Transmural expression of ion channels and transporters in human nondiseased and end-stage failing hearts

Electrophysiological determinants of arrhythmic susceptibility upon endocardial and epicardial pacing in guinea-pig heart

AIM

Endocardial pacing instituted to treat symptomatic bradycardia may nevertheless promote tachyarrhythmia in some pacemaker-implanted patients. We sought to determine the contributing electrophysiological mechanisms.

METHODS

Left ventricular (LV) monophasic action potential duration (APD(90)) and effective refractory periods were determined in perfused guinea-pig hearts along with volume-conducted ECG recordings during epicardial and endocardial stimulations.

RESULTS

Consistent with electrotonic modulation of repolarization, APD(90) at a given (either epicardial or endocardial) recording site tended to be longer while pacing from the ipsilateral LV site as compared to stimulations applied at the opposite side of ventricular wall. As a result, the intrinsic transmural repolarization gradient was amplified during endocardial pacing while being significantly reduced upon epicardial stimulations. The maximum slope of APD(90) restitution was greater upon endocardial than epicardial pacing. The excitability was found to recur at earlier repolarization time point at endocardium than epicardium, thereby contributing to increased endocardial critical intervals for re-excitation. Premature extrasystolic beats could have been elicited at shorter coupling stimulation intervals and propagated with greater transmural conduction delay upon endocardial than epicardial stimulations. Endocardial site exhibited lower ventricular fibrillation thresholds and greater inducibility of tachyarrhythmia upon extrasystolic stimulations as compared to epicardium.

CONCLUSION

Arrhythmic susceptibility in guinea-pig heart is greater during endocardial than epicardial pacing because of greater transmural APD(90) dispersion, steeper electrical restitution slopes, greater critical intervals for LV re-excitation and slower transmural conduction of the earliest premature ectopic beats. Further studies are warranted to determine whether these effects may contribute to proarrhythmia in paced human patients.

Impact of KChIP2 on Cardiac Electrophysiology and the Progression of Heart Failure

Electrophysiological remodeling of cardiac potassium ion channels is important in the progression of heart failure. A reduction of the transient outward potassium current (I_to) in mammalian heart failure is consistent with a reduced expression of potassium channel interacting protein 2 (KChIP2, a K_V4 subunit). Approaches have been made to investigate the role of KChIP2 in shaping cardiac I_to, including the use of transgenic KChIP2 deficient mice and viral overexpression of KChIP2. The interplay between I_to and myocardial calcium handling is pivotal in the development of heart failure, and is further strengthened by the dual role of KChIP2 as a functional subunit on both K_V4 and Ca_V1.2. Moreover, the potential arrhythmogenic consequence of reduced I_to may contribute to the high relative incidence of sudden death in the early phases of human heart failure. With this review, we offer an overview of the insights into the physiological and pathological roles of KChIP2 and we discuss the limitations of translating the molecular basis of electrophysiological remodeling from animal models of heart failure to the clinical setting.

Conduction remodeling in human end-stage nonischemic left ventricular cardiomyopathy

BACKGROUND

Several arrhythmogenic mechanisms have been inferred from animal heart failure models. However, the translation of these hypotheses is difficult because of the lack of functional human data. We aimed to investigate the electrophysiological substrate for arrhythmia in human end-stage nonischemic cardiomyopathy.

METHODS AND RESULTS

We optically mapped the coronary-perfused left ventricular wedge preparations from human hearts with end-stage nonischemic cardiomyopathy (heart failure, n=10) and nonfailing hearts (NF, n=10). Molecular remodeling was studied with immunostaining, Western blotting, and histological analyses. Heart failure produced heterogeneous prolongation of action potential duration resulting in the decrease of transmural action potential duration dispersion (64 ± 12 ms versus 129 ± 15 ms in NF, P<0.005). In the failing hearts, transmural activation was significantly slowed from the endocardium (39 ± 3 cm/s versus 49 ± 2 cm/s in NF, P=0.008) to the epicardium (28 ± 3 cm/s versus 40 ± 2 cm/s in NF, P=0.008). Conduction slowing was likely due to connexin 43 (Cx43) downregulation, decreased colocalization of Cx43 with N-cadherin (40 ± 2% versus 52 ± 5% in NF, P=0.02), and an altered distribution of phosphorylated Cx43 isoforms by the upregulation of the dephosphorylated Cx43 in both the subendocardium and subepicardium layers. Failing hearts further demonstrated spatially discordant conduction velocity alternans which resulted in nonuniform propagation discontinuities and wave breaks conditioned by strands of increased interstitial fibrosis (fibrous tissue content in heart failure 16.4 ± 7.7 versus 9.9 ± 1.4% in NF, P=0.02).

