An ion's view of the potassium channel. The structure of the permeation pathway as sensed by a variety of blocking ions

Mutation in pore-helix modulates interplay between filter gate and Ba2+ block in a Kcv channel pore

The selectivity filter of K+ channels catalyzes a rapid and highly selective transport of K+ while serving as a gate. To understand the control of this filter gate, we use the pore-only K+ channel KcvNTS in which gating is exclusively determined by the activity of the filter gate. It has been previously shown that a mutation at the C-terminus of the pore-helix (S42T) increases K+ permeability and introduces distinct voltage-dependent and K+-sensitive channel closures at depolarizing voltages. Here, we report that the latter are not generated by intrinsic conformational changes of the filter gate but by a voltage-dependent block caused by nanomolar trace contaminations of Ba2+ in the KCl solution. Channel closures can be alleviated by extreme positive voltages and they can be completely abolished by the high-affinity Ba2+ chelator 18C6TA. By contrast, the same channel closures can be augmented by adding Ba2+ at submicromolar concentrations to the cytosolic buffer. These data suggest that a conservative exchange of Ser for Thr in a crucial position of the filter gate increases the affinity of the filter for Ba2+ by >200-fold at positive voltages. While Ba2+ ions apparently remain only for a short time in the filter-binding sites of the WT channel before passing the pore, they remain much longer in the mutant channel. Our findings suggest that the dwell times of permeating and blocking ions in the filter-binding sites are tightly controlled by interactions between the pore-helix and the selectivity filter.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Lithium administered to pregnant, lactating and neonatal rats: entry into developing brain

Background

Little is known about the extent of drug entry into developing brain, when administered to pregnant and lactating women. Lithium is commonly prescribed for bipolar disorder. Here we studied transfer of lithium given to dams, into blood, brain and cerebrospinal fluid (CSF) in embryonic and postnatal animals as well as adults.

Methods

Lithium chloride in a clinically relevant dose (3.2 mg/kg body weight) was injected intraperitoneally into pregnant (E15–18) and lactating dams (birth-P16/17) or directly into postnatal pups (P0–P16/17). Acute treatment involved a single injection; long-term treatment involved twice daily injections for the duration of the experiment. Following terminal anaesthesia blood plasma, CSF and brains were collected. Lithium levels and brain distribution were measured using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry and total lithium levels were confirmed by Inductively Coupled Plasma-Mass Spectrometry.

Results

Lithium was detected in blood, CSF and brain of all fetal and postnatal pups following lithium treatment of dams. Its concentration in pups’ blood was consistently below that in maternal blood (30–35%) indicating significant protection by the placenta and breast tissue. However, much of the lithium that reached the fetus entered its brain. Levels of lithium in plasma fluctuated in different treatment groups but its concentration in CSF was stable at all ages, in agreement with known stable levels of endogenous ions in CSF. There was no significant increase of lithium transfer into CSF following application of Na⁺/K⁺ ATPase inhibitor (digoxin) in vivo, indicating that lithium transfer across choroid plexus epithelium is not likely to be via the Na⁺/K⁺ ATPase mechanism, at least early in development. Comparison with passive permeability markers suggested that in acute experiments lithium permeability was less than expected for diffusion but similar in long-term experiments at P2.

Conclusions

Information obtained on the distribution of lithium in developing brain provides a basis for studying possible deleterious effects on brain development and behaviour in offspring of mothers undergoing lithium therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00285-w.

Exploring the Nature of Cationic Blocker Recognition by the Anthrax Toxin Channel

Obstructing conductive pathways of the channel-forming toxins with targeted blockers is a promising drug design approach. Nearly all tested positively charged ligands have been shown to reversibly block the cation-selective channel-forming protective antigen (PA₆₃) component of the binary anthrax toxin. The cationic ligands with more hydrophobic surfaces, particularly those carrying aromatic moieties, inhibited PA₆₃ more effectively. To understand the physical basis of PA₆₃ selectivity for a particular ligand, detailed information is required on how the blocker structural elements (e.g., positively charged and aromatic groups) influence the molecular kinetics of the blocker/channel binding reactions. In this study, we address this problem using the high-resolution single-channel planar lipid bilayer technique. Several structurally distinct cationic blockers, namely per-6-S-(3-amino) propyl-β-cyclodextrin, per-6-S-(3-aminomethyl) benzyl-α-cyclodextrin, per-6-S-(3-aminomethyl) benzyl-β-cyclodextrin, per-6-S-(3-aminomethyl) benzyl-γ-cyclodextrin, methyltriphenylphosphonium ion, and G0 polyamidoamine dendrimer are tested for their ability to inhibit the heptameric and octameric PA₆₃ variants and PA₆₃F427A mutant. The F427 residues form a hydrophobic constriction region inside the channel, known as the "ϕ-clamp." We show that the cationic blockers interact with PA₆₃ through a combination of forces. Analysis of the binding reaction kinetics suggests the involvement of cation-π, Coulomb, and salt-concentration-independent π-π or hydrophobic interactions in the cationic cyclodextrin binding. It is possible that these blockers bind to the ϕ-clamp and are also stabilized by the Coulomb interactions of their terminal amino groups with the water-exposed negatively charged channel residues. In PA₆₃F427A, only the suggested Coulomb component of the cyclodextrin interaction remains. Methyltriphenylphosphonium ion and G0 polyamidoamine dendrimer, despite being positively charged, interact primarily with the ϕ-clamp. We also show that seven- and eightfold symmetric cyclodextrins effectively block the heptameric and octameric forms of PA₆₃ interchangeably, adding flexibility to the earlier formulated blocker/target symmetry match requirement.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

Constitutive and Synaptic Activation of GIRK Channels Differentiates Mature and Newborn Dentate Granule Cells

Sparse neural activity in the dentate gyrus is enforced by powerful networks of inhibitory GABAergic interneurons in combination with low intrinsic excitability of the principal neurons, the dentate granule cells (GCs). Although the cellular and circuit properties that dictate synaptic inhibition are well studied, less is known about mechanisms that confer low GC intrinsic excitability. Here we demonstrate that intact G protein-mediated signaling contributes to the characteristic low resting membrane potential that differentiates mature dentate GCs from CA1 pyramidal cells and developing adult-born GCs. In mature GCs from male and female mice, intact G protein signaling robustly reduces intrinsic excitability, whereas deletion of G protein-activated inwardly rectifying potassium channel 2 (GIRK2) increases excitability and blocks the effects of G protein signaling on intrinsic properties. Similarly, pharmacological manipulation of GABA_B receptors (GABA_BRs) or GIRK channels alters intrinsic excitability and GC spiking behavior. However, adult-born new GCs lack functional GIRK activity, with phasic and constitutive GABA_BR-mediated GIRK signaling appearing after several weeks of maturation. Phasic activation is interneuron specific, arising primarily from nNOS-expressing interneurons rather than parvalbumin- or somatostatin-expressing interneurons. Together, these results demonstrate that G protein signaling contributes to the intrinsic excitability that differentiates mature and developing dentate GCs and further suggest that late maturation of GIRK channel activity is poised to convert early developmental functions of GABA_B receptor signaling into GABA_BR-mediated inhibition.SIGNIFICANCE STATEMENT The dentate gyrus exhibits sparse neural activity that is essential for the computational function of pattern separation. Sparse activity is ascribed to strong local inhibitory circuits in combination with low intrinsic excitability of the principal neurons, the granule cells. Here we show that constitutive activity of G protein-coupled inwardly rectifying potassium channels (GIRKs) underlies to the hallmark low resting membrane potential and input resistance of mature dentate neurons. Adult-born neurons initially lack functional GIRK channels, with constitutive and phasic GABA_B receptor-mediated GIRK inhibition developing in tandem after several weeks of maturation. Our results reveal that GABA_B/GIRK activity is an important determinant of low excitability of mature dentate granule cells that may contribute to sparse DG activity in vivo.

Modulation of the inwardly rectifying potassium channel Kir4.1 by the pro-invasive miR-5096 in glioblastoma cells

Inwardly rectifying potassium channels (Kir), and especially the barium-sensitive Kir4.1 encoded by KCNJ10, are key regulators of glial functions. A lower expression or mislocation of Kir4.1 is detected in human brain tumors. MicroRNAs participate in the regulation of ionic channels and associated neurologic disorders. Here, we analyze effects of miR-5096 on the Kir4.1 expression and function in two glioblastoma cell lines, U87 and U251. Using whole-cell patch-clamp and western-blot analysis, we show that cell loading with miR-5096 decreases the Kir4.1 protein level and associated K⁺ current. Cell treatment with barium, a Kir4.1 blocker, or cell loading of miR-5096 both increase the outgrowth of filopodia in glioma cells, as observed by time-lapse microscopy. Knocking-down Kir4.1 expression by siRNA transfection similarly increased both filopodia formation and invasiveness of glioma cells as observed in Boyden chamber assay. MiR-5096 also promotes the release of extracellular vesicles by which it increases its own transfer to surrounding cells, in a Kir4.1-dependent manner in U251 but not in U87. Altogether, our results validate Kir4.1 as a miR-5096 target to promote invasion of glioblastoma cells. Our data highlight the complexity of microRNA effects and the role of K⁺ channels in cancer.

