Modulation of Na+/H+ exchange and intracellular pH by protein kinase C and protein phosphatase in blood platelets

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

Effect of complete protein 4.1R deficiency on ion transport properties of murine erythrocytes

Moderate hemolytic anemia, abnormal erythrocyte morphology (spherocytosis), and decreased membrane stability are observed in mice with complete deficiency of all erythroid protein 4.1 protein isoforms (4.1(-/-); Shi TS et al. J Clin Invest 103: 331, 1999). We have examined the effects of erythroid protein 4.1 (4.1R) deficiency on erythrocyte cation transport and volume regulation. 4.1(-/-) mice exhibited erythrocyte dehydration that was associated with reduced cellular K and increased Na content. Increased Na permeability was observed in these mice, mostly mediated by Na/H exchange with normal Na-K pump and Na-K-2Cl cotransport activities. The Na/H exchange of 4.1(-/-) erythrocytes was markedly activated by exposure to hypertonic conditions (18.2 +/- 3.2 in 4.1(-/-) vs. 9.8 +/- 1.3 mmol/10(13) cell x h in control mice), with an abnormal dependence on osmolality (EC(50) = 417 +/- 42 in 4.1(-/-) vs. 460 +/- 35 mosmol/kgH(2)O in control mice), suggestive of an upregulated functional state. While the affinity for internal protons was not altered (K(0.5) = 489.7 +/- 0.7 vs. 537.0 +/- 0.56 nM in control mice), the V(max) of the H-induced Na/H exchange activity was markedly elevated in 4.1(-/-) erythrocytes (V(max) 91.47 +/- 7.2 compared with 46.52 +/- 5.4 mmol/10(13) cell x h in control mice). Na/H exchange activation by okadaic acid was absent in 4.1(-/-) erythrocytes. Altogether, these results suggest that erythroid protein 4.1 plays a major role in volume regulation and physiologically downregulates Na/H exchange in mouse erythrocytes. Upregulation of the Na/H exchange is an important contributor to the elevated cell Na content of 4.1(-/-) erythrocytes.

Role of ion channels and exchangers in mechanical stretch-induced cardiomyocyte hypertrophy

We have previously reported that stretching of cardiomyocytes activates the phosphorylation cascade of protein kinases, including Raf-1 kinase and mitogen-activated protein (MAP) kinases, followed by an increase in protein synthesis partly through enhanced secretion of angiotensin II and endothelin-1. Membrane proteins, such as ion channels and exchangers, have been postulated to first receive extracellular stimuli and evoke intracellular signals. The present study was performed to determine whether mechanosensitive ion channels and exchangers are involved in stretch-induced hypertrophic responses. Neonatal rat cardiomyocytes cultured on expandable silicone dishes were stretched after pretreatment with a specific inhibitor of stretch-sensitive cation channels (gadolinium and streptomycin), of ATP-sensitive K+ channels (glibenclamide), of hyperpolarization-activated inward channels (CsCl), or of the Na+-H+ exchanger (HOE 694). Pretreatment with gadolinium, streptomycin, glibenclamide, and CsCl did not show any inhibitory effects on MAP kinase activation by mechanical stretch. HOE 694, however, markedly attenuated stretch-induced activation of Raf-1 kinase and MAP kinases by approximately 50% and 60%, respectively, and attenuated stretch-induced increase in phenylalanine incorporation into proteins. In contrast, HOE 694 did not inhibit angiotensin II-and endothelin-1-induced Raf-1 kinase and MAP kinase activation. These results suggest that among many mechanosensitive ion channels and exchangers, the Na+-H+ exchanger plays a critical role in mechanical stress-induced cardiomyocyte hypertrophy.

