Thrombin-induced calcium movements in platelet activation

Pericellular Ca(2+) recycling potentiates thrombin-evoked Ca(2+) signals in human platelets

We have previously demonstrated that Na(+)/Ca(2+) exchangers (NCXs) potentiate Ca(2+) signaling evoked by thapsigargin in human platelets, via their ability to modulate the secretion of autocoids from dense granules. This link was confirmed in platelets stimulated with the physiological agonist, thrombin, and experiments were performed to examine how Ca(2+) removal by the NCX modulates platelet dense granule secretion. In cells loaded with the near-membrane indicator FFP-18, thrombin stimulation was observed to elicit an NCX-dependent accumulation of Ca(2+) in a pericellular region around the platelets. To test whether this pericellular Ca(2+) accumulation might be responsible for the influence of NCXs over platelet function, platelets were exposed to fast Ca(2+) chelators or had their glycocalyx removed. Both manipulations of the pericellular Ca(2+) rise reduced thrombin-evoked Ca(2+) signals and dense granule secretion. Blocking Ca(2+)-permeable ion channels had a similar effect, suggesting that Ca(2+) exported into the pericellular region is able to recycle back into the platelet cytosol. Single cell imaging with extracellular Fluo-4 indicated that thrombin-evoked rises in extracellular [Ca(2+)] occurred within the boundary described by the cell surface, suggesting their presence within the open canalicular system (OCS). FFP-18 fluorescence was similarly distributed. These data suggest that upon thrombin stimulation, NCX activity creates a rise in [Ca(2+)] within the pericellular region of the platelet from where it recycles back into the platelet cytosol, acting to both accelerate dense granule secretion and maintain the initial rise in cytosolic [Ca(2+)].

Molecular and functional characterization of the human platelet Na(+) /Ca(2+) exchangers

BACKGROUND AND PURPOSE The Na(+) /Ca(2+) exchanger is a bi-directional transporter that plays an important role in maintaining the concentration of cytosolic Ca(2+) ([Ca(2+) ](i) ) of quiescent platelets and increasing it during activation with some, but not all, agonists. There are two classes of Na(+) /Ca(2+) exchangers: K(+) -independent Na(+) /Ca(2+) exchanger (NCX) and K(+) -dependent Na(+) /Ca(2+) exchanger (NCKX). Platelets have previously been shown to express NCKX1. However, initial studies from our laboratory suggest that NCX may also play a role in platelet activation. The objective of this study was to determine if the human platelet expresses functional NCXs. EXPERIMENTAL APPROACH RT-PCR, DNA sequencing and Western blot analysis were utilized to characterize the human platelet Na(+) /Ca(2+) exchangers. Their function during quiescence and collagen-induced activation was determined by measuring [Ca(2+) ](i) with calcium-green/fura-red in response to: changes in the Na(+) and K(+) gradient, NCX pharmacological inhibitors (CBDMB, KB-R7943 and SEA0400) and antibodies specific to extracellular epitopes of the exchangers. KEY RESULTS Human platelets express NCX1.3, NCX3.2 and NCX3.4. The NCXs operate in the Ca(2+) efflux mode in resting platelets and also during their activation with thrombin but not collagen. Collagen-induced increase in [Ca(2+) ](i) was reduced with the pharmacological inhibitors of NCX (CBDMB, KB-R7943 or SEA0400), anti-NCX1 and anti-NCX3. In contrast, anti-NCKX1 enhanced the collagen-induced increase in [Ca(2+) ](i) . CONCLUSIONS AND IMPLICATIONS Human platelets express K(+) -independent Na(+) /Ca(2+) exchangers NCX1.3, NCX3.2 and NCX3.4. During collagen activation, NCX1 and NCX3 transiently reverse to promote Ca(2+) influx, whereas NCKX1 continues to operate in the Ca(2+) efflux mode to reduce [Ca(2+) ](i) .

Effect of cerebrocrast on the function of human platelets and release of the arachidonic acid from plasma membrane

Diabetes mellitus (DM) is accompanied by several cardiovascular complications such as coronary artery disease, atherosclerosis, hypertension, cerebral and myocardial infarction, etc. DM induces the alteration of platelet functions including activation, hyperaggregation, adhesiveness, and formation of thrombi. Release of AA from phospholipids of the PM, synthesis of TxA(2),PGE(2), activity of PLA(2), and PLC are increased in the platelets of the DM patients. Stimulation of PLA(2) activity and accumulation of bioactive metabolites such as AA, its oxygenated derivatives, prostaglandins and PAF can evoke glucose production, also. In this study we explored the effect of the 1,4-dihydropyridine compound cerebrocrast at a low concentration (10(-6)-10(-8)M) on the level of intracellular calcium in unstimulated human platelets and those stimulated with thrombin as well as release of [(3)H] AA from phospholipids of platelet PM. Cerebrocrast at a concentration of 10(-6) M decreased the basal level of intracellular calcium concentration (platelets were loaded with Fura-2) in unstimulated as well as in thrombin stimulated platelets. Cerebrocrast at concentrations of 10(-6), 10(-7), 10(-8) M inhibited release of [(3)H] AA from phospholipids of platelet PM. We conclude that blockade of human platelet activation with cerebrocrast can prevent aggregation, adhesion and formation of thrombi. The inhibition of [(3)H] AA release from phospholipids of platelet PM can prevent formation of eicosanoids such as TxA(2), PGG(2), and PGH(2) plus AA oxygenated derivatives. These effects of cerebrocrast are very significant in the treatment of DM-evoked cardiovascular complications.