Molecular and functional characterization of inwardly rectifying K+ currents in murine proximal colon

KEY POINTS

Interstitial cells of Cajal (ICC) from murine colonic muscles express genes encoding inwardly rectifying K+ channels. Transcripts of Kcnj2 (Kir2.1), Kcnj4 (Kir2.3), Kcnj14 (Kir2.4), Kcnj5 (Kir3.4), Kcnj8 (Kir 6.1) and Kcnj11 (Kir6.2) were found in colonic ICC. A conductance with properties consistent with Kir2 channels was observed in ICC but not in smooth muscle cells (SMC). Despite expression of gene transcripts, G-protein gated K+ channel (Kir3) and KATP (Kir6) currents were not resolved in ICC. KATP is a conductance prominent in SMC. Kir2 antagonist caused depolarization of freshly dispersed ICC and colonic smooth muscles, suggesting that this conductance is active under resting conditions in colonic muscles. The conclusion of the present study is that ICC express the Ba2+ -sensitive, inwardly rectifying K+ conductance in colonic muscles. This conductance is most probably a result of heterotetramers of Kir2 gene products, with this regulating resting potentials and the excitability of colonic muscles.

ABSTRACT

Membrane potentials of gastrointestinal muscles are important because voltage-dependent Ca2+ channels in smooth muscle cells (SMC) provide the Ca2+ that triggers contraction. Regulation of membrane potential is complicated because SMC are electrically coupled to interstitial cells of Cajal (ICC) and PDGFRα+ cells. Activation of conductances in any of these cells affects the excitability of the syncytium. We explored the role of inward rectifier K+ conductances in colonic ICC that might contribute to regulation of membrane potential. ICC expressed Kcnj2 (Kir2.1), Kcnj4 (Kir2.3), Kcnj14 (Kir2.4), Kcnj5 (Kir3.4), Kcnj8 (Kir 6.1) and Kcnj11 (Kir6.2). Voltage clamp experiments showed activation of inward current when extracellular K+ ([K+ ]o ) was increased. The current was inwardly rectifying and inhibited by Ba2+ (10 μm) and ML-133 (10 μm). A similar current was not available in SMC. The current activated in ICC by elevated [K+ ]o was not affected by Tertiapin-Q. Gβγ, when dialysed into cells, failed to activate a unique, Tertiapin-Q-sensitive conductance. Freshly dispersed ICC showed no evidence of functional KATP . Pinacidil failed to activate current and the inward current activated by elevated [K+ ]o was insensitive to glibenclamide. Under current clamp, ML-133 caused the depolarization of isolated ICC and also that of cells impaled with microelectrodes in intact muscle strips. These findings show that ICC, when isolated freshly from colonic muscles, expressed a Ba2+ -sensitive, inwardly rectifying K+ conductance. This conductance is most probably a result of the expression of multiple Kir2 family paralogues, and the inwardly rectifying conductance contributes to the regulation of resting potentials and excitability of colonic muscles.

Impact of Dendrimer Terminal Group Chemistry on Blockage of the Anthrax Toxin Channel: A Single Molecule Study

Nearly all the cationic molecules tested so far have been shown to reversibly block K⁺ current through the cation-selective PA₆₃ channels of anthrax toxin in a wide nM-mM range of effective concentrations. A significant increase in channel-blocking activity of the cationic compounds was achieved when multiple copies of positively charged ligands were covalently linked to multivalent scaffolds, such as cyclodextrins and dendrimers. Even though multivalent binding can be strong when the individual bonds are relatively weak, for drug discovery purposes we often strive to design multivalent compounds with high individual functional group affinity toward the respective binding site on a multivalent target. Keeping this requirement in mind, here we perform a single-channel/single-molecule study to investigate kinetic parameters of anthrax toxin PA₆₃ channel blockage by second-generation (G2) poly(amido amine) (PAMAM) dendrimers functionalized with different surface ligands, including G2-NH₂, G2-OH, G2-succinamate, and G2-COONa. We found that the previously reported difference in IC₅₀ values of the G2-OH/PA₆₃ and G2-NH₂/PA₆₃ binding was determined by both on- and off-rates of the reversible dendrimer/channel binding reaction. In 1 M KCl, we observed a decrease of about three folds in k o n and a decrease of only about ten times in t r e s with G2-OH compared to G2-NH₂. At the same time for both blockers, k o n and t r e s increased dramatically with transmembrane voltage increase. PAMAM dendrimers functionalized with negatively charged succinamate, but not carboxyl surface groups, still had some residual activity in inhibiting the anthrax toxin channels. At 100 mV, the on-rate of the G2-succinamate binding was comparable with that of G2-OH but showed weaker voltage dependence when compared to G2-OH and G2-NH₂. The residence time of G2-succinamate in the channel exhibited opposite voltage dependence compared to G2-OH and G2-NH₂, increasing with the cis-negative voltage increase. We also describe kinetics of the PA₆₃ ion current modulation by two different types of the "imperfect" PAMAM dendrimers, the mixed-surface G2 75% OH 25% NH₂ dendrimer and G3-NH₂ dendron. At low voltages, both "imperfect" dendrimers show similar rate constants but significantly weaker voltage sensitivity when compared with the intact G2-NH₂ PAMAM dendrimer.

Superficial Layer-Specific Histaminergic Modulation of Medial Entorhinal Cortex Required for Spatial Learning

The medial entorhinal cortex (MEC) plays a crucial role in spatial learning and memory. Whereas the MEC receives a dense histaminergic innervation from the tuberomamillary nucleus of the hypothalamus, the functions of histamine in this brain region remain unclear. Here, we show that histamine acts via H1Rs to directly depolarize the principal neurons in the superficial, but not deep, layers of the MEC when recording at somata. Moreover, histamine decreases the spontaneous GABA, but not glutamate, release onto principal neurons in the superficial layers by acting at presynaptic H3Rs without effect on synaptic release in the deep layers. Histamine-induced depolarization is mediated via inhibition of Kir channels and requires the activation of protein kinase C, whereas the inhibition of spontaneous GABA release by histamine depends on voltage-gated Ca(2+) channels and extracellular Ca(2+). Furthermore, microinjection of the H1R or H3R, but not H2R, antagonist respectively into the superficial, but not deep, layers of MEC impairs rat spatial learning as assessed by water maze tasks but does not affect the motor function and exploratory activity in an open field. Together, our study indicates that histamine plays an essential role in spatial learning by selectively regulating neuronal excitability and synaptic transmission in the superficial layers of the MEC.

Inward rectifier potassium (Kir) current in dopaminergic periglomerular neurons of the mouse olfactory bulb

Dopaminergic (DA) periglomerular (PG) neurons are critically placed at the entry of the bulbar circuitry, directly in contact with both the terminals of olfactory sensory neurons and the apical dendrites of projection neurons; they are autorhythmic and are the target of numerous terminals releasing a variety of neurotransmitters. Despite the centrality of their position, suggesting a critical role in the sensory processing, their properties -and consequently their function- remain elusive. The current mediated by inward rectifier potassium (Kir) channels in DA-PG cells was recorded by adopting the perforated-patch configuration in thin slices; IKir could be distinguished from the hyperpolarization-activated current (I h ) by showing full activation in <10 ms, no inactivation, suppression by Ba(2+) in a typical voltage-dependent manner (IC50 208 μM) and reversal potential nearly coincident with EK. Ba(2+) (2 mM) induces a large depolarization of DA-PG cells, paralleled by an increase of the input resistance, leading to a block of the spontaneous activity, but the Kir current is not an essential component of the pacemaker machinery. The Kir current is negatively modulated by intracellular cAMP, as shown by a decrease of its amplitude induced by forskolin or 8Br-cAMP. We have also tested the neuromodulatory effects of the activation of several metabotropic receptors known to be present on these cells, showing that the current can be modulated by a multiplicity of pathways, whose activation in some case increases the amplitude of the current, as can be observed with agonists of D2, muscarinic, and GABAA receptors, whereas in other cases has the opposite effect, as it can be observed with agonists of α1 noradrenergic, 5-HT and histamine receptors. These characteristics of the Kir currents provide the basis for an unexpected plasticity of DA-PG cell function, making them potentially capable to reconfigure the bulbar network to allow a better flexibility.

