Multiple potential molecular contributors to atrial hypocontractility caused by atrial tachycardia remodeling in dogs

BACKGROUND

Atrial fibrillation impairs atrial contractility, inducing atrial stunning that promotes thromboembolic stroke. Action potential (AP)-prolonging drugs are reported to normalize atrial hypocontractility caused by atrial tachycardia remodeling (ATR). Here, we addressed the role of AP duration (APD) changes in ATR-induced hypocontractility.

METHODS AND RESULTS

ATR (7-day tachypacing) decreased APD (perforated patch recording) by ≈50%, atrial contractility (echocardiography, cardiomyocyte video edge detection), and [Ca(2+)](i) transients. ATR AP waveforms suppressed [Ca(2+)](i) transients and cell shortening of control cardiomyocytes; whereas control AP waveforms improved [Ca(2+)](i) transients and cell shortening in ATR cells. However, ATR cardiomyocytes clamped with the same control AP waveform had ≈60% smaller [Ca(2+)](i) transients and cell shortening than control cells. We therefore sought additional mechanisms of contractile impairment. Whole-cell voltage clamp revealed reduced I(CaL); I(CaL) inhibition superimposed on ATR APs further suppressed [Ca(2+)](i) transients in control cells. Confocal microscopy indicated ATR-impaired propagation of the Ca(2+) release signal to the cell center in association with loss of t-tubular structures. Myofilament function studies in skinned permeabilized cardiomyocytes showed altered Ca(2+) sensitivity and force redevelopment in ATR, possibly due to hypophosphorylation of myosin-binding protein C and myosin light-chain protein 2a (immunoblot). Hypophosphorylation was related to multiple phosphorylation system abnormalities where protein kinase A regulatory subunits were downregulated, whereas autophosphorylation and expression of Ca(2+)-calmodulin-dependent protein kinase IIδ and protein phosphatase 1 activity were enhanced. Recovery of [Ca(2+)](i) transients and cell shortening occurred in parallel after ATR cessation.

CONCLUSIONS

Shortening of APD contributes to hypocontractility induced by 1-week ATR but accounts for it only partially. Additional contractility-suppressing mechanisms include I(CaL) current reduction, impaired subcellular Ca(2+) signal transmission, and altered myofilament function associated with abnormal myosin and myosin-associated protein phosphorylation. The complex mechanistic basis of the atrial hypocontractility associated with AF argues for upstream therapeutic targeting rather than interventions directed toward specific downstream pathophysiological derangements.

Left-to-right atrial inward rectifier potassium current gradients in patients with paroxysmal versus chronic atrial fibrillation

BACKGROUND

Recent evidence suggests that atrial fibrillation (AF) is maintained by high-frequency reentrant sources with a left-to-right-dominant frequency gradient, particularly in patients with paroxysmal AF (pAF). Unequal left-to-right distribution of inward rectifier K(+) currents has been suggested to underlie this dominant frequency gradient, but this hypothesis has never been tested in humans.

METHODS AND RESULTS

Currents were measured with whole-cell voltage-clamp in cardiomyocytes from right atrial (RA) and left (LA) atrial appendages of patients in sinus rhythm (SR) and patients with AF undergoing cardiac surgery. Western blot was used to quantify protein expression of I(K1) (Kir2.1 and Kir2.3) and I(K,ACh) (Kir3.1 and Kir3.4) subunits. Basal current was ≈2-fold larger in chronic AF (cAF) versus SR patients, without RA-LA differences. In pAF, basal current was ≈2-fold larger in LA versus RA, indicating a left-to-right atrial gradient. In both atria, Kir2.1 expression was ≈2-fold greater in cAF but comparable in pAF versus SR. Kir2.3 levels were unchanged in cAF and RA-pAF but showed a 51% decrease in LA-pAF. In SR, carbachol-activated (2 μmol/L) I(K,ACh) was 70% larger in RA versus LA. This right-to-left atrial gradient was decreased in pAF and cAF caused by reduced I(K,ACh) in RA only. Similarly, in SR, Kir3.1 and Kir3.4 proteins were greater in RA versus LA and decreased in RA of pAF and cAF. Kir3.1 and Kir3.4 expression was unchanged in LA of pAF and cAF.

