Atrial Atrial Action Potential Research Articles

A critical role of retinoic acid concentration for the induction of a fully human-like atrial action potential phenotype in hiPSC-CM

Retinoic acid (RA) induces an atrial phenotype in human induced pluripotent stem cells (hiPSCs), but expression of atrium-selective currents such as the ultrarapid (IKur) and acetylcholine-stimulated K+ current is variable and less than in the adult human atrium. We suspected methodological issues and systematically investigated the concentration dependency of RA. RA treatment increased IKur concentration dependently from 1.1± 0.54pA/pF (0 RA) to 3.8± 1.1, 5.8± 2.5, and 12.2± 4.3 at 0.01, 0.1, and 1μM, respectively. Only 1μM RA induced enough IKur to fully reproduce human atrial action potential (AP) shape and a robust shortening of APs upon carbachol. We found that sterile filtration caused substantial loss of RA. We conclude that 1μM RA seems to be necessary and sufficient to induce a full atrial AP shape in hiPSC-CM in EHT format. RA concentrations are prone to methodological issues and may profoundly impact the success of atrial differentiation.

Regional and temporal progression of atrial remodeling in angiotensin II mediated atrial fibrillation

Atrial fibrillation (AF) is associated with electrical and structural remodeling in the atria; however, the regional and temporal progression of atrial remodeling is incompletely understood. The objective of this study was to investigate the regional and temporal progression of atrial remodeling leading to changes in AF susceptibility in angiotensin II (Ang II) mediated hypertension. Mice were infused with Ang II for 3, 10 or 21days. AF susceptibility and atrial electrophysiology were studied in vivo using intracardiac electrophysiology. Right and left atrial myocyte electrophysiology was studied using patch-clamping. Atrial fibrosis was assessed histologically. P wave duration and atrial effective refractory period increased progressively from 3 to 21days of Ang II. AF susceptibility tended to be increased at 10days of Ang II and was elevated at 21days of Ang II. Left, but not right, atrial AP upstroke velocity and Na+ current were reduced at 10 and 21days of Ang II. Left atrial action potential (AP) duration increased progressively from 3 to 21days of Ang II due to reductions in repolarizing K+ current. Right atrial AP prolongation was increased only after 21days of Ang II. Left and right atrial fibrosis developed progressively from 3 to 21days, but increases were larger in the left atrium. In conclusion, Ang II mediated atrial electrical and structural remodeling develop earlier and more extensively in the left atrium compared to the right atrium, providing insight into how atrial remodeling leads to enhanced AF susceptibility in Ang II mediated hypertension.

Abstract P1084: Mature Ipsc-derived Atrial Cardiomyocytes Show That Saturated Fatty Acids Differentially Modulate Atrial Remodeling In Obesity-induced Atrial Fibrillation Compared To Mono And Poly-unsaturated Fatty Acids.

Background: Increased free fatty acids (FFA) and FA infiltration, both major hallmarks of obesity can individually modulate changes leading to AF. For example, mass spectrometry on obese mouse heart tissues have shown that saturated FAs (SFA), monounsaturated FAs (MUFA), and polyunsaturated FAs (PUFA) are all significantly increased compared to controls. The highest increase was shown to be in linoleic acid (LA), a PUFA followed by palmitic acid (PA, SFA) and oleic acid (OA, MUFA). Although reports suggest that some FAs induce ventricular arrhythmias in mice, the differential electrophysiological effects of SFAs, MUFAs, and PUFAs on the atria remain unclear. Objective: To determine the individual roles of SFAs, MUFAs, and PUFAs in mediating atrial remodeling in obesity-induced AF using mature human induced pluripotent stem-derived atrial cardiomyocytes (iPSC-aCMs). Methods: We compared iPSC-aCMs and HL-1 cells treated with bovine serum albumin (BSA) with palmitic acid (PA), oleic acid (OA) and LA cells (600 uM each) for 24 hours and 1 week. Ion channel function were assessed using optical voltage mapping and whole-cell patch clamping, and we assessed atrial remodeling using qPCR, and western blotting. Results: iPSC-aCMs treated with PA, OA, and LA displayed higher expression of FABP3 and CPT1A suggesting increased FA metabolism (Figure A-C). In HL1 cells, protein expression of Cpt1a was increased in all groups after FA treatment (Figure D). PA differentially modulated expression of Kv7.1 and NCX1 in HL1 cells (Figure E-F) and ion channel mRNA expression in iPSC-aCMs (Figure G-L). After 1 week PA and LA treatment b-adrenergic signaling was significantly increased in iPSC-aCMs while OA decreased it compared to controls (Figure M-O). Lastly, 24-hour PA, OA, LA treatment differentially modulated the atrial action potential (AP). While PA and LA significantly shorten the action potential, there was no significant changes to the AP after OA treatment. Conclusions: Using mature iPSC-aCMs, we showed for the first time that SFA, MUFA, and PUFA not only increased FA metabolism but differentially affected b-adrenergic signaling and ion channel remodeling leading to differential effects on the atrial AP. Our findings may have important implications for the management of obesity-induced AF in patients.

