Electrophysiological remodeling in hypertrophy and heart failure.
Explore the source record for details and available documents.
Action Potentials↗
Biomedical subjects
G F Tomaselli
Publications and source records attributed to G F Tomaselli.
Cellular basis of ventricular arrhythmias and abnormal automaticity in heart failure.
The high incidence of sudden death in heart failure may reflect an increased propensity to abnormal repolarization and long Q-T interval-related arrhythmias. If so, cells from failing hearts would logically be expected to exhibit a heightened susceptibility to early afterdepolarizations (EAD). We found that midmyocardial ventricular cells isolated from dogs with pacing-induced heart failure exhibited an increased action potential duration and many more EAD than cells from nonpaced controls; this was the case both under basal conditions (P < 0.01) and after lowering external K(+) concentration ([K(+)](o)) to 2 mM and exposing cells to cesium (3 mM; P < 0.05). An unexpected finding was the occurrence of spontaneous depolarizations (SD, >5 mV) from the resting potential that were not coupled to prior action potentials. These SD were observed in 20% of failing cells (n = 5 of 25) under basal ionic conditions but in none of the normal cells (n = 0 of 27, P < 0.05). The net inward current that underlies SD is not triggered by Ca(2+) oscillations and thus differs fundamentally from the currents that underlie delayed afterdepolarizations. We conclude that cardiomyopathic canine ventricular cells are intrinsically predisposed to EAD and SD. Because EAD have been linked to the pathogenesis of torsade de pointes, our results support the hypothesis that sudden death in heart failure often arises from abnormalities of repolarization. The frequent occurrence of SD points to a novel cellular mechanism for abnormal automaticity in heart failure.
Action Potentials↗
Mechanistic link between lidocaine block and inactivation probed by outer pore mutations in the rat micro1 skeletal muscle sodium channel.
1. Mutations that disrupt Na+ channel fast inactivation attenuate lidocaine (lignocaine)-induced use dependence; however, the pharmacological role of slower inactivation processes remains unclear. In Xenopus oocytes, tryptophan substitution in the outer pore of the rat skeletal muscle channel (micro1-W402) alters partitioning among fast- and slow-inactivated states. We therefore examined the effects of W402 mutations on lidocaine block. 2. Recovery from inactivation exhibited three kinetic components (IF, fast; IM, intermediate; IS, slow). The effects of W402A and W402S on IF and IS differed, but both mutants (with or without beta1 subunit coexpression) decreased the amplitude of IM. In wild-type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use-dependent block was attenuated in both W402A and W402S. 3. To examine the pharmacological role of IS relative to IM, drug-exposed beta1-coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (INa) after a 1 s hyperpolarization interval that allowed recovery from IM but not IS, suggesting that lidocaine affinity for IS is low. 4. Both W402 mutations reduced occupancy of IM in drug-free conditions, and also induced resistance to use-dependent block. We propose that lidocaine-induced use dependence may involve an allosteric conformational change in the outer pore.
Animals↗
Molecular basis of transient outward potassium current downregulation in human heart failure: a decrease in Kv4.3 mRNA correlates with a reduction in current density.
BACKGROUND: Despite advances in medical therapy, congestive heart failure remains a major cause of death in the developed world. A disproportionate number of the deaths of patients with heart failure are sudden and presumed to be arrhythmic. Heart failure in humans and in animal models is associated with prolongation of the action potential duration (APD), the result of downregulation of K+ currents-prominently, the Ca2+-independent transient outward current (Ito). The mechanism for the reduction of Ito in heart failure is unknown. The K+ channel alpha-subunit Kv4.3, a homolog of the Drosophila Shal family, is most likely to encode all or part of the native cardiac Ito in humans. METHODS AND RESULTS: We used ribonuclease protection assays and whole-cell electrophysiological recording to study changes in the level of Kv4.3 mRNA and Ito in human tissues and isolated ventricular myocytes, respectively. We found that the level of Kv4.3 mRNA decreased by 30% in failing hearts compared with nonfailing controls. Furthermore, this reduction correlated with the reduction in peak Ito density measured in ventricular myocytes isolated from adjacent regions of the heart. There was no significant change in the steady-state level of any other mRNA studied (HERG, Kv1.4, Kir2.1, Kvss1.3, and the alpha1C subunit of the Ca2+ channel). mRNAs encoding Kv1.2, Kv1.5, and Kv2.1 were found in low abundance in human ventricle. CONCLUSIONS: These data provide further support for the hypothesis that Kv4.3 encodes all or part of the native cardiac Ito in humans and that part of the downregulation of this current in heart failure may be transcriptionally regulated.
