External Cation Concentration Research Articles

Trk1, the sole potassium-specific transporter in Candida glabrata, contributes to the proper functioning of various cell processes.

Candida glabrata is a haploid yeast that is considered to be an emergent pathogen since it is the second most prevalent cause of candidiasis. Contrary to most yeasts, this species carries only one plasma membrane potassium transporter named CgTrk1. We show in this work that the activity of this transporter is regulated at the posttranslational level, and thus Trk1 contributes to potassium uptake under very different external cation concentrations. In addition to its function in potassium uptake, we report a diversity of physiological effects related to this transporter. CgTRK1 contributes to proper cell size, intracellular pH and membrane-potential homeostasis when expressed in Saccharomyces cerevisiae. Moreover, lithium influx experiments performed both in C. glabrata and S. cerevisiae indicate that the salt tolerance phenotype linked to CgTrk1 can be related to a high capacity to discriminate between potassium and lithium (or sodium) during the transport process. In summary, we show that CgTRK1 exerts a diversity of pleiotropic physiological roles and we propose that the corresponding protein may be an attractive pharmacological target for the development of new antifungal drugs.

Thermal stability and transition temperatures of monovalent salts of high-acyl gellan gels

Summary Thermogravimetry/derivative thermogravimetry (TG/DTG), rheometry and differential scanning calorimetry (DSC) were used to study the thermal stability and determine the transition temperatures of the sodium and potassium salts of high-acyl gellan (HAG) in the presence of 0–100 mm NaCl and KCl, respectively. TG/DTG revealed the potassium gellan (KHAG) gels to be more stable than those of sodium gellan (NaHAG), regardless of external cation concentration. Rheometry and DSC showed the melting (Tm) and gelling (Tg) temperatures to increase with cation concentration. The DSC peak temperatures showed thermal hysteresis contrary to rheometry. In most cases, DSC revealed KHAG to exhibit higher Tm and Tg than NaHAG. Consequently, thermal characteristics of NaHAG and KHAG gels depend on the size of the external cation and its ability to coordinate water molecules. Cation salts of HAG exhibit significantly lower transition temperatures than the commercial preparation from which they were produced.

Suppression of sensitivity to drugs and antibiotics by high external cation concentrations in fission yeast.

BackgroundPotassium ion homeostasis plays an important role in regulating membrane potential and therefore resistance to cations, antibiotics and chemotherapeutic agents in Schizosaccharomyces pombe and other yeasts. However, the precise relationship between drug resistance in S. pombe and external potassium concentrations (particularly in its natural habitats) remains unclear. S. pombe can tolerate a wide range of external potassium concentrations which in turn affect plasma membrane polarization. We thus hypothesized that high external potassium concentrations suppress the sensitivity of this yeast to various drugs.MethodsWe have investigated the effect of external KCl concentrations on the sensitivity of S. pombe cells to a wide range of antibiotics, antimicrobial agents and chemotherapeutic drugs. We employed survival assays, immunoblotting and microscopy for these studies.ResultsWe demonstrate that KCl, and to a lesser extent NaCl and RbCl can suppress the sensitivity of S. pombe to a wide range of antibiotics. Ammonium chloride and potassium hydrogen sulphate also suppressed drug sensitivity. This effect appears to depend in part on changes to membrane polarization and membrane transport proteins. Interestingly, we have found little relationship between the suppressive effect of KCl on sensitivity and the structure, polarity or solubility of the various compounds investigated.ConclusionsHigh concentrations of external potassium and other cations suppress sensitivity to a wide range of drugs in S. pombe. Potassium-rich environments may thus provide S. pombe a competitive advantage in nature. Modulating potassium ion homeostasis may sensitize pathogenic fungi to antifungal agents.

Clustering of the K+ channel GORK of Arabidopsis parallels its gating by extracellular K+.