Sex differences in atrial electrical properties

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Swiss Heart Foundation Background Pronounced sex differences are known in the incidence and recurrence rates of atrial fibrillation (AF). In general, men carry a higher risk for AF - particularly at younger age - during which women are protected. Purpose In this project, we aimed to investigate electrophysiological properties of the atria that may underlie sex differences in AF development in the younger population. Methods First, we assessed sex differences in P-wave morphology (duration: PWD; area: PWA, amplitude) in 12-lead ECG in healthy volunteers (men 31.3±1.6 y, n=10; women 29.8±1.6 y, n=10) and wildtype (WT) rabbits (3-6 months, males n=12, females n=15). We investigated sex differences in atrial action potential (AP) in WT rabbits of both sexes using sharp electrode recording technique (males n=5, females n=6). To elucidate underlying mechanisms, sex differences on atrial inwardly rectifying potassium current IK1 at 37°C were evaluated using whole-cell patch-clamp technique in isolated atrial rabbit cardiomyocytes; to evaluate the long-term effect of sex hormones on IK1, we incubated the isolated cardiomyocytes for 24h with 3 or 10µM of 17β-estradiol (EST) or of dihydrotestosterone (DHT). Results Analyzing ECG parameters (figure 1), we observed a prolonged PWD and an increased PWA both in healthy men (p<0.0001) and in male rabbits (p<0.0001) as compared to healthy women or female rabbits, which may theoretically be due to a slower conduction velocity or longer AP duration in males. Intracellular AP recordings (figure 2A) showed a more negative atrial resting membrane potential (RMP, males -83.07±0.3mV vs. females -74.33±2.6mV, p<0.05) and a prolonged atrial AP duration (APD10, 18.20±3.6ms vs. 10.06±1.9ms, p=0.06; APD50, 36.60±6.6ms vs. 20.06±1.9ms, p<0.05; APD90, 91.00±10.3ms vs. 74.00±4.0ms, p=0.13) in males as compared to females. As highlighted in figure 2B, male atrial cardiomoycytes had smaller IK1 current densities, which could be due to high DHT concentration; in line with these findings (figure 2C) 24h DHT incubation of atrial cardiomyocytes from females reduced IK1, mimicking the sex differences observed. Conclusions Sex impacts on atrial electrophysiology, resulting in sex differences in P-wave morphology, AP parameters and ionic current densities. Delayed (early) repolarization in males may be causatively linked to the observed changes in P-wave morphology. The reduction of IK1 observed in males may be due to testosterone, since this reduction was observed even in female cardiomyocytes when incubated with DHT.

Immediate and Delayed Response of Simulated Human Atrial Myocytes to Clinically-Relevant Hypokalemia.