CONCLUSIONS

Conduction disorder resulting from the anisotropic downregulation of Cx43 expression, the reduction of Cx43 phosphorylation, and increased fibrosis is likely to be a critical component of arrhythmogenic substrate in patients with nonischemic cardiomyopathy.

Biophysical properties of slow potassium channels in human embryonic stem cell derived cardiomyocytes implicate subunit stoichiometry

Human embryonic stem cells (hESCs) are an important cellular model for studying ion channel function in the context of a human cardiac cell and will provide a wealth of information about both heritable arrhythmias and acquired electrophysiological disorders. However, detailed electrophysiological characterization of the important cardiac ion channels has been so far overlooked. Because mutations in the gene for the I(Ks) α subunit, KCNQ1, constitute the majority of long QT syndrome (LQT-1) cases, we have carried out a detailed biophysical analysis of this channel expressed in hESCs to establish baseline I(Ks) channel biophysical properties in cardiac myocytes derived from hESCs (hESC-CMs). I(Ks) channels are heteromultimeric proteins consisting of four identical α-subunits (KCNQ1) assembled with auxiliary β-subunits (KCNE1). We found that the half-maximal I(Ks) activation voltage in hESC-CMs and in myocytes derived from human induced pluripotent stems cells (hiPSC-CMs) falls between that of KCNQ1 channels expressed alone and with full complement of KCNE1, the major KCNE subunit expressed in hESC-CMs as shown by qPCR analysis. Overexpression of KCNE1 by transfection of hESC-CMs markedly shifted and slowed native I(Ks) activation implying assembly of additional KCNE1 subunits with endogenous channels. Our results in hESC-CMs, which indicate an I(Ks) subunit stoichiometry that can be altered by variable KCNE1 expression, suggest the possibility for variable I(Ks) function in the developing heart, in different tissues in the heart, and in disease. This establishes a new baseline for I(Ks) channel properties in myocytes derived from pluripotent stem cells and will guide future studies in patient-specific hiPSCs.

Organotypic slice culture from human adult ventricular myocardium

AIMS

Cardiovascular research requires complex and functionally intact experimental models. Due to major differences in the cellular and subcellular composition of the myocardium between species, the use of human heart tissue is highly desirable. To enhance the experimental use of the human myocardium, we established methods for the preparation of vital tissue slices from the adult ventricular myocardium as well as conditions for their long-term preservation in organotypic culture.

METHODS AND RESULTS

Human ventricular heart samples were derived from surgical specimens excised during a therapeutic Morrow myectomy and cut into 300 μm thick slices. Slices were either characterized in acute experiments or cultured at a liquid-air interface. Viability and functionality were proven by viability staining, enzyme activity tests, intracellular potential recordings, and force measurements. Precision-cut slices showed high viability throughout 28 days in culture and displayed typical cardiomyocyte action potential characteristics, which enabled pharmacological safety testing on the rapid component of the delayed rectifier potassium current (I(Kr)) and ATP-dependent potassium channels throughout the whole culture period. Constant expression of major ion channels was confirmed by quantitative PCR. Acute slices developed excitation-dependent contractions with a clear preload dependency and a β-adrenergic response. Contractility and myosin light chain expression decreased during the first days in culture but reached a steady state with reactivity upon β-adrenergic stimulation being preserved.

CONCLUSION

Organotypic heart slices represent a multicellular model of the human myocardium and a novel platform for studies ranging from the investigation of molecular interactions to tissue engineering.

