CELL AND SARCOMERE CONTRACTILE PERFORMANCE FROM THE SAME CARDIOCYTE USING VIDEO MICROSCOPY

The relationship between whole cell and sarcomere contractile performa nce from within the same myocyte remains unclear. In the present study , the dynamic properties of whole cell and sarcomere contractile perfo rmance were examined from the same myocyte by computer-assisted video microscopy. Isolated canine left ventricular myocytes were field stimu lated at 1 Hz, and whole cell and sarcomere contractile performance wa s measured in the unloaded unattached state (n = 16) and after attachm ent to a basement membrane substrate (n = 18). Whole cell and sarcomer e contractile measurements were obtained immediately on initiation of electrical stimulation as well as at steady state, after which measure ments were repeated in the presence of 25 nM isoproterenol. Video-micr oscopic images of whole cell and sarcomere contractions were obtained at final magnifications of X1,100 and X5,500, respectively. By use of a 240-Hz high-scan-rate charge-coupled device camera and a video-based edge-detection system synchronized with the camera video output, the myocyte and sarcomere motion data were digitized. Steady-state percent age and velocity of shortening for whole cells and sarcomeres were 4.7 5 +/- 0.30% and 56.50 +/- 2.37 mum/s and 8.63 +/- 0.60% and 2.24 +/- 0 .46 mum/s, respectively, for the attached myocytes and 8.63 +/- 0.48% and 71.38 +/- 6.14 mum/s and 11.73 +/- 3.22% and 2.72 +/- 0.62 mum/s, respectively, for the unattached myocytes. With the initiation of elec trical stimulation, the extent of the shortening-velocity of relengthe ning relationship increased in a linear fashion for the attached (whol e cell, r = 0.87; sarcomere, r = 0.90; both P < 0.001) and unattached myocytes (whole cell, r = 0.83; sarcomere, r = 0.88; both P < 0.001). In all experiments, isoproterenol significantly increased the slope of these linear relationships (P < 0.01). Furthermore, the relationship between whole cell and sarcomere velocity of shortening was highly lin ear (r > 0.91, P < 0.001). In summary, this study demonstrated that th e video-based edge-detection technique could be adapted to measure cel l and sarcomere contractile performance from the same myocyte. Further more, a significant linear relationship exists between whole cell and sarcomere contractile dynamics with alterations in both load and inotr opic state.