Outward Potassium Channel Research Articles

Characterization of ion channels on subesophageal ganglion neurons from Chinese tarantula Ornithoctonus huwena: Exploring the myth of the spider insensitive to its venom

Chinese tarantula Ornithoctonus huwena is one of the most venomous spiders distributing in the hilly areas of southern China. In this study, using whole-cell patch-clamp technique we investigated electrophysiological and pharmacological properties of ion channels from tarantula subesophageal ganglion neurons. It was found that the neurons express multiple kinds of ion channels at least including voltage-gated calcium channels, TTX-sensitive sodium channels and two types of potassium channels. They exhibit pharmacological properties similar to mammalian subtypes. Spider calcium channels were sensitive to ω-conotoxin GVIA and diltiazem, two well-known inhibitors of mammalian neuronal high-voltage-activated (HVA) subtypes. 4-Aminopyridine and tetraethylammonium could inhibit spider outward transient and delayed-rectifier potassium channels, respectively. Huwentoxin-I and huwentoxin-IV are two abundant toxic components in the venom of Ornithoctonus huwena. Interestingly, although in our previous work they inhibit HVA calcium channels and TTX-sensitive sodium channels from mammalian sensory neurons, respectively, they fail to affect the subtypes from spider neurons. Moreover, the crude venom has no effect on delayed-rectifier potassium channels and only slightly reduces transient outward potassium channels with an IC50 value of ∼51.3 mg/L. Therefore, our findings provide important evidence for ion channels from spiders having an evolution as self-defense and prey mechanism.

The neurotoxic effects of hydrogen peroxide and copper in Retzius nerve cells of the leech Haemopis sanguisuga.

ABSTRACTOxidative stress and the generation of reactive oxygen species (ROS) play an important role in cellular damage. Electrophysiological analyses have shown that membrane transport proteins are susceptible to ROS. In the present study, oxidative stress was induced in Retzius nerve cells of the leech Haemopis sanguisuga by bath application of 1 mM of hydrogen peroxide (H2O2) and 0.02 mM of copper (Cu) for 20 min. The H2O2/Cu(II) produced considerable changes in the electrical properties of the Retzius nerve cells. Intracellular recording of the resting membrane potential revealed that the neuronal membrane was depolarized in the presence of H2O2/Cu(II). We found that the amplitude of action potentials decreased, while the duration augmented in a progressive way along the drug exposure time. The combined application of H2O2 and Cu(II) caused an initial excitation followed by depression of the spontaneous electrical activity. Voltage-clamp recordings revealed a second effect of the oxidant, a powerful inhibition of the outward potassium channels responsible for the repolarization of action potentials. The neurotoxic effect of H2O2/Cu(II) on the spontaneous spike electrogenesis and outward K+ current of Retzius nerve cells was reduced in the presence of hydroxyl radical scavengers, dimethylthiourea and dimethyl sulfoxide, but not mannitol. This study provides evidence for the oxidative modification of outward potassium channels in Retzius nerve cells. The oxidative mechanism of the H2O2/Cu(II) system action on the electrical properties of Retzius neurons proposed in this study might have a wider significance, referring not only to leeches but also to mammalian neurons.

Isopimaric Acid - A Promiscuous Ion Channel Modulator and a Potential Drug Candidate Against Atrial Fibrillation

Atrial fibrillation is a common cardiac arrhythmia in adulthood, and it is not well controlled by the present-day drugs. Previously, resin acids has been shown to open voltage-gated potassium channels and reduce cellular excitability in dorsal-root ganglion neurons. In this study we explored the effects of a resin acid, Isopimaric acid (IPA), on excitability in cardiomyocytes. Whole-cell patch-clamp recordings showed that 1-25 µmol/L IPA reduced the action potential frequency by up to 50 % in spontaneously beating mouse atrial HL-1 cells. The effect of IPA on six different voltage-gated ion channels were investigated; most voltage-dependent parameters of ion-channel gating were shifted in negative direction along the voltage axis, consistent with an hypothesis that a lipophilic and negatively charged compound binds to the lipid membrane close to the positively charged voltage sensor of the ion channels. The major finding was that IPA inactivated sodium channels and L- and T-type calcium channels more than it activated these channels. It also activated the rapid delayed rectifier potassium channel, the transient outward potassium channel, but had no effect of the pace-maker f-channel. Computer simulations of all IPA effects on the ion currents were consistent with a reduced excitability. They also showed that the effect on the sodium channel played a large role for IPAs ability to reduce the action potential frequency. Finally, induced arrhythmia in the HL-1 cells was shown to be reversed by IPA. In conclusion, low concentrations of IPA reduced the action potential frequency and reversed arrhythmic firing via a new pharmacological mechanism. These findings can form the basis for a new pharmacological strategy to treat atrial fibrillation.

