Controls on na influx in corn roots

We have investigated the effects of hyperpolarization and depolarization, and the presence of K⁺ and/or Ca²⁺, on ²²Na⁺ influx into corn (Zea mays L.) root segments. In freshly excised root tissue which is injured, Na⁺ influx is unaffected by hyperpolarization with fusicoccin, or depolarization with uncoupler (protonophore), or by addition of K⁺. However, added Ca²⁺ suppresses Na⁺ influx by 60%. In washed tissue which has recovered, Na⁺ influx is doubled over that of freshly excised tissue, and the influx is increased by fusicoccin and suppressed by uncoupler. This energy-linked component of Na⁺ influx is completely eliminated by low concentrations of K⁺, leaving the same level and kind of Na⁺ influx seen in freshly excised roots. The K⁺-sensitive energy linkage appears to be by the carrier for active K⁺ influx. Calcium is equally inhibitory to Na⁺ influx in washed as in fresh tissue. Other divalent cations are only slightly less effective. Net Na⁺ uptake was about 25% of ²²Na⁺ influx, but proportionately the response to K⁺ and Ca²⁺ was about the same.

The constancy of K⁺-insensitive Na⁺ influx under conditions known to hyperpolarize and depolarize suggests that if Na⁺ transport is by means of a voltage-sensitive channel, the rise or fall of channel resistance must be proportional to the rise or fall in potential difference. The alternative is a passive electroneutral exchange of ²²Na⁺ for endogenous Na⁺. The data suggest that an inwardly directed Na⁺ current is largely offset by an efflux current, giving both a small net uptake and isotopic exchange.

     

Effects of Inhibitors of Protein and Nucleic Acid Synthesis on the Development of Ion Uptake Mechanisms in Beetroot Slices (Beta vulgaris)

Mechanisms for the uptake of K⁺, Na⁺ and Cl^- develop sequentially in thin slices of beetroot tissue washed under aerobic conditions. Actinomycin D inhibited or prevented the development of K⁺, Na⁺ and Cl^- uptake mechanisms when added to freshly cut slices, but had no effect on net ion uptake when added after the development of the ion uptake mechanisms. The use of puromycin as a specific inhibitor of protein synthesis was unsatisfactory as it caused leakage of pigments and excessive loss of ions from the disks. Cycloheximide prevented the development of ion uptake mechanisms when added at the start of the experiment, but when added after the development of ion uptake mechanisms its inhibitory effect did not become apparent until after a certain time interval which varied from 3 hours for Cl^- to 25 hours for K⁺ uptake in the same experiment. p-Fluorophenylalanine caused an appreciable shortening of the time required for the development of Na⁺ and K⁺ uptake capabilities, while it completely prevented the development of a Cl^- uptake mechanism. p-Fluorophenylalanine-induced early uptake of Na⁺ and K⁺, however, was followed by periods of net leakage of these ions. It is suggested that the development of ion uptake mechanisms depends on the production of m-RNA, which appears to be relatively stable after its synthesis. The synthesis and decay characteristics of specific proteins required for the ion uptake mechanisms appear to differ for each ion species.     

Effect of sodium on potassium fluxes at the cell membrane and vacuole membrane of red beet

Slices of red beet (Beta vulgaris) washed for 5 to 6 days are known to accumulate Na⁺ in preference to K⁺ from solutions containing both ions. The present work, using ion concentrations of 1.0 mm or less, with Ca²⁺ added in some cases, shows that Na⁺ strongly inhibits K⁺ influx at the cell membrane (plasmalemma) while K⁺ efflux is increased to a lesser extent. This result from compartmental analysis is confirmed by short (15-minute) influx experiments, which indicate an immediate inhibitory effect of Na⁺ on K⁺ influx at the cell membrane. It is concluded that cation selectivity, even when Na⁺ is favored for uptake, is primarily determined at the cell membrane. Nevertheless, a high level of K⁺ in the cytoplasm is maintained during Na⁺ influx, by an inhibition of K⁺ transfer to the vacuole.     