Exogenous diacylglycerols synergize with PAF with human platelets, but inhibit PAF-induced responses of rabbit platelets

To investigate whether diacylglycerol (DAG) has a role in reversible platelet aggregation induced by low concentrations of platelet-activating factor (PAF), we attempted to use the DAG kinase inhibitor, R59022, to prevent rapid conversion of DAG to phosphatidic acid. However, we found that R59022 inhibited the binding of [3H]PAF to human and rabbit platelets and to rabbit platelet membranes. We then investigated whether exogenous, cell-penetrating DAGs (1,2-dihexanoyl-sn-glycerol (DHG) and 1-oleoyl-2-acetyl-sn-glycerol (OAG)) act synergistically with low concentrations of PAF that alone induce only reversible aggregation. Platelets were isolated and labeled with [14C]serotonin. DHG (25-75 microM) caused slow, weak aggregation and some release of [14C]serotonin with human, but not rabbit, platelets. OAG (25-75 microM) did not aggregate either species' platelets. Phosphorylation of pleckstrin by DHG was more transient in rabbit platelets than previously observed with human platelets. Both DHG and OAG synergistically potentiated PAF-induced aggregation of human platelets, but, paradoxically, concurrently inhibited the PAF-induced increase in intracellular Ca2+ ([Ca2+]i): potentiation decreased upon incubation with DAGs before PAF addition. In contrast, DHG strongly inhibited PAF-induced aggregation of rabbit platelets; inhibition decreased upon preincubation. OAG, added with PAF, slightly potentiated aggregation of rabbit platelets: upon preincubation, OAG progressively inhibited. Effects of DHG and OAG on PAF-induced increases in [Ca2+]i in rabbit platelets followed a similar pattern; thus, with rabbit platelets, inhibition of the [Ca2+]i increase may at least partially account for inhibition of PAF-induced aggregation by exogenous DAGs. Results with human platelets are consistent with stimulation of protein kinase C by DAGs, and then metabolism of DAGs and/or negative feedback by DAGs, but results with rabbit platelets indicate both an unexpected species difference and a difference between the effects of DHG and OAG on PAF-induced platelet aggregation.

Effect of administered ethanol on protein kinase C in human platelets

There are numerous reports of the effect of ethanol on protein kinase C (PKC) in animals or with in vitro systems. However, the effect of ethanol on PKC in humans has not been extensively investigated despite the large number of studies involving PKC and human platelets. In this study, we administered ethanol to human volunteers and determined the level of PKC before and after a 0.4 g/kg dose of ethanol. We studied Native Americans and Caucasians of both sexes. There was an increases in PKC activity 60 min after ethanol administration. There were no ethnic, age, nor gender differences detected, nor was there any correlation between family history of alcoholism and the basal or stimulated platelet PKC levels. Neither was there any correlation of basal or stimulated PKC activity with the genotypes for ADH2, ADH3, ALDH2, CYP2E1, and CYP1A2.

Chelerythrine inhibits Na(+)-H+ exchange in platelets from spontaneously hypertensive rats

Hypertension has been associated with increased activity of the Na(+)-H+ exchanger. To study the role played by protein kinase C in this process, we used chelerythrine, a potent and specific inhibitor of the kinase. After an acid load by ammonium chloride preincubation, platelets isolated from spontaneously hypertensive rats showed a faster and larger increase in intracellular pH than platelets from Wistar-Kyoto rats. The initial rate of intracellular pH recovery was 2.46 +/- 0.26 pH units per minute in spontaneously hypertensive rats and 1.74 +/- 0.19 in Wistar-Kyoto rats. For protein kinase C inhibition, platelets were incubated for 30 minutes with 10 mumol/L chelerythrine. This treatment induced a significant reduction in the recovery rate only in spontaneously hypertensive rat platelets, indicating that a pathway involving protein kinase C participates in the prestimulation of the exchanger in cells from this rat strain. Addition of chelerythrine reduced the baseline intracellular pH of platelets. No significant difference was found between the decrease of steady-state intracellular pH induced by chelerythrine in either rat strain. These findings indicate that this model of hypertension is characterized by increased Na(+)-H+ activity mediated by protein kinase C stimulation.