Action Potentials↗
Structure and function of voltage-gated sodium channels.
1. Sodium channels mediate fast depolarization and conduct electrical impulses throughout nerve, muscle and heart. This paper reviews the links between sodium channel structure and function. 2. Sodium channels have a modular architecture, with distinct regions for the pore and the gates. The separation is far from absolute, however, with extensive interaction among the various parts of the channel. 3. At a molecular level, sodium channels are not static: they move extensively in the course of gating and ion translocation. 4. Sodium channels bind local anaesthetics and various toxins. In some cases, the relevant sites have been partially identified. 5. Sodium channels are subject to regulation at the levels of transcription, subunit interaction and post-translational modification (notably glycosylation and phosphorylation).
Amino Acid Sequence↗
Phenotypic characterization of a novel long-QT syndrome mutation (R1623Q) in the cardiac sodium channel.
BACKGROUND: A heritable form of the long-QT syndrome (LQT3) has been linked to mutations in the cardiac sodium channel gene (SCN5A). Recently, a sporadic SCN5A mutation was identified in a Japanese girl afflicted with the long-QT syndrome. In contrast to the heritable mutations, this externally positioned domain IV, S4 mutation (R1623Q) neutralized a charged residue that is critically involved in activation-inactivation coupling. METHODS AND RESULTS: We have characterized the R1623Q mutation in the human cardiac sodium channel (hH1) using both whole-cell and single-channel recordings. In contrast to the autosomal dominant LQT3 mutations, R1623Q increased the probability of long openings and caused early reopenings, producing a threefold prolongation of sodium current decay. Lidocaine restored rapid decay of the R1623Q macroscopic current. CONCLUSIONS: The R1623Q mutation produces inactivation gating defects that differ mechanistically from those caused by LQT3 mutations. These findings provide a biophysical explanation for this severe long-QT phenotype and extend our understanding of the mechanistic role of the S4 segment in cardiac sodium channel inactivation gating and class I antiarrhythmic drug action.
Animals↗
Comparison of adenosine effects on atrioventricular node reentry and atrioventricular reciprocating tachycardias.
BACKGROUND: Adenosine is an established first line therapy for the treatment of narrow complex tachycardias. The two most common etiologies of paroxysmal supraventricular tachycardia (SVT) are atrioventricular node reentry tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT). HYPOTHESIS: We postulated that adenosine might have different effects on the termination of AVNRT vs. AVRT, and that these differences might assist in the noninvasive differentiation between these diagnoses. METHODS: Fifty-nine patients referred for the diagnosis and treatment of SVT were included in the study. All patients had SVT induced during electrophysiology testing, and each patient received adenosine during SVT. The adenosine dose, time to tachycardia termination, and site of tachycardia termination were recorded. Seventeen patients required isoproterenol administration to initiate SVT. This subset of patients was compared with those not requiring isoproterenol. RESULTS: There was no statistically significant difference in the adenosine dose or time to tachycardia termination when comparing patients with AVNRT with those with AVRT. All patients with AVNRT had termination of tachycardia in the antegrade direction with final activation in the atria. Patients requiring isoproterenol for tachycardia initiation experienced tachycardia termination significantly faster than those not requiring isoproterenol, although there was no difference in the dose of adenosine required for termination. CONCLUSION: These data demonstrate that patients with dual AV node physiology and AVNRT do not have altered sensitivity to adenosine compared with patients with AVRT and normal AV nodes. Further investigation will be required to determine the clinical utility of the significantly shorter time to tachycardia termination for patients receiving isoproterenol.
Adenosine↗
Mechanism of anode break stimulation in the heart.
Anodal stimulation is routinely observed in cardiac tissue, but only recently has a mechanism been proposed. The bidomain cardiac tissue model proposes that virtual cathodes induced at sites distant from the electrode initiate the depolarization. In contrast, none of the existing cardiac action potential models (Luo-Rudy phase I and II, or Oxsoft) predict anodal stimulation at the single-cell level. To determine whether anodal stimulation has a cellular basis, we measured membrane potential and membrane current in mammalian ventricular myocytes by using whole-cell patch clamp. Anode break responses can be readily elicited in single ventricular cells. The basis of this anodal stimulation in single cells is recruitment of the hyperpolarization-activated inward current I(f). The threshold of activation for I(f) is -80 mV in rat cells and -120 mV in guinea pig or canine cells. Persistent I(f) "tail" current upon release of the hyperpolarization drives the transmembrane potential toward the threshold of sodium channels, initiating an action potential. Time-dependent block of the inward rectifier, I(K1), at hyperpolarized potentials decreases membrane conductance and thereby potentiates the ability of I(f) to depolarize the cell on the break of an anodal pulse. Inclusion of I(f), as well as the block and unblock kinetics of I(K1), in the existing Luo-Rudy action potential model faithfully reproduces anode break stimulation. Thus active cellular properties suffice to explain anode break stimulation in cardiac tissue.
