Uptake of Cd2+ and Fe2+ by excised roots of Tamarix aphylla

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.     

Mechanism of Cd2+ resistance in Euglena gracilis

The characteristics of Cd²⁺ accumulation by Euglena gracilis L. strain Z have been studied using sensitive and resistant cells. In both strains Cd²⁺ is mainly absorbed by a temperature- and light-dependent process. Resistance to Cd²⁺ is associated with a lower accumulation of Cd²⁺ and with a decreased affinity for Cd²⁺. Gel filtration on Sephadex G75 of the soluble fraction shows that resistance is not linked to an induction of metallothioneins.     

Puccinellia tenuiflora maintains a low Na+ level under salinity by limiting unidirectional Na+ influx resulting in a high selectivity for K+ over Na+

Puccinellia tenuiflora is a useful monocotyledonous halophyte that might be used for improving salt tolerance of cereals. This current work has shown that P. tenuiflora has stronger selectivity for K⁺ over Na⁺ allowing it to maintain significantly lower tissue Na⁺ and higher K⁺ concentration than that of wheat under short- or long-term NaCl treatments. To assess the relative contribution of Na⁺ efflux and influx to net Na⁺ accumulation, unidirectional ²²Na⁺ fluxes in roots were carried out. It was firstly found that unidirectional ²²Na⁺ influx into root of P. tenuiflora was significantly lower (by 31–37%) than in wheat under 100 and 150 m m NaCl. P. tenuiflora had lower unidirectional Na⁺ efflux than wheat; the ratio of efflux to influx was similar between the two species. Leaf secretion of P. tenuiflora was also estimated, and found the loss of Na⁺ content from leaves to account for only 0.0006% of the whole plant Na⁺ content over 33 d of NaCl treatments. Therefore, it is proposed that neither unidirectional Na⁺ efflux of roots nor salt secretion by leaves, but restricting unidirectional Na⁺ influx into roots with a strong selectivity for K⁺ over Na⁺ seems likely to contribute to the salt tolerance of P. tenuiflora .     

Excretion of ions (Cd2+, Li+, Na+ and Cl−) by Tamarix aphylla

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 .     

Effects of Cd2+ and EDTA on young sugar beets (Beta vulgaris). I. Cd2+ uptake and sugar accumulation

Sugar beets ( Beta vulgaris L. cv. Monohill) grown in a complete nutrient solution, were treated with Cd²⁺ (5 or 50 μ M ) and/or EDTA (10 or 100 μ M ) in different combinations. The Cd contents of five-week-old roots and shoots were determined by atomic absorption spectrophotometry, and the sucrose, glucose and fructose contents were measured enzymatically. The Cd²⁺ uptake in both roots and shoots shows a linear relationship to the concentration of free Cd²⁺ in the nutrient solution. This uptake is diminished in the presence of EDTA, suggesting that the Cd-EDTA complex is unable to penetrate the membranes. The contents of glucose, fructose and sucrose in both roots and shoots decrease with increasing uptake of free Cd²⁺. This may be a secondary effect caused by the inhibition of photosynthesis in the presence of Cd²⁺. EDTA reduces the inhibition of Cd²⁺ on sugar formation and accumulation. In the presence of EDTA alone the sugar content increases somewhat. EDTA slightly influences the dry weights of whole plants. The ratio roots:whole plants increases. Cd²⁺ (≤ 50 μ M ) increases the dry matter portion of roots by ca 30%, but not that of shoots.     

Ca2+ dependence and La3+ interference of ultraviolet radiationinduced K+ efflux from rose cells

A low fluence of ultraviolet radiation (UV) causes cultured cells of Rosa damascena Mill cv. Gloire de Guilan to lose intracellular K⁺. This effect required the presence of Ca²⁺ in the medium. A reduction in the concentration of free Ca²⁺ to 10⁻⁵ M with ethyleneglycol-bis-(β-aminoethyl-ether)-N.N.N',N'-tetraacetic acid (EGTA) buffer inhibited the UV-stimulated efflux; this was correlated with a discharge of the membrane potential and a stimulation of the leakage of K⁺ from unirradiated cells. All the same effects were seen with La³⁺ at 0.2 m M. At 0.02 m M La³⁺, the UV-stimulated efflux of K⁺ was blocked without concomitant effects on the membrane potential or K⁺ efflux from control cells. It is suggested that removal of Ca²⁺ blocks or masks the UV-induced leakage of K⁺ by destabilizing the plasma membrane. In addition, La³⁺ may specifically inhibit the UV-stimulated opening of K⁺ or anion channels.     

