CL−-Fluxe,Akkumulation und Transport in abgeschnittenen Mais-Wurzeln

Summary Fluxes, accumulation and transport rates of Cl^- in excised corn roots were investigated.Flux equilibrium in 5×10^-4M KCl was not reached within an experimental period of 28 hr.Transport of Cl^- (5×10^-4M KCl) through the xylem of 4–6 cm long excised corn roots had a lag of 1–2 hr. From 6 to 28 hr rates of accumulation and transport were nearly constant (5×10^-4M KCl). The velocity of volume-flow within the xylem was 1.5–2 cm hr^-1 (5×10^-4M KCl). ³⁶Cl^--efflux through the cut end of roots preloaded in K³⁶Cl of low concentration exhibits a discontinuity which is explained by addition of two fluxes: efflux of ions concentrated in the xylem (and symplasm) plus efflux from the vacuoles through the xylem.Accumulation of Cl^- by excised roots approaches a maximum level (Jackson and Edwards, 1966). Influx rates remain constant while efflux rates increase with time. It is shown in this paper that the flux of Cl^- through the xylem becomes a large proportion of the influx across the plasmalemma. Flux rates suggest that more than 50% of the Cl^- ions transported to the xylem passed through a vacuole (5×10^-4M KCl; 20–28 hr).     

Fluxes and compartmentation of potassium and chloride in the green alga Mougeotia

Summary Some ionic relations of the filamentous green alga Mougeotia sp. have been analyzed under different light conditions. Data from influx and efflux measurements using ⁸⁶Rb⁺ and ³⁶Cl^- fit the model of three cellular compartments (cell wall, cytoplasm, vacuole) in series. This result is remarkable, since in a Mougeotia cell at least two thirds of the cytoplasmic compartment are occupied by the cell-filling, flat and nearly rectangular chloroplast which is axially oriented. The chloroplast is concluded to be part of the cytoplasmic flux compartment.Photosynthetically saturating irradiances of continuous white light enhance the active and passive fluxes of K⁺ and Cl^- at the plasmalemma by a factor of 3. Photosystem II is responsible for the light-dependent increase of the uptake of Cl^- (³⁶Cl^-) whereas the uptake of K⁺ (⁸⁶Rb⁺) depends additionally on energy from photosystem I.Ion flux measurements performed after irradiations with red and far-red, respectively, show that the fluxes of K⁺ and Cl^- across the plasmalemma are not affected by the state of phytochrome.     

Cation Effects on Chloride Fluxes and Accumulation Levels in Barley Roots

Accumulation of Cl^- by excised barley roots, as of K⁺, approaches a maximum level at which the ion influx and efflux rates become equal. The rate of Cl^- influx at this equilibrium is close to the initial rate while the efflux rate increases with time from zero to equality with influx. The Cl^- fluxes are independent of simultaneous exchange flux of the cations, but depend on the nature and concentration of the salt solutions from which they originate. The Cl^- content at equilibrium, however, is largely independent of the external concentrations. The approach to equilibrium reflects the presence of the cation. Cl^- flux equilibrium is attained more rapidly in KCl than in CsCl or CaCl₂. This is presumably an effect of much slower distribution of Cs⁺ and Ca⁺⁺ than of K⁺ within the roots. Accumulated Cs⁺ appears to form a barrier to ion movement primarily within the outermost cells, thereby reducing influx and ultimately efflux rates of both Cl^- and cations. Slow internal mixing and considerable self-exchange of the incoming ions suggest internal transport over a series of steps which can become rate-limiting to the accumulation of ions in roots.     

Fluxes and compartmentation of K+, Na+ and Cl−, and action of auxins in suspension-cultured Petroselinum cells

