Cd2+ uptake, Cd2+ binding and loss of cell K+ by a Cd-sensitive and a Cd-resistant strain of Saccharomyces cerevisiae

Abstract Uptake of Cd²⁺ into Cd-resistant cells was approximately four times lower than in Cd-sensitive cells of Saccharomyces cerevisiae . Binding of Cd²⁺ to the yeast cells increased during incubation of the cells in the presence of Cd²⁺. The increase in the binding was much higher for wild-type cells than for Cd-resistant cells. This increased binding is ascribed to permeabilization of part of the cells. There is no single relation between the relative rate of K⁺ efflux and the cellular Cd content as has been found previously for wild-type cells. The rates of K⁺ efflux were much less than those found for the wild-type cells. Only with short incubation periods of the cells with Cd²⁺ was the same dependence found between the efflux of K⁺ and the cellular Cd content for both types of cell. The discrepancies found after extended incubation of the cells with Cd²⁺ are ascribed to the fact that Cd-provoked K⁺ release proceeds via an all-or-nothing process and that K⁺ released from permeabilized cells can be reaccumulated in still intact cells. The latter proceeds more efficiently in Cd-resistant cells than in wild-type cells.     

Kinetics of efflux of K+ from cultured rose cells treated with ultraviolet light

Irradiation of cultured rose ( Rosa damascena Mill. cv. Gloire de Guilan) cells with ultraviolet light caused of loss of K⁺, which occurred with sigmoid kinetics. The kinetics of loss of K⁺ were not changed when the extracellular concentration of K⁺ was held constant during the period of efflux. Furthermore, the rate of loss of K⁺ was approximately the same even though the K⁺ concentration in the medium was increased from 0.1 to 10 m M . The kinetics of uptake of the lipophilic methyltriphenylphosphonium cation, an indicator of the plasma membrane potential, were linear throughout the period of K⁺ efflux, suggesting that the starting and stopping of K⁺ efflux do not reflect a passive response to changes in the membrane potential of the cells. The results are interpreted in terms of activation and inactivation of an efflux channel or pump for K⁺.     

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.     

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 ⁺.     

Sodium ion transport in Neurospora crassa

Na⁺ influx and efflux in Neurospora crassa RL21a can be studied separately to calculate net Na⁺ movements. In the absence of external K⁺, Na⁺ influx was independent of the K⁺ content of the cells, but when K⁺ was present, the inhibition of Na⁺ influx by external K⁺ was higher the higher the K⁺ content. Efflux depended on the K⁺ and Na⁺ content, and on the history of the cells. Efflux was higher the higher the Na⁺ and K⁺ contents, and, in low-K⁺ cells, the efflux was also higher in cells grown in the presence of Na⁺ than when Na⁺ was given to cells grown in the absence of Na⁺. Addition of K⁺ to cells in steady state with external Na⁺ resulted in a net Na⁺-loss. In cells grown without Na⁺ this loss was a consequence of the inhibition of Na⁺ influx. In Na⁺-grown cells, addition of K⁺ inhibited Na⁺ influx and increased Na⁺ efflux.     

EFFECTS OF ELEVATED [K+]0 ON THE RELEASE OF NEUROTRANSMITTERS FROM CORTICAL SYNAPTOSOMES: EFFLUX OR SECRETION?

Abstract— Elevated K⁺₀ elicited a substantial Ca-independent efflux of accumulated GABA from cortical synaptosomal fractions. Efflux from tissue labelled with either NE or choline was affected considerably less by elevated K⁺ pulses in the absence of calcium. K-facilitated Ca-dependent efflux was large for all three of the accumulated substances. K-dependent (Ca-independent) efflux of accumulated GABA was associated with all subcellular fractions exhibiting GABA accumulation whereas K-facilitated Ca-dependent efflux was restricted to fractions containing synaptosomes. Eighty per cent of both GABA accumulation and K-dependent efflux was, however, recovered in a purified synaptosomal fraction. Alanine slightly decreased GABA accumulation, but % K-dependent efflux was not affected.
Elevated K⁺, in the absence of calcium, released GABA from accumulated pools in preference to endogenous pools, whereas the Ca-dependent efflux, facilitated by K⁺, was similar for both accumulated and endogenous GABA.
The Ca-independent efflux of accumulated GABA increased linearly with log [K⁺]₀ between 10 and 70 mM-K⁺ in sodium-containing media. Prior treatment with veratridine or Na-free medium substantially decreased the Ca-independent but not the Ca-dependent GABA efflux produced by elevated K⁺ pulses.
The Ca-dependent and Ca-independent efflux of accumulated GABA in response to elevated K⁺ pulses is suggested to arise not only via different flux mechanisms but also from different GABA pools. The Ca-dependent efflux is interpreted to reflect stimulus-secretion coupling processes whereas the Ca-independent efflux may reflect membrane transport processes.     

