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1.
Perception of salt stress in plant cells induces a change in the free cytosolic Ca2+, [Ca2+]cyt, which transfers downstream reactions toward salt tolerance. Changes in cytosolic H+ concentration, [H+]cyt, are closely linked to the [Ca2+]cyt dynamics under various stress signals. In this study, salt‐induced changes in [Ca2+]cyt, and [H+]cyt and vacuolar [H+] concentrations were monitored in single protoplasts of rice (Oryza sativa L. indica cvs. Pokkali and BRRI Dhan29) by fluorescence microscopy. Changes in cytosolic [Ca2+] and [H+] were detected by use of the fluorescent dyes acetoxy methyl ester of calcium‐binding benzofuran and acetoxy methyl ester of 2′, 7′‐bis‐(2‐carboxyethyl)‐5‐(and‐6) carboxyfluorescein, respectively, and for vacuolar pH, fluorescent 6‐carboxyfluorescein and confocal microscopy were used. Addition of NaCl induced a higher increase in [Ca2+]cyt in the salt‐tolerant cv. Pokkali than in the salt‐sensitive cv. BRRI Dhan29. From inhibitor studies, we conclude that the internal stores appear to be the major source for [Ca2+]cyt increase in Pokkali, although the apoplast is more important in BRRI Dhan29. The [Ca2+]cyt measurements in rice also suggest that Na+ should be sensed inside the cytosol, before any increase in [Ca2+]cyt occurs. Moreover, our results with individual mesophyll protoplasts suggest that ionic stress causes an increase in [Ca2+]cyt and that osmotic stress sharply decreases [Ca2+]cyt in rice. The [pH]cyt was differently shifted in the two rice cultivars in response to salt stress and may be coupled to different activities of the H+‐ATPases. The changes in vacuolar pH were correlated with the expressional analysis of rice vacuolar H+‐ATPase in these two rice cultivars.  相似文献   

2.
Summary Using intracellular microelectrode technique, we investigated the changes in membrane voltage (V) of cultured bovine pigmented ciliary epithelial cells induced by different extracellular solutions. (1)V in 213 cells under steady-state conditions averaged –46.1±0.6 mV (sem). (2) Increasing extracellular K+ concentration ([K+] o ) depolarizedV. Addition of Ba2+ could diminish this response. (3) Depolarization on doubling [K+] o was increased at higher [K+] o (or low voltage). (4) Removing extracellular Ca2+ decreasedV and reduced theV amplitude on increasing [K+] o . (5)V was pH sensitive. Extra-and intracellular acidification depolarizedV; alkalinization induced a hyperpolarization.V responses to high [K+] o were reduced at acidic extracellular pH. (6) Removing K o + depolarized, K o + readdition after K+ depletion transiently hyperpolarizedV. These responses were insensitive to Ba2+ but were abolished in the presence of ouabain or in Na+-free medium. (7) Na+ readdition after Na+ depletion transiently hyperpolarizedV. This reaction was markedly reduced in the presence of ouabain or in K+-free solution but unchanged by Ba2+. It is concluded that in cultured bovine pigmented ciliary epithelial cells K+ conductance depends on Ca2+, pH and [K+] o (or voltage). An electrogenic Na+/K+-transport is present, which is stimulated during recovery from K+ or Na+ depletion. This transport is inhibited by ouabain and in K+-or Na+-free medium.  相似文献   

