The ATPase activity in isometric and shortening skeletal muscle fibres

Muscle proteins utilise the hydrolysis of ATP to provide the energy for force development and the production of mechanical work. We have developed a technique with high sensitivity and time resolution to probe as directly as possible the link between ATPase activity, force development and muscle shortening. The ATPase activity was recorded in real time during contraction and shortening of permeabilised muscle fibres of rabbit skeletal muscle by measuring fluorescence changes associated with the binding of inorganic phosphate, a product of ATPase activity, to a genetically engineered phosphate binding protein labelled with a coumarin fluorophore. The muscle shortening velocity was found to affect directly the ATPase activity, with up to a five-fold increase during shortening at moderate velocities, and a decrease in activity during slow stretch.