Biomedical subjects

Stephen Zicha

Publications and source records attributed to Stephen Zicha.

8 recordsLinked to original sources

PubMed2006

Potassium channel subunit remodeling in rabbits exposed to long-term bradycardia or tachycardia: discrete arrhythmogenic consequences related to differential delayed-rectifier changes.

BACKGROUND: Sustained heart rate abnormalities produce electrical remodeling and susceptibility to arrhythmia. Uncontrolled tachycardia produces heart failure and ventricular tachyarrhythmia susceptibility, whereas bradycardia promotes spontaneous torsade de pointes (TdP). This study compared arrhythmic phenotypes and molecular electrophysiological remodeling produced by tachycardia versus bradycardia in rabbits. METHODS AND RESULTS: We evaluated mRNA and protein expression of subunits underlying rapid (IKr) and slow (IKs) delayed-rectifier and transient-outward K+ currents in ventricular tissues from sinus rhythm control rabbits and rabbits with AV block submitted to 3-week ventricular pacing either at 60 to 90 bpm (bradypaced) or at 350 to 370 bpm (tachypaced). QT intervals at matched ventricular pacing rates were longer in bradypaced than tachypaced rabbits (eg, by approximately 50% at 60 bpm; P<0.01). KvLQT1 and minK mRNA and protein levels were downregulated in both bradypaced and tachypaced rabbits, whereas ERG was significantly downregulated in bradypaced rabbits only. Kv4.3 and Kv1.4 were downregulated by tachypacing only. Patch-clamp experiments showed that IKs was reduced in both but IKr was decreased in bradypaced rabbits only. Continuous monitoring revealed spontaneous TdP in 75% of bradypaced but only isolated ventricular ectopy in tachypaced rabbits. Administration of dofetilide (0.02 mg/kg) to mimic IKr downregulation produced ultimately lethal TdP in all tachypaced rabbits. CONCLUSIONS: Sustained tachycardia and bradycardia downregulate IKs subunits, but bradycardia also suppresses ERG/IKr, causing prominent repolarization delays and spontaneous TdP. Susceptibility of tachycardia/heart failure rabbits to malignant tachyarrhythmias is induced by exposure to IKr blockers. These results point to a crucial role for delayed-rectifier subunit remodeling in TdP susceptibility associated with rate-related cardiac remodeling.

Animals↗

PubMed2005

Atrial fibrillation-associated minK38G/S polymorphism modulates delayed rectifier current and membrane localization.

BACKGROUND: Atrial fibrillation (AF) is a common acquired arrhythmia with multi-factorial pathogenesis. Recently, a single nucleotide polymorphism (SNP, A/G) at position 112 in the KCNE1 gene, resulting in a glycine/serine amino acid substitution at position 38 of the minK peptide, was associated with AF occurrence (AF more frequent with minK38G); however, the functional effect of this SNP is unknown. METHODS AND RESULTS: We used patch clamp recording, confocal microscopy and protein biochemistry to study the effect of this SNP on delayed-rectifier current expression and mathematical simulation to identify potential functional consequences. The density of slow delayed rectifier current (I(Ks)) resulting from co-expression with KvLQT1 was smaller with minK38G (e.g. at +10 mV: 50+/-7 pA/pF in Chinese hamster ovary (CHO) cells, 45+/-14 pA/pF for COS-7 cells) compared to minK38S (93+/-17 pA/pF, 104+/-23 pA/pF, respectively, P<0.05 for each). I(Ks) kinetics and voltage-dependence were unaffected. Currents resulting from co-expression of human ether-a-go-go-related gene (HERG) were similar for minK38G and minK38S, e.g. upon repolarization from +10 to -50 mV: tail currents 23+/-4 pA/pF versus 22+/-5 pA/pF (P=ns). KvLQT1 membrane immunofluorescence was less in CHO cells co-expressing minK38G versus minK38S, and surface expression of KvLQT1, as determined by labelling with streptavidin/biotin, was increased with minK38S co-expression. Computer simulations with a human atrial action potential model predicted that the minK38G SNP would slightly prolong the atrial action potential and reduce the frequency for alternans behaviour. In the presence of reduced repolarization reserve, these effects were enhanced and under specific conditions early afterdepolarizations occurred. CONCLUSIONS: The minK38G isoform is associated with reduced I(Ks), likely due to decreased KvLQT1 membrane expression. This study reveals a novel amino acid determinant of the minK-KvLQT1 interaction, and if the role of minK38G in AF is confirmed, would suggest mechanistic heterogeneity in genetic determinants of AF.