3.
Salinity causes changes in cytosolic Ca2+, [Ca2+]cyt, Na+, [Na+]cyt and pH, pHcyt, which induce specific reactions and signals. Reactions causing a rebalancing of the physiological homeostasis of the cytosol could result in plant resistance and growth. Two wheat cultivars, Triticum aestivum, Seds1 and Vinjett, were grown in nutrient solution for 7 days under moderate salinity (0 and 50 mM NaCl) with and without extra addition of 5 mM CaSO4 to investigate the seedling‐ion homeostasis under salinity. In the leaf protoplasts [Ca2+]cyt, [Na+]cyt and pHcyt were detected using acetoxymethyl esters of the ion‐specific dyes, Fura 2, SBFI and BCECF, respectively, and fluorescence microscopy. In addition, both cultivars were grown for 3 weeks at 0, 50 and 125 mM NaCl with, or without, extra addition of 5 mM CaSO4 to detect overall Na+ and Ca2+ concentrations in leaves and salinity effects on dry weights. In both cultivars, salinity decreased [Ca2+]cyt, while at extra Ca2+ supplied, [Ca2+]cyt increased. The [Ca2+]cyt increase was accompanied by increase in the overall Ca2+ concentrations in leaves and decrease in the overall Na+ concentration. Moreover, irrespective of Ca2+ treatment under salinity, the cultivars reacted in different ways; [Na+]cyt significantly increased only in cv. Vinjett, while pHcyt increased only in cv. Seds1. Even at rather high total Na+ concentrations, the cytosolic concentrations were kept low in both cultivars. It is discussed whether the increase of [Ca2+]cyt and pHcyt can contribute to salt tolerance and if the cytosolic changes are due to changes in overall Ca2+ and Na+ concentrations.  相似文献   

4.
Activation of Na+-H+ exchange in rat thymocytes was found to be followed by an increase in free cytoplasmic Ca2+ concentration ([Ca2+]i). We determined whether the change in [Ca2+]i was secondary to the uptake of Na+, or to the cytoplasmic alkalinization that result from activation of the antiport. Increasing intracellular [Na+] by treating the cells with ouabain or gramicidin failed to affect [Ca2+]i. In contrast, procedures that increased the cytoplasmic pH, such as addition of monensin or NH3, significantly elevated [Ca2+]i. These results suggest an important role of cytoplasmic pH in the control of [Ca2+]i in lymphocytes.  相似文献   

5.
In ovarian follicles of Drosophila melanogaster, ion substitution experiments revealed that K+ is the greatest contributor (68%) in setting oocyte steady‐state potential (Em), while Mg2+ and a metabolic component account for the rest. Because of the intense use made of Drosophila ovarian follicles in many lines of research, it is important to know how changes in the surrounding medium, particularly in major diffusible ions, may affect the physiology of the cells. The contributions made to the Drosophila oocyte membrane potential (Em) by [Na+]o, [K+]o, [Mg2+]o, [Ca2+]o, [Cl?]o, and pH (protons) were determined by substitutions made to the composition of the incubation medium. Only K+ and Mg2+ were found to participate in setting the level of Em. In follicles subjected to changes in external pH from the normal 7.3 to either pH 6 or pH 8, Em changed rapidly by about 6 mV, but within 8 min had returned to the original Em. Approximately half of all follicles exposed to reduced [Cl?]o showed no change in Em, and these all had input resistances of 330 kΩ or greater. The remaining follicles had smaller input resistances, and these first depolarized by about 5 mV. Over several minutes, their input resistances increased and they repolarized to a value more electronegative than their value prior to reduction in [Cl?]o. Together, K+ and Mg2+ accounted for up to 87% of measured steady‐state potential. Treatment with sodium azide, ammonium vanadate, or chilling revealed a metabolically driven component that could account for the remaining 13%. © 2009 Wiley Periodicals, Inc.  相似文献   