Lack of negative slope in I-V plots for BK channels at positive potentials in the absence of intracellular blockers

Large-conductance, voltage- and Ca(2+)-activated K(+) (BK) channels display near linear current-voltage (I-V) plots for voltages between -100 and +100 mV, with an increasing sublinearity for more positive potentials. As is the case for many types of channels, BK channels are blocked at positive potentials by intracellular Ca(2+) and Mg(2+). This fast block progressively reduces single-channel conductance with increasing voltage, giving rise to a negative slope in the I-V plots beyond about +120 mV, depending on the concentration of the blockers. In contrast to these observations of pronounced differences in the magnitudes and shapes of I-V plots in the absence and presence of intracellular blockers, Schroeder and Hansen (2007. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.200709802) have reported identical I-V plots in the absence and presence of blockers for BK channels, with both plots having reduced conductance and negative slopes, as expected for blockers. Schroeder and Hansen included both Ca(2+) and Mg(2+) in the intracellular solution rather than a single blocker, and they also studied BK channels expressed from α plus β1 subunits, whereas most previous studies used only α subunits. Although it seems unlikely that these experimental differences would account for the differences in findings between previous studies and those of Schroeder and Hansen, we repeated the experiments using BK channels comprised of α plus β1 subunits with joint application of 2.5 mM Ca(2+) plus 2.5 mM Mg(2+), as Schroeder and Hansen did. In contrast to the findings of Schroeder and Hansen of identical I-V plots, we found marked differences in the single-channel I-V plots in the absence and presence of blockers. Consistent with previous studies, we found near linear I-V plots in the absence of blockers and greatly reduced currents and negative slopes in the presence of blockers. Hence, studies of conductance mechanisms for BK channels should exclude intracellular Ca(2+)/Mg(2+), as they can reduce conductance and induce negative slopes.

Interactions of high-affinity cationic blockers with the translocation pores of B. anthracis, C. botulinum, and C. perfringens binary toxins

Cationic β-cyclodextrin derivatives were recently introduced as highly effective, potentially universal blockers of three binary bacterial toxins: anthrax toxin of Bacillus anthracis, C2 toxin of Clostridium botulinum, and iota toxin of Clostridium perfringens. The binary toxins are made of two separate components: the enzymatic A component, which acts on certain intracellular targets, and the binding/translocation B component, which forms oligomeric channels in the target cell membrane. Here we studied the voltage and salt dependence of the rate constants of binding and dissociation reactions of two structurally different β-cyclodextrins (AmPrβCD and AMBnTβCD) in the PA(63), C2IIa, and Ib channels (B components of anthrax, C2, and iota toxins, respectively). With all three channels, the blocker carrying extra hydrophobic aromatic groups on the thio-alkyl linkers of positively charged amino groups, AMBnTβCD, demonstrated significantly stronger binding compared with AmPrβCD. This effect is seen as an increased residence time of the blocker in the channels, whereas the time between blockages characterizing the binding reaction on-rate stays practically unchanged. Surprisingly, the voltage sensitivity, expressed as a slope of the logarithm of the blocker residence time as a function of voltage, turned out to be practically the same for all six cases studied, suggesting structural similarities among the three channels. Also, the more-effective AMBnTβCD blocker shows weaker salt dependence of the binding and dissociation rate constants compared with AmPrβCD. By estimating the relative contributions of the applied transmembrane field, long-range Coulomb, and salt-concentration-independent, short-range forces, we found that the latter represent the leading interaction, which accounts for the high efficiency of blockage. In a search for the putative groups in the channel lumen that are responsible for the short-range forces, we performed measurements with the F427A mutant of PA(63), which lacks the functionally important phenylalanine clamp. We found that the on-rates of the blockage were virtually conserved, but the residence times and, correspondingly, the binding constants dropped by more than an order of magnitude, which also reduced the difference between the efficiencies of the two blockers.

Hyaluronan export through plasma membranes depends on concurrent K+ efflux by K(ir) channels

Hyaluronan is synthesized within the cytoplasm and exported into the extracellular matrix through the cell membrane of fibroblasts by the MRP5 transporter. In order to meet the law of electroneutrality, a cation is required to neutralize the emerging negative hyaluronan charges. As we previously observed an inhibiting of hyaluronan export by inhibitors of K⁺ channels, hyaluronan export was now analysed by simultaneously measuring membrane potential in the presence of drugs. This was done by both hyaluronan import into inside-out vesicles and by inhibition with antisense siRNA. Hyaluronan export from fibroblast was particularly inhibited by glibenclamide, ropivacain and BaCl₂ which all belong to ATP-sensitive inwardly-rectifying K_ir channel inhibitors. Import of hyaluronan into vesicles was activated by 150 mM KCl and this activation was abolished by ATP. siRNA for the K⁺ channels K_ir3.4 and K_ir6.2 inhibited hyaluronan export. Collectively, these results indicated that hyaluronan export depends on concurrent K⁺ efflux.

Minimal art: or why small viral K(+) channels are good tools for understanding basic structure and function relations

Some algal viruses contain genes that encode proteins with the hallmarks of K(+) channels. One feature of these proteins is that they are less than 100 amino acids in size, which make them truly minimal for a K(+) channel protein. That is, they consist of only the pore module present in more complex K(+) channels. The combination of miniature size and the functional robustness of the viral K(+) channels make them ideal model systems for studying how K(+) channels work. Here we summarize recent structure/function correlates from these channels, which provide insight into functional properties such as gating, pharmacology and sorting in cells.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Properties of the inner pore region of TRPV1 channels revealed by block with quaternary ammoniums

The transient receptor potential vanilloid 1 (TRPV1) nonselective cationic channel is a polymodal receptor that activates in response to a wide variety of stimuli. To date, little structural information about this channel is available. Here, we used quaternary ammonium ions (QAs) of different sizes in an effort to gain some insight into the nature and dimensions of the pore of TRPV1. We found that all four QAs used, tetraethylammonium (TEA), tetrapropylammonium (TPrA), tetrabutylammonium, and tetrapentylammonium, block the TRPV1 channel from the intracellular face of the channel in a voltage-dependent manner, and that block by these molecules occurs with different kinetics, with the bigger molecules becoming slower blockers. We also found that TPrA and the larger QAs can only block the channel in the open state, and that they interfere with the channel's activation gate upon closing, which is observed as a slowing of tail current kinetics. TEA does not interfere with the activation gate, indicating that this molecule can reside in its blocking site even when the channel is closed. The dependence of the rate constants on the size of the blocker suggests a size of around 10 A for the inner pore of TRPV1 channels.

Amphiphilic blockers punch through a mutant CLC-0 pore

Intracellularly applied amphiphilic molecules, such as p-chlorophenoxy acetate (CPA) and octanoate, block various pore-open mutants of CLC-0. The voltage-dependent block of a particular pore-open mutant, E166G, was found to be multiphasic. In symmetrical 140 mM Cl(-), the apparent affinity of the blocker in this mutant increased with a negative membrane potential but, paradoxically, decreased when the negative membrane potential was greater than -80 mV, a phenomenon similar to the blocker "punch-through" shown in many blocker studies of cation channels. To provide further evidence of the punch-through of CPA and octanoate, we studied the dissociation rate of the blocker from the pore by measuring the time constant of relief from the block under various voltage and ionic conditions. Consistent with the voltage dependence of the effect on the steady-state current, the rate of CPA dissociation from the E166G pore reached a minimum at -80 mV in symmetrical 140 mM Cl(-), and the direction of current recovery suggested that the bound CPA in the pore can dissociate into both intracellular and extracellular solutions. Moreover, the CPA dissociation depends upon the Cl(-) reversal potential with a minimal dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential. That the shift of the CPA-dissociation rate follows the Cl(-) gradient across the membrane argues that these blockers can indeed punch through the channel pore. Furthermore, a minimal CPA-dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential suggests that the outward blocker movement through the CLC-0 pore is more difficult than the inward movement.

Trans-channel interactions in batrachotoxin-modified skeletal muscle sodium channels: voltage-dependent block by cytoplasmic amines, and the influence of mu-conotoxin GIIIA derivatives and permeant ions

External mu-conotoxins and internal amine blockers inhibit each other's block of voltage-gated sodium channels. We explore the basis of this interaction by measuring the shifts in voltage-dependence of channel inhibition by internal amines induced by two mu-conotoxin derivatives with different charge distributions and net charges. Charge changes on the toxin were made at residue 13, which is thought to penetrate most deeply into the channel, making it likely to have the strongest individual interaction with an internal charged ligand. When an R13Q or R13E molecule was bound to the channel, the voltage dependence of diethylammonium (DEA)-block shifted toward more depolarized potentials (23 mV for R13Q, and 16 mV for R13E). An electrostatic model of the repulsion between DEA and the toxin simulated these data, with a distance between residue 13 of the mu-conotoxin and the DEA-binding site of approximately 15 A. Surprisingly, for tetrapropylammonium, the shifts were only 9 mV for R13Q, and 7 mV for R13E. The smaller shifts associated with R13E, the toxin with a smaller net charge, are generally consistent with an electrostatic interaction. However, the smaller shifts observed for tetrapropylammonium than for DEA suggest that other factors must be involved. Two observations indicate that the coupling of permeant ion occupancy of the channel to blocker binding may contribute to the overall amine-toxin interaction: 1), R13Q binding decreases the apparent affinity of sodium for the conducting pore by approximately 4-fold; and 2), increasing external [Na(+)] decreases block by DEA at constant voltage. Thus, even though a number of studies suggest that sodium channels are occupied by no more than one ion most of the time, measurable coupling occurs between permeant ions and toxin or amine blockers. Such interactions likely determine, in part, the strength of trans-channel, amine-conotoxin interactions.

