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1.
Using confocal microscopy, X‐ray microanalysis and the scanning ion‐selective electrode technique, we investigated the signalling of H 2O 2, cytosolic Ca 2+ ([Ca 2+] cyt) and the PM H +‐coupled transport system in K +/Na + homeostasis control in NaCl‐stressed calluses of Populus euphratica. An obvious Na +/H + antiport was seen in salinized cells; however, NaCl stress caused a net K + efflux, because of the salt‐induced membrane depolarization. H 2O 2 levels, regulated upwards by salinity, contributed to ionic homeostasis, because H 2O 2 restrictions by DPI or DMTU caused enhanced K + efflux and decreased Na +/H + antiport activity. NaCl induced a net Ca 2+ influx and a subsequent rise of [Ca 2+] cyt, which is involved in H 2O 2‐mediated K +/Na + homeostasis in salinized P. euphratica cells. When callus cells were pretreated with inhibitors of the Na +/H + antiport system, the NaCl‐induced elevation of H 2O 2 and [Ca 2+] cyt was correspondingly restricted, leading to a greater K + efflux and a more pronounced reduction in Na +/H + antiport activity. Results suggest that the PM H +‐coupled transport system mediates H + translocation and triggers the stress signalling of H 2O 2 and Ca 2+, which results in a K +/Na + homeostasis via mediations of K + channels and the Na +/H + antiport system in the PM of NaCl‐stressed cells. Accordingly, a salt stress signalling pathway of P. euphratica cells is proposed. 相似文献
2.
Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H 2O 2 accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D ( CmRBOHD) under ABA-induced stress. Both melatonin and H 2O 2 induced the accumulation of cytoplasmic-free Ca 2+ ([Ca 2+] cyt) in response to ABA, while blocking of Ca 2+ influx channels attenuated melatonin- and H 2O 2-induced ABA tolerance. CmRBOHD overexpression induced [Ca 2+] cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin-induced [Ca 2+] cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H 2O 2 and Ca 2+ attenuated ABA-induced K + efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA-induced K + efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD-dependent H 2O 2 production that triggers [Ca 2+] cyt accumulation and subsequently inhibits K + efflux and delays cell death/leaf senescence in response to ABA. 相似文献
3.
Low-light (LL) intensity is a primary abiotic stressor that negatively influences turf grass quality. In the present experiment, we studied the effect of exogenous Ca 2+ (0, 10, 50, 100, and 200 mM) on the antioxidant system, the accumulation of MDA and proline, the content of photosynthetic pigments in plant leaves in order to investigate whether exogenous Ca 2+ treatment improves LL tolerance in tall fescue ( Festuca arundinacea Schreb.). We have found that LL significantly reduced a number of growth parameters (plant height, leaf width, leaf fresh weight, root fresh weight, leaf dry weight, and root dry weight), chlorophyll (Chl) a and Chl b contents, and carotenoid (Car) levels, while considerably enhancing electrolyte leakage (EL), MDA accumulation, calcium (Ca 2+) concentration, and generation of reactive oxygen species (ROS), such as hydrogen peroxide (H 2O 2) and superoxide radical (O 2 ·? ). Moreover, LL significantly induced the activities of antioxidant enzymes, such as peroxidase (POD) and catalase (CAT), and slightly increased the activity of superoxide dismutase (SOD) in tall fescue leaves. In contrast, POD and SOD activities declined considerably while CAT activity significantly increased in plant roots under LL stress. The application of 50 mM Ca 2+ significantly improved the aforementioned growth parameters, the content of photosynthetic pigments, and further enhanced the activities of POD, SOD, and CAT, but decreased electrolyte leakage and MDA and H 2O 2 levels in the leaves and roots of tall fescue under LL stress. These results suggest that Ca 2+ is likely involved in a resistance to LL by regulating antioxidant enzyme action in tall fescue leaves and roots. 相似文献
4.