Functional identification of a GORK potassium channel from the ancient desert shrub Ammopiptanthus mongolicus (Maxim.) Cheng f.

A GORK homologue K(+) channel from the ancient desert shrub Ammopiptanthus mongolicus (Maxim.) Cheng f. shows the functional conservation of the GORK channels among plant species. Guard cell K(+) release through the outward potassium channels eventually enables the closure of stomata which consequently prevents plant water loss from severe transpiration. Early patch-clamp studies with the guard cells have revealed many details of such outward potassium currents. However, genes coding for these potassium-release channels have not been sufficiently characterized from species other than the model plant Arabidopsis thaliana. We report here the functional identification of a GORK (for Gated or Guard cell Outward Rectifying K(+) channels) homologue from the ancient desert shrub Ammopiptanthus mongolicus (Maxim.) Cheng f. AmGORK was primary expressed in shoots, where the transcripts were regulated by stress factors simulated by PEG, NaCl or ABA treatments. Patch-clamp measurements on isolated guard cell protoplasts revealed typical depolarization voltage gated outward K(+) currents sensitive to the extracelluar K(+) concentration and pH, resembling the fundamental properties previously described in other species. Two-electrode voltage-clamp analysis in Xenopus lavies oocytes with AmGORK reconstituted highly similar characteristics as assessed in the guard cells, supporting that the function of AmGORK is consistent with a crucial role in mediating stomatal closure in Ammopiptanthus mongolicus. Furthermore, a single amino acid mutation D297N of AmGORK eventually abolishes both the voltage-gating and its outward rectification and converts the channel into a leak-like channel, indicating strong involvement of this residue in the gating and voltage dependence of AmGORK. Our results obtained from this anciently originated plant support a strong functional conservation of the GORK channels among plant species and maybe also along the progress of revolution.

Cold-Inducible RNA-Binding Protein Regulates Cardiac Repolarization by Targeting Transient Outward Potassium Channels

Cold-inducible RNA-binding protein (CIRP) is constitutively expressed at low levels across various tissues. It is rapidly upregulated by multiple stresses, underlying a general role for CIRP in organic adaptations to pathophysiological conditions. However, the role of CIRP in the heart remains unclear. To examine the biofunctions of CIRP in the mammalian heart. Rats with targeted disruption of Cirp were generated using the TALEN (transcription activator-like effector nucleases)-based genome editing technique. The Cirp-knockout rats had structurally and functionally normal hearts. Resting ECG recordings revealed a short rate-corrected QT (QTc) interval in Cirp-null rats without any abnormalities in PR interval, RR interval or QRS waves as compared to wild-type animals. The shortened QTc interval from Cirp ablation was tightly linked to an abbreviated action potential duration in cardiac myocytes, which was attributable to increased transient outward potassium current (Ito). Furthermore, our findings uncovered that CIRP protein selectively bonded to KCND2 and KCND3 mRNAs encoding the functional α-subunits of Ito channel proteins. CIRP deficiency did not change the transcriptional activity of KCND2 or KCND3, but it facilitated their translation. Cirp knockout had no effect on the functional expression of ion channels other than Ito channels. CIRP modulates cardiac repolarization by negatively adjusting the expression and function of Ito channels. Our study may open a window to decipher the potential function of RNA-binding proteins in bioelectric activity.

