Fig. 4.

Representative tension records of fully and partially activated myofibrils at 10℃. Myofibrils were activated from pCa 9.00 to pCa 4.50 or 6.50 by switching of the flow pipette as described in text. Activation occurs at time zero. Arrows indicate the point of activation/relaxation. The tension level prior to activation was taken as the baseline. Measurements of kact and krel were made by fitting a single exponential to the tension record. Data filtered using a low pass filter (half-power point = 2, Sigma Plot 9.0). a Records shown are of a CNTL myofibril (heavy trace), and a mixREC myofibril (medium trace) fully activated at pCa 4.50. Tensions were 262 and 204 mN mm⁻² respectively. b Same records as in a, but tensions have been normalized for comparison, and the time axis expanded. Exponential fits to the CNTL (solid line, R² = 0.996) and mixREC (dashed line, R² = 0.998) data give kact rates of 5.58 and 4.91 s⁻¹, respectively. c Same records as in a, but tensions have been normalized for comparison, and the time axis expanded. Exponential fits to the CNTL (solid line, R² = 1.00) and mixREC (dashed line, R² = 0.98) data give krel rates of 23 and 19 s⁻¹, respectively. d Records shown are of a CNTL myofibril (heavy trace), and a mixREC myofibril (medium trace), partially activated to pCa 6.50. Tensions were 51 and 210 mN mm⁻², respectively. e Same records as in d, but tensions have been normalized for comparison and the time axis expanded. Exponential fits to the CNTL (solid line, R² = 0.94) and mixREC (dashed line, R² = 1.00) data give kact rates of 1.07 and 3.79 s⁻¹, respectively. f Same records as in c, but tensions have been normalized for comparison and the time axis expanded. Exponential fits to the CNTL (solid line, R² = 0.98) and mixREC (dashed line, R² = 0.95) data give k_rel rates of 33 and 35 s⁻¹, respectively; 173 × 197 mm (150 × 150 DPI)