Thrombin-induced changes of intracellular [Ca2+] and pH in human platelets. Cytoplasmic alkalinization is not a prerequisite for calcium mobilization

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+)].

Freeze-dried rehydrated human blood platelets regulate intracellular pH

BACKGROUND

Long-term storage of platelets (PLTs) in the dry state would greatly improve options for PLT storage. Whether trehalose-loaded freeze-dried and rehydrated PLTs could regulate intracellular pH (pHi) was evaluated.

STUDY DESIGN AND METHODS

Previously it was shown that human PLTs can be successfully preserved by freeze-drying with trehalose. Trehalose-loaded freeze-dried rehydrated PLTs and fresh control PLTs were labeled with the pH dye BCECF-AM. pHi was measured in resting cells, cells acidified with nigericin, and cells treated with thrombin. The sodium-proton pump was blocked by treatment with 5-(N-methyl-N-isobutyl)amiloride (MIA).

RESULTS

The pHi of rehydrated PLTs is the same as that of fresh control PLTs, 7.27+/-0.03 (SD; n=5) and 7.27+/-0.02 (n=5), respectively. Nigericin treatment of cells showed that the recovery in pHi was Na+-dependent and followed Michaelis-Menten kinetics. The Vmax values (DeltapH/9 sec) were 0.21+/-0.039 (n=3) and 0.22+/-0.025 (n=3) for rehydrated and control PLTs, respectively. The exchange constants were 17.7+/-2.3 mmol per L (n=3) and 17.0+/-1.9 mmol per L (n=3) for rehydrated and control PLTs, respectively. Treatment of cells with MIA showed that NHE1 remained sensitive to the inhibitor after freeze-drying and rehydration.

CONCLUSION

The results show that the pHi regulation system is largely preserved during freeze-drying and rehydration of PLTs.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.

Resting and thrombin-stimulated cytosolic calcium in platelets of patients with alcoholic withdrawal, bipolar manic disorder and chronic schizophrenia

Cytosolic calcium concentration ([Ca2+]i) in platelets during resting state and when stimulated by thrombin were measured in 7 alcoholic dependent patients in the state of withdrawal (AW) who were receiving diazepam, 7 bipolar manic patients (BM) who were receiving haloperidol, 15 drug free chronic schizophrenic patients (CS) and 26 normal controls (NC). Resting [Ca2+]i in these groups were quite similar at (mean +/- SEM) 112 +/- 20 nM, 127 +/- 18 nM, 103 +/- 16 nM and 106 +/- 8 nM respectively. Increase in platelet [Ca2+]i in response to 0.1 U/ml thrombin was expressed as delta[Ca2+]i and its percentage over resting [Ca2+]i as %[Ca2+]i. Both delta[Ca2+]i and %[Ca2+]i were significantly higher (p = 0.006 and 0.0045 respectively, ANOVA, Waller-Duncan) in AW (433 +/- 71 nM, 417 +/- 58%) than the other groups: NC (223 +/- 25 nM, 225 +/- 23%), BM (309 +/- 38 nM, 260 +/- 31%), and CS (261 +/- 34 nM, 280 +/- 29%) respectively. In vitro incubation of platelets from NC with diazepam or haloperidol did not affect the resting [Ca2+]i and %[Ca2+]i. The enhanced [Ca2+]i response to thrombin in platelets of AW is unlikely to be due to diazepam. It may indicate an abnormality in platelets during the withdrawal phase. Treatment with haloperidol resulted in slightly higher [Ca2+]i in platelets of BM. Platelet [Ca2+]i in drug-free CS was not different from NC.

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.