Effect of K+ and H+ stress and role of Ca2+ in the regulation of intracellular K+ concentration in mung bean roots

The effects of external K⁺, H⁺ and Ca2⁺ concentrations on the intracellular K+ concentration, [K⁺]i, and the K⁺-ATPase activity in 2-day-old mung bean roots [ Vigna mungo (L.) Hepper] were investigated. [K⁺]i, in mung bean roots was markedly decreased by external K⁺ or H⁺ stress and did not recover the initial value even after the stress was removed. This decrease in [K⁺]i, gradually disappeared with the addition of (Ca2⁺. Ca2⁺ may offset the harmful effects of ion stress. Ca2⁺ seems to have two effects on K+ transport; control of K⁺ permeability and activation of K⁺ uptake, although K⁺-ATPase activity was inhibited by Ca2⁺ concentrations higher than 10–4 M. We suggest that Ca2⁺ activates K⁺ uptake indirectly through the acidification of the cytoplasm.     

Stimulation of proton extrusion by K+ and divalent cations (Ni2+, Co2+ Zn2+) in maize root segments

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⁺.     

Binding of Cd2+, Ni2+, Cu2+ and Zn2+ by isolated envelopes of Pseudomonas fluorescens

Abstract Cell envelopes of Pseudomonas fluorescens , cytoplasmic membrane, peptidoglycan and outer membrane were obtained from a fractionation procedure and tested for their metal binding capacity. Isolated envelopes (cytoplasmic membrane, peptidoglycan and outer membrane) were chemically modified and functional carboxyl groups transformed to electropositive amine groups, using carbodiimide ethylenediamine. Transformation of carboxyl groups was evaluated by measuring total amine groups in all fractions (modified or not). Using equilibrium dialysis and Scatchard plots for the data, we have established that isolated unmodified cell envelopes (cytoplasmic membrane, peptidoglycan and outer membrane) possess at least two types of metal binding sites with different association constants ( K _a and K '_a). Introduction of positive charges into the bacterial envelopes resulted in the disappearance of one type of metal binding site which had the highest association constant value for Ni²⁺, Cu²⁺ and Zn²⁺. All fractions, modified or not, always presented at least two types of binding sites with different association constants for Cd²⁺.     

Cation-stimulated H+ efflux by intact roots of barley

Abstract. Rates of proton extrusion and potassium (⁸⁶Rb) influx by intact roots of barley ( Hordeum vulgare cvs . Fergus, Conquest and Betzes) plants were simultaneously measured in short-term (15min) experiments. The nature and extent of apparent coupling between these ion fluxes was explored by manipulating conditions of temperature, pH and cation composition and concentration during flux determinations. In addition, the influence of salt status upon these fluxes was examined. At low K⁺ concentrations (0.01 to 1 mol m⁻³), H⁺ efflux and K⁺ influx were strongly correlated in both low- and high-K⁺ roots, although K⁺: H⁺ exchange stoichiometries were almost consistently greater than 2:1. At higher concentrations (1 to 5 mol m⁻³), H⁺ efflux was either reduced or remained unchanged while K⁺ influxes increased. In the presence of Na₂SO₄, rates of H⁺ extrusion demonstrated similar cation dependence, although below 10 mol m⁻³ Na₂SO₄, H⁺ fluxes were generally 50% lower than in equivalent concentrations of K₂SO₄. These observations are considered in the context of current hypotheses regarding the mechanisms of k⁺/H⁺ exchange.     

Promoting effect of fusicoccin on Na+ efflux in barley roots: evidence for a Na+−H+ antiport

Abstract. The effect of fusicoccin (FC) on the K⁺stimulated Na⁺ efflux in root cells of Na⁺ loaded barley roots was studied. FC (0.02 mM) stimulated Na⁺ efflux in the presence of K⁺ and its effect was synergistic with that of K⁺, in a similar way as its effect on proton extrusion. Decreasing the pH of the elution medium promoted Na⁺ efflux and partially replaced the effect of FC. As FC is known to increase the electrochemical proton gradient at the plasmalemma level, these results are consistent with the hypothesis that Na⁺ is extruded in exchange for H⁺. A further support to this view came from the finding that Na⁺ efflux was also promoted by a lipophilic cation, tributylbenzylammonium (TBBA ⁺), which stimulates H ⁺ extrusion and is generally accepted not to enter the cells by means of the same carrier as K ⁺.     

