Stimulation by thrombin increases the cytosolic free Na+ concentration in human platelets. Studies with the novel fluorescent cytosolic Na+ indicator sodium-binding benzofuran isophthalate

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Sodium-Calcium Exchanger Reverse Mode Sustains Dichotomous Ion Fluxes Required for Procoagulant COAT Platelet Formation

Procoagulant collagen-and-thrombin (COAT)-activated platelets represent a subpopulation of activated platelets, which retain a coat of prohemostatic proteins and express phosphatidylserine on their surface. Dichotomous intracellular signaling generating procoagulant platelet activity instead of traditional aggregating endpoints is still not fully elucidated. It has been demonstrated that secondary messengers such as calcium and sodium play a critical role in platelet activation. Therefore, we developed a flow cytometric analysis to investigate intracellular ion fluxes simultaneously during generation of aggregating and procoagulant platelets. Human platelets were activated by convulxin-plus-thrombin. Cytosolic calcium, sodium, and potassium ion fluxes were visualized by specific ion probes and analyzed by flow cytometry. We observed high and prolonged intracellular calcium concentration, transient sodium increase, and fast potassium efflux in COAT platelets, whereas aggregating non-COAT platelets rapidly decreased their calcium content, maintaining higher cytosolic sodium, and experiencing lower and slower potassium depletion. Considering these antithetical patterns, we investigated the role of the sodium-calcium exchanger (NCX) during convulxin-plus-thrombin activation. NCX inhibitors, CBDMB and ORM-10103, dose-dependently reduced the global calcium mobilization induced by convulxin-plus-thrombin activation and dose-dependently prevented formation of procoagulant COAT platelets. Our data demonstrate that both NCX modes are used after convulxin-plus-thrombin-induced platelet activation. Non-COAT platelets use forward-mode NCX, thus pumping calcium out and moving sodium in, while COAT platelets rely on reverse NCX function, which pumps additional calcium into the cytosol, by extruding sodium. In conclusion, we described for the first time the critical and dichotomous role of NCX function during convulxin-plus-thrombin-induced platelet activation.

Genome-wide methylation profiling in archival formalin-fixed paraffin-embedded tissue samples

New technologies allow for genome-scale measurement of DNA methylation. In an effort to increase the clinical utility of DNA methylation as a biomarker, we have adapted a commercial bisulfite epigenotyping assay for genome-wide methylation profiling in archival formalin-fixed paraffin-embedded pathology specimens. This chapter takes the reader step by step through a biomarker discovery experiment to identify phenotype-correlated DNA methylation signatures in routine pathology specimens.

Lidocaine increases intracellular sodium concentration through a Na+-H+ exchanger in an identified Lymnaea neuron

BACKGROUND

The intracellular sodium concentration ([Na(+)]in) is related to neuron excitability. For [Na(+)]in, a Na(+)-H(+) exchanger plays an important role, which is affected by intracellular pH ([pH]in). However, the effect of lidocaine on [pH]in and a Na(+)-H(+) exchanger is unclear. We used neuron from Lymnaea stagnalis to determine how lidocaine affects [pH]in, Na(+)-H(+) exchanger, and [Na(+)]in.

METHODS

Intracellular sodium imaging by sodium-binding benzofuran isophthalate and intracellular pH imaging by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein were used to measure [Na(+)]in and [pH]in. Measurements for [Na(+)]in were made in normal, Na(+) free saline, with modified extracellular pH, and a Na(+)-H(+) exchanger antagonist [(5-N-ethyl-N-isopropyl amiloride, N-methylisopropylamiloride, and 5-(N,N-hexamethylene)-amiloride) pretreatment trials. Furthermore, [Na(+)]in and [pH]in were recorded simultaneously. From 0.1 to 10 mM, lidocaine, mepivacaine, bupivacaine, prilocaine, and QX-314 were evaluated.

RESULTS

Lidocaine, mepivacaine, and prilocaine increased the [Na(+)]in in a dose-dependent manner. In contrast, QX-314 did not change the [Na(+)]in at each dose. In the Na(+) free saline or in the presence of each Na(+)-H(+) exchanger antagonist, lidocaine failed to increase [Na(+)]in. Lidocaine, mepivacaine, and prilocaine induced a significant decrease in [pH]in below baseline with an increase in [Na(+)]in. In contrast, QX-314 did not change the [pH]in. These results demonstrated that lidocaine increases [Na(+)]in through Na(+)-H(+) exchanger activated by intracellular acidification, which is induced by the proton trapping of lidocaine. This [Na(+)]in increase and [pH]in change induces cell toxicity.

CONCLUSION

Lidocaine increases the [Na(+)] through a Na(+)-H(+) exchanger by proton trapping.

A store-operated nonselective cation channel in human lymphocytes

1. Agonist interaction with phospholipase C-linked receptors at the plasma membrane can elicit both Ca2+ and Na+ influxes in lymphocytes. While Ca2+ influx is mediated by Ca2+ release-activated Ca2+ (CRAC) channels, the pathway responsible for Na+ influx is largely unknown. 2. We show that thapsigargin, ionomycin, ADP-ribose and IP3 activated a nonselective cation channel in lymphocytes that had a slightly outwardly rectifying I-V relationship, and a single channel conductance of 23.1 pS. We termed this channel a Ca2+ release-activated nonselective cation (CRANC) channel. 3. On activation in cell-attached configuration, switching to an inside-out configuration abolished CRANC channel activity. 4. Transfection of Jurkat T cells with antisense oligonucleotides for LTRPC2 reduced capacitative Ca2+ entry. 5. These results suggest that CRANC channels are responsible for the Na+ influx as well as a portion of the Ca2+ influx in lymphocytes induced by store depletion, that sustained activation of CRANC channels requires some property of the environment of a cell depleted of its Ca2+ stores; and that LTRPC2 protein is a likely component of the CRANC channel.

An hypothesis on the consolidation and PGE1-induced deconsolidation of a platelet plug

Consolidation is the final stage in haemostasis in which a platelet plug blocking a bleeding area of a vessel: (a) becomes impermeable to circulating plasma proteins and (b) contracts to resist blood pressure.

