IK Density Research Articles

Long-QT mutation p.K557E-Kv7.1: dominant-negative suppression of IKs, but preserved cAMP-dependent up-regulation.

Mutations in KCNQ1, encoding for Kv7.1, the α-subunit of the IKs channel, cause long-QT syndrome type 1, potentially predisposing patients to ventricular tachyarrhythmias and sudden cardiac death, in particular, during elevated sympathetic tone. Here, we aim at characterizing the p.Lys557Glu (K557E) Kv7.1 mutation, identified in a Dutch kindred, at baseline and during (mimicked) increased adrenergic tone. K557E carriers had moderate QTc prolongation that augmented significantly during exercise. IKs characteristics were determined after co-expressing Kv7.1-wild-type (WT) and/or K557E with minK and Yotiao in Chinese hamster ovary cells. K557E caused IKs loss of function with slowing of the activation kinetics, acceleration of deactivation kinetics, and a rightward shift of voltage-dependent activation. Together, these contributed to a dominant-negative reduction in IKs density. Confocal microscopy and western blot indicated that trafficking of K557E channels was not impaired. Stimulation of WT IKs by 3'-5'-cyclic adenosine monophosphate (cAMP) generated strong current up-regulation that was preserved for K557E in both hetero- and homozygosis. Accumulation of IKs at fast rates occurred both in WT and in K557E, but was blunted in the latter. In a computational model, K557E showed a loss of action potential shortening during β-adrenergic stimulation, in accordance with the lack of QT shortening during exercise in patients. K557E causes IKs loss of function with reduced fast rate-dependent current accumulation. cAMP-dependent stimulation of mutant IKs is preserved, but incapable of fully compensating for the baseline current reduction, explaining the long QT intervals at baseline and the abnormal QT accommodation during exercise in affected patients.

GDF15 regulates Kv2.1-mediated outward K+ current through the Akt/mTOR signalling pathway in rat cerebellar granule cells

GDF15 (growth/differentiation factor 15), a novel member of the TGFβ (transforming growth factor β) superfamily, plays critical roles in the central and peripheral nervous systems, but the signal transduction pathways and receptor subtypes involved are not well understood. In the present paper, we report that GDF15 specifically increases the IK (delayed-rectifier outward K+ current) in rat CGNs (cerebellar granule neurons) in time- and concentration-dependent manners. The GDF15-induced amplification of the IK is mediated by the increased expression and reduced lysosome-dependent degradation of the Kv2.1 protein, the main α-subunit of the IK channel. Exposure of CGNs to GDF15 markedly induced the phosphorylation of ERK (extracellular-signal-regulated kinase), Akt and mTOR (mammalian target of rapamycin), but the GDF15-induced IK densities and increased expression of Kv2.1 were attenuated only by Akt and mTOR, and not ERK, inhibitors. Pharmacological inhibition of the Src-mediated phosphorylation of TGFβR2 (TGFβ receptor 2), not TGFβR1, abrogated the effect of GDF15 on IK amplification and Kv2.1 induction. Immunoprecipitation assays showed that GDF15 increased the tyrosine phosphorylation of TGFβRII in the CGN lysate. The results of the present study reveal a novel regulation of Kv2.1 by GDF15 mediated through the TGFβRII-activated Akt/mTOR pathway, which is a previously uncharacterized Smad-independent mechanism of GDF15 signalling.

A590T mutation in KCNQ1 C-terminal helix D decreases IKs channel trafficking and function but not Yotiao interaction

