An update on atrial fibrillation in 2014: From pathophysiology to treatment

Atrial fibrillation (AF) is the most frequently encountered cardiac arrhythmia. The trigger for initiation of AF is generally an enhanced vulnerability of pulmonary vein cardiomyocyte sleeves to either focal or re-entrant activity. The maintenance of AF is based on a "driver" mechanism in a vulnerable substrate. Cardiac mapping technology is providing further insight into these extremely dynamic processes. AF can lead to electrophysiological and structural remodelling, thereby promoting the condition. The management includes prevention of stroke by oral anticoagulation or left atrial appendage (LAA) occlusion, upstream therapy of concomitant conditions, and symptomatic improvement using rate control and/or rhythm control. Nonpharmacological strategies include electrical cardioversion and catheter ablation. There are substantial geographical variations in the management of AF, though European data indicate that 80% of patients receive adequate anticoagulation and 79% adequate rate control. High rates of morbidity and mortality weigh against perceived difficulties in management. Clinical research and growing experience are helping refine clinical indications and provide better technical approaches. Active research in cardiac electrophysiology is producing new antiarrhythmic agents that are reaching the experimental clinical arena, inhibiting novel ion channels. Future research should give better understanding of the underlying aetiology of AF and identification of drug targets, to help the move toward patient-specific therapy.

The positive frequency-dependent electrophysiological effects of the IKur inhibitor XEN-D0103 are desirable for the treatment of atrial fibrillation

Background

Selective inhibitors of K_v1.5 channels are being developed for the treatment of atrial fibrillation (AF).

Objectives

The purpose of this study was to investigate the effects of the highly selective K_v1.5 inhibitor XEN-D0103 on human atrial action potentials (APs) at high excitation rates and to assess safety.

Methods

Intracellular APs (stimulation rates 1–5 Hz) were measured in right atrial trabeculae from patients in sinus rhythm (SR), chronic AF (cAF; AF of >6 months duration), and paroxysmal AF (pAF). The safety and tolerability of XEN-D0103 were tested in a double-blind, randomized, placebo-controlled phase 1 study.

Results

Depending on its concentration, XEN-D0103 elevated the plateau potential. At 1 Hz, XEN-D0103 (3 µM) shortened action potential duration at 90% repolarization (APD₉₀) and effective refractory period (ERP) in SR preparations, but prolonged these parameters in cAF preparations. In SR and pAF preparations, the shortening effects on APD₉₀ and ERP turned into prolongation at high rates. In cAF trabeculae, XEN-D0103 prolonged APD₉₀ and ERP at 2 and 3 Hz. At high rates, more SR and pAF preparations failed to capture excitation in the presence of the drug than in its absence. XEN-D0103 (10 µM) did not significantly affect human ventricular APs. Even with plasma concentrations reaching 7000 ng/mL, XEN-D0103 did not increase ∆∆QTcF (QT interval corrected by the Fridericia formula) in the analysis of electrocardiograms of healthy volunteers, and no subjects receiving an active treatment had a QT or QTcF interval >450 ms, or increase in QTcF from baseline >30 ms.

Conclusion

APD prolongation and suppression of APs by XEN-D0103 at high stimulation rates in SR and pAF tissue, but not cAF, could be of therapeutic benefit for reducing AF burden. This concept needs to be confirmed in clinical trials.

Use of vitamin K antagonists for secondary stroke prevention depends on the treating healthcare provider in Germany - results from the German AFNET registry

BACKGROUND

Anticoagulation using vitamin K antagonists (VKAs) significantly reduces the risk of recurrent stroke in stroke patients with atrial fibrillation (AF) and is recommended by guidelines.

METHODS

The German Competence NETwork on Atrial Fibrillation established a nationwide prospective registry including 9,574 AF patients, providing the opportunity to analyse AF management according to German healthcare providers.

