Mechanism of rectification in inward-rectifier K+ channels

Spontaneous slow wave oscillations in extracellular field potential recordings reflect the alternating dominance of excitation and inhibition

In the resting state, cortical neurons can fire action potentials spontaneously but synchronously (Up state), followed by a quiescent period (Down state) before the cycle repeats. Extracellular recordings in the infragranular layer of cortex with a micro-electrode display a negative deflection (depth-negative) during Up states and a positive deflection (depth-positive) during Down states. The resulting slow wave oscillation (SWO) has been studied extensively during sleep and under anaesthesia. However, recent research on the balanced nature of synaptic excitation and inhibition has highlighted our limited understanding of its genesis. Specifically, are excitation and inhibition balanced during SWOs? We analyse spontaneous local field potentials (LFPs) during SWOs recorded from anaesthetised rats via a multi-channel laminar micro-electrode and show that the Down state consists of two distinct synaptic states: a Dynamic Down state associated with depth-positive LFPs and a prominent dipole in the extracellular field, and a Static Down state with negligible (≈ 0 mV $ \approx 0{\mathrm{\;mV}}$ ) LFPs and a lack of dipoles extracellularly. We demonstrate that depth-negative and -positive LFPs are generated by a shift in the balance of synaptic excitation and inhibition from excitation dominance (depth-negative) to inhibition dominance (depth-positive) in the infragranular layer neurons. Thus, although excitation and inhibition co-tune overall, differences in their timing lead to an alternation of dominance, manifesting as SWOs. We further show that Up state initiation is significantly faster if the preceding Down state is dynamic rather than static. Our findings provide a coherent picture of the dependence of SWOs on synaptic activity. KEY POINTS: Cortical neurons can exhibit repeated cycles of spontaneous activity interleaved with periods of relative silence, a phenomenon known as 'slow wave oscillation' (SWO). During SWOs, recordings of local field potentials (LFPs) in the neocortex show depth-negative deflection during the active period (Up state) and depth-positive deflection during the silent period (Down state). Here we further classified the Down state into a dynamic phase and a static phase based on a novel method of classification and revealed non-random, stereotypical sequences of the three states occurring with significantly different transitional kinetics. Our results suggest that the positive and negative deflections in the LFP reflect the shift of the instantaneous balance between excitatory and inhibitory synaptic activity of the local cortical neurons. The differences in transitional kinetics may imply distinct synaptic mechanisms for Up state initiation. The study may provide a new approach for investigating spontaneous brain rhythms.

Seeing spermine blocking of K+ ion movement through inward rectifier Kir2.2 channels

Inwardly rectifier potassium (Kir) channels are a major potassium channel sub-class whose function is regulated by ligand-dependent gating and highly voltage-dependent block by polyamines. With molecular dynamics simulations over previously unattainable timescales, Jogini et al. (J. Gen. Physiol. https://doi.org/10.1085/jgp.202213085) provide unprecedented visualization of K+ conduction through open Kir2.2 channels and of the molecular details of channel block by spermine.

Adenosine and Adenosine Receptors: Advances in Atrial Fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in the world. Because the key to developing innovative therapies that limit the onset and the progression of AF is to fully understand the underlying molecular mechanisms of AF, the aim of the present narrative review is to report the most recent advances in the potential role of the adenosinergic system in the pathophysiology of AF. After a comprehensive approach describing adenosinergic system signaling and the mechanisms of the initiation and maintenance of AF, we address the interactions of the adenosinergic system's signaling with AF. Indeed, adenosine release can activate four G-coupled membrane receptors, named A₁, A_2A, A_2B and A₃. Activation of the A_2A receptors can promote the occurrence of delayed depolarization, while activation of the A₁ receptors can shorten the action potential's duration and induce the resting membrane's potential hyperpolarization, which promote pulmonary vein firing, stabilize the AF rotors and allow for functional reentry. Moreover, the A_2B receptors have been associated with atrial fibrosis homeostasis. Finally, the adenosinergic system can modulate the autonomous nervous system and is associated with AF risk factors. A question remains regarding adenosine release and the adenosine receptors' activation and whether this would be a cause or consequence of AF.

Mg2+ Is a Missing Link in Plant Cell Ca2+ Signalling and Homeostasis-A Study on Vicia faba Guard Cells

Hyperpolarization-activated calcium channels (HACCs) are found in the plasma membrane and tonoplast of many plant cell types, where they have an important role in Ca²⁺-dependent signalling. The unusual gating properties of HACCs in plants, i.e., activation by membrane hyperpolarization rather than depolarization, dictates that HACCs are normally open in the physiological hyperpolarized resting membrane potential state (the so-called pump or P-state); thus, if not regulated, they would continuously leak Ca²⁺ into cells. HACCs are permeable to Ca²⁺, Ba²⁺, and Mg²⁺; activated by H₂O₂ and the plant hormone abscisic acid (ABA); and their activity in guard cells is greatly reduced by increasing amounts of free cytosolic Ca²⁺ ([Ca²⁺]_Cyt), and hence closes during [Ca²⁺]_Cyt surges. Here, we demonstrate that the presence of the commonly used Mg-ATP inside the guard cell greatly reduces HACC activity, especially at voltages ≤ −200 mV, and that Mg²⁺ causes this block. Therefore, we firstly conclude that physiological cytosolic Mg²⁺ levels affect HACC gating and that channel opening requires either high negative voltages (≥−200 mV) or displacement of Mg²⁺ away from the immediate vicinity of the channel. Secondly, based on structural comparisons with a Mg²⁺-sensitive animal inward-rectifying K⁺ channel, we propose that the likely candidate HACCs described here are cyclic nucleotide gated channels (CNGCs), many of which also contain a conserved diacidic Mg²⁺ binding motif within their pores. This conclusion is consistent with the electrophysiological data. Finally, we propose that Mg²⁺, much like in animal cells, is an important component in Ca²⁺ signalling and homeostasis in plants.

Computational Insights Into Voltage Dependence of Polyamine Block in a Strong Inwardly Rectifying K+ Channel

Inwardly rectifying potassium (K_IR) channels play important roles in controlling cellular excitability and K⁺ ion homeostasis. Under physiological conditions, K_IR channels allow large K⁺ influx at potentials negative to the equilibrium potential of K⁺ but permit little outward current at potentials positive to the equilibrium potential of K⁺, due to voltage dependent block of outward K⁺ flux by cytoplasmic polyamines. These polycationic molecules enter the K_IR channel pore from the intracellular side. They block K⁺ ion movement through the channel at depolarized potentials, thereby ensuring, for instance, the long plateau phase of the cardiac action potential. Key questions concerning how deeply these charged molecules migrate into the pore and how the steep voltage dependence arises remain unclear. Recent MD simulations on GIRK2 (=Kir3.2) crystal structures have provided unprecedented details concerning the conduction mechanism of a K_IR channel. Here, we use MD simulations with applied field to provide detailed insights into voltage dependent block of putrescine, using the conductive state of the strong inwardly rectifying K⁺ channel GIRK2 as starting point. Our µs long simulations elucidate details about binding sites of putrescine in the pore and suggest that voltage-dependent rectification arises from a dual mechanism.