KCNQ1 encodes the α subunit of the voltage-gated channel that mediates the cardiac slow delayed rectifier K+ current (IKs). Here, we report a KCNQ1 allele encoding an A590T mutation [KCNQ1(A590T)] found in a 39-year-old female with a mild QT prolongation. A590 is located in the C-terminal α helical region of KCNQ1 that mediates subunit tetramerization, membrane trafficking, and interaction with Yotiao. This interaction is known to be required for the proper modulation of IKs by cAMP. Since previous studies reported that mutations in the vicinity of A590 impair IKs channel surface expression and function, we examined whether and how the A590T mutation affects the IKs channel. Electrophysiological measurements in HEK-293T cells showed that the A590T mutation caused a reduction in IKs density and a right-shift of the current–voltage relation of channel activation. Immunocytochemical and immunoblot analyses showed the reduced cell surface expression of KCNQ1(A590T) subunit and its rescue by coexpression of the wild-type KCNQ1 [KCNQ1(WT)] subunit. Moreover, KCNQ1(A590T) subunit interacted with Yotiao and had a cAMP-responsiveness comparable to that of KCNQ1(WT) subunit. These findings indicate that the A590 of KCNQ1 subunit plays important roles in the maintenance of channel surface expression and function via a novel mechanism independent of interaction with Yotiao.

Differential effects of genetically-encoded Gβγ scavengers on receptor-activated and basal Kir3.1/Kir3.4 channel current in rat atrial myocytes

Opening of G-protein-activated inward-rectifying K+ (GIRK, Kir3) channels is regulated by interaction with βγ-subunits of Pertussis-toxin-sensitive G proteins upon activation of appropriate GPCRs. In atrial and neuronal cells agonist-independent activity (Ibasal) contributes to the background K+ conductance, important for stabilizing resting potential. Data obtained from the Kir3 signaling pathway reconstituted in Xenopus oocytes suggest that Ibasal requires free Gβγ. In cells with intrinsic expression of Kir3 channels this issue has been scarcely addressed experimentally. Two Gβγ-binding proteins (myristoylated phosducin — mPhos — and Gαi1) were expressed in atrial myocytes using adenoviral gene transfer, to interrupt Gβγ-signaling. Agonist-induced and basal currents were recorded using whole cell voltage-clamp. Expression of mPhos and Gαi1 reduced activation of Kir3 current via muscarinic M2 receptors (IK(ACh)). Inhibition of IK(ACh) by mPhos consisted of an irreversible component and an agonist-dependent reversible component. Reduction in density of IK(ACh) by overexpressed Gαi1, in contrast to mPhos, was paralleled by substantial slowing of activation, suggesting a reduction in density of functional M2 receptors, rather than Gβγ-scavenging as underlying mechanism. In line with this notion, current density and activation kinetics were rescued by fusing the αi1-subunit to an Adenosine A1 receptor. Neither mPhos nor Gαi1 had a significant effect on Ibasal, defined by the inhibitory peptide tertiapin-Q. These data demonstrate that basal Kir3 current in a native environment is unrelated to G-protein signaling or agonist-independent free Gβγ. Moreover, our results illustrate the importance of physiological expression levels of the signaling components in shaping key parameters of the response to an agonist.

Electrophysiology of human cardiac atrial and ventricular telocytes

Telocytes (TCs) with exceptionally long cellular processes of telopodes have been described in human epicardium to act as structural supporting cells in the heart. We examined myocardial chamber-specific TCs identified in atrial and ventricular fibroblast culture using immunocytochemistry and studied their electrophysiological property by whole-cell patch clamp. Atrial and ventricular TCs with extended telopodes and alternating podoms and podomers that expressed CD34, c-Kit and PDGFR-β were identified. These cells expressed large conductance Ca2+-activated K+ current (BKCa) and inwardly rectifying K+ current (IKir), but not transient outward K+ current (Ito) and ATP-sensitive potassium current (KATP). The active channels were functionally competent with demonstrated modulatory response to H2S and transforming growth factor (TGF)-β1 whereby H2S significantly inhibited the stimulatory effect of TGF-β1 on current density of both BKCa and IKir. Furthermore, H2S attenuated TGF-β1-stimulated KCa1.1/Kv1.1 (encode BKCa) and Kir2.1 (encode IKir) expression in TCs. Our results show that functionally competent K+ channels are present in human atrial and ventricular TCs and their modulation may have significant implications in myocardial physiopathology.