A partial sequence of ionic changes associated with the acrosome reaction of Strongylocentrotus purpuratus

Relationships among several of the ion movements associated with the acrosome reaction of S. purpuratus were investigated. Egg jelly initiates ⁴⁵Ca²⁺ and ²²Na⁺ uptake, and K⁺ and H⁺ efflux. H⁺ efflux and ²²Na⁺ uptake occur with approximately equivalent stoichiometries as rapidly as the appearance of acrosomal rods, perhaps reflecting a linked process. Most K⁺ loss, as measured either by ⁴²K⁺ efflux or K⁺-ion-selective electrodes, occurs after the acrosome reaction is complete. Since an elevation of seawater K⁺ (from 10 to 15 mM) or the addition of 0.5 mM tetraethylammonium (TEA), an inhibitor of K⁺ channels, inhibits the acrosome reaction half-maximally, K⁺ movements or alterations of K⁺-dependent membrane potentials may regulate the triggering by jelly. Most, but not all, of the ⁴⁵Ca²⁺ influx is inhibited with a mixture of 10 μM FCCP, 1 mM CN^?, and 2 μg/ml oligomycin, suggesting that the mitochondria store most of the Ca²⁺. The extracellular Na⁺ concentration affects Ca²⁺ fluxes: sperm placed into 5 mM Na⁺ seawater have enhanced ⁴⁵Ca²⁺ uptake, but do not undergo the acrosome reaction, unless 30 mM Na⁺ is also added. Low Na⁺ concentrations lead to spontaneous triggering, by allowing for both Ca²⁺ influx and Na⁺-dependent H⁺ efflux. At least one early Ca²⁺ requirement precedes the Na⁺ and H⁺ movements, as inferred from attempts at reversing the inhibitors of jelly induction of the acrosome reaction. When sperm are incubated with jelly in the absence of Ca²⁺, then washed and incubated with jelly in the presence of Ca²⁺, the acrosome reaction is triggered only upon the second incubation. However, when sperm are mixed with jelly in the presence of the other inhibitors (verapamil, TEA, 5 mM Na⁺ seawater, low pH, or elevated K⁺), they are altered so that even upon subsequent washing, jelly-mediated triggering is no longer possible. This suggests the existence of an intermediate state in the reaction pathway, that follows an event for which Ca²⁺ is required, but that precedes the Na⁺ and H⁺ movements, which are inhibited by all inhibitors of the acrosome reaction. These data are used to develop a partial sequence of ionic changes associated with the triggering mechanism.     

Potassium and sodium absorption kinetics in roots of two tomato species : lycopersicon esculentum and lycopersicon cheesmanii

Excised roots of the tomato species, Lycopersicon esculentum Mill. cv Walter (the commercial species) and of Lycopersicon cheesmanii ssp. minor (Hook.) C.H. Mull. (a wild species from the Galapagos Islands), were used in comparative studies of their absorption of K⁺ and Na⁺. Uptake of ⁸⁶Rb-labeled K⁺ and ²²Na-labeled Na⁺ by excised roots of `Walter' and L. cheesmanii varied as a function of genotype and tissue pretreatment with or without K<sup>+</sup>. Excised roots of `Walter' consistently absorbed more ⁸⁶Rb-labeled K⁺ than those of L. cheesmanii. Absorption of K⁺ from solutions ranging from 0.01 to 0.2 millimolar KCl showed saturation kinetics in both K⁺-pretreated and K⁺-depleted roots of `Walter,' and for K<sup>+</sup>-depleted roots of L. cheesmanii. K<sup>+</sup>-pretreated roots of L. cheesmanii had exceedingly low rates of K<sup>+</sup> uptake with strikingly different, linear kinetics. Pretreatment with K<sup>+</sup> caused a decrease in rates of K<sup>+</sup> uptake in both genotypes. Potassium depleted roots of L. cheesmanii absorbed Na<sup>+</sup> at a greater rate than those of `Walter,' whereas K⁺-pretreated roots of `Walter' absorbed Na⁺ at a greater rate than those of L. cheesmanii. The results confirm and extend previous conclusions to the effect that closely related genotypes may exhibit widely different responses to the two alkali cations, K⁺ and Na⁺.     