RESULTS

On enrolment, 896 (9.4 %) patients reported a prior ischaemic stroke or transient ischaemic attack. Stroke patients were significantly older, more likely to be female, had a higher rate of cardiovascular risk factors, and more frequently received anticoagulation (almost exclusively VKA) than patients without prior stroke history. Following enrolment, 76.4 % of all stroke patients without VKA contraindications received anticoagulation, which inversely associated with age (OR 0.95 per year; 95 % CI 0.92-0.97). General practitioners/internists (OR 0.40; 95 % CI 0.21-0.77) and physicians working in regional hospitals (OR 0.47; 95 % CI 0.29-0.77) prescribed anticoagulation for secondary stroke prevention less frequently than physicians working at university hospitals (reference) and office-based cardiologists (OR 1.40; 95 % CI 0.76-2.60). The impact of the treating healthcare provider was less evident in registry patients without prior stroke.

CONCLUSIONS

In the AFNET registry, anticoagulation for secondary stroke prevention was prescribed in roughly three-quarters of AF patients, a significantly higher rate than in primary prevention. We identified two factors associated with withholding oral anticoagulation in stroke survivors, namely higher age and-most prominently-treatment by a general practitioner/internist or physicians working at regional hospitals.

Late Sodium Current in Human Atrial Cardiomyocytes from Patients in Sinus Rhythm and Atrial Fibrillation

Slowly inactivating Na⁺ channels conducting “late” Na⁺ current (I_Na,late) contribute to ventricular arrhythmogenesis under pathological conditions. I_Na,late was also reported to play a role in chronic atrial fibrillation (AF). The objective of this study was to investigate I_Na,late in human right atrial cardiomyocytes as a putative drug target for treatment of AF. To activate Na⁺ channels, cardiomyocytes from transgenic mice which exhibit I_Na,late (ΔKPQ), and right atrial cardiomyocytes from patients in sinus rhythm (SR) and AF were voltage clamped at room temperature by 250-ms long test pulses to -30 mV from a holding potential of -80 mV with a 100-ms pre-pulse to -110 mV (protocol I). I_Na,late at -30 mV was not discernible as deviation from the extrapolated straight line IV-curve between -110 mV and -80 mV in human atrial cells. Therefore, tetrodotoxin (TTX, 10 μM) was used to define persistent inward current after 250 ms at -30 mV as I_Na,late. TTX-sensitive current was 0.27±0.06 pA/pF in ventricular cardiomyocytes from ΔKPQ mice, and amounted to 0.04±0.01 pA/pF and 0.09±0.02 pA/pF in SR and AF human atrial cardiomyocytes, respectively. With protocol II (holding potential -120 mV, pre-pulse to -80 mV) TTX-sensitive I_Na,late was always larger than with protocol I. Ranolazine (30 μM) reduced I_Na,late by 0.02±0.02 pA/pF in SR and 0.09±0.02 pA/pF in AF cells. At physiological temperature (37°C), however, I_Na,late became insignificant. Plateau phase and upstroke velocity of action potentials (APs) recorded with sharp microelectrodes in intact human trabeculae were more sensitive to ranolazine in AF than in SR preparations. Sodium channel subunits expression measured with qPCR was high for SCN5A with no difference between SR and AF. Expression of SCN8A and SCN10A was low in general, and lower in AF than in SR. In conclusion, We confirm for the first time a TTX-sensitive current (I_Na,late) in right atrial cardiomyocytes from SR and AF patients at room temperature, but not at physiological temperature. While our study provides evidence for the presence of INa,late in human atria, the potential of such current as a target for the treatment of AF remains to be demonstrated.

