Nigericin forms highly stable complexes with lithium and cesium

Nigericin is a monocarboxylic polyether molecule described as a mobile K⁺ ionophore unable to transport Li⁺ and Cs⁺ across natural or artificial membranes. This paper shows that the ion carrier molecule forms complexes of equivalent energy demands with Li⁺, Cs⁺, Na⁺, Rb⁺, and K⁺. This is in accordance with the similar values of the complex stability constants obtained from nigericin with the five alkali metal cations assayed. On the other hand, nigericinalkali metal cation binding isotherms show faster rates for Li⁺ and Cs⁺ than for Na⁺, K⁺, and Rb⁺, in conditions where the carboxylic proton does not dissociate. Furthermore, proton NMR spectra of nigericin-Li⁺ and nigericin-Cs⁺ complexes show wide broadenings, suggesting strong cation interaction with the ionophore; in contrast, the complexes with Na⁺, K⁺, and Rb⁺ show only clear-cut chemical shifts. These latter results support the view that nigericin forms highly stable complexes with Li⁺ and Cs⁺ and contribute to the explanation for the inability of this ionophore to transport the former cations in conditions where it catalyzes a fast transport of K⁺>Rb⁺>Na⁺.Part of the results of this paper were presented at the 14th International Congress of Biochemistry in Prague, Czechoslovakia.     

BIOELECTRIC POTENTIALS IN VALONIA : II. EFFECTS OF ARTIFICIAL SEA WATERS CONTAINING LiCl,CsCl, RbCl,OR NH4Cl

In their influence on the P.D. across the protoplasm of Valonia macrophysa, Kütz., Li⁺ and Cs⁺ resemble Na⁺, while Rb⁺ and NH₄ ⁺ resemble K⁺. The apparent mobilities of the ions in the external surface layer of Valonia protoplasm increase in the order: Cs⁺, Na⁺, Li⁺ < Cl^- < Rb⁺ < K⁺ < NH₄ ⁺.     

A study of Li+-selective permeation through lipid bilayer membranes mediated by a new ionophore (AS701)

The neutral, noncyclic, imide and ether containing ionophore AS701, has been developed as Li⁺-selective molecule, to be used potentially as an aid in the Li⁺-therapy of manic-depressive illness. The present report is a characterization of this molecule in neutral lipid bilayer membranes. This ionophore was found to the bilayers Li⁺-selective, acting as a selective carrier of monovalent cations. In addition, this molecule was found to be capable of acting as a selective carrier of monovalent anions. For both types of ions, the rate-limitting step in the process of permeation was found to be the diffusion of the carrier-ion complex through the membrane. The membrane-permeating species were found to be 2 : 1 carrier-ion complexes, carrying either a monovalent cation or a monovalent anion. The selectivity sequences among the ions studied being: Li⁺(1) > ClO₄^?(0.7) > Na⁺(0.07) > K⁺(0.016) > Rb⁺(0.0095) > Cs⁺(0.0083) > Cl^?(0.001). Mg²⁺ and SO₄^2? were found to be impermeant (under present experimental conditions). This sequence shows that the AS701 molecule has low selectivity for ions present in biological media, among those studied (i.e. Na⁺, K⁺, Mg²⁺, Cl^2? and SO₄^2?). This indicates that these ions will not interfere in the Li⁺ permeability induced by this carrier in vivo, and that the carrier will not interfere in the normal transport processes of these ions.     