CONCLUSION

Our results support the hypothesis that a left-to-right gradient in inward rectifier background current contributes to high-frequency sources in LA that maintain pAF. These findings have potentially important implications for development of atrial-selective therapeutic approaches.

Differential Alterations of Receptor Densities of Three Muscarinic Acetylcholine Receptor Subtypes and Current Densities of the Corre-sponding K+ Channels in Canine Atria with Atrial Fibrillation Induced by Experimental Congestive Heart Failure

Parasympathetic tone and congestive heart failure (CHF) are two of promoting factors in initiation and perpetuation of atrial fibrillation (AF). Recent studies indicate co-existence of multiple muscarinic acetylcholine receptor subtypes (mAChRs) that mediate several distinct K+ currents in the heart; inward rectifier K+ current IKACh by the M2, and two delayed rectifier K+ currents IKM3 and IK4AP by the M3 and M4 receptors, respectively. We studied the alterations of atrial mAChRs and their coupled K+ channels in the setting of AF in dogs with ventricular tachypacing-induced CHF. Whole-patch-clamp recordings showed that the current densities of IKACh (induced by 1 mM acetylcholine) and IK4AP (induced by 1 mM 4-aminopyridine) were ñ45% and ñ55% lower, respectively, while that of IKM3 (induced by 10 mM choline) was ñ75% higher, at a plateau voltage of 0 mV in atrial myocytes from CHF than those from healthy hearts. In healthy hearts, IKACh comprised >60%, and IKM3 and IK4AP <30%, of the total outward K+ currents mediated by mAChRs at depolarized potentials (between -20 mV and +50 mV). In AF atria of CHF dogs, however, the contribution of IKM3 increased to approximately 50%, exceeding those of IKACh or IK4AP. Western blot analyses with atrial membrane protein samples indicated that receptor densities of the M2 and M4 subtypes decreased by approximately 33% and approximately 22%, respectively, whereas that of the M3 subtype increased by approximately 2.3 folds, in parallel to the alterations of the corresponding K+ currents. We conclude that differential alterations of mAChR subtypes underlie differential alterations of their coupled K+ channels in AF atria and these differential alterations may contribute to atrial remodeling in AF induced in the setting of CHF.

Nuclear-delimited angiotensin receptor-mediated signaling regulates cardiomyocyte gene expression

Angiotensin-II (Ang-II) from extracardiac sources and intracardiac synthesis regulates cardiac homeostasis, with mitogenic and growth-promoting effects largely due to altered gene expression. Here, we assessed the possibility that angiotensin-1 (AT1R) or angiotensin-2 (AT2R) receptors on the nuclear envelope mediate effects on cardiomyocyte gene expression. Immunoblots of nucleus-enriched fractions from isolated cardiomyocytes indicated the presence of AT1R and AT2R proteins that copurified with the nuclear membrane marker nucleoporin-62 and histone-3, but not markers of plasma (calpactin-I), Golgi (GRP-78), or endoplasmic reticulum (GM130) membranes. Confocal microscopy revealed AT1R and AT2R proteins on nuclear membranes. Microinjected Ang-II preferentially bound to nuclear sites of isolated cardiomyocytes. AT1R and AT2R ligands enhanced de novo RNA synthesis in isolated cardiomyocyte nuclei incubated with [alpha-(32)P]UTP (e.g. 36.0 +/- 6.0 cpm/ng of DNA control versus 246.4 +/- 15.4 cpm/ng of DNA Ang-II, 390.1 +/- 15.5 cpm/ng of DNA L-162313 (AT1), 180.9 +/- 7.2 cpm/ng of DNA CGP42112A (AT2), p < 0.001). Ang-II application to cardiomyocyte nuclei enhanced NFkappaB mRNA expression, a response that was suppressed by co-administration of AT1R (valsartan) and/or AT2R (PD123177) blockers. Dose-response experiments with Ang-II applied to purified cardiomyocyte nuclei versus intact cardiomyocytes showed greater increases in NFkappaB mRNA levels at saturating concentrations with approximately 2-fold greater affinity upon nuclear application, suggesting preferential nuclear signaling. AT1R, but not AT2R, stimulation increased [Ca(2+)] in isolated cardiomyocyte nuclei. Inositol 1,4,5-trisphosphate receptor blockade by 2-aminoethoxydiphenyl borate prevented AT1R-mediated Ca(2+) release and attenuated AT1R-mediated transcription initiation responses. We conclude that cardiomyocyte nuclear membranes possess angiotensin receptors that couple to nuclear signaling pathways and regulate transcription. Signaling within the nuclear envelope (e.g. from intracellularly synthesized Ang-II) may play a role in Ang-II-mediated changes in cardiac gene expression, with potentially important mechanistic and therapeutic implications.