Effects of temperature on calcium-sensitive fluorescent probes

The effect of temperature on the binding equilibria of calcium-sensing dyes has been extensively studied, but there are also important temperature-related changes in the photophysics of the dyes that have been largely ignored. We conducted a systematic study of thermal effects on five calcium-sensing dyes under calcium-saturated and calcium-free conditions. Quin-2, chlortetracycline, calcium green dextran, Indo-1, and Fura-2 all show temperature-dependent effects on fluorescence in all or part of the range tested (5-40 degrees C). Specifically, the intensity of the single-wavelength dyes increased at low temperature. The ratiometric dyes, because of variable effects at the two wavelengths, showed, in general, a reduction in the fluorescence ratio as temperature decreased. Changes in viscosity, pH, oxygen quenching, or fluorescence maxima could not fully explain the effects of temperature on fluorescence. The excited-state lifetimes of the dyes were determined, in both the presence and absence of calcium, using multifrequency phase-modulation fluorimetry. In most cases, low temperature led to prolonged fluorescence lifetimes. The increase in lifetimes at reduced temperature is probably largely responsible for the effects of temperature on the physical properties of the calcium-sensing dyes. Clearly, these temperature effects can influence reported calcium concentrations and must therefore be taken into consideration during any investigation involving variable temperatures.

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.

Regulation of phospholipase D activity by neutral lipids in egg-yolk phosphatidylcholine small unilamellar vesicles and by calcium ion in aqueous medium

Hydrolysis activity of phospholipase D from Streptomyces chromofuscus (PLD) was studied in small unilamellar vesicles (SUV) of egg yolk phosphatidylcholine (PC). The enzyme was associated with PC-SUV in a Ca(2+)-dependent manner. Both apparent maximum velocity, Vmax(app), and reciprocal of apparent Michaelis constant, i.e., apparent binding constant, 1/Km(app), increased with Ca2+ concentration, and the maximum values of these kinetic parameters were obtained at about 20 microM Ca2+. Incorporation of 1,2-diacylglycerol (DAG), cholesterol (Chol) or alpha-tocopherol (Toc) into PC-SUV induced shift of the antisymmetric PO2- stretching band of PC to lower frequency. The neutral lipids in SUV brought about increase of the Vmax(app) value (Yamamoto et al. (1993) Biochim. Biophys. Acta 1145, 293-297). On the basis of these findings we discussed the regulation of PLD activity in terms of the Ca(2+)-dependent complex formation of PLD with SUV, and the enhancement of susceptibility of the P-O bond in PC molecule by neutral lipids.

Distribution of plasma membrane Ca(2+)-ATPase and inositol 1,4,5-trisphosphate receptor in human platelet membranes

Human platelet plasma membranes were prepared by the glycerol lysis method of Harmon et al. [Harmon JT. Greco NJ. Jamieson GA. (1992) Isolation of human platelet plasma membranes by glycerol lysis. Meth. Enzymol., 215, 32-36]. The membranes were observed to contain a Ca(2+)-ATPase with different properties than those of internal membranes. The specific activity of Ca(2+)-ATPase was lower in plasma membranes (10-40 nmol ATP hydrolyzed/min/mg), but the ATPase was less sensitive to thapsigargin (41% inhibition at 500 nM) and more sensitive to vanadate (50% inhibition at 4 microM) than the Ca(2+)-ATPase in internal platelet membranes. The plasma membranes contained a Ca(2+)-ATPase detectable by monoclonal and polyclonal antibodies against erythrocyte Ca(2+)-ATPase that had a molecular mass of 144 kD. However, an anti-peptide antibody against an N-terminal sequence of the inositol 1,4,5-trisphosphate receptor recognized this protein in internal membranes, but not plasma membranes.

Platelet calcium homeostasis is abnormal in patients with severe arteriosclerosis

To evaluate platelet calcium homeostasis in a typical thrombosis-prone clinical condition, 14 patients with severe arteriosclerosis and 11 healthy control subjects were studied. Platelet intracellular free calcium concentration ([Ca2+]i) was evaluated by means of the fluorescent probe fura 2 under resting conditions and after challenge with 0.05, 0.1, and 0.5 U/mL thrombin (final concentrations). Three different concentrations of extracellular ionized calcium ([Ca2+]e) were used: 1 mmol/L, 1 mumol/L, and < 1 nmol/L. Resting platelet [Ca2+]i was significantly higher (P < .001) in patients than in control subjects. After addition of 0.05 U/mL thrombin, the relative increase of [Ca2+]i was lower in patients than in control subjects in each of the three [Ca2+]e conditions (P = .05 at 1 mmol/L, P = .02 at 1 mumol/L, and P = .04 at < 1 nmol/L). After addition of 0.1 U/mL thrombin, the relative increase of [Ca2+]i was lower in patients than in control subjects under two [Ca2+]e conditions, 1 mumol/L and < 1 nmol/L (P = .04 and P = .03 respectively). With 0.5 U/mL thrombin, a trend toward lower values in patients than in control subjects was observed, reaching statistical significance (P = .03) only at < 1 nmol/L [Ca2+]e. These results suggest that calcium homeostasis is abnormal in platelets from patients with severe arteriosclerosis and probably reflects a chronic activation.