Ecosystem changes in Galápagos highlands by the invasive tree Cinchona pubescens

Background and aims

Salinity is an increasing problem for agricultural production worldwide. Understanding how Na⁺ enters plants is important if reducing Na⁺ influx, a key component of the regulation of Na⁺ accumulation in plants and improving salt tolerance of crop plants, is to be achieved. Our previous work indicated that two distinct low-affinity Na⁺ uptake pathways exist in the halophyte Suaeda maritima. Here, we report the external NaCl concentration at which uptake switches from pathway 1 to pathway 2 and the kinetics of the interaction between external K⁺ concentration and Na⁺ uptake and accumulation in S. maritima in order to determine the roles of K⁺ transporters or channels in low-affinity Na⁺ uptake.

Methods

Na⁺ influx, Na⁺ and K⁺ accumulations in S. maritima exposed to various concentrations of NaCl (0–200 mM) were analyzed in the absence and presence of the inhibitors TEA and Ba⁺ (5 mM TEA or 3 mM Ba²⁺) or KCl (0, 10 or 50 mM).

Results

Our earlier proposal was confirmed and extended that there are two distinct low-affinity Na⁺ uptake pathways in S. maritima: pathway 1 might be mediated by a HKT-type transporter under low salinity conditions and pathway 2 by an AKT1-type channel or a KUP/HAK/KT type transporter under high salinity conditions. The external NaCl concentration at which two distinct low-affinity Na⁺ uptake switches from pathway 1 to pathway 2, the ‘turning point’, is between 90 and 95 mM. Over a short period (12 h) of Na⁺ and K⁺ treatments, a low concentration of K⁺ (10 mM) facilitated Na⁺ uptake by S. maritima under high salinity (100–200 mM NaCl), whether or not the plants had been subjected to a longer (3 d) period of K⁺ starvation. The kinetics suggests that low concentration of K⁺ (10 mM) might activate AKT1-type channels or KUP/HAK/KT-type transporters under high salinity (100–200 mM NaCl).

Conclusions

The turning-point of external NaCl concentrations for the two low-affinity Na⁺ uptake pathways in Suaeda maritima is between 90 and 95 mM. A low concentration of K⁺ (10 mM) might activate AKT1 or KUP/HAK/KT and facilitate Na⁺ uptake under high salinity (100–200 mM NaCl). The kinetics of K⁺ on Na⁺ uptake and accumulation in S maritima are also consistent with there being two low-affinity Na⁺ uptake pathways.     

Coordination of AtHKT1;1 and AtSOS1 facilitates Na+ and K+ homeostasis in Arabidopsis thaliana under salt stress

Reducing Na⁺ accumulation and maintaining K⁺ stability in plant is one of the key strategies for improving salt tolerance. AtHKT1;1 and AtSOS1 are not only the salt tolerance determinants themselves, but also mediate K⁺ uptake and transport indirectly. To assess the contribution of AtHKT1;1 and AtSOS1 to Na⁺ homeostasis and K⁺ nutrition in plant, net Na⁺ and K⁺ uptake rate, Na⁺ and K⁺ distributions in Arabidopsis thaliana wild type (WT), hkt1;1 mutant (athkt1;1) and sos1 mutant (atsos1) were investigated. Results showed that under 2.5 mM K⁺ plus 25 or 100 mM NaCl, athkt1;1 shoot concurrently accumulated more Na⁺ and less K⁺ than did WT shoot, suggesting that AtHKT1;1 was critical for controlling Na⁺ and K⁺ distribution in plant; while atsos1 root accumulated more Na⁺ and absorbed lower K⁺ than did WT root, implying that AtSOS1 was determiner of Na⁺ excretion and K⁺ acquisition. Under 0.01 mM K⁺, athkt1;1 absorbed lower Na⁺ than did WT with 100 mM NaCl, suggesting that AtHKT1;1 is involved in Na⁺ uptake in roots; while atsos1 shoot accumulated less Na⁺ than did WT shoot no matter with 25 or 100 mM NaCl, implying that AtSOS1 played a key role in controlling long-distance Na⁺ transport from root to shoot. We present a model in which coordination of AtHKT1;1 and AtSOS1 facilitates Na⁺ and K⁺ homeostasis in A. thaliana under salt stress: under the normal K⁺, the major function of AtHKT1;1 is Na⁺ unloading and AtSOS1 is mainly involved in Na⁺ exclusion, whereas under the low K⁺, AtHKT1;1 may play a dominant role in Na+ uptake and AtSOS1 may be mainly involved in Na⁺ loading into the xylem.     

