Cesium toxicity in Arabidopsis

Cesium (Cs) is chemically similar to potassium (K). However, although K is an essential element, Cs is toxic to plants. Two contrasting hypotheses to explain Cs toxicity have been proposed: (1) extracellular Cs+ prevents K+ uptake and, thereby, induces K starvation; and (2) intracellular Cs+ interacts with vital K(+)-binding sites in proteins, either competitively or noncompetitively, impairing their activities. We tested these hypotheses with Arabidopsis (Arabidopsis thaliana). Increasing the Cs concentration in the agar ([Cs](agar)) on which Arabidopsis were grown reduced shoot growth. Increasing the K concentration in the agar ([K](agar)) increased the [Cs](agar) at which Cs toxicity was observed. However, although increasing [Cs](agar) reduced shoot K concentration ([K](shoot)), the decrease in shoot growth appeared unrelated to [K](shoot) per se. Furthermore, the changes in gene expression in Cs-intoxicated plants differed from those of K-starved plants, suggesting that Cs intoxication was not perceived genetically solely as K starvation. In addition to reducing [K](shoot), increasing [Cs](agar) also increased shoot Cs concentration ([Cs](shoot)), but shoot growth appeared unrelated to [Cs](shoot) per se. The relationship between shoot growth and [Cs](shoot)/[K](shoot) suggested that, at a nontoxic [Cs](shoot), growth was determined by [K](shoot) but that the growth of Cs-intoxicated plants was related to the [Cs](shoot)/[K](shoot) quotient. This is consistent with Cs intoxication resulting from competition between K+ and Cs+ for K(+)-binding sites on essential proteins.

Identification and characterization of a nitrate transporter gene in Neurospora crassa

The Neurospora crassa genome database was searched for sequence similarity to crnA, a nitrate transporter in Aspergillus nidulans. A 3.9-kb fragment (contig 3.416, subsequence 183190-187090) was cloned by PCR. The gene coding for this nitrate transporter was termed nit-10. The nit-10 gene specifies a predicted polypeptide containing 541 amino acids with a molecular mass of 57 kDa. In contrast to crnA, which is clustered together with niaD, encoding nitrate reductase, and niiA, encoding nitrite reductase, nit-10 is not linked to nit-3 (nitrate reductase), nit-6 (nitrite reductase), or to nit-2, nit-4 (both are positive regulators of nit-3), or nmr (negative regulator of nit-3) in Neurospora crassa. A nit-10 rip mutant failed to grow in the medium when nitrate (< 10 mM) was used as the sole nitrogen source, but grew similarly to wild type when nitrate concentration was 10 mM or higher. In addition, it showed strong sensitivity to cesium in the presence of nitrate and resistance to chlorate in the presence of alanine, proline, or hypoxanthine. The expression of nit-10 required nitrate induction and was subject to repression by nitrogen metabolites such as glutamine. Expression of nit-10 also required functional products of nit-2 and nit-4. The half-life of nit-10 mRNA was determined to be approximately 2.5 min.

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)(+).

Acquired long QT syndrome and monomorphic ventricular tachycardia after alternative treatment with cesium chloride for brain cancer

Individuals searching for symptomatic relief or a potential cure are increasingly seeking and using nontraditional therapies for their various diseases. Little is known about the potential adverse effects that patients may encounter while undergoing these alternative treatments. Cesium chloride is an unregulated agent that has been reported to have antineoplastic properties. Cesium chloride is advertised as an alternative agent for many different types of cancers and can be purchased easily on the Internet. Recently, QT prolongation and polymorphic ventricular tachycardia were reported in several patients taking cesium chloride as alternative treatment for cancer. We report acquired QT prolongation and sustained monomorphic ventricular tachycardia in a patient who self-initiated and completed a course of cesium chloride as adjunctive treatment for brain cancer.

Ion Binding Affinity in the Cavity of the KcsA Potassium Channel

The hydrophobic cell membrane interior presents a large energy barrier for ions to permeate. Potassium channels reduce this barrier by creating a water-filled cavity at the middle of their ion conduction pore to allow ion hydration and by directing the C-terminal "end charge" of four alpha-helices toward the water-filled cavity. Here we have studied the interaction of monovalent cations with the cavity of the KcsA K(+) channel using X-ray crystallography. In these studies, Tl(+) was used as an analogue for K(+) and the total ion-stabilization energy for Tl(+) in the cavity was estimated by measuring its binding affinity. Binding affinity for the Na(+) ion was also measured, revealing a weak selectivity ( approximately 7-fold) favoring Tl(+) over Na(+). The structures of the cavity containing Na(+), K(+), Tl(+), Rb(+), and Cs(+) are compared. These results are consistent with a fairly large (more negative than -100 mV) electrostatic potential inside the cavity, and they also imply the presence of a weak nonelectrostatic component to a cation's interaction with the cavity.

Impurity of recombinant adeno-associated virus type 2 affects the transduction characteristics following subretinal injection in the rat

We recently reported that different purification methods of recombinant adeno-associated virus type 2 (rAAV2) affect the transduction characteristics following subretinal injection. In this study, we examined the roles of contaminant proteins from the HEK-293 cells and helper adenovirus, inactivation of helper adenovirus and cell stress induced by DNA-damaging agents in rAAV-mediated retinal transduction. Our results showed that contaminating factors/proteins resulting from the helper E1 deleted adenovirus are possibly responsible for efficient RPE transduction. Future studies of these factors will undoubtedly lead to development of new therapeutic approaches to PR- and RPE-specific retinal diseases.

The Occupancy of Ions in the K+ Selectivity Filter: Charge Balance and Coupling of Ion Binding to a Protein Conformational Change Underlie High Conduction Rates

Potassium ions diffuse across the cell membrane in a single file through the narrow selectivity filter of potassium channels. The crystal structure of the KcsA K+ channel revealed the chemical structure of the selectivity filter, which contains four binding sites for K+. In this study, we used Tl+ in place of K+ to address the question of how many ions bind within the filter at a given time, i.e. what is the absolute ion occupancy? By refining the Tl+ structure against data to 1.9A resolution with an anomalous signal, we determined the absolute occupancy of Tl+. Then, by comparing the electron density of Tl+ with that of K+, Rb+ and Cs+, we estimated the absolute occupancy of these three ions. We further analyzed how the ion occupancy affects the conformation of the selectivity filter by analyzing the structure of KcsA at different concentrations of Tl+. Our results indicate that the average occupancy for each site in the selectivity filter is about 0.63 for Tl+ and 0.53 for K+. For K+, Rb+ and Cs+, the total number of ions contained within four sites in the selectivity filter is about two. At low concentrations of permeant ion, the number of ions drops to one in association with a conformational change in the selectivity filter. We conclude that electrostatic balance and coupling of ion binding to a protein conformational change underlie high conduction rates in the setting of high selectivity.