The mechanism of cation permeation in rabbit gallbladder

Summary The questions underlying ion permeation mechanisms, the types of experiments available to answer these questions, and the properties of some likely permeation models are examined, as background to experiments designed to characterize the mechanism of alkali cation permeation across rabbit gallbladder epithelium. Conductance is found to increase linearly with bathing-solution salt concentrations up to at least 400mm. In symmetrical solutions of single alkali chloride salts, the conductance sequence is K⁺>Rb⁺>Na⁺>Cs⁺∼Li⁺. The current-voltage relation is linear in symmetrical solutions and in the presence of a single-salt concentration gradient up to at least 800 mV. The anion/cation permeability ratio shows little change with concentration up to at least 300mm. Ca⁺⁺ reduces alkali chloride single-salt dilution potentials, the magnitude of the effect being interpreted as an inverse measure of cation equilibrium constants. The equilibrium-constant sequence deduced on this basis is K⁺>Rb⁺>Na⁺∼Cs⁺∼Li⁺. These results suggest (1) that the mechanism of cation permeation in the gallbladder is not the same as that in a macroscopic ion-exchange membrane; (2) that cation mobility ratios are closer to one than are equilibrium-constant ratios; (3) that the rate-limiting step for cation permeation is in the membrane interior rather than at the membrane-solution interface; and (4) that the rate-controlling membrane is one which is sufficiently thick that it obeys microscopic electroneutrality.     

Effects of alkali ions on the circadian leaf movements of Oxalis regnellii

The influence of alkali ions on the circadian leaf movements of Oxalis regnellii Mig. was investigated. Ions were given to the oscillating system via the transpiration stream of cut stalks in nutrient medium. Chloride solutions of Rb⁺, Cs⁺, Na⁺ and K⁺ were tested and the results compared to previously published LiCl-results. The period of the circadian leaf movements was unaffected by a continual addition of Na⁺ or K⁺ to the nutrient medium (at least up to 40 mM). Rb⁺, in the concentration of 2.5 or 5 mM, caused a shortening of the period when applied continuously. Rb⁺ concentrations up to 60 mM were tested. Cs⁺ ions caused only lengthenings of the circadian period. Cs⁺ concentrations up to 40 mM were tested. Cs⁺ resembled Li⁺ in producing period lengthenings, but was not as effective as Li⁺ when compared on a concentration basis. Toxicity of the effective ions was in the following order: Li⁺Cs⁺Rb⁺, Rb⁺ pulses (50 mM, 4 h) phase-shifted the rhythm and caused advances. A phase response curve was determined and the maximum steady state advances were of the order of 1 h. The dual effect of the Rb⁺ ions is discussed and is assumed to be due to two counteracting processes, exemplified by Rb⁺-sensitive ATPase-controlled pumping processes and protein synthesis. For comparison, the effects of Rb⁺ and Li⁺ in human depressive disorders is also discussed in relation to their influence on circadian systems. It is emphasized that Rb⁺ and K⁺ behave differently and are not interchangeable in their action on circadian systems.     

Specific cation effect on quenching reactions of excited tris(α,α′-diimine) ruthenium(II) and chromium(III) complexes by cyanide complexes in aqueous solutions

Specific salt effects were studied on the quenching reaction of excited [Ru(NN)₃]²⁺ (NN=2,2′-bipyridine(bpy), 1,10-phenanthrorine(phen)) and [Cr(bpy)₃]³⁺ by [Cr(CN)₆]³⁻, [Fe(CN)₆]³⁻ and [Ni(CN)₄]²⁻ in aqueous solutions as a function of alkali metal ions which were added for adjustment of ionic strength. The quenching rate constants in [Ru(NN)₃]²⁺-[Cr(CN)₆]³⁻ and [Cr(bpy)₃]³⁺-[Cr(CN)₆]³⁻ systems are changed by the cations as Li⁺>Na⁺>K⁺≈Rb⁺≈Cs⁺. On the other hand, the rate constants in [Ru(NN)₃]²⁺-[Fe(CN)₆]³⁻ and [Ru(NN)₃]²⁺-[Ni(CN)₄]²⁻ systems, which are diffusion-controlled reactions, are not varied by the alkali metal cations. The obtained order (Li⁺>Na⁺>K⁺≈Rb⁺≈Cs⁺) of the quenching rate constant is quite different from salt effects, Li⁺<Na⁺<K⁺<Rb⁺<Cs⁺, which have been obtained in the electron transfer reactions between complex anions.     

