Fluorescence digital image analysis of serotonin-induced calcium oscillations in single blood platelets

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Assays using digital fluorescence: 1985-1998

Luminescence continues to provide comprehensive literature surveys which will be published in most issues. These are a continuation of the literature surveys begun in 1986 in the Journal of Bioluminescence and Chemiluminescence which, up until 1998, encompassed more than 6000 references cited by year or specialized topic. With this newly named journal these searches are expanding to reflect the journal's wider scope. In future we will cover all fundamental and applied aspects of biological and chemical luminescence and include not only bioluminescence and chemiluminescence but also fluorescence, time resolved fluorescence, electrochemiluminescence, phosphorescence, sonoluminescence, lyoluminescence and triboluminescence. The compilers would be pleased to receive any comments from the readership. Contact by e-mail: L.J. Kricka: larry_kricka@path1a.med.upenn.edu or P.E. Stanley: Stanley@LUMIWEB.COM Copyright 1999 John Wiley & Sons, Ltd.

Recombinant rhodostomin substrates induce transformation and active calcium oscillation in human platelets

Platelet activation has been a focus of numerous studies in normal and abnormal states. Morphological changes and calcium signals found with activated platelets in vitro have been well characterized. However, the rate of cell spreading on substrates and the frequency of calcium oscillation within individual platelets upon activation have not yet been reported. In this study, we first examined the ability of a recombinant fusion protein of rhodostomin (GST-rhodostomin), a snake disintegrin containing an Arg-Gly-Asp (RGD) motif, to activate platelets when GST-rhodostomin served as a substrate. Four aspects of platelet activities induced by immobilized GST-rhodostomin and fibrinogen were analyzed in parallel. Examinations of (1) translocation of P-selectin from intracellular compartments to the plasma membrane, (2) platelet adhesion to and spreading on substrates, (3) platelet contact pattern on substrates, and (4) the degree of phosphorylation of focal adhesion kinase in platelets indicated that GST-rhodostomin was a better substrate for platelet activation than fibrinogen. Analysis of the rate of platelet spreading on GST-rhodostomin was examined by time-lapsed video microscopy. The spreading rate averaged 0.43 micrometer/minute, while cell spreading averaged 0.22 micrometer/minute when platelets were plated on fibrinogen and treated with thrombin. A newly developed method, using time-lapsed microscopy and the Metamorph program, was used to analyze calcium signals within platelets. We found that platelets on GST-rhodostomin evoked calcium oscillation at a frequency of 4.77 spike/cell/minute vs 2.76 spike/cell/minute on fibrinogen. The results of cell spreading and calcium oscillation were consistent with the results of microscopic and biochemical assays. We therefore conclude that the determination of the rate of platelet spreading and the frequency of calcium oscillation within platelets performed in this study provides more quantitative parameters for measuring platelet activities. Our results also suggest that GST-rhodostomin might potentially be used as a probe to dissect the molecular mechanisms underlying the kinetic processes of platelet activation.

Reversible and irreversible intracellular Ca2+ spiking in single isolated human platelets

1. We have developed conditions that permit long duration recordings of [Ca2+]i in single, isolated human platelets and studied the reversibility of Ca2+i spiking following activation by physiological and artificial stimuli. 2. Fura-2-loaded platelets were immobilized at the tip of a saline-filled glass pipette using gentle suction. 'Contact' activation of Ca2+i spiking was observed in a proportion (11 %) of platelets, which continued for the duration of each recording (range 8-45 min). 3. Platelets that displayed constant, resting Ca2+i levels were used to test the effects of agonists. ADP (10 microM) increased [Ca2+]i in the form of either one to two spikes followed by an elevated plateau level (60 % of cells) or multiple Ca2+ spikes of irregular amplitude (40 % of cells). ADP-induced Ca2+i mobilization was completely reversible and repeatable. 4. Thrombin (1 u ml-1) evoked Ca2+i spiking in the majority (88 %) of platelets tested, which was not inhibited by perfusion of agonist-free saline throughout the recording period (range 8-67 min). 5. The clear difference in the reversibility of activation by different stimuli may reflect the distinct roles of individual agonists in haemostasis and have important consequences in the design of treatments for thrombosis.