Animals↗

PubMed2005

Sinus node dysfunction and hyperpolarization-activated (HCN) channel subunit remodeling in a canine heart failure model.

BACKGROUND: The hyperpolarization-activated cation current I(f) contributes significantly to sinoatrial node pacemaker function and possibly to ectopic arrhythmogenesis. Little is known about the expression of corresponding hyperpolarization-activated cyclic nucleotide-gated (HCN) channel subunits in normal hearts and HCN remodeling by diseases, like congestive heart failure (CHF), associated with disturbances of cardiac rhythm. METHODS AND RESULTS: We assessed expression of HCN1, 2 and 4 in normal mongrel dogs and dogs subjected to 2-week ventricular tachypacing-induced CHF. Competitive RT-PCR, Western blot and immunohistochemistry were used to quantify HCN subunit mRNA and protein expression in the right atrium (RA) and sinoatrial node. CHF approximately doubled sinus node recovery time, indicating suppressed sinus node pacemaker function. HCN expression under control conditions was HCN4 > HCN2 >> HCN1. HCN2 and HCN4 expression was greater at both protein and mRNA levels in sinoatrial node than RA. CHF significantly decreased sinus node HCN expression at both mRNA and protein levels (HCN2 by 78% and 82%; HCN4 by 42% and 77%, respectively). RA HCN2 expression was unaltered by CHF, but HCN4 was significantly upregulated (by 209%). CONCLUSIONS: HCN4 is the dominant subunit in canine sinoatrial node and RA; strong sinus node HCN expression likely contributes to its pacemaker function; downregulation of HCN4 and HCN2 expression contribute to CHF-induced sinus node dysfunction; and upregulation of atrial HCN4 may help to promote atrial arrhythmia formation. These findings provide novel information about the molecular basis of normal and disease-related impairments of cardiac impulse formation.

Animals↗

PubMed2005

Novel targets for cardiac antiarrhythmic drug development.

Cardiac arrhythmias remain a major source of morbidity and mortality in developed countries. Antiarrhythmic drug therapy was traditionally the mainstay of cardiac arrhythmia treatment; however, drug therapy of cardiac arrhythmias has been plagued by incomplete efficacy and by potentially serious adverse reactions, of which the most worrisome has been a potential for malignant proarrhythmia and related effects to increase cardiac mortality. This article reviews the principal arrhythmia mechanisms and their ionic determinants, and discusses potential innovative approaches to new antiarrhythmic drug development, including the consideration of novel ionic targets, potential biophysical approaches and non-channel components involved in composing the arrhythmic substrate.

Action Potentials↗

PubMed2004

Transmural expression of transient outward potassium current subunits in normal and failing canine and human hearts.

The transient outward current (I(to)), an important contributor to transmural electrophysiological heterogeneity, is significantly remodelled in congestive heart failure (CHF). The molecular bases of transmural I(to) gradients and CHF-dependent ionic remodelling are incompletely understood. To elucidate these issues, we studied mRNA and protein expression of Kv4.3 and KChIP2, the principal alpha and beta subunits believed to form I(to), in epicardial and endocardial tissues and in isolated cardiomyocytes from control dogs and dogs with CHF induced by 240 beats min(-1) ventricular tachypacing. CHF decreased I(to) density in both epicardium and endocardium (by 73 and 55% at +60 mV, respectively), without a significant change in relative current density (endocardium/epicardium 0.11 control, 0.17 CHF). There were transmural gradients in mRNA expression of both Kv4.3 (endocardium/epicardium ratio 0.3 under control conditions) and KChIP2 (endocardium/epicardium ratio 0.2 control), which remained in the presence of CHF (Kv4.3 endocardium/epicardium ratio 0.4; KChIP2 0.4). There were qualitatively similar protein expression gradients in human and canine cardiac tissues and isolated canine cardiomyocytes; however, the KChIP2 gradient was only detectable with a highly selective monoclonal antibody and closely approximated the I(to) density gradient. Kv4.3 mRNA expression was reduced by CHF, but KChIP2 mRNA was not significantly changed. CHF decreased Kv4.3 protein expression in canine cardiac tissues and cardiomyocytes, as well as in terminally failing human heart tissue samples, but KChIP2 protein was not down-regulated in any of the corresponding sample sets. We conclude that both Kv4.3 and KChIP2 may contribute to epicardial-endocardial gradients in I(to), and that I(to) down-regulation in human and canine CHF appears due primarily to changes in Kv4.3.