Concealed automaticity from an island of atrial myocardium post cavotricuspid ablation

We report the case of a patient with paroxysmal atrial fibrillation in whom 2 previous cavotricuspid isthmus (CTI) ablations were performed for recurrent type I counterclockwise atrial flutter. One year after the last CTI ablation, the patient underwent pulmonary vein isolation for AF and reassessment of conduction block in the CTI was performed during the procedure. While mapping the CTI, activations were documented within the CTI that were dissociated from both right atrial and ventricular activity during sinus rhythm and pacing maneuvers. This dissociated activity was confined to a region delimited by the 2 previous ablation lines, the tricuspid annulus and the inferior vena cava. These findings suggest that an island of atrial myocardium with automatic activity was created within the CTI by previous ablation lines.

New antiarrhythmic drugs for treatment of atrial fibrillation

Inadequacies in current therapies for atrial fibrillation have made new drug development crucial. Conventional antiarrhythmic drugs increase the risk of ventricular proarrhythmia. In drug development, the focus has been on favourable multichannel-blocking profiles, atrial-specific ion-channels, and novel non-channel targets (upstream therapy). Molecular modification of the highly effective multichannel blocker, amiodarone, to improve safety and tolerability has produced promising analogues such as dronedarone, although this drug seems less effective than does amiodarone. Vernakalant, an atrial-selective drug with reduced proarrhythmic risk, might be useful for cardioversion in atrial fibrillation. Ranolazine, another atrial-selective agent initially developed as an antianginal, has efficacy for atrial fibrillation and is being tested in prospective clinical trials. So-called upstream therapy with angiotensin-converting enzyme and angiotensin-receptor inhibitors, statins, or omega-3 fatty acids and fish oil that target atrial remodelling could be effective, but need further clinical validation. We focus on the basic and clinical pharmacology of newly emerging antiarrhythmic drugs and non-traditional approaches such as upstream therapy for atrial fibrillation.

Mechanisms by which adenosine restores conduction in dormant canine pulmonary veins

BACKGROUND

Adenosine acutely reconnects pulmonary veins (PVs) after radiofrequency application, revealing "dormant conduction" and identifying PVs at risk of reconnection, but the underlying mechanisms are unknown.

METHODS AND RESULTS

Canine PV and left-atrial (LA) action potentials were recorded with standard microelectrodes and ionic currents with whole-cell patch clamp before and after adenosine perfusion. PVs were isolated with radiofrequency current application in coronary-perfused LA-PV preparations. Adenosine abbreviated action potential duration similarly in PV and LA but significantly hyperpolarized resting potential (by 3.9+/-0.5%; P<0.05) and increased dV/dt(max) (by 34+/-10%) only in PV. Increased dV/dt(max) was not due to direct effects on I(Na), which was reduced similarly by adenosine in LA and PV but correlated with resting-potential hyperpolarization (r=0.80). Adenosine induced larger inward rectifier K(+)current (I(KAdo)) in PV (eg, -2.28+/-0.04 pA/pF; -100 mV) versus LA (-1.28+/-0.16 pA/pF). Radiofrequency ablation isolated PVs by depolarizing resting potential to voltages positive to -60 mV. Adenosine restored conduction in 5 dormant PVs, which had significantly more negative resting potentials (-57+/-6 mV) versus nondormant (-46+/-5 mV, n=6; P<0.001) before adenosine. Adenosine hyperpolarized both, but more negative resting-potential values after adenosine in dormant PVs (-66+/-6 mV versus -56+/-6 mV in nondormant; P<0.001) were sufficient to restore excitability. Adenosine effects on resting potential and conduction reversed on washout. Spontaneous recovery of conduction occurring in dormant PVs after 30 to 60 minutes was predicted by the adenosine response.