HYPOTHESIS

The impermeabilization step of consolidation is accomplished through fluid uptake by the platelets from a hydrated intercellular glue formed during thrombin activation. Dehydration occurs through inhibition of the Na+,K+-ATPase of platelets with sodium and water uptake. However, and uniquely, due to the high cellular density of the platelet plug, access of peripheral plasma fluids to the plug is limited forcing the platelets to take up preferentially the fluid of interplatelet space. The increased adhesion properties of the dehydrated glue simultaneously furthers a decreased hydraulic permeability and an improved coupling of the contractile forces among platelets. In 'Deconsolidation', the fluid uptake process can be reversed and amplified by agents that increase cAMP, reactivating the Na+,K+-ATPase and expressing CFTR or equivalent Cl- secretory channels that force the extrusion of fluid from the platelets, with rehydration of the intercellular polymer and a large increase in the interplatelet space.

Intracellular [Na+], Na+ pathways, and fluid transport in cultured bovine corneal endothelial cells

The mechanism of fluid transport across corneal endothelium remains unclear. We examine here the relative contributions of cellular mechanisms of Na+ transport and the homeostasis of intracellular [Na+] in cultured bovine corneal endothelial cells, and the influence of ambient Na+ and HCO3- on the deturgescence of rabbit cornea. Bovine corneal endothelial cells plated on glass coverslips were incubated for 60 min with 10 microm of the fluorescent Na+ indicator SBFI precursor in HCO3- HEPES (BH) Ringer's solution. After loading, cells were placed in a perfusion chamber. Indicator fluorescence (490 nm) was determined with a Chance-Legallais time-sharing fluorometer. Its voltage output was the ratio of the emissions excited at 340 and 380 nm. For calibration, cells were treated with gramicidin D. For fluid transport measurements, rabbit corneas were mounted in a Dikstein-Maurice chamber, and stromal thickness was measured with a specular microscope. The steady-state [Na+]i in BH was 14.36+/-0.38 mM (n = mean+/-s.e.). Upon exposure to Na+ -free BH solution (choline substituted), [Na+]i decreased to 1.81+/-0.20mM (n = 19). When going from Na+ -free plus 100 microm ouabain to BH plus ouabain, [Na+]i increased to 46.17+/-2.50 (n = 6) with a half time of 1.26+/-0.04 min; if 0.1 microm phenamil plus ouabain were present, it reached only 21.78+/-1.50mm. The exponential time constants (min-1) were: 0.56+/-0.04 for the Na+ pump; 0.39+/-0.01 for the phenamil sensitive Na+ channel; and 0.17+/-0.02 for the ouabain-phenamil-insensitive pathways. In HCO3- free medium (gluconate substituted), [Na+]i was 14.03+/-0.11mM; upon changing to BH medium, it increased to 30.77+/-0.74 mm. This last [Na+]i increase was inhibited 66% by 100 microm DIDS. Using BH medium, corneal thickness remained nearly constant, increasing at a rate of only 2.9+/-0.9 microm hr-1 during 3 hr. However, stromal thickness increased drastically (swelling rate 36.1+/-2.6 microm hr-1) in corneas superfused with BH plus 100 microm ouabain. Na+ -free, HCO3- free solution and 100 microm DIDS also led to increased corneal swelling rates (17.7+/-3.6, 14.4+/-1.6 and 14.9+/-1.2 microm hr-1, respectively). The present results are explained by the presence of a DIDS-inhibitable Na+-HCO3- cotransporter and an epithelial Na+ channel, both previously found in these cells. On the other hand, the quantitative picture presented here appears a novelty. The changes we observe are consistent with pump-driven rapid exchange of intracellular Na+, and recirculation of fully 70% of the Na+ pump flux via apical Na+ channels.

Effect of W07-toxin on gut physiological response in mice

A number of unknown secretogenic factor(s) from Vibrio cholerae have been implicated to play a role in inducing cholera-like symptoms observed in patients. The present study has been carried out on the novel W07-toxin (pI 5.2) from V. cholerae W07, an epidemic cholera strain devoid of the ctx gene. The toxin showed maximum binding to GM(1) and interacted with a 20 kDa glycoprotein present on the cell membrane of mice enterocytes in a GM(1) specific manner. The analysis of biochemical parameters in enterocytes triggered with this toxin revealed a significant increase in intracellular calcium concentration and a massive secretion of Cl(-). However, no absorption of Na(+) was observed under the same condition. This toxin also elevated the level of cyclic adenosine 3',5'-monophosphate (cAMP) as well as protein kinase A (PKA). Thus, the novel toxin, although distinct from cholera-toxin, showed some functional homology to it and may be one of the key players inducing electrolyte imbalance within intestinal cells in the cholera-like symptoms associated with V. cholerae W07.

Beta-alanine protection against hypoxic liver injury in the rat

Liver hypoxia still represents an important cause of liver injury during shock and liver transplantation. We have investigated the protective effects of beta-alanine against hypoxic injury using isolated perfused rat livers and isolated rat hepatocyte suspensions. Perfusion with hypoxic Krebs-Henseleit buffer increased liver weight and caused a progressive release of lactate dehydrogenase (LDH) in the effluent perfusate. The addition of 5 mmol/l beta-alanine to the perfusion buffer completely prevented both weight increase and LDH leakage. These findings were confirmed by histological examinations showing that beta-alanine blocked the staining by trypan blue of either liver parenchymal and sinusoidal cells. Studies performed in isolated hepatocytes revealed that beta-alanine exerted its protective effects by interfering with Na+ accumulation induced by hypoxia. The addition of gamma-amino-butyric acid, which interfered with beta-alanine uptake by the hepatocytes or of Na+/H+ ionophore monensin, reverted beta-alanine protection in either hepatocyte suspensions or isolated perfused livers. We also observed that liver receiving beta-alanine were also protected against LDH leakage and weight increase caused by the perfusion with an hyposmotic (205 mosm) hypoxic buffer obtained by decreasing NaCl content from 118 to 60 mmol/l. This latter effect was not reverted by blocking K+ efflux from hepatocyte with BaCl(2) (1mmol/l). Altogether these results indicated that beta-alanine protected against hypoxic liver injury by preventing Na+ overload and by increasing liver resistance to osmotic stress consequent to the impairment of ion homeostasis during hypoxia.