Trans-channel interactions in batrachotoxin-modified rat skeletal muscle sodium channels: kinetic analysis of mutual inhibition between mu-conotoxin GIIIA derivatives and amine blockers

R13X derivatives of mu-conotoxin GIIIA bind externally to single sodium channels and block current incompletely with mean "blocked" durations of several seconds. We studied interactions between two classes of blockers (mu-conotoxins and amines) by steady state, kinetic analysis of block of BTX-modified Na channels in planar bilayers. The amines cause all-or-none block at a site internal to the selectivity filter. TPrA and DEA block single Na channels with very different kinetics. TPrA induces discrete, all-or-none, blocked events (mean blocked durations, approximately 100 ms), whereas DEA produces a concentration-dependent reduction of the apparent single channel amplitude ("fast" block). These distinct modes of action allow simultaneous evaluation of block by TPrA and DEA, showing a classical, competitive interaction between them. The apparent affinity of TPrA decreases with increasing [DEA], based on a decrease in the association rate for TPrA. When an R13X mu-conotoxin derivative and one of the amines are applied simultaneously on opposite sides of the membrane, a mutually inhibitory interaction is observed. Dissociation constants, at +50 mV, for TPrA ( approximately 4 mM) and DEA ( approximately 30 mM) increase by approximately 20%-50% when R13E (nominal net charge, +4) or R13Q (+5) is bound. Analysis of the slow blocking kinetics for the two toxin derivatives showed comparable decreases in affinity of the mu-conotoxins in the presence of an amine. Although this mutual inhibition seems to be qualitatively consistent with an electrostatic interaction across the selectivity filter, quantitative considerations raise questions about the mechanistic details of the interaction.

Compatible solutes reduce ROS-induced potassium efflux in Arabidopsis roots

Reactive oxygen species (ROS) are known to be primarily responsible for the impairment of cellular function under numerous abiotic and biotic stress conditions. In this paper, using non-invasive microelectrode ion flux measuring (MIFE) system, we show that the application of a hydroxyl radical (OH*)-generating Cu2+/ascorbate (Cu/a) mixture to Arabidopsis roots results in a massive, dose-dependent efflux of K+ from epidermal cells in the elongation zone. Pharmacological experiments suggest that both outward-rectifying K+ channels and non-selective cation channels (NSCCs) mediate such effluxes. Low (5 mM) concentrations of compatible solutes (glycine betaine, proline, mannitol, trehalose or myo-inositol) significantly reduces OH*-induced K+ efflux, similar to our previous reports for NaCl-induced K+ efflux. Importantly, a significant reduction in K+ efflux was found using osmolytes with no reported free radical scavenging activity, as well as those for which a role in free radical scavenging has been demonstrated. This indicates that compatible solutes must play other (regulatory) roles, in addition to free radical scavenging, in mitigating the damaging effects of oxidative stress.

Inhibition of Astroglial Inwardly Rectifying Kir4.1 Channels by a Tricyclic Antidepressant, Nortriptyline

The inwardly rectifying K(+) (Kir) channel Kir4.1 is responsible for astroglial K(+) buffering. We examined the effects of nortriptyline, a tricyclic antidepressant (TCA), on Kir4.1 channel currents heterologously expressed in HEK293T cells, using a whole-cell patch-clamp technique. Nortriptyline (3-300 microM) reversibly inhibited Kir4.1 currents in a concentration-dependent manner, whereas it marginally affected neuronal Kir2.1 currents. The inhibition of Kir4.1 channels by nortriptyline depended on the voltage difference from the K(+) equilibrium potential (E(K)), with greater potency at more positive potentials. Blocking kinetics of the drug could be described by first-order kinetics, where dissociation of the drug slowed down and association accelerated as the membrane was depolarized. The dissociation constant (K(d)) of nortriptyline for Kir4.1 inhibition was 28.1 microM at E(K). Other TCAs, such as amitriptyline, desipramine, and imipramine, also inhibited Kir4.1 currents in a similar voltage-dependent fashion. This study shows for the first time that nortriptyline and related TCAs cause a concentration-, voltage-, and time-dependent inhibition of astroglial K(+)-buffering Kir4.1 channels, which might be involved in therapeutic and/or adverse actions of the drugs.

TRPV1 regulators mediate gentamicin penetration of cultured kidney cells

Transient receptor potential (TRP) receptors are, typically, calcium-permeant cation channels that transduce environmental stimuli. Both kidney epithelial and inner ear sensory cells express TRPV1, are mechanosensors and accumulate the aminoglycoside antibiotic gentamicin. Recently, we showed that Texas Red-conjugated gentamicin (GTTR) enters kidney cells via an endosome-independent pathway. Here, we used GTTR to investigate this non-endocytotic mechanism of gentamicin uptake. In serum-free buffers, GTTR penetrated MDCK cells within 30 s and uptake was modulated by extracellular, multivalent cations (Ca2+, La3+, Gd3+) or protons. We verified the La3+ modulation of GTTR uptake using immunocytochemical detection of unconjugated gentamicin. Membrane depolarization, induced by high extracellular K+ or valinomycin, also reduced GTTR uptake, suggesting electrophoretic permeation through ion channels. GTTR uptake was enhanced by the TRPV1 agonists, resiniferatoxin and anandamide, in Ca2+-free media. Competitive antagonists of the TRPV1 cation current, iodo-resiniferatoxin and SB366791, also enhanced GTTR uptake independently of Ca2+, reinforcing these antagonists' potential as latent agonists in specific situations. Ruthenium Red blocked GTTR uptake in the presence or absence of these TRPV1-agonists and antagonists. In addition, GTTR uptake was blocked by RTX in the presence of more physiological levels (2 mM) of Ca2+. Thus gentamicin enters cells via cation channels, and gentamicin uptake can be modulated by regulators of the TRPV1 channel.

Revisiting voltage-dependent relief of block in ion channels: a mechanism independent of punchthrough

Voltage dependence of block was investigated in a simple model for permeation in a multiion pore. Internal blocker could bind to three states of the open channel that differed in the locations and number of permeant ion bound; blocker dissociation occurs exclusively to the internal solution, and the blocker does not itself enter the electric field. By changing the relative stability of blocker binding to these three states, block displayed voltage dependence with relief of block at high potentials. Similar patterns of block could also be generated in more detailed models of ion permeation. These results illustrate that the observation of relief of block at high potentials is not a sufficient criterion for establishing that a blocker is permeant in a channel that has a complex permeation cycle.

5-hydroxytryptamine action in the rat olfactory bulb: In vitro electrophysiological patch-clamp recordings of juxtaglomerular and mitral cells

The olfactory bulb, first relay of olfactory pathways, is densely innervated by serotoninergic centrifugal fibers originating from the raphe nuclei. Although serotonin innervation was reported to be involved in olfactory learning in mammals, the action of this neurotransmitter on its putative cellular targets has been never described through unitary recordings. This lack of data initiated the present study where the effects of 5HT on juxtaglomerular and mitral cells are analyzed using whole-cell recordings on olfactory bulb slices. Serotonin depolarizes 34% of 525 JG cells. A multivariate statistical analysis of juxtaglomerular cells characteristics shows that the serotonin responsive cell group can be individualized regarding their tonic discharge-mode in response to a direct current injection, their lower expression of hyperpolarization-activated cation current and their low membrane capacities. The use of ion channel blockers and ramp voltage protocol indicate that serotoninergic depolarization of juxtaglomerular cells may be due to a nonselective cation current with a reversal potential of -44 mV. Pharmacological tests with serotonin receptor antagonists and agonists reveal that 5HT action on juxtaglomerular cells would be mainly mediated by 5HT2C receptors. In mitral cells, serotonin acts on 49.1% of the 242 tested cells, inducing two types of responses. A first subset of mitral cells (26.8%, n=65) were hyperpolarized by serotonin. This response would be indirect and mediated by action of GABA on GABAA receptors since it was antagonized by bicuculline. The involved GABAergic neurons are hypothesized to be juxtaglomerular and granular cells, on which serotonin would act mainly via 5HT2C and via 5HT2A receptors respectively. The second subset of mitral cells (22.3%, n=54) were directly depolarized by serotonin acting through 5HT2A receptors. Our data on serotonin action on juxtaglomerular cells and mitral cells reveal a part of functional mechanisms whereby serotonin can act on olfactory bulb network. This is expected to enrich the understanding of its determining role in olfactory learning.