6.
Several aspects of Mg2+ homeostasis were investigated in cultured chicken heart cells using the fluorescent Mg2+ indicator, FURAPTRA. The concentration of cytosolic Mg2+ ([Mg2+]i) is 0.48 ± 0.03 mM (n = 31). To test whether a putative Na/Mg exchange mechanism controls [Mg2+]i below electrochemical equilibrium, we manipulated the Na+ gradient and assessed the effects on [Mg2+]i. When extracellular Na+ was removed, [Mg2+]i increased; this increase was not altered in Mg-free solutions, but was attenuated in Ca-free solutions. A similar increase in [Mg2+]i, which was dependent upon extracellular Ca2+, was observed when intracellular Na+ was raised by inhibiting the Na/K pump with ouabain. These results do not provide evidence for Na/Mg exchange in heart cells, but they suggest that Ca2+ can modulate [Mg2+]i. In addition, removing extracellular Na+ caused a decrease in intracellular pH (pHi), as measured by pH-sensitive microelectrodes, and this acidification was attenuated when Cat+ was also removed from the solution. These results suggest that Ca2+ and H+ interact intracellularly. Since changes in the Na+ gradient can also alter pHi, we questioned whether pH can modulate [Mg2+]i. pHi was manipulated by the NH4Cl prepulse method. NH4 +-evoked changes in pHi, as measured by the fluorescent indicator BCECF, were accompanied by opposite changes in [Mg2+]i; [Mg2+]i changed by –0.16 mM/unit pH. These NH4 +-evoked changes in [Mg2+]i were not caused by movements of Mg2+ or Ca2+ across the sarcolemma or by changes in cytosolic Ca2+. Additionally, pHi was manipulated by changing extracellular pH (pHo). When pHo was decreased from 7.4 to 6.3, pHi decreased by 0.64 units and [Mg2+]i increased by 0.12 mM; in contrast, when pHo was raised from 7.4 to 8.3, pHi increased by 0.6 units and [Mg2+]i did not change significantly. The results of our investigations suggest that Ca 2+ and H+ can modulate [Mg2+]i, probably by affecting cytosolic Mg2+ binding and/or subcellular Mg2+ transport and that such redistribution of intracellular Mg2+ may play an important role in Mg2+ homeostasis in cardiac cells.  相似文献   

7.
Summary Mn2+ binding to phosphatidylserine (PS) vesicles was measured by EPR as a function of [Na+] and pH. At nearly physiological monovalent salt concentration the apparent Mn2+ affinity (K a) increased monitonically over the pH range 5.7–8.35, withK a roughly [H+]–1 above pH 7.3. It was found, moreover, thatK a fell off more rapidly with added NaCl at pH 6.1 than at pH 7.87. Qualitatively, these results are consistent with two types of Mn2+-PS binding: (i) simple adsorption and (ii) adsorption with the release of an amino proton from PS. The existence of Mn2+-induced H+ displacement from PS was verified through titration measurements, employing a pH electrode.When H+ displacement is taken into account, the variation inK a with [Na+] observed at pH 6.1 is found to be in reasonably good agreement with that expected from the Gouy-Chapman-Stern theory of ionic binding to charged surfaces.  相似文献   

8.
Summary Sulphate uptake by rabbit ileal brush border membrane vesicles was stimulated by a transmembrane sodium gradient ([Na+] o >[Na+] i ), but not by a similar potassium gradient.35SO 4 2– influx (J oi SO4 ) from outside (o) to inside (i) these vesicles was a hyperbolic function of [SO 4 2– ] o and the affinity constant for anion transport was strongly influenced by [Na+] o (100mm Na+,K t SO4 =0.52mm SO 4 2– ; 10mm Na+,K t SO4 =4.32mm SO 4 2– ).J t SO4 was a sigmoidal function of [Na+] o at pH 7.4 for both low (0.2m) and high (4.0mm) [SO 4 2– ] o . The Na+-dependency ofJ t SO4 was examined at pH 6.0, 7.4, and 8.0 (same pH inside and outside). At pH 6.0 and 7.4 sigmoidal Na+-dependentJ t SO4 exhibited nonlinear Eadie-Hofstee plots indicative of a transport mechanism capable of binding a variable number of sodium ions over the [Na+] o range used. Hill plots of anion transport under these conditions displayed slopes near unity at low [Na+] o and slopes approximating 2.0 at higher cation concentrations. At pH 8.0, Na+-dependentJ t SO4 was hyperbolic and showed linear Eadie-Hofstee and Hill plots, the latter with a single slope near 1.0. When a H+ gradient was imposed across the vesicle wall (pH i =8.0, pH o =6.0), Na+-dependentJ t SO4 was hyperbolic and significantly increased at each [Na+] o over values observed using bilateral pH 8.0. In contrast, a H+ gradient oriented in the opposite direction (pH i =6.0, pH o =8.0) led to Na+-dependentJ t SO4 that was sigmoidal and significantly lower at each [Na+] o than values found using bilateral pH 6.0. Electrogenicity ofJ t SO4 at pH 8.0 for both high and low [Na+] o was demonstrated by using a valinomycin-induced transmembrane electrical potential difference. At pH 6.0, electrogenicJ t SO4 occurred only at low [Na+] o (5mm); anion transfer was electroneutral at 50mm Na+. A model is proposed for proton regulation of sodium sulphate cotransport where flux stoichiometry is controlled by [H+] i and sodium binding affinity is modified by [H+] o . Preliminary experiments with rabbit proximal tubular brush border membrane vesicles disclosed similarJ t SO4 kinetic properties and a common transport mechanism may occur in both tissues.  相似文献   