Na(+) channel effects of remacemide and desglycinyl-remacemide in rat cortical synaptosomes

The effects of the novel anticonvulsant, remacemide hydrochloride and its active metabolite, desglycinyl-remacemide, on veratridine-induced Na(+) influx in rat cortical synaptosomes were investigated and compared to established Na(+) channel blocking antiepileptic drugs. Remacemide and desglycinyl-remacemide reduced veratridine-stimulated Na(+) influx to 30.7% (IC(50)=160.6 microM) and 13.2% (IC(50)=85.1 microM) of control, respectively. Carbamazepine, phenytoin and lamotrigine similarly reduced Na(+) influx to 20.1% (IC(50)=325.9 microM), 79.8% and 27.9% (IC(50)=23.0 microM) of control, respectively. Resting internal Na(+) concentrations were significantly increased by desglycinyl-remacemide (1 and 10 microM) and, conversely, decreased by desglycinyl-remacemide and carbamazepine (both 1000 microM). These studies support previous electrophysiological investigations, which suggest that remacemide and desglycinyl-remacemide exert their antiepileptic effects, at least in part, by an inhibitory action on voltage-gated Na(+) channels. Desglycinyl-remacemide may have an additional action on Na(+) homeostasis that merits further exploration.

Involvement of anion exchange in the hypoxia/reoxygenation-induced changes in pH(i) and

The involvement of Cl(-)/HCO(3)(-) exchange in hypoxia/reoxygenation-induced changes in pH(i) and Ca(2+) concentration ([Ca(2+)](i)) was examined in rat ventricular myocytes. During 10-min hypoxia, the initial pH(i) (7.21+/-0.04) fell to below 6.8. Subsequent reperfusion with reoxygenated buffer returned this acidic pH(i) to the neutral range with increases in [Ca(2+)](i). These responses were reduced by the removal of Cl(-) or HCO(3)(-) and by the addition of anion exchange inhibitors, SITS (4-acetamido-4'isothiocyanato-stilbene-2,2'disulfonic acid) and DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulfonic acid), while inhibitors for the Cl(-) channel and Na(+)/K(+)/2Cl(-) cotransport were without effects. The hypoxia-induced acidification was attenuated by protein kinase C inhibitors, calphostin C and chelerythrine, but not by a protein kinase A inhibitor, KT5720. Under normoxic condition, protein kinase C activation induced a SITS-sensitive acidification. Furthermore, in electrically driven rat papillary muscle, SITS and DIDS improved the recovery of developed tension during the reoxygenation. These results suggest that the hypoxia-induced decrease in pH(i) is mediated at least in part by anion exchange stimulation through protein kinase C activation, and this exchange takes part in the reoxygenation-induced Ca(2+) overload as well as contractile dysfunction.

Alterations of Na(+) homeostasis in hepatocyte reoxygenation injury

Reperfusion injury represents an important cause of primary graft non-function during liver transplantation. However, the mechanism responsible for cellular damage during reoxygenation has not yet been completely understood. We have investigated whether changes in intracellular Na(+) distribution might contribute to cause hepatocyte damage during reoxygenation buffer after 24 h of cold storage. Hepatocyte reoxygenation resulted in a rapid increase in cellular Na(+) content that was associated with cytotoxicity. Na(+) accumulation and hepatocyte death were prevented by the omission of Na(+) from the incubation medium, but not by the addition of antioxidants. Blocking Na(+)/H(+) exchanger and Na(+)/HCO(3)(-) co-transporter by, respectively, 5-(N,N-dimethyl)-amiloride or omitting HCO(3)(-) from the reoxygenation medium significantly decreased Na(+) overload and cytotoxicity. Stimulation of ATP re-synthesis by the addition of fructose also lowered Na(+) accumulation and cell death during reoxygenation. A significant protection against Na(+)-mediated reoxygenation injury was evident in hepatocytes maintained in an acidic buffer (pH 6.5) or in the presence of glycine. The cytoprotective action of glycine or of the acidic buffer was reverted by promoting Na(+) influx with the Na(+)/H(+) ionophore monensin. Altogether, these results suggest that Na(+) accumulation during the early phases of reoxygenation might contribute to liver graft reperfusion injury.

Cell cycle-related changes in the conducting properties of r-eag K+ channels

Release from arrest in G2 phase of the cell cycle causes profound changes in rat ether-à-go-go (r-eag) K+ channels heterologously expressed in Xenopus oocytes. The most evident consequence of the onset of maturation is the appearance of rectification in the r-eag current. The trigger for these changes is located downstream of the activation of mitosis-promoting factor (MPF). We demonstrate here that the rectification is due to a voltage-dependent block by intracellular Na+ ions. Manipulation of the intracellular Na+ concentration indicates that the site of Na+ block is located approximately 45% into the electrical distance of the pore and is only present in oocytes undergoing maturation. Since the currents through excised patches from immature oocytes exhibited a fast rundown, we studied CHO-K1 cells permanently transfected with r-eag. These cells displayed currents with a variable degree of block by Na+ and variable permeability to Cs+. Partial synchronization of the cultures in G0/G1 or M phases of the cell cycle greatly reduced the variability. The combined data obtained from mammalian cells and oocytes strongly suggest that the permeability properties of r-eag K+ channels are modulated during cell cycle-related processes.