Molecular origin of the cation selectivity in OmpF porin: single channel conductances vs. free energy calculation

Ion current through single outer membrane protein F (OmpF) trimers was recorded and compared to molecular dynamics simulation. Unidirectional insertion was revealed from the asymmetry in channel conductance. Single trimer conductance showed particularly high values at low symmetrical salt solution. The conductance values of various alkali metal ion solutions were proportional to the monovalent cation mobility values in the bulk phase, LiCl<NaCl<KCl<RbCl approximately CsCl, but the conductance differences were quantitatively larger than conductivity differences in bulk solutions. Selectivity measurements at low concentration showed that OmpF channels favored permeation of alkali metal ions over chloride and suggested size preference for smaller cations. These results suggest that there are specific interactions between the permeating cation and charged residues lining the channel walls. This hypothesis was supported by computational study which predicted that monovalent cations bind to Asp113 at low concentration. Here, free energy calculations revealed that the affinity of the alkali metal ions to its binding site increased with their atomic radii, Li(+) approximately Na(+)<K(+) approximately Rb(+) approximately Cs(+). A detailed inspection of both experimental and computational results suggested that stronger binding at the central constriction of the channel increases the translocation rate of cations under applied voltage by increasing their local concentration relative to the bulk solution.

Monovalent cations contribute to T-type calcium channel (Cav3.1 and Cav3.2) selectivity

Low voltage-activated (LVA) Ca2+ channels regulate chemical signaling by their ability to select for Ca2+. Whereas Ca2+ is the main permeating species through Ca2+ channels, Ca2+ permeation may be modified by abundant intra- and extracellular monovalent cations. Therefore, we explored monovalent cation regulation of LVA Ca2+ permeation in the cloned T-type Ca2+ channels alpha1G (Cav3.1) and alpha1H (Cav3.2). In physiological [Ca2+], the reversal potential in symmetrical Li+ was 19 mV in alpha1G and 18 mV in alpha1H, in symmetrical Cs+ the reversal potential was 36 mV in alpha1G and 37 mV in alpha1H, and in the bi-ionic condition with Li+ in the bath and Cs+ in the pipette, the reversal potential was 46 mV in both alpha1G and alpha1H. When Cs+ was used in the pipette, replacement of external Cs+ with Li+ (or Na+) shifted the reversal potential positive by 5-6 mV and increased the net inward current in alpha1G. Taken together the data indicate that in physiological [Ca2+], external Li+ (or Na+) permeates more readily than external Cs+, resulting in a positive shift of the reversal potential. We conclude that external monovalent cations dictate T-type Ca2+ channel selectivity by permeating through the channel. Similar to Li+, we previously reported that external [H+] can regulate T-type Ca2+ channel selectivity. Alpha1H's selectivity was more sensitive to external pH changes compared to alpha1G. When Cs+ was used in the pipette and Li+ was used in the bath external acidification from pHo 7.4 to 6.0 caused a negative shift of the reversal by 8 mV in alpha1H. Replacement of internal Cs+ with Li+ reduced the pH-induced shift of the reversal potential to 2 mV. We conclude that, similar to other external monovalent cations, H+ can modify T-type Ca2+ channel selectivity. However, in contrast to external monovalent ions that readily permeate, H+ regulate T-type Ca2+ channel selectivity by increasing the relative permeability of the internal monovalent cation.

Ca2+ -activated nonselective cation channels in rat neonatal atrial myocytes

A nonselective cation channel activated by intracellular Ca(2+) was identified in inside-out membrane patches taken from cultured rat atrial myocytes. Ca(2+) (0.01-1.00 m M) reversibly activated the channel in a concentration-dependent manner. The channel often showed a quick and irreversible rundown within a few minutes after patch excision. The I-V relationship of the channel was linear between -100 and +100 mV. The single channel conductance was 26.0 +/- 0.5 pS and its open probability was weakly voltage-dependent. Ion-substitution experiments showed that the channel was permeable to monovalent cations (P(x)/P(Cs): Li(+) (1.5) = K(+) (1.5) > Na(+) (1.2) > Rb(+) (1.1) > Cs(+) (1.0)) but not to Cl(-) (P(Cl)/P(Cs) < 0.01) and Ca(2+) (P(Ca)/P(Cs) = 0.02 +/- 0.01).

Mechanisms of adrenergic control of sino-atrial node discharge

Among the mechanisms proposed for the increase in discharge of sino-atrial node (SAN) by norepinephrine (NE) are an increase in the hyperpolarization-activated current I(f) and in the slow inward current I(Ca,L). If I(f) is the primary mechanism, cesium (a blocker of I(f)) should eliminate the positive chronotropic effect of NE. If I(Ca,L), is involved, [Ca(2+)](o) should condition NE effects. We studied the electrophysiological changes induced by NE in isolated guinea pig SAN superfused in vitro with Tyrode solution (both SAN dominant and subsidiary pacemaker mechanisms are present) as well as with high [K(+)](o), higher Cs(+) or Ba(2+) (only the dominant pacemaker mechanism is present). In Tyrode solution, NE (0.5-1microM) increased the SAN rate and adding Cs(+) (approximately 12 mM) caused a decaying voltage tail during diastole in subsidiary pacemakers. NE enhanced the Cs(+)-induced tail, and increased the rate but less than in Tyrode solution. In higher [Cs(+)](o) (15- 18 mM), Ba(2+) (1 mM) or Ba(2+) plus Cs(+) (10 mM) dominant action potentials (not followed by a tail) were present and NE accelerated them as in Tyrode solution. In high [K(+)](o), NE increased the rate in the absence and presence of Cs(+), Ba(2+) or Ba(2+) plus Cs(+). In these solutions, NE increased the overshoot and maximum diastolic potential of dominant action potentials (APs) and increased the rate by steepening diastolic depolarization and shifting the threshold for upstroke to more negative values. High [Ca(2+)](o) alone increased the rate and NE enhanced this action, whereas low [Ca(2+)](o) reduced or abolished the increase in rate by NE. In SAN quiescent in high [K(+)](o) plus indapamide, NE induced spontaneous discharge by decreasing the resting potential and initiating progressively larger voltage oscillations. Thus, NE increases the SAN rate by acting primarily on dominant APs in a manner consistent with an increase of I(Ca,L) and I(K) and under conditions where I(f) is either blocked or not activated. NE INITIATES spontaneous discharge by inducing voltage oscillations unrelated to I(f).

Is there an A-type K+ current in guinea pig ventricular myocytes?

A unique transient outward K(+) current (I(to)) has been described to result from the removal of extracellular Ca(2+) from ventricular myocytes of the guinea pig (15). This study addressed the question of whether this current represented K(+)-selective I(to) or the efflux of K(+) via L-type Ca(2+) channels. This outward current was inhibited by Cd(2+), Ni(2+), Co(2+), and La(3+) as well as by nifedipine. All of these compounds were equally effective inhibitors of the L-type Ca(2+) current. The current was not inhibited by 4-aminopyridine. Apparent inhibition of the outward current by extracellular Ca(2+) was shown to result from the displacement of the reversal potential of cation flux through L-type Ca(2+) channels. The current was found not to be K(+) selective but also permeant to Cs(+). The voltage dependence of inactivation of the outward current was identical to that of the L-type Ca(2+) current. It is concluded that extracellular Ca(2+) does not mask an A-type K(+) current in guinea pig ventricular myocytes.

