Cyclic nucleotides attenuate thrombin-evoked alterations in parameters of platelet Na/H antiport. The role of cytosolic Ca

Inhibitory mechanisms of activated matrix metalloproteinase-9 on platelet activation

The intracellular mechanisms underlying the signaling pathways of activated matrix metalloproteinase-9 (MMP-9) in platelets are not yet completely understood. Therefore, the aim of this study was to further examine the effects of activated MMP-9 in preventing platelet aggregation. In this study, activated MMP-9 time-dependently (3-60 min) inhibited platelet aggregation in washed human platelet suspensions stimulated by agonists. However, activated MMP-9 had no significant effect on the binding of FITC-triflavin to the platelet glycoprotein IIb/IIIa complex. Triflavin is a specific antagonist of the glycoprotein IIb/IIIa complex purified from snake venom. Moreover, activated MMP-9 (21 and 90 ng/ml) markedly decreased the fluorescence intensity of platelet membranes tagged with diphenylhexatriene. The thrombin-evoked increase in pHi was inhibited in the presence of activated MMP-9 (21 and 90 ng/ml). In addition, activated MMP-9 (21 and 90 ng/ml) markedly reduced the electron spin resonance (ESR) signal intensity of hydroxyl radicals in collagen (1 mug/ml)-activated platelets. These results indicate that the antiplatelet activity of activated MMP-9 may involve the following pathways: (1) activated MMP-9 may initially induce conformational changes in platelet membranes and hydroxyl radical formation, leading to inhibition of platelet aggregation; and (2) activated MMP-9 also inhibits the Na(+)/H(+) exchanger, leading to reduced intracellular Ca(2+) mobilization, and ultimately to inhibition of platelet aggregation. This study further provides new insights concerning the effects of activated MMP-9 on platelet aggregation.

Inhibitory mechanisms of low concentrations of oxidized low-density lipoprotein on platelet aggregation

The intracellular mechanisms underlying oxidized low-density lipoprotein (oxLDL)-signaling pathways in platelets are not yet completely understood. Therefore, the aim of this study was to further examine the effects of oxLDL in prevention of platelet aggregation. In this study, oxLDL concentration-dependently (40-120 microg/ml) inhibited platelet aggregation in human platelet-rich plasma stimulated by agonists. Moreover, oxLDL (40 and 80 microg/ml) markedly decreased the fluorescence intensity of platelet membranes tagged with diphenylhexatriene. Rapid phosphorylation of a protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by PDBu (150 nM). This phosphorylation was markedly inhibited by oxLDL (40 and 80 microg/ml) in phosphorus-32-labeled platelets. In addition, oxLDL (40 and 80 microg/ml) markedly increased levels of cyclic AMP and cyclic AMP-induced vasodilator-stimulated phosphoprotein (VASP) Ser(157) phosphorylation. The thrombin-evoked increase in pHi was inhibited in the presence of oxLDL (40 and 80 microg/ml). These results indicate that the antiplatelet activity of oxLDL may involve the following pathways. (1) oxLDL may initially induce conformational changes in platelet membranes, leading to inhibition of the activation of protein kinase C, followed by inhibition of P47 protein phosphorylation, and intracellular Ca(2+) mobilization. (2) oxLDL also activated formation of cyclic AMP and cyclic AMP-induced VASP Ser(157) phosphorylation, resulting in inhibition of the Na(+)/H(+)exchanger; this leads to reduced intracellular Ca(2+) mobilization, and ultimately to inhibition of platelet aggregation. This study further provides new insights concerning the effects of low concentrations of oxLDL on platelet aggregation.

Involvement of the antiplatelet activity of magnesium sulfate in suppression of protein kinase C and the Na+/H+ exchanger

