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1.
Mg2+- and Ca2+-uptake was measured in dark-grown oat seedlings ( Avena sativa L. cv. Brighton) cultivated at two levels of mineral nutrition. In addition the stimulation of the ATPase activity of the microsomal fraction of the roots by Mg2+ was measured. Ca2+-uptake by the roots was mainly passive. Mg2+-uptake mainly active; the passive component of Mg2+-uptake was accompanied by Ca2+-efflux up to 60% of the Ca2+ present in the roots.
In general Mg2+ -uptake of oat roots was biphasic. The affinity of the second phase correspond well with that of the Mg2+-stimulation of the ATPase activity, in low-salt roots as well as in high-salt roots and in roots of plants switched to the other nutritional condition. Linear relationships were observed when [phase 2] Mg2+-uptake was plotted against Mg2+-stimulation of the ATPase activity of the microsomal fraction of the roots. In 5 days old high-salt plants 1 ATP (hydrolysed in the presence of Mg2+ J corresponded with active uptake of a single Mg2+ ion, but in older high-salt roots and in low-salt roots more ATP was hydrolysed per net uptake of a Mg2+ ion. The results are discussed against the background of regulation of the Mg2+-level of the cytoplasm of root cells by transport of Mg2+ by a Mg2+-ATPase to the vacuole, to the xylem vessels, and possibly outwards.  相似文献   

2.
Six cultivars of barley ( Hordeum vulgare L., cvs Salve, Nürnberg II, Bomi, Risø 1508, Mona and Sv 73 608) were exposed for three weeks to combinations of high and low mineral supply and differential root/shoot temperature. For all the parameters tested [fresh and dry weights, contents and levels of N, K+, Ca2+ and Mg2+, and influx of Rb+(86Rb)] the cultivar differences were influenced by the mineral supply, the root temperature and the age of the plants.
The cultivar differences in N nutrition of three-week-old plants could partly be attributed to variation in root size, uptake of N and in use-efficiency of the element. The cultivar variation in root-shoot partitioning of N was small, except when low mineral supply was combined with a low root temperature. Similarly, cultivar differences in contents of K+, Ca2+ and Mg2+ were influenced by variation in uptake, use-efficiency and root/shoot partitioning of the elements. Low root temperature increased cultivar variation in K+, Ca2+ and Mg2+ partitioning.
The modern cultivar Salve was compared with Nürnberg II, which is derived from a German land race. Nürnberg II performed better than Salve when low root temperature and restricted mineral supply were combined. Otherwise Salve grew better, partly due to a more efficient use of N.
Two high-lysine lines, Risø 1508 and Sv 73 608, were compared with their mother lines Bomi and Mona. The differences obtained revealed no general effect of the high-lysine genes on growth and mineral nutrition of up to three-week-old barley plants.  相似文献   

3.
Abstract: Accumulation of intracellular Ca2+ is known to be critically important for the expression of NMDA receptor-mediated glutamate neurotoxicity. We have observed, however, that glutamate can also increase the neuronal intracellular Mg2+ concentration on activation of NMDA receptors. Here, we used conditions that elevate intracellular Mg2+ content independently of Ca2+ to investigate the potential role of Mg2+ in excitotoxicity in rat cortical neurons in vitro. In Ca2+-free solutions in which the Na+ was replaced by N -methyl- d -glucamine or Tris (but not choline), which also contained 9 m M Mg2+, exposure to 100 µ M glutamate or 200 µ M NMDA for 20 min produced delayed neuronal cell death. Neurotoxicity was correlated to the extracellular Mg2+ concentration and could be blocked by addition of NMDA receptor antagonists during, but not immediately following, agonist exposure. Finally, we observed that rat cortical neurons grown under different serum conditions develop an altered sensitivity to Mg2+-dependent NMDA receptor-mediated toxicity. Thus, the increase in intracellular Mg2+ concentration following NMDA receptor stimulation may be an underestimated component critical for the expression of certain forms of excitotoxic injury.  相似文献   

