403/+ cardiomyocytes have higher end-diastolic and end-systolic stiffness. (a) Representative data of effective cell length measurements (distance between the CF tips at the cell-attached end) of a cardiomyocyte undergoing voltage stimulation and contraction at 1 Hz, with the application of preload. Initially, the cell is at resting length (time segment R); increasing levels of preload are applied from time segments P1–P4. Dotted lines and dots indicate end-diastolic (blue) and end-systolic (red) lengths, respectively, and correspond to EDL and ESL in . (b) End-diastolic (blue) and end-systolic (red) points at rest (R) and per level of preload (P1–P4) are plotted as normalized force against normalized effective cell length. End-diastolic force-length relation (EDFLR, blue) and end-systolic force-length relation (ESFLR, red) are obtained from linear regressions of end-diastolic and end-systolic points, respectively. (c) EDFLRs of +/+ (solid circles, solid lines) and 403/+ (open circles, dotted lines) cardiomyocytes. The slope of EDFLR for 403/+ cardiomyocytes (0.36 mN/mm2) is steeper than that for +/+ cardiomyocytes (0.22 mN/mm2), indicating that 403/+ cardiomyocytes have higher end-diastolic stiffness. (d) Similarly, ESFLR of 403/+ cardiomyocytes (slope = 0.50 mN/mm2) is steeper than that for +/+ cardiomyocytes (slope = 0.37 mN/mm2), indicating higher end-systolic stiffness.