Synthesis and ionophoric activities of functionalized bis(choloyl) conjugates with a rigid core

Three bis(choloyl) conjugates bearing a rigid p-phenylenediamine/p-bis(aminomethyl)benzene linker and amino/acetamido groups were synthesized, and fully characterized on the basis of ¹H NMR, ESI-MS and HRMS. Their ionophoric activities were investigated by means of pH discharge assay. The results indicate that these conjugates exhibit potent ionophoric activities across egg-yolk l-α-phosphatidylcholine (EYPC)-based liposomal membranes, via a cation/proton antiport mechanism. They show moderate ion selectivity among alkali metal ions. Of the three conjugates, the ones having amino groups transport alkali metal ions in the order of Na⁺ > Li⁺ > K⁺ ≈ Rb⁺ ≈ Cs⁺, whereas the one having acetamido groups functions in the order of Li⁺ > Na⁺ > K⁺ ≈ Rb⁺ ≈ Cs⁺.     

pH gradient across the lysosomal membrane generated by selective cation permeability and donnan equilibrium

The pH within isolated Triton WR 1339-filled rat liver lysosomes was determined by measuring the distribution of [¹⁴C]methylamine between the intra- and extralysosomal space. The intralysosomal pH was found to be approximately one pH unit lower than that of the surrounding medium. Increasing the extralysosomal cation concentration lowered the pH gradient by a cation exchange indicating the presence of a Donnan equilibrium. The lysosomal membrane was found to be significantly more permeable to protons than to other cations. The relative mobility of cations through the lysosomal membrane is H⁺ ? Cs⁺ > Rb⁺ > K⁺ Na⁺ > Li⁺ ? Mg²⁺, Ca²⁺. The presented data suggest that the acidity within isolated Triton WR 1339-filled lysosomes is maintained by: (1) a Donnan equilibrium resulting from the intralysosomal accumulation of nondifussible anions and (2) a selective permeability of the lysosomal membrane to cations.     

Negative conductance caused by entry of sodium and cesium ions into the potassium channels of squid axons

Internal Cs⁺, Na⁺, Li⁺, and, to a lesser degree, Rb⁺ interfere with outward current through the K pores in voltage clamped squid axons. Addition of 100 mM NaF to the perfusion medium cuts outward current for large depolarizations about in half, and causes negative conductance over a range of membrane voltages. For example, suddenly reducing membrane potential from +100 to +60 mv increases the magnitude of the outward current. Internal Cs⁺ and, to a small extent, Li⁺, also cause negative conductance. Na⁺ ions permeate at least 17 times less well through the K pores than K⁺, and Cs⁺ does not permeate measurably. The results strongly suggest that K pores have a wide and not very selective inner mouth, which accepts K⁺, Na⁺, Li⁺, Cs⁺, tetraethylammonium ion (TEA⁺), and other ions. The diameter of the mouth must be at least 8 A, which is the diameter of a TEA⁺ ion. K⁺ ions in the mouths probably have full hydration shells. The remainder of the pore is postulated to be 2.6–3.0 A in diameter, large enough for K⁺ and Rb⁺ but too small for Cs⁺ and TEA⁺. We postulate that Na⁺ ions do not enter the narrower part of the pore because they are too small to fit well in the coordination cages provided by the pore as replacements for the water molecules surrounding an ion.     