Striatal Kir2 K+ channel inhibition mediates the antidyskinetic effects of amantadine

Levodopa-induced dyskinesia (LID) poses a significant health care challenge for Parkinson's disease (PD) patients. Amantadine is currently the only drug proven to alleviate LID. Although its efficacy in treating LID is widely assumed to be mediated by blockade of N-methyl-D-aspartate (NMDA) glutamate receptors, our experiments demonstrate that at therapeutically relevant concentrations, amantadine preferentially blocks inward-rectifying K+ channel type 2 (Kir2) channels in striatal spiny projection neurons (SPNs) - not NMDA receptors. In so doing, amantadine enhances dendritic integration of excitatory synaptic potentials in SPNs and enhances - not antagonizes - the induction of long-term potentiation (LTP) at excitatory, axospinous synapses. Taken together, our studies suggest that the alleviation of LID in PD patients is mediated by diminishing the disparity in the excitability of direct- and indirect-pathway SPNs in the on state, rather than by disrupting LTP induction. This insight points to a pharmacological approach that could be used to effectively ameliorate LID and improve the quality of life for PD patients.

Desensitization of NMDA channels requires ligand binding to both GluN1 and GluN2 subunits to constrict the pore beside the activation gate

N-methyl-D-aspartate (NMDA) receptor channels are activated by glutamate (or NMDA) and glycine. The channels also undergo desensitization, which denotes decreased channel availability, after prolonged exposure to the activating ligands. Glycine apparently has a paradoxical negative effect on desensitization, as the increase in ambient glycine in concentrations required for channel activation would increase sustained NMDA receptor currents. We hypothesized that this classical "glycine-dependent desensitization" could be glycine-dependent activation in essence. By performing electrophysiological recordings and biophysical analyses with rat brain NMDA receptors heterogeneously expressed in Xenopus laevis oocytes, we characterized that the channel opened by "only" NMDA (in nominally glycine-free condition probably with the inevitable nanomolar glycine) would undergo a novel form of deactivation rather than desensitization, and is thus fully available for subsequent activation. Moreover, external tetrapentylammonium ions (TPentA), tetrabutylammonium ions, and tetrapropylammonium ions (TPA, in higher concentrations) block the pore and prohibit channel desensitization with a simple "foot-in-the-door" hindrance effect. TpentA and TPA have the same voltage dependence but show different flow dependence in binding affinity, revealing a common binding site at an electrical distance of ~0.7 from the outside yet differential involvement of the flux-coupling region in the external pore mouth. The smaller tetraethylammonium ion and the larger tetrahexylammonium and tetraheptylammonium ions may block the channel but could not affect desensitization. We conclude that NMDA receptor desensitization requires concomitant binding of both glycine and glutamate, and thus movement of both GluN1 and GluN2 subunits. Desensitization gate itself embodies a highly restricted pore reduction with a physical distance of ~4 Å from the charged nitrogen atom of bound tetraalkylammonium ions, and is located very close to the activation gate in the bundle-crossing region in the external pore vestibule.

Revealing the ultrastructure of the membrane pores of intact Serratia marcescens cells by atomic force microscopy

This study aimed to characterize the surface ultrastructure of intact Serratia marcescens cells under physiological conditions. Topographic information of membrane pores of the cells was obtained by atomic force microscope (AFM). Three types of membrane pores (CH-1-Pore A, CH-1-Pore B and CH-1-Pore C) were observed and the spatial arrangements of membrane-spanning subunits in membranes were defined. High-resolution images revealed that the doughnut-shaped structures of CH-1-Pore A and CH-1-Pore B were composed of six-to-eight and four transmembrane subunits. The inverted teepee-shaped structure of CH-1-Pore C was segmented into two transmembrane subunits straddling a single funnel-like pore. This study, to the best of authors' knowledge, represents the first direct characterization of the surface ultrastructure of the membrane pores of Serratia marcescens CH-1 cells at the nanometer scale and offers new prospects of mapping membrane pores on intact prokaryotic cells.

Potassium channel selectivity filter dynamics revealed by single-molecule FRET

Potassium (K) channels exhibit exquisite selectivity for conduction of K⁺ ions over other cations, particularly Na⁺. High-resolution structures reveal an archetypal selectivity filter (SF) conformation in which dehydrated K⁺ ions, but not Na⁺ ions, are perfectly coordinated. Using single-molecule FRET (smFRET), we show that the SF-forming loop (SF-loop) in KirBac1.1 transitions between constrained and dilated conformations as a function of ion concentration. The constrained conformation, essential for selective K⁺ permeability, is stabilized by K⁺ but not Na⁺ ions. Mutations that render channels nonselective result in dilated and dynamically unstable conformations, independent of the permeant ion. Further, while wild-type KirBac1.1 channels are K⁺ selective in physiological conditions, Na⁺ permeates in the absence of K⁺. Moreover, whereas K⁺ gradients preferentially support ⁸⁶Rb⁺ fluxes, Na⁺ gradients preferentially support ²²Na⁺ fluxes. This suggests differential ion selectivity in constrained versus dilated states, potentially providing a structural basis for this anomalous mole fraction effect.

Domain insertion permissibility-guided engineering of allostery in ion channels

Allostery is a fundamental principle of protein regulation that remains hard to engineer, particularly in membrane proteins such as ion channels. Here we use human Inward Rectifier K⁺ Channel Kir2.1 to map site-specific permissibility to the insertion of domains with different biophysical properties. We find that permissibility is best explained by dynamic protein properties, such as conformational flexibility. Several regions in Kir2.1 that are equivalent to those regulated in homologs, such as G-protein-gated inward rectifier K⁺ channels (GIRK), have differential permissibility; that is, for these sites permissibility depends on the structural properties of the inserted domain. Our data and the well-established link between protein dynamics and allostery led us to propose that differential permissibility is a metric of latent allosteric capacity in Kir2.1. In support of this notion, inserting light-switchable domains into sites with predicted latent allosteric capacity renders Kir2.1 activity sensitive to light.

Allostery is a fundamental principle of protein regulation that remains challenging to engineer. Here authors screen human Inward Rectifier K + Channel Kir2.1 for permissibility to domain insertions and propose that differential permissibility is a metric of latent allosteric capacity in Kir2.1.

Polyamines and potassium channels: A 25-year romance

Potassium channels that exhibit the property of inward rectification (Kir channels) are present in most cells. Cloning of the first Kir channel genes 25 years ago led to recognition that inward rectification is a consequence of voltage-dependent block by cytoplasmic polyamines, which are also ubiquitously present in animal cells. Upon cellular depolarization, these polycationic metabolites enter the Kir channel pore from the intracellular side, blocking the movement of K⁺ ions through the channel. As a consequence, high K⁺ conductance at rest can provide very stable negative resting potentials, but polyamine-mediated blockade at depolarized potentials ensures, for instance, the long plateau phase of the cardiac action potential, an essential feature for a stable cardiac rhythm. Despite much investigation of the polyamine block, where exactly polyamines get to within the Kir channel pore and how the steep voltage dependence arises remain unclear. This Minireview will summarize current understanding of the relevance and molecular mechanisms of polyamine block and offer some ideas to try to help resolve the fundamental issue of the voltage dependence of polyamine block.

Linkage analysis reveals allosteric coupling in Kir2.1 channels

Inwardly rectifying potassium (Kir) channels experience strong (blocking) and weak (intrinsic) rectification. Linkage analysis in the form of a conductance Hill plot is a sensitive method of resolving allosteric interactions between the pore and mediators of the Kir gating process.

