Changes in both calcium pool size and morphology of human platelets incubated in various concentrations of calcium ion. Calcium-specific bleb formation on platelet-membrane surface

An innovative flow cytometric approach for small-size platelet microparticles: Influence of calcium

Microparticles (MPs) are small submicron membrane-derived vesicles shed from a variety of cells and they have been implicated in different disorders. Accordingly, understanding of physiological characteristics of MPs and improvement of methods of their quantification are important for further advance in the field. Although flow cytometry is the most widely applied technique for MP analysis, it is limited by lack of adequate standardisation. Annexin V (AnV), which binds surface phos-phatidylserine (PS) with high affinity, has been long regarded as a marker of MPs, but AnV binding is Ca2+-dependent and it is unclear how [Ca2+] concentrations could affect AnV binding to MPs and its enumeration. MPs from citrated and heparinised plasma were labelled with AnV, anti-CD42b and quantified using an Apogee A50 flow cytometer. The small-size MP gate was defined with the use of size beads (from 0.1 to 0.5 μm) and confirmed with an in vitro assessment of platelet stimulation. Biotinylated anti-CD42b antibodies were then bound to streptavidin conjugated with different fluorochromes, leading to an amplified signal of platelet MPs (PMPs). Moderate increase of [Ca2+] concentrations in the annexin V staining buffer allows initial plasma recalcification and more accurate MP quantification in citrated plasma. Thrombin stimulation of platelet-free plasma containing only MPs did not produce any changes in the concentration of AnV+ MPs, but decreased the anti-CD42b binding. The results also indicate that prolonged storage and thrombin induce the release of AnV+ MPs whereas PS exposure in pre-existent MPs is not affected by thrombin. In conclusion, we present a sensitive protocol for the analysis of circulating and in vitro induced small-size PMPs that might contribute to future cardiovascular and clinical research.

Note: The editorial process for this article was fully handled by Prof. Christian Weber, Editor-in-Chief.

Enhancement of human platelet activation by the combination of low concentrations of collagen and rabbit anticardiolipin antibodies

Low concentrations of collagen and anticardiolipin antibodies (ACLA), which were raised in rabbits by immunization with cardiolipin (CL), co-operatively activated human gel-filtrated platelets (GFP). GFP activated by adding ACLA 5 min prior to collagen (ACLA + Col) showed strong responses in cytosolic Ca2+ mobilization and cell aggregation; the responses decreased after 1 min, however, when collagen was added prior to ACLA (Col + ACLA). Col + ACLA was 30% less effective than the ACLA + Col in: (1) the phosphorylation of pleckstrin and myosin light chain; and (2) the secretion of alpha- and dense granules. Indomethacin inhibited Ca2+ mobilization, pleckstrin phosphorylation and cell aggregation in platelets stimulated by ACLA + Col. The thromboxane B2 level in platelets induced by ACLA + Col was similar to that stimulated by low concentrations of collagen alone. ACLA + Col increased the activities of phospholipase C (PLC) as determined by formation of phosphatidic acid (PA), whereas indomethacin and adenosine 2',5'-diphosphate, an antagonist of the ADP P2Y1 receptor, inhibited PA formation. These results suggest that ACLA, thromboxane A2 derived from the collagen pathway and secreted ADP co-operatively augment PLC activity and lead to platelet aggregation.

Potentiation of ganodermic acid S on prostaglandin E(1)-induced cyclic AMP elevation in human platelets

Ganodermic acid S (GAS), isolated from the Chinese medicinal fungus Ganoderma lucidum (Fr.) Karst (Polyporaceae), exhibits inhibitory effects on platelet responses to various aggregating agonists. Our study demonstrated that GAS also participated in potentiating the response of human gel-filtered platelets to prostaglandin (PG) E(1). GAS at <20 microM did not show any significant change of basal cyclic AMP level in gel-filtered platelets. However, GAS potentiated the PGE(1)-evoked cyclic AMP level in a bell-shaped, concentration-dependent manner. The agent at 7.5 microM enhanced the level up to 1.8-fold of that evoked by PGE(1) alone. Collagen did not inhibit the PGE(1)-induced cyclic AMP level in platelets pretreated with GAS at 6 to 7.5 microM. In the presence of 7.5 microM GAS, the agent enhanced the inhibition of PGE(1) on platelet response to collagen in: phosphorylation of myosin light chain and pleckstrin; alpha-granule secretion; cell aggregation and protein-tyrosine phosphorylation. In addition, the agent along with PGE(1) almost abolished the dense-granule secretion and thromboxane (TX) B(2) formation. The results suggest that GAS played an additional role in potentiating the PGE(1)-induced cyclic AMP synthesis. GAS and PGE(1) inhibited additively the platelet response to collagen.

