Figure 1.
Thermal activation of fast skeletal myofibrils immersed in relaxing solution. (A) Schematic illustration of the experimental system. A rabbit psoas myofibril was immersed in relaxing solution on a coverslip. Temperature was directly increased by an IR laser beam (λ = 1,455 nm), which elicits sarcomere shortening as a function of the distance from the heat source (as shown by the red-yellow gradient; see the color bar on the right). (B) Changes in temperature (ΔT) generated by IR-laser irradiation. Three levels of gradients were adjusted by neutral density filters: 40%, 25%, and 15% for high, medium, and low powers, respectively. Data expressed as mean ± SEM. Error bars are <0.2°C (within plots). n = 3 for all power levels. (C) Left: Phase-contrast images of a skeletal myofibril in the relaxing solution (pCa 9). Top, middle, and bottom show images before, during, and 1 s after heating for 2 s, respectively. Sarcomeres within the yellow-outlined rectangle were analyzed. Pink and blue arrows in each image indicate the positions of Z-disks along a myofibril analyzed. Imaging was performed at 30 fps. Right: Intensity profiles of four sarcomeres along a myofibril shown left. Starting point on the x-axis (i.e., 0 µm) indicates the position of the Z-disk shown by a pink arrow before heating. The heat source was ∼28 µm away from the myofibril. ΔT was 17.1°C (cf. temperature gradient for high power in B). (D) Relationship of the temperature during heating versus changes in sarcomere length (ΔSL). ΔSL was calculated as SL before heating minus SL during heating. Each plot in the graph indicates the average shortening of sequentially connected four to six sarcomeres along a myofibril. n = 20, 10, and 11 myofibrils at 31.6 ± 0.1°C, 38.9 ± 0.3°C, and 41.5 ± 0.2°C, respectively. Data expressed as mean ± SEM. P is determined by Dunnett’s multiple comparison test. *P < 0.05. NS, not statistically significant. (E) Arrhenius plot for ΔSL. ΔSL is expressed in logarithm. Average values from D were used. ΔSL = 1.08 × 1026 exp (−1.65 × 105/RT) (r2 = 1.00). T, absolute temperature. R, gas constant. r2, coefficient of determination. Experiments were performed at 25 ± 1°C.

Thermal activation of fast skeletal myofibrils immersed in relaxing solution. (A) Schematic illustration of the experimental system. A rabbit psoas myofibril was immersed in relaxing solution on a coverslip. Temperature was directly increased by an IR laser beam (λ = 1,455 nm), which elicits sarcomere shortening as a function of the distance from the heat source (as shown by the red-yellow gradient; see the color bar on the right). (B) Changes in temperature (ΔT) generated by IR-laser irradiation. Three levels of gradients were adjusted by neutral density filters: 40%, 25%, and 15% for high, medium, and low powers, respectively. Data expressed as mean ± SEM. Error bars are <0.2°C (within plots). n = 3 for all power levels. (C) Left: Phase-contrast images of a skeletal myofibril in the relaxing solution (pCa 9). Top, middle, and bottom show images before, during, and 1 s after heating for 2 s, respectively. Sarcomeres within the yellow-outlined rectangle were analyzed. Pink and blue arrows in each image indicate the positions of Z-disks along a myofibril analyzed. Imaging was performed at 30 fps. Right: Intensity profiles of four sarcomeres along a myofibril shown left. Starting point on the x-axis (i.e., 0 µm) indicates the position of the Z-disk shown by a pink arrow before heating. The heat source was ∼28 µm away from the myofibril. ΔT was 17.1°C (cf. temperature gradient for high power in B). (D) Relationship of the temperature during heating versus changes in sarcomere length (ΔSL). ΔSL was calculated as SL before heating minus SL during heating. Each plot in the graph indicates the average shortening of sequentially connected four to six sarcomeres along a myofibril. n = 20, 10, and 11 myofibrils at 31.6 ± 0.1°C, 38.9 ± 0.3°C, and 41.5 ± 0.2°C, respectively. Data expressed as mean ± SEM. P is determined by Dunnett’s multiple comparison test. *P < 0.05. NS, not statistically significant. (E) Arrhenius plot for ΔSL. ΔSL is expressed in logarithm. Average values from D were used. ΔSL = 1.08 × 1026 exp (−1.65 × 105/RT) (r2 = 1.00). T, absolute temperature. R, gas constant. r2, coefficient of determination. Experiments were performed at 25 ± 1°C.

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