Kinetic coupling of phosphate release, force generation and rate-limiting steps in the cross-bridge cycle.

A basic goal in muscle research is to understand how the cyclic ATPase activity of cross-bridges is converted into mechanical force. A direct approach to study the chemo-mechanical coupling between P release and the force-generating step is provided by the kinetics of force response induced by a rapid change in [P]. Classical studies on fibres using caged-P discovered that rapid increases in [P] induce fast force decays dependent on final [P] whose kinetics were interpreted to probe a fast force-generating step prior to P release. However, this hypothesis was called into question by studies on skeletal and cardiac myofibrils subjected to P jumps in both directions (increases and decreases in [P]) which revealed that rapid decreases in [P] trigger force rises with slow kinetics, similar to those of calcium-induced force development and mechanically-induced force redevelopment at the same [P]. A possible explanation for this discrepancy came from imaging of individual sarcomeres in cardiac myofibrils, showing that the fast force decay upon increase in [P] results from so-called sarcomere 'give'. The slow force rise upon decrease in [P] was found to better reflect overall sarcomeres cross-bridge kinetics and its [P] dependence, suggesting that the force generation coupled to P release cannot be separated from the rate-limiting transition. The reasons for the different conclusions achieved in fibre and myofibril studies are re-examined as the recent findings on cardiac myofibrils have fundamental consequences for the coupling between P release, rate-limiting steps and force generation. The implications from P-induced force kinetics of myofibrils are discussed in combination with historical and recent models of the cross-bridge cycle.