Voltage-dependent magnesium block of adenosine-triphosphate-sensitive potassium channel in guinea-pig ventricular cells

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Inward and outward currents of native and cloned K(ATP) channels (Kir6.2/SUR1) share single-channel kinetic properties

Background

Methods

Results

Conclusions

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Outward K(ATP) channel currents are not grouped in bursts regardless of membrane potential.

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No differences in the number of kinetic states could be seen for single channels between native K(ATP) channels, Kir6.2ΔC26 alone and co-expressed with SUR1 for outward currents.

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K(ATP) channel open time for outward currents corresponds to burst duration for inward currents.

Characterization of Magnesium Sulfate as an Antiarrhythmic Agent

Background

Recently, intravenous magnesium therapy has been used for the treatment of ventricular arrhythmias, but data to establish a causal link between the electrophysiological properties and the antiarrhythmic actions are lacking.

Methods and Results

The acute antiarrhythmic effect of magnesium sulfate was assessed using epinephrine-, digitalis-, and coronary ligation-induced canine ventricular arrhythmia models. The intravenous administration of magnesium sulfate (100 mg/kg) reduced the incidence of the ventricular arrhythmias of all models. The antiarrhythmic effect on the epinephrine-induced arrhythmia was potent and long-lasting, while those on the other arrhythmia models were weak and transient. The direct cardiovascular effects were assessed using the canine isolated, blood-perfused sinus node, papillary muscle, and atrioventricular node preparations. The intracoronary administration of magnesium sulfate (0.1–30 mg) suppressed sinoatrial automaticity and ventricular contraction, while it increased atrio-His and His-ventricular conduction time, coronary blood flow, and the duration of monophasic action potential in a dose-dependent manner. The effects on His-ventricular conduction and monophasic action potential duration were less potent compared with the other cardiovascular effects.

Conclusion

These results suggest that magnesium sulfate possesses multiple electrophysiological properties and that the effects related to the calcium channel inhibition may be the most relevant for the antiarrhythmic actions.

Synaptic cleft microenvironment influences potassium permeation and synaptic transmission in hair cells surrounded by calyx afferents in the turtle

KEY POINTS

In central regions of vestibular semicircular canal epithelia, the [K⁺ ] in the synaptic cleft ([K⁺ ]_c ) contributes to setting the hair cell and afferent membrane potentials; the potassium efflux from type I hair cells results from the interdependent gating of three conductances. Elevation of [K⁺ ]_c occurs through a calcium-activated potassium conductance, G_BK , and a low-voltage-activating delayed rectifier, G_K(LV) , that activates upon elevation of [K⁺ ]_c . Calcium influx that enables quantal transmission also activates I_BK , an effect that can be blocked internally by BAPTA, and externally by a Ca_V 1.3 antagonist or iberiotoxin. Elevation of [K⁺ ]_c or chelation of [Ca²⁺ ]_c linearizes the G_K(LV) steady-state I-V curve, suggesting that the outward rectification observed for G_K(LV) may result largely from a potassium-sensitive relief of Ca²⁺ inactivation of the channel pore selectivity filter. Potassium sensitivity of hair cell and afferent conductances allows three modes of transmission: quantal, ion accumulation and resistive coupling to be multiplexed across the synapse.

ABSTRACT

In the vertebrate nervous system, ions accumulate in diffusion-limited synaptic clefts during ongoing activity. Such accumulation can be demonstrated at large appositions such as the hair cell-calyx afferent synapses present in central regions of the turtle vestibular semicircular canal epithelia. Type I hair cells influence discharge rates in their calyx afferents by modulating the potassium concentration in the synaptic cleft, [K⁺ ]_c , which regulates potassium-sensitive conductances in both hair cell and afferent. Dual recordings from synaptic pairs have demonstrated that, despite a decreased driving force due to potassium accumulation, hair cell depolarization elicits sustained outward currents in the hair cell, and a maintained inward current in the afferent. We used kinetic and pharmacological dissection of the hair cell conductances to understand the interdependence of channel gating and permeation in the context of such restricted extracellular spaces. Hair cell depolarization leads to calcium influx and activation of a large calcium-activated potassium conductance, G_BK , that can be blocked by agents that disrupt calcium influx or buffer the elevation of [Ca²⁺ ]_i , as well as by the specific K_Ca 1.1 blocker iberiotoxin. Efflux of K⁺ through G_BK can rapidly elevate [K⁺ ]_c , which speeds the activation and slows the inactivation and deactivation of a second potassium conductance, G_K(LV) . Elevation of [K⁺ ]_c or chelation of [Ca²⁺ ]_c linearizes the G_K(LV) steady-state I-V curve, consistent with a K⁺ -dependent relief of Ca²⁺ inactivation of G_K(LV) . As a result, this potassium-sensitive hair cell conductance pairs with the potassium-sensitive hyperpolarization-activated cyclic nucleotide-gated channel (HCN) conductance in the afferent and creates resistive coupling at the synaptic cleft.

Hearing loss mutations alter the functional properties of human P2X2 receptor channels through distinct mechanisms

Activation of P2X2 receptor channels by extracellular ATP is thought to play important roles in cochlear adaptation to elevated sound levels and protection from overstimulation. Each subunit of a trimeric P2X2 receptor is composed of intracellular N and C termini, a large extracellular domain containing the ATP binding site and 2 transmembrane helices (TM1 and TM2) that form a cation permeable pore. Whole-exome sequencing and linkage analysis have identified 3 hP2X2 receptor mutations (V60L, D273Y, and G353R) that cause dominantly inherited progressive sensorineural hearing loss (DFNA41). Available structures of related P2X receptors suggest that these 3 mutations localize to TM1 (V60L), TM2 (G353R), or the β-sheet linking the TMs to the extracellular ATP binding sites (D273Y). Previous studies have concluded that the V60L and G353R mutants are nonfunctional, whereas the D273Y mutant has yet to be studied. Here, we demonstrate that both V60L and G353R mutations do form functional channels, whereas the D273Y mutation prevents the expression of functional channels on the cell membrane. Our results show that the V60L mutant forms constitutively active channels that are insensitive to ATP or the antagonist suramin, suggesting uncoupling of the pore and the ligand binding domains. In contrast, the G353R mutant can be activated by ATP but exhibits alterations in sensitivity to ATP, inward rectification, and ion selectivity. Collectively, our results demonstrate that the loss of functional P2X2 receptors or distinct alterations of its functional properties lead to noise-induced hearing loss, highlighting the importance of these channels in preserving hearing.

Stargazin and cornichon-3 relieve polyamine block of AMPA receptors by enhancing blocker permeation

Polyamine block of AMPA-type ionotropic glutamate receptors is attenuated by auxiliary proteins stargazin and cornichon-3. Brown et al. show that relief from block is due to enhanced blocker permeation and present a modified model of permeant channel block to account for their experimental findings.

Most ligand- and voltage-gated ion channels assemble as signaling complexes consisting of pore-forming and auxiliary subunits. In the mammalian brain, AMPA-type ionotropic glutamate receptors (AMPARs) coassemble with several families of auxiliary subunits that regulate channel gating as well as ion channel block and permeation. Previous work has shown that auxiliary proteins stargazin (or γ2) and cornichon-3 (CNIH-3) attenuate the cytoplasmic polyamine channel block of AMPARs, although the underlying mechanism has yet to be established. Here, we show that γ2 and CNIH-3 relieve channel block by enhancing the rate of blocker permeation. Surprisingly, the relative permeability of the polyamine spermine (Spm) through the pore of the AMPAR-γ2 or -CNIH-3 complexes is considerably more than AMPARs expressed alone. Spm permeability is comparable to that of Na⁺ for the GluA2-γ2 complex and four times greater than Na⁺ with GluA2 + CNIH-3. A modified model of permeant channel block fully accounts for both the voltage- and time-dependent nature of Spm block. Estimates of block rate constants reveal that auxiliary subunits do not attenuate block by shifting the location of the block site within the membrane electric field, and they do not affect the blocker’s ability to reach it. Instead, γ2 and CNIH-3 relieve channel block by facilitating the blocker’s exit rates from the open channel. From a physiological perspective, the relief of channel block exerted by γ2 and CNIH-3 ensures that there is unfettered signaling by AMPARs at glutamatergic synapses. Moreover, the pronounced ability of AMPARs to transport polyamines may have an unexpected role in regulating cellular polyamine levels.

