Ca2+ mobilization can occur independent of acceleration of Na+/H+ exchange in thrombin-stimulated human platelets

Intracellular free Ca2+ [( Ca2+]i) and pH (pHi) were measured simultaneously by dual wavelength excitation in thrombin-stimulated human platelets double-labeled with the fluorescent probes fura2 and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein to determine the relationship between changes in [Ca2+]i and pHi, respectively. At 37 degrees C, thrombin (0.5 or 0.1 units/ml) increased [Ca2+]i with no detectable lag period to maximum levels within 13 s followed by a slow return to resting levels. There was a transient decrease in pHi within 9 s that was immediately followed by an alkalinization response, attributable to activation of Na+/H+ exchange, that raised pHi above resting levels within 22 s. At 10-15 degrees C, thrombin-induced changes in [Ca2+]i and pHi were delayed and therefore better resolved, although no differences in the magnitude of changes in [Ca2+]i and pHi were observed. However, the increase in [Ca2+]i had peaked or was declining before the alkalinization response was detected, suggesting that Ca2+ mobilization occurs before activation of Na+/H+ exchange. In platelets preincubated with 5-(N-ethyl-N-isopropyl)amiloride or gel-filtered in Na+-free buffer (Na+ replaced with N-methyl-D-glutamine) to inhibit Na+/H+ exchange, thrombin stimulation caused a rapid, sustained decrease in pHi. Under these conditions there was complete inhibition of the alkalinization response, whereas Ca2+ mobilization was only partially inhibited. Nigericin (a K+/H+ ionophore) caused a rapid acidification of more than 0.3 pH unit that was sustained in the presence of 5-(N-ethyl-N-isopropyl)amiloride. Subsequent stimulation with thrombin resulted in slight inhibition of Ca2+ mobilization. These data show that, in human platelets stimulated with high or low concentrations of thrombin, Ca2+ mobilization can occur without a functional Na+/H+ exchanger and in an acidified cytoplasm. We conclude that Ca2+ mobilization does not require activation of Na+/H+ exchange or preliminary cytoplasmic alkalinization.     

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

We have studied the effects of thrombin (0.1 U/ml) on intracellular Ca2+ ([Ca2+]i) and pH (pHi) in human platelets loaded with fluorescent indicators. Thrombin produced a transient decrease of pHi which reached its maximum within 15-25 seconds (s) and was followed by a sustained alkalinization which brought pHi above the resting value. [Ca2+]i increased transiently peaking at 5-10 s. The late alkalinization induced by thrombin was antagonized by ethylisopropylamiloride, an inhibitor of Na+-H+ exchange, and by sphingosine, an inhibitor of protein kinase C, with little effect on the [Ca2+]i transient. The early acidification was not inhibited by these treatments. We conclude tha the thrombin-induced changes of [Ca2+]i and pHi are mediated by different mechanisms. The late alkalinization is due to activation of Na+/H+ exchange mediated by protein kinase C and, contrarily to previous proposals (Siffert, W. and Akkerman, J.W.N. (1987) Nature 325, 456-458), it is not necessary for calcium mobilization from intracellular stores.     

Regulation of intracellular pH in human platelets. Effects of thrombin, A23187, and ionomycin and evidence for activation of Na+/H+ exchange and its inhibition by amiloride analogs

