*2.3. Current Recordings*

HiPSC-CMs in EHT were isolated with collagenase II for 5 h (200 U/mL, Worthington, LS004176 dissolved in HBSS-Puffer without Mg2<sup>+</sup> or Ca2<sup>+</sup>, Gibco 14175-053 and 1 mM HEPES; pH 7,4), and re-plated on gelatin-coated coverslips for 24–48 h in order to maintain adherence under perfusion. Outward K<sup>+</sup> currents were measured at 37 ◦C, using the whole-cell configuration of the patch clamp technique. Axopatch 200B amplifier (Axon Instruments, Foster City, CA, USA) and ISO2 software were used for data acquisition and analysis (MFK, Niedernhausen, Germany). Heat-polished pipettes were pulled from borosilicate filamented glass (Hilgenberg, Malsfeld, Germany). Tip resistances were 2.5–5 MΩ when filled with pipette solution. Seal resistances were 2–4 GΩ. The cells were investigated in a small perfusion chamber placed on the stage of an inverse microscope. Application of drugs was performed by a system for rapid solution changes (Cell Micro Controls, Virginia Beach, VA, USA; ALA Scientific Instruments, Long Island, NY, USA) [19]. The experiments were performed with the following bath solution (in mM): NaCl 120, KCl 5.4, HEPES 10, CaCl2 2, MgCl2 1 and glucose 10 (pH 7.4, adjusted with NaOH). Outward currents were elicited by 1000 ms depolarizing test pulses from −80 to +70 mV (0.2 Hz). The pipette solution included (in mM): DL-Aspartate potassium salt 80, KCl 40, NaCl 8, HEPES 10, Mg-ATP 5, Tris-GTP 0.1, EGTA 5 and CaCl2 4.4, pH 7.4, adjusted with KOH [20].

#### *2.4. AP Recordings*

To record APs in intact EHT and in left ventricular trabeculae we used sharp microelectrodes as reported previously [5,9,21]. Microelectrode tip resistances were 20–50 MΩ when filled with 3 mM KCl. APs were elicited by field stimulation at 1 Hz, 0.5 ms stimulus duration and 50% above threshold intensity. The following bath solution was used (in mM): NaCl 125, KCl 5.4, MgCl2 0.6, CaCl2 1, NaH2PO4 0.4, NaH2CO3 22 and glucose 5.5 and was equilibrated with O2–CO2 (95:5). The experiments were performed at 37 ◦C.

#### *2.5. Molecular Biology*

Total RNA was extracted from snap frozen LV and EHT using an RNAeasy mini Kit (Qiagen, Valencia, CA, USA). RNA concentration was determined per fluorometric quantitation with QubitTM (Thermo Fisher Scientific; Waltham, MA, USA) according to the manufacturer's instructions. Of total

RNA 50 ng was used for expression analysis by nanoString nCounter® SPRINT Profiler according to the manufacturer's instructions. Raw data were analyzed with nSolverTM Data Analysis Software including background subtraction using negative controls and normalization to two housekeeping genes (GAPDH and PGK1).