Analysis of a Ca2+-activated K+ channel that mediates hyperpolarization via the thrombin receptor pathway

Dami human leukemia cells express G protein-coupled thrombin receptors that operate through the phospholipase C pathway. When these receptors are activated by alpha-thrombin or by thrombin receptor-activating peptide, an elevation in cytosolic Ca2+ concentration develops that is accompanied by hyperpolarization of the plasma membrane. This transitory phase of hyperpolarization is primarily mediated by inwardly rectifying, Ca2+-activated K+ channels that have an inward conductance of approximately 24 pS. In cell-attached patches the channels open within seconds after superfusion of the cell with thrombin receptor-activating peptide. In inside-out patches, perfusion of submicromolar Ca2+ onto the cytosolic surface of the membrane is sufficient to activate the channels. In outside-out patches, channel opening can be blocked by nanomolar concentrations of charybdotoxin. The function of these intermediate-sized inwardly rectifying, Ca2+-activated K+ channels has not been established; however, by analogy with other cell systems, they may serve to regulate cell volume during cellular activation or to increase the electromotive drive that sustains Na+ and/or Ca2+ influx through ligand-gated cation channels.

Differential stimulation of the Na+/H+ exchanger determines chloroquine uptake in Plasmodium falciparum

Here we describe the identification and characterization of a physiological marker that is associated with the chloroquine-resistant (CQR) phenotype in the human malarial parasite Plasmodium falciparum. Single cell in vivo pH measurements revealed that CQR parasites consistently have an elevated cytoplasmic pH compared to that of chloroquine-sensitive (CQS) parasites because of a constitutively activated Na+/H+ exchanger (NHE). Together, biochemical and physiological data suggest that chloroquine activates the plasmodial NHE of CQS parasites, resulting in a transitory phase of rapid sodium/hydrogen ion exchange during which chloroquine is taken up by this protein. The constitutively stimulated NHE of CQR parasites are capable of little or no further activation by chloroquine. We propose that the inability of chloroquine to stimulate its own uptake through the constitutively activated NHE of resistant parasites constitutes a minimal and necessary event in the generation of the chloroquine-resistant phenotype.

The rotavirus enterotoxin NSP4 mobilizes intracellular calcium in human intestinal cells by stimulating phospholipase C-mediated inositol 1,4,5-trisphosphate production

Rotavirus infection is the leading cause of severe diarrhea in infants and young children worldwide. The rotavirus nonstructural protein NSP4 acts as a viral enterotoxin to induce diarrhea and causes Ca2+-dependent transepithelial Cl- secretion in young mice. The cellular basis of this phenomenon was investigated in an in vitro cell line model for the human intestine. Intracellular calcium concentration ([Ca2+]i) was monitored in fura-2-loaded HT-29 cells using microscope-based fluorescence imaging. NSP4 (1 nM to 5 microM) induced both Ca2+ release from intracellular stores and plasmalemma Ca2+ influx. During NSP4-induced [Ca2+]i mobilization, [Na+]i homeostasis was not disrupted, demonstrating that NSP4 selectively regulated extracellular Ca2+ entry into these cells. The ED50 of the NSP4 effect on peak [Ca2+]i mobilization was 4.6 +/- 0.8 nM. Pretreatment of cells with either 2.3 x 10(-3) units/ml trypsin or 4.4 x 10(-2) units/ml chymotrypsin for 1-10 min abolished the NSP4-induced [Ca2+]i mobilization. Superfusing cells with U-73122, an inhibitor of phospholipase C, ablated the NSP4 response. NSP4 induced a rapid onset and transient stimulation of inositol 1,4,5-trisphosphate (IP3) production in an IP3-specific radioreceptor assay. Taken together, these results suggest that NSP4 mobilizes [Ca2+]i in human intestinal cells through receptor-mediated phospholipase C activation and IP3 production.

Potential-dependent block of human delayed rectifier K+ channels by internal Na+

The delayed rectifier K+ currents in differentiated human SH-SY5Y neuroblastoma cells were characterized with tight-seal recording techniques. Activation and inactivation parameters were measured. At high positive potentials, the current showed a marked rectification, causing a region of negative slope conductance in the current vs. potential curve. The rectification depended markedly on the pipette Na+ concentration. Without Na+, no rectification was observed, whereas with high Na+ (20-60 mM), a marked rectification was always observed. Tail current measurements showed a fast ( < 400 microseconds) block of K+ currents in the presence of internal Na+. With 60 mM Na+ in the pipette 8% of the K+ current was blocked at 0 mV, 27% at +20 mV, and 82% at +100 mV. Similar degrees of block were often seen with 30 mM Na+ in the pipette. The submembrane Na+ concentration in intact cells was estimated, on the basis of the reversal of Na+ current, to be approximately 15 mM. Single-channel K+ currents, in the cell-attached configuration, showed a conductance of approximately 20 pS at 40-60 mV above rest but showed rectification at high potentials.

Lymphocytic Na(+)-H+ exchange increases after an oral glucose challenge

The effects of oral glucose challenge on plasma glucose concentration, plasma insulin concentration, arterial blood pressure, cytosolic pH (pHi), cytosolic free Na+ concentration ([Na+]i), and cellular Na(+)-H+ exchange activity were investigated in 16 healthy subjects. The pHi, [Na+]i, and Na(+)-H+ exchange activity were measured in intact lymphocytes by using the fluorescent dye technique. The oral glucose challenge significantly increased plasma glucose, plasma insulin, and the lymphocytic Na(+)-H+ exchange activity, measured as change of pHi per second (control [0 hours], 5.20 +/- 0.53 x 10(-3) dpHi/s; 1 hour after glucose administration, 8.28 +/- 1.07 x 10(-3) dpHi/s; 2 hours after glucose administration, 8.15 +/- 1.18 x 10(-3) dpHi/s; P = .002). The lymphocytic Na(+)-H+ exchange was significantly correlated with plasma glucose concentration (r = .357, P = .041). During steady state euglycemic hyperinsulinemic clamp, the Na(+)-H+ exchange activity was not significantly changed compared with baseline values. The study shows that changes of blood glucose levels can induce an acute increase in Na(+)-H+ exchange activity. Systolic blood pressure and Na(+)-H+ exchange activity were significantly (P < .001) but weakly correlated during an oral glucose challenge.