Bioelectric gene and reaction networks: computational modelling of genetic, biochemical and bioelectrical dynamics in pattern regulation

Gene regulatory networks (GRNs) describe interactions between gene products and transcription factors that control gene expression. In combination with reaction-diffusion models, GRNs have enhanced comprehension of biological pattern formation. However, although it is well known that biological systems exploit an interplay of genetic and physical mechanisms, instructive factors such as transmembrane potential (Vmem) have not been integrated into full GRN models. Here we extend regulatory networks to include bioelectric signalling, developing a novel synthesis: the bioelectricity-integrated gene and reaction (BIGR) network. Using in silico simulations, we highlight the capacity for Vmem to alter steady-state concentrations of key signalling molecules inside and out of cells. We characterize fundamental feedbacks where Vmem both controls, and is in turn regulated by, biochemical signals and thereby demonstrate Vmem homeostatic control, Vmem memory and Vmem controlled state switching. BIGR networks demonstrating hysteresis are identified as a mechanisms through which more complex patterns of stable Vmem spots and stripes, along with correlated concentration patterns, can spontaneously emerge. As further proof of principle, we present and analyse a BIGR network model that mechanistically explains key aspects of the remarkable regenerative powers of creatures such as planarian flatworms. The functional properties of BIGR networks generate the first testable, quantitative hypotheses for biophysical mechanisms underlying the stability and adaptive regulation of anatomical bioelectric pattern.

ATP-sensitive inwardly rectifying potassium channel modulators alter cardiac function in honey bees

ATP-sensitive inwardly rectifying potassium (K_ATP) channels couple cellular metabolism to the membrane potential of the cell and play an important role in a variety of tissue types, including the insect dorsal vessel, making them a subject of interest not only for understanding invertebrate physiology, but also as a potential target for novel insecticides. Most of what is known about these ion channels is the result of work performed in mammalian systems, with insect studies being limited to only a few species and physiological systems. The goal of this study was to investigate the role that K_ATP channels play in regulating cardiac function in a model social insect, the honey bee (Apis mellifera), by examining the effects that modulators of these ion channels have on heart rate. Heart rate decreased in a concentration-dependent manner, relative to controls, with the application of the K_ATP channel antagonist tolbutamide and K_ATP channel blockers barium and magnesium, whereas heart rate increased with the application of a low concentration of the K_ATP channel agonist pinacidil, but decreased at higher concentrations. Furthermore, pretreatment with barium magnified the effects of tolbutamide treatment and eliminated the effects of pinacidil treatment at select concentrations. The data presented here confirm a role for K_ATP channels in the regulation of honey bee dorsal vessel contractions and provide insight into the underlying physiology that governs the regulation of bee cardiac function.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Mechanism for attenuated outward conductance induced by mutations in the cytoplasmic pore of Kir2.1 channels

Outward currents through Kir2.1 channels regulate the electrical properties of excitable cells. These currents are subject to voltage-dependent attenuation by the binding of polyamines to high- and low-affinity sites, which leads to inward rectification, thereby controlling cell excitability. To examine the effects of positive charges at the low-affinity site in the cytoplasmic pore on inward rectification, we studied a mutant Kir channel (E224K/H226E) and measured single-channel currents and streaming potentials (V_stream), the latter provide the ratio of water to ions queued in a single-file permeation process in the selectivity filter. The water-ion coupling ratio was near one at a high K⁺ concentration ([K⁺]) for the wild-type channel and increased substantially as [K⁺] decreased. On the other hand, fewer ions occupied the selectivity filter in the mutant at all [K⁺]. A model for the Kir channel involving a K⁺ binding site in the wide pore was introduced. Model analyses revealed that the rate constants associated with the binding and release to and from the wide-pore K⁺ binding site was modified in the mutant. These effects lead to the reduced contribution of a conventional two-ion permeation mode to total conductance, especially at positive potentials, thereby inward rectification.

Multiscale computational analysis of the bioelectric consequences of myocardial ischaemia and infarction

Ischaemic heart disease is considered as the single most frequent cause of death, provoking more than 7 000 000 deaths every year worldwide. A high percentage of patients experience sudden cardiac death, caused in most cases by tachyarrhythmic mechanisms associated to myocardial ischaemia and infarction. These diseases are difficult to study using solely experimental means due to their complex dynamics and unstable nature. In the past decades, integrative computational simulation techniques have become a powerful tool to complement experimental and clinical research when trying to elucidate the intimate mechanisms of ischaemic electrophysiological processes and to aid the clinician in the improvement and optimization of therapeutic procedures. The purpose of this paper is to briefly review some of the multiscale computational models of myocardial ischaemia and infarction developed in the past 20 years, ranging from the cellular level to whole-heart simulations.

Barium chloride impaired Kir2.1 inward rectification in its stably transfected HEK 293 cell lines

Kir2.1 channel is a typical inward rectified channel with little outward currents when the membrane depolarized. Barium blocks the inward Kir2.1 currents in a voltage-dependent manner. However, in this study we found that barium would impair the rectification and open Kir2.1 outward currents at a depolarized voltage, causing increment of outward current amplitudes by 43±7% (n=5, P<0.01) after 200s barium application. In the meanwhile, a higher barium concentration did block the outward currents by 17.5±4.3% (n=4, P<0.01) and temporarily twisted current upward tendency. The increment was likely barium specific since both calcium and Kir2.1 specific blocker, Chloroethylclonidine (CEC), did not enhance the current amplitudes. The rectification of Kir2.1 was not recovered by washing barium off, which suggested a non-competitive mechanism. Since the currents occurred at phase 1, 2 of cardiac action potential, it would likely shorten the action potential plateau and it would decrease QT duration in electrocardiography (ECG).

Divalent cations activate TRPV1 through promoting conformational change of the extracellular region

Divalent cations Mg²⁺ and Ba²⁺ selectively and directly potentiate transient receptor potential vanilloid type 1 heat activation by lowering the activation threshold into the room temperature range. We found that Mg²⁺ potentiates channel activation only from the extracellular side; on the intracellular side, Mg²⁺ inhibits channel current. By dividing the extracellularly accessible region of the channel protein into small segments and perturbing the structure of each segment with sequence replacement mutations, we observed that the S1–S2 linker, the S3–S4 linker, and the pore turret are all required for Mg²⁺ potentiation. Sequence replacements at these regions substantially reduced or eliminated Mg²⁺-induced activation at room temperature while sparing capsaicin activation. Heat activation was affected by many, but not all, of these structural alternations. These observations indicate that extracellular linkers and the turret may interact with each other. Site-directed fluorescence resonance energy transfer measurements further revealed that, like heat, Mg²⁺ also induces structural changes in the pore turret. Interestingly, turret movement induced by Mg²⁺ precedes channel activation, suggesting that Mg²⁺-induced conformational change in the extracellular region most likely serves as the cause of channel activation instead of a coincidental or accommodating structural adjustment.

