FIGURE 4.

Force kinetics of cardiac myofibrils exchanged with NBD-cTn after rapid changes in [Ca²⁺]. (A) At t = 0 s, the [Ca²⁺] pCa was increased from >8 to 4.6. The Ca²⁺-induced force development (gray transient) is fitted by a monoexponential function (black line), which gives a rate constant of k_ACT of 0.7 s⁻¹. (B) At t = 0 s, the [Ca²⁺] was rapidly decreased from pCa 4.6 to pCa >8. The force decay after the Ca²⁺ reduction (gray transient) is fitted by a biphasic function (black line) that includes a linear and an exponential term to obtain the three kinetic parameters k_LIN = 0.35 s⁻¹, t_LIN = 0.38 s, and k_REL = 3.8 s⁻¹.