Differential effects of ganodermic acid S on the thromboxane A2-signaling pathways in human platelets

Ganodermic acid S (GAS) [lanosta-7,9(11),24-triene-3beta,15alpha-diacetoxy-26-oic acid], isolated from the Chinese medicinal fungus Ganoderma lucidum (Fr.) Karst (Polyporaceae), exerted a concentration-dependent inhibition on the response of human gel-filtered platelets (GFP) to U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxyprostaglandin F2alpha), a thromboxane (TX) A2 mimetic. GAS at 2 microM inhibited 50% of cell aggregation. GAS at 7.5 microM inhibited 80% of Ca2+ mobilization, 40% of phosphorylation of myosin light chain and pleckstrin, 80% of alpha-granule secretion, and over 95% of aggregation. GAS also strongly inhibited U46619-induced diacylglycerol formation, arachidonic acid release, and TXB2 formation. An immunoblotting study of protein-tyrosine phosphorylation showed that GAS inhibited the formation of phosphotyrosine proteins at the steps involving the engagement of integrin alphaIIbbeta3 and aggregation. However, GAS did not inhibit U46619-induced platelet shape change or the inhibitory effect of U46619 on the prostaglandin E1-evoked cyclic AMP level in GFP. It is concluded that GAS inhibits platelet response to TXA2 on the receptor-Gq-phospholipase Cbeta1 pathway, but not on the receptor-G1 pathway.

Predominant inhibition of ganodermic acid S on the thromboxane A2-dependent pathway in human platelets response to collagen

Ganodermic acid S (GAS), a membrane acting agent, exerts multiple effects on human platelet function (C.N. Wang et al. (1991) Biochem. J. 277, 189-197). The study reported how GAS affected the response of human gel-filtered platelets (GFP) to collagen. The agent inhibited cell aggregation by prolonging lag and shape change periods and decreasing the initial cell aggregation rate. However, the inhibitory efficiency was less than its inhibition on GFP response to U46619, a thromboxane (TX) A2 mimetic. In the agent-effect on biochemical events, GAS effectively inhibited Ca2+ mobilization, phosphorylation of myosin light chain, dense granule secretion and TXB2 generation. The inhibitions might originate from blocking Ca2+ mobilization of the TXA2-dependent pathway. GAS partially decreased the phosphorylation of most phosphotyrosine proteins from early activation to the integrin alphaIIbbeta3-regulated steps. The agent did not affect the phosphorylation of three proteins at the steps regulated by integrin alphaIIbbeta3. The results suggest that GAS inhibits the collagen response predominantly on the TXA2-dependent signaling, and the tyrosine kinase-dependent pathway in collagen response plays a major role in aggregation.

Characterization of the antiplatelet effects of (2S)-5-methoxy-6-methylflavan-7-ol from Draconis Resina

(2S)-5-methoxy-6-methylflavan-7-ol (MMF) was purified from Draconis Resina and its in vitro effects on various aspects of platelet reactivity were examined. Results indicated that MMF dose dependently inhibited aggregation of washed rabbit platelets induced by collagen, arachidonic acid, ADP, U46619 or platelet-activating factor (PAF), with IC50) values of 17.2, 49.8, 179.8, 109.6, and 189.2 microM, respectively. Concomitantly, MMF also dose dependently suppressed ATP release by platelets activated by these stimulants. The increase in intracellular free calcium ([Ca2+]i), elicited by these activating agents, was inhibited by MMF as reflected by fura-2 fluorescence measurements. However, MMF had no effects on the cyclic AMP level of platelets. In addition, MMF inhibited the arachidonic acid-induced thromboxane B2 and prostaglandin D2 formation in intact platelet suspensions or homogenized platelet lysates. This study provided evidence that MMF is an antiplatelet agent whose activity is likely related to cyclooxygenase inhibition and suppression of [Ca2+]i increase.

