Measuring Cardiomyocyte Electrophysiology

In cardiomyocytes freshly isolated from patient ventricular tissue, we simultaneously measured membrane potential and calcium transients using perforated patch whole-cell current-clamp combined with fluorescence recordings after loading with the Ca²⁺-sensitive fluorescent dye Fluoforte (Enzo Life Sciences, Farmingdale, NY, USA), as previously described [31 (link)]: fluorescence was measured at 515 ± 10 nm during excitation at 490 ± 8 nm. For hiPSC-CMs, we used whole-cell current clamp to measure APs; the pipette solution contained (in × 10⁻³ M) 115 K methanesulfonate, 25 KCl, 10 (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) (HEPES), 3 MgCl₂, and cells were perfused with Tyrode buffer containing 1.8 × 10⁻³ M CaCl₂. APs were elicited with short depolarizing current pulses (< 3 ms) at 1 Hz frequency. Action potentials were analyzed for MDP (mV), amplitude (mV), and action potential duration (ADP50 and APD90, ms) using the Clampfit 10.7 software (Molecular Devices, San Jose, CA, USA).