Intracellular pH (pHi) of human platelets was measured with the fluorescent dye 2',7'-bis(carboxyethyl)5,6-carboxyfluorescein under various conditions. Stimulation by thrombin at 23 degrees C caused a biphasic change in pHi (initial pHi 7.09); a rapid fall of 0.01-0.04 units (correlated with the rise of [Ca2+]i measured with quin2) followed after 10-15 s by a sustained rise of 0.1-0.15 units pHi. The fall of pHi and [Ca2+]i mobilization was reduced by early (5 s) addition of hirudin, but the later elevated pHi was not reversed by hirudin added after 30 s, although this strips thrombin from receptors and rapidly returns [Ca2+]i to basal levels. In Na+-free medium, or in presence of the Na+/H+ antiport inhibitors, 5-(N,N-dimethyl)amiloride (DMA) or 5-(N-ethyl-N-isopropyl)amiloride (EIPA), thrombin caused a greater fall of pHi (0.22-0.26 units) that was sustained. DMA or EIPA could also reverse the alkalinization response to thrombin. Ca2+ ionophores (ionomycin, A23187) decreased platelet pHi by 0.02-0.15 units, but without an increase of pHi comparable to that following thrombin; DMA and EIPA enhanced the fall of pHi (0.14-0.33 units). Cytoplasmic acidification produced by nigericin (K+/H+ ionophore) was followed by return towards normal that was abolished by Na+/H+ antiport inhibitors. The phorbol diester phorbol 12-myristate 13-acetate had little effect on resting pHi but increased the rate of recovery 2-3-fold after cytoplasmic acidification by nigericin, ionomycin, or sodium propionate. These results indicate that elevation of [Ca2+]i by thrombin enhances H+ production, but the subsequent alkalinization is independent of receptor occupancy or elevated [Ca2+]i and stimulation of the Na+/H+ antiporter by thrombin probably involves some mechanism apart from regulation by H+ and protein kinase C.     

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

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

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.     

Secondary signals mediated by GPIIb/IIIa in thrombin-activated platelets

We have previously found that stimulation of aequorin-loaded platelets by thrombin produced a two-peaked increase in intracellular free calcium concentration ([Ca2+]i), and the development of the second peak of [Ca2+]i was closely related with the aggregation. In this report, we studied the interrelationship between the GPIIb/IIIa complex, aggregation, cytoskeletons and [Ca2+]i of platelets. The pretreatment of the platelets with dihydrocytochalasin B (4 microM), an actin polymerization inhibitor, did not inhibit aggregation and TXB2 production, but did inhibit both actin polymerization and the second peak of [Ca2+]i increase induced by thrombin, suggesting that actin polymerization and the second peak of [Ca2+]i are interrelated. GRGDSP (100 microM), a synthetic anti-adhesive peptide, has already been reported to inhibit platelet aggregation and the second peak of [Ca2+]i induced by thrombin. It also inhibited actin polymerization and TXB2 production, suggesting that aggregation was important for not only the generation of the second peak of [Ca2+]i but also for actin polymerization and TXB2 production. PGI2 (5 nM) did not abolish but only delayed aggregation, TXB2 production, actin polymerization and the second peak of [Ca2+]i increase. These findings suggest that the secondary signals are caused by aggregation (fibrinogen-binding to the GPIIb/IIIa) in thrombin-aggregated platelets, which results in the TXA2 production and the secondary peak of [Ca2+]i increase, and the latter was dependent on actin polymerization.     

Two thrombin-activated Ca2+ channels in human platelets.

The regulation of extracellular Ca2+ entry into fura-2-loaded human platelets was examined following stimulation with thrombin. In the presence of external Ca2+, stimulation of platelets with thrombin resulted in a rapid increase, followed by a plateau, in intracellular Ca2+ concentration ([Ca2+]i). Pretreatment with wortmannin, a specific inhibitor of myosin light chain kinase, suppressed only the plateau phase and had no effect on the initial rapid increase in [Ca2+]i. In Ca(2+)-free EGTA buffer, thrombin induced a transient and relatively small increase in [Ca2+]i caused by Ca2+ release from internal stores. When Ca2+ was added subsequently to the Ca(2+)-free medium within 10 min after thrombin activation, a marked increase in [Ca2+]i was seen, reflecting thrombin-stimulated external Ca2+ entry. With the Ca(2+)-free medium, wortmannin did not affect either the Ca2+ mobilization from the internal stores or the rapid external Ca2+ entry at early time points (within 5 s) after thrombin stimulation, whereas it significantly inhibited Ca2+ entry when Ca2+ was added later (at 3 min). Wortmannin inhibition of this late Ca2+ entry and that of 20-kDa myosin light chain phosphorylation after thrombin stimulation were dose- and preincubation time-dependent and correlated well with each other. These results suggest that two different channels are responsible for Ca2+ entry in human platelets at the early and late phases of thrombin stimulation and that the channel responsible for the late phase of Ca2+ entry may be activated by a mechanism involving myosin light chain kinase.     