Electrophysiological response of cultured trabecular meshwork cells to synthetic ion channels

The response of living cells of the trabecular meshwork to synthetic ion channels is described. The THF-gramicidin hybrids THF-gram and THF-gram-TBDPS as well as a linked gA-TBDPS and gramicidin A were applied to cultured ocular trabecular meshwork cells. THF-gram application (minimal concentration, 10(-8) M; saturation, 10(-7) M) led to an additional conductance which displayed characteristics of weak Eisenman-I-selective cation channels, no cell destruction, an asymmetric change of the inward/outward currents, and higher current densities using Cs(+) as charge carrier compared to Na(+) and K(+). Linked-gA-TBDPS showed at 10(-12) M increases of the membrane conductance comparable to gA at 10(-7) M and a much faster response of the cells. Thus, THF-gramicidin hybrids form a basis for the use of synthetic ion channels in biological systems, which eventually may lead to new therapeutic approaches.

Analysis of the K+ current profile of mature rat oligodendrocytes in situ

Previous studies have reported that mature oligodendrocytes (OLGs) in vitro display various voltage-dependent K+ currents while in situ OLGs show only voltage-independent K+ currents. Given this discrepancy and the lack of information on myelinating OLG ion channel expression in situ, we characterized mature OLG currents in myelinating corpus callosum slices from 17 to 36-day old rats. OLGs were recorded in cell-attached and whole-cell patch-clamp configurations, displayed morphology typical of mature OLGs, and stained positive for myelin basic protein. OLGs displayed large voltage-independent currents that decayed during the voltage pulse and small voltage-activated outward currents. The latter were blocked by TEA, activated between -40 and -50 mV, and decayed slowly. The former were composed of large voltage-independent, time-dependent Ba2+ (1 mM)-sensitive currents, and voltage-dependent Cs+ (5 mM) and Ba2+ (100 mM)-sensitive currents that reversed near the K+ equilibrium potential and inactivated at hyperpolarized potentials, identifying them as inwardly rectifying K+ currents. Inwardly rectifying single-channel K+currents could be recorded in the cell-attached configuration. The estimated single-channel slope conductance was 30 pS. The steady-state open probability was voltage-dependent and declined from 0.9 to 0.5 between -80 and -150 mV. Overall, mature OLGs in situ possess time- and also voltage-dependent K+ currents, which may facilitate clearance of K+ released during axonal firing.

Modulation of the human polycystin-L channel by voltage and divalent cations

Polycystin-L (PCL) is highly homologous in sequence and membrane topology to polycystin-2, the product of the second gene responsible for autosomal dominant polycystic kidney disease (ADPKD). PCL and polycystin-2 were recently shown to be Ca2+-permeable, Ca2+-activated cation channels. Further characterization of polycystins will help in the understanding of cystogenesis and pathogenesis of ADPKD. In the present study, we expressed human PCL in Xenopus oocytes and studied its function utilizing patch-clamp and two-electrode voltage clamp techniques. In addition to its permeability to Ca2+, K+ and Na+, PCL was highly permeable to NH4+ and Cs+ with a permeability ratio NH4+:Cs+:Na+ of 2.2:1.02:1. Voltage modulation of channel properties was studied using cell-attached (C-A) and excised inside-out (I-O) patches. In the C-A mode, the open probability (NP(o)) of PCL at negative potentials (NP(o)=0.22) was higher than at positive potentials (NP(o)=0.05). The mean open time averaged 31.6 ms at negative potentials, and 6.2 ms at positive potentials; single-channel activity exhibited bursts with a mean interburst time of 178 ms. Using I-O patches under symmetrical ionic conditions, single-channel inward conductance was significantly larger than outward conductance, indicating a slight inward rectification. External Mg2+ inhibited the PCL channel currents. The inhibitory effect was voltage-dependent and substantially reduced by depolarization. The time course of inactivation depended on external calcium concentration but was independent of voltage and peak current. This study shows that although PCL is not a voltage-gated channel, its channel activity and inhibition by Mg2+ are modulated by membrane potential.

Distinct properties of CRAC and MIC channels in RBL cells

In rat basophilic leukemia (RBL) cells and Jurkat T cells, Ca(2+) release-activated Ca(2+) (CRAC) channels open in response to passive Ca(2+) store depletion. Inwardly rectifying CRAC channels admit monovalent cations when external divalent ions are removed. Removal of internal Mg(2+) exposes an outwardly rectifying current (Mg(2+)-inhibited cation [MIC]) that also admits monovalent cations when external divalent ions are removed. Here we demonstrate that CRAC and MIC currents are separable by ion selectivity and rectification properties: by kinetics of activation and susceptibility to run-down and by pharmacological sensitivity to external Mg(2+), spermine, and SKF-96365. Importantly, selective run-down of MIC current allowed CRAC and MIC current to be characterized under identical ionic conditions with low internal Mg(2+). Removal of internal Mg(2+) induced MIC current despite widely varying Ca(2+) and EGTA levels, suggesting that Ca(2+)-store depletion is not involved in activation of MIC channels. Increasing internal Mg(2+) from submicromolar to millimolar levels decreased MIC currents without affecting rectification but did not alter CRAC current rectification or amplitudes. External Mg(2+) and Cs(+) carried current through MIC but not CRAC channels. SKF-96365 blocked CRAC current reversibly but inhibited MIC current irreversibly. At micromolar concentrations, both spermine and extracellular Mg(2+) blocked monovalent MIC current reversibly but not monovalent CRAC current. The biophysical characteristics of MIC current match well with cloned and expressed TRPM7 channels. Previous results are reevaluated in terms of separate CRAC and MIC channels.

Direct measurement of the association constant of HER2/neu antisense oligonucleotide to its target RNA sequence using a molecular beacon

A molecular beacon approach was developed to directly determine the association constant of RNA-DNA hybrid formation. The molecular beacon was composed of a 15-nt loop structure containing the antisense sequence that can hybridize with the AUG translational start site of the HER2/neu gene, which is overexpressed in a significant proportion of breast, ovarian, and lung tumors. The equilibrium association constant (Ka) of DNA binding to the RNA oligonucleotide was 6.4 +/- 0.14 x 10(7) M(-1) in the presence of 150 mM NaCl at 22 degrees C. The free energy change (AG) associated with RNA-DNA hybrid formation was -10.7 kcal/mole. The melting temperature (Tm) of RNA-DNA hybrid was 64.4 degrees C +/- 1 degree C in the presence of 150 mM NaCl. The RNA-DNA hybrid was more stable than the corresponding DNA-DNA duplex in 150 mM NaCl, as judged by both Ka and Tm data. We also determined the Ka, deltaG, and Tm values of RNA-DNA and DNA-DNA duplex formation in the presence of three monovalent cations, Li+, K+, and Cs+. The feasibility of this method was also investigated using a phosphorothioate molecular beacon. The information generated through this new approach for thermodynamic measurements might be useful for the design of oligonucleotides for antisense therapeutics.

