Figure 1: Schematic illustration indicating the structure of a cardiac sarcomere associated with T-tubules. As discussed in [33], T-tubules and Z-disks are considered to run in parallel in cardiac muscle, causing Ca2+ sparks at/near the Z-disks. Thick and thin filaments, and titin are shown in this illustration (for simplicity, only two titin molecules per half thick filament are shown) (cf. [37] for electron microscopic images of titin molecules in the sarcomere). Also, troponin and tropomyosin are bound to the thin filaments. As described in detail in earlier papers (e.g., [12, 13]), I-band titin is in a contracted state at the slack SL; straightening of the tandem Ig segment and, then, extension of the PEVK and N2B segments are thought to occur (resulting in passive force generation) in response to stretch. In the lower part of the figure, the antibodies we used in the present work and the structure of the QD (Qdot 655 Invitrogen, Carlsbad, CA, USA) are shown (number indicating the emission wavelength). The anti-α-actinin antibody-QDs bind to Z-disks, and anti-α 1B-adrenergic receptor (AR) antibody-QDs bind to the T-tubules.