Probing the pore of the auditory hair cell mechanotransducer channel in turtle

Hair cell mechano-electric transducer (MET) channels play a pivotal role in auditory and vestibular signal detection, yet few data exist regarding their molecular nature. Present work characterizes the MET channel pore, a region whose properties are thought to be intrinsically determined. Two approaches were used. First, the channel was probed with antagonists of candidate channel subtypes including: cyclic nucleotide-gated channels, transient receptor potential channels and gap-junctional channels. Eight new antagonists were identified. Most of the effective antagonists had a partially charged amine group predicted to penetrate the channel pore, antagonizing current flow, while the remainder of the molecule prevented further permeation of the compound through the pore. This blocking mechanism was tested using curare to demonstrate the open channel nature of the block and by identifying methylene blue as a permeant channel blocker. The second approach estimated dimensions of the channel pore with simple amine compounds. The narrowest diameter of the pore was calculated as 12.5 +/- 0.8 A and the location of a binding site approximately 45% of the way through the membrane electric field was calculated. Channel length was estimated as approximately 31 A and the width of the pore mouth at < 17 A. Each effective antagonist had a minimal diameter, measured about the penetrating amine, of less than the pore diameter, with a direct correlation between IC(50) and minimal diameter. The IC(50) was also directly related to the length of the amine side chains, further validating the proposed pore blocking mechanism. Data provided by these two approaches support a hypothesis regarding channel permeation and block that incorporates molecular dimensions and ion interactions within the pore.

Mutation-induced blocker permeability and multiion block of the CFTR chloride channel pore

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel is blocked by a broad range of anions that bind tightly within the pore. Here we show that the divalent anion Pt(NO₂)₄²⁻ acts as an impermeant voltage-dependent blocker of the CFTR pore when added to the intracellular face of excised membrane patches. Block was of modest affinity (apparent K_d 556 μM), kinetically fast, and weakened by extracellular Cl⁻ ions. A mutation in the pore region that alters anion selectivity, F337A, but not another mutation at the same site that has no effect on selectivity (F337Y), had a complex effect on channel block by intracellular Pt(NO₂)₄²⁻ ions. Relative to wild-type, block of F337A-CFTR was weakened at depolarized voltages but strengthened at hyperpolarized voltages. Current in the presence of Pt(NO₂)₄²⁻ increased at very negative voltages in F337A but not wild-type or F337Y, apparently due to relief of block by permeation of Pt(NO₂)₄²⁻ ions to the extracellular solution. This “punchthrough” was prevented by extracellular Cl⁻ ions, reminiscent of a “lock-in” effect. Relief of block in F337A by Pt(NO₂)₄²⁻ permeation was only observed for blocker concentrations above 300 μM; as a result, block at very negative voltages showed an anomalous concentration dependence, with an increase in blocker concentration causing a significant weakening of block and an increase in Cl⁻ current. We interpret this effect as reflecting concentration-dependent permeability of Pt(NO₂)₄²⁻ in F337A, an apparent manifestation of an anomalous mole fraction effect. We suggest that the F337A mutation allows intracellular Pt(NO₂)₄²⁻ to enter deeply into the CFTR pore where it interacts with multiple binding sites, and that simultaneous binding of multiple Pt(NO₂)₄²⁻ ions within the pore promotes their permeation to the extracellular solution.

Coupled movement of permeant and blocking ions in the CFTR chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel pore is blocked in a voltage-dependent manner by a broad range of anionic substances added to the cytoplasmic side of the membrane. Here we investigate the origin of the voltage dependence of block by intracellular Au(CN)2-, a highly permeant lyotropic anion which also acts as a high-affinity blocker of Cl- permeation. Not only the affinity, but also the voltage dependence of block by intracellular Au(CN)2- ions is strongly dependent on extracellular Cl- concentration; following replacement of most extracellular Cl- by glucose or by impermeant anions, block by Au(CN)2- shows greatly weakened voltage dependence. This suggests that coupled movement of Au(CN)2- and Cl- ions within the pore contributes to the voltage dependence of block. This explanation requires that interactions between different anions take place within the pore, implying simultaneous binding of multiple anions to intrapore sites. Other anions are able to substitute for extracellular Cl- and interact with intracellular Au(CN)2- ions. Analysis of the effects of different extracellular anions on the apparent affinity and voltage dependence of block by intracellular Au(CN)2- ions suggests that extracellular anions do not need to permeate through the channel in order to destabilize Au(CN)2- binding within the pore, implying that this destabilizing effect results from binding to an externally accessible site in the permeation pathway. We propose that multiple anions can bind simultaneously within the CFTR channel pore, and that repulsive interactions between bound anions speeds anion exit from the pore.

Voltage-dependent blockade of connexin40 gap junctions by spermine

The effects of spermine and spermidine, endogenous polyamines that block many forms of ion channels, were investigated in homotypic connexin (Cx)-40 gap junctions expressed in N2A cells. Spermine blocked up to 95% of I(j) through homotypic Cx40 gap junctions in a concentration- and transjunctional voltage (V(j))-dependent manner. V(j) was varied from 5 to 50 mV in 5-mV steps and the dissociation constants (K(m)) were determined from spermine concentrations ranging from 10 micro M to 2 mM. The K(m) values ranged from 4.9 mM to 107 micro M for 8.6 < or = V(j) < or = 37.7 mV, within the physiological range of intracellular spermine for V(j) > or = 20 mV. The K(m) values for spermidine were > or = 5 mM. Estimates of the electrical distance (delta) for spermine (z = +4) and spermidine (z = +3) were 0.96 and 0.76 respectively. Cx40 single channel conductance was 129 pS in the presence of 2-mM spermine and channel open probability was significantly reduced in a V(j)-dependent manner. Similar concentrations of spermine did not block I(j) through homotypic Cx43 gap junctions, indicating that spermine selectively blocks Cx40 gap junctions. This is contrary to our previous findings that large tetraalkylammonium ions, also known to block several forms of ion channels, block junctional currents (I(j)) through homotypic connexin Cx40 and Cx43 gap junctions.

Na+ block and permeation in a K+ channel of known structure

The effects of intracellular Na(+) were studied on K(+) and Rb(+) currents through single KcsA channels. At low voltage, Na(+) produces voltage-dependent block, which becomes relieved at high voltage by a "punchthrough" mechanism representing Na(+) escaping from its blocking site through the selectivity filter. The Na(+) blocking site is located in the wide, hydrated vestibule, and it displays unexpected selectivity for K(+) and Rb(+) against Na(+). The voltage dependence of Na(+) block reflects coordinated movements of the blocker with permeant ions in the selectivity filter.

Block of the ryanodine receptor channel by neomycin is relieved at high holding potentials

In this study we have investigated the actions of the aminoglycoside antibiotic neomycin on K+ conductance in the purified sheep cardiac sarcoplasmic reticulum (SR) calcium-release channel (RyR). Neomycin induces a concentration- and voltage-dependent partial block from both the cytosolic and luminal faces of the channel. Blocking parameters for cytosolic and luminal block are markedly different. Neomycin has a greater affinity for the luminal site of interaction than the cytosolic site: zero-voltage dissociation constants (Kb(0)) are respectively 210.20 +/- 22.80 and 589.70 +/- 184.00 nM for luminal and cytosolic block. However, neomycin also exhibits voltage-dependent relief of block at holding potentials >+60 mV when applied to the cytosolic face and a similar phenomenon may occur with luminal neomycin at high negative holding potentials. These observations indicate that, under appropriate conditions, neomycin is capable of passing through the RyR channel.

Ionic blockade of the rat connexin40 gap junction channel by large tetraalkylammonium ions

The rat connexin40 gap junction channel is permeable to monovalent cations including tetramethylammonium and tetraethylammonium ions. Larger tetraalkyammonium (TAA(+)) ions beginning with tetrabutylammonium (TBA(+)) reduced KCl junctional currents disproportionately. Ionic blockade by tetrapentylammonium (TPeA(+)) and tetrahexylammonium (THxA(+)) ions were concentration- and voltage-dependent and occurred only when TAA(+) ions were on the same side as net K(+) efflux across the junction, indicative of block of the ionic permeation pathway. The voltage-dependent dissociation constants (K(m)(V(j))) were lower for THxA(+) than TPeA(+), consistent with steric effects within the pore. The K(m)-V(j) relationships for TPeA(+) and THxA(+) were fit with different reaction rate models for a symmetrical (homotypic) connexin gap junction channel and were described by either a one- or two-site model that assumed each ion traversed the entire V(j) field. Bilateral addition of TPeA(+) ions confirmed a common site of interaction within the pore that possessed identical K(m)(V(j)) values for cis-trans concentrations of TPeA(+) ions as indicated by the modeled I-V relations and rapid channel block that precluded unitary current measurements. The TAA(+) block of K(+) currents and bilateral TPeA(+) interactions did not alter V(j)-gating of Cx40 gap junctions. N-octyl-tributylammonium and -triethylammonium also blocked rCx40 channels with higher affinity and faster kinetics than TBA(+) or TPeA(+), indicative of a hydrophobic site within the pore near the site of block.