[Ca2+]i oscillations and [Ca2+]i waves in rat megakaryocytes

ATP activated [Ca2+]i oscillations were measured in single rat megakaryocytes using fluorescence ratio microscopy. With increasing ATP concentration the duration of the [Ca2+]i oscillations increased, however, there was considerable variation from cell to cell in the absolute value of the peak [Ca2+]i and the frequency and duration of the oscillations. This variation depended, in part, on the level of Fura-2 loading suggesting that megakaryocytes are sensitive to buffering of [Ca2+]i by Fura-2. Agents, that increase the level of intracellular cGMP (sodium nitroprusside and 8-pCPT-cGMP) or cAMP (prostacyclin, IBMX, forskolin and 8-bromo-cAMP) inhibited [Ca2+]i oscillations. Despite the large cell to cell variation in the patterns of [Ca2+]i oscillations, reapplication of the agents that elevated cAMP or cGMP inhibited the oscillations similarly. Using video rate fluorescence ratio imaging we found that the agonist-induced [Ca2+]i oscillations were the result of a well-defined [Ca2+]i wave, which spread across the cell with an average speed of about 35 microns/s, during the rising phase of each oscillatory spike. After reaching a peak, [Ca2+]i decreased uniformly across the whole cell during the falling phase of the spike. Analysis of the temperature dependence of [Ca2+]i waves showed that the rate of [Ca2+]i decay exhibited a strong temperature dependence (Q10 approximately 4), whereas, the rate of rise exhibited a weak temperature dependence (Q10 approximately 1.3), suggesting, that the rate limiting process for [Ca2+]i wave propagation in rat megakaryocytes is the rate of [Ca2+]i diffusion.

Variability of the thrombin- and ADP-induced Ca2+ response among human platelets measured using fluo-3 and fluorescent videomicroscopy

The intracellular free Ca2+ concentration ([Ca2+]cyt) of individual human platelets localized between siliconized glass cover slips was determined at rest and after stimulation with thrombin and ADP using the Ca2+ indicator fluo-3 (0.97 +/- 0.30 mmol/l cell volume) with fluorescence video microscopy. Resting [Ca2+]cyt in the presence of 2 mM external Ca2+ showed only small inter-platelet variability ([Ca2+]cyt = 86 +/- 30 (S.D.) nM). Resting [Ca2+]cyt of individual fluo-3-loaded platelets measured as a function of time had a S.D. of 10 nM or 12% (S.D./mean). Individual platelets showed no affinity for the siliconized support and their [Ca2+]cyt showed no tendency to oscillate in either the resting or in the activated state. When 0.2 U/ml thrombin or 20 microM ADP were added, all platelets showed a characteristic Ca2+ transient whereby [Ca2+]cyt increased to peak values within 8-12 sec and then declined. The Ca2+ transients measured with fluo-3 were in approximate synchrony but peak [Ca2+]cyt values showed large inter-platelet variability. The ensemble average peak [Ca2+]cyt for thrombin and ADP were 672 +/- 619 (S.D.) nM and 640 +/- 642 (S.D.) nM, respectively. Thus inter-platelet variations (S.D./mean) were 92% or 100% as large as the average measured values. Mathematically-constructed averages of the single platelet experiments agreed reasonably well with platelet-averaged values obtained in parallel experiments with stirred platelet suspensions in a plastic cuvette, measured with a conventional spectrofluorometer. Peak [Ca2+]cyt values reflecting dense tubular Ca2+ release alone (external Ca2+ removed) also showed large interplatelet variation (171 +/- 105 (S.D.) nM with thrombin and 183 +/- 134 (S.D.) nM with ADP). Dense tubular Ca2+ release induced by cyclopiazonic acid (a dense tubular Ca2+-ATPase inhibitor) gave peak [Ca2+]cyt of 289 +/- 170 nM. Thus the size of the dense tubular Ca2+ pool has an inter-platelet variation of 59% (S.D./mean). Variability of the dense tubular pool size accounts for some, but not all, of the large interplatelet variation in peak (Ca2+]cyt seen with thrombin and ADP activation.