Pharmacological Modulation of Mucosa-Related Impairment of β-Adrenoceptor-Mediated Relaxation in Human Detrusor

Objectives: The mucosa of human detrusor strips impairs catecholamine-induced relaxation. In order to elucidate which signal transduction pathways are involved in this cross talk between the mucosa and detrusor, we have studied the effects of several pharmacological agonists and antagonists on noradrenaline-mediated relaxation in intact and mucosa-denuded detrusor strips. Patients and Methods: Strips of detrusor tissue were obtained from patients who had undergone cystectomy for bladder cancer and were set up for force measurement. KCl- or carbachol-precontracted strips were relaxed with increasing concentrations of noradrenaline in the absence and in the presence of nitric oxide synthase inhibitor, L-NAME; P2X-receptor antagonist, PPADS; ETA-receptor antagonist, BQ-123; ETB-receptor antagonist, BQ-788; cyclooxygenase inhibitor, diclofenac; AT1-receptor antagonist, candesartan; and NK1-receptor antagonist, L-703,606. Results: In intact strips, KCl-stimulated force was enhanced by all blockers; carbachol-stimulated force increased with L-703,606. In denuded strips, only L-NAME augmented the KCl-stimulated contraction. Noradrenaline relaxed the precontracted detrusor strips to a significantly larger extent and at lower concentrations in denuded than in intact strips. L-NAME, PPADS and BQ-123/BQ-788 had little effect on noradrenaline-induced relaxation, whereas diclofenac, candesartan and L-703,606 sensitized intact carbachol-stimulated detrusor strips to noradrenaline-induced relaxation. Conclusion: Inhibition of the noradrenaline-induced relaxation of precontracted human detrusor strips by the mucosa is attenuated by diclofenac, candesartan and L-703,606 suggesting the involvement of prostanoids, angiotensin and neurokinin pathways. Further experiments are required to unravel the exact mechanisms.

β-Adrenoceptor-Mediated Relaxation of Carbachol-Pre-Contracted Mouse Detrusor

Aims: To study the β-adrenoceptor subtypes involved in the relaxation responses to (-)-isoprenaline in carbachol-pre-contracted (CCh) mouse detrusor muscle with intact and denuded mucosa. Methods: Isolated muscle strips from the urinary bladder of male C57BL6 mice or β2-adrenoceptor knockout mice were pre-contracted with CCh, 1 µM and relaxed with increasing concentrations of the β-adrenoceptor (β-AR) agonist (-)-isoprenaline and forskolin. For estimating the β-AR subtypes involved, subtype-selective receptor blockers were used, that is, CGP 20712A (β1-ARs), ICI 118,551 (β2-ARs), and L748,337 (β3-ARs). Results: Unlike in KCl-pre-contracted muscle, the mucosa did not affect the sensitivity of the relaxation response to (-)-isoprenaline in CCh-pre-contracted murine detrusor strips. Increasing concentrations of (-)-isoprenaline produced a biphasic concentration-relaxation response without any difference both during the presence and absence of mucosa. The relaxation fraction produced by low (-)-isoprenaline concentrations was mediated by β2-AR as evidenced by a shift of the concentration-response curve to higher concentrations with ICI 118,551, but not with CGP 20712A and L748,337, and by the absence of this fraction in β2-AR-KO mice. The relaxation response with low sensitivity to (-)-isoprenaline was not affected by any of the β-AR subtype-selective blockers and was the only response detected in detrusor strips from β2-AR-KO mice. Conclusions: In CCh-pre-contracted mouse detrusor, β2-ARs are responsible for the relaxation component with high sensitivity to (-)-isoprenaline as indicated by the conversion of a biphasic into a monophasic CRC with ICI 118,551 or by its absence in β2-AR KO mice. The mucosa does not impair relaxation under these conditions.

Interaction of DPP10a with Kv4.3 channel complex results in a sustained current component of human transient outward current Ito