CONCLUSIONS

Adenosine selectively hyperpolarizes canine PVs by increasing I(KAdo). PVs with dormant conduction show less radiofrequency-induced depolarization than nondormant veins, allowing adenosine-induced hyperpolarization to restore excitability by removing voltage-dependent I(Na) inactivation and explaining the restoration of conduction in dormant PVs.

A high-fat diet increases risk of ventricular arrhythmia in female rats: enhanced arrhythmic risk in the absence of obesity or hyperlipidemia

Obesity increases the incidence of cardiac arrhythmias and impairs wound healing. However, it is presently unknown whether a high-fat diet affects arrhythmic risk or wound healing before the onset of overt obesity or hyperlipidemia. After 8 wk of feeding a high-fat diet to adult female rats, a nonsignificant increase in body weight was observed and associated with a normal plasma lipid profile. Following ischemia/reperfusion injury, scar length (standard diet 0.29 +/- 0.09 vs. high-fat 0.32 +/- 0.13 cm), thickness (standard diet 0.047 +/- 0.02 vs. high-fat 0.059 +/- 0.01 cm), and collagen alpha(1) type 1 content (standard diet 0.21 +/- 0.04 vs. high-fat 0.20 +/- 0.04 arbitrary units/mm(2)) of infarcted hearts were not altered by the high-fat diet. However, the mortality rate was greatly increased 24 h postinfarction (from 5% to 46%, P < 0.01 for ischemia/reperfusion rats; from 20% to 89%, P < 0.0001, in complete-occlusion rats) in high-fat fed rats, in association with a higher prevalence of ventricular arrhythmias. Ventricular arrhythmia inducibility was also significantly increased in noninfarcted rats fed a high-fat diet. In the hearts of rats fed a high-fat diet, connexin-40 expression was absent, connexin-43 was hypophosphorylated and lateralized, and neurofilament-M immunoreactive fiber density (standard diet 2,020 +/- 260 vs. high-fat diet 2,830 +/- 250 microm(2)/mm(2)) and tyrosine hydroxylase protein expression were increased (P < 0.05). Thus, in the absence of overt obesity and hyperlipidemia, sympathetic hyperinnervation and an aberrant pattern of gap junctional protein expression and regulation in the heart of female rats fed a high-fat diet may have contributed in part to the higher incidence of inducible cardiac arrhythmias.

Voltage-clamp-based methods for the detection of constitutively active acetylcholine-gated I(K,ACh) channels in the diseased heart

Vagal nerve stimulation can promote atrial fibrillation (AF) that requires activation of the acetylcholine (ACh)-gated potassium current I(K,ACh). In chronic AF (cAF), I(K,ACh) shows strong activity despite the absence of ACh or analogous pharmacological stimulation. This receptor-independent, constitutive I(K,ACh) activity is suggested to represent an atrial-selective anti-AF therapeutic target, but the underlying molecular mechanisms are unknown. This chapter provides an overview of the voltage-clamp techniques that can be used to study constitutive I(K,ACh) activity in atrial myocytes and summarizes briefly the current knowledge about the potential underlying mechanism(s) of constitutive I(K,ACh) activity in diseased heart.

Changes in connexin expression and the atrial fibrillation substrate in congestive heart failure

RATIONALE

Although connexin changes are important for the ventricular arrhythmic substrate in congestive heart failure (CHF), connexin alterations during CHF-related atrial arrhythmogenic remodeling have received limited attention.

OBJECTIVE

To analyze connexin changes and their potential contribution to the atrial fibrillation (AF) substrate during the development and reversal of CHF.