Potassium-selective inward rectifier (Kir) channels are a class of membrane proteins necessary for maintaining stable resting membrane potentials, controlling excitability, and shaping the final repolarization of action potentials in excitable cells. In addition to the strong inward rectification of the ionic current caused by intracellular blockers, Kir2.1 channels possess “weak” inward rectification observed in inside-out patches after prolonged washout of intracellular blockers. The mechanisms underlying strong inward rectification have been attributed to voltage-dependent block by intracellular Mg²⁺ and polyamines; however, the mechanism responsible for weak rectification remains elusive. Hypotheses include weak voltage-dependent block and intrinsic voltage-dependent gating. Here, we performed a conductance Hill analysis of currents recorded with a double-ramp protocol to evaluate different mechanisms proposed for weak inward rectification of Kir2.1 channels. Linkage analysis in the form of a Hill plot revealed that the ramp currents could be best explained by allosteric coupling between a mildly voltage-dependent pore gate (gating charge ∼0.18 e_o) and a voltage sensor (gating charge ∼1.7 e_o). The proposed voltage sensor stabilized the closing of the pore gate (coupling factor ∼31). We anticipate that the use of linkage analysis will broaden understanding of functional coupling in ion channels and proteins in general.

External K+ dependence of strong inward rectifier K+ channel conductance is caused not by K+ but by competitive pore blockade by external Na

Strong inward rectifier K⁺ (sKir) channels determine the membrane potentials of many types of excitable and nonexcitable cells, most notably the resting potentials of cardiac myocytes. They show little outward current during membrane depolarization (i.e., strong inward rectification) because of the channel blockade by cytoplasmic polyamines, which depends on the deviation of the membrane potential from the K⁺ equilibrium potential (V - E_K) when the extracellular K⁺ concentration ([K⁺]_out) is changed. Because their open-channel conductance is apparently proportional to the "square root" of [K⁺]_out, increases/decreases in [K⁺]_out enhance/diminish outward currents through sKir channels at membrane potentials near their reversal potential, which also affects, for example, the repolarization and action-potential duration of cardiac myocytes. Despite its importance, however, the mechanism underlying the [K⁺]_out dependence of the open sKir channel conductance has remained elusive. By studying Kir2.1, the canonical member of the sKir channel family, we first show that the outward currents of Kir2.1 are observed under the external K⁺-free condition when its inward rectification is reduced and that the complete inhibition of the currents at 0 [K⁺]_out results solely from pore blockade caused by the polyamines. Moreover, the noted square-root proportionality of the open sKir channel conductance to [K⁺]_out is mediated by the pore blockade by the external Na⁺, which is competitive with the external K⁺ Our results show that external K⁺ itself does not activate or facilitate K⁺ permeation through the open sKir channel to mediate the apparent external K⁺ dependence of its open channel conductance. The paradoxical increase/decrease in outward sKir channel currents during alternations in [K⁺]_out, which is physiologically relevant, is caused by competition from impermeant extracellular Na^.

Role of renal vascular potassium channels in physiology and pathophysiology

The control of renal vascular tone is important for the regulation of salt and water balance, blood pressure and the protection against damaging elevated glomerular pressure. The K⁺ conductance is a major factor in the regulation of the membrane potential (V_m ) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by V_m via its effect on the opening probability of voltage-operated Ca²⁺ channels (VOCC) in VSMC. When K⁺ conductance increases V_m becomes more negative and vasodilation follows, while deactivation of K⁺ channels leads to depolarization and vasoconstriction. K⁺ channels in EC indirectly participate in the control of vascular tone by endothelium-derived vasodilation. Therefore, by regulating the tone of renal resistance vessels, K⁺ channels have a potential role in the control of fluid homoeostasis and blood pressure as well as in the protection of the renal parenchyma. The main classes of K⁺ channels (calcium activated (K_Ca ), inward rectifier (K_ir ), voltage activated (K_v ) and ATP sensitive (K_ATP )) have been found in the renal vessels. In this review, we summarize results available in the literature and our own studies in the field. We compare the ambiguous in vitro and in vivo results. We discuss the role of single types of K⁺ channels and the integrated function of several classes. We also deal with the possible role of renal vascular K⁺ channels in the pathophysiology of hypertension, diabetes mellitus and sepsis.

Electrophysiological Signature of Homomeric and Heteromeric Glycine Receptor Channels

Glycine receptors are chloride-permeable, ligand-gated ion channels and contribute to the inhibition of neuronal firing in the central nervous system or to facilitation of neurotransmitter release if expressed at presynaptic sites. Recent structure-function studies have provided detailed insights into the mechanisms of channel gating, desensitization, and ion permeation. However, most of the work has focused only on comparing a few isoforms, and among studies, different cellular expression systems were used. Here, we performed a series of experiments using recombinantly expressed homomeric and heteromeric glycine receptor channels, including their splice variants, in the same cellular expression system to investigate and compare their electrophysiological properties. Our data show that the current-voltage relationships of homomeric channels formed by the α2 or α3 subunits change upon receptor desensitization from a linear to an inwardly rectifying shape, in contrast to their heteromeric counterparts. The results demonstrate that inward rectification depends on a single amino acid (Ala(254)) at the inner pore mouth of the channels and is closely linked to chloride permeation. We also show that the current-voltage relationships of glycine-evoked currents in primary hippocampal neurons are inwardly rectifying upon desensitization. Thus, the alanine residue Ala(254) determines voltage-dependent rectification upon receptor desensitization and reveals a physio-molecular signature of homomeric glycine receptor channels, which provides unprecedented opportunities for the identification of these channels at the single cell level.

A synergistic blocking effect of Mg²⁺ and spermine on the inward rectifier K⁺ (Kir2.1) channel pore

Inward rectifier K⁺ channels (Kir2.1) exhibit an extraordinary rectifying feature in the current–voltage relationship. We have previously showed that the bundle–crossing region of the transmembrane domain constitutes the crucial segment responsible for the polyamine block. In this study, we demonstrated that the major blocking effect of intracellular Mg²⁺ on Kir2.1 channels is also closely correlated with K⁺ current flow, and the coupled movements of Mg²⁺ and K⁺ seem to happen in the same flux–coupling segment of the pore as polyamines. With a preponderant outward K⁺ flow, intracellular Mg²⁺ would also be pushed to and thus stay at the outermost site of a flux–coupling segment in the bundle–crossing region of Kir2.1 channels to block the pore, although with a much lower apparent affinity than spermine (SPM). However, in contrast to the evident possibilities of outward exit of SPM through the channel pore especially during strong membrane depolarization, intracellular Mg²⁺ does not seem to traverse the Kir2.1 channel pore in any case. Intracellular Mg²⁺ and SPM therefore may have a synergistic action on the pore–blocking effect, presumably via prohibition of the outward exit of the higher–affinity blocking SPM by the lower–affinity Mg²⁺.