Although plasma electrolyte levels are quickly and precisely regulated in the mammalian cardiovascular system, even small transient changes in K+, Na+, Ca2+, and/or Mg2+ can significantly alter physiological responses in the heart, blood vessels, and intrinsic (intracardiac) autonomic nervous system. We have used mathematical models of the human atrial action potential (AP) to explore the electrophysiological mechanisms that underlie changes in resting potential (Vr) and the AP following decreases in plasma K+, [K+]o, that were selected to mimic clinical hypokalemia. Such changes may be associated with arrhythmias and are commonly encountered in patients (i) in therapy for hypertension and heart failure; (ii) undergoing renal dialysis; (iii) with any disease with acid-base imbalance; or (iv) post-operatively. Our study emphasizes clinically-relevant hypokalemic conditions, corresponding to [K+]o reductions of approximately 1.5 mM from the normal value of 4 to 4.5 mM. We show how the resulting electrophysiological responses in human atrial myocytes progress within two distinct time frames:(i) Immediately after [K+]o is reduced, the K+-sensing mechanism of the background inward rectifier current (IK1) responds. Specifically, its highly non-linear current-voltage relationship changes significantly as judged by the voltage dependence of its region of outward current. This rapidly alters, and sometimes even depolarizes, Vr and can also markedly prolong the final repolarization phase of the AP, thus modulating excitability and refractoriness.(ii) A second much slower electrophysiological response (developing 5–10 minutes after [K+]o is reduced) results from alterations in the intracellular electrolyte balance. A progressive shift in intracellular [Na+]i causes a change in the outward electrogenic current generated by the Na+/K+ pump, thereby modifying Vr and AP repolarization and changing the human atrial electrophysiological substrate.In this study, these two effects were investigated quantitatively, using seven published models of the human atrial AP. This highlighted the important role of IK1 rectification when analyzing both the mechanisms by which [K+]o regulates Vr and how the AP waveform may contribute to “trigger” mechanisms within the proarrhythmic substrate. Our simulations complement and extend previous studies aimed at understanding key factors by which decreases in [K+]o can produce effects that are known to promote atrial arrhythmias in human hearts.

Heart failure-induced atrial remodelling promotes electrical and conduction alternans.

Heart failure (HF) is associated with an increased propensity for atrial fibrillation (AF), causing higher mortality than AF or HF alone. It is hypothesized that HF-induced remodelling of atrial cellular and tissue properties promotes the genesis of atrial action potential (AP) alternans and conduction alternans that perpetuate AF. However, the mechanism underlying the increased susceptibility to atrial alternans in HF remains incompletely elucidated. In this study, we investigated the effects of how HF-induced atrial cellular electrophysiological (with prolonged AP duration) and tissue structural (reduced cell-to-cell coupling caused by atrial fibrosis) remodelling can have an effect on the generation of atrial AP alternans and their conduction at the cellular and one-dimensional (1D) tissue levels. Simulation results showed that HF-induced atrial electrical remodelling prolonged AP duration, which was accompanied by an increased sarcoplasmic reticulum (SR) Ca2+ content and Ca2+ transient amplitude. Further analysis demonstrated that HF-induced atrial electrical remodelling increased susceptibility to atrial alternans mainly due to the increased sarcoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ reuptake, modulated by increased phospholamban (PLB) phosphorylation, and the decreased transient outward K+ current (Ito). The underlying mechanism has been suggested that the increased SR Ca2+ content and prolonged AP did not fully recover to their previous levels at the end of diastole, resulting in a smaller SR Ca2+ release and AP in the next beat. These produced Ca2+ transient alternans and AP alternans, and further caused AP alternans and Ca2+ transient alternans through Ca2+→AP coupling and AP→Ca2+ coupling, respectively. Simulation of a 1D tissue model showed that the combined action of HF-induced ion channel remodelling and a decrease in cell-to-cell coupling due to fibrosis increased the heart tissue’s susceptibility to the formation of spatially discordant alternans, resulting in an increased functional AP propagation dispersion, which is pro-arrhythmic. These findings provide insights into how HF promotes atrial arrhythmia in association with atrial alternans.

Inverse remodelling of K2P3.1 K+ channel expression and action potential duration in left ventricular dysfunction and atrial fibrillation: implications for patient-specific antiarrhythmic drug therapy.

Atrial fibrillation (AF) prevalence increases with advanced stages of left ventricular (LV) dysfunction. Remote proarrhythmic effects of ventricular dysfunction on atrial electrophysiology remain incompletely understood. We hypothesized that repolarizing K2P3.1 K+ channels, previously implicated in AF pathophysiology, may contribute to shaping the atrial action potential (AP), forming a specific electrical substrate with LV dysfunction that might represent a target for personalized antiarrhythmic therapy. A total of 175 patients exhibiting different stages of LV dysfunction were included. Ion channel expression was quantified by real-time polymerase chain reaction and Western blot. Membrane currents and APs were recorded from atrial cardiomyocytes using the patch-clamp technique. Severely reduced LV function was associated with decreased atrial K2P3.1 expression in sinus rhythm patients. In contrast, chronic (c)AF resulted in increased K2P3.1 levels, but paroxysmal (p)AF was not linked to significant K2P3.1 remodelling. LV dysfunction-related suppression of K2P3.1 currents prolonged atrial AP duration (APD) compared with patients with preserved LV function. In individuals with concomitant LV dysfunction and cAF, APD was determined by LV dysfunction-associated prolongation and by cAF-dependent shortening, respectively, consistent with changes in K2P3.1 abundance. K2P3.1 inhibition attenuated APD shortening in cAF patients irrespective of LV function, whereas in pAF subjects with severely reduced LV function, K2P3.1 blockade resulted in disproportionately high APD prolongation. LV dysfunction is associated with reduction of atrial K2P3.1 channel expression, while cAF leads to increased K2P3.1 abundance. Differential remodelling of K2P3.1 and APD provides a basis for patient-tailored antiarrhythmic strategies.