Barium Evokes Glutamate Release from Rat Brain Synaptosomes by Membrane Depolarization: Involvement of K+, Na+, and Ca2+ Channels

Abstract: During K⁺ -induced depolarization of isolated rat brain nerve terminals (synaptosomes), 1 m M Ba²⁺ could substitute for 1 m M Ca²⁺ in evoking the release of endogenous glutamate. In addition, Ba²⁺ was found to evoke glutamate release in the absence of K⁺-induced depolarization. Ba²⁺ (1–10 m M ) depolarized synaptosomes, as measured by voltage-sensitive dye fluorescence and [³H]-tetraphenylphosphonium cation distribution. Ba²⁺ partially inhibited the increase in synaptosomal K⁺ efflux produced by depolarization, as reflected by the redistribution of radiolabeled ⁸⁶Rb⁺. The release evoked by Ba²⁺ was inhibited by tetrodotoxin (TTX). Using the divalent cation indicator fura-2, cytosolic [Ca²⁺] increased during stimulation by approximately 200 n M , but cytosolic [Ba²⁺] increased by more than 1 μ M . Taken together, our results indicate that Ba²⁺ initially depolarizes synaptosomes most likely by blocking a K⁺ channel, which then activates TTX-sensitive Na⁺ channels, causing further depolarization, and finally enters synaptosomes through voltage-sensitive Ca²⁺channels to evoke neurotransmitter release directly. Though Ba²⁺-evoked glutamate release was comparable in level to that obtained with K⁺-induced depolarization in the presence of Ca²⁺, the apparent intrasynaptosomal level of Ba²⁺ required for a given amount of glutamate release was found to be several-fold higher than that required of Ca²⁺.     

Hypoxia-Induced Catecholamine Release and Intracellular Ca2+ Increase via Suppression of K+ Channels in Cultured Rat Adrenal Chromaffin Cells

Abstract: Hypoxia (5% O₂) enhanced catecholamine release in cultured rat adrenal chromaffin cells. Also, the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) increased within 3 min in ∼50% of the chromaffin cells under hypoxic stimulation. The increase depended on the presence of extracellular Ca²⁺. Nifedipine and ω-conotoxin decreased the population of the cells that showed the hypoxia-induced [Ca²⁺]_i increase, showing that the Ca²⁺ influx was attributable to L- and N-type voltage-dependent Ca²⁺ channels. The membrane potential was depolarized during the perfusion with the hypoxic solution and returned to the basal level following the change to the normoxic solution (20% O₂). Membrane resistance increased twofold under the hypoxic condition. The current-voltage relationship showed a hypoxia-induced decrease in the outward K⁺ current. Among the K⁺ channel openers tested, cromakalim and levcromakalim, both of which interact with ATP-sensitive K⁺ channels, inhibited the hypoxia-induced [Ca²⁺]_i increase and catecholamine release. The inhibitory effects of cromakalim and levcromakalim were reversed by glibenclamide and tolbutamide, potent blockers of ATP-sensitive K⁺ channels. These results suggest that some fractions of adrenal chromaffin cells are reactive to hypoxia and that K⁺ channels sensitive to cromakalim and glibenclamide might have a crucial role in hypoxia-induced responses. Adrenal chromaffin cells could thus be a useful model for the study of oxygen-sensing mechanisms.     

Demonstration of a K+/H+ exchange activity in a photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

Abstract Washed cells of Rhodopseudomonas sphaeroides forma sp. denitrificans , grown under photodenitrifying conditions, exhibited K⁺ uptake dependent on the transmembrane proton gradient (Δ pH). These cells also acidified the suspension medium in response to K⁺ pulses both aerobically and anaerobically in light and in the dark. The results indicate that the photodenitrifier has a reversible K⁺/H⁺ exchange activity which reflects its role in regulating the intracellular K⁺ concentration, as well as intracellular pH. The acidification of the external medium resulting from K⁺ pulses was inhibited by carbonyl cyanide- m -chlorophenylhydrazone (CCCP) indicating that the antiporter is energy-dependent. Addition of KCl to washed cells depolarized the membrane potential (Δψ) with a concomitant increase in ΔpH, indicating that the K⁺/H⁺ antiporter was electrogenic.     

Growth, contents of K+ and kinetics of K+(86Rb) uptake in barley cultured at different low supply rates of potassium

Plants of barley ( Hordeum vulgare L. cv. Salve) were grown with 6.5–35% relative increase of K⁺ supply per day (RKR) using a special computer-controlled culture unit. After a few days on the culture solution the plants adapted their relative growth rate (RGR) to the rate of nutrient supply. The roots of the plants remained in a low salt status irrespective of the rate of nutrient supply, whereas the concentration of K⁺ in shoots increased with RKR. Both V_max and K_m for K⁺(⁸⁶Rb) influx increased with RKR. It is concluded that with a continuous and stable K⁺ stress, the K⁺ uptake system is adjusted to provide an effective K⁺ uptake at each given RKR. Allosteric regulation of K⁺ influx does not occur and efflux of K⁺ is very small.     

Long-term effects of abscisic acid on K+ transport in young wheat plants of different K+ status

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.     