Chronic sympathetic activation promotes downregulation of β-adrenoceptor-mediated effects in the guinea pig heart independently of structural remodeling and systolic dysfunction

It is uncertain if downregulation of β-adrenoceptor signaling pathway is promoted by an enhanced adrenergic tone at an early stage of cardiac disease, or it develops secondary to detrimental local myocardial changes in advanced heart failure. We examined the integrity of β-adrenoceptor signaling pathway upon chronic infusion of isoproterenol, a β-adrenoceptor agonist, at a dose producing no structural left ventricular (LV) remodeling and systolic dysfunction. Subcutaneous isoproterenol infusion (400 μg kg(-1) h(-1) over 16 days) to guinea pigs using osmotic minipumps produced no change in cardiac weights, LV internal dimensions, myocyte cross-sectional area, extent of interstitial fibrosis, and basal contractile function. Isolated, perfused heart preparations from isoproterenol-treated guinea pigs exhibited attenuated responsiveness to acute β-adrenoceptor stimulation, as evidenced by reduced LV developed pressure increase, less shortening of LV epicardial monophasic action potential and effective refractory period, and less myocardial cyclic adenosine monophosphate elevation, in response to isoproterenol exposure, when compared to saline-treated controls. Pharmacological responses to forskolin, an activator of the adenylate cyclase catalytic subunit, were well preserved in isoproterenol-treated hearts. Downregulation of β-adrenoceptor-mediated effects upon chronic isoproterenol infusion was associated with markedly reduced stimulatory G-protein α-subunit (G(sα)) myocardial expression levels. No change in expression levels of β-adrenoceptors, G-protein-coupled receptor kinase 2, inhibitory G-protein α-subunit (G(iα2)), and Ca(v)1.2 and K(v)7.1 ion channels was determined in isoproterenol-treated hearts. We therefore conclude that sustained adrenergic overstimulation may promote downregulation of myocardial β-adrenoceptor-mediated effects independently of structural LV remodeling and systolic failure, an effect attributed to β-adrenoceptor uncoupling from adenylate cyclase due to reduced G(sα)-protein expression.

A novel nonsense variant in Nav1.5 cofactor MOG1 eliminates its sodium current increasing effect and may increase the risk of arrhythmias

BACKGROUND

The protein MOG1 is a cofactor of the cardiac sodium channel, Nav1.5. Overexpression of MOG1 in Nav1.5-expressing cells increases sodium current markedly. Mutations in the genes encoding Nav1.5 and its accessory proteins have been associated with cardiac arrhythmias of significant clinical impact. We sought to investigate whether MOG1 is implicated in cardiac arrhythmias.

METHODS

We performed a genetic screening of the MOG1-encoding gene (gene symbol RANGRF, alias MOG1) in 220 Danish patients with cardiac arrhythmia. Of the 220, 197 were young patients with lone atrial fibrillation and 23 were patients with Brugada syndrome. The effect of one variant was investigated functionally by patch-clamping CHO-K1 cells coexpressing Nav1.5 with MOG1.

RESULTS

We uncovered a novel heterozygous nonsense variant, c.181G>T (p.E61X), that, however, was also present in control subjects, albeit at a lower frequency (1.8% vs 0.4%, P = 0.078). Electrophysiological investigation showed that the p.E61X variant completely eliminates the sodium current-increasing effect of MOG1 and thereby causes loss of function in the sodium current. When mimicking heterozygosity by coexpression of Nav1.5 with wild-type MOG1 and p.E61X-MOG1, no current decrease was seen.

CONCLUSIONS

Our screening of Nav1.5 cofactor MOG1 uncovered a novel nonsense variant that appeared to be present at a higher frequency among patients than control subjects. This variant causes MOG1 loss of function and therefore might be disease causing or modifying under certain conditions.