Fast network oscillations induced by potassium transients in the rat hippocampus in vitro

Brief pressure ejection of solutions containing potassium, caesium or rubidium ions into stratum radiatum of the CA1 or CA3 regions of the hippocampal slice evoked a fast network oscillation. The activity evoked lasted approximately 3-25 s with the predominant frequency component being in the gamma frequency range (30-80 Hz), although beta frequency (15-30 Hz) and ultrafast (> 80 Hz) components could also be seen. The gamma frequency component of the oscillation remained constant, even when large changes in power occurred, and was synchronous across the CA1 region. Measurements with potassium ion-sensitive electrodes revealed that the network oscillation was accompanied by increases (0.5 to 2.0 mM) in the extracellular potassium concentration [K+]o. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 50 microM) had no significant effect but the alpha-amino-3-hydroxy-5-methyl-4-isooxazolepropionic acid (AMPA)/kainate receptor antagonist 2,3,-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide disodium (NBQX; 20 microM) caused a significant reduction (86.7 +/- 4.5 %) in the power in the gamma frequency range. Residual rhythmic activity, presumably arising in the interneuronal network, was blocked by the GABA(A) receptor antagonist bicuculline. The putative gap junction blocker octanol caused a decrease in the power of the gamma frequency component of 75.5 +/- 5.6 %, while carbenoxolone produced a reduction of only 14 +/- 42 %. These experiments demonstrate that a modest increase in exogenous [K+]o in the hippocampus in vitro is sufficient to evoke a fast network oscillation, which is an emergent property of the synaptically and electrically interconnected neuronal network.

Ionic currents in isolated and in situ squid Schwann cells

Ionic currents from Schwann cells isolated enzymatically from the giant axons of the squids Loligo forbesi, Loligo vulgaris and Loligo bleekeri were compared with those obtained in situ. Macroscopic and single channel ionic currents were recorded using whole-cell voltage and patch clamp. In the whole-cell configuration, depolarisation from negative holding potentials evoked two voltage-dependent currents, an inward current and a delayed outward current. The outward current resembled an outwardly rectifying K+ current and was activated at -40 mV after a latent period of 5-20 ms following a step depolarisation. The current was reduced by externally applied nifedipine, Co2+ or quinine, was not blocked by addition of apamin or charibdotoxin and was insensitive to externally applied L-glutamate or acetylcholine. The voltage-gated inward current was activated at -40 mV and was identified as an L-type calcium current sensitive to externally applied nifedipine. Schwann cells were impaled in situ in split-open axons and voltage clamped using discontinuous single electrode voltage clamp. Voltage dependent outward currents were recorded that were kinetically identical to those seen in isolated cells and that had similar current-voltage relations. Single channel currents were recorded from excised inside-out patches. A single channel type was observed with a reversal potential close to the equilibrium potential for K+ (E(K)) and was therefore identified as a K+ channel. The channel conductance was 43.6 pS when both internal and external solutions contained 150 mM K+. Activity was weakly dependent on membrane voltage but sensitive to the internal Ca2+ concentration. Activity was insensitive to externally or internally applied L-glutamate or acetylcholine. The results suggest that calcium channels and calcium-activated K+ channels play an important role in the generation of the squid Schwann cell membrane potential, which may be controlled by the resting intracellular Ca2+ level.

Conformational changes of the nucleotide site of the plasma membrane Ca2+-ATPase probed by fluorescence quenching

Fluorescence quenching by the water-soluble ions I(-) and Cs(+) was used to probe solvent accessibility and polarity of the nucleotide/fluorescein isothiocyanate binding pocket of the purified soluble Ca(2+)-ATPase from plasma membranes. The E(1).Ca.CaM conformer was the least accessible state studied, presenting the lowest suppression constant (K(q)) for both I(-) (K(q) = 6.7 M(-)(1)) and Cs(+) (K(q) = 0.7 M(-)(1)). Accessibility to I(-) was similar for the E(2).VO(4) and E(1).Ca states (K(q) = 7.13 and 7.5 M(-)(1), respectively), whereas E(2) was slightly more accessible (K(q) = 9.1 M(-)(1)). The phosphorylated state E(2)-P presented the highest accessibility, with a K(q) of 16.5 M(-)(1), very near the K(q) of 20.3 M(-)(1) for free FITC. I(-) was unequivocally a better fluorescence quencher, being usually nearly 3-fold as efficient as Cs(+), as indicated by the K(q)(I(-))/K(q)(Cs(+)) ratio (R(q)). The advent of a positive charge cluster on the nucleotide/fluorescein binding pocket in different states was suggested by the increase in R(q), which reached a value as high as 9.5 for the E(1).Ca.CaM conformer. These results indicate (i) a very high water accessibility of the nucleotide/fluorescein pocket for E(2)-P that (ii) is more restricted on the free E(2) state and (iii) becomes rather lower for the E(1).Ca states. Additionally, a positive charge effect of amino acids on the nucleotide site, possibly related to ATP binding and phosphoryl transfer, appears in these E(1).Ca states, being absent in the phosphorylated and nonphosphorylated E(2) states.

Channels involved in transient currents unmasked by removal of extracellular calcium in cardiac cells

In cardiac cells that lack macroscopic transient outward K(+) currents (I(to)), the removal of extracellular Ca(2+) can unmask "I(to)-like" currents. With the use of pig ventricular myocytes and the whole cell patch-clamp technique, we examined the possibility that cation efflux via L-type Ca(2+) channels underlies these currents. Removal of extracellular Ca(2+) and extracellular Mg(2+) induced time-independent currents at all potentials and time-dependent currents at potentials greater than -50 mV. Either K(+) or Cs(+) could carry the time-dependent currents, with reversal potential of +8 mV with internal K(+) and +34 mV with Cs(+). Activation and inactivation were voltage dependent [Boltzmann distributions with potential of half-maximal value (V(1/2)) = -24 mV and slope = -9 mV for activation; V(1/2) = -58 mV and slope = 13 mV for inactivation]. The time-dependent currents were resistant to 4-aminopyridine and to DIDS but blocked by nifedipine at high concentrations (IC(50) = 2 microM) as well as by verapamil and diltiazem. They could be increased by BAY K-8644 or by isoproterenol. We conclude that the I(to)-like currents are due to monovalent cation flow through L-type Ca(2+) channels, which in pig myocytes show low sensitivity to nifedipine.

[The efficient accumulation of cesium ions by Rhodococcus cells]

Bacteria of the genus Rhodococcus were found to be able to accumulate cesium by means of active transport and nonspecific sorption on the cell surface structures. The maximum removal (up to 97%) of cesium from a medium with ammonium acetate was observed at 28 degrees C, pH 7.8-8.6, and an equimolar content (0.2 mM) of potassium and cesium ions in the medium. The most active cesium-accumulating Rhodococcus sp. strains can be used for purification of industrial wastewaters contaminated with radionuclides.

