Background-Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K ϩ -current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy. Methods and Results-The whole-cell K ϩ currents I TO1 , I K1 , I Kr , and I Ks were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9Ϯ1 weeks of AVB and controls with sinus rhythm. I TO1 density and kinetics and I K1 outward current were not different between chronic AVB and control cells. I Kr had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I Kr density was similar in LV myocytes but smaller in RV myocytes (Ϫ45%) of chronic AVB versus control. For I Ks , voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I Ks densities of LV (Ϫ50%) and RV (Ϫ55%) cells were significantly lower in chronic AVB than control. Conclusions-Significant downregulation of delayed rectifier K ϩ current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects. (Circulation. 1999;100:2455-2461.)

In guinea-pig ventricular myocytes, in which the deactivation of slowly activating delayed rectifier potassium current (IKs) is slow, IKs can be increased by rapid pacing as a result of incomplete deactivation and subsequent current accumulation. Whether accumulation of IKs occurs in dogs, in which the deactivation is much faster, is still unclear. In this study the conditions under which accumulation occurs in canine ventricular myocytes were studied with regard to its physiological relevance in controlling action potential duration (APD). At baseline, square pulse voltage clamp experiments revealed that the accumulation of canine IKs could occur, but only at rather short interpulse intervals (< 100 ms). With action potential (AP) clamp commands of constant duration (originally recorded at rate of 2 Hz), an accumulation was only found at interpulse intervals close to 0 ms. Transmembrane potential recordings with high-resistance microelectrodes revealed, however, that at the fastest stimulation rates with normally captured APs (5 Hz) the interpulse interval exceeded 50 ms. This suggested that no IKs accumulation occurs, which was supported by the lack of effect of an IKs blocker, HMR 1556 (500 nM), on APD. In the presence of the beta-adrenergic receptor agonist isoproterenol (isoprenaline, 100 nM) the accumulation with AP clamp commands of constant duration was much more pronounced and a significant accumulating current was found at a relevant interpulse interval of 100 ms. HMR 1556 prolonged APD, but this lengthening was reverse rate dependent. AP clamp experiments in a physiologically relevant setting (short, high rate APs delivered at a corresponding rate) revealed a limited accumulation of IKs in the presence of isoproterenol. In conclusion, a physiologically relevant accumulation of IKs was only observed in the presence of isoproterenol. Block of IKs, however, led to a reverse rate-dependent prolongation of APD indicating that IKs does not have a dominant role at short cycle lengths.

Background-In large mammals and humans, the contribution of I Ks to ventricular repolarization is still incompletely understood. Methods and Results-In

Background-The ventricular action potential exhibits regional heterogeneity in configuration and duration (APD). Across the left ventricular (LV) free wall, this is explained by differences in repolarizing K ϩ currents. However, the ionic basis of electrical nonuniformity in the right ventricle (RV) versus the LV is poorly investigated. We examined transient outward (I TO1 ), delayed (I Ks and I Kr ), and inward rectifier K ϩ currents (I K1 ) in relation to action potential characteristics of RV and LV midmyocardial (M) cells of the same adult canine hearts. Methods and Results-Single RV and LV M cells were used for microelectrode recordings and whole-cell voltage clamping. Action potentials showed deeper notches, shorter APDs at 50% and 95% of repolarization, and less prolongation on slowing of the pacing rate in RV than LV. I TO1 density was significantly larger in RV than LV, whereas steady-state inactivation and rate of recovery were similar. I Ks tail currents, measured at Ϫ25 mV and insensitive to almokalant (2 mol/L), were considerably larger in RV than LV. I Kr , measured as almokalant-sensitive tail currents at Ϫ50 mV, and I K1 were not different in the 2 ventricles. Conclusions-Differences in Kϩ currents may well explain the interventricular heterogeneity of action potentials in M layers of the canine heart. These results contribute to a further phenotyping of the ventricular action potential under physiological conditions. (Circulation. 1999;99:206-210.)

Rationale:The mutation A341V in the S6 transmembrane segment of KCNQ1, the ␣-subunit of the slowly activating delayed-rectifier K ؉ (I Ks ) channel, predisposes to a severe long-QT1 syndrome with sympathetictriggered ventricular tachyarrhythmias and sudden cardiac death.Objective: Several genetic risk modifiers have been identified in A341V patients, but the molecular mechanisms underlying the pronounced repolarization phenotype, particularly during ␤-adrenergic receptor stimulation, remain unclear. We aimed to elucidate these mechanisms and provide new insights into control of cAMPdependent modulation of I Ks . Methods and Results:We characterized the effects of A341V on the I Ks macromolecular channel complex in transfected Chinese hamster ovary cells and found a dominant-negative suppression of cAMP-dependent Yotiao-mediated I Ks upregulation on top of a dominant-negative reduction in basal current. Phosphomimetic substitution of the N-terminal position S27 with aspartic acid rescued this loss of upregulation. Western blot analysis showed reduced phosphorylation of KCNQ1 at S27, even for heterozygous A341V, suggesting that phosphorylation defects in some (mutant) KCNQ1 subunits can completely suppress I Ks upregulation. Functional analyses of heterozygous KCNQ1 WT:G589D and heterozygous KCNQ1 WT:S27A, a phosphorylation-inert substitution, also showed such suppression. Immunoprecipitation of Yotiao with KCNQ1-A341V (in the presence of KCNE1) was not different from wild-type. Key Words: ion channels Ⅲ long-QT syndrome Ⅲ potassium Ⅲ torsade de pointes T he slowly activating delayed-rectifier K ϩ current (I Ks ) contributes importantly to cardiac repolarization. In large mammals, including humans, it has a small amplitude under basal isolated-myocyte conditions but forms a sizable repolarization reserve that is recruited when the action potential duration (APD) prolongs and during ␤-adrenergic receptor (␤AR) stimulation. 1,2 I Ks is carried by a macromolecular channel complex consisting of a homotetramer of pore-forming ␣-subunits encoded by KCNQ1 (Kv7.1), KCNE1 ␤-subunits, 3,4 and the regulatory A-kinase anchoring protein Yotiao, which binds to the KCNQ1 C-terminus. 5 There are also multiple interactions with other proteins. During ␤AR stimulation, when cAMP levels rise, phosphorylation of KCNQ1 at N-terminal position S27 is controlled by protein kinase A (PKA) and protein phosphatase 1 that are localized to the complex by Yotiao, thereby providing local control of I Ks enhancement. 5 Anchored PKA also phosphorylates Yotiao itself, thereby further enhancing I Ks . 6 An intact C-terminus of KCNE1 is critical for the PKA-dependent upregulation of I Ks . 7 Congenital defects (long-QT syndrome types 1 and 5; LQT1, LQT5; Jervell and Lange-Nielsen syndrome), pharmacological inhibition, 8 and acquired loss of I Ks , 9 can all result in QT-interval prolongation and enhanced susceptibility to ventricular tachyarrhythmias, notably torsade de pointes. These arrhythmias occur predominantly during conditions of exerc...