Diverse Kir expression contributes to distinct bimodal distribution of resting potentials and vasotone responses of arterioles

The resting membrane potential (RP) of vascular smooth muscle cells (VSMCs) is a major determinant of cytosolic calcium concentration and vascular tone. The heterogeneity of RPs and its underlying mechanism among different vascular beds remain poorly understood. We compared the RPs and vasomotion properties between the guinea pig spiral modiolar artery (SMA), brain arterioles (BA) and mesenteric arteries (MA). We found: 1) RPs showed a robust bimodal distribution peaked at -76 and -40 mV evenly in the SMA, unevenly at -77 and -51 mV in the BA and ~-71 and -52 mV in the MA. Ba(2+) 0.1 mM eliminated their high RP peaks ~-75 mV. 2) Cells with low RP (~-45 mV) hyperpolarized in response to 10 mM extracellular K(+), while cells with a high RP depolarized, and cells with intermediate RP (~-58 mV) displayed an initial hyperpolarization followed by prolonged depolarization. Moderate high K(+) typically induced dilation, constriction and a dilation followed by constriction in the SMA, MA and BA, respectively. 3) Boltzmann-fit analysis of the Ba(2+)-sensitive inward rectifier K(+) (Kir) whole-cell current showed that the maximum Kir conductance density significantly differed among the vessels, and the half-activation voltage was significantly more negative in the MA. 4) Corresponding to the whole-cell data, computational modeling simulated the three RP distribution patterns and the dynamics of RP changes obtained experimentally, including the regenerative swift shifts between the two RP levels after reaching a threshold. 5) Molecular works revealed strong Kir2.1 and Kir2.2 transcripts and Kir2.1 immunolabeling in all 3 vessels, while Kir2.3 and Kir2.4 transcript levels varied. We conclude that a dense expression of functional Kir2.X channels underlies the more negative RPs in endothelial cells and a subset of VSMC in these arterioles, and the heterogeneous Kir function is primarily responsible for the distinct bimodal RPs among these arterioles. The fast Kir-based regenerative shifts between two RP states could form a critical mechanism for conduction/spread of vasomotion along the arteriole axis.

Inward rectifiers and their regulation by endogenous polyamines

Inwardly-rectifying potassium (Kir) channels contribute to maintenance of the resting membrane potential and regulation of electrical excitation in many cell types. Strongly rectifying Kir channels exhibit a very steep voltage dependence resulting in silencing of their activity at depolarized membrane voltages. The mechanism underlying this steep voltage dependence is blockade by endogenous polyamines. These small multifunctional, polyvalent metabolites enter the long Kir channel pore from the intracellular side, displacing multiple occupant ions as they migrate to a stable binding site in the transmembrane region of the channel. Numerous structure-function studies have revealed structural elements of Kir channels that determine their susceptibility to polyamine block, and enable the steep voltage dependence of this process. In addition, various channelopathies have been described that result from alteration of the polyamine sensitivity or activity of strongly rectifying channels. The primary focus of this article is to summarize current knowledge of the molecular mechanisms of polyamine block, and provide some perspective on lingering uncertainties related to this physiologically important mechanism of ion channel blockade. We also briefly review some of the important and well understood physiological roles of polyamine sensitive, strongly rectifying Kir channels, primarily of the Kir2 family.

Gold nanoparticle-spermidine complex blocks the inward rectifier potassium channel

A previous study showed that negatively charged gold nanoparticles block ion pores by binding to the sulfur group of the cysteine loop of the ion channel when small molecules like amine lead the nanoparticles inside the ion pore. Cells were voltage clamped at -100 mV. Subsequently a bath application of 30 μM Ach produced a current followed by the extracellular application of 100 mM spermidine and 50 nM of nanoparticle complex. Peak amplitude was then recorded. The addition of Ach (30 uM) reversed the effect, and we recorded inhibition of the peak amplitude. We also recorded electrocardiogram (EKG) and the atria effective refractory period (AERP) after treatment with the complex in the atrium of a rabbit heart in a Langendorff apparatus. Upon external application of the complex, the Ach-activated current was blocked by 48.8% ± 3.1% with 82.7% ± 3.1% reversal. In recording the EKG and the AERP after the addition of the complex including 30 mM spermidine with 50 nM nanoparticles, the complete resolution of atrial fibrillation at 50 s and the elongation of AERP from 46 to 52 was observed, which unveils a new class 3 anti arrythmic agent using gold nanoparticles with spermidine. Negatively charged gold nanoparticles (0.8 nm) block ion pores after penetrating the cell membrane with spermidine, thus entering the cells with a polyamine transporter and acting at the intracellular face of the channel via binding to the sulfur group of the human inward rectifying potassium channel- I(KAch).

Multi-ion versus single-ion conduction mechanisms can yield current rectification in biological ion channels

There is clear evidence that the net magnitude of negative charge at the intracellular end of inwardly rectifying potassium channels helps to generate an asymmetry in the magnitude of the current that will pass in each direction. However, a complete understanding of the physical mechanism that links these charges to current rectification has yet to be obtained. Using Brownian dynamics, we compare the conduction mechanism and binding sites in rectifying and non-rectifying channel models. We find that in our models, rectification is a consequence of asymmetry in the hydrophobicity and charge of the pore lining. As a consequence, inward conduction can occur by a multi-ion conduction mechanism. However, outward conduction is restricted, since there are fewer ions at the intracellular entrance and outwardly moving ions must cross the pore on their own. We pose the question as to whether the same mechanism could be at play in inwardly rectifying potassium channels.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Interactions of external K+ and internal blockers in a weak inward-rectifier K+ channel

We investigated the effects of changing extracellular K(+) concentrations on block of the weak inward-rectifier K(+) channel Kir1.1b (ROMK2) by the three intracellular cations Mg(2+), Na(+), and TEA(+). Single-channel currents were monitored in inside-out patches made from Xenopus laevis oocytes expressing the channels. With 110 mM K(+) in the inside (cytoplasmic) solution and 11 mM K(+) in the outside (extracellular) solution, these three cations blocked K(+) currents with a range of apparent affinities (K(i) (0) = 1.6 mM for Mg(2+), 160 mM for Na(+), and 1.8 mM for TEA(+)) but with similar voltage dependence (zδ = 0.58 for Mg(2+), 0.71 for Na(+), and 0.61 for TEA(+)) despite having different valences. When external K(+) was increased to 110 mM, the apparent affinity of all three blockers was decreased approximately threefold with no significant change in the voltage dependence of block. The possibility that the transmembrane cavity is the site of block was explored by making mutations at the N152 residue, a position previously shown to affect rectification in Kir channels. N152D increased the affinity for block by Mg(2+) but not for Na(+) or TEA(+). In contrast, the N152Y mutation increased the affinity for block by TEA(+) but not for Na(+) or Mg(2+). Replacing the C terminus of the channel with that of the strong inward-rectifier Kir2.1 increased the affinity of block by Mg(2+) but had a small effect on that by Na(+). TEA(+) block was enhanced and had a larger voltage dependence. We used an eight-state kinetic model to simulate these results. The effects of voltage and external K(+) could be explained by a model in which the blockers occupy a site, presumably in the transmembrane cavity, at a position that is largely unaffected by changes in the electric field. The effects of voltage and extracellular K(+) are explained by shifts in the occupancy of sites within the selectivity filter by K(+) ions.

Scanning the topography of polyamine blocker binding in an inwardly rectifying potassium channel

Steeply voltage-dependent inward rectification of Kir (inwardly rectifying potassium) channels arises from blockade by cytoplasmic polyamines. These polycationic blockers traverse a long (>70 Å) pore, displacing multiple permeant ions, en route to a high affinity binding site that remains loosely defined. We have scanned the effects of cysteine modification at multiple pore-lining positions on the blocking properties of a library of polyamine analogs, demonstrating that the effects of cysteine modification are position- and blocker-dependent. Specifically, introduction of positively charged adducts results in two distinct phenotypes: either disruption of blocker binding or generation of a barrier to blocker migration, in a consistent pattern that depends on both the length of the polyamine blocker and the position of the modified cysteine. These findings reveal important details about the chemical basis and specific location of high affinity polyamine binding.

