol (PEG) coatings for a constant slow release profile. This PEG-coated PLA/chitosan co-matrix may target drug combinations having synergestic effects for prolonged periods to treat restenosis.

5-Fluorouracil-loaded chitosan coated polylactic acid microspheres as biodegradable drug carriers for cerebral tumours

The development of injectable microspheres for anticancer drug delivery into the brain is a major challenge. The possibility of entrapping 5-fluorouracil (5-FU) in chitosan coated monodisperse biodegradable microspheres with a mean diameter of 10-25 um was demonstrated. An emulsion of 5-FU (in water) and polylactic acid (PLA) dissolved in acetone-dichloromethane mixture was poured into an aqueous solution of chitosan (or poly-vinyl alcohol) with stirring using a high-speed homogenizer, for the formation of microspheres. 5-FU recovery in microspheres ranged from 44-66% depending on the polymer and emulsification systems used for the preparation. Scanning electron microscopy revealed that the chitosan coated microspheres had less surface micropores compared to PVA based preparations. The drug release behaviour from microspheres suspended in phosphate buffered saline exhibited a biphasic pattern. The amount of drug release was much higher initially (approximately 25%), followed by a constant slow release profile for a 30 days period of study. This chitosan coated PLA/PLGA microsphere formulation may have potential for the targeted delivery of 5-FU to treat cerebral tumours.

Colchicine encapsulation within poly(ethylene glycol)-coated poly(lactic acid)/poly(epsilon-caprolactone) microspheres-controlled release studies

Smooth muscle cell proliferation plays a major role in the genesis of restenosis after angioplasty or vascular injury. Local delivery of agents capable of modulating vascular responses have the potential to prevent restenosis. However, the development of injectable microspheres for maintaining high tissue levels of drugs at the site of vascular injury is a major challenge. We demonstrated the possibility of entrapping an antiproliferative agent, colchicine, in polyethylene glycol (PEG)-coated biodegradable microspheres composed of poly(lactic acid)/poly(epsilon-caprolactone) blends, with a mean diameter of 3-6 microm. A solution of colchicine and blends of polylactic acid (PLA)/polycaprolactone (PCL) dissolved in acetone-dichloromethane mixture was poured into an aqueous solution of PEG (or polyvinyl alcohol) with stirring by a high-speed homogenizer to form microspheres. Colchicine recovery in microspheres ranged from 30-50% depending on the emulsification system and the ratio of polymer blends used for the preparations. Scanning electron microscopy revealed that the PLA/PCL microspheres were spherical in shape and had a smooth surface texture. Results of in vitro release studies showed that it is possible to control the colchicine release by choosing the appropriate particle size, loading, and PLA/PCL composition. Water permeability through the PLA membrane was greater, when compared with PCL blends. The amount of drug release also was much higher (58.3%) in PLA compared with PCL (39.3%) microspheres, for 30 days. Therefore, we concluded that the drug release from the microspheres followed a diffusion mechanism where bulk erosion and surface deposition were negligible. These PEG-coated PLA/PCL microspheres may have potential for targeting antiproliferative agents for prolonged periods to treat restenosis.

Use of plasma glow for surface-engineering biomolecules to enhance bloodcompatibility of Dacron and PTFE vascular prosthesis

The search for a nonthrombogenic material having patency to be used for small diameter vascular graft applications continues to be a field of extensive investigation. The purpose of the present study was to examine whether surface modification of polytetra fluoroethylene (PTFE, Teflon) and polyethylene-terephthalate (Dacron) vascular grafts might extend graft biocompatibility without modifying the graft structure. A series of surface coatings were prepared by modifying the argon plasma-treated PTFE and Dacron grafts with collagen IV and laminin and subsequently immobilizing bioactive molecules like PGE1, heparin or phosphatidyl choline via the carbodiimide functionalities. Surface analysis by Fourier transform infrared spectroscopy-attenuated total reflectance revealed the presence of new functional groups on the modified graft surfaces. In vitro studies showed that fibrinogen adsorption and platelet adhesion on modified grafts were significantly reduced. This study proposes that surface grafting of matrix components (collagen-type IV and laminin) and subsequent immobilization of bioactive molecules (PGE1, heparin or phosphatidyl choline) changed the surface conditioning of vascular grafts and subsequently improved their biocompatibility. However, more detailed in vivo studies are needed to confirm these observations.