Possible Involvement of K^＋/Na^＋ in Assessing the Seed Vigor Index

More substances leaked from a higher-vigor seed sample than from a lower-vigor sample. This indicates that, in some cases, electric conductivity does not represent seed vigor level very well, especially for high-vigor seeds. Results from germination, germination index, leachate conductivity, and the ratio of K^＋/Na^＋ from three-seed lots of Chinese cabbage （Brassica pekinensis （Louv.） Rupr） showed that K^＋/Na^＋ correlated well with germination and germination index. The ability of K^＋/Na^＋ to indicate well changes in vigor was further supported by investigation in soybean （Glycine max （L.） Merr.） seeds and another cultivar of Chinese cabbage seeds. Thus, seed leakage of K^＋/Na^＋ can accurately indicate seed vigor, whereas the conductivity test failed to do so. Furthermore, K^＋/Na^＋ showed up bigger quantitative differences in vigor level than did the conductivity test. This findings provide a more sensitive and accurate index for the assessment of seed vigor. The mechanisms of Na^＋ and K^＋ ion transport are also discussed.     

Involvement of Na+,K+-ATPase in the Mitogenic Effect of Insulin-Like Growth Factor-I on Cultured Rat Astrocytes

Abstract: We have previously reported that insulin/insulin-like growth factor (IGF)-I induced the α1 isoform of Na⁺,K⁺-ATPase in cultured astrocytes. In this study the effects of insulin/IGF-I on Na⁺,K⁺-ATPase activity and cell proliferation were examined in astrocytes cultured under the various conditions, to test the possible involvement of the enzyme activity in the mitogenic action of IGF-I on astrocytes. Insulin increased Na⁺,K⁺-ATPase activity and stimulated cell proliferation in subconfluent astrocytes (cultured for 7–14 days in vitro). In contrast, these effects were not observed in confluent cells (cultured for 28 days). Furthermore, insulin stimulated neither the enzyme activity nor [³H]thymidine incorporation in astrocytes preincubated in fetal calf serum-free medium for 2 days (quiescent cells) and treated with dibutyryl cyclic AMP (differentiated cells). The increases in Na⁺,K⁺-ATPase activity and expression of the α1 mRNA preceded the mitogenic effect. ¹²⁵I-IGF-I binding experiment showed that all the cells used here had similar binding characteristics. The insulin-induced increase in enzyme activity was not affected by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), and it was observed even in Ca²⁺-free medium. The stimulation by IGF-I of [³H]thymidine incorporation was attenuated by ouabain and a low external K⁺ level. These findings suggest that stimulation of Na⁺,K⁺-ATPase activity is involved in the mitogenic action of IGF-I on cultured astrocytes.     

Effects of amiodarone on K+, internal pH and Ca2+ homeostasis in Saccharomyces cerevisiae

In this study, amiodarone, at very low concentrations, produced a clear efflux of K⁺. Increasing concentrations also produced an influx of protons, resulting in an increase of the external pH and a decrease of the internal pH. The K⁺ efflux resulted in an increased plasma membrane potential difference, responsible for the entrance of Ca²⁺ and H⁺, the efflux of anions and the subsequent changes resulting from the increased cytoplasmic Ca²⁺ concentration, as well as the decreased internal pH. The Δ tok1 and Δ nha1 mutations resulted in a smaller effect of amiodarone, and Δ trk1 and Δ trk2 showed a higher increase of the plasma membrane potential. Higher concentrations of amiodarone also produced full inhibition of respiration, insensitive to uncouplers and a partial inhibition of fermentation. This phenomenon appears to be common to a large series of cationic molecules that can produce the efflux of K⁺, through the reduction of the negative surface charge of the cell membrane, and the concentration of this cation directly available to the monovalent cation carriers, and/or producing a disorganization of the membrane and altering the functioning of the carriers, probably not only in yeast.     

A Charybdotoxin-Sensitive, Ca2+-Activated K+ Channel with Inward Rectifying Properties in Brain Microvascular Endothelial Cells: Properties and Activation by Endothelins

Abstract: A charybdotoxin-sensitive, Ca²⁺-activated K⁺ channel was identified in cultured rat brain capillary endothelial cells by using conventional single-channel recording techniques and ⁸⁶Rb⁺-influx and efflux experiments. Channel activity was dependent on the presence of Ca²⁺ on the cytosolic face of the membrane with a threshold concentration of 100 n M . It was inhibited by charybdotoxin (IC₅₀ 30 n M ) and quinine (IC₅₀ 0.1 m M ) but not by apamin. K(Ca) channels showed unusual inward rectifying properties under asymmetrical ionic conditions. They were activated by endothelin-1 (EC₅₀ 0.7 n M ) and endothelin-3 (EC₅₀ 7–10 n M ). The actions of endothelins were prevented by BQ-123 ( K _i = 8 n M ) in a competitive fashion, hence suggesting the involvement of an ET_A-receptor subtype. The channel activity was unaffected by cyclic AMP- or cyclic GMP-elevating agents. The possible role of the intermediate conductance, Ca²⁺-activated K⁺ channels for mediating K⁺ movements across the blood-brain barrier is discussed.