Association of localized Ca2+ gradients with redistribution of glycoprotein IIb-IIIa and F-actin in activated human blood platelets

We monitored the intracellular distribution of ionized free Ca2+ concentration ([Ca2+]i) in individual human platelets by digital imaging fluorescence microscopy with fura 2 during platelet activation induced by surface contact or a soluble platelet agonist (thrombin). Contact of platelets with glass resulted in pseudopod formation and spreading, accompanied by a nonuniform rise in [Ca2+]i. The rise in [Ca2+]i was maximal during pseudopod formation. Locally elevated [Ca2+]i was frequently found in pseudopodia and at the edge and core of spread platelets. This pattern was faithfully duplicated by the local pattern of distribution of the cytoskeletal components F-actin, gelsolin, and surface glycoproteins (GP) IIb-IIIa but not by calmodulin. Platelets stimulated by thrombin also showed an inhomogeneous rise in [Ca2+]i, which was well correlated with the staining of F-actin and GPIIb-IIIa. Cytochalasin D, an inhibitor of actin polymerization, inhibited the inhomogeneous increase or redistribution of F-actin and GPIIb-IIIa but did not inhibit the rise in mean [Ca2+]i. These observations suggest that a localized change in [Ca2+]i may be associated with cytoskeletal reorganization and redistribution of GPIIb-IIIa in activated platelets.

Endothelin-1 evoked an increase and oscillations in cytosolic calcium concentration in adherent single human platelets and increased GMP-140 (P-selectin) in platelet suspension

The effect of endothelin-1 (ET-1) on cytosolic calcium ion concentration ([Ca2+]i) in adherent single human blood platelet was determined by fluorescence digital imaging microscopy using Fura-2 as calcium probe. A dose dependent increase and oscillatory changes in [Ca2+]i in single platelets were evoked by ET-1 as with thrombin. Half and 1 microM of ET-1 increased (p < 0.01) the [Ca2+]i in single platelets from a resting level of 83 +/-3.4 nM to 120 +/- 13 nM and 240 +/- 20 nM respectively. The ET-1 induced increase in [Ca2+]i was suppressed by 1 mM EGTA, a calcium chelating agent in the medium. ET-1 increased the production of IP3 (quantitified by IP3 3H-radioreceptor assay kit) in platelets significantly (p < 0.05) in a dose dependent way. We measured the GMP-140 (P-selectin) level in the supernatant of human platelet suspensions incubated with thrombin and ET-1. Both thrombin and ET-1 increased the secretion of soluble GMP-140 in the supernatant of platelet suspensions. Therefore, we suggested that ET-1 increased [Ca2+]i in platelets by both calcium influx and IP3 mediated Ca2+ release resulting in activation and release of GMP-140.

Regulation of cytosolic calcium by collagen in single human platelets

1. There is controversy in the literature as to whether collagen is able to induce directly a rise in cytosolic calcium concentration ([Ca2+]i) in human platelets. We have addressed this question by observing the cytosolic calcium response of single fura-2-loaded human platelets settling onto a collagen-coated surface using dynamic fluorescence ratio imaging. 2. Following a short lag phase after adherence to collagen fibres, platelets underwent a rapid rise in cytosolic calcium from basal values of 80 +/- 13 nM (n = 24) to a peak of 475 +/- 42 nM (n = 24) which was sustained for the remaining period of the experiment. 3. The tyrphostin protein tyrosine kinase inhibitor, ST271, reduced substantially the proportion of platelets which exhibited a rise in [Ca2+]i on adherence to collagen and transformed the response in remaining cells to one of oscillations. 4. In contrast, and as a control for collagen, laminin-coated surfaces induced adherence of human platelets without elevating intracellular [Ca2+]; the cells however remained responsive to ADP. 5. We conclude that collagen directly induces a rise in cytosolic calcium in single human platelets through a tyrosine kinase-mediated pathway.

