Differential inhibition of transient outward currents of Kv1.4 and Kv4.3 by endothelin

Regulation of Kv1.4 potassium channels by PKC and AMPK kinases

Over the last years extensive kinase-mediated regulation of a number of voltage-gated potassium (Kv) channels important in cardiac electrophysiology has been reported. This includes regulation of Kv1.5, Kv7.1 and Kv11.1 cell surface expression, where the kinase-mediated regulation appears to center around the ubiquitin ligase Nedd4-2. In the present study we examined whether Kv1.4, constituting the cardiac I_to,s current, is subject to similar regulation. In the epithelial Madin-Darby Canine Kidney (MDCK) cell line, which constitutes a highly reproducible model system for addressing membrane targeting, we find, by confocal microscopy, that Kv1.4 cell surface expression is downregulated by activation of protein kinase C (PKC) and AMP-activated protein kinase (AMPK). In contrast, manipulating the activities of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) and serum and glucocorticoid-regulated kinase 1 (SGK1) were without effect on channel localization. The PKC and AMPK-mediated downregulation of Kv1.4 membrane surface localization was confirmed by two-electrode voltage clamp in Xenopus laevis oocytes, where pharmacological activation of PKC and AMPK reduced Kv1.4 current levels. We further demonstrate that unlike related Kv channels, Kv1.4 current levels in Xenopus laevis oocytes are not reduced by co-expression of Nedd4-2, or the related Nedd4-1 ubiquitin ligase. In conclusion, we demonstrate that the surface expression of Kv1.4 is downregulated by the two kinases AMPK and PKC, but is unaffected by PI3K-SGK1 signaling, as well as Nedd4-1/Nedd4-2 activity. In the light of previous reports, our results demonstrate an impressive heterogeneity in the molecular pathways controlling the surface expression of highly related potassium channel subunits.

Molecular variability elicits a tunable switch with discrete neuromodulatory response phenotypes

Recent single cell studies show extensive molecular variability underlying cellular responses. We evaluated the impact of molecular variability in the expression of cell signaling components and ion channels on electrophysiological excitability and neuromodulation. We employed a computational approach that integrated neuropeptide receptor-mediated signaling with electrophysiology. We simulated a population of neurons in which expression levels of a neuropeptide receptor and multiple ion channels were simultaneously varied within a physiological range. We analyzed the effects of variation on the electrophysiological response to a neuropeptide stimulus. Our results revealed distinct response patterns associated with low versus high receptor levels. Neurons with low receptor levels showed increased excitability and neurons with high receptor levels showed reduced excitability. These response patterns were separated by a narrow receptor level range forming a separatrix. The position of this separatrix was dependent on the expression levels of multiple ion channels. To assess the relative contributions of receptor and ion channel levels to the response profiles, we categorized the responses into six phenotypes based on response kinetics and magnitude. We applied several multivariate statistical approaches and found that receptor and channel expression levels influence the neuromodulation response phenotype through a complex though systematic mapping. Our analyses extended our understanding of how cellular responses to neuromodulation vary as a function of molecular expression. Our study showed that receptor expression and biophysical state interact with distinct relative contributions to neuronal excitability.

Atrial remodelling in atrial fibrillation: CaMKII as a nodal proarrhythmic signal

CaMKII is a serine-threonine protein kinase that is abundant in myocardium. Emergent evidence suggests that CaMKII may play an important role in promoting atrial fibrillation (AF) by targeting a diverse array of proteins involved in membrane excitability, cell survival, calcium homeostasis, matrix remodelling, inflammation, and metabolism. Furthermore, CaMKII inhibition appears to protect against AF in animal models and correct proarrhythmic, defective intracellular Ca(2+) homeostasis in fibrillating human atrial cells. This review considers current concepts and evidence from animal and human studies on the role of CaMKII in AF.

