Cation flux in the ehrlich ascites tumor cell evidence for Na-for-Na and K-for-K exchange diffusion

In a previous study, evidence was presented for an external Na⁺-dependent, ouabain-insensitive component of Na⁺ efflux and an external K⁺-dependent component of K⁺ efflux in the Ehrlich ascites tumor cell. Evidence is now presented that these components are inhibited by the diuretic furosemide and that under conditions of normal extracellular Na⁺ and K⁺ they represent Na⁺-for-Na⁺ and K-⁺for-K⁺ exchange mechanisms. Using ⁸⁶Rb to monitor K⁺ movements, furosemide is shown to inhibit an ouabain-insensitive component of Rb⁺ influx and a component of Rb⁺ efflux, both representing approx. 30% of the total fux. Inhibition of Rb⁺ efflux is greatly reduced by removal of extracellular K⁺. Furosemide does not alter steady-state levels of intracellular K⁺ and it does not prevent cells depleted of K⁺ by incubation in the cold from regaining K⁺ upon warming. Using ²²Na to monitor Na⁺ movements, furosemide is shown to inhibit an ouabain-insensitive component of unidirectional Na⁺ efflux which represents approx. 22% of total Na⁺ efflux. Furosemide does not alter steady-state levels of intracellular Na⁺ and does not prevent removal of intracellular Na⁺ upon warming from cells loaded with Na⁺ by preincubation in the cold. The ability of furosemide to affect unidirectional Na⁺ and K⁺ fluxes but not net fluxes is consistent with the conclusion that these components of cation movement across the cell membrane represent one-for-one exchange mechanisms. Data are also presented which demonstrate that the uptake of α-aminoisobutyrate is not affected by furosemide. This indicates that these components of cation flux are not directly involved in the Na⁺-dependent amino acid transport system A.     

Generation of plasma membrane potential by the Na+-pump coupled to proton extrusion

Lettré cells maintain a plasma membrane potential near — 60mV, yet are scarcely depolarized by 80 mM Rb⁺ and are relatively impermeable to ⁸⁶Rb⁺. They are depolarized by ouabain without a concomitant change in intracellular cation content. Addition of K⁺ to cells suspended in a K⁺ free medium, or of Na⁺ to cells in a Na⁺ free medium, hyperpolarizes the cells. They contain electroneutral transport mechanisms for Na⁺, K⁺ and H⁺ which can function as Na⁺:K⁺ and Na⁺:H⁺ exchanges. It is concluded that plasma membrane potential of Lettré cells, in steady-state for Na⁺ and K⁺, is produced by an electrogenic Na⁺ pump sustained by electroneutral exchanges, and restricted by anion leakage.     

Cation/proton antiport systems in Escherichia coli.

Three distinct systems which function as proton/cation antiports have been identified in $\text{E.}\text{coli}$ by the ability of the ions to dissipate the ΔpH component of the protonmotive force in everted vesicles. System I exchanges H⁺ for K⁺, Rb⁺ or Na⁺; System II has Na⁺ and Li⁺ as substrates; and System III catalyzes proton exchange for Ca²⁺, Mn²⁺ or Sr²⁺.     

Plasma membrane potential of Lettré cells does not depend on cation gradients but on pumps

Summary The plasma membrane potential of Lettré cells has been determined with the optical indicator oxonol-V and found to be –57 mV at 37°C (range –20 to –80 mV depending on the physiological condition of the cells). Increasing extracellular K⁺ does not depolarize cells: even in the presence of 155mM K⁺ the potential is –41 mV; membrane potential is also insensitive to the chemical gradient of Na⁺,Mg²⁺, Ca²⁺ or Cl^–. Ouabain depolarizes the cells; H⁺ efflux from cells is stimulated by extracellular Na⁺. We propose that in Lettré cells the plasma membrane potential is generated by electrogenic cation pumps. The balancing fluxes of Na⁺ and K⁺ are mainly through electroneutral cation exchanges (Na⁺/K⁺ and Na⁺/H⁺) and the magnitude of the potential is limited by organic anion leaks. Such a mechanism may operate in other biological membranes also.     