ACTIVE POTASSIUM TRANSPORT AND [Na++ K+]ATPase ACTIVITY IN CULTURED GLIOMA AND NEUROBLASTOMA CELLS

—The ouabain-sensitive K⁺ uptake and ATPase activities of cultured glioma and neuroblastoma cells were studied. Both cell lines showed ouabain-sensitive K⁺ uptake which correlated with the level of [Na⁺+ K⁺]ATPase activity found in the respective total cell homogenate. The glioma cells had a 2.1-fold higher rate of K⁺ uptake than neuroblastoma cells, and a 2.4-fold higher [Na⁺+ K⁺]ATPase activity. In the presence of ouabain neuroblastoma cells released K⁺ and took up Na⁺ in a 1:1 ratio. These results are compared and contrasted with similar studies on brain tissue and isolated cells. It is suggested that the cultured cell lines may serve as good models for the cation transport properties of their tissue counterparts.     

Water and electrolyte balance in protoscoleces of Echinococcus granulosus incubated in vitro: general procedures for the determination of water,sodium, potassium and chloride in protoscoleces

Reisin I.L. and Rotunno C.A. 1981. Water and electrolyte balance in protoscoleces of Echimcoccus granulosus incubated in vitro: General procedures for the determination of water, sodium, potassium and chloride in protoscoleces. International Journal for Parasitology11: 399–404. Protoscoleces of E. granulosus (sheep strain) were incubated in vitro at 37°C in Ringer Krebs solution (RKS) for up to 3 h. When they were briefly washed in sucrose 0.3 M at 4°C, the water and electrolyte contents were: 1.768 ± 0.034 mlg^?1 d.w. for water content and 123 ± 2, 209 ± 2 and 78 ± 2 μmolg^?1 d.w. for Na⁺, K⁺ and Cl^? respectively. When protoscoleces were not washed in sucrose solution but were spun down from RKS, the K⁺ content suffered a very small change but larger values for Na⁺ and Cl⁺ contents were obtained. These higher Na⁺ and Cl^? contents are attributed to the RKS ions retained in the trapping space. The steady state distribution of Na⁺ and K⁺ in the protoscoleces incubated at 37°C indicates the activity of an active transport mechanism.     

Energized transport of potassium ions in the absence of valinomycin by cytochrome c oxidase-reconstituted vesicles

Valinomycin-independent energized uptake of K⁺ was observed in cytochrome c oxidase reconstituted proteoliposome. The rate of K⁺ influx was proportoinal to the magnitude of electron flux. The energized uptake of K⁺ was abolished by p-trifluoromethoxycarbonylcyanide phenylhydrazone or by nigericin. Using the safranine fluorescence technique, it was demonstrated that even in the absence of valinomycin, liposomes and proteoliposomes reconstituted with cytochrome c oxidase are able to discriminate between Na⁺ and K⁺ and show a preference for K⁺ in the presence of excess Na⁺.     