High environmental salt elicits an increase in cytosolic Ca 2+ ([Ca 2+] cyt) in plants, which is generated by extracellular Ca 2+ influx and Ca 2+ release from intracellular stores, such as vacuole and endoplasmic reticulum. This study aimed to determine the physiological mechanisms underlying Ca 2+ release from vacuoles and its role in ionic homeostasis in Populus euphratica. In vivo Ca 2+ imaging showed that NaCl treatment induced a rapid elevation in [Ca 2+] cyt, which was accompanied by a subsequent release of vacuolar Ca 2+. In cell cultures, NaCl-altered intracellular Ca 2+ mobilization was abolished by antagonists of inositol (1, 4, 5) trisphosphate (IP 3) and cyclic adenosine diphosphate ribose (cADPR) signaling pathways, but not by slow vacuolar (SV) channel blockers. Furthermore, the NaCl-induced vacuolar Ca 2+ release was dependent on extracellular ATP, extracellular Ca 2+ influx, H 2O 2, and NO. In vitro Ca 2+ flux recordings confirmed that IP 3, cADPR, and Ca 2+ induced substantial Ca 2+ efflux from intact vacuoles, but this vacuolar Ca 2+ flux did not directly respond to ATP, H 2O 2, or NO. Moreover, the IP 3/cADPR-mediated vacuolar Ca 2+ release enhanced the expression of salt-responsive genes that regulated a wide range of cellular processes required for ion homeostasis, including cytosolic K + maintenance, Na + and Cl − exclusion across the plasma membrane, and Na +/H + and Cl −/H + exchanges across the vacuolar membrane. 相似文献
5.
茉莉酸类物质(JAs)作为与昆虫啃噬及损伤相关的植物激素和信号分子在植物防御反应中起重要作用,但是茉莉酸引起的早期防御反应的机理仍不清楚。该研究以拟南芥叶片保卫细胞为材料,结合非损伤微测(NMT)及激光共聚焦技术探讨了茉莉酸诱导的保卫细胞中质膜H+-ATPase与H2O2积累的调控关系。结果表明:茉莉酸甲酯(MeJA)处理导致H+迅速跨膜外排和H2O2积累,H+外排和H2O2积累能够被钒酸钠抑制,而二苯基碘(DPI)处理则对MeJA诱导的H+跨膜外排无显著影响。研究结果证明,在MeJA诱导的早期信号事件中,质膜H+-ATPase的激活先于H2O2的产生。 相似文献
6.
Salinity is among the environmental factors that affect plant growth and development and constrain agricultural productivity. Salinity stress triggers increases in cytosolic free Ca 2+ concentration ([Ca 2+] i) via Ca 2+ influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS). It is well established that ROS also triggers increases in [Ca 2+] i. However, the relationship and interaction between salinity stress-induced [Ca 2+] i increases and ROS-induced [Ca 2+] i increases remain poorly understood. Using an aequorin-based Ca 2+ imaging assay we have analyzed [Ca 2+] i changes in response to NaCl and H 2O 2 treatments in Arabidopsis thaliana. We found that NaCl and H 2O 2 together induced larger increases in [Ca 2+] i in Arabidopsis seedlings than either NaCl or H 2O 2 alone, suggesting an additive effect on [Ca 2+] i increases. Following a pre-treatment with either NaCl or H 2O 2, the subsequent elevation of [Ca 2+] i in response to a second treatment with either NaCl or H 2O 2 was significantly reduced. Furthermore, the NaCl pre-treatment suppressed the elevation of [Ca 2+] i seen with a second NaCl treatment more than that seen with a second treatment of H 2O 2. A similar response was seen when the initial treatment was with H 2O 2; subsequent addition of H 2O 2 led to less of an increase in [Ca 2+] i than did addition of NaCl. These results imply that NaCl-gated Ca 2+ channels and H 2O 2-gated Ca 2+ channels may differ, and also suggest that NaCl- and H 2O 2-evoked [Ca 2+] i may reduce the potency of both NaCl and H 2O 2 in triggering [Ca 2+] i increases, highlighting a feedback mechanism. Alternatively, NaCl and H 2O 2 may activate the same Ca 2+ permeable channel, which is expressed in different types of cells and/or activated via different signaling pathways. 相似文献
7.