Effects of L- and D-arginine and some related esters on the cytosolic mechanisms of alpha-thrombin-induced human platelet activation

1. In Fura-2 preloaded human platelets, the increase in cytosolic calcium induced by alpha-thrombin was reduced by some L- and D-arginine ester compounds the IC50 (microM) values of which were 7.4 for TAEE, 56.9 for BAEE, 77.6 for TAME, 560 for T(d)AME, 656.3 for L-ArgOMe and 2206.7 for D-ArgOMe. alpha-tosyl-L-Arginine, L- and D-arginine were inactive. 2. The inhibitory activity of the L-arginine esters was not modified when platelets were pretreated with 100 microM N omega-monomethyl-L-arginine. 3. The L-arginine esters did not increase cyclic GMP content in platelets either in the presence or absence of indomethacin and apyrase at rest and after alpha-thrombin stimulation. 4. The kinetic parameters of platelet Na+/H+ antiporter (amiloride-inhibitable, evaluated after cytosolic nigericin-induced acidification) were modified by L- and D-arginine esters, while the native amino acids were ineffective. 5. The inhibitory effects of the L- and D-arginine esters on platelet activation appear to be mainly due to their inhibitory effect on Na+/H+ antiporter.

Intracellular Ca2+ mobilization and not calcium influx promotes phorbol ester-stimulated thromboxane A2 synthesis in human platelets

Phorbol esters, potent activators of protein kinase C (PKC), greatly enhance the release of arachidonic acid and its metabolites (TXA2, HETES, HHT) by Ca2+ ionophores in human platelets. In this paper, we report the relationship between intracellular Ca2+ mobilization and external calcium influx into platelets and the ability of PMA plus A23187 to promote thromboxane A2 (TXA2) synthesis. The enhanced levels of TXA2 due to the synergistic stimulation of the platelets with A23187 and phorbol esters are not affected significantly by the presence of external Ca2+ or the calcium-chelator EGTA. PKC inhibitors, staurosporine and sphingosine, abolished phorbol myristate acetate (PMA) potentiation of TXA2 production which strongly supports the role of PKC in the synergism. Platelet aggregation is more sensitive to PMA and external calcium than TXA2 formation. PMA increased TXA2 production as much as 4-fold at low ionophore concentrations. The A23187-induced rise in [Ca2+]i was reduced by pretreatment of human platelets with phorbol esters, both in the presence and absence of EGTA, and staurosporine reversed this inhibitory effect. These results indicate that the synergistic stimulation of TXA2 production by A23187 and phorbol esters is promoted by intracellular Ca2+ mobilization and not by external calcium influx. Our data also suggest that PKC is involved in the regulation of Ca2+ mobilization from some specific intracellular stores and that PKC may also stimulate the Ca(2+)-dependent phospholipase A2 at suboptimal Ca2+i concentrations.

Sphingosine enhances platelet aggregation through an increase in phospholipase C activity by a protein kinase C-independent mechanism

Sphingosine (a potent inhibitor of protein kinase C) at 5-10 microM, which are concentrations lower than those that inhibit this enzyme activity, enhanced the aggregation of rabbit platelets induced by low concentrations of U46619, platelet-activating factor, thrombin and arachidonic acid, whereas H-7 and staurosporine, other protein kinase C inhibitors, failed to do so. Of the sphingosine analogues which also inhibit protein kinase C, psychosine and lyso-GM3 did not show such an enhancing effect. Sphingosine promoted both Ins(1,4,5)P3 formation and an increase in the cytoplasmic free Ca2+ concentration in response to all the agonists used. Furthermore, the hydrolytic action of exogenously added phospholipase C (from Clostridium perfringens) on platelet membrane phospholipids was dose-dependently enhanced by pretreatment of the platelets with sphingosine. These results imply that sphingosine, at relatively low concentrations, brings about hyperaggregability of the platelets by the agonists employed, probably owing to enhancement of the phospholipase C activity. Such an effect appears to be induced by a mechanism independent of protein kinase C inhibition. We suggest that sphingosine might act as a positive modulator for the stimulus-response coupling in the platelets.

Cytochrome P-450 may link intracellular Ca2+ stores with plasma membrane Ca2+ influx

