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.     

A comparison of NH4+ and NO3– net fluxes along roots of rice and maize

Net fluxes of NH₄⁺ and NO₃^– along adventitious roots of rice ( Oryza sativa L.) and the primary seminal root of maize ( Zea mays L.) were investigated under nonperturbing conditions using ion-selective microelectrodes. The roots of rice contained a layer of sclerenchymatous fibres on the external side of the cortex, whereas this structure was absent in maize. Net uptake of NH₄⁺ was faster than that of NO₃^– at 1 mm behind the apex of both rice and maize roots when these ions were supplied together, each at 0·1 mol m^–3. In rice, NH₄⁺ net uptake declined in the more basal regions, whereas NO₃^– net uptake increased to a maximum at 21 mm behind the apex and then it also declined. Similar patterns of net uptake were observed when NH₄⁺ or NO₃^– was the sole nitrogen source, although the rates of NO₃^– net uptake were faster in the absence of NH₄⁺. In contrast to rice, rates of NH₄⁺ and NO₃^– net uptake in the more basal regions of maize roots were similar to those near the root apex. Hence, the layer of sclerenchymatous fibres may have limited ion absorption in the older regions of rice roots.     

Effects of external NaCI, and of changes in turgor pressure and volume, on K+ concentrations in Chlorella emersonii

Summary. Soluble potassium concentrations were determined for the slightly vacuolated, unicellular, walled alga Chlorella emersonii. Sap of cells grown in 1 mol m⁻³ NaCI contained 140 mol m⁻³ K⁺ and sap of cells grown in 125, 200, and 335 mol m⁻³ NaCI contained 160-180 mol m⁻³ K ⁺.
The possible regulation of K ⁺ concentrations by a system of lurgor and volume maintenance was investigated by supplying 3-0-methylglucose. This solute accumulates to 85-230 mol m⁻³ in C. emersonii , but is not metabolized. Accumulation of 3-0-methylglucose increased the volume of cells grown at both low and high NaCI by about 10%. Furthermore, accumulation of 3-0-methylglucose also increased turgor pressures of cells grown in 1 and 125 mol m⁻³ NaCI by 0.3 and 0.2 MPa, respectively. (Similar measurements were not attempted for cells grown in 200 and 335 mol m⁻³ NaCI, because of the insensitivity of available methods to measure turgor pressure of cells exposed to high external osmotic pressures.)
At all NaCI concentrations, the K ⁺ concentrations of cells which had accumulated 3-0-methylglucose were only 10-20 mol m⁻³ lower than K⁺ in cells which had not been supplied with 3-0-methylglucose. In contrast, accumulation of 3-0-methylglucose greatly decreased concentrations of the endogenous osmotic solutes, proline and sucrose, which accumulated in cells grown in 125 mol m⁻³ and higher NaCI concentrations.
It is concluded that K⁺ concentrations in Chlorella emersonii are not controlled by a system of turgor and volume maintenance.     

Fluxes of Na+, K+ and Cl-, and osmotic adjustment in Lycopersicon pimpinellifolium and L. esculentum during short- and long-term exposures to NaCl

NaCl (140 m M ) was applied to 14-day-old plants of salt-sensitive Lycopersicon esculentum Mill. cv. Volgogradskij and its wild relative L. pimpinellifolium Mill. accession PE-2. Changes in the relative growth rate of whole plant, and in the levels of inorganic and organic solutes in leaves, stems and roots were followed for 15 days after the application. Short-term salt exposure (4–6 days of salinization) resulted in enhanced relative growth rates for L. pimpinellifolium , but did not affect growth of L. esculentum , After 6 days of salinization, the relative growth rates of both species decreased significantly; leading to practically comparable growth rates for them by day 15. In all parts of both species, the contribution of organic solutes to the osmotic potential (Ψ_s) gradually decreased from 30% on day 0 to a value lower than 5% on day 4. In L. pimpinellifolium , compared to L. esculentum , short-term salt exposure resulted in (1) a higher percentage of adjustment of Ψ_s; and (2) increases in Na⁺ and K⁺ uptake rates, and in the levels of organic acids and proline (the level of which reached that of sugars, i.e., 10 μmol g^-1 dry weight. Conversely, in L. esculentum , drastic reductions of K⁺ uptake rates and organic acid levels occurred already on day 1. During long-term salt exposure, both species were able to adjust osmotically and both exhibited decreases in organic acid levels as well as in K⁺ uptake and accumulation rates in all parts. The results are discussed in an attempt to explain the adaptive responses during short-term salt exposure and the metabolic dysfunctions that lead to growth decrease after long-term exposure to salt.     

