Human tissue samples
Cardiac samples were obtained from 6 organ donors (2 male) at the University of Kentucky (Table 1). Their mean age was 45.5 (range 10-61) years. As previously described in detail (Blair et al., 2016), hearts were passed to a researcher as soon as they were excised from the body, immediately placed in ice cold saline slush and transported back to the laboratory where tissue samples (~500 mg) were snap frozen in liquid nitrogen and stored at -150°C within ~20 minutes. Mid-myocardial sections of the left ventricular free wall were shipped overnight on dry ice to Washington State University and stored at -80°C for 1-3 weeks, until they were dissected for mechanics and biochemical experiments. All procedures were approved by the University of Kentucky Institutional Review Board.