METHODS AND RESULTS

Three groups of dogs were studied: CHF induced by 2-week ventricular tachypacing (240 bpm, n=15); CHF dogs allowed a 4-week nonpaced recovery interval after 2-week tachypacing (n=16); and nonpaced sham controls (n=19). Left ventricular (LV) end-diastolic pressure and atrial refractory periods increased with CHF and normalized on CHF recovery. CHF caused abnormalities in atrial conduction indexes and increased the duration of burst pacing-induced AF (DAF, from 22+/-7 seconds in control to 1100+/-171 seconds, P<0.001). CHF did not significantly alter overall atrial connexin (Cx)40 and Cx43 mRNA and protein expression levels, but produced Cx43 dephosphorylation, increased Cx40/Cx43 protein expression ratio and caused Cx43 redistribution toward transverse cell-boundaries. All of the connexin-alterations reversed on CHF recovery, but CHF-induced conduction abnormalities and increased DAF (884+/-220 seconds, P<0.001 versus control) remained. The atrial fibrous tissue content increased from 3.6+/-0.7% in control to 14.7+/-1.5% and 13.3+/-2.3% in CHF and CHF recovery, respectively (both P<0.01 versus control), with transversely running zones of fibrosis physically separating longitudinally directed muscle bundles. In an ionically based action potential/tissue model, fibrosis was able to account for conduction abnormalities associated with CHF and recovery.

CONCLUSIONS

CHF causes atrial connexin changes, but these are not essential for CHF-related conduction disturbances and AF promotion, which are rather related primarily to fibrotic interruption of muscle bundle continuity.

Consequences of Atrial or Ventricular Tachypacing on the Heat Shock Proteins (HSP) level of Expression and Phosphorylation

BACKGROUND

Uncontrolled atrial fibrillation (AF) results in complex changes in the cardiomyocyte electrical and contractile functioning that promote atrial remodeling and the continuation of AF. Recently there has been a growing interest in understanding the role of heat shock proteins (HSPs), which are cytoprotective molecular chaperones, in the pathophysiology of AF. Several groups have examined HSP expression in patients with AF but have yielded mixed results. To allow for better consistency and reproducibility between subjects, we utilized canine models to reproduce AFpromoting conditions to better investigate the role of HSPs in the pathophysiology of AF.

METHODS

AF promoting conditions were simulated in canine models with fifteen adult mongrel dogs (20.6 to 36.0 kg) divided into three groups: (1) Control (n=5), (2) two week ventricular tachypacing (VTP) induced congestive heart failure (CHF) (n=5), and (3) one week atrial tachypacying (ATP) (n=5). Quick frozen right atrial free wall tissue samples were used for protein isolation and were analyzed via Western blotting with data was expressed as a relative ratio and were analyzed using a two-tailed, unpaired ttest and significance was set at p < 0.05. The expression levels of HSP 90, 70, and 25 were studied along with the phosphorylation status of HSP27 at serine-78.

RESULTS

We first examined the effects of the ATP and CHF heart models on the expression of a select group of HSPs via Western Blot. We found that there was no significant difference in levels of expression of HSP 90, 70, or 25 when either ATP or CHF models were compared to control canines. The phosphorylation status of HSP27 was significantly decreased in the CHF canine model when compared to control (p < 0.0111) and it tended towards a decrease in the ATP canine model when compared to control (p=0.0923).

CONCLUSION

This study showed that even though the expression levels of HSPs may remain constant, there are protein phosphorylation and dephosphorylation events that occur in AF that may have important consequences in its pathophysiology. It is therefore necessary to investigate the full scale of HSP modifications during AF and AF-promoting conditions.

Animal models for atrial fibrillation: clinical insights and scientific opportunities

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. A variety of animal models have been used to study the pathophysiology of AF, including molecular basis, ion-current determinants, anatomical features, and macroscopic mechanisms. In addition, animal models play a key role in the development of new therapeutic approaches, whether drug-based, molecular therapeutics, or device-related. This article discusses the various types of animal models that have been used for AF research, reviews the principle mechanisms governing atrial arrhythmias in each model, and provides some guidelines for model selection for various purposes.