Ryanoid modification of the cardiac muscle ryanodine receptor channel results in relocation of the tetraethylammonium binding site

The interaction of ryanodine and derivatives of ryanodine with the high affinity binding site on the ryanodine receptor (RyR) channel brings about a characteristic modification of channel function. In all cases, channel open probability increases dramatically and single-channel current amplitude is reduced. The amplitude of the ryanoid-modified conductance state is determined by structural features of the ligand. An investigation of ion handling in the ryanodine-modified conductance state has established that reduced conductance results from changes in both the affinity of the channel for permeant ions and the relative permeability of ions within the channel (Lindsay, A.R.G., A. Tinker, and A.J. Williams. 1994. J. Gen. Physiol. 104:425-447). It has been proposed that these alterations result from a reorganization of channel structure induced by the binding of the ryanoid. The experiments reported here provide direct evidence for ryanoid-induced restructuring of RyR. TEA+ is a concentration- and voltage-dependent blocker of RyR in the absence of ryanoids. We have investigated block of K+ current by TEA+ in the unmodified open state and modified conductance states of RyR induced by 21-amino-9alpha-hydroxyryanodine, 21-azido-9alpha-hydroxyryanodine, ryanodol, and 21-p-nitrobenzoylamino-9alpha-hydroxyryanodine. Analysis of the voltage dependence of block indicates that the interaction of ryanoids with RyR leads to an alteration in this parameter with an apparent relocation of the TEA+ blocking site within the voltage drop across the channel and an alteration in the affinity of the channel for the blocker. The degree of change of these parameters correlates broadly with the change in conductance of permeant cations induced by the ryanoids, indicating that modification of RyR channel structure by ryanoids is likely to underlie both phenomena.

Permeation of large tetra-alkylammonium cations through mutant and wild-type voltage-gated sodium channels as revealed by relief of block at high voltage

Many large organic cations are potent blockers of K(+) channels and other cation-selective channels belonging to the P-region superfamily. However, the mechanism by which large hydrophobic cations enter and exit the narrow pores of these proteins is obscure. Previous work has shown that a conserved Lys residue in the DEKA locus of voltage-gated Na(+) channels is an important determinant of Na(+)/K(+) discrimination, exclusion of Ca(2+), and molecular sieving of organic cations. In this study, we sought to determine whether the Lys(III) residue of the DEKA locus interacts with internal tetra-alkylammonium cations (TAA(+)) that block Na(+) channels in a voltage-dependent fashion. We investigated block by a series of TAA(+) cations of the wild-type rat muscle Na(+) channel (DEKA) and two different mutants of the DEKA locus, DEAA and DERA, using whole-cell recording. TEA(+) and larger TAA(+) cations block both wild-type and DEAA channels. However, DEAA exhibits dramatic relief of block by large TAA(+) cations as revealed by a positive inflection in the macroscopic I-V curve at voltages greater than +140 mV. Paradoxically, relief of block at high positive voltage is observed for large (e.g., tetrapentylammonium) but not small (e.g., TEA(+)) symmetrical TAA(+) cations. The DEKA wild-type channel and the DERA mutant exhibit a similar relief-of-block phenomenon superimposed on background current rectification. The results indicate: (a) hydrophobic TAA(+) cations with a molecular diameter as large as 15 A can permeate Na(+) channels from inside to outside when driven by high positive voltage, and (b) the Lys(III) residue of the DEKA locus is an important determinant of inward rectification and internal block in Na(+) channels. From these observations, we suggest that hydrophobic interfaces between subunits, pseudosubunits, or packed helices of P-region channel proteins may function in facilitating blocker access to the pore, and may thus play an important role in the blocking and permeation behavior of large TAA(+) cations and potentially other kinds of local anesthetic molecules.

Regulation of transient Na+ conductance by intra- and extracellular K+ in the human delayed rectifier K+ channel Kv1.5

1. Significant Na+ conductance has been described in only a few native and cloned K+ channels, but has been used to characterize inactivation and K+ binding within the permeation pathway, and to refine models of K+ flux through multi-ion pores. Here we use Na+ permeation of the delayed rectifier K+ channel Kv1.5 to study extra- and intracellular K+ (K+o and K+i, respectively) regulation of conductance and inactivation, using whole-cell recording from human embryonic kidney (HEK)-293 cells. 2. Kv1.5 Na+ currents in the absence of K+o and K+i were confirmed by: (i) resistance of outward Na+ currents to dialysis by K+-free solutions; (ii) tail current reversal potential changes with Na+o with a slope of 55.8 mV per decade; (iii) block by 4-aminopyridine (50 % at 50 microM), and resistance to Cl- channel inhibition. 3. Na+ currents were transient followed by a small sustained current. An envelope test confirmed that activated Kv1.5 channels conducted Na+, and that rapid current decay reflected C-type inactivation. Sustained currents ( approximately 13 % of peak) represented Na+ flux through inactivated Kv1.5 channels. 4. K+o could modulate the maximum available Na+ conductance in the stable cell line while channels were closed. Before the first pulse of a train, increasing K+o concentration increased the subsequent Na+ conductance from approximately 15 (0 mM K+o) to 30 nS (5 mM K+o), with a Kd of 23 microM. Repeated low rate depolarizations in Na+i/Na+o solutions induced a use-dependent loss of Kv1.5 channel Na+ conductance, distinct from that caused by C-type inactivation. K+o binding that sensed little of the electric field could prevent this secondary loss of available Kv1.5 channels with a Kd of 230 microM. These two effects on conductance were both voltage independent, and had no effect on channel inactivation rate. 5. K+o concentrations >= 0.3 mM slowed the inactivation rate in a strongly voltage-dependent manner. This suggested it could compete for binding at a K+ site or sites deeper in the pore, as well as restoring the Na+ conductance. K+i was able to modulate the inactivation rate but was unable to affect conductance. 6. Mutation of arginine 487 in the outer pore region of the channel to valine (R487V) greatly reduced C-type inactivation in Na+ solutions, caused loss of channel use dependence, and prevented any conductance increase upon the addition of 0.1 mM K+o. Our results confirm the existence of a high affinity binding site at the selectivity filter that regulates inactivation, and also reveals the presence of at least one additional high affinity outer mouth site that predominantly regulates conductance of resting channels, and protects channels activated by depolarization when they conduct Na+.

Spermine and arcaine block and permeate N-methyl-D-aspartate receptor channels

Polyamines such as spermine are thought to be endogenous regulators of NMDA (N-methyl-D-aspartate)-type glutamate receptors. Polyamine block of NMDA receptors was studied in excised outside-out patches from rat hippocampal neurons and Xenopus oocytes expressing recombinant receptors. Extracellular spermine and arcaine reduced NMDA single-channel conductance in a voltage-dependent manner, with partial relief of block evident at large inside negative membrane potentials. Reducing extracellular Na+ concentration increased the apparent affinities for spermine and arcaine, indicating strong interaction between spermine and permeant ions. Internal spermine also blocked NMDA channels in a voltage-dependent manner, with relief of block evident at large inside positive potentials. The Woodhull model of channel block by an impermeant ion adequately described the actions of external spermine from -60 to +60 mV, but failed for more negative potentials. Eyring rate theory for a permeable blocker with two barriers and one binding site adequately described the voltage-dependent block and relief from block by both external and internal spermine over the range of -120 to +60 mV. These findings indicate that polyamines block and permeate neuronal NMDA receptor channels from the extracellular and intracellular sides, although sensitivity to internal spermine is probably too low to be physiologically relevant.

Blockade of HERG channels expressed in Xenopus laevis oocytes by external divalent cations

We have investigated actions of various divalent cations (Ba2+, Sr2+, Mn2+, Co2+, Ni2+, Zn2+) on human ether-a-go-go related gene (HERG) channels expressed in Xenopus laevis oocytes using the voltage clamp technique. All divalent cations inhibited HERG current dose-dependently in a voltage-dependent manner. The concentration for half-maximum inhibition (Ki) decreased at more negative potentials, indicating block is facilitated by hyperpolarization. Ki at 0 mV for Zn2+, Ni2+, Co2+, Ba2+, Mn2+, and Sr2+ was 0.19, 0.36, 0. 50, 0.58, 2.36, and 6.47 mM, respectively. The effects were manifested in four ways: 1) right shift of voltage dependence of activation, 2) decrease of maximum conductance, 3) acceleration of current decay, and 4) slowing of activation. However, each parameter was not affected by each cation to the same extent. The potency for the shift of voltage dependence of activation was in the order Zn2+ > Ni2+ >/= Co2+ > Ba2+ > Mn2+ > Sr2+, whereas the potency for the decrease of maximum conductance was Zn2+ > Ba2+ > Sr2+ > Co2+ > Mn2+. The kinetics of activation and deactivation were also affected, but the two parameters are not affected to the same extent. Slowing of activation by Ba2+ was most distinct, causing a marked initial delay of current onset. From these results we concluded that HERG channels are nonselectively blocked by most divalent cations from the external side, and several different mechanism are involved in their actions. There exist at least two distinct binding sites for their action: one for the voltage-dependent effect and the other for reducing maximum conductance.

