In vivo tracking of individual sarcomeres along myofibrils in LV myocytes of mice at various systolic states. (A) Top: Confocal image of a myocyte expressing α-actinin-AcGFP in the left ventricle of a mouse at ΔLVP 107.7 mm Hg (same myocyte as in Figs. 1 A and 3 A). Within the yellow-outlined rectangles, 8 and 24 sequentially connected sarcomeres were analyzed for M1 and M2, respectively. Left and right arrowheads indicate sarcomere nos. 1 and 8 (or 24), respectively. Bottom: Color diagram showing CI between average and individual SLs during six cardiac cycles for M1 and M2. Average of six cycles is shown. (B) Top: Same as in A, with ΔLVP at 90.4 mm Hg (∼0.5 mm distal to the myocyte in A from the same mouse). Within the yellow-outlined rectangles, five, five, and six sequentially connected sarcomeres were analyzed for M1, M2, and M3, respectively. Arrowheads indicate sarcomere nos. 1 and 5 (or 6), respectively. Bottom: Same as in A, during six cardiac cycles for M1, M2, and M3. Average of six cycles is shown. (C) Confocal image of a myocyte expressing α-actinin-AcGFP in the left ventricle of a mouse at ΔLVP 87.2 mm Hg (∼0.5 mm distal to the myocyte shown in Fig. S6 A). Within the yellow-outlined rectangles, seven, six, and seven sequentially connected sarcomeres were analyzed for M1, M2, and M3, respectively. Arrowheads indicate sarcomere nos. 1 and 7 (or 6), respectively. Bottom: Color diagram showing CI between average and individual SLs during seven cardiac cycles for M1, M2, and M3. Average of seven cycles is shown. (D) Same as in Cat ΔLVP 70.5 mm Hg (∼0.5 mm distal to the myocyte in C or that in Fig. S6 A from the same mouse). Within the yellow-outlined rectangles, six, seven, and six sequentially connecting sarcomeres were analyzed for M1, M2, and M3, respectively. Arrowheads indicate sarcomere nos. 1 and 6 (or 7), respectively. Bottom: Same as in C, during eight cardiac cycles for M1, M2, and M3. Average of eight cycles is shown. In A–D, CI is shown based on a color scale (i.e., from 1 [dark blue] to −1 [dark red]). Scale bars, 10 µm. (E) Relationship between ΔLVP and the fraction of contributing sarcomeres in myofibrils from mice at various ΔLVP values. Closed circles, myofibrils in Fig. S15 A (135.7 mm Hg); closed squares, myofibrils in A (107.7 mm Hg), Fig. 3 A (96.4 mm Hg) and B (90.4 mm Hg); closed diamonds, myofibrils in C (87.2 mm Hg) and D (70.5 mm Hg) and Fig. S6 A (60.5 mm Hg); open circles, myofibrils in Fig. S6 B (40.6 mm Hg); open squares, myofibrils in Fig. S18 A (7.2 mm Hg); open diamonds, myofibrils in Fig. S18 B (4.7 mm Hg). Contributing sarcomeres are defined as those with CI >0.2 in the average data of three to eight cardiac cycles. A significant linear relationship was observed (R = 0.75; P < 0.001). (F) Relationship between ΔLVP and the average values of CI (Avg. CI) in myofibrils from mice with various ΔLVP values. Symbols, same as in E. Error bars indicate SD of Avg. CI (compare Fig. 4 D). A significant linear relationship (R = 0.89; P < 0.001) was observed. In E and F, two myofibrils were analyzed for ΔLVP 107.7, 96.4, 60.5, 40.6, and 7.2 mm Hg, and three myofibrils were analyzed for 135.7, 90.4, 87.2, 70.5, and 4.7 mm Hg (see above for corresponding figures).