Simulation of the undiseased human cardiac ventricular action potential: model formulation and experimental validation

Cellular electrophysiology experiments, important for understanding cardiac arrhythmia mechanisms, are usually performed with channels expressed in non myocytes, or with non-human myocytes. Differences between cell types and species affect results. Thus, an accurate model for the undiseased human ventricular action potential (AP) which reproduces a broad range of physiological behaviors is needed. Such a model requires extensive experimental data, but essential elements have been unavailable. Here, we develop a human ventricular AP model using new undiseased human ventricular data: Ca(2+) versus voltage dependent inactivation of L-type Ca(2+) current (I(CaL)); kinetics for the transient outward, rapid delayed rectifier (I(Kr)), Na(+)/Ca(2+) exchange (I(NaCa)), and inward rectifier currents; AP recordings at all physiological cycle lengths; and rate dependence and restitution of AP duration (APD) with and without a variety of specific channel blockers. Simulated APs reproduced the experimental AP morphology, APD rate dependence, and restitution. Using undiseased human mRNA and protein data, models for different transmural cell types were developed. Experiments for rate dependence of Ca(2+) (including peak and decay) and intracellular sodium ([Na(+)](i)) in undiseased human myocytes were quantitatively reproduced by the model. Early afterdepolarizations were induced by I(Kr) block during slow pacing, and AP and Ca(2+) alternans appeared at rates >200 bpm, as observed in the nonfailing human ventricle. Ca(2+)/calmodulin-dependent protein kinase II (CaMK) modulated rate dependence of Ca(2+) cycling. I(NaCa) linked Ca(2+) alternation to AP alternans. CaMK suppression or SERCA upregulation eliminated alternans. Steady state APD rate dependence was caused primarily by changes in [Na(+)](i), via its modulation of the electrogenic Na(+)/K(+) ATPase current. At fast pacing rates, late Na(+) current and I(CaL) were also contributors. APD shortening during restitution was primarily dependent on reduced late Na(+) and I(CaL) currents due to inactivation at short diastolic intervals, with additional contribution from elevated I(Kr) due to incomplete deactivation.

Effects of KATP channel openers diazoxide and pinacidil in coronary-perfused atria and ventricles from failing and non-failing human hearts

This study compared the effects of ATP-regulated potassium channel (K(ATP)) openers, diazoxide and pinacidil, on diseased and normal human atria and ventricles. We optically mapped the endocardium of coronary-perfused right (n=11) or left (n=2) posterior atrial-ventricular free wall preparations from human hearts with congestive heart failure (CHF, n=8) and non-failing human hearts without (NF, n=3) or with (INF, n=2) infarction. We also analyzed the mRNA expression of the K(ATP) targets K(ir)6.1, K(ir)6.2, SUR1, and SUR2 in the left atria and ventricles of NF (n=8) and CHF (n=4) hearts. In both CHF and INF hearts, diazoxide significantly decreased action potential durations (APDs) in atria (by -21±3% and -27±13%, p<0.01) and ventricles (by -28±7% and -28±4%, p<0.01). Diazoxide did not change APD (0±5%) in NF atria. Pinacidil significantly decreased APDs in both atria (-46 to -80%, p<0.01) and ventricles (-65 to -93%, p<0.01) in all hearts studied. The effect of pinacidil on APD was significantly higher than that of diazoxide in both atria and ventricles of all groups (p<0.05). During pinacidil perfusion, burst pacing induced flutter/fibrillation in all atrial and ventricular preparations with dominant frequencies of 14.4±6.1 Hz and 17.5±5.1 Hz, respectively. Glibenclamide (10 μM) terminated these arrhythmias and restored APDs to control values. Relative mRNA expression levels of K(ATP) targets were correlated to functional observations. Remodeling in response to CHF and/or previous infarct potentiated diazoxide-induced APD shortening. The activation of atrial and ventricular K(ATP) channels enhances arrhythmogenicity, suggesting that such activation may contribute to reentrant arrhythmias in ischemic hearts.

Screening of KCNN3 in patients with early-onset lone atrial fibrillation

AIMS

The aim of this study was to screen KCNN3 encoding the small-conductance calcium-activated K+ channel (SK3) in lone atrial fibrillation patients. Atrial fibrillation (AF) is the most common cardiac arrhythmia. A genome-wide association study has recently associated an intronic single-nucleotide polymorphism (SNP) in KCNN3 with lone AF.