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.     

Lack of inhibition of thrombin-induced rise in intracellular Ca2+ levels and 5-hydroxytryptamine secretion by 1-oleoyl-2-acetylglycerol in human platelets

The effect of 1-oleoyl-2-acetylglycerol (OAG) on the thrombin-induced rise in intracellular Ca2+ levels ([Ca2+]i) and 5-hydroxy[14C]tryptamine ([14C]5HT) secretion was studied. In washed human platelets prelabelled with [14C]5HT and quin 2, OAG (10-50 micrograms/ml) induced no significant aggregation, [14C]5HT secretion or rise in [Ca2+]i in the presence or absence of fibrinogen. However, addition of OAG (10-50 micrograms/ml) 10 s to 5 min before or 10-60 s after addition of threshold concentrations of thrombin (less than 0.03 U/ml) resulted in a significant potentiation of aggregation and [14C]5HT secretion without any effect on the thrombin-induced rise in [Ca2+]i. Both EGTA, which abolished the latter and creatine phosphate/creatine phosphokinase, the ADP scavenger, totally inhibited the aggregation but only partially reduced [14C]5HT secretion in response to thrombin plus OAG. At higher concentrations of thrombin, neither the rise in [Ca2+]i nor the extent of [14C]5HT secretion was significantly altered by OAG addition. The results demonstrate that, unlike phorbol esters, OAG has no inhibitory effect on thrombin-induced [Ca2+]i mobilisation but can synergize with low concentrations of thrombin in potentiating [14C]5HT secretion even at basal [Ca2+]i.     

Calcium/calmodulin transduces thrombin-stimulated secretion: studies in intact and minimally permeabilized human umbilical vein endothelial cells

Thrombin stimulates cultured endothelial cells (EC) to secrete stored von Willebrand factor (vWF), but the signal transduction pathways are poorly defined. Thrombin is known to elevate the concentration of intracellular calcium ([Ca2+]i) and to activate protein kinase C (PKC) in EC. Since both calcium ionophores and phorbol esters release vWF, both second messenger pathways have been postulated to participate in vWF secretion in response to naturally occurring agonists. We find that in intact human EC, vWF secretion stimulated by either thrombin or by a thrombin receptor activating peptide, TR(42-55), can be correlated with agonist-induced elevations of [Ca2+]i. Further evidence implicating calcium in the signal transduction pathway is suggested by the finding that MAPTAM, a cell-permeant calcium chelator, in combination with the extracellular calcium chelator EGTA, can inhibit thrombin-stimulated secretion. In contrast, the observation that staurosporine (a pharmacological inhibitor of PKC) blocks phorbol ester- but not thrombin-stimulated secretion provides evidence against PKC-mediated signal transduction. To examine further the signal transduction pathway initiated by thrombin, we developed novel conditions for minimal permeabilization of EC with saponin (4-8 micrograms/ml for 5-15 min at 37 degrees C) which allow the introduction of small extracellular molecules without the loss of large intracellular proteins and which retain thrombin-stimulated secretion. These minimally permeabilized cells secrete vWF in response to exogenous calcium, and EGTA blocks thrombin-induced secretion. Moreover, in these cells, thrombin- stimulated secretion is blocked by a calmodulin-binding inhibitory peptide but not by a PKC inhibitory peptide. Taken together, these findings demonstrate that thrombin-stimulated vWF secretion is transduced by a rise in [Ca2+]i and provide the first evidence for the role of calmodulin in this process.     

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.     