Transport of ⁸⁶Rb⁺/K⁺, ²²Na⁺, ³⁶Cl^?, and [³H]indole acetic acid (IAA) has been studied on suspension-cultured cells of the parsley, Petroselinum crispum (Mill) Nym. By compartmental analysis two intracellular compartments of K⁺, Na⁺, and Cl^? have been identified and ascribed to the cytoplasm and vacuole; half-times of exchange were around 200 s and 5 h, respectively. According to the Ussing-Teorell flux equation, active transport is required for the influx into the cytoplasm at the plasmalemma (K⁺, Cl^?) and the tonoplast (K⁺, Na⁺, Cl^?). The plasmalemma permeability pattern, P_K:P_Na:P_Cl=1.00:0.24:0.38, features an increased chloride permeability compared with cells from higher plant tissues. IAA uptake showed an exponential timecourse, was half-maximal after 10 min, and a linear function of the IAA concentration from 10^?9 to 10^?5 M. IAA and 2,4-dichlorophenoxy acetic acid reduce the apparent influx of K⁺, Na⁺, Cl^? during the initial 30 min after addition and subsequently accelerate both in- and efflux of these ions. We discuss that auxins could affect the ion fluxes in a complex way, e.g. by protonophorous activity and by control of the hypothetical proton pump.     

The effect of a hyposmotic shock and purinergic agonists on K(Rb) efflux from cultured human breast cancer cells

The effect of a hyposmotic shock and extracellular ATP on the efflux of K⁺(Rb⁺) from human breast cancer cell lines (MDA-MB-231 and MCF-7) has been examined. A hyposmotic shock increased the fractional efflux of K⁺(Rb⁺) from MDA-MB-231 cells via a pathway which was unaffected by Cl⁻ replacement. Apamin, charybdotoxin or removing extracellular Ca²⁺ had no effect on volume-activated K⁺(Rb⁺) efflux MDA-MB-231 cells. An osmotic shock also stimulated K⁺(Rb⁺) efflux from MCF-7 cells but to a much lesser extent than found with MDA-MB-231 cells. ATP-stimulated K⁺(Rb⁺) efflux from MDA-MB-231 cells in a dose-dependent fashion but had little effect on K⁺(Rb⁺) release from MCF-7 cells. ATP-stimulated K⁺(Rb⁺) efflux was only inhibited slightly by replacing Cl⁻ with NO₃⁻. Removal of external Ca²⁺ during treatment with ATP reduced the fractional efflux of K⁺(Rb⁺) in a manner suggesting a role for cellular Ca²⁺ stores. Charybdotoxin, but neither apamin nor iberiotoxin, inhibited ATP-stimulated K⁺(Rb⁺) release from MDA-MB-231 cells. Suramin inhibited the ATP-activated efflux of K⁺(Rb⁺). UTP also stimulated K⁺(Rb⁺) efflux from MDA-MB-231 cells whereas ADP, AMP and adenosine were without effect. A combination of an osmotic shock and ATP increased the fractional efflux of K⁺(Rb⁺) to a level greater than the sum of the individual treatments. It appears that the hyposmotically-activated and ATP-stimulated K⁺ efflux pathways are separate entities. However, there may be a degree of ‘crosstalk’ between the two pathways.     

Chloride transport in Anacystis nidulans

Summary Anacystis nidulans will take up and accumulate chloride ions. When the external concentration was 0.2 mM Cl^- the level in the cells was 2.8 mM Cl^- and under these conditions the flux across the cell surface was in the region of 10^-13equiv Cl^-·sec^-1·cm^-2. It is suggested that this Cl^- influx is active and operates against an electrochemical potential gradient estimated to be 117 mV or 2.68 kcal/mole. The uptake of ³⁶Cl was inhibited by low temperatures and there was a net loss of Cl^- from the cells with the level decreasing towards the equilibrium value as estimated from K⁺ distribution. Although the active influx of Cl^- was often stimulated by light this was not always the case. Dark storage treatment and regulation of the chlorophyll a/phycocyanin ratios as well as total pigment content of the cells did not clarify the conditions which brought about light stimulation. Moreover, the metabolic inhibitors CCCP and CMU and also the use of anaerobic conditions did not clearly indicate the relationship between the influx mechanism and light-dark metabolism and no firm conclusions could be made about the nature of the energy source. The variation in the degree of light stimulation probably reflects the fact that in this procaryotic organism the photosynthetic and respiratory units are located on the same membrane systems and are in very close proximity to the probable site of the Cl^- pump, the plasmalemma.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - CMU 3-(4-chlorophenyl)-1,1-dimethylurea     

Evidence for differential action of indoleacetic acid upon ion fluxes in single cells of Petroselinum sativum

Summary The apparent influx of ³⁶Cl^- and ⁸⁶Rb⁺/K⁺ into cells from the higher plant Petroselinum sativum has been measured during the presence and absence in the culture medium of indolacetic acid (IAA) which is an essential auxin of these cells. While 10^-5 M IAA did not significantly affect the influx of ⁸⁶Rb⁺/K⁺, it substantially reduced that of ³⁶Cl^-, i.e. by a factor 0.25 within 30 min. This differential action of IAA, which holds for a reasonable range of external pH, is assumed to bear on current hypotheses that the primary events of auxin action involve plasmalemma functions.     