Molecular analysis of the mechanism of potassium uptake through the TRK1 transporter of Saccharomyces cerevisiae

The TRK-HKT family of K⁺ transporters mediates K⁺ and Na⁺ uptake in fungi and plants. In this study, we have investigated the molecular mechanism involved in the movement of alkali cations through the TRK1 transporter of Saccharomyces cerevisiae. The model that best explains the activity of ScTRK1 is a cotransport of two K⁺ or Rb⁺, both of which bind the two binding sites of ScTRK1 with very high affinities in K⁺-starved cells. Na⁺ can be transported in the same way but it exhibits a much lower affinity for the second binding site. Therefore, only at critical concentration ratios between K⁺ and Na⁺, or Rb⁺ and Na⁺, the transporter takes up Na⁺ together with K⁺ or Rb⁺. Mutation analyses suggest that the two binding sites are located in the P fragment of the first MPM motif of the transporter, and that Gln⁹⁰ is involved in these binding sites. ScTRK1 can be in two states, medium or high affinity, and we have found that Leu⁹⁴⁹ is involved in the oscillation of the transporter between these two states. ScTRK1 mediates active K⁺ uptake. This is not Na⁺-coupled and direct coupling of ScTRK1 to a source of chemical energy seems more probable than K⁺-H⁺ cotransport.     

Interaction of phosphate with monovalent cation uptake in yeast

The uptake of monovalent cations by yeast via the monovalent cation uptake mechanism is inhibited by phosphate. The inhibition of Rb⁺ uptake shows saturation kinetics and the phosphate concentration at which halfmaximal inhibition is observed is equal to the K_m of phosphate for the sodiumindependent phosphate uptake mechanism. The kinetic coefficients of Rb⁺ and Tl⁺ uptake are affected by phosphate: the maximal rate of uptake is decreased and the apparent affinity constants for the translocation sites are increased.In the case of Na⁺ uptake, the inhibition by phosphate may be partly or completely compensated by stimulation of Na⁺ uptake via a sodium-phosphate cotransport mechanism.Phosphate effects a transient stimulation of the efflux of the lipophilic cation dibenzyldimenthylammonium from preloaded yeast cells and a transient inhibition of dibenzyldimethylammonium eptake. Possibly, the inhibition of monovalent cation uptake in yeast can be explained by a transient depolarization of the cell membrane by phosphate.     

Evidence for Na-Independent HCO(3) Uptake by the Cyanobacterium Synechococcus leopoliensis

At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na⁺. The rate of photosynthesis greatly exceeded the CO₂ supply rate and indicated that HCO₃⁻ was taken up by a Na⁺-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na⁺ and exceeded the CO₂ supply rate by 8 to 45 times. The apparent photosynthetic affinity (K_½) for DIC was high (6-40 micromolar) and was not markedly affected by Na⁺ concentration, whereas with air-grown cells K_½ (DIC) decreased by more than an order of magnitude in the presence of Na⁺. Lithium, which inhibited Na⁺-dependent HCO₃⁻ uptake in air-grown cells, had little effect on Na⁺-independent HCO₃⁻ uptake by standing culture cells. A component of total HCO₃⁻ uptake in standing culture cells was also Na⁺-dependent with a K_½ (Na⁺) of 4.8 millimolar and was inhibited by lithium. Analysis of ¹⁴C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na⁺-independent CO₂ transport system. The conversion from Na⁺-independent to Na⁺-dependent HCO₃⁻ uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO₃⁻ for subsequent photosynthetic fixation.     

Evidence for Hormonal Regulation of the Selectivity of Ion Uptake by Plant Cells

Kinetin exerted opposing effects on the uptake of K⁺ and of Na⁺ by leaf discs of Helianthus annuus: The absorption of K⁺ was stimulated and that of Na+ was inhibited. The K⁺/Na⁺ ratio in kinetin-treated discs was 80–100 % higher than in control tissue. Kinetin also promoted K⁺ uptake by detached cotyledons which had been removed from light-grown seedlings. On the other hand, no clear effect on the absorption of Na⁺ by these cotyledons could be established. Benzyladenine brought about a significant elevation in the K⁺/Na⁺ ratio in attached cotyledons. It is concluded that cytokinins bring about a change in the selectivity of the cells of sunflower leaves and cotyledons towards K⁺ and Na⁺, such that the ‘affinity’ of the cells for pottasium is increased, as compared to the ‘affinity’ for sodium. The possible significance of such a change in selectivity is discussed.     