Calcium-activated potassium channels in human platelets

1. The effect of intracellular [Ca2+] ([Ca2+]i) on human platelet ion channels was studied using the nystatin whole-cell patch clamp recording technique. 2. Ionomycin-induced increases in [Ca2+]i rapidly activated a voltage-independent K(+)-selective channel with a slope conductance of 30 pS in 154 mM K+ saline. The single-channel conductance decreased in proportion to the square root of the external K+ concentration such that the estimated conductance in 5 mM K+ was approximately 5 pS. 3. The peak current under conditions expected to increase [Ca2+]i to micromolar levels indicated that each platelet possesses a small number (5-7) of 30 pS Ca(2+)-dependent K+ channels (KCa channels). 4. Spontaneous [Ca2+]i spiking was observed in many patch-clamped platelets using fura-2 fluorescence measurements. Each Ca2+ spike triggered up to five KCa channels at any one time. KCa channels were not active at resting levels of [Ca2+]i. 5. The results suggest that platelet KCa channels are not active under resting conditions but may have an important role in determining the membrane potential during Ca2+ signalling.

Activated neutrophil by endothelin-1 caused tissue damage in human umbilical cord

Immunostaining of human neutrophils incubated with endothelin-1 (ET-1) showed intense and spreading pattern of anti human granulocyte elastase within the cytosol. That reflected neutrophil activation followed by the release of granule contents by ET-1. In contrast, PBS (phosphate buffered saline) treated neutrophils showed localized and faintly stained granules. Intracellular calcium in fura-2 loaded neutrophils was measured at 340/380 nm. A dose and time dependent increase in intracellular calcium by ET-1 occurred in human single neutrophil. Elastase activity assay was done with chromogenic substrate S2484. ET-1 induces dose and time dependent increase in elastase activity in neutrophil suspensions like ionophore A23187. A similar time dependent increase in elastase activity was retained even after repeated wash and ET-1 treatment. That confirmed the viability of most of the neutrophils after each treatment. In umbilical cord preparations, ET-1 treated neutrophils could migrate from the venous lumen into the tissue matrix of the umbilical cords. Hematoxylin and eosin staining revealed a massive tissue destruction in ET-1 activated neutrophil treated cords when compared to sham control and untreated neutrophil injected cords. Immunostaining with monoclonal anti human elastase revealed an intense staining in former sections when compared to the others. We suggest that ET-1 activated neutrophil might play a major role in endothelial injury and tissue damage in conditions with high blood level of endothelin.

Role of calcium ion in platelet serotonin uptake regulation

It is generally accepted that intracellular Ca2+ is a key substance in the intracellular signal transducing mechanism of platelets. We investigated the possibility that extracellular and/or intracellular Ca2+ might regulate the transport activity of serotonin (5-HT) into platelets. We found that extracellular Ca2+ chelation with EGTA caused inhibition of 5HT uptake activity, which was recovered by extracellulary applied excess Ca2+. Intracellular Ca2+ chelation with acetoxymethyl bis(O-aminophenoxy)ethane-N,N,N'-tetraacetate (BAPTA-AM) did not, however, have any inhibitory effect on 5HT uptake activity in the presence of extracellular Ca2+. In the absence of extracellular Ca2+, BAPTA-AM significantly inhibited 5-HT uptake. The restorative effect of Ca2+ on 5-HT transport into EGTA-treated platelets was mimicked by Ba2+, but not by Sr2+. It was antagonised by inorganic Ca2+ channel antagonist including Ni2+, La3+ and Gd3+, but not by organic Ca2+ channel blockers including verapamil, nifedipine, diltiazem, omega-conotoxin GVIA and omega-agatoxin IVA. Furthermore, 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester hydrochloride (TMB-8), an intracellular Ca2+ antagonist, was found to inhibit the restorative effect of Ca2+. These results have led to the suggestion that depletion of intracellular Ca2+ pool(s) by EGTA might result in a reduction of 5-HT uptake activity. Thus, the intracellular Ca2+ pool(s) susceptible to EGTA might have a regulatory role in maintaining 5-HT transport into blood platelets.