Potassium Transport in Corn Roots : III. Perturbation by Exogenous NADH and Ferricyanide

It has recently been reported that plasmalemma electron transport may be involved in the generation of H⁺ gradients and the uptake of ions into root tissue. We report here on the influence of extracellular NADH and ferricyanide on K⁺ (⁸⁶Rb⁺) influx, K⁺ (⁸⁶Rb⁺) efflux, net apparent H⁺ efflux, and O₂ consumption in 2-centimeter corn (Zea mays [A632 × Oh43]) root segments and intact corn roots. In freshly excised root segments, NADH had no effect on O₂ consumption and K⁺ uptake. However, after the root segments were given a 4-hour wash in aerated salt solution, NADH elicited a moderate stimulation in O₂ consumption but caused a dramatic inhibition of K⁺ influx. Moreover, net apparent H⁺ efflux was significantly inhibited following NADH exposure in 4-hour washed root segments.

Exogenous ferricyanide inhibited K⁺ influx in a similar fashion to that caused by NADH, but caused a moderate stimulation of net H⁺ efflux. Additionally, both reagents substantially altered K⁺ efflux at both the plasmalemma and tonoplast.

These complex results do not lend themselves to straightforward interpretation and are in contradiction with previously published results. They suggest that the interaction between cell surface redox reactions and membrane transport are more complex than previously considered. Indeed, more than one electron transport system may operate in the plasmalemma to influence, or regulate, a number of transport functions and other cellular processes. The results presented here suggest that plasmalemma redox reactions may be involved in the regulation of ion uptake and the `wound response' exhibited by corn roots.

     

The effect of the uncoupler carbonyl cyanide m-chlorophenylhydrazone on K+ transport,ATP level and intracellular pH of Chlorella fusca

1. Low concentrations of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) induced net K⁺ uptake by Chlorella fusca, optimal concentrations being 3 μM CCCP in the light and 1 μM CCCP in the dark. Higher concentrations increasingly stimulated K⁺ release. 2. Measurements of the unidirectional K⁺ fluxes showed that CCCP-induced net K⁺ uptake in the light was mainly a consequence of an inhibition of efflux. In the dark, influx was slightly stimulated in addition. 3. In conditions of CCCP-induced net K⁺ uptake, the ATP level was decreased by less than 10%. With higher CCCP concentrations it fell drastically. 4. By means of the 5,5-dimethyloxazolidine-2,4-dione distribution technique, an acidification of the cell interior on the addition of CCCP was found. 5. It is concluded that uncoupler-induced net K⁺ uptake is due to an enhanced proton leakage into the cell across the plasmalemma. Intracellular acidification by this process stimulates ATP-dependent K⁺/H⁺ exchange which, in itself, is not affected at low uncoupler concentrations.     

Bioelectric effects of ions microinjected into the giant axon of Loligo

1. A technique is described for recording the bioelectric activity of the squid giant axon during and following alteration of the internal axonal composition with respect to ions or other substances. 2. Experimental evidence indicates that the technique as described is capable of measuring changes in local bioelectric activity with an accuracy of 10 to 15 per cent or higher. 3. Alterations of the internal K⁺ or Cl^- concentrations do not cause the change in resting potential expected on the basis of a Donnan mechanism. 4. The general effect of microinjection of K⁺ Rb⁺, Na⁺, Li⁺, Ba⁺⁺, Ca⁺⁺, Mg⁺⁺, or Sr⁺⁺ is to cause decrease in spike amplitude, followed by propagation block. 5. The resting potential decreases when the amplitude of the spike becomes low and block is incipient. 6. The decrease in resting potential and spike amplitude may be confined to the immediate vicinity of the injection. 7. At block, the resting potential decreases up to 50 per cent, but injection of small quantities of divalent cations may cause much larger localized depolarization. 8. The blocking effectiveness of K⁺, Na⁺, and Ca⁺⁺ expressed as reciprocals of the relative amounts needed to cause block is approximately 1:5:100. Rb⁺ has the same low effectiveness as does K⁺. Li⁺ resembles Na⁺. Ba⁺⁺ and Mg⁺⁺ are approximately as effective as Ca⁺⁺. 9. Microinjection of Na⁺ may cause marked prolongation of the spike at the injection site as well as decrease in its amplitude. 10. The anions used (Cl^-, HCO₃^-, NO₃^-, SO₄^-, aspartate, and glutamate) do not seem to exert specific effects. 11. A tentative explanation is offered for the insensitivity of the resting potential to changes in the axonal ionic composition. 12. New data are presented on the range of variation, in a large sample, of the magnitude of the resting potential and spike amplitude.     