Interactions of alginates with univalent cations

Interactions of alginate with univalent cations in solution have been investigated by circular dichroism (c.d.) and rheological measurements. Poly-l-guluronate chain-segments show substantial enhancement (～ 50%) of c.d. ellipticity in the presence of excess of K⁺, with smaller changes for other univalent cations: Li⁺ < Na⁺ < K⁺ > Rb⁺ > Cs⁺ > NH₄⁺. The maximum c.d. change is attained by 0.3m, with no further increase at higher concentrations of cation. No significant dependence on polymer concentration is observed. Spectral changes for poly-d-mannuronate and heterotypic chain-sequences are much smaller. For intact alginates, the magnitude of c.d. change varies almost linearly with poly-l-guluronate content. Difference spectra (c.d. with excess of univalent counterion minus c.d. in distilled water) can be fitted accurately to two Gaussian bands at 211 and 198 nm, assigned to carboxyl n → π^* and π → π^* transitions, respectively. The perturbations induced by Li⁺, K⁺, Rb⁺, Cs⁺, and NH₄⁺ show a clear family relationship, and are mainly in the π → π^* spectral region. With Na⁺, by contrast, c.d. change is largely confined to the n → π^* transition, and is similar to that previously reported for intermolecular (“egg-box”) binding of divalent cations, consistent with results of rheological studies which indicate Na⁺-induced association of poly-l-guluronate chain-sequences. These associations are further enhanced on freezing and thawing. This combined evidence is interpreted in terms of three modes of interaction between univalent cations and alginate chains in solution: (a) ion-pair formation with carboxyl groups of mannuronate and isolated guluronate residues; (b) specific site-binding to contiguous guluronate residues; and (c) co-operative “egg-box” binding, particularly of Na⁺, between poly-l-guluronate chain-sequences.     

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.     

L'induction du rythme endogène de sporulation chez Aspergillus niger: Rõles du rapport glucose/potassium et de micro-éléments

In Aspergillus niger Van Tieghem cultivated on a synthetic medium, the induction of an endogenous rhythm of sporulation and its perpetuation depend on the glucose K⁺ balance in the medium, an excess of one of them suppressing the oscillations. In its inducing effect on the rhythm K⁺ is partially replaced by Rb⁺, but not by Na⁺, Li⁺ or Cs⁺. While the glucose K⁺ balance is favourable for the manifestation of the rhythm, the addition of increasing levels of Na⁺, Li⁺ or Cs⁺ do not modify the period length. Nevertheless, at 0.3 M of Na⁺ or Li⁺ and 0.03 M of Cs⁺ rhythm disappears. The amplitude of oscillations depends on the level of the micro-elements furnished, especially on Mn²⁺. EDTA (1 × 10^?3M) inhibits the rhythm.     

Gustatory salt rejection by Culiseta inornata

Gustatory salt rejection thresholds were determined behaviourally for the mosquito Culiseta inornata. The cation sequence of decreasing effectiveness in causing rejection for monovalent salts is K⁺ = Na⁺ > Li⁺ > Cs⁺ > Rb⁺. The anion sequence of decreasing effectiveness for sodium halides is Cl⁻ > Br⁻ > F⁻ > I⁻. The sequence for divalent cations is Sr²⁺ > Ba²⁺ > Ca²⁺ > Mg²⁺. A possible mechanism to account for the nonadditive effects of monovalent and divalent salts on rejection thresholds is discussed.     

Cation binding to brain plasma membranes: An evaluation of the use of anionic fluorescent probes

Cation binding to brain plasma membranes has been studied using anionic sulfonate fluorescent probes. Ion affinity sequences follow the order Mg²⁺ > Ca²⁺ ? K⁺ > Cs⁺ > Na⁺ > Li⁺. The order of effectiveness, in increasing probe fluorescence, is the reverse of the affinity sequence for ions of the same charge. The affinity orders for erythrocyte membranes and dipalmitoyl lecithin are Mg²⁺ > Ca²⁺ ? Cs⁺ > K⁺ > Na⁺ > Li⁺ and Mg²⁺ > Ca²⁺ ? Li⁺ > Na⁺ > K⁺ > Cs⁺. These sequence variations are related to the differences in the nature of the ion binding sites. Heterogeneity in ion binding sites is demonstrated. Evidence is presented for the role of proteins in binding hydrophobic probes. The problem of separating specific conformational effects on ion binding from nonspecific charge neutralization effects is discussed. Pyrene excimer fluoresence rules out the possibility of extensive changes in mobility in the lipid phase on cation binding. Tetrodotoxin has been shown to inhibit Li⁺-, Na⁺-, and K⁺-induced fluorescence enancements of 1-anilino-8-naphthalene sulfonate bound to brain membranes.     

