CARDIAC QUICK-RELEASE CONTRACTION MECHANOENERGETICS ANALYSIS USING A CARDIAC-MUSCLE CROSS-BRIDGE MODEL

Huxley's sliding filament cross-bridge muscle model coupled with paral lel and series elastic components was simulated to examine the conflic ting reports on the amount; of energy saved by quick release at the pe ak contraction time. Cross-bridge energy utilization was determined by considering the ATP hydrolysis for the cross-bridge cycling. The quic k-release cases were simulated by letting the muscle fiber suddenly sh orten to the resting fiber length at peak systole, and then the contra ction was allowed to continue at the resting length. Simulation result s demonstrated that, using realistic parameter values, typically simil ar to 15% of the muscle fiber energy is used after peak systole (and s imilar to 30% of the crossbridge energy), but this is also a function of the muscle fiber properties characterized by cross-bridge associati on and dissociation rate constants. Increasing the kinetic rate consta nts, the series elasticity, the initial fiber length, or the time of p eak intracellular calcium will increase the amount of energy left, whi ch may explain some of the discrepancies in the literature. Cardiac mu scle hypertrophy will increase the fraction of muscle fiber energy lef t after peak systole to similar to 30%. The strongest indicator of the percent energy left at peak systole was the time the fiber reached pe ak systole, and as the fiber reached peak systole faster, the amount o f energy saved by quick release increased.