The G protein-gated potassium current I(K,ACh) is constitutively active in patients with chronic atrial fibrillation

Background— The molecular mechanism of increased background inward rectifier current (IK1) in atrial fibrillation (AF) is not fully understood. We tested whether constitutively active acetylcholine (ACh)-activated IK,ACh contributes to enhanced basal conductance in chronic AF (cAF). Methods and Results— Whole-cell and single-channel currents were measured with standard voltage-clamp techniques in atrial myocytes from patients with sinus rhythm (SR) and cAF. The selective IK,ACh blocker tertiapin was used for inhibition of IK,ACh. Whole-cell basal current was larger in cAF than in SR, whereas carbachol (CCh)-activated IK,ACh was lower in cAF than in SR. Tertiapin (0.1 to 100 nmol/L) reduced IK,ACh in a concentration-dependent manner with greater potency in cAF than in SR (−logIC50: 9.1 versus 8.2; P<0.05). Basal current contained a tertiapin-sensitive component that was larger in cAF than in SR (tertiapin [10 nmol/L]-sensitive current at −100 mV: cAF, −6.7±1.2 pA/pF, n=16/5 [myocytes/patients] versus SR, −1.7±0.5 pA/pF, n=24/8), suggesting contribution of constitutively active IK,ACh to basal current. In single-channel recordings, constitutively active IK,ACh was prominent in cAF but not in SR (channel open probability: cAF, 5.4±0.7%, n=19/9 versus SR, 0.1±0.05%, n=16/9; P<0.05). Moreover, IK1 channel open probability was higher in cAF than in SR (13.4±0.4%, n=19/9 versus 11.4±0.7%, n=16/9; P<0.05) without changes in other channel characteristics. Conclusions— Our results demonstrate that larger basal inward rectifier K+ current in cAF consists of increased IK1 activity and constitutively active IK,ACh. Blockade of IK,ACh may represent a new therapeutic target in AF

Functional modulation of the transient outward current Ito by KCNE beta-subunits and regional distribution in human non-failing and failing hearts

Objectives: The function of Kv4.3 (KCND3) channels, which underlie the transient outward current I,, in human heart, can be modulated by several accessory subunits such as KChIP2 and KCNE1-KCNE5. Here we aimed to determine the regional expression of Kv4.3, KChIP2, and KCNE mRNAs in non-failing and failing human hearts and to investigate the functional consequences of subunit coexpression in heterologous expression systems. Methods: We quantified mRNA levels for two Kv4.3 isoforms, Kv4.3-S and Kv4.3-L, and for KChIP2 as well as KCNE1-KCNE5 with real-time RT-PCR. We also studied the effects of KCNEs on Kv4.3 + KChIP2 current characteristics in CHO cells with the whole-cell voltage-clamp method. Results: In non-failing hearts, low expression was found for KCNE1, KCNE3, and KCNE5, three times higher expression for KCNE2, and 60 times higher for KCNE4. Transmural gradients were detected only for KChIP2 in left and right ventricles. Compared to non-failing tissue, failing hearts showed higher expression of Kv4.3-L and KCNE1 and lower of Kv4.3-S, KChIP2, KCNE4, and KCNE5. In CHO cells, Kv4.3 + KChIP2 currents were differentially modified by co-expressed KCNEs: time constants of inactivation were shorter with KCNE1 and KCNE3-5 while time-to-peak was decreased, and V-0.5 of steady-state inactivation was shifted to more negative potentials by all KCNE subunits. Importantly, KCNE2 induced a unique and prominent 'overshoot' of peak current during recovery from inactivation similar to that described for human I-to while other KCNE subunits induced little (KCNE4,5) or no overshoot. Conclusions: All KCNEs are expressed in the human heart at the transcript level. Compared to It. in native human myocytes, none of the combination of KChIP2 and KCNE produced an ideal congruency in current characteristics, suggesting that additional factors contribute to the regulation of the native I-to channel

Electrophysiological and arrhythmogenic effects of 5-hydroxytryptamine on human atrial cells are reduced in atrial fibrillation