Modeling the Pancreatic α-Cell: Dual Mechanisms of Glucose Suppression of Glucagon Secretion

The mechanism by which glucose induces insulin secretion in β-cells is fairly well understood. Despite years of research, however, the mechanism of glucagon secretion in α-cells is still not well established. It has been proposed that glucose regulates glucagon secretion by decreasing the conductance of either outward ATP-dependent potassium channels (K(ATP)) or an inward store-operated current (SOC). We have developed a mathematical model based on mouse data to test these hypotheses and found that both mechanisms are possible. Glucose metabolism closes K(ATP) channels, which depolarizes the cell but paradoxically reduces calcium influx by inactivating voltage-dependent calcium and sodium channels and decreases secretion. Glucose metabolism also activates SERCA pumps, which fills the endoplasmic reticulum and hyperpolarizes the cells by reducing the inward current through SOC channels and again suppresses glucagon secretion. We find further that the two mechanisms can combine to account for the nonmonotonic dependence of secretion on glucose observed in some studies, an effect that cannot be obtained with either mechanism alone.

Enhanced large conductance K+channel activity contributes to the impaired myogenic response in the cerebral vasculature of Fawn Hooded Hypertensive rats

Recent studies have indicated that the myogenic response (MR) in cerebral arteries is impaired in Fawn Hooded Hypertensive (FHH) rats and that transfer of a 2.4 megabase pair region of chromosome 1 (RNO1) containing 15 genes from the Brown Norway rat into the FHH genetic background restores MR in a FHH.1(BN) congenic strain. However, the mechanisms involved remain to be determined. The present study examined the role of the large conductance calcium-activated potassium (BK) channel in impairing the MR in FHH rats. Whole-cell patch-clamp studies of cerebral vascular smooth muscle cells (VSMCs) revealed that iberiotoxin (IBTX; BK inhibitor)-sensitive outward potassium (K+) channel current densities are four- to fivefold greater in FHH than in FHH.1(BN) congenic strain. Inside-out patches indicated that the BK channel open probability (NPo) is 10-fold higher and IBTX reduced NPo to a greater extent in VSMCs isolated from FHH than in FHH.1(BN) rats. Voltage sensitivity of the BK channel is enhanced in FHH as compared with FHH.1(BN) rats. The frequency and amplitude of spontaneous transient outward currents are significantly greater in VSMCs isolated from FHH than in FHH.1(BN) rats. However, the expression of the BK-α and -β-subunit proteins in cerebral vessels as determined by Western blot is similar between the two groups. Middle cerebral arteries (MCAs) isolated from FHH rats exhibited an impaired MR, and administration of IBTX restored this response. These results indicate that there is a gene on RNO1 that impairs MR in the MCAs of FHH rats by enhancing BK channel activity.

Prolonged leptin treatment increases transient outward K+ current via upregulation of Kv4.2 and Kv4.3 channel subunits in adult rat ventricular myocytes

Circulating leptin levels are elevated in obesity and hyperleptinaemia has been postulated to be an independent risk factor for the development of cardiovascular diseases. Although many studies have been published on the mechanisms involved in the effects of leptin on cardiac function and pathological remodeling, scarce information is currently available analyzing the influence of prolonged leptin treatment on ionic cardiac channels remodeling in adult ventricular myocytes. Enzymatically isolated adult rat ventricular myocytes were treated with leptin or vehicle for 48h. Real-Time RT-PCR were used to analyze mRNA expression of Kir2.1, Cav1.2, Cav 3.1, Kv4.2 and Kv4.3 α-subunits and KChIP2 auxiliary subunit. The fast transient outward potassium channels (Itof) α-subunits Kv4.2, Kv4.3 and KChIP2 were analyzed by Western-blot. The fast transient outward potassium current and the action potentials were recorded in isolated myocytes by the whole-cell patch-clamp technique. Leptin treatment induced an up-regulation of Kv4.2, Kv4.3 and KChIP2 subunits mRNA expression. However, transcriptional levels of Kir2.1, Cav1.2, or Cav3.1 α-subunit channels were unmodified by leptin. Protein expression levels of Kv4.2, Kv4.3 and KChIP2 subunits were also increased by leptin. The electrophysiological study showed that leptin increases the fast transient outward potassium current amplitudes and densities shortening action potential duration. In addition, leptin activated Akt signaling in cardiomyocytes and this mechanism was involved in the effect of leptin on Itof channels. In conclusión, leptin increases both the expression and function of Itof channels in adult ventricular myocytes and this mechanism involves Akt signaling. Altogether these data suggest that leptin could exert beneficial or detrimental effects depending on the initial ventricular myocyte repolarizing reserve.