Modulation of C-type inactivation by K+ at the potassium channel selectivity filter

With prolonged or repetitive activation, voltage-gated K+ channels undergo a slow (C-type) inactivation mechanism, which decreases current flow through the channel. Previous observations suggest that C-type inactivation results from a localized constriction in the outer mouth of the channel pore and that the rate of inactivation is controlled by the-rate at which K+ leaves an unidentified binding site in the pore. We have functionally identified two K+ binding sites in the conduction pathway of a chimeric K+ channel that conducts Na+ in the absence of K+. One site has a high affinity for K+ and contributes to the selectivity filter mechanism for K+ over Na+. Another site, external to the high-affinity site, has a lower affinity for K+ and is not involved in channel selectivity. Binding of K+ to the high-affinity binding site slowed inactivation. Binding of cations to the external low-affinity site did not slow inactivation directly but could slow it indirectly, apparently by trapping K+ at the high-affinity site. These data support a model whereby C-type inactivation involves a constriction at the selectivity filter, and the constriction cannot proceed when the selectivity filter is occupied by K+.

AMPA/kainate receptor activation inhibits neuronal delayed rectifier K+ current via Na+ entry in rat cortical neurons

In the present study we evaluated the modulation of neuronal delayed rectifier K+ current (IK) by activation of ionotropic glutamate receptors. In whole-cell voltage-clamp experiments, an external application of 10-100 microM kainate suppressed the amplitude of IK following an inward shift of holding current. The effect of kainate on IK was eliminated by CN QX, an AMPA/kainate receptor antagonist, indicating that the receptor-mediated cation entry caused IK suppression. When external Na+ was completely replaced by equimolar choline+ or N-methyl-D-glucamine, kainate-induced IK suppression was abolished. Our results suggest that in cultured rat cortical neurons, AMPA/kainate receptor activation leads to an intracellular Na+ increase which blocks delayed rectifier K+ channels. This contributes to feed-forward excitation of neuronal cells in glutaminergic responses.

The interaction of Na+ and K+ in voltage-gated potassium channels. Evidence for cation binding sites of different affinity

Voltage-gated potassium (K+) channels are multi-ion pores. Recent studies suggest that, similar to calcium channels, competition between ionic species for intrapore binding sites may contribute to ionic selectivity in at least some K+ channels. Molecular studies suggest that a putative constricted region of the pore, which is presumably the site of selectivity, may be as short as one ionic diameter in length. Taken together, these results suggest that selectivity may occur at just a single binding site in the pore. We are studying a chimeric K+ channel that is highly selective for K+ over Na+ in physiological solutions, but conducts Na+ in the absence of K+. Na+ and K+ currents both display slow (C-type) inactivation, but had markedly different inactivation and deactivation kinetics; Na+ currents inactivated more rapidly and deactivated more slowly than K+ currents. Currents carried by 160 mM Na+ were inhibited by external K+ with an apparent IC50 <30 microM. K+ also altered both inactivation and deactivation kinetics of Na+ currents at these low concentrations. In the complementary experiment, currents carried by 3 mM K+ were inhibited by external Na+, with an apparent IC50 of approximately 100 mM. In contrast to the effects of low [K+] on Na+ current kinetics, Na+ did not affect K+ current kinetics, even at concentrations that inhibited K+ currents by 40-50%. These data suggest that Na+ block of K+ currents did not involve displacement of K+ from the high affinity site involved in gating kinetics. We present a model that describes the permeation pathway as a single high affinity, cation-selective binding site, flanked by low affinity, nonselective sites. This model quantitatively predicts the anomalous mole fraction behavior observed in two different K+ channels, differential K+ and Na+ conductance, and the concentration dependence of K+ block of Na+ currents and Na+ block of K+ currents. Based on our results, we hypothesize that the permeation pathway contains a single high affinity binding site, where selectivity and ionic modulation of gating occur.

Adjacent asparagines in the NR2-subunit of the NMDA receptor channel control the voltage-dependent block by extracellular Mg2+

1. The voltage-dependent block of N-methyl-D-aspartate (NMDA) receptor channels by extracellular Mg2+ is a critical determinant of its contribution to CNS synaptic physiology. The function of the narrow constriction of the channel in determining the block was investigated by analysing the effects of a set different amino acid substitutions at exposed residues positioned at or near this region. NMDA receptor channels, composed of wild-type and mutant NR1- and NR2A-subunits, were expressed in Xenopus oocytes or human embryonic kidney (HEK) 293 cells. 2. In wild-type channels, the voltage dependence (delta) of the block Mg2+ was concentration dependent with values of delta of integral of 0.82 in 0.07 mM and higher concentrations. Under bionic conditions with high extracellular Mg2+ and K+ as the reference ion, Mg2+ weakly permeated the channel. Over intermediate potentials (approximately -60 to -10 mV), this weak permeability had no apparent effect on the block but at potentials negative to approximately -60mV, it attenuated the extent and voltage dependence of the block. 3. Substitutions of glycine, serine, glutamine or aspartate for the N-site asparagine in the NR1-subunit enhanced the extent of block over intermediate potentials but left the voltage dependence of the block unchanged indicating that structural determinants of the block remained. These same substitutions either attenuated or left unchanged the apparent Mg2+ permeability. 4. In channels containing substitutions of glycine, serine or glutamine for the N-site asparagine in the NR2A-subunit, the block Mg2+ was reduced at negative potentials. Over intermediate potentials, the block was not strongly attenuated except for the glutamine substitution which reduced the voltage dependence of the block to integral of 0.57 in 0.7 mM Mg2+. 5. Equivalent substitutions for the N + 1 site asparagine in the NR2A-subunit strongly attenuated the block over the entire voltage range. In 0.7 mM Mg2+, the voltage dependence of the block was reduced to 0.50 (glycine), 0.53 (serine) and 0.46 (glutamine). 6. Channels containing substitutions of the N-site or N + 1 site asparagines in the NR2A-subunit showed an increased Mg2+ permeability suggesting that these adjacent asparagines form a barrier for inward Mg2+ flux. Changes in this barrier contribute, at least in part, to the mechanism underlying disruption of the block following substitution of these residues. 7. The adjacent NR2A-subunit asparagines are positioned at or near the narrow constriction of the channel. Pore size, however, did not determine how effectively Mg2+ blocks mutant channels. 8. It is concluded that, at the narrow constriction in the NMDA receptor channel, the adjacent NR2A-subunit asparagines, the N-site and N + 1 site, but not the N-site asparagine of the NR1-subunit, form a critical blocking site for extracellular Mg2+. The contribution to the blocking site, in contrast to the prevailing view, is stronger for the N + 1 site than for the N-site asparagine. The block may involve binding of Mg2+ to these residues.

Delayed rectifier current of bullfrog sympathetic neurons: ion-ion competition, asymmetrical block and effects of ions on gating

1. The delayed rectifier (DR) K+ channel pore was probed using different permeant and blocking ions applied intra- and extracellularly. Currents were recorded from bullfrog sympathetic neurons using whole-cell patch-clamp techniques. 2. With intra- and extracellular Cs+ (0 K+), there were large, tetraethylammonium (TEA)-sensitive currents. Adding K+ back to the extracellular solution revealed that the current with Cs+i was K+ selective (permeability ratio PCs/PK = 0.17 +/- 0.02, n = 4) and showed a strong anomalous mole fraction effect. 3. There were also large non-inactivating currents with Na+i and Na+o (0 K+). The current with Na+i was K+ selective (Na+o vs. K+o: PNa/PK = 0.022 +/- 0.005, n = 5), and was TEA sensitive with K+o but not with Na+o. 4. Permeant ions affected gating kinetics. DR currents activated faster in K+ than in Cs+, and activated faster with increasing concentrations of either K+ or Cs+. Deactivation was slowed by increased K+ or Cs+ concentration, with no difference between K+ and Cs+. 5. The pore was also characterized using intracellular blocking ions. A wide variety of monovalent cations (TEA, N-methyl-D-glucamine, arginine, choline, CH3NH3+, Li+, Cs+ and Na+) blocked DR channels from the inside in a voltage-dependent manner: KD at 0 mV was 2.9 mM for TEA and 134-487 mM for the others, at apparent electrical distances (delta) of 0.33-0.79. There was no detectable block by 10 mM Mgi2+. Apart from TEA, the organic cations did not block from the outside. 6. The permeability to Na+ in the absence of K+, and the strong anomalous mole fraction effects observed for Cs+o + K+o mixtures, suggest that DR channels select for K+ using ion-ion competition. The block by large intracellular cations shows that the pore is asymmetrical. The loss of high affinity TEAo block with Na+i and Na+o, and the effects of permeant ions on gating, suggest that channel conformation may be affected by ions in the pore.