Ion permeation properties of a cloned human 5-HT3 receptor transiently expressed in HEK 293 cells

Summary Human 5-HT₃ receptors expressed in HEK 293 cells were studied using patch-clamp techniques. The permeability ratios of cations to Na⁺ were Li⁺, 1.16; K⁺, 1.04; Rb⁺, 1.11; Cs⁺ 1.11; NMDG⁺, 0.04; Ca²⁺, 0.49, and Mg²⁺, 0.37. The permeability sequence of the alkali metal cations was Lⁱ⁺ > Rb⁺ = Cs⁺ > K⁺ > Na⁺. Increased external concentrations of Ca²⁺ or Mg²⁺ decreased 5-HT-induced currents at all potentials tested in a voltage-independent manner. The single-channel conductance of human 5-HT₃ receptors measured by fluctuation analysis of whole-cell currents was 790 ± 100fS. Differences in the basic properties of 5-HT₃ receptors between species may explain interspecies differences in pharmacological properties.     

Low field electric birefringence study of monovalent cation-DNA interactions

The effects of monovalent cations on DNA have been studied using static and dynamic electric birefringence. Kerr's law is obeyed in a limited E range (<30 Vcm_?1) and the steady state birefringence values are close for the different cations. The birefringence kinetics have been analysed in terms of three relaxation times. On a semilogarithmice plot of Δn(t), the tail of the curve is linear over a wide range of time for Na⁺, K⁺, NH₄⁺ and Li⁺. Only for Cs⁺ solution is no linear part found and a much longer relaxation time is determined. This only contributes a small part of the total birefringence. With Cs⁺ this contribution is more field-dependent than for the other cations and we observe a larger molecular flexibility. On the other hand, with Li⁺ a greater stiffness of the DNA molecule appears. The electrical polarizabilities anisotropies decrease in the order: Cs⁺ >NH⁺₄ >K⁺ >Na⁺ >Li⁺. There are no significant differences in the optical anisotropy factors.     

Alkali metal ion selectivity of the hemocyanin channel

Summary The selectivity of the hemocyanin channel was measured for alkali metal ions and ammonium. Permeability ratios relative to K⁺ measured from biionic potentials were: NH ₄ ⁺ (1.52)>Rb⁺ (1.05)>K⁺ (1.0)>Cs⁺ (0.89)>Na⁺ (0.81)>Li⁺ (0.35). Single-channel ion conductance was a saturating function of ion concentration regardless of the cation present in the bathing medium. Maximal conductances were 270, 267, 215, 176, 170 and 37 ps for K⁺, Rb⁺, NH ₄ ⁺ , Cs⁺, Na⁺ and Li⁺, respectively. Current-voltage curves for the different monovalent cations were measured and described using a threebarrier model previously used to explain the voltage dependence of the instantaneous channel conductance (Cecchi, Alvarez & Latorre, 1981). In this way, binding and peak energies were estimated for the different ions. Considering the energy peaks as transition states between the ion and the channel, it is concluded that they follow Eisenman's selectivity sequences XI (cis peak, i.e., Li⁺>Na⁺>K⁺>Rb⁺>Cs⁺; highest field strength), VII (central peak) and II (trans peak). The cis side was that to which hemocyanin was added and was electrically ground. The binding energies, on the other hand, follow Eisenman's series XI for strong electric field sites. Binding of NH ₄ ⁺ to the cis-well suggests that the orientation of the ligands in the site is tetrahedric.     

Short communication. Selectivity of the fast activating vacuolar cation channel

The FV channel dominates the ion conductance of the vacuolar membrane at physiological Ca²⁺ concentrations. Patch-clamp measurements on whole barley (Hordeum vulgare) mesophyll vacuoles and on excised tonoplast patches showed small differences in a selectivity sequence NH4⁺ > K⁺ Rb⁺ Cs⁺ >Na⁺ >Li⁺. Less permeant cations decreased the open probability. The FV channel allows the uptake of small monovalent cations especially NH4⁺ into the vacuole.     