The sustained component of the K(+) outward current in human atrial myocytes is believed to be due to the slowly inactivating ultra-rapid potassium current I Kur and not to the fast inactivating transient outward current Ito. Here we provide evidence for contribution of Ito to this late current due to the effects of dipeptidyl peptidase-like protein (DPP) 10 (DPP10a) interacting with Kv4.3 channels. We studied the late current component of Ito in human atrial myocytes and CHO cells co-expressing Kv4.3 or Kv4.3/KChIP2 (control) and DPP proteins using voltage-clamp technique and a pharmacological approach. A voltage dependent and slowly inactivating late current (43% of peak amplitude) could be observed in atrial myocytes. We found a similar current in CHO cells expressing Kv4.3/KChIP2 + DPP10a, but not in cells co-expressing Kv4.3 + DPP or Kv4.3/KChIP2 + DPP6-S. Assuming that DPP10a influences atrial Ito, we detected DPP10 expression of three alternatively spliced mRNAs, DPP10 protein and colocalization of Kv4.3 and DPP10 proteins in human atrial myocytes. DPP10a did not affect properties of expressed Kv1.5 excluding a contribution to the sustained IKur in atrial cells. To test for the contribution of Kv4-based Ito on sustained K(+) outward currents in human atrial myocytes, we used 4-AP to block IKur, in combination with Heteropoda toxin 2 to block Kv4 channels. We could clearly separate an Ito fraction of about 19% contributing to the late current in atrial myocytes. Thus, the interaction of DPP10a, expressed in human atrium, with Kv4.3 channels generates a sustained current component of Ito, which may affect late repolarization phase of atrial action potentials.

Application of the RIMARC algorithm to a large data set of action potentials and clinical parameters for risk prediction of atrial fibrillation

Ex vivo recorded action potentials (APs) in human right atrial tissue from patients in sinus rhythm (SR) or atrial fibrillation (AF) display a characteristic spike-and-dome or triangular shape, respectively, but variability is huge within each rhythm group. The aim of our study was to apply the machine-learning algorithm ranking instances by maximizing the area under the ROC curve (RIMARC) to a large data set of 480 APs combined with retrospectively collected general clinical parameters and to test whether the rules learned by the RIMARC algorithm can be used for accurately classifying the preoperative rhythm status. APs were included from 221 SR and 158 AF patients. During a learning phase, the RIMARC algorithm established a ranking order of 62 features by predictive value for SR or AF. The model was then challenged with an additional test set of features from 28 patients in whom rhythm status was blinded. The accuracy of the risk prediction for AF by the model was very good (0.93) when all features were used. Without the seven AP features, accuracy still reached 0.71. In conclusion, we have shown that training the machine-learning algorithm RIMARC with an experimental and clinical data set allows predicting a classification in a test data set with high accuracy. In a clinical setting, this approach may prove useful for finding hypothesis-generating associations between different parameters.

Tachycardia-induced silencing of subcellular Ca2+ signaling in atrial myocytes

Atrial fibrillation (AF) is characterized by sustained high atrial activation rates and arrhythmogenic cellular Ca2+ signaling instability; however, it is not clear how a high atrial rate and Ca2+ instability may be related. Here, we characterized subcellular Ca2+ signaling after 5 days of high atrial rates in a rabbit model. While some changes were similar to those in persistent AF, we identified a distinct pattern of stabilized subcellular Ca2+ signaling. Ca2+ sparks, arrhythmogenic Ca2+ waves, sarcoplasmic reticulum (SR) Ca2+ leak, and SR Ca2+ content were largely unaltered. Based on computational analysis, these findings were consistent with a higher Ca2+ leak due to PKA-dependent phosphorylation of SR Ca2+ channels (RyR2s), fewer RyR2s, and smaller RyR2 clusters in the SR. We determined that less Ca2+ release per [Ca2+]i transient, increased Ca2+ buffering strength, shortened action potentials, and reduced L-type Ca2+ current contribute to a stunning reduction of intracellular Na+ concentration following rapid atrial pacing. In both patients with AF and in our rabbit model, this silencing led to failed propagation of the [Ca2+]i signal to the myocyte center. We conclude that sustained high atrial rates alone silence Ca2+ signaling and do not produce Ca2+ signaling instability, consistent with an adaptive molecular and cellular response to atrial tachycardia.