Rectification of rabbit cardiac ryanodine receptor current by endogenous polyamines

The actions of three endogenous polyamines (spermine, spermidine, and putrescine) were defined on Ca2+ release channels (ryanodine receptors, RyRs) isolated from rabbit cardiac sarcoplasmic reticulum. The current-voltage relationship of the RyR channel was N-shaped in the presence of polyamine (1-5 mM). Polyamine blocked conduction near 0 mV, but the blockade was relieved at large potentials. Polyamines acted (blocked) from both sides of the channel. Polyamine efficacy was dependent on current direction and was inversely related to the ion selectivity of the RyR pore. This suggests that polyamine interacts with current-carrying ions in the permeation pathway. The apparent half-block concentration of spermine at 0 mV was < 0.1 mM. The features of polyamine blockade suggest that the polyamines are permeable cationic blockers of the RyR channel. Further, the levels of polyamines found in muscle cells are sufficient to block single RyR channels and thus may alter the sarcoplasmic reticulum Ca2+ release process in situ.

Potassium current inhibition by nonselective cation channel-mediated sodium entry in rat pheochromocytoma (PC-12) cells

Under physiological conditions, nonselective cation (NSC) channels mediate the entry of cations into cells, the most important being Na+ and Ca2+. In contrast to the Ca(2+)-dependent signaling mechanisms, little is known about the consequences and the spatial distribution of intracellular [Na+] elevation. In this study we demonstrate that Na+ entry, during the opening of ATP-activated NSC channels, leads to an inhibition of voltage-dependent K+ currents (IK) in cromaffin-like undifferentiated PC-12 cells. The effect was dependent on the charge carrier as well as on the density of the ATP-activated current. Extracellular alkali cations (Na+, Li+) were more efficient than NH4+ in suppressing IK. Intracellular infusion of Na+ had the same effect as Na+ influx through ATP-activated NSC channels. The inhibition of IK persisted when the total ATP-induced Na+ entry was reduced by membrane depolarization, suggesting a spatial restriction of the required Na+ accumulation. Our results indicate that NSC channels influence the function of other ion channels by changing local intracellular ion concentrations.

Ion permeation and block of M-type and delayed rectifier potassium channels. Whole-cell recordings from bullfrog sympathetic neurons

Ion permeation and conduction were studied using whole-cell recordings of the M-current (I(M)) and delayed rectifier (IDR), two K+ currents that differ greatly in kinetics and modulation. Currents were recorded from isolated bullfrog sympathetic neurons with 88 mM [K+]i and various external cations. Selectivity for extracellular monovalent cations was assessed from permeability ratios calculated from reversal potentials and from chord conductances for inward current. PRb/PK was near 1.0 for both channels, and GRb/GK was 0.87 +/- 0.01 for IDR but only 0.35 +/- 0.01 for I(M) (15 mM [Rb+]o or [K+]o). The permeability sequences were generally similar for I(M) and IDR: K+ approximately Rb+ > NH4+ > Cs+, with no measurable permeability to Li+ or CH3NH3+. However, Na+ carried detectable inward current for IDR but not I(M). Nao+ also blocked inward K+ current for IDR (but not IM), at an apparent electrical distance (delta) approximately 0.4, with extrapolated dissociation constant (KD) approximately 1 M at 0 mV. Much of the instantaneous rectification of IDR in physiologic ionic conditions resulted from block by Nao+. Extracellular Cs+ carried detectable inward current for both channel types, and blocked I(M) with higher affinity (KD = 97 mM at 0 mV for I(M), KD) approximately 0.2 M at 0 mV for IDR), with delta approximately 0.9 for both. IDR showed several characteristics reflecting a multi-ion pore, including a small anomalous mole fraction effect for PRb/PK, concentration-dependent GRb/GK, and concentration-dependent apparent KD's and delta's for block by Nao+ and Cso+. I(M) showed no clear evidence of multi-ion pore behavior. For I(M), a two-barrier one-site model could describe permeation of K+ and Rb+ and block by Cso+, whereas for IDR even a three-barrier, two-site model was not fully adequate.

Methadone block of K+ current in squid giant fiber lobe neurons

Voltage-dependent ionic currents were recorded from squid giant fiber lobe neurons using the whole-cell patch-clamp technique. When applied to the bathing solution, methadone was found to block IK, I Na and I Ca. Both I Na and I Ca were reduced without apparent change in kinetics and exhibited IC(50)'s of 50-100 and 250-500 mu M, respectively, at +10 mV. In contrast, IK was reduced in a time-dependent manner that is well fit by a simple model of open channel block (K(D)= 32+/- or 2 mu M, +60 mV, 10 degrees Celsius). The mechanism of I(K) block was examined in detail and involves a direct action of methadone, a tertiary amine, on K channels rather than an opioid receptor-mediated pathway. The kinetics of I(K) block resemble those reported for internally applied long chain quaternary ammonium (QA) compounds; and recovery from I(K) block is QA-like in its slow time course and strong dependence on holding potential. A quaternary derivative of methadone (N-methyl-methadone) only reproduced the effects of methadone on I(K) when included in the pipette solution; this compound was without effect when applied externally. I(K) block thus appears to involve diffusion of methadone into the cytoplasm and occlusion of the open K channel at the internal QA blocking site by the protonated form of the drug. This proposed mode of action is supported by the pH and voltage dependence of block as well as by the observation that high external K+ speeds the rate of drug dissociation. In addition, the effect of methadone on I(K) evoked during prolonged (300 ms) depolarizations suggests that methadone block may interfere with endogenous K+ channel inactivation. The effects of temperature, methadone stereoisomers, and the methadone-like drugs propoxyphene and nor-propoxyphene on IK block were examined. Methadone was also found to block I(K) in GH3 cells and in chick myoblasts.

Blockade of K+ channels induced by AMPA/kainate receptor activation in mouse oligodendrocyte precursor cells is mediated by Na+ entry

AMPA/kainate receptor activation in cultured oligodendrocyte precursor cells from embryonic mouse cortex leads to a blockade of delayed rectifying K+ currents. In the present study, we provide evidence using the patch-clamp technique in the whole-cell configuration that the mechanism linking kainate receptor activation and K+ conductance blockade is due to the receptor-mediated Na+ entry: 1) The blockade was not observed in Na(+)-free bathing solution nor when intracellular [Na+] was elevated by dialzying the cell with a pipette solution containing high [Na+]. 2) Elevation of intracellular [Na+] alone led to a blockade of outward currents in contrast to cells dialyzed by sucrose. High [Li+]i also reduced the outward currents, and in Li(+)-containing bathing solution the kainate-induced blockade of K+ channels was more pronounced. Probably, Li+ accumulates intracellularly after permeation through the receptor pore due to slower extrusion mechanisms. Experiments with GTP gamma S or GDP beta S and pertussis toxin indicated that GTP-binding protein-mediated mechanisms were not of importance for the kainate-induced K+ conductance blockade. Our data suggest that in glial precursor cells AMPA/kainate receptor activation leads to an intracellular [Na+] increase which blocks delayed rectifying K+ channels.

Repetitive depletion and recovery of intracellular K+ in retinal Müller glial cells during whole-cell voltage-clamp

A procedure was developed allowing repetitive depletion and recovery of K+ from the cell interior of Müller glial cells without patch pipette perfusion. To this end the whole-cell voltage-clamp technique using tight seal pipettes was applied to enzymatically isolated Müller cells of the guinea pig. When K+ was replaced by Cs+ and/or NMDG in the pipette solution voltage-activated outward currents could be generated similar to those found with normal K(+)-containing intracellular solution. This was the result of the distribution of K+ across the cell membrane due to the negative holding potential, i.e., K+ accumulated at the intracellular side of the cell membrane. This distribution should be favored by the low input resistance of Müller cells. K+ could also be removed from the cell interior by depolarizing voltage steps. Simultaneously, K+ influx had to be cut off by using a K(+)-free extracellular solution or blocking K+ channels using Ba2+. This procedure lead to reduction and eventually cessation of the K+ outward currents, indicating extinction of the intracellular K+ pool. The process is reversible, i.e., outward currents can be evoked again by depolarizing voltage steps after renewed application of extracellular K+ and/or removal of Ba2+. Hence, the intracellular K+ pool is filled up again. The described method was applied to demonstrate the dependence of the Müller cell Na+/glutamate transporter on the intracellular K+ concentration.