Induction of Na+ / K+ exchange in swollen heart mitochondria

Heart mitochondria swollen passively in nitrate salts contract in a respiration-dependent reaction which can be attributed to an endogenous cation/H⁺ exchange component (or components). The rate of contraction increases with increased extent of passive swelling in both Na⁺ and K⁺ salts. Since nearly constant internal cation concentrations are maintained during osmotic swelling, this result suggests that both Na⁺/H⁺ and K⁺/H⁺ exchange is enhanced by increased matrix volume. Endogenous Mg²⁺ is also lost with increased matrix volume, and this observation, in conjunction with other evidence available in the literature, suggests that monovalent cation/H⁺ exchanges may be regulated by divalent cations. Passive exchange of Na⁺/K⁺,⁴²K⁺/K⁺, and²⁴Na⁺/Na⁺ can be readily demonstrated in mitochondria swollen in nitrate. All these exchanges are low or not detectable in unswollen control mitochondria, and it appears that they are manifestations of the activated cation/H⁺ component (or components) functioning in the absence of pH.     

Potassium and Sodium Fluxes in the PhytopathogenicFungus Fusarium oxysporum var. lini

Rubidium uptake in potassium-starved cells followed biphasic kinetics in the micromolar and millimolar range and was independent of the temperature. In contrast, Rb⁺ uptake in normal-K⁺ cells followed a monophasic kinetics in the millimolar range and increased at temperatures higher than 30°C. Differences in the K _m values and in the Arrhenius plots of Rb⁺ uptake suggest different uptake systems in K⁺-starved and in normal-K⁺ cells. In addition, the substantial inhibition of Rb⁺ uptake caused by carbonyl cyanide-m-chlorophenyl hydrazone indicates that these systems are strongly dependent on membrane voltage. Lithium (sodium) tolerance, influx, and efflux were separately studied. F. oxysporum was shown to be very tolerant to sodium, while lithium caused a specific toxic effect. Li⁺ uptake in K⁺-starved cells exhibits a monophasic kinetics with low affinity. Li⁺ efflux was not affected by external pH or addition of potassium to the medium, suggesting that a Na⁺/cation antiporter is not involved in this process. Received: 14 March 2000 / Accepted: 5 June 2000     

Thiol-Dependent passive K/Cl transport in sheep red cells: IV. Furosemide inhibition as a function of external Rb+, Na+, and Cl−

Summary The effect of the loop diuretic furosemide (4-chloro-N-furfuryl-5-sulfamoyl-anthranilic acid) on the thiol-dependent, ouabain-insensitive K(Rb)/Cl transport in low K⁺ sheep red cells was studied at various concentrations of extracellular Rb⁺, Na⁺ and Cl^–. In Rb⁺-free NaCl media, 2×10^–3 m furosemide inhibited only one-half of thiol-dependent K⁺ efflux. In the presence of 23mm RbCl, however, the concentration of furosemide to produce 50% K⁺ efflux inhibition (IC₅₀) was 5×10^–5 m. In Rb⁺ containing NaCl media, the inhibitory effect of 10^–3 m furosemide was equal to that caused by NO ₃ ^– replacement of Cl^– in the medium. The apparent synergistic action of furosemide and external Rb⁺ on K⁺ efflux was also seen in the ouabain-insensitive Rb⁺ influx. A preliminary kinetic analysis suggests that furosemide binding alters both maximal K⁺(Rb⁺) transport and apparent external Rb⁺ affinity. In the presence of external Rb⁺, Na⁺ (as compared to choline) exerted a small but significant augmentation of the furosemide inhibition of K⁺(Rb⁺) fluxes. There was no effect of Cl^– on the IC₅₀ value of furosemide. As there is no evidence for coupled Na⁺K⁺ cotransport in low K⁺ sheep red cells, furosemide may modify thiol-dependent K⁺(Rb⁺/Cl flux or Rb⁺ (and to a slight degree Na⁺) modulate the effect of furosemide.     