Effects of sodium and potassium ions on the uptake of noradrenaline and its precursor tyrosine

The effects of the monovalent cations Na⁺ and K⁺ were studied on the uptake of noradrenaline and tyrosine by a crude synaptosomal fraction in vitro. Sodium ions produced opposite effects on the uptake of noradrenaline and the uptake of tyrosine viz. an increase in noradrenaline uptake and a decrease in the uptake of its precursor tyrosine. A low concentration of K⁺ stimulated the uptake of noradrenaline in the presence of Na⁺, while in the absence of Na⁺ K⁺ had no effect. However, the uptake of tyrosine could be stimulated by low K⁺ in the absence of Na⁺. Besides the increased uptake in the absence of Na⁺, a second uptake was found which was Na⁺, K⁺ activated ATPase dependent. The contribution of this uptake system to the total uptake of tyrosine was about 20%. No evidence was obtained for the involvement of a Na⁺, K⁺ activated ATPase in noradrenaline uptake. It is suggested that another ATPase might be involved in the latter uptake system.     

Lyn regulates creatine uptake in an imatinib-resistant CML cell line

BackgroundImatinib mesylate (imatinib) is the first-line treatment for newly diagnosed chronic myeloid leukemia (CML) due to its remarkable hematologic and cytogenetic responses. We previously demonstrated that the imatinib-resistant CML cells (Myl-R) contained elevated Lyn activity and intracellular creatine pools compared to imatinib-sensitive Myl cells.MethodsStable isotope metabolic labeling, media creatine depletion, and Na⁺/K⁺-ATPase inhibitor experiments were performed to investigate the origin of creatine pools in Myl-R cells. Inhibition and shRNA knockdown were performed to investigate the specific role of Lyn in regulating the Na⁺/K⁺-ATPase and creatine uptake.ResultsInhibition of the Na⁺/K⁺-ATPase pump (ouabain, digitoxin), depletion of extracellular creatine or inhibition of Lyn kinase (ponatinib, dasatinib), demonstrated that enhanced creatine accumulation in Myl-R cells was dependent on uptake from the growth media. Creatine uptake was independent of the Na⁺/creatine symporter (SLC6A8) expression or de novo synthesis. Western blot analyses showed that phosphorylation of the Na⁺/K⁺-ATPase on Tyr 10 (Y10), a known regulatory phosphorylation site, correlated with Lyn activity. Overexpression of Lyn in HEK293 cells increased Y10 phosphorylation (pY10) of the Na⁺/K⁺-ATPase, whereas Lyn inhibition or shRNA knockdown reduced Na⁺/K⁺-ATPase pY10 and decreased creatine accumulation in Myl-R cells. Consistent with enhanced uptake in Myl-R cells, cyclocreatine (Ccr), a cytotoxic creatine analog, caused significant loss of viability in Myl-R compared to Myl cells.ConclusionsThese data suggest that Lyn can affect creatine uptake through Lyn-dependent phosphorylation and regulation of the Na⁺/K⁺-ATPase pump activity.General significanceThese studies identify kinase regulation of the Na⁺/K⁺-ATPase as pivotal in regulating creatine uptake and energy metabolism in cells.     