PDE3, but not PDE4, reduces β₁ - and β₂-adrenoceptor-mediated inotropic and lusitropic effects in failing ventricle from metoprolol-treated patients

BACKGROUND AND PURPOSE

PDE3 and/or PDE4 control ventricular effects of catecholamines in several species but their relative effects in failing human ventricle are unknown. We investigated whether the PDE3-selective inhibitor cilostamide (0.3-1 μM) or PDE4 inhibitor rolipram (1-10 μM) modified the positive inotropic and lusitropic effects of catecholamines in human failing myocardium.

EXPERIMENTAL APPROACH

Right and left ventricular trabeculae from freshly explanted hearts of 5 non-β-blocker-treated and 15 metoprolol-treated patients with terminal heart failure were paced to contract at 1 Hz. The effects of (-)-noradrenaline, mediated through β₁ adrenoceptors (β₂ adrenoceptors blocked with ICI118551), and (-)-adrenaline, mediated through β₂ adrenoceptors (β₁ adrenoceptors blocked with CGP20712A), were assessed in the absence and presence of PDE inhibitors. Catecholamine potencies were estimated from -logEC₅₀s.

KEY RESULTS

Cilostamide did not significantly potentiate the inotropic effects of the catecholamines in non-β-blocker-treated patients. Cilostamide caused greater potentiation (P = 0.037) of the positive inotropic effects of (-)-adrenaline (0.78 ± 0.12 log units) than (-)-noradrenaline (0.47 ± 0.12 log units) in metoprolol-treated patients. Lusitropic effects of the catecholamines were also potentiated by cilostamide. Rolipram did not affect the inotropic and lusitropic potencies of (-)-noradrenaline or (-)-adrenaline on right and left ventricular trabeculae from metoprolol-treated patients.

CONCLUSIONS AND IMPLICATIONS

Metoprolol induces a control by PDE3 of ventricular effects mediated through both β₁ and β₂ adrenoceptors, thereby further reducing sympathetic cardiostimulation in patients with terminal heart failure. Concurrent therapy with a PDE3 blocker and metoprolol could conceivably facilitate cardiostimulation evoked by adrenaline through β₂ adrenoceptors. PDE4 does not appear to reduce inotropic and lusitropic effects of catecholamines in failing human ventricle.

Inter-subject variability in human atrial action potential in sinus rhythm versus chronic atrial fibrillation

AIMS

Human atrial electrophysiology exhibits high inter-subject variability in both sinus rhythm (SR) and chronic atrial fibrillation (cAF) patients. Variability is however rarely investigated in experimental and theoretical electrophysiological studies, thus hampering the understanding of its underlying causes but also its implications in explaining differences in the response to disease and treatment. In our study, we aim at investigating the ability of populations of human atrial cell models to capture the inter-subject variability in action potential (AP) recorded in 363 patients both under SR and cAF conditions.

METHODS AND RESULTS

Human AP recordings in atrial trabeculae (n = 469) from SR and cAF patients were used to calibrate populations of computational SR and cAF atrial AP models. Three populations of over 2000 sampled models were generated, based on three different human atrial AP models. Experimental calibration selected populations of AP models yielding AP with morphology and duration in range with experimental recordings. Populations using the three original models can mimic variability in experimental AP in both SR and cAF, with median conductance values in SR for most ionic currents deviating less than 30% from their original peak values. All cAF populations show similar variations in G(K1), G(Kur) and G(to), consistent with AF-related remodeling as reported in experiments. In all SR and cAF model populations, inter-subject variability in I(K1) and I(NaK) underlies variability in APD90, variability in I(Kur), I(CaL) and I(NaK) modulates variability in APD50 and combined variability in Ito and I(Kur) determines variability in APD20. The large variability in human atrial AP triangulation is mostly determined by I(K1) and either I(NaK) or I(NaCa) depending on the model.

CONCLUSION

Experimentally-calibrated human atrial AP models populations mimic AP variability in SR and cAF patient recordings, and identify potential ionic determinants of inter-subject variability in human atrial AP duration and morphology in SR versus cAF.