GORK is the only outward-rectifying Kv-like K+ channel expressed in guard cells. Its activity is tightly regulated to facilitate K+ efflux for stomatal closure and is elevated in ABA in parallel with suppression of the activity of the inward-rectifying K+ channel KAT1. Whereas the population of KAT1 is subject to regulated traffic to and from the plasma membrane, nothing is known about GORK, its distribution and traffic in vivo. We have used transformations with fluorescently-tagged GORK to explore its characteristics in tobacco epidermis and Arabidopsis guard cells. These studies showed that GORK assembles in puncta that reversibly dissociated as a function of the external K+ concentration. Puncta dissociation parallelled the gating dependence of GORK, the speed of response consistent with the rapidity of channel gating response to changes in the external ionic conditions. Dissociation was also suppressed by the K+ channel blocker Ba2+. By contrast, confocal and protein biochemical analysis failed to uncover substantial exo- and endocytotic traffic of the channel. Gating of GORK is displaced to more positive voltages with external K+, a characteristic that ensures the channel facilitates only K+ efflux regardless of the external cation concentration. GORK conductance is also enhanced by external K+ above 1 mm. We suggest that GORK clustering in puncta is related to its gating and conductance, and reflects associated conformational changes and (de)stabilisation of the channel protein, possibly as a platform for transmission and coordination of channel gating in response to external K+.

Cation transport by the neuronal K(+)-Cl(-) cotransporter KCC2: thermodynamics and kinetics of alternate transport modes.

Both Cs(+) and NH(4)(+) alter neuronal Cl(-) homeostasis, yet the mechanisms have not been clearly elucidated. We hypothesized that these two cations altered the operation of the neuronal K(+)-Cl(-) cotransporter (KCC2). Using exogenously expressed KCC2 protein, we first examined the interaction of cations at the transport site of KCC2 by monitoring furosemide-sensitive (86)Rb(+) influx as a function of external Rb(+) concentration at different fixed external cation concentrations (Na(+), Li(+), K(+), Cs(+), and NH(4)(+)). Neither Na(+) nor Li(+) affected furosemide-sensitive (86)Rb(+) influx, indicating their inability to interact at the cation translocation site of KCC2. As expected for an enzyme that accepts Rb(+) and K(+) as alternate substrates, K(+) was a competitive inhibitor of Rb(+) transport by KCC2. Like K(+), both Cs(+) and NH(4)(+) behaved as competitive inhibitors of Rb(+) transport by KCC2, indicating their potential as transport substrates. Using ion chromatography to measure unidirectional Rb(+) and Cs(+) influxes, we determined that although KCC2 was capable of transporting Cs(+), it did so with a lower apparent affinity and maximal velocity compared with Rb(+). To assess NH(4)(+) transport by KCC2, we monitored intracellular pH (pH(i)) with a pH-sensitive fluorescent dye after an NH(4)(+)-induced alkaline load. Cells expressing KCC2 protein recovered pH(i) much more rapidly than untransfected cells, indicating that KCC2 can mediate net NH(4)(+) uptake. Consistent with KCC2-mediated NH(4)(+) transport, pH(i) recovery in KCC2-expressing cells could be inhibited by furosemide (200 microM) or removal of external [Cl(-)]. Thermodynamic and kinetic considerations of KCC2 operating in alternate transport modes can explain altered neuronal Cl(-) homeostasis in the presence of Cs(+) and NH(4)(+).

Phosphate absorption and efflux of three ectomycorrhizal fungi as affected by external phosphate, cation and carbohydrate concentrations