Activity of rubidium and cesium in soybean looper (Lepidoptera: Noctuidae): insect feeding on cotton and soybean measured by elemental markers

Uptake and translocation of the elemental markers rubidium (Rb) and cesium (Cs) within adult soybean looper, Pseudoplusia includens (Walker), were determined using atomic absorption spectrophotometry in the laboratory in various feeding and mating treatments. Neonates were tested to determine marker transfer from male and female adults fed rubidium chloride (RbCl)-treated artificial nectar, cesium chloride (CsCI)-treated artificial nectar, or both. All females contained detectable levels of Rb, Cs, or both, which were obtained either through direct feeding or via spermatophores. Rubidium was present in females at significantly greater levels than Cs. No significant differences in Rb levels were observed between feeding or spermatophore acquisitions. Most neonates had significantly higher levels of Rb than Cs. In a field cage study to evaluate adult feeding and oviposition behavior on blooming cotton and blooming soybean treated with RbCl and CsCl, respectively, more eggs contained Rb than Cs, indicating greater feeding on cotton nectar than soybean nectar, regardless of the host plant upon which eggs were laid. Females laid more eggs on blooming soybean than on blooming cotton. Higher levels of Rb in cotton than Cs in soybean were recorded and may be attributed to initial elemental marker quantities available to the insects. This study provides the support for the generalized observations that soybean looper infestations in soybean can be related to feeding activities by adults in cotton.

Protein phosphatase type 1 regulates ion homeostasis in Saccharomyces cerevisiae

Protein phosphatase type 1 (PP1) is encoded by the essential gene GLC7 in Saccharomyces cerevisiae. glc7-109 (K259A, R260A) has a dominant, hyperglycogen defect and a recessive, ion and drug sensitivity. Surprisingly, the hyperglycogen phenotype is partially retained in null mutants of GAC1, GIP2, and PIG1, which encode potential glycogen-targeting subunits of Glc7. The R260A substitution in GLC7 is responsible for the dominant and recessive traits of glc7-109. Another mutation at this residue, glc7-R260P, confers only salt sensitivity, indicating that the glycogen and salt traits of glc7-109 are due to defects in distinct physiological pathways. The glc7-109 mutant is sensitive to cations, aminoglycosides, and alkaline pH and exhibits increased rates of l-leucine and 3,3'-dihexyloxacarbocyanine iodide uptake, but it is resistant to molar concentrations of sorbitol or KCl, indicating that it has normal osmoregulation. KCl suppresses the ion and drug sensitivities of the glc7-109 mutant. The CsCl sensitivity of this mutant is suppressed by recessive mutations in PMA1, which encodes the essential plasma membrane H(+)ATPase. Together, these results indicate that Glc7 regulates ion homeostasis by controlling ion transport and/or plasma membrane potential, a new role for Glc7 in budding yeast.

Sodium fluxes through nonselective cation channels in the plasma membrane of protoplasts from Arabidopsis roots

The aim of the present work was to characterize Na(+) currents through nonselective cation channels (NSCCs) in protoplasts derived from root cells of Arabidopsis. The procedure of the protoplast isolation was modified to increase the stability of Arabidopsis root protoplasts in low external Ca(2+) by digesting tissue in elevated Ca(2+). Experiments in whole-cell and outside-out modes were carried out. We found that Na(+) currents in Arabidopsis root protoplasts were mediated by cation channels that were insensitive to externally applied tetraethylammonium(+) and verapamil, had no time-dependent activation (permanently opened or completely activated within 1-2 ms), were voltage independent, and were weakly selective for monovalent cations. The selectivity sequence was as follows: K(+) (1.49) > NH(4)(+) (1.24) > Rb(+) (1.15) approximately equal to Cs(+) (1.10) approximately equal to Na(+) (1.00) > Li(+) (0.73) > tetraethylammonium(+) (0.47). Arabidopsis root NSCCs were blocked by H(+) (pK approximately equal to 6.0), Ca(2+) (K(1/2) approximately equal to 0.1 mM), Ba(2+), Zn(2+), La(3+), Gd(3+), quinine, and the His modifier diethylpyrocarbonate. They were insensitive to most organic blockers (nifedipine, verapamil, flufenamate, and amiloride) and to the SH-group modifier p-chloromercuriphenyl sulfonic acid. Voltage-insensitive, Ca(2+)-sensitive single channels were also resolved. Properties of Arabidopsis root NSCCs are discussed and compared with characteristics of similar conductances studied previously in plants and animals. It is suggested that NSCCs present a distinct group of plant ion channels, mediating toxic Na(+) influx to the cell and probably having other important roles in physiological processes of plants.

Activation of ribokinase by monovalent cations

Carbohydrate kinases frequently require a monovalent cation for their activity. The physical basis of this phenomenon is, however, usually unclear. We report here that Escherichia coli ribokinase is activated by potassium with an apparent K(d) of 5 mM; the enzyme should therefore be fully activated under physiological conditions. Cesium can be used as an alternative ion, with an apparent K(d) of 17 mM. An X-ray structure of ribokinase in the presence of cesium was solved and refined at 2.34 A resolution. The cesium ion was bound between two loops immediately adjacent to the anion hole of the active site. The buried location of the site suggests that conformational changes will accompany ion binding, thus providing a direct mechanism for activation. Comparison with structures of a related enzyme, the adenosine kinase of Toxoplasma gondii, support this proposal. This is apparently the first instance in which conformational activation of a carbohydrate kinase by a monovalent cation has been assigned a clear structural basis. The mechanism is probably general to ribokinases, to some adenosine kinases, and to other members of the larger family. A careful re-evaluation of the biochemical and structural data is suggested for other enzyme systems.

I(Ca(TTX)) channels are distinct from those generating the classical cardiac Na(+) current

The Na(+) current component I(Ca(TTX)) is functionally distinct from the main body of Na(+) current, I(Na). It was proposed that I(Ca(TTX)) channels are I(Na) channels that were altered by bathing media containing Ca(2+), but no, or very little, Na(+). It is known that Na(+)-free conditions are not required to demonstrate I(Ca(TTX).) We show here that Ca(2+) is also not required. Whole-cell, tetrodotoxin-blockable currents from fresh adult rat ventricular cells in 65 mm Cs(+) and no Ca(2+) were compared to those in 3 mM Ca(2+) and no Cs(+) (i.e., I(Ca(TTX))). I(Ca(TTX)) parameters were shifted to more positive voltages than those for Cs(+). The Cs(+) conductance-voltage curve slope factor (mean, -4.68 mV; range, -3.63 to -5.72 mV, eight cells) is indistinguishable from that reported for I(Ca(TTX)) (mean, -4.49 mV; range, -3.95 to -5.49 mV). Cs(+) current and I(Ca(TTX)) time courses were superimposable after accounting for the voltage shift. Inactivation time constants as functions of potential for the Cs(+) current and I(Ca(TTX)) also superimposed after voltage shifting, as did the inactivation curves. Neither of the proposed conditions for conversion of I(Na) into I(Ca(TTX)) channels is required to demonstrate I(Ca(TTX)). Moreover, we find that cardiac Na(+) (H1) channels expressed heterologously in HEK 293 cells are not converted to I(Ca(TTX)) channels by Na(+)-free, Ca(2+)-containing bathing media. The gating properties of the Na(+) current through H1 and those of Ca(2+) current through H1 are identical. All observations are consistent with two non-interconvertable Na(+) channel populations: a larger that expresses little Ca(2+) permeability and a smaller that is appreciably Ca(2+)-permeable.