The cochlea magnesium content is negatively correlated with hearing loss induced by impulse noise

OBJECTIVE

Magnesium is proved to attenuate acoustic trauma, and reactive oxygen species (ROS) formation is a critical role that involves hearing loss induced by impulse noise. We aimed to investigate the relationship between the cochlea magnesium content, ROS formation and hearing loss induced by impulse noise.

METHODS

Ninety pigmented guinea pigs were exposed to impulse noise. Auditory thresholds were assessed by sound-evoked auditory brainstem response (ABR) 24h before and 72h after exposure to impulse noise. 4-Hydroxynonenal(HNE) used as a marker of ROS was determined immunohistochemically. The cochlea magnesium content was examined with the method of energy dispersive x-ray analysis, and the cochlea was also detected with scanning electron microscope. The relationship between the cochlea magnesium content, ROS formation and hearing loss was analyzed.

RESULTS

There was loss of outer hair cell cilia accompanying with significant auditory threshold shift after impulse noise exposure. ROS was positive in the organ of Corti of all animals. The cochlea magnesium content was negatively correlated with ROS formation and hearing loss.

CONCLUSIONS

Inhibiting ROS formation is one of the mechanisms for magnesium to reduce acoustic trauma, and difference in cochlea magnesium contents is one of the factors that induce varying degrees of cochlear damage among each individual after acoustic trauma.

Electrodeposition of hydroxyapatite coating on Mg-4.0Zn-1.0Ca-0.6Zr alloy and in vitro evaluation of degradation, hemolysis, and cytotoxicity

A novel biodegradable Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy was successfully produced using a series of metallurgical processes; including melting, casting, rolling, and heat treatment. The hardness and ultimate tensile strength of the alloy sheets increased to 71.2HV and 320 MPa after rolling and then aging for 12 h at 175°C. These mechanical properties were sufficient for load-bearing orthopedic implants. A hydroxyapatite (HA) coating was deposited on the Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy using a novel coating process combining alkali heat pretreatment, electrodeposition, and alkali heat posttreatment. The microstructure, composition, and phases of the Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy and HA coating were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The degradation, hemolysis, and cytocompatibility of the HA-coated and uncoated Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy were studied in vitro. The corrosion potential (E(corr)) of Mg-4.0Zn-1.0Ca-0.6Zr alloy (-1.72 V) was higher than Mg (-1.95 V), Mg-0.6Ca alloy (-1.91 V) and Mg-1.0Ca alloy (-1.97 V), indicating the Mg-Zn-Ca-Zr alloy would be more corrosion resistant. The initial corrosion potential of the HA-coated Mg alloy sample (-1.51 V) was higher than the uncoated sample (-1.72 V). The hemolysis rates of the HA-coated and uncoated Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy samples were both <5%, which met the requirements for implant materials. The HA-coated and uncoated Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy samples demonstrated the same cytotoxicity score as the negative control. The HA-coated samples showed a slightly greater relative growth rate (RGR%) of fibroblasts than the uncoated samples. Both the HA-coated and uncoated Mg-4.0Zn-1.0Ca-0.6Zr (wt %) alloy provided evidence of acceptable cytocompatibility for medical applications.

Calmodulin kinase II and protein kinase C mediate the effect of increased intracellular calcium to augment late sodium current in rabbit ventricular myocytes

An increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) augments late sodium current (I(Na.L)) in cardiomyocytes. This study tests the hypothesis that both Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and protein kinase C (PKC) mediate the effect of increased [Ca(2+)](i) to increase I(Na.L). Whole cell and open cell-attached patch clamp techniques were used to record I(Na.L) in rabbit ventricular myocytes dialyzed with solutions containing various concentrations of [Ca(2+)](i). Dialysis of cells with [Ca(2+)](i) from 0.1 to 0.3, 0.6, and 1.0 μM increased I(Na.L) in a concentration-dependent manner from 0.221 ± 0.038 to 0.554 ± 0.045 pA/pF (n = 10, P < 0.01) and was associated with an increase in mean Na(+) channel open probability and prolongation of channel mean open-time (n = 7, P < 0.01). In the presence of 0.6 μM [Ca(2+)](i), KN-93 (10 μM) and bisindolylmaleimide (BIM, 2 μM) decreased I(Na.L) by 45.2 and 54.8%, respectively. The effects of KN-93 and autocamtide-2-related inhibitory peptide II (2 μM) were not different. A combination of KN-93 and BIM completely reversed the increase in I(Na.L) as well as the Ca(2+)-induced changes in Na(+) channel mean open probability and mean open-time induced by 0.6 μM [Ca(2+)](i). Phorbol myristoyl acetate increased I(Na.L) in myocytes dialyzed with 0.1 μM [Ca(2+)](i); the effect was abolished by Gö-6976. In summary, both CaMKII and PKC are involved in [Ca(2+)](i)-mediated augmentation of I(Na.L) in ventricular myocytes. Inhibition of CaMKII and/or PKC pathways may be a therapeutic target to reduce myocardial dysfunction and cardiac arrhythmias caused by calcium overload.

A novel gain-of-function KCNJ2 mutation associated with short-QT syndrome impairs inward rectification of Kir2.1 currents

AIMS

Short-QT syndrome (SQTS) is a recently recognized disorder associated with atrial fibrillation (AF) and sudden death due to ventricular arrhythmias. Mutations in several ion channel genes have been linked to SQTS; however, the mechanism remains unclear. This study describes a novel heterozygous gain-of-function mutation in the inward rectifier potassium channel gene, KCNJ2, identified in SQTS.

METHODS AND RESULTS

We studied an 8-year-old girl with a markedly short-QT interval (QT = 172 ms, QTc = 194 ms) who suffered from paroxysmal AF. Mutational analysis identified a novel heterozygous KCNJ2 mutation, M301K. Functional assays displayed no Kir2.1 currents when M301K channels were expressed alone. However, co-expression of wild-type (WT) with M301K resulted in larger outward currents than the WT at more than -30 mV. These results suggest a gain-of-function type modulation due to decreased inward rectification. Furthermore, we analysed the functional significance of the amino acid charge at M301 (neutral) by changing the residue. As with M301K, in M301R (positive), the homozygous channels were non-functional, whereas the heterozygous channels demonstrated decreased inward rectification. Meanwhile, the currents recorded in M301A (neutral) showed normal inward rectification under both homo- and heterozygous conditions. Heterozygous overexpression of WT and M301K in neonatal rat ventricular myocytes exhibited markedly shorter action potential durations than the WT alone.

CONCLUSION

In this study, we identified a novel KCNJ2 gain-of-function mutation, M301K, associated with SQTS. Functional assays revealed no functional currents in the homozygous channels, whereas impaired inward rectification demonstrated under the heterozygous condition resulted in larger outward currents, which is a novel mechanism predisposing SQTS.