A prerequisite for symbiotic phosphate transfer in an ectomycorrhizal (ECM) association is hypothesized to be conditions in the interface between both symbiotic partners, that either promote the release of inorganic phosphate (P) from the Hartig net into the interfacial apoplast and/or decrease the fungal reabsorption from this location. To get more information about conditions, which might be involved in the regulation of P efflux or P reabsorption, the effect of various external conditions on 33P-orthophosphate (33P) uptake or efflux by axenic cultures of the ECM basidiomycetes Hebeloma crustliniforme, Amanita muscaria and Laccaria laccata was analysed. In short-time experiments the following external conditions were analysed: an external supply of (1) P in the preculture, (2) cations (0.1-100 mM K, 0.1-50 mM Na, Mg and Ca), and (3) carbohydrates (0.5-50 mM glucose, fructose or sucrose). The P absorption was generally reduced in cultures previously supplied with an abundant P supply and with increased P concentrations in their tissues. The P uptake was also affected by an external supply of cations, whereas carbohydrates had only a slight effect. Compared to Na, Mg and Ca, the P absorption by H. crustuliniforme and L. laccata was increased by 0.1 mM K in the labelling solution but decreased after a supply of 100 mM K and then did not differ from the other cation treatments. Compared to other cations, an addition of 50 mM Ca led to a decrease of P absorption by A. muscaria, whereas 50 mM Mg increased the P uptake by H. crustuliniforme. The P efflux from the fungi was affected by both the cation and carbohydrate concentration of the bathing solution. High concentrations of the monovalent cations K and Na (5 mM or 50 mM) in the bathing solution increased the P efflux by H. crustuliniforme (only Na) and L. laccata (K and Na), but had little effects on A. muscaria. By contrast, the same concentrations of the divalent cation Mg reduced the P efflux from all fungal basidiomycetes. The P efflux from hyphae of H. crustuliniforme and A. muscaria was also affected by the carbohydrate concentration in the bathing solution. Compared to the hexoses, glucose and fructose, the P efflux by both basidiomycetes was stimulated even by low concentrations of sucrose (0.5 mM) in the bathing solution. The effect of carbohydrates on P efflux especially from hyphae of H. crustuliniforme was clearly concentration-dependent and the maximal efflux rates were observed after a supply of 2.5 mM sucrose or fructose or 5 mM glucose in the bathing solution. A further rise of the carbohydrate concentration led to a progressive decrease of P efflux from hyphae of H. crustuliniforme. The results are discussed with respect to the transfer processes between both symbiotic partners in a mycorrhizal association and a possible regulation of these exchange processes.

Characterization of a HKT-type transporter in rice as a general alkali cation transporter.

We report the characterization of rice OsHKT1 (Oryza sativa ssp. indica) homologous to the wheat K+/Na+-symporter HKT1. Expression of OsHKT1 in the yeast strain CY162 defective in K+-uptake restored growth at mM and micro M concentrations of K+ and mediated hypersensitivity to Na+. When expressed in Xenopus oocytes, rice OsHKT1 showed uptake characteristics of a Na+-transporter but mediated transport of other alkali cations as well. OsHKT1 expression was analysed in salt-tolerant rice Pokkali and salt-sensitive IR29 in response to external cation concentrations. OsHKT1 is expressed in roots and leaves. Exposure to Na+, Rb+, Li+, and Cs+ reduced OsHKT1 transcript amounts in both varieties and, in some cases, incompletely spliced transcripts were observed. By in situ hybridizations the expression of OsHKT1 was localized to the root epidermis and the vascular tissue inside the endodermis. In leaves, OsHKT1 showed strongest signals in cells surrounding the vasculature. The repression of OsHKT1 in the two rice varieties during salt stress was different in various cell types with main differences in the root vascular tissue. The data suggest control over HKT expression as a factor that may distinguish salt stress-sensitive and stress-tolerant lines. Differences in transcript expression in space and time in different lines of the same species appear to be a component of ion homeostasis correlated with salt sensitivity and tolerance.

Induction of larval metamorphosis in the ascidian, Halocynthia roretzi by excess potassium ion and by reduced calcium ion

Effects of external cation concentrations on larval metamorphosis in the ascidian, Halocynthia roretzi were examined. Metamorphosis (tail resorption) was induced by 20 mM and 50 mM K+ but, interestingly, was suppressed by 30 mM K+. Low concentrations of Ca2+ (1–5 mM) induced metamorphosis, while high concentrations did not, in contrast to findings in other invertebrates. Moreover, BAPTA-AM, an intracellular Ca2+ chelator, induced metamorphosis. This suggests that a decrease in intracellular Ca2+ of certain cells initiates larval metamorphosis in H. roretzi.