Mucosa of murine detrusor impairs β2 -adrenoceptor-mediated relaxation

AIMS

To investigate the role of the mucosa in (-)-isoprenaline-induced relaxation of mouse detrusor muscle and to characterize the β-adrenoceptor subtypes involved.

METHODS

Isolated intact and mucosa-denuded muscle strips from the urinary bladder of male C57BL6 mice were pre-contracted with KCl (40 mM) and were relaxed with increasing concentrations of the β-adrenoceptor (β-AR) agonist (-)-isoprenaline and forskolin in the presence and absence of the subtype-selective β-AR blockers CGP20712A (β1 -ARs), ICI118,551 (β2 -ARs), and L748,337 (β3 -ARs).

RESULTS

Force development in response to KCl was larger in mucosa-denuded than in intact preparations and was almost completely relaxed with increasing concentrations of (-)-isoprenaline. Mucosa-denuded muscles were about 10-fold more sensitive to (-)-isoprenaline than intact muscles. CGP20712A did not affect the concentration-response curves (CRCs) to (-)-isoprenaline, ICI118,551 shifted the CRC further to the right in denuded than in intact strips so that the difference between them was abolished. Combined exposure to β1 -AR and β2 -AR blocker yielded the same result. L748,337 did not significantly affect the CRC to (-)-isoprenaline but caused additional blockade to ICI118,551 in the presence of intact mucosa.

CONCLUSIONS

The mucosa of mouse detrusor strips impairs KCl-induced force development and reduces the sensitivity to β-AR-induced relaxation. The relaxing response to (-)-isoprenaline as well as the mucosa effect thereupon are mainly mediated by β2 -ARs. A minor involvement of β3 -ARs becomes apparent particularly at high (-)-isoprenaline concentrations.

Small-conductance calcium-activated potassium (SK) channels contribute to action potential repolarization in human atria

AIMS

Small-conductance calcium-activated potassium (SK) channels are expressed in the heart of various species, including humans. The aim of the present study was to address whether SK channels play a functional role in human atria.

METHODS AND RESULTS

Quantitative real-time PCR analyses showed higher transcript levels of SK2 and SK3 than that of the SK1 subtype in human atrial tissue. SK2 and SK3 were reduced in chronic atrial fibrillation (AF) compared with sinus rhythm (SR) patients. Immunohistochemistry using confocal microscopy revealed widespread expression of SK2 in atrial myocytes. Two SK channel inhibitors (NS8593 and ICAGEN) were tested in heterologous expression systems revealing ICAGEN as being highly selective for SK channels, while NS8593 showed less selectivity for these channels. In isolated atrial myocytes from SR patients, both inhibitors decreased inwardly rectifying K(+) currents by ∼15% and prolonged action potential duration (APD), but no effect was observed in myocytes from AF patients. In trabeculae muscle strips from right atrial appendages of SR patients, both compounds increased APD and effective refractory period, and depolarized the resting membrane potential, while only NS8593 induced these effects in tissue from AF patients. SK channel inhibition did not alter any electrophysiological parameter in human interventricular septum tissue.

CONCLUSIONS

SK channels are present in human atria where they participate in repolarization. SK2 and SK3 were down-regulated and had reduced functional importance in chronic AF. As SK current was not found to contribute substantially to the ventricular AP, pharmacological inhibition of SK channels may be a putative atrial-selective target for future antiarrhythmic drug therapy.

Mkk4 is a negative regulator of the transforming growth factor beta 1 signaling associated with atrial remodeling and arrhythmogenesis with age

BACKGROUND

Atrial fibrillation (AF), often associated with structural, fibrotic change in cardiac tissues involving regulatory signaling mediators, becomes increasingly common with age. In the present study, we explored the role of mitogen-activated protein kinase kinase 4 (Mkk4), a critical component of the stress-activated mitogen-activated protein kinase family, in age-associated AF.