9.
Abstract: The effect of replacement of extracellular Na+ with N-methyl-d -glucamine (NMG) on P2 receptor signaling pathways was investigated in dibutyryl cyclic AMP-differentiated NG108-15 cells. Benzoylbenzoic ATP (BzATP) dose-dependently increased the cytosolic Ca2+ concentration ([Ca2+]i) with an EC50 value of 230 µM. Replacement of Na+ with NMG as well as removal of Mg2+ from the bathing buffer potentiated ethidium bromide uptake, [Ca2+]i increase, and 45Ca2+ uptake in response to ATP or BzATP. In contrast, in the presence of 5 mM Mg2+ to limit the amount of ATP4?, replacement of Na+ with NMG had no effect on the ATP-induced [Ca2+]i increase but caused a markedly larger [Ca2+]i increase when the calculated concentration of ATP4? was >10 µM. The calculated EC50 value for ATP4? stimulation of the [Ca2+]i increase was 23 µM in NG108-15 cells. In vascular smooth muscle cells, intracellular Ca2+ release was the major pathway for the ATP-induced [Ca2+]i increase; both removal of Mg2+ and replacement of Na+ with NMG did not affect the action of ATP. These data suggest that ATP4?-promoted pores are antagonized by Na+ and Mg2+ in dibutyryl cyclic AMP-differentiated NG108-15 cells.  相似文献   

10.
The stress conditions of salt-alkalinized soil were simulated to investigate the features and acting factors of salt-alkaline mixed stress, using a natural salt-alkaline tolerant grass Aneurolepidium chinense (Trin.) Kitag. According to the features of salt-alkalinized soil in the northeast of China, various salt-alkali conditions with different salinities and pHs were established by mixing NaCl, NaHCO3, Na2SO4, and Na2CO3, in various proportions. The treatments included a salt concentration range of 50 to 350 mM and pH values from 7.14 to 10.81. Seedlings of A. chinense were stressed under these salt-alkali conditions. Several physiological indices of seedling stress were determined, including survival rate, tillering rate, number of rhizomes, relative growth rate (RGR), proline content, electrolyte leakage rate, and Na+ and K+ content, in order to analyze the characteristics of the stresses due to the salt-alkali mixes and their main stress factors.The results showed that the survival rate, tillering rate, number of rhizomes, RGR, and K+ content of A. chinense decreased with increasing salinity and pH (or alkalinity). Proline and Na+ content and electrolyte leakage rate increased with increasing salinity and pH (or alkalinity). The deleterious effects of a high pH value or salinity alone were significantly less than those of high pH in combination with salinity. This result suggested that for a salt-alkaline mixed stress, a reciprocal enhancement between salt stress and alkali stress was a characteristic feature, and it was most evidently reflected in the survival rate. When salinity was below 125 mM or pH was below 8.8, survival rates were all 100%. However, when salinity was above 125 mM and pH was above 8.8, survival rates sharply declined with the increasing of either salinity or pH.The buffer capacity of the treatment solution was taken as a stress factor in order to simplify the stress factor analysis. The results of the statistical analysis showed that for the stress factors of the salt-alkaline mixed stress, [CO32–] and [HCO3] could be fully represented by the buffer capacity, and [Na+] could be fully represented by salinity, whereas [SO42–] was negligible. Therefore, four factors, salinity, buffer capacity, pH and [Cl], could reflect all of the stress factors. Perfect linear correlations were observed between all physiological indices and four or three stress factors by a stepwise regression analysis. However, the effects of the four stress factors on the physiological indices were significantly different in magnitude. Buffer capacity and salinity were dominant factors for all physiological indices. Thus, it is reasonable to consider the sum of salinity plus buffer capacity as the strength value of salt-alkaline mixed stress. Furthermore, the relationships between different physiological indices and various stress factors were shown to be different.  相似文献   