Intracellular Na+ measurements using sodium green tetraacetate with flow cytometry

Intracellular sodium concentration ([Na+]i) can be measured using SBFI with image analysis or spectrofluorometry. However, SBFI cannot be used with most flow cytometry systems, due to its requirement for dual excitation in the UV range. Recently a new sodium indicator, Sodium Green, was developed which can be used with flow cytometry to measure [Na+]i. Adequate staining of cells with Sodium Green is dependent upon both dye and cell concentration, time and temperature of loading, use of the non-ionic detergent Pluronic, and the cell line used. Both Sodium Green and SBFI give similar calibration curves using the ionophore gramicidin. Sodium Green has a small pH dependence, but this should not affect most applications. Since a fluorescence ratio technique cannot be used with Sodium Green, we used a ratio of dye fluorescence to forward angle light scatter to normalize for cell size variations in the population. The basal [Na+]i obtained using the Sodium Green technique in CHO ISI cells was 19 mM, in reasonable agreement with previous studies using SBFI in other cells.

A novel monovalent cation channel activated by inositol trisphosphate in the plasma membrane of rat megakaryocytes

The activation of a monovalent cation current was studied in rat megakaryocytes using patch clamp techniques combined with photometric measurements of intracellular concentrations of Ca2+ ([Ca2+]i) and Na+. ADP evoked a release of [Ca2+]i and transiently activated a monovalent cation-selective channel, which, at negative potentials and under physiological conditions, would be expected to carry an inward Na+ current. The single channel conductance, estimated by noise analysis from whole cell currents at -50 to -60 mV was 9 picosiemens. Thapsigargin-induced [Ca2+]i increases failed to stimulate the monovalent cation current, suggesting that neither [Ca2+]i nor the depletion of internal Ca2+ stores were activators of this conductance. However, buffering of [Ca2+]i changes with 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid showed that both activation and inactivation of the current were accelerated by a rise in [Ca2+]i. The monovalent cation conductance was activated by internal perfusion with inositol 1,4,5-trisphosphate, both in the presence and in the absence of a rise in [Ca2+]i. Internal perfusion with inositol 2,4,5-trisphosphate, the poorly metabolizable isomer of inositol trisphosphate, similarly activated the monovalent cation current, whereas 1,3,4,5-tetrakisphosphate neither activated a current nor modified the ADP-induced monovalent current. Heparin, added to the pipette, blocked activation of the channel by ADP. The intracellular concentration of Na+, monitored by sodium-binding benzofuran isopthalate, increased by 10-20 mM in response to ADP under pseudophysiological conditions. We conclude the existence of a novel nonselective cation channel in the plasma membrane of rat megakaryocytes, which is activated by IP3 and can lead to increases in cytosolic Na+ after stimulation by ADP.

Dexamethasone-induced sodium influx in human lymphocytes

We evaluated the direct effects of glucocorticoids on intracellular sodium content and cellular transport systems. Cytosolic free sodium concentration ([Na+]i) was measured in intact human lymphocytes using the sodium-sensitive fluorescent dye sodium-binding benzofuran-isophthalate. Administration of dexamethasone for 60 min increased lymphocytic [Na+]i from 17.6 +/- 2.0 mmol/L to 24.3 +/- 3.9 nmol/L (n = 12; P < 0.01). The dexamethasone-induced [Na+]i increase was abolished in the absence of extracellular sodium, by mifepristone and by actinomycin D. The dexamethasone-induced [Na+]i increase was also seen after inhibition of Na+,K(+)-ATPase by 1 mmol/L ouabain. The present results indicate that dexamethasone produces a trans-plasma membrane sodium influx probably by early occurring genomic effects.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Calcium-activated sodium and chloride fluxes modulate platelet volume: role of Ca2+ stores

An increase in cytosolic ionized Ca2+ concentration ([Ca2+]i) initiates volume changes in various types of cells. In response to increases in [Ca2+]i most cell types contract by efflux of K+ and Cl-, whereas platelets expand in response to rises in [Ca2+]i. This study examined the importance of the source of Ca2+, the flux of ions responsible for the volume change, and the role of Ca(2+)-dependent protein kinases in regulating these ionic fluxes. The baseline platelet volume was independent of extracellular Ca2+ but when stimulated by the Ca2+ ionophore A-23187 (50 nM) the volume increased in both the presence and absence of extracellular Ca2+ (1.18 +/- 0.08 vs. 0.83 +/- 0.06 fl, respectively). The increased volume was caused by the gain of Na+ and Cl-. Na+ entered through both conductive and nonconductive (Na+/H+ exchange) pathways, whereas the influx of Cl- was conductive and inhibited by the Cl- channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The Ca(2+)-induced volume change was blocked by both calmodulin and protein kinase inhibitors. Thus the activation of Ca(2+)-dependent protein kinases promotes platelet swelling by stimulating Na+ and Cl- influx.

Thapsigargin-induced [Ca2+]i increase activates sodium influx in human platelets

Using the fluorescent dyes sodium-binding-benzofuran-isophthalate and fura-2 cytosolic free sodium concentration ([Na+]i) and cytosolic free calcium concentration ([Ca2+]i) were investigated in intact human platelets in order to characterize the effect of elevated [Ca2+]i on [Na+]i. Spectrofluorometric studies of [Ca2+]i and [Na+]i in intact platelets were done after specific inhibition of endoplasmic Ca-ATPase by thapsigargin. Thapsigargin increased [Ca2+]i and [Na+]i in platelets. Addition of thapsigargin increased [Na+]i from 23.5 +/- 2.9 mM to 51.6 +/- 11.1 mM (mean +/- S.E., P < 0.05). The thapsigargin induced [Na+]i increase was also seen in the absence of extracellular calcium. In the absence of external sodium the thapsigargin induced [Na+]i increase was abolished, indicating that thapsigargin induced [Na+]i increase was due to sodium influx. Thapsigargin induced sodium influx was blocked after administration of NiCl2. The present results support the idea that the filling state of intracellular calcium stores regulate plasma permeability for sodium.