4.
Abstract: It is well established that ischemia is associated with prolonged increases in neuronal intracellular free calcium levels. Recent data suggest that regulation of calcium uptake and release from the endoplasmic reticulum is important in maintaining calcium homeostasis. The endoplasmic reticulum Mg2+/Ca2+ ATPase is the major mechanism for sequestering calcium in this organelle. Inhibition of this enzyme may play a causal role in the loss of calcium homeostasis. In order to investigate the effect of ischemia on calcium sequestration into the endoplasmic reticulum, microsomes were isolated from control and ischemic whole brain homogenates by differential centrifugation. Calcium uptake was measured by radioactive calcium (45Ca2+) accumulation in the microsomes mediated by Mg2+/Ca2+ ATPase. Ischemia caused a statistically significant inhibition of presteady-state and steady-state calcium uptake. Duration of ischemia was directly proportional to the degree of inhibition. Decreased calcium uptake was shown not to be the result of increased calcium release from ischemic compared with control microsomes nor the result of selective isolation of ischemic microsomes from the homogenate with a decreased capacity for calcium uptake. The data demonstrate that ischemia inhibits the ability of brain microsomes to sequester calcium and suggest that loss of calcium homeostasis is due, in part, to ischemia-induced inhibition of endoplasmic reticulum Mg2+/Ca2+ ATPase.  相似文献   

5.
The uptake of Cd2+ by excised roots of Tamarix aphylla (L.) Karst, was investigated using roots of hydroponically grown plants. The concentration isotherm of Cd2+ uptake approached saturation with a single phase hyperbola. The time course of Cd2+ absorption was generally hyperbolic, with an apparent linear section between 2 and 30 min. The temperature response varied among different temperature ranges: a Q10 of approximately 1.9 was found between 10 and 20°C, but at higher and lower temperatures Q10 values were only 1–1.3. It is concluded that Cd2+ uptake by the roots of T. aphylla at moderate temperatures is mediated by a metabolic process, combined with a passive influx component that becomes dominant at higher and lower temperatures. The distribution of the absorption sites for Cd2+ and for Fe2+ along the roots of T. aphylla was also investigated. Cadmium uptake showed no apparent pattern, whereas a distinct pattern of uptake was observed for Fe2+, with the highest rates at the root tip. Iron absorption was stimulated in the presence of nutrients, whereas that of Cd2+ was inhibited. Adsorption and absorption of Cd2+ were strongly inhibited by Ca2+ and by Mg2+, but were unaffected by Fe2+. Monovalent ions (Na+, K+, Li+) also reduced Cd2+ absorption, but to a lesser extent than Ca2+ and Mg2+. Uptake of Cd+ was reduced at lower pH of the medium. The importance of interfering cations for Cd2+ tolerance of T. aphylla is emphasized.  相似文献   

6.
Abstract: The association of Mg2+ ions with mitochondria isolated from guinea pig cerebral cortex is investigated and resolved into two components, that bound to the surface of both the outer and the inner membranes and that transported into the mitochondrial matrix. When rotenone-treated mitochondria are preincubated in a Mg2+ -containing medium, Mg2+ binding can be measured and actual Mg2+ transport determined after the addition of succinate. Mg2+ uptake as well as retention within mitochondria is an energy-dependent process linked to substrate oxidation. EGTA completely prevents Mg2+ uptake, while the Ca2+ uniporter inhibitor Ruthenium Red, along with prevention of Mg2+ uptake, induces a slow efflux of accumulated Mg2+ ions. These findings suggest that both inward and outward Mg2+ movements follow Ca2+ fluxes across the mitochondrial membrane. Modulation of Mg2+ movements by mitochondria is therefore suggested to occur within nerve terminals.  相似文献   