Multiscale model of dynamic neuromodulation integrating neuropeptide-induced signaling pathway activity with membrane electrophysiology

We developed a multiscale model to bridge neuropeptide receptor-activated signaling pathway activity with membrane electrophysiology. Typically, the neuromodulation of biochemical signaling and biophysics have been investigated separately in modeling studies. We studied the effects of Angiotensin II (AngII) on neuronal excitability changes mediated by signaling dynamics and downstream phosphorylation of ion channels. Experiments have shown that AngII binding to the AngII receptor type-1 elicits baseline-dependent regulation of cytosolic Ca(2+) signaling. Our model simulations revealed a baseline Ca(2+)-dependent response to AngII receptor type-1 activation by AngII. Consistent with experimental observations, AngII evoked a rise in Ca(2+) when starting at a low baseline Ca(2+) level, and a decrease in Ca(2+) when starting at a higher baseline. Our analysis predicted that the kinetics of Ca(2+) transport into the endoplasmic reticulum play a critical role in shaping the Ca(2+) response. The Ca(2+) baseline also influenced the AngII-induced excitability changes such that lower Ca(2+) levels were associated with a larger firing rate increase. We examined the relative contributions of signaling kinases protein kinase C and Ca(2+)/Calmodulin-dependent protein kinase II to AngII-mediated excitability changes by simulating activity blockade individually and in combination. We found that protein kinase C selectively controlled firing rate adaptation whereas Ca(2+)/Calmodulin-dependent protein kinase II induced a delayed effect on the firing rate increase. We tested whether signaling kinetics were necessary for the dynamic effects of AngII on excitability by simulating three scenarios of AngII-mediated KDR channel phosphorylation: (1), an increased steady state; (2), a step-change increase; and (3), dynamic modulation. Our results revealed that the kinetics emerging from neuromodulatory activation of the signaling network were required to account for the dynamical changes in excitability. In summary, our integrated multiscale model provides, to our knowledge, a new approach for quantitative investigation of neuromodulatory effects on signaling and electrophysiology.

Increased excitability of medium-sized dorsal root ganglion neurons by prolonged interleukin-1β exposure is K(+) channel dependent and reversible

KEY POINTS

Neuropathic pain resulting from peripheral nerve injury is initiated and maintained by persistent ectopic activity in primary afferent neurons. Sciatic nerve injury increases the excitability of medium-sized dorsal root ganglion (DRG) neurons. Levels of the inflammatory cytokine interleukin 1β (IL-1β) increase and peak after 7 days. Five to six days of exposure of medium sized DRG neurons to 100 pm IL-1β promotes persistent increases in excitability which abate within 3-4 days of cytokine removal. This is associated with a profound attenuation of K(+) channel currents but only modest increases in function of cyclic nucleotide-sensitive hyperpolarization-activated channels (HCNs) and of voltage-gated Na(+) and Ca(2+) channel currents. It is unlikely, therefore, that direct interaction of IL-1β with DRG neurons is capable of initiating an enduring phenotypic shift in their electrophysiological properties that follows sciatic nerve injury. The findings also underline the importance of K(+) channel modulation in the actions of inflammatory mediators on peripheral neurons.