Magnesium sulfate is widely used to prevent seizures in pregnant women with hypertension. The aim of this study was to examine the inhibitory mechanisms of magnesium sulfate in platelet aggregation in vitro. In this study, magnesium sulfate concentration-dependently (0.6-3.0 mM) inhibited platelet aggregation in human platelets stimulated by agonists. Magnesium sulfate (1.5 and 3.0 mM) also concentration-dependently inhibited phosphoinositide breakdown and intracellular Ca+2 mobilization in human platelets stimulated by thrombin. Rapid phosphorylation of a platelet protein of M(r) 47,000 (P47), a marker of protein kinase C activation, was triggered by phorbol-12-13-dibutyrate (PDBu, 50 nM). This phosphorylation was markedly inhibited by magnesium sulfate (3.0 mM). Magnesium sulfate (1.5 and 3.0 mM) further inhibited PDBu-stimulated platelet aggregation in human platelets. The thrombin-evoked increase in pHi was markedly inhibited in the presence of magnesium sulfate (3.0 mM). In conclusion, these results indicate that the antiplatelet activity of magnesium sulfate may be involved in the following two pathways: (1) Magnesium sulfate may inhibit the activation of protein kinase C, followed by inhibition of phosphoinositide breakdown and intracellular Ca+2 mobilization, thereby leading to inhibition of the phosphorylation of P47. (2) On the other hand, magnesium sulfate inhibits the Na+/H+ exchanger, leading to reduced intracellular Ca+2 mobilization, and ultimately to inhibition of platelet aggregation and the ATP-release reaction.

Inhibitory mechanisms of kinetin, a plant growth-promoting hormone, in platelet aggregation

Kinetin has been shown to have anti-aging effects on several different systems including plants and human cells. The aim of this study was to examine the detailed inhibitory mechanisms of kinetin in platelet aggregation. In this study, kinetin concentration-dependently (50-150 microM) inhibited platelet aggregation in human platelets stimulated by agonists. Kinetin (70 and 150 microM) also concentration-dependently inhibited intracellular Ca2+ mobilization and phosphoinositide breakdown in platelets stimulated by collagen (1 microg/ml). Kinetin (70 and 150 microM) significantly inhibited thromboxane A2 formation stimulated by collagen (1 microg/ml) and arachidonic acid (60 microM) in human platelets. In addition, kinetin (70 and 150 microM) significantly increased the formation of cyclic AMP. Intracellular pH values were measured spectrofluorometrically using the fluorescent probe BCECF-AM in platelets. The thrombin-evoked increase in pHi was markedly inhibited in the presence of kinetin (70 and 150 microM). Rapid phosphorylation of a platelet protein of molecular weight (Mr) 47000 (P47), a marker of protein kinase C activation, was triggered by collagen (1 microg/ml). This phosphorylation was inhibited by kinetin (70 and 150 microM). In conclusion, these results indicate that the anti-platelet activity of kinetin may be involved in the following pathways: kinetin's effects may initially be due to inhibition of the activation of phospholipase C and the Na+/H+ exchanger. This leads to lower intracellular Ca2+ mobilization, followed by inhibition of TxA2 formation and then increased cyclic AMP formation, followed by a further inhibition of the Na+/H+ exchanger, ultimately resulting in markedly decreased intracellular Ca2+ mobilization and phosphorylation of P47. These results suggest that kinetin has an effective anti-platelet effect and that it may be a potential therapeutic agent for arterial thrombosis.

Morphine-potentiated platelet aggregation in in vitro and platelet plug formation in in vivo experiments

The detailed mechanisms underlying morphine-signaling pathways in platelets remain obscure. Therefore, we systematically examined the influence of morphine on washed human platelets. In this study, washed human platelet suspensions were used for in vitro studies. Furthermore, platelet thrombus formation induced by irradiation of mesenteric venules with filtered light in mice pretreated with fluorescein sodium was used for an in vivo thrombotic study. Morphine concentration dependently (0.6, 1, and 5 microM) potentiated platelet aggregation and the ATP release reaction stimulated by agonists (i.e., collagen and U46619) in washed human platelets. Yohimbine (0.1 microM), a specific alpha(2)-adrenoceptor antagonist, markedly abolished the potentiation of morphine in platelet aggregation stimulated by agonists. Morphine also potentiated phosphoinositide breakdown and intracellular Ca(2+) mobilization in human platelets stimulated by collagen (1 microg/ml). Moreover, morphine (0.6-5 microM) markedly inhibited prostaglandin E(1) (10 microM)-induced cyclic AMP formation in human platelets, while yohimbine (0.1 microM) significantly reversed the inhibition of cyclic AMP by morphine (0.6 and 1 microM) in this study. The thrombin-evoked increase in pH(i) was markedly potentiated in the presence of morphine (1 and 5 microM). Morphine (2 and 5 mg/g) significantly shortened the time require to induce platelet plug formation in mesenteric venules. We concluded that morphine may exert its potentiation in platelet aggregation by binding to alpha(2)-adrenoceptors in human platelets, with a resulting inhibition of adenylate cyclase, thereby reducing intracellular cyclic AMP formation followed by increased activation of phospholipase C and the Na(+)/H(+) exchanger. This leads to increased intracellular Ca(2+) mobilization, and finally potentiation of platelet aggregation and of the ATP release reaction.