Evaluation of heparin immobilized chitosan-PEG microbeads for charcoal encapsulation and endotoxin removal

A technique is described to encapsulate activated charcoal for hemoperfusion to be used in an artificial liver support. Activated charcoal was encapsulated within chitosan-PEG matrix and subsequently surface modified with PGE1 or heparin (hep-AC-PEGCB) via the glutaraldehyde functionalities. This novel matrix was used as the supports for perfusion of endotoxin, under a flow rate of 30 ml/mt. Endotoxin adsorption was quantitatively measured by the method of Limulus Amebocyte lysate test. It seems, the hep-AC-PEGCB may be a good adsorbent system for the removal of toxic endotoxin, and the system may be useful for detoxification of blood. The hep-AC-PEGCB matrix had improved biocompatibility as demonstrated from their hemolytic potential and charcoal release. However, further studies are needed to determine their behaviour under clinical conditions.

Evaluation of modified alginate-chitosan-polyethylene glycol microcapsules for cell encapsulation

A bioartificial pancreas, a medical device entrapping islets of Langerhans (islets) in an immunoisolative membrane, has been regarded as one of the most promising approaches to treat insulin-dependent diabetic patients. In this study, various modifications of alginate-chitosan microcapsules were made such as the inclusion of polyethylene glycol (PEG) and the use of crosslinkers such as carbodiimide (EDC) and glutaraldehyde (GA) in the core and onto the microcapsule membrane surface. A characterization of the modified microcapsules in terms of mechanical stability and albumin diffusion as well as their surface properties using SEM was performed. A mild GA treatment greatly enhanced the mechanical stability of the microcapsules, and this treatment did not affect the coating process of chitosan or PEG. The biological response to such microcapsules was evaluated by microencapsulation of red blood cells (RBC) and subsequent observation of their hemoglobin release. The encapsulated RBC in the PEG-GA coated microcapsules were found to be less hemolytic and had improved stability and biocompatibility. The results suggest the possibility of developing biological assist organs by microencapsulation of mammalian cells such as islets or liver cells in immunoisolative microcapsules in the near future.

Surface-immobilized biomolecules on albumin modified porcine pericardium for preventing thrombosis and calcification

The search for a noncalcifying tissue material to be used for valve replacement application continues to be a field of extensive investigation. A series of porcine pericardial membranes was prepared by modifying the glutaraldehyde--treated tissues with albumin and subsequently immobilizing bioactive molecules like PGE1, PGI2 or heparin via the carbodiimide functionalities. The in vitro calcification and collagenase degradation of these modified tissues were studied as a function of exposure time. Furthermore, the biocompatibility aspects of such novel interfaces were established by platelet adhesion and fibrinogen adsorption. The results reported in this article propose that the treatment with antiplatelet agents such as albumin, heparin and prostaglandins (PGE1 or PGI2) change the surface conditioning of pericardial tissues, suggesting a possible role of deposited serum components in affecting mineralization process on bioprosthesis. Therefore, it is worthy to hypothesize that besides inhibiting the accumulation of calcium in the devitalized cells, the early formation of a conditioning layer on the bioprosthesis surface may affect salt precipitations, determining the propensity of the implant to calcify. More detailed studies are needed to understand the involvement of plasma proteins and cellular components of the recipient blood in tissue-associated calcification.