METHODS AND RESULTS

We sequenced the coding region and splice junctions of KCNN3 in 209 early-onset lone AF patients, screening for variations. A group of 208 healthy blood donors with normal ECGs and without cardiac symptoms were used as controls. All patients and controls were of Danish ethnicity. No mutations were found in the coding regions or splice sites of KCNN3. We found one known exonic synonymous SNP (rs1131820) in KCNN3 that was associated with AF. Both the genotype distribution and allele frequencies of SNP rs1131820 were significantly different between the AF cases and controls (PGenotype=0.047 and PAllele=0.027). Being a homozygous carrier of the major allele (GG) vs. the minor allele (AA) of rs1131820 was associated with an odds ratio of 2.85 (95% CI 1.13-7.18, P=0.026) for lone AF.

CONCLUSIONS

In this study of 209 young lone AF patients, we found no mutations in the exons or splice sites of KCNN3, but we found an association between the synonymous SNP rs1131820 in KCNN3 and lone AF.

Na+ channel distribution and electrophysiological heterogeneities in guinea pig ventricular wall

We sought to explore the distribution pattern of Na+ channels across ventricular wall, and to determine its functional correlates, in the guinea pig heart. Voltage-dependent Na+ channel (Nav) protein expression levels were measured in transmural samples of ventricular tissue by Western blotting. Isolated, perfused heart preparations were used to record monophasic action potentials and volume-conducted ECG, and to measure effective refractory periods (ERPs) and pacing thresholds, in order to assess excitability, electrical restitution kinetics, and susceptibility to stimulation-evoked tachyarrhythmias at epicardial and endocardial stimulation sites. In both ventricular chambers, Nav protein expression was higher at endocardium than epicardium, with midmyocardial layers showing intermediate expression levels. Endocardial stimulation sites showed higher excitability, as evidenced by lower pacing thresholds during regular stimulation and downward displacement of the strength-interval curve reconstructed after extrasystolic stimulation compared with epicardium. ERP restitution assessed over a wide range of pacing rates showed greater maximal slope and faster kinetics at endocardial than epicardial stimulation sites. Flecainide, a Na+ channel blocker, reduced the maximal ERP restitution slope, slowed restitution kinetics, and eliminated epicardial-to-endocardial difference in dynamics of electrical restitution. Greater excitability and steeper electrical restitution have been associated with greater arrhythmic susceptibility of endocardium than epicardium, as assessed by measuring ventricular fibrillation threshold, inducibility of tachyarrhythmias by rapid cardiac pacing, and the magnitude of stimulation-evoked repolarization alternans. In conclusion, higher Na+ channel expression levels may contribute to greater excitability, steeper electrical restitution slopes and faster restitution kinetics, and greater susceptibility to stimulation-evoked tachyarrhythmias at endocardium than epicardium in the guinea pig heart.

Nuclear factor kappaB downregulates the transient outward potassium current I(to,f) through control of KChIP2 expression

RATIONALE

The fast transient outward K(+) current (I(to,f)) plays a critical role in early repolarization of the heart. I(to,f) is consistently downregulated in cardiac disease. Despite its importance, the regulation of I(to,f) in disease remains poorly understood.

OBJECTIVE

Because the transcription factor nuclear factor (NF)-κB is activated in cardiac hypertrophy and disease, we studied the role of NF-κB in mediating I(to,f) reductions induced by hypertrophy.

METHODS AND RESULTS

Culturing neonatal rat ventricular myocytes in the presence of phenylephrine (PE) plus propranolol (Pro), to selectively activate α(1)-adrenergic receptors, caused reductions in I(to,f), as well as KChIP2 and Kv4.3 expression, while increasing Kv4.2 expression. Inhibition of NF-κB, via overexpression of a phosphorylation-deficient mutant of IκBα (IκBαSA) prevented PE/Pro-induced reductions in I(to,f) and KChIP2 mRNA, without affecting Kv4.2 or Kv4.3 expression, suggesting NF-κB mediates the I(to,f) reductions by repressing KChIP2. Indeed, overexpression of the NF-κB activator IκB kinase-β also decreased KChIP2 expression and I(to,f) (despite increasing Kv4.2), whereas IκBαSA overexpression elevated KChIP2 and decreased Kv4.2 levels. In addition, the classic NF-κB activator tumor necrosis factor α also induced NF-κB-dependent reductions of KChIP2 and I(to,f). Finally, inhibition of calcineurin did not prevent PE/Pro-induced reductions in KChIP2.