Gating charge immobilization caused by the transition between inactivated states in the Kv1.5 channel

Sustained Na(+) or Li(+) conductance is a feature of the inactivated state in wild-type (WT) and nonconducting Shaker and Kv1.5 channels, and has been used here to investigate the cause of off-gating charge immobilization in WT and Kv1.5-W472F nonconducting mutant channels. Off-gating immobilization in response to brief pulses in cells perfused with NMG/NMG is the result of a more negative voltage dependence of charge recovery (V(1/2) is -96 mV) compared with on-gating charge movement (V(1/2) is -6.3 mV). This shift is known to be associated with slow inactivation in Shaker channels and the disparity is reduced by 40 mV, or approximately 50% in the presence of 135 mM Cs. Off-gating charge immobilization is voltage-dependent with a V(1/2) of -12 mV, and correlates well with the development of Na(+) conductance on repolarization through C-type inactivated channels (V(1/2) is -11 mV). As well, the time-dependent development of the inward Na(+) tail current and gating charge immobilization after depolarizing pulses of different durations has the same time constant (tau = 2.7 ms). These results indicate that in Kv1.5 channels the transition to a stable C-type inactivated state takes only 2-3 ms and results in strong charge immobilization in the absence of Group IA metal cations, or even in the presence of Na. Inclusion of low concentrations of Cs delays the appearance of Na(+) tail currents in WT channels, prevents transition to inactivated states in Kv1.5-W472F nonconducting mutant channels, and removes charge immobilization. Higher concentrations of Cs are able to modulate the deactivating transition in Kv1.5 channels and prevent the residual slowing of charge return.

Biophysical effects of pore mutations of ROMK1

Potassium channels are ubiquitous, being present in all living organisms. These proteins share common structural elements, which confer common functional features. In general, all K+ channels have a high selectivity for K+, and are blocked by cations of similar dimensions, such as Cs+ and Ba2+. Mutations in the pore region tend to lead to either the total loss of function or K+ selectivity. We have made mutations to one of the most highly conserved residues of the pore, glycine-143, of the inward rectifier ROMK1 (Kir1.1), and examined the resulting channel properties in the Xenopus oocyte expression system with a two-electrode voltage clamp. Mutations G143A and G143R resulted in failure to express functional channels. Co-injection of wild-type ROMK1 cRNA with these mutants led to rescue of channel function, which was different from wild-type ROMK1. In both mutants, the sensitivity to Ba2+ and Cs+ was increased, the rate of onset of block by Ba2+ was enhanced, and the selectivity to potassium was reduced. Whereas the crystallographic evidence shows that cations bind to the carbonyl backbone of the pore-lining residues, the present results indicate that the side chains of these amino acids, which face away from the pore lining, also affect permeation.

Cs(+) induces the kdp operon of Escherichia coli by lowering the intracellular K(+) concentration

Cs(+) was found to induce expression of the kdpFABC operon, encoding a high-affinity K(+) uptake system of Escherichia coli. Quantitative expression analyses at the transcriptional and translational levels reveal that CsCl causes much higher induction of kdpFABC than does NaCl. A decrease of the intracellular K(+) concentration is found in cells exposed to CsCl. The results indicate that kdpFABC expression is induced when the intracellular K(+) concentration is lowered. Moreover, the results imply that the signal transduction cascade mediated by KdpD and KdpE is able to integrate multiple signals.

Inwardly rectifying K(+) channels in spermatogenic cells: functional expression and implication in sperm capacitation

To fertilize, mammalian sperm must complete a maturational process called capacitation. It is thought that the membrane potential of sperm hyperpolarizes during capacitation, possibly due to the opening of K(+) channels, but electrophysiological evidence is lacking. In this report, using patch-clamp recordings obtained from isolated mouse spermatogenic cells we document the presence of a novel K(+)-selective inwardly rectifying current. Macroscopic current activated at membrane potentials below the equilibrium potential for K(+) and its magnitude was dependent on the external K(+) concentration. The channels selected K(+) over other monovalent cations. Current was virtually absent when external K(+) was replaced with Na(+) or N-methyl-D-glucamine. Addition of Cs(+) or Ba(2+) (IC(50) of approximately 15 microM) to the external solution effectively blocked K(+) current. Dialyzing the cells with a Mg(2+)-free solution did not affect channel activity. Cytosolic acidification reversibly inhibited the current. We verified that the resting membrane potential of mouse sperm changed from -52 +/- 6 to -66 +/- 9 mV during capacitation in vitro. Notably, application of 0.3-1 mM Ba(2+) during capacitation prevented this hyperpolarization and decreased the subsequent exocytotic response to zona pellucida. A mechanism is proposed whereby opening of inwardly rectifying K(+) channels may produce hyperpolarization under physiological conditions and contribute to the cellular changes that give rise to the capacitated state in mature sperm.

Influx and accumulation of Cs(+) by the akt1 mutant of Arabidopsis thaliana (L.) Heynh. lacking a dominant K(+) transport system

An extensive literature reports that Cs(+), an environmental contaminant, enters plant cells through K(+) transport systems. Several recently identified plant K(+) transport systems are permeable to Cs(+). Permeation models indicate that most Cs(+) uptake into plant roots under typical soil ionic conditions will be mediated by voltage-insensitive cation (VIC) channels in the plasma membrane and not by the inward rectifying K(+) (KIR) channels implicated in plant K nutrition. Cation fluxes through KIR channels are blocked by Cs(+). This paper tests directly the hypothesis that the dominant KIR channel in plant roots (AKT1) does not contribute significantly to Cs(+) uptake by comparing Cs(+) uptake into wild-type and the akt1 knockout mutant of Arabidopsis thaliana (L.) Heynh. Wild-type and akt1 plants were grown to comparable size and K(+) content on agar containing 10 mM K(+). Both Cs(+) influx to roots of intact plants and Cs(+) accumulation in roots and shoots were identical in wild-type and akt1 plants. These data indicate that AKT1 is unlikely to contribute significantly to Cs(+) uptake by wild-type Arabidopsis from 'single-salt' solutions. The influx of Cs(+) to roots of intact wild-type and akt1 plants was inhibited by 1 mM Ba(2+), Ca(2+) and La(3+), but not by 10 microM Br-cAMP. This pharmacology resembles that of VIC channels and is consistent with the hypothesis that VIC channels mediate most Cs(+) influx under 'single-salt' conditions.