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.     

Effects of acetylcholine and caerulein on 86Rb+ efflux in the mouse pancreas. Evidence for a sodium-potassium-chloride cotransport system

The effect of acetylcholine and the cholecystokinin-like peptide, caerulein on the fractional efflux of ⁸⁶Rb⁺ from preloaded isolated segments of mouse pancreas were studied. Both secretagogues evoked a marked transient increase in ⁸⁶Rb⁺ efflux. The removal of Ca²⁺ from the superfusing medium and addition of 10^?4 M EGTA, markedly reduced, but did not abolish the responses to either acetylcholine or caerulein. Furosemide ($\text{10}{}^{\mathrm{?5}}\text{?10}{}^{\mathrm{?3}}\text{M}$) or piretanide (10^?4 M) reduced the basal efflux and inhibited the secretagogue-elicited responses. Stimulus-induced ⁸⁶Rb⁺ outflow was abolished when the Cl^? component of the superfusing solution was replaced by either NO₃^?, SO₄^2? or I^? but not in case of replacement by Br^?, When Na⁺ was replaced with either Li⁺ or choline⁺ both acetylcholine and caerulein failed to elicit any detectable increase in ⁸⁶Rb⁺ outflow. However, when Tris⁺ was substituted for Na⁺, acetylcholine caused a moderate increase in ⁸⁶Rb⁺ efflux which was abolished by either furosemide (10^?4 M) or chloride depletion (nitrate substitution). The removal of extracellular K⁺ or pretreatment with 10^?3 M ouabain had little effect on secretagogue-evoked ⁸⁶Rb⁺ efflux. These results indicate the presence of a diuretic-sensitive Na⁺-K⁺-Cl^? cotransport system in the mouse pancreatic acinar cell membrane.     

Role of external calcium in sodium and chloride transport in the gills of seawater-adaptedMugil capito

Summary When the mulletMugil capito is transferred to medium lacking Ca⁺⁺ (either Ca⁺⁺-free seawater or distilled water) the passive permeability of the gill to Na⁺ and Cl^– is increased and the activating effect of external K⁺ on the Na⁺ and Cl^– effluxes in hyposaline media is inhibited. The permeability of the gill increases progressively in proportion to the time of Ca⁺⁺ deprivation; it declines when Ca⁺⁺ is added again to the external medium. The active mechanisms for ion excretion are not reversible. At external Ca⁺⁺ concentrations from 0.1 to 10 mM the Na⁺ permeability is constant but the activation of Na⁺ efflux by K⁺ shows a maximum at a Ca⁺⁺ concentration of about 1 mM. For activation of Cl^– efflux external bicarbonate must be present, in addition to Ca⁺⁺, suggesting the existence of a Cl^–/HCO ₃ ^– exchange. The mechanism by which Ca⁺⁺ controls the passive branchial permeability is thus probably different from that involved in K⁺ activation of ion excretion. The Ca⁺⁺ effect on the K⁺ sensitive ionic excretory mechanisms seems to be related to intracellular Ca⁺⁺ movements. Thus, on the one hand, substances such as Ruthenium Red and La⁺⁺⁺ which both inhibit Ca⁺⁺ exchange, in media containing Ca⁺⁺ and HCO ₃ ^– also inhibit K⁺ activation of Na⁺ and Cl^– effluxes; on the other hand, the ionophore A 23187, a stimulator of Ca⁺⁺ exchange, when added to these media, activates the Na⁺ and Cl^– effluxes; its maximal effect on the Na⁺ flux occurs at 2 mM Ca⁺⁺.Abbreviations ASW-Ca artificial seawater minus calcium - DW deionised water - DWCa deionised water with 1 mM Ca⁺⁺ added - DWCaHCO ₃ DW with calcium plus bicarbonate - DWHCO ₃ DW with 1 mM sodium bicarbonate added - FW freshwater (tap water) - FWK freshwater with K⁺ added - P. D. potential difference - SW seawater The experiments reported in this paper were done with Jean Maetz who tragically died in August 1977. It is the last report about several years of friendly collaboration     