Revisiting inward rectification: K ions permeate through Kir2.1 channels during high-affinity block by spermidine

Outward currents through Kir2.1 channels play crucial roles in controlling the electrical properties of excitable cells, and such currents are subjected to voltage-dependent block by intracellular Mg²⁺ and polyamines that bind to both high- and low-affinity sites on the channels. Under physiological conditions, high-affinity block is saturated and yet outward Kir2.1 currents can still occur, implying that high-affinity polyamine block cannot completely eliminate outward Kir2.1 currents. However, the underlying molecular mechanism remains unknown. Here, we show that high-affinity spermidine block, rather than completely occluding the single-channel pore, induces a subconducting state in which conductance is 20% that of the fully open channel. In a D172N mutant lacking the high-affinity polyamine-binding site, spermidine does not induce such a substate. However, the kinetics for the transitions between the substate and zero-current state in wild-type channels is the same as that of low-affinity block in the D172N mutant, supporting the notion that these are identical molecular events. Thus, the residual outward current after high-affinity spermidine block is susceptible to low-affinity block, which determines the final amplitude of the outward current. This study provides a detailed insight into the mechanism underlying the emergence of outward Kir2.1 currents regulated by inward rectification attributed to high- and low-affinity polyamine blocks.

Role of vascular potassium channels in the regulation of renal hemodynamics

K+ conductance is a major determinant of membrane potential ( Vm) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by Vm through the action of voltage-operated Ca2+ channels (VOCC) in VSMC. Increased K+ conductance leads to hyperpolarization and vasodilation, while inactivation of K+ channels causes depolarization and vasoconstriction. K+ channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K+ channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K+ channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K+ channels [calcium activated (KCa), inward rectifier (Kir), voltage activated (KV), and ATP sensitive (KATP)] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K+ channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K+ channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K+ channel function, the role of renal vascular K+ channels in the control of salt and water excretion deserves attention.

Voltage profile along the permeation pathway of an open channel

For ion channels, the transmembrane potential plays a critical role by acting as a driving force for permeant ions. At the microscopic level, the transmembrane potential is thought to decay nonlinearly across the ion permeation pathway because of the irregular three-dimensional shape of the channel's pore. By taking advantage of the current structural and functional understanding of cyclic nucleotide-gated channels, in this study we experimentally explore the transmembrane potential's distribution across the open pore. As a readout for the voltage drop, we engineered cysteine residues along the selectivity filter and scanned the sensitivity of their modification rates by Ag(+) to the transmembrane potential. The experimental data, which indicate that the majority of the electric field drops across the selectivity filter, are in good agreement with continuum electrostatic calculations using a homology model of an open CNG channel. By focusing the transmembrane potential across the selectivity filter, the electromotive driving force is coupled with the movement of permeant ions in the filter, maximizing the efficiency of this process.

Locale and chemistry of spermine binding in the archetypal inward rectifier Kir2.1

Polyamine block of inwardly rectifying potassium (Kir) channels underlies their steep voltage dependence observed in vivo. We have examined the potency, voltage dependence, and kinetics of spermine block in dimeric Kir2.1 constructs containing one nonreactive subunit and one cysteine-substituted subunit before and after modification by methanethiosulfonate (MTS) reagents. At position 169C (between the D172 “rectification controller” and the selectivity filter), modification by either 2-aminoethyl MTS (MTSEA) or 2-(trimethylammonium)ethyl MTS (MTSET) reduced the potency and voltage dependence of spermine block, consistent with this position overlapping the spermine binding site. At position 176C (between D172 and the M2 helix bundle crossing), modification by MTSEA also weakened spermine block. In contrast, MTSET modification of 176C dramatically slowed the kinetics of spermine unblock, with almost no effect on potency or voltage dependence. The data are consistent with MTSET modification of 176C introducing a localized barrier in the inner cavity, resulting in slower spermine entry into and exit from a “deep” binding site (likely between the D172 rectification controller and the selectivity filter), but leaving the spermine binding site mostly unaffected. These findings constrain the location of deep spermine binding that underlies steeply voltage-dependent block, and further suggest important chemical details of high affinity binding of spermine in Kir2.1 channels—the archetypal model of strong inward rectification.

Intrinsic versus extrinsic voltage sensitivity of blocker interaction with an ion channel pore

Many physiological and synthetic agents act by occluding the ion conduction pore of ion channels. A hallmark of charged blockers is that their apparent affinity for the pore usually varies with membrane voltage. Two models have been proposed to explain this voltage sensitivity. One model assumes that the charged blocker itself directly senses the transmembrane electric field, i.e., that blocker binding is intrinsically voltage dependent. In the alternative model, the blocker does not directly interact with the electric field; instead, blocker binding acquires voltage dependence solely through the concurrent movement of permeant ions across the field. This latter model may better explain voltage dependence of channel block by large organic compounds that are too bulky to fit into the narrow (usually ion-selective) part of the pore where the electric field is steep. To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block. The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers. Here, we observe that the voltage dependence of block of a cyclic nucleotide-gated channel by a series of intracellular quaternary ammonium blockers, which are too bulky to traverse the narrow ion selectivity filter, gradually vanishes with extreme depolarization, a predicted feature of the extrinsic voltage dependence model. In contrast, the voltage dependence of block by an amine blocker, which has a smaller "diameter" and can therefore penetrate into the selectivity filter, follows a Boltzmann function, a predicted feature of the intrinsic voltage dependence model. Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

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.

Crystal structure of the eukaryotic strong inward-rectifier K+ channel Kir2.2 at 3.1 A resolution

Inward-rectifier potassium (K+) channels conduct K+ ions most efficiently in one direction, into the cell. Kir2 channels control the resting membrane voltage in many electrically excitable cells, and heritable mutations cause periodic paralysis and cardiac arrhythmia. We present the crystal structure of Kir2.2 from chicken, which, excluding the unstructured amino and carboxyl termini, is 90% identical to human Kir2.2. Crystals containing rubidium (Rb+), strontium (Sr2+), and europium (Eu3+) reveal binding sites along the ion conduction pathway that are both conductive and inhibitory. The sites correlate with extensive electrophysiological data and provide a structural basis for understanding rectification. The channel's extracellular surface, with large structured turrets and an unusual selectivity filter entryway, might explain the relative insensitivity of eukaryotic inward rectifiers to toxins. These same surface features also suggest a possible approach to the development of inhibitory agents specific to each member of the inward-rectifier K+ channel family.

Ionic flow enhances low-affinity binding: a revised mechanistic view into Mg2+ block of NMDA receptors

The N-methyl-d-aspartate receptor (NMDAR) channel is one of the major excitatory amino acid receptors in the mammalian brain. Since external Mg(2+) blocks the channel in an apparently voltage-dependent fashion, this ligand-gated channel displays intriguing voltage-dependent control of Na(+) and Ca(2+) permeability and thus plays an important role in synaptic physiology. We found that the essential features of Mg(2+) block could not be solely envisaged by binding of a charged blocker in the membrane electric field. Instead, the blocking effect of Mg(2+) is critically regulated by, and quantitatively correlated with, the relative tendency of outward and inward ionic fluxes. The 'intrinsic' affinity of Mg(2+) to the binding sites, however, is low (in the millimolar range) in the absence of net ionic flow at 0 mV. Besides, extracellular and intracellular Mg(2+) blocks the channel at distinct sites of electrical distances 0.7 and 0.95 from the outside, respectively. The two sites are separated by a high energy barrier for the movement of Mg(2+) (but not Na(+) or the other ions), and functionally speaking, each could accommodate 1.1 and 0.8 coexisting permeating ions, respectively. Mg(2+) block of the ionic flow thus is greatly facilitated by the flux-coupling effect or the ionic flow (the preponderant direction of permeant ion movement) per se, as if the poorly permeable Mg(2+) is 'pushed' against a high energy barrier by the otherwise permeating ions. Extracellular and intracellular Mg(2+) block then is in essence 'use dependent', more strongly inhibiting both Na(+) and Ca(2+) fluxes with stronger tendencies of influx and efflux, respectively. In conclusion, although permeant ions themselves could compete with Mg(2+), the flow or the tendency of movement of the permeant ions may actually enhance rather than interfere with Mg(2+) block, making the unique current-voltage relationship of NMDAR and the molecular basis of many important neurobiological phenomena.