Constitutive expression of a Mg2+-inhibited K+ current and a TRPM7-like current in human erythroleukemia cells

Whole cell patch-clamp experiments were undertaken to define the basal K(+) conductance(s) in human erythroleukemia cells and its contribution to the setting of resting membrane potential. Experiments revealed a non-voltage-activated, noninactivating K(+) current. The magnitude of the current recorded under whole cell conditions was inhibited by an increase in free intracellular Mg(2+) concentration. Activation or inactivation of the Mg(2+)-inhibited K(+) current (MIP) was paralleled by activation or inactivation of a Mg(2+)-inhibited TRPM7-like current displaying characteristics indistinguishable from those reported for molecularly identified TRPM7 current. The MIP and TRPM7 currents were inhibited by 5-lipoxygenase inhibitors. However, inhibition of the MIP current was temporally distinct from inhibition of TRPM7 current, allowing for isolation of the MIP current. Isolation of the MIP conductance revealed a current reversing near the K(+) equilibrium potential, indicative of a highly K(+)-selective conductance. Consistent with this finding, coactivation of the nonselective cation current TRPM7 and the MIP current following dialysis with nominally Mg(2+)-free pipette solution resulted in hyperpolarized whole cell reversal potentials, consistent with an important role for the MIP current in the setting of a negative resting membrane potential. The MIP and TRPM7-like conductances were constitutively expressed under in vivo conditions of intracellular Mg(2+), as judged by their initial detection and subsequent inactivation following dialysis with a pipette solution containing 5 mM free Mg(2+). The MIP current was blocked in a voltage-dependent fashion by extracellular Cs(+) and, to a lesser degree, by Ba(2+) and was blocked by extracellular La(3+) and 2-aminoethoxydiphenyl borate. MIP currents were unaffected by blockers of ATP-sensitive K(+) channels, human ether-à-go-go-related gene current, and intermediate-conductance Ca(2+)-activated K(+) channels. In addition, the MIP current displayed characteristics distinct from conventional inwardly rectifying K(+) channels. A similar current was detected in the leukemic cell line CHRF-288-11, consistent with this current being more generally expressed in cells of leukemic origin.

Phenotypic Manifestations of Mutations in Genes Encoding Subunits of Cardiac Potassium Channels

Since 1995, when a potassium channel gene, hERG (human ether-à-go-go-related gene), now referred to as KCNH2 , encoding the rapid component of cardiac delayed rectifier potassium channels was identified as being responsible for type 2 congenital long-QT syndrome, a number of potassium channel genes have been shown to cause different types of inherited cardiac arrhythmia syndromes. These include congenital long-QT syndrome, short-QT syndrome, Brugada syndrome, early repolarization syndrome, and familial atrial fibrillation. Genotype-phenotype correlations have been investigated in some inherited arrhythmia syndromes, and as a result, gene-specific risk stratification and gene-specific therapy and management have become available, particularly for patients with congenital long-QT syndrome. In this review article, the molecular structure and function of potassium channels, the clinical phenotype due to potassium channel gene mutations, including genotype-phenotype correlations, and the diverse mechanisms underlying the potassium channel gene–related diseases will be discussed.

Control of voltage-gated K+ channel permeability to NMDG+ by a residue at the outer pore

Crystal structures of potassium (K(+)) channels reveal that the selectivity filter, the narrow portion of the pore, is only approximately 3-A wide and buttressed from behind, so that its ability to expand is highly constrained, and the permeation of molecules larger than Rb(+) (2.96 A in diameter) is prevented. N-methyl-d-glucamine (NMDG(+)), an organic monovalent cation, is thought to be a blocker of Kv channels, as it is much larger (approximately 7.3 A in mean diameter) than K(+) (2.66 A in diameter). However, in the absence of K(+), significant NMDG(+) currents could be recorded from human embryonic kidney cells expressing Kv3.1 or Kv3.2b channels and Kv1.5 R487Y/V, but not wild-type channels. Inward currents were much larger than outward currents due to the presence of intracellular Mg(2+) (1 mM), which blocked the outward NMDG(+) current, resulting in a strong inward rectification. The NMDG(+) current was inhibited by extracellular 4-aminopyridine (5 mM) or tetraethylammonium (10 mM), and largely eliminated in Kv3.2b by an S6 mutation that prevents the channel from opening (P468W) and by a pore helix mutation in Kv1.5 R487Y (W472F) that inactivates the channel at rest. These data indicate that NMDG(+) passes through the open ion-conducting pore and suggest a very flexible nature of the selectivity filter itself. 0.3 or 1 mM K(+) added to the external NMDG(+) solution positively shifted the reversal potential by approximately 16 or 31 mV, respectively, giving a permeability ratio for K(+) over NMDG(+) (P(K)(+)/P(NMDG)(+)) of approximately 240. Reversal potential shifts in mixtures of K(+) and NMDG(+) are in accordance with P(K)(+)/P(NMDG)(+), indicating that the ions compete for permeation and suggesting that NMDG(+) passes through the open state. Comparison of the outer pore regions of Kv3 and Kv1.5 channels identified an Arg residue in Kv1.5 that is replaced by a Tyr in Kv3 channels. Substituting R with Y or V allowed Kv1.5 channels to conduct NMDG(+), suggesting a regulation by this outer pore residue of Kv channel flexibility and, as a result, permeability.

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.

Mammalian MagT1 and TUSC3 are required for cellular magnesium uptake and vertebrate embryonic development

Magnesium (Mg(2+)) is the second most abundant cation in cells, yet relatively few mechanisms have been identified that regulate cellular levels of this ion. The most clearly identified Mg(2+) transporters are in bacteria and yeast. Here, we use a yeast complementary screen to identify two mammalian genes, MagT1 and TUSC3, as major mechanisms of Mg(2+) influx. MagT1 is universally expressed in all human tissues and its expression level is up-regulated in low extracellular Mg(2+). Knockdown of either MagT1 or TUSC3 protein significantly lowers the total and free intracellular Mg(2+) concentrations in mammalian cell lines. Morpholino knockdown of MagT1 and TUSC3 protein expression in zebrafish embryos results in early developmental arrest; excess Mg(2+) or supplementation with mammalian mRNAs can rescue the effects. We conclude that MagT1 and TUSC3 are indispensable members of the vertebrate plasma membrane Mg(2+) transport system.

Protective actions of epoxyeicosatrienoic acid: dual targeting of cardiovascular PI3K and KATP channels

Epoxyeicosatrienoic acid(s) (EETs) have been shown to protect cardiovascular tissue against apoptosis dependent on activation of targets such as ATP-sensitive K+ (KATP) channels (sarcolemmal and mitochondrial), calcium-activated K+ channels, extracellular signal-regulated kinase or phosphoinositide 3-kinase (PI3K). We tested if EETs protect human atrial tissue ex vivo from hypoxia/reoxygenation (H/R) injury, and compared our results with myocardium from two rodent species, rats and mice. EETs reduced myocardial caspase 3 activity in all three species and protected against loss of mitochondrial membrane potential in primary cultures of neonatal rat ventricular myocytes submitted to H/R. In addition, EETs protected mouse pulmonary arteries ex vivo exposed to H/R. Myocardium and pulmonary arteries from genetically engineered mice having elevated plasma levels of EETs (Ephx2-/-) exhibited protection from H/R-induced injury over that of wild type controls, suggesting that endogenously produced EETs may have pro-survival effects. Electrophysiological studies in myocytes demonstrated that EETs can stimulate KATP currents even when PI3K is inhibited. Similarly, activation of PI3K/Akt occurred in the presence of the KATP channel blocker glibenclamide. Based upon loss of protection with EETs in the presence of either wortmannin (a PI3K inhibitor) or glibenclamide, simultaneous activation of at least 2 pathways, PI3K and KATP channels respectively, appears to be required for protection. In conclusion, we demonstrate that exogenous and endogenous EETs have powerful pro-survival effects in cardiovascular tissues including diseased human myocardium, mediated by activation of not only one but at least two pathways, PI3K and KATP channels.