The properties of carbachol-activated nonselective cation channels at the single channel level in guinea pig gastric myocytes

We investigated the properties of carbachol (CCh)-activated nonselective cation channels (NSC(CCh)) at the single channel level in the gastric myocytes of guinea pigs using a magnified whole-cell mode or an outside-out mode. The channel activity (NPo) recorded in a magnified whole-cell mode increased with depolarization (from -120 to -20 mV) and had the half activation potential of -81 mV under the symmetrical 140 mM Cs+ condition. The single channel conductance depended upon the extracellular monovalent cations with the order of Cs+ (35 pS) > Na+ (25 pS) > Li+ (21 pS). The channel activities markedly diminished or disappeared when external Cs+ was replaced with Na+ or N-methyl-D-glucamate (NMDG+). With Cs+ and Na+ as external cations, the channel showed a monotonic increase in NPo with the increased mole fraction of Cs+ over Na+, and it had an intermediate conductance value in solution containing 67% Cs+ with 33% Na+. These data suggested that the extracellular monovalent cations regulate the whole-cell current of NSC(CCh) by modulating both the open state probability and the unitary conductance, and there is one binding site for the extracellular cations within the pore.

Potassium uptake with low affinity and high rate in Enterococcus hirae at alkaline pH

Two high-affinity K+ uptake systems, KtrI and KtrII, have been reported in Enterococcus hirae. A mutant, JEMK1, defective in these two systems did not grow at pH 10 in low-K+ medium (less than 1 mM K+), but grew well when supplemented with 10 mM KCl. In this mutant, we found an energy-dependent K+ uptake at pH 10 with a low affinity for K+ (Km of approximately 20 mM) and an extremely high rate [Vmax of 1.6 micromol x min(-1) (mg protein)(-1)]. Rb+ uptake [Km of approximately 40 mM and Vmax of 0.5 micromol x min(-1) (mg protein)(-1)], which was inhibited competitively by K+ and less prominently by Cs+, was also observed. The specificity of this transport is likely to be K+>Rb+>Cs+. This peculiar K+ transport plays a role as a salvage mechanism against defects in high-affinity systems in the K+ homeostasis of this bacterium.

Cytoskeletal interactions determine the electrophysiological properties of human EAG potassium channels

The electrophysiological properties of ether a go-go (EAG) potassium channels are modified during the cell cycle when they are expressed in heterologous systems. In Chinese hamster ovary (CHO) mammalian somatic cells we found that the cell-cycle-dependent modulation of human EAG (hEAG) channels occurs during the M phase. This modulation has three components: reduction in current density, increased sensitivity to block by intracellular sodium, and increased selectivity for potassium ions. In this work, these three properties have been used to define the mitotic phenotype of EAG currents. The signaling pathway leading to such changes of channel properties is unknown. We tested the hypothesis that cytoskeletal interactions might affect the electrophysiological changes observed during the cell cycle. The disruption of actin filaments induces a significant increase in current density, without inducing the cell-cycle-related phenotype. In contrast, disturbance of the microtubules, achieved by pharmacological means or by mechanical excision of the membrane patch, does induce the cell-cycle-related phenotype. Our results demonstrate that hEAG channels establish complex interactions with cytoskeletal elements, and that these interactions strongly influence the properties of the channels. We also conclude that the electrophysiological changes observed during the cell cycle are most likely due to reorganization of the cytoskeleton during the G2/M transition.

Fast removal of synaptic glutamate by postsynaptic transporters

Glutamate transporters are believed to remove glutamate from the synaptic cleft only slowly because they cycle slowly. However, we show that when glutamate binds to postsynaptic transporters at the cerebellar climbing fiber synapse, it evokes a conformation change and inward current that reflect glutamate removal from the synaptic cleft within a few milliseconds, a time scale much faster than the overall cycle time. Contrary to present models, glutamate removal does not require binding of an extracellular proton, and the time course of transporter anion conductance activation differs from that of glutamate removal. The charge movement associated with glutamate removal is consistent with the majority of synaptically released glutamate being removed from the synaptic cleft by postsynaptic transporters.

A Kv3-like persistent, outwardly rectifying, Cs+-permeable, K+ current in rat subthalamic nucleus neurones

A persistent outward K+ current (IPO), activated by depolarization from resting potential, has been identified and characterized in rat subthalamic nucleus (SThN) neurones using whole-cell voltage-clamp recording in brain slices. IPO both rapidly activated (tau = 8 ms at +5 mV) and deactivated (tau = 2 ms at -68 mV), while showing little inactivation. Tail current reversal potentials varied with extracellular K+ concentration in a Nernstian manner. Intracellular Cs+ did not alter either IPO amplitude or the voltage dependence of activation, but blocked transient (A-like) outward currents activated by depolarization. When extracellular K+ was replaced with Cs+, IPO tail current reversal potentials were dependent upon the extracellular Cs+ concentration, indicating an ability to conduct Cs+, as well as K+. IPO was blocked by Ba2+ (1 mM), 4-aminopyridine (1 mM) and tetraethylammonium (TEA; 20 mM), with an IC50 for TEA of 0.39 mM. The IPO conductance appeared maximal (38 nS) at around +27 mV, half-maximal at -13 mV, with the threshold for activation at around -38 mV. TEA (1 mM) blocked the action potential after-hyperpolarization and permitted accommodation of action potential firing at frequencies greater than around 200 Hz. We conclude that IPO, which shares many characteristics of currents attributable to Kv3.1 K+ channels, enables high-frequency spike trains in SThN neurones.

Regulation of the conductance and resting potential by extracellular K+ in frog taste receptor cells

The effect of extracellular K+ on membrane currents was investigated by the patch clamp and fast perfusion techniques in frog (Rana temporaria) taste receptor cells (TRCs). When added to the bath, K+ increased the TRC conductance. The integral current and current fluctuations depended on the K+ concentration (2.5-90 mM) in the manner which suggested extracellular K+ to serve as a ligand activating ionic channels (potassium-activated (PA) channels). The influence of different ions on the PA current reversal potential indicated that the responsible channels are mainly permeable to K+ and H+. Relative permeabilities were estimated as P(H):P(K) = 3600:1. With 110 mM KCl in the patch pipette and 110 mM NaCl in the bath, isolated TRCs exhibited the resting potentials from -75 to -65 mV. When raised from 2.5 to 110 mM, extracellular K+ intensively depolarized TRCs. Membrane potential vs. K+ concentration displayed a slope of about 41 mV per logarithmic unit. This indicates that the K+ permeability of the TRC membrane dominates the other in setting the potential. With 10 mM K+ in the bath, the PA channels were the major contributor to setting the TRC resting potential. External K+ markedly increased the sensitivity of isolated TRCs to bath solution pH due to the activation of the PA channels suggesting their role in sour transduction.

Rubidium and cesium fluxes in muscle as related to the membrane potential

The reduction of membrane potential in frog sartorius muscle produced by rubidium and cesium ions has been studied over a wide concentration range and compared with depolarization occasioned by potassium ions. The constant field theory of passive flux has been used to predict the potential changes observed. The potential data suggest certain permeability coefficient ratios and these are compared with ratios obtained from flux data using radioactive tracers. The agreement of the flux with the potential data is good if account is taken of the inhibition of potassium flux which occurs in the presence of rubidium and cesium ions. A high temperature dependence has been observed for cesium influx (Q(10) = 2.5) which is correlated with the observation that cesium ions depolarize very little at low temperatures. The observations suggest that cesium ions behave more like sodium ions at low temperatures and more like potassium ions at room temperature with respect to their effect on the muscle cell resting potential. The constant field theory of passive ion flux appears to be in general agreement with the experimental results observed if account is taken of the dependence of permeability coefficients on the concentrations of ions used and of possible interactions between the permeabilities of ions.