Osmoregulation by potassium transport in Escherichia coli

Abstract Cell turgor pressure determines the extent of K⁺ accumulation by Escherichia coli cells. K⁺ influx is mediated both by a constitutive system with a low affinity for K⁺ (Trk) and by an inducible high affinity system (Kdp). K⁺ efflux is controlled by as yet unidentified but independent systems. Cell K⁺ concentration may be the link between growth in media of high osmolarity and the concomitant accumulation of compatible solutes such as betaine.     

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.     

The pha gene cluster of Rhizobium meliloti involved in pH adaptation and symbiosis encodes a novel type of K+ efflux system

The fix-2 mutant of Rhizobium meliloti affected in the invasion of alfalfa root nodules (Inf⁻/Fix⁻) is K⁺ sensitive and unable to adapt to alkaline pH in the presence of K⁺. Using directed Tn 5 mutagenesis, we delimited a 6 kb genomic region in which mutations resulted in both Inf⁻/Fix⁻ and K⁺-sensitive phenotypes. In this DNA region, seven open reading frames (ORFs) were identified and the corresponding genes were designated phaA , B , C , D , E , F and G . The putative PhaABC proteins exhibit homology to the subunits of a Na⁺/H⁺ antiporter from an alkalophilic Bacillus strain. Moreover, PhaA and PhaD also show similarity to the ND5 and ND4 subunits of the proton-pumping NADH:ubiquinone oxidoreductase respectively. Computer analysis suggests that all seven proteins are highly hydrophobic with several possible transmembrane domains. Some of these domains were confirmed by generating active alkaline phosphatase fusions. Ion transport studies on phaA mutant cells revealed a defect in K⁺ efflux at alkaline pH after the addition of a membrane-permeable amine. These results suggest that the pha genes of R. meliloti encode for a novel type of K⁺ efflux system that is involved in pH adaptation and is required for the adaptation to the altered environment inside the plant.     

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 .     

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.     

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.     

The influences of calcium and magnesium ions on the exchange of monovalent cations in Neurospora crassa

Low-K⁺, high-Na⁺ cells of strain RL21a of Neurospora crassa , in steady state with 25 m M Na⁺, were used to study K⁺/Na⁺ exchanges in the presence or absence of Ca²⁺ and Mg²⁺. In the presence of Ca²⁺ and Mg²⁺, a low concentration of K⁺ (0.3 m M ) triggered a rapid exchange, but in the absence of the divalents, a high K⁺ concentration (30 m M ) was required to initiate the exchange at a rapid rate. In the absence of Ca²⁺ and Mg²⁺, K⁺ uptake did not occur at low K⁺ concentration, internal K⁺ did not regulate Na⁺ influx in the presence of external K⁺, and the efflux of Na⁺ proceeded at maximum activity at very low-K⁺ contents.     

EFFECTS OF POTASSIUM AND GLUTAMATE ON BRAIN CORTEX SLICES: UPTAKE AND RELEASE OF GLUTAMIC AND OTHER AMINO ACIDS

Abstract— Effects of an increased concentration of K⁺ (55 m m ) in the medium on fluxes of glutamate and other amino acids in the presence and absence of 10 m m -glutamate were studied. The following observations were made:
(1) The efflux of glutamate is slightly increased by excess K⁺. The glutamate efflux is smaller than the potassium fluxes.
(2) The K⁺-induced increase of glutamate efflux is enhanced under anoxia or in glutamate-containing media.
(3) The influx of glutamate is unaffected or slightly increased by excess K⁺.
(4) The efflux of GABA is increased by excess K⁺, both in the absence and in the presence of glutamate.
(5) Efflux of glutamine, leucine and lysine is increased by excess K⁺, but only provided that glutamate is also present in the medium.
(6) Efflux of glutamate and of GABA is increased by addition of 10 m m -glutamate.     