Role of phosphoinositide hydrolysis in astrocyte volume regulation

Astrocytes exposed to hypoosmotic stress swell and subsequently reduce their size to almost their original volume, a phenomenon called regulatory volume decrease (RVD). We found that during hypoosmotic swelling there was a twofold increase in phosphatidylinositol (PI) hydrolysis. This increase was inhibited by the phospholipase C inhibitor, U-73122 (10 microM). Inhibition of PI hydrolysis resulted in blockage of RVD. We also examined whether agents that stimulate PI hydrolysis would enhance RVD. These agents significantly accelerated RVD. The rank order of potency was endothelin (20 nM) > or = norepinephrine (100 microM) > endothelin-3 (7 nM) > thrombin (1 U/ml) > or = ATP (500 microM) > bradykinin (20 microM) > or = carbachol (500 microM), as indicated by RVD rate constants. The extent of PI hydrolysis induced by these agents at the beginning of RVD exhibited a logarithmic relationship with the magnitude of RVD enhancement. Also, there was a linear relationship between the rate of PI hydrolysis and RVD rate constants. Our results suggest that stimulated PI hydrolysis is involved in the regulation of cell volume in astrocytes.

Ca(2+)-dependent aggregation of rabbit platelets induced by maitotoxin, a potent marine toxin, isolated from a dinoflagellate

1. Administration of maitotoxin (MTX), a dinoflagellate toxin, caused aggregation of rabbit washed platelets. The cytosolic Ca2+ concentration ([Ca2+]i), measured by fura-2 fluorescence technique, was also increased by the presence of MTX. Rates of aggregation response and [Ca2+]i-increase were dependent on tested concentrations (3-100 ng ml-1) of the toxin. 2. The MTX-induced platelet aggregation and [Ca2+]i-increase were totally abolished in a Ca(2+)-free solution. The successive administration of Ca2+ in the presence of MTX elicited the aggregation and increase in [Ca2+]i. 3. Ba2+ was capable of substituting for Ca2+ in the MTX-induced platelet aggregation. In the presence of external Ca2+, transition metals, Co2+, Cd2+ and Ni2+, inhibited the aggregation response to MTX. 4. Organic calcium antagonists (verapamil and nifedipine) as well as a cyclo-oxygenase-inhibitor (aspirin) did not apparently inhibit the aggregation response to MTX, except for a high concentration (10(-5) M) of verapamil, while procaine (10 mM) reduced the rate of platelet aggregation. 5. MTX also elicited a release of ATP from platelets, which was abolished in the absence of external Ca2+. 6. In contrast, thrombin 0.5 unit ml-1 could elicit platelet shape change, [Ca2+]i-increase and ATP-release in the absence of external Ca2+. 7. These results suggest that the MTX-induced platelet activation is caused by an enhanced Ca(2+)-influx presumably through voltage-independent Ca2+ channels on the plasma membrane.

Intracellular calcium mobilization in rat platelets is adversely affected by copper deficiency

The influence of copper deficiency on the mobilization of Ca2+ from intracellular stores following ionomycin treatment or thrombin activation of rat platelets was examined using the fluorescent indicator, fura-2, to measure changes in cytosolic Ca2+ concentration ([Ca2+]i). Platelets, obtained from copper-deficient and control rats and loaded with fura-2, were suspended in medium containing 1 mM EGTA and no added Ca2+. The size of the internal Ca2+ pools in the suspended platelets was estimated from the rise in [Ca2+]i following maximal discharge of stored Ca2+ by treatment with 1 microM ionomycin. Peak [Ca2+]i following ionomycin treatment was lower in platelets from copper-deficient rats compared to control rats (148 +/- 27 nM vs. 188 +/- 17 nM), suggesting that the size of the Ca2+ storage pools was decreased by copper deficiency. Furthermore, once internal Ca2+ stores were discharged by ionomycin, [Ca2+]i remained elevated in platelets from copper-deficient rats, but decreased in control rats. These data indicate that copper deficiency may inhibit the efflux of Ca2+ from platelets after its release from internal stores by ionomycin treatment. In platelets from copper-deficient and control rats, stimulation with 0.1 U/ml thrombin led to rapid rise followed by a slow decay in [Ca2+]i. However, peak [Ca2+]i was lower in platelets from copper-deficient rats than in control rats (94 +/- 19 nM vs. 131 +/- 16 nM). These findings imply that by reducing the amount of Ca2+ available for release from intracellular stores, copper deficiency also reduces [Ca2+]i following thrombin activation in the absence of external Ca2+.

Protein kinase C stimulates dense tubular Ca2+ uptake in the intact human platelet by increasing the Vm of the Ca(2+)-ATPase pump: stimulation by phorbol ester, inhibition by calphostin C

The effects of protein kinase C (PKC) on Ca2+ transport were investigated in human intact platelets. The indicator quin2 was used to measure the free cytoplasmic Ca2+ concentration ([Ca2+]cyt) and to search for possible PKC effects on the Ca(2+)-ATPase extrusion pump located in the plasma membrane. The Ca2+ indicator chlorotetracycline (CTC) was used to study PKC effects on the dense tubular Ca(2+)-ATPase uptake pump. The activity of PKC was stimulated by phorbol 12-myristate 13-acetate (PMA) and was inhibited with calphostin C. Neither PKC activation nor inhibition had any effect on [Ca2+]cyt or the Ca2+ extrusion pump. Substantial activation of the dense tubular pump was observed with PMA. In resting platelets bathed in 2 mM external Ca2+ giving [Ca2+]cyt = 102-106 nM, activation of PKC by PMA (100 nM) increases the rate and extent of dense tubular Ca2+ uptake to 1.62 +/- 0.35 and 1.25 +/- 0.3 times control value (respectively). The Vm of the dense tubular pump was measured by using ionomycin to manipulate [Ca2+]cyt. It is shown that PMA increases the Vm by a factor of 1.7 +/- 0.4 but has no effect on the Km value (= 180 nM). An unexpected finding was that PKC activity supports a portion of the basal activity of the dense tubular Ca2+ pump in resting platelets. Preincubation with the inhibitor calphostin C (100 nM) decreases the rate and extent of dense tubular Ca2+ uptake in resting platelets by 38 +/- 5% and 29 +/- 21% (respectively). This is due to a 28 +/- 9% decrease in the Vm of the dense tubular pump. This suggests that there is a low level of stimulation of dense tubular Ca2+ pump mediated by PKC in resting platelets.