Modifications in accessibility of membrane glycoproteins, binding of specific ligands and coagulation factor V during the activation of platelets in blood emerging from bleeding time wounds

Dual color flow cytometric techniques were applied to micro-aliquots of whole blood obtained from bleeding time (BT) wounds. Modifications in platelet activation markers (P-selectin [CD62P]) and lysosomal related protein (UMPS [CD63]), presence of membrane glycoproteins (GPIb [CD42b], GPIIb-IIIa [CD41a], GDIV [CD36], binding of von Willebrand factor (vWF), fibrinogen (Fg) and factor V [FV]) were analyzed in normal donors (n = 10) and in a severe von Willebrand patient (type 3) von Willebrand disease [vWD]). Samples of blood (20 microl) were sequentially removed from BT wound edges for up to 5 min and fixed with paraformaldehyde. Antigens were detected using the corresponding tagged monoclonal antibodies (moAbs) and quantitative results were referred to those found on platelet samples obtained from venous blood obtained from the same individuals. A progressive increase in % of platelets positive for activation dependent antigens (CD62 from 7 +/- 2 to 48 +/- 19% and CD63 from 9 +/- 1 to 44 +/- 8%; initial vs. 4 min) was observed. Accessibility of GPIIb-IIIa epitopes on platelets from BT wounds remained slightly above levels observed in venous blood platelets, despite a progressive increase in the presence of platelets positive for Fgn. Binding of MoAb to GPIV increased at late stages of BT. A moderate decrease in the binding of a moAb to GPIb was observed on platelets obtained at late stages of the BT (14 +/- 9% and 20 +/- 6% at 4 and 5 min, respectively). This apparent decrease in GPIb epitopes paralleled an increased presence of platelets positive for vWF (26 +/- 12 and 38 +/- 15%). Binding of moAb to GPIb always remained above basal levels in platelets obtained from BTs performed in the patient with type 3 vWD. FV levels on platelets coming from the BT persisted at background levels in all the individuals and at all times studied.

Changes in pericardial calcification due to antiplatelet agents: in vitro studies

To develop tissue valves for prolonged use in the cardiovascular system, the complicated process of surface induced calcification must be better understood. Calcification was examined for 60 days on glutaraldehyde treated bovine pericardium (GABP) and enzyme extracted tissues fixed in glutaraldehyde (GATBP) incubated in metastable solutions of calcium phosphate, and the roles of aspirin and persantine in conjunction with vitamins C, B, or E, gentamycin (antibiotic), or pentothal sodium (anesthetic) in the medium were examined. Further, the diffusion of calcium across the GATBP was evaluated using a diffusion cell with 2 compartments. Pericardial calcification was also observed using scanning electron microscopy (SEM) techniques. It seems that the examined antiplatelet agents can modify the pericardial surfaces and subsequently their mineralization processes (GATBP, 31.7 micrograms/mg tissue; in the presence of 5 mg% vitamin C, 13.1 micrograms/mg tissue; in 1.5 mg% aspirin, 17.2 micrograms/mg tissue; and 1 mg% gentamycin, 14.8 micrograms/mg tissue) on exposure with the metastable calcium phosphate solution for 60 days. In addition, these agents may modify calcium transport and interfere with the adsorption at the surface, hence reducing calcium nodulation on GATBP. Scanning electron micrographs also revealed a reduction in calcium deposition on the pericardium due to these antiplatelet agents. It may be hypothesized that the influx of calcium on GATBP may be due to the cellular components or the involvement of plasma proteins like the fibrinogen molecule. The exact mechanism of these changes in the calcification of the pericardium are still unknown. From these in vitro findings, it appears that a combined vitamin therapy with low doses of aspirin may be beneficial for platelet suppression and thereby for prevention of thrombosis and calcification. However, more in vivo studies are needed to develop applications.

Platelet adhesion and spreading on protein-coated surfaces: variations in behavior in washed cells, PRP, and whole blood

Platelet attachment and spreading were monitored on glass and various protein coated glass, under shear with washed platelets, platelet rich plasma (PRP) and whole blood, using fluorescence Optimas imaging system and software. Results showed that the platelet adhesion and spreading were sensitive to the nature of precoated proteins and the type of medium used for introducing platelet suspension for the study. In general, the cell adhesion and spreading were higher with fibrinogen (Fg), fibronectin (Fn), von Willebrand Factor (vWF), and collagen precoated surfaces. In the presence of albumin on the surface, however, platelets could not attach and spread fully when using washed cells. But, the surface attachment and spreading of the cells were higher on albumin substrates on exposure to PRP or whole blood. This may be due to the replacement of precoated albumin by other plasma proteins, like Fg to facilitate the platelet-surface attachment. The composition of this layer determines the extent of platelet activation and the adhesive strength between platelets and polymer surface. These results indicate that multiple adhesion receptors can mediate platelet adhesion and spread to matrix proteins immobilized on surfaces. Further, these studies combined with some of our earlier observations and suggestions propose the need for developing in vitro tests that resemble in vivo conditions.