A role for Gi in control of thrombin receptor-phospholipase C coupling in human platelets

Stimulation of washed human platelets with alpha-thrombin was accompanied by aggregation, formation of inositol phosphates and phosphatidic acid, liberation of arachidonic acid, mobilization of intracellular Ca2+ stores, and influx of Ca2+ from the extracellular medium. Each of these responses was potentiated by a short pretreatment with epinephrine, although alone this agent was ineffective. A prolonged (5 min) stimulation with alpha-thrombin desensitized both phospholipase C and Ca2+ mobilization to a further thrombin challenge. Epinephrine added following thrombin desensitization restored both the ability of thrombin to release Ca2+ stores and stimulate inositol phospholipid hydrolysis. Resensitization was mediated by alpha 2-adrenergic receptors and lasted about 3 min, after which the Ca2+ levels returned again to basal levels. Pretreatment of platelets with phorbol dibutyrate at concentrations which specifically activate protein kinase C increased the rate of desensitization of the thrombin-induced release of Ca2+ stores and abolished the ability of epinephrine to restore the thrombin response. The protein kinase C inhibitor, staurosporine, blocked the inhibitory effect of phorbol ester and also reduced the rate of desensitization of thrombin and subsequent epinephrine action. These results suggest that thrombin activation of protein kinase C phosphorylates and inactivates a signaling protein which is common to both thrombin and alpha 2-adrenergic receptors. This protein is involved in thrombin stimulation of phospholipase C but is not directly stimulatory since epinephrine alone does not activate this enzyme. We searched for a known second messenger protein common to both thrombin and alpha 2-adrenergic receptors which was phosphorylated in intact platelets by protein kinase C in parallel with thrombin-induced desensitization. The alpha subunit of the inhibitory GTP-binding protein, Gi, was the only candidate which fulfilled all of these criteria as shown by immunoprecipitation. Therefore, we suggest that alpha i maintains the thrombin receptor in a state which can couple to phospholipase C when activated with thrombin. This permissive state of alpha i is blocked by phosphorylation by thrombin-activated protein kinase C.     

Glucose-induced increase in cytoplasmic pH in pancreatic beta-cells is mediated by Na+/H+ exchange, an effect not dependent on protein kinase C.

Glucose-induced changes in cytoplasmic pH (pHi) were investigated using pancreatic beta-cells isolated from obese hyperglycemic mice. Glucose, at concentrations above 3-5 mM, depolarized the beta-cell and increased pHi, cytoplasmic free Ca2+ ([Ca2+]i), and insulin release. This increase in pHi was dependent on the presence of extracellular Na+ and was inhibited by 5-(N-ethyl-N-isopropyl) amiloride, a blocker of Na+/H+ exchange. Stimulation of protein kinase C with phorbol ester also induced an alkalinization. However, when protein kinase C activity was down-regulated, glucose stimulation still induced alkalinization. At 20 mM glucose, 10 mM NH4Cl induced a marked rise in pHi, paralleled by repolarization, inhibition of electrical activity, and decreases in both [Ca2+]i and insulin release. Reduction in [Ca2+]i was prevented by 200 microM tolbutamide, but not by 10 mM tetraethylammonium. At 4 mM glucose, NH4Cl induced a transient increase in insulin release, without changing [Ca2+]i. Exposure of beta-cells to 10 mM sodium acetate caused a persistent decrease in pHi, an effect paralleled by a small transient increase in [Ca2+]i. Acidification per se did not change the beta-cell sensitivity to glucose, not excluding that the activity of the ATP-regulated K+ channels may be modulated by changes in pHi.     

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.     

Thrombin activation of the Na+/H+ exchanger in vascular smooth muscle cells. Evidence for a kinase C-independent pathway which is Ca2+-dependent and pertussis toxin-sensitive

The mechanism by which human alpha-thrombin activates the Na+/H+ exchanger was studied in cultured neonatal rat aortic smooth muscle cells. Thrombin (0.4 unit/ml) caused a rapid cell acidification followed by a slow, amiloride-inhibitable alkalinization (0.10-0.14 delta pHi above base line). In protein kinase C down-regulated cells (exposed to phorbol 12-myristate 13-acetate for 24 or 72 h), the delta pHi induced by thrombin was only partially attenuated. This protein kinase C-independent activation of the Na+/H+ exchanger was blocked by pertussis toxin (islet activating protein (IAP)), reducing delta pHi by 50%. IAP did not directly inhibit Na+/H+ exchange activity as assessed by the response to intracellular acid loading. Thrombin also stimulated arachidonic acid release by 2.5 fold and inositol trisphosphate release by 6.2 fold. IAP inhibited both of these activities by 50-60%. Intracellular Ca2+ chelation with 120 microM quin2 prevented the thrombin-induced Ca2+ spike, inhibited thrombin-induced arachidonic acid release by 75%, and inhibited thrombin-induced activation of the Na+/H+ exchanger in protein kinase C-deficient cells by 65%. Increased intracellular [Ca2+] alone was not sufficient to activate the Na+/H+ exchanger, since ionomycin (0.3-1.5 microM) failed to elevate cell pH significantly. 10 microM indomethacin inhibited thrombin-induced delta pHi in both control and protein kinase C down-regulated cells by 30-50%. Thus, thrombin can activate the Na+/H+ exchanger in vascular smooth muscle cells by a Ca2+-dependent, pertussis toxin-sensitive pathway which does not involve protein kinase C.     