Increased intracellular Na+ augments mobilization of Ca2+ from SR in vascular smooth muscle cells

The effect of a rise in intracellular Na+ concentration ([Na+]cyt) on the amount of Ca2+ in intracellular stores was studied in vascular smooth muscle cells from the A7r5 line. The relative amount of stored Ca2+ was estimated in fura 2-loaded cells by the rise in cytosolic free Ca2+ concentration ([Ca2+]cyt) evoked by Ca2+ release from the sarcoplasmic reticulum (SR). To improve the detection of released Ca2+, extrusion of Ca2+ from the cytosol was minimized by using nominally Na+/Ca(2+)-free medium containing 0.5 mM La3+ [for vasoconstrictor experiments, the medium contained 0.5 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and no La3+]. Ca2+ release was triggered by thapsigargin (TG), an SR Ca(2+)-ATPase inhibitor, and by the vasoconstrictors arginine vasopressin (AVP) and serotonin (5-HT). Incubation with 1-3 mM ouabain for 20 min, which raises [Na+]cyt from 4.4 to 9.0 mM, increased "resting" [Ca2+]cyt only slightly (from 87 to 122 nM). However, ouabain greatly augmented the release of Ca2+ evoked by TG [from 639 nM (control) to 1,021 nM], by AVP (from 993 to 1,597 nM), and by 5-HT (from 559 to 1,486 nM). Ouabain-induced augmentation of TG-evoked Ca2+ release was not affected by 10 microM verapamil; this implies that the effect of ouabain was not due to Ca2+ entry through voltage-gated Ca2+ channels. The response to TG was not augmented when ouabain was applied for 20 min in Na(+)-free medium (Na+ replaced by equimolar N-methyl-D-glucamine) to prevent [Na+]cyt from rising.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of intracellular free Na+ concentration of synaptosomes by a fluorescent indicator, Na(+)-binding benzofuran isophthalate: the effect of veratridine, ouabain, and alpha-latrotoxin

A novel fluorescent Na+ indicator, Na(+)-binding benzofuran isophthalate (SBFI), was used to follow changes in the intracellular free Na+ concentration ([Na+]i) of synaptosomes. The dye, when loaded into synaptosomes in the form of its acetoxymethyl ester, was responsive to changes of [Na+]. Calibration was made using the 340/380 nm excitation ratio when the cytoplasmic Na+ concentration was equilibrated with different concentrations of extracellular Na+ in the presence of 2 microM gramicidin D. The basal value of [Na+]i in synaptosomes in the presence of 140 mM extracellular Na+ was found to be 10.9 +/- 1.8 mM. Veratridine, which opens potential-dependent Na+ channels, caused a sudden increase in [Na+]i in a concentration-dependent manner (1-20 microM), whereas the effect of ouabain (20 and 50 microM), the inhibitor of the plasma membrane Na+,K(+)-ATPase, was more gradual. The rise in the fluorescence intensity upon addition of veratridine was prevented completely by 2 microM tetrodotoxin. alpha-Latrotoxin, the black widow spider toxin, caused an increase in the fluorescence intensity, which became evident 1 min after the addition of the toxin. The rate of increase was proportional to the concentration of the toxin (0.19-1.5 nM). This report confirms our earlier finding demonstrating a Na(+)-dependent component in the action of alpha-latrotoxin, and shows that changes in [Na+]i in synaptosomes can be followed by SBFI.

Intracellular free Na+ in resting and activated A7r5 vascular smooth muscle cells

Regulation of intracellular Na+ ([Na+]i) in cultured vascular smooth muscle cells (A7r5 line) was studied with Na(+)-sensitive fluorescent dye sodium-binding benzofuran isophthalate. Digital imaging microscopy was used to study single-cell fluorescence. Na+ was distributed uniformly in cytoplasm and nucleus; mean Na+ concentration in resting cells was 4.4 +/- 0.3 mM in cytoplasmic areas ([Na+]cyt) and 4.5 +/- 0.4 mM in nuclear areas ([Na+]n). Na+ pump inhibition and cell activation evoked uniform changes in [Na+]cyt and [Na+]n. Inhibition of Na+ pump with 1 mM ouabain or K(+)-free medium caused a rise in [Na+]cyt; in the latter case, [Na+]cyt fell rapidly when external K+ was later restored. Exposure to Ca(2+)-free medium also caused [Na+]cyt to rise; this effect was augmented by Na+ pump inhibition and was reversed by 10(-5) M verapamil or nitrendipine or by restoration of external Ca2+. The implication is that this Na+ entry in absence of external Ca2+ is mediated by Ca2+ channels. Activation by 10(-9) M arginine vasopressin (AVP) and 10(-6) M serotonin (5-HT) caused [Na+]cyt to increase, but response to 5-HT was small (0.6 mM on average) and transient, whereas response to AVP was larger (2.4 mM on average) and was maintained as long as AVP was present (to 20 min). AVP and, to a much smaller extent, 5-HT stimulated Na+ influx; this could be detected when Na+ pump was inhibited by ouabain. Both AVP and 5-HT activated the Na+ pump, as detected by ouabain-sensitive decrease in [Na+]cyt when Na+ influx was inhibited. Agonist-evoked increases in [Na+]cyt were dependent on a rise in cytosolic Ca2+ concentration ([Ca2+]cyt); these [Na+]cyt responses were abolished by prolonged exposure to Ca(2+)-free media, when cytoplasmic Ca2+ was chelated with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or when Ca2+ mobilization was blocked with thapsigargin. Raising [Ca2+]cyt with 40 mM K+ or with thapsigargin did not increases in [Na+]cyt. We conclude that 1) AVP- and 5-HT-evoked increases in [Na+]cyt are agonist specific and depend on the balance between stimulated Na+ influx and efflux; 2) AVP and 5-HT activate the Na+ pump; this is, at least in part, independent of agonist-induced rise in [Na+]cyt; and 3) a rise in [Ca2+]cyt is necessary but not sufficient to trigger agonist-evoked rise in [Na+]i.