Growth, photosynthesis, and respiration of Chlorella sorokiniana after N-starvation. Interactions between light, CO2 and NH4+ supply

N-sufficient cells of Chlorella sorokiniana Shihira and Krauss, strain 211/8k, absorbed NH₄⁺ under light plus CO₂ conditions, when growth occurred, but not in darkness or in the absence of CO₂, when growth was inhibited. N-sufficient cells subjected to conditions of N-starvation for a 24-h period showed a marked loss of photosynthetic activity. Upon supply of NH₄⁺, N-starved cells sufflated with CO₂ air exhibited a time-dependent recovery of photosynthetic activity, both when suspended in light and in darkness. By contrast, growth only occurred in cells suspended in light. N-starved cells absorbed NH₄⁺ in darkness, but at a lower rate than in light. All of these data suggest that dark NH₄⁺ uptake is driven by N assimilation to recover from N-starvation and that the light-dependent NH₄⁺ uptake is driven by growth, being then influenced by conditions that affect recovery or growth. Unlike CO₂ conditions, in a CO₂-free atmosphere, absorption of NH₄⁺ by N-starved cells occurred at a higher rate in darkness than in light. Accordingly, resumption of photosynthetic potential after NH₄⁺ supply occurred in darkened cells, but not in illuminated cells. Respiratory activity of N-starved cells was enhanced up to 3-fold by NH₄⁺ and 2-fold by methylammonium, with different patterns, suggesting that respiratory enzymes were affected by N-metabolism, especially through short-term control mechanisms triggered by the expenditure of metabolic energy involved in N-metabolism.     

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 .     

Potential NH4+ and NO3− uptake in seven Sphagnum species

The rate of nitrogen uptake by seven Sphagnum species, which from a gradient from hummock to hollow and from ombrotrophic to minerotrophic conditions, was measured as the decrease in the concentrations of NH₄⁺ and NO₃⁻ from solutions in which capitula were grown under laboratory conditions.
The highest uptake rate was by individuals of each species with large capitula and a high number of ion exchange sites, i.e. lawn species ( S. pulchrum , S. fallax , S. papillosum and S. magellanicum ). On a dry-mass basis, the most effective species were the hummock species ( S. fuscum and S. rubellum ), even though these species have a low dry mass. Hummock species, which occur in high densities and have high potential N-uptake rates on a dry-mass basis, were the most effective species in retaining available nitrogen.     

Separation of metabolic and non-metabolic steps of Rb+ uptake in spring wheat roots with different K+ status

Uptake of Rb⁺ from a complete nutrient solution with 2.0 mM Rb⁺ was studied in roots of spring wheat seedlings ( Triticum aestivum L. cv. Svenno) with different K⁺ levels. The relationship between Rb⁺ uptake and concentration of K⁺ in the roots indicated a negative feedback mechanism operating through allosteric control. The Rb⁺ uptake process in root cells was divided into two steps: (1) binding of the ion in the free space, and (ii) transmembrane transport into the cytoplasm. Metabolic and non-metabolic components of uptake were separated by addition of the metabolic inhibitor 2,4-dinitrophenol (DNP) to the nutrient solution. It is suggested that metabolic Rb⁺ uptake requires energy in two uptake steps (for binding to the carrier entity in the free space and for transmembrane transport) or in one step only (for transmembrane transport), dependent on the K⁺ status of the roots. The change from metabolic to non-metabolic binding in the free space is accomplished by changing the conformational state of the carrier (slow/fast transitions). There may be a hysteretic effect on metabolic Rb⁺ uptake through a slow transition between carrier states. This is superimposed on the negative cooperativity, strengthening further cooperativity at intermediate K⁺ levels in the roots. Non-metabolic Rb⁺ uptake probably consists of two components, a carrier-mediated (facilitated diffusion) and a parallel diffusive component.     

Actual escape area and lateral escape from the xylem of the alkali ions Na+, K+, Rb+ and Cs+ in tomato

Approximation of the total escape area of the xylem in an inbred line of tomato (Ly-copersicon escutentum Mill. cv. Tiny Tim) with help of the frequency distribution of xylem vessel radii provides the possibility to calculate realistic escape constant values from uptake experiments of several elements into tomato stem segments. Comparison of the lateral escape rates of ²⁴Na⁺, ⁴²K⁺, ⁸⁶Rb⁺ and ¹³⁴Cs⁺ indicate that Na⁺ escape is rate-limited by its uptake into a rather constant number of surrounding cells, regardless of changes in the total escape area of the xylem vessels. The escape of K⁺, Rb⁺ and Cs⁺ seems to be proportional to the surface area of the xylem vessels and their escape is apparently controlled by their transport across the cell walls of the transport channels. The calculated small values for the escape rate constants (apparent permeability of the xylem cell walls, ca 2–3 · 10−⁹ m s−⁷) are probably due to the presence of lignin in the xylem cell walls, the discrimination between ions as a result of differing affinities and selectivities and the presence of other solutes in the applied solution.     