Negative regulation of the platelet Na+/H+ exchanger by trimeric G-proteins

Human platelets contain a Na+/H+ exchanger (NHE) that regulates the cytosolic pH. The role of trimeric G-proteins in NHE control was investigated in plasma membrane vesicles by measuring exchange of intravesicular protons for extravesicular Na+. Exchange was saturable, independent of membrane potential and inhibited by ethylisopropyl amiloride (Ki 0.05 micromol.L-1), demonstrating the involvement of NHE-1. The G-protein activators AlF4- and GMP-P(NH)P reduced exchange by increasing the Km for Na+ from 11.3 +/- 2.1 mM to 21.6 +/- 1.4 mM (AlF4-) and 19.8 +/- 1.1 mM (GMP-P(NH)P), leaving Vmax and the Hill coefficient unchanged. This effect was abolished by inhibitors of Gi-proteins (N-ethylmaleimide, holoenzyme- and A-protomer of pertussis toxin) and by an anti-Galpha Ig and GDP(beta)S. Activation of Gi-proteins by mastoparan and its synthetic analogue Mas7 also strongly reduced NHE activity. These data show that in platelets NHE-1 is under negative control of the Gi-family of trimeric G-proteins.

Ca2+ in the dense tubules: a model of platelet Ca2+ load

In this work, we explored the relationship between the freely exchangeable Ca2+ (FECa2+) in the dense tubules (DT) and the sarco(endo)plasmic reticulum (SER) Ca2+-ATPase (SERCA) in circulating human platelets and examined the relationship between blood pressure (BP) and these platelet parameters. Studying platelets from 32 healthy men, we showed that the maximal reaction velocity (Vmax) of the SERCA significantly correlated with FECa2+ in the DT and with the protein expressions of SERCA 2 and 3. BP positively correlated with both the Vmax of the SERCA (r=.462, P=.010) and the FECa2+ sequestered in the DT (r=.492, P=.005). The relationships between these platelet Ca2+ parameters and BP were in part confounded by increased levels of serum triglycerides and diminished HDL cholesterol with a higher BP. No correlation was observed between the resting cytosolic Ca2+ and BP. Collectively, these findings indicate that (1) an increase in the cellular Ca2+ load in platelets is expressed by a higher activity of the SERCA and an increase in the expressions of SERCA 2 and 3 proteins, coupled with an increase in the FECa2+ in the DT, and (2) a higher BP is associated with an increase in platelet Ca2+ load in human beings, expressed by a rise in the FECa2+ in the DT and the upregulation of SERCA activity.

Role of cyclic nucleotides in store-mediated external Ca2+ entry in human platelets

This study explores the role of cyclic nucleotides (i.e. cyclic AMP and cyclic GMP) in store-regulated external Ca2+ entry in human platelets. To stimulate store-regulated external Ca2+ entry, thapsigargin was used to deplete Ca2+ from the dense tubules, and sodium nitroprusside and iloprost respectively were used to stimulate endogenous cyclic GMP and cyclic AMP formation. Pretreatment with sodium nitroprusside and iloprost (a) attenuated the thapsigargin-evoked external Ca2+ entry and (b) reduced the rate of Ca2+ release from the dense tubules. The effects on external Ca2+ entry and Ca2+ release from the dense tubules were exerted independently and were apparently mediated through activation of the respective cyclic nucleotide-dependent protein kinases. Both sodium nitroprusside and iloprost reduced tyrosine kinase phosphorylation of a number of proteins, particularly a 72 kDa protein band. Both agents also attenuated the thapsigargin-evoked tyrosine kinase phosphorylation of the 72 kDa band. Intracellular Ca2+ depletion resulted in a reduction in tyrosine kinase-mediated phosphorylation of a number of protein bands, including the 72 kDa band and the further attenuation of thapsigargin-mediated tyrosine phosphorylation of this band. The effects of the cyclic nucleotides on cellular Ca2+ homoeostasis in thapsigargin-treated platelets were not exerted via acceleration of Ca2+ extrusion or Ca2+ sequestration into the mitochondria. We conclude that cyclic nucleotides participate in store-regulated control of external Ca2+ entry by slowing down the rate of external Ca2+ entry and Ca2+ release from intracellular Ca2+ stores. These effects are apparently mediated via cyclic nucleotide-dependent protein kinases and the attenuation of protein phosphorylation by tyrosine kinases.