Fluxes of Sodium and Potassium in Acetabularia mediterranea

Sodium efflux in Acetabularia mediterranea occurs against agradient of electrochemical potential and is a light-stimulated,temperature-sensitive process; it is not sensitive to the uncouplerCCCP. Sodium influx is stimulated in CCCP and at low temperature.Potassium influx is temperature- and uncoupler-sensitive, butis not light-stimulated. Tracer K efflux shows complex kinetics,which cannot be explained by any arrangement of intracellularcompartments; it appears to be stimulated at low temperatureand is insensitive to light and uncouplers. There is no evidencefor any chemical linkage between fluxes of Na⁺, K⁺, or Cl^–.It is concluded that Na^– efflux at the plasmalemma isan active process, but no consistent explanation can be advancedto account for the results of K⁺ flux measurements.     

Influence of abscisic acid on unidirectional fluxes and intracellular compartmentation of K+ and Na+ in excised barley root segments

Using compartmental analysis, unidirectional fluxes of K⁺ and Na⁺ and their intracellular compartmentation in excised barley (Hordeum distichon L. cv. Kocher-perle) root segments have been measured during a steady state in the presence or absence of ABA. Almost all flux rates were altered in the presence of external ABA, in particular the xylem transport R’ and the plasmalemma influx Ø_oc (see below) were strongly inhibited in the steady state. At the same time the presence of ABA induced a strong increase in the vacuolar K⁺ and Na⁺ content Q_v and a decrease in the cytoplasmic one (Q_c). Since the fluxes of an ion and its vacuolar or, in particular, cytoplasmic concentrations are interrelated, the ratios of fluxes originating from the cytoplasm and the cytoplasmic ion content were taken into account. On this basis ABA had the following effects: a) the secretion of K⁺ or Na⁺ to the xylem vessels was drastically inhibited; b) the plasmalemma K⁺ or Na⁺ efflux Ø_co was moderately stimulated and c) the tonoplast influx Ø_cv of Na⁺ was stimulated, while the tonoplast influx of K⁺ appeared to be unchanged (the decrease in Ø_cv being due to the decreased cytoplasmic K⁺ content). By a similar argument, also the apparent inhibition of the plasmalemma influx Ø_oc of K⁺ and Na⁺ in the steady state merely is an indirect effect of ABA. It only reflects the strong ABA-induced decrease in the xylem transport, that governs the magnitude of Ø_oc in the steady state. The results are discussed with reference to possible regulatory functions of ABA. In this respect it is suggested that – in particular under conditions of stress – ABA might regulate cellular metabolic processes by changing the cytoplasmic K⁺ level.     

Über den lichtgeförderten Influx von Ionen in Blättern von Elodea densa. Vergleich der Influxe von K+- und Cl--Ionen

Summary The light-dependent influxes of K⁺ and Cl^- in detached leaves of Elodea densa were measured using ³⁶Cl^- and ⁴²K⁺ or ⁸⁶Rb⁺ as tracers.The K⁺ and Cl^- influxes were enhanced by light and also in the dark after a preillumination. The rate of influx decayed in the dark according to a first order reaction with a half-time of 25 or 27 sec.DCMU inhibits the light-dependent K⁺ influx more severely in the presence of CO₂ than in its absence in an atmosphere of N₂ containing a trace of oxygen. This is similar to the effect of DCMU on the Cl^- influx. CCCP¹, atebrin (quinacrine) and Dio-9 all affect the influx of K⁺ and Cl^- in a comparable way. CCCP exerts the strongest effect at low light intensities; atebrin and Dio-9 inhibit strongly even at high intensities when the ion influxes are light-saturated. The influence of these two inhibitors in attributed to an effect at the cellular membranes in addition to an effect on photophosphorylation. The effect of CCCP is ascribed to uncoupling of photophosphorylation, as photosynthesis is inhibited by about the same concentration as is ion influx.In far-red light the relative quantum yields of K⁺ and Cl^- influx drop to a similar degree as does the quantum yield of photosynthesis. Estimated values of the quantum requirement of ion influx are given. The quantum requirement in air is higher than in an atmosphere of N₂. It is a function of ion concentration and is lower at higher external concentrations.The results indicate that the K⁺ and Cl^- influxes are partially coupled. The linkage of the ion influxes with the energy sources in the light and a possible contribution of a pseudocyclic photophosphorylation are discussed.     