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.     

Synergistic potentiation of 5-hydroxytryptamine secretion by platelet agonists and phorbol myristate acetate despite inhibition of agonist-induced arachidonate/thromboxane and beta-thromboglobulin release and Ca2+ mobilization by phorbol myristate acetate.

Previous studies have demonstrated an inhibition of agonist-induced inositol phospholipid breakdown and intracellular Ca2+ ([Ca2+]i) mobilization by phorbol esters in platelets. In this study, we have examined the effect of phorbol 12-myristate 13-acetate (PMA) on agonist-induced granule secretion and correlated it with agonist-induced [Ca2+]i mobilization, arachidonate and thromboxane (Tx) release in human platelets. With increasing times of incubation with PMA (10 s-5 min), the rise in [Ca2+]i induced by thrombin and the TxA2 mimetic, U46619, was increasingly inhibited (90-100% with 5 min incubation) and, correlating with this, thrombin-induced [3H]arachidonate, TxB2 and beta-thromboglobulin (beta TG) release were also inhibited. In addition, the conversion of exogenously added arachidonate to TxB2 was inhibited (50-80%) by a 10 s-5 min pretreatment with PMA. However, secretion of 5-hydroxy[14C]tryptamine (5HT) induced by thrombin or U46619 was not inhibited by 10 s-2 min incubations with PMA and, on the contrary, with low agonist concentrations, was potentiated by PMA in the absence of a significant rise in [Ca2+]i or endogenous Tx formation, to levels significantly greater than or equal to the sum of that obtained when agonist and PMA were added separately. With longer times of incubation with PMA (5 min), these synergistic effects became less pronounced as inhibitory effects of PMA on agonist-induced [14C]5HT secretion became apparent. The results indicate that, while PMA may cause an inhibition of agonist-induced [Ca2+]i mobilization resulting in an inhibition of agonist-induced arachidonate, TxB2 and beta TG release, its effects on agonist-induced 5HT secretion may be complicated by [Ca2+]i-independent synergistic effects of agonist and PMA.     

Na+/H+ exchange modulates Ca2+ mobilization in human platelets stimulated by ADP and the thromboxane mimetic U 46619

According to recent observations ADP stimulates platelets via activation of Na+/H+ exchange which increases cytosolic pH (pHi). This event initiates formation of thromboxane A2 (via phospholipase A2) and, thereafter, inositol 1,4,5-trisphosphate (via phospholipase C) which is known to mobilize Ca2+ from intracellular storage sites. We investigated changes in pHi and cytosolic free Ca2+, [Ca2+]i, activating platelets with ADP and the thromboxane mimetic U 46619. We found that ADP (5 microM) increased pHi from 7.15 +/- 0.08 to 7.35 +/- 0.04 (n = 8) in 2'-7'-bis-(carboxyethyl)-5,6-carboxyfluorescein-loaded platelets, whereas thromboxane A2 formation was inhibited by indomethacin. ADP also induced a dose-dependent Ca2+ mobilization in fura2-loaded platelets which again was not affected by indomethacin. [Ca2+]i increased by 54 +/- 10 nM (n = 8) at 1 microM and by 170 +/- 40 nM (n = 7) at 10 microM ADP above the resting value of 76 +/- 12 nM (n = 47). Inhibition of Na+/H+ exchange by ethylisopropylamiloride (EIPA) reduced ADP-induced Ca2+ mobilization by more than 65% in indomethacin-treated platelets. This inhibition could be completely overcome by artificially raising pHi using either NH4Cl or the Na+/H+ ionophore monensin. We found that U 46619 increased pHi by 0.18 +/- 0.05 at 0.1 microM and by 0.29 +/- 0.07 (n = 7) at 1.0 microM above the resting value via an EIPA-sensitive mechanism. In conflict with the proposed role of the Na+/H+ exchange we found that U 46619 raised [Ca2+]i via a mechanism that for more than 50% depended on intact Na+/H+ exchange. Again, artificially elevating pHi restored U 46619-induced Ca2+ mobilization despite the presence of EIPA. Thus, our data show that Na+/H+ exchange is a common step in platelet activation by prostaglandin endoperoxides/thromboxane A2 and ADP and enhances Ca2+ mobilization independently of phospholipase A2 activity.     