An early event in the hypersensitive response of tobacco to Pseudomonas syringae pv syringae is the initiation of a K +/H + response characterized by specific plasma membrane K + efflux, extracellular alkalinization, and intracellular acidification. We investigated the role of calcium in induction of these host responses. Suspension-cultured tobacco cells exhibited a baseline Ca 2+ influx of 0.02 to 0.06 micromole per gram per hour as determined from 45Ca 2+ uptake. Following bacterial inoculation, uptake rates began to increase coincidently with onset of the K +/H + response. Rates increased steadily for 2 to 3 hours, reaching 0.5 to 1 micromole per gram per hour. This increased Ca 2+ influx was prevented by EGTA and calcium channel blockers such as La 3+, Co 2+, and Cd 2+ but not by verapamil and nifedipine. Lanthanum, cobalt, cadmium, and EGTA inhibited the K +/H + response in both suspension-cultured cells and leaf discs and prevented hypersensitive cell death in leaf discs. We conclude that increased plasmalemma Ca 2+ influx is required for the K +/H + and hypersensitive responses in tobacco. 相似文献
8.
Here we examined whether Ca 2+/Calmodulin (CaM) is involved in abscisic acid (ABA)-induced antioxidant defense and the possible relationship between CaM
and H 2O 2 in ABA signaling in leaves of maize ( Zea mays L.) plants exposed to water stress. An ABA-deficient mutant vp5 and its wild type were used for the experimentation. We found that water stress enhanced significantly the contents of CaM
and H 2O 2, and the activities of chloroplastic and cytosolic superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione
reductase (GR), and the gene expressions of the CaM1, cAPX, GR1 and SOD4 in leaves of wild-type maize. However, the increases mentioned above were almost arrested in vp5 plants and in the wild-type plants pretreated with ABA biosynthesis inhibitor tungstate (T), suggesting that ABA is required
for water stress-induced H 2O 2 production, the enhancement of CaM content and antioxidant defense. Besides, we showed that the up-regulation of water stress-induced
antioxidant defense was almost completely blocked by pretreatment with Ca 2+ inhibitors, CaM antagonists and reactive oxygen (ROS) manipulators. Moreover, the analysis of time course of CaM and H 2O 2 production under water stress showed that the increase in CaM content preceded that of H 2O 2. These results suggested that Ca 2+/CaM and H 2O 2 were involved in the ABA-induced antioxidant defense under water stress, and the increases of Ca 2+/CaM contents triggered H 2O 2 production, which inversely affected the contents of CaM. Thus, a cross-talk between Ca 2+/CaM and H 2O 2 may play a pivotal role in the ABA signaling. 相似文献
9.
Abstract: The features of Ca 2+ fluxes, the importance of the Ca 2+ pump‐mediated H +/Ca 2+ exchanges at plasmalemma level, and the possible involvement of Ca 2+‐ATPase activity in ABA‐induced changes of H + fluxes were studied in Egeria densa leaves. The results presented show that, while in basal conditions no net Ca 2+ flux was evident, a conspicuous Ca 2+ influx (about 1.1 ìmol g ?1 FW h ?1) occurred. The concomitant efflux of Ca 2+ was markedly reduced by treatment with 5 íM eosin Y (EY), a specific inhibitor of the Ca 2+‐ATPase, that completely blocked the transport of Ca 2+ after the first 20 ‐ 30 min. The decrease in Ca 2+ efflux induced by EY was associated with a significant increase in net H + extrusion (?ÄH +) and a small but significant cytoplasmic alkalinization. The shift of external [Ca 2+] from 0.3 to 0.2 mM (reducing Ca 2+ uptake by about 30 %) and the hindrance of Ca 2+ influx by La 3+ were accompanied by progressively higher ?ÄH + increases, in agreement with a gradual decrease in the activity of a mechanism counteracting the Ca 2+ influx by an nH +/Ca 2+ exchange. The ABA‐induced decreases in ?ÄH + and pH cyt were accompanied by a significant increase in Ca 2+ efflux, all these effects being almost completely suppressed by EY, in line with the view that the ABA effects on H + fluxes are due to activation of the plasmalemma Ca 2+‐ATPase. These results substantially stress the high sensitivity and efficacy of the plasmalemma Ca 2+ pump in removing from the cytoplasm the Ca 2+ taken up, and the importance of the contribution of Ca 2+ pump‐mediated H +/Ca 2+ fluxes in bringing about global changes of H + fluxes at plasmalemma level. 相似文献
10.
