Experiments were performed as previously described7 with some modification. Contractility was measured in custom-fabricated cell chambers (Ionoptix) mounted on an LSM Zeiss 880 inverted confocal microscope using a 40x oil 1.4 NA objective and transmitted light camera (IonOptix MyoCam-S). Myocytes were maintained in NT solution (for freshly isolated myocytes) or culture medium (without cytochalasin D, for cultured myocytes) at room temperature and electrical field stimulation was provided at 0.5 Hz with a myopacer (IonOptix MYP100) through platinum electrodes lowered into the bath. Sarcomere length was measured optically by Fourier transform analysis (IonWizard, IonOptix). After 10–30s of 0.5 Hz pacing to achieve steady state, five traces were recorded and analyzed. If not specified, contractility data was obtained at room temperature. The number of myocytes and hearts used in each experiments and further details are listed in Supplementary Table 6 .
To test whether the contractile improvement with MT destabilization remains under more physiological conditions, both 0.5 Hz and 1Hz contractions at 37˚C were recorded and analyzed in a small subset of isolated human myocytes (1 NF and 1 failing heart,Supplementary Fig. 5c ).
To test whether the contractile improvement with MT destabilization remains under more physiological conditions, both 0.5 Hz and 1Hz contractions at 37˚C were recorded and analyzed in a small subset of isolated human myocytes (1 NF and 1 failing heart,