Physical determinants of strong voltage sensitivity of K(+) channel block

Strong voltage sensitivity of inward-rectifier K⁺ (Kir) channels has been hypothesized to arise primarily from an intracellular blocker displacing up to five K⁺ ions from the wide, intracellular part of the ion conduction pore outwardly across the narrow ion selectivity filter. The validity of this hypothesis depends on two assumptions: i) that five ion sites are located intracellular to the filter, and ii) that the blocker can force essentially unidirectional K⁺ movement in a pore region generally wider than the combined dimensions of the blocker plus a K⁺ ion. Here, we present a crystal structure of the cytoplasmic portion of a Kir channel with five ions bound, and demonstrate that a constriction near the intracellular end of the pore, acting as a gasket, prevents K⁺ ions from bypassing the blocker. This heretofore unrecognized “gasket” ensures that the blocker can effectively displace K⁺ ions across the selectivity filter to generate exceedingly strong voltage sensitivity.

Evidence for the direct interaction of spermine with the inwardly rectifying potassium channel

The inwardly rectifying potassium channel (Kir) regulates resting membrane potential, K+ homeostasis, heart rate, and hormone secretion. The outward current is blocked in a voltage-dependent manner, upon the binding of intracellular polyamines or Mg2+ to the transmembrane pore domain. Meanwhile, electrophysiological studies have shown that mutations of several acidic residues in the intracellular regions affected the inward rectification. Although these acidic residues are assumed to bind polyamines, the functional role of the binding of polyamines and Mg2+ to the intracellular regions of Kirs remains unclear. Here, we report thermodynamic and structural studies of the interaction between polyamines and the cytoplasmic pore of mouse Kir3.1/GIRK1, which is gated by binding of G-protein betagamma-subunit (Gbetagamma). ITC analyses showed that two spermine molecules bind to a tetramer of Kir3.1/GIRK1 with a dissociation constant of 26 microM, which is lower than other blockers. NMR analyses revealed that the spermine binding site is Asp-260 and its surrounding area. Small but significant chemical shift perturbations upon spermine binding were observed in the subunit-subunit interface of the tetramer, suggesting that spermine binding alters the relative orientations of the four subunits. Our ITC and NMR results postulated a spermine binding mode, where one spermine molecule bridges two Asp-260 side chains from adjacent subunits, with rearrangement of the subunit orientations. This suggests the functional roles of spermine binding to the cytoplasmic pore: stabilization of the resting state conformation of the channel, and instant translocation to the transmembrane pore upon activation through the Gbetagamma-induced conformational rearrangement.

KirBac1.1: it's an inward rectifying potassium channel

KirBac1.1 is a prokaryotic homologue of eukaryotic inward rectifier potassium (Kir) channels. The crystal structure of KirBac1.1 and related KirBac3.1 have now been used extensively to generate in silico models of eukaryotic Kir channels, but functional analysis has been limited to (86)Rb(+) flux experiments and bacteria or yeast complementation screens, and no voltage clamp analysis has been available. We have expressed pure full-length His-tagged KirBac1.1 protein in Escherichia coli and obtained voltage clamp recordings of recombinant channel activity in excised membrane patches from giant liposomes. Macroscopic currents of wild-type KirBac1.1 are K(+) selective and spermine insensitive, but blocked by Ba(2+), similar to "weakly rectifying" eukaryotic Kir1.1 and Kir6.2 channels. The introduction of a negative charge at a pore-lining residue, I138D, generates high spermine sensitivity, similar to that resulting from the introduction of a negative charge at the equivalent position in Kir1.1 or Kir6.2. KirBac1.1 currents are also inhibited by PIP(2), consistent with (86)Rb(+) flux experiments, and reversibly inhibited by short-chain di-c8-PIP(2). At the single-channel level, KirBac1.1 channels show numerous conductance states with two predominant conductances (15 pS and 32 pS at -100 mV) and marked variability in gating kinetics, similar to the behavior of KcsA in recombinant liposomes. The successful patch clamping of KirBac1.1 confirms that this prokaryotic channel behaves as a bona fide Kir channel and opens the way for combined biochemical, structural, and electrophysiological analysis of a tractable model Kir channel, as has been successfully achieved for the archetypal K(+) channel KcsA.

K+ binding in the G-loop and water cavity facilitates Ba2+ movement in the Kir2.1 channel

K+ are selectively coordinated in the selectivity filter and concerted K+ and water movements in this region ensure high conduction rates in K+ channels. In channels with long pores many K+ binding sites are located intracellular to the selectivity filter (inner vestibule), but their contribution to permeation has not been well studied. We investigated this phenomenon by slowing the ion permeation process via blocking inwardly rectifying Kir2.1 channels with Ba2+ in the selectivity filter and observing the effect of K+ in the inner vestibule on Ba2+ exit. The dose-response effect of the intracellular K+ concentration ([K+]i) on Ba2+ exit was recorded with and without intracellular polyamines, which compete with K+ for binding sites. Ba2+ exit was facilitated by the cooperative binding of at least three K+. Site-directed mutagenesis studies suggest that K+ interacting with Ba2+ bound in the selectivity filter were located in the region between selectivity filter and cytoplasmic pore, i.e. the water cavity and G-loop. One of the K+ binding sites was located at residue D172 and another was possibly at M301. This study provides functional evidence for the three K+ binding sites in the inner vestibule previously identified by crystal structure study.

Blocker protection by short spermine analogs: refined mapping of the spermine binding site in a Kir channel

Strongly inwardly rectifying potassium channels are blocked by intracellular polyamines with a uniquely steep voltage dependence. An understanding of the fundamental details underlying the voltage dependence of polyamine block requires a constrained structural description of the polyamine-binding site. With this goal in mind, we previously used a "blocker protection" approach to examine the effects of polyamine occupancy on the rate of MTSEA modification of cysteine residues located at pore-lining sites in a strongly rectifying Kir channel (Kir6.2[N160D]). In the study presented here, we focused this strategy to characterize the effects of polyamine analogs that are similar in size to spermine on the rate of MTSEA modification. The observed protection profile of spermine is identical to that previously reported, with spermine occupancy inhibiting MTSEA modification of residue 157C, which is deep in the Kir pore, but having little effect on modification rates of 164C or 169C, closer to the intracellular side of the inner cavity. Remarkably, slightly longer synthetic spermine analogs (BE-spermine, CGC-11098) significantly increased the protection observed at position 164C. The extended protection profile observed with slightly extended polyamine analogs significantly enhances the resolution of our previous mapping efforts using the blocker protection approach, by eliminating uncertainties regarding the blocked conformations of the much longer polyamines that were used in earlier studies. For all short polyamine analogs examined, modification at the entrance to the inner cavity (169C) was unaffected by blocker occupancy, although blocker dissociation was dramatically slowed by partial modification of this site. These data support the validity of a blocker protection approach for mapping polyamine-binding sites in a Kir pore, and confirm that spermine binds stably at a deep site in the inner cavity of strongly rectifying Kir channels.