Net fluxes of Ca 2+, H + and K + were measured from intact Chara australis cells and from isolated cell walls, using ion-selective microelectrodes. In both systems, a stimulation in Ca 2+ efflux (up to 100 nmol m ?2 s ?1, from an influx of ~40 nmol m ?2 s ?1) was detected as the H + or K + concentration was progressively increased in the bathing solution (pH 7.0 to 4.6 or K + 0.2 to 10mol m ?3, respectively). A Ca 2+ influx of similar size occurred following the reverse changes. These fluxes decayed exponentially with a time constant of about 10 min. The threshold pH for Ca 2+ efflux (pH 5.2) is similar to a reported pH threshold for acid-induced wall extensibility in a closely related characean species. Application of NH 4+ to intact cells caused prolonged H + efflux and also transient Ca 2+ efflux. We attribute all these net Ca 2+ fluxes to exchange in the wall with H + or K +. A theoretical treatment of the cell wall ion exchanges, using the ‘weak acid Donnan Manning’ (WADM) model, is given and it agrees well with the data. The role of Ca 2+ in the cell wall and the effect of Ca 2+ exchanges on the measured fluxes of other ions, including bathing medium acidification by H + efflux, are discussed. 相似文献
11.
Rapid generation of superoxide radicals and accumulation of H 2O 2 is a characteristic early response of plants following perception of insect herbivory signals. Induction of oxidative burst on account of herbivory triggers various defense mechanisms in plants. Response of superoxide and H 2O 2-metabolizing enzymes and secondary metabolites in nine pigeonpea genotypes to Helicoverpa armigera feeding was investigated. Out of nine, four genotypes were found to be moderately resistant, three were intermediate and two were moderately susceptible. In general, H. armigera infestation resulted in increase in superoxide dismutase activity, H 2O 2 and phenolics content and decrease in catalase (CAT) activity in leaves, developing seeds and pod wall of pigeonpea genotypes. Peroxidase activity was found only in leaves. Among genotypes, the increase in phenolic constituents was found greater in moderately resistant genotypes than in moderately susceptible genotypes; this might determine their contribution in providing resistance to genotypes against H. armigera infestation. The capability of moderately resistant genotypes to maintain relatively lower H 2O 2 content and higher CAT activity in pod wall and developing seeds also appeared to determine resistance of genotypes towards H. armigera. Expression of resistance to H. armigera was found to be associated with a negative correlation of H 2O 2-metabolizing enzymes and phenolics with pod damage as well as with negative association between CAT activity and H 2O 2 content. A positive correlation found between H 2O 2 content and pod damage suggested the accumulation of H 2O 2 in response to pod borer attack. In addition, correlation analysis also revealed a positive association between CAT, phenolic compounds and DPPH radical scavenging activity following pod borer attack; this indicated their contribution in resistance mechanisms against H. armigera herbivory. 相似文献
12.