Seasonal changes of cholinergic response in the atrium of Arctic navaga cod (Eleginus navaga).

Fishes of north-temperate latitudes exhibit marked seasonal changes in electrical excitability of the heart partly as an outcome of temperature-dependent changes in the density of major K+ ion currents: delayed rectifiers (IKr, IKs) and background inward rectifier (IK1). In the arctic teleost, navaga cod (Eleginus navaga), IKr and IK1 are strongly up-regulated in winter. The current study tests the hypothesis that the ligand-gated K+ current, the acetylcholine-activated inward rectifier, IKACh, is also modified by seasonal acclimatization in atrial myocytes of navaga. In sinoatrial preparations of the summer-acclimatized (SA) navaga, 10-6 M carbamylcholine chloride (CCh) caused slowing of heart rate, shortening of atrial action potential (AP) duration and a drastic reduction of AP amplitude, eventually resulting in inexcitability. In winter-acclimatized (WA) atria CCh slowed HR and reduced AP duration, but reduction of AP amplitude was modest and never resulted in inexcitability. The difference in cholinergic response between SA and WA navaga is explained by seasonal changes in IKACh density. The peak density of IKACh, induced by 10-5 M CCh, at the common experimental temperature (+6°C) was 0.97±0.28 pA/pF in SA navaga but only 0.183±0.013 pA/pF in WA navaga (a 5.3-fold difference, P<0.05). At acclimatization temperatures of the fish IKACh density was 2.8±0.50 (at +12°C) and 0.11±0.06 pA/pF (at +3°C) (a 26-fold difference, P<0.05) for SA and WA navaga, respectively. Thus, acclimatization to summer induces a drastic up-regulation of the atrial IKACh, which effectively shortens atrial AP. The reverse temperature compensation of the atrial IKACh may be advantageous in summer under variable water temperatures and oxygen concentrations by reducing workload of the heart.

Human atrial cell models to analyse haemodialysis-related effects on cardiac electrophysiology: work in progress.

During haemodialysis (HD) sessions, patients undergo alterations in the extracellular environment, mostly concerning plasma electrolyte concentrations, pH, and volume, together with a modification of sympathovagal balance. All these changes affect cardiac electrophysiology, possibly leading to an increased arrhythmic risk. Computational modeling may help to investigate the impact of HD-related changes on atrial electrophysiology. However, many different human atrial action potential (AP) models are currently available, all validated only with the standard electrolyte concentrations used in experiments. Therefore, they may respond in different ways to the same environmental changes. After an overview on how the computational approach has been used in the past to investigate the effect of HD therapy on cardiac electrophysiology, the aim of this work has been to assess the current state of the art in human atrial AP models, with respect to the HD context. All the published human atrial AP models have been considered and tested for electrolytes, volume changes, and different acetylcholine concentrations. Most of them proved to be reliable for single modifications, but all of them showed some drawbacks. Therefore, there is room for a new human atrial AP model, hopefully able to physiologically reproduce all the HD-related effects. At the moment, work is still in progress in this specific field.