Effects of external cation concentrations on the induction of long-term potentiation at visual cortical inhibitory synapses

Polymorphism of the genes coding for the human histocompatibility leukocyte antigen class II DR and DQ molecules makes the single largest genetic contribution to the risk of developing insulin-dependent diabetes mellitus (IDDM) and can be associated with highly elevated as well as decreased disease frequency. The mechanism of IDDM risk modification by HLA polymorphism is likely to involve differential presentation of autoantigenic peptides by HLA class II proteins. We have generated T cell lines (TCL) with specificity for the IDDM autoantigen 65 kDa glutamic acid decarboxylase (GAD65) from lymphocytes of two patients carrying HLA class II alleles associated with distinct risk of IDDM (DRB1*0101/0401 and 1302/1501). For both patients, TCL generated at various time points all recognized single epitopes mapped as GAD 88–99 and 248–257, respectively. These epitopes are presented by the DRB1*0101 and DRB5*0101, HLA class II molecules associated with a moderately elevated risk of IDDM, or carried in a strongly protective haplotype, respectively. In an HLA/peptide binding assay, epitope GAD 248–257 was shown to possess high affinity for DRB5*0101. This epitope overlaps with a central GAD peptide binding to the high risk allele DQB1*0302 and containing a Coxsackie P2C-identical mimicry sequence, raising the possibility of competition of DRB5*0101 and DQB1*0302 for binding of a central GAD65 fragment.

Cs+ Uptake in Subapical Maize Root Segments: Mechanism and Effects on H+ Release, Transmembrane Electric Potential and Cell pH

bye of Kplant cells involves a high-affinity system predominating atlow external concentration (< 1-0.3 mM) mediating the ca-tion influx against its electrochemical potential (Maathuisand Sanders 1993, Gassmann and Schroeder 1994), anda low affinity system predominating at higher external ca-tion concentrations. Several mechanisms have been pro-posed for the high-affinity system (Poole 1978, Rodriguez-Navarro et al. 1986, Kochian and Lucas 1989, Gibrat et al.1990) but the low-affinity system is largely accepted to be achannel mechanism (Kochian et al. 1985, Schroeder andFang 1991, Kourie and Goldsmith 1992, Gassman et al.1993, Kochian and Lucas 1993, Gassmann and Schroeder1994). It has also been proposed tha

Three cation influx currents activated by purinergic receptor stimulation in rat megakaryocytes.

1. Simultaneous patch clamp and fura-2 fluorescence measurements were used to study ATP-evoked membrane currents and intracellular [Ca2+] ([Ca2+]i) changes in rat megakaryocytes. 2. At negative potentials, under conditions that blocked K+ currents, 20 microM ATP activated a biphasic inward current and a concurrent biphasic increase in [Ca2+]i. The initial [Ca2+]i increase was due to Ca2+ influx whereas the delayed (1.70 +/- 0.13 s, mean +/- S.D.) increase was at least partly due to the release of internal Ca2+ stores. 3. The initial current was activated within 100 ms, inactivated within 1-4 s and was carried by both Na+ and Ca2+. 4. The delayed current was also transient and carried mainly by Na+ when Ca2+ buffering in the pipette was low. This Na+ conductance did not require an increase in [Ca2+]i for activation, but was triggered by inositol 1,4,5-trisphosphate (IP3), or a metabolite of IP3. 5. Buffering of [Ca2+]i changes with BAPTA revealed a third current activated by Ca2+ release from internal stores. This channel was selective for divalent cations with the permeability sequence Ca2+ >> Ba2+ > Mn2+, Mg2+. 6. Adenosine-5'-O-3-thiotriphosphate (ATP gamma S), like ATP, evoked all three influx currents, whereas ADP only stimulated Ca2+ release and the two currents associated with it. Increasing the external divalent cation concentration abolished the ATP-evoked Ca2+ release and delayed currents but not the initial transient current. 7. We conclude that rat megakaryocytes express two types of purinergic receptor. One type, activated by ATP, is closely coupled to a non-selective cation channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetical parameters of monovalent cation uptake in yeast calculated on accounting for the mutual interaction of cation uptake and membrane potential