ABSTRACT

Chronic constriction injury of rat sciatic nerve promotes signs of neuropathic pain. This is associated with an increase in the level of interleukin 1β (IL-1β) in primary afferents that peaks at 7 days. This initial cytokine exposure has been proposed to trigger an enduring alteration in neuronal phenotype that underlies chronic hyper-excitability in sensory nerves, which initiates and maintains chronic neuropathic pain. We have shown previously that 5-6 days of exposure of rat dorsal root ganglia (DRGs) to 100 pm IL-1β increases the excitability of medium-sized neurons. We have now found using whole-cell recording that this increased excitability reverts to control levels within 3-4 days of cytokine removal. The effects of IL-1β were dominated by changes in K(+) currents. Thus, the amplitudes of A-current, delayed rectifier and Ca(2+) -sensitive K(+) currents were reduced by ∼68%, ∼64% and ∼36%, respectively. Effects of IL-1β on other cation currents were modest by comparison. There was thus a slight decrease in availability of high voltage-activated Ca(2+) channel current, a small increase in rates of activation of hyperpolarization-activated cyclic nucleotide-gated channel current (IH ), and a shift in the voltage dependence of activation of tetrodotoxin-sensitive sodium current (TTX-S INa ) to more negative potentials. It is unlikely, therefore, that direct interaction of IL-1β with DRG neurons initiates an enduring phenotypic shift in their electrophysiological properties following sciatic nerve injury. Persistent increases in primary afferent excitability following nerve injury may instead depend on altered K(+) channel function and on the continued presence of slightly elevated levels IL-1β and other cytokines.

Calmodulin-dependent protein kinase II: linking heart failure and arrhythmias

Understanding relationships between heart failure and arrhythmias, important causes of suffering and sudden death, remains an unmet goal for biomedical researchers and physicians. Evidence assembled over the past decade supports a view that activation of the multifunctional Ca(2+) and calmodulin-dependent protein kinase II (CaMKII) favors myocardial dysfunction and cell membrane electrical instability. CaMKII activation follows increases in intracellular Ca(2+) or oxidation, upstream signals with the capacity to transition CaMKII into a Ca(2+) and calmodulin-independent constitutively active enzyme. Constitutively active CaMKII appears poised to participate in disease pathways by catalyzing the phosphorylation of classes of protein targets important for excitation-contraction coupling and cell survival, including ion channels and Ca(2+) homeostatic proteins, and transcription factors that drive hypertrophic and inflammatory gene expression. This rich diversity of downstream targets helps to explain the potential for CaMKII to simultaneously affect mechanical and electrical properties of heart muscle cells. Proof-of-concept studies from a growing number of investigators show that CaMKII inhibition is beneficial for improving myocardial performance and for reducing arrhythmias. We review the molecular physiology of CaMKII and discuss CaMKII actions at key cellular targets and results of animal models of myocardial hypertrophy, dysfunction, and arrhythmias that suggest CaMKII inhibition may benefit myocardial function while reducing arrhythmias.

Neurokinins inhibit low threshold inactivating K+ currents in capsaicin responsive DRG neurons

Neurokinins (NK) released from terminals of dorsal root ganglion (DRG) neurons may control firing of these neurons by an autofeedback mechanism. In this study we used patch clamp recording techniques to determine if NKs alter excitability of rat L4-S3 DRG neurons by modulating K(+) currents. In capsaicin (CAPS)-responsive phasic neurons substance P (SP) lowered action potential (AP) threshold and increased the number of APs elicited by depolarizing current pulses. SP and a selective NK(2) agonist, [betaAla(8)]-neurokinin A (4-10) also inhibited low threshold inactivating K(+) currents isolated by blocking non-inactivating currents with a combination of high TEA, (-) verapamil and nifedipine. Currents recorded under these conditions were heteropodatoxin-sensitive (Kv4 blocker) and alpha-dendrotoxin-insensitive (Kv1.1 and Kv1.2 blocker). SP and NKA elicited a >10 mV positive shift of the voltage dependence of activation of the low threshold currents. This effect was absent in CAPS-unresponsive neurons. The effect of SP or NKA on K(+) currents in CAPS-responsive phasic neurons was fully reversed by an NK(2) receptor antagonist (MEN10376) but only partially reversed by a PKC inhibitor (bisindolylmaleimide). An NK(1) selective agonist ([Sar(9), Met(11)]-substance P) or direct activation of PKC with phorbol 12,13-dibutyrate, did not change firing in CAPS-responsive neurons, but did inhibit various types of K(+) currents that activated over a wide range of voltages. These data suggest that the excitability of CAPS-responsive phasic afferent neurons is increased by activation of NK(2) receptors and that this is due in part to inhibition and a positive voltage shift in the activation of heteropodatoxin-sensitive Kv4 channels.