Platelet lysis and functional perturbation by 13-methyl myristate. The major fatty acid in Flavobacterium ranacida

Flavobacterium ranacida consisted of 75% of 13-methyl myristate in total fatty acids. The acid at > 60 microM caused the lysis of gel-filtered platelets (GFP) in both time- and concentration-dependent manners. Scanning electron microscopy showed that: 1). GFP in 40 microM of the acid changed the morphology to speculate discoid shape at 15 sec, and to ellipsoids after 30 sec; and 2), the cells gradually swelled to spherical forms as the concentration of the acid increased. At nonlytic concentration, the acid inhibited platelet responses to various agonists with differential concentrations. The order of inhibitory potency was U46619 > low dose collagen > ADP-fibrinogen > phorbol ester > high dose collagen. The results demonstrated that 13-methyl myristate exhibited both cell lytic activity and perturbation on membrane function.

Activation of human platelet phospholipases C and A2 by various oxygenated triterpenes

Eight structural analogues of oxygenated triterpenes exerted striking differences in activation of human platelets. They are four pairs of stereoisomers and two pairs of positional isomers with varying: 1) acetoxyl/hydroxyl substituents; 2) the position of the substituents at C-3 and C-15; and 3) the stereochemistry of a substituent at C-3. It required a threshold concentration for each agent to cause the concentration-dependent activation. These triterpenes were hydrophobic with < 20% difference in the partition coefficients between 1-octanol and water. They caused differential effects on: inositol triphosphate production; the increase in [Ca2+]i; diacylglycerol formation; phosphatidic acid accumulation, protein phosphorylations and arachidonate release. These agents activated both phospholipases C and A2. The trend of activating phospholipase C was triterpenes with two acetoxyl substituents > one acetoxyl/one hydroxyl substituents > two hydroxyl substituents. In activating phospholipase A2, triterpenes with two acetoxyl substituents were most effective, whereas the paired isomers with a hydroxyl group at C-15 alpha and an acetoxyl substituent at C-3 failed the activation. The results enable one to discuss the possible structure-activity relationship of various oxygenated triterpenes in the activation of both phospholipases C and A2.

The mode of action of primary bile salts on human platelets

Cholate and its conjugated amide derivatives glycocholate and taurocholate solubilized human platelets differently as studied by the observations on: (1) the change in optical absorbance of platelet suspension, (2) marker leakiness and (3) component solubility. Cholate ruptures the membrane in an all-or-none process, while both conjugated derivatives shed off both proteins and lipids. The shed lipids formed vesicles and could be separated from the proteins. The conjugated salts gradually chop off the cell membrane into pieces causing the cells to become small spheres (1.5 microns in diameter) as revealed by scanning electron microscopy, which also revealed that morphological change of platelet in these bile salts depended on both concentration and incubation period. Platelets at the prelytic-stage concentration of these three salts deformed initially to spiculate disc and finally to a stretched-out flat form. Also, in the prelytic stage of these bile salts, platelets showed inhibited responses to thrombin which did not happen to platelets in deoxycholate (Shiao et al. (1989) Biochim. Biophys. Acta 980, 56-68.).

Snake venom cardiotoxin can rapidly induce actin polymerization in intact platelets

The action of Taiwan cobra (Naja naja atra) venom cardiotoxin on rabbit platelets at 37 degrees C was characterized by observing cytoskeletal alterations and cell lysis. At a concentration of 21.4 microM the toxin produced no cell lysis within 30 s, and less than 5% of the total lactate dehydrogenase activity of intact cells was detected in the suspending medium after the interaction had proceeded for 3 min. The extent of cell lysis was proportional to toxin concentration and interaction time. Before cell lysis, the toxin caused rapid incorporation of actin monomers into cross-linked actin filaments. The actin incorporation could be inhibited by either the presence of cytochalasin B or increased CaCl2 concentration in the suspending medium. However, addition of indomethacin did not influence the toxin-induced cytoskeletal change.