Regulation of Kir channels in bovine retinal pigment epithelial cells by phosphatidylinositol 4,5-bisphosphate

The inwardly rectifying K+ (Kir) current in mammalian retinal pigment epithelial (RPE) cells, which is largely mediated by Kir7.1 channels, is stable in cells dialyzed with MgATP but runs down when intracellular ATP is depleted. A potential mechanism for this rundown is a decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) regeneration by ATP-dependent lipid kinases. Here, we used the whole cell voltage-clamp technique to investigate the membrane PIP2 dependence of Kir channels in isolated bovine RPE cells. When RPE cells were dialyzed with ATP-free solution containing PIP2 (25-50 microM), rundown persisted but was markedly reduced. Removal of Mg2+ from the pipette solution also slowed rundown, indicating that elevated intracellular Mg2+ concentration contributes to rundown. Cell dialysis with the PIP2 scavenger neomycin in MgATP solution diminished Kir current in a voltage-dependent manner, suggesting that it acted at least in part by blocking the Kir channel. Kir current in MgATP-loaded cells was partially inhibited by bath application of quercetin (100 microM), phenylarsine oxide (100 microM), or wortmannin (50 microM), inhibitors of phosphatidylinositol (PI) kinases, and was completely inhibited by cell dialysis with 2 mM adenosine, a PI4 kinase inhibitor. Both LY-294002 (100 microM), an inhibitor of PI3 kinases, and its inactive analog LY-303511 (100 microM) rapidly and reversibly inhibited Kir current, suggesting that these compounds act as direct channel blockers. We conclude that the activity of Kir channels in the RPE is critically dependent on the regeneration of membrane PIP2 by PI4 kinases and that this may explain the dependence of these channels on hydrolyzable ATP.

Gating and inward rectifying properties of the MthK K+ channel with and without the gating ring

In MthK, a Ca2+-gated K+ channel from Methanobacterium thermoautotrophicum, eight cytoplasmic RCK domains form an octameric gating ring that controls the intracellular gate of the ion conduction pore. The binding of Ca2+ ions to the RCK domains alters the conformation of the gating ring, thereby opening the gate. In the present study, we examined the Ca2+- and pH-regulated gating and the rectifying conduction properties of MthK at the single-channel level. The open probability (Po) of MthK exhibits a sigmoidal relationship with intracellular [Ca2+], and a Hill coefficient >1 is required to describe the dependence of Po on [Ca2+], suggesting cooperative Ca2+ activation of the channel. Additionally, intracellular Ca2+ also blocks the MthK pore in a voltage-dependent manner, rendering an apparently inwardly rectifying I-V relation. Intracellular pH has a dual effect on MthK gating. Below pH 7.5, the channel becomes insensitive to Ca2+. This occurs because the gating ring is structurally unstable at this pH and tends to disassemble (Ye, S., Y. Li, L. Chen, and Y. Jiang. 2006. Cell. 126:1161-1173). In contrast, above pH 7.5, a further increase in pH shifts the Po-[Ca2+] relation towards a lower Ca2+ concentration, augments Po at saturating [Ca2+], and activates the channel even in the absence of Ca2+. Channel activity is marked by bursts of rapid openings and closings separated by relatively longer interburst closings. The duration of interburst closing and the burst length are highly Ca2+ and pH dependent, whereas the kinetics of intraburst events is Ca2+ and pH independent. The rapid intraburst openings and closings are also observed with the isolated MthK pore lacking the attached intracellular gating ring. The fast kinetic events, independent of both Ca2+ and pH, therefore appear to be determined by processes occurring within the ion conduction pore, whereas the slow events reflect the gating process controlled by Ca2+ and pH through the gating ring.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Block by internal Mg2+ causes voltage-dependent inactivation of Kv1.5

Internal Mg2+ blocks many potassium channels including Kv1.5. Here, we show that internal Mg2+ block of Kv1.5 induces voltage-dependent current decay at strongly depolarised potentials that contains a component due to acceleration of C-type inactivation after pore block. The voltage-dependent current decay was fitted to a bi-exponential function (tau(fast) and tau(slow)). Without Mg2+, tau(fast) and tau(slow) were voltage-independent, but with 10 mM Mg2+, tau(fast) decreased from 156 ms at +40 mV to 5 ms at +140 mV and tau(slow) decreased from 2.3 s to 206 ms. With Mg2+, tail currents after short pulses that allowed only the fast phase of decay showed a rising phase that reflected voltage-dependent unbinding. This suggested that the fast phase of voltage-dependent current decay was due to Mg2+ pore block. In contrast, tail currents after longer pulses that allowed the slow phase of decay were reduced to almost zero suggesting that the slow phase was due to channel inactivation. Consistent with this, the mutation R487V (equivalent to T449V in Shaker) or increasing external K+, both of which reduce C-type inactivation, prevented the slow phase of decay. These results are consistent with voltage-dependent open-channel block of Kv1.5 by internal Mg2+ that subsequently induces C-type inactivation by restricting K+ filling of the selectivity filter from the internal solution.

Evidence for sequential ion-binding loci along the inner pore of the IRK1 inward-rectifier K+ channel

Steep rectification in IRK1 (Kir2.1) inward-rectifier K⁺ channels reflects strong voltage dependence (valence of ∼5) of channel block by intracellular cationic blockers such as the polyamine spermine. The observed voltage dependence primarily results from displacement, by spermine, of up to five K⁺ ions across the narrow K⁺ selectivity filter, along which the transmembrane voltage drops steeply. Spermine first binds, with modest voltage dependence, at a shallow site where it encounters the innermost K⁺ ion and impedes conduction. From there, spermine can proceed to a deeper site, displacing several more K⁺ ions and thereby producing most of the observed voltage dependence. Since in the deeper blocked state the leading amine group of spermine reaches into the cavity region (internal to the selectivity filter) and interacts with residue D172, its trailing end is expected to be near M183. Here, we found that mutation M183A indeed affected the deeper blocked state, which supports the idea that spermine is located in the region lined by the M2 and not deep in the narrow K⁺ selectivity filter. As to the shallower site whose location has been unknown, we note that in the crystal structure of homologous GIRK1 (Kir3.1), four aromatic side chains of F255, one from each of the four subunits, constrict the intracellular end of the pore to ∼10 Å. For technical simplicity, we used tetraethylammonium (TEA) as an initial probe to test whether the corresponding residue in IRK1, F254, forms the shallower site. We found that replacing the aromatic side chain with an aliphatic one not only lowered TEA affinity of the shallower site ∼100-fold but also eliminated the associated voltage dependence and, furthermore, confirmed that similar effects occurred also for spermine. These results establish the evidence for physically separate, sequential ion-binding loci along the long inner pore of IRK1, and strongly suggest that the aromatic side chains of F254 underlie the likely innermost binding locus for both blocker and K⁺ ions in the cytoplasmic pore.

The glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase, triose-phosphate isomerase, and pyruvate kinase are components of the K(ATP) channel macromolecular complex and regulate its function

The regulation of ATP-sensitive potassium (K(ATP)) channel activity is complex and a multitude of factors determine their open probability. Physiologically and pathophysiologically, the most important of these are intracellular nucleotides, with a long-recognized role for glycolytically derived ATP in regulating channel activity. To identify novel regulatory subunits of the K(ATP) channel complex, we performed a two-hybrid protein-protein interaction screen, using as bait the mouse Kir6.2 C terminus. Screening a rat heart cDNA library, we identified two potential interacting proteins to be the glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and triose-phosphate isomerase. The veracity of interaction was verified by co-immunoprecipitation techniques in transfected mammalian cells. We additionally demonstrated that pyruvate kinase also interacts with Kir6.2 subunits. The physiological relevance of these interactions is illustrated by the demonstration that native Kir6.2 protein similarly interact with GAPDH and pyruvate kinase in rat heart membrane fractions and that Kir6.2 protein co-localize with these glycolytic enzymes in rat ventricular myocytes. The functional relevance of our findings is demonstrated by the ability of GAPDH or pyruvate kinase substrates to directly block the K(ATP) channel under patch clamp recording conditions. Taken together, our data provide direct evidence for the concept that key enzymes involved in glycolytic ATP production are part of a multisubunit K(ATP) channel protein complex. Our data are consistent with the concept that the activity of these enzymes (possibly by ATP formation in the immediate intracellular microenvironment of this macromolecular K(ATP) channel complex) causes channel closure.

