In vivo tracking of individual sarcomeres in an LV myocyte of a mouse with low systolic function. (A) Top: Confocal image of a myocyte expressing α-actinin-AcGFP in a mouse with ΔLVP 60.5 mm Hg. During six cardiac cycles, ΔLVP was 60.5 ± 4.2 mm Hg, and HR was 581 ± 6.6 bpm (Table 1). In 2 myofibrils, 16 and 14 sequentially connected sarcomeres (in yellow-outlined rectangles) were analyzed for M1 and M2, respectively. Numbers indicate longitudinal positions of sarcomeres. Scale bar, 10 µm. See Video 3. Bottom: Color diagram showing CI between average and individual SLs during six cardiac cycles for M1 and M2. Sarcomere numbers are indicated on top and cardiac cycle numbers on left. Data on the average of six cycles are shown on bottom. (B) Top: Same as in A, in a mouse with ΔLVP 40.6 mm Hg. During six cardiac cycles, ΔLVP was 40.6 ± 2.5 mm Hg, and HR was 347 ± 7.9 bpm (Table 1). In 2 myofibrils, 32 and 35 sequentially connected sarcomeres (in yellow-outlined rectangles) were analyzed for M1 and M2, respectively. Numbers indicate longitudinal positions of sarcomeres. Scale bar, 10 µm. See Video 4. Bottom: Color diagram showing CI between average and individual SLs during six cardiac cycles for M1 and M2. Sarcomere numbers are indicated on top, and cardiac cycle numbers on left. Data on the average of six cycles are shown on bottom.