METHODS AND RESULTS

We developed a novel mouse model with a selective inactivation of atrial cardiomyocyte Mkk4 (Mkk4(ACKO)). We characterized and compared electrophysiological, histological, and molecular features of young (3- to 4-month), adult (6-month), and old (1-year) Mkk4(ACKO) mice with age-matched control littermates (Mkk4(F/F)). Aging Mkk4(ACKO) mice were more susceptible to atrial tachyarrhythmias than the corresponding Mkk4(F/F) mice, showing characteristic slow and dispersed atrial conduction, for which modeling studies demonstrated potential arrhythmic effects. These differences paralleled increased interstitial fibrosis, upregulated transforming growth factor beta 1 (TGF-β1) signaling and dysregulation of matrix metalloproteinases in Mkk4(ACKO), compared to Mkk4(F/F), atria. Mkk4 inactivation increased the sensitivity of cultured cardiomyocytes to angiotensin II-induced activation of TGF-β1 signaling. This, in turn, enhanced expression of profibrotic molecules in cultured cardiac fibroblasts, suggesting cross-talk between these two cell types in profibrotic signaling. Finally, human atrial tissues in AF showed a Mkk4 downregulation associated with increased production of profibrotic molecules, compared to findings in tissue from control subjects in sinus rhythm.

CONCLUSIONS

These findings together demonstrate, for the first time, that Mkk4 is a negative regulator of the TGF-β1 signaling associated with atrial remodeling and arrhythmogenesis with age, establishing Mkk4 as a new potential therapeutic target for treating AF.

Carvedilol induces greater control of β2- than β 1-adrenoceptor-mediated inotropic and lusitropic effects by PDE3, while PDE4 has no effect in human failing myocardium

The β-blockers carvedilol and metoprolol provide important therapeutic strategies for heart failure treatment. Therapy with metoprolol facilitates the control by phosphodiesterase PDE3, but not PDE4, of inotropic effects of catecholamines in human failing ventricle. However, it is not known whether carvedilol has the same effect. We investigated whether the PDE3-selective inhibitor cilostamide (0.3 μM) or PDE4-selective inhibitor rolipram (1 μM) modified the positive inotropic and lusitropic effects of catecholamines in ventricular myocardium of heart failure patients treated with carvedilol. Right ventricular trabeculae from explanted hearts of nine carvedilol-treated patients with terminal heart failure were paced to contract at 1 Hz. The effects of (-)-noradrenaline, mediated through β1-adrenoceptors (β2-adrenoceptors blocked with ICI118551), and (-)-adrenaline, mediated through β2-adrenoceptors (β1-adrenoceptors blocked with CGP20712A), were assessed in the absence and presence of the PDE inhibitors. The inotropic potency, estimated from -logEC50s, was unchanged for (-)-noradrenaline but decreased 16-fold for (-)-adrenaline in carvedilol-treated compared to non-β-blocker-treated patients, consistent with the previously reported β2-adrenoceptor-selectivity of carvedilol. Cilostamide caused 2- to 3-fold and 10- to 35-fold potentiations of the inotropic and lusitropic effects of (-)-noradrenaline and (-)-adrenaline, respectively, in trabeculae from carvedilol-treated patients. Rolipram did not affect the inotropic and lusitropic potencies of (-)-noradrenaline or (-)-adrenaline. Treatment of heart failure patients with carvedilol induces PDE3 to selectively control the positive inotropic and lusitropic effects mediated through ventricular β2-adrenoceptors compared to β1-adrenoceptors. The β2-adrenoceptor-selectivity of carvedilol may provide protection against β2-adrenoceptor-mediated ventricular overstimulation in PDE3 inhibitor-treated patients. PDE4 does not control β1- and β2-adrenoceptor-mediated inotropic and lusitropic effects in carvedilol-treated patients.