A KCNQ1 Mutation Causes a High Penetrance for Familial Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and its incidence is expected to grow. A genetic predisposition for AF has long been recognized, but its manifestation in these patients likely involves a combination of rare and common genetic variants. Identifying genetic variants that associate with a high penetrance for AF would represent a significant breakthrough for understanding the mechanisms that associate with disease. Candidate gene sequencing in 5 unrelated families with familial AF identified the KCNQ1 missense mutation p.Arg231His (R231H). In addition to AF, several of the family members have abnormal QTc intervals, syncope or experienced sudden cardiac arrest or death. KCNQ1 encodes the voltage-gated K(+) channel that conducts the slowly activating delayed rectifier K(+) current in the heart. Functional and computational analyses suggested that R231H increases KCNQ1 current (I(KCNQ1)) to shorten the atrial action potential (AP) duration. R231H is predicted to minimally affect ventricular excitability, but it prevented the increase in I(KCNQ1) following PKA activation. The unique properties of R231H appeared to be caused by a loss in voltage-dependent gating. The R231H variant causes a high penetrance for interfamilial early-onset AF. Our study indicates R231H likely shortens atrial refractoriness to promote a substrate for reentry. Additionally, R231H might cause abnormal ventricular repolarization by disrupting PKA activation of IKCNQ1 . We conclude genetic variants, which increase IKs during the atrial AP, decrease the atrial AP duration, and/or shorten atrial refractoriness, present a high risk for interfamilial AF.

Abstract 18901: Direct Evidence that a KCNQ1 Mutation is Linked to Familial Early-onset Atrial Fibrillation

KCNQ1 encodes the voltage-gated α-subunit of the I Ks channel complex (Kv7.1), and KCNQ1 mutations are associated with multiple arrhythmia syndromes. Over the past 12 years, hundreds of different KCNQ1 mutations have been linked to congenital long QT syndrome but only three KCNQ1 variants, each identified in only one family, are linked to familial early-onset AF (&lt;50 years of age). The scant genetic linkage between KCNQ1 and early-onset AF raises the question as to whether these KCNQ1 variants actually cause familial early-onset AF or simply unmask a latent genetic predisposition. We now report that a single KCNQ1 variant, pArg231His or R231H, is identified in five unrelated families with a high penetrance of familial early-onset AF. Whole-cell patch clamp of cells heterologously expressing the obligatory I Ks β-subunit KCNE1 with wild-type KCNQ1 (WT) or WT and R231H (to mimic the patients’ genotypes) showed that cells co-expressing R231H increased current at negative and resting membrane potentials. Additionally, currents recorded from cells using a human atrial action potential (AP) waveform at physiological temperature showed that WT is minimally activated during the atrial AP and co-expression of R231H increases current measured during the atrial AP waveform by greater than 20-fold (WT, n = 10; WT &amp; R231H, n = 12, p&lt;0.0001). Simulations using a computational model of a human atrial AP also predicted that I Ks is normally minimally activated in the atria and R231H would dramatically increase I Ks and shorten the atrial AP. We conclude that KCNQ1 mutations are capable of causing familial early-onset AF by increasing I Ks during the atrial AP and shortening atrial refractoriness.

Human Atrial Action Potential and Ca 2+ Model

Rationale: Understanding atrial fibrillation (AF) requires integrated understanding of ionic currents and Ca 2+ transport in remodeled human atrium, but appropriate models are limited. Objective: To study AF, we developed a new human atrial action potential (AP) model, derived from atrial experimental results and our human ventricular myocyte model. Methods and Results: Atria versus ventricles have lower I K1 , resulting in more depolarized resting membrane potential (≈7 mV). We used higher I to,fast density in atrium, removed I to,slow , and included an atrial-specific I Kur . I NCX and I NaK densities were reduced in atrial versus ventricular myocytes according to experimental results. SERCA function was altered to reproduce human atrial myocyte Ca 2+ transients. To simulate chronic AF, we reduced I CaL , I to , I Kur and SERCA, and increased I K1 ,I Ks and I NCX . We also investigated the link between Kv1.5 channelopathy, [Ca 2+ ] i , and AF. The sinus rhythm model showed a typical human atrial AP morphology. Consistent with experiments, the model showed shorter APs and reduced AP duration shortening at increasing pacing frequencies in AF or when I CaL was partially blocked, suggesting a crucial role of Ca 2+ and Na + in this effect. This also explained blunted Ca 2+ transient and rate-adaptation of [Ca 2+ ] i and [Na + ] i in chronic AF. Moreover, increasing [Na + ] i and altered I NaK and I NCX causes rate-dependent atrial AP shortening. Blocking I Kur to mimic Kv1.5 loss-of-function increased [Ca 2+ ] i and caused early afterdepolarizations under adrenergic stress, as observed experimentally. Conclusions: Our study provides a novel tool and insights into ionic bases of atrioventricular AP differences, and shows how Na + and Ca 2+ homeostases critically mediate abnormal repolarization in AF.