The Na+ and K+ Content of Isolated Torpedo Synaptosomes and Its Effect on Choline Uptake

Abstract: The Na⁺ and K⁺ concentrations in isolated Torpedo marmorata synaptosomes were determined. Synaptosomes made according to the method of Israël et al. have high internal Na⁺ (290 MM) and low internal K⁺ (30 mM) concentrations. Modification of the homogenisation media permitted the isolation of synaptosomes which could maintain transmembrane ion gradients (internal Na⁺, 96 mM; K⁺, 81 mM); 0.1 mM-ouabain abolished these gradients. The trans-membrane Na⁺ gradient started to dissipate after 15 min at 20°C. Inclusion of ATP in the homogenisation medium enabled the synaptosomes to maintain the Na⁺ gradient for about 90 min. The presence of these transmembrane ion gradients stimulated choline uptake sevenfold. It is concluded that (a) by selecting the isolation media, Torpedo synaptosomes can be prepared with transmembrane ion gradients; (b) these gradients are ouabain-sensitive and stimulate choline uptake: (c) the synaptosomes require additional ATP to maintain the ion gradients.     

Regulation of K+ and NO3− fluxes in roots of sunflower (Helianthus annuus) after changes in light intensity

Shoot activity has been reported to affect rates of ion uptake by plant roots in other ways than merely through supply of assimilates. To elucidate the mechanisms by which a signal from the upper part of the plant controls the rate of K⁺ and NO₃⁻ uptake by roots, both uptake of K⁺ and NO₃⁻ and secretion into the xylem of young sunflower plants ( Helianthus annuus L.) were measured after changes in light intensity.
No close correlation was observed between the uptake of NO₃⁻ and that of K⁺; an increase in light intensity produced a much greater stimulation of NO₃⁻ uptake than of K⁺ uptake. On the other hand, secretion of NO₃⁻ into the xylem was tightly coupled to that of K⁺, and this coupling was strongly disturbed by excision of the root. The results suggest the involvement of the K₂-malate shuttle on the regulation by the shoot of K⁺ and NO₃⁻ secretion in the xylem, which is linked to NO₃⁻ uptake, while K⁺ uptake is independent of this regulation mechanism.     

Salinity effects on oxygen consumption, gill Na+, K+-ATPase and ion regulation in juvenile coho salmon

The metabolic response of juvenile coho salmon Oncorhynchus kisutch to different salinities was examined, using whole-animal oxygen consumption rates and gill Na⁺, K⁺-ATPase activities as indicators of osmoregulatory energetics. Coho salmon smolts were acclimated to fresh water (FW), isosmotic salinity (ISO, 10‰) and sea water (SW, 28‰) and were sampled for up to 6 weeks for plasma levels of cortisol, glucose and ions (Na⁺, K⁺, Cl⁻), gill Na⁺, K⁺-ATPase activity and oxygen consumption rates. Following an initial adjustment period, plasma constituents in SW fish returned to near-FW values, indicating that the fish were acclimated to SW by day 21. Gill Na⁺, K⁺-ATPase activities on days 21 and 42 were lowest in ISO, higher in FW and highest in SW. This result is consistent with the idea that less energy would be required to maintain ion balance in an isosmotic environment, where the ionic gradients between extracellular fluid and water would be minimal. Oxygen consumption rates of swimming fish (1 body length s⁻¹), however, did not differ significantly between the three test salinities after 6 weeks. The results of this study suggest that the metabolic response of juvenile salmonids to changes in salinity is dependent on life-history stage (e.g. fry v . smolt), and that oxygen consumption rates do not necessarily reflect osmoregulatory costs.     

An effect of Na+ on K+ uptake in intact seedlings of Zea mays

Models for the regulation of K⁺ uptake in higher plant roots have become more complex as studies have moved from the level of excised low-salt roots to that of intact plants grown under fully autotrophic conditions. In this paper we suggest that some of the differences between the conditions are qualitative, possibly requiring fundamental changes to the model, rather than simply quantitative.
The uptake of K⁺ by low-salt roots of Zea mays L. [(A619 x Oh 43) x A632], was independent of Na⁺ concentration over a wide range. However, independence of Na⁺ was not the case in plants grown on complete nutrient medium in the light: inclusion of Na⁺ in the uptake medium enhanced K⁺ uptake. In the presence of Na⁺, K⁺ uptake rates were similar in whole plants with high root K⁺ contents to rates in excised or intact, low-salt roots.     

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.     