Duality of plasmin effect on cytosolic free calcium in human platelets

Plasmin caused a modest and gradual increase in platelet cytosolic Ca2+, mediated through both Ca2+ mobilization and external Ca2+ entry. This response was associated with accelerated Ca2+ extrusion and protein tyrosine phosphorylation. Plasmin-enhanced external Ca2+ entry and Ca2+ extrusion (but not Ca2+ mobilization) were attenuated by the tyrosine kinase inhibitor, genistein. Plasmin inhibited the thrombin-evoked increase in cytosolic Ca2+ and also inhibited the Ca2+ response to the tethered peptide TRAP-6 of the thrombin receptor. Furthermore, plasmin inhibited the binding of 125I-labeled alpha-thrombin to platelets. The inhibitory effect of plasmin on the thrombin response shared some characteristics with the effect of protein kinase C stimulators but was not reversed by protein kinase C inhibitors. Plasmin did not change platelet cyclic nucleotides. These results suggest a dual effect of plasmin. Plasmin produces a small rise in platelet cytosolic Ca2+ and a tyrosine kinase-dependent enhancement of Ca2+ turnover (external Ca2+ influx and Ca2+ efflux). However, it also attenuates the thrombin-evoked cytosolic Ca2+ response by blocking Ca2+ mobilization and slowing the rate of external Ca2+ influx. The latter feature would result in a plasmin-induced inhibition of thrombogenesis.

Increased calcium stores in platelets from African Americans

Differences in cation transport have been observed between African Americans and whites. These differences may underlie the increased predisposition of African Americans to essential hypertension. To further explore these racial differences, we used platelets as a cellular model for calcium regulation. We measured 45Ca fluxes in platelets from 21 African American and 25 white men. Additionally, using fura 2, we measured cytosolic free calcium levels in resting platelets and platelets treated with ouabain and thrombin. Platelet 45Ca uptake was described by two exchangeable pools: a small, rapidly exchangeable pool and a larger, slowly exchangeable pool. Both pools were larger in platelets from African Americans than from whites (263 versus 185 pmol per 1 x 10(8) platelets for the rapidly exchangeable pool, P < .05; 744 versus 532 pmol per 1 x 10(8) platelets for the slowly exchangeable pool, P < .01). 45Ca washout was described by a rapidly exchangeable pool and a static pool. The former was also higher in platelets from African Americans than from whites (246 versus 202 pmol per 1 x 10(8) platelets, P < .01). The cytosolic free calcium concentrations in resting platelets were lower in African Americans than in whites. After treatment with ouabain and thrombin, the sustained posttransient levels of cytosolic free calcium increased to a greater extent in platelets from African Americans (46.7 nmol/L) than from whites (34.5 nmol/L, P = .033). Platelets from African Americans demonstrate higher intracellular calcium stores than platelets from whites. This racial difference could explain the sensitivity of African Americans to vasoactive agents acting through calcium mobilization from intracellular stores and cytosolic calcium.