The oxidative burst, the rapid production of O 2- and H 2O 2 by plant cells in response to pathogens and Stressors, is a critical step in plant disease resistance and is controlled by
several different elicitor-initiated signaling pathways. While different defense elicitors appear to activate disparate initial
steps in signaling the oxidative burst, all of the elicitors tested thus far appear to stimulate pathways that converge on
the same three core signaling intermediates: 1) the Ca 2+-independent activation of a mitogen-activated protein kinase (MAPK) family member, 2) the influx of Ca 2+ into the cytosol, deriving most critically from an internal compartment, and 3) the Ca 2+-dependent activation of additional protein kinases including a second MAPK homologue and possibly calcium dependent protein
kinases (CDPKs). Data from several recent reports are summarized to place these signaling events into a complete and updated
model of signaling to the plant oxidative burst. 相似文献
13.
The correlation between an increased production of reactive oxygen species (ROS) and an enhanced calcium entry in primed neutrophils
stimulated with fMLP suggests that endogenous ROS could serve as an agonist to reinforce calcium signaling by positive feedback.
This work shows that exogenous H 2O 2 produced a rapid influx of Mn 2+ and an increase of intracellular calcium. The H 2O 2 was insufficient to produce significant changes in the absence of extracellular calcium but addition of Ca 2+ to H 2O 2-treated cells suspended in a free Ca 2+/EGTA buffer resulted in a great increase in [Ca 2+] i reflecting influx of Ca 2+ across the cell membrane. The increase of intracellular calcium was inhibited by Ni 2+, La 3+, and hyperosmotic solutions of mannitol and other osmolytes. This raises the possibility that the secretion of H 2O 2 by activated neutrophils could act as an autocrine regulator of neutrophil function through the activation of calcium entry. 相似文献
14.
Reactive oxygen species (ROS) play critical roles in cell death, diseases, and normal cellular processes. TRPM2 is a member of transient receptor potential (TRP) protein superfamily and forms a Ca 2+-permeable nonselective cation channel activated by ROS, specifically by hydrogen peroxide (H 2O 2), and at least in part via second-messenger mechanisms. Accumulating evidence has indicated that TRPM2 mediates multiple cellular responses, after our finding that Ca 2+ influx via TRPM2 regulates H 2O 2-induced cell death. Recently, we have demonstrated that Ca 2+ influx through TRPM2 induces chemokine production in monocytes and macrophages, which aggravates inflammatory neutrophil infiltration in mice. However, understanding is still limited for in vivo physiological or pathophysiological significance of ROS-induced TRPM2 activation. In this review, we summarize mechanisms underlying activation of TRPM2 channels by oxidative stress and downstream biological responses, and discuss the biological importance of oxidative stress-activated TRP channels. 相似文献
15.
It has been widely suggested that selenium (Se) deficiency play an important role in the pathophysiology of epilepsy. It has been reported that Se provides protection against the neuronal damage in patients and animals with epilepsy by restoring the antioxidant defense mechanism. The neuroprotective effects of topiramate (TPM) have been reported in several studies but the putative mechanism of action remains elusive. We investigated effects of Se and TPM in neuronal PC12 cell by evaluating Ca 2+ mobilization, lipid peroxidation and antioxidant levels. PC12 cells were divided into eight groups namely control, TPM, Se, H 2O 2, TPM + H 2O 2, Se + H 2O 2, Se + TPM and Se + TPM + H 2O 2. The toxic doses and times of H 2O 2, TPM and Se were determined by cell viability assay which is used to evaluate cell viability. Cells were incubated with 0.01 mM TPM for 5 h and 500 nM Se for 10 h. Then, the cells were exposed to 0.1 mM H 2O 2 for 10 h before analysis. The cells in all groups except control, TPM and Se were exposed to H 2O 2 for 15 min before analysis. Cytosolic Ca 2+ release and lipid peroxidation levels were higher in H 2O 2 group than in control, Se and TPM combination groups although their levels were decreased by incubation of Se and TPM combination. However, there is no difference on Ca 2+ release in TPM group. Glutathione peroxidase activity, reduced glutathione and vitamin C levels in the cells were lower in H 2O 2 group than in control, Se and TPM groups although their values were higher in the cells incubated with Se and TPM groups than in H 2O 2 groups. In conclusion, these results indicate that Se induced protective effects on oxidative stress in PC12 cells by modulating cytosolic Ca 2+ influx and antioxidant levels. TPM modulated also lipid peroxidation and glutathione and vitamin C concentrations in the cell system. 相似文献
16.
