FIGURE 4.

Single myofibril activation/relaxation kinetics. Single myofibril activation/relaxation and release-restretch kinetics were measured in human recombinant troponin-exchanged isolated single myofibrils. A summarizes the average rate of force development upon a rapid (<5-ms) switch of bathing [Ca²⁺] from pCa = 10 to pCa = 4.5 (k_Ca; left panel) and the rate of force redevelopment following a rapid release-restretch maneuver during steady-state force development (k_tr). There were no significant differences between the experimental groups, indicating that neither RCM nor PKA/PKC directly affect cross-bridge cycle kinetics. B, original recordings of force relaxation upon a rapid switch of bathing [Ca²⁺] from pCa = 4.5 to pCa = 10. Force relaxation took place in two distinct phases, a slow almost linear decline followed by a rapid pseudo-exponential decline back toward baseline (indicated by the thin horizontal lines); note that the force traces have been offset vertically to aid visualization of the individual traces. Two parameters of the slow force relaxation, k_lin and the T_lin, were measured as indicated; the rate of fast relaxation was estimated by exponential fit of the force data (not shown). C, average slow force relaxation fit parameters (k_lin and T_lin). Experiments were performed at 15 °C. Data are presented as mean ± S.E. *, p < 0.05 versus WT (RCM, n = 7; all other groups, n = 6). Error bars reflect S.E.