Alterations in Ionic Currents and Gap Junctional Coupling by Pan-Histone Deacetylase Inhibition

Dynamic balance of histone acetyl transferase and histone deacetylases (HDACs)is maintained in multiple signaling pathways determining expression and repression of genes. Pan-HDAC inhibitors like Vorinostat(VOR, ZolinzaTM) and Romidepsin (Istodax(r)) have been approved for treatment of peripheral and cutaneous T-cell lymphomas. On rare occasions during clinical trials,HDACi has been associated with symptomatic ECG alterations, cardiac arrhythmias, and even sudden cardiac death for unknown reasons. Rationale of this study is to identify electrophysiological and molecular determinants involved in asymptomatic cardiac abnormalities in patients treated with HDACi during clinical trials. We investigated the effects of VOR on ionic currents and gap junctions responsible for cardiac excitability and propagation. Cultured neonatal mouse ventricular myocytes (NMVM) were treated with 1-5μM VOR overnight and patch clamp experiments were performed to measure the transient (Ito) and steady state (IK) outward K+, peak inward Na+ (INa), and peak inward L-type Ca2+ (ICa,L) current densities. Dose-dependent decreases in INa density and NaV1.5 protein levels occurred along with +10mV shift in INa activation in myocytes treated with 1-5μM VOR. Conversely, ICa,L density increased significantly without concomitant change in CaV1.2 protein expression levels. Moreover, augmentation of ICa,L byVOR was independent of stimulation with 5μM isoproterenol, suggesting discrete acetylation and phosphorylation dependent mechanisms for modulation of cardiac ICa,L However, VOR did not alter any IK or Ito current densities. Also, VOR dose dependently reduced gap junction coupling and protein expression of connexin43. Pan-HDACi treatment significantly reduces Cx43 protein expression and coupling along with salient alterations in INa and ICa,L densities. These ion channel changes present possible etiologies for cardiac abnormalities in patients. VOR being a pan-HDAC inhibitor, we plan to determine the effects of isoform-selective HDACi on ionic currents and APD in human cardiomyocytes derived from induced pluripotent stem cells.

Revisiting K+ Channel-Dependent Electrical Remodeling in the Border Zone

Revisiting K⁺ Channel-Dependent Electrical Remodeling in the Border Zone

The Sialyltransferase, ST3Gal4, Modulates Ventricular Electrical Signaling

Sialic acids impact voltage‐gated Na+ channel (Nav) and K+ channel (Kv) function, but little is understood about how the glycosylation machinery influences electrical signaling. Diseases of glycosylation often cause symptoms consistent with changes in excitability that include arrhythmias. The sialyltransferase, ST3Gal4, is uniformly expressed throughout cardiac chambers and developmental stages. An ST3Gal4 knockout strain was used to investigate the role of sialylation in the cardiac electrical remodeling often associated with diseases of glycosylation. Data indicate ST3Gal4 deletion significantly affects Nav gating with no change in maximum current density. Activation voltages of voltage‐dependent kinetic components of K+ currents (IK), Ito and IKslow, were depolarized, along with a reduction in peak IK density. The ST3Gal4−/− ventricular myocyte refractory period was reduced and action potential waveform alterations were observed. Consistently, ST3Gal4−/− left ventricular epicardium demonstrated a reduced refractory period and was more susceptible to arrhythmias as observed through isolated whole heart optical mapping studies. The data indicate that relatively minor perturbations in the glycome cause significant changes in cardiac electrical signaling, highlighting the role of glycosylation in cardiac function. Additionally, these studies underscore the possible contribution of reduced sialylation as a mediator for the often observed changes in electrical excitability associated with diseases of glycosylation.