Stimulation of dense tubular Ca2+ uptake in human platelets by cAMP

Elevation of intracellular cAMP is shown to increase the rate (V) and maximal extent of Ca2+ uptake by the dense tubules in intact human platelets. Elevation of [cAMP] was accomplished by preincubation with the adenylate cyclase activator forskolin or with dibutyryl-cAMP (Bt2-cAMP). The free concentration of Ca2+ in the dense tubular lumen ([Ca2+]dt) was monitored using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. The free cytoplasmic Ca2+ concentration ([Ca2+]cyt) was monitored in parallel experiments with quin2. Both [Ca2+]cyt and [Ca2+]dt were analyzed in terms of competition between pump and leak mechanisms in the plasma membrane (PM) and dense tubular membrane (DT). When platelets are incubated in media with approx. 1 microM external Ca2+, [Ca2+]cyt is approx. 50 nM and [Ca2+]dt is very low. When 2 mM external Ca2+ is added, [Ca2+]cyt rises to approx. 100 nM and the process of dense tubular Ca2+ uptake can be resolved. Forskolin (10 microM) and Bt2-cAMP increase the rate of dense tubular Ca2+ uptake (V) to 2.1 +/- 0.60 and 1.70 +/- 40 times control values (respectively). The agents also increase the final [Ca2+]dt to 1.70 +/- 0.21 and 1.72 +/- 0.60 times control values (respectively). Titrations with ionomycin (Iono) showed that the increase was due to an increase in the Vm of the dense tubular Ca2+ pump. With [Iono] = 500 nM, [Ca2+]cyt was raised to greater than or equal to 1.0 microM and Vm of the dense tubular pump was elicited. (At [Iono] = 1.0 microM, the final [Ca2+]dt values were degraded 15% due to shunting of Ca2+ uptake.) Analysis showed that forskolin (10 microM) and Bt2-cAMP (1 mM) increase the Vm by a factors of 1.56 +/- 40 and 1.56 +/- 40, respectively. Analysis showed that neither agent changed the Km of the pump significantly from its control value of 180 nM. Neither agent changed the rate constant for passive leakage of Ca2+ across the DT membrane (1.7 min-1).

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.

Cyclic GMP increases the rate of the calcium extrusion pump in intact human platelets but has no direct effect on the dense tubular calcium accumulation system

Sodium nitroprusside (SNP) and other agents that elevate cGMP levels are known to inhibit the aggregation of human platelets. Published data suggest that cGMP attenuation of agonist-induced Ca2+ transients is involved in this effect. The present study shows that elevation of cGMP increases the rate of the Ca2+ extrusion pump located in the plasma membrane (PM) but does not have a direct effect on the Ca2+ accumulating pump of the dense tubules (DT). The study verifies that SNP can specifically elevate the cGMP level in the platelet. The kinetics of the Ca2+ extrusion system were studied in situ in platelets overloaded with the cytoplasmic Ca2+ indicator quin2 according to a published protocol developed in this laboratory. Elevation of cGMP by means of (10 microM) SNP increased the Vm of the Ca(2+)-ATPase pump by 63%, without affecting its Km (66-80 nM) or Hill coefficient (1.6-1.8). Dibutyryl-cGMP (Bt2-cGMP), preincubated for 45 min at 1 mM, increased the Vm by a factor of 2.2 +/- 0.4. The experiments did not give any indication that SNP or Bt2-cGMP change the rate of the Na+/Ca2+ exchanger which makes a minor contribution to Ca2+ extrusion in the studied [Ca2+]cyt range. The rate constant for passive leakage of Ca2+ across the PM was increased by 32 +/- 4% by SNP and 90 +/- 34% by Bt2-cGMP. The net result is that the free Ca2+ in the cytoplasm ([Ca2+]cyt) at 'rest' is lowered from control values of 112 nM to 89 nM or 80 nM, respectively. The kinetics of Ca2+ uptake by the dense tubules were determined in situ using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. Analysis showed that SNP and Bt2-cGMP had no effect on the Vm or Km of the dense tubular pump, and did not affect the rate constant for passive leakage. The agents did decrease resting [Ca2+]dt by 25% or 30%, respectively, but this result can be explained purely in terms of the reduced [Ca2+]cyt. The effects of cGMP (vs. cAMP) on the PM and DT pumps are closely correlated with reported effects of cGMP/cAMP induced phosphorylation of a protein of the molecular weight of the PM pump and a 22 kDa activator of the DT pump. Cyclic AMP increases the rate of both the PM and the DT pumps, whereas cGMP increases the rate of the PM pump only.(ABSTRACT TRUNCATED AT 400 WORDS)

Barbiturates inhibit intracellular Ca2+ rise induced by thrombin in rat platelets