We have studied the mechanism of the regulation of plasma membrane Ca2+ permeability by the degree of filling of the intracellular Ca2+ stores. Using Mn2+ as a Ca2+ surrogate for plasma membrane Ca2+ channels, we found that Mn2+ uptake by rat thymocytes is inversely related to the degree of filling of the intracellular Ca2+ stores. This store-dependent plasma membrane permeability is inhibited by oxygen scavenging, CO, imidazole antimycotics and other cytochrome P-450 inhibitors. The pattern of inhibition is similar to that reported previously for the inhibition of microsomal cytochrome P-450-mediated aryl hydrocarbon hydroxylase activity of lymphocytes. Several calmodulin antagonists, both phenothiazinic (trifluoperazine, fluphenazine and chlorpromazine) and dibenzodiazepinic (clozapine), accelerate Mn2+ uptake by cells with Ca2(+)-filled stores, and this effect is prevented by imidazole antimycotics. Our results suggest that cytochrome P-450 may be the link between the stores and the plasma membrane Ca2+ pathway. We propose a model in which this cytochrome, sited at the stores, stimulates plasma membrane Ca2+ influx. This stimulatory effect is, in turn, prevented by the presence of Ca2+ inside the stores, possibly via a calmodulin-dependent mechanism.

Cell regulation by sphingosine and more complex sphingolipids

Sphingolipids have the potential to regulate cell behavior at essentially all levels of signal transduction. They serve as cell surface receptors for cytoskeletal proteins, immunoglobulins, and some bacteria; as modifiers of the properties of cell receptors for growth factors (and perhaps other agents); and as activators and inhibitors of protein kinases, ion transporters, and other proteins. Furthermore, the biological activity of these compounds resides not only in the more complex species (e.g., sphingomyelin, cerebrosides, gangliosides, and sulfatides), but also in their turnover products, such as the sphingosine backbone which inhibits protein kinase C and activates the EGF-receptor kinase, inter alia. Since sphingolipids change with cell growth, differentiation, and neoplastic transformation, they could be vital participants in the regulation of these processes.

Arachidonic acid-induced calcium influx in human platelets. Comparison with the effect of thrombin

The effects of arachidonic acid and thrombin on calcium movements have been studied in fura-2-loaded platelets by a procedure which allows simultaneous monitoring of the uptake of manganese, a calcium surrogate for Ca2+ channels, and the release of Ca2+ from intracellular stores. Arachidonic acid induced both Ca2+ (Mn2+) entry through the plasma membrane and Ca2+ release from the intracellular stores. The release of Ca2+ was prevented by cyclo-oxygenase inhibitors and mimicked by the prostaglandin H2/thromboxane A2 receptor agonist U46619. Ca2+ (Mn2+) entry required higher concentrations of arachidonic acid and was not prevented by either cyclo-oxygenase or lipoxygenase inhibitors. Several polyunsaturated fatty acids reproduced the effect of arachidonic acid on Ca2+ (Mn2+) entry, but higher concentrations were required. The effects of maximal concentrations of arachidonic acid and thrombin on the uptake of Mn2+ were not additive. Both agonists induced the entry of Ca2+, Mn2+, Co2+ and Ba2+, but not Ni2+, which, in addition, blocked the entry of the other divalent cations. However, arachidonic acid, but not thrombin, increased a Ni2(+)-sensitive permeability to Mg2+. The effect of thrombin but not that of arachidonic acid was prevented either by pretreatment with phorbol ester or by an increase in cyclic-AMP levels. Arachidonic acid also accelerated the uptake of Mn2+ by human neutrophils, rat thymocytes and Ehrlich ascites-tumour cells.

Interrelationship between growth factor-induced activation of the Na+/H(+)-antiporter and mobilization of intracellular Ca2+ in NIH3T3-fibroblasts

Addition of serum growth factors or bombesin to quiescent NIH3T3-fibroblasts leads to a simultaneous mobilization of intracellular Ca2+ and an increase in cytosolic pH which is inhibitable by dimethylamiloride. The mobilization of intracellular Ca2+ is a pH-dependent process with an optimum at pH 7.1. In quiescent cells with a pHi greater than or equal to 6.8, inhibition of the Na+/H(+)-antiporter by dimethylamiloride or reduction of extracellular Na+ attenuates the growth factor induced Ca2(+)-response. It is concluded that the growth factor induced activation of the Na+/H(+)-antiporter facilitates the mobilization of Ca2+ by shifting the internal pH towards the optimum for the Ca2(+)-release.