Effects of temperature and dinitrophenol on the uptake of potassium and sodium ions in Ricinus communis roots

Summary Detopped root systems of Ricinus communis plants were used for the study of the effects of temperature and DNP on the uptake of K and Na ions supplied as KNO₃ and NaNO₃.When K and Na ions were offered together in equivalent concentrations, the steady state uptake rates for K⁺ and Na⁺ at 23 to 25° gave a K⁺/Na⁺ ratio of 3. Increasing the Na⁺ concentration relative to K⁺ 3-fold did not alter the preferential uptake of K⁺. The uptake of K⁺ was more sensitive to temperature in the range 10 to 40° and to the application of DNP at 1.5x10^-4 M than was the uptake of Na⁺. When NaNO₃ was the only salt supplied Na⁺ uptake became more sensitive to DNP than when both K⁺ and Na⁺ nitrates were supplied. Prolonged application of DNP led to net K⁺ efflux from the roots, even when no K⁺ was being supplied to the roots. Net Na⁺ efflux under the influence of DNP occurred only in roots previously grown on Na-containing nutrient medium.The different responses of the K⁺ and Na⁺ uptake processes to temperature and DNP suggest the operation of different uptake mechanisms for K⁺ and Na⁺ These results have been considered in relation to the recent concept of dual mechanisms for the absorption of alkali cations by plant tissues.     

Alkali cation selectivity of the wheat root high-affinity potassium transporter HKT1

The wheat root high-affinity K⁺ transporter HKT1 functions as a sodium-coupled potassium co-uptake transporter. At toxic millimolar levels of sodium (Na⁺), HKT1 mediates low-affinity Na⁺ uptake while potassium (K⁺) uptake is blocked. In roots, low-affinity Na⁺ uptake and inhibition of K⁺ uptake contribute to Na⁺ toxicity. In the present study, the selectivity among alkali cations of HKT1 expressed in Xenopus oocytes and yeast was investigated under various ionic conditions at steady state. The data show that HKT1 is highly selective for uptake of the two physiologically significant alkali cations, K⁺ and Na⁺ over Rb⁺, Cs⁺ and Li⁺. In addition, Rb⁺ and Cs⁺, and an excess of extracellular K⁺ over Na⁺, are shown to partially reduce or block HKT1-mediated K⁺-Na⁺ uptake. Furthermore, K⁺, Rb⁺ and Cs⁺ also effectively reduce outward currents mediated by HKT1, thereby causing depolarizations. In yeast, HKT1 can produce high-affinity Rb⁺ uptake at approximately 15-fold lower rates than for K⁺. Rb⁺ influx in yeast can be mediated by the ability of the yeast plasma membrane proton pump to balance the 35-fold lower HKT1 conductance for Rb⁺. A model for HKT1 activity is presented involving a high-affinity K⁺ binding site and a high-affinity Na⁺ binding site, and competitive interactions of K⁺, Na⁺ and other alkali cations for binding to these two sites. Possible implications of the presented results for physiological K⁺ and Na⁺ uptake in plants are discussed.     

Energy-dependent sodium efflux and sodium-dependent α-aminoisobutyrate transport in purple photosynthetic bacteria

Uptake of alanine and its nonmetabolizable analog α-aminoisobutyric acid (AIB) by the photosynthetic purple sulfur bacterium Chromatium vinosum is stimulated fivefold by Na⁺. Neither Li⁺ nor K⁺ have any stimulatory effect. AIB uptake can be supported by a Na⁺ gradient in the absence of other energy sources. AIB uptake is also accompanied by Na⁺ uptake. These results suggest that AIB is taken up by C. vinosum via a sodium symport. Cells of C. vinosum and the purple nonsulfur bacterium Rhodospirillum rubrum show energy-dependent Na⁺ efflux and Na⁺ uptake can be demonstrated with chromatophores prepared from these bacteria.     

Activation of Rb+ and Na+ uptake into yeast by monovalent cations

⁸⁶Rb⁺ uptake by yeast was not only stimulated by Rb⁺ or K⁺ but also by Na⁺. The uptake of ²²Na⁺ was enhanced by both Rb⁺ and K⁺, but not by Na⁺, which was inhibitory at all concentrations applied. Inhibition of ²²Na⁺ uptake by inactive Na⁺ occurred in two phases: one phase refers to inhibition at low Na⁺ concentrations and the other to inhibition at high Na⁺ concentrations. Our results can be qualitatively described by a two-site transport mechanism, having two cation binding sites, which must be occupied with monovalent cations before transport can occur.