7.
Abstract: We investigated the modulation of (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced increases in intracellular free Ca2+ ([Ca2+]i) and intracellular free Mg2+ ([Mg2+]i) by cyclothiazide and GYKI 52466 using microspectrofluorimetry in single cultured rat brain neurons. AMPA-induced changes in [Ca2+]i were increased by 0.3–100 µ M cyclothiazide, with an EC50 value of 2.40 µ M and a maximum potentiation of 428% of control values. [Ca2+]i responses to glutamate in the presence of N -methyl- d -aspartate (NMDA) receptor antagonists were also potentiated by 10 µ M cyclothiazide. The response to NMDA was not affected, demonstrating specificity of cyclothiazide for non-NMDA receptors. Almost all neurons responded with an increase in [Ca2+]i to both kainate and AMPA in the absence of extracellular Na+, and these Na+-free responses were also potentiated by cyclothiazide. GYKI 52466 inhibited responses to AMPA with an IC50 value of 12.0 µ M . Ten micromolar cyclothiazide significantly decreased the potency of GYKI 52466. However, the magnitude of this decrease in potency was not consistent with a competitive interaction between the two ligands. Cyclothiazide also potentiated AMPA- and glutamate-induced increases in [Mg2+]i. These results are consistent with the ability of cyclothiazide to decrease desensitization of non-NMDA glutamate receptors and may provide the basis for the increase in non-NMDA receptor-mediated excitotoxicity produced by cyclothiazide.  相似文献   

8.
Abstract The level of cGMP in a suspension of Escherichia coli cells increased transiently upon the addition of chemoattractants. Ca2+ (1 mM), but not Mg2+, produced constant tumbling of cells in the presence of the ionophore A23187. The effect was observed either in stationary-state cells, or in a logarithmic culture treated with EDTA to increase permeability by A23187. Under the same conditions, Ca2+ decreased the cytoplasmic level of cGMP. In Phormidium uncinatum , rapid 45Ca2+ accumulation followed a light-dark stimulus, or the addition of tetramethylquinone (TMQ), a chemorepellent. La3+, which increases the reversal rate, also enhanced the level of cytoplasmic Ca2+, presumably by blocking the outward Ca2+ flux. In both E. coli and P. uncinatum Ca2+ inhibited methylaccepting chemotaxis protein (MCP) methylation. It is concluded that cGMP and Ca2+ are secondary messengers in taxis information-processing.  相似文献   

9.
The Ca++ and Mg++ contents of embryonic chick heart were studied by atomic absorption spectrophotometry during a period from 48 h of foetal development until 2-3 days post-hatching. The hearts were isolated and incubated for 40 min at 22°C in three different media aerated with 95% 02-5% C02. The media included: normal Ringer's; Ca+-free Ringer's with 3 mM EGTA; and Ca++-free Ringer's with 3 mM EDTA. At 48 h, the tubular myocardium contained 7-3 mM Ca++ per wet weight which decreased rapidly to 1-2 mM by 10 days of development and remained between 0-9 and 1-1 mM until hatching. The Ca++ content paralleled the changes in Na+ content reported earlier. Treatment with excess chelators, EGTA or EDTA, resulted in removal of 65-75% of the Ca++ content throughout development until the time of hatching, when 50% of the Ca++ became firmly bound. In contrast to the results with Ca++, myocardial Mg++ content rose rapidly from an initial value of 3.2 mM at 48 h to 6.7 mM by the 5th day of development, and then gradually declined throughout the remaining foetal development to 4.8 mM 2-3 days post-hatching. The Mg++ contents closely paralleled changes in K+ content during development, which were reported earlier. Treatment with EGTA and EDTA removed 13-22% and 19-28% of the myocardial Mg++, respectively, during development until just prior to hatching, when only 10-12% could be removed by chelation.  相似文献   