Contributions of HERG K+ current to repolarization of the human ventricular action potential

Action potential repolarization in the mammalian heart is governed by interactions of a number of time- and voltage-dependent channel-mediated currents, as well as contributions from the Na+/Ca2+ exchanger and the Na+/K+ pump. Recent work has shown that one of the K+ currents (HERG) which contributes to repolarization in mammalian ventricle is a locus at which a number of point mutations can have significant functional consequences. In addition, the remarkable sensitivity of this K+ channel isoform to inhibition by a variety of pharmacological agents and clinical drugs has resulted in HERG being a major focus for Safety Pharmacology requirements. For these reasons we and others have attempted to define the functional role for HERG-mediated K+ currents in repolarization of the action potential in the human ventricle. Here, we describe and evaluate changes in the formulations for two K+ currents, IK1 and HERG (or IK,r), within the framework of ten Tusscher model of the human ventricular action potential. In this computational study, new mathematical formulations for the two nonlinear K+ conductances, IK1 and HERG, have been developed based upon experimental data obtained from electrophysiological studies of excised human ventricular tissue and/or myocytes. The resulting mathematical model provides much improved simulations of the relative sizes and time courses of the K+ currents which modulate repolarization. Our new formulation represents an important first step in defining the mechanism(s) of repolarization of the membrane action potential in the human ventricle. Our overall goal is to understand the genesis of the T-wave of the human electrocardiogram.

Low-affinity spermine block mediating outward currents through Kir2.1 and Kir2.2 inward rectifier potassium channels

The outward component of the strong inward rectifier K(+) current (I(Kir)) plays a pivotal role in polarizing the membranes of excitable and non-excitable cells and is regulated by voltage-dependent channel block by internal cations. Using the Kir2.1 channel, we previously showed that a small fraction of the conductance susceptible only to a low-affinity mode of block likely carries a large portion of the outward current. To further examine the relevance of the low-affinity block to outward I(Kir) and to explore its molecular mechanism, we studied the block of the Kir2.1 and Kir2.2 channels by spermine, which is the principal Kir2 channel blocker. Current-voltage relations of outward Kir2.2 currents showed a peak, a plateau and two peaks in the presence of 10, 1 and 0.1 microm spermine, respectively, which was explained by the presence of two conductances that differ in their susceptibility to spermine block. When the current-voltage relations showed one peak, like those of native I(Kir), outward Kir2.2 currents were mediated mostly by the conductance susceptible to the low-affinity block. They also flowed in a narrower range than the corresponding Kir2.1 currents, because of 3- to 4-fold greater susceptibility to the low-affinity block than in Kir2.1. Reducing external [K(+)] shifted the voltage dependences of both the high- and low-affinity block of Kir2.1 in parallel with the shift in the reversal potential, confirming the importance of the low-affinity block in mediating outward I(Kir). When Kir2.1 mutants known to have reduced sensitivity to internal blockers were examined, the D172N mutation in the transmembrane pore region made almost all of the conductance susceptible only to low-affinity block, while the E224G mutation in the cytoplasmic pore region reduced the sensitivity to low-affinity block without markedly altering that to the high-affinity block or the high/low conductance ratio. The effects of these mutations support the hypothesis that Kir2 channels exist in two states having different susceptibilities to internal cationic blockers.

The role of the cytoplasmic pore in inward rectification of Kir2.1 channels

Steeply voltage-dependent block by intracellular polyamines underlies the strong inward rectification properties of Kir2.1 and other Kir channels. Mutagenesis studies have identified several negatively charged pore-lining residues (D172, E224, and E299, in Kir2.1) in the inner cavity and cytoplasmic domain as determinants of the properties of spermine block. Recent crystallographic determination of the structure of the cytoplasmic domains of Kir2.1 identified additional negatively charged residues (D255 and D259) that influence inward rectification. In this study, we have characterized the kinetic and steady-state properties of spermine block in WT Kir2.1 and in mutations of the D255 residue (D255E, A, K, R). Despite minimal effects on steady-state blockade by spermine, D255 mutations have profound effects on the blocking kinetics, with D255A marginally, and D255R dramatically, slowing the rate of block. In addition, these mutations result in the appearance of a sustained current (in the presence of spermine) at depolarized voltages. These features are reproduced with a kinetic model consisting of a single open state, two sequentially linked blocked states, and a slow spermine permeation step, with residue D255 influencing the spermine affinity and rate of entry into the shallow blocked state. The data highlight a “long-pore” effect in Kir channels, and emphasize the importance of considering blocker permeation when assessing the effects of mutations on apparent blocker affinity.

On the mechanism of TBA block of the TRPV1 channel

The transient receptor potential vanilloid 1 (TRPV1) channel is a nonselective cation channel activated by capsaicin and responsible for thermosensation. To date, little is known about the gating characteristics of these channels. Here we used tetrabutylammonium (TBA) to determine whether this molecule behaves as an ion conduction blocker in TRPV1 channels and to gain insight into the nature of the activation gate of this protein. TBA belongs to a family of classic potassium channel blockers that have been widely used as tools for determining the localization of the activation gate and the properties of the pore of several ion channels. We found TBA to be a voltage-dependent pore blocker and that the properties of block are consistent with an open-state blocker, with the TBA molecule binding to multiple open states, each with different blocker affinities. Kinetics of channel closure and burst-length analysis in the presence of blocker are consistent with a state-dependent blocking mechanism, with TBA interfering with closing of an activation gate. This activation gate may be located cytoplasmically with respect to the binding site of TBA ions, similar to what has been observed in potassium channels. We propose an allosteric model for TRPV1 activation and block by TBA, which explains our experimental data.

Crystallographic study of the tetrabutylammonium block to the KcsA K+ channel

K(+) channels play essential roles in regulating membrane excitability of many diverse cell types by selectively conducting K(+) ions through their pores. Many diverse molecules can plug the pore and modulate the K(+) current. Quaternary ammonium (QA) ions are a class of pore blockers that have been used for decades by biophysicists to probe the pore, leading to important insights into the structure-function relation of K(+) channels. However, many key aspects of the QA-blocking mechanisms remain unclear to date, and understanding these questions requires high resolution structural information. Here, we address the question of whether intracellular QA blockade causes conformational changes of the K(+) channel selectivity filter. We have solved the structures of the KcsA K(+) channel in complex with tetrabutylammonium (TBA) and tetrabutylantimony (TBSb) under various ionic conditions. Our results demonstrate that binding of TBA or TBSb causes no significant change in the KcsA structure at high concentrations of permeant ions. We did observe the expected conformational change of the filter at low concentration of K(+), but this change appears to be independent of TBA or TBSb blockade.

