Mechanical and biochemical cycles in muscle contraction

The current biochemical approach to understanding the mechanism by which chemical energy is converted into mechanical energy in muscle is by investigating the kinetics of the reaction of ATP with the isolated contractile proteins in solution with stopped and quenched flow techniques. The results are interpreted in terms of a cycle of hydrolysis of ATP. The current mechanical approach lies through the kinetics of force production or length change in preparations with the contractile proteins intact, by following step changes in length or tension. The results are usually interpreted in terms of mechanical cycles of activity of cross-bridges, in which a cross-bridge progresses through a series of states with different mechanical properties. If the biochemist's and the mechanist's cycles are one and the same, a problem arises over the correspondence between states in the two cycles. One approach to answering this question is through the determination of the effect of pH and of such substances as ATP, ADP and inorganic orthophosphate on the mechanical response. We hope that these will act at known points in the biochemist's cycle and their influence upon the mechanical cycle should enable their location here to be ascertained. The paper deals with the effects of phosphate, ATP and magnesium ions on the mechanical response of insect flight muscle.