5-Hydroxytryptamine and atrial arrhythmogenesis: A “culprit mechanism” or bystander in patients with chronic atrial fibrillation?

Journal of Molecular and Cellular Cardiology 42 (2007) 51 – 53 www.elsevier.com/locate/yjmcc

Editorial

5-Hydroxytryptamine and atrial arrhythmogenesis: A “culprit mechanism” or bystander in patients with chronic atrial fibrillation?

Atrial remodeling refers to any change in atrial function that promotes atrial fibrillation (AF) or occurs as a consequence of AF. Tachycardia-induced atrial remodeling is associated with shorter effective refractory period (ERP) and impaired rate adaptation (electrical remodeling). In contrast, atrial remodeling in the aged heart or during heart failure involves fibrosis leading to heterogeneous conduction slowing and prolongation of ERP (structural remodeling). In humans, atrial electrical and structural remodeling coexist resulting in mixed changes of atrial function precluding a clear distinction between adaptive and maladaptive alterations [1]. AF has been considered to accommodate in the atria by multiple reentry circuits resulting in abnormal conduction and refractoriness, but accumulating evidence points to the importance of focal activity and Ca2+-handling abnormalities leading to Ca2+-overload and triggered activity [1]. Activation of the autonomic nervous system or excessive release of 5-hydroxytryptamine (5-HT) from platelets may initiate AF by causing Ca2+-overload of atrial myocytes [1,2]. In the human atrium, 5-HT exerts positive inotropic, chronotropic and lusitropic effects via Gs-protein-coupled subtype-4 5-HTreceptors (5-HT4). The subsequent generation of cAMP stimulates protein kinase A (PKA). The resulting increase in protein phosphorylation enhances the amplitude of L-type Ca2+ current (ICa,L) and hence elevates intracellular Ca2+ concentration [2]. Thus the stimulation of cardiac 5-HT4-receptors resembles β-adrenoceptor activation and may promote the generation of early and delayed afterdepolarizations that trigger AF. Although the proarrhythmic effects of 5-HT in the atria have been known for a long time [2], the underlying cellular mechanisms are not fully understood. In addition, whether and how 5-HT contributes to atrial arrhythmogenesis in patients with chronic AF (cAF) is unknown. In this issue of the Journal of Molecular and Cellular Cardiology, Pau et al. [3] provide strong evidence for impaired 5-HT responses in the atria of cAF patients. Specifically, the efficacy of 5-HT to increase ICa,L and to prolong action potential duration was about 50% lower in cAF than in SR. The effects were abolished by the selective 5HT4-receptor antagonist GR113808 (1 μM) indicating involvement of 5HT4-receptors only. Cellular arrhythmic depolarizations were induced in 22% of 0022-2828/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.yjmcc.2006.09.014

the myocytes from SR patients, but none were detected in the cAF group. Furthermore, during co-application of 5-HT and β-adrenoceptor agonist isoproterenol arrhythmic activity was observed in 94% of the myocytes in SR but in only 27% of the cells in cAF. Consistent with previous findings [2,4] arrhythmic depolarizations were inducible only in myocytes from patients treated with β-adrenoceptor antagonists. Finally, blockade of L-type Ca2+-channels with nifedipine prevented the 5-HT- and isoproterenol-induced arrhythmic activity suggesting that Ca2+influx is a key factor in atrial arrhythmogeneity. Collectively, these data point to a key role of 5-HT in initiating AF by promoting ectopic activity. However, after arrhythmia-associated atrial remodeling has developed, the proarrhythmic effect of 5-HT is attenuated. Does this study provide new insights for better understanding the role of remodeling during cAF? Why is the chronically fibrillating atrium protected against the arrhythmogenic influences of 5-HT and isoproterenol? Atrial 5-HT4-receptors are expressed at a lower level and activate the cAMP/PKA cascade to a lesser extent than βadrenoceptors [2]. Nevertheless, 5-HT increases human atrial ICa,L to a similar extent as isoproterenol [5], suggesting greater efficacy of the signal effector coupling. The paper of Pau et al. [3] clearly shows that 5-HT signaling is defective in cAF. Unfortunately, the authors do not address the mechanism(s) of impaired 5-HT response. It remains unclear, whether protein abundance of L-type Ca2+-channel subunits [6] and other relevant effectors differ between the patient groups. For instance, a chronic increase in atrial 5-HT levels during cAF could down-regulate 5-HT4-receptors or key intracellular effectors. This notion is supported by a 35% reduction in tissue mRNA levels of 5-HT4-receptors in cAF patients [7]. However, in the absence of receptor data at the protein level, it remains unclear whether modified 5-HT4-receptor function contributes to the impaired 5-HT response. The stimulatory effects of 5-HT on ICa,L are presumably mediated via PKA-dependent channel subunit phosphorylation leading to enhanced channel availability for opening [8]. Therefore, in cAF the reduced potency and efficacy of 5-HT with respect to ICa,L increase and action potential prolongation could also result from an impaired channel subunit phosphorylation due to increased activity either of cAMP-degrading phosphodiesterases or dephosphorylation