Thrombin and NaF, but not epinephrine, raise cytosolic free Na+ in human platelets

We have investigated changes in [Na+]i in SBFI-loaded platelets stimulated at 37 degrees C with thrombin, epinephrine, and NaF. Basal [Na+]i was 4.9 +/- 1.3 mM (n = 70). Stimulation of platelets with thrombin (0.1 U/ml) in the presence of 1 mM extracellular Ca2+ rapidly raised [Na+]i by 27.3 +/- 6 mM (n = 16). Part of this increase (approx. 20-30%) is caused by Na+/H+ exchange, the rest is predominantly due to Na+ influx. Epinephrine (20 microM) failed to change [Na+]i both in the absence and presence of fibrinogen. This is in agreement with earlier reports showing that epinephrine also fails to activate Na+/H+ exchange in human platelets. NaF which activates platelets via a direct effect on GTP-binding proteins induced a slow rise in [Na+]i to 9.5 +/- 2.5 mM (n = 4) and 33.0 +/- 3.6 mM (n = 12) at 10 and 20 mM NaF, respectively. This effect was completely blocked by SK&F 96365, a blocker of receptor-mediated Ca2+ entry. Hence, the NaF-induced increase in [Na+]i is exclusively due to the opening of non-selective cation channels. This latter finding agrees with earlier observations which showed that NaF does not induce activation of Na+/H+ exchange in platelets.

Intracellular free sodium concentrations in GH4C1 cells

In the present investigation, intracellular sodium ([Na+]i) levels were determined in GH4C1 cells using the fluorescent probe SBFI. Fluorescence was determined by excitation at 340 nm and 385 nm, and emission was measured at 500 nm. Intracellular free sodium ([Na+]i) was determined by comparing the ratio 340/385 to a calibration curve. The ratio was linear between 10 and 60 mM Na+. Resting [Na+]i in GH4C1 cells was 26 +/- 6.2 mM (mean +/- SD). In cells incubated in Na(+)-free buffer [Na+]i decreased to 3 +/- 3.6 mM. If Na+/K+ ATPase was inhibited by incubating the cells with 1 mM ouabain, [Na+]i increased to 47 +/- 12.8 mM in 15 min. Stimulating the cells with TRH, phorbol myristyl acetate, or thapsigargin had no effect on [Na+]i. Incubating the cells in Ca(2+)-free buffer rapidly increased [Na+]i. The increase was not inhibited by tetrodotoxin. Addition of extracellular Ca2+, nimodipine, or Ni2+ to these cells immediately decreased [Na+]i, whereas Bay K 8644 enhanced the influx of Na+. In cells where [Na+]i was increased the TRH-induced increase in intracellular free calcium ([Ca2+]i) was decreased compared with control cells. Our results suggest that Na+ enters the cells via Ca2+ channels, and [Na+]i may attenuate TRH-induced changes in [Ca2+]i in GH4C1 cells.

Regulation of the dynamic properties of platelet plasma membrane by intracellular sodium ions

Our previous experiments in human and rat platelets demonstrated that the absence of extracellular Na+ increased the fluorescence anisotropy of TMA-DPH (trimethylamino-diphenylhexatriene, probe preferentially incorporated into the outer leaflet of the plasma membrane). Here we investigated further the in vitro effects of Na+ ions on membrane dynamic properties. Na(+)-dependent changes were reversible and they required about 10-20 min to be induced. They were specifically located in the TMA-DPH environment because they were not observed with diphenylhexatriene (probe non-selectively incorporated into all hydrophobic domains of the cell). To evaluate the possible influence of the intracellular Na+, the effects of sodium replenishment, monensin, ouabain and thrombin on TMA-DPH anisotropy were measured. A rise in intracellular Na+ above the physiological level was associated with unchanged or slightly decreased TMA-DPH anisotropy whereas its decrease was accompanied by a pronounced rise in TMA-DPH anisotropy. Our data indicate that the changes in intracellular Na+ concentration, rather than those in extracellular Na+ concentration, are responsible for the alterations in platelet membrane fluidity probed by TMA-DPH.

Lack of involvement of [Ca2+]i in the external Ca(2+)-independent release of acetylcholine evoked by veratridine, ouabain and alpha-latrotoxin: possible role of [Na+]i

Synaptosomes were challenged by veratridine, ouabain and alpha-latrotoxin, and the release of 14C-acetylcholine (ACh) was measured in the absence of external Ca2+. We wished to test whether Ca2+ mobilized from internal stores triggered the ACh release that was independent of external Ca2+. We found that none of the agents altered the [Ca2+]i in a Ca(2+)-free medium. Buffering the intracellular Ca2+ concentration with BAPTA did not prevent the increase in release of 14C-ACh by veratridine or ouabain in the absence of Ca2+, however, it greatly reduced the release evoked in a Ca(2+)-containing medium. In parallel samples the release of ACh and the change in the internal Na+ concentration ([Na+]i) were measured. It was found that veratridine, ouabain and alpha-latrotoxin all enhanced [Na+]i in a concentration-dependent manner and a good quantitative relationship existed between the increase in [Na+]i and the release of ACh.

Effect of spironolactone on cytosolic free sodium concentration in platelets from hypertensive patients with primary aldosteronism

Cytosolic free sodium concentration ([Na+]i) was investigated in intact platelets from 5 hypertensive patients with primary aldosteronism (unilateral adenoma in 3 patients, and adrenal hyperplasia in 2 patients) and 21 normotensive control subjects. [Na+]i was measured using a novel sodium-sensitive fluorescent dye technique. [Na+]i was significantly decreased in platelets from patients with primary aldosteronism compared to control subjects (21.9 +/- 4.1 mM vs 35.8 +/- 2.2 mM, mean +/- SEM, P < 0.05). After administration of the mineralocorticoid antagonist spironolactone in 4 patients [Na+]i tended to be higher in platelets although the differences did not reach statistical significance (26.3 +/- 7.2 mM vs 18.2 +/- 2.4 mM, P = 0.125). From the present results it may be concluded that intracellular sodium is decreased by aldosterone-induced activation of Na-K-ATPase. That activation may be partly blocked by spironolactone.