Endothelin-1 increased immunoreactive von Willebrand factor in endothelial cells and induced micro thrombosis in rats

This study was performed (i) to investigate the interaction between ET-1 and endothelial cells and (ii) to study the role of ET-1 in in vivo thrombosis. Fura-2AM loaded human umbilical endothelial cell cultures were incubated with 0, 25, 50 and 100 pmol of ET-1 for 24 hours (n = 6) at 37 degrees C. Fura-2 released in the media was measured by spectroflurophotometer at wavelength of 350 nm excitation and 500 nm emission. We found significant (p < 0.01) and dose dependent decrease in Fura-2 release by the cells indicating increased intracellular calcium in HUVEC. Increased calcium by ET-1 was also confirmed at single cell level by fluorescence digital image analysis using Fura-2AM. 5 ml solution of ET-1 (100 pmol/ml) was injected within the venous lumen of umbilical cords (of normal pregnancy) clumped at both ends and incubated at a temperature 37 degrees C for 3 hours (n = 7). We found intensely stained immunoreactive von Willebrand factor (vWF) on the endothelial cells of ET treated umbilical cords when compared with sham control (Umbilical cords incubated with phosphate buffer saline; n = 7). Intravenous ET-1 infusions at a rate of 1 nmol/kg/hour for 2 hours (cases, n = 7) and 5% dextrose infusions (sham control, n = 7) were performed in rats. Aorta, kidney and liver tissues were obtained to perform immunostaining with polyclonal antibody to vWF and fibrinogen. ET-1 treated rat tissues showed intense staining for vWF and fibrinogen intravascularly at hte same site.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of calcium sequestration by mezerein, a protein kinase C activator, in saponized rabbit platelets

1. The saponin-permeabilized platelet was used to examine the effect of mezerein, a moderate activator of protein kinase C (C-kinase), on the sequestration of Ca2+ to its intracellular storage sites. 2. We found that the activation of C-kinase by mezerein causes the potentiation of the Ca2+ sequestration. 3. It was suggested that C-kinase in platelets might function as a negative feedback regulator for the agonist-induced Ca2+ mobilization and might be involved in its oscillation.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Cytoplasmic Ca2+ oscillation in rat megakaryocytes evoked by a novel type of purinoceptor

1. The responses of megakaryocytes isolated from rat bone marrow to externally applied adenosine triphosphate (ATP) were investigated in the whole-cell mode by the use of nystatin perforated patch-clamp technique. 2. ATP at 1-100 microM evoked periodic outward currents at a holding potential of -40 mV. The reversal potential of the currents was close to K+ equilibrium potential (EK) and the K+ channel blockers such as quinine and quinidine suppressed the currents, indicating that the outward currents are predominantly carried by K+. 3. Since it has been reported that adenosine diphosphate (ADP) evoked monophasic K+ current using a conventional whole-cell recording, we compared the results obtained by perforated and conventional patch-clamp techniques. The crucial difference between our results and previous results was due to the intracellular perfusion with internal solution containing a high concentration of EGTA by which both current shape and concentration response were modified. 4. The membrane permeable Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxy methyl ester; BAPTA AM), inhibited the K+ current concentration dependently, suggesting that ATP-induced oscillatory K+ currents are caused by changes in cytoplasmic free Ca2+ concentration ([Ca2+]i). 5. With increasing ATP concentration, the frequency and the maximum amplitude of K+ current oscillation increased and the latency of current, which is the period required to activate the first K+ current after ATP application, decreased. 6. ADP, 2-methylthio-ATP and ATP-gamma-S could also evoke the periodic K+ currents, but adenosine, uridine triphosphate (UTP) and alpha-beta-methylene adenosine 5'-triphosphate (AMP-CPP) failed. 2-Methylthio-ATP was the most potent agonist; next was ADP which showed a 10-30 times stronger effect than ATP. Cross-desensitization was observed between ATP and ADP, but not between ATP or ADP and thrombin. 7. Extracellular Ca2+ was not required for the ATP-induced K+ current activation, indicating that Ca2+ released from intracellular pools induced the oscillatory response. In addition, the agonist potency increased when extracellular Ca2+ concentration ([Ca2+]o) decreased, suggesting that the principal agonists might be ATP4- and ADP3-. 8. The results suggest the presence of a novel subtype of purinoceptor in the megakaryocyte plasma membrane which induces cytoplasmic Ca2+ oscillation and evokes periodic K+ current flux.