10.
Six cultivars of spring barley ( Hordeum vulgare L. cvs Salve, Nümberg II, Bomi, Risø 1508, Mona and Sv 73 608) were grown in water culture for three weeks with various combinations of mineral supply and differential roots/shoot temperatures during the growth period. Most important for growth and accumulation of N, K+, Ca2+ and Mg2+ was the mineral supply, followed by the root temperature and the choice of cultivar. Treatments with low mineral supply or low root temperature induced a uniform reduction in growth and accumulation of the ions studied. The effects of low mineral supply and low root temperature on growth and N accumulation was additive, which indicates that these factors exert their influence independently of each other.
Roots grown at 10°C were smaller and Rb+(86Rb) influx was higher than in roots grown at 20°C. It is suggested that the control of Rb+(86Rb) influx is affected by the root temperature and the age of the plants. The higher 86Rb+ (86Rb) influx into the low temperature roots could not compensate for the smaller root size. However, the lower total mineral accumulation made up for the needs of the smaller plants and cannot explain the reduction in growth.  相似文献   

11.
Purified plasmalemma vesicles were isolated in the presence of 250 m M sucrose from roots of 14-day-old seedlings of winter wheat ( Triticum aestivum L. Martonvásári-8) by phase partitioning of salt-washed microsomal fractions in a Dextran-polyethylene glycol two-phase system, and both Mg2+- and Ca2+-ATPase activities were detected. Orthovanadate-sensitive Mg2+-ATPase activity associated with the inside of right side-out plasmalemma (PM) vesicles (latency 98%) was inhibited 76% by 0.3 m M Ca2+, Ca2+-dependent ATPase activity located partly on the inside and partly on the outside of plasmalemma vesicles (latency 47%) was not affected by Mg2+.
Mg2+-ATPase activity was inhibited by 68% and inhibition of Mg2+ activation by 0.3 m M Ca2+ partly disappeared in the presence of 10 p M tentoxin, a fungal phytotoxin. Mg2+-ATPase activity remained inhibited up to 10 n M tentoxin while at 1 μ M tentoxin Mg2+ activation was as high as without tentoxin. K+-stimulation and vanadate inhibition was increased and decreased, respectively, by 100 p M -10 n M tentoxin. Ca2+-dependent ATPase activity was continuously increased by 1 p M -10 n M tentoxin, but at 1 μ M tentoxin the stimulation disappeared. The effects of p M tentoxin on plasma-lemma Mg2+-ATPase are discussed in relation to its influence on K+ transport in wheat seedlings.  相似文献   

12.
Abstract: Superfused cortical brain slices from neonatal rats demonstrated large increases in levels of NMR-detectable lipids after sample preparation and perfusion with standard artificial CSF. These increases were reduced by an average of 58% by perfusion with buffer with low (no added) Ca2+ or by perfusion in Ca2+-free buffer. Perfusion with buffer with elevated MgSO4 (10 mmol/L) reduced the lipid changes by 47%. A reduction of 88% was observed in samples perfused in buffer with both low Ca2+ and high Mg2+, suggesting a role for Mg2+ in reducing lipolysis distinct from its known ability to block Ca2+ influx.  相似文献   

13.
Abstract: Current literature suggests that a massive influx of Ca2+ into the cells of the CNS induces cell damage associated with traumatic brain injury (TBI). Using an in vitro model for stretch-induced cell injury developed by our laboratory, we have investigated the role of extracellular Ca2+ in astrocyte injury. The degree of injury was assessed by measurement of propidium iodide uptake and release of lactate dehydrogenase. Based on results of in vivo models of TBI developed by others, our initial hypothesis was that decreasing extracellular Ca2+ would result in a reduction in astrocyte injury. Quite unexpectedly, our results indicate that decreasing extracellular Ca2+ to levels observed after in vivo TBI increased astrocyte injury. Elevating the extracellular Ca2+ content to twofold above physiological levels (2 m M ) produced a reduction in cell injury. The reduction in injury afforded by Ca2+ could not be mimicked with Ba2+, Mn2+, Zn2+, or Mg2+, suggesting that a Ca2+-specific mechanism is involved. Using 45Ca2+, we demonstrate that injury induces a rapid influx of extracellular Ca2+ into the astrocyte, achieving an elevation in total cell-associated Ca2+ content two- to threefold above basal levels. Pharmacological elevation of intracellular Ca2+ levels with the Ca2+ ionophore A23187 or thapsigargin before injury dramatically reduced astrocyte injury. Our data suggest that, contrary to popular assumptions, an elevation of total cell-associated Ca2+ reduces astrocyte injury produced by a traumatic insult.  相似文献   