Na+ influx is mediated by Na(+)-K(+)-2Cl- cotransport and Na(+)-H+ exchange in sublingual mucous acini

The intracellular free Na+ concentration ([Na+]i) was studied using dual-wavelength microfluorometry of the fluorescent Na+ indicator sodium-binding benzofuran isophthalate (SBFI) to determine the mechanism(s) by which muscarinic stimulation increases the Na+ content in rat sublingual mucous acini. [Na+]i was 15.5 +/- 0.7 mM in acini superfused with a Na(+)-containing medium (135 mM Na+). Application of ouabain, a Na(+)-K(+)-adenosinetriphosphatase inhibitor, resulted in an increase in [Na+]i (approximately 75% in 10 min), whereas replacement of extracellular Na+ with N-methyl-D-glucamine induced a gradual decrease in [Na+]i (approximately 55% decrease in 5 min). The recovery of [Na+]i in Na(+)-depleted acini was K+ and Cl- dependent and was inhibited by bumetanide (Bum), a specific Na(+)-K(+)-2Cl- cotransport inhibitor, and by 5-(N-methyl-N-isobutyl)amiloride (MIBA), an amiloride derivative that specifically blocks Na(+)-H+ exchange. Stimulation with a muscarinic agonist (10 microM carbachol) resulted in a dramatic increase in the [Na+]i [approximately 180%, half time (t1/2) approximately 1 min] and a net increase in Na+ content, as measured with 22Na+ (approximately 110%, t1/2 approximately 1 min). Both the initial rate of the increase in [Na+]i and the magnitude of the net increase in Na+ content were dramatically blunted by Bum and MIBA. Increasing the intracellular free Ca2+ concentration ([Ca2+]i) with ionomycin, a Ca2+ ionophore, resulted in an increase in [Na+]i. Preventing the [Ca2+]i increase by chelating cytosolic Ca2+ with bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid completely abolished the agonist-induced evaluation in [Na+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intracellular ions on interleukin-1 beta production by lipopolysaccharide-activated human monocytes

Following the observation that interleukin 1 beta (IL-1 beta) production in lipopolysaccharide (LPS)activated monocytes increases in concert with a rise in intracellular pH (pHi), the role of ion transport in IL-1 beta production was investigated. The amiloride analogue 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an inhibitor of the Na(+)-H+ antiporter, inhibited extracellular IL-1 beta. The replacement of Na+ in the culture medium with sucrose or choline chloride also prevented monocyte activation. The sodium ionophore monensin, in doses from 100 pM to 1 microM, potentiated LPS-stimulated extracellular IL-1 beta when compared with LPS alone. In the absence of LPS activation, monensin by itself at 10 nM stimulated IL-1 beta production to 63%. EIPA at 10 microM inhibited the Na+ influx, the rise in pHi, and intra- and extracellular IL-1 beta production in activated monocytes; this inhibition was reversed by 10 nM monensin. In the absence of bicarbonate, or in the presence of 10 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, the pHi of activated monocytes and the total protein synthesis did not change, but the production of IL-1 beta was inhibited. The data suggest that the stimulated influx of Na+ via the Na(+)-H+ antiporter regulates both pHi and IL-1 beta production in LPS-activated monocytes. The requirement for bicarbonate indicates an additional mechanism(s), separate from the modulation of pHi and intracellular Na+.

Heat shock increases cytosolic free Ca2+ concentration via Na(+)-Ca2+ exchange in human epidermoid A 431 cells

This study characterized cytosolic free Ca2+ concentration ([Ca2+]i) in normal and thermally injured human epidermoid A 431 cells. The resting [Ca2+]i in normal cells at 37 degrees C was 87 +/- 5 nM (n = 105). When cells were subjected to hyperthermia (40-50 degrees C), [Ca2+]i increased in a temperature- and time-dependent manner. The maximal increase in cells exposed to 45 degrees C was observed at 20 min; [Ca2+]i returned to normal within 1 h. The heat-induced [Ca2+]i increase depended on the presence of external Ca2+. La3+ and Cd2+ but not Co2+, verapamil, or nifedipine attenuated the heat-induced [Ca2+]i increase. TMB-8 partially blocked the increase in [Ca2+]i but pertussis toxin and cholera toxin pretreatment did not. The magnitude of the heat-induced [Ca2+]i increase or 45Ca2+ uptake depended on the presence of extracellular Na+. Heat treatment reduced the apparent Michaelis constant for external Ca2+ from 490 +/- 91 to 210 +/- 60 microM, whereas the maximal velocity remained the same. The intracellular Na+ concentration decreased 62.5% after heating. The heat-induced [Ca2+]i increase was completely blocked by amiloride (5 microM) and 5'-(N,N-dimethyl)-amiloride (1 microM). These results suggest heat activates the Na(+)-Ca2+ exchange system so as to increase [Ca2+]i and reduce [Na+]i.

Determination of the intravesicular ionized sodium concentration in a cell-free brain membrane vesicle preparation using the fluorescent indicator, SBFI

The intravesicular ionized Na concentration (Nai) was measured using the fluorescent Na indicator, SBFI, in microsacs, a cell-free brain vesicle preparation. SBFI fluorescence was monitored by a dual excitation-wavelength method at the same wavelengths commonly employed for Fura-2 determination of intracellular ionized calcium concentrations (Cai). Calibration of SBFI fluorescence was reliably performed in brain microsacs in situ. Resting Nai was dependent on the extravesicular Na concentration (Nao) and was about 36 mM in the presence of 120 mM extracellular Nao. In the presence of ouabain, an inhibitor of the plasma membrane Na/K-ATPase, Nai increased by 27 mM over 60 s. Nai was also increased by resuspension of microsacs in buffers of low free Ca concentrations (0 to 0.8 mM), indicating that the extravesicular Ca concentration (Cao) is an important regulator of Nai. Alkaloids active at voltage-sensitive Na channels, veratridine and aconitine, also increased Nai. These results demonstrate the presence of homeostatic mechanisms for neuronal Nai regulation and show that Nai can be measured in a cell-free brain vesicle preparation using SBFI.