The sarcomere responses to load changes consists of three distinctive quantifiable velocity dependent kinetics; A novel method for measuring cross-bridge kinetics.

Introduction: Sarcomere responses to load changes determine cardiac systolic and early diastolic functions and are critical in understanding various cardiomyopathies. However, there is no established method to quantify actomyosin cross-bridge (XB) kinetics, and the description of XB kinetics remain controversial. We have proposed that XB kinetics are sarcomer-velocity (V) dependent and hypothesize that the stress response consists of three measurable components: unitary XB force, XB recruitment, and sarcomere length changes.

Methods: Trabeculae were isolated from rat right ventricles. Sarcomere length was measured by laser diffraction (resolution: 3 nm). Ramp stretches (n=35) and releases (n=44) at different velocities (n=7) were applied during steady isometric contraction induced by tetanic stimulation (8-12 Hz) in the presence of cyclopiazonic acid (30 µM, [Ca2+]_o=1.5, 3, 4.5 and 6 mM]).

Results: The stress (σ) response consists of three distinct phases in the phase plot of dσ/dt vs. σ(t), with large difference between the three rate constants, as predicted theoretically. The first phase terminated within 4 msec, and related to the unitary XB dynamics. The second phase, tenfold slower, exhibited a rate constant of g₀+g₁V, where g₀=64.3±6.7sec^-1, and g₁=14.1±6.5 µm^-1. Theoretically, g₀ is the isometric XB depletion rate, and g₁ describes the effect of the velocity on XB depletion rate. The slowest, third phase was directly proportional to the velocity, and the rate of normalized stress change being 10.0±1.2 fold larger than the rate of change of sarcomere length change. Consequently, a 3% sarcomere lengthening can yield a 30% increase in the stress, following the termination of the first two phases involving changes in the force per XB and XB recruitment.

Conclusion: The good agreement between the theoretical and experimental data provides robust framework for quantifying XB kinetics. The XB parameters determine sarcomere power, economy and efficiency, offering valuable insight for diagnosing cardiomyopathies and evaluating therapeutic interventions.