The effect of BCECF on intracellular pH of human platelets

2',7'-Bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) is frequently used for fluorometric determination of intracellular pH (pHi) and its metabolic changes. Studies of BCECF-loaded platelets have reported different pHi values in the range of 6.98 to 7.35, despite the use of the same probe. It is now shown that intracellular BCECF (BCECFi) content affects pHi, and that its over-loading, leads to significantly lower pHi. Different pHi values can be reproduced by changing BCECFi, as reflected by fluorescence intensity. The major loading factors are: the concentration of the probe parent compound, BCECF acetoxymethyl ester (AM), and whether this ester is partly hydrolyzed externally when applied in plasma. When least affected by BCECF, platelet pHi is 7.34. High BCECFi does not affect ATP content, buffer capacity, activation of Na+/H+ exchange by protein kinase C (PKC) and basal PKC activity. On the other hand high BCECFi does inhibit the Na+/H+ exchange rate by over 50%. Since the Na+/H+ exchange strongly affects platelets pHi, it is proposed that this inhibition accounts, at least partly, for the lowered pHi in BCECF over-loaded platelets.

Stimulation of mitogen-activated protein kinase and Na+/H+ exchanger in human platelets. Differential effect of phorbol ester and vasopressin

Treatment of human platelets with phorbol 12-myristate 13-acetate (PMA) and arginine vasopressin (AVP) increase the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Electrophoretic retardation of MAPK mobility on SDS-polyacrylamide gels was used for determination of MAPK phosphorylation. The activity of MAPK was tested in myelin basic protein (MBP)-containing polyacrylamide gels. In this study we compared the PMA and AVP signal transduction pathways leading to the activation of MAPKs and Na+/H+ exchanger (NHE). Both agonists stimulate MAPK and NHE activities in a similar time frame and concentration dependence. The MAPK and NHE activities induced by PMA were inhibited by staurosporine, a potent inhibitor for protein kinase C (PKC), and by MAPK kinase (MEK) inhibitor, PD98059, but were not affected by the tyrosine kinase inhibitor genistein. In contrast, both AVP-induced MAPK and NHE activities were inhibited by genistein and MEK inhibitor but were not affected by staurosporine. Immunoprecipitation studies demonstrate that PMA, but not AVP, enhances the basal phosphorylation of the NHE-1. In this study, MAPKs are suggested to be a part of converging signaling leading to NHE activation by PKC-dependent and AVP-tyrosine kinase-dependent pathways. We propose that the MAPK activation of the NHE-1 does not involve phosphorylation of this exchanger protein. On the other hand, PKC can lead to phosphorylation and to additional activation of the NHE-1 through a MAPK-independent pathway.

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.

Mechanisms of muscarinic receptor-stimulated expression of c-fos in SH-SY5Y cells

In this study, the signal cascade transducing carbachol stimulation into c-fos expression in SH-SY5Y neuroblastoma cells was investigated. 1,2-Diacylglycerol formation and c-fos expression were mediated via stimulation of muscarinic M1 receptors and the first 5 min of receptor stimulation were critical for these events. Application of 1,2-dioctanoylglycerol induced c-fos expression and this, as well as carbachol-stimulated c-fos expression, was inhibited by protein kinase C inhibitors. Increasing the intracellular Ca2+ concentration had only small effects on c-fos expression. There was a dependency on extracellular Ca2+ for maximal c-fos expression and 1,2-diacylglycerol formation. The carbachol-stimulated c-fos expression was potentiated by application of the protein phosphatase inhibitor okadaic acid. These results demonstrate the importance of 1,2-diacylglycerol formation for muscarinic receptor-stimulated, protein kinase C-mediated c-fos expression in the SH-SY5Y cells and that this cascade is counteracted by an okadaic acid-sensitive protein phosphatase.