Selenite nuclear cataractogenesis: a scanning electron microscope study

The sequential changes during selenite nuclear cataractogenesis were examined with a scanning electron microscope (SEM) and correlated with slit lamp observations. A posterior opacity, visible with the slit lamp 1-2 days after injection of sodium selenite, was found to consist of masses of vacuoles in the superficial posterior cortex by SEM. 2-3 days post injection, a biomicroscopic refractile ring around the nucleus was represented by SEM abnormalities suggesting membrane damage and possible loss of cytosol in the perinuclear region. All normal structure in this region was lost by 5 days after injection when the central nucleus had become opaque. SEM also showed evidence for damage in areas which were still clear by slit lamp examination. Changes, characteristic of aging, were found near selenite induced damage in peripheral (younger) fibers.

Interaction of prostaglandins with adenosine diphosphate induced increase in cytosolic free calcium in human platelets

The interaction of prostaglandins with changes in cytosolic Ca2+ concentration ([Ca2+]) and aggregation of human platelets induced by adenosine diphosphate (ADP) were investigated. Cytosolic [Ca2+] was measured with the fluorescent dye Quin2. Addition of ADP (0.25-2.5 mumol l-1) to platelet suspensions produced a dose dependent increase in cytosolic [Ca2+] from a basal level of 51 +/- 1 nmol l-1 to maximum levels exceeding 1 mumol l-1 and induced platelet aggregation. Chelation of extracellular calcium with 100 mumol l-1 EGTA markedly reduced the increase in cytosolic [Ca2+] induced by 0.25 mumol l-1 ADP, while pretreatment with the calcium entry blocker verapamil was without effect. Stimulation of cyclic AMP with prostaglandins (PGD2, PGE1, PGE2, PGI2, but not PGF2 alpha) and forskolin, or incubation with dibutyryl-cAMP, inhibited the rise in cytosolic [Ca2+] and platelet aggregation following ADP. We conclude that prostaglandins inhibit the increase in cytosolic [Ca2+] and aggregation of human platelets induced by ADP, probably by stimulation of cyclic AMP generation, thereby opposing the mechanism by which ADP increases cytosolic [Ca2+] and subsequently induces platelet aggregation.

Activation of calpain I in thrombin-stimulated platelets is regulated by the initial elevation of the cytosolic Ca2+ concentration

The source and concentration of Ca2+ required to activate calpain I were investigated in thrombin-stimulated platelets. The concentration of cytosolic free Ca2+ ([Ca2+]i) was measured in platelets containing fura-2-AM, and exhibited a biphasic response after stimulation with 0.05, 0.1 or 0.5 NIH units of thrombin/ml. An initial transient elevation, which was predominantly dependent upon Ca2+ released from the internal stores into the cytosol, peaked at 15 s after stimulation, and a secondary sustained elevation, which was due to Ca2+ influx, was observed following the initial elevation. Calpain I was present at about 540 ng/10(8) unstimulated platelets, as measured by immunoblotting using rabbit anti-(human calpain I) IgG. Calpain I was activated 10 s after thrombin stimulation, as determined by the appearance of the 78 kDa and 76 kDa forms on immunoblots. The activation ratio of calpain I was calculated as the amount of the 78 + 76 kDa forms as a percentage of the total (80 + 78 + 76 kDa), and was influenced by the extent of the initial transient [Ca2+]i elevation after stimulation. An initial increase in [Ca2+]i of 300 nM was required to achieve the maximal activation (60%) of calpain I, and half-maximal activation occurred at 160 nM- Ca2+]i. These results suggest that the activation of calpain I in platelets is regulated by the initial elevation in Ca2+]i after thrombin stimulation, and does not necessarily require a Ca2+ influx.