11.
MthK is a Ca2+-gated K+ channel whose activity is inhibited by cytoplasmic H+. To determine possible mechanisms underlying the channel’s proton sensitivity and the relation between H+ inhibition and Ca2+-dependent gating, we recorded current through MthK channels incorporated into planar lipid bilayers. Each bilayer recording was obtained at up to six different [Ca2+] (ranging from nominally 0 to 30 mM) at a given [H+], in which the solutions bathing the cytoplasmic side of the channels were changed via a perfusion system to ensure complete solution exchanges. We observed a steep relation between [Ca2+] and open probability (Po), with a mean Hill coefficient (nH) of 9.9 ± 0.9. Neither the maximal Po (0.93 ± 0.005) nor nH changed significantly as a function of [H+] over pH ranging from 6.5 to 9.0. In addition, MthK channel activation in the nominal absence of Ca2+ was not H+ sensitive over pH ranging from 7.3 to 9.0. However, increasing [H+] raised the EC50 for Ca2+ activation by ∼4.7-fold per tenfold increase in [H+], displaying a linear relation between log(EC50) and log([H+]) (i.e., pH) over pH ranging from 6.5 to 9.0. Collectively, these results suggest that H+ binding does not directly modulate either the channel’s closed–open equilibrium or the allosteric coupling between Ca2+ binding and channel opening. We can account for the Ca2+ activation and proton sensitivity of MthK gating quantitatively by assuming that Ca2+ allosterically activates MthK, whereas H+ opposes activation by destabilizing the binding of Ca2+.  相似文献   

12.
The influence of aging and dietary restriction on increase in intracellular free calcium ([Ca2+]i) of CD4+ lymphocytes from Macaca mulatta was examined after stimulation with anti-CD3 mAb. We used a flow cytometric assay with the dye indo-1 and either direct or reciprocal immunofluorescent staining to dientify CD4+ cells. After stimulation with anti-CD3 mAb, intracellular free calcium responses were reduced in CD4+ lymphocytes from old male and female ad libitum fed monkeys compared to young and adult male or female monkeys. Old female monkeys had significantly lower [Ca2+]i than did old male monkeys. The reduced responses were in part related to a decreased percentage of responding cells. Dietary restrition of males over a four-year period did not alter [Ca2+]i response compared to ad libitum fed male monkeys. Female monkeys of all ages (which were restricted only for four months) also had similar [Ca2+]i responses to ad libitum fed controls. Our data suggest that age-related changes in [Ca2+]i responses are similar between humans and M. mulatta, and that over these intervals, no effects of caloric restrictions can be detected. © 1995 Wiley-Liss, Inc.  相似文献   

13.
Sergio de la Fuente 《BBA》2010,1797(10):1727-1735
We have investigated the kinetics of mitochondrial Ca2+ influx and efflux and their dependence on cytosolic [Ca2+] and [Na+] using low-Ca2+-affinity aequorin. The rate of Ca2+ release from mitochondria increased linearly with mitochondrial [Ca2+] ([Ca2+]M). Na+-dependent Ca2+ release was predominant al low [Ca2+]M but saturated at [Ca2+]M around 400 μM, while Na+-independent Ca2+ release was very slow at [Ca2+]M below 200 μM, and then increased at higher [Ca2+]M, perhaps through the opening of a new pathway. Half-maximal activation of Na+-dependent Ca2+ release occurred at 5-10 mM [Na+], within the physiological range of cytosolic [Na+]. Ca2+ entry rates were comparable in size to Ca2+ exit rates at cytosolic [Ca2+] ([Ca2+]c) below 7 μM, but the rate of uptake was dramatically accelerated at higher [Ca2+]c. As a consequence, the presence of [Na+] considerably reduced the rate of [Ca2+]M increase at [Ca2+]c below 7 μM, but its effect was hardly appreciable at 10 μM [Ca2+]c. Exit rates were more dependent on the temperature than uptake rates, thus making the [Ca2+]M transients to be much more prolonged at lower temperature. Our kinetic data suggest that mitochondria have little high affinity Ca2+ buffering, and comparison of our results with data on total mitochondrial Ca2+ fluxes indicate that the mitochondrial Ca2+ bound/Ca2+ free ratio is around 10- to 100-fold for most of the observed [Ca2+]M range and suggest that massive phosphate precipitation can only occur when [Ca2+]M reaches the millimolar range.  相似文献   