Abstract 485: Enhanced Large Conductance Ca+2 -sensitive K+ Channels (bk) Activity Impairs the Myogenic Response in the Cerebral Vasculature of Fawn Hooded Hypertensive (fhh) Rat

Our recent studies have revealed that the myogenic response in cerebral arteries and autoregulation of cerebral blood flow is impaired in FHH and that transfer of a 2.4 Mb region of chromosome 1 from BN into FHH.1 BN congenic strain restores these responses. The present study examined the role of large conductance calcium activated potassium (BK) channel in altering the myogenic response in FHH rats. Whole-cell patch-clamp of cerebral vascular smooth muscle cells (VSMC) revealed a 4.6 fold increase in outward potassium (K) channel current densities (pA/pF) in FHH rats compared to FHH.1 BN congenic strain. Iberiotoxin (IBTX -a selective BK channel inhibitor) sensitive current densities are significantly greater in FHH rats compared with the FHH.1 BN congenic strain (FHH rats: +40mV; pre IBTX 43.1 ± 7.2, after IBTX 11.8 ± 2.2 pA/pF versus pre IBTX 5.6 ± 1 pA/pF and after IBTX 4.1 ± 0.6 pA/pF in the FHH.1 BN congenic strain). In excised patches, the BK channel exhibited similar single-channel slope conductance for FHH and the FHH.1 BN rats (208.9 pS versus 208.7 pS). However, the open channel probability (NP o ) was ~10 fold higher in FHH rats than in FHH.1 BN rats (1μM free (Ca +2 ) i : +40mV: FHH; 0.8 ± 0.04 versus 0.08 ± 0.004 in FHH.1 BN rats). Voltage and Ca 2+ sensitivity of the BK channel is similar in cerebral VSMC isolated from FHH and FHH.1 BN rats. Middle cerebral arterioles (MCA) isolated from FHH rats increased in diameter from 142 ± 16 to 157 ± 19 μm when pressure was increased from 40 to 140 mmHg. In contrast, the diameter of the MCA decreased by 49% in the FHH.1 BN congenic strain from 127 ± 16 to 65 ± 13 μm. Pharmacological block of BK channel by IBTX (100nM) restored myogenic response in FHH rats but had no effect in FHH.1 BN rats. These results indicate that the impaired myogenic response of the cerebral vessels in FHH rats is mediated via a gene and mechanism that enhances BK channel activity.

Classification of potassium and chlorine ionic currents in retinal ganglion cell line (RGC-5) by whole-cell patch clamp

Retinal ganglion cell line (RGC-5) has been widely used as a valuable model for studying pathophysiology and physiology of retinal ganglion cells in vitro. However, the electrophysiological characteristics, especially a thorough classification of ionic currents in the cell line, remain to be elucidated in details. In the present study, we determined the resting membrane potential (RMP) in RGC-5 cell line and then identified different types of ionic currents by using the whole-cell patch-clamp technique. The RMP recorded in the cell line was between -30 and -6 mV (-17.6 ± 2.6 mV, n = 10). We observed the following voltage-gated ion channel currents: (1) inwardly rectifying Cl- current (I Cl,ir), which could be blocked by Zn2+; (2) Ca2+-activated Cl- current (I Cl,Ca), which was sensitive to extracellular Ca2+ and could be inhibited by disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate; (3) inwardly rectifying K+ currents (I K1), which could be blocked by Ba2+; (4) a small amount of delayed rectifier K+ current (I K). On the other hand, the voltage-gated sodium channels current (I Na) and transient outward potassium channels current (I A) were not observed in this cell line. These results further characterize the ionic currents in the RGC-5 cell line and are beneficial for future studies especially on ion channel (patho)physiology and pharmacology in the RGC-5 cell line.