The apical sensory organ of a gastropod veliger is a receptor for settlement cues

On the basis of anatomy and larval behavior, the apical sensory organ (ASO) of gastropod veliger larvae has been implicated as the site of perception of cues for settlement and metamorphosis. Until now, there have been no experimental data to support this hypothesis. In this study, cells in the ASO of veliger larvae of the tropical nudibranch Phestilla sibogae were stained with the styryl vital dye DASPEI and then irradiated with a narrow excitatory light beam on a fluorescence microscope. When its ASO cells were bleached by irradiation for 20 min or longer, an otherwise healthy larva was no longer able to respond to the usual metamorphic cue, a soluble metabolite from a coral prey of the adult nudibranch. The irradiated cells absorbed the dye acridine orange, suggesting that they were dying. When larvae not stained with DASPEI were similarly irradiated, or when stained larvae were irradiated with the light beam focused on other parts of the body, there was no loss of ability to metamorphose. Together these data provide strong support for the hypothesis. Potassium and cesium ions, known to induce metamorphosis in larvae of many marine-invertebrate phyla, continue to induce metamorphosis in larvae that have lost the ability to respond to the coral inducer due to staining and irradiation. These results demonstrate that (1) the ASO-ablated larvae have not lost the ability to metamorphose and (2) the ions do not act only on the metamorphic-signal receptor cells, but at other sites downstream in the metamorphic signal transduction pathway.

Permeant ion regulation of N-methyl-D-aspartate receptor channel block by Mg(2+)

Block of the channel of N-methyl-D-aspartate (NMDA) receptors by external Mg(2+) (Mg(o)(2+)) has broad implications for the many physiological and pathological processes that depend on NMDA receptor activation. An essential property of channel block by Mg(o)(2+) is its powerful voltage dependence. A widely cited explanation for the strength of the voltage dependence of block is that the Mg(o)(2+)-binding site is located deep in the channel of NMDA receptors; Mg(o)(2+) then would sense most of the membrane potential field during block. However, recent electrophysiological and mutagenesis studies suggest that the blocking site cannot be deep enough to account for the voltage dependence of Mg(o)(2+) block. Here we describe the basis for this discrepancy: the magnitude and voltage dependence of channel block by Mg(o)(2+) are strongly regulated by external and internal permeant monovalent cations. Our data support a model in which access to the channel by Mg(o)(2+) is prevented when permeant ion-binding sites at the external entrance to the channel are occupied. Mg(o)(2+) can block the channel only when the permeant ion-binding sites are unoccupied and then can either unblock back to the external solution or permeate the channel. Unblock to the external solution is prevented if external permeant ions bind while Mg(2+) blocks the channel, although permeation is still permitted. The model provides an explanation for the strength of the voltage dependence of Mg(o)(2+) block and quantifies the interdependence of permanent and blocking ion binding to NMDA receptors.

Ion permeation and block of P2X(2) purinoceptors: single channel recordings

P2X(2) purinoceptors are cation-selective channels activated by ATP and its analogues. Using single channel measurements we studied the channel's selectivity for the alkali metal ions and organic monovalent cations NMDG(+), Tris(+), TMA(+), and TEA(+). The selectivity sequence for currents carried by alkali metal ions is: K(+) > Rb(+) > Cs(+) > Na(+) > Li(+), which is Eisenman sequence IV. This is different from the mobility sequence of the ions in free solution suggesting there is weak interaction between the ions and the channel interior. The relative conductance for alkali ions increases linearly in relation to the Stokes radius. The organic ions NMDG(+), Tris(+), TMA(+) and TEA(+) were virtually impermeant. The divalent ions (Mn(2+), Mg(2+), Ca(2+) and Ba(2+)) induced a fast block visible as a reduction in amplitude of the unitary currents. Using a single-site binding model, the divalent ions exhibited an equilibrium affinity sequence of Mn(2+) > Mg(2+) > Ca(2+) > Ba(2+).

Anion channels in chara corallina tonoplast membrane: calcium dependence and rectification

Tonoplast K(+) channels of Chara corallina are well characterized but only a few reports mention anion channels, which are likely to play an important role in the tonoplast action potential and osmoregulation of this plant. For experiments internodal cells were isolated. Cytoplasmic droplets were formed in an iso-osmotic bath solution according to a modified procedure. Ion channels with conductances of 48 pS and 170 pS were detected by the patch-clamp technique. In the absence of K(+) in the bath solution the 170 pS channel was not observed at negative pipette potential values. When Cl(-) on either the vacuolar side or the cytoplasmic side was partly replaced with F(-), the reversal potential of the 48 pS channel shifted conform to the Cl(-) equilibrium potential with similar behavior in droplet-attached and excised patch mode. These results showed that the 48 pS channel was a Cl(-) channel. In droplet-attached mode the channel rectified outward current flow, and the slope conductance was smaller. When Chara droplets were formed in a bath solution containing low (10(-8) m) Ca(2+), then no Cl(-) channels could be detected either in droplet-attached or in inside-out patch mode. Channel activity was restored if Ca(2+) was applied to the cytoplasmic side of inside-out patches. Rectification properties in the inside-out patch configuration could be controlled by the holding pipette potential. Holding potential values negative or positive to the calculated reversal potential for Cl(-) ions induced opposite rectification properties. Our results show Ca(2+)-activated Cl(-) channels in the tonoplast of Chara with holding potential dependent rectification.

A DNA pentaplex incorporating nucleobase quintets

Supramolecular self-assembly is an integral step in the formation of many biological structures. Here we report a DNA pentaplex that derives from a metal-assisted, hydrogen bond-mediated self-assembly process. In particular, cesium ions are found to induce pentameric assembly of DNA bearing the nonstandard nucleobase iso-guanine. The pentaplex was designed by using a simple algorithm to predict nucleobase structural requirements within a quintet motif. The design principles are general and should extend to complexes beyond pentaplex. Structures exhibiting molecularities of five or more were previously accessible to peptides, but not nucleic acids.

Molecular identification of a eukaryotic, stretch-activated nonselective cation channel

Calcium-permeable, stretch-activated nonselective cation (SA Cat) channels mediate cellular responses to mechanical stimuli. However, genes encoding such channels have not been identified in eukaryotes. The yeast MID1 gene product (Mid1) is required for calcium influx in the yeast Saccharomyces cerevisiae. Functional expression of Mid1 in Chinese hamster ovary cells conferred sensitivity to mechanical stress that resulted in increases in both calcium conductance and the concentration of cytosolic free calcium. These increases were dependent on the presence of extracellular calcium and were reduced by gadolinium, a blocker of SA Cat channels. Single-channel analyses with cell-attached patches revealed that Mid1 acts as a calcium-permeable, cation-selective stretch-activated channel with a conductance of 32 picosiemens at 150 millimolar cesium chloride in the pipette. Thus, Mid1 appears to be a eukaryotic, SA Cat channel.