5-Hydroxytryptamine (5-HT) is proarrhythmic in atrial cells from patients in sinus rhythm (SR) via activation of 5-HT&lt;sub&gt;4&lt;/sub&gt; receptors, but its effects in atrial cells from patients with atrial fibrillation (AF) are unknown. The whole-cell perforated patch-clamp technique was used to record L-type Ca&lt;sup&gt;2+&lt;/sup&gt; current (&lt;i&gt;I&lt;/i&gt;&lt;sub&gt;CaL&lt;/sub&gt;), action potential duration (APD) and arrhythmic activity at 37 °C in enzymatically isolated atrial cells obtained from patients undergoing cardiac surgery, in SR or with chronic AF. In the AF group, 5-HT (10 μM) produced an increase in &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;CaL&lt;/sub&gt; of 115 ± 21% above control (&lt;i&gt;n&lt;/i&gt; = 10 cells, 6 patients) that was significantly smaller than that in the SR group (232 ± 33%; &lt;i&gt;p&lt;/i&gt; 0.05; &lt;i&gt;n&lt;/i&gt; = 27 cells, 12 patients). Subsequent co-application of isoproterenol (1 μM) caused a further increase in &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;CaL&lt;/sub&gt; in the AF group (by 256 ± 94%) that was greater than that in the SR group (22 ± 6%; p &#60; 0.05). The APD at 50% repolarisation (APD&lt;sub&gt;50&lt;/sub&gt;) was prolonged by 14 ± 3 ms by 5-HT in the AF group (&lt;i&gt;n&lt;/i&gt; = 37 cells, 14 patients). This was less than that in the SR group (27 ± 4 ms; &lt;i&gt;p&lt;/i&gt; &#60; 0.05; &lt;i&gt;n&lt;/i&gt; = 58 cells, 24 patients). Arrhythmic activity in response to 5-HT was observed in 22% of cells in the SR group, but none was observed in the AF group (p &#60; 0.05). Atrial fibrillation was associated with reduced effects of 5-HT, but not of isoproterenol, on &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;CaL&lt;/sub&gt; in human atrial cells. This reduced effect on &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;CaL&lt;/sub&gt; was associated with a reduced APD&lt;sub&gt;50&lt;/sub&gt; and arrhythmic activity with 5-HT. Thus, the potentially arrhythmogenic influence of 5-HT may be suppressed in AF-remodelled human atrium

Synergistic anti-arrhythmic effects in human atria with combined use of sodium blockers and acacetin

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Developing effective and safe anti-AF drugs remains an unmet challenge. Simultaneous block of both atrial-specific ultra-rapid delayed rectifier potassium (K⁺) current (I Kur ) and the Na⁺ current (I Na ) has been hypothesized to be anti-AF, without inducing significant QT prolongation and ventricular side effects. However, the antiarrhythmic advantage of simultaneously blocking these two channels vs. individual block in the setting of AF-induced electrical remodeling remains to be documented. Furthermore, many I Kur blockers such as acacetin and AVE0118, partially inhibit other K⁺ currents in the atria. Whether this multi-K⁺ -block produces greater anti-AF effects compared with selective I Kur -block has not been fully understood. The aim of this study was to use computer models to (i) assess the impact of multi-K⁺-block as exhibited by many I Kur blokers, and (ii) evaluate the antiarrhythmic effect of blocking I Kur and I Na , either alone or in combination, on atrial and ventricular electrical excitation and recovery in the setting of AF-induced electrical-remodeling. Contemporary mathematical models of human atrial and ventricular cells were modified to incorporate dose-dependent actions of acacetin (a multichannel blocker primarily inhibiting I Kur while less potently blocking Ito, I Kr , and I Ks ). Rate- and atrial-selective inhibition of I Na was also incorporated into the models. These single myocyte models were then incorporated into multicellular two-dimensional (2D) and three-dimensional (3D) anatomical models of the human atria. As expected, application of I Kur blocker produced pronounced action potential duration (APD) prolongation in atrial myocytes. Furthermore, combined multiple K⁺-channel block that mimicked the effects of acacetin exhibited synergistic APD prolongations. Synergistically anti-AF effects following inhibition of I Na and combined I Kur /K⁺-channels were also observed. The attainable maximal AF-selectivity of I Na inhibition was greatly augmented by blocking I Kur or multiple K⁺-currents in the atrial myocytes. This enhanced anti-arrhythmic effects of combined block of Na⁺- and K⁺-channels were also seen in 2D and 3D simulations; specially, there was an enhanced efficacy in terminating re-entrant excitation waves, exerting improved antiarrhythmic effects in the human atria as compared to a single-channel block. However, in the human ventricular myocytes and tissue, cellular repolarization and computed QT intervals were modestly affected in the presence of actions of acacetin and I Na blockers (either alone or in combination). In conclusion, this study demonstrates synergistic antiarrhythmic benefits of combined block of I Kur and I Na , as well as those of I Na and combined multi K⁺-current block of acacetin, without significant alterations of ventricular repolarization and QT intervals. This approach may be a valuable strategy for the treatment of AF