Effects of IGF-1 onIKandIK1Channels via PI3K/Akt Signaling in Neonatal Cardiac Myocytes

Previous studies suggest that sarcolemmal potassium currents play important roles in cardiac hypertrophy. IGF-1 contributes to cardiac hypertrophy via activation of PI3K/Akt signaling. However, the relationships between IGF-1, PI3K/Akt signaling and sarcolemmal potassium currents remain unknown. Therefore, we tested the hypothesis that IGF-1 and PI3K/Akt signaling, independently, decrease sarcolemmal potassium currents in cardiac myocytes of neonatal rats. We compared the delayed outward rectifier (IK) and the inward rectifier (IK) current densities resulting from IGF-1 treatments to those resulting from simulation of PI3K/Akt signaling using adenoviral (Ad) BD110 and wild-type Akt and to those resulting from inhibition of PI3K signaling by LY294002. Ad.BD110 and Ad.Akt decreased IK and these decrements were attenuated by LY 294002. The IGF-1 treatments decreased both IK and IK1 but only the IK decrement was attenuated by LY294002. These findings demonstrate that IGF-1 may contribute to cardiac hypertrophy by PI3K/Akt signal transduction mechanisms in neonatal rat cardiomyocytes. Failure of LY294002 to effectively antagonize IGF-1 induced decrements in IK1 suggests that a signal pathway adjunct to PI3K/Akt contributes to IGF-1 protection against arrhythmogenesis in these myocytes. Our findings imply that sarcolemmal outward and inward rectifier potassium channels are substrates for IGF-1/PI3K/Akt signal transduction molecules.

Effects of 50 Hz Magnetic Fields With Different Intensities Exposure on Transient Outward Potassium Channel of Cortical Neurons*

Despite growing concern about electromagnetic radiation,the biological effects of the frequency magnetic fields remain obscure.The cortical neurons isolated from the mice were exposed to a 50 Hz electromagnetic field(EMF 1 mT,5 mT,10 mT) for 15 min.Using the whole-cell patch clamp technique,the currents of the transient outward potassium channel were recorded off-line,and then the effects of EMF on the channels were investigated.Compared to the control group,a significant inhibition on the IA was reported,and the rate of inhibition were(63.0 ± 2.2)%(1 mT),(55.0 ± 1.7)%(5 mT),(38.0 ± 1.8)%(10 mT),respectively.Moreover,the characteristics of activation and inactivation were both influenced by EMF,because there were a decreasing both on the half activation voltage and the half inactivation voltage.Additionally,different effects on the channels had been found for the different intensities.In the research,the maximum inhibition rate of current was induced by 1 mT,and the changing of the half activation voltage and the half inactivation voltage were the biggest after exposure to 5 mT,and 10 mT EMF increased the slope factor of inactivation.The results indicated that EMF affected the conformational changes of ion channel protein on cell membrane,and further influenced the normal function of ion channels.

ORAL PRESENTATION

ORAL PRESENTATION

Ionic and Cellular Mechanisms Underlying the Development of Acquired Brugada Syndrome in Patients Treated with Antidepressants

Tricyclic antidepressants are known to induce cardiac arrhythmias at therapeutic or supratherapeutic doses. The tricyclic antidepressant, amitriptyline, is reported to induce ST segment elevation in the right precordial electrocardiogram (ECG) leads, thus unmasking Brugada syndrome (BrS). The mechanism by which antidepressants induce the BrS phenotype and associated sudden death is not well established. Action potentials (AP) were simultaneously recorded from epicardial and endocardial sites of isolated coronary-perfused canine right ventricular wedge preparations, together with a transmural pseudo-ECG. Amitriptyline alone (0.2 μM-1 mM) failed to induce a BrS phenotype. NS5806 (8 μM), a transient outward potassium channel current (I(to) ) agonist, was used to produce an outward shift of current mimicking a genetic predisposition to BrS. In the presence of NS5806, a therapeutic concentration of amitriptyline (0.2 μM) accentuated the epicardial AP notch leading to ST-segment elevation of the ECG. All-or-none repolarization at some epicardial sites but not others gave rise to phase-2-reentry and polymorphic ventricular tachycardia (VT) in 6 of 9 preparations. Isoproterenol (100 nM) or quinidine (10 μM) reversed the effects of amitriptyline aborting phase 2 reentry and VT (4/4). Using voltage-clamp techniques applied to isolated canine ventricular myocytes, 0.2 μM amitriptyline was shown to produce use-dependent inhibition of sodium channel current (I(Na) ), without significantly affecting I(to) (n = 5). Our data suggest that amitriptyline-induced inhibition of I(Na) unmasks the Brugada ECG phenotype and facilitates development of an arrhythmogenic substrate only in the setting of a genetic predisposition by creating repolarization heterogeneities that give rise to phase 2 reentry and VT.