CONCLUSIONS

NF-κB regulates KChIP2 and Kv4.2 expression. The reductions in I(to,f) observed following α-adrenergic receptor stimulation or tumor necrosis factor α application require NF-κB-dependent decreases in KChIP2 expression.

Diazoxide maintenance of myocyte volume and contractility during stress: evidence for a non-sarcolemmal K(ATP) channel location

OBJECTIVE

Animal and human myocytes demonstrate significant swelling and reduced contractility during exposure to stress (metabolic inhibition, hyposmotic stress, or hyperkalemic cardioplegia), and these detrimental consequences may be inhibited by the addition of diazoxide (adenosine triphosphate-sensitive potassium channel opener) via an unknown mechanism. Both SUR1 and SUR2A subunits have been localized to the heart, and mouse sarcolemmal adenosine triphosphate-sensitive potassium channels are composed of SUR2A/Kir6.2 subunits in the ventricle and SUR1/Kir6.2 subunits in the atria. This study was performed to localize the mechanism of diazoxide by direct probing of sarcolemmal adenosine triphosphate-sensitive potassium channel current and by genetic deletion of channel subunits.

METHODS

Sarcolemmal adenosine triphosphate-sensitive potassium channel current was recorded in isolated wild-type ventricular mouse myocytes during exposure to Tyrode's solution, Tyrode's + 100 μmol/L diazoxide, hyperkalemic cardioplegia, cardioplegia + diazoxide, cardioplegia + 100 μmol/L pinacidil, or metabolic inhibition using whole-cell voltage clamp (N = 7-12 cells per group). Ventricular myocyte volume was measured from SUR1(-/-) and wild-type mice during exposure to control solution, hyperkalemic cardioplegia, or cardioplegia + 100 μmol/L diazoxide (N = 7-10 cells per group).

RESULTS

Diazoxide did not increase sarcolemmal adenosine triphosphate-sensitive potassium current in wild-type myocytes, although they demonstrated significant swelling during exposure to cardioplegia that was prevented by diazoxide. SUR1(-/-) myocytes also demonstrated significant swelling during exposure to cardioplegia, but this was not altered by diazoxide.

CONCLUSIONS

Diazoxide does not open the ventricular sarcolemmal adenosine triphosphate-sensitive potassium channel but provides volume homeostasis via an SUR1-dependent pathway in mouse ventricular myocytes, supporting a mechanism of action distinct from sarcolemmal adenosine triphosphate-sensitive potassium channel activation.

Mutations in sodium channel β-subunit SCN3B are associated with early-onset lone atrial fibrillation

AIMS

Atrial fibrillation (AF) is the most frequent arrhythmia. Screening of SCN5A-the gene encoding the α-subunit of the cardiac sodium channel-has indicated that disturbances of the sodium current may play a central role in the mechanism of lone AF. We tested the hypothesis that lone AF in young patients is associated with genetic mutations in SCN3B and SCN4B, the genes encoding the two β-subunits of the cardiac sodium channel.

METHODS AND RESULTS

In 192 unrelated lone AF patients, the entire coding sequence and splice junctions of SCN3B and SCN4B were bidirectionally sequenced. Three non-synonymous mutations were found in SCN3B (R6K, L10P, and M161T). Two mutations were novel (R6K and M161T). None of the mutations were present in the control group (n = 432 alleles), nor have any been previously reported in conjunction with AF. All SCN3B mutations affected residues that are evolutionarily conserved across species. Electrophysiological studies on the SCN3B mutation were carried out and all three SCN3B mutations caused a functionally reduced sodium channel current. One synonymous variant was found in SCN4B.