Inhibition of thrombin and SFLLR-peptide stimulation of platelet aggregation, phospholipase A2 and Na+/H+ exchange by a thrombin receptor antagonist

A thrombin receptor has been described that is activated by thrombin cleavage generating a new N-terminus. The newly exposed SFLLR-containing "tethered-ligand" then activates the receptor. In these studies, we used 3-mercapto-propionyl-Phe-Cha-Cha-Arg-Lys-Pro-Asn- Asp-Lys-amide (Mpapeptide) as a thrombin receptor antagonist. This compound was capable of preventing both thrombin- and SFLLR-peptide-induced platelet aggregation with little effect on collagen-induced platelet aggregation. It also prevented thrombin- and SFLLRNP-induced calcium mobilization with little effect on thromboxane receptor-activated platelet Ca2+ mobilization. Platelet membrane GTPase could be activated by peptides that activated the thrombin receptor, and the thrombin receptor antagonist also prevented receptor-stimulated GTPase activity. Platelet phospholipase A2 (PLA2) activity (measured as the release of radiolabeled arachidonic acid) and Na+/H+ exchange activation were stimulated by alpha-thrombin and by SFLLR-containing peptides. Activation of both processes with low concentrations of thrombin required thrombin's anion-binding exosite, as they were not activated by similar concentrations of gamma-thrombin, and the alpha- and zeta-thrombin activation was blocked by peptides mimicking the C-terminal region of hirudin. Stimulation of PLA2 and Na+/H+ exchange by both thrombin and SFLLR-containing peptides was inhibited by the thrombin receptor antagonist Mpa-peptide. These results support the hypothesis that thrombin stimulation of PLA2 activity and Na+/H+ exchange occurs via activation of the thrombin tethered-ligand receptor. Moreover, these data are consistent with the tethered-ligand receptor mediating most actions elicited by low concentrations of alpha-thrombin involved in human platelet activation.

Inhibition of Ca2+ entry by Ca2+ overloading of intracellular Ca2+ stores in human platelets

1. This study examined the effect of overloading human platelet intracellular Ca2+ stores on the rate of agonist-evoked external Ca2+ entry. To overload the Ca2+ stores (presumably dense tubules), external Na(+)-dependent Ca2+ efflux via Na(+)-Ca2+ exchange was inhibited by pretreating the cells with ouabain or Na(+)-free medium. Ca2+ regulation was then examined after exposure to thrombin, ADP and thapsigargin. Cytosolic free Ca2+ levels were monitored using the fluorescent probe fura-2 and external Ca2+ influx was assessed by the rates of extracellular Mn2+ or 45Ca2+ uptake. 2. Both ouabain and Na(+)-free pretreatments caused a slight increase in the resting cytosolic free Ca2+. 3. In 1 mM Ca(2+)-containing medium, Ca(2+)-overloaded platelets showed similar thrombin-evoked cytosolic free Ca2+ responses to those of control platelets. However, in Ca(2+)-free medium, they showed substantially greater thrombin-evoked cytosolic free Ca2+ responses than control platelets. Moreover, increased thrombin-evoked Ca2+ mobilization from Ca2+ storage sites was accompanied by a diminished rate of thrombin-evoked external Ca2+ entry. 4. Similar reductions in the rate of external Ca2+ entry were observed after treatment with ADP and thapsigargin. 5. Protein kinase C inhibitors (calphostin C and staurosporine) failed to reverse the effect of ouabain pretreatment on thrombin-induced changes in the cytosolic free Ca2+ response. 6. Inositol 1,4,5-trisphosphate profiles in ouabain-treated and non-treated platelets were not significantly different. 7. These data indicate that increased Ca2+ in the dense tubules is associated with diminished agonist-evoked external Ca2+ influx.

Role of cyclic GMP in atrial-natriuretic-peptide stimulation of erythrocyte Na+/H+ exchange

Human atrial natriuretic peptide (ANP) fragments ANP-(127-150) or ANP-III and ANP-(127-149) or ANP-II activate Na+/H+ exchange in human erythrocytes at concentrations as low as 1 pM. Both ANP-(127-147) or ANP-I and ANP-(129-150) or des-Ser5, Ser6-ANP-III have no effect on erythrocyte Na+/H+ exchange. ANP-III also produces a time-dependent increase of intraerythrocyte guanosine 3',5'-phosphate (cGMP) concentration. M&B 22,948, a specific inhibitor of cGMP phosphodiesterase, increases Na+/H+ exchange and the intracellular concentration of cGMP. Both 8-bromoguanosine 3',5'-phosphate (8-Br-cGMP) and dibutyryl-cGMP mimic the effect of ANP-III on erythrocyte Na+/H+ exchange. Our data suggest that human erythrocytes possess guanylate-cyclase activity stimulated by ANP-III and that activation of Na+/H+ exchange by this peptide is mediated by cGMP. Human erythrocytes display a high degree of sensitivity to ANP-III or ANP-II and a specificity for ANP-fragment structures just as cells with established ANP-specific receptors.