Role of the furosemide-sensitive Na+/K+ transport system in determining the steady-state Na+ and K+ content and volume of human erythrocytesin vitro andin vivo

Summary To study the physiological role of the bidirectionally operating, furosemide-sensitive Na⁺/K⁺ transport system of human erythrocytes, the effect of furosemide on red cell cation and hemoglobin content was determined in cells incubated for 24 hr with ouabain in 145mm NaCl media containing 0 to 10mm K⁺ or Rb⁺. In pure Na⁺ media, furosemide accelerated cell Na⁺ gain and retarded cellular K⁺ loss. External K⁺ (5mm) had an effect similar to furosemide and markedly reduced the action of the drug on cellular cation content. External Rb⁺ accelerated the Na⁺ gain like K⁺, but did not affect the K⁺ retention induced by furosemide. The data are interpreted to indicate that the furosemide-sensitive Na⁺/K⁺ transport system of human erythrocytes mediates an equimolar extrusion of Na⁺ and K⁺ in Na⁺ media (Na⁺/K⁺ cotransport), a 1:1 K⁺/K⁺ (K⁺/Rb⁺) and Na⁺/Na⁺ exchange progressively appearing upon increasing external K⁺ (Rb⁺) concentrations to 5mm. The effect of furosemide (or external K⁺/Rb⁺) on cation contents was associated with a prevention of the cell shrinkage seen in pure Na⁺ media, or with a cell swelling, indicating that the furosemide-sensitive Na⁺/K⁺ transport system is involved in the control of cell volume of human erythrocytes. The action of furosemide on cellular volume and cation content tended to disappear at 5mm external K⁺ or Rb⁺. Thein vivo red cell K⁺ content was negatively correlated to the rate of furosemide-sensitive K⁺ (Rb⁺) uptake, and a positive correlation was seen between mean cellular hemoglobin content and furosemide-sensitive transport activity. The transport system possibly functions as a K⁺ and waterextruding mechanism under physiological conditiosin vivo. The red cell Na⁺ content showed no correlation to the activity of the furosemide-sensitive transport system.     

Ba2+-inhibitable86Rb+ fluxes across membranes of vesicles from toad urinary bladder

Summary ⁸⁶Rb⁺ fluxes have been measured in suspensions of vesicles prepared from the epithelium of toad urinary bladder. A readily measurable barium-sensitive, ouabain-insensitive component has been identified; the concentration of external Ba²⁺ required for half-maximal inhibition was 0.6mm. The effects of externally added cations on⁸⁶Rb⁺ influx and efflux have established that this pathway is conductive, with a selectivity for K⁺, Rb⁺ and Cs⁺ over Na⁺ and Li⁺. the Rb⁺ uptake is inversely dependent on external pH, but not significantly affected by internal Ca²⁺ or external amiloride, quinine, quinidine or lidocaine. It is likely, albeit not yet certain, that the conductive Rb⁺ pathway is incorporated in basolateral vesicles oriented right-side-out. It is also not yet clear whether this pathway comprises the principle basolateral K⁺ channel in vivo, and that its properties have been unchanged during the preparative procedures. Subject to these caveats, the data suggest that the inhibition by quinidine of Na⁺ transport across toad bladder does not arise primarily from membrane depolarization produced by a direct blockage of the basolateral channels. It now seems more likely that the quinidine-induced elevation of intracellular Ca²⁺ activity directly blocks apical Na⁺ entry.     