Modulation of vincristine sensitivity of human kidney tumor cells by pharmacological agents interfering with intracellular signals. No apparent relationship to changes in cytoplasmic Ca2+ or pH

The effect of substances proposed to modulate intracellular signal systems on growth and sensitivity to vincristine in the human kidney tumor cell line ACHN was investigated and related to changes in cytoplasmic free Ca2+ concentration ([Ca2+]i) and cytoplasmic pH (pHi). Presence during culture of the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol 13-acetate (TPA) had no effect on cell growth but significantly increased the EC50 concentration for vincristine inhibited cell growth. There was no indication for endogenous PKC activity being responsible for basal vincristine insensitivity since it was not affected by the PKC inhibitor H-7. The Ca2+ ionophore ionomycin tended to increase cell growth and induced vincristine resistance, whereas the calmodulin inhibitor W-7 had opposite effects. Presence during culture of the adenylate cyclase activator forskolin did not affect basal cell growth but dose-dependently made the cells more sensitive to vincristine. The modulators of vincristine sensitivity had no immediate effect on pHi, whereas after 3 days of incubation ionomycin and forskolin tended to increase pHi. Ionomycin and forskolin induced an immediate increase in [Ca2+]i which remained after 3 days only for ionomycin, whereas TPA decreased [Ca2+]i, a change which tended to remain after 3 days of incubation. It is concluded that perturbation of the intracellular signal system may affect both cell growth and cytotoxic drug sensitivity. However, there is no apparent relationship between immediate or late changes in [Ca2+]i and pHi and vincristine sensitivity.     

Superoxide anion increases intracellular pH, intracellular free calcium, and arachidonate release in human amnion cells.

We determined the effects of superoxide anion, produced by addition of xanthine oxidase to hypoxanthine, on the intracellular pH (pHi) and intracellular free calcium concentration ([Ca2+]i) and release of arachidonate in human cultured amnion cells. Superoxide anion induced a prompt increase of pHi and subsequent increase of [Ca2+]i. The evoked pHi was inhibited by pretreatment with anion channel blockers but not affected by omission of extracellular Na+ or addition of amiloride. The increase of [Ca2+]i was inhibited significantly by the absence of extracellular calcium or by the addition of a calcium channel blocker, cobalt. NH4Cl, which can generally increase pHi, also increased [Ca2+]i of amnion cells. But the increase of [Ca2+]i induced by the NH4Cl was significantly less than that induced by the amount of superoxide anion causing a similar increase in pHi. These results show that superoxide anion, crossed through anion channel in membrane, increased [Ca2+]i at least partially via increase of pHi and that the calcium mobilization was dependent on both extracellular and intracellular sources. Superoxide anion induced the release of arachidonate in a dose-dependent manner and this induction was inhibited by omission of extracellular calcium. These data suggest that the release of arachidonate was dependent on the increase of [Ca2+]i. We also determined the viability of cells in the presence of superoxide anion by flow cytometry. Superoxide anion at the levels used in these experiments did not change the percentage of viable cells. These findings suggested that superoxide anion may regulate biological functions in amnion cells via pHi, [Ca2+]i mobilization, and the release of arachidonate without damaging the cells.