Oxygen-radical-mediated lipid peroxidation and inhibition of ADP-induced platelet aggregation

The effects of lipid peroxidation on ADP-induced aggregation of washed rat platelets were examined using a oxygen-radical-generating system consisting of H2O2 and ferrous ion. Lipid peroxidation was assessed by measurement of thiobarbituric acid-reactive substances (TBARS). Incubation of the platelets with various concentrations of H2O2 (2-10 mM) in the presence of 10 microM Fe2+ resulted in a decrease of the aggregating capacity and an increase of TBARS value, depending on the concentrations of H2O2. Addition of catalase (0.1 mg/ml) to the incubation medium containing 10 microM Fe2+ and 10 mM H2O2 effectively protected the aggregating capacity, but superoxide dismutase (0.1 mg/ml) did not protect H2O2/Fe(2+)-induced inhibition of the platelet aggregation. The results of kinetic studies on the platelet aggregation with varying ADP and Ca2+ concentrations suggested that treatment of the platelets with H2O2/Fe2+ causes decreases in the binding affinities of ADP and Ca2+ for the platelets. On the basis of these results, change in the aggregating capacity of the platelets by treatment with H2O2/Fe2+ is discussed in relation to lipid peroxidation.

The inhibition of human platelet function by ganodermic acids

Human gel-filtered platelets aggregate at greater than 20 microM-ganodermic acid S [lanosta-7,9(11),24-triene-3 beta, 15 alpha-diacetoxy-26-oic acid] [Wang, Chen, Shiao & Wang (1989) Biochim. Biophys. Acta 986, 151-160]. This study showed that platelets at less than 20 microM-ganodermic acid S displayed both concentration- and time-dependent inhibition of function, in which the agent potency in response to inducers was ADP-fibrinogen greater than collagen greater than thrombin. The agent caused a biphasic time-dependent effect on platelet phosphoinositide metabolism. The first phase involved the decrease in the pool size of phosphoinositide by 10-20%. The second phase, in which both the resynthesis of phosphatidylinositol 4,5-bisphosphate (PIP2) and the decrease of [32P]phosphatidic acid occurred, took place after 30 min. Scanning electron microscopy also revealed a time-dependent morphological change in platelets in the presence of the agent. The cells initially became spiculate discs, then swelled to a 'potato-like' morphology at 60 min. Further studies on the time-dependent inhibition of thrombin response revealed that: (1) the percentage inhibition of cell aggregation was comparable with that occurring with an increase of cytosolic free Ca2+ concentration [( Ca2+]i) or the phosphorylation of marker proteins; (2) [32P]Pi-labelled platelets showed the time-dependent inhibition of thrombin-stimulated PIP2 resynthesis as indicated by first-2-min time-course studies of phosphoinositide interconversion; (3) scanning electron microscopy revealed that the aged platelet population showed an increase in the percentage of non-responding cells on prolonged incubation. The results, taken together, enabled one to discuss a possible mechanism for the time-dependent inhibition by ganodermic acid S of platelet response to thrombin.

The effect of ganodermic acid S on human platelets

Incubation of gel-filtered human platelets in ganodermic acid S (lanosta-7,9(11), 24-trien-3 beta, 15 alpha-diacetoxy-26-oic acid) showed that uptake of the agent by platelets was a simple diffusion process. The agent caused platelet aggregation at concentrations above 20 microM. Above the threshold, the extent of cell aggregation was in a linear relationship to the agent concentration. Below the aggregation threshold, platelets showed neither the resynthesis of [32P] phosphatidylinositol 4,5-bisphosphate ([32P]PIP2) and [32P] phosphatidylinositol 4-phosphate ([32P]PIP) nor the accumulation of [32P] phosphatidic acid ([32P]PA). The results suggested that ganodermic acid S caused the activation of PIP2 hydrolysis. Scanning electron microscopy revealed that the morphology of platelets below the aggregation threshold appeared to be spiculate discoid shape. Above the threshold, the cells rounded up to spiculate irregular forms.