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promoting protein phosphatases. Indeed, total atrial activity of type-1 and type-2A protein phosphatases is higher in cAF than in SR patients [9,10] and is expected to reduce the availability of single ICa,L channels for opening. Thus, the higher activity of protein phosphatases in cAF does not only contribute to the lower basal ICa,L [6,9] but obviously restrains 5-HT to maximally activate ICa,L. The interesting results reported by Pau et al. raise several important issues. Effective suppression of arrhythmia with nifedipine indicates a crucial role of L-type Ca2+-channels in atrial arrhythmogenesis during cAF. This finding is somewhat surprising because stimulation of ICa,L with the channel activator BayK8644 did not produce arrhythmias neither in multicellular atrial preparations nor in atrial myocytes from both patient groups [11,12]. Despite reduced ICa,L amplitude during individual action potentials, fibrillating atria are nevertheless challenged with substantial Ca2+-load due to the high activation frequency (less influx of Ca2+ per beat but more beats per time unit). Notwithstanding Ca2+-channel blockers are not effective in cAF patients, questioning an arrhythmogenic role of ICa,L in cAF. Therefore 5-HT and isoproterenol may produce arrhythmias via mechanisms not related to Ca2+-influx. For instance, abnormal inositol-1,4,5trisphosphate receptor-mediated Ca2+ releases are arrhythmogenic in the atrium [13]. However, it is not proven whether activation of 5-HT4-receptors promotes the generation of inositol-1,4,5-trisphosphate in human atria. Since PKAmediated phosphorylation of sarcoplasmic reticulum proteins (ryanodine receptor channels, RYRs; phospholamban, etc.) and myofilament related proteins (troponin-I and myosin binding protein-C) contributes to the positive inotropic and arrhythmogenic effects of β-adrenoceptor activation, the same may hold true for 5-HT. Mice overexpressing PKA catalytic subunit exhibit hyperphosphorylation of phospholamban and RYRs and develop AF [14]. Thus, the loss of proarrhythmic potential of 5-HT and isoproterenol may result from an increased phosphorylation state of their PKA substrates. For instance, the hyperphosphorylation of RYRs and phospholamban in patients with cAF [10,15] may limit the ability of 5-HT and isoproterenol to further phosphorylate these proteins to the level required for their arrhythmogenic potential. Abnormal Ca2+ dissociations from cardiac myofilaments may promote arrhythmogenic Ca2+ release from the sarcoplasmic reticulum. Hence, the reduced phosphorylation state of myosin binding protein-C [10] and the hyperphosphorylation of troponin-T [16] in cAF patients may reduce the Ca2+ dissociations from the myofilaments below the threshold needed for arrhythmogenic 5-HT and isoproterenol responses. Finally, atrial myocytes lack transverse tubules and the L-type Ca2+-channels in the surface plasmalemma only partially control the activation of RyRs in the sarcoplasmic reticulum. Since myocyte hypertrophy may increase the distance between ICa,L and RYRs and subsequently the number of orphan RYRs, the activation of ICa,L with 5-HT or isoproterenol may produce less arrhythmogenic Ca2+ waves. Further mechanistic investigations in cAF patients using alternative approaches such as combined patch-clamp/