Hydrogen Sulfide Increases Excitability through Suppression of Sustained Potassium Channel Currents of Rat Trigeminal Ganglion Neurons

BackgroundHydrogen sulfide (H2S), an endogenous gaseotransmitter/modulator, is becoming appreciated that it may be involved in a wide variety of processes including inflammation and nociception. However, the role and mechanism for H2S in nociceptive processing in trigeminal ganglion (TG) neuron remains unknown. The aim of this study is to investigate distribution of endogenous H2S synthesizing enzyme cystathionine-β-synthetase (CBS) expression and role of H2S on excitability and voltage-gated potassium channels of TG neurons.MethodsImmunofluorescence studies were carried out to determine whether CBS was co-expressed in Kv1.1 or Kv1.4-positive TG neurons. Whole cell patch clamp recordings were employed on acutely isolated TG neurons from adult male Sprague Dawley rats (6–8 week old). von Frey filaments were used to examine the pain behavioral responses in rats following injection of sodium hydrosulfide.ResultsIn rat TG, 77.3±6.6% neurons were immunoreactive for CBS, 85.1±3.8% for Kv1.1 and 97.8±1.1% for Kv1.4. Double staining showed that all CBS labeled cells were Kv1.1 and Kv1.4 positive, but only 92.2±6.1% of Kv1.1 and 78.2±9.9% of Kv1.4 positive cells contained CBS. Application of H2S donor NaHS (250 μM) led to a significant depolarization of resting membrane potential recorded from TG neurons. NaHS application also resulted in a dramatic reduction in rheobase, hyperpolarization of action potential threshold, and a significant increase in the number of action potentials evoked at 2X and 3X rheobase stimulation. Under voltage-clamp conditions, TG neurons exhibited transient A-type (IA) and sustained outward rectifier K+ currents (IK). Application of NaHS did suppress IK density while did not change IA density of TG neurons (n=6). Furthermore, NaHS, a donor of hydrogen sulfide, produced a significant reduction in escape threshold in a dose dependent manner.ConclusionThese data suggest that endogenous H2S generating enzyme CBS was co-localized well with Kv1.1 and Kv1.4 in TG neurons and that H2S produces the mechanic pain and increases neuronal excitability, which might be largely mediated by suppressing IK density, thus identifying for the first time a specific molecular mechanism underlying pain and sensitization in TG.

Basic Science of Cardiac Resynchronization Therapy

Electromechanical dyssynchrony can markedly worsen heart failure (HF) morbidity and mortality, independent of traditional risk factors.1–4 Depending on the metric used, current estimates of the prevalence of dyssynchrony vary from 25–30% in patients with HF (based on QRS widening) up to 60%, based on tissue Doppler or MRI measures of dyssynchronous contraction of the left ventricle (LV).5,6 Cardiac resynchronization therapy (CRT) or biventricular pacing has emerged as a promising option to treat patients with HF and dyssynchronous contraction.7–9 The past few decades have seen the rise of pharmacotherapy, primarily through agents that antagonize the effect of excessive concentrations of circulating neurohormones, yet, HF-related morbidity and mortality remain high.3–6 Biventricular stimulation has been demonstrated to improve contractile performance in patients with mechanical dyssynchrony acutely and chronically while also prolonging long-term survival—something not yet achieved by drug therapy.10 Although the clinical and mechanical effectiveness of CRT are well described, 30% of patients do not benefit from CRT and clinical criteria to identify CRT nonresponders remain elusive8,11,12 Currently, the most widely used predictor of reverse remodeling is the presence of marked mechanical dyssynchrony before CRT, as indexed by the width of the QRS.13 Mechanical dyssynchrony seems important, yet imaging-based measures have not predicted response well14 and even improvement in dyssynchrony after initiation of CRT only weakly predicts chronic response.15 Limited understanding of the molecular mechanisms underlying reverse cardiac remodeling induced by CRT has hampered the selection of potential responders. In this review, we focus on the electrophysiological aspects and molecular networks underlying the benefits of CRT. We will review how CRT homogenizes regional differences in stress kinase signaling and electric remodeling and then review its global effect on myocyte function and its …

The newly identified K+ channel blocker talatisamine attenuates beta-amyloid oligomers induced neurotoxicity in cultured cortical neurons