Arabidopsis NHX Transporters: Sodium and Potassium Antiport Mythology and Sequestration During Ionic Stress

A crucial prerequisite for plant growth and survival under high salinity is maintenance of Na⁺ and K⁺ balance. Accumulation of Na⁺ and K⁺ in high concentration in the cytosol reduces crop yield. To cope with such imbalance ionic conditions, plants use a number of transporters to maintain Na⁺ and K⁺ homoeostasis inside the cell and regulate plant growth and development. This cation and pH homoeostasis is regulated by monovalent cation/proton antiporters (CPA) that fall in two categories, the CPA1 family that includes Na⁺/H⁺ NHX antiporters, and the CPA2 family that includes Cation/H⁺ (CHX) and K⁺ efflux antiporters (KEA). In this review we highlighted the role of NHX-antiporters in regulation of Na⁺ and K⁺ balance. NHX proteins are required for accurate K⁺ compartmentation. They mediate K⁺ specific vacuolar sequestration, pH adjustment, turgor and osmotic regulation, and play a unique role in stomatal movement and cell expansion.     

High affinity k uptake in maize roots: a lack of coupling with h efflux

We report here on the putative coupling between a high affinity K⁺ uptake system which operates at low external K⁺ concentrations (K_m = 10-20 micromolar), and H⁺ efflux in roots of intact, low-salt-grown maize plants. An experimental approach combining electrophysiological measurements, quantification of unidirectional K⁺(⁸⁶Rb⁺) influx, and the simultaneous measurement of net K⁺ and H⁺ fluxes associated with individual cells at the root surface with K⁺- and H⁺-selective microelectrodes was utilized. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, and WJ Lucas [1987] Plant Physiol 84: 1177-1184) was used to quantify net ion fluxes from the measurement of electrochemical potential gradients for K⁺ and H⁺ ions within the unstirred layer at the root surface. No evidence for coupling between K⁺ uptake and H⁺ efflux could be found based on: (a) extremely variable K⁺:H⁺ flux stoichiometries, with K⁺ uptake often well in excess of H⁺ efflux; (b) dramatic time-dependent variability in H⁺ extrusion when both fluxes were measured at a particular location along the root over time; and (c) a lack of pH sensitivity by the high affinity K⁺ uptake system (to changes in external pH) when net K⁺ uptake, unidirectional K⁺(⁸⁶Rb⁺) influx, and K⁺-induced depolarizations of the membrane potential were determined in uptake solutions buffered at pH values from pH 4 to 8. Based on the results presented here, we propose that high affinity active K⁺ absorption into maize root cells is not mediated by a K⁺/H⁺ exchange mechanism. Instead, it is either due to the operation of a K⁺-H⁺ cotransport system, as has been hypothesized for Neurospora, or based on the striking lack of sensitivity to changes in extracellular pH, uptake could be mediated by a K⁺-ATPase as reported for Escherichia coli and Saccharomyces.     

Inhibition of Na+/K+-ATPase may be one mechanism contributing to potassium efflux and cell shrinkage in CD95-induced apoptosis

To investigate the involvement of K⁺ efflux in apoptotic cell shrinkage, we monitored efflux of the K⁺ congener,⁸⁶ Rb⁺, and cell volume during CD95-mediated apoptosis in Jurkat cells. An anti-CD95 antibody caused apoptosis associated with intracellular GSH depletion, a significant increase in ⁸⁶Rb⁺ efflux, and a decrease in cell volume compared with control cells. Preincubating Jurkat cells with Val-Ala-Asp-chloromethylketone (VAD-cmk), an inhibitor of caspase proteases, prevented the observed ⁸⁶Rb⁺ efflux and cell shrinkage induced by the anti- CD95 antibody. A wide range of inhibitors against most types of K⁺ channels could not inhibit CD95-mediated efflux of⁸⁶ Rb⁺, however, the uptake of⁸⁶ Rb⁺ by Jurkat cells was severely compromised when treated with anti-CD95 antibody. Uptake of⁸⁶ Rb⁺ in Jurkat cells was sensitive to ouabain (a specific Na⁺/K⁺-ATPase inhibitor), demonstrating Na⁺/K⁺-ATPase dependent K⁺ uptake. Ouabain induced significant⁸⁶ Rb⁺ efflux in untreated cells, as well as it seemed to compete with⁸⁶ Rb⁺ efflux induced by the anti-CD95 antibody, supporting a role for Na⁺/K⁺-ATPase in the CD95-mediated⁸⁶ Rb⁺ efflux. Ouabain treatment of Jurkat cells did not cause a reduction in cell volume, although together with the anti-CD95 antibody, ouabain potentiated CD95-mediated cell shrinkage. This suggests that the observed inhibition of Na⁺+/K⁺-ATPase during apoptosis may also facilitate apoptotic cell shrinkage.     