Determination of the Bonding and Valence Distribution in Inorganic Solids by the Maximum Entropy Method

The distribution of valence among the bonds in the bond graph of an inorganic compound is used to calculate an `entropy'. We show that the distribution of valence that maximizes this entropy (ME) is similar, but not identical, to that obtained using the equal-valence rule (EVR) proposed by Brown [Acta Cryst. (1977), B33, 1305–1310]. Since the ME solutions are maximally non-committal with regard to missing information, they give better predictions of the observed valence distributions than the EVR solutions when lattice constraints or electronic anisotropies are present, but worse predictions when these effects are absent. Since valences calculated using ME are necessarily positive, they give significantly better predictions in cases where EVR predicts a negative bond valence. In the absence of electronic distortions the observed bond graph is either the graph with the highest maximum entropy or it has an entropy within 1% of this value. Since the entropy depends on the oxidation states of the atoms, compounds with the same stoichiometry and cation coordination numbers but different atomic valences may adopt different bond graphs and hence different structures.

Microtubule coils versus the surface membrane cytoskeleton in maintenance and restoration of platelet discoid shape

The discoid form of blood platelets is important to their function in hemostasis. Recent studies have suggested that the spectrin-rich surface membrane cytoskeleton and the cytoplasmic, actin-rich cytoskeleton are responsible for discoid shape, shape change, and recovery after activation or chilling. Earlier studies had suggested that circumferential coils of microtubules supported the disc shape of resting platelets and that their repositioning or reassembly restored disc shape after exposure to low temperature. The present study has used the chilling-rewarming model, together with microtubule stabilizing (taxol) and disassembling (vincristine) agents to retest the relative importance of the surface membrane cytoskeleton and circumferential microtubules in platelet discoid shape and its restoration. Washed platelet samples were rested at 37 degrees C and chilled to 4 degrees C; chilled and rewarmed to 37 degrees C for 60 minutes; or chilled, rewarmed, and exposed to the same cycle in the presence or absence of vincristine or taxol and fixed for study by disseminated interference phase contrast microscopy and electron microscopy. Rhodamine-phalloidin and flow cytometry were used to measure changes in actin filament assembly. Chilling caused loss of disc shape, pseudopod extension, disassembly of microtubule coils, and assembly of new actin filaments. Rewarming resulted in restoration of disc shape, pseudopod retraction, disassembly of new actin filaments, and reassembly of circumferential microtubule coils. Vincristine converted discoid platelets to rounded cells that extended pseudopods when chilled and retracted them when rewarmed, leaving spheres that could undergo the same sequence of changes when chilled and rewarmed again. Taxol prevented cold-induced disassembly of microtubules and limited pseudopod formation. Rewarming caused retraction of pseudopods on taxol-treated, discoid cells. Cytochalasin B, an agent that blocks new actin filament assembly, alone or in combination with taxol, inhibited the cold-induced shape change but not dilation of the open canalicular system. Rewarming eliminated open canalicular system dilation and restored lentiform appearance. The results indicate that microtubule coils are the major structural elements responsible for disc shape and its restoration after submaximal stimulation or rewarming of chilled platelets.

Influence of ionized calcium on thrombin-induced down regulation of GPIb/IX receptors on human platelets

The influence of ionized calcium on the down-regulation of GPIb/IX receptors on human platelets was evaluated by flow cytometry using monoclonal antibodies. Addition of EDTA alone to a washed platelet suspension did not cause decreased monoclonal antibody binding to the cells. However, introduction of thrombin to the washed platelets containing EDTA resulted in a marked decrease in binding of monoclonal antibodies to the GPIb/IX receptors. If calcium at 1-3 mmol/L was added to buffered platelets instead of EDTA before thrombin, down-regulation was prevented or significantly reduced. Restoring calcium to EDTA platelets after the thrombin-stimulated down-regulation had been in progress for 1-3 minutes caused reversal of decreased antibody binding by GPIb/IX to near resting levels. Results demonstrate that extracellular calcium is a major factor regulating thrombin-induced down-regulation.