The effects of a number of barbiturates (anesthetic as well as anticonvulsant) on thrombin-induced calcium mobilization were tested in rat platelets using the fluorescent Ca2+ probe Fura-2. All drugs, except barbituric acid and Na-barbital, inhibited the thrombin-induced intracellular Ca2+ rise. Both the uptake of extracellular Ca2+ and the release of calcium from intracellular organelles were affected but influx was inhibited more strongly and at lower concentrations of the drugs (e.g. IC50 of thiopental was 0.83 mM for influx and 1.2 mM for intracellular release). Inhibitory potencies of the various barbiturates were markedly different. Thiopental was the most and barbital the least potent inhibitor. The order of inhibitory potency of the drugs appeared generally to follow their lipid solubility and the order of their hypnotic efficiency, with hexobarbital as the most conspicuous exception. Therefore, barbiturate treatment of cells perturbs agonist-induced calcium mobilization. This effect may be partially linked to their previously reported inhibitory action on two kinases, protein kinase C and phosphatidylinositol 4-phosphate kinase [1, 2].

Effects of dihydropyridines and inorganic calcium blockers on aggregation and on intracellular free calcium in platelets

[Ca2+]i increase is necessary in physiological platelet activity, particularly aggregation and release. The increase of [Ca2+]i observed during platelet activation depends in part on Ca2+ influx from the extracellular medium. The participation of voltage-operated Ca2+ channels as a pathway for Ca2+ entry is controversial. In the present study we have attempted to reinvestigate this problem by measuring aggregation and [Ca2+]i changes in platelets activated by ADP or thrombin and incubated with organic or inorganic blockers of calcium channels. The main findings of the present paper can be summarized as follows: (i) Ni2+, Co2+ and Mn2+, well known inorganic blockers of Ca2+ channels, inhibited platelet aggregation induced by ADP or thrombin in a dose-dependent manner, Ni2+ being the most effective agent. (ii) Thrombin induced a rise in free [Ca2+]i in platelets incubated both in 1 mmol/l Ca(2+)-containing medium and in nominally Ca(2+)-free medium; the rise of free [Ca2+]i was in the first case up to 370 +/- 31 nmol/l and in the second case up to 242 +/- 26 nmol/l, indicating that this observed difference was due to Ca2+ entry from the extracellular medium. Co2+ and Ni2+ abolished that difference by inhibiting Ca2+ influx. (iii) Nisoldipine, nitrendipine and nimodipine (10-50 nmol/l) inhibited in a dose-dependent manner platelet aggregation induced by either ADP or thrombin in platelets incubated in normal-Ca2+ normal-K+ medium, also, aggregation was inhibited to a similar extent in platelets incubated in normal-Ca2+ high-K+ medium. (iv) Nisoldipine--the most effective dihydropyridine to inhibit platelet aggregation--also inhibited Ca2+ influx in platelets incubated in normal-Ca2+ medium, either in normal-K+ or high-K+ media. Our data support the existence of voltage-operated, dihydropyridine-sensitive calcium channels (L-type) and a physiological role for them in platelet function.

Changes in free calcium concentrations in platelets of SHRSP and WKY: its relationship to ATP releasing potencies and platelet aggregation activities upon stimulation of several reagents

To clarify the relationships between free calcium levels, ATP release and aggregation potencies of SHRSP platelets, we examined the platelets from 9-month-old SHRSP and WKY using fura-2AM and luciferine-luciferase. In the absence of extracellular Ca2+, each reagent elevated the free calcium level to the same extent in the samples of both SHRSP and WKY. With regard to ATP release, thrombin and collagen less potentiated the platelet action in SHRSP than WKY, and ATP release was not affected by extracellular Ca2+. Collagen and ADP induced aggregations showed lower activities in SHRSP than WKY. TPA caused higher Ca2+ influx and aggregation activity in SHRSP than WKY in the presence of extracellular Ca2+. These results indicate that Ca2+ release must be followed by ATP release, and ATP release may be less potentiated, while thrombin and TPA induced aggregation is likely to be stimulated in SHRSP platelets, because protein kinase C activity in SHRSP platelets appears to be high.

Ouabain increases the calcium concentration in intracellular stores involved in stimulus-response coupling in human platelets

The effect of ouabain on Ca2+ homeostasis in human platelets was studied using both quin 2 and chlorotetracycline to monitor changes in cytosolic Ca2+ as well as changes in the amount of Ca2+ accumulated in intracellular storage sites. In resting platelets, ouabain induces a concentration- and time-dependent increase in cytosolic Ca2+ concentration and a marked elevation of Ca2+ in the intracellular stores. The amount of Ca2+ mobilized from these stores upon stimulation with thrombin, as well as thrombin-induced secretion of platelet 5-hydroxytryptamine, was increased after preincubation with the glycoside (3 x 10(-6) M). These data show that ouabain induces an elevation of intracellular Ca2+ levels, most likely mediated via Na(+)-Ca2+ exchange, and that this incremental amount of Ca2+ is accumulated in an intracellular store involved in stimulus-response coupling. This may explain the enhanced functional responses of platelets to agonists in the presence of ouabain and suggests a role for Na(+)-Ca2+ exchange in Ca2+ homeostasis of the human platelet.