Myocardial content of selected elements in experimental anthracycline-induced cardiomyopathy in rabbits

Cardiotoxicity represents the main drawback of clinical usefulness of anthracycline antineoplastic drugs. In this study, a content of selected elements (Ca, Mg, K, Se, Fe) in the post-mortem removed samples of the myocardial tissue was studied in three groups of rabbits: 1) control group (i.v. saline; n = 10); 2) daunorubicin-receiving animals (DAU; 3 mg/kg, i.v; n = 11); 3) animals receiving cardioprotective iron-chelating agent dexrazoxane (DEX; 60 mg/kg, i.p.; n = 5) prior to DAU. Drugs were administered once weekly for 10 weeks. 5-7 days after the last administration, cardiac left ventricular contractility (dP/dtmax) was significantly decreased in DAU-treated animals (745 +/- 69 versus 1245 +/- 86 kPa/s in the control group; P < 0.05), while in the DEX + DAU group it was insignificantly increased (1411 +/- 77 kPa/s). Of the myocardial elements' content studied, a significant increase in total Ca against control (16.2 +/- 2.4 versus 10.6 +/- 0.9 microg/g of dry tissue; P < 0.05) was determined in the DAU-group, which was accompanied with significant decreases in Mg and K. In the heart tissue of DEX-pretreated animals, no significant changes of elements' content were found as compared to controls, while the Ca content was in these animals significantly lower than in the DAU group (9.1 +/- 0.4 versus 16.2 +/- 2.4 microg/g; P < 0.05). Hence, in this study we show that systolic heart failure induced by chronic DAU administration is primarily accompanied by persistent calcium overload of cardiac tissue and the protective action of DEX is associated with the restoration of normal myocardial Ca content.

Reduction in noise-induced temporary threshold shift in humans following oral magnesium intake

A previous study demonstrated the prophylactic effects of magnesium on noise-induced permanent threshold shift in humans. For the first time, this study explores the effects of magnesium on temporary threshold shift in 20 human subjects, all men (16-37 years, mean age 21 years). The study was conducted in a double-blind manner on the same subjects tested in three different phases (placebo, magnesium, no-drug). The cochlear changes were assessed using both behavioural (audiograms) and objective (otoacoustic emission) measures. Blood samples were collected at the beginning and end of each phase. Amongst subjects and phases, magnesium intake was associated with significantly lower temporary threshold shift, compared with the other two phases, which were reflected both by the behavioural and cochlear measures. A correlation was found between the blood magnesium levels and temporary threshold shift reduction. No side-effects were associated with the oral ingestion of the magnesium. Magnesium provides significant protection against temporary threshold shift, complementing the previous permanent threshold shift human study. Both human noise-induced hearing loss studies introduced a novel, biological, natural agent for prevention and possible treatment of noise-induced cochlear damage in humans.

Magnesium treatment for sudden hearing loss

Magnesium treatment has been repeatedly shown to reduce the incidence of both temporary and permanent noise-induced hearing loss. We hypothesized that it might also improve the permanent threshold shift in patients with acute-onset hearing loss. In a prospective, randomized, double-blind, placebo-controlled trial, 28 patients with idiopathic sudden sensorineural hearing loss were treated with either steroids and oral magnesium (study group) or steroids and a placebo (control group). Compared to the controls, the magnesium-treated group had a significantly higher proportion of patients with improved hearing (>10 dB hearing level) across all frequencies tested, and a significantly greater mean improvement in all frequencies. Analysis of the individual data confirmed that more patients treated with magnesium experienced hearing improvement, and at a larger magnitude, than control subjects. Magnesium is a relatively safe and convenient adjunct to steroid treatment for enhancing the improvement in hearing, especially in the low-tone range, in patients with sudden sensorineural hearing loss.

Mechanism of Rectification in Inward-Rectifier K+ Channels

Inward rectifiers are a class of K+ channels that can conduct much larger inward currents at membrane voltages negative to the K+ equilibrium potential than outward currents at voltages positive to it, even when K+ concentrations on both sides of the membrane are made equal. This conduction property, called inward rectification, enables inward rectifiers to perform many important physiological tasks. Rectification is not an inherent property of the channel protein itself, but reflects strong voltage dependence of channel block by intracellular cations such as Mg2+ and polyamines. This voltage dependence results primarily from the movement of K+ ions across the transmembrane electric field along the pore, which is energetically coupled to the blocker binding and unbinding. This mutual displacement mechanism between several K+ ions and a blocker explains the signature feature of inward rectifier K+ channels, namely, that at a given concentration of intracellular K+, their macroscopic conductance depends on the difference between membrane voltage and the K+ equilibrium potential rather than on membrane voltage itself.

Na(+) current through KATP channels: consequences for Na(+) and K(+) fluxes during early myocardial ischemia

During early myocardial ischemia, the myocytes are loaded with Na(+), which in turn leads to Ca(2+) overload and cell death. The pathway of the Na(+) influx has not been fully elucidated. The aim of the study was to quantify the Na(+) inward current through sarcolemmal KATP channels (IKATP,Na) in anoxic isolated cardiomyocytes at the actual reversal potential (Vrev) and to estimate the contribution of this current to the Na(+) influx in the ischemic myocardium. IKATP,Na was determined in excised single channel patches of mouse ventricular myocytes and macropatches of Xenopus laevis oocytes expressing SUR2A/Kir6.2 channels. In the presence of K+ ions, the respective permeability ratios for Na(+) to K(+) ions, PNa/PK, were close to 0.01. Only in the presence of Na(+) ions on both sides of the membrane was IKATP,Na similarly large to that calculated from the permeability ratio PNa/PK, indicative of a Na(+) influx that is largely independent of the K+ efflux at Vrev. With the use of a peak KATP channel conductance in anoxic cardiomyocytes of 410 nS, model simulations for a myocyte within the ischemic myocardium showed that the amplitude of the Na(+) influx and K(+) efflux is even larger than the respective fluxes by the Na(+) - K(+) pump and all other background fluxes. These results suggest that during early ischemia the Na(+) influx through KATP channels essentially contributes to the total Na+ influx and that it also balances the K(+) efflux through KATP channels.

Regulation of action potential duration under acute heat stress by I(K,ATP) and I(K1) in fish cardiac myocytes

The mechanism underlying temperature-dependent shortening of action potential (AP) duration was examined in the fish (Carassius carassius L.) heart ventricle. Acute temperature change from +5 to +18 degrees C (heat stress) shortened AP duration from 2.8 +/- 0.3 to 1.3 +/- 0.1 s in intact ventricles. In 56% (18 of 32) of enzymatically isolated myocytes, heat stress also induced reversible opening of ATP-sensitive K+ channels and increased their single-channel conductance from 37 +/- 12 pS at +8 degrees C to 51 +/- 13 pS at +18 degrees C (Q10 = 1.38) (P < 0.01; n = 12). The ATP-sensitive K+ channels of the crucian carp ventricle were characterized by very low affinity to ATP both at +8 degrees C [concentration of Tris-ATP that produces half-maximal inhibition of the channel (K1/2)= 1.35 mM] and +18 degrees C (K1/2 = 1.85 mM). Although acute heat stress induced ATP-sensitive K+ current (IK,ATP) in patch-clamped myocytes, similar heat stress did not cause any glibenclamide (10 microM)-sensitive changes in AP duration in multicellular ventricular preparations. Examination of APs and K+ currents from the same myocytes by alternate recording under current-clamp and voltage-clamp modes revealed that changes in AP duration were closely correlated with temperature-specific changes in the voltage-dependent rectification of the background inward rectifier K+ current IK1. In approximately 15% of myocytes (4 out of 27), IK,ATP-dependent shortening of AP followed the IK1-induced AP shortening. Thus heat stress-induced shortening of AP duration in crucian carp ventricle is primarily dependent on IK1. IK,ATP is induced only in response to prolonged temperature elevation or perhaps in the presence of additional stressors.