Loss of cytosolic K+ through up-regulated delayed rectifier K+ channels play an important role in beta-amyloid (Aβ) induced neurotoxicity. Potent K+ channel blocker, particular specific for IK channels has been suggested as an attractive candidate for the treatment of Alzheimer's disease (AD). Talatisamine is a novel IK channel blocker discovered by virtual screening and electrophysiological characterization. In the present study, we examined the neuroprotective effect of talatisamine against Aβ oligomers induced cytotoxicity in primarily cultured cortical neurons. The neurotoxicity related to K+ loss caused by Aβ40 oligomers included enhanced IK density, increased cell membrane permeability, reduced cell viability, and impaired mitochondrial transmembrane potential. Decreased Bcl-2 and increased Bax level, activation of Caspase-3 and Caspase-9 were also observed after Aβ40 oligomers incubation. Talatisamine (120μM) and TEA (5mM) inhibited the enhanced IK caused by Aβ40 oligomers, attenuated cytotoxicity of Aβ oligomers by restoring cell viability and suppressing K+ loss related apoptotic response. Our results suggested that talatisamine may become a leading compound as IK channel blocker for neuroprotection.

Enhanced Excitability and Down-Regulated Voltage-Gated Potassium Channels in Colonic DRG Neurons from Neonatal Maternal Separation Rats

Irritable bowel syndrome (IBS), characterized mainly by abdominal pain, is a functional bowel disorder. The present study aimed to examine changes in the excitability and the activity of the voltage-gated K+ channel in dorsal root ganglia (DRG) neurons innervating the colon of rats subjected to neonatal maternal separation (NMS). Colonic DRG neurons from NMS rats as identified by FAST DiI™ labeling showed an increased cell size compared with those from nonhandled (NH) rats. Whole cell current-clamp recordings showed that colonic DRG neurons from NMS rats displayed: 1) depolarized resting membrane potential; 2) increased input resistance; 3) a dramatic reduction in rheobase; and 4) a significant increase in the number of action potentials evoked at twice rheobase. Whole cell voltage-clamp recordings revealed that neurons from both groups exhibited transient A-type (IA) and delayed rectifier (IK) K+ currents. Compared with NH rat neurons, the averaged density of IK was significantly reduced in NMS rat neurons. Furthermore, the Kv1.2 expression was significantly decreased in NMS rat colonic DRG neurons. These results suggest that NMS increases the excitability of colonic DRG neurons mainly by suppressing the IK current, which is likely accounted for by the downregulation of the Kv1.2 expression and somal hypertrophy. PerspectiveThis study demonstrates the alteration of delayed rectifier K current and Kv1.2 expression in DRG neurons from IBS model rats, representing a molecular mechanism underlying visceral pain and sensitization in IBS, suggesting the potential of Kv1.2 as a therapeutic target for the treatment of IBS.

Uneven Expression Patterns of KCNQ1 and KCNE Subunits in the Heart Impact on the Function of Slow Delayed Rectifier (IKs) Channels

Background: IKs has 2 major components: pore-forming (KCN)Q1 channel & regulatory (KCN)E1 subunits. Human heart expresses other members of the KCNE family (E2 - E5) that can all associate with Q1 & confer distinctly different channel phenotypes. The expression patterns of Q1 & different KCNE subunits in the heart, and their relation to the IKs channel function, is not clear. Methods: We use immunoblotting to quantify the protein levels of Q1 & E1 - E3 in different regions of the heart, and use patch clamp to quantify IKs current density and gating kinetics in left atrial (LA) and ventricular (LV) myocytes. We use the guinea pig model because of the robust cardiac IKs. Results: Immunoblot data: (1) Q1 protein is more abundant in atria (A) than in ventricles (V) (immunoblot densitometry ratio of A:V ∼ 1:0.4), (2) E1 & E2 proteins are more abundant in V than in A (E1 ratio ∼ 1:2 - 3; E2 ratio ∼ 1:2), (3) the E3 Ab recognizes a specific 20 kDa band in A but not in V. Patch clamp data: (1) IKs current density is much higher in LA than in LV myocytes, (2) IKs half-maximal activation voltage is more negative in LA than in LV myocytes, (3) IKs activates faster in LA than in LV myocytes. Conclusion: Q1 and E3 are more abundant in A than in V, while E1 & E2 have the opposite expression pattern. The uneven protein expression patterns can enhance IKs contribution to atrial action potential repolarization by generating a higher IKs density, that can reach a higher degree of activation in the action potential plateau range, than its counterpart in the ventricles.