Regulation of stimulus-induced calcium transport pathways in human T (Jurkat) lymphoblasts

In human T (Jurkat) lymphoblasts we have studied the calcium signals induced by monoclonal antibodies reacting with the T-cell antigen receptor complex (TCR and CD3). Jurkat cells were preloaded with the fluorescent calcium indicator Indo-1 and the stimulus-induced rise in cytoplasmic free calcium concn was followed in the absence or in the presence of external calcium. The technique allowed the separate investigation of the intracellular calcium release and the external calcium influx processes. The changes in the membrane potential of Jurkat cells were followed simultaneously by using fluorescent indicators. We found that the activation of protein kinase C by phorbol ester (PMA) or by the permeable diacyl glycerol, DiC8, rapidly eliminated the calcium signal, independently of the presence or absence of external calcium, while these treatments did not appreciably change the membrane potential. In contrast, cell membrane depolarization achieved by various treatments selectively blocked the stimulus-induced calcium influx, while did not affect stimulus-induced calcium release from internal stores. The magnitude of the stimulus-induced calcium influx was found to be largely independent of the external calcium concns between about 2-2500 microM. It is demonstrated that the inhibitory effect of membrane depolarization on calcium influx is not simply due to the reduction of the inward calcium gradient under these conditions. These observations indicate a significant down-regulation of the stimulus-induced calcium signal by protein kinase C activation and a selective inhibition of the receptor-operated calcium channels by membrane depolarization.

Membrane depolarization selectively inhibits receptor-operated calcium channels in human T (Jurkat) lymphoblasts

Jurkat lymphoblasts were stimulated by a monoclonal antibody against the CD3 membrane antigen and the evoked calcium signal was followed by the intracellular fluorescent calcium indicator indo-1. The technique applied allowed us to separately investigate the stimulus-induced intracellular calcium release and the calcium-influx pathways, respectively. In the same cells membrane potential was estimated by the fluorescent dye diS-C3-(5). The resting membrane potential of Jurkat lymphoblasts under normal conditions was between -55 and -60 mV. Membrane depolarization, obtained by increasing external K+ concentration, removing external Cl-, or by increasing the Na+/K+ leak permeability with gramicidin or PCMBS, did not induce calcium influx in the resting cells and did not influence the CD3 receptor-mediated internal calcium release, while strongly inhibited the receptor-mediated calcium influx pathway. Half-maximum inhibition of this calcium influx was observed at membrane potential values of about -35 to -40 mV and this inhibition did not depend on the external calcium concentration varied between 5 and 2500 microM. Membrane hyperpolarization by valinomycin did not affect either component of the calcium signal. The observed selective inhibition of the receptor-operated calcium influx pathway by membrane depolarization is probably an important modulator of calcium-dependent cell stimulation.

Membrane depolarization inhibits thrombin-induced calcium influx and aggregation in human platelets

The relationship between thrombin-evoked changes in intracellular calcium concentration [( Ca2+]i) and aggregation was examined in Indo-1-loaded human platelets. The stimulus-induced intracellular calcium release and external calcium influx, as well as platelet aggregation, were studied in the same cell preparation. A close correlation between the sustained high [Ca2+]i level, depending on calcium entry, and the aggregation response was found. Gramicidin, at a concentration high enough to induce membrane depolarization, strongly inhibited the calcium influx and aggregation, but did not influence the thrombin-induced intracellular calcium release. We conclude that calcium influx through depolarization-inhibited calcium channels is a prerequisite of thrombin-induced platelet aggregation.

Collagen-induced platelet activation mainly involves the protein kinase C pathway

This study analyses early biochemical events in collagen-induced platelet activation. An early metabolic event occurring during the lag phase was the activation of PtdIns(4,5)P2-specific phospholipase C. Phosphatidic acid (PtdOH) formation, phosphorylation of P43 and P20, thromboxane B2 (TXB2) synthesis and platelet secretion began after the lag phase, and were similarly time-dependent, except for TXB2 synthesis, which was delayed. Collagen induced extensive P43 phosphorylation, whereas P20 phosphorylation was weak and always lower than with thrombin. The dose-response curves of P43 phosphorylation and granule secretion were similar, and both reached a peak at 7.5 micrograms of collagen/ml, a dose which induced half-maximal PtdOH and TXB2 formation. Sphingosine, assumed to inhibit protein kinase C, inhibited P43 phosphorylation and secretion in parallel. However, sphingosine was not specific for protein kinase C, since a 15 microM concentration, which did not inhibit P43 phosphorylation, blocked TXB2 synthesis by 50%. Sphingosine did not affect PtdOH formation at all, even at 100 microM, suggesting that collagen itself induced this PtdOH formation, independently of TXB2 generation. The absence of external Ca2+ allowed the cleavage of polyphosphoinositides and the accumulation of InsP3 to occur, but impaired P43 phosphorylation, PtdOH and TXB2 formation, and secretion; these were only restored by adding 0.11 microM-Ca2+. In conclusion, stimulation of platelet membrane receptors for collagen initiates a PtdInsP2-specific phospholipase C activation, which is independent of external Ca2+, and might be the immediate receptor-linked response. A Ca2+ influx is indispensable to the triggering of subsequent platelet responses. This stimulation predominantly involves the protein kinase C pathway associated with secretion, and appears not to be mediated by TXB2, at least during its initial stage.