Relation between permeability to potassium and sodium ions and fusicoccin-stimulated hydrogen-ion efflux in barley roots

Summary Measurements are described of fusicoccin (FC)-stimulated H⁺ efflux in barley (Hordeum vulgare L.) roots when K⁺ and Na⁺ concentrations were varied. In low-salt roots H⁺ efflux was stimulated in both 5 mM KCl and NaCl. In salt-saturated roots H⁺ efflux was stimulated more effectively in KCl than in NaCl solution. The stimulation of H⁺ efflux thus is parallel with the selectivity of these different root preparations for K⁺ and Na⁺ and with estimates of permeability ratios (P _Na/P _K) determined from electrical measurements. It is suggested that the results support electrogenic coupling between FC-stimulated H⁺ efflux and cation uptake.     

NhaK,a novel monovalent cation/H<Superscript>+</Superscript> antiporter of <Emphasis Type="Italic">Bacillus subtilis</Emphasis>

Four Na⁺/H⁺ antiporters, Mrp, TetA(L), NhaC, and MleN have so far been described in Bacillus subtilis 168. We identified an additional Na⁺/H⁺ antiporter, YvgP, from B. subtilis that exhibits homology to the cation: proton antiporter-1 (CPA-1) family. The yvgP-dependent complementation observed in a Na⁺(Ca²⁺)/H⁺ antiporter-defective Escherichia coli mutant (KNabc) suggested that YvgP effluxed Na⁺ and Li⁺. In addition, effects of yvgP expression on a K⁺ uptake-defective mutant of E. coli indicated that YvgP also supported K⁺ efflux. In a fluorescence-based assay of everted membrane vesicles prepared from E. coli KNabc transformants, YvgP-dependent Na⁺ (K⁺, Li⁺, Rb⁺)/H⁺ antiport activity was demonstrated. Na⁺ (K⁺, Li⁺)/H⁺ activity was higher at pH 8.5 than at pH 7.5. Mg²⁺, Ca²⁺ and Mn²⁺ did not serve as substrates but they inhibited YvgP antiport activities. Studies of yvgP expression in B. subtilis, using a reporter gene fusion, showed a significant constitutive level of expression that was highest in stationary phase, increasing as stationary phase progressed. In addition, the expression level was significantly increased in the presence of added K⁺ and Na⁺.     

Biochimica et biophysica acta,vol. 644 (1981)

In the presence of an iso-osmotic concentration (0.4 M) of LiCl, the exit of cellular K⁺ and concomitant entry of Li⁺ in the marine bacterium, Vibrio alginolyticus, were enhanced by an increase in the medium pH, with an optimum at about pH 9.6. In addition to alkaline pH, the K⁺ exit in the NaCl medium required the presence of a weak base such as diethanolamine, ethanolamine or methylamine, which is permeable to the membrane in its unprotonated form. No net entry of Na⁺ was detected in this case and the amine accumulated in exchange for K⁺. The K⁺ exit observed at alkaline pH could be explained by the function of a K⁺/H⁺ antiporter. Once the cells were loaded with the amine, their exposure to the NaCl medium in the absence of loaded amine induced the entry of Na⁺. In RbCl or CsCl medium, fast entry of Rb⁺ or Cs⁺ and exit of K⁺ were observed at neutral pH (7.5), and the rate of K⁺ exit increased with the medium pH. From these results, we established a simple method for the replcement of cellular cations with a desired cation (Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺). The present method was found to be applicable also to Escherichia coli.     