14.
Abstract: Increasing extracellular pH from 7.4 to 8.5 caused a dramatic increase in the time required to recover from a glutamate (3 µ M , for 15 s)-induced increase in intracellular Ca2+ concentration ([Ca2+]i) in indo-1-loaded cultured cortical neurons. Recovery time in pH 7.4 HEPES-buffered saline solution (HBSS) was 126 ± 30 s, whereas recovery time was 216 ± 19 s when the pH was increased to 8.5. Removal of extracellular Ca2+ did not inhibit the prolongation of recovery caused by increasing pH. Extracellular alkalinization caused rapid intracellular alkalinization following glutamate exposure, suggesting that pH 8.5 HBSS may delay Ca2+ recovery by affecting intraneuronal Ca2+ buffering mechanisms, rather than an exclusively extracellular effect. The effect of pH 8.5 HBSS on Ca2+ recovery was similar to the effect of the mitochondrial uncoupler carbonyl cyanide p -(trifluoromethoxyphenyl)hydrazone (FCCP; 750 n M ). However, pH 8.5 HBSS did not have a quantitative effect on mitochondrial membrane potential comparable to that of FCCP in neurons loaded with a potential-sensitive fluorescent indicator, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide (JC-1). We found that the effect of pH 8.5 HBSS on Ca2+ recovery was completely inhibited by the mitochondrial Na+/Ca2+ exchange inhibitor CGP-37157 (25 µ M ). This suggests that increased mitochondrial Ca2+ efflux via the mitochondrial Na2+/Ca2+ exchanger is responsible for the prolongation of [Ca2+]i recovery caused by alkaline pH following glutamate exposure.  相似文献   

15.
Arrhenius plots of the maximal velocities for the Ca2+-and Mg2+-dependent ATPase activities found in a plasma membrane-rich microsome fraction isolated from the roots of barley ( Hordeum vulgare L. cv. Conquest) were nonlinear. Arrhenius plot analyses using a relation which produced curvilinear Arrhenius plots accurately fit the data and allowed the calculation of the activation enthalpies and molar heat capacities of activation. The temperature dependence of the computed Km values for the Ca2+- and Mg2+-dependent ATPase activities was complex, with the highest enzyme-substrate affinities being obtained near the barley seedling growth temperature (16°C). Using electron paramagnetic resonance spectroscopy with amphiphilic cationic and anionic spin probes, it was possible to demonstrate that temperature changes and increasing Ca2+ concentrations could alter the mobility of the membrane lipid polar head groups. Inhibition of the ATPase activities by high levels of Ca2+ may result from a Ca2+-induced reduction in the lipid polar head group mobility. The possible role of lipid polar head group-protein interactions in the complex temperature dependence of the barley root ATPase kinetic constants is discussed.  相似文献   

16.
Excretion of minerals by the NaCl-resistant and comparatively cadmium-resistant tree Tamarix aphylla (L.) Karst, was investigated. Cd2+ was excreted by plants exposed for 1–10 days to 9 or 45 μ M Cd2+ solutions. Excretion of this toxic ion increased considerably with time but was less than 5% of the quantities that had been accumulated in the shoots. Excretion of Na+ and Cl was positively correlated with NaCl concentration (1.5, 10, 50 m M ) of the medium. The Na+/Cl ratios of the excrete were positively correlated with the concentration of the treatment solution. Ca2+ excretion decreased with increasing NaCl concentrations of the solution. Excretion of K+ and Mg2+ was only little affected by NaCl. Excretion of Li+ occurred whenever this element was supplied in the uptake solution; daily excretion rates of Li+ increased with time. The ecological significance of excretion is discussed in relation to the low selectivity of the mechanism in T. aphylla .  相似文献   