Alterations of atrial electrophysiology induced by electrolyte variations: combined computational and P-wave analysis

Haemodialysis (HD) therapy represents a unique model to test in vivo, in humans, the effects of changes in plasma ionic concentrations. Episodes of paroxysmal atrial fibrillation (AF) often occur during the treatment. We investigated the effects of HD-induced electrolyte variations on atrial electrophysiology by analysing ECG P-wave duration (PWd), which reflects atrial conduction velocity (CV), and simulated atrial action potential (AP). In 20 end-stage renal disease patients PWd (signal-averaged ECG), heart rate (HR), blood pressure, Na(+), K(+), Ca(2+), and Mg(2+) plasma concentrations were measured before and after HD session. The Courtemanche computational model of human atrial myocyte was used to simulate the atrial AP. AP upstroke duration (AP(ud)), AP duration and atrial cell effective refractory period (ERP) were computed. Extracellular electrolyte concentrations and HR were imposed to the average values measured in vivo. HD decreased K(+) (from 4.9 +/- 0.5 to 3.9 +/- 0.4 mmol/L, P < 0.001) and Mg(2+) (0.92 +/- 0.08 to 0.86 +/- 0.05 mmol/L, P < 0.05), and increased Na(+) (139.8 +/- 3.4 to 141.6 +/- 3.1 mmol/L, P < 0.05) and Ca(2+) (1.18 +/- 0.09 to 1.30 +/- 0.07 mmol/L, P < 0.001) plasma concentrations. PWd systematically increased in all the patients after HD (131 +/- 11 to 140 +/- 12 ms, P < 0.001), indicating an intra-atrial conduction slowing. PWd increments were inversely correlated with K(+) variations (R = 0.73, P < 0.01). Model-based analysis indicated an AP(ud) increase (from 2.58 to 2.94 ms) after HD, coherent with experimental observations on PWd, and a reduction of ERP by 12 ms. Changes of plasma ionic concentrations may lead to modifications of atrial electrophysiology that can favour AF onset, namely a decrease of atrial CV and a decrease of atrial ERP.

An outwardly rectifying anionic background current in atrial myocytes from the human heart

This report describes a hitherto unreported anionic background current from human atrial cardiomyocytes. Under whole-cell patch-clamp with anion-selective conditions, an outwardly rectifying anion current (IANION) was observed, which was larger with iodide than nitrate, and with nitrate than chloride as charge carrier. In contrast with a previously identified background anionic current from small mammal cardiomyocytes, IANION was not augmented by the pyrethroid tefluthrin (10μM); neither was it inhibited by hyperosmolar external solution nor by DIDS (200μM); thus IANION was not due to basal activity of volume-sensitive anion channels. IANION was partially inhibited by the Cl− channel blockers NPPB (50μM) and Gly H-101 (30μM). Incorporation of IANION into a human atrial action potential (AP) simulation led to depression of the AP plateau, accompanied by alterations to plateau inward calcium current, and to AP shortening at 50% but not 90% of complete repolarization, demonstrating that IANION can influence the human atrial AP profile.

Differential positive inotropic effects of the I Kr-blocker dofetilide and the I TO/I Kur-blocker AVE0118 in canine atrial myocytes

Background: Patients undergoing cardioversion of atrial tachyarrhythmias might benefit from positive inotropic stimulation of the atria. The shape of the atrial action potential (AP) is a major determinant of atrial contractility. Newly developed Ito/IKur-blockers prolong early repolarisation and increase the ’plateau potential’ of the atrial AP while IKr-blockers (e.g. dofetilide) prolong final repolarisation without affecting the plateau phase. We compared the inotropic effects of dofetilide and the Ito/IKur-blocker AVE0118 on isolated atrial myocytes. Methods: Shortening transients (edge detection) and transmembrane APs of isolated canine atrial myocytes were recorded under current clamp conditions using the perforated patch technique (37°C). The ’plateau potential’ was defined as the average potential between 10 and 50ms after the upstroke of the AP. The effect of changes of: 1) plateau duration, 2) plateau potential, and 3) the time required for final repolarisation (prolongation of APD90 without change in APD30) on cellular shortening of atrial myocytes was studied in voltage-clamp experiments. Results: Dofetilide (1 M) prolonged APD90 from 318 25ms to 435 35ms but did not significantly change the plateau potential or the shortening of atrial myocytes (7 cells, 4 dogs). In contrast, AVE0118 increased the plateau potential from -5 6mV to 21 8mV and enhanced myocyte shortening from 4.0 1.0% to 9.8 1.3% (9 cells, 5 dogs). APD90 was unchanged. In voltage clamp experiments, prolongation of the plateau increased myocyte shortening from 1.9 0.4% at 50ms to 16.6 1.2% at 600ms. Increasing the plateau potential enhanced shortening from 0.7 0.1% at -20mV to 17.8 1.2% at 60mV. In contrast, prolongation of APD90 by 200ms without changes of plateau potential or duration did not enhance shortening (9 cells, 5 dogs). Conclusion: AVE0118 enhances atrial contractility by increasing the plateau potential due to prolongation of the early repolarisation. Dofetilide does not affect the plateau phase of the AP and therefore does not enhance atrial contractility.