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.

Neonatal rat cardiac fibroblasts express three types of voltage-gated K+ channels: regulation of a transient outward current by protein kinase C

Cardiac fibroblasts regulate myocardial development via mechanical, chemical, and electrical interactions with associated cardiomyocytes. The goal of this study was to identify and characterize voltage-gated K(+) (Kv) channels in neonatal rat ventricular fibroblasts. With the use of the whole cell arrangement of the patch-clamp technique, three types of voltage-gated, outward K(+) currents were measured in the cultured fibroblasts. The majority of cells expressed a transient outward K(+) current (I(to)) that activated at potentials positive to -40 mV and partially inactivated during depolarizing voltage steps. I(to) was inhibited by the antiarrhythmic agent flecainide (100 microM) and BaCl(2) (1 mM) but was unaffected by 4-aminopyridine (4-AP; 0.5 and 1 mM). A smaller number of cells expressed one of two types of kinetically distinct, delayed-rectifier K(+) currents [I(K) fast (I(Kf)) and I(K) slow (I(Ks))] that were strongly blocked by 4-AP. Application of phorbol 12-myristate 13-acetate, to stimulate protein kinase C (PKC), inhibited I(to) but had no effect on I(Kf) and I(Ks). Immunoblot analysis revealed the presence of Kv1.4, Kv1.2, Kv1.5, and Kv2.1 alpha-subunits but not Kv4.2 or Kv1.6 alpha-subunits in the fibroblasts. Finally, pretreatment of the cells with 4-AP inhibited angiotensin II-induced intracellular Ca(2+) mobilization. Thus neonatal cardiac fibroblasts express at least three different Kv channels that may contribute to electrical/chemical signaling in these cells.

Neurokinins enhance excitability in capsaicin-responsive DRG neurons

Neurokinins released by capsaicin-responsive (C-R) dorsal root ganglia neurons (DRG) may control firing in these neurons by an autofeedback mechanism. Here we used patch clamp techniques to examine the effects of neurokinins on firing properties of dissociated DRG neurons of male rats. In C-R neurons that generated only a few action potentials (APs, termed phasic) in response to long depolarizing current pulses (600 ms), substance P (SP, 0.5 microM) lowered the AP threshold by 11.0+/-0.3 mV and increased firing from 1.1+/-0.7 APs to 5.2+/-0.6 APs. In C-R tonic neurons that fire multiple APs, SP elicited smaller changes in AP threshold (6.0+/-0.1 mV reduction) and the number of APs (11+/-1 vs. 9+/-1 in control). The effects of SP were similar to the effect of heteropodatoxin II (0.05 microM) or low concentrations of 4-aminopyridine (50 microM) that block A-type K(+) currents. A selective NK(2) agonist, [betaAla(8)]-neurokinin A (4-10) (0.5 microM), mimicked the effects of SP. The effects of SP in C-R phasic neurons were fully reversed by an NK(2) receptor antagonist (MEN10376, 0.5 microM) but only partially by a protein kinase C (PKC) inhibitor (bisindolylmaleimide, 0.5 microM). An NK(3)-selective agonist ([MePhe(7)]-neurokinin B, 0.5 microM), an NK(1)-selective agonist ([Sar(9), Met(11)]-substance P, 0.5 microM) or activation of PKC with phorbol 12,13-dibutyrate (0.5 microM) did not change firing. Our data suggest that the excitability of C-R phasic afferent neurons is increased by activation of NK(2) receptors and intracellular signaling mediated only in part by PKC.