K+/H+ antiporter in alkaliphilic Bacillus sp. no. 66 (JCM 9763)

A K⁺/H⁺ antiport system was detected for the first time in right-side-out membrane vesicles prepared from alkaliphilic Bacillus sp. no. 66 (JCM 9763). An outwardly directed K⁺ gradient (intravesicular K⁺ concentration, K_in, 100 mM; extravesicular K⁺ concentration, K_out, 0.25 mM) stimulated uphill H⁺ influx into right-side-out vesicles and created the inside-acidic pH gradient (ΔpH). This H⁺ influx was pH-dependent and increased as the pH increased from 6.8 to 8.4. Addition of 100 μM quinine inhibited the H⁺ influx by 75%. This exchange process was electroneutral, and the H⁺ influx was not stimulated by the imposition of the membrane potential (interior negative). Addition of K⁺ at the point of maximum ΔpH caused a rapid K⁺-dependent H⁺ eflux consistent with the inward exchange of external K⁺ for internal H⁺ by a K⁺/H⁺ antiporter. Rb⁺ and Cs⁺ could replace K⁺ but Na⁺ and Li⁺ could not. The H⁺ efflux rate was a hyperbolic function of K⁺ and increased with increasing extravesicular pH (pH_out) from 7.5 to 8.5. These findings were consistent with the presence of K⁺/H⁺ antiport activity in these membrane vesicles. Received: March 20, 1997 / Accepted: May 22, 1997     

Ouabain-resistant Na+, K+ transport system in mouse NIH 3T3 cells

Summary It is shown that the ouabain-resistant (OR) furosemide-sensitive K⁺(Rb⁺) transport system performs a net efflux of K⁺ in growing mouse 3T3 cells. This conclusion is based on the finding that under the same assay conditions the furosemidesensitive K⁺(Rb⁺) efflux was found to be two- to threefold higher than the ouabain-resistant furosemide-sensitive K⁺(Rb⁺) influx. The oubain-resistant furosemide-sensitive influxes of both²²Na and⁸⁶Rb appear to be Cl^– dependent, and the data are consistent with coupled unidirectional furosemide-sensitive influxes of Na⁺, K⁺ and Cl^– with a ratio of 1 1 2. However, the net efflux of K⁺ performed by this transport system cannot be coupled to a ouabain-resistant net efflux of Na⁺ since the unidirectional ouabain-resistant efflux of Na⁺ was found to be negligible under physiological conditions. This latter conclusion was based on the fact that practically all the Na⁺ efflux appears to be ouabainsensitive and sufficient to balance the Na⁺ influx under such steady-state conditions. Therefore, it is suggested that the ouabain-resistant furosemide-sensitive transport system in growing cells performs a facilitated diffusion of K⁺ and Na⁺, driven by their respective concentration gradients: a net K⁺ efflux and a net Na⁺ influx.     

Sodium or potassium efflux ATPase: A fungal, bryophyte, and protozoal ATPase

The K⁺ and Na⁺ concentrations in living cells are strictly regulated at almost constant concentrations, high for K⁺ and low for Na⁺. Because these concentrations correspond to influx-efflux steady states, K⁺ and Na⁺ effluxes and the transporters involved play a central role in the physiology of cells, especially in environments with high Na⁺ concentrations where a high Na⁺ influx may be the rule. In eukaryotic cells two P-type ATPases are crucial in these homeostatic processes, the Na,K-ATPase of animal cells and the H⁺-ATPase of fungi and plants. In fungi, a third P-type ATPase, the ENA ATPase, was discovered nineteen years ago. Although for many years it was considered to be exclusively a fungal enzyme, it is now known to be present in bryophytes and protozoa. Structurally, the ENA (from exitus natru: exit of sodium) ATPase is very similar to the sarco/endoplasmic reticulum Ca²⁺ (SERCA) ATPase, and it probably exchanges Na⁺ (or K⁺) for H⁺. The same exchange is mediated by Na⁺ (or K⁺)/H⁺ antiporters. However, in eukaryotic cells these antiporters are electroneutral and their function depends on a ΔpH across the plasma membrane. Therefore, the current notion is that the ENA ATPase is necessary at high external pH values, where the antiporters cannot mediate uphill Na⁺ efflux. This occurs in some fungal environments and at some points of protozoa parasitic cycles, which makes the ENA ATPase a possible target for controlling fungal and protozoan parasites. Another technological application of the ENA ATPase is the improvement of salt tolerance in flowering plants.     