CONCLUSION

In 192 young lone AF patients, we found three patients with suspected disease-causing non-synonymous mutations in SCN3B, indicating that mutations in this gene contribute to the mechanism of lone AF. The three mutations in SCN3B were investigated electrophysiologically and all led to loss of function in the sodium current, supporting the hypothesis that decreased sodium current enhances AF susceptibility.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Chronic treatment with anabolic steroids induces ventricular repolarization disturbances: cellular, ionic and molecular mechanism

The illicit use of supraphysiological doses of androgenic steroids (AAS) has been suggested as a cause of arrhythmia in athletes. The objectives of the present study were to investigate the time-course and the cellular, ionic and molecular processes underlying ventricular repolarization in rats chronically treated with AAS. Male Wistar rats were treated weekly for 8 weeks with 10mg/kg of nandrolone decanoate (DECA n=21) or vehicle (control n=20). ECG was recorded weekly. Action potential (AP) and transient outward potassium current (I(to)) were recorded in rat hearts. Expression of KChIP2, Kv1.4, Kv4.2, and Kv4.3 was assessed by real-time PCR. Hematoxylin/eosin and Picrosirius red staining were used for histological analysis. QTc was greater in the DECA group. After DECA treatment the left, but not right, ventricle showed a longer AP duration than did the control. I(to) current densities were 47.5% lower in the left but not in the right ventricle after DECA. In the right ventricle the I(to) inactivation time-course was slower than in the control group. After DECA the left ventricle showed lower KChIP2 ( approximately 26%), Kv1.4 ( approximately 23%) and 4.3 ( approximately 70%) expression while the Kv 4.2 increased in 4 ( approximately 250%) and diminished in 3 ( approximately 30%) animals of this group. In the right ventricle the expression of I(to) subunits was similar between the treatment and control groups. DECA-treated hearts had 25% fewer nuclei and greater nuclei diameters in both ventricles. Our results strongly suggest that supraphysiological doses of AAS induce morphological remodeling in both ventricles. However, the electrical remodeling was mainly observed in the left ventricle.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Gap junction heterogeneity as mechanism for electrophysiologically distinct properties across the ventricular wall

Gap junctions are critical to maintaining synchronized impulse propagation and repolarization. Heterogeneous expression of the principal ventricular gap junction protein connexin43 (Cx43) is associated with action potential duration (APD) dispersion across the anterior ventricular wall. Little is known about Cx43 expression patterns and their disparate impact on regional electrophysiology throughout the heart. We aimed to determine whether the anterior and posterior regions of the heart are electrophysiologically distinct. Multisegment, high-resolution optical mapping was performed in canine wedge preparations harvested separately from the anterior left ventricle (aLV; n = 8) and posterior left ventricle (pLV; n = 8). Transmural APD dispersion was significantly greater on the aLV than the pLV (45 +/- 13 vs. 26 +/- 8.0 ms; P < 0.05). Conduction velocity dispersion was also significantly higher (P < 0.05) across the aLV (39 +/- 7%) than the pLV (16 +/- 3%). Carbenoxolone perfusion significantly enhanced APD and conduction velocity dispersion on the aLV (by 1.53-fold and 1.36-fold, respectively), but not the pLV (by 1.27-fold and 1.2-fold, respectively), and produced a 4.2-fold increase in susceptibility to inducible arrhythmias in the aLV. Confocal immunofluorescence microscopy revealed significantly (P < 0.05) greater transmural dispersion of Cx43 expression on the aLV (44 +/- 10%) compared with the pLV wall (8.3 +/- 0.7%), suggesting that regional expression of Cx43 expression patterns may account for regional electrophysiological differences. Computer simulations affirmed that localized uncoupling at the epicardial-midmyocardial interface is sufficient to produce APD gradients observed on the aLV. These data demonstrate that the aLV and pLV differ importantly with respect to their electrophysiological properties and Cx43 expression patterns. Furthermore, local underexpression of Cx43 is closely associated with transmural electrophysiological heterogeneity on the aLV. Therefore, regional and transmural heterogeneous Cx43 expression patterns may be an important mechanism underlying arrhythmia susceptibility, particularly in disease states where gap junction expression is altered.