Effects of Resibufogenin and Cinobufagin on voltage-gated potassium channels in primary cultures of rat hippocampal neurons

Outward delayed rectifier potassium channel and outward transient potassium channel have multiple important roles in maintaining the excitability of hippocampal neurons. The present study investigated the effects of two bufadienolides, Resibufogenin (RBG) and Cinobufagin (CBG), on the outward delayed rectifier potassium current (IK) and outward transient potassium current (IA) in rat hippocampal neurons. RBG and CBG have similar structures and both were isolated from the venom gland of toad skin. RBG inhibited both IK and IA, whereas CBG inhibited IK without noticeable effect on IA. Moreover, at 1μM concentration both RBG and CBG could alter some channel kinetics and gating properties of IK, such as steady-state activation and inactivation curves, open probability and time constants. These findings suggested that IK is probably a target of bufadienolides, which may explain the mechanisms of bufadienolides’ pathological effects on central nervous system.

Regulation of the instantaneous inward rectifier and the delayed outward rectifier potassium channels by Captopril and Angiotensin II via the Phosphoinositide-3 kinase pathway in volume-overload-induced hypertrophied cardiac myocytes

SummaryBackgroundEarly development of cardiac hypertrophy may be beneficial but sustained hypertrophic activation leads to myocardial dysfunction. Regulation of the repolarizing currents can be modulated by the activation of humoral factors, such as angiotensin II (ANG II) through protein kinases. The aim of this work is to assess the regulation of IK and IK1 by ANG II through the PI3-K pathway in hypertrophied ventricular myocytes.Material/MethodsCardiac eccentric hypertrophy was induced through volume-overload in adult male rats by aorto-caval shunt (3 weeks). After one week half of the rats were given captopril (2 weeks; 0.5 g/l/day) and the other half served as control. The voltage-clamp and western blot techniques were used to measure the delayed outward rectifier potassium current (IK) and the instantaneous inward rectifier potassium current (IK1) and Akt activity, respectively.ResultsHypertrophied cardiomyocytes showed reduction in IK and IK1. Treatment with captopril alleviated this difference seen between sham and shunt cardiomyocytes. Acute administration of ANG II (10−6M) to cardiocytes treated with captopril reduced IK and IK1 in shunts, but not in sham. Captopril treatment reversed ANG II effects on IK and IK1 in a PI3-K-independent manner. However in the absence of angiotensin converting enzyme inhibition, ANG II increased both IK and IK1 in a PI3-K-dependent manner in hypertrophied cardiomyocytes.ConclusionsThus, captopril treatment reveals a negative effect of ANG II on IK and IK1, which is PI3-K independent, whereas in the absence of angiotensin converting enzyme inhibition IK and IK1 regulation is dependent upon PI3-K.

Ionic and Cellular Mechanisms Underlying the Development of Acquired Brugada Syndrome in Patients Treated with Antidepressants

Background: Tricyclic antidepressants, such as amitriptyline, is reported to induce ST segment elevation in the right precordial ECG leads, thus unmasking Brugada syndrome (BrS). The mechanism by which antidepressants induce the BrS phenotype and associated sudden death is not well established. Methods and Results: Action potentials (AP) were recorded from epicardial and endocardial sites of coronary-perfused canine right ventricular wedge preparations, together with a transmural pseudo-ECG. Amitriptyline alone (0.2 µM–1 mM) failed to induce a BrS phenotype. In the presence of NS5806 (8 µM), a transient outward potassium channel current (Ito) agonist, a therapeutic concentration of amitriptyline (0.2 µM) accentuated the epicardial AP notch leading to ST-segment elevation of the ECG. All-or-none repolarization at some epicardial sites but not others gave rise to phase-2-reentry and polymorphic ventricular tachycardia (VT) in 6 of 9 preparations. Isoproterenol (100 nM) or quinidine (10 µM) reversed the effects of amitriptyline. Using voltage-clamp techniques applied to isolated cardiomyocytes, 0.2 µM amitriptyline was shown to produce use-dependent inhibition of sodium channel current (INa), without significantly affecting Ito (n=5). Conclusions: Our data suggest that amitriptyline-induced inhibition of INa unmasks the Brugada ECG phenotype and facilitates development of an arrhythmogenic substrate only in the setting of a genetic predisposition by creating repolarization heterogeneities that give rise to phase-2-reentry and VT.

