Background— The molecular mechanism of increased background inward rectifier current ( I K1 ) in atrial fibrillation (AF) is not fully understood. We tested whether constitutively active acetylcholine (ACh)-activated I K,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 I K,ACh blocker tertiapin was used for inhibition of I K,ACh . Whole-cell basal current was larger in cAF than in SR, whereas carbachol (CCh)-activated I K,ACh was lower in cAF than in SR. Tertiapin (0.1 to 100 nmol/L) reduced I K,ACh in a concentration-dependent manner with greater potency in cAF than in SR (−logIC 50 : 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 I K,ACh to basal current. In single-channel recordings, constitutively active I K,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, I K1 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 I K1 activity and constitutively active I K,ACh . Blockade of I K,ACh may represent a new therapeutic target in AF.

Background-Although pharmacological block of the slow, delayed rectifier potassium current (I Ks ) by chromanol 293B, L-735,821, or HMR-1556 produces little effect on action potential duration (APD) in isolated rabbit and dog ventricular myocytes, the effect of I Ks block on normal human ventricular muscle APD is not known. Therefore, studies were conducted to elucidate the role of I Ks in normal human ventricular muscle and in preparations in which both repolarization reserve was attenuated and sympathetic activation was increased by exogenous dofetilide and adrenaline. Methods and Results-Preparations were obtained from undiseased organ donors. Action potentials were measured in ventricular trabeculae and papillary muscles using conventional microelectrode techniques; membrane currents were measured in ventricular myocytes using voltage-clamp techniques. Chromanol 293B (10 mol/L), L-735,821 (100 nmol/L), and HMR-1556 (100 nmol/L and 1 mol/L) produced a Ͻ12-ms change in APD while pacing at cycle lengths ranging from 300 to 5000 ms, whereas the I Kr blockers sotalol and E-4031 markedly lengthened APD.

All KCNEs are expressed in the human heart at the transcript level. Compared to I(to) 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.