Effects of angiotensin II and endothelin-1 on platelet aggregation and cytosolic pH and free Ca2+ concentrations in essential hypertension

The aims of this study were to determine the relations between platelet free calcium concentrations ([Ca2+]i), intracellular pH (pHi), and aggregation and to assess the effects of angiotensin II (Ang II) and endothelin-1 on these platelet parameters in normotensive subjects and hypertensive patients. Seventeen normotensive subjects, 25 untreated hypertensive patients, and 34 treated hypertensive patients were studied. Platelet cytosolic free [Ca2+]i and pHi were measured spectrofluorometrically using specific fluorescent probes (fura 2-AM and BCECF-AM, respectively) in unstimulated and Ang II- and endothelin-1-stimulated platelets. Aggregation was measured by a turbidometric technique. Basal [Ca2+]i (141 +/- 11 nmol/L) and pH (7.16 +/- 0.01) were higher (P < .05) in the untreated hypertensive group compared with the normotensive (118 +/- 9 nmol/L, 7.11 +/- 0.01, respectively) and treated hypertensive (121 +/- 11 nmol/L, 7.12 +/- 0.01, respectively) groups. In the combined normotensive and hypertensive groups, there were significant correlations between [Ca2+]i and mean arterial pressure (r = .75, P < .01), pHi and mean arterial pressure (r = .72, P < .01), [Ca2+]i and pHi (r = .71, P < .01), [Ca2+]i and aggregation (r = .69, P < .02), and pHi and aggregation (r = .56, P < .05). Ang II stimulation significantly increased [Ca2+]i and pHi in the untreated hypertensive and normotensive groups. The net change in [Ca2+]i induced by Ang II was significantly higher (P < .05) in the untreated hypertensive group compared with the other groups (67 +/- 6 nmol/L for the untreated hypertensive group versus 54 +/- 5 and 29 +/- 8 nmol/L for the normotensive and treated hypertensive groups, respectively). In the presence of Ang II, thrombin-induced aggregatory responses were increased in all three groups, but the maximal response was significantly higher in the untreated hypertensive group compared with the other groups (P < .05). Endothelin-1 increased pHi through endothelin A-receptors (effect blocked by the specific antagonist BQ-123) but had no significant effect on [Ca2+]i or aggregation. However, endothelin-1 blunted thrombin-induced platelet aggregation in normotensive subjects but not in hypertensive patients. In conclusion, increased Ang II-stimulated [Ca2+]i and pHi in platelets of essential hypertensive patients may be associated with increased aggregatory responses. The stimulatory effect of endothelin-1 on pHi but not on [Ca2+]i or aggregation suggests that in platelets endothelin-induced signaling pathways other than phospholipase C may be involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison between sodium-hydrogen ion and lithium-hydrogen ion exchange in human platelets

The membrane-coupled exchange of Li+ and/or Na+ for H+ was studied in human platelets measuring intracellular pH(pHi) with a fluorescent indicator. A Li(+)-containing medium restored the internal pH of preacidified platelets to their prior pHi control value. When Na+ was replaced by Li+, similar steady-state values were attained in this system, although it was transported more slowly. The Km and Vmax were both higher with Na+ than with Li+. Exchanges of Li+ or Na+ with H+ were both blocked by ethylisopropylamiloride (EIPA) achieving half-maximal inhibition at submicromolar concentrations. The efflux Li+ or Na+ and resuspended in a choline medium exhibited an influx of H+ sensitive to EIPA. Thrombin, an activator of Na+/H+ exchange, induced a rapid increase in platelet internal pH in the presence of exogenous Li+. Thus: (1) Li+ can be substituted for Na+ in both the forward and the reverse exchange reaction; (2) Li+, while having a higher affinity than Na+ for the external site of the membrane carrier, has a lower Km and (3) Li+ as well as Na+ exchange are activated by thrombin.

Defective signal transduction in platelets from cirrhotics is associated with increased cyclic nucleotides

BACKGROUND

Patients with advanced cirrhosis show defective platelet aggregation, which is dependent, at least in part, on intrinsic platelet abnormalities. The aim of this study was to evaluate the activating and inhibitory pathways of platelet signal transduction in cirrhotic patients.