Structural basis for the decrease in the outward potassium channel current induced by lanthanum

The current of the outward K+ channel in the cell of horseradish treated with La3+ and the direct interaction between La3+ and the K+ channel protein were investigated using the whole-cell patch-clamp technique, molecular dynamics simulation, and quantum chemistry calculation methods. It was found for the first time that La3+ decreases the current of the K+ channel in the horseradish mesophyll cell. The decrease results from the formation of a coordination bond and hydrogen bond between La3+ and the K+ channel protein in the plasma membrane. The direct interaction destroys the native structure of the K+ channel protein, disturbing the function of the K+ channel protein in the cells. The results can provide the theoretical foundation for understanding the interaction between metal ions (especially high-valence metal ions) and the channel protein in organisms, including animal and plant cells.

Propranolol regulates cardiac transient outward potassium channel in rat myocardium via cAMP/PKA after short-term but not after long-term ischemia

It was recently suggested that the antiarrhythmic effect of propranolol, a ss-adrenoceptor antagonist, on ischemic myocardium includes restoration of I(K1) current and Cx43 conductance; however, little is known whether effects on the transient outward current I(to) contribute. A model of myocardial infarction (MI) by ligating the left anterior descending coronary artery was established. Propranolol was given 1 h or daily for 3 months, whole-cell patch-clamp techniques were used to measure I(to). Kv4.2 and PKA levels were analyzed by Western blot and cAMP level was determined by radioimmunoassay. The results showed that propranolol decreased the incidence of arrhythmias induced by acute ischemia and mortality in 3 month MI rats. Propranolol restored the diminished I(to) density and Kv4.2 protein in MI hearts. In addition, neonatal cardiomyocyte pretreatment with propranolol or administrated after hypoxia can resume I(to) density. cAMP/PKA was enhanced in acute MI, the reason of decreased Kv4.2 expression. Treatment with propranolol prevented the increased cAMP/PKA in 1 h MI, whereas propranolol had little effect on decreased cAMP/PKA in 3 months MI. This study demonstrated that both short- and long-term propranolol administrations protect cardiomyocytes against arrhythmias and mortality caused by cardiac ischemia; the involvement of cAMP/PKA signal pathway in the regulation of propranolol on I(to) acted differently along with the ischemic progression.

Comparison of the Effects of the Transient Outward Potassium Channel Activator NS5806 on Canine Atrial and Ventricular Cardiomyocytes

Objective: NS5806 activates the transient outward potassium current (Ito) in canine ventricular cells. We compared the effects of NS5806 on canine ventricular versus atrial tissues and myocytes. Methods: NS5806 (10 μM) was evaluated in arterially-perfused canine right atrial and left ventricular wedges. Atrial and ventricular epi- and endocardial cells were isolated by enzymatic dissociation. Current and voltage-clamp recordings were made in the absence and presence of NS5806. Results: In ventricular wedges NS5806 increased phase 1 repolarization in epi- and midmyocardial cells. A minor effect on conduction and upstroke velocity also was observed. In contrast, application of NS5806 to atrial preparations slowed upstroke velocity and reduced excitability, consistent with sodium channel block. In ventricular myocytes, NS5806 increased the magnitude of Ito by 80% and 16% in epi and endo, respectively (at +40 mV). In atrial myocytes, NS5806 increased peak Ito by 25% and had no effect on the sustained pedestal current, IKur. INa density in atrial myocytes was nearly 100% greater than in endocardial myocytes. NS5806 caused a negative shift in steady-state mid-inactivation (V1/2) for both cell types (73.9±0.27 to −77.3±0.21mV for endocardial and −82.6±0.12 to −85.1±0.11mV for atrial cells). The shift in V1/2 resulted in a reduction of INa in both cell types. However, the more negative V1/2 in atrial cells suggests that atrial cells lose excitability at more depolarized voltages than endocardial cells which may explain the greater reduction of excitability in atrial vs ventricular wedges by NS5806. Conclusion: NS5806 produces a prominent augmentation of Ito with little effect on INa in the ventricles, but a potent inhibition of INa with little augmentation of Ito in atria.