Regulation of IK by PI3 Kinase/Akt Pathway in Cardiac Myocytes

Abnormal regulation of the repolarization process in cardiac myocytes is related to the induction of certain arrhythmias and ventricular dysfunction. Regulation of the repolarizing delayed outward rectifier current (IK) can be modulated by inotropic and inotrophic hormones, such as IGF‐1. The aim of this work was to assess the regulation of IK by the cell‐survival pathway, IGF‐1/PI3 K/Akt, in freshly isolated neonatal rat ventricular myocytes, using patch‐clamp technique in whole‐cell configuration. Cells were stimulated from holding potential of ‐40mV, by 500msec, 10‐mV step voltages to +40mV. IGF‐1 reduced the functional density of IK from 4.1 ± 0.5 pA/pF to 2.6 ± 0.3 pA/pF. IGF‐1 effect was eliminated by the PI3 Kinase inhibitor, LY294002 (3μM). Adenoviral transfection of cardiocytes with a constitutively activated PI3 K, BD110, mimicked the IGF‐1 effect on IK (2.5 ± 0.4 pA/pF), which was abrogated by LY 294002. Similar to BD110, transfection of the cardiac myocytes with wild‐type Akt reduced IK (2.4 ± 0.3 pA/pF), which was completely reversed by LY 294002. We conclude that IK is negatively regulated by IGF‐1 through downstream activation of PI3 Kinase and Akt. Such a mechanism could play an anti‐arrhythmic role during cardiac events that are associated with the activation of the PI3 K/Akt signaling, such as cardiac hypertrophy.

MinK-dependent internalization of the IKs potassium channel

KCNQ1-MinK potassium channel complexes (4alpha:2beta stoichiometry) generate IKs, the slowly activating human cardiac ventricular repolarization current. The MinK ancillary subunit slows KCNQ1 activation, eliminates its inactivation, and increases its unitary conductance. However, KCNQ1 transcripts outnumber MinK transcripts five to one in human ventricles, suggesting KCNQ1 also forms other heteromeric or even homomeric channels there. Mechanisms governing which channel types prevail have not previously been reported, despite their significance: normal cardiac rhythm requires tight control of IKs density and kinetics, and inherited mutations in KCNQ1 and MinK can cause ventricular fibrillation and sudden death. Here, we describe a novel mechanism for this control. Whole-cell patch-clamping, confocal immunofluorescence microscopy, antibody feeding, biotin feeding, fluorescent transferrin feeding, and protein biochemistry techniques were applied to COS-7 cells heterologously expressing KCNQ1 with wild-type or mutant MinK and dynamin 2 and to native IKs channels in guinea-pig myocytes. KCNQ1-MinK complexes, but not homomeric KCNQ1 channels, were found to undergo clathrin- and dynamin 2-dependent internalization (DDI). Three sites on the MinK intracellular C-terminus were, in concert, necessary and sufficient for DDI. Gating kinetics and sensitivity to XE991 indicated that DDI decreased cell-surface KCNQ1-MinK channels relative to homomeric KCNQ1, decreasing whole-cell current but increasing net activation rate; inhibiting DDI did the reverse. The data redefine MinK as an endocytic chaperone for KCNQ1 and present a dynamic mechanism for controlling net surface Kv channel subunit composition-and thus current density and gating kinetics-that may also apply to other alpha-beta type Kv channel complexes.