METHODS

Twelve cirrhotic patients and 12 control subjects participated in this study. Measurements were performed on washed platelets.

RESULTS

Thrombin-stimulated inositol 1,4,5-trisphosphate production was reduced fivefold, and the increase in cytosolic calcium concentration was significantly lower in platelets from cirrhotic patients following stimulation with thrombin, platelet activating factor, or U-46619. In addition, the activity of the platelet Na+/H+ antiporter, evaluated after an acid load, was significantly lower in platelets from cirrhotic patients (0.90 +/- 0.19 vs. 1.37 +/- 0.16 delta pHi/min, P = 0.07). Cirrhotic patients also showed a significantly increased basal intraplatelet content of both 5'-cyclic adenosine monophosphate (cAMP) (2724 +/- 330 vs. 1561 +/- 258 fmol/10(8) platelets, P < 0.05) and 5'-cyclic guanosine monophosphate (cGMP) (217 +/- 18 vs. 159 +/- 29 fmol/10(8) platelets, P < 0.05).

CONCLUSIONS

Our results indicate that in platelets from cirrhotic patients, defective early signal transduction is associated with an increase in platelet cAMP and cGMP, thus revealing new mechanisms contributing to the defective platelet function in this disease.

Na+/H+ exchange activity induced by thrombin is not inhibited by protein kinase inhibitors, staurosporine, K-252a, H-7 and sphingosine, in human platelets

In human platelets stimulated with thrombin (40 mU/ml), Na+/H+ exchange activity [the ethylisopropylamiloride (EIPA)-sensitive increase of cytoplasmic pH (pHc)] and protein kinase C (PKC) activity [phosphorylation of 47 kDa protein (P47), a substrate for PKC] were determined in the presence of protein kinase inhibitors, staurosporine (0.05-1 microM), K-252a (0.5-10 microM), H-7 (100 microM) and sphingosine (20-40 microM). Staurosporine and K-252a completely blocked PKC activity. H-7 and sphingosine reduced the P47 phosphorylation to 64% and 35%, respectively. On the contrary, the thrombin-induced pHc increase was not inhibited by staurosporine, K-252a or H-7. Sphingosine elevated the resting pHc by 0.26-0.42 independently of the Na+/H+ exchanger and inhibited the thrombin-induced pHc increase. However, after the resting pHc elevated by sphingosine had been reduced to the initial level by adding sodium propionate, the thrombin-induced pHc increase was observed again. These results suggested that sphingosine inhibited the thrombin-induced pHc increase by elevating the resting pHc. Thus, we concluded that the Na+/H+ exchanger was activated by thrombin through a pathway independent of PKC.

Activation of phospholipase C and protein kinase C has little involvement in ADP-induced primary aggregation of human platelets: effects of diacylglycerols, the diacylglycerols, the diacylglycerol kinase inhibitor R59022, staurosporine and okadaic acid