The role of Ca 2+ in the initiation and maintenance of contraction has been extensively studies. Many of these studies have focused on how Ca 2+ influx and efflux affect cytoplasmic Ca 2+ (Ca i) and, therefore, contraction in cardiac muscle. However, it has recently become apparent that Ca i itself may play a major role in the control of Ca 2+ influx and efflux from cardiac muscle. Here we review current ideas on the mechanisms underlying Ca 2+ homeostasis in cardiac muscle, with specific attention to how Ca i may control Ca 2+ influx, both under normal and pathological conditions. 相似文献
17.
Washing corn ( Zea mays L.) root tissue in water causes loss of about one-third of the exchangeable Ca 2+ over the first 10 to 15 minutes. Upon transfer to K +-containing solutions, the tissue shows a short period of rapid K + influx which subsequently declines. Addition of 0.1 millimolar Ca 2+ decreases the initial rapid K + influx, but increases the sustained rate of K + and Cl − uptake. It was confirmed (Elzam and Hodges 1967 Plant Physiol 42: 1483-1488) that 0.1 millimolar Ca 2+ is more effective than higher concentrations for the initial inhibition, and that Mg 2+ will substitute. The inhibition arises from a mild shock affect of restoring Ca2+. With 0.1 millimolar Ca2+ net H+ efflux is blocked for 10 to 15 minutes and the cells are depolarized by about 30 millivolts. However, 1 millimolar Ca2+ rapidly produces increased K+ influx and blocks net H+ efflux for only a few minutes; blockage is preceded by a brief net H+ influx which may restore and increase ion transport by reactivating the plasmalemma H+-ATPase. Stimulation of electrogenic H+-pumping with fusicoccin eliminates the shock responses and minimizes Ca2+ effects on K+ influx. Fusicoccin also strongly decreases Ca2+ influx, but has no effect on Ca2+ efflux. Ice temperatures and high pH decreased Ca2+ efflux, but uncoupler and chlorpromazine did not. It is suggested that the inhibitory and promotive actions of Ca2+ are manifested through decreases or increases in the protonmotive force. 相似文献
18.
Malignant mesothelioma (MMe) is a highly aggressive, lethal tumour requiring the development of more effective therapies. The green tea polyphenol epigallocathechin‐3‐gallate (EGCG) inhibits the growth of many types of cancer cells. We found that EGCG is selectively cytotoxic to MMe cells with respect to normal mesothelial cells. MMe cell viability was inhibited by predominant induction of apoptosis at lower doses and necrosis at higher doses. EGCG elicited H 2O 2 release in cell cultures, and exogenous catalase (CAT) abrogated EGCG‐induced cytotoxicity, apoptosis and necrosis. Confocal imaging of fluo 3‐loaded, EGCG‐exposed MMe cells showed significant [Ca 2+] i rise, prevented by CAT, dithiothreitol or the T‐type Ca 2+ channel blockers mibefradil and NiCl 2. Cell loading with dihydrorhodamine 123 revealed EGCG‐induced ROS production, prevented by CAT, mibefradil or the Ca 2+ chelator BAPTA‐AM. Direct exposure of cells to H 2O 2 produced similar effects on Ca 2+ and ROS, and these effects were prevented by the same inhibitors. Sensitivity of REN cells to EGCG was correlated with higher expression of Ca v3.2 T‐type Ca 2+ channels in these cells, compared to normal mesothelium. Also, Ca v3.2 siRNA on MMe cells reduced in vitro EGCG cytotoxicity and abated apoptosis and necrosis. Intriguingly, Ca v3.2 expression was observed in malignant pleural mesothelioma biopsies from patients, but not in normal pleura. In conclusion, data showed the expression of T‐type Ca 2+ channels in MMe tissue and their role in EGCG selective cytotoxicity to MMe cells, suggesting the possible use of these channels as a novel MMe pharmacological target. 相似文献
19.