Kinetical parameters of monovalent cation uptake in yeast are calculated according to a model for mutual interaction of membrane potential and cation uptake. Apparent K m values for monovalent cation uptake obtained from uptake studies are 3–5-times lower than the K m values expected when the membrane potential remains constant at increasing cation concentrations instead of being reduced. The model accounts for varius phenomena as (i) the increase in apparent K m of Rb + uptake accompanying the decrease in maximum rate of uptake found on increasing the cellular K + content, (ii) the decrease in maximum uptake rate and increase in K m in case of simultaneous transport of phosphate and Rb +, (iii) the change in the maximum uptake rate without a change in K m under conditions that the proton pump is effected and (iv) the absence of an increase in K + efflux at high depolarizing external cation concentrations.

Accumulation of zirconium by microalgae and cyanobacteria

The accumulation of zirconium (Zr) as [Zr4-(OH)8(H2O)16]8+ by cyanobacteria and microalgae has been characterized. In all the cyanobacterial and microalgal species examined, accumulation consisted of a single rapid energy-independent phase (“biosorption”) and no energy-dependent accumulation was observed. Biosorption of Zr was concentration-dependent, followed a Freundlich adsorption isotherm, and was dependent on pH, showing decreased accumulation with decreased pH. Prior treatment with Na+, K+, Cs+, Ca2+, Mg+ and Sr2+ (added as chlorides) also decreased Zr accumulation by cyanobacteria and microalgae, probably a result of competition between Zr ions and other cations, including H+, for available binding sites on the cell walls. Zr desorption from microalgae and cyanobacteria was increased by increasing external cation concentrations or by decreasing the pH of the desorption agent.

Ion movements associated with chorion elevation during egg laying in trout (Oncorhynchus mykiss) in fresh water

Within 1 min of transfer from coelomic fluid to fresh water, eggs of rainbow trout (Oncorhynchus mykiss) underwent a transient loss of Na+ and K+ coupled with an elevation of the chorionic envelope. Both mechanisms were blocked by adding a monovalent cation Li+ or K+ (140 mmol·l-1) to the fresh water, but the divalent ion Mg2+ (100 mmol MgCl2·l-1) or elevating the osmotic pressure to 300 mOsmol·l-1 with glycine had no inhibitory effect. The blocking of Na+ loss occurred at external monovalent cation (LiCl) concentrations above 70 mmol·l-1. A 20-s exposure of eggs to fresh water was sufficient to trigger Na+ loss and chorion elevation, even when the eggs were subsequently transferred to fresh water containing 140 mmol LiCl·l-1. Eggs placed in a medium containing 140 mmol LiCl·l-1 and 2 mmol Ca(NO3)2·l-1 showed chorion elevation and associated Na+ loss after addition of calcium ionophore (20 μmol·l-1 A.23187). This activation by calcium ionophore was supressed in a Ca2+-free medium containing 5 mmol EGTA·l-1.

The effects of Agauria salicifolia leaf extract on calcium current and excitation-contraction coupling of isolated frog muscle cells