The primary phase of ADP-induced aggregation of human platelets does not involve appreciable formation of thromboxane A2 or release of granule contents; lack of formation of inositol trisphosphate has also been noted. Because these responses of platelets to ADP differ so markedly from their responses to other aggregating agents, the roles in ADP-induced aggregation of diacylglycerol, protein kinase C, increases in cytosolic [Ca2+], phosphorylation of pleckstrin (47 kDa) and phosphatases 1 and 2a were investigated. Washed human platelets, prelabelled with [14C]5-hydroxytryptamine and suspended in Tyrode solution (2 mM Ca2+, 1 mM Mg2+), were used for comparisons between the aggregation induced by 2-4 microM ADP, in the presence of fibrinogen, and that induced by 0.05 units/ml thrombin. The diacylglycerol kinase inhibitor 6-(2-[(4-fluorophenyl)phenyl-methylene]-1-piperidinylethyl)-7-meth yl-5H-thiazolo[3,2-a]-pyrimidin-5-one (R59022; 25 microM) had no, or only a slight, enhancing effect on ADP-induced aggregation, but potentiated thrombin-induced responses to a much greater extent. 1,2-Dihexanoyl-sn-glycerol or 1-oleoyl-2-acetyl-sn-glycerol (25 microM) added with or 30-90 s before ADP greatly potentiated aggregation without formation of thromboxane; staurosporine, an inhibitor of protein kinase C, reduced this potentiation. Staurosporine (25 nM) did not inhibit ADP-induced aggregation, although it strongly inhibited thrombin-induced aggregation and release of [14C]5-hydroxytryptamine. All these observations indicate little or no dependence of primary ADP-induced aggregation on the formation of diacylglycerol or on the activation of protein kinase C. At 2-4 microM, ADP did not significantly increase the phosphorylation of pleckstrin (studied with platelets prelabelled with [32P]orthophosphate), but 1,2-dihexanoyl-sn-glycerol- induced phosphorylation of pleckstrin was increased by ADP. Surprisingly, the diacylglycerols strongly inhibited the ADP-induced rise in cytosolic [Ca2+] concurrently with potentiation of ADP-induced aggregation; thus the extent of primary aggregation is independent of the level to which cytosolic [Ca2+] rises. Incubation of platelets with 1,2-dihexanoyl-sn-glycerol or 1-oleoyl-2-acetyl-sn-glycerol for several minutes reversed their potentiating effects on aggregation, and inhibition was observed. Incubation of platelets with okadaic acid, an inhibitor of phosphatases 1 and 2a, inhibited ADP- and thrombin-induced aggregation; although the reason for this effect is unknown, it is unlikely to involve inhibition of phospholipase C, since formation of diacylglycerol appears to have little involvement in the primary phase of ADP-induced aggregation.

Vasopressin elevation of Na+/H+ exchange is inhibited by genistein in human blood platelets

The regulation of intracellular Na+ and pHi in human blood platelets is known to be controlled by the function of the Na+/H+ exchanger. The phosphorylation state of the Na+/H+ exchanger which determines the exchanger activity in human blood platelets is regulated by the activities of protein kinases and protein phosphatases. Observations in this study indicate that arginine vasopressin (AVP) that interacts with a V1 receptor, activates the Na+/H+ exchange in human blood platelets through a genistein-inhibited mechanism. The AVP-activated Na+/H+ exchange is probably not regulated by protein kinase C (PKC), since this activation is not inhibited by staurosporine. The multiple ways in which platelet Na+/H+ exchange can be modulated may indicate the critical role played by this exchanger in the homeostasis control of pHi in human blood platelets.

Ligand-affected shift of Na+/H+ exchange pHi set point in human blood platelets, rapidly revealed by a novel approach

The Na+/H+ exchange time-course of BCECF-loaded human platelets, suspended in isotonic media containing NaCl and sodium propionate and activated by intracellular acidification, was measured spectrofluorimetrically. Sequential alkalinization rates decline exponentially as a function of the changing intracellular pH (pHi) and its linear expression (log rate vs. pHi) extrapolates reproducibly to the pHi set point for the Na+/H+ exchange activation. The set point of control platelets (7.28 +/- 0.01) is shifted rapidly (discernibly less than or equal to 30 s) and markedly to alkaline pHi (7.62 +/- 0.03) by PMA, that activates protein kinase C and is shifted to acidic pHi (7.05 +/- 0.01) by staurosporine, which inhibits protein kinases. The addition of 5-N-(3-aminophenyl)amiloride reveals that the alkalinization measured is predominantly Na+/H+ exchange with only a minute contribution (delta pHi = 0.012 +/- 0.002 in 1 min) of an acid loading component, at pHi greater than 7.2. The results support recent studies concluding that the set point indeed reflects the phosphorylation state of the Na+/H+ exchanger.