confocal microscopic imaging are needed to verify these hypotheses. In conclusion, Pau et al. [3] clearly demonstrates that the atrial remodeling does not necessarily promote arrhythmic activity but may also protect the atria against proarrhythmic stimuli. The results also emphasize that remodeling may contain both maladaptive and adaptive components. However, despite our growing knowledge about the cardiac effects of 5-HT, we still do not know how it induces atrial arrhythmias. Additional studies in patients with paroxysmal or postoperative AF are needed to elucidate the underlying mechanisms of 5-HT arrhythmogenesis. Complementary studies in relevant animal models will help to dissect adaptive from maladaptive remodeling with the hope to identify new therapeutic targets for effective cAF treatment.

Acknowledgments The author thanks Ursula Ravens for helpful comments and the German Federal Ministry of Education and Research (BMBF) through the Atrial Fibrillation Competence NETwork (project C4, grant 01Gi0204) and the Deutsche Forschungsgemeinschaft (DOB 769/1-1, 2) for research support. The author has no conflict of interest. References [1] Dobrev D. Electrical remodeling in atrial fibrillation. Herz Cardiovascular Diseases 2006;31(2):108–12. [2] Kaumann AJ, Levy FO. 5-Hydroxytryptamine receptors in the human cardiovascular system. Pharmacol Ther 2006;111:674–706. [3] Pau D, Workman AJ, Kane KA, Rankin AC. Electrophysiological and arrhythmogenic effects of 5-hydroxytryptamine on human atrial cells are reduced in atrial fibrillation. J Mol Cell Cardiol 2007;42:54–62. [4] Pau D, Workman AJ, Kane KA, Rankin AC. Electrophysiological effects of 5-hydroxytryptamine on isolated human atrial myocytes, and the influence of chronic beta-adrenoceptor blockade. Br J Pharmacol 2003;140 (8):1434–41. [5] Di Scala E, Findlay I, Rose S, Aupart M, Argibay J, Cosnay P, et al. High efficiency activation of L-type Ca2+ current by 5-HT in human atrial myocytes. Recept Channels 2004;10(5–6):159–65. [6] Pitt GS, Dun W, Boyden PA. Remodeled cardiac calcium channels. J Mol Cell Cardiol 2006 (Aug 7) [Electronic publication ahead of print]. [7] Grammer JB, Zeng X, Bosch RF, Kühlkamp V. Atrial L-type Ca2+channel, β-adrenoceptor, and 5-hydroxytryptamine type 4 receptor mRNAs in human atrial fibrillation. Basic Res Cardiol 2001; 96: 82–90. [8] Jahnel U, Nawrath H, Rupp J, Ochi R. L-type calcium channel activity in human atrial myocytes as influenced by 5-HT. Naunyn-Schmiedeberg's Arch Pharmacol 1993;348(4):396–402. [9] Christ T, Boknik P, Wöhrl S, Wettwer E, Graf EM, Bosch RF, et al. Reduced L-type Ca2+ current density in chronic human atrial fibrillation is associated with increased activity of protein phosphatases. Circulation 2004;110:2651–7. [10] El-Armouche A, Boknik P, Eschenhagen T, Carrier L, Knaut M, Ravens U, et al. Molecular determinants of altered Ca2+-handling in human chronic atrial fibrillation. Circulation 2006;114:670–80. [11] Schotten U, Ausma J, Stellbrink C, Sabatschus I, Vogel M, Frechen D, et al. Cellular mechanisms of depressed atrial contractility in patients with chronic atrial fibrillation. Circulation 2001;103:691–8. [12] Skasa M, Jungling E, Picht E, Schondube F, Luckhoff A. L-type calcium

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53 human atrial dilatation and atrial fibrillation. J Mol Cell Cardiol 2006 (Aug 8) [Electronic publication ahead of print].

Dobromir Dobrev Department of Pharmacology and Toxicology, Dresden University of Technology, Fetscherstr. 74, 01307 Dresden, Germany E-mail address: [email protected]. Tel.: +49 351 4586279; fax: +49 351 4586315. 18 September 2006