Inhibition of G protein-coupled and ATP-sensitive potassium currents by 2-methyl-3-(3,5-diiodo-4-carboxymethoxybenzyl)benzofuran (KB130015), an amiodarone derivative

2-Methyl-3- (3,5-diiodo-4-carboxymethoxybenzyl) benzofuran (KB130015; KB), a novel compound derived from amiodarone, has been proposed to have antiarrhythmic properties. Its effect on the G protein-coupled inward rectifying K+ current [IK(ACh) or IK(Ado)], ATP-sensitive K+ current [IK(ATP)], and background inward rectifying current (I(K1)) were studied in guinea pig atrial and ventricular myocytes by the whole-cell voltage-clamp technique. Receptor-activated IK(ACh/Ado), induced in atrial myocytes by the stimulation of either muscarinic or Ado receptors was concentration dependently (IC50 value of approximately 0.6-0.8 microM) inhibited by KB. Receptor-independent guanosine 5'-O-(3-thio)triphosphate-induced and background IK(ACh), which contributes to the resting conductance of atrial myocytes, were equally sensitive to KB (IC50 value of approximately 0.9 microM). IK(ATP) induced in atrial myocytes during metabolic inhibition with 2,4-dinitrophenol (DNP) was also suppressed by KB, whereas IK1 measured in ventricular myocytes was insensitive to the drug (KB < or =50 microM). Although being effective when applied from the outside, intracellular application of KB via the patch pipette affected neither IK(ACh) nor IK(ATP). 3,3',5-triodo-L-thyronin, which shares structural groups with KB, did not have an effect on the K+ currents. Consistent with the effects on single myocytes, KB did not depolarize the resting potential but antagonized the shortening of action potential duration by carbamylcholine-chloride or by DNP in multicellular preparations and antagonized the shortening of action potential duration by acetylcholine in single myocytes. It is concluded that KB inhibits IK(ACh) and IK(ATP) by direct drug-channel interaction at a site more easily accessible from extracellular side of the membrane.

Pharmacologic Manipulation of the Labyrinth with Novel and Traditional Agents Delivered to the Inner Ear

We describe the methodology and rationale behind the delivery of therapeutic medicines to the inner ear. The inner ear has long been impervious to pharmacologic manipulation. This is most likely the result of a protective mechanism called the blood-labyrinth barrier, whose function closely resembles that of the blood-brain barrier. This protective barrier impedes the clinician's ability to treat inner ear diseases with systemically administered medications. Since 1935, otolaryngologists have attempted to manipulate the inner ear with transtympanically injected medicines. Success has varied widely, but medicinal ablation of vestibular function can be achieved in this manner. Unfortunately, the auditory system is also at great risk from any medicine that is delivered to the inner ear via the middle ear. Over the past 10 years, significant improvements in drug delivery have allowed for more “titratable” treatment, which has reduced (but not eliminated) the risk of permanent hearing loss. In this article, we discuss both novel and time-tested methods of delivering medicines to the inner ear. We also review the classes of medications that alter inner ear function and the attendant risks of such treatments.

Mg2+-dependent gating and strong inward rectification of the cation channel TRPV6

TRPV6 (CaT1/ECaC2), a highly Ca(2+)-selective member of the TRP superfamily of cation channels, becomes permeable to monovalent cations in the absence of extracellular divalent cations. The monovalent currents display characteristic voltage-dependent gating and almost absolute inward rectification. Here, we show that these two features are dependent on the voltage-dependent block/unblock of the channel by intracellular Mg(2+). Mg(2+) blocks the channel by binding to a site within the transmembrane electrical field where it interacts with permeant cations. The block is relieved at positive potentials, indicating that under these conditions Mg(2+) is able to permeate the selectivity filter of the channel. Although sizeable outward monovalent currents were recorded in the absence of intracellular Mg(2+), outward conductance is still approximately 10 times lower than inward conductance under symmetric, divalent-free ionic conditions. This Mg(2+)-independent rectification was preserved in inside-out patches and not altered by high intracellular concentrations of spermine, indicating that TRPV6 displays intrinsic rectification. Neutralization of a single aspartate residue within the putative pore loop abolished the Mg(2+) sensitivity of the channel, yielding voltage-independent, moderately inwardly rectifying monovalent currents in the presence of intracellular Mg(2+). The effects of intracellular Mg(2+) on TRPV6 are partially reminiscent of the gating mechanism of inwardly rectifying K(+) channels and may represent a novel regulatory mechanism for TRPV6 function in vivo.

Properties of an inwardly rectifying K(+) channel in the basolateral membrane of mouse TAL

We investigated the properties of K(+) channels in the basolateral membrane of the cortical thick ascending limb (CTAL) using the patch-clamp technique. Approximately 34% of cell-attached patches contained an inwardly rectifying K(+) channel (K(+)-to-Na(+) permeability ratio approximately 22), having an inward conductance (G(in)) of 44 pS and an outward conductance (G(out)) of approximately 10 pS (G(in)/G(out) approximately 4). Channel activity (NP(o)) increased with depolarization. When the cytosolic sides of inside-out patches were exposed to an Mg(2+)-free medium, the channel had a G(in) of 50 pS and was weakly inwardly rectifying (G(in)/G(out) approximately 1). Cytosolic Mg(2+) reduced G(out), yielding a G(in)/G(out) of 3.8 at 1.3 mM Mg(2+). Internal Na(+) also yielded a G(in)/G(out) of 1.6 at 20 mM Na(+). Spermine reduced NP(o) on inside-out membrane patches. Sensitivity to spermine at depolarizing voltages [half-maximal inhibitory concentration (K(i)) = 0.2 microM] was much greater than at hyperpolarizing voltages (K(i) = 26 microM). Half-inactivation by 0.5 microM spermine occurred at a clamp potential of 43 mV, with an effective valence of 1.25. A sigmoid relationship between bath pH and NP(o) of inside-out membrane patches was observed, with a pK of 7.6 and a Hill coefficient of 1.8. Intracellular acidification also reduced the NP(o) of cell-attached patches. This channel is probably a major component of K(+) conductance in the CTAL basolateral membrane.