The Effect of Amiodarone on K<sup>+</sup> Channel Current in the Normal and Hypertrophied Rat Heart

The basis for the unique effectiveness of amiodarone treatment on cardiac arrhythmias is incompletely understood. We investigated and compared the effects of amiodarone on K + channels of hypertrophied ventricular myocytes with normal ventricular myocytes in rats. The pressure overload hypertrophy models of rats were established by partial ligation of ascending aorta for 4 weeks. Ventricular myocytes were exposed to 1, 10 and 50µmol/l amiodarone, and whole cell patch-clamp technique was used to study the effects of amiodarone on outward currents, such as delayed rectifier outward K + current (IK), slowly activating delayed rectifier outward K + current (IKs), transient outward K + current (Ito )a nd rectifier K + current (IK1). Compared with the control group, the current density of IK ,I K1 and Ito were all decreased in hypertrophied myocytes group. Hyper-concentration of amiodarone (10 and 50µmol/l) inhibited Ito of the control group (37.5% ± 5.8% and 54.3% ± 5.7%) and the hypertrophied myocytes group (9.3% ± 2.3% and 22.8% ± 3.0%), but had effect on IK1 neither the control nor hypertrophied myocytes group. 10µmol/l amiodarone inhibited IKs 23.3% ± 6.2% in the control group and 50.1% ± 9.8% in the hypertrophied myocytes group. Ito and IKs appeared difference affection of amiodarone’s action on both groups. IKs in hypertrophy group was higher than that in control, whereas Ito was lower in hypertrophy group. We concluded amiodarone do inhibit Ito and IK in rat normal and hypertrophied cardiomyocytes. Amiodarone application should be treated difference between hypertrophied heart and normal heart.

Electrocardiographic interventricular dispersion of repolarization during autonomic adaptation in LQT1 subtype of long QT syndrome

Objectives. In LQT1 subtype of inherited long QT syndrome, repolarization abnormalities originating from defective IKs render patients vulnerable to ventricular arrhythmia during sudden sympathetic activation. Experimental studies show lower IKs density and longer action potential duration in left (LV) than in right (RV) ventricle. We studied interventricular dispersion of repolarization in patients with IKs defect during autonomic tests. Design. We measured interventricular (difference of QT intervals between LV and RV type leads) and transmural electrocardiographic dispersion of repolarization from 25-lead electrocardiograms in nine asymptomatic KCNQ1 mutation carriers (LQT1) and eight controls during rest, Valsalva maneuver, mental stress, sustained handgrip and supine exercise. Results. LQT1 carriers showed increased interventricular dispersion of repolarization (13±9 ms vs. 4±4 ms, p=0.03) during all tests. Valsalva strain increased the difference between the study groups. In LQT1 carriers, interventricular dispersion of repolarization correlated weakly with electrocardiographic transmural dispersion of repolarization. Conclusions. Asymptomatic KCNQ1 mutation carriers exhibit increased and by abrupt sympathetic activation augmented interventricular difference in electrocardiographic repolarization times. Interventricular and transmural repolarization dispersion behave similarly in patients with IKs defect.

Sudden sympathetic activation and electrocardiographic interventricular dispersion of repolarization in type 1 long QT syndrome

Patients with type 1 inherited long QT syndrome (LQT1) experience ventricular arrhythmias during sudden sympathetic activation. Experimental studies show smaller IKs density and consequently longer action potential duration in left (LV) than in right (RV) ventricle creating interventricular dispersion of repolarization (IDR). Increased IDR predisposes LV to initiate torsade de pointes. We hypothesized that sudden sympathetic activation may increase electrocardiographic (ECG) IDR in LQT1 patients. We studied 9 asymptomatic carriers of the same C-terminal KCNQ1 mutation (age 41±12) and 8 healthy controls (44±10) by measuring QT intervals from 25 simultaneous ECG leads over anterior thorax at rest and during mental stress, sustained handgrip and Valsalva maneuver. IDR was defined as difference of QT peak intervals between LV and RV type leads, identified according to QRS morphology. LQT1 carriers showed greater IDR than control subjects (13±9 ms vs. 4±4 ms, P=0.03) at baseline. Mental stress and handgrip did not change IDR in the study groups, whereas Valsalva strain augmented IDR in LQT1 carriers but reduced it in controls (Figure). Asymptomatic KCNQ1 mutation carriers exhibit increased IDR. Abrupt autonomic sympathetic activation augments IDR in LQT1 patients whereas during static sympathetic activity IDR remains stable. By abrupt sympathetic activation increased IDR may predispose patients with IKs defect to torsade de pointes.