Action Potentials↗
Beta-adrenergic blockade accelerates conversion of postoperative supraventricular tachyarrhythmias.
BACKGROUND: Postoperative supraventricular tachyarrhythmia is a common complication of surgery. Because chemical cardioversion is often ineffective, ventricular rate control remains a principal goal of therapy. The authors hypothesized that patients with supraventricular tachyarrhythmia after major noncardiac surgery who receive intravenous beta-adrenergic blockade for ventricular rate control would experience conversion to sinus rhythm at a rate that differs from those receiving intravenous calcium channel blockade. METHODS: The rate of conversion to sinus rhythm at 2 and 12 h after treatment was examined in 64 cases of postoperative supraventricular tachyarrhythmia. After adenosine administration, patients who remained in supraventricular tachyarrhythmia were prospectively randomized to receive either intravenous diltiazem or intravenous esmolol for ventricular rate control (unblinded). Loading and infusion rates were adjusted to achieve equivalent degrees of ventricular rate control. RESULTS: Patients were similar with regard to age and Apache III score. Most patients in both groups had atrial fibrillation (esmolol, 79%; diltiazem, 81%), and none experienced stable conversion with adenosine. Patients randomized to receive esmolol experienced a 59% rate of conversion to sinus rhythm within 2 h of treatment, compared with only 33% for patients randomized to receive diltiazem (intention to treat, P = 0.049; odds ratio, 2.9; 95% confidence interval, 1.046 to 7.8). After 12 h of therapy, the number of patients converting to sinus rhythm increased in both groups (esmolol, 85%; diltiazem, 62%), and the rates of conversion no longer differed significantly. Ventricular rates when supraventricular tachyarrhythmia began and after 2 and 12 h of rate control therapy were similar in the two treatment groups. The in-hospital mortality rate and length of stay in the intensive care unit were not significantly influenced by treatment group. CONCLUSIONS: Among adenosine-resistant patients in the intensive care unit with atrial fibrillation after noncardiac surgery, intravenous esmolol produced a more rapid (2-h) conversion to sinus rhythm than did intravenous diltiazem.
Adrenergic beta-Antagonists↗
Beat-to-beat repolarization lability identifies patients at risk for sudden cardiac death.
INTRODUCTION: Recent studies have implicated repolarization lability in the genesis of malignant ventricular arrhythmias. However, few data exist on assessment of temporal QT interval variability and its relation to arrhythmogenesis. We tested the ability of the QT variability index (QTVI), a measure of beat-to-beat QT interval fluctuations measured on a single ECG lead, to identify patients presenting with malignant ventricular arrhythmias and predict their subsequent occurrences. METHODS AND RESULTS: We measured the QTVI in 95 patients presenting for electrophysiologic study (EPS). The ability of the QTVI to identify patients with sudden cardiac death (SCD) or sustained monomorphic ventricular tachycardia (MVT) on presentation and during follow-up of 23.7+/-14.3 months was compared with spatial QT dispersion, T wave alternans ratio during atrial pacing, MVT inducibility at EPS, signal-averaged ECG, heart rate variability, and ejection fraction. The QTVI was higher in patients with heart disease than in controls (-0.7+/-0.7 vs -1.1+/-0.5, P < 0.05), and higher in patients presenting with SCD than in other patients with heart disease (0.0+/-0.6 vs -0.8+/-0.5, P < 0.05). The QTVI was the only clinical variable that identified patients who presented with SCD (P = 0.004, odds ratio = 12.5) on stepwise, logistic multiple regression. Fourteen patients had arrhythmic events during follow-up. In a Kaplan-Meier analysis of arrhythmic events, QTVI> or =0.1 was a discriminator for higher risk of arrhythmic events (P < 0.05). CONCLUSIONS: (1) This noninvasive measure of temporal repolarization lability identified patients with SCD and predicted arrhythmia-free survival. (2) Further studies are needed to determine the mechanisms that mediate beat-to-beat QT interval variability.
Death, Sudden, Cardiac↗
Isolation and characterization of the human gene encoding Ito: further diversity by alternative mRNA splicing.