Influence of cytochalasin B (CB) on GP Ib distribution after thrombin or TRAP and before surface activation

The receptor for von Willebrand factor (vWF) on human platelets, glycoprotein (GP) Ib/IX, has been shown in our studies to be an immobile complex when stimulated in suspension or on surfaces. Recent investigations have revealed that GP Ib/IX remains immobile on platelets activated in suspension followed by exposure to formvar surfaces that cause the cells to spread. However, since channels of the open canalicular system (OCS) are evaginated back on to the exposed surface during spreading, it was suggested that our study missed the clearance of GP Ib/IX from the exposed surface to internal membranes. The present study has added cytochalasin B after exposure of platelets to thrombin or TRAP in suspension in order to prevent spreading and movement of GP Ib/IX during subsequent exposure to surface activation on formvar grids. Results indicate that GP Ib/IX receptors remain randomly dispersed from edge to edge on platelets activated by thrombin or TRAP in suspension 10 minutes before treatment with CB followed by surface activation. Statistical analysis of the frequency of immunogold particles binding to monoclonal antibodies attached to GP Ib/IX revealed no significant reduction in frequency, translocation from cell edges or concentration of GP Ib/IX receptors in or around channels of the OCS. Results support the concept that GP Ib/IX is not cleared from exposed surfaces to the OCS of platelets activated by thrombin or TRAP and surface activation.

Aggregated-disaggregated, refractory platelets retain sensitivity to ristocetin

The present investigation has used an aggregation-disaggregation-reaggregation model to determine if exposure to potent aggregating agents causing a refractory state in deaggregated platelets results in down-regulation and clearance of GPIb/IX, the receptor for von willebrand factor. Thrombin, the thrombin receptor activating peptide (TRAP) and the thromboxane A2 mimic, U46619, caused irreversible aggregation and secretion when stirred on an aggregometer with plateletrich plasma (PRP). Addition of prostaglandin E1 (PGE1) after the plateau of maximum aggregation was reached caused rapid dissociation of platelet aggregates. Dissociated platelets were refractory to a second exposure to the primary stimulus or to other aggregating agents. Exposure of the refractory cells to epinephrine before the primary agent restored sensitivity resulting in a second wave of irreversible aggregation. Deaggregated, refractory platelets, however, retained their sensitivity to ristocetin. Amounts of the antibiotic causing agglutination of resting PRP also caused agglutination of disaggregated, refractory platelets. Addition of epinephrine to samples of refractory platelets less sensitive to low concentrations of the antibiotic restored their capacity to undergo irreversible ristocetin-induced agglutination. Analysis of the frequency of gold particles associated with monoclonal antibodies directed against GPIb/IX on control platelets and disaggregated, refractory platelets showed no significant difference in the number of receptors. The findings indicate that significant numbers of GPIb/IX receptors remain on platelet surfaces following exposure to potent aggregating agents.

Platelet adhesion to novel phospholipid materials: modified phosphatidylcholine covalently immobilized to silica, polypropylene, and PTFE materials

Based on the premise of achieving blood compatibility through mimicking the chemical constitutents of the biologically insert surface of the unactivated platelet membrane, a process was developed that entails the covalent grafting of modified phosphatidylcholine molecules to materials including silica, polypropylene, and polytetrafluoroethylene (PTFE) polymer films. These materials were characterized using x-ray photoelectron spectroscopy (XPS) and contactangle measurements. The phosphatidylcholine-containing materials (PC materials) were used as substrates in the plateletadhesion assays and were subjected to enzymatic degradation evaluation. Phosphatidylcholine-grafted silica materials do not support platelet adhesion. In addition the number of adherent platelets correlate with the amount of grafted phospholipid present, as indicated by the phosphorus/ carbon ratio obtained by XPS analysis. Platelet adhesion to phosphatidylcholine-grafted polypropylene and PTFE was inhibited 80% and 90%, respectively, when compared with platelet adhesion to unmodified polypropylene and PTFE.

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.