Primary stimuli of icosanoid release inhibit arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. Mechanism of action of hydrogen peroxide and methyl mercury in platelets

Icosanoid formation in platelets depends on the concentration of free arachidonate that is mainly liberated from membrane phospholipids by phospholipase A2. The concentration of free arachidonate is also controlled by the activities of the reacylating enzymes arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. In human platelet microsomes we determined the high enzyme activities of 5.9 nmol.min-1.(10(9) platelets)-1 for the arachidonoyl-CoA synthetase and 37 nmol.min-1.(10(9) platelets)-1 for the lysophospholipid acyltransferase. The activities of these reacylating enzymes were strongly reduced by hydrogen peroxide (H2O2) and methyl mercury that are primary stimuli of arachidonate release in intact platelets. H2O2 inhibited the arachidonoyl-CoA synthetase with an IC50 of 3.3 mmol/l without affecting the lysophospholipid acyltransferase. Sulfhydryl group protection by 3-mercapto-1,2-propanediol did not overcome the inhibition but glutathione prevented the inhibition of the arachidonoyl-CoA synthetase by H2O2. This suggests that glutathione by virtue of the glutathione peroxidase reduces H2O2 rather than that it protects free sulfhydryl groups of the arachidonoyl-CoA synthetase. Methyl mercury left the arachidonoyl-CoA synthetase activity unaffected but inhibited the lysophospholipid acyltransferase activity with an IC50 of 3.4 mumol/l. The inhibition is probably evoked by the blockade of sulfhydryl groups of the lysophospholipid acyltransferase because it disappeared when 3-mercapto-1,2-propanediol was added at a concentration higher than that of methyl mercury. Thrombin as a physiological full agonist, Ca2+ less than or equal to 1 mmol/l, the calcium ionophore A23187 and phorbol 12-myristate 13-acetate (TPA) and 1-oleoyl-2-acetylglycerol as model stimuli of protein kinase C neither influenced arachidonoyl-CoA synthetase nor lysophospholipid acyltransferase. It is concluded that the inhibitory effect of H2O2 and methyl mercury on the arachidonate-reacylating enzymes arachidonoyl-CoA synthetase or lysophospholipid acyltransferase, respectively, are responsible for their capacity to stimulate icosanoid release in intact cells. Thrombin and its intracellular messengers Ca2+ and diacylglycerol do not directly affect arachidonoyl-CoA synthetase and lysophospholipid acyltransferase.

Mechanism and biological significance of the Ha-ras-induced activation of the Na+/H(+)-antiporter

Expression of the transforming Ha-ras oncogene in MMTV-LTR transfected NIH 3T3 cells leads to a growth factor independent activation of the Na+/H(+)-antiporter. The activation of the antiporter is insensitive to the protein kinase inhibitor staurosporine and equally expressed in protein kinase C-depleted cells. It is concluded that the Ha-ras induced activation of the antiporter occurs by a protein kinase C-independent mechanism. An inhibition of the Na+/H(+)-antiporter by dimethylamiloride or a reduction of the extracellular [Na+] concentration results in a depression of the bombesin induced release of Ca2+ from intracellular stores. These results are explained by a steep pH-dependence of the Ca2(+)-mobilizing system which exhibits a maximum at pH 7.1 in the system studied here. Stimulation by growth factors of quiescent cells with a resting pH below 7 results in a shift of the cytosolic pH towards the optimum for the Ca2+ release. In agreement with the proposed interrelationship, pHi and [Ca2+]i rise and peak simultaneously after addition of bombesin to G0 arrested cells.

Na+/H+ exchange and Ca2+ influx

Cell stimulation raised the cytosolic free Ca2+ concentration, [Ca(2+]i and induces activation of Na+/H+ exchange which raises the cytosolic pH, pHi. Recent studies have addressed the question whether Na+/H+ exchange plays a role in Ca(2+) influx and, specifically, whether a rise in pHi alone suffices to open Ca(2+) channels in the plasma membrane. Artificial cytosolic alkalinization can induce Ca(2+) uptake across the plasma membrane of endothelial cells, lymphocytes and smooth muscle cells. Furthermore, inhibition of Na+/H+ exchange reduces agonist-induced Ca(2+) influx in endothelial cells and platelets which supports the concept that pHi may regulate the opening of Ca(2+) channels in the plasma membrane. Although these findings argue in favour of a role of Na+/H+ exchange in Ca(2+) influx, the onset of pHi and Ca(2+) rises, measured with fluorescent indicators, suggests that the increase in [Ca(2+)]i distinctly precedes the increase in pHi. This challenges the concept that alkalinization per se is a sufficient signal for the opening of Ca(2+) channels in the plasma membrane.