The transient outward K+ current (Ito) in the heart is responsible for the initial phase of repolarization and for setting the plateau voltage of the ventricular action potential. Recently, Kv4.3 has emerged as the leading candidate alpha-subunit gene that underlies Ito in larger mammals such as dogs and humans. We have cloned the human Kv4.3 homolog and describe a carboxyl-terminal splice variant that inserts 19 amino acids with a consensus protein kinase C (PKC) phosphorylation site into the protein after the last membrane-spanning segment. The coding region of Kv4.3 is comprised of at least five exons and is located on chromosome 1p13.3. In the basal state the basic biophysical properties of both of the splice variants are identical.
Alternative Splicing↗
Beat-to-beat QT interval variability: novel evidence for repolarization lability in ischemic and nonischemic dilated cardiomyopathy.
BACKGROUND: Dilated cardiomyopathy (DCM) is associated with a high incidence of malignant ventricular arrhythmias and sudden death. Abnormalities in repolarization of ventricular myocardium have been implicated in the development of these arrhythmias. Spatial heterogeneity in repolarization has been studied in DCM, but temporal fluctuations in repolarization in this setting have been largely ignored. We sought to test the hypothesis that beat-to-beat QT interval variability is increased in DCM patients compared with control subjects. METHODS AND RESULTS: Eighty-three patients with ischemic and nonischemic DCM and 60 control subjects served as the study population. Beat-to-beat QT interval variability was measured by automated analysis on the basis of 256-second records of the surface ECG. A QT variability index (QTVI) was calculated for each subject as the logarithm of the ratio of normalized QT variance to heart rate variance. The coherence between heart rate and QT interval fluctuations was determined by spectral analysis. In patients, ejection fractions were assessed by echocardiography or ventriculography, and spatial QT dispersion was determined from the standard 12-lead ECG. DCM patients had greater QT variance than control subjects (60.4+/-63.1 versus 25.7+/-24.8 ms2, P<.0001) despite reduced heart rate variance (6.7+/-7.8 versus 10.5+/-10.4 bpm2, P=.01). The QTVI was higher in DCM patients than in control subjects, with a high degree of significance (-0.43+/-0.71 versus -1.29+/-0.51, P<10[-12]). QTVI did not correlate with ejection fraction or spatial QT dispersion but did depend on New York Heart Association functional class. QTVI did not differ between DCM patients with ischemic and those with nonischemic origin. Coherence between heart rate and QT interval fluctuations at physiological frequencies was lower in DCM patients compared with control subjects (0.28+/-0.14 versus 0.39+/-0.18, P<.0001). CONCLUSIONS: DCM is associated with beat-to-beat fluctuations in QT interval that are larger than normal and uncoupled from variations in heart rate. QT interval variability increases with worsening functional class but is independent of ejection fraction. These data indicate that DCM leads to temporal lability in ventricular repolarization.
Adult↗
Dispersion of ventricular activation and refractoriness in patients with idiopathic dilated cardiomyopathy.
We sought to evaluate the electrophysiologic substrate for ventricular arrhythmias and sudden cardiac death in patients with idiopathic dilated cardiomyopathy. When compared with controls, patients with cardiomyopathy had prolonged activation times, increased dispersion of activation and recovery, and prolonged duration of monophasic action potential recordings at 70%, but not at 90%, of repolarization.
Cardiac Catheterization↗
Proton inhibition of sodium channels: mechanism of gating shifts and reduced conductance.
Extracellular acidosis affects both permeation and gating of the expressed rat skeletal muscle Na+ channel (micro1). Reduction of the extracellular pH produced a progressive decrease in the maximal whole-cell conductance and a depolarizing shift in the whole-cell current-voltage relationship. A smaller depolarizing shift in the steady-state inactivation curve was observed. The pK of the reduction of maximal conductance was 6.1 over the pH range studied. An upper limit estimate of the pK of the shift of the half-activation voltage was 6.1. The relative reduction in the maximal whole-cell conductance did not change with higher [Na+]o. The conductance of single fenvalerate-modified Na+ channels was reduced by extracellular protons. Although the single-channel conductance increased with higher [Na+]o, the maximal conductances at pH 7.6, 7.0 and 6.0 did not converge at [Na+]o up to 280 mm, inconsistent with a simple electrostatic effect. A model incorporating both Na+ and H+ binding in the pore and cation binding to a Gouy-Chapman surface charge provided a robust fit to the single-channel conductance data with an estimated surface charge density of 1e-/439A2. Neither surface charge nor proton block alone suffices to explain the effects of extracellular acidosis on Na+ channel permeation; both effects play major roles in mediating the response to extracellular pH.
Animals↗
Topology of the P segments in the sodium channel pore revealed by cysteine mutagenesis.
