Aim: 

The aim of this work is to define the coupling between chemical and mechanical steps of actomyosin ATPase cycle in muscle.

Methods: 

In Ca²⁺ activated skinned fibres from rabbit psoas (sarcomere length 2.5 mm, temperature 12°C), we have determined the effects of the concentration of inorganic phosphate (Pi) on the parameters of the force-velocity relation (T-V). The rise of [Pi] to 10 mM decreased the isometric plateau force (T₀) by 60% and increased a/T₀ of the T-V relation and the unloaded shortening velocity (V₀) by 40% and 19%, respectively. These effects can be reproduced by an actomyosin ATPase cycle where the increase in [Pi] enhances the probability of motor detachment from actin at an early stage of the working stroke, with the hydrolysis products still bound to the catalytic site. The hypothesis is tested by determining the effect of Pi on the isotonic velocity transient following a stepwise drop in force from T₀ (Piazzesi et al. J Physiol 545:145, 2002).

Results: 

The rate of the early fast shortening (phase 2, due to the synchronous execution of the working stroke by attached motors) is not significantly affected by the increase in [Pi], while its size is reduced by about 2 nm and the subsequent pause in shortening (phase 3) becomes briefer.

Conclusion: 

These results indicate that Pi release is not associated to the execution of the working stroke but to the transitions responsible for working stroke regeneration. Supported by NIH (USA), MIUR and Ministero della Salute (Italy).