Animals↗

PubMed2004

Post-transcriptional alterations in the expression of cardiac Na+ channel subunits in chronic heart failure.

Clinical and experimental evidence has recently accumulated about the importance of alterations of Na(+) channel (NaCh) function and slow myocardial conduction for arrhythmias in infarcted and failing hearts (i.e., heart failure, HF). The present study evaluated the molecular mechanisms of local alterations in the expression of NaCh subunits which underlie Na(+) current (I(Na)) density decrease in HF. HF was induced in five dogs by sequential coronary microembolization and developed approximately 3 months after the last embolization (left ventricle (LV), ejection fraction = 27 +/- 7%). Five normal dogs served as a control group. Ventricular cardiomyocytes were isolated enzymatically from LV mid-myocardium and I(Na) was measured by whole-cell patch-clamp. The mRNA encoding the cardiac-specific NaCh alpha-subunit Na(v)1.5, and one of its auxiliary subunits beta 1 (NaCh beta 1), were analyzed by competitive reverse transcription-polymerase chain reaction. Protein levels of Na(v)1.5, NaCh beta 1 and NaCh beta 2 were evaluated by western blotting. The maximum density of I(Na)/C(m) was decreased in HF (n = 5) compared to control hearts (33.2 +/- 4.4 vs. 50.0 +/- 4.9 pA/pF, mean +/- S.E.M., n = 5, P < 0.05). The steady-state inactivation and activation of I(Na) remained unchanged in HF compared to control hearts. The levels of mRNA encoding Na(v)1.5, and NaCh beta 1 were unaltered in FH. However, Na(v)1.5 protein expression was reduced about 30% in HF, while NaCh beta 1 and NaCh beta 2 protein were unchanged. We conclude that experimental HF in dogs results in post-transcriptional changes in cardiac NaCh alpha-subunit expression.

Animals↗

PubMed2003

Canine ventricular KCNE2 expression resides predominantly in Purkinje fibers.

Mutations in minK-related peptide 1 (MiRP1), the product of the KCNE2 gene, have been associated with malignant ventricular arrhythmia syndromes related to impaired repolarization. MiRP1 interacts with a variety of ion-channel alpha-subunits, dysfunction of which could account for arrhythmia syndromes; however, the observation of very low-level expression of MiRP1 in ventricular tissue has led to doubts about its relevance. The specialized His-Purkinje system plays a key role in cardiac electrophysiology and is an important contributor to ventricular arrhythmias related to abnormal repolarization. We examined the relative abundance of MiRP1 in canine Purkinje versus ventricular tissue and found much greater expression at both mRNA and protein levels in Purkinje tissue. Thus, the cardiac Purkinje system is a strong candidate to play a role in arrhythmic syndromes due to MiRP1 abnormalities.

Animals↗

PubMed2003

Molecular basis of species-specific expression of repolarizing K+ currents in the heart.

There are important species-specific differences in K+ current profiles and arrhythmia susceptibility, but interspecies comparisons of K+ channel subunit expression are lacking. We quantified voltage-gated K+ channel (Kv) subunit mRNA and protein in rabbits, guinea pigs, and humans. Kv1.4, Kv4.2, and Kv4.3 mRNA was present in rabbits but undetectable in guinea pigs. MinK mRNA concentration in guinea pigs was almost threefold greater versus humans and 20-fold versus rabbits. MinK protein expression in guinea pigs was almost twofold that in humans and sixfold that in rabbits. KvLQT1 mRNA concentration was greatest in humans, and protein expression in humans was increased by approximately 2- and approximately 7-fold compared with values in rabbits and guinea pigs, respectively. The ether-a-go-go-related gene (ERG1) mRNA was more concentrated in humans, but ERG1 protein expression could not be compared across species because of epitope sequence differences. We conclude that important interspecies differences in cardiac K+ channel subunit expression exist and may contribute to the following: 1) lack of a transient outward current in the guinea pig (alpha-subunit transcription absent in the guinea pig heart); 2) small slow delayed rectifier current and torsades de pointes susceptibility in the rabbit (low-level minK expression); and 3) large slow component of the delayed rectifier current in the guinea pig (strong minK expression).

Animals↗