Permeant ion effects on external Mg2+ block of NR1/2D NMDA receptors

Voltage-dependent channel block by external Mg2+ (Mg2+(o)) of NMDA receptors is an essential determinant of synaptic function. The resulting Mg2+(o) inhibition of NMDA responses depends strongly on receptor subunit composition: NR1/2A and NR1/2B receptors are more strongly inhibited by Mg2+(o) than are NR1/2C or NR1/2D receptors. Previous work showed that permeant ions have profound effects on Mg2+(o) block of NMDA receptors composed of NR1, NR2A, and NR2B subunits. Whether permeant ions affect Mg2+(o) inhibition of NR1/2C or NR1/2D receptors is unknown. We investigated the effects of permeant ions on Mg2+(o) block of NR1/2D receptors by integrating results from whole-cell recordings, single-channel recordings, and kinetic modeling. Lowering internal [Cs+] caused a voltage-dependent decrease in the Mg2+(o) IC50 and in the apparent Mg2+(o) unblocking rate, and increase in the apparent Mg2+(o) blocking rate (k(+,app)) of NR1/2D receptors. Lowering external [Na+] caused modest voltage-dependent changes in the Mg2+(o) IC50 and k(+,app). These data can be explained by a kinetic model in which occupation of either of two external permeant ion binding sites prevents Mg2+(o) entry into the channel. Occupation of an internal permeant ion binding site prevents Mg2+(o) permeation and accelerates Mg2+(o) unblock to the external solution. We conclude that variations in permeant ion site properties shape the NR2 subunit dependence of Mg2+(o) block. Furthermore, the external channel entrance varies little among NMDA receptor subtypes. Differences in the Mg2+(o) blocking site, and particularly in the selectivity filter and internal channel entrance, are principally responsible for the subunit dependence of Mg2+(o) block.

A Kir2.3-like K+ Conductance in Mouse Cortical Collecting Duct Principal Cells

K(+) channels play an important role in renal collecting duct cell function. The current study examined barium (Ba(2+))-sensitive whole-cell K(+) currents (IKBa) in mouse isolated collecting duct principal cells. IKBa demonstrated strong inward rectification and was inhibited by Ba(2+) in a dose- and voltage-dependent fashion, with the K (d) decreasing with hyperpolarization. The electrical distance of block by Ba(2+) was around 8.5%. As expected for voltage-dependent inhibition, the association constant increased with hyperpolarization, suggesting that the rate of Ba(2+) entry was increased at negative potentials. The dissociation constant also increased with hyperpolarization, consistent with the movement of Ba(2+) ions into the intracellular compartment at negative potentials. These properties are not consistent with ROMK but are consistent with the properties of Kir2.3. Kir2.3 is thought to be the dominant basolateral K(+) channel in principal cells. This study provides functional evidence for the expression of Kir2.3 in mouse cortical collecting ducts and confirms the expression of Kir2.3 in this segment of the renal tubule using reverse-transcriptase polymerase chain reaction. The conductance described here is the first report of a macroscopic K(+) conductance in mouse principal cells that shares the biophysical profile of Kir2.3. The properties and dominant nature of the conductance suggest that it plays an important role in K(+) handling in the principal cells of the cortical collecting duct.

K+ channel selectivity depends on kinetic as well as thermodynamic factors

Potassium channels are necessary for a number of essential biological tasks such as the generation of action potentials and setting the resting membrane potential in cells, both of which require that these channels selectively permit the passage of potassium ions while suppressing the flow of other ions. Generally, this selectivity is attributed to a narrow stretch of the channel known as the selectivity filter. Over this stretch ions are dehydrated, and the backbone oxygen atoms of the protein mimic the ion's loss of coordination by water. However, channels are long pores with spatially distinct ion-binding sites that all must be traversed during ion permeation. We have shown that selectivity of mutant Kir3.2 (GIRK2) channels can be substantially amplified by introducing acidic residues into the cavity, a binding site below the selectivity filter. Here, we carry out electrostatic calculations on homology models to quantify the degree of stabilization that these mutations have on ions in the cavity. We then construct a multiion model of ion permeation to calculate the channel's permeability to potassium relative to sodium. This kinetic model uses rates derived from the electrostatic calculations and demonstrates that nonselective electrostatic stabilization of cations in the cavity can amplify channel selectivity independently of the selectivity filter. This nonintuitive result highlights the dependence of channel properties on the entire channel architecture and suggests that selectivity may not be fully understood by focusing solely on thermodynamic considerations of ion dehydration and the energetics of the selectivity filter.

Electrostatic interactions in the channel cavity as an important determinant of potassium channel selectivity

Potassium channels are membrane proteins that allow the passage of potassium ions at near diffusion rates while severely limiting the flux of the slightly smaller sodium ions. Although studies thus far have focused on the narrowest part of the channel, known as the selectivity filter, channels are long pores with multiple ions that traverse the selectivity filter, the water-filled central cavity, and the rest of the pore formed by cytoplasmic domains. Here, we present experimental analyses on Kir3.2 (GIRK2), a G protein-activated inwardly rectifying potassium (Kir) channel, showing that a negative charge introduced at a pore-facing position in the cavity (N184) below the selectivity filter restores both K(+) selectivity and inward rectification properties to the nonselective S177W mutant channel. Molecular modeling demonstrates that the negative residue has no effect on the geometry of the selectivity filter, suggesting that it has a local effect on the cavity ion. Moreover, restoration of selectivity does not depend on the exact location of the charge in the central cavity as long as this residue faces the pore, where it is in close contact with permeant ions. Our results indicate that interactions between permeant ions and the channel cavity can influence ion selectivity and channel block by means of an electrostatic effect.

Inward rectifying potassium channels facilitate cell-to-cell communication in hamster retractor muscle feed arteries

This study examined whether inward rectifying K+(KIR) channels facilitate cell-to-cell communication along skeletal muscle resistance arteries. With the use of feed arteries from the hamster retractor muscle, experiments examined whether KIRchannels were functionally expressed and whether channel blockade attenuated the conduction of acetylcholine-induced vasodilation, an index of cell-to-cell communication. Consistent with KIRchannel expression, this study observed the following: 1) a sustained Ba2+-sensitive, K+-induced dilation in preconstricted arteries; 2) a Ba2+-sensitive inwardly rectifying K+current in arterial smooth muscle cells; and 3) KIR2.1 and KIR2.2 expression in the smooth muscle layer of these arteries. It was subsequently shown that the discrete application of acetylcholine elicits a vasodilation that conducts with limited decay along the feed artery wall. In the presence of 100 μM Ba2+, the local and conducted response to acetylcholine was attenuated, a finding consistent with a role for KIRin facilitating cell-to-cell communication. A computational model of vascular communication accurately predicted these observations. Control experiments revealed that in contrast to Ba2+, ATP-sensitive- and large-conductance Ca2+activated-K+channel inhibitors had no effect on the local or conducted vasodilatory response to acetylcholine. We conclude that smooth muscle KIRchannels play a key role in facilitating cell-to-cell communication along skeletal muscle resistance arteries. We attribute this facilitation to the intrinsic property of negative slope conductance, a biophysical feature common to KIR2.1- and 2.2-containing channels, which enables them to increase their activity as a cell hyperpolarizes.

Nerve excitability testing and its clinical application to neuromuscular diseases

Non-invasive nerve excitability testing measures the membrane polarization, ion channel function and paranodal/internodal condition of peripheral nerves. This technique has been recently used for various neuromuscular disorders, such as pure motor conduction block in multifocal motor neuropathy, conduction block in carpal tunnel syndrome and Na(+) channel function disorders in diabetic neuropathy, to shed light on their pathophysiology. Here, we review the basics of ion channel functions and membrane properties that influence nerve excitability, the basic principles of nerve excitability testing and the reported findings in various disorders.