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Fig. S1. IKr recorded from iPSC-cardiomyocytes using cesium method. Membrane currents of cardiomyocytes derived from the control and LQT2 cell lines were measured in the conventional whole-cell configuration at room temperature using an Axopatch 200B amplifier and pClamp 9.2 software (Molecular Devices, Sunnyvale, CA, USA). The extracellular solution used in these recordings contained (in mmol/L) 135 CsCl, 1 MgCl2, 10 glucose, 10 HEPES and 10 µmol/L nifedipine (pH 7.4 with CsOH; osmolarity adjusted to 301±3 mOsm). The recording pipettes were filled with a solution containing (in mmol/L): 135 CsCl, 1 MgCl2, 10 EGTA, 10 HEPES (pH 7.2 with CsOH; osmolarity adjusted to 290±3 mOsm). A & B: Whole-cell currents evoked from control (above) and LQT2 (below) human iPS derived cardiomyocytes by 1s depolarizing steps (from −70 to +70 mV). The boxed section of A, the repolarization to the holding potential of −80 mV, is expanded in B to show the inward tail currents. C: The voltage-dependence of activation of the Cs+ current. Tail current amplitudes were divided by the capacitance value (Cs) to give the tail current density which was plotted against the depolarizing voltage. Fitting of a Boltzmann function to the activation curves gave half-maximum activation voltages and slope factors of −39.5±1.5 and 8.1±1.3 mV for control (n=3) and −43.5±1.0 and 5.8±0.3 mV for LQT2 iPS cardiomyocytes (n=4; P>0.05). These values are similar to those obtained with hERG expressing HEK cells and rabbit ventricular myocytes, however the current densities are smaller (Zhang, 2006). The tail current density of control and LQT2 iPS cardiomyocytes after a depolarizing step to +20 mV was respectively 4.1±0.6 pA/pF (n=3) and 2.5±0.5 pA/pF (n=4; P>0.05).