Translocation-independent activation of protein kinase C by platelet-activating factor, thrombin and prostacyclin. Lack of correlation with polyphosphoinositide hydrolysis in rabbit platelets

The relationship between polyphosphoinositide hydrolysis and protein kinase C (PKC) activation was explored in rabbit platelets treated with the agonists platelet-activating factor (PAF), thrombin and 12-O-tetradecanoylphorbol 13-acetate (TPA), and with the anti-aggregant prostacyclin (PGI2). Measurement of the hydrolysis of radiolabelled inositol-containing phospholipids relied upon the separation of the products [3H]inositol mono-, bis- and tris-phosphates by Dowex-1 chromatography. PKC activity, measured in platelet cytosolic and Nonidet-P40-solubilized particulate extracts that were fractionated by MonoQ chromatography, was based upon the ability of the enzyme to phosphorylate either histone H1 in the presence of the activators Ca2+, diacylglycerol and phosphatidylserine, or protamine in the absence of Ca2+ and lipid. Treatment of platelets for 1 min with PAF (2 nM) or thrombin (2 units/ml) led to the rapid hydrolysis of inositol-containing phospholipids, a 2-3-fold stimulation of both cytosolic and particulate-derived PKC activity, and platelet aggregation. Exposure to TPA (200 nM) for 5 min did not stimulate formation of phosphoinositides, but translocated more than 95% of cytosolic PKC into the particulate fraction, and induced a slower rate of aggregation. PGI2 (1 microgram/ml) did not enhance phosphoinositide production, and at higher concentrations (50 micrograms/ml) it antagonized the ability of PAF, but not that of thrombin, to induce inositol phospholipid turnover, even though platelet aggregation in response to both agonists was blocked by PGI2. On the other hand, PGI2 alone also appeared to activate (by 3-5-fold) cytosolic and particulate PKC by a translocation-independent mechanism. The activation of PKC by PGI2 was probably mediated via cyclic AMP (cAMP), as this effect was mimicked by the cAMP analogue 8-chlorophenylthio-cAMP. It is concluded that this novel mechanism of PKC regulation by platelet agonists may operate independently of polyphosphoinositide turnover, and that activation of cAMP-dependent protein kinase represents another route leading to PKC activation.

Thrombin-induced activation of calcium transport pathways and their role in platelet functions

In human platelets thrombin-induced calcium release from intracellular stores, the consequent influx of extracellular calcium, as well as their role in the aggregation and ATP-secretion reactions were examined. In indo-1-loaded platelets intracellular calcium release was studied in the presence of excess EGTA in the incubation medium, while calcium influx was followed after a rapid repletion of external calcium. After thrombin-stimulation both calcium release and calcium influx produced about the same peak levels of cytoplasmic free calcium but in the first case it was only a transient response, while in the latter one a sustained calcium signal was observed. Increased calcium influx could be evoked for several minutes after the addition of thrombin, it was selectively inhibited by Mg2+ (20 mM) and Ni2+ (1 mM) ions, by neomycin and by PCMB, a non-penetrating SH-group reagent. This calcium influx was practically insensitive to organic calcium channel blockers. Thrombin-induced platelet aggregation was only partial in the absence of external calcium, even if excess magnesium was present in the media, while the aggregation response became complete if external calcium was repleted. A significantly reduced aggregation could be seen in calcium-containing media if calcium influx was selectively inhibited. Platelet ATP-secretion under the same conditions did not depend on external calcium or on calcium influx. These data indicate that in thrombin-stimulated platelets the opening of specific plasma membrane calcium channels can be selectively modulated and these channels play a major role in the development of a full-scale aggregation.

Calpain I activation is not correlated with aggregation in human platelets

Calpain-catalysed hydrolysis of platelet substrates such as cytoskeletal and calmodulin-binding proteins, and of protein kinase C, is assumed to contribute to platelet aggregation. We have measured calpain I activation by immunoblotting, and [Ca2+]i (cytoplasmic Ca2+ concn.) by fura-2 fluorescence, in parallel with measurement of aggregation, in stirred human platelets treated at different [Ca2+]ext (extend Ca2+ concns.) with A23187, leupeptin, phorbol ester and thrombin. Hydrolysis of actin-binding protein, and [3H]5-hydroxytryptamine release, were also measured in some cases. A rise in [Ca2+]i, platelet aggregation and calpain activation often occurred together. With some combinations of agonists and [Ca2+]ext, however, this correlation was clearly not maintained. It was shown: (a) that activation of calpain and its hydrolysis of platelet substrates were not strictly necessary conditions for platelet secretion and aggregation; (b) conversely, that calpain activation could occur without aggregation.

Influence of sodium conductances on platelet activation

The effects of extracellular Na+ and tetrodotoxin on resting membrane potential, cytosolic free Ca2+ levels and aggregation of human platelets have been studied. Neither the decrease in extracellular Na+-concentration (from 140 mmol/l to 0 mmol/l) nor the addition of tetrodotoxin (10(-7) to 10(-5) mol/l) modified the platelet membrane potential. Zero extracellular Na+ concentration or the presence of tetrodotoxin in the medium inhibited platelet aggregation; however, K+-depolarized platelets showed an unchanged aggregation induced by ADP or thrombin in media with zero or low extracellular Na+ concentrations or in the presence of tetrodotoxin. Moreover, zero extracellular Na+ concentration or tetrodotoxin inhibited calcium mobilization in platelets during activation induced by thrombin. Hence, voltage-dependent activation linked to Na+ influx appears to be necessary for ADP- and thrombin-induced platelet aggregation under control conditions. Mechanisms for the role of Na+ conductances in platelet function are discussed.

Platelet free calcium concentrations measured with fura-2 are influenced by the transmembrane sodium gradient

The influence of the transmembrane Na+ gradient on the intracellular free calcium concentration, [Ca2+]i, was studied in Sepharose gel-filtered platelets from healthy human subjects, using the Ca-sensitive fluorescent dye, fura-2. Raising the internal Na+ concentration, [Na+]i, by Na+ pump inhibition with 0.05 mM ouabain, without changing external Na+ did not cause a significant increase in [Ca2+]i. Substitution of extracellular Na+ by iso-osmolar sucrose induced a rapid (half-time about 2 min) and significant rise in [Ca2+]i; this effect was amplified in Na-loaded platelets. Partial restitution of external Na+ in these cells with increased [Ca2+]i promoted a significant and rapid Na+ concentration-dependent fall in [Ca2+]i; little decline in [Ca2+]i was observed if K+ was used instead of Na+. These observations represent in vitro evidence for the existence of a Na/Ca exchange mechanism in human platelets that may, in vivo, participate in the control of [Ca2+]i.