The P segments of the voltage-dependent Na+ channel line the outer mouth and selectivity filter of the pore. The residues that form the cytoplasmic mouth of the pore of the channel have not been identified. To study the structure of the inner pore mouth, the presumed selectivity filter residues (D400, E755, K1237, and A1529), and three amino acids just amino-terminal to each of these residues in the rat skeletal muscle Na+ channel, were mutated to cysteine and expressed in tsA 201 cells. These amino acids are predicted (by analogy to K+ channels) to be on the cytoplasmic side of the putative selectivity filter residues. Inward and outward Na+ currents were measured with the whole-cell configuration of the patch-clamp technique. Cysteinyl side-chain accessibility was gauged by sensitivity to Cd2+ block and by reactivity with methanethiosulfonate (MTS) reagents applied to both the inside and the outside of the cell. Outward currents through the wild-type and all of the mutant channels were unaffected by internal Cd2+ (100 microM). Similarly, 1 mM methanethiosulfonate ethylammonium (MTSEA) applied to the inside of the membrane did not affect wild-type or mutant outward currents. However, two mutants amino-terminal to the selectivity position in domain III (F1236C and T1235C) and one in domain IV (S1528C) were blocked with high affinity by external Cd2+. The Na+ current through F1236C and S1528C channels was inhibited by MTSEA applied to the outside of the cell. The accessibility of these mutants to externally applied cysteinyl ligands indicates that the side chains of the mutated residues face outward rather than inward. The K+ channel model of the P segments as protein loops that span the selectivity region is not applicable to the Na+ channel.
Amino Acid Sequence↗
Molecular motions within the pore of voltage-dependent sodium channels.
The pores of ion channel proteins are often modeled as static structures. In this view, selectivity reflects rigidly constrained backbone orientations. Such a picture is at variance with the generalization that biological proteins are flexible, capable of major internal motions on biologically relevant time scales. We tested for motions in the sodium channel pore by systematically introducing pairs of cysteine residues throughout the pore-lining segments. Two distinct pairs of residues spontaneously formed disulfide bonds bridging domains I and II. Nine other permutations, involving all four domains, were capable of disulfide bonding in the presence of a redox catalyst. The results are inconsistent with a single fixed backbone structure for the pore; instead, the segments that line the permeation pathway appear capable of sizable motions.
Amino Acid Sequence↗
Mechanisms of sodium/calcium selectivity in sodium channels probed by cysteine mutagenesis and sulfhydryl modification.
A conserved lysine residue in the "P loop" of domain III renders sodium channels highly selective. Conversion of this residue to glutamate, to mimic the homologous position in calcium channels, enables Ca2+ to permeate sodium channels. Because the lysine-to-glutamate mutation converts a positively charged side chain to a negative one, it has been proposed that a positive charge at this position suffices for Na+ selectivity. We tested this idea by converting the critical lysine to cysteine (K1237C) in mu 1 rat skeletal sodium channels expressed in Xenopus oocytes. Selectivity of the mutant channels was then characterized before and after chemical modification to alter side-chain charge. Wild-type channels are highly selective for Na+ over Ca2+ (PCa/PNa < 0.01). The K1237C mutation significantly increases permeability to Ca2+ (PCa/PNa = 0.6) and Sr2+. Analogous mutations in domains I (D400C), II (E755C), and IV (A1529C) did not alter the selectivity for Na+ over Ca2+, nor did any of the domain IV mutations (G1530C, W1531C, and D1532C) that are known to affect monovalent selectivity. Interestingly, the increase in permeability to Ca2+ in K1237C cannot be reversed by simply restoring the positive charge to the side chain by using the sulfhydryl modifying reagent methanethiosulfonate ethylammonium. Single-channel studies confirmed that modified K1237C channels, which exhibit a reduced unitary conductance, remain permeable to Ca2+, with a PCa/PNa of 0.6. We conclude that the chemical identity of the residue at position 1237 is crucial for channel selectivity. Simply rendering the 1237 side chain positive does not suffice to restore selectivity to the channel.
Amino Acid Sequence↗
Ion channels as enzymes: analogy or homology?
Ion channels render nerve and muscle excitable. A typical channel protein can mediate the passive transfer of millions of ions per second across the membrane. Thus, channels catalyse the transmembrane flux of ions, fulfilling criteria traditionally associated with enzymes. Is this a semantic coincidence, or do channels and enzymes in fact rely upon similar structural principles? A general answer remains elusive given the paucity of crystallographic data on channels. Nevertheless, emerging evidence points to fundamental similarities between the pores of channels and the active sites of enzymes of resolved structure. Shared features include narrow clefts lined by protein loops, and specific binding of transition intermediates during catalysis. The often cited analogies between channels and enzymes might therefore reflect basic design homologies.
Animals↗