Mathematical analysis of canine atrial action potentials: rate, regional factors, and electrical remodeling.

Dogs have been used extensively to study atrial arrhythmias, but there are no published mathematical models of the canine atrial action potential (AP). To obtain insights into the ionic mechanisms governing canine atrial AP properties, we incorporated formulations of K(+), Na(+), Ca(2+), and Cl(-) currents, based on measurements in canine atrial myocytes, into a mathematical model of the AP. The rate-dependent behavior of model APs corresponded to experimental measurements and pointed to a central role for L-type Ca(2+) current inactivation in rate adaptation. Incorporating previously described regional ionic current variations into the model largely reproduced AP forms characteristic of the corresponding right atrial regions (appendage, pectinate muscle, crista terminalis, and atrioventricular ring). When ionic alterations induced by tachycardia-dependent remodeling were incorporated, the model reproduced qualitatively the AP features constituting the cellular substrate for atrial fibrillation. We conclude that this ionic model of the canine atrial AP agrees well with experimental measurements and gives potential insights into mechanisms underlying functionally important electrophysiological phenomena in canine atrium.

Ionic targets for drug therapy and atrial fibrillation-induced electrical remodeling: insights from a mathematical model.

Recent advances in molecular electrophysiology have made possible the development of more selective ion channel blockers for therapeutic use. However, more information is needed about the effects of blocking specific channels on repolarization in normal human atrium and in atrial cells of patients with atrial fibrillation (AF). AF-induced electrical remodeling is associated with reductions in transient outward current (Ito), ultrarapid delayed rectifier current (IKur), and L-type calcium current (ICa,L). Direct evaluation of the results of ion channel depression is limited by the nonspecificity of the available pharmacological probes. Using a mathematical model of the human atrial action potential (AP), we aimed to: (1) evaluate the role of ionic abnormalities in producing AP changes characteristic of AF in humans and (2) explore the effects of specific channel blockade on the normal and AF-modified AP (AFAP). We used our previously developed mathematical model of the normal human atrial AP (NAP) based on directly measured currents. We constructed a model of the AFAP by incorporating experimentally-measured reductions in Ito (50%), IKur (50%), and ICa,L (70%) current densities observed in AF. The AFAP exhibits the reductions in AP duration (APD) and rate-adaption typical of AF. The reduction in ICa,L alone can account for most of the morphological features of the AFAP. Inhibition of Ito by 90% leads to a reduction in APD measured at -60 mV in both the NAP and AFAP. Inhibition of the rapid component of the delayed rectifier (IKr) by 90% slows terminal repolarization of the NAP and AFAP and increases APD by 38% and 34%, respectively. Inhibition of IKur by 90% slows early repolarization and increases plateau height, activating additional IK and causing no net change in APD at 1 Hz in the NAP. In the presence of AF-induced ionic modifications, IKur inhibition increases APD by 12%. Combining IKur and IKr inhibition under both normal and AF conditions synergistically increases APD. In the NAP, altering the model parameters to reproduce other typical measured AP morphologies can significantly alter the response to K(+)-channel inhibition. (1) The described abnormalities in Ito, IKur and ICa,L in AF patients can account for the effects of AF on human AP properties; (2) AP prolongation by IKur block is limited by increases in plateau height that activate more IK; (3) Blockers of IKur may be more effective in prolonging APD in patients with AF; 4) Inhibition of both IKur and IKr produces supra-additive effects on APD. These observations illustrate the importance of secondary current alterations in the response of the AP to single channel blockade, and have potentially important implications for the development of improved antiarrhythmic drug therapy for AF.