Differential modulation of Kv4.2 and Kv4.3 channels by calmodulin-dependent protein kinase II in rat cardiac myocytes

In this work we have combined biochemical and electrophysiological approaches to explore the modulation of rat ventricular transient outward K+ current ( Ito) by calmodulin kinase II (CaMKII). Intracellular application of CaMKII inhibitors KN93, calmidazolium, and autocamtide-2-related inhibitory peptide II (ARIP-II) accelerated the inactivation of Ito, even at low [Ca2+]. In the same conditions, CaMKII coimmunoprecipitated with Kv4.3 channels, suggesting that phosphorylation of Kv4.3 channels modulate inactivation of Ito. Because channels underlying Ito are heteromultimers of Kv4.2 and Kv4.3, we have explored the effect of CaMKII on human embryonic kidney (HEK) cells transfected with either of those Kvα-subunits. Whereas Kv4.3 inactivated faster upon inhibition of CaMKII, Kv4.2 inactivation was insensitive to CaMKII inhibitors. However, Kv4.2 inactivation became slower when high Ca2+ was used in the pipette or when intracellular [Ca2+] ([Ca2+]i) was transiently increased. This effect was inhibited by KN93, and Western blot analysis demonstrated Ca2+-dependent phosphorylation of Kv4.2 channels. On the contrary, CaMKII coimmunoprecipitated with Kv4.3 channels without a previous Ca2+ increase, and the association was inhibited by KN93. These results suggest that both channels underlying Ito are substrates of CaMKII, although with different sensitivities; Kv4.2 remain unphosphorylated unless [Ca2+]i increases, whereas Kv4.3 are phosphorylated at rest. In addition to the functional impact that phosphorylation of Kv4 channels could cause on the shape of action potential, association of CaMKII with Kv4.3 provides a new role of Kv4.3 subunits as molecular scaffolds for concentrating CaMKII in the membrane, allowing Ca2+-dependent modulation by this enzyme of the associated Kv4.2 channels.

Endothelin-1 receptor blockade prevented the electrophysiological dysfunction in cardiac myocytes of streptozotocin-induced diabetic rats

Diabetes mellitus is complicated with the development of cardiac contractile dysfunction and electrical instability, which contributes to high morbidity and mortality in diabetic patients. This study examined the possible roles of enhanced endothelin-1 (ET-1) on diabetes-induced alterations in ventricular myocyte electrophysiology. Type 1 diabetic rats were induced by single dose injection of streptozotocin (STZ) and treated with or without ET-1 receptor antagonist bosentan for 8 wk before myocyte isolation. Action potential, outward K+ currents, and inward Ca2+ currents in ventricular myocytes were recorded using whole-cell patch clamp technique. STZ-injected rats exhibited hyperglycemia, reduced body weight gain, and elevated plasma ET-1 concentration, indicative of diabetes induction. Ventricular myocytes isolated from diabetic rats exhibited prolonged action potential and reduced all three types of outward K+ currents. Resting membrane potential, height of action potential, and L-type Ca2+ current were not altered in diabetic myocytes. In vivo chronic treatment of diabetic rats with bosentan significantly augmented K+ currents and reversed action potential prolongation in ventricular myocytes. On the other hand, bosentan treatment had no detectable effect on the electrophysiological properties in control myocytes. In addition, bosentan had no effect on Ltype Ca2+ currents in both control and diabetic myocytes. Our data suggest that altered electrophysiological properties in ventricular myocytes were largely resulted from augmented ET-1 system in diabetic animals.