Hydrogen peroxide formation in cultured rose cells in response to UV-C radiation

Suspension-cultured rose ( Rosa damascena Mill. cv. Gloire de Guilan) cells irradiated with UV-C (254 nm. 558 J m⁻²) showed a transient production of H₂O₂ as measured by chemiluminescence of luminol in the presence of peroxidase (EC 1.1 1.1.7). The peak concentration of H₂O₂, which occurred at about 60–90 min after irradiation, was 8–9 μ M . The time course for the appearance of H₂O₂ matched that for UV–induced K⁺ efflux. Treatments that inhibited the UV-induced efflux of K⁺, including heat and overnight incubation with cycloheximide and diethylmaleate, also inhibited the appearance of H₂O₂. The converse was not always true, since catalase (EC 1.11.1.6. and salicylhydroxamic acid, which inhibited luminescence, did not stop K⁺ efflux. We conclude that H₂O₂ synthesis depends on K⁺ efflux. Because H₂.O₂ in the extracellular space is required for lignin synthesis in many plant tissues, we suggest that the UV–stimulated production of H₂O₂ is an integral part of a defensive lignin synthesis.     

Passive uptake of K+(86Rb+) in sunflower roots at low external K+ concentrations

Passive fluxes of K⁺ (⁸⁶Rb) into roots of sunflower ( Helianthus annuus L. cv. Uniflorus) were determined at low K⁺ concentration (0.1 and 1.0 mM K⁺) in the ambient solution. Metabolic uptake of K⁺ was inhibited by 10⁻⁴M 2,4-dinitrophenol (DNP). K⁺ (⁸⁶Rb) fluxes were studied both continuously and by time differentiation of uptake. In high K⁺ roots passive uptake was directly proportional to the K⁺ concentration of the uptake solution, indicating free diffusion. This assumption was supported by the fact that passive Rb⁺ uptake was not affected by high K⁺ concentrations. In low K⁺ roots the passive uptake of K⁺ was higher than in high K⁺ roots. The increase was possibly due to carrier-mediated K⁺ transport. As K⁺ effluxes were quantitatively similar to influxes, it is suggested that passive K⁺ fluxes represent exchange diffusion without relation to net K⁺ transport.     

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.     

Net Glutamate Release from Astrocytes Is Not Induced by Extracellular Potassium Concentrations Attainable in Brain

Abstract: Elevated extracellular potassium concentration ([K⁺]_e) has been shown to induce reversal of glial Na⁺-dependent glutamate uptake in whole-cell patch clamp preparations. It is uncertain, however, whether elevated [K⁺]_e similarly induces a net glutamate efflux from intact cells with a physiological intracellular milieu. To answer this question, astrocyte cultures prepared from rat and mouse cortices were incubated in medium with elevated [K⁺]_e (by equimolar substitution of K⁺ for Na⁺), and glutamate accumulation was measured by HPLC. With [K⁺]_e elevations to 60 m M , medium glutamate concentrations did not increase during incubation periods of 5–120 min. By contrast, 45 min of combined inhibition of glycolytic and oxidative ATP production increased medium glutamate concentrations 50–100-fold. Similar results were obtained in both rat and mouse cultures. Studies were also performed using astrocytes loaded with the nonmetabolized glutamate tracer d -aspartate, and parallel results were obtained; no increase in medium d -aspartate content resulted from [K⁺]_e elevation up to 90 m M , whereas a large increase occurred during inhibition of energy metabolism. These results suggest that a net efflux of glutamate from intact astrocytes is not induced by any [K⁺]_e attainable in brain.     

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.     

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⁺.