Cross-Bridge Dynamics is Determined by Two Velocity Dependent Kinetics; Implications on the Adaptive and Synchronous Cardiac Function

Introduction: The cardiac muscle regulates energy consumption to match function with the demands. The length, stress, and velocity of shortening were suggested as possible modulators of cross-bridge (XB) dynamics.
Methods: The study tested the effects of sarcomere length and the shortening velocity (n=9) on XB dynamics in the intact rat trabeculae. Sarcomere length was measured by laser diffraction and ramp shortenings at various velocities were imposed with a fast servomotor.
Results: Stress responses to ramp length changes revealed two distinct kinetics: a fast (3 msec) and a slower phase. The rate coefficients of the two phases were independent of the initial length or stress level but were linearly dependent on the shortening velocity. The fast kinetics determines the force per XB and the second is ascribed to XB cycling and determines the number of strong XBs.
Conclusion: XB dynamics is modulated by the velocity and not by length or stress. These features shed light on theories of muscle contraction, and are essential for the synchronous activity of all XBs and adaptive regulation of function to match the demands.