Involvement of calcium and protein kinase C in the activation of the Na+/H+ exchanger in cultured bovine aortic endothelial cells stimulated by extracellular ATP

We have studied the activation of the Na+/H+ exchanger which leads to the intracellular alkalinization in cultured bovine aortic endothelial cells stimulated by extracellular ATP. The alkalinization induced by ATP was largely dependent on extracellular Ca2+ and the rate of alkalinization was decreased by about 60% in the absence of extracellular Ca2+. ATP caused a rapid and transient increase and a subsequent sustained increase of the intracellular Ca2+ concentration ([Ca2+]i) in the Ca2+ buffer, while only the rapid and transient increase of [Ca2+]i was observed in the absence of extracellular Ca2+. The Ca2+-depleted cells prepared by incubation in Ca2+-free buffer containing 0.1 mM EGTA showed only a slight increase of [Ca2+]i with no alkalinization on stimulation by ATP. The alkalinization was inhibited by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), an inhibitor of protein kinase C, but not by another isoquinoline analogue (HA 1004), which has a less inhibitory effect on the kinase. Phorbol 12-myristate 13-acetate also induced the alkalinization by the activation of the Na+/H+ exchanger. Neither dibutyryl cyclic AMP nor dibutyryl cyclic GMP affected the alkalinization induced by ATP. Treatment of the cells by pertussis and cholera toxins had no effect on the alkalinization. The results suggest that the increase in [Ca2+]i is essential for the ATP-induced activation of the Na+/H+ exchanger in cultured bovine aortic endothelial cells and a protein kinase C-dependent pathway is involved in the activation.

Intracellular Ca2+ potentiates Na+/H+ exchange and cell differentiation induced by phorbol ester in U937 cells

The human cell line U937 differentiates to monocyte macrophage-like cells in response to tumour-promoting phorbol esters. This effect is attributed to activation of protein kinase C. We show here that U937 cell differentiation induced by 12-O-tetradecanoylphorbol 13-acetate (TPA) is associated with cytoplasmic alkalinization. Ethyl-isopropyl-amiloride (EIPA), a potent inhibitor of Na+/H+ exchange, blocked both cytoplasmic alkalinization and cell differentiation. Cell acidification by addition of 2-4 mM sodium propionate also blocked TPA-induced U937 cell differentiation. These results suggest that a sustained cell alkalinization mediated by activation of Na+/H+ exchange is essential for TPA-induced differentiation in U937 cells. The increase of cytoplasmic free calcium concentration ([Ca2+]i) by addition of the calcium ionophore ionomycin enhanced TPA-induced alkalinization by increasing the apparent affinity of the Na+/H+ antiporter for intracellular H+. Treatment with ionomycin also potentiated differentiation of U937 cells induced by TPA. This synergism suggests that [Ca2+]i either potentiates the activation of protein kinase C or triggers additional transducing mechanisms. The key events of this interaction occur during the first 30 min of treatment, even though cell differentiation manifests much later.

Monitoring of the activation of receptor-operated calcium channels in human platelets

Activation of receptor-operated calcium channels has been monitored by measurements of the quenching of the fluorescence of intracellularly trapped fura-2 by Mn entering from the extracellular medium. Release of calcium from intracellular stores was followed simultaneously by measurements of the ratio of the fluorescence excited at 340 and 380 nm. Thrombin, ADP, platelet-activating-factor (PAF) and collagen, all produced both release of calcium from the intracellular stores and uptake of Mn from the extracellular medium. The uptake of Mn, but not the increase of (Ca2+)i, was blocked by nickel. These results suggest the existence of plasma membrane calcium channels which can be activated by the different agonists tested here. The activation of calcium channels was very fast and transient with ADP and PAF, fast and maintained with thrombin, and delayed with collagen.