14.
Abstract— ATP-induced changes in the intracellular Ca2+concentration ([Ca2+]i) in neuroblastoma glioma hybrid NG108–15 cells were studied. Using the fluorescent Ca2+indicator fura-2, we have shown that the [Ca2+]i increased in response to ATP. ATP at 3 mM caused the greatest increase in [Caz+]i, whereas at higher concentrations of ATP the response became smaller. Two nonhydrolyzable ATP analogues, adenosine 5′-thiotriphosphate and 5′-adenylyl-β, γ-imidodiphosphate, could not trigger significant [Ca2+]i change, but they could block the ATP effect. Other adenine nucleotides, including ADP, AMP, α,β-methylene-ATP, β,γ-methylene-ATP, and 2-methylthio-ATP, as well as UTP and adenosine, all had no effect on [Ca2+]i at 3 mM. In the absence of extracellular Ca2+, the effect of ATP was inhibited totally, but could be restored by the addition of Ca2+ to the cells. Upon removal of Mg2+, the maximum increase in [Ca2+]i induced by ATP was enhanced by about 42%. Ca2+-channel blockers partially inhibited the ATP-induced [Ca2+]i rise. The ATP-induced [Ca2+]i rise was not affected by thapsigargin pretreatment, though such pretreatment blocked bradykinin-induced [Ca2+]i rise completely. No heterologous desensitization of [Ca2+]i rise was observed between ATP and bradykinin. The magnitude of the [Ca2+]i rise induced by ATP increased between 1.5 and 3.1 times when external Na+was replaced with Tris, N-methyl-d -glucamine, choline, or Li+. The addition of EGTA or verapamil to cells after their maximum response to ATP immediately lowered the [Ca2+]i to the basal level in Na+-containing or Na+-free Tris solution. Our results suggest that ATP stimulates Ca2+influx via at least two pathways: ion channels that are permeable to Ca2+ and Na+, and pores formed by ATP4-.  相似文献   

15.
Abstract: Synaptosomes can be loaded with mag-fura-2 without significant perturbation of their ATP content by incubation for 10 min at 37°C with 10 µM mag-fura-2 acetoxymethyl ester in Hanks'-HEPES buffer (pH 7.45). The intrasynaptosomal free Mg2+ concentration ([Mg2+]i) was found to be dependent on external Mg2+ concentration, increasing from 0.8 to 1.25 mM when the concentration of Mg2+ in the incubation medium increased from 1 to 8 mM. Dissipation of the Na+ gradient across the plasma membrane of synaptosomes by treatment with the Na+ ionophore monensin (0.2 mM) or with veratridine (0.2 mM) and ouabain (0.6 mM) produced a moderate increase of [Mg2+]i, from 1.0 to 1.2–1.3 mM in an incubation medium containing 5 mM Mg2+. Plasma membrane depolarization by incubation of synaptosomes in a medium containing 68 mM KCl and 68 mM NaCl had no effect on [Mg2+]i. Reversal of the Na+ gradient by incubation of synaptosomes in a medium in which external Na+ was replaced by choline increased [Mg2+]i up to 1.6 and 2.2 mM for extrasynaptosomal Mg2+ concentrations of 1 and 8 mM, respectively. We conclude that a Na+/Mg2+ exchange operates in the plasma membrane of synaptosomes. In the presence of Mg2+ in the incubation medium, extrasynaptosomal ATP, but not ADP or adenosine, increased [Mg2+]i from 1.1 ± 0.1 up to 1.6 ± 0.1 mM. The nonhydrolyzable ATP analogue adenosine 5′-(βγ-imido)triphosphate antagonized the effect of ATP, but had no effect by itself on [Mg2+]i. It is concluded that Mg2+ transport across the plasma membrane of synaptosomes is modulated by the activity of an ecto-ATPase or an ecto-protein kinase.  相似文献   