Effects of statins on vascular function of endothelin-1

1. Although statins have been reported to inhibit the prepro-endothelin-1 (ET-1) gene transcription in endothelial cells, their effects on the vascular function of ET-1 have not been explored. We, therefore, examined the effects of statins on contraction and DNA synthesis mediated by ET-1 in vascular smooth muscle. The effects of statins on contraction induced by ET-1 were compared to those mediated by noradrenaline (NA) and KCl. 2. Simvastatin (SV) induced a concentration-dependent relaxation of tonic contraction mediated by ET-1 (10 nM) (IC50 value of 1.3 microM). The relaxation was also observed in rings precontracted with NA (0.1 microM) and KCl (60 mM). In contrast, pravastatin did not have any effect on the contractions. 3. Endothelial denudation or pretreatment with L-NAME did not prevent the relaxation, but did reduce the relaxant activity of SV. 4. SV prevented Rho activation caused by ET-1 and KCl in aortic homogenates, as assessed by a Rho pulldown assay. 5. The Rho kinase inhibitor HA-1077 mimicked the effects of SV on tonic contractions induced by ET-1, NA and KCl. 6. Pretreatment with the Kv channels inhibitor, 4-aminopyridine, attenuated the ability of SV to relax contractions mediated by ET-1 and NA. 7. In quiescent VSM cells, SV significantly inhibited DNA synthesis and Rho translocation stimulated by ET-1, as assessed by [3H]thymidine incorporation and Western blot, respectively. 8. Inhibition of Rho geranylgeranylation by GGTI-297, or treatment with HA-1077, mimicked the effects of SV on DNA synthesis stimulated by ET-1. 9. The results show that the statin potently inhibits both ET-1-mediated contraction and DNA synthesis via multiple mechanisms. Clinical benefits of statins may result, in part, from their effects on vascular function of ET-1.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.

Angiotensin Receptor Type 1 Forms a Complex with the Transient Outward Potassium Channel Kv4.3 and Regulates Its Gating Properties and Intracellular Localization

We report a novel signal transduction complex of the angiotensin receptor type 1. In this complex the angiotensin receptor type 1 associates with the potassium channel alpha-subunit Kv4.3 and regulates its intracellular distribution and gating properties. Co-localization of Kv4.3 with angiotensin receptor type 1 and fluorescent resonance energy transfer between those two proteins labeled with cyan and yellow-green variants of green fluorescent protein revealed that Kv4.3 and angiotensin receptor type I are located in close proximity to each other in the cell. The angiotensin receptor type 1 also co-immunoprecipitates with Kv4.3 from canine ventricle or when co-expressed with Kv4.3 and its beta-subunit KChIP2 in human embryonic kidney 293 cells. Treatment of the cells with angiotensin II results in the internalization of Kv4.3 in a complex with the angiotensin receptor type 1. When stimulated with angiotensin II, angiotensin receptors type 1 modulate gating properties of the remaining Kv4.3 channels on the cell surface by shifting their activation voltage threshold to more positive values. We hypothesize that the angiotensin receptor type 1 provides its internalization molecular scaffold to Kv4.3 and in this way regulates the cell surface representation of the ion channel.

Basic arrhythmogenic mechanisms in both inherited and acquired long QT syndrome

The heterologous expression system will provide clues for understanding the basic mechanism of arrhythmogenicity in both inherited and acquired long QT syndrome, which are reviewed here, with emphasis on the K+ channels. Endothelin is implicated in the morphological and electrical remodeling of cardiac muscles in heart failure. The effects of endothelin on the transient outward K+ currents (Ito) were compared between Kv1.4 (rich in endocardial muscle) and Kv4.3 (rich in epicardial muscle) channels in the Xenopus oocytes expression system. Both Itos were decreased by stimulation of endothelin receptor ETA coexpressed with the K+ channels. Ito of Kv1.4 was decreased by about 85% after 10(-8) M ET-1, whereas that of Kv4.3 was decreased by about 60%. By mutagenesis experiments, we identified two phosphorylation sites of PKC and CaMKII in Kv1.4 responsible for the decrease in Ito by ET-1. In Kv4.3 we identified a PKC phosphorylation site that is partly responsible for the decrease. Differences in the suppression of Ito could be due to the differences in intracellular signaling including the number of phosphorylation sites. These findings show some of the molecular mechanisms of ventricular arrhythmias in heart failure, resulting in dispersion and prolongation of action potential which elicit reentry and after depolarization.