Kinetics of cardiac sarcomeric processes and rate-limiting steps in contraction and relaxation

The sarcomere is the core structure responsible for active mechanical heart function. It is formed primarily by myosin, actin, and titin filaments. Cyclic interactions occur between the cross-bridges of the myosin filaments and the actin filaments. The forces generated by these cyclic interactions provide the molecular basis for cardiac pressure, while the motion produced by these interactions provides the basis for ejection. The cross-bridge cycle is controlled by upstream mechanisms located in the membrane and by downstream mechanisms inside the sarcomere itself. These downstream mechanisms involve the Ca(2+)-controlled conformational change of the regulatory proteins troponin and tropomyosin and strong cooperative interactions between neighboring troponin-tropomyosin units along the actin filament. The kinetics of upstream and downstream processes have been measured in intact and demembranated myocardial preparations. This review outlines a conceptual model of the timing of these processes during the individual mechanical heart phases. Particular focus is given to kinetic data from studies on contraction-relaxation cycles under mechanical loads. Evidence is discussed that the dynamics of cardiac contraction and relaxation are determined mainly by sarcomeric downstream mechanisms, in particular by the kinetics of the cross-bridge cycle. The rate and extent of ventricular pressure development is essentially subjected to the mechanistic principles of cross-bridge action and its upstream and downstream regulation. Sarcomere relengthening during myocardial relaxation plays a key role in the rapid decay of ventricular pressure and in early diastolic filling.