The aggregation of human platelet induced by ganodermic acid S

Incubation of gel-filtered human platelets in ganodermic acid S (lanosta-7,9(11),24-trien-3 beta,15 alpha-diacetoxy-26-oic acid) showed that within a min 80% of the agent was taken up by the cells. The process of uptake was a simple diffusion, and the partition coefficient was about 10(5). The agent caused platelet aggregation at a concentration above 20 microM. Above the threshold, the extent of cell aggregation was in a linear relationship to the agent concentration. Also, the % of cell aggregation was comparable to the elevation of: (1) cytosolic free Ca2+ concentration [( Ca2+]i); (2) protein phosphorylation; and (3) serotonin release. Also, it was correlated with the change in the interconversion of phosphoinositides. Moreover, platelets in various concentrations of ganodermic acid S appeared to show different time-course profiles in the changes of [32P]phosphoinositides and [32P]phosphatidic acid (PA). Upon addition of the agent, platelets showed an initial increase in all of the [32P]phosphoinositides, and then the level of each kind of phosphoinositide decreased sequentially in phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol (PI). Below the aggregation threshold, platelets showed neither the resynthesis of [32P]PIP2 and [32P]PIP nor the accumulation of [32P]PA. However, at 25 and 50 microM, platelets showed not only the resynthesis of [32P]PIP2 and [32P]PIP but also the accumulation of [32P]PA. Interestingly, at 100 microM ganodermic acid S, platelets did not show the resynthesis of [32P]PIP2 and [32P]PIP. In this case, the level of [32P]PA accumulation and that of [32P]PI decrease were less than those found in platelets at 50 microM ganodermic acid S. The results suggested that ganodermic acid S caused the activation of PIP2 hydrolysis. Scanning electron microscopy (scanning EM) revealed that the morphology of platelets below the aggregation threshold appeared to be spiculate discoid shape. Above the threshold, the cells rounded up to spiculate irregular forms, which showed an elongation of filopodia after prolonged 30-s incubation. In addition, platelets at greater than or equal to 50 microM ganodermic acid S showed the occurrence of membrane vesiculation. Hence, the incorporation of ganodermic acid S into platelet membrane resulted in the change of membrane morphology.

The solubilization and morphological change of human platelets in various detergents

The solubilization of human gel-filtered platelets by octyl glucoside, Triton X-100, dodecylsulfate, and deoxycholate was compared from the analysis of (1) cell lysis, (2) marker leakiness, and (3) component solubility. These analyses all revealed that the effect of detergent concentration on the solubilization of platelets by these detergents was exerted in three stages, i.e., the prelytic, lytic, and complete platelet-lysis stages. These analyses also indicated several differences among platelets in these detergents. (i) The ratio of the platelet-saturation concentration (PSC) to critical micellar concentration (CMC) was about 1/2 for octyl glucoside. Triton X-100 and dodecylsulfate, while it was close to 1 for deoxycholate. (ii) Platelets in octyl glucoside. Triton X-100, and dodecylsulfate all showed parallel curves in cell lysis, protein solubilization and marker leakiness, while the platelet lysis in deoxycholate was identical to the phospholipid solubilization. (iii) The solubility curves of various components in Triton X-100 and deoxycholate were parallel. However, the solubility of cholesterol in octyl glucoside was lower than that of protein and phospholipid. In dodecylsulfate, the solubility of phospholipid and cholesterol was very low in comparison with that of protein. In addition, morphological studies using scanning electron microscopy (scanning EM) revealed that the solubilization by octyl glucoside or Triton X-100 might occur via membrane area expansion. On the other hand, the solubilization by dodecylsulfate or deoxycholate showed membrane vesiculation prior to cell lysis. Moreover, in the prelytic stage, the morphological change in platelets in octyl glucoside showed only concentration dependence by swelling to an ellipsoid and then to a sphere. However, the morphological change in platelets in the other three detergents was dependent not only on the detergent concentration but also on prolonged incubation. Specifically, in Triton X-100, the cells initially changed to spiculate discs and then reached their final shape as swollen discs with surface invagination. In dodecylsulfate and deoxycholate the morphological changes were almost the same. The cell initially deformed in shape to a spiculate disc and finally to a stretched-out flat form. The results are discussed according to the bilayer couple hypothesis. Also, in the prelytic stage, these detergents caused inhibition of the response of platelets to collagen and ADP-fibrinogen.