Efflux und Transport von Cl− und Rb+ in Maiswurzeln. Wirkung von Außenkonzentration,Ca++, EDTA und IES

Summary The efflux of ³⁶Cl and ⁸⁶Rb and the fluxes of these ions into the xylem were investigated using the device shown in Fig. 1.Efflux of ³⁶Cl is stimulated by external KCl while transport into the xylem is inhibited. Stimulation of the efflux appears to be stronger than inhibition of the transport.The stimulation of the efflux of ³⁶Cl was also observed with roots of intact seedlings.Assuming that the mode of transfer of Cl^– into the xylem (flux 3, Fig. 8) is diffusion exhibiting a linear isotherm (Luttge and Laties, 1966), these results suggest that the primary action of external salts is on the efflux across the plasma-lemma (Weigl, 1967, 1968). We were unable, however, to find a linear relationship between concentration and rate of chloride transport to the shoots of intact seedlings.With respect to the mode of ion transfer to the xylem (Weigl and Lüttge, 1965; Luttge and Laties, 1966) we have to be aware of the following facts:A linear isotherm cannot be taken to signify diffusive permeation (Torii and Laties, 1966; Luttge and Laties, 1966). If the Michaelis constant is extremely high relative to the ion concentration, the relationship between the ion concentration and the rate of a metabolic or mediated transport approaches linearity.The isotherm of the transport into the xylem may primarily reflect the difference of two large fluxes (4 and 5; Fig. 8).The transport data of Luttge and Laties (1966) need not be presented as a straight line (Fig. 6).If at high external ion concentrations the ratio of the ion concentration in the exudation sap to the external ion concentration approaches unity, diffusive permeation into the stele is still not proved to be the mode of migration, since at high stelar ion concentration flux 6 tends to become equal to flux 3.Considerations on radial ion transfer into the xylem depend on contemporary knowledge of the location of transport systems. Cl^–-uptake into root tips (2 mm) from solutions of 1–10 mM KCl did not exhibit a linear isotherm. These results are unpublished since the discrepancy to the results of Torii and Laties (1966) may be due to a higher content of vacuoles in our root tips. We feel it unlikely, however, that a linear isotherm of Cl^–-uptake into root tips is adequately explained by assuming that it is due to a lack of vacuoles while the sensibility to inhibitors is assumed to be due to the presence of vacuoles in root tips.Transport of Cl^– into the xylem is susceptible to inhibitors of oxydative phosphorylation, suggesting that this process, even at high external ion concentrations, is dependent on metabolic energy in contrast to the passive efflux from the cortical cells across the plasmalemma into the environment of the root. The precise location of the metabolic step(s) on the pathway of ions from the environment of the root to the xylem is unknown.The observed effects of Ca⁺⁺, EDTA and IAA may be considered in relation to the theory that auxin exerts its influence on growth by altering the diffusion potential across cell membranes (Brauner and Diemer, 1967). Growth is susceptible to the effect of Ca⁺⁺ and EDTA (Adamson, 1962; Setterfield, 1963; Thimann, 1963). Nevertheless, since IAA exerts no influence on ion fluxes in corn roots, it is not clear whether IAA really exerts its influence on growth by altering the diffusion potential across plant cell membranes. We might be dealing with occasional effects of secondary importance.     