IGF‐1 Modulatory Effects on I K and I K1 Through ERK1/2 and PI‐3K During Eccentric Cardiac Hypertrophy

The outward rectifier (IK) and the inward rectifier (IK1) K+ channels, responsible for the action potential repolarization and maintenance of the resting potential respectively, are altered during cardiac hypertrophy. Activation of insulin-like growth factor-1 (IGF-1) during hypertrophy may affect channel activity. The aim was to study the modulatory effect of IGF-1 on IK and IK1 through EK1/2 and PI3K pathways during hypertrophy. Using specific inhibitors for ERK1/2 (PD98059) and PI3K/Akt (LY294002), whole-cell patch-clamp was conducted on sham and aortocaval shunt-induced eccentric cardiac hypertrophy myocytes from adult rats. In sham and shunt cardiocytes respectively: ERK1/2 inhibition caused reduction by 33% and 40% in IK density and 38% and 53% in that of IK1; Akt inhibition caused decrease by 29% and 23% in IK density and 47% and 17% in that of IK1. In the presence of LY294002, IGF-1 reduced IK density by 36% in sham and 23% in hypertrophied myocytes; whereas it had no effect on IK1 in both groups. In the presence of PD98059, IGF-1 had no affect on IK or IK1 in sham with slight 7% decrease in shunt. Thus, IK and IK1 are positively modulated by basal ERK and PI3K activities. ERK and PI3K are both sufficient and necessary for IK1 modulation by IGF-1; whereas only ERK is for IK. Cardiac hypertrophy is associated with a shift towards a larger role of PI3K in the modulation of both K+ channel. Supported in part by grants GM08016-38 NIGMS/NIH and 2G12 RR003048 RCMI, Division of Research Infrastructure, NCRR/NIH.

Enhanced excitability and suppression of A-type K+ current of pancreas-specific afferent neurons in a rat model of chronic pancreatitis

Chronic pancreatitis (CP) is a relatively common disorder, characterized by glandular insufficiency and chronic, often intractable, pain. The mechanism of pain in CP is poorly understood. We have previously developed a model of trinitrobenzene sulphonic acid (TNBS)-induced CP that results in nociceptive sensitization in rats. This study was designed to examine changes in the excitability and alteration of voltage-gated K(+) currents of dorsal root ganglia (DRG) neurons innervating the pancreas. CP was induced in adult rats by an intraductal injection of TNBS. DRG neurons innervating the pancreas were identified by 1,1'-dioleyl-3,3,3',3-tetramethylindocarbocyanine methanesulfonate fluorescence labeling. Perforated patch-clamp recordings were made from acutely dissociated DRG neurons from control and TNBS-treated rats. Pancreas-specific DRG neurons displayed more depolarized resting potentials in TNBS-treated rats than those in controls (P < 0.02). Some neurons from the TNBS-treated group exhibited spontaneous firings. TNBS-induced CP also resulted in a dramatic reduction in rheobase (P < 0.05) and a significant increase in the number of action potentials evoked at twice rheobase (P < 0.05). Under voltage-clamp conditions, neurons from both groups exhibited transient A-type (I(A)) and sustained outward rectifier K(+) currents (I(K)). Compared with controls, the average I(A) but not the average I(K) density was significantly reduced in the TNBS-treated group (P < 0.05). The steady-state inactivation curve for I(A) was displaced by approximately 20 mV to more hyperpolarized levels after the TNBS treatment. These data suggest that TNBS treatment increases the excitability of pancreas-specific DRG neurons by suppressing I(A) density, thus identifying for the first time a specific molecular mechanism underlying chronic visceral pain and sensitization in CP.