Cytosolic alkalinization alone is not sufficient for Ca2+ mobilization, phosphatidic acid formation, and protein phosphorylation in human platelets

One of the earliest events following stimulation of human platelets with thrombin is a rise in the cytosolic pH, pHi, mediated by Na+/H+ exchange, and an increase in the cytosolic free Ca2+ concentration, [Ca2+]i. In the present study we investigated whether an increase in pHi alone, induced by the Na+/H+ ionophore monensin, is sufficient for platelet activation. Although monensin (20 microM) raised pHi from 7.10 +/- 0.05 (n = 21) to 7.72 +/- 0.17 (n = 13), neither Ca2+ influx nor mobilization were detectable upon this treatment in fura2-loaded platelets. In contrast, thrombin (0.05 U/ml) raised pHi to 7.31 +/- 0.10 (n = 10) and increased [Ca2+]i by more than 250 nM both in the presence and absence of extracellular Ca2+. Thrombin also caused the formation of phosphatidic acid and phosphorylation of the 20 kDa and 47 kDa proteins in platelets labeled with 32P. Monensin, however, induced none of these responses. It is concluded that an increase in pHi alone is not a sufficient trigger for platelet activation but enhances intracellular signal transduction in platelets stimulated by natural agonists.

Effects of sodium removal on calcium mobilization and dense granule secretion induced by thrombin in human platelets

Removal of extracellular sodium decreased calcium mobilization from intracellular stores induced by thrombin in aspirin-treated human platelets. ATP and serotonin secretion were also significantly reduced. Secretion was positively correlated with calcium mobilization, but the presence or absence of sodium did not modify the slope of the regression line. Half-maximal secretion was reached when [Ca2+]i was increased by about 0.1 microM. Calcium mobilization induced by the divalent cation ionophore ionomycin was not modified by sodium removal. Secretion induced by ionomycin was much smaller than the thrombin-induced one for the same increases of [Ca2+]i. These results suggest that the presence of external sodium is required for normal thrombin-induced calcium release from the intracellular stores and hence for dense granule secretion. However, secretion cannot be only attributed to the increase of cell [Ca2+]i but also to other process(es) which are not affected by external sodium.

Effect of intracellular Na+ on platelet aggregation and Ca2+ mobilization

The effects of ionophores, which can carry alkali metal cations, on platelet aggregation were examined. At an alkaline extracellular pH, alkali metal cation/H+ exchanger nigericin accelerated aggregation in K+-enriched medium, whereas it rather inhibited aggregation in Na+-enriched medium, even though the intracellular pH was only slightly alkaline. The inhibitory effect of Na+ on platelet aggregation was more clearly shown with the alkali metal cation exchanger gramicidin D. The ionophore had no effect or a slightly accelerative effect on aggregation in K+-enriched medium, whereas it significantly inhibited aggregation induced by thrombin, ADP and platelet activating factor in Na+-enriched medium. Fluorescence studies on fura-2-labeled platelets revealed that in Na+-enriched medium gramicidin D inhibited agonist-induced Ca2+ mobilization both in the presence and absence of extracellular Ca2+. These results suggest that the intracellular Na+ inhibits platelet aggregation by inhibiting Ca2+ mobilization.

Activation of Na+/H+ exchange and Ca2+ mobilization start simultaneously in thrombin-stimulated platelets. Evidence that platelet shape change disturbs early rises of BCECF fluorescence which causes an underestimation of actual cytosolic alkalinization

Although an increase in cytosolic pH (pHi) caused by Na+/H+ exchange enhances Ca2+ mobilization in platelets stimulated by low concentrations of thrombin [Siffert & Akkerman (1987) Nature (London) 325, 456-458], studies using fluorescent indicators for pHi (BCECF) and [Ca2+]i (fura2) suggest that Ca2+ is mobilized while the cytosolic pH decreases. Several lines of evidence indicate that the initial fall in BCECF fluorescence is not due to cytosolic acidification but is caused by a platelet shape change. (1) Pulse stimulation of platelets by successive addition of hirudin (4 unit/ml) and thrombin (0.2 unit/ml) induced a shape change of 43 +/- 8% and a fall in BCECF fluorescence, which both remained unchanged when Na+/H+ exchange was inhibited by ethylisopropylamiloride (EIPA, 100 microM). (2) Increasing the thrombin concentration to 0.4 unit/ml doubled the shape change and the fall in BCECF fluorescence, but again EIPA had no effect on these responses. (3) Treating platelets with 2 microM-ADP induced shape change and a decline in BCECF fluorescence that was unaffected by EIPA. (4) A second addition of thrombin to platelets that had already undergone shape change induced an immediate increase in BCECF fluorescence without a prior decrease. (5) Activation of protein kinase C by 1,2-dioctanoyl-sn-glycerol (DiC8) neither induced shape change nor a decline in BCECF fluorescence; in contrast BCECF fluorescence rapidly increased indicating an immediate cytosolic alkalinization. Concurrent analysis of [Ca2+]i under conditions in which shape change did not interfere with BCECF fluorescence showed that cytosolic alkalinization and Ca2+ mobilization started almost simultaneously. These observations suggest that cytosolic alkalinization is not preceded by a fall in pHi and can support Ca2+ mobilization induced by weak agonists.

