Contractility and Calcium Transients in hiPSC-CM

Contractility and intracellular calcium transient of hiPSC-derived cardiomyocytes were evaluated with the IonOptix myocyte calcium and contractility system (IonOptix, Milton, MA)^{24 (link)}. Briefly, hiPSC-derived cardiomyocytes cultured on gelatin-coated coverslips were loaded with 1 μM fura-2-AM (Sigma-Aldrich, St. Louis, MO), a calcium-sensitive, radiometric fluorescence dye for 15 min, and then transferred to a perfusion chamber mounted on the stage of an inverted microscope (Olympus, IX-51). The cells were then field stimulated via external platinum electrodes using the MyoPacer stimulator (Ionoptix, MA, USA) at 10 V and 1–4 Hz. Contraction of hiPSC-derived cardiomyocytes was assessed using a video-based edge detection system (IonOptix, MA, USA) to continuously measure the movement of a selected point of the hiPSC-derived cardiomyocytes at 240 Hz. The intracellular calcium transient (A340/380 ratio) was recorded with the MyoCam-S camera (Ionoptix, MA, USA) fixed to the inverted microscope. Data acquisition and analysis were performed using the IonWizard 6.3 software (IonOptix, Milton, MA). The indices measured included peak shortening, time-to-peak shortening, time-to-90% re-lengthening, and maximal velocity of shortening/re-lengthening (±dL/dt).