17.
Entry of the divalent cations Ni2+, Co2+ and Zn2+ into cells of maize ( Zea mays L. cv. Dekalb XL 85) root tissue is accompanied by an acidification of the incubation medium, a decrease in both the pH of the cell sap and the level of malate in the cells, and by an inhibition of dark fixation of CO2. K+, on the contrary, induces only a very low acidification of the incubation medium, does not change either the pH of the cell sap or the malate level in the cells, and induces an increase in CO2 dark fixation. Different mechanisms are postulated for the stimulation of proton extrusion by divalent cations and K+.  相似文献   

18.
Abstract: The role of the Na+/Ca2+ exchanger and intracellular nonmitochondrial Ca2+ pool in the regulation of cytosolic free calcium concentration ([Ca2+]i) during catecholamine secretion was investigated. Catecholamine secretion and [Ca2+]i were simultaneously monitored in a single chromaffin cell. After high-K+ stimulation, control cells and cells in which the Na+/Ca2+ exchange activity was inhibited showed similar rates of [Ca2+]i elevation. However, the recovery of [Ca2+]i to resting levels was slower in the inhibited cells. Inhibition of the exchanger increased the total catecholamine secretion by prolonging the secretion. Inhibition of the Ca2+ pump of the intracellular Ca2+ pool with thapsigargin caused a significant delay in the recovery of [Ca2+]i and greatly enhanced the secretory events. These data suggest that both the Na+/Ca2+ exchanger and the thapsigargin-sensitive Ca2+ pool are important in the regulation of [Ca2+]i and, by modulating the time course of secretion, are important in determining the extent of secretion.  相似文献   

19.
When 1 m M spermidine or spermine was included in an absorption solution which contained 20 m M Na+ and 1 m M Rb+, Na+ influx into excised maize roots ( Zea mays L. cv. Golden Cross Bantam) was reduced. Rb+ influx was reduced in the presence of spermidine and uneffected in the presence of spermine when compared with control solutions. When 1 m M Ca2+ replaced the polyamines, Na+ influx was strongly reduced and Rb+ influx was promoted. Rb+ influx from 1 m M Rb+ solutions which did not contain Na+ was also promoted by 1 m M Ca2+, but was inhibited by 1 m M spermidine. This Ca2+ promotion of Rb+ influx could be reversed by 10 times greater concentration of spermidine in the absorption solution. H+ efflux from excised roots was inhibited by spermidine when compared with Ca2+ or control solutions, however, the plasma membrane ATPase was not inhibited by spermidine. It is concluded that external Ca2+ plays two separate roles in membrane function, only one of which can be substituted for by polyamines. The first role, maintenance of membrane integrity, can be substituted for by spermidine or spermine. The second function, maintenance of the Rb+ transport mechanism, is Ca2+ specific and cannot be substituted for by spermidine or spermine. The results of this study are discussed in terms of electrostatic interactions between the plasma membrane and the Ca2+ or polyamines.  相似文献   

20.
The effects of abscisic acid (ABA) on growth, uptake and translocation of potassium ions, K+,Mg2+-ATPase activity and transpiration were investigated in young wheat ( Triticum aestivum L. cv. Martonvásári-8) plants grown at different K+ supplies. Long-term treatment with ABA (10 μ M ) reduced growth in high-K+ plants, but had less effect under low-K+ conditions. K+(86Rb) uptake was inhibited by about 70 and 40% in low- and high-K+ plants, respectively. The stimulation by K+ of the Mg2+-ATPase activity in the root microsomal fraction was lost with ABA treatment. It is suggested that the inhibitory effect of ABA on K+ uptake may be related to this effects on the K+,Mg2+-ATPase. Translocation of K+ to the shoot was inhibited in low-K+ plants only, and it was not affected in high-K+ plants. In parallel to this, ABA treatment reduced transpiration by about 50% in low-K+ plants, whereas a much smaller effect was seen in high-K+ plants. These observations suggest that the regulation by ABA of the stomatal movements is strongly counteracted by high-K+ status.  相似文献   

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