Ca 2+ transport by sarcoplasmic reticulum vesicles was examined by incubating sarcoplasmic reticulum vesicles (0.15 mg/ml) at 37°C in, either normal medium that contained 0.15 M sucrose, 0.1 M KCl, 60 μM CaCl 2, 2.5 mM ATP and 30 mM Tes at pH 6.8, or a modified medium for elimination of ADP formed from ATP hydrolysis by including, in addition, 3.6 mM phosphocreatine and 33 U/ml of creatine phosphokinase. In normal medium, Ca 2+ uptake of sarcoplasmic reticulum vesicles reached a plateau of about 100 nmol/mg. In modified medium, after this phase of Ca 2+ uptake, a second phase of Ca 2+ accumulation was initiated and reached a plateau of about 300 nmol/mg. The second phase of Ca 2+ accumulation was accompanied by phosphate uptake and could be inhibited by ADP. Since, under these experimental conditions, there was no significant difference of the rates of ATP hydrolysis in normal medium and modified medium, extra Ca 2+ uptake in modified medium but not in normal medium could not be explained by different phosphate accumulation in the two media. Unidirectional Ca 2+ influx of sarcoplasmic reticulum near steady state of Ca 2+ uptake was measured by pulse labeling with 45Ca 2+. The Ca 2+ efflux rate was then determined by subtracting the net uptake from the influx rate. At the first plateau of Ca 2+ uptake in normal medium, Ca 2+ influx was balanced by Ca 2+ efflux with an exchange rate of 240 nmol/mg per min. This exchange rate was maintained relatively constant at the plateau phase. In modified medium, the Ca 2+ exchange rate at the first plateau of Ca 2+ uptake was about half of that in normal medium. When the second phase of Ca 2+ uptake was initiated, both the influx and efflux rates started to increase and reached a similar exchange rate as observed in normal medium. Also, during the second phase of Ca 2+ uptake, the difference between the influx and efflux rates continued to increase until the second plateau phase was approached. In conditions where the formation of ADP and inorganic phosphate was minimized by using a low concentration of sarcoplasmic (7.5 μg/ml) and/or using acetyl phosphate instead of ATP, the second phase of Ca 2+ uptake was also observed. These data suggest that the Ca 2+ load attained by sarcoplasmic reticulum vesicles during active transport is modulated by ADP accumulated from ATP hydrolysis. ADP probably exerts its effect by facilitating Ca 2+ efflux, which subsequently stimulates Ca 2+ exchange. 相似文献
20.
A rapid loss of accumulated Ca 2+ is produced by addition of H + to isolated heart mitochondria. The H +-dependent Ca + efflux requires that either (a) the NAD(P)H pool of the mitochondrion be oxidized, or (b) the endogenous adenine nucleotides be depleted. The loss of Ca 2+ is accompanied by swelling and loss of endogenous Mg 2–. The rate of H +-dependent Ca 2+ efflux depends on the amount of Ca 2+ and P i taken up and the extent of the pH drop imposed. In the absence of ruthenium red the H +-induced Ca 2+-efflux is partially offset by a spontaneous re-accumulation of released Ca 2+. The H +-induced Ca 2+ efflux is inhibited when the P i transporter is blocked with N-ethylmaleimide, is strongly opposed by oligomycin and exogenous adenine nucleotides (particularly ADP), and inhibited by nupercaine. The H +-dependent Ca 2+ efflux is decreased markedly when Na + replaces the K + of the suspending medium or when the exogenous K +/H + exchanger nigericin is present. These results suggest that the H +-dependent loss of accumulated Ca 2+ results from relatively nonspecific changes in membrane permeability and is not a reflection of a Ca 2+/H + exchange reaction. 相似文献
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