Thapsigargin-evoked changes in human platelet Ca2+, Na+, pH and membrane potential

1. In this work we explored the effect of thapsigarin on the intracellular Ca2+, pH, Na+ and membrane potential in human platelets. These parameters were monitored using the fluorescent probes fura-2, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein, sodium-binding benzofuran isophthalate, and 3,3'-dipropylthiadicarbocyanine iodide. 2. Thapsigargin caused an increase in the cytosolic Ca2+, coupled with cytosolic alkalinization. Thapsigargin-induced alkalinization was Na(+)-dependent, indicating that thapsigargin stimulated the Na(+)-H+ exchange. 3. Using Mn2+ as a Ca2+ surrogate, we showed that thapsigargin activated Ca2+ channels at relatively low levels of cytosolic Ca2+, suggesting that a rise in cytosolic free Ca2+ is not the signal for the activation of these channels. 4. Thapsigargin-induced increase in the cytosolic free Ca2+ was greater in Na(+)-containing medium than in Na(+)-free medium, suggesting that Na(+)-dependent mechanisms participate in the regulation of platelet cytosolic Ca2+. 5. Thapsigargin not only increased the cytosolic Ca2+, but also elevated the cytosolic free Na+. The latter effect was more pronounced in Ca(2+)-free medium, a finding that may indicate that some of the Na+ enters through Ca2+ entry pathways. 6. Finally, thapsigargin evoked sustained platelet hyperpolarization which was attenuated by charybdotoxin, indicating thapsigargin-induced stimulation of Ca(2+)-sensitive K+ channels. 7. Together these observations demonstrate a multifactorial effect of thapsigargin on platelets that can be utilized to further understand platelet ionic homeostasis.

Regulation of the cytosolic pH set point for activation of the Na+/H+ antiport in human platelets: the roles of the Na+/Ca2+ exchange, the Na(+)-K(+)-2Cl- cotransport and cellular volume

To explore further the mechanisms that regulate the Na+/H+ antiport in human platelets, we examined the effect of Na+ pump inhibition by ouabain and K+ removal from the extracellular medium on parameters of this transport system. Treatment with ouabain resulted in increased cytosolic free Ca2+ and Na+, coupled with an alkaline shift in the cytosolic pH set point for the Na+/H+ antiport. Inhibition of the Na+ pump by the removal of K+ from the medium increased the cytosolic Na+ but not the cytosolic Ca2+; yet this treatment also produced a substantial alkaline shift in the cytosolic pH set point for the Na+/H+ antiport. This effect appeared to relate to a decline in cellular volume and it was attenuated by the Na(+)-K(+)-2 Cl- cotransport inhibitor, bumetanide. These findings indicate: (a) a link between the Na+ pump and the Na+/H+ antiport, mediated by the Na+/Ca2+ exchange and the cytosolic free Ca2+, and (b) a link between the Na+/H+ antiport and the Na(+)-K(+)-2Cl- cotransport through cellular volume.

Platelet sodium-hydrogen antiport in obese and diabetic black women

In this investigation we correlated platelet Na-H antiport parameters with blood pressure and serum lipids in a sample population of non-insulin-dependent diabetic obese, nondiabetic obese, and nondiabetic nonobese black women. Parameters of the Na-H antiport were examined in aspirin-treated platelets. These parameters were not altered in resting or in thrombin-stimulated platelets of diabetic patients. The activity index of platelet Na-H antiport after thrombin stimulation was positively correlated with the blood pressure (systolic blood pressure, r = 0.5320 and p = 0.0001; diastolic blood pressure, r = 0.5123 and p = 0.0017). Lower high density lipoprotein cholesterol levels were associated with an alkaline shift in the cytosolic pH set point for activation of the Na-H antiport. Highly significant correlations were also observed between the total cholesterol/high density lipoprotein cholesterol ratio and the cytosolic pH set point for activation of the Na-H antiport. These correlations were independent of diabetes or the body mass index. Together, these observations indicate that parameters of platelet Na-H antiport are altered with an increase in blood pressure and a decrease in serum high density lipoprotein cholesterol.