Relationships between serotonin induced elevation of intracellular Ca2+ concentration and stimulation of Ca2+ influx in blood platelets

Serotonin (5-HT) caused immediate elevation of intracellular Ca2+ concentration ([Ca2+]i) in blood platelets, and it was completely inhibited by 1 mM EGTA. In Ca2+ replenished platelets, however, 2 mM EGTA did not affect the 5-HT induced elevation of [Ca2+]i when EGTA was applied just before or during the stimulation by 5-HT. At the same concentration 5-HT was also found to enhance Ca2+ influx through the activation of 5-HT2 receptor, but with rather longer latent time. From these results it is suggested that 5-HT induced elevation of [Ca2+]i is caused by mobilization of Ca2+ from intracellular Ca2+ storage sites, but not by direct stimulation of Ca2+ influx. Depletion of such Ca2+ stores might impair the effect of 5-HT on [Ca2+]i. Thus, 5-HT induced augmentation of Ca2+ influx might be secondary to replenishment of the depleted Ca2+ stores which was caused by 5-HT induced internal release of Ca2+. It is concluded that the effects of 5-HT on [Ca2+]i and Ca2+ influx in platelets are manifested sequentially or independently.

Fluorescence digital image analysis of thrombin and ADP induced rise in intracellular Ca2+ concentration of single blood platelets

Cytoplasmic free Ca2+ concentration, [Ca2+]i, was estimated in single rabbit blood platelets by digital imaging microscopy with the use of the specific Ca(2+)-indicator dye Fura-2. Uneven distribution and low level of [Ca2+]i was found in the resting platelet even in the presence of extracellular 1 mM Ca2+. Thrombin at 1 unit/ml immediately caused a transient increase in [Ca2+]i, which was followed by a secondary and sustained increase in [Ca2+]i. The distribution of increased levels of [Ca2+]i was also shown to be uneven within the cell. The presence of 1 mM EGTA in the medium only slightly decreased the initial rise in [Ca2+]i, but completely inhibited the latter phase, a sustained rise in [Ca2+]i. This result shows that the initial rise of [Ca2+]i might not be caused by Ca2+ influx, but might be induced by mobilization of Ca2+ from intracellular Ca2+ storage sites. This speculation is further supported by the fact that the elevated [Ca2+]i induced by thrombin immediately decreased to the base line value when 3 mM EGTA was applied. Thus, thrombin induced elevation of [Ca2+]i is suggested to consist of two different processes, namely the mobilization of Ca2+ from the intracellular storage sites and the successive Ca2+ influx through the receptor activated Ca2+ channels. Stimulation with ADP also caused a rapid elevation of platelet [Ca2+]i, but this effect of ADP was different form that of thrombin. Thus, the ADP induced rise in [Ca2+]i was accompanied by oscillation and was inhibited by extracellular EGTA. Our present experiment is the first report that clearly and directly reveals the differences between the effects of thrombin and ADP on [Ca2+]i of platelets.