Effects of IKur blocker MK-0448 on human right atrial action potentials from patients in sinus rhythm and in permanent atrial fibrillation

Selective blockers of the Kv1.5 channel have been developed for the treatment of atrial fibrillation (AF), but little is known how these atrial-selective drugs affect human action potentials (APs). Therefore we have investigated the Kv1.5 blocker MK-0448 (N-{6-[(1S)-1-(4-fluorophenyl)-2,2-di(pyridin-3-yl)ethyl]pyridin-2-yl}methanesulfon- amide) in right atrial trabeculae from patients in sinus rhythm (SR), permanent AF (>6 months), and intermittent AF. MK-0448 blocked Kv1.5 current in an expression system and concentration-dependently elevated the plateau phase of atrial APs. In SR preparations stimulated at 1 Hz, MK-0448 (3 μM) shortened action potential duration at 90% of repolarization (APD₉₀) and effective refractory period (ERP), but in permanent AF preparations, MK-0448 prolonged APD₉₀ and ERP. The effects of MK-0448 in intermittent AF resembled those in SR preparations. Block of I_Ks is probably more prominent in AF because of reduced repolarization reserve due to AF-induced remodeling.

[New developments in the antiarrhythmic therapy of atrial fibrillation]

Atrial fibrillation often affects elderly people with cardiovascular disease and takes a progressive course with increasing resistance to treatment. For the latter, electrical and structural changes (remodelling) seem to be responsible that are directly related to the high excitatory rate in the atria. Therapeutic strategies for atrial fibrillation consist of (i) treating the underlying cardiovascular disease, (ii) re-establishing sinus rhythm and (iii) reducing ventricular rate. Rapid pharmacological or electrical cardioversion is expected to prevent remodelling. Classical antiarrhythmic drugs are notoriously ineffective and burdened with serious cardiac and extracardiac side effects so that there is an urgent need for effective and safe novel compounds. In this review the three recently introduced drugs dronedarone, vernakalant and ranolazine are discussed with respect to the use in atrial fibrillation. Other new antiarrhythmic agents are still in the developmental phase and aim at atria-selective mechanisms thereby excluding ventricular proarrhythmic effects. The mechanisms of action will be discussed in the context of the present understanding of the pathophysiology of onset and maintenance of atrial fibrillation.

Attenuated response of L-type calcium current to nitric oxide in atrial fibrillation

AIM

Nitric oxide (NO) synthesized by cardiomyocytes plays an important role in the regulation of cardiac function. Here, we studied the impact of NO signalling on calcium influx in human right atrial myocytes and its relation to atrial fibrillation (AF).

METHODS AND RESULTS

Right atrial appendages (RAAs) were obtained from patients in sinus rhythm (SR) and AF. The biotin-switch technique was used to evaluate endogenous S-nitrosylation of the α1C subunit of L-type calcium channels. Comparing SR to AF, S-nitrosylation of Ca(2+) channels was similar. Direct effects of the NO donor S-nitroso-N-acetyl-penicillamine (SNAP) on L-type calcium current (ICa,L) were studied in cardiomyocytes with standard voltage-clamp techniques. In SR, ICa,L increased with SNAP (100 µM) by 48%, n/N = 117/56, P < 0.001. The SNAP effect on ICa,L involved activation of soluble guanylate cyclase and protein kinase A. Specific inhibition of phosphodiesterase (PDE)3 with cilostamide (1 µM) enhanced ICa,L to a similar extent as SNAP. However, when cAMP was elevated by PDE3 inhibition or β-adrenoceptor stimulation, SNAP reduced ICa,L, pointing to cGMP-cAMP cross-regulation. In AF, the stimulatory effect of SNAP on ICa,L was attenuated, while its inhibitory effect on isoprenaline- or cilostamide-stimulated current was preserved. cGMP elevation with SNAP was comparable between the SR and AF group. Moreover, the expression of PDE3 and soluble guanylate cyclase was not reduced in AF.

CONCLUSION

NO exerts dual effects on ICa,L in SR with an increase of basal and inhibition of cAMP-stimulated current, and in AF NO inhibits only stimulated ICa,L. We conclude that in AF, cGMP regulation of PDE2 is preserved, but regulation of PDE3 is lost.