Elevated Levels of Glucose and L-Fucose Reduce 22Na+ Uptake and Whole Cell Na+ Current in Cultured Neuroblastoma Cells

Abstract: Na⁺ flux was studied in cultured neuroblastoma cells grown in medium containing increased glucose or L - fucose concentrations. Chronic exposure of neuroblastoma cells to 30 m M glucose or 30 m M L-fucose caused a decrease in ouabain-sensitive and veratridine-stimulated ²²Na⁺ uptake compared with cells cultured in unsupplemented medium. The Na⁺ current, determined by using whole-cell configuration of the patch clamp, was also decreased in these cells. Tetrodotoxin (3 μ M ), which blocked whole cell Na⁺ currents, also blocked veratridine-stimulated ²²Na⁺ accumulation. Culturing cells in medium containing 30 m M fructose as an osmotic control had no effect on Na⁺ flux. Specific [³H] saxitoxin binding was not affected by 30 m M glucose or 30 m M L-fucose compared with cells grown in unsupplemented medium, suggesting that the number of Na⁺ channels was not decreased. These studies suggest that exposing cultured neuronal cells to conditions that occur in the diabetic milieu alters Na⁺ transport and Na⁺-channel activity.     

ENERGETICS OF AMINO ACID TRANSPORT INTO BRAIN SLICES: EFFECTS OF K+ DEPLETION AND Rb+ OR Cs+ SUBSTITUTION ON AMINO ACID UPTAKE

Abstract— Mouse brain slices were depleted of K⁺ by three 10-min incubations-in oxygenated HEPES-buffered medium lacking glucose and K⁺. Addition of K⁺ or Rb⁺ (or Cs⁺, to a smaller degree) with glucose, or with succinate, malate, and pyruvate (SMP) before incubation at 37°C with ¹⁴C-amino acids restored active low-affinity transport of d -Glu, α-aminoisobutyrate (AIB), GABA, Gly, His, Val, Leu, Lys, and Orn. Ouabain at 1–2μ m with Rb⁺ was more inhibitory with SMP than with glucose, suggesting that the glycoside may affect specific energy coupling to transport. Valinomycin, in contrast, showed no specificity of inhibition of amino acid uptake with glucose or SMP and K⁺ or Rb⁺. Cs⁺ partially restored amino acid uptake, but Li⁺ was less effective than Cs ⁺. NaF at 10 m m with SMP + Rb⁺, or SMP + K⁺ did not inhibit amino acid uptake. Therefore, it was possible to dissociate glycolysis and Na⁺, K ⁺ -ATPase activity from amino acid transport. The ion replacements for K ⁺ that supported active amino acid transport indicate that the specificity of ions in possible ionic gradients for transport energetics should be reexamined.     

Nitroprusside and Cyclic GMP Stimulate Na+-Ca2+ Exchange Activity in Neuronal Preparations and Cultured Rat Astrocytes

Abstract: The effects of nitric oxide (NO)-generating agents on ⁴⁵Ca²⁺ uptake in rat brain slices and cultured rat astrocytes were studied in the presence of monensin, which is considered to drive the Na⁺-Ca²⁺ exchanger in the reverse mode. Sodium nitroprusside (SNP) at >10 µ M increased monensin-stimulated Ca²⁺ uptake in the slices, although it did not affect high K⁺-stimulated Ca²⁺ uptake. Another NO donor, 3-morpholinosydnonimine, was effective. The effect of SNP was antagonized by hemoglobin (50 µ M ), a NO scavenger, and mimicked by 8-bromo-cyclic GMP (100 µ M ). In rat brain synaptosomes, SNP increased monensin-stimulated Ca²⁺ uptake, but it did not affect high K⁺-stimulated Ca²⁺ uptake. 8-Bromocyclic GMP, but not SNP, increased Na⁺-dependent Ca²⁺ uptake significantly in synaptic membrane vesicles in the absence of monensin. In cultured rat astrocytes, SNP and 8-bromo-cyclic GMP increased Ca²⁺ uptake in the presence of ouabain and monensin, which were required for the Ca²⁺ uptake in the cells. These findings suggest that NO stimulates the Na⁺-Ca²⁺ exchanger in neuronal preparations and astrocytes in a cyclic GMP-dependent mechanism.     

The regulation of Mn2+ and Cu2+ uptake in cells of the yeast Candida utilis grown in continuous culture

Abstract Transport of Mn²⁺ was repressed in Candida utilis cells grown in continuous culture in high-Mn²⁺ (100 μM Mn²⁺) medium as compared to cells grown in basic (0.45 μM Mn²⁺) and low-Mn²⁺ (< 0.05 μM Mn²⁺) media. In contrast, no repression of Cu²⁺ uptake occurred in high-Cu²⁺-grown (25 μM Cu²⁺) cells as compared to cells grown in basic medium (0.54 μM Cu²⁺). Cu²⁺-limited cells did not hyperaccumulate Cu²⁺ and there was not significant difference in initial uptake rates for all 3 Cu²⁺ conditions. Mn²⁺ uptake appears to be regulated by a mechanism sensitive to the external Mn²⁺ concentration, whereas Cu²⁺ transport is not governed in this way by the external Cu²⁺.     

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.     

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.