Spiking in cytosolic calcium concentration in single fibrinogen-bound fura-2-loaded human platelets

Fura-2-loaded human platelets were immobilized on a fibrinogen-coated surface and the cytosolic free calcium concentration ([Ca2+]i) was measured in single platelets by low-light-level video-ratio image-processing of the optical probe signal. Some fibrinogen-bound platelets showed repetitive spiking in [Ca2+]i with a mean frequency of about 2/min, which increased to 5/min in the presence of ADP. Other cells showed no activity until the addition of agonist. When immobilized in the presence of prostaglandin I2 and the fibrinogen antagonist Arg-Gly-Asp-Ser, the platelets adhered less firmly to fibrinogen, and in many [Ca2+]i remained low and constant. Subsequent activation of such platelets with ADP evoked oscillations in [Ca2+]i with a peak frequency of about 5/min and which persisted for at least 5 min. These results indicate that human platelets, like many other non-excitable cells, have an elaborate system of calcium signalling involving spiking.

Cyclic AMP stimulates Ca(2+)-ATPase-mediated Ca2+ extrusion from human platelets

The effect of cAMP on active Ca2+ extrusion across the plasma membrane of intact human platelets was studied using quin2, a fluorimetric indicator of free Ca2+ in the cytoplasmic compartment ([Ca2+]cyt). Elevations of cAMP were achieved by incubation with dibutyryl-cAMP or by forskolin, which was found to selectively elevate cAMP without affecting cGMP levels. Progress curves of Ca2+ extrusion from quin2-overloaded platelets were measured. The rate vs. [Ca2+]cyt characteristic was calculated as previously described (Johansson, J.S. and Haynes, D.H. (1988) J. Membr. Biol. 104, 147-163). Forskolin, at a maximally effective concentration of 10 microM, was shown to stimulate Ca2+ extrusion by increasing by a factor of 1.6 +/- 0.5 the Vm of a saturable component, previously identified with a Ca(2+)-Mg(2+)-ATPase located in the plasma membrane. Neither the Km (80 nM) or Hill coefficient (1.7 +/- 0.3) of the Ca(2+)-ATPase was affected. Forskolin had no effect on the linear, non-saturable component of extrusion (previously identified with a Na+/Ca2+ exchanger) over the [Ca2+]cyt range examined (50-1500 nM). Dibutyryl-cAMP (Bt2-cAMP, 1 mM) stimulated the Ca(2+)-Mg(2+)-ATPase component of Ca2+ extrusion by a factor of 2.0 +/- 0.6. Separate experiments showed that 10 microM forskolin reduces the resting [Ca2+]cyt from 112 nM to 96 nM. Mathematical analysis showed that this can be accounted for by the above-mentioned increase in Vm of the pump, countered by a 37-74% increase in the rate constant for passive Ca2+ leakage across the plasma membrane. The results suggest two mechanisms by which prostacyclin-induced elevation of cAMP inhibits platelet aggregation: (a) lowering of resting [Ca2+]cyt and (b) increasing the rate of Ca2+ extrusion after the initial influx or triggered release event.

Effect of concanavalin A on intracellular calcium concentration in single blood platelets

Mobilization of Ca++ was estimated in single rabbit blood platelets with digital imaging microscopy. Concanavalin A (Con A) caused a rapid initial increase in intracellular concentration of Ca++ ([Ca++]i) with a latent time of about 20 sec, followed by a sustained increase in [Ca++]i. This effect of Con A was antagonized by alpha-methyl-D-mannose, which already was shown to antagonize the inhibitory effect of Con A on 5-HT transport, indicating that this effect of Con A was also derived from its binding to cell surface glycoproteins. The presence of EGTA in the medium did not affect the initial rise, but inhibited the latter phase of sustained rise. Thus, Con A induced elevation of [Ca++]i was suggested to consist of two different processes: mobilization of Ca++ from the intracellular storage sites and successive Ca++ influx through Ca++ channels. The effect of Con A on the 5-HT transport was tested in the presence of EGTA, a condition where no Ca++ influx occurs. The results indicate that Con A induced inhibition of 5-HT transport was not influenced by EGTA in the medium. It is suggested that the effect of Con A on 5-HT transport might be exerted through the Ca++ mobilization from its intracellular storage sites.