Association of neonatal hypoxia with lasting changes in left ventricular gene expression: an animal model

OBJECTIVE

Innovations in pediatric cardiovascular surgery have resulted in significant improvements in survival for children with congenital heart disease. In adults with such disease, however, surgical morbidity and mortality remain significant. We hypothesized that hypoxemia in early life causes lasting changes in gene expression in the developing heart and that such changes may persist into later life, affecting the physiology of the adult myocardium.

METHODS

Microarray expression analyses were performed with left ventricular tissue from 10- and 90-day-old rats exposed to hypoxia (inspired oxygen fraction 0.12) for the first 10 days after birth then subsequently reared in ambient air and with tissue from age-matched rats reared entirely in ambient air. Changes in expression of selected genes were confirmed with real-time reverse transcriptase polymerase chain reaction. Left ventricular cardiomyocytes were isolated from adult animals in both groups, and cellular morphology and viability were compared.

RESULTS

Microarray analyses revealed significant changes in 1945 and 422 genes in neonates and adults, respectively. Changes in genes associated with adaptive vascular remodeling and energy homeostasis, as well as regulation of apoptosis, were confirmed by real-time reverse transcriptase polymerase chain reaction. The viability of cardiomyocytes isolated from hypoxic animals was significantly lower than in those from control animals (36.7% +/- 13.3% vs 85.0% +/- 2.9%, P = .024).

CONCLUSIONS

Neonatal hypoxia is associated with significant changes in left ventricular gene expression in both neonatal and adult rats. This may have physiologic implications for the adult myocardium.

Trafficking-deficient long QT syndrome mutation KCNQ1-T587M confers severe clinical phenotype by impairment of KCNH2 membrane localization: evidence for clinically significant IKr-IKs alpha-subunit interaction

BACKGROUND

KCNQ1-T587M is a trafficking-deficient long QT syndrome (LQTS) missense mutation. Affected patients exhibit severe clinical phenotypes that are not explained by the mutant's effects on I(Ks). Previous work showed a KCNH2 and KCNQ1 alpha-subunit interaction that increases KCNH2 membrane localization and function.

OBJECTIVE

We hypothesized that failure of trafficking-deficient KCNQ1-T587M to enhance KCNH2 membrane expression could reduce KCNH2 current versus wild-type KCNQ1 (KCNQ1-WT), contributing to the LQTS phenotype of KCNQ1-T587M carriers.

METHODS

Patch-clamp, protein biochemical studies, confocal imaging, and in vivo transfection of guinea pig cardiomyocytes were performed.

RESULTS

KCNQ1-T587M failed to generate functional current when coexpressed with KCNE1 and caused haploinsufficiency when coexpressed with KCNQ1-WT/KCNE1. Coexpression of KCNQ1-WT with KCNH2 increased I(KCNH2) versus KCNH2 alone (P <.05). Immunoblots and confocal microscopy indicated increased plasma membrane localization of KCNH2 alpha-subunits in cells cotransfected with KCNQ1-WT plasmid, while total KCNH2 protein synthesis and KCNH2 glycosylation remained unaffected, which suggests a chaperone effect of KCNQ1-WT to enhance the membrane localization of KCNH2. KCNH2 also coimmunoprecipitated with KCNQ1-WT. Although KCNQ1-T587M coprecipitated with KCNH2, the mutant was retained intracellularly and failed to increase KCNH2 membrane localization, abolishing the KCNQ1-WT chaperone function and reducing I(KCNH2) upon coexpression substantially compared with coexpression with KCNQ1-WT (P <.05). In vivo transfection of KCNQ1-T587M in guinea pigs suppressed I(Kr) in isolated cardiomyocytes.

CONCLUSION

The trafficking-deficient LQTS mutation KCNQ1-T587M fails to show the chaperoning function that enhances KCNH2 membrane localization with KCNQ1-WT. This novel mechanism results in reduced I(KCNH2), which would be expected to decrease repolarization reserve and synergize with reduced I(KCNQ1) caused directly by the mutation, potentially explaining the malignant clinical phenotype in affected patients.