Activation of ATP-sensitive K(+) channels by epoxyeicosatrienoic acids in rat cardiac ventricular myocytes

1. We examined the effects of epoxyeicosatrienoic acids (EETs), which are cytochrome P450 metabolites of arachidonic acid (AA), on the activities of the ATP-sensitive K(+) (K(ATP)) channels of rat cardiac myocytes, using the inside-out patch-clamp technique. 2. In the presence of 100 microM cytoplasmic ATP, the K(ATP) channel open probability (P(o)) was increased by 240 +/- 60 % with 0.1 microM 11,12-EET and by 400 +/- 54 % with 5 microM 11,12-EET (n = 5-10, P < 0.05 vs. control), whereas neither 5 microM AA nor 5 microM 11,12-dihydroxyeicosatrienoic acid (DHET), which is the epoxide hydrolysis product of 11,12-EET, had any effect on P(o). 3. The half-maximal activating concentration (EC(50)) was 18.9 +/- 2.6 nM for 11,12-EET (n = 5) and 19.1 +/- 4.8 nM for 8,9-EET (n = 5, P = n.s. vs. 11,12-EET). Furthermore, 11,12-EET failed to alter the inhibition of K(ATP) channels by glyburide. 4. Application of 11,12-EET markedly decreased the channel sensitivity to cytoplasmic ATP. The half-maximal inhibitory concentration of ATP (IC(50)) was increased from 21.2 +/- 2.0 microM at baseline to 240 +/- 60 microM with 0.1 microM 11,12-EET (n = 5, P < 0.05 vs. control) and to 780 +/- 30 microM with 5 microM 11,12-EET (n = 11, P < 0.05 vs. control). 5. Increasing the ATP concentration increased the number of kinetically distinguishable closed states, promoting prolonged closure durations. 11,12-EET antagonized the effects of ATP on the kinetics of the K(ATP) channels in a dose- and voltage-dependent manner. 11,12-EET (1 microM) reduced the apparent association rate constant of ATP to the channel by 135-fold. 6. Application of 5 microM 11,12-EET resulted in hyperpolarization of the resting membrane potential in isolated cardiac myocytes, which could be blocked by glyburide. 7. These results suggest that EETs are potent activators of the cardiac K(ATP) channels, modulating channel behaviour by reducing the channel sensitivity to ATP. Thus, EETs could be important endogenous regulators of cardiac electrical excitability.

Cardiovascular actions of magnesium

Intracellular magnesium is an important modulator of calcium and potassium channels in cardiac myocytes. Hypomagnesemia is common in hospitalized patients and may contribute significantly to cardiac morbidity and mortality, particularly in states associated with myocardial ischemia. Therefore, it is important to maintain the plasma magnesium concentration within the normal range in asymptomatic patients and in patients with cardiac disease as prophylaxis against the occurrence of significant arrhythmias.

Analysis of single K(ATP) channels in mammalian dentate gyrus granule cells

ATP-sensitive potassium (K(ATP)) channels are heteromultimer complexes of subunits from members of the inwardly rectifying K(+) channel and the ATP-binding cassette protein superfamilies. K(ATP) channels couple metabolic state to membrane excitability, are distributed widely, and participate in a variety of physiological functions. Understood best in pancreatic beta cells, where their activation inhibits insulin release, K(ATP) channels have been implicated also in postischemia cardio- and neuroprotection. The dentate gyrus (DG) is a brain region with a high density of K(ATP) channels and is relatively resistant to ischemia/reperfusion-induced cell death. Therefore we were interested in describing the characteristics of single K(ATP) channels in DG granule cells. We recorded single K(ATP) channels in 59/105 cell-attached patches from DG granule cells in acutely prepared hippocampal slices. Single-channel openings had an E(K) close to 0 mV (symmetrical K(+)) and were organized in bursts with a duration of 19.3 +/- 1.6 (SE) ms and a frequency of 3.5 +/- 0.8 Hz, a unitary slope conductance of 27 pS, and a low, voltage-independent, probability of opening (P(open), 0.04 +/- 0.01). Open and closed dwell-time histograms were fitted best with one (tau(open) = 1.3 +/- 0.2 ms) and the sum of two (tau(closed,fast) = 2.6 +/- 0.9 ms, tau(closed,slow) = 302.7 +/- 67. 7 ms) exponentials, respectively, consistent with a kinetic model having at least a single open and two closed states. The P(open) was reduced ostensibly to zero by the sulfonylureas, glybenclamide (500 nM, 2/6; 10 microM,11/14 patches) and tolbutamide (20 microM, 4/6; 100 microM, 4/4 patches). The blocking dynamics for glybenclamide included transition to a subconductance state (43.3 +/- 2.6% of control I(open channel)). Unlike glybenclamide, the blockade produced by tolbutamide was reversible. In 5/5 patches, application of diazoxide (100 microM) increased significantly P(open) (0.12 +/- 0.02), which was attributable to a twofold increase in the frequency of bursts (8.3 +/- 2.0 Hz). Diazoxide was without effect on tau(open) and tau(closed,fast) but decreased significantly tau(closed,slow) (24.4 +/- 2.6 ms). We observed similar effects in 6/7 patches after exposure to hypoxia/hypoglycemia, which increased significantly P(open) (0.09 +/- 0.03) and the frequency of bursts (7.1 +/- 1.7 Hz) and decreased significantly tau(closed,slow) (29.5 +/- 1.8 ms). We have presented convergent evidence consistent with single K(ATP) channel activity in DG granule cells. The subunit composition of K(ATP) channels native to DG granule cells is not known; however, the characteristics of the channel activity we recorded are representative of Kir6.1/SUR1, SUR2B-based channels.

Pore block versus intrinsic gating in the mechanism of inward rectification in strongly rectifying IRK1 channels

The IRK1 channel is inhibited by intracellular cations such as Mg(2+) and polyamines in a voltage-dependent manner, which renders its I-V curve strongly inwardly rectifying. However, even in excised patches exhaustively perfused with a commonly used artificial intracellular solution nominally free of Mg(2+) and polyamines, the macroscopic I-V curve of the channels displays modest rectification. This observation forms the basis of a hypothesis, alternative to the pore-blocking hypothesis, that inward rectification reflects the enhancement of intrinsic channel gating by intracellular cations. We find, however, that residual rectification is caused primarily by the commonly used pH buffer HEPES and/or some accompanying impurity. Therefore, inward rectification in the strong rectifier IRK1, as in the weak rectifier ROMK1, can be accounted for by voltage-dependent block of its ion conduction pore by intracellular cations.

Mechanism of IRK1 channel block by intracellular polyamines

Intracellular polyamines inhibit the strongly rectifying IRK1 potassium channel by a mechanism different from that of a typical ionic pore blocker such as tetraethylammonium. As in other K(+) channels, in the presence of intracellular TEA, the IRK1 channel current decreases with increasing membrane voltage and eventually approaches zero. However, in the presence of intracellular polyamines, the channel current varies with membrane voltage in a complex manner: when membrane voltage is increased, the current decreases in two phases separated by a hump. Furthermore, contrary to the expectation for a nonpermeant ionic pore blocker, a significant residual IRK1 current persists at very positive membrane voltages; the amplitude of the residual current decreases with increasing polyamine concentration. This complex blocking behavior of polyamines can be accounted for by a minimal model whereby intracellular polyamines inhibit the IRK1 channel by inducing two blocked channel states. In each of the blocked states, a polyamine is bound with characteristic affinity and probability of traversing the pore. The proposal that polyamines traverse the pore at finite rates is supported by the observation that philanthotoxin-343 (spermine with a bulky chemical group attached to one end) acts as a nonpermeant ionic blocker in the IRK1 channel.

Mechanism of cGMP-gated channel block by intracellular polyamines

Polyamines block the retinal cyclic nucleotide-gated channel from both the intracellular and extracellular sides. The voltage-dependent mechanism by which intracellular polyamines inhibit the channel current is complex: as membrane voltage is increased in the presence of polyamines, current inhibition is not monotonic, but exhibits a pronounced damped undulation. To understand the blocking mechanism of intracellular polyamines, we systematically studied the endogenous polyamines as well as a series of derivatives. The complex channel-blocking behavior of polyamines can be accounted for by a minimal model whereby a given polyamine species (e.g., spermine) causes multiple blocked channel states. Each blocked state represents a channel occupied by a polyamine molecule with characteristic affinity and probability of traversing the pore, and exhibits a characteristic dependence on membrane voltage and cGMP concentration.