16.
The uranium(IV) complexes [U(EDTA)(H2O)2], [U(HOEDTA)]+, and [U(DTPA)]? are well-formed in the pH fange 2–3 ([DTPA]5- = diethylenetriaminepentaacetate; [HOEDTA]3-  N-(2-hydroxyethyl)ethylenediaminetriacetate). Of these, only [U(DTPA)]- is extracted from an aqueous phase at pH 2 by the perchlorate salt of the primary amine, Primene JM-T. As the aqueous phase pH was raised, extraction occurred in all three cases and hydrolysed species may be extracted from EDTA and HOEDTA solutions but [U(DTPA)]? resists hydrolysis. The addition of sulphate had a marked effect on the extraction of U(IV) from EDTA and HOEDTA through the formation of [U(EDTA)(SO4)(H2O)]2- and [U(HOEDTA)(SO4)(H2O)n]?. The equilibrium constant, log β1, for: [(U(EDTA)(H2O)2] 2 [SO4]2? ? [U(EDTA)(SO4)(H2O)]2- 2 H2O was found to be 2.43 ± 0.04 (I = 1 mol dm?3, NaClO4; pH 2.0; 20 °C) from spectrophotometric data.With tri-n-octylphosphine oxide (TOPO) electronic spectroscopy showed that the same U(IV) complex was extracted at pH 2 for Cs2UCl6, U(IV)/ HOEDTA, and U(IV)/DTPA and the aminepoly- carboxylates were aqueous phase masking agents but with [U(EDTA)(H2O)2] oxidation gave a uranyl(VI) organic phase species.Uranium(IV) is strongly extracted from aqueous solutions of HOEDTA at pH 2 or 3 by bis(2-ethyl- hexyl)phosphoric acid (HBEHP) but less so from EDTA and DTPA. Since U(IV) is completely extracted from Cs2UCl6 it could be that the amine- polycarboxylates were aqueous phase masking agents although spectral evidence did not support this.  相似文献   

17.
Summary The effect of lowering intracellular pH on the membrane potential (E m ) of rat thymic lymphocytes was studied using the potential-sensitive dyebis-oxonol. Cells were acid loaded by addition of the electroneutral K+/H+ exchanging ionophore nigericin. Acidification to pH 6.3 in Na+-free solution resulted in a biphasic change inE m : an early transient hyperpolarization followed by a sustained depolarization. These changes were associated with a rise in cytosolic free Ca2+ ([Ca2+] i ). The hyperpolarization was eliminated when the change in [Ca2+] i was prevented using BAPTA, an intracellular Ca2+ chelator. Moreover, a similar hyperpolarization was elicited by elevation of [Ca2+] i at physiological pH i using ionomycin, suggesting involvement of Ca2+-activated K+ channels. In contrast, the depolarization phase could not be mimicked by raising [Ca2+] i with ionomycin. However, intracellular BAPTA effectively inhibited the acidificationinduced depolarization. Inhibition was also obtained by extracellular addition of EGTA or dithiothreitol, even when the external free Ca2+ concentration remained unaltered. These observations suggested a possible role of contaminating trace metals. Cytosolic acidification is envisaged to induce intracellular accumulation of one or more trace metals, which induces the observed changes inE m . Accordingly, similar changes inE m can be induced without acidification by the addition of small amounts of Cu2+ to the medium. The ionic basis of theE m changes induced by acidification and the significance of these observations are discussed.  相似文献   

18.
Summary We and others have shown previously that ATP is secreted from mouse brain synaptosomes following depolarization of the membrane by high [K+] 0 and the time course can be monitored accurately by measuring the light emitted from luciferin-luciferase included in the reaction medium. In the present work we have evaluated the relative importance of [Ca2+] 0 and membrane potential on the ATP secretion process by modelling the time course of ATP release under different conditions. After correction of the records for destruction of released ATP by synaptosomal ecto-ATPase activity, we found that ATP secretion occurs by an apparent first order process. We also established that, in addition to the classical [Ca2+] 0 -dependent mode, ATP secretion also occurred in the absence of extracellular calcium ([Ca2+] 0 < 1 m). Upon lowering the extracellular Ca2+ concentration, both the rate and the extent of ATP secretion decreased. To assess the contribution of membrane potential to the release rate we measured ATP secretion at membrane potentials determined by extracellular [K+] 0 (or [Rb+] 0 ) as defined by the distribution of the carbocyanine dye, diSC3(5). Rate constants computed from measured secretion curves revealed that this parameter was essentially independent of membrane potential in the absence of [Ca2+] 0 . Noise analysis of the light signal showed that the variance increased upon stimulation by high [K+] 0 , suggesting that both modes of secretion are quantal. Thus, we conclude that the rate of ATP secretion from nerve terminals depends upon Ca2+ entry but not on membrane potential, per se  相似文献   