Calcium uptake and release characteristics of the dense tubules of digitonin-permeabilized human platelets

The kinetics of ATP-driven Ca2+ uptake by the dense tubules were studied in digitonin-permeabilized human blood platelets. Digitonin at 3 micrograms/ml was shown capable of permeabilizing the plasma membrane to lactate dehydrogenase and the cytoplasmic Ca2+ indicator Quin2 without increasing the passive permeability of the dense tubular membrane for Ca2+. Experimentation was carried out with platelets treated with 3 micrograms/ml digitonin reisolated and resuspended in detergent-free medium ('digitonin-permeabilized' platelets). Active Ca2+ accumulation, which occurs over a period of minutes, was monitored by the increase in the fluorescence of chlorotetracycline after the addition of Mg-ATP (37 degrees C). The active uptake is inhibited by 15 microM trifluoperazine. The process is saturable with respect to external [Ca2+], with a Km of 180 +/- 5 nM and a Hill coefficient (n) of 1.40 +/- 0.05. Analysis of the maximal uptake in steady state gave similar results (Km = 160 +/- 5 nM, n = 1.50 +/- 0.05). The rate of uptake at [Ca2+] approximately Km is increased when the digitonin-permeabilized platelets are preincubated with 100 nM phorbol 12-myristate 13-acetate. Actively accumulated Ca2+ is rapidly released (less than 1 min) by addition of D-myo-inositol trisphosphate (IP3). The maximal extent of release is 50%; the EC50 for IP3 is approx. 12 microM. The data are compared with findings for fractionated dense tubular membrane vesicles and for the intact platelet.

Deliberate quin2 overload as a method for in situ characterization of active calcium extrusion systems and cytoplasmic calcium binding: application to the human platelet

The objectives of the title were accomplished by a four-step experimental procedure followed by a simple graphical and mathematical analysis. Platelets are (i) overloaded with the indicator quin2 to cytoplasmic concentrations of 2.9 mM and (ii) are exposed to 2 mM external Ca2+ and 1.0 microM ionomycin to rapidly achieve cytoplasmic Ca2+ ([Ca2+]cyt) of ca. 1.5 microM. (iii) The external Ca2+ is removed by EGTA addition, and (iv) the active Ca2+ extrusion process is then monitored as a function of time. Control experiments show that the ionophore shunts dense tubular uptake and does not contribute to the Ca2+ efflux process during phases iii-iv and that the extrusion process is sensitive to metabolic inhibitors. The progress curves for the decline of quin2 fluorescence (resulting from active Ca2+ extrusion) were analyzed as a function of [Ca2+]cyt using a mathematical model involving the probability that an exported Ca2+ was removed from a quin2 complex (vs. a cytoplasmic binding element). The observed rates of decline of quin2 fluorescence at a particular [Ca2+]cyt are dependent upon (i) the absolute rate of the extrusion system (a function of its Km, Vm and Hill coefficient (n)), (ii) the intrinsic Ca2+ buffer capacity of the cytoplasm (a function of the total site concentration ([B]T) and its Kd) and (iii) the buffer capacity of the intracytoplasmic quin2 (a function of its concentration and Kd). The contribution of (iii) was known and varied and was used to determine (ii) and (i) as a function of [Ca2+]cyt. The Ca2+ binding data were verified by 45Ca2+ experimentation.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channels in thrombin-activated human platelet membrane

Platelet-activating factor, 5-hydroxytryptamine, thromboxane A2, adenosine diphosphate and thrombin are known to activate platelets by stimulating calcium entry, but the nature of the entry pathways is unknown. We present the identification of single divalent cation channels from thrombin-activated human platelets. Membrane vesicles from unstimulated and thrombin-stimulated human platelets were incorporated in planar bilayers and unitary currents through single channels were measured. Divalent cation selective channels could only be demonstrated in thrombin-stimulated preparations. These channels share a number of properties in common with voltage-dependent calcium channels--a high degree of selectivity for divalent cations, a single channel conductance of about 10 pS (in 150 mM Ba2+) and sensitivity to blockade by inorganic calcium channel blockers such as Ni2+. In other respects, these channels are different as they are not voltage-dependent and are not blocked by 1,4-dihydropyridine calcium channel antagonists.

Inositol 1,4,5-trisphosphate-induced calcium release from platelet plasma membrane vesicles

A platelet membrane preparation, enriched in plasma membrane markers, took up 45Ca2+ in exchange for intravesicular Na+ and released it after the addition of inositol 1,4,5-trisphosphate (IP3). The possibility that contaminating dense tubular membrane (DTS) vesicles contributed the Ca2+ released by IP3 was eliminated by the addition of vanadate to inhibit Ca+-ATPase-mediated DTS Ca2+ sequestration and by the finding that only plasma membrane vesicles exhibit Na+-dependent Ca2+ uptake. Ca2+ released by IP3 was dependent on low extravesicular Ca2+ concentrations. IP3-induced Ca2+ release was additive to that released by Na+ addition while GTP or polyethylene glycol (PEG) had no effect. These results strongly suggest that IP3 facilitates extracellular Ca2+ influx in addition to release from DTS membranes.