The effects of the ethanolic extract of Agauria salicifolia (AS) previously tested on sodium currents of normal and TTX-treated voltage-clamped skeletal and cardiac muscle cells was investigated on mechanisms involved in the excitation-contraction coupling of the same biological preparations. AS (10 −6 g/l) prolonged the action potential duration whereas the associated contraction was depressed. This is not due to the blocking action of AS on the tubular sodim current since similar results were obtained in the presence of high tetrodotoxin (TTX) concentrations. This is not due to some blocking action of AS on the L-type calcium current since this type of calcium current was enhanced by the extract. In contrast, the cardiac T-type calcium current was decreased by 10 −6 g/l AS. AS exerted a dose-dependent (tested for concentrations ranging from 5 × 10 −10 to 5 × 10 −5 g/l) inhibition of the two components of the contractile response elicited by durable depolarizations, with a prominent effect on the tonic phase. This effect was partially relieved by increasing the external divalent cation (Ca 2+ or Cd 2+) concentration. At the same concentration is shifted the inactivation/potential relationship for tension by 20 mV towards negative potentials. It is concluded that the inhibitory action of AS on excitation-contraction coupling is partly or completely due to the enhancement of the voltage- and Ca 2+-dependent inactivation processes of the voltage-sensor.

Modification of cardiac Na+ channels by anthopleurin-A: effects on gating and kinetics.

We used the whole cell patch clamp technique to investigate the characteristics of modification of cardiac Na+ channel gating by the sea anemone polypeptide toxin anthopleurin-A (AP-A). Guinea pig ventricular myocytes were isolated enzymatically using a retrograde perfusion apparatus. Holding potential was -140 mV and test potentials ranged from -100 to +40 mV (pulse duration 100 or 1000 ms). AP-A (50-100 nM) markedly slowed the rate of decay of Na+ current (INa) and increased peak INa conductance (gNa) by 38 +/- 5.5% (mean +/- SEM, P < 0.001, n = 12) with little change in slope factor (n = 12) or voltage midpoint of the gNa/V relationship after correction for spontaneous shifts. The voltage dependence of steady-state INa availability (h infinity) demonstrated an increase in slope factor from 5.9 +/- 0.8 mV in control to 8.0 +/- 0.7 mV after modification by AP-A (P < 0.01, n = 14) whereas any shift in the voltage midpoint of this relationship could be accounted for by a spontaneous time-dependent shift. AP-A-modified INa showed a use-dependent decrease in peak current amplitude (interpulse interval 500 ms) when pulse duration was 100 ms (-15 +/- 2%, P < 0.01, n = 17) but showed no decline when pulse duration was 100 ms (-3 +/- 1%). This use-dependent effect was probably the result of a decrease in the recovery from inactivation caused by AP-A which had a small effect on the fast time constant of recovery (from 4.1 +/- 0.3 ms in control to 6.0 +/- 1.1 ms after AP-A, P < 0.05) but increased the slow time constant from 66.2 +/- 6.5 ms in control to 188.9 +/- 36.4 ms (P < 0.002, n = 19) after exposure to AP-A. Increasing external divalent cation concentration (either Ca2+ or Mg2+) to 10 mM abolished the effects of AP-A on the rate of INa decay. These results demonstrate that modification of cardiac Na+ channels by AP-A markedly slowed INa inactivation and altered the voltage dependence of activation; these alterations in gating characteristics, in turn, caused an increase in gNa presumably by increasing the number of channels open at peak INa. AP-A slows the rate of recovery of INa from inactivation which is probably the basis for a use-dependent decrease in peak amplitude. Finally, AP-A binding is sensitive to external divalent cation concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of external cation concentration and metabolic inhibitors on membrane potential of human glial cells.

