To monitor mitochondrial ROS and mitochondrial membrane potential, cells were incubated with 25 nM MitoTracker Red CM-H2XRos, λexc 580 nm and λem 600 nm (MTR, Thermo Fisher Scientific), or 25 nM tetramethylrhodamine, λexc 550 nm and λem 580 nm (TMRM, Thermo Fisher Scientific), respectively. Images were collected at four different time points.
To monitor cytosolic Ca2+ transients, cells were loaded with Fluo-4 acetoxymethyl ester, λexc 490 nm and λem 515 nm (Thermo Fisher Scientific), 0.01% w/v pluronic F-127 (Sigma), and 250 µM sulfinpyrazone (Sigma). To evaluate the release of Ca2+ from the sarcoplasmic reticulum (SR), a pulse of 10 mM caffeine was added to the cells.
To monitor mitochondrial Ca2+, hiPSC-CMs were infected with an adenovirus containing mito-GCaMP5G [38 (link)] and 48 h later imaged both at baseline and following a pulse of 10 mM caffeine. Results are expressed as fluorescence intensity ratio Fλ1/Fλ2, in which λ1 (~ 410 nm) and λ2 (~ 480 nm), respectively.