The effect of sarcomere shortening velocity on force generation, analysis, and verification of models for crossbridge...

The study tests the hypothesis that the transition rate (G) of the cardiac cross-bridge (XB) from the strong force generating state to the weak state is a linear function of the sarcomere shortening velocity (V(SL)). Force (F) was measured with a strain gauge in six trabeculae from the rat right ventricle in K-H solution [(Ca]0 = 1.5 mM, 25 degrees C). Sarcomere length (SL) was measured with laser diffraction techniques. Twitch F at constant SL and the F response to shortening at constant V(SL) (0-8 microm/s; deltaSL 50-100 nm) were measured at varied times during the twitch. The F response to shortening consisted of an initial fast exponential decline (tau = 2 ms), followed by a slow decrease of F. The instantaneous difference (deltaF) between the isometric F (F(M)) and F during the slow phase depended on the duration of shortening (deltat), the instantaneous F(M) and V(SL). deltaF = G1 x F(M) x deltat x V(SL) x (1 -V(SL)/V(MAX)), where V(MAX) is the unloaded V(SL) and G1 was 6.15+/-2.12 microm(-1) (mean +/- s.d.; n=6). DeltaF/F(M) was independent of the time onset of shortening. The linear interrelation between deltaF and V(SL) is consistent with the suggested feedback, whereby XB kinetics depends on V(SL). This feedback provides a more universal description of the interrelation between shortening and force, as well as the observed linear relation between energy consumption and the mechanical energy output.