1. The effect on membrane potential (Em) of low external [K+]o, [Na+]o and [Ca2+]o and of metabolic inhibitors was studied in cultured human glial cells (U-787CG) and human glioma cells (Tp-483MG and U-251MG). Whole cells were voltage or current clamped with the tight-seal recording technique. 2. Em was -76 and -80 mV in glial and glioma cells (mean values in U-787CG and U-251MG, respectively) in a reference external solution with 3.0 mM K+. K(+)-free external solution caused a rapid and reversible depolarization of these cells by about 26 and 42 mV (respectively). 3. Block of K+ channels with 1 mM Ba2+ in external solution rapidly depolarized the cells (U-251MG) by about 35 mV. 4. Na(+)-free solutions caused a delayed depolarization by 40-50 mV, which was slowly reversible (in 2 min). 5. Ouabain (1 mM) depolarized the cells by about 4 mV. It did not prevent the effect of K(+)-free solution. 6. Ca(2+)-free external solution rapidly depolarized the cells to Em about -17 mV. The combination of either Na(+)-K(+)-free or Na(+)-Ca(2+)-free solution transiently repolarized the cell, which indicated that the K+ selectivity of the membrane was decreased in both K(+)- and Ca(2+)-free solutions. 7. Metabolic inhibitors (carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP) and 2,4-dinitrophenol (DNP)) rapidly and reversibly depolarized the cells. This effect was not prevented by intracellular perfusion of a strong Ca(2+)-buffering solution. 8. Voltage clamp revealed only minor changes (< 20%) in the leak conductance (g) of cells that were depolarized by the above-mentioned solutions. 9. Positive polarizing current elicited (in some cells) a regenerative depolarization. The threshold for depolarization was less in low external [K+]o. 10. It is concluded (a) that the resting potential of these glial cells depends on ion channels that are K+ selective only in the presence of external Ca2+ and K+ and (b) that this K+ selectivity may require that Em is near the reversal potential for potassium (EK), and (c) that the action of metabolic inhibitors (DNP and FCCP) is different from that in neurones.

Calculation of the influence of outer depolarizing cations on the dynamics of excitable biomembranes

In this paper an electrochemical model is used for a description of the influence of depolarizing ions on the electric behaviour of excitable biomembranes. The model includes a cyclic gating mechanism as instability-generating membrane process. Depending on various external and internal cation concentrations, we found different dynamical model behaviour, namely action potentials, potential oscillations and bistability. Investigating the influence of the cation by computer simulation, a characteristic relationship between the shape of the current—voltage curve and the stability behaviour of the model has been established. The simulations concerned with different model dynamics are compared with observations at excitable Chara algal cells.

Calculation of the influence of outer depolarizing cations on the dynamics of excitable biomembranes

In this paper an electrochemical model is used for a description of the influence of depolarizing ions on the electric behaviour of excitable biomembranes. The model includes a cyclic gating mechanism as instability-generating membrane process. Depending on various external and internal cation concentrations, we found different dynamical model behaviour, namely action potentials, potential oscillations and bistability. Investigating the influence of the cation by computer simulation, a characteristic relationship between the shape of the current-voltage curve and the stability behaviour of the model has been established. The simulations concerned with different model dynamics are compared with observations at excitable Chara algal cells.

Differential adsorption of Al, Ca, and Mg by roots of red spruce (Picea rubens Sarg.).

The surface adsorption characteristics of red spruce (Picea rubens) roots were examined as a function of changes in external acidity and cation concentrations. Root cation exchange capacity varied significantly with changes in pH, increasing from 110 micromol(c) g(-1) at pH 3.5 to 155 micromol(c) g(-1) at pH 4.5, and reaching 250 micromol(c) g(-1) at pH 7.0. In general, Al adsorption by spruce roots was much greater than either Ca or Mg adsorption under the same initial conditions. However, root affinity for the divalent cations was proportionately much more sensitive to pH changes than was root affinity for Al. The fractions of adsorbed Ca and Mg increased by 50 to 100% as pH increased from 3.5 to 4.5, whereas the fraction of adsorbed Al remained relatively constant at both initial pH conditions. Competition experiments at pH 3.5 and 4.5 indicated that Al adsorption was strongly favored over Ca adsorption, except at low Al concentrations (~10 micromol l(-1)), high solution Ca(2+)/Al(3+) ion activity ratios (> 1.5 to 5.0), and at the higher pH. These results suggest that cell wall exchange sites in red spruce roots will tend to become progressively saturated with Al under the prevailing conditions of many acidic forest soils. To the extent that root adsorbed Al interferes with the active uptake of Ca and Mg, this process of competitive cation adsorption can contribute to impaired mineral nutrition in the spruce forest community.