Fig. 3. The contractile activity of GFP-myosin-expressing cells. (A) DIC image of a small area of an R403Q mutant GFP-myosin-expressing cardiomyocyte showing a clearly defined myofibril. Two z-lines spaced 7 sarcomeres apart are marked with small bulls eyes. The fluorescent cell was selected then the DIC image was recorded at 30 frames/second. (B) The positions of the two marked z-lines were tracked over time and the relative length changes plotted. The velocities of the contraction and relaxation phases (dL/dT) were determined and summarized in Table 1. Cells were maintained at 30°C on a heated microscope stage. Movies of this cell and another can be found at http://jcs.biologists.org/supplemental.