Wirkung von K+ auf die Fluxe und den Transport von Na+ in Gerstenwurzeln,K+-stimulierter Na+-Efflux in der Wurzelrinde

Summary Barley roots grown on a nutrient solution containing 1 mM Na⁺ but no K⁺ are capable of a considerable Na⁺ transport via the symplasm of the root and the xylem vessels. K⁺ added to the medium surrounding the root cortex severely inhibits this transport after a lag period at a high rate constant (Fig. 3).It is likely that the fluxes of Na⁺ are changed drastically during this transition from low to high K⁺ status. Although originally limited to steady state fluxes, the extended method of efflux analysis for excised roots (Pitman, 1971) has been applied to the non-steady fluxes which occur upon the addition of K⁺ to the roots. It is shown that besides other changes the efflux of ²²Na⁺ through the cortex of barley roots is stimulated instantaneously (Fig. 5) by the addition of K⁺ and presumably by an influx of K⁺ ions. From this a transient, K⁺-stimulated Na⁺ efflux at the plasmalemma of the cortical cells can be estimated. It amounts to 10.9 moles/g fw · h compared to the control efflux of 3.3 moles/g fw · h without K⁺.The stimulated efflux is attributed to a Na⁺ efflux pump at the plasmalemma and is thus related to the K-Na-selectivity of barley plants. The inhibition of the Na⁺ transport by K⁺ is probably a consequence of this increased efflux of Na⁺ from the symplasm through the root cortex.     

Ion fluxes and phytochrome protons in mung bean hypocotyl segments: I. Fluxes of potassium

K⁺ [⁸⁶Rb⁺] uptake by Phaseolus aureus Roxb. hypocotyl segments cut immediately below the hook is inhibited by the active form of phytochrome (Pfr). Short load-short wash experiments indicate that the inhibition of uptake occurs across the plasmalemma. A maximal inhibition of short term uptake occurs in 10 to 50 millimolar KCI. Low temperature had only a small effect on influx and the inhibition of influx from 50 millimolar KCI. A consideration of the electrochemical gradient for K⁺ suggests that passive K⁺ fluxes may predominate under these conditions. Red light induces small depolarizations of membrane potential in subhook cells. Far red light antagonizes this effect. Pfr inhibits efflux of K⁺[⁸⁶Rb⁺] from subhook segments. This effect is also relatively insensitive to low temperature. This inhibition of efflux may reflect inhibition of a K⁺ -K⁺ exchange process, or reduced passive permeability of the plasmalemma to K⁺. In contrast, Pfr enhances short term uptake of K⁺[⁸⁶Rb⁺] in apical hypocotyl hook segments of Phaseolus aureus Roxb. Short load-short wash experiments indicate that fluxes across the plasmalemma are modified by Pfr. A maximal enhancement of short term influx occurs in 50 millimolar KCI. Influx and the red light enhancement of influx from 50 millimolar KCI are relatively insensitive to low temperature. Pfr also enhances efflux of K⁺[⁸⁶Rb⁺] from preloaded apical hook segments. This increased influx may reflect enhancement of a K⁺ -K⁺ exchange process or increased passive permeability of the plasmalemma to K⁺.     

Flux ratio of valinomycin-mediated K+ fluxes across the human red cell membrane in the presence of the protonophore CCCP

Summary The ratio of valinomycin-mediated unidirectional K⁺ fluxes across the human red cell membrane, has been determined in the presence of the protonophore carbonylcyanidem-chlorophenylhydrazone, CCCP, using the K⁺ net efflux and⁴²K influx. The driving force for the net efflux (V _m –E _K ⁺) has been calculated from the membrane potential, estimated by the CCCP-mediated proton distribution and the Nernst potential for potassium ions across the membrane. An apparent driving potential for the K⁺ net efflux has been calculated from the K⁺ flux ratio, determined in experiments where the valinomycin and CCCP concentrations were varied systematically. This apparent driving force, in conjunction with the actual driving force calculated on basis of the CCCP estimated membrane potential, is used to calculate a flux ratio exponent, which represents an estimate of the deviation of valinomycin-mediated K⁺ transport from unrestricted electrodiffusion, when protonophore is present.In the present work, the flux ratio exponent is found to be 0.90 when the CCCP concentration is 5.0 m and above, while the exponent decreases to about 0.50 when no CCCP is present. The influence of CCCP upon the rate constants in the valinomycin transport cycle is discussed. The significance of this result is that red cell membrane potentials are overestimated, when calculated from valinomycin-mediated potassium isotope fluxes, using a constant field equation.