Ionic mechanisms underlying human atrial action potential properties: insights from a mathematical model.

The mechanisms underlying many important properties of the human atrial action potential (AP) are poorly understood. Using specific formulations of the K+, Na+, and Ca2+ currents based on data recorded from human atrial myocytes, along with representations of pump, exchange, and background currents, we developed a mathematical model of the AP. The model AP resembles APs recorded from human atrial samples and responds to rate changes, L-type Ca2+ current blockade, Na+/Ca2+ exchanger inhibition, and variations in transient outward current amplitude in a fashion similar to experimental recordings. Rate-dependent adaptation of AP duration, an important determinant of susceptibility to atrial fibrillation, was attributable to incomplete L-type Ca2+ current recovery from inactivation and incomplete delayed rectifier current deactivation at rapid rates. Experimental observations of variable AP morphology could be accounted for by changes in transient outward current density, as suggested experimentally. We conclude that this mathematical model of the human atrial AP reproduces a variety of observed AP behaviors and provides insights into the mechanisms of clinically important AP properties.

The shape of human atrial action potential accounts for different frequency-related changes in vitro

We aimed at investigating frequency-related changes of human atrial action potential (AP) in vitro to see whether baseline AP shape might account for different responses to increasing stimulation rates. Human right atrial trabeculae ( n = 48) obtained from adult ( n = 38, mean age 59 ± 8, range 45–72 years) consecutive patients (≅30% of those operated upon by a single surgeon; 1.26 preparations per patient, range 1–2) were superfused in an organ bath with oxygenated (O 2 content 16 ml/l) and modified (NaHCO 3 25.7 mmol/l) Tyrode's solution at 31 °C. Baseline electrophysiology (pacing: 1 ms duration, 2–4 mA current intensity) at cycle length (CL) of 1000 ms was recorded in 90% (43 out of 48) of the preparations. The frequency-related protocol (CL from 1600 to 300 ms) was, however, undertaken in 23 (48%) preparations because 20 (42%) became pacing unresponsive immediately after baseline recordings. No statistical differences were seen when baseline electrophysiological parameters (mean ± SD) were grouped according to late pacing responsiveness ( n = 43 vs. n = 23): respectively, resting membrane potential (RMP) was −74 ± 6 vs. −75 ± 4 mV, maximal upstroke velocity (Vmax) 172 ± 60 vs. 173 ± 39 V/s, AP amplitude (APA) 89 ± 11 vs. 91 ± 8 mV and AP durations were at 30% (APD30%) 10 ± 13 vs. 13 ± 18 ms, 50% (APD50%) 45 ± 79 vs. 62 ± 91 ms and 90% (APD90%) 383 ± 103 vs. 407 ± 108 ms. To classify baseline AP shape, two criteria were adopted: criterion 1 (“objective”), based on APA (cut-off 90 mV) and APD90% (cut-off 500 ms) computed values and criterion 2 (“visual”) derived from the literature. These criteria enabled us to differentiate three AP shape types: type 1 (spike and dome), type 3 (no dome) and type 4 (extremely prolonged). At baseline, the two criteria diagnosed different proportions of AP shape types. There were, however, no intra-type statistical differences among electrophysiological parameters. By criterion 1, analysis of variance (ANOVA) showed significant inter-type differences of RMP, Vmax, APA, APD50 and 90% and by criterion 2 of APA, APD30, 50 and 90%, respectively. To facilitate comparisons with previous published data, criterion 2 was selected to analyse frequency-related changes of AP shape types. At low stimulation rate, ANOVA for repeated measures (with Greenhouse-Geisser \\ ̂ g3 correction) showed inter-type differences for APD30, 50 and 90% ( P = 0.00005). RMP, Vmax, APA and APD90% were overall frequency-related ( P = 0.00005). Inter-type frequency-related differences were however seen only for APD90%. Human atrial AP durations (30, 50 and 90%) enable differentiation among AP shape types (1, 3 and 4). By a standardized use-dependent protocol overall RMP, Vmax, APA and APD90% are frequency-related. AP shape accounts for frequency-related changes of APD90% only. A type 4 AP shape with much prolonged AP duration had a flat frequency dependence. At high stimulation rates, adult type 1 and 3 AP shapes are indistinguishable. Use-dependent and pharmacological investigations in human atrial myocytes need to take AP shape into account.