Sodium transport measured in plasma membrane vesicles isolated from wheat genotypes with differing K+/Na+ discrimination traits

Right-side-out plasma membrane vesicles were isolated from wheat roots using an aqueous polymer two-phase system. The purity and orientation of the vesicles were confirmed by marker enzyme analysis. Membrane potential (Ψ)-dependent ²²Na⁺ influx and sodium/proton (Na⁺/ H⁺) antiport-mediated efflux across the plasma membrane were studied using these vesicles. Membrane potentials were imposed on the vesicles using either K⁺ gradients in the presence of valinomycin or H⁺ gradients. The ΔΨ was quantified by the uptake of the lipophilic cation tetraphenylphosphonium. Uptake of Na⁺ into the vesicles was stimulated by a negative ΔΨ and had a K_m for extrav-esicular Na⁺ of 34.8 ± 5.9 mol m³. The ΔΨ-dependent uptake of Na⁺ was similar in vesicles from roots of hexaploid (cv. Troy) and tetraploid (cv. Langdon) wheat differing in a K⁺/Na⁺ discrimination trait, and was also unaffected by growth in 50 mol m^?3 NaCl. Inhibition of ΔΨ-dependent Na⁺ uptake by Ca²⁺ was greater in the hexaploid than in the tetraploid. Sodium/proton antiport was measured as Na⁺-dependent, amiloride-inhibited pH gradient formation in the vesicles. Acidification of the vesicle interior was measured by the uptake of ¹⁴C-methylamine. The Na⁺/H⁺ antiport had a K_m, for intravesicular Na⁺ of between 13 and 19 mol m^?3. In the hexaploid, Na⁺/H⁺ antiport activity was greater when roots were grown in the presence of 50 mol m^?3NaCl, and was also greater than the activity in salt-grown tetraploid wheat roots. Antiport activity was not increased in a Langdon 4D chromosome substitution line which carries a trait for K⁺/Na⁺ discrimination. It is concluded that neither of the transport processes measured is responsible for the Na⁺/K⁺ discrimination trait located on the 4D chromosome of wheat.     

Studies on the mechanism of K+ transport in yeast.

The effect of uncouplers and diffusible acids on K⁺ transport was studied in yeast.Although the K⁺ transport system seems to depend on ATP to function, the effects of uncouplers are not due primarily to its action on the energy conserving systems of the cell.Other uncouplers with different structures to that of DNP showed also an inhibitory effect on K⁺ transport, which agrees with their reported ability to conduct protons through membranes.Uncouplers, besides inhibiting K⁺ uptake, produce an efflux of this cation; however, the rate of efflux produced is quantitatively important only when the cells have previously taken up the cation; there seems to exist a mechanism which prevents the loss of cations by yeast.In the absence of substrate, at pH 8.5, with 0.5 m KCl, TCS produces the efflux of H⁺, and when ⁸⁶Rb⁺ was used as a substitute for K⁺, an increase of the entrance of the cation could be detected in the presence of the uncoupler. It seems that the effect of the uncoupler depends on the direction of the combined H⁺ and K⁺ gradients, or the electrochemical potential of the cell.As reported by other authors, weak diffusible acids increase the uptake of K⁺ by yeast, and this effect is not due to changes in the metabolism, but to the magnitude of the entrance of the molecules to the yeast cell.It was found that the efflux of the acids (H₂CO₃), on the other hand, can produce an efflux of K⁺, which means that anions are important not only for the entrance of the cations, but for its permanence within the cell as well.The data seem to be in agreement with the hypothesis of the existence of a proton pump, responsible for the creation of an electrochemical potential, involved in K⁺ transport. At low pH, this pump seems to be activated by the transport of K⁺ into the cell.