Leupeptin does not affect the normal signal transduction mechanism in platelets

Calpains are Ca2+-dependent serine proteases that can regulate protein kinase C-mediated cellular events by cleaving the membrane-bound native enzyme to yield an activated cytosolic fragment. Inhibition of calpain by leupeptin may cause enhancement or inhibition of cellular functions depending on the nature of the protein kinase C reaction involved. We have studied the effects of leupeptin on platelet responses (aggregation, secretion, thromboxane B2 formation and intracellular Ca2+ and pH changes) induced by either thrombin, collagen or phorbol 12-myristate 13-acetate (TPA), which are known to activate protein kinase C by different mechanisms. Only thrombin-induced responses were inhibited by leupeptin. This suggests that the inhibitory effect of leupeptin is not due to antagonism of calpain in this system, but to direct interference with the proteolytic effect of thrombin.

Thrombin-induced platelet aggregation is affected by external Na+ independently of the Na+/H+ exchange

Thrombin affects blood platelets by activation of Na+/H+ exchange and induction of aggregation, but the relationship between these effects is under debate. The present study attempts to clarify whether the activation of the exchanger activity is required for platelet aggregation. In apparent support of such a requirement, thrombin-induced aggregation is higher in Na+ medium than in N-methylglucamine+ medium and is inhibited by sphingosine, an inhibitor of protein kinase C known to regulate the Na+/H+ exchanger. However, the inhibition of aggregation by sphingosine occurs in both Na+-containing and Na+-free media, the aggregation is identical in Na+ and K+-containing media, and is not inhibited by 5-N-(3-aminophenyl)amiloride, at a concentration 10-fold higher than its Ki for platelet Na+/H+ exchange. Furthermore, at low concentration (0.005 U/ml) thrombin induces aggregation but does not activate the exchange. It is concluded that the activation of Na+/H+ exchange is not required for thrombin-induced platelet aggregation and that the apparent augmentation of aggregation by Na+ is due to an inhibitory effect of N-methylglucamine+.

Modulation of protein kinase C and diverse cell functions by sphingosine--a pharmacologically interesting compound linking sphingolipids and signal transduction

Sphingosine, the backbone moiety of sphingomyelin, gangliosides and other complex sphingolipids, is a potent inhibitor of protein kinase C in vitro and of cellular events dependent on this enzyme. The systems that have been found, thus far, to be affected by sphingosine encompass various components of host defense system, including the activation of platelets, neutrophils and natural killer cells; the cytolytic activity of pathogens and expression of viral genes; cell growth and differentiation in several cell types, including leukemic and neuronal cells; insulin stimulated hexose transport and metabolism in adipocytes; ion-transport systems in various models; the response of neuronal cells to excitatory compounds; and receptor desensitization. While sphingosine has appeared to be a relatively potent and specific inhibitor of protein kinase C in the systems studied, recent findings with the epidermal growth factor receptor indicate that it may serve as a pleotrophic modulator of cell functions. New strategies for the design of pharmacologically active agents should arise from further studies of the action of long-chain (sphingoid) bases. Furthermore, since free sphingosine is a natural constituent of cells and the levels can be modulated by phorbol esters and other factors, a cycle of complex sphingolipid hydrolysis and resynthesis to regulate the amount of free sphingosine may constitute one mechanism of action of these compounds.