19.
Summary Efflux of42K+ was measured in frog sartorius muscles equilibrated in depolarizing solutions with external K+ concentrations ([K+] o ) between 75 and 300mm and NaCl concentrations of 60, 120, or 240mm. For several combinations of KCl and NaCl, steady-state internal potentials (V i) were the same for different [K+] o . For the range ofV i examined, K+ efflux occurs principally through the K+ inward rectifier channels. When external K+ is removedV i remains constant for 2 to 3 hr because of the high membrane conductance to Cl, but K+ efflux drops by about one order of magnitude.External Ba2+ in the presence or absence of external K+ produces an inhibition of K+ efflux described by a relation of the formu=(u1/(1+C)[Ba2+] o ))+u 2, whereu is the uninhibited fraction of K+ efflux;u 1, u2 andC are constants; andu 1+u2=1.C depends both on [K+] o andV i. When [K+] o 75mm, increasing [K+] o at constantV i reduces Ba2+ sensitivity. For constantV i–30 mV, Ba2+ sensitivity is less when [K+] o =0 than when [K+] o 75mm. When [K+] o =0, Ba2+ sensitivity decreases asV i is made more positive. The dependence of the Ba2+ sensitivity onV i at constant [K+] o is greater when [K+] o =0 than when [K+] o 75mm.Both the activation of K+ efflux by external K+ and the Ba2+ inhibition of K+ efflux can be explained on the basis of two membrane control sites associated with each channel. When both sites are occupied by K+, the channels are in a high flux state. When one or both sites are empty, the channels are in a low, nonzero flux state. When Ba2+ occupies either site, K+ efflux is further reduced. The reduction of Ba2+-sensitivity by increasing [K+] o at high [K+] o is attributable to the displacement of Ba2+ from the control sites by K+. The increased Ba2+ sensitivity produced by going from [K+] o =0 to [K+] o >-75mm whenV i–30 mV is attributable to states in which Ba2+ occupies one site and K+ the other when [K+] o 0. The smallerV i dependence of the Ba2+ sensitivity when [K+] o 75mm compared to [K+] o =0 is attributable to the necessity that Ba2+ displace K+ at the control sites when [K+] o is high but not when [K+] o =0.  相似文献   

20.
Routes to the synthesis of the mixed sulfide-phenylthiolate complex [Pt2(μ-S)(μ-SPh)(PPh3)4]+ have been explored; reaction of [Pt2(μ-S)2(PPh3)4] with excess Ph2IBr proceeds readily to selectively produce this complex, which was structurally characterised as its PF6 salt. Reactions of [Pt2(μ-S)2(PPh3)4] with other potent arylating reagents (1-chloro-2,4-dinitrobenzene and 1,5-difluoro-2,4-dinitrobenzene) also produce the corresponding nitroaryl-thiolate complexes [Pt2(μ-S){μ-SC6H2(NO2)2X}(PPh3)4]+ (X = H, F). The complex [Pt2(μ-S)(μ-SPh)(PPh3)4]+ reacts with Me2SO4 to produce the mixed alkyl/aryl bis-thiolate complex [Pt2(μ-SMe)(μ-SPh)(PPh3)4]2+, but corresponding reactions with the nitroaryl-thiolate complexes are plagued by elimination of the nitroaryl group and formation of [Pt2(μ-SMe)2(PPh3)4]2+. [Pt2(μ-S)(μ-SPh)(PPh3)4]+ also reacts with Ph3PAuCl to give [Pt2(μ-SAuPPh3)(μ-SPh)(PPh3)4]2+.  相似文献   

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