Contribution of residues in second transmembrane domain of ASIC1a protein to ion selectivity

Acid-sensing ion channels (ASICs) are proton-gated cation-selective channels expressed in the peripheral and central nervous systems. The ion permeation pathway of ASIC1a is defined by residues 426–450 in the second transmembrane (TM2) segment. The gate, formed by the intersection of the TM2 segments, localizes near the extracellular boundary of the plasma membrane. We explored the contribution to ion permeation and selectivity of residues in the TM2 segment of ASIC1a. Studies of accessibility with positively charged methanethiosulfonate reagents suggest that the permeation pathway in the open state constricts below the gate, restricting the passage to large ions. Substitution of residues in the intracellular vestibule at positions 437, 438, 443, or 446 significantly increased the permeability to K⁺ versus Na⁺. ASIC1a shows a selectivity sequence for alkali metals of Na⁺>Li⁺>K⁺≫Rb⁺>Cs⁺. Alanine and cysteine substitutions at position 438 increased, to different extents, the relative permeability to Li⁺, K⁺, Rb⁺, and Cs⁺. For these mutants, ion permeation was not a function of the diameter of the nonhydrated ion, suggesting that Gly-438 encompasses an ion coordination site that is essential for ion selectivity. M437C and A443C mutants showed slightly increased permeability to K⁺, Rb⁺, and Cs⁺, suggesting that substitutions at these positions influence ion discrimination by altering molecular sieving. Our results indicate that ion selectivity is accomplished by the contribution of multiple sites in the pore of ASIC1a.     

Stimulating effects of monovalent cations on activator-dissociated and activator-associated states of Ca2+-ATPase in human erythrocytes

The additional activation by monovalent cations of the $\text{(Ca}{}^{\mathrm{2+}}\text{+ Mg}{}^{\mathrm{2+}}\text{)-dependent}$ ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied.The Ca²⁺-ATPase occurs in two different states. In the A-state the enzyme is virtually free of protein activator and the kinetics of Ca²⁺ activation is characterized by low apparent Ca²⁺ affinity and low maximum activity. In the B-state the enzyme is associated with activator and the kinetics is characterized by high Ca²⁺ affinity and high maximum activity.At optimum concentrations of Ca²⁺ the additional activation of the B-state by K⁺, NH₄⁺, Na⁺ and Rb⁺ exceeded the corresponding activations of the A-state, and half-maximum activations by K⁺, NH₄⁺, and Na⁺ were achieved at lower concentrations in the B-state than in the A-state. Li⁺ and Cs⁺ activated the two states almost equally but maximum activation was obtained at lower cation concentrations in the B-state than in the A-state.The activation of the B-state by the various cations decreased in the order $\text{K}{}^{\mathrm{+}}\text{> NH}{}_4{}^{\mathrm{+}}\text{> Na}{}^{\mathrm{+}}\text{= Rb}{}^{\mathrm{+}}\text{> Li}{}^{\mathrm{+}}\text{= Cs}{}^{\mathrm{+}}$. The A-state was activated almost equally by K⁺, Na⁺, NH₄⁺, and Rb⁺ and to a smaller extent by Li⁺ and Cs⁺.At sub-optimum concentrations of Ca²⁺ high concentrations of monovalent cations (100 mM) activated the Ca²⁺-ATPase equally in the A-state and the B-state. In the absence of Ca²⁺ the monovalent cations inhibited the Mg²⁺-dependent ATPase in both types of membranes. This dependence on Ca²⁺ indicates that the monovalent cations interact with the Ca²⁺ sites in the B-state.The results suggest that K⁺ or Na⁺, or both, contribute to the regulation of the Ca²⁺ pump in erythrocytes.